From the bench to emerging new clinical concepts

Abstract

It is well accepted that growth and differentiation of keratinocytes and other cell types are regulated by 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], the active form of vitamin D. Moreover, growing evidence now indicates that the UV-B-mediated cutaneous photosynthesis of vitamin D (and the consecutive cutaneous production of 1,25(OH)₂D₃) represents an evolutionary highly-conserved endocrine system that protects the skin against environmental hazards that may promote skin cancerogenesis, including ultraviolet and ionizing radiation. It is the aim of this review to summarize our present understanding of the cutaneous vitamin D endocrine system (VDES) and its importance for the prevention of skin cancerogenesis.

Photocarcinogenesis of NMSC

Epidemiological data have shown that sunburns are implicated in the pathogenesis of squamous cell carcinoma (SCC),1,2,3 basal cell carcinoma (BCC)3,4 and malignant melanoma (MM).5,6 Today, it is accepted that chronic sun exposure is the most important etiological factor for the formation of non melanoma skin cancer.7 The solar UV-spectrum can be divided into several bands that vary in their physical and biological properties, namely UV-C (wavelength below 280 nm), UV-B (280–315 nm) and UV-A (315–400 nm).8 The predominant part of the short-wave, high-energy and destructive UV-spectrum cannot reach the surface of the earth. This is due to the fact that the ozone layer of the earth's outer atmosphere absorbs the shorter wavelength up to 310 nm (UV-C and part of UV-B radiation).8 The different layers of human skin absorb UV-radiation in a wavelength-dependent manner. Because UV-B radiation is almost completely absorbed by the epidermis, only 20% of UV-B radiation reaches the epidermal basal cell layer or the dermal stratum papillare.8 On the other hand, UV-A radiation penetrates deeper into the dermis and deposits 30–50% of its energy in the dermal stratum papillare. These absorption characteristics explain at least in part why UV-B effects (including skin cancer development) have to be expected predominantly in the epidermis and UV-A effects (including skin ageing, solar elastosis) in the dermis.8 It is well known that DNA represents a capital epidermal chromophore with an absorption maximum at 260 nm. Both UV-A and UV-B radiation are able to induce structural DNA-damage. UV-B radiation induces molecular rearrangements of the DNA resulting in the characteristic formation of specific photoproducts [most importantly cyclobutane pyrimidine dimers (CPD) and 6-4 photoproducts], which are known to be mutagenic. The genotoxic potential of UV-A radiation has been clearly shown to be predominantly due to indirect mechanisms that include oxidative damage. Gene mutations that have been shown to be of importance for the pathogenesis of skin cancer include mutations in the p53 gene [actinic keratosis (AK), SCC], and mutations in the patched (PTCH)/sonic hedgehog pathway (BCC). Mutation-associated inactivation of p53 tumor suppressor gene plays a critical role both for stages of initiation and progression of SCC.9 Analysis of data on gene mutations in human premalignant AK lesions, as well as data from UV-induced carcinogenesis experiments in mice have suggested that the first step involves acquisition of UV-induced mutations in the p53 gene by epidermal keratinocytes.9 This defect diminishes the formation of sunburn cells and enhances cell survival allowing retention of initiated, precancerous keratinocytes.9 Moreover, chronic exposures to solar UV results in the accumulation of p53 mutations in skin, which confer a selective growth advantage to initiated keratinocytes and allow their clonal expansion, leading to formation of AK.9 The expanded cell death-defective clones represent a larger target for additional UV-induced p53 mutations or mutations in other genes, thus enabling progression to carcinomas. Concerning the pathogenesis of BCC, the importance of PTCH, SMOH and TP53 mutations has been demonstrated.10 Suppression of the skin's immune system has been shown to represent another mechanism by which solar UV-radiation induces and promotes skin cancer growth, even at suberythemogenic doses.11 Immunosuppressive properties have been demonstrated for both UV-B and UV-A.11 Moreover, it has been speculated that UV-B-induced production of vitamin D may be involved in UV-B induced immunosuppression.12

