Direct phosphorylation events involved in HIF-α regulation: the role of GSK-3β

Abstract

Hypoxia-inducible factors (HIFs), consisting of α- and β-subunits, are critical regulators of the transcriptional response to hypoxia under both physiological and pathological conditions. To a large extent, the protein stability and the recruitment of coactivators to the C-terminal transactivation domain of the HIF α-subunits determine overall HIF activity. The regulation of HIF α-subunit protein stability and coactivator recruitment is mainly achieved by oxygen-dependent posttranslational hydroxylation of conserved proline and asparagine residues, respectively. Under hypoxia, the hydroxylation events are inhibited and HIF α-subunits stabilize, translocate to the nucleus, dimerize with the β-subunits, and trigger a transcriptional response. However, under normal oxygen conditions, HIF α-subunits can be activated by various growth and coagulation factors, hormones, cytokines, or stress factors implicating the involvement of different kinase pathways in their regulation, thereby making HIF-α-regulating kinases attractive therapeutic targets. From the kinases known to regulate HIF α-subunits, only a few phosphorylate HIF-α directly. Here, we review the direct phosphorylation of HIF-α with an emphasis on the role of glycogen synthase kinase-3β and the consequences for HIF-1α function.

Keywords: GSK-3β; HIF-1; hypoxia; kinase; phosphorylation; tumor suppressor; ubiquitinylation.

Figure 1

Phosphorylation- and hydroxylation-mediated proteasomal degradation of hypoxia-inducible factor 1α (HIF-1α). Notes: HIF-1α is phosphorylated on specific residues (T498, S502, S505, T506, and S510 or S551, T555, and S589) by glycogen synthase kinase 3β, leading to interaction with Fbw7 that serves as the recognition component of an E3 ubiquitin ligase complex also containing Skp1, Cul1, and Rbx1 and results in degradation of HIF-1α via the ubiquitin proteasome pathway. This degradation pathway is blocked by the ubiquitin-specific protease 28 (USP28). In contrast, HIF-1α is hydroxylated on specific residues (P402 and P564) by a family of prolyl hydroxylases (PHDs), leading to interaction with the VHL ubiquitin ligase complex, containing VHL, elongin C, elongin B, Cul2, and Rbx1, and results also in the proteasomal degradation of HIF-1α. In this case, the HIF-1α degradation could be blocked by the ubiquitin-specific protease 20 (USP20). Abbreviations: bHLH, basic helix-loop-helix; PAS A, Per-ARNT-Sim A domain; PAS B, Per-ARNT-Sim B domain; ODD, oxygen-dependent degradation domain; N-TAD, N-terminal transactivation domain; ID, inhibitory domain; C-TAD, C-terminal transactivation domain; Fbw7, F-box and WD-40 protein 7; VHL, von Hippel-Lindau protein; Cul1, Cullin 1; Cul2, Cullin 2; Skp1, S phase kinase associated protein 1; Rbx1, Ring box 1; E2, E2 ubiquitin-conjugating enzyme; Ub – ubiquitin.