Microenvironment driven invasion: a multiscale multimodel investigation

Cancer is a complex, multiscale process, in which genetic mutations occurring at a subcellular level manifest themselves as functional and morphological changes at the cellular and tissue scale. The importance of interactions between tumour cells and their microenvironment is currently of great interest in experimental as well as computational modelling. Both the immediate microenvironment (e.g. cell-cell signalling or cell-matrix interactions) and the extended microenvironment (e.g. nutrient supply or a host tissue structure) are thought to play crucial roles in both tumour progression and suppression. In this paper we focus on tumour invasion, as defined by the emergence of a fingering morphology, which has previously been shown to be dependent upon harsh microenvironmental conditions. Using three different modelling approaches at two different spatial scales we examine the impact of nutrient availability as a driving force for invasion. Specifically we investigate how cell metabolism (the intrinsic rate of nutrient consumption and cell resistance to starvation) influences the growing tumour. We also discuss how dynamical changes in genetic makeup and morphological characteristics, of the tumour population, are driven by extreme changes in nutrient supply during tumour development. The simulation results indicate that aggressive phenotypes produce tumour fingering in poor nutrient, but not rich, microenvironments. The implication of these results is that an invasive outcome appears to be co-dependent upon the evolutionary dynamics of the tumour population driven by the microenvironment.     

Tumor morphology and phenotypic evolution driven by selective pressure from the microenvironment

Emergence of invasive behavior in cancer is life-threatening, yet ill-defined due to its multifactorial nature. We present a multiscale mathematical model of cancer invasion, which considers cellular and microenvironmental factors simultaneously and interactively. Unexpectedly, the model simulations predict that harsh tumor microenvironment conditions (e.g., hypoxia, heterogenous extracellular matrix) exert a dramatic selective force on the tumor, which grows as an invasive mass with fingering margins, dominated by a few clones with aggressive traits. In contrast, mild microenvironment conditions (e.g., normoxia, homogeneous matrix) allow clones with similar aggressive traits to coexist with less aggressive phenotypes in a heterogeneous tumor mass with smooth, noninvasive margins. Thus, the genetic make-up of a cancer cell may realize its invasive potential through a clonal evolution process driven by definable microenvironmental selective forces. Our mathematical model provides a theoretical/experimental framework to quantitatively characterize this selective pressure for invasion and test ways to eliminate it.     

Emergent behaviors from a cellular automaton model for invasive tumor growth in heterogeneous microenvironments

Understanding tumor invasion and metastasis is of crucial importance for both fundamental cancer research and clinical practice. In vitro experiments have established that the invasive growth of malignant tumors is characterized by the dendritic invasive branches composed of chains of tumor cells emanating from the primary tumor mass. The preponderance of previous tumor simulations focused on non-invasive (or proliferative) growth. The formation of the invasive cell chains and their interactions with the primary tumor mass and host microenvironment are not well understood. Here, we present a novel cellular automaton (CA) model that enables one to efficiently simulate invasive tumor growth in a heterogeneous host microenvironment. By taking into account a variety of microscopic-scale tumor-host interactions, including the short-range mechanical interactions between tumor cells and tumor stroma, degradation of the extracellular matrix by the invasive cells and oxygen/nutrient gradient driven cell motions, our CA model predicts a rich spectrum of growth dynamics and emergent behaviors of invasive tumors. Besides robustly reproducing the salient features of dendritic invasive growth, such as least-resistance paths of cells and intrabranch homotype attraction, we also predict nontrivial coupling between the growth dynamics of the primary tumor mass and the invasive cells. In addition, we show that the properties of the host microenvironment can significantly affect tumor morphology and growth dynamics, emphasizing the importance of understanding the tumor-host interaction. The capability of our CA model suggests that sophisticated in silico tools could eventually be utilized in clinical situations to predict neoplastic progression and propose individualized optimal treatment strategies.     

胃癌的肿瘤微环境研究进展

肿瘤微环境是决定肿瘤细胞行为的主要影响因素,有别于正常细胞与其周围组织所形成的微环境,组织缺氧和酸中毒、间质高压形成、大量生长因子和蛋白水解酶的产生及免疫炎性反应等构成了肿瘤组织代谢环境的生物学特征,这种特性在肿瘤的发生、进展、转移中扮演重要的角色。胃癌早期症状不典型、转移迅速、死亡率高,是消化系统最常见的恶性肿瘤,目前,关于肿瘤微环境的研究尚处于起步阶段,对胃癌肿瘤微环境的研究有助于我们进一步认识胃癌发生发展的机制,并为临床诊断、治疗胃癌提供依据。因此,本文就近年来在胃癌肿瘤微环境方面的研究进展作一综述。     

