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Cardiovascular Research 67 (2005) 594 – 603
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Review
Cellular and molecular mechanisms of sex differences in
renal ischemia–reperfusion injury
Ajay Khera,c, Kirstan K. Meldruma, Meijing Wanga,c, Ben M. Tsaia,
Jeffrey M. Pitchera, Daniel R. Meldruma,b,c,*
a
Departments of Surgery, Indiana University School of Medicine, Indianapolis, Indiana, United States
Cellular and Integrative Physiology, Indiana University School of Medicine, Indianapolis, Indiana, United States
c
Indiana Center for Vascular Biology and Medicine, Indiana University School of Medicine, Indianapolis, Indiana, United States
b
Received 7 March 2005; received in revised form 21 April 2005; accepted 3 May 2005
Available online 9 June 2005
Time for primary review 20 days
Abstract
Renal ischemia – reperfusion (I/R) is an important etiopathological mechanism of acute renal failure (ARF). Despite improvements in the
treatment of ARF, it is associated with significant morbidity and mortality. I/R injury also occurs during renal transplantation and leads to
reduced allograft survival. Sex differences have been found in I/R injury in many different organs including the kidney. Women have half the
mortality of men in ARF. In animal models also, females are protected against renal I/R injury. The mechanisms by which sex affects the
outcome to renal I/R injury are being actively investigated. This review will examine the evidence for gender differences in renal I/R injury
and discuss the probable mechanisms by which sex affects the renal response to I/R injury.
D 2005 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.
1. Introduction
Acute renal failure (ARF) caused by renal ischemia/
reperfusion (I/R) is an important clinical problem. Even
though great progress has been made in patient care, there is
still high morbidity and mortality associated with ARF.
Renal I/R injury is also an important determinant of allograft
survival after transplantation [1]. Studies have shown that
early I/R injury can lead to the initiation of inflammatory
cascade, which may result in delayed graft function and
decreased long term renal allograft survival [1,2]. Blocking
the initial inflammation prevents this loss of function [3– 9].
This is similar to the important role of inflammation in
myocardial I/R [10,11]. In the myocardium, sex differences
exist in the inflammatory response and the outcome after I/R
[12 –15] suggesting that the same may be true for renal I/R.
* Corresponding author. 545 Barnhill Drive, Emerson Hall 215, Indianapolis, Indiana 46202, United States. Tel.: +1 317 313 5217; fax: +1 317
274 2940.
E-mail address: [email protected] (D.R. Meldrum).
In addition, sex differences exist in different renal
diseases. Studies suggest that progression of renal disease
is faster in men. A meta-analysis by Neugarten et al. [16]
showed that males had a rapid rate of progression of renal
disease in membranous nephropathy, IgA nephropathy and
autosomal dominant polycystic kidney disease and had
worse outcome in chronic renal disease. As this study did
not look at the baseline differences between genders,
questions have been raised about whether gender is an
independent factor in these differences or that men had
greater risk factors, which led to this disparity. Furthermore, a study evaluating the efficacy of angiotensinconverting enzyme inhibitors on the progression of renal
disease found that after adjusting for baseline variables
females had faster progression instead of slower [17].
However, the mean age of the patients was 52 years and
hence the majority of women may have been postmenopausal. The Modification of Diet in Renal Disease
(MDRD) study showed that females, especially < 52
years, had a slower rate of progression [18]. However,
after adjusting for proteinuria, blood pressure and HDL
0008-6363/$ - see front matter D 2005 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.cardiores.2005.05.005
A. Kher et al. / Cardiovascular Research 67 (2005) 594 – 603
cholesterol the gender differences were no longer significant. In contrast, studies looking at outcomes in ARF
patients have shown that men have twice the mortality of
women and have found that gender is an independent
predictor of mortality in ARF [19 –21].
Consistent with the clinical studies in ARF, animal
studies have also shown females to be protected against
renal I/R injury. A study by Park et al. [22] showed that
males had deterioration of kidney function after bilateral
renal ischemia of 30 min while females were relatively
protected and developed dysfunction only after 60 min of
ischemia. They also showed males to have a higher
mortality rate. Similarly, Muller et al. [23] reported that
595
8% males compared with 75% females survived seven days
after ischemia. In another study, Fekete et al. [24] showed
that females have lower blood urea nitrogen, serum
creatinine and less severe tubular necrosis after renal I/R
compared to males. These studies show that sex differences
in renal I/R injury exist.
