The effect of a single dose of ultraviolet radiation on epidermal melanocytes

Role of alpha-melanocyte stimulating hormone (α-MSH) in modulating the molecular mechanism adopted by melanocytes of Bos indicus under UVR stress

Ultraviolet radiations (UVR) are responsible for a wide variety of acute and chronic effects on the animal skin. However, the effect of UVR-induced oxidative stress and protection through paracrine factors on animal skin has received little attention. We previously demonstrated how heat stress-induced adaptation in Bos indicus melanocytes was dependent on the level of melanin and reduction of apoptosis. Therefore, in the present investigation, the survival mechanisms adopted by melanocytes under UV stress and the role of α-MSH in cell survival under in vitro conditions were studied. After the treatment of melanocyte cells with UVR (using Osram ultravitalux 300 W lamp), analysis of Gene expression using Real-Time PCR was done to study the adopted molecular pathways under stressful conditions. In addition, α-MSH was used to assess its modulating role in cell survival under stress. This study revealed the increase in the expression of genes related to melanogenesis, cell cycle, heat shock proteins, and apoptosis of the cells after UVR stress and demonstrated the role of paracrine factor (α-MSH) in elevating the protection response to stressful conditions like UVR stress by increasing the melanogenesis and decreasing the mitochondrial-mediated apoptosis. Based on the results of the present study, it can be stated that α-MSH can play a pivotal role in the protection of animal skin cells under stressful conditions in climate-changing scenario.

The Expression of Melanogenic Proteins in Korean Skin after Ultraviolet Irradiation

For proper melanin production, several specific enzymes such as tyrosinase, tyrosinase-related protein 1 (TRP-1) and dopachrome tautomerase are required. Their expressions are increased after exposure to UVB. However, it is not known how long tyrosinase and TRP-1 activities continue after UV irradiation in vivo. The purpose of this study is to measure the changes in expressions of tyrosinase, TRP1, and MITF after exposure to UV on skin in a Korean population. We established an immunohistochemical staining protocol for specimens which were obtained from UV-irradiated skin in five healthy Korean males on the 2nd, 5th, 7th, 28th, and 56th days after UV irradiation. Tyrosinase, TRP-1, and MITF expressions increased until 7 days after UV irradiation and then dropped to the basal constitutive level 4 and 8 weeks later. Interestingly, tyrosinase increased prior to TRP-1. This study reveals the time-sequence of melanin-synthesized enzymes and provides important information for the clinical evaluation of the effectiveness of whitening agents.

Protease-activated receptor 2, a receptor involved in melanosome transfer, is upregulated in human skin by ultraviolet irradiation

Previous studies have shown that the protease-activated receptor 2 is involved in skin pigmentation through increased phagocytosis of melanosomes by keratinocytes. Ultraviolet irradiation is a potent stimulus for melanosome transfer. We show that protease-activated receptor 2 expression in human skin is upregulated by ultraviolet irradiation. Subjects with skin type I, II, or III were exposed to two or three minimal erythema doses of irradiation from a solar simulator. Biopsies were taken from nonexposed and irradiated skin 24 and 96 h after irradiation and protease-activated receptor 2 expression was detected using immunohistochemical staining. In nonirradiated skin, protease-activated receptor 2 expression was confined to keratinocytes in the lower one-third of the epidermis. After ultraviolet irradiation protease-activated receptor 2 expression was observed in keratinocytes in the upper two-thirds of the epidermis or the entire epidermis at both time points studied. Subjects with skin type I showed delayed upregulation of protease-activated receptor 2 expression, however, compared with subjects with skin types II and III. Irradiated cultured human keratinocytes showed upregulation in protease-activated receptor 2 expression as determined by immunofluorescence microscopy and Western blotting. Cell culture supernatants from irradiated keratinocytes also exhibited a dose-dependent increase in protease-activated receptor-2 cleavage activity. These results suggest an important role for protease-activated receptor-2 in pigmentation in vivo. Differences in protease-activated receptor 2 regulation in type I skin compared with skin types II and III suggest a potential mechanism for differences in tanning in subjects with different skin types.

