Muscle protein synthesis as the target of androgen action

Induction of androgen deficiency by administration of a long acting GnRH agonist in healthy, young men is associated with decreased rates of 13C-leucine appearance, a measure of proteolysis (Mauras etal. 1998). Lowering of testosterone concentrations in this study (Mauras etal. 1998) is also associated with a significant decrease in nonoxidative leucine disappearance, a marker for whole body protein synthesis. Conversely, testosterone supplementation stimulates the synthesis of mixed skeletal muscle proteins (Brodsky etal. 1996; Ferrando etal. 2002; Urban etal. 1995). All of these studies of protein turnover have been performed in the fasting state in which the net balance between protein synthesis and breakdown is negative. Testosterone administration improves the muscle protein balance and makes it less negative (Ferrando etal. 2002; Urban etal. 1995). However, none of the studies has demonstrated a clear improvement in muscle protein balance into the positive territory, which would indicate net protein accretion. It has been assumed, but never demonstrated, that during the fed state, testosterone administration leads to net protein accretion. Testosterone improves the efficiency of reutilization of amino acids in the muscle (Ferrando etal. 1998). The effects of testosterone administration on muscle protein breakdown have not been studied extensively. A recent study by Ferrando etal. (2002) reported a significant decrease in muscle protein breakdown following testosterone supplementation of older men. In this study, the proteasome peptidase activity was decreased by testosterone administration, a finding consistent with the decrease in muscle protein degradation assessed by using labeled phenylalanine and measurements of arteriovenous differences.

Several observed effects of testosterone administration on body composition and muscle histomorphology are not easily explained by the muscle protein hypothesis. If muscle protein synthesis or degradation were the primary target of androgen action, then a separate mechanism would be required to explain the reduction in fat mass that occurs during androgen administration. Similarly, muscle protein hypothesis does not easily explain the observed increases in the number of myonuclei and satellite cells in the skeletal muscle during androgen treatment. Undoubtedly, muscle fiber hypertrophy could not occur without a net increase in protein accretion; however, it is likely that the increase in muscle protein synthesis is a secondary event in the cascade of molecular processes that culminate in muscle fiber hypertrophy.

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