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Unite de Diabetologie et Nutrition, Universite Catholique de Louvain (M.D., R.V.B., N.P., P.L., J.V., J.-M.K., J.-P.T.), B-1200 Brussels, Belgium; and Department of Pediatrics, University of North Carolina (L.U.), Chapel Hill, North Carolina 27599-7039
Address all correspondence and requests for reprints to: Dr. Jean-Paul Thissen, Unite de Diabetologie et Nutrition, Universite Catholique de Louvain, 54 avenue Hippocrate, B-1200 Brussels, Belgium. E-mail: thissen{at}diab.ucl.ac.be.
In catabolic conditions, atrogin-1/MAFbx, a muscle-specific ubiquitin-ligase required for muscle atrophy, is increased, and concentrations of IGF-I, a growth factor known to have antiproteolytic action, are reduced. To define the relationship between the decline in IGF-I and the induction of atrogin-1/MAFbx, we studied the effect of IGF-I replacement on atrogin-1/MAFbx mRNA in rats fasted for 51 h and in rats made diabetic with streptozotocin (STZ). Fasting produced a 5.8-fold increase in atrogin-1/MAFbx (P < 0.001). This was attenuated to a 2.5-fold increase by injections of IGF-I (P < 0.05 vs. fasting). Animals with STZ-induced diabetes experienced a 15.1-fold increase in atrogin-1/MAFbx (P < 0.001). Normalization of their circulating IGF-I concentrations by IGF-I infusion blunted the induction of atrogin-1/MAFbx to 6.3-fold (P < 0.05 vs. STZ diabetes without IGF-I). To further delineate the regulation of atrogin-1/MAFbx by IGF-I, we studied a model of cultured muscle cells. We observed that IGF-I produced a time- and dose-dependent reduction of atrogin-1/MAFbx mRNA, with a 50% effective dose of 5 nM IGF-I, a physiological concentration. The degradation rate of atrogin-1/MAFbx mRNA was not affected by IGF-I, suggesting that the reduction of atrogin-1/MAFbx mRNA by IGF-I is a transcriptional effect. Exposure of muscle cells in culture to dexamethasone increased atrogin-1/MAFbx mRNA with a 50% effective dose of 10 nM, a pharmacological concentration. In the presence of dexamethasone, IGF-I at physiological concentrations retained its full inhibitory effect on atrogin-1/MAFbx mRNA. We conclude that IGF-I inhibits atrogin-1/MAFbx expression and speculate that this effect might contribute to the antiproteolytic action of IGF-I in muscle.
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