OVERVIEW
Rickets became a public health problem with the movement of the population from the farms to the cities during the Industrial Revolution. Various foods such as cod liver oil and irradiation of other foods including plants were found to prevent or cure this disease, leading eventually to the discovery of the active principle—vitamin D. Vitamin D comes in two forms (D2 and D3) which differ chemically in their side chains. These structural differences alter their binding to the carrier protein vitamin D binding protein (DBP) and their metabolism, but in general the biologic activity of their active metabolites is comparable. Vitamin D3 is produced in the skin from 7-dehydrocholesterol by UV irradiation, which breaks the B ring to form pre-D3. Pre-D3 isomerizes to D3 but with continued UV irradiation to tachysterol and lumisterol. D3 is preferentially removed from the skin, bound to DBP. The liver and other tissues metabolize vitamin D, whether from the skin or oral ingestion, to 25OHD, the principal circulating form of vitamin D. Several enzymes have 25-hydroxylase activity, but CYP2R1 is the most important. 25OHD is then further metabolized to 1,25(OH)2D principally in the kidney, by the enzyme CYP27B1, although other tissues including various epithelial cells, cells of the immune system, and the parathyroid gland contain this enzyme. 1,25(OH)2D is the principal hormonal form of vitamin D, responsible for most of its biologic actions. The production of 1,25(OH)2D in the kidney is tightly controlled, being stimulated by parathyroid hormone (PTH), and inhibited by calcium, phosphate and FGF23. Extrarenal production of 1,25(OH)2D as in keratinocytes and macrophages is under different control, being stimulated primarily by cytokines such as tumor necrosis factor alfa (TNFα) and interferon gamma (IFNg). 1,25(OH)2D reduces 1,25(OH)2D levels in cells primarily by stimulating its catabolism through the induction of CYP24A1, the 24-hydroxylase. 25OHD and 1,25(OH)2D are hydroxylated in the 24 position by this enzyme to form 24,25(OH)2D and 1,24,25(OH)3D, respectively. This 24-hydroxylation is generally the first step in the catabolism of these active metabolites to the final end product of calcitroic acid, although 24,25(OH)2D and 1,24,25(OH)3D have their own biologic activities. CYP24A1 also has 23-hydroxylase activity that leads to a different end product. Different species differ in their ratio of 23-hydroxylase/24-hydroxyase activity in their CYP24A1 enzyme, but in humans the 24-hydroxyase activity predominates. Like CYP27B1, CYP24A1 is widely expressed. CYP24A1 is induced by 1,25(OH)2D in most tissues, which serves as an important feedback mechanism to avoid vitamin D toxicity. In the kidney, PTH inhibits CYP24A1, whereas FGF23, calcium and phosphate stimulates it, just the opposite of the actions of these hormones and minerals on CYP27B1. However, such regulation is not seen in other tissues. In macrophages, CYP24A1 is either missing or defective, so in situations such as granulomatous diseases like sarcoidosis in which macrophage production of 1,25(OH)2D is increased, hypercalcemia and hypercalciuria due to elevated 1,25(OH)2D can occur without the counter regulation by CYP24A1.
The vitamin D metabolites are transported in blood bound to DBP and albumin. Very little circulates as the free form. The liver produces DBP and albumin, production that is decreased in liver disease, and these proteins may be lost in protein losing enteropathies or the nephrotic syndrome. Thus, individuals with liver, intestinal or renal diseases which result in low levels of these transport proteins may have low total levels of the vitamin D metabolites without necessarily being vitamin D deficient as their free concentrations may be normal.
The receptor for 1,25(OH)2D (VDR) is a transcription factor regulating the expression of genes which mediate its biologic activity. VDR is a member of a rather large family of nuclear hormone receptors which includes the receptors for glucocorticoids, mineralocorticoids, sex hormones, thyroid hormone, and vitamin A metabolites or retinoids. The VDR is widely distributed, and is not restricted to those tissues considered the classic target tissues of vitamin D. The VDR upon binding to 1,25(OH)2D heterodimerizes with other nuclear hormone receptors, in particular the family of retinoid X receptors. This complex then binds to special DNA sequences called vitamin D response elements (VDRE) generally within the genes it regulates, although these VDREs can be thousands of base pairs from the transcription start site. There are thousands of the VDREs in hundreds of genes, and the profile of active VDREs (and regulated genes) varies from cell to cell. A variety of additional proteins called coregulators complex with the VDR to activate (coactivators) or inhibit (corepressors) VDR transcriptional activity. Coactivator factors involved in VDR mediated transcription include factors with histone acetylase activity, including steroid receptor coactivator (SRC) 1, SRC 2 and SRC 3, and CREB-binding protein p300, in addition to the SWI-SNF ATP dependent chromatin remodeling complex, methyltransferases and the Mediator complex (aka DRIP), which functions to recruit RNA polymerases. VDR binding sites are associated with sites for other transcription factors such as p63, C-EBPα, C-EBPβ, Runx2 and PU.1, which can cooperate with VDR and VDR coregulators to influence 1,25(OH)2D responses in target cells. Among other functions these coregulators reconfigure the chromatin structure to bring the VDR/VDRE to the transcription start site, explaining how such distant VDR/VDREs can regulate gene transcription. In addition to coactivators there are a number of corepressors. One such corepressor of VDR action in the skin is called hairless, in that its loss or mutation, like that of the VDR, leads to altered hair follicle cycling resulting in baldness. Corepressors typically work by recruiting histone deacetylases (HDAC) or methyl transferases (MT) to the gene which reverses the actions of HAT, leading to a reduction in access to the gene by the transcription machinery. These coregulators can be specific for different genes, and different cells differentially express these coregulators, providing some specificity for the actions of 1,25(OH)2D and VDR.
