Claudin-6: a novel receptor for CPE-mediated cytotoxicity in ovarian cancer.

Claudins are integral tight junction proteins that are responsible for maintaining the integrity of epithelial cell architecture and cell polarity. Claudin-3 and -4 are overexpressed in several cancers and have been shown to act as receptors for the Clostridium perfringens enterotoxin (CPE), a toxin that causes rapid cell lysis. CPE has demonstrated effectiveness in treating several different cancers in mouse models, provided that these cancers express claudin-3 or claudin-4. Here, we show that claudin-3/4 expression is not an absolute requirement for CPE action and, through overexpression and knockdown experiments, we identify claudin-6 as a novel functional receptor for CPE. Indeed, UCI-101, an ovarian cancer cell line highly sensitive to CPE, does not express claudin-3/4 and knockdown of claudin-6 in these cells decreases CPE sensitivity. Moreover, two different ovarian cell lines that are resistant to the effects of CPE can be made sensitive through claudin-6 overexpression. Binding assays show that CPE can indeed bind claudin-6 in cells and that this binding is associated with CPE cytotoxicity. Multicellular tumor spheroids experiments demonstrate that claudin-6 can also be a target of CPE in three-dimensional cultures. Our data establish claudin-6 as a novel receptor for CPE and introduces the possibility of a novel targeted therapeutic for ovarian and other cancers that express claudin-6.

Claudin-1 overexpression in melanoma is regulated by PKC and contributes to melanoma cell motility.

Serial analysis of gene expression followed by pathway analysis implicated the tight junction protein claudin-1 (CLDN1) in melanoma progression. Tight junction proteins regulate the paracellular transport of molecules, but staining of a tissue microarray revealed that claudin-1 was overexpressed in melanoma, and aberrantly expressed in the cytoplasm of malignant cells, suggesting a role other than transport. Indeed, melanoma cells in culture demonstrate no tight junction function. It has been shown that protein kinase C (PKC) can affect expression of claudin-1 in rat choroid plexus cells, and we observed a correlation between levels of activated PKC and claudin expression in our melanoma cells. To determine if PKC could affect the expression of CLDN1 in human melanoma, cells lacking endogenous claudin-1 were treated with 200 nM phorbol myristic acid (PMA). PKC activation by PMA caused an increase in CLDN1 transcription in 30 min, and an increase in claudin-1 protein by 12 h. Inhibition of PKC signaling in cells with high claudin-1 expression resulted in decreased claudin-1 expression. CLDN1 appears to contribute to melanoma cell invasion, as transient transfection of melanoma cells with CLDN1 increased metalloproteinase 2 (MMP-2) secretion and activation, and subsequently, motility of melanoma cells as demonstrated by wound-healing assays. Conversely, knockdown of CLDN1 by siRNA resulted in the inhibition of motility, as well as decreases in MMP-2 secretion and activation. These data implicate claudin-1 in melanoma progression.

Gene expression response to cisplatin treatment in drug-sensitive and drug-resistant ovarian cancer cells.

The molecular pathways activated in response to acute cisplatin exposure, as well as the mechanisms involved in the long-term development of cisplatin-resistant cancer cells remain unclear. Using whole genome oligonucleotide microarrays, we have examined the kinetics of gene expression changes in a cisplatin-sensitive cell line, A2780, and its cisplatin-resistant derivative, ACRP. Both sensitive and resistant cell lines exhibited a very similar response of p53-inducible genes as early as 16 h after treatment. This p53 response was further increased at the 24-h time point. These experiments identify p53 as the main pathway producing a large-scale transcriptional response after cisplatin treatment in these cells containing wild-type p53. Consistent with a role for the p53 response in cisplatin sensitivity, knockdown of the p53 protein with small interfering RNA led to a twofold decrease in cell survival in the resistant cells. In addition, our analysis also allowed the identification of several genes that are differentially expressed between sensitive and resistant cells. These genes include GJA1 (encoding connexin 43 (Cx43)) and TWIST1, which are highly upregulated in cisplatin-resistant cells. The importance of Cx43 in drug resistance was demonstrated through functional analyses, although paradoxically, inhibition of Cx43 function in high expressing cells led to an increase in drug resistance. The pathways important in cisplatin response, as well as the genes found differentially expressed between cisplatin-resistant and -sensitive cells, may represent targets for therapy aimed at reversing drug resistance.

