Parathyroid hormone and parathyroid hormone-related peptide activate the Na+/H+ exchanger NHE-1 isoform in osteoblastic cells (UMR-106) via a cAMP-dependent pathway

NHERF3 is necessary for Escherichia coli heat-stable enterotoxin-induced inhibition of NHE3: differences in signaling in mouse small intestine and Caco-2 cells

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of childhood death from diarrhea and the leading cause of Traveler's diarrhea. E. coli heat-stable enterotoxin (ST) is a major virulence factor of ETEC and inhibits the brush border Na/H exchanger NHE3 in producing diarrhea. NHE3 regulation involves multiprotein signaling complexes that form on its COOH terminus. In this study, the hypothesis was tested that ST signals via members of the Na/H exchanger regulatory factor (NHERF) family of scaffolding proteins, NHERF2, which had been previously shown to have a role, and now with concentration on a role for NHERF3. Two models were used: mouse small intestine and Caco-2/BBe cells. In both models, ST rapidly increased intracellular cGMP, inhibited NHE3 activity, and caused a quantitatively similar decrease in apical expression of NHE3. The transport effects were NHERF3 and NHERF2 dependent. Also, mutation of the COOH-terminal amino acids of NHERF3 supported that NHERF3-NHERF2 heterodimerization was likely to account for this dual dependence. The ST increase in cGMP in both models was partially dependent on NHERF3. The intracellular signaling pathways by which ST-cGMP inhibits NHE3 were different in mouse jejunum (activation of cGMP kinase II, cGKII) and Caco-2 cells, which do not express cGKII (elevation of intracellular Ca²⁺ concentration [Ca²⁺]_i). The ST elevation of [Ca²⁺]_i was from intracellular stores and was dependent on NHERF3-NHERF2. This study shows that intracellular signaling in the same diarrheal model in multiple cell types may be different; this has implications for therapeutic strategies, which often assume that models have similar signaling mechanisms.

Cyclic GMP kinase II (cGKII) inhibits NHE3 by altering its trafficking and phosphorylating NHE3 at three required sites: identification of a multifunctional phosphorylation site

The epithelial brush-border Na(+)/H(+) exchanger NHE3 is acutely inhibited by cGKII/cGMP, but how cGKII inhibits NHE3 is unknown. This study tested the hypothesis that cGMP inhibits NHE3 by phosphorylating it and altering its membrane trafficking. Studies were carried out in PS120/NHERF2 and in Caco-2/Bbe cells overexpressing HA-NHE3 and cGKII, and in mouse ileum. NHE3 activity was measured with 2',7'-bis(carboxyethyl)-S-(and 6)carboxyfluorescein acetoxy methylester/fluorometry. Surface NHE3 was determined by cell surface biotinylation. Identification of NHE3 phosphorylation sites was by iTRAQ/LC-MS/MS with TiO2 enrichment and immunoblotting with specific anti-phospho-NHE3 antibodies. cGMP/cGKII rapidly inhibited NHE3, which was associated with reduced surface NHE3. cGMP/cGKII increased NHE3 phosphorylation at three sites (rabbit Ser(554), Ser(607), and Ser(663), equivalent to mouse Ser(552), Ser(605), and Ser(659)), all of which had to be present at the same time for cGMP to inhibit NHE3. NHE3-Ser(663) phosphorylation was not necessary for cAMP inhibition of NHE3. Dexamethasone (4 h) stimulated wild type NHE3 activity and increased surface expression but failed to stimulate NHE3 activity or increase surface expression when NHE3 was mutated to either S663A or S663D. We conclude that 1) cGMP inhibition of NHE3 is associated with phosphorylation of NHE3 at Ser(554), Ser(607), and Ser(663), all of which are necessary for cGMP/cGKII to inhibit NHE3. 2) Dexamethasone stimulates NHE3 by phosphorylation of a single site, Ser(663). The requirement for three phosphorylation sites in NHE3 for cGKII inhibition, and for phosphorylation of one of these sites for dexamethasone stimulation of NHE3, is a unique example of regulation by phosphorylation.

