Proopiomelanocortin gene product regulation in keratinocytes

Central role of p53 in the suntan response and pathologic hyperpigmentation

UV-induced pigmentation (suntanning) requires induction of alpha-melanocyte-stimulating hormone (alpha-MSH) secretion by keratinocytes. alpha-MSH and other bioactive peptides are cleavage products of pro-opiomelanocortin (POMC). Here we provide biochemical and genetic evidence demonstrating that UV induction of POMC/MSH in skin is directly controlled by p53. Whereas p53 potently stimulates the POMC promoter in response to UV, the absence of p53, as in knockout mice, is associated with absence of the UV-tanning response. The same pathway produces beta-endorphin, another POMC derivative, which potentially contributes to sun-seeking behaviors. Furthermore, several instances of UV-independent pathologic pigmentation are shown to involve p53 "mimicking" the tanning response. p53 thus functions as a sensor/effector for UV pigmentation, which is a nearly constant environmental exposure. Moreover, this pathway is activated in numerous conditions of pathologic pigmentation and thus mimics the tanning response.

Proopiomelanocortin (POMC), the ACTH/melanocortin precursor, is secreted by human epidermal keratinocytes and melanocytes and stimulates melanogenesis

Proopiomelanocortin (POMC) can be processed to ACTH and melanocortin peptides. However, processing is incomplete in some tissues, leading to POMC precursor release from cells. This study examined POMC processing in human skin and the effect of POMC on the melanocortin-1 receptor (MC-1R) and melanocyte regulation. POMC was secreted by both human epidermal keratinocytes (from 5 healthy donors) and matched epidermal melanocytes in culture. Much lower levels of alpha-MSH were secreted and only by the keratinocytes. Neither cell type released ACTH. Cell extracts contained significantly more ACTH than POMC, and alpha-MSH was detected only in keratinocytes. Nevertheless, the POMC processing components, prohormone convertases 1, 2 and regulatory protein 7B2, were detected in melanocytes and keratinocytes. In contrast, hair follicle melanocytes secreted both POMC and alpha-MSH, and this was enhanced in response to corticotrophin-releasing hormone (CRH) acting primarily through the CRH receptor 1. In cells stably transfected with the MC-1R, POMC stimulated cAMP, albeit with a lower potency than ACTH, alpha-MSH, and beta-MSH. POMC also increased melanogenesis and dendricity in human pigment cells. This release of POMC from skin cells and its functional activity at the MC-1R highlight the importance of POMC processing as a key regulatory event in the skin.

Topical drug rescue strategy and skin protection based on the role of Mc1r in UV-induced tanning

Ultraviolet-light (UV)-induced tanning is defective in numerous 'fair-skinned' individuals, many of whom contain functional disruption of the melanocortin 1 receptor (MC1R). Although this suggested a critical role for the MC1R ligand melanocyte stimulating hormone (MSH) in this response, a genetically controlled system has been lacking in which to determine the precise role of MSH-MC1R. Here we show that ultraviolet light potently induces expression of MSH in keratinocytes, but fails to stimulate pigmentation in the absence of functional MC1R in red/blonde-haired Mc1r(e/e) mice. However, pigmentation could be rescued by topical application of the cyclic AMP agonist forskolin, without the need for ultraviolet light, demonstrating that the pigmentation machinery is available despite the absence of functional MC1R. This chemically induced pigmentation was protective against ultraviolet-light-induced cutaneous DNA damage and tumorigenesis when tested in the cancer-prone, xeroderma-pigmentosum-complementation-group-C-deficient genetic background. These data emphasize the essential role of intercellular MSH signalling in the tanning response, and suggest a clinical strategy for topical small-molecule manipulation of pigmentation.

Do adolescent indoor tanners exhibit dependency?

BACKGROUND

Indoor tanning is a common adolescent risk behavior that has been hypothesized to be motivated and maintained by the mood-altering effects of ultraviolet light.

OBJECTIVE

Our purpose was to explore heretofore anecdotal reports that indoor tanning leads to dependency.

METHODS

A telephone interview was conducted among 1275 adolescents, ages 14 to 17 years. Self-reported difficulty in quitting indoor tanning was assessed among 267 adolescents (20.9% of total) who tanned indoors more than once in the previous year in relation to age of initiation, frequency of use, and positive or negative consequences of the practice.

RESULTS

Difficulty in quitting was more likely with younger age at initiation (age 13 years or younger vs ages 16 to 17; odds ratio = 4.3, 95% confidence interval 1.3-14.7) and higher frequency of use (P = .009), even after accounting for positive or negative consequences of indoor tanning and other demographic characteristics.

LIMITATIONS

This was a cross-sectional study design with a limited outcome measure.

CONCLUSION

Although preliminary, our findings for age at initiation and frequency of use in relation to difficulty in quitting indoor tanning are consistent with other potentially addictive behaviors taken up during adolescence.

Hair melanocytes as neuro-endocrine sensors--pigments for our imagination

We are currently experiencing a spectacular surge in our knowledge of skin function both at the organ and organismal levels, much of this due to a flurry of cutaneous neuroendocrinologic data, that positions the skin as a major sensor of the periphery. As our body's largest organ, the skin incorporates all major support systems including blood, muscle and innervation as well as its role in immuno-competence, psycho-emotion, ultraviolet radiation sensing, endocrine function, etc. It is integral for maintenance of mammalian homeostasis and utilizes locally-produced melanocortins to neutralize noxious stimuli. In particular, the cutaneous pigmentary system is an important stress response element of the skin's sensing apparatus; where stimuli involving corticotrophin-releasing hormone (CRH) and proopiomelanocortin (POMC) peptides help regulate pigmentation in the hair follicle and the epidermis. These pigmentary units are organized into symmetrical functional pigmentary units composed of corticotropin-releasing hormone, and the melanocortin POMC peptides melanocyte stimulating hormone, adrenocorticotropic hormone and also the opiate beta-endorphin. These new findings have led to the concept of "self-similarity" of melanocortin systems based on their expression both at the local (skin) and systemic (CNS) levels, where the only major apparent difference appears to be one of scale. This review explores this concept and describes how the components of the CRH/POMC systems may help regulate the human hair follicle pigmentary unit.

Ultraviolet exposure is a reinforcing stimulus in frequent indoor tanners

BACKGROUND

Investigations into tanners' reasons for tanning have focused primarily on the perception of improved appearance. Reported relaxing effects of tanning suggest the possibility of a physiologic effect of UV that drives tanning behavior.

OBJECTIVE

We sought to determine if there is a physiologic reinforcing effect of UV exposure, separate from appearance motivation, that may contribute to tanning behavior.

METHODS

We determined the reinforcing effect of UV light in a series of controlled, blinded, repeated-choice trials of UV carefully designed to separate as cleanly as possible reinforcing effects of UV exposure from other factors including perceived benefits of having a tan. A total of 14 young adults who used tanning beds regularly were exposed to otherwise identical UV and non-UV tanning bed stimuli on Mondays and Wednesdays for 6 weeks. On Fridays, participants had concurrent access to the two beds. The primary dependent variable was the percentage of choice sessions during which more UV than non-UV tanning was chosen.

RESULTS

In all, 12 participants chose additional tanning exposure on Fridays and, of these, 11 consistently used the UV bed for that exposure. Of the total 41 occasions when participants chose to tan on Friday, 39 sessions (95%) were for the UV bed and only two for the non-UV bed. A more relaxed and less tense mood was reported after UV exposure compared with after non-UV exposure (P=.008 and P=.002, respectively).

DISCUSSION

When exposed to UV and non-UV under blinded conditions, frequent tanners can distinguish the two conditions and undertake further UV exposure, indicating that UV is a reinforcing stimulus. The relaxing and reinforcing effects of UV exposure contribute to tanning behavior in frequent tanners and should be explored in greater detail.

