Biological response of multicellular EMT6 spheroids to exogenous lactate

Cancer cell spheroids are a better screen for the photodynamic efficiency of glycosylated photosensitizers

Photodynamic Therapy (PDT) relies on the use of non-toxic photosensitizers that are locally and selectively activated by light to induce cell death or apoptosis through reactive oxygen species generation. The conjugation of porphyrinoids with sugars that target cancer is increasingly viewed as an effective way to increase the selectivity of PDT. To date, in vitro PDT efficacy is mostly screened using two-dimensional monolayer cultures. Compared to monolayer cultures, three-dimensional spheroid cultures have unique spatial distributions of nutrients, metabolites, oxygen and signalling molecules; therefore better mimic in vivo conditions. We obtained 0.05 mm3 spheroids with four different human tumor cell lines (HCT-116, MCF-7, UM-UC-3 and HeLa) with appropriate sizes for screening PDT agents. We observed that detachment from monolayer culture and growth as tumor spheroids was accompanied by changes in glucose metabolism, endogenous ROS levels, galectin-1 and glucose transporter GLUT1 protein levels. We compared the phototoxic responses of a porphyrin conjugated with four glucose molecules (PorGlu4) in monolayer and spheroid cultures. The uptake and phototoxicity of PorGlu4 is highly dependent on the monolayer versus spheroid model used and on the different levels of GLUT1 protein expressed by these in vitro platforms. This study demonstrates that HCT-116, MCF-7, UM-UC-3 and HeLa spheroids afford a more rational platform for the screening of new glycosylated-photosensitizers compared to monolayer cultures of these cancer cells.

The role of necrosis, acute hypoxia and chronic hypoxia in 18F-FMISO PET image contrast: a computational modelling study

Positron emission tomography (PET) using 18F-fluoromisonidazole (FMISO) is a promising technique for imaging tumour hypoxia, and a potential target for radiotherapy dose-painting. However, the relationship between FMISO uptake and oxygen partial pressure ([Formula: see text]) is yet to be quantified fully. Tissue oxygenation varies over distances much smaller than clinical PET resolution (<100 μm versus  ∼4 mm), and cyclic variations in tumour perfusion have been observed on timescales shorter than typical FMISO PET studies (∼20 min versus a few hours). Furthermore, tracer uptake may be decreased in voxels containing some degree of necrosis. This work develops a computational model of FMISO uptake in millimetre-scale tumour regions. Coupled partial differential equations govern the evolution of oxygen and FMISO distributions, and a dynamic vascular source map represents temporal variations in perfusion. Local FMISO binding capacity is modulated by the necrotic fraction. Outputs include spatiotemporal maps of [Formula: see text] and tracer accumulation, enabling calculation of tissue-to-blood ratios (TBRs) and time-activity curves (TACs) as a function of mean tissue oxygenation. The model is characterised using experimental data, finding half-maximal FMISO binding at local [Formula: see text] of 1.4 mmHg (95% CI: 0.3-2.6 mmHg) and half-maximal necrosis at 1.2 mmHg (0.1-4.9 mmHg). Simulations predict a non-linear non-monotonic relationship between FMISO activity (4 hr post-injection) and mean tissue [Formula: see text] : tracer uptake rises sharply from negligible levels in avascular tissue, peaking at  ∼5 mmHg and declining towards blood activity in well-oxygenated conditions. Greater temporal variation in perfusion increases peak TBRs (range 2.20-5.27) as a result of smaller predicted necrotic fraction, rather than fundamental differences in FMISO accumulation under acute hypoxia. Identical late FMISO uptake can occur in regions with differing [Formula: see text] and necrotic fraction, but simulated TACs indicate that additional early-phase information may allow discrimination of hypoxic and necrotic signals. We conclude that a robust approach to FMISO interpretation (and dose-painting prescription) is likely to be based on dynamic PET analysis.

Chemical analysis of multicellular tumour spheroids

Conventional two dimensional (2D) monolayer cell culture has been considered the 'gold standard' technique for in vitro cellular experiments. However, the need for a model that better mimics the three dimensional (3D) architecture of tissue in vivo has led to the development of Multicellular Tumour Spheroids (MTS) as a 3D tissue culture model. To some extent MTS mimic the environment of in vivo tumours where, for example, oxygen and nutrient gradients develop, protein expression changes and cells form a spherical structure with regions of proliferation, senescence and necrosis. This review focuses on the development of techniques for chemical analysis of MTS as a tool for understanding in vivo tumours and a platform for more effective drug and therapy discovery. While traditional monolayer techniques can be translated to 3D models, these often fail to provide the desired spatial resolution and z-penetration for live cell imaging. More recently developed techniques for overcoming these problems will be discussed with particular reference to advances in instrument technology for achieving the increased spatial resolution and imaging depth required.

