FUNCTIONAL STUDIES OF RGS2 AND RGS20 WITH IMPLICATIONS FOR CANCER BIOLOGY

Regulators of G protein signaling (RGS) proteins are key negative regulators of Gα signaling, a branch of G-protein-coupled receptor (GPCR)-mediated signal transduction. Approximately 35% of drugs approved by the Food and Drug Administration (FDA) target GPCRs, so it is not surprising that the discovery of RGS proteins has triggered an interest in them as new drug targets. Even though many studies have been shown the involvement of RGS proteins in cancers, there is still a knowledge gap in understanding function and regulation of RGS proteins in these diseases. Consequently, in this thesis, I explored roles of two RGS proteins that have been implicated in cancers.

RGS2 is proposed to act as a tumor suppressor in many different cancers, such as breast cancer, bladder, and ovarian cancer. Here, we investigated if RGS2 also plays a tumor suppressor role in UM, whose growth is driven by overactivated Gαq/11 signaling. We found that increased expression levels of RGS2 inhibit cell growth of UM 92.1 and Mel-202 cells. Mechanistically, this cell growth inhibition is dependent on the association between RGS2 and Gαq, but independent of its canonical GTPase-accelerating protein (GAP) activity. Furthermore, RGS2 inhibited the Mitogen-activated protein kinases (MAPK) signaling, downstream of Gαq, while leaving Yes-associated protein 1/Transcriptional coactivator with PDZ-binding motif (YAP/TAZ) activation unaffected. These data indicate a tumor suppressor role for RGS2 in UM and proposes RGS2 stabilization as a potential therapeutic targeting strategy. 

In contrast to RGS2, RGS20 contributes to cancer progression, particularly in breast cancer. However, how RGS20 is regulated is understudied. Palmitoylation, a reversible post-translational modification, regulates functions of other RGS proteins, and RGS20 is predicted to be palmitoylated. We provided direct evidence of RGS20 palmitoylation in cells and validated the palmitoylation site as the conserved cysteine (Cys148) in the RGS domain. Our results showed that palmitoylation on this site does not affect its GAP activity and subcellular localization, but it affects the association between RGS20 and active Gαo, and inhibition of Gαo-mediated signaling. This study serves as a foundation for future studies in furthering understating the role of palmitoylation in RGS20 function and its possible implications in cancer biology.