Reciprocal sensitivity of diffuse large B-cell lymphoma cells to Bcl-2 inhibitors BIRD-2 versus venetoclax
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Tamara Vervloessem1, Haidar Akl1,2, Thomas Tousseyn3, Humbert De Smedt1, Jan B. Parys1 and Geert Bultynck1
1KU Leuven, Laboratory of Molecular and Cellular Signaling, Department of Cellular and Molecular Medicine & Leuven Kanker Instituut (LKI), Leuven, Belgium
2Current/Present address: Lebanese University, Department of Biology, Hadath, Lebanon
3KU Leuven, Translational Cell & Tissue Research, Department of Imaging & Pathology, Leuven, Belgium
Geert Bultynck, email: firstname.lastname@example.org
Keywords: apoptosis; anti-apoptotic Bcl-2; B-cell lymphoma; venetoclax; BIRD-2
Received: April 26, 2017 Accepted: November 16, 2017 Published: December 04, 2017
Bcl-2 is often upregulated in cancers to neutralize the BH3-only protein Bim at the mitochondria. BH3 mimetics (e.g. ABT-199 (venetoclax)) kill cancer cells by targeting Bcl-2’s hydrophobic cleft and disrupting Bcl-2/Bim complexes. Some cancers with elevated Bcl-2 display poor responses towards BH3 mimetics, suggesting an additional function for anti-apoptotic Bcl-2 in these cancers. Indeed, Bcl-2 via its BH4 domain prevents cytotoxic Ca2+ release from the endoplasmic reticulum (ER) by directly inhibiting the inositol 1,4,5-trisphosphate receptor (IP3R). The cell-permeable Bcl-2/IP3R disruptor-2 (BIRD-2) peptide can kill these Bcl-2-dependent cancers by targeting Bcl-2’s BH4 domain, unleashing pro-apoptotic Ca2+-release events. We compared eight “primed to death” diffuse large B-cell lymphoma cell lines (DLBCL) for their apoptotic sensitivity towards BIRD-2 and venetoclax. By determining their IC50 using cytometric cell-death analysis, we discovered a reciprocal sensitivity towards venetoclax versus BIRD-2. Using immunoblotting, we quantified the expression levels of IP3R2 and Bim in DLBCL cell lysates, revealing that BIRD-2 sensitivity correlated with IP3R2 levels but not with Bim levels. Moreover, the requirement of intracellular Ca2+ for BIRD-2- versus venetoclax-induced cell death was different. Indeed, BAPTA-AM suppressed BIRD-2-induced cell death, but promoted venetoclax-induced cell death in DLBCL cells. Finally, compared to single-agent treatments, combining BIRD-2 with venetoclax synergistically enhanced cell-death induction, correlating with a Ca2+-dependent upregulation of Bim after BIRD-2 treatment. Our findings suggest that some cancer cells require Bcl-2 proteins at the mitochondria, preventing Bax activation via its hydrophobic cleft, while others require Bcl-2 proteins at the ER, preventing cytotoxic Ca2+-signaling events via its BH4 domain.
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