The renal PTH/PTHrP receptor is down-regulated in rats with chronic renal failure

Parathyroid hormone-PTH1R signaling in cardiovascular disease and homeostasis

Primary hyperparathyroidism (pHPT) afflicts our aging population with an incidence approaching 50 per 100 000 patient-years at a female:male ratio of ~3:1. Decisions surrounding surgical management are currently driven by age, hypercalcemia severity, presence of osteoporosis, renal insufficiency, or hypercalciuria with or without nephrolithiasis. Cardiovascular (CV) disease (CVD) is not systematically considered. This is notable since the parathyroid hormone (PTH) 1 receptor (PTH1R) is biologically active in the vasculature, and adjusted CV mortality risk is increased almost threefold in individuals with pHPT who do not meet contemporary recommendations for surgical cure. We provide an overview of epidemiology, pharmacology, and physiology that highlights the need to: (i) identify biomarkers that establish a healthy 'set point' for CV PTH1R signaling tone; (ii) better understand the pharmacokinetic-pharmacodynamic (PK-PD) relationships of PTH1R ligands in CV homeostasis; and (iii) incorporate CVD risk assessment into the management of hyperparathyroidism.

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.

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

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

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

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

Effects of intermittent treatment with parathyroid hormone (PTH) on osteoblastic differentiation and mineralization of mouse induced pluripotent stem cells in a 3D culture model

BACKGROUND/OBJECTIVES

PTH plays an important role in bone remodeling, and different actions have been reported depending on its administration method. iPSCs are promising as a cell source for regeneration of periodontal tissue due to their ability of proliferation and pluripotency. However, the effects of PTH on iPSCs remain mostly unknown. The purpose of this study was to investigate in vitro effects of parathyroid hormone (PTH) on osteoblastic differentiation of induced pluripotent stem cells (iPSCs) in a 3D culture model.

MATERIALS AND METHODS

Following embryoid body (EB) induction from mouse iPSCs (miPSCs), dissociated cells (miPS-EB-derived cells) were seeded onto atelocollagen sponge (ACS) in osteoblast differentiation medium (OBM). Cell-ACS constructs were divided into three groups: continuous treatment with human recombinant PTH (1-34) (PTH-C), intermittent PTH treatment (PTH-I) or OBM control. To confirm the expression of PTH receptor-1(PTH1R), the expression of Pth1r and cAMP production over time were assessed. Real-time PCR was used to assess the expression of genes encoding osterix (Sp7), runt-related transcription factor 2 (Runx2), collagen type 1 (Col1a1), and osteocalcin (Bglap) at different time points. Mineralization was assessed by von Kossa staining. Histochemical staining was used to analyze alkaline phosphatase (ALP) activity, and immunolocalization of SP7 and BGLAP was analyzed by confocal laser scanning microscopy (CLSM).

RESULTS

On days 7 and 14, expression of the Pth1r in miPS-EB-derived cells was increased in all groups. Production of cAMP, the second messenger of the PTH1R, tended to increase in the PTH-I group compared with PTH-C group on day 14. Expression of Col1a1 in the PTH-I group on day 14 was significantly higher than other groups. There was a time-dependent increase in the expression of Sp7 in all groups. On day 14, the expression level of Sp7 in the PTH-I group was significantly higher than other groups. In von Kossa staining, the PTH-I group showed higher level of staining compared with other groups on day 14, whereas the level was slightly attenuated in the PTH-C group. In histochemical staining, ALP-positive cells were significantly increased in the PTH-I group compared with other groups on day 14. In CLSM analysis, the numbers of SP7- and BGLAP-positive cells showed a gradual increase over time, and on day 14, a significantly greater SP7 expression was observed in the PTH-I group than other groups.

CONCLUSION

These results suggested that the intermittent PTH treatment promotes osteoblastic differentiation and mineralization of miPSCs in the ACS scaffold.

Osteocytes' expression of the PTH/PTHrP receptor has differing effects on endocortical and periosteal bone formation during adenine-induced CKD

Osteocytes play a key role in the pathophysiology of chronic kidney disease (CKD). However, the extent to which osteocytes contribute to abnormalities in bone turnover due to excessive levels of parathyroid hormone (PTH) remains poorly understood. The purpose of this study was to determine the extent to which bone formation and tissue strength during the progression of CKD is modified through osteocytes' response to PTH. Conditional knockout mice targeting osteocytes' expression of the PTH/PTH-related protein type 1 receptor (PPR) were subjected to adenine-induced CKD. After 6-weeks of treatment, adenine-induced CKD was found to reduce bone formation at the periosteal and endocortical surfaces of the tibia. The loss in bone mass corresponded with a significant decrease in structural-level mechanical properties. In knockout mice, the loss of PPR expression in osteocytes further exacerbated the loss in bone formation at the endocortical surface, but inhibited bone loss at the periosteal surface. In general, the effects of adenine-induced CKD were not as extensive in female mice. Collectively, these findings demonstrate that osteocytes' response to PTH under adenine-induced CKD has a unique impact on bone turnover that is specific to the periosteal and endocortical surfaces.

[Studies of the Various Chronic Kidney Failure Rat Models and Hemodialysis Mini-pig Model for the Evaluation of Anti-hyperphosphatemia Drugs]

Animal models of chronic kidney failure (CKF) have been developed for the pharmacodynamic evaluation of various phosphate binders that are used clinically to treat hyperphosphatemia in patients with chronic kidney disease. However, these models represent different disease states and severities, depending on the experimental conditions and are not clearly defined for pharmacological evaluation. In addition, experimental models have not yet been established for artificial dialysis. The purpose of this study was to confirm the utility of the various rat models of CKF and the mini-pig model of hemodialysis as models of hyperphosphatemia for pharmacodynamic evaluation. Various rat models of pre-dialysis CKF (oral adenine dosing, 5/6 resection, and ligation nephrectomy model) were evaluated through determinations of serum and urinary parameters (osmolality, creatinine, and phosphorus), pathological observations of kidney, and the phosphorus-absorbing properties of lanthanum carbonate (La) formulations. The rat and mini-pig models were compared based on each evaluation index. In the oral adenine dosing model, serum phosphorus increased markedly and the area under the serum phosphorus concentration-time curve (phosphorus AUC) decreased in a dose-dependent manner with the administration of La formulations. In contrast, a significant decrease in serum phosphorus AUC, a prolongation of the dialysis interval, and an improvement in dialysis efficiency were observed after administration of La formulations to the mini-pig hemodialysis model. Furthermore, the results of bioequivalence studies between two La formulations (Fosrenol and SW670, a generic formulation) suggested that the rat and mini-pig models are useful and precise as pre-dialysis and dialysis models, respectively.