This review will provide a brief overview of the
pathophysiology of renal I/R injury followed by a discussion of the possible mechanisms of sex differences in
renal I/R. For greater detail on renal I/R injury, the reader
may refer to some excellent reviews that have recently
covered different aspects of the pathogenesis of ischemic
ARF [25 – 33].
A
Epithelium
Actin Cytoskeleton
Na+ /K+- ATPase
Basement
Membrane
Leukocyte
Endothelium
Basement
Membrane
Loss of polarity
Apical Na+ /K+- ATPase
B
Epithelium
Disrupted actin
cytoskeleton
Epithelial
inflammatory cytokine
Leukocyte
inflammatory cytokine
Leukocyte
Endothelial
adhesion molecule
Basement
Membrane
Fig. 1. Schematic illustration of the renal inflammatory response after ischemia – reperfusion. (A) Pre-ischemic kidney with sodium – potassium ATPase (Na+/
K+ ATPase) localized to the basolateral membrane. (B) Post ischemia/reperfusion kidney with disruption of actin cytoskeleton and loss of polarity in epithelial
cells causing apical localization of Na+/K+ ATPase. The injured epithelial and endothelial cells also secrete inflammatory cytokines which upregulate adhesion
molecules. Leukocytes are recruited and activated by the adhesion molecules and inflammatory mediators present. The activated leukocytes also generate
inflammatory cytokines thus further increasing the injury.
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A. Kher et al. / Cardiovascular Research 67 (2005) 594 – 603
2. Renal ischemia reperfusion injury
Renal I/R injury occurs through a complex interaction
between renal hemodynamics, inflammatory mediators,
endothelial and tubular injury. The kidney receives 25%
of the cardiac output but the majority goes to the cortex and
hence even slight changes in perfusion may lead to ischemia
of the medulla. The S3 segment of the proximal tubule in
the outer strip of the outer medulla is the most susceptible to
I/R injury [34]. Differences between these cells and other
cells in the medulla are being investigated to identify
reasons for their increased susceptibility [35].
During I/R injury renal endothelial and parenchymal
cells secrete proinflammatory cytokines (TNF, IL-1, IL-6,
etc), chemokines (MCP, IL-8, etc) and activate complement
[25]. The cytokines and the reactive oxygen species (ROS)
produced by I/R injury upregulate the expression of
adhesion molecules like ICAM, VCAM and P selectin
[27]. The combination of chemokines, cytokines and
adhesion molecules leads to recruitment, activation and
sequestration of leukocytes, which generates further ROS
and cytokines and potentiates the injury (Fig. 1A and B).
Endothelial dysfunction is an important component of
initiating and continuing renal tubular epithelial injury and
contributes to the pathogenesis of ischemic ARF [28].
Endothelial injury may aggravate the inflammatory response
through loss of normal nitric oxide (NO) production due to
inhibition of endothelial nitric oxide synthase (eNOS). NO
reduces leukocyte-induced injury by blocking leukocyte
sequestration and activation. However, I/R also increases
inducible nitric oxide synthase (iNOS), which potentiates
injury [36,37] as the nitric oxide produced reacts with
oxygen radicals to form peroxynitrite [38]. Also, the high
output production by iNOS might suppress eNOS [39]. This
imbalance between the two NOS may be an important
component of renal I/R injury.
Due to I/R injury, there is disruption of the actin
cytoskeleton, loss of tight junctions and adherens junctions
in the proximal tubular epithelium. This causes a change in
the localization of polarized membrane proteins, especially
sodium – potassium ATPase (NKA) (Fig. 1A and B). NKA is
normally localized to the basolateral plasma membrane but
after ischemia appears on the apical plasma membrane [40].
This reduces the efficiency of transcellular Na+ transport
and increases Na+ delivery to the distal tubules leading to
glomerular vasoconstriction and decreased GFR through
tubuloglomerular feedback [25]. In addition, I/R causes
decreased NKA activity, due to ATP depletion, leading to
increased intracellular Na+ concentration, which increases
intracellular Ca2+ and causes increased injury.
