A model of sensitivity and resistance to histone deacetylase inhibitors in diffuse large B cell lymphoma

Abstract

Diffuse large B cell lymphoma (DLBCL) is an aggressive form of non-Hodgkin lymphoma. While the initial treatment strategy is highly effective, relapse occurs in 40% of cases. Histone deacetylase inhibitors (HDACi) are a promising class of anti-cancer drugs but their single agent efficacy against relapsed DLBCL has been variable, ranging from few complete/partial responses to some stable disease. However, most patients showed no response to HDACi monotherapy for unknown reasons. Here we show that sensitivity and resistance to the hydroxamate HDACi, PXD101, can be modeled in DLBCL cell lines. Sensitivity is characterized by G₂/M arrest and apoptosis and resistance by reversible G₁ growth arrest. These responses to PXD101 are independent of several negative prognostic indicators such as DLBCL subtype, BCL2 and MYC co-expression, and p53 mutation, suggesting that HDACi might be used effectively against highly aggressive DLBCL tumors if they are combined with other therapeutics that overcome HDACi resistance. Our investigation of mechanisms underlying HDACi resistance showed that cyclin-dependent kinase inhibitors (CKIs), p21 and p27, are upregulated by PXD101 in a sustained fashion in resistant cell lines concomitant with decreased activity of the cyclin E/cdk2 complex and decreased Rb phosphorylation. PXD101 treatment results in increased association of CKI with the cyclin E/cdk2 complex in resistant cell lines but not in a sensitive line, indicating that the CKIs play a key role in G₁ arrest. The results suggest several treatment strategies that might increase the efficacy of HDACi against aggressive DLBCL.

Keywords: :

• histone deacetylase inhibitor
• lymphoma
• drug resistance
• cell cycle
• cell cycle inhibitors
• MYC
• BCL2

Disclosure of Potential Conflicts of Interest

CLS and LMR receive financial support for basic research studies from Spectrum Pharmaceuticals which holds the commercial license for belinostat (PXD101). However, none of this money was used to support the work reported in the accompanying manuscript. Spectrum Pharmaceuticals had no influence on the study goals, execution, or preparation of the manuscript nor did they provide the drug used in the study.

Acknowledgments

The authors would like to acknowledge the staff of the Flow Cytometry Shared Service at the Arizona Cancer Center for services rendered. We also thank Dr Tracy Brooks (U Mississippi) for assistance with MTS assays and analysis of MYC expression. We are grateful to members of the Lymphoma Research Group at the UA for helpful suggestions and critique during the progress of the study. This work was supported by a pilot project grant from the Southwest Environmental Health Sciences Center (ES006694, PI S Lau) to CLS, an award from the Arizona Biomedical Research Commission to CLS, and a career development award and developmental project award to CLS (Lymphoma SPORE [1 P50 CA-130805-05, PI RI Fisher]).