Intestinal membrane calcium-binding protein. Vitamin D-dependent membrane component of the intestinal calcium transport mechanism

TRPV6 is not required for 1alpha,25-dihydroxyvitamin D3-induced intestinal calcium absorption in vivo

The requirement for TRPV6 for vitamin D-dependent intestinal calcium absorption in vivo has been examined by using vitamin D-deficient TRPV6 null mice and littermate wild-type mice. Each of the vitamin D-deficient animals received each day for 4 days 50 ng of 1,25-dihydroyvitamin D(3) in 0.1 ml of 95% propylene glycol:5% ethanol vehicle or vehicle only. Both the wild-type and TRPV6 null mice responded equally well to 1,25-dihydroxyvitamin D(3) in increasing intestinal calcium absorption. These results, along with our microarray data, demonstrate that TRPV6 is not required for vitamin D-induced intestinal calcium absorption and may not carry out a significant role in this process. These and previous results using calbindin D9k null mutant mice illustrate that molecular events in the intestinal calcium absorption process in response to the active form of vitamin D remain to be defined.

Alkaline phosphatase and ATPases in brush-border membranes of rat jejunum: distinct effects of divalent cations and of some inhibitors

The effects of divalent cations and of some inhibitors on the activities of alkaline phosphatase and ATPase were examined in rat jejunal brush-border membranes (BBM) isolated by tha Ca(2+)-(BBMCa) or the Mg(2+)-precipitation method (BBMMg). Similar results were found in BBMCa and BBMMg though generally higher in BBMCa. Alkaline phosphatase activity was stimulated by 5 mM MgCl2 (30% to 44%), but not by 5 mM CaCl2 or 0.1 mM ZnCl2, at pH 9.5 or 7.4. ATPase activity was equally stimulated by 5 mM MgCl2 and by 5 mM CaCI2 (about 150%). Alkaline phosphatase activity was significantly inhibited by 1 mM vanadate, 5 mM diamox, 5.0 mM L-leucine and 1 mM theophylline. In contrast, Ca(2+)-ATPase and Mg(2+)-ATPase activities were not depressed by those alkaline phosphatase inhibitors, but were inhibited by 0.1 mM trifluoperazine (more than 70%). 0.1 mM ZnCl2 also appeared to be inhibitory to Ca(2+)-ATPase and Mg(2+)-ATPase, but not to alkaline phosphatase activity even in the presence of Ca2+ and Mg2+. These results suggest that Ca(2+)-ATPase and Mg(2+)-ATPase activities of the rat jejunal BBM are not merely manifestations of alkaline phosphatase, but rather belong to (a) distinct enzyme(s).

Transport and binding in calcium uptake by microvillous membrane of human placenta

Calcium entry across the microvillous membrane of the human placental syncytiotrophoblast is the first step in the transfer of this important nutrient to the fetus. Calcium uptake by isolated microvillous membranes was time dependent. Equilibrium uptake was very much greater than could be explained by equilibration of the vesicle space with medium, indicating that calcium is bound to internal sites. Addition of the ionophore A23187 greatly increased the rates of influx and efflux, indicating that transport across the plasma membrane is rate limiting in entry. Concentration dependence data for calcium transport at 4 s fit well to a Michaelis-Menten equation having two saturable sites and diffusion. Calcium entry by both transporters was unaffected by calcium channel blockers but was strongly inhibited by the group II metals. The distinct inhibition constant values for strontium inhibition provided additional evidence for two transporters. Calcium binding fit well to a single-site model saturable in the micromolar range. In vivo, the saturable transport processes may mediate calcium entry into the syncytiotrophoblast and binding may regulate concentration within the placental microvilli.

