Dissemination and growth of cancer cells in metastatic sites

Metastasis suppressor pathways--an evolving paradigm

A greater understanding of the processes of tumor invasion and metastasis, the principal cause of death in cancer patients, is essential to determine newer therapeutic targets. Metastasis suppressor genes, by definition, suppress metastasis without affecting tumorigenicity and, hence, present attractive targets as prognostic or therapeutic markers. This short review focuses on those twelve metastasis suppressor genes for which functional data exist. We also outline newly identified genes that bear promising traits of having metastasis suppressor activity, but for which functional data have not been completed. We also summarize the biochemical mechanism(s) of action (where known), and present a working model assembling potential metastasis suppression pathways.

Membrane type I matrix metalloproteinase usurps tumor growth control imposed by the three-dimensional extracellular matrix

Cancer cells are able to proliferate at accelerated rates within the confines of a three-dimensional (3D) extracellular matrix (ECM) that is rich in type I collagen. The mechanisms used by tumor cells to circumvent endogenous antigrowth signals have yet to be clearly defined. We find that the matrix metalloproteinase, MT1-MMP, confers tumor cells with a distinct 3D growth advantage in vitro and in vivo. The replicative advantage conferred by MT1-MMP requires pericellular proteolysis of the ECM, as proliferation is fully suppressed when tumor cells are suspended in 3D gels of protease-resistant collagen. In the absence of proteolysis, tumor cells embedded in physiologically relevant ECM matrices are trapped in a compact, spherical configuration and unable to undergo changes in cell shape or cytoskeletal reorganization required for 3D growth. These observations identify MT1-MMP as a tumor-derived growth factor that regulates proliferation by controlling cell geometry within the confines of the 3D ECM.

Transforming growth factor beta signaling impairs Neu-induced mammary tumorigenesis while promoting pulmonary metastasis

The influence of transforming growth factor beta (TGF-beta) signaling on Neu-induced mammary tumorigenesis and metastasis was examined with transgenic mouse models. We generated mice expressing an activated TGF-beta type I receptor or dominant negative TGF-beta type II receptor under control of the mouse mammary tumor virus promoter. When crossed with mice expressing activated forms of the Neu receptor tyrosine kinase that selectively couple to the Grb2 or Shc signaling pathways the activated type I receptor increased the latency of mammary tumor formation but also enhanced the frequency of extravascular lung metastasis. Conversely, expression of the dominant negative type II receptor decreased the latency of Neu-induced mammary tumor formation while significantly reducing the incidence of extravascular lung metastases. These observations argue that TGF-beta can promote the formation of lung metastases while impairing Neu-induced tumor growth and suggest that extravasation of breast cancer cells from pulmonary vessels is a point of action of TGF-beta in the metastatic process.

Metastatic potential: generic predisposition of the primary tumor or rare, metastatic variants-or both?

Metastasis is a rare event. Does it arise from rare, variant, highly metastatic cells or does a primary tumor progress to a premalignant state from which metastases arise stochastically without further changes in gene expression? Arguments and evidence have been adduced to support either position. A paper in this month's Cancer Cell and other arguments instead suggest models combining features of both.

A role for LIM kinase in cancer invasion

In this study, we show that LIM kinase 1 (LIMK1), a critical regulator of actin dynamics, plays a regulatory role in tumor cell invasion. We found that the level and activity of endogenous LIMK1 is increased in invasive breast and prostate cancer cell lines in comparison with less invasive cells. Overexpression of LIMK1 in MCF-7 and in MDA-MB-231 human breast cancer cell lines increased their motility, whereas the specific ROCK and Rho inhibitors Y-27632 and C3, respectively, attenuated this effect. In addition, inhibition of LIMK1 activity in the MDA-MB-231 cells by expression of dominant-negative LIMK1 resulted in decreased motility and formation of osteolytic bone lesions in an animal model of tumor invasion. This study shows an important role for LIMK1 signaling in invasion of cancer, demonstrating its potential as a therapeutic molecular target to decrease metastasis.

