Review article

Vitamin D and cancer: a review of molecular mechanisms

James C. Fleet, Marsha Desmet, Robert Johnson, Yan Li


The population-based association between low vitamin D status and increased cancer risk can be inconsistent, but it is now generally accepted. These relationships link low serum 25OHD (25-hydroxyvitamin D) levels to cancer, whereas cell-based studies show that the metabolite 1,25(OH)2D (1,25-dihydroxyvitamin D) is a biologically active metabolite that works through vitamin D receptor to regulate gene transcription. In the present review we discuss the literature relevant to the molecular events that may account for the beneficial impact of vitamin D on cancer prevention or treatment. These data show that although vitamin D-induced growth arrest and apoptosis of tumour cells or their non-neoplastic progenitors are plausible mechanisms, other chemoprotective mechanisms are also worthy of consideration. These alternative mechanisms include enhancing DNA repair, antioxidant protection and immunomodulation. In addition, other cell targets, such as the stromal cells, endothelial cells and cells of the immune system, may be regulated by 1,25(OH)2D and contribute to vitamin D-mediated cancer prevention.

  • apoptosis
  • 1,25-dihydroxyvitamin D
  • 25-hydroxy-vitamin D (25OH D)
  • proliferation
  • transcription
  • vitamin D receptor


Over the past decade, researchers have generated data demonstrating that vitamin D and its metabolites have actions that may be useful for the prevention or treatment of various cancers. This is a new role for vitamin D that is distinct from the traditional role it has in the control of calcium and bone metabolism [1].

The first suggestion that vitamin D might influence cancer came in 1980 when Garland and Garland [2] proposed that the high rate of colon cancer seen in the Northern U.S. compared with the Southern U.S. was due to the UV light-induced production of vitamin D in the skin. Ecological studies have since extended the ‘sunlight’ hypothesis to 18 different types of cancer [3]. Although ecological studies like these are the weakest forms of scientific evidence, several other lines of evidence suggest that vitamin D or its metabolites have a direct inhibitory action on the development and progression of various cancers. Some population-based studies show that low serum 25OHD (25 hydroxyvitamin D) levels are associated with an increased risk of cancers of the colon, [4], breast [5] and prostate [6,7], as well as other cancers [8]. Studies in animals have shown that severe vitamin D deficiency [911], or deletion of the VDR (vitamin D receptor) gene [1214], increase cancer risk. In addition there are a number of studies that show a reduction in cancer (tumour incidence or tumour size) in animals injected with chemical analogues of the vitamin D hormone 1,25(OH)2D (1,25-dihydroxyvitamin D) [1520]. Although there is a growing consensus that vitamin D and its metabolites are important for the control of various cancers, the mechanistic foundation for this protection is still being determined. In the present review we summarize the current thinking on the mechanisms used by vitamin D metabolites to influence the development of cancer.


Vitamin D3 can be produced from 7-dehydrocholesterol when skin is exposed to UVB light. However, regardless of whether vitamin D comes from the skin or the diet, vitamin D3 is transported in the circulation by the DBP (vitamin D-binding protein) [21]. Once delivered to the liver, vitamin D is hydroxylated on its side chain to form 25OHD. This is a stable metabolite whose serum levels are used to assess vitamin D status. The kidney is the primary site where the active form of vitamin D, 1,25(OH)2D, is produced. The circulating levels of 1,25(OH)2D result from CYP27B1 (cytochrome p450 27B1)-mediated conversion of 25OHD in the proximal tubule of the kidney [22]. Renal CYP27B1 gene expression is activated by PTH (parathroid hormone) and suppressed by 1,25(OH)2D and the serum levels of these hormones are inversely related to dietary calcium intake [23]. Once produced in the proximal tubule, 1,25(OH)2D is released into the serum and acts as an endocrine hormone on the intestine, bone and kidney to control calcium metabolism. Although serum 1,25(OH)2D levels do not associate with cancer risk in human studies, this is the metabolite responsible for the anticancer actions of vitamin D at the cellular level. In addition to the CYP27B1 activity found in the kidney, low levels of CYP27B1 protein and message have been observed in many tissues, including cells of the skin, lymph nodes, colon, pancreas, adrenal medulla, brain, placenta [24], in primary PECs (prostate epithelial cells) [25] in the MCF-7 breast cancer cell line [26], and in the colon cancer cell lines HT-29 and Caco-2 [27]. Extrarenal production of 1,25(OH)2D is proposed to be driven by the serum 25OHD level, thereby accounting for why serum levels of this inactive metabolite could associate with cancer risk. This is consistent with studies that show that CYP27B1 operates well below its Km value [28], the fact that 1,25(OH)2D production is seen in cultured non-renal cells treated with 25OHD [29,30] and the observation that targeted disruption of the CYP27B1 gene in Ras-transformed keratinocytes blocks the antiproliferative and prodifferentiating effects of 25OHD in vitro [31]. However, the weakness of this hypothesis is that no direct evidence currently exists to prove that meaningful local production occurs in vivo.

Molecular mechanism of 1,25(OH)2D signalling

The primary molecular action of 1,25(OH)2D is to initiate or suppress gene transcription by binding to the VDR, a member of the steroid hormone receptor superfamily of ligand-activated transcription factors [32] (Figure 1A). The VDR can be found in both the cytoplasm and nucleus of vitamin D target cells, but in many cells it is predominantly a nuclear protein. Binding of 1,25(OH)2D to the VDR promotes association of VDR with the RXR (retinoid X receptor) and this interaction is essential for VDR transcriptional activity. Some data demonstrates that heterodimerization is required for migration of the RXR–VDR–ligand complex from the cytoplasm to the nucleus [3336] where the 1,25(OH)2D–VDR–RXR complex binds to VDREs (vitamin D-response elements) in DNA to initiate gene transcription [32]. However, recent data from studies using ChIP-seq (chromatin immunoprecipitation followed by DNA sequencing) to identify VDR-binding sites throughout the genome show that RXR can be bound to VDR-binding sites in DNA prior to VDR recruitment to those sites [37]. ChIP-seq analysis conducted in osteoblasts [37] and in lymphoblastoid cells [38] also reveal that the great majority of genomic sites occupied by VDR after 1,25(OH)2D treatment are likely to be acting as enhancer elements; only 13–23% of VDR-binding sites are within the classical promoter regions of genes just proximal to the transcription start site (Figure 1B). Regardless, once bound to DNA the VDR–RXR heterodimer recruits protein complexes that alter chromatin structure. For transcriptional activation these proteins form a complex with HAT (histone acetyltransferase) activity {e.g. CBP [CREB (cAMP-response-element-binding protein)-binding protein]/p300 and SRC1 (steroid receptor co-activator 1) [39,40]} as well as ATP-dependent remodelling activity (e.g. the BAF57 subunit of SWI/SNF directly interacts with SRC1 and steroid hormone receptors [41]) to release a higher-order chromatin structure that limits gene transcription. After chromosomal unwinding, the VDR–RXR dimer recruits the mediator complex to the promoter and utilizes it to recruit and activate the basal transcription unit containing RNA polymerase II [42]. For transcriptional repression the VDR–RXR dimer recruits co-repressors such as NCoR (nuclear receptor co-repressor) 1, NCoR2/SMRT (silencing mediator of retinoid and thyroid receptors) and Alien that then recruit histone deacetylases and DNA methyltransferases that alter histone tails in ways that lead to a more compact chromatin structure [43] (Figure 1A). For those interested in more details regarding the molecular activation of VDR and its role in gene transcription, the subject has been reviewed by Pike et al. [37,44]. In addition to the transcriptional regulation mediated through the VDR, there is some evidence that 1,25(OH)2D may work in other novel ways. These mechanisms are less well described but, where relevant, they will be presented in the following sections.