Rab3a and SNARE proteins: potential regulators of melanosome movement

Melanosomes are specialized organelles that undergo a dynamic process of transport along the melanocyte dendrite to the dendrite tip and transfer to keratinocytes. We hypothesized that soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE), which are involved in membrane fusion, and rab3a, a GTP-binding protein involved in exocytosis in neuronal cells and in SNARE complex assembly, may play a part in melanosome transport and transfer. By reverse transcription-polymerase chain reaction we identified transcripts for rab3a, vesicle-associated membrane protein-2, synaptosome-associated proteins of 23 kDa and 25 kDa, and syntaxin-4 in murine melanocytic cells. We also showed that purified melanosome preparations contain rab3a and SNARE, including vesicle-associated membrane protein-2, syntaxin-4, synaptosome-associated proteins 23 kDa and 25 kDa, and the SNARE accessory protein, alpha-soluble N-ethylmaleimide-sensitive factor attachment protein. Ultraviolet radiation is a potent stimulus for melanosome transport and transfer. We show that ultraviolet radiation rapidly suppresses melanosome-associated rab3a expression and that this occurs at the protein and mRNA level. Finally, we show that vesicle-associated membrane protein-2 and synaptosome-associated protein 23 kDa coimmunoprecipitate from purified melanocytic cell membranes, suggesting that they form complexes. The presence of rab3a and SNARE on melanosomes, and of SNARE complexes in melanocytic cell membranes suggests that these proteins play a part in targeting melanosomes to the plasma membrane, to melanosome transfer to keratinocytes, or both.

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

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

Pigmented human skin equivalent: new method of reconstitution by grafting an epithelial sheet onto a non-contractile dermal equivalent

We have established a new protocol for reconstituting a pigmented human skin equivalent (PSE) and have evaluated its functional responses to environmental stimulus, UVB. The PSE is reconstituted by grafting an epithelial sheet consisting of keratinocytes and melanocytes onto a porous non-contractile dermal equivalent populated with mitotically and metabolically active fibroblasts. i) The PSE has a multilayered, well-differentiated epidermis with cuboidal basal cells and highly organised dermis with newly synthesised extracellular matrix components. ii) Ki67-positive proliferating keratinocytes (18.1 +/- 7.4%) were detected on the basal layer of the epidermis. iii) Melanocytes located exclusively within the basal layer were detected by monoclonal antibody against tyrosinase-related protein (TRP-1). iv) After exposure to UVB (100 mJ/cm2 per day) for 7 consecutive days, the intensity of TRP-1 staining was increased in the PSE, showing their functional state, whereas the number of melanocytes was not changed. This non-contractile and functioning new PSE is potentially useful as a model for studying the role of melanocyte-keratinocyte-fibroblast interactions in photoprotection of the skin in more complex cutaneous microenvironment than monolayer culture, and for developing in vitro disease models and therapeutic protocols with genetically altered cells both in epidermis and dermis.

Effect of pituitary and ovarian hormones on human melanocytes in vitro

Normal human melanocytes in culture became enlarged and dendritic after a 2-day incubation with either the pituitary (beta-MSH, a potent analog of alpha-MSH, ACTH, FSH and LH) or the ovarian (estradiol, estriol and progesterone) hormones. Under the same experimental conditions, pituitary hormones also increased both the tyrosinase activity and tyrosinase-related protein-1 (TRP-1) while ovarian hormones increased TRP-1 but not tyrosinase activity. The results suggest that pituitary and ovarian hormones possibly induce hyperpigmentation of the skin by stimulating the melanogenesis in epidermal melanocytes, and that estradiol and progesterone may be involved in the pathogenesis of melasma (chloasma) usually developing between early adulthood and menopause in which a high concentration of serum ovarian hormones was maintained.

