Cellular and molecular mechanisms of secondary hyperparathyroidism

Skeletal and extraskeletal disorders of biomineralization

The physiological process of biomineralization is complex and deviation from it leads to a variety of diseases. Progress in the past 10 years has enhanced understanding of the genetic, molecular and cellular pathophysiology underlying these disorders; sometimes, this knowledge has both facilitated restoration of health and clarified the very nature of biomineralization as it occurs in humans. In this Review, we consider the principal regulators of mineralization and crystallization, and how dysregulation of these processes can lead to human disease. The knowledge acquired to date and gaps still to be filled are highlighted. The disorders of mineralization discussed comprise a broad spectrum of conditions that encompass bone disorders associated with alterations of mineral quantity and quality, as well as disorders of extraskeletal mineralization (hyperphosphataemic familial tumoural calcinosis). Included are disorders of alkaline phosphatase (hypophosphatasia) and phosphate homeostasis (X-linked hypophosphataemic rickets, fluorosis, rickets and osteomalacia). Furthermore, crystallopathies are covered as well as arterial and renal calcification. This Review discusses the current knowledge of biomineralization derived from basic and clinical research and points to future studies that will lead to new therapeutic approaches for biomineralization disorders.

Osteoporosis in Patients with Chronic Kidney Diseases: A Systemic Review

Chronic kidney disease (CKD) is associated with the development of mineral bone disorder (MBD), osteoporosis, and fragility fractures. Among CKD patients, adynamic bone disease or low bone turnover is the most common type of renal osteodystrophy. The consequences of CKD-MBD include increased fracture risk, greater morbidity, and mortality. Thus, the goal is to prevent the occurrences of fractures by means of alleviating CKD-induced MBD and treating subsequent osteoporosis. Changes in mineral and humoral metabolism as well as bone structure develop early in the course of CKD. CKD-MBD includes abnormalities of calcium, phosphorus, PTH, and/or vitamin D; abnormalities in bone turnover, mineralization, volume, linear growth, or strength; and/or vascular or other soft tissue calcification. In patients with CKD-MBD, using either DXA or FRAX to screen fracture risk should be considered. Biomarkers such as bALP and iPTH may assist to assess bone turnover. Before initiating an antiresorptive or anabolic agent to treat osteoporosis in CKD patients, lifestyle modifications, such as exercise, calcium, and vitamin D supplementation, smoking cessation, and avoidance of excessive alcohol intake are important. Managing hyperphosphatemia and SHPT are also crucial. Understanding the complex pathogenesis of CKD-MBD is crucial in improving one’s short- and long-term outcomes. Treatment strategies for CKD-associated osteoporosis should be patient-centered to determine the type of renal osteodystrophy. This review focuses on the mechanism, evaluation and management of patients with CKD-MBD. However, further studies are needed to explore more details regarding the underlying pathophysiology and to assess the safety and efficacy of agents for treating CKD-MBD.

Higher Intra-Dialysis Serum Phosphorus Reduction Ratio as a Predictor of Mortality in Patients on Long-Term Hemodialysis

Background

Rapid shifting between extracellular and intracellular phosphorus can occur during dialysis sessions, which can cause aberrant intracellular signaling in long-term hemodialysis (LTHD) patients. However, the effect of these intra-dialysis fluctuations of phosphorus on clinical outcomes has not been examined. Therefore, we investigated the relationship between intradialysis serum phosphorus reduction ratio (IDSPRR) and mortality in LTHD patients.

Material/Methods

This was a retrospective, observational cohort study to assess the predictive power of IDSPRR (>0.63 vs. ≤0.63) on mortality in a total of 805 LTHD patients. All these fatal events were analyzed using the Cox proportional hazards regression model.

Results

After multivariable analysis, baseline IDSPRR higher than 0.63 was significantly predictive of all-cause mortality (hazard ratio [HR]: 1.58; 95% confidence interval [CI]: 1.10–2.26), but not for cardiovascular (CV) mortality (HR: 1.41; 95% CI: 0.91–2.18). However, when time-varied IDSPRRs were applied, a value greater than 0.63 was not only significantly predictive of all-cause mortality (HR: 1.74, 95% CI: 1.16–2.63), but also CV mortality (HR: 2.04, 95% CI: 1.23–3.40).

