The Heterocellular Emergence of Colorectal Cancer
Authors: Christopher J. Tape
Journal: Trends in cancer (2017)

Solid tumors contain multiple heterotypic cell types and should be considered to be heterocellular systems. Heterotypic cell types can differentially process signals. When different cells interact (via heterocellular signaling) they expand their collective signal-processing potential to achieve emergent heterocellular phenotypes. Cancer is an emergent phenotype that supervenes upon heterocellular signaling. Perturbing heterocellular signaling can destabilize malignant systems as treatments for cancer. Tissues contain multiple different cell types and can be considered to be heterocellular systems. Signaling between different cells allows tissues to achieve phenotypes that no cell type can achieve in isolation. Such emergent tissue-level phenotypes can be said to 'supervene upon' heterocellular signaling. It is proposed here that cancer is also an emergent phenotype that supervenes upon heterocellular signaling. Using colorectal cancer (CRC) as an example, I review how heterotypic cells differentially communicate to support emergent malignancy. Studying tumors as integrated heterocellular systems – rather than as solitary expansions of mutated cells – may reveal novel ways to treat cancer. Tissues contain multiple different cell types and can be considered to be heterocellular systems. Signaling between different cells allows tissues to achieve phenotypes that no cell type can achieve in isolation. Such emergent tissue-level phenotypes can be said to 'supervene upon' heterocellular signaling. It is proposed here that cancer is also an emergent phenotype that supervenes upon heterocellular signaling. Using colorectal cancer (CRC) as an example, I review how heterotypic cells differentially communicate to support emergent malignancy. Studying tumors as integrated heterocellular systems – rather than as solitary expansions of mutated cells – may reveal novel ways to treat cancer. Metazoan tissues are composed of multiple cell types (e.g., epithelial and mesenchymal cells, leukocytes) [1Bianconi E. et al.An estimation of the number of cells in the human body.Ann. Hum. Biol. 2013; 40: 463-471Crossref PubMed Scopus (549) Google Scholar] and can be thought of as heterocellular systems (see Glossary) [2Tape C.J. Systems biology analysis of heterocellular signaling.Trends Biotechnol. 2016; 34: 627-637Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar]. For example, consider the mammalian intestine. Healthy intestinal tissue is a heterocellular system wherein several different cell types collaborate to form a functional organ. Notably, epithelial enterocytes control nutrient uptake [3Peterson L.W. Artis D. Intestinal epithelial cells: regulators of barrier function and immune homeostasis.Nat. Rev. Immunol. 2014; 14: 141-153Crossref PubMed Scopus (1640) Google Scholar], whereas mesenchymal fibroblasts support epithelial renewal [4Powell D.W. et al.Mesenchymal cells of the intestinal lamina propria.Annu. Rev. Physiol. 2011; 73: 213-237Crossref PubMed Scopus (256) Google Scholar], and tissue-resident lymphocytes and myeloid cells patrol against infection [5Mowat A.M. Agace W.W. Regional specialization within the intestinal immune system.Nat. Rev. Immunol. 2014; 14: 667-685Crossref PubMed Scopus (842) Google Scholar]. Tumors also comprise multiple heterotypic cell types. For example, similarly to the healthy colon, colorectal cancer (CRC) tumors contain epithelial cells, mesenchymal fibroblasts, myeloid cells, and lymphocytes [6Tauriello D.V.F. Batlle E. Targeting the microenvironment in advanced colorectal cancer.Trends Cancer. 2016; 2: 495-504Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar]. Like most solid tumors, CRC tumors are therefore not merely homocellular systems or pools of epithelial cells but are integrated heterocellular systems (Figure 1). Heterotypic cells process and interpret signals completely differently [7Miller-Jensen K. et al.Common effector processing mediates cell-specific responses to stimuli.Nature. 