In addition to regulating gene expression, 1,25(OH)2D has a number of non-genomic actions including the ability to stimulate calcium transport across the plasma membrane. The mechanisms mediating these non-genomic actions and their physiologic significance remain unclear. Similarly, it is not clear that all actions of the VDR require the ligand 1,25(OH)2D. The best example of this is the hair loss in animals and subjects with VDR mutations but not in animals and subjects with mutations in CYP27B1, the enzyme producing 1,25(OH)2D. As mentioned, the VDR is widely distributed, and the actions of 1,25(OH)2D are quite varied. The classic target tissues—bone, gut, and kidney—are involved with calcium homeostasis. The mechanisms by which 1,25(OH)2D regulates transcellular calcium transport are best understood in the intestine. Here 1,25(OH)2D stimulates calcium entry across the brush border membrane into the cell, transport of calcium through the cell, and removal of calcium from the cell at the basolateral membrane. Calcium entry at the brush border membrane occurs down a steep electrochemical gradient. It is controlled in large measure by a specific calcium channel called TRPV6 and in humans also by a homologous calcium channel TRPV5. Transport of calcium through the cell is regulated by a class of calcium binding proteins called calbindins. Much of the transport occurs within vesicles that form in the terminal web. Removal of calcium from the cell at the basolateral membrane requires energy and is mediated by the ATP requiring calcium pump or CaATPase (PMCA1b) as well as the sodium/calcium exchange protein (NCX1). 1,25(OH)2D induces TRPV6 and TRPV5, the calbindins, and the CaATPase, but not all aspects of transcellular calcium transport are a function of new protein synthesis. Animals null for calbindin 9k (the major calbindin in mammalian intestine) have little impairment of intestinal calcium transport. Animals null for TRPV6, on the other hand, have a reduction in intestinal calcium transport, but the deficit is not profound. Thus, it is likely that compensatory mechanisms for intestinal calcium transport exist that have yet to be discovered. Similar mechanisms mediate 1,25(OH)2D regulated calcium reabsorption in the distal tubule of the kidney. The proteins involved are homologous but not identical (TRPV5 and Calbindin 28k, for example). The situation in bone, however, is less clear. VDR are found in osteoblasts, the bone forming cells. 1,25(OH)2D promotes the differentiation of osteoblasts and regulates the production of proteins such as collagen, alkaline phosphatase, and osteocalcin thought to be important in bone formation. 1,25(OH)2D also induces RANKL, a membrane bound protein in osteoblasts that enables osteoblasts to stimulate the formation and activity of osteoclasts. Thus 1,25(OH)2D regulates both bone formation and bone resorption. Some evidence suggests that the major effect of 1,25(OH)2D on bone is to provide adequate levels of calcium and phosphate from the intestine. The rickets of patients with a mutated VDR or of mice in which the VDR has been deleted can be prevented/corrected by normalizing serum calcium and phosphate levels by dietary means. On the other hand, normal bone formation is not restored, and with time the VDR null mice become osteoporotic despite the high calcium/phosphate diet. Moreover, the VDR in osteoblasts/osteocytes appears to control bone resorption especially when dietary calcium is limited. Whether subjects with VDR mutations also develop osteoporosis prematurely or fail to maintain serum calcium in times of calcium deficiency has not been reported.
The non-classic actions of 1,25(OH)2D include regulation of cellular proliferation and differentiation, regulation of hormone secretion, and regulation of immune function. The ability of 1,25(OH)2D to inhibit proliferation and stimulate differentiation has led to the development of a number of analogs in the hopes of treating hyperproliferative disorders such as psoriasis and cancer without raising serum calcium. Psoriasis is now successfully treated with several vitamin D analogs. Observational studies are promising with respect to adequate vitamin D nutrition and cancer prevention. However, supplementation with vitamin D of subjects with adequate vitamin D levels to start with has not been shown to decrease cancer incidence but may be beneficial for cancer mortality. 1,25(OH)2D inhibits parathyroid hormone secretion and stimulates insulin secretion. A number of analogs and 1,25(OH)2D itself are currently available for use in the treatment of secondary hyperparathyroidism accompanying renal failure. Epidemiologic evidence indicates that vitamin D deficiency is associated with increased risk of both type 1 and type 2 diabetes mellitus, but prospective clinical trials to demonstrate a role for vitamin D supplementation in preventing the conversion of prediabetes to diabetes has not shown benefit in vitamin D replete individuals. However, there may be benefit in vitamin D deficient patients. The ability of 1,25(OH)2D to regulate immune function is likely part of its efficacy in the treatment of psoriasis. A number of other autoimmune diseases have been found in animal studies to respond favorably to vitamin D and 1,25(OH)2D or its analogs, and epidemiologic evidence linking vitamin D deficiency to increased incidence of these diseases has been reported. Similarly, epidemiologic evidence linking vitamin D deficiency to a number of respiratory illnesses is substantial, including increased risk of COVID-19 infections.