ARS | Publication request: Effects of sex steroids on indices of protein turnover in rainbow trout (Oncorhynchus mykiss) white muscle

Submitted to: General and Comparative Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 15, 2011
Publication Date: November 1, 2011
Citation: Cleveland, B.M., Weber, G.M. 2011. Effects of sex steroids on indices of protein turnover in rainbow trout (Oncorhynchus mykiss) white muscle. General and Comparative Endocrinology. 174:132-142.

Interpretive Summary: In fish, elevations in plasma estrogen in sexually maturing help the liver produce the protein (vitellogenin) that eventually becomes the yolk found in rainbow trout eggs. The amino acid components that eventually become vitellogenin are stored in muscle and are mobilized to the liver during sexual maturation. However, the hormonal signals responsible for the mobilization of the proteins and amino acids are unknown. We determined that estrogen is a hormone that increases protein degradation in muscle. Estrogen-induced decreases in protein synthesis and increases in expression of genes related to protein degradation also occurred. These results suggest that the increase in estrogen during sexual maturation promotes the movement of proteins and amino acids from muscle to the liver for vitellogenin production. Androgens (testosterone and dihydrotestosterone) did not have widespread effects on protein degradation or proteolytic gene expression, suggesting that in fish these hormones do not act directly in muscle to regulate protein mobilization. Determining how sex steroids like estrogen and testosterone affect protein metabolism during sexual maturation will improve our understanding of the maturation and reproductive process in rainbow trout, which is central to the development of strategies that aim to improve reproductive success in these fish.

Technical Abstract: Effects of 17-estradiol (E2), testosterone, and 5a-dihydrotestosterone (DHT) on protein turnover and proteolytic gene expression were determined in rainbow trout (Oncorhynchus mykiss) primary myocytes and white muscle tissue. E2 reduced rates of protein synthesis and increased rates of protein degradation in primary myocytes by 45% and 27%, respectively. DHT reduced rates of protein synthesis by 27%. Testosterone did not affect protein synthesis and neither testosterone nor DHT affected rates of protein degradation. Single injections of E2 increased expression of ubiquitin ligase genes fbxo32, fbxo25, and murf1, and the proteasome subunit psmd6 by 24 hrs after injection. Within the cathepsin-lysosome pathway, E2 increased expression of cathepsins ctsd and ctsl, as well as autophagy-related genes atg4b and lc3b. Additionally, E2 injection up-regulated the expression of casp3 and casp9 caspase genes. Incubation of primary myocytes with E2 also increased expression of ubiquitin ligase genes. Therefore, catabolic effects of E2 on protein turnover result in part from E2-induced increases in proteolytic gene expression directly in muscle. Injection of testosterone increased milli-calpain (capn2) and casp3 expression, and DHT increased ctsd expression in vivo, whereas both androgens up-regulated fbxo32 expression in primary myocytes. These results suggest that effects of androgens on protein turnover in muscle are not driven primarily by direct effects of these hormones in this tissue.