Conclusions

High IDSPRR (>0.63) is independently associated with increased all-cause and CV mortality, which shows the negative effect of rapid intracellular phosphorus-shifting on LTHD patients.

Phosphate binders for preventing and treating chronic kidney disease-mineral and bone disorder (CKD-MBD)

BACKGROUND

Phosphate binders are used to reduce positive phosphate balance and to lower serum phosphate levels for people with chronic kidney disease (CKD) with the aim to prevent progression of chronic kidney disease-mineral and bone disorder (CKD-MBD). This is an update of a review first published in 2011.

OBJECTIVES

The aim of this review was to assess the benefits and harms of phosphate binders for people with CKD with particular reference to relevant biochemical end-points, musculoskeletal and cardiovascular morbidity, hospitalisation, and death.

SEARCH METHODS

We searched the Cochrane Kidney and Transplant Register of Studies up to 12 July 2018 through contact with the Information Specialist using search terms relevant to this review. Studies in the Register are identified through searches of CENTRAL, MEDLINE, and EMBASE, conference proceedings, the International Clinical Trials Register (ICTRP) Search Portal and ClinicalTrials.gov.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) or quasi-RCTs of adults with CKD of any GFR category comparing a phosphate binder to another phosphate binder, placebo or usual care to lower serum phosphate. Outcomes included all-cause and cardiovascular death, myocardial infarction, stroke, adverse events, vascular calcification and bone fracture, and surrogates for such outcomes including serum phosphate, parathyroid hormone (PTH), and FGF23.

DATA COLLECTION AND ANALYSIS

Two authors independently selected studies for inclusion and extracted study data. We applied the Cochrane 'Risk of Bias' tool and used the GRADE process to assess evidence certainty. We estimated treatment effects using random-effects meta-analysis. Results were expressed as risk ratios (RR) for dichotomous outcomes together with 95% confidence intervals (CI) or mean differences (MD) or standardised MD (SMD) for continuous outcomes.

