Truncation of the TGF-beta type II receptor gene results in insensitivity to TGF-beta in human gastric cancer cells

Recapitulating Human Gastric Cancer Pathogenesis: Experimental Models of Gastric Cancer

This review focuses on the various experimental models to study gastric cancer pathogenesis, with the role of genetically engineered mouse models (GEMMs) used as the major examples. We review differences in human stomach anatomy compared to the stomachs of the experimental models, including the mouse and invertebrate models such as Drosophila and C. elegans. The contribution of major signaling pathways, e.g., Notch, Hedgehog, AKT/PI3K is discussed in the context of their potential contribution to foregut tumorigenesis. We critically examine the rationale behind specific GEMMs, chemical carcinogens, dietary promoters, Helicobacter infection, and direct mutagenesis of relevant oncogenes and tumor suppressor that have been developed to study gastric cancer pathogenesis. Despite species differences, more efficient and effective models to test specific genes and pathways disrupted in human gastric carcinogenesis have yet to emerge. As we better understand these species differences, "humanized" versions of mouse models will more closely approximate human gastric cancer pathogenesis. Towards that end, epigenetic marks on chromatin, the gut microbiota, and ways of manipulating the immune system will likely move center stage, permitting greater overlap between rodent and human cancer phenotypes thus providing a unified progression model.

Transforming growth factor-β: an important mediator in Helicobacter pylori-associated pathogenesis

Helicobacter pylori (H.pylori) is a Gram-negative, microaerophilic, helical bacillus that specifically colonizes the gastric mucosa. The interaction of virulence factors, host genetic factors, and environmental factors contributes to the pathogenesis of H. pylori-associated conditions, such as atrophic gastritis and intestinal metaplasia. Infection with H. pylori has recently been recognized as the strongest risk factor for gastric cancer. As a pleiotropic cytokine, transforming growth factor (TGF)-β regulates various biological processes, including cell cycle, proliferation, apoptosis, and metastasis. Recent studies have shed new light on the involvement of TGF-β signaling in the pathogenesis of H. pylori infection. This review focuses on the potential etiological roles of TGF-β in H. pylori-mediated gastric pathogenesis.

Homozygous deletions at 3p22, 5p14, 6q15, and 9p21 result in aberrant expression of tumor suppressor genes in gastric cancer

Homozygous deletion is a frequent mutational mechanism of silencing tumor suppressor genes in cancer. Therefore, homozygous deletions have been analyzed for identification of tumor suppressor genes that can be utilized as biomarkers or therapeutic targets for cancer treatment. In this study, to elucidate potential tumor suppressor genes involved in gastric cancer (GC), we analyzed the entire set of large homozygous deletions in six human GC cell lines through genome- and transcriptome-wide approaches. We identified 51 genes in homozygous deletion regions of chromosomes and confirmed the deletion frequency in tumor tissues of 219 GC patients from The Cancer Genome Atlas database. We evaluated the effect of homozygous deletions on the mRNA level and found significantly affected genes in chromosome bands 9p21, 3p22, 5p14, and 6q15. Among the genes in 9p21, we investigated the potential tumor suppressive effect of KLHL9. We demonstrated that ectopic expression of KLHL9 inhibited cell proliferation and tumor formation in KLHL9-deficient SNU-16 cell line. In addition, we observed that homozygous focal deletions generated truncated transcripts of TGFBR2, CTNNA1, and STXBP5. Ectopic expression of two kinds of TGFBR2-reverse GADL1 fusion genes suppressed TGF-β signaling, which may lead to the loss of sensitivity to TGF-β tumor suppressive activity. In conclusion, our findings suggest that novel tumor suppressor genes that are aberrantly expressed through homozygous deletions may play important roles in gastric tumorigenesis.

TGF-β cascade regulation by PPP1 and its interactors -impact on prostate cancer development and therapy

Protein phosphorylation is a key mechanism by which normal and cancer cells regulate their main transduction pathways. Protein kinases and phosphatases are precisely orchestrated to achieve the (de)phosphorylation of candidate proteins. Indeed, cellular health is dependent on the fine-tune of phosphorylation systems, which when deregulated lead to cancer. Transforming growth factor beta (TGF-β) pathway involvement in the genesis of prostate cancer has long been established. Many of its members were shown to be hypo- or hyperphosphorylated during the process of malignancy. A major phosphatase that is responsible for the vast majority of the serine/threonine dephosphorylation is the phosphoprotein phosphatase 1 (PPP1). PPP1 has been associated with the dephosphorylation of several proteins involved in the TGF-β cascade. This review will discuss the role of PPP1 in the regulation of several TGF-β signalling members and how the subversion of this pathway is related to prostate cancer development. Furthermore, current challenges on the protein phosphatases field as new targets to cancer therapy will be addressed.

