Regulator of G protein Signaling 4 (RGS4): A Novel Inhibitor of Breast Cancer Metastasis

Abstract

Regulator of G protein signaling (RGS) proteins attenuate signaling of G protein‐coupled receptors associated with breast cancer metastasis. Using quantitative real‐time PCR, we found that RGS4 mRNA was selectively and significantly up‐regulated in highly metastatic breast cancer MDA‐MB‐231 and −436 cells. However, despite this high level of RGS4 transcription, RGS4 protein was barely detectable in these cells and additional studies determined that this was due to proteasome‐dependent degradation of RGS4 protein. Proteasome inhibitor‐induced blockade of endogenous RGS4 protein degradation attenuated the migration and invasion of theses cells in transwell assays. Similarly, transient expression of RGS4, but not other RGS proteins, in MDA‐MB‐231 cells also caused marked inhibition of these indices of tumor metastasis. Stable expression of RGS4, but not its function deficient N128A mutant, in MDA‐MB‐231 cells effectively attenuated cancer cell invasiveness both in vitro and in a mouse xenograft model. Other in vitro studies showed that RGS4 selectively inhibits Gi‐dependent activation of small G protein Rac, a key activator of breast cancer metastasis. Finally, immunohistochemistry studies of human breast carcinoma specimens indicated that RGS4 protein levels are down‐regulated in the cancer cells and that this down‐regulation is a relatively late event associated with the invasive phenotype.