Platelet-derived growth factor-BB and thrombin activate phosphoinositide 3-kinase and protein kinase B: Role in mediating airway smooth muscle proliferation

Proliferation of airway smooth muscle results from persistent inflammatory cytokine and growth factor stimulation and is a critical component of airwa y luminal narrowing in chronic asthma. Using primary cultures of bovine tra cheal smooth muscle (BTSM) cells to examine the signaling basis of cell pro liferation, platelet-derived growth factor (PDGF)-BB and thrombin (which ac t through distinct receptor types) were found to induce DNA synthesis in BT SM cells. Mitogen-induced DNA synthesis could be completely inhibited by LY 294002, a selective phosphoinositide 3-kinase (Ptdlns 3-kinase) inhibitor. Exposure of BTSM cells to PDGF-BB or thrombin resulted in rapid activation of Ptdlns 3-kinase and accumulation of phosphoinositide-3,4,5-trisphosphate . Protein kinase B, a novel signaling protein kinase, was identified in BTS M cells and was activated by PDGF-BB and thrombin in a Ptdlns 3-kinase-depe ndent manner; this may underlie mitogen-stimulated activation of p70(s6k). PD98059, a mitogen-activated protein kinase kinase 1 inhibitor, also partia lly inhibited PDGF-BB- and thrombin-stimulated DNA synthesis, indicating a modulatory role for mitogen-activated protein kinase in proliferation. GF10 9203X, Ro 31-8220, calphostin C, and chelerythrine (selective protein kinas e C inhibitors) had no effect on PDGF-BB- or thrombin-stimulated DNA synthe sis, suggesting that, despite abolishment of mitogen-stimulated protein kin ase C activity, cell proliferation stimulated by PDGF-BB and thrombin is pr otein kinase C-independent. These data demonstrate that the Ptdlns 3-kinase /protein kinase B pathway represents a key signaling route in airway smooth muscle proliferation, with the mitogen-activated protein kinase kinase 1/m itogen-activated protein kinase cascade providing a complementary signal re quired for the full mitogenic response.