Regulation by vitamin D metabolites of messenger ribonucleic acid for preproparathyroid hormone in isolated bovine parathyroid cells

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

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

Which vitamin D derivative to prescribe for renal patients

PURPOSE OF REVIEW

It is possible to control the secondary hyperparathyroidism and osteitis fibrosa of patients with chronic kidney disease by calcitriol when given early and in appropriate doses. However, this control is often achieved at the price of unacceptably high plasma calcium and phosphorus levels, the induction of adynamic bone disease, and soft tissue calcification. To avoid these side effects, so-called 'nonhypercalcemic' vitamin D analogs have been developed. Their possible advantages and their precise place in the treatment and prevention of secondary hyperparathyroidism remain a matter of debate.

RECENT FINDINGS

A large US multicenter study showed that the administration of the vitamin D analog paricalcitol to hemodialysis patients, as compared with calcitriol, was associated with better survival. In a subsequent large US multicenter study paricalcitol-treated hemodialysis patients experienced fewer hospitalizations and hospital days compared with calcitriol-treated patients. In a third, smaller study from Japan, regular alfacalcidol users among hemodialysis patients had better cardiovascular survival than nonusers. Finally, in a recent historical control study the mortality of a large hemodialysis patient cohort was analyzed as a function of previous vitamin D treatment. Patients on active vitamin D compounds at any time had a 2-year survival advantage over vitamin D-naive patients. It must be pointed out, however, that all four studies were retrospective in nature.

SUMMARY

The development of vitamin D analogs with less side effects than with calcitriol is of major theoretical interest. Practically speaking, however, we still need to be convinced that this goal can be achieved in chronic kidney disease patients.

Cinacalcet Hydrochloride Is an Effective Treatment for Secondary Hyperparathyroidism in Patients With CKD Not Receiving Dialysis

BACKGROUND

Secondary hyperparathyroidism develops early in patients with chronic kidney disease (CKD). Clinical guidelines from the National Kidney Foundation-Kidney/Disease Outcomes Quality Initiative emphasize the need to control parathyroid hormone (PTH), calcium, and phosphorus levels in patients with CKD not receiving dialysis to reduce poor outcomes. This phase 2 study evaluated the effects of the oral calcimimetic cinacalcet hydrochloride in patients with CKD not on dialysis therapy.

METHODS

A randomized, double-blind, placebo-controlled, 18-week study enrolled adults with an estimated glomerular filtration rate of 15 to 50 mL/min/1.73 m2 (0.25 to 0.83 mL/s/1.73 m2) and an intact PTH (iPTH) level greater than 130 pg/mL (ng/L). Cinacalcet (or placebo) was titrated from 30 to 180 mg once daily to obtain a 30% or greater reduction in iPTH levels from baseline.

RESULTS

Baseline mean iPTH levels were 243 pg/mL (ng/L) in the cinacalcet group (n = 27) and 236 pg/mL (ng/L) in the control group (n = 27). At baseline, 28% of subjects were being administered vitamin D sterols and 43% were being administered phosphate binders or calcium supplements. The addition of cinacalcet significantly decreased iPTH concentrations compared with controls during the efficacy-assessment phase: 56% versus 19% of subjects achieved a 30% or greater reduction in iPTH levels (P = 0.006), and mean iPTH levels decreased by 32% in the cinacalcet group, but increased by 6% in the control group (P < 0.001). Mean serum calcium and phosphorus levels remained within normal range throughout the study. Cinacalcet generally was well tolerated; the most frequent adverse events were gastrointestinal.

CONCLUSION

This preliminary study provides evidence that cinacalcet is efficacious for the treatment of secondary hyperparathyroidism in subjects with CKD not receiving dialysis.

Overproduction of an amino-terminal form of PTH distinct from human PTH(1-84) in a case of severe primary hyperparathyroidism: influence of medical treatment and surgery

OBJECTIVE

Rare patients with severe primary hyperparathyroidism present with large parathyroid tumours, severe hypercalcaemia, very high PTH levels and osteitis fibrosa cystica. Some of these patients display a large amount of C-PTH fragments in circulation and present with a higher C-PTH/I-PTH ratio than seen in less severe cases of primary hyperparathyroidism. We wanted to determine how PTH levels and circulating PTH high-performance liquid chromatography (HPLC) profiles analysed with PTH assays having different epitopes could be affected by medical and surgical treatment in such patients.

DESIGN

A 55-year-old man with severe hypercalcaemia (Ca(2+): 2.01 mmol/l), very high PTH levels (CA-PTH 82.1 and T-PTH 72 pmol/l) caused by a large parathyroid tumour (7.35 g) and accompanied by significant bone involvement (alkaline phosphatase of 185 UI/l and subperiostal bone resorption of hands) was referred to us. Blood was obtained at various time points during his medical treatment, before and after surgery, to measure parameters of calcium and phosphorus metabolism, and of bone turnover. HPLC separations of circulating PTH molecular forms were performed and analysed with PTH assays having 1-4 (CA), 12-18 (T), 26-32 (E) and 65-84 (C) epitopes.

RESULTS

Before surgery, serum Ca2+ was nearly normalized with hydratation, intravenous (IV) pamidronate and oral vitamin D administration. Despite a decrease in Ca2+ to 1.31 mmol/l, CA-PTH and T-PTH levels decreased by half in relation to a threefold increase in basal 1,25-dihydroxyvitamin D [1,25(OH)2D] level (94 to 337 pmol/l). After this initial positive response, hypercalcaemia and elevated CA- and T-PTH levels recurred even if 1,25(OH)2D levels remained elevated. The tumour was removed surgically and proved to be poorly differentiated with nuclear atypia and mitosis. After surgery, the Ca2+ level and PTH secretion normalized. The higher CA-PTH level relative to the T-PTH level observed before surgery in this patient was related to the oversecretion of an amino-terminal (N) form of PTH recognized by PTH assays with (1-4) or (26-32) epitopes but not by the T-PTH assay with a (12-18) epitope. This molecular form represented 50% of CA-PTH measured in this patient, but only 7% in less severe cases of primary hyperparathyroidism. It was unaffected by medical therapy and disappeared after surgery.

CONCLUSION

The relationship between the overexpression of this N-PTH molecular form and severe primary hyperparathyroidism remains unclear. Further studies will be required in these rare patients to see whether N-PTH is a marker of less well differentiated parathyroid tumours and/or relates to the overproduction of C-PTH fragments in the presence of severe hypercalcaemia.

