Inhibition of calcitriol receptor binding to vitamin D response elements by uremic toxins

Treatment for secondary hyperparathyroidism focusing on parathyroidectomy

Secondary hyperparathyroidism (SHPT) is a major problem for patients with chronic kidney disease and can cause many complications, including osteodystrophy, fractures, and cardiovascular diseases. Treatment for SHPT has changed radically with the advent of calcimimetics; however, parathyroidectomy (PTx) remains one of the most important treatments. For successful PTx, removing all parathyroid glands (PTGs) without complications is essential to prevent persistent or recurrent SHPT. Preoperative imaging studies for the localization of PTGs, such as ultrasonography, computed tomography, and 99mTc-Sestamibi scintigraphy, and intraoperative evaluation methods to confirm the removal of all PTGs, including, intraoperative intact parathyroid hormone monitoring and frozen section diagnosis, are useful. Functional and anatomical preservation of the recurrent laryngeal nerves can be confirmed via intraoperative nerve monitoring. Total or subtotal PTx with or without transcervical thymectomy and autotransplantation can also be performed. Appropriate operative methods for PTx should be selected according to the patients' need for kidney transplantation. In the case of persistent or recurrent SHPT after the initial PTx, localization of the causative PTGs with autotransplantation is challenging as causative PTGs can exist in the neck, mediastinum, or autotransplanted areas. Additionally, the efficacy and cost-effectiveness of calcimimetics and PTx are increasingly being discussed. In this review, medical and surgical treatments for SHPT are described.

Bone Fragility in Chronic Kidney Disease Stage 3 to 5: The Use of Vitamin D Supplementation

Frequently silent until advanced stages, bone fragility associated with chronic kidney disease-mineral and bone disease (CKD-MBD) is one of the most devastating complications of CKD. Its pathophysiology includes the reduction of active vitamin D metabolites, phosphate accumulation, decreased intestinal calcium absorption, renal alpha klotho production, and elevated fibroblast growth factor 23 (FGF23) levels. Altogether, these factors contribute firstly to secondary hyperparathyroidism, and ultimately, to micro- and macrostructural bone changes, which lead to low bone mineral density and an increased risk of fracture. A vitamin D deficiency is common in CKD patients, and low circulating 25(OH)D levels are invariably associated with high serum parathyroid hormone (PTH) levels as well as with bone mineralization defects, such as osteomalacia in case of severe forms. It is also associated with a variety of non-skeletal diseases, including cardiovascular disease, diabetes mellitus, multiple sclerosis, cancer, and reduced immunological response. Current international guidelines recommend supplementing CKD patients with nutritional vitamin D as in the general population; however, there is no randomized clinical trial (RCT) evaluating the effect of vitamin D (or vitamin D+calcium) supplementation on the risk of fracture in the setting of CKD. It is also unknown what level of circulating 25(OH)D would be sufficient to prevent bone abnormalities and fractures in these patients. The impact of vitamin D supplementation on other surrogate endpoints, including bone mineral density and bone-related circulating biomarkers (PTH, FGF23, bone-specific alkaline phosphatase, sclerostin) has been evaluated in several RTCs; however, the results were not always translated into an improvement in long-term outcomes, such as reduced fracture risk. This review provides a brief and comprehensive update on CKD-related bone fragility and the use of natural vitamin D supplementation in these patients.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia, or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

Vitamin D Metabolism and Its Role in Mineral and Bone Disorders in Chronic Kidney Disease in Humans, Dogs and Cats

Some differences regarding Vitamin D metabolism are described in dogs and cats in comparison with humans, which may be explained by an evolutionary drive among these species. Similarly, vitamin D is one of the most important regulators of mineral metabolism in dogs and cats, as well as in humans. Mineral metabolism is intrinsically related to bone metabolism, thus disturbances in vitamin D have been implicated in the development of chronic kidney disease mineral and bone disorders (CKD-MBD) in people, in addition to dogs and cats. Vitamin D deficiency may be associated with Renal Secondary Hyperparathyroidism (RSHPT), which is the most common mineral disorder in later stages of CKD in dogs and cats. Herein, we review the peculiarities of vitamin D metabolism in these species in comparison with humans, and the role of vitamin D disturbances in the development of CKD-MBD among dogs, cats, and people. Comparative studies may offer some evidence to help further research about vitamin D metabolism and bone disorders in CKD.

1,25(OH)2D3 Strengthens the Vasculogenesis of Multipotent Mesenchymal Stromal Cells from Rat Bone Marrow by Regulating the PI3K/AKT Pathway

Background

Multipotent mesenchymal stromal cells (MSCs) have recently been reported to promote vasculogenesis by differentiating into endothelial cells and releasing numerous cytokines and paracrine factors. However, due to low cell activity, their potential for clinical application is not very satisfactory. This study aimed to explore the effects and mechanisms of 1,25-dihydroxyvitamin D (1,25(OH)₂D₃) on the vasculogenesis of MSCs.

Methods

MSCs were isolated from the femurs and tibias of rats and characterized by flow cytometry. After treatment with different concentrations of 1,25(OH)₂D₃ (0 µM, 0.1 µM and 1 µM), the proliferation of MSCs was analyzed by Cell Counting Kit-8 (CCK-8), and the migratory capability was measured by Transwell assays and cell scratch tests. Capillary-like structure formation was observed by using Matrigel. Western blotting was used to detect the expression of FLK-1 and vWF to investigate the differentiation of MSCs into endothelial cells. Western blotting and gelatin zymography were used to detect the expression and activities of VEGF, MMP-2 and MMP-9 secreted by MSCs under the influence of 1,25(OH)₂D_3. Finally, the VDR antagonist pyridoxal-5-phosphate (P5P) and the PI3K/AKT pathway inhibitor LY294002 were utilized to test the phosphorylation levels of key kinases in the PI3K/AKT pathway by Western blotting and the formation of capillary-like structures in Matrigel.

Results

The proliferation and migratory capability of MSCs and the ability of MSCs to form a tube-like structure in Matrigel were enhanced after treatment with 1,25(OH)₂D₃. Moreover, MSCs treated with 1,25(OH)₂D₃ showed high expression of vWF and Flk-1. There was a significant increase in the expression of VEGF, MMP-2 and MMP-9 secreted by MSCs treated with 1,25(OH)₂D_3, as well as in the activity of MMP-2 and MMP-9. The phosphorylation level of AKT increased with time after 1,25(OH)₂D₃ treatment, while LY294002 weakened AKT phosphorylation. In addition, the ability to form capillary-like structures was reduced when the VDR and PI3K/AKT pathways were blocked.

Conclusion

This study confirmed that 1,25(OH)₂D₃ treatment can strengthen the ability of MSCs to promote vasculogenesis in vitro, and the mechanism may be related to the activation of the PI3K/AKT pathway.