CB2 cannabinoid receptor activation produces antinociception by stimulating peripheral release of endogenous opioids

CB(2) cannabinoid receptor-selective agonists are promising candidates for the treatment of pain. CB(2) receptor activation inhibits acute, inflammatory, and neuropathic pain responses but does not cause central nervous system (CNS) effects, consistent with the lack of CB(2) receptors in the normal CNS. To date, there has been virtually no information regarding the mechanism of CB(2) receptor-mediated inhibition of pain responses. Here, we test the hypothesis that CB(2) receptor activation stimulates release from keratinocytes of the endogenous opioid beta-endorphin, which then acts at opioid receptors on primary afferent neurons to inhibit nociception. The antinociceptive effects of the CB(2) receptor-selective agonist AM1241 were prevented in rats when naloxone or antiserum to beta-endorphin was injected in the hindpaw where the noxious thermal stimulus was applied, suggesting that beta-endorphin is necessary for CB(2) receptor-mediated antinociception. Further, AM1241 did not inhibit nociception in mu-opioid receptor-deficient mice. Hindpaw injection of beta-endorphin was sufficient to produce antinociception. AM1241 stimulated beta-endorphin release from rat skin tissue and from cultured human keratinocytes. This stimulation was prevented by AM630, a CB(2) cannabinoid receptor-selective antagonist and was not observed in skin from CB(2) cannabinoid receptor-deficient mice. These data suggest that CB(2) receptor activation stimulates release from keratinocytes of beta-endorphin, which acts at local neuronal mu-opioid receptors to inhibit nociception. Supporting this possibility, CB(2) immunolabeling was detected on beta-endorphin-containing keratinocytes in stratum granulosum throughout the epidermis of the hindpaw. This mechanism allows for the local release of beta-endorphin, where CB(2) receptors are present, leading to anatomical specificity of opioid effects.

A fully functional proopiomelanocortin/melanocortin-1 receptor system regulates the differentiation of human scalp hair follicle melanocytes

The proopiomelanocortin (POMC)-derived peptides, ACTH and alpha-MSH, are the principal mediators of human skin pigmentation via their action at the melanocortin-1 receptor (MC-1R). Recent data have demonstrated the existence of a functionally active beta-endorphin/mu-opiate receptor system in both epidermal and hair follicle melanocytes, whereby beta-endorphin can regulate melanogenesis, dendricity, and proliferation in these cells. However, a role for ACTH and alpha-MSH in the regulation of the human follicular pigmentary unit has not been determined. This study was designed to examine the involvement of ACTH and the alpha-MSH/MC-1R system in human follicular melanocyte biology. To address this question we employed RT-PCR and immunohisto/cytochemistry, and a functional role for these POMC peptides was assessed in follicular melanocyte cultures. Human scalp hair follicle melanocytes synthesized and processed POMC. ACTH and alpha-MSH in association with their processing enzymes and MC-1R are expressed in human follicular melanocytes at the message level in vitro and at the protein level both in situ and in vitro. The expression of the POMC/MC-1R receptor system was confined only to subpopulations of poorly and moderately differentiated melanocytes. In addition, functional studies revealed that ACTH and alpha-MSH are able to promote follicular melanocyte differentiation by up-regulating melanogenesis, dendricity, and proliferation in less differentiated melanocyte subpopulations. Thus, these findings suggest a role for these POMC peptides in regulating human hair follicle melanocyte differentiation.

Biphasic expression of two paracrine melanogenic cytokines, stem cell factor and endothelin-1, in ultraviolet B-induced human melanogenesis

Stem cell factor (SCF) and endothelin-1 (ET-1) have been reported to be up-regulated at the protein and gene levels in human epidermis after ultraviolet B (UVB) irradiation and to play central roles in UVB-induced pigmentation. However, little is known about the time sequence of SCF and ET-1 expression in UVB-exposed human epidermis and the coordination of their roles during epidermal pigmentation. To clarify such parameters in UVB-exposed human skin, we measured the expression patterns of SCF and ET-1 (as well as of their corresponding receptors) at the gene level at various times during UVB-induced human pigmentation. When human forearm skin was exposed to UVB radiation at two minimal erythemal doses, the expression of SCF mRNA transcripts was significantly enhanced at 3 days after irradiation with an early decrease and subsequently constant expression of SCF receptor (c-KIT) mRNA transcripts. In contrast, up-regulation of ET-1 and endothelin B receptor (ET(B)R) mRNA expression was synchronized at 5 to 10 days after irradiation in concert with an increased expression of tyrosinase mRNA transcripts and the increase in pigmentation. In parallel the expression of tyrosinase and ET(B)R proteins as well as ET-1 was up-regulated at 7 to 10 days after irradiation, whereas KIT protein decreased at 3 days after irradiation and returned to the nonirradiated control level at 5 days after irradiation. When cultured human melanocytes were treated with human recombinant SCF, ET(B)R protein expression and the binding of (125)I-labeled ET-1 to the ET(B)R were significantly increased, further suggesting the preferential and coordinated role of early expression of SCF in UVB-induced melanogenesis. These findings suggest that SCF/KIT signaling is predominantly involved in the early phase of UVB-induced human pigmentation during which it stimulates the ET-1/ET(B)R linkage that is associated with the later phase of UVB-induced melanogenesis.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

Solar-simulated irradiation evokes a persistent and biphasic IL-1alpha response

Exposure of skin to solar-simulated irradiation generates a multitude of adaptive responses including cytokine transcription, synthesis and secretion. Interleukin-1 (IL-1) is one of the cytokines induced in epidermal cells in response to UV irradiation. It displays a broad range of mitogenic and inflammatory activities including fibroblast proliferation and T-cell activation. There are two forms, IL-1alpha and IL-1beta; and IL-1alpha is the predominant form secreted by epidermal keratinocytes. UV-induced modulations of IL-1alpha message levels have been extensively studied within the first 48 h after irradiation, but longer term changes and impact on IL-1alpha cellular protein levels are virtually unexplored. We now report that cells of keratinocyte origin (SCC 12F) respond to a single physiologic dose of solar-simulated irradiation with both early (8 h) and late (72 h) peaks of IL-1alpha mRNA induction. UV-stimulated IL-1alpha secretion is increased above sham-irradiated control secretion for at least 96 h after irradiation. Our study provides evidence that UV-induced adaptive cutaneous responses persist for at least several days, and suggests that different mechanisms may mediate the early vs. late inductions.

Endothelin-B receptor activation triggers an endogenous analgesic cascade at sites of peripheral injury

Endothelin-1 (ET-1) is a newly described pain mediator that is involved in the pathogenesis of pain states ranging from trauma to cancer. ET-1 is synthesized by keratinocytes in normal skin and is locally released after cutaneous injury. While it is able to trigger pain through its actions on endothelin-A (ET(A)) receptors of local nociceptors, it can coincidentally produce analgesia through endothelin-B (ET(B)) receptors. Here we map a new endogenous analgesic circuit, in which ET(B) receptor activation induces the release of beta-endorphin from keratinocytes and the activation of G-protein-coupled inwardly rectifying potassium channels (GIRKs, also named Kir-3) linked to opioid receptors on nociceptors. These results indicate the existence of an intrinsic feedback mechanism to control peripheral pain in skin, and establish keratinocytes as an ET(B) receptor-operated opioid pool.

