0868 Regulation of sterol regulatory element binding protein-1 in bovine mammary epithelial cells

The objective of this study was to investigate the molecular mechanisms by which nutrients regulate sterol regulatory element binding protein-1 (SREBP1) in bovine mammary epithelial cells (Mac-T). Three models were tested. First, the relationship between SREBP1 and the mechanistic target of rapamycin (mTOR) signaling was tested through mTOR activation/inhibition as well as SREBP1 knockdown by siRNA. Second, the relationship between AMPK and SREBP1 was tested in t10,c12-CLA-treated Mac-T cells. Third, the activation of SREBP1 was tested by glucose supplementation. Results showed that mTOR activation increased SREBP1 protein as well as the lipogenic gene expression by over 50%. While inhibition on mTOR failed to increase SREBP1, siRNA-directed SREBP1 knockdown confirmed that insulin enhanced lipogenic gene transcription through SREBP1. Further examination found that mTOR signaling regulates SREBP1 by preventing its proteosomal degradation. t10,c12-CLA decreased SREBP1 protein and lipogenic gene expression through phosphorylation of AMPK, while inhibition of AMPK phosphorylation partially rescued the SREBP1 reduction. Lastly, low glucose (1 mmol/L) was able to increase mature SREBP1 level by 2.2-fold. Increasing glucose concentration increased SREBP-cleavage activating protein, a key regulator of SREBP1 activation, in a dosage- and time-dependent manner. In conclusion, these results showed that major cellular metabolic regulators play roles in SREBP1 activation and degradation thus regulating lipogenesis in response to the nutrients provided.