Inhibition of LXRalpha signaling by vitamin D receptor: possible role of VDR in bile acid synthesis

Vitamin D levels and lipid profile in children and adolescents: a tight correlation

INTRODUCTION

Vitamin D is an essential hormone for humans, playing an important role in musculoskeletal and calcium homeostasis. Its deficiency/insufficiency seems to contribute to the development of cardiometabolic diseases in adults: this correlation appears less clear for children and adolescents. The aim of this paper was to review literature data on the relationship between vitamin D and lipid profile alterations in pediatric population.

EVIDENCE ACQUISITION

We carried out a comprehensive research in electronic databases, including MEDLINE and PubMed up to December 2022, for cross-sectional or prospective studies that investigated the correlation between serum vitamin D levels and lipid profile in children and adolescents. At the end of the process, 37 articles were included in this review.

EVIDENCE SYNTHESIS

According to our findings, vitamin D deficiency/insufficiency is strongly associated with lower high-density lipoprotein (HDL) cholesterol levels and higher levels of triglycerides and total cholesterol. Data about low-density lipoproteins (LDL) cholesterol are inconsistent. The potential role of vitamin D supplements for the prevention of cardiometabolic disease currently remains a speculation.

CONCLUSIONS

An increasing number of studies shows how hypovitaminosis D in the pediatric age may play a role in the pathogenesis of metabolic disorders and lipid profile alterations. Data regarding the potential role of vitamin D supplements for the prevention of cardiometabolic disease are currently controversial. Further studies are needed to evaluate the causality of this association and to assess the underlying pathogenetic mechanisms.

Vitamin D and Dyslipidemia: Is There Really a Link? A Narrative Review

Nowadays, the interest in the extraskeletal effects of vitamin D is growing. In the literature, its several possible actions have been confirmed. Vitamin D seems to have a regulatory role in many different fields-inflammation, immunity, and the endocrine system-and many studies would demonstrate a possible correlation between vitamin D and cardiovascular disease. In this paper, we deepened the relationship between vitamin D and dyslipidemia by reviewing the available literature. The results are not entirely clear-cut: on the one hand, numerous observational studies suggest a link between higher serum vitamin D levels and a beneficial lipid profile, while on the other hand, interventional studies do not demonstrate a significant effect. Understanding the possible relationship between vitamin D and dyslipidemia may represent a turning point: another link between vitamin D and the cardiovascular system.

The causal relationship between 25-hydroxyvitamin D and serum lipids levels: A bidirectional two-sample mendelian randomization study

Serum vitamin D levels were linked to lipid metabolism in observational studies, but the exact mechanism was unclear. Several studies have attempted to decipher the relationship between 25(OH)D and lipid levels. Conventional observational studies are vulnerable to confounding. Mendelian randomization (MR) analysis can better control for confounding factors and reverse causality, allowing for the inference of causal association. We, therefore, sought to use MR to investigate the possible causal relationship between 25(OH)D and blood lipid levels (HDL cholesterol, LDL cholesterol, triglycerides, and total cholesterol). A bidirectional two-sample Mendelian randomization (MR) was performed on data primarily from European ancestors. In addition, the potential causal effect of lipids on 25(OH)D was assessed by regressor-based multivariate magnetic resonance (MVMR). The single-nucleotide polymorphisms (SNPs) related to 25(OH)D were selected from a large-scale genome-wide association study (GWAS) database named IEU GWAS, and the SNPs associated with the four blood lipids were chosen from UK Biobank (UKB) lipid GWAS. When blood lipids were the outcome, the results of bidirectional two-sample MR demonstrated that 25(OH)D exhibited a negative causal association with TG, TC, and LDL-C: β = - 0.23, 95% CI = -0.28 to -0.19, P<0.001; β = - 0.16, 95% CI: - 0.30 to-0.03, P < 0.05; β = - 0.11, 95% CI: - 0.23 to 0, P < 0.05. There was no causal relationship between 25(OH)D and HDL-C (β = 0.05, 95% CI: - 0.11 to 0.20, P = 0.56). When setting blood lipids as exposure, TG and 25(OH)D, β = -0.13, 95% CI: - 0.15 to -0.10, P < 0.05; TC and 25(OH)D, β = -0.11, 95% CI: - 0.15 to -0.07, P < 0.05; HDL-C and 25(OH)D, β = 0.02, 95% CI: 0 to 0.03, P = 0.07; LDL-C and 25(OH)D, β = -0.08, 95% CI: - 0.11 to -0.05, P < 0.05). Our MVMR study also showed a significant relationship between genetically determined lipid traits and 25(OH)D levels (TG and 25(OH)D, P < 0.05; TC and 25(OH)D, P < 0.05). In all MR analyses, there was no horizontal pleiotropy (all P > 0.05), or statistical heterogeneity. The "Leave-one-out" sensitivity analysis confirmed the stability of our results. MR Studies have shown a bidirectional causal relationship between genetically-determined 25(OH)D levels and serum TG and TC levels. The findings have potential implications for etiological understanding and disease prevention.

Effects of vitamin D supplementation on metabolic parameters, serum irisin and obesity values in women with subclinical hypothyroidism: a double-blind randomized controlled trial

Purpose

Subclinical hypothyroidism is an early, mild form of hypothyroidism that may progress to overt hypothyroidism if untreated. The current study aimed to assess the effects of vitamin D supplementation on hormonal (thyroid stimulating hormone [TSH], triiodothyronine, thyroxine, and free thyroxine) parameters, lipid profiles, serum irisin, and obesity indices in women with subclinical hypothyroidism.

Methods

The present randomized, double-blind, placebo-controlled clinical trial was carried out on 44 women with subclinical hypothyroidism. The participants were allocated to two groups (22 patients in each group) that received vitamin D (50,000 IU/week) or placebo for 12 weeks. Fasting blood samples, anthropometric and body composition measurements, physical activity levels, and dietary intakes were collected at baseline and at the end of the study.

Results

Vitamin D supplementation significantly decreased TSH, total cholesterol, and fat mass percentage, and significantly increased serum vitamin D and irisin levels and fat-free mass percentage compared to the control group (all, p<0.05). Changes in thyroid hormones, other lipid profiles, and anthropometric indices were not significantly different between the groups.

