ARDent about acetylation and deacetylation in hypoxia signalling Rebecca Bilton 1, 2 , Eric Trottier 1 , Jacques Pouysse ´ gur 1 and M. Christiane Brahimi-Horn 1 1 Institute of Signaling, Developmental Biology and Cancer Research CNRS UMR 6543, University of Nice, Centre A. Lacassagne, 33 Avenue Valombrose, 06189 Nice, France 2 Present address: School of Molecular and Biomedical Sciences and the ARC Special Research Centre for the Molecular Genetics of Development, The University of Adelaide, Adelaide, South Australia 5005, Australia Given the key role that the a subunit of the ab heterodimeric transcription factor hypoxia-inducible factor-1 (HIF-1) has in tumourigenesis, and in particular in angiogenesis, a full understanding of its regulation is crucial to the development of cancer therapeutics. Posttranslational acetylation and deacetylation of this subunit by an acetyltransferase called Arrest-defective-1 (ARD1) and by different histone deacetylases (HDACs), respectively, has been suggested as a mechanism. However, conflicting data bring into question the foundations of this mechanism and at present it is not clear what the precise role of these proteins is with respect to HIF. Nonetheless, the observation that small-molecule inhibitors of HDACs have anti-angio- genic activity suggests that acetylation and deacetyla- tion of HIF or HIF modifiers represents a potential target in cancer therapy. Introduction The transcription factor hypoxia-inducible factor (HIF) is a heterodimer of constitutively expressed a and b protein subunits. The a subunit has a very short half-life (5 min- utes) in the presence of oxygen but is stable in low oxygen conditions (hypoxia) [1]. By contrast, the b subunit is not oxygen-regulated. Oxygen deprivation owing to ineffective or insufficient vascularization is often encountered in can- cer and in ischaemic disorders. Hypoxia in tumours is a consequence of the massive cellular expansion that dis- tances cells from the oxygen-carrying vasculature. Under these conditions, HIF modulates transcription of a vast array of genes, including those encoding the vascular endothelial growth factor (vegf , implicated in angiogen- esis), glycolytic enzymes (implicated in metabolism), erythropoietin (epo, erythropoiesis) and Bcl-2/adenovirus EIB 19 kDa-interacting protein (bnip3, apoptosis) [2,3]. HIF thus initiates an adaptive response that re-establishes a nourishing environment to ensure cell survival and growth [4]. In the clinical setting, increased expression of HIF in a wide range of cancers is linked to a poor response to treatment and thus to poor patient prognosis. Given the key role that HIF has in tumourigenesis, under- standing its regulation is a major topic of research. Posttranslational modification is the crux of how the oxygen-dependent instability of the a subunits of HIF is regulated, in particular hydroxylation by oxygen-depen- dent HIF hydroxylases [1,5]. Prolyl hydroxylation by the prolyl hydroxylase domain (PHD) proteins earmarks HIF- a for ubiquitination by the von Hippel-Lindau (VHL) ubi- quitin E3 ligase-containing complex and for consequent destruction by the proteasome system. In addition, aspar- aginyl hydroxylation by Factor inhibiting HIF-1 (FIH) blocks HIF transcriptional activity by abrogating its inter- action with the co-activator CBP/p300. Another form of modification, acetylation by an N a -acetyltransferase termed Arrest-defective-1 (ARD1) protein, has also been described to increase instability of the HIF-1a protein; this occurs in an oxygen-dependent manner through the reg- ulation of the expression level of ARD1 [6]. This report [6] added HIF to the increasing list of transcription factors and transcriptional co-activators that undergo acetylation and deacetylation of lysine residues by histone acetyltrans- ferases (HATs) and/or histone deacetylases (HDACs), respectively [7]; the list includes the tumour suppressor p53 and the related protein p73, p300 (a transcriptional co- activator with HAT activity), the general transcription factors TFIIE and TFIIF, E2F, the oncoprotein c-Myb, the muscle regulator MyoD, the site-specific DNA binding factors EKLF, GATA1 and GATA3, the cell cycle regulator E2F, the high-mobility group-box-containing transcription factor TCF. However, although hydroxylation is now a well-recognized mechanism of regulation of HIF-a stabi- lity, the acetylation hypothesis remains controversial and the role of the ARD1 isoforms in HIF regulation requires clarification. Investigations into the role of HDACs and HDAC inhibitors do suggest a role for acetylation and deacetylation in the regulation of the stability and activity of HIF and thus in downstream HIF-dependent gene acti- vation or repression. Here, we discuss these aspects further. Saccharomyces cerevisiae Ard1p catalyses the transfer of an acetyl group from acetyl-coenzyme A to the very N-terminus of newly synthesized proteins, a common posttranslational modification of cytosolic proteins [8] (Box 1). Mutation of yeast Ard1 leads to defective mitotic cell cycling, – hence its name Arrest-defective-1 – and results in the inability of S. cerevisiae to respond to Opinion TRENDS in Cell Biology Vol.16 No.12 Corresponding author: Brahimi-Horn, M.C. (brahimi@unice.fr). Available online 27 October 2006. www.sciencedirect.com 0962-8924/$ – see front matter ß 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tcb.2006.10.002