Disrupted tubular parathyroid hormone/parathyroid hormone receptor signaling and damaged tubular cell viability possibly trigger postsurgical kidney injury in patients with advanced hyperparathyroidism

Background

Parathyroidectomy (PTX) that alleviates clinical manifestations of advanced hyperparathyroidism, including hypercalcemia and hypophosphatemia, is considered the best protection from calcium overload in the kidney. However, little is known about the relationship between postsurgical robust parathyroid hormone (PTH) reduction and perisurgical renal tubular cell viability. Post-PTX kidney function is still a crucial issue for primary hyperparathyroidism (PHPT) and tertiary hyperparathyroidism after kidney transplantation (THPT).

Methods

As a clinical study, we examined data from 52 consecutive patients (45 with PHPT, 7 with THPT) who underwent PTX in our center between 2015 and 2017 to identify post-PTX kidney injury. Their clinical data, including urinary liver-type fatty acid-binding protein (L-FABP), a tubular biomarker for acute kidney injury (AKI), were obtained from patient charts. An absolute change in serum creatinine level of 0.3 mg/dL (26.5 µmol/L) on Day 2 after PTX defines AKI. Post-PTX calcium supplement dose adjustment was performed to strictly maintain serum calcium at the lower half of the normal range. To mimic post-PTX-related kidney status, a unique parathyroidectomized rat model was produced as follows: 13-week-old rats underwent thyroparathyroidectomy (TPTX) and/or 5/6 subtotal nephrectomy (NX). Indicated TPTX rats were given continuous infusion of a physiological level of 1-34 PTH using a subcutaneously implanted osmotic minipump. Immunofluorescence analyses were performed by polyclonal antibodies against PTH receptor (PTHR) and a possible key modulator of kidney injury, Klotho.

Results

Patients’ estimated glomerular filtration rate (eGFR) did not have any clinically relevant change (62.5 ± 22.0 versus 59.4 ± 21.9 mL/min/1.73 m², NS), whereas serum calcium (2.7 ± 0.18 versus 2.2 ± 0.16 mmol/L, P < 0.0001) and phosphorus levels (0.87 ± 0.19 versus 1.1 ± 0.23 mmol/L, P < 0.0001) were normalized and PTH decreased robustly (181 ± 99.1 versus 23.7 ± 16.8 pg/mL, P < 0.0001) after successful PTX. However, six patients who met postsurgical AKI criteria had lower eGFR and greater L-FABP than those without AKI. Receiver operating characteristics (ROC) analysis revealed eGFR <35 mL/min/1.73 m² had 83% accuracy. Strikingly, L-FABP >9.8 µg/g creatinine had 100% accuracy in predicting post-PTX-related AKI. Rat kidney PTHR expression was lower in TPTX. PTH infusion (+PTH) restored tubular PTHR expression in rats that underwent TPTX. Rats with TPTX, +PTH and 5/6 NX had decreased PTHR expression compared with those without 5/6 NX. 5/6 NX partially cancelled tubular PTHR upregulation driven by +PTH. Tubular Klotho was modestly expressed in normal rat kidneys, whereas enhanced patchy tubular expression was identified in 5/6 NX rat kidneys. This Klotho and expression and localization pattern was absolutely canceled in TPTX, suggesting that PTH indirectly modulated the Klotho expression pattern. TPTX +PTH recovered tubular Klotho expression and even triggered diffusely abundant Klotho expression. 5/6 NX decreased viable tubular cells and eventually downregulated tubular Klotho expression and localization.

Conclusions

Preexisting tubular damage is a potential risk factor for AKI after PTX although, overall patients with hyperparathyroidism are expected to keep favorable kidney function after PTX. Patients with elevated tubular cell biomarker levels may suffer post-PTX kidney impairment even though calcium supplement is meticulously adjusted after PTX. Our unique experimental rat model suggests that blunted tubular PTH/PTHR signaling may damage tubular cell viability and deteriorate kidney function through a Klotho-linked pathway.

Effect of free running wheel exercise on renal expression of parathyroid hormone receptor type 1 in spontaneously hypertensive rats

An active lifestyle is generally recommended for hypertensive patients to prevent subsequent end-organ damage. However, experimental data on long-term effects of exercise on hypertension are insufficient and underlying mechanisms are not well understood. This study was aimed to investigate the effect of exercise on renal expression of parathyroid hormone-related protein (PTHrP) and parathyroid hormone receptor type 1 (PTHR1) in spontaneously hypertensive rats (SHR). Twenty-four rats started free running wheel exercise at the age of 1.5 months (pre-hypertensive state) and proceeded for 1.5, 3.0, 6.0, and 10.0 months. Thirty rats kept under standard housing conditions were used as sedentary controls. Kidney function was assessed by measuring plasma creatinine levels and urine albumin-to-creatinine ratios. Renal expression of PTHrP and PTHR1 was analyzed by qRT-PCR and western blot. Renal expression of PTHR1 was markedly increased between the 6th and 10th months in sedentary rats and this increase was significantly lower in SHRs with high physical activity on mRNA (-30%) and protein level (-27%). At the same time, urine albumin-to-creatinine ratio increased (from 65 to 231 mg/g) but somehow lower in exercise performing SHRs (48-196 mg/g). Our data suggest that enhanced exercise, stimulated by allocation of a free running wheel, is associated with lower PTHR1 expression in SHRs and this may contribute to preserved kidney function.

TGF-Beta Signaling in Bone with Chronic Kidney Disease

Transforming growth factor (TGF)-β signaling is not only important in skeletal development, but also essential in bone remodeling in adult bone. The bone remodeling process involves integrated cell activities induced by multiple stimuli to balance bone resorption and bone formation. TGF-β plays a role in bone remodeling by coordinating cell activities to maintain bone homeostasis. However, mineral metabolism disturbance in chronic kidney disease (CKD) results in abnormal bone remodeling, which leads to ectopic calcification in CKD. High circulating levels of humoral factors such as parathyroid hormone, fibroblast growth factor 23, and Wnt inhibitors modulate bone remodeling in CKD. Several reports have revealed that TGF-β is involved in the production and functions of these factors in bone. TGF-β may act as a factor that mediates abnormal bone remodeling in CKD.

