Thrombin induces DNA synthesis and phosphoinositide hydrolysis in airway smooth muscle by activation of distinct receptors

Chronic airway inflammation induces numerous structural changes of the airways involving hypertrophy and hyperplasia of airway smooth muscle (ASM). Thrombin has been identified in the bronchoalveolar lavage fluid of asthmatic subjects and displays potent bronchoconstrictor and mitogenic activity towards ASM. This study has addressed which proteinase-activated receptors (PARs) and signalling pathways are involved in mediating distinct effects of thrombin. Using cultured bovine tracheal smooth muscle (BTSM) cells as a model system, thrombin stimulated a marked increase in [³H]inositol phosphate ([³H]InsPs) accumulation, which was fully mimicked by a selective PAR1 activating peptide. In contrast, PAR1, PAR2, PAR3 and PAR4 activating peptides were unable to replicate the ability of thrombin to stimulate DNA synthesis as assessed by [³H]thymidine incorporation. Further investigation demonstrated that the mitogenic effect of thrombin did not involve stimulation of PDGF secretion but did involve activation of PDGF or EGF receptors and a G_i/o-dependent activation of phosphoinositide 3-kinase. Thrombin, but not the PAR1, PAR2, PAR3 or PAR4 activating peptides was able to stimulate PtdIns(3,4,5)P₃ mass accumulation. PAR3 antisense oligonucleotides substantially inhibit thrombin-stimulated [³H]thymidine incorporation and PtdIns(3,4,5)P₃ generation but had no effect on thrombin-induced phosphoinositide hydrolysis. These data indicate that while PI hydrolysis and Ca²⁺ mobilisation induced by thrombin operates via PAR1-dependent activation of phospholipase C, phosphoinositide 3-kinase activation and DNA synthesis occurs via a distinct proteinase-activated receptor pathway, possibly involving PAR3.