Growth and radiation sensitivity of the MLS human ovarian carcinoma cell line grown as multicellular spheroids and xenografted tumours

The Exponential-Gompertzian Tumor Growth Model: Data from Six Tumor Cell Lines in Vitro and in Vivo. Estimate of the Transition point from Exponential to Gompertzian Growth and Potential Cinical Implications

The published growth data were examined for six tumor cell lines (FSA, Line 1, MCA-11, EMT6/RO, MGH-U1, MLS) grown in vivo and in vitro as monolayer cultures and as multicell spheroids cultured under different experimental conditions. Serial estimates of tumor sizes were fitted by Gompertzian equations obtained with a non-linear computerized program. When the growth equations of the same tumor growing in different experimental conditions were compared, the Gompertzian parameters α0 (initial specific growth rate) and β (retardation factor) showed a strong linear correlation in all the examined lines, with no exception. This occurrence supports the exponential-Gompertzian growth model, where an early exponential phase (which is virtually not influenced by exogenous factors) is followed by a Gompertzian phase, the characteristics of which are greatly dependent on environmental conditions. The transition between the two phases was estimated to occur when tumor size reached 102–104 cells, depending on the cell line. This kinetic change in tumor growth may be clinically relevant as regards cytotoxic treatments. It could explain some consequences of delays in adjuvant (postoperative) chemotherapy observed in clinical trials on primary breast cancer.

Modeling platinum sensitive and resistant high-grade serous ovarian cancer: development and applications of experimental systems

High-grade serous ovarian cancer remains the most common sub-type of ovarian cancer and, characterized by high degrees of genomic instability and heterogeneity, is typified by a transition from early response to acquired resistance to platinum-based chemotherapy. Conventional models for the study of ovarian cancer have been largely limited to a set of relatively poorly characterized immortalized cell lines and recent studies have called into question the validity of some of these as reliable models. Here, we review new approaches and models systems that take into account advances in our understanding of ovarian cancer biology and advances in the technology available for their generation and study. We discuss primary cell models, 2D, 3D, and organotypic models, and “paired” sample approaches that capture the evolution of chemotherapy failure within single cases. We also overview new methods for non-invasive collection of representative tumor material from blood samples. Adoption of such methods and models will improve the quality and clinical relevance of ovarian cancer research.

Utilizing mitochondrial events as biomarkers for imaging apoptosis

Cells undergoing apoptosis show a plethora of time-dependent changes. The available tools for imaging apoptosis in live cells rely either on the detection of the activity of caspases, or on the visualization of exposure of phosphatidyl serine in the outer leaflet of the cell membrane. We report here a novel method for the detection of mitochondrial events during apoptosis, namely translocation of Bax to mitochondria and release of cytochrome c (Cyt c) using bimolecular fluorescence complementation. Expression of split yellow fluorescent protein (YFP) fragments fused to Bax and Cyt c, resulted in robust induction of YFP fluorescence at the mitochondria of apoptotic cells with very low background. In vivo expression of split YFP protein fragments in liver hepatocytes and intra-vital imaging of subcutaneous tumor showed elevated YFP fluorescence upon apoptosis induction. Thus, YFP complementation could be applied for high-throughput screening and in vivo molecular imaging of mitochondrial events during apoptosis.

Gonadotropin stimulation of MLS human epithelial ovarian carcinoma cells augments cell adhesion mediated by CD44 and by alpha(v)-integrin

OBJECTIVE

The goal of this work was to evaluate the involvement of gonadotropins in the regulation of adhesion of human epithelial ovarian carcinoma. We studied two pathways that were previously implicated in the metastatic implantation of ovarian carcinoma to the peritoneum, namely hyaluronan-CD44 and RGD-integrin mediated adhesion.

METHODS

Two cell lines derived from human epithelial ovarian carcinoma (MLS and OC238) were stimulated with luteinizing hormone (LH) and/or follicle stimulating hormone (FSH). Expression of CD44 was evaluated by Western blotting. Expression of alpha(v)-integrins was studied by RT-PCR and Northern blot. Integrin and CD44 mediated adhesion of the cells was analyzed using culture plates coated either with a thrombin derived RGD containing peptide or fibronectin for integrin mediated adhesion or with hyaluronan for CD44 mediated adhesion.

