Renal CYP27B1 Research Articles

Role of intestinal claudin-2 in calcium transport and kidney stone disease

Background: Kidney stone disease is characterized by intestinal hyperabsorption of calcium and hypercalciuria, leading to the formation of calcium crystals in the kidney. Claudin-2 is a tight junction protein that forms paracellular cation pores and mutations in its gene are associated with kidney stone disease. We have recently shown that mice deficient in Cldn2 are hypercalciuric due to both decreased renal reabsorption and increased intestinal absorption of calcium and develop medullary mineral deposits reminiscent of kidney stone formers. We, therefore, hypothesized that intestinal claudin-2 is important for calcium secretion, especially in the colon, and that loss of claudin-2 results in increased net intestinal calcium absorption, thereby contributing to kidney stone disease. Method: Villin-Cre (Vil1-Cre/1000Gum) mice were crossed to Cldn2 floxed mice to generate intestine-specific claudin-2 knockout mice. Successful knockout of claudin-2 was confirmed at the mRNA and protein level using qPCR, immunofluorescence staining, and western blot. Male and female littermates were characterized for hypercalciuria from age 3-10 weeks. Circulating levels of parathyroid hormone and 1,25-dihydroxyvitamin D (1,25(OH)2D) were measured by ELISA, and mRNA levels of seven paracellular and transcellular calcium transport genes in the ileum, cecum, distal colon, and nine in the kidney assessed by qPCR. Result: In females, Cldn2 expression was knocked out in the intestine but not in the kidney. However, in males, Cldn2 was knocked out in the intestine and also partially knocked out in the kidneys (where villin is also expressed). Knockout females had normal urine calcium, but knockout males had significantly higher urine calcium at weaning (3 weeks of age) compared to wildtype, which was normalized by 4 weeks of age. At 4 weeks, Cldn19 mRNA was downregulated in knockout mice of both sexes. Male knockout mice had a reduction in intestinal Cldn12, Cldn15, Slc8a1, and increased intestinal Cacnad1, increased renal Cyp27b1 expression, and increased circulating 1,25(OH)2D. Conclusion: Our results suggest that intestine-specific loss of claudin-2, which was only observed in villin-Cre, Cldn2 floxed females, does not cause hypercalciuria. Ongoing whole-body calcium balance and everted sac calcium permeability experiments will determine whether this is because intestinal calcium hyperabsorption is masked by compensatory mechanisms. NIH. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Short-term fasting of mice elevates circulating fibroblast growth factor 23 (FGF23).

Phosphate and vitamin D homeostasis are controlled by fibroblast growth factor 23 (FGF23) from bone suppressing renal phosphate transport and enhancing 24-hydroxylase (Cyp24a1), thereby inactivating 1,25(OH)2 D3 . Serum FGF23 is correlated with outcomes in several diseases. Fasting stimulates the production of ketone bodies. We hypothesized that fasting can induce FGF23 synthesis through the production of ketone bodies. UMR106 cells and isolated neonatal rat ventricular myocytes (NRVM) were treated with ketone body β-hydroxybutyrate. Mice were fasted overnight, fed ad libitum, or treated with β-hydroxybutyrate. Proteins and further blood parameters were determined by enzyme-linked immunoassay (ELISA), western blotting, immunohistochemistry, fluorometric or colorimetric methods, and gene expression by quantitative real-time polymerase chain reaction (qRT-PCR). β-Hydroxybutyrate stimulated FGF23 production in UMR106 cells in a nuclear factor kappa-light-chain enhancer of activated B-cells (NFκB)-dependent manner, and in NRVMs. Compared to fed animals, fasted mice exhibited higher β-hydroxybutyrate and FGF23 serum levels (based on assays either detecting C-terminal or intact, biologically active FGF23 only), cardiac, pancreatic, and thymic Fgf23 and renal Cyp24a1 expression, and lower 1,25(OH)2 D3 serum concentration as well as renal Slc34a1 and αKlotho (Kl) expression. In contrast, Fgf23 expression in bone and serum phosphate, calcium, plasma parathyroid hormone (PTH) concentration, and renal Cyp27b1 expression were not significantly affected by fasting. Short-term fasting increased FGF23 production, as did administration of β-hydroxybutyrate, effects possibly of clinical relevance in view of the increasing use of FGF23 as a surrogate parameter in clinical monitoring of diseases. The fasting state of patients might therefore affect FGF23 tests.

