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Keywords:

  • dermal fibroblast;
  • skin cancer;
  • vitamin D

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. The role of vitamin D in skin cancer development
  5. Development of cutaneous carcinogenesis
  6. Non-melanoma skin cancer, tumour stroma and angiogenesis
  7. Significance of the vitamin D receptor
  8. Conclusions and future perspectives
  9. Declarations
  10. References

Objectives  Very limited information is available on the role of vitamin D in skin carcinogenesis. For most individuals, skin cancer can be readily managed with surgery; however, some patients may face life-threatening neoplasia. Sun exposure, specifically UV radiation, is a causative agent for development of skin cancer, though, somewhat ironically, sunlight through the production of vitamin D may have protective effect against some skin cancers. This review focuses on the development and progression of cutaneous carcinogenesis and the role of vitamin D in the prevention of the initiation and progression of lethal skin cancers.

Key findings  Vitamin D is involved in regulation of multiple signalling pathways that have implications in carcinogenesis. Skin cancer metastasis depends on the tumour microenvironment, where vitamin D metabolites play a key role in prevention of certain molecular events involved in tumour progression. The vitamin D receptor (VDR) is a well-known potent regulator of cellular growth and differentiation.

Summary  The VDR's possible involvement in cell death, tumour microenvironment and angiogenesis makes it a candidate agent for cancer regulation.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. The role of vitamin D in skin cancer development
  5. Development of cutaneous carcinogenesis
  6. Non-melanoma skin cancer, tumour stroma and angiogenesis
  7. Significance of the vitamin D receptor
  8. Conclusions and future perspectives
  9. Declarations
  10. References

There are different types of skin cancers originating in the epidermis and/or the dermis of the skin that can be classified as melanoma skin cancer (MSC) or non-melanoma skin cancer (NMSC). Australia has the highest incidence of skin cancer in the world. These figures are rising very quickly according to the Australian Bureau of Statistics (ABS). In Australia, every year, more than 374 000 people are diagnosed with NMSC (ABS). These numbers significantly contribute to the globally estimated diagnosis of NMSC of 2.5 million people per year (World Health Organization, WHO).

Ultraviolet (UV) irradiance is the major causative factor for these cancers. Numerous studies have shown a direct association between solar irradiance and an increased risk of skin cancers.[1–7] This is of major significance to Australians as MSC and NMSC are common and important public health concerns.

Clinical studies provide strong evidence supporting the importance of diminished plasma vitamin D levels and the incidence of colon, breast, prostate and skin cancers.[3,8–16] Sun exposure is a requirement for adequate endogenous vitamin D formation and is critical for skeletal health.[1,17,18] On the other hand, unprotected and excessive sun exposure is the most significant risk factor for the development of skin cancers, such as melanoma and NMSC, because solar irradiance acts as a major promoter of skin carcinogenesis.[18,19] It has been suggested that vitamin D reduces the risk of mortality from a variety of cancers, presumably by signalling via the vitamin D receptor (VDR). Studies reporting that the VDR induces cellular differentiation and inhibits proliferation.[20–22] Numerous studies have identified that vitamin D metabolites play a crucial role in the development or prevention of cutaneous carcinogenesis.[23–28]

It remains unclear whether an adequate vitamin D level helps to prevent cutaneous carcinogenesis or perhaps even reverse the earliest signs of it. The principal roles of the key metabolites of the cutaneous vitamin D pathway remain to be discovered, as does whether they play a crucial role in the development of NMSCs. In this review, we critique the present literature in an attempt to understand the significance of local cutaneous vitamin D synthesis and its influence on NMSC development and progression.

The role of vitamin D in skin cancer development

  1. Top of page
  2. Abstract
  3. Introduction
  4. The role of vitamin D in skin cancer development
  5. Development of cutaneous carcinogenesis
  6. Non-melanoma skin cancer, tumour stroma and angiogenesis
  7. Significance of the vitamin D receptor
  8. Conclusions and future perspectives
  9. Declarations
  10. References