Regulation of human epidermal melanocyte biology by beta-endorphin

beta-Endorphin is an opioid peptide cleaved from the precursor pro-hormone pro-opiomelanocortin, from which other peptides such as adrenocorticotropic hormone, beta-lipotropic hormone, and alpha-melanocyte-stimulating hormone are also derived. alpha-Melanocyte-stimulating hormone and adrenocorticotropic hormone are well documented to regulate human skin pigmentation via action at the melanocortin-1 receptor. Whereas plasma beta-endorphin is reported to increase after exposure to ultraviolet radiation, to date a functional role for beta-endorphin in the regulation of human epidermal melanocyte biology has not been demonstrated. This study was designed to examine the involvement of the beta-endorphin/mu-opiate receptor system in human epidermal melanocytes. To address this question we employed reverse transcription-polymerase chain reaction, and immunohistochemistry/cytochemistry and immunoelectron microscopy using beta-endorphin and mu-opiate receptor specific antibodies. A functional role for beta-endorphin was assessed in epidermal melanocyte cultures by direct stimulation with the peptide. This study demonstrated the expression of mu-opiate receptor mRNA in cultured epidermal melanocytes, as well as mRNA for pro-opiomelanocortin. In addition, we have shown that beta-endorphin and mu-opiate receptor are expressed at the protein level in situ in glycoprotein100-positive melanocytes. The expression of both beta-endorphin and mu-opiate receptor correlated positively with their differentiation status in vitro. Furthermore, immunoelectron microscopy studies revealed an association of beta-endorphin with melanosomes. Functional studies showed that beta-endorphin has potent melanogenic, mitogenic, and dendritogenic effects in cultured epidermal melanocytes deprived of any exogenous supply of pro-opiomelanocortin peptides. Thus, we report that human epidermal melanocytes express a fully functioning beta-endorphin/mu-opiate receptor system. In the absence of any data showing cross-talk between the mu-opiate receptor and the melanocortin-1 receptor, we conclude that the beta-endorphin/mu-opiate receptor system participates in the regulation of skin pigmentation.

Downregulation of NF-kappaB activation in human keratinocytes by melanogenic inhibitors

BACKGROUND

Exposure of skin cells, particularly keratinocytes to various nuclear factor-kappaB (NF-kappaB) activators (e.g. tumor necrosis factor-alpha, interleukin-1, lipopolysaccharides, and ultraviolet light) leads to phosphorylation and degradation of the inhibitory protein, IkappaB. Liberated NF-kappaB is translocated into the nucleus where it can change or alter expression of target genes, resulting in the secretion of extracellular signaling molecules including melanotrophic factors affecting melanocyte.

OBJECTIVE

In order to demonstrate the possible role of NF-kappaB activation on the synthesis of melanotrophic factors from the keratinocytes, the activities of NF-kappaB induced by melanogenic inhibitors (MIs) were determined in human HaCaT keratinocytes transfected with pNF-kappaB-SEAP-NPT plasmid. Transfectant cells released the secretory alkaline phosphatase (SEAP) as a transcription reporter in response to the NF-kappaB activity and contain the neomycin phosphotransferase (NPT) gene for the dominant selection marker for geneticin resistance.

METHODS

MIs such as niacinamide, kojic acid, hydroquinone, resorcinol, arbutin, and glycolic acid were preincubated with transfectant HaCaT cells for 3 h and then ultraviolet B (UVB) was irradiated. NF-kappaB activation was measured with the SEAP reporter gene assay using a fluorescence detection method.

RESULTS

Of the MIs tested, kojic acid (IC(50)=60 microM) was found to be the most potent inhibitor of UVB-upregulating NF-kappaB activation in transfectant HaCaT cells, which is followed by niacinamide (IC(50)=540 microM). Pretreatment of the transfectant HaCaT cells with the MIs, especially kojic acid and niacinamide, effectively lowered NF-kappaB binding measured by electrophoretic mobility shift assay. Furthermore, these two inhibitors remarkably reduced the secretion level of IL-6, one of melanotrophic factors, triggered by UV-radiation of the HaCaT cells.

CONCLUSION

These observations suggest that MIs working at the in vivo level might act partially through the modulation of the synthesis of melanotrophic factors in keratinocyte.

Inhibition of the elicitation phase of contact hypersensitivity by thymidine dinucleotides is in part mediated by increased expression of interleukin-10 in human keratinocytes

The production of immunomodulatory cytokines such as interleukin-10 (IL-10) from keratinocytes and other target cells in the skin plays a crucial role in UV-induced immunosuppression. Substantial evidence supports an association between DNA damage and immunomodulation. It is also known that small DNA fragments such as thymidine dinucleotides (pTpT) can mimic several UV-induced effects, including inhibition of the induction phase of the contact hypersensitivity response and up-regulation of tumor necrosis factor-alpha (TNF-alpha). To determine whether pTpT also induces IL-10 secretion by keratinocytes, and by inference whether IL-10 production after UV irradiation is a response to DNA damage, we compared the effects of pTpT with those of UV irradiation on primary human keratinocyte cultures. Subconfluent cultures of primary human keratinocytes were treated either with 10 micro M or 100 micro M pTpT or diluent alone, or exposed to solar-simulated light (100 J/m2 of UVB) or sham irradiated. An increase in IL-10 mRNA expression was observed 6-24 h after irradiation and at 24-48 h after treatment with pTpT. Detection of secreted IL-10 protein coincided with up-regulation of IL-10 gene expression at 48 and 72 h as determined by ELISA. Conditioned media from human keratinocytes treated with pTpT, like that from irradiated cells, significantly inhibited lymphocyte proliferation in the allogeneic-mixed lymphocyte reaction (MLR) assay. To determine whether pTpT mimics the suppressive influence of UVB on the elicitation phase of contact hypersensitivity, believed to result largely from IL-10 release, we compared the effects of topical application of pTpT with those of UVB irradiation on C57Bl/6 mice sensitized with dinitrofluorobenzene. Sensitized mice treated with pTpT or UVB irradiation showed markedly suppressed elicitation of ear-swelling responses. These results demonstrate that increased keratinocyte IL-10 mRNA level and IL-10 protein release are among the effects of pTpT and support the hypothesis that pTpT treatment triggers many of the biologic effects of UV irradiation by mimicking UV-induced DNA damage. Finally, regardless of mechanism, the data suggest that topical treatment with pTpT may provide a novel means of suppressing contact hypersensitivity or other lymphocyte-mediated reactions in skin.

Ethnic variation in tyrosinase and TYRP1 expression in photoexposed and photoprotected human skin

The relative expression of a number of key mediators of human pigmentation including tyrosinase, tyrosinase related protein-1 (TYRP1), endothelin-1 and adrenocorticotrophic hormone (ACTH) proteins were analysed and quantified in immunohistochemically stained skin sections using semiquantitative computer assisted image analysis. Comparisons were made between a range of different ethnic skin types including European, Chinese, Mexican, Indian and African at both chronically photoexposed and photoprotected sites. Melanocyte number varied little with ethnicity except in the European group which had 60-80% more melanocytes than other skin types (P < 0.01, n = 10; Student Neuman-Keuls). However, melanocyte number was increased approximately twofold in chronically photoexposed skin of all ethnic groups (P < 0.001, n = 48; paired t-test). Tyrosinase protein expression in melanocytes did not vary with ethnicity, but TYRP1 protein was significantly elevated (approximately 2.6-fold) in darkly pigmented African and Indian skin types compared with lightly pigmented Mexican, Chinese and European skin types. In melanocytes from chronically photoexposed skin, there was a modest but significant increase in the expression of tyrosinase protein (approximately 1.2-fold, P < 0.001, n = 48; paired t-test), together with a significant and slightly larger increase in the expression of TYRP1 protein (approximately 1.6-fold, P < 0.005, n = 48; paired t-test). In contrast, the expression of endothelin-1 and ACTH showed no significant variation with either ethnicity or photoexposure. These data are consistent with the view that maintenance of a chronically hyperpigmented phenotype in chronically photoexposed human skin is largely the result of a stable increase in the number of tyrosinase positive melanocytes at these sites. Moreover, the observed ethnic variation in TYRP1 protein expression suggests that TYRP1 may play a significant role in mediating ethnic differences in melanogenesis and constitutive skin pigmentation in vivo.

