RhoA/Phosphatidylinositol 3-Kinase/Protein Kinase B/Mitogen-Activated Protein Kinase Signaling after Growth Arrest–Specific Protein 6/Mer Receptor Tyrosine Kinase Engagement Promotes Epithelial Cell Growth and Wound Repair via Upregulation of Hepatocyte Growth Factor in Macrophages

Growth arrest-specific protein 6 (Gas6)/Mer signaling modulates cytokine secretion and helps to regulate the immune response and apoptotic cell clearance. Signaling pathways activating an epithelial growth program in macrophages are still poorly defined. We report that Gas6/Mer/RhoA signaling can induce the production of epithelial growth factor HGF in macrophages which ultimately promotes epithelial cell proliferation and wound repair. The RhoA/Akt/ mitogen-activated protein (MAP) kinases, including, p38 MAP kinase, extracellular signal-regulated protein kinase (ERK), and Jun NH2-terminal kinase (JNK), axis in RAW 264.7 cells was identified as Gas6/Mer downstream signaling pathway for the up-regulation of HGF mRNA and protein. Conditioned medium from RAW 264.7 cells that had been exposed to Gas6 or apoptotic cells enhanced LA-4 epithelial cell proliferation and wound closure. Co-treatment with an HGF receptor-blocking antibody or c-Met antagonist was found to down-regulate this enhancement. Inhibition of Mer with siRNA or the RhoA/Rho kinase pathway by RhoA siRNA or Rho kinase pharmacological inhibitor suppressed Gas6-induced HGF mRNA and protein expression in macrophages and blocked epithelial cell proliferation and wound closure induced by the conditioned medium. Our data provide evidence that macrophages can be re-programmed by Gas6 to promote epithelial proliferation and wound repair via HGF, which is induced by Mer/RhoA/Akt/MAP kinases pathway. Thus, defects in Gas6/Mer/RhoA signaling in macrophages may delay tissue repair after injury to the alveolar epithelium.