
[Cancer Research 60, 2579-2583, May 15, 2000]
© 2000 American Association for Cancer Research
Influence of Nitric Oxide Synthase II Gene Disruption on Tumor Growth and Metastasis1
Qian Shi,
Qinghua Xiong,
Bailiang Wang,
Xiangdong Le,
Nasim A. Khan and
Keping Xie2
Departments of Gastrointestinal Medical Oncology and Digestive Diseases [Q. S., Q. X., B. W., X. L., N. A. K., K. X.] and Cancer Biology [K. X.], The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030
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ABSTRACT
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The relationship between nitric oxide (NO) synthase II (NOS II)
expression and the metastatic ability of tumor cells is inconclusive.
We determined the role of host NOS II expression in the growth and
metastasis of the B16-BL6 murine melanoma and M5076 murine ovarian
sarcoma cell lines. The cells were either s.c. or i.v. injected into
syngeneic wild-type (NOS II+/+) and NOS II-null (NOS
II-/-) C57BL/6 mice. Both cell lines produced slightly
larger s.c. tumors in NOS II-/- mice than in NOS
II+/+ mice. However, B16- BL6 cells produced more and
larger experimental lung metastases in NOS II+/+ mice than
in NOS II-/- mice, whereas M5076 cells produced fewer and
smaller experimental lung metastases in NOS II+/+ mice than
in NOS II-/- mice. After activation with IFN-
and
lipopolysaccharide, macrophages isolated from NOS II+/+
C57BL/6 mice produced NO-dependent cytotoxicity in sarcoma cells,
whereas macrophages from NOS II-/- C57BL/6 mice did not.
In contrast, activated macrophages produced little to no NO-mediated
cytotoxicity in melanoma cells. Immunostaining analyses indicated that
NOS II expression was apparent in the metastases growing in NOS
II+/+ mice and correlated with increased cell proliferation
in B16-BL6 lung metastases but with decreased cell proliferation in
M5076 liver metastases. Our data suggest that disruption of host NOS II
expression enhanced the growth and metastasis of NO-sensitive tumor
cells but suppressed the metastasis of NO-resistant tumor cells,
proposing that host-derived NO may differentially modulate tumor
progression.
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Introduction
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NO is a potent biological molecule that mediates a diverse
array of activities, including vasodilatation, neurotransmission, iron
metabolism, and immune defense (1)
. Increasing evidence
suggests that NO has a pleiotropic effect on diverse aspects of tumor
biology. For example, NO is a potential endogenous carcinogen because
it causes DNA damage (2
, 3)
. Also, increased
tumor-associated NO production may alter tumor blood supply by changing
vascular tone and/or formation, thereby influencing tumor progression
(4
, 5) . In addition, tumor-associated NO can be
contributed by tumor and/or host cells (e.g., macrophages)
that infiltrate tumors.
NOS3
II expression and NO production within tumor cells can directly or
indirectly influence the fate of the tumor cells themselves (3
, 5
, 6)
. For example, overproduction of endogenous NO is
autocytotoxic through the induction of apoptosis (6)
and
suppresses tumor growth and metastasis, whereas low production of NO
may protect tumor cells from apoptosis and promote tumor growth
(5)
. Expression of NOS II in tumor cells has also been
implicated as an important factor in cancer metastasis. Expression of
the NOS II gene inversely correlates with the metastatic
ability of human colon cancer (7)
and K-1735 murine
melanoma cells (8)
. Conversely, tumor-related NOS II
activity correlates with more advanced human tumors of the breast
(9)
and central nervous system (10)
. In fact,
NOS II expression directly correlates with the metastatic potential of
UV-2237 murine fibrosarcoma cells (11)
. Transfection
experiments have shown that overexpression of the NOS II
gene inhibits metastasis of human renal cell carcinoma, K-1735 murine
melanoma, and UV-2237 murine fibrosarcoma, in part by accelerating cell
death (6
, 11)
, whereas low expression of NOS II promotes
the growth of human colon cancer cells, although it is not clear
whether the metastatic ability of tumor cells is affected (5
, 12)
. In summary, the effects of endogenous NO on tumor cells are
output dependent and cell type specific (6)
, often
depending on the p53 functional status of the tumor cells exposed to NO
(3
, 11
, 13)
.
