Targeting Survivin in Cancer Therapy: Fulfilled Promises and Open Questions (2007)

CN05-04 Survivin, a member of the IAP family, is involved in essential functions including cell division, apoptosis, cellular stress response and surveillance checkpoints. To exert its function in cell division, survinin localizes to the mitotic apparatus, and its physical association with INCENP, Aurora B and Borealin/Dasra B is required to target the chromosomal passenger complex to the kinetochores, properly form the bipolar spindle, and complete cytokinesis. Such a function of preservation of genome fidelity and regulation of microtubule dynamics requires a sharp cell-cycle-dependent transcription of the survivin gene during the mitotic phase as well as post-translational modifications. This pathway seems to be dominant in normal cells and constitutes the primary function of survivin in adult tissues. Other non-cell-cycle-dependent mechanisms driving survivin gene transcription independent of mitosis have been described; they involve tissue patterning circuits (Wnt/β-catenin), cytokine activation (STAT3), co-stimulatory messages (OX-40) and pleiotropic signaling mechanisms (AkT, NFkB) that are operative during development and generally upregulated in cancer cells. Such non-cell-cycle-dependent pathways are thought to be dominant in tumors. The fraction of survivin produced through these non-cell-cycle-dependent mechanisms mediates apoptosis inhibition through intermolecular cooperation with cofactors (HBXIB, X-IAP), leading to the formation of complexes that inhibit caspase-9 processing. Moreover, subcellular compartmentalization of survivin in mitochondria seems to play a role in the antiapoptotic function of the protein. A physical interaction with the molecular chaperone Hsp90, which involves the Hsp90 ATPase domain and the survivin BIR domain, was shown to be essential for the stability and function of survivin. It has been recently found that survivin has a nuclear export signal and that in cancer cells the antiapototic and mitotic roles of survivin can be separated through mutation of its nuclear export signal, which abrogates the cytoprotective activity of the protein but still allows mitosis to proceed.  Survivin expression in normal tissues is developmentally regulated, and the protein was found to be absent or low in most terminally differentiated tissues. However, recent studies have attributed a role to survivin in regulating the function of normal adult cells including vascular endothelial cells, hematopoietic progenitor cells, polymorphonuclear cells, T cells and erythroid cells. Although survivin is expressed in normal tissues characterized by self renewal and proliferation, its expression is significantly lower than in transformed cells. In fact, several studies have demonstrated strong survivin expression in most human solid tumor types and hematologic malignancies. Expression of survivin has also been detected in a variety of benign and preneoplastic lesions. Growing evidence suggests that survivin expression in cancer cells is associated with clinicopathologic variables of aggressiveness and may represent a prognostic marker for patient outcome.  There is evidence that survivin plays an important role in the drug-resistant phenotype of human cancer cells. It has been shown that survivin overexpression is associated to taxol resistance. In this context, it has been reported that taxol-induced microtubule stabilization and mitotic arrest increase the expression of survivin, which engenders a cell survival pathway to counteract taxol-induced apoptosis. An increase in survivin expression has also been seen in cancer cells resistant to cisplatin, flutamide and TRAIL. As regards radiation, survivin was found to act as a constitutive radioresistance factor in cancer cells. It is noteworthy that survivin can contribute to radiation resistance also by promoting the survival of tumor vascular endothelial cells.  Considerable effort has been made to validate survivin as a new therapeutic target. In this context, a collection of different approaches to counteract survivin in tumor cells has been proposed with the dual aim to inhibit tumor growth potential and to enhance tumor cell response to apoptosis-inducing anticancer agents. Molecular antagonists including antisense oligonucleotides, ribozymes and small interfering RNAs have been extensively used. Results of the studies consistently showed that survivin inhibition was able to reduce cell proliferation and induce caspase-dependent apoptosis in tumor cell lines of different histologic origin, as well as decrease the formation of new tumors and the growth of already established lesions in nude mice. Moreover, survivin knock-down enhanced the apoptotic response of tumor cells to several stimuli, including treatment with cisplatin, vincristine, doxorubicin, topotecan, 17-allylamino-17-demethoxygeldanamycin, TNF-alpha, APO2L/TRAIL and radiation. In many instances, survivin-depleted cells were delayed in mitosis and accumulated in prometaphase with misaligned chromosomes, or they became polyploid.  