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.

Interaction of collagen molecules from the aspect of fibril formation: acid-soluble, alkali-treated, and MMP1-digested fragments of type I collagen

Collagen type I extracted with acid or digested with pepsin forms fibrils under physiological conditions, but this ability is lost when the collagen is treated with alkaline solution or digested with matrix metalloproteinase 1 (MMP1). When acid-soluble collagen was incubated with alkali-treated collagen, the fibril formation of acid-soluble collagen was inhibited. At 37 degrees C, at which alkali-treated collagen is denatured, the lag time was prolonged but the growth rate of fibrils was not affected. At 30 degrees C, at which the triple helical conformation of alkali-treated collagen is retained, the lag time was prolonged and the growth rate reduced. Heat-denatured alkali-treated collagen and MMP1-digested fragments have no inhibitory effect on the fibril formation of acid-soluble collagen. This means that the triple helical conformation and the molecular length are important factors in the interaction of collagen molecules and that alkali-treated collagen acts as a competitive inhibitor for fibril formation of collagen. We found that alkali-treated collagen and MMP1-digested fragments form fibrils that lack the D periodic banding pattern and twisted morphology under acidic conditions at the appropriate ionic strength. We also calculated the relative strengths of hydrophobic and electrostatic interactions between collagen molecules. When the hydrophobic interaction between linear collagen molecules was considered, we found a pattern of periodic maximization of the interactive force including the D period. On the other hand, the electrostatic interaction did not show the periodic pattern, but the overall interaction score affected fibril formation.

Alkali-treated collagen retained the triple helical conformation and the ligand activity for the cell adhesion via alpha2beta1 integrin

Alkaline treatment is a good method for extracting collagen with high recovery even from an aged animal specimen. However, the properties of collagen treated under alkaline conditions have not been well established yet. By the treatment with a solution of 3% sodium hydroxide and 1.9% monomethylamine, the isoelectric point of type I collagen was lowered from 9.3 to 4.8 because of the conversions of Asn and Gln to Asp and Glu. With the acidification of the pI, the denaturation temperature of the collagen was decreased from 42 to 35 degrees C after 20 d treatment, but the collagen-specific triple helical conformation was maintained. Human keratinocytes and fibroblasts adhered to the alkali-treated collagen via the collagen receptor integrin alpha2beta1. This indicates that the alkali-treated collagen maintained its property as a biological adherent molecule. Unlike acid-soluble collagen, alkali-treated collagen lost the ability to form fibrils at neutral pH under physiological conditions. This ability was lost even after 4 h of alkaline treatment, when the denaturation temperature of the collagen did not change. On the other hand, the alkali-treated collagen formed a fibrous precipitate with a uniform diameter of 50-70 nm under acidic conditions at 30 degrees C.