Ubiquitin‐proteasome‐dependent muscle proteolysis responds slowly to insulin release and refeeding in starved rats

Abstract

The central role of the ubiquitin‐proteasome system in the loss of skeletal muscle protein in many wasting conditions has been well established. However, it is unclear what factors are responsible for the suppression of this system during periods of protein gain. Thus, the aim of these studies was to examine the short‐term effects of insulin release and nutrients on skeletal muscle protein turnover in young rats starved for 48 h, and then infused intravenously with amino acids (AA), or fed an oral diet. Forty‐eight hours of starvation (i.e. prolonged starvation in young rats) decreased muscle protein synthesis and increased proteasome‐dependent proteolysis. Four‐hour AA infusion and 4 h of refeeding increased plasma insulin release and AA concentrations, and stimulated muscle protein synthesis, but had no effect on either total or proteasome‐dependent proteolysis, despite decreased plasma corticosterone concentrations. Both muscle proteasome‐dependent proteolysis and the rate of ubiquitination of muscle proteins were not suppressed until 10 h of refeeding. The temporal response of these two measurements correlated with the normalised expression of the 14‐kDa E2 (a critical enzyme in substrate ubiquitination in muscle) and the expression of the MSS1 subunit of the 19S regulatory complex of the 26S proteasome. In contrast, the starvation‐induced increase in mRNA levels for 20S proteasome subunits was normalised by refeeding within 24 h in muscle, and 6 h in jejunum, respectively. In conclusion, unlike protein synthesis, skeletal muscle proteasome‐dependent proteolysis is not acutely responsive in vivo to insulin, AA, and/or nutrient intake in refed starved rats. This suggests that distinct and perhaps independent mechanisms are responsible for the nutrient‐dependent regulation of protein synthesis and ubiquitin‐proteasome‐dependent proteolysis following a prolonged period of catabolism. Furthermore, factors other than the expression of ubiquitin‐proteasome pathway components appear to be responsible for the suppression of skeletal muscle proteasome‐dependent proteolysis by nutrition.

Gastrocnemius transcriptome analysis reveals domestication induced gene expression changes between wild and domestic chickens

“…In this system, proteins are first marked for degradation by chains of the polypeptide cofactor ubiquitin, and then recognized by the 26S proteasome which can degrade ubiquitinated proteins to small peptides [17,18]. In our study we observed a number of ubiquitin-related genes with different expression levels in wild and domestic chicken breeds (Table 2), indicating possible selection pressure on this specific function of genes.…”

USP19 is a ubiquitin-specific protease regulated in rat skeletal muscle during catabolic states

“…4B). However, this return to baseline occurred only on day 2 of refeeding, which was 1 day after the normalization of rates of protein breakdown (17) but was simultaneous to the recovery of muscle mass, which increased to levels similar to those of fed controls on day 2 (Fig. 4A).…”

Effects of fasting and refeeding on expression of MAFbx and MuRF1 in chick skeletal muscle

“…MAFbx and MuRF1 were first demonstrated in muscle atrophy, and were found to be overexpressed in most states involving muscle loss, including paralysis, starvation, diabetes, sepsis, renal failure, and glucocorticoid excess [1]. Previous studies suggested that many genes related to the ubiquitinproteasome system were upregulated by starvation, including 14-kD E2, calpain large subunits, and 20S proteasome subunits [4,9]. The current study demonstrated that, in accord with other components of the ubiquitin-proteasome system, the E3 ligases MAFbx and MuRF1 were significantly induced in chick skeletal muscle at both the RNA and protein levels following starvation for 12 and 24 h. Refeeding normalized the expression of these two genes.…”

“…MuRF1 is a muscle-specific RING-finger protein that binds the Cys-His zinc-binding motif [3]. MAFbx and MuRF1 are specifically expressed in cardiac and skeletal muscles and have vital roles in protein degradation during muscle wasting [4]. Numerous studies have demonstrated that the expression of these two genes is upregulated during processes that involve muscle wasting, including muscle atrophy, injury, burns, aging, chronic illness, and starvation [1, 5,6].…”