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*
Department of Pediatrics, Hospital Central de Asturias and School of
Medicine, Instituto Universitario de Oncología
del Principado de Asturias, University of Oviedo, Oviedo, Asturias,
Spain.
Department of Anatomy, Hospital Central de Asturias and School of
Medicine, Instituto Universitario de Oncología
del Principado de Asturias, University of Oviedo, Oviedo, Asturias,
Spain.
Correspondence to Dr. Fernando Santos, Pediatría, Facultad de Medicina, C/Julián Clavería s/n, 33006 Oviedo, Asturias, Spain. Phone and Fax: +34-98-510-3585, E-mail: fsantos{at}correo.uniovi.es
| Abstract |
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| Introduction |
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Ischemia remains the leading cause of ARF in adults (5), and prerenal ARF related to perinatal asphyxia accounts for the majority of cases of ARF in newborns (2). Thus, experimental models of ischemic ARF have been used widely to gain an understanding of the cellular consequences of ischemic injury and to test new therapeutic agents.
The hallmark of ischemic cell injury is cellular ATP depletion (6), which rapidly causes marked and complex functional and structural changes in renal epithelial cells, especially those of the proximal tubule (7). As a result, the polarity and physiologic functions of the tubular cell are lost. Depending on the severity and the duration of the ischemic insult, the damaged tubular cell may recover, either directly or through an undifferentiated cell intermediate, or die by necrosis or apoptosis (7).
Theoretically, the therapeutic use of growth factors may exert a beneficial effect on this ischemia-induced chain of cellular events because growth factors are, in general, mitogenic and antiapoptotic. On the basis of this assumption, several studies have reported that exogenous administration of growth factors improves the outcome of animals with ARF (8,9,10,11).
The use of insulin-like growth factor I (IGF-I) may have some additional advantages in the treatment of ARF. Besides its mitogenic and antiapoptotic properties, IGF-I mediates most of the systemic anabolic effects of growth hormone (GH), increases renal blood flow and GFR (12), and is widely expressed in kidney (13). Accordingly, IGF-I has elicited substantial interest as a possible therapeutic agent in ARF. A number of reports, although not all (14), have found that administration of exogenous IGF-I is of some benefit in rats with ischemic ARF (15,16,17,18,19). Despite these promising results in animals, a multicenter clinical trial failed to demonstrate that recombinant human IGF-I administration improves the course of established ARF in humans (20). Likewise, recent clinical studies (21) showed that administration of GH increases the mortality rate of critically ill patients, but the mechanism responsible for this harmful effect remains to be determined.
In light of these clinical reports and of unpublished data from our laboratory indicating that administration of exogenous IGF-I worsened the survival rate of rats with severe ischemic ARF, we designed the study presented here to provide some insight on the mechanism responsible for this potential deleterious effect of IGF-I treatment.
| Materials and Methods |
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Preliminary Study
ARF rats were assigned to receive three 0.1-ml doses of 50 µg/100 g body
wt of recombinant human IGF-I (Pharmacia Upjohn, Stockholm, Sweden) or an
equivalent volume of vehicle (saline) subcutaneously at 0, 8, and 16 h after
declamping. A group of sham-operated rats that were treated with vehicle were
used as controls. Tail-vein blood samples were obtained daily for 7 d for
serum creatinine and serum urea nitrogen measurements, using a Kodak Ektachem
DT60 analyzer (Rochester, NY). The mortality rate of the three groups of rats
is shown in Figure 1. In the
IGF-Itreated ARF group, 89% of rats died (33 of 37), whereas ARF rats
that received vehicle had significantly (P < 0.05, Kaplan Meier
analysis followed by log rank test) less mortality (17 of 27 rats [63%]).
Serum creatinine and urea nitrogen profiles in the three groups of rats are
shown in Figure 2. An apparent
beneficial effect of IGF-I treatment may be observed from day 3 on.
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Short-Term Study
ARF rats were assigned to receive three 0.1-ml doses of 50 µg/100 g body
wt of recombinant human IGF-I (Pharmacia Upjohn, Stockholm, Sweden) or vehicle
(saline) subcutaneously at 0, 8, and 16 h postischemia. Sham-operated rats
that were treated with vehicle were used as controls. The number of rats per
group varied so as to obtain five rats at the moment of killing. Two days
after the ischemic insult, rats were anesthetized and killed by
exsanguination, via abdominal aorta. Both kidneys were collected for
histologic study or RNA analysis. Tail-vein blood samples were drawn daily for
determination of serum concentrations of creatinine and urea nitrogen.
