Tissue transglutaminase regulates β-catenin signaling through a c-Src-dependent mechanism

Transglutaminase 2 in diabetes mellitus: Unraveling its multifaceted role and therapeutic implications for vascular complications

Diabetes, a severe metabolic disease characterized by chronic hypoglycemia, poses debilitating and life-threatening risks of microvascular and macrovascular complications, including blindness, kidney failure, heart attacks, and limb amputation. Addressing these complications is paramount, urging the development of interventions targeting diabetes-associated vascular dysfunctions. To effectively combat diabetes, a comprehensive understanding of the pathological mechanisms underlying complications and identification of precise therapeutic targets are imperative. Transglutaminase 2 (TGase2) is a multifunctional enzyme implicated in the pathogenesis of diverse diseases such as neurodegenerative disorders, fibrosis, and inflammatory conditions. TGase2 has recently emerged as a key player in both the pathogenesis and therapeutic intervention of diabetic complications. This review highlights TGase2 as a therapeutic target for diabetic complications and explores TGase2 inhibition as a promising therapeutic approach in their treatment.

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

Discovery and Characterization of PROTACs Targeting Tissue Transglutaminase (TG2)

Tissue transglutaminase (TG2) is a multifunctional enzyme involved in the cross-linking of extracellular matrix proteins, formation of complexes with fibronectin (FN) and integrins, and GTP hydrolysis. TG2 is activated in several pathological conditions, including cancer. We recently described a novel series of ligands that bind to TG2 and inhibit its interaction with FN. Because TG2 acts via multiple mechanisms, we set out to pursue a targeted protein degradation strategy to abolish TG2's myriad functions. Here, we report the synthesis and characterization of a series of VHL-based degraders that reduce TG2 in ovarian cancer cells in a proteasome-dependent manner. Degradation of TG2 resulted in significantly reduced cancer cell adhesion and migration in vitro in scratch-wound and migration assays. These results strongly indicate that further development of more potent and in vivo efficient TG2 degraders could be a new strategy for reducing the dissemination of ovarian and other cancers.

DOT1 L Regulates Ovarian Cancer Stem Cells by Activating β-catenin Signaling

Cancer stem cells (CSC) represent a population of cancer cells responsible for tumor initiation, chemoresistance, and metastasis. Here, we identified the H3K79 methyltransferase disruptor of telomeric silencing-1-like (DOT1L) as a critical regulator of self-renewal and tumor initiation in ovarian CSCs. DOT1 L was upregulated in ovarian CSCs versus non-CSCs. shRNA-mediated DOT1 L knockdown decreased the aldehyde dehydrogenase (ALDH)+ cell population, impaired the tumor initiation capacity (TIC) of ovarian CSCs, and blocked the expression of stemness-associated genes. Inhibition of DOT1L's methyltransferase activity by the small-molecule inhibitor (DOT1Li) EPZ-5676 also effectively targeted ovarian CSCs. Integrated RNA-sequencing analyses of ovarian cancer cells in which DOT1 L was knocked down versus control cells and of ovarian CSCs versus non-CSCs, identified Wnt signaling as a shared pathway deregulated in both CSCs and in DOT1L-deficient ovarian cancer cells. β-catenin, a key transcription factor regulated by Wnt, was downregulated in ovarian cancer cells in which DOT1 L was knocked down and upregulated in DOT1 L overexpressing ovarian cancer cells. Chromatin immunoprecipitation (ChIP) revealed enrichment of the H3K79Me3 mark at the β-catenin promoter, suggesting that its transcription is regulated by DOT1L. Our results suggest that DOT1 L is critical for the self-renewal and TIC of ovarian CSCs by regulating β-catenin signaling. Targeting DOT1 L in ovarian cancer could be a new strategy to eliminate CSCs.

IMPLICATIONS

This study found that the histone methyltransferase DOT1 L regulates the self-renewal and tumor initiation capacity of ovarian CSCs and suggests DOT1 L as a new cancer target.

Crosstalk Between β-CATENIN-Mediated Cell Adhesion and the WNT Signaling Pathway

Cell adhesion and stable signaling regulation are fundamental ways of maintaining homeostasis. Among them, the Wnt/β-CATENIN signaling plays a key role in embryonic development and maintenance of body dynamic homeostasis. At the same time, the key signaling molecule β-CATENIN in the Wnt signaling can also function as a cytoskeletal linker protein to regulate tissue barriers, cell migration, and morphogenesis. Dysregulation of the balance between Wnt signaling and adherens junctions can lead to disease. How β-CATENIN maintains the independence of these two functions, or mediates the interaction and balance of these two functions, has been explored and debated for a long time. In this study, we will focus on five aspects of β-CATENIN chaperone molecules, phosphorylation of β-CATENIN and related proteins, epithelial mesenchymal transition, β-CATENIN homolog protein γ-CATENIN and disease, thus deepening the understanding of the Wnt/β-CATENIN signaling and the homeostasis between cell adhesion and further addressing related disease problems.

Type 2 transglutaminase in the nucleus: the new epigenetic face of a cytoplasmic enzyme

One of the major mysteries in science is how it is possible to pack the cellular chromatin with a total length of over 1 m, into a small sphere with a diameter of 5 mm "the nucleus", and even more difficult to envisage how to make it functional. Although we know that compaction is achieved through the histones, however, the DNA needs to be accessible to the transcription machinery and this is allowed thanks to a variety of very complex epigenetic mechanisms. Either DNA (methylation) or post-translational modifications of histone proteins (acetylation, methylation, ubiquitination and sumoylation) play a crucial role in chromatin remodelling and consequently on gene expression. Recently the serotonylation and dopaminylation of the histone 3, catalyzed by the Transglutaminase type 2 (TG2), has been reported. These novel post-translational modifications catalyzed by a predominantly cytoplasmic enzyme opens a new avenue for future investigations on the enzyme function itself and for the possibility that other biological amines, substrate of TG2, can influence the genome regulation under peculiar cellular conditions. In this review we analyzed the nuclear TG2's biology by discussing both its post-translational modification of various transcription factors and the implications of its epigenetic new face. Finally, we will focus on the potential impact of these events in human diseases.

