Importance of apical membrane delivery of 1,25-dihydroxyvitamin D3 to vitamin D-responsive gene expression in the colon

Sulfated vitamin D metabolites represent prominent roles in serum and in breastmilk of lactating women

BACKGROUND

Concentrations of vitamin D (VitD) and 25-hydroxyvitamin D (25OHD) in breastmilk are low despite the essential role of VitD for normal infant bone development, yet additional metabolic forms of vitamin D may be present. This study evaluates the contribution of sulfated vitamin D metabolites, vitamin D3-sulfate (VitD3-S) and 25-hydroxyvitamin D3-sulfate (25OHD3-S) for lactating women and assesses the response to high-dose VitD3 supplementation.

METHODS

Serum and breastmilk were measured before and after 28 days with 5000 IU/day VitD3 intake in 20 lactating women. Concentrations of VitD3-S and 25OHD3-S in milk, and 25OHD2, 25OHD3, 25OHD3-S, VitD3 and VitD3-S in serum were determined by mass spectrometry.

RESULTS

Baseline vitamin D status was categorized as sufficient (mean ± SD serum 25OHD3 69 ± 19 nmol/L), and both serum VitD3 and 25OHD3 increased following supplementation (p < 0.001). 25OHD3-S was 91 ± 19 nmol/L in serum and 0.47 ± 0.09 nmol/L in breastmilk. VitD3-S concentrations were 2.92 ± 0.70 nmol/L in serum and 6.4 ± 3.9 nmol/L in breastmilk. Neither sulfated metabolite significantly changed with supplementation in either serum or breastmilk.

CONCLUSIONS

Sulfated vitamin D metabolites have prominent roles for women during lactation with 25OHD3-S highly abundant in serum and VitD3-S distinctly abundant in breastmilk. These data support the notion that 25OHD3-S and VitD3-S may have physiological relevance during lactation and nutritional usage for nursing infants.

Validation of the 24,25-dihydroxyvitamin D3 to 25-hydroxyvitamin D3 ratio as a biomarker of 25-hydroxyvitamin D3 clearance

The formation of 24,25-dihydroxyvitamin D (24,25(OH)₂D) from 25-hydroxyvitamin D (25(OH)D) is the primary mechanism for the metabolic clearance of 25(OH)D, and is regulated by tissue-level vitamin D activity. The ratio of 24,25(OH)₂D₃ to 25(OH)D₃ in blood (vitamin D metabolite ratio, VDMR) is postulated to be a marker of 25(OH)D₃ clearance, however this has never been tested. We measured baseline 24,25(OH)₂D₃ and 25(OH)D₃ concentrations in 87 participants by liquid chromatography-tandem mass spectrometry. Following an infusion of deuterated 25(OH)D₃, blood samples for each participant were collected over 56 days and analyzed for deuterated vitamin D metabolites. 25(OH)D₃ clearance and the deuterated metabolite-to-parent AUC ratio (ratio of the AUC of deuterated 24,25(OH)₂D₃ to that of deuterated 25(OH)D₃) were calculated. We compared the VDMR with these two measures using correlation coefficients and linear regression. Participants had a mean age of 64 ± 11years, 41 % were female, 30 % were self-described Black, 28 % had non-dialysis chronic kidney disease (CKD) and 23 % had kidney failure treated with hemodialysis. The VDMR was strongly correlated with 25(OH)D₃ clearance and the deuterated metabolite-to-parent AUC ratio (r = 0.51 and 0.76, respectively). Adjusting for 25(OH)D₃ clearance or the deuterated metabolite-to-parent AUC ratio in addition to clinical covariates, lower VDMR was observed in participants with CKD and kidney failure than in healthy controls; in Black than White participants; and in those with lower serum albumin. Our findings validate the VDMR as a measure of 25(OH)D₃ clearance. This relationship was biased by characteristics including race and kidney disease, which warrant consideration in studies assessing the VDMR.

Vitamin D and Gut Health

Vitamin D is a conditionally required nutrient that can either be obtained from skin synthesis following UVB exposure from the diet. Once in the body, it is metabolized to produce the endocrine hormone, 1,25 dihydroxyvitamin D (1,25(OH)2D), that regulates gene expression in target tissues by interacting with a ligand-activated transcription factor, the vitamin D receptor (VDR). The first, and most responsive, vitamin D target tissue is the intestine. The classical intestinal role for vitamin D is the control of calcium metabolism through the regulation of intestinal calcium absorption. However, studies clearly show that other functions of the intestine are regulated by the molecular actions of 1,25(OH)2 D that are mediated through the VDR. This includes enhancing gut barrier function, regulation of intestinal stem cells, suppression of colon carcinogenesis, and inhibiting intestinal inflammation. While research demonstrates that there are both classical, calcium-regulating and non-calcium regulating roles for vitamin D in the intestine, the challenge facing biomedical researchers is how to translate these findings in ways that optimize human intestinal health.

