We are studying cellular signaling pathways involved in the generation of human cancer. In general, disruption of these pathways alters the ability of a cell to control its proliferation as well as the initiation of programmed cell death (apoptosis). We are focusing on three key signaling pathways that regulate both cellular proliferation and apoptosis: the Myc transcription factor, the Ras signaling protein, and the G1 cyclin dependent kinase (Cdk)/retinoblastoma(Rb)/E2F pathway. While each of these pathways has been extensively studied over the past decade, the nature of their interrelations remains elusive. Since these pathways are deregulated in the majority of all human tumors, we want to understand how they network and synergize to precisely control cellular proliferation versus cell death. This information will contribute to our understanding of the complex nature of cancer progression, and facilitate the generation of meaningful therapies.
We have recently uncovered several important links between these three pathways. We have shown that Myc affects the G1 Cdk/Rb/E2F cell cycle control pathway by directly activating expression of E2F transcription factors. We have also found that activation of Ras leads to aberrant stabilization and oncogenic accumulation of the Myc protein. This finding provides an important mechanism to help explainthe synergistic nature of coexpression of Myc and Ras for both normal cell growth as well as cellular transformation. Specifically, we have shown that activation of Ras regulates two conserved phosphorylation sites in Myc, which have opposing roles in controlling Myc stability (see figure 1). Serine 62 phosphorylation by Ras-activated ERK stabilizes Myc, while Threonine 58 phosphorylation by GSK3b, which is inhibited by Ras-activated PI(3)K, stimulates Mycubiquitin-mediated degradation. Threonine 58 phosphorylation stimulates Myc degradation by recruiting the Pin1 prolyl-isomerase, which catalyzes a cis to trans isomer change at Proline 63 allowing removal of the stabilizing Serine 62 phosphate by the conformation sensitive protein phosphatase 2A-B56a (PP2A-B56a). Phosphorylation at Threonine 58 also recruits the Myc E3 ubiquitin ligase, SCFFbw7, which then directs Myc poly-ubiquitination and degradation.
Importantly, our research has shown that this complex signaling pathway controlling Myc degradation can be disrupted in human cancer leading to high-level onocgenic expression of Myc. We are currently targeting this pathway using gene therapy and small molecule approaches to destabilize Myc for cancer therapeutic purposes. A number of research tools have aided our investigations. These include the ability to generate recombinant adenoviruses to express our favorite genes and specific adaptor molecules to target these recombinant adenoviruses to cancer cells. We have also generated gene knock-in mice in which we can turn on expression of different stability mutants of Myc in any tissue to explore the role of deregulated Myc stability in the generation of cancer in vivo. Finally, we have generated sophisticated software tools to evaluate microarray data that improve normalization and that create gene signatures for predictive modeling. We are using these software tools in a large collaborative study to predict colon cancer outcome and response to chemotherapy.