REVIEW ARTICLE
The subtlesidetohypoxiainduciblefactor(HIFa) regulation
Rebecca L. Bilton and Grant W. Booker
Department of Molecular Biosciences, The University of Adelaide, Australia
The transcription factorhypoxiainduciblefactor a-subunit
(HIFa) is pivotal in the cellular response tothe stress of
hypoxia. Post-translational modification of HIFa by
hydroxylase enzymes has recently been identified as a key
Ôoxygen sensingÕ mechanism within the cell. The absence of
the substrate oxygen prevents the hydroxylases from modi-
fying HIFa during hypoxia and allows dramatic up-regula-
tion of both HIFa protein stability and transcriptional
activation capability. In addition to this oxygen-dependent
response, increased HIFa protein levels and/or enhanced
transcriptional activity during normoxic conditions can be
stimulated by various receptor-mediated factors such as
growth-factors and cytokines (insulin, insulin-like growth
factor 1 or 2, endothelial growth factor, tumour necrosis
factor a, angiotensin-2). Oncogenes are also capable of
HIFa activation. This induction is generally less intense than
that stimulated by hypoxia and although not fully elucida-
ted, appears to occur via hypoxia-independent, receptor-
mediated signal pathways involving either phosphatidyl
-inositol-3-kinase/Akt or mitogen activated protein kinase
(MAPK) pathways, depending on the cell-type. Activation
of Akt increases HIFa protein synthesis in the cell and results
in increased HIFa protein and transcriptional activity.
MAPK also activates HIFa protein synthesis and addi-
tionally may potentiate HIF1a transcriptional activity via a
separate mechanism that does not necessarily require protein
stabilization. Here we review the mechanisms and function
of receptor-mediated signals in the multifaceted regulation
of HIFa.
Keywords:HIFa; growth factor; oncogene; PI3K; MAPK.
Introduction
Spurred-on by the discovery of their involvement in the
pathophysiology of many disease states including cerebral
and pulmonary ischemia, cancer tumourigenesis and
malignancy [1], the bHLH-PAS domain-containing
hypoxia-inducible transcription factor (HIF) family have
become a popular focus for research in the decade since the
HIF1a gene was first characterized [2]. This family includes
the regulatory a-subunits HIF1a and HIF2a that are both
able to bind to their constitutively expressed b-subunit,
ARNT, to form a functional HIF complex. The induction
of HIFa by hypoxia (low physiological levels of oxygen) is
dramatic and has been shown to regulate the transcription
of over 40 downstream target genes, including glycolytic
enzymes, glucose transporters and vascular endothelial
growth factor [3]. Regulation of HIFa is complex and
involves multiple mechanisms of control at the level of
protein degradation and hence protein stabilization, nuclear
translocation and transcriptional activation (Fig. 1). When
stimulated by hypoxia, these mechanisms combine
co-operatively to induce maximal HIF activation. Recently
the Ôoxygen sensorsÕ monitoring this hypoxic response were
identified as prolyl- and asparaginyl-hydroxylase enzymes
[4–6], which during normoxia (normal physiological levels
of oxygen) mediate the rapid degradation of HIFa protein
and prevent transcriptional recruitment of the cofactor
CBP/p300, respectively. These enzymes and the mechanisms
involved in their activation of HIFa upon stimulus by
hypoxia are reviewed elsewhere [7,8].
As elucidation of the hypoxic HIFa signalling pathway
continues, another sideto HIFa biology has quietly emerged.
Zelzer and coworkers [9] were the first to demonstrate that
the growth-factors insulin and insulin-like growth factor-1
(IGF-1) activate HIF1 and that this has subsequently been
shown to occur through pathways separate to that employed
by the classical hypoxic pathway (Fig. 1). The list of
Correspondence to G. Booker, Department of Molecular Biosciences,
The University of Adelaide, North Terrace, Adelaide,
SA 5005, Australia.