Calcium Homeostasis in Human Placenta: Role of Calcium‐Handling Proteins

The human placenta is a transitory organ, representing during pregnancy the unique connection between the mother and her fetus. The syncytiotrophoblast represents the specialized unit in the placenta that is directly involved in fetal nutrition, mainly involving essential nutrients, such as lipids, amino acids, and calcium. This ion is of particular interest since it is actively transported by the placenta throughout pregnancy and is associated with many roles during intrauterine life. At term, the human fetus has accumulated about 25-30 g of calcium. This transfer allows adequate fetal growth and development, since calcium is vital for fetal skeleton mineralization and many cellular functions, such as signal transduction, neurotransmitter release, and cellular growth. Thus, there are many proteins involved in calcium homeostasis in the human placenta.

Modulation of monocarboxylic acid transporter-1 kinetic function by the cAMP signaling pathway in rat brain endothelial cells

MCT1 (monocarboxylic acid transporter 1) facilitates bidirectional monocarboxylic acid transport across membranes. MCT1 function and regulation have not been characterized previously in cerebral endothelial cells but may be important during normal cerebral energy metabolism and during brain diseases such as stroke. Here, by using the cytoplasmic pH indicator 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein-acetoxymethyl ester, the initial rates of monocarboxylate-dependent cytoplasmic acidification were measured as an indication of MCT1 kinetic function in vitro using the rat brain endothelial cell (RBE4) model of blood-brain transport. The initial rate of L-lactate-dependent acidification was significantly inhibited by 5-10-min incubations with agonists of intracellular cAMP-dependent cell signaling pathways as follows: dibutyryl cAMP, forskolin, and isoproterenol. Isoproterenol reduced V(max) but did not affect K(m) values. The effects of forskolin were completely reversed by the protein kinase A inhibitor H89, whereas H89 alone increased transport rates. Cytoplasmic cAMP levels, measured by radioimmunoassay, were increased by forskolin or isoproterenol, and the effect of isoproterenol was inhibited by propranolol. MCT1-independent intracellular pH control mechanisms did not contribute to the forskolin or H89 effects on MCT1 kinetic function as determined with amiloride, monocarboxylate-independent acid loading, or the transport inhibitor alpha-cyano-4-hydroxycinnamate. The data demonstrate the direct modulation of MCT1 kinetic function in cerebral endothelial cells by agents known to affect the beta-adrenergic receptor/adenylyl cyclase/cAMP/protein kinase A intracellular signaling pathway.

Calcitonin receptor-stimulating peptide-1 regulates ion transport and growth of renal epithelial cell line LLC-PK1

Calcitonin receptor-stimulating peptide-1 (CRSP-1) is a peptide recently identified from porcine brain by monitoring the cAMP production through an endogenous calcitonin (CT) receptor in the renal epithelial cell line LLC-PK(1). Here we investigated the effects of CRSP-1 on the ion transport and growth of LLC-PK(1) cells. CRSP-1 inhibited the growth of LLC-PK(1) cells with a higher potency than porcine CT. CRSP-1 enhanced the uptake of (22)Na(+) into LLC-PK(1) cells more strongly than did CT and slightly reduced the (45)Ca(2+) uptake. The enhancement of the (22)Na(+) uptake was abolished by 5-(N-ethyl-N-isopropyl) amiloride, a strong Na(+)/H(+) exchanger (NHE) inhibitor for NHE1, even at a concentration of 1x10(-8)M, although other ion transporter inhibitors did not affect the (22)Na(+) uptake. These results indicate that CRSP-1 enhances the (22)Na(+) uptake by the specific activation of NHE1. Taken together, CRSP-1 is considered to be a new regulator for the urinary ion excretion and renal epithelial cell growth.

