From the
||Renal Division, Department of Medicine,
Brigham and Women's Hospital, Boston, Massachusetts 02115, the
Renal Unit, Medical Services, and
¶Cardiovascular Research Center, Massachusetts
General Hospital, Charlestown, Massachusetts 02129, the
Department of Medicine, Harvard Medical School,
Boston, Massachusetts 02114, and the
**Harvard-Massachusetts Institute of Technology
Division of Health Sciences and Technology, Boston and Cambridge,
Massachusetts 02139
Received for publication, February 19, 2003
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ABSTRACT |
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INTRODUCTION |
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Nitric oxide down-regulates inflammatory reactions, which are important
contributors to ischemia/reperfusion injury
(611).
NO regulates neutrophil recruitment by inhibiting the expression of adhesion
molecules in the vascular endothelium resulting in increased blood flow to
ischemic regions (12). NO
production is dependent on the activities of NOS enzymes, whose expression is,
in turn, modulated by signaling pathways implicated in inflammation, such as
NF-B and MAPKs
(911,
13,
14). NO donors protect the
kidney in diverse models of renal failure, including ischemic renal failure
(15,
16), obstructive nephropathy
(17), or renal allografts
(18). Inhibition of NO
synthesis increases susceptibility to kidney ischemia
(16,
19,
20). By contrast, some studies
have demonstrated that inhibition of NOS protects organs against ischemia
(20,
21).
iNOS/NO has been implicated in protection induced by preconditioning. Takano et al. (22) reported in the heart that iNOS enzymes are implicated in protection 24 h after ischemic preconditioning induced by short episodes of ischemia and reperfusion (22). An iNOS inhibitor eliminated the infarct-sparing effect of preconditioning afforded by isoflurane or halothane anesthesia 24 h previously (23). Prior brain ischemia protects isolated aortic ring reactivity in an iNOS-dependent manner (24). Thus, while these and other data suggest that expression of iNOS plays a role in the protection induced by mild sublethal ischemic preconditioning against a second exposure to ischemia/reperfusion 24 or 48 h later in heart, brain, and kidney (5, 14, 2527), little is known in any organ about the role of NOS or other molecules in preconditioning when the initial insult results in prolonged long term protection, as defined by protection afforded for longer than 48 h.
In our studies we characterized the time characteristics of the protective
effect of ischemic preconditioning, whether the protective mechanisms differ
according to the strength of ischemic preconditioning, and whether ischemic
preconditioning in the kidney is dependent upon NOS expression. Our results
indicate that prior ischemic preconditioning protects the kidney from
ischemia/reperfusion insults up to 12 weeks later with the degree of
protection decreased as length of time between ischemic periods increases.
Thirty minutes of ischemic preconditioning results in a sustained increase in
iNOS expression and sustained damage to the kidney as reflected by
-smooth muscle actin (SMA) accumulation. Pharmacological inhibition of
NO synthesis or genetic deletion of the iNOS gene, but not the
eNOS gene, increases mouse kidney susceptibility to ischemia and
mitigates the protection afforded by 30 min but not 15 min of ischemic
preconditioning. Fifteen minutes of ischemia does not lead to increases in
iNOS or eNOS expression. Thus increases of iNOS expression account for an
important component of long term ischemic preconditioning in the kidney when
the initial ischemia results in persistent tissue injury. While others have
studied the role of iNOS in preconditioning in other organs as indicated
above, our data go beyond these studies in a number of ways. No one
previously, in any organ, has reported nor explored the mechanisms of
preconditioning that persist up to 12 weeks after the initial event.
Furthermore we report persistent long term renal interstitial changes after
ischemia in the mouse kidney, and for the first time implicates these changes
in long term protection against subsequent ischemia. We have thus identified
iNOS as an important factor in prolonged protection that follows ischemic
preconditioning. This role of iNOS depends upon the initial ischemic time in
the mouse kidney.
