NIH Public Access
Author Manuscript
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
NIH-PA Author Manuscript
Published in final edited form as:
Alcohol Clin Exp Res. 2009 July ; 33(7): 1220–1230. doi:10.1111/j.1530-0277.2009.00946.x.
Nitric oxide mediated intestinal injury is required for alcoholinduced gut leakiness and liver damage
Yueming Tang1, Christopher B. Forsyth1,5, Ashkan Farhadi1,3, Jayanthi Rangan1, Shriram
Jakate4, Maliha Shaikh1, Ali Banan1,2, Jeremy Z. Fields1, and Ali Keshavarzian1,2,3
1 Department of Internal Medicine (Division of Digestive Diseases and Nutrition), Rush University,
Chicago IL
NIH-PA Author Manuscript
2
Department of Pharmacology, Rush University, Chicago IL
3
Department of Molecular Biophysics and Physiology, Rush University, Chicago IL
4
Department of Pathology, Rush University, Chicago IL
5
Department of Biochemistry, Rush University, Chicago IL
Abstract
Background—Alcoholic liver disease (ALD) requires endotoxemia and is commonly associated
with intestinal barrier leakiness. Using monolayers of intestinal epithelial cells as an in vitro
barrier model, we showed that ethanol-induced intestinal barrier disruption is mediated by iNOS
(inducible nitric-oxide synthase) upregulation, NO (nitric oxide) overproduction, and oxidation/
nitration of cytoskeletal proteins. We hypothesized that iNOS inhibitors (L-NAME, L-NIL) in
vivo will inhibit the above cascade and liver injury in an animal model of alcoholic steatohepatitis
(ASH).
Methods—Male Sprague-Dawley rats were gavaged daily with alcohol (6 g/kg/day) or dextrose
for 10 weeks ± L-NAME, L-NIL or vehicle. Systemic and intestinal NO levels were measured by
nitrites and nitrates in urine and tissue samples, oxidative damage to the intestinal mucosa by
protein carbonyl and nitrotyrosine, intestinal permeability by urinary sugar tests, and liver injury
by histological inflammation scores, liver fat, and myeloperoxidase activity.
NIH-PA Author Manuscript
Results—Alcohol caused tissue oxidation, gut leakiness, endotoxemia and ASH. L-NIL and LNAME, but not the D-enantiomers, attenuated all steps in the alcohol-induced cascade including
NO overproduction, oxidative tissue damage, gut leakiness, endotoxemia, hepatic inflammation
and liver injury.
Conclusions—The mechanism we reported for alcohol-induced intestinal barrier disruption in
vitro – NO overproduction, oxidative tissue damage, leaky gut, endotoxemia and liver injury –
appears to be relevant in vivo in an animal model of alcohol-induced liver injury. That iNOS
inhibitors attenuated all steps of this cascade suggests that prevention of this cascade in alcoholics
will protect the liver against the injurious effects of chronic alcohol and that iNOS may be a useful
target for prevention of ALD.
Keywords
intestinal hyperpermeability; inducible nitric-oxide synthase (iNOS); L-NIL; oxidative stress;
endotoxemia; alcoholic liver disease
Corresponding Author: Ali Keshavarzian, M.D., Professor of Medicine, Professor of Pharmacology and Molecular Biophysics and
Physiology, Director, Digestive Diseases and Nutrition, Rush University Medical Center, 1725 W. Harrison, Suite 206, Chicago, IL
60612, Phone - 312-563-3890, Fax - 312-563-3883, Ali_Keshavarzian@rush.edu.
Tang et al.
Page 2
Introduction
NIH-PA Author Manuscript
The intestinal epithelium is a highly selective barrier that permits the absorption of nutrients
from the gut lumen into the circulation, but, normally, restricts the passage of harmful and
potentially toxic compounds such as products of the luminal microbiota (Clayburgh et al.,
2004; Hollander, 1992; Keshavarzian et al., 1999). Disruption of intestinal barrier integrity
(leaky gut) may lead to the penetration of luminal bacterial products such as endotoxin, into
the mucosa and then into the systemic circulation and initiate local inflammatory processes
in the intestine and even in distant organs (Clayburgh et al., 2004; Hollander, 1992;
Keshavarzian et al., 1999). Indeed, disrupted intestinal barrier integrity has been implicated
in a wide range of illnesses such as inflammatory bowel disease, systemic disease such as
cancer, and even hepatic encephalopathy (Clayburgh et al., 2004; Hollander, 1992;
Keshavarzian et al., 2001; Keshavarzian and Fields, 2003; Keshavarzian et al., 1994;
Keshavarzian et al., 1999; Mathurin et al., 2000; Sawada et al., 2003; Turner et al., 1997).
NIH-PA Author Manuscript
Several studies, including our own, indicate that EtOH disrupts the functional and structural
integrity of intestinal epithelial cells and results in hyperpermeability of intestinal cell
monolayers and gut leakiness (Banan et al., 1999; Banan et al., 2000; Banan et al., 2001;
Keshavarzian et al., 2001; Keshavarzian and Fields, 2000; Keshavarzian and Fields, 2003;
Keshavarzian et al., 1994; Keshavarzian et al., 1999; Keshavarzian et al., 1996; Robinson et
al., 1981; Tang et al., 2008).
We also found, using monolayers of Caco-2 cells as an in vitro model of gut barrier
function, that oxidative stress plays an important role in EtOH-induced loss of intestinal
barrier integrity (Banan et al., 2000; Banan et al., 2001; Banan et al., 2007).
NIH-PA Author Manuscript
One endogenous oxidant in particular, nitric Oxide (NO), appeared to be involved. At
normal levels, NO is a key mediator of intestinal cell and barrier function (Alican and
Kubes, 1996; Kubes, 1992; Lopez-Belmonte and Whittle, 1994; Unno et al., 1996; Unno et
al., 1997a; Unno et al., 1995). When NO is present in excess, however, the result is barrier
dysfunction (Colgan, 1998; Invernizzi et al., 1997; Unno et al., 1997b) including EtOHinduced barrier dysfunction (Banan et al., 1999; Banan et al., 2000). Many studies (Chow et
al., 1998; Greenberg et al., 1994; Lancaster, 1992; Sisson, 1995) found that chronic EtOH
raises NO levels and that EtOH-induced cytotoxicity is mediated via excess levels of NO
and its metabolite, peroxynitrite (ONOO−). Our previous studies (Banan et al., 1999; Banan
et al., 2000) showed that EtOH upregulates iNOS and increases NO and ONOO− in Caco-2
cells. Because monolayers of these intestinal epithelial cells constitute a model of the gut
barrier, our in vitro data suggest that the main mechanism by which NO overproduction
induces intestinal barrier dysfunction is oxidation and nitration of cytoskeletal proteins
(Banan et al., 1999; Banan et al., 2000). However, this mechanism, which involves
excessive NO signaling, needs to be investigated in vivo.
Accordingly, we hypothesized that inhibition of iNOS activity will prevent EtOH-induced
intestinal barrier dysfunction in an animal model of alcoholic steatohepatitis (ASH), and will
do so by inhibiting EtOH-induced production of excess NO and the oxidative injury to the
intestinal epithelium that ensues. To test this hypothesis, and to study the role of iNOS in
EtOH-induced oxidative injury, gut leakiness, and liver damage in an animal model of ASH,
we used a nonselective inhibitor (NG-nitro-L-arginine methyl ester, L-NAME) and a
selective inhibitor (L-N6-(1-iminoethyl)-lysine, L-NIL) of iNOS, which have been studied in
different models of intestinal inflammation (Kawachi et al., 1999; Krieglstein et al., 2001;
Obermeier et al., 1999).
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 3
Materials and Methods
Animal Subjects
NIH-PA Author Manuscript
Male Sprague-Dawley rats (250–300 g at intake) were obtained from Harlan (Indianapolis,
IN). During experiments, each rat was given either alcohol or an isocaloric amount of
dextrose in a liquid diet intragastrically 2 times each day. The ethanol dose was gradually
increased every 2 to 3 days from 2 g/kg/day to a maximum of 6 g/kg/day by 2 weeks. Study
Day 1 was defined as the first day rats received 6 g/kg/day alcohol. The dextrose dose for
control rats was isocaloric to the amount of EtOH given. L-NAME (20 mg/kg) or D-NAME
(20 mg/kg) or L-NIL (20 mg/kg) or D-NIL (20 mg/kg) were given by gastric gavage in two,
equally divided portions to both dextrose and alcohol fed rats. D-NAME and D-NIL are the
inactive enantiomers of L-NAME and L-NIL and were used as controls. Doses were chosen
from either the literature (Kawachi et al., 1999; Krieglstein et al., 2001; Obermeier et al.,
1999) or from our preliminary data. All agents were purchased from Sigma (St. Louis, MO).
