Rac1 GTPase and the Rac1 exchange factor Tiam1 associate with Wnt-responsive promoters to enhance beta-catenin/TCF-dependent transcription in colorectal cancer cells

The Roles of RAC1 and RAC1B in Colorectal Cancer and Their Potential Contribution to Cetuximab Resistance

Colorectal cancer (CRC) is one of the most diagnosed cancers and a leading contributor to cancer-related deaths in the United States. Clinically, standard treatment regimens include surgery, radiation, and chemotherapy; however, there has been increasing development and clinical use of targeted therapies for CRC. Unfortunately, many patients develop resistance to these treatments. Cetuximab, the first targeted therapy approved to treat advanced CRC, is a monoclonal antibody that targets the epidermal growth factor receptor and inhibits downstream pathway activation to restrict tumor cell growth and proliferation. CRC resistance to cetuximab has been well studied, and common resistance mechanisms include constitutive signal transduction through downstream protein mutations and promotion of the epithelial-to-mesenchymal transition. While the most common resistance mechanisms are known, a proportion of patients develop resistance through unknown mechanisms. One protein predicted to contribute to therapy resistance is RAC1, a small GTPase that is involved in cytoskeleton rearrangement, cell migration, motility, and proliferation. RAC1 has also been shown to be overexpressed in CRC. Despite evidence that RAC1 and its alternative splice isoform RAC1B play important roles in CRC and the pathways known to contribute to cetuximab resistance, there is a need to directly study the relationship between RAC1 and RAC1B and cetuximab resistance. This review highlights the recent studies investigating RAC1 and RAC1B in the context of CRC and suggests that these proteins could play a role in resistance to cetuximab.

RASAL2 mediated the enhancement of YAP1/TIAM1 signaling promotes malignant phenotypes of pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is characterized by a high incidence of metastasis and dismal prognosis. As a member of Gas-Gap gene, RASAL2 is involved in the hydrolysis of RAS-GTP to RAS-GDP and abnormal expression in human cancers. Here we firstly described the function of RASAL2 on PDAC to enrich the knowledge of RAS family.We interestingly observed that RASAL2 expression was upregulated in PDAC at both mRNA and protein levels, and high expression of RASAL2 predicted a poor prognosis in PDAC patients. Additionally, RASAL2 promoted malignant behaviors of PDAC in vitro and in vivo. To determine the mechanistic roles of RASAL2 signaling and its potential as a therapeutic target in PDAC, we clarified that RASAL2 could accumulate the TIAM1 expression in different level through inhibiting YAP1 phosphorylation, increased TIAM1 mRNA expression and suppressed ubiquitination of TIAM1 protein. In conclusion, RASAL2 enhances YAP1/TIAM1 signaling and promotes PDAC development and progression.

Cross-Talk between Wnt Signaling and Src Tyrosine Kinase

Src, a non-receptor tyrosine kinase, was first discovered as a prototype oncogene and has been shown to critical for cancer progression for a variety of tissues. Src activity is regulated by a number of post-translational modifications in response to various stimuli. Phosphorylations of Src Tyr419 (human; 416 in chicken) and Src Tyr530 (human; 527 in chicken) have been known to be critical for activation and inactivation of Src, respectively. Wnt signaling regulates a variety of cellular functions including for development and cell proliferation, and has a role in certain diseases such as cancer. Wnt signaling is carried out through two pathways: β-catenin-dependent canonical and β-catenin-independent non-canonical pathways as Wnt ligands bind to their receptors, Frizzled, LRP5/6, and ROR1/2. In addition, many signaling components including Axin, APC, Damm, Dishevelled, JNK kinase and Rho GTPases contribute to these canonical and non-canonical Wnt pathways. However, the communication between Wnt signaling and Src tyrosine kinase has not been well reviewed as Src regulates Wnt signaling through LRP6 tyrosine phosphorylation. GSK-3β phosphorylated by Wnt also regulates Src activity. As Wnt signaling and Src mutually regulate each other, it is noted that aberrant regulation of these components give rise to various diseases including typically cancer, and as such, merit a closer look.

Rac1 regulates nucleus pulposus cells degeneration and promotes the progression of intervertebral disc degeneration

Nucleus Pulposus Cells (NPCs) dysfunction is considered as an important event related to intervertebral disc degeneration (IVDD). In the present study, tandem mass spectrometry (TMT) was used to detect total protein expression of NP in patients with IVDD and healthy controls. Bioinformatic analysis was performed to identify differentially expressed proteins that may be involved in the degeneration of NP. The results show that Rac1 may be a key protein involved in the degeneration of NP via Wnt/β-catenin pathway activation. We investigated the influence of Rac1 on IVDD degeneration and associated mechanisms. This study has the potential to advance understanding of the mechanism of occurrence of degenerative NP tissues and provide novel strategies for slowing IVDD progression.

Cytoskeleton Dynamics in Peripheral T Cell Lymphomas: An Intricate Network Sustaining Lymphomagenesis

Defects in cytoskeleton functions support tumorigenesis fostering an aberrant proliferation and promoting inappropriate migratory and invasive features. The link between cytoskeleton and tumor features has been extensively investigated in solid tumors. However, the emerging genetic and molecular landscape of peripheral T cell lymphomas (PTCL) has unveiled several alterations targeting structure and function of the cytoskeleton, highlighting its role in cell shape changes and the aberrant cell division of malignant T cells. In this review, we summarize the most recent evidence about the role of cytoskeleton in PTCLs development and progression. We also discuss how aberrant signaling pathways, like JAK/STAT3, NPM-ALK, RhoGTPase, and Aurora Kinase, can contribute to lymphomagenesis by modifying the structure and the signaling properties of cytoskeleton.

Dendrimer Architectonics to Treat Cancer and Neurodegenerative Diseases with Implications in Theranostics and Personalized Medicine

Integration of diagnostic and therapeutic functions in a single platform namely theranostics has become a cornerstone for personalized medicine. Theranostics platform facilitates noninvasive detection and treatment while allowing the monitoring of disease progression and therapeutic efficacy in case of chronic conditions of cancer and Alzheimer's disease (AD). Theranostic tools function by themselves or with the aid of carrier, viz. liposomes, micelles, polymers, or dendrimers. The dendrimer architectures (DA) are well-characterized molecular nanoobjects with a large number of terminal functional groups to enhance solubility and offer multivalency and multifunctional properties. Various noninvasive diagnostic tools like magnetic resonance imaging (MRI), computed tomography (CT), gamma scintigraphy, and optical techniques have been accomplished utilizing DAs for simultaneous imaging and drug delivery. Obstacles in the formulation design, drug loading, payload delivery, biocompatibility, overcoming cellular membrane and blood-brain barrier (BBB), and systemic circulation remain a bottleneck in translational efforts. This review focuses on the diagnostic, therapeutic and theranostic potential of DA-based nanocarriers in treating cancer and neurodegenerative disorders like AD and Parkinson's disease (PD), among others. In view of the inverse relationship between cancer and AD, designing suitable DA-based theranostic nanodrug with high selectivity has tremendous implications in personalized medicine to treat cancer and neurodegenerative disorders.