Tissue transglutaminase activates integrin-linked kinase and β-catenin in ovarian cancer

Ovarian cancer (OC) is the most lethal gynecological cancer. OC cells have high proliferative capacity, are invasive, resist apoptosis, and tumors often display rearrangement of extracellular matrix (ECM) components, contributing to accelerated tumor progression. The multifunctional protein tissue transglutaminase (TG2) is known to be secreted in the tumor microenvironment (TME), where it interacts with fibronectin (FN) and the cell surface receptor β1 integrin. However, the mechanistic role of TG2 in cancer cell proliferation is unknown. Here, we demonstrate TG2 directly interacts with and facilitates the phosphorylation and activation of the integrin effector protein integrin-linked kinase (ILK) at Ser246. We show TG2 and p-Ser246-ILK form a complex that is detectable in patient-derived OC primary cells grown on FN-coated slides. In addition, we show co-expression of TGM2 and ILK correlates with poor clinical outcome. Mechanistically, we demonstrate TG2-mediated ILK activation causes phosphorylation of glycogen synthase kinase-3α/β (GSK-3α/β), allowing β-catenin nuclear translocation and transcriptional activity. Furthermore, inhibition of TG2 and ILK using small molecules, neutralizing antibodies, or shRNA-mediated knockdown block cell adhesion to the FN matrix, as well as the Wnt receptor response to the Wnt-3A ligand, and ultimately, cell adhesion, growth, and migration. In conclusion, we demonstrate TG2 directly interacts with and activates ILK in OC cells and tumors, and define a new mechanism which links ECM cues with β-catenin signaling in OC. These results suggest a central role of TG2/FN/integrin clusters in ECM rearrangement and indicate downstream effector ILK may represent a potential new therapeutic target in OC.

circFBXO7/miR-96-5p/MTSS1 axis is an important regulator in the Wnt signaling pathway in ovarian cancer

BACKGROUND

CircRNAs are a novel class of evolutionarily conserved noncoding RNA molecules that form covalently closed continuous loop structures without 5' caps and 3' poly(A) tails. Accumulating evidence suggests that circRNAs play important regulatory roles in cancer and are promising biomarkers for cancer diagnosis and prognosis, as well as targets for cancer therapy. In this study, we identify and explore the role of a novel circRNA, circFBXO7, in ovarian cancer.

METHODS

rRNA-depleted RNA-sequencing was performed to identify differentially expressed circRNAs between ovarian cancerous and normal tissues. qRT-PCR and single-molecule RNA in-situ hybridization was used to quantify circFBXO7 expression in tumor tissues. The association of circFBXO7 expression with patient prognosis was evaluated by Kaplan-Meier survival analysis. The biological function of circFBXO7 was also investigated using loss-of-function and gain-of-function assays in vivo and in vitro. Luciferase reporter and TOP/FOP-Flash reporter assays were then conducted together with RNA immunoprecipitation and western blot to assess the circFBXO7/miR-96-5p/MTSS1/Wnt/β-catenin axis.

RESULTS

circFBXO7 was downregulated in ovarian cancer which was associated with poor prognosis. Biologically, circFBXO7 overexpression significantly suppressed ovarian cancer cell proliferation, migration, and invasion in vitro, and inhibited tumor growth and metastasis in vivo, whereas its knockdown exerted an opposite role. Mechanistically, circFBXO7 functioned as a competing endogenous RNA for miR-96-5p to regulate the expression of MTSS1. Consequently, downregulation of MTSS1 led to excessive accumulation of β-catenin and increased phosphorylation of GSK3β, leading to the translocation of β-catenin to the nucleus, thereby activating the Wnt/β-catenin signaling pathway and ultimately promoting ovarian cancer progression.

CONCLUSIONS

Our findings indicate that circFBXO7 acts as a bone fide tumor suppressor in ovarian cancer and that the circFBXO7/miR-96-5p/MTSS1 axis is an important regulator in the Wnt/β-catenin signaling pathway which may provide a promising target for ovarian cancer therapy.

The Outside-In Journey of Tissue Transglutaminase in Cancer

Tissue transglutaminase (TG2) is a member of the transglutaminase family that catalyzes Ca²⁺-dependent protein crosslinks and hydrolyzes guanosine 5′-triphosphate (GTP). The conformation and functions of TG2 are regulated by Ca²⁺ and GTP levels; the TG2 enzymatically active open conformation is modulated by high Ca²⁺ concentrations, while high intracellular GTP promotes the closed conformation, with inhibition of the TG-ase activity. TG2’s unique characteristics and its ubiquitous distribution in the intracellular compartment, coupled with its secretion in the extracellular matrix, contribute to modulate the functions of the protein. Its aberrant expression has been observed in several cancer types where it was linked to metastatic progression, resistance to chemotherapy, stemness, and worse clinical outcomes. The N-terminal domain of TG2 binds to the 42 kDa gelatin-binding domain of fibronectin with high affinity, facilitating the formation of a complex with β-integrins, essential for cellular adhesion to the matrix. This mechanism allows TG2 to interact with key matrix proteins and to regulate epithelial to mesenchymal transition and stemness. Here, we highlight the current knowledge on TG2 involvement in cancer, focusing on its roles translating extracellular cues into activation of oncogenic programs. Improved understanding of these mechanisms could lead to new therapeutic strategies targeting this multi-functional protein.

Metastasis suppressor 1 controls osteoblast differentiation and bone homeostasis through regulating Src-Wnt/β-catenin signaling

Metastasis suppressor 1 (MTSS1) plays an inhibitory role in tumorigenesis and metastasis of a variety of cancers. To date, the function of MTSS1 in the differentiation of marrow stromal progenitor cells remains to be explored. In the current study, we investigated whether and how MTSS1 has a role in osteoblast differentiation and bone homeostasis. Our data showed that MTSS1 mRNA was upregulated during osteoblast differentiation and downregulated in the osteoblastic lineage cells of ovariectomized and aged mice. Functional studies revealed that MTSS1 promoted the osteogenic differentiation from marrow stromal progenitor cells. Mechanistic explorations uncovered that the inactivation of Src and afterward activation of canonical Wnt signaling were involved in osteoblast differentiation induced by MTSS1. The enhanced osteogenic differentiation induced by MTSS1 overexpression was attenuated when Src was simultaneously overexpressed, and conversely, the inhibition of osteogenic differentiation by MTSS1 siRNA was rescued when the Src inhibitor was supplemented to the culture. Finally, the in vivo transfection of MTSS1 siRNA to the marrow of mice significantly reduced the trabecular bone mass, along with the reduction of trabecular osteoblasts, the accumulation of marrow adipocytes, and the increase of phospho-Src-positive cells on the trabeculae. No change in the number of osteoclasts was observed. This study has unraveled that MTSS1 contributes to osteoblast differentiation and bone homeostasis through regulating Src-Wnt/β-catenin signaling. It also suggests the potential of MTSS1 as a new target for the treatment of osteoporosis.

