Cancer and Metastasis Reviews

Several lysosomal proteinases including the cysteine proteinase cathepsin B have been implicated in malignant progression of tumors. Many investigators have demonstrated correlations between increased activity of cathepsin B and increased metastatic capability of animal tumors or malignancy of human tumors. Such increases in cathepsin B activity in malignant tumors may reflect alterations in synthesis, in activation and processing, and/or in intracellular trafficking and delivery as well as in the endogenous inhibitors of cathepsin B. Increases in mRNA transcripts for cathepsin B have been observed in both murine and human tumors and multiple transcripts for cathepsin B have been identified, but an association of multiple transcripts with malignancy has not been confirmed. Cathepsin B precursors found in human malignant ascites fluid do not possess mannose-rich carbohydrates suggesting that a defect in the post translational processing of carbohydrate moieties on tumor cathepsin B may be responsible for the release of cathepsin B observed in many tumor systems. However, the intracellular trafficking of cathepsin B responsible for its association with plasma membrane/endosomal systems and for its release willrequire further study as both latent, precursor forms of cathepsin B and native forms of cathepsin B are involved. We speculate that malignant tumor cells adherent to basement membrane are capable of forming a digestive microenvironment in which lysosomal proteinases such as cathepsin B function optimally, a microenvironment similar to that formed between adherent osteoclasts and bone. One of the endogenous cysteine proteinase inhibitors, stefin A, also is affected by malignancy. Reduced expression (mRNA and protein) of stefin A is found as well as a reduction in its inhibitory capacity against cysteine proteinases. The data to date at both the molecular and protein levels supporting a functional role(s) for cathepsin B and its endogenous inhibitors in cancer progression are only correlative. Experimental approaches utilizing well-defined model systems in conjunction with genetic manipulation of cathepsin B and its endogenous inhibitors are needed to provide convincing evidence that cathepsin B has an important role in cancer.

In brain, breast, and other common human tumors there is a correlation between expression of the transcriptional activator hypoxia-inducible factor 1 (HIF-1) and tumor grade and vascularization. HIF-1 stimulates angiogenesis by activating transcription of the gene encoding vascular endothelial growth factor (VEGF). HIF-1 is a heterodimer consisting of a constitutively-expressed HIF-1β subunit and an O2– and growth factor-regulated HIF-1α subunit. Recent studies have demonstrated that HIF-1α expression is increased in tumor relative to normal tissue by two mechanisms. First, decreased intratumoral O2 concentrations provide a physiological stimulus. Second, genetic alterations that activate oncogene products or inactivate tumor suppressor gene products increase HIF-1α expression and/or HIF-1 transcriptional activity independent of the O2 concentration. Taken together, these recent data suggest that increased HIF-1 activity provides a molecular basis for VEGF-induced angiogenesis and other adaptations of cancer cells to hypoxia that are critical for establishment of a primary tumor and its progression to the lethal phenotype.

Current models suggest that at a certain but yet undefined time point of tumour development malignant cells with an aggressive phenotype start to disseminate via the blood stream into distant organs. This invasive phenotype appears to be associated with an epithelial–mesenchymal transition (EMT), which enables detachment of tumour cells from a primary site and migration. The reverse process of mesenchymal–epithelial transition (MET) might play a crucial role in the further steps of metastasis when circulating tumour cells (CTCs) settle down in distant organs and establish (micro-)metastasis. Nevertheless, the exact mechanisms and interplay of EMT and MET are only partially understood and their relevance in cancer patients is unclear. Research groups have just started to apply EMT-related markers in their studies on CTCs in cancer patients. In the present review, we summarize and discuss the current state of investigations on CTCs in the context of research on EMT/MET.

Gastrointestinal stromal tumors (GISTs) present 80% of gastrointestinal tract mesenchymal tumors, with systemic chemotherapy and radiotherapy being unable to improve survival of patients with advanced disease. The identification of activating mutations in either KIT cell surface growth factor receptor or platelet-derived growth factor receptor alpha, which lead to ligand-independent signal transduction, paved the way for the development of novel agents that selectively inhibit key molecular events in disease pathogenesis. The development of imatinib mesylate in the treatment of metastatic GIST represents a therapeutic breakthrough in molecularly targeted strategies, which crucially improved patients’ prognosis while its usefulness in adjuvant and neoadjuvant setting is under study. Sunitinib malate is available in the second-line setting, with ongoing studies evaluating its role in an earlier disease stage, while other targets are under intense investigation in order to enrich the therapeutical armamentarium for this disease. GIST phenotype seems to be an essential indicator of treatment response; thus, obtaining genotype information of each patient may be critical in order to tailor individualized treatment strategies and achieve maximal therapeutic results.

