Opinion: migrating cancer stem cells - an integrated concept of malignant tumour progression

The dissemination of tumour cells is the prerequisite of metastases and is correlated with a loss of epithelial differentiation and the acquisition of a migratory phenotype, a hallmark of malignant tumour progression. A stepwise, irreversible accumulation of genetic alterations is considered to be the responsible driving force. But strikingly, metastases of most carcinomas recapitulate the organization of their primary tumours. Although current models explain distinct and important aspects of carcinogenesis, each alone can not explain the sum of the cellular changes apparent in human cancer progression. We suggest an extended, integrated model that is consistent with all aspects of human tumour progression - the 'migrating cancer stem (MCS)-cell' concept.

Invasion and metastasis in colorectal cancer: epithelial-mesenchymal transition, mesenchymal-epithelial transition, stem cells and beta-catenin

Invasion by colorectal carcinomas is characterized by an epithelial-mesenchymal transition (EMT)-like dedifferentiation of the tumor cells. However, a redifferentiation towards an epithelial phenotype, resembling a mesenchymal-epithelial transition, is detectable in metastases. This indicates that malignant progression is based on dynamic processes, which cannot be explained solely by irreversible genetic alterations, but must be additionally regulated by the tumor environment. The main oncoprotein in colorectal cancer is the Wnt pathway effector beta-catenin, which is overexpressed due to mutations in the APC tumor suppressor in most cases. EMT of the tumor cells is associated with a nuclear accumulation of the transcriptional activator beta-catenin, which is reversed in metastases. Nuclear beta-catenin is involved in two fundamental processes in embryonic development: EMT and stem cell formation. Accumulating data demonstrate that aberrant nuclear expression of beta-catenin can also confer these two abilities to tumor cells, thereby driving malignant tumor progression.

The transcriptional repressor ZEB1 promotes metastasis and loss of cell polarity in cancer

Invasion and metastasis are the hallmarks of malignant tumor progression and the main cause of death in cancer. The embryonic program "epithelial-mesenchymal transition" (EMT) is thought to trigger invasion by allowing tumor cell dissemination. Here, we describe that the EMT-inducing transcriptional repressor ZEB1 promotes colorectal cancer cell metastasis and loss of cell polarity. Thereby, ZEB1 suppresses the expression of cell polarity factors, in particular of Lgl2, which we found reduced in colorectal and breast cancers. We further show that retention of Lgl2 expression is critical for the epithelial phenotype and that its loss might be involved in metastasis. Thus, by linking EMT, loss of polarity, and metastasis, ZEB1 is a crucial promoter of malignant tumor progression.

A transient, EMT-linked loss of basement membranes indicates metastasis and poor survival in colorectal cancer

BACKGROUND & AIMS

Loss of the basement membrane (BM) is considered an important step toward tumor malignancy. However, the BM is still expressed in most typical colorectal adenocarcinomas; nevertheless, these tumors can invade and develop metastases. The aim of this study was to investigate the role, mechanisms, and clinical relevance of BM turnover in malignant colorectal cancer (CRC) progression.

METHODS

Expression of BM components and their transcriptional regulation and clinical relevance were investigated in human CRCs and cell lines.

RESULTS

Our data show new aspects in BM turnover in CRCs with impact on malignant tumor progression: (1) The BM is still expressed in the main tumor mass of most colorectal adenocarcinomas, but selectively lost at invasive regions of the tumor in many cases. (2) Selective loss of the BM at the invasive front has high clinical and tumor biologic relevance for distant metastasis and survival. (3) The BM is reexpressed in metastases, indicating that its loss is transient and regulated by environmental factors. (4) This transient loss is not only due to proteolytic breakdown but to a down-regulated synthesis and linked to an epithelial-mesenchymal transition (EMT) in tumor cells, and, thereby, zinc-finger-enhancer protein 1 (ZEB1) is the crucial transcriptional repressor of BM components in CRCs.

CONCLUSIONS

A transient BM loss at the invasive front is correlated with increased distant metastasis and poor patient survival, indicating its tumor biologic relevance and usefulness as a prognostic marker. Targeting ZEB1 might be a promising therapeutic option to prevent metastasis.