Two main gene therapy approaches targeting survivin have been successfully developed. The first one involves the use of the survivin gene promoter to drive the expression of cytotoxic genes in tumor cells, whereas the second one is based on the delivery of dominant-negative survivin mutants to tumor cells. Specifically, transfection/infection of cancer cells with a mutant carrying a cysteine 84_ alanine (Cys84Ala) mutation in the survivin BIR domain or a phosphorylation-defective threonine 34 AE alanine (Thr34Ala) mutant was able to enhance spontaneous and drug-induced apoptosis in several cancer cell lines, as well as inhibit the formation of new tumors and the growth of already established tumors in nude mice.  Small molecule antagonists of survivin have also been developed. In the context of a strategy focused on pharmacologic inhibition of mitotic phosphorylation of survivin on Thr34 to accelerate protein destruction, CDK inhibitors such as purvalanol A, flavopiridol and NU6140 were tested in tumor cells arrested at mitosis with taxol, which induces hyperphosphorylation of survivin on Thr34. Sequential administration of CDK inhibitors resulted in escape from the mitotic block imposed by taxol, marked activation of mitochondrial-dependent apoptosis, and anticancer activity in vivo. The cell-permeable peptidomimetic shepherdin, identified in a structure-based rational screening for antagonists of the survivin-Hsp90 complex, in addition to counteracting survivin-Hsp90 interaction, was able to destabilize several Hsp90 client proteins and induce selective cell death via apoptotic and nonapoptotic mechanisms in tumor cells. Moreover, the systemic administration of shepherdin inhibited human tumor growth in mice without inducing toxicity. More recently, the nonpeptidic small molecule 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) was rationally identified to bind the Hsp90 N-terminal domain, mimicking the chemical and conformational properties of shepherdin. AICAR was shown to destabilize several Hsp90 client proteins in vivo, and to exhibit antiproliferative and proapoptotic activity in multiple tumor cell lines while not affecting proliferation of normal human fibroblasts. Again, YM155, a small molecule that selectively inhibits survivin gene transcription, showed a marked antiproliferative activity and induced tumor regression in lymphoma, prostate cancer and non-small cell lung cancer xenografts.  Based on its immunologic properties, survivin is also an attractive target for novel immunotherapies against cancer. Several phase I trials based on the administration of survivin-directed autologous cytotoxic T lymphocytes or survivin peptides have been completed, and the treatment proved to be safe and devoid of significant adverse effects. Results from studies exploiting different strategies to interfere with survivin have provided direct and convincing evidence that targeting the survivin network inhibits tumor growth potential in vitro and in vivo and increases spontaneous and treatment-induced apoptosis, thus validating survivin as a promising molecular target for cancer therapy. The applicability of survivin-targeted strategies for the clinical treatment of human tumors is currently under investigation, as the first survivin inhibitors have recently entered phase I-II clinical trials. Although the results of these trials are not yet available, survivin inhibitors may represent a novel type of targeted drug, inasmuch as they specifically interfere with defined molecular pathways of tumor cell maintenance and at the same time are applicable to different tumor types independent of their genetic makeup. Although the anti-survivin therapies developed thus far have not shown any major systemic toxicity in animal models and appear to be extremely promising, the possibility that survivin disruption could affect normal cell function, mainly the hematopoietic and immune systems, cannot be excluded. In this context, a better understanding of the effects exerted by survivin on normal versus malignant cells will be important to identify strategies that maximally disrupt survivin in cancer cells while minimally affecting normal tissues. Moreover, in accord with the recent view suggesting that the two main functions of survivin, i.e., spindle monitoring at mitosis and ability to counteract apoptosis, are differentially exploited in normal and tumor cells, the identification of points at which these functions can be separated could open new possibilities for strategies aimed at eliminating cancer cells while preserving normal cell viability.