Fax: + 61 88303 4348, Tel.: + 61 88303 3090,
E-mail: grant.booker@adelaide.edu.au
Abbreviations: Akt, serine/threonine kinase (also known as protein
kinase B); ARNT, aryl-hydrocarbon receptor nuclear translocation;
bHLH-PAS, basic helix-loop-helix period-ARNT-single-minded;
CBP, CREB binding protein; CO, carbon monoxide; C-TAD,
C-terminal transcriptional activation domain; EGF, epidermal growth
factor; eIF-4E, eukaryotic initiation factor 4E; 4E-BP1, eIF-4E
binding protein 1; FGF-2, fibroblast growth factor-2; FIH-1, factor
inhibiting HIF; FRAP, FKBP(FK506 binding protein) rapamycin
associated-binding protein (also known as mTOR, mammalian target
of rapamycin); HER2
NEU
, heregulin-2 or EGF stimulated receptor
tyrosine kinase; HGF, hepatocyte growth factor; HIFa,hypoxia
inducible factor-1 or ) 2 a subunit; HRE, hypoxic response element;
IGF-1/IGF-2, insulin-like growth factor-1 or -2; IL-1b, interleukin-1b;
JNK, c-Jun amino-terminal kinase; MAPK, mitogen activated protein
kinase; MEK, MAPK kinase; NO, nitric oxide; p70
S6K
, p70 S6 kinase;
PDGF, platelet derived growth factor; PI3K, phosphatidyl-inositol
3-kinase; PTEN, phosphatase and tensin homolog; ROS, reactive
oxygen species; TGF-1b, transforming growth factor-1b;TNFa,
tumour necrosis factor-a; VHL, von Hippel Lindau protein.
(Received 15 October 2002, revised 6 December 2002,
accepted 3 January 2003)
Eur. J. Biochem. 270, 791–798 (2003) Ó FEBS 2003 doi:10.1046/j.1432-1033.2003.03446.x
receptor-mediated factors that stimulate HIFa currently
includes many growth-factors, cytokines and circulatory
factors such as PDGF, EGF, FGF-2, IGF-2, TGF-1b,
HGF, TNFa,IL-1b, angiotensin-2 and thrombin [10–16]. In
addition, oncogenes (HER2
NEU
, Ras, v-Src) [17–19], and
mutations in the tumour suppressor PTEN [20], have also
been shown to affect HIF1a activity through these same
signalling pathways. Other HIFa stimuli include signalling
intermediates such as NO [21,22] and the in vitro phenomena
of cell culture confluence [22,23].
While the degree of observable HIFa protein or tran-
scriptional activation varies with each stimulus and cell-type
[9,22,24], as a generalization, the magnitude of the receptor-
mediated HIFa response in vitro is far less than the dramatic
induction caused by hypoxia. Treatment of L8 or ARPE
cells with insulin for example, results in twofold to sixfold
inductions of an HRE-luciferase reporter [9,24] (Fig. 2). An
exception to this trend, however, can be found in vascular
smooth muscle cells, where several stimulatory factors
increased the amount of protein observed to levels signifi-
cantly greater than those induced by hypoxia [13]. Whilst
apparently minor in comparison tothe in vitro induction by
hypoxia, the gene expression changes resulting from the
receptor-mediated pathways are nonetheless important.
These stimuli often elicit small changes in housekeeping
functions that accumulate over extended periods of time [25].
Receptor-mediated HIFa regulation has been shown to
occur via two well characterized signalling pathways, the
Ras/MEK/MAPK and PI3K/Akt/FRAP kinase cascades
[24,26,27] (Fig. 3). Although the end result is enhanced
HIFa protein levels and/or transcriptional activation even
under normoxic conditions, the molecular mechanisms
involved must differ from those of hypoxia, as low oxygen
tension and activation of the MAPK and Akt pathways can
co-operate to enhance the induction of HIFa activity [20,
27–29]. There is some evidence to suggest that co-operation
of these kinase pathways with hypoxia may occur via the
hypoxic generation of reactive oxygen species (ROS) as an
intermediate signalling step [30] (reviewed in [31]). This may
also be the pathway by which some of the stimulatory
factors such as thrombin, angiotensin and IL-1b influence
HIFa, as their ability to activate HIF1a was blocked by
ROS inhibitors and antioxidants [13,14,32]. In this way,
Fig. 2. Fold induction of pTK-HRE luciferase reporter construct in
stably transfected 3T3L1 adipocyte cells. Treatments include dipyridyl
(100 n
M
), serum free, insulin (100 n
M
), IGF-1 (5 n
M
) and IGF-2
(5 n
M
). Data normalized using pTK-Renilla construct as a transfection
control.