Pathogenesis of parathyroid dysfunction in end-stage kidney disease

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

1,25-dihydroxyvitamin D3 stimulated protein kinase C phosphorylation of type VI adenylyl cyclase inhibits parathyroid hormone signal transduction in rat osteoblastic UMR 106-01 cells

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) treatment of osteoblastic cells was shown previously to attenuate Parathyroid hormone (PTH) response by inhibiting adenylyl cyclase (AC) activity. In this study, we have investigated the mechanism by which 1,25(OH)(2)D(3) inhibits AC in rat osteoblastic UMR 106-01 cells. 1,25(OH)(2)D(3) treatment inhibited both PTH and forskolin-stimulated AC activity by 25%-50% within 12 min in a concentration-dependent manner suggesting a direct inhibition of the AC enzyme. Treatment with 25(OH)D(3) had no effect on basal or stimulated AC activity. We determined the profile of AC subtypes expressed in UMR cells and found AC VI to be the dominant subtype accounting for 50% of AC mRNA. Since AC VI can be inhibited by protein kinase C (PKC) phosphorylation, we examined 1,25(OH)(2)D(3) activation of various PKC isoforms. 1,25(OH)(2)D(3) increased the membrane translocation of PKC-betaI, -delta, and -zeta with a concomitant increase in PKC activity. The translocation of PKC-betaI and -delta was blocked by the PLC inhibitor U73122 whereas that of PKC-zeta was abolished by the PI-3 kinase inhibitor wortmannin. The attenuation of cAMP production by 1,25(OH)(2)D(3) was antagonized by the PKC inhibitors Go6850, calphostin C, and wortmannin, but not by a calmodulin kinase II (CaMKII) inhibitor. Treatment with 1,25(OH)(2)D(3) for 20 min increased AC VI phosphorylation by 10.8-fold and this was blocked partially by Go6850 and partially by wortmannin but was unaffected by CaMKII inhibitor. These results demonstrate that 1,25(OH)(2)D(3) activation of PKC isoforms leads to phosphorylation of AC VI and inhibition of PTH-activation of this pathway in osteoblasts.

Seasonal modifications in blood pressure are mainly related to interdialytic body weight gain in dialysis patients

BACKGROUND

Longitudinal studies in dialysis patients have identified seasonal variations in blood pressure that may follow climatic parameters such as external temperature and humidity. We aimed to assess the participation of interdialytic body weight gain variations in the seasonal profile of blood pressure.

METHODS

Ninety-nine stable patients (40 F/59 M), 52.9 +/- 1.5 years old, dialyzed in a single satellite dialysis unit between January 7, 1991, and September 30, 1998 were studied. Supine systolic and diastolic blood pressure, body weight, and interdialytic body weight gain were determined at every one of the 38,769 dialyses included, and studied along with climatology data obtained from Météo, France.

RESULTS

Blood pressure varied throughout the year, following a cyclic pattern. It increased from the autumn months toward winter, and decreased toward the spring and warmer months. Systolic and diastolic blood pressures were strongly correlated with interdialytic body weight gain (r= 0.925; P < 0.0001 and r= 0.888; P= 0.0001, respectively). Blood pressure was also correlated with the climatic factors: rainfall (r= 0.786; P < 0.003 and r= 0.784; P < 0.003), humidity (r= 0.701; P= 0.011 and r= 0.699; P < 0.012), and day light span (r=-0712; P < 0.01, and r=-0.658; P < 0.02, respectively). Multivariate regression analyses taking blood pressure as a dependent variable retained interdialytic body weight gain as the first variable in the model.

CONCLUSION

Our results establish a strong link between blood pressure variations and interdialytic body weight gain, showing the important participation of volume state in modulating blood pressure in this group of patients.

Regulation of stanniocalcin 1 and 2 expression in the kidney by klotho gene

The klotho gene product and stanniocalcin (STC) 1 and 2 are recently identified molecules implicated in calcium and phosphorus homeostasis. In the present study, we investigated the regulation of STC1 and STC2 gene expression in the kidney by klotho gene expression. Mice deficient in klotho expression (klotho mice) have hypercalcemia and hyperphosphatemia, and increased renal gene expression of STC1 and STC2 compared with wild-type mice. Administration of vitamin D or CaCl(2) to wild-type mice causes upregulation of STC1 but STC2 gene expression is not altered significantly. On the other hand, treatment of klotho mice with low phosphorus diet results in partial decrease in STC2 gene expression with normalization of hyperphosphatemia. These findings indicate that klotho gene expression plays a crucial role in the regulation of renal stanniocalcin gene expression in vivo, at least partly, through the control of circulating calcium and phosphate concentrations.

Disposition and metabolism of F6-1alpha,25(OH)2 vitamin D3 and 1alpha,25(OH)2 vitamin D3 in the parathyroid glands of rats dosed with tritium-labeled compounds

26,26,26,27,27,27-Hexafluoro-1alpha,25(OH)2 vitamin D3, a hexafluorinated analog of 1alpha,25(OH)2 vitamin D3, has been reported to be several times more potent than the parent compound with respect to some vitamin D actions. The reason for enhanced biological activity in the bones, kidneys, and small intestine appears to be related to F6-1alpha,25(OH)2 vitamin D3 metabolism to ST-232 (26,26,26,27,27,27-hexafluoro-1alpha,23S,25-trihydroxyvitamin D3), a bioactive 23S-hydroxylated form that is resistant to further metabolism. We compared the disposition and metabolism of [1beta-3H]F6-1alpha,25(OH)2 vitamin D3 and [1beta-3H]1alpha,25(OH)2 vitamin D3 in parathyroid glands of rats intravenously administered with labeled compounds at a dose of 10 microg/kg. In the [1beta-3H]F6-1alpha,25(OH)2 vitamin D3-dosed group, radioactivity was highly detected in the kidneys, parathyroid glands, and the small intestine. The radioactivity in the parathyroid glands remained high until 48 h postdosing, with values of 2.5, 8.4, and 14.6 times higher at 6, 24, and 48 h postdosing than after dosing with [1beta-3H] 1alpha,25(OH)2 vitamin D3. In the group given [1beta-3H]F6-1alpha,25(OH)2 vitamin D3, the unchanged compound was mainly detected with a small amount of ST-232 at 6 h postdosing. At the 24- and 48-h time points, over half of the radioactivity was observed as ST-232, and additionally, ST-233, the 23-oxo form, accounted for a small amount at the 48-h time point. The present study demonstrated local retention of [1beta-3H]F6-1alpha,25(OH)2 vitamin D3 and the bioactive metabolite ST-232 in parathyroid glands after intravenous administration. The findings may indicate one of the reasons for the higher potency of F6-1alpha,25(OH)2 vitamin D3 than 1alpha,25(OH)2 vitamin D3 in parathyroid.

Trace elements and vitamins in maintenance dialysis patients

Although protein-energy malnutrition occurs commonly in patients with end-stage renal disease undergoing maintenance dialysis treatment, it is not the only form of malnutrition that may exist in these individuals. They may also suffer from deficiencies of micronutrients, particularly trace elements and vitamins. More commonly occurring vitamin deficiencies in maintenance dialysis patients include those for vitamin C (ascorbate), folate, vitamin B6 (pyridoxine), and 1,25-dihydroxycholecalciferol (calcitriol). Among trace elements, deficiencies may occur more commonly for iron, zinc, and possibly selenium, whereas toxicities are more common with aluminum and possibly copper. Evidence suggests that there is an abnormally high prevalence of antioxidant deficiency in maintenance dialysis patients, especially because a low intake of protein and energy may be associated with inadequate ingestion of antioxidant vitamins (ie, vitamins E and C and carotenoids). Thus, some micronutrient deficiencies in maintenance dialysis patients may contribute to the development of atherosclerotic cardiovascular disease. Dietary requirements for vitamins and trace elements in maintenance dialysis patients are reviewed and the recommended daily intakes are discussed.

New vitamin D analogs

BACKGROUND

1,25-(OH)2D3 (calcitriol) controls parathyroid gland growth and suppresses the synthesis and secretion of parathyroid hormone. Because of this, 1,25-(OH)2D3 has been used successfully for the treatment of secondary hyperparathyroidism, which almost always accompanies renal failure. However, the potent effect of 1,25-(OH)2D3 on intestinal calcium and phosphorus absorption and bone mineral mobilization often leads to the development of hypercalcemia and hyperphosphatemia precluding 1,25-(OH)2D3 therapy.

