An examination of the effects of hypoxia, acidosis, and glucose starvation on the expression of metastasis-associated genes in murine tumor cells

Mitochondrial Protein Cox7b Is a Metabolic Sensor Driving Brain-Specific Metastasis of Human Breast Cancer Cells

Distant metastases are detrimental for cancer patients, but the increasingly early detection of tumors offers a chance for metastasis prevention. Importantly, cancers do not metastasize randomly: depending on the type of cancer, metastatic progenitor cells have a predilection for well-defined organs. This has been theorized by Stephen Paget, who proposed the “seed-and-soil hypothesis”, according to which metastatic colonization occurs only when the needs of a given metastatic progenitor cell (the seed) match with the resources provided by a given organ (the soil). Here, we propose to explore the seed-and-soil hypothesis in the context of cancer metabolism, thus hypothesizing that metastatic progenitor cells must be capable of detecting the availability of metabolic resources in order to home in a secondary organ. If true, it would imply the existence of metabolic sensors. Using human triple-negative MDA-MB-231 breast cancer cells and two independent brain-seeking variants as models, we report that cyclooxygenase 7b (Cox7b), a structural component of Complex IV of the mitochondrial electron transport chain, belongs to a probably larger family of proteins responsible for breast cancer brain tropism in mice. For metastasis prevention therapy, this proof-of-principle study opens a quest for the identification of therapeutically targetable metabolic sensors that drive cancer organotropism.

Metabolites of the Oral Microbiome: Important Mediators of Multi-Kingdom Interactions

The oral cavity hosts over 700 different microbial species that produce a rich reservoir of bioactive metabolites critical to oral health maintenance. Over the last two decades, new insights into the oral microbiome and its importance in health and disease have emerged mainly due to the discovery of new oral microbial species using next-generation sequencing (NGS). This advancement has revolutionized the documentation of unique microbial profiles associated with different niches and health/disease states within the oral cavity and the relation of the oral bacteria to systemic diseases. However, less work has been done to identify and characterize the unique oral microbial metabolites that play critical roles in maintaining equilibrium between the various oral microbial species and their human hosts. This article discusses the most significant microbial metabolites produced by these diverse communities of oral bacteria that can either foster health or contribute to disease. Finally, we shed light on how advances in genomics and genome mining can provide a high throughput platform for discovering novel bioactive metabolites derived from the human oral microbiome to tackle emerging human infections and systemic diseases.

Cancer metabolism in space and time: Beyond the Warburg effect

Altered metabolism in cancer cells is pivotal for tumor growth, most notably by providing energy, reducing equivalents and building blocks while several metabolites exert a signaling function promoting tumor growth and progression. A cancer tissue cannot be simply reduced to a bulk of proliferating cells. Tumors are indeed complex and dynamic structures where single cells can heterogeneously perform various biological activities with different metabolic requirements. Because tumors are composed of different types of cells with metabolic activities affected by different spatial and temporal contexts, it is important to address metabolism taking into account cellular and biological heterogeneity. In this review, we describe this heterogeneity also in metabolic fluxes, thus showing the relative contribution of different metabolic activities to tumor progression according to the cellular context. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

Mesenchymal-epithelial signalling in tumour microenvironment: role of high-mobility group Box 1

Glucose deprivation, hypoxia and acidosis are characteristic features of the central core of most solid tumours. Myofibroblasts are stromal cells present in many such solid tumours, including those of the colon, and are known to be involved in all stages of tumour progression. HMGB1 is a nuclear protein with an important role in nucleosome stabilisation and gene transcription; it is also released from immune cells and is involved in the inflammatory process. We report that the microenvironmental condition of glucose deprivation is responsible for the active release of HMGB1 from various types of cancer cell lines (HT-29, MCF-7 and A549) under normoxic conditions. Recombinant HMGB1 (10 ng/ml) triggered proliferation in myofibroblast cells via activation of PI3K and MEK1/2. Conditioned medium collected from glucose-deprived HT-29 colon cancer cells stimulated the migration and invasion of colonic myofibroblasts, and these processes were significantly inhibited by immunoneutralising antibodies to HMGB1, RAGE and TLR4, together with specific inhibitors of PI3K and MEK1/2. Our data suggest that HMGB1 released from cancer cells under glucose deprivation is involved in stimulating colonic myofibroblast migration and invasion and that this occurs through the activation of RAGE and TLR4, resulting in the activation of the MAPK and PI3K signalling pathways. Thus, HMGB1 might be released by cancer cells in areas of low glucose in solid tumours with the resulting activation of myofibroblasts and is a potential therapeutic target to inhibit solid tumour growth.

Metabolic changes associated with tumor metastasis, part 1: tumor pH, glycolysis and the pentose phosphate pathway

Metabolic adaptations are intimately associated with changes in cell behavior. Cancers are characterized by a high metabolic plasticity resulting from mutations and the selection of metabolic phenotypes conferring growth and invasive advantages. While metabolic plasticity allows cancer cells to cope with various microenvironmental situations that can be encountered in a primary tumor, there is increasing evidence that metabolism is also a major driver of cancer metastasis. Rather than a general switch promoting metastasis as a whole, a succession of metabolic adaptations is more likely needed to promote different steps of the metastatic process. This review addresses the contribution of pH, glycolysis and the pentose phosphate pathway, and a companion paper summarizes current knowledge regarding the contribution of mitochondria, lipids and amino acid metabolism. Extracellular acidification, intracellular alkalinization, the glycolytic enzyme phosphoglucose isomerase acting as an autocrine cytokine, lactate and the pentose phosphate pathway are emerging as important factors controlling cancer metastasis.

Interactions between osteopontin and vascular endothelial growth factor: Implications for cancer

For this comprehensive review, 257 publications with the keywords "osteopontin" or "OPN" and "vascular endothelial growth factor" or "VEGF" in PubMed were screened (time frame from year 1996 to year 2014). 37 articles were excluded because they were not focused on the interactions between these molecules, and papers relevant for transformation-related phenomena were selected. Osteopontin (OPN) and vascular endothelial growth factor (VEGF) are characterized by a convergence in function for regulating cell motility and angiogenesis, the response to hypoxia, and apoptosis. Often, they are co-expressed or one molecule induces the other, however, in some settings OPN-associated pathways and VEGF-associated pathways are distinct. Their relationships affect the pathogenesis in cancer, where they contribute to progression and angiogenesis and serve as markers for poor prognosis. The inhibition of OPN may reduce VEGF levels and suppress tumor progression. In vascular pathologies, these two cytokines mediate remodeling, but may also perpetuate inflammation and narrowing of the arteries. OPN and VEGF are elevated and contribute to vascularization in inflammatory diseases.

