The type II TGF-ß receptor phosphorylates Tyr182 in the type I receptor to activate downstream Src signaling.

Transforming growth factor-ß (TGF-ß) signaling has important roles during embryonic development and in tissue homeostasis. TGF-ß ligands exert cellular effects by binding to type I (TßRI) and type II (TßRII) receptors and inducing both SMAD-dependent and SMAD-independent intracellular signaling pathways, the latter of which includes the activation of the tyrosine kinase Src. We investigated the mechanism by which TGF-ß stimulation activates Src in human and mouse cells. Before TGF-ß stimulation, inactive Src was complexed with TßRII. Upon TGF-ß1 stimulation, TßRII associated with and phosphorylated TßRI at Tyr182. Binding of Src to TßRI involved the interaction of the Src SH2 domain with phosphorylated Tyr182 and the interaction of the Src SH3 domain with a proline-rich region in TßRI and led to the activation of Src kinase activity and Src autophosphorylation. TGF-ß1-induced Src activation required the kinase activities of TßRII and Src but not that of TßRI. Activated Src also phosphorylated TßRI on several tyrosine residues, which may stabilize the binding of Src to the receptor. Src activation was required for the ability of TGF-ß to induce fibronectin production and migration in human breast carcinoma cells and to induce α-smooth muscle actin and actin reorganization in mouse fibroblasts. Thus, TGF-ß induces Src activation by stimulating a direct interaction with TßRI that depends on tyrosine phosphorylation of TßRI by TßRII.

The ubiquitin-ligase TRAF6 and TGFß type I receptor form a complex with Aurora kinase B contributing to mitotic progression and cytokinesis in cancer cells.

BACKGROUND: Transforming growth factor ß (TGFß) is overexpressed in several advanced cancer types and promotes tumor progression. We have reported that the intracellular domain (ICD) of TGFß receptor (TßR) I is cleaved by proteolytic enzymes in cancer cells, and then translocated to the nucleus in a manner dependent on the endosomal adaptor proteins APPL1/2, driving an invasiveness program. How cancer cells evade TGFß-induced growth inhibition is unclear. METHODS: We performed microarray analysis to search for genes regulated by APPL1/2 proteins in castration-resistant prostate cancer (CRPC) cells. We investigated the role of TßRI and TRAF6 in mitosis in cancer cell lines cultured in 10% FBS in the absence of exogenous TGFß. The molecular mechanism of the ubiquitination of AURKB by TRAF6 in mitosis and the formation of AURKB-TßRI complex in cancer cell lines and tissue microarrays was also studied. FINDINGS: During mitosis and cytokinesis, AURKB-TßRI complexes formed in midbodies in CRPC and KELLY neuroblastoma cells. TRAF6 induced polyubiquitination of AURKB on K85 and K87, protruding on the surface of AURKB to facilitate its activation. AURKB-TßRI complexes in patient's tumor tissue sections correlated with the malignancy of prostate cancer. INTERPRETATION: The AURKB-TßRI complex may become a prognostic biomarker for patients with risk of developing aggressive PC. FUNDING: Swedish Medical Research Council (2019-01598, ML; 2015-02757 and 2020-01291, CHH), the Swedish Cancer Society (20 0964, ML), a regional agreement between Umeå University and Region Västerbotten (ALF; RV-939377, -967041, -970057, ML). The European Research Council (787472, CHH). KAW 2019.0345, and the Kempe Foundation SMK-1866; ML. National Microscopy Infrastructure (NMI VR-RFI 2016-00968).

Intracellular trafficking of transforming growth factor ß receptors.

Transforming growth factor ß (TGFß) family members signal via heterotetrameric complexes of type I (TßRI) and type II (TßRII) dual specificity kinase receptors. The availability of the receptors on the cell surface is controlled by several mechanisms. Newly synthesized TßRI and TßRII are delivered from the Golgi apparatus to the cell surface via separate routes. On the cell surface, TGFß 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 TGFß signaling, localization of the activated receptor complexes on the early endosomes promotes TGFß-induced Smad activation. Caveolae-mediated endocytosis, which is widely regarded as a mechanism that facilitates the degradation of TGFß receptors, has been shown to be required for TGFß signaling via non-Smad pathways. The importance of proper control of TGFß 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 TGFß receptors together with the regulation of their expression, posttranslational modifications and down-regulation, ensure proper regulation of TGFß signaling.

CIN85 modulates TGFß signaling by promoting the presentation of TGFß receptors on the cell surface.

Members of the transforming growth factor ß (TGFß) family initiate cellular responses by binding to TGFß receptor type II (TßRII) and type I (TßRI) serine/threonine kinases, whereby Smad2 and Smad3 are phosphorylated and activated, promoting their association with Smad4. We report here that TßRI interacts with the SH3 domains of the adaptor protein CIN85 in response to TGFß stimulation in a TRAF6-dependent manner. Small interfering RNA-mediated knockdown of CIN85 resulted in accumulation of TßRI in intracellular compartments and diminished TGFß-stimulated Smad2 phosphorylation. Overexpression of CIN85 instead increased the amount of TßRI at the cell surface. This effect was inhibited by a dominant-negative mutant of Rab11, suggesting that CIN85 promoted recycling of TGFß receptors. CIN85 enhanced TGFß-stimulated Smad2 phosphorylation, transcriptional responses, and cell migration. CIN85 expression correlated with the degree of malignancy of prostate cancers. Collectively, our results reveal that CIN85 promotes recycling of TGFß receptors and thereby positively regulates TGFß signaling.