#3325 EXTRARENAL CYP27B1 ACTIVITY CONTRIBUTES TO THE CIRCULATING POOL OF 1,25(OH)2D

Abstract Background and Aims Alterations of vitamin D metabolism are ubiquitous in patients with chronic kidney disease (CKD) and a key feature of the syndrome of CKD-mineral and bone disorder. A central paradigm in vitamin D metabolism is that the kidneys are the only relevant source of activation of vitamin D for systemic effects. However, various cells and tissues exhibit CYP27B1 activity and are capable of producing 1.25(OH)2D, which could contribute to the circulating pool [1]. This longitudinal, observational study challenges the dogma of the kidneys as the only source of 1.25(OH)2D, by investigating vitamin D metabolites in anephric patients free of active vitamin D therapy. Method Anephric patients free of active vitamin D therapy were included at time of kidney transplantation. A subset had a second study visit at 12 months post-transplant. Liquid chromatography–tandem mass spectrometry was used to measure 25(OH)D2/3, 1,25(OH)2D3 and 24,25(OH)2D3. Metabolic ratios of vitamin D metabolites over precursors were used as measures of CYP24A1 (24,25(OH) 2D3/ 25(OH)D3) and CYP27B1(1,25(OH)2D/ 25(OH)D3) activity. Biointact PTH (1-84) was measured by an in-house immunoradiometric assay [2] and FGF23 was measured by ELISA (Kainos Laboratories, Tokyo, Japan). Interleukin 6 (IL6), tumor necrosis factor-Α (TNF- Α) and interferon-Γ (IFN-Γ) were measured using an electrochemiluminescence multiplex immunoassay (Human Proinflammatory 1 V-plex, MSD, Maryland, US). Two independent laboratories performed confirmatory analyses of 1,25(OH)2D. Results All anephric patients (n = 38) had detectable (>2.5 pg/mL) levels of 1,25(OH)2D, with a median value of 13.5 pg/mL (range 3.9 to 35.7 pg/ml). There was a significant correlation between levels of 25(OH)D and 1,25(OH)2D (r = 0.583, p≪0.001), with 25(OH)D-levels explaining 34% of the variation in 1,25(OH)2D (Fig. 1). 1,25(OH)2D levels were not significantly correlated to PTH (rho = 0.279, p = 0.09) or FGF23 (rho = –0.280, p = 0.09), nor to any of the inflammatory markers; IL6 (rho = 0.068, p = 0.69), TNF-Α (rho = –0.264, p = 0.11), IFN-Γ (rho = –0.001, p = 0.99). To validate our findings, anonymized samples of 17 anephric patients and 17 CKD stage 5D with residual renal function were sent to two independent laboratories. The presence of 1,25(OH)2D was confirmed in all anephric patients. Twenty-five anephric patients underwent a successful kidney transplantation and were free of active vitamin D therapy at a study visit 12 months post-transplant. After kidney transplantation, 25(OH)D-level decreased, while levels of 1,25(OH)₂D and 24,25(OH) ₂D substantially increased. The metabolic ratios expressing CYP24A1 and CYP27B1 activity both increased (Fig. 2). At 12 months post-transplant, 1,25(OH)₂D-levels correlated with levels of biointact PTH (rho = 0.603, p = 0.002), but not with levels of FGF23 (rho = –0.262, p = 0.21). Conclusion Extrarenal CYP27B1 activity contributes to the circulating active vitamin D pool and may sustain low-normal levels of active vitamin D, even in anephric individuals. While renal CYP27B1 activity is hormonally regulated, extrarenal CYP27B1 activity seems to be mainly substrate dependent.

A parathyroid hormone/salt-inducible kinase signaling axis controls renal vitamin D activation and organismal calcium homeostasis.

The renal actions of parathyroid hormone (PTH) promote 1,25-vitamin D generation; however, the signaling mechanisms that control PTH-dependent vitamin D activation remain unknown. Here, we demonstrated that salt-inducible kinases (SIKs) orchestrated renal 1,25-vitamin D production downstream of PTH signaling. PTH inhibited SIK cellular activity by cAMP-dependent PKA phosphorylation. Whole-tissue and single-cell transcriptomics demonstrated that both PTH and pharmacologic SIK inhibitors regulated a vitamin D gene module in the proximal tubule. SIK inhibitors increased 1,25-vitamin D production and renal Cyp27b1 mRNA expression in mice and in human embryonic stem cell-derived kidney organoids. Global- and kidney-specific Sik2/Sik3 mutant mice showed Cyp27b1 upregulation, elevated serum 1,25-vitamin D, and PTH-independent hypercalcemia. The SIK substrate CRTC2 showed PTH and SIK inhibitor-inducible binding to key Cyp27b1 regulatory enhancers in the kidney, which were also required for SIK inhibitors to increase Cyp27b1 in vivo. Finally, in a podocyte injury model of chronic kidney disease-mineral bone disorder (CKD-MBD), SIK inhibitor treatment stimulated renal Cyp27b1 expression and 1,25-vitamin D production. Together, these results demonstrated a PTH/SIK/CRTC signaling axis in the kidney that controls Cyp27b1 expression and 1,25-vitamin D synthesis. These findings indicate that SIK inhibitors might be helpful for stimulation of 1,25-vitamin D production in CKD-MBD.

Intestinal Vitamin D Receptor Is Dispensable for Maintaining Adult Bone Mass in Mice With Adequate Calcium Intake.