Rats received chow ad lib and were weighed daily. Intestinal permeability was measured
just before sacrifice. Sacrifice was done by CO2 inhalation, followed immediately by cardiac
puncture (for blood draw) and harvesting of intestine and liver. All animal protocols and
practices were reviewed and approved in advance by the Rush University Institutional
Animal Care and Use Committee.
NIH-PA Author Manuscript
Intestinal permeability
We used an oral sugar test to assess intestinal permeability as we described (Farhadi et al.,
2006; Farhadi et al., 2003; Keshavarzian et al., 2001). After an 8 h fast, rats were given,
intragastrically, 2.0 ml of a solution containing 107 mg/kg lactulose, 30 mg/kg mannitol, 15
mg/kg sucralose, and 570 mg/kg sucrose. Rats were housed individually in metabolic cages
and urine samples were collected over 5 h. To promote urine output, each rat was
subcutaneously injected with 10 ml of lactated Ringer’s solution, just prior to sugar
administration. Urinary sugar levels were measured by gas chromatography as we reported
(Farhadi et al., 2006; Farhadi et al., 2003; Keshavarzian et al., 2001).
Plasma Endotoxin
Blood was collected from rats at the time of sacrifice. Serum samples were then analyzed for
endotoxin by a kit (Kinetic-QLC; Whittaker Bioproducts) following the protocol from the
manufacturer.
Liver Injury
a.
NIH-PA Author Manuscript
Liver Histology. Formalin fixed liver tissues were stained with H&E. Blinded
assessment was done by a GI pathologist (SJ). Histological slides were assessed for
liver disease by grading steatosis, necrosis, inflammation & fibrosis (Keshavarzian
et al., 2001). Our scoring system was based on one widely used for the scoring of
liver disease (Kleiner et al., 2005). Severity of steatosis (% of liver cells containing
fat) was scored 1 to 4, corresponding to the fraction of liver cells with fat. These
were, respectively, <25%, 26–50%, 51–75%, and >75%. Necrosis was quantified
as the number of necrotic foci/mm2 and number of Councilman Bodies per high
power field (HPF) and severity of necrosis was graded on a scale of 0–4. Severity
of inflammation was graded on a scale of 0–4 based on the extent [number of
inflammatory foci per HPF and number of HPF with inflammatory foci] and
density of inflammatory foci (from a few inflammatory cells to a dense
inflammatory infiltrate). Severity of fibrosis was scored on a scale of 0 to 4 using
trichrome staining. However, the fibrosis score in all rats was zero and thus fibrosis
stage was not included in the histology score. A total histological score (including
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 4
NIH-PA Author Manuscript
fatty liver grade) and necroinflammatory score (inflammation and necrosis score
representing the presence and severity of steatohepatitis) was then calculated. The
maximum histology score and necroinflammatory scores were 12 and 8
respectively. When assessing slides for pathology studies, at least 3 different
sections were studied for each rat. Alcoholic steatohepatitis (ASH) was defined as
the presence in the liver of inflammatory cell infiltration, spotty necrosis, and liver
cell necrosis.
b. Liver Fat Content. Total liver fat was measured gravimetrically as previously
described (Folch et al., 1957) and used by us (Keshavarzian et al., 2001).
Serum alanine aminotransferase (ALT)—ALT was measured in blinded serum
samples by the Rush University Medical Center clinical laboratories and the data provided
as Units/dl ± S.E.
NO Levels in Intestinal Mucosa and Urine—Nitrite and nitrate in urine and tissue
samples were measured (umol/ml or umol/mg) using Nitrate/Nitrite Colorimetric Assay Kits
(Cayman Chemical, Ann Arbor, MI).
Quantitative Slot-Immunoblotting for oxidation (carbonylation) and nitration
(nitrotyrosination)
NIH-PA Author Manuscript
Oxidation and nitration of mucosal proteins were assessed by measuring protein carbonyl
and protein nitrotyrosine formation using a slot-blotting method we previously described
(Banan et al., 2004; Keshavarzian and Fields, 2003).
Data and Statistical Analysis—Data are presented as mean ± S.E. For parametric
analyses of two groups, we used Student’s t-test; ANOVA was used when we compared >2
groups. Least Standard Deviation (LSD) was used for post-hoc analysis and a paired t-test
was used for comparison of paired data such as data on intestinal permeability. P< 0.05 was
regarded as significant.
Results
EtOH-induced NO2/NO3 overproduction in urine and colonic mucosa was prevented by
iNOS inhibitors
NIH-PA Author Manuscript
To confirm, in an in vivo model, our in vitro data that EtOH induces iNOS activation and
oxidative stress (Banan et al., 1999; Banan et al., 2000; Banan et al., 2001), we measured
total NO levels in the urine from dextrose-fed and EtOH-fed rats at week 10. Total NO
production in urine was increased 8.5 fold from 81 ± 11 to 690 ± 75 μmol/mg (p<0.05) in
EtOH fed rats compared with dextrose fed control rats (Fig. 1a). The EtOH-induced increase
in NO was inhibited in rats that were fed, in addition to EtOH, a nonselective iNOS
inhibitor, L-NAME (46% decrease to 410 ± 35, p<0.05, Fig. 1a) or a selective iNOS
inhibitor, L-NIL (98% decrease to 90 ± 8, p<0.05, Fig. 1a). D-NAME and D-NIL had no
effect on EtOH-induced NO production.
To determine if EtOH not only affects systemic NO production, but also affects NO
production in the intestinal epithelium, we measured NO production in the colonic mucosa
in our rats. EtOH induced a 4.1 fold increase in NO production in colonic mucosa (from 850
± 89 to 3548 ± 350 μmol/mg; p<0.05) (EtOH-fed rats compared with dextrose-fed control
rats) (Fig. 1b). The EtOH-induced increase in colonic tissue NO was inhibited by a
nonselective iNOS inhibitor, L-NAME (64% decrease to 1800 ± 160, p<0.05, Fig. 1b) or a
selective iNOS inhibitor, L-NIL (97% decrease to 910 ± 98, p<0.05, Fig. 1b).
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 5
iNOS inhibitors prevented EtOH-induced increases in oxidative tissue damage in intestinal
tissues
NIH-PA Author Manuscript
Our in vitro data suggested that EtOH-induced NO overproduction was required for EtOHinduced oxidative damage in monolayers of intestinal epithelial cells (Banan et al., 1999;
Banan et al., 2000; Banan et al., 2001). Because daily chronic alcohol feeding induced NO
overproduction in intestinal tissues in our rats, we determined whether alcohol also causes
oxidative damage in intestinal tissues and whether an iNOS inhibitor can prevent oxidative
tissue damage. To measure changes in oxidative tissue damage, we used the same markers
in vivo that we had in vitro – protein-nitrotyrosines and protein-carbonyls. The abundance of
nitrotyrosine and carbonyl epitopes were determined by slot blot immunostaining and
quantitative densitometry in tissue samples obtained after sacrifice from rat duodenum,
jejunum, ileum, and colon mucosa. Chronic alcohol feeding markedly elevated tissue protein
nitration (nitrotyrosine levels; Fig. 2) (e.g., 4.6 fold in colon) and protein oxidation (protein
carbonyl levels; Fig. 3) (e.g., 4.3 fold in colon) in intestine of alcohol-fed rats. iNOS
inhibitors significantly inhibited this effect of EtOH (Fig. 2 and Fig. 3). For example, in
colon, L-NAME inhibited EtOH-induced increases in nitrotyrosines by 58% and carbonyls
by 53%; for L-NIL, the corresponding inhibitions were 63% and 72%.
iNOS inhibitors prevented intestinal hyperpermeability induced by chronic alcohol
consumption
NIH-PA Author Manuscript
To determine if EtOH-induced oxidative damage contributes to EtOH-induced disruption of
the intestinal barrier and if iNOS inhibitors can prevent EtOH-induced hyperpermeability by
blocking EtOH-induced increases in oxidative damage, we measured intestinal permeability
in rats using an oral sugar test. Daily alcohol feeding for 10 weeks disrupted intestinal
barrier integrity in rats (Fig. 4). Urinary lactulose (an index of small bowel permeability)
was significantly higher (~7 fold) in alcohol-fed rats than in dextrose fed rats (controls) (Fig.