Alkaline Phosphatases in the Complex Chronic Kidney Disease-Mineral and Bone Disorders

Alkaline phosphatases (APs) remove the phosphate (dephosphorylation) needed in multiple metabolic processes (from many molecules such as proteins, nucleotides, or pyrophosphate). Therefore, APs are important for bone mineralization but paradoxically they can also be deleterious for other processes, such as vascular calcification and the increasingly known cross-talk between bone and vessels. A proper balance between beneficial and harmful activities is further complicated in the context of chronic kidney disease (CKD). In this narrative review, we will briefly update the complexity of the enzyme, including its different isoforms such as the bone-specific alkaline phosphatase or the most recently discovered B1x. We will also analyze the correlations and potential discrepancies with parathyroid hormone and bone turnover and, most importantly, the valuable recent associations of AP's with cardiovascular disease and/or vascular calcification, and survival. Finally, a basic knowledge of the synthetic and degradation pathways of APs promises to open new therapeutic strategies for the treatment of the CKD-Mineral and Bone Disorder (CKD-MBD) in the near future, as well as for other processes such as sepsis, acute kidney injury, inflammation, endothelial dysfunction, metabolic syndrome or, in diabetes, cardiovascular complications. However, no studies have been done using APs as a primary therapeutic target for clinical outcomes, and therefore, AP's levels cannot yet be used alone as an isolated primary target in the treatment of CKD-MBD. Nonetheless, its diagnostic and prognostic potential should be underlined.

Association of Parameters of Mineral Bone Disorder with Mortality in Patients on Hemodialysis according to Level of Residual Kidney Function

BACKGROUND AND OBJECTIVES

The relationship between mineral and bone disorders and survival according to residual kidney function status has not been previously studied in patients on hemodialysis. We hypothesized that residual kidney function, defined by renal urea clearance, modifies the association between mineral and bone disorder parameters and mortality.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The associations of serum phosphorus, albumin-corrected calcium, intact parathyroid hormone, and alkaline phosphatase with all-cause mortality were examined across three strata (<1.5, 1.5 to <3.0, and ≥3.0 ml/min per 1.73 m2) of baseline residual renal urea clearance using Cox models adjusted for clinical characteristics and laboratory measurements in 35,114 incident hemodialysis patients from a large United States dialysis organization over the period of 2007-2011.

RESULTS

A total of 8102 (23%) patients died during the median follow-up of 1.3 years (interquartile range, 0.6-2.3 years). There was an incremental mortality risk across higher serum phosphorus concentrations, which was pronounced among patients with higher residual renal urea clearance (Pinteraction=0.001). Lower concentrations of serum intact parathyroid hormone were associated with higher mortality among patients with low residual renal urea clearance (i.e., <1.5 ml/min per 1.73 m2), whereas higher concentrations showed a higher mortality risk among patients with greater residual renal urea clearance (i.e., ≥1.5 ml/min per 1.73 m2; Pinteraction<0.001). Higher serum corrected total calcium and higher alkaline phosphatase concentrations consistently showed higher mortality risk (Ptrend<0.001 for both) irrespective of residual renal urea clearance strata (Pinteraction=0.34 and Pinteraction=0.53, respectively).

CONCLUSIONS

Residual kidney function modified the mortality risk associated with serum phosphorus and intact parathyroid hormone among incident hemodialysis patients. Future studies are needed to examine whether taking account for residual kidney function into the assessment of mortality risk associated with serum phosphorus and intact parathyroid hormone improves patient management and clinical outcomes in the hemodialysis population.

Uremic Toxicity and Bone in CKD

Patients with chronic kidney disease (CKD), especially those on dialysis treatment, are at high risk of bone fracture. In CKD-mineral and bone disorder (CKD-MBD), secondary hyperparathyroidism in patients with advanced CKD induces bone abnormalities, and skeletal resistance to parathyroid hormone (PTH) starts in the early stages of kidney disease. Uremic toxins such as indoxyl sulfate and p-cresyl sulfate reduce the expression of PTH receptor as well as PTH-induced cyclic adenosine 3',5' monophosphate production in osteoblasts. CKD also impairs bone strength, especially quality. In a rat model, kidney damage reduces the bone-storage modulus and changes the cortical bone chemical composition with or without hyperparathyroidism. The oral charcoal adsorbent AST-120 improves CKD-induced bone abnormalities as blood levels of indoxyl sulfate decrease. Uremic osteoporosis, a new concept of CKD-related bone fragility, is a main cause of CKD-induced bone abnormalities, particularly impaired bone quality. There is limited information about the effect and safety of anti-osteoporotic drugs for patients with CKD, especially those on dialysis, but the use of AST-120 and renin-angiotensin system inhibitors may modulate bone quality and decrease the incidence of fracture. Thus, the management of CKD-MBD plus use of other therapeutic interventions for uremic osteoporosis is necessary to prevent bone fragility in patients with CKD.

Changing bone patterns with progression of chronic kidney disease

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

Parathyroid hormone metabolism and signaling in health and chronic kidney disease

Circulating parathyroid hormone (PTH) shows a complex relationship with hard outcomes in subjects with chronic kidney disease (CKD). Moreover, intervention studies directly targeting PTH failed to yield unequivocal results. Disturbed PTH metabolism, posttranslational modifications of PTH, and end-organ hyporesponsiveness to PTH may explain the poor performance of PTH as an outcome biomarker and precise target of therapy in the setting of CKD, at least in the gray middle target zone. PTH fragments accumulate in CKD patients and may exert effects that are distinct from, if not opposite to biointact (1-84)PTH. Posttranslational modification of PTH and especially oxidation may alter the interaction of PTH with its receptor. Its clinical relevance, however, remains a matter of ongoing debate. Less controversial is the issue of end-organ hyporesponsiveness to PTH. This phenomenon, formally referred to as PTH resistance, has long been recognized in CKD, but factors and mechanisms contributing to it remain poorly defined. Subsequent evidence identified downregulation of the PTH receptor and competing downstream signals as underlying pathophysiologic mechanisms. End-organ hyporesponsiveness to PTH in CKD, along with important analytical and biological variability, renders defining the PTH target range in CKD challenging. Although this may still be accomplished at the population level, it may prove to be very difficult at the individual level. This is a disillusioning thought in an era of personalized medicine. Parallel to the search of a functional and readily available assay quantifying PTH signaling tone or sensitivity, additional biomarkers (or a panel of biomarkers) should be formally evaluated.