Citrate concentrations increase with hypoperfusion in pediatric diffuse intrinsic pontine glioma

Citrate, a tricarboxylic acid cycle intermediate, is present in high concentrations in pediatric diffuse intrinsic pontine gliomas (DIPG). Since citrate increases during hypoxia in animal studies, we hypothesized that it accumulates in DIPG when hypoperfused. Relative tumor blood volumes (rTBV) were determined, using dynamic susceptibility contrast-enhanced magnetic resonance imaging, in twelve children [median age 8.2 (range 3.2-14.5) years] with DIPG and compared to citrate concentrations measured with in vivo proton magnetic resonance spectroscopy ((1)H MRS). Tissue perfusion and metabolite concentration were assessed at initial presentation and over the clinical course, yielding 36 and 46 perfusion and MR spectroscopy datasets, respectively. At presentation, DIPG blood volume was 60 ± 27 % of that measured for normal cerebellum. Citrate, which is not detectable in normal brain tissue, was present in DIPG at concentrations of 3.81 ± 1.44 mmol/kg tissue. Over the course of the disease and treatment, rTBV increased and citrate decreased (both p < 0.05) with an inverse correlation (p = 0.028). Citrate accumulation is associated with tissue hypoperfusion in DIPG.

Understanding the pharmacological properties of a metabolic PET tracer in prostate cancer

Generally, solid tumors (>400 mm(3)) are inherently acidic, with more aggressive growth producing greater acidity. If the acidity could be targeted as a biomarker, it would provide a means to gauge the pace of tumor growth and degree of invasiveness, as well as providing a basis for predicting responses to pH-dependent chemotherapies. We have developed a (64)Cu pH (low) insertion peptide (pHLIP) for targeting, imaging, and quantifying acidic tumors by PET, and our findings reveal utility in assessing prostate tumors. The new pHLIP version limits indiscriminate healthy tissue binding, and we demonstrate its targeting of extracellular acidification in three different prostate cancer models, each with different vascularization and acid-extruding protein carbonic anhydrase IX (CAIX) expression. We then describe the tumor distribution of this radiotracer ex vivo, in association with blood perfusion and known biomarkers of acidity, such as hypoxia, lactate dehydrogenase A, and CAIX. We find that the probe reveals metabolic variations between and within tumors, and discriminates between necrotic and living tumor areas.

Mitochondrial alterations during carcinogenesis: a review of metabolic transformation and targets for anticancer treatments

Mitochondria play important roles in multiple cellular processes including energy metabolism, cell death, and aging. Regulated energy production and utilization are critical in maintaining energy homeostasis in normal cells and functional organs. However, mitochondria go through a series of morphological and functional alterations during carcinogenesis. The metabolic profile in transformed cells is altered to accommodate their fast proliferation, confer resistance to cell death, or facilitate metastasis. These transformations also provide targets for anticancer treatment at different levels. In this review, we discuss the major modifications in cell metabolism during carcinogenesis, including energy metabolism, apoptotic and autophagic cell death, adaptation of tumor microenvironment, and metastasis. We also summarize some of the main metabolic targets for treatments.

Ultrasensitive detection of cancer biomarkers in the clinic by use of a nanostructured microfluidic array

Multiplexed biomarker protein detection holds unrealized promise for clinical cancer diagnostics due to lack of suitable measurement devices and lack of rigorously validated protein panels. Here we report an ultrasensitive electrochemical microfluidic array optimized to measure a four-protein panel of biomarker proteins, and we validate the protein panel for accurate oral cancer diagnostics. Unprecedented ultralow detection into the 5-50 fg·mL(-1) range was achieved for simultaneous measurement of proteins interleukin 6 (IL-6), IL-8, vascular endothelial growth factor (VEGF), and VEGF-C in diluted serum. The immunoarray achieves high sensitivity in 50 min assays by using off-line protein capture by magnetic beads carrying 400,000 enzyme labels and ~100,000 antibodies. After capture of the proteins and washing to inhibit nonspecific binding, the beads are magnetically separated and injected into the array for selective capture by antibodies on eight nanostructured sensors. Good correlations with enzyme-linked immunosorbent assays (ELISA) for protein determinations in conditioned cancer cell media confirmed the accuracy of this approach. Normalized means of the four protein levels in 78 oral cancer patient serum samples and 49 controls gave clinical sensitivity of 89% and specificity of 98% for oral cancer detection, demonstrating high diagnostic utility. The low-cost, easily fabricated immunoarray provides a rapid serum test for diagnosis and personalized therapy of oral cancer. The device is readily adaptable to clinical diagnostics of other cancers.

Lactic acidosis triggers starvation response with paradoxical induction of TXNIP through MondoA

Although lactic acidosis is a prominent feature of solid tumors, we still have limited understanding of the mechanisms by which lactic acidosis influences metabolic phenotypes of cancer cells. We compared global transcriptional responses of breast cancer cells in response to three distinct tumor microenvironmental stresses: lactic acidosis, glucose deprivation, and hypoxia. We found that lactic acidosis and glucose deprivation trigger highly similar transcriptional responses, each inducing features of starvation response. In contrast to their comparable effects on gene expression, lactic acidosis and glucose deprivation have opposing effects on glucose uptake. This divergence of metabolic responses in the context of highly similar transcriptional responses allows the identification of a small subset of genes that are regulated in opposite directions by these two conditions. Among these selected genes, TXNIP and its paralogue ARRDC4 are both induced under lactic acidosis and repressed with glucose deprivation. This induction of TXNIP under lactic acidosis is caused by the activation of the glucose-sensing helix-loop-helix transcriptional complex MondoA:Mlx, which is usually triggered upon glucose exposure. Therefore, the upregulation of TXNIP significantly contributes to inhibition of tumor glycolytic phenotypes under lactic acidosis. Expression levels of TXNIP and ARRDC4 in human cancers are also highly correlated with predicted lactic acidosis pathway activities and associated with favorable clinical outcomes. Lactic acidosis triggers features of starvation response while activating the glucose-sensing MondoA-TXNIP pathways and contributing to the “anti-Warburg” metabolic effects and anti-tumor properties of cancer cells. These results stem from integrative analysis of transcriptome and metabolic response data under various tumor microenvironmental stresses and open new paths to explore how these stresses influence phenotypic and metabolic adaptations in human cancers.

Solid tumors usually have many differences in their chemical environments, such as low oxygen, depletion of glucose, high acidity (low pH), and accumulation of lactate, from normal tissues. These changes are usually called tumor microenvironmental stresses. In this study, we have used microarrays to compare the transcriptional response and metabolic adaptation in response to these different stresses seen in the tumor microenvironments. Through these comparisons, we have found that lactic acidosis triggers a starvation response, highly similar to glucose deprivation, even in the presence of abundant nutrients and oxygen. Even the cells seem to be starved; cells under lactic acidosis have decreased glucose uptake. We found this unexpected biological behavior was due to the paradoxical induction of a glucose-sensing Mondo-TXNIP pathway. The activation of this novel anti-tumor pathway under lactic acidosis contributes to the anti-Warburg effect and the restriction of cell growth in tumorigenesis by limiting nutrient availability and its inactivation may be required for tumor progression under these microenvironmental stresses.