METHODS

This has led to the development of vitamin D analogs that retain the suppressive action on PTH and parathyroid gland growth, but that have less calcemic and phosphatemic activity. Currently, two analogs, 19-nor-1,25-(OH)2D2 and 1,alpha(OH)D2, are being used for the treatment of secondary hyperparathyroidism in the United States, and two are being used in Japan, 22-oxa-calcitriol and 1,25-(OH)2-26,27F6 D3.

RESULTS

All four analogs suppressed PTH, but had less calcemic and phosphatemic activity than 1,25-(OH)2D3. In rats, 19-nor-1,25-(OH)2D2 has been shown to be less calcemic and phosphatemic compared to 1,alpha(OH)D2.

CONCLUSION

Therapeutic doses of 19-nor-1,25-(OH)2D2 could produce a lower Ca x P product compared to 1,alpha(OH)D2, which could be an important consideration in patient treatment. Further studies are necessary to define these differences and to understand the mechanisms behind the differential actions of vitamin D analogs.

Paricalcitol versus calcitriol in the treatment of secondary hyperparathyroidism

BACKGROUND

Management of secondary hyperparathyroidism has included the use of active vitamin D or vitamin D analogs for the suppression of parathyroid hormone (PTH) secretion. Although, these agents are effective, therapy is frequently limited by hypercalcemia, hyperphosphatemia, and/or elevations in the calcium-phosphorus (Ca x P) product. In clinical studies, paricalcitol was shown to be effective at reducing PTH concentrations without causing significant hypercalcemia or hyperphosphatemia as compared to placebo. A comparative study was undertaken in order to determine whether paricalcitol provides a therapeutic advantage to calcitriol.

METHODS

A double-blind, randomized, multicenter study comparing the safety and effectiveness of intravenous paricalcitol and calcitriol in suppressing PTH concentrations in hemodialysis patients was performed. A total of 263 randomized patients were enrolled at domestic and international sites. Following the baseline period, patients with serum Ca x P < 75, and a PTH level > or =300 pg/mL were randomly assigned to receive either paricalcitol or calcitriol in a dose-escalating fashion for up to 32 weeks. Dose adjustments were based on laboratory results for PTH, calcium, and Ca x P. The primary end point was the greater than 50% reduction in baseline PTH. Secondary end points were the occurrence of hypercalcemia and elevated Ca x P product.

RESULTS

Paricalcitol-treated patients achieved a > or =50% reduction from baseline PTH significantly faster than did the calcitriol-treated patients (P = 0.025) and achieved a mean reduction of PTH into a desired therapeutic range (100 to 300 pg/mL) at approximately week 18, whereas the calcitriol-treated patients, as a group, were unable to achieve this range. Moreover, paricalcitol-treated patients had significantly fewer sustained episodes of hypercalcemia and/or increased Ca x P product than calcitriol patients (P = 0.008).

CONCLUSION

Paricalcitol treatment reduced PTH concentrations more rapidly with fewer sustained episodes of hypercalcemia and increased Ca x P product than calcitriol therapy.

RM, Paula FJ. Fisiopatologia da osteoporose induzida por glicocorticóide

O hipercortisolismo crônico é a causa mais freqüente de osteoporose secundária, acometendo principalmente o osso trabecular. Aproximadamente 30-35% dos pacientes com síndrome de Cushing apresentam fraturas de vértebras por compressão e o risco de fraturas de colo de fêmur é aumentado em 50% nessa população. Vários mecanismos têm sido propostos para explicar a ocorrência de osteoporose nessa condição, como a ação direta dos glicocorticóides nas paratireóides e nas células ósseas, alterações na produção de prostaglandinas, citocinas, interleucinas, alterações na secreção do hormônio do crescimento (GH), do fator insulina símile-I (IGF-I) e esteróides gonadais. Resultados controversos têm sido apresentados quanto à alteração na secreção do PTH nesta situação, onde níveis normais e elevados têm sido descritos. A elevação da secreção de PTH pode ser secundária a distúrbios do metabolismo mineral induzidos pelo hipercortisolismo, como diminuição na absorção intestinal, aumento da excreção renal de cálcio, diminuição no número de receptores paratireoideanos para a 1,25(OH)2D3, anormalidades no limiar de sensibilidade do cálcio (set point) para a secreção do PTH e alteração na sua atividade. Nesta revisão, são discutidos diversos aspectos fisiopatológicos e possíveis mecanismos envolvidos na associação entre hipercortisolismo e osteoporose.

Parathyroid hormone (PTH), PTH-derived peptides, and new PTH assays in renal osteodystrophy

Parathyroid hormone (PTH), PTH-derived peptides, and new PTH assays in renal osteodystrophy. Reliable measurements of parathyroid hormone (PTH) concentrations in serum or plasma are critical for the appropriate diagnosis and management of patients with renal osteodystrophy. With the introduction of second generation immunometric assays for PTH, it is now possible to measure exclusively full-length, biologically active PTH(1-84). In contrast, first generation immunometric assays that have been used widely for many years detect not only PTH(1-84), but also other large amino-terminally-truncated, PTH-derived peptides. This development will require a careful re-evaluation of PTH measurements, as determined by either first or second generation immunometric assays, and their relationship to bone histology and bone remodeling rates in patients with end-stage renal disease (ESRD). Such information is essential for proper clinical management, but only limited bone biopsy data are available to guide the interpretation of PTH results using second generation PTH assays. The different performance characteristics of first and second generation immunometric PTH assays also makes it possible to quantify the plasma levels of amino-terminally-truncated, PTH-derived peptides, which may accumulate disproportionately in patients with ESRD. Recent experimental evidence indicates that one or more of these peptides can modify bone cell activity and skeletal remodeling, possibly by interacting with a PTH receptor distinct from the type I PTH receptor that binds to the amino-terminal portion of PTH and mediates the classical biological actions of the hormone. The putative C-PTH receptor interacts with mid- and/or carboxyterminal regions of PTH and other amino-terminally-truncated PTH-derived peptides; signaling through it may contribute to the skeletal resistance to PTH that characterizes ESRD. The current review discusses certain aspects of the molecular structure of PTH and its interaction with various receptors, briefly comments about selected components of PTH secretion, highlights recent technical advances in PTH assays, and summarizes the effects of various PTH-derived peptides on bone cells and on skeletal metabolism.

Differential effects of 19-nor-1,25-(OH)(2)D(2) and 1alpha-hydroxyvitamin D(2) on calcium and phosphorus in normal and uremic rats

BACKGROUND

Calcitriol, 1,25-(OH)(2)D(3) (1,25D), the most active metabolite of vitamin D, has been used in the treatment of secondary hyperparathyroidism (SH) because it controls parathyroid gland growth and suppresses parathyroid hormone (PTH) synthesis and secretion. Due to the calcemic and phosphatemic actions of 1,25D, two analogs with potentially less side effects, 19-nor-1,25-(OH)(2)D(2) (19-nor) and 1alpha(OH)D(2) (1alphaD(2)) are currently being used in the treatment of SH.

METHODS

This study compares the effects of these two analogs on calcium (Ca) and phosphorus (P) metabolism in normal, uremic, and parathyroidectomized (PTX) rats. Using doses of 50 to 250 ng of 19-nor or 1alphaD(2), experiments were conducted in normal and uremic rats.