Histamine stimulates normal human melanocytes in vitro: one of the possible inducers of hyperpigmentation in urticaria pigmentosa

Isolated normal human melanocytes became enlarged and more dendritic in association with an increase in the activity of tyrosinase and the amount of b-locus protein when they were cultured with 0.1-10 microM histamine in vitro. However, histamine did not exert a proliferative effect on them. The stimulatory effect of histamine was observable even 6 h after starting the treatment. This stimulation seems not to be pharmacologically mediated through histamine receptors, because it was inhibited neither by pyrilamine, a histamine H-1 antagonist, nor by cimetidine, a H-2 antagonist. Imidazole derivatives that are rapidly metabolized from histamine in vivo and in vitro also stimulated the melanocytes. We propose that high concentrations of histamine and its imidazole metabolites continuously produced in the lesions of urticaria pigmentosa are probable causative factors of its characteristic skin pigmentation.

Biology of hypopigmentation

A review of the basics of pigment cell biology is followed by a discussion of the characteristics of several disorders of hypopigmentation. By determining such features as inheritance pattern, time of onset (congenital, childhood, adulthood), natural history (stable vs progressive), type of pigment loss (diffuse or circumscribed), distribution of lesions (generalized vs localized), degree of pigment loss (incomplete or complete), number of melanocytes, if any, in biopsy specimens of affected areas, type of melanocytic dysfunction, and associated inflammation or infection, one can classify the disorders of hypopigmentation. The proposed pathophysiology for each disorder of hypomelanosis is presented.

pH-dependent interconversion of two forms of tyrosinase in human skin

1. We have shown that the characteristic lag in cresolase activity of human skin tyrosinase at inhibitory concentration of tyrosine was absent at all pH values studied, i.e. pH 5.2, 5.7, 6.2 and 6.8, if the enzyme solubilized at low pH was used as the source of enzyme, but the same enzyme when dialysed against buffers of various pH values showed linear activity only at pH 5.2 and was not inhibited by excess tyrosine, whereas at higher pH values it exhibited a lag and inhibition by excess tyrosine. 2. However, the enzyme solubilized in buffer/detergent, pH 6.8, when dialysed against buffer of the same pH showed linear activity at pH 5.2 and non-linear activity at pH 6.8. 3. The water/detergent-solubilized enzyme from human skin melanosomes showed linear activity even at inhibitory concentrations of tyrosine at pH 5.2 and 6.8 up to 2 h, but acceleration of rate was observed after 2 h for the enzyme measured at pH 6.8. 4. After dialysis of the water/detergent-solubilized enzyme against double-glass-distilled water, it still exhibits linear activity at inhibitory concentration of tyrosines at pH 6.8 for the first 2 h, but the same enzyme when dialysed against 0.02 M-sodium phosphate buffer, pH 6.8, exhibits negligible activity up to 1/2 h, in contrast with considerable activity before dialysis during the same interval of time, but without any loss of activity at later intervals of incubation time. 5. On the basis of these results, it is concluded that the enzyme exists in at least two interconvertible forms, one without lag and inhibition by excess tyrosine and the other with lag and inhibition by excess tyrosine. These two forms are interconvertible only by gradual change in pH over a period of hours.

Stimulation of human melanocytes by vitamin D3 possibly mediates skin pigmentation after sun exposure

We found an increased amount of immunoreactive tyrosinase in human melanocytes after 6-d culturing with vitamin D3 (cholecalciferol). Most of these melanocytes became more dendritic and swollen in a fashion similar to that noted in the skin after ultraviolet irradiation. However, 7-dehydrocholesterol (pro-vitamin D3) or 1 alpha,25-dihydroxy-vitamin D3 (activated vitamin D3) were found to have little effect on the same system. Because vitamin D3 is known to be photochemically converted from pro-vitamin D3 in the skin by ultraviolet irradiation, the mechanism of human skin pigmentation after ultraviolet irradiation, thus far unknown, may be at least partly explained by this stimulating effect of vitamin D3 on melanocytes.

