Tumor Metastasis: Molecular Insights and Evolving Paradigms

Metastases represent the end products of a multistep cell-biological process termed the invasion-metastasis cascade, which involves dissemination of cancer cells to anatomically distant organ sites and their subsequent adaptation to foreign tissue microenvironments. Each of these events is driven by the acquisition of genetic and/or epigenetic alterations within tumor cells and the co-option of nonneoplastic stromal cells, which together endow incipient metastatic cells with traits needed to generate macroscopic metastases. Recent advances provide provocative insights into these cell-biological and molecular changes, which have implications regarding the steps of the invasion-metastasis cascade that appear amenable to therapeutic targeting. Metastases represent the end products of a multistep cell-biological process termed the invasion-metastasis cascade, which involves dissemination of cancer cells to anatomically distant organ sites and their subsequent adaptation to foreign tissue microenvironments. Each of these events is driven by the acquisition of genetic and/or epigenetic alterations within tumor cells and the co-option of nonneoplastic stromal cells, which together endow incipient metastatic cells with traits needed to generate macroscopic metastases. Recent advances provide provocative insights into these cell-biological and molecular changes, which have implications regarding the steps of the invasion-metastasis cascade that appear amenable to therapeutic targeting. Whereas surgical resection and adjuvant therapy can cure well-confined primary tumors, metastatic disease is largely incurable because of its systemic nature and the resistance of disseminated tumor cells to existing therapeutic agents. This explains why > 90% of mortality from cancer is attributable to metastases, not the primary tumors from which these malignant lesions arise (Gupta and Massagué, 2006Gupta G.P. Massagué J. Cancer metastasis: building a framework.Cell. 2006; 127: 679-695Abstract Full Text Full Text PDF PubMed Scopus (1128) Google Scholar, Steeg, 2006Steeg P.S. Tumor metastasis: mechanistic insights and clinical challenges.Nat. Med. 2006; 12: 895-904Crossref PubMed Scopus (739) Google Scholar). Thus, our ability to effectively treat cancer is largely dependent on our capacity to interdict—and perhaps even reverse—the process of metastasis. These clinical realities have been appreciated for decades. Yet, only recently have molecular and cell-biological details of the mechanisms underlying metastasis emerged. We focus here on the tumors arising in epithelial tissues—carcinomas—which together constitute ∼80% of life-threatening cancers. We highlight recent discoveries, discuss their conceptual implications, and consider their potential clinical utility. Taken together, these advances have established new paradigms that are likely to guide future research on metastasis, as well as the development of new diagnostic and therapeutic strategies. The metastases spawned by carcinomas are formed following the completion of a complex succession of cell-biological events—collectively termed the invasion-metastasis cascade—whereby epithelial cells in primary tumors: (1) invade locally through surrounding extracellular matrix (ECM) and stromal cell layers, (2) intravasate into the lumina of blood vessels, (3) survive the rigors of transport through the vasculature, (4) arrest at distant organ sites, (5) extravasate into the parenchyma of distant tissues, (6) initially survive in these foreign microenvironments in order to form micrometastases, and (7) reinitiate their proliferative programs at metastatic sites, thereby generating macroscopic, clinically detectable neoplastic growths (the step often referred to as "metastatic colonization") (Figure 1) (Fidler, 2003Fidler I.J. The pathogenesis of cancer metastasis: the 'seed and soil' hypothesis revisited.Nat. Rev. Cancer. 