Cancer biomarker discovery: the entropic hallmark

Background

It is a commonly accepted belief that cancer cells modify their transcriptional state during the progression of the disease. We propose that the progression of cancer cells towards malignant phenotypes can be efficiently tracked using high-throughput technologies that follow the gradual changes observed in the gene expression profiles by employing Shannon's mathematical theory of communication. Methods based on Information Theory can then quantify the divergence of cancer cells' transcriptional profiles from those of normally appearing cells of the originating tissues. The relevance of the proposed methods can be evaluated using microarray datasets available in the public domain but the method is in principle applicable to other high-throughput methods.

Methodology/Principal Findings

Using melanoma and prostate cancer datasets we illustrate how it is possible to employ Shannon Entropy and the Jensen-Shannon divergence to trace the transcriptional changes progression of the disease. We establish how the variations of these two measures correlate with established biomarkers of cancer progression. The Information Theory measures allow us to identify novel biomarkers for both progressive and relatively more sudden transcriptional changes leading to malignant phenotypes. At the same time, the methodology was able to validate a large number of genes and processes that seem to be implicated in the progression of melanoma and prostate cancer.

Conclusions/Significance

We thus present a quantitative guiding rule, a new unifying hallmark of cancer: the cancer cell's transcriptome changes lead to measurable observed transitions of Normalized Shannon Entropy values (as measured by high-througput technologies). At the same time, tumor cells increment their divergence from the normal tissue profile increasing their disorder via creation of states that we might not directly measure. This unifying hallmark allows, via the the Jensen-Shannon divergence, to identify the arrow of time of the processes from the gene expression profiles, and helps to map the phenotypical and molecular hallmarks of specific cancer subtypes. The deep mathematical basis of the approach allows us to suggest that this principle is, hopefully, of general applicability for other diseases.

Revisiting the seed and soil in cancer metastasis

Metastasis remains the overwhelming cause of death for cancer patients. During metastasis, cancer cells will leave the primary tumor, intravasate into the bloodstream, arrest at a distant organ, and eventually develop into gross lesions at the secondary sites. This intricate process is influenced by innumerable factors and complex cellular interactions described in 1889 by Stephen Paget as the seed and soil hypothesis. In this review, we revisit this seed and soil hypothesis with an emerging understanding of the cancer cell (i.e. seed) and its microenvironment (i.e. soil). We will provide background to suggest that a critical outcome of the seed-soil interaction is resistance of the stresses that would otherwise impede metastasis.

The tumor microenvironment and metastatic disease

The microenvironment of solid tumors is a heterogeneous, complex milieu for tumor growth and survival that includes features such as acidic pH, low nutrient levels, elevated interstitial fluid pressure (IFP) and chronic and fluctuating levels of oxygenation that relate to the abnormal vascular network that exists in tumors. The metastatic potential of tumor cells is believed to be regulated by interactions between the tumor cells and their extracellular environment (extracellular matrix (ECM)). These interactions can be modified by the accumulation of genetic changes and by the transient alterations in gene expression induced by the local tumor microenvironment. Clinical and experimental evidence suggests that altered gene expression in response to the hypoxic microenvironment is a contributing factor to increased metastatic efficiency. A number of genes that have been implicated in the metastatic process, involving angiogenesis, intra/extravasation, survival and growth, have been found to be hypoxia-responsive. The various metastatic determinants, genetic and epigenetic, somatic and inherited may serve as precedents for the future identification of more genes that are involved in metastasis. Much research has focused on genetic and molecular properties of the tumor cells themselves. In the present review we discuss the epigenetic and physiological regulation of metastasis and emphasize the need for further studies on the interactions between the pathophysiologic tumor microenvironment and the tumor extracellular matrix.

Adaptive landscapes and emergent phenotypes: why do cancers have high glycolysis?

Investigating the causes of increased aerobic glycolysis in tumors (Warburg Effect) has gone in and out of fashion many times since it was first described almost a century ago. The field is currently in ascendance due to two factors. Over a million FDG-PET studies have unequivocally identified increased glucose uptake as a hallmark of metastatic cancer in humans. These observations, combined with new molecular insights with HIF-1alpha and c-myc, have rekindled an interest in this important phenotype. A preponderance of work has been focused on the molecular mechanisms underlying this effect, with the expectation that a mechanistic understanding may lead to novel therapeutic approaches. There is also an implicit assumption that a mechanistic understanding, although fundamentally reductionist, will nonetheless lead to a more profound teleological understanding of the need for altered metabolism in invasive cancers. In this communication, we describe an alternative approach that begins with teleology; i.e. adaptive landscapes and selection pressures that promote emergence of aerobic glycolysis during the somatic evolution of invasive cancer. Mathematical models and empirical observations are used to define the adaptive advantage of aerobic glycolysis that would explain its remarkable prevalence in human cancers. These studies have led to the hypothesis that increased consumption of glucose in metastatic lesions is not used for substantial energy production via Embden-Meyerhoff glycolysis, but rather for production of acid, which gives the cancer cells a competitive advantage for invasion. Alternative hypotheses, wherein the glucose is used for generation of reducing equivalents (NADPH) or anabolic precursors (ribose) are also discussed.

Acidic extracellular pH promotes experimental metastasis of human melanoma cells in athymic nude mice

Extracellular pH (pH(e)) is lower in many tumors than in the corresponding normal tissue. The significance of acidic pH(e) in the development of metastatic disease was investigated in the present work. Human melanoma cells (A-07, D-12, and T-22) were cultured in vitro at pH(e) 6.8 or 7.4 (control) before being inoculated into the tail vein of BALB/c nu/nu mice for formation of experimental pulmonary metastases. Cell invasiveness was studied in vitro by using Matrigel invasion chambers and angiogenesis was studied in vivo by using an intradermal assay. Protein secretion was measured by ELISA and immunocapture assays. Cells cultured at acidic pH(e) showed increased secretion of proteinases and proangiogenic factors, enhanced invasive and angiogenic potential, and enhanced potential to develop experimental metastases. Acidity-induced metastasis was inhibited by treatment with the general matrix metalloproteinase (MMP) inhibitor GM6001, the general cysteine proteinase inhibitor E-64, or blocking antibody against vascular endothelial growth factor-A (VEGF-A) or interleukin-8 (IL-8). Our study indicates that acidic pH(e) promotes experimental pulmonary metastasis in A-07, D-12, and T-22 human melanoma cells by a common mechanism involving acidity-induced up-regulation of the proteolytic enzymes MMP-2, MMP-9, cathepsin B, and cathepsin L and acidity-induced up-regulation of the proangiogenic factors VEGF-A and IL-8. One consequence of this observation is that treatment strategies involving deliberate tumor acidification to improve the efficacy of chemotherapy, photodynamic therapy, and hyperthermia should be avoided. Moreover, the possibility that the pH(e) of the primary tumor may be an important prognostic parameter for melanoma patients merits clinical investigation.

