p21(WAF1) and transforming growth factor-alpha mediate dietary phosphate regulation of parathyroid cell growth

Silver jubilee: 25 years of the first demonstration of the direct effect of phosphate on the parathyroid cell

Although phosphorus is an essential element for life, it is not found in nature in its native state but rather combined in the form of inorganic phosphates (PO₄^3-), with tightly regulated plasma levels that are associated with deleterious effects and mortality when these are out of bounds. The growing interest in the accumulation of PO₄^3- in human pathophysiology originated in its attributed role in the pathogenesis of secondary hyperparathyroidism (SHPT) in chronic kidney disease. In this article, we review the mechanisms by which this effect was justified and we commemorate the important contribution of a Spanish group led by Dr. M. Rodríguez, just 25 years ago, when they first demonstrated the direct effect of PO₄^3- on the regulation of the synthesis and secretion of parathyroid hormone by maintaining the structural integrity of the parathyroid glands in their original experimental model. In addition to demonstrating the importance of arachidonic acid (AA) and the phospholipase A2-AA pathway as a mediator of parathyroid gland response, these findings were predecessors of the recent description of the important role of PO₄^3- on the activity of the calcium sensor-receptor, and also fueled various lines of research on the importance of PO₄^3- overload not only for the pathophysiology of SHPT but also in its systemic pathogenic role.

Mechanism of calcitriol regulating parathyroid cells in secondary hyperparathyroidism

A common consequence of chronic renal disease is secondary hyperparathyroidism (SHPT) and is closely related to the mortality and morbidity of uremia patients. Secondary hyperparathyroidism (SHPT) is caused by excessive PTH production and release, as well as parathyroid enlargement. At present, the mechanism of cell proliferation in secondary hyperparathyroidism (SHPT) is not completely clear. Decreased expression of the vitamin D receptor (VDR) and calcium-sensing receptor (CaSR), and 1,25(OH)2D3 insufficiency all lead to a decrease in cell proliferation suppression, and activation of multiple pathways is also involved in cell proliferation in renal hyperparathyroidism. The interaction between the parathormone (PTH) and parathyroid hyperplasia and 1,25(OH)2D3 has received considerable attention. 1,25(OH)2D3 is commonly applied in the therapy of renal hyperparathyroidism. It regulates the production of parathormone (PTH) and parathyroid cell proliferation through transcription and post-transcription mechanisms. This article reviews the role of 1,25(OH)2D3 in parathyroid cells in secondary hyperparathyroidism and its current understanding and potential molecular mechanism.

Role of Calcimimetics in Treating Bone and Mineral Disorders Related to Chronic Kidney Disease

Renal osteodystrophy is common in patients with chronic kidney disease and end-stage renal disease and leads to the risks of fracture and extraosseous vascular calcification. Secondary hyperparathyroidism (SHPT) is characterized by a compensatory increase in parathyroid hormone (PTH) secretion in response to decreased renal phosphate excretion, resulting in potentiating bone resorption and decreased bone quantity and quality. Calcium-sensing receptors (CaSRs) are group C G-proteins and negatively regulate the parathyroid glands through (1) increasing CaSR insertion within the plasma membrane, (2) increasing 1,25-dihydroxy vitamin D3 within the kidney and parathyroid glands, (3) inhibiting fibroblast growth factor 23 (FGF23) in osteocytes, and (4) attenuating intestinal calcium absorption through Transient Receptor Potential Vanilloid subfamily member 6 (TRPV6). Calcimimetics (CaMs) decrease PTH concentrations without elevating the serum calcium levels or extraosseous calcification through direct interaction with cell membrane CaSRs. CaMs reduce osteoclast activity by reducing stress-induced oxidative autophagy and improving Wnt-10b release, which promotes the growth of osteoblasts and subsequent mineralization. CaMs also directly promote osteoblast proliferation and survival. Consequently, bone quality may improve due to decreased bone resorption and improved bone formation. CaMs modulate cardiovascular fibrosis, calcification, and renal fibrosis through different mechanisms. Therefore, CaMs assist in treating SHPT. This narrative review focuses on the role of CaMs in renal osteodystrophy, including their mechanisms and clinical efficacy.

Phosphate balance during dialysis and after kidney transplantation in patients with chronic kidney disease

PURPOSE OF REVIEW

In patients with chronic kidney disease (CKD), hyperphosphatemia is associated with several adverse outcomes, including bone fragility and progression of kidney and cardiovascular disease. However, there is a knowledge gap regarding phosphate balance in CKD. This review explores its current state, depending on the stage of CKD, dialysis modalities, and the influence of kidney transplantation.

RECENT FINDINGS

Adequate phosphate control is one of the goals of treatment for CKD-mineral and bone disorder. However, ongoing studies are challenging the benefits of phosphate-lowering treatment. Nevertheless, the current therapy is based on dietary restriction, phosphate binders, and optimal removal by dialysis. In the face of limited adherence, due to the high pill burden, adjuvant options are under investigation. The recent discovery that intestinal absorption of phosphate is mostly paracellular when the intraluminal concentration is adequate might help explain why phosphate is still well absorbed in CKD, despite the lower levels of calcitriol.

SUMMARY

Future studies could confirm the benefits of phosphate control. Greater understanding of the complex distribution of phosphate among the body compartments will help us define a better therapeutic strategy in patients with CKD.

Molecular Mechanisms of Parathyroid Disorders in Chronic Kidney Disease

Secondary hyperparathyroidism (SHP) is a common complication of chronic kidney disease (CKD) that induces morbidity and mortality in patients. How CKD stimulates the parathyroid to increase parathyroid hormone (PTH) secretion, gene expression and cell proliferation remains an open question. In experimental SHP, the increased PTH gene expression is post-transcriptional and mediated by PTH mRNA–protein interactions that promote PTH mRNA stability. These interactions are orchestrated by the isomerase Pin1. Pin1 participates in conformational change-based regulation of target proteins, including mRNA-binding proteins. In SHP, Pin1 isomerase activity is decreased, and thus, the Pin1 target and PTH mRNA destabilizing protein KSRP fails to bind PTH mRNA, increasing PTH mRNA stability and levels. An additional level of post-transcriptional regulation is mediated by microRNA (miRNA). Mice with parathyroid-specific knockout of Dicer, which facilitates the final step in miRNA maturation, lack parathyroid miRNAs but have normal PTH and calcium levels. Surprisingly, these mice fail to increase serum PTH in response to hypocalcemia or uremia, indicating a role for miRNAs in parathyroid stimulation. SHP often leads to parathyroid hyperplasia. Reduced expressions of parathyroid regulating receptors, activation of transforming growth factor α-epidermal growth factor receptor, cyclooxygenase 2-prostaglandin E2 and mTOR signaling all contribute to the enhanced parathyroid cell proliferation. Inhibition of mTOR by rapamycin prevents and corrects the increased parathyroid cell proliferation of SHP. This review summarizes the current knowledge on the mechanisms that stimulate the parathyroid cell at multiple levels in SHP.