Role of glucuronidated 25-hydroxyvitamin D on colon gene expression in mice

25-Hydroxyvitamin D₃-3β-glucuronic acid (25OHD-Gluc) is produced in the liver and is a constituent of human blood and bile. Bacterial glucuronidases (GUS) in mammalian digestive microbiota cleave glucuronide conjugates, such as 25OHD-Gluc, and release the free aglycone (i.e., 25OHD) inside the intestinal lumen. We hypothesized that 25OHD-Gluc would elicit a VDR-dependent mRNA response in the colon after cleavage by gut microbiota. The activity of 25OHD-Gluc was investigated by measuring expression of cytochrome P450 24A1 (Cyp24) mRNA both in vitro and in vivo. In cell culture, Caco2 cells responded to 25OHD-Gluc, whereas HT29 cells did not. When coincubated with GUS, both cell lines elicited a robust response as indicated by a 5 Ct (32-fold) increase in Cyp24 mRNA. In vitamin D-sufficient mice, we found that both oral and subcutaneous administration of 1 nmol 25OHD-Gluc induced expression of Cyp24 mRNA in the colon whereas 25OHD did not. In contrast, 25OHD, but not 25OHD-Gluc, was active in the duodenum. When the jejunum was surgically ligated to block flow of digesta to the colon, neither oral nor subcutaneous administration of 2 nmol 25OHD-Gluc was able to induce expression of Cyp24 in the colon. Our findings suggest that 25OHD-Gluc, a vitamin D metabolite found in bile, induces VDR-mediated responses in the colon by crossing the apical membrane of the colon epithelium.NEW & NOTEWORTHY We found that 25OHD-Gluc, an endogenously produced metabolite, is delivered to the colon via bile to induce vitamin D-mediated responses in the colon.

Targeting 1,25(OH)2D-mediated calcium absorption machinery in proximal colon with calcitriol glycosides and glucuronides

High intestinal calcium (Ca) absorption efficiency is associated with high peak bone mass in adolescents and reduced bone loss in adulthood. Transepithelial intestinal Ca absorption is mediated by 1,25-dihydroxyvitamin D (1,25(OH)₂D, calcitriol) through the vitamin D receptor (VDR). Most research on Ca absorption focuses on the proximal small intestine but evidence shows that large intestine plays a crucial role in whole body Ca homeostasis. We directly assessed and compared Ca absorption capacity at the proximal colon and duodenum using in situ ligated loops (2 mM Ca, 10 min). In C57BL/6 J mice, the proximal colon (26.2 ± 3.7 %) had comparable ability to absorb Ca as the duodenum (30.0 ± 6.7 %). In VDR knockout (KO) mice, Ca absorption efficiency was reduced by 67 % in duodenum and 48 % in proximal colon. These data suggest that large intestine could be targeted to improve Ca absorption and protect bone in at risk-groups (e.g. bariatric patients). Glycoside forms of calcitriol found in Solanum Glaucophyllum (Sg) leaf are biologically inert but can be activated in the colon upon bacterial cleavage of the glycosides. We conducted a study to test whether Sg leaf, as well as a novel, synthetic 1,3-diglucuronide form of calcitriol (1,3-diG) could target the proximal colon and upregulate genes involved in Ca absorption (i.e. Trpv6, S100g). 13-week-old female C57BL6/J mice were fed AIN93 G diet containing increasing levels of one of the two compounds for 2 weeks (delivering 0, 0.25, 0.5, 1, or 2 ng calcitriol equivalent per day). Both compounds induced a dose-dependent upregulation of Cyp24a1 and Trpv6 gene expression in the proximal colon. 1,3-diG also induced S100g gene expression in the proximal colon. Duodenal expression of Trpv6 was upregulated at higher doses of 1,3-diG but not Sg leaf. These data suggest that both glycosylated and glucuronidated calcitriol could be used to target the proximal colon but that dosing must be optimized to limit systemic effects that could cause hypercalcemia. Future studies will test the translational potential of these compounds to determine if they can increase Ca absorption at proximal colon and whether this can help protect bone.

