Vascular Remodeling in the Bone Marrow Leukemic Niche: A Therapeutic Target?

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Leukemia stem cells (LSCs) are at the apex of the acute myeloid leukemia (AML) cellular hierarchy and have the capability of unlimited self-renewal and of initiating disease. The quiescent fraction of LSCs provides a reservoir of self-renewing cells that sustain leukemia growth, prevent clonal exhaustion, and are treatment resistant; thus eliminating LSCs is the “holy grail” of anti- leukemia treatment. AML blasts profoundly modify the bone marrow (BM) niche by causing loss of non-permeable arteriolar vessels in the endosteal marrow and enrichment of permeable, fenestrated sinusoid vessels in the central marrow. The remodeled BM niche is permissive of LSC expansion and leukemia growth, yet the fine molecular mechanisms of this vascular remodeling remain to be fully elucidated. MicroRNAs (miRNAs) are small non-coding RNAs that target messenger RNAs and regulate protein levels. miR-126 plays an important role in quiescence, self-renewal and drug resistance of AML LSCs. Recently we showed that miR-126 is mostly expressed in the Sca-1+ endothelial cells (ECs) of arteriolar vessels, which are responsible for supplying miR-126 in the BM niche. Under normal conditions, miR-126 supply from Sca-1+ ECs regulates the homeostasis and activity of hematopoietic stem cells (HSCs). We discovered that AML blast-secreted TNFα down-regulates miR-126 in Sca-1+ ECs and causes a loss of arteriolar vessels. This results in a decreased supply of miR-126 to LSCs, which then engage the cell cycle and induce leukemia growth. We also made the “key” observation that forcing miR-126 down- regulation below the already decreased levels in the BM leukemic niche (hereafter referred to as “miR-126 deprivation”), leads to further loss of arterioles which harms LSCs but not normal HSCs. Restoring BM arteriolar vascularization in AML mice by neutralization of TNFα favors quiescent LSC expansion rather than having an antileukemic effect by increasing endothelial miR-126 supply to these cells. Thus, the central hypothesis of this proposal is that the understanding of the cellular and molecular basis of TNFα-induced miR-126 downregulation and its impact on BM vascular remodeling in AML will allow us to design novel miR-126 deprivation-based treatments that will eliminate homeostatic support to LSCs, rendering them vulnerable to anti-leukemic therapies. Therefore, we propose the following Specific Aims (SAs): SA#1: To prove the central role of the TNFα/miR-126 axis in vascular remodeling of the BM leukemic niche in AML. SA#2: Define the molecular mechanisms of the TNFα/miR-126 axis in the vascular remodeling of the BM leukemic niche in AML. SA#3: Therapeutic targeting of the leukemic vascular niche by a miR- 126 inhibitor in combination with commonly used antileukemic therapies.