Occurrence of Oligosialic Acids on Integrin α5 Subunit and Their Involvement in Cell Adhesion to Fibronectin

Abstract

Integrin α5β1, a major fibronectin receptor, functions in a wide variety of biological phenomena. We have found that α2-8-linked oligosialic acids with 5 ≤ degree of polymerization (DP) ≤ 7 occur on integrin α5 subunit of the human melanoma cell line G361. The integrin α5 subunit immunoprecipitated with anti-integrin α5 antibody reacted with the monoclonal antibody 12E3, which recognizes oligo/polysialic acid with DP ≥ 5 but not with the polyclonal antibody H.46 recognizing oligo/polysialic acid with DP ≥ 8. The occurrence of oligosialic acids was further demonstrated by fluorometric C7/C9 analysis on the immunopurified integrin α5 subunit. Oligosialic acids were also found in the α5 subunit of several other human cells such as foreskin fibroblast and chronic erythroleukemia K562 cells. These results suggest the ubiquitous modification with unique oligosialic acids occurs on the α 5 subunit of integrin α5β1. The adhesion of human melanoma G361 cells to fibronectin was mainly mediated by integrin α5β1. Treatment of cells with sialidase from Arthrobacter ureafaciens cleaving α2-3-, α2-6-, and α2-8-linked sialic acids inhibited adhesion to fibronectin. On the other hand, N-acetylneuraminidase II, which cleaves α2-3 and α2-6 but not α2-8 linkages, showed no inhibitory activity. After the loss of oligosialic acids, integrin α5β1 failed to bind to fibronectin-conjugated Sepharose, indicating that the oligosialic acid on the α5 subunit of integrin α5β 1 plays important roles in cell adhesion to fibronectin.