Oral 25-Hydroxycholecalciferol Acts as an Agonist in the Duodenum of Mice and as Modeled in Cultured Human HT-29 and Caco2 Cells

BACKGROUND

25-Hydroxycholecalciferol [25(OH)D] is the predominant circulating metabolite of vitamin D and serves as the precursor for 1α,25-dihydroxycholecalciferol [1,25(OH)2D], the hormonally active form. The presence of 1α-hydroxylase (1α-OHase) in the intestine suggests that 1,25(OH)2D can be produced from 25(OH)D, but the effects of oral 25(OH)D on the intestine have not been determined.

OBJECTIVES

We investigated the acute intestinal response to orally consumed 25(OH)D in mice by assessing mRNA induction of cytochrome p450 family 24 subfamily A member 1 (Cyp24), a vitamin D-dependent gene. The mechanism of action then was determined through in vitro analyses with Caco2 and HT-29 cells.

METHODS

Adult male C57BL6 mice were given a single oral dose of 40, 80, 200, or 400 ng 25(OH)D (n = 4 per dose) or vehicle (n = 3), and then killed 4 h later to evaluate the duodenal expression of Cyp24 mRNA by qPCR and RNA in situ hybridization. The 25(OH)D-mediated response was also evaluated with Caco2 and HT-29 cells by inhibition assay and dose-response analysis. A cytochrome p450 family 27 subfamily B member 1 (CYP27B1) knockdown of HT-29 was created to compare the dose-response parameters with wild-type HT-29 cells.

RESULTS

Oral 25(OH)D induced expression of Cyp24 mRNA in the duodenum of mice with 80 ng 25(OH)D by 3.3 ± 0.8 ΔΔCt compared with controls (P < 0.05). In vitro, both Caco2 and HT-29 cells responded to 25(OH)D treatment with 200-fold and 175-fold greater effective concentration at 50% maximal response than 1,25(OH)2D, yet inhibition of 1α-OHase and knockdown of CYP27B1 had no effect on the responses.

CONCLUSIONS

In mice, orally consumed 25(OH)D elicits a vitamin D-mediated response in the duodenum. In vitro assessments suggest that the response from 25(OH)D does not require activation by 1α-OHase and that 25(OH)D within the intestinal lumen acts as a vitamin D receptor agonist.

Localization of the 1,25-dihydroxyvitamin d-mediated response in the intestines of mice

1,25-Dihydroxyvitamin D₃ (1,25(OH)₂D) elicits a transcriptional response in the intestines. Assessments of this response are often derived from crude tissue homogenates and eliminate the ability to discriminate among different cell types. Here, we used an RNA in situ hybridization assay, RNAScope (Advanced Cell Diagnostics, Newark, CA), to identify the cells in the intestine that respond to 1,25(OH)₂D with expression of cytochrome P450 family 24 subfamily A member 1 (Cyp24a1) mRNA. Mice were gavaged with a single bolus dose of 1,25(OH)₂D to target the duodenum or a glucuronic acid conjugate of 1,25(OH)₂D, β-G-1,25(OH)₂D, to target the colon. QRT-PCR analysis of Cyp24a1 mRNA verified that the 1,25(OH)₂D-induced responses were present. RNAScope revealed that the mRNA response present after six hours is limited to mature enterocytes exposed to the intestinal lumen in both the duodenum and colon. No detectable expression was observed in goblet cells, lamina propria, muscularis mucosa muscle, submucosa and submucosal lymphoid follicles, or tunica muscularis. Our findings have identified epithelial enterocytes to be the intestinal targets for 1,25(OH)₂D in both the duodenum and colon.

Vitamin D Axis in Inflammatory Bowel Diseases: Role, Current Uses and Future Perspectives

Increasing evidence supports the concept that the vitamin D axis possesses immunoregulatory functions, with vitamin D receptor (VDR) status representing the major determinant of vitamin D’s pleiotropic effects. Vitamin D promotes the production of anti-microbial peptides, including β-defensins and cathelicidins, the shift towards Th2 immune responses, and regulates autophagy and epithelial barrier integrity. Impairment of vitamin D-mediated pathways are associated with chronic inflammatory conditions, including inflammatory bowel diseases (IBD). Interestingly, inhibition of vitamin D pathways results in dysbiosis of the gut microbiome, which has mechanistically been implicated in the development of IBD. Herein, we explore the role of the vitamin D axis in immune-mediated diseases, with particular emphasis on its interplay with the gut microbiome in the pathogenesis of IBD. The potential clinical implications and therapeutic relevance of this interaction will also be discussed, including optimizing VDR function, both with vitamin D analogues and probiotics, which may represent a complementary approach to current IBD treatments.

