miR-26b promoter analysis reveals regulatory mechanisms by lipid-related transcription factors in goat mammary epithelial cells

MicroRNA (miRNA) regulate protein abundance and control diverse aspects of cellular processes and biological functions associated with lipid metabolism. MiR-26b and its host gene CTDSP1 regulate triacylglycerol synthesis by synergistically suppressing the insulin-induced gene 1 (INSIG1); however, the direct regulators of miR-26b expression remain unknown. In the present study, we characterized the activity of a novel putative promoter region in miR-26b. Results revealed that promoter activity and miR-26b expression are dynamically regulated by different transcription factors including peroxisome proliferator-activated receptor gamma (PPARG), sterol regulatory element binding transcription factor 1 (SREBF1), and liver X receptor α (LXRα). Two binding sites for the SREBF1 (SRE1 and SRE3) and the PPARG (peroxisome proliferator response element 1 and 2; PPRE1 and PPRE2), respectively, were identified in the miR-26b promoter, which demonstrated that those binding sites are responsible for the activation by PPARG and SREBF1. In silico analysis and site-directed mutagenesis of LXRα binding elements (LXRE) and SREBF1 binding elements (SRE) revealed that the effects of Ad-LXRα + T0901317 requires the presence of SRE, whereas potential LXRE had no effects on miR-26b expression. This suggested that regulation of miR-26b by LXRα is indirectly via an SRE, and miR-26b is regulated by transcription factors dually through DNA methylation and directly through binding to its promoter, all of which implies that regulation of miR-26b in ruminant mammary epithelial cells results from various mechanisms. In conclusion, we demonstrate a novel dual-regulatory mechanism whereby transcription factors regulate the expression of miR-26b. Overall, these findings contribute to our understanding of the interactions between specific promoter elements and the control of transcription and translation of miRNA.