Green tea polyphenol (–)-epigallocatechin-3-gallate promotes the rapid protein kinase C- and proteasome-mediated degradation of Bad: implications for neuroprotection

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The aim of the present study was to gain a deeper insight into the cell signaling pathways involved in the neuroprotection/neurorescue activity of the major green tea polyphenol (–)-epigallocatechin-3-gallate (EGCG). EGCG (1 μM) caused an immediate (30 min) down-regulation (∼40%) of Bad protein levels, and a more pronounced reduction after 24 h (55%) in the human neuroblastoma cell line SH-SY5Y. Co-treatment with EGCG and the protein synthesis inhibitor cycloheximide prominently shortened Bad half-life, with as little as 30% of the Bad protein content remaining after 2 h, suggesting an effect of EGCG on Bad protein degradation. Accordingly, the proteasome inhibitors MG-132 and lactacystin damped Bad down-regulation by EGCG. The general protein kinase C (PKC) inhibitor GF109203X, or the down-regulation of conventional and novel PKC isoforms, abolished EGCG-induced Bad decline. However, no inhibition was seen with the cell-permeable myristoylated pseudosubstrate inhibitor of the atypical PKCζ isoform. The enforced expression of Bad for up to 72 h rendered the cells more susceptible to serum deprivation-induced cell death, whereas EGCG treatment significantly improved cell viability (up to 1.6-fold). The present study reveals a novel pathway in the neuroprotective mechanism of the action of EGCG, which involves a rapid PKC-mediated degradation of Bad by the proteasome.

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