Mechanism by which insulin stimulates protein synthesis in chick embryo fibroblasts

It has been known that insulin raises the rate of incorporation of [3H]leucine into the total protein of hormone-deficient chick embryo fibroblasts by approximately 1.5-fold. The elevation is not dependent upon the production of new messenger ribonucleic acids (mRNAs). Evidence is now presented in support of the following points: the greater labelling is due to more rapid polypeptide synthesis, not to an increase in the specific activity of leucyl-tRNA; the enhanced synthesis derives largely or entirely from a speeding up of the process of initiation, rather than that of elongation or termination; and the 1.5-fold stimulation is due to the elevated rates of formation of at least many of the fibroblast proteins. The hormone was shown before to stimulate posttranscriptionally and highly preferentially for formation of ribosomal proteins in the resting chick embryo cells. The question has been asked here whether insulin increases the production of total cell ribosomal protein by chemically altering preformed mRNAs. Results obtained by translating messages from deprived and hormone-treated cells in wheat germ and reticulocyte preparations do not support a mechanism involving covalent modification of preformed mRNAs. The observations, coupled with those previously made with inhibitors of translation, lead us to suggest that insulin stimulates protein synthesis in the resting chick embryo cells by activating limiting components of the initiation system. The effects of the hormone are greatest with messages, such as those for the ribosomal proteins, that have low affinities for the limiting initiation components.