The modulatory effect of taurine on benzo (a) pyrene-induced hepatorenal toxicity

Abstract

Toxicities linked with Benzo (a) pyrene B[a]P exposure, particularly in liver and kidney have been reported in both animals and humans. Taurine (2-aminoethane sulfonic acid) is an intracellular β-amino acid reported to elicit hepatorenal protective functions. However, the modulatory effect of taurine on hepatorenal toxicity associated with exposure to B[a]P has not been reported. This study evaluated the effects of taurine on the hepatorenal toxicities induced in cohorts of rats exposed to B[a]P. Experimental rats were treated as follows: B[a]P (10 mg/kg); co-treated cohorts –B[a]P (10 mg/kg) plus taurine (100 or 200 mg/kg) for 4 successive weeks. Results show that co-dosing with taurine significantly (P < 0.05) improved B[a]P-induced distortion of oxidative stress markers (catalase, superoxide dismutase, glutathione S-transferase, glutathione peroxidase, total sulphydryl, reduced glutathione, lipid peroxidation and xanthine oxidase), renal function (urea and creatinine) and liver function marker enzymes (alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase and gamma glutamyl transferase). Moreover, taurine effectively mitigated increase in myeloperoxidase activity, levels of reactive oxygen and nitrogen species, nitric oxide and interleukin-1β in kidney and liver of rats treated with B[a]P. In conclusion, taurine modulates hepatorenal toxicity in B[a]P-exposed rats by suppressing hepatic and renal damage indices, oxidative injury and inflammatory stress.

Introduction

The survival and normal physiological state of humans depend to a large extent on their genetic composition and their interaction with the environment. Through these interactions different toxicants which are detrimental to the health of the organisms get into the system and elicit different pathologies. Examples of such toxicants are polycyclic aromatic hydrocarbon (PAH). PAHs may be formed from direct pyrolysis of organic materials, roasting of fat, meat etc., or by partial burning of charcoal, thereby depositing PAH in grilled meat and fish [1]. Humans extensively metabolize PAHs producing extremely reactive metabolic by-products that can interact and damage deoxyribonucleic acids (DNA), potentially resulting in mutations [2]. Risk assessments of PAHs in food conducted by JECFA and EFSA [2, 3] concurred that serious health associated effect from exposure to PAHs results in the onset of carcinogenesis. Also, since most PAHs are genotoxic, it is difficult to ascertain a no-observed-adverse-effect level for PAHs.

Over 30 PAHs are identified so far; including benzo[a]pyrene B[a]P. B[a]P is a human carcinogen, categorized under the Group 1 carcinogens by the International Agency for the Research on cancer—IARC [4]. Contamination of the environmental by B[a]P is of global concern, and the primary environmental sources include cigarette smoke, diesel exhaust, products from industrial incineration, partial burning of carbonaceous resources for energy generation and barbeque. B[a]P is itemized as a harmful substance in 1980 by the US Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). B[a]P is present at harmful waste locations on the national priorities list (NPL), and ranked high on the priority list of hazardous substances for CERCLA [5].

The field of chemoprevention and nutraceuticals has opened a new vista in the prevention and management of toxicities and diseases caused by the interaction of animals with toxicants such as B[a]P in the environment. Example of such nutraceutical is taurine (2-aminoethane sulfonic acid; TAU) which has been shown to elicit beneficial role to the normal physiology of cells and in preventing pathologies [6]. TAU can be assimilated exogenously from diet or via biosynthesis from the cysteine sulfonic acid pathway in the liver [7]. The nephro- and neuro-protective activities and the antioxidative and anti-inflammatory properties of taurine have been reported [8–10]. Furthermore, taurine is reported to augment cellular osmoregulation and detoxification, stabilize biological membrane, and modulate ion flux and enhances cardiovascular functions [11, 12]. Although, TAU is not a ligand for the aryl hydrocarbon receptor (AhR), disturbance in TAU metabolism, associated with increase oxidative stress and AhR activation has been reported in HepaRG cell lines exposed to polychlorinated biphenyl (PCB)-126 [13]. This is indicative of a role for TAU in reducing oxidative stress and perhaps downregulating AhR in cells exposed to chemical carcinogens including B[a]P. Although a report suggests that BaP toxicity maybe independent of the canonical AhR pathway [14], however increased oxidative stress is involved in AhR-dependent toxicity [15].