Secondary Hyperparathyroidism: Pathogenesis and Latest Treatment

The classic pathogenesis of secondary hyperparathyroidism (SHPT) began with the trade-off hypothesis based on parathyroid hormone hypersecretion brought about by renal failure resulting from a physiological response to correct metabolic disorder of calcium, phosphorus, and vitamin D. In dialysis patients with failed renal function, physiological mineral balance control by parathyroid hormone through the kidney fails and hyperparathyroidism progresses. In this process, many significant genetic findings have been established. Abnormalities of Ca-sensing receptor and vitamin D receptor are associated with the pathogenesis of SHPT, and fibroblast growth factor 23 has also been shown to be involved in the pathogenesis. Vitamin D receptor activators (VDRAs) are widely used for treatment of SHPT. However, VDRAs have calcemic and phosphatemic effects that limit their use to a subset of patients, and calcimimetics have been developed as alternative drugs for SHPT. Hyperphosphatemia also affects progression of SHPT, and control of hyperphosphatemia is, therefore, thought to be fundamental for control of SHPT. Currently, a combination of a VDRA and a calcimimetic is recognized as the optimal strategy for SHPT, and for other outcomes such as reduced cardiovascular disease and improved survival. The latest findings on the pathogenesis and treatment of SHPT are summarized in this review.

Effect of uremic toxin-indoxyl sulfate on the skeletal system

Chronic kidney disease-mineral bone disorders (CKD-MBD) exhibit abnormalities in the circulating mineral levels, vitamin D metabolism, and parathyroid function that contribute to the formation of a bone lesion. The uremic toxin, indoxyl sulfate (IS), accumulates in the blood in cases of renal failure and leads to bone loss. The bone and renal responses to the action of the parathyroid hormone (PTH) are progressively decreased in CKD in spite of increasing PTH levels, a condition commonly called PTH resistance. There is a high prevalence of low bone turnover or adynamic bone disease in the early stages of CKD. This could be due to the inhibition of bone turnover, such as in PTH resistance, reduced active vitamin D levels, diabetes, aluminum, and, increased IS. With an increase in IS, there is a decrease in the osteoblast Wnt/b-catenin signaling and increase in the expression of Wnt signaling inhibitors, such as sclerostin and Dickkopf-1 (DKK1). Thus, a majority of early CKD patients exhibit deterioration of bone quality owing to the action of IS, this scenario could be termed uremic osteoporosis. However, this mechanism is complicated and not fully understood. With progressive deterioration in the renal function, IS accumulates along with persistent PTH secretion, potentially leading to high-turnover bone disease because high serum PTH levels have the ability of overriding peripheral PTH resistance and other inhibitory factors of bone formation. Finally, it leads to deterioration in bone quantity with prominent bone resorption in end stage renal disease. Uremic toxins adsorbents may decelerate oxidative stress and improve bone health in CKD patients. This review article focuses on IS and bone loss in CKD patients.

Uremic Toxicity: Present State of the Art

The uremic syndrome is a complex mixture of organ dysfunctions, which is attributed to the retention of a myriad of compounds that under normal condition are excreted by the healthy kidneys (uremic toxins). In the area of identification and characterization of uremic toxins and in the knowledge of their pathophysiologic importance, major steps forward have been made during recent years. The present article is a review of several of these steps, especially in the area of information about the compounds that could play a role in the development of cardiovascular complications. It is written by those members of the Uremic Toxins Group, which has been created by the European Society for Artificial Organs (ESAO). Each of the 16 authors has written a state of the art in his/her major area of interest.

Vitamin D, a modulator of musculoskeletal health in chronic kidney disease

The spectrum of activity of vitamin D goes beyond calcium and bone homeostasis, and growing evidence suggests that vitamin D contributes to maintain musculoskeletal health in healthy subjects as well as in patients with chronic kidney disease (CKD), who display the combination of bone metabolism disorder, muscle wasting, and weakness. Here, we review how vitamin D represents a pathway in which bone and muscle may interact. In vitro studies have confirmed that the vitamin D receptor is present on muscle, describing the mechanisms whereby vitamin D directly affects skeletal muscle. These include genomic and non-genomic (rapid) effects, regulating cellular differentiation and proliferation. Observational studies have shown that circulating 25-hydroxyvitamin D levels correlate with the clinical symptoms and muscle morphological changes observed in CKD patients. Vitamin D deficiency has been linked to low bone formation rate and bone mineral density, with an increased risk of skeletal fractures. The impact of low vitamin D status on skeletal muscle may also affect muscle metabolic pathways, including its sensitivity to insulin. Although some interventional studies have shown that vitamin D may improve physical performance and protect against the development of histological and radiological signs of hyperparathyroidism, evidence is still insufficient to draw definitive conclusions.

Secondary Hyperparthyroidism: Pathogenesis, Diagnosis, Preventive and Therapeutic Strategies

Uremic secondary hyperparathyroidism is a multifactorial and complex disease often present in advanced stages of chronic kidney disease. The accumulation of phosphate, the increased FGF23 levels, the reduction in active vitamin D production, and the tendency to hypocalcemia are persistent stimuli for the development and progression of parathyroid hyperplasia with increased secretion of PTH. Parathyroid proliferation may become nodular mainly in cases of advanced hyperparathyroidism. The alterations in the regulation of mineral metabolism, the development of bone disease and extraosseous calcifications are essential components of chronic kidney disease-mineral and bone disorder and have been associated with negative outcomes. The management of hyperparathyroidism includes the correction of vitamin D deficiency and control of serum phosphorus and PTH without inducing hypercalcemia. An update of the leading therapeutic tools available for the prevention and clinical management of secondary hyperparathyroidism, its diagnosis, and the main mechanisms and factors involved in the pathogenesis of the disease will be described in this review.

Active vitamin D treatment in CKD patients raises serum sclerostin and this effect is modified by circulating pentosidine levels

BACKGROUND AND AIMS

1,25(OH)₂Vitamin D increases the expression of the sclerostin gene. Whether vitamin D receptor activation (VDRA) influences serum sclerostin in CKD and whether compounds interfering with VDRA like Advanced Glycosylation End Products (AGEs) may alter the sclerostin response to VDRA is unknown.

METHODS AND RESULTS

Eighty-eight stage G3-4 CKD patients randomly received 2 μg paricalcitol (PCT)/day (n = 44) or placebo (n = 44) for 12 weeks. Sclerostin, a major AGE compound like pentosidine, and bone mineral disorder biomarkers were measured at baseline, at 12 weeks and 2 weeks after stopping the treatments. At baseline, in the whole study population sclerostin correlated with male gender (P = 0.002), BMI (P < 0.001), waist circumference (P < 0.001), serum pentosidine (P = 0.002) and to a weaker extent, with diabetes (P = 0.04), 1,25(OH)₂Vitamin D (r = 0.22, P = 0.04) and serum phosphate (r = -0.26, P = 0.01). Sclerostin increased during PCT treatment (average + 15.7 pg/ml, 95% CI: -3.0 to +34.3) but not during placebo (P = 0.03) and the PCT effect was abolished 2 weeks after stopping this drug. The increase in sclerostin levels induced by PCT was modified by prevailing pentosidine levels (P = 0.01) and was abolished by statistical adjustment for simultaneous changes in PTH but not by FGF23 changes.

