The effects of protein kinase-C agonists on parathyroid hormone release and intracellular free Ca2+ in bovine parathyroid cells

Discovery and Development of Calcimimetic and Calcilytic Compounds

The extracellular calcium receptor (CaR) is a G protein-coupled receptor (GPCR) and the pivotal molecule regulating systemic Ca²⁺ homeostasis. The CaR was a challenging target for drug discovery because its physiological ligand is an inorganic ion (Ca²⁺) rather than a molecule so there was no structural template to guide medicinal chemistry. Nonetheless, small molecules targeting this receptor were discovered. Calcimimetics are agonists or positive allosteric modulators of the CaR, while calcilytics are antagonists and all to date are negative allosteric modulators. The calcimimetic cinacalcet was the first allosteric modulator of a GPCR to achieve regulatory approval and is a first-in-class treatment for secondary hyperparathyroidism in patients on dialysis, and for hypercalcemia in some forms of primary hyperparathyroidism. It is also useful in treating some rare genetic diseases that cause hypercalcemia. Two other calcimimetics are now on the market (etelcalcetide) or under regulatory review (evocalcet). Calcilytics stimulate the secretion of parathyroid hormone and were initially developed as treatments for osteoporosis. Three different calcilytics of two different chemotypes failed in clinical trials due to lack of efficacy. Calcilytics are now being repurposed and might be useful in treating hypoparathyroidism and several rare genetic diseases causing hypocalcemia. The challenges ahead for medicinal chemists are to design compounds that select conformations of the CaR that preferentially target a particular signalling pathway and/or that affect the CaR in a tissue-selective manner.

Calcium-sensing receptor (CaSR): pharmacological properties and signaling pathways

In this article we consider the mechanisms by which the calcium-sensing receptor (CaSR) induces its cellular responses via the control (activation or inhibition) of signaling pathways. We consider key features of CaSR-mediated signaling including its control of the heterotrimeric G-proteins Gq/11, Gi/o and G12/13 and the downstream consequences recognizing that very few CaSR-mediated cell phenomena have been fully described. We also consider the manner in which the CaSR contributes to the formation of specific signaling scaffolds via peptide recognition sequences in its intracellular C-terminal along with the origins of its high level of cooperativity, particularly for Ca(2+)o, and its remarkable resistance to desensitization. We also consider the nature of the mechanisms by which the CaSR controls oscillatory and sustained Ca(2+)i mobilizing responses and inhibits or elevates cyclic adenosine monophosphate (cAMP) levels dependent on the cellular and signaling context. Finally, we consider the diversity of the receptor's ligands, ligand binding sites and broader compartment-dependent physiological roles leading to the identification of pronounced ligand-biased signaling for agonists including Sr(2+) and modulators including l-amino acids and the clinically effective calcimimetic cinacalcet. We note the implications of these findings for the development of new designer drugs that might target the CaSR in pathophysiological contexts beyond those established for the treatment of disorders of calcium metabolism.

New concepts in calcium-sensing receptor pharmacology and signalling

The calcium-sensing receptor (CaR) is the key controller of extracellular calcium (Ca(2+)(o)) homeostasis via its regulation of parathyroid hormone (PTH) secretion and renal Ca(2+) reabsorption. The CaR-selective calcimimetic drug Cinacalcet stimulates the CaR to suppress PTH secretion in chronic kidney disease and represents the world's first clinically available receptor positive allosteric modulator (PAM). Negative CaR allosteric modulators (NAMs), known as calcilytics, can increase PTH secretion and are being investigated as possible bone anabolic treatments against age-related osteoporosis. Here we address the current state of development and clinical use of a series of positive and negative CaR modulators. In addition, clinical CaR mutations and transgenic mice carrying tissue-specific CaR deletions have provided a novel understanding of the relative functional importance of CaR in both calciotropic tissues and those elsewhere in the body. The development of CaR-selective modulators and signalling reagents have provided us with a more detailed appreciation of how the CaR signals in vivo. Thus, both of these areas of CaR research will be reviewed.

