Hypoxia: a Review

Abstract

Tissue hypoxia occurs where there is an imbalance between oxygen supply and consumption. Growing evidence from experimental and clinical studies points to the fundamental and patho-physiologic role of hypoxia in cancer, ischemic tolerance, and stroke. Hypoxia-induced changes in ion homeostasis, erythropoiesis, angiogenesis, proliferation and differentiation. This review outlines hypoxia effect at molecular level and describes briefly hypoxia role in the physiological and pathological conditions.

Full text

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
____________________________________________________________________ ________________________________________________________________________________________________________ _________
43
Hypoxia: a Review
Kambiz Gilany*, Mohtaram Vafakhah.
Reproductive Biotechnology Research Center, Avicenna Research Institute, Shaheed Beheshti University of Medical
Sciences, Tehran, Iran.
*Corresponding author: e-mail address: k.gilany@avicenna.ac.ir (K.Gilany)
ABSTRACT
Tissue hypoxia occurs where there is an imbalance between oxygen supply and consumption.
Growing evidence from experimental and clinical studies points to the fundamental and patho-
physiologic role of hypoxia in cancer, ischemic tolerance, and stroke. Hypoxia-induced changes in
ion homeostasis, erythropoiesis, angiogenesis, proliferation and differentiation. This review outlines
hypoxia effect at molecular level and describes briefly hypoxia role in the physiological and
pathological conditions.
Keywords: Chronic Hypoxia; HIF; Ischemic Tolerance; Stroke; Cancer
1. INTRODUCTION
In the life of aerobic organisms, oxygen is an
essential element. The central role of oxygen is due
to the fact that it is the final acceptor of electrons in
the mitochondrial respiratory chain. This allows the
ultimate process of oxidative phosphorylation and
the generation of cellular energy, in the form of
adenosine triphosphate (ATP). ATP is used in most
reactions that are necessary to maintain cellular
viability. Under normoxia a cell continuously
maintains a high and constant ratio of cellular
ATP/ADP ratio in order to survive. The dependence
of cells on a high constant ATP/ADP ratio means a
dependence on oxygen. Therefore, a reduction of the
normal oxygen supply (hypoxia) will have
consequences on the cell viability [1,2].
Hypoxia is encountered not only in different
conditions including the patho-physiological
conditions, such as atherosclerosis, obstructive sleep
apnea, mountain sickness, ischemic diseases (stroke)
and cancer, but also in physiological processes, such
as embryonic development [3-6]. Different terms are
given in the literature about the reduction of oxygen
supply. The term hypoxemia is defined as a reduced
oxygenation of the blood. Hypoxia is defined as a
decrease in the oxygen supply to a level insufficient
to maintain cellular function. Hypoxia-ischemia
stands for the processes of hypoxia combined with
ischemia. Ischemia differs from hypoxia in that it is
not only a decrease in the oxygen supply; it also
involves a reduction of the blood flow which leads to
a decrease of nutrient supply and an accumulation of
metabolic products, including CO2, lactic acid and
ammonia [7-9].
Additionally, hypoxia response can be divided in
different time scales, including an acute, an
intermediate and a chronic response, and in different
levels of oxygen concentration, including a moderate
(5-8% O2) and an anoxic level (<1 O2) (normoxia is
21% O2) [3,10,11]. The brain is regarded as the most
hypoxia-sensitive organ because of its need for a
high oxygen supply, whereas the skeletal muscle is
amongst the most hypoxia-tolerant [12].
The first part of this review will provide a broad
description of responses to chronic moderate
hypoxia. In the second part of the review, the role of
hypoxia in the physiological and pathological
conditions will be described.
2. Sensing hypoxia
Hypoxia orchestrates a multitude of processes of
molecular pathway responses. However, in the
higher organisms, the cellular oxygen sensor itself is
unknown [1,13]. Several mechanisms have been
proposed as to how a cell senses the lack of oxygen.
The traditional mechanism of hypoxia sensing
involves a heme protein (Figure 1). This protein has
been suggested because most proteins capable of
binding O2 contain iron, which usually is in the
center of a heme moiety [13].
Hypoxia could be detected by a reversible binding of
O2 at the heme site, which causes an allosteric shift
in the hemoprotein, inactive (oxyform) to active
(deoxy) form [15]. There are many kinds of heme
containing oxygen binding proteins, but no real
candidate has been found yet [13].
Another mechanism, better known as the
“membrane hypothesis” or “membrane model”,
involves ion channels. It is reported that the ionic
currents/conductance are inhibited during hypoxia in
the O2-sensitive channels, K+- selective, Ca2+ and
Na+ channels.

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
____________________________________________________________________ ________________________________________________________________________________________________________ _________
44
Figure 1. The activity of a hemoprotein is determined by the presence or absence of bound oxygen [14].
In addition, these channels are considered to
be O2-sensitive because their modulation by O2
occurs without known modifications in cytosolic
variables such as pH, [Ca2+] or ATP [1,16].
However, how O2 interacts with the channels is
not known. It is suggested that the channels’
gates are changed either by direct allosteric]
interactions (where the sensor switches
conformation between deoxy and oxy form) or
by means of a mediator [1,15,17,18]. Another
problem that occurs in this model is the fact that
the O2-sensitive ionic currents/conductance are
either inhibited or increased in the same channel
type in different cells [1,18].
The mitochondrion itself has been suggested
to be the site of hypoxia sensing. This is based
on the fact that the mitochondria binds O2 and
represents the primary site of oxygen
consumption in the cell [19]. Furthermore,
experiments of agents mimicking hypoxia have
shown an inhibition of mitochondrial function
(oxidative phoshorylation) [20]. The main
problem in “the mitochondrial hypothesis” lies
in how the mitochondria can detect differences
in the O2 supply [15]. But the mitochondrial
involvement is an attractive proposal since it
provides a link between O2 ensing and
metabolism.
However, the search for an oxygen sensor is
wide open and none of the different suggested
mechanisms can be excluded.
3. Responses to chronic moderate hypoxia
Under chronic moderate hypoxia
multicellular organisms trigger a multitude of
cellular responses in order to survive and
maintain the oxygen homeostasis in function of
time (Figure 2). Here the most important
responses will be described.
Figure 2. Main responses to hypoxia

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
____________________________________________________________________ ________________________________________________________________________________________________________ _________
45
3.1. Ion homeostasis
The acute response to hypoxia is believed
to be significant disturbances of the ionic
homeostasis. It is suggested that the ion
channels are some of the first proteins to sense
the low oxygen level, as mentioned earlier
(hypotheses of oxygen sensing). The obvious
choice of the ion channels comes from the fact
that cells spend much of their ATP production
to maintain the ionic gradient. Under
normoxic condition, it is estimated that up to
60% of the ATP production is used by the ion-
motive ATPases, such as the Na+/K+ ATPase
and Ca2+-ATPase [21].
During hypoxia a decrease in the
intracellular ATP/ADP ratios causes an
enhanced cellular K+ efflux and Na+ and Ca2+
influx [22]. With the decreased activity of the
K+ channels, the membrane depolarizes and
activates the voltage-gated Ca2+ channels,
causing an increase in the concentration of
intracellular Ca2+. The overload of intracellular
Ca2+ causes among other things (1) changes in
the mitochondrial metabolisms, (2) an
activation of lipases and proteases which leads
to membrane damage and a release of free
fatty acids and proteins, (3) an activation of
endonuclease and (4) a generation of reactive
oxygen species (ROS) [22-25]. The bridge
between the acute (depolarization) and chronic
(changes in gene and protein expression)
responses to hypoxia is believed to be this
increase in the intracellular Ca2+. Under
chronic hypoxia, changes in the gene
expression are regulated by the oxygen-
regulated transcription factor HIF-1 (see 3.2).
The key protein, which links the acute and
chronic responses, is believed to be the CaM
kinase II. The increase in the intracellular Ca2+
levels, and its subsequence binding to
Calmodulin, leads to an activation of CaM
kinase II, which phosphorylates the co-
activator (p300) of the HIF-1 complex. This
phosphorylation induces the HIF-1
transcriptional activity [26].
Because of the limited information about
the effect of hypoxia on Na+, the exact
contribution of Na+ under hypoxia is not
known [27]. It is proposed that the increased
intracellular Na+ concentration during hypoxia
contributes to the increase in the intracellular
Ca2+ concentration by reversing the Na+/Ca2+
exchanger [28,29]. However, under chronic
hypoxia, a decreased and an increased
expression of the Na+ channels have been
reported [30].
3.2. Hypoxia-inducible factors (HIFs)
While triggering ion channels is an acute
response, at the molecular level, the hronic
response to hypoxia involves changes in the
gene expression [14]. The essential step in this
process is an activation and stabilization of the
hypoxia-inducible factors (HIFs) [31]. HIFs
are universally used as transcriptional
regulators, which are “turned on” by chronic
hypoxia. The HIF family comprises three
members, HIF-1, HIF-2 and HIF-3, with the
function of HIF-3 poorly understood. HIF-1 is
ubiquitously expressed, whereas HIF-2 is only
expressed in endothelial cells and in the
kidney, heart, lungs and small intestine [32-
34]. HIF-1 transcriptional complex was the
first transcription factor to be discovered. It
binds to DNA at hypoxia response elements
(HRE) in the enhancer or promoter region of
target genes [35]. HIF-1 complex is a
heterodimer consisting of an inducible O2-
regulated HIF-1α subunit and a constitutively
expressed HIF-1β [31].
The HIF-1 activity is tightly regulated, (for
reviews see: [4,13,36-39]). During normoxia,
HIF-1α is subjected to multiple modes of post-
translational modifications (Figure 3). HIF-1α
is hydroxylated on two proline residues in the
oxygen-dependent degradation domain by the
oxygen-sensitive prolyl hydroxylase domain
(PHD) proteins. This modification leads to an
interaction with the ubiquitin E3-containing
ligase von Hippel Lindau complex (pVHL),
which targets HIF-1α by attaching
polyubiquitin chains for proteosomal
degradation. Furthermore, HIF-1α is
hydroxylated by another oxygen-sensitive
enzyme, Factor Inhibiting HIF-1 (FIH-1), on
the asparagine residue in the C-terminal
transcriptional activation domain to prevent
interaction with the transcriptional co-activator
CBP/p300, and thereby represses the
transactivational activity of HIF-1. These post-
translational modifications keep HIF-1 in an
unstable and inactive state. The half-life time
of HIF-1α is only a few minutes (<5 minutes)
under normoxia [1]. During hypoxia the PHDs
and the FIH-1 become inactive. The lack of
hydroxylation results in stable HIF-1α, which
will dimerize with HIF-1β. The heterodimer is
then nuclear translocated and binds to the
HRE on the target genes. Since FIH-1 is
inactive, the co-activator CBP/p300 is
recruited for activation of the transcription.
It should be mentioned that the mechanism
responsible for the post-translational
modifications of HIF-1α is not known. But it
is suggested that PHD might be involved in

