Effect of the phorbol ester TPA on PTH secretion. Evidence for a role for protein kinase C in the control of PTH release

International Union of Basic and Clinical Pharmacology. CVIII. Calcium-Sensing Receptor Nomenclature, Pharmacology, and Function

The calcium-sensing receptor (CaSR) is a class C G protein-coupled receptor that responds to multiple endogenous agonists and allosteric modulators, including divalent and trivalent cations, L-amino acids, γ-glutamyl peptides, polyamines, polycationic peptides, and protons. The CaSR plays a critical role in extracellular calcium (Ca2+ o) homeostasis, as demonstrated by the many naturally occurring mutations in the CaSR or its signaling partners that cause Ca2+ o homeostasis disorders. However, CaSR tissue expression in mammals is broad and includes tissues unrelated to Ca2+ o homeostasis, in which it, for example, regulates the secretion of digestive hormones, airway constriction, cardiovascular effects, cellular differentiation, and proliferation. Thus, although the CaSR is targeted clinically by the positive allosteric modulators (PAMs) cinacalcet, evocalcet, and etelcalcetide in hyperparathyroidism, it is also a putative therapeutic target in diabetes, asthma, cardiovascular disease, and cancer. The CaSR is somewhat unique in possessing multiple ligand binding sites, including at least five putative sites for the "orthosteric" agonist Ca2+ o, an allosteric site for endogenous L-amino acids, two further allosteric sites for small molecules and the peptide PAM, etelcalcetide, and additional sites for other cations and anions. The CaSR is promiscuous in its G protein-coupling preferences, and signals via Gq/11, Gi/o, potentially G12/13, and even Gs in some cell types. Not surprisingly, the CaSR is subject to biased agonism, in which distinct ligands preferentially stimulate a subset of the CaSR's possible signaling responses, to the exclusion of others. The CaSR thus serves as a model receptor to study natural bias and allostery. SIGNIFICANCE STATEMENT: The calcium-sensing receptor (CaSR) is a complex G protein-coupled receptor that possesses multiple orthosteric and allosteric binding sites, is subject to biased signaling via several different G proteins, and has numerous (patho)physiological roles. Understanding the complexities of CaSR structure, function, and biology will aid future drug discovery efforts seeking to target this receptor for a diversity of diseases. This review summarizes what is known to date regarding key structural, pharmacological, and physiological features of the CaSR.

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.

Signal transduction by protein kinase C in mammalian cells

1. The past two decades have witnessed great advances in our understanding of the role of protein kinase C (PKC) in signal transduction. The Ca(2+)-activated, phospholipid-dependent protein kinase discovered by Nishizuka's group in 1977 is now a family of at least 11 isoforms. Protein kinase C isoforms exist in different proportions in a host of mammalian cells and each isoform has a characteristic subcellular distribution in each cell type. 2. Stimulation of a specific PKC isoform often causes redistribution of the isoform from one subcellular compartment to another compartments where it complexes with and phosphorylates a specific protein substrate. 3. The interaction of a specific PKC isoform with its protein substrate may directly activate a specific function of the cell or may trigger a cascade of protein kinases that ultimately stimulates a specific response in differentiated cells or regulates growth and proliferation in undifferentiated 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.

Parathyroid hormone gene expression in hypophosphatemic rats

Phosphate is central to bone metabolism and we have therefore studied whether parathyroid hormone (PTH) is regulated by dietary phosphate in vivo. Weanling rats were fed diets with different phosphate contents for 3 wk: low phosphate (0.02%), normal calcium (0.6%), normal phosphate (0.3%), and calcium (0.6%); high phosphate (1.2%), high calcium (1.2%). The low phosphate diet led to hypophosphatemia, hypercalcemia, and increased serum 1,25(OH)2D3 together with decreased PTH mRNA levels (25 +/- 8% of controls, P < 0.01) and serum immunoreactive PTH (4.7 +/- 0.8: 22.1 +/- 3.7 pg/ml; low phosphate: control, P < 0.05). A high phosphate diet led to increased PTH mRNA levels. In situ hybridization showed that hypophosphatemia decreased PTH mRNA in all the parathyroid cells. To separate the effect of low phosphate from changes in calcium and vitamin D rats were fed diets to maintain them as vitamin D-deficient and normocalcemic despite the hypophosphatemia. Hypophosphatemic, normocalemic rats with normal serum 1,25(OH)2D3 levels still had decreased PTH mRNAs. Nuclear transcript run-ons showed that the effect of low phosphate was posttranscriptional. Calcium and 1,25(OH)2D3 regulate the parathyroid and we now show that dietary phosphate also regulates the parathyroid by a mechanism which remains to be defined.