Stimulation of human breast carcinoma cell invasiveness and urokinase plasminogen activator activity by glucose deprivation

Glucose deprivation has been shown to increase the invasive and metastatic potential of tumour cells. In the present study, we determined whether the enhanced tumour cell invasiveness resulting from glucose deprivation is linked to increased activity of enzymes required for extracellular matrix degradation. Results of in vitro invasion assays revealed that the invasiveness of human MDA-MB-231 and MCF-7 breast carcinoma cells and MCF-10A1 normal breast cells was, respectively, 3.9-, 2.9-, and 2.1-fold higher when they were incubated under glucose-deprivation (0.2 mM glucose) than when incubated under physiological blood glucose levels (5 mM). This effect of glucose deprivation on invasion correlated with increased urokinase plasminogen activator (uPA) and plasmin activity. Glucose deprivation did not increase the levels of gelatinase and plasminogen activator inhibitor-1 secretion, or the expression of cell-associated uPA receptor. To determine whether the increased invasiveness resulting from glucose deprivation is causally linked to increased uPA activity, invasion assays were conducted using MDA-MB-231 cells incubated in 0.2 mM or 5 mM glucose in the presence of a neutralising anti-uPA antibody. Results revealed that the anti-uPA antibody significantly inhibited invasion in a dose-dependent manner and to a much greater extent in cells incubated in 0.2 mM glucose than in cells incubated in 5 mM glucose. These results suggest that low glucose levels in malignant cancers increase tumour cell invasiveness by stimulating uPA and plasmin activity.

Tumor oxygenation status as a prognostic marker

Tumor oxygenation status is an independent prognostic indicator in cancer because it influences tumor progression and treatment outcome. Its quantitative value is determined by a number of tumor vascular parameters such as microvascular density, blood flow, blood volume, blood oxygen saturation, tumor tissue pO2, and resistance to oxygen diffusion within the tumor. Over the past several years, considerable time and effort have been invested into developing techniques to effectively and reliably measure the oxygenation status of a tumor. The measurement and interpretation of data obtained with currently available methods is complicated by the heterogeneity in tumor oxygenation. Currently available techniques can be broadly classified into direct invasive methods, direct non-invasive methods, and measurement of surrogate endogenous markers of tumor oxygenation. Of these methods, the Eppendorf pO2 histograph is considered the 'gold standard' and even so has several limitations. Given the importance of tumor oxygenation status in therapy and in predicting disease progression, it is imperative that reliable, globally usable, and technically simplistic methods be developed to yield a consistent, comprehensive, and reliable profile of tumor oxygenation. Until newer more reliable techniques are developed, existing independent techniques or appropriate combinations of techniques should be optimized and validated using known endpoints in tumor oxygenation status and/or treatment outcomes.

Constitutive and inducible expression and regulation of vascular endothelial growth factor

Vascular endothelial growth factor (VEGF), which was originally discovered as vascular permeability factor, is critical to human cancer angiogenesis through its potent functions as a stimulator of endothelial cell survival, mitogenesis, migration, differentiation and self-assembly, as well as vascular permeability, immunosuppression and mobilization of endothelial progenitor cells from the bone marrow into the peripheral circulation. Genetic alterations and a chaotic tumor microenvironment, such as hypoxia, acidosis, free radicals, and cytokines, are clearly attributed to numerous abnormalities in the expression and signaling of VEGF and its receptors. These perturbations confer a tremendous survival and growth advantage to vascular endothelial cells as manifested by exuberant tumor angiogenesis and a consequent malignant phenotype. Understanding the regulatory mechanisms of both inducible and constitutive VEGF expression will be crucial in designing effective therapeutic strategies targeting VEGF to control tumor growth and metastasis. In this review, molecular regulation of VEGF expression in tumor cells is discussed.

Expression of hypoxia-inducible factor 1alpha and cathepsin D in pituitary adenomas

Hypoxia-inducible factor (HIF)-1alpha is a crucial transcription factor involved in the adaptive response to hypoxia, whereas cathepsin D, which regulates angiostatin in several cancer cell lines, has been reported to be upregulated by HIF-1alpha. In order to determine the involvement of angiogenesis in pituitary adenomas, we studied the expression of both HIF-1alpha and cathepsin D in tissues from 58 patients (39 women, 19 men, ranging in age from 20 to 78 yr), sorted by histological group, and assayed by double immunohistochemistry. HIF-1alpha immunoreactivity, confined to the nucleoplasm, was present in both tumor and vascular endothelial cells. There was no difference in microvascular density (p = 0.7761) by histotype. ACTH-producing adenomas showed the lowest level of HIF-1alpha, whereas prolactin (PRL)-producing adenomas and HIF-1alpha-positive microvessels showed the highest (p < 0.001). In contrast, the lowest expression of cathepsin D was observed in PRL-producing adenomas, whereas the highest expression was detected in ACTH-producing adenomas (p < 0.0001). Imaging analysis with fluorescence double immunohistochemistry showed that HIF-1alpha-negative tumor cells did not express significantly higher levels of cathepsin D. In these poorly vascularized tumors, the hypoxic marker HIF-1alpha may not downregulate cathepsin D. The mechanisms of tumor angiogenesis and cell invasion in pituitary adenomas may differ from those in other tumor cells.

Microenvironmental influences on mutagenesis in mammary epithelial cells

Tumor progression may be viewed as an evolutionary process at the cellular level. Because blood supply to solid tumors is inadequate, the cancer cells face a hostile microenvironment characterized by hypoxia or anoxia, acidic extracellular pH and nutrient deficiencies. It has been proposed that these factors result in increased levels of spontaneous mutagenesis and thereby contribute to tumor progression. We have examined spontaneous mutagenesis in vitro and in vivo, using previously characterized cell lines (mammary epithelial cells [ME] and mammary fibroblast cells [MFib]) from the mammary gland of the BigBluetrade mark rat, carrying a transgene construct suitable for the detection of mutations. Cells were exposed in vitro to control conditions, low pH, or to glucose deprivation, under normoxic or hypoxic culture conditions, and were also grown as xenografted tumors in immune-deficient mice. We examined cell survival and mutant frequency/spectrum at the cII locus. Significant increases in mutant frequency were observed in ME cells exposed to hypoxia alone or in combination with no glucose; the latter condition also resulted in reduced clonogenic survival. Cells grown as xenografts and then recovered and expanded in culture also had elevated frequencies of spontaneous mutations. We observed a shift in the spontaneous mutation spectrum between the ME cells and the MET cells (cultured in vitro or isolated from mouse xenograft tumors). These results support the concept that the tumor microenvironment contributes to tumor progression by enhancing spontaneous mutagenesis, that different cell types from the same organ can respond differently to these stresses and that differences in microenvironment may influence the types of mutations that arise.

