Multicellular spheroids. A review on cellular aggregates in cancer research

A spheroid toxicity assay using magnetic 3D bioprinting and real-time mobile device-based imaging

An ongoing challenge in biomedical research is the search for simple, yet robust assays using 3D cell cultures for toxicity screening. This study addresses that challenge with a novel spheroid assay, wherein spheroids, formed by magnetic 3D bioprinting, contract immediately as cells rearrange and compact the spheroid in relation to viability and cytoskeletal organization. Thus, spheroid size can be used as a simple metric for toxicity. The goal of this study was to validate spheroid contraction as a cytotoxic endpoint using 3T3 fibroblasts in response to 5 toxic compounds (all-trans retinoic acid, dexamethasone, doxorubicin, 5′-fluorouracil, forskolin), sodium dodecyl sulfate (+control), and penicillin-G (−control). Real-time imaging was performed with a mobile device to increase throughput and efficiency. All compounds but penicillin-G significantly slowed contraction in a dose-dependent manner (Z’ = 0.88). Cells in 3D were more resistant to toxicity than cells in 2D, whose toxicity was measured by the MTT assay. Fluorescent staining and gene expression profiling of spheroids confirmed these findings. The results of this study validate spheroid contraction within this assay as an easy, biologically relevant endpoint for high-throughput compound screening in representative 3D environments.

Resveratrol induces chemosensitization to 5-fluorouracil through up-regulation of intercellular junctions, Epithelial-to-mesenchymal transition and apoptosis in colorectal cancer

5-Fluorouracil (5-FU), a common chemotherapeutic agent used for the treatment of colorectal cancer (CRC), by itself has inadequate response rates; highlighting the need for novel and improved treatment regimens for these patients. Resveratrol, a naturally-occurring polyphenol, has been linked with chemosensitizing potential and anticancer properties; however, the underlying mechanisms for these effects remain poorly understood. The effect of resveratrol in parental CRC cell lines (HCT116, SW480) and their corresponding isogenic 5-FU-chemoresistant derived clones (HCT116R, SW480R) was examined by MTT assays, intercellular junction formation and apoptosis by electron- and immunoelectron microscopy, nuclear factor-kappaB (NF-κB) and NF-κB regulated gene products by western blot analysis in a 3D-alginate microenvironment. Resveratrol blocked the proliferation of all four CRC cell lines and synergized the invasion inhibitory effects of 5-FU. Interestingly, resveratrol induced a transition from 5-FU-induced formation of microvilli to a planar cell surface, which was concomitant with up-regulation of desmosomes, gap- and tight junctions (claudin-2) and adhesion molecules (E-cadherin) expression in HCT116 and HCT116R cells. Further, resveratrol significantly attenuated drug resistance through inhibition of epithelial-mesenchymal transition (EMT) factors (decreased vimentin and slug, increased E-cadherin) and down-regulation of NF-κB activation and its translocation to the nucleus and abolished NF-κB-regulated gene end-products (MMP-9, caspase-3). Moreover, this suppression was mediated through inhibition of IκBα kinase and IκBα phosphorylation and degradation. Our results demonstrate that resveratrol can potentiate the anti-tumor effects of 5-FU on CRC cells by chemosensitizing them, inhibiting an EMT phenotype via up-regulation of intercellular junctions and by down-regulation of NF-κB pathway.

Role of iron oxide core of polymeric nanoparticles in the thermosensitivity of colon cancer cell line HT-29

PURPOSE

In this study the effect of PLGA polymeric nanoparticles as a 5-fluorouracil (5-FU) carrier with and without iron oxide core and hyperthermia were investigated on the level of DNA damage in a spheroid culture model of HT-29 colon cancer cell lines by alkaline comet assay.

MATERIALS AND METHODS

First, HT-29 colon cancer cells were cultured in vitro as spheroids with a mean diameter of 100 µm. The spheroids were then treated with different concentrations of 5-FU or nanoparticles as 5-FU carriers with and without an iron oxide core for one volume-doubling time of the spheroids (71 h) and hyperthermia at 43 °C for 1 h. Finally, the effect of treatment on viability and level of DNA damage was examined using trypan blue dye exclusion assay and alkaline comet assay, respectively.

RESULTS

Results showed that hyperthermia in combination with 5-FU or nanoparticles as 5-FU carriers significantly induced the most DNA damage as compared with the control group. The extent of DNA damage following treatment with 5-FU-loaded nanoparticles combined with hyperthermia was significantly more than for 5-FU combined with hyperthermia. In comparison to the effect of 5-FU-loaded nanoparticles with the iron oxide core and 5-FU-loaded nanoparticle without the iron oxide core, the nanoparticles with the iron oxide core combined with hyperthermia induced more DNA damage than the nanoparticles without the iron oxide core.

CONCLUSIONS

According to this study, hyperthermia is a harmful agent and nanoparticles are effective delivery vehicles for drugs into colon cancer cells. The iron oxide filled nanoparticles increased the effect of the hyperthermia. All these factors have a significant role in the treatment of colorectal cancer cells.

Time-lapse imaging assay using the BioStation CT: a sensitive drug-screening method for three-dimensional cell culture

Three-dimensional (3D) cell culture is beneficial for physiological studies of tumor cells, due to its potential to deliver a high quantity of cell culture information that is representative of the cancer microenvironment and predictive of drug responses in vivo. Currently, gel-associated or matrix-associated 3D cell culture is comprised of intricate procedures that often result in experimental complexity. Therefore, we developed an innovative anti-cancer drug sensitivity screening technique for 3D cell culture on NanoCulture Plates (NCP) by employing the imaging device BioStation CT. Here, we showed that the human breast cancer cell lines BT474 and T47D form multicellular spheroids on NCP plates and compared their sensitivity to the anti-cancer drugs trastuzumab and paclitaxel using the BioStation CT. The anticancer drugs reduced spheroid migration velocity and suppressed spheroid fusion. In addition, primary cells derived from the human breast cancer tissues B58 and B61 grown on NCP plates also exhibited similar drug sensitivity. These results were in good agreement with the conventional assay method using ATP quantification. We confirmed the antitumor effects of the drugs on cells seeded in 96-well plates using the BioStation CT imaging technique. We expect this method to be useful in research for new antitumor agents and for drug sensitivity tests in individually-tailored cancer treatments.

