Effect of the Proteasome Inhibitor, Bortezomib, on Histone Modifications in Human Leukemic Cell Lines

Histone-modifying enzymes and histone post-translational modifications (PTMs) play key roles in the organization (euchromatin versus heterochromatin) and function (active versus silenced genes) of chromatin. The abundance and activity of these enzymes, along with their associated histone PTMs, are often altered in cancer cells, leading to deregulated gene expression. The expression of the KMT2A-MLLT3 protein, resulting from a chromosomal translocation in mixed-lineage leukemia (MLL), a subtype of acute myeloid leukemia, augments transcription elongation, promoting the expression of HOXA9 and MEIS1, genes that play critical roles in MLL development. Bortezomib, a proteasome inhibitor, has been effective at treating various cancers. In this study, we compared the impact of bortezomib on histone PTMs in the MLL cell line MOLM-13 and the chronic myeloid leukemic (CML) cell line K562. We report that MOLM-13 had a greater level of histone H2B monoubiquitinated at lysine 120 (H2BK120ub) and histone H3 dimethylated at lysine 79 (H3K79me2) (modifications involved in elongation) and similar levels of histone H2A monoubiquitinated at lysine 119 (H2AK119ub). Bortezomib treatment resulted in significant reductions in H2BK120ub and H2AK119ub levels, as well as in transcript levels of genes involved in MLL development.