Effect of calcitriol on insulin secretion in uraemia

Cardiovascular Consequences of Uremic Metabolites: an Overview of the Involved Signaling Pathways

The crosstalk of the heart with distant organs such as the lung, liver, gut, and kidney has been intensively approached lately. The kidney is involved in (1) the production of systemic relevant products, such as renin, as part of the most essential vasoregulatory system of the human body, and (2) in the clearance of metabolites with systemic and organ effects. Metabolic residue accumulation during kidney dysfunction is known to determine cardiovascular pathologies such as endothelial activation/dysfunction, atherosclerosis, cardiomyocyte apoptosis, cardiac fibrosis, and vascular and valvular calcification, leading to hypertension, arrhythmias, myocardial infarction, and cardiomyopathies. However, this review offers an overview of the uremic metabolites and details their signaling pathways involved in cardiorenal syndrome and the development of heart failure. A holistic view of the metabolites, but more importantly, an exhaustive crosstalk of their known signaling pathways, is important for depicting new therapeutic strategies in the cardiovascular field.

Disturbances in Insulin-Glucose Metabolism in Patients With Advanced Renal Disease With and Without Diabetes

CONTEXT

Use of insulin in patients with diabetes and advanced chronic kidney disease (CKD; stages 4 to 5) is challenging and shows great variability among individuals. We explored the mechanisms underlying this variability.

EVIDENCE ACQUISITION

PubMed was searched for articles in English from 1960 to 2018 for advanced CKD and diabetes, glucose and insulin metabolism, insulin clearance, secretion and resistance, plasma insulin concentration, glycemic control, hypoglycemia, insulin dosage, and continuous glucose monitoring (CGM) in CKD.

EVIDENCE SYNTHESIS

The evidence shows that in most patients the daily dose of insulin needs to be significantly reduced with a high degree of variability; in some the dose remains unchanged, and rarely it is increased. The premise that the marked reduction in insulin requirement is essentially attributable to decreased insulin clearance by kidneys leading to prolongation of its plasma half-life, elevated blood insulin concentration, and hypoglycemia is not entirely correct. Other factors including decreases in food intake, insulin secretion, insulin clearance by peripheral tissues, and renal gluconeogenesis play important roles. There is also heightened resistance to insulin due to metabolic acidosis, uremic toxins, inflammatory state, and vitamin D deficiency. Importantly, the magnitude of changes in each of these factors varies between individuals with the same degree of CKD.

CONCLUSIONS

In the presence of diabetes with advanced CKD, the insulin regimen should be individualized based on knowledge of the daily glucose patterns. The use of CGM is promising for safer glycemic control in patients with advanced CKD and diabetes and helps prevent extremes of hypoglycemia and hyperglycemia.

Vitamin D receptor activation by paricalcitol and insulin resistance in CKD

BACKGROUND AND AIMS

The nature of the link (causal vs non-causal) between low 1,25-OH vitamin D and insulin resistance (IR) in patients with chronic kidney disease (CKD) remains elusive. We have now made a post hoc analysis of the effect of vitamin D receptor activation by paricalcitol on IR in the complete dataset of a double-blind, randomized, placebo controlled trial, the Paricalcitol and ENdothelial fuNction in chronic kidneY disease (PENNY).

METHODS AND RESULTS

Eighty-eight patients with stage 3-4 CKD were randomized (1:1) to receive 2 μg/day paricalcitol or matching placebo for 12 weeks. IR was measured by five IR indices: the homeostasis model assessment of insulin resistance (HOMA-IR), the quantitative insulin sensitivity check index (QUICKI), the McAuley index, the HOMA corrected for adiponectin (HOMA-AD) and the Leptin-adiponectin ratio (LAR). As compared to placebo, paricalcitol produced the expected small rise in serum calcium (+0.07 mmol/L, P = 0.01) and phosphate (+0.08 mmol/L, P = 0.034) and the expected parathyroid hormone suppression (-96 pg/ml, P < 0.001). However, the drug largely failed to affect the five indices of IR which remained unchanged both in the active and the placebo arm (paricalcitol vs placebo, P ranging from 0.25 to 0.62) and no effect modification of paricalcitol on IR by vitamin D or other parameters was registered.

