Protein Breakdown in Muscle from Burned Rats Is Blocked by Insulin-Like Growth Factor I and Glycogen Synthase Kinase-3β Inhibitors

Abstract

We reported previously that IGF-I inhibits burn-induced muscle proteolysis. Recent studies suggest that activation of the phosphotidylinositol 3-kinase (PI3K)/Akt signaling pathway with downstream phosphorylation of Forkhead box O transcription factors is an important mechanism of IGF-I-induced anabolic effects in skeletal muscle. The potential roles of other mechanisms in the anabolic effects of IGF-I are less well understood. In this study we tested the roles of mammalian target of rapamycin and glycogen synthase kinase-3beta (GSK-3beta) phosphorylation as well as MAPK- and calcineurin-dependent signaling pathways in the anticatabolic effects of IGF-I by incubating extensor digitorum longus muscles from burned rats in the presence of IGF-I and specific signaling pathway inhibitors. Surprisingly, the PI3K inhibitors LY294002 and wortmannin reduced basal protein breakdown. No additional inhibition by IGF-I was noticed in the presence of LY294002 or wortmannin. Inhibition of proteolysis by IGF-I was associated with phosphorylation (inactivation) of GSK-3beta. In addition, the GSK-3beta inhibitors, lithium chloride and thiadiazolidinone-8, reduced protein breakdown in a similar fashion as IGF-I. Lithium chloride, but not thiadiazolidinone-8, increased the levels of phosphorylated Foxo 1 in incubated muscles from burned rats. Inhibitors of mammalian target of rapamycin, MAPK, and calcineurin did not prevent the IGF-I-induced inhibition of muscle proteolysis. Our results suggest that IGF-I inhibits protein breakdown at least in part through a PI3K/Akt/GSK3beta-dependent mechanism. Additional experiments showed that similar mechanisms were responsible for the effect of IGF-I in muscle from nonburned rats. Taken together with recent reports in the literature, the present results suggest that IGF-I inhibits protein breakdown in skeletal muscle by multiple mechanisms, including PI3K/Akt-mediated inactivation of GSK-3beta and Foxo transcription factors.

IGF-1 is downregulated in experimental cancer cachexia

“…This possibility also is consistent with a recent report showing that restoration of normal atrogin-1 expression induced in fasted or diabetic rats by treatment with IGF-1 is not associated with recovery of muscle mass (25). In addition, the reduction of protein degradation exerted by GSK-3␤ inhibitors in muscles isolated from burned rats has been reported to occur without changes in atrogin-1 mRNA levels (30). Finally, inhibition of protein wasting by PTX ϩ FRT is paralleled by restoration of MuRF1 mRNA levels, without changes of atrogin-1 expression.…”

Glucocorticoids and Skeletal Muscle

“…Interestingly, while on one hand activated GSK3β has been implicated as an important mediator of glucocorticoid induced muscle atrophy [ 145 ], this kinase also directly phosphorylates the GR [ 146 ]. This results in increased activation of downstream targets such as GILZ; however, the effect of GSK3β phosphorylation of the GR appears to be highly target gene specifi c. Since these studies were conducted in nonmuscle cell lines, it will be interesting to examine GSK3β/GR interactions in the context of skeletal muscle atrophy in that it could serve as a selective feed forward mechanism to amply GR actions in the face of decreased insulin or other growth factor stimulation.…”

IGF-I does not prevent myotube atrophy caused by proinflammatory cytokines despite activation of Akt/Foxo and GSK-3β pathways and inhibition of atrogin-1 mRNA

“…The use of the in vitro C 2 C 12 myotube model allowed uncovering of the signaling pathway by which IGF-I inhibits the atrogin-1 gene expression. Dissection of this pathway has revealed that IGF-I stimulates Akt by phosphorylation, which causes in turn the phosphorylation of the forkhead transcription factors (Foxo) and glycogen synthase kinase-3␤ (GSK-3␤) (14,28) and, hence, their inactivation (36,38). These two factors, in their dephosphorylated state, indeed play a cardinal role in the control of the E3 ubiquitin ligase atrogin-1 expression (13,36,38) by activating its transcription.…”