I/R injury causes cell death by both necrosis and
apoptosis. Recently it has been shown that smaller insults
lead to apoptosis while larger insults lead to necrosis. Our
group has shown that TNF is involved in apoptosis after
renal I/R and that p38 mitogen activated protein kinase
(MAPK) and nuclear factor kappa B are crucial for TNF
production and TNF mediated apoptosis [3,5,6,8,9]. Two
pathways of apoptosis have been clearly delineated each of
which leads to activation of the downstream effector
caspase-3. One is receptor dependent and initiated by
activation through death domains (TNF receptor associated
and Fas associated), which activate caspase-8 followed by
caspase-3. The other pathway is through mitochondrial
release of cytochrome c, which activates caspase-9 and then
caspase-3. The cells have many molecules that regulate
these apoptotic pathways. Important among them are
phosphatidylinositol 3 kinase (PI3K)/Akt and members of
the Bcl-2 family.
3. Mechanisms of gender differences in renal response to
I/R injury
In comparison to myocardial I/R, gender differences in
renal I/R have not been that extensively studied. Certain
mechanisms for these sex differences in renal I/R have been
proposed but the exact mechanism remains to be determined. However, it is a field of study under active
investigation and the rest of the review will focus on the
possible mechanisms.
Sex differences likely exist at several levels, but the
primary factor may be sex hormones. In renal I/R, Muller et
al. [23] have reported that castration and sexual immaturity
did not affect the I/R injury produced in females but
decreased the injury produced in males suggesting that
testosterone may play a bigger role in these gender
differences than estrogen. Park et al. [22] showed similar
results with castration of males and females, in addition they
showed that testosterone administration increased renal I/R
injury in females, ovariectomized females and castrated
males while administration of estrogen provided protection
to males. Thus these studies suggest that testosterone or the
ratio of testosterone/estrogen may be the important determinant of these sex differences in renal I/R injury.
Studies are currently investigating the molecular mechanisms of these gender differences. The cellular mechanisms
and signaling pathways that have been implicated in these
sex differences are discussed below.
3.1. Mitogen activated protein kinases
Mitogen activated protein kinase (MAPK) is a family of
Ser/Thr protein kinases that regulate many cellular processes. They are activated by upstream kinases referred to as
MAP kinase kinase (MAPKK or MEK) which themselves
are activated by MAP kinase kinase kinases (MAPKKK or
MEKK). This sequence of phosphorylation causes amplification of the signal. The MAPK family is divided into 3
subfamilies: extracellular regulated kinase (ERK), c-Jun Nterminal kinase (JNK) and p38. These MAPKs are activated
by I/R injury not only in the kidney but also other organs.
ERK promotes cell survival while JNK and p38 lead to cell
A. Kher et al. / Cardiovascular Research 67 (2005) 594 – 603
death. Some studies postulate that it may be the balance
between the two (ERK versus JNK/p38) that determines the
fate of the cell [41]. Renal I/R causes necrosis predominantly of the proximal tubules (PT), especially the S3
portion [34], while the thick ascending limb (TAL), distal
convoluted tubules and collecting ducts are relatively
spared. There is increased ERK phosphorylation in TAL
but not in PT cells while JNK is activated in both these cells
[35]. Hence it has been speculated that this difference in the
response may be the cause for the relative difference in the
survival of these cells. Other studies have also shown that
renal I/R induces JNK [42]and inhibition of JNK decreases
the development of ARF [43], which is consistent with other
studies showing that JNK promotes cell death [44,45]. In
addition, ERK inhibition decreased survival in TAL cells
while ERK activation increased it in PT cells [35]. Studies
on renal ischemic preconditioning [46] and transient ureteric
obstruction [47] have shown that previous ischemia or
ureteric obstruction provides protection against subsequent
ischemic injury. These studies have shown that these
interventions decrease the production of JNK and p38
caused by the ischemic insult while I/R induced production
of ERK is not affected. Hence, providing further evidence
that it maybe the balance between ERK and JNK/p38 which
is the determinant of I/R injury. Similar results have also
been found with chemical preconditioning using cyclosporine and FK506 [48].