Rapid stimulation of calcium uptake by isolated rat enterocytes by 1,25(OH)2D3

Evidence is accumulating that 1,25(OH)2D3 may stimulate calcium transport from the intestinal lumen extremely rapidly by a mechanism which appears independent of de novo protein synthesis. To investigate this rapid action of 1,25(OH)2D3, the rate of calcium uptake by isolated enterocytes from duodena of young rats was determined in vitro as the uptake of 45Ca from 1-15 min. Prior in vitro exposure of cells to 1,25(OH)2D3 (100 pM) for 20 min significantly increased the rate of calcium uptake (p less than 0.001), an effect unaltered by 50 microM cycloheximide. Incubation with 100 pM 1-alpha-hydroxyvitamin D3 produced the same effect (p less than 0.01). In contrast, exposure to 10 pM 1,25(OH)2D3, as well as to 100 pM or to 1,000 pM 25-hydroxyvitamin D3 induced no significant change. Because both 1,25(OH)2D3 and starvation may stimulate key enzymes in polyamine metabolism, we investigated the effects of (i) difluoromethyl-ornithine (CHF2-Orn), a specific irreversible inhibitor of ornithine decarboxylase and (ii) varying the timing of feeding prior to sacrifice. Both in vitro CHF2-Orn and feeding prior to sacrifice significantly decreased the baseline rate of calcium uptake (p less than 0.05) and reduced the effect of 1,25(OH)2D3. Increased duration of starvation significantly increased the baseline rate of calcium uptake (p less than 0.02) without changing the increment in rate of calcium uptake induced by 1,25(OH)2D3. The study suggests (i) that the early action of 1,25(OH)2D3 on the influx process of intestinal calcium transport may involve a different molecular specificity from that involved in the genomic action of 1,25(OH)2D3 and (ii) that changes in polyamine metabolism may play a part in this process.

On the molecular mechanism of intestinal calcium transport

The intestinal absorption of calcium is certainly a complex process, dependent on several factors of which vitamin D, via 1,25(OH)2D3, is the major controlling hormone. The efficiency of calcium absorption is a function of calcium status and calcium need. As the body's demand for calcium increases, the process commonly termed, adaptation, is activated in which the synthesis of 1,25(OH)2D3 from precursor is increased, resulting in the stimulation of the rate of calcium absorption. The increased demand for calcium might result from the ingestion of a diet deficient in calcium, from growth, pregnancy, lactation and egg shell formation in the laying hen. Accomapanying the change in calcium absorptive efficiency are molecular modifications of the transporting enterocytes, some mentioned herein and elsewhere (Wasserman & Chandler, 1985; Wasserman, 1980; Wasserman et al., 1984). Highly correlated with the rate of calcium absorption under a wide variety of conditions is the concentration of the vitamin D-induced calcium-binding protein, calbindin-D28K (avian type) and calbindin-D9K (mammalian intestinal type). The role of calbindin-D in this transport process is not precisely known but is considered to act at the present time as a cytosolic facilitator of Ca2+ diffusion from the brush border membrane to the basolateral membrane. In addition to the induction of calbindin-D synthesis, 1,25(OH)2D3 exerts other effects on the intestinal epithelium that can have consequences on the calcium absorptive process. Some of these effects are summarized in Figure 14. Vitamin D-dependent reactions might be either direct effects of 1,25(OH)2D3 or indirect effects due to elevated intracellular Ca2+ concentrations. These include changes in the fluidity of the brush border membrane, an increase in microvillar alkaline phosphatase-low affinity Ca-activated ATPase activity, an association of calmodulin with the 105 kD brush border cytoskeletal protein and, following calbindin D synthesis, the binding of calbindin D to a 60 kD brush border protein and to microtubules. The latter has been suggested to be related to the proposed transfer of Ca2+ by an endocytotic-exocytotic mechanism. In addition, a vitamin D-dependent intestinal membrane calcium-binding protein has been identified (Kowarski & Schachter, 1980). Playing into this multi-component system is a stimulation of cyclic nucleotide synthesis by 1,25(OH)2D3 which, through activation of cyclic nucleotide-dependent protein kinases, might modify membrane Ca2+ "channels" by phosphorylation reactions.4+ Intracellular organelles, i.e., the endoplasmic reticulum, mitochondria, the Golgi apparatus, are potent sequesters of Ca2+ and could contribute to the protection of the cell from excessively high Ca2+ concentrations by transiently storing absorbed Ca2+.