RESULTS

In uremic rats, 19-nor did not increase plasma Ca or P while 1alphaD2 caused a dose-dependent increase in both. In addition, while the Ca x P product remained unchanged in 19-nor-treated rats, it increased progressively with 1alphaD(2)administration. In metabolic studies in normal rats treated with vehicle, 10 ng of 1,25D, 100 ng of 19-nor or 100 ng 1alphaD(2), intestinal calcium absorption and urinary calcium excretion were significantly higher in 1alphaD(2)-treated rats compared to those receiving 19-nor. Similar results were seen for intestinal phosphorus absorption and urinary phosphorus excretion. Finally, the skeletal response to these two analogs was tested in PTX rats fed a calcium-deficient diet and treated daily with 100 ng of 19-nor or 1alphaD(2). The increase in plasma calcium in 1alphaD2-treated rats was markedly higher than in those receiving 19-nor. Similar results were seen in plasma phosphorus when these studies were repeated using a phosphorus-deficient diet.

CONCLUSIONS

These studies demonstrate that when given in large doses to rats 19-nor is less calcemic and phosphatemic than 1alphaD(2). The lower Ca x P product in 19-nor treated rats may be an important consideration in patient therapy. Further studies in patients are necessary to define the clinical applicability of these differences.

Mechanisms of secondary hyperparathyroidism

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

Failure of high doses of calcitriol and hypercalcaemia to induce apoptosis in hyperplastic parathyroid glands of azotaemic rats

BACKGROUND

Whether calcitriol administration, which is used to treat secondary hyperparathyroidism in dialysis patients, induces regression of parathyroid-gland hyperplasia remains a subject of interest and debate. If regression of the parathyroid gland were to occur, the presumed mechanism would be apoptosis. However, information on whether high doses of calcitriol can induce apoptosis of parathyroid cells in hyperplastic parathyroid glands is lacking. Consequently, high doses of calcitriol were given to azotaemic rats and the parathyroid glands were evaluated for apoptosis.

METHODS

Rats were either sham-operated (two groups) or underwent a two-stage 5/6 nephrectomy (three groups). For the first 4 weeks, all rats were given a high (1.2%) phosphorus (P) diet to stimulate parathyroid gland growth and then were changed to a normal (0.6%) P diet for 2 weeks. At week 7, three of the five groups were given high doses of calcitriol (500 pmol/100 g body weight) intraperitoneally every 24 h during 72 h before sacrifice. The five groups during week 7 were: (i) normal renal function (NRF)+0.6% P diet; (ii) NRF+0.6% P+calcitriol; (iii) renal failure (RF)+0.6% P; (iv) RF+1.2% P+calcitriol; and (v) RF+0.6% P+calcitriol. Parathyroid glands were removed at sacrifice and the TUNEL stain was performed to detect apoptosis.

RESULTS

At sacrifice, the respective serum calcium values in calcitriol-treated groups (groups 2, 4, and 5) were 15.52+/-0.26, 13.41+/-0.39 and 15.12+/-0.32 mg/dl. In group 3, PTH was 178+/-42 pg/ml, but in calcitriol-treated groups, PTH values were suppressed, 8+/-1 (group 2), 12+/-2 (group 4), and 7+/-1 pg/ml (group 5). Despite, the severe hypercalcaemia and marked PTH suppression in calcitriol-treated groups, the percentage of apoptotic cells in the parathyroid glands was very low (range 0.08+/-0.04 to 0.25+/-0.20%) and not different among the five groups.

CONCLUSIONS

We found no evidence in hyperplastic parathyroid glands that apoptosis could be induced in azotaemic rats by the combination of high doses of calcitriol and severe hypercalcaemia despite the marked reduction in PTH levels that was observed.

Association of decreased calcium-sensing receptor expression with proliferation of parathyroid cells in secondary hyperparathyroidism

BACKGROUND

The down-regulation of both calcium-sensing receptor (CaSR) and vitamin D receptor (VDR) in parathyroid (PT) glands of secondary hyperparathyroidism (HPT) caused by chronic renal failure has been associated with PT hormone hypersecretion as well as PT hypergrowth. To clarify the predominance of decreased expression of CaSR and VDR in the high proliferative activity of PT glands, we examined the relationship between the expression of both receptors and proliferative activity in human PT glands.

METHODS

Serial sections of 56 PT glands, including 52 glands from secondary HPT and 4 normal PT glands resected together with thyroid carcinoma, were examined immunohistochemically with specific antibodies against CaSR, VDR, and Ki67. The Ki67-positive cell number was counted and expressed as the Ki67 score. The CaSR and VDR expressions were semiquantitatively analyzed.

RESULTS

The expressions of both CaSR and VDR were markedly decreased in PT glands of secondary HPT, while the Ki67 score was significantly higher than it was in normal controls. When hyperplastic glands were classified into two subgroups, with [N(+)] or without [N(-)] nodular formation, CaSR expression was significantly decreased in N(+), while VDR expression was not different. Multiple regression analyses revealed that the decreased expression of CaSR could contribute significantly to the high proliferative activity, even if VDR expression was taken into account.

CONCLUSION

The decrease in CaSR expression is associated with the high proliferative activity of PT glands in secondary HPT, independently of the decreased VDR expression. These findings provide a new insight into the pathogenesis of PT hyperplasia, which is refractory to vitamin D therapy in patients with severe secondary HPT.

The in vitro effect of calcitriol on parathyroid cell proliferation and apoptosis

Calcitriol treatment is used to reduce parathyroid hormone levels in azotemic patients with secondary hyperparathyroidism (HPT). Whether long-term calcitriol administration reduces parathyroid gland size in patients with severe secondary hyperparathyroidism is not clear. The aim of the study was to evaluate in vitro the effect of calcitriol on parathyroid cell proliferation and apoptosis in normal parathyroid glands and in adenomatous and hyperplastic human parathyroid glands. Freshly harvested parathyroid glands from normal dogs and hyperplastic and adenomatous glands from patients with secondary (2 degrees) and primary (1 degree) HPT undergoing parathyroidectomy were studied. Flow cytometry was used to quantify the cell cycle and apoptosis of parathyroid cells. Apoptosis was also evaluated by DNA electrophoresis and light and electron microscopy. In normal dog parathyroid glands, culture with calcitriol (10(-10) to 10(-7) M) for 24 h produced a dose-dependent inhibitory effect on the progression of cells into the cell cycle and into apoptosis. When glands from patients with 2 degrees HPT were cultured for 24 h, only high calcitriol concentrations (10(-7) M) inhibited the progression through the cell cycle and the induction of apoptosis. In parathyroid adenomas (1 degrees HPT), even a high concentration of calcitriol (10(-7) M) had no significant effect on the cell cycle or apoptosis. The present study shows that in vitro, calcitriol inhibits in a dose-dependent manner in normal parathyroid glands both parathyroid cell proliferation and apoptosis. However, in secondary hyperplasia, only high concentrations of calcitriol inhibited cell proliferation and apoptosis. In 1 degree HPT, even high concentrations of calcitriol had no effect. Because calcitriol simultaneously inhibits both cell proliferation and apoptosis, a reduction in the parathyroid gland mass may not occur as a direct effect of calcitriol treatment.

