Masamitsu Tanaka

Tumor metastasis suppressor nm23H1 regulates Rac1 GTPase by interaction with Tiam1

The putative tumor metastasis suppressor nm23H1 was originally identified in murine melanomas by subtraction cloning. It displays nucleoside diphosphate kinase activity and regulates cellular events, including growth and development. Recently nm23H1 has been reported to also act as a GTPase-activating protein of the Ras-related GTPase Rad. We attempted to determine whether nm23H1 also regulates Rho-family GTPases. Although we were unable to detect a direct association between nm23H1 and Rho-family GTPases, nm23H1 was shown to be associated with a Rac1-specific nucleotide exchange factor, Tiam1, by interaction with its amino-terminal region in extracts from the cells expressing exogenous Tiam1 and from native tissue. Overexpression of nm23H1 inhibited the Tiam1-induced production of GTP-bound Rac1 and activation of c-Jun kinase. On the other hand, forced overexpression of the wild type, but not the kinase-inactivated mutant of nm23H1, converted the GDP-bound forms of Rac1, Cdc42, and RhoA to their GTP-bound forms in vitro by its nucleoside diphosphate kinase activity, but nm23H1 alone apparently did not produce the GTP-bound form of these GTPases in vivo. These results suggest that nm23H1 negatively regulates Tiam1 and inhibits Rac1 activation in vivo. Moreover, adhesion-stimulated membrane ruffles of Rat1 fibroblasts were reduced by overexpression of nm23H1. Based on these observations, we concluded that we had identified a function of nm23H1 as a regulator of Rac1 and that it may be related to the effect of nm23H1 as a tumor metastasis suppressor.

EphA2 Phosphorylates the Cytoplasmic Tail of Claudin-4 and Mediates Paracellular Permeability

Eph receptors and ephrin ligands are widely expressed in epithelial cells and mediate cell-cell interaction. EphA2 is expressed in various cancer tissues and cell lines. Although the mechanism of action of EphA2 is unknown, its expression correlates with progression of the malignant phenotype of cancerous tissues. Here, we have shown that EphA2 modulates the localization and function of claudin-4, a constituent of tight junctions. EphA2 associates with claudin-4 via their extracellular domains. This association, in turn, leads to phosphorylation of the cytoplasmic carboxyl terminus of claudin-4 at Tyr-208. The tyrosine phosphorylation of claudin-4 attenuates association of claudin-4 with ZO-1, decreasing integration of claudin-4 into sites of cell-cell contact and enhancing paracellular permeability. These results indicate that EphA2 moderates the function of tight junctions via phosphorylation of claudin-4.

alphaPIX nucleotide exchange factor is activated by interaction with phosphatidylinositol 3-kinase.

p21-activated kinase (PAK) is a common effector protein of the small GTPases Cdc42 and Rac, leading to the activation of downstream mitogen activated protein kinases. PAK also mediates polarized cytoskeletal changes induced by these GTPases. The recently identified PAK-interacting exchange factor (PIX) acts as a guanine nucleotide exchange factor on Rac, and colocalizes with PAK in a focal complex, but little is known about the associated signaling cascades, including upstream activators of PIX. In this study, we show that one of the isoforms of PIX, αPIX, is activated by signaling cascades from the platelet-derived growth factor (PDGF) receptor and EphB2 receptor, and from integrin-induced signaling through phosphatidylinositol 3-kinase (PI3-kinase). αPIX is activated by forming a complex with these receptors either via association with PAK and Nck, or direct association with the p85 regulatory subunit of PI3-kinase. Synthetic phosphoinositide and membrane targeted PI3-kinase augmented the αPIX activity in vivo. In Xenopus, aggregates of mesodermal cells derived from embryos microinjected with αPIX significantly increased the peripheral spreading on fibronectin substrate in response to PDGF through PI3-kinase. These results indicate that αPIX is activated by PI3-kinase, and is involved in the receptor mediated signaling leading to the activation of the kinase activity of PAK, and the migration of mesodermal cells on extracellular matrix.

