Nanoparticles for Targeting Intratumoral Hypoxia: Exploiting a Potential Weakness of Glioblastoma

Inference of transcriptional networks that regulate transitions into physiologic or pathologic cellular states remains a central challenge in systems biology. A mesenchymal phenotype is the hallmark of tumor aggressiveness in human malignant glioma but the regulatory programs responsible for implementing the associated molecular signature are largely unknown. Here, we show that reverse-engineering and unbiased interrogation of a glioma-specific regulatory network reveal the transcriptional module that activates expression of mesenchymal genes in malignant glioma. Two transcription factors (C/EBPβ and Stat3) emerge as synergistic initiators and master regulators of mesenchymal transformation. Ectopic co-expression of C/EBPβ and Stat3 reprograms neural stem cells along the aberrant mesenchymal lineage whereas elimination of the two factors in glioma cells leads to collapse of the mesenchymal signature and reduces tumor aggressiveness. In human glioma, expression of C/EBPβ and Stat3 correlates with mesenchymal differentiation and predicts poor clinical outcome. These results reveal that activation of a small regulatory module is necessary and sufficient to initiate and maintain an aberrant phenotypic state in cancer cells. Show more

Glioblastomas are highly lethal cancers that contain cellular hierarchies with self-renewing cancer stem cells that can propagate tumors in secondary transplant assays. The potential significance of cancer stem cells in cancer biology has been demonstrated by studies showing contributions to therapeutic resistance, angiogenesis and tumor dispersal. We recently reported that physiologic oxygen levels differentially induce hypoxia inducible factor-2alpha (HIF2alpha) levels in cancer stem cells. HIF1alpha functioned in proliferation and survival of all cancer cells but also was activated in normal neural progenitors suggesting a potentially restricted therapeutic index while HIF2alpha was essential in only in cancer stem cells and was not expressed by normal neural progenitors demonstrating HIF2alpha is a cancer stem cell specific target. We now extend these studies to examine the role of hypoxia in regulating tumor cell plasticity. We find that hypoxia promotes the self-renewal capability of the stem and non-stem population as well as promoting a more stem-like phenotype in the non-stem population with increased neurosphere formation as well as upregulation of important stem cell factors, such as OCT4, NANOG and c-MYC. The importance of HIF2alpha was further supported as forced expression of non-degradable HIF2alpha induced a cancer stem cell marker and augmented the tumorigenic potential of the non-stem population. This novel finding may indicate a specific role of HIF2alpha in promoting glioma tumorigenesis. The unexpected plasticity of the non-stem glioma population and the stem-like phenotype emphasizes the importance of developing therapeutic strategies targeting the microenvironmental influence on the tumor in addition to cancer stem cells. Show more

HIF-2alpha promotes von Hippel-Lindau (VHL)-deficient renal clear cell carcinoma (RCC) tumorigenesis, while HIF-1alpha inhibits RCC growth. As HIF-1alpha antagonizes c-Myc function, we hypothesized that HIF-2alpha might enhance c-Myc activity. We demonstrate here that HIF-2alpha promotes cell-cycle progression in hypoxic RCCs and multiple other cell lines. This correlates with enhanced c-Myc promoter binding, transcriptional effects on both activated and repressed target genes, and interactions with Sp1, Miz1, and Max. Finally, HIF-2alpha augments c-Myc transformation of primary mouse embryo fibroblasts (MEFs). Enhanced c-Myc activity likely contributes to HIF-2alpha-mediated neoplastic progression following loss of the VHL tumor suppressor and influences the behavior of hypoxic tumor cells. Show more