Melanomas are very aggressive neoplasms with notorious resistance to therapeutics. It was recently proposed that the remarkable phenotypic plasticity of melanoma cells allows for the rapid development of both resistance to chemotherapeutic drugs and invasive properties. Indeed, the capacity of melanoma cells to form distant metastases is the main cause of mortality in melanoma patients. Therefore, the identification of the mechanism controlling melanoma phenotype is of paramount importance. In the present report, we show that deletion of microphthalmiaassociated transcription factor (MITF), the master gene in melanocyte differentiation, is sufficient to increase the metastatic potential of mouse and human melanoma cells. MITF silencing also increases fibronectin and Snail, two mesenchymal markers that might explain the increased invasiveness in vitro and in vivo. Furthermore, ablation of this population by Forskolin-induced differentiation or MITF-forced expression significantly decreases tumour and metastasis formation, suggesting that eradication of low-MITF cells might improve melanoma treatment. Moreover, we demonstrate that a hypoxic microenvironment decreases MITF expression through an indirect, hypoxia-inducible factor 1 (HIF1)a-dependant transcriptional mechanism, and increases the tumourigenic and metastatic properties of melanoma cells. We identified Bhlhb2, a new factor in melanoma biology, as the mediator of hypoxia/HIF1a inhibitory effect on MITF expression. Our results reveal a hypoxia-HIF1a-BHLHB2-MITF cascade controlling the phenotypic plasticity in melanoma cells and favouring metastasis development. Targeting this pathway might be helpful in the design of new anti-melanoma therapies.

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We report a case of fibroblastic rheumatism (FR). Only eight other cases of this recently described entity have been reported previously. FR is characterized by polyarthralgia and joint stiffness without joint destruction, associated with cutaneous nodules and sclerodactyly. Histology shows an increase in the number of fibroblasts and marked dermal fibrosis. Rheumatological and skin manifestations may improve with corticosteroid therapy. In our patient, immunohistochemical studies of involved and uninvolved skin showed an increase in fibronectin and tenascin deposition. In the dermis, the hyperplastic cells had phenotypic features of muscle, suggesting myofibroblastic differentiation. Ultrastructural study showed an increase in active fibroblastic cells with features of myofibroblasts. A hyperproliferative capacity was observed in fibroblasts cultured from involved skin. Biochemical studies of the production of collagen and non-collagen proteins were performed on these cultured cells, and showed a reduction in collagen and non-collagen protein synthesis by FR fibroblasts. Thus, FR appears to differ from other fibrotic skin diseases such as scleroderma, in that dermal fibrosis may be due predominantly to fibroblast proliferation with myofibroblastic differentiation without any increase in collagen synthesis.