Figure 1 Summary of vitamin D-mediated gene transcription through the VDR

(A) Schematic diagram showing alternative models for binding of VDR–RXR dimers to DNA and the regulation of gene transcription by vitamin D. In model A, RXR (yellow hexagon) is resident. VDR (green rectangle) binds after being activated by ligand, releasing any co-repressor complexes if they are present. In model B, VDR and RXR dimerize then bind to DNA. Both model A and B lead to the recruitment of additional co-activators, e.g. p300/CBP (pink star) and SRC-1 (orange oval) for histone acetylation, ATP-dependent chromatin remodelling factors like SWI/SNF, the mediator complex (dark purple rectangle) for recruitment and activation of RNA polymerase II (light blue oval). Model C shows that the vitamin D–VDR complex can also recruit co-repressor complexes with histone deacetylase activity (purple oval) and DNA methyltransferase activity (green circle) to gene promoters. This will suppress gene transcription. (B) Schematic diagram showing the distribution of VDR-binding sites in the genome of osteoblasts [211] and lymphoblastoid cells [38] relative to the transcription start site (TSS) of a gene. Values are given as a percentage of total VDR-binding sites and were determined by chromatin immunoprecipitation coupled to DNA arrays (ChIP-chip) or ChIP-seq.


Many believe that the target cells for the anticancer action of vitamin D are tumour cells and the normal cell types within tissues that transform into tumour cells. In this context the best-studied role of 1,25(OH)2D is its growth-inhibitory effects on proliferating epithelial cells. Colston et al. [45] first showed a dose-dependent decrease in growth rate of melanoma cells treated with 1,25(OH)2D. The growth-inhibitory property of 1,25(OH)2D has since been reported in tumour-derived cells from other tissues, including the colon [46], breast [47] and prostate [48]. The VDR is essential for 1,25(OH)2D-mediated growth inhibition. In both mouse epidermal keratinocytes [49] and SaOS-2 osteosarcoma cells [50] 1,25(OH)2D-induced growth arrest is lost when VDR is absent. Also, antisense oligonucleotides that reduce cellular VDR levels can block 1,25(OH)2D-induced growth arrest in ALVA-31 prostate cancer cells [51] and overexpression of VDR enhanced 1,25(OH)2D-induced growth arrest in several prostate cancer cell lines [52,53]. Recently we reported that PEC-specific deletion of VDR in the mouse increased PEC proliferation and reduced PEC apoptosis [54]. In contrast, Costa et al. [55] found that although siRNA (small interfering RNA)-mediated VDR knockdown was nearly complete in MCF-7 cells (90% lower), the growth-inhibitory effect of 1,25(OH)2D was not significantly reduced by anti-VDR siRNA, suggesting that there also may be VDR-independent mechanisms for this action.

Cell-cycle regulators

Many investigators have looked for direct effects of 1,25(OH)2D on the expression of genes that control cell growth. An example of this is 1,25(OH)2D-mediated transcriptional regulation of the gene encoding the CDK (cyclin-dependent kinase) inhibitor p21 in the myelomonocytic cell line U937 (CDKN1A) [56]. A number of other studies have shown that CDK inhibitors such as p21 or p27 increase, and that cell-cycle-regulatory proteins such as cyclins decrease, coincident with 1,25(OH)2D-induced growth arrest [5760]. In addition, 1,25(OH)2D-mediated growth arrest of prostate cancer cell lines can be inhibited by antisense RNA or siRNA against p21 [58,61]. In contrast, 1,25(OH)2D has a minimal effect on p21 mRNA levels in MCF-7 cells even though the cells growth arrest in response to treatment [62]. In addition, 1,25(OH)2D increased p21 protein, but not p21 mRNA, in LNCaP cells [63]. This suggests that the growth-inhibitory effect of 1,25(OH)2D is not mediated by VDR-mediated transcription activation of the p21 gene promoter in some cell types. However, Thorne et al. [64] recently found that histone modification patterns at three distinct VDRE-containing sites in the p21 gene promoter (at −7 kb, −4.5 kb and −2.1 kb) are important for vitamin D-regulated expression of this gene in immortalized, but non-transformed, RWPE1 PECs. In particular, they found that histone signatures associated with vitamin D-mediated gene activation were enriched in G1 and S phase cells, suggesting a more robust role for induction of p21 in the early phases of the cell cycle.

FoxO (forkhead box O) proteins are tumour suppressors that control cell proliferation [65]. The function of several FoxO family members is inhibited by MAPK (mitogen-activated protein kinase)-mediated phosphorylation. Recently An et al. [66] showed that 1,25(OH)2 D treatment regulates binding of FoxO3a and FoxO4 to DNA-regulatory regions by stimulating a direct interaction between VDR, FoxO3a or FoxO4, and the FoxO regulators Sirt1 (sirtuin 1; a class III histone deacetylase) and protein phosphatase 1. Sirt1 and protein phosphatase 1 promote nuclear retention of FoxO proteins by counteracting MAPK-mediated phosphorylation. Consistent with an essential role for these interactions, 1,25(OH)2D-mediated growth arrest is blocked in SCC25 cells treated with siRNA against FoxO3a.

Microarray analyses have revealed other potential vitamin D gene targets related to cell-cycle control. In primary prostate cancer cells, expression of CDK1 mRNA (encoding a protein required for cell-cycle progression) was suppressed after 24 h of 1,25(OH)2D treatment [67] and transcripts for cell-cycle-inhibitor proteins such as RBL2 (Rb-like protein p130) and RBBP6 (Rb-binding protein 6) were up-regulated by 1,25(OH)2D in the MCF-7 and MDA-MB-231 breast cancer cell lines [68]. A number of transcripts encoding cell-cycle-control proteins appear to be indirectly regulated by 1,25(OH)2D. For example, in RWPE1 PECs, cell-cycle-control genes generally do not change until 24 or 48 h after treatment [69]. Consistent with this, cyclin A, cyclin B and cyclin F mRNA levels were down-regulated in SCC25 cells [70], and cyclin F mRNA was reduced in SW480-ADH cells only after 48 h of treatment with 1,25(OH)2D or the vitamin D analogue EB1089.

IGF (insulin-like growth factor) signaling

1,25(OH)2D may also indirectly influence the growth rate of cells by interfering with the action of growth factors that stimulate proliferation or by increasing the production of those that promote cell differentiation. In MCF-7 cells, IGF-stimulated cell growth was inhibited by vitamin D analogues and this effect was associated with increased release of IGFBP3 (IGF-binding protein 3) into the medium [71]. IGFBP3 is known to limit the proproliferative anti-apoptotic actions of IGF1 and IGF2 by binding to them and limiting their ability to interact with cell-surface receptors. 1,25(OH)2D and vitamin D analogues also induce accumulation of IGFBP3 in prostate cancer cells and primary PECs and this subsequently inhibits IGF2 action [72,73]. Up-regulation of IGFBP3 mRNA levels by 1,25(OH)2D has also been seen in microarray analysis of LNCaP prostate cancer cells [74] and the immortalized PEC line RWPE1 [69]. A putative VDRE was identified in the IGFBP3 gene promoter and characterized by EMSA (electrophoretic mobility-shift assay) and ChIP, indicating that the 1,25(OH)2D-induced increase in IGFBP3 levels is likely to be direct [75]. Further evidence that IGFBP3 is a critical mediator of the growth inhibitory properties of 1,25(OH)2D comes from studies showing that antisense oligonucleotides against IGFBP3 abolish 1,25(OH)2D-mediated growth arrest in LNCaP cells [76]. Nickerson and Huynh [77] have also shown that a 14-day treatment with the vitamin D analogue EB1089 increased the prostate expression of many IGFBP isoforms, including IGFBP3, and this was associated with reduced prostate size in rats. In addition to an effect on IGFBP3 mRNA levels, microarray analyses of 1,25(OH)2D treated SW480-ADH cells also show up-regulation of IGFBP2 and IGFBP6 transcripts, but the functional impact of this regulation has not been explored [78].

