Department of Endocrinology and Nephrology, University Hospital Benjamin Franklin, Free University of Berlin, Germany and 1 Department of Pathology, University of Leiden, The Netherlands
Correspondence and offprint requests to: Markus Ketteler, MD, Department of Endocrinology and Nephrology, University Hospital Benjamin Franklin, Hindenburgdamm 30, D-12203 Berlin, Germany. E-mail: gmkett{at}aol.com.
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Abstract |
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Methods. Anti-Thy-1-glomerulonephritis was induced in male SpragueDawley rats by single-bolus injection of monoclonal ER4-antibodies. Rats were treated with difluoromethylornithine (0.52% in the drinking water), a selective inhibitor of the rate-limiting enzyme of polyamine synthesis, ornithine decarboxylase (ODC). Mesangial cell proliferation and matrix expansion were evaluated in PAS-stained kidney tissues. Glomerular TGF-ß and biglycan-mRNA-expression were determined by Northern blot analysis and albuminuria was measured using a competitive ELISA. Data were compared to untreated controls.
Results. Though complete inhibition of ODC activity was achieved at any time point, difluoromethlornithine treatment had no significant effect on albuminuria, glomerular matrix protein expression and mesangial cell count in this model.
Conclusions. The acute upregulation of glomerular ODC activity above baseline in anti-Thy1-glomerulonephritis is not pathophysiologically important for disease development however, biological effects of available polyamine pools cannot be excluded by our study.
Keywords: arginine metabolism; glomerulonephritis; mesangial proliferation; nitric oxide; ornithine decarboxylase; polyamines
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Introduction |
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Arginase produces L-ornithine from L-arginine and, by competing with nitric oxide synthases for their common substrate, may locally alter the magnitude of NO-synthesis [3,4,8,10]. L-ornithine is the substrate for polyamine synthesis (putrescine, spermidine, spermine). Polyamines are small cationic molecules and essential for tissue growth in most living organisms [12,13]. ODC is the rate-limiting enzyme of polyamine synthesis and shares expression characteristics with some immediate early genes, while the proto-oncogene c-myc is involved in the control of ODC transcription [1214]. Polyamines were shown to play a major role in the induction and regulation of cell differentiation and proliferation in a large number of cells including mesangial cells in vitro [12,13,1517]. Studies performed in cultured glomerular mesangial cells conclusively demonstrated that ODC activity is important for mesangial cell proliferation and may be controlled by an array of growth factors including platelet-derived growth factor (PDGF) [15,16]. Recently, a potential role for ODC in the induction of apoptosis was also described [18,19].
Upregulation of the expression and activity of ODC associated with increases of tissue polyamine levels was previously described in a number of models of tissue injury and repair including some models of kidney diseases [2024]. Biphasic upregulation of ODC in the early phase of anti-Thy-1-glomerulonephritis (anti-Thy-1 GN) was also observed in vivo with the second peak coinciding with the onset of mesangial cell proliferation and growth-factor overexpression [10]. Further, substrate depletion by feeding L-arginine-deficient diets to nephritic rats ameliorated the disease course in this model while it remained unclear which pathway of L-arginine metabolism was affected by this treatment modality [9].
Mesangial cell proliferation and apoptosis are key factors in determining the disease development of mesangioproliferative anti-Thy-1 GN. Since increased polyamine synthesis was suggested to play a significant role in this context, difluoromethlornithine (DFMO)-induced inhibition of ODC activity seemed an appropriate tool to directly evaluate the pathophysiological role of acute polyamine synthesis in the outcome of this model. The influence of this treatment approach on albuminuria, mesangial cell proliferation and glomerular matrix expansion was subsequently determined in a time course experiment and compared with untreated controls. Effects of DFMO-treatment in rat models of experimental glomerulonephritis were previously not reported.
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Methods |
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Rats (n=4 per group) were intravenously injected into the tail vein with a monoclonal anti-Thy-1-antibody (ER4-hybridoma, 1 mg/kg body weight). DFMO was administered in the drinking water in the following concentrations:
Increasing doses of DFMO were administered to ensure effective inhibition of polyamine synthesis, since upregulation of ODC gene transcription was previously reported as a counter-regulatory feedback mechanism in the presence of enzyme inhibition [24]. Twenty-four-hour urines were collected in metabolic cages on days 1, 3 and 7. Rats were sacrificed under ether anaesthesia at 6 h, on day 3 and on day 7 following disease induction. After renal perfusion with 25 ml ice-cold phosphate-buffered saline (PBS), kidneys were excised. Cortex slices of the left kidneys were formalin-fixed or snap-frozen in ODC buffer, respectively. The remainder of the kidneys were subjected to glomerular isolation using graded sieving as previously described [9,10].