MAIN RESULTS

We included 104 studies involving 13,744 adults. Sixty-nine new studies were added to this 2018 update.Most placebo or usual care controlled studies were among participants with CKD G2 to G5 not requiring dialysis (15/25 studies involving 1467 participants) while most head to head studies involved participants with CKD G5D treated with dialysis (74/81 studies involving 10,364 participants). Overall, seven studies compared sevelamer with placebo or usual care (667 participants), seven compared lanthanum to placebo or usual care (515 participants), three compared iron to placebo or usual care (422 participants), and four compared calcium to placebo or usual care (278 participants). Thirty studies compared sevelamer to calcium (5424 participants), and fourteen studies compared lanthanum to calcium (1690 participants). No study compared iron-based binders to calcium. The remaining studies evaluated comparisons between sevelamer (hydrochloride or carbonate), sevelamer plus calcium, lanthanum, iron (ferric citrate, sucroferric oxyhydroxide, stabilised polynuclear iron(III)-oxyhydroxide), calcium (acetate, ketoglutarate, carbonate), bixalomer, colestilan, magnesium (carbonate), magnesium plus calcium, aluminium hydroxide, sucralfate, the inhibitor of phosphate absorption nicotinamide, placebo, or usual care without binder. In 82 studies, treatment was evaluated among adults with CKD G5D treated with haemodialysis or peritoneal dialysis, while in 22 studies, treatment was evaluated among participants with CKD G2 to G5. The duration of study follow-up ranged from 8 weeks to 36 months (median 3.7 months). The sample size ranged from 8 to 2103 participants (median 69). The mean age ranged between 42.6 and 68.9 years.Random sequence generation and allocation concealment were low risk in 25 and 15 studies, respectively. Twenty-seven studies reported low risk methods for blinding of participants, investigators, and outcome assessors. Thirty-one studies were at low risk of attrition bias and 69 studies were at low risk of selective reporting bias.In CKD G2 to G5, compared with placebo or usual care, sevelamer, lanthanum, iron and calcium-based phosphate binders had uncertain or inestimable effects on death (all causes), cardiovascular death, myocardial infarction, stroke, fracture, or coronary artery calcification. Sevelamer may lead to constipation (RR 6.92, CI 2.24 to 21.4; low certainty) and lanthanum (RR 2.98, CI 1.21 to 7.30, moderate certainty) and iron-based binders (RR 2.66, CI 1.15 to 6.12, moderate certainty) probably increased constipation compared with placebo or usual care. Lanthanum may result in vomiting (RR 3.72, CI 1.36 to 10.18, low certainty). Iron-based binders probably result in diarrhoea (RR 2.81, CI 1.18 to 6.68, high certainty), while the risks of other adverse events for all binders were uncertain.In CKD G5D sevelamer may lead to lower death (all causes) (RR 0.53, CI 0.30 to 0.91, low certainty) and induce less hypercalcaemia (RR 0.30, CI 0.20 to 0.43, low certainty) when compared with calcium-based binders, and has uncertain or inestimable effects on cardiovascular death, myocardial infarction, stroke, fracture, or coronary artery calcification. The finding of lower death with sevelamer compared with calcium was present when the analysis was restricted to studies at low risk of bias (RR 0.50, CI 0.32 to 0.77). In absolute terms, sevelamer may lower risk of death (all causes) from 210 per 1000 to 105 per 1000 over a follow-up of up to 36 months, compared to calcium-based binders. Compared with calcium-based binders, lanthanum had uncertain effects with respect to all-cause or cardiovascular death, myocardial infarction, stroke, fracture, or coronary artery calcification and probably had reduced risks of treatment-related hypercalcaemia (RR 0.16, CI 0.06 to 0.43, low certainty). There were no head-to-head studies of iron-based binders compared with calcium. The paucity of placebo-controlled studies in CKD G5D has led to uncertainty about the effects of phosphate binders on patient-important outcomes compared with placebo.It is uncertain whether the effects of binders on clinically-relevant outcomes were different for patients who were and were not treated with dialysis in subgroup analyses.

AUTHORS' CONCLUSIONS

In studies of adults with CKD G5D treated with dialysis, sevelamer may lower death (all causes) compared to calcium-based binders and incur less treatment-related hypercalcaemia, while we found no clinically important benefits of any phosphate binder on cardiovascular death, myocardial infarction, stroke, fracture or coronary artery calcification. The effects of binders on patient-important outcomes compared to placebo are uncertain. In patients with CKD G2 to G5, the effects of sevelamer, lanthanum, and iron-based phosphate binders on cardiovascular, vascular calcification, and bone outcomes compared to placebo or usual care, are also uncertain and they may incur constipation, while iron-based binders may lead to diarrhoea.

Elemental calcium intake associated with calcium acetate/calcium carbonate in the treatment of hyperphosphatemia

Background

Calcium-based and non-calcium-based phosphate binders have similar efficacy in the treatment of hyperphosphatemia; however, calcium-based binders may be associated with hypercalcemia, vascular calcification, and adynamic bone disease.

Scope

A post hoc analysis was carried out of data from a 16-week, Phase IV study of patients with end-stage renal disease (ESRD) who switched to lanthanum carbonate monotherapy from baseline calcium acetate/calcium carbonate monotherapy. Of the intent-to-treat population (N=2520), 752 patients with recorded dose data for calcium acetate (n=551)/calcium carbonate (n=201) at baseline and lanthanum carbonate at week 16 were studied. Elemental calcium intake, serum phosphate, corrected serum calcium, and serum intact parathyroid hormone levels were analyzed.

