Identification of a novel growth factor-responsive gene in vascular smooth muscle cells

We have employed differential screening of a rat aortic smooth muscle cell cDNA library to isolate a cDNA clone, SM-20, whose nucleotide and deduced amino acid sequences are distinct from those reported previously. SM-20 encodes an mRNA of approximately 2.5-3.0 kilobase pairs which is present at low levels in quiescent vascular smooth muscle cells. SM-20 levels increase within 1 h of treatment with growth agonists (serum, platelet-derived growth factor, angiotensin II), isoproterenol, and forskolin. Cycloheximide induces high levels of SM-20 mRNA and superinduces it in the presence of serum, suggesting that the increase in SM-20 mRNA levels is not dependent on protein synthesis and is part of the primary response to growth agonists. SM-20 is expressed at high levels in tissues and cells derived from muscle (smooth, skeletal, and cardiac) and nerve (brain, PC12 cells) and is not expressed in 3T3 fibroblasts or rat 6 fibroblasts. SM-20 encodes a new member of the immediate early gene family and is unusual in that it is expressed in vascular smooth muscle, but not in fibroblasts.