Anti-cancer therapies targeting the tumor stroma

For anti-tumor therapy different strategies have been employed, e.g., radiotherapy, chemotherapy, or immunotherapy. Notably, these approaches do not only address the tumor cells themselves, but also the tumor stroma cells, e.g., endothelial cells, fibroblasts, and macrophages. This is of advantage, since these cells actively contribute to the proliferative and invasive behavior of the tumor cells via secretion of growth factors, angiogenic factors, cytokines, and proteolytic enzymes. In addition, tumor stroma cells take part in immune evasion mechanisms of cancer. Thus, approaches targeting the tumor stroma attract increasing attention as anti-cancer therapy. Several molecules including growth factors (e.g., VEGF, CTGF), growth factor receptors (CD105, VEGFRs), adhesion molecules (alphavbeta3 integrin), and enzymes (CAIX, FAPalpha, MMPs, PSMA, uPA) are induced or upregulated in the tumor microenvironment which are otherwise characterized by a restricted expression pattern in differentiated tissues. Consequently, these molecules can be targeted by inhibitors as well as by active and passive immunotherapy to treat cancer. Here we discuss the results of these approaches tested in preclinical models and clinical trials.     

Front Instabilities and Invasiveness of Simulated Avascular Tumors

We study the interface morphology of a 2D simulation of an avascular tumor composed of identical cells growing in an homogeneous healthy tissue matrix (TM), in order to understand the origin of the morphological changes often observed during real tumor growth. We use the Glazier–Graner–Hogeweg model, which treats tumor cells as extended, deformable objects, to study the effects of two parameters: a dimensionless diffusion-limitation parameter defined as the ratio of the tumor consumption rate to the substrate transport rate, and the tumor-TM surface tension. We model TM as a nondiffusing field, neglecting the TM pressure and haptotactic repulsion acting on a real growing tumor; thus, our model is appropriate for studying tumors with highly motile cells, e.g., gliomas. We show that the diffusion-limitation parameter determines whether the growing tumor develops a smooth (noninvasive) or fingered (invasive) interface, and that the sensitivity of tumor morphology to tumor-TM surface tension increases with the size of the dimensionless diffusion-limitation parameter. For large diffusion-limitation parameters, we find a transition (missed in previous work) between dendritic structures, produced when tumor-TM surface tension is high, and seaweed-like structures, produced when tumor-TM surface tension is low. This observation leads to a direct analogy between the mathematics and dynamics of tumors and those observed in nonbiological directional solidification. Our results are also consistent with the biological observation that hypoxia promotes invasive growth of tumor cells by inducing higher levels of receptors for scatter factors that weaken cell-cell adhesion and increase cell motility. These findings suggest that tumor morphology may have value in predicting the efficiency of antiangiogenic therapy in individual patients.     

通过模拟胚胎微环境逆转肿瘤的研究进展

肿瘤的发生和发展源于一小部分具有自我更新能力的肿瘤干细胞。胚胎干细胞也具有自我更新和多向分化的特性。胚胎干细胞特异的基质微环境能够提供干细胞正常生长的调控分子,在细胞不断更新的情况下,使增殖和分化达到平衡。受胚胎干细胞调节的基质或胚胎微环境作用于肿瘤细胞,可以使肿瘤细胞获得更多的分化表型,显著降低其恶性程度,抑制肿瘤细胞的侵袭行为。进一步的分子机制研究发现,在肿瘤细胞中高表达的Nodal蛋白会抑制肿瘤细胞分化,而胚胎干细胞分泌的糖基化Lefty蛋白可以负反馈调节Nodal蛋白的作用,从而降低肿瘤细胞的恶性程度。利用组织工程来模拟胚胎干细胞微环境,保留Lefty蛋白,从而逆转肿瘤的方法具有广阔的前景。     

Exploring the Competition between Proliferative and Invasive Cancer Phenotypes in a Continuous Spatial Model

Tumor is characterized by extensive heterogeneity with respect to its microenvironment and its genetic composition. We extend a previously developed monoclonal continuous spatial model of tumor growth to account for polyclonal cell populations and investigate the interplay between a more proliferative and a more invasive phenotype under different conditions. The model simulations demonstrate a transition from the dominance of the proliferative to the dominance of the invasive phenotype resembling malignant tumor progression and show a time period where both subpopulations are abundant. As the dominant phenotype switches from proliferative to invasive, the geometry of tumor changes from a compact and almost spherical shape to a more diffusive and fingered morphology with the proliferative phenotype to be restricted in the tumor bulk and the invasive to dominate at tumor edges. Different micro-environmental conditions and different phenotypic properties can promote or inhibit invasion demonstrating their mutual importance. The model provides a computational framework to investigate tumor heterogeneity and the constant interplay between the environment and the specific characteristics of phenotypes that should be taken into account for the prediction of tumor evolution, morphology and effective treatment.     

