DNA damage enhances melanogenesis

Induction of apoptosis by telomere 3' overhang-specific DNA

Telomeres are tandem repeats of a specific TTAGGG nucleotide sequence at the ends of chromosomes. Telomere shortening is proposed to act as a biological clock and cancer prevention mechanism by inducing a nonproliferative, senescent phenotype after a limited number of cellular divisions. Recent evidence also suggests that telomere disruption can trigger apoptosis in certain cell types, mimicking a major cellular response to DNA damage. Here, we show that addition of DNA oligonucleotides homologous to the telomere 3' overhang sequence causes lymphocytic (Jurkat) cells to undergo apoptosis, as described for lymphocytes following telomere loop disruption. We further implicate the p53 tumor suppressor and transcription factor, as well as the p53 homolog p73 and the E2F1 transcription factor, in mediating the apoptotic response. We propose that exposure of the telomere 3' overhang due to opening of the normal telomere loop structure is a physiologic signal for these DNA damage-like responses in vivo and that oligonucleotides partially or completely homologous to the telomere overhang mimic this signal in the absence of DNA damage or telomere disruption.

Nobiletin as a tyrosinase inhibitor from the peel of Citrus fruit

A tyrosinase inhibitor was isolated from the peel of Citrus fruit by activity-guided fractionation, and identified as 3',4',5,6,7,8-hexamethoxyflavone (nobiletin) by comparison with reported spectral data. Nobiletin (IC50 of; 46.2 microM) exhibited more potency than Kojic acid (IC50; 77.4 microM) used as a positive control, and it was found to be potentially an effective inhibitor of the production of melanin.

Human placental lipid induces mitogenesis and melanogenesis in B16F10 melanoma cells

A hydroalcoholic extract of fresh term human placenta was found to be mitogenic as well as melanogenic on B16F10 mouse melanoma in an in vitro culture. The extract, a reservoir of a large number of bioactive molecules, was resolved to get the lipid fraction. Its activity was evaluated on B16F10 mouse melanoma by assessing the change in cellular morphology, growth and melanin induction. The lipid fraction, placental total lipid fraction (PTLF) tested in the study employed doses of 0 01 to 200 microg/ml; optimum growth and melanization accompanied by morphological changes were recorded at 10 and 100 microg/ml respectively. At intermediate doses growth and melanization were found to show a pattern of change over between growth and melanization and finally reached at an inverse relation at the respective optimal dose of response. Compared with defined sphingolipids, C(2) ceramide and sphingosine-1-phosphate, the results were mostly corroborative. The duality of biological response of sphingolipids as reported in numerous studies was comparable for the PTLF suggesting that its active component is a sphingolipid and showing its use for pigment recovery in vitiligo.

DNA oligonucleotide treatment corrects the age-associated decline in DNA repair capacity

Age-related decline in DNA repair capacity (DRC) is associated with decreased constitutive levels of p53 and other nucleotide excision repair proteins. To determine whether pretreatment of cells with small DNA oligonucleotides compensates for decreased DRC in the elderly, fibroblasts from donors of different ages were pretreated with thymidine dinucleotide (pTT), a 5' phosphorylated 9 base oligonucleotide (p9mer) or diluent alone for 48 h, then UV-irradiated with solar-simulated light. Western blot analysis revealed age-associated decreases of 40%-80% between newborn and old adult donor cells in the constitutive protein levels of p53, p21, XPA, RPA, ERCC1, and PCNA. Treatment with pTT or p9mer up-regulated these proteins by 200%-650% at 24, 48, and 72 h. Moreover, pretreatment with oligonucleotides significantly increased the removal rate of photoproducts as determined by reacting DNA with thymine dimer-specific antibodies: 40+/-5% vs. 20+/-9% and 15+/-11% remained after 24 h in diluent, pTT and p9mer treated cells, respectively. Oligonucleotide-treated adult cells removed thymine dimers at least as rapidly as diluent treated newborn cells, demonstrating that pTT and p9mer completely corrected the age-associated decrease in DRC. Our studies suggest that topical oligonucleotide treatment may enhance DRC in older adults and thus reduce the carcinogenic risk from solar UV irradiation in this age group.

Furocoumarin photolysis: chemical and biological aspects

Furocoumarins (psoralens) undergo photolysis when subjected to UVA radiation in solution. Several products are formed, depending on the molecular structure and experimental conditions. Some products are the result of anoxic mechanisms (cyclodimerisation, addition of solvent), others of oxic pathways, leading to oxidised products. The mechanisms thought to underlie photolysis are described and the possible biological relevance of the photoproducts and intermediates is discussed.