This study aims to evaluate the modulatory role of taurine on B[a]P-induced hepatorenal toxicity using Wistar rats as a model. We assessed the effects of taurine on B[a]P-induced distortions of hepato-renal function, biomarkers of hepato-renal antioxidant status and nitrosative stress, reactive oxygen and nitrogen species (RONS) level, inflammatory mediators and apoptosis markers. Moreover, we evaluated the ability of taurine to ameliorate B[a]P-induced histopathological alterations within rat’s kidney and liver. We observed that taurine elicits potent hepatorenal protective effects against B[a]P-induced toxicity.

Materials and Methods

Chemicals

Benzo (a) pyrene and taurine were obtained from Sigma-Aldrich, USA. Aspartate Aminotransferase (AST), Alanine Aminotransferase (ALT), Gamma glutamyl transferase (GGT), Urease and Creatinine kits were obtained from Randox™ Laboratories, Crumlin, UK. Other reagents used were of analytical grade and obtained from the British Drug Houses (Poole, Dorset, UK). Enzyme-linked immunosorbent assay (ELISA) kits for evaluating interleukins (IL-10 and IL-1β) levels were procured from E-labscience Biosciences (Beijing, China).

Maintenance of experimental animals

Fifty (50) Wistar rats—Rattus norvegicus—(100–150 g) used for the experiment were purchased from the Experimental Animal Unit, in the Department of biochemistry, University of Ibadan. The rats were housed in plastic cages placed in a well-ventilated experimental animal facility under a 12-h light: 12-h dark photocycle. Prior to experimentation, rats were acclimatized for 2 weeks and were fed with standard rat chow and allowed uninhibited access to clean drinking water. Animals were humanely cared for in accordance to the ‘Guide for the Care and Use of Laboratory Animals’ prepared by the National Academy of Science and published by the National Institute of Health. Also, the experiment followed the approval by the University of Ibadan Animal Care and Use Research Ethics Committee (ACUREC) UI-ACUREC/20/038.

Treatment protocol of rats with B[a]P and taurine

The rats were randomly divided into experimental cohorts (n = 10 each) and each cohort dosed for 4 weeks [16], as detailed below per os (p.o.):

Rat cohort 1: Control—dosed with corn oil alone p.o. at 2 ml/kg.

Rat cohort 2: Benzo (a) pyrene (B[a]P)—dosed with B[a]P alone p.o. at 10 mg/kg.

Rat cohort 3: Taurine (TAU)—dosed with Tau alone p.o. at 200 mg/kg p.

Rat cohort 4: B[a]P + TAU₁—dosed p.o. with B[a]P (10 mg/kg) + TAU₁ at 100 mg/kg.

Rat cohort 5: B[a]P + TAU₂—dosed p.o. with B[a]P (10 mg/kg) + TAU₂ at 200 mg/kg.

Doses of B[a]P and taurine were selected from Liang et al. [17] and Adedara et al. [18], respectively. Stock solutions of B[a]P (100 mg/ml) and taurine (100 mg/ml) prepared every other day were used in dosing the rats per os. The gavage volumes for taurine (100 and 200 mg/kg) and B[a]P (10 mg/kg) were 100, 200 and 100 μl from the specific stock solution, respectively. B[a]P and taurine stock solutions were individually prepared in corn oil, before experimental rats are treated by gavage. Cohorts of rats that received B[a]P and taurine were first dosed with B[a]P before dosing with taurine within 30 min.