Quantitatively, the major source of tumor-associated NO may be host
cells (e.g., macrophages) that infiltrate the tumors
(1
, 6 , 14)
. It is known that both activated macrophages
and endothelial cells may produce cytotoxic levels of NO in
vitro. However, how this source of NO may influence metastasis is
unclear. Macrophage- and endothelium-derived NO may prevent tumor
growth and metastasis, presumably by killing tumor cells passing
through vascular lumens (6)
. Alternatively, macrophages
may promote tumor growth and metastasis by releasing NO, which is known
to induce immune suppression, vasodilatation, and angiogenesis
(1
, 15)
. In the present study, we found that B16-BL6 mouse
melanoma cells were resistant to cytotoxicity mediated by macrophages
derived from both NOS II+/+ and NOS
II-/- mice, whereas M5076 sarcoma cells were
highly sensitive to NO-dependent cytotoxicity mediated by macrophages
derived from NOS II+/+ mice. Consistent with
their differential sensitivity to NO-mediated cytotoxicity in
vitro, B16-BL6 cells produced more and larger metastases in
syngeneic NOS II+/+ mice than in NOS
II-/- mice, whereas M5076 cells produced fewer
and smaller metastases in syngeneic NOS II+/+
mice than in NOS II-/- mice. Therefore,
physiological expression of host NOS II appears to negatively regulate
the growth and metastasis of NO-sensitive tumor cells, whereas
NO-resistant tumor cells may escape from or even usurp this
physiological expression of host NOS II.
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Materials and Methods
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Reagents.
Eagles MEM, HBSS, and fetal bovine serum were purchased from M. A.
Bioproducts (Walkersville, MD). Mouse recombinant IFN-
(specific
activity, 1 x 107 units/mg
protein) was purchased from Genzyme (Cambridge, MA). Phenol-extracted
Salmonella LPS and AG were purchased from Sigma Chemical Co.
(St. Louis, MO). [3H]Thymidine (specific
activity, 2 Ci/mmol) was purchased from ICN Biomedicals, Inc. (Costa
Mesa, CA). All reagents used in tissue cultures were free of endotoxins
as determined using the Limulus amebocyte lysate assay
(sensitivity limit, 0.125 ng/ml), which was purchased from Associates
of Cape Cod (Woods Hole, MA).
Tumor Cell Lines and in Vitro Culture Conditions.
The B16-BL6 murine melanoma and M5076 murine ovarian sarcoma
cell lines were provided by Dr. Isaiah J. Fidler (The University of
Texas M. D. Anderson Cancer Center). The original M5076 cell line was
established in the laboratory of Dr. W. F. Duning (Papananicolaou
Cancer Research Institute, Miami, FL) and found to produce
organ-specific liver metastases after i.v. injection into syngeneic
C57BL/6 mice (16)
. The B16-BL6 line was established using
in vitro selection and was shown to be highly invasive and
to produce lung metastases after i.v. injection into syngeneic C57BL/6
mice (17)
. Both cell lines are reported to be
nonimmunogenic (18
, 19) . All tumor cell lines were
cultured in tissue culture in MEM supplemented with 10% fetal bovine
serum, sodium pyruvate, nonessential amino acids,
L-glutamine, and vitamins (CMEM; Flow
Laboratories, Rockville, MD). Cell cultures were maintained in plastic
flasks and incubated in 5% CO2-95% air at
37°C. Cultures were free of Mycoplasma.
Growth and Metastasis.
To prepare tumor cells for inoculation, cells in exponential
growth phase were harvested by brief exposure to a 0.25%
trypsin/0.02% EDTA solution (w/v). Cell viability was determined by
trypan blue exclusion, and only single-cell suspensions of >95%
viability were used. To determine tumorigenic ability, tumor cells were
injected s.c. into syngeneic NOS II+/+ or NOS
II-/- C57BL/6 mice (Jackson Laboratory, Bar
Harbor, ME). Latency of tumor formation and tumor diameters were
measured. To determine metastatic ability, 0.2 ml of the tumor cell
suspensions was injected into the lateral tail veins of unanesthetized
mice. The mice were killed 21 days after injection, and their pulmonary
and hepatic metastatic nodules were counted using a dissecting
microscope.
