Parathyroid cell proliferation in normal and chronic renal failure rats. The effects of calcium, phosphate, and vitamin D

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.

Hyperphosphatemia of chronic kidney disease

Observational studies have determined hyperphosphatemia to be a cardiovascular risk factor in chronic kidney disease. Mechanistic studies have elucidated that hyperphosphatemia is a direct stimulus to vascular calcification, which is one cause of morbid cardiovascular events contributing to the excess mortality of chronic kidney disease. This review describes the pathobiology of hyperphosphatemia that develops as a consequence of positive phosphate balance in chronic kidney disease and the mechanisms by which hyperphosphatemia acts on neointimal vascular cells that are stimulated to mineralize in chronic kidney disease. The characterization of hyperphosphatemia of chronic kidney disease as a distinct syndrome in clinical medicine with unique disordered skeletal remodeling, heterotopic mineralization and cardiovascular morbidity is presented.

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.

Parathyroid hormone is inversely related to endothelin-1 in patients on haemodialysis

AIM

Parathyroid hormone secretion is mainly influenced by hypocalcaemia, hyperphosphataemia and vitamin D deficiency. However, previous in vitro and in vivo studies showed that endothelin-1 can influence parathyroid hormone secretion. This study was aimed at evaluating this relationship in vivo in uraemic patients.

METHODS

Parathyroid hormone and endothelin-1 plasma concentrations were measured in 67 haemodialysed patients. Patients with history of cardiovascular diseases and those with parathyroid adenoma were excluded.

RESULTS

Plasma levels of endothelin-1 were found to be inversely related to those of parathyroid hormone (P < 0.04) The multiple regression analysis, carried out considering parathyroid hormone as a dependent variable, and including age, sex, blood pressure, calcium x phosphorus product, and endothelin-1, demonstrated that the independent correlates of parathyroid hormone were endothelin-1 (beta = -0.276; P = 0.015), and calcium x phosphorus product (beta = 0.417; P < 0.0001).

CONCLUSION

For the first time in vivo, we demonstrated an inverse independent relationship between endothelin-1 and parathyroid hormone in haemodialysed patients. Because both endothelin-1 and parathyroid hormone are endowed with well-known harmful actions on cardiovascular apparatus, whether such inverse relation may really influence the natural history of cardiovascular damage due to secondary hyperparathyroidism remains to be elucidated.

Role of dietary phosphorus and degree of uremia in the development of renal bone disease in rats

The remnant kidney rat model has been extensively used for the evaluation of bone changes due to uremia. The present study aimed to assess the effect of the dietary phosphorus availability and of the severity of renal failure on bone histomorphometric changes and various biochemical markers over time in this model. Chronic renal failure (CRF) was induced in male Wistar rats by 5/6th nephrectomy. Half of the number of animals received a standard rat diet (STD) (0.67% P, containing low bioavailable phosphorus of plant origin); the other animals were fed a high phosphorus diet (HPD) (0.93% P, containing inorganic phosphorus with high bioavailability). Every two weeks, blood and urine samples were collected. At sacrifice after 6 or 12 weeks, bone samples were taken for the measurement of histological and histodynamic parameters. Serum creatinine measurements indicated the development of mild to moderate renal failure in both diet groups. Phosphaturia was unexpectedly low in all animals that received the STD, indicating relative phosphorus depletion despite the normal dietary phosphorus content. In the HPD CRF group, a decrease in calcemia and a rise in phosphatemia were seen after 12 weeks of CRF, which were more pronounced in animals with higher serum creatinine. Serum iPTH levels were distinctly increased in CRF rats fed a HPD, especially those with more pronounced renal failure. Serum osteocalcin and to a lesser extend tartrate-resistant acid phosphatase and urinary pyridinoline and deoxypyridinoline crosslinks were higher in the CRF animals compared to the shams, particularly in the animals of the HPD group with more pronounced CRF. In both diet groups, the CRF animals had significantly higher amounts of osteoid compared to shams. Only the animals that received a HPD developed distinct histological signs of secondary hyperparathyroidism (sHPTH), that is, an increased bone formation rate, mineral apposition rate, osteoblast perimeter, and eroded perimeter. Again, this effect was most prominent in rats with more severe CRF. In conclusion, data of the present study indicate that in experimental studies using the remnant kidney rat model, both the dietary phosphorus bioavailability and the degree of renal failure in the development of hyperparathyroidism should be considered.

Mineral metabolism and bone abnormalities in children with chronic renal failure

Abnormalities in mineral metabolism and changes in skeletal histology may contribute to growth impairment in children with chronic renal failure. Hyperphosphatemia, hypocalcemia, metabolic acidosis, alterations in vitamin D and IGF synthesis and parathyroid gland dysfunction play significant roles in the development of secondary hyperparathyroidism and subsequently, bone disease in renal failure. The recent KDIGO conference has made recommendations to consider this as a systemic disorder (chronic kidney disease-mineral bone disorder) and to standardize bone histomorphometry to include bone turnover, mineralization and volume (TMV). The use of DXA to assess bone mass is controversial in children with chronic renal failure. Questions arise regarding the accuracy of bone measurements and difficulty in data interpretation especially in children with renal failure who are not only growth retarded but often have pubertal delay and osteosclerosis. The validity and feasibility of new modalities of skeletal imaging which can detect changes in both trabecular and cortical bone are currently being investigated in children. The management of mineral abnormalities and bone disease in chronic renal failure is multifactorial. To manage hyperphosphatemia, dietary phosphate restriction accompanied by intake of calcium-free and metal-free phosphate binding agents are widely utilized. Vitamin D analogs remain the primary therapy for secondary hyperparathyroidism, although the use of the less hypercalcemic agents is preferred due to concerns of calciphylaxis and vascular calcification. Future clinical studies are needed to evaluate the long-term effects of calcimimetic agents and bisphosphonate therapy in children with chronic renal failure.

New strategies for the treatment of hyperparathyroidism incorporating calcimimetics

BACKGROUND

Hyperparathyroidism (HPT), characterised by increased parathyroid hormone (PTH) secretion and parathyroid hyperplasia, can be caused by physiologic defects in the parathyroid gland (primary HPT [PHPT]) or as a consequence of declining renal function (secondary HPT [SHPT]).

OBJECTIVE

To review the safety and efficacy of cinacalcet in the treatment of SHPT and PHPT.