Terminal sacrifice of rats, serum and tissue preparation

On the 29th day, experimental rats’ final body weights were obtained, venous blood was drawn via the orbital venous plexus from the side of the eye balls into anticoagulant free tubes. The Experimental rats were subsequently sacrificed by carbon dioxide (CO₂) asphyxiation [19, 20] at the end of the experimental procedure and death confirmed by cervical dislocation. Subsequently, liver and kidney were harvested quickly, weighed and processed for histological and biochemical evaluations. Serum was prepared by subsequently centrifuging (4°C; 3000 × g; 10 min) clotted blood. The prepared serum was aliquoted and stored until needed and probed for the assessment of biomarkers of liver and kidney function.

Evaluation of biomarkers of hepatorenal function

Serum samples acquired by centrifugation (3000 × g; 10 min) of whole blood allowed to clot on ice were used for the kit-based assessment of serum creatinine and urea levels as well as hepatic transaminases—AST, ALT, GGT and ALP activities. The assay kits were products of Randox™ Laboratories (Crumlin, UK) and all assay procedure were in strict compliance with the manufacturer’s instructions.

Evaluation of biomarkers of hepatorenal antioxidant status and nitrosative stress from prepared tissue homogenates

Tissue from the examined organs (kidney and liver) were homogenized in Tris–HCl (pH 7.4; 50 mM) buffer and subsequently centrifuged (12 000 × g, 15 min at 4 °C) using an Eppendorf 5604R cold centrifuge (Hamburg, Germany). The obtained supernatants were subsequently processed for the different biochemical assays performed. The total protein concentration of the liver and kidney tissues was assessed according to the method of Bradford and as subsequently reported [21, 22].

Antioxidant enzymes activities of liver and kidney were assayed following standard methods. Superoxide dismutase (SOD) activity was assessed according to the method of Misra and Fridovich, and as subsequently reported [23, 24]. About 2.5 ml carbonate buffer (0.05 M; pH 10.2) was added to test aliquots of sample followed by 0.3 ml adrenaline. The absorbance (at 480 nm) of the ensuing mixture was recorded for 150 s at 30-s interval. Catalase (CAT) activity was determined according to the method of Claiborne and as previously reported [25, 26]; by monitoring the changes in absorbance (240 nm) of the reactants (test samples and reagents) for 5 min. The activity of glutathione-S-transferase (GST) was determined by Habig method as previously reported [27, 28]. The activity of GST was assessed as the GSH/CDNB conjugate formed per min per milligram protein; following the method of Rotruck et al., and as subsequently reported [29, 30]. To assay for glutathione (GSH), the standard method of Jollow et al. was utilized and as earlier reported [31, 32].

Furthermore, lipid peroxidation (LPO) was assessed according to method of Buege and Aust as previously reported [33, 34]. Briefly, test samples, Tris–KCl buffer and trichloroacetic acid were mixed and allowed to react. Afterwards, thiobarbituric acid was added and the reaction allowed to stand for 45 min in a hot water bath. Once cooled, the reaction mixture was centrifuged and the absorbance of the supernatant measured against a reference blank at 532 nm.

The myeloperoxidase (MPO) activity, a biomarker of polymorphonuclear leukocyte accumulation, was estimated spectrophotometrically by the modified method described by Trush and previously reported [35, 36]. In the presence of H₂O₂, MPO catalyzes the oxidation of o-dianisidine to produce a brown colored product with an absorbance of 470 nm. Nitric oxide (NO) was determined by the method of Green et al., as previously reported [37, 38]. Concentrations of NO₃⁻ and NO₂⁻, estimated in the serum, serves as a marker of NO production. Nitrites in serum samples were measured using the Griess reaction: Samples (0.5 ml) + Griess reagent (0.5 ml) were incubated for 20 min at room temperature. The OD at 550 nm obtained by spectrophotometry serves as a measure of nitrite concentration, this was estimated in contrast with a standard solution of known sodium nitrite concentrations OD obtained at 550 nm as well. All biochemical analyses were done using a multiplate reader—Multimodal M384 SpectraMax™ (Molecular Devices, San Jose, USA)—except for SOD and CAT activities, which were analyzed using 752S UV–vis Spectrophotometer (Ningbo, China).

