Temporary effects of circulating Amadori products on glomerular filtration properties in the normal mouse

Amadori albumin in diabetic nephropathy

Nonenzymatic glycation of macromolecules in diabetes mellitus (DM) is accelerated due to persistent hyperglycemia. Reducing sugar such as glucose reacts non enzymatically with free €-amino groups of proteins through series of reactions forming Schiff bases. These bases are converted into Amadori product and further into AGEs. Non enzymatic glycation has the potential to alter the biological, structural and functional properties of macromolecules both in vitro and in vivo. Studies have suggested that amadori as well as AGEs are involved in the micro-macro vascular complications in DM, but most studies have focused on the role of AGEs in vascular complications of diabetes. Recently putative AGE-induced patho-physiology has shifted attention from the possible role of amadori-modified proteins, the predominant form of the glycated proteins in the development of the diabetic complications. Human serum albumin (HSA), the most abundant protein in circulation contains 59 lysine and 23 arginine residues that could, in theory be involved in glycation. Albumin has dual nature, first as a marker of intermediate glycation and second as a causative agent of the damage of tissues. Among the blood proteins, hemoglobin and albumin are the most common proteins that are glycated. HSA with a shorter half life than RBC, appears to be an alternative marker of glycemic control as it can indicate blood glucose status over a short period (2-3 weeks) and being unaffected by RBCs life span and variant haemoglobin, anemia etc which however, affect HbA1c. On the other hand, Amadori albumin may accumulate in the body tissues of the diabetic patients and participate in secondary complications. Amadori-albumin has potential role in diabetic glomerulosclerosis due to long term hyperglycaemia and plays an important role in the pathogenesis of diabetic nephropathy. This review is an approach to compile both the nature of glycated albumin as a damaging agent of tissues and as an intermediate diagnostic marker and its potential role in diabetic nephropathy.

Processing Advanced Glycation End Product-Modified Albumin by the Renal Proximal Tubule and the Early Pathogenesis of Diabetic Nephropathy

Diabetes is characterized by increased quantities of circulating proteins modified by advanced glycation end products (AGEs). Proteins filtered at the glomerulus and presented to the renal proximal tubule are likely to be highly modified by AGEs. The proximal tubule binds, takes up, and catabolizes AGE-modified albumin by pathways different from those of unmodified albumin. These differences were examined in polarized, electrically resistant proximal tubular cells grown in monolayer culture. In patients with type 1 diabetes, urinary excretion of a lysosomal enzyme predicted the development of nephropathy.

Glomerular handling of native albumin in the presence of circulating modified albumins by the normal rat kidney

Persistent hyperglycemia, as occurring in diabetes, induces changes in circulating as well as in structural proteins. These changes involve substitution of lysine residues by glucose adducts resulting in early Amadori products that evolve into toxic and active substances, the advanced glycation end adducts. In previous studies, we demonstrated that early glycated (Amadori) albumin infused into the circulation of normal animals induces transitory alterations of glomerular filtration. Attempting to elucidate the mechanisms underlying these changes, various molecular modifications were introduced in vitro to serum albumin. Glycation, acetylation, carboxymethylation, methylation, and succinylation, involving either a few or a significant number of amino acid residues, produced heavier and more anionic albumin molecules compared with the native one. Native and each of the modified albumin molecules were injected intravenously into normal rats, followed, 30 min later, by hapten-tagged native BSA. Changes in glomerular filtration were evaluated by morphometrical analysis of gold immunolabelings. Compared with native albumin, all the modified forms of albumin induced a deeper penetration of the tracer through the glomerular basement membrane revealing alterations in glomerular permselectivity. This was more evident for severely modified albumin molecules which displayed high labelings in the urinary space and endocytic compartments of proximal tubule epithelial cells. These results indicate that modifications of serum albumin, even minimal, as those occurring in early diabetes, could immediately affect the permselectivity properties of the glomerular wall leading, with time, to severe glomerulopathies.

Identification of Amadori-modified plasma proteins in type 2 diabetes and the effect of short-term intensive insulin treatment

OBJECTIVE

Growing evidence supports that nonenzymatic glycation products may cause hyperglycemia-induced diabetes complications. Amadori-modified proteins are the intermediate products of nonenzymatic glycation and constitute the forms of glycated proteins in diabetes. The objective of the current study was to utilize two-dimensional gel electrophoresis, Western blot, and mass spectrometry to identify Amadori-modified plasma proteins in type 2 diabetic patients with poor glycemic control and assess the impact of short-term insulin treatment on the glycation of these proteins.

RESEARCH DESIGN AND METHODS

We compared eight type 2 diabetic subjects (aged 56 +/- 3 years and BMI 29.7 +/- 0.9 kg/m(2)) with an average diabetes duration of 8.5 years (range 3-19) with equal numbers of weight-matched (aged 56 +/- 2 years and BMI 30.1 +/- 10.0 kg/m(2)) and lean (aged 58 +/- 2 years and BMI 25 +/- 00.5 kg/m(2)) nondiabetic subjects who have no first-degree relatives with diabetes. Two separate blood samples were collected from the type 2 diabetic subjects, one following 2 weeks of withdrawal of all antidiabetic medications (T(2)D-; plasma glucose 12.6 +/- 1.0 mmol/l) and another following 10 days of intensive insulin treatment (T(2)D+; plasma glucose 5.5 +/- 0.2 mmol/l). Plasma proteins were separated using single and two-dimensional gel electrophoresis. Western blot analysis was performed, and several proteins, which reacted with the Amadori-antibody (1-deoxyfructosyl lysine), were identified by tandem mass spectrometry.

RESULTS

No significant differences in the glycation of proteins between the obese and lean groups were noted, but type 2 diabetic patients had several proteins with higher glycation than the control groups. We identified 12 plasma proteins with reduced reaction to the anti-Amadori antibody upon intensive insulin treatment. A significant (P < 0.03) difference in Amadori modification was observed between the T(2)D- and control subjects for all these proteins except the Ig light chain. Insulin treatment reduced Amadori modification of albumin (23.2%, P < 0.02), fibrin (34.6%, P < 0.001), Ig heavy chain constant region (20.7%, P < 0.05), transferrin (25.4%, P < 0.04), and Ig light chain (13%, P < 0.02). In addition, Western blot analysis of two-dimensional gel electrophoresis identified alpha-fibrinogen precursor, beta-fibrinogen precursor, fibrinogen gamma-B chain precursor, hemopexin, vitamin D binding protein, and serine protease inhibitor as proteins with a reduced reaction to anti-Amadori antibody upon intensive insulin treatment.

CONCLUSIONS

The current approach offers the opportunity to identify Amadori modification of many proteins that may cause functional alterations and offers the potential for monitoring short-term glycemic control in diabetic patients.