Tyrosinase gene expression is regulated by p53

Tyrosinase, the rate-limiting enzyme for melanin synthesis, is induced after ultraviolet irradiation as part of the tanning response, the major recognized photoprotective response of human skin. Other DNA-damaging agents and DNA fragments such as thymidine dinucleotides also induce tyrosinase gene expression. Moreover, like ultraviolet light they also activate p53. To determine whether p53 activation is required for this increased tyrosinase expression, we employed two experimental systems: (i) a human melanoma line (WM35) known to express wild-type p53 versus WM35 cells engineered to express a transcriptionally inactive dominant-negative p53 (WM35-p53DN) or the empty vector alone (WM35-pCMV7) and (ii) mice with wild-type p53 versus p53 knockout mice. In WM35-p53DN cells, the baseline p53 protein level was higher than in WM35 or WM35-pCMV7 cells, and tyrosinase transcripts were lower. After ultraviolet irradiation, in all cell lines the p53 protein level increased within the first 24 h, as expected; and at 24 h tyrosinase mRNA levels were decreased. Consistent with the literature, these data in combination suggest that increased p53 protein level downregulates tyrosinase mRNA. In WM35 and WM35-pCMV7 cells at 48 and 72 h, however, whereas p53 levels remained elevated, tyrosinase mRNA levels compared to pre-irradiation levels tripled, whereas in WM35-p53DN cells levels remained below baseline. In thymidine-dinucleotide-treated WM35 and WM35-pCMV7 cells there was a comparable upregulation of tyrosinase mRNA within 24 h that persisted through 72 h, but there was no upregulation of tyrosinase mRNA in WM35-p53DN cells any time after ultraviolet irradiation or thymidine dinucleotide treatment. In ear skin of p53 wild-type mice, topical application of thymidine dinucleotide induced a 4-5-fold increase in epidermal melanin content after 3 wk, but in p53 knockout mice thymidine dinucleotide application caused no detectable increase in melanin. Together, these data demonstrate that p53 activation increases tyrosinase mRNA level and subsequently pigmentation. The data further suggest that tanning is part of a p53-mediated adaptive response of mammalian skin to DNA damage from ultraviolet irradiation.

Biological consequences of cyclobutane pyrimidine dimers

In the skin many molecules may absorb ultraviolet (UV) radiation upon exposure. In particular, cellular DNA strongly absorbs shorter wavelength solar UV radiation, resulting in various types of DNA damage. Among the DNA photoproducts produced the cyclobutane pyrimidine dimers (CPDs) are predominant. Although these lesions are efficiently repaired in the skin, this CPD formation results in various acute effects (erythema, inflammatory responses), transient effects (suppression of immune function), and chronic effects (mutation induction and skin cancer). The relationships between the presence of CPD in skin cells and the subsequent biological consequences are the subject of the present review.

Skin pigmentation enhancers

The highest incidences of cancer are found in the skin, but endogenous pigmentation is associated with markedly reduced risk. Agents that enhance skin pigmentation have the potential to reduce both photodamage and skin cancer incidence. The purpose of this review is to evaluate agents that have the potential to increase skin pigmentation. These include topically applied substances that simulate natural pigmentation: dihydroxyacetone and melanins; and substances that stimulate the natural pigmentation process: psoralens with UVA (PUVA), dimethylsulfoxide (DMSO), L-tyrosine, L-Dopa, lysosomotropic agents, diacylglycerols, thymidine dinucleotides, DNA fragments, melanocyte stimulating hormone (MSH) analogs, 3-isobutyl-1-methylxanthine (IBMX), nitric oxide donors, and bicyclic monoterpene (BMT) diols. These agents are compared with regards to efficacy when administered to melanoma cells, normal human epidermal melanocytes, animal skin, and human skin. In addition, mechanisms of action are reviewed since these may reveal issues related to both efficacy and safety. Both dihydroxyacetone and topically applied melanins are presently available to the consumer, and both of these have been shown to provide some photoprotection. Of the pigmentation stimulators, only PUVA and MSH analogs have been tested extensively on humans, but there are concerns about the safety and side effects of both. At least some of the remaining pigmentation stimulators under development have the potential to safely induce a photoprotective tan.

Local therapy for cutaneous and systemic lupus erythematosus: practical and theoretical considerations

Local therapeutic measures should be maximized in the management of lupus erythematosus (LE), particularly for patients with forms of LE-specific skin disease such as SCLE and CCLE. Local therapy consists of sun protection and locally-applied pharmacologic agents, e.g. topical/intralesional corticosteroids. Patients should be advised to avoid direct sun exposure, wear lightweight tightly woven clothing and broad-brimmed hats, and use broad-spectrum, water-resistant sunscreens. There are several modalities that enhance permeability of local therapeutics through the stratum corneum that could be applied to topical therapy of cutaneous LE, e.g. polar solvent, liposomal encapsulation, ethosomal system, iontophoresis, electroporation and sonophoresis. As novel and more effective modalities to deliver pharmacologic agents across the stratum corneum become available, therapeutics that have been experimental and untested thus far may become part of the local armamentarium in the near future. In addition, physical/contact therapy as well as dermatosurgical methods can be invaluable in enhancing self-image and quality of life of patients with disfiguring cutaneous LE lesions.

Detection of DNA adducts of benzo[a]pyrene using immunoelectrophoresis with laser-induced fluorescence. Analysis of A549 cells

Detection of benzo[a]pyrene diol epoxide (BPDE)-damaged DNA in a human lung carcinoma cell line (A549) has been performed using free zone affinity capillary electrophoresis with laser-induced fluorescence (LIF). Using BPDE as a model carcinogenic compound, the speed, sensitivity and specificity of this technique was demonstrated. Under free zone conditions, an antibody bound adduct was baseline-resolved from an unbound adduct in less than 2 min. The efficiencies of separation were in excess of 6 x 10(5) and 1 x 10(6) plates per meter for the antibody-bound and unbound adducts, respectively. Separation using a low ionic strength buffer permitted the use of a high electric field (830 V/cm) without the loss of resolving power. Using LIF detection, a concentration detection limit of roughly 3 x 10(-10) M was achieved for a 90-mer oligonuleotide containing a single BDPE. The use of formamide in the incubation buffer to enhance denaturing of DNA did not affect the stability of the complex between the antibody and the adducts. Using a fluorescently labeled BPDE-modified DNA adduct probe, a competitive assay was established to determine the levels of BPDE-DNA adducts in A549 cells.

