Reduced blood cell counts are a common feature of acute myeloid leukemia (AML), a cancer that affects bone marrow and results in abnormal cell proliferation. The primary function of bone marrow is to regulate hematopoietic stem and progenitor cells (HSPC) in response to disease or injury. Recent xenograft model data indicate that AML cells invade the bone marrow and suppress HSPC differentiation, although the precise mechanism is unclear. Drawing on their own previous research, Huan et al. (2015) investigated both direct and indirect participation exosomes released by AML cells in hematopoietic function inhibition associated with leukemia.1
To do this, the team employed xenografts (Molm-14 and HL-60 cell lines), qRT-PCR (NanoDrop 2000c spectrophotometer, Thermo Scientific), tandem mass tag reagents (TMT10plex, Thermo Scientific) and proteomics techniques (Orbitrap Fusion Tribrid mass spectrometer and Proteome Discoverer software, Thermo Scientific). They report that leukemia cells, including AML exosomes, inhibit critical stromal retention factors (Scf, Cxcl12). This downregulation redistributes the HSPC from the bone marrow to the peripheral blood. They further observed that AML exosomes participate in direct modulation of HSPC function in vitro in both murine progenitors and human cord blood. This includes marked changes in transcription factor expression linked with HSPC function (c-Kit, c-Myb, Dnmt1, Pcna, Hoxa9, E2f3, Paics, NF-kB1 and NF-kB2). Similarly, in vivo experiments confirmed this with downregulated expression of transcription factors that act as HSPC stromal regulators in AML xenografts (c-Myb, Hoxa9, E2f3 and Ship1). They also explored extramedullary myeloid tumors and determined that AML exosomes from these tumors dysregulate HSPC function by transferring human transcripts to bone marrow cells.
In terms of proteomics, Huan et al. identified 282 differentially expressed proteins between exosome-treated and vehicle-treated cells. Of these, 161 demonstrated upregulation, and 121 demonstrated downregulation. They used functional annotation (DAVID) to determine that 27% of the upregulated proteins linked to immune function. Other categories of note were acetylation, cell proliferation and antioxidant activity. Looking at downregulated proteins, the team noted that 31% participated in protein synthesis. Using the STRING database, they reported downregulation of known epigenetic hematopoiesis regulators (Dnmt1, Hells and Pcna).
The researchers indicate that, taken together, their findings suggest that AML exosomes dysregulate crucial factors for hematopoiesis. They postulate that these could even emerge as therapeutic targets, potentially allowing treatments to minimize HSPC loss associated with acute myeloid leukemia.
1. Huan, J., et al. (2015) “Coordinate regulation of residual bone marrow function by paracrine trafficking of AML exosomes,” Leukemia, 1-11, doi:10.1038/leu.2015.163.