Role of Coactivator Associated Arginine Methyltransferase 1 (CARM1) in the Regulation of the Biological Function of 1,25-Dihydroxyvitamin D3.

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3), the hormonally active form of vitamin D, activates the nuclear vitamin D receptor (VDR) to mediate the transcription of target genes involved in calcium homeostasis as well as in non-classical 1,25(OH)2D3 actions. In this study, CARM1, an arginine methyltransferase, was found to mediate coactivator synergy in the presence of GRIP1 (a primary coactivator) and to cooperate with G9a, a lysine methyltransferase, in 1,25(OH)2D3 induced transcription of Cyp24a1 (the gene involved in the metabolic inactivation of 1,25(OH)2D3). In mouse proximal renal tubule (MPCT) cells and in mouse kidney, chromatin immunoprecipitation analysis demonstrated that dimethylation of histone H3 at arginine 17, which is mediated by CARM1, occurs at Cyp24a1 vitamin D response elements in a 1,25(OH)2D3 dependent manner. Treatment with TBBD, an inhibitor of CARM1, repressed 1,25(OH)2D3 induced Cyp24a1 expression in MPCT cells, further suggesting that CARM1 is a significant coactivator of 1,25(OH)2D3 induction of renal Cyp24a1 expression. CARM1 was found to act as a repressor of second messenger-mediated induction of the transcription of CYP27B1 (involved in the synthesis of 1,25(OH)2D3), supporting the role of CARM1 as a dual function coregulator. Our findings indicate a key role for CARM1 in the regulation of the biological function of 1,25(OH)2D3.

Sodium zirconium cyclosilicate for the management of chronic hyperkalemia in kidney disease, a novel agent.

INTRODUCTION: Hyperkalemia is a common finding in patients with advanced kidney disease for multiple reasons. Renin-Angiotensin-Aldosterone-System Inhibitors (RAASi) that are indicated for slowing down progression of kidney disease are often associated with hyperkalemia which becomes a limiting factor in their use and titration to the maximum dose. Having a safe, effective, tolerable, and affordable potassium binder can help optimize RAAS inhibition in the setting of kidney disease. AREAS COVERED: Although sodium polystyrene sulfonate has been a mainstay of acute management of hyperkalemia for decades, evidence regarding its efficacy is limited, and its chronic use is not routinely recommended for concerns regarding toxicity. The concern of gastrointestinal (GI) adverse effects with sodium polystyrene sulfonate has spurred the development of alternatives. Sodium zirconium cyclosilicate (SZC) is a promising agent that selectively binds potassium in the gut and eliminates it, while being safe for chronic use based on 1 year of data. Even though we do not have head-to-head studies among the three currently available binders, SZC stands out in rapidity of onset and efficacy. EXPERT OPINION: In this review, we summarize the general management of hyperkalemia, including new agents. We review the pre-clinical and clinical data relating to sodium zirconium cyclosilicate.

1,25-Dihydroxyvitamin D3 and dietary vitamin D reduce inflammation in mice lacking intestinal epithelial cell Rab11a.

A number of studies have examined the effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2 D3 ) on intestinal inflammation driven by immune cells, while little information is currently available about its impact on inflammation caused by intestinal epithelial cell (IEC) defects. Mice lacking IEC-specific Rab11a a recycling endosome small GTPase resulted in increased epithelial cell production of inflammatory cytokines, notably IL-6 and early onset of enteritis. To determine whether vitamin D supplementation may benefit hosts with epithelial cell-originated mucosal inflammation, we evaluated in vivo effects of injected 1,25(OH)2 D3 or dietary supplement of a high dose of vitamin D on the gut phenotypes of IEC-specific Rab11a knockout mice (Rab11aΔIEC ). 1,25(OH)2 D3 administered at 25 ng, two doses per mouse, by intraperitoneal injection, reduced inflammatory cytokine production in knockout mice compared to vehicle-injected mice. Remarkably, feeding mice with dietary vitamin D supplementation at 20,000 IU/kg spanning fetal and postnatal developmental stages led to improved bodyweights, reduced immune cell infiltration, and decreased inflammatory cytokines. We found that these vitamin D effects were accompanied by decreased NF-κB (p65) in the knockout intestinal epithelia, reduced tissue-resident macrophages, and partial restoration of epithelial morphology. Our study suggests that dietary vitamin D supplementation may prevent and limit intestinal inflammation in hosts with high susceptibility to chronic inflammation.