Fig. 3. Schematic representation of the molecular interactions controlling
receptor-mediated signals leading to HIFa dependent transcription of
downstream target genes. Arrows indicate activating steps, truncated
arrows indicate inhibitory effects and dotted lines indicate possible
interactions for which only limited evidence is available.
Fig. 1. Schematic overview of the receptor-mediated and hypoxic signal
pathways and the mechanisms they employ to activate HIF and induce
transcription of downstream target genes. The larger arrow highlights
the greater magnitude of the response derived from hypoxic signals
relative to receptor-mediated signals.
792 R. L. Bilton and G. W. Booker (Eur. J. Biochem. 270) Ó FEBS 2003
ROS may then recruit the MAPK or Akt pathways to
activate HIFa via ligand-independent activation of various
growth-factor receptors, such as the EGF receptor [33].
To add further complexity to normoxic HIFa regulation,
the particular kinase pathway employed and its action upon
HIFa may differ depending on cell type or specific stimulus.
It is important to remember that different cell-types may
express a different combination of signalling proteins and
may therefore respond tothe same stimuli to a lesser or
greater extent. This includes the Akt and MAPK pathways,
which are not always active in every cell type [34]. The use of
different pathways occurs in the hepatoma cell-line HepG2
where both TNFa and IL-1b were shown to increase DNA
binding by HIF, but only IL-1b was able to increase the
observed HIF1a protein levels [12]. This suggests a different
mechanism of action for each cytokine in this cell type. In
contrast to these findings, the two cytokines were reported
to act in the same manner to increase transcription of
HIF1a mRNA by twofold to threefold resulting in increased
protein levels in synovial fibroblasts [16]. Furthermore, the
differences in stimuli-induced signalling are highlighted by
the example of IGF-1 stimulation in different cell types.
IGF-1 stimulation allows visualization of HIF1a protein
and increases transcriptional activity of HIF1a via activa-
tion of MAPK in mouse embryo fibroblasts [35], whereas in
the U373 glioblastoma cell line these effects require Akt [20].
Both kinase pathways are reported to additively increase
HIF1a translation and thus protein level in HCT116 colon
carcinoma cells [26]. Finally, the family members HIF1a
and HIF2a are both responsive to receptor-mediated
stimuli, but not necessarily the same stimuli within a single
cell type, even when both homologues are coexpressed [29].
Whilst confusing, this complexity and cross-talk between
signalling pathways is not uncommon for growth factor
stimuli. Dependant on cell type and signal intensity,
stimulation of receptors by insulin can alternatively activate
either MAPK or Akt, and this can result in the completely
disparate outcomes of proliferation (mitogenic) or glucose
uptake (metabolic) [34].
Phosphorylation
Upon polyacrylamide gel electrophoresis, HIFa protein
migrates as a diffuse band consistent with an approximate
20 kDa increase in molecular mass from its predicted size of
104 kDa [14,36,37,41]. This broad band contains phos-
phorylated species of HIFa, as treatment with a phospha-
tase returns the protein to its predicted size [41]. Several
deletion studies have failed to identify specific residues,
which when phosphorylated, alter hypoxic induction of
HIF1a [4,5,38]. However, a report from Gradin et al.[39]
suggests the presence of an oxygen-independent, ubiqui-
tously phosphorylated residue that may play a role in
providing structural support tothe active protein. Although
phosphorylation was not shown directly, mutational ana-
lysis identified the threonine residue 844 of HIF2a,located
near the hydroxylase targeted asparagine 851, as being
important for the function of the C-terminal transcriptional
activation domain (C-TAD), spanning residues 824–874,
regardless of the oxygen state. Phosphorylation or intro-
duction of a negatively charged residue at position 844
allows binding of CBP in a mammalian two-hybrid system
[39]. It is feasible that phosphorylation of other residues also
act in a similar manner to stabilize HIFa postinduction and
this may efficiently maintain the structure of the newly
formed active transcriptional complex. Support for this
proposal is provided by the finding that HIFa is phosphor-
ylated postinduction after a short lag period of up to a
minute [40]. The kinase(s) and upstream regulatory signals
involved in either ubiquitous or stimuli-induced phosphory-
lation have not yet been fully elucidated.