Effect of the vitamin D analogues paricalcitol and calcitriol on bone mineral in vitro

Paricalcitol (19-nor-1,25-dihydroxyvitamin D(2)), a new vitamin D analogue, recently became available for the treatment of hyperparathyroidism in patients with end-stage renal disease. It is safe and effective in suppressing parathyroid hormone, with apparently less propensity for hypercalcemia than calcitriol (1, 25-dihydroxyvitamin D(3)). However, the mechanism of action on bone has not been fully elucidated. This study compares the effects of paricalcitol and calcitriol on the bone mineral. Neonatal (5- to 7-day-old) mouse calvariae were incubated in the absence or presence of either paricalcitol or calcitriol for 48 hours, and calcium flux, osteocalcin and acid and alkaline phosphatase activity, and interleukin-6 (IL-6) release were determined. Increasing concentrations of both calcitriol and paricalcitol increased calcium efflux. At lower concentrations, paricalcitol had no effect on acid phosphatase activity; however, at 10(-8) mol/L, paricalcitol caused a significant increase similar to that of calcitriol at 10(-9) mol/L. Increasing concentrations of paricalcitol had no effect on alkaline phosphatase activity, whereas calcitriol (10(-8) mol/L) caused significant inhibition. At low concentrations, paricalcitol had no effect on osteocalcin release; however, at 10(-8) mol/L, both compounds significantly increased osteocalcin production. Neither compound had an effect on IL-6 release. These data show that: (1) at low concentrations, both compounds induce a similar calcium efflux from cultured bone; (2) at low concentrations, paricalcitol has no effect on osteocalcin or acid and alkaline phosphatase activity; (3) at greater concentrations, paricalcitol and calcitriol have similar effects on acid phosphatase and osteocalcin activity; (4) calcitriol, but not paricalcitol, inhibits alkaline phosphatase release; and (5) the bone-resorbing effect of both compounds is independent of IL-6 release. Thus, although both compounds have similar effects on calcium efflux from bone, at therapeutic concentrations, paricalcitol does not seem to inhibit osteoblast activity. This may explain, in part, the lower calcemic effect of paricalcitol.

Genetic ablation of parathyroid glands reveals another source of parathyroid hormone

The parathyroid glands are the only known source of circulating parathyroid hormone (PTH), which initiates an endocrine cascade that regulates serum calcium concentration. Glial cells missing2 (Gcm2), a mouse homologue of Drosophila Gcm, is the only transcription factor whose expression is restricted to the parathyroid glands. Here we show that Gcm2-deficient mice lack parathyroid glands and exhibit a biological hypoparathyroidism, identifying Gcm2 as a master regulatory gene of parathyroid gland development. Unlike PTH receptor-deficient mice, however, Gcm2-deficient mice are viable and fertile, and have only a mildly abnormal bone phenotype. Despite their lack of parathyroid glands, Gcm2-deficient mice have PTH serum levels identical to those of wild-type mice, as do parathyroidectomized wild-type animals. Expression and ablation studies identified the thymus, where Gcm1, another Gcm homologue, is expressed, as the additional, downregulatable source of PTH. Thus, Gcm2 deletion uncovers an auxiliary mechanism for the regulation of calcium homeostasis in the absence of parathyroid glands. We propose that this backup mechanism may be a general feature of endocrine regulation.

Nutritional osteomalacia: substantial clinical improvement and gain in bone density posttherapy

A 52-yr-old white female presented with worsening low back and hip pain, associated with lower limb proximal muscle weakness and a waddling gait. Her laboratory evaluation revealed hypocalcemia, hypophosphatemia, a very low 25-hydroxyvitamin D level of less than 5 ng/mL, and a bone mineral density in the osteoporotic range. Her laboratory studies were consistent with osteomalacia, although this diagnosis was not established by histomorphometry. She avoided dairy products, spent little time outdoors, and when she went out, she covered her face, arms, and legs. She was on no medication. Her workup for malabsorption including sprue was negative. She was treated with calcium plus high-dose vitamin D 600,000 IU intramuscularly twice witihin 2 mo and had an impressive clinical improvement. Her difficulty with ambulation improved within 1 wk of start of therapy. Her bone mineral density increased by 40% at the spine and 35% at the hip at 4 mo of therapy, by 63% and 39% at 10 mo, and by 62% and 52% at 15 mo at these sites, respectively. Treatment of osteomalacia is extremely rewarding, with dramtic clinical improvement and normalization of bone mineral density.

New analogs of vitamin D3

Calcitriol, the most active metabolite of vitamin D, controls parathyroid gland growth and suppresses the synthesis and secretion of parathyroid hormone (PTH). However, because of its potent effects on intestinal calcium absorption and bone mobilization, calcitriol treatment can induce hypercalcemia, often precluding its use at therapeutic doses. Hyperphosphatemia is also a persistent problem among patients undergoing chronic hemodialysis and can be aggravated by therapeutic doses of calcitriol. Several pharmaceutical companies were able to modify the side-chain of the 1,25(OH)2D3, allowing some of these new analogs to retain the action on the parathyroid glands while decreasing their hypercalcemic and hyperphosphatemic effects. The structure-activity relationship for ligand-mediated transcriptional regulation has been studied in detail. In some analogs the serum binding protein (DBP) plays a key role in determining the pharmacokinetics of the vitamin D compound. The affinity to DBP for 22-oxacalcitriol (OCT), an analog of calcitriol for the treatment of secondary hyperparathryoidism, is approximately 300-400 times lower than that of calcitriol and the analog is rapidly cleared from the circulation. The mechanisms for the selectivity of 19-nor-1,25(OH)2D2 (paricalcitol) (Zemplar) another analog of calcitriol, is clearly different from OCT. Although the mechanisms of action is not completely known, it does appear that paricalcitol down-regulates the VDR in the intestine. It is likely that the unique biological profiles of vitamin D analogs in vivo are due to multiple mechanisms. Understanding the molecular basis of the analog selectivity will not only provide an explanation for their unique actions but allow intelligent design of more effective analogs in the future.

Regulation of the parathyroid hormone gene by vitamin D, calcium and phosphate

Secondary hyperparathyroidism is a frequent complication of chronic renal failure resulting in severe bone disease. Secondary hyperparathyroidism is composed of increased in parathyroid hormone (PTH) synthesis and secretion due to an increase in PTH gene expression and parathyroid cell proliferation. PTH gene expression is regulated by calcium, phosphate and 1,25-dihydroxy vitamin D (1,25(OH)2D). 1,25(OH)2D3 injected to rats leads to a dramatic decrease in PTH gene transcription without any increase in serum calcium. Hypocalcemia leads to a large increase in PTH mRNA levels which is post-transcriptional. Hypophosphatemia leads to a marked decrease in PTH gene expression that is also post-transcriptional. The mechanisms of the post-transcriptional effects of calcium and phosphate on the PTH gene have shown to be due to changes in protein-RNA interactions at the PTH mRNA 3'-UTR. Hypocalcemia leads to increased binding of parathyroid cytosolic proteins to the PTH mRNA 3'-UTR and hypophosphatemia to decreased binding of these proteins to the PTH mRNA 3'-UTR. The binding of the parathyroid proteins stabilizes the PTH RNA in an in vitro degradation assay. In rats with experimental uremia due to 5/6 nephrectomy, there is an increase in PTH mRNA levels due to a decrease in degradation of the PTH RNA as determined by this assay. The characterization of the parathyroid cytosolic proteins that interact with the PTH mRNA 3'-UTR may lead to a clearer understanding of how changes in serum calcium and phosphate result in secondary hyperparathyroidism.

