Exposure to valproic acid and 5,7-dimethoxycoumarin induces pheomelanogenesis in the human melanoma G-361 cells, as demonstrated by Py-GC/MS/MS study

• Pyrolysis products of melanin from the human melanoma G-361 cell line are characteristic of pure eumelanin.
• After the cell exposure to valproic acid and 5,7-dimethoxycoumarin, the pyrolytic markers of pheomelanin were detected in the pigment pyrolysate.
• Valproic acid and 5,7-dimethoxycoumarin induce pheomelanin synthesis in the treated cells.
• The changes in melanin structure are indicative of the tumor cell differentiation to normal cells.

Melanogenesis is one of the main markers of the differentiation of normal and neoplastically transformed melanocytes. Valproic acid (VPA) and 5,7-dimethoxycoumarin (DMC), the potential anti-cancer drugs with differentiating activity, were reported to enhance melanin synthesis in melanoma cells. The aim of this study was to examine if the stimulation with VPA and DMC affects the structure of melanin synthesized by the human melanoma G-361 cell line. G-361 cells were cultured with VPA, DMC, and a mixture of the two agents for 7 days. Melanin isolated from the treated and untreated cells were pyrolysed, and the thermal degradation products were analyzed by gas chromatography/tandem mass spectrometry with a triple quadrupole instrument operating in a multiple reaction monitoring mode. After the stimulation with VPA and DMC, the sulfur containing pyrolytic markers of pheomelanin were detected among the thermal degradation products of the pigment isolated from the tumor cells. The levels of pheomelanin markers were the highest after the cell treatment with the combination of both agents. No sulfur compounds were present in the pyrolysate of melanin from the untreated control cells. We have concluded that VPA and DMC are able to induce pheomelanogenesis, and thus alter the structure of melanin produced in the human melanoma G-361 cells.