Effect of raloxifene and its interaction with human PTH on bone formation

We studied the of effects raloxifene alone or in combination with human PTH (hPTH) 1-34 in mineralizing cultures of SaOS-2 cells. Raloxifene (10(-8)-10(-6) M) increased bone nodule formation in cultures of SaOS-2 cells when added intermittently from day 8 to day 17. A single 24-h treatment with 10(-8) M hPTH (1-34) at day 8 reduced the nodule area by 75.6% at day 17, and raloxifene added concomitantly with hPTH (1-34) reduced this inhibitory effect in a dose-dependent manner. Raloxifene also reduced the hPTH (1-34)-induced inhibition of alkaline phosphatase (ALP) activity. The 10-fold stimulation of c-fos mRNA expression by hPTH (1-34) was not influenced by raloxifene co-treatment. The protein kinase A (PKA) inhibitor 6-22 amide (1.7 nM) and the protein kinase C (PKC) inhibitor-bisindolylmaleimide 1 (10 nM) did not influence the separate effects of PTH and raloxifene on mineralized bone nodule formation. This is the first report on the interaction of PTH and raloxifene in an osteoblast culture system.

Inhibition of apical Cl-/OH- exchange activity in Caco-2 cells by phorbol esters is mediated by PKCepsilon

The present studies were undertaken to examine the possible regulation of apical membrane Cl-/OH- exchanger in Caco-2 cells by protein kinase C (PKC). The effect of the phorbol ester phorbol 12-myristate 13-acetate (PMA), an in vitro PKC agonist, on OH- gradient-driven 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive 36Cl uptake in Caco-2 cells was assessed. The results demonstrated that PMA decreased apical Cl-/OH- exchanger activity via phosphatidylinositol 3-kinase (PI3-kinase)-mediated activation of PKCepsilon. The data consistent with these conclusions are as follows: 1) short-term treatment of cells for 1-2 h with PMA (100 nM) significantly decreased Cl-/OH- exchange activity compared with control (4alpha-PMA); 2) pretreatment of cells with specific PKC inhibitors chelerythrine chloride, calphostin C, and GF-109203X completely blocked the inhibition of Cl-/OH- exchange activity by PMA; 3) specific inhibitors for PKCepsilon (Ro-318220) but not PKCalpha (Go-6976) significantly blocked the PMA-mediated inhibition; 4) specific PI3-kinase inhibitors wortmannin and LY-294002 significantly attenuated the inhibitory effect of PMA; and 5) PI3-kinase activators IRS-1 peptide and phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P(3)] mimicked the effects of PMA. These findings provide the first evidence for PKCepsilon-mediated inhibition of Cl-/OH- exchange activity in Caco-2 cells and indicate the involvement of the PI3-kinase-mediated pathways in the regulation of Cl- absorption in intestinal epithelial cells.

Reversible suppression of in vitro biomineralization by activation of protein kinase A

Parathyroid hormone (PTH-(1-34)) potently suppresses apatite deposition in osteoblastic cultures. These inhibitory effects are mediated through signaling events following PTH receptor binding. Using both selective inhibitors and activators of protein kinase A (PKA), this study shows that a transient activation of PKA is sufficient to account for PTH's inhibition of apatite deposition. This inhibition is not a result of reduced cell proliferation, reduced alkaline phosphatase activity, increased collagenase production, or lowering medium pH. Rather, data suggest a functional relationship between matrix assembly and apatite deposition in vitro. Bone sialoprotein (BSP) and apatite co-localize in the extracellular matrix of mineralizing cultures, with matrix deposition of BSP temporally preceding that of apatite. Transient activation of PKA by either PTH-(1-34) or short term cAMP analog treatment blocks the deposition of BSP in the extracellular matrix without a significant reduction in the total amount of BSP synthesized and secreted. This effect is reversible after allowing the cultures to recover in the absence of PKA activators for several days. Thus, a transient activation of PKA may suppress mineral deposition in vitro as a consequence of altering the assembly of an extracellular matrix permissive for apatite formation.

