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NDT Advance Access originally published online on August 25, 2006
Nephrology Dialysis Transplantation 2006 21(12):3428-3434; doi:10.1093/ndt/gfl469
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© The Author [2006]. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Urinary tumour necrosis factor-{alpha} excretion independently correlates with clinical markers of glomerular and tubulointerstitial injury in type 2 diabetic patients

Juan F. Navarro1–3, Carmen Mora2,3, Mercedes Muros2–4 and Javier García1

1Nephrology Service, University Hospital, Santa Cruz de Tenerife, Spain 2Research Unit, University Hospital Nuestra Señora de Candelaria, Santa Cruz de Tenerife, Spain, 3Centre for Biological Research, Spanish National Research Council, Madrid and 4Clinical Biochemistry Service, Universitary Hospital Nuestra Señora de Candelaria, Santa Cruz de Tenerife, Spain

Correspondence and offprint requests to: Juan F. Navarro, MD, FASN, Nephrology Service, Hospital Nuestra Señora de Candelaria, Carretera del Rosario, 145, 38010 Santa Cruz de Tenerife, Tenerife, Spain. Email: jnavgon{at}gobiernodecanarias.org



   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 References
 
Background. Inflammation is a potential factor in the development and progression of diabetic nephropathy. The aim of this study was to analyse the relationship between the pro-inflammatory cytokine tumour necrosis factor-{alpha} (TNF{alpha}) and clinical markers of glomerular and tubulointerstitial damage [urinary albumin excretion (UAE) and urinary N-acetyl-ß-glucosaminidase (UNAG), respectively] in a large group of type 2 diabetic patients.

Methods. A total of 160 diabetic patients and 32 healthy controls were included in the study. High-sensitive C-reactive protein (hs-CRP) as well as serum and urinary levels of TNF{alpha} were measured. UAE and UNAG were determined by 24-h urine collection.

Results. Serum hs-CRP and TNF{alpha} were significantly higher in diabetic than in control subjects, as well as UAE and UNAG. Diabetic patients had increased urinary TNF{alpha} compared to non-diabetics [14.5 (2–29) vs 4 (0.8–12), P < 0.001]. Serum hs-CRP and TNF{alpha} in diabetics with increased UAE were elevated compared to diabetics having normoalbuminuria. Urinary TNF{alpha} was also higher in diabetic subjects with micro- or macroalbuminuria than in patients with normal UAE [10.5 (4–20) and 18 (9–29) vs 7 (2–18) pg/mg, P < 0.0001, respectively]. Multiple regression analysis showed that urinary TNF{alpha} (P < 0.0001), hs-CRP (P < 0.0001), serum TNF{alpha} (P < 0.01) and HbA1c (P < 0.05) were independent of and significantly associated with UAE, whereas duration of diabetes (P < 0.001), urinary TNF{alpha} (P < 0.01), HbA1c (P = 0.01), hs-CRP (P < 0.05) and serum creatinine (P < 0.05) were associated with UNAG.

Conclusions. In patients with type 2 diabetes, urinary TNF{alpha} excretion is elevated and correlates with severity of renal disease in terms of both glomerular and tubulointerstitial damage, suggesting a significant role for TNF{alpha} in the pathogenesis and progression of renal injury in diabetes mellitus.

Keywords: albuminuria; cytokines; diabetes mellitus; diabetic nephropathy; tumour necrosis factor-{alpha}



   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 References
 
Urinary albumin excretion (UAE) is the classical marker of diabetic nephropathy (DN). However, tubulointerstitial involvement is also a major feature of renal lesion in diabetes, and more important, tubulointerstitial injury is a critical factor in the development and progression of renal dysfunction [1]. N-acetyl-ß-glucosaminidase (NAG) is a lysosomal enzyme which originates from the proximal tubular cells. Increased urinary NAG excretion (UNAG) occurs in cases of significant tubular lesion in different diseases, including diabetes mellitus. Therefore, whereas UAE is a marker for glomerular injury, UNAG is widely used as a marker for tubular renal function [2].

Nowadays, there is growing evidence that activated innate immunity and inflammation are relevant factors in the pathogenesis of diabetes, with convincing data that type 2 diabetes includes an inflammatory component [3]. Regarding diabetic complications, experimental and clinical studies have demonstrated that DN exhibits signs of inflammation, with pro-inflammatory cytokines being suggested as important factors in the development of renal injury [4–7].

