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Tumor Biology |
1-Adrenoceptor Antagonists Induce Prostate Cancer Cell Apoptosis Via an
1-Adrenoceptor-independent Action1
Division of Urology, Departments of Surgery, Biochemistry, & Molecular Biology, University of Maryland School of Medicine, Baltimore, Maryland 21201
| ABSTRACT |
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1-adrenoceptor antagonists, doxazosin and terazosin, exhibit a potent apoptotic effect against prostate tumor epithelial cells, whereas tamsulosin, a sulfonamide-based
1-adrenoceptor antagonist, was ineffective in inducing a similar apoptotic effect against prostate cells (Cancer Res., 60: 45504555, 2000). In this study, to identify the precise molecular mechanism underlying this apoptosis induction, we examined whether doxazosin and terazosin (both piperazinyl quinazolines) affect prostate growth via an
1-adrenoceptor-independent action. Transfection-mediated overexpression of
1-adrenoceptor in human prostate cancer cells, DU-145 (that lack
1-adrenoceptor), did not alter the ability of prostate cancer cells to undergo apoptosis in response to quinazolines. Significantly enough, there was no modification of the apoptotic threshold of the androgen-sensitive prostate cancer cells, LNCaP, to either quinazoline-based
1-agonist by androgens. Furthermore, human normal prostate epithelial cells exhibited a very low sensitivity to the apoptotic effects of doxazosin compared with that observed for the malignant prostate cells. These findings provide the first evidence that the apoptotic activity of the quinazoline-based
1-adrenoceptor antagonists (doxazosin and terazosin) against prostate cancer cells is independent of: (a) their capacity to antagonize
1-adrenoceptors; and (b) the hormone sensitivity status of the cells. This may have potential therapeutic significance in the use of quinazoline-based
1-adrenoceptor antagonists (already in clinical use for the treatment of hypertension and benign prostate hyperplasia) for the treatment of androgen-independent human prostate cancer. | INTRODUCTION |
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Within the normal prostate gland, homeostasis is maintained by a unique balance between the rates of proliferation and apoptosis, such that neither overgrowth nor involution of the gland occurs (7) . Compelling evidence suggests that the tumorigenic growth of the prostate depends on the evasion of normal homeostatic control mechanisms, because of an increase in cell proliferation and a decrease in apoptotic death (5 , 6) . Thus, enhancing the apoptotic process emerges as a significant therapeutic target for the effective elimination of androgen-dependent and androgen-independent prostate cancer cells (3) . Characteristically, because increased bcl-2 expression correlates with development of androgen-independent prostate cancer, an apoptotic defect that renders prostatic tumors resistant to therapy (8 , 9) , recent efforts have targeted key apoptosis regulators to enhance the therapeutic response with minimal toxicity (3) . Modulation of molecular pathways of apoptosis execution will have significant effects on prostate tumor vascularity and sensitivity to other therapeutic modalities (such as radiotherapy).
With continued intensified search for the identification of apoptosis modifiers as therapeutic targets for prostate cancer treatment, attention has recently been directed toward a group of existing drugs:
1-adrenoceptor antagonists that are members of the quinazoline chemical class. Doxazosin and terazosin are long-acting selective
1-adrenoceptor antagonists that are clinically used to provide acute relief of the obstructive symptoms associated with BPH3
(10, 11, 12)
and for the treatment of hypertension via reduction of total peripheral resistance by selective postsynaptic
1-blockade (13)
. Tamsulosin, a recently developed member of this class of
1-adrenoceptor antagonists, is selective for the predominant
1a-adrenoceptor subtype but has a different sulfonamide-related structure, and its potent therapeutic efficacy has been demonstrated in clinical trials (14)
. The therapeutic benefit of
1-adrenoceptor antagonists in the treatment of BPH has been historically attributed to a change in the periurethral tone of the prostate (15)
via a direct action on
1-adrenoceptors present in the prostate smooth muscle (16)
.