Association of Dishevelled with Eph tyrosine kinase receptor and ephrin mediates cell repulsion

Eph tyrosine kinase receptors and their membrane‐bound ligands, ephrins, are presumed to regulate cell–cell interactions. The major consequence of bidirectional activation of Eph receptors and ephrin ligands is cell repulsion. In this study, we discovered that Xenopus Dishevelled (Xdsh) forms a complex with Eph receptors and ephrin‐B ligands and mediates the cell repulsion induced by Eph and ephrin. In vitro re‐aggregation assays with Xenopus animal cap explants revealed that co‐expression of a dominant‐negative mutant of Xdsh affected the sorting of cells expressing EphB2 and those expressing ephrin‐B1. Co‐expression of Xdsh induced the activation of RhoA and Rho kinase in the EphB2‐overexpressed cells and in the cells expressing EphB2‐stimulated ephrin‐B1. Therefore, Xdsh mediates both forward and reverse signaling of EphB2 and ephrin‐B1, leading to the activation of RhoA and its effector protein Rho kinase. The inhibition of RhoA activity in animal caps significantly prevents the EphB2‐ and ephrin‐B1‐mediated cell sorting. We propose that Xdsh, which is expressed in various tissues, is involved in EphB and ephrin‐B signaling related to regulation of cell repulsion via modification of RhoA activity.

EPHA2/EFNA1 expression in human gastric cancer

The erythropoietin‐producing hepatocellular (EPH)A2 receptor, tyrosine kinase, is overexpressed and phosphorylated in several types of human tumors and has been associated with malignant transformation. A recent report, however, indicated that stimulation of the EPHA2 receptor ligand, ephrinA1 (EFNA1), inhibits the growth of EPHA2‐expressing breast cancer. The authors examined the expression of EPHA2 and EFNA1 using semiquantitative reverse transcription‐polymerase chain reaction (RT‐PCR) in four gastric cancer cell lines and 49 primary gastric cancer samples, as well as in normal gastric tissue. EPHA2 was more highly expressed in tumor tissue than in normal tissue in 27 cases (55%). EFNA1 was overexpressed in tumor tissue in 28 cases (57%). No significant correlation was detected between the expression levels and histologic features such as tumor size, age, vessel invasion, or lymph node involvement. However, EPHA2 overexpression was more prominent in macroscopic type 3 and 4 tumors than in type 1 or 2 advanced gastric cancer. The authors observed EPHA2 expression in three of the four gastric cancer cell lines (AGS, KATO3, and MKN74) that were examined. In one cell line, TMK1, EPHA2 expression was barely detectable using northern blotting, RT‐PCR, and western blotting. In contrast, EFNA1 was detected in all cell lines. In the gastric cancer cell lines that endogenously expressed EPHA2, stimulation with ephrinA1‐Fc led to decreased EPHA2 protein expression and increased EPHA2 phosphorylation. Finally, the growth of EPHA2‐expressing cells was inhibited by repetitive stimulation with soluble ephrinA1‐Fc. Taken together, these findings suggest that EPHA2 and EFNA1 expression may influence the behavior of human gastric cancer. (Cancer Sci 2005; 96: 42–48)

Tiam1 mediates neurite outgrowth induced by ephrin‐B1 and EphA2

Bidirectional signals mediated by Eph receptor tyrosine kinases and their membrane‐bound ligands, ephrins, play pivotal roles in the formation of neural networks by induction of both collapse and elongation of neurites. However, the downstream molecular modules to deliver these cues are largely unknown. We report here that the interaction of a Rac1‐specific guanine nucleotide‐exchanging factor, Tiam1, with ephrin‐B1 and EphA2 mediates neurite outgrowth. In cells coexpressing Tiam1 and ephrin‐B1, Rac1 is activated by the extracellular stimulation of clustered soluble EphB2 receptors. Similarly, soluble ephrin‐A1 activates Rac1 in cells coexpressing Tiam1 and EphA2. Cortical neurons from the E14 mouse embryos and neuroblastoma cells significantly extend neurites when placed on surfaces coated with the extracellular domain of EphB2 or ephrin‐A1, which were abolished by the forced expression of the dominant‐negative mutant of ephrin‐B1 or EphA2. Furthermore, the introduction of a dominant‐negative form of Tiam1 also inhibits neurite outgrowth induced by the ephrin‐B1 and EphA2 signals. These results indicate that Tiam1 is required for neurite outgrowth induced by both ephrin‐B1‐mediated reverse signaling and EphA2‐mediated forward signaling.