UV Exposure and MM

Numerous epidemiologic investigations analyzing solar UV-exposure parameters have convincingly shown an association between the development of MM and short-term intense UV-exposure, particularly sunburns in childhood.6,13 It has been constantly demonstrated by numerous investigators that the incidence of MM increases with decreasing latitude towards the equator.14,15 In contrast to short-term intense exposure, more chronic and less intense exposure has not been found to be a risc factor for the development of MM. In fact, it has been found to be protective in many studies.16–19 Grass and Bopp have analyzed the MM mortality rates in different functional groups.19 They concluded that indoor working males (including employees with commercial oder technical education and graduates) had an increased risk, affirming hereby the association between melanoma risk and intermittent solar UV-exposure. On the other hand, outdoor workers with chronic solar UV-exposure appeared slightly protected.19

Vitamin D Metabolism in the Human Body

Vitamin D, the precursor of the biologically active vitamin D metabolite 1,25-dihydroxyvitamin D [1,25(OH)₂D, calcitriol] can be absorbed from the diet or synthesized in the skin under the influence of UV-B radiation from 7-dehydrocholesterol (7-DHC). Under living conditions in most countries in Europe and in the US, approximately 90% of the needed vitamin D have to be synthesized in the skin and only about 10% are taken up by the diet.34 All in all, nine enzymatic reactions are involved in the photochemical cutaneous synthesis of vitamin D, hereunder four photoreversible reactions and one non reversible phototransformation.20 While vitamin D₂ (ergocalciferol) can be found in plants, vitamin D₃ (cholecalciferol) is photochemically synthesized under the influence of UVB radiation in the skin of animals and humans. The biologically active vitamin D metabolite 1,25(OH)₂D, that circulates in the blood, is synthesized from vitamin D by a well characterized biochemical reaction cascade. First, it is hydroxylated in the liver in C-25 position by a cytochrome P450 enzyme, the vitamin D-25-hydroxylase (CYP27A1), before it gets hydroxylated a second time in the kidney in C-1 position by another cytochrome P450 enzyme, the renal 25-hydroxyvitamin D-1α-hydroxylase (CYP27B1).

While the production of 1,25(OH)₂D₃ in the kidney is regulated by a feedback-mechanism of the hormone itself, as well as by parathyroid hormone (PTH), calcium and phosphate, extra-renal CYP27B1 activity is regulated by other factors.21 As an example, macrophage CYP27B1 activity is regulated by pathogen-associated peptides such as lipopolysaccharide (LPS) and by cytokines like interferon γ (IFNγ), interleukin-2 (IL-2) or tumor necrosis factor α (TNFα).21 In the 1970s it was concluded that the kidney was the sole source of 1,25(OH)₂D₃ production. However, more recent in vitro trials and studies on anephric animals humans demonstrated that many cell types including human keratinocytes, monocytes, macrophages osteoblasts, prostate and colon cells, express the enzymatic machinery for the synthesis of 1,25(OH)₂D₃ (i.e. CYP27B1), and are able to synthesize 1,25(OH)₂D₃.21–23 In keratinocytes, studies could prove the presence of 1α-hydroxylase (CYP27A1) and 25-hydroxylase (CYP27B1).24,25 According to these findings, the keratinocyte is the only cell type known until today, that is able to synthesize 1,25(OH)₂D₃ from 7-dehydrocholesterol. From a present-day perspective, 1,25(OH)₂D₃ built in extrarenal tissues is not released into the blood circulation and is not involved in the regulation of bone- and calcium-metabolism, but rather in local regulation of various cellular functions including proliferation and differentiation.

