Myosin light chain diphosphorylation is enhanced by growth promotion of cultured smooth muscle cells

The characteristics of actively growing smooth muscle cells (a variant, SM-3) were compared with those of growth-arrested cells with regard to response of myosin light chain (MLC) phosphorylation. Augmented MLC phosphorylation, in particular diphosphorylation, was observed in actively growing cells when stimulated with 30 μM prostaglandin F(2α) (PGF(2α)). The maximum level of diphosphorylation in growing cells was significantly higher than that in growth-arrested cells. The MLC diphosphorylation was sensitive to protein kinase C down-regulation by phorbol dibutylate and pretreatment by the protein kinase inhibitors, staurosporine (30 nM) and isoquinoline sulphonamide HA 1077 (20 μM). The actively growing cells contained larger amounts of protein kinase C than growth-arrested cells. The phosphorylation sites of mono- and diphospho-MLC were determined to be MLC kinase-dependent sites (Thr¹⁸, Ser¹⁹). The PGF(2α) concentration/response curves of MLC diphosphorylation were shifted to the left and upwards in the presence of the protein phosphatase inhibitor calyculin A. These results suggest that PGF₂ stimulation of actively growing SM-3 cells augments MLC kinase-dependent MLC diphosphorylation. Protein kinase C is involved indirectly in this reaction, possibly through MLC phosphatase-sensitive regulatory mechanisms.