Vitamin D: newly discovered actions require reconsideration of physiologic requirements

Abstract

Vitamin D is not just for preventing rickets and osteomalacia. Recent findings in animal experiments, epidemiologic studies and clinical trials indicate that adequate vitamin D levels are important for cancer prevention, controlling hormone levels and regulating the immune response. Although 25 hydroxyvitamin D (25OHD) levels >10 ng/ml can prevent rickets and osteomalacia, these levels are not sufficient to provide these more recently discovered clinical benefits. Rather, levels of 25OHD >30 ng/ml are generally recommended. Determining optimal levels of 25OHD and the amount of vitamin D supplementation required to achieve those levels for the numerous actions of vitamin D will only be established with additional trials. In this review, these newer applications are summarized and therapeutic considerations are provided.

Figure 1. Vitamin D production and metabolism

Vitamin D exists in two forms, vitamin D₂ and vitamin D₃. In each case vitamin D is produced from a precursor by ultraviolet B radiation (v). (a) Vitamin D₂ is made in plants and yeast from ergosterol, whereas (b) vitamin D₃ is made in the skin from 7-dehydrocholesterol. These forms of vitamin D differ in their side chains: vitamin D₂ has a double bond between C22-23 and a methyl group at C24 that vitamin D₃ lacks. However, vitamin D by itself is not biologically active but must first be converted to 25OHD (the major circulating form of vitamin D) and then to 1,25(OH)₂D, the most biologically active metabolite. (c) The 25-hydroxylation occurs primarily in the liver, although other tissues have the requisite enzymatic activity. The major 25-hydroxylases in the liver include the mitochondrial enzyme CYP27A1 and the microsomal enzyme CYP2R1 (among others). 25OHD is then converted to its active form 1,25(OH)₂D primarily in the kidney by the enzyme CYP27B1. As for the 25-hydroxylases, numerous tissues in addition to the kidney express CYP27B1. (i) In the kidney 1,25(OH)₂D production is stimulated by PTH and low serum levels of calcium and phosphate, but inhibited by FGF23. Also in the kidney and elsewhere is the enzyme CYP24 which converts 25OHD to 24,25(OH)₂D and 1,25(OH)₂D to 1,24,25(OH)₃D. Although these metabolites have been shown to have biologic activity, the 24-hydroxylation is also the first step in catabolizing vitamin D metabolites. (ii) The regulation of CYP24 by PTH, FGF23, calcium, and phosphate is essentially the reverse of that of CYP27B1, at least in the kidney, and 1,25(OH)₂D is a potent inducer of its expression. Regulation of these enzymes in non-renal tissues differs from that in the kidney.

Figure 1. Vitamin D production and metabolism

Vitamin D exists in two forms, vitamin D₂ and vitamin D₃. In each case vitamin D is produced from a precursor by ultraviolet B radiation (v). (a) Vitamin D₂ is made in plants and yeast from ergosterol, whereas (b) vitamin D₃ is made in the skin from 7-dehydrocholesterol. These forms of vitamin D differ in their side chains: vitamin D₂ has a double bond between C22-23 and a methyl group at C24 that vitamin D₃ lacks. However, vitamin D by itself is not biologically active but must first be converted to 25OHD (the major circulating form of vitamin D) and then to 1,25(OH)₂D, the most biologically active metabolite. (c) The 25-hydroxylation occurs primarily in the liver, although other tissues have the requisite enzymatic activity. The major 25-hydroxylases in the liver include the mitochondrial enzyme CYP27A1 and the microsomal enzyme CYP2R1 (among others). 25OHD is then converted to its active form 1,25(OH)₂D primarily in the kidney by the enzyme CYP27B1. As for the 25-hydroxylases, numerous tissues in addition to the kidney express CYP27B1. (i) In the kidney 1,25(OH)₂D production is stimulated by PTH and low serum levels of calcium and phosphate, but inhibited by FGF23. Also in the kidney and elsewhere is the enzyme CYP24 which converts 25OHD to 24,25(OH)₂D and 1,25(OH)₂D to 1,24,25(OH)₃D. Although these metabolites have been shown to have biologic activity, the 24-hydroxylation is also the first step in catabolizing vitamin D metabolites. (ii) The regulation of CYP24 by PTH, FGF23, calcium, and phosphate is essentially the reverse of that of CYP27B1, at least in the kidney, and 1,25(OH)₂D is a potent inducer of its expression. Regulation of these enzymes in non-renal tissues differs from that in the kidney.