The metabolism of 1,25(OH)₂D, circulating in the blood and present in various tissues, to calcitroic acid results from another well characterized biochemical reaction cascade. The reduction of 1,25(OH)₂D₃ to 24,25-dihydroxycholecalciferol in the kidney (and in other tissues) and consecutive reactions are effectuated by a third cytochrome P450 enzyme, the 1,25-dihydroxyvitamin D-24-hydroxylase (CYP24A1). 24,25-dihydroxycholecalciferol has only a slight biological activity. According to present knowledge 1,25(OH)₂D₃ has a 100 to 1000-fold higher biological activity as compared to other natural vitamin D metabolites.26

Physiological and Pharmacological Effects of 1,25-dihydroxyvitamin D₃ on Skin Cells

Inhibition of proliferation and induction of differentiation in keratinocytes.

Numerous in vitro and in vivo studies demonstrate dose-dependent effects of vitamin D analogs on cell proliferation and differentiation. At lower concentrations (< 10⁻⁸ M), 1,25(OH)₂D₃ promotes proliferation of keratinocytes in vitro, at higher pharmacological doses (≥ 10⁻⁸ M) keratinocyte proliferation is inhibited.27 In psoriatic skin, immunohistochemical and biochemical analyses have clearly shown antiproliferative and differentiation-inducing effects in epidermal keratinocytes along with treatment with vitamin D analogs in vivo.28,29 It has been demonstrated that the immunohistochemical staining pattern for various markers of epidermal proliferation [e.g. proliferating cell nuclear antigen (PCNA), Ki-67-antigen] and differentiation (e.g. involucrin, transglutaminase K, filaggrin, cytokeratin 10) changes in lesional psoriatic skin along with topical treatment with vitamin D analogs almost completely to the staining pattern characteristic for nonlesional psoriatic or normal skin.28,29 Although the mechanisms that underly the antiproliferative and differentiation-inducing effects of vitamin D analogs on keratinocytes are not completely understood, it is well known that these effects are at least in part genomic and mediated via the nuclear vitamin D receptor (VDR). It has been shown that keratinocytes from VDR-deficient mice do not respond in vitro to the antiproliferative effects of vitamin D analogs. In lesional psoriatic skin, the clinical improvement correlates with an increase of VDR mRNA in vitamin D analog-treated skin areas.30 However, not all patients with psoriasis respond to treatment with vitamin D analogs. It has been demonstrated that “responders” can be discriminated from “non-responders” by an increase in VDR mRNA in treated skin areas.30 These data suggest that the ability of 1,25(OH)₂D₃ to regulate keratinocyte growth is closely linked to the expression of VDR. The target genes of topical 1,25(OH)₂D₃ that are responsible for its therapeutical efficacy in psoriasis are still unknown. Mayor candidates for 1,25(OH)₂D₃ target genes that are responsible for the 1,25(OH)₂D₃-induced terminal differentiation in keratinocytes are distinct cell cycle associated proteins (i.e. INK4 family), including p21/WAF-1.

Immunmodulatory Effects in the Skin

During the last years, important new immunomodulatory effects of vitamin D analogs have been characterized.23,31–33 It has been demonstrated that various cell types involved in immunologic reactions (e.g. monocytes, T-and B-lymphocytes, Langerhans cells) do not only express VDR, but moreover possess the enzymatic machinery (CYP27B1) for the local synthesis of calcitriol.31 Today, the local synthesis of 1,25(OH)₂D₃ in immune cells is considered to be of high importance for the regulation and control of various immune responses. 1,25(OH)₂D₃ inhibits activation of T-cells and induces the generation of CD25+/CD4+ regulatory T-cells.32 In dendritic cells, 1,25(OH)₂D₃ inhibits maturation and induces a phenotype that promotes tolerance and inhibits immunity after stimulation with antigen.32,33 In dendritic cells, 1,25(OH)₂D₃ suppresses expression of MHC II molecules and of costimulatory molecules including CD40, CD80 and CD86.33 In these cells, production of IL-10 is stimulated and production of IL-12 inhibited, leading to suppression of T-cell activation.33