4a, p<0.05). Urinary sucralose (an index of whole gut [small bowel + large bowel]
permeability) was also significantly increased (~5 fold) in alcohol-fed rats (Fig. 4b, p<0.05).
iNOS inhibitors significantly inhibited alcohol-induced gut leakiness and markedly
decreased urinary lactulose and sucralose in alcohol fed rats (Fig. 4, p<0.05). L-NAME
caused 83% and 100% inhibition of the EtOH-induced increase in urinary lactulose and
sucralose, respectively; for L-NIL, the corresponding inhibitions were 100% and 80%.
iNOS inhibitors prevented EtOH-induced increases in serum endotoxin
NIH-PA Author Manuscript
Since leaky gut can result in endotoxemia, we determined whether iNOS inhibitors also
prevent EtOH-induced increases in serum endotoxin. Feeding of alcohol to rats for 10 weeks
significantly increased serum endotoxin levels 7 fold, from 0.08 ± 0.02 to 0.58 ± 0.11 EU/ml
(Fig. 5, p<0.05). L-NAME reduced this increase by 46%) to 0.35 ± 0.02 EU/ml in alcohol
fed rats (Fig. 5, p<0.05); L-NIL reduced the increase by 84% to 0.16 ± 0.02 EU/ml. Both
agents thus attenuated alcohol-induced endotoxemia.
iNOS inhibitors prevented EtOH-induced increases in inflammatory damage and in fat
content in liver
Because EtOH-induced endotoxemia is an important factor in development of ALD (CriadoJimenez et al., 1995; Hunt and Goldin, 1992; McClain and Cohen, 1989; Purohit et al.,
2008), we determined whether iNOS inhibitors prevent EtOH-induced liver damage. To
study the effect of iNOS inhibitors on EtOH-induced steatohepatitis, we measured hepatic
MPO activity, fat content and serum ALT and also assessed liver tissue histologically in rats
fed EtOH with or without iNOS inhibitors.
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 6
NIH-PA Author Manuscript
Feeding of alcohol to rats for 10 weeks increased MPO activity about 18 fold from 2.1 ±
0.03 to 38 ± 4.11 Units/mg tissue (Fig. 6a, p<0.05). iNOS inhibitors significantly decreased
hepatic MPO activity in alcohol fed rats. For L-NIL, the inhibition was 92% (to 5.1 ± 0.7
Units/mg tissue); for L-NAME, inhibition was 86% (7.2 ± 0.6; Fig. 6a, p<0.05).
Chronic EtOH consumption by rats significantly increased inflammatory injury to the liver;
the inflammatory score increased from zero to 2.5+0.31 (Fig. 6b). iNOS inhibitors (L-NIL;
L-NAME) reduced the EtOH-induced increase in inflammatory score to zero (100%
inhibition; Fig. 6b; p<0.05). Histological scores were assessed for liver injury by grading
steatosis, necrosis, inflammation & fibrosis. EtOH increased histology scores from zero to
11 ± 1.2 (Fig. 6c). iNOS inhibitors significantly reduced the EtOH-induced increase in
histology score. L-NIL reduced it to 2.1 ± 0.3 (−81%); L-NAME reduced it to 3.1 ± 0.4
(−72%; Fig. 6c).
EtOH increased liver fat content (~2 fold), from 4.1 ± 0.5 to 9.2 ± 1.2 (Fig. 6d). iNOS
inhibitors reduced fat content to 4.7 ± 0.5 (−88%) and 5.6 ± 0.6 (−71%) for L-NIL and LNAME, respectively (Fig. 6d).
NIH-PA Author Manuscript
To further study the effect of iNOS inhibitors on the severity of alcoholic steatohepatitis, we
measured serum ALT levels in rats fed EtOH with or without iNOS inhibitors. Chronic
EtOH consumption by rats significantly increased ALT from 85 ± 20 to 152 ± 25 U/L (Fig.
7). iNOS inhibitors significantly reduced serum ALT levels to 91 ± 15 U/L and 96 ± 16 U/L
for L-NIL and L-NAME, respectively (Fig. 7, p<0.05).
Comparison of the effect of specific iNOS inhibition by L-NIL and total inhibition of NOS by
L-NAME on intestinal barrier function and severity of steatohepatitis
The basal level of NO in the urine of dextrose fed rats was 81 ± 11 μmol/mg. Inhibition of
NOS by a non-specific NOS inhibitor, L-NAME, significantly decreased total urinary NO
levels in dextrose fed rats (52 ± 8 μmol/mg, p<0.05 compared to vehicle treated rats);
inhibition by the specific iNOS inhibitor L-NIL had no significant effect (78 ± 10 μmol/mg).
This indicates that iNOS does not significantly contribute to NO production in control rats.
In contrast, iNOS appears to be the key source of the EtOH-induced increase in NO levels
because, in alcohol fed rats, iNOS inhibition by L-NIL significantly decreased NO levels in
both urinary and colonic mucosa. Both L-NAME and L-NIL decreased NO levels in the
urine and in colonic mucosa of alcohol fed rats. However, the inhibitory effect of L-NIL was
significantly greater than L-NAME (98% vs 46% for urine NO; 97% vs 64% for colonic
mucosa NO; p<0.05, Fig. 1).
NIH-PA Author Manuscript
Discussion
Because alcoholic liver disease (ALD), including alcoholic steatohepatitis (ASH), has high
morbidity and mortality but no satisfactory therapy (Burbige et al., 1984; Galambos, 1972;
Grant et al., 1988; Maher, 2002; O’Connor and Schottenfeld, 1998), our broad long-term
research objective has been to develop effective therapies to prevent or reverse liver disease
in alcoholics. Since endotoxemia is an essential co-factor for the development of ALD
(Criado-Jimenez et al., 1995; Hunt and Goldin, 1992; McClain and Cohen, 1989; Purohit et
al., 2008), we traced the origins of endotoxemia back to leakiness of the intestinal barrier –
alcoholics with liver disease have more gut leakiness than alcoholics without liver disease
(Keshavarzian et al., 1999). However, it is not yet established that endotoxemia is a
prerequisite for development of ASH. Moreover, the mechanism by which EtOH causes
intestinal barrier hyperpermeability is still not clear.
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 7
NIH-PA Author Manuscript
To answer these key questions regarding mechanisms, we turned to an animal model of
alcoholic steatohepatitis. Our experimental model of ASH, which involves daily
administration of EtOH chronically (10 to 12 weeks) to rats by gavage, has been validated in
our previous studies (Keshavarzian et al., 2001). In those studies we showed that alcoholic
steatohepatitis can be prevented by protecting intestinal barrier integrity from disruption
(Keshavarzian et al., 2001). Recently, to determine whether endotoxemia occurs prior to
development of ASH and whether gut leakiness and endotoxemia are one of the key cofactors for development of alcoholic steatohepatitis, we studied time courses for
development of gut hyperpermeability, endotoxemia, and liver injury and showed that gut
leakiness and endotoxemia occurred several weeks prior to development of ASH
(Keshavarzian A et al., 2009). These data show that gut leakiness and endotoxemia cannot
be the consequence of ALD and support the notion that gut leakiness causes endotoxemia,
which leads to alcoholic steatohepatitis and serious ALD.