Mouse models of gastric cancer

Animal models have greatly enriched our understanding of the molecular mechanisms of numerous types of cancers. Gastric cancer is one of the most common cancers worldwide, with a poor prognosis and high incidence of drug-resistance. However, most inbred strains of mice have proven resistant to gastric carcinogenesis. To establish useful models which mimic human gastric cancer phenotypes, investigators have utilized animals infected with Helicobacter species and treated with carcinogens. In addition, by exploiting genetic engineering, a variety of transgenic and knockout mouse models of gastric cancer have emerged, such as INS-GAS mice and TFF1 knockout mice. Investigators have used the combination of carcinogens and gene alteration to accelerate gastric cancer development, but rarely do mouse models show an aggressive and metastatic gastric cancer phenotype that could be relevant to preclinical studies, which may require more specific targeting of gastric progenitor cells. Here, we review current gastric carcinogenesis mouse models and provide our future perspectives on this field.

Elevated serum level and gene polymorphisms of TGF-beta1 in gastric cancer

Transforming growth factor (TGF)-beta1, as a candidate tumor marker, is currently of interest. In this study, serum TGF-beta1 levels in gastric cancer (GC) patients and healthy volunteers were measured using enzyme-linked immunosorbent assay (ELISA). In addition, single nucleotide polymorphisms (SNPs) of the TGF-beta1 gene at codon 10 and codon 25 were identified by means of amplification refractory mutation system-polymerase chain reaction (ARMS-PCR) and sequence analysis. Our results indicated that serum concentrations of TGF-beta1 in GC patients were significantly higher than those in the control, and positively correlated with tumor mass, invasion, metastasis, and clinical stage. The serum TGF-beta1 levels of patients recovering from radical resection were markedly lower than those before surgery. Meanwhile, no deoxyribonucleic acid (DNA) sequence variation at codon 25 of the TGF-beta1 gene was found and a TGF-beta1 gene polymorphism at codon 10 did not show obvious correlations with either TGF-beta1 expression or clinicopathological parameters of GC. Our evidence suggested that serum concentration of TGF-beta1 might be a novel tumor marker for GC and the polymorphisms of TGF-beta1 gene did not play a role as a determinant of serum TGF-beta1 concentration or as a genetic risk factor in the gastric carcinogenesis and progression.

Microsatellite unstable colorectal cancer cell lines with truncating TGFβRII mutations remain sensitive to endogenous TGFβ

Disruptions to the TGFbeta signalling pathway have been implicated in most human adenocarcinomas. As cancers progress, many acquire resistance to the growth-suppressing properties of TGFbeta while retaining sensitivity to its tumour-promoting effects. Microsatellite unstable colorectal cancers (MSI-H CRCs) possess truncating mutations in the type II TGFbeta receptor (TGFbetaRII) gene that have been assumed to render these tumours insensitive to TGFbeta. However, numerous reports of TGFbetaRII bypass exist and this study was thus undertaken in order to clarify the true extent of TGFbeta sensitivity in MSI-H CRCs. Using stimulation with exogenous TGFbeta, we demonstrated that, while MSI-H CRCs are capable of binding soluble TGFbeta, two out of three cell lines examined remain refractory to its signalling effects. In contrast, use of a specific inhibitor of the type I TGFbeta receptor (TGFbetaRI) revealed that all remain sensitive to signalling by endogenously produced TGFbeta. Specifically, autocrine signalling via TGFbetaRI mediates constitutive activation of Smad2 as well as repression of Erk signalling. Real-time PCR confirmed that these effects are sufficient to affect the expression level of various TGFbeta-modulated genes. An invasion assay revealed that autocrine TGFbetaRI signalling also promotes the invasion capacity of MSI-H CRCs to an extent similar to that seen in their non-MSI-H counterparts. Independent TGFbetaRI signalling, however, has no effect on the rate of proliferation of MSI-H CRC cells. Together, these results demonstrate that MSI-H CRC cell lines are not completely refractory to TGFbeta, despite lacking functional TGFbetaRII. In addition to clarifying the true consequences of natural TGFbetaRII loss and the independent function of TGFbetaRI, our results highlight the selective nature of TGFbeta resistance developed by cancers.

