Increased plasma fibronectin concentration in diabetic patients with microalbuminuria

Markers of diabetic nephropathy

It is well established that the detection of microalbuminuria in a patient with diabetes mellitus indicates the presence of glomerular involvement in early renal damage. Recent studies have demonstrated that there is also a tubular component to renal complications of diabetes, as shown by the detection of renal tubular proteins and enzymes in the urine. In fact, tubular involvement may precede glomerular involvement, as several of these tubular proteins and enzymes are detectable even before the appearance of microalbuminuria. This review looks at the studies reported so far on serum and urinary markers of diabetic nephropathy, both glomerular and tubular, and their roles in the early detection of renal damage. The advantages and disadvantages of some of these markers are also discussed. The markers reviewed include (1) glomerular--transferrin, fibronectin, and other components of glomerular extracellular matrix, and (2) tubular--low molecular weight proteins (beta 2 microglobulin, retinol binding protein, alpha 1 microglobulin, urine protein 1), other proteins such as Tamm-Horsfall protein, beta 2 glycoprotein-1, urinary enzymes (N-acetyl-beta-D-glucosaminidase, cholinesterase, gamma glutamyltranspeptidase, alanine aminopeptidase), and tubular brush-border antigen.

Urinary fibronectin excretion in streptozotocin-diabetic rats

Previous investigations performed in human diabetics demonstrate an increase in their urinary fibronectin excretion which was already present in subjects without microalbuminuria and which was elevated prior to functional restrictions. The present study was performed to examine whether in an experimental model these data obtained in men can be confirmed using an animal experimental model, and to further study pathomechanisms of diabetic nephropathy in rats. Fibronectin levels in serum and urine, and renal functional properties such as creatinine clearance, urinary albumin and protein excretion were studied in rats rendered diabetic with streptozotocin and compared with values of control and insulin treated animals for 5 months. Diabetic animals demonstrated the same creatinine clearance, but slightly decreased albumin and total protein excretion rates compared to controls and insulin "treated", euglycaemic animals. Diabetic rats showed a significantly increased excretion following day 42 compared to controls and insulin "treated" group. Concerning serum fibronectin, there was no significant difference between control, diabetic and insulin "treated" animals. The urinary fragment pattern of fibronectin was analyzed qualitatively by immunoblotting pattern and consisted of two main bands (M(r) 66,000 and 45,000). These bands were not altered in controls, insulin "treated" and diabetic rats, independent of the stage of renal involvement in diabetes. Present data provide evidence that fibronectin excretion is elevated in diabetic animals prior to functional restrictions, confirming results obtained in human diabetics. Therefore, determination of urinary fibronectin can serve as a more sensitive indicator for renal involvement in diabetes mellitus than microalbuminuria or changes in glomerular filtration rate. Urinary excretion may therefore serve as an early marker for the renal involvement in diabetes before the onset of clinical symptoms.

Nonenzymatic glycation of fibronectin impairs adhesive and proliferative properties of human vascular smooth muscle cells

Nonenzymatic glycation of proteins is involved in the pathogenesis of diabetes vascular complications. Extracellular matrix proteins are a prominent target for nonenzymatic glycation because of their slow turnover rates. The aim of this study was to investigate the influence of human fibronectin (F) nonenzymatic glycation on adhesion and proliferation of cultured human vascular smooth muscle cells (hVSMC). Incubation of human F with 500 mmol/L D-glucose at 37 degrees C induced a time-dependent increase in fluorescence detectable at 440 nm after excitation at 363 nm. Nonenzymatic glycation did not affect binding of F itself to the plates. Adhesion of hVSMC to F increased with the increase of incubation time of the cells on the protein from 30 minutes up to 120 minutes and remained stable thereafter. Adhesion to glycated fibronectin (GF) was reduced in comparison to control F at all the different adhesion times. Adhesion of hVSMC to GF was reduced when F was exposed to glucose for 4, 9, or 28 days (P=.0417 to .0025), but not when F was exposed for 1 day. Adhesion of hVSMC to GF was reduced compared with adhesion to nonglycated F at all coating concentrations from 0.2 to 10 micrograms/mL (P=.05 to .014). Thus, nonenzymatic glycation of F impairs adhesion of hVSMC in vitro. Proliferation of hVSMC on F increased with increasing concentrations of the protein as coating agent (ANOVA:P<.0001 for both nonglycated F and GF). Proliferation with F glycated for 4, 9, and 28 days was reduced at concentrations of 1, 3, and 10 micrograms/mL as compared with proliferation with nonglycated F (P=.0253 to .0001). Proliferation on F glycated for only 1 day was not significantly reduced. When the number of hVSMC plated on control F was reduced by 25% to take into account the reduced adhesion, the number of cells that proliferated on F was still reduced. In conclusion, nonenzymatic glycation of F impairs adhesive and proliferative properties of hVSMC.

Plasma free N-terminal fibronectin 30-kDa domain as a marker of endothelial dysfunction in type 1 diabetes mellitus

The plasma free N-terminal fibronectin 30-kDa domain was measured in 44 type 1 diabetic patients and in 20 healthy subjects. A significantly raised mean concentration of a free N-terminal fibronectin 30-kDa domain was found in plasma of diabetic patients with proliferative retinopathy as compared with healthy persons (P less than 0.001). A positive correlation was observed between free N-terminal fibronectin 30-kDa domain and von Willebrand factor in plasma of all examined subjects (r = 0.62, P less than 0.01). A similar correlation was present between 30-kDa domain and albuminuria (r = 0.56, P less than 0.01). However, no relationship was found between fibronectin 30-kDa domain and control of diabetes as assessed by fructosamine concentration. The free N-terminal fibronectin 30-kDa domain may be used as a marker of actual endothelial cell dysfunction in diabetes.