The RUNX3 tumor suppressor upregulates Bim in gastric epithelial cells undergoing transforming growth factor beta-induced apoptosis

Helicobacter pylori CagA targets gastric tumor suppressor RUNX3 for proteasome-mediated degradation

Chronic infection with cagA-positive Helicobacter pylori is the strongest risk factor for the development of gastric adenocarcinoma. The cagA gene product CagA is injected into gastric epithelial cells and disturbs cellular functions by physically interacting with and deregulating a variety of cellular signaling molecules. RUNX3 is a tumor suppressor in many tissues, and it is frequently inactivated in gastric cancer. In this study, we show that H. pylori infection inactivates the gastric tumor suppressor RUNX3 in a CagA-dependent manner. CagA directly associates with RUNX3 through a specific recognition of the PY motif of RUNX3 by a WW domain of CagA. Deletion of the WW domains of CagA or mutation of the PY motif in RUNX3 abolishes the ability of CagA to induce the ubiquitination and degradation of RUNX3, thereby extinguishing its ability to inhibit the transcriptional activation of RUNX3. Our studies identify RUNX3 as a novel cellular target of H. pylori CagA and also reveal a mechanism by which CagA functions as an oncoprotein by blocking the activity of gastric tumor suppressor RUNX3.

TGFbeta signalling: a complex web in cancer progression

The distortion of growth factor signalling is the most important prerequisite in tumour progression. Transforming growth factor-beta (TGFbeta) signalling regulates tumour progression by a tumour cell-autonomous mechanism or through tumour-stroma interaction, and has either a tumour-suppressing or tumour-promoting function depending on cellular context. Such inherent complexity of TGFbeta signalling results in arduous, but promising, assignments for developing therapeutic strategies against malignant tumours. As numerous cellular context-dependent factors tightly maintain the balance of TGFbeta signalling and contribute to the regulation of TGFbeta-induced cell responses, in this Review we discuss how they maintain the balance of TGFbeta signalling and how their collapse leads to tumour progression.

Role of RUNX3 in bone morphogenetic protein signaling in colorectal cancer

Bone morphogenetic proteins (BMPs), members of the transforming growth factor-beta (TGF-beta) superfamily, are multifunctional cytokines regulating a broad spectrum of biological functions. Recent studies show the presence of BMP receptor 1a mutations in juvenile polyposis and frequent Smad4 mutations in colon cancer, suggesting that aberrations in BMP signaling play an important role in intestinal cancer pathogenesis. However, the exact molecular mechanisms remain poorly understood. The Runt domain transcription factor RUNX3 is an integral component of signaling pathways mediated by TGF-beta and BMPs. RUNX3 is a gastric and colon tumor suppressor, functioning downstream of TGF-beta. Recently, we showed the tumor-suppressive effects of RUNX3 by its ability to attenuate beta-catenin/T-cell factors (TCFs) transactivation in intestinal tumorigenesis. Here, we explore the molecular basis of the tumor-suppressive function of the BMP pathway through RUNX3 in colorectal carcinogenesis. BMP exerted a growth-suppressive effect in HT-29, a human colorectal cancer cell line. c-Myc oncogene was found to be downregulated by BMP and/or RUNX3. We show that upregulation of RUNX3 by BMP reduces c-Myc expression. Evidence is presented suggesting that RUNX3 downregulates c-Myc expression by two parallel pathways-directly at the transcriptional level and through attenuation of beta-catenin/TCFs, downstream of BMPs in colorectal cancer cells.

RUNX3 modulates DNA damage-mediated phosphorylation of tumor suppressor p53 at Ser-15 and acts as a co-activator for p53

Although it has been shown that the gastric tumor suppressor RUNX3 has a growth inhibitory activity, the precise molecular mechanisms behind RUNX3-mediated tumor suppression remained unclear. In this study, we found that RUNX3 is closely involved in DNA damage-dependent phosphorylation of tumor suppressor p53 at Ser-15 and acts as a co-activator for p53. The small interference RNA-mediated knockdown of RUNX3 inhibited adriamycin (ADR)-dependent apoptosis in p53-proficient cells but not in p53-deficient cells in association with a significant reduction of p53-target gene expression as well as phosphorylation of p53 at Ser-15. In response to ADR, RUNX3 was induced to accumulate in the cell nucleus and co-localized with p53. Immunoprecipitation experiments demonstrated that RUNX3 forms a complex with p53 in cells. In vitro pulldown assays revealed that the COOH-terminal portion of p53 is required for the interaction with RUNX3. Forced expression of RUNX3 enhanced p53-mediated transcriptional activation. Additionally, RUNX3 had an ability to induce the phosphorylation of p53 at Ser-15, thereby promoting p53-dependent apoptosis. Intriguingly, RUNX3 interacted with phosphorylated forms of ataxia telangiectasia-mutated in response to ADR; however, it did not affect the extent of DNA damage. From the clinical point of view, coordinated p53 mutation and decreased expression of RUNX3 in 105 human lung adenocarcinomas were significantly associated with the poor outcome of patients (p = 0.0203). Thus, our present results strongly suggest that RUNX3 acts as a novel co-activator for p53 through regulating its DNA damage-induced phosphorylation at Ser-15 and also provide a clue to understanding the molecular mechanisms underlying RUNX3-mediated tumor suppression.

