Hypoxia-Inducible Factors Regulate Tumorigenic Capacity of Glioma Stem Cells

Abstract

Summary Glioblastomas are lethal cancers characterized by florid angiogenesis promoted in part by glioma stem cells (GSCs). As hypoxia regulates angiogenesis, we examined hypoxic responses in GSCs. We now demonstrate that hypoxia-inducible factor HIF2α and multiple HIF-regulated genes are preferentially expressed in GSCs in comparison to nonstem tumor cells and normal neural progenitors. In tumor specimens, HIF2α co-localizes with cancer stem cell markers. Targeting HIFs in GSCs inhibits self-renewal, proliferation and survival in vitro, and attenuates tumor initiation potential of GSCs in vivo. Analysis of a molecular database reveals that HIF2A expression correlates with poor glioma patient survival. Our results demonstrate that GSCs differentially respond to hypoxia with distinct HIF induction patterns and HIF2α may represent a promising target for anti-glioblastoma therapies. Significance Recent evidence supports the presence of cancer stem cell populations that contribute to tumor progression through preferential resistance to radiation and chemotherapy, and promotion of tumor angiogenesis, invasion, and metastasis. Therefore, the elucidation of molecular regulators of cancer stem cells may translate into improved anti-neoplastic therapies. Our work demonstrates that cancer stem cells derived from glioblastomas differentially respond to hypoxia with a distinct induction of HIF2α. We find that HIFs are critical to cancer stem cell maintenance and angiogenic drive, and that expression of HIF2α is significantly associated with poor glioma patient survival. These data further suggest that anti-angiogenic therapies can be designed to target cancer stem cell specific molecules involved in neoangiogenesis, including HIF2α and its regulated factors.

The HIF-2α-Driven Pseudo-Hypoxic Phenotype in Tumor Aggressiveness, Differentiation, and Vascularization

“…Interestingly, HIF-2α is specifically expressed in brain tumor stem cells but not in neural progenitor cells, in contrast to HIF-1α, which is expressed in both cell types. As in neuroblastoma, a proportion of the HIF-2α positive cells are located adjacent to blood vessels in the tumor specimens, indicating that HIF-2α is expressed by a small but significant number of tumor cells, also in non-hypoxic regions (Pietras et al 2008;Pietras et al 2009;Li et al 2009). Finally, analyzing gliomas at the mRNA level, HIF2A, but not HIF1A expression, correlates with poor patient survival ).…”

“…Glioblastoma multiforme (GBM) is characterized by a rich vascular network (Hossman and Bloink 1981;Blasberg et al 1983;Groothuis et al 1983) and intratumoral necrosis (Raza et al 2002). Both HIF-1α and HIF-2α proteins are expressed in human glioblastomas (Jensen 2006;Li et al 2009), with HIF-1α expression being mostly concentrated to areas of necrosis and at the tumor margin (Zagzag et al 2000). Studies on a small set of brain tumors have suggested HIF-1α protein to correlate positively to brain tumor grade and vascularity (Zagzag et al 2000).…”

“…The HIF-2α + neuroblastoma and glioma stem/tumor initiating cells strongly express VEGF (Holmquist-Mengelbier et al 2006;Bao et al 2006;Calabrese et al 2007;Pietras et al 2008;Pietras et al 2009;Li et al 2009) and the perivascular location of phenotypically similar cells in tumor specimens might implicate that tumor stem cells actively take part in the process of tumor vascularization. The hypothesis is supported by the observation that knockdown of HIF-2α in neuroblastoma and glioma stem/tumor initiating cells results in poorly vascularized tumors.…”

“…One striking feature of both neuroblastoma and glioma stem/tumor initiating cells is their pseudo-hypoxic phenotype due to high expression of active HIF-2α at physiological oxygen tensions (Holmquist-Mengelbier et al 2006;Pietras et al 2008;Pietras et al 2009;Li et al 2009). As a consequence, several genes considered to be hypoxia-regulated are highly expressed at non-hypoxic conditions in these HIF-2α + tumor cells, thus creating a pseudohypoxic phenotype, presumably similar to that in a majority of CCRCCs.…”

“…Importantly, and the reason for the focus on HIF-2α in this review, HIF-1α and HIF-2α are differently regulated in tumors such as neuroblastoma, breast cancer, non-small cell lung carcinoma (NSCLC) and glioma and seem to have different impact on tumor behavior and patient outcome in these tumors (Holmquist-Mengelbier et al 2006;Helczynska et al 2008;Heddleston et al 2009;Kim et al 2009b;Li et al 2009;Noguera et al 2009). …”