Histologic Examination. The kidneys were removed, stripped of their capsules, cut longitudinally into halves, fixed in 10% formaldehyde, and embedded in Paraplast (Fluka, Madrid, Spain). Sections of 3 µm were obtained and mounted on glass slides. Different sets of slides were stained with hematoxylin and eosin, periodic acid-Schiff, and naphthol AS-D chloroacetate esterase stains.
Scoring of Renal Damage. Hematoxylin and eosin and periodic acid-Schiff samples were graded by an independent observer using a scoring system that considered the pathologic changes consistent with acute tubular necrosis: peritubular hyperemia at the corticomedullary junction, protein casts, and intratubular cell shedding (22). Grades were added together and divided by 3 to give a final score of 0 to 3. At least 12 microscopic fields per section were chosen randomly to be scored for each parameter.
Neutrophil Infiltration. Neutrophil infiltration was evaluated using naphthol AS-D chloroacetate esterase staining by counting the number of neutrophils present at the corticomedullary junction and at the renal medulla. Sixty microscopic fields per rat were studied. Data are expressed as neutrophils per mm2 of tissue.
Macrophage Infiltration. Macrophage infiltration was assessed by immunohistochemistry using a mouse monoclonal antibody against ED-1 (Serotec, Oxford, UK; 1:100). After incubation with the primary antibody, sections were incubated with peroxidase-labeled sheep antimouse antibody (Amersham, Little Chalfont, Bucks, UK; 1:100). The number of macrophages was counted at the corticomedullary junction and at the renal medulla. Sixty microscopic fields per rat were analyzed. Data are expressed as macrophages per mm2 of tissue.
Apoptosis. Morphologic criteria were used to identify apoptotic cells, which were counted in 20 high-power fields per section (x400) at the corticomedullary junction. Characteristics of apoptosis included cellular rounding and shrinkage, eosinophilic cytoplasm, nuclear chromatin compaction especially along the nuclear envelope in a crescentic manner, membrane-bound cellular blebbing, and formation of apoptotic bodies (23).
DNA Synthesis. Sixty minutes before killing, rats received an intraperitoneal injection of 100 mg/kg body wt of 5-Bromo-2'-deoxyuridine (BrdU; Sigma, St. Louis, MO) in saline (10 mg/ml). Tissue samples were obtained and processed as stated above. BrdU-labeled nuclei were identified as described elsewhere (24). A labeling index (100 x tubular cells labeled with BrdU/total number of tubular cells) was calculated. At least 1 x 103 cells per section were studied.
RNase Protection Assay. Probes. Antisense rat IGF-I receptor RNA probe was synthesized using the EcoRI linearized plasmid pGEM-3 (Promega, Madison, WI), which contains a 265-bp HindIII fragment of the rat IGF-I receptor. Antisense rat IGF-I RNA probe was synthesized from a 376-bp EcoRI/HindIII fragment cloned into the pGEM-3 vector. Both plasmids were used as a template for synthesis of 32P-UTP-labeled RNA probes with SP6 RNA polymerase according to the instructions given by the manufacturer (SP6/T7 Transcription Kit; Boehringer Mannheim GmbH, Mannheim, Germany). A commercially available linearized plasmid into which a cDNA fragment of the rat cyclophilin gene had been subcloned was purchased from Ambion (pTRI-cyclophilin-Rat; Ambion Inc., Austin, TX), and a cyclophilin cRNA was synthesized.
Solution Hybridization. Total RNA was extracted from 50 to 100 mg of kidney tissue of each rat according to the method of Chomczynski and Sacchi (25). A solution hybridization assay was used to quantify IGF-I and IGF-I receptor. In brief, the probes were labeled and hybridized to rat renal RNA using a ribonuclease protection assay kit (RPA II; Ambion) and following the indications given by the manufacturer. The protected fragments were separated on an 8-mM/L urea/6% polyacrylamide denaturing gel. The gels were dried and exposed to Hiperfilm-MP (Amersham Ibérica, Madrid, Spain) with two intensifying screens at 7°C. The radioactivity of the hybridized probes was measured by densitometric scanning of the x-ray films with a Bioimage densitometer (Millipore, Ann Arbor, MI) coupled to a computer with Analysis 1-D program (1D Manager; TDI, Madrid, Spain). Data for IGF-I and IGF-I receptor were adjusted for the corresponding cyclophilin reading and expressed as arbitrary densitometric units. For each gel, the measurement of sham-operated rats was assigned a value of 1 and the results of ARF and ARF+IGF-I groups were estimated in relation to this value.
