Effects of androgens on gene expression in muscle 428

Abstract

The mechanism by which androgens increase muscle mass in animals and in man is unclear. We investigated the effect of androgen on expression of IGF-1, MyoD, myogenin, and androgen receptor (AR) in cultured rodent C2C12 myoblasts. Differentiation of cells was induced by changing culture medium to lower fetal bovine serum concentration after 24 hours (2% vs. standard 10%). Morphological characteristics on light microscopy served to identify differentiated cells. Undifferentiated and differentiated cells were treated with dihydrotestosterone, DHT (10-100 nM) for 48 hours. RNA was extracted and radiolabeled RT-PCR was performed to detect the expression of various genes. Samples from cells not exposed to androgen served as controls, and apparent gene expression was compared by densitometry. There was no effect of androgens on expression of the myogenic factors IGF-I, MyoD1, and myogenin in either differentiated or undifferentiated myocytes with the DHT regimens used. AR was expressed in cultured muscle cells, with increased expression seen in differentiated C2C12 cells in the basal state. DHT treatment resulted in down-regulation of AR in undifferentiated cells. AR expression was also detected in adult murine skeletal muscle. Dystrophic (dy) murine muscle had approximately 3-5 fold lower AR expression compared to that of control mice, despite equal endogenous circulating testosterone levels. We speculate that deficient androgenic activity in dy muscle, contributes to muscle wasting. We conclude that androgens act via the androgen receptor to promote myoblast differentiation. The genes which mediate this process may not include IGF-1, MyoD, or myogenin.Partial grant support provided by Genentech Foundation and by E. Lilly Co.