Multicellular spheroids. A review on cellular aggregates in cancer research

Evaluation of the Hsp90 inhibitor NVP-AUY922 in multicellular tumour spheroids with respect to effects on growth and PET tracer uptake

BACKGROUND

Molecular targeting has become a prominent concept in cancer treatment and heat shock protein 90 (Hsp90) inhibitors are suggested as promising anticancer drugs. The Hsp90 complex is one of the chaperones that facilitate the refolding of unfolded or misfolded proteins and plays a role for key oncogenic proteins such as Her2, Raf-1, Akt/PKB, and mutant p53. NVP-AUY922 is a novel low-molecular Hsp90 inhibitor, currently under clinical development as an anticancer drug. Disruption of the Hsp90-client protein complexes leads to proteasome-mediated degradation of client proteins and cell death. The aim of the current study was to use a combination of the multicellular tumour spheroid (MTS) model and positron emission tomography (PET) to investigate the effects of NVP-AUY922 on tumour growth and its relation to PET tracer uptake for the selection of appropriate PET tracer. A further aim was to evaluate the concentration and time dependence in the relation between growth inhibition and PET tracer uptake as part of translational imaging activities.

METHODS

MTS of two breast cancer cell lines (MCF-7 and BT474), one glioblastoma cell line (U87MG) and one colon carcinoma cell line (HCT116) were prepared. Initially, we investigated MTS growth pattern and (3)H-thymidine incorporation in MTS after continuous exposure to NVP-AUY922 in order to determine dose response. Then the short-term effect of the drug on the four PET tracers 2-[(18)F] fluoro-2-deoxyglucose (FDG), 3'-deoxy-3'-fluorothymidine (FLT), methionine and choline was correlated to the long-term effect (changes in growth pattern) to determine the adequate PET tracer with high predictability. Next, the growth inhibitory effect of different dose schedules was evaluated to determine the optimal dose and time. Finally, the effect of a 2-h exposure to the drug on growth pattern and FDG/FLT uptake was evaluated.

RESULTS

A dose-dependent inhibition of growth and decrease of (3)H-thymidine uptake was observed with 100% growth cessation in the dose range 7-52 nM and 50% (3)H-thymidine reduction in the range of 10-23 nM, with the most pronounced effect on BT474 cells. The effect of the drug was best detected by FLT. The results suggested that a complete cessation of growth of the viable cell volume was achieved with about 50% inhibition of FLT uptake 3 days after continuous treatment. Significant growth inhibition was observed at all doses and all exposure time spans. Two-hour exposure to NVP-AUY922 generated a growth inhibition which persisted dose dependently up to 10 days. The uptake of FDG per viable tumour volume was reduced by just 25% with 300 nM treatment of the drug, whereas the FLT uptake decreased up to 75% in correlation with the growth inhibition and recovery.

CONCLUSIONS

Our results indicate a prolonged action of NVP-AUY922 in this cell culture, FLT is a suitable tracer for the monitoring of the effect and a FLT PET study within 3 days after treatment can predict the treatment outcome in this model. If relevant in vivo, this information can be used for efficient planning of animal PET studies and later human PET trial.

Microenvironment driven invasion: a multiscale multimodel investigation

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

Spheroid-based drug screen: considerations and practical approach

Although used in academic research for several decades, 3D culture models have long been regarded expensive, cumbersome and unnecessary in drug development processes. Technical advances, coupled with recent observations showing that gene expression in 3D is much closer to clinical expression profiles than those seen in 2D, have renewed attention and generated hope in the feasibility of maturing organotypic 3D systems to therapy test platforms with greater power to predict clinical efficacies. Here we describe a standardized setup for reproducible, easy-handling culture, treatment and routine analysis of multicellular spheroids, the classical 3D culture system resembling many aspects of the pathophysiological situation in human tumor tissue. We discuss essential conceptual and practical considerations for an adequate establishment and use of spheroid-based drug screening platforms and also provide a list of human carcinoma cell lines, partly on the basis of the NCI-DTP 60-cell line screen, that produce treatable spheroids under identical culture conditions. In contrast to many other settings with which to achieve similar results, the protocol is particularly useful to be integrated into standardized large-scale drug test routines as it requires a minimum number of defined spheroids and a limited amount of drug. The estimated time to run the complete screening protocol described herein--including spheroid initiation, drug treatment and determination of the analytical end points (spheroid integrity, and cell survival through the acid phosphatase assay)--is about 170 h. Monitoring of spheroid growth kinetics to determine growth delay and regrowth, respectively, after drug treatment requires long-term culturing (> or =14 d).

Radiation quality-dependent bystander effects elicited by targeted radionuclides

The efficacy of radiotherapy may be partly dependent on indirect effects, which can sterilise malignant cells that are not directly irradiated. However, little is known of the influence of these effects in targeted radionuclide treatment of cancer. We determined bystander responses generated by the uptake of radioiodinated iododeoxyuridine ([*I]IUdR) and radiohaloanalogues of meta-iodobenzylguanidine ([*I]MIBG) by noradrenaline transporter (NAT) gene-transfected tumour cells. NAT specifically accumulates MIBG. Multicellular spheroids that consisted of 5% of NAT-expressing cells, capable of the active uptake of radiopharmaceutical, were sterilised by treatment with 20 kBqmL(-1) of the alpha-emitter meta-[211At]astatobenzylguanidine ([211At]MABG). Similarly, in nude mice, retardation of the growth of tumour xenografts containing 5% NAT-positivity was observed after treatment with [131I]MIBG. To determine the effect of subcellular localisation of radiolabelled drugs, we compared the bystander effects resulting from the intracellular concentration of [131I]MIBG and [131I]IUdR (low linear energy transfer (LET) beta-emitters) as well as [123I]MIBG and [123I]IUdR (high LET Auger electron emitters). [*I]IUdR is incorporated in DNA whereas [*I]MIBG accumulates in extranuclear sites. Cells exposed to media from [131I]MIBG- or [131I]IUdR-treated cells demonstrated a dose-response relationship with respect to clonogenic cell death. In contrast, cells receiving media from cultures treated with [123I]MIBG or [123I]IUdR exhibited dose-dependent toxicity at low dose but elimination of cytotoxicity with increasing radiation dose (i.e. U-shaped survival curves). Therefore radionuclides emitting high LET radiation may elicit toxic or protective effects on neighbouring untargeted cells at low and high dose respectively. It is concluded that radiopharmaceutical-induced bystander effects may depend on LET of the decay particles but are independent of site of intracellular concentration of radionuclide.