RAC1 plays an essential role in estrogen receptor alpha function in breast cancer cells

The activity of Rho family GTPase protein, RAC1, which plays important normal physiological functions, is dysregulated in multiple cancers. RAC1 is expressed in both estrogen receptor alpha (ER)-positive and ER-negative breast cancer (BC) cells. However, ER-positive BC is more sensitive to RAC1 inhibition. We have determined that reducing RAC1 activity, using siRNA or EHT 1864 (a small molecule Rac inhibitor), leads to rapid ER protein degradation. RAC1 interacts with ER within the ER complex and RAC1 localizes to chromatin binding sites for ER upon estrogen treatment. RAC1 activity is important for RNA Pol II function at both promoters and enhancers of ER target genes and ER-regulated gene transcription is blocked by EHT 1864, in a dose-dependent manner. Having identified that RAC1 is an essential ER cofactor for ER protein stability and ER transcriptional activity, we report that RAC1 inhibition could be an effective therapeutic approach for ER-positive BC.

RhoA GTPase phosphorylated at tyrosine 42 by src kinase binds to β-catenin and contributes transcriptional regulation of vimentin upon Wnt3A

In the Wnt canonical pathway, Wnt3A has been known to stabilize β-catenin. In the non-canonical Wnt signaling pathway, Wnt is known to activate Rho GTPases. The correlation between canonical and non-canonical pathways by Wnt signaling, however, has not been well elucidated. Here, we identified that Wnt3A promoted superoxide generation, leading to Tyr42 phosphorylation of RhoA through activations of c-Src and Rho-dependent coiled coil kinase 2 (ROCK2) and phosphorylation of p47phox, a component of NADPH oxidase. Wnt3A also induced accumulation of β-catenin along with activations of RhoA and ROCK1. Concurrently, ROCK1 was able to phosphorylate GSK-3β at Ser9, which phosphorylated Src at Ser51 and Ser492 residues, leading to Src inactivation through dephosphorylation of Tyr416 during the late period of Wnt3A treatment. Meanwhile, p-Tyr42 RhoA bound to β-catenin via the N-terminal domain of β-catenin, thereby leading to the nuclear translocation of p-Tyr42 RhoA/β-catenin complex. Notably, p-Tyr42 RhoA as well as β-catenin was associated with the promoter of Vim, leading to increased expression of vimentin. In addition, stomach cancer patients harboring higher expressed p-Tyr42 Rho levels revealed the much poorer survival probability. Therefore, we propose that p-Tyr42 RhoA is crucial for transcriptional regulation of specific target genes in the nucleus by binding to their promoters and involved in tumorigenesis.

Mechanisms and consequences of dysregulation of the Tiam family of Rac activators in disease

The Tiam family proteins - Tiam1 and Tiam2/STEF - are Rac1-specific Guanine Nucleotide Exchange Factors (GEFs) with important functions in epithelial, neuronal, immune and other cell types. Tiam GEFs regulate cellular migration, proliferation and survival, mainly through activating and directing Rac1 signalling. Dysregulation of the Tiam GEFs is significantly associated with human diseases including cancer, immunological and neurological disorders. Uncovering the mechanisms and consequences of dysregulation is therefore imperative to improving the diagnosis and treatment of diseases. Here we compare and contrast the subcellular localisation and function of Tiam1 and Tiam2/STEF, and review the evidence for their dysregulation in disease.

Clostridioides difficile Toxin A-Induced Wnt/β-Catenin Pathway Inhibition Is Mediated by Rac1 Glucosylation

Clostridioides difficile toxin A (TcdA) has been shown to inhibit cellular Wnt signaling, the major driving force behind the proliferation of epithelial cells in colonic crypts, likely through the inhibition of β-catenin nuclear translocation. Herein, we aimed to advance the understanding of this mechanism by replicating the findings in vivo and by investigating the specific role of Rac1, a member of the Rho GTPase family, on the inhibition of the Wnt-induced β-catenin nuclear translocation triggered by TcdA. To investigate the effects of TcdA on the Wnt/β-catenin pathway in vivo, we injected the ileal loops of C57BL/6 mice with TcdA [phosphate-buffered saline (PBS) as the control] to induce C. difficile disease-like ileitis. After 4 h post-injection, we obtained ileum tissue samples to assess Wnt signaling activation and cell proliferation through Western blotting, immunohistochemistry, and qPCR. To assess the role of Rac1 on Wnt signaling inhibition by TcdA, we transfected rat intestinal epithelial cells (IEC-6) with either a constitutively active Rac1 plasmid (pcDNA3-EGFP-Rac1-Q61L) or an empty vector, which served as the control. We incubated these cells with Wnt3a-conditioned medium (Wnt3a-CM) to induce Wnt/β-catenin pathway activation, and then challenged the cells with TcdA. We assessed Wnt signaling activation in vitro with TOP/FOPflash luciferase assays, determined nuclear β-catenin translocation by immunofluorescence, measured cyclin D1 protein expression by Western blotting, and quantified cell proliferation by Ki67 immunostaining. In vivo, TcdA decreased β-catenin, cyclin D1, and cMYC expression and inhibited the translocation of β-catenin into the nucleus in the ileum epithelial cells. In addition, TcdA suppressed cell proliferation and increased Wnt3a expression, but did not alter Rac1 gene expression in the ileum tissue. In vitro, constitutively active Rac1 prevented Wnt signaling inhibition by enabling the β-catenin nuclear translocation that had been blocked by TcdA. Our results show that TcdA inhibits Wnt/β-catenin pathway in vivo and demonstrate that this inhibition is likely caused by a Rac1-mediated mechanism.

DOCK6 promotes chemo- and radioresistance of gastric cancer by modulating WNT/β-catenin signaling and cancer stem cell traits

Gastric cancer (GC) is the third leading cause of cancer-related mortality worldwide and prognosis after potentially curative gastrectomy remains poor. Administration of GC-targeting molecules in combination with adjuvant chemo- or radiotherapy following surgical resection has been proposed as a potentially effective treatment option. Here, we have identified DOCK6, a guanine nucleotide exchange factor (GEF) for Rac1 and CDC42, as an independent biomarker for GC prognosis. Clinical findings indicate the positive correlation of higher DOCK6 expression with tumor size, depth of invasion, lymph node metastasis, vascular invasion, and pathological stage. Furthermore, elevated DOCK6 expression was significantly associated with shorter cumulative survival in both univariate and multivariate analyses. Gene ontology analysis of three independent clinical GC cohorts revealed significant involvement of DOCK6-correlated genes in the WNT/β-catenin signaling pathway. Ectopic expression of DOCK6 promoted GC cancer stem cell (CSC) characteristics and chemo- or radioresistance concomitantly through Rac1 activation. Conversely, depletion of DOCK6 suppressed CSC phenotypes and progression of GC, further demonstrating the pivotal role of DOCK6 in GC progression. Our results demonstrate a novel mechanistic link between DOCK6, Rac1, and β-catenin in GCCSC for the first time, supporting the utility of DOCK6 as an independent marker of GC.

Association between variation of circulating 25-OH vitamin D and methylation of secreted frizzled-related protein 2 in colorectal cancer

BACKGROUNDS

Colorectal cancer (CRC) results from the accumulation of epigenetic and genetic changes in colon cells during neoplasic transformation, which the activation of Wingless (Wnt) signaling pathway is a common mechanism for CRC initiation. The Wnt pathway is mainly regulated by Wnt antagonists, as secreted frizzled-related protein (SFRP) family. Indeed, SFRP2 is proposed as a noninvasive biomarker for CRC diagnosis. Vitamin D also antagonizes Wnt signaling in colon cancers cells. Several studies showed that vitamin D was able to alter DNA methylation, although this mechanism is not yet clear. Therefore, the aim of this study was to find an association between circulating 25-OH vitamin D (30th percentile of vitamin D) and the SFRP2 methylation.

