Suppression of cellular invasion by activated G-protein subunits Gαo, Gαi1, Gαi2, and Gαi3 and sequestration of Gβγ

It was shown previously that platelet-activating factor receptors (PAF-Rs) inhibit invasiveness of colonic and kidney epithelial cells induced by the src and Met oncogenes via a pertussis toxin-sensitive mechanism. Therefore, Madin-Darby canine kidney (MDCKts.src) cells were stably transfected with constitutively activated forms of G alphao, G alpha i1, G alpha i2, G alpha i3 (AG alphao/i), two G beta gamma sequestering proteins [C-terminal end of beta -adrenergic receptor kinase (ct-beta ARK) and the Gat subunit of retinal G-protein transducin], and G beta1-G gamma2 subunits alone or in combination. Cellular invasion induced by src, Met, and leptin was abrogated by the AG alphao/i, ct-beta ARK, and Gat-positive clones, but was induced by coexpression of G alpha1 gamma2. In contrast, invasion stimulated by the trefoil factors (TFFs) pS2 and intestinal trefoil factor in MDCKts.src cells or human colonic epithelial cells PCmsrc and HCT8/S11 was insensitive to PAF, AG alphao, AG alpha i1, and AG alpha i2, but was abolished by AG alpha i3 and the protease-activated receptor-1 (PAR-1) agonist thrombin receptor-activating peptide. Depletion of free G beta gamma heterodimers by ct-beta ARK resulted in a remarkable decrease of cellular adhesion and spreading on collagen matrix. Our data demonstrate the following: 1) PAF-Rs impair cellular invasion induced by src, Met, and leptin via the activation of G alphao and G alpha i1 to -3; 2) invasion induced by TFFs is selectively inhibited by PAR-1 receptors and G alpha i3 activation; and 3) G beta gamma dimers are required as positive effectors of invasion pathways induced by oncogenes and epigenetic factors. Thus, redistribution of G alphao/G alphai and G beta/gamma heterotrimeric G-proteins by PAF-R and PAR-1 exert differential functions on positive and negative signaling pathways involved in cellular invasion and may serve as potential targets for anticancer therapy.