Regulation of growth and phenotype of normal human melanocytes in culture

Regulation of epidermal growth factor receptor expression in human melanocytes

The epidermal growth factor receptor (EGFR) and its ligand, transforming growth factor alpha (TGFalpha), are reportedly involved in autocrine growth of melanoma cells. The signal pathway has also been implicated in early events of transformation, suggesting a function for EGFR in normal cells. This study reports the presence of EGFR in cultured melanocytes and examines some cellular responses to TGFalpha. Western analysis revealed 170 kDa bands in extracts of cultured neonatal human melanocytes, corresponding to the receptor Mr. Protein expression was more pronounced in cells during active growth. EGFR were less evident in cultures populated predominantly by melanized cells, indicating that receptor expression became reduced in differentiating cells. Immunocytochemistry confirmed these observations and also showed that EGFR reactivity was predominantly localized in the cell body but absent from dendrites. Addition of TGFalpha to early cultures induced a rapid increase in phosphotyrosine signal of the 170 kDa protein. Longer treatment (24-48 h) increased the intensity of the EGFR signal, suggesting that receptors had been upregulated. However, inclusion of TGFalpha in cultures did not result in an increase in cell numbers when compared to controls. The observations provide evidence of the existence of a receptor-mediated pathway in melanocytes which has transforming potential in vivo.

Pigmented human skin equivalent--as a model of the mechanisms of control of cell-cell and cell-matrix interactions

The melanin pigment system in human skin is extraordinarly well developed and assures the photoprotection of the skin against harmful solar radiation. Specific cell-cell interactions between one melanocytes and keratinocytes play a fundamental role in the regulation of melanogenesis and melanin pigementation, the two key elements of this system, giving rise to the concept of a structural, functional collaborative 'epidermal melanin unit,' Early experiments strongly suggested that melanocyte growth and differentiation are regulated by paracrine factors from keratinocytes and other skin cells. In addition, co-culture studies with keratinocytes has shown that the extracellular matrix acts as a local environmental signal for dendrite formation and melanogenesis. Attempts to reconstruct pigmented human skin in vitro have made great progress over the last decade. The behavior of cells in these pigmented human skin equivalents closely resembles that in vivo, and the cells can still respond to appropriate extrinsic regulatory stimuli such as ultraviolet radiation. Keratinocytes and fibroblasts have been shown to be active partners in the regulation of melanocyte distribution, viability and other differentiation functions, presumably by direct contact and the effects of various soluble paracrine factors. By reproducing cell-cell and cell-matrix interactions, these culture systems provide a promising experimental model for investigating regulation of the skin pigmentary system and the role of photoprotection against harmful solar radiation.

A simplified method for growth of human microvascular endothelial cells results in decreased senescence and continued responsiveness to cytokines and growth factors

Human dermal microvascular endothelial cells are used to analyze the functions of microvascular endothelium in vitro. However, the low yield and short lifespan of these cells in culture has limited the types of analysis that could be performed. Human microvascular endothelial cells are typically grown in media containing supplements such as dibutyryl cyclic AMP, hydrocortisone, bovine brain extract, and antifungal agents, each of which increase the complexity of experimental design and interpretation of results. In the present study, endothelial cells were transferred after Ulex europeus-I selection into a simplified medium consisting of Endothelial Basal Medium 131, 10% fetal bovine serum, and 2 ng/ml basic fibroblast growth factor and analyzed over 3 mo. The human microvascular endothelial cells expressed the endothelial markers von Willebrand factor, CD31, P-selectin, and E-selectin. In addition, the cells showed increased proliferation in the presence of basic fibroblast growth factor (0.5 ng/ml) or vascular endothelial cell growth factor (10 ng/ml). Tumor necrosis factor-alpha-induced expression of E-selectin was similar in cells at Passages 3, 6, and 12, indicating that the cells maintained responsiveness to cytokines over several weeks. Furthermore, the endothelial cells attained a typical cobblestone morphology with increased cellular density and also formed capillarylike tubes on Matrigel. In summary, the human dermal microvascular endothelial cells display the expected endothelial characteristics when grown for several passages and, therefore, provide a valuable in vitro model for human microvascular endothelium.