Impact of opioid use disorders on outcomes and readmission following cardiac operations.

OBJECTIVE: While opioid use disorder (OUD) has been previously associated with increased morbidity and resource use in cardiac operations, its impact on readmissions is understudied. METHODS: Patients undergoing coronary artery bypass grafting and valve repair or replacement, excluding infective endocarditis, were identified in the 2010-16 Nationwide Readmissions Database. Using International Classification of Diseases 9/10, we tabulated OUD and other characteristics. Multivariable regression was used to adjust for differences. RESULTS: Of an estimated 1 978 276 patients who had cardiac surgery, 5707 (0.3%) had OUD. During the study period, the prevalence of OUD increased threefold (0.15% in 2010 vs 0.53% in 2016, parametric trend<0.001). Patients with OUD were more likely to be younger (54.0 vs 66.0 years, p<0.001), insured by Medicaid (28.2 vs 6.2%, p<0.001) and of the lowest income quartile (33.6 vs 27.1%, p<0.001). After multivariable adjustment, OUD was associated with decreased mortality (1.5 vs 2.7%, p=0.001). Although these patients had similar rates of overall complications (36.1 vs 35.1%, p=0.363), they had increased thromboembolic (1.3 vs 0.8%, p<0.001) and infectious (4.1 vs 2.8%, p<0.001) events, as well as readmission at 30 days (19.0 vs 13.2%, p<0.001). While patients with OUD had similar hospitalisation costs ($50 766 vs $50 759, p=0.994), they did have longer hospitalisations (11.4 vs 10.3 days, p<0.001). CONCLUSION: The prevalence of OUD among cardiac surgical patients has steeply increased over the past decade. Although the presence of OUD was not associated with excess mortality at index hospitalisation, it was predictive of 30-day readmission. Increased attention to predischarge interventions and care coordination may improve outcomes in this population.

Analysis of 1,25-Dihydroxyvitamin D3 Genomic Action Reveals Calcium-Regulating and Calcium-Independent Effects in Mouse Intestine and Human Enteroids.

Although vitamin D is critical for the function of the intestine, most studies have focused on the duodenum. We show that transgenic expression of the vitamin D receptor (VDR) only in the distal intestine of VDR null mice (KO/TG mice) results in the normalization of serum calcium and rescue of rickets. Although it had been suggested that calcium transport in the distal intestine involves a paracellular process, we found that the 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]-activated genes in the proximal intestine associated with active calcium transport (Trpv6, S100g, and Atp2b1) are also induced by 1,25(OH)2D3 in the distal intestine of KO/TG mice. In addition, Slc30a10, encoding a manganese efflux transporter, was one of the genes most induced by 1,25(OH)2D3 in both proximal and distal intestine. Both villus and crypt were found to express Vdr and VDR target genes. RNA sequence (RNA-seq) analysis of human enteroids indicated that the effects of 1,25(OH)2D3 observed in mice are conserved in humans. Using Slc30a10-/- mice, a loss of cortical bone and a marked decrease in S100g and Trpv6 in the intestine was observed. Our findings suggest an interrelationship between vitamin D and intestinal Mn efflux and indicate the importance of distal intestinal segments to vitamin D action.

Multidetector computed tomography in transcatheter aortic valve replacement: an update on technological developments and clinical applications.

INTRODUCTION: Transcatheter aortic valve replacement (TAVR) has revolutionized the treatment of patients with underlying sever aortic valve stenosis across all spectrum of the disease. CT imaging is so crucial to the pre procedural planning, to incorporate the information from the CT imaging in the decision making intraprocedurally and to predict and identity the post procedural complications.Areas covered: In this article, we review available studies on CT role in TAVR procedure and provide update on the technological developments and clinical applications.Expert opinion: CT imaging, with its high resolution, and in particular its utilization in aortic annular measurements, bicuspid aortic valve assessment, hypoattenuated leaflet thickening and valve in valve therapy proved to be the ideal approach to study the mechanisms of aortic stenosis, detection of high-risk anatomy, more accurate risk stratification and thus to allow a personalized catheter based intervention of the affected patients.

A Ceftazidime-Avibactam-Resistant and Carbapenem-Susceptible Klebsiella pneumoniae Strain Harboring blaKPC-14 Isolated in New York City.