Effects of tumour acidification with glucose+MIBG on the spontaneous metastatic potential of two murine cell lines

In addition to hypoxia, acidic extracellular pH (pH_e) is recognised as one of the microenvironmental characteristics of solid tumours. A number of studies have examined ways to increase tumour acidity in order to improve tumour-specific targeting of certain drugs and the effectiveness of hyperthermia. However, previous data have shown that exposure of murine tumour cells to acid conditions in culture can enhance their metastatic potential when injected subsequently into mice, raising the concern that deliberate tumour acidification might increase the probability of metastasis. In this study, we examined the effects of in vivo tumour acidification and hypoxia on the spontaneous metastatic potential of the murine KHT-C fibrosarcoma and B16F1 melanoma cell lines. A tumour-specific increase in extracellular acidity, demonstrated by measurements with pH electrodes, was achieved by daily intraperitoneal injections of meta-iodo-benzylguanidine (MIBG) and/or glucose. This method of tumour acidification during tumour growth did not significantly enhance the spontaneous metastatic potential of the two murine cell lines.

Hypoxia Enhances Metastatic Efficiency by Up-Regulating Mdm2 in KHT Cells and Increasing Resistance to Apoptosis

Tumor hypoxia has been reported to be a negative prognostic factor in a number of tumor sites. Both clinical and experimental studies have suggested a positive correlation between tumor hypoxia and increased metastatic efficiency; however, the mechanisms are not understood. In this study, the mechanisms of hypoxia-enhanced metastasis have been investigated in murine KHT fibrosarcoma and SCC VII cells. We have observed that hypoxia-pretreated KHT-C cells have a higher survival rate than control KHT-C cells after being arrested in mouse lungs. cDNA microarray analysis revealed many hypoxia-regulated genes, most of which have been reported to be involved in cell survival and growth. Among these genes, we have confirmed the up-regulation of Mdm2 by hypoxia and have demonstrated that this up-regulation is p53 independent. The up-regulation of Mdm2 by hypoxia is associated with decreased p53 protein and inhibition of the transactivation of p53 downstream proapoptotic genes. Overexpression of Mdm2 or suppression of p53 by transient transfection increased metastatic efficiency in KHT-C cells. These data suggest that hypoxia can increase tumor cell metastatic efficiency by rendering the tumor cells less sensitive to stress-induced cell death, e.g., through modifying the levels of Mdm2 and p53.

Regulation of the pro-angiogenic microenvironment by carboxyamido-triazole

Anti-angiogenic agents regulate tumor growth by inhibiting endothelial cell proliferation and invasion. Carboxyamido-triazole (CAI), an inhibitor of non-voltage-operated calcium entry and calcium influx-mediated pathways, has angiogenesis and invasion inhibitory activity. We hypothesized that CAI may express its anti-angiogenic effects through negative regulation of pro-angiogenic cytokine production and/or function. In vivo, orally administered CAI prevented A2058 human melanoma xenograft growth and concomitantly resulted in a marked reduction in circulating vascular endothelial growth factor (VEGF) and interleukin-8 (IL-8). In vitro, A2058 cell secretion of VEGF was inhibited by CAI treatment under limiting micronutrient conditions that approximate the tumor microenvironment, media restriction, and acidification to pH 6.8 (P=0.0003 and P=0.0006, respectively). VEGF and HIF-1alpha message and protein were also reduced by CAI treatment. Oral CAI treatment reduced vascular ingrowth in vivo into VEGF-containing Matrigel plugs. Commensurate with those findings, human umbilical vein endothelial cell (HUVEC) migration towards VEGF was reduced below background by exposure to CAI in the migration chamber (P<0.0001). An 88% reduction in circulating IL-8 concentration was measured in CAI-treated animals. However, IL-8 protein secretion and gene expression were increased by CAI treatment in culture (P< or =0.01), where CAI caused a dose-dependent acidification of the culture milieu (P< or =0.005). This paradox suggests that IL-8 production in vitro may be more sensitive to ambient pH than cytosolic calcium. These observations suggest that CAI inhibition of tumor cell VEGF production and endothelial cell response to VEGF results in disruption of signaling between the tumor and its microenvironment, causing a net anti-angiogenic effect.

Regulation of cancer metastasis by stress pathways

The presence of activated oncogenes and/or inactivated tumor suppressor genes may result in constitutive activation of multiple transcription factors. This may be especially true in the early stages of tumor development. At advanced stages, however, uncontrolled tumor growth and the consequent development of a stress microenvironment, such as hypoxia, acidosis, and free radical overproduction, may further alter the activity of these transcription factors. Abnormal activation of and interplay between these factors lead to aberrant expression of multiple metastasis-related proteins and confer a tremendous survival and growth advantage to emerging metastatic variants. Understanding the expression and regulation of these molecules may shed more light on the biology of cancer metastasis as well as suggest new preventive and therapeutic approaches.

Increased cell density decreases cysteine proteinase inhibitor activity and increases invasive ability of two prostate tumor cell lines

The ability of a cancer cell to metastasis to a distant site is partly dependent on the secretion of matrix degrading enzymes. The lysosomal cysteine proteinases, cathepsins B and L, have been shown to be secreted by a number of cancer cells and have been implicated in metastasis. Cathepsins B and L are regulated by a class of inhibitors known as the cystatins; aberrant cystatin activity has also been shown in a number of cancer cells. Two prostate cancer cell lines, PC3 and DU145, and a normal prostate epithelial cell (NPC) culture were used to determine the importance of cathepsins L+B and cysteine proteinase inhibitor (CPI) activity in the ability of each cell line to invade the reconstituted basement membrane, Matrigel. Cathepsin L+B and CPI activities were evident in the cell extract and conditioned media of PC3, DU145 and NPC; however, only the cancer cell lines PC3 and DU145 exhibited invasive ability. Invasive ability was partially inhibited following exposure of PC3 and DU145 cells to the CPI, E-64. Since environmental factors such as cell-cell interactions are responsible for mediating the expression of a number of genes involved in metastasis, the effects of cell density on cathepsin and CPI activities and invasive ability were also determined. CPI activity decreased and invasive ability increased with increasing cell density. We conclude that cathepsin L+B plays a significant role in the invasive ability of the two prostate cancer cell lines, PC3 and DU145. This may be due to decreased regulation by endogenous CPIs whose activity diminishes at high cell densities.