Modulation of normal human melanocyte dendricity by growth-promoting agents

Dendrite formation and extension, which comprise a characteristic morphology of human normal melanocytes in the skin, represent one of the functional activities of melanocytes, the ability to transfer melanosomes into neighboring keratinocytes. However, the morphology of the melanocyte in vitro is usually quite different from that observed in vivo. it is probably due to the hyperproliferative condition of the melanocytes in culture. No studies have ever compared the effects of a single factor on both dendricity and proliferation at the same time. Therefore, we have compared the effects of six growth-promoting agents commonly used for melanocyte cultures on dendrite formation and proliferation. The addition of agents that increase the intracellular levels of cyclic adenosine monophosphate (cAMP)--dibutyryl cyclic adenosine monophosphate (db cAMP; 1 mM) or isobutylmethyl xanthine (IBMX; 0.1 mM)--had a strong effect on dendrite formation and a negative effect on proliferation. This was especially true with db cAMP. In the presence of 2% or 5% of heat-inactivated fetal bovine serum (FBS), dendrite formation was significantly increased as was proliferation. The number of dendrites was decreased in the culture with 12-o-tetradecanoylphorbol-13-acetate (TPA), but cell growth was slightly increased. With human recombinant basic fibroblast growth factor (bFGF) (0.5, 1.0 ng/ml) in the presence of bovine pituitary extract (BPE) (60 micrograms/ml), cell growth was increased. With 2 ng/ml of bFGF, however, a strong inhibitory effect on proliferation was observed. However, dendrite formation was constant at all concentrations of bFGF tested (0.5, 1.0 or 2.0 ng/ml) with BPE (30 or 60 micrograms/ml). In this study, we have demonstrated that dendrite formation was suppressed by the reagents that stimulate melanocyte proliferation, and vice versa, with the only exception being heat-inactivated FBS. Both dendrite formation and proliferation were induced by the heat-inactivated FBS. This approach is crucial to the development of an adequate culture system for proliferation and/or dendrite formation of normal human melanocytes. It is necessary to keep these aspects in mind as we further investigate the biology of melanocytes, especially the cell-to-cell interactions between melanocytes and keratinocytes, involved in melanogenesis and melanin pigmentation in vivo. This study also provides practical and important information for a future reconstitutive skin system composed of melanocytes, keratinocytes, and fibroblasts in a single culture medium.

Melanogenesis is coupled to murine anagen: toward new concepts for the role of melanocytes and the regulation of melanogenesis in hair growth

Hair is actively pigmented only when it grows: the melanogenic activity of follicular melanocytes (MC) is strictly coupled to the anagen stage of the hair cycle. In catagen, melanin formation is switched off and is absent throughout telogen. The appearance of pigmentation is preceded, and further accompanied by, a time-frame - restricted, differential pattern of tyrosinase transcription, translation, and enzyme activities during the development of anagen follicles. In this speculative review, we argue that signals required for melanin synthesis and pigment transfer to bulb keratinocytes (KC) are intrinsic to the skin, rather than coming from the serum. First, the proopiomelanocortin (POMC) gene is expressed and translated during anagen, but is below the level of detectability in telogen; POMC is a precursor protein for adrenocorticotropin and melanotropins, which are potent regulators of MC proliferation and differentiation. Second, fibroblasts and KC produce factors that affect MC proliferation and differentiation. We suggest that signals regulating follicular MC activity partially derive from cutaneous cells expressing POMC. Vice versa, MC transfer to surrounding KC pigment granules with potent bioregulatory properties. MC also produce and secrete various signal molecules that can regulate mesenchymal and epithelial cell functions. Anagen-associated melanogenesis and the cyclic production of a pigmented hair shaft result from programmed and tightly coordinated epithelial-mesenchymal-neuroectodermal interactions, in which MC may act not only as pigmentary, but also as hair growth-regulatory cells.

Leukotriene C4 and TGF-alpha are stimulators of human melanocyte migration in vitro

Human vitiligo is a disease of melanocyte destruction that leads to areas of depigmentation in the skin. The major form of treatment for vitiligo is photochemotherapy using psoralens and UVA radiation (PUVA), which induces the slow migration of pigment cells from hair follicles and normal skin into the depigmented areas. Our hypothesis is that immune cytokines and inflammatory mediators released as a result of the photochemotherapy are signals for melanocyte migration. We have developed an in vitro assay that quantitates the movement of individual cultured melanocytes over a 72-h period using time-lapse photography. Using this assay we found that both LTC4 and TGF-alpha were stimulators of melanocyte migration in vitro. The LTC4 effect was greater and lasted for the entire 72-h experimental period, whereas the TGF-alpha effect was significant only during the first 24 h of the experiment.