Chapter 10 The Cancer Cell–Leukocyte Fusion Theory of Metastasis
Introduction
It is abundantly clear that metastasis—the migration of cancer cells from their site of origin to distant organs and tissues—is what makes cancer so deadly. If we just had to manage the primary tumor and not the spread, cancer survival would soar. The primary tumor can be surgically removed, the local area can be treated by radiation and the patient can receive preventive chemotherapy. If spread has not occurred, there is usually an excellent outcome. But once the cancer cells enter the vasculature or lymphatics and disseminate throughout the body, treatment is far more problematic. Not only do the metastatic cancer cells migrate to virtually anywhere (lungs, brain, bone marrow, liver), but they are usually devilishly resistant to chemotherapy and radiation. Treatments to combat metastases often become palliative and no longer curative. Mortality occurs when vital organs fail due to tumor burden.
It is therefore surprising that so little is known about the onset of metastasis. To our knowledge, the century‐old theory of cancer cell fusion with tumor‐associated leucocytes such as macrophages is really the only complete theory we have—potentially explaining most if not all aspects of metastasis and particularly its initiation (reviewed in Aggarwal, 2004, Pawelek, 2005, Pawelek et al., 2006, Pawelek and Chakraborty, 2008). In this theory, metastasis is virtually a second disease imposed on the primary tumor cell. While the primary cell is notable for its deregulated cell cycle, it has little propensity to migrate away from its site of origin. The fusion theory states that acquisition of a metastatic phenotype occurs when a healthy migratory leucocyte fuses with a primary tumor cell. The resultant hybrid adapts the white blood cell's natural ability to migrate around the body, all the while continuing to go through the uncontrolled cell division of the original cancer cell. A metastatic cell emerges, which like a white blood cell can migrate from the epithelium into the mesoderm, enter the circulatory system, and travel to lymph nodes and distant organs.
The fusion theory was first proposed in the early 1900s and has attracted a lot of scientific interest over the years. Its roots lie in the pioneering work of Theodore and Marcella Boveri on aberrant chromosome numbers and abnormal mitoses in sea urchin eggs and the remarkable insights of German pathologist Otto Aichel as to how this might relate to tumor progression (Aichel, 1911, Boveri, 2008). The Boveri's observed that sea urchin eggs experimentally fertilized with two sets of spermatozoa underwent abnormal mitosis. They later proposed that deregulated growth of cancer cells might also be a result of chromosome imbalance (Boveri, 2008). This work motivated Aichel to first propose fusion and hybridization as a mechanism for the imbalance of chromosomes in human cancer, suggesting that a combination of extra chromosomes and the “qualitative differences” in chromosomes from the two cell types could lead to the metastatic phenotype (Aichel, 1911). In his 1911 article “About cell fusion with qualitatively abnormal chromosome distribution as cause for tumor formation,” Aichel exhorted future scientists to “study chromosomes from all angles” (Aichel, 1911). Decades later, the same hypothesis—that metastasis is caused by leucocyte–tumor cell fusion—was proposed independently by Mekler, 1968, Mekler, 1971) and by Goldenberg (Goldenberg, 1968, Goldenberg and Gotz, 1968). Several laboratories have now reported that hybrids produced by fusion in vitro or in vivo were aneuploid and of higher metastatic potential (Pawelek, 2005, Pawelek et al., 2006, Pawelek and Chakraborty, 2008). In 1984, LaGarde and Kerbel summarized the emerging concepts (Lagarde and Kerbel, 1984):
[Tumor cell hybridization] can lead to major changes in gene expression. These processes can lead to the evolution of subpopulations of tumor cells having major losses or gains in their malignant aggressiveness and therefore represents a large‐scale genetic mechanism capable of generating genotypic and phenotypic diversification … If the normal host cell happens to be a lymphoreticular‐hematopoietic cell, it could donate this phenotype to cell types which otherwise do not normally express metastatic traits.
