Mechanisms of the inhibitory effects of epigallocatechin-3 gallate on platelet-derived growth factor-BB-induced cell signaling and mitogenesis

An enhanced activity of receptor tyrosine kinases (RTKs), such as the platelet-derived growth factor (PDGF) alpha-receptor (PDGF-Ralpha) or the PDGF beta-receptor (PDGF-Rbeta), is involved in the development of proliferative diseases. We have previously demonstrated that green tea catechins containing a galloyl group in the third position of the catechin structure interfere with PDGF-BB-induced mitogenic signaling pathways by inhibiting tyrosine phosphorylation of the PDGF-Rbeta. However, the underlying cellular and molecular mechanisms are unknown. Using human vascular smooth muscle cells (VSMC) and porcine endothelial cells (AEC) stably transfected with PDGF-Ralpha and -beta, respectively, we demonstrate that EGCG preferably inhibited PDGF-BB isoform-mediated signal transduction pathways and cell proliferation. To elucidate cellular and molecular mechanisms of the inhibitory effects of EGCG, we studied the distribution of incorporated EGCG into cellular compartments after subcellular fractionation. Interestingly, most (85%) of the EGCG was found in the cytoplasmic fraction, whereas only ~2% was found within the cell plasma membranes. However, no alteration of membrane fluidity has been observed after treatment of VSMC with 50 microM EGCG. Binding studies with [125I]-PDGF-BB on EGCG-treated VSMC demonstrated that the specific binding of PDGF-BB was completely abolished. Moreover, when [125I]-PDGF-BB was incubated with VSMC in the presence of EGCG, a 50% reduction of cellular [125I]-PDGF-BB binding was observed. Our findings suggest that plasma membrane incorporated EGCG or soluble EGCG directly interacts with PDGF-BB, thereby preventing specific receptor binding.