Immunopurified 25-hydroxyvitamin D 1 alpha-hydroxylase and 1,25-dihydroxyvitamin D 24-hydroxylase are closely related but distinct enzymes

The Role of Intestinal Cytochrome P450s in Vitamin D Metabolism

Vitamin D hydroxylation in the liver/kidney results in conversion to its physiologically active form of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. 1,25(OH)2D3 controls gene expression through the nuclear vitamin D receptor (VDR) mainly expressed in intestinal epithelial cells. Cytochrome P450 (CYP) 24A1 is a catabolic enzyme expressed in the kidneys. Interestingly, a recently identified mutation in another CYP enzyme, CYP3A4 (gain-of-function), caused type III vitamin D-dependent rickets. CYP3A are also expressed in the intestine, but their hydroxylation activities towards vitamin D substrates are unknown. We evaluated CYP3A or CYP24A1 activities on vitamin D action in cultured cells. In addition, we examined the expression level and regulation of CYP enzymes in intestines from mice. The expression of CYP3A or CYP24A1 significantly reduced 1,25(OH)2D3-VDRE activity. Moreover, in mice, Cyp24a1 mRNA was significantly induced by 1,25(OH)2D3 in the intestine, but a mature form (approximately 55 kDa protein) was also expressed in mitochondria and induced by 1,25(OH)2D3, and this mitochondrial enzyme appears to hydroxylate 25OHD3 to 24,25(OH)2D3. Thus, CYP3A or CYP24A1 could locally attenuate 25OHD3 or 1,25(OH)2D3 action, and we suggest the small intestine is both a vitamin D target tissue, as well as a newly recognized vitamin D-metabolizing tissue.

Vitamin D-Mediated Hypercalcemia: Mechanisms, Diagnosis, and Treatment

Hypercalcemia occurs in up to 4% of the population in association with malignancy, primary hyperparathyroidism, ingestion of excessive calcium and/or vitamin D, ectopic production of 1,25-dihydroxyvitamin D [1,25(OH)₂D], and impaired degradation of 1,25(OH)₂D. The ingestion of excessive amounts of vitamin D₃ (or vitamin D₂) results in hypercalcemia and hypercalciuria due to the formation of supraphysiological amounts of 25-hydroxyvitamin D [25(OH)D] that bind to the vitamin D receptor, albeit with lower affinity than the active form of the vitamin, 1,25(OH)₂D, and the formation of 5,6-trans 25(OH)D, which binds to the vitamin D receptor more tightly than 25(OH)D. In patients with granulomatous disease such as sarcoidosis or tuberculosis and tumors such as lymphomas, hypercalcemia occurs as a result of the activity of ectopic 25(OH)D-1-hydroxylase (CYP27B1) expressed in macrophages or tumor cells and the formation of excessive amounts of 1,25(OH)₂D. Recent work has identified a novel cause of non-PTH-mediated hypercalcemia that occurs when the degradation of 1,25(OH)₂D is impaired as a result of mutations of the 1,25(OH)₂D-24-hydroxylase cytochrome P450 (CYP24A1). Patients with biallelic and, in some instances, monoallelic mutations of the CYP24A1 gene have elevated serum calcium concentrations associated with elevated serum 1,25(OH)₂D, suppressed PTH concentrations, hypercalciuria, nephrocalcinosis, nephrolithiasis, and on occasion, reduced bone density. Of interest, first-time calcium renal stone formers have elevated 1,25(OH)₂D and evidence of impaired 24-hydroxylase-mediated 1,25(OH)₂D degradation. We will describe the biochemical processes associated with the synthesis and degradation of various vitamin D metabolites, the clinical features of the vitamin D-mediated hypercalcemia, their biochemical diagnosis, and treatment.

Genetic mutation in the human 25-hydroxyvitamin D3 1alpha-hydroxylase gene causes vitamin D-dependent rickets type I

Vitamin D is deeply involved in a wide variety of biological events such as calcium homeostasis, bone formation and cellular differentiation. An active form of vitamin D, 1alpha,25(OH)2D3, serves as a vitamin D receptor (VDR)-specific ligand to activate the expression of a particular set of target genes. 1Alpha,25(OH)2D3, is biosynthesized from cholesterol, and at the final biosynthesis step, 25-hydroxyvitamin D3 1alpha-hydroxylase [1alpha(OH)ase] in kidney conducts 1alpha-hydroxylation of 25(OH)2D3. This enzymatic activity is under multihormonal regulation and critical for the biosynthesis. Molecular cloning of 1alpha(OH)ase from several species has revealed that this enzyme belongs to a member of the cytochrome P450 enzyme superfamily, with highest homologies to the P450 hydroxylases for vitamin D derivatives. The renal gene expression is strictly regulated at the transcriptional level through its gene promoter by PTH and calcitonine (positive) and 1alpha,25(OH)2D3 (negative). Most importantly in clinical aspects, genetic mutations in this gene to abolish the enzymatic activity are now shown to cause the one of three kinds of hereditary rickets, vitamin D-dependent rickets type I.

