Metabolites of 1 alpha, 25-dihydroxyvitamin D3 in rat bile

Calcitroic Acid-A Review

Calcitroic acid was isolated and characterized almost four decades ago, but little is known about this important vitamin D metabolite. Four reported synthetic strategies to generate calcitroic acid are presented that highlight the scientific progress in the field of chemistry directed to vitamin D analog synthesis. The most recent synthesis described the generation of calcitroic acid with an overall yield of 12.8% in 13 steps. The endogenous formation of calcitroic acid has been demonstrated in perfused rat kidney using 24,25,26,27-tetranor-1,23(OH)₂D₃. Although, the majority of vitamin D metabolism is mediated by 24-hydoxylase (CYP24A1), it is not clear why the formation of calcitroic acid was not observed in the presence of recombinant CYP24A1 enzyme. Furthermore, it is not known if enzyme 1α-hydroxylase (CYP27B1) can convert calcioic acid into calcitroic acid. In addition to the lack of research investigating the endogenous formation of calcitroic acid, the physiological role of calcitroic acid remains unknown. Only a few reports mentioned the biological activity of calcitroic acid in connection with the vitamin D receptor (VDR). When administered subcutaneously, calcitroic acid has anthracitic properties and elevates calcium blood levels when administered intravenously. In vitro, calcitroic acid at higher concentrations has been shown to bind VDR and induce gene transcription. However, these studies were not carried out in cells derived from target organs of calcitroic acid such as kidney, liver, and intestine. We can conclude that our current knowledge of calcitroic acid is limited, and more studies are needed to identify its physiological role.

Vitamin D, disease and therapeutic opportunities

The discovery of the vitamin D endocrine system and a receptor for the hormonal form, 1α,25-dihydroxyvitamin D(3), has brought a new understanding of the relationship between vitamin D and metabolic bone diseases, and has also established the functions of vitamin D beyond the skeleton. This has ushered in many investigations into the possible roles of vitamin D in autoimmune diseases, cardiovascular disorders, infectious diseases, cancers and granuloma-forming diseases. This article presents an evaluation of the possible roles of vitamin D in these diseases. The potential of vitamin D-based therapies in treating diseases for which the evidence is most compelling is also discussed.

Evolution of our understanding of vitamin D

The development of our understanding of the function of vitamin D from its discovery in the second and third decades of the 20(th) century to its hormonal activation of its nuclear receptor and to its present position of an important factor in public health has been traced. The key discoveries of the conversion of vitamin D to its hormonal form, its regulation, and the evolving picture of its molecular mechanism of action are presented. The recognition of its role beyond mineralization of the skeleton to its role in skin, the immune system, and its protective role in some forms of malignancy represent more recent developments. The evolution of derivatives of 1alpha,25-dihydroxyvitamin D(3) as therapeutic agents suggests a richness of therapeutic potential. All of this nevertheless illustrates that much more remains to be discovered and applied to our armaments for preventing and treating disease.

Synthesis and Structure-Activity Relationships of Bioactive Compounds Using Sterols

Sterols are widely and abundantly distributed in nature. It is convenient to utilize them for the preparation of useful compounds such as pharmaceuticals with steroid and secosteroid skeletons. This paper describes the synthesis and structure-activity relationships of naturally occurring active forms of vitamin D analogues, sterols having neurite outgrowth activity, and liver X receptor agonist. The active form of vitamin D(4) showed similar biological activities but had higher affinity to the vitamin D-binding protein compared with the corresponding vitamins D(2) and D(3). This shows that the active form of vitamin D(4) is a good candidate for an agent to replace the active forms of vitamins D(2) and D(3). In the course of screening for low molecular-weight compounds that exhibit neurite outgrowth activity in the culture broth, we found that the natural product dictyosterol showed strong activity. From screening of the analogues, it was found that the double bond between C22 and C23 in the side chain of the sterol is essential for its activity. Ergost-22-ene-1alpha,3beta-diol was found to serve as a stronger liver X receptor agonist than 24(S), 25-epoxycholesterol, which regulates the expression of genes involved in lipid metabolism. Structure-function study showed that the 1alpha-hydroxyl group, the saturated steroid structure, and the double bond between C22 and C23 are needed to function as a liver X receptor agonist.

