Vitamin D production in the skin under the influence of sunlight (UVB) is maximized at levels of sunlight exposure that do not burn the skin. Further metabolism of vitamin D to its major circulating form (25(OH)D) and hormonal form (1,25(OH)2D) takes place in the liver and kidney, respectively, but also in other tissues where the 1,25(OH)2D produced serves a paracrine/autocrine function: examples include the skin, cells of the immune system, parathyroid gland, intestinal epithelium, prostate, and breast. Parathyroid hormone, FGF23, calcium and phosphate are the major regulators of the renal 1-hydroxylase (CYP27B1, the enzyme producing 1,25(OH)2D); regulation of the extra renal 1-hydroxylase differs from that in the kidney and involves cytokines. The major enzyme that catabolizes 25(OH)D and 1,25(OH)2D is the 24-hydroxylase; like the 1-hydroxylase it is tightly controlled in the kidney in a manner opposite to that of the 1-hydroxylase, but like the 1-hydroxylase it is widespread in other tissues where its regulation is different from that of the kidney. Vitamin D and its metabolites are carried in the blood bound to vitamin D binding protein (DBP) and albumin--for most tissues it is the free (i.e., unbound) metabolite that enters the cell; however, DBP bound metabolites can enter some cells such as the kidney and parathyroid gland through a megalin/cubilin mechanism. Most but not all actions of 1,25(OH)2D are mediated by the vitamin D receptor (VDR). VDR is a transcription factor that partners with other transcription factors such as retinoid X receptor that when bound to 1,25(OH)2D regulates gene transcription either positively or negatively depending on other cofactors to which it binds or interacts. The VDR is found in most cells, not just those involved with bone and mineral homeostasis (i.e., bone, gut, kidney) resulting in wide spread actions of 1,25(OH)2D on most physiologic and pathologic processes. Animal studies indicate that vitamin D has beneficial effects on various cancers, blood pressure, heart disease, immunologic disorders, but these non-skeletal effects have been difficult to prove in humans in randomized controlled trials. Analogs of 1,25(OH)2D are being developed to achieve specificity for non-skeletal target tissues such as the parathyroid gland and cancers to avoid the hypercalcemia resulting from 1,25(OH)2D itself. The level of vitamin D intake and achieved serum levels of 25(OH)D that are optimal and safe for skeletal health and the non-skeletal actions remain controversial, but are likely between an intake of 800-2000IU vitamin D in the diet and 20-50ng/ml 25(OH)D in the blood. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text, WWW.ENDOTEXT.ORG.
reflected in you sylvia day pdf free 87
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 vitamin D metabolites are transported in blood bound primarily to vitamin D binding protein (DBP) (85-88%) and albumin (12-15%) (99-101). DBP concentrations are normally 4-8µM, well above the concentrations of the vitamin D metabolites, such that DBP is only about 2% saturated. DBP has high affinity for the vitamin D metabolites (Ka=5x108M-1 for 25OHD and 24,25(OH)2D, 4x107M-1 for 1,25(OH)2D and vitamin D), such that under normal circumstances only approximately 0.03% 25OHD and 24,25(OH)2D and 0.4% 1,25(OH)2D are free (100-102). Conditions such as liver disease and nephrotic syndrome resulting in reduced DBP and albumin levels will lead to a reduction in total 25OHD and 1,25(OH)2D levels without necessarily affecting the free concentrations (103) (figure 3). Similarly, DBP levels are reduced during acute illness, potentially obscuring the interpretation of total 25OHD levels (104). Earlier studies with a monoclonal antibody to measure DBP levels suggested a decreased level in African Americans consistent with their lower total 25OHD levels, but these results were not confirmed using polyvalent antibody-based assays (105). Vitamin D intoxication can increase the degree of saturation sufficiently to increase the free concentrations of 1,25(OH)2D and so cause hypercalcemia without necessarily raising the total concentrations (106).
The vitamin D metabolites bound to DBP are in general not available to most cells. Thus, the free or unbound concentration is that which is critical for cellular uptake as postulated by the free hormone hypothesis. Support for the concept that the role of DBP is to provide a reservoir for the vitamin D metabolites but that it is the free concentration that enters cells and exerts biologic function comes from studies in mice in which DBP has been deleted and in humans in which the gene is mutated. In DBP knockout mice the vitamin D metabolites are presumably all free and/or bioavailable. These mice do not show evidence of vitamin D deficiency unless placed on a vitamin D deficient diet despite having very low levels of serum 25OHD and 1,25(OH)2D (107). Tissue levels of 1,25(OH)2D were found to be normal in the DBP knockout mice as were markers of vitamin D action such as expression of intestinal TRPV6, calbindin 9k, PMCA1b, and renal TRPV5 (108). Recently a family in which a large deletion of the coding portion of the DBP gene (and adjacent NPFFR2 gene) has been reported (109). The proband had normal calcium, phosphate and PTH levels with vitamin D supplementation despite very low levels of 25OHD, 24,25(OH)2D, and 1,25(OH)2D that were not responsive to massive doses of vitamin D (oral or parenteral). The free 25OHD was nearly normal. The carrier sibling had vitamin D metabolite levels between those of the proband and the normal sibling. Thus, both the studies in DBP null mice and humans support the free hormone hypothesis while also supporting the role of DBP as a circulating reservoir for the vitamin D metabolites. Therefore, there is currently a debate as to whether the free concentration of 25OHD, for example, is a better indicator of vitamin D nutritional status than total 25OHD, given that DBP levels, and hence total 25OHD levels, can be influenced by liver disease, nephrotic syndrome, pregnancy, and inflammatory states (110,111). However, certain tissues such as the kidney, placenta, and parathyroid gland express the megalin/cubilin complex which is able to transport vitamin D metabolites bound to DBP into the cell. This is critical for preventing renal losses of the vitamin metabolites (112) and may be important for vitamin D metabolite transport into the fetus and regulation of PTH secretion. Indeed, mice lacking the megalin/cubilin complex have poor survival with evidence of osteomalacia indicating its role in vitamin D transport into critical cells involved with vitamin D signaling
Correlation of total 25OHD (A) and 1,25(OH)2D (C) levels to DBP; lack of correlation of free 25OHD (B) and 1,25(OH)2D (D) levels to DBP. Data from normal subjects (open triangles), subjects with liver disease (closed triangles, open circles), subjects on oral contraceptives (open triangles*), and pregnant women (open squares) are included. These data demonstrate the dependence of total 25OHD and 1,25(OH)2D concentrations on DBP levels which are reduced by liver disease. However, the free concentrations of 25OHD and 1,25(OH)2D are normal in most patients with liver disease. Reprinted with permission from the American Society for Clinical Investigation.
