Abstract B34: Bortezomib induces G2-M arrest through ROS-inducible phosphorylation of ATM-CHK1 in human colon cancer cells

Abstract Colorectal cancer (CRC) is one of the most common cancer types, however, newer drug development has reached a plateau at present. Bortezomib (PS-341, Velcade®) is a proteasome inhibitor and approved in the treatment of hematologic malignancies including multiple myeloma. A few trials of bortezomib alone or combination chemotherapy for CRC patients have been reported, however, the results were not so satisfactory, and it might be due to the lack of understanding on the action mechanism. In this report, we suggest the possibility of using bortezomib as a proteasome inhibitor in human colon cancer cells. Although bortezomib has been known to induce apoptosis and cell cycle arrest in various human cancer cells, the inhibitory mechanism of it was not clear. First, bortezomib induced G2-M arrest. Its treatment causes an increase of intracellular reactive oxygen species (ROS) levels and activation of ATM-CHK1 pathway, but not ATR. Also, activation of that pathway led to inactivation of cdc2. Moreover, treatment with NAC, ROS scavenger, inhibited phosphorylation of ATM and subsequent to decrease of cell number at G2-M phase in response to bortezomib indicating that the increased level of ROS after exposure to bortezomib is a result of ATM phosphorylation. In addition, knockdown of endogenous ATM by RNA interference induced the decreased sensitivity to bortezomib. Therefore, these results suggest that bortezomib induces G2-M arrest through ROS-inducible ATM phosphorylation and shows antitumor activity in human colon cancer cells, implying a potential possibility of further investigations in the treatment of CRC patients. Acknowledgement: This study was supported by grants from the Korea Health 21 R&D Project, Ministry of Health and Welfare and Family Affairs, Korea (A062254).