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Blood, Vol. 95 No. 10 (May 15), 2000:
pp. 3133-3138
HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY
From New York University Medical School, Kaplan Cancer Center, and
Department of Veterans Affairs Medical Center, New York, NY.
Because thrombin-treated tumor cell-induced metastasis increases
tumor nodule volume12 greater than nodule number, we
studied the effect of thrombin on tumor cell growth in vitro and in
vivo (murine B16F10 melanoma, human HCT8 colon carcinoma, DU145
prostate carcinoma). Tumor cell growth was measured after 3 to 7 days
in 1% fetal calf serum (FCS) + RPMI 1640. We found that, whereas
relatively low concentrations of thrombin, 0.1 to 0.5 U/mL (1-5 nmol/L)
enhance tumor cell growth in vitro approximately 2- to 3-fold, higher
concentrations, 0.5 to 1 U/mL (5-10 nmol/L) impaired cell growth
approximately 2- to 4-fold. Impaired cell growth was associated with
cell cycle arrest at G2M and increased pre-Go
DNA, as well as apoptosis, measured by tumor cell binding to Annexin
V and propidium iodide. Apoptosis was reversed with the
general caspase inhibitor, FK-011. The enhancing and inhibiting effects
were specific for thrombin (reversed with inactive
diisopropyl-fluorophosphate [DFP]-thrombin) and mediated via the
protease-activated receptor 1 (PAR-1). PAR-1 activation was
demonstrated by (1) use of a cell line, B16F10, devoid of the 3 other
thrombin receptors, PAR-3, PAR-4, and GPIb; and (2) greater sensitivity
of PAR-1 transfected B16F10 and HCT8 cells to impaired cell
growth/apoptosis, 3- and 14-fold, respectively. Thus, thrombin has a
bimodal effect on PAR-1 in tumor cells: enhanced growth at low
concentration, impaired growth/apoptosis at higher concentration.
(Blood. 2000;95:3133-3138)
Thrombin, a serine protease generated after endothelial
cell damage, is a multifunctional protein involved in a variety of biologic functions, including blood coagulation, platelet adhesion, platelet aggregation, fibroblast and smooth muscle cell mitogenesis, and tumor metastasis.1,3-5 The role of thrombin in
stimulating tumor cell adhesion to platelets,1,4
endothelial cells,5 fibronectin,1,4 and von
Willebrand factor1,4 is well documented, and probably
contributes to the 10- to 156-fold increase in experimental pulmonary
metastasis noted after injection of thrombin-treated syngeneic tumor
cells into the tail vein of mice.1 Because recent studies
indirectly suggested that thrombin may also be enhancing tumor cell
growth, as well as adhesion in an experimental pulmonary metastasis
model,2 we examined the direct effect of thrombin on tumor
cell growth in vitro.
In studying the action of thrombin on tumor cell growth in vitro, we
unexpectedly noted impaired tumor cell growth with concentrations of thrombin at approximately 0.5 to 1 U/mL (5-10 nmol/L) for 72 hours,
in contrast to enhanced tumor cell growth at 0.1 to 0.5 U/mL (1-5 nmol/L) during the same period. In this report, we show that both the
impaired and enhanced tumor cell growth is secondary to activation of
the thrombin PAR-1. We further demonstrate that the impaired tumor cell
growth in vitro is associated with the arrest of cell growth at
G2M in the cell cycle and induction of apoptosis. Materials and methods
Tumor cell lines, tissue culture media, and reagents
Tumor cell growth assays
Reverse transcriptase-polymerase chain reaction The RNAs tested were treated with deoxyribonuclease 1 (Perkin Elmer Cetus, Branchburg, NJ) to eliminate contaminating DNA. RNA was extracted with the ultraspec RNA Isolation System (Biotexc, Houston TX). The primer pair used for PAR-4 was identical to that used by Xu et al12: (sense) 5'-GGYGCCCGCCCTCTATGG at base pairs (bp) 412 to 429 and (antisense) 5'-TCGCGAGGTTCATCAGCA at bp 516 to 533 designed to give a 121-bp product. The primer pair used for murine PAR-4 was (sense) 5'-CTCACTACTGGACTCTGTTTGGTGG at bp 610 to 634 and (antisense) 5'TGGGCACATAGGCTCCATAGAG-3 at bp 988 to 1009 designed to give a 399-bp product.13 The primer pair for murine actin was (sense) 5'-ATGAAGATCCTGACCCGAGCG at bp 490 to 509 and (antisense) 5'-TACTTGCGCTCAGGAGGAGC at bp 913 to 932 designed to give a 443-bp product.