CONCLUSION

Paricalcitol treatment for 12 weeks does not improve IR in patients with stage 3-4 CKD. Low vitamin D receptor activation is not a causal factor for IR in the CKD population.

Study of the effect of vitamin D supplementation on glycemic control in type 2 diabetic prevalent hemodialysis patients

Vitamin D is claimed to have an adjuvant effect on glycemic control by dual action on pancreatic β-cells and insulin resistance. The aim of this study was to assess the possible effect of short-term alfacalcidol supply on glycemic control in type 2 diabetic hemodialysis (HD) patients. Twenty type 2 diabetic HD patients (using diet and oral drugs but not insulin) were randomly selected from our dialysis unit as well as 20 non-diabetic HD patients as control. A third group of 12 healthy subjects were studied as well. All three groups were similar in age, sex, and body mass index. Oral alfacalcidol therapy was administrated daily as recommended by Kidney Disease Outcomes Quality Initiative (K/DOQI) guidelines for 12 weeks guided by monthly serum phosphorus and Cax PO4 product. Corrected total calcium, phosphorus, intact parathyroid hormone, 25-hydroxy vitamin D (25[OH]D), and glucoparameters (fasting blood glucose, glycated hemoglobin [HbA1c%], insulin resistance by homeostatic model assessment, and β-cell function by HOMA-β%) were measured under basal conditions and after 3 months of therapy. 25(OH)D was non-significantly lower in diabetic than non-diabetic HD patients, but significantly lower than healthy subjects at the start of the study. However, vitamin D level increased significantly after 3 months of trial, although the levels did not reach normal values. This vitamin D rise was associated with highly significant improvement in concentrations of fasting blood sugar (FBS), fasting insulin, HbA1c%, and HOMA-β-cell function in diabetic and non-diabetic controls. However, there was a significant rise in insulin resistance after treatment. The percentage of change was evident more in diabetics regarding FBS and 25(OH)D concentration. Adjustment of 25(OH)D level in type 2 diabetic prevalent HD patients may improve, at least with short-term therapy, glycemic control mainly through improving β-cell function.

Effects of Vitamin D Supplementation on C-peptide and 25-hydroxyvitamin D Concentrations at 3 and 6 Months

The link between African-Americans' disproportionate rates of diabetes, obesity and vitamin D deficiency may be marked by C-peptide as an indicator of insulin secretion. We hypothesize that vitamin D supplementation will increase C-peptide, a marker of insulin secretion. During 3 winters from 2007-2010, 328 healthy African-Americans (median age, 51 years) living in Boston, MA were randomized into a 4-arm, double-blind trial for 3 months of placebo, 1000, 2000, or 4000 IU of vitamin D3. The differences in non-fasting C-peptide between baseline and 3 months were -0.44 ng/mL for those receiving placebo, -0.10 ng/mL for those receiving 1000 IU/d, 0 ng/mL for those receiving 2000 IU/d, 1.24 ng/mL for those receiving 4000 IU/d (C-peptide increased 0.42 ng/mL for each additional 1000 IU/d of vitamin D3, p < 0.001). Vitamin D supplementation increased C-peptide in overweight African-Americans and may be compatible with other recommendations for diabetes prevention and management including weight loss and increased physical activity.

The effects of short-term vitamin D supplementation on glucose metabolism in dialysis patients: a systematic review and meta-analysis

PURPOSE

We tested whether short-term vitamin D supplementation improves insulin resistance in patients with kidney disease, a condition with little intrinsic vitamin D activity.

METHODS

PubMed, EMBASE and CENTRAL were searched for relevant observational studies and randomized clinical trials (RCTs). Random-effects models were employed for meta-analysis, and effect sizes were summarized as standardized mean difference (SMD) with 95% confidence intervals. Separate analyses were done for RCTs and non-randomized intervention studies (NRIS).