CONCLUSIONS

VDRA by paricalcitol causes a moderate increase in serum sclerostin in CKD patients. Such an effect is abolished by adjustment for PTH, suggesting that it may serve to counter PTH suppression. The sclerostin rise by PCT is attenuated by pentosidine, an observation in keeping with in vitro studies showing that AGEs alter the functioning of the VDRA.

Effects of the Administration of 25(OH) Vitamin D3 in an Experimental Model of Chronic Kidney Disease in Animals Null for 1-Alpha-Hydroxylase

The final step in vitamin D activation is catalyzed by 1-alpha-hydroxylase (CYP27B1). Chronic kidney disease (CKD) is characterized by low levels of both 25(OH)D3 and 1,25(OH)2D3 provoking secondary hyperparathyroidism (2HPT). Therefore, treatments with active or native vitamin D compounds are common in CKD to restore 25(OH)D3 levels and also to decrease PTH. This study evaluates the dose of 25(OH)D3 that restores parathyroid hormone (PTH) and calcium levels in a model of CKD in CYP27B1-/- mice. Furthermore, we compare the safety and efficacy of the same dose in CYP27B1+/+ animals. The dose needed to decrease PTH levels in CYP27B1-/- mice with CKD was 50 ng/g. That dose restored blood calcium levels without modifying phosphate levels, and increased the expression of genes responsible for calcium absorption (TRPV5 and calbindinD- 28K in the kidney, TRPV6 and calbindinD-9k in the intestine). The same dose of 25(OH)D3 did not modify PTH in CYP27B1+/+ animals with CKD. Blood calcium remained normal, while phosphate increased significantly. Blood levels of 25(OH)D3 in CYP27B1-/- mice were extremely high compared to those in CYP27B1+/+ animals. CYP27B1+/+ animals with CKD showed increases in TRPV5, TRPV6, calbindinD-28K and calbindinD-9K, which were not further elevated with the treatment. Furthermore, CYP27B1+/+ animals displayed an increase in vascular calcification. We conclude that the dose of 25(OH)D3 effective in decreasing PTH levels in CYP27B1-/- mice with CKD, has a potentially toxic effect in CYP27B1+/+ animals with CKD.

Changing bone patterns with progression of chronic kidney disease

It is commonly held that osteitis fibrosa and mixed uremic osteodystrophy are the predominant forms of renal osteodystrophy in patients with chronic kidney disease. Osteitis fibrosa is a high-turnover bone disease resulting mainly from secondary hyperparathyroidism, and mixed uremic osteodystrophy is in addition characterized by a mineralization defect most often attributed to vitamin D deficiency. However, there is ancient and more recent evidence that in early chronic kidney disease stages adynamic bone disease characterized by low bone turnover occurs first, at least in a significant proportion of patients. This could be due to the initial predominance of bone turnover-inhibitory conditions such as resistance to the action of parathyroid hormone (PTH), reduced calcitriol levels, sex hormone deficiency, diabetes, and, last but not least, uremic toxins leading to repression of osteocyte Wnt/β-catenin signaling and increased expression of Wnt antagonists such as sclerostin, Dickkopf-1, and sFRP4. The development of high-turnover bone disease would occur only later on, when serum PTH levels are able to overcome peripheral PTH resistance and the other inhibitory factors of bone formation. Whether FGF23 and Klotho play a direct role in the transition from low- to high-turnover bone disease or participate only indirectly via regulating PTH secretion remains to be seen.

Vascular Calcification Induced by Chronic Kidney Disease Is Mediated by an Increase of 1α-Hydroxylase Expression in Vascular Smooth Muscle Cells

Vascular calcification (VC) is a complication of chronic kidney disease that predicts morbidity and mortality. Uremic serum promotes VC, but the mechanism involved is unknown. A role for 1,25(OH)₂ D₃ in VC has been proposed, but the mechanism is unclear because both low and high levels have been shown to increase it. In this work we investigate the role of 1,25(OH)₂ D₃ produced in vascular smooth muscle cells (VSMCs) in VC. Rats with subtotal nephrectomy and kidney recipient patients showed increased arterial expression of 1α-hydroxylase in vivo. VSMCs exposed in vitro to serum obtained from uremic rats also showed increased 1α-hydroxylase expression. Those increases were parallel to an increase in VC. After 6 days with high phosphate media, VSMCs overexpressing 1α-hydroxylase show significantly higher calcium content and RUNX2 expression than control cells. 1α-hydroxylase null mice (KO) with subtotal nephrectomy and treated with calcitriol (400 ng/kg) for 2 weeks showed significantly lower levels of vascular calcium content, Alizarin red staining, and RUNX2 expression than wild-type (WT) littermates. Serum calcium, phosphorus, blood urea nitrogen (BUN), PTH, and 1,25(OH)₂ D₃ levels were similar in both calcitriol-treated groups. In vitro, WT VSMCs treated with uremic serum also showed a significant increase in 1α-hydroxylase expression and higher calcification that was not observed in KO cells. We conclude that local activation of 1α-hydroxylase in the artery mediates VC observed in uremia. © 2016 American Society for Bone and Mineral Research.

Role of Vitamin D in Cognitive Function in Chronic Kidney Disease

Both vitamin D deficiency and cognitive impairment are common in patients with chronic kidney disease (CKD). Vitamin D exerts neuroprotective and regulatory roles in the central nervous system. Hypovitaminosis D has been associated with muscle weakness and bone loss, cardiovascular diseases (hypertension, diabetes and hyperlipidemia), inflammation, oxidative stress, immune suppression and neurocognitive impairment. The combination of hypovitaminosis D and CKD can be even more debilitating, as cognitive impairment can develop and progress through vitamin D-associated and CKD-dependent/independent processes, leading to significant morbidity and mortality. Although an increasingly recognized comorbidity in CKD, cognitive impairment remains underdiagnosed and often undermanaged. Given the association of cognitive decline and hypovitaminosis D and their deleterious effects in CKD patients, determination of vitamin D status and when appropriate, supplementation, in conjunction with neuropsychological screening, should be considered integral to the clinical care of the CKD population.

Vitamin D prevents hypoxia/reoxygenation-induced blood-brain barrier disruption via vitamin D receptor-mediated NF-kB signaling pathways

Maintaining blood-brain barrier integrity and minimizing neuronal injury are critical components of any therapeutic intervention following ischemic stroke. However, a low level of vitamin D hormone is a risk factor for many vascular diseases including stroke. The neuroprotective effects of 1,25(OH)2D3 (vitamin D) after ischemic stroke have been studied, but it is not known whether it prevents ischemic injury to brain endothelial cells, a key component of the neurovascular unit. We analyzed the effect of 1,25(OH)2D3 on brain endothelial cell barrier integrity and tight junction proteins after hypoxia/reoxygenation in a mouse brain endothelial cell culture model that closely mimics many of the features of the blood-brain barrier in vitro. Following hypoxic injury in bEnd.3 cells, 1,25(OH)2D3 treatment prevented the decrease in barrier function as measured by transendothelial electrical resistance and permeability of FITC-dextran (40 kDa), the decrease in the expression of the tight junction proteins zonula occludin-1, claudin-5, and occludin, the activation of NF-kB, and the increase in matrix metalloproteinase-9 expression. These responses were blocked when the interaction of 1,25(OH) )2D3 with the vitamin D receptor (VDR) was inhibited by pyridoxal 5'-phosphate treatment. Our findings show a direct, VDR-mediated, protective effect of 1,25(OH) )2D3 against ischemic injury-induced blood-brain barrier dysfunction in cerebral endothelial cells.