Ceftazidime-avibactam is a potent antibiotic combination against Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae Here, we describe a unique ceftazidime-avibactam-resistant and carbapenem-susceptible K. pneumoniae strain harboring a novel blaKPC-14 variant. This strain was isolated from a New York City patient in 2003, which predates the introduction of avibactam. Despite resistance to ceftazidime-avibactam, the strain was susceptible to imipenem-relebactam and meropenem-vaborbactam. Comprehensive genomic sequencing revealed that blaKPC-14 is harbored on an ST6 IncN plasmid associated with the early spread of blaKPCIMPORTANCE KPC is currently the most common carbapenemase identified in the United States. More than 40 KPC variants have been described, of which KPC-2 and KPC-3 are the most frequent clinical variants. However, our understanding of the genetic structures and ß-lactam resistance profiles of other novel KPC variants remains incomplete. Here, we report a novel blaKPC variant (blaKPC-14) and the complete genome sequence of blaKPC-14-harboring K. pneumoniae strain BK13048, which is susceptible to carbapenems but resistant to ceftazidime-avibactam. To the best of our knowledge, this is one of the earliest KPC-producing K. pneumoniae strains exhibiting resistance to ceftazidime-avibactam.

EMP2 Is a Novel Regulator of Stemness in Breast Cancer Cells.

Little is known about the role of epithelial membrane protein-2 (EMP2) in breast cancer development or progression. In this study, we tested the hypothesis that EMP2 may regulate the formation or self-renewal of breast cancer stem cells (BCSC) in the tumor microenvironment. In silico analysis of gene expression data demonstrated a correlation of EMP2 expression with known metastasis-related genes and markers of cancer stem cells (CSC) including aldehyde dehydrogenase (ALDH). In breast cancer cell lines, EMP2 overexpression increased and EMP2 knockdown decreased the proportion of stem-like cells as assessed by the expression of the CSC markers CD44+/CD24-, ALDH activity, or by tumor sphere formation. In vivo, upregulation of EMP2 promoted tumor growth, whereas knockdown reduced the ALDHhigh CSC population as well as retarded tumor growth. Mechanistically, EMP2 functionally regulated the response to hypoxia through the upregulation of HIF-1α, a transcription factor previously shown to regulate the self-renewal of ALDHhigh CSCs. Furthermore, in syngeneic mouse models and primary human tumor xenografts, mAbs directed against EMP2 effectively targeted CSCs, reducing the ALDH+ population and blocking their tumor-initiating capacity when implanted into secondary untreated mice. Collectively, our results show that EMP2 increases the proportion of tumor-initiating cells, providing a rationale for the continued development of EMP2-targeting agents.

Glucocorticoid Receptor-Binding and Transcriptome Signature in Cardiomyocytes.

Background An increase in serum cortisol has been identified as a risk factor for cardiac failure, which highlights the impact of glucocorticoid signaling in cardiomyocytes and its influence in the progression of failure. Dexamethasone, a synthetic glucocorticoid, is sufficient for induction of cardiomyocyte hypertrophy, but little is known of the glucocorticoid receptor (GR) genome-binding and -dependent transcriptional changes that mediate this phenotype. Methods and Results In this study using high-resolution sequencing, we identified genomic targets of GR and associated change in the transcriptome after 1 and 24 hours of dexamethasone treatment. We showed that GR associates with 6482 genes in the cardiac genome, with differential regulation of 738 genes. Interestingly, alignment of the chromatin immunoprecipitation and RNA sequencing data show that, after 1 hour, 69% of differentially regulated genes are associated with GR and identify as regulators of RNA pol II-dependent transcription. Conversely, after 24 hours only 45% of regulated genes are associated with GR and involved in dilated and hypertrophic cardiomyopathies as well as other growth-related pathways. In addition, our data also reveal that a majority of genes (76.42%) associated with GR show incremental changes in transcript abundance and are genes involved in basic cellular processes that might be regulated by the dynamics of promoter-paused RNA pol II, as seen in hearts undergoing hypertrophy. In vivo administration of dexamethasone resulted in similar changes in the cardiac transcriptome, as seen in isolated cardiomyocytes. Conclusions Our data reveal genome-wide GR binding sites in cardiomyocytes, identify novel targets and GR-dependent change in the transcriptome that induces and contributes to cardiomyocyte hypertrophy.

Klebsiella pneumoniae ST307 with blaOXA-181, South Africa, 2014-2016.

