Naphthalenesulfonamides such as N-(6-amino-hexyl)-5-chloro-1-naphthalenesulfonamide (W-7) are potent calmodulin (CaM) antagonists and act upon several protein kinases at higher concentration. When the naphthalene ring was replaced by isoquinoline, the derivatives were no longer CaM antagonists but retained the ability to inhibit protein kinases, and some of the derivatives exhibited selective inhibition toward a certain protein kinase. cAMP-dependent, cGMP-dependent, and Ca2+-phospholipid-dependent (protein kinase C) protein kinases were inhibited significantly by addition of 10(-6) M N-[2-(methylamino)ethyl]-5-isoquinoline-sulfonamide (H-8) and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7). H-8 was the most active of the inhibitors in this series and inhibited more markedly cyclic nucleotide dependent protein kinases, than other kinases, while the derivative with the sulfonylpiperazine residue (H-7) was the most potent in inhibiting protein kinase C. Apparent Ki values of H-8 were 0.48 and 1.2 microM for cGMP-dependent and cAMP-dependent protein kinases, respectively, and the Ki value of H-7 for protein kinase C was 6 microM. Both the holoenzyme and the catalytic subunit (or fragment), which is active without an enzyme activator, are susceptible to these compounds with a similar concentration dependency, thereby indicating that the inhibitory effect is attributed to the direct interaction of the compound with the active center of the enzyme but not with the enzyme activator. The inhibitions were freely reversible and of the competitive type with respect to ATP and of the noncompetitive type with respect to the phosphate acceptor.(ABSTRACT TRUNCATED AT 250 WORDS)

ROCK (Rho-kinase), an effector molecule of RhoA, phosphorylates the myosin binding subunit (MBS) of myosin phosphatase and inhibits the phosphatase activity. This inhibition increases phosphorylation of myosin light chain (MLC) of myosin II, which is suggested to induce RhoA-mediated assembly of stress fibers and focal adhesions. ROCK is also known to directly phosphorylate MLC in vitro; however, the physiological significance of this MLC kinase activity is unknown. It is also not clear whether MLC phosphorylation alone is sufficient for the assembly of stress fibers and focal adhesions.We have developed two reagents with opposing effects on myosin phosphatase. One is an antibody against MBS that is able to inhibit myosin phosphatase activity. The other is a truncation mutant of MBS that constitutively activates myosin phosphatase. Through microinjection of these two reagents followed by immunofluorescence with a specific antibody against phosphorylated MLC, we have found that MLC phosphorylation is both necessary and sufficient for the assembly of stress fibers and focal adhesions in 3T3 fibroblasts. The assembly of stress fibers in the center of cells requires ROCK activity in addition to the inhibition of myosin phosphatase, suggesting that ROCK not only inhibits myosin phosphatase but also phosphorylates MLC directly in the center of cells. At the cell periphery, on the other hand, MLCK but not ROCK appears to be the kinase responsible for phosphorylating MLC. These results suggest that ROCK and MLCK play distinct roles in spatial regulation of MLC phosphorylation.

We examined the role of regulatory myosin light chain (MLC) phosphorylation of myosin II in cell migration of fibroblasts. Myosin light chain kinase (MLCK) inhibition blocked MLC phosphorylation at the cell periphery, but not in the center. MLCK-inhibited cells did not assemble zyxin-containing adhesions at the periphery, but maintained focal adhesions in the center. They generated membrane protrusions all around the cell, turned more frequently, and migrated less effectively. In contrast, Rho-associated kinase (ROCK) inhibition blocked MLC phosphorylation in the center, but not at the periphery. ROCK-inhibited cells assembled zyxin-containing adhesions at the periphery, but not focal adhesions in the center. They moved faster and more straight. On the other hand, inhibition of myosin phosphatase increased MLC phosphorylation and blocked peripheral membrane ruffling, as well as turnover of focal adhesions and cell migration. Our results suggest that myosin II activated by MLCK at the cell periphery controls membrane ruffling, and that the spatial regulation of MLC phosphorylation plays critical roles in controlling cell migration of fibroblasts.