METHODS

A total of 67 CRC patients were included in the study. These patients were subdivided into two groups based on their 30th percentile vitamin D (20 patients were below, and 47 participants were above the 30th percentile of vitamin D). We investigated the SFRP2 methylation in peripheral blood mononuclear cells (PBMCs), visceral adipose tissue (VAT), CRC tumor tissue, and adjacent tumor-free area. We also determined the relationship between SFRP2 methylation and methylation of carcinogenic and adipogenic genes. Finally, we tested the effect of vitamin D on the SFRP2 methylation in human colorectal carcinoma cell lines 116 (HCT116) and studied the association of neoadjuvant therapy under the 30th percentile vitamin D with SFRP2 promoter methylation.

RESULTS

SFRP2 methylation in tumor area was decreased in patients who had higher levels of vitamin D. SFRP2 promoter methylation was positively correlated in tumor area with insulin and homeostasis model assessment of insulin resistance (HOMA-IR) but negatively correlated with HDL-c. SFRP2 methylation was also correlated with T cell lymphoma invasion and metastasis 1 (TIAM1) methylation in tumor area and CCAAT/enhancer-binding protein alpha (C/EBPα) in VAT. Treatment with vitamin D did not affect SFRP2 methylation in HCT116 cell line. Finally, neoadjuvant treatment was correlated with higher circulating 25-OH vitamin D and SFRP2 methylation under linear regression model.

CONCLUSION

Our results showed that higher circulating vitamin D is associated with low SFRP2 promoter methylation. Therefore, our results could suggest that vitamin D may have an epigenetic effect on DNA methylation. Finally, higher vitamin D could contribute to an improvement response to neoadjuvant treatment.

Rac1 Signaling: From Intestinal Homeostasis to Colorectal Cancer Metastasis

The small GTPase Rac1 has been implicated in a variety of dynamic cell biological processes, including cell proliferation, cell survival, cell-cell contacts, epithelial mesenchymal transition (EMT), cell motility, and invasiveness. These processes are orchestrated through the fine tuning of Rac1 activity by upstream cell surface receptors and effectors that regulate the cycling Rac1-GDP (off state)/Rac1-GTP (on state), but also through the tuning of Rac1 accumulation, activity, and subcellular localization by post translational modifications or recruitment into molecular scaffolds. Another level of regulation involves Rac1 transcripts stability and splicing. Downstream, Rac1 initiates a series of signaling networks, including regulatory complex of actin cytoskeleton remodeling, activation of protein kinases (PAKs, MAPKs) and transcription factors (NFkB, Wnt/β-catenin/TCF, STAT3, Snail), production of reactive oxygen species (NADPH oxidase holoenzymes, mitochondrial ROS). Thus, this GTPase, its regulators, and effector systems might be involved at different steps of the neoplastic progression from dysplasia to the metastatic cascade. After briefly placing Rac1 and its effector systems in the more general context of intestinal homeostasis and in wound healing after intestinal injury, the present review mainly focuses on the several levels of Rac1 signaling pathway dysregulation in colorectal carcinogenesis, their biological significance, and their clinical impact.

Inhibition of Rac1 activity by NSC23766 prevents cartilage endplate degeneration via Wnt/β-catenin pathway

Cartilage endplate (CEP) degeneration has been considered as one of important factors related to intervertebral disc degeneration (IVDD). Previous researches have showed that Rac1 played a pivotal role in chondrocyte differentiation. However, the effect of Rac1 during the process of CEP degeneration remains unclear. Herein, we explored the effect of Rac1 on CEP degeneration and elucidated the underlying molecular mechanism. We found expression of Rac1‐GTP increased in human‐degenerated CEP tissue and IL‐1β‐stimulated rat endplate chondrocytes (EPCs). Our study revealed that Rac1 inhibitor NSC23766 treatment promoted the expression of collagen II, aggrecan and Sox‐9, and decreased the expression of ADTAMTS5 and MMP13 in IL‐1β‐stimulated rat EPCs. Moreover, we also found that NSC23766 could suppress the activation of Wnt/β‐catenin pathway, suggesting that the beneficial effects of Rac1 inhibition in EPCs are mediated through the Wnt/β‐catenin signalling. Besides, puncture‐induced rats models showed that NSC23766 played a protective role on CEP and disc degeneration. Collectively, these findings demonstrated that Rac1 inhibition delayed the EPCs degeneration and its potential mechanism may be associated with Wnt/β‐catenin pathway regulation, which may help us better understand the association between Rac1 and CEP degeneration and provide a promising strategy for delaying the progression of IVDD.

The Intestinal Tumour Microenvironment

The tumour microenvironment (TME) of intestinal tumours is highly complex and comprises a network of stromal cells, tumour cells, immune cells and fibroblasts, as well as microorganisms. The tumour location, environmental factors and the tumour cells themselves influence the cells within the TME. Immune cells can destroy tumour cells and are associated with better patient prognosis and response to therapy; however, immune cells are highly plastic and easily influenced to instead promote tumour growth. The interaction between local immune cells and the microbiome can lead to progression or regression of intestinal tumours. In this chapter, we will discuss how tumour development and progression can influence, and be influenced by, the microenvironment surrounding it, focusing on immune and fibroblastic cells, and the intestinal microbiota, particularly in the context of colorectal cancer.

TUFT1 Promotes Triple Negative Breast Cancer Metastasis, Stemness, and Chemoresistance by Up-Regulating the Rac1/β-Catenin Pathway

Objectives: Triple negative breast cancer (TNBC) is a subtype of breast cancer with stronger invasion and metastasis, but its specific mechanism of action is still unclear. Tuft1 plays an important regulatory role in the survival of breast cancer cells; however, its role in regulating TNBC metastatic potential has not been well-characterized. Our aim was therefore to systematically study the mechanism of TUFT1 in the metastasis, stemness, and chemoresistance of TNBC and provide new predictors and targets for BC treatment. Methods: We used western blotting and IHC to measure TUFT1and Rac1-GTP expression levels in both human BC samples and cell lines. A combination of shRNA, migration/invasion assays, sphere formation assay, apoptosis assays, nude mouse xenograft tumor model, and GTP activity assays was used for further mechanistic studies. Results: We demonstrated that silencing TUFT1 in TNBC cells significantly inhibited cell metastasis and stemness in vitro. A nude mouse xenograft tumor model revealed that TUFT1 knockdown greatly decreased spontaneous lung metastasis of TNBC tumors. Mechanism studies showed that TUFT1 promoted tumor cell metastasis and stemness by up-regulating the Rac1/β-catenin pathway. Moreover, mechanistic studies indicated that the lack of TUFT1 expression in TNBC cells conferred more sensitive to chemotherapy and increased cell apoptosis via down-regulating the Rac1/β-catenin signaling pathway. Further, TUFT1 expression positively correlated with Rac1-GTP in TNBC samples, and co-expression of TUFT1 and Rac1-GTP predicted poor prognosis in TNBC patients who treated with chemotherapy. Conclusion: Our findings suggest that TUFT1/Rac1/β-catenin pathway may provide a potential target for more effective treatment of TNBC.

