Céline Prunier

Smad7 inhibits the survival nuclear factor κB and potentiates apoptosis in epithelial cells

In this study, we examined the effect of the stable expression of Smad7 in two different cell lines on apoptosis induced by various stimuli including TGF-β, serum withdrawal, loss of cell adhesion (anoikis) and TNF-α. Smad7 increased TGF-β-mediated apoptosis in Mv1Lu cells as well as anoikis and/or serum withdrawal-induced apoptosis in Mv1Lu and MDCK cells. Smad7 markedly decreased the activity of the survival NF-κB transcription factor in MDCK cells. Interestingly, the stable expression of oncogenic Ras in MDCK cells which suppressed Smad7 inhibition of NF-κB also suppressed Smad7 potentiation of serum withdrawal-induced apoptosis and anoikis. In addition, Smad7 inhibited TNF-α stimulation of NF-κB and increased TNF-α-mediated apoptosis in MDCK cells. Our results provide the first evidence that Smad7 induces sensitization of cells to different forms of cell death. They moreover demonstrate that Smad7 inhibits the survival NF-κB factor, providing a potential mechanism whereby Smad7 potentiates cell death.

c-Jun interacts with the corepressor TG-interacting factor (TGIF) to suppress Smad2 transcriptional activity.

The Sma and Mad related (Smad) family proteins are critical mediators of the transforming growth factor-β (TGF-β) superfamily signaling. After TGF-β-mediated phosphorylation and association with Smad4, Smad2 moves to the nucleus and activates expression of specific genes through cooperative interactions with DNA-binding proteins, including members of the winged-helix family of transcription factors, forkhead activin signal transducer (FAST)-1 and FAST2. TGF-β has also been described to activate other signaling pathways, such as the c-Jun N-terminal Kinase (JNK) pathway. Here, we show that activation of JNK cascade blocked the ability of Smad2 to mediate TGF-β-dependent activation of the FAST proteins. This inhibitory activity is mediated through the transcriptional factor c-Jun, which enhances the association of Smad2 with the nuclear transcriptional corepressor TG-interacting factor (TGIF), thereby interfering with the assembly of Smad2 and the coactivator p300 in response to TGF-β signaling. Interestingly, c-Jun directly binds to the nuclear transcriptional corepressor TGIF and is required for TGIF-mediated repression of Smad2 transcriptional activity. These studies thus reveal a mechanism for suppression of Smad2 signaling pathway by JNK cascade through transcriptional repression.

c-Jun Associates with the Oncoprotein Ski and Suppresses Smad2 Transcriptional Activity

The Smad proteins are key intracellular effectors of transforming growth factor-β (TGF-β) cytokines. The ability of Smads to modulate transcription results from a functional cooperativity with the coactivators p300/cAMP-response element-binding protein-binding protein (CBP), or the corepressors TGIF and Ski. The c-Jun N-terminal kinase (JNK) pathway, another downstream target activated by TGF-β receptors, has also been suggested to inhibit TGF-β signaling through interaction of c-Jun with Smad2 and Smad3. Here we show that c-Jun directly interacts with Ski to enhance the association of Ski with Smad2 in the basal state. Interestingly, TGF-β signaling induces dissociation of c-Jun from Ski, thereby relieving active repression by c-Jun. Moreover, activation of JNK pathway suppressed the ability of TGF-β to induce dissociation of c-Jun from ski. Thus, the formation of a c-Jun/Ski complex maintains the repressed state of Smad2-responsive genes in the absence of ligand and participates in negative feedback regulation of TGF-β signaling by the JNK cascade.