It has been concluded that vitamin D analogs may represent promising drugs for the treatment of diseases that are associated with impaired function of T-cells or dendritic cells (experimentally induced allergic encephalomyelitis, collagen-induced arthritis, autoimmune thyreoiditis, diabetes mellitus type I, graft-versus-host reaction).32,33 Furthermore, recent studies show that vitamin d deficiency may promote the pathogenesis of many autoimmune diseases like diabetes mellitus type I and that a sufficient vitamin D serum concentration may reduce the incidence of several autoimmune diseases. At present, a connection between vitamin D and pathogenesis of atopic dermatitis is discussed. Epidemiologic studies have indicated that patients with atopic dermatitis have a lower vitamin D intake as compared to controls.34 It has been demonstrated that vitamin D analogs suppress in vitro IgE-production and IgE-mediated cutaneous reactions.24 These immunomodulatory effects identify vitamin D analogs, most likely new vitamin D analogs with selective immunomodulatory activity, as promising new drugs for the prevention and therapy of inflammatory skin diseases including atopic dermatitis and allergic contact dermatitis. Furthermore, it has been shown that 1,25(OH)₂D represents a direct regulator of antimicrobial innate immune responses.35,36 It induces antimicrobial peptide gene expression in isolated human keratinocytes, monocytes and neutrophils, and, along with LPS synergistically induces cathelicidin antimicrobial peptide (camp) expression in neutrophils. Moreover, it has been reported that the expression of the VDR and the vitamin D-1αOHase (CYP27B1) genes were up-regulated by Toll-like receptor (TLR) activation of human macrophages, leading to induction of cathelicidin and killing of intracellular Mycobacterium tuberculosis. This suggests a link between TLRs and vitamin D-mediated innate immunity.35 New insights from the last years demonstrate that 1,25(OH)₂D₃ induces the expression of the CCR-10 receptor on the surface of T-cells, which leads to a migration of these T-cells towards CCL-27-expressing epidermal keratinocytes.5 This UVB-induced and vitamin D mediated T-cell mobilization from the blood vessels of the dermis into the epidermis characterizes another immunomodulatory effect of vitamin D: an on-demand rising of the T-cell answer in the epidermis.5 The clinical relevance of this function of vitamin D is not totally clarified until now and remains subject of further studies.

Regulation of Apoptosis in Human Keratinocytes by 1,25(OH)₂D₃

1,25(OH)₂D₃ has the potential to induce the neutral Mg²⁺-dependent sphingomyelinase, which hydrolyses sphingomyelin to ceramid.37 Interestingly, ceramid stimulates the prodifferentiating effect from 1,25(OH)₂D₃ on keratinocytes.38 Moreover, it plays an important role in the induction of apoptosis in a variety of cells, including keratinocytes.38 It has been demonstrated that physiological concentrations of 1,25(OH)₂D₃ in keratinocyte cultures do not induce apoptosis. To the contrary, physiological concentrations of 1,25(OH)₂D₃ generate an apoptosis-resistance against ceramides, ultraviolet radiation and tumor necrosis factor-α (TNF-α).39 The cytoprotective/antiapoptotic effect of 1,25(OH)₂D₃ is obviously linked to the development of sphingosine-1-phosphate. This is also clarified by the fact that the antiapoptotic effect of 1,25(OH)₂D₃ can be completely suppressed by addition of the sphingosinkinase-inhibitor N,N-dimethylsphingosine.39 In contrast, pharmacological concentrations of 1,25(OH)₂D₃ (≥ 10⁻⁶ M) do induce apoptosis. Similar effects have been observed in the regulation of the keratinocyte-growth, where, as outlined above, physiological concentrations of 1,25(OH)₂D₃ around 10⁻¹¹ M stimulate cell proliferation, whereas high pharmacological concentrations of 1,25(OH)₂D₃ have a dose-dependent antiproliferative effect.27