B16F10 cells. Bluescript plasmid containing the murine PAR-1 cDNA was kindly provided by Dr S. Coughlin (University of California, San Francisco). An approximately 1.5-kb Xba1 fragment containing the complete coding sequence was inserted into the expression vector pcDNA3 (In Vitrogen). A positive transfectant (S14) was selected by Geneticin resistance and verified by Northern blotting, as well as immunohistochemistry with an antibody against PAR-1 as described previously.2 HCT8 cells. The human wild-type PAR-1 was kindly supplied by Dr S. Coughlin in a pBJ1 plasmid containing the FLAG epitope at the amino terminal end of PAR-1.15 Positive transfectants were selected by Geneticin resistance and verified by Northern blotting, as well as immunoblot. Immunoblotting Sixty micrograms of protein extract from cell lysates was applied to a 12.5% SDS-polyacrylamide gel and electroblotted onto nitrocellulose membranes. Expression of the human PAR-1 transfectants were analyzed by the luminescence method (Amersham Life Science) with an M1-anti-FLAG antibody (Kodak Scientific Imaging System, 1 µg/mL), followed by goat antimouse antibody conjugated to horseradish peroxidase (Sigma).Apoptosis measurements Apoptosis was measured by flow cytometry, using a fluorescein isothiocyanate (FITC) conjugate of Annexin V and propidium iodide (Oncogene Research Products, Cambridge, MA). Annexin V binds to early apoptotic, as well as late apoptotic/necrotic cells. Propidium iodide binds to late apoptotic/necrotic cells. Treated cells were removed with trypsin-EDTA, washed twice with 0.01 mol/L PBS, pH 7.4, centrifuged, and then treated as directed by the manufacturer.
Effect of thrombin exposure time on impaired cell growth B16F10 cells were incubated with 1 U/mL of thrombin for 72 hours or for 3 hours, followed by washing in RPMI media and then incubated for 72 hours. Figure 1 demonstrates that the 72-hour exposure of thrombin resulted in a 70% decrease in cell growth (P = .01), compared with no decrease after 3 hours of thrombin exposure, followed by washing and 72 hours of further incubation, as well as 24 hours of thrombin exposure (data not shown). Impaired cell growth was associated with the rounding of cells that became detached from the plate. Both detached and attached cells were harvested together in the assay. Detached cells were dead as determined by trypan blue exclusion.
Effect of thrombin concentration on cell growth B16F10 cells were next incubated with various concentrations of thrombin for 72 hours. Figure 2A demonstrates a 62% decrease in cell growth at 1 U/mL thrombin (P = .03), compared with a 57% increase in growth at 0.5 U/mL (P = .002), with decreasing concentration resulting in decreased enhancement. An analysis of thrombin concentrations between 0.5 and 1 U/mL revealed a graded decrease in cell growth (data not shown). Similar results were noted with HCT8 cells with a 50% decrease in cell growth at 2 U/mL thrombin and a 63% increase in growth at 0.25 U/mL, Figure 2B. A similar 58% impairment of growth was noted with a megakaryocyte cell line (CHRF) after 1 U/mL thrombin (representative of 2 experiments).
Specificity of thrombin effect on cell growth Specificity of the thrombin effect on B16F10 cell growth was confirmed by competitive inhibition experiments with inactive DFP-thrombin that competes with active thrombin for PAR-1 without activating the receptor. Active thrombin at 1 U/mL impairs tumor cell growth by approximately 50% in Figure 3. Various percentage mixtures of increasing DFP-thrombin with 1 U/mL active thrombin over a 72-hour incubation resulted in enhanced cell growth of 83% at a 1:1 mixture (P < .02) as predicted. Further increases in DFP-thrombin were less effective because less active thrombin was now available for its enhancing effect on cell growth.
Effect of serum concentration on cell growth in the presence of thrombin Impaired cell growth of B16F10 cells ( 50%,
P = .03, 1.07 ± 0.2 vs 0.53 ± 0.02 optical
density) was noted with 1% serum plus 1 U/mL thrombin for 72 hours,
whereas enhanced growth (+ 68%, P = .02, 1.03 ± 0.07
vs 1.73 ± 0.09) was noted in 10% serum with 1 U/mL thrombin.
This protective effect of serum could be overcome with 2 U/mL thrombin
( 48%, P < .01, 1.03 ± .067 vs
0.54 ± 0.067). Data are from 3 different experiments, performed
in quadruplicate.