Bim-targeted cancer therapy: a link between drug action and underlying molecular changes

In the past few years, the pro-apoptotic molecule Bim has attracted increasing attention as a plausible target for tumor therapy. A variety of normal and pathological systems regulated by Bim, dependent on cell type, apoptotic stimulation, and chemotherapeutic agents, have been documented. Bim promotes anoikis of many tumor cells, such as lung cancer, breast cancer, osteosarcoma, and melanoma. Various chemotherapeutic agents use Bim as a mediating executioner of cell death. Hence, Bim suppression supports metastasis and chemoresistance. Imatinib, gefitinib, bortezomib, and Bim protein itself are spotlighted as current and future Bim-targeting therapeutic agents. The potential benefits of Bim-targeted therapies are selectivity of treatment for tumor cells and reduction in tumor-associated phenomena such as chemoresistance and metastasis. Thus, Bim-targeting therapies may provide more effective and unique tumor management modalities in future. This review article discusses all these issues.

Runx3 is required for the differentiation of lung epithelial cells and suppression of lung cancer

Human lung adenocarcinoma, the most prevalent form of lung cancer, is characterized by many molecular abnormalities. K-ras mutations are associated with the initiation of lung adenocarcinomas, but K-ras-independent mechanisms may also initiate lung tumors. Here, we find that the runt-related transcription factor Runx3 is essential for normal murine lung development and is a tumor suppressor that prevents lung adenocarcinoma. Runx3-/- mice, which die soon after birth, exhibit alveolar hyperplasia. Importantly, Runx3-/- bronchioli exhibit impaired differentiation, as evidenced by the accumulation of epithelial cells containing specific markers for both alveolar (that is SP-B) and bronchiolar (that is CC10) lineages. Runx3-/- epithelial cells also express Bmi1, which supports self-renewal of stem cells. Lung adenomas spontaneously develop in aging Runx3+/- mice ( approximately 18 months after birth) and invariably exhibit reduced levels of Runx3. As K-ras mutations are very rare in these adenomas, Runx3+/- mice provide an animal model for lung tumorigenesis that recapitulates the preneoplastic stage of human lung adenocarcinoma development, which is independent of K-Ras mutation. We conclude that Runx3 is essential for lung epithelial cell differentiation, and that downregulation of Runx3 is causally linked to the preneoplastic stage of lung adenocarcinoma.

MicroRNA-130b regulates the tumour suppressor RUNX3 in gastric cancer

AIM

Accumulating evidence indicates that RUNX3 is an important tumour suppressor that is inactivated in many cancer types. This study aimed to assess the role of microRNA (miRNA) in the regulation of RUNX3.

METHODS

Four bioinformatic algorithms were used to predict miRNA binding to RUNX3. The correlation between candidate miRNAs and RUNX3 expression in cell lines was determined by real-time reverse transcriptase quantitative PCR (RT-qPCR) and Western blot. Candidate miRNAs were tested for functional effects through transfection of miRNA precursors and inhibitors, and monitoring cell viability, apoptosis and Bim expression. miRNA and RUNX3 expression, RUNX3 methylation and RUNX3 protein levels were assessed in gastric tissue by RT-qPCR, Methylight analysis and immunohistochemistry, respectively.

RESULTS

Bioinformatics, gene and protein expression analysis in eight gastric cell lines identified miR-130b as the top candidate miRNA for RUNX3 binding. Overexpression of miR-130b increased cell viability, reduced cell death and decreased expression of Bim in TGF-beta mediated apoptosis, subsequent to the downregulation of RUNX3 protein expression. In 15 gastric tumours, miR-130b expression was significantly higher compared to matched normal tissue, and was inversely associated with RUNX3 hypermethylation.

CONCLUSION

Attenuation of RUNX3 protein levels by miRNA may reduce the growth suppressive potential of RUNX3 and contribute to tumourigenesis.

Claudin-1 has tumor suppressive activity and is a direct target of RUNX3 in gastric epithelial cells

BACKGROUND & AIMS

The transcription factor RUNX3 is a gastric tumor suppressor. Tumorigenic Runx3(-/-) gastric epithelial cells attach weakly to each other, compared with nontumorigenic Runx3(+/+) cells. We aimed to identify RUNX3 target genes that promote cell-cell contact to improve our understanding of RUNX3's role in suppressing gastric carcinogenesis.

METHODS

We compared gene expression profiles of Runx3(+/+) and Runx3(-/-) cells and observed down-regulation of genes associated with cell-cell adhesion in Runx3(-/-) cells. Reporter, mobility shift, and chromatin immunoprecipitation assays were used to examine the regulation of these genes by RUNX3. Tumorigenesis assays and immunohistological analyses of human gastric tumors were performed to confirm the role of the candidate genes in gastric tumor development.

RESULTS

Mobility shift and chromatin immunoprecipitation assays revealed that the promoter activity of the gene that encodes the tight junction protein claudin-1 was up-regulated via the binding of RUNX3 to the RUNX consensus sites. The tumorigenicity of gastric epithelial cells from Runx3(-/-) mice was significantly reduced by restoration of claudin-1 expression, whereas knockdown of claudin-1 increased the tumorigenicity of human gastric cancer cells. Concomitant expression of RUNX3 and claudin-1 was observed in human normal gastric epithelium and cancers.

CONCLUSIONS

The tight junction protein claudin-1 has gastric tumor suppressive activity and is a direct transcriptional target of RUNX3. Claudin-1 is down-regulated during the epithelial-mesenchymal transition; RUNX3 might therefore act as a tumor suppressor to antagonize the epithelial-mesenchymal transition.

The regulation of TGFbeta signal transduction

Transforming growth factor beta (TGFbeta) pathways are implicated in metazoan development, adult homeostasis and disease. TGFbeta ligands signal via receptor serine/threonine kinases that phosphorylate, and activate, intracellular Smad effectors as well as other signaling proteins. Oligomeric Smad complexes associate with chromatin and regulate transcription, defining the biological response of a cell to TGFbeta family members. Signaling is modulated by negative-feedback regulation via inhibitory Smads. We review here the mechanisms of TGFbeta signal transduction in metazoans and emphasize events crucial for embryonic development.