In silico estimates of the free energy rates in growing tumor spheroids

The physics of solid tumor growth can be considered at three distinct size scales: the tumor scale, the cell-extracellular matrix (ECM) scale and the sub-cellular scale. In this paper we consider the tumor scale in the interest of eventually developing a system-level understanding of the progression of cancer. At this scale, cell populations and chemical species are best treated as concentration fields that vary with time and space. The cells have chemo-mechanical interactions with each other and with the ECM, consume glucose and oxygen that are transported through the tumor, and create chemical by-products. We present a continuum mathematical model for the biochemical dynamics and mechanics that govern tumor growth. The biochemical dynamics and mechanics also engender free energy changes that serve as universal measures for comparison of these processes. Within our mathematical framework we therefore consider the free energy inequality, which arises from the first and second laws of thermodynamics. With the model we compute preliminary estimates of the free energy rates of a growing tumor in its pre-vascular stage by using currently available data from single cells and multicellular tumor spheroids.

Tumor metabolism of lactate: the influence and therapeutic potential for MCT and CD147 regulation

Tumor metabolism consists of complex interactions between oxygenation states, metabolites, ions, the vascular network and signaling cascades. Accumulation of lactate within tumors has been correlated with poor clinical outcomes. While its production has negative implications, potentially contributing to tumor progression, the implications of the ability of tumors to utilize lactate can offer new therapeutic targets for the future. Monocarboxylate transporters (MCTs) of the SLC16A gene family influence substrate availability, the metabolic path of lactate and pH balance within the tumor. CD147, a chaperone to some MCT subtypes, contributes to tumor progression and metastasis. The implications and consequences of lactate utilization by tumors are currently unknown; therefore future research is needed on the intricacies of tumor metabolism. The possibility of metabolic modification of the tumor microenvironment via regulation or manipulation of MCT1 and CD147 may prove to be promising avenues of therapeutic options.

Energy metabolism transition in multi-cellular human tumor spheroids

It is thought that glycolysis is the predominant energy pathway in cancer, particularly in solid and poorly vascularized tumors where hypoxic regions develop. To evaluate whether glycolysis does effectively predominate for ATP supply and to identify the underlying biochemical mechanisms, the glycolytic and oxidative phosphorylation (OxPhos) fluxes, ATP/ADP ratio, phosphorylation potential, and expression and activity of relevant energy metabolism enzymes were determined in multi-cellular tumor spheroids, as a model of human solid tumors. In HeLa and Hek293 young-spheroids, the OxPhos flux and cytochrome c oxidase protein content and activity were similar to those observed in monolayer cultured cells, whereas the glycolytic flux increased two- to fourfold; the contribution of OxPhos to ATP supply was 60%. In contrast, in old-spheroids, OxPhos, ATP content, ATP/ADP ratio, and phosphorylation potential diminished 50-70%, as well as the activity (88%) and content (3 times) of cytochrome c oxidase. Glycolysis and hexokinase increased significantly (both, 4 times); consequently glycolysis was the predominant pathway for ATP supply (80%). These changes were associated with an increase (3.3 times) in the HIF-1alpha content. After chronic exposure, both oxidative and glycolytic inhibitors blocked spheroid growth, although the glycolytic inhibitors, 2-deoxyglucose and gossypol (IC(50) of 15-17 nM), were more potent than the mitochondrial inhibitors, casiopeina II-gly, laherradurin, and rhodamine 123 (IC(50) > 100 nM). These results suggest that glycolysis and OxPhos might be considered as metabolic targets to diminish cellular proliferation in poorly vascularized, hypoxic solid tumors.

A multiscale model for avascular tumor growth

The desire to understand tumor complexity has given rise to mathematical models to describe the tumor microenvironment. We present a new mathematical model for avascular tumor growth and development that spans three distinct scales. At the cellular level, a lattice Monte Carlo model describes cellular dynamics (proliferation, adhesion, and viability). At the subcellular level, a Boolean network regulates the expression of proteins that control the cell cycle. At the extracellular level, reaction-diffusion equations describe the chemical dynamics (nutrient, waste, growth promoter, and inhibitor concentrations). Data from experiments with multicellular spheroids were used to determine the parameters of the simulations. Starting with a single tumor cell, this model produces an avascular tumor that quantitatively mimics experimental measurements in multicellular spheroids. Based on the simulations, we predict: 1), the microenvironmental conditions required for tumor cell survival; and 2), growth promoters and inhibitors have diffusion coefficients in the range between 10(-6) and 10(-7) cm2/h, corresponding to molecules of size 80-90 kDa. Using the same parameters, the model also accurately predicts spheroid growth curves under different external nutrient supply conditions.

