Pivotal Role of FoxO3 and mTORC1 in the Opposite Effects of SDF-1 and TGF-β on Cell Cycle Progression in CD34⁺ Stem/Progenitor Cells.

Control of cell cycling is a key process in stem cell fate. A balance between cell quiescence and proliferation within the stem cell niche in interaction with microenvironment is critical for sustaining long-term hematopoiesis and for protection against stress. Among growth factors regulating haematopoietic stem/progenitor cell (HSC/HP) cell cycling, TGF-β is known to be a major negative regulator by blocking cell cycle progression during the G₁ phase. Besides its effect on HSC/PH and lymphocyte trafficking, the SDF-1/CXCL12 chemokine is another key regulator which helps maintaining hematopoiesis homeostasis. We have previous demonstrated that SDF-1 acts as a survival and cell cycle promoting factor for CD34⁺ HSC/HP by triggering the G₀-G₁ transition and then regulating early cell cycle phases suggesting that it may act as an antagonist of TGF-β on cell cycle progression (Lataillade et al., Blood 2000 and 2002). The general aim of this study was to investigate the mechanisms of this antagonist effect on cell cycle. This work was performed on CD34⁺ cells purified from the peripheral blood of healthy un-mobilized donors. Since they are mainly in G₀, they allow dissecting the early cell cycle phases including the G₀-G₁ transition. We demonstrated that SDF-1 and TGF-β exert an opposite effect on the expression of cell cycle key regulators such as cyclins and CDKI. We showed that cross-talk between SDF-1 and TGF-β signaling pathways involving PI3K/Akt phosphorylation participates in the control of CD34⁺ cell cycling. We demonstrated a pivotal role of FoxO3 and mTOR in the TGF-β/SDF-1 control of the quiescence/cycling of hematopoietic progenitors. A model integrating a pivotal role for the activation of two Akt substrates, FoxO3 and mTORC1, in the control of CD34⁺ cells quiescence/cycling by TGF-β and SDF-1 is proposed. Altogether, our results shed new light on the intracellular signaling mechanisms of SDF-1 and of its implication, together with TGF-β, in cell cycle promotion in primary CD34⁺ cells. Regarding their opposite effect on cell cycle and survival, we suggest that SDF-1 and TGF-β can be considered as natural mutual competitors, both acting as organizers of the stem cell niche where a balance between HSC hibernation and activation is an essential part of the stemness control.