Accumulation of genetic alterations over a lifetime of excessive sun exposure, viral infections and immune-compromised conditions are the major risk factors for development of NMSC. Sun exposure has been linked to pathological conditions of the skin, including sunburn, skin aging and melanomas, and NMSC. Studies have shown that UVA exposure can cause oxidative stress. Skin acts as a first line of defence against oxidative stress (damage via free radicals). Mice exposed to UVA (340–400 nm) showed increases in squamous cell carcinoma (SCC) formation.[29] It has been demonstrated that UVA can increase the number of melanomas after irradiation by UVB in mice.[30,31] It has been shown that UVA irradiance causes tissue damage by free radical generation and this damage can be prevented by antioxidants.[32] Apart from UV irradiance, a high intake of meat products, saturated fat and dairy products have been reported to be linked to the risk of skin cancer development.[33–35]

The endogenous synthesis of vitamin D and its role in the prevention of cutaneous carcinogenesis is not very well understood and accepted. It is known that vitamin D synthesis in the epidermis has several protective qualities.[36,37] Vitamin D production is initiated in the skin by UVB-induced formation of pre-vitamin D3 (or colecalciferol), which is followed by a multiple-step synthesis of the active hormone calcitriol (1a,25(OH)2D3) in the liver and the kidney. The established function of calcitriol is endocrine regulation of calcium homoeostasis and bone mineralisation. It has been demonstrated that low physiological doses of calcitriol stimulate keratinocyte growth and proliferation.[36–38] Some of the genetic alterations that have been induced by chronic sun exposure in individuals with pre-malignant skin lesions can be reverted via well-balanced and maintained vitamin D supplementation (or its endogenous accumulation under solar irradiance).[38,39] It has been shown that people newly diagnosed with MSCs are less prone to the subsequent development of malignant melanoma if they have sufficient levels of vitamin D.[38,39] An inverse relation between vitamin D levels and risk of malignant melanoma have been identified in numerous studies.[40]

The protective role of vitamin D has been proposed as a result of numerous in-vivo and in-vitro studies.[3,15,19,22,37,41–46] However, it must be emphasised that the biochemical and molecular pathways underpinning vitamin D action are still not fully understood in individuals with NMSC.

Development of cutaneous carcinogenesis

  1. Top of page
  2. Abstract
  3. Introduction
  4. The role of vitamin D in skin cancer development
  5. Development of cutaneous carcinogenesis
  6. Non-melanoma skin cancer, tumour stroma and angiogenesis
  7. Significance of the vitamin D receptor
  8. Conclusions and future perspectives
  9. Declarations
  10. References

NMSCs include pre-cancerous skin lesions such as aktinic keratoses (AKs), in-situ malignancies such as Bowen's disease and invasive types of skin cancers, SCC and basal cell carcinoma (BCC). The history of NMSC involves progression through defined pathological and clinical stages, starting with stratum basale hyperproliferation and subsequent evolution into squamoid neoplasm, such as AKs or SCC in situ, and finally into invasive carcinomas. Cutaneous carcinogenesis occurs via UV irradiance-induced genetic mutations or inherited dispositions. AK is a known precursor in the development of cancerous lesions, mostly SCC, when skin is continuously exposed to the detrimental influence of the sun.

Solar irradiance at different wavelengths is able to penetrate the epidermal (UVB) and dermal (UVA) cell layers where the initial mutation occurs in the cells leading to formation of pre-cancerous lesions. UVB (280–320 nm) is capable of penetrating the epidermal cell layers (in particular stratum papillare and stratum basale)[47] where predisposed keratinocytes that already have a malfunction in vitamin D metabolism can accumulate further mutations (Figure 1).[47,48] These mutations occur in the epidermis, specifically in the keratinocytes of the stratum papillare and stratum basale where UVA has the ability to penetrate deep within the skin to affect the dermis. UVB light can induce DNA damage and it can also generate reactive oxygen species (ROS), whereas UVA exposure induces mostly oxidative stress and ROS production. It is known that for SCC to occur, cumulative UV exposure is required.[39] In addition, the development of BCC is linked to events of extreme and intense UV exposure in life.[39] The precursors of BCC have not been identified despite reports suggesting that BCC initiation has a strong connection with interfollicular basal cells, hair follicles and the sebaceous glands that are mostly found in the dermal layer of the skin.[39,47,49]

image

Figure 1. Vitamin D modulation of cutaneous carcinogenesis. Activation of cutaneous vitamin D synthesis via UVB; subsequent protection from DNA mutations and damage from free radicals. NMSC, non-melanoma skin cancers: AKs, aktinic keratoses, BCC, basal cell carcinoma, SCC, squamous cell carcinoma.

