In our previous studies, the bisindolic
alkaloid voacamine (VOA), isolated from the plant Peschiera fuchsiaefolia, proved to exert a chemosensitizing effect on cultured multidrug resistant (MDR)
osteosarcoma cells exposed to
doxorubicin (DOX). In particular, VOA was capable of inhibiting
P-glycoprotein action in a competitive way, thus explaining the enhancement of the cytotoxic effect induced by DOX on MDR cells. Afterwards, preliminary observations suggested that such an enhancement did not involve the apoptotic process but was due instead to the induction of autophagic cell death. The results of the present investigation demonstrate that the
plant alkaloid VOA is an autophagy inducer able to exert apoptosis-independent cytotoxic effect on both wild-type and MDR
tumor cells. In fact, under treatment condition causing about 50 percent of cell death, no evidence of apoptosis could be revealed by microscopical observations,
Annexin V-FITC labeling and analysis of PARP cleavage, whereas the same cells underwent apoptosis when treated with apoptosis inducers, such as
doxorubicin and
staurosporine. Conversely, VOA-induced autophagy was clearly evidentiated by electron microscopy observations,
monodansylcadaverine staining, LC3 expression, and conversion. These results were confirmed by the analysis of the modulating effects of the pretreatment with autophagy inhibitors prior to VOA administration. In addition, transfection of
osteosarcoma cells with
siRNA against ATG genes reduced VOA cytotoxicity. In conclusion, considering the very debated dual role of autophagy in
cancer cells (protective or lethal, pro- or anti- apoptotic) our findings seem to demonstrate, at least in vitro, that a
natural product able to induce autophagy can be effective against
drug resistant
tumors, either used alone or in association with conventional chemotherapeutics.