Mechanism of increased clearance of glycated albumin by proximal tubule cells

Abstract

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.

Human serum albumin presents isoform variants with altered neonatal Fc receptor interactions

“…Modifications of basic residues via the formation of Amadori products is strongly suggested to affect the tertiary structure conformation of the protein as probed by tryptophan fluorescence . in vitro modified rat and human albumin incubated at 37°C for 21 days in an excess presence of glucose and methylglyoxal showed a reduced binding toward FcRn attributed to significant structural changes using small angle X‐ray scattering . Lys 525 has constantly been found to be a major glycation site of HSA both in vivo and in vitro .…”

“…On the other hand, hypoalbuminemia is commonly observed in disease states [19][20][21] and is associated with inflammation as the combined result of several factors affecting HSA, that is, the increase in capillary permeability leading to an increase of its distribution volume, diminished liver synthesis rate and altered pharmacokinetics decreasing its half-life. 22 Several reports indicate also the change of the PTMs profile of HSA in various pathologies [23][24][25][26][27] but the correlation between observed PTMs and HSA half-life is still under intense investigation to better understand disease states leading to hypoalbuminemia.…”

Proximal tubular epithelial cells preferentially endocytose covalently‐modified albumin compared to native albumin

“…Indeed, a relative increase in urinary excretion of glycated albumin has been described in patients with diabetic nephropathy and macroalbuminuria . Furthermore, the relatively mild albuminuria seen in mice lacking megalin and/or cubulin, is consistent with a lack of tubular uptake of modified albumin while native albumin is handled by a different mechanism, such as the FcRn‐dependent albumin reclamation pathway postulated by a number of researchers . Thus, it would be informative to analyze whether mice lacking megalin and/or cubulin have increased urinary excretion of modified versus native albumin, and if the reclamation pathway for native albumin remains intact in these mice.…”

“…Indeed, glycation of plasma albumin is evident in both diabetic and non‐diabetic individuals, demonstrating that circulating albumin contains a mixture of native and variably modified protein. An interesting observation is that glycated serum albumin is more rapidly eliminated from the circulation compared to native albumin . Indeed, modification by glucose or methylgloxal causes structural changes in albumin with opening of domains and a loss of binding capacity for the neonatal Fc receptor .…”

“…An interesting observation is that glycated serum albumin is more rapidly eliminated from the circulation compared to native albumin . Indeed, modification by glucose or methylgloxal causes structural changes in albumin with opening of domains and a loss of binding capacity for the neonatal Fc receptor . Glycated (modified) forms of albumin can bind to a number of scavenger receptors, which facilitate clearance .…”