Crosstalk between oxidative stress-induced apoptotic and autophagic signaling pathways in Zn(II) phthalocyanine photodynamic therapy of melanoma

Abstract

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.

Keywords: Apoptosis; Autophagy; Cell signaling; Melanoma; Photodynamic therapy; Zinc phthalocyanine.