Inhibition of activator protein 1 activity and cell growth by purified green tea and black tea polyphenols in H-ras-transformed cells: Structure-activity relationship and mechanisms involved

Ras gene mutation, which perpetually turns on the growth signal transductio n pathway, occurs frequently in many cancer types. The mouse epidermal JB6 cell line has been transfected with a mutant H-ras gene to mimic carcinogen esis in vitro. These transformed cells (30.7b Ras 12) are able to grow in s oft agar, exhibiting anchorage independence and high endogenous activator p rotein 1 (AP-1) activity, which can be detected by a stable AP-1 luciferase reporter. The present study investigated the ability of different pure gre en and black tea polyphenols to inhibit this res signaling pathway. The maj or green tea polyphenols (catechins), (-)-epigallocatechin-3-gallate (EGCG) , (-)-epigallocatechin, (-)-epicatechin3-gallate, (-)-epicatechin, and thei r epimers, and black tea polyphenols, theaflavin, theaflavin-3-gallate, the aflavin-3'-gallate, and theaflavin-3,3'-digallate (TFdiG), were compared wi th respect to their ability to inhibit the growth of 30.7b Ras 12 cells and AP-1 activity. All of the tea polyphenols except (-)-epicatechin showed st rong inhibition of cell growth and AP-1 activity. Among the catechins, both the galloyl structure on the B ring and the gallate moiety contributed to the growth inhibition and AP-1 activity; the galloyl structure appeared to have a stronger effect on the inhibitory action than the gallate moiety. Th e epimers of the catechins showed similar inhibitory effects on AP-1 activi ty, The addition of catalase to the incubation of the cells with EGCG or TF diG did not prevent the inhibitory effect on AP-1 activity, suggesting that H2O2 does not play a significant role in the inhibition by tea polyphenols . Both EGCG and TFdiG inhibited the phosphorylation of p44/42 (extracellula r signal-regulated kinase 1 and 2) and c-jun without affecting the levels o f phosphorylated-c-jun-NH2-terminal kinase. TFdiG inhibited the phosphoryla tion of p38, but EGCG did not, EGCG lowered the level of c-jun, whereas TFd iG decreased the level of fra-1. These results suggest that tea polyphenols inhibited AP-1 activity and the mitogen-activated protein kinase pathway, which contributed to the growth inhibition; however, different mechanisms m ay be involved in the inhibition by catechins and theaflavins.