Tumour necrosis factor-{alpha} (TNF{alpha}) is a pleiotropic cytokine that plays an essential role in mediating inflammatory processes. Furthermore, TNF{alpha} is cytotoxic to glomerular, mesangial and epithelial cells, and may induce direct renal damage [8,9]. Studies in animal models of DN have shown that renal expression of TNF{alpha} is increased compared to kidneys of non-diabetic animals [10]. Preliminary clinical reports have found a significant relationship between the urinary excretion of this cytokine and UAE in type 2 diabetic patients [5]. However, the relationship between TNF{alpha} and tubulointerstitial damage has not been previously analysed.

The aim of the present study was to analyse the relationship between TNF{alpha}, one of the most important pro-inflammatory cytokines, and clinical markers of glomerular (UAE) and tubulointerstitial damage (UNAG) in a large group of type 2 diabetic patients.



   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 References
 
A total of 160 diabetic patients (83 men, 77 women; aged 62 ± 9 years) from our out-patient diabetic clinic were included in this study, as well as 32 healthy control subjects (15 men, 17 women; mean age 60 ± 8 years). The protocol was in accordance with the Declaration of Helsinki and the Convention on Human Rights and Biomedicine from the Council of Europe. Informed consent was obtained in all cases.

Patients were categorized as having normoalbuminuria when UAE was persistently <30 mg/day (n = 25). Microalbuminuria was defined as an UAE between 30 and 300 mg/day (n = 60). Finally, patients were categorized as having macroalbuminuria if they had persistent UAE >300 mg/day (n = 75). Subjects with a current acute illness (including infectious diseases), severe proteinuria (UAE >3 g/day), blood pressure (BP) >160/100 mmHg, renal insufficiency (defined as a serum creatinine (Cr) >1.3 mg/dl), present cigarette smoking or medical history of clinical cardiovascular events in the last 6 months were excluded in order to avoid potential confounding factors. Before the definitive inclusion, the possible existence of active immunological diseases, malignancy and infections were investigated. White blood cell count was lower than 10 000/mm3 in all cases. Tumoural markers including carcino-embryonic antigen, {alpha}-fetoprotein, cancer antigen 125 and prostate-specific antigen were negative. Serological tests for antinuclear antibodies, antineuthrophil cytoplasmic antibodies, cryoglobulins, rheumatoid factor, immunoglobulins and complement factors C3 and C4 were negative or within the normal range. Urine cultures and serology to hepatitis B and C and human immunodeficiency virus were also negative.

All diabetic patients were insulin dependent, had signs of diabetic retinopathy and were hypertensive under treatment with the maximum recommended doses of angiotensin-converting enzyme inhibitors (ACEIs) or angiotensin II type 1 receptor antagonists (ARA) for more than 6 months. BP was measured by a mercury sphygmomanometer with the patient in the sitting position after 5 min of rest. Three readings separated by 2 min were taken and the average was used for calculation. The first appearance of sound (phase 1) was used to define systolic BP. The disappearance of sound (phase 5) was used to define diastolic BP. Body mass index (BMI) was calculated as (weight) kg/(height) m2.

Laboratory analysis and cytokine measurements
Blood samples were drawn before breakfast in the morning (between 8 and 11 a.m.), after an 8–12-h overnight fast. Samples were collected in sterile tubes, centrifuged at 3000 g. for 10 min at 4°C, and then stored at –80°C until assayed. The plasma glucose level was measured by an automated enzymatic method. The glycated haemoglobin (HbA1c) concentration was measured by high performance liquid chromatography. The concentration of high-sensitive C-reactive protein (hs-CRP) was measured by ultrasensitive competitive immunoassay (Calbiochem, La Jolla, CA, USA) (interassay coefficient of variation of 8.4%). The serum TNF{alpha} concentration was determined by a high sensitivity quantitative sandwich enzyme immunoassay (R&D systems, Minneapolis, MN, USA) (lower limit of detection, 0.4 pg/ml; intra- and interassay coefficients of variation of the assay, 5.6 and 8.1%, respectively).

