The Role of Metallothioneins in the Differential Pro‐oxidant Effects of (‐)‐Epigallocatechin‐3‐gallate (EGCG) in Oral Cells

Abstract

The tea catechin, (‐)‐Epigallocatechin‐3‐gallate (EGCG), has been shown to exert cancer preventive effects. We and others have found that EGCG induces oxidative stress in oral cancer cells but causes antioxidant responses in normal cells. Here, we examined the role of antioxidant protein metallothionein (MT) in the differential pro‐oxidant effects of EGCG. qPCR analysis showed that EGCG significantly down‐regulated several MT isoforms (MT1E, F, G, X and MT2A) in oral cancer cells (SCC‐25), but up‐regulated several MTs in normal gingival fibroblasts (HGF‐1), particularly MT1G (209‐fold) and MT1X (3.5‐fold). We have previously reported that EGCG suppressed sirtuin 3 (SIRT3), a mitochondrial redox regulator, in cancer cells resulting in oxidative stress and apoptosis. Here, we unexpectedly found that siRNA‐mediated knock‐down of SIRT3 in cancer cells significantly up‐regulated MT1G, MT1X and metal regulated transcription factor 1 (MTF1), reduced oxidative stress, and enhanced cell growth. By contrast, knocking down MTF1 in cancer cells suppressed the mRNA expression of MT1G, MT1X and SIRT3, and increased cell susceptibility to EGCG‐induced growth inhibition. These results suggest MT is a potential target underlying EGCG's selective cytotoxicity, and demonstrate a cross‐talk between MT and SIRT3 signaling which may be mediated by MTF1. (Supported by AICR grant and Penn State University)