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Contrast agents and renal cell apoptosis

Giulia Romano, Carlo Briguori, Cristina Quintavalle, Ciro Zanca, Natalia V. Rivera, Antonio Colombo, Gerolama Condorelli
DOI: http://dx.doi.org/10.1093/eurheartj/ehn197 2569-2576 First published online: 9 May 2008

Abstract

Aims Contrast media (CM) induce a direct toxic effect on renal tubular cells. This toxic effect may have a role in the pathophysiology of contrast nephropathy.

Methods and results We evaluated (i) the cytotoxicity of CM [both low-osmolality (LOCM) and iso-osmolality (IOCM)], of iodine alone, and of an hyperosmolar solution (mannitol 8%) on human embryonic kidney (HEK 293), porcine proximal renal tubular (LLC-PK1), and canine Madin–Darby distal tubular renal (MDCK) cells; and (ii) the effectiveness of various antioxidant compounds [n-acetylcysteine (NAC), ascorbic acid and sodium bicarbonate] in preventing CM cytotoxicity. The cytotoxicity of CM was assessed at different time points, with different methods: cell viability, DNA laddering, flow cytometry, and caspase activation.

Both LOCM and IOCM produced a concentration- and time-dependent increase in cell death as assessed by the different methods. On the contrary, iodine alone and hyperosmolar solution did not induce any significant cytotoxic effect. There was not any significant difference in the cytotoxic effect between LOCM and IOCM. Furthermore, both LOCM and IOCM caused a marked increase in caspase-3 and -9 activities and poly(ADP-ribose) fragmentation, while no effect on caspase-8/-10 was observed, thus indicating that the CM activated apoptosis mainly through the intrinsic pathway. Both CM induced an increase in protein expression levels of pro-apoptotic members of the Bcl2 family (Bim and Bad). NAC and ascorbic acid but not sodium bicarbonate had a dose-dependent protective effect on renal cells after 3 h incubation with high dose (200 mg iodine/mL) of both LOCM and IOCM.

Conclusion Both LOCM and IOCM induce a dose-dependent renal cell apoptosis. NAC and ascorbic acid but not sodium bicarbonate prevent this contrast-induced apoptosis.

  • Contrast media
  • Kidney
  • Apoptosis
  • Prevention

Introduction

Contrast-induced nephropathy (CIN) accounts for 10% of all causes of hospital-acquired renal failure, causes a prolonged in-hospital stay, and represents a powerful predictor of poor early and late outcome.1,2 Haemodynamic changes of renal blood flow, which lead to hypoxia of the renal medulla, and direct toxic effects of contrast media (CM) on renal cells are thought to contribute to the pathogenesis of CIN.3 A predominant toxic effect of CM on renal tubules has been shown in both clinical trials and animal experiments.46 Furthermore, administration of compounds with antioxidant properties [such as N-acetylcysteine (NAC), ascorbic acid, and sodium bicarbonate] has emerged as an effective strategy to prevent CIN.711 Little is known about cellular mechanisms underlying CIN, and, as a consequence, about the mechanism(s) for the protective effect of compounds, such as NAC, ascorbic acid, and sodium bicarbonate.

In the present study, we assessed the apoptotic effect of both iso-osmolar (IOCM) and low-osmolar (LOCM) CM on human embryonic kidney (HEK 293), porcine proximal renal tubular (LLC-PK1), and canine Madin–Darby distal tubular renal (MDCK) cells and determined the role of various antioxidant compounds in preventing CM-induced apoptosis.

Methods

Culture conditions and reagents

Three different cell lines were utilized: (i) human embryonic kidney (HEK 293), which are undifferentiated human renal cells; (ii) porcine proximal renal tubular (LLC-PK1) and canine Madin–Darby renal epithelial (MDCK) cells which have the characteristics of proximal and distal tubule cells, respectively. Cells were grown in a 5% CO2 atmosphere in Dulbecco's Modified Eagle Medium (DMEM) containing 10% heat-inactivated FBS, 2 mM l-glutamine, and 100 U/mL penicillin–streptomycin. Cells were routinely passaged when they reached 80–85% confluent. Media, sera, and antibiotics for cell culture were from Life Technologies, Inc. (Grand Island, NY, USA). Protein electrophoresis reagents were from Bio-Rad (Richmond, VA, USA) and western blotting and ECL reagents (GE Health care, Europe SA). All other chemicals were from Sigma (St Louis, MO, USA).

