A splicing variant of the RON transcript induces constitutive tyrosine kinase activity and an invasive phenotype

Identification of prognostic signatures in breast cancer microarray data using Bayesian techniques

We apply a new Bayesian data analysis technique (latent process decomposition) to four recent microarray datasets for breast cancer. Compared to hierarchical cluster analysis, for example, this technique has advantages such as objective assessment of the optimal number of sample or gene clusters in the data, penalization of overcomplex models fitting to noise in the data and a common latent space of explanatory variables for samples and genes. Our analysis provides a clearer insight into these datasets, enabling assignment of patients to one of four principal processes, each with a distinct clinical outcome. One process is indolent and associated with under-expression across a number of genes associated with tumour growth. One process is associated with over expression of GRB7 and ERBB2. The most aggressive process is associated with abnormal expression of transcription factor genes, including members of the FOX family of transcription factor genes.

Oncogenesis of RON receptor tyrosine kinase: a molecular target for malignant epithelial cancers

Recepteur d'origine nantais (RON) belongs to a subfamily of receptor tyrosine kinases (RTK) with unique expression patterns and biological activities. RON is activated by a serum-derived growth factor macrophage stimulating protein (MSP). The RON gene transcription is essential for embryonic development and critical in regulating certain physiological processes. Recent studies have indicated that altered RON expression contributes significantly to cancer progression and malignancy. In primary tumors, such as colon and breast cancers, overexpression of RON exists in large numbers and is often accompanied by the generation of different splicing variants. These RON variants direct a unique program that controls cell transformation, growth, migration, and invasion, indicating that altered RON expression has the ability to regulate motile/invasive phenotypes. These activities were also seen in transgenic mice, in which targeted expression of RON in lung epithelial cells resulted in numerous tumors with pathological features of human bronchioloalveolar carcinoma. Thus, abnormal RON activation is a pathogenic factor that transduces oncogenic signals leading to uncontrolled cell growth and subsequent malignant transformation. Considering these facts, RON and its variants can be considered as potential targets for therapeutic intervention. Experiments using small interfering RNA and neutralizing monoclonal antibodies demonstrated that suppressing RON expression and activation decreases cancer cell proliferation, increases apoptotic death, prevents tumor formation in nude mice, and reduces malignant phenotypes. Thus, blocking RON expression and activation has clinical significance in reversing malignant phenotypes and controlling tumor growth.

Cell motility is controlled by SF2/ASF through alternative splicing of the Ron protooncogene

Ron, the tyrosine kinase receptor for the Macrophage-stimulating protein, is involved in cell dissociation, motility, and matrix invasion. DeltaRon, a constitutively active isoform that confers increased motility to expressing cells, is generated through the skipping of exon 11. We show that abnormal accumulation of DeltaRon mRNA occurs in breast and colon tumors. Skipping of exon 11 is controlled by a silencer and an enhancer of splicing located in the constitutive exon 12. The strength of the enhancer parallels the relative abundance of DeltaRon mRNA and depends on a sequence directly bound by splicing factor SF2/ASF. Overexpression and RNAi experiments demonstrate that SF2/ASF, by controlling the production of DeltaRon, activates epithelial to mesenchymal transition leading to cell locomotion. The effect of SF2/ASF overexpression is reverted by specific knockdown of DeltaRon mRNA. This demonstrates a direct link between SF2/ASF-regulated splicing and cell motility, an activity important for embryogenesis, tissue formation, and tumor metastasis.

Co-expression of RON and MET is a prognostic indicator for patients with transitional-cell carcinoma of the bladder

Recepteur d'Origine Nantais (RON) is a distinct receptor tyrosine kinase in the c-met proto-oncogene family. We examined the mutational and expression patterns of RON in eight human uroepithelial cell lines. Biological effects of RON overexpression on cancer cells were investigated in vitro, and the prognostic significance of RON and/or c-met protein (MET) expression was analysed in a bladder cancer cohort (n=183). There was no evidence of mutation in the kinase domain of RON. Overexpression of RON using an inducible Tet-off system induced increased cell proliferation, motility, and antiapoptosis. Immunohistochemical analysis showed that RON was overexpressed in 60 cases (32.8%) of primary tumours, with 14 (23.3%) showing a high level of expression. Recepteur d'Origine Nantais expression was positively associated with histological grading, larger size, nonpapillary contour, and tumour stage (all P<0.01). In addition, MET was overexpressed in 82 cases (44.8%). Co-expressed RON and MET was significantly associated with decreased overall survival (P=0.005) or metastasis-free survival (P=0.01) in 35 cases (19.1%). Recepteur d'Origine Nantais-associated signalling may play an important role in the progression of human bladder cancer. Evaluation of RON and MET expression status may identify a subset of bladder-cancer patients who require more intensive treatment.