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
____________________________________________________________________ ________________________________________________________________________________________________________ _________
46
this regulation [40].
It is believed that approximately 1-5% of
the genome is transcriptionally regulated by
hypoxia and many of these genes are known to
be regulated by HIFs [41]. So far, more than
200 target genes have been reported to be
induced by the HIF complex. These genes are
involved in many different biological
processes, including erythropoiesis,
angiogenesis, proliferation, energy metabolism
or apoptosis [38,39,42]. However, the gene
expression pattern in response to the HIF
activation is cell-specific. Hence, the
protective response by the HIF activation in
one cell lineage may not be evident in other
cell types [42].
Figure 3. Regulation of HIF-1α during normoxia and hypoxia. [37].
3. 3. Erythropoiesis
In response to chronic hypoxia, the
capacity of red blood cells to transport oxygen
is up-regulated by the expression of genes
involved in erythropoiesis. Most notably, the
erythropoietin (EPO) gene is increased by
hypoxia, which is required for the formation of
red blood cells. An increase of the number of
the red blood cells enhances the delivery of
oxygen to tissues [43]. Furthermore, iron is
required for heme formation and is the most
limiting factor in erythropoiesis. Hypoxia
increases the expression of the iron-
metabolizing genes, including transferrin,
transferrin receptor and ferroxidase. The
increase of these genes supports the iron
supply to the erythroid tissues [44]. It is well
established that the expression of genes
involved in the erythropoiesis is regulated by
HIF-1. Indeed, oxygen-regulated EPO,
transferring receptor and ferroxidase
expression have been reported to be controlled
by HIF-1 [13, 45].
3.4. Angiogenesis
Chronic hypoxia induces angiogenesis.
Angiogenesis is the process by which new
blood vessels develop from existing
vasculature. Angiogenesis can be defined as a
multistep process, which involves endothelial
cell activation, increased blood vessel
permeability, and local rearrangement of the
basal membrane and extracellular matrix.
Angiogenesis provides a principle mechanism
for the maintenance of an adequate blood flow
to areas of insufficient oxygen supply [46].
However, angiogenesis requires production
and secretion of the so-called angiogenesis
factors, such as vascular endothelial growth
factor (VEGF), platelet-derived growth factor
(PFGF), fibroblast growth factor (FGF), and
interleukin-8 [47]. In keeping with the central
role of HIFs in responses to chronic hypoxia,
it is shown that HIF-1 directly activates the
expression genes involved in angiogenesis,
including VEGF, VEGF receptors FLT-1,
transforming growth factor-β3 (TGF- β3),
angiopoietins and genes involved in matrix
metabolism [48,49]. The most notable and
characteristic angiogenesis factor induced by
HIF-1 is VEGF. It is essential for the
proliferation and migration of vascular
endothelial cells, thereby enabling the
formation of new blood vessels [50]. Several
studies using both HIF mutant cell lines and
murine model systems have shown that HIF
signaling is required for the regulation of
VEGF. However, the relative contribution of
individual HIF family members in the
induction of VEGF expression and

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
____________________________________________________________________ ________________________________________________________________________________________________________ _________
47
angiogenesis process is controversial. The
individual contributions appear to be cell-type
dependent [51, 52]. However, blood flow
under pathophysiological conditions is
controlled by modulations of the vascular tone
through a production of NO (inducible nitric
oxide synthase), CO (heme oxygenase 1),
endothelin 1, adrenomedullin, or an activation
of the α1B-adrenergic receptor, all of which
involve HIF-1 target genes, too. Therefore,
HIF mediates angiogenesis by mechanisms,
far more complex than the simple VEFG
induction [45].
3. 5. Proliferation
Chronic hypoxia induces expressions of the
various growth factors that are known to
promote cell proliferation. This proliferation is
normally involved in initiating cell migration
and regeneration after acute or chronic
hypoxia damage [53]. Several growth factors,
most notably insulin-like growth factor 2
(IGF-2) and transforming growth factor-α, are
HIF-1 target genes. Binding of these growth
factors to their cognate receptors, the insulin-
like growth factor 1 receptor (IGFIR) and
epidermal growth factor receptor (EGFR)
respectively, activates signal transduction
pathways that lead to both HIF-1α expression
and to cell proliferation/survival [48].
Furthermore, it is shown that the p42/p44
mitogen activated protein kinase (MAPK),
which regulates cell proliferation in response
to extracellular growth factors, phosphorylate
HIF-1α and activate transcription of HIF-1
target genes [53].
In addition, phosphatidylinositol 3-OH kinase
(PI3K) activity is also increased in some cell
types under hypoxic conditions. PI3K is one
of the key downstream mediators of many
tyrosine kinase signaling pathways involved in
regulating cell proliferation and suppression of
apoptosis [54].
3.6. Differentiation
Accumulating evidence suggests that
hypoxia promotes cell differentiation in a
variety of cell types. A clear link has been
demonstrated between hypoxia, HIFs and
molecules that are critical for the regulation of
the differentiation of cells, including Notch,
Oct-4 and MYC [55]. Recently, it was
observed that hypoxia directly influences the
Notch signalling pathway activity in the cell
differentiation process [56]. Notch mediates
cell-cell signalling between adjacent cells that
express Notch receptors (Notch 1-4) and
Notch ligands (Delta, Serrate and Lag-2). In
response to ligand presentation from
neighbouring cells, Notch receptors undergo
proteolytic activations to liberate the Notch
intracellular domain (ICD). Subsequently, ICD
forms a complex with DNA binding protein
CSL and co-activators, such as p300/CBP, to
activate Notch target genes. These in turn
negatively regulate the expression or activity
of the differentiation factors [57]. HIF-1α has
been shown to associate physically with ICD,
promoting its stability. A model is proposed in
which HIF-1α interacts with the Notch-CSL
transcriptional complexes at Notch-responsive
promoters in hypoxic cells to control the
differentiation status [56]. However, the
protein “bridging” direct interaction between
HIF-1α and ICD is unknown [58]. Other
molecular pathways underpinning the hypoxic
control of the cell differentiation process
involves OCT4 and MYC transcription factors
which are directly regulated by HIF-2α [55,
59]. However, how HIF-2α leads to the
activation of these transcription factors are
poorly understood. It is shown that when the
Oct-4 locus is an open configuration, HIF-2
complex binds and induces its expression [60].
In the case of MYC, it is postulated that HIF-
2α directly interacts with MYC-MYC-
associated protein X (MAX) complex which
leads to a stabilization of this complex and a
transcriptional activation [59].
A search in the literature shows that a
significant amount of work is performed to
show that hypoxia plays an important role in
the cell differentiation process. However, the
molecular pathways involved in this process
are poorly understood [61].
3.7. Energy metabolism
A key adaptive response to chronic hypoxia
is a switch from oxidative phosphorylation to
anaerobic glycolysis. Under normoxic
conditions, cell energy in the form of ATP is
mainly generated through the oxidative
metabolism of carbohydrates, fats and amino
acids. During hypoxia, ATP generation by
oxidative phosphorylation is arrested. This
will stimulate glycolysis with an increase in
glucose consumption and lactate production
[62,63]. The switch of the respiratory pathway
to anaerobic glycolysis leads to a significant
reduction of the ATP/ADP ratios. Because of
the reduced energy supply, the hypoxic cells
will further response by shutting down the
non-essential energy consuming mechanisms,
such as protein synthesis, and relocate the
energy to more critical functions, such as the
maintenance of the ion homeostasis and
membrane potential [12,21]. Additionally, the
glucose metabolism is regulated by the HIF

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
____________________________________________________________________ ________________________________________________________________________________________________________ _________
48
pathway. HIF-1 complex induces virtually all
genes encoding glucose transports and
glycolytic enzymes [64,65]. It is also
suggested that HIF-1 represses mitochondrial
respiration. This occurs by an induction of
pyruvate dehydrogenase kinase 1 that inhibits
pyruvate dehydrogenase, the enzyme that
converts pyruvate to acetyl-CoA for entry to
the Krebs cycle [66]. In addition, it is
postulated that HIF-1 induces expressions of
some proteins in the mitochondria for a more
efficient transfer of electrons to oxygen [3]. A
switch in the metabolic pathway to anaerobic
glycolysis leads to an increase in the glucose
consumption. The largest store of glucose
equivalents is glycogen.
Indeed, it is observed that during hypoxia,
glycogenolysis is induced [67].
Recently another pathway has been
suggested to play an important role during
hypoxia by regulating the energy metabolism:
the AMP-activated protein kinase (AMPK)
pathway [20,68]. AMPK is regarded as an
energy-sensing enzyme. It is a heterotrimeric
complex composed of a catalytic α-subunit
and regulatory β- and γ-subunits [69,70]. As
mentioned earlier, hypoxia results in a fall in
the cellular energy status. As a consequence,
the enzyme adenylate kinase will catalyze the
reaction: 2 ADP ↔ AMP + ATP in order to
maintain cellular ATP levels. This will results
in a large increase in the AMP levels. AMP
activates the AMPK complex in three
independent ways: 1) allosteric regulation via
the γ-subunit; 2) promotion of phoshorylation
of AMPK by one or more upstream kinases;
and 3) inhibition of dephosphorylation of
AMPK [69].
Figure 4. Key processes of the energy metabolisms that
are regulated by AMPK. The multiple effects of AMPK
regulate some of these processes [69]
Furthermore, the AMPK complex can be
activated by a rise in intracellular Ca2+, which
is seen in hypoxia, through phosphorylation by
a Ca2+-dependent kinase activity [70,71]. The
activated AMPK complex has a lot of
downstream targets in cells or tissues [2]. In
general, stimulation of the AMPK pathway
promotes catabolic pathways that generate
ATP, in order to maintain the ATP supply,
while switching off non-essential ATP
consuming (anabolic) pathways. This is done
by direct phosphorylation of the Regulatory
proteins involved in the process, and by an
indirect effect on gene expressions [72].
Figure 4 shows some of the key effects of
AMPK on the energy metabolism.
3.8. Acidosis
A switch from aerobic to anaerobic glycolysis
decreases the consumption of H+ due to the
lowered production of ATP (by oxidative
phosphorylation) on the one hand, and a
generation of more H+ by other metabolic
reactions, such as the ATPases reaction (ATP
+ H2O ↔ ADP + Pi + H+) on the other hand
[62]. During hypoxia, the cellular pH
homeostasis is disturbed [9]. A gradual
decrease in both extracellular and intracellular
pH is observed [73]. Studies have shown an
approximately drop of 0.8-1.2 pH units [74]. It
is postulated that acidosis might be protective
to hypoxic cells. Acidosis can slow down
some of the enzymatic processes, reduce
energy consumption and ROS production [9].
Additionally, acidosis inhibits ion fluxes
through cellular membrane channels, thereby
reducing the energy required for maintaining
ion gradients across the plasma membrane
[75]. Hypoxia-induced acidosis plays an
important role in the stabilization of HIF-1α.
It is shown that a decrease in pH triggers re-
localization of the von Hippel-Lindau protein
(pVHL) from diffuse nuclear-cytoplasmic
pattern to nucleoli. This nucleolar
sequestration of pVHL stabilizes HIF-1α by
not ubiquitinylating the HIF-1α for protesomal
degradation [76]. HIF-1 complex contributes
to the cells’ surviving the metabolic acidosis
by enhancing several genes of membrane
located transporters (e.g. the H+/lactate
monocarboxylate), exchangers (e.g. the
Na+/H+ exchanger), pumps and ecto-enzymes
(e.g. carbonic anhydrase IX) [3].
3.9. Reactive oxygen species (ROS)
The term reactive oxygen species (ROS)
encompasses wide range of molecules. Free
radicals are chemical species containing one or
more unpaired electrons. The unpaired
electrons of oxygen react to form partially
reduced highly reactive species that are