2007; 448: 604-608Crossref PubMed Scopus (166) Google Scholar, 8Iwamoto N. et al.Context-specific flow through the MEK/ERK module produces cell- and ligand-specific patterns of ERK single and double phosphorylation.Sci. Signal. 2016; 9: ra13Crossref PubMed Scopus (15) Google Scholar]. This cell-specific homocellular signaling enables differentiated cells to achieve distinct phenotypes (Figure 2A, Key Figure). When multiple cell types are combined, heterocellular signaling between cells can take place [2Tape C.J. Systems biology analysis of heterocellular signaling.Trends Biotechnol. 2016; 34: 627-637Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar]. Because each cell type has a different signal-processing capacity, heterocellular signaling can engage signaling pathways that each cell type cannot activate autonomously [9Tape C.J. et al.Oncogenic KRAS regulates tumor cell signaling via stromal reciprocation.Cell. 2016; 165: 910-920Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar]. This signaling expansion enables heterocellular systems to achieve phenotypes beyond those of each cell type in isolation (Figure 2B). For example, myeloid dendritic cells can use major histocompatibility complex (MHC) class-II signal processing to present antigens to lymphoid cytotoxic T cells. In turn, activated T cells can use their unique signaling to launch a cytotoxic immune response against the antigen. Together, the two cell types can achieve adaptive immunity. In isolation they cannot. When several interacting constituents achieve an output beyond the sum of their inputs, an emergent system is formed [10Goldstein J. Emergence as a construct: history and issues.Emergence. 1999; 1: 49-72Crossref Google Scholar]. Such a system requires two core elements: (i) constituent 'nodes' and (ii) interacting 'edges' connecting the nodes. When considering tissue, 'nodes' can be thought of as cells and 'edges' as intercellular signals. For example, several epithelial cells (nodes) can interact via adherens junctions (edges) to form an emergent homocellular epithelium – whereas non-interacting epithelial cells cannot. One way to expand the output of an emergent system is to increase the 'diversity' between nodes. For example, while a homocellular network of interacting epithelial cells can produce an epithelium, a heterocellular system of interacting epithelia, myeloid cells, and lymphocytes can produce epithelium with adaptive immunological surveillance. When different cell types interact to produce tissue-level phenotypes, we can say that heterocellular emergence has occurred (Figure 2C). Heterocellular emergence requires: (i) cell 'nodes', (ii) intercellular signaling 'edges', and (iii) heterotypic 'diversity' between cell nodes. Because heterocellular emergence requires different cell types to communicate with each other, tissue-level phenotypes supervene upon heterocellular signaling. Similarly to healthy tissues, tumors are integrated heterocellular systems that achieve complex phenotypes (e.g., metastasis, immune evasion). We can therefore suppose that malignant phenotypes also supervene upon heterocellular signaling (Figure 2D). Anecdotal examples of heterocellular signaling are widely reported across the tumor microenvironment of many different cancers [11Quante M. et al.The gastrointestinal tumor microenvironment.Gastroenterology. 2013; 145: 63-78Abstract Full Text Full Text PDF PubMed Scopus (110) Google Scholar, 12Quail D.F. Joyce J.A. Microenvironmental regulation of tumor progression and metastasis.Nat. Med. 2013; 19: 1423-1437Crossref PubMed Scopus (4440) Google Scholar]. Nevertheless, despite the explicit heterocellularity of tumors, most cancers are still studied as homocellular pools of cancer cells. For example, CRC has long been considered to be a disease of mutated epithelial tumor cells. As such, most attempts to treat CRC focus on inhibiting epithelial cells directly – with little regard to the tumor microenvironment they occupy in patients. However, recent studies have revealed that CRC tumors are highly heterocellular, and increased heterocellularity leads to poorer survival [13Isella C. et al.Stromal contribution to the colorectal cancer transcriptome.Nat. Genet. 