Conclusion

Our study indicates that vitamin D administration improves serum TSH, total cholesterol, irisin, and body composition in women with subclinical hypothyroidism. More well-designed clinical trials are required to confirm these findings and clarify the effects of vitamin D supplementation on both genders of patients.Clinical trial registration: https://www.irct.ir/trial/57482, Identifier IRCT20100408003664N25.

Association of 25-hydroxyvitamin D levels with lipid profiles in osteoporosis patients: a retrospective cross-sectional study

BACKGROUND

In the literature, scarce data investigate the link between 25-hydroxyvitamin D (25[OH]D) and blood lipids in the osteoporosis (OP) population. 25(OH)D, as a calcium-regulating hormone, can inhibit the rise of parathyroid hormone, increase bone mineralization to prevent bone loss, enhance muscle strength, improve balance, and prevent falls in the elderly. This retrospective cross-sectional study aimed to investigate the association between serum 25(OH)D levels and lipid profiles in patients with osteoporosis, with the objective of providing insight for appropriate vitamin D supplementation in clinical settings to potentially reduce the incidence of cardiovascular disease, which is known to be a major health concern for individuals with osteoporosis.

METHODS

This is a retrospective cross-sectional study from the Affiliated Kunshan Hospital of Jiangsu University, including 2063 OP patients who received biochemical blood analysis of lipids during hospitalization from January 2015 to March 2022. The associations between serum lipids and 25(OH)D levels were examined by multiple linear regression. The dependent variables in the analysis were the concentrations of serum lipoprotein, total cholesterol (TC), triglycerides (TGs), apolipoprotein-A, lipoprotein A, high-density lipoprotein cholesterol and low-density lipoprotein cholesterol (LDL-C). The independent variable was the concentration of blood serum 25(OH)D. At the same time, age, body mass index, sex, time and year of serum analysis, primary diagnosis, hypertension, diabetes, statins usage, beta-C-terminal telopeptide of type I collagen, procollagen type I N-terminal propeptide were covariates. Blood samples were collected in the early morning after the overnight fasting and were analyzed using an automated electrochemiluminescence immunoassay on the LABOSPECT 008AS platform (Hitachi Hi-Tech Co., Ltd., Tokyo, Japan). The generalized additive model was further applied for nonlinear associations. The inception result for smoothing the curve was evaluated by two-piecewise linear regression exemplary.

RESULTS

Our results proved that in the OP patients, the serum 25(OH)D levels were inversely connected with blood TGs concentration, whereas they were positively associated with the HDL, apolipoprotein-A, and lipoprotein A levels. In the meantime, this research also found a nonlinear relationship and threshold effect between serum 25(OH)D and TC, LDL-C. Furthermore, there were positive correlations between the blood serum 25(OH)D levels and the levels of TC and LDL-C when 25(OH)D concentrations ranged from 0 to 10.04 ng/mL. However, this relationship was not present when 25(OH)D levels were higher than 10.04 ng/mL.

CONCLUSIONS

Our results demonstrated an independent relationship between blood lipids and vitamin D levels in osteoporosis patients. While we cannot establish a causal relationship between the two, our findings suggest that vitamin D may have beneficial effects on both bone health and blood lipid levels, providing a reference for improved protection against cardiovascular disease in this population. Further research, particularly interventional studies, is needed to confirm these associations and investigate their underlying mechanisms.

Significance of the Vitamin D Receptor on Crosstalk with Nuclear Receptors and Regulation of Enzymes and Transporters

The vitamin D receptor (VDR), in addition to other nuclear receptors, the pregnane X receptor (PXR) and constitutive androstane receptor (CAR), is involved in the regulation of enzymes, transporters and receptors, and therefore intimately affects drug disposition, tissue health, and the handling of endogenous and exogenous compounds. This review examines the role of 1α,25-dihydroxyvitamin D₃ or calcitriol, the natural VDR ligand, on activation of the VDR and its crosstalk with other nuclear receptors towards the regulation of enzymes and transporters, notably many of the cytochrome P450s including CYP3A4 and sulfotransferase 2A1 (SULT2A1) as well as cholesterol 7α-hydroxylase (CYP7A1). Moreover, the VDR upregulates the intestinal channel, TRPV6, for calcium absorption, LDL receptor-related protein 1 (LRP1) and receptor for advanced glycation end products (RAGE) in brain for β-amyloid peptide efflux and influx, the sodium phosphate transporters (NaP_i), the apical sodium-dependent bile acid transporter (ASBT) and organic solute transporters (OSTα-OSTβ) for bile acid absorption and efflux, respectively, the renal organic anion transporter 3 (OAT3) and several of the ATP-binding cassette protein transporters-the multidrug resistance protein 1 (MDR1) and the multidrug resistance-associated proteins (MRPs). Hence, the role of the VDR is increasingly being recognized for its therapeutic potential and pharmacologic activity, giving rise to drug-drug interactions (DDI). Therapeutically, ligand-activated VDR shows anti-inflammatory effects towards the suppression of inflammatory mediators, improves cognition by upregulating amyloid-beta (Aβ) peptide clearance in brain, and maintains phosphate, calcium, and parathyroid hormone (PTH) balance and kidney function and bone health, demonstrating the crucial roles of the VDR in disease progression and treatment of diseases.

Multiple Targets for Oxysterols in Their Regulation of the Immune System

Oxysterols, or cholesterol oxidation products, are naturally occurring lipids which regulate the physiology of cells, including those of the immune system. In contrast to effects that are mediated through nuclear receptors or by epigenetic mechanism, which take tens of minutes to occur, changes in the activities of cell-surface receptors caused by oxysterols can be extremely rapid, often taking place within subsecond timescales. Such cell-surface receptor effects of oxysterols allow for the regulation of fast cellular processes, such as motility, secretion and endocytosis. These cellular processes play critical roles in both the innate and adaptive immune systems. This review will survey the two broad classes of cell-surface receptors for oxysterols (G-protein coupled receptors (GPCRs) and ion channels), the mechanisms by which cholesterol oxidation products act on them, and their presence and functions in the different cell types of the immune system. Overall, this review will highlight the potential of oxysterols, synthetic derivatives and their receptors for physiological and therapeutic modulation of the immune system.

