Selective induction of an embryonic fibronectin isoform in the rat aorta in vitro

The temporal changes in the expression of fibronectin and other extracellular matrix genes were studied in rat aortic rings incubated in vitro in a serum-free medium. Changes in all forms of fibronectin mRNA increased progressively during the 24-hour incubation period, although an increase in the alternatively spliced form of fibronectin designated EIIIA was most pronounced. Both collagen and elastin mRNA levels decreased markedly during the 24-hour interval, as did alpha-actin mRNA. The increase in the relative amount of the EIIIA isoform after a 24-hour incubation was also shown using ribonuclease protection assays. In situ hybridization showed the distribution of the induced fibronectin mRNA to be within all cell types, including endothelial cells, medial smooth muscle cells, and adventitial fibroblasts. Localization in the media was not uniform and was clearly identified mainly in clusters of cells distributed throughout the media. The early induction of fibronectin mRNA was inhibited by genistein, implicating tyrosine kinase activation as a causative factor in fibronectin expression. The in vitro changes reported may reflect a phenotypic change in vascular cell types that is both similar to and different from the changes reported in vivo under conditions in which vascular injury and repair occur.