Arindam Bhattacharyya

Contribution of p53-mediated Bax transactivation in theaflavin-induced mammary epithelial carcinoma cell apoptosis

Theaflavins, the bioactive flavonoids of black tea, have been demonstrated to inhibit proliferation and induce apoptosis in a variety of cancer cells. However, the contribution of p53 in mammary epithelial carcinoma cell apoptosis by theaflavins remains unclear. It has been reported that p53 triggers apoptosis by inducing mitochondrial outer membrane permeabilization through transcription-dependent and -independent mechanisms. Using wild-type and mutant p53-expressing as well as p53-null cells we found a strong correlation between p53 status and theaflavin-induced breast cancer cell apoptosis. Apoptogenic effect was more pronounced in functional p53-expressing cells in which theaflavins raised p53 protein levels that harmonized with Bax up-regulation and migration to mitochondria. However, in the same cells, when p53-mediated transactivation was inhibited by pifithrin-α, theaflavins not only failed to increase transcription but also to induce apoptosis although p53 up-regulation was not altered. In contrast, Bax over-expression restored back theaflavin-induced apoptosis in pifithrin-α-inhibited/dominant-negative p53-expressing cells. Inhibition of Bax by RNA-interference also reduced theaflavin-induced apoptosis. These results not only indicated the requirement of p53-mediated transcriptional activation of Bax but also its role as down-stream effecter in theaflavin-induced apoptosis. Bax up-regulation resulted in mitochondrial transmembrane potential loss and cytochrome c release followed by activation of caspase cascade. In contrast, mitochondrial translocation of p53 and its interaction with Bcl-2 family proteins or activation of caspase-8 could not be traced thereby excluding the involvement of p53-mediated transcription-independent pathways. Together these findings suggest that in breast cancer cells, p53 promotes theaflavin-induced apoptosis in a transcription-dependent manner through mitochondrial death cascade.

Black tea induces tumor cell apoptosis by Bax translocation, loss in mitochondrial transmembrane potential, cytochrome c release and caspase activation.

Recently the anti‐cancer role of black tea has gained immense importance. Nevertheless, the signaling pathways underlying black tea‐induced tumor cell death are still unknown. Previously we reported that black tea induces Ehrlichs ascites carcinoma (EAC) cell apoptosis by changing the balance between pro‐and anti‐apoptotic proteins. It is now well accepted that many cell death pathways converge at the mitochondria to decrease mitochondrial transmembrane potential (MTP) thereby releasing apoptogenic proteins and resulting in the activation of effecter caspases responsible for the biochemical and morphological alterations associated with apoptosis. The role of pro‐apoptotic protein, Bax, in initiating mitochondrial death cascade has also been established. Here we demonstrate that in culture black tea extract induces EAC apoptosis in a dose‐dependent manner – with IC50 at 100 μg/ml. At this dose, intracellular Bax level increases in EAC followed by its translocation from cytosol to mitochondria resulting in loss in MTP. A search for the downstream pathway further reveals that black tea induces mitochondrial cytochrome c release and activates caspases 9 and 3 by 2 pathways, a) independent of and b) dependent on MTP loss. Interestingly, Black tea‐induced death signal might probably be amplified through mitochondrial membrane depolarization via a feedback activation loop from caspase 3. All these findings indicate that black tea initiates mitochondrial death cascade in EAC cells and thereby results in EAC apoptosis.