Atorvastatin Induces FXR and CYP7A1 Activation as a Result of the Sequential Action of PPARγ/PGC-1α/HNF-4α in Hep3B Cells

Backgrounds/Aims

PPARγ, farnesoid X receptor (FXR) and CYP7A1 are associated with solubility of bile. This study was performed to understand a mechanism and interactions of statin-induced PPARγ, PGC-1α and HNF-4α related to the statin-induced activation of FXR and CYP7A1, and verify whether the mevalonate pathway is involved in the mechanism.

Methods

MTT assays were performed using cultured human Hep3B cells to determine the effect of atorvastatin on the cell proliferation. Expression levels of indicated proteins were measured using Western blotting assays by inhibiting the protein expression or not.

Results

Atorvastatin increased expression of PPARγ, PGC-1α, HNF-4α, FXR, and CYP7A1 in Hep3B cells. PPARγ ligand of troglitazone upregulated the expression of PGC-1α, HNF-4α, FXR, and CYP7A1 in Hep3B cells. Silencing of PPARγ, PGC1α, and HNF4α using respective siRNA demonstrated that atorvastatin-induced FXR and CYP7A1 activation required sequential action of PPARγ /PGC-1α/HNF-4α. The silencing of PPARγ completely inhibited atorvastatin-induced PGC-1α expression, and the PGC1α silencing partially inhibited atorvastatin-induced PPARγ expression. The inhibition of HNF4α did not affect atorvastatin-induced PPARγ expression, but partially inhibited atorvastatin-induced PGC-1α expression. Besides, mevalonate completely reversed the effect of atorvastatin on PPARγ, PGC-1α, HNF-4α, FXR, and CYP7A1.

Conclusions

Atorvastatin induces FXR and CYP7A1 activation as a result of sequential action of PPARγ/PGC-1α/HNF-4α in human hepatocytes. We propose that atorvastatin enhances solubility of cholesterol in bile by simultaneously activating of FXR and CYP7A1.

The Association of Vitamin D Status with Lipid Profile and Inflammation Biomarkers in Healthy Adolescents

Background: The association between vitamin D status and inflammatory biomarkers and lipid profile is not well known, especially in adolescents. Therefore, the aim of the current study is to investigate the association of vitamin D status with serum lipids and inflammatory biomarkers, including IL-10, IL-6, hsCRP, and TNFR-2, in male adolescents. Methods and materials: A sample of seventy-one high school male students, aged 17 years old, from a high school in Tehran were enrolled in the study. They were divided into four groups including group with serum vitamin D below 25 (ng/mL) (SVD < 25; n = 36), 25 and above (ng/mL) (SVD ≥ 25; n = 35), negative-hsCRP (n = 48), and positive-hsCRP (n = 23). Weight, height, body mass index, dietary intake, serum lipids, and inflammatory biomarkers, including IL-10, IL-6, hsCRP, and TNFR-2, were measured. Results: In the (SVD < 25) group, the serum level of TNFR-2 was significantly higher compared to that in the (SVD ≥ 25) group. There was a significant negative association between serum TNFR-2 and vitamin D levels in the whole sample. We found significant lower levels of IL-10 in positive-hsCRP group compared to the negative-hsCRP group. In addition, there was a significant negative correlation between the serum vitamin D level and hsCRP in both hsCRP groups. The HDL level was lower in the (SVD < 25) group compared to that in the (SVD ≥ 25) group. Finally, there was a negative correlation between the serum HDL and hsCRP levels in the positive-hsCRP subjects. Conclusion: Based on the findings it can be concluded that serum vitamin D affects HDL and inflammation status. Although serum levels of HDL and inflammation status are both predictors of metabolic syndrome and cardiovascular disease, further studies are needed to prove it, especially in adolescents.

Steroid Hormone Vitamin D: Implications for Cardiovascular Disease

Understanding of vitamin D physiology is important because about half of the population is being diagnosed with deficiency and treated with supplements. Clinical guidelines were developed based on observational studies showing an association between low serum levels and increased cardiovascular risk. However, new randomized controlled trials have failed to confirm any cardiovascular benefit from supplementation in the general population. A major concern is that excess vitamin D is known to cause calcific vasculopathy and valvulopathy in animal models. For decades, administration of vitamin D has been used in rodents as a reliable experimental model of vascular calcification. Technically, vitamin D is a misnomer. It is not a true vitamin because it can be synthesized endogenously through ultraviolet exposure of the skin. It is a steroid hormone that comes in 3 forms that are sequential metabolites produced by hydroxylases. As a fat-soluble hormone, the vitamin D-hormone metabolites must have special mechanisms for delivery in the aqueous bloodstream. Importantly, endogenously synthesized forms are carried by a binding protein, whereas dietary forms are carried within lipoprotein particles. This may result in distinct biodistributions for sunlight-derived versus supplement-derived vitamin D hormones. Because the cardiovascular effects of vitamin D hormones are not straightforward, both toxic and beneficial effects may result from current recommendations.

Repression of insulin gene transcription by indirect genomic signaling via the estrogen receptor in pancreatic beta cells

The mechanism whereby 17β-estradiol (E2) mediates insulin gene transcription has not been fully elucidated. In this study, exposure of hamster insulinoma (HIT-T15) cells to 5 × 10-9 to 1 × 10-7 M E2 led to a concentration-dependent decrease of insulin mRNA levels. Transient expression of the estrogen receptor (ER) in HIT-T15 cells revealed that estrogen receptor α (ERα) repressed transcription of the rat insulin II promoter in both ligand-dependent and ligand-independent manners. The N-terminal A/B domain of ERα was not required for either activity. However, the repression was absent with mutated ER lacking the DNA-binding domain. Moreover, introducing mutations in the D-box and P-box of the zinc finger of ER (C227S, C202L) also abolished the repression. Deletion of the insulin promoter region revealed that nucleotide positions - 238 to - 144 (relative to the transcriptional start site) were needed for ER repression of the rat insulin II gene. PDX1- and BETA2-binding sites were required for the repression, but an estrogen response element-like sequence or an AP1 site in the promoter was not involved. In conclusion, we found that estrogen repressed insulin mRNA expression in a beta cell line. In addition, the ER suppressed insulin gene transcription in a ligand-independent matter. These observations suggest ER may regulate insulin transcription by indirect genomic signaling.

