Hypoxic niche. Hypoxia has been reported to play a key role in the maintenance and regulation of NSC, leading to the recognition of a “hypoxic niche”.(86,87) In the setting of hypoxia with oxygen and glucose deprivation, NSC upregulate baseline expression of hypoxia-inducible factor (HIF)-1α, which, in turn, upregulates the expression of VEGF as a protective mechanism to stimulate angiogenesis and renormalize the microenvironment.(86) Blockade of VEGF (using a receptor antibody) partly reverses the protection afforded by increased HIF-1α expression.(86)
Multiple reports have also established key regulatory functions of the hypoxic microenvironment on GSC, leading to the recognition of a hypoxic stem cell niche also for GSC.(13,14,88–90) Hypoxia can increase GSC stemness. Expression of several stem cell markers in GSC (e.g. CD133, A2B5, Nestin, Oct4, and Sox2) is upregulated,(13,88–90) whereas the expression of differentiation markers (e.g. glial fibrillary acidic protein) is downregulated.(13,89) The proliferation rate and self-renewal potential of GSC are also significantly increased.(13,14,89) Mechanistic analysis revealed roles for HIF-1α and HIF-2α. In the setting of severe hypoxia (i.e. 1% oxygen), the expression of HIF-1α and HIF-2α is upregulated in GSC.(88–90) The degree of hypoxia influences expression, with milder hypoxia (i.e. 7% oxygen) associated with only minimal HIF-1α expression and moderate upregulation of HIF-2α expression.(13) Upregulation of HIF-1α in hypoxia is a shared molecular response between GSC and both non-stem glioma cells and NSC. The expression of HIF-2α, however, is unique to GSC.(14,88) Expression of HIF-2α, but not HIF-1α, correlates with HGG patient survival.(88) Of note, HIF-2α is expressed by GSC under normoxic conditions.(88,89) Histopathologic evaluation of clinical HGG specimens reveals enrichment of GSC in both vascular and hypoxic, or necrotic, areas.(88,90) In one study,(88) this was determined by colocalization of CD133 and HIF-2α-positive staining, consistent with the GSC expression of HIF-2α in normoxia. These results highlight the presence of an anatomically separate hypoxic niche for GSC.(14,88) Functional studies revealed overlapping yet distinct roles for HIF-1α and HIF-2α. Knockdown of HIF-1α in GSC did not affect stem cell marker expression, but significantly reduced both neurosphere formation(90) and VEGF expression,(88) with downstream effects on endothelial cell proliferation. Knockdown of HIF-2α in GSC significantly diminished stem cell marker expression (consistent with previously described downstream targets, such as Oct4, Sox2, and c-myc), in addition to reducing neurosphere formation and decreasing VEGF expression to a greater extent that HIF-1α, with functional downstream effects on endothelial cells.(88) In non-GSC, knockdown of HIF-2α had no effect on VEGF production, consistent with its largely absent expression.(88) Interestingly, overexpression of HIF-2α in non-GSC reprograms the non-GSC into GSC,(14) highlighting the importance of the hypoxic regulatory microenvironment. In vivo, knockdown of either HIF decreased tumor formation and survival of tumor-bearing mice.(88) Together, these data demonstrate that the hypoxic microenvironment is a distinct anatomic niche enriched with GSC that can be targeted by downregulation of HIF-2α, with downstream effects on stem cell marker expression and VEGF signaling. Blockade of VEGF can downregulate one of the effector mechanisms in the hypoxic niche and so it may be considered an indirect target of the hypoxic niche. Although blockade of VEGF signaling can functionally affect both the hypoxic and perivascular niches, this does not imply that these niches are indistinguishable. The hypoxic niche is a separate entity that merits further attention as a distinct target.