Acetylation of C/EBP alpha inhibits its granulopoietic function

CCAAT/enhancer-binding protein alpha (C/EBP alpha) is an essential transcription factor for myeloid lineage commitment. Here we demonstrate that acetylation of C/EBP alpha at lysine residues K298 and K302, mediated at least in part by general control non-derepressible 5 (GCN5), impairs C/EBP alpha DNA-binding ability and modulates C/EBP alpha transcriptional activity. Acetylated C/EBP alpha is enriched in human myeloid leukaemia cell lines and acute myeloid leukaemia (AML) samples, and downregulated upon granulocyte-colony stimulating factor (G-CSF)-mediated granulocytic differentiation of 32Dcl3 cells. C/EBP alpha mutants that mimic acetylation failed to induce granulocytic differentiation in C/EBP alpha-dependent assays, in both cell lines and in primary hematopoietic cells. Our data uncover GCN5 as a negative regulator of C/EBP alpha and demonstrate the importance of C/EBP alpha acetylation in myeloid differentiation

HSP90 inhibition overcomes ibrutinib resistance in mantle cell lymphoma.

The Bruton tyrosine kinase (BTK) inhibitor ibrutinib induces responses in 70% of patients with relapsed and refractory mantle cell lymphoma (MCL). Intrinsic resistance can occur through activation of the nonclassical NF-κB pathway and acquired resistance may involve the BTK C481S mutation. Outcomes after ibrutinib failure are dismal, indicating an unmet medical need. We reasoned that newer heat shock protein 90 (HSP90) inhibitors could overcome ibrutinib resistance by targeting multiple oncogenic pathways in MCL. HSP90 inhibition induced the complete degradation of both BTK and IκB kinase α in MCL lines and CD40-dependent B cells, with downstream loss of MAPK and nonclassical NF-κB signaling. A proteome-wide analysis in MCL lines and an MCL patient-derived xenograft identified a restricted set of targets from HSP90 inhibition that were enriched for factors involved in B-cell receptor and JAK/STAT signaling, the nonclassical NF-κB pathway, cell-cycle regulation, and DNA repair. Finally, multiple HSP90 inhibitors potently killed MCL lines in vitro, and the clinical agent AUY922 was active in vivo against both patient-derived and cell-line xenografts. Together, these findings define the HSP90-dependent proteome in MCL. Considering the disappointing clinical activity of HSP90 inhibitors in other contexts, trials in patients with MCL will be essential for defining the efficacy of and mechanisms of resistance after ibrutinib failure

Acetylation of C/EBPα inhibits its granulopoietic function

CCAAT/enhancer-binding protein alpha (C/EBPα) is an essential transcription factor for myeloid lineage commitment. Here we demonstrate that acetylation of C/EBPα at lysine residues K298 and K302, mediated at least in part by general control non-derepressible 5 (GCN5), impairs C/EBPα DNA-binding ability and modulates C/EBPα transcriptional activity. Acetylated C/EBPα is enriched in human myeloid leukaemia cell lines and acute myeloid leukaemia (AML) samples, and downregulated upon granulocyte-colony stimulating factor (G-CSF)- mediated granulocytic differentiation of 32Dcl3 cells. C/EBPα mutants that mimic acetylation failed to induce granulocytic differentiation in C/EBPα-dependent assays, in both cell lines and in primary hematopoietic cells. Our data uncover GCN5 as a negative regulator of C/EBPα and demonstrate the importance of C/EBPα acetylation in myeloid differentiation