Abnormalities in metabolism leading to loss of muscle mass contribute to the morbidity and mortality of kidney failure. Our long-term goal is to identify uremia-induced mechanisms causing muscle loss in order to devise novel therapies to combat this problem. We have identified new processes that cause muscle protein losses: 1) caspase-3 activation is the initial step that breaks down the complex structure of muscle yielding substrates that are degraded by the ubiquitin system. 2) The trigger that accelerated muscle loss is decreased activity of IRS-1 associated phosphatidylinositol 3-kinase activity (IRS-1-PI3K). Decreased IRS-1- PI3K stimulates caspase-3 and the ubiquitin system, including the critical enzyme, atrogin-1/MAFbx. 3) Physiological levels of glucocorticoids (GC) are absolutely required to activate protein degradation pathways. Thus, we propose a two hit process: GC and suppressed insulin responses synergistically suppress IRS-1- PI3K activity and initiate muscle proteolysis. We will test the two hit model using experimental models; mice lacking the insulin or IGF-1 receptor in muscle. We will extend our study to uremia using transgenic mice that exhibit activation of the IRS-1-PI3K pathway (e.g., PTEN deletion, Akt or IGF-1). Specifically, we will: 1) test if there is an essential role for both GC and impaired insulin/IGF-1 responses that stimulates muscle proteolysis in mice with deficiency of the insulin or the IGF-1 receptor or both receptors but only in muscle; 2) examine the mechanism for GC- and insulin/IGF-1 deficiency-dependent downregulation of IRS-1-PI3K activity in muscle; and 3) examine how manipulation of PI3K and Akt activities in vivo will change uremia- induced muscle proteolysis using transgenic mice. The significance of our results is that the two-hit model could apply to many conditions because GC and decreased insulin responses are present in many catabolic illnesses,
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