ReviewCan cancer be reversed by engineering the tumor microenvironment?
Introduction
Cancers are commonly thought to result from progressive accumulation of random gene mutations, and most research in this area has therefore sought to identify critical oncogenic genes or proteins. This paradigm led the pharmaceutical industry to focus on development of drugs that target single molecular components encoded or regulated by these genes. It is now clear, however, that cell behaviors are not regulated by a linear series of commands, but rather by networks of molecular interactions that involve positive and negative reinforcement, as well as high levels of cross talk integrated at the whole system (genome-wide gene and protein regulatory network) level [1], [2], [3], [4].
In this type of dynamic regulatory network, switching between different stable states or phenotypes requires that activities of signaling molecules in multiple pathways change in concert [1], [3], [5], [6]. For example, different mitogens commonly activate over 60 genes in common [7], and master switch genes (e.g., myoD, Snail) similarly control the implementation of complex behavioral programs required for cell fate switching by regulating the concerted expression of scores of downstream genes [8], [9]. Adult skin fibroblasts also can be induced to revert to pluripotent embryonic stem cells by simultaneously co-expressing a handful of different transcription factors that, in turn, activate multiple downstream genes [10], [11], [12], [13].
If multiple signaling elements in the cellular regulatory network must be altered simultaneously to change cell phenotype, then various types of environmental stimuli that produce pleiotropic effects in cells (and hence are not commonly thought of as ‘specific’ bioregulators) may contribute to normal and malignant tissue development. This may explain why switching between different cell fates that are critical for cancer (e.g., growth, differentiation, apoptosis, motility) can be triggered in normal and transformed cells, as well as stem cells, by changes in extracellular matrix (ECM) structure and cell shape distortion [14], [15], [16], [17], [18], [19], [20], ‘non-specific’ chemical solvents [6] and electrical ion flows [21], [22], [23], [24] that influence multiple gene activities, as well as by distinct molecular factors or specific gene mutations in epithelial or mesenchymal cells. It also could explain why although a combination of four genes was recently found to be sufficient to induce fibroblasts to revert to embryonic stem cells in two different experimental reports, the four genes utilized were different in each study (only two genes were shared in common) [10], [11], [25]. Perhaps it is for this reason that conventional strategies for the development of anti-cancer therapeutics have been suboptimal, and why many active ‘single target’ drugs (e.g., Glivec) are later discovered to influence multiple signaling pathways simultaneously [26]. Thus, a key obstacle for future progress in the cancer therapeutics field is to develop experimental, theoretical and therapeutic strategies that take into account the structural complexity and system-level nature of cellular regulation [5], [27], [28], [29].
Another major problem restricting forward advance is that cancer is often defined as a disease of cell proliferation. But deregulated growth is not sufficient to make a tissue cancerous; a wart is a simple example. What makes a growing cancer malignant is its ability to break down tissue architecture, invade through disrupted tissue boundaries, and metastasize to distant organ sites. In simplest terms, cancer is a disease of development: it results from loss of the normal controls that direct cells to assemble into tissues, and that hold tissues within their organ confines [30], [31], [32], [33], [34], [35]. This is consistent with the increasing appreciation of the importance of cancer stem cells [36], epithelial–mesenchymal transitions [37], and angiogenesis [38] for tumor formation and metastatic progression. In addition, provocative experiments first carried out over 35 years ago show that certain cancers differentiate and normalize their growth when combined with normal mesenchyme, other embryonic tissues, or with ECMs that are deposited as a result of interactions between these tissues [30], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]. Some human malignant carcinomas also induce host stroma to be tumorigenic in nude mice [49]. Thus, cancer is a developmental disease that involves dysfunction of multi-cellular inductive interactions; it does not result from unregulated growth of a single cell type.