Cytotoxicity and infiltration of human NK cells in in vivo-like tumor spheroids

Background

The complex cellular networks within tumors, the cytokine milieu, and tumor immune escape mechanisms affecting infiltration and anti-tumor activity of immune cells are of great interest to understand tumor formation and to decipher novel access points for cancer therapy. However, cellular in vitro assays, which rely on monolayer cultures of mammalian cell lines, neglect the three-dimensional architecture of a tumor, thus limiting their validity for the in vivo situation.

Methods

Three-dimensional in vivo-like tumor spheroid were established from human cervical carcinoma cell lines as proof of concept to investigate infiltration and cytotoxicity of NK cells in a 96-well plate format, which is applicable for high-throughput screening. Tumor spheroids were monitored for NK cell infiltration and cytotoxicity by flow cytometry. Infiltrated NK cells, could be recovered by magnetic cell separation.

Results

The tumor spheroids were stable over several days with minor alterations in phenotypic appearance. The tumor spheroids expressed high levels of cellular ligands for the natural killer (NK) group 2D receptor (NKG2D), mediating spheroid destruction by primary human NK cells. Interestingly, destruction of a three-dimensional tumor spheroid took much longer when compared to the parental monolayer cultures. Moreover, destruction of tumor spheroids was accompanied by infiltration of a fraction of NK cells, which could be recovered at high purity.

Conclusion

Tumor spheroids represent a versatile in vivo-like model system to study cytotoxicity and infiltration of immune cells in high-throughput screening. This system might proof useful for the investigation of the modulatory potential of soluble factors and cells of the tumor microenvironment on immune cell activity as well as profiling of patient-/donor-derived immune cells to personalize cellular immunotherapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1321-y) contains supplementary material, which is available to authorized users.

Poly(vinyl alcohol)/gelatin Hydrogels Cultured with HepG2 Cells as a 3D Model of Hepatocellular Carcinoma: A Morphological Study

It has been demonstrated that three-dimensional (3D) cell culture models represent fundamental tools for the comprehension of cellular phenomena both for normal and cancerous tissues. Indeed, the microenvironment affects the cellular behavior as well as the response to drugs. In this study, we performed a morphological analysis on a hepatocarcinoma cell line, HepG2, grown for 24 days inside a bioartificial hydrogel composed of poly(vinyl alcohol) (PVA) and gelatin (G) to model a hepatocellular carcinoma (HCC) in 3D. Morphological features of PVA/G hydrogels were investigated, resulting to mimic the trabecular structure of liver parenchyma. A histologic analysis comparing the 3D models with HepG2 cell monolayers and tumor specimens was performed. In the 3D setting, HepG2 cells were viable and formed large cellular aggregates showing different morphotypes with zonal distribution. Furthermore, β-actin and α5β1 integrin revealed a morphotype-related expression; in particular, the frontline cells were characterized by a strong immunopositivity on a side border of their membrane, thus suggesting the formation of lamellipodia-like structures apt for migration. Based on these results, we propose PVA/G hydrogels as valuable substrates to develop a long term 3D HCC model that can be used to investigate important aspects of tumor biology related to migration phenomena.

Prediction of individual response to anticancer therapy: historical and future perspectives

Since the introduction of chemotherapy for cancer treatment in the early 20th century considerable efforts have been made to maximize drug efficiency and at the same time minimize side effects. As there is a great interpatient variability in response to chemotherapy, the development of predictive biomarkers is an ambitious aim for the rapidly growing research area of personalized molecular medicine. The individual prediction of response will improve treatment and thus increase survival and life quality of patients. In the past, cell cultures were used as in vitro models to predict in vivo response to chemotherapy. Several in vitro chemosensitivity assays served as tools to measure miscellaneous endpoints such as DNA damage, apoptosis and cytotoxicity or growth inhibition. Twenty years ago, the development of high-throughput technologies, e.g. cDNA microarrays enabled a more detailed analysis of drug responses. Thousands of genes were screened and expression levels were correlated to drug responses. In addition, mutation analysis became more and more important for the prediction of therapeutic success. Today, as research enters the area of -omics technologies, identification of signaling pathways is a tool to understand molecular mechanism underlying drug resistance. Combining new tissue models, e.g. 3D organoid cultures with modern technologies for biomarker discovery will offer new opportunities to identify new drug targets and in parallel predict individual responses to anticancer therapy. In this review, we present different currently used chemosensitivity assays including 2D and 3D cell culture models and several -omics approaches for the discovery of predictive biomarkers. Furthermore, we discuss the potential of these assays and biomarkers to predict the clinical outcome of individual patients and future perspectives.

Elastic free energy drives the shape of prevascular solid tumors

It is well established that the mechanical environment influences cell functions in health and disease. Here, we address how the mechanical environment influences tumor growth, in particular, the shape of solid tumors. In an in vitro tumor model, which isolates mechanical interactions between cancer tumor cells and a hydrogel, we find that tumors grow as ellipsoids, resembling the same, oft-reported observation of in vivo tumors. Specifically, an oblate ellipsoidal tumor shape robustly occurs when the tumors grow in hydrogels that are stiffer than the tumors, but when they grow in more compliant hydrogels they remain closer to spherical in shape. Using large scale, nonlinear elasticity computations we show that the oblate ellipsoidal shape minimizes the elastic free energy of the tumor-hydrogel system. Having eliminated a number of other candidate explanations, we hypothesize that minimization of the elastic free energy is the reason for predominance of the experimentally observed ellipsoidal shape. This result may hold significance for explaining the shape progression of early solid tumors in vivo and is an important step in understanding the processes underlying solid tumor growth.

Colorectal cancer derived organotypic spheroids maintain essential tissue characteristics but adapt their metabolism in culture

BACKGROUND

Organotypic tumor spheroids, a 3D in vitro model derived from patient tumor material, preserve tissue heterogeneity and retain structural tissue elements, thus replicating the in vivo tumor more closely than commonly used 2D and 3D cell line models. Such structures harbour tumorigenic cells, as revealed by xenograft implantation studies in animal models and maintain the genetic makeup of the original tumor material. The aim of our work was a morphological and proteomic characterization of organotypic spheroids derived from colorectal cancer tissue in order to get insight into their composition and associated biology.

