Effect of Type 2 Diabetes on the Expression and Function of the Renal Proximal Tubule Endocytic Receptors Megalin and Cubilin
Authors: Bryniarski Mark , Morris Marilyn 
Published: November 2023

Type 2 diabetes has a profound effect on the disposition of administered human IgG in the Zucker Diabetic Fatty (ZDF) rat model; there is a 2.5‐fold increase in the total clearance of human IgG and a 100‐fold increase in the renal clearance in 12‐week old diabetic rats when compared to lean controls. When symptoms of diabetic nephropathy are present at 29 weeks of age, the total and renal clearances of human IgG in diabetic ZDF animals increased 3.5‐fold and 300‐fold, respectively, compared to lean control animals. Once‐daily administration of the euglycemic agent pioglitazone to diabetic rats was able to reverse the changes in the total and renal clearances of human IgG at 12 weeks of age; however, pioglitazone treatment was only partially able to reverse the increases in clearance compared to lean controls at 29 weeks of age.

Megalin (LRP2) and cubilin are transport proteins expressed at the brush border of proximal tubule cells. Together, these proteins are responsible for the vast majority of protein reabsorption in the kidney. Impairments in their physiological functions, such as decreased protein expression, have been linked to increases in proteinuria, suggesting a decrease in renal protein reabsorption. Since diabetic kidney disease is associated with proteinuria, the objective of our study was to determine if changes in megalin and/or cubilin protein expression were responsible for the increased IgG renal clearance. ZDF diabetic rats, either untreated or treated daily with 10 mg/kg pioglitazone, and lean controls were sacrificed at 12, 19, and 29 weeks of age. These ages represent early, middle, and late stages of diabetes and were based on routine blood and urine measurements on the animals for blood glucose, blood urea nitrogen, and creatinine‐normalized albuminuria. Assessment of renal cortex megalin and cubilin expression was performed via Western blotting, semi‐quantitative RT‐PCR, and immunohistochemistry.

Our data demonstrate no loss of proximal tubule protein expression of megalin or cubilin in the untreated diabetic rats when compared to lean controls or pioglitazone‐treated animals at any age. However, statistically significant increases in the urinary amounts of the megalin and cubilin substrates albumin and vitamin D binding protein were detected at 15 weeks of age and onward in diabetic urine when compared to urine from age‐matched lean and pioglitazone‐treated rats. These findings were consistent with the higher excretion of administered IgG in ZDF rats, previously reported. Our results suggest that although there is a lack of detectable change in total megalin or cubilin protein expression, the ability of the diabetic kidney to reabsorb protein is diminished in ZDF diabetic rats as early as 15 weeks of age. Future research in this area is critical to understand the functional changes that occur in the diabetic kidney and how these changes affect the renal clearance of therapeutic antibodies.

Support or Funding Information

Support from the Center for Protein Therapeutics, University at Buffalo.