G-Alpha Subunit Abundance and Activity Differentially Regulate β-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 β-catenin signaling. However, the link between G-proteins and β-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 β-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 β-catenin signaling agonist. The results show that Pasteurella multocida toxin (PMT) significantly enhanced LiCl-induced active β-catenin levels in HEK293T cells and mouse embryo fibroblasts. Evaluation of the effect of specific G-alpha proteins on the regulation of β-catenin showed that G_q/11 and G_12/13 knockout cells had significantly higher levels of active and total β-catenin than wild-type cells. The stimulation of active β-catenin by PMT and LiCl was lost upon both constitutive and transient knockdown of G₁₂ and G₁₃ but not G_q Based on our results, we conclude that endogenous G-alpha proteins are negative regulators of active β-catenin; however, PMT-activated G-alpha subunits positively regulate LiCl-induced β-catenin expression in a G_12/13-dependent manner. Hence, G-alpha subunit regulation of β-catenin is context dependent.