Nerve growth factor modulates keratinocyte proliferation in murine skin organ culture

Despite the fact that several cell types residing in or travelling through the skin are targets and/or sources of nerve growth factor (NGF), little is known about the role of NGF in skin development, physiology and disease. Employing a previously defined skin organ culture assay for studying the proliferation of murine keratinocytes in their natural tissue environment, we have assessed the effect of murine NGF (7S) on keratinocyte proliferation in intact skin derived from two defined stages of the murine hair cycle. We found that 10-200 ng/ml NGF stimulated epidermal keratinocyte proliferation in organ-cultured C57 BL-6 mouse skin in the telogen phase of the hair cycle. Follicle keratinocyte proliferation was stimulated by 100 ng/ml NGF in telogen skin organ culture, but this concentration of NGF inhibited both epidermal and follicle keratinocyte proliferation in organ culture of anagen skin. The latter inhibitory effect of NGF was abrogated by co-incubation with neutralizing anti-NGF antibodies or with the protein kinase C inhibitor staurosporine. The proliferation-modulatory effects of NGF were associated with the induction of significant mast cell degranulation, and were inhibited by cromoglycate co-administration. This is the first report of a modulatory, hair cycle-dependent effect of NGF on keratinocyte proliferation in situ, which may require the presence of mast cells. Our study supports the notion of auto- and paracrine functions of NGF in murine skin physiology, which can be further assessed in the physiologically relevant mouse model delineated here.

Murine skin as a target for melatonin bioregulation

Specific binding sites for 3H-melatonin were detected in membrane fractions prepared from C57 BL-6 mouse skin, and were localized to the epidermis and the epithelial bulb of the hair follicle by in situ autoradiography. In skin organ culture, melatonin stimulated DNA synthesis by the epidermal keratinocytes at concentration 0.1-10 nM, while at > or = 1 microM it inhibited tyrosinase activity. We concluded that murine skin is a target for melatonin bioregulation.

Detection of proopiomelanocortin-derived antigens in normal and pathologic human skin

We investigated the presence of proopiomelanocortin (POMC) products in sections of skin from normal subjects and patients with neoplastic and non-neoplastic cutaneous disorders. Antibodies specific against adrenocorticotropin, beta-melanotropin, and beta-endorphin were used for detection and characterization of cell types bearing POMC peptides. POMC products were not observed in sections of normal skin from the corporal (non-scalp) areas (six cases), whereas the hair follicles of scalp skin exhibited positive immunostains that were readily apparent (four cases). POMC products were frequently detected in corporal skin affected by diseases (13 of 26 cases), for example, psoriatic keratinocytes, the inflammatory infiltrate in scarring alopecia, nevocytes, the epithelial cell nests of basal cell carcinoma, and melanoma cells. Further tests were performed in keloids, a primary reactive skin disorder, to evaluate whether POMC accumulation represented a disease-related phenomenon or an expression of normal cutaneous reactivity. POMC products were consistently detected (10 of 11 cases) in the keratinocytes and mononuclear cells at keloid lesions. Thus these observations indicate that POMC products may accumulate locally in lesional skin representing, presumably, a novel cutaneous response to injury. The broad spectrum of POMC products detected suggests that these arise from production in situ (expression of the POMC gene itself) by human skin.

Identification and characterization of two isozymic forms of arylamine N-acetyltransferase in Syrian hamster skin

Arylamine N-acetyltransferase (EC 2.3.1.5) activity was examined using skin from Syrian hamster. Two isozymes of arylamine N-acetyltransferase, designated NAT-1 and NAT-2, were detected on anion-exchange high-performance liquid chromatography analysis. Both enzyme activities had indistinguishable molecular masses (30 kDa), but differed significantly in their specificity toward the aromatic amines including serotonin, dopamine, methoxytryptamine, tryptamine, para-phenetidine, para-aminobenzoic acid, and sulphamethazine. Specifically, NAT-2 but not NAT-1 catalyzed acetylation of dopamine to N-acetyldopamine and acetylation of serotonin to form N-acetylserotonin, a direct precursor of melatonin. The two isozymes were also distinguishable based upon their sensitivity toward methotrexate inhibition (50% inhibiting dose for NAT-1 = 380 microM; NAT-2 > 2 mM). The presence of these two activities in the skin raises new questions about the physiologic role of this enzyme in general and in the skin-specific functions in particular.

