Abstract 1627: Chloroquine-induced inhibition of Arginase-1 promotes the nuclear localization of p53 in colon cancer cell lines

Abstract Emerging studies have identified the overexpression of Arginase-1 (ARG-1) in a variety of human malignancies, including cancer of the colon, leading to the promotion of tumor cell growth and invasion. Inactivation of the p53 tumor suppressor protein is known to involve somatic mutation or binding of viral oncoproteins to the p53 protein. However, several types of malignant and premalignant tissues harbor a genetically wild-type, but transcriptionally inactive, form of p53, often localized in the cytoplasm. The transactivation of the inactive p53 gene is thought to lead to its tumor suppressor function. The present report showed that the treatment of cultured human colon cancer cell lines (HT-29 and HCT-116) with 10 or 20 µM chloroquine (CQ), a known anti-rheumatoid/antimalarial drug, inhibited arginase activity with a concomitant nuclear accumulation of the p53 protein. By using specific antibodies it was demonstrated that the decreased in arginase activity correlated with the reduction of arg-1 levels in the cells. Upon treatment of cells with 20 µM CQ, the levels of arginase activity or arg-1 decreased by 43.5 % or 38.3 % in HT-29 cells and 40.9 or 39.4 % in HCT-116 respectively. Further the treatment of cells with 20 µM CQ for 48 h led to an increased level of p53 in the nucleus in comparison with the control samples (p <0.01). The nuclear:cytosol ratios of p53 levels were 5.3 (control) and 83 (20 µM CQ) in HCT-116 (wild type) cells and 2.6 (control) and 40.9 (20 µM CQ) in the HT-29 (mutant type) cells. These results indicate that the CQ-induced inhibition of arg-1 led to the nuclear accumulation of p53 in colon cancer cells. Thus the inhibition of arg-1 by CQ appears to protect the p53 tumor suppressor function.In an effort to expand our understanding of the molecular link between β-catenin levels, increased nuclear accumulation of p53 and CQ-mediated inhibition of arginase activity, HT-29 cells were co-incubated with CQ (0-50 µM) for 48 h and the level of β-catenin were determined by western blot assay. The results of this study indicate that CQ inhibited the β-catenin protein in a dose dependent manner. This decrease in the β-catenin level corresponded to an increase in the nuclear accumulation of p53 and corresponding diminution of intracellular Arg-1. In conclusion, the data presented herein represent a new and important improvement in understanding the pleiotropic effects of CQ on arg-1, with specific reference to its participation in the enhancement of the nuclear accumulation of p53 with the resultant down regulation of β-catenin protein. Thus this observation that CQ may inhibit the initial step of the arginine-polyamine pathway that results in the enhanced nuclear accumulation of p53 and a concomitant down regulation of β-catenin protein may provide a paradigm for targeted therapy in colorectal cancer chemoprevention and other malignancies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1627. doi:1538-7445.AM2012-1627