Novel HIF-2α interaction with Reptin52 impairs HIF-2 transcriptional activity and EPO secretion

HIF-1 and HIF-2 in cancer: structure, regulation, and therapeutic prospects

Hypoxia, or a state of low tissue oxygenation, has been characterized as an important feature of solid tumors that is related to aggressive phenotypes. The cellular response to hypoxia is controlled by Hypoxia-inducible factors (HIFs), a family of transcription factors. HIFs promote the transcription of gene products that play a role in tumor progression including proliferation, angiogenesis, metastasis, and drug resistance. HIF-1 and HIF-2 are well known and widely described. Although these proteins share a high degree of homology, HIF-1 and HIF-2 have non-redundant roles in cancer. In this review, we summarize the similarities and differences between HIF-1α and HIF-2α in their structure, expression, and DNA binding. We also discuss the canonical and non-canonical regulation of HIF-1α and HIF-2α under hypoxic and normal conditions. Finally, we outline recent strategies aimed at targeting HIF-1α and/or HIF-2α.

Expression, purification, and biophysical analysis of a part of the C-terminal domain of human hypoxia inducible factor-2α (HIF-2α)

Hypoxia inducible factor 2α (HIF-2α) is a member of the basic helix-loop-helix(bHLH)-Per-Arnt-Sim (PAS) family of transcription factors. It is overexpressed in several cancers, associated with poor prognosis of the patients and resistance to treatment. Here, we study the residues 366-704 of the C-terminal end of human HIF-2α, which contains the N-transcriptional activation domain (NTAD), the oxygen-dependent degradation domain (ODD), and a part of the inhibitory domain (IH). An efficient protocol was developed to produce the 366-704 domain of human HIF-2α protein. Subsequently, we analyzed its biophysical characteristics using circular dichroism spectroscopy and size exclusion chromatography showing that the protein forms an antiparallel beta sheet conformation, and a computational model of the HIF-2α structure was produced. Our data offer new structural information for the unique biological properties of HIF-2α.

Regulation of erythroid differentiation in K562 cells by the EPAS1-IRS2 axis under hypoxic conditions

Red blood cells (RBCs) produced in vitro have the potential to alleviate the worldwide demand for blood transfusion. Hematopoietic cell differentiation and proliferation are triggered by numerous cellular physiological processes, including low oxygen concentration (<5%). In addition, hypoxia inducible factor 2α (HIF-2α) and insulin receptor substrate 2 (IRS2) were found to be involved in the progression of erythroid differentiation. However, the function of the HIF-2α-IRS2 axis in the progression of erythropoiesis is not yet fully understood. Therefore, we used an in vitro model of erythropoiesis generated from K562 cells transduced with shEPAS1 at 5% O2 in the presence or absence of the IRS2 inhibitor NT157. We observed that erythroid differentiation was accelerated in K562 cells by hypoxia. Conversely, knockdown of EPAS1 expression reduced IRS2 expression and erythroid differentiation. Intriguingly, inhibition of IRS2 could impair the progression of hypoxia-induced erythropoiesis without affecting EPAS1 expression. These findings indicated that the EPAS1-IRS2 axis may be a crucial pathway that regulates erythropoiesis and that drugs targeting this pathway may become promising agents for promoting erythroid differentiation.

Transcriptional Response to Hypoxia: The Role of HIF-1-Associated Co-Regulators

The Hypoxia Inducible Factor 1 (HIF-1) plays a major role in the cellular response to hypoxia by regulating the expression of many genes involved in adaptive processes that allow cell survival under low oxygen conditions. Adaptation to the hypoxic tumor micro-environment is also critical for cancer cell proliferation and therefore HIF-1 is also considered a valid therapeutical target. Despite the huge progress in understanding regulation of HIF-1 expression and activity by oxygen levels or oncogenic pathways, the way HIF-1 interacts with chromatin and the transcriptional machinery in order to activate its target genes is still a matter of intense investigation. Recent studies have identified several different HIF-1- and chromatin-associated co-regulators that play important roles in the general transcriptional activity of HIF-1, independent of its expression levels, as well as in the selection of binding sites, promoters and target genes, which, however, often depends on cellular context. We review here these co-regulators and examine their effect on the expression of a compilation of well-characterized HIF-1 direct target genes in order to assess the range of their involvement in the transcriptional response to hypoxia. Delineating the mode and the significance of the interaction between HIF-1 and its associated co-regulators may offer new attractive and specific targets for anticancer therapy.

The Underexplored Landscape of Hypoxia-Inducible Factor 2 Alpha and Potential Roles in Tumor Macrophages: A Review

Low tissue oxygenation, termed hypoxia, is a characteristic of solid tumors with negative consequences. Tumor-associated macrophages (TAMs) accumulate in hypoxic tumor regions and correlate with worse outcomes in cancer patients across several tumor types. Thus, the molecular mechanism in which macrophages respond to low oxygen tension has been increasingly investigated in the last decade. Hypoxia stabilizes a group of hypoxia-inducible transcription factors (HIFs) reported to drive transcriptional programs involved in cell survival, metabolism, and angiogenesis. Though both tumor macrophage HIF-1α and HIF-2α correlate with unfavorable tumor microenvironments, most research focuses on HIF-1α as the master regulator of hypoxia signaling, because HIF-1α expression was originally identified in several cancer types and correlates with worse outcome in cancer patients. The relative contribution of each HIFα subunit to cell phenotypes is poorly understood especially in TAMs. Once thought to have overlapping roles, recent investigation of macrophage HIF-2α has demonstrated a diverse function from HIF-1α. Little work has been published on the differential role of hypoxia-dependent macrophage HIF-2α when compared to HIF-1α in the context of tumor biology. This review highlights cellular HIF-2α functions and emphasizes the gap in research investigating oxygen-dependent functions of tumor macrophage HIF-2α.

Vitamin D and Hypoxia: Points of Interplay in Cancer

Vitamin D is a hormone that, through its action, elicits a broad spectrum of physiological responses ranging from classic to nonclassical actions such as bone morphogenesis and immune function. In parallel, many studies describe the antiproliferative, proapoptotic, antiangiogenic effects of calcitriol (the active hormonal form) that contribute to its anticancer activity. Additionally, epidemiological data signify the inverse correlation between vitamin D levels and cancer risk. On the contrary, tumors possess several adaptive mechanisms that enable them to evade the anticancer effects of calcitriol. Such maladaptive processes are often a characteristic of the cancer microenvironment, which in solid tumors is frequently hypoxic and elicits the overexpression of Hypoxia-Inducible Factors (HIFs). HIF-mediated signaling not only contributes to cancer cell survival and proliferation but also confers resistance to anticancer agents. Taking into consideration that calcitriol intertwines with signaling events elicited by the hypoxic status cells, this review examines their interplay in cellular signaling to give the opportunity to better understand their relationship in cancer development and their prospect for the treatment of cancer.