The effects of gemcitabine and TPA on PKC signaling in BG-1 human ovarian cancer cells

Protein kinase C (PKC) signaling pathways play an important role in cell su rvival and anticancer drug-induced apoptosis. We observed in clonogenicity assays of BG-1 human ovarian cancer cells that gemcitabine cytotoxicity was increased synergistically when drug treatment was followed or preceded by a 24-h exposure to 10 nM 12-O-tetradecanoylphorbol-13-acetate (TPA). Coincu bation of 10 nM TPA with pharmacological inhibitors of PKC abrogated the sy nergism of TPA and gemcitabine. These observations prompted further investi gation of PKC signaling events linked to TPA and gemcitabine cytotoxicity i n BG-1 cells. Because PKC isoforms are differentially expressed in various cell types, we determined that BG-1 cells express the alpha, beta, delta, e psilon, and zeta isoforms of PKC. In addition, 1-h exposures to 10 mu M gem citabine triggered cytosol to membrane translocation of PKC isoforms alpha, delta, and epsilon, indicating these isoforms were activated by gemcitabin e. We also explored the PKC mechanism(s) responsible for the synergism of T PA and gemcitabine. and determined that treatment with 10 nM TPA for 24 h i n BG-1 cells: 1) downregulated PKC delta and PKC alpha, without affecting P KC epsilon, 2) did not affect cell cycle distribution into S phase, 3) incr eased extracellular signal-regulated kinase signaling, and 4) increased int racellular alkaline phosphatase activity, a biochemical marker of cellular differentiation. Chronic exposure (24 h) to TPA enhanced gemcitabine cytoto xicity, perhaps by inducing cellular differentiation pathways in BG-1 cells . Therefore, the use of differentiating agents in combination with gemcitab ine may improve its clinical efficacy.