Inhibition of the anti-apoptotic protein MCL-1 severely suppresses human hematopoiesis

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 FDA-approved, BCL-XL and MCL-1 inhibitors are currently in early clinical trials. To predict side effects of therapeutic MCL-1 inhibition on the human hematopoietic system, we used RNAi and the small molecule inhibitor S63845 on cord blood-derived CD34+ cells. Both approaches resulted in almost complete depletion of human hematopoietic stem and progenitor cells. As a consequence, maturation into the different 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 normally in the absence of MCL-1. Combined inhibition of MCL-1 and BCL-XL resulted in synergistic effects with relevant loss of colony-forming HSPCs already at inhibitor concentrations of 0.1 µM each, indicating "synthetic lethality" of the two BH3-mimetics in the hematopoietic system.

BCL-XL expression is essential for human erythropoiesis and engraftment of hematopoietic stem cells.

The anti-apoptotic BCL-2 proteins (BCL-2, BCL-XL, MCL-1, A1, BCL-W) counteract apoptotic signals emerging during development and under stress conditions, and are thus essential for the survival of every cell. While the "BCL-2 addiction" of different cell types is well described in mouse models, there is only limited information available on the role of different anti-apoptotic BCL-2 proteins in a given human cell type. Here we characterize the role of BCL-XL for survival and function of human hematopoietic cells, with the aim to predict hematological side effects of novel BCL-XL-inhibiting BH3-mimetics and to identify hematological malignancies potentially responsive to such inhibitors. Earlier clinical studies have shown that the combined BCL-2/BCL-XL/BCL-W inhibitor, Navitoclax (ABT-263) induces severe thrombocytopenia caused by direct platelet demise and counteracted by increased megakaryopoiesis. In contrast, murine studies have reported important contribution of BCL-XL to survival of late erythroid cells and megakaryocytes. Using lentiviral knockdown, we show that the roles of BCL-XL for human hematopoietic cells are much more pronounced than expected from murine data and clinical trials. Efficient genetic or chemical BCL-XL inhibition resulted in significant loss of human erythroid cells beginning from very early stages of erythropoiesis, and in a reduction of megakaryocytes. Most importantly, BCL-XL deficient human hematopoietic stem cells and multipotent progenitors were reduced in numbers, and they showed a severely impaired capacity to engraft in mice during xenotransplantation. BCL-XL deficiency was fully compensated by BCL-2 overexpression, however, loss of its antagonist BIM did not result in any rescue of human erythroid or stem and progenitor cells. We thus conclude that novel and specific BCL-XL inhibitors might be efficient to treat malignancies of erythroid or megakaryocytic origin, such as polycythemia vera, acute erythroid leukemia, essential thrombocytosis or acute megakaryocytic leukemia. At the same time, it can be expected that they will have more severe hematological side effects than Navitoclax.

Checkpoint kinase 1 is essential for fetal and adult hematopoiesis.

Checkpoint kinase 1 (CHK1) is critical for S-phase fidelity and preventing premature mitotic entry in the presence of DNA damage. Tumor cells have developed a strong dependence on CHK1 for survival, and hence, this kinase has developed into a promising drug target. Chk1 deficiency in mice results in blastocyst death due to G2/M checkpoint failure showing that it is an essential gene and may be difficult to target therapeutically. Here, we show that chemical inhibition of CHK1 kills murine and human hematopoietic stem and progenitor cells (HSPCs) by the induction of BCL2-regulated apoptosis. Cell death in HSPCs is independent of p53 but requires the BH3-only proteins BIM, PUMA, and NOXA. Moreover, Chk1 is essential for definitive hematopoiesis in the embryo. Noteworthy, cell death inhibition in HSPCs cannot restore blood cell formation as HSPCs lacking CHK1 accumulate DNA damage and stop dividing. Moreover, conditional deletion of Chk1 in hematopoietic cells of adult mice selects for blood cells retaining CHK1, suggesting an essential role in maintaining functional hematopoiesis. Our findings establish a previously unrecognized role for CHK1 in establishing and maintaining hematopoiesis.

Concise Review: Cheating Death for a Better Transplant.

Hematopoietic stem cell transplantation is routinely performed for the treatment of various malignant and non-malignant hematological diseases. Successful transplantation depends on the number and fitness of donor stem and progenitor cells, whose quality is substantially influenced by the sampling procedure, sample handling, and cryopreservation. BCL-2 proteins are central to the survival and maintenance of stem and progenitor cells under both, physiological and stress conditions. Transplantation-associated apoptosis of donor cells is mediated by the pro-apoptotic BCL-2 proteins BIM and BMF and prevented by overexpression of their antagonists, BCL-2 and BCL-XL. We have previously reported that deletion of BIM or BMF stabilizes donor stem cell numbers during transplantation and improves cellular fitness and transplantation outcomes, albeit posing a risk for lymphoma and autoimmunity in recipient mice. Short-term apoptosis inhibition in donor cells appears equally effective in improving the outcome of transplantation, but in contrast does not cause pathology. In this review, we discuss the role of BCL-2 proteins in the context of factors that negatively impact donor stem and progenitor cell fitness and viability. Finally, we suggest approaches to improve graft quality and transplantation procedures with an emphasis on the inhibition of BCL-2 regulated apoptosis. Stem Cells 2018;36:1646-1654.

The immunoinhibitory B7-H1 molecule as a potential target in cancer: killing many birds with one stone.

Over expression of B7-H1 (also named PDL-1 or CD 274) molecule in cancer has been linked to worse prognosis and resistance to anti-cancer therapies in several malignancies. In this review, we update on the expression of B7-H1 molecule in solid and hematological malignancies. We also describe the possible mechanisms by which this molecule inhibits/downregulates the immune response to cancer cells. Finally, we highlight current and future potential therapeutic strategies that can be further developed to target this molecule.