MRI of the tumor microenvironment

The microenvironment within tumors is significantly different from that in normal tissues. A major difference is seen in the chaotic vasculature of tumors, which results in unbalanced blood supply and significant perfusion heterogeneities. As a consequence, many regions within tumors are transiently or chronically hypoxic. This exacerbates tumor cells' natural tendency to overproduce acids, resulting in very acidic pH values. The hypoxia and acidity of tumors have important consequences for antitumor therapy and can contribute to the progression of tumors to a more aggressive metastatic phenotype. Over the past decade, techniques have emerged that allow the interrogation of the tumor microenvironment with high resolution and molecularly specific probes. Techniques are available to interrogate perfusion, vascular distribution, pH, and pO(2) nondestructively in living tissues with relatively high precision. Studies employing these methods have provided new insights into the causes and consequences of the hostile tumor microenvironment. Furthermore, it is quite exciting that there are emerging techniques that generate tumor image contrast via ill-defined mechanisms. Elucidation of these mechanisms will yield further insights into the tumor microenvironment. This review attempts to identify techniques and their application to tumor biology, with an emphasis on nuclear magnetic resonance (NMR) approaches. Examples are also discussed using electron MR, optical, and radionuclear imaging techniques.

Oxygen as a regulator of cellular phenotypes in pregnancy and cancer

Cellular phenotype is determined by genetic and microenvironmental factors. There is evidence that tissue oxygenation status is one of the microenvironmental factors regulating cellular behaviour. Both normal and pathological processes such as blastocyst implantation in the uterus, placentation, and rapidly growing tumours occur under conditions characterized by relatively low oxygen levels. In this review, we address the effects of low oxygen concentrations on the phenotype of trophoblast and cancer cells. We provide evidence that oxygenation levels play an important role in the regulation of normal and pathological cellular invasiveness as it occurs during trophoblast invasion of the uterus and in tumour progression and metastasis, drug resistance in cancer, and antitumour activity of natural killer cells of the immune system.

Regulation of vascular endothelial growth factor expression by acidosis in human cancer cells

The influence of acidosis on the expression of the vascular endothelial growth factor (VEGF) gene was determined. FG human pancreatic adenocarcinoma cells were incubated for various time periods in media at a physiologically relevant pH level (6.7-7.4). The expression of VEGF mRNA and protein secretion was inversely correlated with pH in a pH- and time-dependent manner. Transient acidosis also activated the VEGF promoter/enhancer luciferase reporter, which was consistent with an increased VEGF gene transcription rate and VEGF mRNA half-life. These data indicated that acidosis transcriptionally and posttranscriptionally regulates VEGF expression, suggesting that an acidic tumor microenvironment contributes to tumor angiogenesis and progression.

Relationship of hypoxia to metastatic ability in rodent tumours

The relationship between tumour oxygenation in vivo and metastatic potential was investigated in 2 rodent tumour models, KHT-C fibrosarcoma and SCC-VII squamous cell carcinoma. The oxygen status in these rodent tumours transplanted intramuscularly in syngeneic mice was measured using the Eppendorf pO(2)Histograph. The results indicate a considerable heterogeneity in oxygenation between individual tumours within each tumour cell line. At different tumour sizes, animals were killed and lung lobes were examined for macroscopic and microscopic lung metastases. In the KHT-C tumours, a significant increase in early pulmonary metastasis formation was observed in mice with hypoxic primary tumours. Hypoxic SCC-VII tumours did not give rise to enhanced lung metastasis formation despite oxygenation in a range similar to the KHT-C tumours. However, the overall metastasis incidence in the SCC-VII model was very low. The results obtained in the KHT-C model, which show that hypoxic tumours are more likely to metastasize, are in agreement with recent clinical data suggesting that a hypoxic environment might be implicated in metastatic ability of human tumours.

Tumor hypoxia: definitions and current clinical, biologic, and molecular aspects

Tissue hypoxia results from an inadequate supply of oxygen (O(2)) that compromises biologic functions. Evidence from experimental and clinical studies increasingly points to a fundamental role for hypoxia in solid tumors. Hypoxia in tumors is primarily a pathophysiologic consequence of structurally and functionally disturbed microcirculation and the deterioration of diffusion conditions. Tumor hypoxia appears to be strongly associated with tumor propagation, malignant progression, and resistance to therapy, and it has thus become a central issue in tumor physiology and cancer treatment. Biochemists and clinicians (as well as physiologists) define hypoxia differently; biochemists define it as O(2)-limited electron transport, and physiologists and clinicians define it as a state of reduced O(2) availability or decreased O(2) partial pressure that restricts or even abolishes functions of organs, tissues, or cells. Because malignant tumors no longer execute functions necessary for homeostasis (such as the production of adequate amounts of adenosine triphosphate), the physiology-based definitions of the term "hypoxia" are not necessarily valid for malignant tumors. Instead, alternative definitions based on clinical, biologic, and molecular effects that are observed at O(2) partial pressures below a critical level have to be applied.

Hypoxia modulated gene expression: angiogenesis, metastasis and therapeutic exploitation

Tumour hypoxia is the result of an imbalance in oxygen supply and demand. It is an adverse prognostic indicator in cancer as it modulates tumour progression and treatment. Many genes controlling tumour biology are oxygen regulated, and new ones are constantly added to the growing list of hypoxia-induced genes. Of specific importance are hypoxia-responsive transcription factors, as they can modulate the expression of numerous different genes. Similarly, growth factors which govern the formation of new blood vessels or which control blood flow are vitally important for both the maintenance of the primary tumour and metastases at distant sites. The purpose of this review is to present an update of selected issues regarding hypoxia-inducible gene expression and how this affects prognosis, angiogenesis and metastasis. It will conclude by discussing gene therapy as one possible means of exploiting tumour hypoxia for the treatment of cancer.

Hypoxia affects tumor cell invasiveness in vitro: the role of hypoxia-activated ligand HAL1/13 (Ku86 autoantigen)

We have recently identified and characterized a new adhesion ligand, HAL1/13 (hypoxia-activated ligand), which mediates the increase in leukocyte adhesion to endothelium under hypoxic conditions (J. Immunol. 155 (1995) 802-810). The HAL1/13 antigen was cloned and found to be identical to p86 subunit of Ku autoantigen. In this study we demonstrate that exposure of neuroblastoma and breast carcinoma cells to hypoxia results in upregulation of HAL1/13 surface expression, coincident with an increased ability of these tumor cells to invade endothelial monolayers, which could be partially attenuated by the anti-HAL1/13 antibody. Hypoxia also potentiated neuroblastoma and breast carcinoma cell transmigration through Matrigel filters. Anti-HAL1/13 antibody inhibited haptotactic locomotion of hypoxic tumor cells on laminin.

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.

Both increased stability and transcription contribute to the induction of the urokinase plasminogen activator receptor (uPAR) message by hypoxia

Both hypoxia and overexpression of the urokinase plasminogen activator receptor (uPAR) are associated with a poor clinical outcome in human cancers. Hypoxia has been shown to induce uPAR expression in breast cancer cells and to increase their invasion through Matrigel, a phenomenon which can be blocked using an anti-uPAR antibody. We examined expression of uPAR mRNA in MCF7 human breast carcinoma cells under hypoxia and found that an increase in the level of the message could be detected at 1% oxygen but was most marked at 0.2 or 0.05% oxygen with an induction of 9- to 20-fold over baseline. To determine whether changes in RNA stability contributed to this dramatic increase, we used actinomycin D to inhibit transcription and found that the half-life of the message was much longer under hypoxic conditions (approximately 10 h) than during reoxygenation (approximately 2 h). Transient transfections using a luciferase reporter construct containing 2 kbp of the mouse uPAR promoter showed that promoter activity increased up to 5-fold after exposure to 0.2% oxygen. Thus, hypoxic induction of the uPAR message in MCF7 cells is due to both mRNA stabilization and increased transcriptional activation.

