Mesangioproliferative Kidney Diseases and Platelet-Derived Growth Factor–Mediated AXL Phosphorylation

Abstract

Abstract Rationale & Objective IgA nephropathy (IgAN) is a common glomerular disease, with mesangial-cell proliferation as a major feature. There is no disease-specific treatment. Platelet-derived growth factor (PDGF) contributes to the pathogenesis of IgAN. To better understand its pathogenic mechanisms, we assessed PDGF-mediated AXL phosphorylation in human mesangial cells and kidney-tissue biopsy specimens. Study design Immunostaining using human kidney-biopsy specimens and in vitro studies using primary human mesangial cells. Setting & Participants Phosphorylation of AXL was assessed in cultured mesangial cells and 10 kidney-biopsy specimens from 5 patients with IgAN, 3 with minimal change disease (MCD), 1 with membranous nephropathy, and 1 with mesangioproliferative glomerulonephritis (GN). Predictor Glomerular staining for phospho-AXL in kidney-biopsy specimens of patients with mesagioproliferative diseases. Outcomes Phosphorylated AXL detected in biopsy tissues of patients with IgAN and mesangioproliferative GN and in cultured mesangial cells stimulated with PDGF. Analytic approach Student’s t test, Mann-Whitney test, and ANOVA were used to assess significance of mesangial-cell proliferative changes. Results Immunohistochemical staining revealed enhanced phosphorylation of glomerular AXL in IgAN and mesangioproliferative GN, but not in MCD and membranous nephropathy. Confocal-microscopy immunofluorescence analysis indicated that mesangial cells rather than endothelial cells or podocytes expressed phospho-AXL. Kinomic profiling of primary mesangial cells treated with PDGF revealed activation of several protein-tyrosine kinases, including AXL. Immunoprecipitation experiments indicated association of AXL and PDGF-receptor (PDGFR) proteins. An AXL-specific inhibitor (bemcentinib) partially blocked PDGF-induced cellular proliferation and reduced phosphorylation of AXL and PDGFR and the downstream signals (AKT1, ERK1/2). Limitations Small number of kidney-biopsy specimens to correlate the activation of AXL with disease severity. Conclusions PDGF-mediated signaling in mesangial cells involves transactivation of AXL. Finding appropriate inhibitors to block PDGF-mediated transactivation of AXL may provide new therapeutic options for mesangioproliferative kidney diseases, such as IgAN.