Urinary excretion of retinol-binding protein in type 1 (insulin-dependent) diabetic patients with microalbuminuria and clinical diabetic nephropathy

Spot urine protein/creatinine ratio as a reliable estimate of 24-hour proteinuria in patients with immunoglobulin A nephropathy, but not membranous nephropathy

BACKGROUND

Proteinuria is known to be associated with both kidney function deterioration and cardiovascular diseases. While proteinuria estimation from 24-h urine samples has traditionally been considered as the standard method for assessment of the degree of urinary protein excretion, sample collection is associated with several technical problems such as inaccurate collection and the potential spread of drug-resistant pathogens. Therefore, the spot urine protein/creatinine ratio (PCR) assessment is currently recommended as an alternative. While the utility of PCR has been validated, studies on the association between spot urine PCR and 24-h proteinuria (24HP) in patients with chronic glomerular nephritis (CGN) and nephrotic syndrome (NS) are limited. This study aimed to evaluate whether an estimated result from a spot urine PCR could sufficiently approximate the daily urine protein excretion amount from a 24-h urine sample in patients with immunoglobulin A nephropathy (IgAN), minimal change disease (MCD), and membranous nephropathy- nephrotic syndrome (MN-NS).

METHODS

The study participants included 161 patients with IgAN, MCD, or MGN-NS at the Jikei University Kashiwa Hospital and Kanagawa Prefecture Shiomidai Hospital. The correlation between spot urine PCR and a 24-h urine protein was investigated using linear regression analysis with Spearman's correlation (r) coefficient and intraclass correlation coefficient (ICC).

RESULTS

While high correlation coefficients (r = 0.86, P < 0.001) and substantial agreement (ICC: 0.806, P < 0.001) were observed in patients with IgAN, similar correlations were not observed in patients with MCD or MN-NS. In the patients with MCD, r was 0.53 (P < 0.001), which signified a slight correlation, and in the patients with MN-NS, r was 0.289 (P = 0.17), which was not statistically significant.

CONCLUSIONS

This study revealed that spot urine PCR is a reliable estimate of 24HP value in patients with IgAN. In contrast, there is a considerable difference between the daily urine protein excretion amount based on a 24-h urine sample and that which is calculated from spot urine PCR in patients with NS.

Urine AQP5 is a potential novel biomarker of diabetic nephropathy

AIMS

To investigate if urinary AQP5 serves as a new potential biomarker of diabetic nephropathy.

METHODS

Using an AQP5-specific enzyme-linked immunosorbent assay, we measured serum and urine AQP5 first in a cohort consisting of normal controls (n=26) and patients with diabetes mellitus (n=25) or diabetic nephropathy (n=33) and then in a validation cohort possessing normal controls (n=10), patients with diabetes mellitus (n=10) or diabetic nephropathy (n=14), and patients with chronic kidney disease of unknown etiology (n=10). We used various statistical methods including Pearson's correlation coefficient, ANOVA with Holm-Sidak test, Receiver Operator Curve, and multiple logistic regression to analyze the data.

RESULTS

Urine AQP5/creatinine 1) is significantly higher in diabetic nephropathy than in other two groups, and in diabetic nephropathy stage V than in stage III; 2) correlates with serum creatinine, urine albumin, and multiple other known risk factors of the disease; and 3) improves the clinical models in distinguishing diabetic nephropathy from normal controls and diabetic mellitus.

CONCLUSION

Our data suggest that urine AQP5/creatinine may possess diagnostic and prognostic values as a biomarker of diabetic nephropathy.