A provocative challenge for the existing concept on
1-blocker action, stemmed from evidence gathered in this laboratory, suggests that two quinazoline-derived
1-adrenoceptor antagonists, doxazosin and terazosin, have additional effects against cellular growth, which transcend smooth muscle relaxation: both doxazosin and terazosin at pharmacologically relevant doses exert an apoptotic activity in stroma smooth muscle and epithelial cell populations in prostate tissue from BPH patients without affecting their roliferative rate (17
, 18)
. These clinical findings were in full accord with experimental studies using a mouse reconstitution model of prostate hyperplasia, in which doxazosin demonstrated a potent apoptotic effect against oncogene-induced prostate growth (19)
. Furthermore, more recent in vitro studies documented a potent antigrowth effect of the two quinazolines (doxazosin and terazosin), but not tamsulosin, the sulfonamide-based
1-adrenoceptor antagonist, against two highly aggressive, androgen-independent human prostate cancer cell lines, PC-3 and DU-145, via induction of apoptosis (20, 21, 22)
.
Considering the ability of the quinazoline-derived
1-adrenoceptor antagonists, but not the uroselective, high affinity (sulfonamide-based antagonist) tamsulosin, to induce prostate tumor cell apoptosis, in this study, we examined whether the apoptotic effect of terazosin and doxazosin is: (a) an
1-adrenoceptor-mediated mechanism; and (b) dependent on androgens. Our results demonstrate that the quinazoline-driven apoptosis is independent of
1-adrenoceptor action, and DHT does not affect the sensitivity of prostate cancer cells to the apoptotic effects of doxazosin and terazosin. Moreover, there was a minimal cell death effect (by quinazolines) in the normal prostate epithelial cells. The present findings may provide a rationale for advancing these long-acting, quinazoline-based
1-adrenoceptor antagonists toward the development of an effective therapeutic strategy for patients with androgen-independent prostate cancer.
| MATERIALS AND METHODS |
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Drugs
The three
1a-adrenoceptor antagonists used in this study were generously provided by the following pharmaceutical companies that manufacture them, respectively: Doxazosin (Cardura; Doxazosin Mesylate) was provided by Pfizer Pharmaceuticals (New York, NY); Terazosin (Hytrin; Terazosin Hydrochloride) was obtained from Abbott Laboratories (Abbott Park, IL); and tamsulosin (FLOMAX; Tamsulosin Hydrochloride) was provided by Yamanouchi Pharmaceuticals (Tokyo, Japan). DHT (5-androstan-17ß-OL-3-one) was obtained from Sigma Chemical Co. (St. Louis, MO).
Transfection
DU-145 cells were seeded at 1 x 106 cells/100-mm tissue culture dish and after 24 h, were cotransfected with the pcDNA3 plasmid encoding the human
1a-adrenoceptor (a generous gift from Dr. P. Walden, New York University, New York, NY) and the neomycin resistance-encoding plasmid via the calcium phosphate precipitation procedure as described previously (23)
. Neomycin-resistant colonies were selected in the presence of G418 (500 µg/ml; Life Technologies, Inc.), cloned, and expanded into cell lines.
Cell Viability Assay
Subconfluent cultures of cells in six-well plates were exposed in duplicate to increasing concentrations of doxazosin, terazosin, or tamsulosin (1-50 µM). The number of viable cells was assessed 2 days posttreatment using the trypan blue exclusion assay. For the experiments involving treatment with DHT, before treatment with doxazosin, LNCaP cells were grown in charcoal-stripped serum-containing medium in the presence or absence of 1 nM DHT as described previously (24)
. Values are expressed as the percentage of mean cell viability relative to the untreated control cultures.
Cell Proliferation Assays
Rate of DNA Synthesis.
The effect of
1a-adrenoceptor antagonists on the rate of DNA synthesis in normal and malignant human prostate epithelial cells was evaluated using the [3H]thymidine uptake assay, as described previously (20)
.
Cell Cycle Analysis.
Subconfluent cultures of the androgen-sensitive prostate cancer cells LNCaP were treated with doxazosin or terazosin (25 µM) for 48 h. Cells were subsequently harvested and washed with PBS (pH 7.4), and pellets were resuspended with PBS and 70% ethanol. They were stored at -20°C overnight and resuspended (106 cells/ml), at which time 1 mg/ml DNase-free RNase was added. The cell suspension was incubated at 37°C (30 min) and stained overnight at 4°C with 50 µg/ml propidium iodide (Sigma Chemical Co.). Cells were filtered through a 35 µM nylon filter and analyzed using Beckman-Coulter fluorescence-activated cell sorter analyzer. G1, S, and G2-M populations were quantitated using the Multicycle software program (Phoenix Flow Systems). Results are expressed as a percentage of cells in each stage of cell cycle.