Contrast agents

Two different CM were tested: (i) iodixanol (Visipaque®, GE Healthcare Europe; 320 mg iodine/mL) non-ionic, IOCM (290 mOsm/kg of water) and (ii) iobitridol (Xenetix®, Guerbet, France; 250 mg iodine/mL) non-ionic, LOCM (915 mOsm/kg of water).

Experimental design

Experiments were driven in the following phases: (i) assessment of cytotoxicity of both LOCM (iobitridol) and IOCM (iodixanol). In order to assess the impact of contrast dose, two different doses of CM were tested, 100 and 200 mg iodine/mL. The cytotoxicity of CM was tested at 15, 30, 45, 60, 90, 120, 150, 180. The osmolality of DMEM alone was 355 mOsm/L, when compared with 395 mOsm/L for DMEM plus IOCM and 830 mOsm/L for DMEM plus LOCM. In order to clarify the potential major determinants of the cytotoxic effect, we further assessed the effect of iodine alone (by incubation with 100 and 200 mg/mL sodium iodine)12 and hyperosmolality (by incubation in DMEM/8% mannitol, having an osmolality of 830 mOsm/L); (ii) assessment of the effectiveness of various antioxidant compounds (that is, NAC, ascorbic acid, and sodium bicarbonate) in preventing contrast cytotoxicity. Different doses of all tested compounds were utilized, in order to elicit any dose-dependent effect. The doses tested were selected according to the available data in the clinical setting. NAC was tested at 1, 10, and 100 mM.13,14 Ascorbic acid was tested at 2, 4, and 8 mM.15 Sodium bicarbonate was tested at 75, 150, and 300 mM.10 Each concentration was done in triplicate.

Protein isolation and western blotting

Cellular pellets from a singular cell line at time were washed twice with cold PBS and resuspended in JS buffer (HEPES 50 mM, NaCl 150 nM, 1% glycerol, 1% Triton X-100, 1.5 mM MgCl2, 5 mM EGTA) containing Proteinase Inhibitor Cocktail (Roche). Solubilized proteins were incubated for 1 h on ice. After centrifugation at 13 200 rpm for 10 min at 4°C, lysates were collected as supernatants. Eighty micrograms of sample extract were resolved on a 12% SDS-polyacrylamide gel using a mini-gel apparatus (Bio-Rad Laboratories, Richmond, CA, USA) and transferred to Hybond-C extra nitrocellulose (GE Healthcare Europe). Membrane was blocked for 1 h with 5% non-fat dry milk in TBS containing 0.05% Tween-20 and incubated over night at 4°C with specific antibodies. The following antibodies were used for immunoblotting: anti-pro-caspase-3 (recognizing only the inactive pro-caspase-3) (cell signalling), anti-beta Actin (Sigma), anti-PARP (Sigma), anti-Bim (Santa Cruz), anti Bad (Santa cruz), and anti-Caspase-9, -10, and -8 from Stressgen. Washed filters were then incubated for 60 min with HRP-conjugated anti-rabbit or anti-mouse secondary antibodies (GE Healthcare, Europe) and visualized using chemioluminescence detection (GE Healthcare Europe). The activation of caspase was followed by the disappearance of the band corresponding to the inactive pro-caspase enzyme, utilizing a specific antibody that recognizes this form.

