HIF2α inhibition promotes p53 pathway activity, tumor cell death, and radiation responses

Abstract

Approximately 50% of cancer patients receive radiation treatment, either alone or in combination with other therapies. Tumor hypoxia has long been associated with resistance to radiation therapy. Moreover, the expression of hypoxia inducible factors HIF1␣ and/or HIF2␣ correlates with poor prognosis in many tumors. Recent evidence indicates that HIF1␣ expression can enhance radiation-induced apoptosis in cancer cells. We demonstrate here that HIF2␣ inhibition promotes tumor cell death and, in contrast to HIF1␣, enhances the response to radiation treatment. Specifically, inhibiting HIF2␣ expression augments p53 activity, increases apoptosis, and reduces clonogenic survival of irradiated and non-irradiated cells. Moreover, HIF2␣ inhibition promotes p53-mediated responses by disrupting cellular redox homeostasis, thereby permitting reactive oxygen species (ROS) accumulation and DNA damage. These results correlate with altered p53 phosphorylation and target gene expression in untreated human tumor samples and show that HIF2␣ likely contributes to tumor cell survival including during radiation therapy.M any cellular responses to hypoxia are mediated by the hypoxia inducible factors (HIFs). These transcription factors promote the expression of over 200 genes (1) and are heterodimers consisting of either HIF1␣ or HIF2␣ bound to the HIF␤/ARNT subunit. While ARNT is constitutively expressed, both HIF␣ subunits are regulated by O 2 availability. Under normoxia, the von Hippel-Lindau (VHL) E3 ligase complex targets HIF␣ subunits for proteasomal degradation (2). When O 2 levels decline, the HIF␣ subunits are stabilized, bind ARNT, and activate the expression of target genes providing hypoxic adaptations.Solid tumors are characterized by oncogenic signaling and hypoxic microenvironments that promote HIF␣ accumulation (3). Moreover, HIF1␣ and/or HIF2␣ expression has been associated with increased tumor vascularization and poor prognosis of numerous cancers such as breast, ovarian, and non-small cell lung cancer (2, 4). Of note, in mouse xenograft models, HIF2␣ (and not HIF1␣) expression is crucial for growth of clear cell renal cell carcinoma (ccRCC) (5, 6) and neuroblastoma (7) tumors.TP53 is a tumor suppressor that is mutated or silenced in a majority of human cancers (8). It coordinates many cellular stress responses by regulating genes involved in DNA repair, cell cycle arrest, and apoptosis. Following stress stimuli, p53 is activated through a variety of post-translational modifications, including phosphorylation on serine 15 (9). For example, the ataxia telangiectasia mutated (ATM) and checkpoint kinase 2 (Chk2) kinases directly phosphorylate p53 in response to DNA damage, resulting in its activation (9). Although many tumors select for TP53 mutations, p53 pathway inhibition can also contribute to tumor progression (10).With the emergence of HIF inhibitors (11,12) and their potential use in cancer therapy, it is important to accurately predict the response of HIF␣-expressing tumor cells to treatment. HIF1␣ appears to enh...

Therapeutic efficacy of combined vaccination against tumor pericyte-associated antigens DLK1 and DLK2 in mice

“…24 Since tumor VN is also characterized by a loss of hypoxia (and a return to normoxia) within the TME, 24,25 we analyzed treated vs. control tumors for expression of the hypoxiaassociated biomarker HIF-2a. [25][26][27] We determined that tumors in mice vaccinated with lvDLK1 or (more so) lvDLK1 C lvDLK2 displayed significant reductions in HIF-2a transcript levels vs. control-untreated tumors (Fig. 3D), consistent with therapeutic VN in these treatment cohorts.…”

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

“…The fact that Hif2a itself is regulated by ROS may indicate a role as a primary sensor of oxidative stress. In support, ROS accumulation and improved response to radiation therapy by HIF2A inhibition was recently described in human tumor cells (Bertout et al 2009). These findings may implicate a role for HIF2A in radiation and chemotherapy resistance in tumor and possibly normal stem cells.…”

Derivation of neural precursor cells from human ES cells at 3% O2 is efficient, enhances survival and presents no barrier to regional specification and functional differentiation

“…19,40 We propose that this transient HIF-1a stabilisation represents an adaptive response to the low O 2 , whereas HIF-2a maintains a physiological response. 19 This later appearance and persistence of HIF-2a may also contribute to the increased numbers of NPCs generated at 3% O 2 : HIF-2a inhibits the p53 pathway and also regulates SOD1, SOD2, GPX1 and CATALASE expression, 41 so is well placed to modulate the survival of NPCs. Moreover, HIF-2a appears to have a critical role in the proliferation of neuroblastoma cells, 41 and the ability of our system to isolate the downstream effects of HIF-2a from HIF-1a could provide further insights into this observation, as well as into the mechanisms of maintenance of endogenous NSCs.…”

“…19 This later appearance and persistence of HIF-2a may also contribute to the increased numbers of NPCs generated at 3% O 2 : HIF-2a inhibits the p53 pathway and also regulates SOD1, SOD2, GPX1 and CATALASE expression, 41 so is well placed to modulate the survival of NPCs. Moreover, HIF-2a appears to have a critical role in the proliferation of neuroblastoma cells, 41 and the ability of our system to isolate the downstream effects of HIF-2a from HIF-1a could provide further insights into this observation, as well as into the mechanisms of maintenance of endogenous NSCs. In addition to our finding of efficient neural conversion and tri-lineage differentiation we also observed that 3% O 2 allowed long-term maintenance of healthy, mixed differentiated cultures for over 3 months, in the absence of any exogenous neurotrophins that are typically required in cultures differentiated at 20% O 2 for considerably shorter periods such as to 28 days.…”