METHODS

Studies indexed in NLM/PubMed investigating the safety, efficacy, and pharmacokinetics of cinacalcet for PHPT and SHPT and supporting preclinical evidence.

RESULTS/CONCLUSION

Recent evidence has demonstrated the efficacy of the calcimimetic cinacalcet in the treatment of PHPT and SHPT. Compared with traditional therapies such as vitamin D sterols and phosphate binders, cinacalcet treatment can allow an increased proportion of patients with SHPT to improve Kidney Disease Outcomes Quality Initiative (KDOQI) Bone Metabolism and Disease laboratory parameter target attainment. Recent evidence suggests that improvements in these biochemical parameters with cinacalcet can translate into improved morbidity and mortality. Cinacalcet lowers PTH and calcium in patients following renal transplantation, and also normalises serum calcium in patients with PHPT. Ongoing studies are focusing and future studies are likely to focus on the effect of cinacalcet on clinical outcomes and on novel strategies for the integration of cinacalcet with traditional therapies to improve serum PTH and mineral metabolism control.

The parathyroid is a target organ for FGF23 in rats

Phosphate homeostasis is maintained by a counterbalance between efflux from the kidney and influx from intestine and bone. FGF23 is a bone-derived phosphaturic hormone that acts on the kidney to increase phosphate excretion and suppress biosynthesis of vitamin D. FGF23 signals with highest efficacy through several FGF receptors (FGFRs) bound by the transmembrane protein Klotho as a coreceptor. Since most tissues express FGFR, expression of Klotho determines FGF23 target organs. Here we identify the parathyroid as a target organ for FGF23 in rats. We show that the parathyroid gland expressed Klotho and 2 FGFRs. The administration of recombinant FGF23 led to an increase in parathyroid Klotho levels. In addition, FGF23 activated the MAPK pathway in the parathyroid through ERK1/2 phosphorylation and increased early growth response 1 mRNA levels. Using both rats and in vitro rat parathyroid cultures, we show that FGF23 suppressed both parathyroid hormone (PTH) secretion and PTH gene expression. The FGF23-induced decrease in PTH secretion was prevented by a MAPK inhibitor. These data indicate that FGF23 acts directly on the parathyroid through the MAPK pathway to decrease serum PTH. This bone-parathyroid endocrine axis adds a new dimension to the understanding of mineral homeostasis.

Chronic kidney disease mineral and bone disorder in children

Childhood and adolescence are crucial times for the development of a healthy skeletal and cardiovascular system. Disordered mineral and bone metabolism accompany chronic kidney disease (CKD) and present significant obstacles to optimal bone strength, final adult height, and cardiovascular health. Decreased activity of renal 1 alpha hydroxylase results in decreased intestinal calcium absorption, increased serum parathyroid hormone levels, and high-turnover renal osteodystrophy, with subsequent growth failure. Simultaneously, phosphorus retention exacerbates secondary hyperparathyroidism, and elevated levels contribute to cardiovascular disease. Treatment of hyperphosphatemia and secondary hyperparathyroidism improves growth and high-turnover bone disease. However, target ranges for serum calcium, phosphorus, and parathyroid hormone (PTH) levels vary according to stage of CKD. Since over-treatment may result in adynamic bone disease, growth failure, hypercalcemia, and progression of cardiovascular calcifications, therapy must be carefully adjusted to maintain optimal serum biochemical parameters according to stage of CKD. Newer therapeutic agents, including calcium-free phosphate binding agents and new vitamin D analogues, effectively suppress serum PTH levels while limiting intestinal calcium absorption and may provide future therapeutic alternatives for children with CKD.

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.

Secondary hyperparathyroidism disease stabilization following calcimimetic therapy

Standard therapy for secondary hyperparathyroidism (SHPT) includes dietary calcium supplementation, active vitamin D, and phosphate binders; however, these are often insufficient to allow patients to achieve their serum parathyroid hormone (PTH), calcium and calcium-phosphorus product (Ca × P) targets. Recent preclinical studies have demonstrated that treatment with type II calcimimetics that increase the sensitivity of the calcium-sensing receptor (CaR) to calcium can reverse the alterations in CaR and vitamin D receptor expression and parathyroid cell proliferation that are associated with SHPT. These data suggest that calcimimetic treatment could stabilize disease progression and improve maintenance of treatment goals. In clinical trials involving SHPT patients, the calcimimetic cinacalcet has been shown to decrease PTH, calcium, phosphorus and Ca × P. Significant improvements were seen regardless of initial disease severity, and benefits were maintained over the course of long-term therapy (up to 4 years), indicating effective disease stabilization. In conclusion, preclinical and clinical data provide both theoretical and empirical support for the use of calcimimetics in moderate and advanced SHPT to effectively stabilize disease.

Vitamin D Deficiency and Secondary Hyperparathyroidism Among Patients with Chronic Kidney Disease

BACKGROUND

Chronic kidney disease (CKD) is associated with morbid complications that lead to high mortality and costs. Vitamin D deficiency and secondary hyperparathyroidism (SHPT) are frequent complications of CKD.

METHODS

We reviewed the current literature regarding the prevalence, diagnosis, complications, and management of vitamin D deficiency and SHPT among patients with CKD.

RESULTS

There is a high prevalence of vitamin D deficiency among adolescents and adults in the United States (age and gender dependent). Patients with CKD or those who are dialysis-dependent are much more likely to have low levels of vitamin D in comparison to those without kidney disease. In order to avoid significant complications including SHPT and musculoskelatal diseases, vitamin D needs to be measured routinely by primary care physicians and nephrologists. In the majority of CKD patients, SHPT is not diagnosed until late, leading to advanced cardiovascular and bone diseases.

CONCLUSIONS

In conclusion, current management of vitamin D deficiency and SHPT is suboptimal. Early diagnosis of vitamin D deficiency and SHPT are integral to optimal management of CKD, and additional research is needed in this area.

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.