Apoptosis Evaluation
The morphological appearance of apoptosis was assessed using the Hoechst staining as described previously (24)
. Briefly, cells were seeded in six-well plates (in triplicate) at 105 cells/well, and at subconfluency, they were treated with one of the three antagonists, doxazosin, terazosin, or tamsulosin (25 µM), for 48 h. After treatment, cells were fixed in 4% paraformaldehyde/PBS and stained with 10 µg/ml Hoechst 33342 dye (Sigma Chemical Co.)/0.1% Triton X-100. Fixed cells were incubated overnight at 4°C and visualized under a 365 nM UV light microscope (Zeiss Axiovert 10). Quantitative analysis was performed by counting the green fluorescent (apoptosis positive) cells under x400 magnification from three independent fields. Values are expressed as the percentage of apoptotic cells relative to the total number of cells per field (the average number of cells that were positive for apoptosis varied from 240/field, whereas the total number of cells counted per field was
50100).
Protein Analysis
Western Blot Analysis.
LNCaP cells were treated with either doxazosin or terazosin (25 µM) for 1, 2, or 3 days. Cells from treated and control untreated cultures were lysed in 150 mM NaCl, 50 mM Tris (pH 8.0), 0.5% deoxycholic acid, and 1% NP40 with 1 mM phenylmethyl sulfonyl fluoride. Total cell lysates were electrophoretically analyzed through a 15% (w/v) polyacrylamide gel, and proteins were subsequently transferred onto a Hybond-P membrane (Amersham Pharmacia Biotech, Piscataway, NJ). Western blot analysis was performed using the rabbit polyclonal antibody against VEGF (Santa Cruz Biotechnology, Santa Cruz, CA) and the rabbit polyclonal antibody against PSA (DAKO, Carpenteria, CA).
-Actin expression was determined using the monoclonal-actin antibody (Oncogene Research, Cambridge, MA) as a loading and normalizing control. Protein detection was achieved using the Enhanced Chemilluminesence System (Amersham International, Arlington Heights, IL). Values for specific protein expression are expressed as fold-change, relative to
-actin expression (loading control) using the SCION Image analysis.
ELISA Assay for PSA.
LNCaP cells were treated with doxazosin, terazosin, or tamsulosin (at 25 µM) for 24 h as described above. Supernatants from treated and nontreated LNCaP cells were assayed for PSA expression using an ELISA assay. PSA monoclonal antibody (Fitzgerald Industries, Concord, MA) was used as the capture antibody (5 µg/ml). Supernatants were then added after blocking in 1% BSA/PBS (1 h at room temperature). The antirabbit PSA (1 µg/ml; Fitzgerald Industries) and the Goat F(ab')2 antirabbit IgG-biotin conjugate (1 µg/ml; Biosource International, Camarillo, CA) were subsequently applied (1 h). After washing, ExtrAvidin Alkaline Phosphatase Conjugate 1:1000 (Sigma Chemical Co.) was applied, and PSA protein was detected using p-nitrophenyl phosphate (Sigma Chemical Co.) according to the manufacturers instructions. Values represent the percentage of PSA expression relative to the untreated control samples.
Statistical Analysis
Statistical analyses of the numerical data were performed using ANOVA in the Graph Pad Prism program. All values are represented as averages ±SE. Values were considered statistically significant at P < 0.05.
| RESULTS |
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1a-adrenoceptor, human prostate cancer cells DU-145, which totally lack
1a-adrenoceptor expression (20)
, were engineered to overexpress
1a-adrenoceptor, and cloned transfectants were evaluated in vitro. Fig. 1
1a-adrenoceptor transfectant clones. Clone-32 was selected for additional experiments as it expressed a band at Mr 30,000, indicative of
1a-AR reactivity (Fig. 1
1a-adrenoceptor to exogenous norepinephrine was documented by comparatively analyzing it with that of prostate smooth muscle cell line SMC-1 (20)
in the presence or absence of phenoxybenzamine (1 µM; an irreversible inhibitor of
1a-adrenoceptor binding). We found that the DU-145 Clone-32
1a-AR-transfectant cell line exhibited similar biding capacity to norepinephrine as the smooth muscle cell line (data not shown).