RESULTS

MLS cells stimulated with either LH or FSH showed increased adhesion to culture plates coated with hyaluronan, as well as to culture plates coated with fibronectin or with a thrombin derived RGD containing peptide. In these cells, gonadotropin stimulation led to induced expression of the integrin subunit alpha(v) and CD44, the cell surface hyaluronan receptor. On the other hand, OC238 cells showed no expression of the integrin subunit alpha(v) and no hormonal effect on the expression of CD44. Accordingly, adhesion of OC238 cells on either RGD or CD44 was not affected by hormonal stimulation.

CONCLUSION

Elevated levels of gonadotropins may in some cases facilitate peritoneal metastatic dissemination of ovarian cancer by increasing cell adhesion, the first essential step in the invasion process.

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.

Differential effects of the insulin-like growth factor I receptor on radiosensitivity and spontaneous necrosis formation of human glioblastoma cells grown in multicellular spheroids

The purpose of this study is to investigate how the insulin-like growth factor I receptor (IGF-IR) affects cellular radiosensitivity when cells are cultured under different growth conditions. For this, A7(R) and A7(puro) cells were established from human glioblastoma GB A7 cells. The former were derived from the parent cells by stable cotransfection with plasmids carrying human IGF-IR cDNA and a puromycin resistance gene and the latter had the marker gene alone. The cells were either grown exponentially in monolayer cultures or grown in multicellular spheroids as an in vitro model for solid tumors. Spheroids were formed in the two different methods, liquid-overlay (LOC) and spinner (SPC) cultures. Although the growth rate of both cell lines in monolayer was exactly the same, the growth rate of A7(R) spheroids formed in LOC was higher than that of A7(puro) spheroids. A central necrosis region was histologically observed in A7(puro) spheroids, but the corresponding region in A7(R) spheroids was almost completely filled with intact cells in both LOC and SPC spheroids. Both cell lines showed the same radiosensitivity in monolayer cultures in terms of cell viability and clonogenic cell survival. When the spheroids formed in LOC were X-irradiated, the radiosensitivity of A7(R) and A7(puro) cells assayed for cellular clonogenicity was also the same. However, in the spheroids formed in SPC, A7(R) cells were significantly more radiosensitive than A7(puro) cells. The results indicate that overexpression of the IGF-IR could induce radiosensitization of human tumor cells in spheroids while inhibiting spontaneous necrosis formation. This may open a possibility to explore the novel function of the IGF-IR.

A non-parametric method for the analysis of experimental tumour growth data

Analysis of tumour growth is required to investigate the biology of tumours and to determine the effects of new anti-tumour therapies. A non-parametric mathematical method for the analysis of a set of experimental tumour growth data is described. The method is based on the similarity between time series of tumour size measurements (e.g. tumour volume), similarity being defined as the Euclidean distance between data measured for each tumour at the same time. Subsets of similar time series are found for a given population of tumours. A biologically meaningful parameter H has been derived which is a measure of the scattering of experimental volume samples. The method has been applied to the analysis of the growth of (i) untreated multicellular tumour spheroids obtained with different cell lines and (ii) spheroids treated with cytotoxic drugs (immunotoxins). Results are compared with those previously obtained by applying the classical Gompertz growth model to the analysis of treated and untreated spheroids.

Oscillating growth patterns of multicellular tumour spheroids

The growth kinetics of 9L (rat glioblastoma cell line) and U118 (human glioblastoma cell line) multicellular tumour spheroids (MTS) have been investigated by non-linear least square fitting of individual growth curves with the Gompertz growth equation and power spectrum analysis of residuals. Residuals were not randomly distributed around calculated growth trajectories. At least one main frequency was found for all analysed MTS growth curves, demonstrating the existence of time-dependent periodic fluctuations of MTS volume dimensions. Similar periodic oscillations of MTS volume dimensions were also observed for MTS generated using cloned 9L cells. However, we found significant differences in the growth kinetics of MTS obtained with cloned cells if compared to the growth kinetics of MTS obtained with polyclonal cells. Our findings demonstrate that the growth patterns of three-dimensional tumour cell cultures are more complex than has been previously predicted using traditional continuous growth models.