Molecular Actions of Glucocorticoids in Cartilage and Bone During Health, Disease, and Steroid Therapy

Cartilage and bone are severely affected by glucocorticoids (GCs), steroid hormones that are frequently used to treat inflammatory diseases. Major complications associated with long-term steroid therapy include impairment of cartilaginous bone growth and GC-induced osteoporosis. Particularly in arthritis, GC application can increase joint and bone damage. Contrarily, endogenous GC release supports cartilage and bone integrity. In the last decade, substantial progress in the understanding of the molecular mechanisms of GC action has been gained through genome-wide binding studies of the GC receptor. These genomic approaches have revolutionized our understanding of gene regulation by ligand-induced transcription factors in general. Furthermore, specific inactivation of GC signaling and the GC receptor in bone and cartilage cells of rodent models has enabled the cell-specific effects of GCs in normal tissue homeostasis, inflammatory bone diseases, and GC-induced osteoporosis to be dissected. In this review, we summarize the current view of GC action in cartilage and bone. We further discuss future research directions in the context of new concepts for optimized steroid therapies with less detrimental effects on bone.

Adynamic bone disease: from bone to vessels in chronic kidney disease

Adynamic bone disease (ABD) is a well-recognized clinical entity in the complex chronic kidney disease (CKD)-mineral and bone disorder. Although the combination of low intact parathyroid hormone (PTH) and low bone alkaline phosphatase levels may be suggestive of ABD, the gold standard for precise diagnosis is histomorphometric analysis of tetracycline double-labeled bone biopsies. ABD essentially is characterized by low bone turnover, low bone volume, normal mineralization, and markedly decreased cellularity with minimal or no fibrosis. ABD is increasing in prevalence relative to other forms of renal osteodystrophy, and is becoming the most frequent type of bone lesion in some series. ABD develops in situations with reduced osteoanabolic stimulation caused by oversuppression of PTH, multifactorial skeletal resistance to PTH actions in uremia, and/or dysregulation of Wnt signaling. All may contribute not only to bone disease but also to the early vascular calcification processes observed in CKD. Various risk factors have been linked to ABD, including calcium loading, ageing, diabetes, hypogonadism, parathyroidectomy, peritoneal dialysis, and antiresorptive therapies, among others. The relationship between low PTH level, ABD, increased risk fracture, and vascular calcifications may at least partially explain the association of ABD with increased mortality rates. To achieve optimal bone and cardiovascular health, attention should be focused not only on classic control of secondary hyperparathyroidism but also on prevention of ABD, especially in the steadily growing proportions of diabetic, white, and elderly patients. Overcoming the insufficient osteoanabolic stimulation in ABD is the ultimate treatment goal.

Ubiquitination-deubiquitination balance dictates ligand-stimulated PTHR sorting

Parathyroid hormone receptors (PTHR) are promptly internalized upon stimulation by activating (PTH[1-84], PTH[1-34]) and non-activating (PTH[7-84], PTH[7-34]) ligands. Here, we characterized the mechanism regulating the sorting of internalized receptors between recycling and degradative pathways. PTHR recycles faster after challenge with PTH(1-34) than with PTH(7-34). PTHR recycling is complete by 2 h after PTH(1-34) stimulation, but incomplete at this time in cells treated with PTH(7-34). The slower and incomplete recycling induced by PTH(7-34) is due to proteasomal degradation. Both PTH(1-34) and PTH(7-34) induced PTHR polyubiquitination. Ubiquitination by PTH(1-34) was transient, whereas receptor ubiquitination after PTH(7-34) was sustained. PTH(1-34), but not PTH(7-34), induced expression of the PTHR-specific deubiquitinating enzyme USP2. Overexpression of USP2 prevented PTH(7-34)-induced PTHR degradation. We conclude that PTH(1-34) promotes coupled PTHR ubiquitination and deubiquitination, whereas PTH(7-34) activates only ubiquitination, thereby leading to PTHR downregulation. These findings may explain PTH resistance in diseases associated with elevated PTH(7-84) levels.

The role of the liver and kidneys in the pharmacokinetics of subcutaneously administered teriparatide acetate in rats

Teriparatide acetate, a synthetic polypeptide fragment consisting of human parathyroid hormone residues 1-34 [hPTH(1-34)], is a bone anabolic agent used to treat osteoporosis. The present study was conducted to characterise the pharmacokinetics of teriparatide acetate in rats after subcutaneous administration. Teriparatide was rapidly absorbed into the circulation and eliminated immediately. No intact teriparatide was detected in the urine. To elucidate the mechanism of teriparatide metabolism, we performed in vivo and in vitro studies using the radiolabelled bioactive analogue, [(125)I]-[Nle(8,18),Tyr(34)]-hPTH(1-34). After subcutaneous administration, the concentration of analogue metabolites increased in the plasma time-dependently. The concentration in the kidneys was more than 3-fold the concentration in the liver. In vitro analyses suggested that kidney radioactivity was associated with degraded bioactive analogue. In model rats, renal failure, but not hepatic failure, affected the pharmacokinetics of teriparatide acetate, which accounted for the decrease in the clearance of teriparatide. In conclusion, our results suggest that after subcutaneous administration of teriparatide acetate, teriparatide is rapidly absorbed and distributed to the liver or kidneys, where it is immediately degraded. The kidneys play a particularly important role in the distribution and metabolism of teriparatide, but not its excretion.

NHERF1 regulates parathyroid hormone receptor desensitization: interference with beta-arrestin binding

Type 1 parathyroid hormone receptor (PTH1R) activation, desensitization, internalization, and recycling proceed in a cyclical manner. The Na(+)/H(+) exchange regulatory factor 1 (NHERF1) is a cytoplasmic adapter protein that regulates trafficking and signaling of several G protein-coupled receptors (GPCRs) including the PTH1R. The mineral ion wasting and bone phenotype of NHERF1-null mice suggests that PTH1R may interact with NHERF1. The objective of this study was to examine the effect of NHERF1 on PTH1R desensitization. Using rat osteosarcoma T6-N4 cells expressing the endogenous PTH1R, in which NHERF1 expression could be induced by tetracycline, PTH1R desensitization was assessed by measuring adenylyl cyclase activity after successive PTH challenges. PTH1R-mediated adenylyl cyclase responses were desensitized by repetitive PTH challenges in a concentration-dependent manner, and desensitization was inhibited by NHERF1. NHERF1 blocked PTH-induced dissociation of the PTH1R from Galpha(s). Blocking PTH1R endocytosis did not mitigate PTH1R desensitization. Reducing constitutive NHERF1 levels in human osteosarcoma SAOS2 cells, which express both endogenous PTH1R and NHERF1, with short hairpin RNA directed against NHERF1 restored PTH1R desensitization. Mutagenesis of the PDZ-binding domains or deletion of the NHERF1 MERM domain demonstrated that both are required for inhibition of receptor desensitization. A phosphorylation-deficient PTH1R exhibited reduced desensitization and interaction with beta-arrestin2 compared with wild-type PTH1R. NHERF1 inhibited beta-arrestin2 binding to wtPTH1R but had no effect on beta-arrestin2 association with pdPTH1R. Such an effect may protect against PTH resistance or PTH1R down-regulation in cells harboring NHERF1.