Cell-death quantification

Cell death was evaluated with the CellTiter 96® AQueous One Solution Cell Proliferation Assay (Promega, Madison, WI, USA), according to the manufacturer's protocol. The assay is based on reduction of 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt (MTS) to a coloured product that is measured spectrophotometrically. Cells were plated in 96-well plates in triplicate, stimulated, and incubated at 37°C in a 5% CO2 incubator. Iobitridol, iodixanol, NAC, sodium bicarbonate, and ascorbic acid were used in vitro at the doses and time indicated. Metabolically active cells were detected by adding 20 µL of MTS to each well. After 30 min of incubation, the plates were analysed on a Multilabel Counter (Bio-Rad, Richmond, VA, USA). DNA laddering was also used to confirm the apoptotic death induced by CM. Briefly, after CM exposure, the cells were harvested with 500 µL of DNA lysis buffer [5 mM Tris–HCl (pH 7.5), 20 mM EDTA (pH 8.0), 0.5% NP40], and were incubated on ice for 20 min. After centrifugation at 13 200 rpm for 30 min, the DNA was then extracted with phenol chloroform isoamyl alcohol and finally precipitated with the addition of 1.25 mL of cold ethanol 100% and 50 µL sodium acetate (pH 5.2) on dry ice for 20 min. The precipitates were centrifuged (30 min, 13 200 rpm, 4°C), dried at room temperature, solubilized in 10 µl of TE, and then incubated with RNase A for 30 min at 37°C. The DNA samples were finally separated on 1.5% agarose gel containing ethidium bromide (Sigma, St Louis). The gel was photographed under UV light. Apoptosis was also analysed via propidium iodide incorporation in permeabilized cells by flow cytometry. The cells (2 × 105) were washed in PBS and resuspended in 200 µL of a solution containing 0.1% sodium citrate, 0.1% Triton X-100, and 50 µg/mL propidium iodide (Sigma). Following incubation at 4°C for 30 min in the dark, nuclei were analysed with a Becton Dickinson FACScan flow cytometer. Cellular debris was excluded from analyses by raising the forward scatter threshold, and the DNA content of the nuclei was registered on a logarithmic scale. The percentage of elements in the hypodiploid region was calculated.

Statistical analysis

Continuous variables are expressed as mean values ± SD. We performed a multiple comparison test using the information derived by performing one-way analysis of variance (ANOVA) test on groups of independent variables having cell viability as our dependent variable. In an ANOVA, we compared the means of several groups to test the hypothesis that they are all the same, against the general alternative that they are not all the same. However, since the alternative hypothesis may be too general and more information is needed about which pairs of means are significantly different, and which are not, we used the multiple comparison procedure, which allows us to comparing all group mean pairs at the same time. Throughout the analysis, we have specified a significance level α = 0.001 and we performed priori comparisons on the outputs derived from ANOVA test. Also, main focus was given on the ANOVA outputs where the F test resulted significantly. We performed the priori comparisons using the Bonferroni t method for both orthogonal and non-orthogonal comparisons to reduce multiplicity between group comparisons. The Bonferroni t method increases the critical F value needed for the comparison to be declared significant. Data were analysed with SPSS 13.0 (Chicago, IL, USA) for Windows.

Results

Effects of contrast medium on cell viability

As shown in Figure 1A, both LOCM and IOCM produced a concentration-dependent decrease in cell viability as assessed by MTS assay. This effect was identical in all the three renal cell lines utilized (Figure 2). Indeed, the amount of cell death was significantly higher with 200 mg iodine/mL than with 100 mg iodine/mL of contrast at each time-point of the experiment (P = 0.0025; by paired t-test with α = 0.001). There was a significant interaction between time of exposure and the effect of CM on cell viability (P = 9.51 × 10−5; F = 5.93 by ANOVA model with α = 0.001). The toxic effect of CM was further evaluated by DNA laddering (Figure 1B) and propidium staining and FACS analysis (Figure 1C). Both methods confirmed that exposure of cells to LOCM and IOCM induces apoptosis of renal cells.

