In this report, we showed that steamed ginseng extract (S4h) treatment induced mitochondrial damage, ROS accumulation, and apoptosis in colorectal cancer cells. We further characterized the relationship between the effects of S4h and summarized our results in a model (). Our results show that S4h-induced apoptosis in colorectal cancer cells is, at least in part, mediated through the mitochondrial pathway. In addition, S4h treatment also induced ROS accumulation in these colorectal cancer cells, which protected these cells from undergoing apoptosis, at least in part, by activation of the NF-κB pathway.
A working model for American ginseng-induced apoptosis in colorectal cancer cells
This model is supported by the observations that S4h-induced NF-κB reporter activation in colorectal cancer cells was inhibited either by the NF-κB pathway inhibitor PS1145 or by antioxidants such as NAC and Vitamin C, or by the expression of the ROS-scavenging enzyme catalase. In contrast, inhibition of NF-κB activation by PS1145 did not affect S4h-induced ROS (data not shown). These observations support the idea that S4h-induced NF-κB activation is mediated by ROS generation. Furthermore, decreasing the level of ROS by treatment with NAC or expression of catalase had the effect of increasing S4h-induced apoptosis in colorectal cancer cells, similar to inhibition of NF-κB activation by the PS1145 inhibitor, indicating NF-κB activation mediates the key anti-apoptotic effect of ROS in these cells. Finally, protecting the mitochondria by expression of Bcl-XL decreased both S4h-induced ROS and apoptosis, indicating that S4h-induced ROS and apoptosis are at least partially caused by mitochondria damage.
ROS can be induced by a wide variety of extracellular stimuli and can potentially promote either cell survival or cell death. Why will ROS induced by S4h in colorectal cancer cells promote cell survival instead of cell death? While we do not yet know the precise mechanisms involved, previous studies on the distinct roles of NF-κB on cell death or cell survival in response to TNF-α or UV offered some clues [34
]. It was shown that TNF-α activated both JNK signaling and NF-κB. Activation of NF-κB in response to TNF-α signaling blocked the prolonged JNK activation and promotes cell survival. In contrast, UV induced robust and prolonged JNK activation as well as delayed activation of NF-κB. Activation of NF-κB in this case promotes UV-induced cell death by cooperating with the JNK pathway to upregulate the proapoptotic Fas ligand. It should be pointed out that both TNF and UV can also induce ROS, which potentially contribute to the life or death outcome of these stimuli. Therefore whether NF-κB promoted cell survival or death in response to different stimuli is determined by the interactions of the network of pro-survival and pro-death signaling pathways activated by these stimuli. By analogy, it is likely that the interactions of the network of pro-survival and pro-death signaling pathways activated by ginseng and ROS determined the role of ROS in cell survival. Of course further studies will be needed to test this idea and to elucidate the detailed mechanism involved.
The observation that increased level of ROS is mostly caused by S4h-induced mitochondria damage in colorectal cancer cells also explains an apparent paradox: while it was reported that American ginseng contains antioxidants and steaming of American ginseng slightly increased its antioxidant content [35
], treatment of colorectal cancer cells with S4h led to increased cellular ROS due to mitochondrial damage. The observation that steamed ginseng simultaneously activates the apoptosis pathway and the ROS/NF-κB-mediated survival pathway in colorectal cancer cells could potentially provide us with new tools to increase the anti-tumor effect of steamed ginseng by the co-treatment of steamed ginseng with ROS scavengers, such as the commonly-used antioxidants NAC and Vitamin C, or with inhibitors of the NF-κB pathway. In this respect, the anti-tumor effect of steamed ginseng appears to be different from those of conventional cancer therapies such as radiation or doxorubicin. It is generally believed that ROS induced by these conventional anti-cancer treatment promotes cancer cell killing and contributes to the therapeutic effect [37
Both ginseng and antioxidants such as vitamin C are generally safe and commonly used as dietary supplements. Our in vitro observations that antioxidants such as vitamin C can enhance the anticancer activity of ginseng raise the possibility that combinations of ginseng and antioxidant can potentially be used as chemoprevention agents to prevent the development of colorectal cancers. Further studies will be needed to test such possibilities.