An update on the therapeutic potential of vitamin D analogues

The review provides an evaluation of the therapeutic potential of vitamin D analogues in the context of the current understanding of vitamin D biochemistry, molecular biology and physiology. Vitamin D activity results from several circulating and intracellular physiological metabolites acting simultaneously through at least three receptors. Common analogues are reviewed. Although most vitamin D analogues have traditionally been analogues of 1,25-dihydroxyvitamin D, it may be better to deliver high doses of base vitamin or (analogues) of 25-hydroxyvitamin D. This would permit physiological endocrine, paracrine and autocrine vitamin D metabolism. Agonists or antagonists of tissue-specific vitamin D metabolic pathways could be coadministered. The importance of measuring endogenous vitamin D metabolites during in vivo studies and the pitfalls of extending data across species and time are emphasised. Human vitamin D analogue trials should include direct comparison against the related endogenous metabolite.

26-Hydroxylation of 1α,25-Dihydroxyvitamin D3 Does Not Occur under Physiological Conditions

The 26-hydroxylation of 1alpha,25-dihydroxyvitamin D3 in rats in vitro and in vivo was studied under physiological conditions. Incubation of 1alpha,25-dihydroxy-[26,27-3H]vitamin D3 with rat kidney or rat liver homogenate showed formation of a metabolite that was identified as 1alpha,25(S),26-trihydroxy-[26,27-3H]vitamin D3 by comigration on three different HPLC systems and a periodate cleavage reaction. This metabolite was not generated by hydroxylation of 1alpha,25-dihydroxy-[26,27-3H]vitamin D3 itself but by an enzymatic conversion of a precursor that was formed nonenzymatically in substantial amounts upon storage of 1alpha,25-dihydroxy-[26,27-3H]vitamin D3 in ethanol at -20 degrees C under argon for more than three weeks. An in vivo metabolism study in rats dosed with a physiological dose of 1alpha,25-dihydroxy-[26,27-3H]vitamin D3 confirmed the absence of 26-hydroxylation of the hormone. As expected at 6 h postinjection of purified 1alpha,25-dihydroxy-[26,27-3H]vitamin D3, 1alpha,24(R),25-trihydroxy-[26,27-3H]vitamin D3, as well as traces of (23S,25R)-1alpha,25-dihydroxy-[3H]vitamin D3-lactone were detected and identified on straight phase and reverse phase HPLC in serum, kidney, and liver.

A phase one study of the hepatic arterial administration of 1,25-dihydroxyvitamin D3 for liver cancers

BACKGROUND AND AIMS

It is well established that exposure to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) inhibits the proliferation of human colorectal cancer and hepatoma cell lines, both in vitro and in vivo. However, clinical trials of the administration of 1,25(OH)2D3 and analogs for the treatment of malignancy have been limited by the development of hypercalcemia. 1,25-dihydroxyvitamin D3 is principally excreted in bile following hepatic catabolism. This suggested the hypothesis that hepatic regional administration may allow high doses of 1,25(OH)2D3 to be administered for the treatment of liver cancers without producing hypercalcemia, caused by a clinically significant first pass effect. This phase one study investigates the effect of hepatic regional administration of 1,25(OH)2D3 on serum calcium levels, together with other markers of renal and liver function.

METHODS

Six subjects with hepatic colorectal cancer metastases and one with primary hepatocellular cancer were given continuous hepatic arterial infusions of 1,25(OH)2D3, for periods of 1-4 weeks. Blood samples were taken regularly and assayed for calcium levels, liver function tests and urea and electrolyte levels.

RESULTS

Patients remained normocalcemic at dosages of up to 10 mcg/day. No patient experienced any side-effects from the treatment.

CONCLUSIONS

Administration of 1,25(OH)2D3 as a continuous hepatic arterial infusion allows a high dosage to be administered without inducing hypercalcemia. This route of administration may allow the potential of 1,25(OH)2D3 in the treatment of hepatic cancers to be realized.

