Abolishes Inhibition of Proliferation of Rat Vascular Smooth Muscle Cells by Serum

We have investigated the requirement for c-myc downregulation in the growth arrest of vascular smooth muscle cells (VSMCs). Rat VSMCs were infected with a retrovirus vector directing constitutive expression of either the complete human c-Myc protein (VSM-myc cells) or the c-Myc deletion mutant D106-143, which is inactive in cotransformation and autosuppression assays (VSM-D106-143 myc cells). Clones of transfected VSM-myc cells were isolated that constitutively expressed a range of levels of c-Myc protein from that observed in normal proliferating VSMCs to approximately seven times normal. The growth rates of these clones and their responses to growth inhibitors were then assessed. VSM-myc clones possessed a shorter mean intermitotic time than normal cells, which was inversely correlated (P<.05) with the level of c-Myc protein expressed. VSM-myc cells also expressed lower levels of a-smooth muscle actin mRNA and protein and exhibited an altered morphology. The proliferation of normal VSMCs and VSM-D106-143 myc cells was inhibited by serum reduction (0.5% fetal calf serum) and also by treatment with interferon-y (100 IU/mL), heparin (50 ,ug/mL), 8-bromocAMP (0.1 mmol/L), or 8-bromo-cGMP (0.1 mmol/L). In contrast, proliferation of VSM-myc cells was not inhibited by any of these agents, even if present at 10-fold higher concentrations. However, =75% of VSM-myc cells expressing levels of c-Myc protein seen in normal proliferating VSMCs underwent apoptosis after 4 days of serum reduction or treatment with interferon-y. The results show that constitutive c-myc expression induces continuous cell proliferation, reduction in a-smooth muscle actin expression and apoptosis in VSMCs. We conclude that downregulation of c-myc is a prerequisite for growth arrest and subsequent survival of VSMCs. Conversely, deregulated c-myc expression may be important in the proliferation and death of VSMCs -characteristics of the pathogenesis of atherosclerosis. (Circ Res. 1994;74:525-536.)