Passing the baton: the HIF switch

doi:10.1016/j.tibs.2012.06.004

Trends in Biochemical Sciences

Volume 37, Issue 9, September 2012, Pages 364-372

https://doi.org/10.1016/j.tibs.2012.06.004 Get rights and content

Hypoxia is an inadequate oxygen supply to tissues and cells, which can restrict their function. The hypoxic response is primarily mediated by the hypoxia-inducible transcription factors, HIF-1 and HIF-2, which have both overlapping and unique target genes. HIF target gene activation is highly context specific and is not a reliable indicator of which HIF-α isoform is active. For example, in some cell lines, the individual HIFs have specific temporal and functional roles: HIF-1 drives the initial response to hypoxia (<24 h) and HIF-2 drives the chronic response (>24 h). Here, we review the significance of the HIF switch and the relation between HIF-1 and HIF-2 under both physiological and pathophysiological conditions.

Section snippets

Hypoxia and the HIFs in human physiology and disease

Physiological tissue oxygen tensions are significantly lower than ambient oxygen tensions as a result of the dramatic decrease in blood oxygen content as it travels from the lungs throughout the body (Table 1) [1]. Oxygen gradients play an important and beneficial role in mammalian physiology; low oxygen or hypoxia provides the required extracellular stimulus for proper embryogenesis and wound healing, and maintains the pluripotency of stem cells. Hypoxia that involves oxygen tensions below the

HIF regulation

Under aerobic conditions, HIF-1/2α is hydroxylated by specific prolyl hydroxylases (PHDs) at two conserved proline residues (P402/P564 and P405/P531 for human HIF-1α and HIF-2α, respectively) situated within the oxygen-dependent degradation domain (ODD) in a reaction requiring oxygen, 2-oxoglutarate, ascorbate, and iron (Fe²⁺) as a cofactor. HIF-1/2α hydroxylation facilitates binding of von Hippel–Lindau protein (pVHL) to the HIF-1/2α ODD [7]. pVHL forms the substrate recognition module of an

Outcomes of HIF-1 versus HIF-2 activation

Since its identification over a decade ago, HIF-1α has been described as the master regulator of the hypoxic response and the crucial node in ensuring survival during hypoxic stress [3]. HIF-2α was initially identified as the endothelial PAS domain protein (EPAS1), an endothelium-specific HIF-α isoform, and was therefore considered to have a more specialized function than HIF-1α [17]. However, HIF-2α is also expressed in many other tissues including brain, heart, lung, kidney, liver, pancreas,

Different temporal and functional roles of HIF-1 versus HIF-2

The temporal regulation of HIF-1/2 is largely mediated by the oxygen-dependent hydroxylases that regulate HIF-1/2α stability and activity. The actions of the PHDs on different HIF isoforms are generally not equivalent; PHD2 has relatively more influence on HIF-1α than HIF-2α, and PHD3 has relatively more influence on HIF-2α than HIF-1α [26]. HIF-2α is also hydroxylated at a much lower efficiency than HIF-1α by both the PHDs and FIH-1, which can result in the stabilization and activation of

HIFs in vascular development

During early embryonic development, the rapid cellular proliferation in gastrulating embryos results in physiological hypoxia that is necessary for the patterning of the embryonic vascular system [30]. The HIFs are activated by this hypoxic microenvironment, and also by nonhypoxic stimuli such as the renin–angiotensin system, growth factors, and immunogenic cytokines, all of which play important roles in the regulation of placental development and maturation. Embryonic blood vessels form

HIFs in bone development

Bone formation occurs via two different mechanisms: intramembranous and endochondral ossification [51]. Intramembranous ossification occurs during the formation of the flat skull bones and involves the differentiation of mesenchymal cells directly into osteoblasts. Endochondrial ossification occurs during the development of most other bones, and involves a two-stage mechanism, whereby mesenchymal cells become chondrocytes, the primary cell type of cartilage, which form an avascular and highly

HIFs in stem cells and cancer

Tumor hypoxia is of major clinical significance because it promotes both tumor progression and resistance to therapy [59]. In addition to promoting tumor cell survival by shifting cells towards anaerobic metabolism, neovascularization and resistance to apoptosis, hypoxia drives other responses that contribute to tumor aggressiveness, such as increased genetic instability, invasion, metastasis and de-differentiation, largely through activation of the HIFs (Figure 3a) [60]. Elevated levels of

Mediators of the HIF switch

Recent studies have revealed the existence of HIF switches: mechanisms capable of directly changing HIF-α isoform dependency (Table 3). For example, the Hsp70/CHIP axis promotes the specific degradation of HIF-1α during diabetes-associated hypoxia and hyperglycemia, resulting in diabetic complications that are associated with an impaired hypoxic response and cell death [16]. Another HIF switch is the histone deacetylase SIRT1, which deactylates HIF-2α. HIF-2α is acetylated during hypoxia, and

