Chemoresistant Leukemia-Initiating Cell Expansion Is Inhibited By Targeting Oncogenic Self-Renewal

Leukemia is often maintained by a relatively rare subset of tumor-initiating cells, which may be resistant to traditional chemotherapy. Targeting these leukemic stem cells is expected to eliminate or reduce acquired resistance to chemotherapy and disease recurrence, but this goal has been elusive. It's also not clear whether eliminating the 'root' of cancer is sufficient or whether targeting both 'root' and 'branch' (i.e. LSCs and their bulk progeny) is necessary. To test this, we established and characterized a mouse model with clearly defined populations of rare leukemic stem cells (LSCs), abundant leukemic blast cell progeny, and normal hematopoietic stem/progenitor cells (HSPCs). This was achieved by simultaneously activating the Wnt/β-catenin and PI3K/Akt signaling pathways in HSPCs to drive stem cell self-renewal-resulting in successive expansion of HSPCs, LSCs, and leukemic blasts. Functional analysis showed a 1,300-fold increase in leukemia-initiating activity of sorted LSCs compared to bulk blast cells or sorted HSPCs. Since Akt activates β-catenin by phosphorylation at serine 552 and thus represents a molecular link mediating the synergistic, self-renewal promoting activity of these pathways, we sought to specifically target pS⁵⁵²-β-catenin. Unexpectedly, high-throughput screening (HTS) and subsequent validation assays found that doxorubicin (DXR) inhibits pS⁵⁵²-β-catenin with minimal effects on total β-catenin.

DXR exhibits the broadest spectrum of anti-cancer activity known and has been employed as a standard chemotherapeutic agent for decades, but severe side-effects limit its use. We found that while standard chemotherapeutic treatment reduced the bulk leukemic blast cells as expected, it also induced pS⁵⁵²-β-catenin uniquely in LSCs and stimulated LSC expansion. Using long-circulating nanoparticles (NanoDXR) to deliver very low, sustained doses of DXR, we repurposed DXR as a targeted therapy for pS⁵⁵²-β-catenin inhibition rather than a broadly cytotoxic agent. In vivo, this treatment reduced pS⁵⁵²-β-catenin levels in LSCs, prevented LSC expansion, essentially eliminated LSC tumorigenic activity in transplant recipients, and was accompanied by recovery of HSPCs and substantially increased median survival from 44.5 days to 139 days.

Our data reveals a dynamic relationship between rare LSCs and bulk leukemic blast cells in their response to cytotoxic chemotherapy and show how oncogenic self-renewal in chemoresistant LSCs can be targeted while sparing normal HSPCs. In distinguishing the unique properties of LSCs and their progeny, we find that both populations must be differentially targeted at both the 'root' and 'branch' of leukemia and can be effectively reduced or eliminated while allowing for recovery of normal HSPCs.