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Solid tumors are often characterized by insufficient oxygen supply (hypoxia), as a result of inadequate vascularization, which cannot keep up with the rapid growth rate of the tumor. Tumor hypoxia is a negative prognostic and predictive factor and is associated with a more aggressive phenotype in various tumor entities. Activation of the hypoxic response in tumors, which is centered around the hypoxia-inducible transcription factors (HIFs), has been causally linked to neovascularization, increased radio- and chemoresistance, altered cell metabolism, genomic instability, increased metastatic potential, and tumor stem cell characteristics. Thus, the hypoxic tumor microenvironment represents a main driving force for tumor progression and a potential target for therapeutic interventions. Here, we describe several methods for the analysis of tumor hypoxia and the hypoxic response in vivo in tumor xenograft models. These methods can be applied to various tumor models, including brain tumor xenotransplants, and allow simultaneously determining the extent and distribution of hypoxia within the tumor, analyzing HIF levels by immunohistochemistry and immunoblot, and quantifying the expression of HIF target genes in tumor tissue. The combination of these approaches provides an important tool to assess the role of the hypoxic tumor microenvironment in vivo.

Cultured tumor cells are a central tool in cancer research and have provided fundamental insights in tumor biology. Recent evidence, however, indicates that classically established cell lines from different tumors, including glioblastoma, do not fully reflect the genotypes and phenotypes of the respective primary tumors. By contrast, primary cells, isolated from human tumor samples and maintained in serum-free spheroid cultures at low passage under defined growth factor conditions, reproduce key aspects of tumor cell physiology much more faithfully. Among the tumor cell characteristics that are better represented in primary glioblastoma cell cultures is the self-renewal and differentiation potential of the tumor cells. Indeed, a large body of evidence from the past decade indicates that glioblastomas and other tumors are composed of a hierarchy of heterogeneous types of cells, which are generated and maintained by cells that share characteristics of stem cells. This cancer stem cell/tumor initiating cell population is optimally preserved and maintained in primary glioblastoma cultures. Here, we describe a method for the isolation and culture of primary tumor cells from human glioblastomas in serum-free conditions, which allows the routine generation and proper maintenance of tumor cells as spheroid cultures. Such primary tumor cultures can serve as a model of choice for the study of the mechanisms behind key aspects of glioblastoma biology, including tumorigenicity, stem cell hierarchy, invasion, and therapeutic resistance.

Solid tumours are exposed to microenvironmental factors such as hypoxia that normally inhibit cell growth. However, tumour cells are capable of counteracting these signals through mechanisms that are largely unknown. Here we show that the prolyl hydroxylase PHD3 restrains tumour growth in response to microenvironmental cues through the control of EGFR. PHD3 silencing in human gliomas or genetic deletion in a murine high-grade astrocytoma model markedly promotes tumour growth and the ability of tumours to continue growing under unfavourable conditions. The growth-suppressive function of PHD3 is independent of the established PHD3 targets HIF and NF-κB and its hydroxylase activity. Instead, loss of PHD3 results in hyperphosphorylation of epidermal growth factor receptor (EGFR). Importantly, epigenetic/genetic silencing of PHD3 preferentially occurs in gliomas without EGFR amplification. Our findings reveal that PHD3 inactivation provides an alternative route of EGFR activation through which tumour cells sustain proliferative signalling even under conditions of limited oxygen availability.

Glioma growth and progression depend on a specialized subpopulation of tumour cells, termed tumour stem cells. Thus, tumour stem cells represent a critical therapeutic target, but the molecular mechanisms that regulate them are poorly understood. Hypoxia plays a key role in tumour progression and in this study we provide evidence that the hypoxic tumour microenvironment also controls tumour stem cells. We define a detailed molecular signature of tumour stem cell genes, which are overexpressed by tumour cells in vascular and perinecrotic/hypoxic niches. Mechanistically, we show that hypoxia plays a key role in the regulation of the tumour stem cell phenotype through hypoxia-inducible factor 2alpha and subsequent induction of specific tumour stem cell signature genes, including mastermind-like protein 3 (Notch pathway), nuclear factor of activated T cells 2 (calcineurin pathway) and aspartate beta-hydroxylase domain-containing protein 2. Notably, a number of these genes belong to pathways regulating the stem cell phenotype. Consistently, tumour stem cell signature genes are overexpressed in newly formed gliomas and are associated with worse clinical prognosis. We propose that tumour stem cells are maintained within a hypoxic niche, providing a functional link between the well-established role of hypoxia in stem cell and tumour biology. The identification of molecular regulators of tumour stem cells in the hypoxic niche points to specific signalling mechanisms that may be used to target the glioblastoma stem cell population.

Glioma growth and progression depend on a specialized subpopulation of tumour cells, termed tumour stem cells. Thus, tumour stem cells represent a critical therapeutic target, but the molecular mechanisms that regulate them are poorly understood. Hypoxia plays a key role in tumour progression and in this study we provide evidence that the hypoxic tumour microenvironment also controls tumour stem cells. We define a detailed molecular signature of tumour stem cell genes, which are overexpressed by tumour cells in vascular and perinecrotic/hypoxic niches. Mechanistically, we show that hypoxia plays a key role in the regulation of the tumour stem cell phenotype through hypoxia-inducible factor 2α and subsequent induction of specific tumour stem cell signature genes, including mastermind-like protein 3 (Notch pathway), nuclear factor of activated T cells 2 (calcineurin pathway) and aspartate beta-hydroxylase domain-containing protein 2. Notably, a number of these genes belong to pathways regulating the stem cell phenotype. Consistently, tumour stem cell signature genes are overexpressed in newly formed gliomas and are associated with worse clinical prognosis. We propose that tumour stem cells are maintained within a hypoxic niche, providing a functional link between the well-established role of hypoxia in stem cell and tumour biology. The identification of molecular regulators of tumour stem cells in the hypoxic niche points to specific signalling mechanisms that may be used to target the glioblastoma stem cell population. [ABSTRACT FROM PUBLISHER]