Fibronectin metabolism by human mesangial cells: effects of collagens

In the present study we assessed whether the fibronectin (FN) metabolism of human mesangial cells (HMC) in culture is influenced by the contact of HMC with collagens type I and IV. HMC were grown on collagen gels or on collagen-coated surfaces (collagen films). FN concentrations were measured by enzyme-linked immunosorbent assay; FN synthesis was measured by metabolic labeling and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In addition, the structure of matrix FN was examined by immunofluorescence microscopy. Compared with cells grown on plastic, HMC on collagen gels or collagen films accumulated greater amounts of FN in the cell matrix, and in these cultures, matrix FN was organized into a complex mesh of fibers. FN fiber formation was more prominent in cells adherent to collagen IV than in cells adherent to collagen I, and these fibers were observed as early as day 1 in culture. HMC adherent to plastic deposited matrix FN as patches and only occasionally as FN fibers localized to the periphery of the cell. The accumulation of FN in the matrix of HMC on collagen was not due to an increased rate of FN synthesis. In fact, HMC on collagen gels synthesized less FN than HMC on plastic. The present results indicate that the accumulation of FN in the matrix of HMC on collagen is due to the fact that this FN is less likely to be released into the supernatant than the matrix FN produced by HMC on plastic. The decreased FN synthesis demonstrated by HMC on collagen gels was associated with an overall decrease in protein synthesis but was not associated with a decrease in FN mRNA levels. Finally, FN isolated from HMC on collagen gels contained a unique 90-kDa gelatin-binding FN fragment. In conclusion, collagens have effects on the synthesis, localization, organization, and catabolism of FN produced by HMC in culture. In particular, collagen IV, the collagen normally present in the glomerular mesangium, appears to influence uniquely the organization of mesangial matrix FN.