Research Grants Awarded
HBXIP regulation of cell division and apoptosis in breast cancer cells
Genetic instability represents one of the six hallmarks of cancer . Aggressive breast cancers are typically characterized by aneuploidy , indicating they have developed defects in chromosome segregation as one of the manifestations of a genetic instability phenotype. Thus, knowledge of the proteins that control chromosome segregation during cell division may contribute to an understanding of mechanisms of genetic instability, as well as possibly revealing new targets for drugs that arrest cell division and promote eradication of malignant cells. The goal of this proposal is to study the cellular protein HBXIP in the context of breast cancer. HBXIP (HBx-interacting protein) was first discovered as a cellular target of HBx, an oncoprotein produced by Hepatitis B Virus (HBV), a virus that causes over 1 million cancer deaths annually due to liver cancer . Our recent work indicates that HBXIP regulates centrosome dynamics, controlling the splitting of centrosomes and their migration to opposite poles of dividing cells  .Without HBXIP, cells arrest with monopolar spindles, failing to form the bipolar spindles required for proper chromosome alignment at metaphase and subsequent segregation of chromosomes into daughter cells. Conversely, over-expression of HBXIP produces multipolar spindles, leading to aberrant sorting of chromosomes. Interestingly, HBXIP deficiency not only results in mitotic arrest due to failure to form bipolar spindles, but the majority of these arrested cells eventually succumb to apoptosis, consistent with prior data indicating that defective centrosome regulation activates a mitotic checkpoint that signals for cell autodestruction . We have found that HBXIP is commonly over-expressed in human breast cancers, providing rationale for studying this protein in the context of breast malignancy. In seeking to understand the molecular mechanism of HBXIP, we have identified the chromosomal passenger protein Survivin and the microtubule-associated motor protein Dynein-Light Chain as HBXIP-interacting proteins. Taken together, these observations have led us to hypothesize that HBXIP plays an important role in regulation of cell division, chromosome segregation, and apoptosis in breast cancer cells. The following aims are proposed: Aim #1: Determine the domains within HBXIP responsible for interactions with Survivin, itself (HBXIP-HBXIP self interaction), and Dynein light-chain (LC). Aim #2: Determine the functional importance of HBXIP interaction with Survivin, itself, and Dynein LC in cell division of human breast cancer. Aim #3: Characterize the apoptotic mechanism induced by HBXIP deficiency in dividing human breast cancer cells. Aim #4: Explore the therapeutic value of targeting HBXIP in a preclinical model of breast cancer. With respect to the technical approach, mutagenesis studies will be performed to construct mutant HBXIP proteins, which fail to bind Survivin, HBXIP, or Dynein-LC.
One of the cardinal characteristics of cancer is genetic instability. Cancer cells, including breast cancers, become genetically unstable, resulting in progressive accumulation in the malignant cells of DNA mutations that promote tumor progression and facilitate emergence of aggressive cancer cells that are resistant to chemotherapy. Genetic instability can be caused by many mechanisms, with chromosome segregation defects representing a common problem in breast cancer. In normal human cells, with each cell division, the DNA within the 46 chromosomes is replicated and the resulting 92 chromosomes are then divided in an orderly manner between the two daughter cells, each with a perfectly replicated copy of the 46 chromosomes. In many cancers, in contrast, this process of chromosome segregation is sloppy, with chromosome being distributed aberrantly. HBXIP is a cellular protein that was first identified as a target of a cancer-causing gene product produced by the Hepatitis B Virus, a virus that causes over 1 million cancer deaths annually due to liver cancer. We have identified a novel role for the cellular protein HBXIP in controlling chromosome segregation and cell division, and we have obtained evidence that HBXIP is produced at excessive levels in breast cancers. Our preliminary data also show that experimentally reducing HBXIP in breast cancer cells arrests cell division and causes breast cancer cells to die. The goal of this application is to gain a better understanding of how HBXIP functions as regulator of chromosome segregation, in hopes that this knowledge might be exploited to find new and more effective ways of inhibiting breast cancer cell division and eradicating breast cancer cells.