Response of Chinese Hamster V79 Multicellular Spheroids Exposed to High-Energy Carbon Ions

Chinese hamster V79-379A spheroids 200 +/- 30 microm (+/- SD) in diameter were irradiated in agitated medium in different oxygen atmospheres with (1) 227 MeV/nucleon (12)C(+6) ions (plateau region) to model tissue in the entrance channel during therapy, (2) carbon ions in the extended Bragg peak modeling tissue in the target volume, or (3) X rays as a reference modality. Cell survival curves were similar for modes (1) and (3), indicating the absence of a contact effect and the presence of a pronounced oxygen effect with oxygen enhancement ratios (OERs) of 2.8 and 2.9, respectively. In contrast, the oxygen effect was substantially smaller in mode (2) with an OER of 1.4. Under normal or restricted oxygen supply conditions (external pO(2) = 145 or 0 mmHg), the relative biological effectiveness (RBE) was 2.1 or 4.3, respectively, for Bragg-peak irradiation. This modality induced apoptosis and a dose-dependent accumulation of cells in G(2)/M phase even at pO(2) = 0 mmHg. The volume ratios of treated to untreated spheroids exhibited cyclic variations after heavy-particle treatment that were not directly attributable to cell cycle synchronization. In summary, the results suggest that carbon ions in the extended Bragg peak are more effective than conventional X rays, particularly under hypoxic conditions.

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.

Phosphorous metabolites and steady-state energetics of transformed fibroblasts during three-dimensional growth

Rat1-T1 and MR1 spheroids represent separate transformed phenotypes originated from the same rat fibroblasts that differ in three-dimensional (3D) growth kinetics, histological structure, and oxygenation status. In the present study, (31)P-NMR spectroscopy of perfused spheroid suspensions was used to investigate cellular energetics relative to 3D growth, development of necrosis, and cell cycle distribution. Both spheroid types were characterized by a remarkably low amount of free (inorganic) phosphate (P(i)) and a low phosphocreatine peak. The ratio of nucleoside triphosphate (NTP) to P(i) ranged between 1.5 and 2.0. Intracellular pH, NTP-to-P(i) ratio, and NTP/cell remained constant throughout spheroid growth, being unaffected by the emergence of oxygen deficiency, cell quiescence, and necrosis. However, a 50% decrease in the ratio of the lipid precursors phosphorylcholine and phosphorylethanolamine (PC/PE) was observed with increasing spheroid size and was correlated with an increased G(1)/G(0) phase cell fraction. In addition, the ratio of the phospholipid degradation products glycerophosphorylcholine and glycerophosphorylethanolamine (GPC/GPE) increased with spheroid diameter in Rat1-T1 aggregates. We conclude that changes in phospholipid metabolism, rather than alterations in energy-rich phosphates, reflect cell quiescence in spheroid cultures, because cells in the inner oxygen-deficient zones seem to adapt their energy metabolism to the environmental conditions before necrotic cell destruction.

Metabolic mapping with bioluminescence: basic and clinical relevance

This review is focused on metabolic mapping in biological tissue with quantitative bioluminescence and single photon imaging. Metabolites, such as ATP, glucose and lactate, can be imaged quantitatively and within microscopic dimensions in cryosections from shock frozen biological specimens using enzyme reactions and light emission by luciferases. The technique has been applied in numerous targets and models of experimental biomedical research, such as multicellular spheroids, various organs of laboratory animals in a physiological or pathophysiological state, and even in plant seeds. Among numerous other aspects, data obtained with this method have contributed to the elucidation of mechanisms that are involved in the development of necrosis in multicellular spheroids. The combination of the bioluminescence technique with immunohistochemistry, autoradiography or in situ hybridization can considerably reduce ambiguities in the interpretation of the experimental results. Although, an invasive technique, bioluminescence imaging has been used most intensively in clinical oncology using tumor biopsies taken at the first diagnosis of the disease. It has been shown for squamous cell carcinomas of the head and neck and of the uterine cervix that accumulation of high levels of lactate in the primary lesions is associated with a high risk of metastasis formation and a reduced overall and disease-free patient survival. Thus, metabolic imaging can provide additional information on the degree of malignancy and the prognosis of tumors which may help the oncologist in improving specific treatment approaches for each individual malignant disease. Last but not least, metabolic mapping in clinical oncology has stimulated a number of investigations in basic cancer research on mechanisms that underlie the correlation between tumor metabolism and malignancy.

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.