RESULTS

Seventeen studies (5 RCTs and 12 NRIS) were included. The meta-analysis population (n = 131) was mostly middle aged (40-50 years), male and non-diabetic, and on hemodialysis. The duration (4-12 weeks) and type of supplementation varied between studies. Among RCTs, compared to placebo, vitamin D supplementation was associated with significant decrease in fasting glucose [SMD -1.13, (-2.11 to -0.11)] and PTH levels [SMD -1.50, (-2.95 to -0.04)] but no difference in fasting insulin levels [SMD 1.32, (-0.15 to 2.79)]. Among NRIS, there was only a significant decrease in PTH levels [SMD -1.68, (-2.55 to -0.82)] between pre- and post-vitamin D treatment levels.

CONCLUSIONS

Short-term (4-12 weeks) supplementation with vitamin D is associated with lower fasting glucose levels in ESRD with no change in fasting insulin levels. However, the findings from this study are limited by the studies that were used in the meta-analysis, which were mostly small, used multiple different vitamin D compounds and dosing regimens, and had large heterogeneity, and funnel plots showed that there was a dearth of studies with null or negative finding. Therefore, larger RCTs need to be performed to answer this important clinical question.

Regulation of calcitriol biosynthesis and activity: focus on gestational vitamin D deficiency and adverse pregnancy outcomes

Vitamin D has garnered a great deal of attention in recent years due to a global prevalence of vitamin D deficiency associated with an increased risk of a variety of human diseases. Specifically, hypovitaminosis D in pregnant women is highly common and has important implications for the mother and lifelong health of the child, since it has been linked to maternal and child infections, small-for-gestational age, preterm delivery, preeclampsia, gestational diabetes, as well as imprinting on the infant for life chronic diseases. Therefore, factors that regulate vitamin D metabolism are of main importance, especially during pregnancy. The hormonal form and most active metabolite of vitamin D is calcitriol. This hormone mediates its biological effects through a specific nuclear receptor, which is found in many tissues including the placenta. Calcitriol synthesis and degradation depend on the expression and activity of CYP27B1 and CYP24A1 cytochromes, respectively, for which regulation is tissue specific. Among the factors that modify these cytochromes expression and/or activity are calcitriol itself, parathyroid hormone, fibroblast growth factor 23, cytokines, calcium and phosphate. This review provides a current overview on the regulation of vitamin D metabolism, focusing on vitamin D deficiency during gestation and its impact on pregnancy outcomes.

Influence of vitamin D treatment on transcriptional regulation of insulin-sensitive genes

BACKGROUND

Obesity is a risk factor for diabetes and metabolic syndrome, which are characterized by insulin resistance. Inflammation is a co-morbid condition associated with obesity. Vitamin D, besides being a transcriptional regulator, is an inflammation suppressor. However, the role of vitamin D in alleviating obesity-induced insulin resistance is still not well understood.

METHODS

The influence of vitamin D treatment on the transcriptional level of insulin receptor (IR), insulin receptor substrate (IRS-1), glucose transporter type 4 (GLUT-4), and vitamin D receptor (VDR) in insulin target tissues of liver, adipose, and muscle of mice fed on a high-fat diet (HFD) or low-fat diet (LFD) was studied by quantitative RT-PCR.

RESULTS

A gradual weight reduction was observed in HFD-fed mice treated with vitamin D compared to a steady weight increase in control animals (P<0.01). In HFD mice, vitamin D decreased VDR expression to 0.5-fold in muscle (P=0.002), and increased it to 3.6-fold in the liver (P<0.001); however, VDR transcription was unaltered in adipose tissue. Similarly, vitamin D did not influence tissue expression of IR in either LFD- or HFD-fed mice. Muscle IRS-1 transcription level was upregulated to 2.4-fold (P=0.005) in HFD mice, whereas it was reduced to 0.15-fold in liver tissue (P<0.001). Vitamin D treatment had no effect on GLUT-4 transcript levels in any of the tissues under HFD conditions.

CONCLUSION

Vitamin D treatment influenced the expression of insulin-sensitive genes in a tissue-specific fashion. On the basis of the present findings, vitamin D does not aid glucose transport across cells of liver and adipose tissues, the major insulin-sensitive tissues, in HFD-fed mice; however, it appears to enhance the intracellular mechanisms of insulin action mediated by IRS-1 and VDR in muscle tissue.