Gene expression profiling of pulmonary fibrosis identifies Twist1 as an antiapoptotic molecular "rectifier" of growth factor signaling

Idiopathic pulmonary fibrosis (IPF) is a progressive and typically fatal lung disease. To gain insight into IPF pathogenesis, we performed gene expression profiling of IPF lungs. Twist1, a basic helix-loop-helix protein, was found among the most consistently and highly up-regulated genes and was expressed in nuclei of type II epithelial cells, macrophages, and fibroblasts in IPF lungs. We studied the function of Twist1 in fibroblasts further, because they are the major effector cells in this disease and persist despite an ambient proapoptotic environment. Twist1 was induced by the profibrotic growth factors (GFs) basic fibroblast growth factor, platelet-derived growth factor, and epidermal growth factor in primary rat lung fibroblasts (RLFs). Suppression of Twist1 expression resulted in decreased RLF accumulation due to increased apoptosis, whereas Twist1 overexpression protected RLFs against several apoptotic stimuli. Addition of platelet-derived growth factor in combination with other GFs led to an increase in proliferation. When Twist1 was depleted, GFs continued to act as mitogens but caused a marked increase in cell death. The increase in apoptosis under basal or growth factor-stimulated conditions was partly mediated by up-regulation of the proapoptotic Bcl-2 family members, Bim and PUMA. These findings indicate that Twist1 promotes survival and accumulation of fibroblasts by shaping their responsiveness to growth factor stimulation. We propose that Twist1 represents one of the factors that promotes pathogenic accumulation of fibroblasts in fibrotic lung disease.

Runt-related transcription factor RUNX3 is a target of MDM2-mediated ubiquitination

The p14(ARF)-MDM2-p53 pathway constitutes an effective mechanism for protecting cells from oncogenic stimuli such as activated Ras and Myc. Importantly, Ras activation induces p14(ARF) and often occurs earlier than p53 inactivation during cancer development. Here, we show that RUNX3, a tumor suppressor in various tumors including stomach, bladder, colon, and lung, is stabilized by Ras activation through the p14(ARF)-MDM2 signaling pathway. RUNX3 directly binds MDM2 through its Runt-related DNA-binding domain. MDM2 blocks RUNX3 transcriptional activity by interacting with RUNX3 through an acidic domain adjacent to the p53-binding domain of MDM2 and ubiquitinates RUNX3 on key lysine residues to mediate nuclear export and proteasomal degradation. Our data indicate that the lineage-specific tumor suppressor RUNX3 and the ubiquitous p53 protein are both principal responders of the p14(ARF)-MDM2 cell surveillance pathway that prevents pathologic consequences of abnormal oncogene activation.

Ectopic runx2 expression in mammary epithelial cells disrupts formation of normal acini structure: implications for breast cancer progression

The transcription factor Runx2 is highly expressed in breast cancer cells compared with mammary epithelial cells and contributes to metastasis. Here we directly show that Runx2 expression promotes a tumor cell phenotype of mammary acini in three-dimensional culture. Human mammary epithelial cells (MCF-10A) form polarized, growth-arrested, acini-like structures with glandular architecture. The ectopic expression of Runx2 disrupts acini formation, and electron microscopic ultrastructural analysis revealed the absence of lumens. Characterization of the disrupted acini structures showed increased cell proliferation (Ki-67 positive cells), decreased apoptosis (Bcl-2 induction), and loss of basement membrane formation (absence of beta(4) integrin expression). In complementary experiments, inhibition of Runx2 function in metastatic MDA-MB-231 breast cancer cells by stable expression of either short hairpin RNA-Runx2 or a mutant Runx2 deficient in subnuclear targeting resulted in reversion of acini to more normal structures and reduced tumor growth in vivo. These novel findings provide direct mechanistic evidence for the biological activity of Runx2, dependent on its subnuclear localization, in promoting early events of breast cancer progression and suggest a molecular therapeutic target.

Molecular pathology of RUNX3 in human carcinogenesis

A major goal of molecular biology is to elucidate the mechanisms underlying cancer development and progression in order to achieve early detection, better diagnosis and staging and novel preventive and therapeutic strategies. We feel that an understanding of Runt-related transcription factor 3 (RUNX3)-regulated biological pathways will directly impact our knowledge of these areas of human carcinogenesis. The RUNX3 transcription factor is a downstream effector of the transforming growth factor-beta (TGF-beta) signaling pathway, and has a critical role in the regulation of cell proliferation and cell death by apoptosis, and in angiogenesis, cell adhesion and invasion. We previously identified RUNX3 as a major gastric tumor suppressor by establishing a causal relationship between loss of function and gastric carcinogenesis. More recently, we showed that RUNX3 functions as a bona fide initiator of colonic carcinogenesis by linking the Wnt oncogenic and TGF-beta tumor suppressive pathways. Apart from gastric and colorectal cancers, a multitude of epithelial cancers exhibit inactivation of RUNX3, thereby making it a putative tumor suppressor in human neoplasia. This review highlights our current understanding of the molecular mechanisms of RUNX3 inactivation in the context of cancer development and progression.

Is Runx a linchpin for developmental signaling in metazoans?

