The principal biological effects of vitamin D on calcium metabolism, mediated by 1α,25(OH)2D3 are as follows: in the intestine to stimulate the absorption of dietary calcium and to some extent phosphate; in the skeleton to promote both the mineralization of bone matrix and to stimulate osteoclast cell bone resorption (both calcium and phosphate); and in the kidney to reduce the urinary excretion of phosphate and calcium (i.e., to stimulate the renal tubular reabsorption of these two ions). Of these three systems, the most thoroughly studied biochemically is the intestinal Ca2+ absorption process.
Figure 1 presents a schematic model describing the process of 1α,25(OH)2D3 mediated intestinal calcium absorption. Both the 9 kDa CaBP (mammals) and the 28-kDa (birds) CaBP species are soluble proteins found exclusively in the cytosol of intestinal and kidney cells, where they constitute 1–3% of the soluble cellular proteins. The intracellular concentration of Ca2+ in all cells should be maintained no higher than ≈10−7 M (or 0.1 μM) so that Ca2+ at these low concentrations can function as a second messenger. Thus, when dietary Ca2+ is moved across the brush border through the TRPV5 and TRPV6 calcium channels and enters the cytoplasm of the cell, in the absence of the calcium binding protein(s), the intracellular Ca2+ con centration could rise to >500 μM. Thus, the function of the calbindin is to bind all the entering Ca2+ very tightly and to move by diffusion across the cell to the basal lateral membrane where the Ca2+ is off-loaded from the calbindin and pumped by the plasma mem brane Ca2+ ATPase (PMCA1b) into the extracellular fluid and then the blood compartment. The levels of PMCA1b are regulated by 1α,25(OH)2D3 acting through the VDR.
Fig1. Schematic model describing the actions of 1α,25(OH)2D3 in the intestine in stimulating intestinal calcium transport. In this model, the blood transport protein, vitamin D binding protein (DBP) delivers 1α,25(OH)2D3 to the basal lateral membrane of the intestinal cell, where the secosteroid crosses by diffusion and then moves to the cell nucleus and binds to the vitamin D receptor (VDR). VDR with bound 1α,25(OH)2D3 is responsible for upregulating the genes for cytosolic calbindin D9K (mammals) or calbindin 28K (birds) and TRPV6 (a Ca2+ channel located in the brush border membrane). In the intestine TRPV5 is constitutively expressed and is functional in the absence of 1α,25(OH)2D3. Thus Ca2+ present in the lumen of the intestine (light blue dots) enters across the intestinal brush border via the Ca2+ channels. The “free” Ca2+ then binds tightly to the calbindin D9K or D28K (Kd is 0.1 μM) and crosses the interior of the cell to the basal lateral membrane where a calcium pump (plasma membrane Ca2+ ATPase or PMCA), moves the Ca2+ released from the CaBP across the basal lateral membrane where it enters the cytosol and then blood compartment. Thus the 1α,25(OH)2D3 mediated intestinal Ca2+ transport process contributes Ca2+ to elevate the serum calcium concentration. An alternate Ca2+ absorption pathway, the paracellular pathway, involves the movement of Ca2+ between adjacent intestinal cells. The paracellular pathway may provide 10–15% of the intestinal absorption of Ca2+.
The effect of 1α,25(OH)2D3 on stimulating the intestinal absorption of phosphate is known, but the mechanisms are not understood. Similarly, the detailed effects of 1α,25(OH)2D3 on increasing the kidney’s proximal tubule reabsorption of phosphate and the distal tubule reabsorption of Ca2+ are also known but without a clear mechanistic insight. 1α,25(OH)2D3 also has an important effect on increasing the secretion of the phosphaturic hormone, FGF23 (see Figure 2).
Fig2. Schematic of binding of Fibroblast Growth Factor 23 (FGF23) to its receptor in a proximal kidney cell, resulting in the reduction of gene expression of two sodium and phosphate transporters and also the 25(OH)D3-1α-hydroxylase. The FGF23 receptor requires the presence of a protein cofactor known as klotho to generate its signal transduction signals. Generation of biological responses by FGF23 requires the interaction on the surface of the proximal (basal lateral) side of the kidney cell of FGF23 binding to a Klotho-FGF receptor dimeric complex creating formation of a trimeric complex. This trimeric complex then activates phosphorylation of the FGF receptor and activation of the intracellular signal activating the ERK kinase pathway. This then leads to the reduction of gene expression of the NaPi-2a and NaPi-2c electrogenic phosphate transporters and also the expression of the 25(OH)D3-1α-hydroxylase.