Melanocytes as "sensory" and regulatory cells in the epidermis

Epidermal melanocytes (MC) are pigment-producing and secretorily active cells of neural crest origin that communicate directly with multiple targets. Here, we propose that normal epidermal MC also are "sensory" and regulatory cells operating in the context of a regulatory network for the maintenance of human epidermal homeostasis. Altered regulatory MC functions may play a role in selected skin diseases, and racial pigmentation may affect cutaneous functions.

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.

On the potential role of proopiomelanocortin in skin physiology and pathology

Pro-opiomelanocortin (POMC) is the precursor of ACTH, alpha-MSH and beta-endorphin, neuropeptides with multiple regulatory functions. Both the pituitary gland and peripheral tissues such as mammalian skin are capable of generating POMC-derived neuropeptides. Mammalian skin is also a target for POMC products; their possible roles in skin physiology and pathology are discussed in this communication.

Patterns of cell death: the significance of apoptosis for dermatology

Development, function, remodelling, and senescence of multicellular organisms depend on the coordinated occurrence of physiological, actively induced cell death in two major patterns: terminal differentiation and programmed cell death (apoptosis). Apoptosis is a highly selective form of "cell suicide" with characteristic morphological and biochemical features: chromatin condensation, formation of apoptotic bodies, and DNA fragmentation by activation of endonucleases. Here, we outline the current understanding of apoptosis and its subtypes, discuss their biological functions, and delineate why apoptosis is relevant to the skin and its diseases. We distinguish apoptosis from necrosis, and discuss the regulation of apoptosis by selected genes, hormones, growth factors and cytokines. The epidermis and the regressing hair follicle offer interesting models for studying the as yet ill-understood biology of epithelial cell apoptosis. The selective manipulation of cell death programs may become part of the therapeutic arsenal of clinical dermatology.

Glucocorticoid effect on hair growth initiation: a reconsideration

It has been demonstrated by various workers in the past that glucocorticosteroids block hair growth. Using the mouse model for studying hair growth induction we reexamined the effect of topically applied steroids on hair growth to establish at what stage the steroid block acts. In accord with studies by others, we found that these steroids block hair growth at the point of anagen initiation, but that once the steroid applications are stopped, hair growth starts. Since steroid withdrawal alone did not induce hair growth, it is clear that these steroids do not block, either spontaneous or manipulated, hair growth induction, but they do block, the apparent next step, i.e., hair formation. Moreover, since hair growth could be induced even while the animals were being treated with the steroid, the induction step appears independent of the steroid block. These studies and those of others lead us to conclude that these steroids block the expression of hair-forming genes, but do not interfere with the signal(s) that initiates those genes. This system appears to be ideal for identifying the signals (perhaps, genes) responsible for initiating hair growth.

Is alopecia areata an autoimmune-response against melanogenesis-related proteins, exposed by abnormal MHC class I expression in the anagen hair bulb?

The etiology of alopecia areata (AA), a putative autoimmune disease characterized by sudden hair loss, has remained obscure. It is not understood, how the characteristic inflammatory infiltrate that selectively attacks anagen hair follicles in AA is generated. We hypothesize that this reflects an unexplored form of autoimmunity, a cytotoxic T cell attack on rhythmically synthesized autoantigens normally sequestered by a lack or very low level of MHC class I (MHC I)-expression, and suggest the following mechanism of AA pathogenesis: Microtrauma, neurogenic inflammation, or microbial antigens cause a localized breakdown of MHC I-"negativity" in the proximal anagen hair bulb via proinflammatory cytokines. This exposes autoantigens derived from melanogenesis-related proteins (MRP-DP), which are only generated during anagen, and triggers two successive waves of autoimmune responses: CD8+ cytotoxic T cells initiate AA after recognizing MRP-DP abnormally presented by MHC I molecules on hair matrix melanocytes and/or keratinocytes; a secondary attack, carried by CD4+ T cells and antigen presenting cells, is then mounted against MHC class II--presented additional autoantigens exposed by damaged melanocytes and keratinocytes. The latter causes most of the follicular damage, and extrafollicular disease, and depends greatly on the immunogenetic background of affected individuals. This unifying hypothesis explains the clinical heterogeneity and all salient features of AA, and argues that only the unlikely coincidence of multiple predisposing events triggers AA. The suppression of MHC I--expression and synthesis of MRP in the hair bulb, and the "tolerization" of MRP-DP autoreactive CD8+ T cells may be promising strategies for treating AA.