Determination of Nitrite Concentration.
NO production was determined by measuring nitrite accumulation
in culture supernatants using a microplate assay with Griess reagent,
as described previously (11)
. In brief, 50-µl culture
samples were harvested from conditioned medium and allowed to react
with an equal volume of Griess reagent (1.0% sulfanilamide, 0.1%
naphthylethylene diamine dihydrochloride, and 2.5%
H3PO4) at room temperature
for 10 min. The absorbance at 540 nm was monitored using a microplate
reader. The nitrite concentration was determined using sodium nitrite
as a standard.
Macrophage-mediated Cytotoxicity.
Mouse peritoneal exudate macrophages were collected by
peritoneal lavage from mice given an i.p. injection of 1.5 ml of
thioglycollate broth (Baltimore Biological Laboratories, Cockeysville,
MD) 3.5 days before harvesting (20)
. Purified cultures of
mouse macrophages were incubated at 37°C for 18 h with 0.2 ml of
medium alone or containing 10 units/ml IFN-
and 0.01 µg/ml LPS.
After incubation, the macrophage cultures were thoroughly washed, and
1 x 104
[3H]thymidine-labeled B16-BL6 and M5076 target
cells were added to achieve a population density of 2500 macrophages
and 250 tumor cells/mm2. At this population
density, untreated macrophages were not cytotoxic to tumor cells
(20)
. After a 48-h incubation, the cultures were washed
twice with PBS, and adherent viable cells were lysed with 0.1 ml of 0.1
N NaOH. The lysates were harvested using a Harvester 96
(Tomtec, Orange, CT) and counted using a liquid scintillation counter.
Maximal in vitro macrophage-mediated cytotoxicity in this
assay was obtained after 48 h of incubation with target cells,
assessed by measuring the release of radioactivity from DNA of target
cells as described previously (20)
, and calculated as
follows: cytotoxicity (%) = (A - B)/A x 100, where
A is the cpm in cultures of control macrophages and target
cells and B is the cpm in cultures of test macrophages and
target cells.
Cytotoxicity Mediated by Tumor Cells Producing NO.
C4.L8 cells that were stably transfected with a full-length NOS
II gene and constitutively produced NO or C4.S2 cells that were
stably transfected with a truncate NOS II gene and did not
produce NO (6)
were plated into 96-well plates
(2.5 x 104/well) and incubated
for 18 h in medium alone or with 2 mM AG (a
specific NOS II inhibitor).
[3H]Thymidine-labeled B16-BL6 or M5076 target
cells (1 x 104) were added in the
absence or presence of 2 mM AG. After a 48-h
incubation, cytotoxicity against B16-BL6 and M5076 cells was determined
as described above.
Immunohistochemistry.
Tissue sections (5-µm thick) of formalin-fixed, paraffin-embedded
lung and liver specimens were processed using a standard procedure.
Sections were stained for infiltration macrophages (F4/80 antibody;
Ref. 21
), NOS II (21)
, and PCNA
(22)
. The sections were also stained for apoptotic cells
using a terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick
end labeling (23)
. A positive reaction was indicated by a
reddish-brown precipitate in the cytoplasm (F4/80 and NOS II) or in the
nucleus (PCNA and terminal deoxynucleotidyl transferase-mediated
dUTP-biotin nick end labeling).
Statistical Analyses.
The in vitro data were analyzed for significance using
Students t test (two-tailed).
