PTH Release Research Articles

Sustained release of PTH(1–34) from PLGA microspheres suppresses osteoarthritis progression in rats

We previously reported that PTH(1–34) inhibits the terminal differentiation of articular chondrocytes and, in turn, suppresses the progression of osteoarthritis (OA). However, this treatment requires an injection of PTH(1–34) once every 3days over the treatment period. In this study, we studied the effect of sustained administration of PTH(1–34) in a papain-induced OA rat model. We developed an effective controlled-release system for prolonging the treatment duration of an intra-articular injection for OA treatment in rats. The effects of released PTH(1–34) from PLGA(65:35)-encapsulated PTH(1–34) microspheres (PTH/PLGA) on papain-induced OA in rat knees were studied. Microsphere morphology was observed in vitro by scanning electron microscopy, and microsphere size was determined with a particle size analyzer. The PTH(1–34) encapsulation efficiency and release profile, as well as the toxicity of PTH/PLGA, were examined. The bioactivity of released PTH(1–34) was tested by examining cAMP levels in MC3T3E1 cells. In vivo, we evaluated the changes of localized GAG, Col II, and Col X in the articular cartilage of rat knees. Our results demonstrated that the surface of the PLGA microspheres was smooth, and the size of the microspheres was in the range of 51–127μm. PTH/PLGA microspheres sustainably released PTH(1–34) for 19days with a concentration range of 0.01–100nM that covered the expected concentration of 10nM at 37°C. The cAMP levels of MC3T3E1 cells were elevated in the response to released PTH(1–34) from PTH/PLGA microspheres, indicating that the released PTH(1–34) is bioactive. Most importantly, intra-articular treatment with either PTH(1–34) (0.1–100nM) 3days/injection or PTH/PLGA microspheres (15days/injection) for 5weeks revealed the similar effect on suppressing papain-induced OA changes (decreasing GAG and Col II and increasing Col X) in rat knee cartilage. The effect of PTH/PLGA microspheres on suppressing OA progression was similar to that of a once-every-three-day injection of PTH(1–34), indicating that both the sustained and intermittent action of PTH(1–34) effectively suppress OA progression. The developed PLGA microspheres with sustained release and long-term effect may be potent carriers for PTH(1–34) used to treat early OA.

Smoking and hyperparathyroidism in patients with end-stage renal disease (ESRD)

Smoking is associated with hyperparathyroidism in the elderly general population and nicotine, the main component of tobacco smoke, stimulates PTH release in experimental models. Although smoking is a persisting problem in patients with end-stage renal disease (ESRD), the association between smoking and PTH has never been specifically examined in these patients. We investigated the relationship between smoking and hyperparathyroidism in a well-characterized group of 161 nondiabetic dialysis patients. Sixty-four patients (40%) were smokers. Heavy smokers had higher intact PTH (median: 280 pg/mL) and PTH1-84 (188 pg/mL) than light smokers (180 pg/mL and 95 pg/mL) and nonsmokers (169 pg/mL and 95 pg/mL). In a multiple regression analysis, smoking was independently associated with intact PTH (ß=0.29, p=0.002) and PTH1-84 (ß=0.29, p=0.002). Fifty-six of 161 patients (35%) were classified as having hyperparathyroidism. In a multiple logistic regression model the odds of hyperparathyroidism were about 4 times higher in heavy smokers (odds ratio 3.88, 95% CI 1.16-12.92, p=0.027) than in nonsmokers. In dialysis patients heavy smoking is independently associated with high levels of intact PTH and PTH1-84. Further observational, mechanistic and interventional studies are needed to assess the nature (causal or noncausal) of these links in ESRD.

Direct Suppression of PTH Gene Expression by the Vitamin D Prohormones Doxercalciferol and Calcidiol Requires the Vitamin D Receptor

