Differential availability/processing of decorin precursor in arterial and venous smooth muscle cells

The existence of specific differentiation markers for arterial smooth muscle (SM) cells is still a matter of debate. A clone named MM1 was isolated from a library of monoclonal antibodies to adult porcine aorta, which in vivo binds to arterial but not venous SM cells, except for the pulmonary vein. MM1 immunoreactivity in Western blotting involved bands in the range of M(r) 33-226 kDa, in both arterial and venous SM tissues. However, immunoprecipitation experiments revealed that MM1 bound to a 100-kDa polypeptide that was present only in the arterial SM extract. By mass spectrometry analysis of tryptic digests from MM1-positive 130- and 120-kDa polypeptides of aorta SM extract, the antigen recognized by the antibody was identified as a decorin precursor. Using a crude decorin preparation from this tissue MM1 reacted strongly with the 33-kDa polypeptide and this pattern did not change after chondroitinase ABC treatment. In vitro, decorin immunoreactivity was found in secreted grainy material produced by confluent arterial SM cells, although lesser amounts were also seen in venous SM cells. Western blotting of extracts from these cultures showed the presence of the 33-kDa band but not of the high-molecular-weight components, except for the 100-kDa monomer. The 100/33-kDa combination was more abundant in arterial SM cells than in the venous counterpart. In the early phase of neointima formation, induced by endothelial injury of the carotid artery or vein-to-artery transposition, the decorin precursor was not expressed, but it was up-regulated in the SM cells of the media underlying the neointima in both models. Collectively, these data suggest a different processing/utilization of the 100-kDa monomer of proteoglycan decorin in arterial and venous SM cells, which is abolished after vein injury.