Identification of differentially expressed genes involved in the formation of multicellular tumor spheroids by HT-29 colon carcinoma cells

The multicellular tumor spheroid (MCTS) model represents a suitable in vitro model recreating in vivo tumor formation. The aim of this study was to identify differentially expressed genes that could potentially serve as predictive gene markers for MCTS and be involved in the formation of MCTS. Using the suppression subtractive hybridization (SSH) method, we identified ERBB2/HER2-interacting protein (Erbin), Tumor rejection gp96 (Tr-gp96), 12S ribosomal RNA (12S rRNA), ATP synthase, Kruppel-like transcription factor 5 (KLF5), transcription factor-like 5 (TCFL5), and the dual-specificity phosphatase 11 (DUSP11) to be overexpressed in 3-day-old HT-29 colon carcinoma MCTSs compared to HT-29 colon carcinoma cells grown in monolayer. We could also confirm overexpression of these genes in HT-29 MCTSs and in MCTSs formed by the human glioblastoma tumor cell lines U343 MG, U373 MG, and DBTRG 05 MG. Knockdown of KLF5, Erbin, DUSP11, and TCFL5 was effectively achieved after transfection of HT-29 cells with the appropriate short-interfering RNAs (siRNAs), and correlated with a significant inhibition of MCTS formation in the case of KLF5, Erbin, and TCFL5 siRNAs. We suggest that KLF5, Erbin, and TCFL5 are essential for MCTS formation and play a key role in the development of tumor diseases.

Experimental anti-tumor therapy in 3-D: spheroids--old hat or new challenge?

PURPOSE

To give a state-of-the-art overview on the promise of three-dimensional (3-D) culture systems for anticancer drug development, with particular emphasis on multicellular tumor spheroids (MCTS).

RESULTS AND CONCLUSIONS

Cell-based assays have become an integral component in many stages of routine anti-tumor drug testing. However, they are almost always based on homogenous monolayer or suspension cultures and thus represent a rather artificial cellular environment. 3-D cultures--such as the well established spheroid culture system--better reflect the in vivo behavior of cells in tumor tissues and are increasingly recognized as valuable advanced tools for evaluating the efficacy of therapeutic intervention. The present article summarizes past and current applications and particularly discusses technological challenges, required improvements and recent progress with the use of the spheroid model in experimental therapeutics, as a basis for sophisticated drug/therapy screening. A brief overview is given focusing on the nomenclature of spherical 3-D cultures, their potential to mimic many aspects of the pathophysiological situation in tumors, and currently available protocols for culturing and analysis. A list of spheroid-forming epithelial cancer cell lines of different origin is provided and the recent trend to use spheroids for testing combination treatment strategies is highlighted. Finally, various spheroid co-culture approaches are presented that have been established to study heterologous cell interactions in solid tumors and thereby are able to reflect the cellular tumor environment with increasing accuracy. The intriguing observation that in order to retain certain tumor initiating cell properties, some primary tumor cell populations must be maintained exclusively in 3-D culture is mentioned, adding a new but fascinating challenge for future therapeutic campaigns.

A reliable tool to determine cell viability in complex 3-d culture: the acid phosphatase assay

Cell-based assays are more complex than cell-free test systems but still reflect a highly artificial cellular environment. Incorporation of organotypic 3-dimensional (3-D) culture systems into mainstream drug development processes is increasingly discussed but severely limited by complex methodological requirements. The objective of this study was to explore a panel of standard assays to provide an easy-handling, standardized protocol for rapid routine analysis of cell survival in multicellular tumor spheroid-based antitumor drug testing. Spheroids of 2 colon carcinoma cell lines were characterized for evaluation. One of the assay systems tested could reliably be used to determine cell viability in spheroids. The authors verified that the acid phosphatase assay (APH) is applicable for single spheroids in 96-well plates, does not require spheroid dissociation, and is linear and highly sensitive for HT29 and HCT-116 spheroids up to diameters of 650 microm and 900 microm, consisting of 40,000 and 80,000 cells, respectively. Treatment of HT29 and HCT-116 cells with 5-fluorouracil, Irinotecan, and C-1311 revealed critically reduced drug efficacies in 3-D versus monolayer culture, which is discussed in light of literature data. The experimental protocol presented herein is a small but substantial contribution to the establishment of sophisticated 3-D in vitro systems in the antitumor drug screening scenario.

Screening for compounds that induce apoptosis of cancer cells grown as multicellular spheroids

Screening and initial characterization of anticancer drugs are typically performed using monolayer cultures of tumor cells. It is well established that such monolayer cultures do not represent the characteristics of 3-dimensional solid tumors. The multicellular tumor spheroid model is of intermediate complexity between in vivo tumors and in vitro monolayer cultures and would be more suitable for drug screening. The authors describe a procedure in which multicellular spheroids are used to screen for compounds that induce tumor cell apoptosis. Multicellular spheroids were generated in 96-well plates, and apoptosis was determined using the M30-Apoptosense enzyme-linked immunosorbent assay method. A Z' factor of approximately 0.5 was observed for HCT116 colon carcinoma spheroids using staurosporine to induce apoptosis. This procedure is attractive for secondary screening of hits from larger cell-based screens.

Modelling the formation of necrotic regions in avascular tumours

The mechanisms underlying the formation of necrotic regions within avascular tumours are not well understood. In this paper, we examine the relative roles of nutrient deprivation and of cell death, from both the proliferating phase of the cell cycle via apoptosis and from the quiescent phase via necrosis, in changing the structure within multicellular tumour spheroids and particularly the accumulation of dead cell material in the centre. A mathematical model is presented and studied that accounts for nutrient diffusion, changes in cell cycling rates, the two different routes to cell death as well as active motion of cells and passive motion of the dead cell material. In studying the accumulation of dead cell matter we do not distinguish between the route by which each was formed. The resulting mathematical model is examined for a number of scenarios. Results show that in many cases the size of the necrotic core is closely correlated with low levels in nutrient concentration. However, in certain cases, particularly where the rate of necrosis is large, the resulting necrotic core can lead to regions of non-negligible nutrient concentration-dependent upon the mode of cell death.

Role of E-cadherin in the induction of apoptosis of HPV16-positive CaSki cervical cancer cells during multicellular tumor spheroid formation

Multicellular tumor spheroids (MCTS) are three dimensional cell culture systems induced by suspension culture. MCTS are widely used in cancer research because of their similarity to solid tumors. CaSki cells are derived from a metastatic cervical cancer containing human papillomavirus 16 (HPV16). Cell death of CaSki cells in MCTS has been previously reported, and our model is used to better characterize the mechanisms of cell death of HPV16-positive keratinocytes. In this study, we found that apoptosis of CaSki cells was induced by suspension culture along with the formation of MCTS after 24 h of incubation. In suspended CaSki cells, monoclonal antibodies blocking E-cadherin function inhibited MCTS formation and suppressed suspension-induced apoptosis in a dose-dependent manner. Western blot for E-cadherin detected upregulation of the authentic 120 kDa band from MCTS of CaSki cells as well as a shorter 100 kDa band. Addition of EGF, whose receptor is known to form a complex with E-cadherin, abrogated apoptosis of suspended CaSki cells in a dose-dependent manner. These findings suggest that E-cadherin-dependent cell-cell contact, directly or indirectly, mediates the signal to undergo apoptosis of CaSki cells during MCTS formation, and thus provides new information on the role of E-cadherin in cervical cancer cell apoptosis.

