TRPV6 is not required for 1α,25-dihydroxyvitamin D ₃ -induced intestinal calcium absorption in vivo

Abstract

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.A primary function of vitamin D is to markedly increase intestinal absorption of calcium and phosphate (1). During the 1950s, this absorption was shown to be primarily an active calcium transport process and an independent active phosphate transport process (2, 3). With the discovery of the vitamin D endocrine system came the understanding that this process was directly stimulated by the hormonal form of vitamin D, 1␣,25-dihydroxyvitamin D 3 (1,25-(OH) 2 D 3 ) (4). Because this hormonal form of vitamin D is regulated in response to the need for calcium, it became clear that the endogenous factor discovered by Nicolaysen and Egg-Larsen is the vitamin D endocrine system producing the final vitamin D hormone, 1,25-(OH) 2 D 3 (4, 5). There is still debate whether vitamin D further influences the diffusional component of calcium absorption, taking place at high intestinal levels of calcium (6, 7).The molecular mechanism underlying active calcium transport in response to vitamin D began unraveling with the discovery of calbindin D 9k by Wasserman and colleagues (8). Schachter and colleagues (9) also found a transporter responsive to vitamin D. Others have reported that vitamin D stimulates the basal lateral membrane calcium ATPase believed to be a calcium transporter (10, 11). These components have been put together in a diagrammatic fashion to present the current hypothesis of how 1,25-(OH) 2 D 3 stimulates active intestinal calcium absorption. TRPV6 is a calcium channel protein clearly induced by 1,25-(OH) 2 D 3 (12). Calcium entering through this channel is believed to associate with calbindin D 9k , which serves as a shuttle for calcium, presenting it to the basal lateral membrane calcium ATPase. This step provides the energy input for the transfer of calcium against a concentration gradient. All three of these genes are clearly under the influence of the active form of vitamin D (13). Unfortunately not all evidence is currently in support of this hypothesis. Transgenic mice in which calbindin D 9k has been eliminated have shown that calbindin D 9k is not required for 1,25-(OH) 2 D 3 -stimulated intestinal calcium absorption (14, 15). Fo...

1,25-Dihydroxyvitamin D3 Controls a Cohort of Vitamin D Receptor Target Genes in the Proximal Intestine That Is Enriched for Calcium-regulating Components

“…Second, this idea is reinforced by the fact that although numerous investigations over many years have identified multiple components that are regulated by 1,25(OH) 2 D 3 and that contribute to calcium uptake, they have failed to identify a single component that upon deletion fully recapitulates the state of vitamin D deficiency. Trpv6 expression, for example, exerts a significant impact on calcium transport independent of its regulation by 1,25(OH) 2 D 3 , yet it fails upon deletion to block intestinal response to 1,25(OH) 2 D 3 (15,29). Overexpression of calbindin D9K, however, has no impact on intestinal calcium absorption, and Pmca1 has not been examined.…”

“…Moreover, many of these genes have been genetically deleted in mice, providing the opportunity to assess whether they yield a vitamin D deficiency-type phenotype, although the lack of a complete vitamin D deficiency phenotype in mice containing deletions of S100g, Trpv6, or their combination and the more complex contributions of a gene network suggest that individual deletions may not reveal this type of deficiency phenotype (14,29). A number of genes regulated by 1,25(OH) 2 D 3 in addition to S100g, Trpv6, Atp2b1, and Cldn2 have been suggested to be involved in calcium regulation, although their primary function may be different.…”

“…Finally, Trpv6, a member of the transient receptor potential vanilloid family of genes and located in the brush border membranes of the intestinal epithelia (27), has been identified as a potential mediator of calcium uptake into the enterocyte. While retaining many of the characteristics necessary for a true transepithelial calcium transporter, genetic deletion of this gene was similarly unable to prevent the induction of calcium uptake through the intestine by 1,25(OH) 2 D 3 , although deletion of this gene did compromise overall calcium uptake by this tissue (28,29). In contrast to Atp2b1 and S100g, however, contemporary studies of Trpv6 have identified the locations of enhancer elements upstream of the gene's transcription start site (TSS) that mediate up-regulation by 1,25(OH) 2 D 3 (30,31).…”

Calcium Homeostasis and Regulation of Calbindin-D9kby Glucocorticoids and Vitamin D as Bioactive Molecules

“…This previous study together with microarray results indicated that TRPV6 is not required for vitamin D-induced intestinal calcium absorption and may not carry out a significant role in this process. These results obtained from CaBP-9k null and other calcium transport mutant mice illustrated that exact molecular events in the intestinal calcium transport in response to the active form of vitamin D are warranted to be elucidated (Kutuzova et al, 2008).…”

“…On the other hand, the requirement of TRPV6 for vitamin D-dependent intestinal calcium absorption in vivo was examined in vitamin D-deficient TRPV6 null and WT mice (Kutuzova et al, 2008). This previous study together with microarray results indicated that TRPV6 is not required for vitamin D-induced intestinal calcium absorption and may not carry out a significant role in this process.…”