Protease-activated receptor-1 in human lung fibroblasts mediates a negative feedback downregulation via prostaglandin E2. Among the four protease-activated receptors (PARs), PAR-1 plays an important role in normal lung functioning and in the development of lung diseases, including fibrosis. We compared the expression and functional activity of PARs in normal and fibrotic human lung fibroblasts. Both normal and fibrotic cells express PAR-1, -2, and -3, with PAR-2 showing the lowest level. There

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... as no significant difference between normal and fibrotic fibroblasts in expression levels of PAR-1 and PAR-3, whereas a fourfold higher expression level of PAR-2 was observed in fibrotic cells compared with normal cells. Ca 2ϩ imaging studies revealed apparently only PAR-1-induced Ca 2ϩ signaling in lung fibroblasts. PAR-1 agonists, thrombin and synthetic activating peptide, induced concentrationdependent Ca 2ϩ mobilization with EC50 values of 5 nM and 1 M, respectively. The neutrophil protease cathepsin G produced a transient Ca 2ϩ response followed by disabling PAR-1, whereas elastase did not affect Ca 2ϩ level. PAR-1 activation by thrombin or receptor-activating peptide downregulated expression of all three PARs in lung fibroblasts, with maximal effect at 3-6 h, whereas expression returned toward basal level after 24 h. Furthermore, PAR-1 agonists dose dependently increased PGE2 secretion from lung fibroblasts and induction of cyclooxygenase-2 expression. We then found that PGE2 downregulated expression of all three PARs. The effect of PGE2 was continuously growing with time. Furthermore, PGE2 exerts its effect through the EP2 receptor that was confirmed using the selective EP2 agonist butaprost. This novel autocrine feedback mechanism of PGE2 in lung fibroblasts seems to be an important regulator in lung physiology and pathology. thrombin; cyclooxygenase; cyclic adenosine 5Ј-monophosphate; prostanoid * E. Sokolova and Z. Grishina contributed equally to this work.