2015; 47: 312-319Crossref PubMed Scopus (433) Google Scholar, 14Calon A. et al.Stromal gene expression defines poor-prognosis subtypes in colorectal cancer.Nat. Genet. 2015; 47: 320-329Crossref PubMed Scopus (713) Google Scholar, 15Trinh A. et al.Practical and robust identification of molecular subtypes in colorectal cancer by immunohistochemistry.Clin. Cancer Res. 2016; (Published online December 9, 2016)https://doi.org/10.1158/1078-0432.CCR-16-0680Crossref PubMed Scopus (107) Google Scholar, 16Fessler E. Medema J.P. Colorectal cancer subtypes: developmental irigin and microenvironmental regulation.Trends Cancer. 2016; 2: 505-518Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar]. To treat cancer more effectively, we must understand how heterotypic (diverse) cells (nodes) interact (edges) to achieve malignant phenotypes. Using CRC as an example, I discuss here how heterotypic cells interact to form an emergent malignant system – and how this integrated topology can be targeted to treat cancer. The healthy colon is lined by a single sheet of continually renewing epithelial cells. This physical barrier separates the intestinal lumen (containing nutrients, commensal microflora, and pathogens) from subepithelial tissue. The healthy colonic epithelium is organized into a repetitive crypt structure in which pluripotent intestinal epithelial stem cells either self-renew at the crypt base or differentiate into absorptive enterocytes and goblet cells along the crypt [3Peterson L.W. Artis D. Intestinal epithelial cells: regulators of barrier function and immune homeostasis.Nat. Rev. Immunol. 2014; 14: 141-153Crossref PubMed Scopus (1640) Google Scholar]. Colonic malignancy is driven by oncogenic mutations in epithelial cells. Common CRC mutations include loss of APC function (∼80%), loss of p53 (TP53, ∼60%), and hyperactivation of KRAS (∼40%) [17Cancer Genome Atlas Network Comprehensive molecular characterization of human colon and rectal cancer.Nature. 2012; 487: 330-337Crossref PubMed Scopus (5904) Google Scholar]. These mutations occur as a 'big bang' event in a limited subset of epithelial cells [18Sottoriva A. et al.A big bang model of human colorectal tumor growth.Nat. Genet. 2015; 47: 209-216Crossref PubMed Scopus (644) Google Scholar] and seem to drive malignant phenotypes by rewiring cell-autonomous signaling in epithelial cells. For example, loss of APC hyperactivates β-catenin signaling [19Fodde R. et al.signal transduction and genetic instability in colorectal cancer.Nat. Rev. Cancer. 2001; 1: 55-67Crossref PubMed Scopus (763) Google Scholar], whereas KRAS point mutations hyperactivate the RAF–MEK–MAPK cascade [20Downward J. Targeting RAS signalling pathways in cancer therapy.Nat. Rev. Cancer. 2003; 3: 11-22Crossref PubMed Scopus (2517) Google Scholar]. This deregulated cell-autonomous signaling promotes epithelial proliferation and self-renewal. Thus, when viewed from a homocellular perspective, emergent malignant phenotypes appear to supervene upon deregulated cell-autonomous epithelial signaling. However, this homocellular view ignores the explicit heterocellularity of metazoan biology. Both in healthy and diseased tissue, colonic epithelia always interact with intestinal mesenchymal cells, lymphocytes, and myeloid cells. Each of these heterotypic cell types processes signals differently from epithelial cells, and can subsequently facilitate unique heterocellular phenotypes. Consequently, an emergent model of cancer must include heterocellular signaling between all cell types in the tumor microenvironment (Figure 3). Nestled beneath the healthy colon epithelia is the lamina propria. This heterocellular structure contains multiple leukocyte lineages and mesenchymal fibroblasts [4Powell D.W. et al.Mesenchymal cells of the intestinal lamina propria.Annu. Rev. Physiol. 