A novel homozygous deletion at chromosomal band 6q27 in an ovarian cancer cell line delineates the position of a putative tumor suppressor gene.

Chromosomal band 6q27 is believed to contain a tumor suppressor gene important in the development of several cancer types, including ovarian cancer. However, repeated efforts to identify a tumor suppressor gene in this region have been unsuccessful. Because homozygous deletions have been useful in the positional cloning of a number of tumor suppressor genes, we initiated a systematic search for such deletions in ovarian cancer cell lines using 6q microsatellite markers. One of the cell lines, OV167, was found to contain an 80 kb homozygous deletion encompassing marker D6S193 at 6q27 but excluding nearby marker D6S297. No known genes were present in the deleted region. Because the homozygous deletion might affect the expression of nearby genes, we analyzed the expression of the two closest known genes flanking the deletion, RNASE6PL and RSK-3. The expression of these genes were unaffected by the homozygous deletion, suggesting that the functional target of the deletion is located between these two genes. A search of the region against expressed sequence tag (EST) databases revealed that it contained four sets of expressed sequences. The first expressed sequences were derived from a LINE repetitive element and were considered unlikely to represent a tumor suppressor gene. The other expressed sequence tags identified did not show homology to known genes and are currently being investigated. This data may significantly reduce the magnitude of the search for the 6q tumor suppressor gene as it suggests a small area as a prime target for investigation.

Coordinately up-regulated genes in ovarian cancer.

A better understanding of the molecular circuitry in normal ovarian tissues and in ovarian cancer will likely provide new targets for diagnosis and therapy. Recently, much has been learned about the genes expressed in ovarian cancer through studies with cDNA arrays and serial analysis of gene expression. However, these methods do not allow highly quantitative analysis of gene expression on a large number of specimens. Here, we have used quantitative real-time RT-PCR in a panel of 39 microdissected ovarian carcinomas of various subtypes to systematically analyze the expression of 13 genes, many of which were previously identified as up-regulated in a subset of ovarian cancers by serial analyses of gene expression. The genes analyzed are glutathione peroxidase 3 (GPX3), apolipoprotein J/clusterin, insulin-like growth factor-binding protein 2, epithelial cell adhesion molecule/GA733-2, Kop protease inhibitor, matrix gla protein, tissue inhibitor of metalloproteinase 3, folate receptor 1, S100A2, signal transducer and activator of transcription 1, secretory leukocyte protease inhibitor, apolipoprotein E, and ceruloplasmin. All of the genes were found overexpressed, some at extremely high levels, in the vast majority of ovarian carcinomas irrespective of the subtype. Interestingly, GPX3 was found at much higher levels in tumors with clear cell histology and may represent a biomarker for this subtype. Some of the genes studied here may thus represent targets for early detection ovarian cancer. The gene expression patterns were not associated with age at diagnosis, stage, or K-ras mutation status in ovarian cancer. We find that several genes are coordinately regulated in ovarian cancer, likely representing the fact that many genes are activated as part of common signaling pathways or that extensive cross-talk exists between several pathways in ovarian cancer. A statistical analysis shows that genes commonly up-regulated in ovarian cancer may result from the aberrant activation of a limited number of pathways, providing promising targets for novel therapeutic strategies.

Large-scale serial analysis of gene expression reveals genes differentially expressed in ovarian cancer.