Parathyroid hormone regulation of the rat collagenase-3 promoter by protein kinase A-dependent transactivation of core binding factor alpha1

Previously we showed that the activator protein-1 site and the runt domain binding site in the collagenase-3 promoter act cooperatively in response to parathyroid hormone (PTH) in the rat osteoblastic osteosarcoma cell line, UMR 106-01. Our results of the expression pattern of core binding factor alpha1 (Cbfa1), which binds to the runt domain site, indicated that there is no change in the levels of Cbfa1 protein or RNA under either control conditions or after PTH treatment. The importance of posttranslational modification of Cbfa1 in the signaling pathway for PTH-induced collagenase-3 promoter activity was analyzed. PTH stimulation of collagenase-3 promoter activity was completely abrogated by protein kinase A (PKA) inhibition. To determine the role of PKA activity with respect to Cbfa1 activation (in addition to its known activity of phosphorylating cAMP-response element-binding protein to enhance c-fos promoter activity), we utilized the heterologous Gal4 transcription system. PTH stimulated the transactivation of activation domain-3 in Cbfa1 through the PKA site. In vitro phosphorylation studies indicated that the PKA site in the wild type activation domain-3 is a substrate for phosphorylation by PKA. Thus, we demonstrate that PTH induces a PKA-dependent transactivation of Cbfa1, and this transactivation is required for collagenase-3 promoter activity in UMR cells.

Insulin-like growth factor I rapidly enhances acid efflux from osteoblastic cells

Insulin-like growth factor I (IGF-I) is thought to stimulate bone resorption indirectly through a primary effect on osteoblasts, which in turn activate osteoclasts by as-yet-unidentified mechanisms. Small decreases in extracellular pH (pHo) dramatically increase the resorptive activity of osteoclasts. Our purpose was to characterize the effect of IGF-I on acid production by osteoblastic cells. When confluent, UMR-106 osteoblast-like cells and rat calvarial cells acidified the compartment beneath them. Superfusion with IGF-I caused a further decrease in pHo. To investigate the mechanism, we monitored acid efflux from subconfluent cultures. IGF-I rapidly increased net efflux of H+ equivalents in a concentration-dependent manner. IGF-II (10 nM) evoked a smaller response than IGF-I (10 nM). The response to IGF-I was partially dependent on extracellular Na+, but not glucose, and exhibited little if any desensitization. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, abolished the response to IGF-I but not to parathyroid hormone. Thus IGF-I enhances acid efflux from osteoblastic cells, via a signaling pathway dependent on activation of phosphatidylinositol 3-kinase. In vivo, acidification of the compartment between the osteogenic cell layer and the bone matrix may affect diverse processes, including mineralization and osteoclastic bone resorption.

Recombinant human endothelin-converting enzyme ECE-1b is located in an intracellular compartment when expressed in polarized Madin-Darby canine kidney cells

Endothelin-converting enzyme (ECE) is a phosphoramidon-sensitive membrane-bound metalloprotease responsible for the conversion of big-endothelins into endothelins [Yanagisawa, Kurihara, Kimura, Tomobe, Kobayashi, Mitsui, Yazaki, Goto and Masaki (1988) Nature (London) 332, 411-415]. Several distinct isoforms of ECE have been cloned and identified. ECE-1a, b and c have the same ectodomain and differ only by their cytosolic tails [Schweizer, Valdenaire, Nelböck, Deuschle, Edwards, Stumpf and Löffler (1997) Biochem. J. 328, 871-877]. The ectodomain common to ECE-1 a, b and c shares extensive sequence similarities with neprilysin, a major kidney brush border metallopeptidase. To study the sorting of ECE in polarized cells, ECE-1bcDNA was expressed by transfection in polarized Madin-Darby canine kidney (MDCK) cells. Cell-surface biotinylation and immunofluorescence studies showed that ECE-1b is not expressed on the cell-surface but was rather located in intracellular compartments that could also be labelled with anti-Rab-5 and Rab-7 antibodies and was thus tentatively identified as early and late endosomes. Similar results were also obtained when ECE-1b was expressed in non-polarized Chinese hamster ovary cells for comparison purposes. When MDCK or Chinese hamster ovary transfected cells were pre-treated with the ECE inhibitor phosphoramidon, a 3-fold increase in the level of ECE-1b was observed both by Western blotting and by enzymic activity. However, no change in the level of neprilysin or the beta-chain of meprin, two apical membrane metallopeptidases, was observed in MDCK cells transfected under similar conditions. Northern blotting showed that the increase in the level of ECE-1b was not owing to changes in the ECEmRNA transcription rate or stability. Rather, pulse-chase experiments followed by immunoprecipitation showed a decrease in the rate of degradation of ECE-1b in phosphoramidon-treated cells. Half-lives were determined to be 2.8 and 7.5 h for non-treated and phosphoramidon-treated cells, respectively. Confocal microscopy showed accumulation of ECE-1b immunoreactive material in the lysosomes of phosphoramidon-treated cells. Taken together, these results suggest that ECE-1b turns over very rapidly between endosomal and lysosomal compartments and that lysosomal degradation of the enzyme is slowed down by phosphoramidon.

