Protein kinase C-dependent diacylglycerol formation is mediated via Ca2+/calmodulin in parotid cells

Mechanisms Underlying Activation of α₁-Adrenergic Receptor-Induced Trafficking of AQP5 in Rat Parotid Acinar Cells under Isotonic or Hypotonic Conditions

Defective cellular trafficking of aquaporin-5 (AQP5) to the apical plasma membrane (APM) in salivary glands is associated with the loss of salivary fluid secretion. To examine mechanisms of α₁-adrenoceptor (AR)-induced trafficking of AQP5, immunoconfocal microscopy and Western blot analysis were used to analyze AQP5 localization in parotid tissues stimulated with phenylephrine under different osmolality. Phenylephrine-induced trafficking of AQP5 to the APM and lateral plasma membrane (LPM) was mediated via the α_1A-AR subtype, but not the α_1B- and α_1D-AR subtypes. Phenylephrine-induced trafficking of AQP5 was inhibited by ODQ and KT5823, inhibitors of nitric oxide (NO)-stimulated guanylcyclase (GC) and protein kinase (PK) G, respectively, indicating the involvement of the NO/ soluble (c) GC/PKG signaling pathway. Under isotonic conditions, phenylephrine-induced trafficking was inhibited by La³⁺, implying the participation of store-operated Ca²⁺ channel. Under hypotonic conditions, phenylephrine-induced trafficking of AQP5 to the APM was higher than that under isotonic conditions. Under non-stimulated conditions, hypotonicity-induced trafficking of AQP5 to the APM was inhibited by ruthenium red and La³⁺, suggesting the involvement of extracellular Ca²⁺ entry. Thus, α_1A-AR activation induced the trafficking of AQP5 to the APM and LPM via the Ca²⁺/ cyclic guanosine monophosphate (cGMP)/PKG signaling pathway, which is associated with store-operated Ca²⁺ entry.

cGMP antagonizes angiotensin-mediated phosphatidylcholine hydrolysis and C kinase activation in mesangial cells

Previous studies from this laboratory have shown that in cultured rat mesangial cells (MC), angiotensin II (ANG II) mediates its effects via activation of phosphatidylinositol-specific phospholipase C (PI-PLC) and phosphatidylcholine-specific phospholipase C (PC-PLC) and phospholipase D (PC-PLD). In addition, guanosine 3',5'-cyclic monophosphate (cGMP)-elevating maneuvers that stimulate particulate and soluble guanylate cyclase [atrial natriuretic factor (ANF) and sodium nitroprusside (SNP), respectively] antagonize ANG II-mediated PI-PLC activation. The current study explored whether cGMP impairs ANG II-mediated PC-PLC and PLD activity. The ANG II-stimulated release of the water-soluble metabolites of PC breakdown (phosphorylcholine and choline) was blocked by ANF and SNP. ANG II-stimulated phosphatidic acid and phosphatidylethanol formation were significantly reduced by ANF and SNP, confirming that cGMP blunted PLD activity. The inhibitory effect of cGMP on PLD could be reversed by N-(2-[methylamino]ethyl)-5-isoquinolinesulfonamide, a blocker of cGMP-dependent protein kinase. In parallel experiments, ANF and SNP abrogated sustained diacylglycerol (DAG) accumulation derived from ANG II stimulation of PC hydrolysis, confirming that cGMP diminished PC-PLC activity. Inhibition of PC-derived DAG accumulation by cGMP was associated with a concomitant decrement in ANG II-mediated translocation of protein kinase C (PKC) activity from the cytosol to the membrane. In summary, in MC, cGMP antagonizes ANG II-mediated PC hydrolysis, DAG formation, and PKC activation. We propose that cGMP-mediated inhibition of phospholipid metabolism and PKC translocation plays an important role in MC vasorelaxation.

