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Scott Department of Urology [S. A. T., G. Y., S. E., T. L. T., T. S., L. L., A. G., T. C. T.], Department of Pathology [M. I.], Department of Medicine [K. D. M.], Department of Cell and Molecular Biology [T. C. T.], and Department of Radiology [T. C. T.], Baylor College of Medicine, Houston, Texas 77030, and Veterans Affairs Medical Center [T. L. T., M. I., T. C. T.], Houston, Texas 77030
| ABSTRACT |
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| Introduction |
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To analyze the expression of cav-1 in androgen-insensitive prostate cancer we determined the pattern of expression for cav-1 in relevant human prostate cancer tissues. We also investigated the possibility that cav-1 is secreted by prostate cancer cells and that secreted cav-1 could influence metastatic progression.
Our results establish cav-1 as an autocrine/paracrine factor that is highly expressed in androgen-insensitive prostate cancer. We further demonstrate the potential for cav-1 as a therapeutic target for this disease.
| Materials and Methods |
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Cell Culture.
Mouse prostate cancer cell lines derived from primary tumors (148-1PA and 151-1PA) or metastatic deposits (178-2BMA, 148-1LMD, 151-2LMB, 151-2LMC, and 151-1LM1), were cultured as described previously (6
, 10)
. The growth media for the different cells was in 10% fetal bovine serum as follows: RPMI 1640 for LNCaP; MEM-NEAA for DU145; F12K for PC3; F12K supplemented with heparin and endothelial cell growth supplement for human umbilical vascular endothelial cell; and DMEM for human intestinal smooth muscle and all mouse cells. The human cav-1 cDNA in pcDNA3.1 was transfected into LNCaP cells with Tfx reagent (Promega, Madison, Wisconsin; Ref. 7
). For conditioned medium preparation, subconfluent cultures were washed three times with PBS and incubated with SFM for 24 h; the media was collected and contamination of membranous cav-1 from cell debris was minimized by centrifugation at 1,000 x g and then at 100,000 x g. Conditioned media for in vitro viability assays (see below) was concentrated x20 and treated with cav-1 antibody or IgG (10 µg/ml) and incubated for 4 h at 4°C.
Western Blot Analysis.
Conditioned media was collected and centrifuged as described above and 1 ml concentrated by TCA precipitation. The precipitate was redissolved in 70 µl of SDS sample buffer and 30 µl were loaded per well. Proteins obtained from lysed cells and from TCA-precipitated conditioned media were separated by 12% SDS-PAGE, and transferred to nitrocellulose membranes. Membranes were prestained with Ponceau S before blocking to verify even loading. The membranes were blotted with rabbit polyclonal cav-1 antibody (Santa Cruz Biotechnology, Santa Cruz, CA) for 24 h at 4°C, with shaking. After incubation with horseradish peroxidase-conjugated secondary antibodies (ICN/CAPPEL, Aurora, OH), the binding was detected by enhanced chemiluminescence, Super Signal (Pierce Chemical Co., Rockford, IL).
Cell Viability Assays.
The viability by MTT was measured according to the previously described method (6)
. The cell viability was also measured after the initial period of incubation by an ATP-based assay using the Packard ATPLite-M kit (Packard, Meridian, CT) according to the manufacturers procedure, and the cell viability was expressed as the amount of light production/cell number in each well. For the clonogenic assay, 200 cells were treated in SFM or conditioned media as described in the cell viability assay. The medium was then carefully removed, and cells were trypsinized and reseeded to 10-cm plates with complete medium and grown for 1015 days. Colonies were stained with MTT in the culture medium, and the numbers of colonies were counted using Advanced Colony Counting software (NucleoTech Corp., Hayward, CA). Each experiment was repeated three to five times.
In Vivo Metastasis Analysis.
Orthotopic tumors of 1782BMA in male mice were generated by injecting 5000 cells directly into the dorso-lateral prostate. The mice were subsequently treated with control rabbit IgG or rabbit polyclonal cav-1 antibody via i.p. injections (10 µg/animal every other day). After 3 weeks, the tumors were removed and the wet-weight determined. Metastatic activity to lung was evaluated by counting Bouins fixative-stained lungs under a dissecting microscope. Femur bones and pelvic lymph nodes were removed and processed for paraffin sections after a routine procedure. Five-µm sections were cut and stained with H&E for morphological evaluation. To better identify metastatic cancer cells in bone marrow, the adjacent sections were immunostained using the standard avidin-biotin complex procedures (MOM kit; Vector Laboratories, Burlington, CA) in conjunction with a monoclonal antibody to cytokeratin 8 (Dako Corp., Carpinteria, CA). The metastatic cancer cells were counted and data expressed as cancer cell number/microscopic field of bone marrow area. Pelvic lymph nodes metastatic cancer cells were labeled with cytokeratin-18 antibody. The incidence of positive lymph nodes was recorded for each animal and the percentages of metastatic cancer deposits in individual lymph nodes were also measured.
| Results and Discussion |
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2 test). Cav-1 positivity was demonstrated in 62% of the metastatic specimens from patients who had not been treated with hormone therapy, and this frequency was also significantly increased to 82% of metastases from patients treated with hormones (P < 0.05; Mann-Whitney test).