Alternative splicing of TGF-betas and their high-affinity receptors T beta RI, T beta RII and T beta RIII (betaglycan) reveal new variants in human prostatic cells

Background

The transforming growth factors (TGF)-β, TGF-β1, TGF-β2 and TGF-β3, and their receptors [TβRI, TβRII, TβRIII (betaglycan)] elicit pleiotropic functions in the prostate. Although expression of the ligands and receptors have been investigated, the splice variants have never been analyzed. We therefore have analyzed all ligands, the receptors and the splice variants TβRIB, TβRIIB and TGF-β2B in human prostatic cells.

Results

Interestingly, a novel human receptor transcript TβRIIC was identified, encoding additional 36 amino acids in the extracellular domain, that is expressed in the prostatic cancer cells PC-3, stromal hPCPs, and other human tissues. Furthermore, the receptor variant TβRIB with four additional amino acids was identified also in human. Expression of the variant TβRIIB was found in all prostate cell lines studied with a preferential localization in epithelial cells in some human prostatic glands. Similarly, we observed localization of TβRIIC and TGF-β2B mainly in the epithelial cells with a preferential localization of TGF-β2B in the apical cell compartment. Whereas in the androgen-independent hPCPs and PC-3 cells all TGF-β ligands and receptors are expressed, the androgen-dependent LNCaP cells failed to express all ligands. Additionally, stimulation of PC-3 cells with TGF-β2 resulted in a significant and strong increase in secretion of plasminogen activator inhibitor-1 (PAI-1) with a major participation of TβRII.

Conclusion

In general, expression of the splice variants was more heterogeneous in contrast to the well-known isoforms. The identification of the splice variants TβRIB and the novel isoform TβRIIC in man clearly contributes to the growing complexity of the TGF-β family.

Transforming growth factor-beta pathway disruption and infiltration of colorectal cancers by intraepithelial lymphocytes

AIMS

Colorectal cancers deficient in DNA mismatch repair (MMR) are often characterized by the presence of numerous intraepithelial lymphocytes (IELs). These CD8+ T cells selectively express CD103, which is upregulated locally by transforming growth factor (TGF)-beta, and adhere to E-cadherin expressed by mucosal epithelia. Many of these cancers also possess inactivating mutations in the type II TGF-beta receptor and are believed to be insensitive to TGF-beta. The present study aimed to explore whether such refractoriness to TGF-beta is an independently contributing factor to IEL retention.

METHODS AND RESULTS

A panel of colorectal cancers enriched for DNA MMR deficiency was examined by immunohistochemistry to explore the expression levels and localization of various components in the TGF-beta signalling pathway. Logistic regression was then carried out in order to identify predictors of elevated lymphocytic infiltration independent of DNA MMR status. Increases in Smad4 expression, tumour cell proliferation and TGF-beta secretion each emerged as independent predictors of marked lymphocyte infiltration.

CONCLUSIONS

These results strongly support the hypothesis that refractoriness to normal TGF-beta signalling in colorectal cancers plays a role in the retention of lymphocytes within tumour epithelium. Since IEL infiltration is an independent predictor of favourable prognosis, the TGF-beta pathway may represent an important therapeutic target.

Proliferation and Cdk4 expression in microsatellite unstable colon cancers with TGFBR2 mutations

Approximately 15% of human colon cancers have microsatellite instability (MSI) and carry frameshift mutations in a polyadenine tract (BAT-RII) in the type II transforming growth factor beta (TGF-beta) receptor (TGFBR2), a required component of the TGF-beta receptor. The BAT-RII mutations in MSI colon cancers make the tumors resistant to the effects of TGF-beta. In cultured epithelial cells, TGF-beta can inhibit cell proliferation and induce apoptosis, and in vitro it can regulate the expression of a variety of cyclins, cyclin-dependent kinases (cdks) and cdk inhibitors. These effects are context- and tissue type-dependent, raising questions about which of these in vitro effects of TGF-beta signaling inactivation contribute to the formation of primary colon cancer. Thus, this study sought to determine the pathogenetically relevant effects of TGFBR2 inactivation in primary MSI colon cancers with mutant BAT-RII. Colon cancers with mutant BAT-RII were found to have increased proliferation compared to cancers with wild-type BAT-RII. Assessment of cdk4, cyclin D1 and p27(kip1) expression revealed that only cdk4 expression was increased in the cancers with mutant BAT-RII. In order to determine if TGFBR2 inactivation was the cause of these changes, TGFBR2 was reconstituted in an MSI colon cancer cell line, resulting in decreased proliferation and decreased cdk4 expression and kinase activity. These results suggest that TGFBR2 mutations in primary colon cancers may be responsible for the increased proliferation and cdk4 expression in these tumors and provide evidence that deregulation of cdk4 is a pathogenic in vivo consequence of TGFBR2 inactivation in primary colon cancer.

