Investigating the effect of hypoxia on the JmjC histone lysine demethylase KDM4A
Keywords: KDM4A
2OG oxygenase
Epigenetics
Histone demethylase
Hypoxia

The JmjC-histone lysine demethylases (JmjC-KDMs) are epigenetic regulators responsible for the demethylation of methylated lysine residues on the N-terminal histone tails. As Fe2+ and 2-oxoglutarate dependent oxygenases (2OG oxygenases), the JmjC-KDMs possess an absolute requirement for molecular oxygen and are related to the cellular oxygen sensing HIF hydroxylases, PHD2 and FIH. Several JmjC-KDMs are known HIF target genes, hence are upregulated in hypoxia. Moreover, a number of JmjC-KDMs have been shown to have differential oxygen dependences, while aberrant histone methylation has been observed in both hypoxic cells and disease states such as cancer and cardiovascular disease.
The work described in this thesis aimed to investigate the impact of hypoxia on the JmjC-KDM, KDM4A. In vitro kinetic analyses revealed a Kmapp(O2) for recombinant KDM4A of 173 ± 23 μM, which is higher than reported values for the 2OG oxygenases C-P4H, mPAHX and even FIH, and approaching those evaluated for the key oxygen sensor PHD2 (230-1746 μM). These results indicate that KDM4A activity is highly sensitive to oxygen availability, and has the biochemical potential to act as an oxygen sensor in the context of epigenetic regulation.
Subsequent investigation of the cellular oxygen dependence of KDM4A, and found that the activity of ectopically expressed KDM4A in U2OS cells demonstrates a graded response to oxygen. Importantly, this trend correlates with the in vitro results, providing further evidence that hypoxia may impact upon epigenetic regulation by the JmjC-KDMs.
The various factors that may contribute to the hypoxic inhibition of KDM4A were investigated both in vitro and in cells. The results of these studies suggested that altered concentrations of TCA cycle intermediates, comprising reduced levels of the 2OG oxygenase co-substrate 2OG and increased concentrations of the reported inhibitor 2HG, are likely to only minimally affect the activity of KDM4A in hypoxia. Interestingly, the 2OG oxygenase inhibitor IOX1 possessed increased inhibitory potency against KDM4A under conditions of low oxygen, implying that the use of mixed-mode inhibitors against KDM4A may be of therapeutic benefit in hypoxic disease states. This may be of particular pertinence to cardiac hypertrophy (CH), in which KDM4A activity is reported to have pathophysiological consequences. In a collaboration with Dr Tim McKinsey (University of Colorado, Denver), the KDM4 inhibitor CCT1 was tested in a phenotypic screen of cardiomyocyte hypertrophy, the results of which further support a role for KDM4A in this disease, and suggest that the use of small-molecule inhibitors of KDM4A may be a viable therapeutic strategy in CH.
Finally, the effect of reactive oxygen species, levels of which may be increased in hypoxia, on KDM4A activity was explored. Recombinant KDM4A was found to be acutely sensitive to inhibition by hydrogen peroxide (H2O2) when compared to the HIF hydroxylases PHD2 and FIH. These results imply that KDM4A may act as a sensor of oxidative stress at the chromatin level, and further investigation in a more biologically relevant context is proposed.
Overall, the work described herein demonstrates that the activity of KDM4A is sensitive to oxygen availability, a phenomenon that is likely to have significant implications for epigenetic regulation in hypoxia and the expression of KDM4A-regulated genes in ischaemic disease states.