Following up on our previous findings that the skin possesses steroidogenic activity from progesterone, we now show widespread cutaneous expression of the full cytochrome P450 side-chain cleavage (P450scc) system required for the intracellular catalytic production of pregnenolone, i.e. the genes and proteins for P450scc enzyme, adrenodoxin, adrenodoxin reductase and MLN64. Functionality of the system was confirmed in mitochondria from skin cells. Moreover, purified mammalian P450scc enzyme and, most importantly, mitochondria isolated from placenta and adrenals produced robust transformation of 7-dehydrocholesterol (7-DHC; precursor to cholesterol and vitamin D3) to 7-dehydropregnenolone (7-DHP). Product identity was confirmed by comparison with the chemically synthesized standard and chromatographic, MS and NMR analyses. Reaction kinetics for the conversion of 7-DHC into 7-DHP were similar to those for cholesterol conversion into pregnenolone. Thus, 7-DHC can form 7-DHP through P450scc side-chain cleavage, which may serve as a substrate for further conversions into hydroxy derivatives through existing steroidogenic enzymes. In the skin, 5,7-steroidal dienes (7-DHP and its hydroxy derivatives), whether synthesized locally or delivered by the circulation, may undergo UVB-induced intramolecular rearrangements to vitamin D3-like derivatives. This novel pathway has the potential to generate a variety of molecules depending on local steroidogenic activity and access to UVB.

The skin has developed a hierarchy of systems that encompasses the skin immune and local steroidogenic activities in order to protect the body against the external environment and biological factors and to maintain local homeostasis. Most recently it has been established that skin cells contain the entire biochemical apparatus necessary for production of glucocorticoids, androgens and estrogens either from precursors of systemic origin or, alternatively, through the conversion of cholesterol to pregnenolone and its subsequent transformation to biologically active steroids. Examples of these products are corticosterone, cortisol, testosterone, dihydrotesterone and estradiol. Their local production can be regulated by locally produced corticotropin releasing hormone (CRH), adrenocorticotropic hormone (ACTH) or cytokines. Furthermore the production of glucocorticoids is affected by ultraviolet B radiation. The level of production and nature of the final steroid products are dependent on the cell type or cutaneous compartment, e.g., epidermis, dermis, adnexal structures or adipose tissue. Locally produced glucocorticoids, androgens and estrogens affect functions of the epidermis and adnexal structures as well as local immune activity. Malfunction of these steroidogenic activities can lead to inflammatory disorders or autoimmune diseases. The cutaneous steroidogenic system can also have systemic effects, which are emphasized by significant skin contribution to circulating androgens and/or estrogens. Furthermore, local activity of CYP11A1 can produce novel 7 -steroids and secosteroids that are biologically active. Therefore, modulation of local steroidogenic activity may serve as a new therapeutic approach for treatment of inflammatory disorders, autoimmune processes or other skin disorders. In conclusion, the skin can be defined as an independent steroidogenic organ, whose activity can affect its functions and the development of local or systemic inflammatory or autoimmune diseases.

We define previously unrecognized in vivo pathways of vitamin D(3) (D3) metabolism generating novel D3-hydroxyderivatives different from 25-hydroxyvitamin D(3) [25(OH)D3] and 1,25(OH)(2)D3. Their novel products include 20-hydroxyvitamin D(3) [20(OH)D3], 22(OH)D3, 20,23(OH)(2)D3, 20,22(OH)(2)D3, 1,20(OH)(2)D3, 1,20,23(OH)(3)D3, and 17,20,23(OH)(3)D3 and were produced by placenta, adrenal glands, and epidermal keratinocytes. We detected the predominant metabolite [20(OH)D3] in human serum with a relative concentration ∼20 times lower than 25(OH)D3. Use of inhibitors and studies performed with isolated mitochondria and purified enzymes demonstrated involvement of the steroidogenic enzyme cytochrome P450scc (CYP11A1) as well as CYP27B1 (1α-hydroxylase). In placenta and adrenal glands with high CYP11A1 expression, the predominant pathway was D3 → 20(OH)D3 → 20,23(OH)(2)D3 → 17,20,23(OH)(3)D3 with further 1α-hydroxylation, and minor pathways were D3 → 25(OH)D3 → 1,25(OH)(2)D3 and D3 → 22(OH)D3 → 20,22(OH)(2)D3. In epidermal keratinocytes, we observed higher proportions of 22(OH)D3 and 20,22(OH)(2)D3. We also detected endogenous production of 20(OH)D3, 22(OH) D3, 20,23(OH)(2)D3, 20,22(OH)(2)D3, and 17,20,23(OH)(3)D3 by immortalized human keratinocytes. Thus, we provide in vivo evidence for novel pathways of D3 metabolism initiated by CYP11A1, with the product profile showing organ/cell type specificity and being modified by CYP27B1 activity. These findings define the pathway intermediates as natural products/endogenous bioregulators and break the current dogma that vitamin D is solely activated through the sequence D3 → 25(OH)D3 → 1,25(OH)(2)D3.

