Protein Kinase C-<TEX>$eta$</TEX> Is Induced In Ionizing Irradiation Induced Pigmentation

Cutaneous hyperpigmentation is a well-known consequence of both acute and chronic X-irradiation, although the molecular mechanisms involved are not well understood. Recently, protein kinase C-<TEX>$eta$</TEX> (PKC-<TEX>$eta$</TEX>) was shown to activate tyrosinase, a key and the rate-limiting enzyme in melanogenesis [1]. In this study, we have investigated its role in mediating ionizing radiation-induced pigmentation by exposing cultured human melanocytes to X-irradiation. Increased tyrosinase activity after the 4 Gys exposure was observed within 48 hrs and total melanin content doubled after 7 days. Interestingly, tyrosinase mRNA level was not affected by X-irradiation. However, there was a 2-3 fold increase in PKC-<TEX>$eta$</TEX> mRNA after 48 hours of irradiation, coinciding with the increase in tyrosinase activity. This induction was not due to non-specific heat generated during the irradiation because when melanocytes were incubated at 4<TEX>$0^{circ}C$</TEX>, there was no induction of PKC-<TEX>$eta$</TEX> mRNA. Taken together, these data suggest that X-irradiation induces cutaneous hyperpigmentation, at least in part, by up-regulating the level of PKC-<TEX>$eta$</TEX>.