Cytoprotective Effects of 1,25(OH)₂D₃

The development of many types of skin cancer, including basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), is induced by UV-exposure.16 Whereas the total UV-dose added up in the exposed skin area during lifetime is responsible for the genesis of SCC,7 the origin of BCC is presumed both in the total UV-dose added up in the exposed skin area during lifetime and in higher intermittent UV-doses.3 In contrast, the development of malignant melanoma may be promoted as a result of higher intermittent UV-exposure in early childhood (0–6 years).13 Counted among these are already scattered suberythemal solar exposures and above all, mild and heavy sunburns. They lead to the appearance of multiple additional naevi, which are considered as a risk factor for the development of malignant melanoma in adults.13 xcessive UV-exposure, especially UVB with a wavelength range between 290 and 320 mn, is consequently a major risk factor in the development of skin cancer. On the other hand, sufficient doses of UVB are needed for the photochemical synthesis of vitamin D in the skin.40 This observation led to the speculation that the cutaneous vitamin D synthesis may act as an evolutionary highly conserved natural protection system against the hazardous effects of UVB radiation in the skin.1

During the last years, many studies confirmed that 1,25(OH)₂D₃ protects human keratinocytes at least in part against UVB-induced cell damage (Table 1).1,41–43 The underlying mechanisms are manifold and mainly associated with the suppression of UVB-induced apoptosis (reduction of cytochrom c-releasing, decreased PARP-cleavage41). The research group around De Haes et al. demonstrated cytoprotective effects of 1,25(OH)₂D₃ in UVB-irradiated keratinocytes using morphological and colorimetric vitality-assays.44 A significant cytoprotective effect resulted after a pretreatment with 1,25(OH)₂D₃ (≥ 10⁻⁸ M) for at least 8 hours. In this study, relatively high doses of 1,25(OH)₂D₃ (10⁻⁶ M respectively 10⁻⁷ M) were necessary to obtain a considerable cytoprotective effect.44 Using an ELISA that detects DNA-fragmentation they showed that pretreatment of keratinocytes with 1,25(OH)₂D₃ (1 µM) over 24 hours suppresses the UVB-induced apoptosis up to 55–70%,44 and reduces the mitochondrial cytochrome c release, one of the markers of UVB-induced apoptosis, up to 90%.44 Two important mediators of the UV-response, namely the activation of the c-Jun-NH₂-terminal kinase and the production of interleukin-6, are also reduced about 30% respective 75–90% by a pretreatment with 1,25(OH)₂D₃ (1 µM).44 Moreover, the UVB-induced cleavage of the Poly-(ADP-Ribose)-Polymerase (PARP) is inhibited, and methallothionein (MT)-mRNA is induced after pretreatment of keratinocytes with 1,25(OH)₂D₃ (1 µM for 24 h).44 MT is an antioxidans and acts as a radical catcher after UV-radiation.44 Moreover, 1,25(OH)₂D₃ protects keratinocytes against apoptosis by induction of anti-apoptotic proteins including Bcl-2 and activation of the MEK/ERK- and PI-3K/Akt-metabolic pathways.44 In line with these findings, experiments of Tremezaygues et al. using colony-forming-unit culture proliferation assays prove that pretreatment with 1,25(OH)₂D₃ (10⁻⁷ M for 48 h) protects human keratinocytes against the hazardous effects of a single irradiation with 100J/cm² UV-B. In this study, the number of cell colonies counted after a growth period for 7 days c-Jun-NH₂-terminal as twice as high in 1,25(OH)₂D₃-pretreated cells as compared to controls that were not treated with 1,25(OH)₂D₃.1 Furthermore, using WST-1- and crystal violet-based proliferation assays, it could be demonstrated that 1,25(OH)₂D₃ (10⁻⁷ M) has a photoprotective effect after irradiation of keratinocytes with ascending doses of UVB-light (100–1000 J/cm²).1 However, it has to be noted that it depends on many other factors whether effects of 1,25(OH)₂D₃ on apoptosis are beneficial or negative for the human body.