ApaI polymorphism of vitamin D receptor affects health-related quality of life in patients with primary sclerosing cholangitis

Background

Polymorphisms of vitamin D receptor (VDR) contribute to the pathogenesis of multiple autoimmune conditions.

Methods

We investigated the incidence of VDR polymorphisms (rs1544410-BsmI; rs7975232-ApaI; rs731236-TaqI) in a group of patients with primary sclerosing cholangitis (PSC, n = 275) and in healthy controls (n = 376). Additionally, correlations of the VDR polymorphisms with clinical and biochemical factors of the disease were analysed.

Results

The genotype and allele distributions of these polymorphisms in PSC patients were similar to those observed in controls. However, the ApaI polymorphism was associated with an impaired health-related quality of life (HRQoL). The generic SF-36 questionnaire showed that the Role-Physical (p = 0.01), Role-Emotional (p = 0.01), Physical Component Summary (p = 0.01) and Mental Component Summary (p = 0.003) scores were significantly affected. Similarly, the disease-specific questionnaires, PBC-40 and PBC-27, demonstrated that carriers of the C allele suffered from more severe Itch (p = 0.03 assessed by PBC-40 and PBC-27), more Fatigue (p = 0.02 assessed by PBC-40 and PBC-27) and Impaired Cognitive Capacity (p = 0.04 and p = 0.03). Correspondingly, individuals who were AA homozygotes (non-carriers of the C allele of ApaI) had higher summary scores for the Physical (p = 0.01) and Mental Components (p = 0.006) measured with SF-36. Moreover, they experienced less itch (p = 0.03) and less Fatigue (p = 0.03) and had better Cognitive Abilities (p = 0.04) as assessed by the PBC-40 and PBC-27 questionnaires. No associations between other VDR polymorphisms and clinical or laboratory findings were made.

Conclusion

In summary, this study is the first to show that the ApaI polymorphisms in VDR may exert an effect on disease-related symptoms and quality of life in patients with PSC.

Bile Acids, Nuclear Receptors and Cytochrome P450

This review summarizes the importance of bile acids (BA) as important regulators of various homeostatic mechanisms with detailed focus on cytochrome P450 (CYP) enzymes. In the first part, synthesis, metabolism and circulation of BA is summarized and BA are reviewed as physiological ligands of nuclear receptors which regulate transcription of genes involved in their metabolism, transport and excretion. Notably, PXR, FXR and VDR are the most important nuclear receptors through which BA regulate transcription of CYP genes involved in the metabolism of both BA and xenobiotics. Therapeutic use of BA and their derivatives is also briefly reviewed. The physiological role of BA interaction with nuclear receptors is basically to decrease production of toxic non-polar BA and increase their metabolic turnover towards polar BA and thus decrease their toxicity. By this, the activity of some drug-metabolizing CYPs is also influenced what could have clinically relevant consequences in cholestatic diseases or during the treatment with BA or their derivatives.

The Associations of Serum Lipids with Vitamin D Status

Aims

Vitamin D deficiency has been associated with some disorders including cardiovascular diseases. Dyslipidemia is a major risk factor for cardiovascular diseases. However, data about the relationships between vitamin D and lipids are inconsistent. The relationship of vitamin D and Atherogenic Index of Plasma (AIP), as an excellent predictor of level of small and dense LDL, has not been reported. The objective of this study was to investigate the effects of vitamin D status on serum lipids in Chinese adults.

Methods

The study was carried out using 1475 participants from the Center for Physical Examination, 306 Hospital of PLA in Beijing, China. Fasting blood samples were collected and serum concentrations of 25(OH)D, total cholesterol (TC), triglyceride (TG), high density lipoprotein cholesterol (HDL-C) and low density lipoprotein cholesterol (LDL-C) were measured. AIP was calculated based on the formula: log [TG/HDL-C]. Multiple linear regression analysis was used to estimate the associations between serum 25(OH)D and lipids. The association between the occurrences of dyslipidemias and vitamin D levels was assessed by multiple logistic regression analysis. Confounding factors, age and BMI, were used for the adjustment.

Results

The median of serum 25(OH)D concentration was 47 (27–92.25) nmol/L in all subjects. The overall percentage of 25(OH)D ≦ 50 nmol/L was 58.5% (males 54.4%, females 63.7%). The serum 25(OH)D levels were inversely associated with TG (β coefficient = -0.24, p < 0.001) and LDL-C (β coefficient = -0.34, p < 0.001) and positively associated with TC (β coefficient = 0.35, p < 0.002) in men. The associations between serum 25(OH)D and LDL-C (β coefficient = -0.25, p = 0.01) and TC (β coefficient = 0.39, p = 0.001) also existed in women. The serum 25(OH)D concentrations were negatively associated with AIP in men (r = -0.111, p < 0.01) but not in women. In addition, vitamin D deficient men had higher AIP values than vitamin D sufficient men. Furthermore, the occurrences of dyslipidemias (reduced HDL-C, elevated TG and elevated AIP) correlated with lower 25(OH)D levels in men, whereas the higher TC and LDL-C associated with higher 25(OH)D levels in women.

Conclusion

It seems that the serum 25(OH)D levels are closely associated with the serum lipids and AIP. Vitamin D deficiency may be associated with the increased risk of dyslipidemias, especially in men. The association between vitamin D status and serum lipids may differ by genders.

Relationship between 25(OH)D levels and circulating lipids in African American adolescents

BACKGROUND

Vitamin D deficiency is commonly seen among African American adolescents. Lipid levels during childhood are excellent predictors of adult dyslipidemia and atherosclerosis. There is a paucity of data on the relationship between 25 hydroxy vitamin D [25(OH)D] levels and lipids among African American adolescents. The objective of this study was to determine if there is an association between 25(OH)D levels and circulating lipids in African American adolescents residing in midwestern United States.

METHODS

African American adolescents residing in Rochester, MN (latitude 44°N), USA, underwent measurements of 25(OH)D and lipids following overnight fast. Pearson's correlation test, linear regression model and scatter plots were used to explore the association between 25(OH)D levels and lipids.

