H89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide) is a compound characterized in vitro as a potent and selective inhibitor of protein kinase A (PKA). In this study, we found that H89 reduced the phosphorylation of the myosin regulatory light chain (MRLC) at Thr-18/Ser-19 and induced disassembly of stress fibers in HeLa cells. In addition, we found that H89 induced not only reduction of the MRLC phosphorylation but also cell growth inhibition in several human cancer cell lines. Recently H89 has been found to inhibit Rho-kinase with potency similar to or greater than that for inhibition of PKA. Indeed, the effects of H89 on both the MRLC phosphorylation and actin cytoskeleton organization were nearly identical to those of Rho-kinase inhibitor Y-27632. However, unlike H89, Y-27632 did not affect cell growth of HeLa cells. Further, when the myosin phosphatase targeting subunit 1 (MYPT1) expression was silenced by RNA interference in HeLa cells, the suppressive effect of H89 on the MRLC phosphorylation was not affected, while H89-induced cell growth inhibition was blocked. These results suggest that H89-induced reduction of the MRLC phosphorylation results from inhibition of Rho-kinase and that H89-induced cell growth inhibition is independent of reduction of the MRLC phosphorylation.
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