Park et al. [22] found that JNK is activated with ischemia
but more so in males than females. Castration reduced JNK
activation in males but did not alter it in females. This
suggests that testosterone may play a more important role in
JNK activation. In support, they found that testosterone
administration to females increased JNK activation. However, estrogen administration to males lowered JNK
activation (Table 1). They also found that I/R increases
ERK phosphorylation and it is higher in females than in
males. Ovariectomy reduced ERK activation while castration did not alter it, suggesting that estrogen may be more
important in mediating sex differences in ERK activation.
Neudling et al. [49] have shown that in cardiomyocytes 17
beta estradiol caused a rapid and transient ERK activation
while it caused a rapid and sustained rise in JNK
phosphorylation. These rapid effects of estrogen are thought
to occur through the nongenomic action of sex steroids.
Kousteni et al. [50] have described these nongenomic
actions in the bone. They showed that estrogen through
597
Fig. 2. Possible mechanisms by which estrogen provides protection from
renal ischemia/reperfusion (I/R) injury. Plain lines indicate activation or
increase while dashed/interrupted lines indicate inhibition or decrease ( – ).
(A) Estrogen acts through estrogen receptor (ER) to activate Src and
phophatidylinositol 3-kinase (PI3K). (B) Src activates extracellular
regulated kinase (ERK), which increases endothelial nitric oxide synthase
(eNOS) and is anti-apoptotic. (C) PI3K activates Akt, which is antiapoptotic, and increases eNOS that leads to increased nitric oxide (NO)
production. (D) NO protects by causing vasodilation directly and by
decreasing endothelin production. (E) Estrogen also acts as an antioxidant
to decrease the oxidative stress and hence maintains sodium – potassium
ATPase (NKA) activity preventing the I/R induced increase in intracellular
sodium ([Na+]) concentration. The increased [Na+] concentration leads to
increased intracellular calcium ([Ca2+]), which produces cellular injury.
the estrogen receptor activated the Src/Shc/ERK pathway
and mediated the antiapoptotic effect of estrogen (Fig. 2A).
Interestingly, they also showed that this effect could be
mediated by estrogen or androgen receptors irrespective of
whether the ligand is estrogen or androgen. Migliaccio et al.
[51] have found similar findings in other cell lines. The role
of these pathways in I/R injury and in mediating the gender
differences still needs to be defined.
Our group has shown sex differences in p38 MAPK
production in myocardial I/R injury [15]. We found that
females had lower p38 MAPK activation after myocardial I/
R and this was associated with lower proinflammatory
cytokine production and better recovery of myocardial
function. In addition, we have shown that testosterone
increases the p38 MAPK activation produced by I/R [14].
Table 1
Effects of estrogen and testosterone on mechanisms involved in I/R injury
Mechanism
Estrogen
Testosterone
Mitogen activated protein kinases (MAPK) [14,22]
Endothelial nitric oxide synthase (eNOS) [22]
Endothelin [89]
ATP sensitive potassium channels (KATP) [98,102]
Akt [22,114]
Increases ERK activation, decreases JNK activation
Increases eNOS activity
Decreases production
Activates KATP channels
Increases the sustained activation of Akt
Increases JNK and p38 activation
Decreases eNOS activity
Activates KATP channels
Decreases the sustained activation of Akt
ERK indicates extracellular regulated kinase, JNK is c-Jun N-terminal kinase and p38 is p38 mitogen activated protein kinase.
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A. Kher et al. / Cardiovascular Research 67 (2005) 594 – 603
Similarly, Angele et al. [52] have shown that testosterone is
responsible for the increased p38 MAPK activation in male
macrophages after trauma-hemorrhage. As p38 MAPK
mediates I/R induced TNF production and TNF induced
apoptosis, therefore, sex differences in p38 MAPK may be
responsible for the gender differences noted in renal I/R.
These studies show that there are sex differences in
MAPK activation after I/R and they may mediate the gender
differences in renal I/R injury. However, more studies are
needed to further delineate the involvement of these
pathways.