RESULTS

Morphological analysis showed that spheroids were of about 250 μm in size and varied in structure, while the spheroid cells differed in shape and size and were tightly packed together by desmosomes and tight junctions. Our proteomic data revealed significant alterations in protein expression in organotypic tumor spheroids cultured as primary explants compared to primary colorectal cancer tissue. Components underlying cellular and tissue architecture were changed; nuclear DNA/ chromatin maintenance systems were up-regulated, whereas various mitochondrial components were down-regulated in spheroids. Most interestingly, the mesenchymal cells appear to be substantial component in such cellular assemblies. Thus the observed changes may partly occur in this cellular compartment. Finally, in the proteomics analysis stem cell-like characteristics were observed within the spheroid cellular assembly, reflected by accumulation of Alcam, Ctnnb1, Aldh1, Gpx2, and CD166. These findings were underlined by IHC analysis of Ctnnb1, CD24 and CD44, therefore warranting closer investigation of the tumorigenic compartment in this 3D culture model for tumor tissue.

CONCLUSIONS

Our analysis of organotypic CRC tumor spheroids has identified biological processes associated with a mixture of cell types and states, including protein markers for mesenchymal and stem-like cells. This 3D tumor model in which tumor heterogeneity is preserved may represent an advantageous model system to investigate novel therapeutic approaches.

Organ-on-a-chip platforms for studying drug delivery systems

Novel microfluidic tools allow new ways to manufacture and test drug delivery systems. Organ-on-a-chip systems - microscale recapitulations of complex organ functions - promise to improve the drug development pipeline. This review highlights the importance of integrating microfluidic networks with 3D tissue engineered models to create organ-on-a-chip platforms, able to meet the demand of creating robust preclinical screening models. Specific examples are cited to demonstrate the use of these systems for studying the performance of drug delivery vectors and thereby reduce the discrepancies between their performance at preclinical and clinical trials. We also highlight the future directions that need to be pursued by the research community for these proof-of-concept studies to achieve the goal of accelerating clinical translation of drug delivery nanoparticles.

3D tumor models: history, advances and future perspectives

ABSTRACT: 

Evaluation of cancer therapeutics by utilizing 3D tumor models, before clinical studies, could be more advantageous than conventional 2D tumor models (monolayer cultures). The 3D systems mimic the tumor microenvironment more closely than 2D systems. The following review discusses the various 3D tumor models present today with the advantages and limitations of each. 3D tumor models replicate the elements of a tumor microenvironment such as hypoxia, necrosis, angiogenesis and cell adhesion. The review introduces application of techniques such as microfluidics, imaging and tissue engineering to improve the 3D tumor models. Despite their tremendous potential to better screen chemotherapeutics, 3D tumor models still have a long way to go before they are used commonly as in vitro tumor models in pharmaceutical industrial research.

Rapid uptake of glucose and lactate, and not hypoxia, induces apoptosis in three-dimensional tumor tissue culture

The spatial arrangement of cellular metabolism in tumor tissue critically affects the treatment of cancer. However, little is known about how diffusion and cellular uptake relate to intracellular metabolism and cell death in three dimensions. To quantify these mechanisms, fluorescent microscopy and multicellular tumor cylindroids were used to measure pH and oxygen profiles, and quantify the distribution of viable, apoptotic and necrotic cells. Spheroid dissociation, enzymatic analysis, and mass spectrometry were used to measure concentration profiles of glucose, lactate and glutamine. A mathematical model was used to integrate these measurements and calculate metabolic rate parameters. It was found that large cylindroids, >500 μm in diameter, contained apoptotic and necrotic cells, whereas small cylindroids contained apoptotic but not necrotic cells. The center of cylindroids was found to be acidic but not hypoxic. From the edge to the center, concentrations of glucose, lactate and glutamine decreased rapidly. Throughout the cell masses lactate was consumed and not produced. These measurements indicate that apoptosis was the primary mechanism of cell death; acidity was not caused by lactic acid; and cell death was caused by depletion of carbon sources and not hypoxia. The mathematical model showed that the transporter enzymes for glucose and lactate were not saturated; oxygen uptake was limited by intracellular metabolism; and oxygen uptake was not limited by membrane-transport or diffusion. Unsaturated transmembrane uptake may be the cause of both proliferative and apoptotic regimes in cancer. These results suggest that transporter enzymes are excellent targets for treating well oxygenated tumors.

Vital imaging of multicellular spheroids

Cell behavior is significantly different in two-dimensional and three-dimensional culture conditions, and a number of methods have been developed to establish and study three-dimensional cellular arrays in vitro. When grown under nonadherent conditions, many types of cells form structures called multicellular spheroids (MCSs), which have been popular models to study cell behavior in a three-dimensional environment. The histoarchitecture of MCSs derived from malignant cells resembles that of tumors, and there is rapidly increasing interest in using these structures to more accurately understand the dynamics of cancer cells in situ, including their responses to chemotherapeutics. Confocal microscopy is an extremely useful method to investigate cell behavior in MCSs due to its ability to more clearly image fluorescent probes at some depth in three-dimensional structures. This chapter describes some basic approaches toward visualizing a variety of fluorescent probes in MCSs.

Spheroid formation of human thyroid cancer cells in an automated culturing system during the Shenzhou-8 Space mission

Human follicular thyroid cancer cells were cultured in Space to investigate the impact of microgravity on 3D growth. For this purpose, we designed and constructed a cell container that can endure enhanced physical forces, is connected to fluid storage chambers, performs media changes and cell harvesting automatically and supports cell viability. The container consists of a cell suspension chamber, two reserve tanks for medium and fixative and a pump for fluid exchange. The selected materials proved durable, non-cytotoxic, and did not inactivate RNAlater. This container was operated automatically during the unmanned Shenzhou-8 Space mission. FTC-133 human follicular thyroid cancer cells were cultured in Space for 10 days. Culture medium was exchanged after 5 days in Space and the cells were fixed after 10 days. The experiment revealed a scaffold-free formation of extraordinary large three-dimensional aggregates by thyroid cancer cells with altered expression of EGF and CTGF genes under real microgravity.