RESULTS

25(OH)D levels <30 ng/mL were seen in 21/24 (87%) of the subjects. 25(OH)D levels were inversely correlated with total cholesterol (r=-0.42; p=0.040) and with non-high-density lipoprotein (non-HDL) cholesterol (r=-0.42; p=0.040 ). These associations remained statistically significant after adjustment for age, gender and adiposity. In the multivariate linear regression model, and after adjusting for BMI Z-score, each 1 ng/mL increase in 25(OH)D was associated with a decrease in total cholesterol of 1.38 mg/dL (95% CI: -2.63, -0.14, p=0.030) and with a decrease in non-HDL cholesterol of 1.14 mg/dL (95% CI: -2.09, -0.18, p=0.020).

CONCLUSIONS

25(OH)D levels were inversely correlated with total cholesterol and non-HDL cholesterol levels in African American adolescents residing in midwestern United States. Further studies with larger sample sizes are needed to determine if low vitamin D status in African American adolescents is a potential modifiable risk factor for cardiovascular disease.

Vitamin D modulates biliary fibrosis in ABCB4-deficient mice

Purpose

Impaired vitamin D receptor signaling represents an aggravating factor during liver injury, and recent studies suggest that vitamin D might exert a protective role in chronic hepatobiliary diseases. We hypothesized that vitamin D supplementation would ameliorate liver fibrosis in ATP-binding cassette transporter B4 knockout (Abcb4^−/−) mice as a preclinical model of sclerosing cholangitis.

Methods

Abcb4^−/− and wild-type mice were fed a regular chow diet (600 IU vitamin D/kg food) or diets with lower (100 IU/kg) and higher (2,400 IU/kg) vitamin D concentrations for 12 weeks. Serum 25-hydroxyvitamin D concentrations were measured by chemiluminescence immunoassays. Liver injury and biliary fibrosis were assessed by liver enzyme activities, histopathology and hepatic collagen contents. Hepatic mRNA expression of markers for fibrosis, vitamin D and bile acid metabolism were analyzed by quantitative PCR.

Results

Different vitamin D concentrations were observed depending on genotype and diet group, with Abcb4^−/− mice on the control diet showing lower vitamin D concentrations compared to wild-type mice. Abcb4^−/− animals on the low vitamin D diet demonstrated the most advanced liver fibrosis and highest hepatic collagen contents. Feeding Abcb4^−/− mice a high vitamin D diet enriched serum vitamin D levels, lowered liver enzyme activities, altered expression levels of profibrogenic genes and ameliorated, in part, liver injury.

Conclusions

This is the first report to demonstrate that fibrogenesis in the established Abcb4^−/− model is influenced by vitamin D supplementation. Since vitamin D modulates sclerosing cholangitis in vivo, we speculate that sufficient vitamin D intake might improve liver damage and induce antifibrotic effects in chronic cholestasis in humans.

Electronic supplementary material

The online version of this article (doi:10.1007/s12072-014-9548-2) contains supplementary material, which is available to authorized users.

Vitamin D supplementation and lipid profile: what does the best available evidence show?

Vitamin D supplements have increasingly been used for the treatment and prevention of osteoporosis. Historically, effects of the vitamin on the cardiovascular (CV) system have been proposed and demonstrated in the literature, including benefits on serum lipids. Although observational studies support an association between increased serum vitamin D levels and a favorable lipid profile, interventional studies have shown no effects. This review presents and analyzes all the related randomized controlled trials (RCTs) identified in the literature from 1987 to present. A systematic literature search was conducted via MEDLINE, Cochrane Library and EMBASE and, out of 19 relevant RCTs identified, only one reported benefits of vitamin D supplementation on lipid profile parameters, while the rest showed no effects or even adverse outcomes, which are highlighted by the only meta-analysis in the field. Attempts to explain the paradox of beneficial findings of observational studies versus discouraging results of interventional studies have been made and the most popular suggests that high serum vitamin D concentrations may not be the cause of good health but its outcome instead, as healthy people are more likely to stay outdoors longer and have better eating habits. For definitive answers to be given, large, well-designed RCTs need to be conducted that will take into account and adjust for dietary consumption as well as serum calcium and parathyroid hormone levels, both of which have been shown to be associated with the CV system. Until then, recommendations for vitamin D supplementation should not change.

Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol

BACKGROUND & AIMS

Little is known about the effects of the vitamin D receptor (VDR) on hepatic activity of human cholesterol 7α-hydroxylase (CYP7A1) and cholesterol metabolism. We studied these processes in mice in vivo and mouse and human hepatocytes.

METHODS

Farnesoid X receptor (Fxr)(-/-), small heterodimer partner (Shp)(-/-), and C57BL/6 (wild-type control) mice were fed normal or Western diets for 3 weeks and were then given intraperitoneal injections of vehicle (corn oil) or 1α,25-dihydroxyvitamin D3 (1,25[OH]2D3; 4 doses, 2.5 μg/kg, every other day). Plasma and tissue samples were collected and levels of Vdr, Shp, Cyp7a1, Cyp24a1, and rodent fibroblast growth factor (Fgf) 15 expression, as well as levels of cholesterol, were measured. We studied the regulation of Shp by Vdr using reporter and mobility shift assays in transfected human embryonic kidney 293 cells, quantitative polymerase chain reaction with mouse tissues and mouse and human hepatocytes, and chromatin immunoprecipitation assays with mouse liver.

RESULTS

We first confirmed the presence of Vdr mRNA and protein expression in livers of mice. In mice fed normal diets and given injections of 1,25(OH)2D3, liver and plasma concentrations of 1,25(OH)2D3 increased and decreased in unison. Changes in hepatic Cyp7a1 messenger RNA (mRNA) correlated with those of Cyp24a1 (a Vdr target gene) and inversely with Shp mRNA, but not ileal Fgf15 mRNA. Similarly, incubation with 1,25(OH)2D3 increased levels of Cyp24a1/CYP24A1 and Cyp7a1/CYP7A1 mRNA in mouse and human hepatocytes, and reduced levels of Shp mRNA in mouse hepatocytes. In Fxr(-/-) and wild-type mice with hypercholesterolemia, injection of 1,25(OH)2D3 consistently reduced levels of plasma and liver cholesterol and Shp mRNA, and increased hepatic Cyp7a1 mRNA and protein; these changes were not observed in Shp(-/-) mice given 1,25(OH)2D3 and fed Western diets. Truncation of the human small heterodimer partner (SHP) promoter and deletion analyses revealed VDR-dependent inhibition of SHP, and mobility shift assays showed direct binding of VDR to enhancer regions of SHP. In addition, chromatin immunoprecipitation analysis of livers from mice showed that injection of 1,25(OH)2D3 increased recruitment of Vdr and rodent retinoid X receptor to the Shp promoter.

