Melanocyte Homeostasis Research Articles

Membrane‐bound Kit ligand regulates melanocyte adhesion and survival, providing physical interaction with an intraepithelial niche

Morphogenesis, as illustrated by melanocyte migration and homing to the skin, requires cadherin adhesion, integrin-dependent migration and Kit-ligand growth factor signaling. However, it is not known how Kit ligand regulates integrin or cadherin-dependent intraepidermal melanocyte behavior. To answer this question, we developed specific 2-dimensional (2D) and 3D culture systems analyzing how soluble or immobilized Kit-ligand-regulated melanocyte migration on vitronectin and laminin, or within a monolayer of kidney epithelial cells. In a 2D system, soluble Kit ligand stimulated integrin-dependent melanoblast migration and chemotaxis and accelerated integrin turnover. In contrast, immobilized, but not soluble, Kit ligand, enhanced integrin-dependent melanocyte spreading on suboptimal laminin concentrations. In 3D, membrane-bound Kit ligand induced intraepithelial melanocyte proliferation, survival, and tight adhesion to epithelial cells, while cleavable Kit ligand was less effective. In contrast, melanocyte motility was independent of membrane-bound Kit ligand, but increased in the presence of the cleavable Kit-ligand isoform. Transmembrane-dimerization or basolateral-targeting mutants of Kit ligand altered intraepithelial melanocyte localization, survival, and adhesion to epithelial cells. These data and the identification of c-kit/Kit-ligand clusters at cell contacts suggest that membrane-bound Kit ligand captures cell surface-expressed c-kit, providing mechanical anchoring and survival signaling within intraepithelial niches, and thereby defining a new mechanism for melanocyte homeostasis and requirement for environmental niches.

Abstract 2106: Long non-coding RNA signatures for melanoma detection in humans

Abstract Melanoma, an aggressive carcinoma that originates in the basal layer of human epidermis, has the highest mortality rate of skin cancers in industrialized countries. Unfortunately, current treatments for metastatic melanoma provide mostly palliative benefits to only a small proportion of patients. Despite the obvious need for improvement in assessment and treatment of metastatic melanoma, the intricate mechanisms underlying melanoma transformation remain unresolved and poorly understood. Thus, there is considerable interest in understanding the molecular basis of initiation and progression of human melanomas. Recent evidence indicates that long non-coding RNAs (lncRNAs), a subclass of noncoding RNA (ncRNA), are associated with carcinogenesis in melanoma as well as several other human cancers. The elucidation of the molecular function of these RNA species promises to bridge gaps in our knowledge of melanoma development in humans. The expression of selected lncRNAs (CR618740 and AF091731) in a panel of melanoma cell lines (pigmented and non-pigmented), melanocytes, and keratinocytes were assayed by DNA microarray (Ncode arrays from Lifetechnologies) and further confirmed by real-time polymerase chain reaction (qRT-PCR). Next, cell lines results were tested with patient samples. Our results indicate that the differential expression of lncRNAs is an indicator of the development of melanoma, and dysregulated expression of these markers may be involved in the transition of melanocytes to melanoma. Furthermore, the presence of significant correlation between melanoma cell line characteristics (stage, cell phenotype, types of mutations and others) and lncRNA expression data may imply novel causal connection between lncRNA expression and melanoma development. Importantly, this work will investigate the new possibility that lncRNAs play an essential role in cellular regulation and represent a potential platform for melanoma progression. Thus, unraveling the differential lncRNA expression in melanoma could initiate novel understanding of melanocyte homeostasis, tissue organization, and melanoma genesis in humans. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2106. doi:1538-7445.AM2012-2106

The Three-Dimensional Human Skin Reconstruct Model: a Tool to Study Normal Skin and Melanoma Progression