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Tumour initiation and progression in BCC and SCC are predominantly driven by accumulated DNA mutations and the tumour microenvironment. Recent studies indicate that vitamin D metabolites and VDRs are the strongest determinants in the progression of cutaneous carcinogenesis.[21,36,49–51] It has been demonstrated that VDR mRNA and protein are strongly expressed in BCC tumours compared with normal keratinocytes.[49] These findings indicate that the VDR could be involved in the growth regulation of these tumours. Other reports suggest that lack of VDR expression is associated with reduction in epidermal differentiation in mice and humans.[23,24,51,52] Epidermal keratinocytes are the primary source for vitamin D synthesis. Multiple functions of the skin are mediated by vitamin D via its receptor. This involves suppression of proliferation, stimulation of differentiation, promotion of immune response and regulation of neoplastic formation and development.[24,25,38,45,53,54]

Non-melanoma skin cancer, tumour stroma and angiogenesis

  1. Top of page
  2. Abstract
  3. Introduction
  4. The role of vitamin D in skin cancer development
  5. Development of cutaneous carcinogenesis
  6. Non-melanoma skin cancer, tumour stroma and angiogenesis
  7. Significance of the vitamin D receptor
  8. Conclusions and future perspectives
  9. Declarations
  10. References

Dermal fibroblasts synthesise and organise skin connective tissue as well as play a major role in tissue stroma formation.[55,56] As for other tumours, skin cancer development depends on the interactions between stromal and tumour cells.[4,31,54,57–59] For example, BCC tumours do not metastasise due to the lack of angiogenic and other growth factors (such as those of the epidermal growth factor (EGF) family) that regulate cell cycle or regeneration of the epidermis from the stratum basale to the stratum corneum. These growth factors are released by stromal cells and are necessary to promote further tumour progression and metastasis.

The highly vascular dermal layers contain an arrangement of collagen fibres, hair follicles and associated sebaceous glands.[39,60] The hypothesis that dermal fibroblasts produce an inactive form of vitamin D3 that can be activated by epidermal keratinocytes has been tested in in-vitro studies by Vantieghem et al.[25,60] Their study showed that upon UVB irradiation, cultured dermal fibroblast cells produced inactive hormone 25(OH) D3, or calcidiol, the major circulating form of vitamin D. 25(OH)D3 is synthesised in the liver and its measurement is used to assess vitamin D status in the body. 25(OH)D3 was converted into active hormone by cultured epidermal keratinocytes when media from irradiated fibroblasts cells was transferred to epidermal keratinocytes cultures.[45]

It is yet to be established whether dermal fibroblasts provide the vitamin D precursor (25(OH)D3) for cutaneous vitamin D synthesis by keratinocytes (into the active hormone 1a,25(OH)2D3) (Figure 2). It has been reported that 1a,25(OH)2D3 in physiological concentrations promotes keratinocyte survival and minimises DNA mutagenesis, whereas in the dermis, physiological doses of calcitriol have inhibitory actions on fibroblast proliferation. Pro-vitamin D (or 25(OH)D3), which is produced by dermal fibroblasts, is possibly distributed by the rich network of blood vessels flowing through this dermal layer. Failure to synthesise calcidiol in dermal fibroblasts may be associated with vitamin D deficiency and related disorders.

image

Figure 2. Cutaneous vitamin D metabolism and its mediation of tumour stroma formation. 25(OH)D3 protective effect on keratinocytes and dermal tissue. 1a-Hydroxylase is downregulated in the dermis: supplementation of 25(OH)D3 for systemic synthesis of 1,25a(OH)2D3. 7DHC, 7 dehydrocholesterol; 25(OH)D3, 25-hydroxyvitamin D3 (calcidiol); 1a25(OH)D3, 1a,25-dihydroxyvitamin D3 (calcitriol); 1a-Hydroxylase, CYP27B1.