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EXPERIMENTAL PROCEDURES |
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Kidneys were harvested at the times indicated in the figure legends. Kidneys were snap frozen in liquid nitrogen for myeloperoxidase (MPO) activity and Western blot analysis or were rinsed in PBS and fixed in 4% paraformaldehyde for histological analysis.
Renal Functional ParametersSeventy microliters of blood were taken from the retroorbital vein plexus at the times indicated in the figures. Plasma creatinine concentration was measured using a Beckman Creatinine Analyzer.
ImmunocytochemistryAfter perfusion via the left ventricle with 30 ml of PBS for 2 min at 37 °C and then PLP (4% paraformaldehyde, 75 mM L-lysine, 10 mM sodium periodate) fixative for 5 min, kidneys were excised and placed in PLP overnight at 4 °C. Kidneys were then washed and stored in PBS containing 0.02% sodium azide at 4 °C. Fixed tissue was washed with PBS three times for 5 min each, placed overnight in PBS containing 30% sucrose, embedded in oxytetracycline compound (Sakura FineTek, Torrance, CA), frozen in liquid nitrogen, and then cut into 5-µm sections using a cryotome. Sections were mounted on Fisher Superfrost Plus (Fisher) microscope slides, dried in air, and stored at -20 °C.
For staining with phalloidin, which stains the actin cytoskeleton, sections were dried, washed in PBS for 10 min, incubated in blocking buffer (PBS containing 3% nonfat dry milk) for 40 min at room temperature, and washed three times in PBS for 5 min each. The section was incubated in blocking buffer containing TRITC-labeled phalloidin (Sigma, 1:500) for 20 min at room temperature, washed twice in PBS containing 1.9% NaCl, once in PBS and mounted with a 1:1 mixture of Vectashield (Vector Laboratories) and 0.3 M Tris-HCl, pH 8.9.
To detect kidney injury molecule-1 (Kim-1), whose expression is markedly
up-regulated in dedifferentiated proximal tubular cells in the outer stripe
after ischemia or ureteral obstruction
(30,
31), sections were dried,
washed in PBS, incubated in blocking buffer (PBS containing 2% bovine serum
albumin) for 20 min at room temperature, incubated with rabbit polyclonal
anti-Kim-1 antibody (1:1000) overnight at 4 °C, and then washed with PBS.
The sections were incubated with FITC-labeled anti-rabbit IgG for 40 min at
room temperature, washed with PBS three times for 10 min each, and mounted
with Vectashield. To detect -smooth muscle actin, sections were boiled
with 10 mM sodium citrate buffer, at pH 6.0, for 15 min and then
washed with PBS. Monoclonal anti-
-smooth muscle actin antibody, 1:250,
Sigma, was used as primary antibody. Subsequent procedures were carried out as
described above with the exception that the secondary antibody was goat
anti-mouse IgG antibody conjugated with FITC. Images were viewed on a Nikon
FXA epifluorescence microscope.
MPO ActivityMPO activity, an index of tissue leukocyte infiltration, was measured in 1.5 and 24 h postischemic kidneys as previously described (6, 32). Activity was normalized to protein concentration.
Western Blot AnalysisProteins were extracted from kidneys
as previously described (33).
Protein samples were separated on 7.5 or 10% SDS-PAGE gels and then
transferred to an Immobilon membrane (Millipore, Bedford, MA). Membranes were
incubated with rabbit polyclonal anti-HSP-27 or 25 (Upstate Biotechnology),
monoclonal anti--smooth muscle actin (Sigma) or mouse monoclonal
anti-iNOS or -eNOS (BD Transduction Laboratories) antibodies. As a positive
control for iNOS expression, mouse macrophage RAW 264.7 cells were stimulated
with interferon-
(10 ng/ml) and lipopolysaccaride (1 µg/ml) for 12
h. Macrophage cell lysates were provided by BD Transduction Laboratories.
Secondary antibodies, conjugated with horseradish peroxidase (Santa Cruz
Biotechnology), were detected with the ECL system (Amersham Biosciences).
StatisticsAll results are expressed as mean ± S.E. A p value of <0.05 was taken as statistically significant. Each group consisted of more than four animals as indicated in the figure legends.