Figure 2. The mechanism of action of 1,25(OH)2D

1,25(OH)₂D is a sterol hormone, and shares the property of other steroid hormones, thyroid hormone, and retinoic acid as a ligand for a transcription factor, which in the case of 1,25(OH)₂D is the vitamin D receptor (VDR). VDR is a member of the nuclear hormone receptor family. When 1,25(OH)₂D binds to VDR it is transported into the nucleus where it partners with another nuclear hormone receptor, most often the retinoid X receptor (RXR). These heterodimers bind to regions of the genes they regulate at specified sequences called vitamin D response elements (VDRE). Binding to the VDREs is accompanied by the formation of large complexes that can facilitate the expression of the targeted gene (coactivators) or inhibit its expression (cosuppressors). There are a number of different types of coactivator complexes that function either to expose the gene for transcription by acetylating the histones that otherwise conceal the DNA, or by bridging the gap between the VDRE and the initiation complex, stimulating transcription by activating the RNA polymerase as shown in this figure. Corepressors block this process in part by deacetylating the histones, although other modifications of these histones such as methylation have also been reported.

Figure 3. Hormonal regulation by and of 1,25(OH)₂D

(a) 1,25(OH)₂D inhibits (red lines) the production of PTH and renin while stimulating (green arrows) that of FGF23 and insulin. PTH and FGF23 in turn are major regulators of 1,25OH)₂D production by the kidney, PTH stimulating (arrow) while FGF23 inhibits (line). (b) Insulin may also stimulate 1,25(OH)₂D production, perhaps through insulin like growth factor (IGF)-I receptor, although this possibility has received limited evaluation.

Figure 4. Regulation of immunity by 1,25(OH)₂D

(a) Adaptive immunity. Antigen presenting cells, primarily dendritic cells and macrophages but in some cases other cells like keratinocytes, when activated induce the proliferation and differentiation of T cells (CD4 and CD8) along different pathways depending on a variety of conditions including cell context and type of antigen activating the system. (i)The dendritic cell is shown stimulating (ii) the CD4 cell to differentiate into one of 4 different types of T helper (Th) cell. 1,25(OH)₂D influences this process by inhibiting the development of Th1 and Th17 cells, while promoting that of Th2 and Treg cells. (b) Innate immunity. The innate immune response is triggered by a wide variety of environmental stimuli including various infectious organisms. These act through toll like receptors (TLR). Many cells including professional immune cells and a number of epithelial cells express TLRs and are capable of mounting the innate immune response. Several of these TLRs, when activated, induce VDR and CYP27B1, which given adequate substrate (25OHD), results in 1,25(OH)₂D production within the cell. 1,25(OH)₂D in turn binds to the VDR to induce antimicrobial peptides such as cathelicidin capable of killing the invading organism. The innate immune response is also critical for activating the adaptive immune response, but the role of 1,25(OH)₂D in this process has received limited study.

Figure 4. Regulation of immunity by 1,25(OH)₂D

(a) Adaptive immunity. Antigen presenting cells, primarily dendritic cells and macrophages but in some cases other cells like keratinocytes, when activated induce the proliferation and differentiation of T cells (CD4 and CD8) along different pathways depending on a variety of conditions including cell context and type of antigen activating the system. (i)The dendritic cell is shown stimulating (ii) the CD4 cell to differentiate into one of 4 different types of T helper (Th) cell. 1,25(OH)₂D influences this process by inhibiting the development of Th1 and Th17 cells, while promoting that of Th2 and Treg cells. (b) Innate immunity. The innate immune response is triggered by a wide variety of environmental stimuli including various infectious organisms. These act through toll like receptors (TLR). Many cells including professional immune cells and a number of epithelial cells express TLRs and are capable of mounting the innate immune response. Several of these TLRs, when activated, induce VDR and CYP27B1, which given adequate substrate (25OHD), results in 1,25(OH)₂D production within the cell. 1,25(OH)₂D in turn binds to the VDR to induce antimicrobial peptides such as cathelicidin capable of killing the invading organism. The innate immune response is also critical for activating the adaptive immune response, but the role of 1,25(OH)₂D in this process has received limited study.