A comparison of the melanocyte response to narrow band UVA and UVB exposure in vivo

The visible cutaneous pigmentary response to ultraviolet-A (UVA) is immediate and, following sufficient exposure, may persist, whereas ultraviolet-B (UVB)-induced pigmentation appears after a delay of several days. We compared the in vivo response of melanocytes to single and multiple exposures of narrow band UVA and UVB irradiation which produced visibly equal increases in pigmentation. Using a xenon-mercury source matched to a monochromator, human volunteers were exposed to 304 (+/- 5) and 365 (+/- 10) nm radiation. Biopsies were performed 1, 7, and 14 days after irradiation. For each biopsy, the number of melanocytes per square millimeter of epidermis was determined using L-3,4-dihydroxyphenylalanine (dopa)- and tyrosine-incubated split epidermal preparations. Vertical sections were also examined. At days 7 and 14, after both 304 and 365 nm radiation, melanocytes were more intensely dopa-positive than in unirradiated controls, and demonstrated enlarged perikarya and a greater number of enlarged dendrites. Following both 304 and 365 nm radiation the number of dopa-positive melanocytes was increased at days 7 and 14 by 44% and 58%, respectively. Tyrosine positivity, an indicator of enhanced tyrosinase activity and increased melanin formation, was absent in controls and at day 1, and became positive in all but one sample at day 7 and day 14. Therefore, one day after UVA exposure, visible pigmentation but not tyrosinase activity was increased. At day 7, the number of tyrosine-positive melanocytes approximately equaled the number of dopa-positive melanocytes. Although UVA and UVB induce different pigmentary responses, their effects on melanocyte number and function were indistinguishable.

Sensitivity of mouse Skh:HR-2 to ultraviolet radiation: melanocyte inactivation

The hairless mouse, Skh:HR-2, was exposed to doses of ultraviolet (UV) radiation known to induce skin pigmentation. Three parameters associated with perturbations in skin pigmentation were monitored following UV exposure. These include spectroscopy (skin darkness), histology (melanocyte density), and biochemistry (melanin). Within 90 min of UV exposure, the skin became lighter. This was associated with a reduction of quantifiable melanin and the inactivation of epidermal melanocytes.

Ultrastructural demonstration of chemical modification of melanogenesis in hairless mouse skin

We investigated chemical and physical modifications of the genetically determined ultrastructure of melanosomes. The flank skin of hairless mice was treated with ultraviolet energy (UV) shorter than 320 nm or with a combination of a photosensitizer and UV (PUVA treatment). All melanosomes in the induced melanocytes and those in resident melanocytes in the ear skin showed eumelanogenesis, although the degree of melanin deposition differed considerably according to the induction process. Eumelanogenesis was most advanced in the resident melanocytes while PUVA-induced melanocytes showed more immature premelanosomes. We then topically applied 4-tertiary butyl catechol on the skin. The depigmenting agent caused an appearance of pheomelanosomes. The alteration in melanogenesis was seen most distinctly in premelanosomes of the PUVA-induced cells. Altered ultrastructure was also observed in matured melanosomes; this change was most apparent in the resident melanocytes. These findings indicate that cells with eumelanogenesis may undergo pheomelanogenesis. The present study demonstrated effects of chemicals on genetically determined function of melanocytes by quantitative analysis of melanosome ultrastructure.

The epidermal melanocyte population in the skin of ultraviolet-irradiated crested newt

The response of the epidermal melanocyte population to repeated ultraviolet (UV) exposure (wavelength spectrum 275-350 nm) has been investigated in the crested newt, Triturus cristatus carnifex. The effects of different doses of UV light were studied. The animals were killed 7 months after the first UV exposure. Only a slight decrease in the number of pigment cells was found after 85 sequential irradiations with a total dose of 1.3 x 10(5) J/m2, whereas striking decreases were observed when the same total dose was fractionated into 14 exposures or when a double dose was given in 57 exposures. The relationship between the square roots of the epidermal melanocyte densities and single doses appeared to be roughly linear, at least over the range of doses administered. The main factor in melanocyte damage seemed to be the single dose of irradiation rather than the cumulative dose administered. Decreased melanin content of the keratinocytes was observed in most irradiated animals.