Difficulties in the detection, diagnosis, and treatment of ovarian cancer result in an overall low survival rate of women with this disease. A better understanding of the pathways involved in ovarian tumorigenesis will likely provide new targets for early and effective intervention. Here, we have used serial analysis of gene expression (SAGE) to generate global gene expression profiles from various ovarian cell lines and tissues, including primary cancers, ovarian surface epithelia cells, and cystadenoma cells. The profiles were used to compare overall patterns of gene expression and to identify differentially expressed genes. We have sequenced a total of 385,000 tags, yielding >56,000 genes expressed in 10 different libraries derived from ovarian tissues. In general, ovarian cancer cell lines showed relatively high levels of similarity to libraries from other cancer cell lines, regardless of the tissue of origin (ovarian or colon), indicating that these lines had lost many of their tissue-specific expression patterns. In contrast, immortalized ovarian surface epithelia and ovarian cystadenoma cells showed much higher similarity to primary ovarian carcinomas than to primary colon carcinomas. Primary tissue specimens therefore appeared to be a better model for gene expression analyses. Using the expression profiles described above and stringent selection criteria, we have identified a number of genes highly differentially expressed between nontransformed ovarian epithelia and ovarian carcinomas. Some of the genes identified are already known to be overexpressed in ovarian cancer, but several represent novel candidates. Many of the genes up-regulated in ovarian cancer represent surface or secreted proteins such as claudin-3 and -4, HE4, mucin-1, epithelial cellular adhesion molecule, and mesothelin. Interestingly, both apolipoprotein E (ApoE) and ApoJ, two proteins involved in lipid homeostasis, are among the genes highly up-regulated in ovarian cancer. Selected serial analysis of gene expression results were further validated through immunohistochemical analysis of ApoJ, claudin-3, claudin-4, and epithelial cellular adhesion molecule in archival material. These experiments provided additional evidence of the relevance of our findings in vivo. The publicly available expression data reported here should stimulate and aid further research in the field of ovarian cancer.

A frequent deletion polymorphism on chromosome 22q13 identified by representational difference analysis of ovarian cancer.

Sizable homozygous deletions (>100 bp) of genomic DNA in cancer cells are typically interpreted as an indication of the location of a tumor suppressor gene. In an effort to identify novel ovarian growth-suppressing genes, we performed representational difference analysis (RDA) of ovarian cancer cells. One of the RDA probes identified a 276-bp region of chromosome 22q deleted in 47% of the ovarian cancer cell lines examined. This small deletion was also found in the genomic DNA of 25% of colon cancer cell lines examined and, surprisingly, in 18% of the blood DNA samples from healthy controls. The deleted allele, which was named u22q, has a frequency of approximately 50% in the population and is in Hardy-Weinberg equilibrium with the intact allele. The deleted DNA sequence is flanked by direct repeats and likely originated through a slipped mispairing mechanism. The deletion did not encompass known transcripts or expressed sequence tags. It therefore appears likely that u22q represents a common polymorphism, often hemizygous in ovarian cancer because of a high rate of LOH of chromosome 22q. These findings provide an example of a sizable homozygous deletion that does not appear to be associated with disease. Such a finding provides a cautionary tale for positional cloning projects initiated exclusively on the basis of the identification of homozygous deletions. The possibility that the deletions in question may be constitutive should always be considered since it is probable that the genome contains a large number deletions/insertions of various sizes.

Rare activation of the TCF/beta-catenin pathway in ovarian cancer.

OBJECTIVE: The activation of the T cell factor/beta-catenin pathway is a crucial event in colon cancer initiation. A recent report describing the presence of beta-catenin mutations in endometrioid ovarian cancer suggested that the TCF/beta-catenin pathway may be generally activated in ovarian cancer. We therefore undertook to determine the frequency of activation of this pathway in ovarian cancer cell lines using a functional screen. METHODS: We functionally screened a series of ovarian cancer cell lines for the presence of constitutive TCF/beta-catenin-mediated transcriptional activity using a reporter assay. Lines possessing such activity were subjected to mutational and gel-shift analysis, as well as sensitivity to the introduction of dominant-negative TCF or APC alleles. A cDNA harboring a beta-catenin point mutation found in an ovarian cancer line was incorporated into an expression plasmid for functional analysis. RESULTS: Constitutive TCF/beta-catenin transcriptional activity was detected in 21% (4 of 19) of ovarian lines studied, while 32% (6 of 19) exhibited greater than twofold repression. One of the constitutively active lines, UCI107, harbored an activating beta-catenin point mutation, which was shown to be capable of inducing TCF/beta-catenin transcriptional activity in transiently transfected 293 cells. A second active line, SW626, was shown to harbor an inactivating APC mutation and may in fact be of colonic origin. The third and fourth lines harbored neither an APC nor a beta-catenin mutation. Gel-shift analysis, together with the absence of sensitivity to dominant-negative TCF, indicated that the reporter activity exhibited by the latter two cell lines may not be due to a TCF/beta-catenin transcriptional complex. CONCLUSIONS: These results indicate that genuine constitutive activation of the TCF/beta-catenin pathway is infrequent in ovarian cancer, but that constitutive transcriptional repression from TCF sites is more common in this tumor type.