Loss of host tissue transglutaminase boosts antitumor T cell immunity by altering STAT1/STAT3 phosphorylation in ovarian cancer

BACKGROUND

Tissue transglutaminase (TG2), an enzyme overexpressed in cancer cells, promotes metastasis and resistance to chemotherapy. Its distinct effects in cancer versus the host compartments have not been elucidated.

METHODS

Here, by using a TG2^-/- syngeneic ovarian cancer mouse model, we assessed the effects of TG2 deficiency in the host tissues on antitumor immunity and tumor progression. Multicolor flow cytometry was used to phenotype immune cell populations in the peritoneal environment. Cancer cells recovered from malignant ascites were characterized by RNA sequencing, proliferation, and apoptosis assays.

RESULTS

We observed that host TG2 loss delayed tumor growth and ascites accumulation and caused increased infiltration of CD8⁺ T cells and decreased numbers of myeloid cells in the peritoneal fluid. Tumor antigen-specific CD8⁺ T cell cytotoxic responses were enhanced in ascites from TG2^-/- versus TG2^+/+ mice and CD8⁺ T cell depletion caused accelerated ascites accumulation in TG2^-/- mice. CD8⁺ T cells from tumor-bearing TG2^-/- mice displayed an effector T cell phenotype, differentiated toward effector memory (T_em). Mechanistically, absence of TG2 augmented signals promoting T cell activation, such as increased cytokine-induced STAT1 and attenuated STAT3 phosphorylation in T cells. Additionally, immune-suppressive myeloid cell populations were reduced in the peritoneal milieu of TG2^-/- tumor-bearing mice. In response to the more robust immune response caused by loss of TG2, cancer cells growing intraperitoneally exhibited an interferon-γ(IFN-γ) responsive gene signature and underwent apoptosis. In human specimens, stromal, not tumor, TG2 expression correlated indirectly with numbers of tumor-infiltrating lymphocytes.

CONCLUSIONS

Collectively, our data demonstrate decreased tumor burden, increased activation and effector function of T cells, and loss of immunosuppressive signals in the tumor microenvironment of TG2^-/- mice. We propose that TG2 acts as an attenuator of antitumor T cell immunity and is a new immunomodulatory target.

Knockdown of PAK1 Inhibits the Proliferation and Invasion of Non-Small Cell Lung Cancer Cells Through the ERK Pathway

The p21-activated kinase (PAK) family of serine/threonine kinases plays a pivotal role in various human tumors, as supported by our previous report on the overexpressed PAK isoforms in non-small cell lung cancer (NSCLC). To better understand the role of PAKs in tumorigenesis, the authors examined PAK1 expression patterns and its significance in NSCLC. It was demonstrated by immunohistochemical staining that PAK1 was increased and localized in the cytoplasm in 151 of 207 cases. High levels of PAK1 expression correlated with a histologic type of tumor (squamous cell carcinoma), tumor node metastasis stage, and lymph nodal status. We also examined the biological role of PAK1 in lung cancer cell lines transfected with PAK1-small interfering RNA. Decreased expression of PAK1 inhibited lung cancer cell proliferation and invasion, which is the major cause of lung cancer malignancy. Downregulated expression of PAK1 hampered rapidly accelerated fibrosarcoma/mitogen-activated extracellular signal-regulated kinase/extracellular signal-regulated kinase pathway activity but did not affect Wnt/β-catenin signaling. Our findings suggest that PAK1 is an important oncogene in NSCLC, as decreased expression of PAK1 inhibited the proliferation and invasion of NSCLC cells by blocking the ERK pathway. These results provide evidence for using PAK1 inhibition as potential anticancer therapy.

Advanced glycation end products induce endothelial hyperpermeability via β-catenin phosphorylation and subsequent up-regulation of ADAM10

Endothelial hyperpermeability is the initial event in the development of diabetic microvascular complications, and advanced glycation end products (AGEs) are suggested to cause much of the endothelial hyperpermeability associated with diabetes mellitus, but the molecular mechanism remains to be characterized. β-catenin reportedly plays dual functions in maintaining normal endothelial permeability by serving both as an adhesive component and a signal transduction component. Here, we found that AGEs induced the phosphorylation of β-catenin at residues Y654 and Y142 and the endothelial hyperpermeability was reversed when the two residues were blocked. In mechanism, phosphorylation of Y654 was blocked by Src inactivation, whereas phosphorylation of Y142 was reduced by a focal adhesion kinase inhibitor. β-catenin Y654 phosphorylation induced by AGEs facilitated the dissociation of vascular endothelial (VE)-cadherin/β-catenin and the impairment of adherens junctions (AJs), whereas β-catenin Y142 phosphorylation favoured the dissociation of β-catenin and α-catenin. Further investigation revealed that β-catenin Y142 phosphorylation was required for AGEs-mediated β-catenin nuclear translocation, and this nuclear-located β-catenin subsequently activated the TCF/LEF pathway. This pathway promotes the transcription of the Wnt target, ADAM10 (a disintegrin and metalloprotease 10), which mediates VE-cadherin shedding and leads to further impairment of AJs. In summary, our study showed the role of β-catenin Y654 and Y142 phosphorylation in AGEs-mediated endothelial hyperpermeability through VE-cadherin/β-catenin/α-catenin dissociation and up-regulation of ADAM10, thereby advancing our understanding of the underlying mechanisms of AGEs-induced microvascular hyperpermeability.

The Biological and Biomechanical Role of Transglutaminase-2 in the Tumour Microenvironment

Transglutaminase-2 (TG2) is the most highly and ubiquitously expressed member of the transglutaminase enzyme family and is primarily involved in protein cross-linking. TG2 has been implicated in the development and progression of numerous cancers, with a direct role in multiple cellular processes and pathways linked to apoptosis, chemoresistance, epithelial-mesenchymal transition, and stem cell phenotype. The tumour microenvironment (TME) is critical in the formation, progression, and eventual metastasis of cancer, and increasing evidence points to a role for TG2 in matrix remodelling, modulation of biomechanical properties, cell adhesion, motility, and invasion. There is growing interest in targeting the TME therapeutically in response to advances in the understanding of its critical role in disease progression, and a number of approaches targeting biophysical properties and biomechanical signalling are beginning to show clinical promise. In this review we aim to highlight the wide array of processes in which TG2 influences the TME, focussing on its potential role in the dynamic tissue remodelling and biomechanical events increasingly linked to invasive and aggressive behaviour. Drug development efforts have yielded a range of TG2 inhibitors, and ongoing clinical trials may inform strategies for targeting the biomolecular and biomechanical function of TG2 in the TME.

