The inflammation that occurs during rheumatoid arthritis or atherosclerosis is characterized by the release of large amounts of sPLA2 (group IIA secretory phospholipase A2). We have shown previously that the sPLA2 promoter in SMC (smooth-muscle cells) is activated by interleukin-1β and cAMP-signalling pathways, through the interplay of multiple transcription factors [Antonio, Brouillet, Janvier, Monne, Bereziat, Andreani, and Raymondjean (2002) Biochem. J. 368, 415–424]. In the present study, we have investigated the regulation of sPLA2 gene expression in rat aortic SMCs by oxysterols. We found that oxysterol ligands that bind to the LXR (liver X receptor), including 25-HC (25-hydroxycholesterol) and 22(R)-HC, cause the accumulation of sPLA2 mRNA and an increased enzyme activity. Transient transfection experiments demonstrated that the sPLA2 promoter is synergistically activated by 22(R)-HC in combination with 9-cis-retinoic acid, a ligand for the LXR heterodimeric partner RXR (retinoid X receptor). Promoter activity was also increased in a sterol-responsive fashion when cells were co-transfected with LXRα/RXRα or LXRβ/RXRα. Mutagenesis studies and gel mobility-shift assays revealed that LXR/RXR heterodimers regulate sPLA2 transcription directly, by interacting with a degenerated LXRE (LXR response element) at position [−421/−406] of the sPLA2 promoter. Chromatin immunoprecipitation revealed the in vivo occupancy of LXR on the sPLA2 promoter. In addition, the orphan nuclear receptor LRH-1 (liver receptor homologue-1) potentiated the sterol-dependent regulation of the sPLA2 promoter by binding to an identified promoter element (TCAAGGCTG). Finally, we have demonstrated that oxysterols act independent of interleukin-1β and cAMP pathways to activate the sPLA2 promoter. In the present study, we have identified a new pathway activating sPLA2 gene expression in SMCs.
- chromatin immunoprecipitation
- 9-cis-retinoic acid
- secretory phospholipase A2
- liver X receptor
- smooth-muscle cell
Abbreviations used: C/EBP, CCAAT-enhancer-binding protein; ChIP, chromatin immunoprecipitation; 9cRA, 9-cis-retinoic acid; DMEM, Dulbecco's modified Eagle's medium; EMSA, electrophoretic mobility-shift assay; Ets, erythroblastosis virus twenty-six; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; HC, hydroxycholesterol; HDL and LDL, high- and low-density lipoproteins; ox-LDL, oxidized LDL; IL-1β, interleukin-1β; LRH-1, liver receptor homologue-1; LRHE, LRH-1-responsive element; LXR, liver X receptor; LXRE, LXR response element; NF-1, nuclear factor 1; RXR, retinoid X receptor; RT, reverse transcriptase; SMC, smooth-muscle cell; sPLA2, group IIA secretory phospholipase A2.
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