Calcium-dependent biosynthesis of platelet-activating factor by submandibular gland cells

PAF, a putative mediator of oral inflammation

PAF, or platelet-activating factor, is a family of structurally related phospholipids (1-O-alkyl/acyl/alkenyl-2-acetyl-sn-glycero-3-phosphocholine) which possesses a wide spectrum of potent pro-inflammatory actions. These phospholipids are synthesized by a diverse array of cells, including neutrophilic polymorphonuclear leukocytes (PMN), platelets, mast cells, monocytes/macrophages, vascular endothelial cells, and lymphocytes. PAF targets these and other cells via specific, G-protein-coupled receptors to initiate intracrine, autocrine, paracrine, and juxtacrine cell activation. Of importance, these unique acetylated phospholipids are frequently synthesized in concert with pro-inflammatory lipid mediators derived from arachidonic acid. Since PAF synergizes with these and other mediators to amplify the inflammatory response, it seems likely that PAF plays an integral, perhaps pivotal, role in acute and chronic inflammatory processes. PAF is present in the mixed saliva of dentate, but not edentulous, human subjects. The levels of PAF in mixed saliva or in gingival crevicular fluid and tissues are significantly increased during oral inflammatory conditions such as periodontitis and mucositis. Interestingly, the levels of salivary PAF correlate with the extent/severity of these oral diseases. These observations suggest that PAF may participate in pathophysiologic events during the course of oral inflammation. The availability of specific PAF receptor antagonists and human recombinant PAF-acetylhydrolase (PAF-AH), a plasma enzyme which rapidly destroys PAF, should provide clinical tools for the investigation of the role of PAF in these and other inflammatory disorders; and perhaps, ultimately, some of these reagents may prove to be therapeutically useful in the treatment and management of these conditions.

Cold ischemia in the absence of alloreactivity induces chronic transplant nephropathy through a process mediated by the platelet-activating factor

BACKGROUND

Ischemia-reperfusion injury is considered a risk factor for the development of chronic transplant nephropathy (CTN) although the mechanisms that mediate its effects have not been completely established. We have previously shown that treatment with a platelet-activating factor (PAF) receptor antagonist (UR12670) protected kidneys from the progression to chronic nephropathy induced by warm ischemia. Here we examine the contribution of cold ischemia to the development of late functional and structural kidney changes in rats subjected to syngeneic renal transplantation and the role of PAF in this chronic nephropathy.

SUBJECTS AND METHODS

Lewis rats were used as kidney donors and recipients, which were transplanted either immediately or after a cold ischemia period of 5 hr. Contralateral nephrectomy was performed on the seventh day after transplantation. Cyclosporine was administered for 15 days after transplantation. Groups were as follows: Sy, immediate transplantation; SyI, transplantation after 5 hr of cold ischemia; SyIUr, transplantation after 5 hr of cold ischemia plus UR12670 from the transplantation day to the end of the study, at 24 weeks. Serum creatinine, creatinine clearance, and proteinuria were determined every 4 weeks. Urinary

Mediation by platelet-activating factor of 12-hydroxyeicosatetraenoic acid-induced cytosolic free calcium concentration elevation in neutrophils

12(R)-hydroxyeicosatetraenoic acid (HETE) shows biphasic increase in cytosolic free calcium concentration ([Ca2+]i) in rabbit and human neutrophils; the initial transient phase and the continuous falling phase. 12(S)-HETE was less potent in both species. BN50739, a platelet-activating factor (PAF) receptor antagonist, inhibited both phases of 12(R)-HETE-induced [Ca2+]i rise but did not affect leukotriene B4 (LTB4)-induced [Ca2+]i rise. N-tosyl-L-phenylalanine chloromethyl ketone (TPCK), a PAF synthesis inhibitor, and manoalide, a phospholipase A2 inhibitor, reduced 12(R)-HETE-induced [Ca2+]i rise. These blockers inhibited the continuous phase of [Ca2+]i rise induced by N-formyl-methionyl-leucyl-phenylalanine (FMLP) with little effect on the initial phase. It had no significant effect on LTB4-induced [Ca2+]i rise. SC-41930, a LTB4-receptor antagonist, did not block 12-HETE-induced [Ca2+]i rise. In 12(R)-HETE-, FMLP- and LTB4-stimulated cells, accumulations of cell-associated PAF and released PAF were detected but not in unstimulated cells. BN50739 did not affect the accumulation of cell-associated PAF and release of PAF in 12(R)-HETE-stimulated cells. These results suggest that 12(R)-HETE-induced and partially, FMLP-induced, but not LTB4-induced [Ca2+]i rise are mediated by PAF, which is produced and released by stimulation of the cells by 12(R)-HETE and FMLP, respectively.