2003; 3: 453-458Crossref PubMed Scopus (1578) Google Scholar). As discussed below, many of these complex cell-biological events are orchestrated by molecular pathways operating within carcinoma cells. Importantly, cell-nonautonomous interactions between carcinoma cells and nonneoplastic stromal cells also play vital roles throughout the invasion-metastasis cascade (Figure 2). Deregulation of these intrinsic and extrinsic signaling cascades allows incipient metastatic carcinoma cells to generate high-grade, life-threatening malignancies.Figure 2Stromal Cells Play Vital Roles during the Invasion-Metastasis CascadeShow full captionMetastatic progression is not an exclusively cell-autonomous process. Indeed, carcinoma cells enlist nonneoplastic stromal cells to aid in each step of the invasion-metastasis cascade. Examples of the roles of stromal cells during metastasis are illustrated. Carcinoma cells are depicted in red. Angptl4, angiopoietin-like 4; CSF-1, colony-stimulating factor 1; EGF, epidermal growth factor; IL-4, interleukin 4; MMP-9, matrix metalloproteinase 9; OPN, osteopontin; SDF-1, stromal cell-derived factor 1.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Metastatic progression is not an exclusively cell-autonomous process. Indeed, carcinoma cells enlist nonneoplastic stromal cells to aid in each step of the invasion-metastasis cascade. Examples of the roles of stromal cells during metastasis are illustrated. Carcinoma cells are depicted in red. Angptl4, angiopoietin-like 4; CSF-1, colony-stimulating factor 1; EGF, epidermal growth factor; IL-4, interleukin 4; MMP-9, matrix metalloproteinase 9; OPN, osteopontin; SDF-1, stromal cell-derived factor 1. Local invasiveness involves entry of cancer cells that have resided within a well-confined primary tumor into the surrounding tumor-associated stroma and thereafter into the adjacent normal tissue parenchyma. In order to invade the stroma, carcinoma cells must first breach the basement membrane (BM), a specialized ECM that plays vital roles in organizing epithelial tissues, in part by separating their epithelial and stromal compartments. In addition to structural roles played by the BM, components of this ECM contain a repository of tethered growth factor molecules that can be liberated by carcinoma-secreted proteases. Moreover, the BM also plays crucial roles in signal transduction events within carcinoma cells via pathways initiated by integrin-mediated cell-matrix adhesions, leading to alterations in cell polarity, proliferation, invasiveness, and survival (Bissell and Hines, 2011Bissell M.J. Hines W.C. Why don't we get more cancer? A proposed role of the microenvironment in restraining cancer progression.Nat. Med. 2011; 17: 320-329Crossref PubMed Scopus (268) Google Scholar). Emerging evidence indicates that the precisely controlled tissue architecture of normal epithelium serves as an intrinsic barrier to invasiveness that must be overcome by incipient metastatic carcinoma cells before they can develop into overt malignancies. For example, in the mammary gland, myoepithelial cells oppose invasion by helping to maintain BM integrity; indeed, coimplantation with myoepithelial cells reversed the invasiveness of breast carcinoma xenografts (Hu et al., 2008Hu M. Yao J. Carroll D.K. Weremowicz S. Chen H. Carrasco D. Richardson A. Violette S. Nikolskaya T. Nikolsky Y. et al.Regulation of in situ to invasive breast carcinoma transition.Cancer Cell. 2008; 13: 394-406Abstract Full Text Full Text PDF PubMed Scopus (189) Google Scholar). Similarly, in ovarian carcinomas, the mesothelial cell layer that lines peritoneal and pleural organs serves as an obstacle to further dissemination that can be overcome by carcinoma cell-exerted, myosin-dependent traction forces that physically displace mesothelial cells (Iwanicki et al., 2011Iwanicki M.P. Davidowitz R.A. Ng M.R. Besser A. Muranen T. Merritt M. Danuser G. Ince T. Brugge J.S. Ovarian cancer spheroids use myosin-generated force to clear the mesothelium.Cancer Discov. 