Title: A functional characterization of G3BPs
Description: RasGAP SH3 Binding Proteins (G3BPs) are a family of proteins involved in Ras signal transduction, deubiquitylation of certain targets, and stress granules. A review of the current literature demonstrates that G3BPs are quite promiscuous and can interact with many proteins. It is speculated that their role may be to coordinate mitogenic signaling with protein biosynthesis. Elevated levels of G3BPs can be found in a number of cancers and proliferative disorders. We began research on G3BPs with the discovery of a novel interaction between the tumor suppressor p53 and its negative regulator, MDM2. Using a proteomic approach, we found both G3BP1 and G3BP2 bind to p53 in vitro and in vivo . High expression of G3BPs leads to the redistribution of p53 from the nucleus to the cytoplasm. The G3BP2 isoform additionally associated with MDM2. G3BP2 expression resulted in significant reduction in MDM2-mediated p53 ubiquitylation and degradation as well as MDM2 self ubiquitylation. Regardless, expression of shRNA targeting either G3BP1 or G3BP2 in human cancer cell lines resulted in marked upregulation of p53 levels and activity.

To further investigate its role in cancer, we focused on characterizing the phenotype of cells over expressing G3BP2. High expression of G3BP2 has been detected in breast cancer. However, it is unknown if its expression is simply a byproduct of increased proliferative potential or if G3BP2 can actively contribute to the tumor phenotype. We created stable MCF7 breast cancer cell lines that constitutively expressed G3BP2. When under serum free conditions, endogenous levels of G3BP2 are virtually undetectable. Constitutively expressing G3BP2 cells evaded apoptosis and survived under serum free conditions significantly longer than cells lacking the construct. We conclude that G3BPs may play a critical role in tumorigenesis. Our results suggest that both G3BP isoforms act as negative regulators of p53. G3BP2 may play a distinct role in giving cells an advantage to survive in nutrient poor environments. Both isoforms are upregulated in cancers and differences may be tissue specific or due to distinct cellular mechanisms. Future work will uncover their differences and better define their roles in cancer.