84
that hypoxia induces autophagy in tumor cells through AMP kinase, which is activated by hypoxia independently of HIF1α, as discussed previously in the O 2 sensing
section [172].
4.2.1.3 Cell Pluripotency and Differentiation
Vasculogenesis takes place in low O 2 environments, such as the early development
of the embryo, EPC regeneration in the BM, or EPC attachment and differentiation
into mature ECs at neovascularization sites. All of these processes rely on pluripotent/unipotent cells differentiating into the endothelium, where O 2 tension is a crucial parameter regulating their differentiation characteristics. As already discussed,
EPC regeneration in the BM depends on cellular dynamics between the osteoblastic niche (low O 2 ) and vascular niche (high O 2 ); HSCs are quiescent in the osteoblastic niche and differentiate into EPCs in the vascular niche before joining the
circulation [111]. Therefore, it is important to understand the effect of O 2 tension
on the differentiation of cells into EPCs/ECs as a primary step of vasculogenesis.
Hypoxia enhances human embryonic stem cell (hESC) pluripotency via the upregulation of Oct-4, NANOG, and SOX-2, which are pluripotent markers [45, 64, 69,
116]. HIF2α is responsible for the overexpression of Oct-4, SOX-2, and NANOG,
while HIF3α also plays a role in the process by inducing HIF2α transcription [45,
69]. Prasad et al. demonstrated that hypoxic conditions (5% O 2 ) prevent the spontaneous differentiation of hESCs, whereas the inhibition of Notch activation
revoked this effect, suggesting that hypoxia-induced pluripotency occurs via Notch
signaling [182]. On the other hand, the efficiency of the process of reprogramming
mouse and human somatic cells into induced pluripotent stem cells (iPSC) was
shown to be improved in 5% O 2 cultures, compared to atmospheric O 2 cultures
[243]. In contrast, other studies have demonstrated that hypoxia induces the expression of early cardiac genes in spontaneously differentiating embryoid bodies (EBs)
[125, 168]. In a more recent study, Lopez et al. showed that EPCs/ECs can be
obtained from hESCs more efficiently when cultured in 5% O 2 , compared to previous methods that induce EB formation in atmospheric O 2 [181]. Additionally, simply priming EBs in hypoxic conditions mediated suppression of pluripotent marker
Oct-4 and upregulated VEGF [140]. When hPSCs are differentiated toward an
endothelial lineage, hypoxia has also been shown to enhance EC differentiation
through changes in the early stages (mesodermal specification) of EC lineage commitment. Interestingly this affect, which was dependent on low O 2 tension, was
driven by production of ROS [135]. More in-depth investigations have uncovered
the specific role of NADPH oxidase 2 (Nox2)-produced ROS in upregulating Notch
signaling to facilitate differentiation toward arterial endothelial cells [119]. Another
group showed a biphasic regulation of EC fate specification via a HIF1α-mediated
pathway in mESCs. Hypoxia led to upregulation of the transcription factor Etv2 in
the early stages of differentiation, which resulted in development of endothelial
progenitor cells. Continued exposure to hypoxia led to HIF1α-induced upregulation of Notch1 signaling and formation of functional arterial endothelial cells, with
M. R. Blatchley et al.
that hypoxia induces autophagy in tumor cells through AMP kinase, which is activated by hypoxia independently of HIF1α, as discussed previously in the O 2 sensing
section [172].
4.2.1.3 Cell Pluripotency and Differentiation
Vasculogenesis takes place in low O 2 environments, such as the early development
of the embryo, EPC regeneration in the BM, or EPC attachment and differentiation
into mature ECs at neovascularization sites. All of these processes rely on pluripotent/unipotent cells differentiating into the endothelium, where O 2 tension is a crucial parameter regulating their differentiation characteristics. As already discussed,
EPC regeneration in the BM depends on cellular dynamics between the osteoblastic niche (low O 2 ) and vascular niche (high O 2 ); HSCs are quiescent in the osteoblastic niche and differentiate into EPCs in the vascular niche before joining the
circulation [111]. Therefore, it is important to understand the effect of O 2 tension
on the differentiation of cells into EPCs/ECs as a primary step of vasculogenesis.
Hypoxia enhances human embryonic stem cell (hESC) pluripotency via the upregulation of Oct-4, NANOG, and SOX-2, which are pluripotent markers [45, 64, 69,
116]. HIF2α is responsible for the overexpression of Oct-4, SOX-2, and NANOG,
while HIF3α also plays a role in the process by inducing HIF2α transcription [45,
69]. Prasad et al. demonstrated that hypoxic conditions (5% O 2 ) prevent the spontaneous differentiation of hESCs, whereas the inhibition of Notch activation
revoked this effect, suggesting that hypoxia-induced pluripotency occurs via Notch
signaling [182]. On the other hand, the efficiency of the process of reprogramming
mouse and human somatic cells into induced pluripotent stem cells (iPSC) was
shown to be improved in 5% O 2 cultures, compared to atmospheric O 2 cultures
[243]. In contrast, other studies have demonstrated that hypoxia induces the expression of early cardiac genes in spontaneously differentiating embryoid bodies (EBs)
[125, 168]. In a more recent study, Lopez et al. showed that EPCs/ECs can be
obtained from hESCs more efficiently when cultured in 5% O 2 , compared to previous methods that induce EB formation in atmospheric O 2 [181]. Additionally, simply priming EBs in hypoxic conditions mediated suppression of pluripotent marker
Oct-4 and upregulated VEGF [140]. When hPSCs are differentiated toward an
endothelial lineage, hypoxia has also been shown to enhance EC differentiation
through changes in the early stages (mesodermal specification) of EC lineage commitment. Interestingly this affect, which was dependent on low O 2 tension, was
driven by production of ROS [135]. More in-depth investigations have uncovered
the specific role of NADPH oxidase 2 (Nox2)-produced ROS in upregulating Notch
signaling to facilitate differentiation toward arterial endothelial cells [119]. Another
group showed a biphasic regulation of EC fate specification via a HIF1α-mediated
pathway in mESCs. Hypoxia led to upregulation of the transcription factor Etv2 in
the early stages of differentiation, which resulted in development of endothelial
progenitor cells. Continued exposure to hypoxia led to HIF1α-induced upregulation of Notch1 signaling and formation of functional arterial endothelial cells, with
M. R. Blatchley et al.