Direct measurement of tumor cell number at varying concentrations of thrombin Figure 4 demonstrates a 2.1-fold increase in B16F10 cell number with 0.5 U/mL thrombin after 7 days of exposure, with no increase in cell number using 1 U/mL.
Thrombin-induced impaired tumor cell growth operates through the
PAR-1 receptor and requires intact thrombin.
B16F10 cells only contain the PAR-1 thrombin receptor. GPIb and PAR-3
were not detectable by RT-PCR in a previous analysis,2 and
PAR-4 was not detectable by RT-PCR in this study, under conditions in
which it is detectable in mouse spleen cells (Figure
5). Negative results were also obtained in
HCT8 cells under conditions in which it was detectable in human
platelets and CHRF cells (data not shown). Nevertheless, the 14mer
thrombin receptor activation peptide (TRAP) had no impaired growth
effect at concentrations as high as 1 mmol/L (data not shown).
Differential activation of cells by thrombin and TRAP have been
reported.16,17 This is compatible with thrombin operating
through another receptor or mechanism. However, this was not supported
by experiments demonstrating the absence of the newly described PAR-4
thrombin receptor in both cell lines.
Thrombin-induced tumor cell growth is associated with cell cycle
arrest at G2M.
Figure 8 demonstrates "high-dose"
thrombin-induced cell cycle arrest of B16F10 and DU145 cells at
G2M of 2.2- and 3.3-fold respectively, with enhanced
pre-Go DNA of approximately 12- and 8-fold, respectively,
at 36 hours (compared with no thrombin treatment). The increase in
pre-Go DNA suggested the induction of apoptosis. This was
confirmed by Annexin V assay and caspase inhibition experiments (see
below).
Thrombin-induced impaired tumor cell growth is associated with the
induction of apoptosis.
Figure 9 demonstrates apoptosis and cell
death at 1.5 U/mL thrombin. Note the absence of apoptosis at 9 hours of
incubation, onset at 19 hours and induction of 30% cell death at 24 hours of incubation.
Effect of caspase inhibitor on apoptosis of B16F10 cells induced by
1U/mL thrombin.
Figure 10 demonstrates prevention of
thrombin-induced apoptosis of B16F10 cells with the caspase inhibitor,
FK-011, which inhibits all caspases. Note the inhibition of cell growth
with 2 U/mL of thrombin of 79% (P = .03) and prevention of
this effect with 40 µmol/L, as well as 80 µmol/L FK-011
(P < .01), to 57% and 77% of control, respectively.
Similar results were noted with DU145 cells in which cell growth was
inhibited 80% with 1.5 U/mL of thrombin (under similar conditions of
incubation as with B16F10 cells), which was partially corrected with 40 to 80 µmol/L FK-011 to 36% inhibition (P = .001, 2 experiments performed in quadruplicate).
Our studies demonstrate for the first time, a role for thrombin in
the induction of tumor cell impaired growth and apoptosis at higher
thrombin concentrations (0.5-1 U/mL) than those required for tumor cell
mitogenesis in 4 tumor cell lines studied: B16F10, HCT8, CHRF, and
DU145 cells. This is documented by (1) impaired growth by MTT and
Calcein assays at 72 hours; (2) absent thrombin stimulation of cell
growth at 7 days of assay, compared with control cells; (3) cell cycle
arrest at G2M with enhanced pre-Go DNA
accumulation; (4) Annexin V binding at 19 hours with 30% cell death at
24 hours; and (5) inhibition of thrombin-induced impaired cell growth
and apoptosis with caspase inhibitor FK-011, which inhibits all
caspases.10 Interestingly, cell cycle arrest at
G2M has recently been reported to precede the induction of
apoptosis by irradiation of lymphoblastic leukemia cells18
and vinblastine treatment of human small-cell lung carcinoma, MS1
cells,19 suggesting that arrest at this stage facilitates
the induction of apoptosis.
Submitted May 17, 1999; accepted December 22, 1999.
Supported by NIH grant HL-13336-26 and grants from the Helen
Polonsky Research Fund and the Dorothy and Seymour Weinstein Research Fund.
Reprints: Simon Karpatkin, New York University Medical School,
550 First Ave, New York, NY 10016; e-mail:
simon.karpatkin{at}med.nyu.edu.
The publication costs of this
article were defrayed in part by
page charge payment. Therefore,
and solely to indicate this fact,
this article is hereby marked
"advertisement"
in accordance with 18 U.S.C.
section 1734.
Presented at the 40th annual meeting of the American Society of
Hematology, Miami, FL, December 1998.
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