Ron tyrosine kinase receptor regulates papilloma growth and malignant conversion in a murine model of skin carcinogenesis

Recent studies demonstrate that the receptor tyrosine kinase (TK) Ron is tumorigenic when overexpressed and plays a role in regulating skin homeostasis. We hypothesized that Ron signaling promotes skin carcinogenesis. To test this hypothesis, mice deficient in the TK domain of Ron (TK(-/-) mice) were crossed with v-Ha-ras (Tg.AC) transgenic mice; the resulting TK(-/-) Tg.AC(+/-) mice, and their controls, were utilized in a model of chemically induced Ras-mediated skin carcinogenesis. The mice were treated with 2.5 microg of 12-O-tetradecanoylphorbol-13-acetate applied weekly to the shaved back of 36 control (TK(+/+) Tg.AC(+/-)) and 35 experimental (TK(-/-) Tg.AC(+/-)) mice. In an analysis of the resulting papillomas, a reduction in cellular proliferation and papilloma volume was found in the TK(-/-) Tg.AC(+/-) mice compared to controls. Further, Ron protein expression was upregulated during papilloma formation. Ablation of Ron signaling resulted in partial defects in MAPK and Akt signaling that may account for the decreased papilloma growth in the TK(-/-) Tg.AC(+/-) mice. The papilloma-bearing mice were monitored for the occurrence of malignant skin tumors and other malignant tumor types for a period of 48 weeks. Loss of Ron receptor signaling significantly reduced the percent of papillomas that underwent malignant conversion as well as the number of mice developing other malignant tumor types. In conclusion, these studies demonstrate that Ron signaling augments papilloma growth and malignant conversion in vivo.

Alterations of pre-mRNA splicing in cancer

Recent genomewide analyses of alternative splicing (AS) indicate that up to 70% of human genes may have alternative splice forms, suggesting that AS together with various posttranslational modifications plays a major role in the production of proteome complexity. Splice-site selection under normal physiological conditions is regulated in the developmental stage in a tissue type-specific manner by changing the concentrations and the activity of splicing regulatory proteins. Whereas spliceosomal errors resulting in the production of aberrant transcripts rarely occur in normal cells, they seem to be an intrinsic property of cancer cells. Changes in splice-site selection have been observed in various types of cancer and may affect genes implicated in tumor progression (for example, CD44, MDM2, and FHIT) and in susceptibility to cancer (for example, BRCA1 and APC). Splicing defects can arise from inherited or somatic mutations in cis-acting regulatory elements (splice donor, acceptor and branch sites, and exonic and intronic splicing enhancers and silencers) or variations in the composition, concentration, localization, and activity of regulatory proteins. This may lead to altered efficiency of splice-site recognition, resulting in overexpression or down-regulation of certain splice variants, a switch in splice-site usage, or failure to recognize splice sites correctly, resulting in cancer-specific splice forms. At least in some cases, changes in splicing have been shown to play a functionally significant role in tumorigenesis, either by inactivating tumor suppressors or by gain of function of proteins promoting tumor development. Moreover, cancer-specific splicing events may generate novel epitopes that can be recognized by the host's immune system as cancer specific and may serve as targets for immunotherapy. Thus, the identification of cancer-specific splice forms provides a novel source for the discovery of diagnostic or prognostic biomarkers and tumor antigens suitable as targets for therapeutic intervention.

RNA-mediated gene silencing of the RON receptor tyrosine kinase alters oncogenic phenotypes of human colorectal carcinoma cells

Altered expression of receptor tyrosine kinases contributes to tumorigenic behaviors of epithelial cancers. In this study, the pathogenic roles of receptor tyrosine kinase RON (recepteur d'origine nantais) in regulating oncogenic phenotypes in colorectal epithelial cells were studied. Increased expression of RON and its variants resulted in colony formation and motile activities of colonic epithelial AA/C1 cells as evident in soft-agar and migration assays, respectively. These results suggest that overexpression of wild-type RON mediates the transformed phenotypes in immortalized colon epithelial cells. In colorectal cancer cells (HT-29, HCT116, and SW620) that naturally express RON, the RON gene expression was silenced by RNA interference. The introduction of RON-specific small interfering (si) RNA significantly affected cancer cell proliferation, motility, and led to increased apoptotic cell death. Focus-forming activities and anchorage-independent growth of colon cancer cells were also dramatically reduced. Moreover, it was demonstrated in tumor growth assays that silencing RON gene expression significantly reduces tumorigenic activities of SW620 cells in vivo. By analysing signaling proteins involved in colon carcinogenesis, we found that the effect of RON-specific siRNA is associated with diminished expression of beta-catenin, a critical component in the Wnt signaling pathway. Taken together, our results demonstrate that altered expression of RON in colon cancer cells is required to maintain tumorigenic phenotypes. Thus, silencing RON gene expression could have potential to reverse malignant activities of colon tumors in vivo.