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3)-mediated intestinal calcium (Ca) absorption supplies Ca for proper bone mineralization during growth. We tested whether vitamin D receptor (VDR)-mediated 1,25(OH)2D3 signaling is critical for adult Ca absorption and bone by using mice with inducible Vdr gene knockout in the whole intestine (villin-CreERT2+/- × Vdrf/f, WIK) or in the large intestine (Cdx2-CreERT2+/- ×Vdrf/f, LIK). At 4-month-old, Vdr alleles were recombined (0.05 mg tamoxifen/g BW, intraperitoneally [i.p.], 5 days) and mice were fed diets with either 0.5% (adequate) or 0.2% (low) Ca. Ca absorption was examined after 2 weeks while serum 1,25(OH)2D3, bone mass, and bone microarchitecture were examined after 16 weeks. Intestinal and renal gene expression was measured at both time points (n = 12/genotype/diet/time point). On the 0.5% Ca diet, all phenotypes in WIK and LIK mice were similar to the controls. Control mice adapted to the 0.2% low-Ca diet by increasing renal Cyp27b1 mRNA (3-fold), serum 1,25(OH)2D3 level (1.9-fold), and Ca absorption in the duodenum (Dd, + 131%) and proximal colon (PCo, + 28.9%), which prevented bone loss. In WIK mice, low-Ca diet increased serum 1,25(OH)2D3 (4.4-fold) but Ca absorption remained unaltered in the Dd and PCo. Consequently, significant bone loss occurred in WIK mice (e.g., cortical thickness, Ct.Th, -33.7%). LIK mice adapted to the low-Ca diet in the Dd but not the PCo, and the effect on bone phenotypes was milder (e.g., Ct.Th, -13.1%). Our data suggest intestinal VDR in adult mice prevents bone loss under low Ca intake but is dispensable under adequate calcium intake.

Ca Minerals and Oral Bioavailability of Pb, Cd, and As from Indoor Dust in Mice: Mechanisms and Health Implications.

Elevating dietary calcium (Ca) intake can reduce metal(loid)oral bioavailability. However, the ability of a range of Ca minerals to reduce oral bioavailability of lead (Pb), cadmium (Cd), and arsenic (As) from indoor dust remains unclear. This study evaluated the ability of Ca minerals to reduce Pb, Cd, and As oral bioavailability from indoor dust and associated mechanisms. A mouse bioassay was conducted to assess Pb, Cd, and As relative bioavailability (RBA) in three indoor dust samples, which were amended into mouse chow without and with addition of , , Ca gluconate, Ca lactate, Ca aspartate, and Ca citrate at . The mRNA expression of Ca and phosphate (P) transporters involved in transcellular Pb, Cd and As transport in the duodenum of mice was quantified using real-time polymerase chain reaction. Serum 1,25-Dihydroxyvitamin D3 [], parathyroid hormone (PTH), and renal CYP27B1 activity controlling synthesis were measured using ELISA kits. Metal(loid) speciation in the feces of mice was characterized using X-ray absorption near-edge structure (XANES) spectroscopy. In general, mice exposed to each of the Ca minerals exhibited lower Pb-, Cd-, and As-RBA for three dusts. However, RBAs with the different Ca minerals varied. Among minerals, mice fed dietary did not exhibit lower duodenal mRNA expression of Ca transporters but did have the lowest Pb and Cd oral bioavailability at the highest Ca concentration (; 51%-95% and 52%-74% lower in comparison with the control). Lead phosphate precipitates (e.g., chloropyromorphite) were observed in feces of mice fed dietary . In comparison, mice fed organic Ca minerals (Ca gluconate, Ca lactate, Ca aspartate, and Ca citrate) had lower duodenal mRNA expression of Ca transporters, but Pb and Cd oral bioavailability was higher than in mice fed . In terms of As, mice fed Ca aspartate exhibited the lowest As oral bioavailability at the highest Ca concentration (; 41%-72% lower) and the lowest duodenal expression of P transporter (88% lower). The presence of aspartate was not associated with higher As solubility in the intestine. Our study used a mouse model of exposure to household dust with various concentrations and species of Ca to determine whether different Ca minerals can reduce bioavailability of Pb, Cd, and As in mice and elucidate the mechanism(s) involved. This study can contribute to the practical application of optimal Ca minerals to protect humans from Pb, Cd, and As coexposure in the environment. https://doi.org/10.1289/EHP11730.

LBODP016 Lithocholic Acid Induced 25-hydroxyvitamin D-1alpha Hydroxylase (cyp27b1) Expression And Activities In Vitro And In Vivo