The Runt domain (Runx) is a 128 amino acid sequence motif that defines a metazoan family of sequence-specific DNA binding proteins, which appears to have originated in concert with the intercellular signaling systems that coordinate multicellular development in animals. In the model organisms where they have been studied (fruit fly, mouse, sea urchin, and nematode) Runx genes are essential for normal development, and in humans they are causally associated with a variety of cancers, manifesting both oncogenic and tumor suppressive attributes. During development Runx proteins support both cell proliferation and differentiation, and function in both transcriptional activation and repression. Runx function is thus context-dependent, with the context provided genetically by cis-regulatory sequence architecture and epigenetically by development. This context dependency makes it difficult to formulate reductionistic generalizations concerning Runx function in normal and carcinogenic development. However, a growing body of literature links Runx function to each of the major intercellular signaling systems in animals, suggesting that the general function of Runx transcription factors may be to potentiate and govern genomic responsiveness to developmental signaling.

Runx1 is a co-activator with FOXO3 to mediate transforming growth factor beta (TGFbeta)-induced Bim transcription in hepatic cells

Transforming growth factor beta (TGFbeta) regulates essential cellular functions such as cellular proliferation, differentiation, and apoptosis. The Bcl-2 family of proteins has been implicated as mediators of TGFbeta-induced apoptosis. We demonstrated previously that TGFbeta induces the expression of Bim (Bcl-2-interacting mediator of cell death), a member of the BH3-only family of pro-apoptotic Bcl-2 proteins, to induce cell death in B-lymphocytes. Here, we investigated the mechanism of TGFbeta-mediated Bim expression in two hepatocyte cell lines that undergo apoptosis with TGFbeta, AML-12 and Hep3B. We show that TGFbeta induces Bim protein and mRNA levels, and its expression is sufficient to induce cell death. Gene array results revealed that Runx1, a member of the Runx family of transcription factors, was induced by TGFbeta, and this induction was confirmed at the mRNA and protein levels. Interestingly, TGFbeta specifically induced the expression of Runx1 protein from an internal ribosome entry site (IRES)-dependent, cap-independent, mRNA transcript, and its overexpression was sufficient to induce hepatocyte apo pto sis. Deletion and mutation analyses of the murine Bim promoter identified a putative forkhead binding element, at position -174 to -168 from the transcription start site, as the mediator of Runx1 induction. Co-immunoprecipitation, electrophoretic mobility shift assays, and chromatin immunoprecipitation assays demonstrated that Runx1 does not bind directly to the identified forkhead binding element but rather binds the transcriptional regulator FOXO3, which occupies this site. Finally, small interfering RNA knockdown of Runx1 or FOXO3 decreased TGFbeta-induced Bim expression. Our results support a mechanism in which TGFbeta stimulates Bim transcription by up-regulating Runx1 expression, which binds FOXO3, and the two cooperate in the transcriptional induction of Bim.

'Runxs and regulations' of sensory and motor neuron subtype differentiation: implications for hematopoietic development

Runt-related (RUNX) transcription factors are evolutionarily conserved regulators of a number of developmental mechanisms. RUNX proteins often control the balance between proliferation and differentiation and alterations of their functions are associated with different types of cancer and other human pathologies. Moreover, RUNX factors control important steps during the developmental acquisition of mature phenotypes. A number of investigations are beginning to shed light on the involvement of RUNX family members in the development of the nervous system. This review summarizes recent progress in the study of the roles of mammalian RUNX proteins during the differentiation of sensory and motor neurons in the peripheral and central nervous system, respectively. The implications of those findings for RUNX-mediated regulation of hematopoietic development will also be discussed.

RUNX3 inactivation in colorectal polyps arising through different pathways of colonic carcinogenesis

OBJECTIVES

We hypothesized that RUNX3 inactivation by promoter hypermethylation in colorectal polyps is an early molecular event in colorectal carcinogenesis.

METHODS

RUNX3 protein expression was analyzed immunohistochemically in 50 sporadic colorectal polyps comprising 19 hyperplastic polyps (HPs), 14 traditional serrated adenomas (TSAs), and 17 sporadic traditional adenomas (sTAs) as well as in 19 familial adenomatous polyposis (FAP) samples from 10 patients showing aberrant crypt foci (ACF) (n=91), small adenomas (SmAds) (n=40), and large adenomas (LAds) (n=13). In addition, we assessed the frequency of promoter hypermethylation of RUNX3 by methylation-specific PCR (MSP) in all the 50 sporadic polyps as well as 38 microdissected FAP polyps comprising ACF, SmAds, and LAds obtained from 7 FAP samples. A total of 12 normal colon samples were also included for RUNX3 MSP analysis.

RESULTS

Compared to normal colon (2 of 12, 16%) and sTAs (3 of 17, 18%), HPs (15 of 19, 79%) and TSAs (8 of 14, 57%) displayed significant inactivation of RUNX3 (P<0.05). In FAP, RUNX3 inactivation was more frequently seen in ACF (78 of 91, 86%), SmAds (25 of 40, 62%), and LAds (6 of 13, 46%) compared to normal mucosa (0 of 19, 0%) in the same samples (all P<0.05). Promoter hypermethylation of RUNX3 was significantly higher in colorectal polyps (64 of 87, 74%) compared to normal colon (2 of 12, 16%) (P=0.001). Serrated polyps such as HPs (17 of 19, 89%) and TSAs (12 of 14, 86%) were significantly more methylated than sTAs (7 of 17, 44%) (P=0.004). RUNX3 hypermethylation was observed in 28 of the total 38 (74%) FAP polyps. Overall, RUNX3 promoter methylation correlated with inactivation of RUNX3 expression in sporadic (27 of 36, 75%) (P=0.022) and FAP (21 of 28, 75%) (P=0.021) polyps.

