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AntiCancer, Inc., San Diego, California 92111 [M. Y., P. J., F-X. S., S. H., E. B., R. M. H.]; Department of Surgery, University of California, San Diego, California 92103-8220 [M. Y., S. H., A. R. M., R. M. H.]; and Department of Surgery, Yokohama City University School of Medicine, Yokohama, Japan [M. Y., S. H., T. C., H. S.]
| ABSTRACT |
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| Introduction |
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Early experimental models involving implanting human prostate cancer cells s.c. in athymic nude mouse failed to yield metastatic disease (3) . With the androgen-independent PC-3 human prostate cancer cell line as a model, which originated from a bone metastasis (4) , osseous metastasis could be induced in nude mice by injecting tumor cells i.v. with concomitant occlusion of the inferior cava or by intracardiac implantation (5, 6, 7) .
In orthotopic transplant models of human prostate cancer, Stephenson et al. (8) , Fu et al. (9) , Pettaway et al. (6) , Saito et al. (10) , Rembrink et al. (11) , and An et al. (12) have observed prostate cancer metastasis but only in the lymph nodes and the lung. Thalmann et al. (13) reported a spontaneous bone metastasis model of androgen-independent human prostate cancer LNCaP-derived sublines. The animals developed bone metastasis in 10 and 21.5% of intact and castrated hosts, respectively, after orthotopic injection of cell suspensions (13) . These results provided some useful information to recognize the biological behavior of prostate cancer.
The early stages of tumor progression and micrometastasis formation have been difficult to visualize in current models due to the inability to identify small number of tumor cells against a background of many host tissues. We have developed new models of human and animal cancer by transfer of the Aequorea victoria jellyfish GFP3 gene to tumor cells, which enables visualization of fluorescent tumors and metastases at the microscopic level in fresh viable tissue after transplantation (14, 15, 16, 17, 18, 19) .
Here, high expression of GFP in the tumor cells in the surgical orthotopic implantation (SOI) model of androgen-independent PC-3 human prostate cancer has revealed the very extensive and widespread skeletal and multiorgan metastatic potential of human prostate cancer. This new model, reflecting the clinical situation, should be very useful for study of the biology of androgen-independent prostate cancer that is metastatic to the bone and for development of therapy for this disease.
| Materials and Methods |
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Cell Culture, Vector Production, Transfection, and Subcloning.
PT67, an NIH3T3-derived packaging cell line, expressing the 10 Al viral envelope, was purchased from Clontech. PT67 cells were cultured in DMEM (Irvine Scientific, Santa Ana, CA) supplemented with 10% heat-inactivated fetal bovine serum (Gemini Bio-products, Calabasas, CA). For vector production, packaging cells (PT67), at 70% confluence, were incubated with a precipitated mixture of N-[1-(2,3-dioleoyloxyl)propyl]-N,N,N-trimethylammoniummethyl sulfate reagent (Boehringer Mannheim) and saturating amounts of pLEIN plasmid for 18 h. Fresh medium was replenished at this time. The cells were examined by fluorescence microscopy 48 h posttransfection. For selection, the cells were cultured in the presence of 200-1000 µg/ml G418 (potency, >45%; Life Technologies, Inc., Grand Island, NY) for 7 days.
GFP Gene Transduction of PC-3 Cells.
For GFP gene transduction, 20% confluent PC-3 cells were incubated with a 1:1 precipitated mixture of retroviral supernatants of PT67 cells and Hams F-12 K (Life Technologies, Inc.) containing 7% fetal bovine serum (Gemini Bio-products) for 72 h. Fresh medium was replenished at this time. PC-3 cells were harvested by trypsin-EDTA 72 h posttransduction and subcultured at a ratio of 1:15 into selective medium that contained 200 µg/ml G418. The level of G418 was increased to 1000 µg/ml stepwise. The brightest PC-3 clones expressing GFP (PC-3-GFP) were selected, combined, and then amplified and transferred by conventional culture methods.
Doubling Time of Stable GFP Clones.
PC-3-GFP or nontransduced cells were seeded at 1.5 x 104 in 35-mm culture dishes. The cells were harvested and counted every 24 h using a hemocytometer (Reichert Scientific Instruments, Buffalo, NY). The doubling time was calculated from the cell growth curve over a period of 10 days.
s.c. Tumor Growth.
Three 6-week-old BALB/c nu/nu male mice were injected s.c. with a single dose of 5 x 106 PC-3-GFP cells selected by high fluorescence, as described above. Cells were first harvested by trypsinization and washed three times with cold serum-free medium and then injected in a total volume of 0.2 ml within 40 min of harvesting. The nude mice were sacrificed to harvest the tumor fragments 6 weeks after tumor cell injection.
SOI.
Two tumor fragments (1 mm3) from a high GFP-fluorescent s.c. tumor from a single animal were implanted by SOI in the dorsolateral lobe of the prostate in each of five nude mice. After proper exposure of the bladder and prostate following a lower midline abdominal incision, the capsule of the prostate was opened, and the two tumor fragments were inserted into the capsule. The capsule was then closed with an 8-0 surgical suture. The incision in the abdominal wall was closed with a 6-0 surgical suture in one layer (7
, 8)
. The animals were kept under isoflurane anesthesia during surgery. All procedures of the operation described above were performed with a 7x magnification microscope (Olympus).
Analysis of Metastases.
