Extracellular Vesicles and Chemotherapy Resistance in the AML Microenvironment

Abstract

Extracellular vesicle (EV) trafficking provides for a constitutive mode of cell-cell communication within tissues and between organ systems. Different EV subtypes have been identified that transfer regulatory molecules between cells, influencing gene expression, and altering cellular phenotypes. Evidence from a range of studies suggests that EV trafficking enhances cell survival and resistance to chemotherapy in solid tumors. In acute myeloid leukemia (AML), EVs contribute to the dynamic crosstalk between AML cells, hematopoietic elements and stromal cells and promote adaptation of compartmental bone marrow (BM) function through transport of protein, RNA, and DNA. Careful analysis of leukemia cell EV content and phenotypic outcomes provide evidence that vesicles are implicated in transferring several known key mediators of chemoresistance, including miR-155, IL-8, and BMP-2. Here, we review the current understanding of how EVs exert their influence in the AML niche, and identify research opportunities to improve outcomes for relapsed or refractory AML patients.

Keywords: acute myeloid leukemia; bone marrow microenvironment; chemoresistance; extracellular vesicles; stroma.

Figure 1

EV mediated transfer of chemoresistance between leukemia cells in the BM microenvironment. (A) Diagram of the BM microenvironment, composed of the hematopoietic niche (right) stromal compartment (left). Hematopoietic Stem and Progenitor Cell (HSPC) give rise to Common Myeloid Progenitors (CMP), Granulo-Monocytic Progenitor Cells (GMP), Erythroblasts (EB), Megakaryocytes (Mk), and many other cell types that populate the cells of the blood. In the stromal compartment, Mesenchymal Stromal Cells (MSC) give rise to Osteoprogenitor Cells (OPC) and Osteoblasts (OB), together these cells function to form bone and regulate hematopoiesis in part through EV-mediated signaling. (B) Expansion of leukemic cells results in microenvironmental dysregulation. EV trafficking between AML cells transfers regulatory factors that induce resistance to chemotherapy. (C) Chemo-experienced AML cells shed EVs containing NMP1, SRSF1, and SRSF9, which increase apoptosis resistance through upregulation of BCL-2 and NPM1 in unexperienced recipient AML cells. (D) EVs from chemo-experienced AML cells also contain miR-19b and−20a, which reduce TGF-β signaling and increase Akt signaling and the expression of MRP1 chemo-efflux pump in recipient AML cells.

Figure 2

EV trafficking between leukemia and stromal cells reduces sensitivity to chemotherapeutics. (A) Bidirectional trafficking between leukemia and stromal cells leads to alterations in both the cellular composition and secretome of the stromal compartment. (B) In CML, EV-mediated signaling upregulates the secretion of IL-8 from BM stromal cells. Increased IL-8 in the leukemic microenvironment increases CML cell adhesion, motility, and survival through CXCR1 and CXCR2 engagement, and promotes resistance to etoposide. (C) AML cells from the nutrient-depleted leukemic microenvironment exhibit marked endoplasmic reticulum (ER) stress and upregulation of BMP-2. AML-EVs transfer BMP-2 and ER-stress to stromal MSCs and OPCs, activating the unfolded protein response pathway (UPR). UPR activation induces osteogenic differentiation in MSCs and causes increased apoptosis in osteoprogenitor cells, altering the cellular composition of the BM, AML survival, and response to chemotherapy.

Figure 3

EV mediated resistance to immunotherapy. (A) AML EVs contain numerous immunosuppressive ligands (TRAIL, FASL, MICA/B) that reduce natural killer (NK) cell reactivity through receptor mediated binding. This EV-mediated signaling interferes with cell-based therapy, diminishing cytotoxic killing of tumor cells following adoptive transfer of NK cells. (B) EVs in CLL contain surface CD20, which acts as a decoy by sequestering Rituximab (anti-CD20) and preventing therapeutic antibodies from binding and opsonizing the tumor cells. (C) AML cells release EVs that contain the immunosuppressive ligand PD-L1. The transfer of PD-L1 via EVs reduces T cell activation in response to TCR stimulus, while also acting as decoys that compete with checkpoint inhibitor binding and prevent therapeutic antibodies from reaching their intended target.