Vitamin D compounds regulate PTH at the transcriptional level, presumably via binding to the vitamin D receptor (VDR), but the exact mechanism is presently unclear. We recently reported that the several vitamin D prohormones with low VDR affinity suppressed PTH, even when their activation was inhibited, raising the possibility that their actions may be VDR independent. To test this hypothesis, we developed a novel organ culture that allowed the assessment of activities of the prohormones on PTH release from wild-type and VDR-null thyroparathyroid explants. The cultures remained viable with respect to PTH release for at least 2 weeks. Full suppression of PTH by the native vitamin D hormone, 1α,25-dihydroxyvitamin D(3) [1α,25 (OH)(2)D(3)], required 2 days, consistent with a transcriptional mechanism, and was reversible, indicating that reduced PTH was not attributable to cell death. Inhibition of PTH release by 1α,25 (OH)(2)D(3) and two prohormones, 25-hydroxyvitamin D(3) and 1α-hydroxyvitamin D(2), was observed in explants from wild-type mice but not in those from VDR-null mice. These findings 1) are the first direct demonstration of the role of the VDR in regulation of PTH by 1α,25(OH)(2)D(3), 2) confirm that the suppressive actions of the vitamin D prohormones are mediated by the VDR, and 3) introduce a novel organ culture model that allows the ex vivo study of the function of parathyroid glands from transgenic animals.

Sustained activation of renal N-methyl-D-aspartate receptors decreases vitamin D synthesis: a possible role for glutamate on the onset of secondary HPT.

N-methyl-D-aspartate (NMDA) receptors (NMDAR) are tetrameric amino acid receptors that act as membrane calcium channels. The presence of the receptor has been detected in the principal organs responsible for calcium homeostasis (kidney, bone, and parathyroid gland), pointing to a possible role in mineral metabolism. The aim of this study was to test the effect of NMDAR activation in the kidney and on 1,25(OH)₂D₃ synthesis. We determined the presence of NMDAR subunits in HK-2 (human kidney cells) cells and proved its functionality. NMDA treatment for 4 days induced a decrease in 1α-hydroxylase levels and 1,25(OH)₂D₃ synthesis through the activation of the MAPK/ERK pathway in HK-2 cells. In vivo administration of NMDA for 4 days also caused a decrease in blood 1,25(OH)₂D₃ levels in healthy animals and an increase in blood PTH levels. This increase in PTH induced a decrease in the urinary excretion of calcium and an increase in urinary excretion of phosphorous and sodium as well as in diuresis. Bone turnover markers also increased. Animals with 5/6 nephrectomy showed low levels of renal 1α-hydroxylase as well as high levels of renal glutamate compared with healthy animals. In conclusion, NMDAR activation in the kidney causes a decrease in 1,25(OH)₂D₃ synthesis, which induces an increase on PTH synthesis and release. In animals with chronic kidney disease, high renal levels of glutamate could be involved in the downregulation of 1α-hydroxylase expression.

New aminopropandiol derivatives as orally available and short-acting calcium-sensing receptor antagonists

Synthesis and structure–activity relationship studies on a new aminopropandiol class of derivatives as calcium-sensing receptor antagonists are described. Modification of the phenolic moiety of a calcilytic compound NPS 2143 led to the identification of an orally available compound (R,R)-31 which demonstrated a rapid and transient stimulation of PTH release in rats.

The effect of oxotremorine on in-vitro secretion of parathyroid hormone.

The effect of the muscarine-like acting parasympathomimetic drug oxotremorine and its antagonist atropine on PTH release was tested using single cell suspensions obtained from a) primary parathyroid adenomas, and b) secondary hyperplastic parathyroid tissue from patients undergoing chronic hemodialysis. For both cell-types we found a dose-dependent suppression of PTH release with oxotremorine. The suppressive activity could be antagonized with atropine, suggesting that parasympathomimetic nerval impulses play a suppressive role in the regulation of PTH release. Muscarine-receptors appear to be responsible for this mechanism.

The secretion of parathyroid hormone and its fragments from dispersed cells of adenomatous parathyroid tissue at different calcium concentrations.

The effect of varying calcium concentrations on PTH release from dispersed cells of adenomatous parathyroid tissues (n = 15) was studied. During high calcium concentrations PTH secretion was inhibited up to 62.5% as compared to low calcium concentrations. There was no correlation between iPTH secretion rate and suppressibility. Each adenoma had a different iPTH secretion rate. Three adenomas showed a high suppressibility (28.0%, 53.8%, and 62.5%). The supernates of their media were chromatographed and examined by midregion and C-terminal specific PTH-RIAs. Carboxyl-terminal PTH fragments were found to be released by two adenomas. There was no evidence for the release of midregion PTH fragments. Comparison of incubation media revealed that the adenoma with the lowest suppressibility released the highest amount of intact PTH (per 100,000 cells/ml). This adenoma exclusively secreted intact PTH, whereas PTH fragments were only released from two adenomas with higher suppressibility. In these media the ratio of PTH fragments to intact PTH increased with the calcium content of the media. The ratio was also dependent on the suppressibility of the adenomas; i.e. the ratio was greater when the suppressibility was higher. This suggests that in adenomas the intracellular hormone degradation is responsible for the degree of PTH suppression. As the suppressibility of PTH secretion is reduced in hyperfunctioning parathyroid tissue, diminished intracellular hormone degradation seems to be involved in the pathogenesis of primary hyperparathyroidism.