Designer peptide antagonist of the leptin receptor with peripheral antineoplastic activity

The obesity hormone leptin has been implicated in the development and progression of different cancer types, and preclinical studies suggest that targeting leptin signaling could be a new therapeutic option for the treatment of cancer, especially in obese patients. To inhibit pro-neoplastic leptin activity, we developed leptin receptor (ObR) peptide antagonists capable of blocking leptin effects in vitro and in vivo. Our lead compound (Allo-aca), however, crosses the blood-brain-barrier (BBB), inducing undesirable orexigenic effects and consequent weight gain. Thus, redesigning Allo-aca to uncouple its central and peripheral activities should produce a superior compound for cancer treatment. The aim of this study was to generate novel Allo-aca analogs and test their biodistribution in vivo and anti-neoplastic activity in vitro in breast and colorectal cancer cells. Examination of several Allo-aca analogs resulted in the identification of the peptidomimetic, d-Ser, that distributed only in the periphery of experimental animals. d-Ser inhibited leptin-dependent-proliferation of ObR-positive breast and colorectal cancer cells in vitro at 1nM concentration without exhibiting any partial agonistic activity. d-Ser efficacy was demonstrated in monolayer and three-dimensional cultures, and its antiproliferative action was associated with the inhibition of several leptin-induced pathways, including JAK/STAT3, MAPK/ERK1/2 and PI3K/AKT, cyclin D1, and E-cadherin. In conclusion, d-Ser is the first leptin-based peptidomimetic featuring peripheral ObR antagonistic activity. The novel peptide may serve as a prototype to develop new therapeutics, particularly for the management of obesity-related cancers.

Multiparameter analyses of three-dimensionally cultured tumor spheroids based on respiratory activity and comprehensive gene expression profiles

Multicellular spheroids of human breast cancer cells (MCF-7) formed with two different three-dimensional (3D) culture methods were evaluated in detail on the basis of respiratory activity and high-throughput gene expression analysis. The spheroids formed with poly(dimethylsiloxane) (PDMS) microwell arrays indicated significant restriction of the spheroid size, whereas their respiratory activity was 2-fold greater than that formed with the hanging drop culture method. Fluidigm BioMark dynamic array was used for comprehensive and quantitative real-time polymerase chain reaction (qRT-PCR) analysis on the samples whose respiratory activity had been measured. Genes involved in cellular senescence and glucose metabolism indicated significantly higher values for the PDMS microwell culture method than for the hanging drop culture method (P<0.05). Interestingly, samples formed with the PDMS microwell culture method showed stronger responses for glycolysis than those formed with the hanging drop method. These results illustrate the power of multiparameter analysis to characterize multicellular spheroids cultured in different microenvironments even if they have the same morphology.

3D pancreatic carcinoma spheroids induce a matrix-rich, chemoresistant phenotype offering a better model for drug testing

Background

Pancreatic ductal adenocarcinoma (PDAC) is the fourth most common cause of cancer related death. It is lethal in nearly all patients, due to an almost complete chemoresistance. Most if not all drugs that pass preclinical tests successfully, fail miserably in the patient. This raises the question whether traditional 2D cell culture is the correct tool for drug screening. The objective of this study is to develop a simple, high-throughput 3D model of human PDAC cell lines, and to explore mechanisms underlying the transition from 2D to 3D that might be responsible for chemoresistance.

Methods

Several established human PDAC and a KPC mouse cell lines were tested, whereby Panc-1 was studied in more detail. 3D spheroid formation was facilitated with methylcellulose. Spheroids were studied morphologically, electron microscopically and by qRT-PCR for selected matrix genes, related factors and miRNA. Metabolic studies were performed, and a panel of novel drugs was tested against gemcitabine.

Results

Comparing 3D to 2D cell culture, matrix proteins were significantly increased as were lumican, SNED1, DARP32, and miR-146a. Cell metabolism in 3D was shifted towards glycolysis. All drugs tested were less effective in 3D, except for allicin, MT100 and AX, which demonstrated effect.

Conclusions

We developed a high-throughput 3D cell culture drug screening system for pancreatic cancer, which displays a strongly increased chemoresistance. Features associated to the 3D cell model are increased expression of matrix proteins and miRNA as well as stromal markers such as PPP1R1B and SNED1. This is supporting the concept of cell adhesion mediated drug resistance.

Opportunities and challenges for use of tumor spheroids as models to test drug delivery and efficacy

Multicellular spheroids are three dimensional in vitro microscale tissue analogs. The current article examines the suitability of spheroids as an in vitro platform for testing drug delivery systems. Spheroids model critical physiologic parameters present in vivo, including complex multicellular architecture, barriers to mass transport, and extracellular matrix deposition. Relative to two-dimensional cultures, spheroids also provide better target cells for drug testing and are appropriate in vitro models for studies of drug penetration. Key challenges associated with creation of uniformly sized spheroids, spheroids with small number of cells and co-culture spheroids are emphasized in the article. Moreover, the assay techniques required for the characterization of drug delivery and efficacy in spheroids and the challenges associated with such studies are discussed. Examples for the use of spheroids in drug delivery and testing are also emphasized. By addressing these challenges with possible solutions, multicellular spheroids are becoming an increasingly useful in vitro tool for drug screening and delivery to pathological tissues and organs.

Avascular tumour growth dynamics and the constraints of protein binding for drug transportation

The potential for the use of in-silico models of disease in progression monitoring is becoming increasingly recognised, as well as its contribution to the development of complete curative processes. In this paper we report the development of a hybrid cellular automaton model to mimic the growth of avascular tumours, including the infusion of a bioreductive drug to study the effects of protein binding on drug transportation. The growth model is operated within an extracellular tumour microenvironment. An artificial Neural Network based scheme was implemented that modelled the behaviours of each cell (proliferation, quiescence, apoptosis and/or movement) based on the complex heterogeneous microenvironment; consisting of oxygen, glucose, hydrogen ions, inhibitory factors and growth factors. To validate the growth model results, we conducted experiments with multicellular tumour spheroids. These results showed good agreement with the predicted growth dynamics. The outcome of the avascular tumour growth model suggested that tumour microenvironments have a strong impact on cell behaviour. To address the problem of cellular proteins acting as resistive factors preventing efficient drug penetration, a bioreactive drug (tirapazamine) was added to the system. This allowed us to study the drug penetration through multicellular layers of tissue after its binding to cellular proteins. The results of the in vitro model suggested that the proteins reduce the toxicity of the drug, reducing its efficacy for the most severely hypoxic fractions furthest from a functional blood vessel. Finally this research provides a unique comparison of in vitro tumour growth with an intelligent in silico model to measure bioreductive drug availability inside tumour tissue through a set of experiments.