Increased receptor stimulation elicits differential calcium-sensing receptor(T888) dephosphorylation

The calcium-sensing receptor (CaR) elicits oscillatory Ca(2+)(i) mobilization associated with dynamic, inhibitory protein kinase C-mediated phosphorylation of CaR(T888). While modest CaR stimulation elicits Ca(2+)(i) oscillations, greater stimulation either increases oscillation frequency or elicits sustained responses by an unknown mechanism. Here, moderate CaR stimulation (2.5 mm Ca(2+)(o), 10 min) increased CaR(T888) phosphorylation (160-kDa mature receptor) 5-fold in CaR stably transfected HEK-293 cells, whereas 3-5 mm Ca(2+)(o) treatments were without apparent effect. Treatment with 2 mm Ca(2+)(o) caused sustained CaR(T888) phosphorylation (> or = 20 min) and oscillatory Ca(2+)(i) mobilization. However, 5 mm Ca(2+)(o) increased CaR(T888) phosphorylation only briefly while eliciting sustained Ca(2+)(i) mobilization, suggesting that greater CaR activation induces rapid CaR(T888) dephosphorylation, thus permitting sustained Ca(2+)(i) responses. Indeed, 5 mm Ca(2+)(o) stimulated protein phosphatase 2A activity and induced CaR(T888) dephosphorylation following acute phorbol ester pretreatment, the latter effect being mimicked by CaR-positive allosteric modulators (NPS-R467 and l-Phe). Finally, the phosphatase inhibitor calyculin-A reversed CaR-induced inhibition of parathyroid hormone secretion from bovine parathyroid slices and normal human parathyroid cells, demonstrating the physiological importance of phosphorylation status on parathyroid function. Therefore, high Ca(2+)(o)-stimulated protein kinase C acts in concert with high Ca(2+)(o)-induced phosphatase activity to generate and maintain CaR-induced Ca(2+)(i) oscillations via the dynamic phosphorylation and dephosphorylation of CaR(T888).

Protein Kinase C-mediated Phosphorylation of the Calcium-sensing Receptor Is Stimulated by Receptor Activation and Attenuated by Calyculin-sensitive Phosphatase Activity

The agonist sensitivity of the calcium-sensing receptor (CaR) can be altered by protein kinase C (PKC), with CaR residue Thr(888) contributing significantly to this effect. To determine whether CaR(T888) is a substrate for PKC and whether receptor activation modulates such phosphorylation, a phospho-specific antibody against this residue was raised (CaR(pT888)). In HEK-293 cells stably expressing CaR (CaR-HEK), but not in cells expressing the mutant receptor CaR(T888A), phorbol ester (PMA) treatment increased CaR(pT888) immunoreactivity as observed by immunoblotting and immunofluorescence. Raising extracellular Ca(2+) concentration from 0.5 to 2.5 mM increased CaR(T888) phosphorylation, an effect that was potentiated stereoselectively by the calcimimetic NPS R-467. These responses were mimicked by 5 mM extracellular Ca(2+) and abolished by the calcilytic NPS-89636 and also by PKC inhibition or chronic PMA pretreatment. Whereas CaR(T888A) did exhibit increased apparent agonist sensitivity, by converting intracellular Ca(2+) (Ca(2+)(i)) oscillations to sustained plateau responses in some cells, we still observed Ca(2+)(i) oscillations in a significant number of cells. This suggests that CaR(T888) contributes significantly to CaR regulation but is not the exclusive determinant of CaR-induced Ca(2+)(i) oscillations. Finally, dephosphorylation of CaR(T888) was blocked by the protein phosphatase 1/2A inhibitor calyculin, a treatment that also inhibited Ca(2+)(i) oscillations. In addition, calyculin/PMA cotreatment increased CaR(T888) phosphorylation in bovine parathyroid cells. Therefore, CaR(T888) is a substrate for receptor-induced, PKC-mediated feedback phosphorylation and can be dephosphorylated by a calyculin-sensitive phosphatase.

Protein Kinase C Modulates Agonist-sensitive Release of Ca2+ from Internal Stores in HEK293 Cells Overexpressing the Calcium Sensing Receptor

This study examined the mechanism of Ca2+ entry and the role of protein kinase C (PKC) in Ca2+ signaling induced by activation of the calcium sensing receptor (CaR) in HEK293 cells stably expressing the CaR. We demonstrate that influx of Ca2+ following CaR activation exhibits store-operated characteristics in being associated with Ca2+ store depletion and inhibited by 2-aminoethoxydiphenyl borate. Inhibition of PKC with GF109203X, Go6983, or Go6976 and down-regulation of PKC activity enhanced the release of Ca2+ from internal stores in response to the polyvalent cationic CaR agonist neomycin, whereas activation of PKC with acute 12-O-tetradecanoylphorbol-13-acetate treatment decreased the release. In contrast, overexpression of wild type PKC-alpha or -epsilon augmented the neomycin-induced release of Ca2+ from internal stores, whereas dominant negative PKC-epsilon strongly decreased the release, but dominant negative PKC-alpha had little effect. Prolonged treatment of cells with 12-O-tetradecanoylphorbol-13-acetate effectively down-regulated immunoreactive PKC-alpha but had little effect on the expression of PKC-epsilon. Together these results indicate that diacylglycerol-responsive PKC isoforms differentially influence CaR agonist-induced release of Ca2+ from internal stores. The fundamentally different results obtained when overexpressing or functionally down-regulating specific PKC isoforms as compared with pharmacological manipulation of PKC activity indicate the need for caution when interpreting data obtained with the latter approach.

