Chromatin regulation of MHC class II expression
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In the eukaryotic cell, DNA is assembled into higher order structure to allow the packaging of genetic material into the confined space within the cell nucleus. Chromatin remodeling must occur too allow transcription factors binding and the activation of gene expression. Class II transactivator (CIITA) is the master regulator of MHC class II expression and it has been shown to participate in chromatin remodeling at the MHC class II promoter. We found that in several tumor cell lines (Colon 26 and RJ2.2.5), MHC class II expression is silenced and can be induced by treatment with the histone deacetylase inhibitor Trichostatin A (TSA) in the absence of CIITA. Although Colon 26 contains an intrinsic, active IFN-[gamma] pathway, TSA pretreatment inhibited the IFN-[gamma] induced STAT-1 phosphorylation suggesting a different TSA activation pathway from that induced by IFN-[gamma]. Chromatin immunoprecipitation (ChIP) assays demonstrated similar patterns of histone modification changes upon IFN-[gamma] (CIITA-dependent) and TSA (CIITA-independent) stimulation. Both IFN-[gamma] and TSA treated cells demonstrated an enhancement of markers associated with active gene expression (Acetylated histone H3 and H4 and trimethylated H3 lysine 4) at the MHC class II promoter while the level of the repressive state marker, trimethylated H3 lysine 9, was decreased. Unlike other histone modifications, enhanced trimethylated H3 lysine 36 was detected at the 3'region but not the promoter of the MHC class II gene in both IFN-[gamma] inducible (HeLa) and TSA inducible (RJ2.2.5) cell lines. Such differential localization of the histone modifications suggests that each mark may be involved in the different phases of the transcription process. H3K79me2 remained unchanged and at low level after IFN-[gamma] (HeLa) or TSA (RJ2.2.5) treatment when compared to the MHC class II expressing cell line Raji. However, H3K79 methylation can be rescued by the reconstitution of CIITA expression in the CIITA deficient cell line RJ2.2.5 by stable transfection. Together, our data suggests that TSA treatment can replace the function of CIITA in activating MHC class II expression. Although IFN-[gamma] and TSA induced MHC class II expression through different regulatory pathways, both resulted in similar patterns of histone modifications and chromatin remodeling. Lastly, CIITA may be crucial for the recruitment of histone methyltransferase to the MHC class II gene.