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Department of Bioactive Molecules, National Institute of Infectious Diseases, Tokyo 162-8640, Japan

Anchorage-independent growth is a hallmark of oncogenic transformation. We reported that the mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) inhibitor U0126 inhibited anchorage-independent growth of Ki-ras-transformed rat fibroblasts by simultaneously blocking both extracellular signal-regulated kinase (ERK) and mammalian target of rapamycin (mTOR)-p70^S6K pathways. Here, we examined the effects of U0126 on the growth of eight human breast cancer cell lines. U0126 selectively repressed anchorage-independent growth of MDA-MB231 and HBC4 cells, two lines with constitutively activated ERK. Loss of contact with substratum triggers apoptosis in many normal cell types, a phenomenon termed anoikis. U0126 sensitized MDA-MB231 and HBC4 to anoikis, i.e., upon treatment with U0126, cells deprived of anchorage entered apoptosis, whereas adherent cells remained viable. Another MEK inhibitor PD98059 also induced anoikis sensitivity in MDA-MB231 cells but not in HBC4 cells. However, HBC4 cells were sensitized to anoikis when PD98059 was combined with the mTOR inhibitor rapamycin. To study the biochemical basis for induction of anoikis sensitivity, we examined the effects of the MEK inhibitors on ERK and p70^S6K pathways in anchored versus nonanchored cells. As in Ki-ras-transformed rat fibroblasts, U0126 reduced activation of both ERK and p70^S6K in MDA-MB231 and HBC4 cells, irrespective of anchorage. PD98059, in anchored cells, was more selective for the ERK pathway and did not significantly block the p70^S6K pathway. Removal of anchorage substantially sensitized p70^S6K to PD98059 in MDA-MB231 cells, whereas p70^S6K in suspended HBC4 cells remained fairly refractory. U0126 was either without effect or less inhibitory on p70^S6K in MDA-MB453 and SKBR3, two cell lines in which anoikis sensitivity was not induced. Thus, susceptibility of the p70^S6K pathway to MEK inhibitors appeared to be an important determinant of anoikis sensitivity. The results indicate that concurrent inhibition of MEK-ERK and mTOR-p70^S6K pathways induces apoptosis in MDA-MB231 and HBC4 cells when cells are deprived of anchorage but not when anchored. Inhibitors of MEK-ERK and mTOR-p70^S6K pathways may provide a therapeutic strategy to selectively target neoplasms proliferating at ectopic locations, with acceptable effects on normal cells in their proper tissue context.

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