title: Direct inhibition of the ubiquitin-proteasome pathway by ester bond-containing green tea polyphenols is associated with increased expression of sterol regulatory element-binding protein 2 and LDL receptor.
abstract: Green tea has been shown to lower plasma cholesterol, associated with up-regulation of the low-density lipoprotein receptor (LDLR) although the responsible molecular mechanism is unknown. Previously, we reported that ester bond-containing green tea polyphenols (GTPs), such as (-)-epigallocatechin-3-gallate [(-)-EGCG], potently inhibit the tumor cellular proteasome activity, which may contribute to the cancer-preventative effect of green tea. In the current study, we hypothesize that the proteasome is a heart disease-associated molecular target of GTPs. We have shown that ester bond-containing GTPs, including (-)-EGCG, potently inhibit the proteasomal activity in intact hepatocellular carcinoma HepG2 and cervical carcinoma HeLa cells, as evident by accumulation of ubiquitinated proteins and three natural proteasome targets (p27, IkappaB-alpha and Bax). (-)-EGCG selectively inhibits the chymotrypsin-like, but not trypsin-like, activity of the proteasome. Associated with proteasome inhibition by ester bond-containing GTPs, there was a significant, time- and concentration-dependent increase in levels of the cleaved, activated, but not the precursor, form of sterol regulatory element-binding protein 2 (SREBP-2), an essential factor for LDLR transcription. Subsequently, LDL receptor expression was increased dramatically in HepG2 and HeLa cells treated with (-)-EGCG. Our results suggest that ester bond-containing GTPs inhibit ubiquitin/proteasome-mediated degradation of the active SREBP-2, resulting in up-regulation of LDLR. This identified molecular mechanism may be related to the previously reported cholesterol-lowering and heart disease-preventative effects of green tea.
authors.affiliation: Drug Discovery Program, H Lee Moffitt Cancer Center and Research Institute, University of South FLorida, Tampa, FL 33612, USA.
authors.forename: Deborah J
authors.initials: DJ
authors.lastname: Kuhn
authors.forename: Audrey C
authors.initials: AC
authors.lastname: Burns
authors.forename: Aslamuzzaman
authors.initials: A
authors.lastname: Kazi
authors.forename: Q Ping
authors.initials: QP
authors.lastname: Dou
mesh_terms: D028241:Camellia sinensis / Q000378:metabolism; D002392:Catechin / Q000031:analogs & derivatives* / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D004952:Esters; D006367:HeLa Cells; D006801:Humans; D009097:Multienzyme Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D011973:Receptors, LDL / Q000235:genetics / Q000378:metabolism*; D051782:Sterol Regulatory Element Binding Protein 2; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D025801:Ubiquitin / Q000378:metabolism*
chemical_list: D004268:DNA-Binding Proteins; D004952:Esters; D009097:Multienzyme Complexes; D011973:Receptors, LDL; C497390:SREBF2 protein, human; D051782:Sterol Regulatory Element Binding Protein 2; D014157:Transcription Factors; D025801:Ubiquitin; D002392:Catechin; C045651:epigallocatechin gallate; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex