Inhibition of cell growth is a primary method of treating leukemia; however, the blockade of the cell cycle may prevent the efficacy of chemotherapeutic agents, which mainly target the proliferative phase of tumor cells. When most tumor cells are blocked at the quiescent phase, they may evade the killing powers of chemotherapeutics and may ultimately form micro residual disease (MRD). We hypothesize that leukemic MSCs may provide a niche for tumor stem cells, in which K562

cells back up the proliferation and self-renewal potential. These tumor cells may then be the source of relapse. Constitutive activation of Akt, one downstream target of PI3K, is also believed to promote proliferation and increase cell survival, leading to cancer FK228 progression[21]. The PI3K-Akt signal pathway is involved in the

antiapoptotic find more activity of tumor cells and culminates in the phosphorylation of the BCL-2 family member, Bad, thereby suppressing apoptosis and promoting cell survival. Akt phosphorylates Bad both in vitro and in vivo, and blocks Bad-induced cell death [22]. The PI3K-Akt-Bad pathway may represent a form of general antiapoptotic machinery, although there is insufficient evidence to support this hypothesis at present. We determined the expression levels of Akt, p-Akt, Bad, p-Bad proteins in K562 cells after inoculation with MSCs. Under the condition of K562 cells alone, there was a basal expression of p-Akt, and p-Bad, which might have been related to the bcr/abl SB202190 price fusion protein-activated PI3K-Akt signal pathway. In addition, the

expression of p-Akt and p-Bad was increased after coculture with leukemic MSCs. The addition of the specific inhibitor LY294002, which competes with PI3K for ATP binding sites [23], resulted in a dramatic decrease in levels of both phosphorylated proteins, while no obvious difference in Akt and Bad expression was observed among the three groups. Abiraterone Hence, we showed that the PI3K-Akt pathway was activated after coculture with MSCs. The pro-apoptotic molecule, Bad, was then phosphorylated and exerted inhibitory effects on starvation-induced apoptosis. Taken together, serum deprivation appears to mimic the effects of an adverse HM for leukemia cells. MSCs of leukemia patients can retard the cell cycles of K562 cells, inhibiting their proliferation and reducing their apoptosis. Consequently, MSCs protect leukemia cells against adverse conditions like serum deprivation and ultimately sustain their viability. The activation of the PI3K-Akt-Bad signaling pathway seems to be involved in the protective machinery. Therefore, approaches that block the activation of this signaling pathway may in turn remove this shielding and consequently may prove to be of benefit in the effective treatment of leukemia. Acknowledgements This work is supported by grants of 863 projects from the Ministry of Science & Technology of China (2006AA02A110 for H.Z, L.