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The skin-damaging events of sun exposure mostly occur via accumulation of free radicals. ROS-mediated oxidative damage can alter DNA, proteins and lipids and cause numerous genetic alterations.[53] Some of these genetic alterations have direct correlation with skin cancer development. The p53 tumour suppressor gene is well known as a regulator of cell cycle and programmed cell death. Mutations in, or loss of, the p53 gene have been detected in all types of skin cancers. This suggests that these alterations are linked to tumour formation. Study showed that inactivation of the p53 triggers the induction of skin cancer by UV irradiation.[53]

Vitamin D has been reported to be involved in the prevention of some molecular events that control the initiation and promotion of NMSC (such as alterations in expression of p53, p21, p27, Ras, hedgehog (Hg) pathway, patched family gene (particularly protein patched homologue 1 – PTCH1) and numerous other genes).[26,39,48,61,62] Epigenetic regulation of gene expression within the cell is crucial for the prevention of tumour initiation and progression. An epigenetic modification involves DNA methylation of cytosine residues of CpG dinucleotides. These epigenetic alterations could play a role in regulating vitamin D metabolism. It was demonstrated that decreased methylation of VDR CpG islands was associated with enhanced expression of the VDR and protection against chemically induced colon cancer in a mouse model.[63] The role of vitamin D in the prevention of skin cancer requires a deeper understanding as it is a naturally produced compound in the human body that could influence the progression of skin cancers.[13,21,31,50]

Studies show the importance of the tumour environment, most commonly the underlying or surrounding stroma.[64,65] Less emphasis has been placed on the influence of vitamin D on growth related to angiogenesis in the stroma and metastatic potential of the tumour. The signalling pathways and local communication between dermal tumour cells and their neighbours within the epithelium at tumour boundaries remain to be elucidated.

Both metabolites of vitamin D, 25(OH)D3 and 1a,25(OH)2D3, can play a vital regulatory role in the cell cycle.[22,34,66] It is well established that 1a,25(OH)2D3 influences cellular growth and differentiation.[24,36,44,67,68] 1a,25(OH)2D3 acts in the prevention and treatment of malignancy via both anti-proliferative and pro-differentiating mechanisms. Cultured normal keratinocytes respond to 1a,25(OH)2D3 with a reduction in proliferation and an increase in differentiation. In human and animal studies, 1,25a(OH)2D3 showed a stimulatory effect on keratinocyte differentiation when administered orally or topically.[69]

Numerous in-vitro studies have shown that 1,25a(OH)2D3 inhibits the growth of breast, colon and prostate cancer cell lines.[10,12,14,17,20,41] Zinser et al. reported that in cultured keratinocytes and malignant melanocytes calcitriol induced cell cycle arrest.[70] The ability of keratinocytes to accumulate high levels of vitamin D provides protection against the increase in ROS and induction of cell death which occur as a result of UVA exposure.[19,22,36,42] 1,25a(OH)2D3 enhances the anti-apoptotic effect associated with cellular survival in primary keratinocyte cultures.[25] The influence of 25(OH)D3 on cell proliferation and differentiation has been demonstrated by in-vitro studies.[68,71] The anti-proliferative actions of vitamin D in the form of calcitriol has been found to be weaker compared with the effect of 1,25a(OH)2D3.[68] Tuohimaa et al. demonstrated in in-vitro studies that 25(OH)D3 significantly inhibits growth of human prostatic stromal cells at a high physiological concentration (250 nm) whereas pharmacological (toxic) concentrations of calcitriol (10 nm) were required to achieve the same effect.[68] Based on these studies we speculate that 25(OH)D3 produced endogenously by cutaneous fibroblasts may have a preventative or even a defensive role against skin carcinogenesis.

In addition to these studies, vitamin D has been found to regulate cell cycle progression by induction of p21 and p27 proteins that interact with cyclin and cyclin-dependent kinase (CDK) complexes.[22,72,73] Anti-proliferative effects of 1,25a(OH3)D3 and 25(OH)D3 have been demonstrated in in-vivo and in-vitro studies in mice and humans.[21,22,45,48,70,72–74] Cultured malignant melanocytes and keratinocytes have a rapid anti-proliferative response to vitamin D treatment.[36] In addition to the inhibitory actions on malignant keratinocytes, calcitriol demonstrates a strong suppressive effect on the expression of proto-oncogenes, such as c-myc, c-jun and c-fos,[22,68] enhanced expression of the pro-apoptotic protein p53[48,69] and prevention of p120 protein downregulation, which prevents tumour cell migration and inhibition of angiogenesis.[75]

