CLEC12B Is a Melanocytic Gene Regulating the Color of the Skin.

Pigmentation of the human skin is a complex process regulated by many genes. However, only a few have a profound impact on melanogenesis. Transcriptome analysis of pigmented skin compared with analysis of vitiligo skin devoid of melanocytes allowed us to unravel CLEC12B as a melanocytic gene. We showed that CLEC12B, a C-type lectin receptor, is highly expressed in melanocytes and that its expression is decreased in dark skin compared with that in white skin. CLEC12B directly recruits and activates SHP1 and SHP2 through its immunoreceptor tyrosine-based inhibitory motif domain and promotes CRE-binding protein degradation, leading to the downregulation of the downstream MITF pathway. CLEC12B ultimately controls melanin production and pigmentation in vitro and in a model of reconstructed human epidermis. The identification of CLEC12B in melanocytes shows that C-type lectin receptors exert function beyond immunity and inflammation. It also provides insights into the understanding of melanocyte biology and regulation of melanogenesis.

Dual Covalent Inhibition of PKM and IMPDH Targets Metabolism in Cutaneous Metastatic Melanoma.

Overcoming acquired drug resistance is a primary challenge in cancer treatment. Notably, more than 50% of patients with BRAFV600E cutaneous metastatic melanoma (CMM) eventually develop resistance to BRAF inhibitors. Resistant cells undergo metabolic reprogramming that profoundly influences therapeutic response and promotes tumor progression. Uncovering metabolic vulnerabilities could help suppress CMM tumor growth and overcome drug resistance. Here we identified a drug, HA344, that concomitantly targets two distinct metabolic hubs in cancer cells. HA344 inhibited the final and rate-limiting step of glycolysis through its covalent binding to the pyruvate kinase M2 (PKM2) enzyme, and it concurrently blocked the activity of inosine monophosphate dehydrogenase, the rate-limiting enzyme of de novo guanylate synthesis. As a consequence, HA344 efficiently targeted vemurafenib-sensitive and vemurafenib-resistant CMM cells and impaired CMM xenograft tumor growth in mice. In addition, HA344 acted synergistically with BRAF inhibitors on CMM cell lines in vitro. Thus, the mechanism of action of HA344 provides potential therapeutic avenues for patients with CMM and a broad range of different cancers. SIGNIFICANCE: Glycolytic and purine synthesis pathways are often deregulated in therapy-resistant tumors and can be targeted by the covalent inhibitor described in this study, suggesting its broad application for overcoming resistance in cancer.

Innate lymphocyte-induced CXCR3B-mediated melanocyte apoptosis is a potential initiator of T-cell autoreactivity in vitiligo.

T-cells play a crucial role in progression of autoimmunity, including vitiligo, yet the initial steps triggering their activation and tissue damage remain unknown. Here we demonstrate increased presence of type-1 innate lymphoid cells (NK and ILC1)-producing interferon gamma (IFNγ) in the blood and in non-lesional skin of vitiligo patients. Melanocytes of vitiligo patients have strong basal expression of chemokine-receptor-3 (CXCR3) isoform B which is directly regulated by IFNγ. CXCR3B activation by CXCL10 at the surface of cultured human melanocytes induces their apoptosis. The remaining melanocytes, activated by the IFNγ production, express co-stimulatory markers which trigger T-cell proliferation and subsequent anti-melanocytic immunity. Inhibiting the CXCR3B activation prevents this apoptosis and the further activation of T cells. Our results emphasize the key role of CXCR3B in apoptosis of melanocytes and identify CXCR3B as a potential target to prevent and to treat vitiligo by acting at the early stages of melanocyte destruction.

Melanocytes Sense Blue Light and Regulate Pigmentation through Opsin-3.

