Sterol regulatory element binding protein-1 (SREBP-1)c promoter: Characterization and transcriptional regulation by mature SREBP-1 and liver X receptor a in goat mammary epithelial cells

Sterol regulatory element binding protein-1 (SREBP-1) is a key transcription factor that regulates lipogenesis in rodent liver. Two isoforms (SREBP-la and SREBP-1c) of SREBP-1 are transcribed by an alternative promoter on the same gene (SREBF1), and the isoforms differ only in their first exon. Although the regulatory effects of SREBP-1 on lipid and milk fat synthesis have received much attention in ruminants, SREBP-1c promoter and its regulatory mechanisms have not been characterized in the goat. In the present study, we cloned and sequenced a 2,012-bp fragment of the SREBP-lc 5'-flanking region from goat genomic DNA. A luciferase reporter assay revealed that SREBP-1c is transcriptionally activated by the liver X receptor alpha (LXR alpha) agonist T0901317, and is decreased by SREBP-1 small interfering (si)RNA. A 5' deletion analysis revealed a core promoter region located 395 to +1 bp upstream of the transcriptional start site (TSS). Site-directed mutagenesis of LXR alpha binding elements (LXRE1 and LXRE2) and sterol regulatory elements (SRE1 and SRE2) revealed that the full effects of T0901317 require the presence of both LXRE and SRE. We also characterized a new SRE (SRE1) and demonstrated a direct role of SREBP-1 (auto loop regulation) in maintaining its basal transcription activity. Results suggest that goat SREBP-1c gene is transcriptionally regulated by mature SREBP-1 (autoloop circuit regulation) and LXRot in goat mammary epithelial cells.