Findings

Of the 551 patients with calcium acetate dose data, 271 (49.2%) had an elemental calcium intake of at least 1.5 g/day at baseline, and 142 (25.8%) had an intake of at least 2.0 g/day. Mean (95% confidence interval [CI]) serum phosphate levels were 6.1 (5.89, 6.21) mg/dL at baseline and 6.2 (6.04, 6.38) mg/dL at 16 weeks; mean (95% CI) corrected serum calcium levels were 9.3 (9.16, 9.44) mg/dL and 9.2 (9.06, 9.34) mg/dL, respectively. Of the 201 patients with calcium carbonate dose data, 117 (58.2%) had an elemental calcium intake of at least 1.5 g/day, and 76 (37.8%) had an intake of at least 2.0 g/day. Mean (95% CI) serum phosphate levels were 5.8 (5.52, 6.06) mg/dL at baseline and 5.8 (5.53, 6.05) mg/dL at week 16; mean (95% CI) corrected serum calcium levels were 9.7 (9.15, 10.25) mg/dL and 9.2 (9.06, 9.34) mg/dL, respectively.

Conclusion

Calcium acetate/calcium carbonate phosphate binders, taken to control serum phosphate levels, may result in high levels of elemental calcium intake. This may lead to complications related to calcium balance.

Association between serum pregnancy-associated plasma protein-A and bicarbonate in hemodialysis patients

BACKGROUND

Acidosis is associated with protein-energy malnutrition, inflammation, and bone disease, and low bicarbonate levels have been implicated in higher mortality rates in chronic kidney disease. Recently, the concentration of serum pregnancy-associated plasma protein-A (PAPP-A) has become accepted as a prognostic marker in hemodialysis patients. This study determined the relationship between PAPP-A and bicarbonate levels in these patients.

METHODS

The study enrolled 65 hemodialysis patients (41 males, 24 females) and 26 control subjects (11 males, 15 females). Serum PAPP-A, intact parathormone (iPTH), calcium, phosphorus (P), and bicarbonate levels were measured. Correlations between PAPP-A and bicarbonate, iPTH, calcium, and phosphorus were evaluated.

RESULTS

Median PAPP-A levels were significantly higher in hemodialysis patients [15.1 (<0.03-158.8) ng/ml] than in control subjects [6.6 (<0.03-16.4) ng/ml] (P < 0.05). There were statistically significant correlations between serum PAPP-A and bicarbonate, iPTH, and P in hemodialysis patients but not in control subjects.

CONCLUSION

Elevation of serum PAPP-A has been found in hemodialysis patients and its significant correlation with bicarbonate suggests that it may be a prognostic factor.

Effects of gum arabic (Acacia senegal) on renal function in diabetic mice

BACKGROUND/AIMS

Gum arabic (GA) is a Ca(2+)-, Mg(2+)- and K(+)-rich dietary fiber used for the treatment of patients with chronic kidney disease in Middle Eastern countries. In healthy mice, GA treatment increases creatinine clearance, renal ADH excretion, as well as intestinal and renal excretion of Mg(2+) and Ca(2+). GA decreases plasma Pi concentration, urinary Pi and Na(+) excretion. The present study explored the effects of GA on renal function in diabetic mice.

METHODS

Metabolic cage experiments were performed on Akita mice (akita(+/-)), which spontaneously develop insulin deficiency and thus hyperglycemia. Plasma and urinary concentrations of Na(+), K(+) and Ca(2+) were measured by flame photometry (AFM 5051, Eppendorf, Germany), creatinine by the Jaffé method, phosphate photometrically, urea by an enzymatic method, glucose utilizing a glucometer and an enzymatic kit, aldosterone using an RIA, urinary albumin fluorometrically, and blood pressure by the tail-cuff method.

RESULTS

GA (10% in drinking water) significantly increased urinary excretion of Ca(2+) and significantly decreased plasma phosphate and urea concentrations, urinary flow rate, urinary Na(+), phosphate and glucose excretion, blood pressure and proteinuria.

CONCLUSIONS

GA treatment decreases blood pressure and proteinuria in diabetic mice and may thus prove beneficial in diabetic nephropathy.

