Oxidative stress generated by irradiation of a zinc(II) phthalocyanine induces a dual apoptotic and necrotic response in melanoma cells

Abstract

Melanoma is an aggressive form of skin carcinoma, highly resistant to traditional therapies. Photodynamic therapy (PDT) is a non-invasive therapeutic procedure that can exert a selective cytotoxic activity toward malignant cells. In this work we evaluated the effect of a cationic zinc(II) phthalocyanine (Pc13) as photosensitizer on a panel of melanoma cells. Incubation with Pc13 and irradiation induced a concentration and light dose-dependent phototoxicity. In order to study the mechanism underlying Pc13-related cell death and to compare the effect of different doses of PDT, the most sensitive melanoma B16F0 cells were employed. By confocal imaging we showed that Pc13 targeted lysosomes and mitochondria. After irradiation, a marked increase in intracellular reactive oxygen species was observed and a complete protection from Pc13 phototoxicity was reached in the presence of the antioxidant trolox. Acridine orange/ethidium bromide staining showed morphological changes indicative of both apoptosis and necrosis. Biochemical hallmarks of apoptosis, including a significant decrease in the expression levels of Bcl-2, Bcl-xL and Bid and mitochondrial membrane permeabilization, were observed at short times post irradiation. The consequent release of cytochrome c to cytosol and caspase-3 activation led to PARP-1 cleavage and DNA fragmentation. Simultaneously, a dose dependent increase of lactate dehydrogenase in the extracellular compartment of treated cells revealed plasma membrane damage characteristic of necrosis. Taken together, these results indicate that a dual apoptotic and necrotic response is triggered by Pc13 PDT-induced oxidative stress, suggesting that combined mechanisms of cell death could result in a potent alternative for melanoma treatment.