Assembly of exogenous fibronectin by fibronectin-null cells is dependent on the adhesive substrate

The role of endogenously synthesized fibronectin (FN) in assembly was studied with cells lacking or expressing FN. Cells were cultured as homogeneous or mixed populations on surfaces coated with different matrix proteins. Compared with FN-expressing cells, FN-null cells poorly assembled exogenous plasma FN (pFN) when adhered to vitronectin or the recombinant cell-binding domain (III_7-10) of FN. Vitronectin had a suppressive effect that was overcome by co-adsorbed pFN or laminin-1 but not by soluble FN. In co-cultures of FN-expressing cells and FN-null cells, endogenous FN was preferentially assembled around FN-expressing cells regardless of the adhesive ligand. If the adhesive ligand was vitronectin, exogenous pFN assembled preferentially around cells expressing cellular FN or recombinant EDa- or EDa+ FN. In co-cultures on vitronectin of FN-null cells and β₁ integrin subunit-null cells, fibrils of cellular FN and pFN were preferentially deposited by FN-null (β₁-expressing) cells immediately adjacent to (FN-secreting) β₁-null cells. In co-cultures on vitronectin of FN-null cells and β₁-null cells expressing a chimera with the extracellular domain of β₁ and the cytoplasmic domain of β₁, preferential assembly was by the chimera-expressing cells. These results indicate that the adhesive ligand is a determinant of FN assembly by cells not secreting endogenous FN (suppressive if vitronectin, non-suppressive but non-supportive III_7-10, supportive if pFN or laminin-1) and suggest that efficient interaction of freshly secreted cellular FN with a β₁ integrin, presumably α₅β₁, substitutes for integrin-mediated adherence to a preformed matrix of laminin-1 or pFN to support assembly of FN.