Abstract Our previous study reported that oleanolic acid, a natural agonist for bile acid membrane receptor Takeda G protein coupled receptor 5 (TGR5), could exert bone protective effects, increase circulating 1, 25(OH)2D3 level and induce renal CYP27B1 expression and activities. However, it is unclear if bile acids, as the endogenous agonist of bile acid receptors, could regulate CYP27B1 expression and activities in kidney and bone tissues. The present study aimed to characterize the effects of lithocholic acid (LCA), a secondary bile acid, on CYP27B1 expression and activities in vitro using human proximal tubule (HKC-8) and osteosarcoma (MG-63) and in vivo using mature (6-month-old) male C57BL/6 mice. HKC-8 and MG-63 cells were treated with either LCA (10-10 to 10-5 M), PTH (10-7M), or its vehicle for 4 hours. For MG-63 cells, LCA significantly induced CYP27B1 protein expression at 10-10 and 10-9M (P<0. 001) and CYP27B1 mRNA expression at 10-9M (P<0. 001). For HKC-8 cells, LCA significantly induced CYP27B1 protein expression at 10-10 to 10-6M (P<0. 01) and CYP27B1 mRNA expression at 10-10M (P<0. 001). In addition, LCA significantly induced CYP27B1 activity in MG-63 cells (10-10M) (P<0. 05) and in HKC-8 cells (10-6M) (P<0. 01). For the in vivo study, male C57BL/6 mice were fed with 25mg/kg LCA or its vehicle for three consecutive days and sacrificed 5 hours after the last gavage. Serum, kidney, and iliac crests were collected for subsequent analysis. LCA significantly induced circulating 1,25(OH)2D3 levels (P<0. 01) and CYP27B1 protein expression in both kidney and iliac crests in mice (P <0. 05). In addition, the time-dependent change (2, 5, 8 hours) in serum 1,25(OH)2D3 levels were determined upon feeding the male mice with a single dose of 25mg/kg LCA. LCA significantly increased serum 1,25(OH)2D3 levels in male mice at 2 and 5 hours (P <0. 01 and P <0. 05, respectively), and serum 1,25(OH)2D3 levels returned to basal level by 8 hours. LCA significantly increased CYP27B1 protein expression in kidney in mice at 8 hours after treatment (vs. control group, P<0. 05). In summary, our results showed that LCA significantly induced circulating 1,25(OH)2D3 levels, possibly by its actions to induce CYP27B1 expressions and activities in renal and extra-renal (bone) tissues. The study indicated that bile acids and other TGR5 ligands might play an important role in the regulation of vitamin D metabolism. Presentation: No date and time listed

Rapid genomic changes by mineralotropic hormones and kinase SIK inhibition drive coordinated renal Cyp27b1 and Cyp24a1 expression via CREB modules

Vitamin D metabolism centers on kidney regulation of Cyp27b1 by mineralotropic hormones, including induction by parathyroid hormone (PTH), suppression by fibroblast growth factor 23 (FGF23) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), and reciprocal regulations for Cyp24a1. This coordinated genomic regulation results in production of endocrine 1,25(OH)2D3, which, together with PTH and FGF23, controls mineral homeostasis. However, how these events are coordinated is unclear. Here, using invivo chromatin immunoprecipitation sequencing in mouse kidney, we demonstrate that PTH activation rapidly induces increased recruitment of phosphorylated (p-133) CREB (pCREB) and its coactivators, CBP (CREB-binding protein) and CRTC2 (CREB-regulated transcription coactivator 2), to previously defined kidney-specific M1 and M21 enhancers near the Cyp27b1 gene. At distal enhancers of the Cyp24a1 gene, PTH suppression dismisses CBP with only minor changes in pCREB and CRTC2 occupancy, all of which correlate with decreased genomic activity and reduced transcripts. Treatment of mice with salt-inducible kinase inhibitors (YKL-05-099 and SK-124) yields rapid genomic recruitment of CRTC2 to Cyp27b1, limited interaction of CBP, and a transcriptional response for both Cyp27b1 and Cyp24a1 that mirrors the actions of PTH. Surprisingly, we find that 1,25(OH)2D3 suppression increases the occupancy of CRTC2 in the M1 enhancer, a novel observation for CRTC2 and 1,25(OH)2D3 action. Suppressive actions of 1,25(OH)2D3 and FGF23 at the Cyp27b1 gene are associated with reduced CBP recruitment at these CREB-module enhancers that disrupts full PTH induction. Our findings show that CRTC2 contributes to transcription of both Cyp27b1 and Cyp24a1, demonstrate salt-inducible kinase inhibition as a key modulator of vitamin D metabolism, and provide molecular insight into the coordinated mechanistic actions of PTH, FGF23, and 1,25(OH)2D3 in the kidney that regulate mineral homeostasis.

Importance of Extra-Renal CYP24A1 Expression for Maintaining Mineral Homeostasis

Calcium homeostasis involves a complex interplay between kidneys, parathyroid glands, intestine and bone. Specifically, 1,25(OH)2D3 is a key calciotropic hormone which stimulates intestinal calcium absorption. A growing body of evidence suggests that circulating levels of 1,25(OH)2D3 depend not only on its synthesis under the action of PTH in the kidneys, but also its catabolism by 24-hydroxylase, herein referred to as CYP24A1. The clinical importance of CYP24A1 has been demonstrated by human loss-of-function mutations, which lead to severe hypercalcemia due to exaggerated levels of 1,25(OH)2D3. Despite its growing importance, little is known about its tissue-specific contributions to normal vitamin D metabolism. To explore the physiology of CYP24A1 and delineate renal-specific effects of CYP24A1 in calcium metabolism, we generated a mouse with constitutive kidney-specific deletion of Cyp24a1 (Six2Cre-Cyp24flox). Six2 marks the nephron progenitor population throughout nephrogenesis. We hypothesized that hypercalcemia as seen in CYP24A1 inactivating mutations is related to lack of both renal and extrarenal expression, and that renal deletion does not lead to severe hypercalcemia. To confirm Cyp24a1 deletion, we measured mRNA expression in the kidney using qPCR and RNA in situ hybridization. All mice were fed a standard commercial rodent diet and followed longitudinally for five months with interval calcium measurements. At time of termination, serum PTH levels were measured along with vitamin D-dependent calcium transporters as a functional measure of 1,25(OH)2D3 action. Cyp24a1 expression was significantly knocked down in total kidneys from Six2Cre-Cyp24flox mice as compared to intestinal expression suggesting successful gene deletion. Compared to age-matched wildtype controls, Six2Cre-Cyp24flox mice were mildly but persistently hypercalcemic (diff between means= 0.46 mg/dL, p-value: 0.03, n=8 per group). As expected, 1,25D-dependent calcium transporters in the kidney (Calb1, Trpv5, Slc8a1, Atp2b1) and intestine (Trpv6, s100g) were all increased, consistent with increased systemic 1,25(OH)2D3 activity. PTH levels were appropriately suppressed in the Six2Cre-Cyp24flox mice (diff between means=83 pg/mL, p-value 0.2, n=9 control, n=3 exp) as were renal cyp27b1 mRNA expression. These data suggest that renal CYP24A1 is important for systemic 1,25(OH)2D3 regulation, but the lack of severe hypercalcemia supports critical contributions of extra-renal CYP24A1.