Carbachol-induced phosphatidylcholine hydrolysis and choline efflux in rat submandibular gland involves phospholipase D activation and is modulated by protein kinase C and calcium

The role of calcium and protein kinase C activation in carbachol-induced choline efflux from submandibular glands was investigated. The participation of phospholipase D in this signal transduction pathway was demonstrated by the formation of [14C]phosphatidylethanol in [14C]lysophosphatidylcholine-labelled submandibular gland cells treated with carbachol or noradrenaline in the presence of ethanol. Chelation of the intracellular calcium with BAPTA/AM reduced the carbachol stimulated outflow of [3H]choline. The calcium ionophore A23187 in a high concentration (10 microM) increased the basal [3H]choline outflow, but decreased the carbachol-induced outflow. Removal of the extracellular calcium enhanced the carbachol-stimulated outflow, which returned to control when calcium was re-added to the medium. Activation of protein kinase C by phorbol-12,13-dibutyrate (100 nM) or 1-oleyl-2-acetyl-sn-glycerol (20 microM) was without effect per se, but enhanced the carbachol-mediated outflow of [3H]choline. Phorbol-12,13-dibutyrate in combination with 1 microM A23187 induced a small efflux of [3H]choline. A 2 h treatment with phorbol-12,13-dibutyrate (1 microM), causing down-regulation of protein kinase C, significantly decreased the carbachol-stimulated [3H]choline outflow. In conclusion, elevation of intracellular calcium levels and protein kinase C activation are of importance for the carbachol-stimulated outflow of [3H]choline. Inflow of calcium, if anything, reduces the carbachol-stimulated outflow of [3H]choline. Since phosphatidylcholine breakdown generates diacylglycerol and this could lead to activation of protein kinase C, activation of this signal transduction pathway may be important for the protein content of the saliva and for the known trophic effects of parasympathetic innervation.

Translocation of the alpha-isozyme of protein kinase C during stimulation of rat parotid acinar cells by phorbol ester and carbachol

Protein kinase C activity was detected in the cytosolic fraction of quiescent parotid acinar cells; the particulate fraction contained a much smaller proportion of the enzyme. Protein kinase C activity was increased in the membrane fraction and decreased in the cytosol after exposure of intact cells to phorbol 12-myristate 13-acetate (PMA) or the muscarinic-receptor agonist carbachol. The effect of PMA was potentiated by a subthreshold concentration of ionomycin. Immunoblot analysis with anti-protein kinase C antibodies revealed that the protein kinase C-alpha isoform is expressed in rat parotid cells. Other Ca(2+)-dependent isoforms were not detected. Further, agonist stimulation caused the redistribution of protein kinase C-alpha from cytosol to a membrane fraction. Agonists may promote parotid acinar cell activity, including amylase secretion, by increasing the affinity of protein kinase C-alpha for the membrane fraction, presumably via a rise in Ca2+ and diacylglycerol derived from polyphosphoinositide hydrolysis.

Breakdown of membrane choline-phospholipids induced by endogenous and exogenous muscarinic agonist is potentiated by VIP in rat submandibular gland

The outflow of tritium from rat submandibular gland fragments, pre-labelled with [3H]choline, following electrical or pharmacological stimulation was studied. Electrical stimulation of the tissue increased the outflow of tritium in a frequency dependent manner. Atropine treatment decreased the electrically-induced release, indicating that the outflow did not reflect acetylcholine from nerve endings, but was largely brought about by postsynaptic receptors. In agreement with this hypothesis, treatment with noradrenaline or carbachol induced a dose dependent increase in tritium outflow from the gland fragments which could be blocked by prazosin or atropine, respectively. Moreover, analysis of the tissue-associated tritium revealed an incorporation primarily in the lipid fraction of the tissue (almost 80%), of which about 90% was in phosphatidylcholine, indicating that this was the source of the tritium outflow. Pre-incubation with vasoactive intestinal polypeptide (VIP), which coexists with acetylcholine in the parasympathetic neurons innervating the submandibular gland, increased the carbachol-induced tritium overflow significantly. The effect of VIP could be imitated by the adenylyl cyclase stimulator forskolin, which increased the carbachol-stimulated tritium efflux in a dose dependent manner. Taken together, our results suggests that muscarinic- and alpha 1-receptor agonists may activate a phospholipase coupled to phosphatidylcholine hydrolysis in the rat submandibular gland. Endogenous acetylcholine released from parasympathetic nerve endings appear to activate this mechanism. Furthermore, VIP treatment, and the concomitant cAMP-accumulation, potentiates the acetylcholine induced phosphatidylcholine hydrolysis, demonstrating a new type of interaction between the classical transmitter acetylcholine and the co-stored neuropeptide VIP.

