Regulation of gene expression by the immunoproteasome (P5049)

Abstract

Background: Proteasomes play crucial roles in regulating fundamental cellular processes in eukaryotic cells. Catalytic subunits β1, β2 and β5 of the proteasome are replaced by LMP2, MECL-1 and LMP7 to form the immunoproteasome (IPr). We have shown that the IPr modulates mRNA levels of several clusters of genes in dendritic cells, but the precise mechanism by which it regulates gene expression is unknown. Results: By comparing transcriptome of Lmp7+/+/Mecl1+/+ (WT) and Lmp7-/-/Mecl1-/- (dKO) thymocytes by microarray, we found that the IPr affects the expression of 50 transcripts. The genes source of these transcripts are clustered in the genome, and are enriched in cell cycle-associated functions. Moreover, an overlap of 2% is seen by comparing with transcripts modulated by the IPr in dendritic cells. Chymotrypsin-like activity is higher in dKO than in WT cells, due to β5 overexpression. Furthermore, ubiquitinated proteins levels are similar between WT and dKO cells whereas free ubiquitin is lower in dKO cells. We next investigated levels of ubiquitin on histones, since it represent an important reservoir of ubiquitin and it can regulate transcriptional activity. Our results show that H2B, but not H2A, ubiquitination is increased in dKO cells. Conclusion: The effect of IPr on transcription is tissue-specific and could be due to increased ubiquitinated H2B levels. While proteasomes clearly regulate transcription, the impact of IPr on transcription has never been investigated.