SM-20, an angiotensin II-responsive protein, is a novel marker for human arterial smooth muscle cells (SMC).

We have recently identified SM-20, a novel 3.0 kb growth factor-reponsive mRNA, not expressed in fibroblasts, but present in cardiac, skeletal, and smooth muscle. In this report, affinity purified polyclonal antibodies were raised against a 230 amino acid portion of the putative SM-20 peptide. Western blot analysis of rat aortic SMC lysates revealed a single protein species migrating at ≈40 kD. SM-20 was not detected in concentrates of the culture medium. Immunocytochemistry localized SM-20 protein to the cytoplasm. Like its mRNA, SM-20 protein was induced ≈3-fold 1-2 hrs after exposure to growth factors in cultured SMC. SM-20 antigen was also detected in the SMC of the tunica media of normal rat aorta, but not in the endothelium or adventitial fibroblasts. After aortic balloon injury, SM-20 protein was restricted to the SMC of the media and neointima. A similar distribution, in the SMC of the media and neointima but not in the adventitia, was seen in sections of human coronary arteries and atheromas. All cells stained by SM-20 antibody were costained with an antibody directed against smooth muscle α-actin. Not all cells staining for α-actin costained for SM-20, suggesting that SM-20 may identify a subpopulation of SMC. SM-20 antigen was not detected in cells costaining with a monocyte specific antibody, KP-1. SM-20 thus represents a novel growth factor-inducible protein whose expression in the normal and atherosclerotic vessel wall is specific to the SMC.