Current treatment options for secondary hyperparathyroidism in patients with stage 3 to 4 chronic kidney disease and vitamin D deficiency

Introduction: Secondary hyperparathyroidism (SHPT) represents a complication of chronic kidney disease (CKD). Vitamin D system is altered since early CKD, and vitamin D deficiency is an established trigger of SHPT. Although untreated SHPT may degenerate into tertiary hyperparathyroidism with detrimental consequences in advanced CKD, best treatments for counteracting SHPT from stage 3 CKD are still debated. Enthusiasm on prescription of vitamin D receptor activators (VDRA) in non-dialysis renal patients, has been mitigated by the risk of low bone turnover and positive calcium-phosphate balance. Nutritional vitamin D is now suggested as first-line therapy to treat SHPT with low 25(OH)D insufficiency. However, no high-grade evidence supports the best choice between ergocalciferol, cholecalciferol, and calcifediol (in its immediate or extended-release formulation).Areas covered: The review discusses available data on safety and efficacy of nutritional vitamin D, VDRA and nutritional therapy in replenishing 25(OH)D deficiency and counteracting SHPT in non-dialysis CKD patients.Expert opinion: Best treatment for low 25(OH)D and SHPT remains unknown, due to incomplete understanding of the best homeostatic, as mutable, adaptation of mineral metabolism to CKD progression. Nutritional vitamin D and nutritional therapy appear safest interventions, whenever contextualized with single-patient characteristics. VDRA should be restricted to uncontrolled SHPT by first-line therapy.

Upregulated LRRC55 promotes BK channel activation and aggravates cell injury in podocytes

Podocyte injury is a common hallmark in various glomerular diseases. The level of LRRC55 was increased in podocytes of patients with focal segmental glomerulosclerosis (FSGS), diabetic nephropathy (DN), and membranous nephropathy (MN). Upregulated LRRC55 and increased intracellular Ca2+ led to BK channel activation and the loss of intracellular potassium, resulting in apoptosome formation and caspase-3 activation in angiotensin II (Ang II)-treated podocytes. Knockout of Lrrc55 or the BK channel prevented the BK current and ameliorated podocyte injury in Ang II-treated mice. Upstream, NFATc3 regulated the expression of LRRC55. Increased LRRC55 expression in podocytes was also evident in animal models of FSGS, DN, and MN. Treatment with losartan or LRRC55 siRNA suppressed LRRC55 expression, prevented BK channel activation, and attenuated podocyte injury in animal models of FSGS, DN, and MN. In conclusion, upregulated LRRC55 promotes BK channel activation and aggravates cell injury in podocytes in FSGS, DN, and MN. LRRC55 inhibition may represent a new therapeutic approach for podocyte injury.

Phosphate as a Signaling Molecule

Phosphorus plays a vital role in diverse biological processes including intracellular signaling, membrane integrity, and skeletal biomineralization; therefore, the regulation of phosphorus homeostasis is essential to the well-being of the organism. Cells and whole organisms respond to changes in inorganic phosphorus (Pi) concentrations in their environment by adjusting Pi uptake and altering biochemical processes in cells (local effects) and distant organs (endocrine effects). Unicellular organisms, such as bacteria and yeast, express specific Pi-binding proteins on the plasma membrane that respond to changes in ambient Pi availability and transduce intracellular signals that regulate the expression of genes involved in cellular Pi uptake. Multicellular organisms, including humans, respond at a cellular level to adapt to changes in extracellular Pi concentrations and also have endocrine pathways which integrate signals from various organs (e.g., intestine, kidneys, parathyroid glands, bone) to regulate serum Pi concentrations and whole-body phosphorus balance. In mammals, alterations in the concentrations of extracellular Pi modulate type III sodium-phosphate cotransporter activity on the plasma membrane, and trigger changes in cellular function. In addition, elevated extracellular Pi induces activation of fibroblast growth factor receptor, Raf/mitogen-activated protein kinase/ERK kinase (MEK)/extracellular signal-regulated kinase (ERK) and Akt pathways, which modulate gene expression in various mammalian cell types. Excessive Pi exposure, especially in patients with chronic kidney disease, leads to endothelial dysfunction, accelerated vascular calcification, and impaired insulin secretion.

Parathyroid Cell Proliferation in Secondary Hyperparathyroidism of Chronic Kidney Disease

Secondary hyperparathyroidism (SHP) is a common complication of chronic kidney disease (CKD) that correlates with morbidity and mortality in uremic patients. It is characterized by high serum parathyroid hormone (PTH) levels and impaired bone and mineral metabolism. The main mechanisms underlying SHP are increased PTH biosynthesis and secretion as well as increased glandular mass. The mechanisms leading to parathyroid cell proliferation in SHP are not fully understood. Reduced expressions of the receptors for calcium and vitamin D contribute to the disinhibition of parathyroid cell proliferation. Activation of transforming growth factor-α-epidermal growth factor receptor (TGF-α-EGFR), nuclear factor kappa B (NF-kB), and cyclooxygenase 2- prostaglandin E2 (Cox2-PGE2) signaling all correlate with parathyroid cell proliferation, underlining their roles in the development of SHP. In addition, the mammalian target of rapamycin (mTOR) pathway is activated in parathyroid glands of experimental SHP rats. Inhibition of mTOR by rapamycin prevents and corrects the increased parathyroid cell proliferation of SHP. Mice with parathyroid-specific deletion of all miRNAs have a muted increase in serum PTH and fail to increase parathyroid cell proliferation when challenged by CKD, suggesting that miRNA is also necessary for the development of SHP. This review summarizes the current knowledge on the mechanisms of parathyroid cell proliferation in SHP.

Severe chronic kidney disease environment reduced calcium-sensing receptor expression in parathyroid glands of adenine-induced rats even without high phosphorus diet

Background

Chronic kidney disease (CKD) disrupts mineral homeostasis and its main underlying cause is secondary hyperparathyroidism (SHPT). We previously reported that calcium-sensing receptor (CaSR) mRNA and protein expression in parathyroid glands (PTGs) significantly decreased in a CKD rat model induced by a 5/6 nephrectomy that were fed a high phosphorus diet. However, there was a significant difference in the severity of CKD between high phosphorus and adequate phosphorus diet groups. Thus, it was unclear whether CKD environment or the high phosphorus diet influenced CaSR expression, and the underlying mechanism remains largely unknown.

Methods

CKD was induced in rats with 0.75% adenine-containing diet. CKD and control rats were maintained for 5 days and 2 weeks on diets with 0.7% or 1.3% phosphorus. For gene expression analysis, quantitative real-time polymerase chain reaction was performed with TaqMan probes. Protein expression was analyzed by immunohistochemistry.

Results

PTG CaSR expression significantly decreased in the presence of a severe CKD environment, even without the high phosphate load. Ki67 expressing cells in PTGs were significantly higher only in the CKD rats fed a high phosphorus diet. Furthermore, among the many genes that could affect CaSR expression, only vitamin D receptor (VDR) and glial cells missing 2 (Gcm2) showed significant changes. Moreover, Gcm2 was significantly reduced at an early stage without significant changes in serum calcium, phosphorus and 1,25(OH)₂ vitamin D, and there was no significant reduction in CaSR and VDR expressions. Then, significantly elevated Ki67-positive cell numbers were also only observed in the early CKD PTGs with high-phosphorus diets.