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Results and Discussion
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NO is a pleiotropic molecule; therefore, it is not surprising that
tumor-associated NO may have diverse effects on tumor progression
(1, 2, 3, 4, 5, 6)
. The apparently opposing roles of NO have been
attributed to many factors, including NOS isoforms, expression levels,
measurement methods, and heterogeneous tumor tissues or cell lines
(6)
. Moreover, tumor-associated NO represents a mixture of
both tumor and host infiltration cell-derived NOS activities (1
, 6
, 9
, 13) . The functional NOS II status of tumor-associated
macrophages or other infiltration cells may differ from tumor to tumor,
and different tumor cells apparently express different levels of NOS II
as well (6)
. Therefore, the ultimate effect of
tumor-associated NOS II activity on tumor growth and metastasis may be
dictated by multiple sources and levels of NOS II expression. In
general, macrophages produce much higher levels of NOS II expression
than do tumor cells or other host cells and thus become a dominant
source of NO production (1
, 6
, 9
, 11)
. To determine the
role of host NOS II in tumor growth and metastasis, we used the
well-characterized B16-BL6 melanoma and M5076 sarcoma cell lines, which
are syngeneic to C57BL/6 mice (16
, 17)
.
In the first set of experiments, we measured the growth and metastasis
of both B16-BL6 melanoma and M5076 sarcoma cells in syngeneic NOS
II+/+ and NOS II-/-
C57BL/6 mice. B16-BL6 or M5076 cells (5 x 105 cells/mouse) were injected s.c. into the
mice. The average latency period for B16-BL6 and M5076 cells was 9 and
12 days, respectively, in NOS II+/+ C57BL/6 mice
but 7 and 10 days, respectively, in NOS II-/-
C57BL/6 mice. Consistent with the slightly earlier tumor onset in NOS
II-/- C57BL/6 mice, the average tumor size was
also slightly larger in NOS II-/- C57BL/6 mice
than in NOS II+/+ C57BL/6 mice (Table 1)
. However, the difference in both tumor latency and tumor size was not
statistically significant.
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Table 1 Tumor growth and metastasis of B16-BL6 murine melanoma and M5076 murine
ovarian sarcoma cells in syngeneic mice with or without functional NOS
II gene
B16-BL6 cells (5 x 104 cells/mouse) were
injected s.c. or i.v. into groups of C57BL/6 mice
(n = 10); M5076 cells were injected s.c.
(5 x 105 cells/mouse) or i.v. (5 x 103 cells/mouse) into groups of C57BL/6 mice
(n = 10). Tumor diameters were measured 15
days after tumor injection. Number and size of lung metastases
(B16-BL6) and liver metastases (M5076) were determined 21 days after
tumor injection. The experiment was repeated one time and similar
results were obtained.
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To determine the extent of experimental lung metastasis, B16-BL6 cells
(5.0 x 104 cells/mouse) and M5076
cells (5.0 x 103
cells/mouse)
were injected i.v. into syngeneic NOS II+/+ and
NOS II-/- C57BL/6 mice. As shown in Table 1
,
B16-BL6 cells produced fewer and smaller experimental lung metastases
in NOS II-/- C57BL/6 mice than in NOS
II+/+ C57BL/6 mice. In sharp contrast, M5076
cells produced more and larger experimental liver metastases in NOS
II-/- C57BL/6 mice than in NOS
II+/+ C57BL/6 mice. Therefore, we found that
disruption of the host NOS II gene differentially affected
the growth and metastasis of B16-BL6 and M5076 cells.
Because host macrophages may be the major source of host cell-derived
NOS II activity in the tumor (1
, 6
, 9)
, we sought to
determine the role of macrophage NOS II disruption in altered tumor
growth and metastasis. Early work suggested that NO derived from
macrophages produced cytostasis in target tumor cells
(14)
. Recent studies have shown that overactivated
macrophages can also lead to NO-dependent cytolysis of B16-F10 murine
melanoma cells (20)
. Additionally, in cell coculture
systems, macrophage-derived NO has induced apoptosis in P815 cells
(24)
and Meth A tumor cells (25)
. Similarly,
activated microvessel endothelial cells can also produce NO-dependent
lysis of tumor cells (26)
. This has been further supported
by results of several recent studies. For example,
LPS/cytokine-activated endothelial cells were shown to express NOS II
and produce NO, which lead to thymocyte apoptosis (27)
.