Structure-function relationship of the extracellular calcium-sensing receptor

The extracellular calcium-sensing receptor (CaR) originally cloned from bovine parathyroid gland is a G protein-coupled receptor. The physiological relevance of the cloned CaR for sensing and regulating the extracellular calcium concentration has been established by identifying hyper- and hypocalcemic disorders resulting from inactivating and activating mutations, respectively, in the CaR. The cloned CaR has been stably or transiently expressed in human embryonic kidney cells and significant progress has been made in elucidating its regulation and activation process using physiological, biochemical and molecular biological methods. A large collection of naturally occurring CaR mutations offers a valuable resource for studies aimed at understanding the structure-function relationships of the receptor, including functional importance of CaR dimerization. In turn, characterization of these naturally occurring mutations has clarified the pathogenesis of clinical conditions involving abnormalities in the CaR, such as familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism.

Protein kinase C (PKC) phosphorylation of the Ca2+ o-sensing receptor (CaR) modulates functional interaction of G proteins with the CaR cytoplasmic tail

The extracellular calcium (Ca(2+)(o))-sensing receptor (CaR) activates Ca(2+) influx independent of the release of intracellular Ca(2+) stores. The latter can be negatively regulated by protein kinase C (PKC) through phosphorylation of Thr-888 of the CaR. In this study, we substituted Thr-888 with various amino acid residues or a stop codon to understand how PKC phosphorylation of the CaR inhibits receptor-mediated release of intracellular Ca(2+) stores. Substitutions of Thr-888 with hydrophobic and hydrophilic amino acid residues had various effects on CaR-mediated release of intracellular Ca(2+) stores as well as activation of Ca(2+) influx. Several point mutations, such as T888D, had marked negative effects on CaR-mediated release of intracellular Ca(2+) stores but not on phorbol myristate acetate-insensitive activation of Ca(2+) influx. Presumably, the negatively charged aspartate mimics phospho-threonine. Interestingly, truncating the receptor at 888 had an even more pronounced negative effect on CaR-elicited release of intracellular Ca(2+) stores without significantly affecting CaR-mediated activation of Ca(2+) influx. Therefore, truncation at position 888 of the CaR affects the activity of the receptor in a manner that resembles PKC phosphorylation of the CaR. This in turn suggests that PKC phosphorylation of the CaR prevents G protein subtypes from interacting with the region of the receptor critical for releasing Ca(2+) stores, which is missing in the truncated receptor.

Extracellular calcium sensing and extracellular calcium signaling

The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

Protein kinase C activation blocks calcium receptor signaling in Xenopus laevis oocytes

We examined whether calcium receptor (CaR) signaling is affected by protein kinase C (PKC) activation by assessing the effects of phorbol-12-myristate-13-acetate (PMA) on 45Ca2+ efflux from Xenopus laevis oocytes expressing wild-type (WT) and mutant bovine parathyroid CaRs. Raising extracellular [Ca2+] ([Ca2+]0) from 0.5 to 5.5 mM increased 45Ca efflux (26 +/- 3-fold) in oocytes expressing full-length and C-terminally truncated receptor (amino acid 1-895). These increases in 45Ca efflux were blocked by 88 +/- 3% after PMA treatment for 20 min. Three consensus PKC phosphorylation sites (Thr-647, Ser-795, and Thr-889) were mutated in the context of the full-length and truncated CaR. PMA treatment inhibited high [Ca2+]0-induced responses in oocytes expressing the Ser795Ala CaR (1-895), Thr889Ala CaR (1-895), and Ser795Ala/Thr889Ala CaR (1-895) by 30-40% compared with untreated controls (P < 0.05). A triple mutant of the full-length CaR demonstrated similarly reduced susceptibility to inhibition of 45Ca efflux by PMA. Thus, these sites are important in mediating the effects of PKC activation on CaRs, but other residues and effector molecules are likely to participate in the effects of PKC on CaR-induced signal transduction in target cells.