Vitamin D-dependent active calcium transport: the role of CaBP

Transepithelial calcium transport in the intestine involves an active and a passive route. The active route is totally vitamin D-dependent, transcellular, and is largely expressed in the proximal intestine. Of the three steps involved in transcellular transport--entry into the mucosal cell, intracellular movement, and extrusion at the basolateral pole of the cell--neither entry nor extrusion appears rate-limiting in the absence of vitamin D, even though both are enhanced as a result of the action of the vitamin D. However, intracellular calcium movement inside the mucosal cell can match the experimental Vm of transcellular transport only in the presence of the vitamin D-dependent calcium-binding protein (CaBP, Mr = 8.8kDa). CaBP is thought to act as the equivalent of a calcium ferry by amplifying the intracellular movement of calcium. Thus, the major action of vitamin D on cellular calcium transport is via its hormonal product, CaBP, which amplifies intracellular calcium movement by raising total and free calcium levels in the transporting cell.

Comparison of calbindin D-28K immunoreactivity in superficial pineal bodies of mongolian gerbil and rat

Immunocytochemical reaction for demonstration of calbindin D-28K has been performed in superficial pineal bodies of the Mongolian gerbil (Meriones unguiculatus) and the rat. Whereas in the Mongolian gerbil there were no clearly expressed calbindin immunoreactive cells, these were numerous in the rat pineal body. Here the calbindin-positive cells - probably pinealocytes - were disposed along capillaries. In view of the role of calbindin in binding and transporting calcium and regulating its intracellular levels, the absence of this protein in the gerbil pineal body has been interpreted as signifying the inability of pinealocytes to eliminate intracellular calcium with possible consequent formation of acervuli.

The metabolism and mechanism of action of 1,25-dihydroxyvitamin D3

Much has been learned about the formation of the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3. Information concerning its formation and catabolism has allowed a clear understanding of factors involved in the maintenance of plasma concentrations of the hormone. The effects of 1,25-dihydroxyvitamin D3 on calcium transporting cells in the intestine are marked and well defined. The tissue (intestinal tissue) is easily isolated and manipulated and hence, this is an ideal tissue in which to examine the mechanism of divalent cation transport. The mechanism by which 1,25-dihydroxyvitamin D3 brings about this effect should help in understanding sterol hormone action.

1,25-Dihydroxyvitamin D3-stimulated mRNAs in rat small intestine

The technique of differential hybridization has been employed to study gene expression associated with vitamin D action on the mammalian intestine. A cDNA library consisting of 10(6) independent recombinants was constructed from poly(A)+ RNA extracted from vitamin D-deficient rats given 1,25-dihydroxyvitamin D3. A survey of 20,000 clones resulted in identification of four distinct cDNAs whose corresponding mRNAs are significantly increased 12 h after an intrajugular dose of 1,25-dihydroxyvitamin D3 given to vitamin D-deficient rats. DNA sequence analysis identified these mRNAs as mitochondrial ATP synthetase, vitamin D-dependent calcium binding protein, cytochrome oxidase subunit I, and cytochrome oxidase subunit III. The time course of response of three of these mRNAs was similar, with maximum values at 12 h after dosing, while that of cytochrome oxidase subunit I showed two peaks at 6 and 18 h following a single dose of 1,25-dihydroxyvitamin D3. The levels of all four mRNAs were elevated in rats supplied with vitamin D when hypocalcemia was produced by dietary calcium restriction.

The liver plasma membrane Ca2+ pump: hormonal sensitivity

The liver plasma membrane Ca2+ pump is supposed to extrude cytosolic calcium out of the cell. This system has now been well defined on the basis of its plasma membrane origin, its high affinity Ca2+ -stimulated ATPase activity, its Ca2+ transport activity, its phosphorylated intermediate. The liver calcium pump appears to be a target of hormonal action since it has been shown that glucagon and calcium mobilizing hormones namely alpha 1-adrenergic agonists, vasopressin, angiotensin II inhibit this system. The present review details the mechanism of calcium pump inhibition by glucagon and points out its difference from the inhibition process induced by calcium mobilizing hormones. We conclude that the inhibitory action of the Ca2+ mobilizing hormones and glucagon on the liver plasma membrane Ca2+ pump might play a key role in the actions of these hormones by prolonging the elevation in cytosolic free Ca2+.