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RESULTS |
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Renal ischemia/reperfusion results in disruption of the actin cytoskeleton (6, 34). We evaluated the effect of ischemic preconditioning on postischemic actin cytoskeleton changes using immunocytochemistry techniques. Kidney sections were stained for phalloidin to identify the actin cytoskeleton (35). There is normal apical phalloidin staining in the proximal tubules of the sham-operated kidneys (Fig. 2A). Twenty-four hours after 30 min of ischemia, severe disruption of the actin cytoskeleton is observed in the non-preconditioned kidney (Fig. 2B). The disruption is particularly severe in the S3 segment proximal tubular cells in the outer stripe of the outer medulla (Fig. 2). At 8 days or 6 or 12 weeks after ischemia, the disruption of the actin cytoskeleton has largely reversed (Fig. 2, C, E, and G). At 24 h after a second period of ischemia imposed on day 8, the changes in the actin cytoskeleton were much less pronounced in the preconditioned kidneys than in the non-preconditioned kidneys (compare Fig. 2, D and B). With increasing time between ischemic insults the postischemic disruption of the actin cytoskeleton increases (Fig. 2).
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We evaluated the effect of 30 min of ischemic preconditioning on Kim-1 expression after subsequent ischemia as an additional measure of injury to the proximal nephron. We wanted to evaluate whether there are effects of ischemia that persisted well beyond the time that serum creatinine returned to normal. Kim-1 is markedly up-regulated in the S3 segment of the proximal tubule in the outer stripe of the outer medulla after ischemia and ureteral obstruction (30, 31), In the normal kidney there is no staining with an anti-Kim-1 antibody (Fig. 3). Ischemia results in the marked expression of Kim-1 in the S3 segment of the proximal tubules (Fig. 3). Kim-1 expression peaked 24 h after ischemia and reperfusion and then decreased over time. Increased expression of Kim-1 was seen for at least 3 weeks after 30 min of ischemia suggesting there was persistent tubular injury (Fig. 3) despite the absence of any increase in plasma creatinine (Fig. 1). When a second ischemic insult was imposed either 3 or 6 weeks after the first the postischemic expression of Kim-1 in the preconditioned kidney was much lower when compared with the expression normally seen 24 h after ischemia in the non-preconditioned kidney (Fig. 3).
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Thirty Minutes of Ischemic Preconditioning Reduces Postischemic Tissue MPO ActivityTo evaluate whether protection against ischemic injury could be related to postischemic leukocyte infiltration, the extent of tissue leukocyte infiltration was determined by tissue MPO activity. When 30 min of ischemia was induced on day 8, MPO levels markedly increased in the non-preconditioned kidneys 1.5 and 24 h later (Fig. 4). By comparison there was no increase of MPO activity in the preconditioned kidneys (Fig. 4).
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Thirty Minutes of Ischemia Increases Endothelial and Inducible NOS Protein ExpressionEndothelial or iNOS protein expression was evaluated by Western blot analysis. Thirty minutes of ischemia results in a significant increase in the expression of eNOS and iNOS in whole kidney lysate (Fig. 5). Sham-surgery does not result in a change in the expression of eNOS and iNOS (Fig. 5). The increased expression of eNOS and iNOS after ischemia/reperfusion persists for 12 weeks (Fig. 5). The expression of iNOS peaks at 1 week after ischemia and gradually diminishes over time, but remains above baseline levels for 12 weeks (Fig. 5, A and C).
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Since the sustained long term iNOS expression suggests the presence of a
persistent stimulus, we evaluated whether the initial ischemic period resulted
in long term changes in the kidney, which were not severe enough to cause
measurable functional changes in plasma creatinine. -Smooth muscle
actin expression is a characteristic feature of renal fibrosis
(36). Thirty minutes of
ischemia results in an increase of
-smooth muscle actin expression in
the interstitium 1, 6, and 12 weeks later
(Fig. 6A). This
increase in renal
-smooth muscle actin expression is also manifest on
Western blots taken from kidneys at 1, 3, 6, and 12 weeks after 30 min of
bilateral ischemia (Fig.