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.

Discovery and Structure−Activity Relationships of Trisubstituted Pyrimidines/Pyridines as Novel Calcium-Sensing Receptor Antagonists

The trisubstituted pyrimidine 1 was identified through high-throughput screening as a novel calcium-sensing receptor (CaSR) antagonist. Small molecule CaSR antagonists and/or negative allosteric modulators have the potential to act as an anabolic agent for the treatment of osteoporosis. The investigation of structure-activity relationships around 1 resulted in the identification of 18c and 18d, which showed efficacy at promoting PTH release in vivo and exhibited improved potency and solubility over the original lead 1.

Hyperparathyroidism in a diabetic patient on dialysis.

A 58-year-old male, who had type II diabetes and was on haemodialysis (HD) for 3 years, was referred for management of refractory hyperparathyroidism in 2006. In 2003, he started HD due to diabetic renal failure, after which vitamin D analogue together with oral calcium carbonate and sevelamer hydrochloride was administered, so that serum calcium and phosphate levels were controlled properly. Serum intact parathyroid hormone (iPTH) levels were <200 pg/dl. However, in 2006 serum calcium began to rise above 11 mg/dl and iPTH reached 400 pg/dl despite intravenous vitamin D administration. On admission, corrected calcium was 13.3 mg/dl, phosphate 6.2 mg/dl, alkaline phosphatase 1070 IU/l and iPTH 1640 pg/ml. 99mTc-methoxyisobutylisonitril (MIBI) scintigraphy revealed one enlarged functional parathyroid gland in the right lower region of the neck, which was shown as a irregularly enhanced nodule with calcification by a computed tomography (CT) scan (Figure ​(Figure1a–c).1a–c). Surgical parathyroidectomy (PTx) was performed and two parathyroid glands were removed. The right lower gland weighed 4000 mg and the other was almost normal without hyperplasia. Histology of the enlarged gland showed monotonous cellularity accompanied by all three typical features: breach of the glandular capsule, abnormal mitoses and cellular invasion into blood vessels, although the presence of at least two of the three is enough for diagnosis of parathyroid carcinoma (Figure ​(Figure2a–c).2a–c). The patient is now free of vitamin D and phosphate binder and no evidence of either local resistance or remote metastasis is found. Fig. 1 Radiological study for cervical mass lesion. CT scan revealed the nodule with calcification at the right lower region of the neck (red arrow) in the axial (a) and reformatted frontal image (b). 99mTc-MIBI parathyroid scintigraphy (c) showed only one hot ... Fig. 2 Histopathological assessment. (a) Breach of the glandular capsule, (b) abnormal mitosis (red arrow) and (c) cellular invasion into blood vessels were compatible with parathyroid carcinoma (a: haematoxylin and eosin; original magnification, ×100; ... In general, few diabetic patients on dialysis develop secondary parathyroidism with the indication of PTx, partly because PTH release is inhibited by hyperglycaemia or insulin deficiency [1,2]. Therefore, when a relatively rapid rise in PTH is detected early on the maintenance HD especially in diabetic patients, the possibility of parathyroid carcinoma should be taken into consideration, even though rare [3]. Conflict of interest statement. None declared.