Local injection of a selective endothelin-B receptor agonist inhibits endothelin-1-induced pain-like behavior and excitation of nociceptors in a naloxone-sensitive manner

We showed previously that subcutaneous injection of the injury-associated peptide mediator endothelin-1 (ET-1) into the rat plantar hindpaw produces pain behavior and selective excitation of nociceptors, both through activation of ET(A) receptors likely on nociceptive terminals. The potential role of ET(B) receptor activation in these actions of ET-1-has not been examined. Therefore, in these experiments, we studied the effect of blocking or activating ET(B) receptors on ET-1-induced hindpaw flinching and excitation of nociceptors in rats. An ET(B) receptor-selective antagonist, BQ-788 (3 mm), coinjected with ET-1 (200 microm) reduced the time-to-peak of flinching and significantly enhanced the average maximal flinch frequency (MFF). In contrast, coinjection of an ET(B) receptor selective agonist, IRL-1620 (100 or 200 microm), with ET-1 reduced the average MFF and the average total number of flinches. Interestingly, this unexpected inhibitory effect of IRL-1620 was prevented by the nonselective opioid receptor antagonist naloxone (2.75 mm). To confirm these inhibitory actions, we studied the effects of IRL-1620 on ET-1-induced spike responses in single, physiologically characterized nociceptive C-fibers. IRL-1620 suppressed spike responses to ET-1 in all (n = 12) C-units, with mean and maximum response frequencies of 0.08 +/- 0.02 and 1.5 +/- 0.4 impulses/sec versus 0.32 +/- 0.07 and 4.17 +/- 0.17 impulses/sec for ET-1 alone. In additional support of the behavioral results, coinjection of naloxone (2.75 mm) completely prevented this inhibitory action of IRL-1620. These results establish that ET(B) receptor activation inhibits ET-1-induced pain behavior and nociception in a naloxone-sensitive manner and point to a previously unrecognized dual modulation of acute nociceptive signaling by ET(A) and ET(B) receptors in cutaneous tissues.

Biochemical characterization of endothelin-converting enzyme-1alpha in cultured skin-derived cells and its postulated role in the stimulation of melanogenesis in human epidermis

The vasoconstrictive peptide endothelin-1 (ET-1) is expressed in human epidermis at the gene and protein levels and plays an important role in stimulating pigmentation via its increased secretion by keratinocytes following ultraviolet B (UVB) irradiation. However, one or more biological mechanisms underlying the secretion of ET-1 by keratinocytes in human skin have never been evaluated. In mammalian endothelial cells, a membrane-bound neutral metalloproteinase, termed endothelin-converting enzyme (ECE), catalyzes the specific cleavage of the inactive precursor Big ET to produce mature active ET, which leads in turn to the secretion of ET by those cells. To clarify the potential involvement of ECE in the processing and secretion of ET-1 by human keratinocytes, we synthesized the N-terminal peptide of human ECE-1alpha and generated a rabbit polyclonal antibody (alphaPEPT6) that specifically recognizes ECE-1alpha. Reverse transcription PCR and Western blotting analysis revealed that significant expression of ECE-1 transcripts and ECE-1alpha protein occurs in human keratinocytes. When ECE activity was assayed in extracts of human keratinocytes at pHs ranging from 5.0 to 8.0, the enzymatic profile had an optimal neutral pH of 7.0 and was sharply pH-dependent. Furthermore, when extracts of human keratinocytes were treated with alphaPEPT6, ECE activity was significantly reduced compared with extracts treated with the prebleed serum of alphaPEPT6, which supports the notion that ECE-1alpha is a major metalloproteinase with ECE activity in human keratinocytes. The exogenous addition of the pro-inflammatory cytokine interleukin-1alpha significantly increased expression of ECE-1 transcripts in cultured human keratinocytes, which suggests an association with post-inflammatory hyperpigmentation. Collectively, our results demonstrate for the first time that ECE-1alpha is expressed at significant levels in various types of human skin cells (including keratinocytes) and that it plays a constitutive role in the processing and UVB-inducible secretion of ET-1 by human keratinocytes, which leads to the stimulation of pigmentation in the epidermis.

Increase of pro-opiomelanocortin mRNA prior to tyrosinase, tyrosinase-related protein 1, dopachrome tautomerase, Pmel-17/gp100, and P-protein mRNA in human skin after ultraviolet B irradiation

In ultraviolet-induced tanning, the protein levels of various gene products critical for pigmentation (including tyrosinase and tyrosinase-related protein-1) are increased in response to ultraviolet B irradiation, but changes in mRNA levels of these factors have not been investigated in vivo. We have established an in situ hybridization technique to investigate mRNA levels of pro-opiomelanocortin, tyrosinase, tyrosinase-related protein-1, dopachrome tautomerase, P-protein, Pmel-17/gp100, and microphthalmia-associated transcription factor, and have analyzed the changes in mRNA levels in the ultraviolet B-exposed skin in vivo. The right or left forearm of each volunteer was irradiated with ultraviolet B, and skin biopsies were obtained at 2 and 5 d postirradiation. mRNA level of pro- opiomelanocortin was increased 2 d after ultraviolet B irradiation, and returned to a near-basal level after 5 d, whereas the mRNA levels of tyrosinase, tyrosinase-related protein-1, dopachrome tautomerase, P-protein, and Pmel-17/gp100 showed some or no increase at 2 d, but were significantly increased 5 d after ultraviolet B irradiation. Microphthalmia-associated transcription factor mRNA was slightly increased on days 2 and 5 after ultraviolet B irradiation. Our results suggest that the mechanism of the tanning response of human skin may involve the transcriptional regulation of certain pigmentary genes, and that pro-opiomelanocortin-derived melanocortins such as alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone may play a part in regulating these genes in vivo.

Role of endogenous opioids in the effects of light on mood and behavior

Skin cells produce endogenous opioids in response to light. This results in increased skin and plasma levels of endogenous opioids. Increased plasma levels of endogenous opioids may improve mood and affect behavior. The author suggests that improvement in mood after exposure to light may be related to the light-induced increase in levels of endogenous opioids. The author speculates that genetic factors may affect the response of skin cells to light: different people may have different genetically determined responses to natural or artificial light. The author also suggests that light may affect endogenous opioids via several different mechanisms.

Cell-density-regulated chemotactic responsiveness of keratinocytes in vitro

Keratinocytes represent the main constituents of the epidermis and have been found to play a regulatory role in a variety of inflammatory skin diseases. The functional activity of keratinocytes is highly heterogeneous, and depends on the cell localization in the epidermal architecture, and the maturation or differentiation state of the cells. Spontaneously proliferating HaCaT cells, showing several similarities to basal epidermal keratinocytes, were found to respond to external chemoattractants, including the chemokines RANTES (regulated on activation normal T cell expressed and secreted) and interleukin-8 and the mu-opioid agonist DAMGO ([d-ala2, N-Me-Phe4, Gly-ol5]enkephalin) in migration assays. The chemotactic responsiveness was highly dependent on the cell density of the monolayer, with greatest chemotactic activity at the highest cell density. Whereas RANTES was found to be the most potent chemoattractant, constitutive RANTES production was also detected in the HaCaT cultures. We found an inverse correlation between constitutive RANTES production and chemotactic responsiveness toward external RANTES, suggesting a possible functional down-modulation of the RANTES receptors, CC chemokine receptor 1 and CC chemokine receptor 5, during culture. Results from confocal laser scanning microscopy showed reduced CC chemokine receptor 1, but not CC chemokine receptor 5, expression by HaCaT cells at low cell densities, which was abolished in the presence of neutralizing antibodies against RANTES. The total CC chemokine receptor 1 pool (surface and intracellular receptors), however, showed no significant change during in vitro culture. Chemotactic responsiveness toward RANTES was directly correlated with the level of CC chemokine receptor 1 surface expression. Taken together these results show that with keratinocyte proliferation and the progressive increase in cell density there are dramatic alterations in keratinocyte function.