Differential effects of intermittent PTH(1-34) and PTH(7-34) on bone microarchitecture and aortic calcification in experimental renal failure

PTH(1-84) and PTH(7-84) are elevated in chronic kidney disease (CKD). These peptides, as their shorter analogs PTH(1-34) and PTH(7-34) both promote PTH receptor (PTH1R) internalization but only PTH(1-34) and PTH(1-84) activate the receptor. Here, we examined the effects of intermittent administration of PTH(1-34) and PTH(7-34) on mineral ion metabolism, bone architecture, and vascular calcification in rats with experimental CKD. CKD with or without parathyroidectomy (PTX) was established by 5/6 nephrectomy (NPX) in rats. Animals were divided into 4 groups: Sham PTX+ sham NPX (Sham); PTX+ sham NPX (PTX); Sham PTX+NPX (NPX); PTX+NPX (PTX/NPX). Rats were treated with single daily doses of 40 microg/kg PTH(1-34), PTH(7-34), or vehicle. Creatinine was higher in NPX and Ca lower in PTX and PTX/NPX groups than in Sham or NPX rats. Plasma phosphate was higher in PTX, NPX and PTX/NPX than in Sham rats. PTH(1-34) was more hypercalcemic than PTH(7-34) in PTX rats. Fractional bone volume in rats treated with PTH(1-34) increased significantly in all groups compared to that of vehicle treatment. In addition, trabecular number, thickness and volumetric bone density increased in rats treated with PTH(1-34). In contrast, PTH(1-34) diminished vascular calcification. Bone and renal PTH1R mRNA expression was reduced as much or more in PTX/NPX rats as in NPX alone, whereas PTH(7-34) had no effect on PTH1R expression. Renal but not bone PTH1R mRNA increased in response to PTH(1-34). These findings suggest that PTH(1-34) exerts greater hypercalcemic and anabolic effects in parathyroidectomized and/or nephrectomized rats than does PTH(7-34). There was no evidence for significant bone or vascular actions of PTH(7-34). We conclude that PTH(1-34) protects against vascular calcification and bone demineralization in experimental renal failure.

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.

NHERF1 Regulates Parathyroid Hormone Receptor Membrane Retention without Affecting Recycling

Na/H exchange regulatory factor-1 (NHERF1) is a PDZ protein that regulates trafficking of several G protein-coupled receptors. The phenotype of NHERF1-null mice suggests that the parathyroid hormone (PTH) receptor (PTH1R) is the principal GPCR interacting with NHERF1. The effect of NHERF1 on receptor recycling is unknown. Here, we characterized NHERF1 effects on PTH1R membrane tethering and recycling by radio-ligand binding and recovery after maximal receptor endocytosis. Using Chinese hamster ovary cells expressing the PTH1R, where NHERF1 expression could be induced by tetracycline, NHERF1 inhibited PTH1R endocytosis and delayed PTH1R recycling. NHERF1 also inhibited PTH-induced receptor internalization in MC4 osteoblast cells. Reducing constitutive NHERF1 levels in HEK-293 cells with short hairpin RNA directed against NHERF1 augmented PTH1R endocytosis in response to PTH. Mutagenesis of the PDZ-binding domains or deletion of the MERM domain of NHERF1 demonstrated that both are required for inhibition of endocytosis and recycling. Likewise, an intact COOH-terminal PDZ recognition motif in PTH1R is needed. The effect of NHERF1 on receptor internalization and recycling was not associated with altered receptor expression or binding, activation, or phosphorylation but involved beta-arrestin and dynamin. We conclude that NHERF1 inhibits endocytosis without affecting PTH1R recycling in MC4 and PTH1R-expressing HEK-293 cells. Such an effect may protect against PTH resistance or PTH1R down-regulation in certain cells harboring NHERF1.

Expression of bone type 1 PTH receptor in rats with chronic renal failure

Some researchers have speculated that a decrease in bone type 1 PTH receptor (PTH1R) may be among the causes of "skeletal resistance" in chronic renal failure (CRF). Indeed, the down-regulation of PTH1R mRNA has been identified in uremic bones. However, few studies have identified the patterns of PTH1R protein expression. In this article we compare the bone expression of PTH1R protein and mRNA under control and CRF conditions. Sprague-Dawley rats underwent 5/6 nephrectomies (Nx) or sham operations (control), and were killed 16 weeks later. Blood urea nitrogen (BUN), serum Cr, P, and parathyroid hormone (PTH) were higher in the Nx group than in the controls, while serum Ca and 1,25(OH)(2)D(3) were lower in the Nx group. Immunohistochemical images of lumbar bone samples were analyzed by an image processing system. PTH1R was essentially identified in all osteoblasts. The expression of osteoblast PTH1R protein was quantified based on the gray value of PTH1R staining. The mean gray scale of osteoblasts was 25% lower in Nx rats than in control rats (P < 0.01), whereas osteoblast cell counts and cell sizes were not significantly different between the two groups. Thus, down-regulation of PTH1R protein expression under the CRF condition appeared likely. Total RNA extracted from the bone samples was reverse transcribed for real-time polymerase chain reaction (PCR). PTH1R mRNA expression was 33% lower in the Nx group than in the control group in the quantitative PCR analysis (P < 0.05). Our findings suggested that osteoblast PTH1R expression is down-regulated at both the protein and mRNA levels in the steady state of CRF.