Liver metastasis of cancer facilitated by chemokine receptor CCR6

When injected subcutaneously, mouse plasmacytoma (MOPC315) grew rapidly in situ, and metastatic cells became detectable first in the lymph nodes (LNs) and bone marrow, and later in the liver and lungs. We studied MOPC315 cell migration by tracking metastatic cells labelled with green fluorescent protein (GFP). We measured the levels of their chemokine receptor mRNA (by semiquantitative and real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), because chemokines can regulate organ predilection of metastasis. Freshly sorted metastatic cells and tumour cell lines derived from the liver of BALB/c mice overexpressed functional CCR6 and CCR7 molecules compared with primary tumour. Preincubation with the CCR6 ligand (CCL20) induced liver-sorted tumour cells to preferentially colonize the liver, demonstrating an association between liver metastasis and CCR6 expression in the mouse. Because the liver is a common site for metastasis, second only to draining LNs, we wished to ascertain whether this finding could be generalized, i.e. whether other cancers can use the similar mechanism of metastasis to the liver, and whether it holds true for humans. We found that CCR6 is overexpressed in small liver metastases of colon, thyroid and ovarian carcinomas compared with normal liver. Because human liver constitutively expresses CCL20, it could attract and select CCR6+ cancer cells. We suggest that chemotaxis via CCR6 might be a common mechanism by which malignant cancers metastasize to the liver. As metastasis in patients with cancer poses the biggest peril for survival, inhibition of CCR6 signalling, either during or after medical or surgical treatment, might be useful in preventing liver metastasis.

A multigenic program mediating breast cancer metastasis to bone

We investigated the molecular basis for osteolytic bone metastasis by selecting human breast cancer cell line subpopulations with elevated metastatic activity and functionally validating genes that are overexpressed in these cells. These genes act cooperatively to cause osteolytic metastasis, and most of them encode secreted and cell surface proteins. Two of these genes, interleukin-11 and CTGF, encode osteolytic and angiogenic factors whose expression is further increased by the prometastatic cytokine TGF beta. Overexpression of this bone metastasis gene set is superimposed on a poor-prognosis gene expression signature already present in the parental breast cancer population, suggesting that metastasis requires a set of functions beyond those underlying the emergence of the primary tumor.

Tumour-cell invasion and migration: diversity and escape mechanisms

Cancer cells possess a broad spectrum of migration and invasion mechanisms. These include both individual and collective cell-migration strategies. Cancer therapeutics that are designed to target adhesion receptors or proteases have not proven to be effective in slowing tumour progression in clinical trials--this might be due to the fact that cancer cells can modify their migration mechanisms in response to different conditions. Learning more about the cellular and molecular basis of these different migration/invasion programmes will help us to understand how cancer cells disseminate and lead to new treatment strategies.

Apoptosis, proliferation, differentiation: in search of the order

In search of the order, we are tempted to universally link cell death, proliferation, differentiation, and senescence. Current models (classical, conflicting signal and quantitative signal models) are restricted, precisely because they attempt to hardware a plethora of end-points of cellular responses. By defining each cellular process in molecular term, one can disconnect proliferation (CDK activation), apoptosis (caspase activation), and differentiation (tissue function genes expression), even though these responses are linked by upstream signal transduction pathways. These ambivalent pathways (e.g. mitogen-activated pathways) simultaneously transduce opposite signals (for growth arrest and cycling, for cell death and survival), which are ultimately translated in all possible combinations of cellular responses. When depicted in multidimensional axis, this universal model may also include invasiveness, senescence, metastatic and angiogenic responses and even such integral characteristics as malignant transformation.