Ultraviolet B radiation enhances a phytochrome-B-mediated photomorphogenic response in Arabidopsis

Ultraviolet B radiation (UV-B, 290-315 nm) can cause damage and induce photomorphogenic responses in plants. The mechanisms that mediate the photomorphogenic effects of UV-B are unclear. In etiolated Arabidopsis seedlings, a daily exposure to 2.5 h of UV-B enhanced the cotyledon opening response induced by a subsequent red light (R) pulse. An R pulse alone, 2.5 h of UV-B terminated with a far-red pulse, or 2.5 h of continuous R caused very little cotyledon opening. The enhancing effect of UV-B increased with fluence rate up to approximately 7.58 micromol m(-2) s(-1); at higher fluence rates the response to UV-B was greatly reduced. The phyA, phyA cry1, and cry1 cry2 mutants behaved like the wild type when exposed to UV-B followed by an R pulse. In contrast, phyB, phyB cry1, and phyB phyA mutants failed to open the cotyledons. Thus, phytochrome B was required for the cotyledon opening response to UV-B --> R treatments, whereas phytochrome A and cryptochromes 1 and 2 were not necessary under the conditions of our experiments. The enhancing effect of low doses of UV-B on cotyledon opening in uvr1 uvr2 and uvr1 uvr3 mutants, deficient in DNA repair, was similar to that found in the wild type, suggesting that this effect of UV-B was not elicited by signals derived from UV-B-induced DNA lesions (cyclobutane pyrimidine dimers and 6-4 photoproducts). We conclude that low doses of UV-B, perceived by a receptor system different from phytochromes, cryptochromes, or DNA, enhance a de-etiolation response that is induced by active phytochrome B.

Fluoroquinolones as chemical tools to define a strategy for photogenotoxicity in vitro assessment

Today's lifestyle is often associated with frequent exposure to sunlight, but some xenobiotics used in drugs, cosmetics or food chemicals can produce adverse biological effects when irradiated. In particular, they can increase the risk of photogenotoxicity already due to UV radiation itself. There is thus a need to design appropriate approaches in order to obtain relevant data at the molecular and cellular level in this field. For ethical and practical reasons, in vitro models can be very convenient at least for first evaluation tests. Here, we propose a strategy based on complementary experiments to study the photogenotoxic potential of a compound. The fluoroquinolones BAYy3118 and lomefloxacin were used as standards to demonstrate the performance of each test: photoinduced interaction with supercoiled circular DNA, photomutagenicity in the yeast Saccharomyces cerevisae, induction of DNA photodamage in cultured human skin cells as revealed by comet assay, and finally induction of specific phototoxic stress responses such as p53 activation or melanogenesis stimulation. Such a strategy should help to ensure the safety of products likely to undergo environmental sunlight exposure.

Effects of wine phenolics and sorghum tannins on tyrosinase activity and growth of melanoma cells

In this study, three different phenolic (anthocyanin, other flavonoid, and phenolic acid) fractions from wine and a condensed tannin preparation from sorghum were tested for their effects on melanogenesis of normal cells and growth of human melanoma cells. The wine phenolic fractions decreased melanogenic activity (tyrosinase activity) at concentrations that resulted in a slight variation in melanocyte viability. Sorghum tannins, however, increased melanogenic activity, although no increase was found in total melanin at the concentrations that least affect melanocyte viability. Incubation of human melanoma cells with the wine fractions and sorghum tannins resulted in a decrease in colony formation, although the effect was not dose dependent in all cases. These results suggest that all of these phenolic fractions have potential as therapeutic agents in the treatments of human melanoma, although the mechanisms by which cellular toxicity is effected seem to be different among the fractions.

Stimulation of melanogenesis by DNA oligonucleotides: effect of size, sequence and 5' phosphorylation

It has been shown that the small DNA fragment thymidine dinucleotide, (pTpT) induces photoprotective responses in cultured cells and intact skin. These responses include increased melanogenesis, enhanced DNA repair, and induction of TNF-alpha, and are accomplished, at least in part, through the induction and activation of the p53 tumor suppressor and transcription factor. Here it is reported that other, but not all, larger oligonucleotides induce the pigmentation response even more efficiently than pTpT. A 9 base oligonucleotide (p9mer) stimulated pigmentation in Cloudman S91 murine melanoma cells to 6-times the level of control cells while a 5 base oligonucleotide (p5mer#1) was inactive. In addition, the p9mer increased p21 mRNA levels and inhibited cell proliferation to a greater degree than did pTpT, consistent with the presumptive mechanism of action involving p53. Smaller, truncated versions of the p9mer also stimulated pigmentation, although to a lesser extent than did the p9mer. The ability of these oligonucleotides to stimulate pigmentation was highly dependent on the presence of a 5' phosphate group on the molecule, which was shown by confocal microscopy and fluorescent activated cell sorter (FACS) analysis to greatly facilitate the uptake of these oligonucleotides into the cells. Although the melanogenic activity of the oligonucleotides was directly related to increased length and 5' phosphorylation, nucleotide sequence is also critical because a p20mer was efficiently internalized yet was a poor inducer of pigmentation.

