Anti-Inflammatory Chromatinscape Suggests Alternative Mechanisms of Glucocorticoid Receptor Action

Area of Science:

• Immunology and Molecular Biology
• Pharmacology and Therapeutics

Background:

• Glucocorticoids (GCs) are widely used anti-inflammatory drugs, but their precise mechanisms of action remain incompletely understood.
• Most research investigates GC effects before inflammatory triggers, unlike clinical scenarios where GCs are administered during active inflammation.

Purpose of the Study:

• To elucidate the mechanistic effects of late-onset glucocorticoid treatment on macrophage responses during inflammation.
• To compare transcriptional and chromatin accessibility changes induced by early versus late glucocorticoid receptor (GR) activation.

Main Methods:

• Macrophages were treated with Dexamethasone (Dex) either before or after lipopolysaccharide (LPS) stimulation.
• Transcriptional profiling and chromatin accessibility (chromatin-scape) analysis were performed to assess gene expression and GR binding.
• Investigated the interaction between GR and inflammatory transcription factors like NF-κB and AP-1.

Main Results:

• Late GR activation yielded gene expression profiles similar to pre-activation but mitigated disruption of metabolic genes.
• GR chromatin occupancy did not correlate with Dexamethasone-regulated gene expression, challenging the 'trans-repression by tethering' model.
• GR activation globally blocked NF-κB and AP-1 binding to chromatin and induced their inhibitory factors.

Conclusions:

• Late GC administration, timed for clinical relevance, effectively modulates inflammatory responses.
• GR's anti-inflammatory action involves genome-wide inhibition of inflammatory transcription factor binding rather than solely direct gene repression.
• These findings offer novel insights into the 'inflamed epigenome' and GR mechanisms for therapeutic intervention.