**2. Content**

#### **2.1 Vitamin D, skin production and metabolism**

Vitamin D or calciferol refers to cholecalciferol or vitamin D3 and ergocalciferol or vitamin D2. D3 is produced in the skin as a result of ultraviolet irradiation of 7-dehydrocholesterol (7-DHC) and D2 is produced by ultraviolet irradiation of the plant sterol ergosterol(1).

Vitamin D was discovered in the 1900's as a result of research efforts geared towards the treatment of the disease, rickets. Within the last decade, vitamin D has become a popular topic in medical research as investigators aim to elucidate the role it plays in both maintaining health and contributing to the onset of disease.

Most people obtain their vitamin D requirement from sunlight exposure (2) in addition to smaller amounts obtained through the diet since very few foods naturally contain vitamin D.

7-DHC absorbs ultraviolet B (UVB) radiation and optimum wavelengths for vitamin D3 production are between 295 nm and 300 nm with a peak at 297 nm(3). Levels of 7-DHC have been observed to decline with age, which might negatively impact vitamin D3 synthesis in the skin (2). Vitamin D3 produced in the skin or ingested from the diet can be stored in body fat and later released into circulation. Vitamin D3 is sequestered deep into body fat, making it less bioavailable in obese individuals(4). Vitamin D is biologically inert and must be hydroxylated in the liver to form 25-hydroxyvitamin D [25(OH)D or calcidiol], which is the

and carbohydrate status in psoriasis patients. A review of the published studies will be used to accomplish this task. In our previously published studies, the serum concentrations of 25(OH)D, 1,25(OH)2D, PTH, calcium and creatinine were measured before and after phototherapy in Caucasian patients with moderate to severe active plaque psoriasis. Bone mineral density (BMD) was examined using dual-energy X-ray absorptiometry (DEXA) at the hip and lumbar spine in a group of postmenopausal women with psoriasis. Lipid and

We found that UVB/heliotherapy improved the psoriasis score and lipid and carbohydrate status of the patients, increased serum 25(OH)D synthesis and reduced serum PTH concentrations. Vitamin D production in psoriasis patients increased less with narrowband UVB than with broadband UVB phototherapy. There was no correlation between the dose of UVB and the increase in 25(OH)D. The ratio of low density lipoprotein (LDL) and high density lipoprotein (HDL) cholesterol decreased, and the levels of glycosylated hemoglobin A1c (HbA1c) also decreased in psoriasis patients during heliotherapy. Postmenopausal women with psoriasis had higher BMD than age-matched controls, a finding that could be

The changes in serum concentrations of vitamin D metabolite 25(OH)D were not related to the degree of improvement in psoriasis severity. This can be explained by the fact that 25(OH)D is biologically inert. It is unclear if the serum 25(OH)D level is linked to the level of the active form of vitamin D3 (1,25(OH)2D) present in the skin. It has been suggested that cutaneous conversion of 25(OH)D to 1,25(OH)2D does not play a role because the amount of free 25(OH)D3 that penetrates the cell membrane of epidermal keratinocytes is too small to produce sufficient amounts of 1,25(OH)2D. Therefore, of great interest will be the study of

Vitamin D or calciferol refers to cholecalciferol or vitamin D3 and ergocalciferol or vitamin D2. D3 is produced in the skin as a result of ultraviolet irradiation of 7-dehydrocholesterol (7-DHC) and D2 is produced by ultraviolet irradiation of the plant sterol ergosterol(1).