Concluding remarks

Much progress has been made towards understanding the complex regulation of the HIFs in both physiological and pathophysiological processes. It is evident that hypoxia, as a complex microenvironmental stimulus that can vary both in intensity and duration, requires a sliding scale response that is largely provided by the intricate interplay between HIF-1 and HIF-2. Developmentally, HIF-1 plays a central role in early vascular and bone development; a role that is later assumed by HIF-2 as oxygen

Acknowledgments

The authors would like to acknowledge NIH grants CA095060 and CA098920 to GP.

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Hypoxia-inducible factors (HIFs) are transcription factors that reprogram the transcriptome for cells to survive hypoxic insults and oxidative stress. They are important during embryonic development and reprogram the cells to utilize glycolysis when the oxygen levels are extremely low. This metabolic change facilitates normal cell survival as well as cancer cell survival. The key feature in survival is the transition between acute hypoxia and chronic hypoxia, and this is regulated by the transition between HIF-1 expression and HIF-2/HIF-3 expression. This transition is observed in many human cancers and endothelial cells and referred to as the HIF Switch. Here we discuss the mechanisms involved in the HIF Switch in human endothelial and cancer cells which include mRNA and protein levels of the alpha chains of the HIFs. A major continuing effort in this field is directed towards determining the differences between normal and tumor cell utilization of this important pathway, and how this could lead to potential therapeutic approaches.
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A general feature of cancer is hypoxia, determined as low oxygen levels. Low oxygen levels may cause cells to alter in ways that contribute to tumor growth and resistance to treatment. Hypoxia leads to variations in cancer cell metabolism, angiogenesis and metastasis. Furthermore, a hypoxic tumor microenvironment might induce immunosuppression. Moreover, hypoxia has the potential to impact cellular processes, such as autophagy. Autophagy refers to the catabolic process by which damaged organelles and toxic macromolecules are broken down. The abnormal activation of autophagy has been extensively recorded in human tumors and it serves as a regulator of cell growth, spread to other parts of the body, and resistance to treatment. There is a correlation between hypoxia and autophagy in human malignancies. Hypoxia can regulate the activity of AMPK, mTOR, Beclin-1, and ATGs to govern autophagy in human malignancies. Furthermore, HIF-1α, serving as an indicator of low oxygen levels, controls the process of autophagy. Hypoxia-induced autophagy has a crucial role in regulating the growth, spread, and resistance to treatment in human malignancies. Hypoxia-induced regulation of autophagy can impact other mechanisms of cell death, such as apoptosis. Chemoresistance and radioresistance have become significant challenges in recent years. Hypoxia-mediated autophagy plays a crucial role in determining the response to these therapeutic treatments.
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Transcriptional activity of the hypoxia inducible factor (HIF) relies on the formation of a heterodimer composed of an oxygen-regulated α-subunit and a stably expressed β-subunit. Heterodimeric HIF activates expression by binding to RCGTG motifs within promoters of hypoxia-activated genes. Some hypoxia targets also possess an adjacent HIF ancillary sequence (HAS) reported to increase transcription but whose function remains obscure. Here, we investigate the contribution of the HAS element to the hypoxia response and its mechanism of action, using the HAS-containing prolyl 4-hydroxylase subunit α1 (P4HA1) as a gene model in NIH/3T3 mouse embryonic fibroblasts and HEK293 human embryonic kidney cells. Our HIF overexpression experiments demonstrate that the HAS motif is essential for full induction by hypoxia and that the presence of the tandem HAS/HIF, as opposed to HIF-only sequences, provides HIF proteins with the capacity to form complexes of stoichiometry beyond the classical heterodimer, likely tetramers, to cooperatively potentiate hypoxia-induced transcription. We also provide evidence of the crucial role played by the Fα helix of the PAS-B domain of the HIF1β subunit to support the interaction between heterodimers. Functional analysis showed that human genes containing the HAS/HIF motifs are better responders to hypoxia, and their promoters are enriched for specific transcription factor binding sites. Gene ontology enrichment revealed a predominance of HAS/HIF in genes primarily related to tissue formation and development. Our findings add an extra level of regulation of the hypoxia/HIF signaling through multimerization of HIF proteins on regulatory elements containing the HAS/HIF motifs.
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Prodrug is a potential regime to overcome serious adverse events and off-target effects of chemotherapy agents. Among various prodrug activators, hypoxia stands out owing to the generalizability and prominence in tumor micro-environment. However, existing hypoxia activating prodrugs generally face the limitations of stringent structural requirements, the lack of feedback and the singularity of therapeutic modality, which is imputed to the traditional paradigm that recognition groups must be located at the terminus of prodrugs. Herein, a multifunctional nano-prodrug Mal@Cy-NTR-CB has been designed. In this nano-prodrug, a self-destructive tether is introduced to break the mindset, and achieves the activation by hypoxia of chemotherapy based on Chlorambucil (CB), whose efficacy can be augmented and traced by photodynamic therapy (PDT) and fluorescence from Cyanine dyes (Cy). In addition, Maleimide (Mal) carried by the nano-shells can regulate glutathione (GSH) content, preventing ¹O₂ scavenging, so as to realize PDT sensitization. Experiments demonstrate that Mal@Cy-NTR-CB specifically responds to hypoxic tumors, and achieve synchronous activation, enhancement and feedback of chemotherapy and PDT, inhibiting the tumor growth effectively. This study broadens the design ideas of activatable prodrugs and provides the possibility of multifunctional nano-prodrugs to improve the generalization and prognosis in precision oncology.
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Trends in Biochemical Sciences

