Defining the target specificity of ABT-737 and synergistic antitumor activities in combination with histone deacetylase inhibitors

Abstract

The apoptotic and therapeutic activities of the histone deacetylase inhibitor (HDACi) vorinostat are blocked by overexpresssion of Bcl-2 or Bcl-X L . Herein, we used the small molecule inhibitor ABT-737 to restore sensitivity of E-myc lymphomas overexpressing Bcl-2 or Bcl-X L to vorinostat and valproic acid (VPA). Combining low-dose ABT-737 with vorinostat or VPA resulted in synergistic apoptosis of these cells. ABT-737 was ineffective against E-myc/Mcl-1 and E-myc/A1 cells either as a single agent or in combination with HDACi. However, in contrast to the reported binding specificity data, E-myc/Bcl-w lymphomas were insensitive to ABT-737 used alone or in combination with HDACi, indicating that the regulatory activity of ABT-737 is restricted to Bcl-2 and Bcl-X L . E-myc lymphomas that expressed Bcl-2 throughout the tumorigenesis process were especially sensitive to ABT-737, while those forced to overexpress Mcl-1 were not. This supports the notion that tumor cells "addicted" to ABT-737 target proteins (ie, Bcl-2 or Bcl-X L ) are likely to be the most sensitive target cell population. Our studies provide important preclinical data on the binding specificity of ABT-737 and its usefulness against primary hematologic malignancies when used as a single agent and in combination with HDACi. (Blood.

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

“…However, we do not exclude that A1 may play a similar role as Mcl-1 in certain subpopulations, because it was shown to be up-regulated, together with Bcl-x L , in response to cell-cell interactions within the lymph node microenvironment and be the reason of acquired resistance to Perhaps the most important and unexpected outcome from our study, confirmed in different lymphoid cell types, is that increasing Bcl-2, but not Bcl-x L or Bcl-w, sensitizes cells to ABT-737 even though it binds tightly to all 3 in cell-free assays. 6,7,44 Of note, protection of lymphoid cells from ABT-737 by Bcl-w was previously reported, 45 although this study also reported that both Bcl-2 and Bcl-x L -overexpressing E-Myc lymphoma cells were sensitive to ABT-737. Interestingly, CD4 ϩ 8 ϩ thymocytes, which are highly sensitive to many apoptotic stimuli (eg, DNA damage, glucocorticoids) 46 were the cell type most resistant to ABT-737 treatment in WT mice.…”

A Bim-targeting strategy overcomes adaptive bortezomib resistance in myeloma through a novel link between autophagy and apoptosis

“…59 Thus, the MOAs of HDACIs (eg, Bim upregulation) and BH3 mimetics (eg, unleashing Bim from Bcl-2/Bcl-xL) provide a specific rationale for combining these two classes of agents. Although we and other groups have described synergistic interactions between HDACIs and BH3 mimetics in various hematopoietic malignant cells, 29,[60][61][62] the ability of such a Bim-targeting strategy to re-prime bortezomibresistant MM cells toward death has not previously been examined. Significantly, HDACIs, particularly when combined with BH3 mimetics, increased Bim expression and sharply increased BH3-mimetic lethality in bortezomib-resistant cells.…”

A dual role for Hdac1: oncosuppressor in tumorigenesis, oncogene in tumor maintenance

“…16 For the generation of Em-myc lymphomas knocked down for Hdac1 and Hdac2, fetal livers were harvested from E13.5-E15.5 Em-myc embryos (in the C57BL/6/ CD45.2 strain) and were transduced with pLMSshRNAmiR30.SCR, pLMSshRNAmiR30.Hdac1 or pLMSshRNAmiR30Hdac2 constructs as detailed previously. 17,18 Irradiated recipient PTPRCA/CD45.1 mice 19 were injected with unsorted fetal liver cells (1 3 10 6 GFP 1 /mouse) and were euthanized once lymphoma developed, at which point the lymph nodes, spleen, and thymus were harvested and were stored in liquid nitrogen for further analysis. Animals were checked periodically for clinical signs of disease (by inspection, and periodic blood analysis using the AcT™ 5diff Beckman Coulter, Brea, CA).…”