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1From the Departments of Ophthalmology and 2Endocrinology, Leiden University Medical Center, Leiden, The Netherlands.
| Abstract |
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METHODS. A luciferase (Luc)-positive human uveal melanoma cell line (OCM-1 FRT/luc) was established. Tumor cells were inoculated into the anterior chamber of murine eyes for induction of orthotopic growth or into the left heart ventricle to mimic hematogenous micrometastatic spread. Development of metastases and tumor growth was monitored weekly by whole-body bioluminescent reporter imaging (BLI).
RESULTS. Injection of cancer cells into the anterior chamber of the eye of mice closely mimicked orthotopic tumor growth of uveal melanoma. Tumor progression could be quantitatively monitored 3 weeks after inoculation of 105 OCM-1 FRT/luc cells. Of the mice injected, 83% exhibited a detectable tumor within 5 weeks. Intracardiac injection of tumor cells resulted in metastatic growth, especially in bone. Mice had bone (maxillofacial region and femora) and visceral (lung and mediastinum) metastases after 4 to 6 weeks. OCM-1 FRT/luc cells may also have a propensity to colonize the eye after intracardiac inoculation.
CONCLUSIONS. BLI enables continuous quantitative monitoring in the same animal of growth kinetics for each tumor and its metastases. This model will accelerate the understanding of the pathogenesis and treatment of uveal melanoma and metastasis.
Liver and other metastases are early or late events in the evolution of metastasis and can be detected by liver function analysis, ultrasound scan of the liver, or postmortem examination.3 It is important to note that formation of micrometastases is the pathologic basis for the occurrence of clinically overt metastasis. Therefore, therapeutic strategies designed to interfere with initial events in organ colonization and metastasis are needed. Current in vivo models4 5 6 lack the sensitivity that is necessary to detect the initial processes, such as the angiogenic switch,7 essential for tumor progression. Furthermore, disturbed levels of liver enzymes in blood are an indirect measure of tumor burden. More sensitive methods of detecting and monitoring tumor and metastatic growth directly are needed. A commonly used animal model for induction of tumor growth involves orthotopic inoculation of human uveal melanoma cell lines into the anterior chamber of the eye of immunodeficient (nu/nu) mice.4 5 Intracardiac injection of tumor cells in immunodeficient mice can be used as a model of experimental metastasis.8 9
Expression in vivo of reporter genes encoding bioluminescent or fluorescent proteins can be detected externally by sensitive detection systems.10 Cancer cell lines stably transfected either with the firefly luciferase (Luc) or the green fluorescent protein (GFP) have been used to monitor local tumor growth and metastasis in living mice.11 12 13
We investigated the possibility of whole-body bioluminescent reporter imaging (BLI) to visualize in vivo the development of local tumor growth and metastasis after inoculation of luciferase-transfected OCM-1 cells (OCM-1 FRT/luc),4 a human uveal melanoma cell line, into the anterior chamber of the eye (orthotopic site) and to study experimental metastasis in immunodeficient (BALBc nu/nu) mice in vivo.8 14
In this study, BLI allowed sensitive, noninvasive, and quantitative localization and monitoring of orthotopic and metastatic growth of uveal melanoma in vivo.
| Materials and Methods |
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For surgical manipulation, mice were anesthetized with an intraperitoneal injection of a mixture of ketamine hydrochloride (0.66 mg/kg body weight) and xylazine (0.13 mg/kg body weight) in PBS. Mice were killed by CO2 at the first sign of distress or after a 12-week observation period.
Human Uveal Melanoma Cell Line
OCM-1 cells were obtained from primary uveal melanoma.15 They grew as monolayer in 10 mL/dish Dulbeccos modified Eagles medium (Invitrogen-Gibco, Gronningen, The Netherlands) supplemented with 10% FCS (Hyclone, Logan, UT), 100 IU/mL penicillin (Invitrogen-Gibco), and 100 µg/mL streptomycin (Invitrogen-Gibco). Cell cultures were incubated at 37°C in a humidified atmosphere and a CO2 content of 5% in air.
Establishment of Stable Transfectants Expressing the Luciferase Reporter Gene
OCM-1 cells were transfected with 1 µg Flp-in construct (Fig. 1 , Invitrogen-Gibco), using a commercial reagent (FuGENE-6) according to the manufacturers protocol (Roche Biochemicals, Almere, The Netherlands). For better selection of stable transfectants, the zeocin-resistant gene (original vector) was replaced by a blasticidine-resistant gene (blasticidine, 3.0 µg/mL; Invitrogen-Gibco). Stable blasticidine-resistant cell clones were isolated, subcloned, and tested for the presence of a single FRT cassette by Southern blot analysis. Only those clones containing one Flp construct in the genomic DNA were used. Subsequently, an FRT-positive clone (E11) from OCM-1 was used for the generation of a luciferase-expressing cell line. Cotransfection was performed between the FRT+ tumor clone E11 with the Flp-in expression vector pcDNA5/FRT and the Flp recombinase expression vector pOG44. The gene of interest (GOI) was integrated into the FRT site, by using 1 µg of the construct CMV-luciferase FRT (Fig. 1 , firefly luciferase cDNA) and the transfection reagent (FuGENE-6; Roche Biochemicals).16 Stable transfectants were selected with hygromycin (400 µg/mL, Invitrogen). Subsequently, the OCM-1 E11 FRT/luc was tested for expression of luciferase activity per cell (Fig. 2) . The clone was cultured up to passage 15. Expression of luciferase in OCM-1 FRT/luc cells remained stable over more than 10 passages. This luciferase expression OCM-1 clone was therefore used for in vivo evaluation by BLI.
