| Mitochondrial apoptosis is tightly regulated by the BCL2 family, and its dysregulation has been implicated in cancer development and tumor resistance to chemotherapies. BH3 mimetics are small molecules discovered or designed to mimic---and thereby disrupt---the binding of pro-apoptotic BH3-only proteins (e.g., NOXA and BAD) to the hydrophobic groove of anti-apoptotic proteins (e.g., BCL2, BCL-XL, and MCL1), thereby promoting apoptosis. The BH3 mimetic ABT-737 inhibits BCL2 and BCLXL but not MCL1 and kills cells in a BAX/BAK-dependent manner. The ability of other BH3 mimetics to inhibit anti-apoptotic proteins has been questioned because they kill cells independent of BAX/BAK. To first address this concern, we developed a cell-based assay to assess the on-target efficacy of BH3 mimetics. We show that ABT-737 is the only BH3 mimetic that inhibits BCL2 in cells, as visualized by the displacement of BAD. Other BH3 mimetics do not inhibit BCL2 but inhibit MCL1 through the induction of NOXA due to activation of the endoplasmic reticulum stress response. Furthermore, when ABT-737 is combined with a BH3 mimetic that induces NOXA, cells undergo rapid BAX/BAK-dependent apoptosis, suggesting that BH3 mimetics target BCL2 family members to induce cell death.;BH3 mimetics also combine with chemotherapeutic drugs to induce apoptosis in cells. For example, vinblastine is a microtubule-interfering agent that causes accumulation of cells in mitosis prior to apoptosis. When combined with ABT-737 however, vinblastine kills 100% of NB4 human leukemia cells within 6 h from all phases of the cell cycle. Vinblastine also induces the up-regulation of NOXA and the displacement of BAD from BCL2. These events may work synergistically with BH3 mimetics to induce acute apoptosis, highlighting the therapeutic potential of BH3 mimetics as an adjuvant to conventional chemotherapy.;This thesis provides the molecular basis for determining functional inhibition of anti-apoptotic proteins in cell-based systems and implicates novel regulatory pathways involving drug-induced NOXA. With insights gained from understanding how BH3-only proteins relay apoptotic signals to the mitochondria, we further propose how BCL2 family members can be pharmacologically manipulated to target cancer cells for death and how patients can be stratified for combination therapy based on their BCL2 profiles. |