In situ expression of corticotropin-releasing hormone (CRH) and proopiomelanocortin (POMC) genes in human skin

Systemic stresses induce corticotropin-releasing hormone (CRH) expression in hypothalamus. CRH is released to the pituitary gland, where it stimulates proopiomelanocortin (POMC) production acting via the CRH receptor (CRH-R). CRH and POMC peptides are also detected in sites outside of the central nervous system (CNS), such as the skin. However, it has not been elucidated whether these peptides detected in the skin are derived from CNS or are produced locally. Using immunohistochemical and in situ reverse-transcription (RT)-PCR techniques, we demonstrated coexpression of CRH and POMC mRNAs in the epidermis and pilosebaceous units of the human skin. This coexpression was confirmed by the combination of laser-capture microdissection (LCM) with RT-PCR, analyzing mRNA expressions in captured sebaceous cells. Immunoreactivities and expressions of CRH and POMC mRNAs were strong in inflammatory lesions, melanocytic nevus, seborrheic keratosis, and also in the periphery of the benign tumor. These findings suggest that CRH and POMC peptides are produced locally in the skin and are regulated by inflammatory cells as well as by autocrine mechanisms. The skin may have "a local stress response system," whose activity is mediated by CRH and POMC peptides, in an equivalent to hypothalamus-pituitary adrenal axis.

Presence of immunoreactive beta-endorphin in human skin

The production and its induction by ultraviolet radiation (UVR) of proopiomelanocortin (POMC)-derived peptides by keratinocytes has been reported, albeit not consistently. Recently we demonstrated that only under specific culturing conditions human keratinocytes are capable of producing a beta-endorphin (betaE)-like peptide with the characteristics of beta-lipotropin (betaLPH). Here the presence and UV-induction of betaE-immunoreactivity (betaE-IR) in keratinocytes in human skin in vivo was investigated. betaE-IR was detectable by immunohistochemistry in keratinocytes of the follicular matrix and to some extent in cells of sweat ducts, but was absent from epidermal keratinocytes. Absence of betaE-IR was confirmed by radioimmunoassay of HPLC-fractionated extracts of normal epidermis. Repeated exposure to solar-simulated UVR had no effect. This investigation is the first to demonstrate the presence of betaE-immunoreactive material in the follicular matrix of corporal hairs and in duct cells of sweat glands. The possible meaning of these results is discussed.

The role of alpha-MSH as a modulator of cutaneous inflammation

Among various neuropeptides such as substance P, calcitonin gene-related peptide and others, alpha-melanocyte-stimulating hormone (alpha-MSH) was found to be produced in the skin. Moreover, melanocortin receptor 1 (MC-1R), which is specific for alpha-MSH and ACTH, is expressed in the skin on keratinocytes, dendritic cells, macrophages and endothelial cells. In monocytes, macrophages and dendritic cells alpha-MSH inhibits the production and activity of immunoregulatory and proinflammatory cytokines such as IL-2, IFN-gamma, TNF-alpha and IL-1. It downregulates the expression of costimulatory molecules such as CD86 and CD40 and induces the production of suppressor factors such as the cytokine synthesis inhibitory factor IL-10. On endothelial cells alpha-MSH is capable of downregulating the LPS-induced expression of adhesion molecules such as vascular cell adhesion molecule (VCAM) and E-selectin. Moreover, the LPS-induced activation of transcription factors such as NF kappa B is downregulated by alpha-MSH. In a mouse model i.v. or topical application of alpha-MSH was found to inhibit the induction phase as well as the effector phase of contact hypersensitivity (CHS) reactions and to induce hapten-specific tolerance. These findings indicate that the production of immunosuppressing neuropeptides such as alpha-MSH by epidermal cells may play an essential role during the pathogenesis of immune and inflammatory reactions in the skin.

The paracrine role of stem cell factor/c-kit signaling in the activation of human melanocytes in ultraviolet-B-induced pigmentation

The interaction of stem cell factor with its receptor, c-kit, is well known to be critical to the survival of melanocytes. Little is known about the role(s) of the stem cell factor/c-kit interaction in epidermal pigmentation, however. To clarify whether the stem cell factor/c-kit signaling has a paracrine role in ultraviolet-B-induced pigmentation, we determined whether the exposure of human keratinocytes, melanocytes, and the epidermis to ultraviolet B light stimulates the expression of stem cell factor or c-kit at the gene and/or protein levels. We further examined whether interrupting the binding of stem cell factor to c-kit by subepidermal injection of a monoclonal antibody to c-kit affects ultraviolet-B-induced pigmentation in brownish guinea pig skin. When human keratinocytes and melanocytes in culture were exposed to ultraviolet B light, transcripts of stem cell factor and c-kit (as assessed by reverse transcription polymerase chain reaction) and expression of those proteins (by enzyme-linked immunosorbent assay and western blotting) increased significantly and peaked at a dose of 20-40 mJ per cm2. In ultraviolet-B-exposed human epidermis, stem cell factor transcripts and protein expression were also markedly enhanced compared with the nonexposed epidermis. Immunohistochemistry with antibodies to stem cell factor revealed an increased staining in the ultraviolet-B-exposed epidermis, which was accompanied by a slight epidermal hyperplasia. In the course of ultraviolet-B-induced pigmentation of brownish guinea pig skin, the subepidermal injection of c-kit inhibitory antibodies completely abolished the induction of pigmentation in the ultraviolet-B-exposed area, and there was no increase in the number of dihydroxyphenylalanine-positive melanocytes. These findings indicate that the stem cell factor/c-kit signaling is critically involved in the biologic mechanism of ultraviolet-B-induced pigmentation.

Expression of proopiomelanocortin peptides in human dermal microvascular endothelial cells: evidence for a regulation by ultraviolet light and interleukin-1

Proopiomelanocortin peptides such as alpha-melanocyte-stimulating hormone and adrenocorticotropin are expressed in the epidermal and dermal compartment of the skin after noxious stimuli and are recognized as modulators of immune and inflammatory reactions. Human dermal microvascular endothelial cells mediate leukocyte-endothelial interactions during cutaneous inflammation by the expression of cellular adhesion molecules and cytokines such as interleukin-1. This study addresses the hypothesis that human dermal microvascular endothelial cells express both proopiomelanocortin and prohormone convertases, which are required to generate proopiomelanocortin peptides. Semiquantitative reverse transcriptase polymerase chain reaction and northern blot studies revealed a constitutive expression of proopiomelanocortin mRNA by human dermal microvascular endothelial cells in vitro that was time- and concentration-dependently upregulated by interleukin-1 beta. Furthermore, irradiation of human dermal microvascular endothelial cells with ultraviolet A1 (30J per cm(2)) or ultraviolet B (12.5 mJ per cm(2)) enhanced proopiomelanocortin expression as well as the production and release of the proopiomelanocortin peptides adrenocorticotropin and alpha-melanocyte-stimulating hormone. In addition to proopiomelanocortin, prohormone convertase 1 mRNA expression was detected by reverse transcriptase polymerase chain reaction in unstimulated human dermal microvascular endothelial cells and was augmented after exposure to alpha-melanocyte- stimulating hormone, interleukin-1 beta, or irradiation with ultraviolet. These findings demonstrate that human dermal microvascular endothelial cells express proopiomelanocortin and prohormone convertase 1 required for the generation of adrenocorticotropin. Additionally, human dermal microvascular endothelial cells express mRNA for the prohormone convertase 2 binding protein 7B2. Taken together these findings indicate that human dermal microvascular endothelial cells upon stimulation express both proopiomelanocortin and prohormone convertases required for the generation of alpha-melanocyte-stimulating hormone. As proopiomelanocortin peptides were found to regulate the production of human dermal microvascular endothelial cell cytokines and adhesion molecules and to have a variety of anti-inflammatory properties these peptides may significantly contribute to the modulation of skin inflammation. J Invest Dermatol 115:1021-1028 2000

Neuroendocrinology of the skin

The classical observations of the skin as a target for melanotropins have been complemented by the discovery of their actual production at the local level. In fact, all of the elements controlling the activity of the hypothalamus-pituitary-adrenal axis are expressed in the skin including CRH, urocortin, and POMC, with its products ACTH, alpha-MSH, and beta-endorphin. Demonstration of the corresponding receptors in the same cells suggests para- or autocrine mechanisms of action. These findings, together with the demonstration of cutaneous production of numerous other hormones including vitamin D3, PTH-related protein (PTHrP), catecholamines, and acetylcholine that share regulation by environmental stressors such as UV light, underlie a role for these agents in the skin response to stress. The endocrine mediators with their receptors are organized into dermal and epidermal units that allow precise control of their activity in a field-restricted manner. The skin neuroendocrine system communicates with itself and with the systemic level through humoral and neural pathways to induce vascular, immune, or pigmentary changes, to directly buffer noxious agents or neutralize the elicited local reactions. Therefore, we suggest that the skin neuroendocrine system acts by preserving and maintaining the skin structural and functional integrity and, by inference, systemic homeostasis.

