The Mediator complex kinase module is necessary for fructose regulation of liver glycogen levels through induction of glucose-6-phosphatase catalytic subunit (G6pc).

Abstract Objective Liver glycogen levels are dynamic and highly regulated by nutrient availability as the levels decrease during fasting and are restored during the feeding cycle. However, feeding in the presence of fructose in water suppressed glycogen accumulation in the liver by up-regulating the expression of glucose-6-phosphatase catalytic subunit (G6pc) gene although the exact mechanism is unknown. We generated liver specific knockout of MED13 mice that lack the transcriptional Mediator complex kinase module to examine its effect on transcriptional activation of inducible target gene expression, such as the ChREBP- and FOXO1-dependent control of the G6pc gene promoter. Methods The relative changes in liver expression of lipogenic and gluconeogenic genes as well as glycogen levels were examined in response to fed standard low fat laboratory chow supplemented with water, or water containing sucrose or fructose in control (Med13fl/fl) and liver-specific MED13 knockout (MED13-LKO) mice. Results Although MED13 deficiency had no significant effect on constitutive gene expression, all dietary inducible gene transcripts were significantly reduced despite the unchanged insulin sensitivity in MED13-LKO mice as compared to the control mice. G6pc gene transcription displayed the most significant difference between Med13fl/fl and MED13-LKO mice particularly when fed with fructose. Following fasting that depleted liver glycogen, feeding induced the restoration of glycogen levels except in the presence of fructose. MED13 deficiency rescued the glycogen accumulation defect in the presence of fructose. This is resulted from the suppression of G6pc expression and thus G6PC enzymatic activity. Among two transcriptional factors that regulate G6pc gene expression, FOXO1-binding to the G6pc promoter was not affected, whereas ChREBP-binding was dramatically reduced in MED13-LKO hepatocytes. In addition, there was a marked suppression of both FOXO1 and ChREBP-β transcriptional activities in MED13-LKO hepatocytes. Conclusions Taken together, our data suggest that the kinase module of the Mediator complex is necessary for the transcriptional activation of metabolic genes, such as G6pc, and has an important role in regulation of the glycogen levels in the liver through altering transcription factor binding and activity at the G6pc promoter.