Phosphate binders for preventing and treating bone disease in chronic kidney disease patients

BACKGROUND

Phosphate binders are widely used to lower serum phosphorus levels in people with chronic kidney disease (CKD) but their impact in CKD remains controversial.

OBJECTIVES

To review the effects of various phosphate binders on biochemical and patient-level end-points in CKD stages 3 to 5D.

SEARCH STRATEGY

In March 2010 we searched MEDLINE, EMBASE, the Cochrane Renal Group's Specialised Register and CENTRAL for relevant studies.

SELECTION CRITERIA

Randomised controlled trials (RCTs) or quasi-RCTs that assessed the effects of various phosphate binders in adults with CKD.

DATA COLLECTION AND ANALYSIS

Two authors independently reviewed search results and extracted data. Results were expressed as mean differences (MD) for continuous outcomes and risk ratios (RR) for dichotomous outcomes with 95% confidence intervals (CI) using a random-effects model.

MAIN RESULTS

Sixty studies (7631 participants) were included. There was no significant reduction in all-cause mortality (10 studies, 3079 participants: RR 0.73, 95% CI 0.46 to 1.16), or serum calcium by phosphorus (Ca x P) product with sevelamer hydrochloride compared to calcium-based agents. There was a significant reduction in serum phosphorus (16 studies, 3126 participants: MD 0.23 mg/dL, 95% CI 0.04 to 0.42) and parathyroid hormone (PTH) (12 studies, 2551 participants; MD 56 pg/mL, 95% CI 26 to 84) but a significant increase in the risk of hypercalcaemia (12 studies, 1144 participants: RR 0.45, 95% CI 0.35 to 0.59) with calcium-based agents compared to sevelamer hydrochloride. There was a significant increase in the risk of adverse gastrointestinal events with sevelamer hydrochloride in comparison to calcium salts (5 studies, 498 participants: RR 1.58, 95% CI 1.11 to 2.25). Compared with calcium-based agents, lanthanum significantly reduced serum calcium (2 studies, 122 participants: MD -0.30 mg/dL, 95% CI -0.64 to -0.25) and the Ca x P product, but not serum phosphorus levels. The effects of calcium acetate on biochemical end-points were similar to those of calcium carbonate. The phosphorus lowering effects of novel agents such as ferric citrate, colestilan and niacinamide were only reported in a few studies.

AUTHORS' CONCLUSIONS

Available phosphate-binding agents have been shown to reduce phosphorus levels in comparison to placebo. However, there are insufficient data to establish the comparative superiority of novel non-calcium binding agents over calcium-containing phosphate binders for patient-level outcomes such as all-cause mortality and cardiovascular end-points in CKD.

Involvement of GCMB in the transcriptional regulation of the human parathyroid hormone gene in a parathyroid-derived cell line PT-r: effects of calcium and 1,25(OH)2D3

Expression of the PTH gene is known to be under strict tissue-specific control and is also regulated by extracellular calcium and 1,25(OH)(2)D. However, the precise mode of transcriptional regulation remains to be elucidated, because of the unavailability of appropriate cell lines derived from the parathyroid gland. We tried to identify the transcription factor(s) regulating the human PTH gene transcription using the PT-r cell line. We found that PT-r cells endogenously express PTH and several parathyroid-related genes. Using the cells, we investigated the transcriptional regulation of human PTH gene. We found that GCMB binds to the PTH gene 5'-promoter (-390/-383 bp) and positively regulates its transcription. On the other hand, 1,25(OH)(2)D(3), and, in the presence of the calcium sensing receptor, high extracellular calcium, exerted inhibitory effects on PTH gene expression. These results indicate that GCMB and vitamin D receptor are involved in the positive and negative regulation of PTH gene expression, respectively. Our data also suggest that PT-r cells retain some of the characteristics of parathyroid cells.

