Involvement of Intracellular Ca²⁺ and Nitric Oxide in the Hydroxyurea-Induced Melanogenesis

The molecular mechanism underlying hyperpigmentation as an adverse side effect of the anticancer drug, hydroxyurea (HU), was investigated in B16 melanoma cells. HU increased the melanin content and intracellular Ca²⁺ concentration in a dose-dependent manner. These effects were significantly reduced by treatment with the intracellular Ca²⁺ release blockers, dantrolene and 2-aminoethoxidiphenylborate (2-APB). Additionally, HU induced the production of nitric oxide (NO) in a time- and dose-dependent manner, and this was significantly blocked by inhibitors of the increase in intracellular Ca²⁺ levels(bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid/acetoxymethyl ester, dantrolene, and 2- APB). Furthermore, the HU-induced increases in NO and melanin production were significantly suppressed by NG-nitro- L-arginine methyl ester and 2-(4-carboxy-2-phenyl)-4,4,5,5-tetramethylinidazoline-1-oxyl-3-oxide, an NO synthase inhibitor and an NO scavenger, respectively. These results suggest that HU-induced melanogenesis through NO and an increase in intracellular Ca²⁺ levels may be involved in the mechanism of its hyperpigmentation effect.