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Overexpression of inhibitors of apoptosis (IAP) family members in human breast and ovarian cancer models of non-MDR1 taxane resistance

Four breast cancer cell lines (MCF-7, BT-549, MDA-MB-231 and T-47D) and four ovarian cancer cell lines (1A9/A2780, ES-2, MES-OV and OVCAR-3) were selected for taxane resistance by exposing cells to either docetaxel or paclitaxel in the presence of the P-glycoprotein inhibitor, PSC-833 (valspodar, 2 μM). All of these co-selected variants are MDR1/ABCB1(-), and the resistance to taxanes is not transporter-mediated. We have previously reported elevated class III β-tubulin (TUBB3), reduced BRCA1, elevated CDKN1A (p21), and altered epithelial-mesenchymal transition (EMT) genes in the majority of the non-MDR1 taxane variants. Inhibitors of apoptosis (IAP) proteins directly bind and inhibit several caspases, and may play a critical role in determining cell fate after exposure to chemotherapeutic agents. In this study, we profiled the expression of IAP proteins and found elevated content in this panel of taxane-resistant human breast and ovarian cancer cell lines. In both the paclitaxel- (TP) and docetaxel-selected (TxTP) ovarian cancer cell lines we observed significantly overexpressed cIAP1 (BIRC2), cIAP2 (BIRC3), XIAP (BIRC4), and Livin (BIRC7) relative to parental cell lines. Expression of cIAP2 was undetectable in OVCAR-3 and its taxane resistant cell lines, OVCAR-3/TP20 and OVCAR-3/TxTP5. Elevated cIAP1 and XIAP levels were detected in the human breast cancer variant BT-549/TxTP50, and Livin was overexpressed in all taxane-resistant breast cancer cell models. Survivin (BIRC5) is highly expressed in cancers and has been associated with taxane resistance via its effects on apoptosis and the cell cycle. We observed reduced Survivin content in the majority of the variants established in our laboratory except for the ovarian ES-2/TP80 cell line where we found slightly elevated expression resulting from paclitaxel selection relative to its parental control. In addition to these alterations in IAP content, we observed elevated Bcl-2 in ovarian (MES-OV/TP40 and MES-OV/TxTP50) and breast (MCF-7/TxTP50, BT-549/TxTP50, and MDA231/TxTP50) cancer cell lines at the transcript level by rt-PCR and confirmed by immunoblotting. Specific gene silencing by RNAi and treatment with small molecule inhibitors will test the functional significance of IAP alterations in these models of taxane resistance.

IAP ; apoptosis ; taxane ; non-MDR ; drug resistance

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