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Chemoresistance of ovarian cancer has been previously attributed to the expression and activation of Bcl-2 family proteins. BH3-mimetic molecules possessing potential anticancer activity are able to inhibit antiapoptotic Bcl-2 family proteins. AT101 (R-(-)-gossypol), a natural BH3-mimetic molecule, has shown anti-tumor activity as a single agent and in combination with standard anticancer therapies in a variety of tumor models. Here, we report the effect of AT101 on apoptosis in cisplatinresistant ovarian cancer cells and identify the major molecular events that determine sensitivity. AT101 induced cell apoptosis by activating Bax through a conformational change, translocation, and oligomerization. The inhibition of Bax expression only partially prevented caspase-3 cleavage. However, the gene silencing of Bax had no effect on mitochondrial Smac release. Further experiments demonstrated that Smac reduction inhibited caspase-3 activation and attenuated cell apoptosis. More importantly, the inhibition of Smac or overexpression of XIAP attenuated Bax activation in ovarian cells. Furthermore, our data indicate that the Akt-p53 pathway is involved in the regulation of Smac release. Taken together, our data demonstrate the role of Smac and the molecular mechanisms of AT101-induced apoptosis of chemoresistant ovarian cancer cells. Our findings suggest that AT101 not only triggers Bax activation but also induces mitochondrial Smac release. Activated Smac can enhance Bax-mediated cellular apoptosis. Therefore, Smac mediates Bax activation to determine the threshold for overcoming cisplatin resistance in ovarian cancer cells. [ABSTRACT FROM AUTHOR]

In this study, we investigated the antitumor activity of a novel coumarin derivative, 5,7-dihydroxy-4-methyl-6-(3-methylbutanoyl)-coumarin (DMAC), on colorectal carcinoma. DMAC treatment resulted in substantial proapoptotic activity against colon cancer HCT116 and LoVo cells. Induction of apoptotic characteristics, including cellular shrinkage, chromatin condensation, and Annexin V detection, was observed following DMAC treatment. Mechanistically, we observed that DMAC elicited induction of proteolytic cascade activation including cleavage of caspase-3 and poly ADP-ribose polymerase (PARP) expression and loss of the antiapoptotic proteins, Mcl-1 and Bcl-XL, accompanied by an increase in expression of the proapoptotic protein, Bak. In addition, suppressing c-Jun N-terminal protein kinase (JNK), but not extracellular-regulated protein kinase (ERK) or p38, substantially diminished DMAC-induced cell death and caspase-3 and PARP cleavage. However, pretreatment with antioxidants, including N-acetyl-l-cysteine (NAC) and diphenylene iodonium (DPI), failed to protect against DMAC-elicited apoptosis. Pretreatment with the JNK inhibitor, SP600125, suppressed DMAC-induced JNK phosphorylation, which was accompanied by a reversal of Bcl-XL expression. Moreover, combining DMAC treatment with the conventional anticancer drugs, 5-FU and CPT-11, considerably enhanced their therapeutic efficacies. Structural-activity relationship analyses further revealed that an alkylation substitution at position 6 of the coumarin ring was critical for inducing apoptosis, and the phenyl group at position 4 might have enhanced its bioactivity. Our data showed that DMAC can be used as part of a promising strategy to enhance therapeutic efficacies, and could be used to develop an approach for structure-based drug design for cancer treatment. [ABSTRACT FROM AUTHOR]