Comparison between fractionated high dose rate irradiation and continuous low dose rate irradiation in spheroids

Recent interest in clinical brachytherapy focuses on the possible radiobiological equivalence between fractionated high dose rate (HDR) and continuous low dose rate (LDR) irradiations. This study is designed to compare the radiobiological effects between the two in vitro using multicellular spheroids of human tumor. Both HDR and LDR irradiations were delivered by 137Cs source, the dose rates of which were as 1.18 Gy/min and 5.5 mGy/min, respectively. Fractionated HDR irradiation of various fraction sizes was applied twice a day. We found that: (1) The fractionated HDR irradiation (8 Gy/2 fr/day) was more effective radiobiologically than continuous LDR irradiation (8 Gy/day) and the ratio of radiobiological effects of these irradiations was estimated as 0.82, based on the 50% spheroid cure dose (SCD50); (2) the radiobiological effectiveness was independent of the fraction size of HDR irradiation administrated, and the repair of sublethal damage (SLD) was absent, suggesting that the sparing effect of fractionated HDR irradiations was absent in spheroids. Our findings could provide important information for the clinical usage of the fractionated HDR radiotherapy to replace continuous LDR radiotherapy.

Analysis of subcutaneous angiogenesis by gradient echo magnetic resonance imaging

The goal of this study was to develop an experimental method for noninvasive analysis of angiogenesis, namely the sprouting of capillaries from existing blood vessels. Angiogenesis was assayed in the subcutaneous vasculature of nude mice in a region of 3 x 3 cm that included in its center a defined angiogenic stimulus. Angiogenic stimuli included agarose beads containing angiogenic growth factors, multicellular tumor spheroids, and dermal incisions. Highly significant correlation (r = 0.905, P = 0.0001) was found between the apparent vessel density determined by gradient echo MRI and the density of blood-containing vessels determined postmortem. The functionality of the neovasculature was demonstrated in mice breathing alternatingly carbogen or 95% air/5% CO2. Large signal enhancement with carbogen breathing corresponded to regions of high vessel density. The assay reported here can be applied for the study of dermal wound healing, primary vascularization of subcutaneous implants, and for measuring the activity of angiogenic and antiangiogenic agents.

Loss of ovarian function promotes angiogenesis in human ovarian carcinoma

We show here that elevated levels of gonadotropins (luteinizing hormone and follicle stimulating hormone), as found in menopause or after ovariectomy, promote growth of human ovarian carcinoma by induction of tumor angiogenesis. Human epithelial ovarian cancer tumors progressed faster in ovariectomized mice. This induced growth could be attributed to the elevated levels of gonadotropins associated with loss of ovarian function because direct administration of gonadotropins also was effective in promoting tumor progression in vivo. On the other hand, gonadotropins had no direct effect on the proliferation of human ovarian cancer cells in vitro. Using MRI, we demonstrated that ovariectomy significantly (P < 0.02) induces neovascularization of human ovarian carcinoma spheroids implanted in nude mice. Moreover, conditioned medium of gonadotropin-treated human ovarian carcinoma cells showed increased mitogenic activity to bovine endothelial cells, and this activity could be blocked by neutralizing antibodies against luteinizing hormone and against vascular endothelial growth factor. Accordingly, gonadotropin stimulation resulted in a dose-dependent-induced expression of vascular endothelial growth factor in monolayer culture as well as in the outer proliferating cells of human ovarian cancer spheroids. These results demonstrate the significance of the elevated levels of gonadotropins, as found in menopause and in all ovarian cancer patients, on the progression of ovarian cancer and could explain the protective effect of estrogen replacement therapy. Based on these results, we suggest that hormonal therapy aimed at lowering the circulating levels of gonadotropins may possibly prolong remission in ovarian cancer by extending tumor dormancy.

Apoptosis, energy metabolism, and fraction of radiobiologically hypoxic cells: a study of human melanoma multicellular spheroids

The magnitude of the fraction of radiobiologically hypoxic cells in tumours is generally believed to reflect the efficiency of the vascular network. Theoretical studies have suggested that the hypoxic fraction might also be influenced by biological properties of the tumour cells. Quantitative experimental results of cell energy metabolism, hypoxia- induced apoptosis, and radiobiological hypoxia are reported here. Human melanoma multicellular spheroids (BEX-c and WIX-c) were used as tumour models to avoid confounding effects of the vascular network. Radiobiological studies showed that the fractions of hypoxic cells in 1000-microM spheroids were 32 +/- 12% (BEX-c) and 2.5 +/- 1.1% (WIX-c). The spheroid hypoxic volume fractions (28 +/- 6% (BEX-c) and 1.4 +/- 7% (WIX-c)), calculated from the rate of oxygen consumption per cell, the cell packing density, and the thickness of the viable rim, were similar to the fractions of radiobiologically hypoxic cells. Large differences between tumours in fraction of hypoxic cells are therefore not necessarily a result of differences in the efficiency of the vascular network. Studies of monolayer cell cultures, performed to identify the biological properties of the BEX-c and WIX-c cells leading to this large difference in fraction of hypoxic cells, gave the following results: (1) WIX-c showed lower cell surviving fractions after exposure to hypoxia than BEX-c, (2) WIX-c showed higher glucose uptake and lactate release rates than BEX-c both under aerobic and hypoxic conditions, and (3) hypoxia induced apoptosis in WIX-c but not in BEX-c. These observations suggested that the difference between BEX-c and WIX-c spheroids in fraction of hypoxic cells resulted partly from differences in cell energy metabolism and partly from a difference in capacity to retain viability under hypoxic stress. The induction of apoptosis by hypoxia was identified as a phenomenon which has an important influence on the magnitude of the fraction of radiobiologically hypoxic cells in multicellular spheroids.