UAE was determined by 24-h urine collections. After collection, the samples were centrifuged at 3000 r.p.m. for 10 min and the supernatant was stored at –20°C. Urinary albumin was quantified by immunoturbidimetry (coefficient of variation 5.5%). Enzyme-linked immunosorbent assay (ELISA) was used for the detection of urinary TNF{alpha}. Urinary NAG was measured using a colorimetric assay with 3-cresolsulfonphthaleyn-N-acetyl-ß-D-glucosaminide as a substrate (NAG colour test, Boehringer Mannheim), which is hydrolysed by NAG with the release of 3-cresolfulfonphthaleyn sodium salt, and measured photometrically at 580 nm. Urinary levels of TNF{alpha} and NAG were related to the concomitant urinary Cr concentration content in order to compensate for variations in urinary concentration, and expressed as NAG/Cr (U/g) and TNF{alpha}/Cr (pg/mg).

Statistical analysis
Results are presented as mean ± SD, except for UAE, UNAG, hs-CRP and TNF{alpha}, which are presented as median and range. Before analysis, these variables were logarithmically transformed. Differences in parameters between diabetic patients and control subjects were analysed by the Student's t-test and the Mann–Whitney U-test when appropriate. Comparison of patients stratified by albuminuria status was performed by one-way analysis of variance and Kruskal–Wallis analysis of variance. Post hoc comparisons were made using the Bonferroni procedure. Correlation between variables was calculated using the Pearson's and the Spearman correlation tests. Partial correlation analysis was performed to determine the extent to which a relationship was altered after adjusting for the other variables. Forward stepwise multiple regression analysis was performed to determine the independent association between relevant patient characteristics and clinical parameters as potential predictor variables (age, sex, duration of diabetes, BMI, systolic and diastolic BP, HbA1c, serum Cr, hs-CRP and serum and urinary TNF{alpha}) and UAE and UNAG as the dependent variables. Colinearity was excluded by the analysis of the eigenvalue. P-values <0.05 were considered statistically significant.



   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 References
 
General characteristics and clinical parameters of healthy non-diabetic controls and diabetic patients stratified by albuminuria status are depicted in Table 1. Serum concentrations of the inflammatory parameters hs-CRP and TNF{alpha} as well as urinary TNF{alpha} were significantly higher in diabetic patients than in control subjects: 5.2 (0.8–9.3) mg/l, 5.9 (0.7–18) pg/ml and 14.5 (2–29) pg/mg vs 1.9 (0.2–3.6) mg/l, 2.3 (0.7–5) pg/ml and 4 (0.8–12) pg/mg, P < 0.001, respectively. Concerning urinary measurements, UAE and UNAG were also higher in diabetics than in healthy controls: 210 (6–2630) mg/day and 6.3 (1.7–88) U/g vs 13 (6–26) mg/day and 4.3 (0.8–11) U/g, P < 0.001 and P < 0.01, respectively.


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  		Table 1. General and clinical parameters in healthy controls and type 2 diabetic patients stratified by albuminuria status

 
Diabetic patients with normoalbuminuria had higher systolic and diastolic BP as well as higher levels of glucose and HbA1c than healthy non-diabetic individuals (Table 1). Regarding inflammatory parameters, the only difference between both groups was the urinary TNF{alpha} excretion, which was significantly higher in normoalbuminuric diabetic patients than in controls: 7 (2–18) vs 4 (0.8–12) pg/mg, respectively (P < 0.001). When diabetic patients were stratified by albuminuria status (Table 1), a significant difference was observed in serum levels of hs-CRP and TNF{alpha} between patients with increased urinary albumin excretion in comparison with subjects with normoalbuminuria. Regarding urinary TNF{alpha}, the excretion of this cytokine in subjects with micro- or macroalbuminuria was also significantly higher than in individuals with a normal UAE. A similar finding was observed in patients with macroalbuminuria with respect to subjects with microalbuminuria. Serum and urinary levels of TNF{alpha} and hs-CRP in control subjects and diabetic patients are presented in Figure 1. Patients on the upper quartile for these inflammatory parameters had more albuminuria and UNAG than patients on the lower quartile (Table 2). Thus, UAE and UNAG in patients on the 75th percentile for urinary TNF{alpha} excretion were 900 (167–2630) mg/day and 11.2 (2.3–88.2) U/g, respectively, whereas in patients on the 25th percentile albuminuria was 42 (6–220) mg/day and UNAG was 4.6 (1.7–9.2) U/g (P < 0.001).


Figure 1
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  		Fig. 1. Serum concentrations of hs-CRP (mg/l) (white bars), and serum (pg/ml) (grey bars) and urinary (pg/mg) (black bars) levels of TNF{alpha} in healthy control individuals and type 2 diabetic patients stratified according albuminuria status: normoalbuminuria (NAB), microalbuminuria (MicrAB) and macroalbuminuria (MacrAB).

