Bcl-2, Bcl-xL, and Bcl-w are not equivalent targets of ABT-737 and navitoclax (ABT-263) in lymphoid and leukemic cells

Abstract

The BH3-mimetic ABT-737 and an orally bioavailable compound of the same class, navitoclax (ABT-263), have shown promising antitumor efficacy in preclinical and early clinical studies. Although both drugs avidly bind Bcl-2, Bcl-x L , and Bcl-w in vitro, we find that Bcl-2 is the critical target in vivo, suggesting that patients with tumors overexpressing Bcl-2 will probably benefit. In human non-Hodgkin lymphomas, high expression of Bcl-2 but not Bcl-x L predicted sensitivity to ABT-263. Moreover, we show that increasing Bcl-2 sensitized normal and transformed lymphoid cells to ABT-737 by elevating proapoptotic Bim. In striking contrast, increasing Bcl-x L or Bcl-w conferred robust resistance to ABT-737, despite also increasing Bim. Cell-based protein redistribution assays unexpectedly revealed that ABT-737 disrupts Bcl-2/Bim complexes more readily than Bcl-x L /Bim or Bcl-w/Bim complexes. These results have profound implications for how BH3-mimetics induce apoptosis and how the use of these compounds can be optimized for treating lymphoid malignancies. (Blood. 2012;119(24):5807-5816) IntroductionDefects in the mitochondrial apoptotic pathway regulated by the Bcl-2 family of proteins play a major role in cancer development and in conferring chemoresistance. 1 Within the family, Bax and Bak are essential for mitochondrial membrane permeabilization and cell death. 2 Prosurvival proteins (Bcl-2, Bcl-x L , Bcl-w, Mcl-1, A1) oppose Bax and Bak and ensure mitochondrial integrity and cell survival. 1 These prosurvival proteins also interact with distant relatives that share only 1 Bcl-2 Homology region, BH3, that is critical for their proapoptotic function. The BH3-only proteins such as Bim, Bad, Puma, and Noxa act as stress sensors and relieve the inhibition of Bax and Bak by the prosurvival proteins.The clinical efficacy of most anticancer therapeutics primarily reflects their ability to induce apoptosis. Resistance to conventional anticancer therapeutics (eg, etoposide) is often because of a failure to activate BH3-only proteins, for example because of mutation of the tumor suppressor p53, which is critical for transcriptional induction of Puma and Noxa after DNA damage. 3 Overexpression of prosurvival Bcl-2 proteins, or silencing of BH3-only protein expression, are also associated with inferior therapeutic outcomes. 4,5 BH3-mimetic drugs, such as ABT-737, were developed to directly counter such apoptotic blocks. ABT-737 binds avidly to Bcl-2, Bcl-x L , and Bcl-w, but not Mcl-1 or A1. 6,7 In preclinical studies, it demonstrated single agent efficacy against tumors with low Mcl-1 or A1 levels, such as follicular lymphoma (FL), chronic lymphocytic leukemia (CLL), and small cell lung carcinoma (SCLC). ABT-737 shows limited toxicity toward normal cells, although there is transient reduction of platelets and lymphocytes. 6,8 , an orally bioavailable compound in the same class with similar target specificity, also exhibited efficacy in various cancer-derived cell lines both in vitro and in vivo, 6,9,10 and is undergoing p...

Elevated Mcl-1 inhibits thymocyte apoptosis and alters thymic selection

“…24,25 Following exposure to cytotoxic agents, the level of BH3-only proteins increased in both WT and Mcl-1tg thymocytes, in a pattern consistent with previous findings. 26 Thus, for the DNA-damaging agent etoposide, there was a marked increase in Puma, Bim and Noxa (Figures 2b and f); for PMA, an increase in Bim and Puma; for ionomycin, an increase in Bim and Puma proteins and Noxa RNA; and for dexamethasone, an increase in Bim (Figure 2e).…”

The BCL-2 protein family, BH3-mimetics and cancer therapy

“…BH3-mimetic compounds bind directly to and block the BCL-2 pro-survival proteins and thereby elicit apoptosis even in cells lacking upstream activators of BH3-only proteins, such as the tumour-suppressor p53, which is critical for transcriptional induction of Puma and Noxa However, as ABT-737 and ABT-263 bind only a subset of the BCL-2-like pro-survival proteins, overexpression of MCL1 or A1/BFL-1, both of which are not inhibited by these compounds, has the potential to confer resistance to therapy. 164 Some cancers (such as CLL) with very high expression of BCL-2 (and possibly also cancers expressing high levels of BCL-XL and/or BCL-W) respond robustly to the existing BH3-mimetics when used as single agents. However, in order to maximise treatment efficacy, in many cancers these BH3-mimetics are probably best employed in combination with drugs known to activate BH3-only proteins, such as BIM or PUMA, that can potently neutralise MCL-1 and/or A1.…”

Decreased mitochondrial priming determines chemoresistance of colon cancer stem cells

“…ABT-737 and the highly related ABT-263 both inhibit BCL2, BCLXL and BCLW [19][20][21] and were shown to be effective in killing cancer cells in vitro and in vivo 21 with a preference for BCL2. 19,22 As BCL2 protein expression is often upregulated in hematopoietic cancers, it represents a promising target, which was supported by high efficacy of these BH3 mimetics in animal experiments. 21 However, in vivo efficacy is limited due to thrombocytopenia, which relates to a dependence of platelets on BCLXL for survival.…”