Survivin was initially identified as a member of the inhibitor of apoptosis protein family (Li et al., 1998
). Overexpression of survivin is observed in a large portion of human cancers and, in many cases, correlates with poor prognosis (Altieri, 2003
). Survivin appears to have two distinct biological roles. First, survivin may exhibit anti-apoptotic activity only under certain experimental conditions although this is controversial (Li et al., 1998
; Tamm et al., 1998
). Elevation of survivin levels have been implicated in radioresistance in cancers (Rodel et al., 2005
; Wang and Greene, 2005
), whereas downregulation of survivin sensitizes cancer cells to genotoxic agents. The mechanistic role of survivin in apoptosis is currently unclear.
Second, more recent studies indicate that survivin is involved in chromosome segregation during mitosis (Skoufias et al., 2000
; Uren et al., 2000
). Survivin acts as an integral component of the chromosome passenger protein complex, which also includes Aurora kinase B, INCENP, TD-60 and Borealin. Survivin is localized to the kinetochore following chromosome condensation and becomes relocated to the midbody after chromosome segregation and until cytokinesis (Skoufias et al., 2000
; Uren et al., 2000
). Survivin has an essential role in modulating the formation of the mitotic spindle, which modulates accurate chromosome segregation during mitosis. Overexpression of survivin might corrupt the spindle checkpoint control and contribute to chromosome instability and aneuploidy.
Loss of survivin functions may affect microtubule dynamics during mitosis. siRNA depletion of survivin increased the number of microtubules nucleated at centrosomes and promoted microtubule instability (Rosa et al., 2006
). Cells microinjected with a polyclonal antibody to survivin exhibit defective spindles and depletion of microtubules (Giodini et al., 2002
). Clearly, the chromosomal passenger complex is required for microtubule stabilization and spindle assembly during mitosis (Sampath et al., 2004
Our previous studies indicated that activation of the ErbB family receptor tyrosine kinases, such as epidermal growth factor receptor, leads to upregulation of survivin (Wang and Greene, 2005
). Modulation of survivin levels by epidermal growth factor receptor is dependent on the PI-3 kinase pathway but not the mitogen-activated protein kinase pathway. Similarly, the signaling events initiated by p185Her2/neu
and ErbB3 also cause increase of survivin levels (Asanuma et al., 2005
; Xia et al., 2006
). In some cells, disabling p185Her2/neu
has been suggested to facilitate cell death in a survivin-dependent manner (Xia et al., 2006
The structure of survivin has been determined crystallographically (Chantalat et al., 2000
; Verdecia et al., 2000
) and by nuclear magnetic resonance (NMR) (Sun et al., 2005
). The N-terminal region of survivin contains a zinc-binding fold similar to the classic baculovirus inhibitor of apoptosis protein repeat (BIR) motif, which binds to phosphorylated histone and is involved in recruiting the chromosome passenger complex (CPC) proteins to the chromosome (Kelly et al., 2010
; Wang et al., 2010
; Yamagishi et al., 2010
). The survivin BIR domain consists of a three-stranded b-sheet and four a-helices (Chantalat et al., 2000
; Verdecia et al., 2000
) and the survivin protein forms a dimer that resembles a ‘bow tie’. The N-terminal region contains the structural elements involved in dimerization and subcellular localization to the kinetochore and the midbody (Li and Ling, 2006
). In addition, mutations of the aminoacid residues in this region affects the function of survivin (Li et al., 1998
). Ubiquitination of survivin in the N-terminal region modulates localization as well as degradation (Vong et al., 2005
). A number of survivin splice variants have been identified, such as survivindelta Ex3 lacking exon 3 and survivin-2B retaining a part of intron 2 as a cryptic exon. Of note, the N-terminal region, which we have focused on, is shared by the variant forms (Li and Ling., 2006
; Noton et al., 2006
More recently, the structure of a survivin-borealin-INCENP core complex has been defined (Jeyaprakash et al., 2007
). Borealin and INCENP associate with the c-terminal helical domain of survivin to form a three-helical bundle of 1:1:1 stoichiometry. The interactions of the core components are essential for central spindle and midbody localization of the complex. Both survivin and borealin bind to the N-terminus of INCENP (corresponding to the amino-acid residues 1–58 of human INCENP), which is sufficient for targeting to the centromere (Ainsztein et al., 1998
). Survivin is required for localization of the chromosome passenger protein complex to the centromere.
The dual role of survivin in influencing some forms of apoptosis and as a critical element in mitosis makes it an attractive target for cancer therapy. Resistance to apoptosis is seen in most human cancers. Developing molecular interventions to commit tumor cells to the cell death pathways is therefore a general strategy for pharmaceutical therapy (Nicholson, 2000
). Moreover, one of the widely explored approaches to develop cancer therapeutics is to induce aberrant mitosis in tumor, which causes cell death. For example, taxanes, a class of compounds that blocks mitosis by stabilizing tubulin polymerization and interfering with the formation of the mitotic spindle, have been used to treat various forms of human cancer. However, because microtubule polymerization is involved in a large variety of physiological processes, these drugs cause profound side effects.
Survivin is selectively overexpressed in many human cancers but its levels are low in normal tissues and undetectable in non-dividing cells (Wang and Greene, 2005
). Indeed, ablation of survivin function can cause mitotic arrest and death to dividing cells, including a variety of cancer cell lines. Therefore, small molecules that target and disable survivin function represent a rational targeted-therapeutic approach that interferes with both apoptosis and mitosis in cancer cells.
In this study, we use small molecules to probe a discreet survivin function. Using a recently developed algorithm that identifies cavity interacting small molecules at critical allosteric sites, we successfully used the compounds to disrupt survivin functions related to metaphase. We have performed biological studies to examine the effects of these molecules on cell viability, mitosis and in vivo tumor growth. Our results identify a new targeted therapeutic approach to affect metaphase mitotic events in human cancers.