ADP-ribosylation factor 6 regulates actin cytoskeleton remodeling in coordination with Rac1 and RhoA

In this study, we have documented an essential role for ADP-ribosylation fa ctor 6 (ARF6) in cell surface remodeling in response to physiological stimu lus and in the down regulation of stress fiber formation. We demonstrate th at the G-protein-coupled receptor agonist bombesin triggers the redistribut ion of ARF6- and Rac1-containing endosomal vesicles to the cell surface. Th is membrane redistribution was accompanied by cortical actin rearrangements and was inhibited by dominant negative ARF6, implying that bombesin is a p hysiological trigger of ARF6 activation. Furthermore, these studies provide a new model for bombesin-induced Rac1 activation that involves ARF6-regula ted endosomal recycling. The bombesin-elicited translocation of vesicular A RF6 was mimicked by activated G alpha q and was partially inhibited by expr ession of RGS2, which down regulates Gq function. This suggests that Gq fun ctions as an upstream regulator of ARF6 activation. The ARF6-induced periph eral cytoskeletal rearrangements were accompanied by a depletion of stress fibers. Moreover, cells expressing activated ARF6 resisted the formation of stress fibers induced by lysophosphatidic acid. We show that the ARF6-depe ndent inhibition of stress fiber formation was due to an inhibition of RhoA activation and was overcome by expression of a constitutively active RhoA mutant. The latter observations demonstrate that activation of ARF6 down re gulates Rho signaling. Our findings underscore the potential roles of ARF6, Rac1, and RhoA in the coordinated regulation of cytoskeletal remodeling.