CONCLUSIONS

Activation of the VDR represses hepatic SHP to increase levels of mouse and human CYP7A1 and reduce cholesterol.

Pravastatin activates the expression of farnesoid X receptor and liver X receptor alpha in Hep3B cells

BACKGROUND

Statins are suggested to preserve gallbladder function by suppressing pro-inflammatory cytokines and preventing cholesterol accumulation in gallbladder epithelial cells. They also affect cross-talk among the nuclear hormone receptors that regulate cholesterol-bile acid metabolism in the nuclei of hepatocytes. However, there is controversy over whether or how statins change the expression of peroxisome proliferator-activated receptor (PPAR)alpha, PPARgamma, liver X receptor alpha (LXRalpha), farnesoid X receptor (FXR), ABCG5, ABCG8, and 7alpha-hydroxylase (CYP7A1) which are directly involved in the cholesterol saturation index in bile.

METHODS

Human Hep3B cells were cultured on dishes. MTT assays were performed to determine the appropriate concentrations of reagents to be used. The protein expression of PPARalpha and PPARgamma was measured by Western blotting analysis, and the mRNA expression of LXRalpha, FXR, ABCG5, ABCG8 and CYP7A1 was estimated by RT-PCR.

RESULTS

In cultured Hep3B cells, pravastatin activated PPARalpha and PPARgamma protein expression, induced stronger expression of PPARgamma than that of PPARalpha, increased LXRalpha mRNA expression, activated ABCG5 and ABCG8 mRNA expression mediated by FXR as well as LXRalpha, enhanced FXR mRNA expression, and increased CYP7A1 mRNA expression mediated by the PPARgamma and LXRalpha pathways, together or independently.

CONCLUSION

Our data suggested that pravastatin prevents cholesterol gallstone diseases via the increase of FXR, LXRalpha and CYP7A1 in human hepatocytes.

Illuminating liver fibrosis with vitamin D

Hepatic fibrosis results from the accumulation of extracellular matrix-producing myofibroblasts in the liver. The mechanisms leading to the activation of hepatic stellate cells (HSCs) into myofibroblasts have been well described. By contrast, few molecular pathways leading to myofibroblast deactivation have been documented. Recently, the vitamin D-VDR axis has been shown to modulate HSC activity through a complex mechanism involving epigenetic modifications induced by the SMAD pathway.

Recycling rate of bile acids in the enterohepatic recirculation as a major determinant of whole body 75SeHCAT retention

Measurement of the whole body retention of orally administered (75)SeHCAT is used to investigate patients with unexplained diarrhoea. Retention values of <15 % at 7 days post-administration are taken to indicate bile acid malabsorption (BAM). Whilst idiopathic BAM is frequently diagnosed with (75)SeHCAT, functional and morphological studies of the terminal ileum rarely show any abnormality, so the disorder may be more appropriately termed bile acid diarrhoea (BAD). In addition to malabsorption, excess bile acid may reach the colon, where the events leading to diarrhoea take place, as a result firstly of increased bile acid synthesis and secondly of an increased recycling rate of bile acids. Increased recycling has been largely ignored as a cause of BAD, but, as shown in this study, can readily result in excess bile acids reaching the colon even when ileal absorption efficiency is normal (i.e. 95-97 %). There needs to be a re-evaluation of the causes of BAD in patients without a history of previous intestinal resection or evidence of ileal pathology, such as Crohn's disease.

Theoretical basis of a beneficial role for vitamin D in viral hepatitis

Abnormal bone metabolism and dysfunction of the calcium-parathyroid hormone-vitamin D axis have been reported in patients with viral hepatitis. Some studies suggested a relationship between vitamin D and viral hepatitis. Genetic studies have provided an opportunity to identify the proteins that link vitamin D to the pathology of viral hepatitis (i.e., the major histocompatibility complex class II molecules, the vitamin D receptor, cytochrome P₄₅₀, the renin-angiotensin system, apolipoprotein E, liver X receptor, toll-like receptor, and the proteins regulated by the Sp1 promoter gene). Vitamin D also exerts its effects on viral hepatitis via non-genomic factors, i.e., matrix metalloproteinase, endothelial vascular growth factor, prostaglandins, cyclooxygenase-2, and oxidative stress. In conclusion, vitamin D could have a beneficial role in viral hepatitis. Calcitriol is best used for viral hepatitis because it is the active form of the vitamin D₃ metabolite.

Effect of vitamin D3 in reducing metabolic and oxidative stress in the liver of streptozotocin-induced diabetic rats

Diabetes mellitus is a growing health problem worldwide and is associated with severe liver complications. The aim of the present study is to analyse the status of metabolic and free-radical-scavenging enzymes and second messengers in the liver of streptozotocin (STZ)-induced diabetic rats, and to determine the hepatoprotective role of vitamin D(3). All studies were performed using the liver of adult male Wistar rats. Gene expression studies were carried out using real-time PCR with specific probes. Second messenger levels were determined using (3)H-labelled Biotrak assay kits, and glucose uptake assay with D-[(14)C]glucose. The present results show that there was a decrease in hepatic glucose uptake, malate dehydrogenase activity, glycogen content, inositol triphosphate (IP(3)) and cyclic GMP levels, and superoxide dismutase, glutathione peroxidase, phospholipase C, cyclic AMP-responsive element-binding protein, vitamin D receptor (VDR) and insulin receptor (INSR) gene expression in the diabetic rats when compared with the controls (all P < 0·05), while cyclic AMP levels and GLUT2 expression were increased (P < 0·05). Treatment of the diabetic rats with vitamin D(3) and insulin reversed the altered parameters to near control values. In conclusion, the data suggest a novel role of vitamin D(3) in restoring impaired liver metabolism in STZ-induced diabetic rats by regulating glucose uptake, storage and metabolism. We demonstrated that the restoring effect of vitamin D(3) is mediated through VDR modulation, thereby improving signal transduction and controlling free radicals in the liver of diabetic rats. These data suggest a potential role for vitamin D(3) in the treatment of diabetes-associated hepatic complications.