Statistical Analyses
Data are given as mean ± SEM. Comparisons were made by one-way ANOVA
at a level of significance of 95%. Student-Newman-Keuls test was performed for
comparisons between two groups.
| Results |
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Pathological Changes
Forty-eight hours after reperfusion, widespread necrosis, sloughing of the
proximal straight tubules with obstructing casts, different degrees of brush
border loss, and detached tubular cells were seen in kidney samples from ARF
and ARF+IGF-I rats (Figure 3).
As shown in Table 2, IGF-I
administration increased (P < 0.05) the score of renal damage in
comparison with ARF rats.
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DNA Synthesis
Proliferative activity of tubular cells, as assessed by the percentage of
BrdU-labeled cells, was much higher in ARF and ARF+IGF-I groups; no difference
was found between the two groups of rats with renal failure
(Table 2).
Neutrophil Infiltration
Neutrophils accumulated at the corticomedullary junction and the renal
medulla in both groups of clamped rats
(Table 3,
Figure 3). As compared with the
ARF group, the number of infiltrating neutrophils was significantly
(P < 0.05) higher in the IGF-I-treated rats; the difference was
even more marked at the renal medulla.
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Macrophage Infiltration
The number of macrophages was higher (P < 0.05) in ARF and
ARF+IGF-I rats at the corticomedullary junction as well as at the medulla
(Table 3) in comparison with
the controls. There was no difference between the two groups of rats with
renal failure.
Apoptosis
Similarly increased numbers of apoptotic cells were found in ARF (42.9
± 7.0 per mm2) and ARF+IGF-I (47.6 ± 9.9 per
mm2) as compared with sham-operated rats (8.2 ± 2.9 per
mm2; P < 0.05).
Renal Expression of IGF-I mRNA
RNase protection assay for IGF-I mRNA in kidney tissue revealed a band of
376 bp. A representative experiment is shown in
Figure 4. IGF-I mRNA levels
were significantly (P < 0.05) reduced in the two groups of rats
with renal failure in comparison with sham-operated rats (ARF, 0.1 ±
0.0; ARF+IGF-I, 0.2 ± 0.1; sham, 1.0 ± 0.0). There was no
difference between the ARF and ARF+IGF-I groups.
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Renal Expression of IGF-I Receptor mRNA
RNase protection assay for IGF-I receptor mRNA in kidney tissue revealed a
band of mRNA of 265 bp. A representative experiment is shown in
Figure 5. IGF-I receptor mRNA
levels were significantly (P < 0.05) reduced in the two groups of
rats with renal failure in comparison with sham-operated rats (ARF, 0.6
± 0.2; ARF+IGF-I, 0.5 ± 0.1; sham, 1.0 ± 0.0). No
difference was found between the ARF and ARF+IGF-I groups.
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| Discussion |
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Our short-term study was designed to shed light on the mechanism responsible for the increased mortality induced by IGF-I treatment. In agreement with the findings on day 2 of the preliminary 1-wk experiment, the serum concentrations of urea nitrogen and creatinine were equivalent in the ARF and ARF+IGF-I groups. However, the histologic damage was more marked in the kidneys of rats that were treated with IGF-I (Table 2). Thus, it can be assumed that if the rats had been allowed to live longer, then a greater risk of death, resulting from a more severe renal lesion, had existed in the IGF-I treated group.