Transglutaminase-2 regulates Wnt and FoxO3a signaling to determine the severity of osteoarthritis

Transglutaminase 2 (TG2), also known as tissue transglutaminase, is a calcium-dependent enzyme that has a variety of intracellular and extracellular substrates. TG2 not only increases in osteoarthritis (OA) tissue but also affects the progression of OA. However, it is still unclear how TG2 affects cartilage degradation in OA at the molecular level. Surgically induced OA lead to an increase of TG2 in the articular cartilage and growth plate, and it was dependent on TGFβ1 in primary chondrocytes. The inhibition of TG2 enzymatic activity with intra-articular injection of ZDON, the peptide-based specific TG2 inhibitor, ameliorated the severity of surgically induced OA as well as the expression of MMP-3 and MMP-13. ZDON attenuated MMP-3 and MMP-13 expression in TGFβ- and calcium ionophore-treated chondrocytes in a Runx2-independent manner. TG2 inhibition with ZDON suppressed canonical Wnt signaling through a reduction of β-catenin, which was mediated by ubiquitination-dependent proteasomal degradation. In addition, TG2 activation by a calcium ionophore enhanced the phosphorylation of AMPK and FoxO3a and the nuclear translocation of FoxO3a, which was responsible for the increase in MMP-13. In conclusion, TG2 plays an important role in the pathogenesis of OA as a major catabolic mediator that affects the stability of β-catenin and FoxO3a-mediated MMP-13 production.

A Precision Strategy to Cure Renal Cell Carcinoma by Targeting Transglutaminase 2

In a recent report, no significance of transglutaminase 2 (TGase 2) was noted in the analyses of expression differences between normal and clear cell renal cell carcinoma (ccRCC), although we found that knock down of TGase 2 induced significant p53-mediated cell death in ccRCC. Generally, to find effective therapeutic targets, we need to identify targets that belong specifically to a cancer phenotype that can be differentiated from a normal phenotype. Here, we offer precise reasons why TGase 2 may be the first therapeutic target for ccRCC, according to several lines of evidence. TGase 2 is negatively regulated by von Hippel-Lindau tumor suppressor protein (pVHL) and positively regulated by hypoxia-inducible factor 1-α (HIF-1α) in renal cell carcinoma (RCC). Therefore, most of ccRCC presents high level expression of TGase 2 because over 90% of ccRCC showed VHL inactivity through mutation and methylation. Cell death, angiogenesis and drug resistance were specifically regulated by TGase 2 through p53 depletion in ccRCC because over 90% of ccRCC express wild type p53, which is a cell death inducer as well as a HIF-1α suppressor. Although there have been no detailed studies of the physiological role of TGase 2 in multi-omics analyses of ccRCC, a life-long study of the physiological roles of TGase 2 led to the discovery of the first target as well as the first therapeutic treatment for ccRCC in the clinical field.

Wnt/β-catenin signalling in ovarian cancer: Insights into its hyperactivation and function in tumorigenesis

Epithelial ovarian cancer (EOC) is the deadliest female malignancy. The Wnt/β-catenin pathway plays critical roles in regulating embryonic development and physiological processes. This pathway is tightly regulated to ensure its proper activity. In the absence of Wnt ligands, β-catenin is degraded by a destruction complex. When the pathway is stimulated by a Wnt ligand, β-catenin dissociates from the destruction complex and translocates into the nucleus where it interacts with TCF/LEF transcription factors to regulate target gene expression. Aberrant activation of this pathway, which leads to the hyperactivity of β-catenin, has been reported in ovarian cancer. Specifically, mutations of CTNNB1, AXIN, or APC, have been observed in the endometrioid and mucinous subtypes of EOC. In addition, upregulation of the ligands, abnormal activation of the receptors or intracellular mediators, disruption of the β-catenin destruction complex, inhibition of the association of β-catenin/E-cadherin on the cell membrane, and aberrant promotion of the β-catenin/TCF transcriptional activity, have all been reported in EOC, especially in the high grade serous subtype. Furthermore, several non-coding RNAs have been shown to regulate EOC development, in part, through the modulation of Wnt/β-catenin signalling. The Wnt/β-catenin pathway has been reported to promote cancer stem cell self-renewal, metastasis, and chemoresistance in all subtypes of EOC. Emerging evidence also suggests that the pathway induces ovarian tumor angiogenesis and immune evasion. Taken together, these studies demonstrate that the Wnt/β-catenin pathway plays critical roles in EOC development and is a strong candidate for the development of targeted therapies.

MiR-532-3p suppresses colorectal cancer progression by disrupting the ETS1/TGM2 axis-mediated Wnt/β-catenin signaling

The expression panel of plasma microRNA defined miR-532-3p as a valuable biomarker for colorectal adenoma (CRA). However, its expression pattern and function in colorectal cancer (CRC) have remained unclear. The present study investigated the expression levels of miR-532-3p and found that it was in situ downregulated both in CRA and CRC. Moreover, it functioned as a sensitizer for chemotherapy in CRC by inducing cell cycle arrest and early apoptosis via its activating effects on p53 and apoptotic signaling pathways. In addition, miR-532-3p was found to restrain cell growth, metastasis, and epithelial–mesenchymal transition (EMT) phenotype of CRC. A study on the mechanism behind these effects revealed that miR-532-3p directly binds to 3′UTR regions of ETS1 and TGM2, ultimately repressing the canonical Wnt/β-catenin signaling. Further investigation showed that TGM2 was transcriptionally regulated by ETS1 and ETS1/TGM2 axis served as a vital functional target of miR-532-3p in suppressing CRC progression. To conclude, miR-532-3p mimics could act as potential candidate for molecular therapy in CRC through inactivation of the canonical Wnt/β-catenin signaling and enhancement of chemosensitivity.

Small Molecules Target the Interaction between Tissue Transglutaminase and Fibronectin

Tissue transglutaminase (TG2) is a multifunctional protein with enzymatic, GTP-ase, and scaffold properties. TG2 interacts with fibronectin (FN) through its N-terminus domain, stabilizing integrin complexes, which regulate cell adhesion to the matrix. Through this mechanism, TG2 participates in key steps involved in metastasis in ovarian and other cancers. High-throughput screening identified several small molecule inhibitors (SMI) for the TG2/FN complex. Rational medicinal chemistry optimization of the hit compound (TG53) led to second-generation analogues (MT1-6). ELISA demonstrated that these analogues blocked TG2/FN interaction, and bio-layer interferometry (BLI) showed that the SMIs bound to TG2. The compounds also potently inhibited cancer cell adhesion to FN and decreased outside-in signaling mediated through the focal adhesion kinase. Blockade of TG2/FN interaction by the small molecules caused membrane ruffling, delaying the formation of stable focal contacts and mature adhesions points and disrupted organization of the actin cytoskeleton. In an in vivo model measuring intraperitoneal dissemination, MT4 and MT6 inhibited the adhesion of ovarian cancer cells to the peritoneum. Pretreatment with MT4 also sensitized ovarian cancer cells to paclitaxel. The data support continued optimization of the new class of SMIs that block the TG2/FN complex at the interface between cancer cells and the tumor niche.

