Psoralen activity and binding sites in melanotic and amelanotic human melanoma cells

The biological activity and specific binding sites of 8-methoxypsoralen (8- MOP) are assayed using two I human melanoma cell lines, melanotic SK-Mel 28 and amelanotic C32TG. Long-term (72 hr) treatment with 8-MOP at a concentr ation of 10(-4) M results in an increase in melanogenesis and a decrease in proliferation, similar in both cell lines. Daily exposure of these cells t o ultraviolet A (UVA) irradiation (1.28 mJ/cm(2)) does not enhance the resp onse to the compound. Daily pulse application (30 min daily) of 8-MOP does not promote any response. However, in combination with UVA, 8-MOP pulse tre atment becomes as effective as the long-term treatment. A decrease in cell proliferation in the constant presence of 8-MOP is not coupled with apoptos is, since no increase in the number of apoptotic nuclei was observed after the treatment. The flow cytometry indicates that 8-MOP arrests the cells at the G0/G1 phase, irrespective of the presence or absence of UVA light. In view of the lack of epidermal growth factor (EGF) receptors in both cell li nes, it is not likely that such an arrest is associated with the down-regul ation of EGF receptors by 8-MOP. It is noted that this compound elicits a b iphasic cell response, since cell proliferation increases after the first 2 4-hr treatment, whereas it decreases in the subsequent 48 hr and thereafter . Competition binding assays using H-3-8-MOP disclosed: 1) the specific bin ding of the compound in both cell lines occurs in the presence or absence o f UVA light, and 2) a higher binding rate at low concentrations of the comp ound is in SK-Mel 28 (72%) rather than C32TG (58%) cells. The competition a ssays in the presence of UVA suggest a possible occurrence of covalent bind ings between psoralen and receptor, as DNA covalent binding accounted to on ly 3-5% of the total binding in both cell lines.