Protein kinase C isoform expression and activity alter paclitaxel resistance in vitro

The aim of this study was to assess the relationship of protein kinase C (PKC) isoform expression and functional activity to the development of multidrug resistance in gynecologic malignancies.

Paclitaxel-resistant subclones (T30 and T30-Res) of the Mes-sa human uterine sarcoma cell line were selected through exposure to paclitaxel in vitro. Indices of relative drug resistance were determined by the MTT (3-[4,5-dimethyl-2-thiazolyl]-2, 5-diphenyl-2H-tetrazolium bromide) assay. Differences in the expression pattern of PKC isoforms were assessed by Western blot of cell lysates. Finally, the influence of PKC activity (i.e., translocation to the plasma membrane, confirmed by Western blot of plasma membrane bound protein) on resistance to paclitaxel was examined with the MTT assay in cells preincubated with PMA.

The indices of relative paclitaxel resistance of Mes-sa, Mes-sa-T30, and Mes-sa-T30-Res were 1-, 5-, and 11-fold, respectively. Five (alpha, gamma, iota, lambda, and mu) of the 11 known PKC isoforms were detected in all cell lysates. Only PKC-alpha and PKC-gamma expression increased with increasing indices of paclitaxel resistance. Interestingly, PMA induction of PKC activity reversed resistance to paclitaxel in all cell lines by 2- to 3-fold, and this reversal of drug resistance was associated with a time-dependent translocation of PKC-alpha and PKC-gamma to the plasma membrane compartment.

Increased expression of only the PKC-alpha and PKC-gamma isoforms correlates with increasing levels of paclitaxel resistance in Mes-sa cells in this in vitro experimental model. However, increased functional activity of these and other PKC isoforms leads to reversal in paclitaxel resistance. Therefore, PKC activating mechanisms normally present in primary tumor cells may be compromised in drug-resistant clones.