 

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  		Table 2. Urinary albumin and N-acetyl-ß-glucosaminidase (NAG) excretion according to lower (25th) and higher (75th) percentiles for inflammatory parameters in diabetic patients

 
Unadjusted linear regression analysis showed that UNAG excretion was significant and positively associated with urinary TNF{alpha} concentration (R = 0.52), diabetes duration (R = 0.51), albuminuria (R = 0.48), serum levels of hs-CRP (R = 0.46) and HbA1c (R = 0.42) (P < 0.001 in all cases), as well as with serum Cr (R = 0.27), serum TNF{alpha} (R = 0.18) and SBP (R = 0.17) (P < 0.05 in all cases). Regarding UAE, this parameter was significantly correlated with urinary TNF{alpha} excretion (R = 0.64), hs-CRP (R = 0.52), duration of diabetes (R = 0.41) and HbA1c (R = 0.29) (P < 0.001 in all cases), as well as with serum Cr (R = 0.25, P < 0.01), serum glucose (R = 0.21, P < 0.01) and serum TNF{alpha} (R = 0.20, P < 0.05). The associations between urinary TNF{alpha} with albuminuria and UNAG remained significant both in patients with microalbuminuria (R = 0.55 and R = 0.56, respectively, P < 0.001) and in patients with macroalbuminuria (R = 0.31, P < 0.01 and R = 0.53, P < 0.001, respectively). However, these parameters were not significantly related in subjects with normal UAE. Serum levels of TNF{alpha} and hs-CRP were not related to UAE or UNAG excretion in diabetic patients. Likewise, urinary levels of TNF{alpha} were not significantly correlated with the serum concentration of this cytokine in any group of diabetic subjects. Finally, serum, but not urinary, TNF{alpha} levels were significantly correlated with BMI (R = 0.16, P < 0.05), whereas only urinary TNF{alpha} was significantly associated with HbA1c (R = 0.46, P < 0.01).

After adjusting for the effect of other variables by partial correlation analysis, the previous associations between urinary TNF{alpha} and UAE and UNAG remained significant, but not the relationship between serum TNF{alpha} and UNAG (Table 3). Finally, forward stepwise multiple regression analysis was performed to examine the associations between UAE and UNAG (dependent variables) on the one hand and relevant patient characteristics and clinical parameters (i.e. age, gender, time of diabetes, systolic and diastolic BP, serum Cr, HbA1c, hs-CRP and serum and urinary TNF{alpha}) as the independent variables on the other. The result showed that urinary TNF{alpha} (P < 0.0001), hs-CRP (P < 0.0001), serum TNF{alpha} (P < 0.01) and HbA1c (P < 0.05) were independent of and significantly associated with UAE according to the following function (adjusted R2 = 0.63, P < 0.001): UAE = 0.31 + (0.09 x urinary TNF{alpha}) + (0.24 x hs-CRP) + (0.06 x serum TNF{alpha}) + (0.17 x HbA1c). Concerning UNAG, duration of diabetes (P < 0.001), urinary TNF{alpha} (P < 0.01), HbA1c (P = 0.01), hs-CRP (P < 0.05) and serum Cr (P < 0.05) were independent of and significantly associated with this variable according to the following expression (adjusted R2 = 0.44, P < 0.001): UNAG = –28.6 + (0.26 x time of diabetes) + (0.23 x urinary TNF{alpha}) + (0.17 x HbA1c) + (0.17 x hs-CRP) + (0.15 x serum Cr).


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  		Table 3. Significant associations between the independent variables albuminuria and urinary NAG excretion and other parameters by partial correlation analysis

 


   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
References
 
In the present study, we analysed the serum levels and urinary TNF{alpha} excretion in type 2 diabetic patients and healthy non-diabetic controls. Our results show that serum and urinary concentrations of this cytokine are enhanced in diabetic subjects, increasing as renal damage progresses. More importantly urinary TNF{alpha} is significant and independently related to clinical markers of both glomerular (UAE: R = 0.49, P < 0.0001) and tubulointerstitial (UNAG: R = 0.24, P < 0.01) injury. Works in animal models as well as other smaller clinical studies have reported interesting data about the importance of TNF{alpha} in the setting of DN. Our results highlight the possible role of TNF{alpha}, one of the main pro-inflammatory cytokines, in the development and progression of renal injury in diabetic patients.