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Induction of Metastasis by Intracardiac Injection of Cancer Cells
A single-cell suspension of OCM-1/luc cells (1 x 105/100 µL PBS) was inoculated into the left heart ventricle (n = 7) according to the method described by Arguello et al.,18 with modifications.8 14 Development of metastases was monitored weekly by BLI. During the entire experimental period of intracardiacally inoculated mice, none of the metastases detected by BLI showed a decline in bioluminescent activity. After 12 weeks, all mice were killed, and metastases were dissected for immunochemistry.
Bioluminescent Reporter Imaging
We used an imaging unit (IVIS Imaging System 100; Xenogen Corp., Alameda, CA) that consisted of an intensified 25-mm square, back-thinned, back-illuminated, charge-coupled device camera fitted to a light-tight chamber and equipped with an F0.95/16.5-mm lens. The imaging system is managed by analysis software (Living Image, ver. 2.20; Xenogen Corp.).
For detection of luciferase-expressing cells, mice were anesthetized as described earlier. Thirty microliters of a 250-mM aqueous solution of luciferin8 10 11 12 13 (D-luciferin-Na; Molecular Probes, Leiden, the Netherlands) was injected intraperitoneally before photon recording began. Mice were placed in the light-tight chamber. Photon emission was then integrated for 20 seconds, creating pseudoimages.
The bioluminescent signal was quantified by measuring the amount of highlighted pixels in the area shaped around each site of photon emission with the aid of the software (Living Image ver. 2.20; Xenogen Corp.).
Histochemistry
After enucleation, eyes were immediately fixed in 5 mL of 4% paraformaldehyde (Lommerse Pharma, Oss, The Netherlands). After 24 hours, the eyes were dehydrated for 2 hours in 70% ethanol, 1 hour in 90%, and 30 minutes in 99% and processed for paraffin embedding. Goldner trichrome (to demonstrate the presence and maturation of newly formed bone), hematoxylin-eosin (H&E), and periodic acid-Schiff (PAS) staining were performed on 5-µm serial sections of tumorous eyes and metastases. Bone metastases were selectively excised and fixed overnight in at 4°C. After decalcification in EDTA acid, the sections were processed for paraffin embedding.
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| Discussion |
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Several groups have used a model of inoculation with uveal melanoma cells4 5 17 in which histologic staining of tumor sections and liver enzyme tests were used to analyze therapeutic results. These in vivo models, with in vivo/ex vivo methods, have several limitations. First, several groups of animals must be killed at different time points of an experiment to retrieve temporal information. Second, positive blood samples are a late event in metastatic disease. Third, monitoring of tumor growth in the anterior chamber can be difficult when edema develops in the cornea or a tumor starts to grow in the posterior segment of the eye. For these reasons, BLI is time saving and results in generation of more data per experimental series, leading to statistically sound results obtained more rapidly and with the use of fewer animals.11
BLI provides an opportunity to detect a small number of malignant cells rapidly, over the entire body of the mouse. The high sensitivity and specificity for detection of cells in all compartments of an intact animal frequently uncovers biological phenomena such as tumor escape mechanisms and patterns of metastasis.11 This capability enables the study of metastatic and minimal residual disease states in animal models.8 In the present study, BLI of uveal melanoma cells in the anterior chamber of the eye allowed detailed anatomic information, continuous noninvasive sensitive monitoring, and precise quantification of tumor growth in vivo.
Furthermore, micrometastases are difficult to detect because of the lack of clinical signs in mice. Induction of metastasis by intracardiac injection of uveal melanoma cells will provide the opportunity to explore preferential sites for metastasis. According to the literature, the OCM-1 cell line is capable of metastasizing in vivo.4 17 In our study, after intracardiac injection, the OCM-1 FRT/luc cell line induced metastases in the maxillofacial region, the eye, and the hind legs. These findings support the concept that organ specificity of metastasis is not due to differences in blood perfusion, but primarily depends on local interactions between cancer cells and the organ-specific microenvironment.20
This OCM-1 FRT/luc model provides a valuable experimental setting for preclinical evaluation of the in vivo antitumor activity of investigational agents in the same animal over time. Also responses at the different metastatic sites in the same animal can be easily tested.
Many gene-profiling studies have been published in the past years,21 22 reporting different genes that can play an important role in the metastatic potential of uveal melanoma. Another advantage of this model is the opportunity to explore the pathogenic role of these different genes of interest by transfecting the OCM-1 FRT cells.
In conclusion, we have established a new model of intraocular and intracardiac injection of luciferase-reporterpositive transfected cancer cells. Luciferase-positive uveal melanoma cells coupled to BLI detection allowed continuous noninvasive monitoring in vivo of tumor growth and metastasis. This model will facilitate studies of the molecular mechanisms involved in early stages of tumor development and the development of new therapies.
| Acknowledgements |
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| Footnotes |
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Submitted for publication March 3, 2004; revised April 28 and July 6, 2004; accepted July 9, 2004.
Disclosure: I.C. Notting, None; J.T. Buijs, None; I. Que, None; R.E Mintardjo, None; G. van der Horst, None; M. Karperien, None; G.S.O.A. Missotten, None; M.J. Jager, None; N.E. Schalij-Delfos, None; J.E.E. Keunen, None; G. van der Pluijm, None
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C.
1734 solely to indicate this fact.
Corresponding author: Irene C. Notting, Department of Ophthalmology, Leiden University Medical Center, Albinusdreef 2, 2300 RC Leiden, The Netherlands; i.c.notting{at}lumc.nl.
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