Thioredoxin Reductase As A Cellular Target For The Anticancer Activity Of Natural Compounds

Mammalian thioredoxin reductase (TrxR) is a selenium-containing enzyme, and plays vital roles in regulating cellular redox balance as well as redox-mediated signal transduction. Primarily via interacting with its native substrate thioredoxin (Trx), TrxR controls cell proliferation, differention, and death. In the past years, accumulating evidence supports that overactivation of TrxR is closely related to tumorigenesis, suggesting that TrxR is a potential target for cancer chemotherapy. In this thesis, we investigated the interaction of some natural compounds with potent anticancer activity, including gambogic acid (GA), shikonin (SHK), and parthenolide (PTL) with TrxR in vitro and in cells. The main contents and results are as follows:1. A brief review of the structure and function of TrxR was presented in Chapter1. The recently reported TrxR inhibitors were also summarized.2. GA demonstrates potent anticancer activity in numerous types of human cancer cells, and has entered phase II clinical trials. We discovered that GA may interact with TrxR to elicit oxidative stress, and eventually induce apoptosis in human hepatocellular carcinoma SMMC-7721cells. GA primarily targets the Sec residue in the antioxidant enzyme TrxR to inhibit its Trx-reduction activity, leading to accumulation of reactive oxygen species (ROS) and collapse of the intracellular redox balance. Importantly, overexpression of functional TrxRl in cells attenuates the cytotoxicity of GA, while knockdown of TrxRl sensitizes cells to GA. Targeting TrxR by GA thus discloses a previously unrecognized mechanism underlying the biological action of GA, and provides a depth insight for further developing GA as potential agent in treatment of cancer.3. SHK, a major active component of the Chinese herbal plant Lithospermum erythrorhizon, has been applied for centuries in traditional Chinese medicine. Although shikonin demonstrates potent anticancer efficacy in numerous types of human cancer cells, the cellular targets of shikonin have not been fully defined. We reported here that SHK may interact with the cytosolic thioredoxin reductase (TrxRl) to induce ROS-mediated apoptosis in human promyelocytic leukemia HL-60cells. SHK primarily targets the Sec residue in TrxRl to inhibit its physiological function, but further shifts the enzyme to an NADPH oxidase to generate superoxide anions, which leads to accumulation of ROS and collapse of the intracellular redox balance. Importantly, overexpression of functional TrxRl attenuates the cytotoxicity of SHK, while knockdown of TrxRl sensitizes cells to SHK treatment. Targeting TrxR1by SHK thus discloses a previously unrecognized mechanism underlying the biological activity of SHK, and provides an in-depth insight in understanding the action of SHK in treatment of cancer.4. PTL, an active ingredient from the traditional medical herb feverfew (Tanacetum parthenium), belongs to the family of sesquiterpene lactones containing a reactive a-methykene-y-lactone ring and epoxide group, which could potentially conjugate protein thiols through a Michael addition reaction. We have demonstrated that PTL is a potent TrxR inhibitor in vitro, and induce apoptosis by inhibition of TrxR activity in HeLa cells. Interestingly, knockdown of TrxRl sensitizes cells to PTL treatment in HeLa cells. These results showed that PTL significantly suppresses cell growth in parallel with direct inhibition of TrxRl activity. Therefore, the TrxR inhibition could be considered as a novel mechanism contributing to aiticancer effects of PTL.