Transforming growth factor β (TGFβ) is a pluripotent cytokine promoting epithelial cell plasticity during morphogenesis and tumour progression. TGFβ binding to type II and type I serine/threonine kinase receptors (TβRII and TβRI) causes activation of different intracellular signaling pathways. TβRI is associated with the ubiquitin ligase tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6). Here we show that TGFβ, via TRAF6, causes Lys63-linked polyubiquitination of TβRI, promoting cleavage of TβRI by TNF-alpha converting enzyme (TACE), in a PKCζ-dependent manner. The liberated intracellular domain (ICD) of TβRI associates with the transcriptional regulator p300 to activate genes involved in tumour cell invasiveness, such as Snail and MMP2. Moreover, TGFβ-induced invasion of cancer cells is TACE- and PKCζ- dependent and the TβRI ICD is localized in the nuclei of different kinds of tumour cells in tissue sections. Thus, our data reveal a specific role for TβRI in TGFβ mediated tumour invasion.

Bone morphogenetic proteins (BMP) are polypeptide growth factors that regulate cell differentiation and proliferation. BMPs bind to type I and type II serine/threonine kinase receptors to initiate intracellular signalling. BMPR-II is the type II receptor, its mutations lead to hereditary pulmonary hypertension, and knockout of Bmpr-II results in early embryonic lethality. To identify novel interacting proteins and explore signalling pathways that can be initiated by BMPR-II, we performed glutathione-S-transferase (GST) pull-down assays with BMPR-II protein constructs fused to GST and extracts of mouse myoblast C2C12 cells. We generated three constructs which contain different parts of the cytoplasmic region of BMPR-II: full-length cytoplasmic part of BMPR-II, only the kinase domain, or only the C-terminal tail of BMPR-II. Proteins which formed complexes with these BMPR-II constructs were analyzed by two-dimensional gel electrophoresis (2-D GE), and specifically interacting proteins were identified by matrix-assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOF-MS). We identified 33 interacting proteins; 11 proteins interacted with the C-terminal tail of BMPR-II, 4 with full-length BMPR-II, and 18 with a short form of the receptor with a deleted tail. Fourteen proteins have assigned functions in various signalling processes, suggesting links of BMP signalling to regulation of MAP kinase pathway, apoptosis, transcription, PKCss, and PKA. Five of the identified proteins are components of the cytoskeleton, and four are enzymes involved in metabolism, e.g., processing of estrogens or lipids. We confirmed interaction of PKC beta and CtBP with BMPR-II using immunodetection. We showed that the C-terminal tail of BMPR-II provides binding sites for a number of regulatory proteins that may initiate Smad-independent signalling.

Transforming growth factor β (TGFβ) family members signal via heterotetrameric complexes of type I (TβRI) and type II (TβRII) dual specificity kinase receptors. The availability of the receptors on the cell surface is controlled by several mechanisms. Newly synthesized TβRI and TβRII are delivered from the Golgi apparatus to the cell surface via separate routes. On the cell surface, TGFβ receptors are distributed between different microdomains of the plasma membrane and can be internalized via clathrin-and caveolae-mediated endocytic mechanisms. Although receptor endocytosis is not essential for TGFβ signaling, localization of the activated receptor complexes on the early endosomes promotes TGFβ-induced Smad activation. Caveolae-mediated endocytosis, which is widely regarded as a mechanism that facilitates the degradation of TGFβ receptors, has been shown to be required for TGFβ signaling via non-Smad pathways. The importance of proper control of TGFβ receptor intracellular trafficking is emphasized by clinical data, as mislocalization of receptors has been described in connection with several human diseases. Thus, control of intracellular trafficking of the TGFβ receptors together with the regulation of their expression, posttranslational modifications and down-regulation, ensure proper regulation of TGFβ signaling.