Parathyroid hormone responses of cyclic AMP-, serum- and phorbol ester-responsive reporter genes in osteoblast-like UMR-106 cells

Parathyroid hormone (PTH) and PTH-related protein interact with a G protein-coupled receptor linked to the activation of adenylyl cyclase and phospholipase C signaling pathways. Regulation by PTH of the expression of three distinct, stably transfected luciferase reporter genes responsive to cAMP (CRE-luc), serum (SRE-luc) and phorbol ester (TRE-luc) has been studied in rat osteoblast-like UMR-106 cells. Maximal 43-fold induction of CRE-luc expression occurred in response to 100 nM rat (r)PTH(1-34) (EC50=0.44 nM), but SRE-luc and TRE-luc remained unaffected. Maximal 2.8- and 3.4-fold inductions of SRE-luc by 10 ng/ml EGF and 100 nM phorbol ester (PMA) were suppressed with 100 nM rPTH(1-34) (IC50=0.04 and 0.15 nM, respectively). Similarly, 7.3-fold induction of TRE-luc by 100 nM PMA was inhibited to 50% with 100 nM rPTH(1-34) (IC50=0.5 nM). Activation of mitogen-activated protein kinase by EGF and PMA was also suppressed by rPTH(1-34). 1 mM 8-Br-cAMP and 0.1 mM forskolin mimicked all the effects of rPTH(1-34). In conclusion, the regulation of target genes by PTH in osteoblast-like UMR-106 cells is mediated by the activation of the cAMP/protein kinase A signaling pathway.

A new action of parathyroid hormone. receptor-mediated stimulation of extracellular acidification in human osteoblast-like SaOS-2 cells

The major physiological function of parathyroid hormone (PTH) is the maintenance of Ca2+/Pi homeostasis via the parathyroid hormone/parathyroid hormone-related protein receptor (PTHR) in kidney and bone. An important consequence of PTHR activation in bone is enhanced local acidification of the extracellular space. Agonist activation of some seven transmembrane-domain receptors increases the extracellular acidification rate (ECAR). We utilized microphysiometry to investigate PTH-stimulated, receptor-mediated increases in ECAR in human osteoblast-like SaOS-2 cells. PTH-(1-34) elicited a large, acute, dose-dependent increase in ECAR with an EC50 of about 2 nM. The PTH-induced increase in ECAR was specific to cells expressing the PTHR and was inhibited by PTHR antagonists. Rapid, partial, homologous desensitization of the PTH-induced increase in ECAR was observed. Incubation of SaOS-2 cells with 8-bromo-cyclic AMP neither mimicked nor abrogated the PTH effect, and PTH stimulated an acute increase in ECAR in cAMP-resistant SaOS-2 Ca#4A cells. Stimulation of ECAR by PTH was independent of transient increases in cytosolic free calcium. Both inhibition and down-regulation of PKC reduced the PTH-induced increase in ECAR. Inhibition of Na+/H+ exchange did not affect the PTH-induced ECAR response. We conclude that PTH caused a receptor-mediated, concentration-dependent, increase in ECAR, which was not dependent on the cAMP/PKA signaling pathway or the Na+/H+ exchanger but involved the action of PKC. Thus, acid production in bone, a physiologically important action of PTH, is not confined to osteoclasts as previously considered but is also mediated by osteoblasts.