1,25-dihydroxyvitamin D3 rapidly alters the morphology of the duodenal mucosa of rachitic chicks: Evidence for novel effects of 1,25-dihydroxyvitamin D3

When rachitic chicks are given 1,25-dihydroxyvitamin D3 in amounts as low as 50 ng/bird, the appearance of the duodenal mucosa is altered within 2 h of the administration of the hormone. The changes are most readily apparent on scanning electron microscopy and include: a more plump appearance of villi with loss of furrows and pits on their surfaces, elongation of villi and a smoother, more uniform microvillus surface. These changes occur within 2 h of the administration of the hormone and persist for as long as 24 h. The morphological change precedes the increase in calcium absorption induced by 1,25-dihydroxyvitamin D3 in the intestine. These observations suggest that 1,25-dihydroxyvitamin D3 may play an important part in maintenance of the structure of the duodenal mucosa.

Biosynthesis and compartmentalization of rat-intestinal vitamin-D-dependent calcium-binding protein

We have purified the primary translation product of rat intestinal vitamin-D-dependent calcium-binding protein mRNA from wheat germ and ascites cell-free systems. We show that calcium-binding protein is neither synthesized as a larger percursor nor likely to be exported from the intestinal epithelium. Our conclusions are based on the following observations. (1) The primary translation product, NH2-terminally labeled with formyl[35S]methionine, comigrates with the mature cytoplasmic protein during electrophoresis through denaturing gels. (2) It does not possess a cleavable signal peptide sequence or internal signal equivalent as judged by co- and post-translational cleavage assays in vitro. (3) The NH2 terminus of the cell-free product is acetylated. (4) Comparison of the NH2-terminal amino acid sequences of the primary translation product and cyanogen bromide peptides obtained from the blocked, purified cytoplasmic protein. The kinetics of calcium-binding protein mRNA accumulation and decay in rachitic intestinal epithelium after primary and secondary stimulation with 1,25-dihydroxycholecalciferol (calcitriol) were studied using the cell-free translation system. The results are reminiscent of other steroid-hormone-inducible systems. Both the rate of mRNA accumulation and the peak response were greater after secondary stimulation.

Rat duodenal calcium-binding protein messenger RNA: induction by 1,25-dihydroxyvitamin D3

To extend our previous observations on the regulation of CaBP biosynthesis by 1,25-dihydroxyvitamin D3, we have studied the specific mRNA encoding this protein in vitamin D-deficient and in vitamin D-repleted rats as well as the rate of its induction after a single injection of 1,25(OH)2D3 to vitamin D-deficient animals. The CaBP-mRNA was quantified by translation in a cell-free reticulocyte lysate system. CaBP-mRNA activity and cytoplasmic CaBP (measured by radioimmunoassay), dramatically decreased in rats previously fed a vitamin D-free diet for 5 weeks but neither parameter was zero. In vitamin D-deficient rats, a single injection of 1,25(OH)2D3 led to an increase in CaBP-mRNA activity within 2 h. This CaBP-mRNA activity peaked at about 4-6 h and thereafter declined to low value by 48 h, and the changes in mRNA activity always preceded the changes in cytosolic CaBP concentration. These results indicate that the induction of CaBP biosynthesis results from a 1,25(OH)2D3-induced increase in the levels of total cellular CaBP-mRNA activity and are, therefore, consistent with a transcriptional regulation of CaBP biosynthesis by 1,25(OH)2D3. This study also shows that the production of many other proteins seem to be under the control of vitamin D3.

A high-affinity (Ca2+ + Mg2+)-ATPase in plasma membranes of rat ascites hepatoma AH109A cells

