UVB Stimulates the Expression of Endothelin B Receptor in Human Melanocytes via a Sequential Activation of the p38/MSK1/CREB/MITF Pathway Which Can Be Interrupted by a French Maritime Pine Bark Extract through a Direct Inactivation of MSK1

Abstract

Melanogenesis is the physiological process by which melanin is synthesized to protect the skin from UV damage. While paracrine interactions between keratinocytes and melanocytes are crucial for regulating epidermal pigmentation, the endothelin (EDN)-endothelin B-receptor (EDNRB) interaction is one of the key linkages. In this study, we found that a single exposure of normal human melanocytes (NHMs) with UVB stimulates the expression of EDNRB and its upstream transcription factor microphthalmia-associated transcription factor (MITF) at the transcriptional and translational levels. That stimulation can be abrogated by post-irradiation treatment with a French maritime pine bark extract (PBE). UVB stimulated the phosphorylation of p38 and c-jun N-terminal kinase (JNK), but not ERK, followed by the increased phosphorylation of MSK1 and CREB. The post-irradiation treatment with PBE did not affect the increased phosphorylation of p38 and JNK, but distinctly abrogated the phosphorylation of MSK1 and CREB. Post-irradiation treatment with the MSK1 inhibitor H89 significantly down-regulated the increased gene expression of MITF and EDNRB in UVB-exposed NHMs. Our findings indicate for the first time that the increased expression of MITF that leads to the up-regulation of melanocyte-specific proteins in UVB-exposed NHMs is mediated via activation of the p38/MSK1/CREB pathway but not the ERK/RSK/CREB pathway. The mode of action by PBE demonstrates that interrupting MSK1 activation is a new target for antioxidants including PBE which can serve as anti-pigmenting agents in a reactive oxygen species-depletion-independent manner.

Signaling Cascades Activated by UVB in Human Melanocytes Lead to the Increased Expression of Melanocyte Receptors, Endothelin B Receptor and c‐KIT

“…A single exposure of NHM s to UVB stimulates endothelin B receptor ( EDNRB ) expression . A: RT‐PCR of EDNRB transcripts, B: Western blotting of EDNRB protein.…”

“…As RSK, which functions upstream of CREB, is not activated due to inactivation of the upstream signaling molecule ERK in UVB‐exposed NHMs and the activated p38 cannot directly phosphorylate CREB , it was of considerable interest to determine which protein kinases that function downstream of p38 are activated to directly phosphorylate CREB. In UVB‐exposed human primary keratinocytes, the activated p38 was known to phosphorylate and then activate nuclear kinase mitogen‐ and stress‐activated kinase (MSK)1 which can directly phosphorylate CREB and NFkBp65 in the nucleus during the NFkB signaling pathway .…”

“…In UVB‐exposed human primary keratinocytes, the activated p38 was known to phosphorylate and then activate nuclear kinase mitogen‐ and stress‐activated kinase (MSK)1 which can directly phosphorylate CREB and NFkBp65 in the nucleus during the NFkB signaling pathway . Therefore, we next determined whether UVB radiation stimulates the phosphorylation of MSK1 in NHMs and/or if oligomeric proanthocyanidin (OPA), which can serve as an inhibitor of MSK1 activation (phosphorylation) when treated postirradiation , can abrogate the increased phosphorylation of MSK1 in concert with interruption of the increased CREB phosphorylation. Western blotting analyses revealed that the phosphorylation of Thr581 (Fig.…”

“…Stimulatory effect of UVB irradiation on the phosphorylation of MSK 1/ NF kBp65 and the inhibitory effect of oligomeric proanthocyanidin ( OPA ) on that increase . A: Phosphorylation of MSK1 at Thr581, B:Phosphorylation of MSK1 at Ser376, C: Phosphorylation of NFkBp65 at Ser276, D: Phosphorylation of CREB at Ser133.…”

“…However, there have been few studies describing the direct effect of UVB on NHMs in terms of the expression patterns of melanocyte‐specific proteins and the intercellular signaling cascades that are activated. In this review, we summarize the intracellular signaling mechanisms underlying the UVB‐induced expression of two important melanogenic receptors, EDNRB and c‐KIT, in NHMs, which have been reported by our research groups .…”