MAPK
Direct phosphorylation of HIFa by MAPK has been
reported by several groups. These researchers showed that
activated recombinant or endogenous MAPK was able to
phosphorylate either full-length HIFa or a C-TAD-fusion
product when supplied as a substrate [27,41,42]. In those
studies employing HIF1a-fusion proteins expressed in
COS-7 cells, the region targeted by MAPK was shown to
lie within residues 786–826 of the C-TAD [27] or residues
531–826 spanning both the inhibitory domain and C-TAD
[42]. Although Sodhi et al. [42] identified up to eight serine
residues within this inhibitory region that contain adjacent
proline residues that may serve as putative consensus target
sites for the MAPK family, the specific HIFa residue(s) that
are phosphorylated by MAPK have yet to be identified. A
proposed function for phosphorylation leading to increased
HIF transcriptional activity is through the derepression of
the inhibitory domain that lies between the two transcrip-
tional activation domains of the HIF a-subunit [42].
Regions within this inhibitory domain have been shown
to be important for the interaction of HIFa with factor
inhibiting HIF-1 (FIH-1) [43,44] identified as the asparagine
hydroxylase [45]. In the three-dimensional structure, these
regions may form part of the FIH-1 recognition site. One
possible explanation for the observed derepression of the
HIFa inhibitory domain, is that phosphorylation of
residues within this domain may prevent docking of FIH,
and thus prevent the subsequent asparagine hydroxylation.
This would result in a derepression of transcriptional
activity, as CBP/p300 would be able to associate with HIFa.
As mentioned previously, hypoxia is able to activate
MAPK in some cell lines [27–29]. However, activation of
MAPK in hypoxia is not necessarily required for HIF1a
activation and this appears dependent on cell-type. In
fibroblasts, MAPK activation was blocked by application
of the MEK inhibitor PD 098059 and yet the activation of
HIF1a induced by hypoxia remained unaffected [35]. In
contrast, it was found that PD 098059 treatment of HT42
and Rat-1 fibroblast cell-lines moderately decreased HIF
transcriptional activity in both normoxia and hypoxia
[18,36]. It is possible that the reduction in HIF transcrip-
tional activity in these latter studies was due to inhibition of
MAPK activity that may have been stimulated by factors
present in the culture media. Indeed, HIF transcriptional
ability induced by receptor-mediated stimuli is impaired
when the MEK chemical inhibitors PD 098059 and U0126,
or dominant-negative MAPK mutants were applied to cells
in culture [18,29]. Other kinase family members such as the
stress kinases p38a,p38c and JNK have also been shown to
be involved in signalling to HIFa in several cell-types [42]. In
most reports documenting a role for MAPK in HIF
Ó FEBS 2003 Kinase pathways influence HIFa (Eur. J. Biochem. 270) 793
function, no change tothe observable level of HIFa protein
expression, protein stability, rate of protein degradation or
DNA-binding ability were observed [27,29,41]. This indi-
cates that effects due to MAPK signalling, in most cell-types
studied to date, do not precede HIFa protein expression or
stabilization and instead improve HIF transcriptional
activity. Possibilities for MAPK action upon HIF tran-
scriptional ability include recruitment of cofactors to the
active transcriptional complex, or a direct MAPK phos-
phorylation of HIFa residue(s). Phosphorylation may
improve HIF transcriptional activity by derepression of
the HIFa inhibitory domain or simply favouring a confor-
mation that supports the active domain. Given the varia-
tions encountered so far within the characterization of the
receptor-mediated signalling pathway, it is not surpri-
sing that, in a few cell types, the up-regulation of HIF
transcriptional activity via MAPK activity has been attri-
buted to an increase in observable HIF1a protein [22,24]. In
this way, MAPK may act via a similar mechanism to Akt to
improve HIFa protein synthesis. The effects of MAPK on
protein stability or transcriptional activation need not
necessarily be mutually exclusive.