Comparing scaffold-free and fibrin-based adipose-derived stromal cell constructs for adipose tissue engineering: an in vitro and in vivo study

Success of adipose tissue engineering for soft tissue repair has been limited by insufficient adipogenic differentiation, an unfavorable host response, and insufficient vascularization. In this study, we examined how scaffold-free spheroid and fibrin-based environments impact these parameters in human adipose-derived stromal cell (ASC)-based adipose constructs. ASCs were differentiated in spheroids or fibrin-based constructs. After 7 days, conditioned medium was collected and spheroids/fibrin-based constructs were either harvested or implanted subcutaneously in athymic mice. Following 7 days of implantation, the number of blood vessels in fibrin-based constructs was significantly higher than in spheroids (93±45 vs. 23±11 vessels/mm(2)), and the inflammatory response to fibrin-based constructs was less severe. The reasons for these results were investigated further in vitro. We found that ASCs in fibrin-based constructs secreted significantly higher levels of the angiogenic factors VEGF and HGF and lower levels of the inflammatory cytokine IL-8. Furthermore, ASCs in fibrin-based constructs secreted significantly higher levels of leptin and showed a 2.5-fold upregulation of the adipogenic transcription factor PPARG and a fourfold to fivefold upregulation of the adipocyte-specific markers FABP4, perilipin, and leptin. These results indicate that fibrin-based ASC constructs are potentially more suitable for ASC-based adipose tissue reconstruction than scaffold-free spheroids.

Advances in the formation, use and understanding of multi-cellular spheroids

INTRODUCTION

Developing in vitro models for studying cell biology and cell physiology is of great importance to the fields of biotechnology, cancer research, drug discovery, toxicity testing, as well as the emerging fields of tissue engineering and regenerative medicine. Traditional two-dimensional (2D) methods of mammalian cell culture have several limitations and it is increasingly recognized that cells grown in a three-dimensional (3D) environment more closely represent normal cellular function due to the increased cell-to-cell interactions, and by mimicking the in vivo architecture of natural organs and tissues.

AREAS COVERED

In this review, we discuss the methods to form 3D multi-cellular spheroids, the advantages and limitations of these methods, and assays used to characterize the function of spheroids. The use of spheroids has led to many advances in basic cell sciences, including understanding cancer cell interactions, creating models for drug discovery and cancer metastasis, and they are being investigated as basic units for engineering tissue constructs. As so, this review will focus on contributions made to each of these fields using spheroid models.

EXPERT OPINION

Multi-cellular spheroids are rich in biological content and mimic better the in vivo environment than 2D cell culture. New technologies to form and analyze spheroids are rapidly increasing their adoption and expanding their applications.

Loss of cancer drug activity in colon cancer HCT-116 cells during spheroid formation in a new 3-D spheroid cell culture system

Clinically relevant in vitro methods are needed to identify new cancer drugs for solid tumors. We report on a new 3-D spheroid cell culture system aimed to mimic the properties of solid tumors in vivo. The colon cancer cell lines HCT-116 wt and HCT-116 wt/GFP were grown as monolayers and for 3 or 6 days on 96-well NanoCulture® plates to form spheroids. Expression of surface markers, genes and hypoxia were assessed to characterize the spheroids and drug induced cytotoxicity was evaluated based on fluorescein diacetate (FDA) conversion by viable cells to fluorescent fluorescein or by direct measurement of fluorescence of GFP marked cells after a 72 h drug incubation. The cells reproducibly formed spheroids in the NanoCulture® plates with tight cell-attachment after 6 days. Cells in spheroids showed geno- and phenotypical properties reminiscent of hypoxic stem cells. Monolayer cultured cells were sensitive to standard and investigational drugs, whereas the spheroids gradually turned resistant. Similar results for cytotoxicity were observed using simplified direct measurement of fluorescence of GFP marked cells compared with FDA incubation. In conclusion, this new 3-D spheroid cell culture system provides a convenient and clinically relevant model for the identification and characterization of cancer drugs for solid tumors.

Antitumor efficacy following the intracellular and interstitial release of liposomal doxorubicin

pH-triggered lipid-membranes designed from biophysical principles are evaluated in the form of targeted liposomal doxorubicin with the aim to ultimately better control the growth of vascularized tumors. We compare the antitumor efficacy of anti-HER2/neu pH-triggered lipid vesicles encapsulating doxorubicin to the anti-HER2/neu form of an FDA approved liposomal doxorubicin of DSPC/cholesterol-based vesicles. The HER2/neu receptor is chosen due to its abundance in human breast cancers and its connection to low prognosis. On a subcutaneous murine BT474 xenograft model, superior control of tumor growth is demonstrated by targeted pH-triggered vesicles relative to targeted DSPC/cholesterol-based vesicles (35% vs. 19% decrease in tumor volume after 32 days upon initiation of treatment). Superior tumor control is also confirmed on SKBR3 subcutaneous xenografts of lower HER2/neu expression. The non-targeted form of pH-triggered vesicles encapsulating doxorubicin results also in better tumor control relative to the non-targeted DSPC/cholesterol-based vesicles (34% vs. 41% increase in tumor volume). Studies in BT474 multicellular spheroids suggest that the observed efficacy could be attributed to release of doxorubicin directly into the acidic tumor interstitium from pH-triggered vesicles extravasated into the tumor but not internalized by cancer cells. pH-triggered liposome carriers engineered from gel-phase bilayers that reversibly phase-separate with lowering pH, form transiently defective interfacial boundaries resulting in fast release of encapsulated doxorubicin. Our studies show that pH-triggered liposomes release encapsulated doxorubicin intracellularly and intratumorally, and may improve tumor control at the same or even lower administered doses relative to FDA approved liposomal chemotherapy.