The world pandemic of vitamin D deficiency could possibly be explained by cellular inflammatory response activity induced by the renin-angiotensin system

This review attempts to show that there may be a relationship between inflammatory processes induced by chronic overstimulation of the renin-angiotensin system (RAS) and the worldwide deficiency of vitamin D (VitD) and that both disorders are probably associated with environmental factors. Low VitD levels represent a risk factor for several apparently different diseases, such as infectious, autoimmune, neurodegenerative, and cardiovascular diseases, as well as diabetes, osteoporosis, and cancer. Moreover, VitD insufficiency seems to predispose to hypertension, metabolic syndrome, left ventricular hypertrophy, heart failure, and chronic vascular inflammation. On the other hand, inappropriate stimulation of the RAS has also been associated with the pathogenesis of hypertension, heart attack, stroke, and hypertrophy of the left ventricle and vascular smooth muscle cells. Because VitD receptors (VDRs) and RAS receptors are almost distributed in the same tissues, a possible link between VitD and the RAS is even more plausible. Furthermore, from an evolutionary point of view, both systems were developed simultaneously, actively participating in the regulation of inflammatory and immunological mechanisms. Changes in RAS activity and activation of the VDR seem to be inversely related; thus any changes in one of these systems would have a completely opposite effect on the other, making it possible to speculate that the two systems could have a feedback relationship. In fact, the pandemic of VitD deficiency could be the other face of increased RAS activity, which probably causes lower activity or lower levels of VitD. Finally, from a therapeutic point of view, the combination of RAS blockade and VDR stimulation appears to be more effective than either RAS blockade or VDR stimulation individually.

A pilot study of active vitamin D administration and insulin resistance in African American patients undergoing chronic hemodialysis

OBJECTIVE

Insulin resistance (IR) is associated with increased cardiovascular risk in multiple patient populations, including those undergoing chronic hemodialysis (CHD). Active vitamin D deficiency has been proposed to play a role in the extent of IR observed in patients with CHD. We postulated that administration of paracalcitol, an active vitamin D medication, influences IR in patients with CHD.

DESIGN AND METHODS

This was a pilot randomized controlled trial. Ten prevalent CHD patients receiving a stable dose of paracalcitol were recruited. Paracalcitol was withheld for 8 weeks in all patients (phase I). Parathyroid hormone levels were managed with the calcium-sensing receptor agonist cinacalcet. At week 8, patients were randomized to continue cinacalcet or to restart paracalcitol for 8 weeks (phase II). The primary outcome was the change in IR measured by the glucose disposal rate (GDR) using hyperinsulinemic euglycemic clamp (HEGC) method. Secondary outcomes included changes in IR between groups in indirect indices of IR, biomarkers of inflammation, and adipokine levels.

RESULTS

The mean age was 49 years (range, 46-57 years) and 40% of patients were women. There was no detectable change in the GDR at the end of phase I (P = .7) when compared with baseline values. There was also no statistically significant difference in GDR between groups at the end of phase II (P = .9). No changes were observed in indirect indices of IR, adipokine levels, or biomarkers of inflammation in either phase.

CONCLUSION

The results of this pilot study suggest that withdrawal of paracalcitol over 8 to 16 weeks and replacement for 8 weeks after withdrawal does not influence IR measured by HEGC in patients receiving CHD.

Paricalcitol does not improve glucose metabolism in patients with stage 3-4 chronic kidney disease