Hypoxia promotes invasive growth by transcriptional activation of the met protooncogene

Hypoxia unleashes the invasive and metastatic potential of tumor cells by largely unknown mechanisms. The Met tyrosine kinase, a high affinity receptor for hepatocyte growth factor (HGF), plays a crucial role in controlling invasive growth and is often overexpressed in cancer. Here we show that: (1) hypoxia activates transcription of the met protooncogene, resulting in higher levels of Met; (2) hypoxic areas of tumors overexpress Met; (3) hypoxia amplifies HGF signaling; (4) hypoxia synergizes with HGF in inducing invasion; (5) the proinvasive effects of hypoxia are mimicked by Met overexpression; and (6) inhibition of Met expression prevents hypoxia-induced invasive growth. These data show that hypoxia promotes tumor invasion by sensitizing cells to HGF stimulation, providing a molecular basis to explain Met overexpression in cancer.

Role of calcium in E-selectin induced phenotype of T84 colon carcinoma cells

The adhesion of cancer cells to the endothelium during the metastatic process involves the interaction of specific cell-cell adhesion receptors on the cell surface. E-selectin on endothelial cells and sialyl Lewis X carbohydrate component on tumor cells are mainly implicated in the adhesion of colon carcinoma cells to the endothelium of target organ. In this paper we show that binding of E-selectin to T84 colon tumor cells causes approximately a twofold increase in intracellular calcium concentration. In particular, using two inhibitors of receptor operated calcium channels, CAI and SK&F 96365, we present evidences that the augmentation in cytoplasmic calcium originates from ionic influx from extracellular sources. Furthermore, we demonstrated that modulation of [Ca2+]i by engagement of E-selectin receptor starts signal transduction pathways that affect cell spreading, tyrosine phosphorylation signaling, and cancer cell motility.

Basic and translational advances in cancer metastasis: Nm23

Cancer metastasis is a significant contributor to breast cancer patient morbidity and mortality. To develop new anti-metastatic therapies, we need to understand the biological and biochemical mechanisms of metastasis. Toward these efforts, we and others have studied metastasis suppressor genes, which halt metastasis in vivo without affecting primary tumor growth. The first metastasis suppressor gene confirmed was nm23, also known as NDP kinase. Using in vitro assays, nm23 overexpression resulted in reduced anchorage-independent colonization in response to TGF-beta, reduced invasion and motility in response to multiple factors, and increased differentiation. We hypothesize that the mechanism of action of Nm23 in metastasis suppression involves diminished signal transduction, downstream of a particular receptor. We hypothesize that a histidine protein kinase activity of Nm23 underlies its suppression of metastasis, and identify candidate substrates. This review also discusses therapeutic options on the basis of reexpression of metastasis suppressors.

Metastasis suppressors alter the signal transduction of cancer cells

Tumour metastasis is a significant contributor to death in cancer patients. Eight metastasis-suppressor genes that reduce the metastatic propensity of a cancer cell line in vivo without affecting its tumorigenicity have been identified. These affect important signal-transduction pathways, including mitogen-activated protein kinases, RHO, RAC and G-protein-coupled and tyrosine-kinase receptors. So how might we use this knowledge to improve the treatment of patients with cancer?

Molecular pathology of tumor metastasis III. Target array and combinatorial therapies

Therapy of tumor progression and the metastatic disease is the biggest challenge of clinical oncology. Discovery of the diverse molecular pathways behind this complex disease outlined an approach to better treatment strategies. The development of combined cytotoxic treatment protocols has produced promising results but no breakthrough in the clinical management of metastatic disease. The multiple - specific and non-specific pathways and cellular targets of tumor progression are outlined in this review. Such an approach, individually designed for various cancer types, may have a better chance to treat or even cure cancer patients with progressive disease.

Expression pattern of CXC chemokine receptor-4 is correlated with lymph node metastasis in human invasive ductal carcinoma

BACKGROUND

The stromal cell-derived factor-1/CXC chemokine receptor-4 (SDF-1/CXCR4) signal has been shown to be important in various immunological reactions. Recent studies have suggested that CXCR4 is expressed in certain cancer cells and that they use this chemokine receptor efficiently for metastasis formation.