Conclusions

Our data suggest that the cause of the decreased PTG CaSR expression is the reduction in VDR and Gcm2 expression; Gcm2 may play a role in the onset and progression of SHPT.

Prostaglandin E2 receptor EP2 mediates the effect of cyclooxygenase 2 on secondary parathyroid hyperplasia in end-stage renal disease

BACKGROUND

Secondary hyperparathyroidism (SHPT) in patients with end-stage renal disease (ESRD) is characterized by hyperplasia of the parathyroid glands (PTGs), while the underlying mechanism is not completely understood. Previously we demonstrated a relationship between cyclooxygenase 2 (COX2) overexpression and parathyroid hyperplasia and here we investigate the role of COX2 downstream metabolic product prostaglandin E2 (PGE2) and its receptor EP2 in the pathogenesis of SHPT.

METHODS

PTGs isolated from ESRD patients with advanced SHPT were used to test the expression of COX2-microsomal prostaglandin E synthase-1 (mPGES-1)-EP2 pathway. A diffuse proliferative section of the PTGs was used for tissue culture and treated with high phosphate (HPi) medium, COX2-PGE2-EP2 pathway inhibitors or agonists. EP2 short hairpin RNA (shRNA) lentivirus was locally applied to treat an SHPT rat model.

RESULTS

In PTGs isolated from ESRD patients, enhanced immunoactivities of COX2, mPGES-1 and EP2 were observed. In primary cultured PTG tissues, HPi induced intact parathyroid hormone (iPTH) secretion, proliferating cell nuclear antigen (PCNA) expression and COX2 activity, while COX2 and EP2 inhibitors attenuated hyperparathyroidism promoted by HPi. Furthermore, PGE2 or EP2 agonist (butaprost) directly stimulated hyperparathyroidism, whereas EP2 receptor antagonist or cyclic adenosine monophosphate inhibitor attenuated the hyperparathyroidism promoted by PGE2 or butaprost. EP2 shRNA treatment significantly reduced excessive expressions of EP2 and PCNA in the PTGs of nephrectomy rats fed an HPi diet, diminished the size of PTGs and downregulated serum iPTH levels.

CONCLUSIONS

The COX2 downstream PGE2 and its receptor EP2 may play an important role in HPi-induced parathyroid hyperplasia and may serve as a potential therapeutic target for SHPT in ESRD.

Exploring the Link between Serum Phosphate Levels and Low Muscle Strength, Dynapenia, and Sarcopenia

Emerging evidences addressed an association between phosphate and muscle function. Because little attention was focused on this issue, the objective of our study was to explore the relationship of phosphate with muscle strength, dynapenia, and sarcopenia. From the National Health and Nutrition Examination Survey, a total of 7421 participants aged 20 years or older were included in our study with comprehensive examinations included anthropometric parameters, strength of the quadriceps muscle, and appendicular lean masses. Within the normal range of serum phosphate, we used quartile-based analyses to determine the potential relationships of serum phosphate with dynapenia, and sarcopenia through multivariate regression models. After adjusting for the pertinent variables, an inverse association between the serum phosphate quartiles and muscle strength was observed and the linear association was stronger than other anthropometric parameters. Notably, the significant association between phosphate and muscle strength was existed in >65 years old age group, not in 20-65 years old. The higher quartiles of phosphate had higher likelihood for predicting the presence of dynapenia rather than sarcopenia in entire population. Our study highlighted that higher quartiles of phosphate had significant association with lower muscle strength and higher risks for predicting the presence of dynapenia.

Hyperphosphatemia in Dialysis Patients: The Therapeutic Role of Lanthanum Carbonate

Phosphate overload is a dramatic consequence in end-stage renal disease (ESRD) patients. Recent studies have well documented that abnormalities in mineral and bone metabolism in these patients are associated with increased cardiovascular morbidity and mortality. Elevated serum phosphate and calcium-phosphate product levels play an important role in the pathogenesis of secondary hyperparathyroidism and extra-skeletal calcification in dialysis patients. Furthermore, inorganic phosphate may cause vascular calcification directly through a real “ossification” of the tunica media in the vasculature of ESRD patients.

The “classical” treatment of secondary hyperparathyroidism and hyperphosphatemia in ESRD patients consists of either calcium- or aluminum-based phosphate binders and calcitriol administration. Unfortunately, this “old generation” therapy is not free of complications. This review paper suggests that new calcium- and aluminum-free phosphate binders, such as lanthanum carbonate, can be used to treat hyperphosphatemia and secondary hyperparathyroidism in ESRD patients.

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.

Phosphorylation of Ribosomal Protein S6 Mediates Mammalian Target of Rapamycin Complex 1-Induced Parathyroid Cell Proliferation in Secondary Hyperparathyroidism

Secondary hyperparathyroidism is characterized by increased serum parathyroid hormone (PTH) level and parathyroid cell proliferation. However, the molecular pathways mediating the increased parathyroid cell proliferation remain undefined. Here, we found that the mTOR pathway was activated in the parathyroid of rats with secondary hyperparathyroidism induced by either chronic hypocalcemia or uremia, which was measured by increased phosphorylation of ribosomal protein S6 (rpS6), a downstream target of the mTOR pathway. This activation correlated with increased parathyroid cell proliferation. Inhibition of mTOR complex 1 by rapamycin decreased or prevented parathyroid cell proliferation in secondary hyperparathyroidism rats and in vitro in uremic rat parathyroid glands in organ culture. Knockin rpS6(p-/-) mice, in which rpS6 cannot be phosphorylated because of substitution of all five phosphorylatable serines with alanines, had impaired PTH secretion after experimental uremia- or folic acid-induced AKI. Uremic rpS6(p-/-) mice had no increase in parathyroid cell proliferation compared with a marked increase in uremic wild-type mice. These results underscore the importance of mTOR activation and rpS6 phosphorylation for the pathogenesis of secondary hyperparathyroidism and indicate that mTORC1 is a significant regulator of parathyroid cell proliferation through rpS6.

Pathophysiology of parathyroid hyperplasia in chronic kidney disease: preclinical and clinical basis for parathyroid intervention

Secondary hyperparathyroidism is characterised by excessive secretion of parathyroid hormone and parathyroid hyperplasia, resulting in both skeletal and extraskeletal consequences. Recent basic and clinical studies have brought considerable advances in our understanding of the pathophysiology of parathyroid hyperplasia and have also provided practical therapeutic approaches, especially with regard to indications for parathyroid intervention. In this context, it is quite important to recognize the development of nodular hyperplasia, because the cells in nodular hyperplasia are usually resistant to calcitriol treatment. Patients with nodular hyperplasia should undergo parathyroid intervention including percutaneous ethanol injection therapy (PEIT). Selective PEIT of the parathyroid gland is an effective approach in which the enlarged parathyroid gland with nodular hyperplasia is 'selectively' destroyed by ethanol injection, and other glands with diffuse hyperplasia are then managed by medical therapy. With a more focused attention to applying parathyroid intervention, we can expect significant improvement in the management of secondary hyperparathyroidism in dialysis patients.

