Definition
We can define as diabetic nephropathy all alterations in the kidney, especially glomerular sclerosis and interstizial tubular fibrosis, that arise in diabetic patients and can lead to renal failure.
Epidemiology
This nephropathy is particularly associated with diabetes mellitus type 2. The prevalence of the disease increases over time from the onset of the diabetes up to 20% after 20-25 years. As far as gender difference across all ages is concerned, the incidence and rate of progression of most non-diabetic renal diseases are markedly higher in men compared with age-matched women. These observations suggest that the female sex may be renoprotective. In the setting of diabetes, however, this female protection against the development and progression of renal disease is diminished. (Diabetic nephropathy is associated with decreased circulating estradiol levels and imbalance in the expression of renal estrogen receptors 2005 Dec)
Symptoms
In its clinical presentation, the renal disease associated with diabetes presents several stages:
- Stage I or pre-clinical: is characterized by the hypertrophy of the glomerulus. At this stage both blood and urine tests are negative for the majority of patients.
- Stage II: characterized by microalbuminuria <300mg/24h. Even at this stage the albumin test done with sticks, which have a colour change in case of positive results, is negative.
- Stage III or overt: characterized by sclerosis of the mesangium, tubular necrosis, oliguria and significant proteinuria.
- Stage IV or uremic: in all cases there is high blood pressure and the patients need a
dialysis treatment due to kidney failure. The transplant is almost never recommended because of old age of the patients and of the presence of micropoliangiopatia and diabetic neuropathy.
Diagnosis
To obtain a diagnosis of diabetic nephropathy the following laboratory tests can be performed:
- Standard urine test
- Evaluation of the presence of albumin in the urine sample, using sticks that show a color change in case of positivity
- Evaluation of the albumin / creatinine ratio
- Evaluation of VGF by measuring creatinine cleareance, which can be tested also indirectly by knowing plasma concentration of creatinine as follows:
CL = k* [ (140-age) * weight] / creatinine concentration ([micromoles / liter]) with k = 1.224 for men and k = 1.04 for women
- Renal biopsy
- Imaging
Pathogenesis
Role of hyperglycemia
Hyperglycemia has a clear role in the establishment of the glomerular alteration and it's important in the setting of the disease for many reasons. First of all the increase in glucose blood concentration leads to an increase of filtered glucose load. Glucose is absorbed in symport with sodium in the renal tubule which allows an absorption of water by osmosis and a consequent increase in blood volume and pressure. Consequently there is a greater atrial distension, which stimulates the secretion of atrial natriuretic peptide. The latter causes vasodilation of the arteriole afferent to the glomerulus.
In addition there is a reduction of the sodium load at the macula densa, which triggers the
tubule-glomerular feedback resulting in the release of renin. Renin determines the activation of angiotensin system, which is involved in the vasoconstriction of arteriole efferent to the glomerulus. Hyperfiltration for glomerular hypertension follows, which results in an alteration of the MBG. In addition, glucose is able to interact with the MBG,inducing the glycation of proteins. The net result is a reduction of negative electrical charges of the MBG, which allows the filtration of albumin (a blood protein with relevant negative electrical charges).
There are also suggestion which assert that high glucose filtered load increased expression of VEGF at the mRNA and protein levels, identified by semi-quantitative RT-PCR and western blotting. Vascular endothelial growth factor (VEGF) is known to be primarily involved in neoangiogenesis and increased endothelial permeability. Calphostin-C and PMA (inhibitor of PKC system) inhibited glucose-induced increases in VEGF expression at the mRNA and protein levels. In conclusion, high glucose can directly increase VEGF expression via a PKC-dependent mechanism. These results suggest that VEGF could be a potential mediator of glomerular hyperfiltration and proteinuria in diabetic nephropathy.
(Expression of vascular endothelial growth factor in response to high glucose in rat mesangial cells 2000 Jun)
Lastly hyperglycemia induces apoptosys of renal cell by producing reactive oxygen species (ROS) which lead to considerable cellular damage and to a point of no return in apoptosis when insufficient cytoprotective and ROS scavenging molecules are available.
(Apoptosis in diabetes 2009 October )
Role of proteinuria
Proteinuria, which is initially selective, is the main factor involved in the progression of the disease. The filtered proteins should be tubularly absorbed, but because of the overload, the tubule is unable to make a full compensation. This gradually leads to tubular interstitial fibrosis.
