Increased migration in late G1 phase in cultured smooth muscle cells

Migration and proliferation of smooth muscle cells (SMC) contribute to neointimal formation after arterial injury. However, the relation between migration and proliferation in these cells is obscure. To discriminate between migration and proliferation, we employed a migration assay of SMC at different phases of the cell cycle. Serum-deprived SMC were synchronized in different phases of the cell cycle by addition of serum for various periods of time. Migration induced by platelet-derived growth factor B-chain homodimer was maximal in SMC that were predominantly in the late G(1) (G(1b)) phase. In addition, in nonsynchronized SMC, 65-75% of SMC that had migrated were in the G1b phase. Phosphorylated myosin light chain was enriched around the cell periphery in SMC in the G(1b) phase compared with SMC in the other cell cycle phases. Interestingly, the Triton X-100-insoluble fraction of myosin was remarkably decreased in G(1b)-enriched SMC. These findings suggest that migratory activity of SMC may be coupled with the G(1b) phase. The phosphorylation and retention of myosin might explain some of the properties responsible for increased migration.