The beneficial role of vitamin D in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in the elderly individuals and is associated with progressive neurodegeneration of the human neocortex. Patients with AD have a high prevalence of vitamin D deficiency, which is also associated with low mood and impaired cognitive performance in older people. Genetic studies have provided the opportunity to determine which proteins link vitamin D to AD pathology (ie, the major histocompatibility complex class II molecules, vitamin D receptor, renin-angiotensin system, apolipoprotein E, liver X receptor, Sp1 promoter gene, and the poly(ADP-ribose) polymerase-1 gene). Vitamin D also exerts its effect on AD through nongenomic factors, that is, L-type voltage-sensitive calcium channels, nerve growth factor, the prostaglandins, cyclooxygenase 2, reactive oxygen species, and nitric oxide synthase. In conclusion, vitamin D clearly has a beneficial role in AD and improves cognitive function in some patients with AD. Calcitriol, 1 α,25-dihydroxyvitamin D3, is best used for AD because of its active form of vitamin D(3) metabolite and its receptor in the central nervous system.

Vitamin D and the vitamin D receptor in liver pathophysiology

Vitamin D through the vitamin D nuclear receptor (VDR) plays a key role in mineral ion homeostasis. The liver is central in vitamin D synthesis, however the direct involvement of the vitamin D-VDR axis on the liver remains to be evaluated. In this review, we will describe vitamin D metabolism and the mechanisms of homeostatic control. We will also address the associations between the vitamin D-VDR axis and pathological liver entities, such as non-alcoholic fatty liver disease, autoimmune liver disease, viral hepatitis and liver cancer. The link between liver diseases and the vitamin D-VDR axis will be discussed in light of evidences arising from in vitro and in vivo studies. Finally, we will consider the therapeutic potential of the vitamin D-VDR axis in liver diseases.

Comparative effects of doxercalciferol (1α-hydroxyvitamin D₂) versus calcitriol (1α,25-dihydroxyvitamin D₃) on the expression of transporters and enzymes in the rat in vivo

Effects of 1.28 nmol/kg doxercalciferol [1α(OH)D₂], a synthetic vitamin D₂ analog that undergoes metabolic activation to 1α,25-dihydroxyvitamin D₂, the naturally occurring, biologically active form of vitamin D₂, on rat transporters and enzymes were compared with those of 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃, active form of vitamin D₃; 4.8 and 6.4 nmol/kg] given on alternate days intraperitoneally for 8 days. Changes were mostly confined to the intestine and kidney where the vitamin D receptor (VDR) was highly expressed: increased intestinal Cyp24 and Cyp3a1 messenger RNA (mRNA) and a modest elevation of apical sodium-dependent bile salt transporter (Asbt) and P-glycoprotein (P-gp) protein; increased renal VDR, Cyp24, Cyp3a9, Mdr1a, and Asbt mRNA, as well as Asbt and P-gp protein expression; and decreased renal PepT1 and Oat1 mRNA expression. In comparison, 1α(OH)D₂ treatment exerted a greater effect than 1,25(OH)₂D₃ on Cyp3a and Cyp24 mRNA. However, the farnesoid X receptor -related repressive effects on liver Cyp7a1 were absent because intestinal Asbt, FGF15 and portal bile acid concentrations were unchanged. Rats on the alternate day regimen showed milder changes and lessened signs of hypercalcemia and weight loss compared with rats receiving daily injections (similar or greater amounts of 0.64-2.56 nmol/kg daily ×4) described in previous reports, showing that the protracted pretreatment regimen was associated with milder inductive and lesser toxic effects in vivo.

Effects of 1alpha,25-dihydroxyvitamin D3 on transporters and enzymes of the rat intestine and kidney in vivo

1alpha,25-Dihydroxyvitamin D3 (1,25(OH)2D3), the natural ligand of the vitamin D receptor (VDR), was found to regulate bile acid related transporters and enzymes directly and indirectly in the rat intestine and liver in vivo. The kidney is another VDR-rich target organ in which VDR regulation on xenobiotic transporters and enzymes is ill-defined. Hence, changes in protein and mRNA expression of nuclear receptors, transporters and enzymes of the rat intestine and kidney in response to 1,25(OH)2D3 treatment (0 to 2.56 nmol/kg/day intraperitoneally in corn oil for 4 days) were studied. In the intestine, protein and not mRNA levels of Mrp2, Mrp3, Mrp4 and PepT1 in the duodenum and proximal jejunum were induced, whereas Oat1 and Oat3 mRNA were decreased in the ileum after 1,25(OH)2D3 treatment. In the kidney, VDR, Cyp24, Asbt and Mdr1a mRNA and protein expression increased significantly (2- to 20-fold) in 1,25(OH)2D3-treated rats, and a 28-fold increase of Cyp3a9 mRNA but not of total Cy3a protein nor Cyp3a1 and Cyp3a2 mRNA was observed, implicating that VDR played a significant, renal-specific role in Cyp3a9 induction. Additionally, renal mRNA levels of PepT1, Oat1, Oat3, Ostalpha, and Mrp4, and protein levels of PepT1 and Oat1 were decreased in a dose-dependent manner, and the approximately 50% concomitant reduction in FXR, SHP, HNF-1alpha and HNF-4alpha mRNA expression suggests the possibility of cross-talk among the nuclear receptors. It is concluded that the effects of 1,25(OH)2D3 changes are tissue-specific, differing between the intestine and kidney which are VDR-rich organs.