Hair shaft elongation, follicle growth, and spontaneous regression in long-term, gelatin sponge-supported histoculture of human scalp skin

In order to better understand the molecular mechanisms of human hair growth control and to test hair growth-modulatory drugs, appropriate in vitro models are required. Here, we report the long-term growth, shaft elongation, and spontaneous regression of human hair follicles in histoculture of intact scalp skin. Human scalp skin with abundant hair follicles in various stages of the hair growth cycle was grown for up to 40 days in a gelatin sponge-supported histoculture system at the air/liquid interface. Isolated follicles placed in the gelatin-sponge matrix also supported hair shaft elongation, with the hair follicle cells remaining proliferative and viable for very long periods. Hair shaft elongation occurred mainly during the first 10 days of histoculture of both intact skin and isolated follicles. However, hair follicles were viable and follicle keratinocytes continued to incorporate [3H]thymidine for up to several weeks after shaft elongation had ceased as shown by fluorescent-dye double staining, measured by confocal laser scanning microscopy, and by histological autoradiography of [3H]thymidine incorporation, respectively. Hair follicles could continue their cycle in histoculture; for example, apparent spontaneous catagen induction was observed both histologically and by the actual regression of the hair follicle. In addition, vellus follicles were shown to be viable at day 40 after initiation of culture. In the histocultured human scalp we demonstrated the association of mast cells with anagen follicles and macrophages with catagen follicles, suggesting a role of these cells in the hair cycle. This histoculture technique should serve as a powerful tool for future hair research in the human system as well as a screening assay for compounds that can perturb the hair cycle.

[New perspectives in hair research: in search of the "biological clock" of the hair cycle]

The hair follicle is not only a clinically and commercially important tissue, but can also serve as a fascinating model for studying epithelial-mesenchymal-neuroectodermal interactions. Recently, this has led to a renaissance of hair research, which this speculative review attempts to outline. The unanswered key questions in hair biology are defined before we focus on the crucial search for the "biological clock" that governs the hair cycle. Specifically, we delineate the clinical importance of understanding the mechanisms of catagen induction and propose an immunological mechanism of catagen induction. New research trends are discussed against the background of their potential significance for improving the still frustrating management of hair diseases.

Proopiomelanocortin expression in the skin during induced hair growth in mice

We demonstrate for the first time a hair cycle-dependent gene and protein expression of proopiomelanocortin in mouse skin in vivo. Northern blot detected POMC mRNA with an apparent size of 0.9 kb in anagen but not telogen skin. Western blot emphasized a specific protein of 30-33 kDa recognized by anti beta-endorphin in late but not early anagen or telogen skin. By immunocytochemistry, beta-endorphin antigen was localized in the sebaceous gland in a hair cycle dependent manner.

Melanotropic activity of gamma MSH peptides in melanoma cells

We studied the pigmentary activity of the peptides gamma 1, gamma 2 and gamma 3 melanocyte stimulating hormone (MSH), which differ in the structure of their C-termini, using hamster and mouse melanoma cell lines responsive to beta-MSH by increasing tyrosinase activity. Gamma 1-MSH alone or in combination with beta-MSH had no effect on either cell line. Gamma 2-MSH alone was biologically inactive but potentiated beta-MSH stimulation of tyrosinase activity. Gamma 3-MSH at high concentration (10 microM) induced tyrosinase activity and dendrite formation in the hamster melanoma line. When added together with beta-MSH, gamma 3-MSH partially inhibited the tyrosinase activity response to beta-MSH. Thus, gamma-MSH peptides have low intrinsic melanotropic activity in mammalian melanoma cells; the specific pigmentary responses appear to be affected by the structure of the C-terminal portion.

Does prolactin play a role in skin biology and pathology?

Despite its extensive repertoire of biological activities, which include the growth- and osmoregulation of epithelial tissues as well as immunoregulatory properties, the potential significance of the pituitary hormone prolactin (PRL) for human skin biology and pathology has yet to be fully appreciated. In this essay, the hypothesis is presented that PRL acts as a neuroendocrine modulator of skin epithelial cell proliferation and of the skin immune system by forming a 'prolactin-circuit' between the central nervous system and the skin. Binding to specific skin receptors, modulation of cytokine release in the skin, and stimulation of somatomedin release by mesenchymal cells are among the suggested pathways by which PRL could affect epithelial cell growth in the skin. Potential feedback signals, arising from the skin and modifying pituitary PRL release, are briefly outlined. Centering on the role of PRL in both psoriasis and hair growth as models for studying the proposed PRL-skin connection, clinical and experimental evidence in support of this theory is discussed in the context of a 'neuroimmune-dermatological' perspective.