Minimizing bone abnormalities in children with renal failure

Renal osteodystrophy (ROD), a metabolic bone disease accompanying chronic renal failure (CRF), is a major clinical problem in pediatric nephrology. Growing and rapidly remodeling skeletal systems are particularly susceptible to the metabolic and endocrine disturbances in CRF. The pathogenesis of ROD is complex and multifactorial. Hypocalcemia, phosphate retention, and low levels of 1,25 dihydroxyvitamin D(3) related to CRF result in disturbances of bone metabolism and ROD. Delayed diagnosis and treatment of bone lesions might result in severe disability. Based on microscopic findings, renal bone disease is classified into two main categories: high- and low-turnover bone disease. High-turnover bone disease is associated with moderate and severe hyperparathyroidism. Low-turnover bone disease includes osteomalacia and adynamic bone disease. The treatment of ROD involves controlling serum calcium and phosphate levels, and preventing parathyroid gland hyperplasia and extraskeletal calcifications. Serum calcium and phosphorus levels should be kept within the normal range. The calcium-phosphorus product has to be <5 mmol(2)/L(2) (60 mg(2)/dL(2)). Parathyroid hormone (PTH) levels in children with CRF should be within the normal range, but in children with end-stage renal disease PTH levels should be two to three times the upper limit of the normal range. Drug treatment includes intestinal phosphate binding agents and active vitamin D metabolites. Phosphate binders should be administered with each meal. Calcium carbonate is the most widely used intestinal phosphate binder. In children with hypercalcemic episodes, sevelamer, a synthetic phosphate binder, should be introduced. In children with CRF, ergocalciferol (vitamin D(2)), colecalciferol (vitamin D(3)), and calcifediol (25-hydroxyvitamin D(3)) should be used as vitamin D analogs. In children undergoing dialysis, active vitamin D metabolites alfacalcidol (1alpha-hydroxy-vitamin D(3)) and calcitriol (1,25 dihydroxyvitamin D(3)) are applied. In recent years, a number of new drugs have emerged that hold promise for a more effective treatment of bone lesions in CRF. This review describes the current approach to the diagnosis and treament of ROD.

Mutation of phenylalanine-34 of parathyroid hormone disrupts NHERF1 regulation of PTH type I receptor signaling

Internalization of the PTH type I receptor (PTH1R) is regulated in a cell- and ligand-specific manner. We previously demonstrated that the sodium/proton exchanger regulatory factor type 1 (NHERF1; EBP50) is pivotal in determining the range of peptides that internalize the PTH1R. Antagonist PTH fragments can internalize the PTH1R in some kidney and bone cell models. PTH(7-34), which binds to, but does not activate, the PTH1R, internalizes the PTH1R in kidney distal tubule (DT) cells, where NHERF1 is not expressed. The effect of antagonist PTHrP peptides has not, to this point, been assessed. PTH1R internalization was measured by real-time confocal fluorescence microscopy of DT cells stably expressing 105 EGFP-tagged PTH1R/cell. PTHrP(7-34) internalized the PTH1R in a manner indistinguishable from PTH(7-34). Introduction of NHERF1 into DT cells, however, blocked PTH(7-34)-, but not PTHrP(7-34)-, induced PTH1R internalization. To delineate the sequences within PTHrP that determine whether PTH1R internalization is affected by NHERF1, chimeric PTH/PTHrP fragments were tested for their ability to induce PTH1R internalization. PTH(7-21)/PTHrP (22-34), PTH(7-32)/PTHrP(33-34), and PTH(7-33)/PTHrP(34) at 1 microM each internalized the PTH1R 50-70% in a NHERF1-independent manner. When the C terminus of PTHrP was replaced with homologous amino acids from PTH, NHERF1 inhibited PTH1R internalization. It was determined that simply mutating F34 to A in PTH induced PTH1R internalization in a NHERF1-independent manner. None of the chimeric peptides activated the PTH1R but all effectively competed for 1 nM PTH(1-34) in cyclic AMP assays. In addition, all chimeric peptides competed for radiolabeled PTH(1-34) in binding assays in DT cells. PTH(1- 34) and PTHrP(7-34), but not PTH(7-34), efficiently recruited beta-arrestin1 to plasma membrane PTH1Rs. We, therefore, conclude that PTH(1-34) and PTHrP(7-34) induce a conformational change in the PTH1R that promotes arrestin binding and dissociates NHERF1 from PTH1R internalization.

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.

The need for reliable serum parathyroid hormone measurements

Serum parathyroid hormone (PTH) is a recognized marker of bone remodeling in patients with renal osteodystrophy. However, identification of N-terminal truncated PTH fragments and a new form of PTH that interfere with second-generation PTH assays may be responsible for the great variability of PTH values and the difficulties of implementing the recommendations of the National Kidney Foundation/Kidney Disease Outcomes Quality Initiative.

Cinacalcet HCl: A Novel Treatment for Secondary Hyperparathyroidism Caused by Chronic Kidney Disease

Secondary hyperparathyroidism (SHPT) develops as a result of impaired calcium homeostasis when the failing kidneys disturb the complicated interactions between parathyroid hormone (PTH), calcium, phosphorus, and vitamin D. Twelve years ago, the calcium-sensing receptor (CaR) of the parathyroid gland was first cloned and identified as the principal regulator of PTH secretion. The activation of the CaR by small changes in extracellular calcium (ec(Ca2+)) regulates PTH, calcitonin secretion, urinary calcium excretion, and ultimately, bone turnover. The CaR became an ideal target for the development of calcimimetics, which are able to amplify its sensitivity to ec(Ca2+) suppressing PTH secretion. Cinacalcet HCl, a first-in-class calcimimetic, approved in both the United States and the European Union, offers a new therapeutic approach to the treatment of SHPT. The efficacy of cinacalcet HCl in treating SHPT in dialysis patients (n = 1,136) was studied in three similarly designed phase III clinical trials comparing patients receiving standard SHPT therapy plus cinacalcet HCl or plus placebo. Cinacalcet HCl, dosed from 30 to 180 mg/day, significantly reduced PTH while simultaneously lowering calcium, phosphorus, and calcium-phosphorus product in each of the three studies. Respective to the National Kidney Foundation-Kidney Disease Outcomes and Quality Initiative (NKF-K/DOQI) recommended targets for bone and mineral metabolism, 41% of cinacalcet HCl-treated patients achieved both PTH and calcium-phosphorus product targets, compared with only 6% in the placebo group. Results from 2 recent phase IIIb studies (TARGET and CONTROL) conducted in the United States also showed that cinacalcet HCl can significantly reduce or maintain reduction in PTH while simultaneously lowering calcium, phosphorus, and calcium-phosphorus product. In addition, patients taking vitamin D at baseline of these 2 trials were able to see significant mean reductions in vitamin D dose. Further assessment of cinacalcet HCl trial data has shown some important effects in SHPT patient clinical outcomes. A combined post-hoc analysis of clinical events using data from 4 (n = 1,184) cinacalcet HCl phase II and III studies suggests that treatment with cinacalcet HCl has a beneficial effect on relative risks of parathyroidectomy, fracture, and hospitalization for cardiovascular complications. Nausea and vomiting occurred more often in patients taking cinacalcet HCl than in those taking a placebo. There were also transient episodes of hypocalcemia in 5% of cinacalcet HCl patients versus 1% of placebo patients. However, these episodes were rarely associated with symptoms. The development of calcimimetics has already changed the treatment of SHPT in renal patients. Its effectiveness on the control of PTH secretion, along with simultaneous reductions in calcium, phosphorus, and calcium-phosphorus product, give this agent an advantage over traditional therapies in all levels of severity of SHPT.

