Ultraviolet B Radiation Acts through the Nitric Oxide and cGMP Signal Transduction Pathway to Stimulate Melanogenesis in Human Melanocytes

Abstract

Ultraviolet B (UVB) radiation is the main physiological stimulus for human skin pigmentation; however, the molecular mechanisms underlying this process are still unclear. Recently, nitric oxide (NO) and cGMP have been involved in mediation of skin erythema induced by UVB. Therefore, we investigated the role of NO and cGMP in UVB-induced melanogenesis. In this study, we demonstrated that UVB stimulation of melanogenesis was mimicked by exogenous NO donors. Additionally, we showed that NO stimulated cGMP synthesis and that cGMP was also a potent stimulator of melanogenesis. Furthermore, the inhibition of the melanogenic effect of NO by guanylate cyclase inhibitor demonstrated that NO mediated its effect through the activation of guanylyl cyclase. Interestingly, 1 min after UVB irradiation, we observed a significant increase in cGMP content in melanocytes. The effects of UVB on cGMP production and on melanogenesis were blocked by both guanylate cyclase and NO synthase inhibitors. Additionally, inhibition of cGMP-dependent kinase also prevented the stimulation of melanogenesis by UVB and NO. Therefore, we concluded that NO and cGMP production is required for UVB-induced melanogenesis and that cGMP mediated its melanogenic effects mainly through the activation of cGMP-dependent kinase.Epidermal melanin is responsible for skin darkening and is synthesized by melanocytes as the result of a cascade of enzymatic reactions. Tyrosinase, which converts tyrosine to dopaquinone, is the rate-limiting enzyme involved in melanin synthesis and represents the major regulatory step in melanogenesis. Numerous stimuli are able to alter melanogenesis of cultured pigmented cells; vitamin D metabolites (1), retinoids (2, 3), melanocyte-stimulating hormone (4 -6), forskolin, cholera toxin, isobutylmethylxanthine (7,8), diacylglycerol analogs (9, 10), and UV irradiation (11)(12)(13)(14). Until now the molecular mechanism involved in the induction of melanogenesis by these agents was not fully elucidated. A role for the cAMP pathway has been proposed on the basis that adenylate cyclase activators such as melanocyte-stimulating hormone, forskolin, and cholera toxin or phosphodiesterase inhibitors (e.g. isobutylmethylxanthine) can stimulate melanogenesis (6). Additionally, the protein kinase C (PKC) 1 pathway was thought to be involved in the regulation of melanogenesis. Indeed, 1-oleyl-2-acetyl-glycerol, a PKC activator, stimulates melanogenesis (9, 10, 15), and a direct correlation between the level of PKC activity in melanocytes and the activity of tyrosinase has been shown (16). However, induction of melanogenesis by UVB or by 1-oleyl-2-acetyl-glycerol is unaffected by RO 31-8220, a PKC inhibitor (17), leading to the conclusion that protein kinase C does not play a pivotal role in the control of melanogenesis. In humans, only UVB radiation represents an established physiological stimulus of melanogenesis, and despite many efforts to identify the molecular events triggered by UVB radiation, the mechanisms underlying UVB-induced melanogene...

Unanticipated role of melanin in causing carcinogenic cyclobutane pyrimidine dimers

“…5 Inhibition of melanin synthesis significantly reduces the generation of dark CPDs. Additionally, UV radiation (UVR) activates inducible nitric oxide synthase (iNOS) and NADPH oxidase (NOX), which generate nitric oxide and superoxide, respectively 6,7 (Fig. 1).…”

“…We also observed that UVA is able to solubilize melanin polymer 2 and the resulting melanin derivatives and monomers can diffuse intracellularly, potentially entering the nucleus. In addition, peroxynitrite and nitric oxide can diffuse across membranes and there are reports of NOX4 in the nucleus, 7 which would generate superoxide. This series of events would lead to the generation of dioxetanes on the melanin derivatives in the nucleus.…”

Cyclic AMP a Key Messenger in the Regulation of Skin Pigmentation

“…In human melanocytes, NO donors and cGMP analogs stimulate melanogenesis. Furthermore, UV radiation increases both NO and cGMP production, and the effects of UV on melanogenesis can be blocked by guanylate cyclase and NO synthase inhibitors (29). These data strongly suggest that NO and cGMP production are required for UVB-induced melanogenesis.…”

Stimulation of melanogenesis by glycyrrhizin in B16 melanoma cells

“…Active PKC then phosphorylates tyrosinase, leading to its activation (Park and Gilchrest, 1999). Recently, there are reports that cGMP signal transduction pathway stimulates melanogenesis (Romero-Graillet et al, 1996;Romero-Graillet et al, 1997). The signal transduction pathway in GR-mediated stimulation of melanogenesis requires further study.…”