The shorter wavelengths of the visible light spectrum have been recently reported to induce a long-lasting hyperpigmentation but only in melano-competent individuals. Here, we provide evidence showing that OPN3 is the key sensor in melanocytes responsible for hyperpigmentation induced by the shorter wavelengths of visible light. The melanogenesis induced through OPN3 is calcium dependent and further activates CAMKII followed by CREB, extracellular signal-regulated kinase, and p38, leading to the phosphorylation of MITF and ultimately to the increase of the melanogenesis enzymes: tyrosinase and dopachrome tautomerase. Furthermore, blue light induces the formation of a protein complex that we showed to be formed by tyrosinase and dopachrome tautomerase. This multimeric tyrosinase/tyrosinase-related protein complex is mainly formed in dark-skinned melanocytes and induces a sustained tyrosinase activity, thus explaining the long-lasting hyperpigmentation that is observed only in skin type III and higher after blue light irradiation. OPN3 thus functions as the sensor for visible light pigmentation. OPN3 and the multimeric tyrosinase/tyrosinase-related protein complex induced after its activation appear as new potential targets for regulating melanogenesis but also to protect dark skins against blue light in physiological conditions and in pigmentary disorders.

Implication of REDD1 in the activation of inflammatory pathways.

In response to endotoxemia, the organism triggers an inflammatory response, and the visceral adipose tissue represents a major source of proinflammatory cytokines. The regulation of inflammation response in the adipose tissue is thus of crucial importance. We demonstrated that Regulated in development and DNA damage response-1 (REDD1) is involved in inflammation. REDD1 expression was increased in response to lipopolysaccharide (LPS) in bone marrow derived macrophages (BMDM) and in epidydimal adipose tissue. Loss of REDD1 protected the development of inflammation, since the expression of proinflammatory cytokines (TNFα, IL-6, IL-1ß) was decreased in adipose tissue of REDD1-/- mice injected with LPS compared to wild-type mice. This decrease was associated with an inhibition of the activation of p38MAPK, JNK, NF-κB and NLRP3 inflammasome leading to a reduction of IL-1ß secretion in response to LPS and ATP in REDD1-/- BMDM. Although REDD1 is an inhibitor of mTORC1, loss of REDD1 decreased inflammation independently of mTORC1 activation but more likely through oxidative stress regulation. Absence of REDD1 decreases ROS associated with a dysregulation of Nox-1 and GPx3 expression. Absence of REDD1 in macrophages decreases the development of insulin resistance in adipocyte-macrophage coculture. Altogether, REDD1 appears to be a key player in the control of inflammation.

Transcriptional Analysis of Vitiligo Skin Reveals the Alteration of WNT Pathway: A Promising Target for Repigmenting Vitiligo Patients.

Vitiligo affects 1% of the worldwide population. Halting disease progression and repigmenting the lesional skin represent the two faces of therapeutic challenge in vitiligo. We performed transcriptome analysis on lesional, perilesional, and non-depigmented skin from vitiligo patients and on matched skin from healthy subjects. We found a significant increase in CXCL10 in non-depigmented and perilesional vitiligo skin compared with levels in healthy control skin; however, neither CXCL10 nor other immune factors were deregulated in depigmented vitiligo skin. Interestingly, the WNT pathway, which is involved in melanocyte differentiation, was altered specifically in vitiligo skin. We demonstrated that oxidative stress decreases WNT expression/activation in keratinocytes and melanocytes. We developed an ex vivo skin model and confirmed the decrease activation of the WNT pathway in human skin subjected to oxidative stress. Finally, using pharmacological agents that activate the WNT pathway, we treated ex vivo depigmented skin from vitiligo patients and successfully induced differentiation of resident stem cells into pre-melanocytes. Our results shed light on the previously unrecognized role of decreased WNT activation in the prevention of melanocyte differentiation in depigmented vitiligo skin. Furthermore, these results support further clinical exploration of WNT agonists to repigment vitiligo lesions.

Endothelial Cells Promote Pigmentation through Endothelin Receptor B Activation.