Expression of L1-CAM and ADAM10 in human colon cancer cells induces metastasis

L1-CAM, a neuronal cell adhesion receptor, is also expressed in a variety of cancer cells. Recent studies identified L1-CAM as a target gene of beta-catenin-T-cell factor (TCF) signaling expressed at the invasive front of human colon cancer tissue. We found that L1-CAM expression in colon cancer cells lacking L1-CAM confers metastatic capacity, and mice injected in their spleen with such cells form liver metastases. We identified ADAM10, a metalloproteinase that cleaves the L1-CAM extracellular domain, as a novel target gene of beta-catenin-TCF signaling. ADAM10 overexpression in colon cancer cells displaying endogenous L1-CAM enhanced L1-CAM cleavage and induced liver metastasis, and ADAM10 also enhanced metastasis in colon cancer cells stably transfected with L1-CAM. DNA microarray analysis of genes induced by L1-CAM in colon cancer cells identified a cluster of genes also elevated in a large set of human colon carcinoma tissue samples. Expression of these genes in normal colon epithelium was low. These results indicate that there is a gene program induced by L1-CAM in colon cancer cells that is also present in colorectal cancer tissue and suggest that L1-CAM can serve as target for colon cancer therapy.

Down-regulation of the homeodomain factor Cdx2 in colorectal cancer by collagen type I: an active role for the tumor environment in malignant tumor progression

The homeobox transcription factor Cdx2 specifies intestinal development and homeostasis and is considered a tumor suppressor in colorectal carcinogenesis. However, Cdx2 mutations are rarely found. Invasion of colorectal cancer is characterized by a transient loss of differentiation and nuclear accumulation of the oncoprotein beta-catenin in budding tumor cells. Strikingly, this is reversed in growing metastases, indicating that tumor progression is a dynamic process that is not only driven by genetic alterations but also regulated by the tumor environment. Here we describe a transient loss of Cdx2 in budding tumor cells at the tumor host interface, and reexpression of Cdx2 in metastases. Cell culture experiments show that collagen type I, through beta(1) integrin signaling, triggers a transient transcriptional down-regulation of Cdx2 and its intestine-specific target gene sucrase isomaltase, associated with a loss of differentiation. These data indicate an active role for the tumor environment in malignant tumor progression.

Beta-catenin activates a coordinated expression of the proinvasive factors laminin-5 gamma2 chain and MT1-MMP in colorectal carcinomas

In colorectal carcinomas, loss-of-function mutations of the adenomatous polyposis coli (APC) tumor suppressor gene lead to a nuclear accumulation of the oncogenic transcriptional activator beta-catenin, predominantly at the invasive front within the tumor host interface. Various identified genes activated by beta-catenin are associated with tumor invasion. One prerequisite for malignant tumor invasion is the ability of tumor cells to migrate. We recently described the gamma2 chain of laminin as another beta-catenin target gene. Fragments of the laminin gamma2 chain, resulting from cleavage by the membrane type 1 matrix metalloproteinase (MT1-MMP), are strong inducers of epithelial cell migration. We here show a coordinated expression of nuclear beta-catenin, its target gene and MT1-MMP substrate laminin gamma2 chain, as well as MT1-MMP in tumor cells at invasive regions of colorectal carcinomas. We further demonstrate that MT1-MMP expression is regulated by beta-catenin/TCF through a TCF binding site in its promoter. These results suggest that nuclear beta-catenin activates the coordinated expression of the interacting proinvasive proteins laminin gamma2 chain and MT1-MMP, thereby leading to a promigratory activity at the invasive front of colorectal cancers. This further supports an important role of beta-catenin for invasion and metastasis of colorectal carcinomas.

