Insulin inhibition of protein degradation in cell monolayers

Protein degradation has been measured in confluent monolayers of eleven lines of contact‐inhibited cells and ten transformed lines as the rate of release of trichloroacetic acid‐soluble radioactivity after prelabeling cell protein with [3H]leucine. Insulin, at concentrations from 10−12 M to 10−6 M, has been added at the beginning of the 4‐hour degradation period to detect selective effects of this hormone as an inhibitor of the inducible proteolysis occurring in serumfree medium. In addition insulin binding measurements have been performed on selected cell lines in an attempt to relate receptor properties to insulin action. Substantial effects of insulin are found in most cells with a selective inhibition at low insulin concentrations noted in several of the transformed lines. The difference in insulin sensitivity is not entirely definitive because temperature‐sensitive transformation mutants of NRK cells are not more sensitive to insulin at a temperature where they show the transformed phenotype. Although insulin receptors on different cell lines have similar binding properties, two of the hepatomas used, H35 and MH1C1, show inhibition of protein degradation at insulin concentrations where receptor occupancy is extremely low. Calvarial osteoblast‐like cells have a high rate of protein degradation which can be reduced by growth factors but not by insulin. The lack of an insulin response is a consequence of poor insulin binding to the cells. Insulin binds to the osteogenic sarcoma cells in substantial amounts. However, its normal action to inhibit the induced proteolysis is restricted because with these cells no increase of proteolysis occurs in serum‐free medium. Generally higher rates of protein degradation are observed in the contact‐inhibited lines than the transformed cells. We suggest that this difference may provide a selective growth advantage to transformed cells.