Biosynthesis of type I and type III collagens by cultured uterine smooth muscle cells

Cells derived from human endometrium were grown in culture. Over 90% of the cells in the culture were identified as smooth muscle cells, similar to those found in the endometrial junction. Cells grown to confluency after the 10th population doubling level, 4th passage, were incubated for 24 h at 37°C with 10 μCi per flask of l-[2,3-³H]proline, and the radiolabeled collagens thus synthesized were characterized. Collagen plus procollagen (“collagen”) in the culture medium contained about 82% of the total radioactivity incorporated into proteins. In the cell layer, however, “collagen” accounted for only about 30% of the total radioactivity in proteins. About 64% of the “collagen” in the medium was identified as Type I collagen and 36% as Type III collagen. In the cell layer, 74% of the radioactivity was in Type I collagen and 26% in Type III collagen. About 50% of the total Type I “collagen” was isolated from the culture medium and indeed was recovered as collagen, that is very little procollagen was present. In contrast, of the total Type III “collagen” in the culture medium, about 60% was isolated and that fraction was almost all in the form of Type III procollagen. These differences in the amounts of Type III and Type I procollagens isolated from the culture medium suggest possible differences, in culture, of the activities of procollagen peptidases. The biosynthetic ratio of Type I to Type III collagen (3/1) obtained in this study is similar to that reported for smooth muscle cells from human aorta. The uterine cell culture system may offer an excellent model for the exquisitely controlled regulation by hormones of collagen metabolism of the uterus.