Non-Smad signaling pathways

Transforming growth factor-beta (TGF beta) is a key regulator of cell fate during embryogenesis and has also emerged as a potent driver of the epithelial-mesenchymal transition during tumor progression. TGF beta signals are transduced by transmembrane type I and type II serine/threonine kinase receptors (T beta RI and T beta RII, respectively). The activated T beta R complex phosphorylates Smad2 and Smad3, converting them into transcriptional regulators that complex with Smad4. TGF beta also uses non-Smad signaling pathways such as the p38 and Jun N-terminal kinase (JNK) mitogen-activated protein kinase (MAPK) pathways to convey its signals. Ubiquitin ligase tumor necrosis factor (TNF)-receptor-associated factor 6 (TRAF6) and TGF beta-associated kinase 1 (TAK1) have recently been shown to be crucial for the activation of the p38 and JNK MAPK pathways. Other TGF beta-induced non-Smad signaling pathways include the phosphoinositide 3-kinase-Akt-mTOR pathway, the small GTPases Rho, Rac, and Cdc42, and the Ras-Erk-MAPK pathway. Signals induced by TGF beta are tightly regulated and specified by post-translational modifications of the signaling components, since they dictate the subcellular localization, activity, and duration of the signal. In this review, we discuss recent findings in the field of TGF beta-induced responses by non-Smad signaling pathways.