2011; 73: 213-237Crossref PubMed Scopus (256) Google Scholar]. The healthy intestine is entirely reliant on heterocellular signaling between epithelial cells and fibroblasts [21Crosnier C. et al.Organizing cell renewal in the intestine: stem cells, signals and combinatorial control.Nat. Rev. Genet. 2006; 7: 349-359Crossref PubMed Scopus (567) Google Scholar]. Notably, fibroblasts are maintained by epithelial Hedgehog signaling [22van den Brink G.R. Hedgehog signaling in development and homeostasis of the gastrointestinal tract.Physiol. Rev. 2007; 87: 1343-1375Crossref PubMed Scopus (237) Google Scholar] and, in turn, fibroblasts secrete Wnt ligands and BMP antagonists to support the epithelial stem cell niche [4Powell D.W. et al.Mesenchymal cells of the intestinal lamina propria.Annu. Rev. Physiol. 2011; 73: 213-237Crossref PubMed Scopus (256) Google Scholar, 23Gregorieff A. et al.Expression pattern of Wnt signaling components in the adult intestine.Gastroenterology. 2005; 129: 626-638Abstract Full Text Full Text PDF PubMed Scopus (309) Google Scholar]. Neither cell type alone is sufficient to maintain intestinal homeostasis – but together they perpetually achieve heterocellular emergence. Given the intimate relationship between epithelial and mesenchymal cells in the healthy colon, it is perhaps unsurprising that fibroblasts are also involved in colonic malignancy. When colonic epithelial cells acquire oncogenic mutations (such as loss of APC), proximal fibroblasts proliferate [24Cui G. et al.Progressive cellular response in the lamina propria of the colorectal adenoma–carcinoma sequence.Histopathology. 2009; 54: 550-560Crossref PubMed Scopus (31) Google Scholar]. This is accompanied by increased expression of the myofibroblast marker αSMA and secretion of matrix metalloproteinases [25Hawinkels L.J. et al.Interaction with colon cancer cells hyperactivates TGF-beta signaling in cancer-associated fibroblasts.Oncogene. 2014; 33: 97-107Crossref PubMed Scopus (174) Google Scholar]. Increased myofibroblast abundance drives epithelial liver metastasis and predicts CRC recurrence after surgery [26Tsujino T. et al.Stromal myofibroblasts predict disease recurrence for colorectal cancer.Clin. 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Cancer Res. 2008; 14: 6405-6413Crossref PubMed Scopus (51) Google Scholar]. Collectively, mesenchymal fibroblasts are a major marker of poor prognosis in CRC [13Isella C. et al.Stromal contribution to the colorectal cancer transcriptome.Nat. Genet. 2015; 47: 312-319Crossref PubMed Scopus (433) Google Scholar, 14Calon A. et al.Stromal gene expression defines poor-prognosis subtypes in colorectal cancer.Nat. Genet. 2015; 47: 320-329Crossref PubMed Scopus (713) Google Scholar]. Because oncogenic mutations only occur in epithelial cells, malignant mesenchymal phenotypes must supervene upon heterocellular interactions. To achieve this, oncogenic epithelial cells need to differentially communicate with fibroblasts (or, to use network parlance, 'malignant edges' must form between epithelial and mesenchymal nodes). Although little is known regarding specific heterocellular interactions in CRC, several studies have revealed that colonic epithelial cells can activate fibroblasts via TGF-β [25Hawinkels L.J. et al.Interaction with colon cancer cells hyperactivates TGF-beta signaling in cancer-associated fibroblasts.Oncogene. 2014; 33: 97-107Crossref PubMed Scopus (174) Google Scholar, 31Jiang L. et al.EpCAM-dependent extracellular vesicles from intestinal epithelial cells maintain intestinal tract immune balance.Nat. Commun. 2016; 7: 13045Crossref PubMed Scopus (71) Google Scholar] and poor-prognosis mesenchymal CRC subtypes display TGF-β-induced epithelial expression signatures. In addition to TGF-β, epithelial IL-33 also activates fibroblasts [32Maywald R.L. et al.IL-33 activates tumor stroma to promote intestinal polyposis.Proc. Natl. Acad. Sci. U. S. A. 2015; 112: E2487-E2496Crossref PubMed Scopus (115) Google Scholar]. 