Synthesis of undulin by rat liver fat-storing cells: comparison with fibronectin and tenascin.
Fat-storing cells (FSCs) are known to synthesize various components of the hepatic extracellular matrix and thereby play an important role during liver fibrogenesis. The aim of our study was to investigate the synthesis of undulin, a recently described connective tissue protein belonging to the fibronectin-tenascin superfamily of glycoproteins, by fat-storing cells in primary culture. SDS-PAGE analysis of immunoprecipitates from cell layer lysates or media pulse-labeled with radioactive methionine revealed undulin-specific bands A (270 kDa), B1 (190 kDa), and B2 (180 kDa) after reduction. A single undulin-specific transcript was detected at about 7 kb. Undulin synthesized by cell-free translation revealed two polypeptides migrating about 5000 Da below the B1 and B2 subunits. Treatment of FSCs with tunicamycin created two novel bands slightly below the B2 chain. Since the electrophoretic patterns of undulin chains recovered by cell-free translation and tunicamycin treatment of cells were very similar we suggest that N-glycosylation is the major post-translational processing event. Newly synthesized undulin was detected after 30 min of pulse labeling in the cell layer fraction and was secreted into the medium at a slower rate than fibronectin. In contrast to fibronectin and tenascin, undulin was already synthesized by freshly isolated FSCs and during the early stage of primary FSC culture ("resting" cells), supporting the hypothesis that undulin is associated with a differentiated mesenchyma. However, in analogy to fibronectin and tenascin, undulin was also synthesized by "activated" FSCs, indicating that undulin might also be of importance in dedifferentiated tissues.