A Novel Protein Tyrosine Kinase NOK that Shares Homology with Platelet- Derived Growth Factor/Fibroblast Growth Factor Receptors Induces Tumorigenesis and Metastasis in Nude Mice

Receptor protein tyrosine kinases (RPTKs) play important roles in the regulation of a variety of cellular processes including cell migration, proliferation, and protection from apoptosis. Here, we report the identification and characterization of a novel RPTK-like molecule that has a critical role in induction of tumorigenesis and metastasis and is termed Novel Oncogene with Kinase-domain (NOK). NOK contains a putative single transmembrane domain and a conserved intracellular tyrosine kinase domain that shares homology with members of the platelet-derived growth factor/fibroblast growth factor receptor superfamily. NOK was exclusively located in the cytoplasm. NOK mRNAs were detected in limited human organs and expressed with the highest abundance in the prostate. A variety of tumor cells also expressed the NOK mRNAs. We demonstrated that NIH3T3 and BaF3 cells could be strongly transformed by the expression of the NOK gene as examined by colony formation experiment. In addition, BaF3 cells with the stable expression of NOK induced rapid tumorigenesis in nude mice. Interestingly, these NOK-expressing tumor cells could promptly invade and spread into various distinct organs and form metastatic foci, eventually leading to the rapid death of these animals. Moreover, molecular mechanism studies indicated that NOK could concomitantly activate both MAP kinase and phosphatidylinositol 3'-kinases (PI3K) pathways in stable BaF3 cells. Thus, our results both in vitro and in vivo suggest that NOK is a novel oncogene with the capacity of promoting cell transformation, tumorigenesis, and metastasis.

Oncogenic and invasive potentials of human macrophage-stimulating protein receptor, the RON receptor tyrosine kinase

The product of the RON (recepteur d'origine nantais) gene belongs to the MET proto-oncogene family, a distinct subfamily of receptor tyrosine kinases. The ligand of RON was identified as macrophage-stimulating protein (MSP), a member of the plasminogen-related growth factor family. RON is mainly expressed in cells of epithelial origin and is required for embryonic development. In vitro RON activation results in epithelial cell dissociation, migration and matrix invasion, suggesting that RON might be involved in the pathogenesis of certain epithelial cancers in vivo. Indeed, recent studies have shown that RON expression is significantly altered in several primary human cancers, including those of the breast and colon. Truncation of the RON protein has also been found in primary tumors from the gastrointestinal tract. These alterations lead to constitutive activation of RON that causes cell transformation in vitro, induces neoplasm formation in athymic nude mice, and promotes tumor metastasis into the lung. Studies employing transgenic models further demonstrated that over-expression of RON in lung epithelial cells results in multiple tumor formation with features of large cell undifferentiated carcinoma. The oncogenic activities of RON are mediated by RON-transduced signals that promote unbalanced cell growth and transformation leading to tumor development. Thus, abnormal accumulation and activation of RON could play a critical role in vivo in the progression of certain malignant human epithelial cancers.

The Ron proto-oncogene product is a phenotypic marker of renal oncocytoma

The proto-oncogene product Ron is the receptor for macrophage stimulating protein, a scatter factor that stimulates cell proliferation, prevents apoptosis, and induces an invasive cell phenotype. We investigated the expression of Ron, Ki-67 (proliferation index), p53, and bcl-2 (proapoptotic and antiapoptotic proteins, respectively) in 50 renal tumors (19 clear cell carcinomas, 18 oncocytomas, 7 papillary cell carcinomas, 5 chromophobe cell carcinomas, and 1 carcinoma with sarcomatoid areas). In addition, we studied Ron in normal kidney and in the renal carcinoma cell line Caki-1. Immunostaining and Western blot showed Ron in normal kidney and in all oncocytomas but never in renal cell carcinomas or in Caki-1. In addition, Western blot showed that Ron was expressed in phosphorylated, i.e., active, form. Bcl-2 was strongly expressed in oncocytomas, whereas Ki-67 and p53 were much less expressed in oncocytomas than in carcinomas. These results indicate in Ron a marker that differentiates oncocytoma from the other renal epithelial tumors. We therefore think that Ron may prove to be a new tool for a sound and precise diagnosis of oncocytoma, a benign tumor that cannot always be distinguished from carcinomas at histologic examination. The overexpression of bcl-2, but not p53 in oncocytoma, suggests that the MSP/Ron system sustains the growth of oncocytoma by opposing apoptosis.

C to A single nucleotide polymorphism in intron 18 of the human MST1R (RON) gene that maps at 3p21.3

The MST1R (RON) gene, that maps at 3p21.3, encodes a protein tyrosine kinase receptor comprised of an extra-cellular domain that contains the ligand binding pocket and an intracellular region where the kinase domain is located. It controls cell survival and motility programs related to invasive growth. With the single strand conformation polymorphism (SSCP) method, a C to A nucleotide polymorphism (SNP) was found in intron 18 of the gene. The SNP has a frequency of 0.28 among African-American, 0.25 among Caucasian CEPH and 0.09 among Asian healthy individuals. During these studies, an alternatively spliced cDNA of MST1R, lacking exon 19, was also found that may result from this change.