Klebsiella pneumoniae sequence type (ST) 307 is an emerging global antimicrobial drug-resistant clone. We used whole-genome sequencing and PCR to characterize K. pneumoniae ST307 with oxacillinase (OXA) 181 carbapenemase across several private hospitals in South Africa during 2014-2016. The South Africa ST307 belonged to a different clade (clade VI) with unique genomic characteristics when compared with global ST307 (clades I-V). Bayesian evolution analysis showed that clade VI emerged around March 2013 in Gauteng Province, South Africa, and then evolved during 2014 into 2 distinct lineages. K. pneumoniae ST307 clade VI with OXA-181 disseminated over a 15-month period within 42 hospitals in 23 cities across 6 northeastern provinces, affecting 350 patients. The rapid expansion of ST307 was most likely due to intrahospital, interhospital, intercity, and interprovince movements of patients. This study highlights the importance of molecular surveillance for tracking emerging antimicrobial clones.

PU.1 and epigenetic signals modulate 1,25-dihydroxyvitamin D3 and C/EBPα regulation of the human cathelicidin antimicrobial peptide gene in lung epithelial cells.

LL-37, the only known human cathelicidin which is encoded by the human antimicrobial peptide (CAMP) gene, plays a critical role in protection against bacterial infection. We previously demonstrated that cathelicidin is induced by 1,25-dihydroxyvitamin D3 (1,25(OH) 2 D 3 ) in human airway epithelial cells with a resultant increase in bactericidal activity. In this study we identify key factors that co-operate with 1,25(OH) 2 D 3 in the regulation of CAMP. Our results show for the first time that PU.1, the myeloid transcription factor (which has also been identified in lung epithelial cells), co-operates with the vitamin D receptor and CCAAT/enhancer binding protein α (CEBPα) to enhance the induction of CAMP in lung epithelial cells. Our findings also indicate that enhancement of 1,25(OH) 2 D 3 regulation of CAMP by histone deacetylase inhibitors involves co-operation between acetylation and chromatin remodeling through Brahma-related gene 1 (BRG1; a component of the SWItch/sucrose nonfermentable [SWI/SNF] complex). BRG1 can be an activator or repressor depending on BRG1-associated factors. Protein arginine methyltransferase 5 (PRMT5), a methlytransferase which interacts with BRG1, represses 1,25(OH) 2 D 3 induced CAMP in part through dimethylation of H4R3. Our findings identify key mediators involved in the regulation of the CAMP gene in lung epithelial cells and suggest new approaches for therapeutic manipulation of gene expression to increase the antibacterial capability of the airway.

Transgenic Expression of the Vitamin D Receptor Restricted to the Ileum, Cecum, and Colon of Vitamin D Receptor Knockout Mice Rescues Vitamin D Receptor-Dependent Rickets.

Although the intestine plays the major role in 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] action on calcium homeostasis, the mechanisms involved remain incompletely understood. The established model of 1,25(OH)2D3-regulated intestinal calcium absorption postulates a critical role for the duodenum. However, the distal intestine is where 70% to 80% of ingested calcium is absorbed. To test directly the role of 1,25(OH)2D3 and the vitamin D receptor (VDR) in the distal intestine, three independent knockout (KO)/transgenic (TG) lines expressing VDR exclusively in the ileum, cecum, and colon were generated by breeding VDR KO mice with TG mice expressing human VDR (hVDR) under the control of the 9.5-kb caudal type homeobox 2 promoter. Mice from one TG line (KO/TG3) showed low VDR expression in the distal intestine (<50% of the levels observed in KO/TG1, KO/TG2, and wild-type mice). In the KO/TG mice, hVDR was not expressed in the duodenum, jejunum, kidney, or other tissues. Growth arrest, elevated parathyroid hormone level, and hypocalcemia of the VDR KO mice were prevented in mice from KO/TG lines 1 and 2. Microcomputed tomography analysis revealed that the expression of hVDR in the distal intestine of KO/TG1 and KO/TG2 mice rescued the bone defects associated with systemic VDR deficiency, including growth plate abnormalities and altered trabecular and cortical parameters. KO/TG3 mice showed rickets, but less severely than VDR KO mice. These findings show that expression of VDR exclusively in the distal intestine can prevent abnormalities in calcium homeostasis and bone mineralization associated with systemic VDR deficiency.

