Phenotypic effects of overexpression of PKC_β1 in rat liver epithelial cells

We have used a previously described retroviral expression vector pMV7‐PKC_β1 to develop derivatives of two rat liver epithelial cell lines, K16 and K22, that stably express about tenfold‐higher PKC activity than control cells. Despite these high levels of PKC, these cells did not exhibit gross morphologic changes, anchorage‐independent growth, or tumorigenicity. K16PKC‐4 and K22PKC‐2, two lines with the highest PKC enzyme activity, were studied further in terms of several responses to the phorbol ester tumor promoter TPA. When treated with 100 ng/ml of TPA, the control K16MV7 and K22MV7 cells displayed a slight change in morphology, whereas the K16PKC‐4 and K22PKC‐2 cells displayed a marked change in morphology. Northern blot analyses demonstrated that TPA induced increased levels of fos, myc, phorbin, and ODC RNAs in control K16MV7 and K22MV7 cells, with maximum induction occurring at about 0.5, 1, 8, and 8 h, respectively. In K16PKC‐4 and K22PKC‐2 cells, TPA induction of phorbin and ODC RNAs was markedly enhanced, but this was not the case for myc and fos RNAs. In addition, the levels of myc RNA were constitutively higher in both K16PKC‐4 and K22PKC‐2 cells than in the control cells. Taken together, these results provide direct evidence that PKC plays a critical role in modulating the expression of myc, phorbin, and ODC RNAs. On the other hand, overexpression of PKC_β1 is not itself sufficient to cause cell transformation.