Radiobiological characterization of human tumor cell multilayers after conventional and particle irradiation

The goal of this study was to establish planar multilayers from human tumor cells (WiDr and SiHa) as a model for irradiation of solid tumors. In addition to using conventional X rays (250 kV) as a reference standard, multilayers were tested for their suitability in cell survival studies with heavy-ion irradiation ((12)C(6+)) in the plateau and the extended Bragg peak with a scanned ion beam. Multilayers of both cell lines showed decreased survival compared to the corresponding monolayers after both X and heavy-ion irradiation. This multicellular sensitization effect is in contrast to the multicellular resistance or contact effect commonly described in the literature. Flow cytometry measurements showed an arrest of irradiated SiHa cells in G(2)/M phase. In contrast to the transient arrest of the monolayers, the multilayers stayed in a prolonged arrest. After Bragg-peak irradiation of monolayers, the arrest time was increased by 12-24 h, and more cells were arrested than with X rays. For multilayers, there were no differences between G(2) arrest after X rays and heavy ions for the entire observation period.

Head and neck squamous cell carcinoma cell lines: established models and rationale for selection

BACKGROUND

Head and neck squamous cell carcinoma (HNSCC) cell lines are important preclinical models in the search for novel and targeted therapies to treat head and neck cancer. Unlike many other cancer types, a wide variety of primary and metastatic HNSCC cell lines are available. An easily accessible guide that organizes important characteristics of HNSCC cell lines would be valuable for the selection of appropriate HNSCC cell lines for in vitro or in vivo studies.

METHODS

A literature search was performed.

RESULTS

Cell growth and culture parameters from HNSCC cell lines were catalogued into tables or lists of selected characteristics. Methods for establishing cancer cell lines and basic cell culture maintenance techniques were reviewed.

CONCLUSIONS

A compendium of HNSCC cell line characteristics is useful for organizing the accumulating information regarding cell line characteristics to assist investigators with the development of appropriate preclinical models.

Activated stress response pathways within multicellular aggregates utilize an autocrine component

Multicellular aggregates (spheroids) of primary human foreskin fibroblasts (HFF-2) and a glioblastoma cell line (T98G) entered and exited from long term (2 weeks) metabolic arrest utilizing an autocrine response. Cytokine production (specifically IFN-gamma) activated a Gadd45alpha/p38 pathway that led to increased AP-1 (c-jun and ATF3) transcription factor levels, augmenting cytokine production in an autocrine fashion. Whereas HFF-2 aggregates were capable of surviving long term arrest and recovery during NF-kappaB inhibition independent of JNK activation, T98G aggregates were not. Such endogenous processes are not easily observed with adherent monolayer cell culturing systems, strongly suggesting that more emphasis needs to be placed on determining the operational signal transduction cascades within multicellular aggregates. Extracellular inputs such as spheroid formation, arrest, and regrowth as monolayers invoke intracellular signaling responses converging at the AP-1 transcription factor level. Variations in responses are both cell type and transformation state dependent and require an autocrine cytokine component. The data are discussed in relation to the wounding response and avascular tumor growth mechanisms.

An immersed boundary framework for modelling the growth of individual cells: an application to the early tumour development

A biomechanical approach in modelling the growth and division of a single fully deformable cell by using an immersed boundary method with distributed sources is presented, and its application to model the early tumour development is discussed. This mathematical technique couples a continuous description of a viscous incompressible cytoplasm with the dynamics of separate elastic cells, containing their own point nuclei, elastic plasma membranes with membrane receptors, and individually regulated cell processes. This model enables one to focus on the biomechanical properties of individual cells and on communication between cells and their microenvironment, simultaneously allowing for the formation of clusters or sheets of cells that act together as one complex tissue. Several examples of early tumours growing in various geometrical configurations and with distinct conditions of their initiation and progression are also presented to show the strength of our approach in modelling different topologies of the growing tissues in distinct biochemical conditions of the surrounding media.

Role of soluble factors and three-dimensional culture in in vitro differentiation of intestinal macrophages

AIM: To examine the factor(s) involved in differentiation of intestinal macrophages (IMACs) using a recently established in vitro model.

METHODS: To test whether soluble or membrane bound factors induce IMAC-differentiation, freshly elutriated monocytes (MO) were incubated with conditioned media or cell membranes of intestinal epithelial cells (IEC) or cultured with IEC in transwell systems. To determine the importance of an active migration of MO, three-dimensional aggregates from a 1:1-mixture of MO and IEC were examined by immunohistochemistry and flow cytometry. Apoptosis was examined by caspase-3 Western blots. Extracellular matrix production in differentiation models was compared by immunohistochemistry.

RESULTS: IMAC differentiation was observed in a complex three-dimensional co-culture model (multicellular spheroid, MCS) with IEC after migration of MO into the spheroids. By co-culture of MO with conditioned media or membrane preparations of IEC no IMAC differentiation was induced. Co-culture of MO with IEC in transwell-cultures, with the two cell populations separated by a membrane also did not result in intestinal-like differentiation of MO. In contrast to IEC-spheroids with immigrating MO in mixed MCS of IEC and MO only a small subpopulation of MO was able to survive the seven day culture period.

CONCLUSION: Intestinal-like differentiation of MO in vitro is only induced in the complex three-dimensional MCS model after immigration of MO indicating a role of cell-matrix and/or cell-cell interactions during the differentiation of IMACs.

Modeling of oxygen transport across tumor multicellular layers

PURPOSE

Tumor oxygen level plays a major role in the response of tumors to different treatments. The purpose of this study was to develop a method of determining oxygen transport properties in a recently developed 3D model of tumor parenchyma, the multicellular layer (MCL).

METHODS

OCM-1 human choroidal melanoma cells were grown as 3D MCLs on collagen-coated culture plate inserts. A recessed-cathode oxygen microelectrode was used to measure oxygen tension (PO(2)) profiles across 8 different MCL from the free surface to the insert membrane. The profiles were fitted to four different one-dimensional diffusion models: 1-, 2-, and 3-region models with uniform oxygen consumption (q) in each region and a modified 3-region model with a central region where q=0 and PO(2)=0.

RESULTS

Depending upon the presence of a central region of anoxia, the PO(2) profiles were fitted best by either the two-region model or the modified 3-region model. Consumption of tumor cells near the insert membrane was higher than that of cells close to the free surface (33.1+/-13.6 x 10(-4) vs. 11.8+/-6.7 x 10(-4) mm Hg/mum(2), respectively).

CONCLUSIONS

The model is useful for determining oxygenation and consumption in MCL, especially for cell lines that cannot be grown as spheroids. In the future, this model will permit the study of parameters important in tumor oxygenation in vitro.

