On the relationship between vitamin D action and actinomycin-sensitive processes

TRPV1 channels as a newly identified target for vitamin D

Vitamin D is known to elicit many biological effects in diverse tissue types and is thought to act almost exclusively upon its canonical receptor within the nucleus, leading to gene transcriptional changes and the subsequent cellular response. However, not all the observed effects of vitamin D can be attributed to this sole mechanism, and other cellular targets likely exist but remain to be identified. Our recent discovery that vitamin D is a partial agonist of the Transient Receptor Potential Vanilloid family 1 (TRPV1) channel may provide new insights as to how this important vitamin exerts its biological effects either independently or in addition to the nuclear vitamin D receptor. In this review, we discuss the literature surrounding this apparent discrepancy in vitamin D signaling and compare vitamin D with known TRPV1 ligands with respect to their binding to TRPV1. Furthermore, we provide evidence supporting the notion that this novel vitamin D/TRPV1 axis may explain some of the beneficial actions of this vitamin in disease states where TRPV1 expression and vitamin D deficiency are known to overlap. Finally, we discuss whether vitamin D may also act on other members of the TRP family of ion channels.

Homeostatic control of plasma calcium concentration

Due to the importance of Ca2+ in the regulation of vital cellular and tissue functions, the concentration of Ca2+ in body fluids is closely guarded by an efficient feedback control system. This system includes Ca(2+)-transporting subsystems (bone, and kidney), Ca2+ sensing, possibly by a calcium-sensing receptor, and calcium-regulating hormones (parathyroid hormone [PTH], calcitonin [CT], and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]). In humans and birds, acute Ca2+ perturbations are handled mainly by modulation of kidney Ca2+ reabsorption and by bone Ca2+ flow under PTH and possibly CT regulation, respectively. Chronic perturbations are also handled by the more sluggish but economic regulatory action of 1,25(OH2)D3 on intestinal calcium absorption. Peptide hormone secretion is modulated by Ca2+ and several secretagogues. The hormones' signal is produced by interaction with their respective receptors, which evokes the cAMP and phospholipase C-IP3-Ca2+ signal transduction pathways. 1,25 (OH)2D3 operates through a cytoplasmic receptor in controlling transcription and through a membrane receptor that activates the Ca2+ and phospholipase C messenger system. The calciotropic hormones also influence processes not directly associated with Ca2+ regulation, such as cell differentiation, and may thus affect the calcium-regulating subsystems also indirectly.

Transfer ribonucleic acid methylases of bone. Studies on vitamin A and D deficiency

Methods were devised for the assay of tRNA methylases of rat bone. The activities of bone tRNA methylases are similar to those from other mammalian tissues. However, unlike reports on liver methylases, no inhibitors were found in the supernatant fraction from pH5 precipitate of bone extracts. The effects of vitamins A and D on the methylation of tRNA by cell-free extracts of rat bone were studied. Deficiency of either vitamin resulted in a decrease in the rate and extent of tRNA methylation, whereas the administration of vitamin A to hypovitaminotic-A rats and vitamin D to hypovitaminotic-D rats increased the rate and extent of tRNA methylation. These effects appear to be apart from changes in ribonuclease activity or in concentrations of calcium or magnesium. No evidence of inhibitors of tRNA methylases was found in bone extracts from vitamin-deficient rats nor of activators in bone extracts from deficient rats given vitamin A or D. The pattern of tRNA methylation under conditions of vitamin A or D deficiency was not changed, suggesting a generalized cellular deficiency. It was of significance to find that the specificity for methylation of specific bases in tRNA was different after the administration of vitamin A as contrasted with the effects of vitamin D. The possible significance of tRNA methylation to the biochemical action of the vitamins on bone is discussed.

Mechanism of action of 1,25-dihydroxycholecalciferol on intestinal calcium transport

The prior administration of actinomycin D prevents the metabolism of [(3)H]25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, a metabolite of vitamin D(3) that is effective in the stimulation of intestinal calcium transport. In this paper, the question of whether the response of intestinal calcium transport to 1,25-dihydroxycholecalciferol is sensitive to actinomycin D was examined. While the response of intestinal transport to physiological amounts of 25-hydroxycholecalciferol is blocked by actinomycin D, the response of intestinal calcium transport to 1,25-dihydroxycholecalciferol is insensitive to the antibiotic. These results suggest that 1,25-dihydroxycholecalciferol, or a further metabolite thereof, is the metabolically active form of vitamin D in the intestine, that it functions by a process not involving transcription of DNA, and that the step sensitive to actinomycin D in the action of vitamin D on the intestine does not occur in the intestine, but is the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol in the kidney.