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The pro-malignant role of mesenchymal cells is not limited to the primary tumor because liver fibroblasts also support epithelial cancer cells in the metastatic niche [36Nakagawa H. et al.Role of cancer-associated stromal fibroblasts in metastatic colon cancer to the liver and their expression profiles.Oncogene. 2004; 23: 7366-7377Crossref PubMed Scopus (190) Google Scholar]. These observations support the hypothesis that mutated epithelial cells use the alternative signal-processing capacity of mesenchymal cells to achieve emergent malignant phenotypes in CRC. While focusing on epithelial–mesenchymal heterocellular interactions provides more insight than studying epithelial cells alone, mesenchymal fibroblasts are only one of many non-epithelial cell types in CRC tumors. For a more complete heterocellular perspective, interactions between epithelial cells, fibroblasts, and tissue-resident leukocytes cells must also be considered. The gastrointestinal tract is a major site of host–pathogen interaction. As a result, healthy intestinal tissue is richly populated with leukocytes that patrol against infection [5Mowat A.M. Agace W.W. Regional specialization within the intestinal immune system.Nat. Rev. Immunol. 2014; 14: 667-685Crossref PubMed Scopus (842) Google Scholar]. Macrophages are the most profuse leukocyte in the colon [5Mowat A.M. Agace W.W. Regional specialization within the intestinal immune system.Nat. Rev. Immunol. 2014; 14: 667-685Crossref PubMed Scopus (842) Google Scholar], and their abundance increases further during CRC tumorigenesis [24Cui G. et al.Progressive cellular response in the lamina propria of the colorectal adenoma–carcinoma sequence.Histopathology. 2009; 54: 550-560Crossref PubMed Scopus (31) Google Scholar]. Similarly to myofibroblast expansion, heterocellular signaling also drives the recruitment of myeloid macrophages. For example, epithelial cancer cells produce IL-10 [37Zhang Y. et al.Crosstalk between colon cancer cells and macrophages via inflammatory mediators and CD47 promotes tumour cell migration.Eur. J. Cancer. 2013; 49: 3320-3334Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar] and CCL2 [38Popivanova B.K. et al.Blockade of a chemokine, CCL2, reduces chronic colitis-associated carcinogenesis in mice.Cancer Res. 2009; 69: 7884-7892Crossref PubMed Scopus (160) Google Scholar] to recruit tumor-associated macrophages (TAMs). In turn, TAMs express SIRPα, which can reciprocally increase epithelial migration via CD47 [37Zhang Y. et al.Crosstalk between colon cancer cells and macrophages via inflammatory mediators and CD47 promotes tumour cell migration.Eur. J. Cancer. 2013; 49: 3320-3334Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar]. 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Thus, both in the primary tumor and in distant metastasis, tumor immunogenicity supervenes upon heterocellular interactions between epithelial cancer cells, myeloid cells, and lymphocytes. Although typically considered as hubs for signal processing and ECM production, it is becoming increasingly clear that mesenchymal fibroblasts play a major role in regulating both innate and adaptive immune responses. For example, myofibroblast abundance correlates with the increased presence of cytotoxic T cells in CRC [27Ueno H. et al.Histological categorisation of fibrotic cancer stroma in advanced rectal cancer.Gut. 2004; 53: 581-586Crossref PubMed Scopus (131) Google Scholar]. This can result from fibroblast Toll-like receptor (TLR) activation, which upregulates proinflammatory cytokines that can recruit lymphocytes and myeloid cells into a tumor [54Pang G. et al.GM-CSF, IL-1 alpha, IL-1 beta, IL-6, IL-8, IL-10, ICAM-1 and VCAM-1 gene expression and cytokine production in human duodenal fibroblasts stimulated with lipopolysaccharide, IL-1 alpha and TNF-alpha.Clin. Exp. Immunol. 1994; 96: 437-443Crossref PubMed Scopus (141) Google Scholar]. Intestinal fibroblasts can also act as non-professional antigen-presenting cells (APCs) to the adaptive immune system [55Saada J.I. et al.Subepithelial myofibroblasts are novel nonprofessional APCs in the human colonic mucosa.J. Immunol. 