Age-dependent alterations in Ca2+homeostasis: role of TRPV5 and TRPV6

Aging is associated with alterations in Ca2+homeostasis, which predisposes elder people to hyperparathyroidism and osteoporosis. Intestinal Ca2+absorption decreases with aging and, in particular, active transport of Ca2+by the duodenum. In addition, there are age-related changes in renal Ca2+handling. To examine age-related changes in expression of the renal and intestinal epithelial Ca2+channels, control (TRPV5+/+) and TRPV5 knockout (TRPV5−/−) mice aged 10, 30, and 52 wk were studied. Aging of TRPV5+/+mice resulted in a tendency toward increased renal Ca2+excretion and significantly decreased intestinal Ca2+absorption, which was accompanied by reduced expression of TRPV5 and TRPV6, respectively, despite increased serum 1,25(OH)2D3levels. Similarly, in TRPV5−/−mice the existing renal Ca2+loss was more pronounced in elder animals, whereas the compensatory intestinal Ca2+absorption and TRPV6 expression declined with aging. In both mice strains, aging resulted in a resistance to 1,25(OH)2D3and diminished renal vitamin D receptor mRNA levels, whereas serum Ca2+levels remained constant. Furthermore, 52-wk-old TRPV5−/−mice showed severe hyperparathyroidism, whereas PTH levels in elder TRPV5+/+mice remained normal. In 52-wk-old TRPV5−/−mice, serum osteocalcin levels were increased in accordance with the elevated PTH levels, suggesting an increased bone turnover in these mice. In conclusion, downregulation of TRPV5 and TRPV6 is likely involved in the impaired Ca2+(re)absorption during aging. Moreover, TRPV5−/−mice likely develop age-related hyperparathyroidism and osteoporotic characteristics before TRPV5+/+mice, demonstrating the importance of the epithelial Ca2+channels in Ca2+homeostasis.

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.

Direct maxacalcitol injection into hyperplastic parathyroids improves skeletal changes in secondary hyperparathyroidism

Direct maxacalcitol (OCT) injection into a parathyroid gland (PTG) ameliorates several important etiologic factors of resistance to medical treatments for secondary hyperparathyroidism (s-HPT): the upregulations of vitamin D receptor (VDR) and Ca-sensing receptor (CaSR) in PTGs and the regression of PTG hyperplasia by the induction of apoptosis. In this study, we evaluated the bone histomorphology on the basis of maintaining these effects in advanced s-HPT. Five/six nephrectomized Sprague-Dawley rats were fed a high-phosphorus and low-calcium diet for 8 weeks. These rats were divided into four treatment groups: (1) basic uremic (at the baseline), (2) direct OCT single injection into PTGs (DI-OCT) followed by OCT intravenous administration for 4 weeks (IV-OCT), (3) direct vehicle injection and IV-OCT, and (4) no treatment for an additional 4 weeks. The effects of these treatments on serum intact-parathyroid hormone (PTH) level, PTG weight, VDR and CaSR expression levels in PTGs, and bone histomorphometric parameters were investigated. In the DI-OCT+IV-OCT group, the significant decrease in serum intact-PTH level was maintained by the following IV-OCT. A significant decrease in PTG weight and the upregulations of VDR and CaSR expression levels in PTGs were also observed. Bone histomorphometric analysis showed significant improvements in osteitis fibrosa in both cancellous and cortical bones. However, these findings were not observed in the other groups. These results suggest that osteitis fibrosa caused by advanced s-HPT can be successfully reversed by a control of PTH at an appropriate level through the improvement of PTG hyperplasia as induced by DI-OCT+IV-OCT.

Development of renal bone disease

Renal osteodystrophy (ROD) develops as the early stages of chronic renal failure (CRF) and covers a spectrum of bone changes observed in the uraemic patient, which extend from high remodelling bone disease (frequently known as osteitis fibrosa) to low turnover, or adynamic disease. Between these two extremes there are also cases of bone mineralization compromised in variable degrees, as is the case of 'mixed bone disease' and osteomalacia. The dynamic process of bone remodelling is compromised in CRF, and a positive or negative bone balance can be observed in uraemic patients. In addition to the classic modulators of bone remodelling, like parathyroid hormone, calcitriol and calcitonin, other factors were recently identified as significant modulators of osteoblast and osteoclast activation in uraemic patients. In fact, different cytokines and growth factors, acting at an autocrine or paracrine level, seem to play a relevant role in the bone and mineral changes observed in uraemia. Recently, observations have been made of the development of more sensitive and specific techniques to assay different biochemical markers of bone turnover and mineral metabolism. Analogously, new contributions of conventional bone histology, bone immunocytochemistry and molecular biology, which enabled the understanding of some etiopathogenic mechanisms of ROD, were observed.

Direct injection of calcitriol or its analog into hyperplastic parathyroid glands induces apoptosis of parathyroid cells

Hyperplasia of the parathyroid gland (PTG) is associated not only with excessive secretion of parathyroid hormone (PTH) but also with changes in the parathyroid cell (PTC) characteristics (i.e. hyperproliferative activity, and low contents of vitamin D and calcium-sensing receptors). Control of PTG hyperplasia is most important in the management of secondary hyperparathyroidism, but the advanced stage of hyperplasia is considered irreversible. In the present study, dialysis patients with PTG hyperplasia underwent direct injection of calcitriol or maxacalcitol (OCT) into the PTG. Ultrasonography showed that this treatment had significantly reduced PTG volume and tissue analysis using the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) method and DNA electrophoresis indicated that cellular apoptosis had been induced. The mechanism of apoptosis was evaluated in detail in uremic rats fed a high-phosphate diet. OCT or its vehicle was directly injected into the rats' PTGs. In the PTGs treated by OCT, there was a significantly increased number of TUNEL-positive PTCs and DNA electrophoresis revealed the characteristic ladder pattern of DNA fragmentation, both findings indicative of apoptosis. There was also a significant upregulation of both vitamin D and Ca-sensing receptors in the PTCs and a clear shift of the Ca-PTH response curve to the left and downward. None of these findings was observed in the PTGs treated by vehicle. This novel treatment is successful in causing regression of PTG hyperplasia. Thus, it is expected to significantly reduce the PTH level and ameliorate the abnormal bone turnover and mineral metabolism.

Parathyroid Hormone 7-84 Induces Hypocalcemia and Inhibits the Parathyroid Hormone 1-84 Secretory Response to Hypocalcemia in Rats with Intact Parathyroid Glands

Biologic effects of large C-terminal parathyroid hormone (PTH) fragments, opposite to those of N-terminal PTH, have been demonstrated. C-terminal PTH fragments are co-secreted with N-terminal PTH from the parathyroids. The aim of our study was to examine whether C-terminal PTH 7-84 regulates secretion of PTH 1-84 and affects the expression of genes of relevance for parathyroid function, PTH, calcium-sensing receptor (CaR), PTH type 1 receptor (PTHR1), and PTH-related peptide (PTHrP) genes in rat parathyroid glands. PTH 7-84 induced a significant decrease in plasma Ca2+ in rats with intact parathyroid glands. Despite the reduction of plasma Ca2+, no stimulation of PTH 1-84 secretion took place. Furthermore, the PTH 1-84 secretory response to EGTA-induced acute and severe hypocalcemia was significantly inhibited by PTH 7-84. During recovery from hypocalcemia, plasma Ca2+ levels were significantly lower in the PTH 7-84-treated group, as compared with the vehicle group, and at the same time plasma PTH 1-84 levels were significantly suppressed. The expression of PTH, CaR, PTHR1, and PTHrP genes in the rat parathyroid glands was not affected by PTH 7-84. The peripheral metabolism of PTH 1-84 was not affected by PTH 7-84. PTH 7-84 did not cross-react with the rat bioactive PTH 1-84 assay. In normal rats with intact parathyroid glands, PTH 7-84 inhibited the PTH 1-84 secretory response to hypocalcemia and induced a significant decrease in plasma Ca2+. These effects of PTH 7-84 on PTH 1-84 secretion and on plasma Ca2+ levels were not associated with significant changes in PTH, PTHR1, CaR, and PTHrP gene expressions in the rat parathyroid glands. It is hypothesized that PTH 7-84 regulates PTH secretion via an autocrine/paracrine regulatory mechanism.

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 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.

Can hyperparathyroid bone disease be arrested or reversed?

Parathyroid hyperplasia, oversecretion of parathyroid hormone (PTH), and hyperparathyroid bone disease are characteristic features of chronic uremia; they develop early in the course of uremia and often in a progressive way. This review focuses on the potential for arrest or regression of hyperparathyroid-induced bone disease. For this purpose, the review addresses investigations that have used bone histology and not investigations that indirectly attempted to demonstrate changes in the skeleton by measurements of bone mineral density or laboratory indices of bone turnover, other than PTH. A prerequisite for inducing regression of the hyperparathyroid bone disease is a significant suppression of PTH secretion or reversal of hyperparathyroidism and uremia. It is concluded, on the basis of paired bone biopsy studies in patients with established hyperparathyroid bone disease, that bone histology can be improved or normalized after treatment that diminishes PTH levels. Oversuppression of PTH levels, however, might lead to adynamic bone disease.

Cinacalcet hydrochloride: calcimimetic for the treatment of hyperparathyroidism

Mineral metabolism disorders, including those related to secondary hyperparathyroidism, affect a large number of patients with chronic kidney disease and are associated with increased relative risk of morbidity and mortality in hemodialysis patients. The traditional therapy of secondary hyperparathyroidism based on vitamin D compounds and calcium-based phosphate binders is often limited by the increase of serum calcium and phosphorus levels limiting the dose that can be given safely, and thus, preventing the attainment of treatment targets. Cinacalcet hydrochloride (Sensipar^®, Mimpara^®, Parareg^®) is the first in a new class of therapeutic agents, the calcimimetics, that increase the sensitivity of calcium-sensing receptors to the extracellular calcium ions, thus lowering parathyroid hormone production and release, decreasing serum calcium and phosphorous concentrations simultaneously. Different randomized, double-blind, placebo-controlled trials evaluated the safety and ability of cinacalcet hydrochloride treatment to improve achievement of target levels of parathyroid hormone, calcium, phosphorus and calcium phosphorus product in dialysis patients. Cinacalcet hydrochloride has also demonstrated to be effective in reducing parathyroid hormone and serum calcium concentrations in patients with primary hyperparathyroidism. On the basis of available data, calcimimetics represent an important innovation and will change the management of mineral metabolism disorders in patients with chronic kidney disease and primary hyperparathyroidism.

Calcimimetic R-568 Decreases Extraosseous Calcifications in Uremic Rats Treated with Calcitriol

Calcimimetics decrease parathyroid hormone (PTH) levels in uremic patients with secondary hyperparathyroidism without increasing serum calcium (Ca). The aim of this study was to evaluate the effect of calcimimetic R-568 alone or in combination with calcitriol on vascular and other soft tissue calcifications in uremic rats with secondary hyperparathyroidism. Sham-operated and 5/6 nephrectomized Wistar rats were studied. 5/6 Nephrectomized rats were treated with vehicle, calcitriol (80 ng/kg every other day), R-568 (1.5 and 3 mg/kg per d), and both calcitriol and R-568 1.5 mg/kg, as above. Rats were killed after 14 or 56 d of treatment. Blood was drawn for biochemical measurements. Aortic, heart, kidney, lung, and stomach tissue samples were processed for histopathology and measurement of tissue Ca and phosphorus content. PTH concentrations were significantly reduced by all treatments. Treatment with calcitriol induced significant vascular calcification (aortic Ca increased to 4.2+/-1.2 mg/g at day 14 and to 11.4+/-0.7 mg/g at day 56; P<0.05 versus vehicle). Treatment with R-568 did not induce vascular calcification. Concurrent administration of R-568 with calcitriol reduced the aortic Ca (1.9+/-0.2 mg/g at day 14 and 7.5+/-1.4 mg/g at day 56) in relation to calcitriol alone. Soft tissue calcifications mirrored aortic mineralizations. Survival was significantly (P<0.001) reduced in calcitriol-treated rats, and mortality was attenuated (P=0.01) by concurrent treatment with R-568. In uremic rats, R-568 reduces elevated PTH levels without inducing vascular calcification, prevents calcitriol-induced vascular calcification, and decreases mortality.

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.

Prevention and treatment of renal osteodystrophy in children on chronic renal failure: European guidelines

Childhood renal osteodystrophy (ROD) is the consequence of disturbances of the calcium-regulating hormones vitamin D and parathyroid hormone (PTH) as well as of the somatotroph hormone axis associated with local modulation of bone and growth cartilage function. The resulting growth retardation and the potentially rapid onset of ROD in children are different from ROD in adults. The biochemical changes of ROD as well as its prevention and treatment affect calcium and phosphorus homeostasis and are directly associated with the development of cardiovascular disease in pediatric renal patients. The aims of the clinical and biochemical surveillance of pediatric patients with CRF or on dialysis are prevention of hyperphosphatemia, avoidance of hypercalcemia and keeping the calcium phosphorus product below 5 mmol(2)/l(2). The PTH levels should be within the normal range in chronic renal failure (CRF) and up to 2-3 times the upper limit of normal levels in dialysed children. Prevention of ROD is expected to result in improved growth and less vascular calcification.