2011; 1: 144-157Crossref PubMed Scopus (48) Google Scholar). Moreover, modulation of ECM stiffness, achieved by altering collagen crosslinking, affects breast carcinoma progression via altered integrin signaling (Levental et al., 2009Levental K.R. Yu H. Kass L. Lakins J.N. Egeblad M. Erler J.T. Fong S.F. Csiszar K. Giaccia A. Weninger W. et al.Matrix crosslinking forces tumor progression by enhancing integrin signaling.Cell. 2009; 139: 891-906Abstract Full Text Full Text PDF PubMed Scopus (690) Google Scholar). At a cell-biological level, most types of carcinomas can invade as cohesive multicellular units through a process termed "collective invasion." Alternatively, individual tumor cells may invade via two distinct programs: the protease-, stress-fiber-, and integrin-dependent "mesenchymal invasion" program or the protease-, stress-fiber-, and integrin-independent, Rho/ROCK-dependent "amoeboid invasion" program (Friedl and Wolf, 2003Friedl P. Wolf K. Tumour-cell invasion and migration: diversity and escape mechanisms.Nat. Rev. Cancer. 2003; 3: 362-374Crossref PubMed Scopus (1158) Google Scholar). Indeed, differential expression of molecules that enable either mesenchymal or amoeboid invasion can be observed in signatures of local invasiveness derived from mammary carcinoma models (Wang et al., 2004Wang W. Goswami S. Lapidus K. Wells A.L. Wyckoff J.B. Sahai E. Singer R.H. Segall J.E. Condeelis J.S. Identification and testing of a gene expression signature of invasive carcinoma cells within primary mammary tumors.Cancer Res. 2004; 64: 8585-8594Crossref PubMed Scopus (230) Google Scholar). Tumor cells can apparently interconvert between these various invasion strategies in response to changing microenvironmental conditions. This has caused some to propose that robust suppression of single-cell invasion requires concomitant inhibition of the mesenchymal and amoeboid invasion programs (Friedl and Wolf, 2003Friedl P. Wolf K. Tumour-cell invasion and migration: diversity and escape mechanisms.Nat. Rev. Cancer. 2003; 3: 362-374Crossref PubMed Scopus (1158) Google Scholar). Indeed, certain regulators of invasion function as pleiotropically acting factors that simultaneously modulate components of both pathways. For example, the microRNA (miRNA) miR-31 inhibits breast cancer invasion via concurrent suppression of key effectors of both the mesenchymal (such as integrin α5) and amoeboid (such as RhoA) invasion programs (Valastyan et al., 2009Valastyan S. Reinhardt F. Benaich N. Calogrias D. Szász A.M. Wang Z.C. Brock J.E. Richardson A.L. Weinberg R.A. A pleiotropically acting microRNA, miR-31, inhibits breast cancer metastasis.Cell. 2009; 137: 1032-1046Abstract Full Text Full Text PDF PubMed Scopus (461) Google Scholar). The single-cell invasion pathways cited above are clearly incompatible with one critical element of epithelial tissue organization, specifically the E-cadherin-mediated intercellular junctions that knit together epithelial cell sheets and prevent dissociation of individual epithelial cells from their neighbors. In order to overcome this and other obstacles to invasion, carcinoma cells may co-opt a cell-biological program known as epithelial-mesenchymal transition (EMT), which is critical for multiple aspects of normal embryonic morphogenesis. The EMT program, which involves dissolution of adherens and tight junctions and a loss of cell polarity, dissociates the cells within epithelial cell sheets into individual cells that exhibit multiple mesenchymal attributes, including heightened invasiveness (Thiery et al., 2009Thiery J.P. Acloque H. Huang R.Y.J. Nieto M.A. Epithelial-mesenchymal transitions in development and disease.Cell. 2009; 139: 871-890Abstract Full Text Full Text PDF PubMed Scopus (2097) Google Scholar). EMT programs are orchestrated by a set of pleiotropically acting transcription factors, including Slug, Snail, Twist, ZEB1, and ZEB2, which organize entrance into a mesenchymal state by suppressing expression of epithelial markers and inducing expression of other markers associated with the mesenchymal state (Thiery et al., 2009Thiery J.P. Acloque H. Huang R.Y.J. Nieto M.A. Epithelial-mesenchymal transitions in development and disease.Cell. 2009; 139: 871-890Abstract Full Text Full Text PDF PubMed Scopus (2097) Google Scholar). Indeed, several of these transcription factors directly repress levels of E-cadherin, the keystone of the epithelial state. Certain miRNAs, notably those belonging to the miR-200 family, also regulate EMT programs. One important mechanism by which miR-200 promotes an epithelial phenotype involves its ability to posttranscriptionally suppress expression of the ZEB1 and ZEB2 EMT-inducing transcription factors. Acting in the opposite direction, ZEB1 