EFFECTS OF INCREASED EXPRESSION OF PROTEIN-KINASE-C ON RADIATION-INDUCED CELL-TRANSFORMATION

The in vitro oncogenic transformation of C3H 10T1/2 cells by ionizing radiation is known to be enhanced by the tumor promoter 12-O-tetradeca noylphorbol-13-acetate (TPA). It is also known that the activation of protein kinase C (PKC) by TPA is an important step in its tumor-promot ing effect. In the present study, we examined the effects of overexpre ssion of a specific isoform of PKC, PKCbeta 1 on gamma-ray-induced tra nsformation of 10T1/2 cells. In addition, the effects of overexpressio n of PKCbeta 1 on the malignant phenotype of a previously transformed 10T1/2 cell line were also evaluated. Derivatives of 10T1/2 cells that stably overexpress PKCbeta 1 were obtained by transduction with the r etroviral expression vector pMV7 carrying the rat PKCbeta 1 cDNA seque nce. We found that the parental 10T1/2 cells and a control cell line 1 0T1/2 MV7, which carried only the pMV7 vector without the cDNA insert, expressed dose-dependent transformation frequencies when exposed to g amma-rays. On the other hand, concurrently treated PKC-overexpressing cells that had an 11-fold increase in enzyme activity (PKC-4 cells) fa iled to yield any morphologically identifiable foci. Cell lines that e xpressed lower levels of PKCbeta 1 were partially resistant to transfo rmation by gamma-rays. Clonogenic survival data indicated that this ob servation was not due to radioresistance per; se. Thus, overexpression of PKCbeta 1 did not appear to function as an endogenous substitute f or TPA in promoting radiation-induced transformation. Furthermore, ove rexpression of PKC did not reverse the transformation phenotypes in tu morigenic 10T1/2 cells once it was established. These findings are dis cussed with respect to the specific roles of individual isoforms of PK C in growth control.