Tubular proteinuria in patients with HNF1α mutations: HNF1α drives endocytosis in the proximal tubule

Hepatocyte nuclear factor 1α (HNF1α) is a transcription factor expressed in the liver, pancreas, and proximal tubule of the kidney. Mutations of HNF1α cause an autosomal dominant form of diabetes mellitus (MODY-HNF1A) and tubular dysfunction. To gain insights into the role of HNF1α in the proximal tubule, we analyzed Hnf1a-deficient mice. Compared with wild-type littermates, Hnf1a knockout mice showed low-molecular-weight proteinuria and a 70% decrease in the uptake of β2-microglobulin, indicating a major endocytic defect due to decreased expression of megalin/cubilin receptors. We identified several binding sites for HNF1α in promoters of Lrp2 and Cubn genes encoding megalin and cubilin, respectively. The functional interaction of HNF1α with these promoters was shown in C33 epithelial cells lacking endogenous HNF1α. Defective receptor-mediated endocytosis was confirmed in proximal tubule cells from these knockout mice and could be rescued by transfection of wild-type but not mutant HNF1α. Transfection of human proximal tubule HK2 cells with HNF1α was able to upregulate megalin and cubilin expression and to increase endocytosis of albumin. Low-molecular-weight proteinuria was consistently detected in individuals with HNF1A mutations compared with healthy controls and patients with non-MODY-HNF1A diabetes mellitus. Thus, HNF1α plays a key role in the constitutive expression of megalin and cubilin, hence regulating endocytosis in the proximal tubule of the kidney. These findings provide new insight into the renal phenotype of individuals with mutations of HNF1A.

Renoprotection of Danshen Injection on streptozotocin-induced diabetic rats, associated with tubular function and structure

ETHNOPHARMACOLOGICAL RELEVANCE

Danshen Injection, the aqueous extracts of Radix Salvia miltiorrhiza (S. miltiorrhiza), is one of the most commonly used traditional Chinese herbs in chronic renal failure treatment. In present study, the mechanism of the renoprotective effect of Danshen Injection was analyzed on streptozocin (STZ)-induced diabetic rats.

MATERIALS AND METHODS

Diabetic experimental model was established in male Sprague-Dawley (SD) rats by intraperitoneal injection of STZ. Rats with blood glucose concentration of higher than 300 mg/dl were intraperitoneally administered with Danshen Injection at a dose of 0.78 ml/kgday. The blood glucose, 24h urinary protein excretion, serum creatinine (sCr), blood urea nitrogen (BUN), advanced glycation end products (AGEs), lipid peroxide (LPO), antioxidant enzyme of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), transforming growth factor-β1 (TGF-β1), and histomorphological changes in kidney of diabetic rats were analyzed during the course of Danshen Injection administration, as well as the tubular function index of albumin reabsorption of fluorescein isothiocyanate labeled bovine serum albumin (FITC-BSA).

RESULTS

The intraperitoneal administration of Danshen Injection could ameliorate the physiological dysfunctions of increased 24h urinary protein excretion((48.21 ± 8.04)%), sCr((39.4 ± 3.7)%), and BUN((43.37 ± 6.74)%), alleviate the ultrastructural abnormalities of hypertrophy, matrix expansion, and fibrosis in glomerulus, decrease the TGF-β1 expression, AGEs and LPO accumulation, and increase the activity of SOD and GSH-Px in kidney of diabetic rats, but did not significantly influence the blood glucose. Besides these, the Danshen Injection administration also partly restored the decrease of megalin expression in tubules and reabsorptive function of FITC-BSA, in diabetic rats.

CONCLUSION

The renoprotection of Danshen Injection on diabetic rats was associated with the preservation of tubular function and structure from the hyperglycemia induced toxicities of inappropriate cytokines secretion, oxidative stress, advanced glycation stress, and megalin expression deletion.

Contrasting effects of type 2 and type 1 diabetes on plasma RBP4 levels: the significance of transthyretin

Retinol-binding protein 4 (RBP4) is the principle carrier of retinol in the human plasma, which circulates as a complex with transthyretin (TTR), a homotetrameric thyroxine transport protein. Although this complex formation is thought to prevent glomerular filtration of RBP4, it also stabilizes the quaternary structure of TTR. Recent studies indicate elevated plasma levels of RBP4 in type 2 diabetes (T2D). In contrast, reduced RBP4 levels were observed in type 1 diabetes (T1D). Herein, we critically examine the probable mechanisms involved in the regulation of RBP4 and TTR levels during T2D and T1D. The available evidences point to the involvement of pancreatic factors in regulating the expression of both RBP4 and TTR. It appears that during T1D, TTR levels are reduced and it exists predominantly as a monomer that may interfere its interaction with RBP4 resulting in its loss through glomerular filtration. However, plasma TTR levels remain high under T2D conditions and thus reducing glomerular filtration of RBP4. Therefore, the plasma TTR levels appear to be an important determinant of plasma RBP4 levels in these two diabetic conditions.