Expression of p53, Ki67, epidermal growth factor receptor, transforming growth-factorα, and p21 in primary and secondary hyperparathyroidism

BACKGROUND

Secondary hyperparathyroidism (SH) is major problem in chronic renal failure. There are studies to examine proliferation and apoptosis associated biomarkers expressions in parathyroid lesions to reveal specific features. In this study, we evaluated the expression of some growth factors and their receptors in parathyroid gland of patients with SH or primary hyperparathyroidism (PH).

MATERIALS AND METHODS

A total of 49 patients had been operated for PH and 26 for SH. Parathyroid tissue samples were evaluated histopathologically and immunohistochemically using antibodies to human p53, Kİ-67, anti-human p21, antitransforming growth factor (TGF) α, CPP32 (caspase 3), and epidermal growth factor receptor (EGFR).

RESULTS

Adenoma was higher in PH compared with SH as 48/49 and 3/26, respectively (P = 0.000). Parathyroid hyperplasia was found in 23/26 patients with SH and 1/49 patient with PH. In parathyroid tissue there were no difference between PH and SH for p53, Ki-67, caspase, EGFR expressions; while there were significantly difference for TGFα (P = 0.047) and borderline significant difference for p21 (P = 0.06) expressions.

CONCLUSION

Adenoma was priority present in PH patients, hyperplasia was present in SH. There were no differences between primary and SH or adenoma and hyperplasia for expressions of cycline-dependent kinase inhibitor p21, p53, EGFR, Ki67, caspase; while TGFα expression was found to be different.

The induction of C/EBPβ contributes to vitamin D inhibition of ADAM17 expression and parathyroid hyperplasia in kidney disease

BACKGROUND

In secondary hyperparathyroidism (SHPT), enhanced parathyroid levels of transforming growth factor-α (TGFα) increase EGF receptor (EGFR) activation causing parathyroid hyperplasia, high parathyroid hormone (PTH) and also reductions in vitamin D receptor (VDR) that limit vitamin D suppression of SHPT. Since anti-EGFR therapy is not an option in human SHPT, we evaluated ADAM17 as a therapeutic target to suppress parathyroid hyperplasia because ADAM17 is required to release mature TGFα, the most potent EGFR-activating ligand.

METHODS

Computer analysis of the ADAM17 promoter identified TGFα and C/EBPβ as potential regulators of the ADAM17 gene. Their regulation of ADAM17 expression, TGFα/EGFR-driven growth and parathyroid gland (PTG) enlargement were assessed in promoter-reporter assays in A431 cells and corroborated in rat and human SHPT, using erlotinib as anti-EGFR therapy to suppress TGFα signals, active vitamin D to induce C/EBPβ or the combination.

RESULTS

While TGFα induced ADAM17-promoter activity by 2.2-fold exacerbating TGFα/EGFR-driven growth, ectopic C/EBPβ expression completely prevented this vicious synergy. Accordingly, in advanced human SHPT, parathyroid ADAM17 levels correlated directly with TGFα and inversely with C/EBPβ. Furthermore, combined erlotinib + calcitriol treatment suppressed TGFα/EGFR-cell growth and PTG enlargement more potently than erlotinib in part through calcitriol induction of C/EBPβ to inhibit ADAM17-promoter activity, mRNA and protein. Importantly, in rat SHPT, the correction of vitamin D deficiency effectively reversed the resistance to paricalcitol induction of C/EBPβ to suppress ADAM17 expression and PTG enlargement, reducing PTH by 50%.

CONCLUSION

In SHPT, correction of vitamin D and calcitriol deficiency induces parathyroid C/EBPβ to efficaciously attenuate the severe ADAM17/TGFα synergy, which drives PTG enlargement and high PTH.

Phosphate balance in ESRD: diet, dialysis and binders against the low evident masked pool

Phosphate metabolism is crucial in the pathophysiology of secondary hyperparathyroidism and vascular calcification. High phosphate levels have been consistently associated with unfavorable outcomes in dialysis patients, but several limitations are still hampering a resolutive definition of the optimal targets of phosphate serum levels to be achieved in this cohort. Nonetheless, hyperphosphatemia is a late marker of phosphate overload in humans. Clinical nephrologists routinely counteract the positive phosphate balance in dialysis patients through nutritional counseling, stronger phosphate removal by dialysis and prescription of phosphate binders. However, the superiority against placebo of phosphate control by diet, dialysis or binders in terms of survival has never been tested in dedicated randomized controlled trials. The present review discusses this conundrum with particular emphasis on the rationale supporting the value of a simultaneous intervention against phosphate overload in dialysis patients via the improvement of dietary intakes, dialysis efficiency and an individualized choice of phosphate binders.

Cinacalcet HCl prevents development of parathyroid gland hyperplasia and reverses established parathyroid gland hyperplasia in a rodent model of CKD

BACKGROUND

Secondary hyperparathyroidism (sHPT) represents an adaptive response to progressively impaired control of calcium, phosphorus and vitamin D in chronic kidney disease (CKD). It is characterized by parathyroid hyperplasia and excessive synthesis and secretion of parathyroid hormone (PTH). Parathyroid hyperplasia in uremic rats can be prevented by calcium-sensing receptor (CaSR) activation with the calcimimetic cinacalcet (Sensipar®/Mimpara®); however, it is unknown, how long the effects of cinacalcet persist after withdrawal of treatment or if cinacalcet is efficacious in uremic rats with established sHPT.

METHODS

We sought to determine the effect of cinacalcet discontinuation in uremic rats and whether cinacalcet was capable of influencing parathyroid hyperplasia in animals with established sHPT.

RESULTS

Discontinuation of cinacalcet resulted in reversal of the beneficial effects on serum PTH and parathyroid hyperplasia. In rats with established sHPT, cinacalcet decreased serum PTH and mediated regression of parathyroid hyperplasia. The cinacalcet-mediated decrease in parathyroid gland size was accompanied by increased expression of the cyclin-dependent kinase inhibitor p21. Prevention of cellular proliferation with cinacalcet occurred despite increased serum phosphorus and decreased serum calcium.

CONCLUSIONS

The animal data provided suggest established parathyroid hyperplasia can be reversed by modulating CaSR activity with cinacalcet and that continued treatment may be necessary to maintain reductions in PTH.

