Transcriptional regulation of lipid synthesis in bovine mammary epithelial cells by sterol regulatory element binding protein-1

The objective of this study was to investigate the transcriptional regulation of lipid synthesis by sterol regulatory element binding protein-1 (SREBP-1) in bovine mammary epithelial cells. In the current study, bovine mammary epithelial (MAC-T) cells cultured in insulin- and prolactin-containing medium were treated with a transfection reagent as control, a nontargeting small interfering (si)RNA sequence (100 nM) as a negative control, or an SREBP-1-specific siRNA (100 nM) for 48 h. The mRNA expression of SREBP-1 was decreased more than 90% by siRNA. Precursor and mature forms of SREBP-1 protein were undetectable in cells treated with SREBP-1 siRNA. Fatty acid synthesis and fatty acid uptake, measured using isotope incorporation, were reduced significantly in cells treated with SREBP-1 siRNA compared with controls. Transcript abundance of acyl-CoA synthetase short-chain family member 2, acetyl-CoA carboxylase, fatty acid synthetase, and isocitrate dehydrogenase 1 (key enzymes of de novo lipogenesis) was decreased by 40 to 65% with SREBP-1 siRNA, in agreement with acetate incorporation data. The mRNA levels of fatty acid binding protein 3 and stearyl-CoA desaturase 1 (proteins responsible for intracellular fatty acid trafficking and long-chain fatty acid modification) were decreased 76 and 60%, respectively, by SREBP-1 siRNA treatment compared with controls. The mRNA expression of mitochondrial glycerol-3-phosphate acyltransferase and lipin 1 (involved in triglyceride synthesis) was significantly decreased in cells treated with SREBP-1 siRNA compared with control cells. However, the expression of milk fat globule membrane proteins measured did not differ among treatments. In conclusion, SREBP-1 plays an important role in integrated regulation of lipid synthesis in bovine mammary epithelial cells through regulation of key enzymes.