A research team at Inselspital, Bern University Hospital and the University of Bern has identified and tested the use of an agent that can effectively inhibit the proliferation of leukemia stem cells.
Metoclopramide (MPR), used as an anti-emetic medication, interrupts the unique CD93 signaling pathway that only leukemia stem cells use to proliferate. This opens up a therapeutic approach using MPR to selectively eliminate leukemia stem cells.
Chronic myeloid leukemia (CML) results from a degeneration of the hematopoietic stem cells (leukemia stem cells), thereby leading to the uncontrolled formation of specific white blood cells, the so-called granulocytes. Research work at the Department of Medical Oncology at the Inselspital, Bern University Hospital and the University of Bern focused therefore on identifying the signaling pathways and control mechanisms of the leukemia stem cell.
A promising approach is provided by working with MPR, an anti-emetic medication commonly used to treat nausea and vomiting.
The exact role of the surface molecule CD93 (cluster of differentiation 93) in controlling the proliferation of leukemia stem cells was analyzed and documented, initially in animal experiments and subsequently in experiments with leukemia stem cells from patients.
This revealed a distinct regulatory function of CD93 in leukemia stem cells. To begin with, the effect was demonstrated in vivo in animal experiments. It was further shown that the control function only applies to leukemia stem cells, not to normal hematopoietic stem cells.
Furthermore, it was demonstrated that the anti-emetic MPR interrupts the signaling pathway that stimulates cell proliferation of leukemia stem cells in vitro and also, in animal experiments, visibly improves survival with CML by blocking the proliferation of leukemia stem cells. This provides strong evidence that MPR may also show positive results in treating CML in humans.
The study presented in this publication involved exceptionally extensive research. This is also true with regard to the interdisciplinary teams participating from the Department of Medical Oncology, the Department for Biomedical Research, the Institute of Cell Biology and the Department of Orthopedic Surgery and Hematology at the Bern University Hospital and the University of Bern. Prof. Dr. sc. nat. Carsten Riether explains:
“In order to develop a new, promising approach to combat CML, contributions from numerous disciplines were necessary and different research approaches had to be pursued. In a screening procedure, we elicited the candidate Metoclopramide and were subsequently able to demonstrate its effect on the CD93 signaling pathway in both in-vitro and in-vivo experiments.”
The research infrastructure in Bern is optimally designed for such major projects. The expertise in fundamental research at the Department for Biomedical Research (DBMR) and in clinical research at the University Hospital are closely linked and can rapidly produce sound results.
What are the next research activities?
The results have pinpointed CD93 as a specific regulator responsible for leukemia stem cell proliferation. This identifies a promising pathway to targeting leukemia stem cells. Further studies must now prove the clinical effect and relevance. Prof. Adrian Ochsenbein says “Thanks to this pool of expertise, we were able to identify Metoclopramide as a promising candidate for CML therapy.
And with the broad-based research infrastructure and our excellent national and international network, we are hopefully in a position to present clinical results within a reasonable timescale.”
Leukemia stem cells (LSCs) have been characterized as the leukemia-initiating cells in acute myeloid leukemia (AML), chronic myeloid leukemia (CML), and other hematological neoplasms (Holyoake and Vetrie, 2017; Lapidot et al., 1994). In CML, LSCs are found in the majority of patients in the lineage-negative (Lin−) CD34+CD38− fraction of bone marrow (BM) cells, which is phenotypically similar to normal hematopoietic stem cells (HSCs) (Holyoake and Vetrie, 2017; Riether et al., 2015a).
The introduction of BCR-ABL1-targeting tyrosine kinase inhibitors (TKIs) has revolutionized the treatment of CML. In chronic phase, CML patients often reach deep molecular remissions by treatment with first- or second-generation TKIs (Bhatia et al., 2003; Chu et al., 2011).
A subgroup of these patients can successfully discontinue TKI therapy and maintain a treatment-free remission (Laneuville, 2017). However, TKI-insensitive LSCs persist in the majority of patients over a prolonged time period (Holyoake and Vetrie, 2017).