Paricalcitol in management of chronic kidney disease–mineral and bone disorder

Mineral and bone disorders in chronic kidney disease (CKD) is a systemic disorder of mineral and bone metabolism due to CKD manifested by either one or a combination of the following: abnormalities of calcium, phosphorus, PTH, or vitamin D metabolism (secondary hyperparathyroidism); abnormalities in bone turnover, mineralization, volume, linear growth, or strength; or vascular or other soft tissue calcification. Decreasing 1,25(OH)2D (calcitriol) and rising parathyroid hormone (PTH) levels occur on early stages of CKD. Secondary hyperparathyroidism contributes to the high morbidity and mortality noted in this population. Long-term decompensation of secondary hyperparathyroidism in patients with impaired renal function leads to irreversible changes in multiple organ systems, resistance to conservative treatment and the requirement for surgical intervention. Suppress of renal CYP27B1 and the calcitriol deficiency play a major role in the development of mineral and bone disorders in CKD, thus VDR activators are widely used for management of secondary hyperparathyroidism. These medications are effective in suppression of PTH and demonstrate the positive effects on bone metabolism. There is evidence of pleiotropic effects of VDR activators that are crucial for the prevention of renal fibrosis and extraskeletal calcification. This review focuses on the involvement of vitamin D in the pathogenesis of mineral and bone disorders and the role of paricalcitol in their correction. The efficacy of paricalcitol in patients with various stages of CKD has been evaluated in a large number of observational and randomized clinical trials, the comparative effectiveness of paricalcitol therapy has been summarized in several metanalyses.

A chromatin-based mechanism controls differential regulation of the cytochrome P450 gene Cyp24a1 in renal and non-renal tissues

Cytochrome P450 family 27 subfamily B member 1 (CYP27B1) and CYP24A1 function to maintain physiological levels of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in the kidney. Renal Cyp27b1 and Cyp24a1 expression levels are transcriptionally regulated in a highly reciprocal manner by parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and 1,25(OH)2D3 In contrast, Cyp24a1 regulation in nonrenal target cells (NRTCs) is limited to induction by 1,25(OH)2D3 Herein, we used ChIP-Seq analyses of mouse tissues to identify regulatory regions within the Cyp24a1 gene locus. We found an extended region downstream of Cyp24a1 containing a cluster of sites, termed C24-DS1, binding PTH-sensitive cAMP-responsive element-binding protein (CREB) and a cluster termed C24-DS2 binding the vitamin D receptor (VDR). VDR-occupied sites were present in both the kidney and NRTCs, but pCREB sites were occupied only in the kidney. We deleted each segment in the mouse and observed that although the overt phenotypes of both cluster deletions were unremarkable, RNA analysis in the C24-DS1-deleted strain revealed a loss of basal renal Cyp24a1 expression, total resistance to FGF23 and PTH regulation, and secondary suppression of renal Cyp27b1; 1,25(OH)2D3 induction remained unaffected in all tissues. In contrast, loss of the VDR cluster in the C24-DS2-deleted strain did not affect 1,25(OH)2D3 induction of renal Cyp24a1 expression yet reduced but did not eliminate Cyp24a1 responses in NRTCs. We conclude that a chromatin-based mechanism differentially regulates Cyp24a1 in the kidney and NRTCs and is essential for the specific functions of Cyp24a1 in these two tissue types.

Acute Calcitriol‐Mediated PTH Suppression is Attenuated by High Phosphate Diet in Experimental Model of CKD