Vitamin D was discovered in the 1900's as a result of research efforts geared towards the treatment of the disease, rickets. Within the last decade, vitamin D has become a popular topic in medical research as investigators aim to elucidate the role it plays in both

Most people obtain their vitamin D requirement from sunlight exposure (2) in addition to smaller amounts obtained through the diet since very few foods naturally contain vitamin D. 7-DHC absorbs ultraviolet B (UVB) radiation and optimum wavelengths for vitamin D3 production are between 295 nm and 300 nm with a peak at 297 nm(3). Levels of 7-DHC have been observed to decline with age, which might negatively impact vitamin D3 synthesis in the skin (2). Vitamin D3 produced in the skin or ingested from the diet can be stored in body fat and later released into circulation. Vitamin D3 is sequestered deep into body fat, making it less bioavailable in obese individuals(4). Vitamin D is biologically inert and must be hydroxylated in the liver to form 25-hydroxyvitamin D [25(OH)D or calcidiol], which is the

carbohydrate status were assessed in patients treated with heliotherapy.

related to their higher body weight, levels of physical activity and UVB exposure.

UVB induced local effects on vitamin D synthesis and metabolism in psoriatic skin.

**2.1 Vitamin D, skin production and metabolism** 

maintaining health and contributing to the onset of disease.

**2. Content** 

major circulating metabolite. Further hydroxylation into 1,25-dihydroxyvitamin D [1,25(OH)2D or calcitriol] occurs primarily in the kidneys (Figure 1). Hydroxylation in the kidneys is stimulated by parathyroid hormone (PTH) and suppressed by phosphate. Homeostatic mechanisms include parathyroid activity, serum calcium and serum 1,25(OH)2D itself. Conversion of vitamin D to 25(OH)D is mediated by the enzyme vitamin D-25-hydroxylase (CYP27A1). The synthesis and degradation of calcitriol are regulated by the enzymes 25(OH)D-1-α-hydroxylase (CYP27B1) and 25(OH)2D-24-hydroxylase (CYP24A1), respectively. The combined activity of these enzymes is an important factor in determining the circulating concentrations of 25(OH)D, and 1,25(OH)2D(1). In addition to the kidney, other tissues and cells, including keratinocytes and immune cells, contain these enzymes and are able to convert 25(OH)D to active 1,25(OH)2D(5).

Besides being an essential steroid for calcium homeostasis and skeletal health, vitamin D also plays a role in regulation of cellular growth, cell proliferation and cell differentiation. Vitamin D also regulates the immune system, controls cancer cell growth and plays a role in the regulation of blood pressure(6). These effects are mediated through the intracellularly located vitamin D receptor (VDR). VDR is a member of the steroid, estrogen and retinoid receptor gene family of proteins that mediate transcriptional activities of the respective ligands. The VDR complex interacts with vitamin D responsive elements on the target gene. Alterations in calcitriol levels and polymorphisms of the VDR gene have been shown to be associated with several malignant and autoimmune diseases including psoriasis vulgaris(7).

25(OH)D is used clinically to measure vitamin D status. The cut-off level for serum 25(OH)D, which is used as a diagnostic marker for vitamin D deficiency, has varied over the years(8-10). The early biochemical changes in vitamin D insufficiency include a rise in serum PTH, which begins to increase as serum 25(OH)D levels fall below 30 ng/ml or 75 nmol/l(9). This level of 25(OH)D has become the suggested cut-off point for vitamin D deficiency or inadequacy(9, 11-13). At the present time, there is no clear consensus regarding levels of 25(OH)D for optimal health but levels of > 50 nmol/l (20 ng/ml)(14) and > 75 nmol/l (30 ng/ml) have been based on considering the outcomes of bone health, fracture prevention and colorectal cancer(15,16). Sun exposure is the strongest factor influencing 25(OH)D. The serum concentrations of 25(OH)D vary seasonally, with maximum and minimum values in the late summer and winter respectively(17). The extent of this seasonal variation depends on factors such as latitude, skin pigmentation, clothing, and the use of sunscreen(18).

Currently, limited data is available on the role of vitamin D deficiency in the pathogenesis or outcomes of psoriasis. The lack of conclusive data combined with vitamin D's immunomodulatory role, warrants further research investigating the role of vitamin D insufficiency in chronic diseases as well as monitoring 25(OH)D levels in children and adults of all ages as a part of routine physical examinations.