Neprilysin, a novel target for ultraviolet B regulation of melanogenesis via melanocortins

Compelling evidence suggest a role for melanocortins in the regulation of melanogenesis by ultraviolet radiation. Within the epidermis, melanocytes and keratinocytes produce alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone. The persistence and the strength of the biologic signal delivered by these peptides depend on their local concentration, which is controlled by the rate of peptide production and by the rate of its degradation. In this study, we investigated the mechanism of melanocortin degradation by melanocytes and the effect of ultraviolet on this process. We have focused our attention on a neutral endopeptidase, neprilysin, which has been implicated in the ending of numerous peptidergic signals. We have shown that this enzyme is expressed at the surface of human melanocytes. Interestingly, its activity and its expression are dramatically downregulated by ultraviolet B treatment. Moreover, in the presence of phosphoramidon, a stable inhibitor of neprilysin, we observed an increased efficiency of alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone to stimulate both tyrosinase activity and microphthalmia expression. Taken together, these data indicate that neprilysin expressed by melanocytes has a physiologic role in the regulation of melanogenesis by proopiomelanocortin peptide. Further, its downregulation by ultraviolet B irradiation shed light on a new and appealing mechanism of ultraviolet B induced melanogenesis via the control of melanocortins degradation.

Cytoplasmic dynein colocalizes with melanosomes in normal human melanocytes

BACKGROUND

Melanocytic dendrites consist of a central core of microtubules and a subcortical actin network. Several studies provide arguments supporting the hypothesis that actin-based and microtubule-based motor proteins co-operate in melanosome transport towards the dendrite tips. Melanosomes can move bidirectionally along microtubules in vitro, and in murine melanocytes, they move towards the cell periphery and back again. Microtubules have a fast-growing plus end and a slow-growing minus end. Microtubule-associated motor proteins move unidirectionally either towards the plus or towards the minus end. However, it is not known which motor protein is responsible for minus end-directed movement of melanosomes.

OBJECTIVES

We aimed to investigate the in vitro expression of the minus end-directed motor protein cytoplasmic dynein in normal human epidermal melanocytes, keratinocytes and dermal fibroblasts.

METHODS

Reverse transcription-polymerase chain reaction and Northern blot analysis were used. In addition, an attempt to obtain insight into the subcellular localization of cytoplasmic dynein, immunofluorescence studies and immunogold electron microscopic studies were performed.

RESULTS

The three different forms of cytoplasmic dynein heavy chain were expressed in all studied skin cells. Immunofluorescence staining showed similar punctate distributions for dynein heavy chain 1 and dynein heavy chain 2 in melanocytes, with accentuation in the perinuclear area and dendrite tips. Double labelling with a melanosome marker showed apparent co-localization of both dynein heavy chains 1 and 2 with melanosomes in the perinuclear area and dendrite tips. For the dynein intermediate chain of 74 kDa, again a punctate staining pattern was seen with intense centrosomal staining. A close association of dynein intermediate chain 74 and alpha-tubulin with the melanosome surface was detected using immunogold electron microscopy.

CONCLUSIONS

The colocalization of different subunits of the cytoplasmic dynein complex with melanosomes is consistent with the hypothesis that this motor protein supports minus end-directed melanosome movement along microtubules.

Corticotropin releasing hormone and proopiomelanocortin involvement in the cutaneous response to stress

The skin is a known target organ for the proopiomelanocortin (POMC)-derived neuropeptides alpha-melanocyte stimulating hormone (alpha-MSH), beta-endorphin, and ACTH and also a source of these peptides. Skin expression levels of the POMC gene and POMC/corticotropin releasing hormone (CRH) peptides are not static but are determined by such factors as the physiological changes associated with hair cycle (highest in anagen phase), ultraviolet radiation (UVR) exposure, immune cytokine release, or the presence of cutaneous pathology. Among the cytokines, the proinflammatory interleukin-1 produces important upregulation of cutaneous levels of POMC mRNA, POMC peptides, and MSH receptors; UVR also stimulates expression of all the components of the CRH/POMC system including expression of the corresponding receptors. Molecular characterization of the cutaneous POMC gene shows mRNA forms similar to those found in the pituitary, which are expressed together with shorter variants. The receptors for POMC peptides expressed in the skin are functional and include MC1, MC5 and mu-opiate, although most predominant are those of the MC1 class recognizing MSH and ACTH. Receptors for CRH are also present in the skin. Because expression of, for example, the MC1 receptor is stimulated in a similar dose-dependent manner by UVR, cytokines, MSH peptides or melanin precursors, actions of the ligand peptides represent a stochastic (predictable) nonspecific response to environmental/endogenous stresses. The powerful effects of POMC peptides and probably CRH on the skin pigmentary, immune, and adnexal systems are consistent with stress-neutralizing activity addressed at maintaining skin integrity to restrict disruptions of internal homeostasis. Hence, cutaneous expression of the CRH/POMC system is highly organized, encoding mediators and receptors similar to the hypothalamic-pituitary-adrenal (HPA) axis. This CRH/POMC skin system appears to generate a function analogous to the HPA axis, that in the skin is expressed as a highly localized response which neutralizes noxious stimuli and attendant immune reactions.

Spatiotemporal expression, distribution, and processing of POMC and POMC-derived peptides in murine skin

In murine skin, after depilation-induced anagen, there was a differential spatial and temporal expression of pro-opiomelanocortin (POMC) mRNA, of the POMC-derived peptides beta-endorphin, ACTH, beta-MSH, and alpha-MSH, and of the prohormone convertases PC1 and PC2 in epidermal and hair follicle keratinocytes and in the cells of sebaceous units. Using a combination of in situ hybridization histochemistry and immunohistochemistry, we found cell-specific variations in the expression of POMC mRNA that were consistent with immunoreactivities for POMC-derived peptides. Cells that contained POMC peptide immunoreactivity (IR) also expressed POMC mRNA, and where the IR increased there was a parallel increase in mRNA. The levels of PC1-IR and PC2-IR also showed cell-specific variations and were present in the same cells that contained the POMC peptides. Based on the cleavage specificities of these convertases and on the spatial and temporal expression of the convertases and of ACTH, beta-endorphin, beta-MSH, and alpha-MSH, we can infer that the activities of PC1 and PC2 are responsible for the cell-specific differential processing of POMC in murine skin.

