AML Associated Mesenchymal Stroma Cells Support Growth of AML Cells As a Result of Activated Notch Signaling and This Can be Targeted By Dexamethasone

Introduction: Acute myeloid leukemia (AML) is a hematological malignancy characterized by poor prognosis. Interaction with bone marrow mesenchymal stromal cells (BM-MSC) supports leukemic cells to evade chemotherapy-induced apoptosis. Of particular interest, massive parallel transcriptome sequencing of MSCs from mice and patients with preleukemic syndromes identified inflammatory signaling in inducing genotoxic stress, leukemogenesis, and clinical outcome. Recently, Dexamethasone (Dex), an anti-inflammatory drug, demonstrated antileukemic activity and improved clinical outcome in AML patients. These observations intrigued us to further investigate leukemia-BM niche-associated signaling and to identify signaling cascades supporting leukemic cells and BM-MSC interaction.

Methods: BM-MSC from AML patients at diagnosis, or in remission were cultured alone or together with human AML cell lines (Kasumi-1 and M-07e) in direct contact or separated by transwells. Following transplantation experiments, BM-MSC were analyzed by microarray analysis. Gene set enrichment analysis (GSEA) was used to identify the hallmark gene sets overrepresented in AML MSC. The functional characterization of BM-MSC and AML cell interaction was investigated using in vitro (cell proliferation and apoptosis) and in vivo (leukemia development, drug response, and survival) analysis.

Results: To functionally evaluate the role of BM-MSC, we co-cultured MSCs from AML (AML-MSC) or non-leukemic (non-AML-MSC) patients with Kasumi-1 or M-07e cells. AML-MSCs significantly supported proliferation of AML cells by altering cell cycle distribution from G0/G1 to S phase (30.5 vs 51.3%, p<0.01). Strikingly, the cell cycle profile was unchanged when the cells were separated by a transwell. Consistently, in vivo, MLL-AF9 leukemic cells (Lin^-cells transduced with an MLL-AF9 oncogene) and AML-MSCs succumbed to leukemia only when cotransplanted (40 vs 143 days, p<0.05).

To gain further insights into leukemic cell-induced changes in MSC, we performed gene expression profiling in MSCs sorted from secondary transplanted leukemic (AML-MSC, n=3) and healthy (HD-MSC, n=3) mice. GSEA analysis identified hallmark-Notch signaling to be significantly overrepresented in AML-MSC as compared to controls (p<0.005, NES: 1.7). Increased Notch-1 and -2 expression could be verified at both the mRNA and protein levels in AML-MSCs. To functionally determine the contribution of Notch activation in MSC, we ectopically expressed the Notch intracellular domain (Notch-ICN) in a murine stromal cell line, MS-5 (Notch-ICN). Underscoring a role for Notch signaling in AML-MSC interaction, Notch-ICN expression significantly supported the proliferation of both MLL-AF9 leukemic cells and MSCs.

A role for Notch signaling in the modulation of the inflammatory response has been previously reported. Dex an anti-inflammatory corticosteroid enhanced clinical outcome in AML patients. We, therefore, investigated whether Notch-dependent AML-MSC interaction could be therapeutically targeted using Dex. Interestingly, Dex treatment affected Notch1 expression (52%; p<0.005) and proliferation of leukemia cells (63%; p<0.001). Finally, we evaluated the therapeutic efficacy of Dex by treating mice transplanted with MLL-AF9 leukemic cells. Interestingly, mice treated with Dex exhibited significantly prolonged survival (73 vs 21 days, p<0.01) and had a lower percentage of blasts (50%, p<0.01) and MSC compared to leukemic mice receiving vehicle treatment. Our data thus identified Dex mediated abrogation of AML-MSC interaction is in part mediated by affecting Notch signaling.

Conclusion: Our study demonstrated distinct functional differences between AML and non-AML-MSC and direct AML-MSC interaction as an indispensable factor in influencing cell proliferation, leukemogenesis, and survival in mice. We further identified and confirmed the role of Notch signaling in AML-MSC interaction. Therapeutically, Dex abrogated AML-MSC interaction, decreasing AML cell viability, and leukemic mice survival. Further studies are warranted to better understand how aberrant Notch signaling in MSC contributes to AML development and whether the use of Dex alone or in combination with Notch inhibitors could circumvent chemoresistance and relapse in AML patients.