Rosmarinic acid sensitizes cell death through suppression of TNF-α-induced NF-κB activation and ROS generation in human leukemia U937 cells

Because tumor necrosis factor-alpha (TNF-α) is well-known to induce inflammatory responses, thus its clinical use is limited in cancer treatment. Rosmarinic acid (RA), a naturally occurring polyphenol flavonoid, has been reported to inhibit TNF-α-induced NF-κB activation in human dermal fibroblasts. However, the precise mechanisms of RA have not been well elucidated in TNF-α-mediated anti-cancer therapy. In this study, we found that RA treatment significantly sensitizes TNF-α-induced apoptosis in human leukemia U937 cells through the suppression of nuclear transcription factor-kappaB (NF-κB) and reactive oxygen species (ROS). Activation of caspases in response to TNF-α was markedly increased by RA treatment. However, pretreatment with the caspase-3 inhibitor, z-DEVD-fmk, was capable of significantly restoring cell viability in response to combined treatment. RA also suppressed NF-κB activation through inhibition of phosphorylation and degradation of IκBα, and nuclear translocation of p50 and p65. This inhibition was correlated with suppression of NF-κB-dependent anti-apoptotic proteins (IAP-1, IAP-2, and XIAP). RA treatment also normalized TNF-α-induced ROS generation. Additionally, ectopic Bcl-2 expressing U937 reversed combined treatment-induced cell death, cytochrome c release into cytosol, and collapse of mitochondrial potential. These results demonstrated that RA inhibits TNF-α-induced ROS generation and NF-κB activation, and enhances TNF-α-induced apoptosis.