Akt
The serine/threonine kinase Akt has also been identified as a
signalling intermediate downstream of the receptor-medi-
ated factors that alter HIFa regulation. Unlike stimulation
by MAPK, Akt activity increases HIF transcriptional
activation by increasing the pool of available HIFa protein
within the cell. The use of chemical inhibitors such as
wortmannin and LY 294002 that block the phosphatidyl-
inositol 3-kinase (PI3K) family of enzymes, or dominant
negative mutants of the PI3K/Akt pathway were shown to
inhibit factor- or hypoxia-stimulated HIF1a protein accu-
mulation as detected by Western blot [10,46]. A reduction in
the levels of observable HIFa protein resulted in loss of
DNA binding ability of HIF and failure to up-regulate the
transcription of reporter constructs or endogenous down-
stream target genes [10,46]. Similarly over-expression of
members of the PI3K/Akt pathway or inhibition of PTEN,
a negative regulator of Akt, resulted in increased levels of
HIF1a protein, DNA binding or transcriptional activity in
many cell types [18,20] (Fig. 3). It was initially proposed
that the observed increase in HIFa protein was due to
enhanced stability, possibly through inhibition of the
proteosomal degradation machinery that is active in norm-
oxic conditions [20,28].
Recently several publications have clarified the role that
Akt plays in HIFa biology, linking an Akt signal to an
increased rate of HIFa protein synthesis [17,24,26]. When
stimulated by heregulin, IGF-1 or insulin, activation of the
PI3K/Akt/FRAP pathway was shown to increase de novo
protein synthesis, as shown by inhibition with the transla-
tion inhibitor cycloheximide and pulse chase experiments
[17,24,26]. FRAP, also known as mTOR, de-represses the
translational regulatory protein eIF-4E by phosphorylating
and inactivating its binding protein 4E-BP1 [25]. FRAP also
activates p70
S6K
whichinturnisabletoactivatethe40S
ribosomal protein S6 [25] (Fig. 3). Thus in a wortmannin-,
LY 294002- and rapamycin-sensitive manner, activation of
eIF-4E and p70
S6K
results in the increased translation from
HIF1a mRNA [17,24,26]. Interestingly, Fukuda et al.
identified a novel role for MAPK in HCT116 cells as its
activation was also shown to alter HIF1a protein synthesis
[26]. Unlike other reports (see above) that document
enhanced HIF transcriptional activity upon MAPK stimu-
lation with no alteration to HIFa protein levels, activation
ofMAPKinthesecellswasshowntoleadtoatransient
activation of eIF-4E and its effects on HIF1a protein
synthesis were additive to those of PI3K/Akt/FRAP [26]
(Fig. 3). Enhanced levels of HIFa synthesis may explain the
previous reports for which activation of MAPK resulted in
an increase in observed HIFa protein levels [22,24]. The
increase in HIFa protein synthesis appears to be relatively
gene specific since the translation of control luciferase-
reporter or ARNT mRNA was not altered [17]. In addition,
over-expression of both FRAP and eIF-4E have been
previously shown to disproportionately increase the trans-
lation of specific target genes [25,47]. This mechanism of
increased translation is in contrast to that employed by
hypoxic stimuli for which it has been repeatedly shown, for
most cell types, that there is no alteration of either HIFa
mRNA levels or the rate of de novo protein synthesis when
oxygen levels are limiting [17,48].
Increased translation of HIFa mRNA ultimately leads to
an increase in the HIFa protein pool, thus explaining initial
reports that observed increased HIFa protein in response to
stimulatory factors. Given that the prolyl (and presumably
the asparaginyl) hydroxylase enzymes are believed not to be
at high concentrations within the cell [49], increasing the
availability of their HIFa substrate may easily titrate them
out. As well as overwhelming the HIFa degradation
mechanisms, substrate saturation also relieves the transcrip-
tional repression due tothe asparagine hydroxylase, FIH-1.
Thus it is plausible that even small increases in total HIFa
protein via up-regulated translation could saturate one or
both of these enzymes. Overwhelming the hydroxylase
enzymes may enable a small portion of the total HIFa
protein translated to escape the normoxic suppression and
degradation pathways. The presence of even a small amount
of active protein will result in some HIF-target gene
transcription, although the level of down-stream target
genes transcribed may be much less than the maximal
inductions possible in hypoxia [9,14]. Although receptor-
mediated signals have been shown to significantly improve
the observable levels of HIFa protein and/or ability to bind
DNA [13,24], this does not necessarily always correlate with
large increases in transcriptional activation. In addition,
over-expressed HIFa protein is not fully activated in
normoxia [50]. This lack of full transcriptional activity,
even in the presence of high levels of protein, could be either
because cofactors are limiting in these cells, or that there are
differences in the substrate saturation levels between the
different hydroxylases. In concluding this section on the
mechanism(s) of action of PI3K/Akt, it must be noted that
although induction of the PI3K/Akt/FRAP pathway
played no role in alteration of HIF1a ubiquitination,
VHL interaction, proteosomal-degradation or protein sta-
bility in ARPE-19, MCF-7 or HCT116 cell lines [17,24,26],
up-regulated translation may not be the only mechanism for
protein stabilization in all cell types. Inhibition of HIFa
protein degradation, possibly by interruption of the
hydroxylase function, cannot yet be ruled out as a
794 R. L. Bilton and G. W. Booker (Eur. J. Biochem. 270) Ó FEBS 2003
mechanism of action for some receptor-mediated stimuli.