Role of Vitamin D Deficiency in Chronic Kidney Disease

Chronic kidney disease (CKD) has been recognized as a significant public health problem, with approximately 20 million Americans, or approximately 11% of the adult population, currently living with CKD. A significant source of morbidity associated with CKD is the development of disturbances of mineral metabolism, which occurs in virtually all patients during the progression of their disease, and is associated with bone loss and fractures, cardiovascular disease, immune suppression, and increased mortality. As kidney disease develops, there is decreased functional renal mass and a tendency to retain phosphorus. The reduction in functional renal mass and the retained phosphorus act to reduce renal 1alpha-hydroxylase activity and thus the renal production of calcitriol. Further compensation to maintain normal serum calcium and phosphorus homeostasis includes increased production and release of PTH and potentially other phosphaturic factors, such as fibroblast growth factor-23 (FGF23). This increase in FGF23 contributes to maintain normal serum phosphate independent of PTH but may worsen calcitriol deficiency by also inhibiting renal 1alpha-hydroxylase activity. The decrease in calcitriol also results in promoting further hyperparathyroidism and parathyroid gland hyperplasia, because calcitriol normally inhibits the production of prepro-PTH and parathyroid cell proliferation.

Alterations in calcium metabolism in patients on long-term bisphosphonate therapy for metastatic breast cancer (MBC): A potential therapeutic target?

1044 Background: The effect of long-term potent BP use on calcium homeostasis in MBC pts is poorly understood. These effects and the need for calcium and Vitamin D supplementation may have significant implications for pt management in both the metastatic and adjuvant settings. Methods: We evaluated serum calcium (sCa), PTH, and 25(OH)D levels and their inter-relationships in 30 MBC pts after prolonged BP use for bone metastases (BM), compared with a control group (CG) of 1484 normal women without bone or mineral disease (JCEM 2003;88:185). The MBC pts were prescribed oral calcium (1g/d) and vitamin D (400IU/d) for 1 month prior to analysis. To determine whether MBC pts differed from CG with respect to the sCa-PTH correlation, logistic regression was performed with sCa, log-transformed PTH and an interaction term (Ca × lnPTH). Results: Mean age of the two groups did not differ [MBC group (58.5 ± 2.0 yr) vs. CG (55.2 ± 0.4 yr, p &gt;.05)]. VitD-deficiency (25vitD &lt; 40 nM) was equally prevalent in the two groups [MBC (23%) vs. CG (25%)]. sCa was significantly higher in MBC pts (2.473 ± 0.019 mM) compared to CG (2.377 ± 0.003, p &lt;0.001). However, PTH was not decreased in MBC pts (4.60 ± 0.36 pM), compared to CG (4.56 ± 0.06, t-test, p=0.91). Logistic regression was highly significant (p&lt;0.0001), as were coefficients for both explanatory variables (sCa, p &lt;0.0001 and lnPTH, p = 0.009) and their interaction (p=0.011). Conclusion: In pts with BM and prolonged BP use there appears to be a shift in the calcium set-point such that PTH release is activated by higher levels of sCa than in controls. We are currently studying whether prophylaxis for relative vit D deficiency in this population can not only correct this shift but also improve pt symptoms No significant financial relationships to disclose.

Impaired parathyroid hormone response to hypocalermic stimuli in a patient with hypomagnesemic hypocalcemia

Magnesium (Mg) deficiency sometimes causes hypocalcemia with impaired PTH secretion although the precise mechanism remains unclear. We examined the PTH secretion in response to physiological hypocalcemic stimuli in a patient with hypomagnesemic hypocalcemia. We adopted sodium bicarbonate infusion test, which we recently developed, to evaluate the PTH response to acute decrease in plasma ionized Ca. The results showed that, before Mg replacement and when the patient was mildly hypocalcemic, absolutely no PTH release to hypocalcemic stimuli was observed. In contrast, the plasma Ca was promptly normalized following the start of Mg replacement, and brisk PTH response to hypocalcemic stimuli was obtained during the same test carried out a week after the Mg replacement. The data in this case thus suggest that: a) the acute regulation of PTH release by plasma ionized Ca is lost in the patient with hypomagnesemic hypocalcemia, and b) Mg deficiency itself is likely to be a primary cause of this disorder because the hormone response was clearly restored after shortterm Mg replacement alone.

