Therapeutic Targeting of the Interleukin 1 Receptor Accessory Protein (IL1 RAP) Reveals Novel Oncogenic Functions in Acute Myeloid Leukemia
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Leukemia stem cells (LSC) are the defective counterparts of hematopoietic stem and progenitor cells (HSPC) that can give rise to acute myeloid leukemia (AML). LSC have been implicated as drivers of relapse in AML as they are generally chemotherapy resistant and thus are not eliminated. Our lab has previously performed gene expression profiling to identify genes dysregulated in HSPCs from AML patient samples compared to immunophenotypically matched cells from healthy control samples. The interleukin-1 receptor accessory protein (IL1 RAP) was one of the few genes whose expression was commonly upregulated across multiple hematopoietic stem and progenitor compartments and AML subtypes, whereas expression was minimal or absent in healthy HSPC. The IL1RAP protein was validated to be overexpressed in AML HSPC, and since it spans the plasma membrane, it has potential to be an easily targetable molecule. However, the functions of IL1RAP in AML have yet to be described. Previous investigation by our lab showed that higher expression of IL1 RAP correlates with poor prognosis in AML. Additionally, shRNA-mediated knockdown of IL1RAP significantly decreased clonogenic growth and increased apoptosis in AML cell lines. These studies indicate a functional role for IL1RAP in AML. In this study, we characterized previously undescribed roles of IL1RAP in AML and healthy hematopoiesis as well as its efficacy as a therapeutic target in AML. As the hematopoietic system is compromised in AML, it is often the case that patients do not have a fully functioning immune system. Therefore, our study aimed to apply therapeutic strategies that do not rely on the assistance of immune effectors for elimination of malignant cells, but rather that exploit functional dependence of AML cells on IL1RAP. These therapeutic strategies served also as a tool for determining cell-intrinsic functions of IL1RAP. We found that targeting of IL1RAP via RNA interference, genetic deletion, or antibodies directed at its extracellular domain inhibited AML cell growth and viability in vitro and in vivo, with minimal perturbation of healthy hematopoietic function or viability. These effects were seen in particular mutational subsets of AML, specifically those lacking activating mutations in the receptor tyrosine kinase FLT3. Incubation of FLT3 wildtype AML cell lines and primary human patient samples with IL1RAP shRNAs and antibodies inhibited growth, promoted differentiation and led to apoptosis in vitro. IL1 RAP shRNAs and in vivo antibody treatment were effective in eliminating human AML cells in xenograft mouse models, and Ill rap deletion inhibited progression of AML in a mouse genetic model of the disease. Using phospho-immunoblotting, phospho-flow cytometry analysis, and cytokine-stimulated growth of AML cells, we investigated the signaling pathways affected by shRNAmediated targeting, antibody-mediated targeting, and genetic deletion of IL1 RAP in AML cells. We found that the role of IL1RAP is not restricted to the canonical IL-1 receptor pathway, but that IL1RAP mediates signaling and proliferation through FLT3 and c-KIT, two receptor tyrosine kinases with known key roles in AML pathogenesis. Coimmunoprecipitation and Fbrster resonance energy transfer (FRET) experiments demonstrated interactions between IL1RAP and these receptors in AML cell lines, indicating a mechanism for how targeting IL1RAP dysregulates the FLT3 and c-KIT signaling pathways and leads to AML cell death. Overall, our study provides a new mechanistic basis for the efficacy of IL1RAP targeting in AML and reveals novel roles for this protein in the pathogenesis of the disease.