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1-blockers but not the sulfonamide-derived tamsulosin (20
, 21)
. In this experiment, the effect of the three drugs (doxazosin, terazosin, and tamsulosin) on the cell viability and rate of DNA synthesis was examined in the androgen-sensitive human prostate cancer cells LNCaP (Fig. 3)
80% cell death. In a similar pattern, terazosin treatment also resulted in a significant loss of cell viability (40%), whereas tamsulosin had virtually no effect on cell death in LNCaP cells. Fig. 3B
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1-antagonists, the effect of and DHT on doxazosin-induced cell death in LNCaP cells was examined. Fig. 4A
1-adrenoceptor antagonists, cells were treated with doxazosin alone (25 µM), doxazosin in the presence of DHT, or DHT alone (1 nM). As shown in Fig. 4B
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1-adrenoceptor antagonists in normal prostate epithelial cells are shown on Table 1
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1-adrenoceptor antagonists, doxazosin, terazosin, and tamsulosin. As shown in Table 3
1-blocker, tamsulosin, at any of the treatment doses (Table 3)
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1-antagonists in androgen-sensitive prostate cancer cells, we subsequently evaluated the effect of doxazosin on the cellular expression of VEGF and PSA protein in LNCaP cells. Elevated VEGF has been associated with prostate cancer progression (25)
and has become a target for antiangiogenesis therapy in androgen-responsive prostatic tumors (26)
. As shown in Fig. 6
1-adrenoceptor antagonist.
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| DISCUSSION |
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1-adrenoceptor antagonists, doxazosin and terazosin, is selectively targeted at apoptosis in androgen-independent and androgen-sensitive prostate cancer cells. Interestingly enough, normal prostate epithelial cells seem to be relatively protected from this quinazoline-mediated apoptotic effect. Our data established that neither of the two quinazoline-derived
1-adrenoceptor antagonists evaluated exerted a significant effect on cell cycle progression of prostate cancer cells. These observations are in accord with our previous clinically gathered data (17
, 18
, 27)
, as well as experimental studies by others (19)
, that doxazosin and terazosin inhibit benign and malignant prostate growth via induction of apoptosis (over the normal therapeutic dose range) without affecting the rate of cell proliferation. However, this is in contrast with a recent report demonstrating that doxazosin inhibits G1-S transition in human coronary smooth muscle cells (28)
, lending support to the possibility that cell type specificity exists with regards to the cellular process targeting of the
1-adrenoceptor antagonist antigrowth function.
Of major mechanistic significance was the observation that this apoptotic effect against prostate cancer cells was independent of the ability of the two
1-adrenoceptor antagonists to antagonize the
1a-adrenoceptor binding in prostate cancer cells. Our results are consistent with previous studies in other cellular systems, demonstrating that doxazosin inhibits growth and migration of the human vascular smooth muscle cells (29)
and human coronary smooth muscle cells (28)
, independent of an antagonistic effect on
1-adrenoceptor. Moreover, this action is in accord with our previous observations that documented that preincubation with phenoxybenzamine, an irreversible inhibitor of
1-adrenoceptor binding (that inactivates
1-adrenoceptors in vascular smooth muscle cells), did not affect the cellular response to doxazosin-induced apoptosis in prostate cancer cells (20)
. Whereas these data provide an intriguing mechanistic insight, the precise cellular pathway that signals this effect remains to be elucidated. One could speculate that the apoptotic activity of the quinazoline-derived
1-adrenoceptor antagonists could reflect either a direct effect on cellular dynamics or could arise secondarily to an action on cellular apoptotic factors, such as transforming growth factor-ß.
