Riproximin (Rpx) is a type II ribosome inactivating protein, which was extracted and purified from the seeds of Ximenia americana. Previous studies demonstrated cytotoxicity of Rpx against a variety of cell lines originating from solid and non-solid cancers. In this study, we investigated the mechanistic aspects of Rpx in selected human and rat colorectal cancer (CRC) cell lines. Cytotoxic levels of Rpx were determined by MTT assay, while cytostatic and apoptotic effects were investigated by flow cytometry and nuclear staining procedures. Effects of Rpx exposure on colony formation/migration of CRC cells and expressional modulations in anticancer/stress-related genes were also studied. Rpx showed significant and comparable levels of cytotoxicity in CRC cells as determined by inhibitory concentration (IC) values. Similar inhibitory effects were found for clonogenicity, while more pronounced inhibition of migration was observed in response to Rpx exposure. Profound arrest in S phases of the cell cycle was noted especially in primary CRC cells. Apoptotic effects were more prominent in rat CRC cells as indicated by Annexin V-FITC assay and Hoechst 33342 nuclear staining. Rpx exposure induced significantly increased levels of the IL24/MDA-7, a well characterized anticancer gene, in all CRC cells. In addition, following Rpx treatment, high expression levels of growth arrest and DNA damage (GADD family) genes were also observed. Increased expression of two additional GADD genes (34 and 153) only in rat CRC cells (CC531) conferred higher sensitivity towards Rpx and subsequent anti-proliferative/apoptotic effects as compared to human CRC cells (SW480 and SW620). The present investigation indicates the anticancer potential of Rpx in CRC and favor further evaluation of this natural compound as therapeutic agent.
