Src-mediated ligand release-independent EGFR transactivation involves TGF-β-induced Smad3 activation in mesangial cells

A great deal of evidence highlighted the pathophysiologic importance of TGF-beta 1/Smad3 pathway in masangial extracellular matrix (ECM) accumulation, but some alternative signaling pathways are also involved. TGF-beta was shown recently to induce rapid and transient epidermal-like growth factor receptor (EGFR) transactivation and subsequent fibronectin expression via heparin-binding epidermal-like growth factors (HB-EGF) release and binding in mesangial cells, which is independent of Smad2 activation. However, whether TGF-beta could induce persistent EGFR transactivation remains to be identified. The present study demonstrates that in addition to transient EGFR transactivation, TGF-beta 1 can also induce continuous EGFR transactivation by a non-ligand dependent pathway in rat mesangial cells. This sustained EGFR transactivation is mainly due to Src kinase-mediated persistent EGFR tyrosine phosphorylation at Y845 rather than Y1173. TGF-beta 1 induced early Smad3 phosphorylation is independent of transient EGFR transactivation and ERK1/2 activation initiated by HB-EGF release, whereas Src mediated chronic EGFR transactivation and ERK1/2 activation participate in Smad3 activation in a relatively modest and delayed manner. Therefore, the present study further clarifies the mechanisms of EGFR trans activation in the TGF-beta initiated ECM upregulation and raises the possibility that targeting EGFR may provide a viable alternative strategy for inhibiting TGF-beta in chronic kidney disease. (C) 2017 Published by Elsevier Inc.