Vitamin D, calcium homeostasis and aging.

Osteoporosis is characterized by low bone mass and microarchitecture deterioration of bone tissue, leading to enhanced bone fragility and consequent increase in fracture risk. Evidence is accumulating for an important role of calcium deficiency as the process of aging is associated with disturbed calcium balance. Vitamin D is the principal factor that maintains calcium homeostasis. Increasing evidence indicates that the reason for disturbed calcium balance with age is inadequate vitamin D levels in the elderly. In this article, an overview of our current understanding of vitamin D, its metabolism, and mechanisms involved in vitamin D-mediated maintenance of calcium homeostasis is presented. In addition, mechanisms involved in age-related dysregulation of 1,25(OH)2D3 action, recommended daily doses of vitamin D and calcium, and the use of vitamin D analogs for the treatment of osteoporosis (which remains controversial) are reviewed. Elucidation of the molecular pathways of vitamin D action and modifications that occur with aging will be an active area of future research that has the potential to reveal new therapeutic strategies to maintain calcium balance.

25-Hydroxyvitamin D3 24-Hydroxylase: A Key Regulator of 1,25(OH)2D3 Catabolism and Calcium Homeostasis.

One of the most pronounced effects of the hormonally active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), is increased synthesis of 25-hydroxyvitamin D3 24-hydroxylase (CYP24A1), the enzyme responsible for the catabolism of 1,25(OH)2D3. Thus, 1,25(OH)2D3 regulates its own metabolism, protecting against hypercalcemia and limiting the levels of 1,25(OH)2D3 in cells. This chapter summarizes the catalytic properties of CYP24A1, the recent data related to the crystal structure of CYP24A1, the findings obtained from the generation of mice deficient for the Cyp24a1 gene as well as recent data identifying a causal role of a genetic defect in CYP24A1 in certain patients with idiopathic infantile hypercalcemia. This chapter also reviews the regulation of renal and placental CYP24A1 as well as the genomic mechanisms, including coactivators, repressors, and epigenetic modification, involved in modulating 1,25(OH)2D3 regulation of CYP24A1. We conclude with future research directions related to this key regulator of 1,25(OH)2D3 catabolism and calcium homeostasis.

Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects.

1,25-Dihydroxvitamin D3 [1,25(OH)2D3] is the hormonally active form of vitamin D. The genomic mechanism of 1,25(OH)2D3 action involves the direct binding of the 1,25(OH)2D3 activated vitamin D receptor/retinoic X receptor (VDR/RXR) heterodimeric complex to specific DNA sequences. Numerous VDR co-regulatory proteins have been identified, and genome-wide studies have shown that the actions of 1,25(OH)2D3 involve regulation of gene activity at a range of locations many kilobases from the transcription start site. The structure of the liganded VDR/RXR complex was recently characterized using cryoelectron microscopy, X-ray scattering, and hydrogen deuterium exchange. These recent technological advances will result in a more complete understanding of VDR coactivator interactions, thus facilitating cell and gene specific clinical applications. Although the identification of mechanisms mediating VDR-regulated transcription has been one focus of recent research in the field, other topics of fundamental importance include the identification and functional significance of proteins involved in the metabolism of vitamin D. CYP2R1 has been identified as the most important 25-hydroxylase, and a critical role for CYP24A1 in humans was noted in studies showing that inactivating mutations in CYP24A1 are a probable cause of idiopathic infantile hypercalcemia. In addition, studies using knockout and transgenic mice have provided new insight on the physiological role of vitamin D in classical target tissues as well as evidence of extraskeletal effects of 1,25(OH)2D3 including inhibition of cancer progression, effects on the cardiovascular system, and immunomodulatory effects in certain autoimmune diseases. Some of the mechanistic findings in mouse models have also been observed in humans. The identification of similar pathways in humans could lead to the development of new therapies to prevent and treat disease.

C/EBPα and the Vitamin D Receptor Cooperate in the Regulation of Cathelicidin in Lung Epithelial Cells.