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
____________________________________________________________________ ________________________________________________________________________________________________________ _________
49
classified as ROS, including superoxide (O2),
hydrogen peroxide (H2O2), hydroxyl radical,
and peroxynitrite. Under normoxia, ROS can
be generated at several cell sites and
organelles, though the major site of ROS
production is the mitochondrial electron
transfer chain (ETC) [77]. It is estimated that
under normoxia up to 2% of the electron flow
on the ETC leads to ROS production [37].
ROS has also been suggested to be an essential
participant in cell signaling, acting as a second
messenger [78-80]. Under hypoxia, it is
unclear how ROS is formed. Therefore, it has
become the most debated subject of cells or
tissues exposed to hypoxia. Controversial
results of the generation of ROS during
hypoxia have been published. Investigators
have observed both decreases and increases of
ROS levels in the cells or tissues exposed to
hypoxia [37,81,82]. Inconsistencies in studies
on the direction and degree of ROS level
production come from the differences in cell
types and sub-cellular compartments
examined. Additionally, there are no direct
methods for measuring intracellular ROS
levels [83]. However, it seems that it is
accepted that exposure of cells to a chronic
moderate hypoxia leads to a relative increase
in ROS generation [84]. Increases in cellular
ROS levels are regarded as toxic. Cells
possess several antioxidant systems, such as
the enzyme superoxide dismutase and non-
enzymatic antioxidant NADPH-coupled
reactions, to protect themselves from these
toxic species [85,86]. If one accepts that the
increase of the ROS levels during hypoxia
comes from the mitochondria site, it is
postulated that it plays an important role in
HIF-regulation [19,37,83]. It is shown that
HIF-1 activation directly correlates with
changes in ROS [87]. But it is not known how
mitochondrial ROS regulate HIF-1 stability. It
is proposed that the PHDs might be the key
element. It is suggested that ROS triggers an
unknown signal transduction cascade, which
results in post-translational modifications of
PHDs. These unknown post-translational
modifications will make these proteins
inactive. Inactivation of the PHDs leads to a
stabilization of HIF-1α [19,68,83].
3.10. Cell death
Paradoxically, cell adaptation to hypoxia
leads not only to cell proliferation/survival but
also to cell death in some circumstances.
When the protective adaptive mechanism,
initiated by HIF-1 is not sufficient, cell death
occurs [33]. Hypoxia induces cell death by a
number of HIF-1-mediated and –independent
pathways [53]. It is demonstrated that chronic
moderate hypoxia alone is not sufficient to
cause cell death [88,89]. A cell death requires
a combination of factors/events, such as an
increased level of Ca2+, a generation of ROS, a
change in the cellular energy levels and
acidosis etc. [9,88,90-92]. There are different
forms of cell death, including necrosis,
apoptosis and autophagy. There are extensive
reviews about these cell death mechanisms,
which will not be discussed here [90,91,93].
Necrosis is defined as a passive form of cell
death, since it can occur in absence of ATP
[94]. It is believed to be caused by a disruption
of cellular ionic gradients in association with a
reduced ATP/ADP ratio [95]. In contrast,
apoptosis is an energy dependent and delayed
form of cell death that occurs as the result of
an activation of a genetic program [89,96].
The autophagy has been described as an
alternative form of programmed cell death, a
non-apoptotic form. During this process, the
cell “eats itself” [93].
When and which form of cell death takes
place during the hypoxic condition is debated.
It is suggested that an increase of ionic Ca2+
leads to a rapid or a slow consumption of
ATP. It is postulated that a rapid use of ATP
leads to necrosis, whereas a slow use of ATP
leads to apoptosis [8,97]. Furthermore, it has
recently been suggested that the autophagy
form of cell death can be involved in hypoxia.
This form of cell death has been postulated to
represent an early survival mechanism. In this
strategy, cells switch to a catabolic metabolic
program in which cellular constituents are
degraded for energy production [3].
HIF-1 contribution to cell death is also
debated. It is discussed that HIF-1 can either
be anti-apoptotic or pro-apoptotic, but it
depends on the cell type and experimental
conditions [33, 94]. Under chronic hypoxia,
HIF-1 induces apoptosis. It is shown that HIF-
1 increases the expression of various pro-
apoptotic members of the BCL-2 (B-cell
lymphoma-2) family [11,94,98]. The BCL-2
family is separated in three classes: the first
class inhibits apoptosis, the second class
promotes apoptosis, and the third class can
bind and regulate the anti-apoptotic BCL-2
proteins to promote apoptosis [99]. HIF-1
complex has been shown to specifically induce
the second class members of the BCL-2 family
[98,100]. These classes lead to
permeabilization of the outer mitochondrial
and the subsequent release of apoptogenic
molecules, such as cytochrome c, which leads
to an activation of the caspase proteins.
Caspases, which are cysteinyl aspartate

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
50
proteases, in turn cleave a series of substrates,
activate DNases and orchestrate the demolition
of the cell [99].
Furthermore, chronic hypoxia induces the
stabilization of tumor suppressor p53 protein
[101,102]. Traditionally, p53 protein controls
cellular homeostasis by affecting cell cycle
progression and apoptosis. Under normoxia,
p53 has a very short half-life and the protein is
often at an undetectable level. Cell exposure to
stress such as DNA damage or chronic
hypoxia, leads to a stabilization of p53. As a
consequence, p53 becomes activated as a
transcription factor and promotes transcription
of genes involved in cell cycle regulation,
apoptotic event etc. [103]. It is argued that a
direct interaction between p53 and HIF-1α
leads to p53 stabilization, which results in an
inhibition of HIF-1 dependent transcription
[33,104,105]. Furthermore, it is proposed that
p53 leads to a targeting of HIF-1α for
ubiquitination and subsequent proteasomal
degradation [106]. However, a search in the
literature shows that a unifying picture
concerning hypoxia and cell death is lacking
and how cells balance between adaptation and
cell death is still an unanswered question.
4. Physological and Pathological responses
to hypoxia
Hypoxia has been implicated in a range of
pathological and physiological conditions, and
it can be harmful or beneficial, depending on
the circumstances. In the following text the
most well known hypoxic conditions will be
described.
4.1. Detection of hypoxia
Chronic hypoxia is a strong prognostic factor
for the outcome of various diseases. Currently,
the use of 2-nitromidazole drugs that
specifically bind to hypoxic cells has been
largely advocated, pimonidazole and EF5
being the most well known [107]. Reductive
enzymes metabolize these drugs in the
presence of oxygen, while when oxygen is
absent, the extra electrons are not removed,
and the drugs are converted to highly reactive
free radical molecules that covalently bind to
protein and DNA. The drug-protein adduct can
then be detected by specific antibodies
[5,108]. However, the disadvantage of these
drugs is that they have to be administered
before sampling the tissue. Therefore, the
search for possible surrogate markers for
hypoxia is still growing.
4.2. Hypoxia preconditioning/ Ischaemic
tolerance
Although hypoxia is correlated with
pathological conditions, in the recent years it
has been used for preconditioning stimuli.
Preconditioning is defined as a stressful but
non-damaging stimulus to cells, tissues or
organisms to promote a transient adaptive
response, so that injury resulting from
subsequent exposure to a harmful stimulus is
reduced [109].
Hypoxic preconditioning, or hypoxia-
induced tolerance, refers to a brief period of
hypoxia that protects against an otherwise
lethal insult (for example, stroke) minutes,
hours or days later. Hypoxic preconditioning
protects the brain, heart and retina against
several types of injury, including ischemia
[110]. It is proposed that the protective
response induced by hypoxic preconditioning
is due, at least in part, to hypoxic induction of
HIF isoforms and HIF isoforms target genes.
Indeed, several studies have shown systemic
hypoxia, which produced hypoxic
preconditioning and protected the brain against
ischemia, increased the levels of HIF-1α in
neonatal and adult rodent brains [111,112].
Additionally, hypoxic preconditioning can also
be achieved by known chemical inducers of
HIF-1α such as CoCl2 and desferrioxamine,
suggesting that the preconditioning
phenomenon is mainly mediated by HIF-
activity [113]. Although, hypoxia leads to an
increase level of HIF, it is probably the HIF
target genes that provide the protection against
subsequent ischemia and other types of injury.
Indeed, induction of some of these genes
products, such as EPO and VEGF, has been
shown to protect the brain against ischemia on
their own. In particular, it is shown that EPO
both protects the brain against ischemia and
produce an EPO-mediated preconditioning
[114, 115]. Furthermore, it is reported that an
overexpression of glucose transporter 1, which
is important for the glucose metabolism,
protects cells in vitro and in vivo against
ischemia and other types of injury [116].
However, hypoxic preconditioning is more
complex than the involvement of the
transcription factor HIF. Indeed, studies have
shown that other transcription factors and their
target gene products might participate in this
response, including activating protein 1 (AP1),
the cyclic AMP-response-element-
binding(CREB), nuclear factor-κB (NF-κB),
early growth response 1 and the redox-
regulated transcriptional activator SP1 [117-
120]. Moreover, preconditioning can also be
induced by hyperoxia, oxidative stress,