It is well recognized that the photocarcinogenesis of skin cancer is mainly due to mutations resulting from insufficient repaired DNA-photoproducts.43 The most established DNA-photoproducts caused by UV radiation are CPDs. Recent laboratory investigations show that 1,25(OH)₂D₃ reduces the number of CPDs in human keratinocytes after UVB-radiation.1,43 A study from Gupta et al. describes a reduction of the number of CPDs after a 24-hours-pretreatment with 1,25(OH)₂D₃ (10⁻⁹ M) followed by an irradiation of the cells with 200 mJ/cm² UVB, as compared to controls that were not treated with 1,25(OH)₂D₃.42 In line with these results, Tremezaygues et al. demonstrated that a 48-hours pretreatment of keratinocytes with 1,25(OH)₂D₃ (10⁻⁷ M) has a protective effect on the cells, even after irradiation with higher doses of UVB (100 J/cm² and 1000J/cm²).1 Complementing the results of Gupta et al., Trémezaygues et al. showed that pretreatment of HaCaT-keratinocytes with 1,25(OH)₂D₃ results not only in a reduction of the number of formed CPDs but also in a quicker reparation of the CPDs, as compared to controls that were not treated with 1,25(OH)₂D₃.1

Regarding the influence of vitamin D metabolites on the development of ionizing radiation damage, several investigations were performed during the last years. The characteristic radiation damage after ionizing radiation is the double strand break (DSB). The histone protein H2AX is phosphorylated in position 139 at the carboxyterminus as an answer to a double strand break—the result is γH2AX. This phosphorylated histone protein conduces to a recruition and retention of various repair proteins at the site of the DSB. It can therefore be considered as a marker for DSBs. Recent studies show a reduction of the immunoreactivity for γH2AX caused by ionizing radiation after pretreatment of the cells with 1,25(OH)₂D₃. A significant cytoprotective effect was proved after a 48-hour pretreatment of the cells with 1,25(OH)₂D₃ in a concentration of 10⁻⁷ M.1

To put it in a nutshell, the current literature supports the concept of a cytoprotective effect of 1,25(OH)₂D₃, which clinical potential has not seemed to have exhausted yet.

Antioxidative Effects

As outlined above, 1,25(OH)₂D₃ has a photoprotective effect on keratinocytes in vitro. It induces the production of the protein metallothionein in keratinocytes, whose antioxidative potential has been described several times.45,41 This may represent an important mechanism, protecting the cells against the UVB-induced synthesis of reactive oxygen radicals.

VDES and Skin Cancer: Malignant Melanoma

25(OH)D serum levels and melanoma risk and prognosis.

The association of 25(OH)D serum levels with risk and prognosis of malignant melanoma has been investigated 2009 by Newton-Bishop in a retrospective study of 271 patients with melanoma (Table 2).46 Nonrelapsers had higher mean 25(OH)D levels than relapsers (49 vs 46 nmol/l). In this study, higher 25(OH)D levels were associated with lower Breslow thickness at diagnosis and were independently protective of relapse and death: the hazard ratio for relapse-free survival was 0.79 for a 20 nmol/L increase in serum level. The cohort study provides evidence that higher 25(OH)D levels, at diagnosis, are associated with both thinner tumors and better survival from melanoma, independent of Breslow thickness.46 Nürnberg et al. also found out in another study that among the patients with malignant melanoma, significantly reduced serum 25(OH)D levels were found in the stage IV patients as compared to stage I patients.47 Furthermore, those with low 25(OH)D serum levels (<10 ng/ml) developed earlier distant metastatic disease compared to those with higher 25(OH) D serum levels (>20 ng/ml).47 Patients with melanoma, and those at high risk of melanoma, should seek to ensure vitamin D sufficiency. Additional studies are needed to establish optimal serum levels for melanoma-patients.

VDR-polymorphisms and melanoma risk and prognosis.