TGFβ (transforming growth factor β) signalling pathway

TGFβ2 is essential for the maintenance of tissue homoeostasis and is an anti-proliferative factor in normal epithelial cells and at early stages of carcinogenesis [79]. For example, the TGFβ/SMAD4 signalling axis constrains prostate cancer growth and metastatic progression in Pten (phosphatase and tensin homologue deleted on chromosome 10)-null mice [80]. Short-term 1,25(OH)2D or vitamin D analogue treatment (<12 h) increased expression of TGFβ2 mRNA in MCF-7 cells, MDA-MB-231 cells [68], MCF10CA1a cells [81] and primary prostate cancer cells [67]. Consistent with this induction, Wu et al. [82] used deletion/mutation analysis in reporter gene assays and EMSAs to identify and characterize two VDREs in the TGFβ2 promoter.

In addition to TGFβ2 expression, 1,25(OH)2D and its analogue EB1089 induce expression of TGFβ1 and TGFβ receptors in MCF-7 breast cancer cells and immortalized mammary epithelial cells (185A1 cells) through a mechanism that appears to require SMAD3 as a co-activator [83]. Also, negative regulators of TGFβ availability, LTBP (latent TGFβ-binding protein) 1 and LTBP2 were significantly suppressed by 1,25(OH)2D treatment in OVCAR3 cells [84] and primary prostate cancer cells respectively [67]. The early response of these genes to 1,25(OH)2D suggests that some genes whose protein products control TGFβ signalling may be direct targets of 1,25(OH)2D/VDR.

Another TGFβ superfamily member is GDF15 (growth differentiation factor 15). Forced expression of GDF15 in PC-3 cells decreased cell proliferation, soft agar clone formation and xenograft tumour growth [85]. The level of GDF15 mRNA is up-regulated by 1,25(OH)2D in LNCaP cells [74] and GDF15 has been shown to be a direct VDR target gene that is indispensible for 1,25(OH)2D-mediated growth inhibition [85]. In contrast, the effects of 1,25(OH)2D on TGFβ family member mRNA levels (e.g. TGFBR1, SMAD6 and TGFβ1) are only seen after prolonged treatment in other cell types and this suggests that the effect may be indirect [70,78].

BMPs (bone morphogenic proteins) are another group of growth factors belonging to the TGFβ superfamily that play pivotal roles in regulating tissue morphogenesis; BMP signalling is often dysregulated in cancer (e.g. colon cancer [86]). The mRNA level of several BMP forms are regulated by 1,25(OH)2D or vitamin D analogue treatment in primary prostate cancer cells (BMP6 [67]), MCF10AT1 cells (BMP2 and BMP6 [81]) and squamous cell carcinoma lines (TGFβ1 and BMP2A [70]).

Wnt/β-catenin signalling

An alternative hypothesis has emerged to explain how vitamin D mediates cell growth arrest: disruption of β-catenin function, the terminal mediator of Wnt signalling. In the cytoplasm, β-catenin is found in association with APC (adenomatous polyposis coli). Activation of Wnt signalling leads to the accumulation of β-catenin and its release from APC. This free β-catenin translocates to the nucleus, binds with the TCF (transcription factor) 4 on DNA, and activates transcription of genes whose protein products control proliferation (e.g. c-Myc and cyclin D1) [87]. Mutations in the APC gene that disrupt APC–β-catenin interactions are common in colon cancer [88]. 1,25(OH)2D can block β-catenin-mediated gene transcription in cultured SW480-ADH [89], Caco-2 and HT-29 colon cancer cells [90] by inducing binding of VDR to β-catenin, an event that subsequently reduces the formation of the TCF4–β-catenin transcriptional complex [89]. Consistent with these cell-based observations, Xu et al. [91] found that thrice weekly injections of 1,25(OH)2D and 1,25(OH)2D analogues for 12 weeks reduced polyp number and load in APCmin mice and this was associated with reduced expression of β-catenin target genes in the small intestine and colon. Shah et al. [92] used a mammalian two-hybrid assay in HEK (human embryonic kidney)-293 cells to show that the AF (activation function)-2 domain of VDR interacts with the C-terminus of β-catenin; this interaction may also be enhanced by acetylation of Lys671/672 of β-catenin. 1,25(OH)2D-mediated events may also indirectly influence β-catenin function through increased production of E-cadherin, a membrane protein that can bind β-catenin and prevent its nuclear accumulation. However, 1,25(OH)2D treatment can repress β-catenin-mediated gene transcription even in SKBR-3 cells lacking the E-cadherin gene (CDH1) [92]. Thus these data demonstrate that E-cadherin up-regulation is not the only mechanism for 1,25(OH)2D-mediated repression of β-catenin signalling.

In addition to its impact on VDR–β-catenin interactions, 1,25(OH)2D can also influence expression of known regulators of Wnt signalling, e.g. inhibition of the Wnt activator DKK (dickkopf)-4 [93] and up-regulation of the Wnt antagonist DKK-1 [94]. Evidence for cross-talk between vitamin D and the Wnt signalling pathway has also been observed in vivo. In APCmin/+ mice 1,25(OH)2D injections decreased nuclear β-catenin, TCF1, CD44 and c-Myc levels in tumour-free colonic and small intestine tissue [91].

Collectively these data suggest that vitamin D may decrease colon tumour cell proliferation by interfering with the Wnt signalling pathway. Still, many gaps exist in our understanding of this VDR-mediated growth arrest mechanism. For example, it is not clear whether 1,25(OH)2D can interfere with β-catenin function through VDR in cells other than colonocytes and mammary gland cells lines [95]. In addition, it is not clear if all β-catenin–DNA-binding sites are equally inhibited by VDR. Recent ChIP (chromatin immunoprecipitation)-chip studies show that at some TCF4/β-catenin-binding sites CDX2 (caudal type homeobox 2) is required for TCF4 binding and activation of gene transcription [96]. This shows that there are subgroups of β-catenin-binding sites in DNA, but it is not clear whether binding of β-catenin to these different types of sites are equally affected by 1,25(OH)2D and VDR.


Several groups have reported that 1,25(OH)2D influences apoptosis in MCF-7 breast cancer cells [97] and a variety of colon cancer cell lines [98]. Pan et al. [99] found that 1,25(OH)2D treatment promoted apoptosis in the undifferentiated gastric cancer cell line HGC-27 through a mechanism that depends upon VDR-mediated PTEN up-regulation. PTEN is a tumour-suppressor gene that negatively regulates the anti-apoptotic Akt signalling pathway. Also, in the colorectal cancer cell line MIP101, 1,25(OH)2D treatment increased basal and chemotherapy-induced apoptosis by a mechanism that was sensitive to SPARC (secreted protein acidic and rich in cysteine)-induced VDR synthesis [100], suggesting that the regulation may be transcriptionally mediated. Blutt et al. [101] observed that 6 days of 1,25(OH)2D treatment induced apoptosis in LNCaP cells and this was accompanied by down-regulation of the anti-apoptotic proteins Bcl-2 and Bcl-XL.