Patients with chronic kidney disease are often insulin resistant and glucose intolerant--abnormalities that promote cardiovascular disease. Administration of 1,25-dihydroxyvitamin D (calcitriol) has improved glucose metabolism in patients with end-stage renal disease. We conducted a randomized, placebo-controlled clinical trial to test whether paricalcitol, a 1,25-dihydroxyvitamin D analog, changes glucose tolerance in earlier stages of chronic kidney disease. In a crossover design, 22 nondiabetic patients with estimated glomerular filtration rates of stage 3-4 chronic kidney disease and fasting plasma glucose of 100-125 mg/dl were given daily oral paricalcitol for 8 weeks and matching placebo for 8 weeks, separated by an 8-week washout period. The order of interventions was random and blinded to both participants and investigators. Paricalcitol significantly reduced serum concentrations of parathyroid hormone, 1,25-dihydroxyvitamin D, and 25-hydroxyvitamin D while significantly increasing serum concentrations of fibroblast growth factor-23 and 24,25-dihydroxyvitamin D. Paricalcitol, however, had no significant effect on glucose tolerance (the primary outcome measure), insulin sensitivity, beta-cell insulin response, plasma free fatty acid suppression, or urinary F2-isoprostane excretion. Thus, despite substantial effects on vitamin D metabolism, paricalcitol did not improve glucose metabolism in nondiabetic patients with stage 3-4 chronic kidney disease.

Effect of activated vitamin D on glucoparameters in HCV seropositive and seronegative patients on chronic hemodialysis

INTRODUCTION

Many studies support the role of vitamin D in the pathogenesis of both types of diabetes. Pancreatic tissues express the vitamin D receptor (VDR) and vitamin D-binding protein; some allelic variations in genes involved in vitamin D metabolism and VDR are associated with glucose intolerance, defective insulin secretion, and sensitivity. Epidemiological links have been established between type 2 diabetes mellitus (DM) and hepatitis C virus (HCV) infection.

AIM

To explore the possible therapeutic potential of pharmacologic doses of 1-α-hydroxy vitamin D therapy in improving pancreatic β-cell function in HCV seropositive hemodialysis (HD) patients.

PATIENTS AND METHODS

Twenty HCV seropositive HD patients and 20 HCV seronegative patients as control group were randomly selected from HD units. 1-α-Hydroxy vitamin D therapy was administrated in the dose ranged from 0.25 to 0.5 μg/day for 3 months. Corrected total serum calcium, phosphorus, intact parathyroid hormone (iPTH), 25-hydroxy vitamin D [25(OH) vitamin D], 1,25-dihydroxy vitamin D, and glucoparameters [fasting blood glucose, glycohemoglobin test (HbA1c%), homeostatic model assessment (HOMA)-insulin resistance, and HOMA-β-cell function% (B%)] were measured under basal conditions and after 3 months of therapy.

RESULTS

There was highly significant improvement in the concentrations of fetal bovine serum (FBS), serum insulin, HbA1c%, 25(OH) vitamin D, and HOMA-β-cell function in HCV seropositive and HCV seronegative groups after oral 1-alphacalcidiol therapy (p < 0.001). Positive correlation exists between the percentage increase in serum insulin and that in HOMA-β-cell function versus 25(OH) vitamin D (p < 0.021 and p < 0.027, respectively) in HCV negative group.

CONCLUSION

1-α-Hydroxy vitamin D oral therapy may improve glycemic control in HCV seropositive and HCV seronegative HD patients.

Measurement of insulin resistance in chronic kidney disease

PURPOSE OF REVIEW

Insulin resistance is a known complication of end-stage renal disease that also appears to be present in earlier stages of chronic kidney disease (CKD). It is a risk factor for cardiovascular disease and an important potential therapeutic target in this population. Measurement of insulin resistance is reviewed in the context of known pathophysiologic abnormalities in CKD.

RECENT FINDINGS

Insulin resistance in CKD is due to a high prevalence of known risk factors (e.g. obesity) and to unique metabolic abnormalities. The site of insulin resistance in CKD is localized to skeletal muscle. Estimates based on fasting insulin concentration may not adequately capture insulin resistance in CKD because they largely reflect hepatic defects and because CKD impairs insulin catabolism. A variety of dynamic tests are available to directly measure insulin-mediated glucose uptake.

SUMMARY

Insulin resistance may be an important therapeutic target in CKD. Complementary methods are available to assess insulin resistance, and each method has unique advantages, disadvantages, and levels of complexity. These characteristics, and the likelihood that CKD alters the performance of some insulin resistance measurements, must be considered when designing and interpreting clinical studies.