RORα and RORγ are expressed in human skin cells that produce the noncalcemic 20-hydroxyvitamin D3 [20(OH)D3] and 20,23-dihydroxyvitamin D3 [20,23(OH)2D3]. Chinese hamster ovary (CHO) cells stably expressing a Tet-on RORα or RORγ expression vector and a ROR-responsive element (RORE)-LUC reporter, and a mammalian 2-hybrid model examining the interaction between the ligand binding domain (LBD) of RORα or RORγ with an LBD-interacting LXXLL-peptide, were used to study ROR-antagonist activities. These assays revealed that 20(OH)D3 and 20,23(OH)2D3 function as antagonists of RORα and RORγ. Moreover, 20(OH)D3 inhibited the activation of the promoter of the Bmal1 and G6pase genes, targets of RORα, and 20(OH)D3 and 20,23(OH)2D3 inhibited Il17 promoter activity in Jurkat cells overexpressing RORα or RORγ. Molecular modeling using crystal structures of the LBDs of RORα and RORγ revealed docking scores for 20(OH)D3, 20,23(OH)2D3 and 1,25(OH)2D3 similar to those of the natural ligands, predicting good binding to the receptor. Notably, 20(OH)D3, 20,23(OH)2D3, and 1,25(OH)2D3 inhibited RORE-mediated activation of a reporter in keratinocytes and melanoma cells and inhibited IL-17 production by immune cells. Our study identifies a novel signaling pathway, in which 20(OH)D3 and 20,23(OH)2D3 act as antagonists or inverse agonists of RORα and RORγ, that opens new possibilities for local (skin) or systemic regulation.-Slominski, A. T., Kim, T.-K., Takeda, Y., Janjetovic, Z., Broz˙yna, A. A., Skobowiat, C., Wang, J., Postlethwaite, A., Li, W., Tuckey, R. C., Jetten, A. M. RORα and ROR γ are expressed in human skin and serve as receptors for endogenously produced noncalcemic 20-hydroxy- and 20,23-dihydroxyvitamin D.

SummaryHuman skin expresses elements of the hypothalamo-pituitary-adrenal (HPA) axis including proopiomelanocortin (POMC), corticotropin releasing hormone (CRH), the CRH receptor-1 (CRH-R1), key enzymes of corticosteroid synthesis and synthesizes glucocorticoids. Expression of these elements is organized in functional, cell type-specific regulatory loops, which imitate the signaling structural hierarchy of the HPA axis. In melanocytes and fibroblasts CRH-induced CRH-R1 stimulation upregulates POMC expression and production of ACTH through activation of cAMP dependent pathway(s). Melanocytes respond with enhanced production of cortisol and corticosterone, which is dependent on POMC activity. Fibroblasts respond to CRH and ACTH with enhanced production of corticosterone, but not cortisol, which is produced constitutively. Organcultured human scalp hair follicles also show a fully functional HPA axis equivalent, including cortisol synthesis and secretion and negative feedback regulation by cortisol on CRH expression. Thus differential, CRH-driven responses of skin reproduce key features of the central HPA axis at the tissue/single cell levels.