The casitas b-lineage lymphoma protein (c-Cbl) was initially identified as an E3 ubiquitin ligase involved in regulation of tyrosine kinase signalling. Previous studies showed that global knockout of c-Cbl in mice protects against high fat-induced obesity and insulin resistance by increasing energy expenditure (Diabetes 55:708-15, 2006). We hypothesized that skeletal muscle is an important organ responsible for these observed metabolic effects induced by c-Cbl deficiency. The present study first examined the relationship of obesity with c-Cbl protein expression in skeletal muscle, liver and adipose tissue in C57 male mice. The results showed that c-Cbl content was only significantly increased in muscle (by ~ 35%, p<0.05) in high-fat fed mice where obesity (as indicated by more 80% of BW gain and 3-fold increase in epididymal fat mass, both p<0.01) and glucose intolerance (AUC 1450 vs AUC 710, p<0.01) were evident. We next investigated the specific effect of c-Cbl on energy metabolism in C2C12 muscle cell line by knocking down the expression of c-Cbl. Using a specific short-hairpin RNA, expression of c-Cbl protein was reduced by 60% to 70%. This resulted in significant increases in oxygen consumption basally and during FCCP-stimulated maximal respiration (by ~20%, p2-fold p<0.05). These data suggest that skeletal muscle is likely to be a major site of c-Cbl-mediated energy metabolism for the whole body. While its up-regulation in muscle is associated with obesity down-regulation of c-Cbl promotes energy expenditure in muscle cells with the involvement of AMPK activation and possibly myogenesis. Further studies are under way to investigate whether the increased oxygen consumption by c-Cbl knockdown may protect muscle cells against lipid-induced insulin resistance.
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