Toxicity of phorbol esters for human epithelial cells expressing a mutant ras oncogene

Cytotoxic action of phorbol esters on human pancreatic cancer cells

We previously showed that phorbol esters are cytotoxic to human thyroid epithelial cells expressing a mutant RAS oncogene. Here we explore the generality of this finding using cells derived from pancreatic cancer, which, like thyroid, shows a high frequency of RAS mutation, but is a much greater cause of cancer mortality. The response to phorbol myristate acetate (PMA) and related agents was assessed on a panel of 9 pancreatic cancer cell lines, using a range of assays for cell growth and death in vitro and in vivo. In most lines, PMA induced non-apoptotic cell death which was, surprisingly, independent of its "classic" target, protein kinase C. With 24 hr exposure, the IC(50) in the most sensitive line (Aspc-1) was <1 ng/ml (1.6 nM), with survival undetectable at concentrations >/=>/=16 nM, and after only 1 hr exposure the IC(50) was still </=</=16 nM. Interestingly, the efficacy of a second phorbol ester, phorbol dibutyrate, was much lower, and the PMA analogue bryostatin-1, which is in clinical trials against other tumour types, was totally inactive. Pre-treatment of Aspc-1 cells with PMA before subcutaneous inoculation into nude mice prevented, or greatly retarded, subsequent xenograft tumour growth. Furthermore, treatment of established tumours with a single peri-tumoral injection of PMA induced extensive cell death and arrested tumour development. Taken together with recent Phase 1 clinical studies, these data suggest that activity against pancreatic cancer will be attainable by systemic administration of PMA, and point to potential novel therapeutic targets for this highly aggressive cancer.

Induction of cell death by stimulation of protein kinase C in human epithelial cells expressing a mutant ras oncogene: a potential therapeutic target

Ras oncogene activation is a key genetic event in several types of human cancer, making its signal pathways an ideal target for novel therapies. We previously showed that expression of mutant ras sensitizes human thyroid epithelial cells to induction of cell death by treatment with phorbol 12-myristate 13-acetate (PMA) and other phorbol esters. We have now investigated further the nature and mechanism of this cell death using both primary and cell line models. The cytotoxic effect of PMA could be blocked by bisindolylmaleimide (GF 109203X), a well-characterized inhibitor of c and n protein kinase C (PKC) isoforms, and by prior down-regulation of PKC, indicating that it is mediated by acute stimulation, rather than down-regulation. Western analysis identified two candidate isoforms--alpha and epsilon--both of which showed PMA-induced subcellular translocation, either or both of which may be necessary for PMA-induced cell death. Immunofluorescence showed that PMA induced a rapid nuclear translocation of p42 MAP kinase of similar magnitude in the presence or absence of mutant ras expression. Cell death exhibited the microscopic features (chromatin condensation, TdT labelling) and DNA fragmentation typical of apoptosis but after a surprising lag (4 days). Taken together with recent models of ras-modulated apoptosis, our data suggest that activation of the MAPK pathway by PMA tips the balance of pro- and anti-apoptotic signals generated by ras in favour of apoptosis. The high frequency of ras mutations in some cancers, such as cancer of the pancreas, which are refractory to conventional chemotherapy, together with the potential for stimulating PKC by cell-permeant pharmacological agents, makes this an attractive therapeutic approach.