Porphyromonas gingivalis lipopolysaccharide-induced cytosolic phospholipase A₂ activation interferes with salivary mucin synthesis via platelet activating factor generation

Liberation of arachidonate from membrane phospholipids by cytosolic phospholipase A₂ (cPLA₂) upon cell activation is considered the key step in generation of platelet-activating factor (PAF), a potent lipid messenger recognized as the most proximal mediator of inflammatory events triggered by bacterial infection. Here, we report on the role of cPLA₂ in the disturbances in salivary mucin synthesis evoked by the lipopolysaccharide (LPS) of a periodontopathic bacterium, P. gingivalis. Using mucous cells of sublingual gland, we show that P. gingivalis LPS detrimental effect on salivary mucin synthesis, associated with up-regulation in PAF and endothelin-1 (ET-1) generation, was subject to suppression by a specific inhibitor of cPLA₂, MAFP. Moreover, the LPS-induced changes in mucin synthesis and ET-1 generation were countered by PAF receptor antagonist, BN52020. The inhibition by PAF antagonist of the LPS-induced reduction in mucin synthesis was countered by wortmannin, an inhibitor of PI3K, as well as by ERK inhibitor, PD98059. The blockade of ERK caused also inhibition of the LPS-induced cPLA₂ activation and amplification in the impedance capacity of PAF antagonist on the LPS-induced ET-1 generation, while the inhibitor of PI3K had no effect. The findings are the first to demonstrate that P. gingivalis LPS detrimental effect on salivary mucin synthesis involves ERK-dependent cPLA₂ activation that leads to up-regulation in PAF production and ET-1 generation. We also show that PAF receptor activation is a critical prerequisite for the LPS-induced ET-1 production.