Figure 6.

The TGF-β non-SMAD signaling pathways. (A) The ERK-MAP kinase pathway. First step for the activation of the TGF-β-induced ERK pathway is the phosphorylation of ShcA by the activated type I receptor. TGF-β-induced tyrosine phosphorylation of ShcA promotes formation of ShcA/Grb2/Sos complex and Ras activation. This leads to the sequential activation of Raf, MEK1/2 and finally ERK1/2. Activated ERK1/2 phosphorylate transcription factors (TF), thus contributing to TGF-β-induced transcriptional responses. Activated ERK1/2 can also phosphorylate SMADs at the linker region to regulate their activity. (B) The p38/JNK and NF-κB pathway (via TAK1). Upon ligand binding, TGF-β receptor complexes interact with TRAF6 promoting its autoubiquitylation. SMAD6 can inhibit TRAF6 ubiquitylation and activation by recruiting the deubiquitylating enzyme A20. TRAF6 activates TAK1 via Lys63-linked polyubiquitylation and activated TAK1 in turn activates through phosphorylation MAP kinase kinases (MKKs) MKK4, MKK3 and MKK6. MKKs activate their downstream kinases JNK and p38, which can then phosphorylate their target transcription factors (TF) in order to regulate transcription. SMAD7 enhances the activation of the p38 pathway as it acts as a scaffolding protein for TAK1, MKK3 and p38. Activated JNK and p38 phosphorylate also SMADs at the linker region, thus regulating SMAD-dependent transcriptional responses as well. Finally, TAK1 activates also IKK, which eventually leads to the activation of NF-κB signaling. (C) The PI3K/AKT/mTOR pathway. TGF-β promotes PI3K/AKT activation via direct interaction of the p85 subunit of PI3K (not shown) with TGF-β receptors. TGF-β-induced autoubiquitylation of TRAF6 results in recruitment and phosphorylation of AKT. TGF-β via PI3K, promotes also activation of mTORC2, which in turn can also phosphorylate and activate AKT promoting cell survival. Moreover, activated AKT prevents phosphorylation of SMAD3, thus attenuating SMAD3-dependent signaling. (D) TGF-β signaling by type I receptor intracellular domain signaling. The transmembrane metalloprotease TACE, promotes ectodomain cleavage of type I receptor, which is then followed by TRAF6-mediated ubiquitylation of the cytoplasmic domain of type I receptor and recruitment of presenilin-1 (PS1), part of the γ-secretase complex. PS1 proteolytically cleaves the ubiquitylated intracellular cytoplasmic domain of the receptor (TGFβRI ICD), which is released into the cytoplasm. Then, TGFβRI ICD translocates to the nucleus where it associates with other co-factors (not shown) and induces the expression of target genes. (E) MAP kinase pathway activation via TRAF4. The MAP kinase pathway can also be activated via TRAF4, another E3 ligase that upon ligand binding is recruited to the receptor complex, gets autoubiquitylated and then activates TAK1 via polyubiquitylation, eventually leading to activation of the p38 pathway. At the same time, TRAF4 targets SMURF2 for polyubiquitylation and subsequent degradation, thus contributing to the stability of TGF-β type I receptor. (F) The JAK-STAT pathway. STAT3 gets phosphorylated and activated by JAK (which interacts with the type I receptor) in response to TGF-β in order to regulate the expression of subset of TGF-β target genes. (G) The Rho-(like) GTPase pathway activation. TGF-β induces activation of RhoA GTPase (via both SMAD-independent and SMAD-dependent mechanisms), which eventually results in actin cytoskeleton reorganization and formation of stress fibers. Additionally, Par6, a regulator of cell polarity, once phosphorylated by TGF-β type II receptor, recruits Smurf1 E3 ligase that targets RhoA for degradation, eventually leading to tight junction dissociation. Upon TGF-β stimulation, Rho-like proteins Cdc42 and Rac1 are also activated and promote actin reorganization via activation of PAK2.