Protein kinase C phosphorylation of threonine at position 888 in Ca2+o-sensing receptor (CaR) inhibits coupling to Ca2+ store release

Previous studies in parathyroid cells, which express the G protein-coupled, extracellular calcium-sensing receptor (CaR), showed that activation of protein kinase C (PKC) blunts high extracellular calcium (Ca2+o)-evoked stimulation of phospholipase C and the associated increases in cytosolic calcium (Ca2+i), suggesting that PKC may directly modulate the coupling of the CaR to intracellular signaling systems. In this study, we examined the role of PKC in regulating the coupling of the CaR to Ca2+i dynamics in fura-2-loaded human embryonic kidney cells (HEK293 cells) transiently transfected with the human parathyroid CaR. We demonstrate that several PKC activators exert inhibitory effects on CaR-mediated increases in Ca2+i due to release of Ca2+ from intracellular stores. Consistent with the effect being mediated by activation of PKC, the inhibitory effect of PKC activators on Ca2+ release can be blocked by a PKC inhibitor. The use of site-directed mutagenesis reveals that threonine at amino acid position 888 is the major PKC site that mediates the inhibitory effect of PKC activators on Ca2+ mobilization. The effect of PKC activation can be maximally blocked by mutating three PKC sites (Thr888, Ser895, and Ser915) or all five PKC sites. In vitro phosphorylation shows that Thr888 is readily phosphorylated by PKC. Our results suggest that phosphorylation of the CaR is the molecular basis for the previously described effect of PKC activation on Ca2+o-evoked changes in Ca2+i dynamics in parathyroid cells.

Regulation of extracellular calcium-activated cation currents by cAMP in parathyroid cells

Parathyroid cells express Ca2+-sensing receptors that couple changes in the extracellular Ca2+ concentration ([Ca2+]o) to increases in the intracellular free Ca2+ concentration ([Ca2+]i) and to the suppression of parathyroid hormone secretion. Using whole cell patch clamping, we previously identified voltage-independent Ca2+-conducting currents in bovine parathyroid cells that increased with rising [Ca2+]o and were blocked by Cd2+ and nifedipine. Because cAMP-dependent phosphorylation regulates dihydropyridine-sensitive Ca2+ channels in other systems, we tested whether cAMP modulates these currents. At 0.7 mM Ca2+, nonselective Ca2+-conducting currents were suppressed by 30-50% when the recording pipette was perfused with cAMP. High-[Ca2+]o-induced increases in membrane currents were also abrogated. The effects of cAMP were reversible and dose dependent (3 x 10(-9) to 3 x 10(-3) M) and required ATP in the pipette solution. Perfusion of the cell interior with the catalytic subunit of protein kinase A mimicked the effects of cAMP, as did perfusion of the bath with the adenylate cyclase activator forskolin. These findings support the idea that cAMP-dependent phosphorylation suppresses high-[Ca2+]o-induced cation currents and may play a role in regulating ion fluxes in parathyroid cells.

Cloning and characterization of a calcium-sensing receptor from the hypercalcemic New Zealand white rabbit reveals unaltered responsiveness to extracellular calcium

The extracellular Ca2+ (Ca(0)2+)-sensing receptor (CaR) recently cloned from mammalian parathyroid, kidney, brain, and thyroid plays a central role in maintaining near constancy of Ca(0)2+. We previously showed that the hypercalcemia normally present in New Zealand white rabbits is associated with an elevated set point for Ca(02+)-regulated PTH release (the level of Ca(0)2+ half-maximally inhibiting hormonal secretion). This observation suggested an alteration in the Ca(02+)-sensing mechanism in the rabbit parathyroid, a possibility we have now pursued by isolating and characterizing the rabbit homolog of the CaR. The cloned rabbit kidney CaR (RabCaR) shares a high degree of overall homology (> 90% amino acid identity) with the bovine, human, and rat CaRs, although it differs slightly in several regions of the extracellular domain potentially involved in binding ligands. By Northern analysis and/or immunohistochemistry, a similar or identical receptor is also expressed in parathyroid, thyroid C cells, small and large intestine, and in the thick ascending limb and collecting ducts of the kidney. When expressed transiently in HEK293 cells and assayed functionally through CaR agonist-evoked increases in Ca(i)2+, the rabbit CaR shows apparent affinities for Ca(0)2+, Mg(0)2+, and Gd(0)3+ that are indistinguishable from those observed in studies carried out concomitantly using the human CaR. Therefore, at least as assessed by its ability to increase Ca(i)2+ when expressed in HEK293 cells, the intrinsic functional properties of the rabbit CaR cannot explain the hypercalcemia observed in vivo in the New Zealand white rabbit.