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
51
inflammatory cytokines, anaesthetics and
metabolic inhibitors [109].
Nonetheless, the best current research
strategy in order to obtain preconditioning
aims to mimic the hypoxic response by
increasing HIF-1 activity [121]. Induction of
HIF-1 could occur either specifically, with
targeted inducers, through gene therapy or
through the action of hypoxia mimetics [122].
4.3. Stroke/Cerebral Ischemia and the
potential role of neuroglobin
Stroke (hypoxia-ischemia) occurs when
cerebral blood flow to the brain is interrupted.
It restricts the delivery of substrates,
particularly oxygen and glucose, and impairs
the energetics required to maintain ionic
gradients. With energy failure, membrane
potential is lost and neurons and glia
depolarize. Subsequently, the voltage-
dependent Ca2+ channels become activated and
cause accumulation of intra-neuronal free Ca2+
[123]. A prolonged elevation of intracellular
Ca2+ leads to the catabolic process of vital
molecules and irreversible death of neuronal
cells through multiple mechanisms that
involve the activation of Ca2+-dependent
effector proteins, such as calpains,
endonucleases and caspases [124].
Furthermore, ROS are produced by the Ca2+-
dependent enzymes, such as nitric-oxide
synthase, phospholipase A2 and
cyclooxygenase [125,126]. The important role
of ROS in cell damage associated with stroke
is understood by the fact that even delayed
treatment with free-radical scavengers can be
effective in experimental ischemia [127]. In
the initial phase of a stroke, an induction of
HIF-1 is reported. However, this induction
seems to be by a hypoxia-independent
pathway, e.g. via cytokines [110,121].
Furthermore, HIF-1 is a marker for chronic
hypoxia, where failure of ion homeostasis and
an increase in intracellular Ca2+ concentration
is an acute hypoxia effect [53]. Therefore, the
induction of HIF-1 during stroke remains
unclear. Nevertheless, the disturbance of ion
homeostasis plays an important role in the
pathogenic of the stroke; other mechanisms,
such as inflammation and peri-infarct
depolarizations, are involved in the complex
sequence of the pathological events that
evolve over time and space [126].
The brain also activates neuroprotective
mechanisms in an attempt to counteract the
damaging effects of excitotoxicity [128]. By
administrating EF5, a hypoxia marker (see
4.1), in an experimental animal model, it is
shown that the brain is hypoxic during the first
few hours of recovery, but the tissue is no
longer hypoxic after 2 days [129]. This
hypoxic period is believed to play an
important role in protecting the brain cells
from further damage. It is postulated that
hypoxia induced HIF-1 levels lead to
proportional increases in the HIF-1 target
genes product during recovery [130]. It is well
documented that during recovery, HIF-1 target
genes, such as EPO, VEGF and IGF 2,
involved in the erythropoiesis, angiogenesis
and proliferation processes respectively, are
over-expressed [110]. Additionally, it is
suggested that HIF might be important in
affecting cell survival and recovery through a
regulation of the cellular antioxidant capacity.
It is believed that the most damaging effect
during recovery (reperfusion) is caused by an
increased generation of ROS [42,131].
Moreover, neuroglobin (Ngb), a recently
discovered protein, is suggested to be the key
mediator of hypoxic-ischemic injury-repair
coupling in the brain [132]. Ngb is a
monomeric heme-protein, which binds oxygen
reversibly, and is preferentially expressed in
the neurons of the central and peripheral
nervous systems (CNS, PNS) [133]. The
intracellular globins play an important role in
oxygen homeostasis of the animal cells, e.g.
myoglobin facilitates oxygen diffusion to the
mitochondria and stores oxygen for short or
long term periods of hypoxia [134]. We have
recently shown that Ngb is upregulated in the
human neuroblastoam cell line under hypoxic
condition [135]. Additionally, another group
has shown that Ngb is upregulated in mice
brain exposed to hypoxia-ischemia condition
[136]. However, we and the other group were
not able to show statistically significant
upreglulation of Ngb in mice brain under
hypoxic condition [137, 138]. Therefore,
upregulation of Ngb in mice brain under
hypoxic condition remains an open question.
The mechanism for hypoxic induction of Ngb
is unknown. However, both HIF-dependent
and independent mechanisms of induction has
been suggested [132]. It is postulated that Ngb
protects neurons from hypoxic and ischemic
cell death [139]. However, since Ngb is a
relatively newcomer, it remains unknown how
Ngb protects neurons [140]. Several
neuroprotective mechanisms have been
proposed for Ngb. It is suggested that Ngb
might have an Mb-like function in the oxygen
supply to the respiratory chain, either by
facilitating oxygen diffusion or by providing a
short term oxygen store [141]. We suggest that

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
52
Ngb can function as a ROS scavenger during
oxidative injury. Our conclusion is based on
firstly, an increase survival of cell over-
expression Ngb under H2O2 stress and
secondly, the observation that in eyes a
negative correlation of Ngb and H2O2 levels in
hypoxia/reoxygenation studies is present [137,
142]. Additionally, other studies have shown
Ngb is an efficient scavenger of NO. and
peroxynitrite [143,144]. Furthermore, it has
been suggested that Ngb can interact with Rho
GTPases as a GDP-dissociation inhibitor
(GDI). Rho GTPases are GDP-bound and
maintained inactive in the cytosol, complexed
with a GDI. Activation of Rho GTPases
requires GDI dissociation, replacement of
GDP with GTP, and intracellular translocation
of GTPases from the cytoplasma to the plasma
membrane. Binding of Ngb to Rho GTpases
keep this protein in the inactive form [145].
Recently, it has been demonstrated that this
interaction leads to inhibition of death
signaling [139]. Additionally, it is suggested
that Ngb inhibits apoptosis by inactivating
Cytochrome c (Cyt c). It is shown that Ngb
ferrous (Fe2+) can reduce Cyt c ferric (Fe3+),
thereby inactivating Cyt c in the apoptotic
pathway [146].
Based on the current observation that Ngb
expression is induced by neuronal hypoxia,
cerebral ischemia, ischemia/reperfusion, and
probably other neurodegenerative diseases, it
is suggested that Ngb has the potential to be a
neuroprotective molecule [132].
4.4. Hypoxia and cancer
It was more than 50 years ago that it was
first reported that human tumors grew under a
condition which was termed “chronic
hypoxia” [147]. It is described that hypoxia
contributes to selecting cancer cells resistant to
apoptosis and mediates resistance to
chemotherapy and radiotherapy [148].
Therefore, a correction of hypoxia before
radiation therapy is routine, by using blood
transfusion to increase the haemoglobin
concentration in patients, which results in a
better response to the therapy [149].
Nowadays, hypoxia in human cancers, such as
the head and neck, cervical or breast cancer, is
associated with increased metastasis and poor
survival in patients [150, 151]. As earlier
mentioned, cells undergo a variety of
responses to the chronic hypoxic condition,
which involves different pathways. The
earliest pathway was noted 70 years ago:
cancer cells shift from oxidative
phosphorylation to anaerobic glycolysis. This
process is known as the Warburg effect and
involves a decreased mitochondrial respiration
and an increased lactate production, even in
the presence of oxygen [152]. Furthermore, it
has been known for a long time that
tumorigenesis involves multiple mechanisms,
including angiogenesis, proliferation,
metastasis, differentiation [153, 154].
However, a better understanding of the
regulation of the multiple steps in
tumorigenesis was first recognized in the early
1990s when HIF-1 was identified. Today, it is
well established that HIFs play an important
role in tumour progression. Studies have
reported that HIF-1α, HIF-2α and HIF-3 α are
overexpressed in human cancers, and that the
level of expression is correlated with
tumorigenesis and patient mortality [155-157].
Furthermore, in support of HIF-1’s role in
tumour progression, it is reported that genetic
or pharmacological inhibition of HIF-1 in
animal model systems manifests a decrease in
tumorigenesis and an increase in survival [158,
159]. Additionally, recent studies have
provided evidence indicating that HIF-1
mediates resistance to chemotherapy and
radiation [160,161]. Therefore, with a growing
understanding of the HIF-1 pathway, the
regulation of its transcriptional activity has
become an attractive goal for therapeutic
targeting in cancer. Currently, different
approaches have been used to inhibit HIF-1α
gene transcription: through inhibition of the
ability of HIF-1α to interact with proteins that
modulate its activity, or through inhibition of
signal transduction pathways [162]. There are
several approved therapeutic agents reported
which are capable of inhibit HIF-1 activity,
including Trastuzumab (Herceptin), Imatinib
(Glivec), Camptothecin. Although, the
anticancer effect of these agents might be due,
in part, to their inhibition of HIF-1, none of
these drugs specifically target HIF-1. The lack
of selectivity increases the difficulty in
correlating molecular and clinical responses in
patients, but it does not disqualify these drugs
as potential anticancer agents [155].
4.5. Hypoxia and cancer stem cell
In recent years, it has been observed that
hypoxia plays an important role in the
differentiation of stem cells to cancer stem
cells. Stem cells are undifferentiated cells,
generally characterized by their functional
capacity to both self-renew and to generate a
large number of differentiated progeny cells
[163]. Because of their exceptional properties,
stem cells have the potential to be used for