From 47 to 61 days (Table 1)
after orthotopic implantation, the performance status in the mice began to decrease, at which time the animals were sacrificed and autopsied. The orthotopic primary tumor and all major organs as well as the whole skeleton were explored. The fresh samples were sliced at
1 mm thickness and observed directly under fluorescence microscopy without any processing.
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| Results and Discussion |
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Bone and Visceral Metastasis Visualized by GFP after Orthotopic Tumor Progression.
Five of five mice developed strongly fluorescent orthotopic tumors (Fig. 1b)
. Three of five tumors metastasized to the skeletal system. The skeletal metastasis included the skull, rib, pelvis, femur, and tibia. Fig. 2
shows examples of tumor metastases in the skeletal system visualized by GFP. Fig. 2a
shows the skull involved with metastatic prostate tumor. Fig. 2b
shows the surface of the rib that is involved with tumor. In Fig. 2c
, no metastatic lesions can be seen in the rib under bright-field microscopy. Fig. 2d
shows the rib in the same field as Fig. 2c
, under fluorescence microscopy where a strongly GFP-fluorescent metastasis can be visualized in the rib. Fig. 2, e and f
, shows the prostate tumor metastasis in the pelvis, which is one of the most common metastatic sites of clinical prostate cancer. Fig. 2g
shows the surface of the epiphysis of the femur. No metastatic lesions are visualized on the surface under bright-field microscopy. Fig. 2h
shows the same field as Fig. 2g
under fluorescence microscopy where a strongly GFP-fluorescent metastasis can be visualized on the surface of the epiphysis of the femur.
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As mentioned above, RT-PCR was necessary (20
, 24)
to detect prostate cancer cells in the bone marrow; in some cases, bone scans were used for detection (7)
. Detection of bone and other metastases is of much lower resolution in these models than this model, in which a single GFP-expressing cell can be visualized in live fresh tissue, as seen in Fig. 3
.
In this study, we established a spontaneous bone metastasis mouse model of human prostate cancer with the combination of GFP expression and SOI technique. In this model, the GFP expression of the SOI-implanted PC-3 cell line enabled metastases to be visualized throughout the skeletal system and to other important organs as well. Extensive and widespread skeletal metastases, visualized by GFP expression, were found in 60% of the animals explored. Metastases in the transplanted animals were also found in the lung, pleural membrane, liver, kidney, adrenal gland, brain, and spinal cord. Thus, the metastatic pattern of human prostate cancer PC-3-GFP accurately reproduces the clinical course of advanced metastatic androgen-independent prostate cancer. In this investigation, the extensive metastatic process in the skeleton and other organs, including the micrometastases in brain and spinal cord, was revealed by GFP fluorescence. These data demonstrate the far-reaching malignancy of this tumor, and the metastatic sites match the clinical situation. Prostate cancer mainly metastasizes to lymph nodes and bone in patients. But it also metastasizes to the other organs, such as lung, liver, pancreas, adrenal gland, large bowel, ureters, central nervous system, and so on (25 , 26) .
Here, we found micrometastasis in the kidney by GFP visualization. This may initiate further study of occult micro-metastasis of prostate cancer in the kidney. This may be a factor in the transfer of cancer to patients who are recipients of kidney and other transplants (27) .
Such a high incidence of skeletal and other metastases could not have been previously visualized before the development of the GFP-SOI model described here.
It is possible that growth of the primary GFP-PC-3 orthotopic tumor, previously grown s.c. could have influenced the metastasis pattern, growth, or dormancy of the GFP-PC-3 cells due to angiogenic or other effects as was previously observed with the Lewis lung carcinoma (28) . GFP expression could allow the effects of the primary tumor on the metastases to be even seen in the live animal, because GFP expression is visualized in live tissue. Future experiments will address this point by resection of the primary tumor at various time points.
Although autofluorescence exists in various tissues, it does not interfere with the visualization of GFP-expressing tumor cells in vivo since tissue autofluorescence is much weaker than GFP and has emission at various wavelengths depending on the tissue. For example, in this study, the brain and spinal cord, Fig. 3, f and g
, respectively, show a high background of autofluorescence, but even single GFP-expressing tumor cells can still be distinguished clearly. Thus, GFP visualization of tumor metastasis has higher resolution and is much more facile than the traditional cumbersome pathological examination procedures, such as histology and immunohistochemistry or intricate molecular techniques such as RT-PCR.
The present study revealed that the bone microenvironment in the mouse provides a highly fertile soil for human prostate cancer matching the clinical situation (25 , 26) . Future studies, taking advantage of GFP expression, will be performed to determine the clonogenicity of the bone metastasis. The PC-3-GFP model also revealed, for the first time, the extensive spontaneous liver metastasis potential of this tumor. This new metastatic model will be useful for studying the mechanism and developing therapy of skeletal and other metastases in prostate cancer.
| FOOTNOTES |
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1 This study was supported in part by National Cancer Institute Small Business Innovative Research Grant R44 CA53963. ![]()
2 To whom requests for reprints should be addressed, at AntiCancer, Inc., 7917 Ostrow Street, San Diego, CA 92111. Phone: (619) 654-2555; Fax: (619) 268-4175; E-mail: all{at}anticancer.com ![]()
3 The abbreviations used are: GFP, green fluorescent protein; SOI, surgical orthotopic implantation; RT-PCR, reverse transcription-PCR. ![]()
Received 10/27/98. Accepted 1/ 4/99.
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