2006; 177: 5968-5979Crossref PubMed Scopus (94) Google Scholar, 56Fletcher A.L. et al.Lymph node fibroblastic reticular cells directly present peripheral tissue antigen under steady-state and inflammatory conditions.J. Exp. 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However, intestinal myofibroblasts can also suppress the proliferation of helper T cells [57Pinchuk I.V. et al.PD-1 ligand expression by human colonic myofibroblasts/fibroblasts regulates CD4+ T-cell activity.Gastroenterology. 2008; 135: 1228-1237Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar], induce colonic regulatory T cells (Tregs) [58Pinchuk I.V. et al.Human colonic myofibroblasts promote expansion of CD4+ CD25high Foxp3+ regulatory T cells.Gastroenterology. 2011; 140: 2019-2030Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar], and secrete anti-inflammatory IL-11 [59Calon A. et al.Dependency of colorectal cancer on a TGF-beta-driven program in stromal cells for metastasis initiation.Cancer Cell. 2012; 22: 571-584Abstract Full Text Full Text PDF PubMed Scopus (742) Google Scholar, 60Putoczki T.L. et al.Interleukin-11 is the dominant IL-6 family cytokine during gastrointestinal tumorigenesis and can be targeted therapeutically.Cancer Cell. 2013; 24: 257-271Abstract Full Text Full Text PDF PubMed Scopus (292) Google Scholar] – and can be considered to be immunosuppressive [61Jones S. et al.The antiproliferative effect of mesenchymal stem cells is a fundamental property shared by all stromal cells.J. Immunol. 2007; 179: 2824-2831Crossref PubMed Scopus (217) Google Scholar]. As a result, the exact role of fibroblasts in regulating CRC immunogenicity is still under investigation. Cancer immunogenicity is a complex heterocellular process. This emergent phenotype can only be achieved by a heterocellular system composed of epithelial, myeloid, lymphoid, and mesenchymal cells. No individual cell type possesses the signal-processing capacity to achieve such emergence on its own. Given the important role of both myofibroblast activation and cancer immunogenicity in CRC prognosis, this provides further evidence that tumors can only be understood as interconnected heterocellular systems (Figure 4). If malignant phenotypes supervene upon heterocellular signaling, then perturbing heterocellular nodes and/or edges could present a powerful approach to treat cancer. Although our knowledge of CRC as a heterocellular system is deeply incomplete, attempts to disrupt heterocellular signaling are already underway. Several studies have focused on disrupting epithelial–mesenchymal interactions [6Tauriello D.V.F. Batlle E. Targeting the microenvironment in advanced colorectal cancer.Trends Cancer. 2016; 2: 495-504Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar]. Notably, perturbation of FAP+ fibroblasts can inhibit CRC progression [62Santos A.M. et al.Targeting fibroblast activation protein inhibits tumor stromagenesis and growth in mice.J. Clin. 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For example, inhibition of CCL5 signaling between T lymphocytes and macrophages can reduce epithelial CRC metastasis [65Halama N. et al.Tumoral immune cell exploitation in colorectal cancer metastases can be targeted effectively by anti-CCR5 therapy in cancer patients.Cancer Cell. 2016; 29: 587-601Abstract Full Text Full Text PDF PubMed Scopus (281) Google Scholar]. This finding suggests that interactions between discrete myeloid and lymphocyte lineages can regulate the emergent malignant phenotype of epithelial cells – further emphasizing the interconnected nature of heterocellular emergence. Although CRC tumors do not demonstrate dramatic responses to immune-checkpoint inhibitors (as observed for melanoma and lung cancer [66Drake C.G. et al.Breathing new life into immunotherapy: review of melanoma, lung and kidney cancer.Nat. Rev. Clin. Oncol. 2014; 11: 24-37Crossref PubMed Scopus (329) Google Scholar]), disrupting epithelial–lymphocyte interactions has shown some promise in CRC patients. For example, mismatch-repair (MMR)-deficient (also known as microsatellite instable, MSI) epithelial CRC cells express higher levels of PD-L1, have a higher neo-antigen load [67Llosa N.J. et al.The vigorous immune microenvironment of microsatellite instable colon cancer is balanced by multiple counter-inhibitory checkpoints.Cancer Discov. 