Conversion of vitamin D₃ to 1α,25‐dihydroxyvitamin D₃ in human skin equivalents

Abstract

These results demonstrate for the first time that human keratinocytes under in vivo-like conditions have the capacity of the enzymatic hydroxylation of vitamin D3 to hormonally active calcitriol (1alpha,25(OH)2D3). Supplementation of the culture medium with bovine serum albumin (BSA) up to 1.5%) (w/v) amplifies the conversion of vitamin D3 to 1alpha,25(OH)2D3. The maximum turnover rate of this reaction at 780 nM vitamin D3 in presence of 1.0% (w/v) BSA amounts to approximately 3 pmol 1alpha,25(OH)2D3 per 10(6) cells after 6 h of incubation. The hydroxylation of vitamin D3 to 1alpha,25(OH)2D3 is inhibited by the P-450 oxidase inhibitor ketoconazole. The generation of 1alpha,25(OH)2D3 from vitamin D3 has an apparent Michaelis constant (Km) of 2.3x10(-6) M. The intrinsic conversion of vitamin D3 to biologically active 1alpha,25(OH)2D3 may be of importance for the regulation of proliferation and differentiation of keratinocytes.

Vitamin D inhibits the activation of stress-activated protein kinases by physiological and environmental stresses in keratinocytes

“…The effective hormone concentrations are of the order of 10 9 M, higher than the circulating concentrations of 1,25(OH) 2 D 3 . However, such concentrations may well be attained in epidermal cells, the site of autocrine production of 1,25(OH) 2 D 3 (Lehmann et al 2000, Schuessler et al 2001. These results are consistent with previous findings that the hormone protects epidermal cells in vivo and in vitro from UVinduced damage (Lee & Youn 1998, Mason & Holliday 2000 and from apoptosis caused by chemotherapy (Hanada et al 1995, Schilli et al 1998.…”

“…Epidermal cells are regularly exposed to marked variations in ambient temperature, environmental stressors that affect the cellular redox state and osmotic balance, and to physiological stressors such as inflammatory cytokines. Epidermal keratinocytes contain the machinery needed to produce the hormonal form of vitamin D, 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ) from its initial precursor 7-dehydrocholesterol (Lehmann et al 2000, Schuessler et al 2001. These cells also contain the nuclear vitamin D receptor that mediates the effects of the hormone on keratinocytes (Bikle & Pillai 1993).…”

Inhibition of poly(adenosine diphosphate-ribose) polymerase by the active form of vitamin D

“…In 1998 Lehmann et al demonstrated for the first time that the human HaCaT cell line could metabolize both vitamin D and 1α-hydroxyvitamin D3 into 1α,25-dihydroxyvitamin D3 (33,34) and later demonstrated that this occurred not only in the HaCaT cell line but also in primary human skin cells (18). In our studies, vitamin D treatment of HaCaT cells inhibited peroxynitrite-induced PARP activation.…”

“…Additional studies have demonstrated that this vitamin is also involved in a host of cellular processes including cell differentiation, cell proliferation and immune function (16). Vitamin D, the biologically inactive moiety, is converted into the active form 1α,25-dihydroxyvitamin D3 by hydroxylation reactions, which occur primarily in the liver and kidney (16), but have also been described in keratinocytes (17)(18)(19)(20). Vitamin D has been shown to be anti-inflammatory (16) and able to protect against various inflammatory disease states including rheumatoid arthritis (21), type I diabetes (22), inflammatory bowel disease (23), transplant rejection (24) and multiple sclerosis (25).…”

A Novel Pathway for Hormonally Active Calcitriol

“…The physiological importance of this pathway however is unclear because only trace amounts of calcitriol were synthesized from calcidiol by skin ex vivo [10]. Previously, we were able to demonstrate that in cultures of human keratinocytes biologically inactive D 3 can be converted to 1α,25(OH) 2 D 3 [11, 12]. Since the skin is both the site of D 3 synthesis and a target organ for 1α,25(OH) 2 D 3 it is important to know whether UVB-induced photolysis of 7-DHC with consecutive synthesis of D 3 can influence the epidermal production 1α,25(OH) 2 D 3 .…”