Induction of Apoptosis in Estrogen Receptor-Negative Breast Cancer Cells by Natural and Synthetic Cyclopentenones: Role of the I B Kinase/Nuclear Factor- B Pathway

Nuclear factorB (NFB), a transcription factor with a critical role in promoting inflammation and cell survival, is constitutively activated in estrogen-receptor (ER)-negative breast cancer and is considered a potential therapeutic target for this type of neoplasia. We have previously demonstrated that cyclopentenone prostaglandins are potent inhibitors of NFB activation by inflammatory cytokines, mitogens, and viral infection, via direct binding and modification of the subunit of the I B kinase complex (IKK). Herein, we describe the NFB-dependent anticancer activity of natural and synthetic cyclopentenone IKK inhibitors. We demonstrate that the natural cyclopentenone 15-deoxyprostaglandin J2 (15d-PGJ2) is a potent inhibitor of constitutive I B-kinase and NFB activities in chemotherapy-resistant ER-negative breast cancer cells. 15d-PGJ2-induced inhibition of NFB function is rapidly followed by down-regulation of NFB-dependent antiapoptotic proteins cIAPs 1/2, Bcl-XL, and cellular FLICE-inhibitory protein, leading to caspase activation and induction of apoptosis in breast cancer cells resistant to treatment with paclitaxel and doxorubicin. We then demonstrate that the cyclopentenone ring structure is responsible for these activities, and we identify a new synthetic cyclopentenone derivative, 3-tert-butyldimethylsilyloxy-5-(E)-iso-propylmethylenecyclopent-2-enone (CTC35), as a potent NFB inhibitor with proapoptotic activity in ER-negative breast cancer cells. The results open new perspectives in the search for novel proapoptotic molecules effective in the treatment of cancers presenting aberrant NFB regulation. Apoptosis is an essential physiological process required for embryonic development, immune system function, and the maintenance of tissue homeostasis in multicellular organisms (Hengartner, 2000). When the decision to undergo apoptosis is made in response to physiological signals, a proteolytic cascade involving different caspases is triggered in the suicidal cell, which ultimately results in activation of nucleases that degrade chromosomal DNA (Hengartner, 2000). One of the major regulators of life or death decisions is the nuclear factorB (NFB) (Karin and Lin, 2002; Karin et al.,