The ability of the skin to locally synthesise 25(OH)D3 and 1,25a(OH)2D3 (Figure 2), independently from the general systemic vitamin D pathway[22,36] is not completely understood. In circulation 25(OH)D3 binds to the vitamin D binding protein (VDBP) to reach targeted tissues and possibly promotes formation of intracellular 1,25a(OH)2D3 in a tissue-specific manner.[41,43,66,76,77] It has been reported that the primary function of localised skin 1,25a(OH)2D3 is protection from UV-induced DNA mutations and repair of the mutation sites.[18,46] It also has been shown in in-vitro studies that 1,25a(OH)2D3 promotes differentiation of epidermal keratinocytes.[10,13,26,44,67,78] The prevention of mutations in keratinocytes in turn prevents further progression of carcinogenesis in the form of BCC, SCC and malignant melanoma. Localised cutaneous vitamin D formation may promote resistance to angiogenesis and metastasis as well as possibly have an inhibitory effect on the development of skin tumours. Cellular uptake and formation of 25(OH)D3 and localized cutaneous synthesis of 1,25a(OH)2D3 may differ in tumour cells compared with non-tumour cells.

Measurement of circulatory calcidiol levels is the most common way to determine overall vitamin D status. However, it does not reflect the status of locally produced vitamin D, which is often stored in skin, bone, muscle, breast, prostate, intestine and other tissues.[12,22] The optimal level of serum 25(OH)D3 is yet to be determined but many studies suggest a level above 50 ng/ml as favourable.[3,12,13,66,79,80] It has been reported that decreased circulatory levels of 25(OH)D3 are associated with a greater risk of development of lung, colorectal, prostate, mammary and skin cancers.[12,22,45,66,68] There is a link between low circulatory calcidiol status and increased incidence and mortality from gastrointestinal cancers.[81] Clinical studies have suggested that lower circulatory vitamin D levels in cancer patients are also linked to poor cancer prognosis and survival.[3,19,41,82,83] Mice with impaired vitamin D metabolism are more likely to develop skin cancers than control mice with adequate vitamin D homoeostasis.[21,70,74,84,85]

Correlation between reduced levels of vitamin D and increased incidence of skin cancer may be associated with the role of vitamin D in cellular homoeostasis and function at the biochemical and molecular levels.[22,45,69,80] Recent research examined the possibility of an inverse relation between risk of developing of skin cancers and vitamin D uptake. The finding showed the connection between low dietary vitamin D intake and increases in melanoma risk through a population-based case-control study.[40] Another study determined the relationship between levels of vitamin D3 and non-melanoma skin cancer (NMSC); in a case-controlled study in elderly men, it was found that high 25(OH)D levels may be associated with a reduced risk of NMSC.[86]

A failure at any given metabolic step of vitamin D synthesis may contribute to vitamin D deficiency and possibly influence skin cancer development and progression. The possible alterations in localised synthesis of vitamin D metabolites and altered serum levels of 25(OH)D3 and 1,25a(OH)2D3 may be associated with the development and progression of skin carcinogenesis. It is not yet fully understood whether localised synthesis of 25(OH)D3 and 1,25a(OH)2D3 in the skin contributes to vitamin D inhibition of cutaneous carcinogenesis. Therefore, studies teasing out the mechanism of vitamin D action in skin cancer tissue are warranted.

Significance of the vitamin D receptor

  1. Top of page
  2. Abstract
  3. Introduction
  4. The role of vitamin D in skin cancer development
  5. Development of cutaneous carcinogenesis
  6. Non-melanoma skin cancer, tumour stroma and angiogenesis
  7. Significance of the vitamin D receptor
  8. Conclusions and future perspectives
  9. Declarations
  10. References

The biological actions of vitamin D are mediated by the VDR. While studies show that VDRs regulates cell differentiation, proliferation and apoptosis, the physiological significance of this regulation remains to be established. Vitamin D minimises the risk of mortality from a broad range of cancers, via its signalling mechanisms via the VDR which can eventually lead to cellular proliferation and differentiation.[24,44,45,52] 1a,25(OH)D3 is the biologically active ligand for the VDR. Since 1,25(OH)2D3 promotes differentiation in keratinocytes, it could prevent potential malignancy in these cells.[47]

