Asselin E, Mills G B, Tsang B K
Department of Obstetrics and Gynaecology, University of Ottawa, Loeb Health Research Institute, The Ottawa Hospital, Ontario, Canada.
Cancer Res. 2001 Mar 1;61(5):1862-8.
Chemoresistance is a major hurdle for successful cancer therapy. Although multiple mechanisms have been implicated to be involved in cisplatin resistance, recent evidence has suggested that X-linked inhibitor of apoptosis protein (XIAP) may be a key determinant in chemosensitivity in ovarian cancer. Cell fate is determined by a balance between cell survival and apoptotic signaling. Whereas phosphatidylinositol 3-kinase (PI 3-K) and XIAP are believed to be important cell survival factors in human ovarian surface epithelial cancer cells, if and how they interact to confer resistance to chemotherapy is not known. In the present study, we have investigated the role of XIAP in the regulation of the PI 3-K/Akt survival pathway in chemosensitive (A2780-s, OV2008, and OVCAR-3) and resistant (A2780-cp) ovarian cancer cell lines and the nature of this interaction in cell death/survival signaling. Cisplatin decreased XIAP protein levels and induced Akt cleavage and apoptosis in chemosensitive, but not in resistant, ovarian cancer cells. Cisplatin also induced cleavage of caspase-9 and caspase-3, a process blocked by XIAP overexpression. Pretreatment of ovarian cancer cells and their whole cell lysate with tetrapeptide inhibitors of caspases in vitro significantly decreased Akt cleavage induced by cisplatin and exogenous active caspase-3. Adenoviral sense XIAP cDNA expression increased XIAP protein levels and increased Akt phosphorylation, indicative of activation of Akt and, likely, of PI 3-K. This was associated with a decrease in cisplatin-induced apoptosis. In a cell line (OVCAR-3) where basal phosphorylated Akt levels were high, XIAP overexpression failed to increase further the level of this phosphoprotein. XIAP down-regulation induced Akt cleavage and apoptosis, and treatment of whole cell lysate with human recombinant active caspase-3 resulted in a similar pattern of Akt cleavage. In the presence of the PI 3-K inhibitor (LY294002), XIAP overexpression failed to block cisplatin-induced apoptosis and to induce Akt phosphorylation, suggesting that the site of action of XIAP is upstream of Akt in this cell survival pathway. Taken together, the results indicate that XIAP prevents apoptosis through a PI 3-K-dependent inhibition of the caspase cascade. These results demonstrate a novel mechanism by which XIAP regulates apoptosis and the possible involvement of the PI 3-K/Akt survival pathway in XIAP-mediated chemoresistance of ovarian cancer cells.
化疗耐药是癌症治疗成功的主要障碍。尽管多种机制被认为与顺铂耐药有关，但最近的证据表明，X连锁凋亡抑制蛋白（XIAP）可能是卵巢癌化疗敏感性的关键决定因素。细胞命运由细胞存活和凋亡信号之间的平衡决定。虽然磷脂酰肌醇3激酶（PI 3-K）和XIAP被认为是人类卵巢表面上皮癌细胞中重要的细胞存活因子，但它们是否以及如何相互作用以赋予化疗耐药性尚不清楚。在本研究中，我们研究了XIAP在化疗敏感（A2780-s、OV2008和OVCAR-3）和耐药（A2780-cp）卵巢癌细胞系中对PI 3-K/Akt存活途径的调节作用以及这种相互作用在细胞死亡/存活信号中的性质。顺铂降低了化疗敏感卵巢癌细胞中XIAP蛋白水平并诱导Akt裂解和凋亡，但在耐药卵巢癌细胞中未出现这种情况。顺铂还诱导了caspase-9和caspase-3的裂解，XIAP过表达可阻断这一过程。在体外，用caspase的四肽抑制剂预处理卵巢癌细胞及其全细胞裂解物可显著降低顺铂和外源性活性caspase-3诱导的Akt裂解。腺病毒正义XIAP cDNA表达增加了XIAP蛋白水平并增加了Akt磷酸化，表明Akt以及可能的PI 3-K被激活。这与顺铂诱导的凋亡减少有关。在基础磷酸化Akt水平较高的细胞系（OVCAR-3）中，XIAP过表达未能进一步增加这种磷酸化蛋白的水平。XIAP下调诱导Akt裂解和凋亡，用人重组活性caspase-3处理全细胞裂解物导致类似的Akt裂解模式。在存在PI 3-K抑制剂（LY294002）的情况下，XIAP过表达未能阻断顺铂诱导的凋亡并诱导Akt磷酸化，这表明在该细胞存活途径中XIAP的作用位点在Akt的上游。综上所述，结果表明XIAP通过PI 3-K依赖性抑制caspase级联反应来预防凋亡。这些结果证明了XIAP调节凋亡的新机制以及PI 3-K/Akt存活途径可能参与XIAP介导的卵巢癌细胞化疗耐药。