Gene deregulation in gastric cancer

Despite its decreasing frequency in the Western world during recent decades, gastric cancer is still one of the leading causes of cancer-related deaths worldwide. Due to the oligosymptomatic course of early gastric cancer, most cases are diagnosed in the advanced stages of the disease. The curative potential of current standard treatment continues to be unsatisfactory, despite multimodal approaches involving surgery, chemotherapy and radiotherapy. Novel therapeutics including small molecules and monoclonal antibodies are being developed and have been partially introduced into clinical use in connection with neoplastic diseases such as chronic myeloid leukemia, non-Hodgkin's lymphoma and colorectal cancer. Thorough understanding of the changes in gene expression occurring during gastric carcinogenesis may help to develop targeted therapies and improve the treatment of this disease. Novel molecular biology techniques have generated a wealth of data on up- and down-regulation, activation and inhibition of specific pathways in gastric cancer. Here, we provide an overview of the different aspects of aberrant gene expression patterns in gastric cancer.

Biology of SNU cell lines

SNU (Seoul National University) cell lines have been established from Korean cancer patients since 1982. Of these 109 cell lines have been characterized and reported, i.e., 17 colorectal carcinoma, 12 hepatocellular carcinoma, 11 gastric carcinoma, 12 uterine cervical carcinoma, 17 B-lymphoblastoid cell lines derived from cancer patients, 5 ovarian carcinoma, 3 malignant mixed Mllerian tumor, 6 laryngeal squamous cell carcinoma, 7 renal cell carcinoma, 9 brain tumor, 6 biliary tract, and 4 pancreatic carcinoma cell lines. These SNU cell lines have been distributed to biomedical researchers domestic and worldwide through the KCLB (Korean Cell Line Bank), and have proven to be of value in various scientific research fields. The characteristics of these cell lines have been reported in over 180 international journals by our laboratory and by many other researchers from 1987. In this paper, the cellular and molecular characteristics of SNU human cancer cell lines are summarized according to their genetic and epigenetic alterations and functional analysis.

Association between endogenous gene expression and growth regulation induced by TGF-β1 in human gastric cancer cells

AIM: To investigate the association between endogenous gene expression and growth regulation including proliferation and apoptosis induced by transforming growth factor-β1 (TGF-β1) in human gastric cancer (GC) cells.

METHODS: Reverse transcription polymerase chain reaction (RT-PCR) was performed to detect the main components of the TGF-β1/Smads signal pathway in human poorly differentiated GC cell line BGC-823. Localization of Smad proteins was also determined using immunofluorescence. Then, the BGC-823 cells were cultured in the presence or absence of TGF-β1 (10 ng/mL) for 24 and 48 h, and the effects of TGF-β1 on proliferation and apoptosis were measured by cell growth curve and flow cytometry (FCM) analysis. The ultrastructural features of BGC-823 cells with or without TGF-β1 treatment were observed under transmission electron microscope. The apoptotic cells were visualized by means of the terminal deoxynucleotidyl transferase (TdT)-mediated dTUP in situ nick end-labeling (TUNEL) method. Meanwhile, the expression levels of endogenous p15,p21 and Smad7 mRNA and the corresponding proteins in the cells were detected at 1, 2 and 3 h after culture in the presence or absence of TGF-β1 (10 ng/mL) by semi-quantitative RT-PCR and Western blot, respectively.

RESULTS: The TGF-β1/Smad signaling was found to be intact and functional in BGC-823 cells. The growth curve revealed the most evident inhibition of cell proliferation by TGF-β1 at 48 h, and FCM assay showed G1 arrest accompanied with apoptosis induced by TGF-β1. The typical morphological changes of apoptosis were observed in cells exposed to TGF-β1. The apoptosis index (AI) in TGF-β1-treated cells was significantly higher than that in the untreated controls (10.7±1.3% vs 0.32±0.06%, P<0.01). The levels of p15,p21 and Smad7 mRNA and corresponding proteins in cells were significantly up-regulated at 1 h, but gradually returned to basal levels at 3 h following TGF-β1 (10 ng/mL) treatment.

CONCLUSION: TGF-β1 affects both proliferation and apoptosis of GC cells through the regulation of p15 and p21, and induces transient expression of Smad 7 as a negative feedback modulation of TGF-β1 signal. Our results suggest a novel functional role of p21 as an accelerant of TGF-β1-mediated apoptosis in GC cells.

Expression of transforming growth factor type II receptor in gastric carcinoma tissue

AIM: To investigate the expression of transforming growth factor type II receptor (TGFIIR) in the gastric carcinoma tissue.