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) and the vitamin D receptor (VDR) have been reported to have an important role in the regulation of innate immunity. We earlier reported that the antimicrobial peptide cathelicidin is induced by 1,25(OH)2D3 in normal human bronchial epithelial cells with a resultant increase in antimicrobial activity against airway pathogens. In this study, we demonstrate that C/EBP alpha (C/EBPα) is a potent enhancer of human cathelicidin antimicrobial peptide (CAMP) gene transcription in human lung epithelial cells. In addition we found that C/EBPα functionally cooperates with VDR in the regulation of CAMP transcription. A C/EBP binding site was identified at -627/-619 within the CAMP promoter, adjacent to the vitamin D response element (VDRE; -615/-600). Mutation of this site markedly attenuated the transcriptional response to C/EBPα as well as to 1,25(OH)2D3, further indicating cooperation between these two factors in the regulation of CAMP. ChIP analysis using 1,25(OH)2D3 treated human lung epithelial cells showed C/EBPα and VDR binding to the CAMP promoter. C/EBPα has previously been reported to cooperate with Brahma (Brm), an ATPase that is component of the SWI/SNF chromatin remodeling complex. We found that dominant negative Brm significantly inhibited C/EBPα as well as 1,25(OH)2D3 mediated induction of CAMP transcription, suggesting the functional involvement of Brm. These findings define novel mechanisms involving C/EBPα, SWI/SNF, and 1,25(OH)2D3 in the regulation of CAMP in lung epithelial cells. These mechanisms of enhanced activation of the CAMP gene in lung epithelial cells suggest potential candidates for the development of modulators of innate immune responses for adjunct therapy in the treatment of airway infections.

Novel mechanism of negative regulation of 1,25-dihydroxyvitamin D3-induced 25-hydroxyvitamin D3 24-hydroxylase (Cyp24a1) Transcription: epigenetic modification involving cross-talk between protein-arginine methyltransferase 5 and the SWI/SNF complex.

The SWI/SNF chromatin remodeling complex facilitates gene transcription by remodeling chromatin using the energy of ATP hydrolysis. Recent studies have indicated an interplay between the SWI/SNF complex and protein-arginine methyltransferases (PRMTs). Little is known, however, about the role of SWI/SNF and PRMTs in vitamin D receptor (VDR)-mediated transcription. Using SWI/SNF-defective cells, we demonstrated that Brahma-related gene 1 (BRG1), an ATPase that is a component of the SWI/SNF complex, plays a fundamental role in induction by 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) of the transcription of Cyp24a1 encoding the enzyme 25-hydroxyvitamin D3 24-hydroxylase involved in the catabolism of 1,25(OH)2D3. BRG1 was found to associate with CCAAT-enhancer-binding protein (C/EBP) ß and cooperate with VDR and C/EBPß in regulating Cyp24a1 transcription. PRMT5, a type II PRMT that interacts with BRG1, repressed Cyp24a1 transcription and mRNA expression. Our findings indicate the requirement of the C/EBP site for the inhibitory effect of PRMT5 via its methylation of H3R8 and H4R3. These findings indicate that the SWI/SNF complex and PRMT5 may be key factors involved in regulation of 1,25(OH)2D3 catabolism and therefore in the maintenance of calcium homeostasis by vitamin D. These studies also define epigenetic events linked to a novel mechanism of negative regulation of VDR-mediated transcription.

Vitamin D biology revealed through the study of knockout and transgenic mouse models.

Early studies identifying vitamin D as an antirachitic factor led to studies in vitamin D-deficient models that resulted in a basic understanding of the mechanism of action of vitamin D. Recent studies using genetically modified mice have provided important new insight into the physiological role of vitamin D at target tissues and the functional significance of vitamin D target proteins, as well as the functional significance of proteins involved in the transport and metabolism of vitamin D. Studies using these mice have played an increasingly important role in elucidating the mechanisms involved in the control of calcium homeostasis and have provided evidence for a role of vitamin D in extraskeletal health.

Vitamin D: metabolism.

The biologically active metabolite of vitamin D, 1,25(OH)(2)D(3), affects mineral homeostasis and has numerous other diverse physiologic functions including effects on growth of cancer cells and protection against certain immune disorders. This article reviews the role of vitamin D hydroxylases in providing a tightly regulated supply of 1,25(OH)(2)D(3). The role of extrarenal 1α(OH)ase in placenta and macrophages is also discussed, as well as regulation of vitamin D hydroxylases in aging and chronic kidney disease. Understanding specific factors involved in regulating the hydroxylases may lead to the design of drugs that can selectively modulate the hydroxylases. The ability to alter levels of these enzymes would have therapeutic potential for the treatment of various diseases, including bone loss disorders and certain immune diseases.