Vitamin D Reduces Colitis- and Inflammation-Associated Colorectal Cancer in Mice Independent of NOD2

Inflammatory bowel disease (IBD) patients are at increased risk of developing colorectal cancer (CRC). Vitamin D (vD) induces NOD2 gene expression, enhancing immunity, while deficiency impairs intestinal epithelial integrity, increasing inflammation. This study investigated the effect of vD on CRC in colitis, and if preventive benefits are mediated via NOD2. Inflammation-associated CRC was induced by treating C57BL/6J and Nod2^-/- mice with azoxymethane (AOM) and dextran sodium sulfate (DSS) cycles (×3). vD-deficient mice displayed more severe colitis compared to vD-supplemented mice, with greater weight loss, higher colitis activity index, increased colonic weight/length ratios, and lower survival rates. Increased histological inflammation score and increased IL-6 were observed in the mucosa of vD-deficient mice. Overall incidence of colonic tumors was not significantly different between vD-deficient and vD-supplemented mice. Higher tumor multiplicity was observed in vD-deficient vs vD-supplemented groups (both mouse strains). After AOM/DSS treatment, decreased plasma 25(OH)D₃ levels and downregulation of vD target genes Cyp24 and Vdr were observed in both mice strains (vD-deficient or vD-supplemented diet), compared to saline-treated controls on the vD-deficient diet. In conclusion, vD supplementation reduced colitis severity and decreased the number of inflammation-associated colorectal tumors in both C57BL/6J and Nod2^-/- mice, independent of NOD2.

The differential effects of 1,25-dihydroxyvitamin D3 on Salmonella-induced interleukin-8 and human beta-defensin-2 in intestinal epithelial cells

Salmonellosis or Salmonella, one of the most common food-borne diseases, remains a major public health problem worldwide. Intestinal epithelial cells (IECs) play an essential role in the mucosal innate immunity of the host to defend against the invasion of Salmonella by interleukin (IL)-8 and human β-defensin-2 (hBD-2). Accumulated research has unravelled important roles of vitamin D in the regulation of innate immunity. Therefore, we investigated the effects of 1,25-dihydroxyvitamin D3 (1,25D3) on Salmonella-induced innate immunity in IECs. We demonstrate that pretreatment of 1,25D3 results in suppression of Salmonella-induced IL-8 but enhancement of hBD-2, either protein secretion and mRNA expression, in IECs. Furthermore, 1,25D3 enhanced Salmonella-induced membranous recruitment of nucleotide oligomerization domain (NOD2) and its mRNA expression and activation of protein kinase B (Akt), a downstream effector of phosphoinositide 3-kinase (PI3K). Inhibition of the PI3K/Akt signal counteracted the suppressive effect of 1,25D3 on Salmonella-induced IL-8 expression, while knock-down of NOD2 by siRNA diminished the enhanced hBD-2 expression. These data suggest differential regulation of 1,25D3 on Salmonella-induced IL-8 and hBD-2 expression in IECs via PI3K/Akt signal and NOD2 protein expression, respectively. Active vitamin D-enhanced anti-microbial peptide in Salmonella-infected IECs protected the host against infection, while modulation of proinflammatory responses by active vitamin D prevented the host from the detrimental effects of overwhelming inflammation. Thus, active vitamin D-induced innate immunity in IECs enhances the host's protective mechanism, which may provide an alternative therapy for invasive Salmonella infection.

Targeted delivery of 1,25-dihydroxyvitamin D3 to colon tissue and identification of a major 1,25-dihydroxyvitamin D3 glycoside from Solanumglaucophyllum plant leaves