and ZEB2 transcriptionally repress miR-200 family members, thereby establishing a double-negative-feedback loop that operates as a bistable switch, reinforcing the residence of cells in either the mesenchymal or epithelial state (Thiery et al., 2009Thiery J.P. Acloque H. Huang R.Y.J. Nieto M.A. Epithelial-mesenchymal transitions in development and disease.Cell. 2009; 139: 871-890Abstract Full Text Full Text PDF PubMed Scopus (2097) Google Scholar). Ultimately, loss of the BM barrier allows direct invasion by carcinoma cells of the stromal compartment. Active proteolysis, effected principally by matrix metalloproteinases (MMPs), drives this loss. In normal tissue, the activity of MMPs is carefully controlled via transcriptional and posttranslational mechanisms. Carcinoma cells have devised numerous means by which to derail the normally tight control of MMP activity, almost invariably leading to enhanced MMP function. While degrading the BM and other ECM that lie in the path of invading tumor cells, MMP-expressing cells also liberate growth factors that are sequestered there, thereby fostering cancer cell proliferation (Kessenbrock et al., 2010Kessenbrock K. Plaks V. Werb Z. Matrix metalloproteinases: regulators of the tumor microenvironment.Cell. 2010; 141: 52-67Abstract Full Text Full Text PDF PubMed Scopus (866) Google Scholar). Once invading carcinoma cells have dissolved the BM, they enter the stroma. Here, they are confronted with a variety of tumor-associated stromal cells, whose composition is governed by the state of tumor progression. As primary tumor progression proceeds, the stroma becomes increasingly "reactive" and acquires many of the attributes of the stroma of tissues that are in the midst of wound healing or are chronically inflamed (Grivennikov et al., 2010Grivennikov S.I. Greten F.R. Karin M. Immunity, inflammation, and cancer.Cell. 2010; 140: 883-899Abstract Full Text Full Text PDF PubMed Scopus (1384) Google Scholar). Accordingly, tumor cells invading into a reactive stroma encounter fibroblasts and myofibroblasts, endothelial cells, adipocytes, and various bone marrow-derived cells such as mesenchymal stem cells, as well as macrophages and other immune cells (Joyce and Pollard, 2009Joyce J.A. Pollard J.W. Microenvironmental regulation of metastasis.Nat. Rev. Cancer. 2009; 9: 239-252Crossref PubMed Scopus (952) Google Scholar). These stromal cells are capable of further enhancing the aggressive behaviors of carcinoma cells through various types of heterotypic signaling. For example, breast cancer invasiveness can be stimulated through the secretion of interleukin-6 (IL-6) by adipocytes present in the local microenvironment (Dirat et al., 2011Dirat B. Bochet L. Dabek M. Daviaud D. Dauvillier S. Majed B. Wang Y.Y. Meulle A. Salles B. Le Gonidec S. et al.Cancer-associated adipocytes exhibit an activated phenotype and contribute to breast cancer invasion.Cancer Res. 2011; 71: 2455-2465Crossref PubMed Scopus (82) Google Scholar). Furthermore, stromal CD4+ T-lymphocytes promote mammary carcinoma invasion by stimulating tumor-associated macrophages (TAMs) to activate epidermal growth factor receptor (EGFR) signaling in the carcinoma cells (DeNardo et al., 2009DeNardo D.G. Barreto J.B. Andreu P. Vasquez L. Tawfik D. Kolhatkar N. Coussens L.M. CD4(+) T cells regulate pulmonary metastasis of mammary carcinomas by enhancing protumor properties of macrophages.Cancer Cell. 2009; 16: 91-102Abstract Full Text Full Text PDF PubMed Scopus (370) Google Scholar). Similarly, secretion of IL-4 by breast cancer cells triggers cathepsin protease activity in TAMs, further augmenting carcinoma cell invasiveness (Gocheva et al., 2010Gocheva V. Wang H.W. Gadea B.B. Shree T. Hunter K.E. Garfall A.L. Berman T. Joyce J.A. IL-4 induces cathepsin protease activity in tumor-associated macrophages to promote cancer growth and invasion.Genes Dev. 2010; 24: 241-255Crossref PubMed Scopus (163) Google Scholar). These findings provide examples of the bidirectional interactions that occur between tumor cells and the nearby stroma: carcinoma cells stimulate the formation of an inflamed stroma, and the latter reciprocates by enhancing the malignant traits of the carcinoma cells, thereby establishing a