Epidermal growth factor enhances cisplatin-induced apoptosis by a caspase 3 independent pathway

Purpose: Activation of the epidermal growth factor (EGF) receptor has previously been shown to increase the sensitivity of cancer cells to DNA-damaging agents, including cisplatin, UV-B, and gamma -radiation. We now investigated the mechanisms by which EGF enhances the sensitivity of human ovarian cancer cells to cisplatin. Results: The effect of EGF on cisplatin sensitivity could not be entirely explained by alterations in the cellular detoxification of cisplatin by glutathione or DNA repair of transcribed genes, as assessed by a plasmid reactivation assay. Furthermore, EGF did not affect the levels of several proteins that regulate apoptotic pathways, including bcl2, bclX(L), bax and p53. Cisplatin treatment resulted in activation of caspase 3 and subsequent cleavage of specific substrates containing the DEVD (Asp-Glu-Val-Asp) amino acid sequence, including PARP (poly(ADP-ribose) polymerase). The EGF-mediated increase in cisplatin-induced apoptosis, however, did not result in increased caspase 3 activity. Moreover, apoptosis induced by cisplatin alone was completely inhibited by the caspase 3 inhibitor DEVD-CHO, whereas cell death induced by combined treatment with cisplatin and EGF was not prevented by inhibition of caspase 3. Conclusion: Our results suggest that, although cisplatin alone induces apoptosis by a caspase 3 dependent pathway, EGF enhances cisplatin-induced cell death by activating an apoptotic pathway that is independent of caspase 3.