Topically applied eicosapentaenoic acid protects against local immunosuppression induced by UVB irradiation, cis-urocanic acid and thymidine dinucleotides

UVB-induced immunosuppression, a promoter of photocarcinogenesis, involves the formation of pyrimidine dimers and cis-urocanic acid (cis-UCA), but reactive oxygen species (ROS) also plays an important role. Eicosapentaenoic acid (EPA) can inhibit photocarcinogenesis, but due to its polyunsaturated nature it is susceptible to oxidative damage by ROS. The antioxidant defense system may therefore be challenged upon ultraviolet-B (UVB) irradiation in the presence of EPA. We investigated whether topically applied EPA in mice could protect against local immunosuppression (contact hypersensitivity response to dinitrofluorobenzene) induced by UVB radiation (1.5 J/cm2), or topically applied cis-UCA (150 nmol/cm2) or thymidine dinucleotides (pTpT) (5 nmol/cm2). The influence of EPA on epidermal lipid peroxidation and antioxidant status was also measured. UVB irradiation, cis-UCA and pTpT all caused 70% immunosuppression. Topical pretreatment of mice with EPA partially protected against immunosuppression; the EPA dose needed to accomplish this was 10 nmol/cm2 for UVB irradiation, 100 nmol/cm2 for cis-UCA and 1000 nmol/cm2 for pTpT. Higher EPA doses caused higher UVB-induced lipid peroxidation and lower vitamin C levels. Glutathione only decreased with the highest EPA dose whereas vitamin E was not decreased after UVB irradiation. In conclusion, topically applied EPA protects against UVB-, cis-UCA- and pTpT-induced immunosuppression and maintenance of an adequate antioxidant defense seems to be an important prerequisite for the protective action by EPA.

A role for NF-kappaB-dependent gene transactivation in sunburn

Exposure of skin to ultraviolet (UV) radiation is known to induce NF-kappaB activation, but the functional role for this pathway in UV-induced cutaneous inflammation remains uncertain. In this study, we examined whether experimentally induced sunburn reactions in mice could be prevented by blocking UV-induced, NF-kappaB-dependent gene transactivation with oligodeoxynucleotides (ODNs) containing the NF-kappaB cis element (NF-kappaB decoy ODNs). UV-induced secretion of IL-1, IL-6, TNF-alpha, and VEGF by skin-derived cell lines was inhibited by the decoy ODNs, but not by the scrambled control ODNs. Systemic or local injection of NF-kappaB decoy ODNs also inhibited cutaneous swelling responses to UV irradiation. Moreover, local UV-induced inflammatory changes (swelling, leukocyte infiltration, epidermal hyperplasia, and accumulation of proinflammatory cytokines) were all inhibited specifically by topically applied decoy ODNs. Importantly, these ODNs had no effect on alternative types of cutaneous inflammation caused by irritant or allergic chemicals. These results indicate that sunburn reactions culminate from inflammatory events that are triggered by UV-activated transcription of NF-kappaB target genes, rather than from nonspecific changes associated with tissue damage.

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.

Transcriptional activation of tyrosinase and TRP-1 by p53 links UV irradiation to the protective tanning response

We are exposed constantly to potentially harmful compounds and radiations. Complex adaptive protective responses have evolved to prevent such agents causing cellular damage, including potentially oncogenic mutation. The p53 tumour suppressor appears to have a role in co-ordinating such responses: it is activated by diverse insults and it acts as a transcriptional regulator of downstream genes that facilitate cellular adaptation. Ultraviolet (UV) light is a particularly potent inducer of p53 expression. In addition, UV light induces the production of melanin as a protection against further irradiation-induced damage. This study shows that the promoters of the genes coding for the enzymes crucial in melanin biosynthesis, namely tyrosinase and tyrosinase-related protein-1 (TRP-1), are activated by wild-type p53. Both promoters have p53-responsive elements and are activated in vivo in a dose-dependent manner by wild-type p53, as well as by the p53 homologues p73alpha and p63alpha.

Mutation spectrum of 4-nitroquinoline N-oxide in the lacI transgenic Big Blue Rat2 cell line

This paper describes the spectrum of mutations induced by 4-nitroquinoline N-oxide (4-NQO) in the lacI target gene of the transgenic Big Blue Rat2 cell line. There are only a few report for the mutational spectrum of 4-NQO in a mammalian system although its biological and genetic effects have been well studied. Big Blue Rat2 cells were treated with 0.03125, 0.0625 or 0.125 microg/ml of 4-NQO, the highest concentration giving 85% survival. Our results indicated that the mutant frequency (MF) induced by 4-NQO was dose-dependent with increases from three- to seven-fold. The DNA sequence analysis of lacI mutants from the control and 4-NQO treatment groups revealed an obvious difference in the spectra of mutations. In spontaneous mutants, transition (60%) mutations, especially G:C-->A:T transition (45%), were most frequent. However, the major type of base substitution after treatment of 4-NQO was transversions (68.8%), especially G:C-->T:A (43.8%), while only 25% of mutants were transitions. These results are consistent with those produced by 4-NQO in other systems and the transgenic assay system will be a powerful tool to postulate more accurately the mechanism of chemical carcinogenesis involved.

Do sunscreens increase or decrease melanoma risk: an epidemiologic evaluation

Ultraviolet adiation is an important cause of melanoma, so the use of sunscreen lotions has been advocated for melanoma prevention. Several arguments have been raised in opposition to this inference. Sunscreen use may interfere with cutaneous vitamin D synthesis, which some have hypothesized may lower melanoma risk. Sunscreen users may compensate for their sunscreen use by staying out much longer in the sun, or may use sunscreen lotions inconsistantly. Published melanoma case-control studies have not consistantly demonstrated a protective effect of sunscreens; however, these studies do not provide strong evidence, ultraviolet radiation is a known cause of melanoma, and ultraviolet B may be particularly potent, so on balance the evidence supports continued advocacy of sunscreen lotion use as part of an overall sun-protection regimen. Uncertainty will remain, however, until the action spectrum of melanoma is convincingly demonstrated or the methodologic limitations of existing epidemiologic evidence are overcome. The latter may require another decade or more of experience with sunscreen use.