Shining D' light on chronic kidney disease: mechanisms that may underpin the cardiovascular benefit of vitamin D

Hypovitaminosis D is a significant health-care burden worldwide, particularly in susceptible populations such as those with chronic kidney disease (CKD). Recent epidemiological studies have identified that both higher serum vitamin D concentrations and use of vitamin D supplements may confer a survival benefit both in terms of all-cause and cardiovascular mortality. There is potential to investigate this inexpensive therapy for the CKD population, which suffers excessive cardiovascular events, although the mechanisms explaining this link have yet to be fully elucidated. This review discusses potential mechanisms identified in the basic science literature that may provide important insights into how vitamin D may orchestrate a change in cardiovascular risk profile through such diverse mechanisms as inflammation, atherogenesis, glucose homeostasis, vascular calcification, renin-angiotensin regulation and alterations in cardiac physiology. Where available, the clinical translation of these concepts to intervention trials in the CKD population will be reviewed.

Mitochondrial localization of vitamin D receptor in human platelets and differentiated megakaryocytes

BACKGROUND

Like other steroid hormones, vitamin D elicits both transcriptional events and rapid non genomic effects. Vitamin D receptor (VDR) localization and mechanisms of VDR-triggered non genomic responses are still controversial. Although anticoagulant effects of vitamin D have been reported and VDR signalling has been characterized in monocytes and vascular cells, nothing is known about VDR expression and functions in human platelets, anucleated fragments of megakaryocytes which are known targets of other steroids.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we characterized the expression and cellular localization of VDR in human platelets and in a megakaryocyte lineage. Human platelets and their TPA-differentiated precursors expressed a classical 50 kDa VDR protein, which increased with megakaryocytes maturation. By biochemical fractionation studies we demonstrated the presence of the receptor in the soluble and mitochondrial compartment of human platelets, and the observation was confirmed by immunoelectron microscopy analysis. Similar localization was found in mature megakaryocytes, where besides its classical nuclear localization the receptor was evident as soluble and mitochondria resident protein.

CONCLUSIONS

The results reported here suggest that megakaryocytopoiesis and platelet activation, which are calcium-dependent events, might be modulated by a mitochondrial non genomic activity of VDR. These data open challenging future studies on VDR physiological role in platelets and more generally in mitochondria.

The role of 25-hydroxyvitamin D deficiency in promoting insulin resistance and inflammation in patients with chronic kidney disease: a randomised controlled trial

Background

Approximately 50% of patients with stage 3 Chronic Kidney Disease are 25-hydroxyvitamin D insufficient, and this prevalence increases with falling glomerular filtration rate. Vitamin D is now recognised as having pleiotropic roles beyond bone and mineral homeostasis, with the vitamin D receptor and metabolising machinery identified in multiple tissues. Worryingly, recent observational data has highlighted an association between hypovitaminosis D and increased cardiovascular mortality, possibly mediated via vitamin D effects on insulin resistance and inflammation. The main hypothesis of this study is that oral Vitamin D supplementation will ameliorate insulin resistance in patients with Chronic Kidney Disease stage 3 when compared to placebo. Secondary hypotheses will test whether this is associated with decreased inflammation and bone/adipocyte-endocrine dysregulation.

Methods/Design

This study is a single-centre, double-blinded, randomised, placebo-controlled trial. Inclusion criteria include; estimated glomerular filtration rate 30-59 ml/min/1.73 m²; aged ≥18 on entry to study; and serum 25-hydroxyvitamin D levels <75 nmol/L. Patients will be randomised 1:1 to receive either oral cholecalciferol 2000IU/day or placebo for 6 months. The primary outcome will be an improvement in insulin sensitivity, measured by hyperinsulinaemic euglycaemic clamp. Secondary outcome measures will include serum parathyroid hormone, cytokines (Interleukin-1β, Interleukin-6, Tumour Necrosis Factor alpha), adiponectin (total and High Molecular Weight), osteocalcin (carboxylated and under-carboxylated), peripheral blood mononuclear cell Nuclear Factor Kappa-B p65 binding activity, brachial artery reactivity, aortic pulse wave velocity and waveform analysis, and indirect calorimetry. All outcome measures will be performed at baseline and end of study.