Expression and characterization of recombinant rat parathyroid hormone-related peptide (1-141) and an amino-terminally-truncated analogue (38-141)

We have synthesized and purified recombinant parathyroid hormone related peptide (PTHrP (1-141)) and PTHrP (38-141) using an E. coli system that requires minimal purification. The cDNAs encoding PTHrP (1-141) and PTHrP (35-141) respectively were inserted into the multiple cloning site of the pTrcHis-B bacterial expression plasmid. The PTHrP encoded sequences were thereby fused at their NH2-termini to six histidine residues within the fusion protein. The recombinant plasmids were transfected into E. coli cells and PTHrP synthesis was induced by addition of 1 mM isopropyl-beta-D-thiogalactopyranoside (IPTG) at 37 degrees C. The recombinant fusion proteins were purified by binding of the histidine residues to a nickel column followed by gradient elusion and dialysis. PTHrP (1-141) was released from its fusion protein by cyanogen bromide cleavage, whereas PTHrP (38-141) was released by enzymatic digestion with enterokinase. This rapid isolation method resulted in pure PTHrP (1-141) and (38-141) as judged by SDS-polyacrylamide gel electrophoresis and NH2-terminal sequence analysis. PTHrP (1-141) stimulated cAMP accumulation and mobilized intracellular calcium ([Ca2+]i) in UMR106 osteoblast-like cells, and stimulated phosphate transport in OK/E renal cells, whereas PTHrP (38-141) was inert in these bioassays. Availability of PTHrP and its NH2-terminally truncated analogue, which lacks the sequence necessary for its hypercalcemic actions, will enable their biological activities to be examined in greater detail.

5-HT1A receptor activates Na+/H+ exchange in CHO-K1 cells through Gialpha2 and Gialpha3

5-HT1A receptors couple to many signaling pathways in CHO-K1 cells through pertussis toxin-sensitive G proteins. The purpose of this study was to determine which members of the Gi/o/z family mediate 5-HT1A receptor-activated Na+/H+ exchange as measured by microphysiometry of cell monolayers. The method was extensively validated, showing that proton efflux was sodium-dependent, inhibited by amiloride analogs, and activated by growth factors, phorbol ester, calcium ionophore, and hypertonic stress. 5-HT and the specific agonist (+/-)-8-hydroxy-2-(di-N-propylamino)tetralin hydrobromide rapidly stimulated proton efflux that was blocked by a specific receptor antagonist, amiloride analogs or pertussis toxin. The activation by 5-HT depended upon extracellular sodium and could be demonstrated under conditions of imposed intracellular acid load, as well as in the presence and absence of glycolytic substrate. Acceleration of proton efflux was not inhibited by sequestration of G protein betagamma-subunits, a maneuver that blocked 5-HT1A receptor activation of mitogen-activated protein kinase. Transfection of Gzalpha and pertussis toxin-resistant mutants of Goalpha and Gialpha1 did not reverse the blockade induced by pertussis toxin. In contrast, pertussis toxin-resistant mutants of Gialpha2 and Gialpha3 "rescued" the ability of 5-HT to increase proton efflux after pertussis toxin treatment. These experiments demonstrate clearly that Gialpha2 and Gialpha3 can specifically mediate rapid agonist-induced acceleration of Na+/H+ exchange.