Transglutaminase 2 takes center stage as a cancer cell survival factor and therapy target

Transglutaminase 2 (TG2) has emerged as a key cancer cell survival factor that drives epithelial to mesenchymal transition, angiogenesis, metastasis, inflammation, drug resistance, cancer stem cell survival and stemness, and invasion and migration. TG2 can exist in a GTP-bound signaling-active conformation or in a transamidase-active conformation. The GTP bound conformation of TG2 contributes to cell survival and the transamidase conformation can contribute to cell survival or death. We present evidence suggesting that TG2 has a role in human cancer, summarize what is known about the TG2 mechanism of action in a range of cancer types, and discuss TG2 as a cancer therapy target.

Targeting the Microenvironment in High Grade Serous Ovarian Cancer

Cancer⁻stroma interactions play a key role in cancer progression and response to standard chemotherapy. Here, we provide a summary of the mechanisms by which the major cellular components of the ovarian cancer (OC) tumor microenvironment (TME) including cancer-associated fibroblasts (CAFs), myeloid, immune, endothelial, and mesothelial cells potentiate cancer progression. High-grade serous ovarian cancer (HGSOC) is characterized by a pro-inflammatory and angiogenic signature. This profile is correlated with clinical outcomes and can be a target for therapy. Accumulation of malignant ascites in the peritoneal cavity allows for secreted factors to fuel paracrine and autocrine circuits that augment cancer cell proliferation and invasiveness. Adhesion of cancer cells to the mesothelial matrix promotes peritoneal tumor dissemination and represents another attractive target to prevent metastasis. The immunosuppressed tumor milieu of HGSOC is permissive for tumor growth and can be modulated therapeutically. Results of emerging preclinical and clinical trials testing TME-modulating therapeutics for the treatment of OC are highlighted.

Tissue Tranglutaminase Regulates Interactions between Ovarian Cancer Stem Cells and the Tumor Niche

Cancer progression and recurrence are linked to a rare population of cancer stem cells (CSC). Here, we hypothesized that interactions with the extracellular matrix drive CSC proliferation and tumor-initiating capacity and investigated the functions of scaffold protein tissue transglutaminase (TG2) in ovarian CSC. Complexes formed by TG2, fibronectin (FN), and integrin β1 were enriched in ovarian CSC and detectable in tumors. A function-inhibiting antibody against the TG2 FN-binding domain suppressed complex formation, CSC proliferation as spheroids, tumor-initiating capacity, and stemness-associated Wnt/β-catenin signaling. Disruption of the interaction between TG2 and FN also blocked spheroid formation and the response to Wnt ligands. TG2 and the Wnt receptor Frizzled 7 (Fzd7) form a complex in cancer cells and tumors, leading to Wnt pathway activation. Protein docking and peptide inhibition demonstrate that the interaction between TG2 and Fzd7 overlaps with the FN-binding domain of TG2. These results support a new function of TG2 in ovarian CSC, linked to spheroid proliferation and tumor-initiating capacity and mediated through direct interactions with Fzd7. We propose this complex as a new stem cell target.Significance: These findings reveal a new mechanism by which ovarian CSCs interact with the tumor microenvironment, promoting cell proliferation and tumor initiation. Cancer Res; 78(11); 2990-3001. ©2018 AACR.

Reduced expression of FILIP1L, a novel WNT pathway inhibitor, is associated with poor survival, progression and chemoresistance in ovarian cancer

Filamin A interacting protein 1-like (FILIP1L) is an inhibitor of the canonical WNT pathway. WNT/β-catenin signaling and its downstream pathway, epithelial-to-mesenchymal transition (EMT), play a key role in ovarian cancer metastasis and chemoresistance. To study the clinical implications of FILIP1L in regulating the WNT/β-catenin pathway, the expression of FILIP1L, β-catenin, SNAIL and SLUG was analyzed by immunohistochemistry on tissue microarrays of 369 ovarian samples ranging from normal to metastatic. In addition, the results were validated in mouse model and in vitro cell culture. In the present study, we demonstrated that FILIP1L expression was inversely correlated with poor prognosis, stage and chemoresistance in ovarian cancer. Notably, low FILIP1L expression was independent negative prognostic factor with respect to overall and disease-free survival. FILIP1L inhibited peritoneal metastases in orthotopic mouse model. FILIP1L knockdown induced chemoresistance in ovarian cancer cells and this phenotype was rescued by simultaneous knockdown of FILIP1L and SLUG, an EMT activator. We also demonstrated that FILIP1L regulates β-catenin degradation. FILIP1L co-localizes with phospho-β-catenin and increases phospho-β-catenin at the centrosomes, destined for proteosomal degradation. Finally, we showed that FILIP1L regulates EMT. Overall, these findings suggest that FILIP1L promotes β-catenin degradation and suppresses EMT, thereby inhibiting metastases and chemoresistance. Our study provides the first clinical relevance of FILIP1L in human cancer, and suggests that FILIP1L may be a novel prognostic marker for chemotherapy in ovarian cancer patients. Further, the modulation of FILIP1L expression may have the potential to be a target for cancer therapy.