Serum TNF{alpha} has been reported to be elevated in diabetic subjects, and even in patients with only impaired glucose tolerance, compared to healthy individuals [11,12]. In the present work, diabetic patients had approximately a 2.5-fold higher serum TNF{alpha} than non-diabetic individuals. However, there were no significant differences in serum TNF{alpha} between diabetic subjects with normoalbuminuria and non-diabetic controls. In fact, serum TNF{alpha} concentrations were increased only in diabetic patients with micro- or macroalbuminuria. Previous works have reported similar results, and furthermore, they showed a significant relationship between serum TNF{alpha} and UAE [5,13,14]. In our work, after multivariate adjusted analysis, serum TNF{alpha} was significantly related to UAE, but not to UNAG.

In the present study, there were no significant differences in BMI among diabetic patients after stratification by albuminuria status, whereas diabetic subjects with micro- or macroalbuminuria had higher levels of HbA1c than patients with normal UAE. After regression analysis, serum but not urinary TNF{alpha} levels were significantly correlated with BMI, whereas only urinary TNF{alpha} was significantly associated with HbA1c. These findings are in agreement with the results of previous studies, and suggest that serum levels of TNF{alpha} are essentially determined by the systemic production of this adipocytokine by adipose tissue, unrelated to metabolic factors such as HbA1c [15,16]. On the contrary, urinary concentration of TNF{alpha} represents the local production of this cytokine within the kidney, which would be significantly related to metabolic control [17].

The local actions and implications of intrarenal TNF{alpha} are extremely interesting. In 1991, Hasegawa et al. [4] reported that macrophages incubated with glomerular basement membranes from diabetic rats produced greater amounts of TNF{alpha} than did macrophages incubated with membranes from non-diabetic animals. These authors suggested for the first time the potential implication of this cytokine in the pathogenesis of DN. TNF{alpha} is a pleiotropic cytokine playing an essential role in the molecular basis of inflammatory and immunological reactions. Furthermore, TNF{alpha} is a potential causative agent implicated in glomerular and tubulointerstitial damage in the setting of diabetes.

Recent experimental works have demonstrated that TNF{alpha} gene expression is increased in diabetic rats as compared to control rats, both in glomerular and tubulointerstitial structures [10,18–20]. It is important to note that this cytokine may be produced in the diabetic kidney by infiltrating cells (mainly macrophages) as well as intrinsically by renal cells (endothelial, mesangial, glomerular and tubular epithelial cells) [21,22]. Within the glomerulus, TNF{alpha} displays a number of effects that are relevant to the manifestations observed at the initial stages and during the progression of glomerular injury. TNF{alpha} is implicated in the disbalance between vasodilatory and vasoconstrictive mediators, which may result in alterations of glomerular blood flow and glomerular filtration rate [23]. Additionally, this cytokine is cytotoxic to glomerular, mesangial and epithelial cells, and may induce direct renal damage [8,24]. TNF{alpha} is able to promote the local generation of reactive oxygen species, which affects the barrier function of the glomerular capillary wall resulting in enhanced albumin permeability, independently of haemodynamic factors or effects of recruited inflammatory cells [9]. Studies in diabetic rats have demonstrated that UAE significantly correlates with renal cortical mRNA levels and urinary TNF{alpha} excretion, and moreover, increased TNF{alpha} concentrations in urine as well as in renal interstitial fluid preceded the rise in UAE [10,19]. Regarding tubulointerstitial damage, exposure of tubular epithelial cells to TNF{alpha} increased the synthesis and secretion of lymphocyte and neutrophil chemoattractant factors [25] as well as the cell surface expression of intercellular adhesion molecule-1 [26,27], which has been implicated in the development of renal injury in diabetes [27]. Finally, TNF{alpha} has stimulatory effects on sodium uptake by proximal tubule cells [28], contributing to sodium retention and renal hypertrophy, typical alterations that occur during the early stage of DN [20]. The results of these studies, as well as the findings in the present work, suggest that urinary TNF{alpha} may reflect the intrarenal activity of this cytokine, which may be related to the development and progression of renal injury in diabetes.