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)

The release of parathyroid hormone and the exocytosis of a proteoglycan are modulated by extracellular Ca2+ in a similar manner

Secretion of parathyroid hormone (PTH) is regulated in part by a classical "stimulus-secretion" pathway responsive to catecholamines. The primary physiological modulator of PTH exocytosis in parathyroid cells, however, is extracellular free Ca2+. Ca(2+)-modulated PTH release exhibits several characteristics suggestive of constitutive secretion. The aim of this work was to obtain further information about the possible intracellular origins of Ca(2+)-modulated exocytosis in parathyroid cells. Freshly dissociated bovine parathyroid cells labeled with [35S]sulfate synthesized a soluble chondroitin/dermatan sulfate proteoglycan (M(r) approximately 90-150 K) that was secreted into the medium. The export of [35S]sulfated proteoglycan satisfied several criteria that generally define constitutive release: 1) export is detected in the medium shortly (7-15 min) after a 5-min pulse, 2) there is minimal intracellular storage after equilibrium labeling (because of combined processes of rapid release and intracellular degradation), and 3) there is insensitivity to stimulation with isoproterenol, a known secretagogue in parathyroid cells. Nevertheless, the increase in extracellular Ca2+ from 0.5 to 2.0 mM reduced the export of the [35S]sulfated proteoglycan from 60% of total labeled to 30%. In addition, a secreted pool of immunoreactive PTH and [35S]sulfated proteoglycan was modulated by external Ca2+ to the same degree and sensitivity, although isoproterenol was more effective in stimulating the release of PTH than that of proteoglycan. Together, our experimental results show that in the parathyroid cell extracellular Ca2+ modulates negatively the export of both PTH and proteoglycan, a putative marker for constitutive secretion. We further suggest that a portion of newly synthesized PTH also enters this pathway, whereas another portion proceeds to an isoproterenol-releasable compartment from which the proteoglycan is largely excluded.

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)

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.

Regulation of protein kinase C by extracellular calcium in bovine parathyroid cells

Regulation of protein kinase C in the parathyroid gland was investigated by testing the effects of phorbol ester, exogenous phospholipase C, and low and high calcium concentrations on enzyme activity. Treatment of bovine parathyroid cells with phorbol ester, which activates protein kinase C directly, and with phospholipase C, which produces diacylglycerol, an activator of protein kinase C, significantly stimulated protein kinase C activity. Both agents also enhanced the release of parathyroid hormone. Acute exposure of bovine parathyroid cells to low extracellular calcium (0.5 mM) caused a 5- to 6-fold increase in protein kinase C activity associated with the particulate fraction. In contrast, high extracellular calcium (1.75 mM and 2.5 mM) markedly decreased membrane protein kinase C activity. These data suggest that the effects of extracellular calcium on parathyroid hormone secretion are due, at least in part, to regulation of protein kinase C activity in the parathyroid-cell membrane.

Dopamine receptors: molecular structure and function

Two distinct categories of dopamine receptors, termed D1 and D2, have been identified on the basis of pharmacological and biochemical criteria. Some of the progress made in our understanding of the subunit structure, function and signal transduction properties of these important membrane proteins are reviewed.

High extracellular Ca2+ and Mg2+ stimulate accumulation of inositol phosphates in bovine parathyroid cells