Most in vitro studies in experimental skin biology have been done in 2-dimensional (2D) monocultures, while accumulating evidence suggests that cells behave differently when they are grown within a 3D extra-cellular matrix and also interact with other cells (1-5). Mouse models have been broadly utilized to study tissue morphogenesis in vivo. However mouse and human skin have significant differences in cellular architecture and physiology, which makes it difficult to extrapolate mouse studies to humans. Since melanocytes in mouse skin are mostly localized in hair follicles, they have distinct biological properties from those of humans, which locate primarily at the basal layer of the epidermis. The recent development of 3D human skin reconstruct models has enabled the field to investigate cell-matrix and cell-cell interactions between different cell types. The reconstructs consist of a "dermis" with fibroblasts embedded in a collagen I matrix, an "epidermis", which is comprised of stratified, differentiated keratinocytes and a functional basement membrane, which separates epidermis from dermis. Collagen provides scaffolding, nutrient delivery, and potential for cell-to-cell interaction. The 3D skin models incorporating melanocytic cells recapitulate natural features of melanocyte homeostasis and melanoma progression in human skin. As in vivo, melanocytes in reconstructed skin are localized at the basement membrane interspersed with basal layer keratinocytes. Melanoma cells exhibit the same characteristics reflecting the original tumor stage (RGP, VGP and metastatic melanoma cells) in vivo. Recently, dermal stem cells have been identified in the human dermis (6). These multi-potent stem cells can migrate to the epidermis and differentiate to melanocytes.

The Three-Dimensional Human Skin Reconstruct Model: a Tool to Study Normal Skin and Melanoma Progression

Most in vitro studies in experimental skin biology have been done in 2-dimensional (2D) monocultures, while accumulating evidence suggests that cells behave differently when they are grown within a 3D extra-cellular matrix and also interact with other cells (1-5). Mouse models have been broadly utilized to study tissue morphogenesis in vivo. However mouse and human skin have significant differences in cellular architecture and physiology, which makes it difficult to extrapolate mouse studies to humans. Since melanocytes in mouse skin are mostly localized in hair follicles, they have distinct biological properties from those of humans, which locate primarily at the basal layer of the epidermis. The recent development of 3D human skin reconstruct models has enabled the field to investigate cell-matrix and cell-cell interactions between different cell types. The reconstructs consist of a "dermis" with fibroblasts embedded in a collagen I matrix, an "epidermis", which is comprised of stratified, differentiated keratinocytes and a functional basement membrane, which separates epidermis from dermis. Collagen provides scaffolding, nutrient delivery, and potential for cell-to-cell interaction. The 3D skin models incorporating melanocytic cells recapitulate natural features of melanocyte homeostasis and melanoma progression in human skin. As in vivo, melanocytes in reconstructed skin are localized at the basement membrane interspersed with basal layer keratinocytes. Melanoma cells exhibit the same characteristics reflecting the original tumor stage (RGP, VGP and metastatic melanoma cells) in vivo. Recently, dermal stem cells have been identified in the human dermis (6). These multi-potent stem cells can migrate to the epidermis and differentiate to melanocytes.

RXRα Ablation in Epidermal Keratinocytes Enhances UVR-Induced DNA Damage, Apoptosis, and Proliferation of Keratinocytes and Melanocytes

We show here that keratinocytic nuclear receptor retinoid X receptor-α (RXRα) regulates mouse keratinocyte and melanocyte homeostasis following acute UVR. Keratinocytic RXRα has a protective role in UVR-induced keratinocyte and melanocyte proliferation/differentiation, oxidative stress-mediated DNA damage, and cellular apoptosis. We discovered that keratinocytic RXRα, in a cell-autonomous manner, regulates mitogenic growth responses in skin epidermis through secretion of heparin-binding EGF-like growth factor, GM-CSF, IL-1α, and cyclooxygenase-2 and activation of mitogen-activated protein kinase pathways. We identified altered expression of several keratinocyte-derived mitogenic paracrine growth factors such as endothelin 1, hepatocyte growth factor, α-melanocyte stimulating hormone, stem cell factor, and fibroblast growth factor-2 in skin of mice lacking RXRα in epidermal keratinocytes (RXRα(ep-/-) mice), which in a non-cell-autonomous manner modulated melanocyte proliferation and activation after UVR. RXRα(ep-/-) mice represent a unique animal model in which UVR induces melanocyte proliferation/activation in both epidermis and dermis. Considered together, the results of our study suggest that RXR antagonists, together with inhibitors of cell proliferation, can be effective in preventing solar UVR-induced photocarcinogenesis.