Evaluation of RONS level

Measurement of reactive oxygen and reactive nitrogen species (RONS) level was accomplished according to the modified protocol of Owumi and Dim and as previously reported [26, 38]. RONS readily oxidize dichlorodihydrofluorescin diacetate (DCFH-DA) to DCF. DCF fluorescence emission ensuing from DCFH-DA oxidation was evaluated at an excitation (488 nm) and emission (525 nm) wavelength for 10 min. The rates of formation (DCF) are represented as percentage over the control group.

Estimation of biomarkers inflammatory response

ELISA-based measurement of Interleukin (IL-1β and IL-10) levels were performed by procedure using commercially available assay kits from Elabscience Biotechnologies (Beijing, China) on a Molecular Devices SpectraMax™ multiplate reader (CA, USA) following the manufacturer’s manual. Specifically, the following biomarkers of inflammatory response were assayed biochemically:

Histopathological examination of liver and kidney

Kidney and liver samples were processed for histological examination following Bancroft and Gamble’s methods and reported in literature [39, 40]. Specimens from the kidney and liver were fixed in a solution of buffered formaldehyde (10%) for 3 days. Subsequent to sequential specimen dehydration, representative samples were embedded in paraffin. 4–5-μm section were then sliced out using a microtome and afterward stained with hematoxylin and eosin (H&E). The study slides were then blinded beforehand and examination under a Carl Zeiss Axio light microscope (Jean, Germany), for identification of histopathological anomalies and scored by a pathologist. Representative images were captured upon examination using a Zeiss Axiocam 512 camera (Jena, Germany) attached to the microscope by a pathologist oblivious of the various treatment cohorts the slides were prepared from.

Statistical analysis

The analysis of the data generated from this study was performed by one-way analysis of variance (ANOVA) followed by post-hoc test (Bonferroni) using GraphPad Prism version 8.3.0 for Mac (www.graphpad.com; GraphPad, CA, USA). Statistically significant differences were set at values of P < 0.05. The results are expressed as the mean ± SD of replicates.

Results

Effects of B[a]P and taurine treatment on rats’ relative organ weight

Results of the body weight gain and relative organ—liver and kidney—of experimental animals are presented in Table 1. Relative to control, cohort of rat treated with B[a]P alone significantly (P < 0.05) exhibited decreases in body weight gain which was restored above control in rats co-treated with taurine at 100 and significantly (P < 0.05) increased at 200-mg/kg body weight.

Table 1.

Effect of B[a]P and TAU on body weight gain and relative organ weight of rats treated for 28 consecutive days

	Control	B[a]P	TAU	B[a]P + TAU1	B[a]P + TAU2
Initial body weight (IBW; g)	151.0 ± 9.91	170.40 ± 16.41	155.60 ± 14.48	170.00 ± 10.88	164.00 ± 15.69
Final body weight (FBW; g)	170.8 ± 17.58*	182.70 ± 12.40	179.60 ± 18.14*	189.90 ± 11.46*	185.30 ± 18.66
Weight change (g)	19.80	12.30	24.00	19.90	21.30*
Liver weight (g)	4.21 ± 0.56	5.12 ± 0.60	4.47 ± 0.69	5.14 ± 0.57	4.94 ± 0.53
Kidney weight (g)	0.94 ± 0.09	1.04 ± 0.08	0.86 ± 0.16	0.98 ± 0.12	1.01 ± 0.12
Relative liver weight (%)	2.39	2.93	2.35	2.68	2.60
Relative kidney weight (%)	0.53	0.59	0.45	0.51	0.53

Notes: B[a]P: 10 mg/kg of Benzo (a) pyrene; TAU₁: 100 mg/kg of Taurine; TAU₂: 200 mg/kg of Taurine. n = 10. Data represent the mean ± SD of 10 rats.

