EGCG Stabilizes p27kip1 in E2-Stimulated MCF-7 Cells through Down-Regulation of the Skp2 Protein

Loss of p27(Kip1) is associated with a poor prognosis in breast cancer. According to previous findings, a decrease in p27(Kip1) levels is mainly the result of enhanced proteasome-dependent degradation mediated by its specific ubiquitin ligase subunit S-phase kinase protein 2 (Skp2). Epigallocatechin-3-gallate (EGCG), the main constituent of green tea, was found to stabilize p27(Kip1) levels in breast cancer, but whether this effect is mediated through changes in Skp2 expression remains unclear. Here we investigated the mechanisms involved in EGCG's growth inhibition of estrogen-responsive human breast cancer MCF-7 cells. In our results, EGCG increased p27(Kip1) and decreased Skp2 in a time- and dose-dependent manner, suggesting that p27(Kip1) and Skp2 may be involved in the growth inhibition by EGCG in estrogen-stimulated MCF-7 cells. Interestingly, mRNA levels of p27(Kip1) and Skp2 did not significantly change in estrogen-stimulated MCF-7 cells after EGCG treatments. Moreover, overexpression of Skp2 in MCF-7 cells prevented accumulation of p27(Kip1) and promoted resistance to the antiproliferative effects of EGCG. This suggests that the down-regulation of the F-box protein Skp2 is the mechanism underlying p27(Kip1) accumulation. Furthermore, both tamoxifen and paclitaxel significantly and synergistically enhanced the growth inhibition of MCF-7 cells by EGCG through the down-regulation of Skp2 protein. However, the down-regulation of Skp2 was not always correlate with the up-regulation of p27, suggesting that EGCG-dependent Skp2 down-regulation can influence cell growth in several ways. The therapeutic strategies designed to reduce Skp2 may therefore play an important clinical role in treatment of breast cancer cells. (Endocrinology 149: 5972-5983, 2008)