Actin depolymerization via the beta-adrenoceptor in airway smooth muscle cells: a novel PKA-independent pathway

Actin is a major functional and structural cytoskeletal protein that mediat es such diverse processes as motility, cytokinesis, contraction, and contro l of cell shape and polarity. While many extracellular signals are known to mediate actin filament polymerization, considerably less is known about si gnals that mediate depolymerization of the actin cytoskeleton. Human airway smooth muscle cells were briefly exposed to isoproterenol, forskolin, or t he cAMP-dependent protein kinase A (PKA) agonist stimulatory diastereoisome r of adenosine 3', 5'-cyclic monophosphate (Sp-cAMPS). Actin polymerization was measured by concomitant staining of filamentous actin with FITC-phallo idin and globular actin with Texas red DNase I. Isoproterenol, forskolin, o r Sp-cAMPS induced actin depolymerization, indicated by a decrease in the i ntensity of filamentous/globular fluorescent staining. The PKA inhibitor Rp diastereomer of adenosine 3', 5'-cyclic monophosphothioate (Rp-cAMPS) comp letely inhibited forskolin-stimulated depolymerization, whereas it only par tially inhibited isoproterenol-induced depolymerization. The protein tyrosi ne kinase inhibitors genistein or tyrphostin A23 also partially inhibited i soproterenol-induced actin depolymerization. In contrast, the combination o f Rp-cAMPS and either tyrosine kinase inhibitor had an additive effect at i nhibiting isoproterenol-induced actin depolymerization. These results sugge st that both PKA-dependent and -independent pathways mediate actin depolyme rization in human airway smooth muscle cells.