potentially self-amplifying positive-feedback loop. Detailed characterizations of stromal cells provide further evidence of their critical roles in enabling the malignant behavior of carcinoma cells. For example, microarray profiling of the tumor-associated stroma derived from breast cancer patients reveals characteristic expression signatures associated with metastatic outcome (Finak et al., 2008Finak G. Bertos N. Pepin F. Sadekova S. Souleimanova M. Zhao H. Chen H. Omeroglu G. Meterissian S. Omeroglu A. et al.Stromal gene expression predicts clinical outcome in breast cancer.Nat. Med. 2008; 14: 518-527Crossref PubMed Scopus (575) Google Scholar). Additionally, an expression signature that typifies the transcriptional response of cultured fibroblasts to serum and thus reflects one component of wound-healing responses correlates with increased risk of metastatic recurrence in human breast, gastric, and lung carcinomas (Chang et al., 2004Chang H.Y. Sneddon J.B. Alizadeh A.A. Sood R. West R.B. Montgomery K. Chi J.T. van de Rijn M. Botstein D. Brown P.O. Gene expression signature of fibroblast serum response predicts human cancer progression: similarities between tumors and wounds.PLoS Biol. 2004; 2: E7Crossref PubMed Scopus (479) Google Scholar). Such observations are consistent with the role of an increasingly activated stroma in driving malignant behavior in closely apposed carcinoma cells, but they hardly prove causality. Instead, such evidence has begun to emerge from experimental models. For example, perturbation of Hedgehog signaling or caveolin-1 specifically within the stroma alters tumor progression in neighboring carcinoma cells (Olive et al., 2009Olive K.P. Jacobetz M.A. Davidson C.J. Gopinathan A. McIntyre D. Honess D. Madhu B. Goldgraben M.A. Caldwell M.E. Allard D. et al.Inhibition of Hedgehog signaling enhances delivery of chemotherapy in a mouse model of pancreatic cancer.Science. 2009; 324: 1457-1461Crossref PubMed Scopus (822) Google Scholar, Goetz et al., 2011Goetz J.G. Minguet S. Navarro-Lérida I. Lazcano J.J. Samaniego R. Calvo E. Tello M. Osteso-Ibáñez T. Pellinen T. Echarri A. et al.Biomechanical remodeling of the microenvironment by stromal caveolin-1 favors tumor invasion and metastasis.Cell. 2011; 146: 148-163Abstract Full Text Full Text PDF PubMed Scopus (121) Google Scholar). Independent of the detailed mechanisms of stromal-epithelial interactions within primary carcinomas, it is clear that entry of neoplastic cells into the stroma provides abundant opportunities for tumor cells to directly access the systemic circulation and thereby disseminate to distant sites. Intravasation involves locally invasive carcinoma cells entering into the lumina of lymphatic or blood vessels. Although lymphatic spread of carcinoma cells is routinely observed in human tumors and represents an important prognostic marker for disease progression, dissemination via the hematogenous circulation appears to represent the major mechanism by which metastatic carcinoma cells disperse (Gupta and Massagué, 2006Gupta G.P. Massagué J. Cancer metastasis: building a framework.Cell. 2006; 127: 679-695Abstract Full Text Full Text PDF PubMed Scopus (1128) Google Scholar). Intravasation can be facilitated by molecular changes that promote the ability of carcinoma cells to cross the pericyte and endothelial cell barriers that form the walls of microvessels. For example, the transcriptional modulator amino-terminal enhancer of split (Aes) inhibited the intravasation of colon carcinoma cells by impairing trans-endothelial invasion through Notch-dependent mechanisms (Sonoshita et al., 2011Sonoshita M. Aoki M. Fuwa H. Aoki K. Hosogi H. Sakai Y. Hashida H. Takabayashi A. Sasaki M. Robine S. et al.Suppression of colon cancer metastasis by Aes through inhibition of Notch signaling.Cancer Cell. 2011; 19: 125-137Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar). Conversely, the cytokine-transforming growth factor-β (TGFβ) enhances mammary carcinoma intravasation, ostensibly by increasing carcinoma cell penetration of microvessel walls or augmenting invasiveness more generally (Giampieri et al., 2009Giampieri S. Manning C. Hooper S. Jones L. Hill C.S. Sahai E. Localized and reversible TGFbeta signalling switches