Formation of fatty acid esterified vitamin D3 in rat skin by exposure to ultraviolet radiation

Vitamin D esters are the major form of vitamin D produced by UV irradiation in mice

The primary source of vitamin D3 for humans is that produced in skin by ultraviolet irradiation. Ultraviolet (UV) B (UVB, 280-310 nm) light causes the isomerization of 7-dehydrocholesterol (7-DHC) to pre-vitamin D3 that is thermally isomerized to vitamin D3. In addition to free vitamin D3, it has been previously reported that esterified vitamin D3 is also found in the skin of rats irradiated with UVB. We found that a large fraction of the vitamin D3 precursor, 7-dehydrocholesterol is in the esterified form. Following UVB irradiation, vitamin D3 esters represent the majority of tissue vitamin D3, comprising approximately 80% in mice. Examination of vitamin D3 ester transport from skin in DBP^-/- mice demonstrated that skin vitamin D3 ester content decreased only in the presence of DBP. No significant binding of vitamin D3 esters by serum was observed and no vitamin D3 esters were detectable in mouse serum after UVB treatment, indicating that the esters are hydrolyzed prior to transport into the circulation. The significance of vitamin D3 esters and their hydrolysis is the subject of current investigation.

Metabolism of selective 20-epi-vitamin D3 analogs in rat osteosarcoma UMR-106 cells: Isolation and identification of four novel C-1 fatty acid esters of 1α,25-dihydroxy-16-ene-20-epi-vitamin D3

Analogs of 1α,25-dihydroxyvitamin D₃ (S1) with 20-epi modification (20-epi analogs) possess unique biological properties. We previously reported that 1α,25-dihydroxy-20-epi-vitamin D₃ (S2), the basic 20-epi analog is metabolized into less polar metabolites (LPMs) in rat osteosarcoma cells (UMR-106) but not in a perfused rat kidney. Furthermore, we also noted that only selective 20-epi analogs are metabolized into LPMs. For example, 1α,25-dihydroxy-16-ene-20-epi-vitamin D₃ (S4), but not 1α,25-dihydroxy-16-ene-23-yne-20-epi-vitamin D₃ (S5) is metabolized into LPMs. In spite of these novel findings, the unequivocal identification of LPMs has not been achieved to date. We report here on a thorough investigation of the metabolism of S4 in UMR-106 cells and isolated two major LPMs produced directly from the substrate S4 itself and two minor LPMs produced from 3-epi-S4, a metabolite of S4 produced through C-3 epimerization pathway. Using GC/MS, ESI-MS and ¹H NMR analysis, we identified all the four LPMs of S4 as 25-hydroxy-16-ene-20-epi-vitamin D₃-1-stearate and 25-hydroxy-16-ene-20-epi-vitamin D₃-1-oleate and their respective C-3 epimers. We report here for the first time the elucidation of a novel pathway of metabolism in UMR-106 cells in which both 1α,25(OH)₂-16-ene-20-epi-D₃ and 1α,25(OH)₂-16-ene-20-epi-3-epi-D₃ undergo C-1 esterification into stearic and oleic acid esters.

Vitamin D: Photobiological and Ecological Aspects

Vitamin D was discovered as a result of its ability to cure rickets, but recently a wide range of other functions for it in the human body has been suggested. Vitamin D is not a vitamin in the strict sense as it can be synthesised in the human body following exposure of the skin to ultraviolet radiation. Provitamin D (7-dehydrocholesterol) is converted to previtamin D which is further modified by a series of reactions to the active form, 1,25-dihydroxyvitamin D. This Chapter summarises the discovery of vitamin D and reviews the chemistry and photochemistry of its precursors, transformations and metabolites. The production of vitamin D in various human populations is described, and how to assess vitamin D status. The skeletal and non-skeletal effects of vitamin D are discussed, particularly its role in immunomodulation with consequences for protection against a variety of human diseases. The Chapter concludes with evolutionary aspects, the occurrence and role of vitamin D in the plant kingdom, biogeographical considerations, and the nonphotochemical production of vitamin D in certain plants.

UVB-induced production of 1,25-dihydroxyvitamin D3 and vitamin D activity in human keratinocytes pretreated with a sterol delta7-reductase inhibitor

The skin fulfills an important role in the vitamin D photo-endocrine system. Epidermis is not only the site of vitamin D3 photoproduction. In addition, epidermal keratinocytes contain the vitamin D receptor (VDR) and possess 25-hydroxylase and 1alpha-hydroxylase activity indicating that all components of the vitamin D system are present. We investigated whether these components cooperate in inducing vitamin D activity upon treatment with physiological UVB doses. Upon irradiation, 24-hydroxylase mRNA was induced in keratinocytes pretreated with a sterol Delta7-reductase inhibitor (BM15766) whereby the 7-dehydrocholesterol content increased by 300-fold. Transfection experiments with a vitamin D response element containing construct confirmed VDR-dependent gene activation. Furthermore, the UVB-dependent induction of 24-hydroxylase was blocked by the cytochrome-P450 inhibitor ketoconazole. The 24-hydroxylase inducing photoproduct was transferable to unirradiated keratinocytes by medium and cellular homogenates of UVB-irradiated, BM15766-pretreated cells and was identified as 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] by high-performance liquid chromatography with tandem mass spectrometric detection. Addition of vitamin D binding protein blunted UVB-induced 24-hydroxylase suggesting the possibility of a paracrine or autocrine role for 1,25(OH)2D3. In conclusion, epidermal keratinocytes can produce vitamin D3, convert it to 1,25(OH)2D3 and respond to it upon UVB irradiation in the absence of exogenous 7-dehydrocholesterol and therefore contain a unique and complete photo-endocrine vitamin D system.

