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Modulation of cell death in the tumor microenvironment.

The microenvironment of solid human tumors is characterized by heterogeneity in oxygenation. Hypoxia arises early in the process of tumor development because rapidly proliferating tumor cells outgrow the capacity of the host vasculature. Formation of solid tumors thus requires coordination of angiogenesis with continued tumor cell proliferation. However, despite such neovascularization, hypoxia is persistent and frequently found in tumors at the time of diagnosis. Tumors with low oxygenation have a poor prognosis, and strong evidence suggests this is because of the effects of hypoxia on malignant progression, angiogenesis, metastasis, and therapy resistance. The presence of viable hypoxic cells is likely a reflection of the development of hypoxia tolerance resulting from modulation of cell death in the microenvironment. This acquired feature has been explained on the basis of clonal selection-the hypoxic microenvironment selects cells capable of surviving in the absence of normal oxygen availability. However, the persistence and frequency of hypoxia in solid tumors raises a second potential explanation. We suggest that stable microregions of hypoxia may play a positive role in tumor growth. Although hypoxia inhibits cell proliferation and in tumor cells will eventually induce cell death, hypoxia also provides angiogenic and metastatic signals. The development of hypoxia tolerance will thus allow prolonged survival in the absence of oxygen and generation of a persistent angiogenic signal. We will discuss the concept of hypoxia tolerance and review mechanisms used by cancer cells to acquire this phenotype. The concept of hypoxia tolerance has important implications for current and future therapeutic approaches. Most therapeutic efforts to combat hypoxia have focused on targeting the presence of hypoxia itself. Our hypothesis predicts that targeting the biological responses to hypoxia and the pathways leading to hypoxia tolerance may also be attractive therapeutic strategies.

Apoptosis↗

The tumor microenvironment as a determinant of drug response and resistance.

Classically, studies of drug resistance in cancer have focused on the molecular biology of single cancer cells. These types of studies have provided important information regarding certain drug resistance mechanisms, including mechanisms that reduce intracellular drug accumulation, alter or repair drug-induced damage, and reduce drug-induced apoptosis. While these cellular mechanisms undoubtedly contribute to the overall phenomenon of drug resistance, it is now evident that the tumor cell microenvironment also influences how a tumor cell behaves and responds to cytotoxic drugs or radiation. Two different forms of tumor cell-environmental interaction may explain how some tumor cells survive initial drug exposure and eventually express classical mechanisms of drug resistance. The first form involves soluble mediators, such as interleukins, that are secreted by non-tumor, stromal cells. Interleukin-6 (IL-6) is a classical example of how a soluble mediator secreted by the tumor microenvironment is capable of enhancing tumor cell survival and perhaps blocking apoptosis. The second form of tumor cell-environment interaction requires direct cell contact and has been given the term cell-adhesion-mediated drug resistance (CAM-DR). In this case, binding extracellular matrix ligands in the tumor microenvironment may activate cell adhesion molecules, such as the integrins, and these interactions result in the activation of signal transduction pathways that block drug-induced apoptosis. Interrupting the tumor cell-environment interactions or the associated signal transduction pathways may represent a new approach for the treatment of cancer. Copyright 1999 Harcourt Publishers Ltd.

Journal Article↗

Expression of CD146 adhesion molecules (MUC18 or MCAM) in the thymic microenvironment.

The thymic microenvironment is thought to play a critical role in T-lymphocyte development, providing signals both via cell surface molecules such as adhesion molecules and soluble molecules. The present investigation is focused on immunoelectron microscopical analysis of distribution patterns of CD146 adhesion molecules (MUC18 or MCAM) in the microenvironment of normal human thymus, using the pre-embedding indirect immunoperoxidase technique. The anti-CD146 monoclonal antibody (mAb) revealed strong membraneous labelling of immature thymocytes at both the light and electron microscopical level. Proliferating thymocytes, most of the epithelial cells, macrophages, endothelial cells and smooth musle cells of small vessels and capillaries showed both membraneous and cytoplasmic labelling with anti-CD146 mAb as was demonstrated by electron microscopy. In contrast, these cells displayed a strong cytoplasmic immunoreactivity at the light microscopical level. The extracellular matrix was also stained with the anti-CD146 mAb. No labelling was observed in interdigitating cells. Interestingly, the CD146 molecule was strongly expressed on apical and lateral membranes of endothelial cells as was demonstrated electron microscopically. This selective CD146 labelling of capillary endothelium mainly localized at the cortico-medullary junction may be manifestations of lymphocyte transmembrane migration and lymphocyte homing. In conclusion, the present study suggests that CD146 is expressed by most elements of the microenvironment of normal human thymus. Therefore, it may be a pan-antigen which is essential for the maintenance of thymic architecture and function.