BackgroundChronic kidney disease (CKD) patients have impaired phosphate and calcium excretion as well as vitamin D deficiency, which exacerbates secondary hyperparathyroidism (SHPT). A decrease in renal CYP27B1 conversion of calcifediol leads to reduced levels of calcitriol. SHPT leads to increased bone resorption and is a risk factor for vascular calcification and cardiovascular disease in the CKD population. The effectiveness of calcitriol in managing SHPT is problematic, as significant resistance develops with chronic therapy; a condition that can lead to parathyroidectomy.AimDetermine the time course and impact of increasing dietary phosphate on calcitriol‐induced suppression of PTH in experimental CKD.MethodsIn Sprague Dawley rats (N=10), a 0.25% adenine 0.5% phosphate diet was fed for 4–5 weeks to generate stable CKD. A single oral calcitriol dose (160ng/kg) was given pre‐CKD, and at 1, 3, and 5 (off adenine) weeks of CKD induction (0.5% dietary phosphate). From 7–8 weeks rats were given increased dietary phosphate (1.0%) and the response to calcitriol was re‐assessed. Changes in circulating PTH, fibroblast growth factor 23 (FGF‐23), calcium and phosphate in response to the biweekly sham or calcitriol dosing were assessed at 0, 4, 8 and 24 hours.ResultsIn CKD rats, PTH increased from 80±25.8 pg/mL to 221±66.7 pg/mL (week 5) and FGF‐23 from 421±26.0 pg/mL to 6803±1510.2 pg/mL (week 5). The two week 1% phosphate markedly increased PTH to 1342±310 pg/mL (8 weeks) and FGF‐23 to 14490±4980.8 pg/mL (7 weeks). In CKD rats on a low phosphate diet, the biweekly calcitriol dose reduced PTH by up to 44% and raised FGF‐23 by 82% whereas in rats on high dietary phosphate PTH was no longer responsive to calcitriol suppression.ConclusionResponses to calcitriol therapy were attenuated with increasing dietary phosphate and increasing CKD severity. This study supports the importance of dietary phosphate restriction in the CKD population to limit the impact of PTH responsiveness and mineral bone disease in this population.SignificanceCharacterization of calcitriol resistance will help to develop new dosing strategies for vitamin D receptor agonists in this population and prevent SHPT and parathyroidectomies.Support or Funding InformationFunded by the Canadian Institutes of Health ResearchThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Preclinical Prevention Trial of Calcitriol: Impact of Stage of Intervention and Duration of Treatment on Oral Carcinogenesis

The anticancer activity of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3 or calcitriol) has been widely reported in preclinical models. However, systematic investigation into the chemopreventive potential of calcitriol against the spectrum of oral carcinogenesis has not been performed. To address this gap in knowledge, we conducted a preclinical prevention trial of calcitriol in the 4-nitroquinoline-1-oxide (4NQO) oral carcinogenesis model. C57BL/6 mice were exposed to the carcinogen 4NQO in drinking water for 16 weeks and randomized to control (4NQO only) or calcitriol arms. Calcitriol (0.1 μg i.p, Monday, Wednesday, and Friday) was administered for (i) 16 weeks concurrently with 4NQO exposure, (ii) 10 weeks post completion of 4NQO exposure, and, (iii) a period of 26 weeks concurrent with and following 4NQO exposure. Longitudinal magnetic resonance imaging (MRI) was performed to monitor disease progression until end point (week 26). Correlative histopathology of tongue sections was performed to determine incidence and multiplicity of oral dysplastic lesions and squamous cell carcinomas (SCC). Vitamin D metabolites and calcium were measured in the serum using liquid chromatography-mass spectrometry (LC–MS/MS) and colorimetric assay, respectively. Renal CYP24A1 (24-hydroxylase) and CYP27B1 (1α-hydroxylase) expression was measured by quantitative reverse transcription polymerase chain reaction (qRT-PCR). Immunostaining of tongue sections for vitamin D receptor (VDR), CYP24A1, and Ki67 was also performed. Non-invasive MRI enabled longitudinal assessment of lesions in the oral cavity. Calcitriol administered concurrently with 4NQO for 16 weeks significantly (P < .001) decreased the number of premalignant lesions by 57% compared to 4NQO only controls. Mice treated with calcitriol for 26 weeks showed highest renal CYP24A1, lowest serum 1,25(OH)2D3 levels and highest incidence of invasive SCC. Immunohistochemistry revealed increased VDR, CYP24A1 and Ki67 staining in dysplastic epithelia compared to normal epithelium, in all four groups. Collectively, our results show that the effects of calcitriol on oral carcinogenesis are critically influenced by the stage of intervention and duration of exposure and provide the basis for exploring the potential of calcitriol for prevention of OSCC in the clinical setting.

Oleanolic Acid and Ursolic Acid Improve Bone Properties and Calcium Balance and Modulate Vitamin D Metabolism in Aged Female Rats.

Oleanolic acid (OA) and ursolic acid (UA) are the major chemical constituents in Fructus Ligustri Lucidi (FLL), a kidney-tonifying Chinese herb that is previously shown to improve bone properties and enhance calcium balance in aged female rats. The present study was designed to study if OA and UA act as the active ingredients in FLL to exert the positive effects on bone and mineral metabolism in aged rats. Aged (13-month-old) Sprague-Dawley female rats were randomly assigned to four groups with oral administration of drug or vehicle treatment for 12 weeks: medium calcium diet (MCD, 0.6% calcium), high calcium diet (HCD, 1.2% calcium), MCD + FLL (700 mg/kg/day), MCD + OA (23.6 mg/kg/day) + UA (8.6 mg/kg/day). A group of mature (3-month-old) female rats fed with MCD was included as positive control. The results demonstrated that FLL and OA+UA increased bone mineral density and improved microarchitectural properties of aged female rats. The osteoprotective effects of FLL and OA+UA might be, at least in part, associated with their actions on enhancing calcium balance and suppressing age-induced secondary hyperparathyroidism in aged female rats. FLL and OA+UA also significantly induced renal CYP27B1 protein expression and OA+UA treatment decreased CYP24A1 mRNA and protein expressions in aged female rats. In addition, FLL and OA+UA significantly increased the promoter activity, mRNA and protein expressions of renal CYP27B1 in vitro in human proximal tubule HKC-8 cells. The present findings suggest that OA+UA can be regarded as the active ingredients of FLL and might be a potential drug candidate for prevention and treatment of osteoporosis.