Recently, Chan and coworkers (2002) have shown that
HIFa protein was increased by expression of the v-Src and
RasG12V onco-proteins, as well as constitutively active
Akt, however, there were variations in the amount of HIFa
proline-hydroxylation detected for each stimulus [51]. The
antibody generated during this work, which targets the
hydroxylated proline 564 of HIF1a (531 of HIF2a) [51], will
prove a valuable tool in delineating the exact hydroxylation
status of HIFa protein during all types of stimuli.
Nitric oxide, carbon monoxide and cell
confluence
Nitric oxide (NO) has been shown to increase HIFa protein
levels, DNA binding and transcriptional activity in endo-
thelial, smooth muscle, Hep3B and LLC-PK
1
cells during
normoxia [21,52]. Paradoxically, it has also been reported
that NO can also have the completely opposite effect
of inhibition of both basal- and hypoxia-induced expression
of HIF target genes in endothelial cells [53]. Inhibition of
hypoxia-induced DNA-binding activity by carbon monox-
ide (CO) or NO exposure was also seen in several other cell
types [54,55], although reduced HIFa protein expression
was only observed in one case [55]. These inhibitory effects
may be stimuli specific as CO did not prevent the
stabilization of HIF1a protein and transcriptional activity
induced by either cobalt chloride or the iron-chelator
desferrioxamine [55]. The differences in these findings
indicate that cell type, concentration of NO or CO stimuli
and cellular oxygen status are important experimental
considerations and suggest that CO and NO may mediate
their effects through multiple targets within the HIF
pathway, possibly dependent on their concentration. To
further complicate matters, the genes that produce these NO
and CO species, inducible-nitric oxide synthase and heme
oxygenase, are regulated by HIF [56,57] and the growth-
factor insulin is able to stimulate NO production via a
PI3K-dependant pathway [58]. Although the mechanism(s)
via which NO or CO affect HIF remain unclear, one
proposal is that they bind tothe hydroxylase enzymes [59],
and activate HIF in normoxia [53]. As analogues of
molecular oxygen, they may bind to hydroxylases but not
participate in the hydroxylation reaction. Thus in normal
oxygen conditions, the activity of hydroxylases, and thus
protein degradation may be prevented [59]. However, this
cannot explain how in low oxygen concentrations, during
hypoxia, NO or CO are able to prevent the hypoxic
stabilization and activation of HIFa [53].
Nitric oxide was identified as a signalling intermediate
between HIF and the stimulus of increased cell confluence
[22]. When the density of prostate cells in culture was
increased, levels of HIF1a protein increased concomitantly
via a nitric oxide and Ras/MAPK dependant signalling
pathway [22]. In addition, increased transcription of the
HIF target gene vascular endothelial growth factor was
observed in dense cultures of human glioblastoma cells
(U87) and fibrosarcoma cells (HT1080) [23]. This is in
contrast to several findings which document a decrease in
HIF1a protein expression and consequently reduced DNA
binding activity in prostate cancer cells grown at high
density (90%) compared to low density (50%) during both
hypoxic and normoxic conditions [10,60]. It also conflicts
with the finding that stimulation of HIF1a by insulin was
only possible when cells were cultured at low density [11]
that suggests the capacity for induced up-regulation of
HIFa is prevented at high density. Given these completely
disparate results, there can be no consensus currently made
as to a mechanism for cell-density mediated effects on HIFa
and clearly this area requires more research. However, the
phenomenon of confluence is an important consideration
during in vitro cell-based assays, particularly because the
effects of confluence may be due in part to localized
hypoxia. Confluence should be carefully monitored during
analysis of HIFa activation by each nonhypoxic stimulus so
the mechanism by which that stimulus contributes to HIFa
can be clearly defined. Finally, the contribution that density
makes to HIFa activity in vivo within tissues is unknown.