Sense and sensibility: the use of cell death biomarker assays in high-throughput anticancer drug screening and monitoring treatment responses

Cell-based screening allows identification of biologically active compounds, for example, potential anticancer drugs. In this review, various screening assays are discussed in terms of what they measure and how this affects interpretation and relevance. High-throughput (HT) assays of viability based on the reduction of exogenous substrates do not always reflect viability or cell number levels. Membrane integrity assays can be used for HT quantification of cell death, but are non-specific as to the death mode. Several HT assays monitor end point apoptosis. Screening libraries at a single concentration (micromolar) can prevent detection of potent apoptosis inducers, as high concentrations may induce mainly necrosis. Using monolayer cultures limits the significance of cell-based screening as the properties of monolayer cells differ from tumours in vivo. Spheroid cultures are more physiological, but are impractical for screening by conventional methods. The authors have developed an assay quantifying accumulation of a caspase-cleaved protein specific for epithelial cells. It provides an integrated measure of apoptosis in two- and three-dimensional cultures and can be used as a blood biomarker assay for tumour apoptosis in vivo.

Rapid generation of single-tumor spheroids for high-throughput cell function and toxicity analysis

Spheroids are widely used in biology because they provide an in vitro 3-dimensional (3D) model to study proliferation, cell death, differentiation, and metabolism of cells in tumors and the response of tumors to radiotherapy and chemotherapy. The methods of generating spheroids are limited by size heterogeneity, long cultivation time, or mechanical accessibility for higher throughput fashion. The authors present a rapid method to generate single spheroids in suspension culture in individual wells. A defined number of cells ranging from 1000 to 20,000 were seeded into wells of poly-HEMA-coated, 96-well, round-or conical-bottom plates in standard medium and centrifuged for 10 min at 1000 g. This procedure generates single spheroids in each well within a 24-h culture time with homogeneous sizes, morphologies, and stratification of proliferating cells in the rim and dying cells in the core region. Because a large number of tumor cell lines form only loose aggregates when cultured in 3D, the authors also performed a screen for medium additives to achieve a switch from aggregate to spheroid morphology. Small quantities of the basement membrane extract Matrigel, added to the culture medium prior to centrifugation, most effectively induced compact spheroid formation. The compact spheroid morphology is evident as early as 24 h after centrifugation in a true suspension culture. Twenty tumor cell lines of different lineages have been used to successfully generate compact, single spheroids with homogenous size in 96-well plates and are easily accessible for subsequent functional analysis.

A new organotypic model to study cell interactions in the intestinal mucosa

BACKGROUND

Recently, we demonstrated that freshly elutriated monocytes differentiate into macrophages with a phenotype similar to that of intestinal macrophages in a three-dimensional model of intestinal epithelial cells. Here we describe a more organotypic model to study cell interactions in the intestinal mucosa.

METHODS

Primary intestinal fibroblasts and freshly elutriated blood monocytes (ratio 1:1) were embedded in collagen type I gels and cultured for 5 days. At day 5, intestinal epithelial cells (HT-29) were seeded on top of the collagen gels. After another 7 days collagen gels were harvested and fixed for immunohistochemical analysis. Cryosections of the aggregates were prepared and staining for monocyte/macrophage markers and basement membrane compounds was performed. Cell interactions inside the aggregates were examined by electron microscopy.

RESULTS

Intestinal fibroblasts contracted the collagen gels which formed stable three-dimensional aggregates within the first 5 days of culture. Intestinal epithelial cells formed a monolayer on top of the gels about 3 days after seeding. Intestinal fibroblasts were distributed randomly over the aggregate. Monocytes inside aggregates were localized in the vicinity to epithelial cells by positive staining for CD68. Typical monocyte/macrophage specific markers such as CD14, CD16, CD11b, CD11c and the co-stimulatory molecules CD80 and CD86 were down-regulated or not detectable on these cells after co-culture in three-dimensional aggregates. Omission of epithelial cells from the model was followed by impaired differentiation of intestinal macrophages.

CONCLUSION

In the three-dimensional organotypic cell culture model monocytes differentiate into intestinal-like macrophages when co-cultured with control intestinal fibroblasts and intestinal epithelial cells. Intestinal epithelial cells may be necessary for differentiation of intestinal macrophages.

Three-dimensional characterization of cell clusters using synchrotron-radiation-based micro-computed tomography

Micro-computed tomography with the highly intense, monochromatic X rays produced by the synchrotron is a superior method to nondestructively measure the local absorption in three-dimensional space. Because biological tissues and cells consist mainly of water as the surrounding medium, higher absorbing agents have to be incorporated into the structures of interest. Even without X-ray optics such as refractive lens, one can uncover the stain distribution with the spatial resolution of about 1 mum. Incorporating the stain at selected cell compartments, for example, binding to the RNA/DNA, their density distribution becomes quantified. In this communication, we demonstrate that tomograms obtained at the beamlines BW2 and W2 (HASYLAB at DESY, Hamburg, Germany) and 4S (SLS, Villigen, Switzerland) clearly show that the RNA/DNA-stained HEK 293 cell clusters have a core of high density and a peripheral part of lower density, which correlate with results of optical microscopy. The inner part of the clusters is associated with nonvital cells as the result of insufficient oxygen and nutrition supply. This necrotic part is surrounded by (6 +/- 1) layers of vital cells.

Multicellular tumour spheroid as a model for evaluation of [18F]FDG as biomarker for breast cancer treatment monitoring

Background

In order to explore a pre-clinical method to evaluate if [¹⁸F]FDG is valid for monitoring early response, we investigated the uptake of FDG in Multicellular tumour spheroids (MTS) without and with treatment with five routinely used chemotherapy agents in breast cancer.

Methods

The response to each anticancer treatment was evaluated by measurement of the [¹⁸F]FDG uptake and viable volume of the MTSs after 2 and 3 days of treatment.

Results

The effect of Paclitaxel and Docetaxel on [¹⁸F]FDG uptake per viable volume was more evident in BT474 (up to 55% decrease) than in MCF-7 (up to 25% decrease).

Doxorubicin reduced the [¹⁸F]FDG uptake per viable volume more noticeable in MCF-7 (25%) than in BT474 MTSs.

Tamoxifen reduced the [¹⁸F]FDG uptake per viable volume only in MCF-7 at the highest dose of 1 μM.

No effect of Imatinib was observed.

Conclusion

MTS was shown to be appropriate to investigate the potential of FDG-PET for early breast cancer treatment monitoring; the treatment effect can be observed before any tumour size changes occur.

The combination of PET radiotracers and image analysis in MTS provides a good model to evaluate the relationship between tumour volume and the uptake of metabolic tracer before and after chemotherapy. This feature could be used for screening and selecting PET-tracers for early assessment of treatment response.