Dietary nitrogen and calcium modulate CYP27B1 expression in young goats

In livestock, feeding a reduced nitrogen (N) diet is favored for economic and ecological reasons. Ruminants cope more easily with a reduced N diet than monogastric species. However, changes in mineral homeostasis such as a reduction in 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) concentrations, calcium (Ca), and IGF1 levels were observed in goats kept on a reduced N diet. The decrease in 1,25-(OH)2D3 occurred even during a simultaneous reduction in dietary N and Ca, whereas a solitary Ca reduction stimulated 1,25-(OH)2D3 synthesis. The aim of the present study was to examine the effects of N- and/or Ca-reduced diets on the expression of 24-hydroxylase (CYP24A1), 1-alpha-hydroxylase (CYP27B1), vitamin D receptor (VDR), retinoid X receptor alpha (RXRα), IGF1 receptor (IGF1R), Klotho, and fibroblast growth factor receptor 1c (FGFR1c) in kidneys of young goats. Four groups were kept on a control diet, an N-reduced diet, a Ca-reduced diet or an N- and a Ca-reduced diet. Renal expression of CYP24A1 was not affected, whereas CYP27B1 expression was significantly diminished in the N-reduced diet fed goats (P < 0.05) and significantly elevated with the Ca reduction (P < 0.001). The VDR expression was not modified, whereas RXRα (P < 0.05) and Klotho expression (P < 0.001) were stimulated during Ca reduction. The IGF1R (P < 0.05) and FGFR1c (P < 0.05) expression were enhanced with the N reduction. From these data, it can be concluded that the downregulation of renal CYP27B1 expression observed with dietary N reduction is probably mediated by a complex interaction between the somatotropic axis and the Klotho/FGF signaling pathway in young goats.

Potencies of vitamin D analogs, 1α-hydroxyvitamin D3 , 1α-hydroxyvitamin D2 and 25-hydroxyvitamin D3 , in lowering cholesterol in hypercholesterolemic mice in vivo.

Vitamin D3 and the synthetic vitamin D analogs, 1α-hydroxyvitamin D3 [1α(OH)D3 ], 1α-hydroxyvitamin D2 [1α(OH)D2 ] and 25-hydroxyvitamin D3 [25(OH)D3 ] were appraised for their vitamin D receptor (VDR) associated-potencies as cholesterol lowering agents in mice in vivo. These precursors are activated in vivo: 1α(OH)D3 and 1α(OH)D2 are transformed by liver CYP2R1 and CYP27A1 to active VDR ligands, 1α,25-dihydroxyvitamin D3 [1,25(OH)2 D3 ] and 1α,25-dihydroxyvitamin D2 [1,25(OH)2 D2 ], respectively. 1α(OH)D2 may also be activated by CYP24A1 to 1α,24-dihydroxyvitamin D2 [1,24(OH)2 D2 ], another active VDR ligand. 25(OH)D3 , the metabolite formed via CYP2R1 and or CYP27A1 in liver from vitamin D3 , is activated by CYP27B1 in the kidney to 1,25(OH)2 D3 . In C57BL/6 mice fed the high fat/high cholesterol Western diet for 3weeks, vitamin D analogs were administered every other day intraperitoneally during the last week of the diet. The rank order for cholesterol lowering, achieved via mouse liver small heterodimer partner (Shp) inhibition and increased cholesterol 7α-hydroxylase (Cyp7a1) expression, was: 1.75nmol/kg 1α(OH)D3 >1248nmol/kg 25(OH)D3 (dose ratio of 0.0014) >>1625nmol/kg vitamin D3 . Except for 1.21nmol/kg 1α(OH)D2 that failed to lower liver and plasma cholesterol contents, a significant negative correlation was observed between the liver concentration of 1,25(OH)2 D3 formed from the precursors and liver cholesterol levels. The composite results show that vitamin D analogs 1α(OH)D3 and 25(OH)D3 exhibit cholesterol lowering properties upon activation to 1,25(OH)2 D3 : 1α(OH)D3 is rapidly activated by liver enzymes and 25(OH)D3 is slowly activated by renal Cyp27b1 in mouse.

