Sensitization of human melanoma cells by tamoxifen to apoptosis induction by pancratistatin, a nongenotoxic natural compound

The objective of this study was to determine the efficacy of the natural compound pancratistatin (PST), isolated from the Hymenocallis littoralis, in human melanoma cells. Melanoma is an aggressive form of skin cancer that is commonly fatal if not diagnosed in its early stage of development. Melanoma is resistant to many treatments, thus drastically limiting chemotherapy options for this cancer. We have shown that exposure to PST induces apoptosis in human melanoma within 72 h using Hoechst staining. Interestingly tamoxifen (TAM), an estrogen receptor antagonist, sensitizes these cells to apoptosis induction by PST as observed with Hoechst and annexin-V staining. This cotreatment did not affect the viability of normal noncancerous human fibroblasts. Both of these compounds have been shown to target the mitochondria synergistically, as indicated by higher levels of reactive oxygen species generation from isolated mitochondria. PST alone and in combination with TAM shows depolarization of the mitochondrial membrane potential as shown by JC-1 staining. Melanoma drug resistance was not observed after posttreatment recuperation, as cells displayed apoptotic morphology up to 96 h after drug-free media replacement. Our results indicate that TAM alone does not induce apoptosis in this cell line, but sensitizes the mitochondria, thereby enhancing the effect of PST exposure. In conclusion, combination of two nongenotoxic compounds offers a novel treatment regime for this notoriously resilient form of skin cancer.