肿瘤酸性微环境的研究进展

研究表明,肿瘤转移是恶性肿瘤的临床治疗失败的根本原因。肿瘤转移不仅取决于肿瘤细胞自身的特性,还涉及其与肿瘤酸性微环境之间的相互作用。肿瘤微环境构成非常复杂,可促进肿瘤的增生、转移、侵袭,以及逃避宿主免疫监视和治疗耐药性。肿瘤细胞的生存依赖于在酸性微环境条件下的适应,肿瘤细胞可以通过一些离子交换体维持酸性微环境,缺氧的肿瘤组织酸化可以释放蛋白酶如纤维蛋白酶及MMPs降解细胞外基质、上调VEGF基因表达促进肿瘤新生血管生成等促进肿瘤侵袭转移。近年来,影响肿瘤微环境的因素已经成为癌症研究领域中的新兴话题。     

Nonlinear simulation of tumor growth

 We study solid tumor (carcinoma) growth in the nonlinear regime using boundary-integral simulations. The tumor core is nonnecrotic and no inhibitor chemical species are present. A new formulation of the classical models [18,24,8,3] is developed and it is demonstrated that tumor evolution is described by a reduced set of two dimensionless parameters and is qualitatively unaffected by the number of spatial dimensions. One parameter describes the relative rate of mitosis to the relaxation mechanisms (cell mobility and cell-to-cell adhesion). The other describes the balance between apoptosis (programmed cell-death) and mitosis. Both parameters also include the effect of vascularization. Our analysis and nonlinear simulations reveal that the two new dimensionless groups uniquely subdivide tumor growth into three regimes associated with increasing degrees of vascularization: low (diffusion dominated, e.g., in vitro), moderate and high vascularization, that correspond to the regimes observed in vivo. We demonstrate that critical conditions exist for which the tumor evolves to nontrivial dormant states or grows self-similarly (i.e., shape invariant) in the first two regimes. This leads to the possibility of shape control and of controlling the release of tumor angiogenic factors by restricting the tumor volume-to-surface-area ratio. Away from these critical conditions, evolution may be unstable leading to invasive fingering into the external tissues and to topological transitions such as tumor breakup and reconnection. Interestingly we find that for highly vascularized tumors, while they grow unbounded, their shape always stays compact and invasive fingering does not occur. This is in agreement with recent experimental observations [30] of in vivo tumor growth, and suggests that the invasive growth of highly-vascularized tumors is associated to vascular and elastic anisotropies, which are not included in the model studied here. Received: 1 May 2002 / Revised version: 26 August 2002 / Published online: 18 December 2002 Current address: Department of Biomedical Engineering, and Department of Mathematics, University of California at Irvine, Irvine CA 92697. e-mail: cristini@math.uci.edu; lowengrb@math.umn.edu. Key words or phrases: Tumor growth – Linear stability analysis – Self-similarity – Boundary-integral simulations     

Long-term Intravital Immunofluorescence Imaging of Tissue Matrix Components with Epifluorescence and Two-photon Microscopy

Besides being a physical scaffold to maintain tissue morphology, the extracellular matrix (ECM) is actively involved in regulating cell and tissue function during development and organ homeostasis. It does so by acting via biochemical, biomechanical, and biophysical signaling pathways, such as through the release of bioactive ECM protein fragments, regulating tissue tension, and providing pathways for cell migration. The extracellular matrix of the tumor microenvironment undergoes substantial remodeling, characterized by the degradation, deposition and organization of fibrillar and non-fibrillar matrix proteins. Stromal stiffening of the tumor microenvironment can promote tumor growth and invasion, and cause remodeling of blood and lymphatic vessels. Live imaging of matrix proteins, however, to this point is limited to fibrillar collagens that can be detected by second harmonic generation using multi-photon microscopy, leaving the majority of matrix components largely invisible. Here we describe procedures for tumor inoculation in the thin dorsal ear skin, immunolabeling of extracellular matrix proteins and intravital imaging of the exposed tissue in live mice using epifluorescence and two-photon microscopy. Our intravital imaging method allows for the direct detection of both fibrillar and non-fibrillar matrix proteins in the context of a growing dermal tumor. We show examples of vessel remodeling caused by local matrix contraction. We also found that fibrillar matrix of the tumor detected with the second harmonic generation is spatially distinct from newly deposited matrix components such as tenascin C. We also showed long-term (12 hours) imaging of T-cell interaction with tumor cells and tumor cells migration along the collagen IV of basement membrane. Taken together, this method uniquely allows for the simultaneous detection of tumor cells, their physical microenvironment and the endogenous tissue immune response over time, which may provide important insights into the mechanisms underlying tumor progression and ultimate success or resistance to therapy.     