Discussion

To date, no randomised controlled trial has been performed in pre-dialysis CKD patients to study the correlation between vitamin D status with supplementation, insulin resistance and markers of adverse cardiovascular risk. We remain hopeful that cholecalciferol may be a safe intervention, with health benefits beyond those related to bone-mineral homeostasis.

Trial registration

Australian and New Zealand Clinical Trials Registry ACTRN12609000246280.

Vitamin D and glucose metabolism in chronic kidney disease

PURPOSE OF REVIEW

Vitamin D deficiency has been linked with impaired glucose metabolism, an established risk factor for cardiovascular disease. Effects of vitamin D may be particularly important in chronic kidney disease, because vitamin D metabolism is substantially impaired in this setting. The present review examines the potential role of vitamin D in improving glucose metabolism and clinical health outcomes in chronic kidney disease.

RECENT FINDINGS

Impaired glucose metabolism is common in chronic kidney disease. Short-term studies on maintenance hemodialysis patients have shown that vitamin D treatment improves insulin secretion and sensitivity. Intervention studies on people without end-stage renal disease have yielded mixed results. Mounting evidence from observational studies links vitamin D treatment with decreased mortality.

SUMMARY

Vitamin D therapy holds promise for improving health outcomes in chronic kidney disease. Improved glucose metabolism is one potential mechanism through which vitamin D may exert beneficial effects. However, further data from clinical trials are needed to test whether vitamin D has clinically relevant long-term effects on glucose metabolism and overall clinical outcomes.

Vitamin D

The vitamin D endocrine system plays an essential role in calcium homeostasis and bone metabolism, but research during the past two decades has revealed a diverse range of biological actions that include induction of cell differentiation, inhibition of cell growth, immunomodulation, and control of other hormonal systems. Vitamin D itself is a prohormone that is metabolically converted to the active metabolite, 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. This vitamin D hormone activates its cellular receptor (vitamin D receptor or VDR), which alters the transcription rates of target genes responsible for the biological responses. This review focuses on several recent developments that extend our understanding of the complexities of vitamin D metabolism and actions: the final step in the activation of vitamin D, conversion of 25-hydroxyvitamin D to 1,25(OH)(2)D in renal proximal tubules, is now known to involve facilitated uptake and intracellular delivery of the precursor to 1alpha-hydroxylase. Emerging evidence using mice lacking the VDR and/or 1alpha-hydroxylase indicates both 1,25(OH)(2)D(3)-dependent and -independent actions of the VDR as well as VDR-dependent and -independent actions of 1,25(OH)(2)D(3). Thus the vitamin D system may involve more than a single receptor and ligand. The presence of 1alpha-hydroxylase in many target cells indicates autocrine/paracrine functions for 1,25(OH)(2)D(3) in the control of cell proliferation and differentiation. This local production of 1,25(OH)(2)D(3) is dependent on circulating precursor levels, providing a potential explanation for the association of vitamin D deficiency with various cancers and autoimmune diseases.

Calcitriol improves streptozotocin-induced diabetes and recovers bone mineral density in diabetic rats

Vitamin D analogs exert a preventative effect on experimental diabetes, but whether or not they are able to halt progress of established diabetes is not yet known. Moreover, it is widely accepted that diabetes may induce osteoporosis, but the efficacy of vitamin D on diabetic osteoporosis is not clear. In order to help clarify these issues, we have tested the efficacy of calcitriol streptozotocin-induced diabetes. Streptozotocin (60 mg/Kg body weight) was injected in 3-month-old Wistar rats, randomly distributed into two groups: vehicle (olive oil) treated diabetic rats (D) and diabetic rats treated with 1.25-(OH)2D3 250 mg, three times a week (DT). Control animals (C) were treated with vehicle alone. The experiment lasted 8 weeks. The histology of the pancreata was evaluated. Blood glucose and calcium and phosphate in serum and urine were measured. Finally, bone mineral density (BMD) of tibia and lumbar vertebrae were evaluated. After 8 weeks, diabetes persisted in 85% of the diabetic rats (D group), but in only 45% of vitamin D-treated group (DT). At the end of the experiment, DT animals were separated into two groups, those still remaining diabetic (DT-NR) and reversed animals (DT-R). Moreover, bone loss was observed in diabetic animals (D), whereas BMD of DT-R rats showed similar values to those of controls (C). Our results suggest that 1.25(OH)2D3 improves diabetes and, as such, may recover BMD in streptozotocin-induced diabetic rats.