CONCLUSIONS

Our data suggest that RUNX3 inactivation due to promoter hypermethylation in colorectal polyps represents an early event in colorectal cancer (CRC) progression. In addition, epigenetic RUNX3 inactivation is a frequent event in the serrated colonic polyps as well as in the ACF of FAP polyps.

Transforming growth factor beta (TGFbeta)-induced apoptosis: the rise & fall of Bim

Transforming growth factor beta (TGFbeta) regulates essential cellular functions such as cellular proliferation, differentiation and apoptosis. Multiple apoptotic mediators and signaling pathways have been implicated in TGFbeta-induced apoptosis. Bim, a BH3-only protein, is critical for apoptosis in a variety of cell types. In resting cells, BimEL expression levels, the major and most abundant isoform, are controlled by Erk1/2-mediated phosphorylation, which targets BimEL for ubiquitination and degradation. We previously reported that TGFbeta induces the expression of the pro-apoptotic protein Bim through a Smad3-dependent mechanism to induce cell death in B-lymphocytes. A number of studies have shown TGFbeta to cause transcriptional induction of Bim in many cell types. Recently, we demonstrated that, in addition to its transcriptional effects on Bim, TGFbeta induces a MAPK phosphatase (MKP), MKP2/DUSP4, to rapidly increase BimEL levels by inactivation of Erk1/2, resulting in dephosphorylation and escape of BimEL from ubiquitin-mediated degradation. Our findings are of importance not only in the context that we implicate TGFbeta to increase BimEL levels through both an immediate post-translational regulatory mechanism and a long-term effect through transcriptional induction, but also in the context of implicating MKPs as regulatory players in apoptosis. Here we summarize these recent findings and their significance to our understanding of how TGFbeta mediates apoptosis, and we explore the possible regulatory mechanisms controlling Bim expression levels.

Runx3 expression in gastrointestinal tract epithelium: resolving the controversy

We reported earlier that RUNX3 is expressed in human and mouse gastrointestinal tract (GIT) epithelium and that it functions as a tumor suppressor in gastric and colorectal tissues. However, there have been conflicting reports describing the absence of Runx3 in GIT epithelial cells. A part of the controversy may be derived from the use of a specific antibody by other groups (referred to as G-poly). Here, we show further evidence to support our earlier observations and provide a possible explanation for this apparent controversy. We generated multiple anti-RUNX3 monoclonal antibodies and found that RUNX3 antibodies recognizing the RUNX3 N-terminal region (residues 1-234) react with RUNX3 in gastric epithelial cells, whereas those recognizing the C-terminal region (beyond residue 234) did not. G-poly primarily recognizes the region beyond 234 and hence, is unable to detect Runx3 in this tissue.

Subnuclear targeting of the Runx3 tumor suppressor and its epigenetic association with mitotic chromosomes

Runx proteins are tissue-specific transcriptional scaffolds that organize and assemble regulatory complexes at strategic sites of target gene promoters and at intranuclear foci to govern activation or repression. During interphase, fidelity of intranuclear targeting supports the biological activity of Runx1 and Runx2 proteins. Both factors regulate genes involved in cell cycle control and cell growth (e.g., rRNA genes), as well as lineage commitment. Here, we have examined the subcellular regulatory properties of the third Runx member, the tumor suppressor protein Runx3, during interphase and mitosis. Using in situ cellular and biochemical approaches we delineated a subnuclear targeting signal that directs Runx3 to discrete transcriptional foci that are nuclear matrix associated. Chromatin immunoprecipitation results show that Runx3 occupies rRNA promoters during interphase. We also find that Runx3 remains associated with chromosomes during mitosis and localizes with nucleolar organizing regions (NORs), reflecting an interaction with epigenetic potential. Taken together, our study establishes that common mechanisms control the subnuclear distribution and activities of Runx1, Runx2, and Runx3 proteins to support RNA polymerase I and II mediated gene expression during interphase and mitosis.

E1A physically interacts with RUNX3 and inhibits its transactivation activity

The adenoviral gene, termed early region 1A (E1A), is crucial for transformation and has been used very effectively as a tool to determine the molecular mechanisms that underlie the basis of cellular transformation. pRb, p107, p130, p300/CBP, p400, TRRAP, and CtBP were identified to be E1A-binding proteins and their roles in cellular transformation have been established. Although the major function of E1A is considered to be the regulation of gene expression that is critical for differentiation and cell cycle exit, one of the most significant questions relating to E1A transformation is how E1A mediates this regulation. RUNX3 is a transcription factor that was first described as a gastric cancer tumor suppressor but is now known to be involved in many different cancers. Exogenous expression of RUNX3 strongly inhibits the growth of cells. Here, we show that the adenovirus oncoprotein E1A interacts with RUNX3 in vitro and in vivo. RUNX3 interacts with the N-terminus (amino acids 2-29) of E1A, which is known to interact with p300/CBP, p400, and TRRAP. E1A interacts directly with the Runt domain of RUNX3 but does not interfere with CBFbeta-RUNX3 interactions. In addition, E1A inhibits the transactivation activity of RUNX3 on the p21(WAF1/CIP1) promoter. Consistent with these observations, the growth inhibition induced by RUNX3 is reduced by E1A. These results demonstrate that E1A specifically binds to RUNX3 and inactivates its transactivation activity. We propose that one of the mechanisms for the oncogenic activity of E1A is the inhibition of RUNX3, similar to that of RB and p300/CBP.