A standardized protocol for assessing regulators of pigmentation

Varied effects of chemical or biological compounds on mammalian pigmentation have been reported by many groups, but to date, no standardized method has established necessary and/or optimal parameters for testing such agents. A standardized method has been developed to screen compounds with potential effects on pigmentation. The protocol comprises basic parameters to analyze melanogenic effects and allows for further characterization of candidate compounds, providing important insights into their mechanism of action. In this protocol (termed STOPR, for standardized testing of pigmentation regulators), compounds are initially screened using purified tyrosinase and are then tested on melanocytes in culture. After treatment of melanocytes with potentially bioactive compounds, cell proliferation and viability, total melanin accumulated, and melanogenic potential are measured. This protocol is an important first step in characterizing chemical regulation of effects on melanogenesis. When bioactive candidate compounds are identified, testing may proceed for pharmacological or otherwise commercial applications in coculture and/or organ culture models followed by in vivo testing. As an application of this method, results for compounds known to stimulate and/or inhibit melanogenesis (including arbutin, hydroquinone, kojic acid, melanocyte-stimulating hormone, and thymidine dimers) as well as some commercial skin whiteners are reported.

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.

Age-associated decreases in human DNA repair capacity: Implications for the skin

Multiple pathways are involved in accurate synthesis and distribution of DNA during replication, repair and maintenance of genomic integrity. An increased error rate, abovethe spontaneous mutation baseline, has been implicated in carcinogenesis and aging. Moreover, cytogenetic abnormalities are increased in Down's, Edwards', Patau's, and Klinefelter's syndromes with increasing maternal age, and in Marfan's and Apert's syndromes with paternal age. In response to DNA damage, multiple overlapping systems of DNA repair have evolved, preferentially repairing the transcribed strand within transcriptionally-active regions of the genome. These include direct reversal of dimers and specific adducts and pathways for base excision, nucleotide excision, and mismatch repair. A consensus has emerged that some DNA repair capacities decline with organism age, contradictory reports notwithstanding. As is the case for inborn defects in humans, knockout mice lacking components of nucleotide excision repair or DNA-damage checkpoint arrest have increased frequencies of skin and internal cancers, whereas mice overexpressing DNA repair genes have fewer spontaneous cancers. Oxidative stress and resultant free radicals can damage genomic and mitochondrial DNA; damage increases with age but decreases with caloric restriction. We review recent studies of long-lived C. elegans mutants which appear to involve metabolic attenuation, the role of telomere shortening and telomerase in cellular senescence, and the genetic bases of progeroid syndromes in humans. Finally, we discuss roles of extrinsic and intrinsic factors in skin aging, and their association with DNA damage, emphasizing preventive and protective measures and prospects for intervention by modulating DNA repair pathways in the skin.

Enhanced repair of benzo(a)pyrene-induced DNA damage in human cells treated with thymidine dinucleotides

The small DNA fragment thymidine dinucleotide (pTpT) stimulates photoprotective responses in mammalian cells and intact skin. These responses include increased melanogenesis (tanning) and enhanced repair of DNA damage induced by ultraviolet (UV) light. Here we show that pTpT treatment of human keratinocytes enhances their repair of DNA damaged by the chemical carcinogen benzo(a)pyrene (BP), as determined by increased expression of a transfected BP-damaged reporter plasmid containing the chloramphenicol acetyltransferase (CAT) gene. The pTpT-enhanced repair of this BP-damaged plasmid is accomplished at least in part through activation of the p53 tumor suppressor protein and transcription factor, because p53-null H1299 cells showed enhanced repair only if previously transfected with a p53-expression vector. To elucidate the mechanism of this enhanced DNA repair, we examined the expression of p21 and proliferating cell nuclear antigen (PCNA), proteins known to be regulated by p53, as well as the XPA protein, which is mutated in the inherited repair-deficient disorder xeroderma pigmentosum (XP) group A and is necessary for the recognition of UV-induced DNA photoproducts. The p53, PCNA and XPA proteins were all up-regulated within 48 h after the addition of pTpT. Taken together, these data demonstrate that pTpT-enhanced repair of DNA damaged by either UV irradiation or chemical mutagens can be achieved in human cells by exposure to small DNA fragments at least in part through the activation of p53 and increased expression of p53-regulated genes.

Bicyclic monoterpene diols induce differentiation of S91 melanoma and PC12 pheochromocytoma cells by a cyclic guanosine-monophosphate-dependent pathway

Previously, we showed that 5-norbornene-2,2-dimethanol (5-NBene-2,2-DM) is an effective inducer of melanogenesis in cultured cells and guinea-pig skin [Brown et al. (1998) J. Invest. Dermatol., 110:428-437]. This study shows that 2,3-cis/exo-pinanediol (2,3-cs/ex-PinD) is a more effective inducer of melanogenesis than 5-NBene-2,2-DM in S91 mouse melanoma cells. Furthermore, 2,3-cs/ex-PinD appears to penetrate guinea-pig skin better than 5-NBene-2,2-DM and to induce higher levels of pigmentation. Both 5-NBene-2,2-DM and 2,3-cs/ex-PinD induce synthesis of nitric oxide (NO) in S91 cells, and the melanogenic activity of both compounds is reduced by inhibitors of the NO/cyclic guanosine monophosphate (cGMP)/protein kinase(PK) G signaling pathway, but not by inhibitors of the PKC or PKA pathways. Thus, these bicyclic monoterpene diols appear to induce melanogenesis by the same pathway in S91 cells as that shown previously for ultraviolet radiation in melanocytes (Romero-Graillet et al. (1996) J. Biol. Chem., 271:28052-28056). These compounds also induce NO synthesis, neurite outgrowth, and tyrosine hydroxylase activity in PC12 pheochromocytoma cells. Neurite outgrowth in PC12 cells is blocked by the guanylate cyclase inhibitor, LY83583 (6-anilino-2,8-quinolinequinone), indicating that, similar to S91 cells, the induction of morphological differentiation of PC12 cells by bicyclic monoterpene diols is regulated by a cGMP-dependent pathway.

