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Hypoxia-inducible transcription factors (HIFs) are fundamental to the cellular adaptation to low oxygen levels but how they interact with chromatin and efficiently activate their target genes is unclear. Using genome-wide mutagenesis in human cancer cells, we define genes required for HIF transcriptional activation, and identify a requirement for the Histone 3 lysine 4 (H3K4) methyltransferase SET1B. Loss of SET1B leads to a selective reduction in HIF transcriptional activity in hypoxia, with SET1B driving expression of genes involved in angiogenesis rather than glycolysis, resulting in impaired tumour establishment in SET1B deficient xenografts. Mechanistically, we show that SET1B is itself oxygen regulated, accumulates on chromatin in hypoxia, and is recruited to HIF target genes through HIF-1α. Accordingly, we show that the hypoxic induction of H3K4me3 at specific HIF targets is both HIF and SET1B dependent, and when impaired, decreases promoter acetylation and gene expression. Together, these findings reveal SET1B as a determinant of site-specific histone methylation and provide insight into how HIF target genes are differentially regulated.

Hypoxia-inducible transcription factors (HIFs) are fundamental to the cellular adaptation to low oxygen levels but it is unclear how they interact with chromatin and activate their target genes. Here we use genome-wide mutagenesis to identify genes involved in HIF transcriptional activity, and define a requirement for the histone H3 lysine 4 (H3K4) methyltransferase SET1B. SET1B loss leads to a selective reduction in transcriptional activation of HIF target genes, resulting in impaired cell growth, angiogenesis, and tumor establishment in SET1B-deficient xenografts. Mechanistically, we show that SET1B accumulates on chromatin in hypoxia, and is recruited to HIF target genes by the HIF complex. The selective induction of H3K4 trimethylation at HIF target loci is both HIF- and SET1B-dependent, and when impaired correlates with decreased promoter acetylation and gene expression. Together, these findings reveal SET1B as a determinant of site-specific histone methylation and provide insight into how HIF target genes are differentially regulated.

Hypoxia-inducible transcription factors (HIFs) facilitate cellular adaptations to low-oxygen environments. However, it is increasingly recognised that HIFs may be activated in response to metabolic stimuli, even when oxygen is present. Understanding the mechanisms for the crosstalk that exists between HIF signalling and metabolic pathways is therefore important. This review focuses on the metabolic regulation of HIFs by small molecule metabolites and iron, highlighting the latest studies that explore how tricarboxylic acid (TCA) cycle intermediates, 2-hydroxyglutarate (2-HG) and intracellular iron levels influence the HIF response through modulating the activity of prolyl hydroxylases (PHDs). We also discuss the relevance of these metabolic pathways in physiological and disease contexts. Lastly, as PHDs are members of a large family of 2-oxoglutarate (2-OG) dependent dioxygenases that can all respond to metabolic stimuli, we explore the broader role of TCA cycle metabolites and 2-HG in the regulation of 2-OG dependent dioxygenases, focusing on the enzymes involved in chromatin remodelling.

Paredes et al. (1) describe polymerase-δ interacting protein 2 (Poldip2) as a novel regulator of mitochondrial lipoylation through stabilization of Ac-CoA synthetase medium-chain family member 1 (ACSM1). We have several concerns with their proposed model based on the following reasons. Prior mammalian and yeast biochemical studies are not consistent with a significant physiological role for lipoate scavenging in eukaryotes (2, 3). Genetic depletion or germline mutations in de novo lipoic acid synthesis enzymes (LIAS, LIPT1, and LIPT2) result in loss of mitochondrial lipoylation, respiration, and developmental defects in mammals, which are not reversed with exogenous lipoic acid (2 … [↵][1]1To whom correspondence may be addressed. Email: dieckman{at}email.arizona.edu, alexander.kastaniotis{at}oulu.fi, or jan33{at}cam.ac.uk. [1]: #xref-corresp-1-1

The combined effects of intracellular lactate and proton accumulation on cell volume, Vc, were investigated in resting Rana temporaria striated muscle fibres. Intracellular lactate and H+ concentrations were simultaneously increased by exposing resting muscle fibres to extracellular solutions that contained 20–80 mm sodium lactate. Cellular H+ and lactate entry was confirmed using pH-sensitive electrodes and 1H-NMR, respectively, and effects on Vc were measured using confocal microscope xz-scanning. Exposure to extracellular lactate up to 80 mm produced significant changes in pH and intracellular lactate (from a pH of 7.24 ± 0.03, n = 8, and 4.65 ± 1.07 mm, n = 6, respectively, in control fibres, to 6.59 ± 0.03, n = 4, and 26.41 ± 0.92 mm, n = 3, respectively) that were comparable to those observed following fatiguing stimulation (6.30–6.70 and 18.04 ± 1.78 mm, n = 6, respectively). Yet, the increase in intracellular osmolarity expected from such an increase in intracellular lactate did not significantly alter Vc. Simulation of these experimental results, modified from the charge difference model of Fraser & Huang, demonstrated that such experimental manoeuvres produced changes in intracellular [H+] and [lactate] comparable to those observed during muscle fatigue, and accounted for this paradoxical conservation of Vc through balancing negative osmotic effects resulting from the net cation efflux that would follow a titration of intracellular membrane-impermeant anions by the intracellular accumulation of protons. It demonstrated that with established physiological values for intracellular buffering capacity and the permeability ratio of lactic acid and anionic lactate, PLacH: PLac−, this would provide a mechanism that precisely balanced any effect on cell volume resulting from lactate accumulation during exercise.