3.2. Nitric oxide
Nitric oxide (NO) plays an important role in renal
vascular tone and hemodynamics [53,54]. NO is produced
from L-arginine by nitric oxide synthase (NOS). There are 3
NOS: neuronal NOS (nNOS), endothelial NOS (eNOS) and
inducible NOS (iNOS). Of these, nNOS and eNOS are
calcium dependent (cNOS) while iNOS is calcium independent (ciNOS). The beneficial effect of NO in renal I/R
injury has been shown by the protective effect of NO donors
[55 –57]. Sex differences in NO have been shown in the
kidney in different animal models [58 – 60]. Females have
higher eNOS mRNA and protein expression compared to
males [61,62]. Also, ovariectomy reduced eNOS while
estrogen replacement increased it.
Park et al. [22] showed that after renal I/R ciNOS
activity increased in both males and females while cNOS
activity decreased in males but increased in females. This
difference in cNOS activity may be an important component of the gender difference noted. They also showed that
testosterone decreased the cNOS activity post I/R while
estrogen increased it. Similarly, many studies in other
organs have also shown that estrogen and testosterone
modulate NOS expression and NO production [49,63 – 66].
Estrogen activates eNOS in a biphasic manner [67]. The
initial increase is mediated by ERK [49,68] while the latter
occurs through phophatidylinositol 3-kinase (PI3K) [67]
(Fig. 2B and C). Both these pathways are activated
through the rapid, nongenomic actions of estrogen. ERK
is probably activated through the Src/Shc/ERK pathway
while estrogen receptor alpha interacts with a subunit of
PI3K and stimulates it leading to activation of both eNOS
and Akt. Thus, estrogen may mediate the gender differences in renal I/R through differences in NOS expression
and NO production.
3.3. Sodium –potassium ATPase
Elevated intracellular calcium causes cellular injury
during renal I/R [69]. An increase in intracellular Na+
concentration correlates with the increased Ca2+. Accumulation of Na+ is caused by inhibition of sodium – potassium
ATPase (NKA) activity, which occurs due to decreased ATP
production. The increased Na+ leads to Na+/Ca2+ exchange
by sodium – calcium exchanger (NCX) and hence Ca2+
overload (Fig. 2E). Sex differences in renal NKA have been
shown by Fekete et al. [24]. They found that the mRNA
expression of NKA a1 subunit was higher in females and
this difference was further increased after I/R. I/R lead to a
decrease in mRNA expression of NKA a1 subunit in both
genders but it was more severe in males. However, protein
levels of NKA a1 and NKA activity were similar between
genders in the control groups but after I/R males had a
greater decrease in both. In addition, estradiol has been
shown to stimulate NKA in the myocardium and antagonizes the depression in NKA caused by ischemia [70]. This
suggests that differences in NKA activity might mediate sex
differences in renal I/R injury.
Further support for the role of NKA in mediating sex
differences is provided by studies on NCX in the heart.
NCX countertransports three Na+ ions for one Ca2+ ion.
NCX can function in either direction depending on the
transmembrane gradients of the ions and the membrane
potential. NCX normally works in the calcium removal
mode but in ischemia there is an increase in intracellular
Na+ and change in the membrane potential, which leads to
reversing of the NCX [71]. Isolated hearts of transgenic
mice overexpressing NCX had greater I/R injury than wild
type mice [72]. However, female transgenic mice were
relatively protected from the increased I/R injury compared
to male transgenics. Sugishita et al. [73] studied myocytes
isolated from NCX overexpressing mice and found lower
intracellular calcium after metabolic inhibition in females.
Females had lower intracellular sodium suggesting that the
decreased calcium in females might be due to this. Due to
the stoichiometry of NCX, exchanges 3 sodium ions for 1
calcium ion, even small differences in intracellular sodium
would lead to larger differences in calcium. Interestingly,
they also found that estrogen could reduce the increase in
intracellular sodium and calcium produced by metabolic
inhibition. In another study, Sugishita et al. [74] showed that
tamoxifen did not block the estrogen mediated inhibition of
the rise in Ca2+. Also estrone, estriol, alpha and beta
estrogen produced the same results while testosterone did
not, suggesting that these effects might be mediated by
antioxidant mechanisms due to the hydroxyl group at the C3
position of the A ring of the steroid molecule. Oubain (NKA
inhibitor) and KB-R7943 (NCX inhibitor) blocked these
protective effects. This suggests that the antioxidant effect
helps maintain NKA function and this decreases Na+
accumulation, which leads to decreased Ca2+ entry through
NCX and hence decreased injury (Fig. 2E).
