Alpha-melanocyte-stimulating hormone counteracts the suppressive effect of UVB on Nrf2 and Nrf-dependent gene expression in human skin.
2009

Abstract
Human skin is constantly exposed to UV light, the most ubiquitous environmental stressor. Here, we investigated the expression and regulation of Nrf1-3, transcription factors crucially involved in protection against oxidative stress in human skin cells in vitro, ex vivo, and in situ. In particular, we examined whether alpha-MSH, a UV-induced peptide, is capable of modulating Nrf2 and Nrf-dependent gene expression. Nrf1, -2, and -3 were found to be expressed in various cutaneous cell types in vitro. Surprisingly, UVB irradiation at physiological doses (10 mJ/cm(2)) reduced Nrf2 and Nrf-dependent gene expression in normal keratinocytes and melanocytes in vitro as well as ex vivo in skin organ cultures. alpha-MSH alone significantly increased Nrf2 as well as Nrf-dependent heme oxygenase-1, gamma-glutamylcysteine-synthetase, and glutathione-S-transferase Pi gene expression in both keratinocytes and melanocytes. This effect of alpha-MSH occurred at physiological doses and was due to transcriptional induction, mimicked by the artificial cAMP inducer forskolin, and blocked by protein kinase A pathway inhibition. In silico promoter analysis of Nrf2 further identified several putative binding sites for activator protein 1 and cAMP response element-binding protein, transcription factors typically activated by alpha-MSH. Importantly, alpha-MSH prevented or even overcompensated the UVB-induced suppression of Nrf2 and Nrf-dependent genes not only in normal keratinocytes and melanocytes in vitro but also in skin organ cultures. These findings, for the first time, show regulation of Nrf2 and Nrf-dependent genes by alpha-MSH. Our data also highlight a novel facet in the cytoprotective and antioxidative effector mechanisms of alpha-MSH and perhaps of related melanocortin peptides (Melanotan II).

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Impact of ?-MSH oxidation on its modulatory effect on UVB-mediated reduction of Nrf2 expression in NHM and proposed molecular mechanism of the modulatory effect of ?-MSH. A, RNA expression analysis of Nrf2 in NHM exposed to UVB, ?-MSH plus UVB, or oxidized (ox.) ?-MSH plus UVB. Cells were preincubated with ?-MSH (10?6 m) or oxidized ?-MSH at the same dose for 24 h, exposed to UVB (10 mJ/cm2), and harvested after 2 h. Nrf2 mRNA expression was monitored by real-time RT-PCR. *, P < 0.05; **, P < 0.01 vs. UVB plus ?-MSH. B, Schematic illustration depicting the proposed role of ?-MSH as an endogenous protector against UVB-induced oxidative stress via induction of Nrf2 and Nrf-dependent genes or directly by scavenging ROS in keratinocytes and melanocytes. ARE, Antioxidant response element; CRE, cAMP response element; TRE, 12-O-tetradecanoylphorbol-13-acetate response element.