Helenalin bypasses Bcl-2-mediated cell death resistance by inhibiting NF-κB and promoting reactive oxygen species generation

Abstract

Evasion of cell death by overexpression of anti-apoptotic proteins, such as Bcl-2, is commonly observed in cancer cells leading to a lack of response to chemotherapy. Hence, there is a need to find new chemotherapeutic agents that are able to overcome chemoresistance mediated by Bcl-2 and to understand their mechanisms of action. Helenalin, a sesquiterpene lactone (STL), induces cell death and abrogates clonal survival in a highly apoptosis-resistant Bcl-2 overexpressing Jurkat cell line as well as in two other Bcl-2 overexpressing solid tumor cell lines (mammary MCF-7; pancreatic L6.3pl). This effect is not achieved by directly affecting the mitochondria-protective function of Bcl-2 in the intrinsic pathway of apoptosis since Bcl-2 overexpressing Jurkat cells do not show cytochrome c release and dissipation of mitochondrial membrane potential upon helenalin treatment. Moreover, helenalin induces an atypical form of cell death with necrotic features in Bcl-2 overexpressing cells, neither activating classical mediators of apoptosis (caspases, AIF, Omi/HtrA2, Apaf/apoptosome) nor ER-stress mediators (BiP/GRP78 and CHOP/GADD153), nor autophagy pathways (LC3 conversion). In contrast, helenalin was found to inhibit NF-κB activation that was considerably increased in Bcl-2 overexpressing Jurkat cells and promotes cell survival. Moreover, we identified reactive oxygen species (ROS) and free intracellular iron as mediators of helenalin-induced cell death whereas activation of JNK and abrogation of Akt activity did not contribute to helenalin-elicited cell death. Our results highlight the NF-κB inhibitor helenalin as a promising chemotherapeutic agent to overcome Bcl-2-induced cell death resistance.