Similar to the role of NCX in myocardial I/R injury,
studies with NCX inhibitors have shown that NCX plays an
important role in renal I/R injury [75,76]. In addition,
heterozygous knockouts of NCX were relatively protected
against renal I/R injury [77,78]. Thus, estrogen might
through its antioxidant effects maintain NKA function,
mediate differences in intracellular sodium and calcium
concentrations and finally protect against renal I/R injury.
A. Kher et al. / Cardiovascular Research 67 (2005) 594 – 603
3.4. Endothelin
Endothelin (ET)-1, a potent vasoconstrictor, is elevated
in the plasma in patients with ARF and in the kidney in
animal models of ischemia induced ARF [79 –81]. This
increase occurs especially in the peritubular capillaries and
may be responsible for tubular necrosis [80]. Administration
of ET-1 antibodies or ET A receptor antagonists but not ET
B receptor antagonists decreased the renal dysfunction
produced by I/R [81 – 85].
Muller et al. [23] showed gender differences in recovery
from renal I/R injury and that these differences were
abolished by administration of LU 135252 (a selective
ETA receptor antagonist) suggesting that ET might mediate
the gender differences in renal I/R injury. Estrogen has been
shown to decrease ET-1 production [86 – 88]. Takaoka et al.
[89] studied male rats with estrogen administration and they
found that estrogen dose dependently reduced renal injury
and dysfunction and also decreased the ET-1 overproduction
caused by I/R injury. However, the mechanism by which
estrogen reduces endothelin production is not defined.
The possible mechanisms may include estrogen increasing NO which downregulates endothelin or that estrogen
decreases Ca2+ overload and hence decreases the endothelin
production. Different NO donors have shown that increased
NO inhibits the increased endothelin-1 production caused
by I/R [90,91] (Fig. 2D). Jeong et al. [91] showed that
sodium nitroprusside (a NO donor) given before renal
ischemia decreased endothelin production and provided
protection from I/R. In addition, decreased ET-1 production
has been observed in studies using NCX inhibitors and in
mice heterozygous for NCX undergoing renal I/R
[75,76,78]. This suggests that intracellular Ca2+ overload
leads to increased ET-1 production. In support, studies have
shown that increase in intracellular Ca2+ through reverse
NCX does occur in vascular endothelial cells [92] and that
increased Ca2+ does induce expression of ET-1 [93].
3.5. Adenosine and ATP sensitive potassium channels
During hypoxia, ischemia or inflammation adenosine is
produced locally and mediates a variety of functions. These
heterogeneous effects are mediated through multiple adenosine receptors localized in different regions of the kidney.
Four G protein-coupled receptors have been identified: A1,
A2A, A2B, and A3. A1 and possibly A3 receptor activation
produce preconditioning and provide protection, possibly by
increasing mitochondrial ATP sensitive potassium (KATP)
channel activity [94,95]. A2A agonists also provide protection from I/R injury, which is correlated with an inhibition
of neutrophil oxidase activity, neutrophil accumulation,
endothelial adhesion molecule expression and cytokine
production [96]. A2A receptor also mediates the vasodilation
produced by adenosine and increases renal blood flow and
glomerular filtration rate. In renal ischemia, A1 and A2A
receptor activation is protective [96,97].
599
Estrogen has been shown to decrease infarct size in
cardiac I/R through mitochondrial KATP channels [98].
Estrogen has also been used in coronary angioplasty patients
and reduced myocardial ischemia caused by balloon
inflation, possibly through KATP channels [99]. The opening
of these channels results in potassium influx, which
decreases the driving force for calcium uptake and decreases
calcium induced injury. Similar to its role in cardiac I/R,
KATP channels are protective in renal I/R injury. Diazoxide
(a KATP channel opener) provided protection in renal I/R
[100]. These results suggest that estrogen may activate KATP
channels and provide protection from renal I/R through it.
In contrast, testosterone has been shown to block
adenosine mediated vasodilation [101]. However, Er et al.