A comparative analysis of the interactions of the E6 proteins from cutaneous and genital papillomaviruses with p53 and E6AP in correlation to their transforming potential

Common necessity for all papillomaviruses is to induce DNA synthesis in quiescent cells. This is commonly achieved by the E7 gene product, which interferes with the function of members of the retinoblastoma family controlling transition from the G1-phase to the S-phase of the cell cycle. Uncontrolled entry into S-phase activates, however, negative growth control signals which have to be bypassed to achieve production of progeny viruses. In addition to inherent activities of the E7 protein, high risk genital types encode an E6 protein that overcomes p53-mediated G1-arrest and apoptosis in concert with the cellular factor E6AP by targeting p53 for the enhanced ubiquitin-dependent degradation. The key question, which of these functions of genital E6 and E7 proteins is responsible for the carcinogenic phenotype, is still not completely answered. In contrast to high risk genital types no immortalizing or transforming activities have been found for the E7 proteins of the high risk cutaneous HPV8 and 47. On the other hand the ability of the E6 protein to transform established rodent fibroblasts seems to be a property shared by high risk genital and cutaneous types. To examine the existence of a common E6-mediated transforming pathway for both virus groups we compared the properties of the cutaneous E6 proteins with already known functions of E6 proteins of genital viruses. For this we analyzed the E6 proteins of low nak and high risk cutaneous and genital papillomaviruses with respect to cell transformation, to their abilities to bind, degradate, and influence the activity of human p53, and to bind E6AP. The results of our study demonstrate a clear lack of interaction between the transforming E6 proteins of HPV1 and HPV8 and both cellular proteins p53 and E6AP. In contrast, we found E6AP-independent binding of HPV16 E6 and HPV6 E6 to p53, although both proteins were different in their transforming potential. Of all four proteins investigated, only HPV16 E6 was able to bind to p53 and E6AP and to induce degradation of the p53 protein in the reticulocyte system. When we investigated in frame deletion mutants of the E6 protein of HPV16 for their abilities to bind to p53 or E6AP, degradate, and inhibit the transactivation function of p53 and to transform rodent fibroblasts, no correlation between the different activities could be found. Mutants still able to bind p53 and E6AP lacked transforming ability and other mutants that were transformation-competent were deficient in p53 and E6AP bindings.

Evidence for acid-induced loss of Cdx2 expression in duodenal gastric metaplasia

Gastric metaplasia in the duodenum (GMD) is characterized by transdifferentiation of intestinal epithelial cells into gastric foveolar cells within the duodenal mucosa. GMD is often associated with duodenal ulceration. Higher duodenal acidity due to increased gastric acid output into the duodenum has been implicated in the development of GMD. Intestinal development and homeostasis are controlled by the homeobox transcription factor Cdx2, which is considered to be the master regulator of intestinal differentiation. Using immunohistochemistry, the present study shows that GMD is associated with loss of expression of Cdx2 and its target gene product sucrase-isomaltase. Quantitative RT-PCR experiments using the intestinal cell line Caco2 revealed that Cdx2 and sucrase-isomaltase were down-regulated and gastric mucins MUC5AC and MUC6 were up-regulated under acidic culture conditions. Thus, it is suggested that increased acid exposure leads to GMD by impairing the transcription of Cdx2 and subsequently that of its intestine-specific target genes.

Wnt/FZD signaling and colorectal cancer morphogenesis

Malignant progression of colorectal carcinomas is characterized by an epithelial-mesenchymal transition (EMT)-like de-differentiation of the invading tumor cells. However a re-differentiation towards an epithelial phenotype, resembling a mesenchymal-epithelial transition (MET), is detectable in metastases. This indicates that malignant progression is based on dynamic processes, which can not be explained solely by irreversible genetic alterations, but must be additionally regulated by the tumor environment. The main oncoprotein in colorectal cancer is the Wnt-pathway effector beta-catenin, which in most cases is overexpressed due to mutations in the adenomatous polyposis coli (APC) tumor suppressor. EMT of tumor cells is associated with a nuclear accumulation of the transcriptional activator beta-catenin, which is reversed in metastases. Nuclear beta-catenin is involved in two fundamental processes in embryonic development: EMT and stem cell formation. Accumulating data demonstrate that aberrant nuclear expression of beta-catenin can also confer these two abilities to tumor cells, indicating the crucial role of aberrant Wnt-signaling for malignant tumor progression.