6B).
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Effect of L-Arginine or L-NNA on Ischemia/Reperfusion InjuryTo evaluate whether NO/NOS can modulate the extent of ischemic renal injury, we examined the effect of L-arginine and L-NNA in BALB/c male mice. Treatment with L-arginine or L-NNA does not change plasma creatinine levels (Fig. 7A). Twenty-four hours after a second procedure of 30 min of ischemia, plasma creatinine levels in the non-preconditioned mice treated with L-NNA 30 min prior to and subsequent to ischemia are higher than in the mice treated with L-arginine or vehicle (Fig. 7A). Treatment of ischemia-preconditioned animals with L-NNA, prior to and subsequent to a second ischemic period, partially mitigates the protection afforded by preconditioning. The increase in plasma creatinine, however, does not reach levels seen in the non-preconditioned mice treated with vehicle or L-NNA (Fig. 7A). Three days after ischemia 75% of the L-NNA-treated non-preconditioned (S/I) mice died, whereas all vehicle- or L-arginine-treated mice survived (Fig. 7B). Treatment of preconditioned mice with L-NNA 30 min prior to and 30 min after the second ischemia results in increased MPO activity 24 h after ischemia, although the increase of MPO activity does not reach levels seen in the non-preconditioned kidney (S/I) treated with L-NNA or vehicle (Fig. 7C). There was no increase of kidney MPO activity 24 h after sham operation whether or not mice were treated with L-NNA.
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Effect of eNOS, iNOS Gene Deletion, or L-NIL on Ischemia/Reperfusion InjuryTo examine which forms of NOS enzymes contribute to the protection afforded by ischemic preconditioning, we evaluated ischemic preconditioning in eNOS -/-, eNOS +/+, iNOS -/- and iNOS +/+ mice. We use both 25- and 30-min periods of ischemia so that we could be sure that an effect was not masked by too much injury after 30 min of ischemia. In iNOS -/- mice, 25 min of bilateral ischemia significantly increased plasma creatinine levels 24 h after ischemia (Fig. 8A). In contrast, there is a small increase in plasma creatinine levels in iNOS +/+ mice (Fig. 8A). Thirty minutes of bilateral ischemia markedly increases plasma creatinine levels in both iNOS -/- and iNOS +/+ mice 24 h after ischemia (Fig. 8B). Twenty-four hours after either 25 or 30 min of ischemia the plasma creatinine levels are significantly higher in iNOS -/- mice than in iNOS +/+ mice (Fig. 8, A and B). Sham-operation in both iNOS -/- and iNOS +/+ does not result in a change in plasma creatinine levels (Fig. 8, A and B). Eight days after 30 min of ischemic preconditioning survival rate is 84.6% in iNOS +/+ (n = 23) and 44.4% in iNOS -/- (n = 27) mice, respectively (Fig. 8C). All mice survived for 8 days after 25 min of initial ischemia (data not shown). Eight days after either the 25 or 30 min of bilateral ischemia or sham-operation, the animals were subjected to 30 min of bilateral ischemia (Fig. 8, A and B). Thirty minutes of bilateral ischemia on day 8 results in an increase in plasma creatinine levels in both iNOS -/- and iNOS +/+ mice which are preconditioned by 25 min of bilateral ischemia on day 0, but the postischemic increase in plasma creatinine is less in iNOS +/+ than iNOS -/- mice (Fig. 8A). Thirty minutes of bilateral ischemic preconditioning of iNOS +/+ mice completely prevents the increase of plasma creatinine induced by 30 min of bilateral ischemia on day 8 (Fig. 8B), whereas in iNOS -/- mice there is only partial protection of the kidney against this second insult (Fig. 8B). On day 9 postischemic tissue MPO activity is greater in the iNOS -/- than in the iNOS +/+ mice 24 h after a second ischemic period (I/I) (Fig. 8D). Treatment with L-NIL, a specific inhibitor of iNOS, 30 min prior to and after the second procedure mitigates the kidney protection afforded by 30 min of prior ischemia (Fig. 8E).