Phosphorylation of eEF1A1 at Ser300 by TßR-I results in inhibition of mRNA translation.

BACKGROUND: Transforming growth factor ß (TGF-ß) is a potent inhibitor of cell proliferation that regulates cell functions by activating specific serine/threonine kinase receptors on the cell surface. Type I TGF-ß receptor (TßR-I) is essential for TGF-ß signaling, and substrates of TßR-I provide insights into molecular mechanisms of TGF-ß signaling. RESULTS: Here we identify eukaryotic elongation factor 1A1 (eEF1A1) as a novel substrate of TßR-I. We show that TßR-I phosphorylates eEF1A1 at Ser300 in vitro and in vivo. Ser300 was found to be important for aminoacyl-tRNA (aa-tRNA) binding to eEF1A1. Ser300 phosphorylation or mutations of Ser300 correlate with inhibition of protein synthesis in vitro and in vivo. We show that mimicking eEF1A1 phosphorylation at Ser300 results in inhibition of cell proliferation, and that mutations of Ser300 affect TGF-ß dependency in inhibition of protein synthesis and cell proliferation. Increased expression of eEF1A has been reported to enhance carcinogenesis. An analysis of human breast cancer cases revealed a decrease of eEF1A1 phosphorylation at Ser300 in malignant tumor cells as compared to epithelial cells in noncancerous tissues. CONCLUSIONS: Phosphorylation of eEF1A1 by TßR-I is a novel regulatory mechanism that provides a direct link to regulation of protein synthesis by TGF-ß, as an important component in the TGF-ß-dependent regulation of protein synthesis and cell proliferation.

Interactome of transforming growth factor-beta type I receptor (TbetaRI): inhibition of TGFbeta signaling by Epac1.

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.

Glycoproteome profiling of transforming growth factor-beta (TGFbeta) signaling: nonglycosylated cell death-inducing DFF-like effector A inhibits TGFbeta1-dependent apoptosis.

Transforming growth factor-beta (TGFbeta) is a potent regulator of cell growth, differentiation, and apoptosis. TGFbeta binds to specific serine/threonine kinase receptors, which leads to activation of Smad-dependent and Smad-independent signaling pathways. O-Glycosylation is a dynamic PTM which has been observed in many regulatory proteins, but has not been studied in the context of TGFbeta signaling. To explore the effect of TGFbeta1 on protein O-glycosylation in human breast epithelial cells, we performed analyses of proteins which were affinity purified with Helix pomatia agglutinin (HPA). HPA lectin allowed enrichment of proteins containing GalNAc and GlcNAc linked to serine and threonine residues. Using 2-DE and MALDI-TOF-MS, we identified 21 HPA-precipitated proteins, which were affected by treatment of cells with TGFbeta1. Among these proteins, regulators of cell survival, apoptosis, trafficking, and RNA processing were identified. We found that TGFbeta1 inhibited the appearance of cell death-inducing DFF-like effector A (CIDE-A) in 2-D gels with HPA-precipitated proteins. CIDE-A is a cell death activator which promotes DNA fragmentation. We observed that TGFbeta1 did not affect expression of CIDE-A, but inhibited its glycosylation. We found that deglycosylation of CIDE-A correlated with enhanced nuclear export of the protein, and that high level of nonglycosylated CIDE-A inhibited TGFbeta1-dependent cell death. Thus, inhibition of the glycosylation of CIDE-A may be a mechanism to protect cells from apoptosis.

Phosphoproteome profiling of transforming growth factor (TGF)-beta signaling: abrogation of TGFbeta1-dependent phosphorylation of transcription factor-II-I (TFII-I) enhances cooperation of TFII-I and Smad3 in transcription.

Transforming growth factor-beta (TGFbeta) signaling involves activation of a number of signaling pathways, several of which are controlled by phosphorylation events. Here, we describe a phosphoproteome profiling of MCF-7 human breast epithelial cells treated with TGFbeta1. We identified 32 proteins that change their phosphorylation upon treatment with TGFbeta1; 26 of these proteins are novel targets of TGFbeta1. We show that Smad2 and Smad3 have different effects on the dynamics of TGFbeta1-induced protein phosphorylation. The identified proteins belong to nine functional groups, e.g., proteins regulating RNA processing, cytoskeletal rearrangements, and proteasomal degradation. To evaluate the proteomics findings, we explored the functional importance of TGFbeta1-dependent phosphorylation of one of the targets, i.e., transcription factor-II-I (TFII-I). We confirmed that TGFbeta1 stimulated TFII-I phosphorylation at serine residues 371 and 743. Abrogation of the phosphorylation by replacement of Ser371 and Ser743 with alanine residues resulted in enhanced complex formation between TFII-I and Smad3, and enhanced cooperation between TFII-I and Smad3 in transcriptional regulation, as evaluated by a microarray-based measurement of expression of endogenous cyclin D2, cyclin D3, and E2F2 genes, and by a luciferase reporter assay. Thus, TGFbeta1-dependent phosphorylation of TFII-I may modulate TGFbeta signaling at the transcriptional level.