There is now considerable evidence to support these concepts.
The pathways of invasion and metastasis have been under intense scientific scrutiny and much is now known about the steps involved (Chambers et al., 2002, Gupta et al., 2005). However, the actual genesis of metastatic cells from within populations of nonmetastatic cells of the primary tumor is not understood. What are the initiating mechanisms that cause a carcinoma or melanoma cell in the epithelium to free its adhesions to neighboring cells, adapt a migratory phenotype, cross the basal lamina into the dermis, intravasate into the blood circulatory system or lymphatics, extravasate, and form new tumors in lymph nodes and distant tissues or organs? The long‐standing view is essentially Darwinian: the unstable cancer genome combined with host selective pressures generates metastatic cells in the otherwise nonmetastatic primary tumor (Fidler and Kripke, 1977, Nowell, 1976). This view continues to provide the best framework for envisioning tumor progression. Yet it is difficult to imagine how this might occur through successive, stepwise mutations since generation of a metastatic phenotype would require activation and silencing of very large numbers of genes in the primary tumor cell (Gupta et al., 2005). One solution to this problem lies in the activation of master regulatory genes that control multiple pathways and initiate prometastatic cascades (Ma et al., 2007). This has been highlighted in reports that master regulators of epithelial‐mesenchymal transition (EMT) in development, such as Snail, Slug, SPARC, Twist, and others, play analogous roles in invasion and metastasis where they activate mesoderm‐associated pathways of cellular adhesion and migration (Gupta et al., 2005, Ma et al., 2007). For example, in breast cancer, Twist activates microRNA‐10b that in turn causes increased expression of the prometastatic gene RHOC with increased metastatic potential of the affected cells (Ma et al., 2007). However, the mechanisms through which master regulators such as Twist are themselves upregulated in cancer are not understood. We propose that at least in some cases this could be initiated by fusion of cancer cells with bone marrow‐derived cells (BMDCs). While a transition from epithelial to mesodermal gene expression is indeed a characteristic of invasion and metastasis, the expressed genes are often remarkably similar to those associated with migratory BMDCs such as macrophages and other myeloid‐lineage cells (2 s, 4, 17). Fusion of migratory BMDCs and cancer cells with coexpression of both fusion partner genomes provides a potential explanation for this phenomenon as first proposed by Munzarova et al. (1992).
In our opinion, the fusion theory comes closer to a unifying explanation of tumor progression than any yet proposed. Fusion represents a nonmutational mechanism that could explain the aberrant gene expression patterns associated with malignant cells. Studies of macrophage–tumor cell fusions have demonstrated that genes from both parental partners are expressed in hybrid cells (Chakraborty et al., 2001a). Gene expression in such cells reflects combinations of myeloid lineage genes along with those of the cancer cell lineage, all in a background of deregulated cell division. In fact, many molecules and traits associated with tumor progression are expressed by healthy myeloid lineage cells, for example, angiogenesis, motility, chemotaxis and tropism, immune signaling, matrix degradation and remodeling, responses to hypoxia, and multidrug resistance to chemotherapy (Pawelek, 2005, Pawelek et al., 2006). Tumor fusion could also account for aneuploidy and genetic rearrangements in metastatic cells (Duelli and Lazebnik, 2007, Aggarwal, 2004, Pawelek, 2005, Pawelek et al., 2006, Pawelek and Chakraborty, 2008). It is further possible that tumor cell–BMDC fusions are a source of cancer stem cells (Bjerkvig et al., 2005).
This chapter reviews the molecular and cellular pathways activated following fusion of tumor cells with BMDCs, their expression in macrophages and other BMDCs, and their similarities to those governing tumor progression in animal and human cancer.
