Cell genetic evidence of correlation of intracellular translocation of protein kinase C (PKC) and PKC-mediated phosphorylation of 80-kDa protein with mitogenic action of tumor promoters.

Recently, we isolated a series of 3T3-L1 cell variants that are unable to respond to mitogenic stimulation by the tumor promoter, 12-O-tetradecanoylphorbol acetate (TPA). Since protein kinase C (PKC) is the major receptor for TPA and appears to play a key role in cellular proliferation, we have examined the distribution of PKC in the parental 3T3-L1 cells and the variant VT-1 cells. PKC was located predominantly in the cytosol of growth-arrested confluent 3T3-L1 cells, and upon TPA treatment it was rapidly translocated into the plasma membrane. In contrast, PKC was located predominantly in the plasma membrane of confluent VT-1 variant cells and was no longer activated by TPA. Two-dimensional gel analysis showed that a Mr 80,000 acidic protein (80-kDa protein) was rapidly phosphorylated in 3T3-L1 cells upon TPA treatment, whereas phosphorylation of this protein was barely detected in VT-1 cells. In growing cultures, the majority of PKC was found in the plasma membrane of both cell lines, and no change occurred upon TPA treatment. Hydroxyapatite column chromatography revealed the presence of alpha-type PKC as the major component in both cell lines. These results suggest that the intracellular translocation of alpha-type PKC and the PKC-mediated phosphorylation of the 80-kDa protein may be involved in the mechanism of mitogenic signal transfer.