EXPRESSION OF VARIANT FIBRONECTINS IN WOUND-HEALING - CELLULAR SOURCEAND BIOLOGICAL-ACTIVITY OF THE EIIIA SEGMENT IN RAT HEPATIC FIBROGENESIS

We have examined the cell-specific expression of two fibronectin isofo rms, EIIIA and EIIIB, during experimental hepatic fibrosis induced by ligation of the biliary duct. At the mRNA level, EIIIA and EIIIB were undetectable in normal liver but expressed early in injury, preceding fibrosis. The cellular sources of these changes were determined by fra ctionating the liver at various time points after bile duct ligation i nto its constituent cell populations and extracting RNA from the fresh isolates. EIIIA-containing fibronectin mRNA was undetectable in norma l sinusoidal endothelial cells but increased rapidly within 12 h of in jury. By contrast, the EIIIB form was restricted to hepatic lipocytes (Ito or fat-storing cells) and appeared only after a lag of 12-24 h: i t was minimal in sinusoidal endothelial cells. Both forms were minimal in hepatocytes. At the protein level, EIIIA-containing fibronectin wa s markedly increased within two days of injury and exhibited a sinusoi dal distribution. Secretion of this form by endothelial cells was conf irmed in primary culture. Matrices deposited in situ by endothelial ce lls from injured liver accelerated the conversion (''activation'') of normal lipocytes to myofibroblast-like cells, and pretreatment of matr ices with monoclonal antibody to the EIIIA segment blocked this respon se. Finally, recombinant fibronectin peptide containing the EIIIA segm ent was stimulatory to lipocytes in culture. We conclude that expressi on of EIIIA fibronectin by sinusoidal endothelial cells is a critical early event in the liver's response to injury and that the EIIIA segme nt is biologically active, mediating the conversion of lipocytes to my ofibroblasts.