Study on the metabolites of 1alpha,25-dihydroxyvitamin D4

Three major metabolites of 1alpha,25-dihydroxyvitamin D(4) were isolated from the bile of rat and the structures were elucidated on the basis of spectral data and the periodate oxidative cleavage of the diol structures of the metabolites. One of the metabolites was the known calcitroic acid. Another two metabolites were isomers and identified as 9,10-secoergosta-5,7,10(19)-triene-1alpha,3beta,24,25-tetrahydroxy-26-oic acid and 9,10-secoergosta-5,7,10(19)-triene-1alpha,3beta,24,25-tetrahydroxy-28-oic acid. It was found that 1alpha,25-dihydroxyvitamin D(4) is metabolized in a similar manner in vivo to that of 1alpha,25-dihydroxyvitamin D(2) but differently from 1alpha,25-dihydroxyvitamin D(3).

Metabolism of vitamin D(3) by human CYP27A1

Human vitamin D(3) 25-hydroxylase (CYP27A1) cDNA was expressed in Escherichia coli, and its enzymatic properties were revealed. The reconstituted system containing the membrane fraction prepared from the recombinant E. coli cells was examined for the metabolism of vitamin D(3). Surprisingly, at least eight forms of metabolites including the major product 25(OH)D(3) were observed. HPLC analysis and mass spectrometric analysis suggested that those metabolites were 25(OH)D(3), 26(OH)D(3), 27(OH)D(3), 24R,25(OH)(2)D(3), 1alpha, 25(OH)(2)D(3, )25,26(OH)(2)D(3) (25,27(OH)(2)D(3)), 27-oxo-D(3) and a dehydrogenated form of vitamin D(3). These results suggest that human CYP27A1 catalyzes multiple reactions and multiple-step metabolism toward vitamin D(3). The K(m) and V(max) values for vitamin D(3) 25-hydroxylation and 25(OH)D(3) 1alpha-hydroxylation were estimated to be 3.2 microM and 0.27 (mol/min/mol P450), and 3.5 microM and 0.021 (mol/min/mol P450), respectively. These kinetic studies have made it possible to evaluate a physiological meaning of each reaction catalyzed by CYP27A1.

Antagonistic action of novel 1alpha,25-dihydroxyvitamin D3-26, 23-lactone analogs on differentiation of human leukemia cells (HL-60) induced by 1alpha,25-dihydroxyvitamin D3

We examined the effects of two novel 1alpha,25-dihydroxyvitamin D3-26,23-lactone (1alpha,25-lactone) analogues on human promyelocytic leukemia cell (HL-60) differentiation using the evaluation system of the vitamin D nuclear receptor (VDR)/vitamin D-responsive element (DRE)-mediated genomic action stimulated by 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) and its analogues. We found that the 1alpha,25-lactone analogues (23S)-25-dehydro-1alpha-hydroxyvitamin-D3-26,23-lactone (TEI-9647), and (23R)-25-dehydro-1alpha-hydroxyvitamin-D3-26,23-lactone (TEI-9648) bound much more strongly to the VDR than the natural (23S, 25R)-1alpha,25(OH)2D3-26,23-lactone, but did not induce cell differentiation even at high concentrations (10(-6) M). Intriguingly, the differentiation of HL-60 cells induced by 1alpha,25(OH)2D3 was inhibited by either TEI-9647 or TEI-9648 but not by the natural lactone. In contrast, retinoic acid or 12-O-tetradecanoylphorbol-13-acetate-induced HL-60 cell differentiation was not blocked by TEI-9647 or TEI-9648. In separate studies, TEI-9647 (10(-7) M) was found to be an effective antagonist of both 1alpha,25(OH)2D3 (10(-8) M) mediated induction of p21(WAF1, CIP1) in HL-60 cells and activation of the luciferase reporter assay in COS-7 cells transfected with cDNA containing the DRE of the rat 25(OH)D3-24-hydroxylase gene and cDNA of the human VDR. Collectively the results strongly suggest that our novel 1alpha,25-lactone analogues, TEI-9647 and TEI-9648, are specific antagonists of 1alpha, 25(OH)2D3 action, specifically VDR/DRE-mediated genomic action. As such, they represent the first examples of antagonists, which act on the nuclear VDR.