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
53
developmental biology, drug screening,
functional genomics applications, and
regenerative medicine. Stem cells reside in
specialized cellular contexts called stem-cell
niches, which are defined as particular
locations or microenvironments that provide
the signals and physical support to maintain
stem cells. Cancer stem cells are cancer-
initiating cells that can self-renew and
generate distinct cell types. These cells are
capable of indefinite self-renewal and can give
rise to rapidly dividing transit amplifying cells
that have a limited capacity for self-renewal
and whose progeny differentiate to produce
the bulk of the tumor [164, 165]. The finding
that many cancers are maintained by a small
population of stem cells has extremely
important implications for both understanding
and treating cancer. A major question is how
these stem cells arise. The source of cancer
stem cells is not entirely clear and may differ
depending on the specific disease. They can
arise from normal stem cells that have
sustained a mutation to make them cancerous
[164]. In contrast, cancer stem cells can be
derived from more differentiated cells that
have undergone a mutation or epigenetic
changes that give them stem cell properties
[166]. It is believed that hypoxia promotes a
generation of cancer stem cells. Some of the
effects of hypoxia on the generation of cancer
stem cells are mediated by the HIF proteins. It
is striking that HIFs activate several key stem
cell genes and pathways; thereby tumor
hypoxia may contribute to the conversion of
differentiated tumor cells into cancer stem
cells [167]. It is interesting that two
transcription factor markers for stem cells
differentiation, Oct-4 and c-Myc, are directly
activated by HIF- 2α. The activity of c-Myc, a
prominent oncogene, is modulated by HIF.
Through a binding to the transcription factor
Sp1 under hypoxic conditions, HIF-1α
antagonizes c-Myc activity and inhibits c-Myc
dependent cell-cycle progression [168]. In
contrast, HIF-2α potentiates c-Myc activity by
enhancing its physical association with Sp1,
Miz1, and Max [169]. Oct-4 transcription
factor, which is not expressed in normal
differentiated somatic cells, is also expressed
in a variety of cancer cell lines and is induced
by hypoxia in a HIF-2α expressing renal
carcinoma cell line [170]. It has been
demonstrated that inducible expressions of
Oct-4 in transgenic mice produce reversible
epithelial dysplasia, a characteristic of
premalignant lesions [171]. Together, these
data suggest that the Oct-4 locus, which is not
expressed in normal differentiated somatic
cells, may promote the proliferation of
undifferentiated progenitor and/or stem cells,
thereby contributing to tumor growth.
However, it is not well-known to what extent
Oct-4 contributes to the growth of human
tumors. In addition, HIF regulation of ATP-
binding cassette (ABC) glycoprotein activity
may also contribute to cancer stem cells
formation. Some cancer stem cells express
ABC glycoprotein transporters at the cell
surface, a trait shared with normal
hematopoietic stem cells. These transporters
remove chemotherapeutic drugs and promote
the multidrug resistance (MDR), observed in a
large number of cancer cell lines [172].
Finally, HIFs have been shown to activate the
Notch signaling pathway that controls stem
cells self renewal and multipotency and
appears to have both oncogenic and tumor
suppressor effects in different contexts [173,
174]. HIF-1α may also activate the expression
of c-Myc by regulating Notch signaling [175].
These discoveries could help in developing
novel therapeutics for cancer treatment. The
fact that cancer can grow from malignant cells
with stem cell properties strongly support the
idea that eradicating cancer stem cells should
be an important goal in curing cancer.
Therefore, the HIF pathways are even more
attractive as targets of therapeutic intervention
[156]. Inhibiting HIF activity could
promotecancer stem cells differentiation,
thereby reducing their ability to repopulate
tumors after chemo and radiation therapies.
5. DISCUSSION
Our understanding on the role of hypoxia in
physiology and pathophysiology has increased
several folds in recent years, thanks to
identification of HIF which acts as a master
regulator coordinating oxygen homeostasis.
This review has summarized the current
understanding of cell responses to chronic
hypoxia and conditions in which hypoxia can
be harmful or beneficial. Many questions need
to be answered about how cells sense hypoxia
and activate the oxygen-regulated pathways,
and how the pathways are integrated.
Significant studies are done to understand the
hypoxia HIF-dependent responses, but limited
researches available which have focused on
the hypoxia HIF-independent responses.
Despite the increase in knowledge of HIF
activation, mechanisms that contribute to the
positive and negative regulation of HIF are
poorly understood. More research is required
to determine the ratio of the pro-survival

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
54
pathways and cell death pathways that are
activated in response to hypoxia and how these
are regulated. A search in literature shows that
hypoxia effects are analyzed under different
conditions, such as cell types, duration of
exposure to hypoxia, etc. It is well established
that different cell or tissue types response
differently to hypoxia. Therefore, the
interpretations of the results in the literature
are become more complicated. In order to
obtain a unifying picture of hypoxia response a
standardization of the experimental is needed.
Finally, it is postulated that up to 5% of
genome is transcriptionally regulated by
hypoxia. Until identification of all these genes
a complete picture of response to hypoxia will
remain unclear.
REFERENCE
1. Lopez-Barneo J, Pardal R and Ortega-Saenz
P. Cellular mechanism of oxygen sensing.
Annu. Rev. Physiol 2001; 63: 259-287.
2. Hardie D. G. Minireview: the AMP-
activated protein kinase cascade: the key
sensor of cellular energy status. Endocrinology
2003; 144: 5179-5183.
3. Brahimi-Horn M. C and Pouyssegu J.
Oxygen, a source of life and stress. FEBS Lett
2007; 581:3582-3591.
4. Brahimi-Horn M. C, Chiche J and
Pouyssegur J. Hypoxia signaling control
metabolic demand. Curr. Opin. Cell Biol
2007; 19: 223-229.
5. Sluimer J. C, Gasc J. M, van Wanroij J. L,
Kisters N, Groeneweg M, Sollewijn G,
Cleutjens J. P, van den Akker L. H, Corvol P,
Wouters B. G, Daemen M. J and Bijnens A. P.
Hypoxia. Hypoxia-inducible transcription
factor, and macrophages in human
atherosclerotic plaques are correlated with
intraplaque angiogenesis. J. Am. Coll. Cardiol
2008; 51: 1258-1265.
6. Li J, Bosch-Marce M, Nanayakkara A,
Savransky V, Fried S. K, Semenza G. L and
Polotsky V. Y. Altered metabolic responses to
intermittent hypoxia in mice with partial
deficiency of hypoxia-inducible factor-1α.
Physiol Genomics 2006; 25: 450-457.
7. Sharp F. R, Ran R, Lu A, Tang Y, Strauss
K. I, Glass T, Ardizzone T and Bernaudin M.
Hypoxic preconditioning protects against
ischemic brain injury. NeuroRx 2004; 1: 26-
35.
8. Biagas K. Hypoxic-ischemic brain injury:
advancements in the understanding of
mechanisms and potential avenues for therapy.
Curr. Opin. Pediatr 1999; 11: 223-228.
9- Zauner A, Daugherty WP, Bullock MR,
Warner DS. Brain oxygenation and energy
metabolism: part |-biological function and
pathophysiology. Neurosurgery 2002;51(2):
289-301.
10. Toescu E. C. Hypoxia sensing and
pathways of cytosolic ca2+ increases. Cell
Calcium 2004; 36: 187-199.
11. Lee K, Roth R. A and LaPres J. J.
Hypoxia, drug therapy and toxicity.
Pharmacol. Ther 2007; 113: 229-246.
12. Boutilier R. G. Mechanisms of cell
survival in hypoxia and hypothermia. J. Exp.
Biol 2001; 204: 3171-3181.
13. Wenger R. H. Mammalian oxygen sensing,
signaling and gene regulation. J. Exp. Biol
2000; 203:1253-1263.
14. Semenza G. L. Perspectives on oxygen
sensing. Cell 1999; 98: 281-284.
15. Chandel N. S and Schumacker P. T.
Cellular oxygen sensing by mitochondria: old
questions, new insight. J. Appl. Physiol 2000;
88: 1880-1889.
16. Kemp P. J. Novel partners and
mechanisms in oxygen sensing: detecting
acute changes in oxygen: will the real sensor
please stand up? Exp. Physiol 2006; 91: 829-
834.
17. Lopez-Barneo J. Oxygen-sensitive ion
channels: how ubiquitous are they? Trends
Neurosci 1994; 17: 133-135.
18. Kemp P. J and Peers C. Oxygen sensing by
ion channels. Essays Biochem 2007; 43: 77-
90.
19. Guzy R. D and Schumacker P. T. Noval
partners and mechanisms in oxygen sensing:
oxygen sensing by mitochondria at complex:
the paradox of increased reactive oxygen
species during hypoxia. Exp. Physiol 2006;
91: 807-819.
20. Wyatt C. N and Evans A. M. AMP-
activated protein kinase and
chemotransduction in the carotid body. Respir.
Physiol Neurobiol 2007; 157: 22-29.
21. Erecinska M and Silver I. A. Tissue
oxygen tension and brain sensitivity to
hypoxia. Respir. Physiol 2001; 128: 263-276.
23. Bickler P. E and DonohoeP. H. Adaptive
responses of vertebrate neurons to hypoxia. J.
Exp. Biol 2002; 205: 3579-3586.
24. Corbucci G. G, Marchi A, Lettieri B and
Luongo C. muscle catabolic mechanisms:
from disuse atrophy to cachexia. Minerva
Anestesiol 2005; 71: 727-740.
25. Seta K. A, Yuan, Y, Spicer Z, Lu G,
Bedard J, Ferguson T. K, Pathrose P, Cole-
Strauss A, Kaufhold A and Millhorn D. E. The
role of calcium in hypoxia-induced signal