TIAM1 promotes chemoresistance and tumor invasiveness in colorectal cancer

Accumulating evidence suggests that cancer cells with stem cell-like features have higher resistance to chemotherapeutic agents. Herein, we identified T-lymphoma invasion and metastasis-inducing protein-1 (TIAM1) as one of the Wnt-signaling associated genes which drives self-renewal and its expression is upregulated by cancer associated fibroblasts (CAFs). TIAM1 expression was assessed in resected colorectal cancer (CRC) tissues from 300 patients who did or did not respond to chemotherapy. siRNA and CRISPR/Cas9 was used to examine whether the inhibition of TIAM1 affects chemosensitivity of CRC. We demonstrate that stemness through Wnt signaling regulates chemosensitivity and this phenomenon occurs exclusively in cancer stem cells. Subsequently, we established patient-derived CAFs and tested whether the drug sensitivity of CRC cell lines is altered with CAF-derived conditioned medium. High-TIAM1 expression correlated significantly with poor prognosis of CRC patients, and was overexpressed in patients who did not respond to chemotherapy. We demonstrated that the inhibition of TIAM1 enhanced sensitivity to chemotherapeutic drugs and reduced tumor invasiveness in a series of experiments in vitro. Moreover, CAF-derived conditioned media increased stemness and chemoresistance in CRC cell lines through TIAM1 overexpression. In addition, we validated TIAM1 associated drug sensitivity using a xenograft model. We have demonstrated that TIAM1 is overexpressed in CRC tumors from patients who did not respond to chemotherapeutic drugs and levels of TIAM1 expression served as an independent prognostic factor. Mechanistically, CAFs enhanced CRC chemoresistance through TIAM1 overexpression. Collectively, these results suggest that TIAM1 regulates chemosensitivity in tumors and stroma and thus may be an attractive therapeutic target.

The prognostic value of Tiam1 correlates with its roles in epithelial-mesenchymal transition progression and angiogenesis in lung adenocarcinoma

Background

Tiam1 has been identified as an oncogene and acts as an activator of GTPase Rac. Tiam1 was reported to be a promoter of cancer progression in various cancer types, while in lung adenocarcinoma, its mechanism of action is poorly understood.

Materials and Methods

Immunohistochemistry staining and Western blot assay were used to determine Tiam1 expression in lung adenocarcinoma tissues, and its association with prognosis was determined by statistical analysis. We depleted Tiam1 in both A549 and H1975 cancer cell lines. Carboxyfluorescein diacetate succinimidyl ester staining and colony formation assays were used to evaluate its impact on cell proliferation ability after depletion. Transwell migration assay and wound healing assays were performed to determine its impact on migration ability of both cell lines. Western blot assay and immunofluorescence staining were used to analyze the association between Tiam1 and epithelial-mesenchymal transition (EMT) progression. Tube formation assay and vasculogenic mimicry assay were used to show the impact of Tiam1 depletion on cancer angiogenesis.

Results

In this study, we demonstrated that Tiam1 overexpression in lung adenocarcinoma was significantly associated with advanced tumor grade and poor prognosis. In vitro assays indicated that Tiam1 depletion significantly inhibited cell proliferation, colony formation, and migration capacities in A549 and H1975 cells. Further investigations revealed that Tiam1 plays an important role in EMT program enhancement, angiogenesis, and accelerated tumor progression. Notably, Tiam1 depletion in cancer cells strongly inhibited human umbilical vein endothelial cell angiogenesis and vasculogenic mimicry capacities of both cancer cell lines.

Conclusion

Tiam1 overexpression is associated with lung adenocarcinoma progression and may indicate poor prognosis. Tiam1 accelerated tumor progression due to EMT and angiogenesis enhancement. Our data may provide a novel therapeutic target for lung adenocarcinoma.

Opposing Regulation of Cancer Properties via KRT19-Mediated Differential Modulation of Wnt/β-Catenin/Notch Signaling in Breast and Colon Cancers

Although Keratin 19 (KRT19) has been reported as a tumor cell marker and found to interact with other proteins that modulate cancer properties, its role in cancer prognosis remains to be fully elucidated. We found that KRT19 expression was increased in both colon and breast cancer, but that knockdown of KRT19 showed opposing effects on cancer properties. In colon cancer, KRT19 knockdown resulted in suppression of cancer via downregulation of Wnt/Notch signaling without altering NUMB transcription. In breast cancer, KRT19 knockdown led to an increase in cancer properties because of attenuated Wnt and enhanced Notch signaling. In colon cancer, KRT19 interacted with β-catenin but not with RAC1, allowing the LEF/TCF transcription factor to bind primarily to the LEF1 and TCF7 promoter regions, whereas in breast cancer, KRT19 interacted with the β-catenin/RAC1 complex and led to apparent upregulation of NUMB expression and NUMB-mediated suppression of Notch signaling. These results reveal a novel differential role of KRT19 in carcinogenesis, due to differential modulation of Wnt/β-catenin/Notch signaling crosstalk through various interactions of KRT19 with only β-catenin or with the β-catenin/RAC1 complex, which might have implications for clinical cancer research.

Engineering of Human Corneal Endothelial Cells In Vitro

Human corneal endothelial cells are responsible for controlling corneal transparency, however they are notorious for their limited proliferative capability. Thus, damage to these cells may cause irreversible blindness. Currently, the only way to cure blindness caused by corneal endothelial dysfunction is via corneal transplantation of a cadaver donor cornea with healthy corneal endothelium. Due to severe shortage of donor corneas worldwide, it has become paramount to develop human corneal endothelial grafts in vitro that can subsequently be transplanted in humans. Recently, we have reported effective expansion of human corneal endothelial cells by reprogramming the cells into progenitor status through use of p120-Kaiso siRNA knockdown. This new reprogramming approach circumvents the need of using induced pluripotent stem cells or embryonic stem cells. Successful promotion of this technology will encourage scientists to re-think how "contact inhibition" can safely be perturbed to our benefit, i.e., effective engineering of an in vivo-like tissue while successful maintaining the normal phenotype. In this review, we present current advances in reprogramming corneal endothelial cells in vitro, detail the methods to successful engineer human corneal endothelial grafts, and discuss their future clinical applications to cure corneal blindness.

The Tiam1 guanine nucleotide exchange factor is auto-inhibited by its pleckstrin homology coiled-coil extension domain

T-cell lymphoma invasion and metastasis 1 (Tiam1) is a Dbl-family guanine nucleotide exchange factor (GEF) that specifically activates the Rho-family GTPase Rac1 in response to upstream signals, thereby regulating cellular processes including cell adhesion and migration. Tiam1 contains multiple domains, including an N-terminal pleckstrin homology coiled-coiled extension (PHn-CC-Ex) and catalytic Dbl homology and C-terminal pleckstrin homology (DH-PHc) domain. Previous studies indicate that larger fragments of Tiam1, such as the region encompassing the N-terminal to C-terminal pleckstrin homology domains (PHn-PHc), are auto-inhibited. However, the domains in this region responsible for inhibition remain unknown. Here, we show that the PHn-CC-Ex domain inhibits Tiam1 GEF activity by directly interacting with the catalytic DH-PHc domain, preventing Rac1 binding and activation. Enzyme kinetics experiments suggested that Tiam1 is auto-inhibited through occlusion of the catalytic site rather than by allostery. Small angle X-ray scattering and ensemble modeling yielded models of the PHn-PHc fragment that indicate it is in equilibrium between "open" and "closed" conformational states. Finally, single-molecule experiments support a model in which conformational sampling between the open and closed states of Tiam1 contributes to Rac1 dissociation. Our results highlight the role of the PHn-CC-Ex domain in Tiam1 GEF regulation and suggest a combinatorial model for GEF inhibition and activation of the Rac1 signaling pathway.