RUNX3 attenuates beta-catenin/T cell factors in intestinal tumorigenesis

In intestinal epithelial cells, inactivation of APC, a key regulator of the Wnt pathway, activates beta-catenin to initiate tumorigenesis. However, other alterations may be involved in intestinal tumorigenesis. Here we found that RUNX3, a gastric tumor suppressor, forms a ternary complex with beta-catenin/TCF4 and attenuates Wnt signaling activity. A significant fraction of human sporadic colorectal adenomas and Runx3(+/-) mouse intestinal adenomas showed inactivation of RUNX3 without apparent beta-catenin accumulation, indicating that RUNX3 inactivation independently induces intestinal adenomas. In human colon cancers, RUNX3 is frequently inactivated with concomitant beta-catenin accumulation, suggesting that adenomas induced by inactivation of RUNX3 may progress to malignancy. Taken together, these data demonstrate that RUNX3 functions as a tumor suppressor by attenuating Wnt signaling.

Runx transcription factors in neuronal development

Runt-related (Runx) transcription factors control diverse aspects of embryonic development and are responsible for the pathogenesis of many human diseases. In recent years, the functions of this transcription factor family in the nervous system have just begun to be understood. In dorsal root ganglion neurons, Runx1 and Runx3 play pivotal roles in the development of nociceptive and proprioceptive sensory neurons, respectively. Runx appears to control the transcriptional regulation of neurotrophin receptors, numerous ion channels and neuropeptides. As a consequence, Runx contributes to diverse aspects of the sensory system in higher vertebrates. In this review, we summarize recent progress in determining the role of Runx in neuronal development.

Expression of RUNX3 in salivary adenoid cystic carcinoma: implications for tumor progression and prognosis

Runt-related transcription factor-3 (RUNX3), being a tumor suppressor gene in gastric cancer, plays an important role in inhibiting cellular growth by participating in the transforming growth factor-beta-dependent apoptosis. The aim of this study was to determine the expression of RUNX3 in normal salivary glands and adenoid cystic carcinomas (ACCs), comparing the results with clinicopathological factors and patient survival. The quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis and Western blot analysis revealed the expression of RUNX3 both in normal salivary glands and ACCs. Nuclear and cytoplasmic immunoreactivities against RUNX3 in ductal luminal cells and acinous cells, but immunonegative in myoepithelial cells, were detected in normal salivary glands. In ACC, the RUNX3 immunostaining was shown in the cytoplasm of tumor cells; however, no nuclear location of RUNX3 was found. Lower RUNX3 expression showed significant correlation to distant metastasis and histological growth pattern (P = 0.009 and P = 0.025, respectively). On univariate analysis, low level of RUNX3 immunolabeling (P = 0.012), stage T4 (P = 0.017), lymph node involvement (P = 0.007), and distant metastasis (P < 0.001) were significantly associated with decreased overall survival. Multivariate analysis showed only distant metastasis had an independent prognostic effect on overall survival (P = 0.043). Our results demonstrate the expression of RUNX3 in normal salivary glands and salivary ACCs. The low level of RUNX3 protein in salivary ACCs might play a pivotal role in tumor progression and have prognostic values in ACCs.

Novel roles for MLH3 deficiency and TLE6-like amplification in DNA mismatch repair-deficient gastrointestinal tumorigenesis and progression

DNA mismatch repair suppresses gastrointestinal tumorgenesis. Four mammalian E. coli MutL homologues heterodimerize to form three distinct complexes: MLH1/PMS2, MLH1/MLH3, and MLH1/PMS1. To understand the mechanistic contributions of MLH3 and PMS2 in gastrointestinal tumor suppression, we generated Mlh3^−/−;Apc^1638N and Mlh3^−/−;Pms2^−/−;Apc^1638N (MPA) mice. Mlh3 nullizygosity significantly increased Apc frameshift mutations and tumor multiplicity. Combined Mlh3;Pms2 nullizygosity further increased Apc base-substitution mutations. The spectrum of MPA tumor mutations was distinct from that observed in Mlh1^−/−;Apc^1638N mice, implicating the first potential role for MLH1/PMS1 in tumor suppression. Because Mlh3;Pms2 deficiency also increased gastrointestinal tumor progression, we used array-CGH to identify a recurrent tumor amplicon. This amplicon contained a previously uncharacterized Transducin enhancer of Split (Tle) family gene, Tle6-like. Expression of Tle6-like, or the similar human TLE6D splice isoform in colon cancer cells increased cell proliferation, colony-formation, cell migration, and xenograft tumorgenicity. Tle6-like;TLE6D directly interact with the gastrointestinal tumor suppressor RUNX3 and antagonize RUNX3 target transactivation. TLE6D is recurrently overexpressed in human colorectal cancers and TLE6D expression correlates with RUNX3 expression. Collectively, these findings provide important insights into the molecular mechanisms of individual MutL homologue tumor suppression and demonstrate an association between TLE mediated antagonism of RUNX3 and accelerated human colorectal cancer progression.

Approximately one million people every year are diagnosed with colorectal cancer worldwide, and about five hundred thousand of these people subsequently perish from the disease. Colorectal cancer is thought to develop through a series of early and later stages (called cancer initiation and progression, respectively). Deaths from colorectal cancer are particularly tragic because the disease can usually be cured if discovered before full-blown progression. However, our knowledge of how these tumors progress remains very limited. DNA mismatch repair is known to be an important process in preventing ∼15% of colorectal cancer initiation. In this study we describe how two of these genes (Mlh3 and Pms2) that have partial functional redundancy and therefore individually are rarely mutated are also important in preventing colorectal cancer progression. Additionally, we describe a new gene (Tle6-like) that, when overactive, makes these cancers progress more rapidly. The overall goal of this study is to understand colorectal cancer progression better so that we can come up with new ways to block it at the later stage.

Enhancer of zeste homologue 2 (EZH2) down-regulates RUNX3 by increasing histone H3 methylation

Overexpression of enhancer of zeste homologue 2 (EZH2) occurs in various malignancies and is associated with a poor prognosis, especially because of increased cancer cell proliferation. In this study we found an inverse correlation between EZH2 and RUNX3 gene expression in five cancer cell lines, i.e. gastric, breast, prostate, colon, and pancreatic cancer cell lines. Chromatin immunoprecipitation assay showed an association between EZH2 bound to the RUNX3 gene promoter, and trimethylated histone H3 at lysine 27, and HDAC1 (histone deacetylase 1) bound to the RUNX3 gene promoter in cancer cells. RNA interference-mediated knockdown of EZH2 resulted in a decrease in H3K27 trimethylation and unbound HDAC1 and an increase in expression of the RUNX3 gene. Restoration of RUNX3 expression was not associated with any change in DNA methylation status in the RUNX3 promoter region. RUNX3 was repressed by histone deacetylation and hypermethylation of a CpG island in the promoter region and restored by trichostatin A or/and 5-aza-2'-deoxycytidine. Immunofluorescence staining confirmed restoration of expression of the RUNX3 protein after knockdown of EZH2 and its restoration resulted in decreased cell proliferation. In vivo, an inverse relationship between expression of the EZH2 and RUNX3 proteins was observed at the individual cell level in gastric cancer patients in the absence of DNA methylation in the RUNX3 promoter region. The results showed that RUNX3 is a target for repression by EZH2 and indicated an underlying mechanism of the functional role of EZH2 overexpression on cancer cell proliferation.

E2F1-regulated microRNAs impair TGFbeta-dependent cell-cycle arrest and apoptosis in gastric cancer

Deregulation of E2F1 activity and resistance to TGFbeta are hallmarks of gastric cancer. MicroRNAs (miRNAs) are small noncoding RNAs frequently misregulated in human malignancies. Here we provide evidence that the miR-106b-25 cluster, upregulated in a subset of human gastric tumors, is activated by E2F1 in parallel with its host gene, Mcm7. In turn, miR-106b and miR-93 regulate E2F1 expression, establishing a miRNA-directed negative feedback loop. Furthermore, upregulation of these miRNAs impairs the TGFbeta tumor suppressor pathway, interfering with the expression of CDKN1A (p21(Waf1/Cip1)) and BCL2L11 (Bim). Together, these results suggest that the miR-106b-25 cluster is involved in E2F1 posttranscriptional regulation and may play a key role in the development of TGFbeta resistance in gastric cancer.

Runx2-mediated activation of the Bax gene increases osteosarcoma cell sensitivity to apoptosis

The Runx family of transcription factors regulate cell growth and differentiation, and control the expression of target genes involved in cell fate decisions. We examined the role of the bone-related member of this family, Runx2, in regulating apoptosis via modulation of the Bcl2 family of genes in the osteosarcoma cell line Saos2. Our data demonstrate that Runx2 directly binds to two Runx-specific regulatory elements on the human bax promoter thereby inducing Bax expression. Furthermore, bone morphogenetic protein-induced or vector-mediated expression of Runx2 resulted in upregulation of Bax expression, and subsequent increased sensitivity of Saos2 cells to apoptosis. Finally, the observed upregulation of Bax expression and increased apoptosis were Runx2 dependent as Runx2 loss of function abrogated these effects. Our study provides the first evidence for Bax as a direct target of Runx2, suggesting that Runx2 may act as a proapoptotic factor in osteosarcoma cells.

TGF-beta signaling: a tale of two responses

Transforming growth factor-beta (TGF-beta) regulates a wide variety of cellular processes including cell growth, apoptosis, differentiation, migration, and extracellular matrix production among others. The canonical signaling pathway induced by the TGF-beta receptor complex involves the phosphorylation of Smad proteins which upon activation accumulate in the nucleus and regulate transcription. Interestingly, the cellular response to TGF-beta can be extremely variable depending on the cell type and stimulation context. TGF-beta causes epithelial cells to undergo growth arrest and apoptosis, responses which are critical to suppressing carcinogenesis, whereas it can also induce epithelial-mesenchymal transition and mediate fibroblast activation, responses implicated in promoting carcinogenesis and fibrotic diseases. However, TGF-beta induces all these responses via the same receptor complex and Smad proteins. To address this apparent paradox, during the last few years a number of additional signaling pathways have been identified which potentially regulate the different cellular responses to TGF-beta. The identification of these signaling pathways has shed light onto the mechanisms whereby Smad and non-Smad pathways collaborate to induce a particular cellular phenotype. In this article, we review TGF-beta signaling in epithelial cells and fibroblasts with a focus on understanding the mechanisms of TGF-beta versatility.

Genetically engineered mice: a new paradigm to study gastric physiology

PURPOSE OF REVIEW

To summarize key aspects from recent research as well as review articles on the topic of genetic mouse models, particularly in knockout mice, that have considerably contributed to understanding the pathways and mechanisms underlying gastric physiology.

RECENT FINDINGS

A series of knockout mouse models has proven to be invaluable in elucidating the mechanism and validating the current model of acid secretion. The interaction between the gastrin-histamine and cholecystokinin-somatostatin pathways was identified using the genetic approach as being critical in regulating acid secretion. Curiously, neither ghrelin nor ghrelin receptor knockout mice displayed the expected lean phenotype. Importantly, the study of obestatin in GPR39 knockout mice could be misleading, as zinc rather than obestatin is the endogenous ligand for GPR39. The physiological roles of ghrelin and obestatin have yet to be confirmed using knockout mouse models.