Proliferative activity and tumorigenic conversion: impact on cellular metabolism in 3-D culture

Oxygen consumption, glucose, lactate, and ATP concentrations, as well as glucose and lactate turnover rates, have been studied in a three-dimensional carcinogenesis model of differently transformed rat embryo fibroblasts (spontaneously immortalized Rat1 and myc-transfected M1, and the ras-transfected, tumorigenic descendants Rat1-T1 and MR1) to determine metabolic alterations that accompany tumorigenic conversion. Various bioluminescence techniques, thymidine labeling, measurement of PO(2) distributions with microelectrodes, and determination of cellular oxygen uptake rates (Qc(O(2))) have been applied. In the ras-transfected, tumorigenic spheroid types, the size dependencies of some of the measured parameters exhibited sharp breaks at diameters of approximately 830 microm for Rat1-T1 and approximately 970 microm for MR1 spheroids, respectively, suggesting that some fundamental change in cell metabolism occurred at these characteristic diameters (denoted as "metabolic switch"). Qc(O(2)) decreased and lactate concentration increased as functions of size below the characteristic diameters. Concomitantly, glucose and lactate turnover rates decreased in MR1 spheroids and increased in Rat1-T1. Spheroids larger than the characteristic diameters (exhibiting cell quiescence and lactate accumulation) showed an enhancement of Qc(O(2)) with size. Systematic variations in the ATP and glucose levels in the viable cell rim were observed for Rat1-T1 spheroids only. Proliferative activity, Qc(O(2)), and ATP levels in small, nontumorigenic Rat1 and M1 aggregates did not differ systematically from those recorded in the largest spheroids of the corresponding ras transfectants. Unexpectedly, respiratory activity was present not only in viable but also in the morphologically disintegrated core regions of M1 aggregates. Our data suggest that myc but not ras transfection exerts major impacts on cell metabolism. Moreover, some kind of switch has been detected that triggers profound readjustment of tumor cell metabolism when proliferative activity begins to stagnate, and that is likely to initiate some other, yet unidentified energy-consuming process.

Hyperlactacidaemia in isolated hyperthermic perfusion of tumour bearing rat limbs: a study of feasibility using a novel infusion solution

PURPOSE

In a methodological study the applicability of hyperlactacidaemia in isolated hyperthermic perfusion of tumour-bearing rat limbs was investigated.

METHODS

In 50 Sprague Dawley rats, DS-sarcoma growth was initiated on the right food dorsum by subcutaneous injection of 0.5 ml ascites cells. In the anaesthetized animals isolated limb perfusion was performed under steady state conditions for 60min using a miniature equipment. Thereafter tumour volume was measured daily. (a) Investigation of feasability: 40 rats were allocated to four groups. Group I: Normothermic perfusion at 38 degrees C, n = 10; Group II: Hyperthermic perfusion at 40-41 degrees C, n = 10; Group III: Normothermic perfusion at 38 degrees C and hyperlactacidaemia of 10 mmol/l, n = 10; Group IV: Hyperthermic perfusion at 40-41 degrees C and hyperlactacidaemia of 10 mmol/l, n = 10. (b) Investigation of survival and histological changes: In group V hyperthermic perfusion at 40-41 degrees C and hyperlactacidaemia of 10 mmol/l, n = 10 was performed. After the animals had died, hip disarticulation of the tumour-bearing limb was performed for histological examination.

RESULTS

Normothermic and hyperthermic perfusion of tumour-bearing rat limbs using miniature equipment was feasible and tolerated by the animals. Regional hyperlactacidaemia of 10 mmol/l could be maintained throughout the perfusions. After combined treatment with hyperthermia and hyperlactacidaemia, tumour volume decreased and extensive tumour necrosis occurred, while in other animals aggressive tumour growth with bone infiltration could be observed.

CONCLUSIONS

The present study demonstrates the applicability of hyperlactacidaemia in hyperthermic isolated limb perfusion in the rat and proved a tumour growth delay due to an induction of tumour necrosis thereafter. Further investigations in other tumour entities and experimental models are required to confirm this impressive therapeutic effect of hyperthermia in combination with hyperlactacidaemia.

Variable presence of hypoxia in M006 human glioma spheroids and in spheroids and xenografts of clonally derived sublines

Recently we reported the variable presence of hypoxia adjacent to necrosis in human glioma lines grown as subcutaneous tumours in severe combined immunodeficient (SCID) mice. To assess the basis for this observation, we examined the pattern of oxygenation in M006 and M006XLo glioma spheroids. We found a wide range of binding of [3H]misonidazole to cells adjacent to the necrotic core, analogous to the patterns seen in xenografts, indicating substantial differences in the central oxygen tension of the spheroids. Clonal selection was used to isolate single cell-derived sublines of the M006XLo line. Some sublines gave spheroids that showed narrow distributions of [3H]misonidazole binding to the cells adjacent to necrosis, whereas other sublines showed a range of binding similar to that seen in spheroids of the parent line. After additional passages in monolayer culture, clonal sublines occasionally gave rise to spheroids in which the mean oxygen tension of cells adjacent to necrosis differed substantially from that of the initial spheroids. No relationship was evident between the thickness of the rim of viable cells and the presence or absence of central hypoxia, over a wide range of rim thickness. These results indicate that different oxygenation characteristics of glioma spheroids and tumour microregions are unlikely to arise from stable genetic variants coexisting in the parent line.

