Crosstalk of the Wnt/β-Catenin Signaling Pathway in the Induction of Apoptosis on Cancer Cells

Abstract

The Wnt/β-catenin signaling pathway plays a major role in cell survival and proliferation, as well as in angiogenesis, migration, invasion, metastasis, and stem cell renewal in various cancer types. However, the modulation (either up- or downregulation) of this pathway can inhibit cell proliferation and apoptosis both through β-catenin-dependent and independent mechanisms, and by crosstalk with other signaling pathways in a wide range of malignant tumors. Existing studies have reported conflicting results, indicating that the Wnt signaling can have both oncogenic and tumor-suppressing roles, depending on the cellular context. This review summarizes the available information on the role of the Wnt/β-catenin pathway and its crosstalk with other signaling pathways in apoptosis induction in cancer cells and presents a modified dual-signal model for the function of β-catenin. Understanding the proapoptotic mechanisms induced by the Wnt/β-catenin pathway could open new therapeutic opportunities.

Keywords: apoptosis; cancer cells; crosstalk; signaling; β-catenin.

Figure 1

Non-canonical and canonical Wnt pathways. In the non-canonical Wnt/PCP and Wnt/Ca²⁺ pathway (A), the interaction of Wnt/Fzd/ROR and RYK promotes the activation of small GTPases like Rac1, Cdc42, and RhoA, which activate JNK and ROCK, inducing cell polarity and cytoskeletal reorganization through the expression of the target gene induced by the AP-1 transcriptional factor (PCP pathway). The interaction of Wnt/Fzd with G proteins induces the activation of PLC, promoting the cleavage of PIP₂ to DAG and PIP₃, causing the activation of PKC and calcium release by IP₃ from the endoplasmic reticulum. Calcium activates CAMK, with the subsequent transactivation of NFAT and the regulation of EMT, cell invasion, and migration (Wnt/Ca²⁺ pathway). With respect to the canonical Wnt/β-catenin pathway (B), in the absence of Wnt ligands, the pathway is turned off. Cytosolic β-Catenin is phosphorylated by a Destruction Complex formed by APC, Axin, CKI, and GSK3β, which lead to its proteasomal degradation. In the nucleus, the transcriptional factors TCF/LEF bind the transcriptional repressors Groucho/HDAC. On the other hand, when activated, Wnt binds Fzd/ Lrp5/6, activating Dvl, which recruits Axin, favoring the dissociation and inactivation of the Destruction Complex and an ensuing cytoplasmic accumulation (and a nuclear accumulation at latter times) of β-catenin; in the nucleus, β-catenin interacts with TCF/LEF and transcriptional coactivators like Bcl-9, CBP, and p300 to transcribe its target genes, regulating cell survival, proliferation, EMT transition, differentiation, cell cycle arrest, and apoptosis. Continue arrows (↓) indicate activation; arrows with (⊥) indicate inhibition.

Figure 2

The crosstalk of the Wnt/β-catenin pathway with the Hippo, TGFβ, EGFR, VEGF, c-Met, and Notch signaling pathways modulates cancer progression and apoptosis, depending on the cellular context. An activation of the canonical Wnt pathway induces the stability of cytoplasmic β-catenin when the Wnt ligand binds FZ/Lrp5/6. This leads to a phosphorylation of Dvl, which inactivates the Destruction Complex, allowing an increase in the levels of free cytoplasmic β-catenin, which is then translocated to the nucleus, bound to TCF/LEF and transcription activators, to induce genic expression. On the other hand, the Hippo signaling increases β-catenin cytoplasmic levels by inactivating YAP/TAZ via SAV/MST1/2/LATS 1/2. When phosphorylated, YAP/TAZ activates the destruction complex via Dvl inhibition and sequestration of β-catenin. However, β-catenin also inhibits YAP/TAZ via Dvl and proteasomal activity, blocking the genic expression induced by YAP/TAZ. The Hippo signaling is activated via CD44/NF2 and p53. The TGFβ signaling pathway can also induce the cytoplasmic stability of β-catenin through smad 7. In turn, smad 7 interacts with TAK1 and MKK3, leading to the activation of p38 and the subsequent activation of AKT, inactivation of GSK 3β, and stabilization of β-catenin. The EGF pathway also can induce an increase in free cytoplasmic β-catenin by promoting the phosphorylation (inactivation) of α-catenin bound to E-cadherin through ERK activation. The VEGFR pathway also can increase the cytoplasmic levels of β-catenin through eNOS activation, which induces the S-nitrosylation of β-catenin, promoting its dissociation from α-catenin and its nuclear translocation. In addition, after the HGF ligand binds its receptor, c-MET promotes the phosphorylation of c-Met-associated β-catenin and a subsequent release of free cytoplasmic β-catenin. However, the activation of the Notch pathway by its Jagger ligand can decrease the cytoplasmic levels of β-catenin through the induction of DKK and Yap/Taz transcripts, which inhibit the Wnt/β-catenin pathway. Conversely, the Wnt/β-catenin pathway inhibits the Notch pathway through Dvl and DP1. Additionally, β-catenin induces the activation of the pro-apoptotic transcriptional factor FOXO. Depending on the cellular context, this could lead either to a carcinogenic process or a cell death process on cancer cells. Continue arrows (↓) indicate activation, arrows with (⊥) indicate inhibition.

