Abnormal adipogenic signaling in the bone marrow mesenchymal stem cells contributes to supportive microenvironment for leukemia development.

BACKGROUND: Acute myeloid leukemia (AML) is an aggressive hematological malignancy, associated with unfavorable patient outcome, primarily due to disease relapse. Mesenchymal stem cells (MSCs) residing in the bone marrow (BM) niche are the source of mesenchyma-derived subpopulations, including adipocytes, and osteocytes, that are critical for normal hematopoiesis. This study aimed to characterize BM-derived adipocyte/osteocyte fractions and their crosstalk with AML cells as a potential mechanism underlying leukemogenesis. METHODS: BM cell subpopulations derived from primary AML patients were evaluated using humanized ex-vivo and in-vivo models, established for this study. The models comprised AML blasts, normal hematopoietic stem and progenitor cells and mesenchymal stromal subpopulations. ELISA, FACS analysis, colony forming unit assay, whole exome sequencing and real-time qPCR were employed to assess the differentiation capacity, genetic status, gene expression and function of these cell fractions. To explore communication pathways between AML cells and BM subpopulations, levels of signaling mediators, including cytokines and chemokines, were measured using the ProcartaPlex multiplex immunoassay. RESULTS: The study revealed deficiencies in adipogenic/osteogenic differentiation of BM-MSCs derived from AML patients, with adipocytes directly promoting survival and clonogenicity of AML cells in-vitro. In whole exome sequencing of BM-MSC/stromal cells, the AHNAK2 gene, associated with the stimulation of adipocyte differentiation, was found to be mutated and significantly under-expressed, implying its abnormal function in AML. The evaluation of communication pathways between AML cells and BM subpopulations demonstrated pronounced alterations in the crosstalk between these cell fractions. This was reflected by significantly elevated levels of signaling mediators cytokines/chemokines, in AML-induced adipocytes/osteocytes compared to non-induced MSCs, indicating abnormal hematopoiesis. Furthermore, in-vivo experiments using a fully humanized 3D scaffold model, showed that AML-induced adipocytes were the dominant component of the tumor microenvironment, providing preferential support to leukemia cell survival and proliferation. CONCLUSIONS: This study has disclosed direct contribution of impaired functional, genetic and molecular properties of AML patient-derived adipocytes to effective protection of AML blasts from apoptosis and to stimulation of their growth in vitro and in vivo, which overall leads to disease propagation and relapse. The detected AHNAK2 gene mutations in AML-MSCs point to their involvement in the mechanism underlying abnormal adipogenesis. Video Abstract.

Abnormal morphological and functional nature of bone marrow stromal cells provides preferential support for survival of acute myeloid leukemia cells.

Hematopoietic progenitors, residing in the bone marrow (BM) niche, are supported by mesenchymal stromal cells (MSCs). Cytogenetic and molecular aberrations in these progenitors lead to acute myeloid leukemia (AML). The BM-MSC role in leukemogenesis is not fully elucidated. In the current study, an ex-vivo system of patient's own stroma (POS), best mimicking the in-vivo BM niche, has been developed aiming to unravel interactions and crosstalk between MSCs and AML cells. POS derived from AML patients at diagnosis (Dx), relapse (Rx) and remission (Rm) was compared to healthy donor MSCs in terms of their morphology, growth pattern, support of leukemia cell viability and cytokine profile. Compared to control MSCs, POS (Dx/Rx, Rm) demonstrated a reduced proliferation rate (35%), significantly slower expansion, enlarged cell area (3-4-fold) and provided preferential support to leukemic cells of the same individual. Cytokine profiling showed significantly higher secreted phosphoprotein-1 (SPP1) expression in Dx/Rx and Rm POS compared to healthy MSCs. Additionally, the angiopoietin-1 expression was elevated in Dx/Rx POS with a further increase in the AML cell presence. In conclusion, the fact that POS derived in active disease and remission exhibited similar morphological and functional characteristics, might imply the involvement of the BM niche in leukemogenesis.