In addition, this new method gives a possibility to assess quickly, and in vitro, a good preclinical profile of existing and newly developed anti-cancer drugs.

A new, fast and semi-automated size determination method (SASDM) for studying multicellular tumor spheroids

Background

Considering the width and importance of using Multicellular Tumor Spheroids (MTS) in oncology research, size determination of MTSs by an accurate and fast method is essential. In the present study an effective, fast and semi-automated method, SASDM, was developed to determinate the size of MTSs. The method was applied and tested in MTSs of three different cell-lines. Frozen section autoradiography and Hemotoxylin Eosin (H&E) staining was used for further confirmation.

Results

SASDM was shown to be effective, user-friendly, and time efficient, and to be more precise than the traditional methods and it was applicable for MTSs of different cell-lines. Furthermore, the results of image analysis showed high correspondence to the results of autoradiography and staining.

Conclusion

The combination of assessment of metabolic condition and image analysis in MTSs provides a good model to evaluate the effect of various anti-cancer treatments.

A More Aggressive Breast Cancer Spheroid Model Coupled to an Electronic Capillary Sensor System for a High-Content Screening of Cytotoxic Agents in Cancer Therapy: 3-Dimensional In Vitro Tumor Spheroids as a Screening Model

One major problem in cancer therapy is the immortality of tumor cells showing an active telomerase, which is responsible for the elongation of the telomeres after each cellular division and the knocking down of apoptotic suppressors. A further phenomenon occurring during cancer therapies is the problem of multicellular resistance. To develop therapeutic anticancer approaches inducing cellular apoptosis, gene-modified biological in vitro systems were established and evaluated for drug screening in a capillary system for a real-time, impedimertic monitoring. Multicellular spheroids of the human breast cancer cell line T-47D clone 11 were transfected with 1) antisense caspase-3 cDNA expression vectors for knocking down the main cell death molecule and 2) sense Bcl-xl cDNA expression vectors for overexpressing the apoptotic suppressor, resulting in more aggressive tumor models. These gene-modified tumor spheroids less sensitive for apoptosis were developed for screening drugs such as methotrexate in tumor spheroid–based biosensor systems via impedance spectroscopy. In this report, it is demonstrated that this could successfully exhibit that this real-time monitoring system with tumor spheroids positioned in a capillary system with a 4-electrode configuration is the most efficient high-content screening module for impedimetric measurements of physiological alterations during gene modification and drug application.

Sustainable use of marine resources: cultivation of sponges

Among all metazoan phyla, sponges are known to produce the largest number of bioactive compounds, some of them metabolites with human therapeutic value. Therefore, an increasing interest in basic cell biology research up to biochemical engineering can be observed aiming at the production of sponge metabolites under completely controlled conditions. One major obstacle is the limited availability of larger quantities of defined sponge material--the so-called supply problem. In this chapter, different approaches used so far for producing sponge biomass by in situ aquaculture as well as some significant progress in the maintenance of sponges in aquaria are reviewed. These approaches are mainly based on old methods for producing commercial bath sponges as well as on experience in maintaining sponges in public aquaria and on the usage of artificial substrates for a natural-like colonization structure. In recent years, great efforts have been made to set up in vitro culture systems for the cultivation of sponge cells. One of the major advantages of cell cultures is the possibility to control and manipulate the cultivation conditions depending on the sponge species and the target metabolite. Up to now, monolayer cultures of dissociated sponge cells have been shown in a few cases to produce the desired product. However, to date, no continuously growing sponge cell line has been established. Organotypic culture systems, which maintain or mimic the natural tissue structure, have been developed in recent years and demonstrate a promising way towards the biotechnology of sponges. Successful attempts to produce sponge metabolites using the three-dimensional growing primmorphs are given. The use of sponge fragments, another three-dimensional approach, has reappeared and has also been successfully used as an in vitro approach as well as for the biotechnological production of boreal sponge tissue.

The use of 3-D cultures for high-throughput screening: the multicellular spheroid model

Over the past few years, establishment and adaptation of cell-based assays for drug development and testing has become an important topic in high-throughput screening (HTS). Most new assays are designed to rapidly detect specific cellular effects reflecting action at various targets. However, although more complex than cell-free biochemical test systems, HTS assays using monolayer or suspension cultures still reflect a highly artificial cellular environment and may thus have limited predictive value for the clinical efficacy of a compound. Today's strategies for drug discovery and development, be they hypothesis free or mechanism based, require facile, HTS-amenable test systems that mimic the human tissue environment with increasing accuracy in order to optimize preclinical and preanimal selection of the most active molecules from a large pool of potential effectors, for example, against solid tumors. Indeed, it is recognized that 3-dimensional cell culture systems better reflect the in vivo behavior of most cell types. However, these 3-D test systems have not yet been incorporated into mainstream drug development operations. This article addresses the relevance and potential of 3-D in vitro systems for drug development, with a focus on screening for novel antitumor drugs. Examples of 3-D cell models used in cancer research are given, and the advantages and limitations of these systems of intermediate complexity are discussed in comparison with both 2-D culture and in vivo models. The most commonly used 3-D cell culture systems, multicellular spheroids, are emphasized due to their advantages and potential for rapid development as HTS systems. Thus, multicellular tumor spheroids are an ideal basis for the next step in creating HTS assays, which are predictive of in vivo antitumor efficacy.

Tumour eradication by antiangiogenic therapy: analysis and extensions of the model by Hahnfeldt et al. (1999)

The model proposed by Hahnfeldt et al. (1999) describes the growth of a tumour assuming that tumour growth is strictly controlled by the evolution of the vascular network that supplies oxygen and nutrients to tumour cells. Consequently, it provides a framework to represent the effects of antiangiogenic therapies. In this paper, some possible modifications of that model are proposed, and conditions that guarantee the eradication of the tumour under a regimen of periodic antiangiogenic therapy are derived. The model variants considered assume the potential doubling time of the vasculature to be constant, and subdivide the endothelial cell pool, which is involved in angiogenesis, in resting and proliferating cells allowing for a more detailed description of drug effects.

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.

Gene transfer and expression of enhanced green fluorescent protein in variant HT-29c cells

AIM: To study the expression of enhanced green fluorescent protein (EGFP) gene in retrovirally transduced variant HT-29 cells.

METHODS: The retroviral vector prkat EGFP/neo was constructed and transfected into the 293T cell using a standard calcium phosphate precipitation method. HT-29c cells (selected from HT-29 cells) were transduced by a retroviral vector encoding the GEFP gene. The fluorescence intensity of colorectal carcinoma HT-29c cells after transduced with the EGFP bearing retrovirus was visualized using fluorescence microscope and fluorescence activated cell sorter (FACS) analysis. Multiple biological behaviors of transduced cells such as the proliferating potential and the expression of various antigens were comparatively analyzed between untransduced and transduced cells in vitro. EGFP expression of the fresh tumor tissue was assessed in vivo.

