Abstract
NRAS proteins are central regulators of proliferation, survival, and self-renewal in leukemia. Previous work demonstrated that the effects of oncogenic NRAS in mediating proliferation and self-renewal are mutually exclusive within leukemia subpopulations and that levels of oncogenic NRAS vary between highly proliferative and self-renewing leukemia subpopulations. These findings suggest that NRAS activity levels may be important determinants of leukemic behavior. To define how oncogenic NRAS levels affect these functions, we genetically engineered an acute myeloid leukemia (AML) cell line, THP-1, to express variable levels of NRASG12V. We replaced the endogenous NRASG12D gene with a tetracycline-inducible and dose-responsive NRASG12V transgene. Cells lacking NRASG12V oncoprotein were cell-cycle arrested. Intermediate levels of NRASG12V induced maximal proliferation; higher levels led to attenuated proliferation, increased G1 arrest, senescence markers, and maximal self-renewal capacity. Higher levels of the oncoprotein also induced self-renewal and mitochondrial genes. We used mass cytometry (CyTOF) to define the downstream signaling events that mediate these differential effects. Not surprisingly, we found that the levels of such canonical RAS-effectors as pERK and p4EBP1 correlated with NRASG12V levels. β-Catenin, a mediator of self-renewal, also correlated with NRASG12V levels. These signaling intermediates may mediate the differential effects of NRASG12V in leukemia biology. Together, these data reveal that oncogenic NRAS levels are important determinants of leukemic behavior explaining heterogeneity in phenotypes within a clone. This system provides a new model to study RAS oncogene addiction and RAS-induced self-renewal in AML. Implications: Different levels of activated NRAS may exert distinct effects on proliferation and self-renewal.