Figure 3

Extrinsic and intrinsic apoptosis pathway and its regulation via p53. Under cellular stress, the extrinsic pathway is started by ligand binding to death receptors, including TNFα, Fas and TRAIL; this leads to the autoactivation of caspases-8 and -10, which in turn promote the catalytic activation of the effector caspase-3. Another target of caspase-8 is the pro-apoptotic protein Bid, which is hydrolyzed to tBid, inducing Bax oligomerization and mitochondria depolarization with release of cyt c. Along with the activation of caspase-9, these events amplify the apoptotic pathway. The intrinsic pathway involves the permeabilization of the mitochondrial external membrane, which facilitates the cytosolic release of pro-apoptotic proteins like SMAC/Diablo and cyt c, which are otherwise confined within the intermembrane space. cyt c binds the Apaf-1 protein, which in turn binds and activates caspase-9, responsible for the activation of apoptosis executioners: Caspases-3, -6, and -7. On the other hand, SMAC/Diablo inhibits IAPs, which bind and neutralize caspases-8 and -10. Another protein involved in apoptosis regulation is p53, which transcriptionally activates pro-apoptotic genes and inhibits anti-apoptotic genes, directly inhibiting Bcl_XL and Bcl-2 in the mitochondria, favoring apoptosis. Continue arrows (↓) indicate activation, arrows with (⊥) indicate inhibition.

Figure 4

Suggested pathway initiated by β-catenin to promote the induction of apoptotic cell death on cancer cells. β-catenin can be stabilized by the Wnt/β-catenin pathway and/or the EGF and TGF-β signaling pathways. β-catenin can bind and activate TCF4/LEF, which may activate the transcription of p53 and c-myc. C-myc can increase the genic expression of p14^ARF, fas, Trail, fasR, and DR4/5. FasR and DR4/5 activate the apoptotic extrinsic pathway, which is initiated by the binding of their respective ligands. This leads to the autoactivation of caspases-8 and -10, which in turn promote the catalytic activation of the effector caspase-3. Another target of caspase-8 is the pro-apoptotic protein Bid, which is hydrolyzed to tBid, inducing Bax oligomerization and mitochondrial depolarization with release of cyt c. Along with the activation of caspase-9, these events amplify the apoptotic pathway. On the other hand, p14^ARF inhibits mdm2, which induces the ubiquitination and ensuing degradation of p53. p53 can induce apoptosis via transactivation of pro-apoptotic genes such as Noxa, Puma, Bax, and Bid, which inhibit the Bcl-2 and Bcl_XL anti-apoptotic proteins. p53 also acts by directly inhibiting Bcl_XL and Bcl-2 in the mitochondria, inducing the permeabilization of the mitochondrial outer membrane, with the ensuing release of cyt c y the activation of apoptosis intrinsic pathway. Furthermore, p53 induces c-myc genic expression. Continue arrows (↓) indicate activation, arrows with (⊥) indicate inhibition.