Activation of ß- and α2-adrenergic receptors stimulate tubulin polymerization and promote the association of Gßγ with microtubules in cultured NIH3T3 cells.

Microtubules (MTs) constitute a crucial part of the cytoskeleton and are essential for cell division and differentiation, cell motility, intracellular transport, and cell morphology. Precise regulation of MT assembly and dynamics is essential for the performance of these functions. Although much progress has been made in identifying and characterizing the cellular factors that regulate MT assembly and dynamics, signaling events in this process is not well understood. Gßγ, an important component of the G protein-coupled receptor (GPCR) signaling pathway, has been shown to promote MT assembly in vitro and in cultured NIH3T3 and PC12 cells. Using the MT depolymerizing agent nocodazole, it has been demonstrated that the association of Gßγ with polymerized tubulin is critical for MT assembly. More recently, Gßγ has been shown to play a key role in NGF-induced neuronal differentiation of PC12 cells through its interaction with tubulin/MTs and modulation of MT assembly. Although NGF is known to exert its effect through tyrosine kinase receptor TrkA, the result suggests a possible involvement of GPCRs in this process. The present study was undertaken to determine whether agonist activation of GPCR utilizes Gßγ to promote MT assembly. We used isoproterenol and UK 14,304, agonists for two different GPCRs (ß- and α2-adrenergic receptors, respectively) known to activate Gs and Gi respectively, with an opposing effect on production of cAMP. The results demonstrate that the agonist activation of ß- and α2-adrenergic receptors promotes the association of Gßγ with MTs and stimulates MT assembly in NIH3T3 cells. Interestingly, the effects of these two agonists were more prominent when the cellular level of MT assembly was low (30% or less). In contrast to MT assembly, actin polymerization was not affected by isoproterenol or UK 14, 304 indicating that the effects of these agonists are limited to MTs. Thus, it appears that, upon cellular demand, GPCRs may utilize Gßγ to promote MT assembly. Stimulation of MT assembly appears to be a novel function of G protein-mediated signaling.