Biosynthesis of chick type VI collagen. II: Processing and secretion in fibroblasts and smooth muscle cells.

The biosynthesis of type VI collagen was studied in "matrix-free" chick embryo smooth muscle cells and fibroblasts. Omission of ascorbate from the culture affected to a great extent the secretion in fibroblasts but had a very minor effect on smooth muscle cells. Quantitative analysis of the secretion process in continuous time course and in pulse-chase experiments confirmed that fibroblasts and smooth muscle cells secreted type VI collagen with the same chain composition but with different kinetics: after 4 h of chase more than 60% of the labeled type VI collagen was present in the culture medium of fibroblasts, whereas at the same time interval less than 25% was secreted by smooth muscle cells. The different kinetics depends on intrinsic properties of the cells, since it was detected also in adherent cells. However, even in fibroblasts, secretion of type VI collagen was much slower than secretion of fibronectin, of which more than 50% was already in the cell medium after 1 h of chase. Treatment of the cells with inhibitors of hydroxylation and glycosylation caused a shift in mobility that revealed a size heterogeneity in the Mr = 260,000 subunit. No evidence of processing was observed in chick cells for any of the subunits that were synthesized and secreted uncleaved. In addition, after several days of chase the Mr of the subunits of type VI collagen isolated from the matrix remained unchanged, thus excluding that in the chick even a partial or incomplete processing takes place.