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When 30 min of bilateral ischemia is induced in the eNOS +/+ or eNOS -/- mice, the levels of plasma creatinine markedly increased to equivalent levels at 24 and 48 h after ischemia in mice of both genotypes (Fig. 8F). There are no differences in the levels of plasma creatinine between eNOS +/+ and eNOS -/- at any time point (Fig. 8F). Sham-operation in both eNOS -/- and eNOS +/+ does not result in a change in plasma creatinine levels (Fig. 8F). When the animals are subjected to 30 min of bilateral ischemia on day 8, significant postischemic increases of plasma creatinine levels were not seen in either eNOS -/- and eNOS +/+ animals (Fig. 8F).
Thirty Minutes of Ischemia Increases HSP-25 ExpressionWe previously reported that increased HSP-25 expression is associated with the late phase of ischemic preconditioning (2, 6). When the levels of HSP-25 expression were evaluated by Western blot analysis, 30 min of bilateral ischemia increased HSP-25 expression, and the increase in HSP-25 protein expression was sustained for at least 12 weeks after the preconditioning in BALB/c male mice (Fig. 9A). After peaking 24 h after ischemia, the increased expression of HSP-25 decreased over time (Fig. 9A). The level of expression at 8 days after 30 min of bilateral ischemia is elevated to nearly equivalent levels in iNOS -/- or iNOS +/+ mice, at a time when there are major differences in functional changes in response to subsequent ischemia between iNOS +/+ and iNOS -/- mice (Fig. 9B).
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Effect of Fifteen Minutes of Ischemic Preconditioning on Postischemic
Renal Function, Postischemic Disruption of Actin Cytoskeleton, Expression of
iNOS, eNOS, -Smooth Muscle Actin, or HSP-25 or Response to
L-NIL TreatmentSince 15 min of prior ischemia does not
result in an increase in plasma creatinine and partially protects the kidney
from ischemia (2), we evaluated
whether the protection afforded by 15 min of ischemia involves NO production
or expression of iNOS protein. Fifteen minutes of prior bilateral ischemia
results in partial protection of the kidney from 30 min of bilateral ischemia
8 days later (Fig.
10A). Fifteen minutes of bilateral ischemia does not
increase plasma creatinine (Fig.
10A), disrupt the actin cytoskeleton
(Fig. 10B), increase
the expression of iNOS or eNOS protein
(Fig. 10, C and
D), or result in fibrosis as reflected by
-smooth
muscle actin expression (Fig. 10,
E and G). Fifteen minutes of ischemia results in
an increase of HSP-25 expression, and the increased HSP-25 expression is
sustained for 8 days after the ischemia
(Fig. 10F). In
contrast to the affect of L-NIL observed with 30 min of prior
ischemia (Fig. 8E),
the protection afforded by 15 min of prior ischemia was not mitigated by
L-NIL (Fig.
10H).
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DISCUSSION |
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Ischemia/reperfusion results in enhanced leukocyte-endothelial adhesion interactions in the small vessels of the outer medulla (7). These interactions can alter blood flow characteristics in the outer stripe of the outer medulla, and further impair oxygen supply to the proximal straight tubule, the major site of injury in this model of ischemic renal failure (39, 40). Our laboratory has reported that treatment with anti-neutrophil serum or anti-intercellular adhesion molecule-1 (ICAM-1) antibody or deletion of ICAM-1 from mice protects animals from ischemic renal failure (32, 41). Recently, we also observed preconditioning due to prior ureteral obstruction, which is associated with a reduced subsequent postischemic leukocyte infiltration and congestion in the outer medulla (6). While our observations do not distinguish between leukocyte trapping as a cause or effect of the protection, they suggest that the reduced postischemic inflammatory response may be responsible for the protection.