Regulation of parathyroid hormone messenger RNA levels by protein kinase A and C in bovine parathyroid cells

Secretion of parathyroid hormone (PTH) is regulated by Ca2+ as well as by protein kinases A and C. In this study we report that protein kinases A and C regulate PTH messenger RNA levels in vitro in dispersed bovine parathyroid cells. Incubation of bovine parathyroid cells with cholera toxin (10(-9) M), which activates adenylate cyclase and indirectly stimulates protein kinase A, increased PTH mRNA levels about 2-fold after 3 and 7 h incubation, but not at 24 h. Incubation with pertussis toxin (5 x 10(-9) M), which blocks the high-calcium-mediated inhibition of cyclic adenosine monophosphate accumulation in these cells, also reversed the inhibition of PTH mRNA levels at high Ca2+ (2.0 mM) with a marked increase in PTH mRNA levels. Pertussis toxin also increased PTH mRNA at a low extracellular Ca2+ concentration (0.7 mM) (4-fold increase) and a normal concentration (1.25 mM) (2-fold increase). Inhibition of protein kinase C both by staurosporine (1 x 10(-8) M) and by prolonged incubation with the phorbol ester phorbol 12-myristate 13-acetate (PMA) (1 x 10(-7) M), decreased PTH mRNA levels at 24 h, reaching approximately 40% and 5% of control, respectively. Staurosporine and PMA had no effect on PTH mRNA levels at 3 h. The inactive phorbol ester, phorbol 12-13-dibutyrate (PDBu), had no effect on PTH mRNA levels at 1 and 24 h. There were no changes in a control gene 18S RNA in these studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase C modulates hormone secretion regulated by extracellular polycations in bovine parathyroid cells

1. The role of protein kinase C (PKC) in the regulation of parathyroid hormone (PTH) secretion was examined in dissociated bovine parathyroid cells. 2. Increasing the concentration of extracellular Ca2+ from 0.5 to 2 mM inhibited PTH secretion by 60-80%. Similar depressive effects on secretion were obtained by increasing the concentration of extracellular Mg2+ from 1 to 7 mM or by adding La3+ (to 40 microM). The PKC activator phorbol myristate acetate (PMA) depressed PTH secretion at the lower and potentiated secretion at the higher concentrations of extracellular Ca2+, Mg2+ or La3+. The inhibitory effect of PKC on secretion correlated positively with the magnitude of the inhibitory effect elicited by elevated extracellular Ca2+. 3. The stimulatory effects of PKC activators on PTH secretion were reversed completely and the inhibitory effects were reversed partially by the PKC inhibitor staurosporine. Staurosporine alone did not affect secretion at low (0.5 mM) or high (2 mM) concentrations of extracellular Ca2+ but it did depress secretion at intermediate concentrations (around 1 mM) of extracellular Ca2+. 4. The stimulatory effects of PKC activators on secretion were overcome by increases in the concentration of extracellular Ca2+ (to 5 or 10 mM) or La3+ (to 100 microM). In contrast, increasing the concentration of extracellular Mg2+ to 11 or 19 mM did not alleviate the potentiating effects of PKC activators. The different results obtained with Ca2+ and Mg2+ could not be explained by their different effects on cytosolic Ca2+ and suggests that different cations can have varying degrees of efficacy to activate functional responses linked to the Ca2+ receptor on bovine parathyroid cells. 5. PTH secretion stimulated by isoprenaline was not affected by PKC activators or staurosporine. Similarly, the inhibitory effects of extracellular ATP gamma S on secretion were unaffected by PKC activators. These results show that PKC activators affect specifically PTH secretion regulated by extracellular polycations. 6. The stimulatory effect of PKC activators on secretion parallels its inhibitory effects on [Ca2+]i and inositol trisphosphate formation, showing that PKC blunts the mechanisms associated with extracellular Ca(2+)-induced inhibition of secretion. The specificity of these actions suggests that PKC acts at a very early step of stimulus-secretion coupling in parathyroid cells, specific to that used by extracellular polycations and perhaps involving the Ca2+ receptor.