Role of the cytoskeleton in extracellular calcium-regulated PTH release

The calcium-sensing receptor (CaR) mediates the effects of extracellular calcium ([Ca 2+] o) on PTH release, such that increasing levels of [Ca 2+] o inhibit PTH secretion through poorly defined mechanisms. In the present studies, immunocytochemical analysis demonstrated that F-actin, PTH, CaR, and caveolin-1 are colocalized at the apical secretory pole of PT cells, and subcellular fractionation of PT cells showed these proteins to be present within the secretory granule fraction. High [Ca 2+] o caused F-actin, PTH, and caveolin-1 to move to the apical pole of the cells. Depolymerization of F-actin by cytochalasin reduced the actin network and induced redistribution of actin/caveolin-1 to a dispersed pattern within the cell. The F-actin-severing compounds, latrunculin and cytochalasin, significantly increased PTH secretion, while the actin polymerizing agent, jasplakinolide, substantially inhibited PTH secretion. We have demonstrated that in polarized PT cells, the F-actin cytoskeleton is involved in the regulation of PTH secretion and is critical for inhibition of PTH secretion by high calcium.

Potential Role of Pancreatic and Enteric Hormones in Regulating Bone Turnover

Potential Role of Pancreatic and Enteric Hormones in Regulating Bone Turnover

Peripheral type benzodiazepine receptor in human parathyroid glands: Up-regulation in adenoma

In this study we report the presence of peripheral benzodiazepine receptors (PBRs) in human parathyroid glands and describe the effect of their benzodiazepine type ligands on parathyroid cell function. PBR binding features in normal parathyroid tissue were characterized and compared to parathyroid adenoma, using a specific and selective ligand for PBR, [3H] 1-(2-chlorophenyl)-N-methyl-N-(1-methyl-propyl)-3-isoquinoline-carboxamide ([3H]PK11195). Affinity and density of [3H]PK11195 binding sites in homogenate membrane preparations from adenomatous and normal tissues were determined. Parathyroid adenoma showed a statistically significant 2.2 fold increase of [3H]PK11195 binding sites, while the affinity remained unchanged. Our results represent the first evidence of PBRs in parathyroid glands and suggest for them a role in influencing PTH release. A clear trend of PBR up-regulation in parathyroid adenoma was also found.

Morphological Changes in the Parathyroid Gland of Rats with Humoral Hypercalcaemia of Malignancy

Morphological changes in parathyroid chief cells were investigated in rats with humoral hypercalcaemia of malignancy (HHM), induced by intraperitoneal inoculation with a rat pulmonary adenocarcinoma cell line, IP-B12. Inoculation with IP-B12 cells resulted in the production of parathyroid hormone-related protein (PTHrP); thus, IP-B12-bearing rats developed hypercalcaemia, with elevated plasma PTHrP concentrations and decreased serum PTH, at 5 to 7 weeks after the inoculation. Morphometrical analysis revealed an increase in the cytoplasmic area of chief cells in IP-B12-bearing rats, as compared with controls. Ultrastructurally, although the number of mature storage granules in the chief cells was similar in controls and IP-B12-bearing animals, degraded granules were significantly increased in tumour-bearing rats. In addition, the chief cells of IP-B12-bearing rats showed a trend towards decreased tortuosity of the cytoplasmic membrane, with simple interdigitations between adjacent cells. These findings suggested that under HHM conditions, PTH granules in the chief cells were maintained in the form of storage granules and were hydrolysed, resulting in the suppression of PTH release into the blood circulation. Serum PTH concentrations may be regulated by inhibition of secretion rather than suppression of granular synthesis in hypercalcaemic rats. This is the first description of morphological changes in the parathyroid gland of rats with HHM.

Beta1-adrenergic blockade augments pulsatile PTH secretion in humans.

Pulsatile peptide hormone secretion provides efficient control of specific end organ functions. To test the hypothesis that sympathetic neuronal activity drives synchronous pulsatile PTH release from the parathyroid glands, we investigated the acute effects of beta1-adrenergic receptor blockade on PTH secretion patterns in a single-blinded study in nine healthy adults. Plasma PTH levels were determined at 1-min intervals. After a 75-min baseline period, seven subjects received a continuous intravenous infusion of the short-acting beta1-adrenergic receptor blocker esmolol for 105 min. After a 30-min washout period, esmolol was infused for another 30 min. Two additional subjects were randomized to receive solvent infusions. PTH secretion characteristics were analyzed by multiparameter deconvolution analysis, and the orderliness of plasma PTH fluctuations by Approximate Entropy statistics. BP slightly decreased during esmolol infusion, whereas heart rate, ionized calcium, phosphate, magnesium, and plasma and urine catecholamines remained unchanged. Esmolol increased mean plasma PTH by 33 +/- 8% (P < 0.01), due to a preferential increase in the pulsatile PTH secretion component (+129 +/- 44%, P < 0.02). The increased pulsatile PTH secretion was mediated by an augmented PTH burst mass (+117 +/- 42%, P < 0.01), whereas burst frequency remained unchanged. The regularity of PTH fluctuations was not affected by the beta-adrenergic blockade. The effects were reproducible during the second esmolol infusion. The authors conclude that the sympathetic nervous system has a modulating effect on pulsatile PTH secretion. Selective beta-1 adrenergic blockade acutely increases plasma PTH by augmenting the mass of hormone secreted per burst, but it does not alter the rhythmicity of pulsatile PTH release.