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.

Acute regulation of parathyroid hormone by dietary phosphate

Secondary hyperparathyroidism in chronic renal failure is stimulated by dietary phosphate (P(i)) loading and ameliorated by dietary P(i) restriction. We investigated the rapidity of the response of serum parathyroid hormone (PTH) to changes in dietary P(i). When uremic rats adapted to a high P(i) diet (HPD) were fed a single meal of low P(i) diet (LPD), plasma PTH fell 80% within 2 h; plasma P(i) fell 1 mg/dl with no change in plasma ionized Ca (ICa). When uremic rats on the HPD were gavaged with LPD, PTH fell 60% within 15 min; plasma P(i) fell by 3.0 mg/dl with no change in total plasma Ca. However, HPD gavage increased PTH by 80% within 15 min with no change in plasma P or Ca, suggesting that the response may be independent of altered plasma P(i). Duodenal infusion of sodium P(i) increased PTH twofold within 10 min, with no change in ICa but an increase in plasma P(i), whereas duodenal infusion of NaCl had no effect on any of these parameters. Intravenous infusion of sodium phosphate also increased PTH within 10 min with no change in plasma ICa; intravenous NaCl had no effect. Additionally, duodenal infusion of phosphonoformate, a nonabsorbable phosphate analog, increased PTH fourfold within 5 min, but did not change plasma P or ICa. These findings indicate that oral P(i) increases PTH release in vivo more rapidly than previously reported; this response may be from both plasma phosphate and an additional signal arising from the gastrointestinal tract.

Regulation of PTH synthesis and secretion relevant to the management of secondary hyperparathyroidism in chronic kidney disease

Regulation of PTH synthesis and secretion relevant to the management of secondary hyperparathyroidism in chronic kidney disease. Small decreases in serum Ca(++) and more prolonged increases in serum phosphate (P) stimulate the parathyroid (PT) to secrete parathyroid hormone (PTH), while 1,25(OH)(2)-vitamin D(3) decreases PTH synthesis and secretion. A prolonged decrease in serum Ca(++) and 1,25(OH)(2)D(3), or increase in serum P, such as in patients with chronic renal failure, leads to the appropriate secondary increase in serum PTH. This secondary hyperparathyroidism involves increases in PTH gene expression, synthesis, and secretion, and, if chronic, to proliferation of the parathyroid cells. A low serum Ca(++) leads to an increase in PTH secretion, PTH mRNA stability, and parathyroid cell proliferation. Pi also regulates the parathyroid in a similar manner. The effect of Ca(++) on the parathyroid is mediated by a membrane Ca(2+) receptor (CaR). 1,25(OH)(2)D(3) decreases PTH gene transcription. Ca(2+) and P regulate the PTH gene post-transcriptionally by regulating the binding of parathyroid cytosolic proteins, trans factors, to a defined cis sequence in the PTH mRNA 3'-untranslated region (UTR), thereby determining the stability of the transcript. The parathyroid trans factors and cis elements have been defined.

Pharmacological and clinical properties of calcimimetics: calcium receptor activators that afford an innovative approach to controlling hyperparathyroidism

Circulating levels of calcium ion (Ca2+) are maintained within a narrow physiological range mainly by the action of parathyroid hormone (PTH) secreted from parathyroid gland (PTG) cells. PTG cells can sense small fluctuations in plasma Ca2+ levels by virtue of a cell surface Ca2+ receptor (CaR) that belongs to the superfamily of G protein-coupled receptors (GPCR). Compounds that activate the CaR and inhibit PTH secretion are termed 'calcimimetics' because they mimic or potentiate the effects of extracellular Ca2+ on PTG cell function. Preclinical studies with NPS R-568, a first generation calcimimetic compound that acts as a positive allosteric modulator of the CaR, have demonstrated that oral administration decreases serum levels of PTH and calcium, with a leftward shift in the set-point for calcium-regulated PTH secretion in normal rats. NPS R-568 also suppresses the elevation of serum PTH levels and PTG hyperplasia and can improve bone mineral density (BMD) and strength in rats with chronic renal insufficiency (CRI). Clinical trials with cinacalcet hydrochloride (cinacalcet), a compound with an improved metabolic profile, have shown that long-term treatment continues to suppress the elevation of serum levels of calcium and PTH in patients with primary hyperparathyroidism (1HPT). Furthermore, clinical trials in patients with uncontrolled secondary hyperparathyroidism (2HPT) have demonstrated that cinacalcet not only lowers serum PTH levels, but also the serum phosphorus and calcium x phosphorus product; these are a hallmark of an increased risk of cardiovascular disease and mortality in dialysis patients with end-stage renal disease. Indeed, cinacalcet has already been approved for marketing in several countries. Calcimimetic compounds like cinacalcet have great potential as an innovative medical approach to manage 1HPT and 2HPT.

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.

Sevelamer controls parathyroid hormone-induced bone disease as efficiently as calcium carbonate without increasing serum calcium levels during therapy with active vitamin D sterols

Little is known about the impact of various phosphate binders on the skeletal lesions of secondary hyperparathyroidism (2 degrees HPT). The effects of calcium carbonate (CaCO3) and sevelamer were compared in pediatric peritoneal dialysis patients with bone biopsy-proven 2 degrees HPT. Twenty-nine patients were randomly assigned to CaCO3 (n = 14) or sevelamer (n = 15), concomitant with either intermittent doses of oral calcitriol or doxercalciferol for 8 mo, when bone biopsies were repeated. Serum phosphorus, calcium, parathyroid hormone (PTH), and alkaline phosphatase were measured monthly. The skeletal lesions of 2 degrees HPT improved with both binders, and bone formation rates reached the normal range in approximately 75% of the patients. Overall, serum phosphorus levels were 5.5 +/- 0.1 and 5.6 +/- 0.3 mg/dl (NS) with CaCO3 and sevelamer, respectively. Serum calcium levels and the Ca x P ion product increased with CaCO3; in contrast, values remained unchanged with sevelamer (9.6 +/- 01 versus 8.9 +/- 0.2 mg/dl; P < 0.001, respectively). Hypercalcemic episodes (>10.2 mg/dl) occurred more frequently with CaCO3 (P < 0.01). Baseline PTH levels were 980 +/- 112 and 975 +/- 174 pg/ml (NS); these values decreased to 369 +/- 92 (P < 0.01) and 562 +/- 164 pg/ml (P < 0.01) in the CaCO3 and the sevelamer groups, respectively (NS between groups). Serum alkaline phosphatase levels also diminished in both groups (P < 0.01). Thus, treatment with either CaCO3 or sevelamer resulted in equivalent control of the biochemical and skeletal lesions of 2 degrees HPT. Sevelamer, however, maintained serum calcium concentrations closer to the lower end of the normal physiologic range, thereby increasing the safety of treatment with active vitamin D sterols.