Vitamin D analogs: from renal bone disease to osteoporosis

The history of the use of vitamin D metabolites and analogs in clinical medicine began with their use in renal bone disease following the appreciation that the kidney is the major site of the 1 alpha-hydroxylase of vitamin D. In the presence of advanced renal failure, production of calcitriol is impaired and treatment is viewed conceptually as a hormone replacement therapy. Since then, there has been much interest in the rationale for the use of such compounds in osteoporosis. Arguments have been variously forwarded that osteoporosis is in part due to either a defective 1 alpha-hydroxylase occurring at menopause or in later life or to target tissue resistance to calcitriol; some argue that disturbances in vitamin D metabolism are irrelevant to osteoporosis, being either a consequence of aging or a result, not a cause, of osteoporosis itself. This paper reviews these various issues and explores the differences and similarities between the pathogenesis of renal bone disease and osteoporosis.

Effect of phosphate on parathyroid hormone secretion in vivo

Alterations in phosphate homeostasis play an important role in the development of secondary hyperparathyroidism in renal failure. Until recently, it was accepted that phosphate retention only increased parathyroid hormone (PTH) secretion through indirect mechanisms affecting calcium regulation and calcitriol synthesis. However, recent in vitro studies have suggested that phosphate may directly affect PTH secretion. Our goal was to determine whether in vivo an intravenous phosphate infusion stimulated PTH secretion in the absence of changes in serum calcium. Three different doses of phosphate were infused intravenously during 120 minutes to increase the serum phosphate concentration in dogs. Sulfate was also infused intravenously as a separate experimental control. A simultaneous calcium clamp was performed to maintain a normal ionized calcium concentration throughout all studies. At the lowest dose of infused phosphate (1.2 mmol/kg), serum phosphate values increased to approximately 3 mM, but PTH values did not increase. At higher doses of infused phosphate (1.6 mmol/kg and 2.4 mmol/kg), the increase in serum phosphate to values of approximately 4 mM and 5 mM, respectively, was associated with increases in PTH, even though the ionized calcium concentration did not change. Increases in PTH were not observed until 30-60 minutes into the study. These increases were not sustained, since by 120 minutes PTH values were not different from baseline or controls despite the maintenance of marked hyperphosphatemia. During the sulfate infusion, serum sulfate values increased by approximately 3-fold, but no change in PTH values were observed. In conclusion, an acute elevation in serum phosphate stimulated PTH secretion in the intact animal, but the magnitude of hyperphosphatemia exceeded the physiologic range. Future studies are needed to determine whether PTH stimulation is more sensitive to phosphate loading in states of chronic phosphate retention. Moreover, the mechanisms responsible for the delay in PTH stimulation and the failure to sustain the increased PTH secretion need further evaluation.

Vitamin D receptor interactions with the rat parathyroid hormone gene: synergistic effects between two negative vitamin D response elements

Vitamin D response elements (VDREs) that are required for negative regulation of rat parathyroid hormone (rPTH) gene expression have been characterized. Gel mobility shift assays using DNA restriction enzyme fragments and recombinant proteins for vitamin D and retinoic acid X receptors (VDR/RXR) revealed a sequence between -793 and -779 that bound a VDR/RXR heterodimer with high affinity (VDRE1). Furthermore, a lower affinity site (VDRE2) was detected that acted in combination with VDRE1 to bind a second VDR/RXR complex. As determined by ethylation interference analysis, the nucleotide sequence of VDRE1 consisted of GGTTCA GTG AGGTAC, which is remarkably similar to the sequence of the negative VDRE found in the chicken PTH (cPTH) gene. Using the same technique, VDRE2 was identified between positions -760 and -746 and contained the sequence AGGCTA GCC AGTTCA. Functional analysis was determined by transfection studies with plasmid constructs that expressed the gene for chloramphenicol acetyl transferase (CAT). The ability of the VDREs to regulate gene expression was tested in their native context with the rPTH promoter as well as when positioned immediately upstream from the cPTH promoter. With either plasmid construct, exposure to 10(-8)M 1,25(OH)2D3 resulted in a 60-70% decrease in CAT gene expression when both VDRE1 and VDRE2 were present. Examination of the individual VDREs showed that inhibition by 10(-8) M 1,25(OH)2D3 was only 35-40% when just VDRE1 was present. By itself, VDRE2 was even less effective, as significant inhibition of CAT activity (20%) was observed only in the presence of higher concentrations of 1, 25(OH)2D3 (10(-7)M) or when a plasmid vector that overexpressed the VDR protein was cotransfected. In conclusion, the rPTH gene contains two negative VDREs that act in concert to bind two RXR/VDR heterodimer complexes and that both VDREs are required for maximal inhibition by 1,25(OH)2D3.

No difference in intestinal strontium absorption after an oral or an intravenous 1,25(OH)2D3 bolus in normal subjects. For the European Study Group on Vitamin D in children with renal failure

It has been suggested that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) stimulates intestinal calcium absorption less via the intravenous (iv) than the oral route, because the first avoids direct contact of the drug with the enterocytes. However, no study has addressed the issue directly. This investigation was designed to measure the effect of a single oral or iv dose of 1,25(OH)2D3 on calcium absorption, using stable strontium (Sr) as a surrogate for calcium, and measuring the Sr fractional absorbed dose (FAD%) over 240 minutes after Sr administration. In 10 healthy volunteers, five tests were performed in a cross-over design, with a wash-out period between two consecutive tests: Sr absorption without 1,25(OH)2D3 (test A); Sr absorption immediately after either oral (test B) or iv (test C) 1,25(OH)2D3 (1.5 microg/m2 of body surface area [BSA]); Sr absorption (24 hr after either oral (test D) or iv (test E) 1, 25(OH)2D3 (1.5 microg/m2 BSA). The concurrent administration of 1, 25(OH)2D3 and Sr (tests B and C) did not significantly change the area under the Sr FAD%-time curve with respect to test A (test A: 4090 +/- 345; test B: 4510 +/- 345; test C: 4210 +/- 345), whereas Sr absorption was significantly increased (p < 0.001) when Sr was given 24 hr after either oral or iv 1,25(OH)2D3 (test D: 5710 +/- 345; test E: 5510 +/- 345). It was concluded that 1,25(OH)2D3 is likely to influence calcium absorption significantly only via its genomic effect, independent of its administration route.

Antagonistic effects of vitamin D and parathyroid hormone on lipoprotein lipase in cultured adipocytes

The effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) (calcitriol) and parathyroid hormone (PTH) on synthesis and secretion of lipoprotein lipase (LPL) were studied in 3T3-L1 adipocytes. Expression of the vitamin D receptor was demonstrated by saturation kinetics with radiolabeled calcitriol. Incubation with calcitriol (10(-8) M) for up to 4 d resulted in a time-dependent significant increase in heparin-releasable LPL activity (LPLa) accompanied by a significant increase in LPL mRNA. In contrast, incubation with intact (1-84) PTH (10(-6) to 10(-9) M) produced a time- and dose-dependent significant decrease in LPLa, but no change in LPL mRNA. The effect of PTH (24-h incubation, 10(-8) M) could be prevented by the calcium channel blocker verapamil. Coincubation with both calcitriol and PTH at equimolar concentration (10(-8) M) resulted in an increase in LPLa and LPL mRNA. These data indicate an antagonistic role for calcitriol and PTH in the regulation of LPL, possibly mediated by intracellular calcium, which may contribute to the alterations in lipoprotein metabolism occurring in uremia.

