Biology Reference
In-Depth Information
are recruited to supplement the cells that constitutively synthesize EPO; when
the hypoxic stimulus is removed, the recruited cells return to their non-secre-
tory state [16, 17]. The mediator of this pathway is HIF-1, a transcription acti-
vator of at least 20 genes involved in erythropoiesis and angiogenesis, includ-
ing EPO, transferrin, the vascular endothelial growth factor (VEGF), and a
series of genes central to glucose metabolism. HIF-1 binds to the hypoxia-
response element in the target hypoxia-response genes to activate their tran-
scription [18].
HIF-1 is a heterodimeric transcription factor consisting of two sub-units: the
oxygen-insensitive HIF-1
sub-unit is expressed constitutively at a relatively
constant concentration independent of tissue oxygenation; the oxygen-sensi-
tive HIF-1
β
sub-unit, also expressed constitutively, is degraded rapidly (with
a half-life less than five minutes) in cells supplied with adequate oxygen, but
accumulates when cells are exposed to low-oxygen stress. The transcriptional
activity of the heterodimer HIF-1 depends upon the availability of the HIF-1
α
α
sub-unit [19, 20].
HIF-1
is regulated by two hydroxylases that require molecular oxygen as
a co-substrate: targeted proline hydroxylation mediates HIF-1
α
degradation
[21], and specific asparagine hydroxylation events regulate HIF-1
α
activity
[22] (Fig. 2). Under normoxic conditions, proline residues within the oxygen-
dependent degradation domain of HIF-1
α
are hydroxylated. The hydroxyla-
tion is catalyzed by an Fe
2
-dependent prolylhydroxylase, where oxygen is the
essential co-substrate and appears to be the rate-limiting factor in this reaction
[21]. This event targets HIF-1
α
for proteasomal degradation as the hydroxy-
lated proline sites are recognized by pVHL (von Hippel-Lindau protein): In
the cytoplasm, pVHL is present in a complex with an E3 ubiquitin protein lig-
ase [23]. Under hypoxic conditions, the lack of oxygen prevents proline
hydroxylation and HIF-1
α
is not tagged by pVHL for degradation; HIF-1
α
α
rapidly moves into the nucleus and binds with HIF-1
to form the active
β
heterodimeric HIF-1 transcription factor [24].
A second hypoxia-sensitive site is present in the carboxyl-terminal transac-
tivation domain (CAD) of HIF-1
[25]. In the normoxic state, an asparagine
residue in CAD is hydroxylated by an Fe
2
-dependent arginine hydroxylase,
with oxygen as co-substrate. Arginine hydroxylation prevents the association
of CAD with the transcription co-activators, CBP and P300. In the hypoxic
state and the absence of arginine hydroxylation, CBP and P300 association
with CAD enhances the binding of HIF-1
α
to the hypoxia-responsive element
sites on target genes. This increased transcriptional activity may provide the
“fine tuning” for the enhanced synthesis of EPO, transferrin, VEGF, and other
factors that maintain oxygen homeostasis.
In tumors associated with an absent or mutated pVHL (e.g., in clear cell
renal carcinoma), degradation of hydroxylated HIF-1
α
is decreased, the HIF-1
dimer accumulates and continues to stimulate gene transcription even under
normoxic conditions; the enhanced angiogenesis due to upregulation of VEGF
and the increased glycolytic activity are of major significance in the develop-
α