Altered expression of the RON receptor tyrosine kinase in primary human colorectal adenocarcinomas: generation of different splicing RON variants and their oncogenic potential

The RON receptor tyrosine kinase is a member of the MET proto-oncogene family that has been implicated in regulating motile-invasive phenotypes in certain types of epithelial cancers. The purpose of this study was to determine if RON expression is altered in primary human colorectal adenocarcinomas. Results from immunohistochemical staining showed that RON is highly expressed in the majority of colorectal adenocarcinomas (29/49 cases). Accumulated RON is also constitutively active with autophosphorylation in tyrosine residues. Moreover, three splicing variants of RON, namely RONdelta165, RONdelta160, and RONdelta155 were detected and cloned from two primary colon cancer samples. These RON variants were generated by deletions in different regions in extracellular domains of the RON beta chain. Functional studies showed that expression of RONdelta160 or RONdelta155 in Martin-Darby canine kidney cells resulted in increased cell dissociation (scatter-like activity). RON variants, RONdelta160 and RONdelta155, also exerted the ability to induce multiple focus formation and sustain anchorage-independent growth of transfected NIH3T3 cells. Moreover, NIH3T3 cells expressing RONdelta160 or RONdelta155 formed tumors in athymic nude mice and colonized in the lungs. These data suggest that RON expression is altered in certain primary colon cancers. Abnormal accumulation of RON variants may play a role in the progression of certain colorectal cancers in vivo.

A novel gene encoding a TIG multiple domain protein is a positional candidate for autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD) is a common hereditary renal cystic disease in infants and children. By genetic linkage analyses, the gene responsible for this disease, termed polycystic kidney and hepatic disease 1 (PKHD1), was mapped on human chromosome 6p21.1-p12, and has been further localized to a 1-cM genetic interval flanked by the D6S1714/D6S243 (telomeric) and D6S1024 (centromeric) markers. We recently identified a novel gene in this genetic interval from kidney cDNA, using cloning strategies. The gene PKHD1 (PKHD1-tentative) encodes a novel 3396-amino-acid protein with no apparent homology with any known proteins. We named its gene product "tigmin" because it contains multiple TIG domains, which usually are seen in proteins containing immunoglobulin-like folds. PKHD1 encodes an 11.6-kb transcript and is composed of 61 exons spanning an approximately 365-kb genomic region on chromosome 6p12-p11.2 adjacent to the marker D6S1714. Northern blot analyses demonstrated that the gene has discrete bands with one peak signal at approximately 11 kb, indicating that PKHD1 is likely to have multiple alternative transcripts. PKHD1 is highly expressed in adult and infant kidneys and weakly expressed in liver in northern blot analysis. This expression pattern parallels the tissue involvement observed in ARPKD. In situ hybridization analysis further revealed that the expression of PKHD1 in the kidney is mainly localized to the epithelial cells of the collecting duct, the specific tubular segment involved in cyst formation in ARPKD. These features of PKHD1 make it a strong positional candidate gene for ARPKD.

Constitutive TrkA activity in receptor-overexpressing PC12 clones

We have studied ligand-independent signaling by the nerve growth factor receptor TrkA in PC12 clones, under conditions of receptor overexpression. Our results indicate that TrkA-overexpressing PC12 clones display constitutive receptor activation, involving both the mature, 140-kDa form and the immature, intracellular 110-kDa form of the receptor. Phosphorylation of Tyr 674/675, located in the activation loop domain and reflecting TrkA kinase activity, appears particularly prominent in the immature form of the receptor. Constitutive receptor activation is able to chronically stimulate the PI-3 kinase/Akt as well as the mitogen-activated protein kinase pathways, leading to ligand-independent neurite extension. Under conditions of overexpression, a significant fraction of the receptor is retained intracellularly by thiol-mediated mechanisms. Exposure of the cells to reducing agents promotes translocation of the intracellular pool of the receptor to the plasma membrane and suppresses ligand-independent neurite outgrowth. Our results suggest that the levels of expression of TrkA, both intracellularly and at the cell surface, may act to modulate its kinase activity and generate ligand-independent downstream signaling.

Macrophage-stimulating protein is produced by tubular cells and activates mesangial cells

Until now, hepatocytes have been the only known cell source of macrophage-stimulating protein (MSP), and tissue macrophages have been the cells on which the biologic effects of MSP have been proved. To extend the understanding of the biologic meaning of MSP, it was investigated whether MSP operates in the kidney. MSP protein was evaluated by Western blot in supernatant of cultured human tubular cells (HK2) and human mesangial cells (HMC). MSP mRNA was investigated in HK2 by reverse transcription-polymerase chain reaction (RT-PCR). The expression of the MSP receptor, RON, was evaluated in HMC and HK2 by Western blot. RON mRNA was investigated in HMC by RT-PCR. The expression of MSP and RON in normal human renal tissue was studied by immunohistochemistry. HMC were stimulated with recombinant MSP (rMSP) and HK2 supernatant to study cell growth, migration, and the capacity to invade an artificial collagen matrix and synthesize interleukin-6 (IL-6). HK2 produced MSP and expressed RON in a form that was phosphorylated by rMSP. HMC expressed RON but did not produce MSP. MSP in HK2 supernatant and rMSP induced in HMC phosphorylation of RON, growth, migration, invasion, and IL-6 synthesis. In normal human kidney, tubules expressed MSP and RON. These results indicate a novel field of operation for MSP and suggest a pathogenic role of the MSP/RON system in renal disease. In fact, MSP released by tubular cells may recruit monocytes/macrophages in inflammatory tubulointerstitial disorders. In addition, MSP either circulating or as paracrine product may sustain glomerular mesangioproliferative disease.