Antigens, CD↗

Hurdles to lymphocyte trafficking in the tumor microenvironment: implications for effective immunotherapy.

An important consideration in the development of T cell-based cancer immunotherapy is that effector T cells must efficiently traffic to the tumor microenvironment in order to control malignant progression. T cell trafficking to target tissues is orchestrated by dynamic interactions between circulating lymphocytes and endothelial cells lining blood vessels. It is informative, in this regard, to compare and contrast the molecular mechanisms governing lymphocyte extravasation at distinct vascular sites: (1) high endothelial venules (HEV) of secondary lymphoid organs, which are portals for efficient trafficking of naive and central memory T lymphocytes; (2) non-activated endothelium of normal tissues that mediate relatively low basal levels of trafficking but are rapidly transformed into HEV-like vessels in response to local inflammatory stimuli; and (3) vessels within the intratumoral region and the surrounding peritumoral areas. These vessels can be distinguished by differential expression of hallmark trafficking molecules that function as molecular beacons directing lymphocyte migration across vascular barriers. This article reviews evidence that recruitment of effector T cells to the intratumoral microenvironment is impeded by sub-threshold expression of trafficking molecules on tumor microvessels. Emerging data support the thesis that when considered from the perspective of extravasation, vessels embedded within the intratumoral microenvironment of established tumors do not exhibit stereotypical characteristics of a chronic inflammatory state. A major challenge will be to develop therapeutic approaches to improve trafficking of effector T lymphocytes to tumor sites without skewing the balance in favor of a chronic inflammatory milieu that facilitates tumor maintenance and progression.

Animals↗

Microenvironment-induced cancer metastasis.

PURPOSE: To present and evaluate clinical data suggesting that cancer metastasis may be induced by the microenvironment of the primary tumour and to discuss possible mechanisms of microenvironment-induced metastasis, based on a critical review of relevant data from studies of experimental tumours and cells in culture. CONCLUSIONS: Low oxygen tension in the primary tumour is associated with metastasis in soft tissue sarcoma, cervix carcinoma and carcinoma of the head and neck. Multiple mechanisms may be involved in hypoxia-induced metastasis. Thus, hypoxia followed by reoxygenation may induce point mutations and DNA strand breakage leading to deletions, amplifications and genomic instability. Hypoxia may also provide a physiological pressure in tumours selecting for metastatic cell phenotypes. Moreover, hypoxia may induce a temporary increase in the expression of gene products involved in the metastatic cascade, either through gene amplifications or through normal physiological processes by activating oxygen sensors, hypoxia signal transduction pathways and DNA transcription factors. Low glucose concentration, high lactate concentration and low extracellular pH may induce metastasis by similar mechanisms as hypoxia. Tumour reoxygenation during radiation therapy may promote microenvironment-induced metastasis by rescuing hypoxic or nutritionally deprived metastatic cells from dying. Ionizing radiation can elicit a stress response in tumour cells similar to that elicited by hypoxia. Radiation therapy may therefore adversely affect the rate of metastasis in patients who do not achieve control of the primary tumour by enhancing the expression of gene products of importance in metastasis.

Animals↗

Long-term effects of plutonium-239 and radium-224 on the distribution and performance of pluripotent haemopoietic progenitor cells and their regulatory microenvironment.