Securin (hPTTG1) expression is regulated by beta-catenin/TCF in human colorectal carcinoma

Overexpression of the transcriptional activator β-catenin, mostly owing to loss-of-function mutations of the adenomatous polyposis coli (APC) tumour suppressor gene, is crucial for the initiation and progression of human colorectal carcinogenesis. Securin is a regulator of chromosome separation and its overexpression has been shown to be involved in different tumour-promoting processes, like transformation, hyperproliferation and angiogenesis, and correlates with tumour cell invasion. However, the molecular mechanism leading to securin overexpression in human colorectal cancer is unknown. Here we show a correlated high expression of β-catenin and securin (hPTTG1) in colorectal adenomas and carcinomas and further demonstrate that securin is a target of β-catenin transcriptional activation. This implies that deregulation of the β-catenin/T-cell factor-signalling pathway leads to overexpression of securin in human colorectal cancer, which subsequently may contribute to tumour progression.

Deletion of gpUL132, a structural component of human cytomegalovirus, results in impaired virus replication in fibroblasts

The coding capacity of human cytomegalovirus (HCMV) for glycoproteins by far exceeds that of other herpesviruses. Few of these proteins have been characterized so far. We have investigated the gene product of reading frame UL132. The putative protein product of UL132 is a glycoprotein with a theoretical mass of 29.8 kDa. Transcription analysis revealed that the gene is transcribed with a true late kinetics from the laboratory-adapted strain AD169 and the low-passage isolate TB40E. Two proteins of 22 to 28 kDa and 45 to 60 kDa were detected in virus-infected cells as well as in extracellular virions. The larger protein carried N-linked carbohydrates. Both protein forms were present in laboratory-adapted strains as well as in low-passage isolates of HCMV. Recombinant viruses with the UL132 gene deleted were constructed in the low-passage HCMV isolate PAN as well as the high-passage isolate AD169. Deletion of UL132 from either genome resulted in a pronounced replication deficit with a reduction of approximately 100-fold for HCMV strain AD169. Thus, the protein product of the UL132 reading frame represents a structural viral glycoprotein of HCMV that has an important function for viral replication in tissue culture.

Variable FZD7 expression in colorectal cancers indicates regulation by the tumour microenvironment

Recent evidence shows that a sub-population of Wnt/beta-catenin target genes is specifically induced in different tissue contexts. FZD7 is a putative Wnt/beta-catenin target gene and although it is highly expressed in well-differentiated colorectal cancer tumour cells, its expression is decreased in de-differentiated tumour cells at the invasive front despite elevated Wnt/beta-catenin signalling in this area. This variable expression of FZD7 implicates additional regulation by the microenvironment; however, this has not been investigated. To begin to elucidate the role of extracellular matrix in regulating FZD7 expression, we generated a FZD7 promoter reporter and analysed FZD7 promoter activity in colorectal cancer cells grown on different matrices. We demonstrate that the FZD7 promoter is regulated by beta-catenin in colorectal cancer cells and observed decreased promoter activity in cells grown on fibronectin but not collagen I or collagen IV. Thus, expression of FZD7 in colorectal cancer may be regulated by fibronectin in the microenvironment.

Identification of glycoprotein gpTRL10 as a structural component of human cytomegalovirus

Human cytomegalovirus (HCMV) has a coding capacity for glycoproteins which far exceeds that of other herpesviruses. Few of these proteins have been characterized. We have investigated the gene product(s) of reading frame 10, which is present in both the internal and terminal repeat regions of HCMV strain AD169 and only once in clinical isolates. The putative protein product is a 171-amino-acid glycoprotein with a theoretical mass of 20.5 kDa. We characterized the protein encoded by this reading frame in the laboratory strain AD169 and a recent isolate, TB40E. The results from both strains were comparable. Northern blot analyses showed that the gene was transcribed with early/late kinetics. Two proteins of 22 and 23.5-kDa were detected in virus-infected cells and in cells transiently expressing recombinant TRL10. Both forms contained only high-mannose-linked carbohydrate modifications. In addition, virus-infected cells expressed small amounts of the protein modified with complex N-linked sugars. Image analysis localized transiently expressed TRL10 to the endoplasmic reticulum. Immunoblot analyses as well as immunoelectron microscopy of purified virions demonstrated that TRL10 represents a structural component of the virus particle. Immunoblot analysis in the absence of reducing agents indicated that TRL10, like the other HCMV envelope glycoproteins, is present in a disulfide-linked complex. Sequence analysis of the TRL10 coding region in nine low-passage clinical isolates revealed strain-specific variation. In summary, the protein product of the TRL10 open reading frame represents a novel structural glycoprotein of HCMV and was termed gpTRL10.

