Interferon regulatory factor 1 and histone H4 acetylation in systemic lupus erythematosus

Histone acetylation modulates gene expression and has been described as increased in systemic lupus erythematosus (SLE). We investigated interferon regulatory factor 1 (IRF1) interactions that influence H4 acetylation (H4ac) in SLE. Intracellular flow cytometry for H4 acetylated lysine (K) 5, K8, K12, and K16 was performed. Histone acetylation was defined in monocytes and T cells from controls and SLE patients. RNA-Seq studies were performed on monocytes to look for an imbalance in histone acetyltransferases and histone deacetylase enzyme expression. Expression levels were validated using real-time quantitative RT-PCR. IRF1 induction of H4ac was evaluated using D54MG cells overexpressing IRF1. IRF1 protein interactions were studied using co-immunoprecipitation assays. IRF1- dependent recruitment of histone acetyltransferases to target genes was examined by ChIP assays using p300 antibody. Flow cytometry data showed significantly increased H4K5, H4K8, H4K12, and H4K16 acetylation in SLE monocytes. HDAC3 and HDAC11 gene expression were decreased in SLE monocytes. PCAF showed significantly higher gene expression in SLE than controls. IRF1-overexpressing D54MG cells were associated with significantly increased H4K5, H4K8, and H4K12 acetylation compared to vector-control D54MG cells both globally and at specific target genes. Co-immunoprecipitation studies using D54MG cells revealed IRF1 protein-protein interactions with PCAF, P300, CBP, GCN5, ATF2, and HDAC3. ChIP experiments demonstrated increased p300 recruitment to known IRF1 targets in D54MG cells overexpressing IRF1. In contrast, p300 binding to IRF1 targets decreased in D54MG cells with IRF1 knockdown. SLE appears to be associated with an imbalance in histone acetyltransferases and histone deacetylase enzymes favoring pathologic H4 acetylation. Furthermore, IRF1 directly interacts with chromatin modifying enzymes, supporting a model where recruitment to specific target genes is mediated in part by IRF1.