PTH(1–84)/PTH(7–84): a balance of power

This review considers many new basic and clinical aspects of parathyroid hormone (PTH). We focus especially on the identification of PTH fragments and how they may relate to renal failure, diagnosis, and treatment of secondary hyperparathyroidism and renal osteodystrophy. The biosynthesis and metabolism of PTH, measurement of circulating forms of PTH, the effects of PTH on receptor activation and turnover, the relationship between PTH levels and bone turnover in renal failure in humans, and the involvement of PTH in experimental models of renal failure are discussed. Despite these developments in understanding the etiology of renal failure and the availability of new assays for bioactive PTH, no adequate surrogate for bone biopsy and quantitative bone histomorphometry has been developed.

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.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

PTH revisited11This paper is dedicated to Professor Thomas E. Andreoli. Pigmæi gigantum humeris impositi plusquam ipsi gigantes vident.22Original studies were supported by National Institutes of Health grant DK-54171

Recent investigations of parathyroid hormone (PTH) have advanced our understanding of its circulating forms as well as its action. It is now clear that first-generation immunoradiometric assays of so-called intact "PTH" not only measured full-length PTH(1-84) but also recognized large PTH fragments lacking the amino-terminus. New, second generation assays detect only full-length PTH. Under diverse pathological settings, second generation assays display lower levels of PTH (1-84). By measuring full-length PTH (bioactive PTH) and the combined full-length plus amino-terminal PTH fragments, the amount of non-PTH(1-84) in circulation can be estimated. The primary amino-terminal fragment is likely to be PTH(7-84). A considerable controversy surrounds the pathological significance of PTH(7-84) and its relation to adynamic bone disease. While these findings were emerging, other work uncovered the apparent basis by which PTH receptors signal through cAMP in some instances but through Ca/inositol phosphate in others. This signaling switch is dictated by the cytoplasmic adapter protein NHERF1 (EBP50), which is expressed in a cell-selective fashion. Other provocative findings may provide a means of unifying determinations of PTH(7-84) with the effects of NHERF1 on PTH receptor signaling. These latter studies reveal that in cells expressing NHERF1, PTH(7-84) has no effect on PTH receptor signaling or internalization. However, in cells lacking or expressing low levels of NHERF1, PTH(7-84) internalizes the PTH receptor without accompanying activation. Together, these findings suggest that the accumulation of PTH(7-84) in renal failure may lead to PTH resistance by internalizing and down-regulating PTH receptors.

Effect of 2′-phosphophloretin on renal function in chronic renal failure rats

Hyperhosphatemia and secondary hyperparathyroidism are common and severe complications of chronic renal failure. Therapies to reduce serum phosphate have been shown to reduce serum parathyroid hormone (PTH) and slow the progression of renal failure. The effect of the inhibitor of intestinal phosphate absorption, 2′-phosphophloretin (2′-PP), on serum and urine chemistry, renal histology, and cardiac structure in the uremic rat model of renal failure, 5/6 nephrectomy (5/6 NX), was examined. The effect of 2′-PP on serum phosphate, serum PTH, serum total Ca2+, and ionized Ca2+, Ca2+ × Pi product, urine protein, urine osmolality, and creatinine clearance in 5/6 NX rats was examined. Uremic rats in chronic renal failure were gavaged daily with 25 μM 2′-PP. Over the course of a 5-wk experiment, serum chemistry in untreated uremic rats, 2′-PP-treated uremic rats, and age-matched control rats with normal renal function was determined twice a week. Urine creatinine, urine osmolality, urine phosphate, and urine protein were determined once a week from 24-h collections. 2′-PP reduced serum phosphate 40 ± 3% compared with a 17% increase in untreated uremic control rats. 2′-PP did not alter total serum Ca2+. During 5-wk experiments, serum PTH increased 65 ± 25% in untreated uremic rats and decreased 70 ± 7% in uremic rats treated with 25 μM 2′-PP. Creatinine clearance decreased 20% in untreated uremic rats compared with a 100% increase in 2′-PP-treated uremic rats. Urine protein decreased and urine osmolality increased in uremic rats treated with 2′-PP. The mechanism of the effect of 2′-PP on serum phosphate was inhibition of intestinal phosphate absorption. 2-PP inhibited intestinal phosphate absorption 50% without altering dietary protein absorption or intestinal Ca2+ absorption. Over the course of the 5-wk treatment with 2′-PP, uremic animals treated with 2′-PP had a 2–4% weight gain/wk, similar to the weight gain seen in age-matched control rats with normal renal function.

Special aspects of renal osteodystrophy in children

Renal osteodystrophy represents a spectrum of skeletal lesions that range from high-turnover to low-turnover bone disease. Similar factors are involved in the pathogenesis of renal osteodystrophy in adult and pediatric patients with chronic kidney disease (CKD). However, growth retardation and the development of bone deformities are specific complications that occurred in pediatric patients with CKD. Metabolic acidosis, renal osteodystrophy, malnutrition, and disturbances in the insulin growth factor (IGF)/growth hormone (GH) are among the main factors involved and they are discussed briefly in this article. In addition to disturbances in bone remodeling, longitudinal bone growth occurs at the growth plate cartilage by endochondral ossification. Although young rats with experimental CKD have growth retardation, the characteristics of the growth plate are markedly different between animals with severe secondary hyperparathyroidism and those with calcium-induced adynamic osteodystrophy. These disturbances may suggest potential molecular mechanisms by which endochondral bone formation may be altered in renal failure, consequently leading to growth retardation.

Activation-independent parathyroid hormone receptor internalization is regulated by NHERF1 (EBP50)

Parathyroid hormone (PTH) regulates extracellular calcium homeostasis through the type 1 PTH receptor (PTH1R) expressed in kidney and bone. The PTH1R undergoes beta-arrestin/dynamin-mediated endocytosis in response to the biologically active forms of PTH, PTH-(1-34), and PTH-(1-84). We now show that amino-truncated forms of PTH that do not activate the PTH1R nonetheless induce PTH1R internalization in a cell-specific pattern. Activation-independent PTH1R endocytosis proceeds through a distinct arrestin-independent mechanism that is operative in cells lacking the adaptor protein Na/H exchange regulatory factor 1 (NHERF1) (ezrin-binding protein 50). Using a combination of radioligand binding experiments and quantitative, live cell confocal microscopy of fluorescently tagged PTH1Rs, we show that in kidney distal tubule cells and rat osteosarcoma cells, which lack NHERF1, the synthetic antagonist PTH-(7-34) and naturally circulating PTH-(7-84) induce internalization of PTH1R in a beta-arrestin-independent but dynamin-dependent manner. Expression of NHERF1 in these cells inhibited antagonist-induced endocytosis. Conversely, expression of dominant-negative forms of NHERF1 conferred internalization sensitivity to PTH-(7-34) in cells expressing NHERF1. Mutation of the PTH1R PDZ-binding motif abrogated interaction of the receptor with NHERF1. These mutated receptors were fully functional but were now internalized in response to PTH-(7-34) even in NHERF1-expressing cells. Removing the NHERF1 ERM domain or inhibiting actin polymerization allowed otherwise inactive ligands to internalize the PTH1R. These results demonstrate that NHERF1 acts as a molecular switch that legislates the conditional efficacy of PTH fragments. Distinct endocytic pathways are determined by NHERF1 that are operative for the PTH1R in kidney and bone cells.