Multicellular spheroids: a three-dimensional in vitro culture system to study tumour biology

The growth of tumour cells as three-dimensional multicellular spheroids in vitro has led to important insights in tumour biology, since properties of the in vivo-tumour such as proliferation or nutrient gradients, can be studied under controlled conditions. While this review starts with an update of recent data on spheroid monocultures, especially concerning tumour microenvironment and therapeutic modalities, the main emphasis is put on the spectrum of heterologous cultures which have evolved in previous years. This type of culture includes tumour cell interaction with endothelial, fibroblast or immunocompetent cells. The relation of the spheroid culture model to other types of three-dimensional culture and our critical evaluation and presentation of the technical aspects of growing and analysing spheroids are included in the text. These topics are chosen to help the experimental pathologist design experiments with tumour spheroids and to stimulate discussion.

Mitochondrial function in oncogene-transfected rat fibroblasts isolated from multicellular spheroids

Two mitochondrion-specific fluorochromes, 10-N-nonyl acridine orange (NAO) and rhodamine 123 (Rh123), were used to determine the mechanism responsible for alterations in energy metabolism of transformed rat embryo fibroblast cells isolated from different locations within multicellular spheroids. Accumulation of Rh123 depends on intact mitochondrial membrane potential, whereas NAO is taken up by mitochondria independently of their function and thus represents mitochondrial distribution only. A reproducible selective dissociation procedure was used to isolate cells from different locations within the spheroids. After isolation, cells were simultaneously stained with one mitochondrial stain and the DNA dye Hoechst 33342, and several parameters, including cell volume, were monitored via multilaser-multiparameter flow cytometry. Our data clearly show a decrease in the uptake of Rh123 in cells from the periphery to the inner regions of the tumor spheroids, reflecting a persistent alteration in mitochondrial function. However, NAO staining experiments showed no reduction in the total mitochondrial mass per unit cell volume. Because cells were exposed to stain under uniform conditions after isolation from the spheroid, these data indicate the downregulation of mitochondrial function is associated with cell quiescence rather than a transient effect of reduced nutrient availability. This result, which is in accordance with data from two other cell lines (EMT6 and 9L), might reflect a general phenomenon in multicellular spheroids, supporting the hypothesis that quiescent cells in the innermost viable spheroid layer stably reduce their mitochondrial function, presumably to compensate for lower nutrient supply and/or decreased energy demand.

Glutathione content of V79 cells in two- or three-dimensional culture

The cellular glutathione (GSH) content of two- and three-dimensional cell cultures of V79 hamster lung cells has been studied. As previously described, cells in monolayer cultures show a decrease in GSH when they reach the confluent state. Three-dimensional cell cultures (multicell spheroids) allow a smoother transition from the initial proliferating to the nonproliferating status, and they show a central area of necrosis when a certain diameter is reached. Cellular GSH content in spheroids is variable throughout the culturing period: 1) GSH content (expressed per mg protein) is lower in spheroids with central necrotic areas than in smaller spheroids without necrosis, and 2) results expressed per cell number show a sharp increase around the diameter where necrosis appears. Once a relatively large necrotic area has been established, GSH decreases again to approximately the prenecrotic level. Interestingly, this GSH "peak" is not dependent on the time in culture but on the spheroid size. Acute hypoxia occurs in central areas of spheroids at a much higher size range than those described herein. Thus we suggest a combination of factors, which may include oxidative stress among others, as the explanation for these cellular GSH variations.

Tumour-growth inhibition by induced hyperglycaemia/hyperlactacidaemia and localized hyperthermia