Bone disease in elderly individuals with CKD

Bone disease can lead to significant morbidity and mortality for those who are afflicted by it, irrespective of etiology. Two very prevalent causes of bone disease that contribute to this are osteoporosis and chronic kidney disease (CKD). The modern era has seen important advances in the understanding and management of these processes, but in elderly patients with CKD it remains a complex issue that has yet to be clearly defined. Changes in mineral metabolism that accompany the loss of renal function result in a spectrum of bone disease that occurs concomitantly with bone loss secondary to aging. As such, the traditional paradigms used to manage bone disease may not be appropriate for these patients. With the aging dialysis population, a better understanding of these 2 processes and their interplay deserves more attention.

Elevated serum PTH is independently associated with poor outcomes in older patients with hip fracture and vitamin D inadequacy

To determine whether serum 25(OH)D and/or PTH levels in older patients with hip fracture (HF) could predict short-term clinical outcomes, we conducted a prospective observational study of 287 consecutive HF patients (mean age 81.9 + or - 7.5 [SD] years, 72% females). The prevalence of vitamin D inadequacy (25[OH]D < 80 nmol/l) was 97.1%, that of vitamin D deficiency (25[OH]D < 50 nmol/l) was 79.8%, and that of elevated PTH level (>6.8 pmol/l) was 35.5%. After adjustment for age and sex, PTH was significantly associated with in-hospital mortality (OR = 1.12, 95% CI 10.5-1.20, P < 0.001), myocardial injury (OR = 1.05, 95% CI 1.03-1.15, P = 0.002), prolonged length of stay (LOS > or = 20 days; OR = 1.05, 95% CI 1.01-1.06, P = 0.044), and being discharged to institutional care (OR = 1.07, 95% CI 1.01-1.18, P = 0.48). Secondary hyperparathyroidism (SHPT), but not vitamin D deficiency, was associated with older age, a higher prevalence of trochanteric fracture, coronary artery disease, hypertension, previous stroke, renal impairment, increased levels of serum osteocalcin, bone-specific alkaline phosphatase, and adiponectin as well as a significantly higher in-hospital mortality (11.8 vs. 0.54%, P = 0.001), perioperative myocardial injury (32.7 vs. 22.5%, P = 0.043), LOS > or = 20 days (40.2 vs. 26.9%, P = 0.017), and being discharged to institutional care (29.5 vs. 14.6%, P = 0.019). In multivariate regression analyses, SHPT was strongly associated with in-hospital mortality and LOS > or = 20 days. We conclude that elevated PTH (but not vitamin D deficiency per se) is a strong independent predictor of poor outcomes in older patients.

Benefits and harms of phosphate binders in CKD: a systematic review of randomized controlled trials

BACKGROUND

Phosphate binders are widely used to control serum phosphorus levels in patients with chronic kidney disease (CKD). We analyzed the effects of phosphate binders on biochemical and patient-level end points in patients with CKD.

STUDY DESIGN

Systematic review and meta-analysis by searching MEDLINE (1966 to April 2009), EMBASE (1980 to April 2009), and the Cochrane Renal Group Specialised Register and the Cochrane Central Register of Controlled Trials (CENTRAL).

SETTING & POPULATION

Patients with CKD.

SELECTION CRITERIA FOR STUDIES

Randomized controlled trials.

INTERVENTION

Phosphate binders.

OUTCOMES

Serum phosphorus, calcium, and parathyroid hormone levels; incidence of hypercalcemia; all-cause mortality; adverse effects.

RESULTS

40 trials (6,406 patients) were included. There was no significant decrease in all-cause mortality (10 randomized controlled trials; 3,079 patients; relative risk [RR], 0.73; 95% confidence interval [CI], 0.46 to 1.16), hospitalization, or end-of-treatment serum calcium-phosphorus product levels with sevelamer compared with calcium-based agents. There was a significant decrease in end-of-treatment phosphorus and parathyroid hormone levels with calcium salts compared with sevelamer and a significant decrease in risk of hypercalcemia (RR, 0.47; 95% CI, 0.36 to 0.62) with sevelamer compared with calcium-based agents. There was a significant increase in risk of gastrointestinal adverse events with sevelamer in comparison to calcium salts (RR, 1.39; 95% CI, 1.04 to 1.87). Compared with calcium-based agents, lanthanum significantly decreased end-of-treatment serum calcium and calcium-phosphorus product levels, but with similar end-of-treatment phosphorus levels. Effects of calcium acetate on biochemical end points were similar to those of calcium carbonate. Existing data are insufficient to conclude for a differential impact of any phosphate binder on cardiovascular mortality or other patient-level outcome.