Hormonal Control of Calcium Homeostasis

Calcium homeostasis in the extracellular fluid is tightly controlled and defended physiologically. Hypercalcemia always represents considerable underlying pathology and occurs when the hormonal control of calcium homeostasis is overwhelmed. The major hormones that are responsible for normal calcium homeostasis are parathyroid hormone and 1,25-dihydroxyvitamin D; these hormones control extracellular fluid calcium on a chronic basis. Over- or underproduction of these hormones or the tumor peptide, parathyroid hormone-related peptide, are the major causes of aberrant extracellular fluid calcium concentrations. These hormonal defense mechanisms are reviewed here.

Vitamin D and the parathyroid

Vitamin D's biologically active metabolite, 1,25(OH)2D3, has important effects upon the parathyroid cell that are relevant to both the physiology of mineral metabolism and the regulation of the secondary hyperparathyroidism of chronic renal failure. 1,25(OH)2D3 markedly decreases parathyroid hormone (PTH) gene transcription and thus PTH synthesis and secretion. It also acts to decrease parathyroid cell proliferation. Nonhypercalemic analogs of 1,25(OH)2D3 are being developed that may have a wider therapeutic window than 1,25(OH)2D3 itself. In the situations of chronic hypocalcemia and hypophosphatemia, there are interesting interrelationships between 1,25(OH)2D3 and the post-transcriptional regulation of the PTH gene. In nodular secondary hyperparathyroidism, there is down-regulation of the vitamin D receptor in the parathyroid. Different vitamin D receptor genotypes may be associated with higher levels of serum PTH and a predisposition to autonomous hyperplasia.

Prevention of renal osteodystrophy in predialysis patients

Impaired calcitriol synthesis is one of the major factors contributing to the development of secondary hyperparathyroidism in patients with chronic renal failure. Vitamin D therapy, particularly 1alpha-hydroxyvitamin D3, even in low doses, has been shown to be effective in the treatment of secondary hyperparathyroidism in patients with mild-to-moderate chronic renal failure. Complications associated with calcitriol and alfacalcidol therapy, which include hypercalcemia and progressive deterioration of renal function, have been reported in some patients. The majority of the studies reviewed, however, demonstrated that daily calcitriol and alfacalcidol doses below 0.25 microg are rarely associated with hypercalcemia, hyperphosphatemia, or progressive decline in renal function. In addition, these complications usually resolve with the reduction in dose or discontinuation of the medication. Thus, vitamin D therapy may be valuable in the treatment of patients with mild-to-moderate chronic renal failure who may be at high risk of developing secondary hyperparathyroidism.

Differential effects of 1,25-dihydroxy-vitamin D3 and 19-nor-1,25-dihydroxy-vitamin D2 on calcium and phosphorus resorption in bone

1,25-Dihydroxy-vitamin D3 [1,25-(OH)2D3] suppresses the secretion and synthesis of parathyroid hormone (PTH) and has been used in the treatment of secondary hyperparathyroidism. However, 1,25-(OH)2D3 can induce hypercalcemia, which often precludes its use. Therefore, an analog of 1,25-(OH)2D3 that would retain its therapeutic effects but produce minor effects on calcium and phosphorus metabolism could be an ideal tool for the treatment of secondary hyperparathyroidism. It has been shown that 19-nor-1,25-dihydroxy-vitamin D2 [19-nor-1,25-(OH)2D2], an analog of 1,25-(OH)2D3, can suppress PTH levels in uremic rats at doses that do not affect plasma ionized calcium levels. The experiments presented here, using parathyroidectomized rats fed diets deficient in either calcium (0.02%) or phosphorus (0.02%), were performed to compare the effects of 1,25-(OH)2D3 and 19-nor-1,25-(OH)2D2 on calcium and phosphorus resorption in bone. Parathyroidectomized rats received daily intraperitoneal injections of vehicle, 1,25-(OH)2D3 (100 ng), or 19-nor-1,25-(OH)2D2 (100 or 1000 ng) for 9 d. Plasma calcium and phosphorus levels were monitored during the study, and ionized calcium levels were determined at the end of the study. By 9 d, 1,25-(OH)2D3 (100 ng/d) increased total calcium levels to 12.4+/-0.26 mg/dl, compared with 6.32+/-0.25 mg/dl (P<0.001) in control animals. The same dose of 19-nor-1,25-(OH)2D2 (100 ng/d) was much less potent (9.45+/-0.28 mg/dl, P<0.001). Similar results were seen with ionized calcium levels [19-nor-1,25-(OH)2D2, 3.61+/-0.12 mg/dl; 1,25-(OH)2D3, 5.03+/-0.16 mg/dl; P<0.001]. Ionized calcium levels were also lower in rats receiving the higher dose (1000 ng) of 19-nor-1,25-(OH)2D2 (4.59+/-0.09 mg/dl, P<0.05). Similar results were seen in rats fed the phosphorus-deficient diet. 1,25-(OH)2D3 (100 ng) increased plasma phosphorus levels from 4.30+/-0.39 mg/dl in vehicle-treated rats to 7.43+/-0.26 mg/dl (P<0.001). The same dose of 19-nor-1,25-(OH)2D2 had no effect (5.19+/-0.32 mg/dl), whereas the high dose (1000 ng) increased plasma phosphorus levels (7.31+/-0.24 mg/dl) in a manner similar to that of 1,25-(OH)2D3 (100 ng). Therefore, 19-nor-1,25-(OH)2D2 is approximately 10 times less effective in mobilizing calcium and phosphorus from the skeleton, compared with 1,25-(OH)2D3. With its ability to suppress PTH at noncalcemic doses, 19-nor-1,25-(OH)2D2 is a potential therapeutic tool for the treatment of secondary hyperparathyroidism in chronic renal failure.

In vivo dose-related receptor binding of the vitamin D analogue [3H]-1,25-dihydroxy-22-oxavitamin D3 (OCT) in rat parathyroid, kidney distal and proximal tubules, duodenum, and skin, studied by quantitative receptor autoradiography

1,25-Dihydroxy-22-oxavitamin D3 (OCT) is a new synthetic analogue of 1,25(OH)2D3 with a low calcemic effect. This study utilized quantitative receptor autoradiography to determine the dose-related receptor binding and saturation among the vitamin D target cells: parathyroid chief cells, kidney distal and proximal tubule epithelium, duodenal absorptive epithelium, and epidermal keratinocytes. Rats were injected with 0.25, 0.5, 1.0, 2.0, 4.0, 8.0, or 16.0 microgram/kg bw of [26-3H]-OCT and sacrificed 1 hr afterwards. Then autoradiographs were prepared under identical conditions. In these target cells, nuclear uptake of radioactivity increased with dose and then achieved a plateau. However, their saturation doses showed differences: parathyroid chief cells 1-2 microgram duodenal absorptive epithelium, distal tubule epithelium, and epidermal keratinocytes 4-6 microgram proximal tubule epithelium 8 microgram (per kg bw). In contrast, in nontarget cells, such as liver and duodenal smooth muscle, radioactivity did not concentrate in the nuclei but increased in the cytoplasm with dose, without plateauing. These results provide the first information on the relative saturabilities of various target cell populations with a vitamin D ligand. Parathyroid chief cells required the relatively lowest receptor saturation dose. This suggests a high sensitivity and response to OCT treatment with related therapeutic potential for the regulation of parathyroid function.