Urinary MCP-1 and RBP: independent predictors of renal outcome in macroalbuminuric diabetic nephropathy

BACKGROUND

Albuminuria has been considered a sine qua non condition for the diagnosis of diabetic nephropathy (DN) and has been widely used as a surrogate outcome of chronic kidney disease (CKD). However, recent data suggest that albuminuria may fail as a biomarker in a subset of patients, and the search for novel markers is intense.

METHODS

We analyzed the role of urinary RBP and of serum and urinary cytokines (TGF-beta, MCP-1 and VEGF) as predictors of the risk of dialysis, doubling of serum creatinine or death (primary outcome, PO) in 56 type 2 diabetic patients with macroalbuminuric DN.

RESULTS

Mean follow-up time was 30.7±10 months. Urinary RBP and MCP-1 were significantly higher in patients presenting the PO, whereas no difference was shown for TGF-β or VEGF. In the Cox regression, urinary RBP, MCP-1 and VEGF were positively associated and serum VEGF was inversely related to the risk of the PO. However, after adjustments for creatinine clearance, proteinuria, and blood pressure only urinary RBP (OR 11.6; 95% CI 2.7-49.2, p=0.001 for log RBP) and urinary MCP-1 (OR 11.0; 95% CI 1.6-76.4, p=0.02 for log MCP-1) remained as significant independent predictors of the PO.

CONCLUSION

Urinary RBP and MCP-1 are independently related to the risk of CKD progression in patients with macroalbuminuric DN. Whether these biomarkers have a role in the setting of normoalbuminuria and microalbuminuria in DN should be further investigated.

Significance of urinary full-length and ectodomain forms of megalin in patients with type 2 diabetes

OBJECTIVE

Megalin, an endocytic receptor in proximal tubule cells, is involved in the mechanisms of albuminuria in diabetic nephropathy (DN). To develop efficient novel biomarkers associated with the pathogenesis of DN, we investigated urinary megalin excretion in type 2 diabetes.

RESEARCH DESIGN AND METHODS

Sandwich enzyme-linked immunosorbent assay systems were established with monoclonal antibodies against the NH(2) (amino [A]-megalin assay) and COOH (C-megalin assay) termini of megalin to analyze urinary forms of megalin in 68 patients with type 2 diabetes.

RESULTS

The A-megalin assay mainly detected a megalin ectodomain form in the soluble urinary fraction, whereas the C-megalin assay identified a full-length form in both soluble and insoluble fractions. Urinary C-megalin levels were significantly high in patients with normoalbuminuria, were elevated in line with increased albuminuria, and showed a better association with estimated glomerular filtration rate (eGFR) (<60 mL/min/1.73 m(2)) than did urinary albumin. In contrast, urinary A-megalin levels were increased in patients with normo- and microalbuminuria but not in those with macroalbuminuria. Urinary C-megalin levels were also positively associated with plasma inorganic phosphate and negatively with hemoglobin levels in those showing no features of bleeding and not taking vitamin D analogs, phosphate binders, or erythropoiesis-stimulating agents.

CONCLUSIONS

Urinary full-length megalin excretion as measured by the C-megalin assay is well associated with reduced eGFR and linked to the severity of DN, phosphate dysregulation, and anemia, whereas urinary excretion of megalin ectodomain as measured by the A-megalin assay may be associated with distinctive mechanisms of earlier DN in type 2 diabetes.