Point mutations and overexpression of Ron induce transformation, tumor formation, and metastasis

The receptor tyrosine kinase Ron is a member of the receptor family that includes the proto-oncogene Met and the avian oncogene Sea. The interaction of Ron with its ligand, known as hepatocyte growth factor-like protein (HGFL) or macrophage stimulating protein (MSP), induces crucial cellular responses including invasive growth, proliferation, cell scattering, and branching morphogenesis. Based on the homology and functional similarities between Met and Ron it was hypothesized that Ron may be important in tumor formation and metastasis. To test this hypothesis, wild-type mouse Ron and three mutant forms of Ron containing mutations similar to those found in the Met gene in human hereditary papillary renal carcinoma (HPRC), were expressed in NIH3T3 cells. A transformed phenotype was produced in cell lines expressing either wild-type Ron or the mutated Ron proteins. Further, these cell lines displayed oncogenic potential by exhibiting increased proliferation and constitutive phosphorylation of Ron. These cell lines were also tested for the ability to form solid tumors. Cells expressing wild-type Ron and the three proteins with single amino acid substitutions were highly tumorigenic in vivo. In a model of experimental metastasis, two of the cell lines with altered Ron protein formed highly aggressive tumors in the lungs. These results suggest that Ron may be an aggressive oncogene when either overexpressed or when activated by mutation.

Ligand-independent dimerization and activation of the oncogenic Xmrk receptor by two mutations in the extracellular domain

Overexpression of the oncogenic receptor tyrosine kinase ONC-Xmrk is the first step in the development of hereditary malignant melanoma in the fish Xiphophorus. However, overexpression of its proto-oncogene counterpart (INV-Xmrk) is not sufficient for the oncogenic function of the receptor. Compared with INV-Xmrk, the ONC-Xmrk receptor displays 14 amino acid changes, suggesting the presence of activating mutations. To identify such activating mutations, a series of chimeric and mutant receptors were studied. None of the mutations present in the intracellular domain was found to be involved in receptor activation. In the extracellular domain, we found two mutations responsible for activation of the receptor. One is the substitution of a conserved cysteine (C578S) involved in intramolecular disulfide bonding. The other is a glycine to arginine exchange (G359R) in subdomain III. Either mutation leads to constitutive dimer formation and thereby to activation of the ONC-Xmrk receptor. Besides, the presence of these mutations slows down the processing of the Xmrk receptor in the endoplasmic reticulum, which is apparent as an incomplete glycosylation.

Immunohistochemical analysis of distribution of RON receptor tyrosine kinase in human digestive organs

The immunohistochemical distribution of RON receptor tyrosine kinase in digestive organs of both human fetus and adult, including the esophagus, stomach, duodenum, small intestine, colon, rectum, liver, gallbladder, pancreas, and spleen, was investigated semiquantitively using an affinity-purified rabbit polyclonal antibody. RON was observed to be widely distributed throughout various digestive organs and cell types in humans. The immunoreactivity for RON was observed in the epithelium of the esophagus, small intestine, colon, hepatocytes, Kupffer cells, and splenic macrophages both in the adult and the fetus, suggesting that the MSP/RON signaling pathway possesses the proper biological properties to possibly be involved in morphogenesis or differentiation of cells in these organs and cell types. Several organs differed in immunoreactivity between adult and fetus. No immunoreactive cells were found in the pancreas of adults; however, immunoreactivity was observed in acinar cells and in some of the duct or ductular cells and endocrine cells of the islet of the fetus. Similarly, immunoreactivity was not observed in gastric mucosa except in the intestinal metaplastic cells in adults; however, immunoreactivity was found in the foveolar epithelium of the stomach of the fetus. Although the biological significance of RON in malignancy is unclear, the presence of RON immunoreactivity in the fetus and it lack in the adult may indicate that RON is a oncofetal substance in human pancreas and stomach.

Overexpression and activation of the RON receptor tyrosine kinase in a panel of human colorectal carcinoma cell lines

RON is a receptor tyrosine kinase belonging to the MET proto-oncogene family. The purposes of this study are to determine the expression and activation of RON in a panel of human colon carcinoma cell lines. Western blotting showed that RON is barely detectable in normal and SV-40-transformed colon epithelial cells, but highly expressed and constitutively activated in several colon carcinoma cell lines including Colo201, HT-29, HCT116, and SW837. Moreover, a novel RON variant with a molecular mass of 160 kDa (RONDelta160) was identified from HT-29 cells. The cDNA encoding RONDelta160 has an in-frame deletion of 109 amino acids in the extracellular domain of the RON beta chain, which is caused by splicing out of two exons in the RON mRNA. No mutations were found in the kinase domain of the RON gene in five carcinoma cell lines screened. By expressing RON in colon epithelial cells, we found that RON activation increases cell motile-invasive activities and protects cells against apoptotic death. These data suggest that RON expression and activation are deregulated in colon carcinoma cell lines. By abnormal activation of RON, this receptor and its variant may regulate motile-invasive phenotypes of certain colon carcinoma cells in vivo.