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
55
transduction and gene expression. Cell
Calcium 2004; 36: 331-340.
26. Semenza G. L, Shimoda L. A and
Prabhakar N. R. Novartis. Found. Symp 2006;
272: 2-8.
27. Neubauer J. A and Sunderram J. Invited
review oxygen-sensing neurons in the central
nervous system. J. Appl. Physiol 2004; 96:
367-374.
28. Hammarstrom A. K and Gage P. W.
Hypoxia and persistent sodium current.
European Biophysics Journal with Biophysics
Letters. Eur. Biophys J 2002; 31: 323-330.
29. Pierce G. N and Czubryt M. P. The
contribution of ionic contribution of ionic
imbalance to ischemia/reperfusion-induced
injury. J. Mol. Cell Cardiol 1995; 27: 53-63.
30. Xia Y and Haddad G. G. Effect of
prolonged O2 deprivation on Na+ channels:
differential regulation in adult versus fetal rat
brain.Neuroscience 1999; 94: 1231-1243.
31. Bracken C. P, Whitelaw M. L and Peet D.
J. The hypoxia-inducible factors: key
transcriptional regulators of hypoxic
responses. Cell Mol. Life Sci 2003; 60: 1376-
1393.
32. Semenza G. L. HIF-1, O2, and the 3
PHDs: how animal cells signal hypoxia to the
nucleus. Cell 2001; 107:1-3.
33. Piret J. P, Mottet D, Raes M and Michiels
C. Is HIF-1α a pro- or an anti-apoptotic
protein? Biochem. Pharmacol 2002; 64: 889-
892.
34. Gordan J. D, Simon M. C. Hypoxia-
inducible factors: central regulators of the
tumor phenotype. Curr. Opin. Genet. Dev
2007; 17: 71-77.
35. Wang G. L, Jiang B. H, Rue E. A and
Semenza G. L. Hypoxia-inducible factor 1is a
basic-helix-loop-helix-PAS heterodimer
regulated by cellular O2 tension. Proc. Natl.
Acad. Sci U.S.A 1995; 92: 5510-5514.
36. Marti H. H. Erythropoietin and hypoxic
brain. J. Exp. Biol 2004; 207: 3233-3242.
37. Michiels C, Minet E, Mottet D and Raes
M. Regulation of gene expression by oxygen
NF-kappaB and HIF-1, two extremes. Free
Radic. Biol. Med 2002; 33: 1231-1242.:
38. Lundgren K, Holm C and Landberg G.
Hypoxia and breast cancer: prognostic and
therapeutic implications. Cell Mol. Life Sci
2007; 64: 3233-3247.
39. Kaur B, Khwaja F. W, Severson E. A,
Matheny S. L, Brat D. J and Van Meir E. G.
Hypoxia and the hypoxia-inducible-factor
pathway in glioma growth and angiogenesis.
Neuro. Oncol 2005; 7:134-153.
40. Siddiq A, Aminova L. R and Ratan R. R.
Poly 4-hydroxylase activity-responsive
transcription factors: hydroxylation to gene
expression and neuroprotection. Front Biosci
2008; 13: 2875-2887.
41. Siddiq A, Aminova L. R and Ratan R. R.
Hypoxia inducible factor prolyl 4-hydroxylase
enzymes: center stage in the battle against
hypoxia, metabolic compromise and oxidative
stress. Neurochem. Res 2007; 32: 931-946.
42. Loor G and Schumacker P. T. Role of
hypoxia-inducible factor in cell survival
during myocardial ischemia [ndash]
reperfusion. Cell Death. Differ 2008; 15: 686-
690.
43. Farrell F and Lee A. The erythropoietin
receptor and its expres- sion in tumor cells and
other tissues. Oncologist 2004; 9 (Suppl. 5):
18-30.
44. Ke Q and Costa M. Hypoxia-inducible
factor-1 (HIF-1). Mol. Pharmacol 2006; 70:
1469-1480.
45. Wenger R. H. Cellular adaptation to
hypoxia: O2-sensing protein hydroxylases,
hypoxia-inducible transcription factors, and
O2-regulated gene expression. FASEB J 2002;
16: 1151-1162.
46. Liao D, Johnson R. S. Hypoxia: a key
regulator of angiogenesis in cancer. Cancer
Metastasis Rev 2007; 26: 281-290.
47. Kunz M and Ibrahim S. M. Molecular
responses to hypoxia in tumor cells. Mol
Caner 2003; 2: 23.
48. Semenza G.L. Targeting HIF-1 for cancer
therapy. Nat Rev Cancer 2003; 3: 721-732.
49. Rankin E. B and Giaccia A. J. The role of
hypoxia-inducible factors in tumorigenesis.
Cell Death Differ 2008; 15: 678-685.
50. Pugh C. W and Ratcliffe P. J. Regulation
of angiogenesis by hypoxia: role of the HIF
system. Nat Med 2003; 9(6): 677-684.
51. Raval R. R, Lau K. W, Tran M. G, Sowter
H. M, Mandriota S. J, Li J. L, et al.
Contrasting properties of hypoxia-inducible
factor 1 (HIF-1) and HIF-2 in von hipple-
lindau-associated renal cell carcinoma. Mol
Cell Biol 2005; 25: 5675-5686.
52. Covello K. L, Simon M. C, Keith B.
Targeted replacement of hypoxia-inducible
factor-1α by a hypoxia-inducible factor-2α
knock-in allele promotes tumor growth.
Cancer Res 2005; 65: 2277-2286.
53. Harris A. L. Hypoxia- a key regulatory
factor in tumor growth. Nat Rev Cancer 2002;
2: 38-47.
54. Berra E, Pages G, Pouyssegur J. MAP
kinases and hypoxia in the control of VEGF
expression. Cancer Metastasis Rev 2000; 19:
139-145.

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
56
55. Simon M. C and Keith B. The role of
oxygen availability in embryonic development
and stem cell function. Nat. Rev. Mol. Cell.
Biol 2008; 9: 285-296.
56. Gustafsson M. V, et al. hypoxia requires
Notch signalling to maintain the
undifferentiated cell state. Dev. Cell 2005; 9:
617-628.
57. Bray S.J. Notch signalling: a simple
pathway becomes complex. Nat. Rev. Mol.
Cell. Biol 2006; 7: 678-689.
58. Sainson R. C. A and Harris A. L. hypoxia-
regulated differentiation: lets step it up a
Notch. Notch. Trends Mol. Med 2006; 12(4):
141-143.
59. Dang C. V, Kim J. W, Gao P and Yustein
J. The interplay between MYC and HIF in
cancer. Nat Rev Cancer 2008; 8: 51-56.
60.Covello K, Kehler J, Yu H, Gordan J. D,
Arsham A. M, Hu C. J, Labosky P. A, Simson
M. C and Keith B. HIF-2α regulates OCT-4:
effects of hypoxia on stem cell function,
embryonic development, and tumor growth.
Genes Dev 2006; 20: 557-570.
61. Cejude-Martin P and Johnson R. S. A new
notch in the HIF Belt: How hypoxia impacts
differentiation. Dev. Cell 2005; 9: 575-576.
62. Kristian T. Metabolic stages, mitochondria
and calcium in hypoxia/ ischemic brain
damage. Cell Calcium 2004; 36: 221-233.
63. Hochachka P. W, Buc, L. T, Doll C. J and
Land S. C. Unifying theory of hypoxia
tolerance: molecular/ metabolic defense and
rescue mechanisms for surviving oxygen lack.
Proc. Natl. Acad. Sci. U.S.A 1996; 93: 9493-
9498.
64. Semenza G. L, Roth P. H, Fang H. M and
Wang G. L.Transcriptional regulation of genes
encoding glycolytic enzymes by hypoxia-
inducible factor 1. J. Biol. Chem 1994; 269:
23757-23763.
65. Wang G. L and Semenza G. L. General
involvement of hypoxia-inducible factor 1 in
transcriptional response to hypoxia. Proc. Natl.
Acad. Sci. U.S.A 1993; 90 4304-4308.
66. Kim J. W, Tchernyshyov I, Semenza G. L
and Dang C. V. HIF-1-mediated expression of
pyruvate dehydrogenase kinase: A metabolic
switch required for cellular adaptation to
hypoxia. Cell Metab 2006; 3: 177-185.
67. Gruetter R. Glycogen: The forgotten
cerebral energy store. J. Neurosci. Res 2003;
74: 179-183.
68. Taylor C. T. Mitochondria and cellular
oxygen sensing in the HIF pathway. Biochem.
J 2008; 409: 19-26.
69. Hardie D. G, Hawley S. A and Scott J. W.
AMP-activated protein kinase-development of
the energy sensor concept. J. Physiol 2006;
574: 7-15.
70. Towler M. C and Hardie D. G. AMP-
activated protein kinase in metabolic control
and insulin signaling. Circ. Res 2007; 100:
328-341.
71. Evans A. M, Hardie D. G, Galione A,
Peers C, Kumar P and Wyatt C. N. Novartis.
Found. Symp 2006; 272: 234-252.
72. Hardie D. G, Scott J. W, Pan D. A and
Hudson E. R. Management of cellular energy
by the AMP-activated protein kinase system.
FEBS Lett 2003; 546:113-120.
73. Silver I. A and Erecinska M. J. reported on
the ion homeostasis in rat brain hippocam- pus
during recovery from short-term transient.
Cereb. Blood Flow Metab 1992; 12: 759-772.
74. Yao H and Haddad G. G. Calcium and PH
homeostasis in neurons during hypoxia and
ischemia. Cell Calcium 2004; 36: 247-255.
75. Moody W, Jr. Effects of intracellular H+
on the electrical properties of excitable cells.
Annu. Rev. Neurosci 1984; 7: 257-278.
76. Mekhail K, Gunaratnam L, Bonicalzi M. E
and Lee S. HIF activation by PH-dependent
nucleolar sequestration of VHL. Nat. Cell Biol
2004; 6: 642-647.
77. Fruehauf J. P and Meyskens F. L, Jr.
Reactive oxygen species: A breath of life or
death? Clin. Cancer Res 2007; 13: 789-794.
78. Hensley K, Robinson K. A, Gabbita S. P,
Salsman S and Floyd R. A. Reactive oxygen
species, cell signalling, and cell injury. Free
Radic. Biol. Med 2000; 28:1456-1462.
79. Sauer H, Wartenberg M and Hescheler J.
Reactive oxygen species as intracellular
messengers during cell growth and
differentiation. Cell Physiol. Biochem 2001;
11: 173-186.
80. Thannickal V. J and Fanburg B. L. Heme
oxygenase: colors of defense against cellular
stress. Am. J. Physiol Lung Cell Mol. Physiol
2000; 279: L1005-L1028.
81. Chandel N. S and Budinger G. R. The
cellular basis for diverse responses to oxygen.
Free Radic. Biol. Med 2007; 42: 165-174.
82. Fandrey J and Genius J. Reactive oxygen
species as regulators of oxygen dependent
gene expression. Adv. Exp. Med. Biol 2000;
475: 153-159.
83. Cash T. P, Pan Y and Simon M. C.
Reactive oxygen species and cellular oxygen
sensing. Free Radic. Biol. Med 2007; 43:
1219-1225.
84. Chandel N. S, Maltepe E, Goldwasser E,
Mathieu C. E, Simon M. C and Schumacker P.
T. Mitochondrial reactive oxygen species
trigger hypoxia-induced transcription. Proc.

