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Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, Texas 77030 [S. E. B., T. C. P., L. V. S., N. L. W.], and Departments of Urology and Biostatistics and Epidemiology, Memorial Sloan-Kettering Cancer Center, New York, New York 10021 [M. W. K.]
| ABSTRACT |
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,25-dihydroxyvitamin D3 (calcitriol) analogues to
inhibit cancer growth. We demonstrate here that the calcitriol
analogue, EB1089, extensively inhibits the growth of LNCaP prostate
cancer cells in culture and causes the cells to both accumulate in
G0-G1 and undergo apoptosis. Importantly, we
found that EB1089 inhibits the growth of LNCaP tumor xenografts in nude
mice. Because of these antiproliferative properties in
vivo, EB1089 is a potential new therapeutic agent for the
treatment of prostate cancer. | Introduction |
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10100
nM; Ref. 1
) are unachievable
in vivo because of unacceptable side effects such as
hypercalcemia (2)
. Much effort has been devoted to the
development of analogues of calcitriol that retain antiproliferative
capabilities but do not cause unwanted side effects in vivo.
One promising analogue, EB1089 (3)
, developed by Leo
Pharmaceuticals (Ballerup, Denmark), is a deltanoid derivative
of calcitriol that inhibits LNCaP cell growth at much lower
concentrations than calcitriol (
1 nM; Ref.
4
). We demonstrate here that EB1089, at much lower levels
than those required for calcitriol, virtually irreversibly inhibits the
growth of LNCaP cells and causes a comparable amount of apoptosis and
G0-G1 cell cycle
accumulation. We also show that pharmacological levels of EB1089
injected i.p. substantially reduce the growth of LNCaP tumors in a nude
mouse xenograft model without raising serum calcium levels (a measure
of hypercalcemia) beyond the normal range. This property of EB1089
makes it a promising candidate for the treatment of prostate cancer. | Materials and Methods |
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Cell Culture.
LNCaP cells were plated in six-well dishes at a density of 13,000
cells/well and grown in RPMI 1640 (Life Technologies, Inc.,
Gaithersburg, MD) supplemented with Rehatuin fetal bovine serum
(Intergen Co., Purchase, NY) at 37°C in a humidified atmosphere of
5% CO2 as described by Blutt et al.
(5)
. Cells were treated with 1 nM
EB1089 for 6 days with medium and hormone replaced on day 3, after
which hormone was removed and growth recovery was compared for cells
continuously treated with hormone. Cells were harvested and counted as
reported by Blutt et al. (5)
. All samples were
tested in triplicate, and statistical significance was determined by
ANOVA using SigmaStat (Jandel Scientific, San Rafael, CA). Each
experiment was performed a minimum of three times, and a representative
experiment is shown.
Cell Cycle Analysis.
LNCaP cells were plated in 10-cm dishes at a density of 200,000
cells/dish and treated with EB1089, calcitriol, or vehicle for 6 days
with fresh medium and hormone added on the third day. Cells were pulsed
with 10 µM bromodeoxyuridine (Sigma Chemical Co., St.
Louis, MO) for 15 h before harvesting. Cells were harvested and
processed, and flow cytometry was performed as described by Blutt
et al. (5)
. Samples were run in triplicate, and
statistical significance was determined by ANOVA using SigmaStat. A
minimum of three experiments was performed.
Cell Death Analysis.
LNCaP cells were treated for 6 days, harvested, and fixed using 1%
formaldehyde (TEM grade; Tousimis Research Corp., Rockville, MD)
followed by 70% ethanol. Cells were processed using a DNA
deoxynucleotidyltransferase (terminal transferase) reaction mixture
(Boehringer Mannheim) and incubated with avidin-FITC to identify
positive cells. Total DNA content was measured by counterstaining with
propidium iodide. Flow cytometry was performed as described previously
(5)
. Samples were run in triplicate, and statistical
significance was determined by ANOVA using SigmaStat.
Nude Mouse Tumor Studies.
LNCaP cells at 70% confluency were trypsinized and resuspended in
Matrigel Matrix (Becton Dickinson, San Jose, CA) at a concentration of
one million cells/100 µl matrix. Male athymic nu/nu mice,
6 weeks of age (Harlan Sprague Dawley, Indianapolis, IN), on a
vitamin D-deficient diet supplemented with 1% calcium (Purina Mills
Inc., St. Louis, MO) received injections s.c. with 100 µl of the
cell/Matrigel mix on the dorsal surface. Treatment was begun when
tumors reached a calculated volume of
150 mm3
(length x width x 1/2 the
greater of length or width). Animals received injections i.p. with
either 0.50 µg/kg (equivalent to 3 nM, assuming
uniform body distribution) EB1089 (supplied by Leo Pharmaceuticals)
diluted in sesame oil or sesame oil alone every other day, and weights
and calculated tumor volumes were collected over the course of
treatment. SAS software (Cary, NC) was used to determine statistical
significance using analysis of covariance. Statistical
significance of weight was determined using Students t
test. In the experiment shown, 20 animals received injections, and
after tumors were established, the animals were randomly assigned to
control (n = 10) or treated
(n = 10) groups.
Calcium Measurements.
Serum calcium was measured on a Boehringer Mannheim/Hitachi 704
Analyzer (Boehringer Mannheim) using the system pack for calcium
(
-aminobutyric acid) and following the manufacturers instructions.
