Insulin-mimetic effect of trypsin on the insulin receptor tyrosine kinase in intact adipocytes.

It has previously been demonstrated that the insulin-mimetic agent trypsin stimulates autophosphorylation of purified insulin receptors and activates the insulin receptor tyrosine kinase in vitro. We now report the effects of trypsin on whole cell tyrosine kinase activation and insulin receptor autophosphorylation. Trypsin treatment of intact adipocytes produces a time-dependent stimulation of tyrosine kinase activity as measured in lectin extracts containing the insulin receptor, or specifically immunoprecipitated insulin receptor samples. Trypsin treatment of adipocytes also results in a loss of insulin binding capacity, and a linear correlation exists between loss of binding and stimulation of tyrosine kinase activity. Exposure of adipocytes to trypsin is known to result in a time- and dose-dependent activation of intracellular glycogen synthase. Examination of the time courses of stimulation of tyrosine kinase and glycogen synthase activation in our system indicates that the stimulation of tyrosine kinase activity by trypsin occurs with sufficient rapidity and magnitude to be consistent with a role of phosphorylation in the activation of glycogen synthase. Trypsin has further been demonstrated to stimulate autophosphorylation of the beta-subunit of the insulin receptor in intact adipocytes. Cells prelabeled with [32P]PO4 for 2 h were exposed to trypsin, and receptors were partially purified over wheat germ agglutinin-agarose columns. Receptors were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the beta-subunit was identified by autoradiography. The protein was extracted and hydrolyzed, and the phosphoamino acids were separated by electrophoresis and quantitated. Two- and five-fold increases in phosphotyrosine were observed with 3 and 10 min of trypsin treatment, respectively. We conclude that trypsin-induced cleavage of the insulin receptor alpha-subunit is relevant to the ability of trypsin to activate the insulin receptor tyrosine kinase in intact adipocytes. We further conclude that autophosphorylation of the insulin receptor and activation of its tyrosine kinase by trypsin may be important to the insulin-mimetic anabolic effects of trypsin.