Lipoproteins enhance fibronectin binding to adherent cells

We have identified very low density (VLDL) and low density (LDL) lipoproteins as blood plasma components that enhance the binding and deposition of fibronectin into the extracellular matrices of cultured MG-63 osteosarcoma cells and human fibroblasts. The lipoproteins increased the binding and deposition of iodinated fibronectin by MG-63 cells threefold over control levels. LDL also increased the deposition of multimeric fibronectin into extracellular matrix as assessed by gel electrophoresis and fluorescence microscopy. High density lipoprotein (HDL) and the d > 1.21 g/ml nonlipoprotein fraction had less activity. Enhancement of binding of fibronectin was observed within 15 minutes, when binding was largely reversible. LDL also increased the binding of a fragment containing the 70-kd amino-terminal region of fibronectin that is primarily responsible for the reversible binding of fibronectin to cell layers. LDL had to be present simultaneously with radiolabeled fibronectin to exert an effect on fibronectin binding. LDL enhanced fibronectin binding equally well to normal skin fibroblasts and to familial hypercholesterolemic fibroblasts lacking the LDL receptor. Acetylation of LDL, performed to block its interaction with the LDL receptor, did not diminish the enhancement of fibronectin binding to MG-63 cells. These results indicate that LDL and VLDL interact with fibronectin to potentiate binding to monolayer cells through a pathway that does not involve the LDL receptor.