Present and future pharmacotherapy for osteoporosis

Although neither calcium nor vitamin D has been shown to prevent osteoporosis in postmenopausal women alone, the combination does. Both calcium and vitamin D are commonly used in the treatment of osteoporosis. The estrogens and raloxifene both prevent bone loss in postmenopausal women, and the estrogens probably also decrease the risk of first fracture. There is good evidence that raloxifene prevents further fractures in postmenopausal women who have already had fractures and some evidence that estrogen does as well. Calcitonin increases bone mineral density in early postmenopausal women and men with idiopathic osteoporosis, and also reduces the risk of new fractures in osteoporotic women. The bisphosphonate alendronate prevents bone loss and reduces fractures in healthy and osteoporotic postmenopausal women, and in osteoporotic men. Risedronate is more potent and has fewer upper gastrointestinal side effects than alendronate, and reduces the incidence of fractures in osteoporotic women. Intermittent use of the potent bisphosphonate zoledronate also increases bone mineral density and may become an alternative in the prevention and treatment of osteoporosis. All of the agents discussed above prevent bone resorption, whereas teriparatide increases bone formation and is effective in the treatment of osteoporotic women and men. In the treatment of secondary osteoporosis associated with the use of glucocorticoids to treat inflammation or prevent rejection after transplantation, the bisphosphonates are effective. The agents that have undergone some clinical trialing as new or alternative drugs for the treatment of osteoporosis include tibolone, new SERMs, androgens, growth hormone, insulin-like growth factor-1 and stontium ranelate. The targets/drugs that are being developed to inhibit bone resorption include the OPG/ RANKL/RANK system, cathepsin K inhibitors, vitronectin receptor antagonists, estren, the interleukin-6 and gp130 system, cytokines and growth factors. New drugs/targets to promote bone formation include the commonly used lipid-lowering statins and the calcilytic release of PTH.

Mitogen-activated protein kinase cascade in human normal and tumoral parathyroid cells.

The calcium-sensing receptor (CaR) activation has recently been shown to modulate the ERK1 and ERK2 cascade in different cell lines. The present study investigated this pathway in human normal and tumoral parathyroid cells. In cells from normal parathyroids and almost all hyperplasia increasing extracellular calcium concentrations (Ca(o)(2+)) induced a significant activation of ERK1 and -2, the percent stimulation over basal activity (at 0.5 mM Ca(o)(2+)) being 545 +/- 140 and 800 +/- 205 in normal cells and 290 +/- 71 and 350 +/- 73 in hyperplasia at 1 and 2 mM Ca(o)(2+), respectively. This effect was mediated by CaR because it was mimicked by the receptor agonist gadolinium and neomycin. Basal and Ca(o)(2+)-stimulated ERK1 and -2 activity was nearly abolished by the PKC inhibitor calphostin C, and PKA changes did not affect ERK1 and -2 activity. PI3K blockade by wortmannin, known to prevent G protein betagamma subunit effect on ERK1 and -2, induced a 30% reduction of the Ca(o)(2+)-stimulated ERK1 and -2 activity. Adenomatous cells showed high PKC-dependent ERK1 and -2 activity in resting conditions that was unresponsive to high Ca(o)(2+). A role of MAPK on PTH secretion was suggested by the finding that PD98059, a specific MEK inhibitor, abolished the inhibitory effect of 1.5 mM Ca(o)(2+) on PTH release from normal parathyroid cells. In conclusion, these data first demonstrate that CaR activation, through the PKC pathway and, to a lesser extent, PI3K, increases ERK1 and -2 activity in normal parathyroid cells and this cascade seems to be involved in the modulation of PTH secretion by Ca(o)(2+). Interestingly, this signaling pathway is disrupted in parathyroid tumors.