Soluble CD109 binds TGF-β and antagonizes TGF-β signalling and responses

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine implicated in many diseases, including tissue fibrosis and cancer. TGF-beta mediates diverse biological responses by signalling through type I and II TGF-beta receptors (T beta RI and T beta RII). We have previously identified CD109, a glycosylphosphatidylinositol (GPI)-anchored protein, as a novel TGF-beta co-receptor that negatively regulates TGF-beta signalling and responses and demonstrated that membrane-anchored CD109 promotes TGF-beta receptor degradation via a SMAD7/Smurf2-mediated mechanism. To determine whether CD109 released from the cell surface (soluble CD109 or sCD109) also acts as a TGF-beta antagonist, we determined the efficacy of recombinant sCD109 to interact with TGF-beta and inhibit TGF-beta signalling and responses. Our results demonstrate that sCD109 binds TGF-beta with high affinity as determined by surface plasmon resonance (SPR) and cell-based radioligand binding and affinity labelling competition assays. SPR detected slow dissociation kinetics between sCD109 and TGF-beta at low concentrations, indicating a stable and effective interaction. In addition, sCD109 antagonizes TGF-beta-induced Smad2/3 phosphorylation, transcription and cell migration. Together, our results suggest that sCD109 can bind TGF-beta, inhibit TGF-beta binding to its receptors and decrease TGF-beta signalling and TGF-beta-induced cellular responses.