KRT19 directly interacts with β-catenin/RAC1 complex to regulate NUMB-dependent NOTCH signaling pathway and breast cancer properties

Studies have reported that interactions between keratins (KRTs) and other proteins initiate signaling cascades that regulate cell migration, invasion, and metastasis. In the current study, we found that expression of KRT19 was specifically high in breast cancers and significantly correlated with their invasiveness. Moreover, knockdown of KRT19 led to increased proliferation, migration, invasion, drug resistance, and sphere formation in breast cancer cells via an upregulated NOTCH signaling pathway. This was owing to reduced expression of NUMB, an inhibitory protein of the NOTCH signaling pathway. In addition, we found that KRT19 interacts with β-catenin/RAC1 complex and enhances the nuclear translocation of β-catenin. Concordantly, knockdown of KRT19 suppressed the nuclear translocation of β-catenin as well as β-catenin-mediated NUMB expression. Furthermore, modulation of KRT19-mediated regulation of NUMB and NOTCH1 expression led to the repression of the cancer stem cell properties of breast cancer patient-derived CD133high/CXCR4high/ALDH1high cancer stem-like cells (CSLCs), which showed very low KRT19 and high NOTCH1 expression. Taken together, our study suggests a novel function for KRT19 in the regulation of nuclear import of the β-catenin/RAC1 complex, thus modulating the NUMB-dependent NOTCH signaling pathway in breast cancers and CSLCs, which might bear potential clinical implications for cancer or CSLC treatment.

The guanine nucleotide exchange factor Net1 facilitates the specification of dorsal cell fates in zebrafish embryos by promoting maternal β-catenin activation

Wnt/β-catenin signaling is essential for the initiation of dorsal-ventral patterning during vertebrate embryogenesis. Maternal β-catenin accumulates in dorsal marginal nuclei during cleavage stages, but its critical target genes essential for dorsalization are silent until mid-blastula transition (MBT). Here, we find that zebrafish net1, a guanine nucleotide exchange factor, is specifically expressed in dorsal marginal blastomeres after MBT, and acts as a zygotic factor to promote the specification of dorsal cell fates. Loss- and gain-of-function experiments show that the GEF activity of Net1 is required for the activation of Wnt/β-catenin signaling in zebrafish embryos and mammalian cells. Net1 dissociates and activates PAK1 dimers, and PAK1 kinase activation causes phosphorylation of S675 of β-catenin after MBT, which ultimately leads to the transcription of downstream target genes. In summary, our results reveal that Net1-regulated β-catenin activation plays a crucial role in the dorsal axis formation during zebrafish development.

Tiam1/Rac1 complex controls Il17a transcription and autoimmunity

RORγt is a master transcription factor of Th17 cells and considered as a promising drug target for the treatment of autoimmune diseases. Here, we show the guanine nucleotide exchange factor, Tiam1, and its cognate Rho-family G protein, Rac1, regulate interleukin (IL)17A transcription and autoimmunity. Whereas Tiam1 genetic deficiency weakens IL-17A expression partially and inhibits the development of experimental autoimmune encephalomyelitis (EAE), deletion of Rac1 in T cells exhibits more robust effects on Th17 cells and EAE. We demonstrate Tiam1 and Rac1 form a complex with RORγt in the nuclear compartment of Th17 cells, and together bind and activate the Il17 promoter. The clinical relevance of these findings is emphasized by pharmacological targeting of Rac1 that suppresses both murine and human Th17 cells as well as EAE. Thus, our findings highlight a regulatory pathway of Tiam1/Rac1 in Th17 cells and suggest that it may be a therapeutic target in multiple sclerosis.

Tiam1 is a guanine nucleotide exchange factor for the Rho-family GTPase Rac1. Here, the authors show that nuclear Tiam1 and Rac1 bind to RORγt on the IL-17 promoter, activating its transcription, and that inhibiting Tiam1/Rac1 is beneficial in a mouse model of autoimmunity.

Rac1 augments Wnt signaling by stimulating β-catenin-lymphoid enhancer factor-1 complex assembly independent of β-catenin nuclear import

β-Catenin transduces the Wnt signaling pathway and its nuclear accumulation leads to gene transactivation and cancer. Rac1 GTPase is known to stimulate β-catenin-dependent transcription of Wnt target genes and we confirmed this activity. Here we tested the recent hypothesis that Rac1 augments Wnt signaling by enhancing β-catenin nuclear import; however, we found that silencing/inhibition or up-regulation of Rac1 had no influence on nuclear accumulation of β-catenin. To better define the role of Rac1, we employed proximity ligation assays (PLA) and discovered that a significant pool of Rac1-β-catenin protein complexes redistribute from the plasma membrane to the nucleus upon Wnt or Rac1 activation. More importantly, active Rac1 was shown to stimulate the formation of nuclear β-catenin-lymphoid enhancer factor 1 (LEF-1) complexes. This regulation required Rac1-dependent phosphorylation of β-catenin at specific serines, which when mutated (S191A and S605A) reduced β-catenin binding to LEF-1 by up to 50%, as revealed by PLA and immunoprecipitation experiments. We propose that Rac1-mediated phosphorylation of β-catenin stimulates Wnt-dependent gene transactivation by enhancing β-catenin-LEF-1 complex assembly, providing new insight into the mechanism of cross-talk between Rac1 and canonical Wnt/β-catenin signaling.

WNT10B enhances proliferation through β-catenin and RAC1 GTPase in human corneal endothelial cells

The cornea is the anterior, transparent tissue of the human eye that serves as its main refractive element. Corneal endothelial cells are arranged as a monolayer on the posterior surface of the cornea and function as a pump to counteract the leakiness of its basement membrane. Maintaining the cornea in a slightly dehydrated state is critical for the maintenance of corneal transparency. Adult human corneal endothelial cells are G1-arrested, even in response to injury, leading to an age-dependent decline in endothelial cell density. Corneal edema and subsequent vision loss ensues when endothelial cell density decreases below a critical threshold. Vision loss secondary to corneal endothelial dysfunction is a common indication for transplantation in developed nations. An impending increase in demand for and a current global shortage of donor corneas will necessitate the development of treatments for vision loss because of endothelial dysfunction that do not rely on donor corneas. Wnt ligands regulate many critical cellular functions, such as proliferation, making them attractive candidates for modulation in corneal endothelial dysfunction. We show that WNT10B causes nuclear transport and binding of RAC1 and β-catenin in human corneal endothelial cells, leading to the activation of Cyclin D1 expression and proliferation. Our findings indicate that WNT10B promotes proliferation in human corneal endothelial cells by simultaneously utilizing both β-catenin-dependent and -independent pathways and suggest that its modulation could be used to treat vision loss secondary to corneal endothelial dysfunction.

