Beta-catenin control of T-cell transcription factor 4 (Tcf4) importation from the cytoplasm to the nucleus contributes to Tcf4-mediated transcription in 293 cells

MAGP2 promotes osteogenic differentiation during fracture healing through its crosstalk with the β-catenin pathway

Osteogenic differentiation is important for fracture healing. Microfibrial-associated glycoprotein 2 (MAGP2) is found to function as a proangiogenic regulator in bone formation; however, its role in osteogenic differentiation during bone repair is not clear. Here, a mouse model of critical-sized femur fracture was constructed, and the adenovirus expressing MAGP2 was delivered into the fracture site. Mice with MAGP2 overexpression exhibited increased bone mineral density and bone volume fraction (BV/TV) at Day 14 postfracture. Within 7 days postfracture, overexpression of MAGP2 increased collagen I and II expression at the fracture callus, with increasing chondrogenesis. MAGP2 inhibited collagen II level but elevated collagen I by 14 days following fracture, accompanied by increased endochondral bone formation. In mouse osteoblast precursor MC3T3-E1 cells, MAGP2 treatment elevated the expression of osteoblastic factors (osterix, BGLAP and collagen I) and enhanced ALP activity and mineralization through activating β-catenin signaling after osteogenic induction. Besides, MAGP2 could interact with lipoprotein receptor-related protein 5 (LRP5) and upregulated its expression. Promotion of osteogenic differentiation and β-catenin activation mediated by MAGP2 was partially reversed by LRP5 knockdown. Interestingly, β-catenin/transcription factor 4 (TCF4) increased MAGP2 expression probably by binding to MAGP2 promoter. These findings suggest that MAGP2 may interact with β-catenin/TCF4 to enhance β-catenin/TCF4's function and activate LRP5-activated β-catenin signaling pathway, thus promoting osteogenic differentiation for fracture repair. mRNA sequencing identified the potential targets of MAGP2, providing novel insights into MAGP2 function and the directions for future research.

Human steroid sulfatase induces Wnt/β-catenin signaling and epithelial-mesenchymal transition by upregulating Twist1 and HIF-1α in human prostate and cervical cancer cells

Steroid sulfatase (STS) catalyzes the hydrolysis of estrone sulfate and dehydroepiandrosterone sulfate (DHEAS) to their unconjugated biologically active forms. Although STS is considered a therapeutic target for estrogen-dependent diseases, the cellular functions of STS remain unclear. We found that STS induces Wnt/β-catenin s Delete ignaling in PC-3 and HeLa cells. STS increases levels of β-catenin, phospho-β-catenin, and phospho-GSK3β. Enhanced translocation of β-catenin to the nucleus by STS might activate transcription of target genes such as cyclin D1, c-myc, and MMP-7. STS knockdown by siRNA resulted in downregulation of Wnt/β-catenin signaling. β-Catenin/TCF-mediated transcription was also enhanced by STS. STS induced an epithelial-mesenchymal transition (EMT) as it reduced the levels of E-cadherin, whereas levels of mesenchymal markers such as N-cadherin and vimentin were enhanced. We found that STS induced Twist1 expression through HIFα activation as HIF-1α knockdown significantly blocks the ability of STS to induce Twist1 transcription. Furthermore, DHEA, but not DHEAS is capable of inducing Twist1. Treatment with a STS inhibitor prevented STS-mediated Wnt/β-catenin signaling and Twist1 expression. Interestingly, cancer cell migration, invasion, and MMPs expression induced by STS were also inhibited by a STS inhibitor. Taken together, these results suggest that STS induces Wnt/β-catenin signaling and EMT by upregulating Twist1 and HIF-1α. The ability of STS to induce the Wnt/β-catenin signaling and EMT has profound implications on estrogen-mediated carcinogenesis in human cancer cells.

Wnt pathway activation and ABCB1 expression account for attenuation of proteasome inhibitor-mediated apoptosis in multidrug-resistant cancer cells

Multiple drug resistance (MDR) is a major obstacle to attenuating the effectiveness of chemotherapy to many human malignancies. Proteasome inhibition induces apoptosis in a variety of cancer cells and is recognized as a novel anticancer therapy approach. Despite its success, some multiple myeloma patients are resistant or become refractory to ongoing treatment by bortezomib suggesting that chemoresistant cancer cells may have developed a novel mechanism directed against the proteasome inhibitor. The present study aimed to investigate potential mechanism(s) of attenuation in a MDR cell line, MES-SA/Dx5. We found that compared to the parental human uterus sarcoma cell line MES-SA cells, MES-SA/Dx5 cells highly expressed the ABCB1 was more resistant to MG132 and bortezomib, escaping the proteasome inhibitor-induced apoptosis pathway. The resistance was reversed by co-treatment of MG132 and the ABCB1 inhibitor verapamil. The data indicated that ABCB1 might play a role in the efflux of MG132 from the MES-SA/Dx5 cells to reduce MG132-induced apoptosis. Furthermore, the canonical Wnt pathway was found activated only in the MES-SA/Dx5 cells through active β-catenin and related transactivation activities. Western blot analysis demonstrated that Wnt-targeting genes, including c-Myc and cyclin D1, were upregulated and were relevant in inhibiting the expression of p21 in MES-SA/Dx5 cells. On the other hand, MES-SA cells expressed high levels of p21 and downregulated cyclin D1 and caused cell cycle arrest. Together, our study demonstrated the existence and participation of ABCB1 and the Wnt pathway in an MDR cell line that attenuated proteasome inhibitor-induced apoptosis.