Thymidine dinucleotide mimics the effect of solar simulated irradiation on p53 and p53-regulated proteins

The tumor suppressor protein p53 participates in DNA repair and cell cycle regulation in response to injuries like ultraviolet (UV) irradiation. We have previously reported that the thymidine dinucleotide (pTpT), a common target for DNA photoproduct formation by UV light, mimics many effects of UV irradiation in cultured skin-derived cells, at least in part through the activation of p53. In this report we compare the effects of solar-simulated irradiation and pTpT on p53 and p53-regulated proteins involved in cellular growth arrest and DNA repair in cultured human dermal fibroblasts. We find that, like UV irradiation, pTpT increases the levels of p53, p21, and proliferating-cell nuclear antigen. The magnitude and time course of the inductions are UV dose dependent and consistent with known regulatory interactions among these nuclear proteins. These data confirm and expand previous studies of UV effects on nuclear proteins involved in cell cycle regulation and DNA repair. Our observations suggest that such protective effects can also be induced by pTpT in the absence of initial DNA damage, rendering cells more capable of responding to subsequent DNA damage.

THE DARK SIDE OF THE MYCELIUM: Melanins of Phytopathogenic Fungi

Melanins are darkly pigmented polymers that protect organisms against environmental stress. Even when not directly involved in pathogenesis, fungal melanin is likely required by melanizing phytopathogens for survival in the environment. However, some phytopathogenic fungi that produce melanized appressoria for host invasion require appressorial melanogenesis for pathogenicity. Much less is known about the role melanins play in pathogenesis during infection by other phytopathogens that do not rely on appressoria for host penetration. Here we focus on one such phytopathogenic fungus, Gaeumannomyces graminis var. tritici, the etiologic agent of the devastating root disease of cereals, take-all. This fungus is lightly pigmented in culture, but requires melanin biosynthesis for pathogenesis, perhaps to produce melanized, ectotrophic macrohyphae on roots. However, the constitutively melanized, asexual Phialophora anamorph of G. graminis var. tritici is nonpathogenic. In addition, melanization of G. graminis var. graminis is not required to produce root disease on its rice host. Explanations for these apparent contradictions are suggested, as are other functions for the melanins of phytopathogenic fungi.

Expression of the human XPB/ERCC-3 excision repair gene-homolog in the sponge Geodia cydonium after exposure to ultraviolet radiation

The marine demosponge Geodia cydonium encodes a gene, termed GCXPB, which displays 62% identity to the human XPB/ERCC-3 gene that specifically corrects the repair defect in xeroderma pigmentosum and in Cockayne's syndrome. The cDNA was isolated and characterized the deduced aa sequence, XPB_GEOCY, with the calculated size of 91,541 Da comprises the characteristic domains found in the related helicases. Phylogenetic tree analysis revealed that the sponge sequence is grouped to the metazoan related XPB/ERCC-3 polypeptides. Northern Blot analyses have been performed with sponge samples collected at different depths, thus exposed to different intensities of UV sunlight in the field. The intensity of the 2.6 kb band, corresponding to the transcripts of the sponge GCXPB gene was highest in those biotopes, which are closer to the surface of the sea, lower were the expressions in animals from a cave or from depths of 22 to 35 m. Controlled laboratory studies revealed that after irradiation of specimens with 300 or 1000 J/m2 UVB light a dose-dependent increase of the steady-state level of GCXPB occurs, values up to 29-fold with respect to the controls which were kept in the dark have been determined. In parallel, the DNA integrity in the sponge samples was measured using the sensitive Fast Micromethod assay. The data revealed that the degree of strand DNA breaks paralleled the increase of expression of the GCXPB gene. From these data it is concluded that the XPB/ERCC-3-like gene in the sponge G. cydonium is UV light-inducible and hence might be used as biomarker for UV light exposure in the field.

Human pigmentation genes and their response to solar UV radiation

Identification and characterisation of the genes involved in melanin pigment formation, together with the study of how their action is influenced by exposure to UV radiation, is providing a molecular understanding of the process of skin photoprotection through tanning. The mechanisms underlying this change in epidermal melanin involve both a transcriptional response of the pigmentation genes and post-translational control of the melanin biosynthetic pathway. UV rays are known to interact with numerous molecules within cells, and among these the photochemical reactions involving lipids and DNA are implicated in modulating melanogenesis. The combination of DNA damage, the formation of diacylglycerol, and the action of the melanocyte stimulating hormone receptor are all likely to be involved in UV-induced tanning.

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.

Potentials and limitations of the natural antioxidants RRR-alpha-tocopherol, L-ascorbic acid and beta-carotene in cutaneous photoprotection

Sun exposure has been linked to several types of skin damage including sun burn, photoimmunosuppression, photoaging and photocarcinogenesis. In view of the increasing awareness of the potentially detrimental long term side effects of chronic solar irradiation there is a general need for safe and effective photoprotectants. One likely hypothesis for the genesis of skin pathologies due to solar radiation is the increased formation of reactive oxidants and impairment of the cutaneous antioxidant system. Consequently, oral antioxidants that scavenge reactive oxidants and modulate the cellular redox status may be useful; systemic photoprotection overcomes some of the problems associated with the topical use of sunscreens. Preclinical studies amply illustrate the photoprotective properties of supplemented antioxidants, particularly RRR-alpha-tocopherol, L-ascorbate and beta-carotene. However, clinical evidence that these antioxidants prevent, retard or slow down solar skin damage is not yet convincing. The purpose of this review is to provide the reader with current information on cutaneous pathophysiology of photoxidative stress, to review the literature on antioxidant photoprotection and to discuss the caveats of the photo-oxidative stress hypothesis.