We examined the effects of the divalent cations Ca2+ and Mg2+ on inositol phosphate accumulation in bovine parathyroid cells prelabelled with [3H]inositol to determine whether the high extracellular Ca2+ and Mg2+-evoked transients in cytosolic Ca2+ in these cells might result from increases in cellular IP3 levels. In the presence of Li+, both Ca2+ and Mg2+ produced rapid, 2-6-fold increases in IP3 and IP2 and a linear increase in IP of 6-8-fold at 30 min. Smaller (1.5-2-fold) increases in IP2 and IP3 were evident within 7.5-15 s upon exposure to high (3 mM) Ca2+ in the absence of Li+. The relative potencies of Ca2+ and Mg2+ (Ca2+ 3-fold more potent than Mg2+) in elevating inositol phosphates were similar to those for their effects in inhibiting PTH release. Fluoride (5 and 10 mM) also produced similar increases in inositol phosphate accumulation, presumably through activation of phospholipase C by a guanine nucleotide (G) protein-dependent process. Thus, high extracellular Ca2+ and Mg2+-induced spikes in cytosolic Ca2+ in bovine parathyroid cells may be mediated by increases in IP3, perhaps through a receptor-mediated process linked to phospholipase C by a G-protein.

Down-regulation of protein kinase C is due to an increased rate of degradation

The phorbol-ester-induced loss of protein kinase C that has been documented in many cell types appears to be a critical event in the generation of a cellular refractory state. We have investigated here the synthesis and degradation of the protein kinase C polypeptide in order to determine why its steady-state amounts are depleted in response to phorbol esters. These results indicate that depletion is due to an increased rate of degradation, with no change either in mRNA amounts or in rates of polypeptide synthesis.

Possible involvement of protein kinase C in parathyroid hormone degradation by osteoblast-like rat osteosarcoma cell line UMR106

The effects of 12-O-tetraadecanoyl phorbol-13-acetate (TPA), 1-oleoyl-2-acetyl-glycerol (OAG), and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) on the parathyroid hormone (PTH) degrading activity in a PTH-responsive osteoblast-like rat osteosarcoma cell line UMR106 were investigated to assess the role of Ca2+-activated. Phospholipid dependent protein kinase (protein kinase C) on the degradation of hormones. TPA and OAG, activators of protein kinase C, enhanced the PTH degrading activity dose-dependently, whereas H-7, an inhibitor of protein kinase C, exhibited a dose-dependent inhibition on this activity. These data suggest that protein kinase C activation may enhance PTH degrading activity by UMR106 cells as a possible regulator of PTH degradation.

Cell-specific expression of the human parathyroid hormone gene in rat pituitary cells

The promoter region of the human parathyroid hormone gene was fused to the Escherichia coli neo gene and introduced into GH4C1 rat pituitary and human HeLa cells. Both TATA boxes of the human parathyroid hormone gene accurately directed transcription in GH4C1 cells; the parathyroid hormone promoter was inactive in HeLa cells.

Ouabain and low extracellular potassium inhibit PTH secretion from bovine parathyroid cells by a mechanism that does not involve increases in the cytosolic calcium concentration

We have previously found that high extracellular calcium (Ca++) concentrations inhibit PTH release in association with a threefold to fourfold rise in cytosolic Ca++ concentration. Recent data have also shown that low extracellular potassium (K+) concentration or ouabain also inhibits PTH release to an extent comparable to that seen with high Ca++ and produce a marked rise in the intracellular sodium (Na+) content. These results suggested that low K+ and ouabain might modulate PTH release through increases in cytosolic Ca++ related to alterations in Na+-Ca++-exchange. In the present studies, we have examined further the mechanism(s) by which inhibition of the Na+-K+-ATPase regulates PTH release. Exposure of cells loaded with the Ca++-sensitive dye QUIN-2 to low K+ produced a 10% to 17% increase in cytosolic Ca++ at 0.5 to 1.0 mmol/L extracellular Ca++, which was statistically significant only at 0.75 mmol/L Ca++. In contrast, low K+ caused a statistically significant decrease in cytosolic Ca++ at 1.5 to 2 mmol/L Ca++, while ouabain lowered cytosolic Ca++ significantly by 23% to 46% at all Ca++ concentrations examined (0.5 to 2 mmol/L). Low K+ or ouabain had no effect on cellular levels of ATP or GTP or intracellular pH measured using the pH-sensitive dye BCECF [2', 7'-bis(carboxyethyl)-5,6-carboxyfluorescein]. The inhibition of secretion by low K+ or ouabain, unlike that due to high extracellular Ca++, was not reversed by TPA (12-O-tetradecanoyl phorbol 13-acetate), an activator of protein kinase C. Low K+ did produce a modest (30% to 40%) lowering of agonist-stimulated but not basal cAMP content.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone secretion does not respond to changes of free calcium in electropermeabilized bovine parathyroid cells, but is stimulated with phorbol ester and cyclic AMP

