The AnnexinV stainings reveal that while Myc is necessary for cell cycling, Pim1 allows survival of these proliferating cells. This finding agrees with the previous reports, which indicate that Pim1 is a co-activator PD0325901 chemical structure of Myc and cooperates by an anti-apoptotic action to enhance Myc-driven cell proliferation in a proB-cell line 19 and a human embryonic kidney cell line 22. Verbeek et al. 18 found that Eμ-Pim1/Myc-double-transgenic mice develop a dramatic

prenatal expansion of pre-B cells and early B cells in liver and spleen, but not in BM, Peyer’s patches and lymph nodes. Transplantation of such expanding pre-B- and early B cells from peripheral blood of the double-transgenic mice resulted in the outgrowth of lymphomas within 9 weeks. After transplantation, buy Romidepsin our Pim1/Myc-double-transduced pre-B cells show a population and expansion of pre-B cells comparable to that in Eμ Myc/Pim1 mice also in spleen, LNs and peritoneum upon overexpression of Pim1 and Myc for 4-8 weeks. This cellular expansion was completely reversible upon removal of doxycycline. Hence, additional rare transforming events had no measurable effects on the proliferative expansion of the oncogene-transduced

pre-B cells within the 2 months in the presence of doxycycline after transplantation. If such additional transforming events had occurred in the in vitro or in vivo expanding pre-B- and immature B cells, they either did not become independent of the actions of Pim1 and Myc, or were too rare to become manifest within the two months in vivo. Recently, it has been shown that even in established tumors, constitutive expression of specific oncogenes (and especially Myc) is crucial for tumor survival 30, 31. In contrast to pre-BI cells, which started to propagate in the transplanted host mice upon overexpression of Pim1 and

Myc, in vivo matured sIgM+ B cells in transplanted host mice were not able to expand in vivo upon overexpression of Pim1 and Myc. This suggests that the resting, mature B-cell pools are unaffected by the overexpression of Pim1 and Myc. Even upon ex vivo stimulation of purified IgM+ or CD19+ splenic B cells with polyclonal B-cell activators, proliferation remained limited, regardless of whether Pim1/Myc were Immune system overexpressed or not. This finding rules out the possibility that the mature B cells need an external trigger (such as activation) to enter the cell cycle before overexpression of Myc and Pim1 can maintain cell cycling and enhance survival. Therefore, the capability of B-lineage cells to proliferate in response to Pim1 and Myc overexpression seems to be restricted to a window of B-cell development from pre-BI to immature B cells. It remains to be investigated whether the transformability by Pim1 and Myc extends into earlier stages of hematopoietic development.