Gene expression profiles in 3D tumor analogs indicate compressive strain differentially enhances metastatic potential

Interstitial fluid flow in cancer: implications for disease progression and treatment

As cancer progresses, a dynamic microenvironment develops that creates and responds to cellular and biophysical cues. Increased intratumoral pressure and corresponding increases in interstitial flow from the tumor bulk to the healthy stroma is an observational hallmark of progressing cancers. Until recently, the role of interstitial flow was thought to be mostly passive in the transport and dissemination of cancer cells to metastatic sites. With research spanning the past decade, we have seen that interstitial flow has a promigratory effect on cancer cell invasion in multiple cancer types. This invasion is one mechanism by which cancers can resist therapeutics and recur, but the role of interstitial flow in cancer therapy is limited to the understanding of transport of therapeutics. Here we outline the current understanding of the role of interstitial flow in cancer and the tumor microenvironment through cancer progression and therapy. We also discuss the current role of fluid flow in the treatment of cancer, including drug transport and therapeutic strategies. By stating the current understanding of interstitial flow in cancer progression, we can begin exploring its role in therapeutic failure and treatment resistance.

Role of constitutive behavior and tumor-host mechanical interactions in the state of stress and growth of solid tumors

Mechanical forces play a crucial role in tumor patho-physiology. Compression of cancer cells inhibits their proliferation rate, induces apoptosis and enhances their invasive and metastatic potential. Additionally, compression of intratumor blood vessels reduces the supply of oxygen, nutrients and drugs, affecting tumor progression and treatment. Despite the great importance of the mechanical microenvironment to the pathology of cancer, there are limited studies for the constitutive modeling and the mechanical properties of tumors and on how these parameters affect tumor growth. Also, the contribution of the host tissue to the growth and state of stress of the tumor remains unclear. To this end, we performed unconfined compression experiments in two tumor types and found that the experimental stress-strain response is better fitted to an exponential constitutive equation compared to the widely used neo-Hookean and Blatz-Ko models. Subsequently, we incorporated the constitutive equations along with the corresponding values of the mechanical properties - calculated by the fit - to a biomechanical model of tumor growth. Interestingly, we found that the evolution of stress and the growth rate of the tumor are independent from the selection of the constitutive equation, but depend strongly on the mechanical interactions with the surrounding host tissue. Particularly, model predictions - in agreement with experimental studies - suggest that the stiffness of solid tumors should exceed a critical value compared with that of the surrounding tissue in order to be able to displace the tissue and grow in size. With the use of the model, we estimated this critical value to be on the order of 1.5. Our results suggest that the direct effect of solid stress on tumor growth involves not only the inhibitory effect of stress on cancer cell proliferation and the induction of apoptosis, but also the resistance of the surrounding tissue to tumor expansion.

Elastic free energy drives the shape of prevascular solid tumors

It is well established that the mechanical environment influences cell functions in health and disease. Here, we address how the mechanical environment influences tumor growth, in particular, the shape of solid tumors. In an in vitro tumor model, which isolates mechanical interactions between cancer tumor cells and a hydrogel, we find that tumors grow as ellipsoids, resembling the same, oft-reported observation of in vivo tumors. Specifically, an oblate ellipsoidal tumor shape robustly occurs when the tumors grow in hydrogels that are stiffer than the tumors, but when they grow in more compliant hydrogels they remain closer to spherical in shape. Using large scale, nonlinear elasticity computations we show that the oblate ellipsoidal shape minimizes the elastic free energy of the tumor-hydrogel system. Having eliminated a number of other candidate explanations, we hypothesize that minimization of the elastic free energy is the reason for predominance of the experimentally observed ellipsoidal shape. This result may hold significance for explaining the shape progression of early solid tumors in vivo and is an important step in understanding the processes underlying solid tumor growth.

Modeling tumor microenvironments in vitro

Tumor progression depends critically upon the interactions between the tumor cells and their microenvironment. The tumor microenvironment is heterogeneous and dynamic; it consists of extracellular matrix, stromal cells, immune cells, progenitor cells, and blood and lymphatic vessels. The emerging fields of tissue engineering and microtechnologies have opened up new possibilities for engineering physiologically relevant and spatially well-defined microenvironments. These in vitro models allow specific manipulation of biophysical and biochemical parameters, such as chemical gradients, biomatrix stiffness, metabolic stress, and fluid flows; thus providing a means to study their roles in certain aspects of tumor progression such as cell proliferation, invasion, and crosstalk with other cell types. Challenges and perspectives for deconvolving the complexity of tumor microenvironments will be discussed. Emphasis will be given to in vitro models of tumor cell migration and invasion.

