Depletion of protein kinase C (PKC) by 12-O-tetradecanoylphorbol-13-acetate (TPA) enhances platinum drug sensitivity in human ovarian carcinoma cells.

Down-regulation of protein kinase C (PKC) by 12-Otetradecanoylphorbol-13-acetate (TPA) enhances the sensitivity of human ovarian carcinoma 2008 cells to various types of platinum compounds such as cisplatin (DDP), carboplatin and (-)-(R)-2-aminomethylpyrrolidine (1,1-cyclobutanedicarboxylato)-platinum(II) monohydrate (DWA) by a factor of two- to threefold. TPA enhanced the sensitivity of the DDP-resistant 2008/C13*5.25 subline to each of these three drugs to the same extent as for the 2008 cells. The extent of PKC down-regulation and drug sensitization depended on the duration of TPA exposure; maximum effect was achieved with a 48 h pretreatment. Sensitization was TPA concentration-dependent and was maximal at 0.05 microM TPA. 2008 cells expressed only the PKCalpha and PKCzeta isoforms. Western blot analysis revealed that whereas the expression of PKCalpha was reduced by TPA the level of PKCzeta was not affected. These results suggest that PKCalpha is the isotype responsive to TPA in these cells and that platinum drug sensitivity can be modulated by this isoform alone. In parallel to its effect on PKCalpha, TPA decreased cellular glutathione content by 30 +/- 3 (standard deviation (s.d.) % in 2008 cells and by 41 +/- 3 (s.d.) % in 2008/C13*5.25 cells. TPA also increased accumulation of DDP and DWA by 70%, although this effect was limited to the 2008/C13*5.25 cells. TPA rendered 2008 and 2008/C13*5.25 cells resistant to cadmium chloride by a factor of 3.7 and 3.6-fold respectively, suggesting a significant increase in cellular metallothionein content. Although the mechanism of TPA induced sensitization is not yet fully understood, this study points to a central role for PKCalpha in modulating platinum drug sensitivity.