Lately, the relevance of vitamin D receptor (VDR) gene restriction fragment length polymorphisms for various types of cancer has been analyzed by a great number of studies. It has been assumed that VDR polymorphisms may influence both the risk of cancer occurrence and prognosis. Anyway, at present it is still not possible to make any definitive statements about the importance of the VDR genotype for cancer occurrence. It seems appropriate that interactions with other factors such as calcium and vitamin D intake, 25(OH)D plasma levels and UV radiation exposure play a determined role in cancer occurrence and should not be underestimated.48 Other risk factors such as smoking status, parity status, obesity, energy intake and others are also frequently mentioned as being more or less important for carcinogenesis depending on the VDR genotype.48 Strongest associations with VDR polymorphisms and cancer risk have been reported for breast cancer (Bsm1, Fok1), prostate cancer (Fok1) and malignant melanoma (Fok1).48 In a hospital-based case-control study, Hutchinson et al. found out that polymorphism at the FokI, but not TaqI, RFLP was associated with an altered risk of malignant melanoma (Table 3).49 More importantly, variant alleles were associated with increased Breslow thickness. Thus, homozygosity for variant alleles at both RFLP (ttff genotype combination) was significantly associated with thicker tumors.49 Thus, polymorphisms of the VDR gene, which would be expected to result in impaired function, are associated with susceptibility and prognosis in malignant melanoma.49 These data suggest that 1,25(OH)₂D₃, the ligand of the VDR, may have a protective influence in malignant melanoma. Futrthermore, Halsall et al. tested the hypothesis that the G(-1520)A(-1012) haplotype of the VDR promoter polymorphism might be a greater risk factor for MM than A-1012 alone.50 The A allele of A-1012G was preferentially linked to G of G-1520C and was more frequent in malignant melanoma patients but G of G-1520C was not. Halsall et al. found out that the CA haplotype was a very significant risk factor for malignant melanoma while the CG haplotype was protective.50 There was no effect of GA haplotype. The A allele of A-1012G was more frequent in patients with metastasis than malignant melanoma patients without metastasis, as was the G allele. The GA haplotype was more frequent in patients with metastasis, while frequencies of CA were similar. Halsall et al. suggest that the different roles of the A allele of A-1012G in susceptibility and metastasis risk may be a function of the availability of transcription factors in the differing cellular backgrounds related to susceptibility and progression of malignant melanoma.50

In a melanoma case-control comparisons for six vitamin D receptor (VDR) gene single nucleotide polymorphisms and serum-hydroxyvitamin D₃ levels Randerson-Moore et al. found out that the FokI T allele was associated with increased melanoma risk.51 In a meta-analysis in conjunction with published data from other smaller data sets (total 3769 cases and 3636 controls), the FokI T allele was associated with increased melanoma risk, whereas the BsmI A allele was associated with a reduced risk, in each instance under a parsimonious dominant model.51

Non-Melanoma Skin Cancer (NMSC)

25(OH)D serum levels and NMSC risk.

Tang et al. conducted a nested case-control study in a cohort of elderly men to analyze the association of NMSC with serum levels of 25(OH)D (Table 2).52 Health habit and medical history, including self-reported history of NMSC were recorded and 25(OH)D serum levels were measured.52 Mean age (73 +/− 5), BMI, daily vitamin D and calcium intake were similar in the men with (n = 178) and without NMSC (n = 930).52 Tang et al. found out that higher levels of 25(OH)D were associated with a decreased risk of having a history of NMSC. According to the results of the study, men in the highest quintile of 25(OH)D (>30 ng/mL) had 47% lower odds of NMSC compared to those in the lowest quintile. These results suggest that high 25(OH)D levels may be associated with a reduced risk of NMSC.52

VDR-polymorphisms and NMSC risk.

Analyzing genetic variants of the VDR, Carless et al. found a significant difference in genotype frequencies of the TaqI polymorphism between populations with and without solar keratosis (SK)(Table 4).53 They found a trend for homozygote genotypes to increase the odds of SK, which suggests that intermediate VDR activity is important in protection or that the heterodimer formed by a heterozygous genotype may have an altered binding potential.53 Overall, these analyses indicate that VDR may be important in SK development. Furthermore, Han et al. evaluated genetic polymorphisms in the methylenetetrahydrofolate reductase (MTHFR) gene (C677T and A1298C) and the vitamin D receptor (VDR) gene (Fok1, Bsm1 and Cdx2) with skin cancer risk in a nested case-control study within the Nurses' Health Study [219 melanoma, 286 squamous cell carcinoma (SCC), 300 basal cell carcinoma (BCC) and 873 controls].54 They observed an interaction between the C677T polymorphism and total folate intake on SCC risk; the highest risk was observed among women with TT genotype and low folate intake.54 The VDR Bsm1 BB genotype was significantly associated with an increased SCC risk.54 An interaction between the Bsm1 polymorphism and total vitamin D intake on SCC was observed, with the highest risk seen in women with the BB genotype and high vitamin D intake.54 This study suggests a possible role of the polymorphisms in MTHFR and VDR interacting with dietary intakes of folate and vitamin D in skin cancer development, especially for SCC.54