The importance of 25-hydroxyvitamin D3 1 alpha-hydroxylase gene in vitamin D-dependent rickets

Vitamin D plays a role in a wide variety of biological events such as calcium homeostasis, bone formation and cellular differentiation. An active form of vitamin D acting as a ligand specific vitamin D receptor (VDR), 1 alpha,25(OH)2D3, is biosynthesized from cholesterol, and during this biosynthesis a renal 25-hydroxylation at the final stage by 25-hydroxyvitamin D3 1 alpha-hydroxylase is critical. Recent studies isolated the cDNA encoding 1 alpha-hydroxylase from several species, and revealed that this enzyme belongs to a member of the cytochrome p450 enzyme superfamily, with highest homologies to the p450 hydroxylases for vitamin D derivatives. One of three kinds of hereditary rickets (vitamin D-dependent rickets type I) displays an autosomal recessive trait and clinical features consistent with a defect of 1 alpha-hydroxylase activity, and the genetic analysis of the type I patients identified missense mutations of the 1 alpha(OH)ase gene that results in a loss of this enzymatic activity.

25-Hydroxyvitamin D3 1alpha-hydroxylase and vitamin D synthesis

Renal 25-hydroxyvitamin D3 1alpha-hydroxylase [1alpha(OH)ase] catalyzes metabolic activation of 25-hydroxyvitamin D3 into 1alpha, 25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], an active form of vitamin D, and is inhibited by 1alpha,25(OH)2D3. 1alpha(OH)ase, which was cloned from the kidney of mice lacking the vitamin D receptor (VDR-/- mice), is a member of the P450 family of enzymes (P450VD1alpha). Expression of 1alpha(OH)ase was suppressed by 1alpha, 25(OH)2D3 in VDR+/+ and VDR+/- mice but not in VDR-/- mice. These results indicate that the negative feedback regulation of active vitamin D synthesis is mediated by 1alpha(OH)ase through liganded VDR.

A possible role for CYP27 as a major renal mitochondrial 25-hydroxyvitamin D3 1 alpha-hydroxylase

A mitochondrial cytochrome P450 fraction catalyzing 1 alpha- and 27-hydroxylation but not 24-hydroxylation of 25-hydroxyvitamin D3 was purified from pig kidney. The ratio between the 1 alpha- and 27-hydroxylase activities was the same in all purification steps including a side fraction. Attempts to separate the 1 alpha- and 27-hydroxylase activities were unsuccessful. A monoclonal antibody directed against purified pig liver CYP27 recognized a protein of the same apparent M(r) and immunoprecipitated both the 1 alpha- and 27-hydroxylase activities towards 25-hydroxyvitamin D3 in the purified kidney enzyme fraction as well as in a solubilized, crude cytochrome P450 extract considered to represent the major part of the 25-hydroxyvitamin D3 hydroxylases in kidney mitochondria. Taken together, the results from the purification and the experiments with CYP27 antibody, substrate inhibition, and recombinant expressed human liver CYP27 strongly indicate that CYP27 is able to catalyze 1 alpha-hydroxylation but not 24-hydroxylation of 25-hydroxyvitamin D3 in kidney. In conclusion, the results provide evidence for a role for CYP27 as a major renal mitochondrial 25-hydroxyvitamin D3 1 alpha-hydroxylase.

Human 25-hydroxyvitamin D3-24-hydroxylase, a multicatalytic enzyme

Human 25-hydroxyvitamin D-24-hydroxylase has been expressed in Spodoptera frugiperda (Sf21) insect cells using the previously cloned cDNA in baculovirus (AcNPV-P450cc24). The activity of recombinant h-P450cc24 required adrenodoxin, adrenodoxin reductase, and NADPH. Incubation of this reconstituted system with 25-OH-[26,27-(3)H]D3 substrate produced several metabolites that were resolved on a normal-phase cyano HPLC system. These products exactly comigrated with authentic standards for 24-oxo-25-OH-D3, 23(S),25-(OH)2D3, 24(R),25-(OH)2D3, and 24-oxo-23(S),25-(OH)2D3. The soluble proteins from Sf21 cells infected with wild-type baculovirus produced neither 24,25-(OH)2D3 nor any of the other 25-OH-D3 metabolites. The products were isolated and subjected to a normal-phase amino HPLC for further separation, purification, and characterization. Comigration on two HPLC systems, periodate cleavage reactions, and NaBH4 reduction established clearly the identity of these metabolites. Incubation of recombinant h-P450cc24 with 25-OH-[3 alpha-3H]D3 led to the isolation of an additional product that comigrated with 24,25,26,27-tetranor-23-OH-D3. Treatment of putative 24,25,26,27-tetranor-23-OH-[3 alpha-3H]D3 with acetic anhydride changed its migration on amino HPLC to a less polar position, indicating acetylation of a hydroxyl group(s). These data demonstrate conclusively that h-P450cc24 is a multicatalytic enzyme catalyzing most, if not all, of the reactions in the C-24/C-23 pathway of 25-OH-D3 metabolism. It is likely that this enzyme by itself converts 25-OH-D3 and 1,25-(OH)2D3 to one of its final excretion products.