Cytosolic calcium homeostasis in bovine parathyroid cells and its modulation by protein kinase C

1. The effects of protein kinase C (PKC) activators and inhibitors on the mechanisms regulating cytosolic Ca2+ homeostasis in dissociated bovine parathyroid cells loaded with fura-2 were examined. 2. Stepwise increases in the concentration of extracellular Ca2+ (from 0.5 to 2 or 3 mM) elicited transient followed by sustained increases in the concentration of intracellular free Ca2+ ([Ca2+]i). Cytosolic Ca2+ transients reflected the mobilization of intracellular Ca2+ and influx of extracellular Ca2+ whereas sustained increases in [Ca2+]i resulted from the influx of extracellular Ca2+. Brief (1-2 min) pretreatment with phorbol myristate acetate (PMA) shifted the concentration-response curve for extracellular Ca(2+)-induced cytosolic Ca2+ transients to the right without affecting the maximal response. Cytosolic Ca2+ transients elicited by extracellular Mg2+ were similarly affected by PMA. 3. These effects of PMA were mimicked by various other activators of PKC with the rank order of potency PMA > phorbol dibutyrate > bryostatin , > (-)indolactam V > mezerein. Isomers or analogues of these compounds that do not alter PKC activity (4 alpha-phorbols and (+)indolactam V) did not alter [Ca2+]i. 4. PKC activators depressed evoked increases in [Ca2+]i when influx of extracellular Ca2+ was blocked with Gd3+. Cytosolic Ca2+ transients elicited by extracellular Mg2+ in the absence of extracellular Ca2+ were similarly inhibited by PKC activators. Activation of PKC thus inhibits the mobilization of intracellular Ca2+ elicited by extracellular divalent cations. 5. Increases in the concentration of extracellular Ca2+ caused corresponding increases in the formation of [3H]inositol 1,4,5-trisphosphate ([3H]InsP3). Pretreatment with PMA shifted the concentration-response curve for extracellular Ca(2+)-induced [3H]InsP3 formation to the right without affecting the maximal response. 6. PKC activators also caused some depression of steady-state increases in [Ca2+]i elicited by extracellular Ca2+. In contrast, PMA did not affect increases in [Ca2+]i elicited by ionomycin or thapsigargin. 7. Ba2+ was used to monitor divalent cation influx. PMA decreased the rate of rise of the fluorescent signal elicited by extracellular Ba2+. 8. All these effects of PKC activators on [Ca2+]i were blocked or reversed by staurosporine at concentrations (30-100 nM) that inhibited PKC activity in parathyroid cells. Staurosporine alone potentiated cytosolic Ca2+ responses evoked by submaximal concentrations of extracellular divalent cations. 9. PKC thus depresses both the mobilization of intracellular Ca2+ and the influx of extracellular Ca2+ in parathyroid cells. The effects on [Ca2+]i provide evidence for a Ca2+ receptor on the surface of parathyroid cells that uses transmembrane signalling mechanisms common to some other Ca(2+)-mobilizing receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of high extracellular calcium concentrations on phosphoinositide turnover and inositol phosphate metabolism in dispersed bovine parathyroid cells

We previously showed that high extracellular calcium (Ca2+) concentrations raise the levels of inositol phosphates in bovine parathyroid cells, presumably via the G protein-coupled, "receptor-like" mechanism through which Ca2+ is thought to regulate these cells. To date, however, there are limited data showing Ca(2+)-evoked hydrolysis of phosphoinositides with attendant increases in the levels of the biologically active 1,4,5 isomer of inositol trisphosphate (IP3) that would be predicted to arise from such a receptor-mediated process. In the present studies we used HPLC and TLC, respectively, to quantify the high Ca(2+)-induced changes in various inositol phosphates, including the isomers of IP3, and phosphoinositides in bovine parathyroid cells prelabeled with [3H]inositol. In the absence of lithium, high Ca2+ dose dependently elevated the levels of inositol-1,4,5-trisphosphate [I(1,4,5)P3], with a maximal, 4- to 5-fold increase within 5 s; the levels of inositol 1,3,4-trisphosphate [I(1,3,4)P3] first rose significantly at 5-10 s and remained 5- to 10-fold elevated for at least 30 minutes. These changes were accompanied by reciprocal 29-36% decreases in PIP2 (within 5-10 s, the earliest time points examined), PIP (within 60 s), and PI (within 60 s). These results document that, as in other cells responding to more classic "Ca(2+)-mobilizing" hormones, the high Ca(2+)-evoked increases in inositol phosphates in bovine parathyroid cells arise from the hydrolysis of phosphoinositides, leading to the rapid accumulation of the active isomer of IP3. The latter presumably underlies the concomitant spike in the cytosolic calcium concentration (Ca(i)) in parathyroid cells.

