Study of the melanocytic differentiation applied to vitiligo and melanoma: from the patient to the molecular mechanisms

Background and Previous activities

Vitiligo is an acquired, idiopathic disorder characterized by circumscribed depigmented macules and patches. Vitiligo affects approximately 0.5–2% of the general population worldwide, and it may appear any time from shortly after birth to old-age. Although limited to skin defects, vitiligo has a strong impact on the quality of life of affected people. It has been demonstrated that the impact of vitiligo on professional, social and sexual life is higher than in diabetes, depression, or even some cancers. Thus, vitiligo induces a strong therapeutic demand from affected people but unfortunately actual treatments are far to give truly satisfactory results. Vitiligo is a multifactorial disorder related to both genetic and nongenetic factors. It is generally agreed that there is an absence of functional melanocytes in vitiligo skin and that this loss of histochemically recognizable melanocytes is the result of their destruction. Recent advances showed the crucial role of the immune system in the pathophysiology of this disorder. However, the main problem is to stimulate the proliferation and the differentiation of melanoblasts to repigment the affected areas but the mechanisms involved in such phenomenon remain poorly understood and modest responses are obtained with the actual therapeutic approaches.
The transformation of melanocytic cells is responsible for melanoma, one of the deadliest cancers when it gets to a metastatic stage. The increased incidence of melanoma was higher in the past 10 years than all other cancers except for the lung cancer in women. Its incidence doubles every 10 years and keeps on growing of 5% a year in Caucasian population. Recent advances showed the heterogeneity of melanomas. Depending on the type and localization of melanomas several mutations have been described. Melanoma is highly resistant to radiotherapy and the rate of response to chemotherapies is low and usually is limited in time. Immunotherapy and vaccination protocols give inconsistent results. Therapies targeting activated pathways recently showed encouraging results but resistances developed under this approach and lead to a secondary progression of the disease. Those data urge the need for other therapeutic approaches. The approaches aiming to enhance the differentiation of cancer cells by using retinoic acid and derivatives, although useful in some leukemia and solid cancers, did not show any efficacy in melanoma. A protein, named PRAME, was initially described as a melanoma antigen. PRAME was then reported to be expressed by several solid cancers. PRAME works as an antagonist of the receptor of retinoic acid. Its inhibition restored the sensitivity to retinoic acid. The strong expression of PRAME in most of the melanoma cells probably explains the high resistance of those tumors to retinoic acid treatments.
The importance to better understand the mechanisms of the melanocytic proliferation and differentiation appears clearly. The study of the signaling pathways that regulate those mechanisms should allow identifying new therapeutic targets to regulate melanogenesis and melanocyte differentiation, but also to decrease the proliferation and to play on the differentiation of melanoma cells.
Keeping in mind those objectives we studied the role of the transcriptional factor called SOX9 in melanocytic cells. We first demonstrated that SOX9 is expressed in vitro and in vivo by the melanocytes of the skin and plays a key role in the UVB induced pigmentation. Activated by the cyclic AMP and protein kinase A (PKA), SOX9 increases the expression of the enzymes responsible for the melanogenesis by acting directly on the transcriptional factor MITF. This finally increases the production of melanin within the melanosomes. We also demonstrated that Agouti Signaling Protein (ASP) already known to inhibit the melanogenesis, is able to decrease the expression of SOX9 in melanocytes. This action can explain, at least partially, the still poorly understood mechanism of action of ASP.
Then, we further studied the expression and the role of SOX9 in melanoma cells. We demonstrated that the expression of SOX9 was decreased in most of advanced human melanoma samples. We showed that SOX9 inhibits the proliferation of melanoma cells by acting indirectly (through MITF) but also directly on the p21 promoter. We also demonstrated that SOX9 can restore the sensitivity to retinoic acid by decreasing the expression of the PRAME protein. The over expression of SOX9 strongly reduces the tumorigenicity of melanoma cells in vivo in mouse models and ex vivo in human reconstructed skins. We finally showed that agents capable of increasing the expression and/or activating SOX9, such as the prostaglandin D2 (PGD2) could reproduce its action on the proliferation of melanoma cells and also restore their sensitivity to retinoic acid.
Thus, the use of topical agents capable of regulating SOX9 (including prostaglandins) appears of great interest for treating pigmentary disorders. Such agents could be used to differentiate the melanocyte stem cells and to promote the repigmentation of vitiligo lesions. Concomitantly, the major effect of SOX9 on the proliferation of melanoma cells and its capability to restore retinoic acid sensitivity offer combined therapeutic perspectives not only for melanoma but also to most solid cancers that are resistant to retinoic acid.

Research Projects

Our project aims studying the mechanisms of differentiation of the melanocytic cells with a special focus on vitiligo and melanoma. By using the unique opportunity to work with patient’s samples, we would like to dissect the mechanisms responsible for the differentiation of melanocytes. The ultimate goal is to identify agents that could be used in clinical practice to treat pigmentary disorders and melanoma.

A) Study of the mechanisms involved in the repigmentation of the vitiligo lesions

In vitiligo lesions, the melanocytes are no longer present in the epidermis, however some treatments (mostly UV) can promote the differentiation of melanoblasts to melanocytes that can re-colonize the skin and repigment the affected lesions. The mechanisms implicated in the differentiation of melanocyte stem cells are still poorly understood. Identifying those mechanisms could have a huge impact on the treatment of the vitiligo.
We are using transcriptional analysis of skin samples of vitiligo patients to better understand the mechanisms involved in the pathogenesis of vitiligo and in the differentiation and proliferation of melanocytes progenitors under treatment. We use ex vivo models to test the effects of selected molecules and then dissect the pathways involved. Identifying such molecules could be used for repigmented affected skin of vitiligo patients.

B) Study of the redifferentiation of melanoma cells

We are studying the regulation of PRAME and other proteins involved in the resistance of melanoma cells to retinoic acid. We are testing the optimal combination of agents capable of decreasing this resistance and agonists of retinoic acid receptor to reach an optimal effect on melanoma proliferation.
We are also studying the effect of PGD2 derivatives on melanoma proliferation and differentiation and dissect the pathways involved in those processes.
All those experiments are made on melanoma cells line but also on melanoma cells freshly extracted from melanoma patients for a best accuracy in the results. Then agents are finally tested in murine models.

The restoration of the sensitivity to retinoic acid derivatives, could allow targeting all the cancer cells and forcing them to stop their proliferation and to redifferentiate. If we succeed, we will have a potential curative approach, not only for melanoma but for most of the solid cancers.

Team Publications