Phosphatidylinositol 3-kinase mediates mitogen-induced human airway smoothmuscle cell proliferation

Hypertrophy and hyperplasia of airway smooth muscle (ASM) are important pat hological features that contribute to airflow obstruction in chronic severe asthma. Despite considerable research effort, the cellular mechanisms that modulate ASM growth remain unknown. Recent evidence suggests that mitogen- induced activation of phosphoinositide (PI)-specific phospholipase C (PLC) and PI-dependent calcium mobilization are neither sufficient nor necessary to stimulate human ASM proliferation. In this study, we identify phosphatid ylinositol (PtdIns) 3-kinase as a key regulator of human ASM proliferation. Pretreatment of human ASM with the PtdIns 3-kinase inhibitors wortmannin a nd LY-294002 significantly reduced thrombin- and epidermal growth factor (E GF)-induced DNA synthesis (IC50 similar to 10 nM and similar to 3 mu M, res pectively). In separate experiments, wortmannin and LY-294002 markedly inhi bited PtdIns 3-kinase and 70-kDa S6 protein kinase (pp(70S6k)) activation i nduced by stimulation of human ASM cells with EGF and thrombin but had no e ffect on EGF- and thrombin-induced p42/p44 mitogen-activated protein kinase (MAPK) activation. The specificity of wortmannin and LY-294002 was further suggested by the demonstrated inability of these compounds to alter thromb in-induced calcium transients, total PI hydrolysis, or basal cAMP levels. T ransient expression of constitutively active PtdIns 3-kinase (p110*) activa ted pp(70S6k), whereas a dominant-negative PtdIns 3-kinase (Delta p85) bloc ked EGF- and thrombin-stimulated pp(70S6k) activity. Collectively, these da ta suggest that activation of PtdIns S-kinase is required for the mitogenic effect of EGF and thrombin in human ASM cells. Further investigation of th e role of PtdIns 3-kinase may offer new therapeutic approaches in the treat ment of diseases characterized by smooth muscle cell hyperplasia such as as thma and chronic bronchitis.