Redistribution of melanosomal complexes within keratinocytes following UV-A irradiation: a possible mechanism for cutaneous darkening in man

In contrast to other ultraviolet (UV) wavelengths, UV-A can induce long-term or "true" pigmentation rapidly with little or no latency. The response cannot be clearly separated from immediate pigment darkening and is too rapid in onset to be explained by neomelanogenesis. In order to investigate possible mechanisms for this phenomenon, UV-irradiated skin was examined microscopically and ultrastructurally 18 h postirradiation. Specimens from skin sites tanned by exposure to melanogenic doses of UV-A showed a paradoxical reduction in the degree of basal melanization by light microscopy compared to unirradiated skin. Ultrastructurally, there was migration and dispersion of packaged melanosomes within keratinocytes from their normal, aggregated location around the nucleus towards the periphery of the cell. These changes were not observed in specimens exposed to melanogenic doses of UV-B. We propose that UV-A wavelengths can selectively cause redistribution of melanin-laden organelles within human keratinocytes in vivo and that this phenomenon accounts for the visually observed hyperpigmentation that develops soon after single exposures to these wavelengths. Dispersion of melanosomal complexes may be another mechanism by which UV-radiation (UVR) can induce tanning in human skin.

Effects of aging and chronic sun exposure on melanocytes in human skin

Both aging and sun exposure have well-documented effects on the human melanocyte system. Paired biopsies of habitually exposed and nonexposed skin from adjacent anatomic sites were obtained from 8 donors aged 28 to 80 yr in order to study the combined effect of chronic actinic irradiation and chronologic aging. Density of dopa-positive melanocytes was roughly twofold higher in the exposed than in the nonexposed skin at all ages, suggesting an irreversible effect of sun exposure. Melanocyte density declined approximately 6 to 8% of the surviving population per decade in both sites. Dopa-positivity of individual melanocytes was consistently greater in the chronically exposed skin than in the nonexposed skin of the same subject and did not vary with age. These data strengthen and expand earlier observations of age-related melanocyte changes, and explain the apparent paradox of a generalized increase in pigmentation and simultaneous decrease in melanocyte density which frequently accompany advancing age. In addition, the present study suggests that the principal effect of chronic sun exposure on the human pigmentary system is not premature "aging" as currently recognized histologically, but rather activation and/or proliferation of the exposed melanocytes.

Testosterone and UVL-B stimulation of epidermal melanocytes in rat scrotal skin

The effects of UVL-B and/or testosterone replacemnt therapy are compared in normal and castrated rats in order to determine whether testosterone is required for UVL-B (290-315 nm) stimulation of melanogenesis in the testosterone-dependent epidermal melanocyte system of the scrotal skin of black Long Evans rats. Testosterone is not a prerequisite for UVL-B stimulation of melanocytes as in both castrates and normal animals the melanocytes respond to UVL-B by increases in size, length and number of dendrites (dendriticness), and tyrosinase activity (intensity of Dopa reaction). Addition of testosterone to castrates does enhance the effects of UVL-B. However, UVL-B with or without testosterone cannot maintain normal melanogenesis in rats irradiated immediately after castration nor can it restore normal melanogenesis following long term castration. Bth the amount of UVL nergy/exposure and the number of exposures are important variables in stimulation of the epidermal melanocytes. Administration of a dose of UVL-B to castrates in a single exposure is ineffective, while the same overall dose spread over several exposures increases the size and dendriticness of melanocytes. Testosterone and UVL-B act synergistically in affecting melanogenesis although neither singly nor in combination are they able to fully restore normal melanogenesis.