Morphology and mesangial cell count
Formalin-fixed kidney tissue sections were stained with periodic acid Schiff (PAS). Glomerular matrix expansion was scored in a blinded fashion by two investigators as previously published (matrix score: 1=normal glomerular morphology4=diffuse and complete glomerulosclerosis) [10]. Mesangial cell count was determined by counting cell nuclei in 35 glomerular cross-sections per animal [10]. Swollen nuclei at the 6-h-time point were regarded as severely damaged cells and excluded from counting.
Albuminuria measurements
Rat albuminuria was determined by using a modified competitive ELISA technique [26]. Ninety-six-well-plates (Nunc-Immuno Maxisorp, Denmark) were coated with rat albumin (solution: 0.2 mg/l in 0.1 M sodium bicarbonate) by incubation at 37°C for 3 h and at 4°C for an additional 12 h using 100 µl per well. The plates were subsequently washed three times with buffer A (20 mM diethylmalonic acid, 150 mM sodium chloride, 0.1 mM EDTA, 1 ml Tween 20, 0.5% gelatine; pH 7.4). Twenty-four-hour urines were diluted with buffer A at the following concentrations: 1:50, 1:500, 1:5000 and 1:20000. Aliquots of 50 µl of these diluted samples were pipetted into the wells and 50 µl of antibody conjugate (GAR a/ALB/PO) (BIOGENZIA/Nordic Immunologie, Bochum, Germany) diluted 1:5000 in buffer A were added. After additional washings with buffer A, 200 µl of 0.5 mM TMB were pipetted into each well. The plates were then put on a shaker and incubated for 1 h at room temperature. Extinction was determined every 10 min for 90 min in an automated plate reader at 650 nm. Values were compared to rat albumin standards.
Ornithine decarboxylase activity assay
ODC activity was determined by measuring the conversion of L-[1-14C]ornithine (Amersham International, Little Chalfont, UK) to 14CO2 as previously described [10]. Briefly, cortex slices were homogenized in assay buffer (50 mmol/l Na2HPO4, 0.1 mmol/l EDTA, 5 mmol/l NaF, 0.2 mmol/l pyridoxalphosphate, 2.5 mmol/l DTT, 2 mmol/l PMSF; pH 7.2) on ice and subsequently centrifuged at 14000xg for 25 min. Aliquots of 200 µl of the supernatant were subjected to a 15-ml conical centrifuge tube, 0.25 µCi of radiolabelled L-ornithine and cold L-ornithine (final concentration: 0.33 mmol/l) were added. After incubation for 1 h at 37°C in a water bath, 200 µl of 2 M citric acid was injected into the tube to release all CO2. 14CO2 was trapped on a filter paper soaked with 20 µl NaOH, which was mounted in the top of the tube. After an additional incubation of 45 min at 37°C, filter papers were placed in 5 ml scintillation fluid and subjected to scintillation counting. Protein content of samples was measured by the Bradford method. Linearity of the assay was determined by a standard curve employing bovine ODC (0.1, 0.5, 1 and 2 mU, respectively) under the described assay conditions. ODC activity was calculated as mU per g protein. Interassay background activity ranged between 50 and 250 mU/g protein due to condensation of radiolabelled L-ornithine on the filter paper. Therefore, ODC activity values below 250 mU/g protein were considered as completely suppressed activity.
Northern blot hybridization
Northern blot analysis was performed as previously described [9,10]. Isolated, pooled glomeruli from four rats per group were lysed in 4 M guanidine isothyanate and subsequently ultracentrifuged on a 5.7 M cesium chloride cushion. Following electrophoresis on a 2.2 M formaldehyde/0.9% agarose gel and transfer to a nylon membrane, prehybridization of membranes (5xDenhardt's solution, 5xSSC, 50 mmol/l sodium phosphate (pH 6.5), 0.1 mg/ml herring sperm, 0.1% SDS, 50% formamide) was performed at 42°C for 3 h and membranes were subsequently hybridized overnight using the following cDNA probes after random primer labelling with [32P]CTP (Boehringer Mannheim, Germany): (i) Rat ODC-cDNA (Dr Jaenne, University of Helsinki, Finland) [27]; (ii) Murine TGF-ß-cDNA (Dr Derynck, San Francisco, CA) [28]; (iii) Human Biglycan-cDNA (Dr Fisher, NIH, Bethesda, MD) [29]; and (iv) Rat GAPDH-cDNA (Dr Kondaiah, Dr Sporn, NIH, Bethesda, MD) [30] as a control house-keeping gene. After hybridization, membranes were washed three times in 2xSSC, 0.1% SDS for 5 min at room temperature, and once in 0.1xSSC, 0.1% SDS for 15 min at 50°C. Expression of ODC-, TGF-ß- and biglycan-mRNA was quantitated by phophoimaging technique (FUJI Biolmaging Analyzer (Type BAS-IIIs), Raytest, Berlin, Germany) and by comparison to GAPDH-mRNA-expression. Northern blot experiments were performed twice, and increases of mRNA-expression of >50% vs controls and untreated animals at the same time point respectively, were regarded as significant.
