- Synthetic triterpenoids enhance transforming growth factor beta/Smad signaling. Suh, N., Roberts, A.B., Birkey Reffey, S., Miyazono, K., Itoh, S., ten Dijke, P., Heiss, E.H., Place, A.E., Risingsong, R., Williams, C.R., Honda, T., Gribble, G.W., Sporn, M.B. Cancer Res. (2003)