Smad2 phosphorylation by type I receptor: contribution of arginine 462 and cysteine 463 In the C terminus of Smad2 for specificity.

Transforming growth factor-beta (TGFbeta) is a potent regulator of cell proliferation, differentiation, motility, and apoptosis. TGFbeta binds to and activates serine/threonine kinase receptors that phosphorylate Smad2 and Smad3 intracellular signal transducers at two C-terminal serine residues. Here we show that substitutions of Arg-462 and Cys-463 residues, which are in proximity of the C-terminal serine residues, inhibited TGFbeta type I receptor-dependent phosphorylation of the C-terminal Smad2 peptides and full-length GST-Smad2 proteins in vitro. In vivo, mutation of Arg-462 and Cys-463 inhibited TGFbeta1-stimulated phosphorylation of the C-terminal serine residues in Smad2. Moreover, Smad2 with mutated Arg-462 and Cys-463 was less efficient in activation of the Smad2-responsive activin-responsive element-containing luciferase reporter ARE-luc, as compared with the wild-type protein. Thus, Arg-462 and Cys-463, which are in proximity of the C-terminal serine residues, contribute to recognition and phosphorylation of the C terminus of Smad2 by type I TGFbeta receptor.

Potential role of transforming growth factor beta1 in drug resistance of tumor cells.

Acquired drug resistance of tumor cells is frequently observed in cancer patients undergoing chemotherapy. We studied murine leukemia L1210 cells sensitive and resistant to the cytotoxic action of cisplatin and showed that cisplatin-resistant leukemia cells were also refractory to TGF beta1-dependent growth inhibition and apoptosis. Addressing the question about the mechanisms responsible for the cross-resistance to cisplatin and TGF beta1, we found that cisplatin- and TGF beta1-resistant L1210 cells possessed a decreased expression of type I TGF beta1 receptor, while the expression of type II TGF beta1 receptor was not affected. Western blot analysis of Smad proteins 2, 3, 4, 6, and 7, which participate in signal transduction pathway down-stream of the TGF beta1 receptors, revealed an increased expression of Smad 6, inhibiting TGF beta1 action, only in cisplatin- and TGF beta1-resistant L1210 cells. TGF beta1 and especially the cytotoxic mistletoe agglutinin increased Smad 6 expression in TGF beta1-sensitive but not in TGF beta1-resistant L1210 cells. TGF beta1-resistant L1210 cells also differed from TGF beta1-sensitive cells by the lack of expression of the pro-apoptotic p53 protein and higher level of expression of the anti-apoptotic Bcl-2 protein. Thus, the described co-expression of tumor cell refractoriness to an anti-cancer drug and to the inhibitory cytokine TGF beta1 is accompanied by multiple changes in the TGF beta1 signal transduction pathway and in other regulatory systems of the target cells. Besides, we found that various anti-tumor drugs and cytotoxic plant lectins increased the level of TGF beta1 expression in both TGFbeta1-sensitive and -resistant L1210 cells. A hypothesis is proposed that TGFbeta1 can at least partly mediate the effect of cell-stressing agents and, thus, the development of TGF beta1 resistance may be responsible for the appearance of tumor cell refractoriness to the action of some anti-cancer drugs.

Inhibition of transforming growth factor-beta signaling by low molecular weight compounds interfering with ATP- or substrate-binding sites of the TGF beta type I receptor kinase.

Transforming growth factor-beta (TGFbeta) is a potent regulator of cell proliferation, differentiation, apoptosis, and migration. TGF-beta type I receptor (TbetaR-I), which has intrinsic serine/threonine kinase activity, is a key component in activation of intracellular TGFbeta signaling. We studied two different classes of TbetaR-I inhibitors, i.e., compounds interfering with the ATP-binding site of the kinase and substrate-mimicking peptides. We found that pyridinylimidazole compounds inhibited TbetaR-I kinase at micromolar concentration. A representative compound, SB203580, inhibited in vivo Smad2 phosphorylation by TbetaR-I and affected TGFbeta-dependent transcriptional activation. Peptides mimicking the TbetaR-I phosphorylation sites at the C-terminus of Smad2 also inhibited the autophosphorylation of TbetaR-I and phosphorylation of Smad2 by TbetaR-I in vitro and in vivo, whereas a similar peptide from Smad5 was without effect. The substrate-mimicking peptide, fused to penetratin, inhibited a TGFbeta1-dependent transcriptional response in a luciferase reporter assay and ligand-dependent growth inhibition of Mv1Lu cells. Thus, the substrate-mimetic peptide is a new type of specific inhibitor of the TGFbeta signaling in vivo.

BRCA2 and Smad3 synergize in regulation of gene transcription.

Smad3 is an essential component in the intracellular signaling of transforming growth factor-beta (TGFbeta), 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. TGFbeta1 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.