Oxytocin and lysophosphatidic acid induce stress fiber formation in human myometrial cells via a pathway involving Rho-kinase

The actin cytoskeleton is important for stress fiber formation and contribu tes to the initiation and maintenance of smooth muscle contraction. To dete rmine if oxytocin and lysophosphatidic acid (LPA) induce stress fiber forma tion, cultured human myometrial cells were exposed to oxytocin (10(-5) M) o r LPA (10(-6) M), and filamentous (F) and globular (G) actin pools were sta ined with fluorescein isothiocyanate-phalloidin and Texas red DNase I, resp ectively. The F- to G-actin fluorescent-staining ratio was measured by fluo rescence microscopy. Oxytocin and LPA increased stress fiber formation. as indicated by an increase in the F- to G-actin fluorescent-staining ratio. T he Rho-kinase inhibitor Y-27632 markedly attenuated this increase. Oxytocin -induced stress fiber formation was completely inhibited in the presence of the oxytocin antagonist compound VI. Tyrosine kinase inhibition with tyrph ostin A23 partially blocked the increase induced by oxytocin but had no eff ect on LPA-induced stress fiber formation. Stress fiber formation was not b locked by pertussis toxin, mitogen-activated protein kinase, or protein kin ase C inhibitors. Our results show that human myometrial cells respond to o xytocin and LPA with the formation of stress fibers that may be involved in the maintenance of uterine contractions. Rho-kinase appears to be a key si gnaling factor in this pathway.