Conjugated forms of [3H] alpha, 25-dihydroxyvitamin D3 in rat bile

When small doses of [3H]D3, [3H]25-OHD3 and [3H]alpha, 25-diOHD3 were administered intravenously to rats 6.3 +/- 1.1% (means +/- SEM, n = 4), 9.7 +/- 0.9% (n = 6) and 12.8 +/- 2.6% (n = 8), respectively, of the administered radioactivity was excreted in bile. The radioactive biliary conjugated metabolites were analysed by ion exchange chromatography: in the case of all 3 substrates about 30% of metabolites were found to be cationic on the basis of their being retained on sulphopropyl-Sephadex G-25 (H+-form) when applied in 70% methanol. The balance of the metabolites were neutral and anionic and were analysed on TEAP-Lipidex: in the case of 1 alpha, 25-diOHD3 the following metabolite classes were detected (on the basis of co-elution with authentic standards) (in order of quantitative importance): taurine conjugates, neutral metabolites, monosulphates, glucuronides, carboxylic acids, glycine conjugates and disulphates. Alkaline hydrolysis of the taurine and glycine conjugates yielded products 60% of which now chromatographed as carboxylic acids. Hydrolysis of the glucuronide and monosulphate fractions indicated significant levels of mixed conjugation yielding some products which now chromatographed as glycine and taurine conjugates, respectively. The nature of the cationic conjugates was not elucidated but they had the following properties: they could be hydrolysed by alkali to yield non-cationic radioactive metabolites (these released metabolites were heterogeneous as judged by TEAP-lipidex chromatography); they were partially hydrolysed to non-cationic forms by beta-glucuronidase; and on reverse-phase HPLC they had an elution profile that was significantly different to that of histidyl-, ornithyl- or lysyl-calcitroic acid.

The metabolism and functions of vitamin D

Vitamin D functions by stimulating intestinal calcium and phosphorus absorption, by stimulating bone calcium mobilization, and by increasing renal reabsorption of calcium in the distal tubule. These functions on bone and possibly kidney, but not intestine, require the parathyroid hormone. As a result of these functions, serum calcium and phosphorus concentrations are elevated to supersaturating levels required for the mineralization of bone to prevent rickets, osteomalacia, and hypocalcemic tetany. Recent experiments demonstrate that maintaining serum calcium and phosphorus levels in vitamin D-deficient rats in the normal range results in normal bone growth and mineralization. However, increased calcification results because bone resorption by osteoclasts is a vitamin D-dependent process. Thus, bone resorption, modeling and remodeling must be considered vitamin D-dependent processes. Vitamin D must be metabolized to 25-hydroxyvitamin D3 by the liver and subsequently by the kidney to 1,25-dihydroxyvitamin D3 before function. 1,25-Dihydroxyvitamin D3 is metabolized to a C-23 carboxylic acid (calcitroic acid) but the pathway is unknown. Although 25-hydroxyvitamin D3 is metabolized to 24R,25-dihydroxyvitamin D3, 25,26-dihydroxyvitamin D3 and 25-hydroxyvitamin D3-26,23-lactone, these pathways play no role in the function of vitamin D as shown by appropriate fluoro analogs of 25-hydroxyvitamin D3. 1,25-Dihydroxyvitamin D3 binds to a specific receptor in the intestinal nuclei to elicit a stimulation of calcium transport. 1,25-Dihydroxyvitamin D3 plus the receptor causes transcription of specific genes that code for calcium and phosphorus transport proteins. Only one protein, the calcium binding protein, has been identified as being vitamin D dependent. Two others have been described, but no clear description of the molecular mechanism of action of 1,25-dihydroxyvitamin D3 is yet available.

