| Lysophosphatidic acid (LPA) is a simple endogenous phospholipid released by activated platelets which has mitogenic activity on multiple cell types. Effects of LPA on lung cells suggest that LPA may play a role in airway remodeling seen in diseases like asthma. LPA stimulates mitogenesis of cultured human airway smooth muscle (HASM) cells, and treatment of HASM cells with LPA together with epidermal growth factor (EGF) results in synergistic stimulation of mitogenesis. The integration of growth factor responses to LPA, a G protein-coupled (GPC) mitogen, and EGF, a receptor tyrosine kinase (RTK) mitogen, is the focus of these studies.; Specificity studies indicated that synergism occurs with various other GPC mitogens plus EGF, and with other RTK mitogens plus LPA. Additionally synergism appeared to be a feature of smooth muscle cells. Use of pharmacological inhibitors of signaling pathways implicated mitogen-activated protein kinase (MAPK), protein kinase C, phosphatidylinositol 3-kinase, and p70 ribosomal S6 kinase in mitogenic signaling by both LPA and EGF. Additionally, LPA-mediated mitogenic signaling required Gi activation. Activation of Rho was required for synergistic stimulation.; Use of HASM cells infected with luciferase reporters identified AP-1 as a transcription factor synergistically activated by LPA plus EGF. Additionally, LPA activated CRE, NFAT, NFκB, and SRE; EGF activated CRE, NFAT and SRE; activation of CRE, NFAT and SRE by LPA plus EGF was additive. NFκB activation and synergistic AP-1 activation both required Rho activation, consistent with signaling requirements for mitogenesis.; LPA also induced EGFR upregulation through transcriptional regulation, with properties similar to those for synergism between LPA and EGF. However, LPA did not induce EGFR transactivation in HASM cells. Together these studies implicate Rho activation, AP-1-mediated signaling, and EGFR regulation as potential components of the mechanism of synergism. Because cells respond to multiple stimuli in vivo, these studies shed light on physiologically relevant mitogenic signal integration. Understanding the regulation of airway smooth muscle proliferation should be helpful in preventing airway smooth muscle thickening seen in airway remodeling and asthma. |
