Where does albuminuria come from in diabetic kidney disease?

Role of endocannabinoid CB1 receptors in Streptozotocin-induced uninephrectomised Wistar rats in diabetic nephropathy

Background

The endocannabinoid CB1 receptor is known to have protective effects in kidney disease. The aim of the present study is to evaluate the potential agonistic and antagonistic actions and to determine the renoprotective potential of CB1 receptors in diabetic nephropathy. The present work investigates the possible role of CB1 receptors in the pathogenesis of diabetes-induced nephropathy. Streptozotocin (STZ) (55 mg/kg, i.p., once) is administered to uninephrectomised rats for induction of experimental diabetes mellitus. The CB1 agonist (oleamide) and CB1 antagonist (AM6545) treatment were initiated in diabetic rats after 1 week of STZ administration and were given for 24 weeks.

Results

The progress in diabetic nephropathy is estimated biochemically by measuring serum creatinine (1.28±0.03) (p < 0.005), blood urea nitrogen (67.6± 2.10) (p < 0.001), urinary microprotein (74.62± 3.47) (p < 0.005) and urinary albuminuria (28.31±1.17) (p < 0.0001). Renal inflammation was assessed by estimating serum levels of tumor necrosis factor alpha (75.69±1.51) (p < 0.001) and transforming growth factor beta (8.73±0.31) (p < 0.001). Renal morphological changes were assessed by estimating renal hypertrophy (7.38± 0.26) (p < 0.005) and renal collagen content (10.42± 0.48) (p < 0.001).

Conclusions

From the above findings, it can be said that diabetes-induced nephropathy may be associated with overexpression of CB1 receptors and blockade of CB1 receptors might be beneficial in ameliorating the diabetes-induced nephropathy.

Graphical abstract

Imaging mass spectrometry reveals direct albumin fragmentation within the diabetic kidney

Albumin degradation in the renal tubules is impaired in diabetic nephropathy such that levels of the resulting albumin fragments increase with the degree of renal injury. However, the mechanism of albumin degradation is unknown. In particular, fragmentation of the endogenous native albumin has not been demonstrated in the kidney and the enzymes that may contribute to fragmentation have not been identified. To explore this we utilized matrix-assisted laser desorption/ionization imaging mass spectrometry for molecular profiling of specific renal regions without disturbing distinct tissue morphology. Changes in protein expression were measured in kidney sections of eNOS^-/-db/db mice, a model of diabetic nephropathy, by high spatial resolution imaging allowing molecular localizations at the level of single glomeruli and tubules. Significant increases were found in the relative abundances of several albumin fragments in the kidney of the mice with diabetic nephropathy compared with control nondiabetic mice. The relative abundance of fragments detected correlated positively with the degree of nephropathy. Furthermore, specific albumin fragments accumulating in the lumen of diabetic renal tubules were identified and predicted the enzymatic action of cathepsin D based on cleavage specificity and in vitro digestions. Importantly, this was demonstrated directly in the renal tissue with the endogenous nonlabeled murine albumin. Thus, our results provide molecular insights into the mechanism of albumin degradation in diabetic nephropathy.

Therapy with atorvastatin versus rosuvastatin reduces urinary podocytes, podocyte-associated molecules, and proximal tubule dysfunction biomarkers in patients with type 2 diabetes mellitus: a pilot study

BACKGROUND

Diabetic nephropathy is a severe complication of Type 2 diabetes. Tubular lesions may play an important role in its early stages. The aim of our study was to determine if atorvastatin protects the podocytes and the proximal tubule in patients with Type 2 diabetes.

METHODS

A total of 63 patients with Type 2 diabetes completed this 6-months prospective pilot study. They were randomized to continue rosuvastatin therapy (control group) or to be administered an equipotent dose of atorvastatin (intervention group), and were assessed regarding urinary podocytes, podocyte-associated molecules, and biomarkers of proximal tubule dysfunction.

RESULTS

The patients from the intervention group presented a significant reduction in podocyturia (from 7.0 to 4.0 cells/ml, p < .05), urinary nephrin (from 1.7 to 1.3 mg/g, p < .001), urinary vascular endothelial growth factor (from 262.8 to 256.9, p < .01), urinary alpha1-microglobulin (from 10.0 to 8.3 mg/g, p < .01), urinary kidney injury molecule-1 (from 139.5 to 136.3 ng/g, p < .001), and urinary advanced glycation end-products (from 112.6 to 101.3 pg/ml, p < .001). Podocyturia correlated directly with the podocyte damage biomarkers, proximal tubule dysfunction biomarkers, albumin to creatinine ratio, and advanced glycation end-products, and inversely with the glomerular filtration rate.

CONCLUSIONS

In patients with Type 2 diabetes, atorvastatin exerts favorable effects on the kidney. There is a correlation between the evolution of the podocytes and of the proximal tubule biomarkers, supporting the hypothesis that the glomerular changes parallel proximal tubule dysfunction in the early stages of diabetic nephropathy.