Down-regualtion of miR-106b induces epithelial-mesenchymal transition but suppresses metastatic colonization by targeting Prrx1 in colorectal cancer

Accumulating evidence identified that epithelial-mesenchymal transition (EMT) is acquired during progression to metastatic, but whether it is an absolute requirement is still controversial. MiR-106b has been confirmed to promote cancer cell proliferation; however few studies are available on its functions in EMT and metastasis in colorectal cancer (CRC). In this study, we found that knocking down miR-106b induced EMT conferring migratory and invasive properties. MiR-106b knockdown induced cytoskeletal reorganization through staining intracellular F-actin. The expression of Rho GTPases (Rac1 and Cdc42) and Tiam1 was significantly enforced after miR-106b down-regulation. However, miR-106b knocking down could suppress metastatic colonization in vivo. Correspondingly, over expression of miR-106b obtained an opposite effect. We identified Prrx1 was a direct target of miR-106b through using target prediction algorithms and dual-Luciferase reporter assay. Moreover, Moreover, we also found TGF-β1 could down-regulate miR-106b, and simultaneously miR-106b also influences the expression of TGF-β1, establishing a negative feedback loop to regulate the expression of Prrx1 together. Taken together, these findings demonstrated that miR-106b knockdown could induce EMT which conferring cells migratory and invasive properties but could not accomplish distant metastatic colonization efficiently.

piggyBac insertional mutagenesis screen identifies a role for nuclear RHOA in human ES cell differentiation

The mechanisms regulating human embryonic stem (ES) cell self-renewal and differentiation are not well defined in part due to the lack of tools for forward genetic analysis. We present a piggyBac transposon gain of function screen in human ES cells that identifies DENND2C, which genetically cooperates with NANOG to maintain self-renewal in the presence of retinoic acid. We show that DENND2C negatively regulates RHOA activity, which cooperates with NANOG to block differentiation. It has been recently shown that RHOA exists in the nucleus and is activated by DNA damage; however, its nuclear function remains unknown. We discovered that RHOA associates with DNA and that DENND2C affects nuclear RHOA localization, activity, and DNA association. Our study illustrates the power of piggyBac as a cost-effective, efficient, and easy to use tool for forward genetic screens in human ES cells and provides insight into the role of RHOA in the nucleus.

•

NANOG-sensitized piggyBac screen for regulators of stem cell differentiation

•

DENND2C is a negative regulator of RHOA

•

Genetic cooperation exists between RHOA and NANOG

•

Nuclear RHOA associates with DNA

Rac1 promotes diethylnitrosamine (DEN)-induced formation of liver tumors

To elucidate the function of the Ras-homologous GTPase Rac1 in hepatocarcinogenesis induced by diethylnitrosamine (DEN), mice lacking hepatic Rac1 expression were treated with DEN and compared to the wild-type (WT). Rac1 knock-out (KO) mice were found to have a lower tumor yield as compared to Rac1 proficient mice. The small-sized tumors formed in the absence of Rac1 lack an activated Ras/Raf/mitogen-activated protein kinase pathway, as indicated by the absence of p-ERK expression. Apparently, Rac1 is required for Ras-driven oncogenic pathways. Moreover, tumors in Rac1 deficient mice were glutamine synthase (GS) negative. They displayed a high number of p-H3-positive and cyclinB1 expressing cells, pointing to a defect in mitotic progression. To elucidate the influence of Rac1 on mechanisms of tumor initiation, acute DEN-induced hepatic stress responses were monitored. Rac1 deficiency caused fairly complex, partially time-dependent, alterations in both basal and/or DEN-induced messenger RNA (mRNA) and protein levels of susceptibility-related genes. Basal protein expression of DNA repair factors Brca1 and DNA repair protein RAD51 homolog (Rad51) and the cell cycle regulatory factor p27 was enhanced in the absence of Rac1. Following DEN treatment, p21 mRNA and protein expression was stimulated independent of the Rac1 status. Lack of Rac1 increased mechanisms of the DNA damage response (DDR), as shown by elevated protein levels of p-ATR, p-p53 and γH2AX 24h after DEN treatment. The data show that Rac1 is essential for DEN-stimulated hepatocarcinogenesis. We hypothesize that it promotes tumor initiation by counteracting the elimination of initiated cells and, moreover, alleviates the outgrowth of transformed cells. Hence, pharmacological targeting of Rac1 could be suitable for chemoprevention.

Regulation of Stat5 by FAK and PAK1 in Oncogenic FLT3- and KIT-Driven Leukemogenesis

Oncogenic mutations of FLT3 and KIT receptors are associated with poor survival in patients with acute myeloid leukemia (AML) and myeloproliferative neoplasms (MPNs), and currently available drugs are largely ineffective. Although Stat5 has been implicated in regulating several myeloid and lymphoid malignancies, how precisely Stat5 regulates leukemogenesis, including its nuclear translocation to induce gene transcription, is poorly understood. In leukemic cells, we show constitutive activation of focal adhesion kinase (FAK) whose inhibition represses leukemogenesis. Downstream of FAK, activation of Rac1 is regulated by RacGEF Tiam1, whose inhibition prolongs the survival of leukemic mice. Inhibition of the Rac1 effector PAK1 prolongs the survival of leukemic mice in part by inhibiting the nuclear translocation of Stat5. These results reveal a leukemic pathway involving FAK/Tiam1/Rac1/PAK1 and demonstrate an essential role for these signaling molecules in regulating the nuclear translocation of Stat5 in leukemogenesis.

Naturally occurring neomorphic PIK3R1 mutations activate the MAPK pathway, dictating therapeutic response to MAPK pathway inhibitors

PIK3R1 (p85α regulatory subunit of PI3K) is frequently mutated across cancer lineages. Herein, we demonstrate that the most common recurrent PIK3R1 mutation PIK3R1(R348∗) and a nearby mutation PIK3R1(L370fs), in contrast to wild-type and mutations in other regions of PIK3R1, confers an unexpected sensitivity to MEK and JNK inhibitors in vitro and in vivo. Consistent with the response to inhibitors, PIK3R1(R348∗) and PIK3R1(L370fs) unexpectedly increase JNK and ERK phosphorylation. Surprisingly, p85α R348(∗) and L370fs localize to the nucleus where the mutants provide a scaffold for multiple JNK pathway components facilitating nuclear JNK pathway activation. Our findings uncover an unexpected neomorphic role for PIK3R1(R348∗) and neighboring truncation mutations in cellular signaling, providing a rationale for therapeutic targeting of these mutant tumors.

Nuclear Rac1 regulates the bFGF-induced neurite outgrowth in PC12 cells

Rac1 plays a key role in neurite outgrowth via reorganization of the actin cytoskeleton. The molecular mechanisms underlying Rac1-mediated actin dynamics in the cytosol and plasma membrane have been intensively studied, but the nuclear function of Rac1 in neurite outgrowth has not yet been addressed. Using subcellular fractionation and immunocytochemistry, we sought to explore the role of nuclear Rac1 in neurite outgrowth. bFGF, a strong agonist for neurite outgrowth in PC12 cells, stimulated the nuclear accumulation of an active form of Rac1. Rac1-PBR (Q) mutant, in which six basic residues in the polybasic region at the C-terminus were replaced by glutamine, didn’t accumulate in the nucleus. In comparison with control cells, cells expressing this mutant form of Rac1 displayed a marked defect in extending neurites that was concomitant with reduced expression of MAP2 and MEK-1. These results suggest that Rac1 translocation to the nucleus functionally correlates with bFGF-induced neurite outgrowth. [BMB Reports 2013; 46(12): 617-622]

RhoB controls coordination of adult angiogenesis and lymphangiogenesis following injury by regulating VEZF1-mediated transcription

Mechanisms governing the distinct temporal dynamics that characterize post-natal angiogenesis and lymphangiogenesis elicited by cutaneous wounds and inflammation remain unclear. RhoB, a stress-induced small GTPase, modulates cellular responses to growth factors, genotoxic stress and neoplastic transformation. Here we show, using RhoB null mice, that loss of RhoB decreases pathological angiogenesis in the ischaemic retina and reduces angiogenesis in response to cutaneous wounding, but enhances lymphangiogenesis following both dermal wounding and inflammatory challenge. We link these unique and opposing roles of RhoB in blood versus lymphatic vasculatures to the RhoB-mediated differential regulation of sprouting and proliferation in primary human blood versus lymphatic endothelial cells. We demonstrate that nuclear RhoB-GTP controls expression of distinct gene sets in each endothelial lineage by regulating VEZF1-mediated transcription. Finally, we identify a small-molecule inhibitor of VEZF1-DNA interaction that recapitulates RhoB loss in ischaemic retinopathy. Our findings establish the first intra-endothelial molecular pathway governing the phased response of angiogenesis and lymphangiogenesis following injury.