Similarly, human erythroleukemic K562 cells were shown to undergo
apoptosis after coculture with rodent vascular smooth muscle cells or
endothelial cells expressing NOS II and producing NO in the presence of
IFN-
and tumor necrosis factor-
(28)
. Indeed,
increased production of NO by murine, rat, and human NK cells has been
shown to be responsible, at least in part, for destruction of target
cells by NK cells and NK cell-mediated DNA fragmentation and cell lysis
(6)
.
In the present study, the tumoricidal activity of macrophages obtained
from the peritoneal cavity of NOS II+/+ and NOS
II-/- C57BL/6 mice was determined. Purified
macrophages obtained from NOS II+/+ and NOS
II-/- mice were pretreated with 10 units/ml
IFN-
and 0.01 µg/ml LPS for 18 h, as reported previously
(20)
. The [3H]thymidine-labeled
melanoma and sarcoma cells were then added and incubated for another
48 h in the presence or absence of 2 mM AG.
Cytotoxicity was determined by measuring
[3H]thymidine release. The treatment induced NO
production in NOS+/+ macrophages, which was
inhibited by AG, whereas it did not induce NO production in
NOS-/- macrophages (Fig. 1A
). The preactivated NOS II+/+
macrophages induced significant cytotoxicity in M5076 sarcoma cells
(Fig. 1B
) but not in B16-BL6 melanoma cells (Fig. 1C
), whereas macrophages without activation did not (Fig. 1A
). The cytotoxicity of M5076 cells was associated with the
induction of NO (Fig. 1B
). Both NO production and
cytotoxicity were totally inhibited by the addition of the specific NOS
II inhibitor AG, suggesting that activated macrophages mediated
NO-dependent cytotoxicity. In sharp contrast, the same treatment did
not induce NO production and cytotoxicity by NOS
II-/- macrophages (Fig. 1, B and C
). Similar results were obtained using macrophages isolated
from lung and liver tissues (data not shown). These data clearly
indicate that disruption of NOS II impairs the antitumor ability of
macrophages, which may in part be responsible for the enhanced growth
and metastasis of M5076 sarcoma cells and conversely for the suppressed
metastasis of B16-BL6 melanoma cells. Whether NOS II disruption also
affects the functions of other effector cells (e.g.,
vascular endothelial and NK cells) remains to be determined.
To further confirm the different sensitivity of tumor cell lines to
NO-mediated cytotoxicity, both
[3H]thymidine-labeled B16-BL6 and M5076 cells
were coincubated for 48 h in the presence or absence of 2
mM AG with C4.L8 cells that were stably transfected with a
full-length NOS II gene and constitutively produced NO or
C4.S2 cells that were stably transfected with a truncate NO
II gene and did not produce NO (Fig. 2A
; Ref. 6
). Cytotoxicity was determined as
described above. As shown in Fig. 2B
, significant
cytotoxicity was produced in M5076 cells and was completely inhibited
by AG, whereas no cytotoxicity was produced in B16-BL6 cells.
Furthermore, both B16-BL6 and M5076 cells were incubated for 24 h
with different concentrations of a NO donor, SNAP. M5076 cells were
highly sensitive to SNAP, whereas B16-BL6 were not (Fig. 2C
). Similar results were also obtained using other NO
donors (e.g., sodium nitroprusside, GEA5024, and NOC-12;
data not shown). These data clearly show that these cell lines have a
different intrinsic sensitivity to NO-mediated cytotoxicity. Whether
the cellular functional status, such as that of the p53
tumor suppressor gene, plays a role is currently under investigation.

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Fig. 2. Demonstration of NO sensitivity. A, C4.S2
and C4.L8 cells were plated into 96-well plates (2.5 x 104/well) and incubated for 18 h with medium alone
( ) or with 2 mM AG ( ), and NO production was
determined by measuring nitrite accumulation in the culture
supernatant. [3H]Thymidine-labeled B16-BL6 or M5076
target cells (1 x 104) were added in the
absence ( ) or presence of 2 mM AG ( ).
Bars, SD. B, after a 48-h incubation,
cytotoxicity against B16-BL6 and M5076 cells was determined as
described in "Materials and Methods." Bars, SD.