The secretion of parathyroid hormone (PTH) is suppressed in bovine parathyroid cells by raised extracellular [Ca2+], and 12-0-tetradecanoyl-phorbol-13-acetate (TPA) stimulates the release of PTH from cells suppressed by high extracellular [Ca2+]. Extracellular and cytosolic free [Ca2+] are proportionally related in intact cells. To assess the role of cytosolic free [Ca2+] on PTH secretion, bovine parathyroid cells were rendered permeable by brief exposure to an intense electric field. PTH secretion was comparable at 40 nM, 500 nM, 5 microM, 28 microM, 0.5 mM and 2 mM [Ca2+] (release of total cellular PTH 3.7 +/- 0.5%, 3.9 +/- 0.4%, 3.4% +/- 0.3%, 3.9 +/- 0.4%, 3.1 +/- 0.3%, 3.5 +/- 0.7%, respectively), but the secretion was stimulated twofold (P less than 0.05 vs. control) in a dose and ATP dependent manner with TPA (100 nM) and cyclic AMP (1 mM). As a result, free [Ca2+] in the range of those observed in intact cells during regulation of PTH secretion by changes of extracellular [Ca2+] did not affect the release of PTH in permeabilized cells. The [Ca2+] independent stimulation of PTH release by TPA and cyclic AMP indicates that changes of cytosolic free [Ca2+] may represent a secondary event not related to the regulation of PTH secretion.

Studies and perspectives of protein kinase C

Protein kinase C, an enzyme that is activated by the receptor-mediated hydrolysis of inositol phospholipids, relays information in the form of a variety of extracellular signals across the membrane to regulate many Ca2+-dependent processes. At an early phase of cellular responses, the enzyme appears to have a dual effect, providing positive forward as well as negative feedback controls over various steps of its own and other signaling pathways, such as the receptors that are coupled to inositol phospholipid hydrolysis and those of some growth factors. In biological systems, a positive signal is frequently followed by immediate negative feedback regulation. Such a novel role of this protein kinase system seems to give a logical basis for clarifying the biochemical mechanism of signal transduction, and to add a new dimension essential to our understanding of cell-to-cell communication.

Role of cytosolic calcium in the control of cAMP content by calcium in bovine parathyroid cells

In the present studies we used the calcium (Ca2+)-sensitive dye Quin-2 to determine whether the cytosolic (Cyt) Ca2+ mediates the effects of extracellular (EC) Ca2+ on cAMP accumulation through changes in adenylate cyclase and phosphodiesterase activity in bovine parathyroid cells (bPTC). In dispersed (d) bPTC, increasing the EC Ca2+ from 0.5 to 2 mM produces a rise in the Cyt Ca2+ from 179 to 646 nM which is associated with a 52% inhibition of agonist-stimulated cAMP accumulation. Over this range of free Ca2+ adenylate cyclase activity decreased by approx. half (57%) and phosphodiesterase activity increases 2-fold (101%) suggesting that changes in the Cyt Ca2+ can account for the effects of EC Ca2+ on cAMP through changes in these enzymes. Unlike dbPTC, 4-day-old cultured bPTC show only a 23% suppression of cAMP by high EC Ca2+ and a reduced rise in the Cyt Ca2+ from 0.5 to 3 mM EC Ca2+. Although there is no reduction in the Ca2+ sensitivity of adenylate cyclase, phosphodiesterase activity shows no change at varied free Ca2+. Thus, this diminished Ca2+ sensitivity of phosphodiesterase activity, and the decreased rise in Cyt Ca2+ relative to EC Ca2+ may both contribute to the resistance to the effects of EC Ca2+ on cAMP content in cultured cells. Because in addition to Cyt Ca2+, protein kinase C may also mediate the effects of EC Ca2+ on PTH release, we studied the effects of TPA (12-alpha-tetradecanoylphorbol 13-acetate) on agonist-stimulated cAMP in dbPTC.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell-specific expression of the human parathyroid hormone gene in rat pituitary cells

The promoter region of the human parathyroid hormone gene was fused to the Escherichia coli neo gene and introduced into GH4C1 rat pituitary and human HeLa cells. Both TATA boxes of the human parathyroid hormone gene accurately directed transcription in GH4C1 cells; the parathyroid hormone promoter was inactive in HeLa cells.

