ACC-α (acetyl-CoA carboxylase-α), a key regulator of fatty-acid metabolism, is encoded by mRNAs transcribed from three promoters, PI, PII and PIII, in the ovine genome. Enhanced expression of transcripts encoded by PIII in mammary gland during lactation is associated with alterations in chromatin structure that result in the detection of two DNase I hypersensitive sites, upstream of the start site. The most proximal site, located between −190 and −10, is characterized by the presence of an inverted-CCAAT box, C2 at −167, and E-boxes, E1 and E2, at −151 and −46. Deletion of these motifs, which bind nuclear factor-Y and upstream stimulatory factors respectively in gel-shift assays, attenuates the activity of luciferase reporter constructs in transfected cells. Chromatin immunoprecipitation demonstrated that these transcription factors were associated with PIII in vivo in both lactating and non-lactating mammary tissues. The basic helix–loop–helix-leucine zipper transcription factor, SREBP-1 (sterol-regulated-element-binding protein-1), transactivated PIII reporter constructs in transfected HC11 mammary cells, and this was dependent on the presence of E1, but not on C2 or E2. SREBP-1 was only associated with PIII in chromatin from lactating animals, which was coincident with a 4-fold increase in the precursor (125 kDa) form of SREBP-1 in microsomes and the appearance of the mature form (68 kDa) in the nucleus. SREBP-1 motifs are also present in the proximal region of PII, which is also induced in lactation. This indicates that SREBP-1 is a major developmental regulator of the programme of lipid synthesis de novo in the lactating mammary gland.
- basic helix—loop—helix
- chromatin immunoprecipitation
The nucleotide sequence data reported in this paper has been submitted to EMBL, GenBank® and DDBJ Nucleotide Sequence Databases under the accession number AJ292286.
Abbreviations used: ACC, acetyl-CoA carboxylase; ADD1, adipocyte determination and differentiation-dependent factor 1; bHLH-ZIP, basic helix–loop–helix-leucine zipper; ChIP, chromatin immunoprecipitation; EMSA, electrophoretic mobility-shift assay; E5A, exon 5A; FAS, fatty-acid synthase; HSS, hypersensitive site; NF-Y/CBF, nuclear factor-Y/CCAAT-binding factor; SRE, sterol-regulated element; SREBP-1, SRE-binding protein-1; STAT5, signal transducer and activator of transcription 5; tss, transcription start site; USF, upstream stimulatory factor.
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