Point mutations of the human parathyroid calcium receptor gene are not responsible for non-suppressible renal hyperparathyroidism

The calcium-dependent secretion of parathyroid hormone (PTH) is mediated through an extracellular G protein-coupled calcium receptor (CaR). Inactivating point mutations of this receptor have been found in familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. These diseases feature a decreased calcium sensitivity of the parathyroid glands, resulting in a rightward shift of the Ca2(+)-PTH relationship. Severe non-suppressible renal hyperparathyroidism (rHPT) is often characterized by similar setpoint shifts to the right. Thus, point mutations of the CaR gene could contribute to non-suppressible rHPT. We examined genomic DNA of hyperplastic or mainly nodular tissues of 39 parathyroids from 25 rHPT-patients with resistance to calcitriol therapy. Amplification of the six exons of the CaR gene was followed by single-strand conformation polymorphism (SSCP) analysis. DNA sequencing was performed where band shifts were observed. No point mutations in the coding sequence of the CaR gene were detected using the PCR-SSCP strategy. Point mutations in the coding regions of the CaR gene probably play no role in the evolution of renal HPT and are not responsible for the calcitriol resistance of PTH secretion.

Inhibition of PTH secretion by interleukin-1 beta in bovine parathyroid glands in vitro is associated with an up-regulation of the calcium-sensing receptor mRNA

The principal regulator of parathyroid hormone (PTH) secretion is ionized calcium, but other factors are also known to modulate PTH secretion, such as vitamin D, estrogen, and recently inorganic phosphate. Interleukin-1 (IL-1) possesses a wide variety of biological activities and is produced by leukocytes as well as by various other cells including cells from endocrine tissues and might play a role as a paracrine factor in the control of PTH secretion. We investigated the effect in vitro of IL-1 beta on PTH release, PTHmRNA and the mRNA for the extracellular calcium-sensing receptor (CaR) levels in preparations of bovine parathyroid cells. PTH secretion from cultured parathyroid tissue slices was significantly inhibited in a medium containing IL-1 beta at a concentration of 2000 pg/ml (PTH in % of control: 63.5 +/- 5.3), n=10 (p<0.01). The inhibitory effect of IL-1 beta was not found in preparations of dispersed cells. The inhibitory effect of IL-1 beta could be counteracted by the IL-1 receptor antagonist (IL-1ra), indicating that the inhibitory effect was mediated through the specific IL-1 receptor on the parathyroid cells. IL-1 beta (2000 pg/ml) up-regulated CaRmRNA levels to 180% of control, whereas no change in PTHmRNA was found. IL-1ra abolished the upregulating effect of IL-1 beta on the CaRmRNA. This study demonstrates a direct effect in vitro of IL-1 beta on PTH secretion from bovine parathyroid glands, an effect which may be mediated at least in part through the specific IL-1 receptor causing an upregulation of the calcium-sensing receptor mRNA. IL-1 might therefore play a role as a auto- and/or paracrine factor in the regulation of the PTH secretion.

Targeted ablation of the vitamin D receptor: an animal model of vitamin D-dependent rickets type II with alopecia

Vitamin D, the major steroid hormone that controls mineral ion homeostasis, exerts its actions through the vitamin D receptor (VDR). The VDR is expressed in many tissues, including several tissues not thought to play a role in mineral metabolism. Studies in kindreds with VDR mutations (vitamin D-dependent rickets type II, VDDR II) have demonstrated hypocalcemia, hyperparathyroidism, rickets, and osteomalacia. Alopecia, which is not a feature of vitamin D deficiency, is seen in some kindreds. We have generated a mouse model of VDDR II by targeted ablation of the second zinc finger of the VDR DNA-binding domain. Despite known expression of the VDR in fetal life, homozygous mice are phenotypically normal at birth and demonstrate normal survival at least until 6 months. They become hypocalcemic at 21 days of age, at which time their parathyroid hormone (PTH) levels begin to rise. Hyperparathyroidism is accompanied by an increase in the size of the parathyroid gland as well as an increase in PTH mRNA levels. Rickets and osteomalacia are seen by day 35; however, as early as day 15, there is an expansion in the zone of hypertrophic chondrocytes in the growth plate. In contrast to animals made vitamin D deficient by dietary means, and like some patients with VDDR II, these mice develop progressive alopecia from the age of 4 weeks.

Parathyroid hormone prevents 1,25 (OH)2D3 induced down-regulation of the vitamin D receptor in growth plate chondrocytes in vitro

1,25(OH)2D3 has an antiproliferative effect on growth plate chondrocytes when given in high doses, whereas low doses stimulate chondrocyte proliferation. In the present in vitro study we investigated the effects of parathyroid hormone (PTH) when given concomitantly with 1,25(OH)2D3 on cell proliferation and vitamin D receptor (VDR) regulation. Freshly isolated rat tibial chondrocytes were grown in monolayer cultures or in agarose stabilized suspension cultures (10% charcoal-treated FCS). VDR expression was determined by RT-PCR generating a 297 bp fragment and by binding assays (Scatchard analysis) with [3H]-1,25(OH)2D3. Cell proliferation was measured by [3H]-thymidine incorporation, growth curves in monolayer cultures and by colony formation in agarose-stabilized suspension cultures. Optimal concentration of 1,25(OH)2D3 (10(-12) M) and of PTH fragments [bPTH(1-34) or hPTH(28-48), 10(-10)M] showed additive effects on DNA synthesis of and colony formation by growth plate chondrocytes. This may be explained in part by an up-regulation of VDR by PTH: PTH increased both mRNA expression of VDR and binding capacity. 1,25(OH)2D3 (10(-12) M) induced an up-regulation of the VDR within 24 hours followed by a down-regulation after incubation for more than 24 hours. PTH fragments added concomitantly prevented the down-regulation seen with 1,25(OH)2D3. These findings provide evidence that PTH is a growth promoting hormone that also modulates the effects of 1,25(OH)2D3 by regulating the VDR status of 1,25(OH)2D3 target cells.

Non-familial primary hyperparathyroidism

During recent decades, primary hyperparathyroidism (pHPT) has appeared as one of the more common endocrine disorders. Previously, the disease was the obvious cause of severe, symptomatic bone disease, recurrent renal stones, and sometimes devastating muscular weakness. The condition often progressed rapidly and ultimately ended in renal insufficiency. Today, pHPT is frequently recognized in patients with less obvious symptoms and markedly slower disease progression. However, if thoroughly examined, many of these patients will also present typical symptoms and complications of the disease. Surgery in pHPT has also developed as a highly efficient procedure with low failure rate and few complications. Further, successful operation is likely to decrease the risk of developing long-term disturbances of calcium metabolism and recently recognized cardiovascular complications of the disease. However, in a group of generally elderly patients with especially mild hypercalcemia and no obvious symptoms, disease progression may be slow, and it is possible that some of these patients can be followed safely without surgery. These patients also constitute a majority of cases detected in population surveys. Pathophysiological studies of pHPT have revealed more or less disturbed secretory regulation as a characteristic feature of pathological parathyroid glands, and this accounts principally for the patients' hypercalcemia. This abnormality has been related to decreased expression or capacity of parathyroid cell surface receptors executing a crucial calcium-sensing function. Recent progress has also led to the identification of causes of a growth regulatory disturbance in pathological parathyroid glands. Exploration of molecular mechanisms behind these abnormalities are likely to further unveil disease characteristics and help explain differences in clinical symptoms and disease progression among the patients with pHPT.