Type I collagen is a non-adhesive extracellular matrix for macrophages

Macrophages adhere to a variety of substrata including plastic, glass or an extracellular matrix either in a highly specific manner or through less specific mechanisms. We investigated the effect of type I collagen, the most abundant protein in animal tissues, on the adhesion of macrophages derived from a human monoblastic cell line U937. Macrophages were observed to adhere very weakly to type I collagen and aggregate, whereas they adhered firmly and spread on plastic, bovine serum albumin or fibronectin. On the adhesive substratum, the lower surface of the macrophages was flat and closely apposed to the substratum. In contrast, macrophages adhered on type I collagen at the tip of cell processes. The adhesion of macrophages to plastic, bovine serum albumin or fibronectin was associated with the induction of tyrosine phosphorylation of a variety of proteins including a major protein band at 66 kDa. In contrast, the induction of tyrosine phosphorylation was markedly reduced when the macrophages were cultured on type I collagen. Two members of the src family, Lyn and Hck, were tyrosine phosphorylated in firmly adhered macrophages but not in macrophages cultured on type I collagen. These results suggest that the adhesion of macrophages is associated with the tyrosine phosphorylation of a variety of proteins including Lyn and Hck, and that type I collagen serves as a non-adhesive substratum for macrophages, resulting in an altered signal transduction.