NIH-PA Author Manuscript
To determine the mechanism of alcohol-induced disruption of intestinal barrier integrity, we
first turned to a well-established in vitro model of intestinal barrier function – monolayers of
intestinal (Caco-2) cells. We demonstrated that alcohol-induced overproduction of NO and
the attendant oxidative injury to key proteins, were necessary for dysregulation of the
monolayer barrier and barrier hyperpermeability (Banan et al., 2000; Banan et al., 2001;
Banan et al., 2007; Forsyth et al., 2007). In the current study, we extended our studies of this
mechanism of the effects of EtOH on the intestinal tract to the in vivo situation. Using our
animal model of ASH, we confirmed the mechanism suggested by our in vitro findings. We
showed that chronic daily alcohol feeding of rats for 10 weeks causes overproduction of
nitric oxide (NO) that in turn results in (i) oxidative tissue damage to the intestinal mucosa
and (ii) intestinal hyperpermeability. EtOH-induced leaky gut was associated with
endotoxemia and hepatic inflammation and liver cell injuries in our animal model of
alcoholic steatohepatitis. More importantly, iNOS inhibitors reduced these effects of EtOH
and prevent the downstream cascade of injurious events in the intestine and the liver that
would have otherwise occurred (Fig. 8). We thus established the relevance of the in vitro
mechanism we observed in monolayers of intestinal cells to the in vivo situation. Our
findings could be clinically useful because inhibiting iNOS activation may become a novel
and effective therapeutic strategy for preventing and/or treating ALD. This, of course, would
require a study showing that iNOS inhibitors in ALD patients prevent EtOH-induced
oxidative tissue damage to the intestines, prevent the hyperpermeability, and prevent the
subsequent endotoxemia-mediated inflammation and liver injury.
NIH-PA Author Manuscript
Our studies indicate that EtOH-induced oxidative stress results in gut leakiness by inducing
nitration and oxidation of intestinal mucosa. However, the mechanisms by which NO
induces intestinal epithelial barrier dysfunction are not clearly understood. NO-mediated
oxidation of cellular proteins is due to its metabolite, peroxynitrite, which is a product of the
reaction of NO with superoxide radicals (Banan et al., 2001; Kolios et al., 2004).
Peroxynitrite oxidizes and damages proteins by reacting with amino acid residues such as
cysteine (Banan et al., 2001; Kolios et al., 2004). For example, nitration of phenolic amino
acid residues produces nitrotyrosine, a stable foot-print of peroxynitrite reactions and thus an
index of peroxynitrite formation (Banan et al., 2001; Kolios et al., 2004). Our previous in
vitro studies demonstrated that EtOH increases NO and ONOO− formation, and that this
overproduction of ONOO− causes nitration and carbonylation of cytoskeletal proteins; this
damage then disrupts intestinal integrity (Banan et al., 2000; Banan et al., 2001; Banan et al.,
2007).
Our current findings, which demonstrate the importance of NO in alcohol-induced gut
leakiness and steatohepatitis, are supported by these prior reports. Most (Chow et al., 1998;
Greenberg et al., 1994; Lancaster, 1992; Sisson, 1995) but not all (Neiman and Benthin,
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 8
NIH-PA Author Manuscript
1997; Persson and Gustafsson, 1992) reports indicate that chronic EtOH raises NO levels in
multiple organs. Indeed, many have suggested that EtOH-induced cytotoxicity is mediated
via upregulation of NO and its metabolite, peroxynitrite (ONOO−). For example, NO
appears to be a key mediator of EtOH’s cytotoxic effect on the CNS (Lancaster, 1992).
Increased NO levels in primary cultures of bovine bronchial epithelial cells are responsible
for the harmful effects of EtOH (Sisson, 1995). iNOS upregulation in adrenal glands
(Greenberg et al., 1994) may cause EtOH-induced blunting of the ACTH response to sepsis.
EtOH damage to gastric mucosa appears to be mediated via increased iNOS/NO (Chow et
al., 1998). Finally, EtOH-induced damage to the liver may be NO-mediated (Nanji et al.,
1995). Indeed, our own published in vitro data indicate that EtOH upregulates iNOS and
increases NO and ONOO in Caco-2 cells (Banan et al., 2000; Banan et al., 2001; Banan et
al., 2007).
NIH-PA Author Manuscript
The next question is how chronic alcohol feeding increases NO levels. NO is synthesized
from L-arginine by nitric oxide synthases (NOS). Three isoforms of nitric oxide synthases
have been identified: neuronal (nNOS), endothelial (eNOS), and inducible NOS (iNOS)
(Kolios et al., 2004). nNOS and eNOS are referred to as constitutive NOS (cNOS)(Kolios et
al., 2004). NO production by cNOS modulates several aspects of intestinal physiology and is
considered to be required for maintaining epithelial cell barrier integrity (Collins, 1996;
Takahashi, 2003; Vallance et al., 2004). In contrast, NO produced by iNOS is believed to
occur under pathological conditions such as inflammation and is believed to be harmful to
the integrity of the intestinal barrier. Several studies have shown that iNOS is induced by
bacterial products, microbes and certain cytokines(Kolios et al., 2004) resulting in
production of high levels of NO. For example, several studies have demonstrated
upregulation of NOS activity in the inflamed mucosa of patients with ulcerative colitis and
in animal models of colitis (Vallance et al., 2004). We also showed that alcohol upregulates
iNOS in Caco-2 cell monolayers, resulting in increased levels of NO and ONOO− formation
(Banan et al., 2000; Banan et al., 2001; Banan et al., 2007).
Our data in the present study support the conclusion of these prior reports that iNOS is not a
major source of NO under normal, physiological conditions, but is a primary source of
increased tissue NO under pathological conditions such as alcohol-induced organ
dysfunction. We showed that L-NAME, but not the specific iNOS inhibitor L-NIL,
significantly decreases urinary NO levels in dextrose fed control rats. This finding supports
the notion that cNOS and not iNOS is the major source of NO under normal, physiological
conditions. In contrast, we showed that L-NIL is more potent in preventing alcohol-induced
increases in NO levels in the urine and in colonic tissue, supporting the notion that iNOS is
the major source of elevated NO levels under pathological conditions.
NIH-PA Author Manuscript
Our studies show that inhibition of iNOS not only decreased the severity of EtOH-induced
hepatic inflammation and injury, but also, it markedly attenuated (2 fold) the increase in
liver fat content caused by EtOH and reduced EtOH-induced steatosis by 71 to 88%. The
mechanism by which iNOS inhibitors attenuate alcoholic steatosis is unclear. iNOS
inhibition can potentially reduce steatosis indirectly by inhibiting the endotoxin-cytokine
cascade and/or directly by affecting hepatic lipid homeostasis. Although, our current study
can not differentiate between these two mechanisms, our time course study, using the same
model of alcoholic steatohepatitis as used here, support the direct effects of iNOS inhibition
on hepatic lipid metabolism (Keshavarzian A et al., 2009). We demonstrated that fatty liver
occurs early, within the first 2 weeks of daily alcohol gavage and at least 2 weeks before
significant endotoxemia occurs. Our data support the conclusion of prior studies that
steatosis is primarily due to the direct effects of alcohol on hepatic lipid metabolism and is
not dependent on endotoxin (Hall, 1994). Indeed, it is well established that chronic alcohol
exposure induces hepatic enzymes involved in lipid metabolism and the alcohol-induced
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 9
NIH-PA Author Manuscript
increase in NO levels can inhibit these enzymes including ALDH2, APT synthase and other
mitochondrial proteins and enzymes involved in mitochondrial beta-oxidation of fatty acids
and steatosis (Deng and Deitrich, 2007). Our finding of a marked reduction in alcoholic
steatosis by iNOS inhibition was previously shown for another model of alcoholic
steatohepatitis by McKim et al (McKim et al., 2003) who reported that alcohol–induced
fatty accumulation is significantly attenuated in iNOS knockout mice. Using a specific iNOS
inhibitor (1400W) in wild type mice, they also found similar protective effects against
alcohol-induced steatosis and liver damage (McKim et al., 2003). Endotoxin mediated
changes in hepatic lipid metabolism and fat accumulation can clearly exaggerate the direct
effects of alcohol and worsen alcoholic steatosis. Thus, our findings that iNOS inhibitors
reduce steatosis can be due to their direct hepatic effects and their ability to decrease
endotoxin levels and endotoxin-mediated hepatic inflammatory cascades.