The intact nephron hypothesis: the concept and its implications for phosphate management in CKD-related mineral and bone disorder

Mechanistic understanding of secondary hyperparathyroidism, vascular calcification, and regulation of phosphate metabolism in chronic kidney disease (CKD) has advanced significantly in the past five decades. In 1960, Bricker developed the 'intact nephron hypothesis', opening the door for hundreds of investigations. He emphasized that 'as the number of functioning nephrons decreases, each remaining nephron must perform a greater fraction of total renal excretion'. Phosphate per se, independent of Ca²+ and calcitriol, directly affects the development of parathyroid gland hyperplasia and secondary hyperparathyroidism. Vitamin D receptor, Ca²+ sensing receptor, and Klotho-fibroblast growth factor (FGF) receptor-1 complex are all significantly decreased in the parathyroid glands of patients with CKD. Duodenal instillation of phosphate rapidly decreases parathyroid hormone release without changes in calcium or calcitriol. The same procedure also rapidly increases renal phosphate excretion independently of FGF-23, suggesting the possibility of an 'intestinal phosphatonin'. These observations suggest a possible 'phosphate sensor' in the parathyroid glands and gastrointestinal tract, although as yet there is no proof for the existence of such a sensor. Evidence shows that phosphate has a key role in parathyroid hyperplasia by activating the transforming growth factor-α-epidermal growth factor receptor complex. Thus, control of serum phosphorus early in the course of CKD will significantly ameliorate the pathological manifestations observed during progressive deterioration of renal function.

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.

Development and progression of secondary hyperparathyroidism in chronic kidney disease: lessons from molecular genetics

The identification of the calcium-sensing receptor (CaSR) and the clarification of its role as the major regulator of parathyroid gland function have important implications for understanding the pathogenesis and evolution of secondary hyperthyroidism in chronic kidney disease (CKD). Signaling through the CaSR has direct effects on three discrete components of parathyroid gland function, which include parathyroid hormone (PTH) secretion, PTH synthesis, and parathyroid gland hyperplasia. Disturbances in calcium and vitamin D metabolism that arise owing to CKD diminish the level of activation of the CaSR, leading to increases in PTH secretion, PTH synthesis, and parathyroid gland hyperplasia. Each represents a physiological adaptive response by the parathyroid glands to maintain plasma calcium homeostasis. Studies of genetically modified mice indicate that signal transduction via the CaSR is a key determinant of parathyroid cell proliferation and parathyroid gland hyperplasia. Because enlargement of the parathyroid glands has important implications for disease progression and disease severity, it is possible that clinical management strategies that maintain adequate calcium-dependent signaling through the CaSR will ultimately prove useful in diminishing parathyroid gland hyperplasia and in modifying disease progression.

Activator protein 2alpha mediates parathyroid TGF-alpha self-induction in secondary hyperparathyroidism

In secondary hyperparathyroidism, enhanced expression of TGF-alpha in the parathyroid leads to its own upregulation, generating a feed-forward loop for TGF-alpha activation of its receptor, EGFR receptor (EGFR), which promotes parathyroid hyperplasia. These studies examined the role of activator protein 2alpha (AP2), an inducer of TGF-alpha gene transcription, in the upregulation of parathyroid TGF-alpha in secondary hyperparathyroidism. In rat and human secondary hyperparathyroidism, parathyroid AP2 expression strongly correlated with TGF-alpha levels and with the rate of parathyroid growth, as expected. Furthermore, the increases in rat parathyroid content of AP2 and its binding to a consensus AP2 DNA sequence preceded the increase in TGF-alpha induced by high dietary phosphate. More significant, in A431 cells, which provide a model of enhanced TGF-alpha and TGF-alpha self-induction, mutating the core AP2 site of the human TGF-alpha promoter markedly impaired promoter activity induced by endogenous or exogenous TGF-alpha. Important for therapy, in five-sixths nephrectomized rats fed high-phosphate diets, inhibition of parathyroid TGF-alpha self-induction using erlotinib, a highly specific inhibitor of TGF-alpha/EGFR-driven signals, reduced AP2 expression dosage dependently. This suggests that the increases in parathyroid AP2 occur downstream of EGFR activation by TGF-alpha and are required for TGF-alpha self-induction. Indeed, in A431 cells, erlotinib inhibition of TGF-alpha self-induction caused parallel reductions in AP2 expression and nuclear localization, as well as TGF-alpha mRNA and protein levels. In summary, increased AP2 expression and transcriptional activity at the TGF-alpha promoter determine the severity of the hyperplasia driven by parathyroid TGF-alpha self-upregulation in secondary hyperparathyroidism.

EGFR activation increases parathyroid hyperplasia and calcitriol resistance in kidney disease

Calcitriol, acting through vitamin D receptors (VDR) in the parathyroid, suppresses parathyroid hormone synthesis and cell proliferation. In secondary hyperparathyroidism (SH), VDR content is reduced as hyperplasia becomes more severe, limiting the efficacy of calcitriol. In a rat model of SH, activation of the EGF receptor (EGFR) by TGF-alpha is required for the development of parathyroid hyperplasia, but the relationship between EGFR activation and reduced VDR content is unknown. With the use of the same rat model, it was found that pharmacologic inhibition of EGFR activation with erlotinib prevented the upregulation of parathyroid TGF-alpha, the progression of growth, and the reduction of VDR. Increased TGF-alpha/EGFR activation induced the synthesis of liver-enriched inhibitory protein, a potent mitogen and the dominant negative isoform of the transcription factor CCAAT enhancer binding protein-beta, in human hyperplastic parathyroid glands and in the human epidermoid carcinoma cell line A431, which mimics hyperplastic parathyroid cells. Increases in liver-enriched inhibitory protein directly correlated with proliferating activity and, in A431 cells, reduced VDR expression by antagonizing CCAAT enhancer binding protein-beta transactivation of the VDR gene. Similarly, in nodular hyperplasia, which is the most severe form of SH and the most resistant to calcitriol therapy, higher TGF-alpha activation of the EGFR was associated with an 80% reduction in VDR mRNA levels. Thus, in SH, EGFR activation is the cause of both hyperplastic growth and VDR reduction and therefore influences the efficacy of therapy with calcitriol.

The role of the calcium-sensing receptor in the pathophysiology of secondary hyperparathyroidism

The calcium-sensing receptor (CaR), a seven-transmembrane domain receptor belonging to the G protein-coupled receptor family, is responsible for calcium-mediated signalling initiated at the surface of parathyroid cells that controls the synthesis and secretion of parathyroid hormone (PTH). Expression of the CaR is downregulated in animal models of uraemia and in patients with secondary hyperparathyroidism (SHPT). Cinacalcet is a type II calcimimetic agent that acts as an allosteric modulator of CaR signalling. It has been shown in clinical studies to improve control of serum PTH levels and in preclinical studies to attenuate SHPT disease progression and parathyroid hyperplasia. Cinacalcet represents the first of this novel class of agents and a major advance in the treatment of SHPT.

Direct injection of calcitriol or its analog improves abnormal gene expression in the hyperplastic parathyroid gland in uremia

AIMS

In this study, we investigated the effects of direct injection (DI) of calcitriol or maxacalcitol into the hyperplastic parathyroid gland (PTG) on altered gene expression related to the advanced status of secondary hyperparathyroidism (SHPT).