Section snippets
Cancer Cell Fusion in vivo
From studies in animal and human cancers, there is little doubt that tumor hybrids are generated in vivo and that at least in animals they can be a source of metastases (Aggarwal, 2004, Pawelek, 2005, Pawelek et al., 2006, Pawelek and Chakraborty, 2008). Cancer cells fuse with many cell types in vivo including stromal cells (Jacobsen et al., 2006), epithelial cells (Rizvi et al., 2006), and endothelial cells (Bjerregaard et al., 2006, Mortensen et al., 2004, Bingle et al., 2002). There are more
Tumor‐Associated Macrophages as Candidates for Cancer Cell Fusion Partners
Munzarova et al. (1992) noted that a number of macrophage‐like traits are expressed by metastatic melanoma and other malignancies and proposed that metastatic melanoma cells might be macrophge–melanoma hybrids. For example, Pernick et al. showed that human melanomas are often immunoreactive for macrophage markers such as CD68, alpha‐1‐antitrypsin, HAM56, Mac387, and muramidase (Pernick et al., 1999). In breast cancer, Shabo et al. showed that expression of CD163, a macrophage scavenger
Macrophage–Melanoma Fusion in vitro Generates Altered Gene Expression and a Metastatic Phenotype in vivo
Tumor cell–BMDC fusions might explain how common gene expression patterns emerge for different tumor types. We, and others, have found that when BMDC–tumor cell hybrids were isolated in vitro with no selective pressure other than for growth in drug‐containing media, remarkably high numbers of them exhibited a metastatic phenotype in mice. Further, the most metastatic clones tended to be highly melanized compared to parental melanoma cells or weakly metastatic hybrids (described below) (Fig. 7).
β1,6‐Branched Oligosaccharides and Coarse Vesicles in Putative Human BMT–Tumor Hybrids
Prompted by these observations, β1,6‐branched oligosaccharides (stained with the lectin LPHA) and coarse vesicles were investigated in the two human cases of putative BMT–tumor hybrids in renal cell carcinomas described above (Chakraborty et al., 2004, Yilmaz et al., 2005) (Fig. 8). In both cases, the tumors were LPHA‐positive with a coarse vesicular cytoplasm. In the case of the lymph node metastasis in a child receiving a BMT from his brother (Chakraborty et al., 2004), tumor cells were
β1,6‐Branched Oligosaccharides and Coarse Vesicles are Common in Human Cancers
From these results, we initiated a survey for β1,6‐branched oligosaccharides and coarse vesicles in human cancers (Handerson and Pawelek, 2003). LPHA‐positive coarse vesicles were readily found in all 22 different cancers surveyed, including carcinomas of the lung, colon, breast, ovary, prostate, kidney, liver, and a variety of lymphomas (Handerson and Pawelek, 2003). To illustrate, an in situ cutaneous melanoma filled with coarse melanin is shown (Fig. 9A). When a sequential section was
Considerations for Studying Fusion in vivo
To prove fusion and genomic hybridization requires identification of genes or chromosomes from both of the putative fusion partners in the same cell or cells. Hence fusion has been well‐documented in tumor xenografts in animals where hybrids were identified by the presence of both tumor and host genes. Little is yet known of the extent of cancer cell fusion in humans. While a few human cases have recently been reported (Andersen et al., 2007, Avital et al., 2007, Chakraborty et al., 2004, Cogle
Implications
Two of the hybrid‐associated features described above, enhanced migration and autophagy, could together have important implications for the initiation of metastasis. Remarkably, both features may be activated through GnT‐V‐mediated addition of β1,6‐branched oligosaccharides. For the primary carcinoma or melanoma cell, a migratory phenotype would imply loss of adhesion to adjoining cells in the epidermis, activation of matrix proteinases, induction of chemotaxis and tropism, and major
Acknowledgments
We gratefully acknowledge the many and invaluable contributions of David Bermudes, Jean Bolognia, Douglas Brash, Dennis Cooper, Rossitza Lazova, Lynn Margulis, Josh Pawelek, James Platt, Michael Rachkovsky, Stefano Sodi, and Yesim Yilmaz. Supported in part by a gift from Vion Pharmaceuticals (JP), and a grant from Avon Pharmaceuticals (AC).