Effect of P2X purinergic receptors in tumor progression and as a potential target for anti-tumor therapy

The development of tumors is a complex pathological process involving multiple factors, multiple steps, and multiple genes. Their prevention and treatment have always been a difficult problem at present. A large number of studies have proved that the tumor microenvironment plays an important role in the progression of tumors. The tumor microenvironment is the place where tumor cells depend for survival, and it plays an important role in regulating the growth, proliferation, apoptosis, migration, and invasion of tumor cells. P2X purinergic receptors, which depend on the ATP ion channel, can be activated by ATP in the tumor microenvironment, and by mediating tumor cells and related cells (such as immune cells) in the tumor microenvironment. They play an important regulatory role on the effects of the skeleton, membrane fluidity, and intracellular molecular metabolism of tumor cells. Therefore, here, we outlined the biological characteristics of P2X purinergic receptors, described the effect of tumor microenvironment on tumor progression, and discussed the effect of ATP on tumor. Moreover, we explored the role of P2X purinergic receptors in the development of tumors and anti-tumor therapy. These data indicate that P2X purinergic receptors may be used as another potential pharmacological target for tumor prevention and treatment.

     

Mechanisms and significance of bifunctional NK4 in cancer treatment

Based on the background that hepatocyte growth factor (HGF) and c-Met/HGF receptor tyrosine kinase play a definite role in tumor invasion and metastasis, NK4, four-kringles containing intramolecular fragment of HGF, was isolated as a competitive antagonist for the HGF-c-Met system. Independent of its HGF-antagonist action, NK4 inhibited angiogenesis induced by vascular endothelial cell growth factor and basic fibroblast growth factor, as well as HGF, indicating that NK4 is a bifunctional molecule that acts as an HGF-antagonist and angiogenesis inhibitor. Interestingly, kringle domains in distinct types of proteins, e.g., plasminogen, prothrombin, plasminogen activators, apolipoprotein(a), and HGF, share angioinhibitory actions. In experimental models of distinct types of cancers, NK4 protein administration or NK4 gene therapy inhibited tumor invasion, metastasis, and angiogenesis-dependent tumor growth. Cancer treatment with NK4 may prove to suppress malignant tumors to be 'static' in both tumor growth and spreading, as based on biological characteristics of malignant tumors.     

CXCR4/CXCL12 在肿瘤中的研究进展

研究表明趋化因子及其受体在胚胎发育、干细胞迁移以及各种免疫反应中发挥重要作用,是许多生理及病理过程中细胞运动的重要因素。趋化因子受体CXCR4是一个由352个氨基酸构成的、7次跨膜的G蛋白偶联受体。趋化因子CXCL12为其特异性受体。研究发现,CXCR4/CXCL12在多种肿瘤中都有表达,在肿瘤的生长、血管生成、转移等方面发挥着重要作用。与正常组织相比,肿瘤组织及转移灶CXCR4高表达。因此,对CXCR4/CXCL12轴在肿瘤病生理中的作用机制进行进一步研究,很可能为肿瘤的治疗及对肿瘤转移的预防提供一个新的思路。我们现在就对其在肿瘤病生理中的作用做一综述。     

Exploring cancer stem cell niche directed tumor growth

The finding that only a sub-fraction of tumor cells, so called Cancer Stem Cells (CSC), is endowed with the capacity to initiate new tumors has important consequences for fundamental as well as clinical cancer research. Previously we established by computational modeling techniques that CSC driven tumor growth instigates infiltrative behavior, and perhaps most interesting, stimulates tumor cell heterogeneity. An important question that remains is to what extend CSC functions are intrinsically regulated or whether this capacity is orchestrated by the microenvironment, i.e. a putative CSC niche. Here we investigate how extrinsic regulation of CSC properties affects the characteristics of malignancies. We find that highly invasive growth in tumors dependent on a small subset of cells is not restricted to CSC-driven tumors, but is also observed in tumors where the CSC capacity of tumor cells is completely defined by the microenvironment. Importantly, also the high level of heterogeneity that was observed for CSC-driven tumors is preserved and partially even increased in malignancies with a microenvironmentally orchestrated CSC population. This indicates that invasive growth and high heterogeneity are fundamental properties of tumors fueled by a small population of tumor cells.     