RESULTS: After transduced, HT-29c cells displayed a stable and long-term EGFP expression under the nonselective conditions in vitro. After cells were successively cultured to passage 50 in vitro, EGFP expression was still at a high level. Their biological behaviors, such as expression of tumor antigens, proliferation rate and aggregation capability were not different compared to untransduced parental cells in vitro. In subcutaneous tumors, EGFP was stable and highly expressed.

CONCLUSION: An EGFP expressing retroviral vector was used to transduce HT-29c cells. The transduced cells show a stable and long-term EGFP expression in vitro and in vivo. These cells with EGFP are a valuable tool for in vivo research of tumor metastatic spread.

A new method to prepare multicellular spheroids in cancer cell lines using a thermo-reversible gelation polymer

The purpose of this study is to utilize the thermo-reversible gelation polymer in which the sol-gel transitting phase is reversibly changed by temperature in a three-dimensional culture system. Human cancer cells have been observed to form multicellular spheroids, whereas fibroblasts slowly develop into small spheroids with the culture medium including this polymer. This polymer has some advantages for use as a culture material, as follows: first, cancer cells grow three-dimensionally in the aqueous solution of this polymer; second, it is easy to harvest cells or spheroids in the aqueous solution of this polymer by simply cooling down the temperature; and third, the culture medium including this polymer is so translucent that the cells or spheroids can be observed through a phase-contrast microscope. We thus conclude that this polymer is a very useful material for three-dimensional cultures.

Temozolomide induces apoptosis and senescence in glioma cells cultured as multicellular spheroids

Temozolomide is an alkylating cytostatic drug that finds increasing application in the treatment of melanoma, anaplastic astrocytoma and glioblastoma multiforme. The compound is a prodrug that decomposes spontaneously, independent of an enzymatic activation step. DNA methylation induces futile mismatch repair cycles and depletion of the DNA repair enzyme O(6)-methylguanine-DNA methyltransferase should then initiate programmed cell death. We show drug-dependent inhibition of tumour growth in a three-dimensional cell culture model of the glioma cell lines U87MG and GaMG. Migrational behaviour of the glioblastoma cells remained unaltered. However, coincubation of tumour spheroids with primary brain aggregates showed reduced tumour cell invasion into brain tissue in the presence of temozolomide. This was not achieved by slowing cellular migration, as temozolomide-treated cells displayed no reduced motility. By transferase-mediated dUTP nick-end labelling (TUNEL) of apoptotic nuclei, we found that the drug was able to induce apoptosis throughout the tumour cell spheroids. Apoptosis was highest in the core region of the spheroids. Repetitive application of sublethal doses of temozolomide to multicellular spheroids resulted in the development of drug resistance in GaMG cells. We suggest that temozolomide is a strong initiator of apoptosis in glioblastoma tumour cells in a spheroid cell culture system, when cells are already in a stressful environment.

Mathematical modelling of drug transport in tumour multicell spheroids and monolayer cultures

In this paper we adapt an avascular tumour growth model to compare the effects of drug application on multicell spheroids and on monolayer cultures. The model for the tumour is based on nutrient driven growth of a continuum of live cells, whose birth and death generates volume changes described by a velocity field. The drug is modelled as an externally applied, diffusible material capable of killing cells, both linear and Michaelis-Menten kinetics for drug action on cells being studied. Numerical solutions of the resulting system of partial differential equations for the multicell spheroid case are compared with closed form solutions of the monolayer case, particularly with respect to the effects on the cell kill of the drug dosage and of the duration of its application. The results show an enhanced survival rate in multicell spheroids compared to monolayer cultures, consistent with experimental observations, and indicate that the key factor determining this is drug penetration. An analysis of the large time tumour spheroid response to a continuously applied drug at fixed concentration reveals up to three stable large time solutions, namely the trivial solution (i.e. a dead tumour), a travelling wave (continuously growing tumour) and a sublinear growth case in which cells reach a pseudo-steady-state in the core. Each of these possibilities is formulated and studied, with the bifurcations between them being discussed. Numerical solutions reveal that the pseudo-steady-state solutions persist to a significantly higher drug dose than travelling wave solutions.

Accumulation and Photocytotoxicity of Hypericin and Analogs in Two- and Three-Dimensional Cultures of Transitional Cell Carcinoma Cells¶

The aim of this study was to investigate the in vitro cellular accumulation, distribution and photocytotoxic effect of hypericin in two-dimensional (2-D) and three-dimensional (3-D) cultured RT-112 transitional cell carcinoma cells of the bladder. In addition, two iodinated derivatives of hypericin were incorporated to investigate whether these analogs, with their increased lipophilicity and heavy-atom effect, display a different biological behavior and optimized photodynamic effect. The results indicate that hypericin and mono-iodohypericin behave similarly in terms of cellular accumulation, spheroidal distribution and photocytotoxic effect. In contrast, di-iodohypericin concentrated to a higher extent in monolayers and spheroids, but the accumulation was restricted to the outermost part of the spheroid. An inverse correlation therefore seems to exist between the extent of cellular uptake under 2-D conditions and the penetration of the compounds in multicellular systems. Moreover, a less pronounced photocytotoxic effect was observed for di-iodohypericin in both 2-D and 3-D cell culture systems. It can be concluded that iodinated derivatives of hypericin do not show an increased cytotoxic effect upon irradiation in either monolayers or spheroids. Moreover, this study shows that when new photosensitizers are preclinically developed, the use of 3-D cell aggregates is critical for a correct evaluation of their efficacy.

Reversible model of spheroid formation allows for high efficiency of gene delivery ex vivo and accurate gene assessment in vivo

