Clinical Syndromes of Vitamin D and Phosphate Dysregulation

1

Introduction

Vitamin D was first described in the year 1922 as an essential nutrient promoting calcium deposition and a possible cure for rickets. This description of a new vitamin was preceded by Sir Edward Mellanby’s experiment. He demonstrated that dogs kept on an oatmeal diet develop rickets and this can cured by including cod liver oil in their meals. The fact that these dogs were being deprived of sunlight remained unknown at that time. In 1922, McCollum attributed this property of cod liver oil to a new vitamin as it retained its antirachitic properties even after the vitamin A activity of the oil was destroyed by oxygenating it. In the following years, two independent experiments demonstrated that rachitic children can be cured by exposure to sunlight or artificially produced ultraviolet light. Steenbock and Black, further showed that foods can also be irradiated to impart antirachitic properties. Moreover, Goldblatt and Soames managed to cure rickets in rats by using tissue extracted from the liver of irradiated rats. The structure of vitamin D was finally described in 1931 by Askew et al. Vitamin D ₃ structure was later on described by Windaus et al. A lot more insight has been gained into the structure, function, and physiological role of vitamin D, since its first description.

The role of vitamin D in maintaining skeletal health is well established and accordingly, it is best described as a group of fat-soluble secosteroids involved in the intestinal absorption of calcium, phosphate, iron, and other minerals. The major health problem related to vitamin D deficiency is rickets and has been battled by fortification of food with various forms of vitamin D. However, the magnitude and manifestation of vitamin D deficiency is far more extensive. In utero and during infancy, it can present as growth retardation. In adults, it causes osteomalacia, exacerbation of osteoporosis, and muscle weakness. The importance of vitamin D is now better understood and extends beyond its role in rickets prevention, extending to multiple aspects of human physiology including immune system modulation, cancer physiology regulation via vitamin D receptor (VDR)-mediated signaling pathways, as well as cardiovascular disease (CVD) pathogenesis. Based on the recent advances, it is more appropriate to classify vitamin D as a prohormone rather than a vitamin. Moreover, the term vitamin is a misnomer since it can be endogenously synthesized. This chapter will review the clinical spectrum of vitamin D-related health problems and the recent advances in vitamin D research.

2

Metabolism

There are two major forms of vitamin D viz. vitamin D ₂ (ergocalciferol) and vitamin D ₃ (cholecalciferol). Vitamin D ₃ is the major form which is mainly produced in the skin. Both vitamin D ₂ and vitamin D ₃ are also obtained from dietary sources. Natural sources include salmon fish, cod liver oil, egg yolk, and shiitake mushrooms. Certain foods are fortified with vitamin D in the United States such as milk, orange juice, and yogurt. Vitamin D ₂ is derived from irradiation of yeast ergosterol and used for fortification.

3

Transport

Dietary vitamin D being fat-soluble is rapidly absorbed in the small intestine along with chylomicrons. These are delivered to hepatocytes where they undergo 25-hydroxylation and are added to the circulation to be transported to other tissues. Vitamin D ₃ from the skin also reaches the liver and has the same fate. All vitamin D metabolites are derivatives of cholesterol and hence are lipophilic molecules with low aqueous solubility. They require plasma protein carriers to be transported within the circulation. Vitamin D-binding protein (DBP) is the most important carrier. It binds all metabolites of vitamin D with varying affinity. 25-OH vitamin D has the highest affinity followed by 24,25-dihydroxy vitamin D, 1,25-dihydroxy, and native forms of vitamin D. Greater than 99% of circulating vitamin D metabolites is protein bound mainly to DBP and in lesser amounts to albumin and lipoproteins. Like other hormones, the biological activity of vitamin D correlates with the free hormone concentration with the protein bound portion acting as a buffer system to maintain appropriate levels. This buffer system helps in maintaining normal 1,25-OH vitamin D activity even when DBP levels change such as in case of liver disease, nephrotic syndrome, and estrogen therapy.

As previously mentioned, 25-OH vitamin D undergoes further bioactivation in the proximal tubule cells (PTC) of kidney. However, it does not enter PTC by simple diffusion across the basolateral membrane but by receptor-mediated uptake of DBP at the brush border. Endocytic receptor megalin is involved in this process. Defects in this receptor will cause vitamin D deficiency due to the loss of DBP and its bound metabolites in the urine.

4

Mechanism of Action

Vitamin D functions through a single, nuclear VDR which has been cloned for several species including humans and rats. VDR is very similar in design and functioning to the retinoic acid and thyroid hormone receptor. It is a transcription factor comprising three structural domains. An N-terminal with two zinc fingers that binds to DNA, a C-terminal ligand-binding domain, and a hinge domain joining these. The zinc fingers bind to discrete sites in DNA called vitamin D response elements (VDRE). VDRE’s are repeat sequences of six nucleotides separated by three nonspecified bases and are present in the promoter region of vitamin D-regulated genes. The binding of VDR to VDRE leads to recruitment of coregulatory complexes. These complexes can be both gene- and cell-specific and the type of coregulatory element recruited decides the gene or cellular function affected by VDR activation. This explains the selective action of vitamin D among different cell types. Many genes have been recognized to be regulated by VDR till date; however, CYP24 or 24-hydroxylase enzyme responsible for vitamin D degradation seems to be most closely reagulated.