The present findings suggest that the cellular sensitivity to quinazoline-induced apoptosis in the androgen-sensitive prostate cancer cells, LNCaP, was not modified by androgens (at physiological levels). Interestingly enough, both doxazosin and terazosin treatment reduced PSA expression and secretion. This is consistent with recent clinical studies from our center that documented the ability of terazosin to reduce tissue PSA expression in patients with prostate cancer (27) . Because androgens had no effect on the apoptotic response of LNCaP cells, the data imply that doxazosin-mediated reduction in PSA expression represents a result of the cell killing, rather than being a molecular target for the quinazoline-apoptotic action at the transcriptional level. The effect of quinazoline on VEGF expression is supported by our earlier observations that terazosin inhibits prostate tissue vascularity and VEGF protein levels in clinical specimens of prostate cancer (27) .
The in vivo antitumor efficacy of the
1-adrenoceptor antagonists, against prostate cancer growth via apoptosis induction (20)
, resembles the in vivo effect in the model of intimal hyperplasia (30)
. Considering this evidence, it is tempting to speculate on a potentially clinically relevant concept that there may be substantive analogies between the effects of
1-adrenoceptor antagonists on cell growth in the cardiovascular system and in the prostate gland. Attractive as this argument might be, one should also consider the differential effects of doxazosin (apoptotic versus antiproliferative) demonstrated in the two cell types (present findings and Refs. 28
and 29
, respectively).
In conclusion, the present study indicates that the quinazoline-derived
1-antagonists, doxazosin and terazosin, but not the sulfonamide-based tamsulosin, activate apoptosis in prostate cancer cells via a mechanism independent of androgens and without interfering with cell cycle progression. More significantly, this potent apoptotic activity of doxazosin and terazosin occurs through a novel pathway unrelated to their ability to block the
1-adrenoceptor. Current studies are exploring the apoptotic signaling triggered by the quanizoline-based
1-antagonists, doxazosin and terazosin, within the context of anoikis regulation (31)
. Preliminary evidence suggests that the quinazolines cause severe perturbation in cell attachment to the extracellular matrix leading to induction of anoikis via direct activation of the transforming growth factor-ß signaling pathway.4
These findings highlight the emerging therapeutic significance of the use of the class of quinazoline-derived
1-antagonists, not only for the management of BPH but also as potential antitumor agents for the treatment of androgen-dependent and androgen-independent prostate cancer. Because this apoptotic effect is independent of an action on the
1-adrenoceptor, assuming that the molecular mechanism can be unraveled (32)
, this could represent an exciting new starting point for quinazoline-based drug design for targeting prostate cancer cell apoptosis.
| FOOTNOTES |
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1 Supported by educational grants from Pfizer and Abbott Laboratories. ![]()
2 To whom requests for reprints should be addressed, at Division of Urology, Room 8SD, 22 South Greene Street, University of Maryland Medical Center, Baltimore, MD 21201. Phone: (410) 706-7549; Fax: (410) 706-0311; E-mail: NKyprianou{at}smail.umaryland.edu ![]()
3 The abbreviations used are: BPH, benign prostatic hyperplasia; DHT, dihydrotestosterone; PSA, prostate-specific antigen; VEGF, vascular endothelial growth factor. ![]()
4 I. Anglin and N. Kyprianou. Induction of prostate cell apoptosis by quinazoline-derived
1-adrenoceptor antagonists via TGF-ß signaling and caspase-3 activation, submitted for publication. ![]()
Received 6/28/01. Accepted 11/13/01.
| REFERENCES |
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Adrenoceptor blockade in the treatment of benign prostatic hyperplasia: past, present, and future. Br. J. Urol., 80: 521-532, 1997.[Medline]
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1a-adrenoceptor subtype in the rat, monkey, and human urinary bladder and prostate. J. Urol., 157: 1032-1038, 1997.[Medline]
1-Adrenoceptor antagonists terazosin and doxazosin induce prostate apoptosis without affecting cell proliferation in patients with benign prostatic hyperplasia. J. Urol., 160: 2002-2005, 1999.
1-adrenoceptor antagonists doxazosin and terazosin via induction of apoptosis. Cancer Res., 60: 4550-4555, 2000.
1-adrenoceptor antagonists on cell proliferation and apoptosis in the prostate: therapeutic implications in prostatic disease. Prostate Suppl., 9: 42-46, 2000.[Medline]
1 adrenergic receptor antagonism. J. Cardiovasc. Pharmacol., 31: 833-839, 1998.[Medline]
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