METHODS

Sprague-Dawley rats were 5/6-nephrectomized (uremic) or sham-operated (normal). In each uremic rat, one of the bilateral PTG was treated by DI of calcitriol (PTG(CAL)) or maxacalcitol (PTG(OCT)), and the other gland was treated with control solution (PTG(CONT)). The PTG were evaluated for levels of expression of various mRNA and immunohistochemical staining of proliferating cell nuclear antigen (PCNA).

RESULTS

Significant differences in levels of expression of mRNA and PCNA were confirmed between the uremic and normal groups. In PTG(CAL) and PTG(OCT), expressions of almost all mRNA and PCNA were significantly improved; both agents were able to normalize the abnormalities of the uremic PTG, in contrast to the baseline and individual PTG(CONT). However, the difference in effect between PTG(CAL) and PTG(OCT) was only small.

CONCLUSION

Our results suggest that very high concentrations of calcitriol or maxacalcitol in the PTG improve abnormal gene expression and proliferation activity of parathyroid cells, and might explain the better control of SHPT using the DI technique.

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.

Influence of parathyroid mass on the regulation of PTH secretion

In advanced uremia, parathyroid hormone (PTH) levels should be controlled at a moderately elevated level in order to promote normal bone turnover. As such, a certain degree of parathyroid gland (PG) hyperplasia has to be accepted. No convincing evidence of apoptosis or of involution of PG hyperplasia exists. However, even considerable parathyroid hyperplasia can be controlled when the functional demand for increased PTH levels is abolished. When 20 isogenic PG were implanted into one parathyroidectomized (PTX) rat normalization of Ca(2+) and PTH levels and normal suppressibility of PTH secretion by high Ca(2+) was obtained. Similarly, normal levels of Ca(2+) and PTH and suppressibility of PTH secretion were obtained when Eight isogenic PG from uremic rats were implanted into normal rats or when long-term uremia and severe secondary hyperparathyroidism (sec. HPT) was reversed by an isogenic kidney transplantation. Normalization of PTH levels after experimental kidney transplantation took place despite a persistent decrease of vitamin D receptor (VDR) mRNA and calcium sensing receptor (CaR) mRNA in PG. Thus, in experimental models PTH levels are determined by the functional demand and not by parathyroid mass, per se. When non-suppressible sec. HPT is present in patients referred to PTX, nodular hyperplasia with differences in gene expression between different nodules has been observed in most cases. An altered expression of some autocrine/paracrine factors has been demonstrated in the nodules. Enhanced expression of PTH-related peptide (PTHrP) has been demonstrated in PG from patients with severe secondary HPT. PTHrP has been shown to stimulate PTH secretion in vivo and in vitro. PTH/PTHrP receptor was demonstrated in the parathyroids. The low Ca(2+) stimulated PTH secretion was enhanced by 300% by PTHrP 1-40. The altered quality of the parathyroid mass and not only the increased parathyroid mass, per se, might be responsible for non-controllable hyperparathyroidism in uremia and after kidney transplantation.

Pathogenic mechanisms for parathyroid hyperplasia

Parathyroid hyperplasia is the cause of parathyroid gland enlargement in kidney disease (KD). Hypocalcemia, hyperphosphatemia, and vitamin D deficiency are critical contributors to the worsening of the hyperplastic parathyroid growth induced by KD. Reproduction of the features of human KD in the 5/6 nephrectomized rat model has shown that 80% of the mitogenic signals induced by KD in parathyroid cells that are aggravated by either high phosphate (P) or low calcium (Ca) diets occurred within 5 days after the onset of KD. Enhanced parathyroid expression of the potent growth promoter transforming growth factor alpha (TGFalpha) and its receptor, the epidermal growth factor receptor (EGFR), was identified as the main cause of parathyroid hyperplasia in experimental KD. Indeed, administration of highly specific EGFR-tyrosine kinase inhibitors (TKI), which block downstream signaling from TGFalpha-activated EGFR, completely prevented high P- and low Ca-induced parathyroid hyperplasia in early KD, as well as the severe progression of high P-induced parathyroid growth in established secondary hyperparathyroidism, the latter characterized by marked TGFalpha and EGFR overexpression in the parathyroid glands. More importantly, the suppression of signals downstream from TGFalpha binding to EGFR with EGFR-TKI treatment also revealed that TGFalpha self-upregulation in the parathyroid glands is the main determinant of the severity of the hyperplastic growth, and that enhanced TGFalpha activation of EGFR mediates the reduction in parathyroid vitamin D receptor levels thereby causing resistance to both the antiproliferative and parathyroid hormone-suppressive properties of calcitriol therapy.

Persistent hyperparathyroidism in renal allograft recipients: vitamin D receptor, calcium-sensing receptor, and apoptosis

The phenotypic changes in parathyroid cells after successful renal transplantation remain to be elucidated. We compared 10 diffuse and 11 nodular hyperplastic parathyroid glands from five renal allograft recipients with persistent hyperparathyroidism, with five diffuse and 13 nodular hyperplasia from seven uremic patients on hemodialysis, and 13 normal glands. Comparisons included expressions of both vitamin D receptor (VDR) and calcium-sensing receptor (CaSR), proliferative activity (Ki67), and apoptosis (TUNEL). Immunoreactivity was assessed semiquantitatively and expressed as labeling index. The area/cell was also measured to assess cellular hypertrophy. The labeling indexes of VDR (587+/-71; mean+/-s.e.m.) and CaSR (45.0+/-2.8) in recipients' diffuse hyperplasia were significantly higher than those in uremic diffuse hyperplasia (224+/-44, 29.3+/-2.3, respectively) (P<0.01, each). However, these expressions remained low in recipients' nodular hyperplasia (42+/-8, 11.8+/-1.4, respectively). Ki67 labeling index in recipients' nodular hyperplasia (7+/-1) was significantly smaller than in uremic patients (24+/-6, P<0.01). TUNEL labeling index in recipients' diffuse hyperplasia (30+/-5) was the highest among the groups. The cell volume tended to be smaller in both patterns of hyperplasia in allograft recipients compared with uremic patients. Our results suggest that the phenotypic change in parathyroid cells after renal transplantation depends on the pattern of hyperplasia, where it is normalized only in diffuse hyperplastic glands in which the number of cells also regresses with significant induction of apoptosis.

Basic Science and Dialysis: Parathyroid Growth and Suppression in Renal Failure

In advanced uremia, parathyroid hormone (PTH) levels should be controlled at a moderately elevated level in order to promote normal bone turnover. As such, a certain degree of parathyroid hyperplasia has to be accepted. Uremia is associated with parathyroid growth. In experimental studies, proliferation of the parathyroid cells is induced by uremia and further promoted by hypocalcemia, phosphorus retention, and vitamin D deficiency. On the other hand, parathyroid cell proliferation might be arrested by treatment with a low-phosphate diet, vitamin D analogs, or calcimimetics. When established, parathyroid hyperplasia is poorly reversible. There exists no convincing evidence of programmed parathyroid cell death or apoptosis in hyperplastic parathyroid tissue or of involution of parathyroid hyperplasia. However, even considerable parathyroid hyperplasia can be controlled when the functional demand for increased PTH levels is removed by normalization of kidney function. Today, secondary hyperparathyroidism can be controlled in patients with long-term uremia in whom considerable parathyroid hyperplasia is to be expected. PTH levels can be suppressed in most uremic patients and this suppression can be maintained by continuous treatment with phosphate binders, vitamin D analogs, or calcimimetics. Thus modern therapy permits controlled development of parathyroid growth. When nonsuppressible secondary hyperparathyroidism is present, nodular hyperplasia with suppressed expression of the calcium-sensing receptor (CaR) and vitamin D receptor (VDR) has been found in most cases. An altered expression of some autocrine/paracrine factors has been demonstrated in the nodules. The altered quality of the parathyroid mass, and not only the increased parathyroid mass per se, might be responsible for uncontrollable hyperparathyroidism in uremia and after kidney transplantation.