Laboratory abnormalities in CKD-MBD: markers, predictors, or mediators of disease?

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is characterized by bone abnormalities, vascular calcification, and an array of laboratory abnormalities. The latter classically include disturbances in the parathyroid hormone/vitamin D axis. More recently, fibroblast growth factor 23 (FGF23) and klotho also have been identified as important regulators of mineral metabolism. Klotho deficiency and high circulating FGF23 levels precede secondary hyperparathyroidism in CKD patients. Levels of FGF23 and parathyroid hormone increase along the progression of CKD to maintain mineral homeostasis and to overcome end-organ resistance. It is hard to define when the increase of both hormones becomes maladaptive. CKD-MBD is associated with adverse outcomes including cardiovascular disease and mortality. This review summarizes the complex pathophysiology of CKD-MBD and outlines which laboratory abnormalities represent biomarkers of disease severity, which laboratory abnormalities are predictors of cardiovascular disease, and which laboratory abnormalities should be considered (direct) uremic toxins exerting organ damage. This information may help to streamline current and future therapeutic efforts.

Vitamin D and chronic kidney disease-mineral bone disease (CKD-MBD)

Chronic kidney disease (CKD) is a modern day epidemic and has significant morbidity and mortality implications. Mineral and bone disorders are common in CKD and are now collectively referred to as CKD- mineral and bone disorder (MBD). These abnormalities begin to appear even in early stages of CKD and contribute to the pathogenesis of renal osteodystrophy. Alteration in vitamin D metabolism is one of the key features of CKD-MBD that has major clinical and research implications. This review focuses on biology, epidemiology and management aspects of these alterations in vitamin D metabolism as they relate to skeletal aspects of CKD-MBD in adult humans.

Calcitriol, calcidiol, parathyroid hormone, and fibroblast growth factor-23 interactions in chronic kidney disease

Objective

To review the inter-relationships between calcium, phosphorus, parathyroid hormone (PTH), parent and activated vitamin D metabolites (vitamin D, 25(OH)-vitamin D, 1,25(OH)₂-vitamin D, 24,25(OH)₂-vitamin D), and fibroblast growth factor-23 (FGF-23) during chronic kidney disease (CKD) in dogs and cats.

Data Sources

Human and veterinary literature.

Human Data Synthesis

Beneficial effects of calcitriol treatment during CKD have traditionally been attributed to regulation of PTH but new perspectives emphasize direct renoprotective actions independent of PTH and calcium. It is now apparent that calcitriol exerts an important effect on renal tubular reclamation of filtered 25(OH)-vitamin D, which may be important in maintaining adequate circulating 25(OH)-vitamin D. This in turn may be vital for important pleiotropic actions in peripheral tissues through autocrine/paracrine mechanisms that impact the health of those local tissues.

Veterinary Data Synthesis

Limited information is available reporting the benefit of calcitriol treatment in dogs and cats with CKD.

Conclusions

A survival benefit has been shown for dogs with CKD treated with calcitriol compared to placebo. The concentrations of circulating 25(OH)-vitamin D have recently been shown to be low in people and dogs with CKD and are related to survival in people with CKD. Combination therapy for people with CKD using both parental and activated vitamin D compounds is common in human nephrology and there is a developing emphasis using combination treatment with activated vitamin D and renin-angiotensin-aldosterone-system (RAAS) inhibitors.

Effects of decreased vitamin D and accumulated uremic toxin on human CYP3A4 activity in patients with end-stage renal disease

In patients with end-stage renal disease, not only renal clearance but also hepatic clearance is known to be impaired. For instance, the concentration of erythromycin, a substrate of cytochrome P450 3A4 (CYP3A4), has been reported to be elevated in patients with end-stage renal disease. The purpose of this study is to elucidate the reason for the decrease in hepatic clearance in patients with end-stage renal disease. Deproteinized pooled sera were used to assess the effects of low-molecular-weight uremic toxins on CYP3A4 activity in human liver microsomes and human LS180 cells. Four uremic toxins (3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, hippuric acid, indole-3-acetic acid, and 3-indoxyl sulfate) present at high concentrations in uremic serum were also studied. Simultaneous treatment of uremic serum (less than 10%) or uremic toxins did not affect testosterone 6β-hydroxylation in human liver microsomes. On the other hand, pretreatment of each serum activates CYP3A4 in LS180 cells, and the increased CYP3A4 activity in uremic serum-treated cells was smaller than normal serum-treated cells. In addition, CYP3A4 and CYP24A1 mRNA levels also increased in LS180 cells exposed to normal serum, and this effect was reduced in uremic serum-treated cells and in cells exposed to uremic serum added to normal serum. Furthermore, addition of 1,25-dihydroxyvitamin D to uremic serum partially restored the serum effect on CYP3A4 expression. The present study suggests that the decrease of 1,25-dihydroxyvitamin D and the accumulation of uremic toxins contributed to the decreased hepatic clearance of CYP3A4 substrates in patients with end-stage renal disease.

The vitamin d receptor and T cell function

The vitamin D receptor (VDR) is a nuclear, ligand-dependent transcription factor that in complex with hormonally active vitamin D, 1,25(OH)₂D₃, regulates the expression of more than 900 genes involved in a wide array of physiological functions. The impact of 1,25(OH)₂D₃-VDR signaling on immune function has been the focus of many recent studies as a link between 1,25(OH)₂D₃ and susceptibility to various infections and to development of a variety of inflammatory diseases has been suggested. It is also becoming increasingly clear that microbes slow down immune reactivity by dysregulating the VDR ultimately to increase their chance of survival. Immune modulatory therapies that enhance VDR expression and activity are therefore considered in the clinic today to a greater extent. As T cells are of great importance for both protective immunity and development of inflammatory diseases a variety of studies have been engaged investigating the impact of VDR expression in T cells and found that VDR expression and activity plays an important role in both T cell development, differentiation and effector function. In this review we will analyze current knowledge of VDR regulation and function in T cells and discuss its importance for immune activity.

Vitamin D in kidney disease: pathophysiology and the utility of treatment

CKD is associated with decreased vitamin D metabolites, both the storage form 25(OH)D and the active form 1,25-dihydroxyvitamin D. This contributes to hyperparathyroidism, and increased levels of PTH mobilize minerals from the skeleton and increase the risk for fractures. Treatment with vitamin D sterols efficiently reduces secondary hyperparathyroidism of CKD. Observational studies suggest survival and other potential benefits of vitamin D treatment in the CKD population. These observations need to be verified with controlled prospective trials.