LIMITATIONS

Few long-term studies of the efficacy of phosphate binders on mortality and musculoskeletal morbidity, significant heterogeneity for many surrogate outcomes, and suboptimal reporting of study methods to determine trial quality.

CONCLUSION

Currently, there are insufficient data to establish the comparative superiority of non-calcium-binding agents over calcium-containing phosphate binders for such important patient-level outcomes as all-cause mortality and cardiovascular end points. Additional trials are still required to examine the differential effects of phosphate-binding agents on these end points and the mineral homeostasis pathway.

Vitamin D's role in cell proliferation and differentiation

Vitamin D has pleiotropic effects that go beyond its traditional role in calcium homeostasis. Hundreds of genes with vitamin D receptor response elements directly or indirectly influence cell cycling and proliferation, differentiation, and apoptosis. Vitamin D compounds also have effects on cell function that are nongenomic. The noncalcemic actions of vitamin D influence normal and pathological cell growth, carcinogenesis, immune function, and cardiovascular physiology. This review examines many of the various mechanisms by which vitamin D alters cellular growth and differentiation and explores cell-specific factors that influence responsiveness to vitamin D.

Stimulating parathyroid cell proliferation and PTH release with phosphate in organ cultures obtained from patients with primary and secondary hyperparathyroidism for a prolonged period

The pathogenesis of primary hyperparathyroidism (I degrees -HPT) and secondary hyperparathyroidism (II degrees -HPT) remains to be elucidated. To characterize their pathophysiology, we investigated the effects of calcium and phosphate on cell proliferation and PTH release in an organ culture of parathyroid tissues. Dissected parathyroid tissues obtained from patients with I degrees -HPT (adenoma) or II degrees -HPT (nodular hyperplasia) were precultured on a collagen-coated membrane for 1-4 week. After changing the medium for one containing various concentrations of phosphate, PTH release and [(3)H]thymidine incorporation were studied. In contrast to dispersed parathyroid cells cultured in a monolayer, calcium decreased PTH release in a concentration-dependent manner in parathyroid tissues. Furthermore, when parathyroid tissues obtained from II degrees -HPT were precultured for 1-4 weeks, PTH release and parathyroid cell proliferation were significantly increased in high-phosphate medium. These phosphate effects were also observed to a lesser extent in parathyroid tissues obtained from I degrees -HPT, but there was no significant difference between I degrees -HPT and II degrees -HPT. Microarray analyses revealed that mRNA levels of PTH, CaSR, and VDR were well preserved, and several growth factors (e.g. TGF-beta1-induced protein) were abundantly expressed in II degrees -HPT. Using organ cultures of hyperparathyroid tissues, in which PTH release and CaSR are well preserved for a prolonged period, we have demonstrated that phosphate stimulates parathyroid cell proliferation not only in II degrees -HPT but also in I degrees -HPT. Although the mechanism responsible for phosphate-induced cell proliferation remains to be elucidated, our in vitro findings suggest that both parathyroid tissues preserve to some extent a physiological response system to hyperphosphatemia as observed in normal parathyroid cells.

A slight decrease in renal function further impairs bone mineral density in primary hyperparathyroidism

BACKGROUND

The impairment of renal function can affect the clinical presentation of primary hyperparathyroidism (PHPT), increasing cardiovascular morbidity, fracture rate, and the risk of mortality.

AIM

The aim of the study was to assess the differences in bone status in a series of consecutive patients affected by PHPT without overt renal failure at diagnosis grouped according to creatinine clearance (Ccr).

