C18 ORF1, a Novel Negative Regulator of Transforming Growth Factor- β Signaling

Background: The structure of C18ORF1 is similar to that of TMEPAI. Results: C18ORF1 inhibits TGF- signaling, but not BMP signaling, by its competition with SARA for Smad2/3 binding. Conclusion: C18ORF1 is a surveillant during the steady state of TGF- signaling, although it is helped by TMEPAI to inhibit TGF- signaling in a coordinated manner. Significance: C18ORF1 acts as a gatekeeper that abrogates excessive TGF- signaling. Transforming growth factor (TGF)- signaling is deliberately regulated at multiple steps in its pathway from the extracellular microenvironment to the nucleus. However, how TGF- signaling is activated or attenuated is not fully understood. We recently identified transmembrane prostate androgen-induced RNA (TMEPAI), which is involved in a negative feedback loop of TGF- signaling. When we searched for a family molecule(s) for TMEPAI, we found C18ORF1, which, like TMEPAI, possesses two PY motifs and one Smad-interacting motif (SIM) domain. As expected, C18ORF1 could block TGF- signaling but not bone morphogenetic protein signaling. C18ORF1 bound to Smad2/3 via its SIM and competed with the Smad anchor for receptor activation for Smad2/3 binding to attenuate recruitment of Smad2/3 to the TGF- type I receptor (also termed activin receptor-like kinase 5 (ALK5)), in a similar fashion to TMEPAI. Knockdown of C18ORF1 prolonged duration of TGF--induced Smad2 phosphorylation and concomitantly potentiated the expression of JunB, p21, and TMEPAI mRNAs induced by TGF-. Consistently, TGF--induced cell migration was enhanced by the knockdown of C18ORF1. These results indicate that the inhibitory function of C18ORF1 on TGF- signaling is similar to that of TMEPAI. However, in contrast to TMEPAI, C18ORF1 was not induced upon TGF- signaling. Thus, we defined C18ORF1 as a surveillant of steady state TGF- signaling, whereas TMEPAI might help C18ORF1 to inhibit TGF- signaling in a coordinated manner when cells are stimulated with high levels of TGF-.