Pushing tumor cells towards a malignant phenotype: stimuli from the microenvironment, intercellular communications and alternative roads

The tumor microenvironment produces different types of stimuli capable of endowing tumor cells with an aggressive behavior that is characterized by increased motility, invasiveness and propensity to metastasize, gain of a tumor-initiating phenotype, and drug resistance. The following classes of stimuli have been reported to promote such a malignant phenotype: (i) solid- or fluid-induced stress; (ii) altered composition of the extracellular matrix; (iii) hypoxia and low pH; (iv) innate and adaptive immune responses; (v) antitumor drugs. The simultaneous presence of more than one of these stimuli, as likely occurs in vivo, may lead to synergistic interactions in the induction of malignant traits. In many cases, the gain of a malignant phenotype is not the result of a direct effect of the stimuli on tumor cells but, rather, a stimulus-promoted cross-talk between tumor cells and other cell types within the tumor microenvironment. This cross-talk is mainly mediated by two classes of molecules: paracrine factors and adhesion receptors. Stimuli that promote a malignant phenotype can promote additional outcomes in tumor cells, including autophagy and cell death. We summarize here the available evidence about the variables that induce tumor cells to take one or the other of these roads in response to the same stimuli. At the end of this review, we address some unanswered questions in this domain and indicate future directions of research.

Choline plasmalogens isolated from swine liver inhibit hepatoma cell proliferation associated with caveolin-1/Akt signaling

Plasmalogens play multiple roles in the structures of biological membranes, cell membrane lipid homeostasis and human diseases. We report the isolation and identification of choline plasmalogens (ChoPlas) from swine liver by high performance thin layer chromatography (HPTLC) and high performance liquid chromatography (HPLC)/MS. The growth and viability of hepatoma cells (CBRH7919, HepG2 and SMMC7721) was determined following ChoPlas treatment comparing with that of human normal immortal cell lines (HL7702). Result indicated that ChoPlas inhibited hepatoma cell proliferation with an optimal concentration and time of 25 μmol/L and 24 h. To better understand the mechanism of the ChoPlas-induced inhibition of hepatoma cell proliferation, Caveolin-1 and PI3K/Akt pathway signals, including total Akt, phospho-Akt(pAkt) and Bcl-2 expression in CBRH7919 cells, were determined by western blot. ChoPlas treatment increased Caveolin-1 expression and reduced the expression of phospho-Akt (pAkt) and Bcl-2, downstream targets of the PI3K/Akt pathway. Further cell cycle analysis showed that ChoPlas treatment induced G1 and G1/S phase transition cell cycle arrest. The expression of essential cell cycle regulatory proteins involved in the G1 and G1/S phase transitions, cyclin D, CDK4, cyclin E and CDK2, were also analyzed by western blot. ChoPlas reduced CDK4, cyclin E and CDK2 expression. Taken together, the results indicate that swine liver-derived natural ChoPlas inhibits hepatoma cell proliferation associated with Caveolin-1 and PI3K/Akt signals.

Mechano-transduction in tumour growth modelling

The evolution of biological systems is strongly influenced by physical factors, such as applied forces, geometry or the stiffness of the micro-environment. Mechanical changes are particularly important in solid tumour development, as altered stromal-epithelial interactions can provoke a persistent increase in cytoskeletal tension, driving the gene expression of a malignant phenotype. In this work, we propose a novel multi-scale treatment of mechano-transduction in cancer growth. The avascular tumour is modelled as an expanding elastic spheroid, whilst growth may occur both as a volume increase and as a mass production within a cell rim. Considering the physical constraints of an outer healthy tissue, we derive the thermo-dynamical requirements for coupling growth rate, solid stress and diffusing biomolecules inside a heterogeneous tumour. The theoretical predictions successfully reproduce the stress-dependent growth curves observed by in vitro experiments on multicellular spheroids.

Causes, consequences, and remedies for growth-induced solid stress in murine and human tumors

The presence of growth-induced solid stresses in tumors has been suspected for some time, but these stresses were largely estimated using mathematical models. Solid stresses can deform the surrounding tissues and compress intratumoral lymphatic and blood vessels. Compression of lymphatic vessels elevates interstitial fluid pressure, whereas compression of blood vessels reduces blood flow. Reduced blood flow, in turn, leads to hypoxia, which promotes tumor progression, immunosuppression, inflammation, invasion, and metastasis and lowers the efficacy of chemo-, radio-, and immunotherapies. Thus, strategies designed to alleviate solid stress have the potential to improve cancer treatment. However, a lack of methods for measuring solid stress has hindered the development of solid stress-alleviating drugs. Here, we present a simple technique to estimate the growth-induced solid stress accumulated within animal and human tumors, and we show that this stress can be reduced by depleting cancer cells, fibroblasts, collagen, and/or hyaluronan, resulting in improved tumor perfusion. Furthermore, we show that therapeutic depletion of carcinoma-associated fibroblasts with an inhibitor of the sonic hedgehog pathway reduces solid stress, decompresses blood and lymphatic vessels, and increases perfusion. In addition to providing insights into the mechanopathology of tumors, our approach can serve as a rapid screen for stress-reducing and perfusion-enhancing drugs.

