Regulation of de novo lipogenesis through BAD-dependent glucose signaling The homeostatic balance between hepatic utilization, storage, and production of glucose and fat in fed and fasted states is exquisitely controlled by hormonal and nutrient cues. In the fed state, the liver stores excess glucose first as glycogen then as fat through de novo lipogenesis. How the liver senses glucose to determine its utilization and storage is not fully understood at the molecular level. Emerging evidence indicates that glucose-derived metabolites, including the activity of glucokinase (GK), which catalyzes the first step of hepatic glycolysis, influence de novo lipogenesis by both providing metabolic precursors for lipid synthesis as well as triggering the expression of lipogenic genes. Glucose regulation of de novo lipogenesis is dependent, at least in part, on the Carbohydrate Response Element Binding Protein (ChREBP), a transcription factor that is activated by certain glucose-derived metabolites. My proposed studies test the role of the BCL-2 family protein BAD as an upstream regulator of hepatic glucose signaling and de novo lipogenesis through ChREBP. BAD's modulation of glucose metabolism is mediated by phosphorylation of Ser155, which leads to direct activation of GK. The functional relevance of this interaction is evident from the observations that BAD deficiency or interference with its phosphorylation is associated with reduced hepatic GK activity and glycolysis. I have found that glucose induction of ChREBP activity and lipogenic gene expression is diminished in Bad -/- hepatocytes. These data, together with the known capacity of BAD to activate GK and GK's relevance in ChREBP activation, give rise to the hypothesis that BAD modulates hepatic ChREBP activity and de novo lipogenesis through its ability to regulate GK activity. I will test this hypothesis through the following specific aims:
Aim 1 will interrogate the acute and cell autonomous effect of BAD modifications in primary hepatocytes and the attendant changes in GK on stimulation of ChREBP transcriptional activity by glucose.
Aim 2 will determine the functional and metabolic correlates of BAD-dependent changes in lipogenic gene expression by biochemical measurement of de novo lipogenesis and glucose-dependent changes in lipid profiles in primary hepatocytes following genetic or pharmacologic modification of BAD.
Aim 3 will dissect the mechanistic link between BAD modifications and ChREBP activity by examining alterations in defined post translational modifications of ChREBP in response to glucose, including phosphorylation and acetylation known to modulate ChREBP transcriptional activity. In the fullness of time, these studies will provide an integrated picture of the pathway connecting BAD and GK- dependent glucose metabolism with ChREBP activity and the lipogenic program. Understanding this pathway should yield useful molecular insights into hepatic glucose sensing, nutrient utilization and storage.
Excess glucose is converted to fats in the liver. Elucidating how the liver senses glucose and controls fat synthesis may, in the fullness of time, lead to new therapeutic targets for diseases involving excess fat synthesis including: obesity, type II diabetes and cardio-vascular disease. The proposed studies examine the role of a protein called BAD in fat synthesis from glucose.