Experiments are described which investigate the long-term damage to haemopoietic progenitor cells (CFU-S) and their microenvironment in mouse marrow resulting from the administration of leukaemogenic amounts of plutonium-239 and radium-224. 239Pu (35 Bq g-1 body weight) and 224Ra (555 Bg g-1 body weight) were injected into 10-12-week-old mice, and numbers, proliferative activity and self-renewal capacity of CFU-S were measured at different locations in femoral marrow at intervals over the following 2 years. Parallel measurements were also made of the quality of the haemopoietic microenvironment by ectopic transplantation of bone marrow cells. There was some recovery from the initial effects of 239Pu on CFU-S numbers after 3-6 months, although the recovery was not maintained in all marrow fractions. Following 224Ra administration there was an initial transient increase in CFU-S numbers in the fraction of marrow furthest from bone surfaces but a considerable depression in numbers in other regions of marrow; there was no recovery between 3 and 6 months and subsequent recovery was not complete in all regions of marrow. The differential responses of CFU-S and the haemopoietic microenvironment following 224Ra or 239Pu administration seemed in some ways related to the metabolism of the radionuclides. There was a profound reduction in the ability of marrow to generate ossicles when transplanted under the kidney capsule as a result of the administration of either 224Ra or 239Pu, with only transient recoveries from the effects of 239Pu at 4 days and at 3 months after injection.

Animals↗

Zoledronic acid exerts its antitumor effect in multiple myeloma interfering with the bone marrow microenvironment.

Multiple myeloma (MM) is a B-cell malignancy characterized by an excess of monotypic plasma cells which localize almost exclusively in the bone marrow provoking bone destruction via the activation of the osteoclasts. The bone marrow microenvironment, mainly through stromal cells, is strictly involved in the evolution of the disease supporting MM cell growth and survival [1]. MM plasma cells reside in the bone marrow by binding to adhesion molecule of extracellular matrix (ECM) and stromal cells. The activation of some signaling pathways within the stromal cells increases the production of several cytokines which in turn favors the myeloma cell proliferation and survival [2-6], and enhance the drug resistance by anti-apoptotic mechanisms [1,7-9]. Novel therapeutic agents target not only the myeloma cells but also the interaction between MM cells and the bone marrow microenvironment [8]. Bisphosphonates (Bps) interfere as well with bone microenvironment inhibiting the survival of stromal cells and hampering the contact between plasma and stromal cells. In this review we will revise preclinical evidences, and the potential mechanisms of the antitumor activity of zoledronic acid.

Animals↗

Survival of B lineage leukemic cells: signals from the bone marrow microenvironment.

Stromal cells are an essential component of the bone marrow microenvironment that regulate development of immature hematopoietic progenitor cells. Through production of soluble cytokines, and signaling through adhesion molecule interactions, stromal cells impact survival, proliferation, and differentiation of hematopoietic progenitor cells. Similarities between normal pro-B and pre-B cells and B lineage acute lymphoblastic leukemic (ALL) progenitors have been well characterized which provide a model for investigation of the mechanisms by which ALL cells respond to bone marrow microenvironment signals. In addition to providing survival signals to B lineage ALL during initiation of disease, the bone marrow has long been recognized as a "sanctuary site" for leukemic cells during traditional chemotherapy. In the current review, mechanisms by which stromal cells contribute to leukemic cell survival, and the potential impact on treatment efficacy, are discussed. A growing appreciation of the significance of the bone marrow microenvironment in the progression of ALL, and further investigation of the signaling between leukemic progenitors and stromal cells, may contribute to novel treatment strategies aimed at enhancing sensitivity of ALL cells to currently available chemotherapeutic agents.

Animals↗

Essential microenvironment for thymopoiesis is preserved in human adult and aged thymus.

Normal human thymuses at various ages were immunohistologically examined in order to determine whether adult or aged thymus maintained the microenvironment for the T cell development and thymopoiesis was really ongoing. To analyze the thymic microenvironment, two monoclonal antibodies (MoAb) were employed. One is MoAb to IL-1 receptor (IL-1R) recognizing medullary and subcapsular cortical epithelial cells of normal infant human thymus. The other is UH-1 MoAb recognizing thymic epithelial cells within the cortex, which are negative with IL-1R-MoAb. Thymus of subjects over 20 years of age was split into many fragments and dispersed in the fatty tissue. However, the microenvironment of each fragment was composed of both IL-1R positive and UH-1 positive epithelial cells, and the UH-1 positive portion was populated with lymphocytes showing a follicle-like appearance. Lymphocytes in these follicle-like portions were mostly CD4+CD8+ double positive cells and contained many proliferating cells as well as apoptotic cells. Thus these follicle-like portions in adult and aged thymus were considered to be functioning as cortex as in infant thymus. Proliferative activity of thymocytes in the thymic cortex and the follicle-like portions definitely declined with advance of age, while incidence of apoptotic thymocytes increased with aging.