Penetration of endothelial cell coated multicellular tumor spheroids by iron oxide nanoparticles

Iron oxide nanoparticles are a useful diagnostic contrast agent and have great potential for therapeutic applications. Multiple emerging diagnostic and therapeutic applications and the numerous versatile parameters of the nanoparticle platform require a robust biological model for characterization and assessment. Here we investigate the use of iron oxide nanoparticles that target tumor vasculature, via the tumstatin peptide, in a novel three-dimensional tissue culture model. The developed tissue culture model more closely mimics the in vivo environment with a leaky endothelium coating around a glioma tumor mass. Tumstatin-iron oxide nanoparticles showed penetration and selective targeting to endothelial cell coating on the tumor in the three-dimensional model, and had approximately 2 times greater uptake in vitro and 2.7 times tumor neo-vascularization inhibition. Tumstatin provides targeting and therapeutic capabilities to the iron oxide nanoparticle diagnostic contrast agent platform. And the novel endothelial cell-coated tumor model provides an in vitro microtissue environment to evaluate nanoparticles without moving into costly and time-consuming animal models.

Generation and differentiation of microtissues from multipotent precursor cells for use in tissue engineering

This protocol describes an effective method for the production of spherical microtissues (microspheres), which can be used for a variety of tissue-engineering purposes. The obtained microtissues are well suited for the study of osteogenesis in vitro when multipotent stem cells are used. The dimensions of the microspheres can easily be adjusted according to the cell numbers applied in an individual experiment. Thus, microspheres allow for the precise administration of defined cell numbers at well-defined sites. Here we describe a detailed workflow for the production of microspheres using unrestricted somatic stem cells from human umbilical cord blood and adapted protocols for the use of these microspheres in histological analysis. RNA extraction methods for mineralized microtissues are specifically modified for optimum yields. The duration of running the complete protocol without preparatory cell culture but including 2 weeks of microsphere incubation, histological staining and RNA isolation is about 3 weeks.

Cytocentrifugation: a convenient and efficient method for seeding tendon-derived cells into monolayer cultures or 3-D tissue engineering scaffolds

Tendon and ligament injuries are very common, requiring some 200,000 reconstructions per year in the USA. Autografting can be used to repair these but donor tissue is limited and harvesting leads to morbidity at the graft sites. Tissue engineering has been used to grow simple tissues such as skin, cartilage and bone and due to their low vascularity and simple structure, tendons should be ideal candidates for such an approach. Scaffolds are essential for tissue engineering as they provide structure and signals that regulate growth. However, they present a physical barrier to cell seeding with the majority of the cells congregating at the scaffold surface. To address this we used centrifugation to enhance penetration of tendon-derived cells to the centres of 3-D scaffolds. The process had no apparent deleterious effects on the cells and both plating efficiency and cell distribution improved. After attachment the cells continued to proliferate and deposit a collagenous matrix. Scaffold penetration was investigated using layers of Azowipes allowing the separation and examination of individual leaves. At relatively low g-forces, cells penetrated a stack of 6 Azowipes leaving cells attached to each leaf. These data suggest that cytocentrifugation improves the penetration and homogeneity of tendon derived cells in 3-D and monolayer cultures.

Identification of agents that induce apoptosis of multicellular tumour spheroids: enrichment for mitotic inhibitors with hydrophobic properties

Cell-based anticancer drug screening generally utilizes rapidly proliferating tumour cells grown as monolayer cultures. Hit compounds from such screens are not necessarily effective on hypoxic and slowly proliferating cells in 3-D tumour tissue. The aim of this study was to examine the potential usefulness of 3-D cultured tumour cells for anticancer drug screening. We used colon carcinoma multicellular spheroids containing hypoxic and quiescent cells in core areas for this purpose. Three libraries (∼11 000 compounds) were screened using antiproliferative activity and/or apoptosis as end-points. Screening of monolayer and spheroid cultures was found to identify different sets of hit compounds. Spheroid screening enriched for hydrophobic compounds: median XLogP values of 4.3 and 4.4 were observed for the hits in two independent screening campaigns. Mechanistic analysis revealed that the majority of spheroid screening hits were microtubuli inhibitors. One of these inhibitors was examined in detail and found to be effective against non-dividing cells in the hypoxic centres of spheroids. Spheroid screening represents a conceptually new strategy for anticancer drug discovery. Our findings have implications for drug library design and hit selection in projects aimed to develop drugs for the treatment of solid tumours.

Localization of complexed anticytokeratin 8 scFv TS1-218 to HeLa HEp-2 multicellular tumor spheroids and experimental tumors

Recombinant single-chain fragment variable (scFv) antibodies with specificity to tumor antigens can be used to target tumors in vivo. The approach to use administration of complexes of idiotypic-anti-idiotypic scFvs when targeting tumors has not been tested earlier, and from a theoretical point it could contribute to longer in vivo circulation and improved targeting efficiency by dissociation, when in contact with the target antigen. In this study two models to evaluate the targeting efficiency of such complexes were used. HeLa HEp-2 tumor cells were grown as multicellular tumor spheroids (MCTS) and exposed to the antibody constructs in vitro. The behavior in vivo was tested in an in vivo tumor xenograft model. To increase the size of the anticytokeratin 8 scFv, TS1-218, complexes were formed between TS1-218 and its anti-idiotype, alphaTS1 scFv. The functionality of (125)I-labeled TS1-218 alone and in complex was studied in both models. The uptake patterns were similar in both models. The idiotypic TS1-218 was able to localize to the MCTS and xenografted tumors, both alone and in complex with alphaTS1 scFv. TS1-218 in complex, however, demonstrated a significantly higher uptake than the monomeric TS1-218 in both models (p < 0.0005 and p < 0.0089, respectively). When complexes were administered in vivo, a slower clearance and an increased tumor half-life could be observed. The present investigation indicates that administration of targeting antibodies, with initially blocked antigen-binding sites by complex formation with their anti-idiotypes, may improve targeting efficiency.

