Sensitizing Acute Myeloid Leukemia Cells to Interferon-Induced Differentiation By Inhibiting RIP1/RIP3 Necroptotic Pathway

Background: In addition to inducing apoptosis, tumor necrosis factor-α (TNFα) and FAS-induced death receptor signaling also induce necroptosis, a recently defined type of programed necrosis, through activation of the RIP1/RIP3-kinase pathway. The role of death receptor signaling-induced apoptosis in the development of leukemia has been well documented. However, the role of necroptotic signaling in the pathogenesis of leukemia has not been studied.

Methods: RIP1/RIP3 signaling was inactivated in murine and human acute myeloid leukemia (AML) cells by genetic deletion of RIP1/RIP3 or pharmacologic inhibition using the specific inhibitor Necrostatin-1. The effects of RIP1/RIP3 signal inactivation on the behavior and interferon-γ (IFNγ)-induced differentiation of AML cells were evaluated by morphologic analysis, cell surface markers, in vitro colony-forming ability and in vivo transplantation/leukemogenic capacity. The responses of mice transplanted with Rip1^-/- or Rip3^-/-murine AML to IFNγ treatment were compared to the mice transplanted with wild-type AML.

Results: We found that most types of AML cells produce TNFα and already show basal level activation of RIP1/RIP3 signaling but they do not undergo necroptosis. Using both pharmacological inhibition and genetic deletion techniques, we determined that inactivation of the RIP1/RIP3 signal resulted in partial differentiation of both human AML cell lines and MLL-AF9-transduced murine AML cells as demonstrated by studies of morphology, cell surface markers and colony-forming ability. Rip1 or Rip3 deletion significantly repressed the leukemogenic capacity of murine AML cells in vivo. In addition, inactivation of RIP1/RIP3 signaling significantly enhanced the complete differentiation of AML cells induced by IFNγ. The combination of RIP1/RIP3 inactivation and IFN treatment significantly further attenuated the clonogenic capacity of both primary AML cells and AML cell lines. Such combination treatment also further compromised the leukemogenic ability of AML cells in vivo. Mechanistically, we found that the RIP1/RIP3 pathway regulates the expression of SOCS1, a key negative regulator of IFNγ signaling. Inactivation of RIP1/RIP3 signaling in AML cells renders them hypersensitive to IFNγ -induced monocytic lineage maturation.

Conclusion: Resistance to apoptosis is one of the key mechanisms involved in the development of drug resistance in leukemic cells. RIP1/RIP3-mediated necroptosis induced by TNFα has been proposed to be an alternative therapeutic strategy to treat leukemia. However, our studies suggested that inhibition of RIP1/RIP3-mediated necroptotic signaling might be useful strategy for AML treatment when combined with IFNγ.