breast cancer cells from cohesive to single cell motility.Nat. Cell Biol. 2009; 11: 1287-1296Crossref PubMed Scopus (230) Google Scholar). Additionally, the intravasation of breast carcinoma cells can be enhanced by perivascular TAMs via a positive-feedback loop comprised of the reciprocal secretion of epidermal growth factor (EGF) and colony-stimulating factor-1 (CSF-1) by TAMs and carcinoma cells, respectively (Wyckoff et al., 2007Wyckoff J.B. Wang Y. Lin E.Y. Li J.F. Goswami S. Stanley E.R. Segall J.E. Pollard J.W. Condeelis J. Direct visualization of macrophage-assisted tumor cell intravasation in mammary tumors.Cancer Res. 2007; 67: 2649-2656Crossref PubMed Scopus (316) Google Scholar). The mechanics of intravasation are likely to be strongly influenced by the structural features of tumor-associated blood vessels. Through a variety of mechanisms—many of which converge on the actions of vascular endothelial growth factors (VEGFs)—tumor cells stimulate the formation of new blood vessels within their local microenvironment via the process termed neoangiogenesis. In contrast to blood vessels present in normal tissues, the neovasculature generated by carcinoma cells is tortuous, prone to leakiness, and in a state of continuous reconfiguration (Carmeliet and Jain, 2011Carmeliet P. Jain R.K. Principles and mechanisms of vessel normalization for cancer and other angiogenic diseases.Nat. Rev. Drug Discov. 2011; 10: 417-427Crossref PubMed Scopus (254) Google Scholar). The weak interactions between adjacent endothelial cells that form the tumor-associated microvasculature and the absence of extensive pericyte coverage are likely to facilitate intravasation. In support of this notion, the capacity of cyclooxygenase-2 (COX-2), epiregulin (EREG), MMP-1, and MMP-2 to synergistically promote breast carcinoma intravasation was tied to their ability to stimulate neoangiogenesis and the formation of leaky blood vessels (Gupta et al., 2007aGupta G.P. Nguyen D.X. Chiang A.C. Bos P.D. Kim J.Y. Nadal C. Gomis R.R. Manova-Todorova K. Massagué J. Mediators of vascular remodelling co-opted for sequential steps in lung metastasis.Nature. 2007; 446: 765-770Crossref PubMed Scopus (318) Google Scholar). Once carcinoma cells have successfully intravasated into the lumina of blood vessels, they can disseminate widely through the venous and arterial circulation. Recent technological advances have facilitated detection of circulating tumor cells (CTCs) within the bloodstream of carcinoma patients (Nagrath et al., 2007Nagrath S. Sequist L.V. Maheswaran S. Bell D.W. Irimia D. Ulkus L. Smith M.R. Kwak E.L. Digumarthy S. Muzikansky A. et al.Isolation of rare circulating tumour cells in cancer patients by microchip technology.Nature. 2007; 450: 1235-1239Crossref PubMed Scopus (1174) Google Scholar, Pantel et al., 2008Pantel K. Brakenhoff R.H. Brandt B. Detection, clinical relevance and specific biological properties of disseminating tumour cells.Nat. Rev. Cancer. 2008; 8: 329-340Crossref PubMed Scopus (488) Google Scholar, Stott et al., 2010Stott S.L. Hsu C.H. Tsukrov D.I. Yu M. Miyamoto D.T. Waltman B.A. Rothenberg S.M. Shah A.M. Smas M.E. Korir G.K. et al.Isolation of circulating tumor cells using a microvortex-generating herringbone-chip.Proc. Natl. Acad. Sci. USA. 2010; 107: 18392-18397Crossref PubMed Scopus (359) Google Scholar). CTCs ostensibly represent carcinoma cells that are en route between primary tumors and sites of dissemination and therefore may represent "metastatic intermediates." CTCs in the hematogenous circulation must survive a variety of stresses in order to reach distant organ sites. For example, they would seem to be deprived of the integrin-dependent adhesion to ECM components that is normally essential for cell survival. In the absence of such anchorage, epithelial cells normally undergo anoikis, a form of apoptosis triggered by loss of anchorage to substratum (Guo and Giancotti, 2004Guo W. Giancotti F.G. Integrin signalling during tumour progression.Nat. Rev. Mol. Cell Biol. 2004; 5: 816-826Crossref PubMed Scopus (683) Google Scholar). Some of the signaling events that oversee anoikis responses impinge upon metabolic programs, such as the pentose phosphate pathway and control of glucose uptake (Schafer et al., 2009Schafer Z.T. Grassian A.R. Song L. Jiang Z. Gerhart-Hines Z. Irie H.Y. Gao S. Puigserver P. Brugge J.S. Antioxidant and oncogene rescue of metabolic defects caused by loss of matrix attachment.Nature. 2009; 461: 109-113Crossref PubMed Scopus (194) Google Scholar). Also of interest, the tyrosine kinase TrkB was identified as a suppressor of anoikis whose expression is required for metastatic progression in transformed intestinal epithelial cells (Douma et al., 2004Douma S. Van Laar T. Zevenhoven J. Meuwissen R. Van Garderen E. Peeper D.S. Suppression of anoikis and induction of metastasis by the neurotrophic receptor TrkB.Nature. 2004; 430: 1034-1039Crossref PubMed Scopus (289) Google Scholar). A more nuanced understanding of the lives of CTCs is precluded at present by the dearth of simple facts: we do not know how long cancer cells linger in the circulation. Some have estimated that their dwell time in breast cancer patients may be several hours (Meng et al., 2004Meng S. Tripathy D. Frenkel E.P. Shete S. Naftalis E.Z. Huth J.F. Beitsch P.D. Leitch M. Hoover S. Euhus D. et al.Circulating tumor cells in patients with breast cancer dormancy.Clin. Cancer Res. 2004; 10: 8152-8162Crossref PubMed Scopus (314) Google Scholar). However, given the relatively large diameters of carcinoma cells (20–30 μm) and the luminal diameter of capillaries (∼8 μm), the vast majority of CTCs are likely to become trapped in various capillary beds during their first pass through the circulation (that is, within minutes of intravasation). It is therefore possible that many tumor cells spend only relatively brief periods of time in the bloodstream, allowing CTCs to escape from the circulation long before anoikis alarms are sounded. In addition to stresses imposed by matrix detachment, tumor cells in the circulation must overcome the damage incurred by hemodynamic shear forces and predation by cells of the innate immune system, specifically natural killer cells. Conveniently, carcinoma cells seem to simultaneously evade both of these threats through a single mechanism that depends on co-opting one aspect of normal blood coagulation. More specifically, by forming relatively large emboli via interactions with blood platelets, a process that appears to be mediated by the expression of tissue factor and/or L- and P-selectins by the carcinoma cells, tumor cells are able to both shield themselves from shear forces and evade immune detection (Joyce and Pollard, 2009Joyce J.A. Pollard J.W. Microenvironmental regulation of metastasis.Nat. Rev. Cancer. 2009; 9: 239-252Crossref PubMed Scopus (952) Google Scholar). Thus, platelet-coated tumor cells are better able to persist within the circulation until they arrest at distant tissue sites, an event whose likelihood may be further increased due to the large effective diameter of these microemboli. Despite the theoretical ability of CTCs to disseminate to a wide variety of secondary loci, clinicians have long noted that individual carcinoma types form metastases in only a limited subset of target organs (Figure 3) (Fidler, 2003Fidler I.J. The pathogenesis of cancer metastasis: the 'seed and soil' hypothesis revisited.Nat. Rev. Cancer. 2003; 3: 453-458Crossref PubMed Scopus (1578) Google Scholar). A major unresolved issue concerns whether this tissue tropism simply reflects a passive process whereby CTCs arrest within capillary beds due to the layout of the vasculature and size restrictions imposed by blood vessel diameters or, instead, indicates a capacity of CTCs to actively home to specific organs via genetically templated ligand-receptor interactions between these cells and the luminal walls of the microvasculature. Carcinomas originating from a particular epithelial tissue form detectable metastases in only a limited subset of theoretically possible distant organ sites. Shown here are the most common sites of metastasis for six well-studied carcinoma types. Primary tumors are depicted in red. Thickness of black lines reflects the relative frequencies with which a given primary tumor type metastasizes to the indicated distant organ site. The issue of physical trapping of CTCs in microvessels looms large here. For example, the anatomical layout of the va