Melanoma, a cancer derived from melanocytes is very difficult to treat, especially in advanced cases. There are several encouraging studies of the efficacy of photodynamic therapy (PDT) in melanoma. However, PDT has to overcome the main defense mechanisms like: defects in the apoptotic pathways, pigmentation, sequestration of the photosensitisers (PS) inside melanosomes and increased oxidative stress defense. Two meso-substituted porphyrins, meso-5,10,15,20-tetrakis (4-hydroxyphenyl) porphyrin (THOPP) and meso-5-(4-hydroxyphenyl)-10, 15, 20-tris (4-methoxyphenyl) porphyrin (THOMPP) were used as PS to investigate several mechanisms underlining the PDT anti-melanoma effects, on a lightly pigmented melanoma cell line (WM35), in vitro. γH2AX foci formation (a measure of DNA double strand brakes) was used for the assessment of DNA damage by means of immune-fluorescence and western blot. Cytoskeleton alterations were detected by phalloidin staining. Tyrosinase activity and melanin pigment were quantified by spectrophotometry, tyrosinase protein by western blot, total peroxidase activity by resorurfin reaction (Amplex Red). PDT induced high levels of DNA damage, cytoskeleton alterations and enhanced pigmentogenesis. THOPP mediated PDT was the most efficient. The melanogenesis stimulated by PDT was directly correlated to the PDT induced cellular damage and provided no protection against therapy. Thus, PDT induced melanogenesis combined with severe DNA damage was able to overcome the mechanisms of resistance and increased the efficiency of PDT in WM35 melanoma cells. These results are encouraging for a possible use of PDT, as an adjuvant therapy in lightly pigmented melanomas.
Keywords: Melanogenesis; Melanoma; Meso-substituted porphyrins; Photodynamic therapy; γH2AX.
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