Consistent with an effect of 1,25(OH)2D on the regulation of pro- and anti-apoptotic proteins, a number of studies have found that transcripts for genes encoding proteins that control apoptosis are regulated by 1,25(OH)2D treatment. G0S2 (G0/G1 switch gene 2) is a pro-apoptotic protein whose expression is frequently suppressed in cancer [102]. Expression of G0S2 mRNA was induced after 1,25(OH)2D treatment in SW480-ADH colon cancer cells [78] and by the 1,25(OH)2D analogue EB1089 in SCC25 squamous carcinoma cells [70]. In the chronic myeloid leukaemia cell line K562, 1,25(OH)2D-induced growth arrest and apoptosis is accompanied by lower expression of Bcl2 and Bcl-XL mRNA (anti-apoptotic) and increased Bax mRNA (pro-apoptotic) levels [103], whereas in SW480-ADH cells, 1,25(OH)2D increased mRNA levels for the pro-apoptotic proteins Baxα, Baxγ and Baxδ [78]. Transcript profiling studies show that the mRNA levels for the pro-apoptotic proteins DAP (death-associated protein)-3, CFKAR [CASP8 (caspase 8, apoptosis-related cysteine peptidase) and FADD (Fas-associated death domain)-like apoptosis regulator], and a number of caspases (e.g. caspases 3, 4, 6 and 8) were induced in 1,25(OH)2D-treated MCF-7 and/or MDA-MB-231 cells [68], whereas in MCF10AT1 and MCF10CA1a cells, the vitamin D analogue Ro3582 induced PDCD4 (programmed cell death 4) mRNA levels [81]. Although these transcript level changes suggest that 1,25(OH)2D induces apoptosis by transcriptionally activating or repressing various genes, such direct regulation has not yet been reported for most of these genes.


In the past decade 1,25(OH)2D has been shown to regulate a much wider array of cellular events than previously thought possible. Below we will discuss several of these processes and relate them to the prevention of cancer.


Autophagy is a process used by cells to degrade cytosolic macromolecules and organelles in lysosomes. Although autophagy is generally considered a survival tactic to protect cells during stress (e.g. starvation, pro-oxidant conditions), this process can also be used to trigger the death of cancer cells and to block tumour growth [104]. The first suggestion that 1,25(OH)2D may induce autophagy was indirect. Mathiasen et al. [105] found that 1,25(OH)2D induced cell growth arrest and cell death by a caspase- and p53-independent pathway that could be inhibited by the anti-apoptotic protein Bcl-2. Hoyer-Hansen et al. [106] later found that the vitamin D analogue EB1089 induced autophagy in MCF-7S1 cells that could be enhanced by the Atg (autophagy-related gene) protein beclin-1. In HL-60 leukaemia cells 1,25(OH)2D treatment suppresses anti-autophagic mTOR (mammalian target of rapamycin) activity levels as well as increases the level of the pro-autophagic protein beclin-1. This treatment also increases the interactions between beclin-1 and PI3K (phosphoinositide 3-kinase; a pro-autophagic event) or the anti-apoptotic protein Bcl-XL (leading to reduced apoptosis). Vitamin D-induced autophagy may require a complex interplay with CDK inhibitors [107]; in p19-deficient SCC25 cells 1,25(OH)2D can induce autophagy that can be suppressed by suppression of p27. Despite these promising mechanistic relationships, there is currently no in vivo research to directly connect activation of autophagy to the anti-cancer actions of vitamin D compounds. However, the logic of the argument in favour of vitamin D-induced autophagy as a mechanism for cancer treatment or prevention was recently discussed in an opinion piece by Hoyer-Hansen et al. [108].

Antioxidant defence and DNA repair

Oxidative stress-induced damage of DNA and loss of DNA repair mechanisms contribute to carcinogenesis [109], but these effects can be prevented by induction of antioxidant defence mechanisms that reduce the biological impact of reactive oxygen species. Oxidative DNA damage (measured by the level of 8-hydroxy-2′-deoxyguanosine) is elevated in the distal colonic epithelium of VDR-knockout mice [110] and is reduced in the colon epithelium of humans receiving a daily supplement of 800 I.U. of vitamin D3 [111]. 1,25(OH)2 D has been shown to induce the expression of several enzymes involved in the antioxidant defence system. In primary prostate cancer cells, SW480-ADH, MCF-7, MDA-MB-231 and MCF10AT1 cells, 1,25(OH)2D or vitamin D analogues induce the expression of TXNRD1 (thioredoxin reductase 1) a protein that keeps thioredoxin in the reduced state needed for its role as an antioxidant [67,68,78,81]. In addition, mRNA levels for the essential antioxidant proteins SOD (superoxide dismutase) 1 and SOD2 are induced by 1,25(OH)2D in primary PECs [67] and LNCaP cells [85] respectively. 1,25(OH)2D-induced SOD1 activity has also been seen in the liver of diethylnitrosamine-treated rats and is associated with reduced DNA damage (assessed by comet assay) [112]. 1,25(OH)2D induced G6PD (glucose-6-phosphate dehydrogenase) after treatment in ovarian cancer cells [84], in RWPE1 cells [69] and in cells from benign prostatic hypertrophy, but not in malignant prostate cancer cells (DU 145, CWR22R) [113]. G6PD is an enzyme involved in maintaining reduced glutathione levels in cells. Consistent with a critical role for G6PD in vitamin D-mediated antioxidant protection, Bao et al. [113] showed that G6PD expression is controlled by 1,25(OH)2D in prostate epithelial cells through a VDRE located in the first intron of the gene, that 1,25(OH)2D protected RWPE1 cells against H2O2-induced apoptosis, and this protection was lost in the presence of a non-competitive G6PD inhibitor. It is also possible that vitamin D-mediated protection from pro-oxidant stress is indirect due to the induction of NFE2L2 [nuclear factor (erythroid-derived 2)-like 2], a transcription factor that controls expression of genes for many antioxidant enzyme systems [114]. NFE2L2 expression is down-regulated in prostate cancer and suppression of NFE2L2 promotes prostate tumour development in TRAMP mice [115]. Consistent with a role for NFE2L2 in vitamin D-mediated cancer prevention, a number of NFE2L2 target genes were increased in RWPE1 cells after 1,25(OH)2D treatment, e.g. GPX (glutathione peroxidase) 3, HMOX1 [haem oxygenase (decycling) 1], AKR1C2 (aldo-keto reductase 1C2) and TXNRD1 [69]. Finally, GPX1 was induced by 1,25(OH)2D in SW480-ADH cells [78] and by EB1089 in SCC25 cells [70].

There is some evidence that 1,25(OH)2 D regulates genes for proteins that protect the genome. Akutsu et al. [116] found that the 1,25(OH)2D analogue EB1089 up-regulated GADD45α (growth arrest and DNA-damage-inducible α) mRNA and protein levels in SSC cells. GADD45α is a p53 target gene whose product is involved in DNA repair. It was later shown that the GADD45 gene contains an exonic enhancer element that binds VDR after 1,25(OH)2D treatment leading to increased GADD45 mRNA levels in ovarian cancer cells [117]. 1,25(OH)2D-mediated G2/M arrest in ovarian cancer cells is lost upon deletion of GADD45, suggesting the critical importance of GADD45 induction in vitamin D effects [117]. Other 1,25(OH)2D-regulated transcripts whose protein products may contribute to DNA repair and pro-apoptotic effects of vitamin D have been revealed by microarray analyses. In MCF-7 cells, 1,25(OH)2D induced the expression of mRNAs for p53, RAD23B (RAD23 homologue B), PCNA (proliferating cell nuclear antigen) [68] and DAP-1α [78]. Taken together, it is possible that 1,25(OH)2D directly regulates the expression of a variety of genes whose protein products are involved in DNA damage repair and programmed cell death, thereby offering protection against carcinogenesis.

