BETP degradation simultaneously targets acute myelogenous leukemic stem cells and the microenvironment

Abstract

The antileukemic effect of inhibiting bromodomain and extra-terminal domain-containing (BET-containing) proteins (BETPs) such as BRD4 has largely been largely attributed to transcriptional downregulation of cellular anabolic and antiapoptotic processes, but its effect on the bone marrow microenvironment, a sanctuary favoring the persistence of leukemic stem/progenitor cells, is unexplored. Sustained degradation of BETP with the small-molecule BET proteolysis-targeting chimera (PROTAC) ARV-825 resulted in a marked downregulation of surface CXCR4 and CD44, key proteins in leukemia-microenvironment interactions, in acute myeloid leukemia (AML) cells. Abrogation of surface CXCR4 expression impaired SDF-1α–directed migration and was mediated through transcriptional downregulation of PIM1 kinase, which in turn phosphorylates CXCR4 and facilitates its surface localization. Downregulation of CD44, including isoforms CD44v8–10 impaired cystine uptake, lowered intracellular reduced glutathione, and increased oxidative stress. More important, BETP degradation markedly decreased the CD34+CD38–CD90–CD45RA+ leukemic stem cell population and, alone or in combination with cytarabine, prolonged survival in a mouse model of human leukemia that included AML patient-derived xenografts (AML-PDX). Gene expression profiling and single-cell proteomics confirmed a downregulation of the gene signatures associated with “stemness” in AML and Wnt/β-catenin and Myc pathways. Hence, BETP degradation by ARV-825 simultaneously targets cell-intrinsic signaling, stromal interactions, and metabolism in AML.