transcriptional activation of phosphoenolpyruvate carboxykinase(PEPCK), fructose-1,6-bisphosphatase and glucose-6-phosphatase(G6Pase). Overexpression of PEPCK promotes IR in mice [57].Insulin action represses PEPCK expression via Akt-mediatedFoxO1 phosphorylation. This pathway is repressed by activationof PPARa signaling-mediated induction of the pseudokinase tribbles-homologue 3 (TRB3), and TRB3 expression is associated withIR in patients [58,59]. Other NRs (PGC1a, FXR), glucagon and glucocorticoidsalso mediate expression of PEPCK and G6Pase[55,60]. FoxO1, a transcriptional activator of PEPCK and G6Pase,is both directly and indirectly activated by PGC1a, HNF4a, CREBPand PPARa [61]. PEPCK knock-out mice not only show decreasedgluconeogenesis, but more importantly a decreased removal ofTCA anions, which causes hepatic TAG accumulation and steatosis[62]. In mice, PEPCK overexpression in the striated muscleinterestingly leads to longevity and an impressive increase inmuscle strength and endurance; this phenotype is partiallyexplained by an optimized mitochondrial oxidation of TAG [63].These processes again demonstrate the close inter-relationbetween gluconeogenesis and hepatic lipid metabolism (Fig. 2B).
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