Melanocyte homeostasis in vivo tolerates Rb1 loss in a developmentally independent fashion

There has been uncertainty regarding the precise role that the pocket protein Rb1 plays in murine melanocyte homeostasis. It has been reported that the TAT-Cre mediated loss of exon 19 from a floxed Rb1 allele causes melanocyte apoptosis in vivo and in vitro. This is at variance with other findings showing, either directly or indirectly, that Rb1 loss in melanocytes has no noticeable effect in vivo, but in vitro leads to a semi-transformed phenotype. In this study, we show that Rb1-null melanocytes lacking exon 19 do not undergo apoptosis and survive both in vitro and in vivo, irrespective of the developmental stage at which Cre-mediated ablation of the exon occurs. Further, Rb1 loss has no serious long-term ramifications on melanocyte homeostasis in vivo, with Rb1-null melanocytes being detected in the skin after numerous hair cycles, inferring that the melanocyte stem cell population carrying the Cre-mediated deletion is maintained. Consequently, whilst Rb1 loss in the melanocyte is able to alter cellular behaviour in vitro, it appears inconsequential with respect to melanocyte homeostasis in the mouse skin.

Mechanisms Regulating Skin Pigmentation: The Rise and Fall of Complexion Coloration

Skin pigmentary abnormalities are seen as aesthetically unfavorable and have led to the development of cosmetic and therapeutic treatment modalities of varying efficacy. Hence, several putative depigmenting agents aimed at modulating skin pigmentation are currently being researched or sold in commercially available products. In this review we will discuss the regulation of processes that control skin complexion coloration. This includes direct inhibition of tyrosinase and related melanogenic enzymes, regulation of melanocyte homeostasis, alteration of constitutive and facultative pigmentation and down-regulation of melanosome transfer to the keratinocytes. These various processes, in the complex mechanism of skin pigmentation, can be regulated individually or concomitantly to alter complexion coloration and thus ameliorate skin complexion diseases.

Keratinocyte dysfunction in vitiligo epidermis: cytokine microenvironment and correlation to keratinocyte apoptosis.

Vitiligo is a skin disorder characterized by loss of functional melanocytes. Keratinocytes contribute to melanocyte homeostasis, and keratinocyte alteration may play a role in melanocyte dysfunction in vitiligo. In particular, the release of melanogenic mediators and the level of functioning keratinocytes may affect melanocyte dysfunction in vitiligo epidermis. Keratinocyte-derived mediators involved in pigmentation, analysed by in situ hybridization, and epidermal apoptosis, detected by TUNEL assay and electron microscopy, were evaluated in lesional and perilesional skin biopsies from 15 patients with active vitiligo and in 5 control subjects. Among the melanogenic mediators, stem cell factor (SCF) and endothelin-1 (ET-1) mRNA were significantly reduced in lesional as compared to perilesional epidermis, whereas no difference was observed in mRNA of basic fibroblastic growth factor (bFGF) and granulocyte-monocyte colony stimulating factor (GM-CSF). The expression of mRNA for tumor necrosis factor (TNF)-alpha and interleukin-6 (IL-6), two pro-inflammatory cytokines with an inhibitory effect on pigmentation, was increased in the epidermis from vitiligo biopsies, whereas their expression was practically undetectable in the skin of control subjects. Apoptotic keratinocytes were more abundant in lesional vs. perilesional skin of vitiligo patients and were absent in the epidermis of control subjects. Changes in expression of keratinocyte-derived mediators observed in the present study are consistent with their differential functions in melanocyte regulation. In particular, increased TNF-alpha could contribute to keratinocyte apoptosis, which results in reduced release of melanogenic cytokines and ultimately in melanocyte disappearance.

Dual loss of Rb1 and Trp53 in melanocytes perturbs melanocyte homeostasis and genetic stability in vitro but does not cause melanoma or pigmentation defects in vivo

Dual loss of <i>Rb1</i> and <i>Trp53</i> in melanocytes perturbs melanocyte homeostasis and genetic stability in vitro but does not cause melanoma or pigmentation defects in vivo