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Cav-1 was detected in conditioned media from androgen-insensitive mouse (151-1LM1, 178-2BMA, 151-2LMC, 151-2LMB, and 148-1LMD), and human (DU145, PC3, and TSU-Pr1) prostate cancer cells in variable amounts. In androgen-sensitive, LP-LNCaP cells, cav-1 was not expressed; yet in HP-LNCaP cells that had reduced androgen-sensitivity, cav-1 was expressed and secreted into the medium. In contrast, nonprostatic cells such as endothelial, fibroblast, and smooth muscle had a substantial amount of intracellular cav-1 yet minimal or nondetectable levels of cav-1 in their conditioned media (Fig. 1A)
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We investigated the secretory route for cav-1 by expressing human cav-1 in cav-1-negative LP-LNCaP cells. After transfection, a substantial amount of ectopically expressed cav-1 was detected in the media compared with that in the cell lysate, and cav-1 secretion was increased in response to DHT. Cav-1 was not detected in the media or cell lysate of the vector control-transfected cells, yet all transfected cells excreted prostate-specific antigen into the media in a DHT-regulated fashion (Fig. 1C)
. These results show that cav-1 is secreted by androgen-insensitive mouse and human prostate cancer cells in response to specific steroid hormones. Although we do not provide evidence for the mechanism by which cav-1 enters the secretory pathway, the results show that ectopically expressed cav-1 is secreted by LNCaP cells, and that secreted cav-1 migrates on SDS-PAGE similarly to that derived from endothelial cells and fibroblasts, suggesting that the secreted form is not modified posttranscriptionally. To determine whether cav-1 could also be secreted by human prostate cancer cells in vivo, we fractionated human serum and analyzed various fractions for cav-1. Our results revealed that cav-1 is specifically detected in the serum HDL3 lipoprotein subfraction, and that cav-1 levels may be higher in the serum of prostate cancer patients compared with the serum of normal individuals (Fig. 1D)
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The function of secreted cav-1 was investigated by testing the effects of concentrated conditioned media collected from HP-LNCaP cells on LP-LNCaP cell viability and clonal growth under serum-free conditions. The results indicate that secreted cav-1 was capable of promoting viability, using a standard MTT method (Ref. 6
; Fig. 2A
) or luminescent technique (Packard ATPLite; Fig. 2B
) and of stimulating viability/clonal growth using a clonogenic assay (Fig. 2C)
. To test whether such activities would be specific for the cav-1 molecule, polyclonal cav-1 antibody was added to conditioned media or rabbit IgG as a control. Treatment of the conditioned media with anti-cav-1 antibody reduced the viability significantly (P < 0.001 for MTT and clonogenic assays and P < 0.0001 for ATPLite assay) compared with the IgG-treated medium. We also tested the effect of secreted cav-1 on Tg-induced apoptosis in LP-LNCaP cells. Tg promotes apoptosis (14)
, characterized by caspase activation and the appearance of apoptotic bodies in these cells (data not shown). The results indicated that secreted cav-1 was able to protect the cells from the apoptotic effects of this drug (Fig. 2D)
. These studies revealed that media containing secreted cav-1 generates antiapoptotic activities in prostate cancer cells similar to those elicited after enforced expression of cav-1 within the cell (6
, 7)
.
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The concept of a secreted autocrine/paracrine factor that directly contributes to androgen resistance in prostate cancer is novel and represents an efficient mechanism for maximizing resistance to various proapoptotic stimuli that metastatic cells often encounter during the highly inefficient process of metastasis (20) . Our in vivo studies indicating that cav-1-specific antibody delivered i.p. can suppress malignant progression of androgen-insensitive, cav-1-secreting mouse prostate cancer cells are remarkable. These results not only indicate that secreted cav-1 promotes metastasis in vivo, but also raise the possibility of using cav-1 as a therapeutic target for androgen-insensitive disease. It is conceivable that when combined with anti-androgen therapy or potentially with chemotherapy, cav-1-specific antibody therapy may have greater therapeutic activity. Additional studies will be required to address this issue.
| FOOTNOTES |
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1 This work was supported by Grants CA 50588, CA 68814, and SPORE P50-58204 from the National Cancer Institute and DAMD17-98-1-8575 from the Department of Defense. ![]()
2 To whom requests for reprints should be addressed, at Scott Department of Urology, 6560 Fannin Street, Suite 2100, Houston, TX 77030. Phone: (713) 799-8718; Fax: (713) 799-8712; E-mail: timothyt{at}www.urol.bcm.tmc.edu ![]()
3 The abbreviations used are: cav-1, caveolin-1; Dex, dexamethasone; HDL, high-density lipoprotein; TCA, trichloroacetic acid; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; LP-LNCaP, low-passage LNCaP cells; HP-LNCaP, high-passage LNCaP cells; DHT, dihydrotestosterone; SFM, serum-free medium; Tg, thapsigargin. ![]()
Received 1/31/01. Accepted 3/29/01.
| REFERENCES |
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