Interest in work pursuing developmental contributions and non-genetic causes of cancer waned when the molecular biology revolution surged and the focus shifted almost entirely to genetic causation. But there is now renewed interest in developmental and environmental contributions to cancer growth because of more recent studies that confirm the central role that the tissue microenvironment plays during tumor formation [35], [50], [51], [52]. These experiments show, for example, that carcinogens must act on the connective tissue stroma as well as the epithelium to produce a cancer [53]; mechanical interactions between cancer cells and ECM can accelerate neoplastic transformation [18], [54]; normal tissues can be induced to become cancerous in vivo by altering ECM structure [55]; and stroma from healthy adult animals can prevent neoplastic transformation and encourage normal growth of grafted epithelial cancer cells [48]. The recent clinical approval and use of angiogenesis inhibitors that target the vasculature in the stromal compartment of the tumor, and not the cancer cells themselves, provide additional evidence supporting the potential value of this unconventional view of cancer formation and progression.
Thus, the challenge is to retrace our steps, and to re-explore this old path of investigation in cancer research that was left by the wayside years ago. Specifically, these observations raise the possibility that the production of cancer stem cells, epithelial–mesenchymal transitions, increased angiogenesis and unrestrained cell growth that drive cancer formation may result from deregulation of the tissue microenvironment. Conversely, embryonic tissues may reverse cancerous growth by restoring these normal microenvironmental cues. In this article, we explore these possibilities in greater detail, and briefly discuss the central role that the Rho family of small GTPases plays in this micromechanical control system. The possibility of developing a tissue engineering approach to cancer therapy that involves creation of biomimetic materials that mimic the inductive, cancer-reversing properties of embryonic tissues, is also discussed.
Section snippets
Structural determinants of cancer formation
Epigenetic (non-genetic) factors play an important role in cancer formation. Although the term ‘epigenetic’ has come to be used in a very narrow way (i.e., to refer to stable chromatin modifications), the reality is that there are many other non-genetic contributors to cancer development. For example, while constitutive expression of an oncogene in the beta cells of pancreatic islets stimulates growth and produces pancreatic tumor formation in transgenic mice, these tumors remain in a benign
Cancer as a disease of tissue development
Cancer is a developmental disease because it results from a breakdown of the fundamental rules that govern how cells stably organize within tissues, tissues within organs, and organs within the whole living organism. Uncontrolled cell growth is necessary for cancer formation, but it is not sufficient. It is only when growth becomes autonomous and leads to disorganization of normal tissue architecture that a pathologist can recognize that a normal tissue has undergone ‘neoplastic
A microstructural view of cancer formation
The early observations described above led to the proposal over 25 years ago that local changes in the physical properties of the ECM in the tissue microenvironment might potentially lead to cancer formation [30], [31], [32], [34], [44], [57]. This engineering view of cancer formation was based on the idea that tumors may result from progressive deregulation of normal epithelial–mesenchymal interactions that are required to maintain stable tissue form throughout adult life (Fig. 1). For
Mechanochemical control of cell fate switching by ECM
Studies carried out in the field of Mechanical Biology over the past 20 years have confirmed that cells can be switched between phenotypes critical for neoplastic transformation, including growth, differentiation, motility and apoptosis in the presence of soluble mitogens by mechanically distorting cells and altering cytoskeletal structure. For example, epithelial and endothelial cells generally proliferate on ECM substrates that resist cell traction forces generated in the actin cytoskeleton
Integration of structural and information processing networks
How can mechanical distortion of the cell influence its growth and function? Mechanical strain of the ECM that alters forces transmitted across cell surface integrin receptors can activate various intracellular signaling pathways (e.g., ERK, MAP, Ca++, src, G proteins, etc.) that are also triggered by soluble cytokines when their bind their surface receptors [28], [102], [103]. But cell fate switching cannot be explained in terms of changes in a single signaling pathway. For example, the
Can we devise new therapies that ‘reboot’ cancers to revert to normal tissues?
The studies and concepts reviewed above bring many of the existing assumptions in the Cancer Biology field into question. All scientific fields face conflicting hypotheses and mechanistic explanations during their formative stages, but then one or two theories win out and dominate thereafter. Often the losing theories are incorrect or significantly flawed; however, sometimes they are valid, but the tools do not yet exist to test them properly. The importance of the physicality of the tumor
Acknowledgements
This article reviews work that was supported by NIH. This article is based on a proposal that has been funded by a DoD Breast Cancer Innovator Award (to D.E.I.).
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