PGC-1 promotes insulin resistance in liver through PPAR-alpha-dependent induction of TRB-3.

Insulin resistance is a major hallmark in the development of type 2 diabetes, which is characterized by an impaired ability of insulin to inhibit glucose output from the liver and to promote glucose uptake in muscle. The nuclear hormone receptor coactivator PGC-1 (peroxisome proliferator-activated (PPAR)-gamma coactivator-1) has been implicated in the onset of type 2 diabetes. Hepatic PGC-1 expression is elevated in mouse models of this disease, where it promotes constitutive activation of gluconeogenesis and fatty acid oxidation through its association with the nuclear hormone receptors HNF-4 and PPAR-alpha, respectively. Here we show that PGC-1-deficient mice, generated by adenoviral delivery of PGC-1 RNA interference (RNAi) to the liver, experience fasting hypoglycemia. Hepatic insulin sensitivity was enhanced in PGC-1-deficient mice, reflecting in part the reduced expression of the mammalian tribbles homolog TRB-3, a fasting-inducible inhibitor of the serine-threonine kinase Akt/PKB (ref. 6). We show here that, in the liver, TRB-3 is a target for PPAR-alpha. Knockdown of hepatic TRB-3 expression improved glucose tolerance, whereas hepatic overexpression of TRB-3 reversed the insulin-sensitive phenotype of PGC-1-deficient mice. These results indicate a link between nuclear hormone receptor and insulin signaling pathways, and suggest a potential role for TRB-3 inhibitors in the treatment of type 2 diabetes.