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Ferrata Storti Foundation
Haematologica 2021 Volume 106(12):3136-3148
Inhibition of the anti-apoptotic protein MCL-1 severely suppresses human hematopoiesis
Sheila Bohler,1,2 Sehar Afreen,1 Juncal Fernandez-Orth,1 Eva-Maria Demmerath,1 Christian Molnar,1,2,3 Ying Wu,1,2 Julia Miriam Weiss,1 Venugopal Rao Mittapalli,1 Lukas Konstantinidis,4 Hagen Schmal,4 Mirjam Kunze5 and Miriam Erlacher1,6,7
1Department of Pediatrics and Adolescent Medicine, Division of Pediatric Hematology and Oncology, University Medical Center Freiburg, Faculty of Medicine, University of Freiburg, Freiburg; 2Faculty of Biology, University of Freiburg, Freiburg; 3Spemann Graduate School of Biology and Medicine (SGBM), University of Freiburg, Freiburg; 4Department of Orthopedics and Trauma Surgery, Medical Center, Faculty of Medicine, Albert-Ludwigs-University of Freiburg, Freiburg; 5Department of Obstetrics and Gynecology, University Medical Center Freiburg, Faculty of Medicine, University of Freiburg, Freiburg; 6German Cancer Consortium (DKTK), Freiburg, and 7German Cancer Research Center (DKFZ), Heidelberg, Germany
ABSTRACT
BH3-mimetics inhibiting anti-apoptotic BCL-2 proteins represent a novel and promising class of antitumor drugs. While the BCL-2 inhibitor venetoclax is already approved by the Food and Drug Administration, BCL-XL and MCL-1 inhibitors are currently in early clin- ical trials. To predict side effects of therapeutic MCL-1 inhibition on the human hematopoietic system, we used RNA interference and the small molecule inhibitor S63845 on cord blood-derived CD34+ cells. Both approaches resulted in almost complete depletion of human hematopoi- etic stem and progenitor cells. As a consequence, maturation into the dif- ferent hematopoietic lineages was severely restricted and CD34+ cells expressing MCL-1 shRNA showed a very limited engraftment potential upon xenotransplantation. In contrast, mature blood cells survived nor- mally in the absence of MCL-1. Combined inhibition of MCL-1 and BCL-XL resulted in synergistic effects with relevant loss of colony-form- ing hematopoietic stem and progenitor cells already at inhibitor concen- trations of 0.1 mM each, indicating “synthetic lethality” of the two BH3- mimetics in the hematopoietic system.
Introduction
BH3-mimetics represent a novel and very promising group of anticancer drugs, with venetoclax being the first compound approved by the Food and Drug Administration (FDA).1 They act by directly inhibiting anti-apoptotic BCL-2 pro- teins that prevent the intrinsic apoptosis pathway and thereby ensure survival of every human cell. BCL-2 and its homologs BCL-XL, MCL-1, BFL1/A1 and BCL-W bind to and inhibit BAX and BAK, two downstream pro-apoptotic effector BCL-2 proteins which, upon activation, lead to permeabilization of the outer mitochon- drial membrane.2 As a consequence, cytochrome c is released into the cytosol, a process regarded as a “point of no return” for the initiation of apoptosis. In the cytosol, cytochrome c together with APAF1 and procaspase 9 molecules form a large complex termed an apoptosome, in which caspase 9 is activated. Caspase 9 then activates the effector caspases 3, 6 and 7 which eventually degrade vital cel- lular structures and execute cell death. This whole process is regulated by upstream pro-apoptotic proteins that also belong to the large BCL-2 family but share only the BH3 (BCL2 homology 3) domain with the other members of the anti-apoptotic BCL-2 family. BH3-only proteins are upregulated or activated upon given stress signals and then bind and inhibit the anti-apoptotic BCL-2 proteins. As a consequence, BAX and BAK are released, leading to apoptosis.3 As their name indicates, BH3-mimetics imitate the mode of action of the BH3-only proteins.4 While BH3-only proteins are tightly regulated and only activated upon lethal stress
Hematopoiesis
Correspondence:
MIRIAM ERLACHER
miriam.erlacher@uniklinik-freiburg.de
Received: March 17, 2020. Accepted: November 6, 2020. Pre-published: November 26, 2020.
https://doi.org/10.3324/haematol.2020.252130 ©2021 Ferrata Storti Foundation
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