Calcification and cardiovascular problems in renal failure

Extraosseus calcification has plagued management of renal failure since the beginning of hemodialysis, but the issue has largely been neglected because the impact on survival was thought to be limited. The recent recognition that hyperphosphatemia is a strong predictor of all-cause mortality, and particularly of cardiac mortality, has transformed the situation. Relatively stringent, though difficult to implement, guidelines have been proposed for the management of hyperphosphatemia. Important recent insights document that, for different reasons, both high and low turnover of bone disease increase the risk of vascular calcifications. Vascular calcification impacts cardiac death not only by complicating coronary atherosclerosis, but also by increasing the stiffness of central arteries, impacting on heart function (increased impedance, reduced coronary perfusion). While in the past extraosseous calcification, including vascular calcification, was thought to be a passive process resulting from transgression of a critical Ca x P product, recent studies show that the adverse effect of hyperphosphatemia is also mediated by active processes (e.g., induction of "osteogenic" genetic programs), and is modulated by calcification inhibitors.

Vitamin D analogs: new therapeutic agents for secondary hyperparathyroidism

Patients with chronic renal failure frequently develop secondary hyperparathyroidism, primarily as a result of phosphate retention and low serum 1,25(OH)2D3. Replacement therapy with calcitriol or its synthetic precursor alfacalcidol [1alpha(OH)D3] often produces hypercalcemia, especially when combined with calcium-based phosphate binders. In addition, the natural vitamin D compounds can exacerbate the hyperphosphatemia in patients with chronic renal failure. This combined increase in calcium and phosphate has been correlated with vascular calcification leading to coronary artery disease, the most common cause of mortality in renal patients. Several vitamin D analogs have now been developed that retain the direct suppressive action of calcitriol on the parathyroid glands but have less calcemic activity, thereby offering a safer and more effective means of controlling secondary hyperparathyroidism. Maxacalcitol [22-oxa-1,25(OH)2D3] and falecalcitriol [1,25(OH)2-26,27-F6-D3] are currently available in Japan, and paricalcitol [19-nor-1,25(OH)2D2] and doxercalciferol [1alpha(OH)D2] are available in the US. The mechanisms by which these analogs exert their selective actions on the parathyroid glands are under investigation. The low calcemic activity of maxacalcitol has been attributed to its rapid clearance from the circulation. This prevents sustained effects on intestinal calcium absorption and bone resorption, but still allows a prolonged suppression of parathyroid hormone gene expression. The selectivity of the other analogs is achieved by distinct mechanisms. Understanding how these compounds exert their selective actions on the parathyroid glands will aid in the design of safer, more effective analogs.

Nutritional secondary hyperparathyroidism in rabbits

The present study was designed to document the effect of a low (0.6%) calcium-high (1.2%) phosphorus (LCaHP) diet on the development of parathyroid gland hyperplasia in rabbits and to describe the dynamics of parathyroid function (PTH-Ca2+ curves) in rabbits with nutritional secondary hyperparathyroidism (N2HPT). Parathyroid gland weight, parathyroid cell proliferation (measured as percentage of cells in S-phase), and parathyroid calcium (CaRmRNA) and Vitamin D (VDRmRNA) receptor expression were measured in normal rabbits and in rabbits with N2HPT. The PTH-Ca2+ curve was studied in normal rabbits (Group I) and in rabbits with N2HPT at two stages: 2-3 weeks (Group IIA) and 5-6 weeks (Group IIB) after being fed LCaHP diet. An increase in parathyroid gland weight and percentage of cells in S-phase was detected in the course of N2HPT. After receiving a LCaHP diet for 6 weeks rabbits had decreased levels of CaRmRNA but VDRmRNA remained unchanged. A progressive increase in the concentrations of plasma PTH (Group IIA=167+/-14 pg/ml and Group IIB=377+/-54 pg/ml, P<0.05 versus Group I=27+/-3 pg/ml) was detected in the rabbits fed a LCaHP diet. This was accompanied by an increase in maximal and minimal PTH, reductions in plasma Ca2+ and calcitriol and elevations in plasma phosphate and creatinine. In conclusion, feeding a LCaHPD results in a rapid induction of N2HPT in rabbits. After 6 weeks on the LCaHPD rabbits develop parathyroid hyperplasia characterized by increases in PTH secretion, glandular weight and proliferation and by a decrease in CaRmRNA.

1,25-Dihydroxyvitamin D3 but not cinacalcet HCl (Sensipar/Mimpara) treatment mediates aortic calcification in a rat model of secondary hyperparathyroidism

BACKGROUND

Calcitriol treatment of secondary hyperparathyroidism (HPT) in chronic kidney disease (CKD) patients can lead to increased serum calcium and phosphorus, which have been associated as risk factors for vascular calcification. Cinacalcet HCl (Sensipar/Mimpara) {(alphaR)-(-)-alpha-methyl-N-[3-[3-(trifluoromethylphenyl)propyl]-1-napthalenemethanamine hydrochloride} lowers serum parathyroid hormone (PTH), calcium, phosphorus and calcium-phosphorous (CaxP) product in stage 5 CKD dialysis patients; however, its effects on vascular calcification are unknown.

METHODS

Cinacalcet HCl (10 or 1 mg/kg, p.o. gavage), 1,25-dihydroxyvitamin D(3) (0.1 microg, s.c, calcitriol) or the combination was administered daily for 26 days in a rat model of secondary HPT [5/6 nephrectomy]. After dosing, aortic calcification was determined using the von Kossa staining method. Serum PTH and blood chemistries were determined on days 0, 26 and 0, 14, 26, respectively, prior to and after dosing.

