Pharmacologic Replacement of BIM BH3 Reactivates Apoptosis in Hematologic Cancer and Lymphoproliferative Disease.

Selective targeting of deregulated apoptotic protein networks is a promising pharmacologic strategy for subverting diseases of unrestrained cellular survival, such as cancer. BCL-2 family protein interactions constitute a critical control point for the regulation of apoptosis. Whereas multidomain anti-apoptotic proteins such as BCL-2 guard against cell death, multidomain pro-apoptotic proteins such as BAX constitute a gateway to cell death through mitochondrial damage. The BH3-only proteins function as death sentinels situated throughout the cell, poised to transmit signals of cellular injury to multidomain members. BH3-only proteins deliver their death messages via their conserved alpha-helical BH3 domains. Whereas the indirect activator class of BH3-only proteins (e.g. BAD) counteract anti-apoptotic proteins, the direct activator subgroup (e.g. BIM) is believed to trigger apoptosis both by neutralizing anti-apoptotics and by directly activating the mitochondrial executioners BAX and BAK. The essential roles of BH3-only proteins in maintaining cellular homeostasis is highlighted by the development of autoimmune disease and cancer in mouse models of BH3-only protein deficiency. By inserting hydrocarbon “staples” into native BH3 peptide sequences, we have produced a chemical toolbox of stabilized alpha-helices of BCL-2 domains (SAHBs) to dissect apoptotic signaling pathways in vivo and explore the pharmacodynamic effects of “BH3 replacement” in cancer cells and mouse models of deregulated apoptosis. Whereas SAHBs display high affinity binding to anti-apoptotic targets, BID and BIM SAHBs also directly engage BAX and are thus especially potent in inducing apoptosis of a panel of leukemia and lymphoma cell lines. To evaluate the impact of selective BH3 replacement in vivo, we tested the capacity of BIM SAHB to reactivate apoptosis in the lymphoproliferative disease of Bim-/- mice. Strikingly, Bim-/- mice treated with BIM SAHB displayed marked influx of tingible-body macrophages into the lymphoid infiltrates of affected organs, with scattered cells throughout the infiltrate robustly positive for activated caspase-3, suggestive of SAHB induced apoptosis induction. Our studies highlight the therapeutic potential of BH3 replacement to circumvent apoptotic blockade and restore the death pathway in hematologic cancer and lymphoproliferative disease.