Etoposide is a potent and widely used antineoplastic agent. It is able to induce apoptosis in most cell types. However, very little is known about its mechanism of action. In this study, we demonstrate the cytotoxic signal that induced by etoposide and investigate how etoposide exerts antitumor activity in HPV-16 (+) CaSki cervical carcinoma cells.

Antiproliferation activity was measured in CaSki cell lines by using MTT assays, DNA fragmentation assay. Cell cycle distribution was analyzed using flow cytometry. Expression of proteins involved in the apoptotic pathway was analyzed by Western blotting (WB). Electron microscopic (EM) and biochemical studies (Western blotting, RT-PCR) revealed that non-apoptotic death was associated with autophagosomes/-autolysosomes. These parameters have also been measured in cells treated with 3-methyladenine (an autophagy inhibitor), zVAD-fmk (a pan-caspase inhibitor) and both.

The etoposide induced apoptosis. In cell cycle analysis, etoposide-treated CaSki cells were few induced hypodiploid DNA content, suggesting that apoptotic cell death. EM study revealed that autophagic appearance in the presence of etoposide exhibited by autophagosomes/autolysosomes. It was confirmed by LysoTracker probe and WB against Beclin 1, APG 5, APG 12 and p53. When autophagy was blocked by 3-MA, not only the protein expression of Beclin 1, but also the antitumor effect of etoposide was suppressed. On the other hand, the addition of zVAD-fmk could induce a few etoposide-induced autophagy. And etoposide-treated CaSki cells were rescued by combination of 3-MA and zVAD-fmk.

Our results suggest that etoposide not only initiated apoptosis but ultimately caused cell death through autophagy. In this study, we demonstrate novel features for the action of etoposide in HPV-16 (+) CaSki cervical carcinoma cells. Autophagic cell death induction by some anticancer agents underlines the potential utility of its induction as a new cancer treatment modality.