Figure 1

Effects of contrast media on renal cell viability. (A) HEK 293, LCC-PK1, and MDCK cells were incubated in the presence of either 100 (low dose) or 200 (high dose) mg iodine/mL of iobitridol or iodixanol for the indicated time. Cell viability was then assessed with CellTiter Proliferation Assay. By the ANOVA model and multiple comparison test, there was a significant interaction between cell viability and time of exposure (P < 0.001; F = 285.02) and dose of contrast media (P < 0.001; F = 5.93). (B) HEK 293, LCC-PK1, and MDCK cells were incubated in the presence of 200 mg iodine/mL of iobitridol or iodixanol for 3 h. DNA was extracted and loaded on 1.5% agarose gel. (C) HEK 293, LCC-PK1, and MDCK cells were incubated in the presence of 100 and 200 mg iodine/mL of iobitridol or iodixanol for 24, 48, or 72 h and then DNA fragmentation was measured by flow cytometry. Data represent the mean±SD of two separate experiments performed in triplicate.

Figure 2

Effects of contrast media, iodine, and mannitol on renal cells. HEK 293 (A), LCC-PK1 (B), and MCDK (C) cells were incubated in the presence of 200 mg iodine/mL of iobitridol (IBT) or iodixanol (IXN), of iodine alone (sodium iodine, 100 and 200 mg) and 8% mannitol (hyperosmolar solution) for 3 h. Cell viability was then assessed with CellTiter Proliferation Assay. *P < 0.001 vs. all the other groups; (D) HEK 293, LCC-PK1, and MDCK cells were incubated in the presence of 200 mg iodine/mL of iobitridol or iodixanol for 72 h and then DNA fragmentation was measured by flow cytometry. Data represent the mean±SD of two separate experiments performed in triplicate.

The cytotoxic effect, although maximum at 3 h, was mostly (≈85%) observed already at 15 min of incubation (Figure 1A). In order to better clarify the time-dependency effect, we performed a further control experiment in which cells were exposed for a short period (only 15 min), then washed free of CM, and studied for viability immediately or 3 h later, and compared these effects to those observed upon 3 h of incubation. Interestingly, we found that the cytotoxic effect induced by 15 min of high dose (200 mg iodine/mL) of CM exposure was similar whether it was observed immediately or 3 h later (cell viability: 12 ± 6 vs. 13 ± 6%, respectively; P = 1.0) (data not shown).

There was not any interaction between the cytotoxic effect and the type of contrast used (P = 0.22; F = 1.87 by ANOVA model with α = 0.001). Furthermore, neither sodium iodine alone nor hyperosmolar solution decreased cell viability or induced cell apoptosis (Figure 2). All cell lines were exposed to the same concentrations of CM. On the contrary, in vivo, cell apoptosis was mostly found in the more distal tubular cells (MDCK) which may be exposed to higher concentrations of CM.

Role of caspases in contrast-induced cytotoxicity

To test whether CM stimulate caspase activity, HEK 293 cells were incubated in the presence of either LOCM or IOCM at different time points and then the activation of caspases-8, -10, -3, -9 was assessed by western blot (Figure 3A). Both LOCM and IOCM caused a marked increase in caspase-3 and -9 activities at 7 h of exposure, as assessed by the reduction of the pro-caspase form (Figure 3A). No effect on caspases-8 and -10 was observed, thus indicating that the CM activated apoptosis mainly through the intrinsic, or ‘mitochondrial’, pathway (Figure 3A). This pathway of apoptosis is regulated by Bcl2 family members. Hence, we studied the expression of Bad and Bim, two pro-apoptotic members of the Bcl2 family, after incubation with the CM. Western blotting revealed that exposure to CM induce an increase in both Bad and Bim expression (Figure 3B). Similar results were obtained in the other cell types (data not shown).

Figure 3

Effect of contrast media on caspase activation and on Bcl2 family proteins expression. HEK 293 cells were incubated in the presence of 200 mg iodine/mL of iobitridol or iodixanol for the indicated time. Eight micrograms of protein were loaded onto 12% SDS–PAGE gel. (A) The membranes were incubated with anti-caspase-3, -8, -9, -10 antibodies and visualized by chemioluminescence detection. Contrast media induced an activation of caspases-3 and -9 and not of caspases-8 and -10 as assessed by the reduction of the pro-caspase levels. (B) Membranes were incubated with anti-Bim or anti-Bad antibodies and visualized by chemioluminescence detection. Loading control was controlled with anti-β-actin.

