Microenvironmental Triggers Induce a Chemoresistant, Differentiated Subset of S100A8/A9^high AML Cells Via the Jak/STAT3 Signaling Axis

Introduction: The bone marrow stromal niche can serve as a protective environment in hematological malignancies such as AML. Intensive research of the bidirectional interactions between leukemic blasts and stromal cells already highlighted numerous mechanisms how malignant cells are capable of shaping their local milieu in an opportunistic fashion. However, the complex mechanisms remain to be fully elucidated.

Previously, we found two small intracellular calcium-sensing molecules, S100A8 and S100A9, among the top upregulated genes in primary AML blasts upon stromal contact in an unbiased analysis. They are members of the S100 protein family that can modulate cellular responses such as proliferation, migration, inflammation, and differentiation. Dysregulation of S100 protein expression is described as a common feature in several human cancers. Particularly, expression of S100A8 in leukemic blasts predicts poor survival in de novo AML patients. Thus, we aimed to decipher the underlying pathways of the stroma-mediated S100A8/A9 upregulation as well as its functional consequences.

Methods: We cultured AML cell lines in absence or presence of bone marrow stroma cell-conditioned medium. Subsequent, AML cells were analyzed for alterations in intracellular signaling pathways and metabolic characteristics by means of flow cytometry, fluorescence microscopy and quantitative real-time PCR. Additionally, AML cells were analyzed for their resistance towards chemotherapy under the very same culture conditions. Moreover, matched pairs of patient-derived AML samples from the peripheral blood and the bone marrow were compared for key findings from the in vitro analyses.

Results: The previously seen upregulation of S100A8 and S100A9 in primary AML blasts upon stromal contact could be confirmed in AML cell lines both on the mRNA and the protein level and proved to be contact-independent and reversible. Interleukin 6-mediated triggering of the Jak/STAT3 signaling was shown to be one major pathway of this induction. The S100A8/A9^high subpopulation is characterized by profound metabolic alterations, and by increased surface levels of maturation markers (i.e. CD11b and CD14) both conferring chemoresistance. Strikingly, both differentiation and chemoresistance could be reversed by inhibition of Jak/STAT signaling or metabolic interference. Ultimately, elevated frequencies of S100A8/A9^high AML blasts with increased surface levels of maturation markers could be detected in bone marrow-derived specimen compared to matched samples from the peripheral blood in the same patient.

Conclusion: We could demonstrate that bone marrow stroma-induced S100A8/A9^high AML cells show increased maturation and chemoresistance which both can be targeted by metabolic or oncogenic signaling inhibition representing additional possible targets for further pre-clinical exploration.