Truncation of the extracellular region abrogrates cell contact but retains the growth-suppressive activity of E-cadherin.

E-cadherin has been demonstrated to induce growth suppression and decrease the invasiveness of cancer cells and thus has been proposed to be a tumor suppressor gene. The ability of E-cadherin to mediate cell-cell contact and contact inhibition presumably accounts for its antitumor effects, which are attributed to the extracellular domain of the protein. Here we report that blocking the ability of E-cadherin to mediate contact inhibition by either antagonistic antibodies or expression of a mutant form of E-cadherin with the extracellular region deleted does not abrogate growth suppression. Transfection of the E-cadherin gene into the human prostate cancer cell line TSU.Pr-1 induced cell-cell contact formation, growth suppression, and redistribution of beta-catenin to the cell membrane. Treatment of the E-cadherin transfectant (CAD) with blocking antibodies disrupted cell-cell contact formation but did not influence the growth rate, suggesting that cell-cell interaction is not required for E-cadherin-mediated growth suppression. Similarly, transfection of an E-cadherin construct in which the NH2-terminal (extracellular) region was deleted did not allow cell-cell contact formation but induced growth suppression. In contrast, transfection of an E-cadherin construct in which the COOH-terminal (cytoplasmic) region was deleted did not induce suppression but promoted cell contact formation. In cells expressing E-cadherin lacking the cytoplasmic region, beta-catenin was evenly distributed in the cytoplasm. By contrast, in cells expressing E-cadherin lacking the extracellular region, beta-catenin was cell membrane associated. Growth suppression was always associated with the localization of beta-catenin to the cell membrane. The redistribution of beta-catenin from the cytoplasm to the cell membrane initially suggested the involvement of the Wnt signaling pathway in regulating cell growth. However, only small differences in beta-catenin/T-cell factor signaling were detected in control and E-cadherin-expressing cells, suggesting that the Wnt pathway is not involved. Taken together, these findings suggest that E-cadherin-induced growth inhibition may not be solely attributed to contact inhibition but may involve the redistribution of beta-catenin from the cytoplasm to the cell membrane, and this redistribution may affect growth pathways independent of T-cell factor.

The Yin-Yang of TCF/beta-catenin signaling.