Intestinal calcium absorption, in particular the active component of transcellular calcium absorption, is one of the oldest and best known actions of vitamin D having been first described in vitro by Schachter and Rosen (187) in 1959 and in vivo by Wasserman et al. (188) in 1961. Absorption of calcium from the luminal contents of the intestine involves both transcellular and paracellular pathways. The transcellular pathway dominates in the duodenum and cecum, and this is the pathway primarily regulated by 1,25 dihydroxyvitamin D (1,25(OH)2D) (189), although elements of the paracellular pathway such as the claudins 2 and 12 are likewise regulated by 1,25(OH)2D (reviews in (190,191). Figure 7 shows a model of our current understanding of how this process is regulated by 1,25(OH)2D. Calcium entry across the brush border membrane (BBM) occurs down a steep electrical-chemical gradient and requires no input of energy. Removal of calcium at the basolateral membrane must work against this gradient, and energy is required. This is achieved by the CaATPase (PMCA1b), an enzyme induced by 1,25(OH)2D in the intestine. Calcium movement through the cell occurs with minimal elevation of the intracellular free calcium concentration (192) by packaging the calcium in calbindin containing vesicles (193-195) that form in the terminal web following 1,25(OH)2D administration.
Calmodulin is the major calcium binding protein in the microvillus (212). Its concentration in the microvillus is increased by 1,25(OH)2D; no new calmodulin synthesis is required or observed after 1,25(OH)2D administration (213). Calmodulin is likely to play a major role in calcium transport within the microvillus, and inhibitors of calmodulin block 1,25(OH)2D stimulated calcium uptake by BBMV (214). Within the microvillus calmodulin is bound to a 110kD protein, myosin 1A (myo1A)) (previously referred to as brush border myosin 1). 1,25(OH)2D increases the binding of calmodulin to myo1A in brush border membrane preparations (213), although binding of calmodulin to the myo1A attached to the actin core following detergent extraction of the membrane appears to be reduced (215). The calmodulin/myo1A complex appears late in the development of the brush border, and is found in highest concentration in the same cells of the villus which have the highest capacity for calcium transport (216). Myo1A is located primarily in the microvillus of the mature intestinal epithelial cell, although small amounts have been detected associated with vesicles in the terminal web (217). Thus, the calmodulin/myo1A complex may be responsible for moving calcium out of the microvillus. Its exact role in calcium transport is unclear in that mice null for myo1A do not show reduced intestinal calcium transport(218)). Calbindin is the dominant calcium binding protein in the cytoplasm (212,219), where it appears to play the major role in calcium transport from the terminal web to the basolateral membrane (190). The increase in calbindin levels in the cytosol following 1,25(OH)2D administration is blocked by protein synthesis inhibitors (210). Indeed, calbindin was the first protein discovered to be induced by vitamin D (219). Glenney and Glenney (212) observed that calbindin has a higher affinity for calcium than does calmodulin. The differential distribution of calmodulin and calbindin between microvillus and cytosol combined with the differences in affinity for calcium led Glenney and Glenney (212) to propose that in the course of calcium transport calcium flowed from calmodulin in the microvillus to calbindin in the cytosol with minimal change in the free calcium concentration in either location. However, the role of calbindin in intestinal calcium transport does not appear to be critical in that mice null for calbindin9k grow normally, have normal intestinal calcium transport, and their serum calcium levels and bone mineral content are equivalent to wildtype mice regardless of the calcium content of the diet (220). The CaATPase (PMCA1b) at the basolateral membrane and the sodium/calcium exchanger (NCX1) are responsible for removing calcium from the cell against the same steep electrochemical gradient as favored calcium entry at the brush border membrane (221). Related proteins are found in the renal distal tubule. As its name implies, the extrusion of calcium from the cell by the calcium pump requires ATP. This pump is a member of the PMCA family, and in the intestine the isoform PMCA1b is the major isoform found. This pump is induced by 1,25(OH)2D (222). Calmodulin activates the pump, but calbindin may do likewise (223). Deletion of Pmca1b reduces calcium absorption and blocks 1,25(OH)2D stimulation of such resulting in reduction in growth and bone mineralization (224)., Moreover, the deletion of protein 4.1R, which regulates PMCA1b expression in the intestine, results in decreased intestinal calcium transport (225). The role of NCX is not considered to be as important as PMCA1b for intestinal calcium transport (226). 2ff7e9595c
Comments