Abstract
Shogaols, the major constituents of thermally processed ginger, have been proven to be highly effective anticancer agents. Our group has identified cysteine-conjugated shogaols (M2, M2', and M2″) as the major metabolites of [6]-, [8]-, and [10]-shogaol in human and found that M2 is a carrier of its parent molecule [6]-shogaol in cancer cells and in mice, while being less toxic to normal colon fibroblast cells. The objectives of this study are to determine whether M2' and M2″ behave in a similar manner to M2, in both metabolism and efficacy as anticancer agents, and to further explore the biological pro-apoptotic mechanisms of the cysteine-conjugated shogaols against human colon cancer cells HCT-116 and HT-29. Our results show that [8]- and [10]-shogaol have similar metabolic profiles to [6]-shogaol and exhibit similar toxicity toward human colon cancer cells. M2' and M2″ both show low toxicity against normal colon cells but retain potency against colon cancer cells, suggesting that they have similar activity to M2. We further demonstrate that the cysteine-conjugated shogaols can cause cancer cell death through the activation of the mitochondrial apoptotic pathway. Our results show that oxidative stress activates a p53 pathway that ultimately leads to p53 up-regulated modulator of apoptosis (PUMA) induction and down-regulation of B-cell lymphoma 2 (Bcl-2), followed by cytochrome c release, perturbation of inhibitory interactions of X-linked inhibitor of apoptosis protein (XIAP) with caspases, and finally caspase 9 and 3 activation and cleavage. A brief screen of the markers attenuated by the proapoptotic activity of M2 revealed similar results for [8]- and [10]-shogaol and their respective cysteine-conjugated metabolites M2' and M2″. This study highlights the cysteine-conjugated metabolites of shogaols as novel dietary colon cancer preventive agents.