Conclusions

Growing evidence indicates that the UV-B-mediated cutaneous photosynthesis of vitamin D [and the consecutive cutaneous synthesis of 1,25(OH)₂D₃] represents an evolutionary highly-conserved endocrine system that protects the skin against environmental hazards that may promote skin cancerogenesis, including ultraviolet and ionizing radiation. 1,25(OH)₂D₃ regulates important cellular functions, including cellular growth, in a broad variety of different cell types, including human keratinocytes. Furthermore, 1,25(OH)₂D₃ has immunmodulatory, antioxidative and cytoprotective effects in the skin and plays a role in the regulation of apoptosis. Finally, significant associations with VDR polymorphisms and/or 25-hydroxyvitamin D serum levels have been reported in malignant melanoma as well as in non melanoma skin cancer. The association with Vitamin D intake is less clear and further studies could be useful to clarify the role of diet. In summary, growing evidence now indicates that the cutaneous vitamin D endocrine system is of importance for pathogenesis and progression of MM and NMSC. Pharmacologic modulation of the vitamin D endocrine system, e.g. with vitamin D analogs, may represent a promising new concept for the prevention and/or therapy of skin cancer.

Figures and Tables

Table 1 Cytoprotective effects of 1,25(OH)₂D₃: A selection of studies

Study design	Cell type	Results	Reference
Experimental study	Keratinocytes	Suprression of UVB-induced apoptosis (up to 55–70%) and reduction of mitochondrial cytochrome c release (up to 90%) after pretreatment with 1,25(OH)2D3 (1 µM for 24 h)	44
Experimental study	Keratinocytes	Induction of (MT)-mRNA	44
Experimental study	Keratinocytes	induction of anti-apoptotic proteins (including Bcl-2) and activation of the MEK/ERK- and PI-3K/Akt-metabolic pathways	44
Experimental study	Keratinocytes	↑ proliferation after UV-B treatment	1
Experimental study	Keratinocytes	↑ vitality after UV-B treatment	1
Experimental study	Keratinocytes	↑ DNA-damage (CPD, DSB) after UV-B or ionizing radiation	1
Experimental study	Keratinocytes	quicker reparation of CPDs after UV-B treatment	1

Table 2 VDR polymorphism in malignant melanoma (MM): A selection of studies

Study design	Polymorphism	Results	Reference
Case-control study	Fokl T Taql	Altered risk of MM Risk of MM not altered	49
Case-control study	Fokl T Bsml A	Increased risk of MM Reduced risk of MM	49

Table 3 VDR polymorphisms in non melanoma skin cancer (NMSC): A selection of studies

Study design	Polymorphism	Results	Reference
Unknown	Taql	→ risk of solar keratosis	53
Case-control study	Bsm1	→ SCC risk	54

Table 4 25(OH)-D serum levels and skin cancer risk and prognosis: a selection of studies

Study design	Results	Reference
Retrospective cohort study	(1) Higher 25(OH)D levels protective of MM relapse and death (2) higher 25(OH)D levels associated with lower Breslow thickness at time of diagnosis of MM	46
Case-control study	Among the patients with malignant melanoma, significantly reduced serum 25(OH)D levels were found in stage IV patients as compared to stage I patients	47
Case-control study	Higher levels of 25(OH)D associated with decreased risk of having a history of NMSC	52