ReviewPassing the baton: the HIF switch

Section snippets

Hypoxia and the HIFs in human physiology and disease

HIF regulation

Outcomes of HIF-1 versus HIF-2 activation

Different temporal and functional roles of HIF-1 versus HIF-2

HIFs in vascular development

HIFs in bone development

HIFs in stem cells and cancer

Mediators of the HIF switch

Concluding remarks

Acknowledgments

J. Biol. Chem.

Cancer Cell

Curr. Opin. Genet. Dev.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Blood

Blood

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Dev. Biol.

J. Biol. Chem.

Cancer Cell

Blood

Cancer Cell

Cancer Cell

Cell

Cancer Cell

Cell Stem Cell

Cell Stem Cell

Mol. Cell

J. Biol. Chem.

Why is the partial oxygen pressure of human tissues a crucial parameter? Small molecules and hypoxia

J. Cell. Mol. Med.

Hypoxia signalling in cancer and approaches to enforce tumour regression

Nature

Hypoxia-inducible factor 1 is a basic-helix-loop-helix-PAS heterodimer regulated by cellular O2 tension

Proc. Natl. Acad. Sci. U.S.A.

The hypoxia-associated factor switches cells from HIF-1{alpha}- to HIF-2{alpha}-dependent signaling promoting stem cell characteristics, aggressive tumor growth and invasion

Cancer Res.

Targeting of HIF-alpha to the von Hippel–Lindau ubiquitylation complex by O2-regulated prolyl hydroxylation

Science

Ubiquitination of hypoxia-inducible factor requires direct binding to the beta-domain of the von Hippel–Lindau protein

Nat. Cell Biol.

FIH-1: a novel protein that interacts with HIF-1alpha and VHL to mediate repression of HIF-1 transcriptional activity

Genes Dev.

RACK1 vs. HSP90: competition for HIF-1 alpha degradation vs. stabilization

Cell Cycle

Regulation of tumor angiogenesis by p53-induced degradation of hypoxia-inducible factor 1alpha

Genes Dev.

Hypoxia-associated factor, a novel E3-ubiquitin ligase, binds and ubiquitinates hypoxia-inducible factor 1alpha, leading to its oxygen-independent degradation

Mol. Cell. Biol.

The chaperone-dependent ubiquitin ligase CHIP targets HIF-1alpha for degradation in the presence of methylglyoxal

PLoS ONE

Endothelial PAS domain protein 1 (EPAS1), a transcription factor selectively expressed in endothelial cells

Genes Dev.

Widespread hypoxia-inducible expression of HIF-2alpha in distinct cell populations of different organs

FASEB J.

Contrasting properties of hypoxia-inducible factor 1 (HIF-1) and HIF-2 in von Hippel–Lindau-associated renal cell carcinoma

Mol. Cell. Biol.

Differentiating the functional role of hypoxia-inducible factor (HIF)-1alpha and HIF-2alpha (EPAS-1) by the use of RNA interference: erythropoietin is a HIF-2alpha target gene in Hep3B and Kelly cells

FASEB J.

HIF1alpha and HIF2alpha: sibling rivalry in hypoxic tumour growth and progression

Nat. Rev. Cancer

Silencing hypoxia-inducible factor-1alpha inhibits cell migration and invasion under hypoxic environment in malignant gliomas

Int. J. Oncol.

Iron-regulatory proteins limit hypoxia-inducible factor-2alpha expression in iron deficiency

Nat. Struct. Mol. Biol.

Review
Passing the baton: the HIF switch