Phosphatidylinositol 3-kinase/mammalian target of rapamycin pathway is critical for survival and proliferation of pancreatic cancer stem-like side population cells

Abstract

To isolate and identify the side population (SP) cells in pancreatic cancer and investigate the role of phosphatidylinositol 3-kinase/mammalian target of rapamycin (PI3K/mTOR) pathway in the survival and proliferation of them. Pancreatic cancer cell of the lines PANC-1, BXPC-3, ASPC-1, PC-3, and SW-1990 were cultured. Hoechst 33432 staining and fluorescence-activated cell sorter (FACS) were used to sort the SP cells. Verapamil, inhibitor of ATP binding cassette (ABC) transporter, was added before the Hoechst 33342 inoculation to observe its influence on the SP proportion. Media with LY294002, specific inhibitor of P13K, or rapamycin, specific inhibitor of mammalian target of rapamycin (mTOR), were used to culture PANC-1 cells to observe the survival of cells. Twenty-one NOD-SCID mice were randomly divided into 7 equal groups. Four groups were inoculated subcutaneously with SP cells of the concentrations of 5x10(5), 5x10(4), 5x10(3), or 1x10(3) at the right axillary fossa and with non-SP cells at the left then Hoechst 33342 staining, flow cytometric sorting were used to detect the content of SP cells at the left axillary fossa. The other 3 groups were injected subcutaneously with non-Hoechst 33342 treated cells of the concentrations of 5x10(5), 5x10(4), 5x10(3), or 1x10(3) at the right axillary fossa and PBS at the left axillary fossa. Ten weeks later the mice were killed to undergo pathological examination. All cell lines were found to exhibit verapamil-sensitive SP cells except BXPC-3 cells. The SP cell proportion of the PANC-1 cells was 7.84%. No SP cell was found in the cells treated with verapamil. The colony-formation ability of the SP cells was (43.7+/-3.1)%, significantly higher than those of the non-SP cells and cells without Hoechst 33342 cells [(8.3+/-1.6)% and (10.2+/-1.9)% respectively, both P=0.000]. The tumorigenic ability of the SP cells was 100 times as those of the non-SP cells and Hoechst 33342 un-treated cells. After addition of LY294002 and rapamycin the fractions of the SP cells in the in PANC-1 cells decreased from (7.60+/-0.27)% to (1.90+/-0.22)% and (1.14+/-0.20)% respectively, both P=0.000, and they preferentially inhibited the SP cells rather than non-SP cells. SP cells are enriched in pancreatic cancer stem-like cells. PI3K/mTOR pathway is critical for pancreatic SP cells maintenance that can be selected as a new target for inhibiting cancer stem-like cells.