Parathyroid growth and regression in experimental uremia

Early 1,25-dihydroxyvitamin D(3) (VD(3)) therapy during the course of renal failure prevents the downregulation of VD(3) receptor (VDR), calcium-sensing receptor (CaSR) or p21, and the parathyroid (PT) growth. We hypothesized that VD(3) could restore the decreased expressions of VDR and CaSR, and cause regression in enlarged PT glands. 5/6 nephrectomized rats fed high-phosphorus diet were killed at 1, 3, 5, or 7 days and at 2, 3, 4, 8, or 12 weeks. VD(3)-treated rats were given VD(3) intraperitoneally for 1, 2, 3, or 4 weeks, starting 8 weeks after 5/6 nephrectomy. PT glands were weighed and subjected to immunohistochemical analyses for VDR, CaSR, p21, Ki67, and Tdt-mediated dUTP nick end-labeling (TUNEL) assay. The area per cell was measured as the parameter of cell size. The expression of VDR and p21 began to decrease at day 1, and Ki67 increased at day 3, but decreased thereafter. There was a significant increase in PT gland weight to week 12 with the increase of cell size. VD(3) treatment significantly increased both VDR and CaSR expressions 2 weeks after the start of injection, and reduced the PT gland weight at week 3 with significant increase of TUNEL-positive cells and decrease of cell size. Our results suggest that PT growth in uremic rats involves both PT cell proliferation and hypertrophy, in association with the reduction of VDR, CaSR, and p21 expressions. In addition, VD(3) treatment could reverse PT hyperplasia and hypertrophy via restoration of these proteins.

Effect of endothelin receptor antagonist on parathyroid gland growth, PTH values and cell proliferation in azotemic rats

BACKGROUND

A variety of stimuli are involved in the pathogenesis of parathyroid gland hyperplasia in renal failure. Recently, it was shown that blocking the signal from the endothelin-1 (ET-1) receptor (ET(A)R/ET(B)R) by a non-selective receptor antagonist, bosentan, reduced parathyroid cell proliferation, parathyroid gland hyperplasia and parathyroid hormone (PTH) levels in normal rats on a calcium deficient diet. Our goal was to determine whether in 5/6 nephrectomized (NPX) rats with developing or established hyperparathyroidism, the endothelin receptor blocker, bosentan, reduced the increase in parathyroid cell proliferation, parathyroid gland hyperplasia and PTH values.

METHODS

High (HPD, 1.2%) or normal phosphorus diets (PD) (NPD, 0.6%) were given to 5/6 NPX rats for 15 days (NPX(15)). In each dietary group, one-half the rats were given bosentan (B) i.p. 100 mg/kg/day. The four groups of rats were: (1) NPX(15)-1.2% P; (2) NPX(15)-1.2% P+B; (3) NPX(15)-0.6% P; and (4) NPX(15)-0.6% P+B. In a second study in which hyperparathyroidism was already established in 5/6 NPX rats fed a HPD for 15 days, rats were divided into two groups in which one group was maintained on a HPD and the other group was changed to very low PD (VLPD, <0.05%) for an additional 15 days. In each dietary group, one-half the rats were given bosentan i.p. 100 mg/kg-day. The four groups of rats were: (1) NPX(30)-1.2% P; (2) NPX(30)-1.2% P+B; (3) NPX(30)-0.05% P and (4) NPX(30)-0.05% P+B. Parathyroid cell proliferation was measured by proliferating cell nuclear antigen (PCNA) staining and ET-1 expression by immunohistochemical techniques.

RESULTS

In the study of developing hyperparathyroidism, bosentan reduced ET-1 expression in the parathyroid glands of rats on the NPD and HPD (P<0.05). But only in rats on the NPD did bosentan result in a reduced increase in parathyroid gland weight (P<0.05). In the study of established hyperparathyroidism, in which 5/6 NPX rats were given a HPD for 15 days, bosentan started on day 15 reduced (P<0.05) ET-1 expression in rats maintained for 15 additional days on the HPD or the VLPD. On the VLPD, parathyroid gland weight was less (P<0.05) than that in rats on the HPD sacrificed at 15 or 30 days. Bosentan did not reduce parathyroid cell proliferation or parathyroid gland weight in rats maintained on the HPD or further reduce these parameters beyond that obtained with dietary phosphorus restriction. PTH values were lowest in the VLPD group, intermediate in the NPD group, and highest in the HPD group, but in none of the three groups did bosentan decrease PTH values.

CONCLUSIONS

In azotemic rats with developing hyperparathyroidism, bosentan resulted in a reduced increase in parathyroid gland weight when dietary phosphorus content was normal. Despite a reduction in ET-1 expression in rats on a HPD with developing or established hyperparathyroidism, bosentan did not reduce the increase in parathyroid cell proliferation, parathyroid gland growth or PTH values. Thus, ET-1 blockade with bosentan did not prevent parathyroid gland growth in the azotemic rat.

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.

A critical role for enhanced TGF-α and EGFR expression in the initiation of parathyroid hyperplasia in experimental kidney disease

The parathyroid hyperplasia secondary to kidney disease is associated with enhanced expression of the growth promoter transforming growth factor-α (TGF-α). TGF-α stimulates growth through activation of its receptor, the epidermal growth factor receptor (EGFR), normally expressed in the parathyroid glands. Because enhanced coexpression of TGF-α and EGFR causes aggressive cellular growth, these studies utilized highly specific inhibitors of EGFR tyrosine kinase, a step mandatory for TGF-α-induced EGFR activation, to assess the contribution of growth signals from enhanced expression of TGF-α exclusively or both TGF-α and EGFR to the rapid parathyroid growth induced by kidney disease and exacerbated by high-phosphorus (P) and low-calcium (Ca) diets in rats. The enhancement in parathyroid gland weight and proliferating activity (proliferating cell nuclear antigen/Ki67) induced by kidney disease and aggravated by either high P or low Ca intake, within the first week after 5/6 nephrectomy, in rats, coincided with simultaneous increases (2- to 3-fold) in TGF-α and EGFR content. Conversely, prevention of the increases in both TGF-α and EGFR paralleled the efficacy of either P restriction or high-Ca intake in ameliorating uremia-induced parathyroid hyperplasia. More importantly, suppression of TGF-α/EGFR signaling, through prophylactic administration of potent and highly selective inhibitors of ligand-induced EGFR activation, completely prevented both high-P- and low-Ca-induced parathyroid hyperplasia as well as TGF-α self-upregulation. Thus enhanced parathyroid TGF-α/EGFR expression, self-upregulation, and growth signals occur early in kidney disease, are aggravated by low-Ca and high-P intake, and constitute the main pathogenic mechanism of the severity of parathyroid hyperplasia.

