Structural basis for recruitment of CBP/p300 by hypoxia-inducible factor-1 alpha

Hypoxia and High Altitude

Increased erythropoietin plasma levels and the consequent augmented production of red blood cells is the best known systemic adaptation to reduced oxygen partial pressure (pO2). Intensive research during the last years revealed that the molecular mechanism behind the regulation of erythropoietin is ubiquitous and has far more implications than first thought. Erythropoietin regulation results from the activation of the hypoxia-inducible factor-1 (HIF-1) pathway under hypoxic conditions. HIF-1 is a heterodimer consisting of an oxygen sensitive--HIF-1--and an oxygen-independent subunit--HIF-1beta (also known as the aryl hydrocarbon receptor nuclear translocator--ARNT). In addition to erythropoietin, more than 30 genes are now known to be up-regulated by HIF-1. Recently, the critical involvement of HIF-1alpha post-translational modifications in the cellular oxygen sensing mechanism was discovered. In this review we will focus on the regulation of the HIF-1 pathway and the cellular oxygen sensor and discuss their implications in high altitude hypoxia.

Structural basis for the recognition of the E2F transactivation domain by the retinoblastoma tumor suppressor

Repression of E2F transcription activity by the retinoblastoma (Rb) tumor suppressor through its interaction with the transactivation domain of the E2F transcription factor is one of the central features of G1/S arrest in the mammalian cell cycle. Deregulation of the Rb-E2F interaction results in hyperproliferation, lack of differentiation, and apoptosis, and can lead to cancer. The 2.2-A crystal structure of the Rb pocket complexed with an 18-residue transactivation-domain peptide of E2F-2 reveals that the boomerang-shaped peptide binds to the highly conserved interface between the A-box and the B-box of the Rb pocket in a bipartite manner. The N-terminal segment of the E2F-2 peptide in an extended beta-strand-like structure interacts with helices from the conserved groove at the A-B interface, whereas the C-terminal segment, which contains one 3(10) helix, binds to a groove mainly formed by A-box helices. The flexibility in the middle of the E2F-2 peptide is essential for the tight association of E2F to the Rb pocket. The binding of Rb to the E2F-2 peptide conceals several conserved residues that are crucial for transcription activation of E2F. We provide the structural basis for the Rb-mediated repression of E2F transcription activity without the requirement of histone-modifying enzymes.

Leu-574 of HIF-1alpha is essential for the von Hippel-Lindau (VHL)-mediated degradation pathway

Oxygen homeostasis is crucial for a myriad of developmental, physiological, and pathophysiological processes. Hypoxia-inducible factor 1alpha (HIF-1alpha) plays a pivotal role in response to hypoxia by transcriptionally activating target genes involving oxygen uptake, transport, delivery, and consumption. HIF-1alpha activity is regulated primarily through the ubiquitin-proteasome degradation pathway, which targets the oxygen-dependent degradation domain (ODD) of HIF-1alpha. In particular, the von Hippel-Lindau (VHL) protein complex, an E3 ubiquitin ligase, binds to the ODD upon hydroxylation of HIF-1alpha Pro-564. Here, we show that in vivo VHL interacts with the N-terminal as well as the C-terminal ODD independently, supporting the notion of functional redundancy within the ODD. Moreover, we demonstrate that Leu-574 of HIF-1alpha is essential for VHL binding to the C-terminal ODD. Despite the presence of Pro-564, deletion or mutation of Leu-574 resulted in a loss of VHL binding and a gain of protein stability. Furthermore, the identification of Leu-574 redefines the N-terminal activation domain of HIF-1alpha to be constitutively active. Taken together, this study provides new insight into the mechanisms underlying VHL-mediated HIF-1alpha degradation and transcriptional activation, and a molecular basis for drug targeting.