METHODS: Expression of TGFIIR was studied in 20 cases of normal gastric tissues and 74 gastric carcinoma tissues by immunohistochemical method.

RESULTS: In normal gastric tissue, TGFIIR was mainly expressed in the lower part of the gland, especially in the cytoplasm, and the expresssion was significantly decreased in well and moderately differentiated adenocarcinoma tissues (P < 0.05), and in poorly differeantiated and mucinous or signet ring cell carcinoma tissues (P < 0.01) as compared with normal mucosa.

CONCLUSION: Decreased expression of TGFIIR may play a role in the development of gastric cancer.

DNA hypermethylation in gastric cancer

BACKGROUND

Transcriptional silencing of tumour suppressor genes by DNA hypermethylation plays a crucial role in the progression of gastric cancer. Many genes involved in the regulation of cell cycle, tissue invasion, DNA repair and apoptosis have been shown to be inactivated by this type of epigenetic mechanism.

RESULTS

Recent studies have demonstrated that DNA hypermethylation begins early in cancer progression, and in some cases, may precede the neoplastic process. Ageing is associated with DNA hypermethylation, and may provide a mechanistic link between ageing and cancer. Several reports have indicated that Epstein-Barr virus-related gastric cancer is associated with a high frequency of DNA hypermethylation, suggesting that viral oncogenesis might involve DNA hypermethylation with inactivation of tumour suppressor genes. Hypermethylation of hMLH1 with the resulting loss of its expression is known to cause microsatellite instability, which reflects genomic instability associated with defective DNA mismatch repair genes in the tumour.

CONCLUSIONS

In conclusion, recent studies demonstrate that DNA hypermethylation is a crucial mechanism of inactivation of tumour suppressor genes in gastric cancer. A better understanding of DNA hypermethylation will provide us with new opportunities in the diagnosis and therapy of gastric cancer.

Loss of the Smad3 expression increases susceptibility to tumorigenicity in human gastric cancer

Loss of the tumor suppressive effect of transforming growth factor-beta (TGF-beta) has been commonly found at later stages in carcinogenic progression. Although the genes encoding TGF-beta receptors and Smads have been found genetically altered in certain human cancers, no mutation in Smad3 has been observed. Therefore, suppression of Smad3 expression may mediate key oncogenic properties of TGF-beta. First, we observed that 37.5% of human gastric cancer tissues showed low to undetectable levels of Smad3 and that in nine human gastric cancer cell lines examined, two showed deficient Smad3 expression. Introduction of Smad3 into human gastric cancer cells that did not express Smad3, restored TGF-beta responsiveness: induction of p21 and p15 gene expression, and growth inhibition in response to TGF-beta. Furthermore, these Smad3-expressing cells showed markedly decreased and delayed tumorigenicity in vivo. These findings suggest that Smad3 expression may have a critical role in tumor suppression in the early stages of gastric carcinogenesis.

Aberrant signaling of TGF-beta1 by the mutant Smad4 in gastric cancer cells

TGF-beta1 has been known to suppress the growth of gastric cancer cells. Interestingly, TGF-beta1 treatment increased the proliferation of human gastric cancer cell line, SNU-216 cells, while it reduced the proliferation of other tumor cells including SNU-620 cells. TGF-beta1-mediated down-regulation of c-Myc and induction of p21CIP1 were observed in SNU-620, but there was no change in SNU-216 in response to TGF-beta1. Similarly, TGF-beta1 receptors were upregulated by TGF-beta1 treatment in SNU-620, but they were not responded in SNU-216. By a single strand conformation polymorphism analysis, a repeated insertion of 37 nucleotides in the exon 8 of Smad4, resulting in premature termination at codon 362, was found in SNU-216. Furthermore, this truncated Smad4 functioned as a dominant negative form in TGF-beta1-mediated reporter activity and TGF-beta1 receptor expression. However, the proliferation of tumor cells was not affected by Smad4 mutation, but it was modulated by PD98059. Taken together, a mutation in Smad4 in addition to mitogen-activated protein kinase altered the TGF-beta1-mediated signaling, which is one of key events of gastric tumorigenesis.

Role of TGF-beta 1 and TGF-beta type II receptor in gastric cancer

BACKGROUND

TGF-beta is known as a cell growth inhibitory factor to suppress almost all cells, including the epithelial cell. Unlike normal cells, cancer cells are not affected by TGF-beta growth inhibitory action and the lack of TGF-beta receptor expression or mutation is being reported as its mechanism, which is rarely studied in Korea. Therefore, we investigated this study to clarify the role of TGF-beta I and TGF-beta II receptors in gastric cancer.