Thymidine dinucleotides induce S phase cell cycle arrest in addition to increased melanogenesis in human melanocytes

Although the induction of pigmentation following exposure of melanocytes to ultraviolet light in vivo and in vitro is well documented, the intracellular mechanisms involved in this response are not yet fully understood. Exposure to UV-B radiation leads to the production of DNA damage, mainly cyclobutane pyrimidine dimers, and it was recently suggested that the thymidine dinucleotide pTpT, mimicking small DNA fragments released in the course of excision repair mechanisms, could trigger melanin synthesis. We now report that the thymidine dinucleotide pTpT induces melanogenesis both in human normal adult melanocytes and in human melanoma cells. Thus, the SOS-like response suggested by Gilchrest's work to be evolutionary conserved, based primarily on work in murine cells and guinea pigs, is also apparently present in the human. Thymidine dinucleotide is nontoxic to melanoma cells and does not induce apoptosis in these cells, but induces S phase cell cycle arrest and a proliferation slow down. Because thymidine excess in culture medium leads to the synchronization of cells in S phase, we investigated whether this phenomenon was involved in the increase in melanin synthesis. We show that melanin synthesis is specifically triggered by the dimeric form of the thymidine and not by the monomeric form pT. Thus, our data strongly support that thymidine dinucleotides pTpT mimic at least part of the effects of ultraviolet irradiation, and may hence represent an invaluable model in the study of the molecular events involved in melanogenesis induction triggered through DNA damage.

Regulatory effects of heat on normal human melanocyte growth and melanogenesis: comparative study with UVB

Although energy-rich ultraviolet B (UVB) is considered to be primarily responsible for most of the effects associated with solar radiation, small energy recorded as heat appears to contribute to the biologic effects of solar radiation on the skin. We compared the effects of heat and UVB on normal human melanocyte functions. In monolayer culture the following was found. (i) Heat-treated melanocytes showed an increased dendricity and exhibited a larger cell body compared with nontreated melanocytes. (ii) After multiple treatments with UVB (20 mJ per cm2, 312 nm) or heat (42 degrees C for 1 h) for 3 d, melanocytes had a lower survival than nontreated melanocytes, but they resumed proliferation within 6 d in the same manner as seen in control. (iii) The expression levels of cell cycle regulators, p53 and p21 proteins, were increased after multiple treatments with UVB or heat. (iv) The tyrosinase (dopa-oxidase) activity per cell was increased after the multiple treatments with UVB or heat. (v) The number of dopa-positive melanocytes in coculture with keratinocytes in epithelial sheets was greatly increased by UVB or heat treatments. (vi) Similarly, the increased number of tyrosinase-related protein 1 positive melanocytes was seen in skin equivalents after UVB (100 mJ per cm2) or heat (42 degrees C for 1 h) treatments for 7 d. These results suggest that heat shares significant biologic activities with UVB in melanocyte functions. These results could be considered as one of the protective or adaptive responses of the skin pigmentary system to the environment.

UVB induces atypical melanocytic lesions and melanoma in human skin

A direct causal relationship between ultraviolet (UV) light in the B range and melanoma development has not been demonstrated in humans; this study aims to establish causality. A total of 158 RAG-1 mice, grafted with human newborn foreskin, were separated into four groups and observed for a median of 10 months: 1) no treatment, 2) a single treatment with 7,12-dimethyl(a)benzanthracene (DMBA), 3) UVB irradiation at 500 J/m2 alone, three times weekly, and 4) a combination of DMBA and UVB. Twenty-three percent of 40 normal human skin grafts treated with UVB only and 38% of 48 grafts treated with the combination of DMBA and UVB developed solar lentigines within 5 to 10 months of treatment. Melanocytic hyperplasia was found in 73% of all UVB-treated xenografts. Histological melanocytic changes resembling lentigo and lentigo maligna were seen in several skin grafts treated with both DMBA and UVB. In one graft of an animal treated with a combination of DMBA and UVB, a human malignant melanoma, nodular type, developed. This experimental system demonstrates that chronic UVB irradiation with or without an initiating carcinogen can induce human melanocytic lesions, including melanoma.

Supranuclear melanin caps reduce ultraviolet induced DNA photoproducts in human epidermis

Melanin can form supranuclear caps in human epidermis, suggesting that intracellular melanin reduces ultraviolet transmission to underlying cell nuclei and inhibits the formation of ultraviolet induced DNA photoproducts. The purpose of this study was to determine the photoprotective effect of epidermal melanin. We irradiated normal human skin explants with ultraviolet B and determined the formation of cyclobutane pyrimidine dimers and (6-4)photoproducts in individual epidermal cells by indirect immunofluorescence and by laser cytometry using monoclonal antibodies specific for cyclobutane dimers or for (6-4)photoproducts. We found that epidermal cells with supranuclear melanin caps had significantly less DNA photoproducts (both types) than epidermal cells without supranuclear melanin caps. Moreover, the protection factor against both types of photolesions correlated with melanin concentration in epidermal cells. These results indicate that melanin reduces ultraviolet induced DNA photoproducts in human epidermis in a concentration dependent manner.