2015; 5: 43-51Crossref PubMed Scopus (971) Google Scholar], and subsequently respond better to T lymphocyte PD-1 blockade [68Le D.T. et al.PD-1 blockade in tumors with mismatch-repair deficiency.N. Engl. J. Med. 2015; 372: 2509-2520Crossref PubMed Scopus (6109) Google Scholar]. However, because only 15% of patients are MMR-deficient, much is still unknown regarding epithelial–immune interactions in CRC. Interestingly, high myeloid infiltrate (as seen in CRC) can suppress checkpoint inhibitor efficacy, and inhibition of myeloid-specific PI3Kγ signaling improves T cell cytotoxicity towards epithelial cancer cells [69De Henau O. et al.Overcoming resistance to checkpoint blockade therapy by targeting PI3Kgamma in myeloid cells.Nature. 2016; 539: 443-447Crossref PubMed Scopus (523) Google Scholar]. These results suggest combined inhibition of cell-specific signaling nodes such as myeloid PI3Kγ, lymphocyte PD-1, and mesenchymal TGF-βR1 could provide a powerful way to treat CRC as an interconnected heterocellular system. Collectively, these nascent observations suggest that coordinated inhibition of cell-specific mesenchymal, myeloid, and lymphoid signaling nodes can disrupt malignant emergence in CRC. However, experience from other cancer types should prompt caution. For example, when inhibitors of mesenchymal SMO were used to treat pancreatic ductal adenocarcinoma (PDA), patients actually performed worse [70Garrido-Laguna I. Hidalgo M. Pancreatic cancer: from state-of-the-art treatments to promising novel therapies.Nat. Rev. Clin. Oncol. 2015; 12: 319-334Crossref PubMed Scopus (438) Google Scholar]. This may be because, in addition to supporting pro-tumor fibroblasts, SMO signaling is also essential for antitumor T lymphocyte activity [71de la Roche M. et al.Hedgehog signaling controls T cell killing at the immunological synapse.Science. 2013; 342: 1247-1250Crossref PubMed Scopus (94) Google Scholar]. By focusing only on epithelial–mesenchymal signaling, pro-tumor consequences of the drug in other cell types were overlooked. Future efforts to target heterocellular signaling need to consider the response of all cell types in a tumor – and not only of those thought to contain the hypothesized drug target. The heterocellularity of metazoan life allows tumors to explore a broad range of signaling options. This expanded signaling potential enables cancer to achieve heterocellular phenotypes that no cell type can accomplish in isolation. As a result, tumors cannot be understood by monitoring cancer cells alone – cancer must be studied as an integrated heterocellular system. Our current knowledge of the tumor microenvironment comes from assembling anecdotal heterocellular signaling events. However, to study cancer as a truly integrated heterocellular system, we eventually need to measure not only epithelial–mesenchymal interactions, or how myeloid cells signal to lymphocytes – but how all the cell types simultaneously collaborate to drive tumors (see Outstanding Questions). Although extremely challenging, such integrative studies are now theoretically possible. Biomimetic (e.g., organoid co-cultures) and genetically engineered mouse models provide solid experimental systems to observe heterocellular emergence. These model systems can then be interrogated using new cell-specific signaling technologies that enable researchers to measure heterocellular signaling at the systems-biology level (reviewed in [2Tape C.J. Systems biology analysis of heterocellular signaling.Trends Biotechnol. 2016; 34: 627-637Abstract Full Text Full Text PDF PubMed Scopus (23) Google Scholar]). Methods such as heterocellular multivariate proteomics [72Gauthier N.P. et al.Cell-selective labeling using amino acid precursors for proteomic studies of multicellular environments.Nat. Methods. 