Findings of increased vascularization in melasma lesions and hyperpigmentation in acquired bilateral telangiectatic macules suggested a link between pigmentation and vascularization. Using high-magnification digital epiluminescence dermatoscopy, laser confocal microscopy, and histological examination, we showed that benign vascular lesions of the skin have restricted but significant hyperpigmentation compared with the surrounding skin. We then studied the role of microvascular endothelial cells in regulating skin pigmentation using an in vitro co-culture model using endothelial cells and melanocytes. These experiments showed that endothelin 1 released by microvascular endothelial cells induces increased melanogenesis signaling, characterized by microphthalmia-associated transcription factor phosphorylation, and increased tyrosinase and dopachrome tautomerase levels. Immunostaining for endothelin 1 in vascular lesions confirmed the increased expression on the basal layer of the epidermis above small vessels compared with perilesional skin. Endothelin acts through the activation of endothelin receptor B and the mitogen-activated protein kinase, extracellular signal-regulated kinase (ERK)1/2, and p38, to induce melanogenesis. Finally, culturing of reconstructed skin with microvascular endothelial cells led to increased skin pigmentation that could be prevented by inhibiting EDNRB. Taken together these results demonstrated the role of underlying microvascularization in skin pigmentation, a finding that could open new fields of research for regulating physiological pigmentation and for treating pigmentation disorders such as melasma.

Hypoxia inhibits Cavin-1 and Cavin-2 expression and down-regulates caveolae in adipocytes.

During obesity, a hypoxic state develops within the adipose tissue, resulting in insulin resistance. To understand the underlying mechanism, we analyzed the involvement of caveolae because they play a crucial role in the activation of insulin receptors. In the present study, we demonstrate that in 3T3-L1 adipocytes, hypoxia induces the disappearance of caveolae and inhibits the expression of Cavin-1 and Cavin-2, two proteins necessary for the formation of caveolae. In mice, hypoxia induced by the ligature of the spermatic artery results in the decrease of cavin-1 and cavin-2 expression in the epididymal adipose tissue. Down-regulation of the expression of cavins in response to hypoxia is dependent on hypoxia-inducible factor-1. Indeed, the inhibition of hypoxia-inducible factor-1 restores the expression of cavins and caveolae formation. Expression of cavins regulates insulin signaling because the silencing of cavin-1 and cavin-2 impairs insulin signaling pathway. In human, cavin-1 and cavin-2 are decreased in the sc adipose tissue of obese diabetic patients compared with lean subjects. Moreover, the expression of cavin-2 correlates negatively with the homeostatic model assessment index of insulin resistance and glycated hemoglobin level. In conclusion, we propose a new mechanism in which hypoxia inhibits cavin-1 and cavin-2 expression, resulting in the disappearance of caveolae. This leads to the inhibition of insulin signaling and the establishment of insulin resistance.

Inhibition of melanogenesis by the antidiabetic metformin.

Several reports have demonstrated the inhibitory effect of metformin, a widely used drug in the treatment of type 2 diabetes, on the proliferation of many cancers including melanoma. Recently, it has been shown that metformin is able to modulate the cAMP level in the liver. As cAMP has a crucial role in melanin synthesis and skin pigmentation, we investigated the effect of metformin on melanogenesis both in vitro and in vivo. We showed that metformin led to reduced melanin content in melanoma cells and in normal human melanocytes by decreasing cAMP accumulation and cAMP-responsive element-binding protein phosphorylation. This inhibitory effect is correlated with decreased expression of master genes of melanogenesis, microphthalmia-associated transcription factor, tyrosinase, dopachrome tautomerase, and tyrosinase-related protein 1. Furthermore, we demonstrated that the antimelanogenic effect of metformin is independent of the AMPK pathway. Interestingly, topical application of metformin induced tail whitening in mice. Finally, we confirmed the antimelanogenic effect of metformin on reconstituted human epidermis and on human skin biopsies. These data emphasize the depigmenting effect of metformin and suggest a clinical strategy for using metformin in the topical treatment of hyperpigmentation disorders.

Regulated in development and DNA damage responses -1 (REDD1) protein contributes to insulin signaling pathway in adipocytes.

