Bcl‐2 phosphorylation confers resistance on chronic lymphocytic leukaemia cells to the BH3 mimetics ABT‐737, ABT‐263 and ABT‐199 by impeding direct binding

Abstract

Background and Purpose

Although the ongoing clinical trials of ABT‐263 and ABT‐199 in chronic lymphocytic leukaemia (CLL) have indicated that BH3 mimetics hold considerable promise, understanding the mechanism of CLL resistance to BH3 mimetics remains a challenge.

Experimental Approach

The LD ₅₀ values of ABT‐737, ABT‐263 and ABT‐199 in a number of primary CLL cells from 40 patients, were determined. The levels of Bcl‐2 family proteins, including phosphorylated Bcl‐2 (pBcl‐2) and their interactions were measured by immunoblotting and co‐immunoprecipitation. In vitro binding assays were performed by isothermal titration calorimetry and ELISA. BH3 profiling in isolated mitochondria was analysed.

Key Results

The ratio of (Mcl‐1 + pBcl‐2) to Bcl‐2 expression provided the most significant predictive marker for the cytotoxic potential of ABT‐737, ABT‐263 and ABT‐199 in the panel of CLL samples. Mechanistically, pBcl‐2 inhibited the effects of the ABT compounds on the displacement of Bax and Bim from Bcl‐2, thereby suppressing mitochondrial apoptosis. The ABT compounds exhibited 100–300‐fold lower binding affinity to the glutamic acid, phosphomimetic, mutant of Bcl‐2 (T69E, S70E and S87E; EEE‐Bcl‐2). BH3 peptides exhibited different rank orders of binding affinities to full‐length WT‐Bcl‐2 and full‐length EEE‐Bcl‐2.

Conclusions and Implications

Our study suggested that a structural alteration in the BH3‐binding groove was induced by phosphorylation of Bcl‐2. Our data also provided a framework to overcome resistance of CLL cells to the ABT compounds by combining pBcl‐2 kinase inhibitors with the ABT compounds.