Vitamin D

The vitamin D endocrine systems plays a critical role in calcium and phosphate homeostasis. The active form of vitamin D, 1, 25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], binds with high affinity to a specific cellular receptor that acts as a ligand-activated transcription factor. The activated vitamin D receptor (VDR) dimerizes with another nuclear receptor, the retinoid X receptor (RXR), and the heterodimer binds to specific DNA motifs (vitamin D response elements, VDREs) in the promoter region of target genes. This heterodimer recruits nuclear coactivators and components of the transcriptional preinitiation complex to alter the rate of gene transcription. 1,25(OH)(2)D(3) also binds to a cell-surface receptor that mediates the activation of second messenger pathways, some of which may modulate the activity of the VDR. Recent studies with VDR-ablated mice confirm that the most critical role of 1, 25(OH)(2)D(3) is the activation of genes that control intestinal calcium transport. However, 1,25(OH)(2)D(3) can control the expression of many genes involved in a plethora of biological actions. Many of these nonclassic responses have suggested a number of therapeutic applications for 1,25(OH)(2)D(3) and its analogs.

Oral glucose tolerance test after high-dose i.v. biotin administration in normoglucemic hemodialysis patients

Abnormal glucose metabolism in uremia may result from a complex interplay between decreased insulin secretion and insulin resistance. Recent studies report beneficial effect of biotin administration in glucose metabolism in diabetic animals and in a small number of patients with diabetes mellitus. The aim of the present study was to evaluate the response of oral glucose tolerance test (OGTT) to the i.v. administration of large doses of biotin in hemodialysis patients. Eleven hemodialysis patients aged 56.90 +/- 11.20 (32-76) years on regular hemodialysis thrice a week for 2.72 +/- 1.79 (1-7) years were studied. Fasting venous plasma glucose, glucosylated hemoglobin (%GH), and plasma glucose concentration 2 h after the administration of a 75-g glucose load were measured before, and 2 weeks and 2 months after administration of 50 mg of biotin i.v. postdialysis, and after a 2-month washout period. During the study, dialysis schedule and patients' medication, diet, and dry weight were kept unchanged. OGTT was abnormal in 4 patients before biotin administration and became normal in 3 patients (75%). Our results offer support to the findings of other studies about the beneficial effect of biotin in experimental or clinical diabetes mellitus, and argue for the involvement of biotin in glucose metabolism.

Vitamin D therapy in patients receiving dialysis

Secondary hyperparathyroidism is found in a large proportion, but not all patients on dialysis. Calcitriol controls moderate hyperparathyroidism in most patients but only in a proportion of those with advanced hyperparathyroidism. Patients with nodular parathyroid hyperplasia respond less frequently, presumably because of monoclonal growth and diminished calcitriol-receptor expression by parathyroid cells. In patients with nodular parathyroid hyperplasia, parathyroidectomy is an important alternative to calcitriol treatment. A priori reasoning indicates that prophylactic administration of calcitriol (to prevent parathyroid hyperplasia) is a reasonable option, but currently no controlled evidence for long-term efficacy of this approach without side effects is available. Intermittent administration of calcitriol by intravenous or oral routes is effective and, at least in experimental studies, superior to continuous calcitriol. However, in clinical comparisons, no superiority of intravenous versus oral or daily versus intermittent calcitriol has been documented. Calcitriol treatment must be closely supervised to prevent hypercalcemia, hyperphosphatemia, and excessive suppression of parathyroid hormone. Because of an altered dose response relationship, parathyroid hormone levels should not be completely normalized so as to prevent low bone turnover (adynamic bone lesion).