Photoepicutaneous testing. UV-induced protection against photoxic reactions in normal and vitiliginous skin: a clinical and histological study

The effect of erythemic UV irradiation on the phototoxic reactions caused by topical methoxsalen + UVA exposure was studied on normal skin, normal-looking skin of vitiligo patients, and vitiliginous skin. Although only slight histological changes were detectable 9 days after irradiation with 5 MED of erythemic UV, this pre-irradiation did induce protection against photoxic reactions in all skin types. This protection was clinically equal in all skin types; the slight differences were not statistically significant. Histological evaluation, however, showed a most conspicuous protective effect on vitiliginous skin. In all skin types the influence of UV pre-irradiation was confined to epidermal protection; the dermal phototoxic changes were unaffected.

Number of epidermal melanocytes, hair follicles, and sweat ducts in skin of Solomon Islanders

The number of epidermal appendages and melanocytes in forearm skin of three groups of Solomon Islanders has been determined. There were no significant differences in the number of hair follicles or sweat ducts among Solomon Islander groups, nor between them and a Caucasian group used for comparison. The Nasioi and Kwaio, however, do have a significantly higher melanocyte population than Caucasian groups. This difference may in part be due to the exposure of the Solomon Islander skin used in this study to chronic solar radiation.

Some aspects of melanin biology: 1950-1975

Recent advances in the biology of mammalian pigmentation are reviewed. The multicellular epidermal melanin unit (melanocyte and associated pool of keratinocytes) rather than the melanocyte alone forms the focal point for melanin metabolism within mammalian epidermis. Within an epidermal melanin unit, melanosomes are synthesized by melanocytes and transferred to keratinocytes where they are degraded as they ascend to the epidermal surface. During the past 25 years, technical advances in biology and biochemistry have frosted a multidisciplinary approach to research on mammalian pigmentation. Emphasizing this perspective, we have examined the current state of knowledge of the form and function of epidermal melanin units from the levels of biologic organization ranging from the molecules relevant to melanin synthesis through the skin as a totally intergrated system. To an unusual degree, advances in melanin pigmentation have resulted from the integration of clinical medicine and basic science.

The induction of melanogenesis by ultraviolet light in the pigmentary system of Rhesus monkeys

Except for the face, eyelids, friction surfaces, and lips, the epidermis of the rhesus monkey contains no discernible melanocytes. After ultraviolet irradiation, however, dopa-positive dendritic cells appeared. With daily sequential irradiation, the number of histochemically demonstrable dopa-positive dendritic cells peaked after 30 exposures, then declined to a basal level which was maintained for the duration of the experiment (216 exposures or 43 weeks). Pigment cells can be restimulated by shading part of the irradiated area and then reirradiating after 3 months. While shaded, dopa-positive cells disappeared; but when reirradiated, they reappeared, increased, then declined again to a basal level. These melanocytes, unlike those in other primates, require high threshold levels of irradiation to produce a response, have a definite period during which they are active, and transfer very little melanin to the surrounding keratinocytes. Long-term ultraviolet irradiation has no discernible effect on dermal pigment-containing cells.

Mitotic activity in non-neoplastic melanocytes in vivo as determined by histochemical, autoradiographic, and electron microscope studies

Mitotic figures were demonstrated in the differentiated melanocytes of normal epidermal and nonepidermal tissues without the presence of external stimuli. These dividine melanocytes were present in human and mouse skin, mouse hair, chick feathers, and embryonic chick retinal pigment epithelium. In normal adult human epidermis, dividing melanocytes, though rare, were found in the nonstimulated areas. L-3,4-dihydroxyphenylalanine reaction on the melanocytes during mitosis demonstrated activity of the melanin-forming enzyme, tyrosinase, and ultrastructural studies demonstrated the characteristic melanosomes in variour stages of maturation. Other ultrastructural characteristics of the melanocytes during mitosis, except for the Golgi apparatus, which was smaller and less complex, were similar to those seen in well-differentiated nondividing melanocytes. Autoradiographic studies of thymidine incorporation into mouse skin indicated that 0.7% of epidermal melanocytes, when slightly stimulated, are in the S phase. Thus, in vivo differentiation of non-neoplastic melanocytes (to produce pyrosinase and melanosomes) does not preclude their replication by mitotic division.