Evidence in favor of a severely impaired net intestinal calcium absorption in patients with (early-stage) chronic kidney disease

INTRODUCTION

Calcium and phosphorus are essential to many vital physiological processes. Little is known about the net and fractional intestinal absorption of calcium and phosphorus in patients with chronic kidney disease (CKD) and their clinical and hormonal determinants.

METHODS

Blood and 24-hour urine samples were collected in 20 healthy volunteers (HV) and 72 stable CKD stage 1-4 patients and analyzed for parameters of mineral metabolism including calcidiol, calcitriol, and parathyroid hormone (PTH). Dietary intake was assessed by dietary history.

RESULTS

The 24-hour urinary calcium excretion, as opposed to the phosphorus excretion, showed a stepwise decrease across CKD stages (median of 219, 84, 40, and 22 mg/day in HV and patients with CKD stages 1-2, 3 and 4, respectively). Younger age, high serum calcitriol, and high estimated GFR were associated with a high 24-hour urinary calcium excretion. High serum calcitriol levels and dietary phosphorus intake were associated with a high 24-hour urinary phosphorus excretion. The fractional intestinal calcium absorption, as estimated by the urinary-to-ingested calcium ratio, decreased across CKD stages.

CONCLUSIONS

The 24-hour urinary excretion of calcium, as opposed to phosphorus, is markedly decreased in CKD, even in early-stage disease. This is partly explained by low calcitriol levels and older age. Assuming a neutral calcium balance at the time of urine collection, we infer that net intestinal calcium absorption may be severely impaired in CKD.

Secondary hyperparathyroidism: pathogenesis, disease progression, and therapeutic options

Secondary hyperparathyroidism (SHPT) is a challenge frequently encountered in the management of patients with chronic kidney disease (CKD). Downregulation of the parathyroid vitamin D and calcium-sensing receptors represent critical steps that lead to abnormalities in mineral metabolism: high phosphate, low calcium, and vitamin D deficiency. These imbalances result in parathyroid hyperplasia and contribute to vascular calcification. New studies have established a central role for fibroblast growth factor 23 (FGF-23) in the regulation of phosphate-vitamin D homeostasis. FGF-23 concentration increases in CKD and contributes to SHPT. Achieving current targets for the key mineral parameters in the management of SHPT set by the Kidney Disease Improving Global Outcomes (KDIGO) guidelines can be challenging. This review summarizes the current understanding and evidence supporting strategies for SHPT treatment in CKD patients. Treatment should include a combination of dietary phosphorus restriction, phosphate binders, vitamin D sterols, and calcimimetics. Parathyroidectomy is effective in suitable candidates refractory to medical therapy and the standard against which new approaches should be measured. Future strategies may focus on the stimulation of apoptotic activity of hyperplastic parathyroid cells.

Vitamin D in kidney disease: pathophysiology and the utility of treatment

Vitamin D physiology has gained more importance and publicity than any of its counterparts in the water- and fat-soluble vitamin groups combined. This is partly because vitamin D deficiency is still widely prevalent in the developed world and the beneficial effects are thought to extend beyond the regulation of calcium and phosphorus homeostasis alone. Vitamin D deficiency becomes even more important in the various stages of chronic kidney disease (CKD); CKD itself is also on the increase. How vitamin D physiology is altered in CKD and how the various treatment modalities can alter the morbidity and mortality associated with CKD is the topic of discussion for this article.

Use of 3-hour daily hemodialysis and paricalcitol in patients with severe secondary hyperparathyroidism: A case series

Patients with poor metabolic control receiving conventional hemodialysis are at risk for developing severe secondary hyperparathyroidism. We postulated that daily hemodialysis may be effective at controlling parathyroid hormone (PTH) in the setting of severe secondary hyperparathyroidism by improving the control of hyperphosphatemia and allowing increased use of vitamin D analogs. We present 5 patients with severe secondary hyperparathyroidism (median iPTH=1783 pg/mL) who were treated with 3-hour daily hemodialysis (3 hours x 6 times a week). Daily hemodialysis, at 1 year, was associated with a 70.4% reduction in median PTH (1783 pg/mL [interquartile range: 1321-1983]-472 pg/mL [334, 704], P<0.001). Additionally, there was an increase in paricalcitol dose from 0 mcg/d to 10.8 (2.00, 11.7) mcg/d, a 39% reduction in calcium x phosphorus product (80.3 +/- 26.8-48.9 +/- 14.0, P<0.01), a 52% reduction in serum phosphorus (9.90 +/- 2.34-4.75 +/- 0.79 mg/dL, P<0.0001), and a 17.6% increase in serum calcium (8.18 +/- 2.04-9.62 +/- 0.93 mg/dL, P<0.01). Three-hour daily hemodialysis with the use of high-dose paricalcitol is associated with improved control of severe secondary hyperparathyroidism.

Parathyroid cell resistance to fibroblast growth factor 23 in secondary hyperparathyroidism of chronic kidney disease

Although fibroblast growth factor 23 (FGF23) acting through its receptor Klotho-FGFR1c decreases parathyroid hormone expression, this hormone is increased in chronic kidney disease despite an elevated serum FGF23. We measured possible factors that might contribute to the resistance of parathyroid glands to FGF23 in rats with the dietary adenine-induced model of chronic kidney disease. Quantitative immunohistochemical and reverse transcription-PCR analysis using laser capture microscopy showed that both Klotho and FGFR1 protein and mRNA levels were decreased in histological sections of the parathyroid glands. Recombinant FGF23 failed to decrease serum parathyroid hormone levels or activate the mitogen-activated protein kinase signaling pathway in the glands of rats with advanced experimental chronic kidney disease. In parathyroid gland organ culture, the addition of FGF23 decreased parathyroid hormone secretion and mRNA levels in control animals or rats with early but not advanced chronic kidney disease. Our results show that because of a downregulation of the Klotho-FGFR1c receptor complex, an increase of circulating FGF23 does not decrease parathyroid hormone levels in established chronic kidney disease. This in vivo resistance is sustained in parathyroid organ culture in vitro.

A new role for vitamin D receptor activation in chronic kidney disease

Vitamin D has proven to be much more than a simple "calcium hormone." The fact that the vitamin D receptor has been found in cells not related to mineral metabolism supports that statement. The interest of nephrologists in vitamin D and its effects beyond mineral metabolism has increased over the last few years, evidencing the importance of this so-called "sunshine hormone." In the present review, we highlight the most recent developments in the traditional use of vitamin D in chronic kidney disease (CKD) patients, namely, the control of secondary hyperparathyroidism (sHPT). Furthermore, we also explore the data available regarding the new possible therapeutic uses of vitamin D for the treatment of other complications present in CKD patients, such as vascular calcification, left ventricular hypertrophy, or proteinuria. Finally, some still scarce but very promising data regarding a possible role of vitamin D in kidney transplant patients also are reviewed. The available data point to a potential beneficial effect of vitamin D in CKD patients beyond the control of mineral metabolism.

