Type VIII collagen. Synthesis by normal and malignant cells in culture.

A novel protein belonging to the collagen family was originally purified from the culture medium of bovine aortic endothelial cells. This endothelial collagen, termed EC, was also found to comprise the major collagen type synthesized by a malignant astrocytoma-derived cell line. Examination of several cell strains derived from normal tissues revealed that EC was not synthesized by all endothelial cells; it was absent from human endothelial cells cultured from both large and small vessels but was present in bovine cells, including those from capillaries. Human foreskin fibroblasts also secreted this protein in small amounts relative to interstitial procollagens, but it was not detected in two human epithelial cell strains. EC was consistently observed in human cell lines derived from several carcinomas and comprised the major collagenous protein secreted by cells cultured from a Ewing's sarcoma. In contrast, malignant or transformed murine cells did not produce EC in vitro. In addition, the protein was not apparent after metabolic labeling of human cells from an epidermoid carcinoma, a fibrosarcoma, and two Wilms' tumors. EC-like proteins were isolated from cell culture medium by ion-exchange chromatography and were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after cleavage with vertebrate collagenase, mast cell protease, and CNBr. In addition to the homologies displayed by comparative peptide mapping, these collagens exhibited other unusual properties that collectively were characteristic of EC from endothelial and astrocytoma-derived cells. These studies support the existence of a novel class of collagenous proteins that are secreted by a wide variety of cells derived from both normal and neoplastic tissues. This class of proteins, which manifests several unusual structural characteristics, has been designated type VIII collagen.