Chronic sun exposure can lead to severe skin disorders such as carcinogenesis. The cell death process triggered by ultraviolet B (UVB) irradiation is crucial because it protects the surrounding tissue from the emergence and the accumulation of cells that bear the risk of becoming transformed. Here, we show that repression of NF-jB and Egr-1 expression drastically inhibits UVB-mediated cell death. Furthermore, we demonstrate that Egr-1 is induced upon UVB irradiation through NF-jB activation and the binding of p65/RelA within the Egr-1 promoter. We show that Egr-1 contributes to the regulation of the Gadd45a and Gadd45b genes, which are involved in the control of cell cycle, DNA repair and apoptosis, by direct binding to their promoter. Our study demonstrates for the first time a signaling cascade involving sequential activation of NF-jB, Egr-1 and Gadd45 to induce UVB-mediated cell death. Failure in the induction of each protagonist of this pathway alters the UVB-mediated cell death process. Therefore, impairment of the cascade could be at the onset of skin carcinogenesis mediated by genotoxic stress.

In melanocytes and melanoma cells α-melanocyte stimulating hormone (α-MSH), via the cAMP pathway, elicits a large array of biological responses that control melanocyte differentiation and influence melanoma development or susceptibility. In this work, we show that cAMP transcriptionally activates Hif1a gene in a melanocyte cell–specific manner and increases the expression of a functional hypoxia-inducible factor 1α (HIF1α) protein resulting in a stimulation of Vegf expression. Interestingly, we report that the melanocyte-specific transcription factor, microphthalmia-associated transcription factor (MITF), binds to the Hif1a promoter and strongly stimulates its transcriptional activity. Further, MITF “silencing” abrogates the cAMP effect on Hif1a expression, and overexpression of MITF in human melanoma cells is sufficient to stimulate HIF1A mRNA. Our data demonstrate that Hif1a is a new MITF target gene and that MITF mediates the cAMP stimulation of Hif1a in melanocytes and melanoma cells. Importantly, we provide results demonstrating that HIF1 plays a pro-survival role in this cell system. We therefore conclude that the α-MSH/cAMP pathway, using MITF as a signal transducer and HIF1α as a target, might contribute to melanoma progression.

SummaryIntegrin receptors and their extracellular matrix ligands provide cues to cell proliferation, survival, differentiation and migration. Here, we show that 21 integrin, when ligated to the basement membrane component laminin-1, triggers a proliferation arrest in primary endothelial cells. Indeed, in the presence of strong growth signals supplied by growth factors and fibronectin, 21 engagement alters assembly of mature focal adhesions by 51 and leads to impairment of downstream signaling and cell-cycle arrest in the G1 phase. Although the capacity of 51 to signal for GTP loading of Rac is preserved, the joint engagement of 21 interferes with membrane anchorage of Rac. Adapting the 'split-ubiquitin' sensor to screen for membrane-proximal 2 integrin partners, we identified the CD9 tetraspanin and further establish its requirement for destabilization of focal adhesions, control of Rac subcellular localization and growth arrest induced by 21 integrin. Altogether, our data establish that 21 integrin controls endothelial cell commitment towards quiescence by triggering a CD9-dependent dominant signaling.