Abstract
The stimulatory and inhibitory effects of epigallocatechin-3-gallate (EGCG) and its related two compounds (luteolin and quercetin) on the phosphorylation of four proteins [bovine myelin basic protein (bMBP), human recombinant tau protein (hrTP), human recombinant vimentin (hrVM) and rat collapsin response mediator protein-2 (rCRMP-2)] by glycogen synthase kinase-3β (GSK-3β) were comparatively determined in vitro. We found that (i) EGCG, not quercetin and luteolin, highly stimulated the GSK-3β-mediated phosphorylation of hrTP and significantly stimulated the phosphorylation of bMBP and hrVM by the kinase; (ii) these three polyphenols inhibited dose-dependently the phosphorylation of rCRMP-2 by GSK-3β; (iii) only EGCG significantly enhanced autophosphorylation of GSK-3β; and (iv) EGCG had a binding-affinity with two basic proteins (bMBP and hrTP) and a low affinity with rCRMP-2 rather than hrVM in vitro. In addition, the binding of EGCG to these two basic proteins induced to highly stimulate their phosphorylation, including novel potent sites for GSK-3β, and to significantly reduce the K(m) value and increase the V(max) value of these two substrate proteins for the kinase in vitro. These results provided here suggest that EGCG acts as an effective stimulator for the GSK-3β-mediated phosphorylation of its binding proteins containing EGCG-inducible phosphorylation sites for the kinase in vitro.
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