Mlx Is the Functional Heteromeric Partner of the Carbohydrate Response Element-binding Protein in Glucose Regulation of Lipogenic Enzyme Genes

Abstract

The expression of genes encoding enzymes involved in de novo triglyceride synthesis (lipogenesis) is transcriptionally induced in the liver in response to increased glucose metabolism. The carbohydrate response element-binding protein (ChREBP) is a newly identified basic helix-loop-helix/leucine zipper transcription factor proposed to regulate the expression of the glucoseresponsive gene pyruvate kinase. This gene contains a carbohydrate response element (ChoRE) consisting of two E box motifs separated by 5 bp that is necessary and sufficient for glucose regulation. We demonstrate that overexpression of ChREBP in primary rat hepatocytes activates other ChoRE-containing promoters in a manner consistent with their ability to respond to glucose. In vitro binding of ChREBP to ChoRE sequences was not detected. Because E box-binding proteins function as obligate dimers, we performed a yeast two-hybrid screen of a mouse liver cDNA library to identify potential heteromeric partners. Mlx (Max-like protein X) was selected as the only basic helix-loop-helix/leucine zipper interaction partner in this screen. When a plasmid expressing either Mlx or ChREBP was cotransfected with a ChoRE-containing reporter plasmid into human embryonic kidney 293 cells, no increase in promoter activity was observed. However, the expression of both proteins dramatically enhanced promoter activity. This activation was observed with reporters containing ChoREs from several different lipogenic enzyme genes. In contrast, reporters containing non-glucose-responsive E box elements were not activated by ChREBP-Mlx expression. In vitro binding of ChREBP to ChoRE-containing oligonucleotides was observed only in the presence of Mlx. ChREBP-Mlx binding discriminated between E box sites that are glucose-responsive and those that are not. We conclude that Mlx is a functional heteromeric partner of ChREBP in regulating the expression of glucose-responsive genes.

The subcellular localization of the ChoRE-binding protein, encoded by the Williams–Beuren syndrome critical region gene 14, is regulated by 14-3-3

“…Hence, 14-3-3 proteins appear to be important regulators of the Mlx-driven transcription factor (this work and 32). The regulation of glucose metabolism by Wbscr14 supports the hypothesis that the diabetic phenotype observed in WBS patients could be related to WBSCR14 hemizygosity (5,6,9,14,57). The hemizygosity of this transcription factor could reduce its ability to respond to high-glucose diet through activation of the expression of enzymes involved in de novo triglycerides synthesis using glucose precursors.…”

“…Conversely, glucagon and fatty acids increase the level of AMP and cAMP in hepatocytes activating AMPK and/or PKA, which results in the phosphorylation of WBSCR14 at residue Ser 568 and inhibition of its DNA-binding activity (5,6,9,12). Recent observations demonstrated that rat WBSCR14 plays also an essential role in the regulation of other lipogenic enzymes, besides LPK, involved in glucose and lipid metabolism (12)(13)(14).…”

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

“…3B). ChREBP is thought to be the primary glucose-responsive transcription factor that governs the expression of glycolytic and lipogenic genes (6,9). Similar to the other lipogenic transcription factors, ACSL3 knockdown suppressed ChREBP reporter gene activity (Fig.…”

Direct Role of ChREBP·Mlx in Regulating Hepatic Glucose-responsive Genes

“…Overexpressing ChREBP in primary hepatocytes induces activity of the ChoRE-containing PK promoter in high glucose conditions but not in low glucose conditions (20). However, ChREBP does not effectively homodimerize or bind to the ChoREs as a homodimer (22,24). Using a yeast-two hybrid screen, Max-like protein X (Mlx) was identified as a bHLH/LZ protein in liver that interacts with the bHLH/LZ domain of ChREBP (24).…”

“…However, ChREBP does not effectively homodimerize or bind to the ChoREs as a homodimer (22,24). Using a yeast-two hybrid screen, Max-like protein X (Mlx) was identified as a bHLH/LZ protein in liver that interacts with the bHLH/LZ domain of ChREBP (24). Mlx is a member of the Myc/Max/Mad family of transcription factors, and similar to Max, it can serve as a common interaction partner of a tran-scription factor network (25)(26)(27).…”