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Journal of Clinical Medicine Research

Original Article

Volume 4, Number 2, April 2012, pages 95-101

Nephrotoxin-Induced Renal Cell Injury Involving Biochemical Alterations and Its Prevention With Antioxidant

Various nephrotoxic agents, such as HgCl₂, glycerol, cisplatin, gentamicin etc., have been known to exert cytotoxic effects on renal tubular cells [1, 2], inducing acute renal cell injury. However, as their cytotoxic action yet remains elusive, the preventive or therapeutic modalities for renal cell injury have not been established. Nevertheless, the accumulating data suggest that the involvement of oxidative stress (generation of oxygen free radicals) could be crucial for such renal cell injury induced by nephrotoxins [3, 4]. In addition, we have recently reported that oxidative stress would play a primary role in renal cell injury with certain nephrotoxins [5]. We were thus encouraged to further expand our study of nephrotoxin-mediated oxidative stress, focusing on biochemical parameters in the different renal cell line.

One of such biochemical parameters is glyoxalase I (Gly-I), a vital enzyme involved in the cellular detoxification process [6], playing a critical role in detoxifying and scavenging cytotoxic agents and metabolites (including free radicals) [7, 8]. Since activation of Gly-I inevitably requires reduced glutathione (GSH) as a cofactor [7], it is also categorized in a family of GSH-dependent enzymes. It is then plausible that Gly-I could be somewhat involved in detoxification of nephrotoxins.

A family of heat shock proteins (Hsps) is another interesting biochemical parameter, consisting of several species [9]. As they are functionally known as stress-response proteins, immediately responding to a variety of stresses exerted on the cells, they have been considered to play an important role in the cellular defense mechanism [10]. Among those Hsps, we are particularly interested in heat shock protein 90 (Hsp90) because it is primarily localized in the distal tubules and collecting ducts in the kidney [11] and would serve to protect renal cells from stress-related assaults [12]. It is thus possible that Hsp90 may also help protect renal cells from certain nephrotoxins.

We proposed that nephrotoxins would exert oxidative stress on renal cells as their primary cytotoxic action, triggering a cascade of biochemical events and leading ultimately to cell death. Accordingly, glycerol (GLC), a nephrotoxin capable of inducing renal cell injury, was chosen for studying its cytotoxic action on renal proximal tubular OK cells [13] in vitro (instead of LLC-PK₁ cells used in our previous study). We first examined if GLC would actually exert oxidative stress on OK cells, and possible effects of GLC (through oxidative stress) were then examined on two biochemical parameters, Gly-I and Hsp90. We also explored if certain antioxidants (e.g. vitamins C/E and GSH) might provide cytoprotection against GLC-mediated oxidative assault. Detailed studies and notable findings are described and discussed herein.

The renal proximal tubular OK cells [13] were maintained in RPMI-1640 medium supplemented with 10% fetal bovine serum, penicillin (100 units/ml), and streptomycin (100 μg/ml). For experiments, OK cells were seeded at 2 × 10⁵ cells/ml in 6-well plates or T-75 flasks and cultured with varying concentrations of glycerol (GLC). Cell number/viability was determined at specified times by the trypan blue exclusion method.

Severity of oxidative stress on the cells was assessed by LPO assay measuring the amount of malondialdehyde (MDA) formed, which was indicative of oxidative damage in the plasma membrane [14]. The detailed procedures were described in the vendor’s protocol (Calbiochem, La Jolla, CA), and the amount of MDA formed was expressed by μM determined from the MDA standards.

Gly-I activity was measured following the method of Ranganathan and Tew [15]. After preparation of the reaction mixture (200 mM imidazole HCl, pH 7.0, 16 mM MgSO₄, 7.9 mM methylglyoxal, 1 mM GSH), the reaction was started by the addition of cell lysates (40 μg). Due to a production of S-D-lactoylglutathione (E₂₄₀ = 3.37 mM^-1· cm^-1), the increase in absorbance at 240 nm was measured with times on a spectrophotometer. Gly-I activity was then expressed by units/mg protein where one unit was defined to catalyze the formation of one μmol of S-D-lactoylglutathione per min.