RESULTS

Calcitriol-treated rats had moderate to marked aortic calcification, whereas no significant calcification was observed in vehicle- or cinacalcet HCl-only treated groups. Co-administration of cinacalcet HCl with calcitriol did not attenuate the calcitriol-mediated increase in CaxP product or calcitriol-mediated aortic calcification. Both calcitriol and cinacalcet HCl therapy significantly reduced serum PTH levels. Calcitriol significantly elevated serum calcium, serum phosphorous and CaxP product above pretreatment levels, or those seen with vehicle or cinacalcet HCl. Cinacalcet HCl (10 or 1 mg/kg) decreased serum ionized calcium and decreased calcitriol-induced hypercalcaemia.

CONCLUSION

Cinacalcet HCl and calcitriol both effectively reduce PTH, albeit via different mechanisms, but unlike calcitriol, cinacalcet HCl did not produce hypercalcaemia, an increased CaxP product or vascular calcification.

Pathogenesis of parathyroid dysfunction in end-stage kidney disease

Small decreases in serum calcium (Ca(2+)) and more-prolonged increases in serum phosphate (Pi) stimulate the parathyroid (PT) to secrete parathyroid hormone (PTH). 1,25-Dihydroxyvitamin D(3) [1,25(OH)(2) D(3)] decreases PTH synthesis and secretion. The prolonged decrease in serum Ca(2+) and 1,25(OH)(2) D(3), or increase in serum Pi, observed in patients with chronic renal failure leads to a secondary increase in serum PTH. This secondary hyperparathyroidism involves increases in PTH gene expression, synthesis, and secretion and, if chronic, to proliferation of the PT cells. A low serum Ca(2+) leads to an increase in PTH secretion, PTH mRNA stability, and PT cell proliferation. Pi also regulates the PT in a similar manner. The effect of Ca(2+) on the PT is mediated by a membrane Ca(2+) receptor. 1,25(OH)(2) D(3) decreases PTH gene transcription. Ca(2+) and Pi regulate the PTH gene post transcriptionally by regulating the binding of PT cytosolic proteins, trans factors, to a defined cis sequence in the PTH mRNA 3'-untranslated region, thereby determining the stability of the transcript. The PT trans factors and cis elements have been defined.

Life-threatening tertiary hyperparathyroidism in the critically ill

BACKGROUND

Tertiary hyperparathyroidism typically occurs in patients who have recovered from renal failure after renal transplantation. This report describes a syndrome of tertiary hyperparathyroidism after recovery from multiple organ failure (MOF) with acute oliguric renal failure (AORF).

METHODS

Six patients with MOF including AORF are presented. Increased parathyroid hormone (PTH) levels were documented as early as 3 weeks after injury or septic insult and remained increased in some patients for several weeks.

RESULTS

The resultant increase in calcium levels led to recurrent bouts of bradycardia, often leading to asystole requiring cardiopulmonary resuscitation. Hypercalcemic-induced bradycardia was refractory to hydration, loop diuresis, atropine, and external pacing. Definitive treatment requires bisphosphonate therapy, which must be repeated until organ function has returned to normal.

CONCLUSIONS

A new syndrome of life-threatening tertiary hyperparathyroidism is described in patients with critical illness. This syndrome probably is being overlooked frequently in critical care units. Early diagnosis and prophylactic treatment with bisphosphonate may preclude the life-threatening cardiac arrhythmias.

Cinacalcet HCl attenuates parathyroid hyperplasia in a rat model of secondary hyperparathyroidism

BACKGROUND

Secondary hyperparathyroidism (HPT) in chronic kidney disease (CKD) is a physiologic response to kidney failure characterized by elevated serum parathyroid hormone (PTH) levels and parathyroid gland enlargement. Calcimimetic agents acting through allosteric modification of the calcium-sensing receptor (CaR) can attenuate parathyroid hyperplasia in rats with secondary HPT. The present study explores the effects of the calcimimetic cinacalcet HCl on parathyroid hyperplasia, apoptosis, and PTH secretion in a rat model of secondary HPT.

METHODS

Cinacalcet HCl was gavaged daily (1, 5, or 10 mg/kg) for 4 weeks starting 6 weeks post-5/6 nephrectomy. After dosing, hyperplasia was determined using parathyroid weight and proliferating cell nuclear antigen (PCNA) immunochemistry. Apoptosis was determined using in situ techniques. Serum PTH((1-34)) and blood chemistries were determined throughout the course of the study.

RESULTS

Administration of cinacalcet HCl (5 or 10 mg/kg) significantly reduced the number of PCNA-positive cells and decreased parathyroid weight compared with vehicle-treated 5/6 nephrectomized rats. There was no difference in apoptosis from cinacalcet HCl-treated or vehicle-treated animals. Serum PTH and blood ionized calcium levels decreased in cinacalcet HCl-treated animals compared with vehicle-treated controls.

CONCLUSION

The results confirm previous work demonstrating that calcimimetic agents attenuate the progression of parathyroid hyperplasia in subtotally nephrectomized rats, extending earlier observations to now include cinacalcet HCl. These results support a role for the CaR in regulating parathyroid cell proliferation. Therefore, cinacalcet HCl may represent a novel therapy for improving the management of secondary HPT.

Biochemical and Cellular Effects of Direct Maxacalcitol Injection into Parathyroid Gland in Uremic Rats

The most important etiological factors of resistance to medical treatments for secondary hyperparathyroidism are the decreased contents of the vitamin D receptor (VDR) and Ca-sensing receptor (CaSR) in parathyroid cells and a severely swollen parathyroid gland (PTG) as a result of hyperplasia. The effects of direct maxacalcitol (OCT) injection into PTG in terms of these factors were investigated in this study. The PTG of Sprague-Dawley rats that were 5/6 nephrectomized and fed a high-phosphate diet were treated by a direct injection of OCT (DI-OCT) or vehicle (DI-vehicle). The changes in serum intact parathyroid hormone (PTH), Ca(2+), and phosphorus levels, in VDR and CaSR expression levels in parathyroid cells, and in Ca(2+)-PTH curves were examined. Apoptosis was analyzed by the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling method and DNA electrophoresis for PTG. DI-OCT markedly decreased serum intact PTH level, and a significant difference in this level between DI-OCT and DI-vehicle was observed. However, serum Ca(2+) and phosphorus levels did not changed markedly in both groups. The upregulations of both VDR and CaSR, the clear shift to the left downward in the Ca(2+)-PTH curve, and the induction of apoptosis after DI-OCT were observed. These findings were not observed in the DI-vehicle-treated rats. Moreover, these effects of DI-OCT were confirmed by the DI-OCT into one PTG and DI-vehicle alone into another PTG in the same rat. DI-OCT may introduce simultaneous VDR and CaSR upregulations and the regression of hyperplastic PTG, and these effects may provide a strategy for strongly suppressing PTH levels in very severe secondary hyperparathyroidism.

