Melanoma is the most aggressive type of
skin cancer, highly resistant to conventional
therapies.
Photodynamic therapy (
PDT) is a minimally invasive treatment modality that combines the use of a
photosensitizer, visible light and molecular
oxygen, leading to oxidative stress in the specific site of irradiation. The cationic
zinc(II) phthalocyanine Pc13 has shown to be a potent
photosensitizer in different
melanoma cell lines. In this study, we explored the intracellular signaling pathways triggered by Pc13
PDT and the role of these cascades in the phototoxic action of Pc13 in human
melanoma A375 cells. Activation of MAPKs p38, ERK, JNK and PI3K-I/AKT was observed
after treatment and prevented by using the
antioxidant trolox. Inhibition of p38 reduced Pc13
phototoxicity, whereas blockage of JNK potentiated cell death. Results obtained indicate that p38 is involved in the cleavage of PARP-1, an important mediator of apoptosis. On the other hand, Pc13 irradiation induced the activation of an autophagic program, as evidenced by enhanced levels of
Beclin-1, LC3-II and GFP-LC3 punctate staining. We also demonstrated that this autophagic response is promoted by JNK and negatively regulated by PI3K-I/AKT pathway. The blockage of autophagy increased Pc13
phototoxicity and enhanced PARP-1 cleavage, revealing a protective role of this mechanism, which tends to prevent apoptotic cell death. Furthermore, reduced susceptibility to treatment and increased activation of autophagy were detected in A375 cells submitted to repeated cycles of Pc13
PDT, indicating that autophagy could represent a mechanism of resistance to
PDT. The efficacy of Pc13
PDT and an improved phototoxic action in combination with
chloroquine were also demonstrated in
tumor spheroids. In conclusion, we showed the interplay between apoptotic and autophagic signaling pathways triggered by Pc13
PDT-induced oxidative stress. Thus, autophagy modulation represents a promising therapeutic strategy to potentiate the efficacy of
PDT in
melanoma.