METHOD

The expression of CXCR4 was evaluated by immunohistochemical study in 79 surgically resected invasive ductal carcinomas, and the relation between the staining pattern and clinicopathological features was examined.

RESULTS

CXCR4 was diffusely and homogeneously expressed in 59 cancers, which were further divided into 28 high-expression and 31 low-expression cancers by their staining intensity. The other 20 cancers showed heterogeneous immunoreactivity in tumor tissue, which was defined as focal type. In comparison with the diffuse type, focal type tumors showed significantly more extensive lymph node metastasis, because the number and extent of metastatic nodes were larger in the focal than the diffuse type. In the diffuse type, the rate of node-positive cases did not show a difference in staining intensity. However, high-CXCR4 tumors showed more extensive nodal metastasis in comparison with low-expression tumors. In contrast, the expression pattern of CXCR4 did not have a significant correlation with hematogeneous metastasis. The overall survival of these patients tended to be better in the diffuse type than in the focal type, although the difference was not statistically significant.

CONCLUSION

The expression pattern of CXCR4 was significantly correlated with the degree of lymph node metastasis in breast cancers. Our data suggest that CXCR4 might be particularly important in facilitating metastasis through the lymphatic system.

The Systemic Progression of Human Cancer: A Focus on the Individual Disseminated Cancer Cell—The Unit of Selection

The metastatic progression of solid tumors is discussed controversially. Because metastasis is usually lethal, it appears as an end point of successive cellular changes. This has led to the prevailing interpretation that genetic changes, in addition to those present in the most advanced clone of the primary tumor, are required to initiate invasion, dissemination, and growth at anatomically distant sites. It has become possible to detect and analyze single disseminated cancer cells at ectopic sites long before metastasis can be diagnosed by standard clinical techniques. Because the finding of single disseminated cancer cells correlates with the subsequent development of distant metastasis, these cells have been identified as the precursors of metastasis. Their direct molecular-genetic characterization, however, shows that dissemination occurs very early in the process of accumulation of genetic changes and suggests that metastases may seldom be derived from the dominant clone of the primary tumor. In contrast, it appears that cancer cell evolution explores a multitude of variant cells from which systemic cancer can develop independently. This review integrates data derived by different approaches into a model of systemic cancer progression.

Motility cues in the tumor microenvironment

It is now increasingly recognized that the microenvironment plays a critical role in the progression of tumors. Perhaps less obvious is the concept that the microenvironment may share responsibility in determining the "malignant" traits of tumor cells, i.e. invasiveness and metastasis. If tumors are tissues, however unbalanced, rather than a collection of "malignant" cells recruiting local resources for the purpose of growth, then it is inevitable that tumor cells will respond to local stimuli. These stimuli include cues for motility and migration, which normally appear in tissues undergoing formation, remodeling or healing. Carcinoma cells are likely to be sensitive to the motility cues that normally regulate epithelial morphogenetic movements such as ingression, delamination, invagination, and tube or sheet migration. "Malignant" tumors, then, can be redefined as those in which these cues arise more frequently or act more effectively. Here, we expand on this view and propose that invasion and metastasis may be the outcome of tumor cell responses to microenvironmental motility cues. Understanding how such motility cues arise and act, both in normal and tumor tissue, should be a high priority in cancer research.

Musculoaponeurotic variations along the course of the median nerve in the proximal forearm

31 cadaver arms have been dissected to study the variations in the anatomy of the muscles and fibrous arches which might cause compression of the median nerve in the forearm. Pronator teres always had a superficial head and usually a deep head. Flexor digitorum superficialis varied greatly in its site of origin. The median nerve might be crossed by two, one or no fibro-aponeurotic arches. Gantzer's muscle, an accessory head of flexor pollicis longus, was present in 45% of cadavers. No ligament of Struthers was found. Possible sites and causes of nerve compression are discussed.