Secretory Non‐Pancreatic Phospholipase A2 and cyclooxygenase‐2 Expression by Tracheobronchial Smooth Muscle Cells

Abstract

Lipid mediators of inflammation, contribute to airway hyper-reactivity in asthma. Since production of lipid mediators is largely regulated by phospholipase A₂ (PLA₂), and since PLA₂ expression in mesenchymal cells is induced by cytokines and other signals, we examined PLA₂ expression by rat tracheobronchial smooth muscle cells (TBSMC). PLA₂ expression in TBSMC cultures was markedly increased by tumour-necrosis factor (TNF)α (130-fold) and interleukin-1β (IL-1β) (7.4-fold). Lipopolysaccharide (LPS; 100 ng/ml) resulted in a 51-fold increase in extracellular PLA₂ activity. PLA, expression by LPS-stimulated or cytokine-stimulated cells was downregulated by dexamethasone. Whereas forskolin or dibutyrl CAMP increased PLA₂ activity, inhibition of protein kinase A but not tyrosine kinase reduced PLA₂ expression. Northern blot analysis showed that TNFα and IL-1β increased both PLA₂ and inducible cyclooxygenase (Cox-2) mRNA transcription. Addition of dexamethasone substantially blunted the increase in PLA₂ and Cox-2 mRNA. In contrast, the level of Cox-1 mRNA was very low and did not change with the various treatments. Since proinflammatory lipid mediators have been implicated in the pathogenesis of asthma and PLA₂ activity regulates generation of these lipid mediators, cytokine-stimulated synthesis and release of PLA₂ by airway smooth cells may contribute to the potentiation of airway inflammation in asthma.