Both Notch1 and Notch2 contribute to the regulation of melanocyte homeostasis

Notch signaling affects a variety of mammalian stem cells, but there has been limited evidence that a specific Notch molecule regulates adult stem cells. Recently, it was reported that the reduced Notch signaling initiated at the embryonic stage results in a gradual hair graying phenotype after birth. Here we demonstrate that the oral administration of a gamma-secretase inhibitor (GSI) to wild-type adult C57/Bl6 mice led to a gradual increase in gray spots, which remained unchanged for at least 20 weeks after discontinuing the GSI. In GSI-treated mice, there was a severe decrease in unpigmented melanocytes in the bulge/subbulge region where melanocyte stem cells are located. While we confirmed that Notch1+/-Notch2+/- double heterozygous mice with a C57/Bl6 background were born with a normal hair color phenotype and gradually turned gray after the second hair cycle, in the c-kit mutant Wv background, Notch1+/- and Notch2+/- mice had larger white spots on the first appearance of hair than did the Wv/+ mice, which did not change throughout life. Notch1+/-Notch2+/-Wv/+ mice had white hair virtually all over the body at the first appearance of hair and the depigmentation continued to progress thereafter. Using a neural crest organ culture system, GSI blocked the generation of pigmented melanocytes when added to the culture during the period of melanoblast proliferation, but not during the period of differentiation. These observations imply roles of Notch signaling in both development of melanocyte during embryogenesis and maintenance of melanocyte stem cells in adulthood, while the degree of requirement is distinct in these settings: the latter is more sensitive than the former to the reduced Notch signaling. Furthermore, Notch1 and Notch2 cooperates with c-kit signaling during embryogenesis, and they cooperate with each other to regulate melanocyte homeostasis after birth.

Stem cell factor affects tumour progression markers in metastatic melanoma cells

Stem cell factor (SCF), next to various relevant biological effects exerted on many cell types, is able to keep melanocyte homeostasis through its receptor c-kit. Only a minority of metastatic melanoma cells (MMC) express c-kit receptor, but c-kit positive MMC move more slowly towards tumour progression and have a more natural tendency to undergo apoptosis. In our study c-kit positive MMC from human melanoma metastases and a c-kit positive human melanoma cell line-SK-MEL-28-showed a clear-cut reduction of cytokines normally up-regulated along melanoma progression after SCF stimulation. SCF was also able to maintain all MMC and SK-MEL-28 cells in a well differentiated status with an increase in organellogenesis and in particular of melanosomes in various degree of differentiation, but it did not induce apoptosis as observed in other in vitro models. The increase of melanosomes matched an increase of tyrosinase production. SCF did not modify the expression of NOS while it enhanced the expression of HLA-DR molecules on MMC membranes. Taken altogether these data stress the biological activity of SCF as a cytokine which is able to maintain MMC in a well differentiated status, and suggest a more in depth evaluation of possible effects of SCF on melanoma cells.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

Functioning methionine sulfoxide reductases A and B are present in human epidermal melanocytes in the cytosol and in the nucleus

Oxidation of methionine residues by reactive oxygen (ROS) in protein structures leads to the formation of methionine sulfoxide which can consequently lead to a plethora of impaired functionality. The generation of methionine sulfoxide yields ultimately a diastereomeric mixture of the S and R sulfoxides. So far two distinct enzyme families have been identified. MSRA reduces methionine S-sulfoxide, while MSRB reduces the R-diastereomer. It has been shown that these enzymes are involved in regulation of protein function and in elimination of ROS via reversible methionine formation besides protein repair. Importantly, both enzymes require coupling to the NADPH/thioredoxin reductase/thioredoxin electron donor system. In this report, we show for the first time the expression and function of both sulfoxide reductases together with thioredoxin reductase in the cytosol as well as in the nucleus of epidermal melanocytes which are especially sensitive to ROS. Since this cell resides in the basal layer of the epidermis and its numbers and functions are reduced upon ageing and for instance also in depigmentation processes, we believe that this discovery adds an intricate repair mechanism to melanocyte homeostasis and survival.

Normal Human Melanocyte Homeostasis as a Paradigm for Understanding Melanoma

Melanocytes, after cell division, separate and migrate along the basement membrane; they extend their dendrites and establish multiple contacts with keratinocytes. Once adhesion is established, keratinocytes control melanocyte growth and expression of cell surface receptors. Most melanomas arise within the epidermis (melanoma in situ) and then invade across the basement membrane. These melanoma cells escape from control by keratinocytes through five major mechanisms: (1) downregulation of receptors important for communication with keratinocytes such as E-cadherin, P-cadherin, and desmoglein, which is achieved through growth factors such as hepatocyte growth factor, platelet-derived growth factor, and endothelin-1 produced by fibroblasts or keratinocytes; (2) upregulation of receptors and signaling molecules important for melanoma cell-melanoma cell and melanoma cell-fibroblast interactions such as N-cadherin, Mel-CAM, and zonula occludens protein-1; (3) deregulation of morphogens such as Notch receptors and their ligands; (4) loss of anchorage to the basement membrane because of an altered expression of cell-matrix adhesion molecules; (5) increased elaboration of metal-loproteinases. Thus, investigating normal melanocyte homeostasis helps us to better define how melanoma cells escape the microenvironment created by epidermal keratinocytes and how they develop new cellular partners in fibroblasts and endothelial cells, which support their growth and invasion.