NIH-PA Author Manuscript
Indeed, the importance of activation of the Kupffer cells induced by gut-derived endotoxin
in the development of alcoholic liver disease is now well established (Nagata et al., 2007). It
is generally accepted that endotoxin can activate Kupffer cells via Toll-like receptors
(TLR-4) resulting in upregulation of the transcription factor NFk-B and increased release of
proinflammatory cytokines and other mediators like TNF-alpha, IL-6, IFN, and NO that are
known to play key roles in the development of ALD (Baraona et al., 2002; Lieber, 2004;
Nagata et al., 2007). Endotoxin-induced TNF-alpha production and iNOS activation in
Kupffer cells may worsen hepatic oxidative stress caused by direct effects of EtOH such as
EtOH induction of CYPE1 and associated lipid peroxidation (Lieber, 2004; Nagata et al.,
2007; Yuan et al., 2006). The synergy between the effect of EtOH and endotoxin on the liver
not only can initiate liver injury, but also, can create a vicious circle that sustains a chronic
necroinflammatory process and hastens the onset of liver failure. Thus, in addition to its well
established role in hepatic inflammation and liver cell injury, the gut-derived endotoxincytokine cascade may also contribute directly to EtOH-induced steatosis. Our observed
beneficial effects of iNOS inhibition, therefore, could be due to attenuation of both
endotoxin-cytokine cascades and prevention of NO-mediated disruption of hepatic lipid
homeostasis.
NIH-PA Author Manuscript
Several studies have demonstrated the importance of NO in alcoholic liver disease.
However, there are conflicting results for the consequence of NOS inhibition on the severity
of liver disease (Nanji et al., 1995; Nanji et al., 2001; Uzun et al., 2005). For example,
Nanji, et al reported that L-NAME worsened EtOH-induced liver injury (Nanji et al., 1995).
In contrast, Uzun et al found that L-NAME decreased the severity of ethanol-induced liver
damage by decreasing oxidative stress and increasing antioxidant status (Uzun et al., 2005).
Our results are consistent with Uzun’s study that L-NAME prevents EtOH-induced NO
overproduction and decreases oxidative stress and liver injury. Furthermore, we showed that
inhibition of iNOS is more effective in alcohol-induced injury, suggesting that iNOS is the
primary source of NO overproduction and oxidative tissue damage caused by alcohol.
Several other studies have confirmed the importance of NO homeostasis in liver injury. It
has been shown by several groups that while iNOS activation is associated with hepatic
necro-inflammation and liver injury (Yuan et al., 2006), eNOS activation can protect liver
against injurious agents. For example, eNOS had a protective effect on the liver injury
induced by carbon tetrachloride (Leung et al., 2008; Tipoe et al., 2008), but iNOS
exacerbated the liver injury (Aram et al., 2008). Also, chronic ethanol and LPS significantly
inhibited eNOS activity, leading to extensive steatohepatitis with extensive focal necrosis
(Karaa et al., 2005; Yuan et al., 2006). Our study confirmed the importance of NO
upregulation in the development of alcoholic steatohepatitis and, for the first time, also
shows that the beneficial effects of NOS inhibition is due, at least in part, to prevention of
alcohol-induced gut leakiness and endotoxemia.
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 10
NIH-PA Author Manuscript
NIH-PA Author Manuscript
The importance of iNOS in alcohol-induced gut leakiness in man is not surprising. Other
studies using iNOS inhibitors have also shown that NO plays an important role in the
regulation of the functions of the human intestinal epithelium (Kawachi et al., 1999;
Krieglstein et al., 2001; Obermeier et al., 1999). Increased NO production has been
described in intestinal inflammation associated with hyperpermeability (Lefer and Lefer,
1999; McCafferty et al., 1999; Vallance et al., 2004). Our studies show that inhibition of
iNOS reduces EtOH-induced gut leakiness. The iNOS inhibitors may directly inhibit iNOS
activation in intestinal epithelial cells. iNOS expression in intestinal epithelial cells was
demonstrated by immunohistochemistry in the epithelial cells of colonic mucosa of patients
with active ulcerative colitis (UC) and infectious colitis (Kolios et al., 1998). Using in situ
hybridization and immunohistochemistry, other studies have demonstrated that iNOS
expression is localized to the surface epithelium and crypts in mucosa from UC patients
(Godkin et al., 1996; Singer et al., 1996). These studies strongly indicate that colonic
epithelial cells are the major source of NO production and iNOS activity in the mucosa of
patients with UC (Godkin et al., 1996; Kolios et al., 1998; Singer et al., 1996). Therefore,
we suspect that iNOS inhibition of EtOH-induced NO overproduction is mainly occurring in
intestinal epithelial cells. This is consistent with a recent study which showed that epithelial
iNOS makes a larger contribution to intestinal inflammation induced by Dextran sodium
sulfate (DSS), although both blood cell-derived and epithelium-derived iNOS contribute to
the mechanism (Krieglstein et al., 2007). Our previous in vitro experiments using
monolayers of intestinal epithelial cells without other types of immune cells demonstrate the
importance of iNOS in intestinal epithelial cell in alcohol-induced intestinal leakiness.
However, our findings and these data do not exclude the possibility that iNOS in nonepithelial cells is also involved in alcohol-induced gut leakiness. Indeed, iNOS is expressed
in a variety of cells including epithelial cells, neutrophils, macrophages, neurons, glia, and
endothelial cells (Krieglstein et al., 2007). Some studies suggest that iNOS from bone
marrow-derived cells plays a critical role in regulating colonic inflammation (Beck et al.,
2007). A limitation of using iNOS inhibitors in vivo is that it is difficult to determine which
cell or tissue source of iNOS is mediating the effects of EtOH. To overcome this problem,
tissue specific iNOS knock out mice may be useful (Beck et al., 2007; Krieglstein et al.,
2007). Indeed, some studies using a tissue-specific iNOS knock out animal model of colitis
demonstrated the relative contribution of bone marrow-derived or epithelial-derived iNOS in
the regulation of colonic inflammation (Beck et al., 2007; Krieglstein et al., 2007). Further
studies in animal and human are now required to determine the cellular source of the
upregulated iNOS in the intestinal mucosa, to determine how iNOS upregulation disrupts
intestinal barrier integrity, and to identify the optimal target for future therapeutic
interventions.
NIH-PA Author Manuscript
The mechanism by which EtOH induces iNOS upregulation in intestinal epithelial cells has
not been fully explored. Our monolayer studies showed that alcohol upregulates iNOS in
Caco-2 cells through NFkB signaling (Banan et al., 2007). Yuan et al also showed that
upregulation of iNOS in the liver is associated with activation of NFkB, TNF-alpha
expression and liver injury (Yuan et al., 2006). Further studies in both in vitro and in vivo
models and in patients with ALD are now required to identify the signaling pathways
involved in alcohol-induced upregulation of iNOS and disruption of barrier function in
intestinal epithelial cells.
In summary, our findings demonstrate that iNOS inhibitors reduce EtOH-induced steatosis
and liver cell injury by preventing oxidative stress-induced intestinal hyperpermeability and
the consequent endotoxemia – at least in our animal model of alcoholic steatohepatitis.
These results indicate that strategies designed to target iNOS could lead to therapeutic
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 11
agents for the treatment and prevention of ALD and of other diseases associated with NO
overproduction and intestinal hyperpermeability.
NIH-PA Author Manuscript
Acknowledgments
This study was supported by NIH grant AA13745 (AK).
References
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Alican I, Kubes P. A critical role for nitric oxide in intestinal barrier function and dysfunction. Am J
Physiol. 1996; 270(2 Pt 1):G225–37. [PubMed: 8779963]
Aram G, Potter JJ, Liu X, Torbenson MS, Mezey E. Lack of inducible nitric oxide synthase leads to
increased hepatic apoptosis and decreased fibrosis in mice after chronic carbon tetrachloride
administration. Hepatology. 2008; 47(6):2051–8. [PubMed: 18506890]
Banan A, Choudhary S, Zhang Y, Fields JZ, Keshavarzian A. Ethanol-induced barrier dysfunction and
its prevention by growth factors in human intestinal monolayers: evidence for oxidative and
cytoskeletal mechanisms. J Pharmacol Exp Ther. 1999; 291(3):1075–85. [PubMed: 10565827]
Banan A, Fields JZ, Decker H, Zhang Y, Keshavarzian A. Nitric oxide and its metabolites mediate
ethanol-induced microtubule disruption and intestinal barrier dysfunction. J Pharmacol Exp Ther.
2000; 294(3):997–1008. [PubMed: 10945852]
Banan A, Fields JZ, Zhang Y, Keshavarzian A. iNOS upregulation mediates oxidant-induced
disruption of F-actin and barrier of intestinal monolayers. Am J Physiol Gastrointest Liver Physiol.
