Phosphatase and tensin homolog (PTEN) is a tumor suppressor that inhibits PI3K/Akt signaling. To examine the effect of PTEN on breast cancer cell proliferation, we expressed PTEN in MDA-MB-468 cells (MDA-MB-468 PTEN) by retroviral infection and tested the cell proliferation rate. We found that the growth rate of MDA-MB-468 PTEN cells was significantly lower than that of MDA-MB-468 control vector cells (MDA-MB-468 vec). When the PI3K/Akt signaling inhibitor LY294002 and the MEK/Erk signaling inhibitor U0126 were applied, LY294002 reduced cell proliferation in MDA-MB-468 PTEN and MDA-MB-468 vec by 20%, while U0126 led to a >60% reduction in MDA-MB-468 PTEN and a 20% reduction in MDA-MB-468 vec cells. FACS analysis demonstrated that the subG0/G1 apoptotic fraction was significantly increased in MDA-MB-468 PTEN cells after U0126 treatment, while LY294002 treatment in both cell lines and U0126 treatment in MDA-MB-468 vec cells led to a modest increase in the apoptotic fraction. This phenomenon was accompanied by the down-regulation of p-Erk. p-Erk levels were significantly lower after U0126 treatment in MDA-MB-468 PTEN cells. Similar results were observed in MDA-MB-231 cells, which express endogenous PTEN. The growth of MDA-MB-231 cells was significantly inhibited after U0126 treatment, compared to LY294002, while PTEN-null ZR-75-1 cells did not show increased sensitivity to U0126 over LY294002. Taken together, these findings suggest that blockade of PI3K/Akt signaling by PTEN may render breast cancer cells more dependent on the MEK/Erk pathway for their proliferation and survival.

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