The present study was undertaken to exploit pathophysiological properties of solid tumours for a tumour-specific therapy. Experiments were carried out on DS-sarcomas implanted s.c. in the hind foot dorsum of Sprague Dawley rats. Treatment strategies included tumour acidification, lactate accumulation and disturbance of the microcirculation by induced systemic hyperglycaemia/hyperlact-acidaemia (15-25/10 mmol/L; for 60 min) as well as localized hyperthermia (water-bath; 43 degrees C, 30 min.). A special infusion solution was developed for the systemic treatment containing glucose, lactic acid and organic buffer without inorganic ions. Growth kinetics of tumour volume and animal survival were taken as endpoints in order to quantify therapeutic efficiency. After a single treatment with combined modalities, i.e., with hyperglycaemia/hyperlactacidaemia and hyperthermia, approximately 50% of the tumours showed complete remission in three independent series of experiments; around 40% of the animals survived more than two months. In the untreated control group, all animals died from the disease within 10-15 days after tumour implantation. The overall effect on tumour volume changes of the combined therapy was supra-additive compared to that of treatment with hyperthermia or hyperglycaemia/hyperlactacidaemia alone. However, treated animals either showed a dramatic response to the combination of treatments with complete tumour remission or hardly responded at all, justifying a subdivision into responders and non-responders. Pathophysiological mechanisms responsible for this behaviour have to be elucidated in future studies. Nevertheless, the present study represents an approach to an efficient tumour therapy with a potential application in clinical oncology in the not too distant future.

On the relation between size of necrosis and diameter of tumor spheroids

PURPOSE

In many previous experimental studies on multicellular tumor spheroids, the spheroid diameter at which central necrosis develops has been determined to be twice the thickness of the viable cell rim measured at a later stage of spheroid growth. This procedure tacitly assumes that there is a linear relation between the diameter of necrosis and that of the whole spheroid over the entire range of emergence and growth of necrosis. However, some experimental investigations have demonstrated that necroses do not grow gradually with spheroid diameter, but show a rapid initial increase, once a few cells have died. The present article offers an explanation for this phenomenon, which is derived from basic diffusion theory.

METHODS AND MATERIALS

A theoretical relation between sizes of spheroids and of their central necroses is developed, which is based on the assumption that formation of necrosis is caused by depletion of substrates or accumulation of metabolic waste products. In a second part, the theoretical model is fitted to experimental data from the literature, and oxygen consumption rate as a function of spheroid size is determined.

RESULTS

It turns out that the model closely mimics the experimentally observed behavior described above. These experimental results, therefore, do not furnish any evidence for assuming other hypotheses of necrosis formation. Resulting O2 consumption rates are well in the range of previously published data. In all cases, approximations to the measured data are better than the corresponding linear squares fits.

CONCLUSION

At least in some tumor cell lines, depletion of substrates or accumulation of waste products can explain formation of necrosis without the assumption of any additional mechanisms. Moreover, the model presented in this article offers an alternative way of determining the turnover rate of a substrate or metabolic waste product provided that depletion/accumulation of this substance represents the cause for necrosis development.

Calibration of misonidazole labeling by simultaneous measurement of oxygen tension and labeling density in multicellular spheroids

To correlate misonidazole concentrations and oxygen pressures (Po2) at identical locations within EMT6/Ro multi-cell spheroids (mean diameters +/- SD: 867 +/- 20 microns), Po2 measurements were performed with oxygen-sensitive microelectrodes during incubation of these spheroids with tritiated misonidazole (10 mg/I; 445 microCi/mg). In each individual spheroid, Po2 profiles were correlated with the corresponding spatial distribution of misonidazole as quantified by conventional autoradiography and grain counting. To compare the oxygenation status of spheroids in the measuring chamber with that of spheroids in spinner culture, misonidazole labeling was performed in both environments following the same protocol. All experiments were conducted in 20% oxygen and BME or in 5% oxygen and DMEM to obtain spheroids with different degrees of oxygenation. Labeled misonidazole was fairly evenly distributed in the outer, better oxygenated regions of EMT6 spheroids. In contrast, there was an accumulation of the labeled substance near central necrosis where low oxygen tensions were measured. Grain densities were similar at corresponding oxygen pressures under both environmental conditions. Except for some scatter, grain density as a function of oxygen pressure showed little variation in the Po2 range of 20-60 mm Hg, but exhibited a steep increase below 10 mm Hg. The findings imply that a substantial rise in local misonidazole labeling indicates a metabolically active tissue region at low Po2 that is not necessarily identical with the radiobiologically hypoxic cell fraction. A comparison of the labeling densities of spheroids in spinner flasks and in the Po2 measuring chamber indicates that oxygenation of spheroids is better in rotation culture than during microelectrode measurements.