Abstract-Ca2ϩ sensitization of vascular smooth muscle (VSM) contraction involves Rho-dependent and Rho-kinasedependent suppression of myosin phosphatase activity. We previously demonstrated that excitatory agonists in fact induce activation of RhoA in VSM. In this study, we demonstrate a novel Ca 2ϩ -dependent mechanism for activating RhoA in rabbit aortic VSM. High KCl-induced membrane depolarization as well as noradrenalin stimulation induced similar extents of sustained contraction in rabbit VSM. Both stimuli also induced similar extents of time-dependent, sustained increases in the amount of an active GTP-bound form of RhoA. Consistent with this, the Rho kinase inhibitors HA1077 and Y27632 inhibited both contraction and the 20-kDa myosin light chain phosphorylation induced by KCl as well as noradrenalin, with similar dose-response relations.

N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and its derivatives are putative calmodulin antagonists that bind to calmodulin and inhibit Ca2+/calmodulin-regulated enzyme activities. Autoradiographic studies using tritiated W-7 showed that this compound penetrates the cell membrane, is distributed mainly in the cytoplasm, and inhibits proliferation of Chinese hamster ovary K1 (CHO-K1) cells. Cytoplasmic [3H]W-7 was excluded completely within 6 hr after removal of [3H]W-7 from the culture medium. N-(6-aminohexyl)-1-naphthalenesulfonamide, an analogue of W-7 that interacts only weakly with calmodulin, proved to be a much weaker inhibitor of cell proliferation. CHO-K1 cells were synchronized by shaking during mitosis and then released into the cell cycle in the presence of 25 microM W-7 or 2.5 mM thymidine for 12 hr. Cell division was observed approximately 6 hr later. The results suggest that the effect of W-7 on cell proliferation might be through selective inhibition of the G1/S boundary phase, which is similar to the effect of excess thymidine. This pharmacological demonstration that cytoplasmic calmodulin is involved in cell proliferation is significant; W-7 and its derivatives may be useful tools for research on calmodulin and cell biology-related studies.

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In smooth muscle, a Rho-regulated system of myosin phosphatase exists; however, it has yet to be established whether Rho kinase, one of the downstream effectors of Rho, mediates the regulation of myosin phosphatase activity in vivo. In the present study, we demonstrate in permeabilized vascular smooth muscle cells (SMCs) that the vasodilator 1-(5-isoquinolinesulfonyl)-homopiperazine (HA-1077), which we show to be a potent inhibitor of Rho kinase, dose dependently inhibits Rho-mediated enhancement of Ca(2+)-induced 20-kDa myosin light chain (MLC(20)) phosphorylation due to abrogating Rho-mediated inhibition of MLC(20) dephosphorylation. By an immune complex phosphatase assay, we found that guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) stimulation of permeabilized SMCs caused a decrease in myosin phosphatase activity with an increase in the extent of phosphorylation of the 130-kDa myosin-binding regulatory subunit (MBS) of myosin phosphatase in a Rho-dependent manner. HA-1077 abolished both of the Rho-mediated events. Moreover, we observed that the pleckstrin homology/cystein-rich domain protein of Rho kinase, a dominant negative inhibitor of Rho kinase, inhibited GTPgammaS-induced phosphorylation of MBS. These results provide direct in vivo evidence that Rho kinase mediates inhibition of myosin phosphatase activity with resultant enhancement of MLC(20) phosphorylation in smooth muscle and reveal the usefulness of HA-1077 as a Rho kinase inhibitor.

Citron kinase is a Rho-effector protein kinase that is related to Rho-associated kinases of ROCK/ROK/Rho-kinase family. Both ROCK and citron kinase are suggested to play a role in cytokinesis. However, no substrates are known for citron kinase. We found that citron kinase phosphorylated regulatory light chain (MLC) of myosin II at both Ser-19 and Thr-18 in vitro. Unlike ROCK, however, citron kinase did not phosphorylate the myosin binding subunit of myosin phosphatase, indicating that it does not inhibit myosin phosphatase. We found that the expression of the kinase domain of citron kinase resulted in an increase in MLC di-phosphorylation. Furthermore, the kinase domain was able to increase di-phosphorylation and restore stress fiber assembly even when ROCK was inhibited with a specific inhibitor, Y-27632. The expression of full-length citron kinase also increased di-phosphorylation during cytokinesis. These observations suggest that citron kinase phosphorylates MLC to generate di-phosphorylated MLC in vivo. Although both mono- and di-phosphorylated MLC were found in cleavage furrows, di-phosphorylated MLC showed more constrained localization than did mono-phosphorylated MLC. Because citron kinase is localized in cleavage furrows, citron kinase may be involved in regulating di-phosphorylation of MLC during cytokinesis.