References (263)
Nuclear factor‐kappaB: The enemy within
Cancer Cell
(2004)- et al.
Modulation of fibronectin gene expression in inflammatory mononuclear phagocytes of rat liver after acute liver injury
J. Hepatol.
(2004) - et al.
Smoldering and polarized inflammation in the initiation and promotion of malignant disease
Cancer Cell
(2005) - et al.
Macrophages express neurotrophins and neurotrophin receptors. Regulation of nitric oxide production by NT‐3
J. Neuroimmunol.
(2001) - et al.
Neoexpression of the c‐met/hepatocyte growth factor‐scatter factor receptor gene in activated monocytes
Blood
(1997) - et al.
Basic fibroblast growth factor mediates its effects on committed myeloid progenitors by direct action and has no effect on hematopoietic stem cells
Blood
(1995) - et al.
Up‐regulation of MET expression by alpha‐melanocyte‐stimulating hormone and MITF allows hepatocyte growth factor to protect melanocytes and melanoma cells from apoptosis
J. Biol. Chem.
(2007) - et al.
Melanocyte receptors: Clinical implications and therapeutic relevance
Dermatol. Clin.
(2007) - et al.
Differential gene expression in genetically matched mouse melanoma cells with different metastatic potential
Gene
(2003) - et al.
Human monocyte x mouse melanoma fusion hybrids express human gene
Gene
(2001)
Current concepts of the biology of human cutaneous malignant melanoma
Adv. Cancer Res.
HIF‐1alpha is essential for myeloid cell‐mediated inflammation
Cell
Microfilaments and microtubules regulate recycling from phagosomes
Exp. Cell Res.
Autophagy promotes tumor cell survival and restricts necrosis, inflammation, and tumorigenesis
Cancer Cell
Monocytes promote natural killer cell interferon gamma production in response to the endogenous danger signal HMGB1
Mol. Immunol.
Glycoprotein glycosylation and cancer progression
Biochim. Biophys. Acta
Pro‐MMP‐9 is a specific macrophage product and is activated by osteoarthritic chondrocytes via MMP‐3 or a MT1‐MMP/MMP‐13 cascade
Exp. Cell Res.
M‐CSF induces the stable interaction of cFms with alphaVbeta3 integrin in osteoclasts
Int. J. Biochem. Cell Biol.
Wide metastatic spreading in infiltrating lobular carcinoma of the breast
Eur. J. Cancer
Enhanced lung colonization and tumorigenicity of fused cells isolated from primary MCA tumors
Cancer Lett.
Prespecification and plasticity: Shifting mechanisms of cell migration
Curr. Opin. Cell Biol.
Structure of sialylated fucosyl lactosaminoglycan isolated from human granulocytes
J. Biol. Chem.
Hepatocyte growth factor‐regulated genes in differentiated RAW 264.7 osteoclast and undifferentiated cells
Gene
A novel type I cytokine receptor is expressed on monocytes, signals proliferation, and activates STAT‐3 and STAT‐5
J. Biol. Chem.
On the ‘human’ nature of highly malignant heterotransplantable tumors of human origin
Eur. J. Cancer
The molecular basis of multidrug resistance in cancer: The early years of P‐glycoprotein research
FEBS Lett.
Hypoxia and hypoxia inducible factors (HIF) as important regulators of tumor physiology
Cancer Treat. Res.
Colony‐stimulating factor‐1 blockade by antisense oligonucleotides and small interfering RNAs suppresses growth of human mammary tumor xenografts in mice
Cancer Res.
A high‐throughput study in melanoma identifies epithelial‐mesenchymal transition as a major determinant of metastasis
Cancer Res.
Autophagy is an immediate macrophage response to Legionella pneumophila
Cell Microbiol.