Hormonal factor
It is conceivable that the loss of female sex as a renoprotective factor in diabetes may be related to the abnormal regulation of sex hormone concentrations. Both clinical and experimental data suggest that diabetes may be associated with an imbalance in estradiol concentration. Even though few studies have directly examined the effects of estrogens in diabetic nephropathy, numerous reports indicate that E2 modulates cellular processes in the kidney that are involved in the pathophysiology of the disease. At first estrogens seem to protect the glomerular epithelial cell from apoptosis. There are some experimental studies on animals which have shown that the administration of estradiol reduces the number of podocytes that proceed to apoptosis, mediated by TGF-beta. But this effect is completely eliminated by the administration of an inhibitor of estrogen receptor. In addition the administration of testosterone allows the apoptosis. The intracellular trasduction pathway was also shown: in fact the administration of an inhibitor of MAP-kinase system, completely deletes, the protective factor of estrogen. This means that the interaction with the receptor ERalpha promotes the phosphorylation and activation of ERK2. The system of MAP-kinase, at least in Drosophila, is a powerful anti-apoptotic system, in fact ERK promotes phosphorylation of HID, which is an anti-IAP protein. So the IAPs proteins are free to act, going to selectively inhibit the caspase pathway. (Prevenzione dell’insufficienza renale cronica: il ruolo di estrogeni e testosterone nell’apoptosi delle cellule epiteliali glomerulari)
Moreover in both the diabetic and nondiabetic kidney, E2 attenuates glomerulosclerosis and tubulointerstitial fibrosis by reducing the synthesis of type I and type IV collagen and increasing expression of matrix metalloproteinases. Furthermore, E2 decreases the expression of transforming growth factor-β, angiotensin AT1 receptor and endothelin-1, all of which stimulate abnormal cell growth and extracellular matrix metabolism and contribute to the vascular dysfunction associated with renal disease. E2 has also been shown to upregulate nitric oxide synthase activity and vascular endothelial growth factor expression in the glomerulus. Both of these factors regulate vascular permeability and thereby may contribute to the loss of glomerular function in progressive chronic kidney disease, including diabetic nephropathy. Selective estrogen receptor modulators have renoprotective effects that are similar to those of E2 in suppressing collagen type I and IV synthesis, promoting matrix metalloproteinase activity, and inhibiting transforming growth factor-β protein expression.There is also a growing body of evidence suggesting that metabolites of E2, 2-methoxyestradiol in particular, mediate the beneficial effects of E2, but without the feminizing side effects. These studies clearly indicate that estrogens have a beneficial effect on the kidney.Future studies are likely to yield important information about the precise role of estrogens in the pathophysiology of diabetic renal disease.
Although a specific role for androgens in diabetes and/or diabetic nephropathy has not been elucidated, many studies show a positive relationship between androgens and insulin resistance or type 2 DM. In both premenopausal and postmenopausal women, hyperandrogenicity is correlated with insulin resistance and type 2 DM, and it has been found to be an independent risk factor for the development of type 2 DM. A large meta-analysis of sex differences in type 2 DM showed that increased testosterone concentrations and decreased concentrations of SHBG, an independent marker of hyperandrogenicity, predicted an increased risk of type 2 DM in postmenopausal women, but that risk of type 2 DM was higher in men with low testosterone and high SHBG concentrations.Consistent with these findings, low SHBG concentrations (and thus high androgen concentrations) predicted development of type 2 DM in premenopausal women, but SHBG was not predictive of type 2 DM in age-matched men suggesting that androgens, like estrogens, may have sex-specific roles in the development of type 2 DM and possibly in the progression of disease complications, such as nephropathy. (Estrogens and the diabetic kidney 2008)
At least sex hormones are involved also in the pathogenesis of the cardiovascular diseases, in fact premenopausal women are protected to some extent from these diseases. The cardiovascular system is regulated by the renin-angiotensin-aldosterone system (RAAS), which in turn, appears to be regulated by sex hormones. In general, estrogen increases angiotensinogen levels and decreases renin levels, angiotensin-converting enzyme (ACE) activity, AT receptor density, and aldosterone production. Estrogen also activates counterparts of the RAAS such as natriuretic peptides, AT receptor density, and angiotensinogen. Progesterone competes with aldosterone for mineralocorticoid receptor. Less is known about androgens, but testosterone seems to increase renin levels and ACE activity. These effects of sex hormones on the RAAS can explain at least some of the gender differences in cardiovascular diseases. Diabetic women lose this protective factor, so the incidence of the disease is about the same for both male and female; in fact both experimental and clinical data suggest that diabetes may be associated with an imbalance in estradiol concentration. (Renin angiotensin system and gender differences in the cardiovascular system 2002 Feb
Gender and the renin-angiotensin-aldosterone system 2010 Dec)
Protective role of antioxidants in the setting of the disease
Oxidative stress occurs in diabetic patients and experimental models of diabetes. The following study examines whether two antioxidants, melatonin and taurine, can ameliorate diabetic nephropathy. Expression of glomerular TGF-beta1 and fibronectin mRNAs and proteinuria were employed as indices of diabetic nephropathy. Experimental diabetes was induced by intravenous injection of streptozotocin 50 mg/kg. Two days after streptozotocin, diabetic rats were assigned to one of the following groups: untreated; melatonin supplement in drinking water; or taurine supplement in drinking water. Four weeks after streptozotocin, diabetic rats exhibited an increase in urinary protein excretion, in glomerular TGF-beta1 mRNA, in glomerular fibronectin mRNAs, in plasma lipid peroxides (LPO), and in urinary LPO excretion above the values in control rats. Chronic administration of melatonin and taurine prevented increases in glomerular TGF-beta1 and fibronectin mRNAs and proteinuria without having effect on blood glucose. Both treatments reduced lipid peroxidation by nearly 50%. The present data demonstrate beneficial effects of melatonin and taurine on early changes in diabetic kidney and suggest that diabetic nephropathy associated with hyperglycemia is largely mediated by oxidative stress. (Melatonin and taurine reduce early glomerulopathy in diabetic rats)
(Pycnogenol modulates apoptosis by suppressing oxidative stress and inflammation in high glucose-treated renal tubular cells 2011 Sep)
Therapy
For the treatment of diabetic nephropathy, it is necessary to check glycemia, hypertension and dyslipidemia. So it is important to reduce body weight, avoid the intake of smoke and alcohol and perform physical activity.
