BCL6 repression of EP300 in human diffuse large B cell lymphoma cells provides a basis for rational combinatorial therapy
Leandro C. Cerchietti, … , Gabriela Chiosis, Ari Melnick
Leandro C. Cerchietti, … , Gabriela Chiosis, Ari Melnick
Published November 1, 2010
Citation Information: J Clin Invest. 2010;120(12):4569-4582. https://doi.org/10.1172/JCI42869.
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Research Article Oncology

BCL6 repression of EP300 in human diffuse large B cell lymphoma cells provides a basis for rational combinatorial therapy

Abstract

B cell lymphoma 6 (BCL6), which encodes a transcriptional repressor, is a critical oncogene in diffuse large B cell lymphomas (DLBCLs). Although a retro-inverted BCL6 peptide inhibitor (RI-BPI) was recently shown to potently kill DLBCL cells, the underlying mechanisms remain unclear. Here, we show that RI-BPI induces a particular gene expression signature in human DLBCL cell lines that included genes associated with the actions of histone deacetylase (HDAC) and Hsp90 inhibitors. BCL6 directly repressed the expression of p300 lysine acetyltransferase (EP300) and its cofactor HLA-B–associated transcript 3 (BAT3). RI-BPI induced expression of p300 and BAT3, resulting in acetylation of p300 targets including p53 and Hsp90. Induction of p300 and BAT3 was required for the antilymphoma effects of RI-BPI, since specific blockade of either protein rescued human DLBCL cell lines from the BCL6 inhibitor. Consistent with this, combination of RI-BPI with either an HDAC inhibitor (HDI) or an Hsp90 inhibitor potently suppressed or even eradicated established human DLBCL xenografts in mice. Furthermore, HDAC and Hsp90 inhibitors independently enhanced RI-BPI killing of primary human DLBCL cells in vitro. We also show that p300-inactivating mutations occur naturally in human DLBCL patients and may confer resistance to BCL6 inhibitors. Thus, BCL6 repression of EP300 provides a basis for rational targeted combinatorial therapy for patients with DLBCL.

Authors

Leandro C. Cerchietti, Katerina Hatzi, Eloisi Caldas-Lopes, Shao Ning Yang, Maria E. Figueroa, Ryan D. Morin, Martin Hirst, Lourdes Mendez, Rita Shaknovich, Philip A. Cole, Kapil Bhalla, Randy D. Gascoyne, Marco Marra, Gabriela Chiosis, Ari Melnick

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Figure 4

RI-BPI synergizes with HDAC inhibitors.

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RI-BPI synergizes with HDAC inhibitors. (A) 7 BCL6-dependent DLBCL cell ...
(A) 7 BCL6-dependent DLBCL cell lines were exposed to 6 concentrations of SAHA (from 0.5 to 10 μM), TSA (from 25 to 400 nM), or vehicle control (DMSO, 0.1% in water) for 48 hours and analyzed for viability. Dose-effect (percentage of dead cells) curves were plotted. Data points represent experimental data for a particular dose effect. The curves were derived using these data points and the Compusyn software. (B) The same cells were treated with 6 concentrations of SAHA or TSA and the combination of these drugs with 6 concentrations of RI-BPI at a constant ratio (concurrent administration). Conservative GI50 isobolograms for the combination of SAHA or TSA with BPI for each drug were plotted. Data points falling on the line indicate an additive effect, points below the line indicate synergy, and points above the line indicate infra-additive effect. The dose values for each GI50 for each cell line are shown in Supplemental Table 1. (C) For the cell lines that were resistant (i.e., the GI50 was higher than the upper dose limit) to 1 or more drugs, a potentiation effect with BPI was calculated. Cells were treated with BPI (10 μM) or SAHA (1 μM) or TSA (100 nM) or the combination of these for 48 hours (sequential schedule BPI→drug). Cell viability was determined and compared with control-treated cells. (D) The 7 DLBCL cell lines were exposed to RI-BPI (10 μM) (white bars), SAHA (1 μM) (light gray bars), RI-BPI plus SAHA (black bars), or vehicle control (dark gray bars, not visible) for 48 hours and analyzed for caspase 7/3 activity. y axis represents caspase 7/3 activity (fold). Data are presented as mean with 95% CI.