Adolescent↗

Relative importance of school bus-related microenvironments to children's pollutant exposure.

Real-time concentrations of black carbon, particle-bound polycyclic aromatic hydrocarbons, nitrogen dioxide, and fine particulate counts, as well as integrated and real-time fine particulate matter (PM2.5) mass concentrations were measured inside school buses during long commutes on Los Angeles Unified School District bus routes, at bus stops along the routes, at the bus loading/unloading zone in front of the selected school, and at nearby urban "background" sites. Across all of the pollutants, mean concentrations during bus commutes were higher than in any other microenvironment. Mean exposures (mean concentration times time spent in a particular microenvironment) in bus commutes were between 50 and 200 times greater than those for the loading/unloading microenvironment, and 20-40 times higher than those for the bus stops, depending on the pollutant. Although the analyzed school bus commutes represented only 10% of a child's day, on average they contributed one-third of a child's 24-hr overall black carbon exposure during a school day. For species closely related to vehicle exhaust, the within- cabin exposures were generally dominated by the effect of surrounding traffic when windows were open and by the bus's own exhaust when windows were closed. Low-emitting buses generally exhibited high concentrations only when traveling behind a diesel vehicle, whereas high-emitting buses exhibited high concentrations both when following other diesel vehicles and when idling without another diesel vehicle in front of the bus. To reduce school bus commute exposures, we recommend minimizing commute times, avoiding caravanning with other school buses, using the cleanest buses for the longest bus routes, maintaining conventional diesel buses to eliminate visible emissions, and transitioning to cleaner fuels and advanced particulate control technologies as soon as possible.

Air Pollutants, Occupational↗

The tumor microenvironment as a determinant of cancer cell survival: a possible mechanism for de novo drug resistance.

The influence of the microenvironment in the pathogenesis and progression of human cancer has traditionally been considered in the context of solid tumors. More recently, evidence has been accumulating to support the role of the bone marrow microenvironment in hematologic malignancies as well, particularly in multiple myeloma. This review focuses on myeloma as a model to demonstrate that the bone marrow microenvironment provides a sanctuary against programmed cell death and promotes tumor cell survival and progression. Additionally, the protective effects of the bone marrow milieu may confer a protection from cytotoxic drugs, allowing the emergence of drug-resistant tumors. These advances may assist in the design of novel therapeutic approaches to enhance the efficacy of standard chemotherapeutic drugs.

Bone Marrow↗

Investigating the microenvironments of inhomogeneous soft materials with multiple particle tracking.

We develop a multiple particle tracking technique for making precise, localized measurements of the mechanical microenvironments of inhomogeneous materials. Using video microscopy, we simultaneously measure the Brownian dynamics of roughly one hundred fluorescent tracer particles embedded in a complex medium and interpret their motions in terms of local viscoelastic response. To help overcome the inherent statistical limitations due to the finite imaging volume and limited imaging times, we develop statistical techniques and analyze the distribution of particle displacements in order to make meaningful comparisons of individual particles and thus characterize the diversity and properties of the microenvironments. The ability to perform many local measurements simultaneously allows more precise measurements even in systems that evolve in time. We show several examples of inhomogeneous materials to demonstrate the flexibility of the technique and learn new details of the mechanics of the microenvironments that small particles explore. This technique extends other microrheological methods to allow simultaneous measurements of large numbers of probe particles, enabling heterogeneous samples to be studied more effectively.

Journal Article↗

Growth factors and chronic wounds: the need to understand the microenvironment.

The care of chronic wounds has become a major health issue in developed countries because of their increasingly elderly populations. There is hope that progress made in understanding and producing growth factors will lead to their successful use to induce faster and better healing of chronic wounds. This report will discuss growth factors in the context of their use in chronic wounds, and will focus on the importance of the wound microenvironment in determining the interactions between growth factors and wounds. We believe that a greater understanding of the chronic wound microenvironment will be of benefit in the optimal use of growth factors. In published studies, we have found that wound fluid taken from acute wounds stimulates fibroblast and endothelial cell proliferation, whereas fluid obtained from chronic non-healing wounds inhibits the growth of fibroblasts, endothelial cells, and keratinocytes. In this report, we describe the effect of these two types of wound fluid on the synthesis of extracellular matrix components. We hypothesize that the chronic wound microenvironment is generally non-conducive to cell growth, and that this may prevent a truly successful use of topical growth factors in chronic wounds. Novel approaches in the delivery of growth factors to wounds may be necessary to overcome these obstacles.

