Structure-activity relationships of synthetic analogs of (-)-epigallocatechin-3-gallate as proteasome inhibitors

Background: Cancer-related molecular targets of green tea polyphenols, such as (-)-epigallocatechin-3-gallate [(-)-EGCG], remain unknown. We previously showed that ()-EGCG is a potent and specific inhibitor of the proteasomal chymotrypsin-like activity in vitro and in vivo. Materials and Methods: EGCG amides and five simple analogs were prepared by enantioselective synthesis. Proteasome inhibition in vitro was measured by fluorogenic substrate assay and in vivo by accumulation of proteasome target proteins (p27, IkappaB-alpha and Bax). Inhibition of tumor cell proliferation was determined by G(1) arrest, DNA fragmentation and colony formation inhibition. Results: EGCG analogs with modifications in the A-ring, C-ring or ester bond inhibit the chymotrypsin-like activity of purified 20S proteasome with altered potencies. However; these compounds were able to potently inhibit the proteasome activity in vivo and also suppress colony formation of prostate cancer LNCaP cells. Some compounds caused G, arrest and DNA fragmentation in leukemia Jurkat T cells. However, these EGCG analogs caused no or little proteasome inhibition in normal or nontransformed cells. Conclusion: The A-ring and gallate ester/amide bond are essential for the proteasome-inhibitory function of (-)-EGCG.