Mechanism for Fatty Acid “Sparing” Effect on Glucose-induced Transcription

Abstract

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 ChREBPoverexpressed 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 of 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.Glucose metabolism in liver is inhibited by administration of fatty acids, the so-called "glucose sparing" effect (1-3). Fatty acids inhibit genes of key enzymes of glycolysis and lipogenesis such as L-pyruvate kinase (L-PK), 1 acetyl-CoA carboxylase, and fatty acid synthetase. L-PK, regulating the flux of metabolites through the pyruvate-phosphoenolpyruvate cycle (4), is known to play an important role in hepatic glucose and lipid metabolism. The activity of L-PK is subject to acute control by covalent modification and allosteric effectors (5). On the other hand, long term control of L-PK is achieved by regulating L-PK gene transcription (5). Fatty acids inhibit transcription of L-PK and other enzymes in glycolysis and lipogenesis pathways, whereas excess glucose induces expression of these genes (6). However, the mechanism of fatty acid inhibition of transcription is not understood. We recently identified a new transcription factor, which binds specifically to the carbohydrate-responsive element of the L-PK gene, and we have termed this new transcription factor, "carbohydrate-responsive element-binding protein" (ChREBP) (7). ChREBP is expressed specifically in liver and is responsive to diet. ChREBP is activated by a high carbohydrate diet and inhibited by a h...

ChREBP Mediates Glucose Repression of Peroxisome Proliferator-activated Receptor α Expression in Pancreatic β-Cells

“…These results suggest that glucose regulates GPDH and PPAR␣ expression by a common signaling pathway involving AMPK and ChREBP. Furthermore, the results indicate that AMPK can counteract glucose-induced ChREBP activity in ␤-cells as previously reported in hepatocytes (39).…”

“…However, the fact that the contribution of the pentose phosphate pathway to total glucose oxidation is very low in islets and purified islet ␤-cells (57,58) suggests that it is not xylulose-5-phosphate but rather another glucose metabolite that activates PP2A in ␤-cells. Furthermore, AMPK has been reported to impair the transcriptional activity of ChREBP in primary mouse hepatocytes and to inhibit the DNA binding activity of mouse ChREBP by phosphorylating ChREBP on Ser 566 in vitro (39). Moreover, the same site was found to be phosphorylated in primary rat hepatocytes in an independent study (59).…”

“…Furthermore, dephosphorylation of ChREBP by PP2A was found to relieve inhibitory phosphorylations by protein kinase A and AMP-activated protein kinase (AMPK), which have been shown to interfere with nuclear localization and DNA binding, respectively (35,36,38,39).…”

Suppression of Long Chain Acyl-CoA Synthetase 3 Decreases Hepatic de Novo Fatty Acid Synthesis through Decreased Transcriptional Activity

“…Recently studies have shown that the acyl-CoA synthetase activity plays a key role in altering AMPK activation (28). Thus, ACSL3 knockdown could potentially alter AMPK activation, which is known to regulate numerous transcription factors to control gene expression (29,30). However, neither total AMPK nor phosphorylation of AMPK at Thr-172 were changed by ACSL3 knockdown (Fig.…”

Stearoyl-CoA desaturase 1 deficiency increases fatty acid oxidation by activating AMP-activated protein kinase in liver

“…Phosphorylation of p300 by AMPK inhibits its interaction with several nuclear receptors, i.e., peroxisome proliferator-activated receptors and thyroid hormone, retinoic acid, and retinoid X receptors, resulting in either activation or repression of gene expression (12). Phosphorylation of carbohydrateresponse-element-binding protein by AMPK blocks DNA binding and mediates inhibition of glucose-induced gene transcription (13). AMPK activation also suppresses the expression of SREBP-1c, a key transcription factor regulating genes of fatty acid and triglyceride biosynthesis (14).…”