Phosphorylation of eEF1A1 by TβR-I is a novel regulatory mechanism that provides a direct link to regulation of protein synthesis by TGF-β, as an important component in the TGF-β-dependent regulation of protein synthesis and cell proliferation.

Transforming growth factor-beta (TGFbeta) is a potent regulator of cell growth, differentiation, and apoptosis. Type I TGFbeta receptor (TbetaRI) is the key receptor for initiation of intracellular signaling by TGFbeta. Here we report proteomics-based identification of proteins that form a complex with TbetaRI. Using 2D-GE and MALDI TOF mass spectrometry, we identified 16 proteins that specifically interacted with a GST-fused TbetaRI Thr204Asp construct with constitutively active serine/threonine kinase. We confirmed interactions of the receptor with cAMP regulated guanine nucleotide exchange factor 1 (Epac1), beta-spectrin, PIASy, and beta-catenin proteins using immunoblotting. Interaction of the receptor with Epac1 required intact kinase activity of TbetaRI but was not affected by deletion of cAMP-binding domain of Epac1. TGFbeta1-induced C-terminal phosphorylation of Smad2 was inhibited in vivo and in vitro in the presence of Epac1. Epac1 inhibited also TGFbeta1/TbetaRI-dependent transcriptional activation, as evaluated by luciferase reporter assays. We observed that expression of Epac1 counteracted TGFbeta/TbetaRI-dependent decrease of cell adhesion and TGFbeta/TbetaRI-induced stimulation of cell migration. Thus, we have reported novel TRI-interacting proteins and have shown that Epac1 inhibited TGFbeta-dependent regulation of cell migration and adhesion.

Smad3 is an essential component in the intracellular signaling of transforming growth factor-b (TGFb), which is a potent inhibitor of tumor cell proliferation. BRCA2 is a tumor suppressor involved in early onset of breast, ovarian and prostate cancer. Both Smad3 and BRCA2 possess transcription activation domains. Here, we show that Smad3 and BRCA2 interact functionally and physically. We found that BRCA2 forms a complex with Smad3 in vitro and in vivo, and that both MH1 and MH2 domains of Smad3 contribute to the interaction. TGFb1 stimulates interaction of endogenous Smad3 and BRCA2 in non-transfected cells. BRCA2 co-activates Smad3-dependent transcriptional activation of luciferase reporter and expression of plasminogen activator inhibitor-1 (PAI-1). Smad3 increases the transcriptional activity of BRCA2 fused to the DNA-binding domain (DBD) of Gal4, and reciprocally, BRCA2 co-activates DBD-Gal4-Smad3. Thus, our results show that BRCA2 and Smad3 form a complex and synergize in regulation of transcription.

The adaptor CIN85 enhances TGFβ-induced signaling and cellular responses to TGFβ by promoting the expression of TGFβ receptors on the surface in a Rab11-dependent manner.

Transmembrane receptors with intrinsic serine/threonine or tyrosine kinase domains regulate vital functions of cells in multicellular eukaryotes, e.g., differentiation, apoptosis, and proliferation. Here, we show that bone morphogenetic protein type II receptor (BMPR-II) which has a serine/threonine kinase domain, and stem cell factor receptor (c-kit) which contains a tyrosine kinase domain form a complex in vitro and in vivo; the interaction is induced upon treatment of cells with BMP2 and SCF. Stem cell factor (SCF) modulated BMP2-dependent activation of Smad1/5/8 and phosphorylation of Erk kinase. SCF also enhanced BMP2-dependent differentiation of C2C12 cells. We found that BMPR-II was phosphorylated at Ser757 upon co-expression with and activation of c-kit. BMPR-II phosphorylation required intact kinase activity of BMPR-II. Abrogation of the c-kit/SCF-dependent phosphorylation of BMPR-II at the Ser757 interfered with the cooperative effect of BMP2 and SCF. Our data suggest that the complex formation between c-kit and BMPR-II leads to phosphorylation of BMPR-II at Ser757, which modulates BMPR-II-dependent signaling.