Stimulation of parathyroid hormone secretion by phorbol esters is associated with a decrease of cytosolic calcium

Unlike in other endocrine systems calcium inhibits parathyroid hormone (PTH) secretion and this inhibition is paralleled by a rise of cytosolic calcium concentration ([Ca]i). Because of evidence that diglyceride levels and protein kinase C activity are also decreased by high extracellular calcium we have investigated the effects of 12-O-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C, on [Ca]i and PTH secretion using dispersed bovine parathyroid cells. At 1.5 mM medium calcium TPA enhanced PTH secretion and caused reduction of [Ca]i from 639 +/- 36 nM (SE) to 335 +/- 21 nM (P less than 0.001); at 0.5 mM calcium TPA was ineffective. Moreover, TPA suppressed the rise of [Ca]i evoked by high extracellular calcium. Thus TPA presumably stimulates PTH secretion via activation of protein kinase C, and the lowering of [Ca]i may TPA presumably stimulates PTH secretion via activation of protein kinase C, and the lowering of [Ca]i may be a secondary event related to diglyceride availability.

12-O-tetradecanoyl phorbol-13-acetate (TPA) stimulates somatostatin release from isolated perfused rat stomach

Effect of TPA (12-O-tetradecanoyl phorbol-13-acetate), a potent tumor promoter, on immunoreactive somatostatin release was investigated using the isolated perfused rat stomach. TPA at the concentration as low as 20nM significantly stimulated the somatostatin release from isolated perfused rat stomach. The integrated net output of somatostatin induced by TPA was dose-dependent in a range of 5 - 50nM TPA. Since TPA is known to activate C-kinase specifically at a low dose (less than 20nM), these findings suggest that C-kinase system may be involved in the regulation of somatostatin release in rat stomach.

Stimulation of porcine jejunal ion secretion in vivo by protein kinase-C activators

Microbial toxins act through cyclic nucleotide dependent (cAMP or cGMP) or cyclic nucleotide independent pathways to cause intestinal ion secretion. To explore the calcium dependent, cyclic nucleotide independent pathway that is postulated to involve protein kinase C activation, we measured protein kinase C activity and phorbol ester binding in isolated intestinal epithelial cells and examined the effects of the C-kinase activators, phorbol myristate acetate, phorbol dibutyrate, and 4-beta-phorbol-12,13-didecanoate, in weaned pig jejunum in vivo. We demonstrated both protein kinase C activity and specific phorbol ester binding in porcine jejunal epithelial cells. Phorbol myristate acetate, phorbol dibutyrate, and 4-beta-phorbol-12,13-didecanoate (10(-5) M) each caused striking secretory responses at 5 h with accumulation of Na+, K+, Cl-, and HCO3- intraluminally. In contrast, 4-alpha-phorbol and 4-alpha-phorbol-12,13-didecanoate, which do not affect protein kinase C, allowed normal net absorption of all electrolytes from the intestinal lumen equivalent to controls with only Ringer's lactate. Time course studies revealed significant secretion within 30 min after exposure to the C-kinase activators. These data suggest an important role for protein kinase C activation in intestinal ion secretion.

Modulation of intracellular Ca2+ in the parathyroid cell. Release of Ca2+ from non-mitochondrial pools by inositol trisphosphate

Stimuli which enhance secretion from parathyroid cells such as low extracellular Ca2+ or Mg2+ are associated with a decrease in the cytosolic Ca2+ concentration as measured by quin2. Current evidence suggests that increased production of inositol 1,4,5-triphosphate (IP3) releases Ca2+ from cellular stores thus increasing cytosolic Ca2+. We used saponin-permeabilized dispersed bovine parathyroid cells to study the effect of IP3 on intracellular Ca2+. IP3 released Ca2+ from these cells in a dose-dependent manner; half-maximal response occurred with 0.3 microM IP3 and maximal response with 1.2 microM IP3. Permeabilized cells incubated in the presence of the mitochondrial inhibitor antimycin A released a similar amount of Ca2+ suggesting that IP3 releases Ca2+ from a non-mitochondrial pool. These results suggest that IP3 regulates cytosolic Ca2+ in this system and may function as a second messenger controlling hormone secretion.