Cdc42 Contributes to Phorbol Ester-Induced Ca²⁺-Independent Contraction of Pulmonary Artery Smooth Muscle

Abstract

We determined the contribution of the Rho family of low molecular GTP-binding proteins to phorbol ester-induced contraction in swine pulmonary artery smooth muscle. In Ca²⁺-free medium containing 1 mM EGTA, 12-deoxyphorbol 13-isobutyrate (DPB, 1 μM), a protein kinase C (PKC) activator, elicited sustained contractions, which were not inhibited by treatment with verapamil, a voltage-dependent Ca²⁺ channel antagonist, and Y27632, a Rho-associated kinase inhibitor. Immunoblot analysis showed three PKC isoforms (α, ε, and ζ) and two Rho GTPases (RhoA and Cdc42) in both cytosolic and the membrane fractions from quiescent strips. DPB (1 μM) significantly induced PKCα and ε to translocate from the cytosolic to the membrane fraction in Ca²⁺-free medium. DPB also elicited the translocation of Cdc42, but not RhoA to the membrane fraction. Similarly, in the experiment for measurement of Rho GTPase activity by pull-down assay, DPB (1 μM) significantly increased the activity of Cdc42 in Ca²⁺-free medium. Norepinephrine (NE, 10 μM) stimulated the redistribution of RhoA from the cytosolic to the membrane fraction in swine pulmonary artery smooth muscle. In contrast, NE did not alter the subcellular distributions of Cdc42 and the PKC isoforms. These results indicate that phorbol ester evokes PKC-mediated Ca²⁺-independent contraction via a Rho GTPase pathway, especially Cdc42, in smooth muscle from swine pulmonary arteries.