Prostaglandin metabolism and action

A variety of studies have shown that prostaglandin signalling stimulates cancer cell growth and cancer progression [118121]. In this context, COX (cyclo-oxygenase) 1 and COX2 are the rate-limiting enzymes in prostaglandin synthesis. In particular, COX2 expression is induced by a variety of mitogens, cytokines and tumour promoters [122] making this pathway a drug target for cancer treatment, e.g. drugs specifically targeting COX2 have been shown to lower the risk of prostate cancer [120]. 1,25(OH)2D has been proposed as a negative regulator of prostaglandin synthesis and signalling, and this is supported by research on the advanced prostate cancer cell lines LNCaP and PC-3 [123]. In these cells, 1,25(OH)2D suppressed COX2 expression, reduced expression of the prostaglandin receptors EP2 and FP, and induced expression of 15-PGDH (hydroxyprostaglandin dehydrogenase 15-NAD), the enzyme that inactivates prostaglandins [74,123]. More importantly, 1,25(OH)2D reduced prostaglandin E2 levels, blocked prostaglandin-mediated induction of c-fos mRNA, and reduced the growth stimulatory effects of prostaglandins and prostaglandin precursors in LNCaP cells [124]. These results suggest the direct regulation of the prostaglandin metabolism and signalling by 1,25(OH)2D and the subsequent attenuation of their signalling activities. In contrast to the findings from advanced prostate cancer cells, COX2 mRNA was induced by 1,25(OH)2D in the immortalized but non-tumorigenic RWPE1 cell line, and 15-PGDH and prostaglandin receptor mRNA levels weren't altered [69]. Consistent with this observation, Moreno et al. [124] found that COX2 mRNA was suppressed and 15-PGDH mRNA was increased in some, but not all, primary prostate epithelial cell lines.


In the past decade adult stem cells have been discussed as potential target cells for accumulating mutations that contribute to carcinogenesis and therefore as cells that would benefit from exposure to cancer prevention agents [125]. Adult stem cells in tissues serve the primary function of replacing cells lost during the normal lifespan of an organ or following tissue injury. The hallmarks of the adult stem cell are its ability for self-renewal and multipotency. Unfortunately, this capacity for self-renewal means that adult stem cells can accumulate first-hit mutations that, although not harmful themselves, could be combined with subsequent mutations to cause cancer. Trosko [126] argued that rather than the induction of ‘immortalization’ of a normal differentiated cell, we should view carcinogenesis as a process that begins by blocking ‘mortalization’ (or the ability to growth arrest and differentiate) of an adult stem cell.

In addition to adult stem cells, a small subset of cells within tumors has the ability to recapitulate the morphologic diversity of a neoplasm when isolated from the parent neoplasm and xenotransplanted into nude mice. These cells have been called ‘cancer stem cells’ and are thought to be the cells from which primary cancers arise or which survive cytotoxic treatments and cause tumor recurrence. Cancer stem cells are also attractive targets for cancer prevention and treatment [127].

The question to be asked in the context of vitamin D signalling and carcinogenesis is ‘are adult stem cells or cancer stem cells targets for 1,25(OH)2D action?’ As discussed above, 1,25(OH)2D regulates a number of important biological processes. These effects would be beneficial for protecting an adult stem cell (e.g. DNA repair and protection from oxidative cellular injury) or for limiting the expansion of cancer stem cells (e.g. cell-cycle arrest, apoptosis). To date, the vast majority of the experimental data exploring the molecular function of 1,25(OH)2D has been collected from in vitro cell culture systems using cancer cell lines or primary cell cultures. Only recently have researchers begun to directly evaluate the impact of 1,25(OH)2D on stem cells. For example, as part of a team headed by Dr Scott Cramer (School of Medicine, Wake Forest University, Winston-Salem, NC, U.S.A.) we have shown that the growth of mouse prostate stem cells [128] is suppressed by 1,25(OH)2D through a VDR-dependent induction of IL (interleukin)-1α production [129]. In the mouse colon, VDR message is predominantly found in the differentiated luminal colonocytes [130]. However, Fedirko et al. [131] found that hTERT (human telomerase reverse transcriptase) labelling in the upper part of the colon crypt was suppressed in people given vitamin D3 and calcium supplements. hTERT is the catalytic subunit of telomerase that marks a slowly cycling population of intestinal stem cells [132], thus vitamin D may suppress the expansion of this cell population and protect them from potential cancer-causing gene mutations. Finally, 1,25(OH)2D and its analogues have been show to regulate the expression of a putative cancer stem cell marker (CD44) in human breast cancer cells in vitro and when they are implanted into immunodeficient mice [133]. Research on the impact of vitamin D on adult and cancer stem cells is certain to expand in the future.


Although most cell-based vitamin D research has focused on the impact of 1,25(OH)2D on either tumour cells or their non-neoplastic progenitors, there is also evidence that other cells that exist in the tissue or tumour microenvironment are targets of vitamin D action. For example, in prostate the communication between epithelial cells and the stromal cells surrounding them is critical for the progression of cancer in that organ [134]. Lou et al. [135] reported that 1,25(OH)2D can suppress the growth of prostate stromal cell lines, but it is not yet clear if it can alter stromal–epithelial cell communication. In the next section we will discuss two additional examples of non-epithelial cell targets of vitamin D action: vascular cells and immune cells.

Inhibition of angiogenesis

Angiogenesis is essential for the expansion of tumour growth and for tumour cell metastasis. It is a multistep multicellular process that depends upon a variety of pro-angiogenic factors including VEGF (vascular endothelial growth factor), bFGF (basic fibroblast growth factor) and PDGF BB (platelet-derived growth factor BB homodimer). 1,25(OH)2D may inhibit the development of the tumour vasculature that is required for the progression of solid tumours and this may occur due to effects on either endothelial or epithelial cells [136140]. 1,25(OH)2D can directly inhibit the proliferation of aortic and tumour-derived endothelial cells, and can stop endothelial cell sprouting and elongation induced by VEGF [136,141]. Consistent with a direct role for vitamin D signalling in endothelial cells, Chung et al. [138] found that prostate tumours from TRAMP mice grew larger, had a greater vascular volume, and had larger vessels when implanted into VDR-knockout mice. Effects of vitamin D on epithelial cells may also influence angiogenesis. One proposed mechanism for the anti-angiogenic effects of 1,25(OH)2D is that it suppresses expression of VEGF family members, the major pro-angiogenic cytokines. 1,25(OH)2D reduces expression of VEGF in normal PECs [142], reduces VEGF mRNA and increases mRNA levels for the anti-angiogenic protein thrombospondin-1 in SW480-ADH colon cancer cells [140], reduces hypoxia-induced VEGF expression in a variety of cancer cell lines [143], and suppresses mRNA levels for VEGFC and the VEGF receptors KDR (kinase insert domain receptor) and NRP1 (neuropilin 1) in RWPE1, a PEC line [69]. The impact of 1,25(OH)2D is partially due to reduced levels and activity of the transcription factor HIF-1 (hypoxia-inducible factor-1); the ability of 1,25(OH)2D to suppress VEGF expression is lost in HCT116 colon cancer cells lacking HIF-1α [143]. VEGF signalling can be suppressed by competitive binding of semaphorins to the VEGF receptor NRP1. In RWPE1 cells, 1,25(OH)2D induced expression of several semaphorin isoforms including SEMA3B, 3F and 6D, and VDR binding to the SEMA3B gene promoter was enriched after 1,25(OH)2D treatment [69]. In prostate cancer cells 1,25(OH)2D can also reduce the mRNA level of the pro-angiogenic cytokine IL-8 by reducing translocation of the p65 subunit of NF-κB (nuclear factor κB) into the nucleus, thereby limiting NF-κB-mediated IL-8 gene transcription [139]. However, the impact of 1,25(OH)2D on VEGF gene regulation has not been entirely consistent. In mouse embryo fibroblasts and human vascular smooth muscle cells 1,25(OH)2D induces VEGFA expression through a VDRE in its promoter [144,145].

