131in increasing mutational rate in tumors by reducing levels of mismatch repair pro-
tein (mutS) and promoting glucose flux through a non-oxidative arm of the pentose
phosphate pathway (Zhong et al. 1999 ). HIF-1α also increases the expression of
VEGF and VEGF receptors (FLT-1 and FLK-1), plasminogen activator inhibitor-1
(PAI-1), angiopoietins (Ang-1 and Ang-2) and matrix metalloproteinases (MMP-2
and MMP-9) thereby promoting angiogenesis and invasion (Semenza 2013 ; Mendez
et al. 2010 ). HIF-1α also reduced the sensitivity of GBM cells towards pro-
differentiation and pro-apoptotic signals such as bone morphogenic proteins (BMPs)
(Pistollato et al. 2009 ; Persano et al. 2012 ). HIF-1α also activates the expression of
multidrug resistance 1 (MDR1) gene which encodes for P-glycoprotein (P-gp),
belonging to a family of ATP binding cassette (ABC) transporters which acts as a
drug efflux pump, thereby reducing intracellular concentration of various chemo-
therapeutics (Chou et al. 2012 ; Chen et al. 2014 ). HIF-1α also stabilizes NF-κB and
contributes to suppressing hypoxia related apoptosis through expression of NF-κB
target genes such as Bax, Bcl-2, Bcl-xL (Gorlach and Bonello 2008 ).
Hypoxia has also been reported to induce expression of stem cell and GSC mark-
ers such as CD133, Oct4 and Sox2. Studies also report differentiation of CD133+
cell population upon exposure of GSCs to normoxic conditions, indicating that
reduced levels of HIF-1α affect their differentiation state (McCord et al. 2009 ).
HIF-1α and HIF-2α share 75% homology but have distinct functions within
cells. HIF-2α is also regulated by PHD hydroxylation at the transcriptional level as
opposed to HIF-1α which is only regulated at the translational level. Genes such as
OCT4, Nanog, Sox2, Serpin B9, and TGF-α are specifically regulated by HIF-2α.
Recent studies have shown that HIF-2α is preferentially expressed in GSCs and
knockdown of HIF-2α reduces self-renewal of GSCs indicating an important role
for HIF-2α in maintaining the stem-like differentiation state of GSCs. Studies have
also reported that the expression of HIF-2α in non-stem GBM cells induce expres-
sion of stem cell markers such as Oct4, myc and Nanog and neurosphere formation.
Expression of non-degradable HIF-2α also increases tumorigenic potential of non-
stem GBM cells in vivo and increases ratio of GSCs to non-stem GBM cells (Li
et al. 2009 ; Heddleston et al. 2009 ). Future studies into the specific role of HIFs in
the maintainance and self-renewal of GSCs and their effect on the GSC microenvi-
ronment such as promoting angiogenesis and invasion could help us better under-
stand the GSC biology and lead to identification of better targets within GSC
microenvironment.
7.4.3 ECM and Paracrine Factors
The components of the extracellular matrix that form a complex of macromolecules
within the tumor cell niche are essential for the survival and migration of GBM
cells. Remodeling of ECM to facilitate processes such as angiogenesis and pro-
survival signals through integrin mediated signaling cascades promotes GBM
growth and progression. Overexpression of basement membrane protein laminin 8
7 Glioblastoma Stem Cells and Their Microenvironment