G-Alpha Subunit Abundance and Activity Differentially Regulate ß-Catenin Signaling.

Heterotrimeric G proteins are signal transduction proteins involved in regulating numerous signaling events. In particular, previous studies have demonstrated a role for G-proteins in regulating ß-catenin signaling. However, the link between G-proteins and ß-catenin signaling is controversial and appears to depend on G-protein specificity. We describe a detailed analysis of a link between specific G-alpha subunits and ß-catenin using G-alpha subunit genetic knockout and knockdown approaches. The Pasteurella multocida toxin was utilized as a unique tool to activate G-proteins, with LiCl treatment serving as a ß-catenin signaling agonist. The results show that Pasteurella multocida toxin (PMT) significantly enhanced LiCl-induced active ß-catenin levels in HEK293T cells and mouse embryo fibroblasts. Evaluation of the effect of specific G-alpha proteins on the regulation of ß-catenin showed that Gq/11 and G12/13 knockout cells had significantly higher levels of active and total ß-catenin than wild-type cells. The stimulation of active ß-catenin by PMT and LiCl was lost upon both constitutive and transient knockdown of G12 and G13 but not Gq Based on our results, we conclude that endogenous G-alpha proteins are negative regulators of active ß-catenin; however, PMT-activated G-alpha subunits positively regulate LiCl-induced ß-catenin expression in a G12/13-dependent manner. Hence, G-alpha subunit regulation of ß-catenin is context dependent.