Ischemia/reperfusion in the kidney activates NOS enzymes (42) and increases the expression of NOS proteins (20, 40, 43), Kim-1 (30, 31), and HSPs (2, 6, 20, 21, 40, 44). In the present study we observed that 30 min of ischemic preconditioning increases the expression of eNOS, iNOS, and HSP-25 and the increased expression of eNOS, iNOS, and HSP-25 is sustained for 12 weeks. The increases in iNOS, Kim-1, and HSP-25 expression are reduced over time postischemia. The sustained expression of these proteins might be related to the presence of irreversible injury and inflammatory responses causing ongoing generation of reactive oxygen species and a persistent dedifferentiation and proliferation of tubular epithelial cells. Since Kim-1 is expressed in dedifferentiated cells (30), the sustained Kim-1 expression might reflect ongoing responses of kidney epithelial cells to persistent inflammatory stimuli and/or repair processes of the kidney epithelial cells after ischemic preconditioning. Recently Basile et al. (45, 46) reported in rats that severe ischemia results in irreversible injury and progresses to chronic renal disease. In the present studies in mice 30 min of ischemia results in renal fibrosis indicating persistence of renal injury. This persistent interstitial response with fibrosis may lead to the persistent increase in iNOS, Kim-1, and HSP-25 expression.
Our data indicate that pharmacological inhibition of NOS proteins by L-NNA enhances the susceptibility to ischemia. Genetic deletion of iNOS increases the kidney susceptibility to ischemia. By contrast gene deletion of eNOS in mice has no effect. L-NIL mitigates the protection afforded by 30 min of prior ischemia. Thus, genetic deletion of iNOS or pharmacological inhibition of NOS proteins by L-NNA or iNOS by L-NIL mitigates the protection afforded by 30 min of ischemic preconditioning, whereas genetic deletion of the eNOS gene does not affect the protection. L-NNA or L-NIL treatment, or iNOS gene deletion, however, does not completely abolish the protection indicating that NO/iNOS is important for ischemic preconditioning but does not account for the entire phenomenon. Furthermore L-NIL has no effect on the protection afforded by 15 min of prior ischemia indicating that the mechanism involved in the protection 8 days after a prior short ischemic period does not involve iNOS.
Many studies have demonstrated that the increased activity of NOS is associated with reduced ischemia/reperfusion-induced injury and an increase of blood flow in the ischemic region (47). By contrast, Ling et al. (21) reported that genetic deletion of the iNOS gene in mice, in part, attenuates postischemic kidney dysfunction through higher postischemic expression of HSP-72 (21). The discrepancy between our results and their report might be due to the different levels of NO production associated with the degree or method of ischemia/reperfusion injury. Noiri et al. (20) reported that treatment with NG-nitro L-arginine methyl ester (L-NAME), an inhibitor of NOS worsens the postischemic renal function, whereas treatment with antisense oligodeoxynucleotides targeting iNOS protects the kidney. NO may have a protective effect due to its anti-apoptotic action and effects to decrease leukocyte-endothelial interactions. Nitric oxide can result in vasodilatation and inhibition of platelet plug formation, as well as reduction of the inflammatory response. In contrast NO can induce injury via lipid peroxidation, DNA damage, and pro-apoptotic effects, which are implicated in ischemia/reperfusion injury (19). Goligorsky et al. (19) demonstrate that cellular effects of NO depend on its concentration, site of release and duration of action. Low levels of NO may be protective but higher levels may be detrimental (19).
Bolli and co-workers (48, 49) suggest that prior short episodes of ischemia/reperfusion in the heart without severe injury increases iNOS expression leading to production of NO. These investigators proposed that the early production of NO stimulates iNOS expression through intracellular signal pathways, and the induction of iNOS protein mediates the late phase protection afforded by ischemic preconditioning in heart. Recently, Bolli et al. (5, 14, 25) have reported that iNOS inhibition by pharmacological or genetic modulation of mice abolished the protection afforded by short episodes ischemia/reperfusion preconditioning at 2448 h in heart. In kidney, Jefayri and colleagues reported that there is increased NO release secondary to increased NOS expression 6 h after 4 cycles of 4 min of ischemia followed by 11 min of reperfusion (43). In comparison to our studies these prior studies are examining relatively short term effects not studied beyond 48 h.
