Mitochondrial dysfunction and mitophagy: the beginning and end to diabetic nephropathy?

Diabetic nephropathy (DN) is a progressive microvascular complication arising from diabetes. Within the kidney, the glomeruli, tubules, vessels and interstitium are disrupted, ultimately impairing renal function and leading to end-stage renal disease (ESRD). Current pharmacological therapies used in individuals with DN do not prevent the inevitable progression to ESRD; therefore, new targets of therapy are urgently required. Studies from animal models indicate that disturbances in mitochondrial homeostasis are central to the pathogenesis of DN. Since renal proximal tubule cells rely on oxidative phosphorylation to provide adequate ATP for tubular reabsorption, an impairment of mitochondrial bioenergetics can result in renal functional decline. Defects at the level of the electron transport chain have long been established in DN, promoting electron leakage and formation of superoxide radicals, mediating microinflammation and contributing to the renal lesion. More recent studies suggest that mitochondrial-associated proteins may be directly involved in the pathogenesis of tubulointerstitial fibrosis and glomerulosclerosis. An accumulation of fragmented mitochondria are found in the renal cortex in both humans and animals with DN, suggesting that in tandem with a shift in dynamics, mitochondrial clearance mechanisms may be impaired. The process of mitophagy is the selective targeting of damaged or dysfunctional mitochondria to autophagosomes for degradation through the autophagy pathway. The current review explores the concept that an impairment in the mitophagy system leads to the accelerated progression of renal pathology. A better understanding of the cellular and molecular events that govern mitophagy and dynamics in DN may lead to improved therapeutic strategies.

Receptor for advanced glycation end-products (RAGE) provides a link between genetic susceptibility and environmental factors in type 1 diabetes

AIMS/HYPOTHESIS

This group of studies examines human genetic susceptibility conferred by the receptor for advanced glycation end-products (RAGE) in type 1 diabetes and investigates how this may interact with a western environment.

METHODS

We analysed the AGER gene, using 13 tag SNPs, in 3,624 Finnish individuals from the FinnDiane study, followed by AGER associations with a high risk HLA genotype (DR3)-DQA1*05-DQB1*02/DRB1*0401-DQB1*0302 (n = 546; HLA-DR3/DR4), matched in healthy newborn infants from the Finnish Type 1 Diabetes Prediction and Prevention (DIPP) Study (n = 373) using allelic analysis. We also studied islets and circulating RAGE in NODLt mice.

RESULTS

The rs2070600 and rs17493811 polymorphisms predicted increased risk of type 1 diabetes, whereas the rs9469089 SNP was related to decreased risk, on a high risk HLA background. Children from the DIPP study also showed a decline in circulating soluble RAGE levels, at seroconversion to positivity for type 1 diabetes-associated autoantibodies. Islet RAGE and circulating soluble RAGE levels in prediabetic NODLt mice decreased over time and were prevented by the AGE lowering therapy alagebrium chloride. Alagebrium chloride also decreased the incidence of autoimmune diabetes and restored islet RAGE levels.

CONCLUSIONS/INTERPRETATION

These studies suggest that inherited AGER gene polymorphisms may confer susceptibility to environmental insults. Declining circulating levels of soluble RAGE, before the development of overt diabetes, may also be predictive of clinical disease in children with high to medium risk HLA II backgrounds and this possibility warrants further investigation in a larger cohort.

Receptor for AGEs (RAGE) blockade may exert its renoprotective effects in patients with diabetic nephropathy via induction of the angiotensin II type 2 (AT2) receptor

AIMS/HYPOTHESIS

The receptor for AGEs (RAGE) contributes to the development and progression of diabetic nephropathy. In this study, we examined whether the protective effects of RAGE blockade are exerted via modulation of the renal angiotensin II type 2 (AT2) receptor.

METHODS

Control and streptozotocin diabetic mice, wild-type or deficient in the AT2 receptor (At2 knockout [KO]) or RAGE (Rage KO), were studied for 24 weeks. Adenoviral overexpression of full-length Rage in primary rat mesangial cells was also used to determine the effects on AT2 production.

RESULTS

With diabetes, Rage-deficient mice had less albuminuria, and an attenuation of hyperfiltration and glomerulosclerosis as compared with diabetic wild-type and At2 KO mice. Renal gene and protein expression of RAGE was elevated with diabetes. Diabetic Rage KO mice had a greater increase in renal AT2 receptor protein than was seen in diabetic wild-type mice. Diabetes-induced increases in renal cytosolic and mitochondrial superoxide generation were prevented in diabetic Rage KO mice, but enhanced in all At2 KO mice. Adenoviral overexpression of RAGE or AGE treatment decreased cell surface AT2 expression, in association with increasing superoxide generation; both were reversed using antioxidants N-acetylcysteine and apocynin, and soluble RAGE in primary mesangial cells.

CONCLUSIONS/INTERPRETATION

RAGE appears to be a common and key modulator of AT2 receptor expression, a finding that would implicate a newly defined RAGE-AT2 axis in the development and progression of diabetic nephropathy.

Role of the AGE crosslink breaker, alagebrium, as a renoprotective agent in diabetes

The biochemical process of advanced glycation appears to play a central role in the development and progression of diabetic vascular complications. A number of strategies to influence this pathway have been designed, one of which involves the putative advanced glycation end-product (AGE) crosslink breaker, alagebrium which has been shown in in vitro studies to cleave preformed AGE crosslinks. This agent has been studied in various models of diabetic complications and has been shown to attenuate diabetic renal disease, cardiac dysfunction, and atherosclerosis. In addition to the ability of alagebrium to reduce tissue levels of AGEs, this drug appears to inhibit activation of certain protein kinase C isoforms. Planned clinical studies in diabetic subjects at risk of complications should assist in determining the role of alagebrium in the prevention, retardation, and reversal of diabetic micro- and macrovascular disease.