Gene expression of vitamin D hydroxylase and megalin in the remnant kidney of nephrectomized rats

BACKGROUND

Regulation of vitamin D hydroxylase genes in the early stage of chronic renal failure is not fully understood. Using nephrectomized rats, we examined changes in mRNA levels of CYP27B1 (25-hydroxyvitamin D3-1 alpha-hydroxylase), CYP24 (25-hydroxyvitamin D3-24-hydroxylase), and vitamin D receptor in relation to megalin, recently found to participate in renal vitamin D metabolism.

METHODS

A rat model of moderate renal failure was induced by 3/4 nephrectomy. Plasma parameters, including vitamin D metabolite concentrations, were measured at weeks 2, 4 and 8, and poly(A)+ RNA extracted from the remnant kidneys was subjected to Northern blot hybridization.

RESULTS

Plasma creatinine concentration at week 2 was 0.40 +/- 0.02 mg/dL in the sham-operated and 0.93 +/- 0.15 mg/dL in the nephrectomized rats, and both values remained constant up to week 8. Plasma concentrations of 25(OH)D3, 1 alpha,25(OH)2D3, and 24,25(OH)2D3 were unchanged between nephrectomized and sham-operated rats at week 8. Intact parathyroid hormone (PTH) increased at week 8 in nephrectomized rats. CYP27B1 mRNA in nephrectomized rats did not vary at week 2, but increased approximately two- and four-fold at weeks 4 and 8, respectively, compared to the sham-operated rats. CYP24 and megalin mRNAs, on the other hand, began to decline as early as at week 2 in nephrectomized rats and kept decreasing throughout the experiment. The expression of vitamin D receptor was modestly but significantly decreased only at week 8.

CONCLUSION

Coordinated and reciprocal alterations of the increase in CYP27B1 mRNA and the decrease in CYP24 mRNA may play a pivotal role in maintaining the plasma level of 1 alpha,25(OH)2D3 in the face of reduced nephron mass and/or megalin expression.

Correlation between histomorphometric parameters of bone resorption and serum type 5b tartrate-resistant acid phosphatase in uremic patients on maintenance hemodialysis

BACKGROUND

Serum tartrate-resistant acid phosphatase 5b (TRACP) is a new marker of potential clinical use to monitor osteoclastic activity and bone resorption rate. The relationship between histomorphometric parameters of bone resorption and serum TRACP was evaluated in 14 chronically dialyzed patients and 6 healthy control subjects.

METHODS

All patients underwent bone biopsies and serum biochemical testing for TRACP, intact parathyroid hormone (iPTH), pyridinoline cross-linked telopeptide domain of type I collagen (ICTP), total calcium, phosphorus, and albumin, which were measured at the time of biopsy.

RESULTS

Bone histological examination showed predominant hyperparathyroid bone disease (HPT) in 6 patients, mixed uremic osteodystrophy in 3 patients, low-turnover osteomalacia in 1 patient, and adynamic bone disease in 4 patients. Mean TRACP activity was 3.25 +/- 0.59 U/L in control subjects. Median TRACP activity was significantly greater in patients with HPT (11.97 +/- 8.92 U/L) than those with other types of renal osteodystrophy (ROD; 2.17 +/- 0.61 U/L). Serum iPTH levels were greatest in all patients with HPT, but also were significantly elevated in 7 of 8 patients with other types of ROD. Serum ICTP levels also were significantly elevated in all patients with HPT and 6 of 8 patients with other types of ROD. Serum TRACP levels correlated more strongly with histological parameters of osteoclasts than those of erosion. Also, correlations between TRACP and histological parameters of osteoclasts were stronger than those of iPTH and ICTP levels.

CONCLUSION

These early results suggest that serum TRACP levels correlate well with histological indices of osteoclasts and may serve as a specific marker for osteoclastic activity in patients with renal bone disease.

Central nervous dysfunction in uremia

The mechanisms of central nervous system dysfunction in uremia are multifactorial and only partially characterized. Studies using sealed presynaptic nerve terminals (synaptosomes) for in vitro ion transport and metabolism of neurotransmitter in chronic renal failure (CRF) neuronal cell culture and in vivo brain structure microdialysis generated significant new information. An increase in total calcium content of the cerebral cortex accompanied by increased levels of cytosolic calcium ([Ca(2+)]i) in synaptosomes are common findings in rats with CRF. Mechanisms leading to the increase in [Ca(2+)]i include increased calcium uptake mediated by parathyroid hormone and decreased activity of Na(+),K(+)-adenosine triphosphatase (ATPase) and Ca(2+)-ATPase of synaptosomes in CRF rats. Moreover, these synaptosomes respond inappropriately to depolarization, which can impair neurotransmitter metabolism. Brain gamma-aminobutyric acid content, norepinephrine, and acetylcholine release uptake and degradation are affected by uremia. These may lead to certain somatic, behavioral, and motor dysfunctions in uremia. Many derangements of the central nervous system in uremia appear to be mediated by secondary hyperparathyroidism of CRF because parathyroidectomy of animals with CRF prevented the increase in basal levels of [Ca(2+)]i and derangements in neurotransmitter metabolism. The role of other neurotoxins, such as guanidinosuccinic acid, are also reviewed.

Bisphosphonates in renal osteodystrophy

The present review considers the role that bisphosphonates might have in patients with renal failure. Although bisphosphonates are widely used to reduce fracture risk in patients with osteoporosis, few studies have documented their effect in patients with renal osteodystrophy. The pathogenesis of bone loss after renal transplantation and the role of the recently identified osteoprotegerin/receptor activating nuclear factor-kappaB system is described. Inhibition of bone resorption may prove beneficial when high bone turnover is present, but there are potential drawbacks to widespread use of bisphosphonates. These issues are discussed, with emphasis placed on reports published within the past 18 months.