Hypoxia-inducible factor asparaginyl hydroxylase (FIH-1) catalyses hydroxylation at the beta-carbon of asparagine-803

Asparagine-803 in the C-terminal transactivation domain of human hypoxia-inducible factor (HIF)-1 alpha-subunit is hydroxylated by factor inhibiting HIF-1 (FIH-1) under normoxic conditions causing abrogation of the HIF-1alpha/p300 interaction. NMR and other analyses of a hydroxylated HIF fragment produced in vitro demonstrate that hydroxylation occurs at the beta-carbon of Asn-803 and imply production of the threo -isomer, in contrast with other known aspartic acid/asparagine hydroxylases that produce the erythro -isomer.

Functional analysis of hypoxia-inducible factor-1 alpha-mediated transactivation. Identification of amino acid residues critical for transcriptional activation and/or interaction with CREB-binding protein

The hypoxia-inducible factor-1 alpha (HIF-1 alpha) is a key regulator of adaptive responses to hypoxia. HIF-1 alpha has two independent transactivation domains (TADs). Whereas the N-terminal TAD (N-TAD) also constitutes a degradation box, the C-terminal TAD (C-TAD) functions in a strictly hypoxia-inducible fashion. Oxygen-dependent hydroxylation of an asparagine residue has recently been reported to regulate C-TAD function by disrupting the interaction with the CH1 domain of the p300/CBP coactivator at normoxia. Here we have performed alanine-scanning mutagenesis of a predicted alpha-helix within the C-TAD of mouse HIF-1 alpha to identify residues important for transactivation and interaction of the C-TAD with transcriptional coactivators. We observed that several hydrophobic residues, Ile(802), Leu(808), Leu(814), Leu(815), and Leu(818), were critical for transactivation and binding to the CH1 domain of CBP in hypoxic cells. Moreover, E812A/E813A and D819A mutations impaired hypoxia-dependent transactivation without disrupting binding to CH1. In the context of full-length HIF-1 alpha, mutation of the leucine residues conferred conformational changes to the protein and significantly reduced the transactivation function as well as functional interaction with the transcriptional coactivators CBP and SRC-1. These mutations also affected intranuclear redistribution of HIF-1 alpha in the presence of CBP, indicating that the integrity of the C-TAD is critical for intracellular localization of mouse HIF-1 alpha.

Molecular basis of the VHL hereditary cancer syndrome

The von Hippel-Lindau hereditary cancer syndrome was first described about 100 years ago. The unusual clinical features of this disorder predicted a role for the von Hippel-Lindau gene (VHL) in the oxygen-sensing pathway. Indeed, recent studies of this gene have helped to decipher how cells sense changes in oxygen availability, and have revealed a previously unappreciated role of prolyl hydroxylation in intracellular signalling. These studies, in turn, are laying the foundation for the treatment of a diverse set of disorders, including cancer, myocardial infarction and stroke.

Hypoxia-inducible factor (HIF) asparagine hydroxylase is identical to factor inhibiting HIF (FIH) and is related to the cupin structural family

Activity of the hypoxia-inducible factor (HIF) complex is controlled by oxygen-dependent hydroxylation of prolyl and asparaginyl residues. Hydroxylation of specific prolyl residues by 2-oxoglutarate (2-OG)-dependent oxygenases mediates ubiquitinylation and proteasomal destruction of HIF-alpha. Hydroxylation of an asparagine residue in the C-terminal transactivation domain (CAD) of HIF-alpha abrogates interaction with p300, preventing transcriptional activation. Yeast two-hybrid assays recently identified factor inhibiting HIF (FIH) as a protein that associates with the CAD region of HIF-alpha. Since FIH contains certain motifs present in iron- and 2-OG-dependent oxygenases we investigated whether FIH was the HIF asparaginyl hydroxylase. Assays using recombinant FIH and HIF-alpha fragments revealed that FIH is the enzyme that hydroxylates the CAD asparagine residue, that the activity is directly inhibited by cobalt(II) and limited by hypoxia, and that the oxygen in the alcohol of the hydroxyasparagine residue is directly derived from dioxygen. Sequence analyses involving FIH link the 2-OG oxygenases with members of the cupin superfamily, including Zn(II)-utilizing phosphomannose isomerase, revealing structural and evolutionary links between these metal-binding proteins that share common motifs.