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102

Chapter 5

directly in isolated murine myotubes. Moreover, urinary PIF excretion is related to weight loss in patients with prostatic and primary gastrointestinal tumors. Mechanistically, PIF expression leads to increased proteasome expression (20S proteasome subunits, MSS1 and p42, another ATPase subunit of the 19S regulator, as well as an increased chymotrypsin-like enzyme activity of the b5 subunits of the proteasome) and activity is likely via inducing NFkB in a ROS-dependent manner. Indeed, studies in animal models of cancer cachexia, as well as in cancer patients, suggest that the ubiquitin proteasome pathway plays the predominant role in the degradation of myofibrillar proteins, particularly in patients with a weight loss of 410%.

Depressed protein synthesis during cancer cachexia in skeletal muscle is related not only to anorexia, as protein synthesis is also depressed in other animal models of cachexia where anorexia is absent. This suggests an underlying defect in the protein synthetic machinery. For example, gastrocnemius muscles from mice bearing the cachexia-inducing tumor MAC16 show activation of protein kinase R (PKR) when the weight loss is 416% and a corresponding increase in phosphorylation of eIF2a (an inhibitor of eIF2Be), which would suppress mRNA translation. Weight loss in mice bearing the MAC16 tumor is also associated with an increased amount of eIF4E bound to 4E-BP1 in gastrocnemius muscle, due to hypophosphorylation of 4E-BP1, resulting in a progressive decrease in the concentration of the active eIF4GeIF4E complex. This would also contribute to a depression in protein synthesis, as would also a decrease in phosphorylation of mTOR and p70S6K1. There is also an increase in the phosphorylation of eEF2, which would also decrease protein synthesis through a decrease in translation elongation.

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