S6K1 Regulates Self-Renewal of Leukemia Initiating Cells and Normal Hematopoietic Stem Cells

The mixed-lineage leukemia (MLL) gene is required for the maintenance of adult hematopoietic stem cells (HSCs) and regulation of progenitor population. Translocations in MLL have been detected in approximately 5-10% of adult acute leukemia patients and in approximately 70% of acute leukemias in infants. Various genes, including AF4, AF5, AF9, ELL and ENL have been identified as partners for translocation in MLL-rearranged leukemia. In hematopoietic cells, expression of MLL fusion proteins result in a block of differentiation. AML patients who successfully undergo treatment but relapse suggest the importance of targeting leukemia initiating cells (LICs) within the MLL leukemia. LICs remain in a quiescent state and are capable of survival despite treatment with chemotherapeutic agents or targeted molecular inhibitors. In mouse models, LICs are defined by the capability of successfully propagating disease upon serial transplantation. In order to prevent disease relapse, identification of molecules, which regulate the self-renewal of LICs, is essential.

The phosphatidylinositol 3-kinase (PI3K)-Akt-mechanistic target of rapamycin complex1 (mTORC1) pathway is a key regulator of self-renewal of both HSCs and LICs. Deletion of Raptor, a subunit of mTORC1, does not affect initiation and progression of acute myeloid leukemia (AML), but Raptor deficiency results in delayed propagation of AML. Upon its activation, mTORC1 phosphorylates and activates p70 ribosomal protein S6 kinase (S6K1) and inhibits the activity of eukaryote translation initiation factor 4E binding protein 1 (4E-BP1). S6K1 has been shown to be hyperactivated in hematopoietic cells expressing oncogenic MLL-AF9 fusion protein.

In our study, we have assessed the role of S6K1 in the initiation, progression and propagation of AML using a genetic model of S6K1 knockout mice (S6K1-/-). We expressed MLL-AF9 fusion oncoprotein in WT and S6K1-/- hematopoietic stem and progenitor cells (HSC/Ps) and transplanted them into lethally irradiated recipients. Recipients of both WT and S6K1-/- HSC/Ps bearing MLL-AF9 displayed high white blood cell (WBC) count, splenomegaly and developed AML. There was no difference in survival between the WT and S6K1-/- recipients. In order to determine whether S6K1 regulates the self-renewal of LICs, we transplanted lethally irradiated mice with cells from WT and S6K1-/- primary recipients who developed AML. Recipients of S6K1 deficient AML cells survived significantly longer compared to controls (n=17/group, p<0.001). S6K1 deficient HSC/Ps expressing MLL-AF9 showed reduced activation of Akt as well as decreased mTORC1 activity, suggesting that deletion of S6K1 results in reduced activation of PI-3K-Akt-mTORC1 pathway both upstream and downstream of mTORC1 which indicates that S6K1 might be involve in a feedback loop within this pathway.

To determine the role of S6K1 in normal HSC development and maintenance, we analyzed bone marrow derived HSCs in WT and S6K1-/- mice. S6K1 deficiency did not alter the frequency of long term HSCs (LT-HSCs) as defined by CD150+ CD48- Lin- Sca1+ c-Kit+ surface markers, but the absolute number of LT-HSCs were significantly reduced in S6K1 deficient mice (p<0.02). The absolute number of multipotent progenitors (MPPs) (p<0.001), common myeloid progenitors (CMPs) (p<0.01) and megakaryocyte-erythroid progenitors (MEPs) (p<0.01) were also significantly reduced in S6K1 deficient mice. Deficiency of S6K1 resulted in reduced quiescence of LT-HSCs (p<0.05). Expression level of p21, an inhibitor of cell cycle progression, was significantly decreased in LT-HSCs derived from S6K1-/- mice compared to LT-HSCs from control group. To study the role of S6K1 in HSCs' function, we performed competitive repopulation assay. Sorted LT-HSCs from bone marrow cells derived from either WT or S6K1-/- mice were transplanted with competitor cells into lethally irradiated recipients. S6K1 deficient LT-HSCs displayed reduced repopulating ability in secondary recipients (n=9-10/group, p<0.001). Expression level of p21 was downregulated in donor-derived HSCs isolated from secondary recipients of S6K1 deficient HSCs compared to control. Overall, our study establishes S6K1 as a critical regulator of self-renewal of both LICs and HSCs. Deficiency of S6K1 in AML cells results in delayed propagation of disease and deficiency of S6K1 in HSCs results in decreased self-renewal potential.