SUMMARY

The knockout mouse continues to serve as an excellent model to dissect the complexity of the mechanism of gastric acid secretion and to study the physiological importance of gastric ghrelin.

Runx1 protects hematopoietic stem/progenitor cells from oncogenic insult

The RUNX1/AML1 gene encodes a transcription factor essential for the generation of hematopoietic stem cells and is frequently targeted in human leukemia. In human RUNX1-related leukemias, the RAS pathway is often concurrently mutated, but the mechanism of the synergism remains elusive. Here, we found that inactivation of Runx1 in mouse bone marrow cells results in an increase in the stem/progenitor cell fraction due to suppression of apoptosis and elevated expression of the polycomb gene Bmi-1, which is important for stem cell self-renewal. Introduction of oncogenic N-RAS into wild-type cells, in contrast, reduced the stem/progenitor cell fraction because of senescence, apoptosis, and differentiation. Such detrimental events presumably occurred because of the cellular fail-safe program, although hyperproliferation was initially induced by an oncogenic stimulus. Runx1 insufficiency appears to impair such a fail-safe mechanism, particularly in the stem/progenitor cells, thereby supporting the clonal maintenance of leukemia-initiating cells expressing an activated oncogene. Disclosure of potential conflicts of interest is found at the end of this article.

Repression of interleukin-4 in T helper type 1 cells by Runx/Cbf beta binding to the Il4 silencer

Interferon γ (IFNγ) is the hallmark cytokine produced by T helper type 1 (Th1) cells, whereas interleukin (IL)-4 is the hallmark cytokine produced by Th2 cells. Although previous studies have revealed the roles of cytokine signaling and of transcription factors during differentiation of Th1 or Th2 cells, it is unclear how the exclusive expression pattern of each hallmark cytokine is established. The DNaseI hypersensitivity site IV within the mouse Il4 locus plays an important role in the repression of Il4 expression in Th1 cells, and it has been named the Il4 silencer. Using Cbfβ- or Runx3-deficient T cells, we show that loss of Runx complex function results in derepression of IL-4 in Th1 cells. Binding of Runx complexes to the Il4 silencer was detected in naive CD4⁺ T cells and Th1 cells, but not in Th2 cells. Furthermore, enforced expression of GATA-3 in Th1 cells inhibited binding of Runx complexes to the Il4 silencer. Interestingly, T cell–specific inactivation of the Cbfβ gene in mice led to elevated serum immunoglobulin E and airway infiltration. These results demonstrate critical roles of Runx complexes in regulating immune responses, at least in part, through the repression of the Il4 gene.

Contribution of reactivated RUNX3 to inhibition of gastric cancer cell growth following suberoylanilide hydroxamic acid (vorinostat) treatment

Vorinostat (suberoylanilide hydroxamic acid, SAHA) represents a new class of highly potent histone deacetylase (HDAC) inhibitors that cause growth arrest, differentiation, and apoptosis of many tumor types in vitro and in vivo. RUNX3, a gastric tumor suppressor, is epigenetically silenced in gastric cancer cells. This study investigates the role of RUNX3 in vorinostat-induced suppression of gastric cancer cell growth. RUNX3 was up-regulated by vorinostat in gastric cancer cell lines not expressing RUNX3. In terms of cell viability, the mean IC(50) of vorinostat in RUNX3-negative cells was significantly lower than that seen in RUNX3-positive cells, indicating that the former are more sensitive to vorinostat in terms of growth arrest than are RUNX3-positive lines. The mechanism underlying this difference was found to be reactivation of RUNX3 expression by vorinostat and concomitant increase in acetylated histone H3 in the promoter region of RUNX3. Using three RUNX3-negative cell lines, we determined the contribution of RUNX3 reactivation to growth inhibition and induction of apoptosis following treatment of cells with vorinostat and found that up-regulated RUNX3 was significantly responsible for tumor suppressive activities.

Frequent silencing of RUNX3 in esophageal squamous cell carcinomas is associated with radioresistance and poor prognosis

Radiotherapy is an effective treatment for some esophageal cancers, but the molecular mechanisms of radiosensitivity remain unknown. RUNX3, a novel tumor suppressor of gastric cancer, functions in transforming growth factor (TGF)-beta-dependent apoptosis. We obtained paired samples from 62 patients with advanced esophageal cancers diagnosed initially as T3 or T4 with image diagnosis; one sample was obtained from a biopsy before presurgical radiotherapy, and the other was resected in surgical specimens after radiotherapy. RUNX3 was repressed in 67.7% cases of the pretreatment biopsy samples and 96.7% cases of the irradiated, resected samples. The nuclear expression of RUNX3 was associated with radiosensitivity and a better prognosis than cytoplasmic or no RUNX3 expression (P<0.003); cytoplasmic RUNX3 expression was strictly associated with radioresistance. RUNX3 was downregulated and its promoter was hypermethylated in all radioresistant esophageal cancer cell lines examined. Stable transfection of esophageal cancer cells with RUNX3 slightly inhibited cell proliferation in vitro, enhanced the antiproliferative and apoptotic effects of TGF-beta and increased radiosensitivity in conjunction with Bim induction. In contrast, transfection of RUNX3-expressing cells with a RUNX3 antisense construct or a Bim-specific small interfering RNA induced radioresistance. Treatment with 5-aza-2'-deoxycytidine restored RUNX3 expression, increased radiosensitivity and induced Bim in both control and radioresistant cells. These results suggest that RUNX3 silencing promotes radioresistance in esophageal cancers. Examination of RUNX3 expression in pretreatment specimens may predict radiosensitivity, and induction of RUNX3 expression may increase tumor radiosensitivity.