Tyrosine kinases and gastric cancer

Carcinoma of the stomach is one of the most prevalent cancer types in the world today. Two major forms of gastric cancer are distinguished according to their morphological and clinicopathological classifications (well differentiated/intestinal type and poorly differentiated/diffuse type), characteristics that could also be attributed to the altered expression of different types of oncogenes or tumor suppressor genes. Significant differences exist for gastric cancer incidence comparing people of different ethnic origins, implicating various genetic and epigenetic factors for gastric oncogenesis. There are only a limited number of molecular markers available for gastric cancer detection and prognostic evaluation, among which are tyrosine kinases. There is convincing evidence that tyrosine kinases are involved in oncogenesis and disease progression for many human cancers. Amplifications of certain tyrosine kinases (c-met, k-sam and erbB2/neu) have been associated with human gastric cancer progression. Alternatively spliced transcripts and enhanced protein-expression levels for some of these tyrosine kinases are correlated with clinical outcomes for gastric cancer patients. With advent of high throughput techniques, it is now possible to detect nearly all expressed tyrosine kinases in a single screen. This increases the chance to identify additional tyrosine kinases as predictive markers for gastric cancers. In this article, we will first review the literature data concerning certain tyrosine kinases implicated in gastric carcinogenesis and then summarize more recent work which provide comprehensive tyrosine kinase profiles for gastric cancer specimens and cell lines. Two new gastric cancer molecular markers (tie-1 and mkk4) have been identified through the use of these profiles and demonstrated effective as clinical prognostic indicators.

Effects of unpaired cysteines on yield, solubility and activity of different recombinant antibody constructs expressed in E. coli

New E. coli vectors based on the pOPE/pSTE vector system [Gene 128 (1993) 97] were constructed to express a single-chain Fv antibody fragment (scFv), a scFv-streptavidin fusion protein and two disulfide bond-stabilized Fv antibody fragments (dsFvs) utilizing different side chain positions for disulfide stabilization. All of these constructs encoded fusion proteins carrying five C-terminal histidine residues preceded by an unpaired cysteine. The influence of this cysteine, which was originally introduced to allow the chemical modification of the fusion proteins, was assessed by exchanging the two amino acids CysIle in front of the carboxy terminal His-tag to SerHis in all constructs. Yield and antigen-binding activity of the antibody constructs were compared after standard lab-scale periplasmic expression in Escherichia coli. The removal of the unpaired cysteine resulted in a significant increase in antigen-binding activity of the crude periplasmic extracts. Further, a three-five fold increase of yield and a significantly improved purity were observed after immobilized metal affinity chromatography (IMAC) with all four constructs.

Requirement of both tyrosine residues 1330 and 1337 in the C-terminal tail of the RON receptor tyrosine kinase for epithelial cell scattering and migration

RON is a receptor tyrosine kinase that mediates cell scattering, migration, and tubular formation. This study focused on the function of two tyrosines, Y1330 and Y1337, in the C-terminus of RON in regulating epithelial cell scattering and migration. Substitution of both tyrosine residues with phenylalanine causes complete loss of cell scattering and migration in kidney 293 cells. In contrast, single mutation of either tyrosine residue has no effect. We found that mutation at Y1330 or Y1337 alone does not significantly affect the association of RON with PI-3 kinase, whereas a double mutation abolishes the recruitment of substrates. RON-mediated cell migration was inhibited by PI-3 kinase inhibitor wortmannin. This effect was also achieved by a dominant inhibitory p85 of PI-3 kinase. We conclude that Y1330 and Y1337 are required for RON-mediated cell motility. By associating with PI-3 kinase, the Y1330-Y1337 docking site plays a critical role in transducing motile signals of RON.

MET(PRC) mutations in the Ron receptor result in upregulation of tyrosine kinase activity and acquisition of oncogenic potential

Ron and Met are structurally related receptor tyrosine kinases that elicit a complex biological response leading to invasive growth. Naturally occurring point mutations activate the Met kinase in papillary renal carcinomas (MET(PRC) mutations). By site-directed mutagenesis, we generated homologous amino acid substitutions in the Ron kinase domain and analyzed the biochemical and biological properties of the mutant receptors. Among the mutations studied, D(1232)H and M(1254)T displayed transforming activity in NIH3T3 cells, inducing focus formation and anchorage-independent growth. The D(1232)H and M(1254)T substitutions resulted in increased Ron autophosphorylation both in vivo and in vitro and constitutive binding to intracellular signal transducers. Both mutations yielded a dramatic increase in catalytic efficiency, indicating a direct correlation between kinase activity and oncogenic potential. Molecular modeling of the Ron D(1232)H mutation suggests that this single amino acid substitution favors the transition of the kinase from the inactive to the active state. These data demonstrate that point mutations can confer transforming activity to the Ron receptor and show that RON is a potential oncogene.