[102] have demonstrated in a cellular model that testosterone decreased ischemia-induced death of cardiomyocytes by
activating KATP channels. Thus, further research is needed
to clarify the effect of testosterone on adenosine and KATP
channels and the role of these mechanisms in renal I/R.
3.6. Apoptosis
I/R injury can lead to cell death by necrosis or apoptosis.
Prolonged renal ischemia leads to necrotic cell death while
in shorter periods of renal ischemia apoptosis is the primary
mode of cell death [103]. Inhibiting renal apoptosis protects
against renal I/R injury [30,104]. Testosterone has been
shown to promote apoptosis in vascular endothelial cells
and renal tubular cells [105,106]. Verzola et al. [106]
showed a dose dependent effect of testosterone on apoptosis
in renal tubular cells after serum deprivation. They also
showed that testosterone upregulated Fas, Fas ligand and
Fas associated death domain. Testosterone also decreased
Bcl-2 while increasing Bax expression. The use of caspase-3
inhibitor, caspase-8 inhibitor or caspase-9 inhibitor reduced
the apoptosis produced by testosterone. These studies
indicate the possible role of testosterone in promoting
apoptosis though further research is needed to delineate the
mechanism of this effect.
Studies suggest that one of the critical regulators of cell
survival is Akt (protein kinase B) [107–109]. Akt is activated
by many different stimuli primarily through phophatidylinositol 3-kinase (PI3K) (Fig. 2C). Akt is present in the cytoplasm
but after phosphorylation (activation) it translocates to the
nucleus and phosphorylates its targets. One such substrate is
forkhead [110], which is proapoptotic in the dephosphorylated
state and is translocated from the nucleus to the cytoplasm on
phosphorylation. Activation of Akt and phosphorylation of
forkhead proteins has been shown after renal I/R injury and in
LLC-PK1 cells undergoing chemical anoxia [111]. Other
potential mechanisms for the antiapoptotic effect of Akt
include phosphorylation of caspase 9 and phosphorylation of
Bcl-XL/Bcl-2 associated death promoter (BAD) [109,112].
Sex differences have been found in Akt in different organs
including the kidney. Park et al. [22] found that female kidneys
have higher basal levels of Akt than males. Castration and
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A. Kher et al. / Cardiovascular Research 67 (2005) 594 – 603
estrogen supplementation of males led to an increase in basal
Akt while ovariectomy and testosterone supplementation of
females reduced basal Akt. Ischemia led to an increase in Akt
but this was transient in the non-protected groups. Similar to
basal Akt levels, sustained Akt activation after ischemia was
produced with estrogen and decreased by testosterone.
Similarly, Camper-Kirby et al. [113] observed gender differences in Akt in cardiomyocytes. They found that women had
higher nuclear localization of phospho-Akt in the myocardium.
They also observed that estradiol administration increased the
nuclear localization of phopho-Akt in neonatal cultured
cardiomyocytes. In addition, Patten et al. [114] showed that
17 beta estradiol administration to ovariectomized female
increased Akt activation and reduced apoptosis after in vivo
coronary artery ligation. In an in vitro model they showed that
the antiapoptotic effect of estradiol was mediated through the
estrogen receptor and PI3K-Akt mediated pathway. Simoncini
et al. [67] have further clarified the mechanism of Akt
activation by estrogen. They showed that estrogen receptor
alpha interacts with a subunit of PI3K (nongenomic action) and
stimulates it leading to activation of Akt and eNOS (Fig. 2C).
4. Summary
Though association of these pathways with gender
differences in renal I/R has been found, further research is
needed to prove their mechanistic involvement in the gender
differences noted. Sex hormones and their effects on
MAPK, NO, NKA, endothelin, adenosine, KATP channels
and Akt may be involved in these gender differences. A
better understanding of these mechanisms may allow
selective targeting and therapeutic manipulations to help
decrease the significant morbidity and mortality caused by
ischemic acute renal failure.
Acknowledgments
This work was supported in part by NIH R01GM070628
(DRM), the Clarian Values Fund (DRM), the Showalter
Trust (DRM), and the Cryptic Masons Medical Research
Foundation (DRM, MW).
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