Uremia is often associated with weight loss, reduced muscle mass and a low serum albumin. Our long-term goal is to identify cellular mechanisms causing protein malnutrition in uremia to improve treatment. In rats with chronic renal failure (CRF), we identified metabolic acidosis as a stimulus for accelerated muscle protein degradation. This proteolytic response was confirmed in patients and is now vigorously treated to improve their nutritional status but the mechanism(s) underlying muscle catabolism is unknown. Factors that we identified as activators of the ubiquitin-proteasome (Ub-P'some) Proteolytic pathway in muscle are a low extracellular pH, glucocorticoids and a low insulin level. Important problems to be solved are: first, myofibrillar proteins are not degraded and adding actin blocks myosin degradation by the Ub-P'some system. Our Preliminary Results indicate that the protease, caspase 3, is induced by acidification and plays a role in the early stages of muscle cell proteolysis by degrading myofibrillar proteins and actin to a stage where degradation proceeds via the Ub-P'some pathway. A second problem is how to integrate multiple signals that can activate the Ub-P'some system. We propose that depressed phoshatidylinositol 3-kinase (PI 3-kinase) is a key step as: we find CRF reduces muscle PI 3-kinase activity and insulin (or IGF- 1) blocks caspase 3 activation in muscle cells. A third problem is to understand how signals that initiate muscle proteolysis invariably stimulate transcription of genes encoding components of the Ub-P'some pathway. Since clinical evidence suggests an important role for TNFalpha in causing muscle wasting in CRF, we will examine the opposing actions between glucocorticoids and a TNFalpha-inducible transcription factor, NF-kB, on transcription of the C3 proteasome subunit to understand the requirement for glucocorticoids in stimulating transcription in many catabolic conditions. Finally, we will examine how TNFalpha influences protein degradation in muscle of CRF rats and in cultured muscle cells. Our results will uncover cellular mechanisms regulating protein turnover in uremia and other catabolic conditions.
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