A three-dimensional cell biology model of human hepatocellular carcinoma in vitro

We established an in vitro 3-D model of metastatic hepatocellular carcinoma (HCC) by culturing MHCC97H cells on molecular scaffolds within a rotating wall vessel bioreactor. Morphological and biochemical analyses revealed that the 3-D HCC model mirrored many clinical pathological features of HCC in vivo, including cancer cell morphology, tissue ultrastructure, protein production and secretion, glucose metabolism, tissue-specific gene expression, and apoptosis. Xenografts into livers of nude mice resulted in tumorigenesis and distant metastasis. This 3-D HCC spheroid is a promising model for HCC tumor biology, anticancer drug screening, and for the establishment of HCC animal models.

Construction of divalent anti-keratin 8 single-chain antibodies (sc(Fv)(2)), expression in Pichia pastoris and their reactivity with multicellular tumor spheroids

Single-chain variable fragments (scFvs) are small monovalent recombinant antibody fragments that retain the specificity of their parent immunoglobulins. ScFvs are excellent building blocks for new and improved immunodiagnostic and therapeutic proteins. However, the monovalency and the rapid renal elimination of scFvs result in poor tumor accumulation and retention. Engineering divalent antibody fragments is an excellent way to address these shortcomings. In this study, covalent divalent single-chain variable fragments (sc(Fv)(2)s), were constructed from the monovalent anti-keratin 8 scFvs, TS1-218 and its mutant, HE1-Q. The scFvs and sc(Fv)(2)s were expressed in the methylotrophic yeast Pichia pastoris, utilizing the alpha-factor secretion signal (α-factor) for extracellular secretion. The immunoreactivity and specificity of the antibody fragments were analyzed with enzyme-linked immunosorbent assay (ELISA) and the uptake and retention of the (125)I labeled antibody fragments were evaluated using HeLa HEp-2 multicellular tumor spheroids (MCTSs). Analysis of the antibody fragments demonstrated that parts of the α-factor remained at the N-terminal of the antibody fragments. Despite incomplete processing of the α-factor, the antibody fragments were functional where the sc(Fv)(2)s gave a three-fold stronger signal in ELISA compared to their scFv counterparts and the mutant antibodies demonstrated a stronger signal than their initial wild types. In addition, the sc(Fv)(2)s DiTS1-218 and DiHE1-Q displayed an approximately two-fold higher uptake and were retained to a larger extent in the MCTS, demonstrating a 3.9 and 9.4-fold increase in half-life respectively compared to their corresponding scFvs. In conclusion, expression in P. pastoris improved the yield 20-fold and facilitated the purification of the antibody fragments. Furthermore, the sc(Fv)(2)s presented a higher functional affinity to K 8 both in ELISA and MCTS compared to the scFvs with DiHE1-Q being the best candidate for further studies.

Tissue engineered tumor models

Many research programs use well-characterized tumor cell lines as tumor models for in vitro studies. Because tumor cells grown as three-dimensional (3-D) structures have been shown to behave more like tumors in vivo than do cells growing in monolayer culture, a growing number of investigators now use tumor cell spheroids as models. Single cell type spheroids, however, do not model the stromal-epithelial interactions that have an important role in controlling tumor growth and development in vivo. We describe here a method for generating, reproducibly, more realistic 3-D tumor models that contain both stromal and malignant epithelial cells with an architecture that closely resembles that of tumor microlesions in vivo. Because they are so tissue-like we refer to them as tumor histoids. They can be generated reproducibly in substantial quantities. The bioreactor developed to generate histoid constructs is described and illustrated. It accommodates disposable culture chambers that have filled volumes of either 10 or 64 ml, each culture yielding on the order of 100 or 600 histoid particles, respectively. Each particle is a few tenths of a millimeter in diameter. Examples of histological sections of tumor histoids representing cancers of breast, prostate, colon, pancreas and urinary bladder are presented. Potential applications of tumor histoids include, but are not limited to, use as surrogate tumors for pre-screening anti-solid tumor pharmaceutical agents, as reference specimens for immunostaining in the surgical pathology laboratory and use in studies of invasive properties of cells or other aspects of tumor development and progression. Histoids containing nonmalignant cells also may have potential as "seeds" in tissue engineering. For drug testing, histoids probably will have to meet certain criteria of size and tumor cell content. Using a COPAS Plus flow cytometer, histoids containing fluorescent tumor cells were analyzed successfully and sorted using such criteria.

Alginate-based microfluidic system for tumor spheroid formation and anticancer agent screening

We demonstrate a microfluidic system for long-term tumor cell culture and drug testing. Three-dimensional cell culture is critical in characterizing anticancer treatments since it may provide a better model than monolayer culture of tumor cells. Breast tumor cells were encapsulated within alginate which was gelled in situ within the microchannels. Tumor spheroid formation was observed several days after cell seeding, and various concentrations of doxorubicin were applied to the encapsulated cell aggregates. Drug effects on cell viability and proliferation were measured. In future, hydrogel-based microfluidic devices can comprise part of systems which replace labor intensive screening platforms currently implemented in the laboratory, and they address a need for improving preclinical testing of cancer cell sensitivity to anti-cancer drugs.

Spatial insulin signalling in isolated skeletal muscle preparations

During in vitro incubation in the absence or presence of insulin, glycogen depletion occurs in the inner core of the muscle specimen, concomitant with increased staining of hypoxia-induced-factor-1-alpha and caspase-3, markers of hypoxia and apoptosis, respectively. The aim of this study was to determine whether insulin is able to diffuse across the entire muscle specimen in sufficient amounts to activate signalling cascades to promote glucose uptake and glycogenesis within isolated mouse skeletal muscle. Phosphoprotein multiplex assay on lysates from muscle preparation was performed to detect phosphorylation of insulin-receptor on Tyr(1146), Akt on Ser(473) and glycogen-synthases-kinase-3 on Ser(21)/Ser(9). To address the spatial resolution of insulin signalling, immunohistochemistry studies on cryosections were performed. Our results provide evidence to suggest that during the in vitro incubation, insulin sufficiently diffuses into the centre of tubular mouse muscles to promote phosphorylation of these signalling events. Interestingly, increased insulin signalling was observed in the core of the incubated muscle specimens, correlating with the location of oxidative fibres. In conclusion, insulin action was not restricted due to insufficient diffusion of the hormone during in vitro incubation in either extensor digitorum longus or soleus muscles from mouse under the specific experimental settings employed in this study. Hence, we suggest that the glycogen depleted core as earlier observed is not due to insufficient insulin action.