Wingless/Wnt signaling directs cell-fate choices during embryonic development. In Drosophila, Wingless signaling mediates endoderm induction and the establishment of segment polarity in the developing embryo. The fly Wingless cascade is strikingly similar to the vertebrate Wnt signaling pathway, which controls a number of key developmental decisions such as dorsal-ventral patterning in Xenopus. Factors of the TCF/LEF HMG domain family (Tcfs) have recently been established as the downstream effectors of the Wingless/Wnt signal transduction pathways. Upon Wingless/Wnt signaling, a cascade is initiated that results in the accumulation of cytoplasmic beta-catenin (or its fly homolog, Armadillo). There is also a concomitant translocation of beta-catenin/Armadillo to the nucleus, where it interacts with a specific sequence motif at the N terminus of Tcfs to generate a transcriptionally active complex. This bipartite transcription factor is targeted to the upstream regulatory regions of Tcf target genes including Siamois and Nodal related gene-3 in Xenopus, engrailed and Ultrabithorax in Drosophila via the sequence-specific HMG box, and mediates their transcriptional activation by virtue of transactivation domains contributed by beta-catenin/Armadillo. In the absence of Wingless/Wnt signals, a key negative regulator of the pathway, GSK3 beta, is activated, which mediates the downregulation of cytoplasmic beta-catenin/Armadillo via the ubiquitin-proteasome pathway. In the absence of nuclear beta-catenin, the Tcfs recruit the corepressor protein Groucho to the target gene enhancers and actively repress their transcription. An additional corepressor protein, CREB-binding protein (CBP), may also be involved in this repression of Tcf target gene activity. Several other proteins, including adenomatous polyposis coli (APC), GSK3 beta, and Axin/Conductin, are instrumental in the regulation of beta-catenin/Armadillo. In APC-deficient colon carcinoma cell lines, beta-catenin accumulates and is constitutively complexed with nuclear Tcf-4. A proportion of APC wild-type colon carcinomas and melanomas also contains constitutive nuclear Tcf-4/beta-catenin complexes as a result of dominant mutations in the N terminus of beta-catenin that render it insensitive to downregulation by APC, GSK3 beta, and Axin/Conductin. This results in the unregulated expression of Tcf-4 target genes such as c-myc. Based on the established role for Tcf-4 in maintaining intestinal stem cells it is likely that deregulation of c-myc expression as a result of constitutive Tcf-4/beta-catenin activity promotes uncontrolled intestinal cell proliferation. This would readily explain the formation of intestinal polyps during colon carcinogenesis. Similar mechanisms leading to deregulation of Tcf target gene activity are likely to be involved in melanoma and other forms of cancer.

A public database for gene expression in human cancers.

A public database, SAGEmap, was created as a component of the Cancer Genome Anatomy Project to provide a central location for depositing, retrieving, and analyzing human gene expression data. This database uses serial analysis of gene expression to quantify transcript levels in both malignant and normal human tissues. By accessing SAGEmap (http://www.ncbi.nlm.nih.gov/SAGE) the user can compare transcript populations between any of the posted libraries. As an initial demonstration of the database's utility, gene expression in human glioblastomas was compared with that of normal brain white matter. Of the 47,174 unique transcripts expressed in these two tissues, 471 (1.0%) were differentially expressed by more than 5-fold (P<0.001). Classification of these genes revealed functions consistent with the biological properties of glioblastomas, in particular: angiogenesis, transcription, and cell cycle related genes.

beta-catenin signaling and cancer.

Since its discovery as a protein associated with the cytoplasmic region of E-cadherin, beta-catenin has been shown to perform two apparently unrelated functions: it has a crucial role in cell-cell adhesion in addition to a signaling role as a component of the Wnt/wg pathway. Wnt/wg signaling results in beta-catenin accumulation and transcriptional activation of specific target genes during development. It is now apparent that deregulation of beta-catenin signaling is an important event in the genesis of a number of malignancies, such as colon cancer, melanoma, hepatocellular carcinoma, ovarian cancer, endometrial cancer, medulloblastoma pilomatricomas, and prostate cancer. beta-catenin mutations appear to be a crucial step in the progression of a subset of these cancers, suggesting an important role in the control of cellular proliferation or cell death. The APC/beta-catenin pathway is highly regulated and includes players such as GSK3-beta, CBP, Groucho, Axin, Conductin, and TCF. c-MYC and cyclin D1 were recently identified as a key transcriptional targets of this pathway and additional targets are likely to emerge. Published 1999 John Wiley & Sons, Inc.

CDX2 is mutated in a colorectal cancer with normal APC/beta-catenin signaling.

The majority of human colorectal cancers have elevated beta-catenin/TCF regulated transcription due to either inactivating mutations of the APC tumor suppressor gene or activating mutations of beta-catenin. Surprisingly, one commonly used colorectal cancer cell line was found to have intact APC and beta-catenin and no demonstrable beta-catenin/TCF regulated transcription. However, this line did possess a truncating mutation in one allele of CDX2, a gene whose inactivation has recently been shown to cause colon tumorigenesis in mice. Expression of CDX2 was found to be induced by restoring expression of wild type APC in a colorectal cancer cell line. These findings raise the intriguing possibility that CDX2 contributes to APC's tumor suppressive effects.

