Loss of class IA PI3K signaling in muscle leads to impaired muscle growth, insulin response, and hyperlipidemia.

The evolutionarily conserved phosphoinositide 3-kinase (PI3K) signaling pathway mediates both the metabolic effects of insulin and the growth-promoting effects of insulin-like growth factor-1 (IGF-1). We have generated mice deficient in both the p85alpha/p55alpha/p50alpha and the p85beta regulatory subunits of class I(A) PI3K in skeletal muscles. PI3K signaling in the muscle of these animals is severely impaired, leading to a significant reduction in muscle weight and fiber size. These mice also exhibit muscle insulin resistance and whole-body glucose intolerance. Despite their ability to maintain normal fasting and fed blood glucose levels, these mice show increased body fat content and elevated serum free fatty acid and triglyceride levels. These results demonstrate that in vivo p85 is a critical mediator of class I(A) PI3K signaling in the regulation of muscle growth and metabolism. Our finding also indicates that compromised muscle PI3K signaling could contribute to symptoms of hyperlipidemia associated with human type 2 diabetes.