Effects of starvation, diabetes and acute insulin treatment on the regulation of polypeptide-chain initiation in rat skeletal muscle

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Abstract
The rate of protein synthesis in skeletal muscle is greatly decreased in response to diabetes and starvation. Analysis of polyribosome profiles indicates that polypeptide-chain initiation is impaired under these conditions. To identify the step in initiation that is affected, the incorporation of [35S]methionyl-tRNAfMet into [35S]methionyl-tRNAfMet.cntdot.40S-ribosomal-subunit initiation complexes was assayed in cell-free extracts based on postmitochondrial supernatants prepared from gastrocnemius muscle. Extracts from either starved or diabetic rats were 30-40% less active in forming these complexes compared with those derived from fed or insulin-maintained controls, respectively. This change could be reversed by treatment of either starved or diabetic rats with insulin in vivo 30 min before death. Formation of 40S initiation complexes by extracts from either fed or starved rats could be stimulated by the addition of exogenous purified initiation factor eIF-2, but extracts from starved or diabetic rats were more sensitive than controls to stimulation by low concentrations of the factor. These results provide evidence for the acute regulation by insulin of protein synthesis in skeletal muscle at the level of polypeptide-chain initiation, and suggest that in this tissue, as in certain other eukaryotic systems, control of initiation appears to be mediated by changes in the activity of initiation factor eIF-2.