Regulation of immune cell function by 1,25(OH)2D

The first line of protection against pathogens and environmental insults is the barrier epithelium of the lung, skin and intestine. There is evidence that 1,25(OH)2D signalling through the VDR helps maintain intestinal barrier function. In VDR-knockout mice, but not wild-type mice, DSS (dextran sulfate sodium) treatment causes the redistribution of the tight junction proteins occludin and ZO (zona occludin)-1 away from the membrane and severely disrupts tight junctions in the colon [146]. This is consistent with earlier work showing that prolonged treatment of SW480-ADH colon cancer cells with 1,25(OH)2D (100 nM, 2–7 days) induced expression of tight junction and adhesion proteins (E-cadherin, occludin, ZO-1 and vinculin) and promoted translocation of ZO-1 to the plasma membrane [89]. In vitamin D-deficient rats, microarray analysis revealed that 1,25(OH)2 D treatment increased the mRNA level of junction proteins such as claudin 3, claudin 17 and RhoA [147]. Ordonez-Moran et al. [148] found that 1,25(OH)2D treatment increased cytosolic calcium leading to the activation of RhoA in SW480-ADH cells. By using dominant-negative RhoA, they were subsequently able to block 1,25(OH)2D-induced localization of occludin to the plasma membrane. Collectively, these data support a mechanism whereby vitamin D signalling regulates the level and localization of tight junction proteins leading to a more secure intestinal barrier. It is not clear whether similar protections are present in other barrier tissues such as skin and lung, nor is it clear what level of vitamin D status is necessary to protect this function.

There are three distinct roles for the immune system in cancer prevention [149]. First, it protects the host against viral infection and suppresses virus-induced tumours. Secondly it suppresses inflammation that facilitates tumorigenesis by effectively eliminating pathogens and limiting the period of inflammation that can promote carcinogenesis [150,151]. Finally, it performs immunosurveillance that eliminates nascent tumour cells in tissues by activating receptors on innate immune cells and lymphocytes of the adaptive immune system. As a result of these functions, the cells of the immune system may be important targets for limiting cancer. However, in the context of cancer, the role of the immune system is not straightforward. Interactions can occur between cancer cells and host immune cells in the tumour microenvironment to create an immunosuppressive network that promotes tumour growth and protects the tumour from immune attack [152]. Thus agents that can suppress inflammation and immune responses prior to tumour formation may be harmful in established tumours where tumour-associated immunosuppression already exists.

A variety of studies have shown that 1,25(OH)2D regulates cells of both the innate and adaptive immune system [153]. However, few of these studies have been conducted in the context of cancer. As a result, it remains to be shown that the effects of vitamin D on immunity are effective in the prevention of cancer or in the tumour environment. However, non-cancer examples give us insight into the impact that vitamin D could have on inflammation and immunity in carcinogenesis.

The innate immune system interacts with vitamin D in several interesting ways. First, a primary role of macrophages is to engulf and kill bacteria. Invading microbes can induce inflammation when barrier tissues become leaky, e.g. during the intestinal condition Crohn's disease or experimental colitis induced by DSS [154]. 1,25(OH)2D can regulate a number of genes in cells of the innate immune system whose protein products are crucial for autophagy and anti-microbial actions. The human cathelicidin (CAMP) and β-defensin (DEFB1) genes are activated by 1,25(OH)2D treatment through a VDRE in their promoters, but these VDREs are not conserved in mice [155,156]. β-Defensin can also be up-regulated by an NF-κB-mediated transcriptional event after NF-κB has been activated through the pattern recognition receptor NOD2 (nucleotide-binding oligomerization domain 2). 1,25(OH)2D can stimulate expression of the NOD2 gene in primary human monocytic cells through a VDRE in a distal enhancer [157]. In addition, the mRNA levels for the antimicrobial protein angiogenin-4 are significantly reduced in the colon of vitamin D-deficient mice and this may increase the severity of DSS-induced colitis [158].

Several cytokines can modulate vitamin D metabolism in macrophages, monocytes and dendritic cells. Pro-inflammatory cytokines such as IFN (interferon) γ [159] and TNFα (tumour necrosis factor α) [160] stimulate production of 1,25(OH)2 D by increasing CYP27B1 expression in monocytes. Inflammatory cytokines and TLR (Toll-like receptor) agonists also increase CYP27B1 and VDR expression in dendritic cells [161]. In contrast, IL-4 produced by Th2 cells activates CYP24 (25-hydroxyvitamin D-24 hydroxylase) expression in monocytes leading to formation of the inactive metabolite 24,25-dihydroxyvitamin D [159]. These regulatory events can influence local vitamin D metabolite levels and these vitamin D metabolites may then modulate the function other cells in the microenvironment. For example, in vitro 1,25(OH)2D reprogrammes dendritic cells to become tolerogenic [161].

In the adaptive immune system, many T-lympocyte subpopulations express the VDR and are vitamin D target cells [162]. However, mice lacking the VDR have no gross abnormalities in the number or type of T-cells present [163] and the function of their mature T-cells is not strongly influenced by VDR deletion [164,165]. This suggests that the VDR does not have a primary function for normal T-cell function, but it does not preclude the possibility that 1,25(OH)2D may be a modulator of T-cell-mediated immune responses. Consistent with the ‘modulator’ hypothesis, 1,25(OH)2D promotes T-cell profiles reflective of immunotolerence and immunosuppression. In vivo, VDR-deficient NOD (non-obese diabetic) mice develop a more aggressive form of autoimmune prostatitis with more severe lesions, a greater lymphoproliferative response against prostate antigen, higher levels of IFNγ secretion, and higher mononuclear cell infiltration into the prostate gland [166]. Similarly, VDR is necessary in CD4+ cells for 1,25(OH)2D to prevent autoimmune T-cell responses that cause experimental autoimmune encephalomyelitis in mice [167]. In vitro, 1,25(OH)2D can suppress NF-κB signalling necessary for T-helper cell activation [168], increase the activity of regulatory T-cells necessary for immunosuppression [169], and block development of Th17 and Th9 cells implicated in the pathogenesis of different types of autoimmunity and inflammatory diseases [170]. The effect of 1,25(OH)2D on Th9 cell development is mediated through IL-10, a cytokine that can be induced by 1,25(OH)2D in B-cells [171] and naïve T-cells [170]. Treatment of Th17 cells with 1,25(OH)2D suppresses IL-17 production indirectly by inducing the expression of CHOP [C/EBP (CCAAT/enhancer-binding protein) homologous protein], a molecule involved in endoplasmic reticulum stress and translational inhibition.