Evidence That Smad2 Is a Tumor Suppressor Implicated in the Control of Cellular Invasion

The Smad2 protein plays an essential role in the transforming growth factor-β (TGF-β) signaling pathway. This pathway mediates growth inhibitory signals from the cell surface to the nucleus. Although Smad2 protein is significantly mutated in human cancers, there is no definitive evidence implicating Smad2 as a tumor-suppressor gene. Here we show that overexpression of the tumor-derived missense mutation Smad2.D450E, an unphosphorylable form of Smad2 found in colorectal and lung cancers, did not abolish the TGF-β-mediated growth arrest, suggesting that resistance to the growth-inhibiting effects of TGF-β exhibited by human tumors cannot be linked to the inactivation of Smad2 protein. In contrast, overexpression of Smad2.D450E induces cellular invasion, and this effect was enhanced by TGF-β. A similar invasive phenotype was obtained in cells expressing another inactivating mutation in Smad2 (Smad2.P445H) found in colorectal cancer. These findings indicate that genetic defects in Smad2 are sufficient to confer the invasion-promoting effect of TGF-β and reveal that TGF-β acts through Smad2 to induce cellular invasion by a novel mechanism that is independent of Smad2 phosphorylation by the activated TGF-β type I receptor.

Mechanism for Mutational Inactivation of the Tumor Suppressor Smad2

ABSTRACT Transforming growth factor β (TGF-β) is a potent natural antiproliferative agent that plays an important role in suppressing tumorigenicity. In numerous tumors, loss of TGF-β responsiveness is associated with inactivating mutations that can occur in components of this signaling pathway, such as the tumor suppressor Smad2. Although a general framework for how Smads transduce TGF-β signals has been proposed, the physiological relevance of alterations of Smad2 functions in promoting tumorigenesis is still unknown. Here, we show that expression of Smad2.P445H, a tumor-derived mutation of Smad2 found in human cancer, suppresses the ability of the Smads to mediate TGF-β-induced growth arrest and transcriptional responses. Smad2.P445H is phosphorylated by the activated TGF-β receptor at the carboxy-terminal serine residues and associates with Smad3 and Smad4 but is unable to dissociate from the receptor. Upon ligand-induced phosphorylation, Smad2.P445H interacts stably with wild-type Smad2, thereby blocking TGF-β-induced nuclear accumulation of wild-type Smad2 and Smad2-dependent transcription. The ability of the Smad2.P445H to block the nuclear accumulation of wild-type Smad2 protein reveals a new mechanism for loss of sensitivity to the growth-inhibitory functions of TGF-β in tumor development.

Differential roles of JNK and Smad2 signaling pathways in the inhibition of c-Myc-induced cell death by TGF-β

The transforming growth factor beta (TGF-β) plays an important role in constraining cellular proliferation, but it is also a potent inducer of programmed cell death or apoptosis. Here, we demonstrate that TGF-β can have an opposite effect, acting as a survival factor to prevent c-Myc-induced cell death in Rat-1 fibroblasts. However, in marked contrast to TGF-β, Smad2, which is a critical intracellular mediator of the TGF-β signaling pathway, functions as an antagonist to induce increased cell death. The protective activity of TGF-β was associated with the activation of c-Jun N-terminal Kinase (JNK) and was not linked to the ability of TGF-β to promote cell cycle progression. Expression of dominant-interfering forms of various components of the JNK signaling pathway, including Rac1, Cdc42, mitogen-activated protein kinase kinase 4 (MKK4), and c-Jun, abolished TGF-β-mediated cell survival. Furthermore, overexpression of the constitutively activated mutant RacL61F37A, which selectively stimulates JNK cascade but not G1 cell cycle progression or actin polymerization, was sufficient to prevent apoptosis induced by c-Myc. These findings describe a differential effect of two separated signaling pathways of TGF-β and indicate for the first time that Smad2 can act as antagonist to suppress TGF-β-dependent cell survival.

Identification of PCTA, a TGIF antagonist that promotes PML function in TGF‐β signalling

The TGIF homoeodomain protein functions as an important negative regulator in the TGF‐β signalling pathway. The inhibitory function of TGIF is executed in part through its ability to sequester the tumour suppressor cytoplasmic promyelocytic leukaemia (cPML) in the nucleus, thereby preventing the phosphorylation of Smad2 by the activated TGF‐β type I receptor. Here, we report on the identification of PCTA (PML competitor for TGIF association), a TGIF antagonist that promotes TGF‐β‐induced transcriptional and cytostatic responses. We provide evidence that PCTA functions in TGF‐β signalling by relieving the suppression of Smad2 phosphorylation by TGIF. Furthermore, we demonstrate that PCTA selectively competes with cPML for TGIF association, resulting in the accumulation of cPML in the cytoplasm, where it associates with SARA and coordinates the access of Smad2 for phosphorylation by the activated TGF‐β type I receptor. Thus, our findings on the mode of action of PCTA provide new and important insights into the molecular mechanism underlying the antagonistic interplay between TGIF and cPML in the TGF‐β signalling network.