The native three-dimensional architecture of carcinomas, which governs numerous autocrine-paracrine interactions related to tumor progression, cannot be faithfully recreated in most in vitro models. Even when the three-dimensional architecture is recreated in artificial scaffolds such as soft agar, this approach does not truly recreate the natural microenvironment of the tumor. Multicellular spheroids can reasonably recreate in vitro the natural three-dimensional architecture of carcinomas, but even the most efficient gene delivery vectors will penetrate only the outer layers of these structures and hence only a small fraction of cells will receive the gene of interest. If the multicellular spheroids are disrupted into a single-cell suspension in order to achieve high transfection efficiency, the single-cell production may have so altered the gene expression profile of the spheroid as to bring into question its present relevancy to in vivo tumor progression. Our laboratory has developed a human-SCID (severe combined immunodeficient) mouse model of inflammatory breast cancer, MARY-X, which grows as tight multicellular spheroids in vitro and as lymphovascular emboli in vivo. The spheroids, which express only low levels of surface sialyl-Lewis(x/a) (sLe(x/a)), are able to form compact homotypic cell clumps mediated by an intact, overexpressed E-cadherin/alpha,beta-catenin axis. The spheroids can be fully disrupted by trypsin proteolysis, anti-E-cadherin antibodies, or Ca(2+) depletion. Of these approaches the disruption with depleted Ca(2+), complete after 30 min, is fully reversible by the readdition of Ca(2+) within 6 hr. This time interval allows for a transfection "window" in which successful gene delivery can be achieved before spheroid reformation. Retroviruses (10(6)-10(7) CFU/ml) carrying the gene encoding either green fluorescent protein (GFP), a dominant-negative E-cadherin mutant (H-2K(d)-E-cad), its control (H-2K(d)-E-cad Delta C25), or alpha-1,3-fucosyltransferase III (FucT-III), an enzyme that increases surface sLe(x/a), were used to transfect either intact (wild-type) or disadhered/readhered (reformed) spheroids. There were marked differences in transfection efficiency in the reformed versus wild-type spheroids. Retroviral transfection of GFP resulted in successful delivery of this reporter gene to only the outer layer of cells of the wild-type spheroids, but to all layers of the reformed spheroids. A single retroviral transfection of H-2K(d)-E-cad, H-2K(d)-E-cad Delta C25, or FucT-III produced evidence of their respective gene expression at 72 hr throughout all layers of the reformed spheroids, but only H-2K(d)-E-cad and FucT-III produced progressive disadherence. Both H-2K(d)-E-cad-MARY-X and FucT-III-MARY-X lost their ability to form lymphovascular emboli in SCID mice. This reversible model of spheroid formation has provided us with insight into the pathogenesis of inflammatory breast carcinoma. If more broadly applied, this model could be used to examine the effects of any gene, using any gene delivery system in the three-dimensional context of native tumoral architecture.

Cell kinetics in tumour cords studied by a model with variable cell cycle length

A mathematical model is developed that describes the proliferative behaviour at the stationary state of the cell population within a tumour cord, i.e. in a cylindrical arrangement of tumour cells growing around a blood vessel and surrounded by necrosis. The model, that represents the tumour cord as a continuum, accounts for the migration of cells from the inner to the outer zone of the cord and describes the cell cycle by a sequence of maturity compartments plus a possible quiescent compartment. Cell-to-cell variability of cycle phase transit times and changes in the cell kinetic parameters within the cord, related to changes of the microenvironment, can be represented in the model. The theoretical predictions are compared against literature data of the time course of the labelling index and of the fraction of labelled mitoses in an experimental tumour after pulse labelling with 3H-thymidine. It is shown that the presence of cell migration within the cord can lead to a marked underestimation of the actual changes along cord radius of the kinetics of cell cycle progression.

Comparative studies on two potential methods for the biotechnological production of sponge biomass

The production of marine sponge biomass is one of the main outstanding goals of marine biotechnology. Due to the increased number of sponge secondary metabolites of economical value the interest in sponge cultivation increased over the last years, too. Therefore, we examined cultivation properties of 11 Mediterranean sponge species. Two methodologies were tested: functional fragment culture and multicell reaggregate culture. The in vitro cultivation of sponge fragments without further dissociation and reaggregation is a method formerly not reported. Reaggregates and functional fragments are promising attempts for culture system development. A broad spectrum of reaggregation properties was found among the species tested. In three species multicell aggregate cultures could be maintained for several months: Petrosia ficiformis, Suberites domuncula and Acanthella acuta. Our results indicate that cellular aggregates or fragments of sponges can be valuable tools in the development of methods for biotechnological production of sponge biomass. Further focus on nutritional demands and the biochemical status of the cells in these kind of cellular associations are needed in order to obtain functional aggregates and fragments.

A heterologous 3-D coculture model of breast tumor cells and fibroblasts to study tumor-associated fibroblast differentiation

The objective of our study was to establish spheroid cocultures as a valid 3-D in vitro model mimicking tumor-fibroblast interactions in scirrhous breast tumors. The experimental setup was designed to verify if in cocultures (a) adherence and migration reflect the invasive potential of breast tumor cells, (b) breast tumor cells induce tumor-associated fibroblast differentiation, and (c) tumor-derived fibroblasts better reflect the in vivo situation than normal skin fibroblasts. Only one (SK-BR-3) out of five tumor cell types showed extensive fibroblast infiltration, MCF-7 cells frequently invaded fibroblast spheroids; BT474, T47D, and ZR-75-1 were noninvasive. While tumor cell invasion was independent of fibroblast origin, tumor-associated myofibroblast differentiation defined by alpha-SMA expression was demonstrated for tumor-derived but not normal skin fibroblasts in coculture indicating that (a) tumor cell invasion and myofibroblast differentiation are autonomous processes and (b) cocultures with tumor-derived fibroblasts resemble advanced stages of desmoplastic carcinomas while cocultures with normal skin fibroblasts rather reflect the early tumor development. The latter is also implied by fibroblast-associated alterations in tumor cell morphology and ECM distribution in the system. By using RNA arbitrarily primed PCR and cells isolated from cocultures by fluorescence-activated and magnetic cell separation, peripheral myelin protein PMP22/SR13 has been identified as a novel candidate with potential relevance in the interaction between tumor cell and normal fibroblast since PMP22 mRNA was significantly reduced in normal skin fibroblasts in coculture with BT474 cells.

Tumor biology and experimental therapeutics

Recent research using multicellular tumor spheroids has resulted in new insights in the regulation of invasion and metastasis, angiogenesis and cell cycle kinetics. The onset and expansion of central necrosis in tumor spheroids has been characterized to be a complex interaction of several mechanisms; in a number of cases, necrosis is not a consequence of hypoxia or anoxia, but emerges as secondary necrosis following an accumulation of apoptosis in spheroids. Recent therapeutically oriented studies have been directed towards novel hypoxic markers, targeted therapy, multicellular-mediated drug resistance, and heavy ion irradiation of spheroids. Research efforts should be enhanced mainly in the fields of tumor tissue modeling by heterotypic three-dimensional (3D) cultures and of apoptotic versus necrotic cell death. Based on the fundamental differences between monolayer and 3D cultures, spheroids should become mandatory test systems in therapeutic screening programs.

Spheroids in radiobiology and photodynamic therapy

Spheroids are tridimensional aggregates of tumor cells coming from one or several cell clones. This model, which mimics the micro-tumors structure and some of their properties, shows oxygen, pH and nutrient gradients inducing a necrotic area in the center of the spheroid. Analysis of spheroids, cultured under static or stirred conditions, can be performed on whole spheroids or dissociated spheroids. The spheroids sensitivity to ionizing radiation and photodynamic therapy can be altered by oxygen status, damage repair, intercellular commmunications and apoptosis induction, as in experimental tumor models. In radiobiology, the similarity of radiation response between spheroids and tumor xenograft bearing mice makes the spheroids to be a good alternative model to in vivo irradiation studies. In photodynamic therapy, spheroids lead to a better understanding of the own tumor response without interactions with vascular system. Finally, despite the quality of spheroid model, only the use of new technology for analysis of spheroid populations will help to increase their experimental use, particularly in preclinical oncology.

