Description of Research Expertise

Research Interests
Our research interests are focused broadly on the molecular pathways governing cellular growth control and their disruption during the initiation and progression of human cancers.

Key words: cancer, tumorigenesis, apoptosis, p53, mdm2, gene regulation, stress-response.

Description of Research
Currently, a major focus of investigation centers on the role of the p53 tumor suppressor gene in mediating programmed cell death (apoptosis). This gene governs a major pathway protecting human cells from malignant transformation , and it represents the most frequently mutated gene in human cancer. It is generally accepted that disruption of p53' apoptosis function directly contributes to tumor progression. Thus, understanding the mechanisms by which p53 acts in the execution of cell death pathways is of considerable importance in cancer biology.

Clearly established is p53's role as a nuclear transcription factor with the ability to activate, or repress, the expression of many genes. Accumulating data, however, now reveal that p53 has a direct cytoplasmic role at mitochondria in activating the apoptotic machinery. We obtained several key pieces of evidence supporting this conclusion. As an example, by analyzing a common sequence polymorphism of p53 at codon 72 (R72 and P72), we found that the arginine form of p53 (R72) kills cells better than the P72 form, in part because R72 more efficiently localizes to mitochondria. Mechanistically, R72 traffics better to the mitochondria because it interacts better with MDM2, a p53-regulator that ubiquitylates p53 and targets it for nuclear export. Ongoing efforts are aimed at defining the overall contribution of mitochondrial p53 to apoptosis induction, and identifying other cellular factors that dictate the relative degree of p53 mitochondrial localization in different cells/tissues.

As part of our efforts at defining the mitochondrial role of the p53 protein, we are identifying mitochondrial p53-interacting proteins. Utilizing affinity chromatography and mass spectrometry, we uncovered an interaction between p53 and the mitochondrial protein BAK. BAK is a critical "gatekeeper" of mitochondrial integrity. When p53 binds BAK it triggers a conformational change in BAK, leading to BAK oligomerization. This, in turn, activates the mitochondrial apoptosis pathway, resulting in release of cytochrome c and other apoptogenic proteins from the mitochondria into the cytoplasm.

Interestingly, some tissues are very senstivie to apoptosis induced by the p53 tumor suppressor protein, while others, such as liver, are quite resistant; however, the molecular basis remains poorly understood. In recent studies, we discovered that p53 activation in liver hepatocytes leads to enhanced expression of a liver-specific protein called IGFBP1 (insulin-like growth factor binding protein-1). Exhibiting a previously unanticipated role, a portion of intracellular IGFBP1 protein localizes to mitochondria where it binds to the proapoptotic protein BAK and hinders BAK activation and apoptosis induction. Interestingly, when IGFBP1 is in a complex with BAK, formation of a proapoptotic p53/BAK complex and apoptosis induction are impaired, both in cultured cells and in liver. In contrast, livers of IGFBP1-deficient mice exhibit spontaneous apoptosis that is accompanied by p53 mitochondrial accumulation and evidence of BAK-oligomerization. These data support the importance of BAK as a mediator of p53's mitochondrial function. The results also identify IGFBP1 as a negative regulator of the BAK-dependent pathway of apoptosis, whose expression integrates the transcriptional and mitochondrial functions of the p53 tumor suppressor protein.

Future studies are aimed at identifying and characterizing those cellular factors that act to inhibit or promote p53-mediated apoptosis, including the mitochondrial actions of this tumor suppressor protein. The goal is to gain new information that may be utilized for the development of strategies and therapeutics in oncology. Our studies, then, have implications for cancer diagnosis and therapy, and should provide new insight concerning critical determinants of growth control in normal and transformed cells.