2p21

ECYT4,HIF2A,HLF,MOP2,PASD2,bHLHe73

Renal clear cell carcinoma

The predominant loss of von Hippel Lindau in clear cell renal cell carcinoma (ccRCC), results in defective targeting of HIF-α proteins for degradation at normoxia (Gnarra et al., 1994). Therefore, both HIF-1α and HIF-2α accumulate irrespective of oxygen levels in VHL-defective cells and are abundantly expressed in cells of ccRCC origin (Krieg et al., 2000). For unknown reasons, the expression of HIF-2α is more prominent than that of HIF-1α in RCC cell lines and tumors and HIF-1α expression is often lost in RCC cell lines (Maxwell et al., 1999; Krieg et al., 2000). Regulation of HIF target genes in RCC cell lines are more dependent on HIF-2α than on HIF-1α and silencing of HIF-2α in VHL-deficient cells suppress tumor growth, suggesting an important role for HIF-2α in renal carcinoma (Kondo et al., 2003; Carroll et al., 2006).

Paraganglioma/pheochromocytoma

Paraganglioma and pheochromocytoma derive from the chromaffin cell lineage of the sympathetic nervous system, and notably, genes involved in the hypoxic response (e.g. VHL and SDH genes) are frequently mutated in these tumors (Neumann et al., 2002). Recently, somatic mutations in the EPAS1 gene itself were discovered in two paraganglioma patients, describing the first cases of EPAS1 mutations in any cancer type (Zhuang et al., 2012). These gain-of-function mutations lead to increased protein half-life and HIF-2α activity, in turn resulting in up-regulation of HIF-2α downstream target genes, presumably explaining the clinical presentation in these patients. In a follow-up study, two additional EPAS1 mutations were discovered in patients presenting with polycythemia and somatostatinoma or paraganglioma (Yang et al., 2013). These novel mutations lead to disruption of the ODD domain-PHD2 interaction and thereby result in less ubiquitination and higher activity of the HIF-2α protein. In another study, 7 out of 41 examined patients with pheochromocytoma or paraganglioma presented with somatic EPAS1 mutations, and interestingly, three of these cases were also accompanied by an exclusive gain of chromosome 2p (Comino-Mendez et al., 2013).

Neuroblastoma

In the childhood tumor neuroblastoma, HIF-2α positive tumor cells have been identified in a perivascular niche, suggesting a non-hypoxic driven expression (Pietras et al., 2008). The HIF-2α positive cells display an immature tumor stem cell-like phenotype and their presence in neuroblastoma specimens correlate to poor overall survival (Holmquist-Mengelbier et al., 2006; Noguera et al., 2009). In neuroblastoma cell lines, HIF-2α is expressed at hypoxic conditions (1% oxygen) and at near end-capillary physiological oxygen levels (5% oxygen) (Jogi et al., 2002; Holmquist-Mengelbier et al., 2006). At prolonged hypoxic conditions, HIF-2α is continuously expressed in contrast to its homologue HIF-1α. In addition, overexpression of HIF-2α in a mouse neuroblastoma cell line promoted in vivo tumor angiogenesis, while mutant HIF-2α cells formed tumors that were highly necrotic (Favier et al., 2007).

Glioma

Knockdown of HIF-2α expression can reduce vascularization but accelerate tumor growth of human glioblastoma cells pointing to a role for HIF-2α as a tumor suppressor in glioblastoma (Acker et al., 2005). In contrast, recent work has focused on HIF-2α as a putative glioblastoma cancer stem cell (CSC) marker. Specifically, HIF-2α protein expression co-localizes with CD133 in a fraction of tumor cells (McCord et al., 2009) and with cancer stem cell markers in glioma specimens (Li et al., 2009). Glioblastoma putative CSCs respond to hypoxia by induction of HIF2-α (Li et al., 2009; Seidel et al., 2010), and inhibiting HIF2-α in glioblastoma CSCs decreases self-renewal, proliferation and survival in vitro and tumor-initiating capacity in vivo (Li et al., 2009). In addition, elevated HIF2A mRNA levels are associated with poor prognosis in glioma patients (Li et al., 2009).

Breast cancer

HIF-2α is expressed and associates with high vascular density, high c-erbB-2 expression and extensive nodal metastasis in breast cancer (Giatromanolaki et al., 2006). In a slightly larger study, HIF-2α was associated with ABCG2 expression, histology grade and Ki67 expression in invasive ductal carcinoma (Xiang et al., 2012). Importantly, in two separate breast cancer cohorts, HIF-2α correlate to reduced recurrence-free survival, breast-cancer specific survival and presence of distal metastasis (Helczynska et al., 2008).

Knockdown of HIF-2α in CD34+ acute myeloid leukemia (AML) cells reduce engraftment ability in irradiated mice (Rouault-Pierre et al., 2013). The HIF-2α deficient cells are more susceptible to apoptosis as a result of increased ROS and ER-induced stress indicating that HIF-2α is important for AML cell survival.

Other tumor types

Expression of the HIF-2α protein has also been reported in other solid tumor types including colorectal cancer (Yoshimura et al., 2004), prostate cancer (Boddy et al., 2005), non-small cell lung cancer (Giatromanolaki et al., 2001), squamous cell head-and-neck cancer (Koukourakis et al., 2002), nodular malignant melanoma (Giatromanolaki et al., 2003) and endometrial adenocarcinoma (Sivridis et al., 2002).

Inflammation

Sites of inflammation are often hypoxic due to vascular damage and large infiltration of cells. In order to operate under this condition, cells of the innate immunity adapt by expressing the HIF proteins (Fang et al., 2009; Imtiyaz and Simon, 2010). HIF-2α has been directly coupled to the regulation of proinflammatory cytokine expression in activated macrophages (Fang et al., 2009; Imtiyaz et al., 2010). Furthermore, HIF-2α has been detected in bone marrow-derived macropahges (BMDMs) and tumor associated macrophages (TAMs) of various human cancers (Talks et al., 2000). Importantly, HIF-2α is essential for TAM migration into tumor lesions (Imtiyaz et al., 2010), which in turn will promote progression and metastasis of tumor cells (Pollard, 2004).

Erythrocytosis, see section on germinal mutations.

Development

The four available HIF2A knockout mice display substantial differences in phenotype, presumably due to strain background. The first knockout mouse was created on a 129/SvJ background, and resulted in embryonic lethality due to circulatory failure during midgestation (Tian et al., 1997). Two of the following knockout studies demonstrated a role for HIF-2α in vascular development. HIF2A deficient embryos from an ICR/129Sv background die in utero and display severe post-vasculogenic defects (Peng et al., 2000), while HIF2A deficient mice on 129/Sv x Swiss background display lowered VEGF levels (Compernolle et al., 2002). The latter mice are embryonically lethal due to respiratory distress syndrome and cardiac failure (Compernolle et al., 2002). HIF-2α is also important in normal hematopoiesis, as demonstrated by creating adult HIF2A knockout mice by crossing of heterozygous 129S6/SvEvTac EPAS1 and heterozygous C57BL/6J EPAS1 knockout mice (Scortegagna et al., 2003b). These adult HIF2A deficient mice suffer from cardiac hypertrophy, hepatomegaly, oxidative stress and pancytopenia (Scortegagna et al., 2003a). In summary, HIF2A knockout studies demonstrate important roles for HIF-2α in catecholamine synthesis, reactive oxygen species (ROS) homeostasis and vascular remodeling during development.