METHODS

23 cases that underwent operations for gastric cancer provided RNA collected from their carcinoma tissues and adjacent normal tissues. We investigated the level of TGF-beta 1 and T beta R-II mRNA expression with semi-quantitatively reverse transcription PCR and analyzed the correlation with prognostic factors, such as tumor size, depth of invasion, tumor differentiation and lymph-node metastasis.

RESULTS

(1) TGF-beta 1 and T beta R-II mRNA were expressed in all carcinoma tissues and adjacent normal tissues of the 23 cases without statistical difference in the level of the expression. (2) The level of TGF-beta 1 mRNA expression was higher in patients with gastric cancer invaded only at the mucosa and submucosa than in patients with gastric cancer invaded over muscular propria, and also higher in the patients without lymph-node metastasis or perineural invasion than in the patients with lymph-node metastasis or perineural invasion. There was no significant correlation between the level of T beta R-II mRNA expression and several parameters, such as age, gender, tumor size, location, differentiation, Lauren's classification and vascular invasion. (3) There was a significant correlation between the level of TGF-beta 1 and T beta R-II mRNA expression in carcinoma tissues.

CONCLUSION

It indicated that TGF-beta 1 mRNA expression in gastric cancer might concern the early stage of gastric carcinogenesis and, unlike the earlier reports, it was higher in patients with early gastric cancer, negative lymph-nodes or negative perineural invasion. Further studies are required to clarify the role of TGF-beta 1 in gastric carcinogenesis with more patients.

Conditional loss of TGF-beta signalling leads to increased susceptibility to gastrointestinal carcinogenesis in mice

BACKGROUND

Downregulation of TGF-beta receptors is implicated in colon cancer development. Inactivation of either of the two transmembrane serine/threonine kinases, TGF-beta1 types I/II receptors, is now implicated in carcinogenesis, especially gastrointestinal carcinogenesis.

METHODS

We generated transgenic mice, called pS2-dnRII or ITF-dnRII, of which the dominant negative mutant of the TGF-beta type II receptor was expressed under the control of tissue-specific promoters, the pS2 promoter for stomach and ITF for intestine. They were either infected with H.pylori (ATCC 43504 strain, CagA+ and VacA+) or administered with azoxymethane to determine the significance of loss of TGF-beta signalling in gastrointestinal carcinogenesis.

RESULTS

Gastric adenocarcinoma developed in pS2-dnRII mice, whereas only chronic active gastritis was noted in wild-type littermates after 36 weeks of H.pylori infection. Mice lacking in TGF-beta signalling specifically in the stomach showed a significantly higher proliferation cell nuclear antigen-labelling index when infected with H.pylori than wild-type littermates (P < 0.01). Development of colonic aberrant crypt foci was provoked in mice by intraperitoneal injections of azoxymethane, and ITF-dnRII mice showed significantly higher incidences of ACF and colon cancers than wild-type littermates.

CONCLUSIONS

Maintaining normal TGF-beta signalling in the gastrointestinal tract seems to be important either for preventing abnormal mucosal proliferation, or for suppressing or retarding carcinogenesis.

A soluble transforming growth factor-beta (TGF-beta ) type I receptor mimics TGF-beta responses

Transforming growth factor-beta (TGF-beta) signaling requires a ligand-dependent interaction of TGF-beta receptors Tau beta R-I and Tau beta R-II. It has been previously demonstrated that a soluble TGF-beta type II receptor could be used as a TGF-beta antagonist. Here we have generated and investigated the biochemical and signaling properties of a soluble TGF-beta type I receptor (Tau beta RIs-Fc). As reported for the wild-type receptor, the soluble Tau beta R-I does not bind TGF-beta 1 on its own. Surprisingly, in the absence of TGF-beta1, the Tau beta RIs-Fc mimicked TGF-beta 1-induced transcriptional and growth responses in mink lung epithelial cells (Mv1Lu). Signaling induced by the soluble TGF-beta type I receptor is mediated via the obligatory presence of both TGF-beta type I and type II receptors at the cell surface since no signal was observed in Mv1Lu-derivated mutants for TGF-beta receptors R-1B and DR-26. The comparison between the structures of TGF-betas and a three-dimensional model of the extracellular domain of Tau beta RI has shown that five residues of the supposed binding site of TGF-beta 1 (Lys(31), His(34), Glu(5), Tyr(91), and Lys(94)) were found with equivalent biochemical properties and similar spatial positions.

