Cross Talk between c-Met and Epidermal Growth Factor Receptor during Retinal Pigment Epithelial Wound Healing

In response to pathologic conditions, retinal pigment epithelial (RPE) cells initiate a wound-healing process and become transformed from a stationary epithelial state to a migratory and proliferative mesenchymal state, leading to the epiretinal membrane formation associated with the development of proliferative vitreoretinopathy (PVR).1 It is thought that activation of several autocrine or paracrine loops by growth factors and their receptors is critical for RPE transformation and PVR progression.2 Prominent among these factors are hepatocyte growth factor (HGF)/scatter factor (SF) and the epidermal growth factor (EGF) family. HGF is involved in cell scattering and migration and from epithelial to mesenchymal transition (EMT).3,4 The EGF receptor tyrosine kinase (RTK) family has been characterized in many cell systems, including RPE,5-7 and is known to participate in a wide variety of biological responses, including cell migration, proliferation, and differentiation. HGF is a multipotential cytokine that has been implicated in diverse events in organ development, tissue maintenance and homeostasis, and wound healing. At the cellular level, HGF can promote other bioactivities, such as junctional breakdown, cell scattering, migration, cell survival, and invasive behavior.8,9 HGF is thought to be synthesized by mesenchymally derived cells, typically fibroblasts, which primarily target epithelial cells in a paracrine manner through c-Met, the only known receptor for HGF that mediates all HGF-induced biological activities.8,10,11 c-Met consists of an α/β heterodimer at the cell surface, with α as an extracellular subunit and β as a subunit containing an extracellular domain, a membrane-spanning domain, and a cytoplasmic tyrosine kinase domain.12 On HGF stimulation, the c-Met receptor is tyrosine phosphorylated; this is followed by the recruitment of a group of signaling molecules, adaptor proteins, or both to its cytoplasmic domain and to its multiple docking sites. This action leads to the activation of several different signaling cascades, including extracellular signal-regulated kinase (ERK) of the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3′-kinase (PI3K), that form a signaling network of intracellular and extracellular responses. Unlike HGF, the EGFR ligand family of growth factors consists of more than 10 members, including EGF13 and HB-EGF.14 These factors act through the stimulation of specific cell-surface receptors of the erbB or EGFR family. There are four related RTKs: EGFR/erbB1, erbB2, erbB3, and erbB4.15-18 Activation of erbBs, similar to c-Met, elicits myriad signaling events, including ERK and PI3K.19-21 EGFR ligand stimulation promotes RPE cell proliferation and survival, signaling through both ERK/MAPK and PI3K pathways.5,6 Recently, HB-EGF has been implicated in driving the uncontrolled wound-healing process of the retina during proliferative retinopathy.7 Although various reactions have been described, wounding or breakdown of the tight junction barrier in vivo results in the availability of circular or otherwise segregated22 growth factors, such as HGF and EGFR ligands to their receptors, leading to the initiation of a wound healing response. Hence, the multiplicity of cell surface receptors activated by endogenous signals is contrasted by the relative uniformity of intracellular signaling pathways triggered by these receptors. In particular, the activation of EGFR and c-Met may elicit similar signal transduction pathways in cells. Thus, cross talk of these growth factor receptors may affect the strength, duration, or both of shared downstream signaling pathways. Whether c-Met and EGFR influence each other’s activity and how the cross talk between these RTKs determines cell signaling remains to be fully explored. Hence, we investigated the role of HGF and HB-EGF in mediating RPE wound healing and the cross talk between these two growth factors using cultured human ARPE-19 cells.