Galpha12 differentially regulates Na+-H+ exchanger isoforms

Activation of several GTPases stimulates Na+-H+ exchange, resulting in an increased efflux of intracellular H+. These GTPases include alpha subunits of the heterotrimeric G proteins Gq and G13, as well as the low molecular weight GTP-binding proteins Ras, Cdc42, and Rho (Hooley, R., Yu, C.-Y., Simon, M., and Barber, D. L. (1996) J. Biol. Chem. 271, 6152-6158). GTPases coupled to the inhibition of Na+-H+ exchange, however, have not been identified. Several neurotransmitters, including somatostatin and dopamine, inhibit Na+-H+ exchange through a guanine-nucleotide-dependent mechanism, suggesting the involvement of a GTPase. In this study we determined that mutational activation of the alpha subunit of G12 inhibits the ubiquitously expressed Na+-H+ exchanger isoform, NHE1. Transient expression of mutationally activated Galpha12 inhibited serum- and Galpha13-stimulated NHE1 activity in HEK293 cells and CCL39 fibroblasts. In addition, in NHE-deficient AP1 cells stably expressing specific NHE isoforms, mutationally activated Galpha12 inhibited NHE1 activity but stimulated activities of the Na+-H+ exchanger (NHE) isoforms NHE2 and NHE3. In contrast, mutationally activated Galpha13, another member of the Galpha12/13 family, stimulated all three NHE isoforms. Although previous studies have identified a parallel action of Galpha12 and Galpha13 in regulating MAP (mitogen-activated protein) kinases and cell growth, these GTPases have opposing effects on NHE1 activity.

Structurally diverse N-terminal peptides of parathyroid hormone (PTH) and PTH-related peptide (PTHRP) inhibit the Na+/H+ exchanger NHE3 isoform by binding to the PTH/PTHRP receptor type I and activating distinct signaling pathways

N-terminal peptides of parathyroid hormone (PTH) and PTH-related peptide (PTHRP) elicit a wide variety of biological responses in target cells, including the inhibition of Na+/H+ exchanger NHE3 activity in renal cells. This response is believed to be mediated by ligand binding to a common receptor (i.e. PTH/PTHRP receptor type I) and activation of cAMP-dependent and/or Ca2+/phospholipid-dependent protein kinases (PKA and PKC, respectively). However, the mechanism of action of these N-terminal peptides is now unclear because of recent data reporting the existence of additional receptor isoforms. Therefore, to directly examine the ligand binding and signaling characteristics of the PTH/PTHRP receptor type I and its ability to elicit a biological response, cDNAs encoding the rat type I receptor and the rat NHE3 isoform were transfected into Chinese hamster ovary (AP-1) cells that lack endogenous expression of these proteins. Competition binding assays using [125I-Tyr36]PTHRP-(1-36)-NH2 radioligand indicated that several biologically active human N-terminal PTH and PTHRP fragments (PTH-(1-34), PTH-(3-34), PTH-(28-42), PTH-(28-48), and PTHRP-(1-34)) were capable of binding to the type I receptor. Both PTH-(1-34) and PTHRP-(1-34) stimulated adenylate cyclase and PKC activities in these cells, whereas PTH-(3-34), PTH-(28-42), and PTH-(28-48) selectively enhanced only PKC activity. PTHRP-(1-16), a biologically inert fragment, was incapable of binding to this receptor and influencing either the PKA or PKC pathway. Furthermore, all the analogues with the exception of PTHRP-(1-16) inhibited NHE3 activity. Inhibition of PKC by the potent antagonist chelerythrine chloride abolished the depression of NHE3 activity by PTH-(3-34), PTH-(28-42), and PTH-(28-48) but did not alleviate the effects of PTH-(1-34). Likewise, antagonism of PKA by H-89 was unable to prevent the inhibition caused by PTH-(1-34). However, inhibition of both PKA and PKC by the nonselective protein kinase antagonist H-7 abolished the reduction of NHE3 activity by PTH-(1-34). These data indicate that discrete N-terminal analogues of PTH and PTHRP can interact with the classical PTH/PTHRP receptor type I and activate PKA and/or PKC. Activation of either signaling pathway independently leads to inhibition of NHE3.

