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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2006;26:1267.)
© 2006 American Heart Association, Inc.

Vascular Biology

Apelin Stimulates Myosin Light Chain Phosphorylation in Vascular Smooth Muscle Cells

Tatsuo Hashimoto; Minoru Kihara; Junji Ishida; Nozomi Imai; Shin-ichiro Yoshida; Yoshiyuki Toya; Akiyoshi Fukamizu; Hitoshi Kitamura; Satoshi Umemura

From the Department of Pathology, Division of Cellular Pathobiology, and Department of Medical Science and Cardiorenal Medicine, Yokohama City University Graduate School of Medicine and School of Medicine, Yokohama, and the Center for Tsukuba Advanced Research Alliance, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan.

Correspondence to Dr Minoru Kihara, Department of Medical Science and Cardiorenal Medicine, Yokohama City University Graduate School of Medicine and School of Medicine, Kanazawa-ku, Yokohama, 236-0004, Japan. E-mail minoru{at}med.yokohama-cu.ac.jp

Objective— Physiological roles of apelin and its specific receptor APJ signaling were investigated in vascular smooth muscle cells (VSMCs). The present study determined whether apelin activates myosin light chain (MLC), a major regulatory event in initiating smooth muscle contraction.

Methods and Results— To assess MLC activation, we performed Western blot and immunohistochemical studies using an antibody against the phospho-MLC. In VSMCs, apelin induces the phosphorylation of MLC in a concentration-dependent manner with a peak at 2 minutes. Pretreatment of VSMCs with pertussis toxin abolishes the apelin-induced phosphorylation of MLC. Inhibition of protein kinase C (PKC) with GF-109203X markedly attenuated the apelin-induced MLC phosphorylation. In addition, methylisobutyl amiloride, a specific inhibitor of the Na⁺/H⁺ exchanger (NHE), and KB-R7943, a potent inhibitor for the reverse mode of the Na⁺/Ca²⁺ exchanger (NCX), significantly suppressed the action of apelin. In wild-type mice, apelin phosphorylates MLC in vascular tissue, whereas it had no response in APJ-deficient mice by Western blot and immunohistochemistry. Apelin-induced phosphorylation of MLC was accompanied with myosin phosphatase target subunit phosphorylation.

Conclusions— These results provide the first evidence to our knowledge for apelin-mediated MLC phosphorylation in vitro and in vivo, which is a potential mechanism of apelin-mediated vasoconstriction.

In the vascular smooth muscles, apelin produced APJ-dependent phosphorylation of myosin light chain. This reaction was accompanied with myosin phosphatase target subunit phosphorylation. The apelin–APJ system may play a role in regulating vascular contraction.

Key Words: apelin • APJ • myosin light chain • myosin phosphatase target subunit • vasoconstriction

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