Effect of glycation of albumin on its binding to renal brush-border membrane vesicles: influence of aging in rats

Mechanism of increased clearance of glycated albumin by proximal tubule cells

Serum albumin is the most abundant plasma protein and has a long half-life due to neonatal Fc receptor (FcRn)-mediated transcytosis by many cell types, including proximal tubule cells of the kidney. Albumin also interacts with, and is modified by, many small and large molecules. Therefore, the focus of the present study was to address the impact of specific known biological albumin modifications on albumin-FcRn binding and cellular handling. Binding at pH 6.0 and 7.4 was performed since FcRn binds albumin strongly at acidic pH and releases it after transcytosis at physiological pH. Equilibrium dissociation constants were measured using microscale thermophoresis. Since studies have shown that glycated albumin is excreted in the urine at a higher rate than unmodified albumin, we studied glucose and methylgloxal modified albumins (21 days). All had reduced affinity to FcRn at pH 6.0, suggesting these albumins would not be returned to the circulation via the transcytotic pathway. To address why modified albumin has reduced affinity, we analyzed the structure of the modified albumins using small-angle X-ray scattering. This analysis showed significant structural changes occurring to albumin with glycation, particularly in the FcRn-binding region, which could explain the reduced affinity to FcRn. These results offer an explanation for enhanced proximal tubule-mediated sorting and clearance of abnormal albumins.

Binding of Amadori glucose-modified albumin by the monocytic cell line MonoMac 6 activates protein kinase C epsilon protein tyrosine kinases and the transcription factors AP-1 and NF-kappaB

An affinity purification procedure is employed for the isolation of FL-specific binding proteins from MM6 cell membranes using magnetobeads coated with glycated polylysine and elution with FL and glycated 6-aminocaproic acid. Two main binding proteins were identified as membrane-bound nucleolin and cellular myosin heavy chain, which are glycosylated. This study shows that in these cells binding of short-term glycated albumin leads to activation of PKC, especially its isoform epsilon and this is linked to translocation of AP-1 and NF-kappaB into the nucleus. Consequently, an increased formation of IL-1ss mRNA is observed. The PKC inhibitor GO6976 prevents all these effects. Glycated albumin also stimulates activation of PTK. The PTK inhibitor genistein prevents activation of AP-1 indicating that PTK is also involved in this process, whereas NF-kappaB translocation is only dependent on PKC activation.

Hemoglobin binding sites on renal brush-border membranes

Prolonged exposure of renal tubules to hemoglobin markedly reduces kidney function and eventually leads to acute renal failure called pigment nephropathy. Intracellular hemoglobin toxicity is one of main pathomechanisms involved in the disease development. However, the process in which hemoglobin is taken up by renal tubular epithelium has not been characterized so far. Isolated renal brush-border membranes of the rat and radioiodinated rat and human hemoglobins were used. Binding properties were examined by the use of rapid filtration technique. Partial isolation of hemoglobin binding proteins was achieved by affinity chromatography. Our experiments showed that both human and rat hemoglobins can be specifically bound to renal brush-border membranes by one class of low affinity (Kd, 7.7 microM) and high capacity (Bmax, 0.18 nmol/mg protein) binding sites. The sites were relatively selective for hemoglobin. Albumin did not compete with hemoglobin. Cationic molecules cytochrome C and lysine exhibited some competition while strong competition of myoglobin was observed. The binding was affected by EGTA indicating a Ca2+ requirement for the interaction. There was a rise in binding in pH 5.4. Fall in binding activity after preincubation of the membranes with peptidases suggested the proteinaceous nature of the binding sites. Affinity chromatography of membrane proteins extract yielded heterogeneous preparation consisting of proteins with molecular masses of 110, 72, 38 and 27 kDa respectively. The existence of binding sites for hemoglobin in renal brush-border membranes strongly suggests that uptake of the protein by tubular epithelia occurs via adsorptive endocytosis. Increased binding of hemoglobin to the membranes under acidic conditions may explain exacerbation of hemoglobinuric acute renal failure in aciduric states.