Impaired Albumin Uptake and Processing Promote Albuminuria in OVE26 Diabetic Mice

The importance of proximal tubules dysfunction to diabetic albuminuria is uncertain. OVE26 mice have the most severe albuminuria of all diabetic mouse models but it is not known if impaired tubule uptake and processing are contributing factors. In the current study fluorescent albumin was used to follow the fate of albumin in OVE26 and normal mice. Compared to normal urine, OVE26 urine contained at least 23 times more intact fluorescent albumin but only 3-fold more 70 kD fluorescent dextran. This indicated that a function other than size selective glomerular sieving contributed to OVE26 albuminuria. Imaging of albumin was similar in normal and diabetic tubules for 3 hrs after injection. However 3 days after injection a subset of OVE26 tubules retained strong albumin fluorescence, which was never observed in normal mice. OVE26 tubules with prolonged retention of injected albumin lost the capacity to take up albumin and there was a significant correlation between tubules unable to eliminate fluorescent albumin and total albuminuria. TUNEL staining revealed a 76-fold increase in cell death in OVE26 tubules that retained fluorescent albumin. These results indicate that failure to process and dispose of internalized albumin leads to impaired albumin uptake, increased albuminuria, and tubule cell apoptosis.

Usefulness of HPLC assay for early detection of microalbuminuria in chronic kidney disease

BACKGROUND

Since the degree and increase of albuminuria predict cardiovascular and renal diseases even within the range of normoalbuminuria. The high-performance liquid chromatography (HPLC) assay has been reported as a useful tool for earlier detection of microalbuminuria than turbidimetric immunoassay (TIA) in diabetes but has not been evaluated for other common diseases that caused chronic kidney disease (CKD).

METHODS

We measured albumin in spot urine by TIA and HPLC assay in 119 patients with diabetes, hypertension, IgA nephropathy in complete remission, or autosomal-dominant polycystic kidney disease whose dipstick screening tests were negative.

RESULTS

There were significant correlations (r = 0.946) between TIA and HPLC assay, and the ratios of urine albumin/creatinine (ACR) measured by HPLC assay were always higher than those measured by TIA. The value of microalbuminuria was highest in IgA nephropathy patients, and higher in diabetic patients with hypertension than in those without hypertension. Fifty-one patients were classified as having normoalbuminuria and 42 as having microalbuminuria by both TIA and HPLC assay. However, 26 patients (21.8%) were classified having as normoalbuminuria by TIA but microalbumnuria by HPLC assay. Three of these patients were reclassified as microalbuminuria by both assays within 1 year.

CONCLUSION

These results suggest that ACR measurements by HPLC assay are better than TIA for early detection and monitoring of microalbuminuria in patients with diabetes and hypertension.

The pathogenic role of the renal proximal tubular cell in diabetic nephropathy

A growing body of evidence indicates that the renal proximal tubular epithelial cell (PTEC) plays an important role in the pathogenesis of diabetic nephropathy (DN). Microalbuminuria that intensifies over time to overt proteinuria, a hallmark of DN, is already known to activate the PTEC to induce tubulointerstitial inflammation. In addition to proteins, a number of diabetic substrates including high glucose per se, advanced glycation end-products and their carbonyl intermediates, angiotensin II, and ultrafiltered growth factors activate a number of signaling pathways including nuclear factor kappa B, protein kinase C, extracellular signal-regulated kinase 1/2, p38, signal transducer and activator of transcription-1 and the generation of reactive oxygen species, to culminate in tubular cell hypertrophy and the accumulation in the interstitium of a repertoire of chemokines, cytokines, growth factors and adhesion molecules capable of orchestrating further inflammation and fibrosis. More recently, the kallikrein-kinin system (KKS) and toll-like receptors (TLRs) in PTECs have been implicated in this process. While in vitro data suggest that the KKS contributes to the progression of DN, there are conflicting in vivo results on its precise role, which may in part be strain-dependent. On the other hand, there are both in vitro and in vivo data to suggest a role for both TLR2 and TLR4 in DN. In this review, we offer a critical appraisal of the events linking the participation of the PTEC to the pathogenesis of DN, which we believe may be collectively termed diabetic tubulopathy.

Treatment of db/db diabetic mice with triptolide: a novel therapy for diabetic nephropathy

BACKGROUND

Current research on the progression of diabetic nephropathy (DN) suggests many important factors; metabolic disturbance, haemodynamic abnormity, chronic inflammation, oxidative stress, innate immune system activation and podocyte lesion. Triptolide, which is active diterpene purified from the traditional Chinese medicine Tripterygium wilfordii Hook F (TwHF), has anti-inflammatory, anti-oxidative, immunosuppressive and podocyte-protective effects. Herein, we investigated the therapeutic effects of triptolide on DN in db/db diabetic mice and studied the potential mechanisms.

METHODS

db/db mice with DN were administrated with triptolide or valsartan. After 4, 8 and 12 weeks of treatment, 24-h urine albumin level, blood biochemical parameters and body weight were measured. Glomerulus area, glomerulus volume to Bowman's capsule volume ratio, podocyte changes and inflammatory and oxidative stress markers were quantitatively determined to evaluate renal lesions.

RESULTS

The albuminuria in db/db diabetic mice was markedly attenuated after triptolide treatment, accompanied with alleviated glomerular hypertrophy and podocyte injury. In addition, the inflammation and oxidative stress in the kidneys were also attenuated, accompanied with improved hyperlipidaemia and obesity. The efficacy increased with the prolonging of triptolide treatment, and the efficacy in high-dose triptolide group was superior to that in the low-dose group. The effect of triptolide on glomerular hypertrophy was similar to valsartan, but the effects of triptolide on renal inflammation and oxidative stress were more profound than those of valsartan.

CONCLUSIONS

Triptolide can dramatically attenuate albuminuria and renal lesion accompanied with dyslipidaemia and obesity in db/db diabetic mice. It is a new drug that exerts comprehensive protective effects on preventing DN progression.