Secondary hyperparathyroidism: Review of the disease and its treatment

BACKGROUND

Most patients with chronic kidney disease (CKD) stage 5 develop secondary hyperparathyroidism (SHPT). SHPT is an adaptive response to CKD and its associated disruptions in the homeostatic control of serum phosphorus, calcium, and vitamin D. The poor control of mineral and parathyroid hormone (PTH) levels characteristic of SHPT is associated with serious clinical consequences.

OBJECTIVE

This review discusses the pathophysiology and consequences of SHPT, as well as the efficacy and limitations of current treatment modalities.

METHODS

Literature searches were conducted using the MEDLINE, EMBASE, and BIOSIS databases. Additional information was obtained from Internet web sites, textbooks, and nephrology congress abstracts.

RESULTS

Patients with uncontrolled SHPT are at higher risk for cardiovascular morbidity and mortality, hospitalization, bone disease, vascular and soft-tissue calcification, and vascular access failure than patients whose mineral and PTH levels are well managed. New National Kidney Foundation Kidney Disease Outcomes Quality Initiative (K/DOQI) targets for calcium, phosphorus, calcium-phosphorus product, and PTH control have recently been published with the aim of improving the management of mineral metabolism in CKD patients. Data from observational studies suggest that the majority of patients currently have PTH and mineral levels outside these target ranges.

CONCLUSIONS

Given the inadequacies of current therapies, novel agents are being developed that may help improve the management of SHPT.

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.

Involvement of protein kinase C-alpha and -epsilon in extracellular Ca2+ signalling mediated by the calcium sensing receptor

The sensing of extracellular Ca(2+) concentration ([Ca(2+)](o)) and modulation of cellular processes associated with acute or sustained changes in [Ca(2+)](o) are cell-type specific and mediated by the calcium sensing receptor (CaR). [Ca(2+)](o) signalling requires protein kinase C (PKC), but the identity and role of PKC isoforms in CaR-mediated responses remain unclear. Here we show that high [Ca(2+)](o) activated PKC-alpha and PKC- in parathyroid cells and in human embryonic kidney (HEK293) cells overexpressing the CaR (HEK-CaR) and that this response correlated with the CaR-dependent activation of mitogen-activated protein kinases ERK1/2. Activation of ERK1/2 by acute high [Ca(2+)](o) required influx of Ca(2+)through Ni(2+)-sensitive Ca(2+)channels and phosphatidylinositol-dependent phospholipase C-beta activity. Inhibition of PKC by co-expression of dominant-negative (DN) mutants of PKC-alpha or - with the CaR attenuated sustained ERK1/2 activation. Overexpression of a PKC phosphorylation site (T888A) mutant CaR in HEK293 cells showed that this site was important for ERK1/2 activation at high [Ca(2+)](o). Activation of ERK1/2 by high [Ca(2+)](o) was not necessary for the [Ca(2+)](o)-regulated secretion of parathyroid hormone (PTH) in dispersed bovine parathyroid cells. These data suggest that the CaR-mediated [Ca(2+)](o) signal leading to regulated PTH secretion that requires diacylglycerol-responsive PKC isoforms is not mediated via the ERK pathway.

Calcium-enriched bread for treatment of uremic hyperphosphatemia

OBJECTIVE

To assess phosphate-binding efficacy of a new food product, bread with unusually high calcium content (Ca-bread).

DESIGN AND SETTING

A randomized parallel group trial in the university hospital outpatient dialysis unit.

PATIENTS

Fifty-three randomly selected uremic patients who met the following inclusion criteria: (1) required maintenance hemodialysis treatment, (2) were not to receive vitamin D throughout the study, (3) were nondiabetic, and (4) were diagnosed with hyperphosphatemia.

INTERVENTION

Fifty-three patients were randomized into 2 groups: control group (n = 26), which received calcium acetate as a phosphate binder throughout the study, and Ca-bread group (n = 27), which, after a 2-week washout period, received Ca-bread containing 2.5% of elemental calcium (by weight), which served as a phosphate binder. Bread was made using wheat flour, calcium carbonate, and fermented buttermilk. The amount of elemental calcium used as a phosphate binder was similar in both groups. Observation of both groups lasted 14 weeks.

RESULTS

Mean serum phosphate concentration at randomization was 2.11 +/- 0.14 mmol/L in the control group and 2.20 +/- 0.13 mmol/L in the Ca-bread group. Mean serum calcium concentration at randomization was 2.12 +/- 0.21 mmol/L in the control group and 2.14 +/- 0.11 mmol/L in the Ca-bread group. The Ca-bread group patients' predialysis phosphate concentration decreased to a mean of 1.67 +/- 0.18 mmol/L (P <.05), and their mean calcium concentration increased to 2.27 +/- 0.11 mmol/L (P = NS). In the control group, neither value changed significantly from the original readings. After the hemodialysis session, the mean serum calcium concentration in the control group and the Ca-bread group increased by 7.5% and 7.9%, respectively (P = NS). Mean phosphate concentration simultaneously decreased to nearly 1/2 its original predialysis value in both groups. Ca-bread group patients saw a decrease in the mean phosphate concentration (from predialysis to postdialysis values) that was 13.8% greater than that of the control group (P = NS).

CONCLUSION

A new form of calcium-containing phosphate binder was developed: Ca-bread with an elemental calcium content of 2.5%. Ca-bread allows for effective amelioration of hyperphosphatemia without inducing hypercalcemia. Furthermore, patient compliance may increase if hyperphosphatemia can be treated by consuming bread with an elevated calcium content.