Proximal tubule cell hypothesis for cardiorenal syndrome in diabetes

Incidence of cardiovascular disease (CVD) is remarkably high among patients with chronic kidney disease (CKD), even in the early microalbuminuric stages with normal glomerular filtration rates. Proximal tubule cells (PTCs) mediate metabolism and urinary excretion of vasculotoxic substances via apical and basolateral receptors and transporters. These cells also retrieve vasculoprotective substances from circulation or synthesize them for release into the circulation. PTCs are also involved in the uptake of sodium and phosphate, which are critical for hemodynamic regulation and maintaining the mineral balance, respectively. Dysregulation of PTC functions in CKD is likely to be associated with the development of CVD and is linked to the progression to end-stage renal disease. In particular, PTC dysfunction occurs early in diabetic nephropathy, a leading cause of CKD. It is therefore important to elucidate the mechanisms of PTC dysfunction to develop therapeutic strategies for treating cardiorenal syndrome in diabetes.

Molecular mechanisms of receptor-mediated endocytosis in the renal proximal tubular epithelium

Receptor-mediated endocytosis is a pivotal function of renal proximal tubule epithelial cells (PTECs) to reabsorb and metabolize substantial amounts of proteins and other substances in glomerular filtrates. The function accounts for the conservation of nutrients, including carrier-bound vitamins and trace elements, filtered by glomeruli. Impairment of the process results in a loss of such substances and development of proteinuria, an important clinical sign of kidney disease and a risk marker for cardiovascular disease. Megalin is a multiligand endocytic receptor expressed at clathrin-coated pits of PTEC, playing a central role in the process. Megalin cooperates with various membrane molecules and interacts with many intracellular adaptor proteins for endocytic trafficking. Megalin is also involved in signaling pathways in the cells. Megalin-mediated endocytic overload leads to damage of PTEC. Further studies are needed to elucidate the mechanism of megalin-mediated endocytosis and develop strategies for preventing the damage of PTEC.

Regulation of megalin expression in cultured proximal tubule cells by angiotensin II type 1A receptor- and insulin-mediated signaling cross talk

Impairment of proximal tubular endocytosis of glomerular-filtered proteins including albumin results in the development of proteinuria/albuminuria in patients with chronic kidney disease. However, the mechanisms regulating the proximal tubular function are largely unknown. This study aimed to investigate the role of angiotensin II type 1A receptor (AT(1A)R)- and insulin-mediated signaling pathways in regulating the expression of megalin, a multiligand endocytic receptor in proximal tubule cells (PTCs). Opossum kidney PTC-derived OK cells that stably express rat AT(1A)R but are deficient in endogenous angiotensin II receptors (AT(1A)R-OK cells) were used for this study. Treatment of the cells with angiotensin II suppressed mRNA and protein expression of megalin at 3- and 24-h incubation time points, respectively. Cellular uptake and degradation of albumin and receptor-associated protein, megalin's endocytic ligands were suppressed 24 h after angiotensin II treatment. The AT(1A)R-mediated decrease in megalin expression was partially prevented by ERK inhibitors. Insulin competed with the AT(1A)R-mediated ERK activation and decrease in megalin expression. Inhibitors of phosphatidylinositol 3-kinase (PI3K), a major component of insulin signaling, also suppressed megalin expression, and activation of the insulin receptor substrate (IRS)/PI3K system was prevented by angiotensin II. Collectively the AT(1A)R-mediated ERK signaling is involved in suppressing megalin expression in the OK cell line, and insulin competes with this pathway. Conversely, the insulin-IRS/PI3K signaling, with which angiotensin II competes, tends to stimulate megalin expression. In conclusion, there is AT(1A)R- and insulin-mediated competitive signaling cross talk to regulate megalin expression in cultured PTCs.