Condition-dependent presence of beta-lipotropin-like peptide in human keratinocytes

This study aimed to characterize the beta-endorphin-immunoreactive material (betaE-IR) detectable in normal human keratinocytes (NHK). The effects of different culturing conditions and UV-irradiation on production of betaE-IR by NHK were assessed by radioimmunoassay and HPLC. All culture systems contained low levels of betaE-IR that was increased in conditioned media after UV-irradiation under certain conditions. NHK grown in nutrient-poor medium contained highest levels of betaE-IR that exhibited beta-lipotropin-like properties after HPLC analysis. The other culturing conditions displayed no authentic betaE-related peptides. Our results indicate that under certain culturing conditions NHK can produce POMC peptides like beta-lipotropin, which can be induced by UV-radiation.

beta-endorphin stimulates cytokeratin 16 expression and downregulates mu-opiate receptor expression in human epidermis

It has been reported that opioid peptides modulate the differentiation of normal human keratinocytes and that mu-opiate receptors are expressed in human epidermis. The regulation of keratinocyte differentiation is particularly important in psoriasis, and one of the markers for hyperproliferative and differentiating skin diseases is cytokeratin 16. The finding that the endogenous mu-opiate receptor ligand beta-endorphin is increased in serum of patients with psoriasis indicates that the mu-opiate system may play an important role in the pathophysiology of the skin. In this study, we addressed the question whether there is a link between mu-opiate receptor regulation and cytokeratin 16 expression in normal and psoriatic skin. Firstly, we demonstrate that beta-endorphin concentrations between 16 and 1000 nM significantly downregulate mu-opiate receptor expression in epidermis of cultured human skin after 48 h. Secondly, we show that beta-endorphin regulates cytokeratin 16 expression in the epidermis of skin organ cultures exposed to 41-125 nM beta-endorphin for 48 h, leading to elevated cytokeratin 16 production. As expected, the expression of cytokeratin 16 was detected primarily in the suprabasal layer. The same pattern was observed in psoriatic lesional skin, i.e., mu-opiate receptor expression was significantly downregulated and cytokeratin 16 expression upregulated. These results suggest that the mu-opiate receptor system and its ligand beta-endorphin are involved in the pathogenesis of psoriasis, especially in terms of differentiation.

Thymidine dinucleotides inhibit contact hypersensitivity and activate the gene for tumor necrosis factor alpha1

DNA is a target for ultraviolet-B-induced inhibition of contact hypersensitivity, and small DNA fragments such as thymidine dinucleotides (pTpT) can simulate several ultraviolet-induced effects. To determine whether pTpT mimics the suppressive influence of ultraviolet-B on contact hypersensitivity, we compared the effects of topical application of pTpT with those of ultraviolet-B irradiation on C57BL/6 mice sensitized to dinitrofluorobenzene. Mice pretreated with pTpT or ultraviolet-B irradiation showed markedly suppressed ear swelling responses to dinitrofluorobenzene challenge. Because tumor necrosis factor alpha mediates ultraviolet-B-induced suppression of contact hypersensitivity, and because pTpT exerts many ultraviolet-mimetic effects by augmenting mRNA and protein levels of effector molecules, we asked if pTpT mimics ultraviolet-B's upregulatory influence on tumor necrosis factor alpha expression. Using transgenic mice carrying a chloramphenicol acetyl transferase reporter linked to the tumor necrosis factor alpha promoter, we examined effects of ultraviolet-B irradiation versus intradermal injection of pTpT on tumor necrosis factor alpha gene transcription. Both treatments induced cutaneous chloramphenicol acetyl transferase activity. Ultra- violet-B or pTpT treatment of cultured dermal fibroblasts from these mice also stimulated chloramphenicol acetyl transferase activity. To determine whether human cells responded similarly, a well- differentiated ultraviolet-responsive human squamous cell carcinoma line was treated with pTpT. pTpT increased tumor necrosis factor alpha mRNA expression and protein secretion in a dose-dependent manner. Our findings expand the spectrum of ultraviolet effects mimicked by pTpT to include inhibition of contact hypersensitivity and activation of the tumor necrosis factor alpha gene. These results support the hypothesis that DNA photoproducts and/or their repair intermediates trigger many of the biologic consequences of ultraviolet irradiation.

An immunocytochemical approach to the study of beta-endorphin production in human keratinocytes using confocal microscopy

Proopiomelanocortin (POMC) is a protein that is posttranslationally processed to yield POMC peptides. The main site of POMC expression is the anterior pituitary lobe but many other sources have been identified. There is evidence that the skin produces POMC peptides, although their roles have not yet been defined. In the skin, regulation of POMC gene expression is known to be hair-cycle dependent, and it is localized to the sebaceous gland. In particular, beta-endorphin, a POMC peptide, has been shown to be modulated by TPA, IL-1 alpha, and ultraviolet radiation in keratinocytes. These results were obtained by examination of POMC mRNA levels using the Northern blot method; beta-endorphin protein production by the Western blot method on cultured cells; and immunocytochemistry for tissue preparations. This report represents an approach to use immunocytochemistry to quantify beta-endorphin production in cultured human keratinocytes. Additionally, we examined whether exposure to 20 mJ ultraviolet B radiation (UVB) and/or UVA could influence beta-endorphin production in these cells. Keratinocytes were grown in monolayers, in serum-free medium, fixed, and incubated with antiserum to whole synthetic beta-endorphin. Fluorescence microscopy was performed with a confocal laser scanning microscope. The integrated level of fluorescence was evaluated in n = 18 +/- 8 individual cells, and this was assumed to be proportional to beta-endorphin content. High variability was observed in the fluorescence intensity among cells. No significant differences between control and UVB- or UVA + UVB-treated cells was found. Similar results were produced by using brefeldin A, a compound that disrupts the secretory pathway, eliminating the possibility that the absence of a difference between beta-endorphin content in the treated and control cells was due to secretion of the peptide into the medium. We conclude that: (1) beta-endorphin or beta-endorphin-like peptides are produced in human keratinocytes and are readily detected by immunocytochemistry; (2) under the conditions tested, UVA and/or UVB did not increase beta-endorphin-like immunoreactivity in these cells.

Cutaneous expression of CRH and CRH-R. Is there a "skin stress response system?"

The classical neuroendocrine pathway for response to systemic stress is by hypothalamic release of corticotropin releasing hormone (CRH), subsequent activation of pituitary CRH receptors (CRH-R), and production and release of proopiomelanocortin (POMC) derived peptides. It has been proposed that an equivalent to the hypothalamic-pituitary-adrenal axis functions in mammalian skin, in response to local stress (see Reference 1). To further define such system we used immunocytochemistry, RP-HPLC separation, and RIA techniques, in rodent and human skin, and in cultured normal and malignant melanocytes and keratinocytes. Production of mRNA for CRH-R1 was documented in mouse and human skin using RT-PCR and Northern blot techniques; CRH binding sites and CRH-R1 protein were also identified. Addition of CRH to immortalized human keratinocytes, and to rodent and human melanoma cells induced rapid, specific, and dose-dependent increases in intracellular Ca2+. The latter were inhibited by the CRH antagonist alpha-helical-CRH(9-41) and by the depletion of extracellular calcium with EGTA. CRH production was enhanced by ultraviolet light radiation and forskolin (a stimulator for intracellular cAMP production), and inhibited by dexamethasone. Thus, evidence that skin cells, both produce CRH and express functional CRH-R1, supports the existence of a local CRH/CRH-R neuroendocrine pathway that may be activated within the context of a skin stress response system.

Expression of functional melanocortin receptors and proopiomelanocortin peptides by human dermal microvascular endothelial cells

Human dermal microvascular endothelial cells (HDMEC) are capable of mediating leukocyte-endothelial interactions by the expression of cellular adhesion molecules and the release of proinflammatory cytokines and chemokines during cutaneous inflammation. Recent studies support the important role for proopiomelanocortin (POMC) peptides, such as alpha-melanocyte stimulating hormone (alpha-MSH), as immunomodulators in the cutaneous immune system. The purpose of the studies described here was to determine whether HDMEC serves as both target and source for POMC peptides. RT-PCR and Northern blot studies demonstrated the constitutive expression of mRNA for the adrenocorticotropin (ACTH) and alpha-MSH-specific melanocortin receptor 1 (MC-1R) in HDMEC, and the microvascular endothelial cell line HMEC-1 that could be upregulated by stimulation with IL-1 beta and alpha-MSH. HDMEC responded to stimulation by alpha-MSH with a dose- and time-dependent synthesis and release of the CXC chemokines, IL-8 and GRO alpha. Likewise, alpha-MSH augmented HDMEC chemokine release induced by TNF or IL-1. HD-MEC were found to constitutively express POMC and prohormone convertase 1 (PC-1); the latter being required to generate ACTH from the POMC prohormone. POMC and PC-1 mRNA expression are increased as a result of stimulation with UVB and UVA1 radiation, IL-1, and alpha-MSH. In addition, UV-radiation is capable of inducing the release of HDMEC, ACTH, and alpha-MSH in a time- and dose-dependent fashion. Thus, these data provide evidence that HDMEC are capable of expressing functional MC-1R, POMC, and PC-1 mRNA; and of releasing POMC peptides with UV light, IL-1, and alpha-MSH as regulatory factors. The expression and regulation of these peptides may be of importance, not only for the autocrine or paracrine regulation of physiologic functions of dermal endothelial cells, but also for the regulation of certain microvascular-mediated cutaneous or systemic inflammatory responses.