Biomarkers of hepatorenal function in rats treated with B[a]P and taurine

Figure 1 displays the results of biomarkers of hepato-renal function in the control, taurine-treated, B[a]P-treated and taurine + B[a]P treated groups. The result shows that there is a statistically significant increase in the activities of serum ALT, AST, GGT and ALP in B[a]P-treated groups compared with the control group. Also, the significant increase in serum ALT, AST, ALP and GGT activities in B[a]P treated group compared with TAU, B[a]P + TAU₁ and B[a]P + TAU₂ treated groups show the modulatory role of taurine in restoring the activities of the enzymes to near that of the control. Serum urea and creatinine levels in the BaP group increased significantly compared with the control group (P < 0.05). Also, a statistically significant change was observed between the BaP treated group compared with taurine, B[a]P + TAU₁ and B[a]P + TAU₂ treated groups (P < 0.05).

Figure 1.

Activities of ALT, AST, GGT and ALP; and levels of Urea and Creatinine in the serum of rats following oral exposure to B[a]P alone, Taurine alone and both B[a]P and Taurine for 28 days. B[a]P: 10 mg/kg of Benzo (a) pyrene; TAU₁: 100 mg/kg of Taurine; TAU₂: 200 mg/kg of Taurine. Data analysis was performed by one-way ANOVA followed by a post-hoc test (Bonferroni). Each bar represents the mean ± SD of 10 rats. Abbreviation: ALP, alkaline phosphatase. ^****Statistically significant difference as compared with the control group at P < 0.05. ^****Statistically significant difference as compared with the B[a]P group at P < 0.05.

Hepatorenal antioxidant enzymes, antioxidant status and LPO in rats treated with B[a]P and taurine

The effects of taurine on antioxidant status and biomarkers of oxidative stress in the kidney and liver of B[a]P exposed rats are shown in Figs 2–4. Relative to control, antioxidant enzymes—CAT, SOD, GPx and GST—activities in the liver and kidney were decreased (P < 0.05) in the BaP treated group. There is significant difference in hepatorenal SOD and CAT activities in group treated with B[a]P alone compared with taurine co-treated groups at the low and high doses—TAU₁ and TAU₂. Similar trend also occurred in the kidney GST and GPx activity as shown in Figs 2 and 3. In the liver, there is also an increase (P < 0.05) in GPx and GST activity in B[a]P and taurine co-treated group dose-dependently, this is more obvious at taurine (200 mg/kg). Hepatorenal level of GSH, and total thiol (TSH) level in rats treated with B[a]P alone were significantly reduced, these reductions in GSH and TSH levels were reversed markedly in rats co treated with B[a]P and taurine in a dose-dependent manner. Figure 4 shows that there is increase (P < 0.05) in the hepatorenal RONS and LPO levels and XO activity in cohorts of rats treated with B[a]P alone compared with the control. Conversely, there was reduction (P < 0.05) in hepatorenal RONS, and LPO level and XO activity in cohorts of rats co-treated with B[a]P and taurine relative to B[a]P alone.

Figure 2.

Activities of SOD and Catalase activities, and TSH level in liver and kidney of rats following oral exposure to B[a]P alone, Taurine alone and both B[a]P and Taurine for 28 days. B[a]P: 10 mg/kg of Benzo (a) pyrene; TAU₁: 100 mg/kg of Taurine; TAU₂: 200 mg/kg of Taurine. Data analysis was performed by one-way ANOVA followed by a post-hoc test (Bonferroni). Each bar represents the mean ± SD of 10 rats. ^****Statistically significant difference as compared with the control group at P < 0.05. ^****Statistically significant difference as compared with the B[a]P group at P < 0.05.

Figure 4.

Levels of RONS and LPO and XO activities in the liver and kidney of rats following oral exposure to B[a]P alone, Taurine alone and both B[a]P and Taurine for 28 days. B[a]P: 10 mg/kg of Benzo (a) pyrene; TAU₁: 100 mg/kg of Taurine; TAU₂: 200 mg/kg of Taurine. Data analysis was performed by one-way ANOVA followed by a post-hoc test (Bonferroni). Each bar represents the mean ± SD of 10 rats. ^****Statistically significant difference as compared with the control group at P < 0.05. ^****Statistically significant difference as compared with the B[a]P group at P < 0.05.

Figure 3.