#### **2.2 The effects of vitamin D in psoriasis**

Vitamin D has pleotropic functions; it acts as a hormone by controlling calcium homeostasis as well as exerting autocrine/paracrine effects on tissues that express CYP27B1 and VDR. Besides its local effects, calcitriol may also act in psoriasis through its immunomodulatory properties by inhibiting T-cell proliferation and Th1 development, modulating antigenpresenting cell (APCs) function, inducing hyporesponsiveness to antigens, inhibiting

UVB and Vitamin D in Psoriasis 125

filaggrin, which are required for CE formation. VDR ligands decreased the expression of proinflammatory cytokines IL-2, IFN-γ, IL-6, IL-8 (37-40) and proliferation of T lymphocytes and keratinocytes. Furthermore, topical calcipotriol increased anti-inflammatory cytokine IL-10 in psoriatic lesions(41), and increased the expression of IL-10 receptor in

Antigen presenting cells (APCs), which play an important role in psoriasis, are one of the major targets of calcitriol-mediated immunosuppressive action (43). VDR ligands prevent the activation, differentiation, maturation and survival of APCs, leading to T cell hyporesponsiveness(44). Calcitriol also increased the expression of IL-10 and decreased the

Several studies have shown that calcipotriol as well as calcitriol and tacalcitol are efficacious, safe and can be used on a long-term basis for psoriasis (43, 46-49). Vitamin D3

Few studies on vitamin D status and its role in psoriasis have been performed or published. Low vitamin D status is associated with an increased risk of cancer, autoimmune, infectious, and inflammatory diseases, although the role of vitamin D status in the pathogenesis of

**3. The effects of phototherapy on vitamin D status in patients with psoriasis**  A similar wavelength spectrum of UVB is responsible for vitamin D synthesis (280-315 nm), which has been successfully used for years to treat psoriasis and other chronic inflammatory

Phototherapy (broadband UVB, narrowband UVB and heliotherapy - treatment with natural sunlight) is an effective treatment, commonly used for widespread psoriasis. Therefore, phototherapy is an excellent option for patients with generalized psoriasis because of its

In addition to standard broadband ultraviolet radiation B (BUVB), (280-315 nm), narrowband phototherapy (NBUVB) (monochromatic UV between 311-312 nm) and heliotherapy (treatment with natural sunlight) have become important treatment modalities for psoriasis. Research suggests that NBUVB is more effective than broadband UVB for reducing PASI scores, as well as being a safer and better tolerated option for patients in comparison to PUVA when taken at suberythemogenic doses (52). Furthermore, these advantages along with the handling ease of the NBUVB lamp led to a reduction in the usage of broadband UVB. However, one drawback of the new lamp is that the radiation times have almost doubled (54). Additionally, several studies indicate that combination therapy using both calcipotriol and UVB radiation illustrate more rapid healing of psoriasis when

Serum levels of 25(OH)D increased during treatment with artificial UV (BUVB and NBUVB) and during heliotherapy(56-59). The increase in 25(OH)D was higher in the BUVB treated patients when compared to the NBUVB (p=0.008) and heliotherapy (p=0.017) treated groups. Low-dose NBUVB treatment significantly increased vitamin D status in

superior systemic safety profile in comparison to systemic and biologic agents (51).

expression of IL-12, two major cytokines that are involved in Th1-Th2 balance(45).

analogs can be used in combination with phototherapy(50).