Quantitative recording of vitality patterns in living multicellular spheroids by confocal microscopy

Fluorescent dyes were used in conjunction with confocal microscopy to record the vitality status of cells in multicellular glioma spheroids. Multicellular spheroids are in vitro models for micrometastases or intravascular microregions of large tumors. With progressing growth three distinct concentric annular shells develop. A rim of proliferating cells in the periphery is followed towards the center by layers of quiescent cells and at a defined spheroid diameter cell death occurs in the central core. Fluorescein diacetate (FDA) and Calcein/AM were used as vital stains and Lucifer Yellow/VS (LYVS) was used as a marker for dead cells. For loading multicellular spheroids with the esterase substrate dyes we used a two step cold incubation technique to avoid dye accumulation in the most peripheral cell layers. Homogenously stained tissue allowed to describe the fluorescence attenuation in depth as a monoexponential decay. An attenuation coefficient C was calculated from calibration experiments to be 12.5 x 10(-3) in vital stained tissue and 17.9 x 10(-3) in lethal stained tissue. Using the respective attenuation coefficient the raw data were corrected for light absorption and scattering in depth. In radial recordings of the vitality status of multicellular glioma spheroids using CLSM-technique we showed that spheroids up to a diameter of 250 microns were homogenously stained with Calcein/AM and FDA. Spheroids larger than 250 microns consist of vital stained cells and unstained cells. They do not show dead cell staining until they reach a diameter of about 400 microns. The thickness of the rim of vital stained cells decreased with increasing diameter of the spheroids to 64 +/- 7 microns in spheroids of a diameter of 550 +/- 25 microns. Thereafter the thickness of the Calcein/AM or FDA stained rim augmented again, reaching 93 +/- 9 microns in spheroids of 700 microns in diameter. The first signs of dead cell staining in the central core occurred at a diameter of 400 +/- 25 microns. The radius of the core increased in an exponential way. The cell layer which was stained neither by vital nor by lethal dyes showed a thickness of 150 microns in spheroids of 550 +/- 25 microns in diameter. Our staining technique and the radial recording of mean field fluorescence signals in living multicellular spheroids will be a valuable tool for experimental cancer research providing a non invasive quantification of cell vitality in living multicellular spheroids.

Quantitative comparison between the gel-film and polyvinyl alcohol methods for dehydrogenase histochemistry reveals different intercellular distribution patterns of glucose-6-phosphate and lactate dehydrogenases in mouse liver

The precise histochemical localization and quantification of the activity of soluble dehydrogenases in unfixed cryostat sections requires the use of tissue protectants. In this study, two protectants, polyvinyl alcohol (PVA) and agarose gel, were compared for assaying the activity of lactate dehydrogenase (LDH) and glucose-6-phosphate dehydrogenase (G6PDH) in normal female mouse liver. Quantification of enzyme activity was determined cytophotometrically in periportal (PP), pericentral (PC) and midzonal (MZ) areas. No coloured reaction product was present in PVA media after the incubation period. In contrast, the agarose gels appeared to be highly coloured after incubation. As a consequence, sections incubated with gel media were less intensely stained than those incubated in PVA-containing media. The specific G6PDH reaction (test minus control) yielded approximately 75% less formazan in sections incubated by the agarose gel method than with the PVA method. Further, the amount of formazan deposits attributable to G6PDH activity was highest in the midzonal and pericentral zones of the liver lobule with PVA media, and Kupffer cells could be discriminated easily because of their high G6PDH activity. Significant zonal differences or Kupffer cells could not be observed when agarose gel films were used for the detection of G6PDH activity. The LDH localization patterns appeared to be more uniform after incubation with both methods: no significant differences in specific test minus control reactions were seen between PP, PC and MZ. However, less formazan production (33%) was detected in sections incubated with agarose gels when compared with those incubated with PVA media.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen consumption rate and mitochondrial density in human melanoma monolayer cultures and multicellular spheroids

Rate of oxygen consumption per cell has been shown in previous studies to decrease with increasing depth in the viable rim of multicellular spheroids initiated from rodent cells, human colon-carcinoma cells, and human glioma cells, due to progressive accumulation of quiescent cells during spheroid growth. The purpose of our work was to determine oxygen-consumption profiles in human melanoma spheroids. Monolayer cultures of 4 lines (BEX-c, COX-c, SAX-c, and WIX-c) and spheroid cultures of 2 lines (BEX-c and WIX-c) were subjected to investigation. Spheroids were initiated from monolayer cell cultures and grown in spinner flasks. Rate of oxygen consumption was measured with a Clarke-type electrode. Mitochondrial density was determined by stereological analysis of transmission electron micrographs. Thickness of viable rim and cell packing density were assessed by light microscopy of central spheroid sections. Cell-cycle distribution was determined by analysis of DNA histograms measured by flow cytometry. Cell volume was measured by an electronic particle counter. Rate of oxygen consumption per cell differed by a factor of approximately 1.8 between the 4 cell lines and was positively correlated to total volume of mitochondria per cell. Rate of oxygen consumption per cell and total volume of mitochondria per cell were equal for monolayer cell cultures, 600-microns spheroids and 1,200-microns spheroids of the same line. Mitochondrial density and location in the cell did not differ between cells at the spheroid surface, in the middle of the viable rim and adjacent to the central necrosis. Cell-cycle distribution, cell volume, and cell-packing density in the outer and inner halves of the viable rim were not significantly different. Consequently, the rate of oxygen consumption per cell in inner regions of the viable rim was probably equal to that at the spheroid surface, suggesting that oxygen diffusion distances may be shorter in some melanomas than in many other tumor types.