The activity of calcium-stimulated and magnesium-dependent adenosinetriphosphatase which possesses a high affinity for free calcium (high-affinity (Ca2+ + Mg2+)-ATPase, EC 3.6.1.3) has been detected in rat ascites hepatoma AH109A cell plasma membranes. The high-affinity (Ca2+ + Mg2+)-ATPase had an apparent half saturation constant of 77 +/- 31 nM for free calcium, a maximum reaction velocity of 9.9 +/- 3.5 nmol ATP hydrolyzed/mg protein per min, and a Hill number of 0.8. Maximum activity was obtained at 0.2 microM free calcium. The high-affinity (Ca2+ + Mg2+)-ATPase was absolutely dependent on 3-10 mM magnesium and the pH optimum was within physiological range (pH 7.2-7.5). Among the nucleoside trisphosphates tested, ATP was the best substrate, with an apparent Km of 30 microM. The distribution pattern of this enzyme in the subcellular fractions of the ascites hepatoma cell homogenate (as shown by the linear sucrose density gradient ultracentrifugation method) was similar to that of the known plasma membrane marker enzyme alkaline phosphatase (EC 3.1.3.1), indicating that the ATPase was located in the plasma membrane. Various agents, such as K+, Na+, ouabain, KCN, dicyclohexylcarbodiimide and NaN3, had no significant effect on the activity of high-affinity (Ca2+ + Mg2+)-ATPase. Orthovanadate inhibited this enzyme activity with an apparent half-maximal inhibition constant of 40 microM. The high-affinity (Ca2+ + Mg2+)-ATPase was neither inhibited by trifluoperazine, a calmodulin-antagonist, nor stimulated by bovine brain calmodulin, whether the plasma membranes were prepared with or without ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetra-acetic acid. Since the kinetic properties of the high-affinity (Ca2+ + Mg2+)-ATPase showed a close resemblance to those of erythrocyte plasma membrane (Ca2+ + Mg2+)-ATPase, the high-affinity (Ca2+ + Mg2+)-ATPase of rat ascites hepatoma cell plasma membrane is proposed to be a calcium-pumping ATPase of these cells.

Characterization of the Ca2+- and Mg2+-dependent ATPases in Electrophorus electroplax microsomes

Electrophorus electroplax microsomes were examined for Ca2+- and Mg2+-dependent ATPase activity. In addition to the previously reported low-affinity ATPase, a high-affinity (Ca2+,Mg2+)-ATPase was found. At low ATP and Mg2+ concentrations (200 microM or less), the high-affinity (Ca2+,Mg2+)-ATPase exhibits an activity of 18 nmol Pi mg-1 min-1 with 0.58 microM Ca2+. At higher ATP concentrations (3 mM), the low-affinity Ca2+-ATPase predominates, with an activity of 28 nmol Pi mg-1 min-1 with 1 mM Ca2+. In addition, Mg2+ can also activate the low-affinity ATPase (18 nmol Pi mg-1 min-1). The high-affinity ATPase hydrolyzes ATP at a greater rate than it does GTP, ITP, or UTP and is insensitive to ouabain, oligomycin, or dicyclohexylcarbodiimide inhibition. The high-affinity enzyme is inhibited by vanadate, trifluoperazine, and N-ethylmaleimide. Added calmodulin does not significantly stimulate enzyme activity; rinsing the microsomes with EGTA does not confer calmodulin sensitivity. Thus the high-affinity ATPase from electroplax microsomes is similar to the (Ca2+,Mg2+)-ATPase reported to be associated with Ca2+ transport, based on its affinity for calcium and its response to inhibitors. The low-affinity enzyme hydrolyzes all tested nucleoside triphosphates, as well as diphosphates, but not AMP. Vanadate and N-ethylmaleimide do not inhibit the low-affinity enzymes. The low-affinity enzyme reflects a nonspecific nucleoside triphosphatase, probably an ectoenzyme.

Serum 1,25-dihydroxyvitamin D levels in subjects with X-linked hypophosphatemic rickets and osteomalacia

In order to determine whether a defect in vitamin D metabolism might play a role in the pathogenesis of X-linked hypophosphatemic rickets and osteomalacia (XLH), we compared the serum 1,25-dihydroxyvitamin D [1,25(OH)2D] level in 52 normal subjects and 37 patients with XLH. In untreated patients, adults were found to have values similar to age-matched controls, while youths had values similar to growth-rate-matched controls but significantly lower than the levels of age-matched controls who were growing at a normal rate. In contrast, treated XLH patients of all ages had serum levels significantly lower than both controls and untreated XLH patients. Further, the serum levels of 1,25(OH)2D in these treated patients had a significant inverse linear correlation with serum 25-(OH)D concentrations. We propose that subjects with XLH have serum 1,25(OH)2D levels within appropriate age- and growth-rate-matched normal ranges. However, in the presence of hypophosphatemia, we would have anticipated elevated levels of 1,25(OH)2D; viewed in this light the serum 1,25(OH)2D levels are inadequate, suggesting the presence of a relative deficiency of this active vitamin D metabolite.