Applications of magnetic resonance in model systems: tumor biology and physiology

A solid tumor presents a unique challenge as a system in which the dynamics of the relationship between vascularization, the physiological environment and metabolism are continually changing with growth and following treatment. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) studies have demonstrated quantifiable linkages between the physiological environment, angiogenesis, vascularization and metabolism of tumors. The dynamics between these parameters continually change with tumor aggressiveness, tumor growth and during therapy and each of these can be monitored longitudinally, quantitatively and non-invasively with MRI and MRS. An important aspect of MRI and MRS studies is that techniques and findings are easily translated between systems. Hence, pre-clinical studies using cultured cells or experimental animals have a high connectivity to potential clinical utility. In the following review, leaders in the field of MR studies of basic tumor physiology using pre-clinical models have contributed individual sections according to their expertise and outlook. The following review is a cogent and timely overview of the current capabilities and state-of-the-art of MRI and MRS as applied to experimental cancers. A companion review deals with the application of MR methods to anticancer therapy.

Modulation of invasive properties of murine squamous carcinoma cells by heterologous expression of cathepsin B and cystatin C

Murine SCC-VII squamous carcinoma cells have the capacity to penetrate reconstituted basement membranes (Matrigel) in vitro. The invasion of Matrigel layers by SCC-VII cells was significantly reduced by E-64, a specific inhibitor of lysosomal cysteine proteinases. The cathepsin-B-selective E-64 derivative, CA-074, inhibited penetration of Matrigel by SCC-VII cells to the same extent, indicating a major role for this particular lysosomal enzyme in extracellular-matrix degradation during squamous-carcinoma-cell invasion. SCC-VII cells were stably transfected with a cDNA encoding human procathepsin B, in an attempt to modulate the invasive properties of the cell line. The transfected cells expressed the heterologous gene, secreted increased amounts of procathepsin B and displayed enhanced invasive potential. In vivo, the activity of cathepsin B is strictly regulated by endogenous inhibitors. SCC-VII cells were therefore also stably transfected with a cDNA encoding human cystatin C, the most potent cysteine-proteinase inhibitor in mammalian tissues. The expression of this transgene resulted in the production of active recombinant cystatin C and a pronounced reduction in Matrigel invasion. These studies demonstrate that the invasive properties of squamous-cell carcinomas can be changed by modulation of the balance between cathepsin B and its endogenous inhibitors, and provide further evidence for the involvement of this lysosomal cysteine proteinase in tumour invasion and metastasis.

Causes and effects of heterogeneous perfusion in tumors

A characteristic of solid tumors is their heterogeneous distribution of blood flow, with significant hypoxia and acidity in low-flow regions. We review effects of heterogeneous tumor perfusion are reviewed and propose a conceptual model for its cause. Hypoxic-acidic regions are resistant to chemo- and radiotherapy and may stimulate progression to a more metastatic phenotype. In normal tissues, hypoxia and acidity induce angiogenesis, which is expected to improve perfusion. However, aggressive tumors can have high local microvessel density simultaneously with significant regions of hypoxia and acidosis. A possible explanation for this apparent contradiction is that the mechanisms regulating growth and adaptation of vascular networks are impaired. According to a recent theory for structural adaptation of vascular networks, four interrelated adaptive responses can work as a self-regulating system to produce a mature and efficient blood distribution system in normal tissues. It is proposed that heterogeneous perfusion in tumors may result from perturbation of this system. Angiogenesis may increase perfusion heterogeneity in tumors by increasing the disparity between parallel low- and high-resistance flow pathways. This conceptual model provides a basis for future rational therapies. For example, it indicates that selective destruction of tumor vasculature may increase perfusion efficiency and improve therapeutic efficacy.

Hypoxia-induced metastasis of human melanoma cells: involvement of vascular endothelial growth factor-mediated angiogenesis

Tumour cells exposed to hypoxia have been shown to up-regulate the expression of vascular endothelial growth factor (VEGF). The purpose of the present work was to investigate whether hypoxia-induced VEGF up-regulation can result in increased metastatic efficiency of human melanoma cells. Two melanoma lines, one showing high (A-07) and the other showing low (D-12) VEGF secretion under aerobic conditions, were included in the study. Cell cultures were exposed to hypoxia (oxygen concentrations < 10 ppm) in vitro and metastatic efficiency, i.e. lung colonization efficiency, as well as transplantability and angiogenic potential were assessed in BALB/c-nu/nu mice Both cell lines showed significantly increased VEGF secretion under hypoxic conditions as measured by enzyme-linked immunosorbent assay The D-12 cells showed increased metastatic efficiency, transplantability and angiogenic potential following exposure to hypoxia. The metastatic efficiency increased with the duration of the hypoxia treatment and decreased with the time after reoxygenation. The A-07 cells on the other hand showed unchanged metastatic efficiency, transplantability and angiogenic potential following exposure to hypoxia. Both cell lines showed significantly decreased metastatic efficiency and angiogenic potential in mice treated with neutralizing antibody against VEGF. These results suggest that (a) VEGF is a limiting factor for the rate of angiogenesis in low but not in high VEGF-expressing melanomas under normoxic conditions and (b) transient hypoxia might promote the development of metastases in low VEGF-expressing melanomas by upregulating the expression of VEGF and hence enhancing the angiogenic potential of the tumour cells.

Regulation of E-cadherin: does hypoxia initiate the metastatic cascade?

The ability of tumours to metastasis is regarded as one of the hallmarks of malignancy. The process through which tumours evolve to achieve this has been termed the metastatic cascade. This cascade has been the subject of much investigation over many years. One of the vital events identified by these investigations is the reduction of adhesion between tumour cells facilitating invasion of the surrounding tissues and vascular channels, ultimately leading to the development of a distant metastasis. E-cadherin and its associated catenin complex have been identified as key molecules in cell adhesion. This review looks at the structure and interaction of the E-cadherin-catenin complex and the factors that appear to regulate E-cadherin expression and thus cell adhesion. From the data gathered, it has become possible to propose the hypothesis that the development of tumour hypoxia is the initiating factor that sets the tumour on the road to metastasis.