Cyclic ADP-ribose induces Ca2+ release from caffeine-insensitive Ca2+ pools in canine salivary gland cells

Cyclic ADP-ribose (cADPR), a novel putative messenger of the ryanodine receptor, was examined regarding its ability to mobilize Ca2+ from intracellular Ca2+ stores in isolated cells of parotid and submandibular glands of the dog. cADPR induced a rapid and transient Ca2+ release in the digitonin-permeabilized cells of salivary glands. cADPR-induced Ca2+ release was inhibited by ryanodine receptor antagonists ruthenium red, ryanodine, benzocaine, and imperatoxin inhibitor but not by the inositol 1,4,5-trisphosphate (IP3)-receptor antagonist heparin. Thapsigargin, at a concentration of 3 to 30 microM, inhibited IP3-induced Ca2+ release, while higher concentrations were required to inhibit cADPR-induced Ca2+ release. Cross-potentiation was observed between cADPR and ryanodine or SrCl2, suggesting that cADPR sensitizes the Ca2+-induced Ca2+ release mechanism. Cyclic AMP plays a stimulatory role on cADPR- and IP3-induced Ca2+ release in digitonin-permeabilized cells. Calmodulin also potentiated cADPR-induced Ca2+ release, but inhibited IP3-induced Ca2+ release. Acetylcholine and ryanodine caused the rise in intracellular free Ca2+ concentration ([Ca2+]i) in intact submandibular and parotid cells. Caffeine did not produce any increase in Ca2+ release or [Ca2+]i rise in any preparation. ADP-ribosyl cyclase activity was found in the centrifuged particulate fractions of the salivary glands. These results suggest that cADPR serves as an endogenous modulator of Ca2+ release from Ca2+ pools through a caffeine-insensitive ryanodine receptor channel, which are different from IP3-sensitive pools in canine salivary gland cells. This system is positively regulated by cyclic AMP and calmodulin.

Inhibition of Na+,K(+)-ATPase by platelet-activating factor in dog submandibular glands

Physiological stimulation of dog submandibular gland has been shown to generate platelet-activating factor (PAF). However, PAF is not released from cells in the tissue. To assess its intracellular activity, the effect of PAF on Na+,K(+)-ATPase was examined in dog submandibular gland cells. PAF inhibited Na+,K(+)-ATPase in membrane preparations, and the inhibitory effect was dependent on the protein concentration in the enzyme preparation. The inhibitory effect of a low concentration of PAF was antagonized by a PAF-receptor antagonist, BN 50,739, but at high concentrations, PAF was not antagonized. Kinetic analysis of PAF inhibition of Na+,K(+)-ATPase suggests that the inhibition of Na+,K(+)-ATPase by PAF is not due to competition by PAF at K(+)- or Na(+)-binding sites on the enzyme, but by complex inhibitory mechanisms. These results suggest that PAF may interact with specific and nonspecific site of action resulting in the inhibition of Na+,K(+)-ATPase. Ouabain increased mucin release from dog submandibular gland cells. Because Na+,K(+)-ATPase and ion exchange pathways are important in the secretory responses of acinar cells, PAF may regulate intracellularly the secretory function of acinar cells by modulating Na+,K(+)-ATPase and ionic homeostasis.

