alpha-Lipoic acid prevents p53 degradation in colon cancer cells by blocking NF-kappa B induction of RPS6KA4

Abstract

alpha-Lipoic acid (alpha-LA) is a biogenic antioxidant that has been used successfully in the treatment of diabetic polyneuropathy and its application to many oxidative stress-associated chronic diseases has increased. In this study, we investigated the effect of alpha-LA on colorectal cancer cell growth and its underlying mechanism. alpha-LA treatment resulted in a marked reduction in the growth of HCT116 colon cancer cells in a dose-dependent manner through the G(1) arrest of the cell cycle and apoptosis induction. alpha-LA treatment significantly increased tumor cell response to various apoptotic stresses, such as etoposide, 5-fluorouracil, UVC, gamma-irradiation, hypoxia, and tumor necrosis factor alpha(TNF alpha). Interestingly, alpha-LA increased p53 protein stability and its apoptosis-enhancing effect was more evident in wild-type p53-carrying cells compared with p53-deficient cells, suggesting that the proapoptotic role of alpha-LA is associated with its p53-stabilizing function. On the basis of our microarray data showing alpha-LA downregulation of the ribosomal protein p90S6K (RPS6KA4), which has been reported to inhibit p53 function, we tested whether alpha-LA regulation of RPS6KA4 is associated with its proapoptotic function. alpha-LA treatment led to a marked reduction in the RPS6KA4 mRNA level in multiple colorectal cancer cells and restoration of RPS6KA4 expression markedly attenuated alpha-LA induction of apoptosis in a p53-dependent manner. In addition, we observed that RPS6KA4 expression is activated by TNF alpha whereas both basal and TNF alpha induction of RPS6KA4 are inhibited by the nuclear factor-kappa B (NF-kappa B) inhibitor BAY11-7082 or transfection of a dominant-negative mutant of NF-kappa B, indicating that NF-kappa B plays a crucial role in RPS6KA4 gene expression. Finally, we found that alpha-LA exerts an inhibitory effect on the nuclear translocation of NF-kappa B triggered by TNF alpha. Collectively, our study shows that alpha-LA suppresses colorectal tumor cell growth at least partially by preventing RPS6KA4-mediated p53 inhibition through blockade of NF-kappa B signaling. (c) 2013 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.