Thyroid hormone action in chronic kidney disease

PURPOSE OF REVIEW

Chronic kidney disease is characterized by multiple abnormalities in the thyroid hormone physiology. In the present review, we will briefly discuss the effects of uremia on thyroid hormone synthesis, metabolism, transport, and action.

RECENT FINDINGS

Uremic toxins have been shown to interfere at various levels of the thyroid hormone action, including thyroid hormone transport across plasma membrane and thyroid hormone receptor activity. These abnormalities could explain the resistance to thyroid hormone action in uremia, at least in some tissues.

SUMMARY

The pathogenesis of thyroid axis abnormalities in uremia is incompletely understood, and its clinical significance remains unclear. The increasing prevalence of chronic kidney disease underscores the need for further efforts to understand the metabolic consequences of uremia and address questions such as the impact of thyroid hormone therapy.

Renal Osteodystrophy in Peritoneal Dialysis: Special Considerations

Bone disease is one of the most challenging complications in patients with chronic kidney disease. Today, it is considered to be part of a complex systemic disorder manifested by disturbances of mineral metabolism and vascular calcifications called chronic kidney disease – mineral bone disorder (CKD-MBD).

The term renal osteodystrophy is reserved to define the specific bone lesion in CKD-MBD, whose spectrum ranges from high turnover to low turnover disease. Phosphate retention, decreased serum calcium, and 1,25-dihydroxy vitamin D synthesis are involved in the pathogenesis of high bone turnover. However, the various therapeutic approaches (calcium supplements, phosphate binders, and vitamin D metabolites, among others), the renal replacement modality (hemodialysis or continuous ambulatory peritoneal dialysis), and the types of patients to whom dialysis is offered (more patients who are diabetic or older, or both) may influence the evolution of the bone disorder. As a result, recent studies have reported a greater prevalence of adynamic forms of renal osteodystrophy, especially in diabetic and peritoneal dialysis patients.

The present article reviews, for patients treated with peritoneal dialysis, the pathophysiologic mechanisms involved in the evolution and perpetuation of this bone disease and the therapeutic modalities for treating and preventing adynamic bone.

Cross-sectional analysis of abnormalities of mineral homeostasis, vitamin D and parathyroid hormone in a cohort of pre-dialysis patients. The chronic renal impairment in Birmingham (CRIB) study

BACKGROUND

Disturbances in mineral and vitamin D metabolism, which affect parathyroid hormone (PTH) synthesis, are well recognized in patients receiving dialysis. However, it is unclear at what stage of chronic kidney disease (CKD) these abnormalities develop.

METHODS

The associations between CKD stages 3 and 5, and alterations of calcium, phosphate, vitamin D and PTH concentrations were assessed in 249 patients (mean age 61 years, 66% male) and 79 age- and sex-matched healthy controls.

RESULTS

As compared to controls, serum phosphate concentrations were elevated among CKD patients (1.40 vs. 1.11 mmol/l; p < 0.0001). And levels of both 25-hydroxyvitamin D (42.1 vs. 60.4 nmol/l; p < 0.0001) and 1,25-dihydroxyvitamin D (58.2 vs. 119.5 pmol/l; p < 0.0001) were lower among patients with CKD, even among those with only stage 3 CKD and despite 73% of patients receiving vitamin D supplements. The ratio of 1,25-dihydroxy- to 25-hydroxyvitamin D was lower than controls, even among patients with stage 3 CKD (p = 0.0001), and this ratio diminished with advancing renal impairment. Concomitant elevations were observed in intact PTH (13.8 vs. 4.2 pmol/l; p < 0.0001) and whole PTH (7.9 vs. 2.7 pmol/l; p < 0.0001).

CONCLUSION

Impaired conversion of 25-hydroxy- to 1,25-dihydroxyvitamin D is an early feature of renal disease, and progresses as renal function deteriorates.

Metabolic Bone Disease in Chronic Kidney Disease

Metabolic bone disease is a common complication of chronic kidney disease (CKD) and is part of a broad spectrum of disorders of mineral metabolism that occur in this clinical setting and result in both skeletal and extraskeletal consequences. Detailed research in that past 4 decades has uncovered many of the mechanisms that are involved in the initiation and maintenance of the disturbances of bone and mineral metabolism and has been translated successfully from "bench to bedside" so that efficient therapeutic strategies now are available to control the complications of disturbed mineral metabolism. Recent emphasis is on the need to begin therapy early in the course of CKD. Central to the assessment of disturbances in bone and mineral metabolism is the ability to make an accurate assessment of the bone disease by noninvasive means. This remains somewhat problematic, and although measurements of parathyroid hormone are essential, recently recognized difficulties with these assays make it difficult to provide precise clinical practice guidelines for the various stages of CKD at the present time. Further research and progress in this area continue to evaluate the appropriate interventions to integrate therapies for both the skeletal and extraskeletal consequences with a view toward improving patient outcomes.

Parathyroid cell growth in patients with advanced secondary hyperparathyroidism: vitamin D receptor, calcium sensing receptor, and cell cycle regulating factors

The parathyroid gland (PTG) is a unique endocrine organ in which the quiescent glandular cells begin to proliferate in response to the demand for maintaining calcium (Ca) homeostasis in the progressive course of renal failure, leading to secondary hypereparathyroidism (SHPT). SHPT is characterized with continuous over-secretion of parathyroid hormone (PTH) and high turn-over bone disease, osteitis fibrosa, and the major factors include a deficiency of active vitamin D, hypocalcemia, and phosphate retention. With long-term end-stage renal failure, SHPT becomes resistant to conventional medical treatment such as phosphate binders and active vitamin D supplementation, and the growth of the PTG accelerates with the pattern of hyperplasia changing from diffuse to nodular type. In this process, the sigmoid curve between extracellular Ca concentration (exCa) and the plasma level of PTH shifts to the upper-rightward, indicating both an absolute increase in PTH secretion and the resistance of PT cells to exCa. Many experimental and human studies have revealed down-regulation of vitamin D receptor (VDR), calcium-sensing receptor (CaSR), and retinoid X receptor (RXR) in PT cells. The sustained proliferation of PT cells after obtaining autonomicity is another characteristic feature of SHPT. In this context, it has been demonstrated that the cell cycle is markedly progressed, where the expression of cyclin-dependent kinase inhibitor (CDKI), p21 and p27, is depressed in a VDR-dependent manner. These pathological features are most evident in nodular hyperplasia, in which monoclonal proliferation is obvious, indicating the phenotypic changes have occured in PT cells. It has been observed by Fukagawa and colleagues that pharmacologically high dose of active vitamin D administered orally can cause small-size PTG hyperplasia to regress in patients with advanced SHPT. Successful renal transplantation may also restore VDR and CaSR expressions in the diffuse type, in association with increasing TUNEL-positive cells. Thus, it is important to vigorously treat SHPT when the PT cell proliferation is in the reversible stage of diffuse hyperplasia.