Macrophages rapidly transfer pathogens from lipid raft vacuoles to autophagosomes
Autophagy
Comparative analysis of integrin expression on monocyte‐derived macrophages and monocyte‐derived dendritic cells
Immunology
Osteoclast nuclei of myeloma patients show chromosome translocations specific for the myeloma cell clone: A new type of cancer‐host partnership?
J. Pathol.
Signal transduction and signal modulation by cell adhesion receptors: The role of integrins, cadherins, immunoglobulin‐cell adhesion molecules, and selectins
Pharmacol. Rev.
Premature chromosome condensation in carcinoma of the bladder: Presumptive evidence for fusion of normal and malignant cells
Cytogenet. Cell Genet.
Tumor x host cell hybrids in the mouse: Chromosomes from the normal cell parent maintained in malignant hybrids tumors
J. Natl. Cancer Inst.
Donor‐derived human bone marrow cells contribute to solid organ cancers developing after bone marrow transplantation
Stem Cells
Morphological and immunophenotypic variations in malignant melanoma
Histopathology
The Snail genes as inducers of cell movement and survival: Implications in development and cancer
Development
Alpha4beta1 integrin (VLA‐4) blockade attenuates both early and late leukocyte recruitment and neointimal growth following carotid injury in apolipoprotein E (‐/‐) mice
J. Vasc. Res.
Characteristics of “hybrid”‐type clonal cell lines obtained from mixed cultures in vitro
JNCI
Lipopolysaccharide activation of the TPL‐2/MEK/extracellular signal‐regulated kinase mitogen‐activated protein kinase cascade is regulated by IkappaB kinase‐induced proteolysis of NF‐kappaB1 p105
Mol. Cell. Biol.
Relationship of matrix metalloproteinases and macrophages to embolization during endoluminal carotid interventions
J. Endovasc. Ther.
The role of tumour‐associated macrophages in tumour progression: Implications for new anticancer therapies
J. Pathol.
Opinion: The origin of the cancer stem cell: Current controversies and new insights
Nat. Rev. Cancer
Syncytin is involved in breast cancer‐endothelial cell fusions
Cell. Mol. Life Sci.
Invasive growth: A MET‐driven genetic programme for cancer and stem cells
Nat. Rev. Cancer
Concerning the origin of malignant tumors. Translated and annotated by Harris, H
J. Cell Sci.
SPARC, a matricellular protein that functions in cellular differentiation and tissue response to injury
J. Clin. Invest.
Cited by (144)
Updated Management of Colorectal Cancer Liver Metastases: Scientific Advances Driving Modern Therapeutic Innovations
2023, Cellular and Molecular Gastroenterology and HepatologyPolyploidy formation in cancer cells: How a Trojan horse is born
2022, Seminars in Cancer BiologyCell-cell fusions and cell-in-cell phenomena in healthy cells and cancer: Lessons from protists and invertebrates
2022, Seminars in Cancer BiologyIntegrin α<inf>2</inf>β<inf>1</inf> as a negative regulator of the laminin receptors α<inf>6</inf>β<inf>1</inf> and α<inf>6</inf>β<inf>4</inf>
2021, MicronCitation Excerpt :During malignant cell invasion and metastasis, basement membranes are often penetrated or dissolved, and cell adhesion to laminin is therefore of great importance in this process (Patarroyo et al., 2002). Integrins α6β1 and α6β4 are the dominant laminin receptors in many cell types (Aumailley et al., 1990), and this significance in adhesion to laminin makes them a major indicator of metastasis (Pawelek and Chakraborty, 2008). For instance, upregulation of integrin α6β1 endows sarcoma cells with the capability to invade basement membranes (Kielosto et al., 2009), while expression of integrin α6β4 is associated with the formation, migration, invasion, and survival of carcinoma cells (Bertotti et al., 2005; Mercurio and Rabinovitz, 2001).
True significance of N-acetylglucosaminyltransferases GnT-III, V and α1,6 fucosyltransferase in epithelial-mesenchymal transition and cancer
2021, Molecular Aspects of Medicine