Fibrillin-1 Is a Novel Ligand of Epidermal Growth Factor (EGF) Receptor That Promotes Mesangial Cell Activation and Glomerulosclerosis

Background: Podocyte injury and mesangial activation are hallmarks of a wide variety of glomerular diseases. Although podocyte damage is thought to be an early event that causes subsequent mesangial activation, the mechanism connecting these two events remain poorly understood. In this study, we demonstrated that injured podocytes trigger mesangial cell activation and proliferation by secreting extracellular matrix glycoprotein fibrillin-1 (FBN1). Methods: Expression of FBN1 was assessed by Western blotting and immunohistochemical staining. Mice with podocyte-specific knockout of FBN1 gene were generated by Cre-LoxP system. The binding of FBN1 to EGF receptor (EGFR) was investigated by molecular docking, co-immunoprecipitation and cellular thermal shift assay. Results: Podocyte FBN1 was upregulated in various animal models of glomerular disease and in patients with proteinuric kidney disorders. FBN1 stimulated mesangial cell activation and proliferation both in vitro and in glomerular mini-organ cultures. Overexpression of FBN1 promoted mesangial cell activation and aggravated glomerulosclerosis in Adriamycin (ADR) nephropathy. Conversely, podocyte-specific knockout of FBN1 gene attenuated mesangial activation and glomerulosclerosis after ADR injury. RNA sequencing, molecular docking simulation and protein interactions analyses demonstrated that FBN1 could directly bind to epidermal growth factor receptor (EGFR), which resulted in a sequential phosphorylation and activation of EGFR, phosphoinositide 3-kinase (PI3K) and AKT kinase. Comparing to EGF, FBN1 triggered a sustained activation of EGFR, PI3K and AKT in comparable doses, but not extracellular signal-regulated kinase-1 and -2 (ERK1/2). Pharmacologic inhibition of EGFR, PI3K or AKT abolished FBN1-induced mesangial cell activation and proliferation in vitro. Furthermore, inhibition of EGFR activation by erlotinib ameliorated FBN1-induced mesangial activation and glomerulosclerosis in vivo. Conclusion: These findings illustrate that extracellular matrix protein FBN1 is a previously unrecognized ligand for EGFR and induces a sustained activation of EGFR/PI3K/AKT signaling, which leads to mesangial cell activation and glomerulosclerosis. Funding: Government Support – Non-U.S.