Fibronectin augments monocyte adhesion to low-density lipoprotein-stimulated mesangial cells

Background. Glomerular monocyte infiltration is an early feature of lipid-mediated renal injury in animal models. Interactions between mesangial and infiltrating mononuclear cells may contribute to the development of glomerular scarring. Methods. Adherence of U-937 monocytes to low-density lipoprotein (LDL)- or tumor necrosis factor alpha (TNF alpha)-prestimulated human mesangial cells was assessed by colorimetry of nuclear staining with crystal violet. Blocking antibodies were added to examine the mechanisms of binding. Adhesion molecule expression and fibronectin synthesis were measured by ELISA. Results. Preincubation of mesangial cells for 24 hours with LDL (100 mu g/ml) or mildly oxidized (minimally modified) LDL (MM-LDL) increased monocyte adhesion by 207% and 240%, respectively, compared with control nonstimulated cells (100%). TNF alpha (100 U/ml) enhanced binding by 335% and up-regulated intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) expression by 505% and 179%, respectively, as compared with MM-LDL (120% and 116%) and LDL, which had no effect. Blocking antibodies to these adhesion molecules inhibited monocyte binding to TNF alpha- and, to a lesser extent, MM-LDL-primed mesangial cells, but had no effect after LDL pretreatment. In contrast to TNF alpha, MM-LDL and LDL increased mesangial cell-associated fibronectin, whereas antibodies to fibronectin inhibited monocyte binding to lipoprotein-stimulated but not TNF alpha-stimulated cells. Conclusions. Although enhanced monocyte adhesion to TNF alpha- and, to a lesser extent, MM-LDL-stimulated mesangial cells is mediated by changes in ICAM-1 and VCAM-1 expression, both LDL and MM-LDL promote similar cellular interactions as a result of increased fibronectin production.