Breast cancer subtypes express distinct receptor repertoires for tumor-associated macrophage derived cytokines

Infiltration of the tumor microenvironment by macrophages is associated with poor outcomes in breast cancer and other solid tumors, however the identity and roles of many of the soluble factors these macrophages produce remains to be elucidated in detail. In addition to producing angiogenic factors (e.g. VEGF), proteases (e.g. MMP9) and immunomodulatory factors (e.g. IL10) which, by modifying the local microenvironment, likely contribute to progression in the majority of solid tumors, we have evaluated the extent to which macrophage cytokines may differentially affect distinct breast cancer subtypes. We identified 23 cytokines produced in a culture model of human tumor-associated macrophages and report that basal and luminal breast cancer cell lines express different repertoires of receptors for these cytokines. These data suggest that tumor-associated macrophages make specific contributions to different breast cancer subtypes and that understanding the importance of these interactions will be crucial to developing subtype-specific therapies targeting the macrophage component of the breast tumor microenvironment.     

跨太平洋生物入侵研究展望

跨太平洋生物入侵是当代最受关注、最具影响的生物学现象之一 ,这一过程导致并促进了新东亚 -北美间断分布格局 (与许多众所周知的古间断分布相对应 )的形成。为了更好地了解这一现象以及相关的生物类群 ,我们探讨了以下几个问题 :1)哪些类型的物种参与或可能会参与跨太平洋生物入侵 ,2 )这些入侵种在入侵之后会发生什么变化以及会导致什么样的后果 ,3)为了有效地监控生物入侵 ,我们应该从哪些方面着手研究入侵种及其原生和非原生生境。为了解决这些问题 ,我们应该对原产地和入侵地的这些物种进行比较研究 ,这些研究包括 :1)遗传学 ,2 )生活史 /形态学 (如 :个体大小、种子大小等 ) ,3)生态学 (如 :生活型 /生长型、传粉媒介和竞争对手等 ) ,4 )在原产地和入侵地的地理分布 (如 :分布区的大小、形状以及纬度等 ) ,5 )物理影响因子 (如土壤、水分和气候等 )。这些研究的目的在于 :1)确定外来种在其原生生境中影响其分布的限制因子 ,2 )了解入侵种能够在入侵地成功的原因 ,3)预测可能进一步发生的生物入侵 ,4 )为有效地监控和管理生物入侵提供资料。     

Ecological photodynamic therapy: New trend to disrupt the intricate networks within tumor ecosystem

As with natural ecosystems, species within the tumor microenvironment are connected by pairwise interactions (e.g. mutualism, predation) leading to a strong interdependence of different populations on each other. In this review we have identified the ecological roles played by each non-neoplastic population (macrophages, endothelial cells, fibroblasts) and other abiotic components (oxygen, extracellular matrix) directly involved with neoplastic development. A way to alter an ecosystem is to affect other species within the environment that are supporting the growth and survival of the species of interest, here the tumor cells; thus, some features of ecological systems could be exploited for cancer therapy. We propose a well-known antitumor therapy called photodynamic therapy (PDT) as a novel modulator of ecological interactions. We refer to this as “ecological photodynamic therapy.” The main goal of this new strategy is the improvement of therapeutic efficiency through the disruption of ecological networks with the aim of destroying the tumor ecosystem. It is therefore necessary to identify those interactions from which tumor cells get benefit and those by which it is impaired, and then design multitargeted combined photodynamic regimes in order to orchestrate non-neoplastic populations against their neoplastic counterpart. Thus, conceiving the tumor as an ecological system opens avenues for novel approaches on treatment strategies.     

Hallmarks of cancer: Interactions with the tumor stroma

Ten years ago, Hanahan and Weinberg delineated six “Hallmarks of cancer” which summarize several decades of intense cancer research. However, tumor cells do not act in isolation, but rather subsist in a rich microenvironment provided by resident fibroblasts, endothelial cells, pericytes, leukocytes, and extra-cellular matrix. It is increasingly appreciated that the tumor stroma is an integral part of cancer initiation, growth and progression. The stromal elements of tumors hold prognostic, as well as response-predictive, information, and abundant targeting opportunities within the tumor microenvironment are continually identified. Herein we review the current understanding of tumor cell interactions with the tumor stroma with a particular focus on cancer-associated fibroblasts and pericytes. Moreover, we discuss emerging fields of research which need to be further explored in order to fulfil the promise of stroma-targeted therapies for cancer.