The role of SREBP-1c and ChREBP in the regulation of lipogenic gene expression

Lipogenesis is the process by which excess carbohydrates are converted into triglycerides, the main storage form of energy in mammals. Feeding a mammal a high carbohydrate diet induces lipogenesis in the liver by increasing transcription of enzymes involved in the process. These lipogenic enzymes are induced by two potential signaling pathways. First, the increase in insulin secretion from pancreatic R-cells leads to an initiation of insulin signaling pathways. Second, the increase in blood glucose levels following a meal stimulates glucose metabolism in the hepatocyte. The goal of this thesis was to evaluate the contributions of specific transcription factors that are activated by these two signaling pathways. The role of sterol regulatory element binding protein-1c (SREBP-1c), a transcription factor known to be induced by a high carbohydrate diet, was first investigated. SREBP-1c mRNA and protein levels were induced in hepatocytes by insulin treatment, but were unaffected by glucose. Further, with the use of an SREBP-1c-expressing adenovirus in hepatocytes, I was able to demonstrate that SREBP-1c does induce expression of the lipogenic genes encoding fatty acid synthase, acetyl-CoA carboxylase, and S₁₄. However, this induction is not sufficient to account for the increase in expression seen with high glucose and insulin treatment. Therefore, an additional transcription factor is required for the induction of lipogenic enzyme genes. Carbohydrate response element binding protein (ChREBP) has emerged as a candidate for a glucose-responsive transcription factor. Overexpression of ChREBP in hepatocytes was able to increase promoter activity of lipogenic enzyme genes above high glucose treatment in reporter assays. The transcription factor Max-like protein X (Mlx) was found to be the predominant ChREBP-interacting protein in liver. Co-expression of ChREBP and Mlx in fibroblasts activated transcription from multiple glucose-responsive sequences, but did not activate similar sequences that do not respond to glucose. Finally, ChREBP and Mlx are able to heterodimerize and bind to an E-box DNA sequence in an electromobility shift assay. I conclude that SREBP-1c and ChREBP are both critical for induction of lipogenic enzyme genes and mediate actions of insulin and glucose, respectively.