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
57
Natl. Acad. Sci. U.S.A. 1998; 95:11715-
11720.
85. Saeed S. A., Shad K. F, Saleem T, Javed F
and Khan M. U. some new prospects in the
understanding of the molecular basis of the
pathogenesis of stroke. Exp. Brain Res 2007;
182:1-10.
86. Clanton T. L. Hypoxia-induced reactive
oxygen species formation in skeletal muscle.
J.Appl. Physiol 2007; 102: 2379-2388.
87. Chandel N. S, McClintock D. S, Felician,
C. E, Wood T. M, Melendez J. A, Rodriguez
A. M and Schumacker P. T. Reactive Oxygen
Species Generated at Mitochondrial Complex
III Stabilize Hypoxia-inducible Factor-1α
during Hypoxia: A MECHANISM OF O2
SENSING. J. Biol. Chem 2000; 275: 25130-
25138.
88. Webster K. A. Discher D. J, Kaiser S,
Hernandez O, Sato B and Bishopric N. H.
Hypoxia-activated apoptosis of cardiac
myocytes requires reoxygenation or a pH shift
and is independent of p53. J. Clin. Invest
1999; 104:, 239-252.
89. Banasiak K. J, Xia Y and Haddad G. G.
Mechanism's underlying hypoxia-induced
neuronal apoptosis. Prog. Neurobiol 2000; 62:
215-249.
90. Buja L. M, Eigenbrodt M. L and
Eigenbrodt E. H. Arch. Apoptosis and
necrosis: Basic types and mechanisms of cell
death.Pathol. Lab Med 1993; 117:1208-1214.
91. Majno G and Joris I. Apoptosis, oncosis,
and necrosis. An overview of cell death. Am.
J. Pathol 1995; 146: 3-15.
92. Sastry P. S and Rao K. S. Apoptosis and
the Nervous System. J. Neurochem 2000; 74:
1-20.
93. Edinger A. L and Thompson C. B. Death
by design: apoptosis, necrosis and autophagy.
Curr. Opin. Cell Biol 2004; 16: 663-669.
94. Greijer A. E and van der W. E. The role of
hypoxia inducible factor 1 (HIF-1) in hypoxia
induced apoptosis. J. Clin. Pathol 2004; 57:
1009-1014.
95. Shi L. C, Wang H. Y and Friedman E.
Involvement of Platelet-Activating Factor in
Cell Death Induced Under
Ischemia/Postischemia-Like Conditions in an
Immortalized Hippocampal Cell Line. J.
Neurochem 1998; 70: 1035-1044.
96. McClintock D. S, Santore M. T, Lee V. Y,
Brunelle J, Budinger G. R, Zong W. X,
Thompson C. B, Hay N and Chandel N. S.
Bcl-2 Family Members and Functional
Electron Transport Chain Regulate Oxygen
Deprivation-Induced Cell Death. Mol. Cell
Biol 2002; 22: 94-104.
97. Eguchi Y, Shimizu S and Tsujimoto Y.
Intracellular ATP Levels Determine Cell
Death Fate by Apoptosis or Necrosis. Cancer
Res 1997; 57: 1835-1840.
98. Saikumar P, Dong Z, Patel Y, Hall K,
Hopfer U, Weinberg J. M and Venkatachalam
M. A. Role of hypoxia-induced Bax
translocation and cytochrome c release in
reoxygenation injury. Oncogene 1998; 17:
3401-3415.
99. Youle R. J and Strasser A. The BCL-2
protein family: opposing activities that
mediate cell death. Nat. Rev. Mol. Cell Biol
2008; 9: 47-59.
100. Gustafsson A. B and Gottlieb R. A. Bcl-2
family members and apoptosis, taken to
heartAm. J. Physiol Cell Physiol 2007; 292:
C45-C51.
101. Graeber T. G, Peterson J. F, Tsai M,
Monica K, Fornace A. J, Jr and Giaccia A. J.
Hypoxia induces accumulation of p53 protein,
but activation of a G1-phase checkpoint by
low-oxygen conditions is independent of p53
status. Mol. Cell Biol 1994; 14: 6264-6277.
102. Sumiyoshi Y, Kakeji Y, Egashira A,
Mizokami K, Orita H and Maehara Y. Over
expression of Hypoxia-Inducible Factor 1α
and p53 is a Marker for an Unfavorable
Prognosis in Gastric Cancer. Clin. Cancer
Resnn 2006; 12: 5112-5117.
103. Schmid T, Zhou J and Brune B. J. p300
relieves p53-evoked transcriptional repression
of hypoxia-inducible factor-1 (HIF-1). Cell
Mol. Med 2004;380: 289-295.
104. Hammond E. M and Giaccia A. J.
Hypoxia-Inducible Factor-1 and p53: Friends,
Acquaintances or Strangers? Clin.Cancer Res
2006; 12: 5007-5009.
105. Hammond E. M and Giaccia A. J. The
role of p53 in hypoxia-induced apoptosis.
Biochem. Biophys. Res. Commun 2005; 331:
718-725.
106. Ravi R, Mookerjee B, Bhujwalla Z. M,
Sutter C. H, Artemov D, Zeng Q, Dillehay L.
E, Madan A, Semenza G. L and Bedi A.
Regulation of tumor angiogenesis by p53-
induced degradation of hypoxia-inducible
factor 1α. Genes Dev 2000; 14:34-44.
107. Michiels C. Physiological and
Pathological Responses to Hypoxia. Am. J.
Pathol 2004; 164: 1875-1882.
108. Evens S. M, Hahn S, Pook D. R, Jenkins
W. T, Chalian A. A, Zhang P, Stevens C,
Weber R, Weinstein G, Benjamin I, Mirza N,
Morgen M, Rubin S, McKenna W. G, Lord E.
M and Koch C. J. Detection of Hypoxia in
Human Squamous Cell Carcinoma by EF5
Binding. Cancer Res 2000; 60: 2018-2024.

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
58
109. Gidday J. M. Cerebral preconditioning
and ischaemic tolerance. Nat. Rev. Neurosci
2006; 7:437-448.
110. Sharp F. R and Bernaudin M. HIF1 and
oxygen sensing in the brain. Nat. Rev.
Neurosci 2004; 5:437-448.
111. Bernaudin M, Nedelec A. S, Divoux D,
MacKenzia E. T, Petit E and Schumann-Bard
P. Normobaric Hypoxia Induces Tolerance to
Focal Permanent Cerebral Ischemia in
Association With an Increased Expression of
Hypoxia-Inducible Factor-1 and Its Target
Genes, Erythropoietin and VEGF, in the Adult
Mouse Brain. J. Cereb. Blood Flow Metab
2002; 22: 393–403.
112. Jones N. M and Bergeron M. Hypoxic
Preconditioning Induces Changes in HIF-1
Target Genes in Neonatal Rat Brain. J. Cereb.
Blood Flow Metab 2001; 21:1105–1114.
113. Prass K, Ruscher K, Karsch M, Isaev N,
Megow D, Priller J, Scharff A, Dirnagl U and
Meisel A. Desferrioxamine Induces Delayed
Tolerance Against Cerebral Ischemia In Vivo
and In Vitro. J. Cereb. Blood Flow Metab
2002; 22: 520-525.
114.Brines M. L, Ghezzi P, Keenan S, Agnello
D, de Lanerolle N. C, Cerami C, Itri, L. M and
Cerami A. From the Cover: Erythropoietin
crosses the blood–brain barrier to protect
against experimental brain injury. Proc. Natl
Acad. Sci. USA 2000; 97: 10526–10531.
115. Prass K, Scharff A, Ruscher K, Lowl D,
Muselmann C, Victorov I, Kapinya K, Dirnagl
U and Meisel A. Hypoxia-induced stroke
tolerance in the mouse is mediated by
erythropoietin. Stroke 2003; 34:1981-1986.
116. Lawrence M. S, H, D. Y, Dash R and
Sapolsky, R. M. Herpes simplex virus vectors
over expressing the glucose transporter gene
protect against seizure-induced neuron loss.
Proc. Natl Acad. Sci USA 1995; 92: 7247–
7251.
117.Kapinya K, Penzel R, Sommer C and
Kiessling M. Temporary changes of the AP-1
transcription factor binding activity in the
gerbil hippocampus after transient global
ischemia, and ischemic tolerance induction.
Brain Res 2000; 872:282–293.
118. Hara T, Hamada J, Yano S, Morioka M,
Kai Y and Ushio Y. CREB is required for
acquisition of ischemic tolerance in gerbil
hippocampal CA1 region. J. Neurochem 2003;
86: 805–814.
119.Blondeau N, Widmann C, Lazdunski M
and Heurteaux C. ivation of the Nuclear
Factor-B Is a Key Event in Brain Tolerance. J.
Neurosci 2001; 21: 4668–4677.
120. Ryu H, Lee J, Zaman K, Kubilis J,
Ferrante R. J, Ross B. D, Neve R and Ratan R.
R. Sp1 and Sp3 Are Oxidative Stress-
Inducible, Antideath Transcription Factors in
Cortical Neurons. J. Neurosci 2003; 23:3597–
3606.
121. Marti H. H. Erythropoietin and the
Hypoxic Brain. J. Exp. Biol 2004; 207: 3233-
3242.
122. Giaccia A, Siim BG, Johnson R. HIF-1 as
a target for drug development. Nat Rev Drug
Discovery 2003; 2:1-9.
123. Acker T and Acker H. Cellular oxygen
sensing need in CNS function: physiological
and pathological implications. Journal of
Experimental Biology 2004; 207:3171-3188.
124.Woon S. J, Kim D. Yand Gwang J. B.
Cellular and Molecular Pathways of Ischemic
Neuronal Death. J. Biochem. Mol. Biol 2002;
35(1):67-86.
125. Knowles R. G and Moncada S. Nitric
oxide synthases in mammals. Biochem. J
1994; 298: 249-258.
126. Dirnagl U, Iadecola C and Moskowitz M.
A. Pathobiology of Ischaemic Stroke: An
Integrated View. Trends Neurosci 1999;
22:391-397.
127.Oliver C. N, Starke-Reed P. E, Stadtman
E. R, Liu G. J, Carney J. M and Floyd R. A.
Oxidative damage to brain proteins, loss of
glutamine synthetase activity, and production
of free radicals during ischemia/reperfusion-
induced injury to gerbil brain. Proc. Natl.
Acad. Sci. USA 1990; 87: 5144–5147.
128. Slevin M, Krupinski J, Kumar P,Gaffney
J and Kumar S. J. Gene activation and protein
expression following ischaemic stroke:
strategies towards neuroprotection. Cell. Mol.
Med 2005; 9(1): 85-102.
129. Chavez J. C and LaManna J. C.
Activation of Hypoxia-Inducible Factor-1 in
the Rat Cerebral Cortex after Transient Global
Ischemia: Potential Role of Insulin-Like
Growth Factor-1. J. Neurosci 2002; 22: 8922–
8931.
130. Ziello J. E, Jovin I. S, Huang Y. Hypoxia-
Inducible Factor (HIF)-1 regulatory pathway
and its potential for therapeutic intervention in
malignancy and ischemia. Yale journal of
biology and medicine 2007; 80: 51-60.
131.Xu W, Chi L, Row B. W, Xu R, Ke Y, Xu
B, Luo C, Kheirandish L, Gozal D and Liu R.
Increased oxidative stress is associated with
chronic intermittent hypoxia-mediated brain
cortical neuronal cell apoptosis in a mouse
model of sleep apnea. Neurosci 2004; 126:
313-323.
132. Greenberg D. A, Jin, K and Khan A. A.
Neuroglobin: an endogenous neuroprotectant.

