PGC-1α Integrates Insulin Signaling, Mitochondrial Regulation, and Bioenergetic Function in Skeletal Muscle

Abstract

The pathophysiology underlying mitochondrial dysfunction in insulin-resistant skeletal muscle is incompletely characterized. To further delineate this we investigated the interaction between insulin signaling, mitochondrial regulation, and function in C2C12 myotubes and in skeletal muscle. In myotubes elevated insulin and glucose disrupt insulin signaling, mitochondrial biogenesis, and mitochondrial bioenergetics. The insulin-sensitizing thiazolidinedione pioglitazone restores these perturbations in parallel with induction of the mitochondrial biogenesis regulator PGC-1␣. Overexpression of PGC-1␣ rescues insulin signaling and mitochondrial bioenergetics, and its silencing concordantly disrupts insulin signaling and mitochondrial bioenergetics. In primary skeletal myoblasts pioglitazone also up-regulates PGC-1␣ expression and restores the insulin-resistant mitochondrial bioenergetic profile. In parallel, pioglitazone upregulates PGC-1␣ in db/db mouse skeletal muscle. Interestingly, the small interfering RNA knockdown of the insulin receptor in C2C12 myotubes down-regulates PGC-1␣ and attenuates mitochondrial bioenergetics. Concordantly, mitochondrial bioenergetics are blunted in insulin receptor knock-out mouse-derived skeletal myoblasts. Taken together these data demonstrate that elevated glucose and insulin impairs and pioglitazone restores skeletal myotube insulin signaling, mitochondrial regulation, and bioenergetics. Pioglitazone functions in part via the induction of PGC-1␣. Moreover, PGC-1␣ is identified as a bidirectional regulatory link integrating insulin-signaling and mitochondrial homeostasis in skeletal muscle.Understanding the pathophysiology initiating the development of insulin resistance should augment our capacity to identify novel therapeutic targets for the prevention and treatment of type 2 diabetes. This biology remains incompletely characterized in part due to the complexity of the interaction of multiple organ systems and the multiplicity of intracellular perturbations within these organs governing the development of insulin resistance. The major peripheral organ systems implicated in insulin resistance include skeletal muscle, adipose tissue, liver, and the immune system. The enhancement of our understanding of this biology will require the combination of reductionist and systems biological approaches.The complexity of the effects of insulin resistance in a single organ is exemplified in skeletal muscle where disruption in glucose uptake (1), insulin signaling (2, 3), glycogen synthesis (4) and in mitochondrial biology (5-7) are evident in insulin-resistant subjects up to two decades before their developing diabetes. A fundamental question arising is whether disruption of skeletal muscle mitochondria is a primary component in this disease pathophysiology or whether it is a consequence of reduced aerobic activity in response to alternate metabolic perturbations associated with insulin resistance and diabetes (8). Although this question has not been definitively answered, increasing mitocho...

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