Molecular mechanism of black tea polyphenols induced apoptosis in human skin cancer cells: involvement of Bax translocation and mitochondria mediated death cascade

Theaflavins (TF) and thearubigins (TR) are the most exclusive polyphenols of black tea. Even though few previous reports showed the anticancer effects of TF through apoptosis, the potential effect of TR has not been appraised. This study investigated the induction of apoptosis in human skin cancer cells after treatment of TF and TR. We report that both TF and TR could exert inhibition of A431 (human epidermoid carcinoma) and A375 (human malignant melanoma) cell proliferation without adversely affecting normal human epidermal keratinocyte cells. Growth inhibition of A375 cells occurred through apoptosis, as evident from cell cycle arrest at G(0)/G(1) phase, increase in early apoptotic cells, externalization of phosphatidylserine and DNA fragmentation. In our pursuit to dissect the molecular mechanism of TF- and TR-induced apoptosis in A375 cells, we investigated whether cell death is being mediated by mitochondria. In our system, Bax translocation to mitochondria persuaded depolarization of mitochondrial membrane potential, cytochrome c release in cytosol and induced activation of caspase-9, caspase-3 and poly (ADP-ribose) polymerase cleavage. Our intricate investigations on apoptosis also explained that TF and TR augmented Bax:Bcl2 ratio, up-regulated the expression of p53 as well as p21 and inhibited phosphorylation of the cell survival protein Akt. Furthermore, TF and TR elicited intracellular reactive oxygen species generation in A375 cells. These observations raise speculations that TF as well as TR might exert chemopreventive effect through cell cycle arrest and induction of apoptogenic signals via mitochondrial death cascade in human skin cancer cells.