Regulation of skin cancer by VDRs occurs through mediation of anti-proliferative genes that are involved in the regulation of the cell cycle.[3,17,87] The binding of 1,25a(OH)2D3 to the nuclear VDR regulates various transcriptional factors that play critical roles in carcinogenesis.[21,22,39,54,67,69,72,76,80] VDR signalling pathways regulate a multitude of genes that control cellular proliferation, differentiation, immune responses and apoptosis. The existing connection between the VDR and other tumour modulators may open important new perspectives for treatment and prevention of melanoma and NMSC.

It is has been documented that several VDR polymorphisms are associated with diminished transcriptional activity, a risk factor for increased incidence and poorer prognosis in melanomas and NMSC.[88,89] VDR gene polymorphisms have also been linked to initiation of cutaneous carcinogenesis such as solar keratosis. The influence of VDR gene polymorphisms on VDR signalling and function requires further investigation.[90]

The alteration in expression of VDRs has a downstream effect on expression of cytochrome P450 hydroxylases.[45,66] Cellular sensitivity to calcitriol and calcidiol largely relies on VDRs. Levels of both VDR mRNA and protein are increased in keratinocytes of BCCs as compared with normal human epidermal keratinocytes.[49,51,91] VDR knockout mice exhibit extremely high rates of skin carcinogenesis.[85] VDR ablation is linked to diminished production of calcidiol and calcitriol in animal models and humans.[27,28,38,43,45,52,70,92]

VDR expression status is important since molecular pathways signalling via vitamin D are effective in suppressing carcinogenesis only if cells express and maintain functional VDRs.[21,93–95] Nuclear VDRs have important pro-differentiative functions in SCC cultures, although cultured SCC tumour cells fail to respond to the pro-differentiating actions of calcitriol, which has great affinity for VDR nuclear receptors.[28] Nuclear VDRs in SCC are blocked by cellular p160/DRIP (vitamin D receptor-interacting protein) complex, which prevents calcitriol binding and the promotion of genetic events required for cellular differentiation.[28] In contrast, the effect of calcidiol has not yet been tested in SCC tumour cells despite its tumour-suppressive effect at physiological concentrations in other cancer cell lines. A better understanding of the role played by VDR regulation in cutaneous cancers may assist in developing novel methods for treatment of difficult forms of skin cancers.

Conclusions and future perspectives

  1. Top of page
  2. Abstract
  3. Introduction
  4. The role of vitamin D in skin cancer development
  5. Development of cutaneous carcinogenesis
  6. Non-melanoma skin cancer, tumour stroma and angiogenesis
  7. Significance of the vitamin D receptor
  8. Conclusions and future perspectives
  9. Declarations
  10. References

In summary, the exact role of vitamin D in skin carcinogenesis is still to be elucidated. Solar irradiance is a causative factor for induction of skin cancers and at the same time is an essential requirement for vitamin D synthesis. This paper highlights the need for continued exploration of the role of vitamin D in development and progression of NMSC. Possible anti-cancer benefits of vitamin D are associated with slower tumour progression and decreased risks of malignancies. The next step in this area of research is detailed understanding of the molecular pathways involved in vitamin D regulation of the tumour microenvironment. By clarifying vitamin D regulation of cutaneous tumours we could improve recovery and survival of patients with skin cancer and maybe even prevent or minimize the occurrence of this condition.

Declarations

  1. Top of page
  2. Abstract
  3. Introduction
  4. The role of vitamin D in skin cancer development
  5. Development of cutaneous carcinogenesis
  6. Non-melanoma skin cancer, tumour stroma and angiogenesis
  7. Significance of the vitamin D receptor
  8. Conclusions and future perspectives
  9. Declarations
  10. References

Conflict of interest

The Author(s) declare(s) that they have no conflicts of interest to disclose.

Funding

This review received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. The role of vitamin D in skin cancer development
  5. Development of cutaneous carcinogenesis
  6. Non-melanoma skin cancer, tumour stroma and angiogenesis
  7. Significance of the vitamin D receptor
  8. Conclusions and future perspectives
  9. Declarations
  10. References