The mechanisms by which lifestyle interventions, such as exercise, benefit diabetic nephropathy are unknown. There are some studies which hypothesized that exercise inhibits early diabetic nephropathy by attenuation of the mitochondrial apoptotic pathway and oxidative damage. Type 2 diabetic and normoglycemic wild-type mice were exercised for an hour everyday at a moderate intensity for 7 week, following which renal function, morphology, apoptotic signaling, and oxidative stress were evaluated. Exercise reduced body weight, albuminuria, and pathological glomerular expansion in DM mice independent of hyperglycemic status. Changes in renal morphology were also related to reduced caspase-3 (main effector caspase in renal apoptosis), caspase-8 (main initiator caspase of the “extrinsic” pathway) activities, and TNF-α expression. Kidneys from diabetic mice also produced higher levels of superoxides and had greater oxidative damage concurrent with downregulation of superoxide dismutase (SOD) 1 and 3. Interestingly, although exercise also increased superoxides, there was also upregulation of multiple SODs that likely inhibited lipid (hydroperoxides) and protein oxidation in diabetic kidneys. In conclusion, exercise can inhibit progression of early diabetic nephropathy independent of hyperglycemia. Reductions in caspase-3 and caspase-8 activities, with parallel improvements in SOD expression and reduced oxidative damage, could underlie the beneficial effects of exercise in diabetic kidney disease. (Moderate exercise attenuates caspase-3 activity, oxidative stress, and inhibits progression of diabetic renal disease in db/db mice 2009 January)
If blood pressure remains high it is necessary to start a drug therapy based on beta-blockers, ACE inhibitors and ARBs. ARBs have a very important role in the therapy of diabetic nephropathy because improve the NOS uncoupling by increasing BH4 bioavailability.
ROS production in the isolated glomeruli of DM rats was mediated via NADPH oxidase and NOS activation, but not via a xanthine oxidase or mitochondrial pathway. ROS production by NADPH oxidase was significantly increased in DM rats compared with control rats and ARBs treatment attenuated ROS production. Bioavailable NO was decreased and ROS production was increased in the glomeruli of DM rats compared with the control rats. Losartan treatment significantly increased NO production and decreased ROS production in the glomeruli of DM rats.These results suggest that treatment of diabetic nephropathy with losartan normalized the imbalance of NO/ROS in the glomeruli. In addition BH4 is an essential cofactor of NOS. ROS such as peroxynitrite or nitrogen dioxide convert BH4 into BH2, and BH4 deficiency decreases NO bioactivity. Serum BH4 and BH2 levels were measured. The BH4 levels were decreased in DM rats compared with control rats, and losartan treatment increased the levels in rats with diabetic nephropathy. In contrast, the BH2 levels were increased in DM rats compared with control rats and losartan treatment decreased the levels in rats with diabetic nephropathy. The key enzyme involved in de novo BH4 synthesis is GTPCH1 (GTP cyclohydrolase 1). The GTPCH1 mRNA expression in glomeruli was not changed in the DM group compared to the Cont group. Losartan treatment also showed no changes in GTPCH1 mRNA expression in DM glomeruli. However, the protein expression was significantly decreased in DM and losartan restored protein expression. These results suggest that GTPCH1 was decreased in the protein level in the isolated glomeruli of DM rats and losartan restored GTPCH1 protein levels and kept serum BH4.
(Angiotensin II type 1 receptor blocker ameliorates uncoupled endothelial nitric oxide synthase in rats with experimental diabetic nephropathy 2008 July)
Moreover serum concentrations of ovarian hormones may provide a new means for predicting future risk of renal complications in diabetes. So the administration of exogenus steroid hormones may be an effective treatment for attenuating the progression of diabetic nephropathy.