alpha-MSH and the regulation of melanocyte function

alpha-MSH, has numerous actions in the skin and by activating the MC1 receptor (MC1-R) on melanocytes it stimulates melanogenesis. Rather than producing large increase in melanin production alpha-MSH acts specifically to stimulate eumelanin synthesis. Although this could be important in determining skin color and tanning there is debate as to the pigmentary significance of alpha-MSH in humans. Circulating levels of alpha-MSH are negligible and although it is produced in the skin by different cell types, including melanocytes, the major skin form is desacetyl alpha-MSH, and this is a weak agonist at MC1-R. Certain ACTH peptides, notably ACTH1-17, are more potent agonists at the MC1-R and, since their skin concentrations exceed those of alpha-MSH, they could serve as natural ligands at this receptor and regulate pigmentary responses in humans. Activation of MC1-R does, however, produce other responses in human melanocytes. Thus, alpha-MSH stimulates melanocyte dendricity and attachment to extracellular matrix proteins. It also protects melanocytes from the damaging effects of oxidative stress, and regulates their production of NO by modulating the induction of iNOS--as it does within macrophages. alpha-MSH clearly affects various aspects of melanocyte behavior and its melanogenic effects could be the consequence of a more fundamental role in the melanocyte. The precise nature of this role is unclear, but it could be part of a generic role that alpha-MSH and other POMC peptides have in skin homeostasis.

Enhanced expression of melanocortin-1 receptor (MC1-R) in normal human keratinocytes during differentiation: evidence for increased expression of POMC peptides near suprabasal layer of epidermis

Immunohistochemical staining of human skin specimen showed the stronger localization of proopiomelanocortin peptides near the suprabasal layer of the epidermis, where keratinocytes are mostly differentiated. To test the possibilities of whether the production of proopiomelanocortin peptides or their receptor-binding activity or both is increased during differentiation of keratinocytes, we treated the cells in culture with Ca2+ to induce their differentiation. The production of proopiomelanocortin peptides and its gene expression were not induced significantly, but the binding ability of melanocortin receptor, as well as its gene expression were stimulated by Ca2+. Ultraviolet B irradiation, an inducer of differentiation, stimulated both proopiomelanocortin production and melanocortin receptor expression. These data show that normal human keratinocytes express melanocortin receptor similar to melanocytes, and that it is induced during differentiation.

Expression of proopiomelanocortin (POMC)-derived melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH) peptides in skin of basal cell carcinoma patients

We proposed that local expression and production of proopiomelanocortin (POMC) peptides may play a role in human skin physiology and pathology, including the development and progression of skin cancers. Reverse transcription polymerase chain reaction (RT-PCR) and Northern blotting hybridization techniques were used to study gene expression. Reversed-phase (RP) high-pressure liquid chromatography (HPLC) separation with subsequent radioimmunoassays were used to identify alpha-melanocyte-stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH) peptides. Immunocytochemistry (IHC) was used to localize ACTH, alpha-MSH, and beta-MSH antigens in skin. RT-PCR, RP-HPLC, and IHC analyses documented the expression of POMC mRNA and production of ACTH and alpha-MSH peptides in lesional and perilesional skin of basal cell carcinoma (BCC) patients and in cultured keratinocytes, which was accompanied by the expression of the MC1-R gene encoding the receptor activated by MSH and ACTH. Thirty specimens were analyzed by IHC. Immunoreactive alpha-MSH, beta-MSH, and ACTH were detected, in 21 of 21, in 11 of 20, and in 6 of 8 of lesional skin, and in 6 of 6, in 5 of 7, and in 6 of 8 perilesional skin specimens analyzed, respectively. Antigen distribution was heterogenous and present in BCC, epidermis, hair follicles, dermal mononuclear cells, and extracellular matrix. We conclude that messenger RNA for POMC, MC1-R, and the peptides MSH and ACTH are produced in skin of BCC patients. Because keratinocytes are a target for MSH and ACTH bioregulation, the production of these peptides is stimulated by UVB, and the peptides can act as immunosupressors, we suggest that MSH and ACTH may facilitate development of BCC.

Increased levels of enkephalin following natural sunlight (combined with salt water bathing at the Dead Sea) and ultraviolet A irradiation

The opioid peptides enkephalins have been shown to modulate inflammatory responses and keratinocyte proliferation and differentiation. Furthermore, increased levels of enkephalin are present in psoriatic lesions. The purpose of the present study was to determine the effect of natural sunlight combined with salt water bathing in the Dead Sea on the methionine-enkephalin (e.n.k.) level in psoriatic skin. Ten patients were treated at the Dead Sea for 4 weeks, and keratotome biopsies were obtained before and after treatment. The amount of enkephalin extracted from the biopsies was measured by radioimmunoassay. Treatment at the Dead Sea resulted in a complete clinical clearance of psoriasis, and immunohistochemical stainings of lesional skin showed that the treatment decreased both epidermal thickness/parakeratosis and the dermal infiltration of CD3- and CD68-positive cells, although the number of CD3- and CD68-positive cells became normal in only two of the 10 cases. However, there was only a slight decrease in the mean enk levels (21%). Furthermore, the level of enk was high in non-lesional psoriatic skin after treatment at the Dead Sea, and immunostaining showed that, in some patients, the treatment induced a mild epidermal hyperplasia and a dermal infiltration of CD3- and CD68-positive cells. Enkephalin-like immunoreactivity was detected in the cytoplasm of both epidermal keratinocytes and dermal infiltrating cells. To determine whether the relatively high skin enk levels after treatment at the Dead Sea was caused by ultraviolet (UV) radiation, normal volunteers were exposed to a single dose of UVA and UVB (2 minimal erythema doses). UVA, but not UVB, irradiation stimulated the mean enk level in the irradiated skin by about sixfold. Furthermore, multiple whole-body UVA irradiations not only resulted in increased skin levels of enk, but also in increased plasma levels. In conclusion, natural sunlight combined with salt water bathing cleared psoriasis without causing a significant decrease in lesional enk levels. Furthermore, non-lesional enk levels were increased. These findings may be the result of a direct stimulatory effect of UVA irradiation on enk formation in the skin. It is possible that the increased circulating levels of enk after UV exposure may contribute to the beneficial effects of UVA irradiation.

Hair cycle-dependent production of ACTH in mouse skin

We investigated the functional determinants of the cutaneous expression of elements of the hypothalamic-pituitary-adrenal axis. In the present work, the presence of adrenocorticotropin (ACTH) peptide in skin of C57/BL6 mouse was demonstrated by reversed-phase HPLC analysis combined with specific radioimmunoassay. ACTH concentration that was low in telogen, increased during anagen in two steps: a rapid phase in anagen I, and a slower rise that reached its peak in anagen VI. Immunofluorescence localized the ACTH antigen to the basal layer of epidermis, outer root sheath of hair follicle and subcutaneous muscle of anagen VI skin. At physiological plasma concentration (10-9 M), ACTH selectively stimulated DNA synthesis in dermis, while pharmacological doses (10-7-10-6 M) inhibited DNA synthesis in both dermis and epidermis. In conclusion, we suggest that local production of ACTH may represent a regulatory element in the control of skin functions including hair growth.