Precancerous lesions upon sporadic activation of beta-catenin in mice

BACKGROUND & AIMS

Inappropriate activation of beta-catenin in adult tissues is associated with a wide variety of cancers, especially in the digestive tract. Classic transgenic and knockout murine models in which beta-catenin is activated in large fields of cells have provided experimental support in favor of a role for this molecule in tumorigenesis. However, these models do not reproduce the sporadic nature of the majority of human cancers, beginning with the activation of an oncogene at random in a single cell.

METHODS

We used the "hit and run" strategy to generate a mouse model in which the expression of an activated form of beta-catenin occurs sporadically in vivo.

RESULTS

Sporadic, multifocal lesions were observed in the stomach of 3% of mice aged 8 months and older. These lesions were associated with loss of Sonic hedgehog (Shh), and a causal relationship between beta-catenin activation and Shh inhibition was established in gastric cells in vitro. No lesion was detected in the intestine or in the liver. In addition, one third of female mutant mice developed benign perimammary papillomas. Mutant mice were also hypersensitive to chemically induced premalignant skin lesions.

CONCLUSIONS

These results challenge the view that activation of beta-catenin induces malignant cancerogenesis, because they show in mice that sporadically activated beta-catenin is sufficient for tumor initiation, yet without further malignant progression, and that it sensitizes cells to environmental hits. This model represents a powerful tool to investigate the interplay between genetic and environmental factors in tumor progression.

Glycoprotein gpTRL10 of human cytomegalovirus is dispensable for virus replication in human fibroblasts

Human cytomegalovirus (HCMV) has a coding capacity for glycoproteins which by far exceeds that of other herpesviruses. Few of these proteins have been characterized. Glycoprotein gpTRL10 represents a structural component of HCMV. The protein has no homologous counterparts in other herpesviruses. We have isolated bacterial artificial chromosomes (BACs) harboring the genome from the low passage clinical isolate PAN and constructed a deletion mutant in reading frame TRL10. Our results show that deletion of gpTRL10 has only minimal effects on viral replication in human fibroblasts.

A reciprocal repression between ZEB1 and members of the miR-200 family promotes EMT and invasion in cancer cells

The embryonic programme 'epithelial-mesenchymal transition' (EMT) is thought to promote malignant tumour progression. The transcriptional repressor zinc-finger E-box binding homeobox 1 (ZEB1) is a crucial inducer of EMT in various human tumours, and was recently shown to promote invasion and metastasis of tumour cells. Here, we report that ZEB1 directly suppresses transcription of microRNA-200 family members miR-141 and miR-200c, which strongly activate epithelial differentiation in pancreatic, colorectal and breast cancer cells. Notably, the EMT activators transforming growth factor beta2 and ZEB1 are the predominant targets downregulated by these microRNAs. These results indicate that ZEB1 triggers an microRNA-mediated feedforward loop that stabilizes EMT and promotes invasion of cancer cells. Alternatively, depending on the environmental trigger, this loop might switch and induce epithelial differentiation, and thus explain the strong intratumorous heterogeneity observed in many human cancers.

Epithelial-mesenchymal and mesenchymal-epithelial transitions during cancer progression

Invasion by colorectal carcinomas is characterized by an epithelial-mesenchymal transition (EMT)-like de-differentiation of the tumor cells. However a re-differentiation towards an epithelial phenotype, resembling a mesenchymal-epithelial transition (MET) is detectable in metastases. This indicates that malignant progression is based on dynamic processes, which can not be explained solely by irreversible genetic alterations, but must be additionally regulated by the tumor environment. The main oncoprotein in colorectal cancer is the Wnt-pathway effector beta-catenin, which is overexpressed due to mutations in the APC tumor suppressor in most cases. EMT of the tumor cells is associated with a nuclear accumulation of the transcriptional activator beta-catenin, which is reversed in metastases. Nuclear beta-catenin is involved in two fundamental processes in embryonic development: EMT and stem cell formation. Accumulating data demonstrate that aberrant nuclear expression of beta-catenin can confere these two abilites also to tumor cells. The unusual combination of EMT with stem cell competence might result in a migrating tumor stem cell, which drives tumor invasion and metastasis.