Radiation physics and genetic targeting: new directions for radiotherapy. The Douglas Lea Lecture 1999

Radiation as a cancer treatment modality is of high physical precision but limited biological specificity. Targeted radiotherapy, the delivery of radiation to cancer cells by radioisotopes conjugated to tumour-seeking targeting agents, is a biologically attractive option but is currently effective for just a few tumour types (neuroblastoma, lymphoma) for which efficacious targeting agents are available. Radiobiological modelling and radiation microdosimetry have provided useful guidelines in choosing treatment strategies for targeted radiotherapy. These considerations generally favour the incorporation of targeted radiotherapy as one component of a multimodal treatment regimen. Very recently, gene therapy techniques have been developed which should enhance the clinical efficacy of both external beam radiation and targeted radiotherapy. Typically, non-harmful viruses are modified to incorporate therapeutic genes which cause altered cellular radiosensitivity or which facilitate the cellular uptake of targeting agents. To achieve specificity, therapeutic genes would be co-transfected with tissue-specific promoter genes causing the therapeutic genes to be expressed in cells of particular types. In laboratory models, our research group are exploring the transfection-mediated uptake of the targeting agents MIBG and sodium iodide. These approaches do not require transfection of every cell in order to cure a tumour-cells which have escaped transfection may be sterilized by radiation cross-fire from transfected neighbours. A new task for radiation microdosimetry is to quantify the cross-fire effect and to compute the efficacies of gene transfection which will be required for tumour cure. In the spirit of Douglas Lea, the analytic approach of physics can be used to illuminate and enhance developments in genetics, to the benefit of medicine.

Amino acid uptake and regulation in multicellular hepatoma spheroids

BACKGROUND

Cancer cells maintained in monolayer tissue culture are frequently used to study tumor biology and nutrient uptake, but there is a concern that this system may not fully reflect clinical tumor physiology. Because cells grown in a 3-D configuration more closely resemble an in vivo environment, a model was developed and characterized for the growth of SK-Hep human hepatoma cells in suspension as multicellular tumor spheroids (MTS). The measurement of nutrient uptake in such a system has never been established.

MATERIALS AND METHODS

SK-Hep cultures were initiated as single cell suspensions and grown as MTS in siliconized spinner flasks. The transport of several individual amino acids (arginine, glutamate, leucine, alpha-(N-methylamino)isobutyric acid (MeAIB), and glutamine (GLN)) was measured in SK-Hep single cell suspensions and MTS (0. 50-0.60 mm diameter) by a radiotracer/rapid filtration technique, as was the regulation of glutamine uptake by phorbol esters. l-[(3)H]GLN uptake was also measured in larger spheroids (0.85-1.5 mm diameter). MTS cellularity was evaluated by histological examination, and single cell integrity after the transport assay was confirmed by scanning electron microscopy (SEM).

RESULTS

SK-Hep MTS displayed gradients of cellular morphology and staining, with central necrosis visible at diameters >0.8 mm. Single cell suspensions endured the rapid filtration technique based on functional Na(+)-dependent uptake rates and SEM analysis. Of all amino acids tested, only GLN transport rates were visibly affected by growth format. In small MTS, Na(+)-dependent GLN uptake was diminished by 40%, but was 40-53% higher in MTS >1 mm displaying central necrosis, when compared to single cell suspensions. Likewise, slight parallel changes in glutamine transporter ATB(0) mRNA levels were observed in Northern blot analysis. Finally, phorbol ester-dependent GLN transport down-regulation (by 40-50%), previously established in SK-Hep monolayers, remained operative in all cell formats tested.

CONCLUSIONS

The data suggest that the tumor microenvironment differentially impacts the uptake of specific nutrients despite the conservation of key regulatory pathways. This MTS technique may prove useful for further studies on the role of nutrient transport in nascent tumor growth.

Effect of an immobilization matrix and capsule membrane permeability on the viability of encapsulated HEK cells

The effect of inclusion of an immobilization matrix and the capsule membrane permeability on the viability, metabolic activity, and proliferation of encapsulated HEK cells was investigated in vitro. In the absence of a matrix, a particular transfected HEK cell line formed a single aggregate in the core of the poly(hydroxyethyl methacrylate-co-methyl methacrylate) (HEMA-MMA) capsule, and the number of live cells decreased significantly with the passage of time. In contrast, co-encapsulation with a 1% (w/v) ultralow gelling temperature agarose matrix promoted the proliferation of the encapsulated cells. The initial number of approximately 200 live cells/capsule doubled 14 d after encapsulation and reached a plateau of approximately 500 live cells/capsule 28 d after encapsulation. The agarose matrix provided uniform distribution of the cells within the capsule core giving rise to multiple aggregates upon proliferation. Reduction of the polymer solution concentration, and hence the increase of the permeability of the capsule membrane, did not have an effect on the extent or rate of proliferation of cells co-encapsulated with agarose, and did not improve the viability of cells that were encapsulated without a matrix. These cells (transfected with the cDNA for human hepatic lipase) served as a model as part of a program evaluating the use of encapsulated cells for gene therapy.

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.

Metabolic imaging in multicellular spheroids of oncogene-transfected fibroblasts

Four rat embryo fibroblast (REF) cell lines with defined oncogenic transformation were used to study the relationship between tumorigenic conversion, metabolism, and development of cell death in a 3D spheroid system. Rat1 (spontaneously immortalized) and M1 (myc-transfected) fibroblasts represent early nontumorigenic transformation stages, whereas Rat1-T1 (T24Ha-ras-transfected Rat1) and MR1 (myc/T24Ha-ras-co-transfected REF) cells express a highly tumorigenic phenotype. Localized ATP, glucose, and lactate concentrations in spheroid median sections were determined by imaging bioluminescence. ATP concentrations were low in the nonproliferating Rat1 aggregates despite sufficient oxygen and glucose availability and lack of lactate accumulation. In MR1 spheroids, a 50% decrease in central ATP preceded the development of central necrosis at a spheroid diameter of around 800 micrometer. In contrast, the histomorphological emergence of cell death at a diameter of around 500 micrometer in Rat1-T1 spheroids coincided with an initial steep drop in ATP. Concomitantly, reduction in central glucose and increase in lactate before cell death were recorded in MR1 but not in Rat1-T1 spheroids. As shown earlier, myc transfection confers a considerable resistance to hypoxia of MR1 cells in the center of spheroids, which is reflected by their capability to maintain cell integrity and ATP content in a hypoxic environment. The data obtained suggest that small alterations in the genotype of tumor cell lines, such as differences in the immortalization process, lead to substantial differences in morphological structure, metabolism, occurrence of cell death, and tolerance to hypoxia in spheroid culture.