The Ron/STK receptor tyrosine kinase is essential for peri-implantation development in the mouse

The Ron/STK receptor tyrosine kinase is a member of the c-Met family of receptors and is activated by hepatocyte growth factor-like protein (HGFL). Ron activation results in a variety of cellular responses in vitro, such as activation of macrophages, proliferation, migration, and invasion, suggesting a broad biologic role in vivo. Nevertheless, HGFL-deficient mice grow to adulthood with few appreciable phenotypic abnormalities. We report here that in striking contrast to the loss of its only known ligand, complete loss of Ron leads to early embryonic death. Embryos that are devoid of Ron (Ron-/-) are viable through the blastocyst stage of development but fail to survive past the peri-implantation period. In situ hybridization analysis demonstrates that Ron is expressed in the trophectoderm at embryonic day (E) 3.5 and is maintained in extraembryonic tissue through E7.5, compatible with an essential function at this stage of development. Hemizygous mice (Ron+/-) grow to adulthood; however, these mice are highly susceptible to endotoxic shock and appear to be compromised in their ability to downregulate nitric oxide production. These results demonstrate a novel role for Ron in early mouse development and suggest that Ron plays a limiting role in the inflammatory response.

Elevated expression of activated forms of Neu/ErbB-2 and ErbB-3 are involved in the induction of mammary tumors in transgenic mice: implications for human breast cancer

To assess the importance of Neu activation during mammary tumorigenesis, altered receptors harboring in-frame deletions within the extracellular domain were expressed in transgenic mice. Females from several independent lines develop multiple mammary tumors that frequently metastasize to the lung. Tumor progression in these strains was associated with elevated levels of tyrosine-phosphorylated Neu and ErbB-3. Consistent with these observations, a survey of primary human breast tumors revealed frequent co-expression of both erbB-2 and erbB-3 transcripts. The ability of altered Neu receptors to induce mammary tumorigenesis in transgenic mice prompted us to examine whether similar mutations occurred in ErbB-2 during human breast cancer progression. Interestingly, an alternatively spliced form of erbB-2, closely resembling spontaneous activated forms of neu, was detected in human breast tumors. The ErbB-2 receptor encoded by this novel transcript harbors an in-frame deletion of 16 amino acids in the extracellular domain and can transform Rat-1 fibroblasts. Together, these observations argue that co-expression of ErbB-2 and ErbB-3 may play a critical role in the induction of human breast tumors, and raise the possibility that activating mutations in the ErbB-2 receptor may also contribute to this process.

Interactions between scatter factors and their receptors: hints for therapeutic applications

The scatter factors, which include hepatocyte growth factor and macrophage stimulating protein, stand out from other cytokines because of their uncommon biological properties. In addition to promoting cell growth and protection from apoptosis, they are involved in the control of cell dissociation, migration into extracellular matrices, and a unique process of differentiation called 'branching morphogenesis'. Through the concerted regulation of these complex phenomena, scatter factors promote development, regeneration, and reconstruction of normal organ architecture. In transformed epithelia, scatter factors can mediate tumor invasive growth, a harmful feature of neoplastic progression in which cancer cells invade surrounding tissues, penetrate across the vascular walls, and eventually disseminate throughout the body, giving rise to systemic metastases. A much-debated issue in basic biology, which has strong implications for experimental medicine, is how to dissociate the favorable effects of growth factors from their adverse ones. Accordingly, to find agonists or antagonists with potential therapeutic applications is a crucial undertaking for current research. Domain-mapping analyses of growth factor molecules can help to isolate specific structural requirements for the induction of selective biological effects. Based on the observation that certain growth factors must undergo posttranslational modifications to exert a full response, it is possible to interfere with their activation mechanisms to modulate their functions. Finally, the identification of cell type-specific coreceptors able to potentiate their activity allows drawing of a functional body map, where some organs or tissues may be more responsive than others to growth factors. This review is focused on how, and to what extent, scatter factors can behave 'well' or 'badly' according to their molecular structure, the way they are activated, and the way they interact with cell surface receptors and coreceptors.

Assembly, sorting, and exit of oligomeric proteins from the endoplasmic reticulum

The endoplasmic reticulum (ER) uses various mechanisms to ensure that only properly folded proteins enter the secretory pathway. For proteins that oligomerize in the ER, the proper tertiary and quaternary structures must be achieved before their release. Although some proteins fold before oligomerization, others initiate oligomerization cotranslationally. Here, we discuss these different strategies and some of the unique problems they present for the ER quality control system. One mechanism used by the ER is thiol retention. Thiol retention operates by monitoring the redox state of specific cysteine residue(s) and was discovered in studies on the assembly of IgM, a complex oligomeric glycoprotein. This system is also involved in retaining other unassembled proteins in the ER. Mutations that result in uneven numbers of cysteine residues can subject yet other proteins to thiol retention, altering their oligomerization status and function. The implications of these results on the effects of thiol retention on protein function and cell fate are discussed.

Mutations in the extracellular domain cause RET loss of function by a dominant negative mechanism

The RET proto-oncogene encodes a tyrosine kinase receptor expressed in neuroectoderm-derived cells. Mutations in specific regions of the gene are responsible for the tumor syndromes multiple endocrine neoplasia types 2A and 2B (MEN 2A and 2B), while mutations along the entire gene are involved in a developmental disorder of the gastrointestinal tract, Hirschsprung's disease (HSCR disease). Two mutants in the extracellular domain of RET, one associated with HSCR disease and one carrying a flag epitope, were analyzed to investigate the impact of the mutations on RET function. Both mutants were impeded in their maturation, resulting in the lack of the 170-kDa mature form and the accumulation of the 150-kDa immature form in the endoplasmic reticulum. Although not exposed on the cell surface, the 150-kDa species formed dimers and aggregates; this was more pronounced in a double mutant bearing a MEN 2A mutation. Tyrosine phosphorylation and the transactivation potential were drastically reduced in single and double mutants. Finally, in cotransfection experiments both mutants exerted a dominant negative effect over protoRET and RET2A through the formation of a heteromeric complex that prevents their maturation and function. These results suggest that HSCR mutations in the extracellular region cause RET loss of function through a dominant negative mechanism.