An interesting finding in our study was the lack of correlation between urinary TNF{alpha} with UAE and UNAG in normoalbuminuric diabetic patients, whereas it is significantly associated with these parameters in diabetic subjects with increased UAE. These findings may suggest two possibilities: (i) the presence of a threshold phenomenon, and therefore, it is very interesting to speculate that by maintaining the urinary TNF{alpha} concentration below a certain level it will be possible to offer significant renoprotection; and (ii) the importance of diabetes duration: in our study, albuminuric diabetic patients had a higher duration of diabetes than normoalbuminuric subjects. Therefore, it is possible to hypothesize that the higher time of diabetes, the higher intrarenal effect of several local factors, such as TNF-{alpha}, which may contribute significantly to the magnitude of renal injury.

Clinical studies analysing the urinary excretion of TNF{alpha} in diabetic patients are scarce. Two previous works measured this parameter in the setting of DN, although the main objective was to study patients with primary glomerulonephritis. In the first work, TNF{alpha} was measured in 6 diabetic subjects, with urinary TNF{alpha} activity being present only in 1 patient [29]. In the second study [30], the median urinary TNF{alpha} in 16 patients with DN was 14 (range 8–52) pg/ml, similar to the values found by us in the present work. Finally, in a preliminary study by our group in 65 diabetic patients [5], urinary TNF{alpha} was significantly correlated with UAE. The present study confirms these preliminary findings in a large group of diabetic patients; in addition, it shows that urinary TNF{alpha} excretion is also significant, direct and independently related to UNAG, a clinical marker of tubulointerstitial damage.

The importance of this association is highlighted by the effects of therapeutic interventions based on the modulation of TNF{alpha}. Pentoxifylline (PTF) is a phosphodiesterase inhibitor with significant immunoregulatory and anti-inflammatory properties. PTF inhibits the accumulation of TNF{alpha} mRNA and the transcription of the TNF{alpha} gene, suppressing the synthesis and production of this cytokine [31]. Recent studies by DiPetrillo et al. [32] and by our group (our unpublished data) in experimental DN have found that PTF administration was able to prevent the increased renal TNF{alpha} expression, synthesis and excretion during diabetes. In addition, inhibition of TNF{alpha} by PTF was associated with a reduction in sodium retention and renal hypertrophy. Together with these experimental observations, clinical trials have shown that PTF reduces urinary protein and NAG excretion in diabetic subjects, both with normal renal function and renal insufficiency [13,33–35]. The antiproteinuric action of PTF was not related to an improvement of metabolic or haemodynamic factors, but interestingly, it was direct and significantly related to a modulation of TNF{alpha} activity [13,35].

Finally, our study may have some potential limitations. First, the differences in several parameters among patients after stratification by albuminuria status. However, these are the actual characteristics of diabetic subjects that attend to out-patient clinics daily. Second, the influence of the blockade of renin–angiotensin system and BP on serum TNF{alpha}. In our study, since ACEI and ARA are able to reduce TNF{alpha} levels, all patients were treated with the maximum recommended doses of these drugs in order to avoid this confounding factor derived from different dosage. On the other hand, there was no significant association between systolic or diastolic BP with TNF{alpha} levels. And third, it is possible to consider that the correlation between urinary TNF{alpha} and albuminuria exists due to broader glomerular damage reflected by UAE. However, an important finding in our study is that urinary TNF{alpha} was not correlated with the serum levels of this cytokine. This strongly suggests that intrarenal TNF{alpha} production is involved in urinary TNF{alpha} excretion, which is not linked to glomerular damage. In addition, recent studies in experimental models of diabetic nephropathy have demonstrated that renal TNF{alpha} gene expression was increased in diabetic rats, with a significant correlation between renal TNF{alpha} expression and urinary TNF{alpha} levels [10]. Moreover, it has been shown that increased TNF{alpha} concentrations in urine as well as in renal interstitial fluid preceded the rise in UAE [16].

In conclusion, the findings of the present study show that in type 2 diabetic patients, urinary TNF{alpha} excretion is elevated and correlates with severity of renal disease in terms of both glomerular and tubulointerstitial damage. This suggests that TNF{alpha} might play a significant role in the pathogenesis and progression of renal injury in diabetes. Prognostic clinical studies will be necessary to evaluate urinary TNF{alpha} excretion as a parameter for monitoring the development and evolution of DN. In addition, the potential relevance of these findings with respect to new preventive and therapeutic strategies in the setting of DN deserves future investigation.



   Acknowledgments
 
This study was supported by Fundación Canaria de Investigación y Salud (FUNCIS) and Asociación Científica para la Investigación Nefrológica (ACINEF).

Conflict of interest statement. None declared.



   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 

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Received for publication: 22. 5.06
Accepted in revised form: 11. 7.06


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