Effects of NAC on contrast-induced cytotoxicity

HEK 293, LLC-PK1, and MDCK cells were pre-incubated with different concentrations of NAC and cell viability was assessed with the cell proliferation assay. We observed a dose-dependent protective effect of NAC on renal cells after 3 h incubation with the high dose (200 mg iodine/mL) of both LOCM and IOCM (P < 0.001; F = 396.22 by ANOVA test; Figure 4A). As compared to baseline, after 3 h of incubation, cell viability was <10% in the CM-treated cells, <25% with the lowest (1 mM) dose of NAC, <30% with the middle (10 mM), and approximately 80% with the highest (100 mM) dose of NAC. There was not any interaction between the protective effect of NAC (for dose 1 and 10 mM) and the type of CM (P = 0.75; F = 0.12 and P = 0.32; F = 1.31, respectively, both by ANOVA test with α = 0.001). However, results for NAC 100 mM with LOCM appears to be slightly better for cell viability when compared with NAC 100 mM with IOCM (P = 0.006; F = 28.22). In order to clarify the mechanism by which NAC prevented contrast-induced apoptosis, we analysed the effect of NAC pre-treatment on Poly(ADP-ribose) (PARP), a final substrate of caspase-3. We found that the CM induced the activation of PARP as assessed by the marked reduction of the 116 kDa PARP pro-form. On the contrary, NAC completely prevented this activation, suggesting that NAC acts through the inhibition of the intrinsic pathway of apoptosis (Figure 4B).

Figure 4

Pre-treatment with NAC and contrast-induced cell death. (A) HEK 293 (open columns), LCC-PK1 (black columns), and MDCK (grey columns) cells were pre-treated for 2 h with different concentration of N-acetylcysteine (NAC) as indicated (1, 10, and 100 mM) and then incubated for 3 h with 200 mg iodine/mL of iobitridol or iodixanol. Cell viability was then assessed with CellTiter Proliferation Assay. NAC protects the cells from contrast-induced cell death in a dose-dependent fashion (HEK 293: P < 0.001; F = 143.51 with iobitridol; P < 0.001; F = 122.43 with iodixanol. LCC-PK1 cells: P < 0.04; F = 12.9 with iobitridol; P < 0.03; F = 14.79 with iodixanol; MDCK cells: P < 0.006; F = 41.19 with iobitridol; P < 0.01; F = 25.24 with iodixanol). *P = <0.001 vs. baseline (CM−/NAC−) and [CM+/NAC 100 mM]; P < 0.05 vs. (CM+/NAC 1 mM) and (CM+/NAC 10 mM); P = 0.003 vs. (CM+/NAC 100 mM); P < 0.03 vs. (CM+/NAC 100 mM); #P < 0.05 vs. baseline (CM−/NAC−). (B) HEK 293 cells were pre-incubated with NAC for 2 h and then incubated in the presence of 200 mg iodine/mL of iobitridol or iodixanol for 3 h. Eighty microgram of protein was loaded onto 10% SDS–PAGE. The membranes were incubated with anti-PARP antibody and visualized by chemioluminescence detection. PARP activation was detected by the reduction of the 116 kDa non-cleaved form of PARP. Loading was controlled with anti-b-actin.

Effects of ascorbic acid on contrast-induced cytotoxicity

We observed a dose-dependent protective effect of ascorbic acid on renal cells exposed after 3 h of incubation with the high dose (200 mg iodine/mL) of both LOCM (HEK 293: P = 2.99 × 10−5; F = 1552.67; LLC-PK1: P = 0.04; F = 10.85; MDCK: P = 0.04; F = 18.57) and IOCM (HEK 293: P = 6.43 × 10−5; F = 933.55; LLC-PK1: P = 0.02; F = 16.29; MDCK: P = 0.01; F = 19.98) (Figure 5A). When compared to baseline, at 3 h of incubation cell viability was <6% in the control group, <15% with the lowest (2 mM) dose of ascorbic acid, and <60% with in both 4 mM and 8 mM doses of ascorbic acid, respectively. There was a significant interaction between the protective effect of ascorbic acid and cell viability for both types of CM (Iobitridol: P = 0.0017, F = 10.09, and Iodixanol: P = 0.0002, F = 16.46, both by the ANOVA model).