Distinct structural domains confer cAMP sensitivity and ATP dependence to the Na+/H+ exchanger NHE3 isoform

Agents known to increase cAMP levels in renal and intestinal epithelia decrease sodium absorption by inhibiting NHE3, an isoform of the Na+/H+ exchanger expressed at high levels in apical membranes of these cells. In contrast, the ubiquitous, housekeeping isoform of the exchanger (NHE1) is stimulated by cAMP in some cell types. Optimal activity of NHE3 as well as NHE1 requires the presence of ATP. To gain insight into the molecular mechanisms of ATP dependence and cAMP regulation of NHE3, a series of mutations were constructed by progressively truncating segments of the C-terminal cytoplasmic domain of the transporter at amino acid positions 684, 638, and 579 (named NHE3delta684, NHE3delta638, and NHE3delta579). In addition, chimeric antiporters were constructed with the N-terminal transmembrane domain of NHE3 linked to the entire cytoplasmic region of NHE1 (chimera NHE3/1) or vice versa (chimera NHE1/3). These constructs were heterologously expressed in antiport-deficient Chinese hamster ovary cells, and their activities were assessed by fluorimetric measurements of intracellular pH and by radioisotope determinations of Na+ influx. Forskolin, which directly stimulates adenylate cyclase, inhibited NHE3 as well as NHE1/3, but not NHE3/1, suggesting that the cytoplasmic domain of NHE3 was sufficient to confer sensitivity to inhibition by cAMP. Forskolin also inhibited the truncated mutant NHE3delta684 to an extent similar to that for wild type NHE3. However, the inhibitory effect was greatly reduced in NHE3delta638 and more profound truncations (NHE3delta579 obliterated the effect of forskolin. These findings suggest that a region found between amino acids 579 and 684 is essential for the cAMP response of NHE3. In contrast, comparable ATP dependence was observed in all exchanger constructs examined. These observations indicate that ATP dependence is conferred by a region of the molecule in or adjacent to the transmembrane domain, which is most conserved between isoforms. It is concluded that different sites, and therefore different mechanisms, underlie inhibition of NHE3 by cAMP and by depletion of ATP.

Plasma membrane Na+/H+ exchanger isoforms (NHE-1, -2, and -3) are differentially responsive to second messenger agonists of the protein kinase A and C pathways

Na+/H+ exchanger (NHE) activity is regulated by several types of receptors directly coupled to distinct classes (i.e. Gs, Gi, Gq, and G12) of heterotrimeric (alpha beta gamma) GTP-binding proteins (G proteins), which, upon activation, modulate production of various second messengers (e.g. cAMP, cGMP, diacylglycerol, inositol trisphosphate, and Ca2+). Recently, four isoforms of the rat Na+/H+ exchanger were identified by molecular cloning. To examine their intrinsic responsiveness to G protein and second messenger stimulation, three of these isoforms, NHE-1, -2, and -3, were stably expressed in mutant Chinese hamster ovary cells devoid of endogenous NHE activity (AP-1 cells). Incubation of cells with either AIF4-, a general agonist of G proteins, or cholera toxin, a selective activator of G alpha s that stimulates adenylate cyclase, accelerated the rates of amiloride-inhibitable 22Na+ influx mediated by NHE-1 and -2, whereas they inhibited that by NHE-3. Similarly, short term treatment with phorbol 12-myristate 13-acetate, which mimics diacylglycerol activation of protein kinase C (PKC), or with agents (i.e. forskolin, 8-(4-chlorophenylthio)-cAMP, and isobutylmethylxanthine) that lead to activation of cAMP-dependent protein kinase (PKA) also stimulated transport by NHE-1 and NHE-2 but depressed that by NHE-3. The effects of phorbol 12-myristate 13-acetate were blocked by depleting cells of PKC or by inhibiting PKC using chelerythrine chloride, confirming a role for PKC in modulating NHE isoform activities. Likewise, the PKA antagonist, H-89, attenuated the effects of elevated cAMPi on NHE-1, -2, and -3, further demonstrating the regulation by PKA. Unlike cAMPi, elevation of cGMPi by treatment with dibutyryl-cGMP or 8-bromo-cGMP had no influence on NHE isoform activities, thereby excluding the possibility of a role for cGMP-dependent protein kinase in these cells. These data support the concept that the NHE isoforms are differentially responsive to agonists of the PKA and PKC pathways.