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

Abstract Text: 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.  

Keywords: AMPK, glucose, mTOR, SREBP1, t10,c12-CLA