Abstract
This study demonstrates that combined treatment with subtoxic doses of Codium extracts (CE), a flavonoid found in many fruits and vegetables, and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), induces apoptosis in TRAIL-resistant colorectal cancer (CRC) cells. Effective induction of apoptosis by combined treatment with CE and TRAIL was not blocked by Bcl-xL overexpression, which is known to confer resistance to various chemotherapeutic agents. While TRAIL-mediated proteolytic processing of procaspase-3 was partially blocked in various CRC cells treated with TRAIL alone, co-treatment with CE efficiently recovered TRAIL-induced caspase activation. We observed that CE treatment of CRC cells did not change the expression of anti-apoptotic proteins and pro-apoptotic proteins, including death receptors (DR4 and DR5). However, CE treatment markedly reduced the protein level of the short form of the cellular FLICE-inhibitory protein (c-FLIPS), an inhibitor of caspase-8, via proteasome-mediated degradation. Collectively, these observations show that CE recovers TRAIL sensitivity in various CRC cells via down-regulation of c-FLIPS.