Advanced glycation of apolipoprotein A-I impairs its anti-atherogenic properties

AIMS/HYPOTHESIS

AGE contribute to the pathogenesis of diabetic complications, including dyslipidaemia and atherosclerosis. However, the precise mechanisms remain to be established. In the present study, we examined whether AGE modification of apolipoprotein A-I (apoA-I) affects its functionality, thus altering its cardioprotective profile.

MATERIALS AND METHODS

The ability of AGE-modified apoA-I to facilitate cholesterol and phospholipid efflux, stabilise ATP-binding cassette transporter A1 (ABCA1) and inhibit expression of adhesion molecules in human macrophages and monocytes was studied.

RESULTS

The ability of AGE-modified apoA-I to promote cholesterol efflux from THP-1 macrophages, isolated human monocytes and from ABCA1-transfected HeLa cells was significantly reduced (>70%) compared with unmodified apoA-I. This effect was reversed by preventing AGE formation with aminoguanidine or reversing AGE modification using the cross-link breaker alagebrium chloride. AGE-modification of HDL also reduced its capacity to promote cholesterol efflux. AGE-apoA-I was also less effective than apoA-I in stabilising ABCA1 in THP-1 cells as well as in inhibiting expression of CD11b in human monocytes.

CONCLUSIONS/INTERPRETATION

AGE modification of apoA-I considerably impairs its cardioprotective, antiatherogenic properties, including the ability to promote cholesterol efflux, stabilise ABCA1 and inhibit the expression of adhesion molecules. These findings provide a rationale for targeting AGE in the management of diabetic dyslipidaemia.

Altered placental oxidative stress status in gestational diabetes mellitus

Oxidative stress has been clearly linked to type 2 diabetes mellitus, however, limited data are available on the involvement of oxidative stress in gestational diabetes mellitus (GDM), a disease of similar pathophysiology. The aim of this study was to investigate the status of placental oxidative stress in healthy pregnant women and women with GDM. The hypothesis to be tested was that tissue markers of oxidative stress are significantly increased in GDM compared to normal placental tissues. Markers of oxidative stress measured were the release of 8-isoprostane (8-epi-prostaglandin F(2alpha)) from human term placental explants (n=11), the activity of the antioxidant enzymes superoxide dismutase and glutathione peroxidase (n=10), and protein carbonyl content (n=12). Placental release of 8-isoprostane was 2-fold greater from women with GDM (P<0.001) compared to healthy pregnant women. Superoxide dismutase activity and protein carbonyl content were elevated in placentae obtained from women with GDM (P<0.04 and P<0.004 respectively), whilst there was no significant difference in the activity of glutathione peroxidase. These data demonstrate the presence of oxidative stress in the placenta from women with GDM, in addition to the induction of a key antioxidant, collectively indicating a state of existing oxidative stress in this condition.

Glucose-induced release of tumour necrosis factor-alpha from human placental and adipose tissues in gestational diabetes mellitus

AIMS

The cytokine tumour necrosis factor-alpha (TNF-alpha) has been implicated in the pathogenesis of insulin resistance in Type 2 diabetes mellitus, but limited data are available in relation to gestational diabetes mellitus (GDM), a disease in which similar biochemical abnormalities exist. We investigated the effect of exogenous glucose on the release of TNF-alpha from placental and adipose (omental and subcutaneous) tissue obtained from normal pregnant women, and women with GDM.

METHODS

Human tissue explants were incubated for up to 24 h and TNF-alpha concentration in the incubation medium quantified by ELISA. The effect of normal (5 mmol/l) and high (15 and 25 mmol/l) glucose concentrations on the release of TNF-alpha was assessed.

RESULTS

In placental and subcutaneous adipose tissues obtained from women with GDM (n = 6), TNF-alpha release was significantly greater under conditions of high glucose compared with normal glucose (placenta, 25 mmol/l 5915.7 +/- 2579.6 and 15 mmol/l 4547.1 +/- 2039.1 vs. 5 mmol/l 1897.1 +/- 545.5; subcutaneous adipose tissue, 25 mmol/l 423.5 +/- 207.0 and 15 mmol/l 278.5 +/- 138.7 vs. 5 mmol/l 65.3 +/- 28.5 pg/mg protein; P < 0.05). In contrast, there was no stimulatory effect of high glucose on TNF-alpha release by tissues obtained from normal pregnant women (n = 6) (placenta, 25 mmol/l 1542.1 +/- 486.2 and 15 mmol/l 4263.3 +/- 2737.7 vs. 5 mmol/l 5422.4 +/- 1599.0; subcutaneous adipose tissue, 25 mmol/l 189.8 +/- 120.4 and 15 mmol/l 124.5 +/- 32.3 vs. 5 mmol/l 217.9 +/- 103.5 pg/mg protein).

CONCLUSIONS

These observations suggest that tissues from patients with GDM release greater amounts of TNF-alpha in response to high glucose. As TNF-alpha has been previously implicated in the regulation of glucose and lipid metabolism, and of insulin resistance, these data are consistent with the hypothesis that TNF-alpha may be involved in the pathogenesis and/or progression of GDM.