Histone deacetylase inhibitors decrease Toll-like receptor-mediated activation of proinflammatory gene expression by...

Area of Science:

• Molecular Biology
• Immunology
• Epigenetics

Background:

• Post-translational modifications of histone proteins, including acetylation, are crucial for regulating chromatin structure and gene expression in eukaryotes.
• Histone deacetylases (HDACs) remove acetyl groups, generally leading to a more condensed chromatin state and reduced transcriptional activity.
• Activation of histone acetyltransferases (HATs) or inhibition of HDACs is typically associated with chromatin opening and enhanced gene transcription.

Purpose of the Study:

• To investigate the effect of HDAC inhibitors on cytokine gene expression in immune cells upon Toll-like receptor (TLR) stimulation.
• To elucidate the underlying molecular mechanisms of HDAC inhibitor action on specific cytokine gene loci, such as interleukin-12 p40 (IL-12p40).

Main Methods:

• Treatment of murine dendritic cells and macrophages with HDAC inhibitors (trichostatin A and suberoylanilide hydroxamic acid) at non-apoptotic concentrations.
• Stimulation of cells with Toll-like receptor (TLR) agonists to induce inflammatory responses.
• Analysis of mRNA and protein levels for cytokines (IL-12p40, tumor necrosis factor-alpha, IL-1 beta) and costimulatory molecules.
• Chromatin immunoprecipitation (ChIP) assays to assess histone acetylation, transcription factor binding (Rel-A, IRF1), and chromatin remodeling at the IL-12p40 locus.
• Assessment of upstream signaling pathways, including nuclear factor-kappaB (NF-κB) and mitogen-activated protein kinase (MAPK) activation.

Main Results:

• HDAC inhibition strongly suppressed the induction of IL-12p40 mRNA and protein in response to TLR stimulation in murine dendritic cells and macrophages.
• TLR-mediated upregulation of costimulatory molecules was also inhibited by HDAC inhibitors.
• While HDAC inhibition increased histone acetylation at the IL-12p40 locus, it abrogated IL-12p40 chromatin remodeling, transcription factor binding, and transcriptional activation.
• Effects on tumor necrosis factor-alpha were observed only after prolonged exposure, and IL-1 beta regulation was unaffected.
• HDAC inhibitors did not affect upstream NF-κB or MAPK activation.

Conclusions:

• HDACs play a positive regulatory role in the expression of a subset of cytokine genes, including IL-12p40, by facilitating transcription factor recruitment.
• Contrary to general belief, HDAC inhibition can suppress, rather than enhance, the expression of certain inflammatory genes by interfering with downstream transcriptional machinery.
• These findings highlight a complex, gene-specific role for HDACs in immune cell activation and cytokine production.