Dysregulated 1,25-dihydroxyvitamin D levels in high-fat diet–induced obesity can be restored by changing to a lower-fat diet in mice

Altered regulation of vitamin D metabolites, 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D (1,25[OH]2D), was observed in high-fat diet (HFD)–induced obesity. We hypothesized that these HFD-induced changes in vitamin D metabolism would be reversed by decreasing fat mass through dietary intervention. Four-week-old C57BL/6J mice were assigned to 1 of 3 experimental diet groups: (1) the LL group was fed a control diet for 31 weeks, (2) the HH group was fed an HFD for 31 weeks, and (3) the HL group was fed HFD for 15 weeks then switched to the control diet for the remaining 16 weeks. The fat mass of the HL group decreased by 15% from the 14th to the 30th week. Serum 1,25(OH)2D level was significantly higher in the HH group than the LL group, whereas that of the HL group was intermediate to the 2 groups. Serum parathyroid hormone and renal 1-hydroxylase (Cyp27b1) mRNA levels, which are known to stimulate renal 1,25(OH)2D production, were significantly higher in the HH group than the LL group. After losing fat mass, the HL group had significantly lower renal Cyp27b1 mRNA levels than the HH group. No differences were found in serum 25-hydroxyvitamin D levels and mRNA levels of hepatic 25-hydroxylases. In adipose tissue, mRNA levels of 25-hydroxylase and vitamin D receptor were elevated in parallel to the adiposity. In conclusion, serum 1,25(OH)2D levels were closely associated with body adiposity, and reducing fat mass by changing to a lower-fat diet can reverse this obesity-associated increase in circulating 1,25(OH)2D levels.

Physiologic and pathophysiologic roles of extra renal CYP27b1: Case report and review

Although the kidney was initially thought to be the sole organ responsible for the production of 1,25(OH)2D via the enzyme CYP27b1, it is now appreciated that the expression of CYP27b1 in tissues other than the kidney is wide spread. However, the kidney is the major source for circulating 1,25(OH)2D. Only in certain granulomatous diseases such as sarcoidosis does the extra renal tissue produce sufficient 1,25(OH)2D to contribute to the circulating levels, generally associated with hypercalcemia, as illustrated by the case report preceding the review. Therefore the expression of CYP27b1 outside the kidney under normal circumstances begs the question why, and in particular whether the extra renal production of 1,25(OH)2D has physiologic importance. In this chapter this question will be discussed. First we discuss the sites for extra renal 1,25(OH)2D production. This is followed by a discussion of the regulation of CYP27b1 expression and activity in extra renal tissues, pointing out that such regulation is tissue specific and different from that of CYP27b1 in the kidney. Finally the physiologic significance of extra renal 1,25(OH)2D3 production is examined, with special focus on the role of CYP27b1 in regulation of cellular proliferation and differentiation, hormone secretion, and immune function. At this point the data do not clearly demonstrate an essential role for CYP27b1 expression in any tissue outside the kidney, but several examples pointing in this direction are provided. With the availability of the mouse enabling tissue specific deletion of CYP27b1, the role of extra renal CYP27b1 expression in normal and pathologic states can now be addressed definitively.

Oleanolic Acid Exerts Osteoprotective Effects and Modulates Vitamin D Metabolism.

Oleanolic acid (OA) is a triterpenoid with reported bone anti-resorption activities. The present study aimed to characterize its bone protective effects in vivo and to study its effects on vitamin D metabolism, both in vivo and in vitro. OA significantly increased bone mineral density, improved micro-architectural properties, reduced urinary Ca excretion, increased 1,25(OH)2D3 and renal CYP27B1 mRNA expression in mature C57BL/6 ovariectomised (OVX) mice. OA also improved bone properties, Ca balance, and exerted modulatory effects on renal CYP27B1 and CYP24A1 expressions in aged normal female Sprague–Dawley rats. In addition, OA significantly increased renal CYP27B1 mRNA and promoter activity, and suppressed CYP24A1 mRNA and protein expressions in human proximal tubule HKC-8 cells. OA exerted bone protective effects in mature OVX mice and aged female rats. This action on bone might be, at least in part, associated with its effects on Ca and vitamin D metabolism. The present findings suggest that OA is a potential drug candidate for the management of postmenopausal osteoporosis.

Effects of Cholecalciferol on Key Components of Vitamin D-Endo/Para/Autocrine System in Experimental Type 1 Diabetes.

Objectives Recent prospective studies have found the associations between type 1 diabetes (T1D) and vitamin D deficiency. We investigated the role of vitamin D in the regulation of 25OHD-1α-hydroxylase (CYP27B1) and VDR expression in different tissues of T1D rats. Design T1D was induced in male Wistar rats by streptozotocin (55 mg/k b.w.). After 2 weeks of T1D, the animals were treated orally with or without vitamin D3 (cholecalciferol; 100 IU/rat, 30 days). Methods Serum 25-hydroxyvitamin D (25OHD) was detected by ELISA. CYP27A1, CYP2R1, CYP27B1, and VDR were assayed by RT-qPCR and Western blotting or visualized by immunofluorescence staining. Results We demonstrated that T1D led to a decrease in blood 25OHD, which is probably due to the established downregulation of CYP27A1 and CYP2R1 expression. Vitamin D deficiency was accompanied by elevated synthesis of renal CYP27B1 and VDR. Conversely, CYP27B1 and VDR expression decreased in the liver, bone tissue, and bone marrow. Cholecalciferol administration countered the impairments of the vitamin D-endo/para/autocrine system in the kidneys and extrarenal tissues of diabetic rats. Conclusions T1D-induced vitamin D deficiency is associated with impairments of renal and extrarenal CYP27B1 and VDR expression. Cholecalciferol can be effective in the amelioration of diabetes-associated abnormalities in the vitamin D-endo/para/autocrine system.