Figure 5

Effects of ascorbic acid and sodium bicarbonate on contrast-induced cell death. HEK 293 (open columns), LCC-PK1 (black columns), and MDCK (grey columns) were pre-treated for 2 h with different concentrations of ascorbic acid (AA) (2, 4, and 8 mM) or sodium bicarbonate (75, 150, and 300 mM) as indicated and then incubated for 3 h with 200 mg iodine/mL of iobitridol or iodixanol. Cell viability was then assessed with CellTiter Proliferation Assay. *P < 0.03 vs. (CM+/AA 2 mM+), P < 0.001 vs. all the other groups.

Effects of sodium bicarbonate on contrast-induced cytotoxicity

We did not find any protective effect of sodium bicarbonate on HEK 293 (LOCM: P = 0.53; F = 0.78; IOCM: P = 0.02; F = 23.02); LCC-PK1 (LOCM: P = 0.09; F = 6; IOCM: P = 0.94; F = 0.06); and MDCK (LOCM: P = 0.88; F = 0.13; IOCM: P = 0.71; F = 0.38) after 3 h of incubation with the high dose of either LOCM or IOCM. Cell viability was quite similar even in the presence of high (300 mM) dose of sodium bicarbonate (Figure 5B). This lack of any protective effect was similar with LOCM and IOCM. There was no difference in pH in the medium from the various groups (Table 1) and there was no effect on cell viability (Iobitridol: P = 0.72, F = 0.33; Iodixanol: P = 0.49, F = 0.73 by the ANOVA model).

View this table:
Table 1

pH in the various treatment groups

GroupIobitridol*Iodixanol**
Contrast media alone7.34 (7.18–7.50)7.25 (7.06–7.50)
Contrast media plus NAC7.29 (6.97–7.60)7.03 (6.96–7.10)
Contrast media plus AA6.90 (6.80–7.01)7.04 (7.06–7.10)
Contrast media plus NaHCO37.30 (7.21–7.40)7.24 (7.08–7.50)
  • Values are expressed as median and interquartile range.

  • NAC, N-acetylcysteine; AA, ascorbic acid; NaHCO3, sodium bicarbonate.

  • *P = 0.57 through the groups by ANOVA test, after transforming pH values into proton H+ concentrations.

  • **P = 0.65 through the groups by ANOVA test after transforming pH values into proton H+ concentrations.

Effects of co-incubation of NAC with ascorbic acid or with sodium bicarbonate

The protective effect of NAC (100 mM) was greater than that of ascorbic acid (8 mM) LOCM: P = 1.25 × 10−8, F = 52.21; and IOCM P = 9.90 × 10−9, F = 54.03 by the ANOVA model; Figure 6). We performed a further experiment to investigate the effect on cell death of 2 h of NAC pre-treatment (100 mM), in the presence of either ascorbic acid (8 mM), or sodium bicarbonate (150 mM) on cell death after 3 h of incubation with the high dose (200 mg iodine/mL) of either LOCM or IOCM. As shown in Figure 6, the combination of NAC with another antioxidant agent was less effective than NAC alone (P = 0.95; F = 0.54 by the ANOVA test).

Figure 6

Effect of NAC, ascorbic acid, and sodium bicarbonate alone and in combinations on contrast-induced cell death. HEK 293 cells were pre-treated for 2 h in the presence of N-acetylcysteine (NAC, 100 mM), ascorbic acid (AA, 8 mM), and sodium bicarbonate (NaHCO3, 150 mM) alone or in combination and then incubated for 3 h with 200 mg iodine/mL of iobitridol or iodixanol. Cell viability was then assessed with CellTiter Proliferation Assay. *P < 0.001 vs. (CM+/AA+), (CM+/NAC+), (CM+/NAC+/AA+), (CM+/NAC+/NaHCO3+), and (CM+/AA+/NaHCO3+); P < 0.001 vs. (CM+/NAC+), (CM+/NAC+/NaHCO3+), and (CM+/NAC+/AA).