Inhibition of the metabolism of 25-hydroxycholecalciferol by actinomycin D and cycloheximide

Actinomycin D (or cycloheximide) administered prior to radioactive 25-hydroxycholecalciferol blocks the metabolism of 25-hydroxycholecalciferol to polar metabolites that accumulate in intestinal tissue, while it does not prevent the 25-hydroxylation of vitamin D(3) in the liver. Actinomycin D given after radioactive 25-hydroxycholecalciferol does not inhibit 25-hydroxycholecalciferol metabolism. These results indicate that 25-hydroxycholecalciferol must interact with the nuclei of cells to bring about the production of an enzyme(s) that converts it to its polar metabolites.

Vitamin D3-stimulated template activity of chromatin from rat intestine

Administration of vitamin D to rats deficient in this vitamin markedly increases the template activity for DNA-dependent RNA synthesis by rat intestinal mucosal chromatin. The maximum stimulation of template activity occurs three hours after a dose of 2000 international units of vitamin D(3) is given. These results support the concept that vitamin D functions by initiating the transcription of DNA into mRNA, which codes for functional protein(s) involved in calcium transport in the small intestine.

Studies on the effect of vitamin D on calcium absorption and transport

1. Mucosal cells of the small intestine obtained from rats deprived of vitamin D or given excessive amounts of the vitamin accumulated significantly more calcium than did cells from control animals. 2. Mucosal cells from vitamin D-deficient rats released less calcium than did cells from normal or hypervitaminotic D animals. 3. Studies in vivo showed that the transfer of (45)Ca from the intestine to the blood was delayed in vitamin D deficiency, but was accelerated in hypervitaminosis D. 4. The findings support the thesis that vitamin D is involved in the release of calcium rather than in its uptake by mucosal cells. 5. Further evidence is presented suggesting that uptake of calcium by intestinal mucosal cells at 0 degrees is primarily passive, whereas at 38 degrees uptake and release are effected by an active process that depends on energy derived from glycolytic activity.

Chromosomal receptor for a vitamin D metabolite

Evidence has been presented for the existence of an acidic protein(s) or protein portion of a more complex molecule which has a high affinity for binding noncovalently a biologically active metabolite of vitamin D. This molecule could be solubilized from the residual chromatin via treatment with either 0.3 M KCl or high pH and has been purified 167-fold over the crude mucosa homogenate. Characterization of the still crude receptor fraction showed that it contains significant amounts of RNA and that it may exist in multiple forms, i.e., a 50,000-70,000 and a >200,000 molecular weight species. The binding capacity of the receptor fraction for the metabolite is saturated after administration of a physiological dose of the parent vitamin D.

Chromosomal receptor for a vitamin D metabolite

Evidence has been presented for the existence of an acidic protein(s) or protein portion of a more complex molecule which has a high affinity for binding noncovalently a biologically active metabolite of vitamin D. This molecule could be solubilized from the residual chromatin via treatment with either 0.3 M KCl or high pH and has been purified 167-fold over the crude mucosa homogenate. Characterization of the still crude receptor fraction showed that it contains significant amounts of RNA and that it may exist in multiple forms, i.e., a 50,000-70,000 and a >200,000 molecular weight species. The binding capacity of the receptor fraction for the metabolite is saturated after administration of a physiological dose of the parent vitamin D.

Restoration of parathyroid responsiveness in vitamin D-deficient rats by parenteral calcium or dietary lactose

The effects of treatment with vitamin D, calcium, or lactose on the responsiveness of vitamin D-deficient rats to parathyroid hormone were compared. In the absence of vitamin D, parenteral calcium or dietary lactose administration resulted in increases in serum calcium concentration although not to the normal values obtained in animals given vitamin D. Dietary lactose also partially restored the low bone calcium content of vitamin D-deficient rats. Untreated vitamin D-deficient rats showed no significant changes in serum calcium concentration after parathyroidectomy or parathyroid extract administration. Vitamin D, lactose, and calcium all restored responsiveness to parathyroid hormone; serum calcium concentration decreased after parathyroidectomy and showed a dose-related increase in response to parathyroid extract. Hence, the unresponsiveness to parathyroid hormone in vitamin D deficiency may be due to a lack of calcium at a local site of action, presumably bone, rather than to the absence of vitamin D as a specific cofactor.