Other cells outside of the traditional immune system have the capacity to respond to and produce immunomodulatory factors. CD14 mRNA is strongly up-regulated by 1,25(OH)2D in the PEC line RWPE1 [69], keratinocytes [172], U937 monocytes [173] and the myeloid cell line HL-60 [174]. CD14 and TLR4 are important for detection of pathogens that have lipopolysaccharide on their cell surface [175] and CD14 has been identified as a crucial factor for vitamin D-induced expression of the antimicrobial peptide cathelicidin in human keratinocytes [172]. Nonn et al. [176] have shown that in normal PECs, 1,25(OH)2D inhibits TNFα-induced IL-6 production through a mechanism that requires direct transcriptional regulation of the MAPK phosphatase 5 gene (DUSP-10). Although these data suggest that vitamin D should suppress immune and inflammatory conditions that promote cancer, very little data explicitly supports this hypothesis, particularly as it relates to the ranges of vitamin D status being discussed as relevant to cancer prevention in humans. However, injections of the vitamin D analogue Ro26-2198 delayed the onset of clinical colitis and reduced the number of dysplastic foci present in mice treated with azoxymethane and DSS [177].


Although we generally focus on the effect that vitamin D has on the development of cancer, it is just as important to understand the impact that cancer has on vitamin D action. In this section we will review studies which indicate that vitamin D signalling is altered in various cancers. This concept is summarized in Figure 2.

Figure 2 A summary of the negative effects of cancer on vitamin D metabolism and action


There are conflicting findings regarding the impact of cancer on VDR protein levels. For example, using IHC (immunohistochemistry) Matusiak et al. [27] found that VDR protein levels decline as a function of colon tumour dedifferentiation, whereas Kure et al. [178] observed that VDR protein levels were overexpressed in colorectal cancer tumours with K-Ras mutations. Unfortunately, a recent study highlights the inadequacy of many anti-VDR antibodies for IHC [179], and therefore IHC-based studies should be viewed with caution. None-the-less, using IHC VDR levels have been reported to decline with the development of cancer in pigmented skin lesions [180], invasive breast tumours [181] and ovarian cancer [182], whereas high VDR levels are associated with reduced risk of lethal prostate cancer [183] and improved overall survival for non-small cell lung carcinoma [184]. Consistent with the hypothesis that cancer reduces VDR protein levels, Xu et al. [91] showed that VDR mRNA and protein expression (by Western blot analysis) were lower in the small intestine and colon tissue of Apcmin/+ mice compared with wild-type mice. Other mutations that cause cancer can also lead to reduced VDR expression; H-Ras transformation in HC-11 mammary cells [185], SV40 (simian virus 40) large T antigen in HME (human mammary epithelial) cells [186] and in K-Ras transformation in RWPE-2 human PECs [187].

A number of factors have been proposed to influence VDR expression in cancer. First, overexpression of SNAIL transcription factors can reduce VDR gene expression and blunt 1,25(OH)2D or vitamin D analogue-induced cell growth arrest and/or gene expression of colon cancer cell lines (e.g. SW-480-ADH, HCT116, Caco-2, LS174T and HT29) [188190]. SNAIL1 is absent from normal tissue and expressed in colon tumours, whereas SNAIL2 is expressed in normal tissue but its levels are increased in colon tumours [190,191]. In colon cancer cell lines and breast cancer cell lines SNAIL1 and 2 can bind to E-boxes in the proximal promoter of the VDR gene and increase recruitment of co-repressors that down-regulate VDR gene expression [190,192]. Another protein regulating VDR gene expression is p53. Overexpression of wild-type p53 in Saos-2 and H1299 cells (both p53-null) increases VDR protein levels by enhancing p53 association with conserved p53-binding sites within intron 1 of the VDR gene [193]. In addition to suppression of VDR gene expression, the mutant form of p53 found in many tumours can interact with VDR, be recruited to VDR-regulated genes, and modulate the expression of these genes in breast and lung cancer cell lines [194]. In this context, mutant p53 converts 1,25(OH)2D treatment from a pro-apoptotic agent into an anti-apoptotic agent, suggesting that in cancers with p53 mutations the therapeutic potential of vitamin D or vitamin D analogues could be limited.

There is also some evidence that the VDR gene may be subject to epigenetic silencing in cancer. Smirnoff and colleagues first observed that in colonic tissue from ovariectomized rats treated with DMH (dimethylhydrazine), CpG island methylation in the VDR gene increased [195]. This was associated with reduced VDR protein levels; both CpG island methylation in the VDR gene and reduced VDR expression could be prevented by oestradiol treatment. In breast tumours and in breast cancer cell lines, CpG island methylation in the −700 bp VDR promoter region was associated with lower VDR expression [196]. Demethylation of the VDR promoter with 5-azacytidine increased both VDR protein levels and potentiated the growth inhibitory response to 1,25(OH)2D in normal and breast cancer cell lines.

VDR mRNA levels are also a target of the microRNA miR125b [197]. Overexpression of miR125b reduced VDR protein levels and attenuated 1,25(OH)2D-induced expression of CYP24 in MCF-7 cells [197]. Similarly, melanoma cell lines with low miR125b levels have higher VDR levels and are more sensitive to 1,25(OH)2D-induced growth suppression than cells with high miR125b levels [198]. miR125b levels are up-regulated in patients with metastatic prostate cancer [199], but are down-regulated in subjects with HER2-overexpressing breast cancers [200]; however, it remains to be determined whether miR125b expression can influence the therapeutic potential of vitamin D for specific cancers.

In addition to the negative impact of cancer on VDR levels, there is also evidence that Ras activation, a common mutation in many cancers, can impair vitamin D-mediated transcriptional activity. This was first shown by Solomon et al. [201] who found that H-Ras-transformed keratinocytes have reduced VDR transcriptional activity due to phosphorylation of the VDR heterodimeric partner RXRα on Ser260. In the K-Ras-transformed human PEC line RWPE2, we have shown that 1,25(OH)2D-mediated transcription is blunted due to phosphorylation of RXRα within the AF-1 domain [187], a process that impairs the ability of the co-activator SRC-1 to bind to RXRα. Collectively, this suggests that the development of cancer may lead to lower responses to 1,25(OH)2D by impairing signalling through the VDR.


The current hypothesis for how high vitamin D status reduces cancer risk is that 1,25(OH)2D is produced and acts locally, i.e. the CYP27B1 enzyme known to produce 1,25(OH)2D from 25OHD in the kidney is present and active in cancer target tissues [202]. Several groups have used cultured cells to show that CYP27B1 activity, and the ability to produce 1,25(OH)2D from 25OHD, is lost as cells develop a more severe cancer phenotype. Hsu et al. [25] found that CYP27B1 was present in normal PECs, but that its activity was reduced in cells isolated from subjects with benign prostatic hypertrophy and nearly absent from cells isolated from subjects with prostate cancer. As a result, while normal cells could respond to treatment with 25OHD by growth arresting, cancer cells with low CYP27B1 expression could not. This observation was confirmed by Chen et al. [30] who also showed that transgenic expression of CYP27B1 confers a 25OHD-mediated growth inhibitory response to LNCaP cells with low CYP27B1 activity. CYP27B1 expression is also absent from metastases from colon tumours in humans [27,203]. However, cancer-associated reductions in CYP27B1 levels are not uniformly observed for all cancers. Friedrich et al. [26] found that CYP27B1 mRNA was increased in breast cancer as compared with normal tissue. More recently Lopes [181] analysed CYP27B1 protein levels by IHC in breast tissue samples and found that CYP27B1 expression did not significantly change between normal and cancer tissue.