Tumor Histology.
Tumors were harvested and fixed in 10% neutral buffered formalin
overnight and placed in 70% ethanol. Tumors were paraffin embedded,
sectioned, placed on poly-lysine slides, and stained with H&E to
visualize cell nuclei and cytoplasm. Images were recorded using a Zeiss
Axioskop light microscope at x10.
Androgen Receptor Staining.
Sections from above were deparaffinized, and antigen was retrieved
using 0.01 M citrate buffer and boiling in a microwave.
Slides were preincubated with horse serum (Vectastain ABC kit; Vector
Laboratories, Inc., Burlingame, CA). Human androgen receptor antibody
AR441 was used to detect AR-positive cells in tumor sections. Positive
staining was visualized using the Vectastain kit reagents and
3,3'-diaminobenzidine (Research Genetics, Huntsville, AL). Slides were
counterstained with hematoxylin and dehydrated. Images were recorded
using a Zeiss Axioskop light microscope at x10.
| Results and Discussion |
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0.05),
this is likely an underestimate of the difference in tumor burden for
two reasons: (a) we are estimating the third dimension.
Shown in Fig. 3, A and B
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To determine which cells were LNCaP cells, we used an antibody specific
for the human AR. Fig. 3, E and F
, illustrate the
AR-positive LNCaP cells within both untreated and treated tumor
sections. The control section (Fig. 3E)
consists of densely
packed AR-positive cells interspaced with some AR-negative cells. The
sections from the treated animal (F) contained islands of
AR-positive cells amid acellular material and regions containing
AR-negative (presumably mouse) cells. Controls (incubated without the
AR antibody) for specificity are shown in Fig. 3, G and H
.
Serum Calcium Levels Are in the Normal Range.
The tumor growth inhibition reported above occurred with no
physically apparent symptoms of hypercalcemia (lethargy or
dehydration), and serum calcium levels in both groups (9.25 ± 0.35 for untreated animals and 10.5 ± 0.60 in
treated animals) were within the normal range of serum calcium for mice
of the age and sex used (Fig. 2B;
Ref. 11
). A
small but insignificant drop (P = 0.09) in
weight was observed in animals treated with EB1089, but this did not
appear to significantly affect the health of the animals (Fig. 2C).
In other unrelated studies, the same dose of EB1089
administered for over 6 weeks did not raise serum calcium levels above
the normal range, and normal mouse weights were maintained.
Conclusions.
Although calcitriol itself and other calcitriol analogues have
been tested in the Dunning tumor model of prostate cancer
(12)
and in a PC3 human prostate cancer cell athymic mouse
model (13)
with some success in inhibiting prostate tumor
growth, hypercalcemia and weight loss were also noted. In contrast, our
findings show that EB1089 can inhibit LNCaP tumor growth without
inducing hypercalcemia. Studies that report the effect of EB1089 in
breast (14)
, pancreatic (15)
, and colon
cancer (16)
models suggest that this compound may be
useful in treating a number of tumors. Two promising studies in humans
demonstrated that calcitriol can stabilize rising PSA levels in
prostate cancer patients with advanced disease, although both studies
were limited by hypercalcemic side effects (17
, 18)
. Of
interest will be whether EB1089 has similar effects in humans without
the incidence of side effects.
Our finding that the treated tumors consist of small islands of cells surrounded by acellular material and mouse-derived cells rather than large masses of tumor cells suggests that one potential mechanism of EB1089 action may be through the inhibition of new blood vessel formation. This inhibition of blood vessel formation could result in cell islands that cannot obtain enough nutrients to proliferate and eventually become necrotic. In fact, in some of our tumor sections from treated animals, we have observed necrotic centers in the cellular islands. Some studies have already reported calcitriol as having significant antiangiogenic properties (19 , 20) , and additional studies will be necessary to elucidate the antiangiogenic properties of EB1089. Our findings of the antiproliferative effects of EB1089 on LNCaP xenografts with no observable calcemic side effects may offer a novel therapeutic option for the treatment of prostate cancer.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by the NIH/National Cancer Institute
Specialized Programs of Research Excellence Grant for Prostate Cancer
CA58204 and USPHS Grant CA75337 ![]()
2 To whom requests for reprints should be
addressed, at Department of Molecular and Cellular Biology, Baylor
College of Medicine, Houston, TX 77030. Phone: (713) 798-6234; Fax:
(713) 790-1275; E-mail: nweigel{at}bcm.tmc.edu ![]()
3 The abbreviations used are: calcitriol,
1
,25-dihydroxyvitamin D3; AR, androgen receptor. ![]()
Received 10/ 5/99. Accepted 1/ 4/00.
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-hydroxylated vitamin D analogs with polyunsaturated side chains Norman A. W. Bouillone R. Thomasset M. eds. . Vitamin D, Gene Regulation, Structure-Function Analysis and Clinical Application, : 192-193, Walter de Gruyer Berlin 1991.
,25-dihydroxyvitamin D3 in human prostate cancer cell line LNCaP involves reduction of cyclin-dependent kinase 2 activity and persistent G1 accumulation. Endocrinology, 139: 1197-1207, 1998.
,25-dihydroxyvitamin D3 in androgen-responsive LNCaP cells. Biochem. Biophys. Res. Commun., 235: 539-544, 1997.[Medline]
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