Tiam1/Rac1 signals contribute to the proliferation and chemoresistance, but not motility, of chronic lymphocytic leukemia cells

Signals from the tumor microenvironment promote the migration, survival, and proliferation of chronic lymphocytic leukemia (CLL) cells. Rho GTPases control various signaling pathways downstream of microenvironmental cues. Here, we analyze the function of Rac1 in the motility and proliferation of CLL cells. We found decreased transcription of the Rac guanine nucleotide exchange factors Tiam1 and Vav1 in unstimulated peripheral blood CLL cells with almost complete loss of Tiam1 but increased transcription of the potential Rac antagonist RhoH. Consistently, stimulation of CLL cells with the chemokine CXCL12 induced RhoA but not Rac1 activation, whereas chemokine-induced CLL cell motility was Rac1-independent. Coculture of CLL cells with activated T cells induced their activation and subsequent proliferation. Here, Tiam1 expression was induced in the malignant cells in line with increased Ki-67 and c-Myc expression. Rac1 or Tiam1 knockdown using siRNA or treatment with the Tiam1/Rac inhibitor NSC-23766 attenuated c-Myc transcription. Furthermore, treatment of CLL cells with NSC-23766 reduced their proliferation. Rac inhibition also antagonized the chemoresistance of activated CLL cells toward fludarabine. Collectively, our data suggest a dynamic regulation of Rac1 function in the CLL microenvironment. Rac inhibition could be of clinical use by selectively interfering with CLL cell proliferation and chemoresistance.

The sirtuins promote Dishevelled-1 scaffolding of TIAM1, Rac activation and cell migration

Rac1-GTPases serve as intermediary cellular switches which conduct transient and constitutive signals from upstream cues, including those from Ras oncoproteins. While the sirtuin1 (SIRT1) deacetylase is overexpressed in several human cancers and has recently been linked to cancer cell motility as a context-dependent regulator of multiple pathways, its role in Rac1 activation has not been reported. Likewise, SIRT2 has been demonstrated to be upregulated in some cancers; however, studies have also reported its role in tumor suppression. Here, we demonstrate that SIRT1 and SIRT2 positively regulate the levels of Rac1-GTP and the activity of T-cell lymphoma invasion and metastasis 1 (TIAM1), a Rac guanine nucleotide exchange factor (GEF). Transient inhibition of SIRT1 and SIRT2 resulted in increased acetylation of TIAM1 whereas chronic SIRT2 knockdown resulted in enhanced acetylation of TIAM1. SIRT1 regulates Dishevelled (DVL) protein levels in cancer cells and DVL along with TIAM1 are known to augment Rac activation; however, SIRT1 or 2 have not been previously linked with TIAM1. We found that diminished sirtuin activity led to the disruption of the DVL1-TIAM1 interaction. We hence propose a model for Rac activation where SIRT1/2 positively modulate the DVL/TIAM1/Rac axis and promote sustained pathway activation.

Pebble/ECT2 RhoGEF negatively regulates the Wingless/Wnt signaling pathway

Wingless (Wg)/Wnt signaling is essential for patterning invertebrate and vertebrate embryos, and inappropriate Wnt activity is associated with a variety of human cancers. Despite intensive study, Wnt pathway mechanisms are not fully understood. We have discovered a new mechanism for regulating the Wnt pathway: activity of a Rho guanine nucleotide exchange factor (GEF) encoded by pebble (pbl) in Drosophila and ECT2 in humans. This RhoGEF has an essential role in cytokinesis, but also plays an unexpected, conserved role in inhibiting Wg/Wnt activity. Loss and gain of pbl function in Drosophila embryos cause pattern defects that indicate altered Wg activity. Both Pbl and ECT2 repress Wg/Wnt target gene expression in cultured Drosophila and human cells. The GEF activity is required for Wnt regulation, whereas other protein domains important for cytokinesis are not. Unlike most negative regulators of Wnt activity, Pbl/ECT2 functions downstream of Armadillo (Arm)/beta-catenin stabilization. Our results indicate GTPase regulation at a novel point in Wg/Wnt signal transduction, and provide new insight into the categorization of ECT2 as a human proto-oncogene.

Neuronal Rac1 is required for learning-evoked neurogenesis

Hippocampus-dependent learning and memory relies on synaptic plasticity as well as network adaptations provided by the addition of adult-born neurons. We have previously shown that activity-induced intracellular signaling through the Rho family small GTPase Rac1 is necessary in forebrain projection neurons for normal synaptic plasticity in vivo, and here we show that selective loss of neuronal Rac1 also impairs the learning-evoked increase in neurogenesis in the adult mouse hippocampus. Earlier work has indicated that experience elevates the abundance of adult-born neurons in the hippocampus primarily by enhancing the survival of neurons produced just before the learning event. Loss of Rac1 in mature projection neurons did reduce learning-evoked neurogenesis but, contrary to our expectations, these effects were not mediated by altering the survival of young neurons in the hippocampus. Instead, loss of neuronal Rac1 activation selectively impaired a learning-evoked increase in the proliferation and accumulation of neural precursors generated during the learning event itself. This indicates that experience-induced alterations in neurogenesis can be mechanistically resolved into two effects: (1) the well documented but Rac1-independent signaling cascade that enhances the survival of young postmitotic neurons; and (2) a previously unrecognized Rac1-dependent signaling cascade that stimulates the proliferative production and retention of new neurons generated during learning itself.

The Cdc42/Rac nucleotide exchange factor protein β1Pix (Pak-interacting exchange factor) modulates β-catenin transcriptional activity in colon cancer cells: evidence for direct interaction of β1PIX with β-catenin

Wnt/β-catenin signaling is highly regulated and critical for intestinal epithelial development and repair; aberrant β-catenin signaling is strongly associated with colon cancer. The small GTPase Rac1 regulates β-catenin nuclear translocation and signaling. Here we tested the hypothesis that β1Pix, a Cdc42/Rac guanine nucleotide exchange factor (GEF), regulates β-catenin-dependent transcriptional activity and cell function. We report the novel observations that β1Pix binds directly to β-catenin, an action requiring both the β1Pix DH and dimerization domains but not β1Pix GEF activity. In human colon cancer cells, activation of β-catenin signaling with LiCl decreased β1Pix/β-catenin association in the cytosol and increased nuclear binding of β-catenin to β1Pix. Nuclear association of β1Pix and β-catenin was independent of Rac1 expression and activation; down- and up-regulating Rac1 expression levels did not alter nuclear β1Pix/β-catenin association. Ectopic β1Pix expression enhanced LiCl-induced β-catenin transcriptional activity. Conversely, siRNA knockdown of β1Pix attenuated both LiCl-induced β-catenin transcriptional activity and colon cancer cell proliferation. Ectopic expression of β1Pix stimulated β-catenin transcriptional activity, whereas β1PixΔ(602-611), which is unable to bind β-catenin, had no effect. Altogether, these findings suggest that β1Pix functions as a transcriptional regulator of β-catenin signaling through direct interaction with β-catenin, an action that may be functionally relevant to colon cancer biology.

