Passing the baton: the HIF switch

Abstract

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 (<24h) and HIF-2 drives the chronic response (>24h). Here, we review the significance of the HIF switch and the relation between HIF-1 and HIF-2 under both physiological and pathophysiological conditions.

Figure 1

The structural domains of hypoxia inducible factor (HIF)-1/2/3α and their transcriptional binding partner, HIF-1β/ARNT (aryl hydrocarbon nuclear translocator)that together, form the HIF-1, HIF-2 and HIF-3 transcriptional complexes, respectively. The basic helix-loop-helix (bHLH) and per-Arnt-SIM (PAS) domains are involved in DNA binding and heterodimerization; the oxygen -dependent degradation (ODD) domain is required for oxygen-dependent hydroxylation and degradation; and the N -terminal and C-terminal transactivation domains (TAD-N and TAD-C, respectively) are required for transcriptional activation. The binding domains of known modulators of HIF-α are depicted, along with the effectsof these interactions on activity of the HIF transcriptional complex( red font indicates inhibitory interactions, green font indicates activating interactions). The von Hippel Lindau protein (pVHL) E3 ligase complex regulates the oxygen-dependent degradation of all three major HIF-α subunits. Factor inhibiting HIF-1 (FIH-1)hydroxylates HIF -2α at a lower efficiency (broken oval) than HIF-1α(unbroken oval). Receptor for activated protein kinase C 1 (RACK1) promotes the degradation of HIF-1α when heat shock protein 90 (Hsp90)is inhibited, such as by Hsp90 inhibitors. The hypoxia-associated factor (HAF)selectively binds to HIF-1α and HIF-2α, mediating degradation and transactivation, respectively. Hsp70 promotes the binding of carboxyl terminus of Hsp70-interacting protein (CHIP)to HIF-1α, resulting in HIF-1α degradation. Sirtuin 1 (SIRT1)deacetylates HIF -2α, resulting inactivation.

Figure 2

The transition from HIF-1- to HIF-2-dependent transcription is dependent on oxygen tension during vascular and bone development. (a) HIF-1 plays a dominant role during vasculogenesis (i), which occurs under intense hypoxia. Although both HIF-1 and HIF-2 play complementary roles during angiogenesis (ii), the final stages of vascular remodeling and stabilization (iii, iv), such as pericyte (pct) and smooth muscle cell (smc) recruitment, occur under less hypoxic conditions and are mainly HIF-2 driven. HIF target genes involved in the various stages of vascular development are indicated in italics. (b) During the initial stage of bone development, HIF-1 is required for mesenchymal cell condensation, chondrocyte proliferation, and synthesis of the cartilaginous extracellular matrix (ECM) (i). Next, proliferation continues to be promoted by HIF-1, while cells within the hypertrophic zone require HIF-2 to undergo hypertrophic differentiation (ii). This is followed by vascular invasion of the hypertrophic zone, which requires both HIF-1 and HIF-2 (iii). Finally, degradation of cartilage and its eventual replacement with bone is promoted by HIF-2 (iv). Major HIF target genes are indicated in italics.

Figure 3

The switch from HIF-1- to HIF-2-dependent transcription during chronic hypoxia in solid tumors. (a) The diffusion limitation of oxygen results in oxygen gradients within solid tumors (i). Acute hypoxia caused by vessel occlusion or rapid tumor growth promotes induction of both HIF-1α and -2α (ii); however, HIF-1 is the major driver of the acute response. This causes activation of the HIF-1 transcriptional program, which promotes alleviation of hypoxia through angiogenesis and/or reperfusion, or (iii) cell death (broken circles), depending on the mutational landscape of the tumor cells. Alternatively, chronic hypoxia (iv) can increase HAF and HIF-2α levels, mediating a switch to HIF-2-dependent transcriptionthat promotes tumor adaptation, proliferation and progression. (b) The temporal regulation of HIF-1α (blue line), HIF-2α (green line) and HAF (red line) in response to prolonged hypoxic exposure. The window in which the HIF-1α to HIF-2α switch occurs is indicated by broken vertical lines.