Possibly it forms part of the basal level of HIF activity and
this may be different in each tissue type, depending on how
tightly packed the cells are. Although HIFa is ubiquitously
expressed within all cells, the level of normoxic HIFa
protein observable and also the capacity for inducible
up-regulation varies in different cell types [61].
A role for receptor-mediated HIFa
in vivo
?
HIFa is a transcription factor with a complex set of multiple
regulatory mechanisms. Activation through various recep-
tor-mediated pathways, to influence only a subset of these
regulatory mechanisms, allows for a moderate induction of
HIFa and consequently a small increase in the transcription
of downstream target genes. Given thesubtle effect upon
HIFa, it is likely that receptor-mediated signalling during
normoxia plays a secondary role tothe induction of HIFa
by hypoxia. It appears more than a coincidence that genes
encoding a number of components of the receptor-mediated
signal pathways are themselves regulated by HIFa or
hypoxia. Inducible nitric oxide synthase [56] and haem
oxygenase-1 [57] contain HRE within their promoters,
whilst other genes, IGF-2 [11], IGF binding proteins-2 and -3
[11], PDGF [62], FGF [63] and TGF-1b [64] may be
indirectly altered by hypoxia or HIF. With this complex
web of autoregulatory feedback, it would seem reasonable
to propose that receptor-mediated activation of HIFa has
arolein vivo, and is not just an in vitro cell culture
phenomenon.
What is the role of receptor-mediated activation of HIFa?
One proposal is that these signals may be important for
stimulation of HIFa for oxygen-independent purposes and
they may also act to enhance hypoxic activation in some
tissue types. There are many situations where expression of
HIF target genes may provide a biological advantage other
than just hypoxic stress. Receptor-mediated signals may
increase the transcription of only a subset of the HIF
responsive genes during this time. These situations may
include cell proliferation, differentiation, development or
inflammatory responses. All of these biological activities
would place an increased Ômetabolic loadÕ on a cell above its
normal metabolic requirements, and interestingly, these are
processes which are often stimulated by, or result in, the
expression of growth-factors and cytokines known to
activate HIFa. For example, HIF1a is one of many genes
whose expression is up-regulated during adipogenesis [65]
Ó FEBS 2003 Kinase pathways influence HIFa (Eur. J. Biochem. 270) 795
and IGF-2 levels are extremely high in embryogenesis [66].
Interestingly, HIF1a has been shown to be stabilized and
activated by the cytokine TNFa during inflammation in
normoxic wounds, allowing increased expression of the HIF
target-gene vascular endothelial growth factor in order to
promote wound healing [67]. It is possible that these receptor-
mediated effects, particularly through HIFa protein synthe-
sis, are only able to occur in active tissues, especially because
the translation initiation factor eIF-4E is only abundant in
nonquiescent cells [25]. Stimulation of HIFa though oxygen-
independent mechanisms could increase expression of genes
that promote angiogenesis, vasodilation, glucose uptake or
glycolysis to provide increased nutrient supply to those
tissues requiring it. Many of these HIF target genes have
other regulatory elements within their promoters and their
expression is a balance between converging signals. This may
be the case with many of the glycolytic genes such as
hexokinase that have glucose or carbohydrate responsive
elements nearby tothe hypoxic response elements that may
combine synergistically to regulate gene expression [68,69].
Further work is required to clarify the molecular details of
the receptor-mediated signalling pathways and their different
effects on HIFa activity. However the activation of HIFa by
receptor-mediated signals has been established. It will also be
important to define the role(s) for these signalling pathways
and to investigate the possibility that this type of receptor-
mediated induction of HIFa may regulate the transcription
of only a subset of HIF responsive genes for specific functions
like regulating the metabolic load of highly active cells.
Acknowledgements
Rebecca Bilton funded by an Australian Postgraduate Award (APA)
and a CSIRO Postgraduate scholarship. Funding from the CSIRO/
Adelaide University Nutrition Trust is gratefully acknowledged. With
thanks to Dr D. Peet for critical reading of this manuscript.
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