Tiam1 transgenic mice display increased tumor invasive and metastatic potential of colorectal cancer after 1,2-dimethylhydrazine treatment

Background

T lymphoma invasion and metastasis 1 (Tiam1) is a potential modifier of tumor development and progression. Our previous study in vitro and in nude mice suggested a promotion role of Tiam1 on invasion and metastasis of colorectal cancer (CRC). In the present study, we generated Tiam1/C1199-CopGFP transgenic mice to investigate the tumorigenetic, invasive and metastatic alterations in the colon and rectum of wild-type and Tiam1 transgenic mice under 1,2-dimethylhydrazine (DMH) treatment.

Methods

Transgenic mice were produced by the method of pronuclear microinlectlon. Whole-body fluorescence imaging (Lighttools, Edmonton, Alberta, Canada), PCR, and immunohistochemical techniques (IHC) were applied sequentially to identify the transgenic mice. The carcinogen DMH (20 mg/kg) was used to induce colorectal tumors though intraperitoneal (i.p.) injections once a week for 24 weeks from the age of 4 weeks on Tiam1 transgenic or non-transgenic mice.

Results

We successfully generated Tiam1/C1199-CopGFP transgenic mice and induced primary tumors in the intestine of both wild type and Tiam1 transgenic mice by DMH treatment. In addition, Tiam1 transgenic mice developed larger and more aggressive neoplasm than wild-type mice. Moreover, immunohistochemical staining revealed that upregulation of Tiam1 was correlated with increased expression of β-Catenin and Vimentin, and downregulation of E-Cadherin in these mice.

Conclusions

Our study has provided in vivo evidence supporting that Tiam1 promotes invasion and metastasis of CRC, most probably through activation of Wnt/β-catenin signaling pathway, in a Tiam1 transgenic mouse model.

RNAi mediated Tiam1 gene knockdown inhibits invasion of retinoblastoma

T lymphoma invasion and metastasis protein (Tiam1) is up-regulated in variety of cancers and its expression level is related to metastatic potential of the type of cancer. Earlier, Tiam1 was shown to be overexpressed in retinoblastoma (RB) and we hypothesized that it was involved in invasiveness of RB. This was tested by silencing Tiam1 in RB cell lines (Y79 and Weri-Rb1) using siRNA pool, targeting different regions of Tiam1 mRNA. The cDNA microarray of Tiam1 silenced cells showed gene regulations altered by Tiam1 were predominantly on the actin cytoskeleton interacting proteins, apoptotic initiators and tumorogenic potential targets. The silenced phenotype resulted in decreased growth and increased apoptosis with non-invasive characteristics. Transfection of full length and N-terminal truncated construct (C1199) clearly revealed membrane localization of Tiam1 and not in the case of C580 construct. F-actin staining showed the interaction of Tiam1 with actin in the membrane edges that leads to ruffling, and also imparts varying invasive potential to the cell. The results obtained from our study show for the first time that Tiam1 modulates the cell invasion, mediated by actin cytoskeleton remodeling in RB.

Wnt and Rho GTPase signaling in osteoarthritis development and intervention: implications for diagnosis and therapy

Wnt and Rho GTPase signaling play critical roles in governing numerous aspects of cell physiology, and have been shown to be involved in endochondral ossification and osteoarthritis (OA) development. In this review, current studies of canonical Wnt signaling in OA development, together with the differential roles of Rho GTPases in chondrocyte maturation and OA pathology are critically summarized. Based on the current scientific literature together with our preliminary results, the strategy of targeting Wnt and Rho GTPase for OA prognosis and therapy is suggested, which is instructive for clinical treatment of the disease.

A novel strategy to increase the proliferative potential of adult human β-cells while maintaining their differentiated phenotype

Our previous studies demonstrated that Wnt/GSK-3/β-catenin and mTOR signaling are necessary to stimulate proliferative processes in adult human β-cells. Direct inhibition of GSK-3, that engages Wnt signaling downstream of the Wnt receptor, increases β-catenin nuclear translocation and β-cell proliferation but results in lower insulin content. Our current goal was to engage canonical and non-canonical Wnt signaling at the receptor level to significantly increase human β-cell proliferation while maintaining a β-cell phenotype in intact islets. We adopted a system that utilized conditioned medium from L cells that expressed Wnt3a, R-spondin-3 and Noggin (L-WRN conditioned medium). In addition we used a ROCK inhibitor (Y-27632) and SB-431542 (that results in RhoA inhibition) in these cultures. Treatment of intact human islets with L-WRN conditioned medium plus inhibitors significantly increased DNA synthesis ∼6 fold in a rapamycin-sensitive manner. Moreover, this treatment strikingly increased human β-cell proliferation ∼20 fold above glucose alone. Only the combination of L-WRN conditioned medium with RhoA/ROCK inhibitors resulted in substantial proliferation. Transcriptome-wide gene expression profiling demonstrated that L-WRN medium provoked robust changes in several signaling families, including enhanced β-catenin-mediated and β-cell-specific gene expression. This treatment also increased expression of Nr4a2 and Irs2 and resulted in phosphorylation of Akt. Importantly, glucose-stimulated insulin secretion and content were not downregulated by L-WRN medium treatment. Our data demonstrate that engaging Wnt signaling at the receptor level by this method leads to necessary crosstalk between multiple signaling pathways including activation of Akt, mTOR, Wnt/β-catenin, PKA/CREB, and inhibition of RhoA/ROCK that substantially increase human β-cell proliferation while maintaining the β-cell phenotype.

Role of rac1 in the proliferation of pancreatic carcinoma cells and possible mechanisms involved

AIM: To investigate the role of rac1 in the proliferation of pancreatic carcinoma cells and to explore the possible mechanisms involved.

METHODS: The influence of rac1 on the proliferation of pancreatic carcinoma cells was detected by cell counting kit-8 (CCK-8) assay and EDU assay. Real-time PCR was employed to measure the expression of target genes of wnt-β-Catenin signaling pathway. Western blot and immunofluorescence were employed to measure the expression of β-Catenin in the cytoplasm and nucleus.

RESULTS: Disruption of rac1 activity by transfection with si-Rac1 or treatment with NSC23766 inhibited cell proliferation. Suppression of rac1 markedly down-regulated the mRNA expression of c-myc and Cyclin D1 and slightly decreased the mRNA expression of c-jun. Rac1 knockdown did not affect β-Catenin stability in the cytoplasm but markedly reduced β-Catenin accumulation in the nucleus. Similar result was also obtained in immunofluorescence experiments.

CONCLUSION: Rac1 amplifies the wnt signaling pathway activity possibly by promoting β-Catenin accumulation in the nucleus and augments wnt target gene transcription in pancreatic carcinoma cells.