REDD1 (Regulated in development and DNA damage response 1) is a hypoxia and stress response gene and is a negative regulator of mTORC1. Since mTORC1 is involved in the negative feedback loop of insulin signaling, we have studied the role of REDD1 on insulin signaling pathway and its regulation by insulin. In human and murine adipocytes, insulin transiently stimulates REDD1 expression through a MEK dependent pathway. In HEK-293 cells, expression of a constitutive active form of MEK stabilizes REDD1 and protects REDD1 from proteasomal degradation mediated by CUL4A-DDB1 ubiquitin ligase complex. In 3T3-L1 adipocytes, silencing of REDD1 with siRNA induces an increase of mTORC1 activity as well as an inhibition of insulin signaling pathway and lipogenesis. Rapamycin, a mTORC1 inhibitor, restores the insulin signaling after downregulation of REDD1 expression. This observation suggests that REDD1 positively regulates insulin signaling through the inhibition of mTORC1 activity. In conclusion, our results demonstrate that insulin increases REDD1 expression, and that REDD1 participates in the biological response to insulin.

Metformin, independent of AMPK, induces mTOR inhibition and cell-cycle arrest through REDD1.

Metformin is a widely prescribed antidiabetic drug associated with a reduced risk of cancer. Many studies show that metformin inhibits cancer cell viability through the inhibition of mTOR. We recently showed that antiproliferative action of metformin in prostate cancer cell lines is not mediated by AMP-activated protein kinase (AMPK). We identified REDD1 (also known as DDIT4 and RTP801), a negative regulator of mTOR, as a new molecular target of metformin. We show that metformin increases REDD1 expression in a p53-dependent manner. REDD1 invalidation, using siRNA or REDD1(-/-) cells, abrogates metformin inhibition of mTOR. Importantly, inhibition of REDD1 reverses metformin-induced cell-cycle arrest and significantly protects from the deleterious effects of metformin on cell transformation. Finally, we show the contribution of p53 in mediating metformin action in prostate cancer cells. These results highlight the p53/REDD1 axis as a new molecular target in anticancer therapy in response to metformin treatment.

Insulin induces REDD1 expression through hypoxia-inducible factor 1 activation in adipocytes.

REDD1 (regulated in development and DNA damage responses) is essential for the inhibition of mTORC1 (mammalian target of rapamycin complex) signaling pathway in response to hypoxia. REDD1 expression is regulated by many stresses such as hypoxia, oxidative stress, and energy depletion. However, the regulation of REDD1 expression in response to insulin remains unknown. In the present study, we demonstrate that in murine and in human adipocytes, insulin stimulates REDD1 expression. Insulin-induced REDD1 expression occurs through phosphoinositide 3-kinase/mTOR-dependent pathways. Moreover, using echinomycin, a hypoxia-inducible factor 1 (HIF-1) inhibitor, and HIF-1alpha small interfering RNA, we demonstrate that insulin stimulates REDD1 expression only through the transcription factor HIF-1. In conclusion, our study shows that insulin stimulates REDD1 expression in adipocytes.

Hypoxia decreases insulin signaling pathways in adipocytes.

OBJECTIVE: Obesity is characterized by an overgrowth of adipose tissue that leads to the formation of hypoxic areas within this tissue. We investigated whether this phenomenon could be responsible for insulin resistance by studying the effect of hypoxia on the insulin signaling pathway in adipocytes. RESEARCH DESIGN AND METHODS: The hypoxic signaling pathway was modulated in adipocytes from human and murine origins through incubation under hypoxic conditions (1% O(2)) or modulation of hypoxia-inducible factor (HIF) expression. Insulin signaling was monitored through the phosphorylation state of several key partners of the pathway and glucose transport. RESULTS: In both human and murine adipocytes, hypoxia inhibits insulin signaling as revealed by a decrease in the phosphorylation of insulin receptor. In 3T3-L1 adipocytes, this inhibition of insulin receptor phosphorylation is followed by a decrease in the phosphorylation state of protein kinase B and AS160, as well as an inhibition of glucose transport in response to insulin. These processes were reversible under normoxic conditions. The mechanism of inhibition seems independent of protein tyrosine phosphatase activities. Overexpression of HIF-1alpha or -2alpha or activation of HIF transcription factor with CoCl(2) mimicked the effect of hypoxia on insulin signaling, whereas downregulation of HIF-1alpha and -2alpha by small interfering RNA inhibited it. CONCLUSIONS: We have demonstrated that hypoxia creates a state of insulin resistance in adipocytes that is dependent upon HIF transcription factor expression. Hypoxia could be envisioned as a new mechanism that participates in insulin resistance in adipose tissue of obese patients.