2001; 280(6):G1234–46. [PubMed: 11352817]
Banan A, Keshavarzian A, Zhang L, Shaikh M, Forsyth CB, Tang Y, Fields JZ. NF-kappaB activation
as a key mechanism in ethanol-induced disruption of the F-actin cytoskeleton and monolayer barrier
integrity in intestinal epithelium. Alcohol. 2007; 41(6):447–60. [PubMed: 17869053]
Banan A, Zhang LJ, Shaikh M, Fields JZ, Farhadi A, Keshavarzian A. Novel effect of NF-kappaB
activation: carbonylation and nitration injury to cytoskeleton and disruption of monolayer barrier in
intestinal epithelium. Am J Physiol Cell Physiol. 2004; 287(4):C1139–51. [PubMed: 15175222]
Baraona E, Zeballos GA, Shoichet L, Mak KM, Lieber CS. Ethanol consumption increases nitric oxide
production in rats, and its peroxynitrite-mediated toxicity is attenuated by
polyenylphosphatidylcholine. Alcohol Clin Exp Res. 2002; 26(6):883–9. [PubMed: 12068258]
Beck PL, Li Y, Wong J, Chen CW, Keenan CM, Sharkey KA, McCafferty DM. Inducible nitric oxide
synthase from bone marrow-derived cells plays a critical role in regulating colonic inflammation.
Gastroenterology. 2007; 132(5):1778–90. [PubMed: 17449036]
Burbige EJ, Lewis DR Jr, Halsted CH. Alcohol and the gastrointestinal tract. Med Clin North Am.
1984; 68(1):77–89. [PubMed: 6361419]
Chow JY, Ma L, Cho CH. Effect of cigarette smoke on ethanol-induced gastric mucosal lesions: the
role of nitric oxide and neutrophils. Eur J Pharmacol. 1998; 342(2–3):253–60. [PubMed: 9548394]
Clayburgh DR, Shen L, Turner JR. A porous defense: the leaky epithelial barrier in intestinal disease.
Lab Invest. 2004; 84(3):282–91. [PubMed: 14767487]
Colgan SP. Nitric oxide and intestinal epithelia: just say NO. Gastroenterology. 1998; 114(3):601–3.
[PubMed: 9496953]
Collins SM. The immunomodulation of enteric neuromuscular function: implications for motility and
inflammatory disorders. Gastroenterology. 1996; 111(6):1683–99. [PubMed: 8942751]
Criado-Jimenez M, Rivas-Cabanero L, Martin-Oterino JA, Lopez-Novoa JM, Sanchez-Rodriguez A.
Nitric oxide production by mononuclear leukocytes in alcoholic cirrhosis. J Mol Med. 1995; 73(1):
31–3. [PubMed: 7633939]
Deng XS, Deitrich RA. Ethanol metabolism and effects: nitric oxide and its interaction. Curr Clin
Pharmacol. 2007; 2(2):145–53. [PubMed: 18690862]
Farhadi A, Keshavarzian A, Fields JZ, Sheikh M, Banan A. Resolution of common dietary sugars from
probe sugars for test of intestinal permeability using capillary column gas chromatography. J
Chromatogr B Analyt Technol Biomed Life Sci. 2006; 836(1–2):63–8.
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 12
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Farhadi A, Keshavarzian A, Holmes EW, Fields J, Zhang L, Banan A. Gas chromatographic method
for detection of urinary sucralose: application to the assessment of intestinal permeability. J
Chromatogr B Analyt Technol Biomed Life Sci. 2003; 784(1):145–54.
Folch J, Lees M, Sloane Stanley GH. A simple method for the isolation and purification of total lipides
from animal tissues. J Biol Chem. 1957; 226(1):497–509. [PubMed: 13428781]
Forsyth CB, Banan A, Farhadi A, Fields JZ, Tang Y, Shaikh M, Zhang LJ, Engen PA, Keshavarzian
A. Regulation of oxidant-induced intestinal permeability by metalloprotease-dependent epidermal
growth factor receptor signaling. J Pharmacol Exp Ther. 2007; 321(1):84–97. [PubMed:
17220428]
Galambos JT. Alcoholic hepatitis: its therapy and prognosis. Prog Liver Dis. 1972; 4:567–88.
[PubMed: 4569010]
Godkin AJ, De Belder AJ, Villa L, Wong A, Beesley JE, Kane SP, Martin JF. Expression of nitric
oxide synthase in ulcerative colitis. Eur J Clin Invest. 1996; 26(10):867–72. [PubMed: 8911859]
Grant BF, Dufour MC, Harford TC. Epidemiology of alcoholic liver disease. Semin Liver Dis. 1988;
8(1):12–25. [PubMed: 3283941]
Greenberg SS, Xie J, Wang Y, Malinski T, Summer WR, McDonough K. Escherichia coli-induced
inhibition of endothelium-dependent relaxation and gene expression and release of nitric oxide is
attenuated by chronic alcohol ingestion. Alcohol. 1994; 11(1):53–60. [PubMed: 7511391]
Hall PD. Pathological spectrum of alcoholic liver disease. Alcohol Alcohol Suppl. 1994; 2:303–13.
[PubMed: 8974350]
Hollander D. The intestinal permeability barrier. A hypothesis as to its regulation and involvement in
Crohn’s disease. Scand J Gastroenterol. 1992; 27(9):721–6. [PubMed: 1411276]
Hunt NC, Goldin RD. Nitric oxide production by monocytes in alcoholic liver disease. J Hepatol.
1992; 14(2–3):146–50. [PubMed: 1500677]
Invernizzi P, Salzman AL, Szabo C, Ueta I, O’Connor M, Setchell KD. Ursodeoxycholate inhibits
induction of NOS in human intestinal epithelial cells and in vivo. Am J Physiol. 1997; 273(1 Pt
1):G131–8. [PubMed: 9252519]
Karaa A, Kamoun WS, Clemens MG. Chronic ethanol sensitizes the liver to endotoxin via effects on
endothelial nitric oxide synthase regulation. Shock. 2005; 24(5):447–54. [PubMed: 16247331]
Kawachi S, Cockrell A, Laroux FS, Gray L, Granger DN, van der Heyde HC, Grisham MB. Role of
inducible nitric oxide synthase in the regulation of VCAM-1 expression in gut inflammation. Am J
Physiol. 1999; 277(3 Pt 1):G572–6. [PubMed: 10484382]
Keshavarzian A, Farhadi A, Forsyth CB, Jakate Shriram, Shaikh M, Banan A, Fields JZ. Evidence that
chronic alcohol exposure promotes intestinal oxidative stress, intestinal hyperpermeability, and
endotoxemia prior to development of alcoholic steatohepatitis in rats. Journal of Hepatology. 2009
in press.
Keshavarzian A, Choudhary S, Holmes EW, Yong S, Banan A, Jakate S, Fields JZ. Preventing gut
leakiness by oats supplementation ameliorates alcohol-induced liver damage in rats. J Pharmacol
Exp Ther. 2001; 299(2):442–8. [PubMed: 11602653]
Keshavarzian A, Fields J. Alcohol: “ice-breaker” yes, “gut barrier-breaker,” maybe. Am J
Gastroenterol. 2000; 95(5):1124–5. [PubMed: 10811316]
Keshavarzian A, Fields J. Alcoholic liver disease: is it an “extraintestinal” complication of alcoholinduced intestinal injury? J Lab Clin Med. 2003; 142(5):285–7. [PubMed: 14647031]
Keshavarzian A, Fields JZ, Vaeth J, Holmes EW. The differing effects of acute and chronic alcohol on
gastric and intestinal permeability. Am J Gastroenterol. 1994; 89(12):2205–11. [PubMed:
7977243]
Keshavarzian A, Holmes EW, Patel M, Iber F, Fields JZ, Pethkar S. Leaky gut in alcoholic cirrhosis: a
possible mechanism for alcohol-induced liver damage. Am J Gastroenterol. 1999; 94(1):200–7.
[PubMed: 9934756]
Keshavarzian A, Jacyno M, Urban G, Winship D, Fields JZ. The role of nitric oxide in ethanolinduced gastrointestinal dysfunction. Alcohol Clin Exp Res. 1996; 20(9):1618–24. [PubMed:
8986213]
Kleiner DE, Brunt EM, Van Natta M, Behling C, Contos MJ, Cummings OW, Ferrell LD, Liu YC,
Torbenson MS, Unalp-Arida A, Yeh M, McCullough AJ, Sanyal AJ. Design and validation of a
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 13
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
histological scoring system for nonalcoholic fatty liver disease. Hepatology. 2005; 41(6):1313–21.
