The homeodomain protein TGIF has been implicated in the negative regulation of TGF-beta signaling. In this study, we report an unexpected role of TGIF in the inhibition of Smad2 phosphorylation, which occurs by a mechanism independent of its association with Smad2. This inhibitory function of TGIF is executed in concert with c-Jun, which facilitates the interaction of TGIF with cPML, resulting in the nuclear sequestration of cPML and the disruption of the cPML-SARA complex. Notably, knockdown of TGIF by siRNA caused increased association of cPML with SARA and cytoplasmic accumulation of cPML. Furthermore, c-Jun(-/-) fibroblasts exhibit enhanced association of cPML with SARA. c-Jun(-/-) fibroblasts also lose their sensitivity to TGIF-mediated disruption of the cPML-SARA complex and of nuclear sequestration of cPML. We suggest that the interaction of TGIF with cPML through c-Jun may negatively regulate TGF-beta signaling through controlling the localization of cPML and, consequently, the assembly of the cPML-SARA complex.

Cell cycle regulation plays a fundamental role in stem cell biology. A balance between quiescence and proliferation of hematopoietic stem cells in interaction with the microenvironment is critical for sustaining long-term hematopoiesis and for protection against stress. We analyzed the molecular mechanisms by which stromal cell-derived factor-1 (SDF-1) exhibited a cell cycle-promoting effect and interacted with transforming growth factor-␤ (TGF-␤), which has negative effects on cell cycle orchestration of human hematopoietic CD34؉ progenitor cells. We demonstrated that a low concentration of SDF-1 modulated the expression of key cell cycle regulators such as cyclins, cyclin-dependent kinase inhibitors, and TGF-␤ target genes, confirming its cell cycle-promoting effect. We showed that a cross-talk between SDF-1-and TGF-␤-related signaling pathways involving phosphatidylinositol 3-kinase (PI3K)/Akt phosphorylation participated in the control of CD34 ؉ cell cycling. We demonstrated a pivotal role of two downstream effectors of the PI3K/Akt pathway, FoxO3a and mammalian target of rapamycin, as connectors in the SDF-1-/TGF-␤-induced control of the cycling/quiescence switch and proposed a model integrating a dialogue between the two molecules in cell cycle progression. Our data shed new light on the signaling pathways involved in SDF-1 cell cycle-promoting activity and suggest that the balance between SDF-1-and TGF-␤-activated pathways is critical for the regulation of hematopoietic progenitor cell cycle status. STEM CELLS 2008;26:3150 -3161 Disclosure of potential conflicts of interest is found at the end of this article.