METHODS

A total of 161 consecutive patients with PHPT were studied. They were divided into two groups based on Ccr. Group A had Ccr 70 ml/min or less (n = 49), and group B had Ccr greater than 70 ml/min (n = 112). PTH, total and ionized serum calcium; urinary calcium and phosphate; serum 25-hydroxyvitamin D3; serum and urinary bone markers; lumbar, forearm, and femoral bone mineral density (BMD) were evaluated.

RESULTS

Patients in group A were older than those in group B (P < 0.0001). PTH levels did not differ in the two groups, whereas both urinary calcium and phosphorus were lower in group A than group B (P < 0.01). Lower BMD was evident in group A at lumbar spine (P < 0.002), forearm (P < 0.0001), and femur (P < 0.01). In asymptomatic PHPT, those with Ccr 70 ml/min or less had lower forearm BMD than patients with higher Ccr (P < 0.00001). When adjusting for age and body mass index in PHPT, BMD at each site persisted being lower (P < 0.05) in group A than group B. In all PHPT subjects, Ccr (beta = 0.29, P < 0.0005), age (beta = -0.27, P < 0.00001), and PTH levels (beta = -0.27, P < 0.0005) were all independently associated with forearm BMD.

CONCLUSIONS

In PHPT a slight decrease in renal function is associated with more severe BMD decrease, independent of age, body mass index, and PTH levels. This association is also present in asymptomatic PHPT and strengthens the National Institutes of Health recommendations for surgery in patients with mild PHPT.

Therapeutic use of calcimimetics

It has long been recognized that the secretion of PTH by chief cells in the parathyroid gland is regulated by extracellular ionized calcium. The molecular mechanism by which extracellular Ca2+ performs this feat was deduced by the cloning of the extracellular calcium-sensing receptor (CaSR) in 1993 in the laboratories of Brown and Hebert. The CaSR is a G protein-coupled cell surface receptor that belongs to family 3 of the GPCR superfamily. The CaSR senses the extracellular ionic activity of the divalent minerals Ca2+ and Mg2+ and translates this information, via a complex array of cellular signaling pathways, to modify cell and tissue function. Genetic studies have demonstrated that the activity of this receptor determines the steady-state plasma calcium concentration in humans by regulating key elements in the calcium homeostatic system. CaSR agonists (calcimimetics) and antagonists (calcilytics) have been identified and have provided both current and potential therapies for a variety of disorders. Calcimimetics can effectively reduce PTH secretion in all forms of hyperparathyroidism. They are likely to become a major therapy for secondary hyperparathyroidism associated with renal failure and for treatment of certain patients with primary hyperparathyroidism. On the therapeutic horizon are calcilytics that can transiently increase PTH and may prove useful in the treatment of osteoporosis.

Causes of secondary hyperparathyroidism in a healthy population: the Tromsø study

Secondary hyperparathyroidism (SHPT) develops as a compensatory mechanism when the body is in calcium deficit. SHPT may be harmful and has been associated with elevated blood pressure. The cause of SHPT could be low calcium intake, reduced intestinal calcium absorption, or increased excretion. However, the relative importance of these factors for the development of SHPT is not known. During the 5th Tromsø study, serum PTH and calcium were measured in 7954 subjects. Then 96 subjects with SHPT (defined as serum PTH above 6.4 pmol/l together with serum calcium below 2.40 mmol/l) and 106 control subjects were examined at follow-up with a food frequency questionnaire, calcium absorption test, measurement of 24-h urinary calcium excretion, and serum vitamin D status. The statistical analyses showed several interactions necessitating subgroup analysis. It was found that the calcium intake was significantly lower in the SHPT group, but only in nonsmoking males; the calcium absorption was nonsignificantly higher in the SHPT group; the serum 25-hydroxyvitamin D levels were significantly lower in the SHPT group but only in nonsmokers; and the 24-h urinary calcium excretion was significantly lower in the SHPT group but only in those not on blood pressure medication. The most frequent cause of SHPT appeared to be low calcium intake (18%) and a low serum 25-hydroxyvitamin D level (18%). However, in most subjects with SHPT all tests were within the normal range, and the cause is therefore probably a combination of several factors.