Cells, Cultured↗

CpG motif acts as a 'danger signal' and provides a T helper type 1-biased microenvironment for DNA vaccination.

The method of delivering a DNA vaccine can influence the type of immune response induced by the vaccine. Application of a DNA vaccine by gene gun typically induces a T helper type 2 (Th2)-type reaction, whereas needle inoculation triggers a Th1 response. In the present study, we found that physical trauma, gold-particle bombardment and the CpG motif can act as 'danger signals' that recruit inflammatory cells to damaged tissues. Analysis of the cytokine profiles of draining lymph nodes or lymph-node-derived mononuclear cells from different groups by real-time reverse transcription-polymerase chain reaction revealed that, while gene-gun-bombardment induced a Th2-type cytokine microenvironment with increased interleukin-4 (IL-4) and IL-10 mRNA expression and almost no increase in IL-12 and interferon-gamma mRNA expression in draining lymph nodes, intradermal injection as well as subcutaneous injection of muscle induced the opposite. We further studied whether the addition of the CpG motif can switch the Th2-type cytokine microenvironment produced by gene-gun bombardment in draining lymph nodes. Results showed that the addition of the CpG motif can increase IL-12 mRNA expression in draining lymph nodes whether induced by intradermal injection, intramuscular injection, or gene-gun bombardment. These data suggest that delivery of the CpG motif induced a Th1-biased microenvironment in draining lymph nodes. Taken together, the CpG motif can act as a 'danger signal' and Th1 immune response enhancer in DNA vaccination. These results may help to explain the mechanism of different types of immune response induced by DNA vaccines delivered by different routes and facilitate the application of DNA vaccines.

Animals↗

Thermostability of a nuclear-targeted luciferase expressed in mammalian cells. Destabilizing influence of the intranuclear microenvironment.

Protein denaturation and aggregation are most likely the cause for the noxious effects of heat shock. There are some indications that the nucleus is one of the most sensitive cellular compartments. To test the possibility that the intranuclear microenvironment might be detrimental to the heat stability of proteins, we compared the in situ thermal stability of a reporter protein localized in the nucleus or in the cytoplasm. A recombinant firefly (Photynus pyralis) luciferase carrying a point mutation in the C-terminal domain remains in the cytoplasm (cyt-luciferase). A nuclear localization sequence was fused to the N-terminal domain of cyt-luciferase; the resulting nuc-luciferase was efficiently targeted to the cell nucleus. In both cases, decreased luciferase activity and solubility were found in lysates from heat-shocked cells. These characteristics were taken as an indication of thermal denaturation in situ. The heat-inactivated luciferases were partially reactivated during recovery after stress, indicating the capacity of both the cytoplasmic and nuclear compartments to reassemble proteins from an aggregated state. Although both the nuc- and the cyt-luciferases were heat inactivated at similar rates in vitro, nuc-luciferase was more susceptible to thermal denaturation in situ compared to cyt-luciferase. This observation suggests that the microenvironment of an intracellular compartment may modulate the thermal stability of proteins. The local concentration might be one element of this microenvironment affecting the heat-stability of proteins. In cells made thermotolerant by a priming shock, the thermal inactivation of the recombinant luciferases occurred at a slower rate during a second challenging stress. However, this decreased thermal sensitivity was less pronounced for the nuc-luciferase (threefold) than for the cyt-luciferase (sevenfold). The nuclear luciferase might become a useful tool to investigate the action of molecular chaperones in the nucleus.

Base Sequence↗

Immune resistance orchestrated by the tumor microenvironment.