Urinary excretion of n-acetyl-beta-D-glucosaminidase and retinol binding protein as alternative indicators of nephropathy in patients with type 1 diabetes mellitus

Diabetic nephropathy (DN) is usually characterized by glomerular dysfunction, with microalbuminuria as an early indicator. Urinary excretion of smaller molecular weight proteins such as n-acetyl-beta-glucosaminidase (beta-NAG) and retinol binding protein (RBP) indicate proximal tubular dysfunction, and may identify diabetic patients at risk of developing diabetic nephropathy. In a trial to assess renal tubular function, urinary excretion of beta-NAG (by colorimetric assay) and RBP (by ELISA) were determined in 59 type 1 diabetic patients (mean age 15 +/- 3.2 yr). Of the 59 patients, 11 were microalbuminuric while 48 had normal urinary albumin excretion (UAE). Patients were compared with 40 matched healthy subjects. Diabetic patients with microalbuminuria (n = 11) had concomitant renal tubular disorder indicated by high urinary beta-NAG in all (100%) and RBP in 10 (90.9%) of them. Meanwhile, patients without microalbuminuria (n = 48) had both tubular markers excreted in urine in significantly higher amounts than controls (mean beta-NAG = 6.88 vs. 3.76 U/g Cr, p < 0.001; RBP = 386.6 vs. 151.8 microg/dL, p < 0.001). Among those patients, 29 (61%) had raised urinary beta-NAG activity, and 39 (82%) had increased loss of RBP in urine. A significant correlation was found between urinary beta-NAG and RBP in normoalbuminuric patients (r = 0.66, p < 0.001), as well as between each of the two tubular markers and HbA1c (r = 0.83, p < 0.001). At 30 and 36 months of follow-up, two out of 48 (4.2%) diabetic patients developed persistent microalbuminuria. Both had elevated baseline HbA1C, and urinary beta-NAG. In conclusion, proximal tubular dysfunction may occur independent of glomerular alteration. Whether tubular markers precede the development of microalbuminuria needs further study.

Urinary excretion of albumin and retinol binding protein in systemic lupus erythematosus

We have measured the urinary excretion of total protein, albumin and retinol binding protein (RBP) in random urine specimens obtained from 40 female patients with systemic lupus erythematosus (SLE). Thirty-three of these patients had no clinical evidence of any renal impairment (non-renal SLE); seven had overt renal disease (renal SLE). RBP:creatinine ratios were significantly higher in non-renal SLE patients compared with female controls (P = 0.002). There was no significant difference between urine total protein concentrations, albumin:creatinine or total protein:creatinine ratios in non-renal SLE patients when compared with controls, despite approximately 20% of these patients having elevated excretion of total protein or albumin. All seven renal SLE patients had elevated albumin:creatinine ratios but only four of them had an increased RBP:creatinine ratio. Of 29 non-renal SLE patients who had urinary total protein concentrations below 0.2 g/L, (i.e. approximating to a negative protein dipstick), 14 had increases in either albumin or RBP:creatinine ratios. Only two patients had increases in both. In the absence of clinical evidence of renal disease, increases in urinary albumin or RBP excretion could indicate subclinical nephropathy and measurements may have a role in the early diagnosis and subsequent monitoring of renal disease in SLE.

High retinol/retinol-binding protein ratio in noninsulin-dependent diabetes mellitus

The authors evaluated serum retinol, retinol-binding protein (RBP), and beta-carotene levels to elucidate the retinoid metabolism in non-insulin-dependent diabetes mellitus (NIDDM). The mean retinol levels by gender (1.83 mumol/L for females and 2.24 mumol/L for males) in diabetics were higher than those (1.31 mumol/L for females and 1.82 mumol/L for males) in control subjects (P < 0.0001, P < 0.01, respectively). The mean retinol/RBP ratios (0.95 for females and 0.97 for males) of diabetics were higher than those of the control subjects (0.60 for females and 0.64 for males) and of male patients having impaired glucose tolerance (0.55) (P < 0.0001). Lipid-lowering medication significantly decreased retinol, with decreasing apolipoprotein C-II but without a commensurate decrease in RBP. The retinol levels had a positive correlation with apolipoprotein C-II in all or normolipidemic patients with diabetes and control subjects. The high retinol/RBP ratio implies that an excessive or free retinol possibly exists in NIDDM. An alternative metabolism of retinol is inferred to underlie NIDDM without direct influences of cholesterol or triglyceride themselves.