Changes in protein kinase C activity in rat calvarial bone cells cultured in a low-calcium environment

This enzyme activity was examined in bone cells cultured for 8-10 days; the calcium concentration was 1.87 +/- 0.05 (n = 10) mM in the control medium and 0.34 +/- 0.02 (n = 10) mM in the low-calcium medium. The activity was significantly lower in the low-calcium group than in the control (p less than 0.01). The cytosolic fraction decreased more than the membranous fraction. After restoration to a regular calcium environment, the protein kinase C activity recovered rapidly to near the control value. The extent of recovery was greater in the membranous than in the cytosolic fraction. These results suggest that the enzyme was inhibited in bone cells placed in a low-calcium environment, while the sensitivity in the membrane was enhanced.

Activation and inhibition of protein kinase C in cultured bovine parathyroid cells: effect on the release of C-terminal fragments of parathyroid hormone

The response of the parathyroid gland to low Ca2+ may be mediated in part by protein kinase C (PKC). We assessed the effect of two PKC activators, SC-9 and SC-10, and one PKC inhibitor, H-7, on Ca(2+)-regulated PTH release and degradation in primary cultures of bovine parathyroid cells. Both SC-9 and SC-10 stimulated PTH release, compared to high Ca2+ alone, in parathyroid cells incubated in high Ca2+, with maximal PTH release of at least twofold occurring at a concentration of either activator of 10 nM (p less than 0.05). We have previously shown that another PKC activator, PMA, not only enhances PTH release in the presence of high Ca2+ but suppresses low Ca(2+)-stimulated PTH secretion. In the present study, neither SC-9 nor SC-10 caused a comparable suppression of PTH release at low Ca2+. However, the PKC inhibitor, H-7 (1 microM), blocked low Ca(2+)-stimulated (compared to the low Ca2+ control) PTH secretion by approximately 50% (p less than 0.01) and did not affect high Ca2+ suppression of PTH secretion. H-7 (1 microM) was able to oppose the stimulation of PTH release by the PKC activators SC-9, SC-10, and PMA at high Ca2+ and negated the PTH release-inhibiting effect of PMA at low Ca2+. Culture medium from these experiments was subjected to reversed-phase HPLC and the eluted fractions analyzed by RIA for the presence of intact and C-terminal fragments of PTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Neomycin interacts with Ca2+ sensing of normal and adenomatous parathyroid cells

Effects of the polyvalent cationic antibiotic neomycin on regulation of the cytoplasmic Ca2+ concentration ([Ca2+]i) were studied in normal and adenomatous human, and bovine parathyroid cells. Parathyroid hormone (PTH) release was also measured in the bovine cells. Elevation of extracellular Ca2+ from 0.5 to 3 mM caused biphasic increase of [Ca2+]i and inhibition of PTH release. In low external Ca2+ neomycin inhibited PTH release and virtually only triggered the [Ca2+]i transient. In contrast [Ca2+]i was lowered and PTH release stimulated by neomycin in the presence of 3.0 mM Ca2+ or 7 mM Mg2+. These actions of Ca2+ and neomycin on [Ca2+]i were qualitatively similar but less pronounced in the adenomatous than normal human parathyroid cells. Some effects of neomycin were thus similar to those induced by other cationic agents interacting with the Ca2+ receptor mechanism on the parathyroid cell surface, whereas others may involve phospholipase C inhibition, protein kinase C activation or a direct reduction of the Ca2+ influx.

Cellular physiology and pathophysiology of the parathyroid glands

This report provides insight into parathyroid gland physiology and the pathophysiology of hyperparathyroidism (HPT). Increases in the extracellular calcium concentration constitute the primary physiological signal for inhibition of parathyroid hormone (PTH) release. Transduction of the external signal into a cellular response involves activation of a cation receptor mechanism on the plasma membrane with rapid rise in the cytoplasmic calcium concentration of the cells. This recently discovered parathyroid calcium receptor has been characterized as a glycoprotein of unusually high molecular weight, which may play a key role in calcium homeostasis since it is also expressed in the kidney and placenta. Binding of external calcium to the receptor is associated with mobilization of intracellular calcium as well as calcium influx into the cells and phosphoinositol hydrolysis. These events rapidly interfere with the release process through essentially unknown mechanisms and probably also at sustained stimulation inhibit PTH gene transcription. The relative calcium insensitivity of the PTH release in HPT is associated with a deranged regulation of cytoplasmic calcium within pathological parathyroid cells. The molecular basis for this disturbance comprises down regulation of the cation receptor, whereby external calcium is translated into abnormally low levels of cytoplasmic calcium and insufficient inhibition of PTH release. Studies on expression of the functionally important cation sensing glycoprotein and its associated cellular signal systems may provide novel means for interference with the pathophysiological derangements of HPT.

