The mechanism by which ultraviolet radiation induces melanogenesis in epidermal melanocytes is unknown. Previous observations that in cultured human melanocytes
1-oleoyl-2-acetylglycerol augmented both basal and ultraviolet radiation-induced melanogenesis, suggested that the responses were mediated via
protein kinase C. However, paradoxically the
phorbol ester TPA was without effect. Therefore, the present study has examined the involvement of
protein kinase C in melanogenesis. Analysis of the
isozyme profile of human melanocytes revealed the presence of
protein kinase C alpha, beta I, epsilon and zeta but not the
isozyme eta. Following exposure to 500 nM TPA for 24 hours,
isozymes alpha, beta I and epsilon were downregulated, but zeta was unaffected. Similar
isozyme profiles were observed in S91 and SKMEL3
melanoma cells. The melanogenic responses to
1-oleoyl-2-acetylglycerol and ultraviolet radiation were unaffected by inhibition of
protein kinase C with
Ro31-8220, or ablation by downregulation with 500 nM TPA, in human melanocytes and
melanoma cells.
1-Oleoyl-2-acetylglycerol had no effect on
protein kinase C activity in human melanocytes, as measured by rapid phosphorylation of the 80 kDa
protein myristoylated alanine-rich C kinase substrate (MARCKS). Ultraviolet radiation induced a small increase in
MARCKS protein phosphorylation but this effect was inhibited by pretreatment for 24 hours with 500 nM TPA, which had no effect on ultraviolet-induced melanogenesis. Overall, these findings indicate that
1-oleoyl-2-acetylglycerol and ultraviolet radiation activate melanogenesis via
protein kinase C-independent pathways.