Inhibition of nitric oxide synthase activity by early and advanced glycation end products in cultured rabbit proximal tubular epithelial cells

Nitric oxide (NO) is important in the regulation of renal tubular function. We have investigated whether glycated proteins could impair the NO production by examining the effects of Amadori products (AP-BSA) and advanced glycation end products (AGE-BSA) on primary cultures of rabbit proximal tubular epithelial (PTE) cells. Nitric oxide synthase activity was assessed by measurement of the conversion of L-arginine to L-citrulline and by production of NO, after short-term (30 min) or long-term (1 or 3 days) incubation. Short incubations of PTE cells with either 200 microg/ml AP-BSA or 40 microg/ml AGE-BSA significantly decreased NO production. AP-BSA (3000 microg/ml) inhibited the Ca(2+)-dependent NOS activity even though above 50 microg/ml it increased Ca(2+)-independent NOS activity. In contrast, 40 microg/ml AGE-BSA inhibited both isoforms of NOS. Longer incubations with 200 microg/ml AP-BSA or 250 microg/ml AGE-BSA decreased NO release and inhibited Ca(2+)-dependent and -independent NOS activities. APs did not affect NO release by S-nitroso-N-acetyl-penicillamine (SNAP), while 250 microg/ml AGEs decreased it. After 3 days incubation, glycation products had no effect on the NOS cell content. Cell viability and proliferation were not modified under these experimental conditions, suggesting that the fall in NO production was not due to there being fewer cells. These data indicate that APs and AGEs directly inhibit NOS activity, and additionally that AGEs quench released NO. Thus, both types of glycated proteins alter the production of NO by PTE cells and could participate in the renal tubule dysfunction associated with aging and diabetes.

Carbonylated proteins in aging and exercise: immunoblot approaches

Protein carbonyls were studied in aging and exercise by immunoblot followed by one- or two-dimensional polyacrylamide gel electrophoresis using antibodies against 2,4-dinitrophenylhydrazones. Proteins of rat kidneys exhibited significant age-related increase in the amount of carbonyl while those of the brain and liver did not. Major carbonylated proteins in the kidney included serum albumin. In nematodes in which protein carbonyls increased with age, one of the carbonylated proteins was identified as vitellogenin, an egg-yolk protein. A possible biological significance of this protein present in abundance even after egg-laying stages is discussed in terms of protection against oxidative stress. Exhaustive exercise induced significant increase in the carbonylation of selected but unidentified proteins in the lung. This oxidative stress might be caused by xanthine oxidase in this tissue and hypoxanthine derived from ATP-depleted muscles. Exercise at high altitude caused higher carbonylation of the skeletal muscle proteins, most notably a protein likely to be actin, than that at sea level but no significant difference was observed in lipid peroxidation. These studies emphasize the value of immunoblot analysis of tissue protein carbonyls in a variety of situations where oxidative stress is likely involved.

Immunocytochemical detection of advanced glycated end products in rat renal tissue as a function of age and diabetes

BACKGROUND

High blood glucose levels play major roles in the pathogenesis of renal diabetic complications through non-enzymatic glycation. For long-lived molecules this leads to formation of advanced glycation end products (AGE), and the renal extracellular matrix appears to be one of the targets for such processes. Using immunocytochemistry, we studied the appearance and deposition of AGE products in renal tissues from normal and diabetic rats at different ages, to evaluate the effects of aging and hyperglycemia.

METHODS

The streptozotocin-injected rat represented our model of hyperglycaemic condition. The immunogold techniques were applied at the light and electron microscope levels using specific monoclonal and polyclonal antibodies against AGE adducts. The results were analyzed by morphometry.

RESULTS

In normoglycemic animals, significant increases in labeling were detected in tubular basement membranes and mesangial matrix at 12 to 15 months of age. In contrast, in diabetic animals, significant increases in labeling were found for all extracellular matrices as soon as after two months of hyperglycemia. Labelings were also detected in cellular compartments, particularly in nuclei that showed increases in diabetic conditions. The labeling was particularly intense in proximal convoluted tubules and their endosomal compartment, due to the reabsorption of urinary AGE products.

CONCLUSION

The presence of AGE products in the renal extracellular matrix of old normoglycemic animals and their rapid appearance in hyperglycemia, indicate that AGE products may participate in the pathogenesis of renal complications. Furthermore, the non-enzymatic glycation is not restricted to extracellular matrices but also affects cellular proteins.