Tissue transglutaminase induces Epithelial-Mesenchymal-Transition and the acquisition of stem cell like characteristics in colorectal cancer cells

Human colon cancer cell lines (CRCs) RKO, SW480 and SW620 were investigated for TG2 involvement in tumour advancement and aggression. TG2 expression correlated with tumour advancement and expression of markers of epithelial-mesenchymal transition (EMT). The metastatic cell line SW620 showed high TG2 expression compared to the primary tumour cell lines SW480 and RKO and could form tumour spheroids under non- adherent conditions. TG2 manipulation in the CRCs by shRNA or TG2 transduction confirmed the relationship between TG2 and EMT. TGFβ1 expression in CRC cells, and its level in the cell medium and extracellular matrix was increased in primary tumour CRCs overexpressing TG2 and could regulate TG2 expression and EMT by both canonical (RKO) and non-canonical (RKO and SW480) signalling. TGFβ1 regulation was not observed in the metastatic SW620 cell line, but TG2 knockdown or inhibition in SW620 reversed EMT. In SW620, TG2 expression and EMT was associated with increased presence of nuclear β-catenin which could be mediated by association of TG2 with the Wnt signalling co-receptor LRP5. TG2 inhibition/knockdown increased interaction between β-catenin and ubiquitin shown by co-immunoprecipitation, suggesting that TG2 could be important in β-catenin regulation. β-Catenin and TG2 was also upregulated in SW620 spheroid cells enriched with cancer stem cell marker CD44 and TG2 inhibition/knockdown reduced the spheroid forming potential of SW620 cells. Our data suggests that TG2 could hold both prognostic and therapeutic significance in colon cancer.

Transglutaminases factor XIII-A and TG2 regulate resorption, adipogenesis and plasma fibronectin homeostasis in bone and bone marrow

Appropriate bone mass is maintained by bone-forming osteoblast and bone-resorbing osteoclasts. Mesenchymal stem cell (MSC) lineage cells control osteoclastogenesis via expression of RANKL and OPG (receptor activator of nuclear factor κB ligand and osteoprotegerin), which promote and inhibit bone resorption, respectively. Protein crosslinking enzymes transglutaminase 2 (TG2) and Factor XIII-A (FXIII-A) have been linked to activity of myeloid and MSC lineage cells; however, in vivo evidence has been lacking to support their function. In this study, we show in mice that TG2 and FXIII-A control monocyte-macrophage cell differentiation into osteoclasts as well as RANKL production in MSCs and in adipocytes. Long bones of mice lacking TG2 and FXIII-A transglutaminases, show compromised biomechanical properties and trabecular bone loss in axial and appendicular skeleton. This was caused by increased osteoclastogenesis, a cellular phenotype that persists in vitro. The increased potential of TG2 and FXIII-A deficient monocytes to form osteoclasts was reversed by chemical inhibition of TG activity, which revealed the presence of TG1 in osteoclasts and assigned different roles for the TGs as regulators of osteoclastogenesis. TG2- and FXIII-A-deficient mice had normal osteoblast activity, but increased bone marrow adipogenesis, MSCs lacking TG2 and FXIII-A showed high adipogenic potential and significantly increased RANKL expression as well as upregulated TG1 expression. Chemical inhibition of TG activity in the null cells further increased adipogenic potential and RANKL production. Altered differentiation of TG2 and FXIII-A null MSCs was associated with plasma fibronectin (FN) assembly defect in cultures and FN retention in serum and marrow in vivo instead of assembly into bone. Our findings provide new functions for TG2, FXIII-A and TG1 in bone cells and identify them as novel regulators of bone mass, plasma FN homeostasis, RANKL production and myeloid and MSC cell differentiation.

Epithelial-mesenchymal transition of ovarian cancer cells is sustained by Rac1 through simultaneous activation of MEK1/2 and Src signaling pathways

Epithelial-mesenchymal transition (EMT) is regarded as a crucial contributing factor to cancer progression. Diverse factors have been identified as potent EMT inducers in ovarian cancer. However, molecular mechanism sustaining EMT of ovarian cancer cells remains elusive. Here, we show that the presence of SOS1/EPS8/ABI1 complex is critical for sustained EMT traits of ovarian cancer cells. Consistent with the role of SOS1/EPS8/ABI1 complex as a Rac1-specific guanine nucleotide exchange factor, depleting Rac1 results in the loss of most of mesenchymal traits in mesenchymal-like ovarian cancer cells while expressing constitutively active Rac1 leads to EMT in epithelial-like ovarian cancer cells. With the aid of clinically tested inhibitors targeting various EMT-associated signaling pathways, we show that only combined treatment of MEK1/2 and Src inhibitors can abolish constitutively active Rac1-led EMT and mesenchymal traits displayed by mesenchymal-like ovarian cancer cells. Further experiments also reveal that EMT can be induced in epithelial-like ovarian cancer cells by co-expressing constitutively active MEK1 and Src rather than either alone. As the activities of Erk and Src are higher in ovarian cancer cells with constitutively active Rac1, we conclude that Rac1 sustains ovarian cancer cell EMT through simultaneous activation of MEK1/2 and Src signaling pathways. Importantly, we demonstrate that combined use of MEK1/2 and Src inhibitors effectively suppresses development of intraperitoneal xenografts and prolongs the survival of ovarian cancer-bearing mice. This study suggests that cocktail of MEK1/2 and Src inhibitors represents an effective therapeutic strategy against ovarian cancer progression.

Essential Role of Transglutaminase 2 in Vascular Endothelial Growth Factor-Induced Vascular Leakage in the Retina of Diabetic Mice

Diabetic retinopathy is predominantly caused by vascular endothelial growth factor (VEGF)-induced vascular leakage; however, the underlying mechanism is unclear. Here we designed an in vivo transglutaminase (TGase) activity assay in mouse retina and demonstrated that hyperglycemia induced vascular leakage by activating TGase2 in diabetic retina. VEGF elevated TGase2 activity through sequential elevation of intracellular Ca(2+) and reactive oxygen species (ROS) concentrations in endothelial cells. The TGase inhibitors cystamine and monodansylcadaverin or TGase2 small interfering RNA (siRNA) prevented VEGF-induced stress fiber formation and vascular endothelial (VE)-cadherin disruption, which play a critical role in modulating endothelial permeability. Intravitreal injection of two TGase inhibitors or TGase2 siRNA successfully inhibited hyperglycemia-induced TGase activation and microvascular leakage in the retinas of diabetic mice. C-peptide or ROS scavengers also inhibited TGase activation in diabetic mouse retinas. The role of TGase2 in VEGF-induced vascular leakage was further supported using diabetic TGase2(-/-) mice. Thus, our findings suggest that ROS-mediated activation of TGase2 plays a key role in VEGF-induced vascular leakage by stimulating stress fiber formation and VE-cadherin disruption.

Transglutaminase is a tumor cell and cancer stem cell survival factor

Recent studies indicate that cancer cells express elevated levels of type II transglutaminase (TG2), and that expression is further highly enriched in cancer stem cells derived from these cancers. Moreover, elevated TG2 expression is associated with enhanced cancer stem cell marker expression, survival signaling, proliferation, migration, invasion, integrin-mediated adhesion, epithelial-mesenchymal transition, and drug resistance. TG2 expression is also associated with formation of aggressive and metastatic tumors that are resistant to conventional therapeutic intervention. This review summarizes the role of TG2 as a cancer cell survival factor in a range of tumor types, and as a target for preventive and therapeutic intervention. The literature supports the idea that TG2, in the closed/GTP-binding/signaling conformation, drives cancer cell and cancer stem cell survival, and that TG2, in the open/crosslinking conformation, is associated with cell death.

