SM-20 IS A NOVEL 40-KD PROTEIN WHOSE EXPRESSION IN THE ARTERIAL-WALL IS RESTRICTED TO SMOOTH-MUSCLE

We have previously reported the identification of a novel cDNA, SM-20, whose corresponding mRNA levels are regulated by growth factors in ra t aortic smooth muscle cell (SMC) culture. Affinity-purified polyclona l and monoclonal antibodies were made against a 230 amino acid region of the SM-20 putative peptide expressed in the bacterial vector, pET-3 b. Western blot analyses of rat and human SMC lysates detected a singl e protein species of approximately 40 kd. SM-20 was not detected in co ncentrates of the culture medium. By immunohistochemistry, SM-20 was l ocalized to filaments in the cytoplasm of cultured SMC. Like SM-20 mRN A, levels of SM-20 protein were increased 1-3 hours after serum stimul ation of rat aortic SMC. In rat tissues, SM-20 antigen was detected in abundance in smooth, cardiac, and skeletal muscle. SM-20 was also det ected in some epithelial cells of the kidney, pancreas, gastrointestin al tract, and skin, but was not found in the parenchyma of the liver, spleen, or testis. In the rat arterial wall, SM-20 antigen was restric ted to the SMC of the media and, after balloon arterial injury, was fo und most abundantly in the neointima. In human atherosclerotic coronar y arteries, SM-20 antigen predominated in the SMC of the intimal plaqu es and was not detected in macrophages, endothelium, or adventitial ce lls. Thus, SM-20 encodes a protein which serves as a novel SMC-specifi c marker within the vessel wall.