Regulation of lipid synthesis by liver X receptor α and sterol regulatory element-binding protein 1 in mammary epithelial cells

The objectives of this experiment were to characterize the roles of the transcription factors liver X receptor alpha (LXR alpha) and sterol regulatory element-binding protein 1 (SREBP1) in the transcriptional regulation of lipid synthesis in a bovine mammary epithelial cell line. Whereas many lipid synthesis genes contain a response element in their promoters for SREBP1, a few also contain a response element for LXR, suggesting that both transcription factors could directly regulate transcription of these genes. However, the promoter of SREBP1 contains a response element for LXR, indicating the additional potential for indirect transcriptional regulation by LXR, through SREBP1, on lipogenic genes. To characterize these effects, small interfering RNA (siRNA) directed against LXRa and SREBP1 were used to knockdown gene expression, and then, in the presence of SREBP1 siRNA, T0901317 (T09) was used to specifically activate LXR alpha. Reducing LXR alpha mRNA abundance in mammary alveolar T cells did not alter mRNA abundance of genes involved in de novo lipogenesis or the rate of de novo lipogenesis, suggesting that LXR alpha is not required for basal transcription of genes required for fatty acid synthesis. Knockdown of SREBP1 reduced the mRNA abundance of acetylcoenzyme A (CoA) carboxylase, fatty acid synthase, diacylglycerol acyltransferase, and stearoyl-CoA desaturase-1, indicating that these genes are regulated in part by SREBP1. When SREBP1 was reduced, T09 increased the mRNA abundance of acetyl-CoA carboxylase, fatty acid synthase, and diacylglycerol acyltransferase, potentially indicating that these genes are directly regulated by LXR. The results of the present study provide insight into the transcriptional regulatory mechanisms involved in lipid synthesis by mammary epithelial cells, and suggest that several transcription factors may be required for full lipogenic activation.