Interestingly, this senescence response is p19Arf-independent in prostate (Chen et al., 2009) but p19Arfdid suppress leukemogenesis afterPtendeletion, indicating tissue-specific functions for p19Arfin tumor suppression. p16Ink4adeficiency,p16Ink4a/p19Arfdeficiency, orp53deficiency all significantly prolonged the ability ofPten-deficient HSCs to give multilineage reconstitution in irradiated mice (Fig. survival by phosphorylating Apatinib diverse substrates (Manning and Cantley, 2007), including the Tuberous Sclerosis Complex (TSC) (Inoki et al., 2002). Phosphorylation by Akt negatively regulates TSC, leading to the activation of the mammalian target of rapamycin (mTOR) kinase (Inoki et al., 2002). mTOR functions in two distinct complexes, mTORC1, which is directly inhibited by rapamycin, and mTORC2, which can be indirectly inhibited by rapamycin (Guertin and Sabatini, 2007;Sarbassov et al., 2006). mTORC1 promotes cell growth and proliferation by activating S6 kinase and inactivating 4EBP1, promoting protein synthesis (Inoki et al., 2002). mTORC2 regulates Akt activation (Guertin and Sabatini, 2007). PI-3kinase signaling is attenuated by Pten, which dephosphorylates PIP3 (Maehama NOS3 and Dixon, 1998), reducing the activation of Akt, mTORC1, and S6 kinase. As a result,Ptendeficiency increases the growth, proliferation, and survival of many cells (Sun et al., 1999) andPtenis commonly deleted in diverse cancers (Di Cristofano and Pandolfi, 2000). PI-3kinase pathway signaling has divergent effects on stem cells. Conditional deletion ofPtenfrom embryonic stem cells and neural stem cells increases cell cycle entry and self-renewal (Gregorian et al., 2009;Groszer et al., 2006;Groszer et al., 2001;Sun et al., Apatinib 1999). In contrast,Ptendeletion from adult HSCs increases cell cycle entry but this leads to rapid HSC depletion (Yilmaz et al., 2006;Zhang et al., 2006). We showed that this depletion was mediated by mTOR activation as rapamycin blocked the depletion ofPten-deficient HSCs (Yilmaz et al., 2006). Subsequent studies ofTsc1-deficient HSCs confirmed that increased PI-3kinase pathway signaling can drive HSCs into cycle but that this leads to mTOR-mediated HSC depletion (Chen et al., 2008;Gan et al., 2008).Pmldeletion also increased HSC cycling and led to mTOR-mediated HSC depletion (Ito et al., 2008). mTOR is thus a Apatinib critical modulator of stem cell maintenance, raising the question of how mTOR activation leads to stem cell depletion. WhilePtendeletion leads to the depletion of normal HSCs, this leads to the generation and expansion of leukemia-initiating cells (Yilmaz et al., 2006). This makes it possible to deplete leukemia-initiating cells while rescuing normal HSC function using rapamycin in mice that are deficient forPtenorPml(Ito et al., 2008;Yilmaz et al., 2006). A sophisticated understanding of PI-3kinase pathway signaling can therefore lead to therapies that eliminate cancer-initiating cells with limited toxicity to normal stem cells. One mechanism by whichPtendeletion and PI-3kinase pathway activation Apatinib could deplete stem cells involves a tumor suppressor response. Sustained oncogenic signals can induce tumor suppressors that cause cellular senescence (Lin et al., 1998;Serrano et al., 1997). Conditional inactivation ofPtenin prostate leads to the induction of p53-mediated senescence, impeding the development of prostate cancer (Chen et al., 2005). These studies raise the question of whether a similar tumor suppressor response occurs afterPtendeletion in stem cells, and whether this suppresses leukemogenesis or depletes HSCs. Another mechanism by whichPtendeletion could deplete HSCs involves the inactivation of FoxO family transcription factors. When localized to the nucleus, FoxO transcription factors promote the expression of enzymes that eliminate reactive oxygen species (ROS). However, activated Akt phosphorylates FoxO proteins, leading to their retention in the cytoplasm (Biggs et al., 1999;Brunet et al., 1999) and increasing ROS levels. HSCs are particularly sensitive to the toxic effects of ROS (Ito et al., 2004;Ito et al., 2006). Deletion ofFoxO1/3/4, orFoxO3aalone, from adult HSCs leads to increased ROS levels and HSC depletion that can be at least partially rescued by the antioxidant N-Acetyl-cysteine (NAC) (Miyamoto et al., 2007;Tothova et al., 2007;Yalcin et al., 2008). The depletion ofTsc1-deficient HSCs is also partly rescued by NAC treatment.
