Type V collagen regulates the assembly of collagen fibrils in cultures of bovine vascular smooth muscle cells

Vascular smooth muscle cells (SMCs), the major cellular constituent of the medial layer of an artery, synthesize the majority of connective tissue pro teins, including fibrillar collagen types I, Iii, and V/XI. Proper collagen synthesis and deposition, which are important for the integrity of the art erial wall, require the antioxidant vitamin C. Vitamin C serves as cofactor for the enzymes prolyl and lysyl hydroxylase, which are responsible for th e proper hydroxylation of collagen. Here, the role of type V collagen in th e assembly of collagen fibrils in the extracellular matrix (ECM) of culture d vascular SMCs was investigated. Treatment of SMCs with vitamin C resulted in a dramatic induction in the levels of the cell-layer associated pepsin- resistant type V collagen, whereas only a minor induction in the levels of types I and III collagen was detected. Of note, the deposition of type V co llagen was accompanied by the formation of striated collagen fibrils in the ECM. Immunohistochemistry demonstrated that type V collagen, but not type I collagen, became masked as collagen fibrils matured. Furthermore, the rel ative ratio of type V to type I collagen decreased as the ECM matured as a function of days in culture, and this decrease was accompanied by an increa se in the diameter of collagen fibrils. Together these results suggest that the masking of type V collagen is caused by its internalization on continu ous deposition of type I collagen on the exterior of the fibril. Furthermor e, they suggest that type V collagen acts as framework for the initial asse mbly of collagen molecules into heterotypic fibrils, regulating the diamete r and architecture of these fibrils. (C) 2000 Wiley-Liss, Inc.