Polymorphisms in the hepatic lipase gene affect plasma HDL-cholesterol levels in a Turkish population

We investigated the effects of single nucleotide polymorphisms (SNPs) of the hepatic lipase gene (LIPC) on plasma HDL-cholesterol (HDL-C) levels in Turks, a population with low levels of HDL-C. All exons and six evolutionarily conserved regions from 28 Turkish subjects were sequenced. We found 51 SNPs, nine of which were novel. Those 51 SNPs and SNPs from the National Center for Biotechnology Information dbSNP were evaluated by bioinformatics approaches. The population frequencies and linkage disequilibrium among SNPs from HapMap were combined with results from transcriptional factor prediction tools and the literature to select SNPs for genotyping. We found that five tagging LIPC SNPs, two reported here for the first time, were significantly associated with plasma HDL-C levels in both men and women (n = 2,612). These results were replicated in a separate Turkish cohort (n = 1,164). Plasma HDL-C levels were higher in subjects homozygous for the minor alleles of rs4775041, rs1800588 (-514C>T), and rs11858164 and lower in subjects homozygous for the minor alleles of rs11856322 and rs2242061. These SNPs seemed to have independent and additive effects on plasma HDL-C levels (1.5-5.2 mg/dl). Hepatic lipase activity in a subset (n = 260) of the main cohort was also significantly associated with all five SNPs. Thus, five LIPC SNPs, two novel, are associated with plasma HDL-C levels and hepatic lipase activity in two cohorts of Turkish subjects.

Serum cholesterol and expression of ApoAI, LXRbeta and SREBP2 in vitamin D receptor knock-out mice

Vitamin D insufficiency has been reported to be associated with increased blood cholesterol concentrations. Here we used two strains of VDR knock-out (VDR-KO) mice to study whether a lack of vitamin D action has any effect on cholesterol metabolism. In 129S1 mice, both in male and female VDR-KO mice serum total cholesterol levels were significantly higher than those in wild type (WT) mice (20.7% (P=0.05) and 22.2% (P=0.03), respectively). In addition, the serum high-density lipoprotein-bound cholesterol (HDL-C) level was 22% (P=0.03), respectively higher in male VDR-KO mice than in WT mice. The mRNA expression levels of five cholesterol metabolism related genes in livers of 129S1 mice were studied using quantitative real-time PCR (QRT-PCR): ATP-binding cassette transporter A1 (ABCA1), regulatory element binding protein (SREBP2), apolipoprotein A-I (ApoAI), low-density lipoprotein receptor (LDLR) and liver X receptor beta (LXRbeta). In the mutant male mice, the mRNA level of ApoAI and LXRbeta were 49.2% (P=0.005) and 38.8% (P=0.034) higher than in the WT mice. These changes were not observed in mutant female mice, but the female mutant mice showed 52.5% (P=0.006) decrease of SREBP2 mRNA expression compared to WT mice. Because the mutant mice were fed with a special rescue diet, we wanted to test whether the increased cholesterol levels in mutant mice were due to the diet. Both the WT and mutant NMRI mice were given the same diet for 3 weeks before the blood sampling. No difference in cholesterol or in HDL-C between WT and mutant mice was found. The results suggest that the food, gender and genetic background have an effect on the cholesterol metabolism. Although VDR seems to regulate some of the genes involved in cholesterol metabolism, its role in the regulation of serum cholesterol seems to be minimal.

Mechanism of vitamin D receptor inhibition of cholesterol 7alpha-hydroxylase gene transcription in human hepatocytes

Lithocholic acid (LCA) is a potent endogenous vitamin D receptor (VDR) ligand. In cholestasis, LCA levels increase in the liver and intestine. The objective of this study is to test the hypothesis that VDR plays a role in inhibiting cholesterol 7alpha-hydroxylase (CYP7A1) gene expression and bile acid synthesis in human hepatocytes. Immunoblot analysis has detected VDR proteins in the nucleus of the human hepatoma cell line HepG2 and human primary hepatocytes. 1alpha, 25-Dihydroxy-vitamin D(3) or LCA acetate-activated VDR inhibited CYP7A1 mRNA expression and bile acid synthesis, whereas small interfering RNA to VDR completely abrogated VDR inhibition of CYP7A1 mRNA expression in HepG2 cells. Electrophoretic mobility shift assay and mutagenesis analyses have identified the negative VDR response elements that bind VDR/retinoid X receptor alpha in the human CYP7A1 promoter. Mammalian two-hybrid, coimmunoprecipitation, glutathione S-transferase pull-down, and chromatin immunoprecipitation assays show that ligand-activated VDR specifically interacts with hepatocyte nuclear factor 4alpha (HNF4alpha) to block HNF4alpha interaction with coactivators or to compete with HNF4alpha for coactivators or to compete for binding to CYP7A1 chromatin, which results in the inhibition of CYP7A1 gene transcription. This study shows that VDR is expressed in human hepatocytes and may play a critical role in the inhibition of bile acid synthesis, thus protecting liver cells during cholestasis.

Calcitriol and TO-901317 interact in human prostate cancer LNCaP cells

Vitamin D receptor (VDR) and liver X receptor (LXR) are nuclear receptors, which regulate gene transcription upon binding of their specific ligands. VDR seems to play a role in the regulation of prostate cancer cell proliferation. ATP-binding cassette transporter A1 (ABCA1) is known to be a target gene of LXR and it has been reported to be inhibited by androgen and to be involved in the regulation of LNCaP proliferation. We find that calcitriol (1 alpha,25(OH)(2)D(3)) inhibits both basal and a LXR agonist, TO-901317, induced ABCA1 mRNA expression but has no effect on the mRNA expression of ATP-binding cassette transporter G1 (ABCG1), LXR alpha nor LXR beta. TO-901317 increases both basal and calcitriol induced 25-hydroxyvitamin D(3)-24-hydroxylase (CYP24) mRNA expression and it slightly but significantly inhibits VDR mRNA expression. The inhibition of ABCA1 by calcitriol appears to be androgen-independent. Cell growth assay shows that when each of calcitriol and 5 alpha-dihydrotestosterone (DHT) was co-treated with ABCA1 blocker, glybenclamide, cell-growth is significantly decreased compared to their own treatments respectively. Our study suggests a possible interaction between calcitriol and TO-901317 in LNCaP cells. Alike DHT, the inhibition of ABCA1 by calcitriol may be involved in its regulation of LNCaP growth.