Heterogeneous response of individual multicellular tumour spheroids to immunotoxins and ricin toxin

The cytoreductive effects of anti-transferrin receptor (anti-TfnR) immunotoxins (ITs) and of ricin toxin against tumour micromasses have been evaluated in a multicellular tumour spheroid (MTS) model. More than 600 (656) MTSs obtained with human breast carcinoma (MCF7) or rat glioblastoma (9L) cell lines were treated individually with ITs or toxin and the effects induced by the treatment were measured for each MTS as volume variation vs time by applying the Gompertz growth model. Two dose-dependent patterns of MTS growth were observed in MTSs of both cell lines in response to IT or toxin treatment: (1) complete inhibition of MTS growth ('sterilisation'); and (2) partial/complete inhibition ('heterogeneous response'). Within the range of IT or toxin concentrations resulting in partial inhibition of MTS growth, the sensitivity of treated MTSs was extremely heterogeneous (the cytoreductive effects varying between 0.1 and 4 logs of cells killed for a given IT or toxin concentration). Analysis of the post-treatment regrowth kinetics indicated that treated non-sterilised and control MTSs reached the same final limiting volumes. However, the doubling time estimated for the surviving cells of treated MCF7 and 9L MTSs ranged between 15 and 50 h, indicating that each MTS had individual growing potential. In conclusion, our results indicate that at substerilising IT concentrations individual heterogenicity of MTSs may greatly influence the cytoreductive potential of ITs. An implication of our study is that the efficacy of an IT treatment in eradicating disseminated micrometastases may not be predictable a priori. The MTS model that we describe in this paper may help in dissecting out factors limiting the effect of ITs in three-dimensional tumours.

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

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

Cytoreductive effects of anti-transferrin receptor immunotoxin in a multicellular tumor spheroid model

We have evaluated the sensitivity to immunotoxins (IT) of monolayer and of 200-250 microns multicellular tumor spheroid (MTS) cultures obtained with human breast (MCF7) and glioblastoma (U118) tumor cells and with rat glioblastoma (9L) cells. Monolayer MCF7 and U118 cells were highly sensitive to antitransferrin receptor (anti-TfnR) ricin A chain (RTA)-IT (Tfn-RTA and MAb OKT9-RTA) treatment in the presence of the intracellular RTA-IT enhancing agent human serum albumin-monensin (HSA-Mo) conjugate. A 790- to 2000-fold higher concentration of anti-TfnR IT was instead required to reduce by 50% the volume of individually treated MCF7 spheroids, as evaluated by applying the Gompertz growth model. Monolayer 9L cells showed 230- to 5700-fold lower sensitivity to Tfn-RTA IT than MCF7 and U118 monolayers, yet 9L spheroid cells were almost as sensitive to anti-TfnR IT as monolayer 9L cultures. Binding studies performed with [125I]-Tfn and FITC-labelled anti-TfnR MAb revealed that 9L monolayers and MTS expressed 4.1-fold and 8.8-fold lower amounts of TfnR than MCF7 monolayers and MTS, respectively. However, Tfn bound to TfnR sites of 9L and of MCF7 cells with comparable affinity. Experiments carried out with the diphtheria toxin mutant CRM107 linked to Tfn confirmed the pattern observed with RTA-IT. Monolayers and spheroids showed no considerable differences in sensitivity to ricin toxin. Collectively, these results indicated that the efficacy of IT against 3-D tumors is heavily influenced by the number of target Ag expressed by the tumor cells, as well as by the affinity of IT/toxin-cell interaction.