Frequent silencing of the GPC3 gene in ovarian cancer cell lines.

GPC3 encodes a glypican integral membrane protein and is mutated in the Simpson-Golabi-Behmel syndrome. Simpson-Golabi-Behmel syndrome, an X-linked condition, is characterized by pre- and postnatal overgrowth as well as by various other abnormalities, including increased risk of embryonal tumors. The GPC3 gene is located at Xq26, a region frequently deleted in advanced ovarian cancers. To determine whether GPC3 is a tumor suppressor in ovarian neoplasia, we studied its expression and mutational status in 13 ovarian cancer cell lines. No mutations were found in GPC3, but its expression was lost in four (31%) of the cell lines analyzed. In an of the cases where GPC3 expression was lost, the GPC3 promoter was hypermethylated, as demonstrated by Southern analysis. Expression of GPC3 was restored by treatment of the cells with the demethylating agent 5-aza-2'-deoxycytidine. A colony-forming assay confirmed that ectopic GPC3 expression inhibited the growth of ovarian cancer cell lines. Our results show that GPC3, a gene involved in the control of organ growth, is frequently inactivated in a subset of ovarian cancers and suggest that it may function as a tumor suppressor in the ovary.

Identification of c-MYC as a target of the APC pathway.

The adenomatous polyposis coli gene (APC) is a tumor suppressor gene that is inactivated in most colorectal cancers. Mutations of APC cause aberrant accumulation of beta-catenin, which then binds T cell factor-4 (Tcf-4), causing increased transcriptional activation of unknown genes. Here, the c-MYC oncogene is identified as a target gene in this signaling pathway. Expression of c-MYC was shown to be repressed by wild-type APC and activated by beta-catenin, and these effects were mediated through Tcf-4 binding sites in the c-MYC promoter. These results provide a molecular framework for understanding the previously enigmatic overexpression of c-MYC in colorectal cancers.

Mutational analysis of the APC/beta-catenin/Tcf pathway in colorectal cancer.

Mutation of the adenomatous polyposis coli (APC) tumor suppressor gene initiates the majority of colorectal (CR) cancers. One consequence of this inactivation is constitutive activation of beta-catenin/Tcf-mediated transcription. To further explore the role of the APC/beta-catenin/Tcf pathway in CR tumorigenesis, we searched for mutations in genes implicated in this pathway in CR tumors lacking APC mutations. No mutations of the gamma-catenin (CTNNG1), GSK-3alpha (GSK3A), or GSK-3beta (GSK3B) genes were detected. In contrast, mutations in the NH2-terminal regulatory domain of beta-catenin (CTNNB1) were found in 13 of 27 (48%) CR tumors lacking APC mutations. Mutations in the beta-catenin regulatory domain and APC were observed to be mutually exclusive, consistent with their equivalent effects on beta-catenin stability and Tcf transactivation. In addition, we found that CTNNB1 mutations can occur in the early, adenomatous stage of CR neoplasia, as has been observed previously with APC mutations. These results suggest that CTNNB1 mutations can uniquely substitute for APC mutations in CR tumors and that beta-catenin signaling plays a critical role in CR tumorigenesis.

Mechanisms underlying nonsteroidal antiinflammatory drug-mediated apoptosis.

Nonsteroidal antiinflammatory drugs (NSAIDs) can inhibit colorectal tumorigenesis and are among the few agents known to be useful for the chemoprevention of neoplasia. Here, we show that the tumor suppressive effects of NSAIDs are not likely to be related to a reduction in prostaglandins but rather are due to the elevation of the prostaglandin precursor arachidonic acid (AA). NSAID treatment of colon tumor cells results in a dramatic increase in AA that in turn stimulates the conversion of sphingomyelin to ceramide, a known mediator of apoptosis. These results have significant implications for understanding and improving colon cancer chemoprevention.