Stimulation of platelet-activating factor synthesis by neurotransmitters in salivary glands

Platelet-activating factor (PAF), a phospholipid mediator exhibiting potent biological activities, has been shown to stimulate amylase release from the pancreas and salivary glands. The capacity of salivary glands for PAF biosynthesis in response to stimulation has also been demonstrated. To elucidate the role of PAF in salivary glands, we studied the regulation of platelet-activating factor synthesis by the autonomic nervous system in canine salivary glands. Acetylcholine and ionomycin stimulated PAF production in dispersed cells from parotid, submandibular, and sublingual glands of dogs. Norepinephrine and phenylephrine, but not isoproterenol, also stimulated PAF production in submandibular gland cells. Norepinephrine-induced PAF production was blocked by phentolamine but not by propranolol. Acetylcholine and norepinephrine increased both the PAF production and liberation of [14C]arachidonic acid from cells pre-labeled with [14C]arachidonic acid in the presence of Ca2+ in the medium. These stimulants increased [14C]arachidonic acid liberation without the accompanying production of PAF in Ca(2+)-deprived medium. No activators or inhibitors of protein kinase C produced or affected acetylcholine-induced PAF production. Lyso-PAF:acetyl-CoA acetyltransferase was activated in the cells treated with acetylcholine, norepinephrine, isoproterenol, and 8Br-cyclic AMP. Deprivation of Ca2+ in the medium markedly reduced acetylcholine-induced activation of the transferase, but little affected norepinephrine-, isoproterenol-, and 8Br-cyclic AMP-induced activation. Dithiothreitol-insensitive cholinephosphotransferase activity was also increased by acetylcholine, norepinephrine, isoproterenol, and 8Br-cyclic AMP, and the deprivation of Ca2+ in the medium further increased the activation of the enzyme activity by these agents. These results suggest that PAF synthesis in canine salivary glands is under the control of muscarinic cholinergic and alpha-adrenergic systems via Ca(2+)-dependent remodeling pathways, and that the independent activation of either phospholipase A2 or acetyltransferase is insufficient for PAF production in submandibular gland cells, i.e., the concurrent activation of these enzymes is required.

The effect of initial periodontal therapy on salivary platelet-activating factor levels in chronic adult periodontitis

Platelet-activating factor (PAF), a potent phospholipid inflammatory mediator, is increased in the mixed saliva of subjects with periodontal disease and correlates with the extent of oral inflammation. The present study was designed to provide a longitudinal evaluation of the effect of initial periodontal therapy (home care instruction, prophylaxis, and scaling/root planing) on salivary PAF levels in chronic adult periodontitis patients (n = 15). Mixed saliva was collected prior to, during, and after initial therapy and was utilized to assess PAF levels after lipid extraction and fractionation as well as to histologically assess the number of polymorphonuclear leukocytes (PMN). PAF activity was determined in bioassay relative to authentic PAF (1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine; 16:0-alkyl-PAF). Initial salivary PAF levels (12.1 +/- 2.8 pmole equivalents of 16:0-alkyl-PAF/ml saliva; mean +/- SE) decreased following supragingival plaque control (9.6 +/- 2.4) and were further reduced following scaling and root planing (5.7 +/- 1.4). In parallel, salivary PMN levels were significantly reduced and clinical estimates of periodontal disease were significantly improved; i.e., there was a decrease in the percentage of sites with both bleeding on probing (from 46.1 +/- 4.6% of sites at pretreatment to 25.9 +/- 2.6% after scaling and root planing) and probing depths > or = 4 mm (from 16.7 +/- 1.9% of sites to 10.3 +/- 1.2%). Thus, initial periodontal therapy reduced salivary PAF levels in concert with improvements in clinical estimates of marginal and submarginal periodontal inflammation suggesting that PAF may participate in inflammatory events during periodontal tissue injury and disease.

