BH3-only proteins are dispensable for apoptosis induced by pharmacological inhibition of both MCL-1 and BCL-XL

Abstract

The impressive selectivity and efficacy of BH3 mimetics for treating cancer has largely been limited to BCL-2 dependent hematological malignancies. Most solid tumors depend on other anti-apoptotic proteins, including MCL-1, for survival. The recent description of S63845 as the first specific and potent MCL-1 inhibitor represents an important therapeutic advance, since MCL-1 is not targeted by the currently available BH3 mimetics, Navitoclax or Venetoclax, and is commonly associated with chemoresistance. In this study, we confirm a high binding affinity and selectivity of S63845 to induce apoptosis in MCL-1-dependent cancer cell lines. Furthermore, S63845 synergizes with other BH3 mimetics to induce apoptosis in cell lines derived from both hematological and solid tumors. Although the anti-apoptotic BCL-2 family members in these cell lines interact with a spectrum of pro-apoptotic BH3-only proteins to regulate apoptosis, these interactions alone do not explain the relative sensitivities of these cell lines to BH3 mimetic-induced apoptosis. These findings necessitated further investigation into the requirement of BH3-only proteins in BH3 mimetic-mediated apoptosis. Concurrent inhibition of BCL-X and MCL-1 by BH3 mimetics in colorectal HCT116 cells induced apoptosis in a BAX- but not BAK-dependent manner. Remarkably this apoptosis was independent of all known BH3-only proteins. Although BH3-only proteins were required for apoptosis induced as a result of BCL-X inhibition, this requirement was overcome when both BCL-X and MCL-1 were inhibited, implicating distinct mechanisms by which different anti-apoptotic BCL-2 family members may regulate apoptosis in cancer.

NOXA-dependent contextual synthetic lethality of BCL-XL inhibition and “osmotic reprogramming” in colorectal cancer

“…A previous study, however, convincingly demonstrated that NOXA-triggered release of BIM from MCL-1 was essential for BAX/BAK activation 44 . Our results resemble findings from another study that showed dispensability of BIM (and other BH3-only proteins) for apoptosis induction when MCL-1 and BCL-XL are inhibited 5 . Whether attenuated cytotoxicity of NaCl/WEHI-539 in PUMA-deficient cells indicates involvement of PUMA in hypertonicity-enforced BCL-XL addiction requires further studies (Fig.…”

“…3a). BAX is essential for MOMP in HCT116 cells and its activation is controlled by interaction with MCL-1 and BCL-XL 5,22,23 . Coimmunoprecipitation experiments, however, failed to demonstrate hypertonicity-induced changes of BCL-XL/MCL-1 interaction with BAX ( Fig.…”

“…BCL-XL/MCL-1) that hamper efficient cancer cell elimination when a single BCL-2-like protein is targeted. Preclinical studies demonstrated that inhibition of two or more BCL-2-like proteins was required to overcome this limitation [4][5][6][7] . Our previous work showed that cancer cells facing a hypertonic environment exhibited a lower threshold for MOMP induction [8][9][10] .…”

BCL-2 family deregulation in colorectal cancer: potential for BH3 mimetics in therapy

“…In a CRC cell line HCT116, treatment with A-1155463 alone was sufficient to induce apoptosis, while MCL-1 inhibitor S63845 alone did not induce any apoptosis. However, combining the two compounds resulted in more pronounced apoptosis even in the absence of all BH3-only proteins, in a BAX dependent manner [171].…”

“…Cells that express high level of NOXA either basally or induced by other therapies show increased sensitivity to BCL-XL inhibition by A-1155463 and ABT-737 [119,169]. This suggests that concurrent treatment with MCL-1 inhibitors might potentiate the effect of BCL-XL inhibition in tumors that either present with high MCL-1 or low NOXA levels [170,171]. In a CRC cell line HCT116, treatment with A-1155463 alone was sufficient to induce apoptosis, while MCL-1 inhibitor S63845 alone did not induce any apoptosis.…”