SBP-1: A Novel Survivin Binding Protein in Breast Cancer

Institution: The Burnham Institute for Medical Research
Investigator(s): Kazuya Okada, M.D., Ph.D. -
Award Cycle: 2001 (Cycle VII) Grant #: 7FB-0052 Award: $32,050
Award Type: Postdoctoral Fellowship
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease

Initial Award Abstract (2001)
Cancers arise due to defects in the amounts and functions of molecules that control the rates of cell production (cell division) and the rates of cell loss as a result of programmed cell death (apoptosis). But, cell division and apoptosis do not exist as separate processes inside cells. There are key proteins that link them together. Inappropriate amounts and subtle changes in activity of these linkage factors appear to distinguish normal from cancer cells. One of the molecules consistently over-produced in tumors is survivin, a protein that is necessary for both tumor cell division and tumor cell survival. Proof-of-concept experiments have demonstrated that if one could inhibit survivin's actions, it would be possible to stop cancer cells from dividing and trigger them to die. To find new strategies for attacking survivin, we have searched for proteins that bind to it. This led us to discover survivin-binding protein-1 (SBP-1).

This project aims to unravel the mechanism by which SBP-1 influences the activity of survivin. The methods that will be employed include transfer of genes producing SBP-1 or experimentally modified forms of SBP-1 into breast cancer cells. Using these artificial mutant cells we can explore the effects of SBP-1 on cell division and cell survival. Analysis will also be performed on human breast cancer specimens to determine whether SBP-1 is produced in malignant cells. In other experiments we will produce antibodies to SBP-1 to visualize its localization in the mitotic structures using microscopy. Studies of the SBP-1 may suggest new approaches for developing novel therapeutics for improved treatment of breast cancer and other kinds of malignancies. Even if targeting SBP-1 and survivin do not directly impact cancer cells, blocking these proteins could enhance the effectiveness of many current therapeutics. And, it is important to develop new tools and biomarkers to utilize in the continuing effort to determine which therapeutic options work best for each woman.

Final Report (2002)
Note: The PI resigned the project after 9 months to take another position.

Survivin is an IAP (Inhibitor of Apoptosis Protein)-family protein which is pathologically over-expressed in most human cancers, and which functions at the cross-roads of cell death and cell cycle regulation. Our current thinking is that Survivin controls a checkpoint associated with chromosome segregation and cell division. Having learned about the recent description of mitotic phosphorylation of Survivin by Cdc2 and about the critical importance of this phosphorylation event for tumor cell division and survival, we decided to undertake a two-hybrid screen with a phospho-mimic T34E mutant of Survivin as bait. The purpose to was identify other proteins that interact with Survivin to better understand its exact function. This approach resulted in the identification of a novel protein, Survivin Binding Protein-1 (SBP-1). SBP-1 protein contains a Cks/Suc1 domain - a structure known to bind Cdk/Cyclin complexes and to regulate kinase activity. We addressed several hypotheses about Survivin-SBP-1 complex, which may suggest new strategies for combating breast cancer. Specific interaction of SBP-1 to Survivin was confirmed, and binding domains of each protein were detected in vitro and in vivo. In vitro kinase assays revealed SBP-1 enhanced phosphorylation of Survivin by cdc2/Cyclin B1 kinase, and does so more strongly than Cks/Suc1. Recently, it has been reported that Cks/Suc1 directs ubiquitin-mediated proteolysis of the phosphorylated p27Kir1 by the protein ubiquitin ligase (E3) SCFskp2. Using SBPI antisense, we demonstrated that SBP-1 also promotes degradation of phosphorylated Survivin through ubiquitination of the protein.

Although this project ended early, we were able to develop important new information on how Survivin and its binding proteins function to regulate cell growth. As these processes and the proteins involved are dissected, then we will in a better position to influence them to control breast cancer tumor growth.