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.

Calcium and phosphorus metabolism in patients who have chronic kidney disease

Disturbances in calcium and phosphorus metabolism are almost invariable consequences of chronic kidney disease (CKD). Because the capacity to regulate calcium and phosphorus metabolism becomes compromised progressively as kidney function declines, calcium and phosphorus homeostasis is disrupted and serum calcium or phosphorus levels are perturbed in many patients with CKD. The level of interest in, and concerns about, abnormalities in calcium and phosphorus metabolism among patients with CKD has increased substantially in recent years. Strategies for clinical management are being revised, and recent recommendations differ substantially from those used previously with a renewed emphasis on safety.

Consequences of hyperphosphatemia in patients with end-stage renal disease (ESRD)

Hyperphosphatemia is invariably present among patients with end-stage renal disease (ESRD) and is becoming an increasingly important clinical entity. Despite concerted efforts by patients, dietitians, and nephrologists to control serum phosphorus, a recent study by Block et al found that more than 60% of patients on hemodialysis in the United States have serum phosphorus levels above the recommended goal of 5.5 mg/dL. Historically, nephrologists have been concerned about the central role of elevated serum phosphorus in the pathogenesis of secondary hyperparathyroidism and extraosseous calcification. However, the consequences of untreated hyperphosphatemia have assumed more importance in the last few years, largely due to recent clinical studies that revealed a more sinister role of elevated serum phosphorus in increasing the mortality risk among patients with ESRD. Hemodialysis patients with serum phosphorus greater than 6.5 mg/dL were reported to have a 27% higher mortality risk than patients with serum phosphorus between 2.4 and 6.5 mg/dL. The pathophysiologic mechanisms by which persistent hyperphosphatemia enhances the mortality risk in dialysis patients are not yet completely understood. However, given that inadequate control of serum phosphorus contributes to elevated calcium-phosphorus product (Ca x P), untreated hyperphosphatemia may play a key role in cardiovascular calcification. In response to these findings, the National Kidney Foundation Kidney Disease Outcome Quality Initiative (K/DOQI) Clinical Practice Guidelines for Bone Metabolism and Disease in Chronic Kidney Disease have recently recommended more stringent levels for controlling serum phosphorus and Ca x P product in order to improve patients' quality of life and longevity.

The calcium-sensing receptor: a key factor in the pathogenesis of secondary hyperparathyroidism

Serum calcium levels are regulated by the action of parathyroid hormone (PTH). Major drivers of PTH hypersecretion and parathyroid cell proliferation are the hypocalcemia and hyperphosphatemia that develop in chronic kidney disease patients with secondary hyperparathyroidism (SHPT) as a result of low calcitriol levels and decreased kidney function. Increased PTH production in response to systemic hypocalcemia is mediated by the calcium-sensing receptor (CaR). Furthermore, as SHPT progresses, reduced expression of CaRs and vitamin D receptors (VDRs) in hyperplastic parathyroid glands may limit the ability of calcium and calcitriol to regulate PTH secretion. Current treatment for SHPT includes the administration of vitamin D sterols and phosphate binders. Treatment with vitamin D is initially effective, but efficacy often wanes with further disease progression. The actions of vitamin D sterols are undermined by reduced expression of VDRs in the parathyroid gland. Furthermore, the calcemic and phosphatemic actions of vitamin D mean that it has the potential to exacerbate abnormal mineral metabolism, resulting in the formation of vascular calcifications. Effective new treatments for SHPT that have a positive impact on mineral metabolism are clearly needed. Recent research shows that drugs that selectively target the CaR, calcimimetics, have the potential to meet these requirements.

Proliferation in hyperplastic human and normal rat parathyroid glands: role of phosphate, calcitriol, and gender

BACKGROUND

Parathyroid gland hyperplasia develops in azotemic patients. A phosphate excess and calcitriol deficiency play critical roles in its development. Our goals were to determine whether differences in serum phosphate values at parathyroidectomy (PTX) in hemodialysis patients with refractory hyperparathyroidism: (1) correlated with parathyroid cell proliferation; and (2) affected the antiproliferative response to in vitro calcitriol. Studies were also performed to determine whether the phosphate concentration in the medium affected the antiproliferative response to calcitriol, and whether a high phosphate diet and calcitriol treatment affected parathyroid cell proliferation and parathyroid hormone (PTH) levels in normal rats.

METHODS

Forty-seven parathyroid glands from 19 hemodialysis patients were obtained at PTX. Flow cytometry was used to determine cell proliferation (percent cells in S phase) in excised parathyroid glands. Similarly, cell proliferation was determined in parathyroid tissue incubated for 24 hours in medium with or without 10(-7) mol/L calcitriol and with 1 or 4 mmol/L phosphate. In normal rats, the effect of 3 days of a high phosphate diet (1.2% P) and calcitriol treatment (100 pmol/kg) on PTH values and cell proliferation was evaluated.

RESULTS

In cells from freshly removed parathyroid glands obtained at PTX from hemodialysis patients, there were no significant correlations between the percent cells in S phase and age, gender, and serum phosphate, calcium, and PTH. While incubation of parathyroid tissue with 10(-7) mol/L calcitriol did reduce cell proliferation (P < 0.001), both the pre-PTX serum phosphate value (P= 0.003) and female gender (P=0.003) were associated with a decreased response to calcitriol. Incubation of parathyroid tissue in medium containing 4 mmol/L phosphate did not increase cell proliferation. In normal rats, a high phosphate diet for 3 days increased cell proliferation (P < 0.05) and PTH levels (P < 0.05), and calcitriol treatment was without effect.

CONCLUSION

Our findings suggest a high phosphate burden, as well as female gender, favor parathyroid cell proliferation and both may reduce the inhibition of parathyroid function by calcitriol.

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.

Pathogenesis and treatment of renal osteodystrophy

Renal osteodystrophy is the term used to describe the many different patterns of the skeletal abnormalities that occur in patients with chronic kidney disease. The main two conditions are osteitis fibrosa, characterized by high bone turnover, increased osteoclastic and osteoblastic activity, and high levels of circulating parathyroid hormone (PTH) and adynamic bone disease characterized by low bone turnover and low levels of circulating PTH. Retention of phosphorus, decreased levels of calcitriol in blood, decreased levels of serum ionized calcium, reduced numbers of vitamin D receptors and calcium sensors in the parathyroid gland, and skeletal resistance to the calcemic action of PTH play a major role in the development of renal osteodystrophy. This review will describe the current approach for the treatment of renal osteodystrophy.