Hair growth inhibition by heparin in mice: a model system for studying the modulation of epithelial cell growth by glycosaminoglycans?

Although it is known that glycosaminoglycans (GAG) can affect hair growth, their role in follicular growth regulation is not yet understood. We have administered one such GAG, heparin, to anagen-induced mice to help to elucidate this role, using the C57 Bl-6 model for murine hair growth studies. Heparin was found to exert dose- and hair cycle-dependent, differential effects on skin epithelial cell functions. Intraperitoneal, but not topical application of heparin inhibited the development of anagen follicles in anagen-induced mice as assessed by morphometry. When the skin of heparin-treated mice was cultured in an organ culture assay, the epidermis showed a significant increase in the synthesis of arginine-rich proteins (ARP), while epithelial bulb, but not epidermal cell proliferation was reduced in comparison with control skin. In mouse-skin organ culture, the effects of direct administration of heparin to the medium on epithelial cell proliferation and ARP synthesis were dose-dependent and varied (inhibition or stimulation of either parameter), depending on the stage of the hair cycle, the cell population in question (epidermal vs. epithelial bulb keratinocytes), and the length of incubation. PAM cell and mouse dermal papilla cell proliferation in vitro was inhibited by heparin. We conclude that heparin may be a useful tool for characterizing the role of GAG in epithelial cell biology and epithelial-mesenchymal interactions in general, and in hair growth in particular.

Differential expression and activity of melanogenesis-related proteins during induced hair growth in mice

In C57 Bl-6 mice, melanogenesis is strictly coupled to the growth phase of the hair cycle (anagen). To further study this phenomenon of concerted developmental and pigmentary activity, we followed the sequence of tyrosinase (key enzyme of melanogenesis) expression and activity and the presence of the melanosomal protein gp 75 during the development of traumatically induced anagen follicles (days 0 = telogen, and days 1-12, after anagen induction studied). In addition to performing Northern and Western blots for tyrosinase, tyrosine hydroxylase activity (THA) and dopa oxidase activity (DOA) were measured. On day 0, DOA was undetectable, and THA was very low. On days 1 and 2, both activities were undetectable; starting from day 3, they increased rapidly, reaching a plateau on days 8 and 12. DO-positive proteins had apparent molecular weights (MW) of 66-68 kD (days 3-12), 72-74 kD (days 5-12), and 130 kD (days 8 and 12). Western blotting emphasized proteins of MW 66-68 kD (tyrosinase), and 73-75 kD (gp 75); tyrosinase was undetectable on day 0, but already present on days 1 and 2; it increased by day 5 and had reached a plateau on days 8 and 12; gp 75 was undetectable on days 0-2; it was present on day 3, increased by day 5, and reached a plateau on days 8 and 12. Northern blot analysis revealed high levels of tyrosinase mRNA on days 5 and 8, low levels on days 1-3, and none on day 0. These data suggest a highly regulated, time frame-restricted, differential pattern of tyrosinase transcription, translation, and enzyme activity during the different stages of the developing murine anagen follicle, possibly as a result of complex interactions between follicular melanocytes and their environment.

The epidermal pentapeptide pyroGlu-Glu-Asp-Ser-GlyOH inhibits murine hair growth in vivo and in vitro

Tissue growth may be controlled by negative feedback mechanisms. Recently, a pentapeptide, pyroGlu-Glu-Asp-Ser-GlyOH ('epidermal pentapeptide', EPP), which slows the growth of mouse epidermis in vivo and of mouse keratinocytes in vitro, was isolated from mouse epidermis. Since inhibitory molecules like EPP might be part of the feedback systems underlying hair growth control, we assessed the effect of synthesized EPP on the growth of hair follicles, using rodent in vivo and in vitro assays. We report for the first time that intraperitoneally injected EPP (30 nmol/animal/day over 6 days) significantly slows the growth of hair follicles in plucking-induced anagen skin of C57 B1-6 mice (as assessed by microscopic morphometry). Using an in vitro organ culture assay, EPP inhibits the incorporation of 3H-thymidine into mouse pelage anagen follicles. That this epidermal-derived peptide affects hair growth raises the possibility that hair growth may be regulated by an inhibition/disinhibition mechanism under participation of EPP-like molecules and that the epidermis may play a role in the control of hair growth.