The Oncoprotein c-Ski Functions as a Direct Antagonist of the Transforming Growth Factor-β Type I Receptor

The oncoprotein c-Ski has been implicated in the negative regulation of transforming growth factor-β (TGF-β) signaling owing to its ability to repress Smad transcriptional activity via recruitment of a transcriptional corepressor complex containing histone deacetylases. However, c-Ski has also been shown to localize to the cytoplasm, raising the interesting possibility that it might disable TGF-β signaling through alternative mechanisms. Here, we provide evidence that c-Ski can restrict TGF-β signaling by interacting directly with the activated TGF-β type I receptor (TβRI). We explored the physiologic relevance of the c-Ski/TβRI interaction and found that it can culminate in a constitutive association of TβRI with a nonfunctional R-Smad/Smad4 complex. Based on these findings, we hypothesize that the interaction between c-Ski and TβRI might interfere with nuclear translocation of the R-Smad/Smad4 complex, thereby attenuating TGF-β signaling. Such a mechanism may play a crucial role in tumor progression, because many tumors that express high levels of c-Ski also display impaired nuclear accumulation of Smads.

The oncogenic TEL/PDGFRβ fusion protein induces cell death through JNK/SAPK pathway

The TEL/PDGFRβ (T/P) fusion protein isolated from patients bearing a t(5;12) translocation is transforming when expressed in haematopoietic cells. To examine the signal transduction events activated by this protein, we measured the effect of T/P on activation of the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) in mouse bone marrow-derived Ba/F3 cells. Significant increase in the activity of JNK/SAPK1 was observed in transient transfection as well as in Ba/F3 cells stably expressing T/P. This activation was abrogated when the T/P-expressing cells were treated with a specific inhibitor of the PDGFRβ tyrosine kinase, indicating that the activity of the PDGFRβ part of the fusion protein was involved in JNK/SAPK activation. Expression of a dominant negative mutant of mitogen-activated protein kinase kinase 4 (MKK4), a direct activator of JNK/SAPK, prevented T/P-induced JNK/SAPK activation. In addition, inhibition of phosphoinositide-3 OH kinase (PI-3 kinase), a promoting survival factor, potentiated the effect of T/P on JNK/SAPK activation. Interestingly, expression of T/P was shown to initiate an apoptotic response that was enhanced by treatment of cells with the PI-3 kinase inhibitor LY294002, suggesting that T/P mediated cell death through activation of JNK/SAPK signalling pathway. Consistent with this hypothesis, expression of the dominant negative mutant of MKK4 decreased T/P-mediated apoptosis, while a dominant-negative mutant of PI-3 kinase enhances cell death. These findings indicate that activation of JNK/SAPK by T/P is related to apoptosis rather than cell proliferation and transformation.

A Model of Partnership Co-Opted by the Homeodomain Protein TGIF and the Itch/AIP4 Ubiquitin Ligase for Effective Execution of TNF-α Cytotoxicity

The homeodomain protein TGIF functions as a negative regulator of multiple classes of transcription factors. Here we report on the characterization of TGIF as an essential component of the tumor necrosis factor alpha (TNF-alpha) cytotoxic program. This proapoptotic role of TGIF does not appear to rely on transcriptional modulation but instead is executed in conjunction with Itch/AIP4, an E3 ubiquitin ligase operating in TNF-alpha-induced apoptosis through its ability to target the caspase antagonist cFlip(L) for degradation. Notably, we found that activation of TNF-alpha signaling induced the association of TGIF with Itch/AIP4, resulting in increased accessibility of cFlip(L) for association and ubiquitination by Itch/AIP4. Moreover, we show that Itch/AIP4 can also stabilize the TGIF protein in response to TNF-alpha by triggering its monoubiquitination at lysine 259, thereby revealing the existence of a functional network that can evolve into a positive feedback loop for ensuring effective execution of the TNF-alpha apoptotic signaling.