Ultraviolet (UV) radiation is the main physiological stimulus for human skin pigmentation. Within the epidermalmelanin unit, melanocytes synthesize and transfer melanin to the surrounding keratinocytes. Keratinocytes produce paracrine factors that affect melanocyte proliferation, dendricity, and melanin synthesis. In this report, we show that normal human keratinocytes secrete nitric oxide (NO) in response to UVA and UVB radiation, and we demonstrate that the constitutive isoform of keratinocyte NO synthase is involved in this process. Next, we investigate the melanogenic effect of NO produced by keratinocytes in response to UV radiation using melanocyte and keratinocyte cocultures. Conditioned media from UV-exposed keratinocytes stimulate tyrosinase activity of melanocytes. This effect is reversed by NO scavengers, suggesting an important role for NO in UVinduced melanogenesis. Moreover, melanocytes respond to NO-donors by decreased growth, enhanced dendricity, and melanogenesis. The rise in melanogenesis induced by NOgenerating compounds is associated with an increased amount of both tyrosinase and tyrosinase-related protein 1.These observations suggest that NO plays an important role in the paracrine mediation of UV-induced melanogenesis. ( J. Clin. Invest. 1997. 99:635-642.)

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...

In their abstract, Davenport et al (1997) state that ''Results consistently demonstrated that all the test sunscreens protected (against immunosuppression) beyond their designated protections factors.'' Protection factors are ratios by definition, and if the above statement by Davenport et al (1997) means anything at all, it is that the sunscreens' immune protection factors were greater than their corresponding sun protection factors (SPF). Chu et al (1998) restate their claim in their recent letter to the Journal of Investigative Dermatology in which they also state that they did not calculate immune protection factors in their original paper but would have to concede, from their own data, that had they done so, as suggested in letters by Gasparro (1998) and Wolf and Kripke (1998), these would have been lower than the corresponding SPF. If the immune protection factor is lower than the SPF, it is just not possible for sunscreens ''to protect beyond their designated protection factors.'' The authors tested products with SPF ranging from 3.6 to 5.7, and defend their original conclusions by arguing that although ex vivo skin exposed to 5 minimal erythema doses (MED) results in 50% immunosuppression, a sunscreen of SPF ϭ 5 completely protects against immunosuppression when skin is exposed to 5 MED. This is hardly surprising as 5 MED with an SPF of 5 is equivalent to 1 MED without a sunscreen, and 1 MED without sunscreen almost certainly does not cause immunosuppression in their model. All the authors can conclude from this analysis is that 1 MED delivered to the skin, with or without sunscreen, is not immunosuppressive. Furthermore, the UV dose-threshold for immunosuppression in Manuscript

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Recently, α6 integrin has been proposed as the epithelial cell receptor for papillomavirus. This study investigated whether α6 integrin is the cellular receptor for bovine papillomavirus type 4 (BPV-4), which is strictly epitheliotropic and infects the mucous epithelium of the upper digestive tract. Primary bovine mucosal keratinocytes from the palate of a foetus (PalK) displayed high levels of α6 integrin ; matched primary fibroblasts from the same biopsy (PalF) expressed almost no α6 integrin. However, BPV-4 bound both PalK and PalF to similar, saturable levels. Native BPV-4 virions infected PalK in vitro, as detected by RT-PCR of E7 RNA. Infection could be blocked by excess virus-like particles (VLPs) and by neutralizing antisera against L1-L2 and L1 VLPs or by denaturation of the virions, supporting the view that infection in vitro mimics the process in vivo. α6 integrin-negative human keratinocyte cell lines were derived from patients affected by junctional epidermolysis bullosa presenting genetic lesions in their hemidesmosomes. The level of α6 integrin expression was determined in these cell lines by in situ immunofluorescence and FACS. Despite the absence of α6 integrin expression by BO-SV cells, they were bound by BPV-4 to similar, saturable levels as normal keratinocytes, KH-SV. Furthermore, BO-SV and KH-SV cells were both infected by BPV-4 to apparently the same extent as PalK cells. These results are consistent with the conclusion that α6 integrin is not the obligatory receptor for a bovine mucosotropic papillomavirus.