The filtration unit of the kidney is the glomerulus, a capillary network supported by mesangial cells and extracellular matrix (ECM). Glomerular function is compromised in
diabetic nephropathy (DN) by uncontrolled buildup of ECM, especially
type IV collagen, which progressively occludes the capillaries. Increased levels of the ECM
protein fibronectin (FN) are also present; however, its role in DN is unknown. Mesangial cells cultured under high
glucose conditions provide a model system for studying the effect of elevated
glucose on deposition of FN and
collagen IV. Imaging of mesangial cell cultures and analysis of
detergent-insoluble matrix show that, under high
glucose conditions, mesangial cells assembled significantly more FN matrix, independent of FN
protein levels. High
glucose conditions induced
protein kinase C-dependent β1
integrin activation, and FN assembly in normal
glucose was increased by stimulation of
integrin activity with Mn(2+).
Collagen IV incorporation into the matrix was also increased under high
glucose conditions and colocalized with FN fibrils. An inhibitor of FN matrix assembly prevented
collagen IV deposition, demonstrating dependence of
collagen IV on FN matrix. We conclude that high
glucose induces FN assembly, which contributes to
collagen IV accumulation. Enhanced assembly of FN might facilitate dysregulated ECM accumulation in DN.