A study into the potential role of Survivin localization in resistance to drug-induced apoptosis

Abstract

The aim of this study was to test the hypothesis that diverting the cytoplasmic subcellular localization of the anti-apoptotic form of Survivin to the nucleus would sensitize cancer cells to chemotherapeutics. Apoptosis is a morphologically and biochemically distinctive form of cell death, critical in the maintenance of tissue homeostasis. Caspases are a family of cysteine proteases that have a vital role in the implementation of apoptosis, and their activity is regulated by Inhibitors of Apoptosis Proteins. Recent studies indicate that one such inhibitor, Survivin, may have dual functions that are specific to its cellular location, including suppression of apoptotis (cytosolic) and regulation of cell division (nuclear). Since both apoptosis and proliferation are altered in cancer, identifying whether these roles for Survivin are dependent on its subcellular localization will inform future approaches to treat chemotherapeutically resistant tumour cells. After initially confirming the specificity of several Survivin antibodies, the distribution of Survivin was examined by immunofluorescence microscopy and sub-cellular fractionation in breast cancer cell lines. The threshold of drug-induced apoptosis was compared in cells over-expressing either wildtype Survivin or a form of Survivin unable to exit the nucleus due to a mutation in its nuclear export sequence. Endogenous Survivin localized to both nucleus and cytoplasm of breast cancer cell lines. Over-expressed Survivin had an anti-apoptotic, protective function. In contrast, cells expressing Survivin with the mutated nuclear export sequence had a lower apoptotic threshold to chemotherapeutic drugs. These results demonstrate for the first time that Survivin is localized to both the nucleus and cytoplasm of breast cancer cell lines. Importantly, the sensitivity of cells to chemotherapeutic drugs was increased when Survivin's localization was restricted to the nucleus, consistent with cytoplasmic Survivin having the anti-apoptotic role. Since clinical studies have shown that nuclear Survivin is a positive prognostic factor in breast cancer patients, the data suggest that strategies to alter Survivin distribution may be useful in the fight against cancer. © 2008 The Author(s).