These quiescent, self-renewing LSCs in the BM are the major cause of relapse after drug discontinuation or by the acquisition of mutations leading to TKI resistance (Holyoake and Vetrie, 2017; Jabbour et al., 2013; Savona and Talpaz, 2008).
CD93 (C1qRp) is a C-type lectin-like type I transmembrane protein (Greenlee et al., 2008; Dean et al., 2000; Nepomuceno et al., 1997).
Cleavage products of the intracellular domain (ICD) of CD93 have been detected in the cytoplasm of human monocytes after activation of protein kinase C (PKC) signaling (Bohlson et al., 2005a; Greenlee et al., 2009). The ICD of CD93 contains a nuclear localization signal that is thought to regulate gene expression by acting as a transcription factor in complex with other transcription factors (Cokol et al., 2000).
CD93 is preferentially expressed on endothelial cells (ECs), platelets, myeloid cells, and early B cell precursors, but not lymphoid cells (Dean et al., 2000, 2001; Løvik et al., 2000; McKearn et al., 1985; Nepomuceno et al., 1997). In addition, CD93 is expressed on multipotent HSCs of the fetal liver and yolk sac (Huang and Auerbach, 1993) but is absent or very low on normal adult HSCs (Kinstrie et al., 2020).
CD93 signaling is involved in many biological processes such as angiogenesis (Khan et al., 2017; Lorenzon et al., 2012; Petrenko et al., 1999), antibody production, and maintenance of long-lived plasma cells (Chevrier et al., 2009), as well as the engulfment of apoptotic cells in vivo (Norsworthy et al., 2004). CD93 was identified as a marker for human CML LSCs that persist after TKI therapy (Kinstrie et al., 2020). In addition, CD93 signaling has been shown to induce proliferation and disease progression in AML LSCs carrying the MLL gene rearrangement (Iwasaki et al., 2015; Saito et al., 2010).
The ligands for CD93 are largely unknown. Initially, CD93 was thought to be a receptor for the complement factor C1q (Nepomuceno et al., 1997; Norsworthy et al., 2004). However, McGreal et al. reported that the CD93 receptor does not bind to C1q (McGreal and Gasque, 2002; McGreal et al., 2002). Instead, the EC-specific extracellular matrix protein multimerin 2 (MMRN2) has been recently identified as a potential ligand for CD93 and other receptors such as C-type lectin-domain-containing 14A (CLEC14A) and CD248 in HEK293 T cells (Khan et al., 2017).
Based on the documented expression of CD93 on LSCs, the aim of this study was to define its function in CML. CD93 is expressed on leukemia stem/progenitor cells (LSPCs), but not on more differentiated leukemia granulocytes. CD93 signaling promotes self-renewal and proliferation of LSCs, leading to disease progression in a murine CML model. RNA sequencing (RNA-seq) analysis reveals that CD93 signaling induces a stem-cell-maintenance- and proliferation-promoting gene expression program.
Interestingly, the ICD of CD93 promotes gene transcription via the transcriptional regulator SCY1-like pseudo-kinase 1 (SCYL1) independently of ligand binding to the extracellular domain of CD93. Genetic ablation of CD93 signaling reduces the frequency of LSCs and prevents CML disease development in mice. Comparable to the results in murine CML, CD93 is expressed on human CML LSPCs, triggers the expression of genes involved in self-renewal and proliferation, and promotes colony formation in vitro.
In a drug library screen, we identify metoclopramide (MCP) as an inhibitor of CD93-signaling in LSCs. MCP treatment reduces murine and human LSCs and prolongs survival of CML mice. These results identify CD93 as an important regulator of stemness of CML LSCs and a potential therapeutic target and the anti-emetic agent MCP as a drug that blocks CD93 signaling in CML.
More information: Carsten Riether et al, Metoclopramide treatment blocks CD93-signaling-mediated self-renewal of chronic myeloid leukemia stem cells, Cell Reports (2021). DOI: 10.1016/j.celrep.2020.108663