Forced Expression of PTEN Confers Sensitivity to MEK/MAPK Inhibitor in MDA-MB-468 cells

초록

Phosphatase and tensin homolg (PTEN) is widely known as an important tumor suppressor that inhibits PI3K/AKT signaling pathway. In an attempt to examine the effect of PTEN reconstruction in PTEN-null MDA-MB-468 breast cancer cells, MDA-MB-468 PTEN cells expressing PTEN were generated by retroviral infection of PTEN expression vector. Proliferation assays revealed that proliferation rate of MDA-MB-468 PTEN cells were significantly lower than MDA-MB-468 vec cells, suggesting that forced expression of PTEN reduces the growth rate of MDA-MB-468 cells. When the effects of PI3K/Akt signaling inhibitor, LY294002 and MEK/MAPK signaling inhibitor, U0126 on cell proliferation were compared both in MDA-MB-468 PTEN and MDA-MB-468 vec cells, treatment with LY294002 resulted in roughly the same 20% reduction of cell proliferation after 96 hours in two cell lines, whereas U0126 treatment led to more than 60% reduction in MDA-MB-468 PTEN cells and only 20% reduction in MDA-MB-468 vec cells. These results indicate that forced expression of PTEN may confer sensitivity to MEK/MAPK inhibitor. When apoptotic cell death of inhibitor treated cells was monitored by flow cytometry of propium iodide-labeled nuclei, subG0/G1 apoptotic fraction was significantly increased in MDA-MB-468 PTEN cells by U0126 treatment for 48 hours, while LY294002 treatment in both cell lines and U0126 treatment in MDA-MB-468 vec cells only led to a modest increase in apoptotic fraction, indicating that the decreased proliferation rate of MDA-MB-468 PTEN cells by U0126 might be due to increase of apoptotic cell death in these cells. When the effects of LY294002 and U0126 on phosphorylation of intracellular effector molecules were investigated, phosphorylation of Akt was completely shut down in MDA-MB-468 PTEN cells and significantly down-regulated in MDA-MB-468 vec cells by LY294002 while U1026 treatment led to a significant decrease MAPK phosphorylation only in MDA-MB-468 PTEN cells. Taken together, these experimental findings may suggest that blockade of functional PI3K/Akt signaling by forced expression of PTEN might render MDA-MB-468 PTEN cells become more dependent on MEK/MAPK pathway for their proliferation and survival.