Estrogen receptor α (ERα) mediates most of the biological effects of estrogen in mammary epithelial cells and stimulates growth signals involving phosphoinositide-3-OH kinase (PI3K)/Akt in breast cancer cells. Phosphatase and tensin homologue (PTEN) is a critical counter-regulator of PI3K signaling and is thus one of the major tumor suppressors in breast cancer. Inhibition of PI3K with an inhibitor, wortmannin, increased the level of PTEN protein in ERα-positive MCF-7 cells, while levels in ERα-negative MDA-MB 231 cells were not altered. In addition, the level of PTEN protein in MCF-7 cells was significantly lower than that in MDA-MB 231 cells, which correlated with high levels of phospho-Akt and phosphatidylinositol-3,4,5,-trisphosphate (PIP3). However, PTEN mRNA expression as measured by real-time PCR showed no differences in either cell line. Notably, the levels of casein kinase 2 (CK2) and phospho-PTEN (Ser380/Thr382/383) in MCF-7 cells were lower than those in MDA-MB 231 cells, indicating that the down-regulation of PTEN protein in MCF-7 cells is caused by low levels of CK2 expression, leading to accelerated PTEN degradation. Collectively, these results suggest that ERα induces the down-regulation of PTEN through PI3K activation in breast cancer cells.