The formation of secondary tumors by circulating cancer cells (blood-borne metastasis) correlates with an increased tendency of the cells to form emboli by aggregation with other tumor cells or with host cells. Although it is evident that cell-cell recognition and adhesion are mediated by cell surface components, the identity of these molecules is only now being unraveled. Over the last decade an increasing number of studies have demonstrated the presence of endogenous carbohydrate-binding proteins on the surface of various normal cells, and it has been proposed that such lectin-like molecules might be involved in intercellular adhesion. We have shown that various tumor cell lines contain endogenous galactose-specific lectins. Lectin activity was detected at the cell surface by the binding of asialofetuin. This glycoprotein also enhanced the aggregation of the tumor cells. After purification by affinity chromatography on immobilized asialofetuin the lectin activity was associated with two proteins of Mr 14.500 and 34,000. By using polyclonal and monoclonal antilectin antibodies in conjunction with various immunologic techniques we have demonstrated that the endogenous lectins are present on the surface of different tumor cells. Quantitation of cell surface lectins by flow cytometric analyses of antilectin antibody binding revealed that among related tumor cells those exhibiting a higher metastatic potential expressed more lectin on their surface. The binding of monoclonal antilectin antibodies to metastatic cells decreased asialofetuin-induced homotypic aggregation in vitro and suppressed the ability of the cells to form lung metastases after intravenous injection in the tail vein of syngeneic mice. These results strongly implicate the tumor cell surface lectins in cell adhesion and metastasis. We propose that such lectins can increase the ability of tumor cells that enter the blood stream to form aggregates with other tumor cells, or to adhere to host cells or the extracellular matrix and thereby increase their metastatic potential. Other contributing components to tumor cell-host cell interactions are cell surface carbohydrate-binding proteins that have been detected on lymphocytes, platelets, macrophages, hepatocytes, and endothelial cells. These lectin-like molecules might recognize and bind carbohydrates expressed on the surface of tumor cells and enhance emboli formation and organ colonization.

To understand the mechanisms of tumour invasion a number of in vitro assays have been developed, in which test cells (invasive or non-invasive) have been confronted with normal cells or tissues (the host) and cultured in various ways. We will discuss here the methods by which invasion can be judged in such assays. Histology is the method that is used successfully by pathologists for the evaluation of tumour invasiveness in vivo. The major difficulty is that the relationship between the non-invasive counterpart and the host tissue is not known in vitro or in vivo. Some authors have attempted to quantify invasion on histologic sections, but a generally accepted method is not yet available. The potential contribution of time lapse cinephotography to the study of the kinetic aspects of invasion is obvious. Optical systems which overcome the relative lack of transparency of three-dimensional tissues, however, do not exist. Electron microscopy has contributed by providing details about the interactions between invasive cells on the one hand and normal cells as well as intercellular matrices on the other hand. Subpopulations of highly invasive and metastatic cells were collected after repeated migration through normal tissues in two-compartment assays. Attempts to evaluate the cytotoxic effect of invasive cells confronting radiolabelled cells or tissues have produced equivocal results. In contrast, measurements of radioactivity in the medium of labelled substrata have demonstrated the lytic effect of invasive cells on the extracellular matrix. It is concluded that further search for better methods to analyze and quantitate invasion is needed. So far, corroborative analysis by various methods seems to be the best policy.

The vascular endothelial growth factor (VEGF) family of polypeptide growth factors regulates a family of VEGF receptor (VEGFR) tyrosine kinases with pleiotropic downstream effects. Angiogenesis is the best known of these effects, but additional VEGF-dependent actions include increased vascular permeability, paracrine/autocrine growth factor release, enhancement of cell motility, and inhibition of apoptosis. In theory, therapeutic inhibition of angiogenesis should reduce tumor perfusion and thus increase tumor hypoxia and chemoresistance, but in clinical practice the VEGF antibody bevacizumab acts as a broad-spectrum chemosensitizer. Since VEGFR expression occurs in many tumor types, such chemosensitization is more readily explained by direct inhibition of tumor cell survival signals than by indirect stromal/vascular effects. The emerging model of anti-VEGF drug action being mediated primarily by tumoral (as distinct from endothelial) VEGFRs has clinically important implications for optimizing the anti-metastatic efficacy of this expanding drug class.

Cancer plasticity is now a recognized new hallmark of cancer which is due to disturbances of cell differentiation programs. It is manifested not only in various forms like the best-known epithelial-mesenchymal transition (EMT) but also in vasculogenic and megakaryocytic mimicries regulated by EMT-specific or less-specific transcription factors such as HIF1a or STAT1/2. Studies in the past decades provided ample data that cancer plasticity can be manifested also in the expression of a vast array of immune cell genes; best-known examples are PDL1/CD274, CD47, or IDO, and we termed it immunogenic mimicry (IGM). However, unlike other types of plasticities which are epigenetically regulated, expression of IGM genes are frequently due to gene amplifications. It is important that the majority of the IGM genes are regulated by interferons (IFNs) suggesting that their protein expressions are regulated by the immune microenvironment. Most of the IGM genes have been shown to be involved in immune escape of cancers broadening the repertoire of these mechanisms and offering novel targets for immunotherapeutics.

Epithelial ovarian cancer (EOC) is often diagnosed in advanced stage with peritoneal dissemination. Recent studies indicate that aberrant accumulation of collagen fibers in tumor stroma has a variety of effects on tumor progression. We refer to remodeled fibrous stroma with altered expression of collagen molecules, increased stiffness, and highly oriented collagen fibers as tumor-associated fibrosis (TAF). TAF contributes to EOC cell invasion and metastasis in the intraperitoneal cavity. However, an understanding of molecular events involved is only just beginning to emerge. Further development in this field will lead to new strategies to treat EOC. In this review, we focus on the recent findings on how the TAF contributes to EOC malignancy. Furthermore, we will review the recent initiatives and future therapeutic strategies for targeting TAF in EOC.