Some have also hypothesized that the enzyme responsible for the degradation of vitamin D metabolites, CYP24, is influenced by cancer. CYP24 has been proposed as a putative oncogene that is amplified in breast tumours [204]. Consistent with this, Anderson et al. [205] reported that CYP24 mRNA levels are higher in colorectal cancer than in adjacent normal colon tissue. Matusiak and Benya [203] subsequently found that CYP24 protein was present in the nuclei of normal tissue, that CYP24 levels were increased in aberrant crypt foci and polyps, and that CYP24 protein was shifted to the cytoplasm in tumours and metastatic colon cancer. Overexpression of CYP24 has also been reported in ovarian, cervical, lung, cutaneous basal cell and squamous cell carcinoma [205207]. This suggests that increased 1,25(OH)2D metabolism may be a feature of advanced cancer. This is supported by studies showing that KTZ, a CYP24 inhibitor, can restore 1,25(OH)2D-mediated growth arrest to prostate cancer cells that are insensitive to the action of 1,25(OH)2D [208]. Recently, Komagata et al. [209] found that CYP24 protein levels are post-transcriptionally suppressed by the miR125b, a microRNA that can bind to the 3′-UTR (untranslated region) of CYP24 mRNA. In addition, they found that CYP24 levels were inversely related to miR125b levels in breast cancer, suggesting that the loss of this regulatory RNA may account for elevated CYP24 levels in cancer. In contrast with other cancers, a microarray dataset in the ONCOMINE database shows that CYP24 expression can be lower in prostate cancer ( Consistent with this, CYP24 mRNA levels are inversely correlated with CYP24 promoter methylation in prostate cancer cell lines with the promoter being unmethylated, and vitamin D induction of CYP24 expression being greatest, in DU145 cells [210].

The overall impact of these cancer-induced changes to vitamin D metabolism and signalling could affect cancer prevention in several ways. The protection provided by high serum 25OHD levels depends upon the level of CYP27B1 in cells. Therefore if CYP27B1 activity is lost, so will the protection due to high vitamin D status. On the other hand, when CYP24 activity is elevated and/or when the VDR level/signalling is reduced, higher cellular levels of 1,25(OH)2D will be needed to induce chemoprotective molecular events. However, it is not clear whether local production of 1,25(OH)2D is sufficient to overcome this cancer-associated insensitivity. Because of the strong effect it has on calcium metabolism, increases in serum 1,25(OH)2D levels are not likely to be an effective means to correct tumour-associated vitamin D resistance. 1,25(OH)2D-treatment-associated hypercalcaemia is a major motivation to produce vitamin D analogues that separate the calcaemic and non-calcaemic effects of 1,25(OH)2D. However, even the effectiveness of vitamin D analogues will be limited in cancers with reduced VDR expression.


There is now a large amount of population-based evidence showing that higher vitamin D status can protect against a variety of cancers. Unfortunately, the mechanisms supporting this association have not yet been resolved. None-the-less, mechanistic explanations for how vitamin D can reduce cancer risk or disrupt the biology of transformed cells are increasing. Although the role of 1,25(OH)2D in cancer has traditionally been linked to the suppression of proliferation and the stimulation of apoptosis, there is now compelling evidence that 1,25(OH)2D regulates other cancer-relevant cellular processes. For example, several lines of evidence suggest that the immunoregulatory actions of vitamin D metabolites are worth exploring for cancer prevention. The breadth of potential mechanisms used by vitamin D to prevent or suppress cancer is given in Figure 3. Still, many gaps remain in our understanding of how vitamin D regulates cell biology relevant to cancer prevention and treatment. Many of the mechanisms proposed for vitamin D and cancer prevention have been studied only in the context of one tissue or one type of cancer and so further studies must be conducted to determine if these mechanisms can be generalized (e.g. VDR-mediated inhibition of β-catenin function has been studied primarily in the colon). Also, although many studies show that 1,25(OH)2D can alter various transcript levels, it is not clear which genes are direct targets for VDR-mediated regulation. Recent advances in global profiling of transcription factor-binding sites with techniques such as ChIP-seq should overcome this uncertainly and help researchers differentiate between the direct and indirect effects of vitamin D in the context of cancer.

Figure 3 A summary of the potential molecular events regulated by 1,25(OH)2 D (VD) relevant to cancer

Another feature of the vitamin D/cancer paradigm that requires additional investigation is that vitamin D metabolism and signalling through the VDR may be disrupted as cancers progress. Until we understand this relationship more clearly, our ability to use vitamin D or vitamin D analogues for cancer prevention or treatment will be limited. Similarly, few controlled animal studies exist that confirm the validity of proposed molecular mechanisms for vitamin D-mediated cancer prevention using the ranges of vitamin D status that are observed in human populations. The lack of such ‘proof of principle’ studies has hampered a broader acceptance of the vitamin D/cancer relationship in the research and medical community. For example, while higher serum 25OHD is associated with reduced cancer risk, and CYP27B1 is expressed in non-renal cells, there is no evidence that the low-level expression of CYP27B1 in tissues leads to meaningful local production of 1,25(OH)2D, and that this mediates the protection from cancer provided by high vitamin D status. Still, even with all of these gaps in our understanding, this is an exciting time for research on vitamin D and cancer.


This work of the authors is supported by a National Institutes of Health award [grant number CA10113] and American Institute for Cancer Research award [grant number 09A098 (to J.C.F.)].


  • 2 James Fleet has acted as a scientific consultant for Johnson & Johnson.

Abbreviations: AF, activation function; APC, adenomatous polyposis coli; BMP, bone morphogenic protein; CBP, CREB (cAMP-response-element-binding protein)-binding protein; CDK, cyclin-dependent kinase; ChIP-seq, chromatin immunoprecipitation followed by DNA sequencing; CHOP, C/EBP (CCAAT/enhancer-binding protein) homologous protein; COX, cyclo-oxygenase; CYP24, 25-hydroxyvitamin D-24 hydroxylase; CYP27B1, cytochrome p450 27B1; DAP, death-associated protein; DKK, dickkopf; DSS, dextran sulfate sodium; EMSA, electrophoretic mobility-shift assay; FoxO, forkhead box O; GADD45α, growth arrest and DNA-damage-inducible α; GDF15, growth differentiation factor 15; G6PD, glucose-6-phosphate dehydrogenase; GPX, glutathione peroxidase; G0S2, G0/G1 switch gene 2; HIF-1, hypoxia-inducible factor-1; hTERT, human telomerase reverse transcriptase; IGF, insulin-like growth factor; IGFBP3, IGF-binding protein 3; IFN, interferon; IHC, immunohistochemistry; IL, interleukin; MAPK, mitogen-activated protein kinase; NCoR, nuclear receptor co-repressor; NF-κB, nuclear factor κB; NFE2L2, nuclear factor (erythroid-derived 2)-like 2; NOD2, nucleotide-binding oligomerization domain 2; NRP1, neuropilin 1; 1,25(OH)2D, 1,25-dihydroxyvitamin D; 25OHD, 25-hydroxyvitamin D; PEC, prostate epithelial cell; 15-PGDH, hydroxyprostaglandin dehydrogenase 15-NAD; PTEN, phosphatase and tensin homologue deleted on chromosome 10; RXR, retinoid X receptor; siRNA, small interfering RNA; Sirt1, sirtuin 1; SOD, superoxide dismutase; SRC1, steroid receptor co-activator 1; TCF, transcription factor; TGFβ, transforming growth factor β; LTBP, latent TGFβ-binding protein; TLR, Toll-like receptor; TNFα, tumour necrosis factor α; TXNRD1, thioredoxin reductase 1; VDR, vitamin D receptor; VDRE, vitamin D-response element; VEGF, vascular endothelial growth factor; ZO, zona occludin


View Abstract