S1, a Novel Pan-BH3 Mimetic, Induces Apoptosis in Mcl-1-Overexpressing Cells Through Bak

Abstract

Abstract. Mcl-1, an anti-apoptotic Bcl-2 homolog that has a structurally divergent BH3-binding pocket, non-redundant action model, and unique characteristic of short life confers complete resistance to the BH3 mimetic ABT-737. Herein, we used S1, previously identified as a Mcl-1/Bcl-2 dual inhibitor and a pure BH3 mimetic, to explore the mechanism of Mcl-1's action and supply a strategy to challenge Mcl-1's protection. Apoptosis assay in SMMC-7721, HCT116, and K562 cells demonstrated that S1 can effectively challenge Mcl-1's anti-apoptotic effect. Notably, we discovered an unexpected dynamic change of Mcl-1 that directly correlates with resistance or commitment to apoptosis induced by both ABT-737 and S1. Co-immunoprecipitation assays demonstrated that Mcl-1 increase results from Bim trafficking from Bcl-2 to Mcl-1, while subsequent Bak released by S1 determines Mcl-1 decrease and full-blown apoptosis. Further experiments using Bak shRNA testified that Bak accounts for S1-induced apoptosis and Mcl-1 decrease. Consistently, Bax-deficient DU145 cells are sensitive to S1, whereas Bak-mutant MKN-28 cells are significantly more resistant. The in vitro model could be extended to an in vivo mouse xenograft model in which Mcl-1 confers resistance by increased protein level, and the release of Bak could serve as a biomarker of apoptosis.[Supplementary materials: available only at http://dx

The putative BH3 mimetic S1 sensitizes leukemia to ABT-737 by increasing reactive oxygen species, inducing endoplasmic reticulum stress, and upregulating the BH3-only protein NOXA

“…Taken together, the rapid increase in ROS and ER stress observed following S1 treatment call into question prior claims that S1 acts as a true BH3 mimetic. However, experiments with S1 in mice found it to be well tolerated and demonstrated anti-tumor activity at 0.3 mg/kg every other day [11]. This data, along with our results showing no apoptosis in normal lymphocytes, suggests that S1 may be well tolerated in vivo .…”

“…S1 was selected for further characterization, due to its robust NOXA induction, minimal toxicity as a single agent, and unique signaling properties (as will be described here). This compound was originally described as an inhibitor of BCL2 and MCL1 [10], which induces apoptosis in multiple cancer cell lines and a mouse xenograft model [11]. However, we report here that it fails to induce apoptosis in CLL cells, which are well known to rely on BCL2 for survival.…”

“…To the best of our knowledge, only one other group is studying S1 as an anti-cancer agent [10, 11, 32]. S1 was originally identified in a cytotoxicity screen, and was found to induce caspase-dependent apoptosis, in BCL2-overexpressing cells.…”

BH3 Mimetics: Status of the Field and New Developments

“…Further characterization of S1 has been provided by recent data. One study described a dynamic change in the interactions of BIM with BCL-2 and MCL-1 leading to BAK release, suggesting that this mechanism might be involved in the effects of S1 in cells overexpressing MCL-1 (40). A second study indicated that autophagy has an important role in S1-triggered cell death through endoplasmic reticulum stress and disruption of the BCL-2/BECLIN 1 interaction (41).…”

BH3 mimetics: Their action and efficacy in cancer chemotherapy

“…S1 [53] BCL-2, MCL-1 Anti-tumor activity of S1 was shown in a mouse liver carcinoma xenograft model [54].…”