N-acetyl-L-cysteine enhances fisetin-induced cytotoxicity via induction of ROS-independent apoptosis in human colonic cancer cells

Oxidative stress or excessive antioxidant levels-caused redox imbalance can alter apoptotic responses, and N-acetyl-L-cysteine (NAC) was able to inhibit H₂O₂-mediated cell death, but unable to prevent apoptosis induced by other chemicals such as etoposide. We now demonstrate that 10 and 20mM NAC, non-toxic concentrations, can enhance fisetin (FIS)-mediated apoptosis in colon cancer cells COLO205. Compared to treatment with FIS alone, combination treatment with NAC increased the expression of cleaved caspase-3 and PAPR protein, and produced greater density of DNA ladders. NAC reduced the mitochondrial membrane potential of FIS-treated COLO205 cells with induction of caspase 9 protein cleavage. DNA ladders induced by FIS+NAC were diminished by adding the caspase 3 inhibitor, DEVD-FMK, and the caspase 9 inhibitor, YVAD-FMK. Combinatorial treatment COLO205 cells with NAC and FIS showed potent inhibition on ERK protein phosphorylation, compared with those from FIS or NAC-treated groups by Western blotting using specific antibodies. Addition of the chemical ERK inhibitors, PD98059 and U0126, significantly inhibited ERK protein phosphorylation, accompanied by induced DNA ladder formation, cleavage of caspase 3 and PARP protein in COLO205 cells. Furthermore, NAC showed an enhancement on a FIS-related chemical chrysin-induced apoptosis of COLO205 cells, and NAC sensitization of colon cancer cells to FIS-induced apoptosis was also identify in colonic cancer cells HCT-116, HT-29, and HCT-15 cells. The evidence to support NAC sensitizing human colon cancer cells to FIS-induced apoptosis was provided, and application of NAC and FIS as a strategy to treat colonic cancer deserved for further in vivo study.