Discussion

The main conclusions of the present study are (i) CM induce dose- and time-dependent renal cell apoptosis through the activation of the intrisinc pathway, (ii) this cytotoxic effect does not seem to be caused by iodine or osmolality ≤830 mOsm/L, and (iii) pre-treatment with NAC and ascorbic acid but not with sodium bicarbonate prevents apoptosis in a dose-dependent fashion.

Contrast media and renal cell apoptosis

Our study confirms that the CM induce renal cell apoptosis.46,1618 In order to strengthen this finding, we used three different renal cell lines, namely, human epithelial cells (HEK 293) and two cell lines with the characteristics of proximal and distal tubule cells [porcine kidney proximal tubular epithelial cells (LLC-PK1) and Madin–Darby canine kidney cells (MDCK)]. The activation of caspase-9 and -3, but not of caspases-8 and -10 observed after exposure to CM supports the concept that CM induce apoptosis through the intrinsic, or ‘mitochondrial’, pathway. This finding was also supported by the activation of PARP, a final substrate of caspase-3. In a rat model of CIN, cellular injury of the renal medulla consisted of extensive DNA fragmentation, which has been attributed to medullary hypoxia.17,19 Yano et al.19 have shown that CM induced apoptosis in the porcine tubular cell line, LLC-PK-1, and that the injuries might be due to de-regulation in Bax/Bcl-2 expression, followed by increases in caspases-9 and -3 activities. In agreement with these previous observations, we found that CM induce an increase of at least two Bcl-2 pro-apoptotic family members, i.e. Bim and Bad.20,21

Role of contrast dose and osmolality

CM induce renal cell apoptosis in a dose- and time-dependent manner.18 Guidelines recommend to limit the volume of CM usage in order to prevent contrast-associated nephrotoxicity.22,23 It has been suggested that using the iodine dose/glomerular filtration rate ratio may be a more expedient way of improving risk assessment of CIN than the more common practice of estimating CM dose from volume alone.23 After intravascular administration of CM in rabbits, a urinary concentration higher than 100 mg/mL of iodine has been measured.12 However, we found that the iodine alone does not cause renal cell apoptosis.

We observed that the cytotoxic effect, although maximum at 3 h, was mostly (≈85%) observed already at 15 min of incubation. This suggests that even a short period of exposure activates the cascade leading to apoptosis and therefore what is being observed at the later time periods represents mostly the cumulative effect of that initial exposure. This finding highlights the importance of strategies limiting the exposure of the kidney to the toxins contained in the contrast agent by generating high urine flow in patients.

The contribution of osmolality to contrast-induced apoptosis is controversial.18,24,25 Although previous studies demonstrated that the cytotoxicity of high-osmolality contrast media (HOCM) is higher than that of LOCM,18 we did not find any difference in the extent of cell injury between IOCM and LOCM. Furthermore, the cytotoxic effect may be related to CM hypertonicity, since equally hyperosmolal but less hypertonic urea solution failed to induce DNA fragmentation.18 Factors other than osmolality may contribute to the toxic effect. Ionicity and/or molecular structure (monomeric or dimeric) may be of importance. Heinrich et al.26 demonstrated that at an equal iodine concentration, no significant differences exist between the direct toxic effects of non-ionic monomeric and dimeric CM on renal proximal tubular cells in vitro. On the contrary, when comparing the data on a molar basis, the dimeric CM showed a significantly stronger effect on the tubular cells than did the non-ionic monomeric CM. This suggests a greater cytotoxic effect of the dimeric CM molecules. In the last generation of CM (which has a non-ionic, dimeric structure), iso-osmolality has been achieved at the price of an increased viscosity. Indeed viscosity is inversely related to osmolality. High viscous CM compromise renal medullary blood flux, renal medullary erythrocyte concentration, and renal medullary pO2.27 Our in vitro experiments allow us to examine the cytotoxic effects of CM on renal cells, eliminating the effects of confounding variables (e.g. hypoxia due to haemodynamic changes or viscosity), which can be found in vivo. Therefore, additional studies are necessary to assess whether molecular structure and/or other components of the CM may induce this cytotoxic effect.