All action of vitamin D cannot be explained by genomic changes. Some actions are too rapid which suggests other mechanisms are involved. A small proportion of VDR present in the membrane is believed to regulate the activity of many kinases, phosphatases, and ion channels. Understanding of these mechanisms is still under investigation and the expanse of cellular effects initiated by VDR activation is still not completely known.

5

Functions of Vitamin D

Vitamin D endocrine system is a network of interaction between kidney, bone, parathyroid gland, and the intestine. The aim of this system is to maintain extracellular calcium levels within the narrow range which is required for maintenance of normal cellular function and skeletal integrity. Calcium levels in plasma are sensed by transmembrane proteins in the parathyroid gland which are coupled with a G-protein system. Decrease in levels activates this system and stimulates secretion of parathyroid hormone (PTH). The major tissues for PTH action are osteoblasts and PTC. In the PTC, PTH increases the levels of 25-hydroxyvitamin D 1-α hydroxylase leading to increased production of 1,25-OH vitamin D. 1,25-OH vitamin D has four major target tissues viz. intestine, bone, kidney, and the parathyroid glands.

Action of vitamin D in the small intestine increases the absorption of dietary calcium from 10%–15% to about 30%–40%. Vitamin D increases the expression of epithelial calcium channels TRPV6 and TRPV5, calcibindin D, and calcium ATPase. TRPV6 and TRPV5 are highly calcium-selective channels and enhance the entry of calcium from the intestinal lumen. Inside the cell, calcibindin D transfers the calcium across the cell and delivers to the basolateral plasma membrane. Calcium ATPase, protein PMCA1b, and sodium-calcium exchanger finally deliver calcium to the bloodstream. The initial calcium uptake is the rate-limiting step in intestinal calcium absorption and highly dependent on vitamin D. In the skeleton, vitamin D is essential for both osteogenesis and osteoclastogenesis. Osteoblasts express a surface ligand called RANKL which can bind to either RANK or a soluble decoy receptor osteoprotegerin (OPG). RANK is present on osteoclast progenitors, and binding of RANKL stimulates the differentiation and maturation of osteoclasts. OPG plays a role in regulating this process by alternatively binding to RANKL and preventing the activation of RANK. 1,25-OH D ₃ increases the expression of RANKL and decreases OPG levels. This is necessary for maximal PTH-induced osteoclast production. It is important to note that 1,25-OH D ₃ in itself does not sustain osteoclastogenesis but is essential for PTH-induced osteoclast production.

Apart from being the source of 1,25-OH D ₃ , kidneys are also an important target organ. PTH receptors in the distal convoluted tubules regulate the reabsorption of filtered calcium. 1,25-OH D ₃ enhances this process by increasing expression of epithelial calcium channels (TRPV5) and calcibindin. This represents a major contribution to the calcium pool as the amount of calcium filtered (∼7g/day) in the kidneys is significant. Other than this, 1,25-OH D ₃ also regulates its own levels by suppressing the expression of 1-α-hydroxylase and increasing 24-hydroxylase in the PTC. 24-hydroxylase converts 25-OH vitamin D to 24,25-OH vitamin D which is inactive. As PTH increases vitamin D production, higher levels of vitamin D in turn regulate PTH synthesis and parathyroid cell growth. This is evident by parathyroid hyperplasia seen in vitamin D deficiency and the role of 1,25-OH D ₃ in treating secondary hyperparathyroidism of chronic kidney disease (CKD).

Apart from calcium, inorganic phosphorus is the other ionic component required for hydroxyapatite formation during mineralization of extracellular matrix. Therefore, as expected phosphate homeostasis goes hand in hand with calcium homeostasis. Undoubtedly, vitamin D plays a major role in the former as well. Action of 1,25-OH D ₃ in the small intestine increases the absorption of phosphorus from 60% to more than 80%. Dietary phosphate is absorbed from the duodenal and jejunal epithelium by both a concentration-dependent passive process and an active sodium-dependent process. The latter is increased by 1,25-OH D ₃ through increased levels of sodium-dependent phosphate transporters. 1,25-OH D ₃ along with PTH also stimulate the production of fibroblast growth factor-23(FGF-23) in bone. It is predominantly produced by osteoblasts and participates in a counterregulatory feedback loop with vitamin D. FGF-23 suppresses 1-α hydroxylase levels in the proximal tubules leading to reduced circulating levels of 1,25-OH D ₃ . FGF-23 inhibits renal reabsorption of phosphate and decreases the transcription of α-klotho protein. α-klotho is a coreceptor for FGF-23, expressed prominently in the distal convoluted tubule. Both FGF-23 and α-klotho levels are deranged in phosphate-related disorders, indicating their importance in maintaining phosphate homeostasis.