It is now little disputed that most if not all cancer cells express antigens that can be recognized by specific CD8(+) T lymphocytes. However, a central question in the field of anti-tumor immunity is why such antigen-expressing tumors are not spontaneously eliminated by the immune system. While in some cases, this lack of rejection may be due to immunologic ignorance, induction of anti-tumor T-cell responses in many patients has been detected in the peripheral blood, either spontaneously or in response to vaccination, without accompanying tumor rejection. These observations argue for the importance of barriers downstream from initial T-cell priming that need to be addressed to translate immune responses into clinical tumor regression. Recent data suggest that the proper trafficking of effector T cells into the tumor microenvironment may not always occur. T cells that do effectively home to tumor metastases are often found to be dysfunctional, pointing toward immunosuppressive mechanisms in the tumor microenvironment. T-cell anergy due to insufficient B7 costimulation, extrinsic suppression by regulatory cell populations, inhibition by ligands such as programmed death ligand-1, metabolic dysregulation by enzymes such as indoleamine-2,3-dioxygenase, and the action of soluble inhibitory factors such as transforming growth factor-beta have all been clearly implicated in generating this suppressive microenvironment. Identification of these downstream processes points to new therapeutic targets that should be manipulated to facilitate the effector phase of anti-tumor immune responses in concert with vaccination or T-cell adoptive transfer.

Animals↗

Detection of functional and dimeric activin A in human marrow microenvironment. Implications for the modulation of erythropoiesis.

Activin A, which was initially recognized as a gonadal protein, was implicated in the modulation of erythropoiesis through a paracrine control in the bone marrow microenvironment. Present studies demonstrate that, in contrast to T lymphocytes and cultured skin fibroblasts, human marrow stromal cells produce a functional and dimeric beta A beta A molecule (i.e., activin A). RT-PCR further indicates that both alpha and beta A mRNAs of inhibin A/activin A are produced in human stromal cells. The level of beta A subunit mRNAs, however, is in large excess over that of alpha subunit mRNAs, suggesting the predominant production of beta A beta A dimers, as well as some inhibin A (alpha beta A). It should be noted, however, that the beta A subunit can form dimeric proteins other than activin A, such as activin AB (beta A beta B) and inhibin A (alpha beta A). Hence, the presence of the beta A subunit may not necessarily indicate the production of the activin A molecule in any tissue. Therefore, a special quantitative sandwich ELISA assay specific for the dimeric beta A beta A molecule was developed for the measurement of activin A. With this assay, production of activin A in marrow stromal cells is found to be greatly enhanced by cytokines and inflammatory mediators such as TNF-alpha, IL-1 alpha, and lipopolysaccharide. These studies thus suggest that inflammatory cytokines are the inducers for activin A, probably serving a role of up-regulating activin A production locally in bone marrow microenvironment. At present, activin A is not known to play any role in inflammatory reaction; this study may thus raise the possibility that activin A performs more functions than are currently recognized. Alternatively, the enhanced production of this molecule in the bone marrow microenvironment may be regarded as a compensatory mechanism in host defenses, countering inflammatory mediators that are known to suppress erythropoiesis.

Activins↗

Remodeling of the microenvironment by aggressive melanoma tumor cells.

A necessity for development and tumor progression is a blood supply formed by vasculogenic and/or angiogenic events, involving the cooperative interactions of cells with their microenvironment. Based on the recent characterization of vasculogenic mimicry by aggressive melanoma cells, particularly their ability to express VE (vascular endothelial)-cadherin, TIE-1, and EphA2, current studies have focused on the molecular signals deposited by these cells as they remodel their microenvironment. The experimental approach utilizes unique three-dimensional collagen matrices preconditioned by metastatic melanoma cells, which contain laminin 5 gamma2 chain-enriched tracks with promigratory cleavage fragments produced by cooperative interactions with specific matrix metalloproteinases (MMPs). The results demonstrate that the collagen matrices preconditioned by the metastatic cells induce poorly aggressive melanoma cells to express, for the first time, key angiogenic/vasculogenic/matrix-remodeling genes. Treatment of aggressive melanoma cells with an MMP inhibitor resulted in the inhibition of vasculogenic mimicry-associated genes in these tumor cells and abrogation of the inductive effects of the preconditioned matrix on poorly aggressive melanoma cells. These observations illustrate the remarkable influence of the microenvironment on the phenotype of melanoma cells and may provide new perspectives on tumor cell plasticity and unique treatment strategies.

Cell Adhesion Molecules↗