Aliphatic and alicyclic diols induce melanogenesis in cultured cells and guinea pig skin

We have found that several aliphatic and alicyclic diols induce melanogenesis in cultured S91 mouse melanoma cells and normal human epidermal melanocytes (NHEM). In addition, these compounds induce melanogenesis when applied to guinea pig skin, with transfer of melanin to keratinocytes and formation of "supranuclear caps," as occurs in naturally pigmented skin. The relative order of potency of some of these diols in NHEM is 5-norbornene-2,2-dimethanol > 3,3-dimethyl-1,2-butanediol > cis-1,2-cyclopentanediol > 2,3-dimethyl-2,3-butanediol > 1,2-propanediol. Following treatment with these diols or 3-isobutyl-1-methylxanthine, melanin and tyrosinase activity are increased within S91 cells and NHEM; however, for cultured NHEM, the largest increases of melanin and tyrosinase occur in an extracellular particulate fraction, shown by electron microscopy to consist almost entirely of stage III and IV melanosomes. These results indicate that cultured NHEM treated with diols export melanosomes in a fashion that is commensurate with natural melanogenic processes. In contrast, S91 mouse melanoma cells exhibit aberrant melanosomal trafficking, in accordance with the known defect in myosin-V mediated melanosomal transport. Both S91 cells and NHEM exhibit morphologic changes and growth arrest indicative of differentiation following treatment with diols. The diols described in this report are candidates for use as cosmeceutical tanning agents.

Enhancement of DNA repair in human skin cells by thymidine dinucleotides: evidence for a p53-mediated mammalian SOS response

Thymidine dinucleotide (pTpT) stimulates melanogenesis in mammalian pigment cells and intact skin, mimicking the effects of UV irradiation and UV-mimetic DNA damage. Here it is shown that, in addition to tanning, pTpT induces a second photoprotective response, enhanced repair of UV-induced DNA damage. This enhanced repair results in a 2-fold increase in expression of a UV-damaged chloramphenicol acetyltransferase expression vector transfected into pTpT-treated skin fibroblasts and keratinocytes, compared with diluent-treated cells. Direct measurement of thymine dimers and (6-4) photoproducts by immunoassay demonstrates faster repair of both of these UV-induced photoproducts in pTpT-treated fibroblasts. This enhanced repair capacity also improves cell survival and colony-forming ability after irradiation. These effects of pTpT are accomplished, at least in part, by the up-regulation of a set of genes involved in DNA repair (ERCC3 and GADD45) and cell cycle inhibition (SDI1). At least two of these genes (GADD45 and SDI1) are known to be transcriptionally regulated by the p53 tumor suppressor protein. Here we show that pTpT activates p53, leading to nuclear accumulation of this protein, and also increases the specific binding of this transcription factor to its DNA consensus sequence.

Nuclear and non-nuclear targets of genotoxic agents in the induction of gene expression. Shared principles in yeast, rodents, man and plants

The interplay between environmental cues and the genetic response is decisive for the development, health and well-being of an organism. For some environmental factors a narrow margin separates beneficial and toxic impacts. With the increasing exposure to UV-B this dichotomy has reached public attention. This review will be concerned with the mechanisms that mediate a cellular genetic response to noxious agents. The toxic stimuli find access to the regulatory network inside cells by interacting at several points with cellular molecules - a process that converts the 'outside information' into 'cellular language'. As a consequence of such interactions, many adverse agents cause massive signal transduction and changes of gene expression. There is an interesting conservation of the mechanisms from yeast to man. An understanding of the genetic programs and of their phenotypic consequences is lagging behind.

The regulation of tyrosinase gene transcription

Tyrosinase is one of the key enzymes essential for melanogenesis. The control of its activity rests in part at the level of transcriptional regulation. The 5' promoter regions of the human, mouse, chicken, quail, snapping turtle, and frog tyrosinase sequences have been isolated and the mechanisms regulating the activity of these sequences are beginning to be elucidated. This review provides an update on the following aspects of tyrosinase gene regulation: basal promoter elements that determine the site of transcription initiation for RNA polymerase II; the cis-acting elements and DNA-binding factors that mediate melanocyte-specific expression of the tyrosinase gene; promoter elements involved in the temporal control of tyrosinase gene expression; additional elements that may be required to achieve wild-type levels of gene expression; and specific elements that may be required for modulation of tyrosinase gene expression in response to humoral factors or external stimuli that are known to influence the amounts of melanin synthesized by fully differentiated melanocytes. The wild type expression of tyrosinase is the result of the interaction of many different factors and it is becoming evident that certain elements and factors play more than one role in this process.

Nitric oxide produced by ultraviolet-irradiated keratinocytes stimulates melanogenesis

Ultraviolet (UV) radiation is the main physiological stimulus for human skin pigmentation. Within the epidermal-melanin unit, melanocytes synthesize and transfer melanin to the surrounding keratinocytes. Keratinocytes produce paracrine factors that affect melanocyte proliferation, dendricity, and melanin synthesis. In this report, we show that normal human keratinocytes secrete nitric oxide (NO) in response to UVA and UVB radiation, and we demonstrate that the constitutive isoform of keratinocyte NO synthase is involved in this process. Next, we investigate the melanogenic effect of NO produced by keratinocytes in response to UV radiation using melanocyte and keratinocyte cocultures. Conditioned media from UV-exposed keratinocytes stimulate tyrosinase activity of melanocytes. This effect is reversed by NO scavengers, suggesting an important role for NO in UV-induced melanogenesis. Moreover, melanocytes respond to NO-donors by decreased growth, enhanced dendricity, and melanogenesis. The rise in melanogenesis induced by NO-generating compounds is associated with an increased amount of both tyrosinase and tyrosinase-related protein 1. These observations suggest that NO plays an important role in the paracrine mediation of UV-induced melanogenesis.