Maréne Landström

Mechanism of TGF-β signaling to growth arrest, apoptosis, and epithelial–mesenchymal transition

Members of the transforming growth factor-beta (TGF-beta) family have important roles during embryogenesis, as well as in the control of tissue homeostasis in the adult. They exert their cellular effects via binding to serine/threonine kinase receptors. Members of the Smad family of transcription factors are important intracellular messengers, and recent studies have shown that the ubiquitin ligase TRAF6 mediates other specific signals. TGF-beta signaling is tightly controlled by post-translational modifications, which regulate the activity, stability, and subcellular localization of the signaling components. The aim of this review is to summarize some of the recent findings on the mechanism of TGF-beta signaling to growth arrest, apoptosis, and epithelial-mesenchymal transition.

The type I TGF-beta receptor engages TRAF6 to activate TAK1 in a receptor kinase-independent manner.

Transforming growth factor-β (TGF-β) is a multifunctional cytokine that regulates embryonic development and tissue homeostasis; however, aberrations of its activity occur in cancer. TGF-β signals through its Type II and Type I receptors (TβRII and TβRI) causing phosphorylation of Smad proteins. TGF-β-associated kinase 1 (TAK1), a member of the mitogen-activated protein kinase kinase kinase (MAPKKK) family, was originally identified as an effector of TGF-β-induced p38 activation. However, the molecular mechanisms for its activation are unknown. Here we report that the ubiquitin ligase (E3) TRAF6 interacts with a consensus motif present in TβRI. The TβRI–TRAF6 interaction is required for TGF-β-induced autoubiquitylation of TRAF6 and subsequent activation of the TAK1–p38/JNK pathway, which leads to apoptosis. TβRI kinase activity is required for activation of the canonical Smad pathway, whereas E3 activity of TRAF6 regulates the activation of TAK1 in a receptor kinase-independent manner. Intriguingly, TGF-β-induced TRAF6-mediated Lys 63-linked polyubiquitylation of TAK1 Lys 34 correlates with TAK1 activation. Our data show that TGF-β specifically activates TAK1 through interaction of TβRI with TRAF6, whereas activation of Smad2 is not dependent on TRAF6.

Non-Smad signaling pathways.

Transforming growth factor-beta (TGFβ) 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. TGFβ signals are transduced by transmembrane type I and type II serine/threonine kinase receptors (TβRI and TβRII, respectively). The activated TβR complex phosphorylates Smad2 and Smad3, converting them into transcriptional regulators that complex with Smad4. TGFβ 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 TGFβ-associated kinase 1 (TAK1) have recently been shown to be crucial for the activation of the p38 and JNK MAPK pathways. Other TGFβ-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 TGFβ 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 TGFβ-induced responses by non-Smad signaling pathways.

The TAK1–TRAF6 signalling pathway

Cellular responses to pathogens, growth factors, cytokines, extra- or intra-cellular stress, is a prerequisite for the cell to adapt to novel and potentially dangerous situations. If the changes in the extra- or intra-cellular milieu causes DNA-damage or revoke a signalling pathway utilized during morphogenesis, the epithelial cells might be forced to undergo programmed cell death (apoptosis) in the benefit for the whole organism or transform to a mesenchymal cell type (epithelial to mesenchymal transition; EMT), in respond to a specific stimuli. An overview is presented over the current knowledge for the key components in signal transduction in homeostasis, inflammation and cancer. A handful of transcription factors are crucial for the determination of the specific cellular responses, where the transforming growth factor-beta (TGF-beta) is an important factor as discussed in this review.

Smad7 mediates apoptosis induced by transforming growth factor β in prostatic carcinoma cells

Transforming growth factor beta (TGF-beta) is an important regulator of apoptosis in some cell types, but the underlying molecular mechanisms are largely unknown. TGF-beta signals through type I and type II receptors and downstream effector proteins, termed Smads. TGF-beta induces the phosphorylation of Smad2 and Smad3 (receptor-activated Smads) which associate with Smad4 and translocate to the nucleus, where they regulate gene transcription [1]. Smad7 protein is induced by TGF-beta1 and has been classified as an inhibitory Smad. Smad7 prevents phosphorylation of receptor-activated Smads, thereby inhibiting TGF-beta-induced signaling responses [1]. Smad7 expression is increased in rat prostatic epithelial cells undergoing apoptosis as a result of castration [2]. Here we have shown that TGF-beta1 treatment or ectopic expression of Smad7 in human prostatic carcinoma cells (PC-3U) induces apoptosis. Furthermore, TGF-beta1-induced apoptosis was prevented by inhibition of Smad7 expression, by antisense mRNA in stably transfected cell lines or upon transient transfection with antisense oligonucleotides in several investigated cell lines. These findings provide evidence for a new effector function for Smad7 in TGF-beta1 signaling.

Inhibitory effects of soy and rye diets on the development of Dunning R3327 prostate adenocarcinoma in rats

Dunning R3327 PAP prostate tumors were transplanted in 125 rats, the rats were divided into five groups, and tumor development was examined for 24 weeks during treatment with diets containing 33% of soy flour (SD), rye bran (RB), heat‐treated rye bran (HRB), or rye endosperm (RE).

TRAF6 ubiquitinates TGFβ type I receptor to promote its cleavage and nuclear translocation in cancer

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.

Interaction between Smad7 and β-Catenin: Importance for Transforming Growth Factor β-Induced Apoptosis

ABSTRACT Members of the transforming growth factor β (TGF-β) and Wnt/wingless superfamilies regulate cell fate during development and tissue maintenance. Here we report that Smad7 interacts with β-catenin and lymphoid enhancer binding factor 1/T-cell-specific factor (LEF1/TCF), transcriptional regulators in Wnt signaling, in a TGF-β-dependent manner. Smad7 was found to be required for TGF-β1-induced accumulation of β-catenin and LEF1 in human prostate cancer (PC-3U) cells as well as in human keratinocytes (HaCaT cells). Moreover, when the endogenous Smad7 was repressed by specific small interfering RNA, TGF-β-induced increase of activated p38, Akt phosphorylated on Ser473, glycogen synthase kinase 3β phosphorylated on Ser9 was prevented, as well as the TGF-β-induced association between β-catenin and LEF1. Notably, the observed physical association of Smad7 and β-catenin was found to be important for TGF-β-induced apoptosis, since suppression of β-catenin expression by small interfering RNA decreased the apoptotic response to TGF-β.

Mechanisms for 2-methoxyestradiol-induced apoptosis of prostate cancer cells

Prostate and breast carcinomas are sex hormone‐related carcinomas, which are known to be associated with an over‐expression of the proto‐oncogene Bcl‐2. Here, we report that 2‐methoxyestradiol (2‐ME), an endogenous metabolite of estrogen that does not bind to nuclear estrogen receptors, effectively induces apoptosis in Bcl‐2‐expressing human prostate and breast carcinoma cells in vitro and in a rat prostate tumor model in vivo. In several cell lines derived from prostate, breast, liver and colorectal carcinomas, 2‐ME treatment led to an activation of c‐Jun N‐terminal kinase (JNK) and phosphorylation of Bcl‐2, which preceded the induction of apoptosis. In summary, our data suggest that 2‐ME induces apoptosis in epithelial carcinomas by causing phosphorylation of JNK, which appears to be correlated with phosphorylation of Bcl‐2.

Soy and rye diets inhibit the development of Dunning R3327 prostatic adenocarcinoma in rats

Two experiments were conducted to investigate the effect of soy and rye on the development of Dunning R3327 prostatic adenocarcinoma in rats.