Starvation promotes Dictyostelium development by relieving PufA inhibition of PKA translation through the YakA kinase pathway

When nutrients are depleted, Dictyostelium cells undergo cell cycle arrest and initiate a developmental program that ensures survival. The YakA protein kinase governs this transition by regulating the cell cycle, repressing growth-phase genes and inducing developmental genes. YakA mutants have a shortened cell cycle and do not initiate development. A suppressor of yakA that reverses most of the developmental defects of yakA- cells, but none of their growth defects was identified. The inactivated gene, pufA, encodes a member of the Puf protein family of translational regulators. Upon starvation, pufA- cells develop precociously and overexpress developmentally important proteins, including the catalytic subunit of cAMP-dependent protein kinase, PKA-C. Gel mobility-shift assays using a 200-base segment of PKA-C's mRNA as a probe reveals a complex with wild-type cell extracts, but not with pufA- cell extracts, suggesting the presence of a potential PufA recognition element in the PKA-C mRNA. PKA-C protein levels are low at the times of development when this complex is detectable, whereas when the complex is undetectable PKA-C levels are high. There is also an inverse relationship between PufA and PKA-C protein levels at all times of development in every mutant tested. Furthermore, expression of the putative PufA recognition elements in wild-type cells causes precocious aggregation and PKA-C overexpression, phenocopying a pufA mutation. Finally, YakA function is required for the decline of PufA protein and mRNA levels in the first 4 hours of development. We propose that PufA is a translational regulator that directly controls PKA-C synthesis and that YakA regulates the initiation of development by inhibiting the expression of PufA. Our work also suggests that Puf protein translational regulation evolved prior to the radiation of metazoan species.

A quantitative analysis of the reduction in oxygen levels required to induce up-regulation of vascular endothelial growth factor (VEGF) mRNA in cervical cancer cell lines

The presence of hypoxia (low oxygen concentrations) in solid tumours correlates with poor prognosis, increased metastasis, and resistance to radiotherapy and some forms of chemotherapy. Malignant cells produce an angiogenesis factor, vascular endothelial growth factor (VEGF), which may increase metastatic ability and is up-regulated in the presence of hypoxia. Clinical data for cancers of the cervix and head and neck relate oxygen levels in the tumour to treatment outcome. This suggests the possibility that the presence of VEGF mRNA might be used as a marker for relevant levels of hypoxia. Suspension cultures of three human cervical cancer cell lines, SiHa, ME-180 and HeLa, were used to investigate up-regulation of VEGF mRNA levels following exposure to precisely defined oxygen concentrations for 2 or 4 h. An oxygen sensor was used to confirm the actual levels of dissolved oxygen present. The oxygen concentrations which caused half-maximal upregulation (the Km value) of VEGF mRNA level in the three cell lines were similar except for one instance (Km at 4 h: SiHa 27.0 +/- 5.7 microM, ME-180 16.8 +/- 3.3 microM, HeLa 13.0 +/- 1.8 microM, SiHa and HeLa P = 0.01). The Km values for the HeLa cell line as measured at 2 h (24.9 +/- 0.8 microM) and 4 h (13.0 +/- 1.8 microM) were significantly different (P < 0.0001). VEGF mRNA half-lives measured in air were consistent with values in the literature (SiHa 59.8 +/- 5.8 min, ME-180 44.4 +/- 7.2 min, HeLa 44.5 +/- 6.3 min). Differences in oxygen consumption at low oxygen concentrations were noted between the different cell lines. Stirring in suspension culture was found to induce VEGF mRNA in SiHa cells. The presence of VEGF mRNA may be a marker for radiobiologic hypoxia.

Small molecule inhibitors of tumor-promoted angiogenesis, including protein tyrosine kinase inhibitors

Angiogenesis is an exciting and promising new area of research. The concept that tumor cells are absolutely dependent upon neovascularization to grow and metastasize has opened the door to a multitude of new approaches and targets for developing anticancer therapies. These potential new antiangiogenic therapies offer the possibility for improved efficacy and reduced toxicity relative to conventional cancer treatments without the possibility of drug resistance. In particular, reports of small molecule inhibitors of tumor-promoted angiogenesis are appearing ever more frequently in the scientific literature. For this reason, the major focus of this review will be to cover small molecule inhibitors of tumor-promoted angiogenesis. The present review concentrates on selected literature, principally from mid-1996 to mid-1998, where there are sufficient biological data to support claims of antiangiogenic activities by small molecules. In addition, a historical background is presented and some of the important issues related to this field are discussed within.

Heterogeneity of tumor oxygenation: relationship to tumor necrosis, tumor size, and metastasis

PURPOSE

Measurements of oxygenation in the transplanted rodent KHT-C and SCC-VII tumors demonstrate significant heterogeneity from tumor to tumor as is observed in human tumors. This finding suggests that heterogeneity in oxygenation between tumors is likely related to factors associated with tumor growth rather than to intrinsic genetic differences. In this study we examined whether measurements of the oxygenation of individual KHT-C tumors were related to necrosis in the tumors or to tumor size and whether the more hypoxic tumors gave rise to more metastases.

METHODS

Tumors were grown in the gastrocnemius muscle of C3H mice and tumor oxygenation was measured at defined sizes (approx. 0.35 g, 1.0 g, and 2.0 g) using an Eppendorf polarographic oxygen probe. Necrosis was assessed by examining histological sections cut from tumors used for the oxygen measurements. Metastasis was assessed by counting macroscopic lung nodules in mice sacrificed when their tumors reached a size of approximately 2 g.

RESULTS

Tumor oxygenation in individual KHT-C tumors became poorer and necrosis became more extensive as the tumors grew larger but, at a size of 0.3-0.4 g, there was no relationship between oxygenation and extent of necrosis. In general, measurements of tumor pO2 at a size of 0.3-0.4 g were predictive of tumor PO2 in the same tumor at a size of about 1 g, but by the time the tumors reached a size of about 2 g they were all very hypoxic. There was a trend suggesting a relationship between macroscopic metastases in the lung and degree of hypoxia in the KHT-C tumors but this was not statistically significant.

CONCLUSION

The results indicate that the heterogeneity of oxygenation seen in KHT-C tumors is not explained by different degrees of necrosis in the individual tumors. The lack of a correlation between increased metastasis formation and increased levels of hypoxia in the KHT-C tumors is not consistent with results reported for human tumors.

Microenvironmental control of gene expression: implications for tumor angiogenesis, progression, and metastasis

Low oxygen tension (hypoxia) is an important prognostic factor in cancer treatment because it affects tumor formation and malignant progression. Many genes governing these complex processes have been found to be oxygen regulated. This article reviews the present knowledge of hypoxia-inducible gene expression and how this affects angiogenesis, progression, and metastasis. Of particular importance are hypoxia-regulated transcription factors because they can modulate expression of countless different genes. Additional genes analyzed in some detail include those encoding angiogenic growth factors, factors controlling blood flow, and those involved in metastasis. Although hypoxia is generally perceived as a hindrance to cancer therapy, it is possibly exploitable because severe oxygen deficiency is tumor specific. Strategies aimed at using the presence of hypoxia in solid tumors include oxygen sensitive chemotherapy and gene therapy.