Cell line-based platforms to evaluate the therapeutic efficacy of candidate anticancer agents

Efforts to discover new cancer drugs and predict their clinical activity are limited by the fact that laboratory models to test drug efficacy do not faithfully recapitulate this complex disease. One important model system for evaluating candidate anticancer agents is human tumour-derived cell lines. Although cultured cancer cells can exhibit distinct properties compared with their naturally growing counterparts, recent technologies that facilitate the parallel analysis of large panels of such lines, together with genomic technologies that define their genetic constitution, have revitalized efforts to use cancer cell lines to assess the clinical utility of new investigational cancer drugs and to discover predictive biomarkers.

NF-kappa B is required for the development of tumor spheroids

Tumor cells cultured in three-dimensional models provide a more realistic and biologically meaningful analysis of the initial phases of cancer development and drug resistance. Several studies have demonstrated that culture of cancer cells in three dimensions induces cellular resistance to a variety of anti-neoplastic drugs by poorly understood mechanisms. The role of the transcription factor NF-kappaB and inhibitors of apoptosis proteins (IAPs) in the onset and development of drug resistance during tumor spheroid growth has not been established. In this work, we found a significant increase in the activity and expression of NF-kappaB and its downstream target XIAP (X-linked IAP) in cancer cells grown as multi-cellular tumor spheroids. Blocking XIAP expression with RNA interference markedly increased the sensitivity of cancer tumor spheroid cells toward anti-neoplastic drugs, indicating a role for IAPs in establishing drug resistance. In turn, inhibition of NF-kappaB by negative dominants suppressed spheroid formation, whereas overexpression of the upstream kinase IkappaBKbeta increased their growth and resistance. The present data suggested that NF-kappaB and its downstream target XIAP were essential for the growth and drug resistance of small avascular tumor.

Multicellular tumor spheroids: an underestimated tool is catching up again

The present article highlights the rationale, potential and flexibility of tumor spheroid mono- and cocultures for implementation into state of the art anti-cancer therapy test platforms. Unlike classical monolayer-based models, spheroids strikingly mirror the 3D cellular context and therapeutically relevant pathophysiological gradients of in vivo tumors. Some concepts for standardization and automation of spheroid culturing, monitoring and analysis are discussed, and the challenges to define the most convenient analytical endpoints for therapy testing are outlined. The potential of spheroids to contribute to either the elimination of poor drug candidates at the pre-animal and pre-clinical state or the identification of promising drugs that would fail in classical 2D cell assays is emphasised. Microtechnologies, in the form of micropatterning and microfluidics, are also discussed and offer the exciting prospect of standardized spheroid mass production to tackle high-throughput screening applications within the context of traditional laboratory settings. The extension towards more sophisticated spheroid coculture models which more closely reflect heterologous tumor tissues composed of tumor and various stromal cell types is also covered. Examples are given with particular emphasis on tumor-immune cell cocultures and their usefulness for testing novel immunotherapeutic treatment strategies. Finally, tumor cell heterogeneity and the extraordinary possibilities of putative cancer stem/tumor-initiating cell populations that can be maintained and expanded in sphere-forming assays are introduced. The relevance of the cancer stem cell hypothesis for cancer cure is highlighted, with the respective sphere cultures being envisioned as an integral tool for next generation drug development offensives.

Nonlinear modelling of cancer: bridging the gap between cells and tumours

Despite major scientific, medical and technological advances over the last few decades, a cure for cancer remains elusive. The disease initiation is complex, and including initiation and avascular growth, onset of hypoxia and acidosis due to accumulation of cells beyond normal physiological conditions, inducement of angiogenesis from the surrounding vasculature, tumour vascularization and further growth, and invasion of surrounding tissue and metastasis. Although the focus historically has been to study these events through experimental and clinical observations, mathematical modelling and simulation that enable analysis at multiple time and spatial scales have also complemented these efforts. Here, we provide an overview of this multiscale modelling focusing on the growth phase of tumours and bypassing the initial stage of tumourigenesis. While we briefly review discrete modelling, our focus is on the continuum approach. We limit the scope further by considering models of tumour progression that do not distinguish tumour cells by their age. We also do not consider immune system interactions nor do we describe models of therapy. We do discuss hybrid-modelling frameworks, where the tumour tissue is modelled using both discrete (cell-scale) and continuum (tumour-scale) elements, thus connecting the micrometre to the centimetre tumour scale. We review recent examples that incorporate experimental data into model parameters. We show that recent mathematical modelling predicts that transport limitations of cell nutrients, oxygen and growth factors may result in cell death that leads to morphological instability, providing a mechanism for invasion via tumour fingering and fragmentation. These conditions induce selection pressure for cell survivability, and may lead to additional genetic mutations. Mathematical modelling further shows that parameters that control the tumour mass shape also control its ability to invade. Thus, tumour morphology may serve as a predictor of invasiveness and treatment prognosis.

Response of tumor spheroids to radiation: modeling and parameter estimation

We propose a spatially distributed continuous model for the spheroid response to radiation, in which the oxygen distribution is represented by means of a diffusion-consumption equation and the radiosensitivity parameters depend on the oxygen concentration. The induction of lethally damaged cells by a pulse of radiation, their death, and the degradation of dead cells are included. The compartments of lethally damaged cells and of dead cells are subdivided into different subcompartments to simulate the delays that occur in cell death and cell degradation, with a gain in model flexibility. It is shown that, for a single irradiation and under the hypothesis of a sufficiently small spheroid radius, the model can be reformulated as a linear stationary ordinary differential equation system. For this system, the parameter identifiability has been investigated, showing that the set of unknown parameters can be univocally identified by exploiting the response of the model to at least two different radiation doses. Experimental data from spheroids originated from different cell lines are used to identify the unknown parameters and to test the predictive capability of the model with satisfactory results.