Statistics
Differences between DFMO-treated and untreated animals at given time points were analysed by unpaired Student's t-test. Open bars indicate standard deviations in the figures.
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Results |
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Discussion |
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Our treatment approach was started with a 3-day DFMO-pretreatment period, and DFMO concentrations were stepwise increased from 0.5 to 2% in the drinking water. Pretreatment was performed to achieve a near maximal effect by reaching a relative glomerular polyamine depletion prior to the induction of disease. The DFMO dose was gradually increased, because it had been previously suggested that ODC inhibition by DFMO can be counteracted by a dose-dependent increase in ODC gene transcription [25]. Indeed, in pretreated controls and at 6 h after disease induction ODC-mRNA was higher in DFMO-treated rats potentially due to this mechanism, but ODC activity was efficiently blocked despite increased gene transcription.
DFMO had been employed as an effective antiproliferative substance in vitro and in some animal models of cancer [12,31]. DFMO has also been previously used in a number of models of kidney diseases [2224,3234]. Early kidney hypertrophy in streptozotocin-induced diabetes mellitus could be prevented by DFMO, but later changes in the development of diabetic nephropathy could not be influenced [24,32]. Gunnia et al. reported amelioration of disease in experimental lupus nephritis in MRL-lpr/lpr mice by DFMO, however, this effect was due to a suppression of defectively activated T-cells in this model and probably not to an influence on intrinsic renal cells [33,34]. In most other models of renal disease, DFMO had no measurable effect on kidney hypertrophy or other pathophysiological features [22,23].
However, DFMO was conclusively shown to be capable of inducing complete growth arrest of cultured mesangial cells [15,16]. These superior effects of DFMO on mesangial cell growth in vitro must be regarded from the experimental protocol, where a gradual cellular polyamine depletion over several days was necessary to achieve such a maximal effect concerning growth inhibition [16].
The results of our study reflect an in vivo resistance to DFMO treatment, which had previously been suggested from studies in human cancer [12,31]. The potential reason for this observation is that the available tissue polyamine pools in the body may be sufficient to account for growth modulatory effects [12,31]. Transport of extracellular polyamines and upregulation of S-adenosylmethionine decarboxylase (AdoMetDC) are also suggested mechanisms from studies on tumour cell growth regulation partially compensating for ODC enzyme inhibition [12,31, 35,36]. To obtain maximal effects on cell proliferation in vivo, it appears likely that both ODC and AdoMetDC inhibition combined with dietary L-arginine and polyamine restriction are necessary as it had been suggested from anticancer studies, but potentially available AdoMetDC inhibitors appear to have significant toxicity and are thus not feasible [31,35]. However, since polyamines are physiologically essential for cells to enter into and progress through the cell cycle, a near-total polyamine depletion could well have severe unwanted effects, which may outweigh a moderate benefit.
Though we did not directly demonstrate changes in tissue polyamine concentrations in this study, previous investigators conclusively showed in different models of kidney diseases that putrescine and spermidine, but not spermine, renal tissue levels are effectively decreased by DFMO-induced suppression of ODC activity [22,23,32,33]. These findings further suggested that the discussed compensatory mechanisms may not be significantly operative in kidney tissues, possibly in contrast to tumour models. The determination of glomerular polyamine concentrations in anti-Thy-1 GN in parallel with RNA extraction and ODC activity assays was not possible, since up to 500 mg of glomeruli per animal would have been necessary per animal to securely enable successful HPLC detection.
Nevertheless, DFMO treatment successfully inhibited the acute increases in ODC activity at any time point in our study. Our conclusion is that acute glomerular upregulation of ODC expression and activity above baseline levels has no pathophysiological relevance in the early phase and during disease development of this particular model. The previously observed beneficial effects of L-arginine-modified diets are thus probably mediated through an influence on nitric oxide, agmatine or L-proline synthesis, or through a non-metabolic or substrate-independent mechanism.
In summary, in vivo inhibition of polyamine synthesis by DFMO treatment had no influence on the disease course of anti-Thy-1 GN in SpragueDawley rats. Therefore, the acutely upregulated ODC activity, which was completely blocked by this treatment, does not appear to be an important disease mechanism in this model. Potentially sufficient growth-modulating effects of available polyamine pools in this model cannot be excluded to explain the lack of therapeutic efficacy of DFMO.
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Acknowledgments |
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References |
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