Vascular myosin expression during cytokinesis, attachment, and hypertrophy

Primary cultures of canine saphenous vein smooth muscle cells downregulate the expression of the two muscle myosin heavy chains (mMHC) and upregulate the expression of a nonmuscle myosin heavy chain (nmMHC) when maintained in medium containing 10% fetal calf serum (FCS). The cellular function and control of these changes in contractile protein expression are not known. The purposes of these experiments were to determine whether the expression of nmMHC was required for cytokinesis, whether cell attachment stimulated nmMHC expression, and whether during changes in total myosin production the relative amounts of the two mMHCs remained constant. Primary cultures were maintained in FCS, in a serum-free defined medium (SFM), or after 2 days in SFM switched to FCS to induce proliferation and changes in myosin expression. nmMHC expression occurred before cytokinesis if cells were placed directly in FCS, whereas it occurred after cytokinesis if growth-arrested cells were exposed to FCS. The position of the cell in the cell cycle was responsible for these differences, and the temporal correlation of cell-cycle progression and nmMHC expression indicated that expression occurred during G1. Cells that remained unattached in the presence of FCS or attached in SFM did not express nmMHC. During net production or loss of mMHC by growth in SFM or FCS, respectively, the relative amounts of the two mMHC remained constant. These results suggest that 1) the expression of nmMHC requires the combined effect of FCS and attachment, 2) it occurs in cells progressing through G1 but is not required for cytokinesis, and 3) during changes in net myosin production, the two mMHC are coregulated.