[PubMed: 15915461]
Kolios G, Rooney N, Murphy CT, Robertson DA, Westwick J. Expression of inducible nitric oxide
synthase activity in human colon epithelial cells: modulation by T lymphocyte derived cytokines.
Gut. 1998; 43(1):56–63. [PubMed: 9771406]
Kolios G, Valatas V, Ward SG. Nitric oxide in inflammatory bowel disease: a universal messenger in
an unsolved puzzle. Immunology. 2004; 113(4):427–37. [PubMed: 15554920]
Krieglstein CF, Anthoni C, Cerwinka WH, Stokes KY, Russell J, Grisham MB, Granger DN. Role of
blood- and tissue-associated inducible nitric-oxide synthase in colonic inflammation. Am J Pathol.
2007; 170(2):490–6. [PubMed: 17255317]
Krieglstein CF, Cerwinka WH, Laroux FS, Salter JW, Russell JM, Schuermann G, Grisham MB, Ross
CR, Granger DN. Regulation of murine intestinal inflammation by reactive metabolites of oxygen
and nitrogen: divergent roles of superoxide and nitric oxide. J Exp Med. 2001; 194(9):1207–18.
[PubMed: 11696587]
Kubes P. Nitric oxide modulates epithelial permeability in the feline small intestine. Am J Physiol.
1992; 262(6 Pt 1):G1138–42. [PubMed: 1616043]
Lancaster FE. Alcohol, nitric oxide, and neurotoxicity: is there a connection?--a review. Alcohol Clin
Exp Res. 1992; 16(3):539–41. [PubMed: 1320808]
Lefer AM, Lefer DJ. Nitric oxide. II. Nitric oxide protects in intestinal inflammation. Am J Physiol.
1999; 276(3 Pt 1):G572–5. [PubMed: 10070031]
Leung TM, Tipoe GL, Liong EC, Lau TY, Fung ML, Nanji AA. Endothelial nitric oxide synthase is a
critical factor in experimental liver fibrosis. Int J Exp Pathol. 2008; 89(4):241–50. [PubMed:
18429990]
Lieber CS. Alcoholic fatty liver: its pathogenesis and mechanism of progression to inflammation and
fibrosis. Alcohol. 2004; 34(1):9–19. [PubMed: 15670660]
Lopez-Belmonte J, Whittle BJ. The involvement of endothelial dysfunction, nitric oxide and
prostanoids in the rat gastric microcirculatory responses to endothelin-1. Br J Pharmacol. 1994;
112(1):267–71. [PubMed: 8032649]
Maher JJ. Alcoholic steatosis and steatohepatitis. Semin Gastrointest Dis. 2002; 13(1):31–9. [PubMed:
11944632]
Mathurin P, Deng QG, Keshavarzian A, Choudhary S, Holmes EW, Tsukamoto H. Exacerbation of
alcoholic liver injury by enteral endotoxin in rats. Hepatology. 2000; 32(5):1008–17. [PubMed:
11050051]
McCafferty DM, Miampamba M, Sihota E, Sharkey KA, Kubes P. Role of inducible nitric oxide
synthase in trinitrobenzene sulphonic acid induced colitis in mice. Gut. 1999; 45(6):864–73.
[PubMed: 10562585]
McClain CJ, Cohen DA. Increased tumor necrosis factor production by monocytes in alcoholic
hepatitis. Hepatology. 1989; 9(3):349–51. [PubMed: 2920991]
McKim SE, Gabele E, Isayama F, Lambert JC, Tucker LM, Wheeler MD, Connor HD, Mason RP,
Doll MA, Hein DW, Arteel GE. Inducible nitric oxide synthase is required in alcohol-induced
liver injury: studies with knockout mice. Gastroenterology. 2003; 125(6):1834–44. [PubMed:
14724835]
Nagata K, Suzuki H, Sakaguchi S. Common pathogenic mechanism in development progression of
liver injury caused by non-alcoholic or alcoholic steatohepatitis. J Toxicol Sci. 2007; 32(5):453–
68. [PubMed: 18198478]
Nanji AA, Greenberg SS, Tahan SR, Fogt F, Loscalzo J, Sadrzadeh SM, Xie J, Stamler JS. Nitric
oxide production in experimental alcoholic liver disease in the rat: role in protection from injury.
Gastroenterology. 1995; 109(3):899–907. [PubMed: 7657120]
Nanji AA, Jokelainen K, Lau GK, Rahemtulla A, Tipoe GL, Polavarapu R, Lalani EN. Arginine
reverses ethanol-induced inflammatory and fibrotic changes in liver despite continued ethanol
administration. J Pharmacol Exp Ther. 2001; 299(3):832–9. [PubMed: 11714866]
Neiman J, Benthin G. Nitric oxide is not increased in alcoholic brain. Alcohol Alcohol. 1997; 32(5):
551–3. [PubMed: 9373696]
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 14
NIH-PA Author Manuscript
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Obermeier F, Kojouharoff G, Hans W, Scholmerich J, Gross V, Falk W. Interferon-gamma (IFNgamma)- and tumour necrosis factor (TNF)-induced nitric oxide as toxic effector molecule in
chronic dextran sulphate sodium (DSS)-induced colitis in mice. Clin Exp Immunol. 1999; 116(2):
238–45. [PubMed: 10337013]
O’Connor PG, Schottenfeld RS. Patients with alcohol problems. N Engl J Med. 1998; 338(9):592–602.
[PubMed: 9475768]
Persson MG, Gustafsson LE. Ethanol can inhibit nitric oxide production. Eur J Pharmacol. 1992;
224(1):99–100. [PubMed: 1451748]
Purohit V, Bode JC, Bode C, Brenner DA, Choudhry MA, Hamilton F, Kang YJ, Keshavarzian A, Rao
R, Sartor RB, Swanson C, Turner JR. Alcohol, intestinal bacterial growth, intestinal permeability
to endotoxin, and medical consequences: summary of a symposium. Alcohol. 2008; 42(5):349–61.
[PubMed: 18504085]
Robinson GM, Orrego H, Israel Y, Devenyi P, Kapur BM. Low-molecular-weight polyethylene glycol
as a probe of gastrointestinal permeability after alcohol ingestion. Dig Dis Sci. 1981; 26(11):971–
7. [PubMed: 7297377]
Sawada N, Murata M, Kikuchi K, Osanai M, Tobioka H, Kojima T, Chiba H. Tight junctions and
human diseases. Med Electron Microsc. 2003; 36(3):147–56. [PubMed: 14505058]
Singer II, Kawka DW, Scott S, Weidner JR, Mumford RA, Riehl TE, Stenson WF. Expression of
inducible nitric oxide synthase and nitrotyrosine in colonic epithelium in inflammatory bowel
disease. Gastroenterology. 1996; 111(4):871–85. [PubMed: 8831582]
Sisson JH. Ethanol stimulates apparent nitric oxide-dependent ciliary beat frequency in bovine airway
epithelial cells. Am J Physiol. 1995; 268(4 Pt 1):L596–600. [PubMed: 7537462]
Takahashi T. Pathophysiological significance of neuronal nitric oxide synthase in the gastrointestinal
tract. J Gastroenterol. 2003; 38(5):421–30. [PubMed: 12768383]
Tang Y, Banan A, Forsyth CB, Fields JZ, Lau CK, Zhang LJ, Keshavarzian A. Effect of alcohol on
miR-212 expression in intestinal epithelial cells and its potential role in alcoholic liver disease.
Alcohol Clin Exp Res. 2008; 32(2):355–64. [PubMed: 18162065]
Tipoe GL, Liong EC, Leung TM, Nanji AA. A voluntary oral-feeding rat model for pathological
alcoholic liver injury. Methods Mol Biol. 2008; 447:11–31. [PubMed: 18369908]
Turner JR, Rill BK, Carlson SL, Carnes D, Kerner R, Mrsny RJ, Madara JL. Physiological regulation
of epithelial tight junctions is associated with myosin light-chain phosphorylation. Am J Physiol.