Biological effects of targeted inactivation of hepatocyte growth factor-like protein in mice

Hepatocyte growth factor-like protein (HGFL) is a liver-derived serum glycoprotein involved in cell proliferation and differentiation, and is proposed to have a fundamental role in embryogenesis, fertility, hematopoiesis, macrophage activation, and tissue repair. To assess the in vivo effects of total loss of HGFL, we generated mice with targeted disruption of the gene resulting in loss of the protein. Disruption of the HGFL gene allowed for normal embryogenesis, and followed a Mendelian pattern of genetic transmission. Mice homozygous for the targeted allele (HGFL-/- mice) are fertile, and grow to adulthood without obvious phenotypic abnormalities in unchallenged animals, except for development of lipid-containing cytoplasmic vacuoles in hepatocytes throughout the liver lobules. These histologic changes are not accompanied by discernible changes in synthetic or excretory hepatic functions. Hematopoiesis appears unaltered, and although macrophage activation is delayed in the absence of HGFL, migration to the peritoneal cavity upon challenge with thioglycollate was similar in HGFL-/- and wild-type mice. Challenged with incision to skin, HGFL-/- mice display normal wound healing. These data demonstrate that HGFL is not essential for embryogenesis, fertility, or wound healing. HGFL-deficient mice will provide a valuable means to assess the role of HGFL in hepatic and systemic responses to inflammatory and infectious stimuli in vivo.

Cysteine and glutathione secretion in response to protein disulfide bond formation in the ER

Protein folding in the endoplasmic reticulum (ER) often involves the formation of disulfide bonds. The oxidizing conditions required within this organelle were shown to be maintained through the release of small thiols, mainly cysteine and glutathione. Thiol secretion was stimulated when proteins rich in disulfide bonds were translocated into the ER, and secretion was prevented by the inhibition of protein synthesis. Endogenously generated cysteine and glutathione counteracted thiol-mediated retention in the ER and altered the extracellular redox. The secretion of thiols might link disulfide bond formation in the ER to intra- and intercellular redox signaling.

Stringent thiol-mediated retention in B lymphocytes and Xenopus oocytes correlates with inefficient IgM polymerization

Thiol-dependent retention mechanisms involving the microsecond chain Cys575 ensure that only polymeric IgM are secreted. B lymphocytes are unable to polymerize IgM and degrade unpolymerized precursors intracellularly. Since several non-lymphoid transfectants secrete hexameric IgM, specific mechanism(s) inhibiting IgM polymerization/secretion may be active in B cells. Here, we show that Xenopus laevis oocytes are also unable to polymerize IgM and retain this isotype via Cys575 as efficiently as B cells. The mechanisms and the hierarchy of the thiol-dependent pre-Golgi retention are conserved in amphibian oocytes, as indicated by the efficient retention of secretory IgA and the slow secretion of unassembled J558 lambda chains. We also show that B cells do not lack any structural component necessary to polymerize IgM: after retention has been weakened by 2-mercaptoethanol, polymerization can occur if oxidizing conditions are restored. Since release from retention can result in polymerization, stringent retention in B cells and oocytes might be at the basis of their common inability to polymerize secretory IgM. Our findings suggest that disulfide interchange reactions in the exocytic compartment can be modulated during B cell differentiation to control IgM secretion.

Constitutive activation of the RON gene promotes invasive growth but not transformation

MET, RON, and SEA are members of a gene family encoding tyrosine kinase receptors with distinctive properties. Besides mediating growth, they control cell dissociation, motility ("scattering"), and formation of branching tubules. While there are transforming counterparts of MET and SEA, no oncogenic forms of RON have yet been identified. A chimeric Tpr-Ron, mimicking the oncogenic form of Met (Tpr-Met) was generated to investigate its transforming potential. For comparison, a chimeric Tpr-Sea was also constructed. Fusion with Tpr induced constitutive activation of the Ron and Sea kinases. While Tpr-Sea was more efficient than Tpr-Met in transformation, Tpr-Ron did not transform NIH 3T3 cells. The differences in the transforming abilities of Tpr-Met and Tpr-Ron were linked to the functional features of the respective tyrosine kinases using the approach of swapping subdomains. Kinetic analysis showed that the catalytic efficiency of Tpr-Ron is five times lower than that of Tpr-Met. Moreover, constitutive activation of Ron resulted in activation of the MAP kinase signaling cascade approximately three times lower than that attained by Tpr-Met. However, constitutive activation of Ron did induce a mitogenic-invasive response, causing cell dissociation, motility, and invasion of extracellular matrices. Tpr-Ron also induced formation of long, unbranched tubules in tridimensional collagen gels. These data show that RON has the potential to elicit a motile-invasive rather than a transformed phenotype.