The analysis of cellular proliferation, apoptosis, and local expression of IGF-I and IGF-I receptor mRNA revealed that IGF-I treatment caused no changes beyond those already induced by ischemia. Thus, it seems that some of the main events linked to the reparative process after the ischemic insult were not modified substantially by exogenous administration of IGF-I. By contrast, the administration of IGF-I markedly enhanced the accumulation of neutrophils in the kidney. In comparison with the control rats, the neutrophil count in untreated ARF rats 48 h postischemia increased by a factor of 180 at the corticomedullary junction and by a factor of 50 at the renal medulla. In ARF rats that were treated with IGF-I, the accumulation of neutrophils was markedly higher, reaching values well over 200 times those of untreated ARF rats both at the corticomedullary junction and the renal medulla. This effect of IGF-I administration was in some way specific for neutrophils because it was not observed for macrophages. To determine whether the neutrophil infiltration was resolved with the recovery from acute renal failure and whether massive neutrophil accumulation found in the ARF+IGF-I group was transient and dependent on IGF-I administration, we also investigated neutrophil infiltration in stored kidney samples of rats from the preliminary study, i.e., rats killed 7 d after clamping. In comparison with the neutrophil infiltration observed on the second day postischemia (Table 3), the number of neutrophils per mm2 of tissue decreased to 7.7 ± 1.4 and 12.7 ± 4.6 at the corticomedullary junction and to 3.2 ± 0.4 and 3.3 ± 0.2 at the renal medulla in ARF (n = 5) and ARF+IGF-I (n = 4), respectively. These figures, although still higher than those observed in sham-operated rats (0.2 ± 0.1 and 0.3 ± 0.1), were markedly lower than those found 48 h after clamping. Moreover, there was no difference between the ARF and ARF+IGF-I groups. The reduction of neutrophil infiltration found 7 d after clamping clearly is consistent with an inflammatory response related to the acute period of renal failure and markedly exacerbated by IGF-I administration.
With the use of indium-labeled neutrophils, it has been shown that the number of neutrophils retained in the kidney is dependent on the duration of renal ischemia and the activity state of the neutrophil (27). The recruitment, activation, and transendothelial migration of neutrophils after ischemic aggression is a complex process that has been investigated extensively in recent years (5,28,29). It is mediated by a variety of molecules such as selectins, integrins, Ig-like adhesion molecules, and chemokines. Several lines of evidence now indicate an important role of this accumulation of neutrophils in renal ischemia/reperfusion injury (5,28,29).
Despite that inflammation is considered to be the most important cause of tissue injury in organs subjected to ischemia, the effect of IGF-I treatment on the inflammatory response has not been analyzed in rats with ischemic ARF. Miller et al. (15) reported the degree of peritubular hyperemia observed in IGF-I treated rats as a criteria of histologic damage. However, specific histochemical staining for neutrophils (chloroacetate esterase reaction) or assays for neutrophil-derived enzymes such as myeloperoxidase were not used. It also is of note that vascular changes found in kidneys of rats with ischemic ARF treated with IGF-I may be the result not only of the effect of ischemia itself but also of the specific renal hemodynamic actions of IGF-I and, therefore, may not be a reliable index of the underlying inflammatory response. Using a murine model, Daemen et al. (30) suggested that apoptosis may be the mechanism that triggers inflammation after ischemia and that IGF-I, by means of its antiapoptotic potential, may prevent reperfusion-induced inflammation and subsequent tissue injury. In the same study, it also is shown that administration of IGF-I fails to prevent loss of kidney function and inflammation when given once apoptosis of renal cells has started. Thus, the timing of IGF-I action may be important, and it may be hypothesized that an earlier administration of IGF-I, before the ischemic insult, and/or the use of a route of administration with a faster absorption of the peptide might have led to a favorable response in our treated rats.
The association between increased mortality and enhanced neutrophil accumulation observed in our IGF-Itreated rats with severe renal failure also supports the contention that the greater risk of death caused by treatment with high doses of GH in critically ill patients (21) may be linked to the hypermetabolic and proinflammatory effects of GH (31). Because IGF-I mediates most of GH actions, it is tempting to speculate that an IGF-Iinduced exacerbated inflammatory response might play a pivotal role in this increased risk of death.
In conclusion, in the study presented here, IGF-I treatment increased mortality rate of rats with severe ischemic ARF. The higher risk of death was associated with a massive accumulation of neutrophils in kidney tissue, indicating that IGF-I administration may have led to an exacerbation of the inflammatory response caused by the process of ischemia-reperfusion. Our study does not provide any clue to the mechanism that is responsible for this IGF-Iinduced proinflammatory effect that, however, might play an important role in the deleterious effect of GH therapy observed in critically ill patients.
| Acknowledgments |
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| References |
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