Suppression of BCL6 function by HDAC inhibitor mediated acetylation and chromatin modification enhances BET inhibitor effects in B-cell lymphoma cells

Abstract

Multiple genetic aberrations in the regulation of BCL6, including in acetyltransferase genes, occur in clinically aggressive B-cell lymphomas and lead to higher expression levels and activity of this transcriptional repressor. BCL6 is, therefore, an attractive target for therapy in aggressive lymphomas. In this study romidepsin, a potent histone deacetylase inhibitor (HDACi), induced apoptosis and cell cycle arrest in Burkitt and diffuse large B-cell lymphoma cell lines, which are model cells for studying the mechanism of action of BCL6. Romidepsin caused BCL6 acetylation at early timepoints inhibiting its function, while at later timepoints BCL6 expression was reduced and target gene expression increased due to chromatin modification. MYC contributes to poor prognosis in aggressive lymphoma. MYC function is reduced by inhibition of chromatin readers of the bromodomain and extra-terminal repeat (BET) family, which includes BRD4. The novel combination of romidepsin and JQ1, a BRD4 inhibitor was investigated and showed synergy. Collectively we suggest that the combination of HDACi and BRD4i should be pursued in further pre-clinical testing. Aggressive lymphomas such as diffuse large B-cell lymphoma (DLBCL) and Burkitt lymphoma (BL) are a heterogeneous group of disorders in terms of clinical behavior, biological characteristics and response to treatments 1. Despite improvements in diagnosis and treatment, non-Hodgkins-lymphomas are still an important cause of morbidity and mortality worldwide. Combination of rituximab with anthracycline-based chemotherapy regimen, such as R-CHOP (Rituximab-cyclophosphamide, doxorubicin, vincristine and prednisone) is effective 2 but overall >40% of patients are not cured of their disease. Thus, it is important to identify new approaches to therapy. Aggressive lymphomas are often derived from germinal center B-cells. Germinal centers are dynamic structures within normal lymph nodes, where B-cells proliferate intensely and undergo somatic hypermutation 3 , a process involving the production of DNA breaks that is essential for the formation of high affinity antibodies. Conditions within the germinal center are therefore believed to predispose to the formation of lymphomas 3. There is a wealth of genetic evidence that BCL6 contributes to the survival of DLBCL and clinical evidence suggests that a proportion of BCL6 expressing DLBCL patients have poor clinical outcomes. The role of BCL6 in Burkitt lymphoma has not been investigated but it is expressed in all cases and is likely to contribute to proliferation and survival. BCL6 is a master transcription factor that is essential for normal germinal center formation 4,5. Enforced expression of BCL6 in mice is sufficient for the development of lymphomas 6. Multiple genetic abnormalities

HDACi mediate UNG2 depletion, dysregulated genomic uracil and altered expression of oncoproteins and tumor suppressors in B- and T-cell lines

“…It should be noted that we also observed downregulation of PCLAF in the non-lymphoma human cell lines, HEK293 (embryonal kidney), HeLa (cervical carcinoma), DLD1 (colon carcinoma) and HaCaT (skin keratinocytes) (Additional file 2: Figure S1), indicating that this might a common effect of HDACi-treatment in several cell types, including cancer cells. Finally, we found a significant downregulation BCL6 (1.5-fold, p = 0.001) in SUDHL5 (Additional file 1: Table S1), in agreement with a very recent study demonstrating romidepsin-mediated depletion of BCL6 in B-cell lymphoma cells [39]. BCL6 is a central mediator of germinal center formation [40] and has been suggested as a therapeutic target in lymphoma [41].…”

Single-Cell Atlas Reveals Fatty Acid Metabolites Regulate the Functional Heterogeneity of Mesenchymal Stem Cells

“…HDACs, a series of critical transcriptional cofactors modulating gene expression by deacetylating histones and transcription factors, participate in stemness maintenance, lineage commitment determination, cell differentiation, and proliferation as well as other activities in normal hematopoiesis (Akimova et al, 2012;Cortiguera et al, 2019). Homozygous deletion of HDAC3 in Prrx1-expressing cells reduced chondrocyte and osteoblast differentiation in vitro (Feigenson et al, 2017).…”

Advances in Epigenetic Cancer Therapeutics

“…The benzamide HDAC inhibitor chidamide induces mitochondrial dysfunction, necroptosis and apoptosis [ 39 ]. The cyclic peptide HDAC inhibitor romidepsin induces cell cycle arrest and apoptosis by increasing the acetylation of BCL6 [ 40 ]. Finally, the fatty acid HDAC inhibitor AR-42 induces cell cycle arrest and apoptosis by inhibiting the AKT/NFκB pathway [ 41 ].…”