Mechanism for fatty acid sparing effect on glucose-induced transcription - Regulation of carbohydrate-responsive element-binding protein by AMP-activated protein kinase

Carbohydrate-responsive element-binding protein (ChREBP) is a new transcription factor that binds to the carbohydrate-responsive element of the L-type pyruvate kinase gene (L-PK). The aim of this study was to investigate the mechanism by which feeding high fat diets results in decreased activity of ChREBP in the liver (Yamashita, H., Takenoshita, M., Sakurai, M:, Bruick, R. K., Henzel, W. J., Shillinglaw, W., Arnot; D., and Uyeda, K. (2001) Proc. Natl. Acad. Sci. U.S.A. 98,9116-9121). We cloned the rat liver ChREBP gene for use throughout this study. Acetate, octanoate, and palmitate inhibited the glucose-induced activation of L-PK transcription in ChREBP-overexpressed hepatocytes. In these hepatocytes, the cytosolic AMP concentration increased 30-fold and AMP-activated protein kinase activity was activated 2-fold. Similarly to the fatty acids, 5-amino-4-imidazolecarboxamide ribotide, a specific activator of AMP-activated protein kinase (AMPK) also inhibited the L-PK transcription activity in ChREBP-overexpressed hepatocytes. Using as a substrate a truncated ChREBP consisting of the C-terminal region, we demonstrated that phosphorylation by AMPK resulted in inactivation of the DNA binding activity. AMPK specifically phosphorylated Ser(568) of ChREBP. A S568A mutant of the ChREBP gene showed tight DNA binding and lost its fatty acid sensitivity, whereas a S568D mutant showed weak DNA binding and inhibited L-PK transcription activity even in the absence oaf fatty acid. These results strongly suggested that the fatty acid inhibition of glucose-induced L-PK transcription resulted from AMPK phosphorylation of ChREBP at Ser(568,) which inactivated the DNA binding activity. AMPK was activated by the increased AMP that was generated by the fatty acid activation.