Physiological, pathological, and structural implications of non-enzymatic protein-protein interactions of the multifunctional human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed member of an enzyme family catalyzing Ca(2+)-dependent transamidation of proteins. It is a multifunctional protein having several well-defined enzymatic (GTP binding and hydrolysis, protein disulfide isomerase, and protein kinase activities) and non-enzymatic (multiple interactions in protein scaffolds) functions. Unlike its enzymatic interactions, the significance of TG2's non-enzymatic regulation of its activities has recently gained importance. In this review, we summarize all the partners that directly interact with TG2 in a non-enzymatic manner and analyze how these interactions could modulate the crosslinking activity and cellular functions of TG2 in different cell compartments. We have found that TG2 mostly acts as a scaffold to bridge various proteins, leading to different functional outcomes. We have also studied how specific structural features, such as intrinsically disordered regions and embedded short linear motifs contribute to multifunctionality of TG2. Conformational diversity of intrinsically disordered regions enables them to interact with multiple partners, which can result in different biological outcomes. Indeed, ID regions in TG2 were identified in functionally relevant locations, indicating that they could facilitate conformational transitions towards the catalytically competent form. We reason that these structural features contribute to modulating the physiological and pathological functions of TG2 and could provide a new direction for detecting unique regulatory partners. Additionally, we have assembled all known anti-TG2 antibodies and have discussed their significance as a toolbox for identifying and confirming novel TG2 regulatory functions.

Cellular Factor XIIIA Transglutaminase Localizes in Caveolae and Regulates Caveolin-1 Phosphorylation, Homo-oligomerization and c-Src Signaling in Osteoblasts

Transglutaminases (TGs) are a family of widely distributed enzymes that catalyze protein crosslinking by forming a covalent isopeptide bond between the substrate proteins. We have shown that MC3T3-E1 osteoblasts express Factor XIII-A (FXIII-A), and that the extracellular crosslinking activity of FXIII-A is involved in regulating matrix secretion and deposition. In this study, we have investigated the localization and potential role of intracellular FXIII-A. Conventional immunofluorescence microscopy and TIRF microscopy analyses showed that FXIII-A co-localizes with caveolin-1 in specialized membrane structures, caveolae, in differentiating osteoblasts. The caveolae-disrupting agent methyl-β-cyclodextrin abolished FXIII-A staining and co-localization with caveolin-1 from the osteoblast plasma membrane. The presence of FXIII-A in caveolae was confirmed by preparing caveolae-enriched cellular fractions using sucrose density gradient ultracentrifugation followed by western blotting. Despite this association of FXIII-A with caveolae, there was no detectable transglutaminase activity in caveolae, as measured by monodansylcadaverine incorporation. TG inhibitor NC9--which can alter TG enzyme conformation--localized to caveolae and displaced FXIII-A from these structures when added to the osteoblast cultures. The decreased FXIII-A levels in caveolae after NC9 treatment increased c-Src activation, which resulted in caveolin-1 phosphorylation, homo-oligomerization and Akt phosphorylation, suggesting cellular FXIII-A has a role in regulating c-Src signaling in osteoblasts.

SET7/9 regulates cancer cell proliferation by influencing β-catenin stability

β-Catenin, which is a key mediator of the wingless-integration site (Wnt)/β-catenin signaling pathway, plays an important role in cell proliferation, cell fate determination, and tumorigenesis, by regulating the expression of a wide range of target genes. Although a variety of posttranslational modifications are involved in β-catenin activity, the role of lysine methylation in β-catenin activity is largely unknown. In this study, su(var)3-9, enhancer-of-zeste, trithorax (SET) domain-containing protein 7 (SET7/9), a lysine methyltransferase, interacted with and methylated β-catenin, as demonstrated both in vitro and in vivo. The interaction and methylation were significantly enhanced in response to H2O2 stimulation. A mutagenesis assay and mass spectrometric analyses revealed that β-catenin was monomethylated by SET7/9 at lysine residue 180. Methylated β-catenin was easily recognized by phosphokinase glycogen synthase kinase (GSK)-3β for degradation. Consistent with this finding, the mutated β-catenin (K180R) that cannot be methylated exhibited a longer half-life than did the methylated β-catenin. The consequent depletion of SET7/9 by shRNA or the mutation of the β-catenin (K180R) significantly enhanced the expression of Wnt/β-catenin target genes such as c-myc and cyclin D1 and promoted the growth of cancer cells. Together, these results provide a novel mechanism by which Wnt/β-catenin signaling is regulated in response to oxidative stress.

Nuclear signaling from cadherin adhesion complexes

The arrival of multicellularity in evolution facilitated cell-cell signaling in conjunction with adhesion. As the ectodomains of cadherins interact with each other directly in trans (as well as in cis), spanning the plasma membrane and associating with multiple other entities, cadherins enable the transduction of "outside-in" or "inside-out" signals. We focus this review on signals that originate from the larger family of cadherins that are inwardly directed to the nucleus, and thus have roles in gene control or nuclear structure-function. The nature of cadherin complexes varies considerably depending on the type of cadherin and its context, and we will address some of these variables for classical cadherins versus other family members. Substantial but still fragmentary progress has been made in understanding the signaling mediators used by varied cadherin complexes to coordinate the state of cell-cell adhesion with gene expression. Evidence that cadherin intracellular binding partners also localize to the nucleus is a major point of interest. In some models, catenins show reduced binding to cadherin cytoplasmic tails favoring their engagement in gene control. When bound, cadherins may serve as stoichiometric competitors of nuclear signals. Cadherins also directly or indirectly affect numerous signaling pathways (e.g., Wnt, receptor tyrosine kinase, Hippo, NFκB, and JAK/STAT), enabling cell-cell contacts to touch upon multiple biological outcomes in embryonic development and tissue homeostasis.

Transglutaminase 2 promotes PDGF-mediated activation of PDGFR/Akt1 and β-catenin signaling in vascular smooth muscle cells and supports neointima formation

BACKGROUND

Phenotypic switch of vascular smooth muscle cells (VSMCs) accompanies neointima formation and associates with vascular diseases. Platelet-derived growth factor (PDGF)-induced activation of PDGFR/Akt1 and β-catenin signaling pathways in VSMCs has been implicated in vessel occlusion. Transglutaminase 2 (TG2) regulates these pathways and its levels are increased in the neointima.

