Proteasome inhibition reprograms chromatin landscape in breast cancer (ATAc-Seq)

The 26S proteasome regulates general protein homeostasis and controls vital cellular processes, including cell division and transcriptional regulation. Cancer cells are dependent on the proper functioning of the proteasome to modulate gene expression networks that promote tumor growth. Proteasome inhibition has emerged as an effective therapeutic strategy, although mechanisms by which this approach achieves its outcomes remain unclear. We performed an integrative analysis of the transcriptome and chromatin landscape of MCF-7 breast cancer cells treated with a model proteasome inhibitor, MG132, to examine the consequences of proteasome inhibition on gene regulation. MG132 treatment initiated dynamic changes in chromatin accessibility at specific loci termed Differentially Open Chromatin Regions (DOCRs). DOCRs with increased accessibility were primarily distal to transcription start sites (TSS), while those with decreased accessibility were promoter proximal and distal to TSS. Promoter proximal DOCRs showed a unique chromatin architecture associated with distinct divergent transcription patterns. Conversely, DOCRs distal to TSS were enriched in oncogenic super enhancers that are predominantly accessible in non-basal compared to basal breast tumor subtypes. These data define distinct chromatin states and RNAPII transcription patterns, revealing molecular mechanisms by which the proteasome modulates the expression of gene networks intrinsic to breast cancer biology.