**2.4 Vitamin D status in patients with psoriasis** 

compared to monotherapy of either treatment (50,55).

keratinocytes(42).

psoriasis is unknown.

skin disorders.

production of IL-2, IL-17, IL-8 and interferon-γ, increasing the production of IL-10 and regulatory T cells(19, 20). Calcitriol has also been suggested to reduce production of interferon-α in some cells(21). Calcitriol is involved in the regulation of antimicrobial peptides cathelicidin and human β-defensin 2 (HBD2), which both participate in the pathogenesis of psoriasis (22). Vitamin D's role in psoriasis is further supported by studies that confirm the link between VDR polymorphism and psoriasis (23, 24). An association between VDR genotypes (Apa1) and the mean age at onset of psoriasis were previously observed (25). Since VDR gene polymorphisms show ethnic variability, concern arises on how to treat psoriasis patients of different populations according to their potentially varied treatment response (26). Moreover, it has been demonstrated that VDR gene polymorphisms may also play a role in partial resistance to calcipotriol therapy (24).

There are few studies on high-dose vitamin D3 in the treatment of psoriasis while systemic administration of 1,25(OH)2D for the treatment of psoriasis might be limited by its toxicity. A number of small trials show the efficacy and safety of vitamin D metabolites in the treatment of psoriasis and psoriatic arthritis (27-29). Systemic calcitriol treatment had an immunomodulatory effect manifested by a short-term temporary decrease in type 1 immune responses and a decrease in disease activity in patients with psoriatic arthropathy (27). Administration of vitamin D3 could be a better option than calcitriol or alphacalcidol since it is safer and less expensive, although more studies are needed to assess its efficacy (21). However, the use of calcitiol in dermatology is hampered by its hypercalcemic activity. There is limited information on the role of vitamin D deficiency in the pathogenesis of psoriasis or the role of vitamin D deficiency in response to treatments with topical or systemic drugs. There is a report of resolution of anti-TNFα-induced psoriasiform lesions by high doses of vitamin D3, in a patient with rheumatoid arthritis and vitamin D deficiency (21). More studies are needed to assess the possible usefulness of high-dose vitamin D3 in the treatment of psoriasis.

#### **2.3 Effects of vitamin D3 analogues in psoriasis**

The observation that keratinocytes and T cells express VDR and that 1,25(OH)2D is a potent stimulator of keratinocyte differentiation provides a potential basis for the clinical use of VDR ligands for the treatment of psoriasis (30, 31). Clinical data that first supported the use of vitamin D analogs was obtained when a patient treated orally with 1-hydroxyvitamin D3 for osteoporosis showed remarkable remission of psoriatic lesions(32). In addition, promising clinical results were obtained in studies using oral 1-hydroxyvitamin D3, oral and topical calcitriol which led to improvement of psoriatic lesions in approximately 70-80% of patients (33). Vitamin D3 analogs (calcipotriol (Dovonex), calcitriol (Silkis) or tacalcitol (Curatoderm)) inhibit proliferation, induce terminal differentiation of human keratinocytes and exhibit immunomodulating properties (33). Differentiation of keratinocytes results in the formation of a cornified envelope (CE) that provides the barrier function of the skin. The expression of involucrin, a component of the CE, and transglutaminase I (TGase I), the enzyme that cross-links the components of CE, was increased by calcitriol and other VDR ligands(35). Treatment of keratinocytes with a medium containing high calcium also stimulated keratinocyte differentiation by increasing the expression of involucrin and TGase I. 1,25(OH)2D also promoted keratinocyte differentiation, at least in part by increasing intracellular calcium and by increased expression of calcium receptors in keratinocytes(36). Calcitriol indirectly induces the expression of keratin 1, involucrin, TGase I, loricrin, and

production of IL-2, IL-17, IL-8 and interferon-γ, increasing the production of IL-10 and regulatory T cells(19, 20). Calcitriol has also been suggested to reduce production of interferon-α in some cells(21). Calcitriol is involved in the regulation of antimicrobial peptides cathelicidin and human β-defensin 2 (HBD2), which both participate in the pathogenesis of psoriasis (22). Vitamin D's role in psoriasis is further supported by studies that confirm the link between VDR polymorphism and psoriasis (23, 24). An association between VDR genotypes (Apa1) and the mean age at onset of psoriasis were previously observed (25). Since VDR gene polymorphisms show ethnic variability, concern arises on how to treat psoriasis patients of different populations according to their potentially varied treatment response (26). Moreover, it has been demonstrated that VDR gene polymorphisms