Melanocytes in conditional Rb–/– mice are normal in vivo but exhibit proliferation and pigmentation defects in vitro

The function of the retinoblastoma tumour suppressor (Rb1), and the pocket protein family in general, has been implicated as an important focal point for deregulation in many of the molecular pathways mutated in melanoma. We have focused on the role of Rb1 in mouse melanocyte homeostasis using gene targeting and Cre/loxP mediated tissue-specific deletion. We show that constitutive Cre-mediated ablation of Rb1 exon 2 prevents the production of Rb1 and recapitulates the phenotype encountered in other Rb1 knockout mouse models. Mice with conditional melanocyte-specific ablation of Rb1 manifest overtly normal pigmentation and are bereft of melanocytic hyperproliferative defects or apoptosis-induced depigmentation. Histologically, these mice have melanocyte morphology and distribution comparable with control littermates. In contrast, Rb1-null melanocytes removed from their in vivo micro-environment and cultured in vitro display some of the characteristics associated with a transformed phenotype. They proliferate at a heightened rate when compared with control melanocytes and have a decreased requirement for mitogens. With progressive culture the cells depigment at relatively early passage and display a gross morphology which, whilst reminiscent of early passage melanocytes, is generally different to equivalent passage control cells. These results indicate that Rb1 is dispensable for in vivo melanocyte homeostasis when its ablation is targeted from the melanoblast stage onwards, however, when cultured in vitro, Rb1 loss increases melanocyte growth but the cells are not fully transformed.

CRH peptides modulate proliferation, melanogenesis and dendri‐city in human follicular melanocytes

Corticotropin‐releasing hormone (CRH) is the most proximal element of the hypothalamic‐pituitary‐adrenal axis (HPA) and is the chief regulator of pituitary POMC gene expression and the subsequent production and secretion of POMC peptides. Previously, our laboratories documented cutaneous expression of CRH, urocortin and functional CRH receptors (CRH‐Rs), suggesting their role in skin physiology and pathology. Human skin predominately expressed CRH‐R1 with CRH‐R2 being expressed primarily in the adnexal structures. While CRH‐R activity has been implicated in the regulation of epidermal cell function, a role for these receptors in human hair biology has not yet been demonstrated. This study was designed to investigate the effects of modified CRH peptides (D‐Glu20)‐CRH, (D‐Pro5)‐CRH and (D‐Pro4)‐urocortin with respective selectivity for CRH‐R1 and CRH‐R2 on behaviour of cultured hair follicle melanocytes (HFMs) derived from scalp of seven normal individuals. HFMs were stimulated with these peptides (10−7−10−10 m) for 72 h. (D‐Glu20)‐CRH (10−8 m) and (D‐Pro5)‐CRH (10−9 and 10−10 m) markedly increased cell dendricity, melanogenesis and proliferation (P &lt; 0.01) compared with unstimulated levels. While (D‐Pro4)‐urocortin failed to stimulate cell dendricity, this peptide did stimulate melanogenesis (10−8 m) (P &lt; 0.01) and exhibited a biphasic proliferative response; stimulating pigment cell division at 10−7 and 10−8 m (P &lt; 0.01) but inhibiting proliferation at 10−9 and 10−10 m (P &lt; 0.01). Here, we demonstrate the existence of functionally active CRH‐Rs in cultured human scalp HFM and show that signalling via these receptors modulates follicular melanocyte dendricity, melanogenesis and proliferation. Thus, activation of CRH‐Rs may have a pivotal role in the regulation of follicular melanocyte homeostasis.

Apoptosis regulators and responses in human melanocytic and keratinocytic cells.