Targeting the β-catenin nuclear transport pathway in cancer

The nuclear localization of specific proteins is critical for cellular processes such as cell division, and in recent years perturbation of the nuclear transport cycle of key proteins has been linked to cancer. In particular, specific gene mutations can alter nuclear transport of tumor suppressing and oncogenic proteins, leading to cell transformation or cancer progression. This review will focus on one such factor, β-catenin, a key mediator of the canonical wnt signaling pathway. In response to a wnt stimulus or specific gene mutations, β-catenin is stabilized and translocates to the nucleus where it binds TCF/LEF-1 transcription factors to transactivate genes that drive tumor formation. Moreover, the nuclear import and accumulation of β-catenin correlates with clinical tumor grade. Recent evidence suggests that the primary nuclear transport route of β-catenin is independent of the classical Ran/importin import machinery, and that β-catenin directly contacts the nuclear pore complex to self-regulate its own entry into the nucleus. Here we propose that the β-catenin nuclear import pathway may provide an opportunity for identification of specific drug targets and inhibition of β-catenin nuclear function, much like the current screening of drugs that block binding of β-catenin to LEF-1/TCFs. Here we will discuss the diverse mechanisms regulating nuclear localization of β-catenin and their potential as targets for anticancer agent development.

The WNT signaling pathway from ligand secretion to gene transcription: molecular mechanisms and pharmacological targets

Wingless/integrase-1 (WNT) signaling is a key pathway regulating various aspects of embryonic development; however it also underlies several pathological conditions in man, including various cancers and fibroproliferative diseases in several organs. Investigating the molecular processes involved in (canonical) WNT signaling will open new avenues for generating new therapeutics to specifically target diseases in which WNT signaling is aberrantly regulated. Here we describe the complexity of WNT signal transduction starting from the processes involved in WNT ligand biogenesis and secretion by WNT producing cells followed by a comprehensive overview of the molecular signaling events ultimately resulting in enhanced transcription of specific genes in WNT receiving cells. Finally, the possible targets for therapeutic intervention and the available pharmacological inhibitors for this complex signaling pathway are discussed.

Specific armadillo repeat sequences facilitate β-catenin nuclear transport in live cells via direct binding to nucleoporins Nup62, Nup153, and RanBP2/Nup358

β-Catenin transduces the Wnt signal from the membrane to nucleus, and certain gene mutations trigger its nuclear accumulation leading to cell transformation and cancer. β-Catenin shuttles between the nucleus and cytoplasm independent of classical Ran/transport receptor pathways, and this movement was previously hypothesized to involve the central Armadillo (Arm) domain. Fluorescence recovery after photobleaching (FRAP) assays were used to delineate functional transport regions of the Arm domain in living cells. The strongest nuclear import/export activity was mapped to Arm repeats R10-12 using both in vivo FRAP and in vitro export assays. By comparison, Arm repeats R3-8 of β-catenin were highly active for nuclear import but displayed a comparatively weak export activity. We show for the first time using purified components that specific Arm sequences of β-catenin interact directly in vitro with the FG repeats of the nuclear pore complex (NPC) components Nup62, Nup98, and Nup153, indicating an independent ability of β-catenin to traverse the NPC. Moreover, a proteomics screen identified RanBP2/Nup358 as a binding partner of Arm R10-12, and β-catenin was confirmed to interact with endogenous and ectopic forms of Nup358. We further demonstrate that knock-down of endogenous Nup358 and Nup62 impeded the rate of nuclear import/export of β-catenin to a greater extent than that of importin-β. The Arm R10-12 sequence facilitated transport even when β-catenin was bound to the Arm-binding partner LEF-1, and its activity was stimulated by phosphorylation at Tyr-654. These findings provide functional evidence that the Arm domain contributes to regulated β-catenin transport through direct interaction with the NPC.

Binary cell fate specification duringC. elegansembryogenesis driven by reiterated reciprocal asymmetry of TCF POP-1 and its coactivatorβ-catenin SYS-1

C. elegans embryos exhibit an invariant lineage comprised primarily of a stepwise binary diversification of anterior-posterior (A-P)blastomere identities. This binary cell fate specification requires input from both the Wnt and MAP kinase signaling pathways. The nuclear level of the TCF protein POP-1 is lowered in all posterior cells. We show here that theβ-catenin SYS-1 also exhibits reiterated asymmetry throughout multiple A-P divisions and that this asymmetry is reciprocal to that of POP-1. Furthermore, we show that SYS-1 functions as a coactivator for POP-1, and that the SYS-1-to-POP-1 ratio appears critical for both the anterior and posterior cell fates. A high ratio drives posterior cell fates, whereas a low ratio drives anterior cell fates. We show that the SYS-1 and POP-1 asymmetries are regulated independently, each by a subset of genes in the Wnt/MAP kinase pathways. We propose that two genetic pathways, one increasing SYS-1 and the other decreasing POP-1 levels, robustly elevate the SYS-1-to-POP-1 ratio in the posterior cell, thereby driving A-P differential cell fates.