Retinoic acid (RA)-inducible gene I (RIG-I) is highly upregulated and functionally implicated in the RA-induced maturation of acute myeloid leukemia (AML) blasts. However, the underlying mechanism and the biological relevance of RIG-I expression to the maintenance of leukemogenic potential are poorly understood. Here, we show that RIG-I, without priming by foreign RNA, inhibits the Src-facilitated activation of AKT-mTOR in AML cells. Moreover, in a group of primary human AML blasts, RIG-I reduction renders the Src family kinases hyperactive in promoting AKT activation. Mechanistically, a PxxP motif in RIG-I, upon the N-terminal CARDs' association with the Src SH1 domain, competes with the AKT PxxP motif for recognizing the Src SH3 domain. In accordance, mutating PxxP motif prevents Rig-I from inhibiting AKT activation, cytokine-stimulated myeloid progenitor proliferation, and in vivo repopulating capacity of leukemia cells. Collectively, our data suggest an antileukemia activity of RIG-I via competitively inhibiting Src/AKT association.

Inactivation of p53 and/or Rb pathways restrains osteoblasts from cell-cycle exit and terminal differentiation, which underpins osteosarcoma formation coupled with dedifferentiation. Recently, the level of p-S6K was shown to independently predict the prognosis for osteosarcomas, while the reason behind this is not understood. Here we show that in certain high-grade osteosarcomas, immature SSEA-4+ tumor cells represent a subset of tumor-initiating cells (TICs) whose pool size is maintained by mTORC1 activity. mTORC1 supports not only SSEA-4+ cell self-renewal through S6K but also the regeneration of SSEA-4+ TICs by SSEA-4− osteosarcoma cell dedifferentiation. Mechanistically, active mTORC1 is required to prevent a likely upregulation of the cell-cycle inhibitor p27 independently of p53 or Rb activation, which otherwise effectively drives the terminal differentiation of SSEA-4− osteosarcoma cells at the expense of dedifferentiation. Thus, mTORC1 is shown to critically regulate the retention of tumorigenicity versus differentiation in discrete differentiation phases in SSEA-4+ TICs and their progeny.