Apoptosis in keratinocytes is required for epidermal turnover, stratum corneum formation, and removal of ultraviolet-damaged premalignant cells. Its role in melanocyte homeostasis and transformation, on the other hand, has not been defined, although apoptosis resistance is a commonly recognized feature of melanoma. We examined the expression of apoptosis regulators in melanocytes, keratinocytes, melanoma, and HaCat cells. Melanocytic cells expressed relatively high levels of Bcl-2, Bcl-X(L), Mcl-1, C-IAP-1, C-IAP-2, XIAP, Livin, and Apaf-1. The only apoptotic regulator that was differentially expressed in melanoma cells and not melanocytes was Survivin, whereas Bax was expressed in melanocytes but not in most melanoma lines. Keratinocytic cells, on the other hand, expressed high levels of FLIP and were relatively deficient in Bcl-2 family proteins. Levels of p53 were highest in HaCat cells and some of the melanoma lines, and barely detectable in melanocytes and keratinocytes. Next, susceptibility of these cells types to apoptosis induced by ultraviolet B, the tyrosine analog 4-tert-butylphenol, and cytotoxic drugs was examined. Melanocytes were relatively resistant to ultraviolet B, whereas keratinocytes were unresponsive to 4-tert-butylphenol. Melanocytes and keratinocytes were generally less susceptible than melanoma lines and HaCat cells to etoposide, cisplatin, and staurosporine. Induction of apoptosis in these cell types was generally associated with decreased levels of Mcl-1, XIAP, and Livin, and increased levels of p53, whereas levels of other apoptotic regulators were unaltered. These results provide insights into the potential roles of apoptosis in the function and transformation of epidermal melanocytes and keratinocytes.

The SCF/KIT Pathway Plays a Critical Role in the Control of Normal Human Melanocyte Homeostasis

During development, the interaction of stem cell factor (SCF) with its receptor, KIT, is critical for the survival of melanocytes. Limited in vivo human studies have suggested a possible activating role of SCF on adult human melanocytes. In order to study the impact of this pathway on normal melanocyte homeostasis, human skin xenografts were treated with serial injections of recombinant human SCF or a KIT-inhibitory antibody (K44.2). On histologic evaluation, SCF injection increased, whereas KIT inhibition decreased the number, size, and dendricity of melanocytes. Immunohistochemical expression of melanocyte differentiation antigens, including tyrosinase-related-protein-1 and gp100/pmel17, was markedly increased by treatment with SCF, and decreased by K44.2 treatment. The number of Ki67-positive melanocytes was increased in the SCF-treated tissue, suggesting a direct proliferative effect of SCF; conversely, treatment with K44.2 resulted in melanocyte loss, which did not appear reversible with prolonged treatment. These findings demonstrate that the SCF/KIT pathway remains critical in adult human skin, and that pharmacologic modulation of this single pathway can control cutaneous melanocyte homeostasis.

Murine cutaneous mastocytosis and epidermal melanocytosis induced by keratinocyte expression of transgenic stem cell factor.

The growth and differentiation of mast cells and melanocytes require stem cell factor (SCF), the ligand for the kit receptor tyrosine kinase. SCF may exist as a membrane-bound or soluble molecule. Abnormalities of the SCF-kit signaling pathway, with increased local concentrations of soluble SCF, have been implicated in the pathogenesis of the human disease cutaneous mastocytosis, but have not yet been shown to play a causal role. To investigate both the potential of SCF to cause mastocytosis and its role in epidermal melanocyte homeostasis, we targeted the expression of SCF to epidermal keratinocytes in mice with two different transgenes controlled by the human keratin 14 promoter. The transgenes contained cDNAs that either produced SCF, which can exist in both membrane-bound and soluble forms, or SCF, which remains essentially membrane bound. Murine epidermal keratinocyte expression of membrane-bound/ soluble SCF reproduced the phenotype of human cutaneous mastocytosis, with dermal mast cell infiltrates and epidermal hyperpigmentation, and caused the maintenance of a population of melanocytes in the interadnexal epidermis, an area where melanocytes and melanin are found in human skin but where they are not typically found in murine skin. Expression of membrane-bound SCF alone resulted in epidermal melanocytosis and melanin production, but did not by itself cause mastocytosis. We conclude, first, that a phenotype matching that of human mastocytosis can be produced in mice by keratinocyte overproduction of soluble SCF, suggesting a potential cause of this disease. Second, we conclude that keratinocyte expression of membrane-bound SCF results in the postnatal maintenance of epidermal melanocytes in mice. Since the resulting animals have skin that more closely approximates human skin than do normal mice, their study may be more relevant to human melanocyte biology than the study of skin of normal mice.

The SCF/KIT pathway plays a critical role in the control of normal human melanocyte homeostasis

The SCF/KIT pathway plays a critical role in the control of normal human melanocyte homeostasis