Correlation of EPHA2 overexpression with high microvessel count in human primary colorectal cancer

Evidence suggests that the erythropoietin‐producing hepatocellular (EPH) receptor tyrosine kinases (RTKs) and their ephrin (EFN) ligands are involved in human carcinogenesis. Expression of two of them, EFNA1 ligand and its receptor, EPHA2, has been proposed to contribute to tumor‐induced neovascularization. Colorectal cancers were examined for expressions of EPHA2 and its ligand EFNA1 by semi‐quantitative RT‐PCR, and double‐immunostained for EPHA2 and CD34. Microvessels in the tumors were counted. Double‐staining was also performed in 25 cases of adenoma with focal cancer for comparison. Trends of overexpression of both EPHA2 and EFNA1 was found in tumor tissue compared to the corresponding normal tissue in the same specimen [22/37 (59.5%) and 25/37 (67.5%), respectively; P=0.100 for EPHA2 and P=0.009 for EFNA1]. Overexpression of EPHA2 and EFNA1 was noted more frequently in the early stage than in the late stage [EPHA2, 15/21 (71.4%) vs. 7/16 (43.8%), P=0.007; EFNA1, 15/21 (71.4%) vs. 10/16 (62.5%), P=0.007]. Both EPHA2 and EFNA1 were more frequently overexpressed in smaller tumors (less than 5 cm) than in larger tumors [EPHA2, 15/21 (71.4%) vs. 7/16 (43.8%), P=0.017; EFNA1, 16/21 (76.2%) vs. 8/16 (50%), P=0.001]. Tumors less than 5 cm in diameter and in stages I and II were significantly more likely to overexpress EPHA2 and EFNA1 (P=0.001 for EPHA2, P=0.001 for EFNA1). Microvessel counts (MVCs) after immuno‐staining for CD34 were significantly correlated (r=0.343, P=0.037) with overexpression of EPHA2. EPHA2‐expressing focal cancer also surrounded microvessels in adenomas with focal cancers. These findings suggest an involvement of EPHA2 in colon carcinogenesis, mainly in stages I and II, and probably through their effect on microvessel induction.

Phosphorylation of ephrin‐B1 via the interaction with claudin following cell–cell contact formation

The interaction of the Eph family of receptor protein tyrosine kinase and its ligand ephrin family induces bidirectional signaling via the cell–cell contacts. Although most previous studies have focused on the function of Eph–ephrin pathways in the neural system and endothelial cells, this process also occurs in epithelial and cancer cells, of which the biological involvement is poorly understood. We show that ephrin‐B1 creates an in vivo complex with adjacent claudin1 or claudin4 via the extracellular domains of these proteins. The cytoplasmic domain of ephrin‐B1 was phosphorylated on tyrosine residues upon the formation of cell–cell contacts, possibly recognizing an intercellular adhesion of claudins. Phosphorylation of ephrin‐B1 induced by claudins was abolished by the treatment with 4‐amino‐5‐(4‐chlorophenyl)‐7–(t‐butyl)pyrazolo[3,4‐d]pyrimidine, an inhibitor of the Src family kinases. Moreover, overexpression of ephrin‐B1 triggered consequent change in the level of cell–cell adhesion depending on its phosphorylation. These results suggest that ephrin‐B1 mediated the cell–cell adhesion of epithelial and cancer cells via a novel Eph receptor‐independent mechanism.

High prevalence of p53 protein overexpression in patients with esophageal cancer in Linxian, China and its relationship to progression and prognosis

Background. Linxian is the highest endemic area of esophageal squamous cell carcinoma (ESCC) in China and one of the highest incidence areas in the world. The relationship of p53 protein accumulation to geographic variation, pathologic findings, and prognosis has not been investigated extensively.

Downregulation of EphA7 by hypermethylation in colorectal cancer

A significant reduction of EphA7 expression in human colorectal cancers was shown using semiquantitative reverse transcription–polymerase chain reaction analysis in 59 colorectal cancer tissues, compared to corresponding normal mucosas (P=0.008), and five colon cancer cell lines. To investigate the mechanism of EphA7 downregulation in colorectal cancer, we examined the methylation status of the 5′CpG island around the translation start site in five colon cancer cell lines using restriction enzymes, methylation-specific PCR, and bisulfite sequencing and found evidence of aberrant methylation. The expression of EphA7 in colon cancer cell lines was restored after treatment with 5-aza-2′-deoxycytidine. Analysis of methylation status in totally 75 tumors compared to clinicopathological parameters revealed that hypermethylation of colorectal cancers was more frequent in male than in female (P=0.0078), and in moderately differentiated than in well-differentiated adenocarcinomas (P=0.0361). There was a tendency that hypermethylation in rectal cancers was more frequent than in colon cancers (P=0.0816). Hypermethylation was also observed in colorectal adenomas. This is the first report describing the downregulation of an Eph family gene in a solid tumor via aberrant 5′CpG island methylation. It provides the evidence that EphA7 gene is involved in human colorectal carcinogenesis.