BACKGROUND: Innate immune cells are major targets of glucocorticoids as anti-inflammatory therapies. Glucocorticoids are metabolic hormones that provide natural feedback regulation of immune function. They are widely prescribed, but use is restricted by side-effects. Much of our knowledge about how glucocorticoids work comes from studies in mice. However, since mice are imperfect models of human macrophage biology, for example in inflammation, whether this knowledge can be directly translated to man is uncertain. We aimed to address this uncertainty.
METHODS: We performed global expression profiling of primary cultured mouse and human macrophages, sampling at six points during 24 h after treatment with glucocorticoids. To assess the mechanism behind the regulation of transcription we also determined the DNA-binding pattern of the nuclear glucocorticoid receptor (GR) using chromatin immunoprecipitation followed by sequencing (ChIP-seq).
FINDINGS: Glucocorticoids initiated a temporal cascade of predominantly induced gene regulation in both species, but with little overlap in the gene sets. Single nucleotide polymorphisms (SNPs) linked to inflammatory disease were enriched near human (but not mouse) glucocorticoid-regulated genes. Using our genes as candidates, we identified eight SNPs reported as low significance that might be of biological relevance. We found that GR bound at candidate enhancers in the vicinity of induced genes and that this was strongly associated with canonical GR-dimer binding motifs. By contrast, promoters of induced genes, and the smaller set of repressed genes, showed no association with GR binding. Sites bound by GR were also not conserved between human and mouse macrophages, due to loss of the GR binding motif, which reflects the divergence in transcription.
INTERPRETATION: We conclude that the response of innate immune cells to glucocorticoids has diverged significantly between species because of the gain or loss of glucocorticoid-responsive enhancers.
FUNDING: Wellcome Trust.