Leaves of the Solanum glaucophyllum (Sg) plant, indigenous to South America, have long been known for their calcinogenic toxicity in ruminant animals. It was determined the leaves contained glycosidic derivatives of 1,25-dihydroxyvitamin D3 (1,25D3) and liberation of the free hormone by rumen bacterial populations elicited a hypercalcemic response. Our interest in the leaves is predicated on the concept that the glycoside forms of 1,25D3 would target release of the active hormone in the lower gut of non-ruminant mammals. This would provide a means of delivering 1,25D3 directly to the colon, where the hormone has been shown to have beneficial effects in models of inflammatory bowel disease (IBD) and colon cancer. We fed mice for 10 days with variable amounts of Sg leaf. Feeding 7-333μg leaf/day produced no changes in plasma Ca(2+) and 1,25D3 concentrations, and only at ≥1000μg leaf/day did these values become significantly elevated compared to controls. Gene expression studies from colon tissue indicated a linear relationship between the amount of leaf consumed and expression of the Cyp24a1 gene. In contrast, Cyp24a1 gene expression in the duodenums and ileums of these mice was unchanged compared to controls. One of the major 1,25D3-glycosides was isolated from leaves following extraction and purification by Sep-Pak cartridges and HPLC fractionation. Ultraviolet absorbance was consistent with modification of the 1-hydroxyl group, and positive ion ESI mass spectrometry indicated a diglycoside of 1,25D3. 2-Dimensional NMR analyses were carried out and established the C1 proton of the A-ring was interacting with a C1' sugar proton, while the C3 proton of the A-ring was linked with a second C1' sugar proton. The structure of the isolated compound is therefore consistent with a β-linked 1,3-diglycoside of 1,25D3. Thus, Sg leaf administered to mice at up to 333 ug/day can elicit colon-specific enhancement of Cyp24a1 gene expression without inducing hypercalcemia, and the 1,3-diglycoside is one of the major forms of 1,25D3 found in the leaf. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

Vitamin D-enhanced duodenal calcium transport

For humans and rodents, duodenum is a very important site of calcium absorption since it is exposed to ionized calcium released from dietary complexes by gastric acid. Calcium traverses the duodenal epithelium via both transcellular and paracellular pathways in a vitamin D-dependent manner. After binding to the nuclear vitamin D receptor, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] upregulates the expression of several calcium transporter genes, e.g., TRPV5/6, calbindin-D9k, plasma membrane Ca(2+)-ATPase1b, and NCX1, thereby enhancing the transcellular calcium transport. This action has been reported to be under the regulation of parathyroid-kidney-intestinal and bone-kidney-intestinal axes, in which the plasma calcium and fibroblast growth factor-23 act as negative feedback regulators, respectively. 1,25(OH)2D3 also modulates the expression of tight junction-related genes and convective water flow, presumably to increase the paracellular calcium permeability and solvent drag-induced calcium transport. However, vitamin D-independent calcium absorption does exist and plays an important role in calcium homeostasis under certain conditions, particularly in neonatal period, pregnancy, and lactation as well as in naturally vitamin D-impoverished subterranean mammals.

Human UGT1A4 and UGT1A3 conjugate 25-hydroxyvitamin D3: metabolite structure, kinetics, inducibility, and interindividual variability

25-Hydroxyvitamin D3 (25OHD3) is used as a clinical biomarker for assessment of vitamin D status. Blood levels of 25OHD3 represent a balance between its formation rate and clearance by several oxidative and conjugative processes. In the present study, the identity of human uridine 5'-diphosphoglucuronyltransferases (UGTs) capable of catalyzing the 25OHD3 glucuronidation reaction was investigated. Two isozymes, UGT1A4 and UGT1A3, were identified as the principal catalysts of 25OHD3 glucuronidation in human liver. Three 25OHD3 monoglucuronides (25OHD3-25-glucuronide, 25OHD3-3-glucuronide, and 5,6-trans-25OHD3-25-glucuronide) were generated by recombinant UGT1A4/UGT1A3, human liver microsomes, and human hepatocytes. The kinetics of 25OHD3 glucuronide formation in all systems tested conformed to the Michaelis-Menten model. An association between the UGT1A4*3 (Leu48Val) gene polymorphism with the rates of glucuronide formation was also investigated using human liver microsomes isolated from 80 genotyped livers. A variant allele dose effect was observed: the homozygous UGT1A4*3 livers (GG) had the highest glucuronidation activity, whereas the wild type (TT) had the lowest activity. Induction of UGT1A4 and UGT1A3 gene expression was also determined in human hepatocytes treated with pregnane X receptor/constitutive androstane receptor agonists, such as rifampin, carbamazepine, and phenobarbital. Although UGT mRNA levels were increased significantly by all of the known pregnane X receptor/constitutive androstane receptor agonists tested, rifampin, the most potent of the inducers, significantly induced total 25OHD3 glucuronide formation activity in human hepatocytes measured after 2, but not 4 and 24 hours, of incubation. Finally, the presence of 25OHD3-3-glucuronide in both human plasma and bile was confirmed, suggesting that the glucuronidation pathway might be physiologically relevant and contribute to vitamin D homeostasis in humans.