A therapeutic target for CKD: activin A facilitates TGFβ1 profibrotic signaling

Background: TGF beta 1 is a major profibrotic mediator in chronic kidney disease (CKD). Its direct inhibition, however, is limited by adverse effects. Inhibition of activins, also members of the TGF beta superfamily, blocks TGF beta 1 profibrotic effects, but the mechanism underlying this and the specific activin(s) involved are unknown. Methods: Cells were treated with TGF beta 1 or activins A/B. Activins were inhibited generally with follistatin, or specifically with neutralizing antibodies or type I receptor downregulation. Cytokine levels, signaling and profibrotic responses were assessed with ELISA, immunofluorescence, immunoblotting and promoter luciferase reporters. Wild-type or TGF beta 1-overexpressing mice with unilateral ureteral obstruction (UUO) were treated with an activin A neutralizing antibody. Results: In primary mesangial cells, TGF beta 1 induces secretion primarily of activin A, which enables longer-term profibrotic effects by enhancing Smad3 phosphorylation and transcriptional activity. This results from lack of cell refractoriness to activin A, unlike that for TGF beta 1, and promotion of TGF beta type II receptor expression. Activin A also supports transcription through regulating non-canonical MRTF-A activation. TGF beta 1 additionally induces secretion of activin A, but not B, from tubular cells, and activin A neutralization prevents the TGF beta 1 profibrotic response in renal fibroblasts. Fibrosis induced by UUO is inhibited by activin A neutralization in wild-type mice. Worsened fibrosis in TGF beta 1-overexpressing mice is associated with increased renal activin A expression and is inhibited to wild-type levels with activin A neutralization. Conclusions: Activin A facilitates TGF beta 1 profibrotic effects through regulation of both canonical (Smad3) and non-canonical (MRTF-A) signaling, suggesting it may be a novel therapeutic target for preventing fibrosis in CKD.