Expression of the 1,25-dihydroxyvitamin D3-24-hydroxylase gene in rat intestine: response to calcium, vitamin D3 and calcitriol administration in vivo

The 25(OH)D3/1,25(OH)2D3 24-hydroxylase (24-hydroxylase) displays an induction profile responsive to vitamin D (D) abundance and is hence only observed in normal extracellular Ca2+ concentrations. However, the participation of Ca2+ in the expression of the 24-hydroxylase gene in vivo is not known. The present studies investigate the role played by the circulating Ca2+ and the D3 and/or 1,25(OH)2D3 status on the 1,25(OH)2D3-mediated inducibility of the 24-hydroxylase gene in rat duodenum. Hypocalcemic D-depleted rats were supplemented with calcium alone to normalize serum Ca2+ without normalizing the D3 status or were acutely or chronically supplemented with D3 or 1,25(OH)2D3. Messenger RNA for the 24-hydroxylase was undetectable in the intestine of hypocalcemic D-depleted rats, and short- or long-term calcium supplementation was completely unsuccessful in inducing its expression. By contrast, acute 1,25(OH)2D3 administration led to significant increases in the levels of expression of the gene which was independent of the calcium intake, the prevailing circulating Ca2+, and the D3 or 1,25(OH)2D3 status. Moreover, 24-hydroxylase gene expression was only found to respond to acutely administered 1,25(OH)2D3, the mRNA levels being unaltered following continuous exposure to physiological or pharmacological doses of the hormone for 7 days. Time-course studies revealed, however, that induction of the gene was clearly apparent early in the 1,25(OH)2D3 supplementation course but gradually faded by 3 days to return to basal uninduced levels by 7 days, suggesting the presence of intestinal adaptation mechanism(s) which down-regulated the responsiveness in the continuous presence of 1,25(OH)2D3. Our data show the lack of effect of calcium alone or in combination with 1,25(OH)2D3 on the in vivo induction of the 24-hydroxylase gene expression in rat intestine. By rapidly reacting to surges in 1,25(OH)2D3 concentrations, the 24-hydroxylase efficiently controls the amount of 1,25(OH)2D3 in intestine as the first step in the biotransformation pathway aimed at the irreversible clearance of the secosteroid hormone.

Human 25-hydroxyvitamin D 24-hydroxylase cytochrome P450 subunit maps to a different chromosomal location than that of pseudovitamin D-deficient rickets

We have cloned part of the human 25-OHD 24-hydroxylase cytochrome P450 (P450cc24) cDNA. The characterized sequence consists of 776 bp of the coding and 720 bp of the 3'-untranslated region interrupted by an intron. In the coding region we found 79.8% similarity in DNA and 87.5% in deduced amino acid sequences between human and rat, with no similarity in the 3'-untranslated region. By Southern blot hybridization of DNA from human-hamster somatic cell hybrids and by in situ immunofluorescence hybridization, we mapped P450cc24 to human chromosome 20q13.1. This location of P450cc24 is different from that of pseudovitamin D-deficient rickets (PDDR), previously assigned to chromosome 12q14 by linkage analysis, thus excluding it as a target of the PDDR mutation. Since it is likely that PDDR is caused by a mutation in the 25-OHD 1 alpha-hydroxylase P450 subunit (P450cc1 alpha) our results do not support the hypothesis that the two cytochromes are encoded by a single gene.

Characterization and regulation of the vitamin D hydroxylases

The metabolism of vitamin D is regulated by three major cytochrome P450-containing h hydroxylases-the hepatic 25-hydroxylase, the renal 1α-hydroxylase, and the renal and intestinal 24-hydroxylase. In the liver, the 25-hydroxylation reaction is catalyzed by microsomal and mitochondrial cytochrome P450cc25. The microsomal P450 accepts electrons from the NADPH-cytochrome P450 reductase, and the mitochondrial P450 accepts electrons from NADPH-ferredoxin reductase and ferredoxin. In the kidney, the 1α- and 24-hydroxylation reactions are catalyzed by mitochondrial cytochromes P450cc1α and P450cc24, respectively. The 24-hydroxylase is also found in vitamin D target tissues such as the intestine. The rat hepatic mitochondrial P450cc25 and the rat renal mitochondrial P450cc24 have been purified, and their cDNAs have been cloned and sequenced. 1,25-Dihydroxyvitamin D, the active metabolite of vitamin D, markedly stimulates renal P450cc24 mRNA and 24-hydroxylase activity in the intact animal and in renal cell lines. This stimulation occurs via a receptor-mediated mechanism requiring new protein synthesis. Despite the availability of a clone, no studies have yet been reported of the regulation of hepatic P450cc25 at the mRNA level. The study of one of the most important enzymes in vitamin D metabolism, the renal 1α-hydroxylase which produces the active metabolite, awaits the definitive cloning of the cDNA for the P450cc1α.