The molecular basis for prevention of colorectal cancer

Colorectal cancer is the second leading cause of cancer death in the United States. Despite aggressive treatment and early screening strategies, the prognosis for patients with advanced disease remains poor. Extensive research examining familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC), two common forms of inherited colorectal cancer, have provided invaluable insights into some of the molecular mechanisms underlying both familial, as well as nonfamilial, colorectal cancer. The molecules involved in these pathways may provide effective targets for prevention and/or treatment of colorectal cancer. These targets include cyclooxygenase-2 (COX-2), peroxisome proliferator-activated receptor (PPAR)-delta, PPAR-gamma, transforming growth factor-beta receptor type II, epidermal growth factor receptor, and inducible-nitrous oxide synthase.

Sequence-specific enhancer binding protein is responsible for the differential expression of ERT/ESX/ELF-3/ESE-1/jen gene in human gastric cancer cell lines: Implication for the loss of TGF-beta type II receptor expression

Transcriptional repression of the TGF-beta type II receptor (RII) is one of the mechanisms leading to TGF-beta resistance. The newly identified epithelium-specific ets transcription factor ERT/ESX/ELF-3/ESE-1/jen binds to the TGF-beta RII promoter and induces promoter activity. The human gastric cancer cell lines, which show undetectable level of TGF-beta RII mRNA, do not express ERT mRNA. To study the molecular mechanisms of loss of ERT expression, we have cloned and characterized the human ERT promoter. DNA transfection experiments and electrophoretic mobility shift assays have revealed the existence of a distinct enhancer element (-186 to -177) which we named ESE (ERT promoter specific element). Deletion of the ESE markedly decreased expression of the target gene. ESE interacts with two distinct nuclear protein complexes, at least one of which appears to be inactivated in a cell line which does not express the ERT mRNA, compared to a cell line expressing the ERT mRNA. These results suggest the possibility that inactivation of the sequence-specific DNA binding protein to the region from -186 to -177 contributes to the loss of ERT expression, leading to the loss of TGF-beta type II receptor mRNA in human gastric cancer cell lines.

Nonsteroidal antiinflammatory drugs, cyclooxygenase-2, and colorectal cancer prevention

Colorectal cancer is the second leading cause of cancer deaths in the United States. Despite proven screening strategies, less than 40% of eligible Americans undergo appropriate screening for colorectal cancer. Research evaluating the underlying defects responsible for hereditary nonpolyposis colorectal cancer and familial adenomatous polyposis has provided insight into some of the molecular mechanisms responsible for familial and sporadic colorectal cancer. The signaling pathways involved in the development of colorectal cancer may provide effective targets for prevention and treatment. These targets include cyclooxygenase-2, peroxisome proliferator activated receptor- delta, peroxisome proliferator activated receptor- gamma, transforming growth factor- beta receptors, and the inducible-nitric oxide synthase.

Antisense to the Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP-1) sensitizes EBV-immortalized B cells to transforming growth factor-beta and chemotherapeutic agents

The Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP-1) is absolutely required for EBV transformation of B cells. LMP-1 mimics a constitutively activated receptor of the tumor necrosis factor receptor family, mediating diverse oncogenic functions that influence growth, differentiation and susceptibility to apoptosis. Given the critical functions of LMP-1 in EBV-associated transformation, it represents a rational therapeutic target for modulation. We used antisense oligodeoxynucleotides targeted to LMP-1 as a strategy to suppress LMP-1 expression and thereby inhibit its functions. In previous studies, we have shown that short-term treatment of EBV-positive lymphoblastoid cell lines (LCLs) with LMP-1 antisense oligodeoxynucleotides can dramatically reduce levels of LMP-1 protein in association with inhibition of proliferation, stimulation of apoptosis, down-regulation of Bcl-2 and Mcl-1 and enhanced sensitivity to the chemotherapeutic agent, etoposide. Here, we provide further evidence of the profound effects of reducing LMP-1 levels using antisense oligodeoxynucleotides in EBV-transformed B cells. We have shown that LMP-1 antisense treatment of LCLs partially restores sensitivity to the anti-proliferative and apoptotic effects of transforming growth factor-beta, a potent negative regulator of normal human B-cell growth, in association with a reduction in cyclin D2 levels. In addition, LMP-1 antisense sensitizes LCLs to chemotherapeutic drugs from diverse classes, including etoposide, vincristine and dexamethasone, by enhancing apoptotic cell death. Finally, the anti-proliferative and apoptotic effects of LMP-1 antisense treatment were observed not only in laboratory-derived LCLs, but also in an EBV-positive cell line derived from an AIDS-related lymphoma. These studies demonstrate that antisense targeting of LMP-1 represents a rational therapeutic strategy for EBV-positive lymphoproliferative disorders.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.