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
59
Curr. Opin. Pharmacol 2008; 8:20-24.
133. Burmester T, Weich B, Reinhardt S and
Hanklen T. A vertebrate globin expressed in
the brain. Nat 2000; 407:520-523.
134.Wittenberg J. B and Wittenberg B. A.
Myoglobin function reassessed. J. Exp. Biol
2003; 206: 2011-2020.
135.Fordel E, Thijs L, Martinet W, Schrijvers
D, Moens L and Dewilde S. Anoxia or oxygen
and glucose deprivation in SH-SY5Y cells: A
step closer to the unraveling of neuroglobin
and cytoglobin functions. Gene 2007; 398:
114-122.
136. Sun Y, Jin K, Mao X.O, Zhu Y,
Greenberg D. A. Neuroglobin is up-regulated
by and protects neurons from hypoxic-
ischemic injury. Proc. Natl. Acad. Sci. USA
2001; 98:15306-15311.
137. Fordel E, Thijs L, Moens L and Dewilde
S. Neuroglobin and cytoglobin expression in
mice. FEBS. J 2007; 274:1312-1317.
138.Li R. C, Lee S. K, Pouranfar F, Brittian K.
R, Clair H. B, Row B. W, Wang Y and Gozal
D. Hypoxia differentially regulates the
expression of neuroglobin and cytoglobin in
rat brain. Brain Res 2006; 1096: 173-179.
139. Khan A. A, Mao X. O, Banwait S,
DerMardirossian C. M, Bokoch G. M, Jin K
and Greenberg D. A. Regulation of hypoxic
neuronal death signaling by neuroglobin.
FASEB. J 2008; 22:1737-1747.
140. Hankeln T, Ebner B, Fuchs C, Gerlach F,
Haberkamp M, Laufs T. L, Roesner A,
Schmidt M, Weich B, Wystub S, Saaler-
Reinhardt S, Reuss S, Bolognesi M, De
Sanctis D, Marden M. C, Kiger L, Moens L,
Dewilde S, Nevo E, Avivi A, Weber R. E,
Fago A, Burmester T. Neuroglobin and
cytoglobin in search of their role in the
vertebrate globin family. J Inorg Biochem
2005; 99:110-119.
141. Burmester T, Gerlach F, Hankeln T.
Hypoxia and the Circulation. Roach, R. C., Ed.
New York: Springer; 2007. Chapter 13.
142. Fordel E, Thijs L, Martinet W, Lenjou M,
Laufs T, Van Bockstaele D, Moens L and
Dewilde S. Neuroglobin and cytoglobin
overexpression protects human SH-SY5Y
neuroblastoma cells against oxidative stress-
induced cell death. Neurosci. Lett 2006;
410:146-151.
143. Herold S, Fago A, Weber R. E, Dewilde
S. and Moens L. The Redox State of the Cell
Regulates the Ligand Binding Affinity of
Human Neuroglobin and Cytoglobin. J. Biol.
Chem 2003; 278(51): 51713-21.
144. Jin K, Mao X. O, Xie L, Khan A. A and
Greenberg D. A. Neuroglobin protects against
nitric oxide toxicityNeurosci. Lett 2008;
430:135-137.
145. Wakasugi K, Kitatsuji C and
Morishima I. Possible neuroprotective
mechanism of human neuroglobin. Ann.
N. Y. Acad. Sci 2005; 1053:220-230.
146. Fago A, Mathews A. J, Dewilde S,
Moens L and Brittain T. J. Inorg. Two
forms of the ferrous protein. Biochem
2006; 100:1339-1343.
147. Thomlinson R and Gray L. Br. J. The
histological structure of some human lung
cancers and the possible implications for
radiotherapy. Canc 1955; 9:539–549.
148. Brown J. M and Giaccia A. J. The Unique
Physiology of Solid Tumors: Opportunities
(and Problems) for Cancer Therapy. Cancer
Res 1998; 58:1408–16.
149. Littlewood T. J. The impact of
hemoglobin levels on treatment outcomes
in patients with cancer. Semin. Oncol
2001; 28:49–53.
150. Hockel M and Vaupel P. Tumor Hypoxia:
Definitions and Current Clinical, Biologic, and
Molecular Aspects. J. Natl Cancer Inst 2001;
93: 266–276.
151. Hockel M, Schlenger K, Hockel S and
Vaupel P. Hypoxic Cervical Cancers with Low
Apoptotic Index Are Highly Aggressive.
Cancer Res 1999; 59: 4525–4528.
152. Bartrons R and Caro J. J. Bioenerg.
Hypoxia, glucose metabolism and the
Warburg s effect. Biomembr 2007; 39:223-
229.
153. Rankin E. B and Giaccia A. J. The role of
hypoxia-inducible factors in tumorigenesis.
Cell Death Differ 2008; 15:678-685.
154. Gatenby R. A and Gillies R. J. Why do
cancers have high aerobic glycolysis? Nat.
Rev. Cancer 2004; 4: 891-899.
155. Kim W. Y and Kaelin W. G. Role of
VHL Gene Mutation in Human Cancer. J.
Clin. Oncol 2004; 22:4991–5004.
156. Semenza G. L. Targeting HIF-1 for
cancer therapy. Nat. Rev. Cancer 2003; 3:721
– 32.
157. Maynard M. A and Oh M. The role of
hypoxia-inducible factors in cancer. Cell. Mol.
Life. Sci 2007; 64:2170-2180.
158.Carmeliet P, Dor Y, Herbert J. M,
Fukumura D, Brusselmans K, Dewerchin M,
et al. Role of HIF-1alpha in hypoxia-mediated
apoptosis, cell proliferation and tumour
angiogenesis. Nat 1998; 394: 485–490.
159. Ryan H. E, Lo J and Johnson R. S. HIF-
1 is required for solid tumor formation and
embryonic vascularization. EMBO J 1998;

Journal of Paramedical Sciences (JPS) Spring 2010 Vol.1, No.2 ISSN 2008-496X
___________________________________________________________ ___________________________________________________________________________________________________ _______________________
60
17:3005–3015.
160. Unruh A, Ressel A, Mohamed H. G,
JohnsonR. S, Nadrowitz R, Richter E,
Katschinski D. M and Wenger R. H. The
hypoxia-inducible factor-1 is a negative
factor for tumor therapy Oncogene 2003;
22:3213–3220.
161. Aebersold D. M, Burri P, Beer K. T,
Laissue J, Djonov V, Greiner R. H, Giaccia A,
Siim B. G and JohnsonR. S. Targeting HIF-1
for cancer therapy. Nat. Rev. Drug Rev 2003;
2: 1-9.
162. Blau H. M, Brazelton T. R and Weimann
J. M. The Evolving Concept of a Stem cell.
Cell 2001; 105:829-841.
163.Semenza G. L. Expression of Hypoxia-
inducible Factor-1 : A Novel Predictive and
Prognostic Parameter in the Radiotherapy of
Oropharyngeal Cancer. Cancer Res 2001; 61:
2911.
164.Huntly B. J and Gilliland D. G.
Leukaemia stem cells and the evolution of
cancer-stem-cell research. Nat. Rev. Cancer
2005; 5:311–321.
165.Reya T, Morrison S. J, Clarke M. F and
Weissman I. L. Stem Cells, Cancer and Cancer
Stem Cells. Nat 2001; 414:105–111.
166.Krivtsov A.V, Twomey D, Feng Z, Stubbs
M.C, Wang Y, Faber J, LevineJ.E, Wang J,
Hahn W.C, Gilliland D.G, Golub T. R and
Armstrong S. A. Transformation from
committed progenitor to leukaemia stem cell
initiated by MLL-AF9.Nat 2006; 442:818–
822.
167. Keith B and Simon M. C. Hypoxia-
Inducible Factors, Stem Cells, and Cancer.
Cell 2007; 129:465-472.
168. Koshiji M and Huang L. E. Dynamic
balancing of the dual nature of HIF-1alpha for
cell survival. Cell Cycle 2004; 3(7):853-4.
169. Gordan J. D, Thompson C. B and Simon
M. C. HIF and c-Myc: Sibling Rivals for
Control of Cancer Cell Metabolism and
Proliferation. Cancer Cell 2007; 12(2):108-
113.
170. Tai M.H, Chang C.C, Kiupel M, Webster
J.D, Olson L.K and Trosko J.E. Oct 4
expression in adult human stem cells: evidence
in support of the stem cell theory of
carcinogenesis. Carcinogenesis 2005; 26: 495–
502.
171. Hochedlinger K, Yamada Y, Beard C and
Jaenisch R. Ectopic Expression of Oct-4
Blocks Progenitor-Cell Differentiation and
Causes Dysplasia in Epithelial Tissues. Cell
2005; 121: 465–477.
172. Comerford K.M, Wallace T.J, Karhausen
J, Louis N.A, Montalto M.C and Colgan S.P.
Hypoxia-inducible Factor-1-dependent
Regulation of the Multidrug Resistance
(MDR1) Gene. Cancer Res 2002; 62: 3387–
3394.
173. Weng A.P and Aster J.C. Multiple niches
for Notch in cancer: context is everything.
Curr. Opin. Genet. Dev 2004;14: 48–54.
174.Wilson A and Radtke F. Mulyiple
functions of notch signaling in self renewing
organs and cancers. FEBS Lett 2006; 580:
2860–2868.
175.Weng A.P, Millholland J.M, Yashiro-
Ohtani Y, Arcangeli M.L, Lau A, Wai C,
Del Bianco C, Rodriguez C.G, Sai H,
Tobias J, Li Y, Wolfe M. S, Shachaf C,
Felsher D, Blacklow S. C, PearW. S and
Aster J. C. C-Myc is an important direct
target of Notch1 in T-cell acute
lymphoblastic leukemia/lymphoma. Genes
Dev 2006; 20: 2096–210.