2013; 10: 768-773Crossref PubMed Scopus (41) Google Scholar, 73Tape C.J. et al.Cell-specific labeling enzymes for analysis of cell–cell communication in continuous co-culture.Mol. 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By treating cancers as integrated heterocellular systems – rather than as simple homocellular expansions of cancer cells – new therapeutic opportunities are already being discovered. Future attempts to treat cancer will likely involve disrupting novel heterocellular interactions across epithelial, mesenchymal, myeloid, and lymphoid cells. Because comparable heterocellular interactions have been reported across many solid tumors – including breast [79Bussard K.M. et al.Tumor-associated stromal cells as key contributors to the tumor microenvironment.Breast Cancer Res. 2016; 18: 84Crossref PubMed Scopus (433) Google Scholar], pancreatic [80Ying H. et al.Genetics and biology of pancreatic ductal adenocarcinoma.Genes Dev. 2016; 30: 355-385Crossref PubMed Scopus (316) Google Scholar], prostate [81Taddei M.L. et al.Microenvironment and tumor cell plasticity: an easy way out.Cancer Lett. 2013; 341: 80-96Abstract Full Text Full Text PDF PubMed Scopus (189) Google Scholar], and ovarian cancers [82Hansen J.M. et al.Targeting the tumour microenvironment in ovarian cancer.Eur. J. Cancer. 2016; 56: 131-143Abstract Full Text Full Text PDF PubMed Scopus (86) Google Scholar] – heterocellular emergence is likely to be a universal feature of cancer.Outstanding QuestionsHow do oncogenic mutations (e.g., loss of APC, KRASG12D, loss of p53) regulate heterocellular signaling?Do heterocellular signals converge on common pathways – or does each patient have a unique heterocellular signaling structure that can be targeted?Are heterocellular signaling pathways distinct across different cancer types – or do all cancers employ similar heterocellular signaling networks?How does heterocellular signaling in the primary tumor differ from that in distant metastases? How do oncogenic mutations (e.g., loss of APC, KRASG12D, loss of p53) regulate heterocellular signaling? Do heterocellular signals converge on common pathways – or does each patient have a unique heterocellular signaling structure that can be targeted? Are heterocellular signaling pathways distinct across different cancer types – or do all cancers employ similar heterocellular signaling networks? How does heterocellular signaling in the primary tumor differ from that in distant metastases? C.J.T. is funded by a Sir Henry Wellcome Fellowship (098847/Z/12/Z). C.J.T. would like to acknowledge Dr Claus Jørgensen and Profs Doug Lauffenburger, Clare Isacke, Jon Pines, and Chris Marshall for essential support and guidance. mutation-driven signaling within the mutated cell. emergence is a process in which complex higher-order features are formed from interactions between multiple lower-order entities. differentiated intestinal epithelial cells that absorb nutrients from the intestinal lumen. a process where complex phenotypes are achieved through interactions between different cell types (e.g., adaptive immunity). communication between different cell types that each have their own unique signal processing capacity. This allows each cell type to expand its signaling options. a population of cells composed of multiple differentiated cell types (e.g., metazoan tissue). communication between cell types that each have the same signal-processing capacity (e.g., epithelial–epithelial cell-junction signaling). a population of cells composed of a single differentiated cell type (e.g., epithelium). sub-epithelial intestinal structure containing fibroblasts and leukocytes. a subset of CRC cells with errors in MMR DNA repair machinery. deregulated signaling in a non-mutated cell driven by mutations in a local mutated cell. a mutation that drives malignant phenotypes. deregulated signaling in a mutated cancer cell as a result of a non cell-autonomous signal returning from a non-mutated cell. an ontological relationship whereby higher-level properties of an emergent system depend on lower-level entities. all cells, matrix, and nutrients in the vicinity of a tumor.