Pannexin1 drives multicellular aggregate compaction via a signaling cascade that remodels the actin cytoskeleton

Pannexin 1 (Panx1) is a novel gap junction protein shown to have tumor-suppressive properties. To model its in vivo role in the intratumor biomechanical environment, we investigated whether Panx1 channels modulate the dynamic assembly of multicellular C6 glioma aggregates. Treatment with carbenoxolone and probenecid, which directly and specifically block Panx1 channels, respectively, showed that Panx1 is involved in accelerating aggregate assembly. Experiments further showed that exogenous ATP can reverse the inhibitive effects of carbenoxolone and that aggregate compaction is sensitive to the purinergic antagonist suramin. With a close examination of the F-actin microfilament network, these findings show that Panx1 channels act as conduits for ATP release that stimulate the P(2)X(7) purinergic receptor pathway, in turn up-regulating actomyosin function. Using a unique three-dimensional scaffold-free method to quantify multicellular interactions, this study shows that Panx1 is intimately involved in regulating intercellular biomechanical interactions pivotal in the progression of cancer.

Stress clamp experiments on multicellular tumor spheroids

The precise role of the microenvironment on tumor growth is poorly understood. Whereas the tumor is in constant competition with the surrounding tissue, little is known about the mechanics of this interaction. Using a novel experimental procedure, we study quantitatively the effect of an applied mechanical stress on the long-term growth of a spheroid cell aggregate. We observe that a stress of 10 kPa is sufficient to drastically reduce growth by inhibition of cell proliferation mainly in the core of the spheroid. We compare the results to a simple numerical model developed to describe the role of mechanics in cancer progression.

Influence of stress on extracellular matrix and integrin biology

Dynamic interactions between cells and extracellular matrix (ECM) through integrins influence most cellular functions. Normal cells, but even more, tumor cells are subjected to different forms of stress, including ischemia, radical oxygen species production, starvation, mechanical stress or genotoxic insults due to anti-cancer drugs or irradiation. In these situations, an adaptative cellular response occurs, integrating a complex network of intracellular signaling modules, which, depending on stress intensity, may result to either damage repair followed by complete restitution of cellular functions, or programmed cell death. Because of its implication in oncogenesis and anti-cancer therapy, cellular stress response has been thoroughly investigated. However, most of these studies have been performed in the context of isolated cells without taking into consideration that most cells are part of the tissue within which they interact with ECM through integrin. Few studies have described the influence of stress on cell-to-ECM interaction. However, one can speculate that, in these conditions, cells could functionally interact with protein microenvironment either to create positive interactions to survive (for example by facilitating protective pathways) or negative interaction to die (for example by facilitating detachment). In this review, we summarize the knowledge relative to the influence of different stress modalities on ECM remodeling, integrin expression and/or function modifications, and possible functional consequences, independently from the cellular model as these findings came from a large variety of cells (mesenchymal, endothelial, muscular, epithelial and glandular) and fields of application (cancer, vascular biology and tissue engineering). Most studies support the general notion that non-lethal stress favors ECM stiffness, integrin activation and enhanced survival. This field opens large perspectives not only in tumor biology but also in anti-cancer therapy by targeting one or several steps of the integrin-mediated signaling pathway, including integrin ligation, or activation of integrin-linked enzymes or integrin adaptors.

Biomechanical forces shape the tumor microenvironment

The importance of the tumor microenvironment in cancer progression is indisputable, yet a key component of the microenvironment--biomechanical forces--remains poorly understood. Tumor growth and progression is paralleled by a host of physical changes in the tumor microenvironment, such as growth-induced solid stresses, increased matrix stiffness, high fluid pressure, and increased interstitial flow. These changes to the biomechanical microenvironment promote tumorigenesis and tumor cell invasion and induce stromal cells--such as fibroblasts, immune cells, and endothelial cells--to change behavior and support cancer progression. This review highlights what we currently know about the biomechanical forces generated in the tumor microenvironment, how they arise, and how these forces can dramatically influence cell behavior, drawing not only upon studies directly related to cancer and tumor cells, but also work in other fields that have shown the effects of these types of mechanical forces vis-à-vis cell behaviors relevant to the tumor microenvironment. By understanding how all of these biomechanical forces can affect tumor cells, stromal cells, and tumor-stromal crosstalk, as well as alter how tumor and stromal cells perceive other extracellular signals in the tumor microenvironment, we can develop new approaches for diagnosis, prognosis, and ultimately treatment of cancer.