Platelet-activating factor mediated potentiation of stimulation-evoked catecholamine release and the rise in intracellular free Ca2+ concentration in adrenal chromaffin cells

The effects of platelet-activating factor (PAF) on catecholamine (CA) release and intracellular free Ca2+ concentration ([Ca2+]i) were studied in cultured bovine adrenal chromaffin cells. PAF (1 nM-1 micron) alone had no effect on [Ca2+]i and basal CA release, but potentiated the [Ca2+]i rise and CA release evoked by acetylcholine (ACh) and by elevated extracellular K+. PAF did not potentiate the responses to caffeine in Ca(2+)-deficient medium or to Bay K 8644. In chromaffin cells pretreated with either BN 50739, tetrodotoxin and amiloride or in Na(+)-deficient medium, PAF failed to potentiate the stimulation-evoked [Ca2+]i rise and CA release. In contrast, neomycin, U 73122, 5-(N-ethyl-N-isopropyl)amiloride or pertussis toxin were ineffective in blocking the potentiating effect of PAF. In a membrane fraction prepared from fresh bovine adrenal medulla, ligand-binding studies using [3H]WEB 2086 identified a PAF-displaceable binding site. These results are consistent with a model in which PAF modulates CA release by activating plasma membrane receptors that can enhance the [Ca2+]i rise via an Na(+)-dependent mechanism.

Salivary PAF levels correlate with the severity of periodontal inflammation

Platelet-activating factor (PAF), a potent phospholipid mediator of inflammation, has been previously identified in inflamed gingival tissues and gingival crevicular fluid. However, the role of PAF in oral pathobiology remains unknown. The purpose of the present study was to evaluate the relationship between salivary PAF levels and the severity of periodontal inflammation. PAF activity in lipid extracts of whole (mixed) saliva collected from 69 untreated subjects immediately prior to routine oral evaluation was determined by platelet bioassay. Significant positive correlations were observed between the level of PAF in saliva and measures of periodontal inflammation, i.e., the percentage of periodontal probing depths greater than 4 mm, the number of periodontal bleeding sites, and the number of histologically identified polymorphonuclear neutrophilic leukocytes (PMN) in saliva. Moreover, when subjects were subdivided into groups on the basis of periodontal probing depths, a significant correlation was observed between salivary PAF levels and the extent of periodontal disease, i.e., PAF levels in saliva progressively increased from the healthiest group to the most severely affected group. Thus, salivary PAF levels correlate with the severity of periodontal inflammation. These results support the hypothesis that this pro-inflammatory phospholipid autacoid may participate in the pathogenesis of periodontal tissue injury and disease.

Calcium dependency of adrenergic and muscarinic cholinergic stimulation of mucin release from dog submandibular gland cells

Stimulation of muscarinic cholinergic, alpha-adrenergic and beta-adrenergic receptors elicited mucin release from dispersed dog submandibular cells. The secretory response to acetylcholine was much more pronounced than to adrenergic agonists, and largely dependent on the presence of extracellular Ca2+, but the dependency on extracellular Na+ was slight. Ionomycin also stimulated mucin release. In rat submandibular cells, neither muscarinic cholinergic agonists nor ionomycin were as effective mucosecretagogues as beta-adrenergic agonists. alpha-Adrenoceptor-mediated release was decreased by chelating extracellular Ca2+ with EGTA. The beta-adrenoceptor-mediated response was diminished by extensive exposure of cells to EGTA, due at least in part to the requirement of Ca2+ for beta-adrenoceptor stimulation of cAMP formation. 8-br-cAMP stimulated 45Ca2+ release from cells preloaded with 45Ca2+. The 8-br-cAMP-induced mucin release was eliminated in ionomycin-pretreated cells, but not inhibited by chelating extracellular Ca2+ and by the treatment of the cells with TMB-8 or in the cells loaded with BAPTA. These results suggest that not only the adrenergic system but also the muscarinic cholinergic system may participate in the regulation of mucin release in dog submandibular gland, and also provide the possibility that, in addition to a cAMP-mediated mechanism, Ca(2+)-dependent mechanisms may be involved in mucosecretion in dog submandibular acini.