Inhibition of parathyroid hormone secretion correlates with increased incorporation of 32P into phosphatidylinositol and lysophosphatidylinositol

We have studied the incorporation of radioactive P (32P) into lipids of bovine parathyroid tissue under conditions of stimulated and inhibited hormone secretion. Utilizing low (0.5 mM) and high (3.0 mM) concentrations of calcium to regulate parathyroid hormone secretion, we initially found that the labeling of the cellular phospholipids with 32P was greater in those tissues incubated in high-calcium medium. Thin-layer chromatography of lipid extracts prepared from tissue incubated in either low- or high-calcium media revealed that the increased incorporation of 32P (high or low) was localized primarily to two phospholipids. To determine whether the increases were due directly to the different calcium concentrations, the experiments were performed in media containing normal calcium concentrations (1.25 mM) and low (0.5) or high (3.0) magnesium concentrations to modulate hormone secretion. The results were identical to those obtained using low and high calcium, indicating that the increased 32P incorporation was not an effect of high calcium but rather correlated with the inhibition of hormone secretion. The use of other secretagogues confirmed this correlation. The identity of the two phospholipids was established, by two-dimensional thin-layer chromatography, to be phosphatidylinositol (PI) and lysophosphatidylinositol (LPI). The correlation of increased 32P incorporation with inhibition of secretion led us next to examine isolated secretory granules from tissues exposed to either high-or low-calcium conditions. Thin-layer chromatography of granule lipid extracts yielded chromatograms containing PI and LPI, and the radioactivity of each was greater in the high-calcium sample than in the low-calcium sample.(ABSTRACT TRUNCATED AT 250 WORDS)

Phorbol esters modulate the high Ca2(+)-stimulated accumulation of inositol phosphates in bovine parathyroid cells

We examined the effects of TPA on the high Ca2(+)-stimulated accumulation of inositol phosphates in bovine parathyroid cells to determine whether protein kinase C modulates phosphoinositide turnover in a fashion similar to that observed in other cell types stimulated by more classic Ca2+ mobilizing hormones. Following exposure of parathyroid cells to TPA (10(-6) M) for 10 or 30 minutes, there was a time- and dose-dependent inhibition of the accumulation of inositol monophosphate (IP), inositol bisphosphate (IP2) and inositol trisphosphate (IP3) stimulated by 3 mM Ca2+. Half the maximal observed inhibition took place at 1-10 nM TPA, with 50-60% inhibition of high Ca2(+)-stimulated accumulation of inositol phosphates at 10(-6) M TPA. The active phorbol ester, 4 beta-phorbol didecanoate, produced similar effects; the inactive derivative, 4 alpha-phorbol didecanoate, was without effect. When parathyroid cells were exposed to TPA (10(-6) M) for varying times and were then incubated with high (3 mM) Ca2+, inhibition of inositol phosphate accumulation was observed with 10 or 30 minutes preincubation. In contrast, preincubation of cells with TPA for 3 or 18 h markedly enhanced the high (3 mM) Ca2(+)-induced increase in inositol phosphates. In cells preincubated with TPA for 18 h, binding sites for [3H]phorbol dibutyrate and total protein kinase C (PKC) activity were reduced by greater than 95% and by 71%, respectively, consistent with downregulation of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical identification of calcitonin gene-related peptide and substance P in nerves of the bovine parathyroid gland

Although peptide neurotransmitters have been shown to modulate hormone secretion in many glands, there are very few studies of neurotransmitters in the parathyroid gland. Bovine parathyroid glands were collected at a local abattoir, fixed with paraformaldehyde, sectioned using a cryostat, and stained by indirect immunohistochemistry for calcitonin gene-related peptide and substance P. We were able to positively identify both neuropeptides. Nerve fibres containing calcitonin gene-related peptide and substance P were identified in contact with the tunica media of arteries and arterioles and dispersed throughout the stroma of the gland. While many of the fibres encircled parenchymal lobules, no intimate contact with the peripheral chief cells was observed. All immunoreactive fibres were found to contain both neuropeptides. Since calcitonin gene-related peptide and substance P are vasodilators, they may increase blood flow within the gland. In addition, the neuropeptides may diffuse from perilobular nerve fibres into the parenchyma, thereby modulating secretion of parathyroid hormone.