Integrin and Arg-Gly-Asp dependence of cell adhesion to the native and unfolded triple helix of collagen type VI

Pepsin-solubilized collagen VI in triple-helical and heat-denatured, unfolded form was shown to promote Mg²⁺- and Mn²⁺-dependent attachment and spreading of various cell lines. On the triple-helical substrate no inhibition of cell adhesion was observed with several synthetic RGD peptides except in the case of A375 melanoma cells. In contrast, adhesion to the unfolded substrate was highly sensitive to RGD inhibition. Nine synthetic peptides were designed according to 10 RGD sequences present in the triple-helical sequence of human collagen α1(VI), α2(VI), and α3(VI) chains. Only one peptide, corresponding to the C-terminal end of α3(VI) chain, showed substantial inhibitory activity, whereas several peptides were active in direct adhesion assays when used as albumin conjugates. Inhibition tests with antibodies to integrin subunits, affinity chromatography, and ligand binding with purified integrins (α1β1, α2β1, αVβ3, and αIIβ3) were used to identify collagen VI receptors. Binding to the triple-helical substrate is mediated by α1β1 and α2β1 integrins. Binding of both integrins to collagen VI was weaker than that to collagens I and/or IV. Recognition of the denatured substrate is mediated by β1 and β3 integrins. Activity was shown for α5β1 and αVβ3 and weakly for αIIbβ3 but not all α subunits possibly involved were identified. Distinct sets of receptors were also involved in A375 cell binding to triple-helical (β1-mediated) and denatured (β3-mediated) collagen VI, even though in this case both interactions could be efficiently inhibited by RGD peptides.