Antioxidant compounds and contrast-induced apoptosis

In the last few years, a number of clinical studies have suggested that NAC may prevent CIN.7,8 Recently, two additional antioxidant strategies have aroused considerable interest: sodium bicarbonate10 and ascorbic acid.11 It has been hypothesized that all these compounds may be effective due to their antioxidant properties. Our study supports the clinical observation of the effectiveness of NAC and ascorbic acid in preventing contrast-induced apoptosis. This effect is dose-dependent: indeed, the greater the dose, the larger the cellular benefit. This finding supports the clinical observation of the dose-dependency of NAC in preventing CIN.8,28 The plasma level of NAC ranges from 10 mM (with a dosing regimen of 600 mg/day) to 100 mM (with a dosing regimen of 1200 mg BID).13,14 Of note, NAC was more effective against contrast-induced apoptosis than ascorbic acid. In contrast, sodium bicarbonate does not prevent contrast-induced apoptosis. However, recent clinical studies suggest that the sodium bicarbonate seems to be effective in preventing CIN.10 This discordance may be explained by alternative mechanisms. We recently demonstrated that the combined prophylactic strategy of sodium bicarbonate plus NAC, but not the combination of ascorbic acid and NAC, is more effective than NAC alone in preventing CIN. We speculated that NAC and ascorbic acid may work through similar pathways while the protective action of bicarbonate may be different in comparison to NAC and, therefore, additive.9 The lack of benefit of the combination of NAC and ascorbic acid in preventing contrast-induced apoptosis observed in the present study supports this hypothesis. Free-radical formation is promoted by an acidic environment typical of distal tubular urine, but is inhibited by the higher pH of normal extracellular fluid.29 It has been hypothesized that alkalinizing renal tubular fluid with bicarbonate10 may reduce injury. At physiologic concentrations, bicarbonate scavenges peroxynitrite and other reactive species generated from nitric oxide.10 In the clinical setting, the higher concentration of HCO3 in the proximal convoluted tubule may (i) buffer the higher production of H+ due to cellular hypoxia and (ii) facilitate Na+ reabsorption through the electrogenic Na/HCO3 co-transporter.29 The result of our in vitro study does not support the former mechanism. It may be that NaHCO3 may facilitate Na+ reabsorption: this would mitigate the increase in sodium delivery to the macula densa induced by CM, an effect that results in vasoconstriction of the afferent arteriola through the oricess of tubuloglomerular feedback. Furthermore, in our in vitro model, NAHCO3 did not raise the pH of the media in comparison to CM alone.

Study limitations

Hizoh et al.18 observed that NAC failed to reduce the DNA fragmentation rate caused by HOCM. This discordance may be explained by (i) the lower dose of NAC utilized (10 mM) and (ii) the use of a HOCM. Additional data are necessary to address the issue of which CM component (other than iodine) and chemical property (such as viscosity) causes renal cell apoptosis. The investigators who evaluated the cellular damage were not blinded to the contrast type, the iodine dose, or the protective strategy attempted.

In conclusion, CM induce apoptosis through the activation of the intrinsic pathway. Pre-treatment with NAC and ascorbic acid but not with sodium bicarbonate prevents apoptosis in a dose-dependent fashion.

Funding

This work was partially supported by funds from Associazione Italiana Ricerca sul Cancro, AIRC (G.C.), MIUR-FIRB (RBIN04J4J7).

Acknowledgements

Giulia Romano is recipient of Fondazione SDN fellowship. We thank M. Fiammetta Romano, MD, for her invaluable help in FACS analysis, and Michael Latronico, PhD, for paper revision.

Conflict of interest: none declared.

Footnotes

  • The first two authors contributed equally to this work.

References

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