G protein beta gamma-subunits inhibit purified adenylate cyclase independent of the activation by Ca2+ and calmodulin

Adenylate cyclase purified by affinity chromatography was activated about 2.5-fold in a Ca(2+)- and calmodulin-dependent fashion. G protein beta gamma-subunits, an inhibitor in the receptor-mediated inhibition of adenylate cyclase, inhibited the purified cyclase by more than 80%. The extent of beta gamma-induced inhibition was not affected by the activation with Ca2+ and calmodulin. Moreover, the prior addition of the beta gamma-subunits to the cyclase did not prevent the subsequent activation of the enzyme by Ca2+ and calmodulin. We conclude that the beta gamma-subunits inhibit adenylate cyclase activity in a calmodulin-independent mode.

Ca2+ ionophore and phorbol ester stimulate diacylglycerol formation and phosphatidylcholine hydrolysis in rat parotid acinar cells

We investigated the effects of A23187 and phorbol 12,13-dibutyrate (PDBu) on sn-1,2-diacylglycerol (DAG) accumulation and phosphatidylcholine (PC) hydrolysis in rat parotid acinar cells. Both A23187 and PDBu, in concentration ranges of 0.001-0.1 microM, stimulated DAG accumulation and PC hydrolysis in a time- and concentration-dependent manner. Treatment with A23187 and PDBu stimulated the release of [3H]choline and [3H]phosphocholine into the medium, indicating [3H]PC hydrolysis is due to the activation of phospholipases C and D; however, [3H]phosphatidylethanolamine hydrolysis was not indicated. These releases were unaffected by the addition of glucose 6-phosphate, a phosphatase inhibitor. Staurosporine, a protein kinase C inhibitor, significantly inhibited the DAG accumulation and the PC hydrolysis stimulated by these agents. Combinations of A23187 and PDBu potentiated the stimulatory effect which each of these agents alone had on DAG accumulation and PC hydrolysis. This mode of action was additive but not synergistic. These results suggest that DAG accumulation induced by A23187 and PDBu is related to the PC hydrolysis mediated via the activation of phospholipases C and D, and that it is not related to phosphatidylethanolamine hydrolysis.

Mechanism of carbachol-stimulated diacylglycerol formation in rat parotid acinar cells

We studied the relationship between phosphoinositide hydrolysis, phosphatidylcholine hydrolysis, and sn-1,2-diacylglycerol (DAG) formation in response to carbachol stimulation in rat parotid acinar cells. Previously, we demonstrated that DAG formation stimulated with 1 microM carbachol was biphasic: the first peak occurred at 5 min and the second one at 20 min. It was also demonstrated that the second peak was regulated in part by a calmodulin/protein kinase C-dependent mechanism. Based on the kinetic analysis of DAG formation and [32P]phosphoinositide breakdown, the first peak of carbachol (1 microM)-stimulated DAG accumulation was found to be related to the breakdown of [32P]phosphatidylinositol 4-monophosphate ([32P]PIP) and [32P]phosphatidylinositol 4,5-bisphosphate ([32P]PIP2). The second peak was found to be related to [32P]PIP2 breakdown. Carbachol stimulated the release of [3H]phosphocholine into the medium, indicating that the predominant pathway for phosphatidylcholine hydrolysis was via phospholipase C. Moreover, carbachol stimulated the release of [3H]choline metabolites in a time- and dose-dependent manner. This agonist slightly stimulated the release of [3H]ethanolamine metabolites. A calmodulin/protein kinase C-dependent mechanism was also studied and was found to be involved in carbachol-stimulated phosphatidylcholine hydrolysis; W-7, a calmodulin inhibitor and staurosporine, a protein kinase C inhibitor, inhibited the carbachol (1-microM)-induced release of [3H]choline metabolites at 20 min in a dose-dependent manner, but did not have inhibitory effects at 5 min. These results suggest that the first peak of DAG accumulation induced by carbachol is predominantly associated with the breakdown [32P]PIP and of [32P]PIP2 and that the second peak is predominantly associated with [32P]PIP2 breakdown and phosphatidylcholine hydrolysis.