Oncogene-associated growth behavior and oxygenation of multicellular spheroids from rat embryo fibroblasts

The basis of the present investigation was the establishment of an oncogene-dependent, genetically determined two-stage carcinogenesis in vitro model as multicellular spheroids. Spheroid formation was achieved with four rat embryo fibroblast cell lines, two of which represent the first step of malignant transformation, known as stage of immortalization. The ras-transfected counterparts of these two parental cell clones represent fully transformed phenotypes. The data obtained show that spheroid volume growth and cellular viability reflect the degree of tumorigenicity in vivo of the different fibroblast types investigated. In addition, ras-transfection alters not only the growth kinetics but also the cellular oxygen metabolism. Furthermore, the results demonstrate very clearly that different fibroblast clones at the same stage of malignant transformation may be characterized by an entirely different growth behavior, morphology and metabolic activity in spheroid culture. This is true, although these cells originate from the same primary cells, differ only in the step of immortalization, and were cultured as spheroids under identical environmental conditions.

Geographical mapping of metabolites in biological tissue with quantitative bioluminescence and single photon imaging

This article features a novel technique for measuring the spatial distribution of metabolites, such as ATP, glucose, and lactate, in rapidly frozen tissue. Concentration values are obtained in absolute terms and with a spatial resolution of single-cell dimension. The method is based on enzymatic reactions that link the metabolite of interest to luciferase with subsequent light emission. Using a specific array, cryosections are brought into contact with the enzymes in a well-defined, reproducible way inducing a distribution of light across the section with an intensity that is proportional to the metabolite concentration. The emitted light can be visualized through a microscope and an imaging photon counting system, and the respective image can be transferred to a computer for image analysis. Measurements in spherical cell aggregates with central necrosis demonstrate a close correlation between the distribution of ATP and of cellular viability at a microregional level. Similarly, ATP and glucose are correlated with the geometrical arrangement of more viable and more necrotic tissue regions in human melanomas xenografted in nude mice. Lactate did not show such a structure-related distribution in these tumours. Structure-related distributions of ATP, glucose, and lactate are found in cervix tumours of patients. In contrast to the heterogeneous distributions in tumours, the distribution patterns were much more homogeneous in normal tissues. Regional differences were present, but were much more gradual than in malignancies. This was illustrated for heart muscle where ATP concentrations were found that agreed with data in the literature, and that showed a decrease in periventricular areas.

Proliferation-associated oxygen consumption and morphology of tumor cells in monolayer and spheroid culture

The oxygen consumption rate, proliferative activity, and morphology of EMT6/Ro mouse mammary sarcoma cells in monolayer and multicellular spheroid culture have been investigated in a comparative study. During the transition of monolayer cells from the exponential into the plateau growth phase, there is a distinct decrease in the cellular volume that is associated with a corresponding decrease in the proliferative and respiratory activity of the cells. The decline in cell volume is mainly due to a decrease in the content of cytoplasm, whereas the size of the nucleus is only slightly reduced. A concomitant decrease in the number of mitochondria per cell obviously accounts for the reduction in cellular oxygen uptake. Despite a continuous decrease of cell proliferation from the surface to interior regions of EMT6 spheroids reflected by a gradient in tritiated thymidine labeling, volume-related oxygen consumption is rather uniform in viable regions of these aggregates. The finding can be explained by the results of the morphometric evaluation showing a uniform volume density of mitochondria, i.e., of oxygen-consuming sites within these spheroids.

Influence of glucose on metabolism and growth of rat glioma cells (C6) in multicellular spheroid culture

The metabolism and growth of rat glioma C6 cells in multicellular spheroid culture depended strongly on the glucose supply. A low glucose level (0.1 g/l) in the culture medium reduced lactate production, increased oxygen consumption and diminished hydrogen ion production under normoxia as well as hypoxia. A high glucose level (10 g/l glucose) increased lactate production, had no significant influence on oxygen consumption and increased the hydrogen ion production under hypoxia. Hydrogen ion release from cells under normoxic and hypoxic conditions could be significantly diminished by amiloride (l mM), indicating the involvement of the Na+/H+ exchanger. The growth of the C6 spheroids was enhanced under low glucose conditions, possibly due to the more physiological extracellular pH in the deeper regions of the spheroids. The growth was inhibited under high glucose conditions, which seemed to be toxic due to a massive hydrogen production giving acidosis. The glucose supply strongly influenced the local hydrogen ion production inside the C6 spheroids and this might in turn lead to changes in the response to different therapeutic modalities.