JmjC (Jumonji C)-domain-containing human histone-lysine demethylase (KDM) enzymes are Fe(II) and 2-oxoglutarate dependent oxygenases involved in epigenetic regulation. The KDM4 sub-family demethylate di-and tri-methylated lysine residues 9 and 36 on Histone 3 (H3K9me2/3 and H3K36me2/3, respectively) regulating transcription (1). Increased methylation of these histone markers in macrophages under hypoxia, a pathological feature of many cardiovascular diseases, has been attributed to attenuation of KDM activity (2). Consistent with this, one enzymatic family member, KDM4E, has been shown to have a graded response to oxygen (3), suggesting an oxygen-sensing role similar to the related HIF hydroxylases (4). Furthermore, global epigenetic changes in cardiomyocytes have been detected in a rat model of cardiac hypertrophy, with levels of the H3K9me3 mark being significantly affected (5). We are therefore investigating the oxygen-dependence of the KDMs, and the consequences for their activity in ischaemic heart disease.
The KDM4 isoform KDM4A has been linked to the development of cardiac hypertrophy in both mice and humans (6) and has therefore been the initial focus of our investigations. An in vitro oxygen-dependence assay on recombinant KDM4A demonstrated that the enzyme activity is highly sensitive to oxygen concentrations, leading us to investigate its response to different oxygen concentrations in cells. Intriguingly, HeLa cells subjected to hypoxia following overexpression of KDM4A retained higher levels of the H3K9me3 mark compared to cells incubated in normoxic conditions, as determined by immunofluorescence. Altered KDM4A activity under hypoxic conditions is likely to have significant implications for global histone methylation levels and thus transcriptional regulation in ischaemic cardiovascular disease states.