C, B16-BL6 and M5076 cells were seeded into 96-well
plates (2.5 x 104/well) and incubated for
18 h. SNAP was then added at final concentrations of 0, 0.05, 0.1,
0.2, 0.4, 0.8, and 1.6 mM. Cytotoxicity was determined
24 h after SNAP addition using
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay as
described previously (8
, 11)
. The experiments were
repeated one time, and similar results were obtained. *,
P < 0.01.
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To provide evidence that host NOS II expression differentially affect
the fate of injected tumor cells, immunostaining was performed on tumor
sections. Macrophage infiltration was apparent in the B16-BL6 lung
metastases in both NOS II+/+ and NOS
II-/- mice (Fig. 3, A and B
). NOS II expression was only detected in
B16-BL6 lung metastases and M5076 liver metastases in NOS
II+/+ mice (Fig. 3, B and G
) but not in NOS II-/- mice (Fig. 3, E and J
), suggesting that NOS II was mainly
expressed in the host infiltration cells. The decreased NOS II
expression was correlated with decreased cell proliferation in B16-BL6
lung metastases (Fig. 3
, F versus C). In contrast, decreased
NOS II expression was correlated with increased cell proliferation
(Fig. 3
, K versus H) and decreased apoptosis (Fig. 3
,
L versus I) in M5076 liver metastases. These data suggested
that host NOS II expression differentially affects the survival and/or
proliferation of tumor cells. It is likely that NOS II disruption and
then decreased physiological level of NO production lead to decreased
blood supply to the growing metastases of B16-BL6 cells resistant to
NO-mediated cytotoxicity (3
, 5)
. To that end, we are
currently investigating the effect of NOS II disruption on B16-BL6
melanoma angiogenesis and the role of organ-specific tumor-host
interaction in NOS II expression and tumor cell survival.

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Fig. 3. Immunohistochemistry. Both B16-BL6 and M5076 cells
(5 x 104 cells/mouse) were injected i.v.
into NOS II+/+ (AC and
GI) or NOS II-/- (DF
and JL) C57BL/6 mice. Lungs and livers were harvested
and processed for staining for macrophages (A and
D), NOS II (B, E, G, and
J), PCNA (C, F, H, and K),
and apoptotic cells (I and L).
Arrowheads, tumor lesions.
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In summary, we found that disruption of the host NOS II gene
differentially affects the metastasis of tumor cells, depending on
their sensitivity to NO-mediated cytotoxicity. NOS II disruption
enhanced the metastasis of NO-sensitive M5076 murine ovarian sarcoma
cells, whereas it suppressed the metastasis of NO-resistant B16-BL6
murine melanoma cells. These data suggest that physiological expression
of the host NOS II gene may differentially modulate tumor
progression (3
, 5
, 6)
. However, it remains to be
determined whether disruption of host NOS II gene expression
influences tumor growth and metastasis by other mechanisms in addition
to impaired production of cytotoxic effectors, such as angiogenesis
(3, 4, 5, 6)
. Collectively, we have demonstrated that host
NOS II gene expression regulates tumor growth and
metastasis, suggesting that the modulation of host NOS II
gene expression can be manipulated to control tumor growth and
metastasis.
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Acknowledgments
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We thank Don Norwood for editorial comments, Judy King for
assistance in the preparation of the manuscript, and the reviewers
highly constructive suggestions.
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FOOTNOTES
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 Supported in part by the University Startup Fund
and Cancer Center Support Core Grant CA 16672 from the NIH (to
K. X.). 
2 To whom requests for reprints should be
addressed, at the Department of Gastrointestinal Medical Oncology and
Digestive Diseases, Box 78, The University of Texas M. D. Anderson
Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030. Phone:
(713) 792-2013; Fax: (713) 745-1163; E-mail: kepxie{at}mail.mdanderson.org 
3 The abbreviations used are: NOS, NO synthase;
AG, aminoguanidine; PCNA, proliferating cell nuclear antigen; LPS,
lipopolysaccharide; NK, natural killer; SNAP,
S-nitroso-N-acetyl-D,L-penicillamine. 
Received 10/29/99.
Accepted 3/31/00.
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