End product inhibition of hepatic 25-hydroxyvitamin D production in the rat: specificity and kinetics

The role of vitamin D metabolites in the regulation of hepatic 25-hydroxyvitamin D production was investigated by examining the effects of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, and 24,25-dihydroxyvitamin D on the synthesis of [25-3H]hydroxyvitamin D by rachitic rat liver homogenates. Production of [25-3H]hydroxyvitamin D was inhibited by 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, but not by 24,25-dihydroxyvitamin D. 25-Hydroxyvitamin D increased the Km of the vitamin D-25-hydroxylase enzyme(s), while 1,25-dihydroxyvitamin D decreased the Vmax with a Ki of 88.7 ng/ml. Inhibition of hepatic 25-hydroxyvitamin D production by 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D may be another control mechanism to regulate circulating vitamin D levels.

1,25 dihydroxyvitamin D-induced inhibition of 3H-25 hydroxyvitamin D production by the rachitic rat liver in vitro

The effect of the vitamin D metabolites 1,25 dihydroxyvitamin D (100 pg/ml) and 25-hydroxyvitamin D (30 ng/ml) on hepatic production of 3H-25 hydroxyvitamin D was investigated using rachitic liver perfusions and homogenates. 1,25 dihydroxyvitamin D inhibited hepatic 3H-25 hydroxyvitamin D production in the liver perfusion (3.6 +/- 0.4 vs 2.0 +/- 0.5 pmol/liver, P less than 0.05) and in liver homogenates (11.9 +/- 0.6 vs 10.1 +/- 0.4 pmol/g liver protein/3 h, P less than 0.02). Inhibition was time and dose dependent. 25-hydroxyvitamin D inhibited production in liver homogenates (11.9 +/- 0.6 vs 9.2 +/- 0.1 pmol/g liver protein/3 h, P less than 0.05) but not in the intact liver (3.6 +/- 0.4 vs 3.4 +/- 0.5 pmol/liver). The data indicate that 1,25 dihydroxyvitamin D is able to feedback regulate the production of its precursor, 25-hydroxyvitamin D. Although 25-hydroxyvitamin D also inhibits its own production in liver homogenates, it failed to alter total production in the intact liver, suggesting that this metabolite may require immediate access to the vitamin D 25-hydroxylase, located on the microsomes and mitochondria, to induce inhibition.

23,24,25-Trihydroxyvitamin D3, 24,25,26-trihydroxyvitamin D3, 24-keto-25-hydroxyvitamin D3, and 23-dehydro-25-hydroxyvitamin D3: new in vivo metabolites of vitamin D3

Four new in vivo metabolites of vitamin D3 were isolated from the blood plasma of chicks given large doses of vitamin D3. The metabolites were isolated by methanol-chloroform extraction and a series of chromatographic procedures. By use of mass spectrometry, ultraviolet absorption spectrophotometry, and specific chemical reactions, the metabolites were identified as 23,24,25-trihydroxyvitamin D3, 24,25,26-trihydroxyvitamin D3, 24-keto-25-hydroxyvitamin D3 and 23-dehydro-25-hydroxyvitamin D3.

Subcellular mechanisms involving vitamin D

During the past 15 years a vitamin D endocrine system has been demonstrated in which vitamin D produced normally in the skin is activated first by conversion in the liver and subsequently in the kidney to a hormonal form, 1,25-(OH)2D3. The production of the hormonal form of vitamin D3, is regulated, and much has been learned regarding the molecular mechanism of the hydroxylations of vitamin D and regarding the physiologic regulators of the 25-OH-D-1-hydroxylase. Much remains to be learned regarding the mechanism whereby the 1-hydroxylase is modulated. 1,25-(OH)2D3 appears to function in the target organs of bone, intestine, kidney, and elsewhere by a nucleus-mediated process. Receptors for 1,25-(OH)2D3 have been clearly demonstrated and characterized in crude form. How the receptor and ligand interact with the nucleus is not clear, nor are the gene products that result from this interaction known. One product, a calcium binding protein, is known but its role in calcium transport is in debate. Although much has been learned in the last decade and a half, much remains to be learned regarding the molecular mechanisms whereby vitamin D brings about its remarkable changes in mineral metabolism.