Regulation of parathyroid function in chronic renal failure

This review summarizes the factors involved in the development of hyperparathyroidism secondary (2nd-HPTH) to chronic kidney disease (CKD). Calcium and calcitriol act on their respective specific parathyroid cell receptors to inhibit parathyroid function. As well as the well-known effect of calcium and calcitriol on parathyroid cell function, there is experimental work that demonstrates that phosphate, changes in pH, PTHrP, estrogens, and some cytokines also have an effect on PTH secretion. These factors are relevant in patients with chronic kidney disease. However, low calcium, vitamin D deficiency, and an accumulation of phosphate due to the decrease in renal function are the main pathogenic factors involved in the pathogenesis of 2nd-HPTH in CKD patients.

The role of vitamin D in left ventricular hypertrophy and cardiac function

The role of vitamin D in left ventricular hypertrophy and cardiac function. Cardiovascular disease is the leading cause of death among patients with end-stage renal disease (ESRD). Traditional cardiac risk factors, as well as other factors specific to the ESRD population such as hyperphosphatemia, elevated calcium and phosphate product, abnormal lipid metabolism, hyperhomocysteinemia, and chronic inflammation play a role in the excessive risk of cardiovascular death in this population. Left ventricular disorders are proven risk factors for cardiac mortality in hemodialysis patients. These disorders are present in incident ESRD patients at rates far above the general population. There is an accumulating body of evidence that suggests that vitamin D plays a role in cardiovascular disease. Abnormal vitamin metabolism, through deficiency of the active form of 1,25-dihydroxyvitamin D(3), and acquired vitamin D resistance through the uremic state, have been shown to be important in ESRD. Vitamin D deficiency has long been known to affect cardiac contractility, vascular tone, cardiac collagen content, and cardiac tissue maturation. Recent studies using vitamin D receptor deficient mice as a model demonstrate a crucial role of vitamin D in regulation of the renin-angiotensin system. Additionally, there is emerging evidence linking treatment with vitamin D to improved survival on hemodialysis and improvement in cardiac function. The emergence of this data is focusing attention on the previously underappreciated nonmineral homeostatic effects of vitamin D that very likely play an important role in the pathogenesis of cardiac disease in ESRD.

VITAMIN D IN HEALTH AND DISEASE: Beyond Minerals and Parathyroid Hormone: Role of Active Vitamin D in End-Stage Renal Disease

Secondary hyperparathyroidism is a common complication of end-stage renal disease (ESRD) that is often treated with activated forms of intravenous vitamin D. The natural course and treatment of secondary hyperparathyroidism in hemodialysis patients is punctuated by episodes of hypercalcemia, hyperphosphatemia, and increased calcium-phosphate product, which in previous studies were linked to increased mortality. Historically these episodes have been attributed to vitamin D, leading some authorities to favor decreased vitamin D use. However, the studies that examined the impact of mineral levels and parathyroid hormone (PTH) on survival did not consistently account for vitamin D therapy itself on hemodialysis patient survival. The current review examines in detail two recent large-scale studies of hemodialysis patients: one that demonstrated a survival advantage of paricalcitol over calcitriol and a second that demonstrated a significant survival advantage of any intravenous vitamin D formulation versus none. In both studies, the effects were independent of mineral and PTH levels, suggesting "nontraditional" actions of vitamin D contributed to the observed survival advantage. Several of these nontraditional actions are reviewed with an emphasis on those that might impact hemodialysis 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.

Thyroid hormone receptor binding to DNA and T3-dependent transcriptional activation are inhibited by uremic toxins

Background

There is a substantial clinical overlap between chronic renal failure (CRF) and hypothyroidism, suggesting the presence of hypothyroidism in uremic patients. Although CRF patients have low T₃ and T₄ levels with normal thyroid-stimulating hormone (TSH), they show a higher prevalence of goiter and evidence for blunted tissue responsiveness to T₃ action. However, there are no studies examining whether thyroid hormone receptors (TRs) play a role in thyroid hormone dysfunction in CRF patients. To evaluate the effects of an uremic environment on TR function, we investigated the effect of uremic plasma on TRβ1 binding to DNA as heterodimers with the retinoid X receptor alpha (RXRα) and on T₃-dependent transcriptional activity.

Results

We demonstrated that uremic plasma collected prior to hemodialysis (Pre-HD) significantly reduced TRβ1-RXRα binding to DNA. Such inhibition was also observed with a vitamin D receptor (VDR) but not with a peroxisome proliferator-activated receptor gamma (PPARγ). A cell-based assay confirmed this effect where uremic pre-HD ultrafiltrate inhibited the transcriptional activation induced by T₃ in U937 cells. In both cases, the inhibitory effects were reversed when the uremic plasma and the uremic ultrafiltrate were collected and used after hemodialysis (Post-HD).

Conclusion

These results suggest that dialyzable toxins in uremic plasma selectively block the binding of TRβ1-RXRα to DNA and impair T₃ transcriptional activity. These findings may explain some features of hypothyroidism and thyroid hormone resistance observed in CRF patients.

Activated injectable vitamin D and hemodialysis survival: a historical cohort study

Patients with ESRD commonly experience secondary hyperparathyroidism, a condition primarily managed with activated injectable vitamin D. The biologic effects of vitamin D, however, are widespread, and it is possible that activated injectable vitamin D alters survival in ESRD. This hypothesis was tested in a historical cohort study of incident hemodialysis patients who lived throughout the United States between January 1996 and December 1999. The primary outcome was 2-yr survival among those who survived for at least 90 d after initiation of chronic hemodialysis. During this period, 51,037 chronic hemodialysis patients survived for at least 90 d from the initiation of hemodialysis, and in the ensuing 2 yr, 37,173 received activated injectable vitamin D and 13,864 did not. At 2 yr, mortality rates were 13.8/100 person-years in the group that received injectable vitamin D compared with 28.6/100 person-years in the group that did not (P < 0.001). Cox proportional hazards analyses adjusting for several potential confounders and examining injectable vitamin D therapy as a time-dependent exposure suggested that compared with patients who did not receive injectable vitamin D, the 2-yr survival advantage associated with the group that did receive injectable vitamin D was 20% (hazard ratio, 0.80; 95% confidence interval, 0.76 to 0.83). The incidence of cardiovascular-related mortality was 7.6/100 person-years in the injectable vitamin D group, compared with 14.6/100 person-years in the non-vitamin D group (P < 0.001). The benefit of injectable vitamin D was evident in 48 of 49 strata examined, including those with low serum levels of intact parathyroid hormone and elevated levels of serum calcium and phosphorus, situations in which injectable vitamin D is often withheld. Repeating the entire analysis using marginal structural models to adjust for time-dependent confounding by indication yielded a survival advantage of 26% (hazard ratio, 0.74; 95% confidence interval, 0.71 to 0.79) associated with the injectable vitamin D group. In this historical cohort study, chronic hemodialysis patients in the group that received injectable vitamin D had a significant survival advantage over patients who did not. Randomized clinical trials would permit definitive conclusions.