The procedures essentially followed the protocol described previously [16]. Briefly, an equal amount of cell lysates (7 μg) obtained from control and agent-treated cells was subjected to 10% SDS-polyacrylamide gel electrophoresis and transferred to a nitrocellulose membrane. The blot (membrane) was incubated with the primary antibody against Hsp90 (anti-Hsp90), followed by an incubation with the secondary antibody conjugate. The immunoreactive protein bands were then detected by chemiluminescence following the manufacturer’s protocol (Kirkegaard and Perry Laboratories, Gaithersberg, MD).

All data were presented as mean ± SD (standard deviation), and statistical differences between groups were assessed with either one-way analysis of variance (ANOVA) or the unpaired Student’s t test. Values of P < 0.05 were considered to indicate statistical significance.

OK cells were cultured with varying concentrations (0 - 3%) of GLC and cell growth/viability was determined in 24 hours. GLC was capable of inducing 38%, 60%, 79%, 95%, and 100% growth reduction with 1%, 1.5%, 2%, 2.5%, and 3% GLC, respectively (Fig. 1). Although no apparent cell death was observed up to 2% GLC, a 95% growth reduction attained with 2.5% GLC was largely attributed to severe cell death. We then used this 2.5% GLC as the most effective concentration in the rest of our study.

Click for large image

Figure 1. Dose-dependent effects of glycerol (GLC) on OK cell growth. Cells were cultured with varying concentrations (0, 0.5, 1, 1.5, 2, 2.5, or 3%) of GLC for 24 hours, and cell numbers were determined as mean ± SD (standard deviation) from three separate experiments (*P < 0.03).

As our previous study [5] suggested a primary role of oxidative stress in renal cell injury induced by nephrotoxins, we further investigated possible alterations in specific biochemical events induced by a nephrotoxin, glycerol (GLC), in renal tubular OK cells in vitro. GLC (2.5%) demonstrated to be highly cytotoxic, inducing severe cell death (95%) in 24 hours. Such cytotoxicity appeared to be primarily due to oxidative stress through GLC, indicated by LPO assay (Table 1). It is thus conceivable that such oxidative stress may trigger a cascade of various biochemical events, leading ultimately to cell death.

We next examined if certain antioxidant(s) might counteract with GLC-mediated oxidative stress. Interestingly, only NAC was found to protect OK cells from GLC oxidative assault (Fig. 2), implying the possible involvement of specific NAC-activated enzyme such as glyoxalase I (Gly-I) [6]. Our study then showed that a 75% loss in Gly-I activity after 6-hour GLC exposure (Fig. 3A) was completely reversed or prevented with NAC (Fig. 3B) and cell viability also remained at nearly 100% (Fig. 2). Thus, these results suggest that Gly-I may somewhat detoxify/diminish GLC cytotoxicity, contributing to NAC-provided cytoprotection.

It is crucial but remains uncertain how GLC would inactivate Gly-I. Yet, we assume that GLC-induced Gly-I inactivation may likely result from an insufficient availability of cellular GSH (synthesized in renal cells) because activation of Gly-I essentially requires GSH (or NAC used in this study) [7]. It is possible that GLC by itself or GLC-mediated oxidative stress may interfere with de novo synthesis of GSH, depriving cellular GSH. For example, γ-glutamylcysteine synthetase [18], a key enzyme involved in GSH synthesis, or other enzymes could be primarily targeted by GLC or free radicals. Further studies are thus required.

Another interesting biochemical alteration through GLC was partial degradation of Hsp90 (Fig. 4). As Hsp90 is shown to play a defensive role, its degradation could result in a weakening or destabilization of the cellular defense system, ultimately leading to renal cell death. It is yet of interest to further explore how Hsp90 would be degraded by GLC. As protein degradation is generally known to be carried out by protease(s), we assume that proteasome, a multicatalytic protease, might be involved in Hsp90 degradation because it has been shown to play a major role in degradation of intracellular proteins including Hsps [19]. Such study is currently underway in our laboratory.

In summary, the cytotoxic action of GLC primarily involves oxidative stress, causing adverse biochemical alterations such as inactivation of Gly-I and partial degradation of Hsp90. This indicates a collapse of the cellular detoxification and defense systems, presumably leading to renal cell death. However, NAC-provided cytoprotection against GLC, restoring Gly-I activity and protecting Hsp90 integrity, may have clinical implications in preventing renal cell injury/death induced by nephrotoxins.

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