OBJECTIVE

The aim of this study was to evaluate the role of TG2 in PDGF/β-catenin signaling cross-talk and assess its contribution to neointima.

METHODS

Aortic VSMCs from wild-type and TG2 knockout mice were tested in vitro for levels of VSMC markers, proliferation, migration and PDGF-induced activation of PDGFR/Akt1 and β-catenin pathways. Neointima in these mice was studied ex vivo in coronary vessels using a heart slice model and in vivo using a carotid artery ligation model.

RESULTS

Genetic deletion of TG2 attenuated the PDGF-induced phenotypic switch of aortic VSMCs, reduced their proliferation and migration rates, and inhibited PDGF-induced activation of PDGFR/Akt1 and β-catenin pathways in both ex vivo and in vivo neointima models. Importantly, genetic deletion of TG2 also markedly attenuated vessel occlusion.

CONCLUSIONS

TG2 promotes neointima formation by mediating the PDGF-induced activation of the PDGFR/Akt1 and β-catenin pathways in VSMCs. This study identifies TG2 as a potential therapeutic target for blocking neointima in blood vessels.

Lack of transglutaminase 2 diminished T-cell responses in mice

Transglutaminase 2 (TG2) has been reported to play a role in dendritic cell activation and B-cell differentiation after immunization. Its presence and role in T cells, however, has not been explored. In the present study, we determined the expression of TG2 on mouse T cells, and evaluated its role by comparing the behaviours of wild-type and TG2(-/-) T cells after activation. In our results, naive T cells minimally expressed TG2, expression of which was increased after activation. T-cell proliferation, expression of activation markers such as CD69 and CD25, and secretions of interleukin-2 and interferon-γ were suppressed in the absence of TG2, presumably due, in part, to diminished nuclear factor-κB activation. These effects on T cells seemed to be reflected in the in vivo immune response, the contact hypersensitivity reaction elicited by 2,4-dinitro-1-fluorobenzene, with lowered peak responses in the TG2(-/-) mice. When splenic T cells from mice immunized with tumour lysate-loaded wild-type dendritic cells were re-challenged ex vivo with the same antigen, the profile of surface markers including CD44, CD62L, and CD127 strongly indicated lesser generation of memory CD8(+) T cells in TG2(-/-) mice. In the TG2(-/-)  CD8(+) T cells, moreover, Eomes expression was markedly decreased. These results indicate possible roles of TG2 in CD8(+) T-cell activation and CD8(+) memory T-cell generation.

β-Catenin-regulated ALDH1A1 is a target in ovarian cancer spheroids

Cancer cells form three dimensional (3D) multicellular aggregates (or spheroids) under non-adherent culture conditions. In ovarian cancer (OC), spheroids serve as a vehicle for cancer cell dissemination in the peritoneal cavity, protecting cells from environmental stress-induced anoikis. To identify new targetable molecules in OC spheroids, we investigated gene expression profiles and networks upregulated in three dimensional (3D) versus traditional monolayer culture conditions. We identified ALDH1A1, a cancer stem cell marker as being overexpressed in OC spheroids and directly connected to key elements of the β-catenin pathway. B-catenin function and ALDH1A1 expression were increased in OC spheroids vs. monolayers and in successive spheroid generations, suggesting that 3D aggregates are enriched in cells with stem cell characteristics. B-catenin knockdown decreased ALDH1A1 expression levels and β-catenin coimmunoprecipitated with the ALDH1A1 promoter, suggesting that ALDH1A1 is a direct β-catenin target. Both siRNA mediated β-catenin knockdown and A37, a novel ALDH1A1 small molecule enzymatic inhibitor described here for the first time, disrupted OC spheroid formation and cell viability (p<0.001). B-catenin knockdown blocked tumor growth and peritoneal metastasis in an OC xenograft model. These data strongly support the role of β-catenin regulated ALDH1A1 in the maintenance of OC spheroids and propose new ALDH1A1 inhibitors targeting this cell population.

Small molecule inhibitors target the tissue transglutaminase and fibronectin interaction

Tissue transglutaminase (TG2) mediates protein crosslinking through generation of ε-(γ-glutamyl) lysine isopeptide bonds and promotes cell adhesion through interaction with fibronectin (FN) and integrins. Cell adhesion to the peritoneal matrix regulated by TG2 facilitates ovarian cancer dissemination. Therefore, disruption of the TG2-FN complex by small molecules may inhibit cell adhesion and metastasis. A novel high throughput screening (HTS) assay based on AlphaLISA™ technology was developed to measure the formation of a complex between His-TG2 and the biotinylated FN fragment that binds TG2 and to discover small molecules that inhibit this protein-protein interaction. Several hits were identified from 10,000 compounds screened. The top candidates selected based on >70% inhibition of the TG2/FN complex formation were confirmed by using ELISA and bioassays measuring cell adhesion, migration, invasion, and proliferation. In conclusion, the AlphaLISA bead format assay measuring the TG2-FN interaction is robust and suitable for HTS of small molecules. One compound identified from the screen (TG53) potently inhibited ovarian cancer cell adhesion to FN, cell migration, and invasion and could be further developed as a potential inhibitor for ovarian cancer dissemination.

Transglutaminase 2 as a novel activator of LRP6/β-catenin signaling

The β-catenin signaling axis is critical for normal embryonic development and tissue homeostasis in adults. We have previously shown that extracellular enzyme transglutaminase 2 (TG2) activates β-catenin signaling in vascular smooth muscle cells (VSMCs). In this study, we provide several lines of evidence that TG2 functions as an activating ligand of the LRP5/6 receptors. Specifically, we show that TG2 synergizes with LRP6 in the activation of β-catenin-dependent gene expression in Cos-7 cells. Interfering with the LRP5/6 receptors attenuates TG2-induced activation of β-catenin in Cos-7 cells. Further, we show that TG2 binds directly to the extracellular domain of LRP6, which is also able to act as a substrate for TG2-mediated protein cross-linking. Furthermore, inhibitors of TG2 protein cross-linking quench the observed TG2-induced β-catenin activation, implicating protein cross-linking as a novel regulatory mechanism for this pathway. Together, our findings identify and characterize a new activating ligand of the LRP5/6 receptors and uncover a novel activity of TG2 as an agonist of β-catenin signaling, contributing to the understanding of diverse developmental events and pathological conditions in which transglutaminase and β-catenin signaling are implicated.