Transforming growth factor-β: pleiotropic role in the regulation of hematopoiesis

Hematopoiesis is a remarkable cell-renewal process that leads to the continuous generation of large numbers of multiple mature cell types, starting from a relatively small stem cell compartment. A highly complex but efficient regulatory network is necessary to tightly control this production and to maintain the hematopoietic tissue in homeostasis. During the last 3 decades, constantly growing numbers of molecules involved in this regulation have been identified. They include soluble cytokines and growth factors, cell–cell interaction molecules, and extracellular matrix components, which provide a multifunctional scaffolding specific for each tissue. The cloning of numerous growth factors and their mass production have led to their possible use for both fundamental research and clinical application.

Release from quiescence of primitive human hematopoietic stem/progenitor cells by blocking their cell-surface TGF-beta type II receptor in a short-term in vitro assay

Genetic alterations of the signaling cascade of transforming growth factor-beta (TGF-beta) are often associated with neoplastic transformation of primitive cells. This demonstrates the key role for this pleiotropic factor in the control of quiescence and cell proliferation in vivo. In the high proliferative potential-quiescent cell (HPP-Q) in vitro assay, the use of TGF-beta1 blocking antibodies (anti-TGF-beta1) allows the detection within two to three weeks of primitive hematopoietic cells called HPP-Q, which otherwise would not grow. However, the possibility of triggering cell proliferation by blocking the cell-surface TGF-beta receptors has not been investigated until now. We have tested here the efficiency of a blocking antibody against TGF-betaRII (anti-TGF-betaRII) on CD34(+)CD38(-) hematopoietic cells, a subpopulation enriched in primitive stem/progenitor cells, and compared its effect with that of anti-TGF-beta1. About twice as many HPP colony-forming cells were detected in the presence of anti-TGF-beta1 or anti-TGF-betaRII, compared to the control (p < 0.02). Moreover, anti-TGF-betaRII was as efficient as anti-TGF-beta1 for activating multipotent HPP-granulocyte erythroid macrophage megakaryocyte and HPP-Mix, bipotent HPP-granulocyte-macrophage (GM) and unipotent HPP-G, HPP-M and HPP-BFU-E. We therefore propose the use of anti-TGF-betaRII to release primitive cells from quiescence in the HPP-Q assay. This strategy could be extended to nonhematopoietic tissues, as TGF-beta1 may be a pleiotropic regulator of somatic stem cell quiescence.

Haploid loss of the tumor suppressor Smad4/Dpc4 initiates gastric polyposis and cancer in mice

The tumor suppressor SMAD4, also known as DPC4, deleted in pancreatic cancer, is a central mediator of TGF-beta signaling. It was previously shown that mice homozygous for a null mutation of Smad4 (Smad4-/-) died prior to gastrulation displaying impaired extraembryonic membrane formation and endoderm differentiation. Here we show that Smad4+/- mice began to develop polyposis in the fundus and antrum when they were over 6 - 12 months old, and in the duodenum and cecum in older animals at a lower frequency. With increasing age, polyps in the antrum show sequential changes from hyperplasia, to dysplasia, in-situ carcinoma, and finally invasion. These alterations are initiated by a dramatic expansion of the gastric epithelium where Smad4 is expressed. However, loss of the remaining Smad4 wild-type allele was detected only in later stages of tumor progression, suggesting that haploinsufficiency of Smad4 is sufficient for tumor initiation. Our data also showed that overexpression of TGF-beta1 and Cyclin D1 was associated with increased proliferation of gastric polyps and tumors. These studies demonstrate that Smad4 functions as a tumor suppressor in the gastrointestinal tract and also provide a valuable model for screening factors that promote or prevent gastric tumorigenesis.

Molecular mechanisms of inactivation of TGF-beta receptors during carcinogenesis

Signals from the TGF-betas are mediated by the TGF-beta receptors and their substrates, the Smad proteins. Inactivation of either of the two transmembrane serine/threonine kinases called the TGF-beta type I and type II receptors is now known to underlie a wide variety of human pathologies including, especially carcinogenesis. Numerous studies have now demonstrated that the TGF-beta receptor complex and its downstream signaling intermediates constitute a tumor suppressor pathway. We review here a specific pathway of mutational inactivation of the TGF-beta type II receptor resulting from microsatellite instability and demonstrate that, by contrast, the most common mechanism of loss of expression of the TGF-beta type II receptor involves transcriptional repression. This provides a new target for therapeutic intervention.