There are few studies on high-dose vitamin D3 in the treatment of psoriasis while systemic administration of 1,25(OH)2D for the treatment of psoriasis might be limited by its toxicity. A number of small trials show the efficacy and safety of vitamin D metabolites in the treatment of psoriasis and psoriatic arthritis (27-29). Systemic calcitriol treatment had an immunomodulatory effect manifested by a short-term temporary decrease in type 1 immune responses and a decrease in disease activity in patients with psoriatic arthropathy (27). Administration of vitamin D3 could be a better option than calcitriol or alphacalcidol since it is safer and less expensive, although more studies are needed to assess its efficacy (21). However, the use of calcitiol in dermatology is hampered by its hypercalcemic activity. There is limited information on the role of vitamin D deficiency in the pathogenesis of psoriasis or the role of vitamin D deficiency in response to treatments with topical or systemic drugs. There is a report of resolution of anti-TNFα-induced psoriasiform lesions by high doses of vitamin D3, in a patient with rheumatoid arthritis and vitamin D deficiency (21). More studies are needed to assess the possible usefulness of high-dose vitamin D3 in

The observation that keratinocytes and T cells express VDR and that 1,25(OH)2D is a potent stimulator of keratinocyte differentiation provides a potential basis for the clinical use of VDR ligands for the treatment of psoriasis (30, 31). Clinical data that first supported the use of vitamin D analogs was obtained when a patient treated orally with 1-hydroxyvitamin D3 for osteoporosis showed remarkable remission of psoriatic lesions(32). In addition, promising clinical results were obtained in studies using oral 1-hydroxyvitamin D3, oral and topical calcitriol which led to improvement of psoriatic lesions in approximately 70-80% of patients (33). Vitamin D3 analogs (calcipotriol (Dovonex), calcitriol (Silkis) or tacalcitol (Curatoderm)) inhibit proliferation, induce terminal differentiation of human keratinocytes and exhibit immunomodulating properties (33). Differentiation of keratinocytes results in the formation of a cornified envelope (CE) that provides the barrier function of the skin. The expression of involucrin, a component of the CE, and transglutaminase I (TGase I), the enzyme that cross-links the components of CE, was increased by calcitriol and other VDR ligands(35). Treatment of keratinocytes with a medium containing high calcium also stimulated keratinocyte differentiation by increasing the expression of involucrin and TGase I. 1,25(OH)2D also promoted keratinocyte differentiation, at least in part by increasing intracellular calcium and by increased expression of calcium receptors in keratinocytes(36). Calcitriol indirectly induces the expression of keratin 1, involucrin, TGase I, loricrin, and

may also play a role in partial resistance to calcipotriol therapy (24).

the treatment of psoriasis.

**2.3 Effects of vitamin D3 analogues in psoriasis** 

filaggrin, which are required for CE formation. VDR ligands decreased the expression of proinflammatory cytokines IL-2, IFN-γ, IL-6, IL-8 (37-40) and proliferation of T lymphocytes and keratinocytes. Furthermore, topical calcipotriol increased anti-inflammatory cytokine IL-10 in psoriatic lesions(41), and increased the expression of IL-10 receptor in keratinocytes(42).

Antigen presenting cells (APCs), which play an important role in psoriasis, are one of the major targets of calcitriol-mediated immunosuppressive action (43). VDR ligands prevent the activation, differentiation, maturation and survival of APCs, leading to T cell hyporesponsiveness(44). Calcitriol also increased the expression of IL-10 and decreased the expression of IL-12, two major cytokines that are involved in Th1-Th2 balance(45).

Several studies have shown that calcipotriol as well as calcitriol and tacalcitol are efficacious, safe and can be used on a long-term basis for psoriasis (43, 46-49). Vitamin D3 analogs can be used in combination with phototherapy(50).