1997; 273(4 Pt 1):C1378–85. [PubMed: 9357784]
Unno N, Menconi MJ, Salzman AL, Smith M, Hagen S, Ge Y, Ezzell RM, Fink MP.
Hyperpermeability and ATP depletion induced by chronic hypoxia or glycolytic inhibition in
Caco-2BBe monolayers. Am J Physiol. 1996; 270(6 Pt 1):G1010–21. [PubMed: 8764209]
Unno N, Menconi MJ, Smith M, Aguirre DE, Fink MP. Hyperpermeability of intestinal epithelial
monolayers is induced by NO: effect of low extracellular pH. Am J Physiol. 1997a; 272(5 Pt
1):G923–34. [PubMed: 9176198]
Unno N, Menconi MJ, Smith M, Fink MP. Nitric oxide mediates interferon-gamma-induced
hyperpermeability in cultured human intestinal epithelial monolayers. Crit Care Med. 1995; 23(7):
1170–6. [PubMed: 7541323]
Unno N, Wang H, Menconi MJ, Tytgat SH, Larkin V, Smith M, Morin MJ, Chavez A, Hodin RA,
Fink MP. Inhibition of inducible nitric oxide synthase ameliorates endotoxin-induced gut mucosal
barrier dysfunction in rats. Gastroenterology. 1997b; 113(4):1246–57. [PubMed: 9322519]
Uzun H, Simsek G, Aydin S, Unal E, Karter Y, Yelmen NK, Vehid S, Curgunlu A, Kaya S. Potential
effects of L-NAME on alcohol-induced oxidative stress. World J Gastroenterol. 2005; 11(4):600–
4. [PubMed: 15641155]
Vallance BA, Dijkstra G, Qiu B, van der Waaij LA, van Goor H, Jansen PL, Mashimo H, Collins SM.
Relative contributions of NOS isoforms during experimental colitis: endothelial-derived NOS
maintains mucosal integrity. Am J Physiol Gastrointest Liver Physiol. 2004; 287(4):G865–74.
[PubMed: 15217783]
Yuan GJ, Zhou XR, Gong ZJ, Zhang P, Sun XM, Zheng SH. Expression and activity of inducible
nitric oxide synthase and endothelial nitric oxide synthase correlate with ethanol-induced liver
injury. World J Gastroenterol. 2006; 12(15):2375–81. [PubMed: 16688828]
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 15
NIH-PA Author Manuscript
Fig. 1.
NIH-PA Author Manuscript
iNOS inhibitors prevented EtOH-induced NO overproduction in urine and colon mucosa.
Total NO (NO2 + NO3) in urine and colonic mucosa from dextrose fed (control) and alcohol
fed (10 weeks) rats was assayed after sacrifice (see Methods). Data are shown as mean for
total NO (umol/mg) ± SE for n=6 for each group. The difference between groups was
analyzed using ANOVA, *: p<0.05 compared to dextrose fed rats (controls), #: p<0.05
compared to alcohol fed rats, &: p<0.05 compared to alcohol + L-NAME group.
NIH-PA Author Manuscript
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 16
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Fig. 2.
iNOS inhibitors prevented EtOH-induced increases in nitration (protein-nitrotyrosination) in
intestinal tissues. Levels of nitrotyrosine epitopes, a marker of tissue nitration, were
determined by slot blotting and quantitative densitometry in tissue (mucosal) samples
obtained after sacrifice from duodenum (a), jejunum (b), ileum (c), and colon (d). Data are
shown as the mean ± SE for each group (n=6/group). The difference between groups was
analyzed using ANOVA, *: p<0.05 compared to dextrose-fed rats (controls),, #: p<0.05
compared to alcohol fed rats (10 weeks). A representative slot blot image for colonic
mucosa is shown in Fig. 2d.
NIH-PA Author Manuscript
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 17
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Fig. 3.
iNOS inhibitors prevented EtOH-induced increases in oxidation (protein-carbonyls) in
intestinal tissues. Levels of the carbonyl epitope, an oxidative stress marker, were
determined by slot blots and quantitative densitometry in tissue (mucosal) samples obtained
after sacrifice from duodenum (a), jejunum (b), ileum (c), and colon (d). Data are shown as
the mean ± SE for each group (n=6). The difference between groups was analyzed using
ANOVA, *: p<0.05 compared to dextrose-fed rats (controls), #: p<0.05 compared to
alcohol-fed rats (10 weeks). A representative slot blot image for colonic mucosa is shown in
Fig. 3d.
NIH-PA Author Manuscript
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 18
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Fig. 4.
NIH-PA Author Manuscript
iNOS inhibitors prevented EtOH-induced increases in intestinal permeability. Intestinal
permeability was assayed by measuring urinary lactulose and sucralose after an oral dose of
these sugars. Urinary lactulose (a marker of small intestine permeability and urinary
sucralose (a marker of whole gut permeability) increased in alcohol fed rats. iNOS inhibitors
(L-NIL or L-NAME) significantly prevented EtOH-induced hyperpermeability. Data are
means of the fraction of the oral dose recovered ± S.E. for N = 6 rats for each group. The
difference between groups was analyzed by ANOVA, *: p<0.05 compared to dextrose-fed
rats (controls), #: p<0.05 compared to alcohol fed rats (10 weeks).
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 19
NIH-PA Author Manuscript
Fig. 5.
NIH-PA Author Manuscript
iNOS inhibitors prevented EtOH-induced increases in serum endotoxin. Serum endotoxin
was determined for control and alcohol-fed rats at 10 weeks after alcohol feeding as
described in Methods. Serum endotoxin levels were significantly higher in alcohol-fed rats.
iNOS inhibitors (L-NIL or L-NAME) prevented EtOH-induced increases in endotoxin
levels. Data are expressed as mean endotoxin Units (EU) per ml serum ± S.E. for N=6 rats
for each group. The difference between groups was analyzed by ANOVA, *: p<0.05
compared to the control group, #: p<0.05 compared to alcohol-fed rats (10 weeks), &:
p<0.05 compared to alcohol + L-NAME group.
NIH-PA Author Manuscript
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 20
NIH-PA Author Manuscript
Fig. 6.
NIH-PA Author Manuscript
iNOS inhibitors prevented EtOH-induced steatohepatitis. EtOH-induced inflammatory
reactions were determined by measuring hepatic myeloperoxidase (MPO) activity (a) and by
calculating histological inflammatory scores (b). Total histological score represents a
combined inflammatory score (i.e. hepatitis) and score for the severity of fat accumulation in
the liver (i.e. steatosis) and thus it represents the severity of steatohepatitis (c). EtOHinduced fatty liver was determined by measuring hepatic fat content (d). Feeding of alcohol
to rats for 10 weeks significantly increased MPO activity, inflammation score, histology
score, and fat content in liver tissue. iNOS inhibitors (L-NIL or L-NAME) significantly
decreased all indices of alcoholic steatohepatitis. Data are expressed as mean ± S.E. for N=6
rats for each group. The difference between groups was analyzed using ANOVA, *: p<0.05
compared to dextrose-fed rats (controls), #: p<0.05 compared to alcohol-fed rats (10 weeks).
NIH-PA Author Manuscript
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 21
NIH-PA Author Manuscript
Fig. 7.
NIH-PA Author Manuscript
iNOS inhibitors prevented EtOH-induced increases in alanine aminotransferase (ALT).
Chronic EtOH consumption by rats significantly increased serum ALT levels, an index of
liver cell injury. iNOS inhibitors significantly inhibited EtOH-induced increases in ALT:
back to 91+15 U/dl by L-NIL and 96+16 U/dl by L-NAME. Data are expressed as mean ±
S.E. for N=6 rats for each group. Differences between groups were analyzed using ANOVA,
*: p<0.05 compared to dextrose-fed rats (controls),, #: p<0.05 compared to alcohol-fed rats
(10 weeks).
NIH-PA Author Manuscript
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.
Tang et al.
Page 22
NIH-PA Author Manuscript
NIH-PA Author Manuscript
Fig. 8.
Our current model for the mechanism by which iNOS inhibitors reduce EtOH-induced
steatohepatitis. They do so by preventing oxidative stress-induced increases in intestinal
hyperpermeability and the endotoxemia that would otherwise result.
NIH-PA Author Manuscript
Alcohol Clin Exp Res. Author manuscript; available in PMC 2010 October 5.