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

Abstract

Growth arrest-specific protein 6 (Gas6)/Mer receptor tyrosine kinase (Mer) signaling modulates cytokine secretion and helps to regulate the immune response and apoptotic cell clearance. Signaling pathways that activate 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 hepatic growth factor (HGF) in macrophages, which ultimately promotes epithelial cell proliferation and wound repair. The RhoA/protein kinase B (Akt)/mitogen-activated protein (MAP) kinases, including p38 MAP kinase, extracellular signal-regulated protein kinase, and Jun NH 2 -terminal kinase axis in RAW 264.7 cells, was identified as Gas6/Mer downstream signaling pathway for the upregulation of HGF mRNA and protein. Conditioned medium from RAW 264.7 cells that had been exposed to Gas6 or apoptotic cells enhanced epithelial cell proliferation of the epithelial cell line LA-4 and wound closure. Cotreatment with an HGF receptor-blocking antibody or c-Met antagonist downregulated this enhancement. Inhibition of Mer with small interfering RNA (siRNA) or the RhoA/Rho kinase pathway by RhoA siRNA or Rho kinase pharmacologic 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 reprogrammed by Gas6 to promote epithelial proliferation and wound repair via HGF, which is induced by the Mer/RhoA/ Akt/MAP kinase pathway. Thus, defects in Gas6/Mer/RhoA signaling in macrophages may delay tissue repair after injury to the alveolar epithelium.

Gas6 Prevents Epithelial-Mesenchymal Transition in Alveolar Epithelial Cells via Production of PGE2, PGD2 and Their Receptors

“…The COX-2/PGE 2 and PGD 2 pathways result in the inhibition of EMT in lung and renal cells [25,34]. Thus, we hypothesized that PGE 2 and PGD 2 secretion by Gas6 mediates anti-EMT effects in an autocrine/paracrine manner.…”

“…These results highlight the importance of the TAM signaling for modulation of the innate immune response. In addition, we reported that macrophages can be reprogrammed by Gas6 to promote EC proliferation and wound repair via HGF, which is induced by the Mer pathway in macrophages [25]. Gas6/Axl signaling plays an important role in the control of lens EC growth and survival [26].…”

Macrophages programmed by apoptotic cells inhibit epithelial-mesenchymal transition in lung alveolar epithelial cells via PGE2, PGD2, and HGF

“…Programming of macrophages by apoptotic cells may influence epithelial cell homeostasis during lung injury 28 42 . Recent studies provide evidence that TGF-β1-induced EMT of alveolar epithelial cells may contribute to the formation of myofibroblasts in murine fibrotic lungs and IPF patient lungs 43 44 45 46 47 48 .…”

“…Mice were cared for and handled in accordance with the National Institute of Health (NIH) Guide for the Care and Use of Laboratory Animals. Mouse pharyngeal aspiration was used to administer a test solution containing bleomycin (5 U/kg body weight in 30 μl) 42 60 . Two days after bleomycin treatment, saline alone or 1 × 10 7 apoptotic or viable Jurkat cells in 50 μl saline were administered intratracheally through pharyngeal aspiration 61 62 .…”

Targeting Tyro3, Axl and MerTK (TAM receptors): implications for macrophages in the tumor microenvironment

“…Shown in RAW 264.7 cells, this increase in HGF production was found to be mediated by MerTK (but not Axl or Tyro3) activation via RhoA-mediated signaling [116, 117]. Gas6/MerTK stimulated HGF production was found involve the same signaling pathway and was able to stimulate wound repair and cell growth of LA-4 epithelial cells [118]. While these studies speculated implications with alveolar macrophages, it remains of interest if these findings can translate to tumor associated macrophages since HGF can promote disease through HGF/c-Met signaling in a number of cancers [119].…”