Dermal fibroblasts pretreated with a sterol Δ7-reductase inhibitor produce 25-hydroxyvitamin D3 upon UVB irradiation

As dermis is a physiological site of vitamin D3 photoproduction, the photo-endocrine vitamin D3 system was studied in dermal fibroblasts. Dermal fibroblasts contain the vitamin D receptor and induce 1alpha,25-dihydroxyvitamin D3-24-hydroxylase [CYP24] mRNA upon stimulation with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. In addition, dermal fibroblasts contain mRNA of the vitamin D3-25-hydroxylases (CYP2R1 and CYP27A1). However, we could not detect any 25-hydroxyvitamin D3 [25OHD3]-1alpha-hydroxylase mRNA in dermal fibroblasts and no CYP24 mRNA was induced upon ultraviolet [UVB] irradiation, even when endogenous 7-dehydrocholesterol content was elevated by pretreatment with the sterol Delta7-reductase inhibitor BM15766. Nevertheless, dermal fibroblasts produce inactive vitamin D3 metabolites that can be activated by epidermal keratinocytes as CYP24 mRNA is induced in epidermal keratinocytes but not in dermal fibroblasts after transfer of medium or cellular suspensions from BM15766-pretreated, UVB-irradiated fibroblasts. This CYP24 induction was UVB-dose dependent and was inhibited by ketoconazole. As revealed in a competitive binding assay, BM15766-pretreated dermal fibroblasts are able to produce 25OHD3 upon UVB irradiation, but no 1,25(OH)2D3 was detected via combined high-performance liquid chromatography radioimmunoassay. The physiological relevance of dermal vitamin D3 photoproduction and its subsequent conversion into 25OHD3 remains elusive.

UVB-induced conversion of 7-dehydrocholesterol to 1alpha,25-dihydroxyvitamin D3 in an in vitro human skin equivalent model

We have previously shown that keratinocytes in vitro can convert biologically inactive vitamin D3 to the hormone calcitriol (1alpha,25-dihydroxyvitamin D3). This study was initiated to test whether the ultraviolet-B-induced photolysis of provitamin D3 (7-dehydrocholesterol), which results in the formation of vitamin D3, can generate calcitriol in an in vivo-like human skin equivalent model made of fibroblasts in a collagen matrix as the dermal component and keratinocytes as the epidermal component. Cultures were preincubated with increasing concentrations of 7-dehydrocholesterol (0.53-5.94 nmol per cm2 human skin equivalent) at 37 degrees C and irradiated with monochromatic ultraviolet B at wavelengths ranging from 285 to 315 nm (effective ultraviolet doses 7.5-45 mJ per cm2). In our in vitro model irradiation with ultraviolet B resulted in a sequential metabolic process with generation of previtamin D3 followed by the time-dependent formation of vitamin D3, 25-hydroxyvitamin D3, and ultimately calcitriol in the femtomolar range. Unirradiated cultures and irradiated cultures without keratinocytes generated no calcitriol. Irradiation of skin equivalents at wavelengths > 315 nm generated no or only trace amounts of calcitriol. The ultraviolet-B-triggered conversion of 7-dehydrocholesterol to calcitriol was strongly inhibited by ketoconazole indicating the involvement of P450 mixed function oxidases. The amount of calcitriol generated was dependent on the 7-dehydrocholesterol concentration, on wavelength, and on ultraviolet B dose. Hence, keratinocytes in the presence of physiologic concentrations of 7-dehydrocholesterol and irradiated with therapeutic doses of ultraviolet B may be a potential source of biologically active calcitriol within the epidermis.

UVB-induced conversion of 7-dehydrocholesterol to 1 alpha,25-dihydroxyvitamin D3 (calcitriol) in the human keratinocyte line HaCaT

We have previously shown that keratinocytes in vitro can convert biologically inactive vitamin D3 to the hormone calcitriol. The present study was initiated to test whether ultraviolet B (UVB)-induced photolysis of provitamin D3 (7-dehydrocholesterol, [7-DHC]) which results in the formation of vitamin D3 also leads to the generation of calcitriol in keratinocytes. Submerged monolayers of HaCaT keratinocytes were preincubated with 7-DHC (25 microM) at 37 degrees C and irradiated with monochromatic UVB at different wavelengths (effective UV-doses: 7.5-60 mJ/cm2), or a narrow-band fluorescent lamp Philips TL-01 (UVB-doses: 125-1500 mJ/cm2). Irradiation with both sources of UVB resulted in the generation of different amounts of previtamin D3 in our in vitro model followed by time-dependent isomerization to vitamin D3 and consecutive formation of calcitriol in the picomolar range. Unirradiated cultures or cultures exposed to wavelengths > 315 nm generated no or only trace amounts of calcitriol. The conversion of vitamin D3 generated after UVB irradiation to calcitriol is inhibited by ketoconazole indicating the involvement of P450 mixed function oxidases in this chemical reaction. The generation of calcitriol was wavelength- and UVB dose dependent and reached approximately 18-fold higher levels after irradiation at 297 nm than at 310 nm (effective UVB dose: 30 mJ/cm2). Hence, keratinocytes may be a potential source of biologically active calcitriol within epidermis, when irradiated with therapeutical doses of UVB.