Impaired release of interleukin-6 from human osteoblastic cells in the uraemic milieu

BACKGROUND

Osteoblast-derived interleukin-6 (IL-6) affects bone metabolism and is linked with a number of pathological states characterized by increased bone resorption, including osteoporosis and renal osteodystrophy. To examine the possibility that uraemia directly influences the release of this cytokine in bone, we have investigated the effect of human uraemic serum on the release of IL-6 from human osteoblast-like cells.

METHODS

Individual serum samples collected from healthy male volunteers or male haemodialysis patients prior to and during a dialysis treatment were assayed for IL-6, interleukin-1beta (IL-1beta) and soluble IL-6 receptor (sIL-6R) using specific enzyme-linked immunosorbent assays. MG-63 and SaOS-2 cells were cultured in media containing pooled sera from both groups and alongside matching charcoal-stripped sera. IL-6 concentrations were determined in harvested cell supernatants after 24 h. In further experiments, media containing individual sera obtained from five patients at regular intervals during their haemodialysis treatment were incubated with MG-63 cells to determine the effects of the dialysis process on IL-6 secretion.

RESULTS

Haemodialysis patients had significantly higher (n = 10, P < 0.001) circulating concentrations of IL-6 (7.0 +/- 1.6 pg/ml) than normal subjects (0.4 +/- 0.1 pg/ml), but there were no significant differences in the concentrations of either IL-1beta or sIL-6R. These serum concentrations did not change significantly during 80 min of dialysis. IL-6 release by MG-63 cells incubated with charcoal-stripped serum from normal or from uraemic subjects was similar. Incubation with untreated sera from normal subjects increased IL-6 release by approximately 6-fold above the charcoal-stripped control, whereas sera from uraemic subjects increased IL-6 release by only approximately 2- to 3-fold (normal vs uraemic of 6878 +/- 595 and 2579 +/- 169 pg/ml, respectively, P < 0.001). Similar results were seen with SaOS-2 cells. Haemodialysis did not restore the capacity of uraemic serum to augment IL-6 release to the same degree as normal serum.

CONCLUSIONS

These data show that the augmentation of IL-6 release from human osteoblastic cells after incubation with normal serum is greater than after uraemic serum. This may indicate the presence of an inhibitor of IL-6 release in uraemic serum that is involved in the deranged bone turnover of uraemic patients.

Regulation of PTH1 receptor expression by uremic ultrafiltrate in UMR 106-01 osteoblast-like cells

BACKGROUND

Homologous down-regulation/desensitization of the parathyroid hormone receptor (PTH1R)/adenylate cyclase system has been demonstrated in uremia, and may contribute to parathyroid hormone (PTH) resistance; however, additional studies have shown that parathyroidectomy fails to normalize the down-regulation of the PTH1R. The present studies were designed to test directly, in vitro, the hypothesis that factors circulating in the uremic environment, other than PTH, decrease the response of osteoblastic cells to PTH.

METHODS

Studies were conducted in confluent cultures of UMR 106-01 osteoblast-like cells. Uremic ultrafiltrate (UUF) was obtained from patients on hemodialysis. Cells were exposed to media containing 50% uremic ultrafiltrate for periods of up to 72 hours. Control cultures were exposed to a buffered salt solution containing a comparable ionic composition to that of the UUF. PTH-stimulated cyclic adenosine monophosphate (cAMP) generation was determined by radioimmunoassay (RIA), PTH binding and PTH1R mRNA levels were determined by radioligand binding and Northern analysis, respectively.

RESULTS

PTH-stimulated cAMP generation from cultures treated with uremic ultrafiltrate for 48 hours was 1385.8 +/- 183.2 pmol/culture/5 minutes, whereas control cultures generated 2389.5 +/- 271 pmol cAMP/culture/5 minutes (P < 0.05). PTH binding was decreased by 30% in cultures incubated with UUF as compared to controls. The decrease in binding induced by UUF was accompanied by a decrease in PTH1R mRNA levels.

CONCLUSION

These findings demonstrate that factors present in UUF decrease PTH-stimulated cAMP generation by a mechanism that involves a decrease in the levels of PTH1R mRNA levels. Thus, the skeletal resistance to PTH in the setting of chronic kidney disease, may be explained, at least in part, by circulating factors other than PTH.

Vitamin D receptor and analogs

In chronic kidney disease (CKD), high circulating levels of parathyroid hormone (PTH) cause osteitis fibrosa, bone loss, and cardiovascular complications that increase morbidity and mortality. Impaired production of 1,25-dihydroxyvitamin D (calcitriol), the hormonal form of vitamin D, is a major contributor to the generation and maintenance of parathyroid hyperplasia and increased synthesis and secretion of PTH. Calcitriol inhibits PTH gene transcription and ameliorates parathyroid hyperplasia by suppressing the expression of and growth signals from the autocrine transforming growth factor alpha (TGFalpha)/epidermal growth factor receptor (EGFR)-growth loop, a main determinant of parathyroid cell proliferation. Calcitriol reduction of parathyroid hyperplasia and serum PTH levels demands a functional vitamin D receptor (VDR). Although VDR is normal in CKD, parathyroid VDR content is reduced markedly. Furthermore, VDR function, as a transcriptional regulator of vitamin D responsive genes, is impaired by several factors including hypocalcemia, hyperphosphatemia, accumulation of uremic toxins, and reduction in cellular levels of the VDR partner, retinoid X receptor. Therapy with calcitriol analogs can overcome the antagonism on calcitriol-VDR actions induced by CKD. Although not all analog formulations are equally effective, they offer a wider therapeutic window in counteracting vitamin D resistance and survival advantage over exclusive calcitriol therapy.

Biochemical and Clinical Evidence for Uremic Toxicity

The uremic syndrome is a mix of clinical features resulting from multiple organ dysfunctions which develop when kidney failure progresses, and is attributed to the retention of solutes, which under normal conditions are excreted by the healthy kidneys into the urine. The most practical classification of uremic solutes is based on their physicochemical characteristics that influence their dialytic removal, in (1) small water soluble compounds, (2) the larger "middle molecules," and (3) the protein bound compounds. Hence, uremic retention is much more complex than originally believed. Among the small water soluble compounds, urea exerts not much toxic activity and is not very representative in its kinetic behavior for many other uremic solutes. Among the middle molecules, many have been recognized to exert biological activity and hence to contribute to the uremic syndrome. Specific dialysis strategies apply large pore membranes to remove those middle molecules and have a beneficial impact on uremic morbidity and mortality. A substantial number of uremic solutes are protein bound. Only recently, a relation between their concentration and clinical status could be demonstrated. Likewise, it was only recently possible to demonstrate more than standard removal with super-flux dialysis membranes. To further improve characterization of uremic solutes and to develop directed therapeutic approaches, further concerted action among various groups of researchers will be needed.