Since the genetic deletion of the eNOS gene in our studies does
not abolish the protection afforded by preconditioning, our results indicate
that the increase of eNOS expression is not required for the late phase of
protection in the kidney. The NO generated from iNOS after the initial
ischemia/reperfusion may react with reactive oxygen species (ROS), such as
superoxide
to
generate other oxidant species such as ONOO- and/or OH·. ROS
have been found to be essential for preconditioning in the heart
(50). In the present studies,
the protection afforded by 30 min of ischemic preconditioning, which increases
iNOS expression, is partially inhibited by inhibition of iNOS, whereas the
protection afforded by 15 min of ischemia, which did not increase iNOS
expression, is not inhibited by treatment with L-NIL. With the
longer period of ischemia there is persistent renal injury, which is not
present with the shorter ischemic periods. Thus the prolonged protection
induced by 30 min of ischemic preconditioning is partially mediated by the
increase in generation of NO by iNOS. This persistent increase in iNOS
expression may be caused by recurrent low grade tissue injury.
As an extension of our previous reports
(2,
6) we observed that enhanced
HSP-25 expression persists for 12 weeks after the initial ischemia. HSPs
confer cytoprotection against ischemia, ATP depletion or reactive oxygen
species in many organs and cultured cells through stabilization of the actin
cytoskeleton and/or reduction of the inflammatory reaction
(5154).
We also reported that the overexpression of HSP-25 protein in renal epithelial
LLC-PK1 cells using adenoviral vectors protects cells from injury due to
oxidants and chemical anoxia
(6). HSPs suppress
cytokine-induced IL-8 and TNF- expression and the translocation of the
p65 subunit of NF-
B, which regulates iNOS expression
(13,
55). In iNOS -/- mice,
however, where the preconditioning effect is mitigated, the expression of
HSP-25 is elevated in both iNOS -/- and iNOS +/+ animals. Fifteen minutes of
ischemic preconditioning also results in an increase of HSP-25 expression and
the increased HSP-25 persists for 8 days later. It is possible that the
residual protection seen in iNOS -/- mice and the protection afforded by short
periods of ischemia are related to up-regulation of HSP-25.
In summary, we have demonstrated that the mouse kidney is profoundly
protected against ischemia/reperfusion injury imposed up to 12 weeks after an
initial 30-min ischemic exposure. Preconditioning occurs in more than one
mouse strain indicating it is not strain-specific. Preconditioning induces
iNOS expression. iNOS inhibition by pharmacological inhibitors or genetic
deletion partially abolishes the protective effects of preconditioning, but
eNOS gene deletion does not. Inducible NOS inhibition does not
mitigate protection 8 days after a shorter 15 min of ischemia under conditions
where the ischemia results in much less chronic interstitial accumulation of
-smooth muscle actin. These results indicate that iNOS plays an
important role in kidney protection afforded by prolonged ischemic
preconditioning which may be explained by chronic interstitial inflammation,
which stimulates iNOS to generate NO, which in turn attenuates postischemic
interactions between leukocytes and endothelium.
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FOOTNOTES |
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To whom correspondence should be addressed: MRB4, Brigham and Women's
Hospital, 75 Francis St., Boston, MA 02115. Tel.: 617-732-6020; Fax:
617-582-6010; E-mail:
joseph_bonventre{at}hms.harvard.edu.
1 The abbreviations used are: HSP, heat shock protein; MPO, myeloperoxidase;
PBS, phosphate-buffered saline; TRITC, tetramethylrhodamine isothiocyanate;
Kim-1, kidney injury molecule-1; L-NIL,
L-N6-(1-iminoethyl) lysine; NOS, nitric-oxide synthase; iNOS,
inducible NOS; eNOS, endothelial NOS; i.p., intraperitoneal; L-NNA,
N-nitro-L-arginine; SMA, smooth muscle actin.
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REFERENCES |
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