Microfluorimetric imaging study of the mechanism of activation of the Na+/H+ antiport by muscarinic agonist in rat mandibular acinar cells

The mechanism of regulation of intracellular pH (pHi) in dispersed acini from the rat mandibular salivary gland has been studied with a microfluorimetric imaging method and the pH probe 2',7'-bis(2-carboxyethyl)-5(and -6)-carboxyfluorescein. The pHi in the TRIS/HEPES-buffered standard solution was 7.29 +/- 0.01. Addition of 1 mumol/l acetylcholine (ACh) or ionomycin caused a sustained increase in the pHi. These agents decreased pHi in the absence of external Na+ or in the presence of amiloride. The rate of pHi recovery from an acid load after NH+4 prepulse was a linear function of pHi and increased as pHi became more acidic. Addition of ACh shifted the relationship towards a more alkaline pHi range. The increase in pHi induced by ACh or ionomycin was not inhibited by the protein kinase C inhibitors staurosporine (10 nM) and 1-(5-isoquinolinesulfonyl)-1-methylpiperazine (50 mumol/l). Addition of 0.1-1 mumol/l phorbol 12-myristate 13-acetate (TPA) had little effect on pHi within 10 min; however, exposure to TPA for 120 min resulted in a significant rise in pHi. In Ca(2+)-free solution with 50 mumol/l 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate, the ACh-induced rise in both pHi and cytosolic Ca2+ concentration was suppressed. ACh and ionomycin caused an increment of amiloride-sensitive acid output into the extracellular fluid, while 20 mumol/l 1-oleoyl-2-acetylglycerol had little effect on it. It was concluded that (a) stimulation with ACh activated the Na+/H+ antiport in the plasma membrane, (b) ACh also stimulated the intracellular acid production but acid extrusion by the Na+/H+ antiport prevented the cell from intracellular acidification, and (c) the major route of signal transduction for the ACh-induced activation of the Na+/H+ antiport was independent of protein kinase C but was dependent on the rise in cytosolic Ca2+ concentration. The implication of the cytosolic acidification and cell volume change in pHi regulation is discussed.

Substance P-induced diacylglycerol formation in rat parotid acinar cells

The mechanisms underlying the ability of substance P, to stimulate the sn-1,2-diacylglycerol (DAG) formation were studied using rat parotid acinar cells. During a 60 s stimulation, 1 microM substance P caused a rapid rise in DAG accumulation at 5 s, whereas a low (0.1 microM) concentration of agonist did not. During long term stimulation for 30 min, DAG accumulation induced by 1 microM substance P reached near maximal levels at 5 min and remained elevated for at least 20 min. In contrast, DAG formation induced by 0.1 microM substance P exhibited a peak at 5 min, gradually declined and returned to near basal levels at 30 min. Furthermore, DAG accumulation in response to substance P at 5 and 20 min increased in a dose-dependent manner. The breakdown of both [32P]phosphatidylinositol 4-monophosphate ([32P]PIP) and [32P]phosphatidylinositol 4,5-bisphosphate ([32P]PIP2) stimulated by 1 microM substance P significantly increased from 5 to 20 min and returned to basal levels by 30 min; however, the breakdown of [32P]PIP2 was greater than that of [32P]PIP. At a low concentration of substance P, [32P]PIP2 breakdown reached maximal levels at 5 min followed by a progressive decrease and returned to basal levels at 30 min, whereas the breakdown of [32P]PIP reached maximal levels at 5 min and returned to near basal levels at 10 min. Both concentrations of substance P caused some [32P]phosphatidylinositol breakdown at 5 min. Changes in [3H]inositol trisphosphate induced by substance P were similar to those in [32P]PIP2. In addition, substance P (1 microM) did not stimulate the release of [3H]choline or [3H]ethanolamine metabolites into the medium. Substance P-induced DAG formation was not inhibited by staurosporine, a protein kinase C inhibitor. These results suggest that DAG formation caused by substance P is closely associated with the hydrolysis of phosphatidylinositides but not that of phosphatidylcholine or phosphatidylethanolamine, and is not regulated by protein kinase C-dependent mechanism(s).