Pentagalloylglucose (5GG), which is isolated from medicinal plants, can arrest the cell cycle at G1 phase through downregulation of cyclin-dependent kinases 2 and 4, and upregulation of the cyclin-dependent kinase inhibitors p27Kip1 and p21Cip1/WAF1 in human breast cancer cells. 5GG also induces apoptosis in human leukemic cells. Yet the mechanisms by which 5GG induces these effects is unclear. We now show that 5GG inhibits the activities of purified 20S and 26S proteasomes in vitro, the 26S proteasome in Jurkat T cell lysates, and chymotrypsin-like activity of the 26S proteasome in intact Jurkat T cells. The turnover of p27Kip1 and p21Cip1/WAF1, which is necessary for cell cycle progression mediated by proteasome degradation, was disrupted by treatment of human Jurkat T cells with 5GG. This was shown by cycloheximide treatment and in vivo pulse-chase labeling experiments and this effect correlated with the arrest of proliferation of Jurkat T cells at G1. Inhibition of the proteasome by 5GG and by the proteasome inhibitor MG132 caused accumulation of ubiquitin-tagged proteins in Jurkat T cells. Addition of 5GG to Jurkat T cells enhanced the stability of the proteasome substrate Bax, and increased cytochrome C release and apoptosis. Our findings suggest a mechanism for the effect of 5GG on the cell cycle in human leukemic cells - that 5GG downregulates proteasome-mediated pathways because it is a proteasome inhibitor.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]