Activities of GST and GPx and GSH levels in the liver and kidney of rats following oral exposure to B[a]P alone, Taurine alone and both B[a]P and Taurine for 28 days. B[a]P: 10 mg/kg of Benzo (a) pyrene; TAU₁: 100 mg/kg of Taurine; TAU₂: 200 mg/kg of Taurine. Data analysis was performed by one-way ANOVA followed by a post-hoc test (Bonferroni). Each bar represents the mean ± SD of 10 rats. GPx: glutathione peroxidase. ^****Statistically significant difference as compared with the control group at P < 0.05. ^****Statistically significant difference as compared with the B[a]P group at P < 0.05.

Effect of B[a]P and taurine treatments on rat hepatorenal mediators of inflammatory

The effects of taurine treatment on inflammatory mediators in B[a]P exposed rats are shown in Figs 5 and 6. Relative to controls, cohorts of rat treated with B[a]P alone showed increase (P < 0.05) in hepatorenal NO levels and MPO activities. Rat cohorts co-treated with taurine, at both doses, exhibited significant decrease in NO levels and MPO activities in a dose-dependently manner (Fig. 5). Pro-inflammatory cytokine IL-1β level in rats’ liver and kidney treated with B[a]P alone was significantly higher than all other groups, conversely the anti-inflammatory cytokine IL-10 was lessened (P < 0.05). Co-treatment with B[a]P and taurine reversed the observed increase of IL-1β in the liver and kidney, but resulted in a consequential taurine dose-related increases in IL-10 (Fig. 6).

Figure 5.

Level of NO and activity of MPO in liver and kidney of rats following oral exposure to B[a]P alone, Taurine alone and both B[a]P and Taurine for 28 days. B[a]P: 10 mg/kg of Benzo (a) pyrene; TAU₁: 100 mg/kg of Taurine; TAU₂: 200 mg/kg of Taurine. Data analysis was performed by one-way ANOVA followed by a post-hoc test (Bonferroni). Each bar represents the mean ± SD of 10 rats. ^****Statistically significant difference as compared with the control group at P < 0.05. ^****Statistically significant difference as compared with the B[a]P group at P < 0.05.

Figure 6.

Levels of IL 1β and IL-10 in liver and kidney of rats following oral exposure to B[a]P alone, Taurine alone and both B[a]P and Taurine for 28 days. B[a]P: 10 mg/kg of Benzo (a) pyrene; TAU₁: 100 mg/kg of Taurine; TAU₂: 200 mg/kg of Taurine. Data analysis was performed by one-way ANOVA followed by a post-hoc test (Bonferroni). Each bar represents the mean ± SD of 10 rats. IL-1β: interleukin 1beta; IL-10: interleukin 10. ^****Statistically significant difference as compared with the control group at P < 0.05. ^****Statistically significant difference as compared with the B[a]P group at P < 0.05.

Histopathological damages in rat’s kidney and liver following treatment with B[a]P and taurine

Representative photomicrographs of the liver and kidney from experimental rats showing the effect of B[a]P and taurine treatment on histological tissue architecture and injury rats are presented in Fig. 7 (×400 magnification). Control liver showed normal hepatocytes, central veins, portal triads and sinusoids while renal tissue showed normal glomeruli, bowman capsule and tubules. Administration of B[a]P did not affect renal histology but induced mild periportal inflammation (black arrow) in the liver. Rats in the BaP + TAU₁ and BaP + TAU₂ groups presented with typical hepatic and renal cyto-architecture.

Figure 7.

Representative histopathological changes of the kidney (K) and liver (L) following treatment of rats with B[a]P and Taurine. 1: Control; 2: B[a]P alone; 3: Taurine alone; 4: B[a]P + TAU₁, 5: B[a]P + TAU₂, Administration of B[a]P induced mild periportal inflammation (black arrow in L2) in the liver but kidneys were not affected. Hepatic and renal architectures in rats treated with B[a]P + TAU₁ and B[a]P + TAU₂ groups appeared typical.

Discussion

The liver is the clearinghouse for xenobiotics and it performs other essential systemic housekeeping functions as well. However, liver is highly susceptible to injury, the consequence of its metabolic function. Liver injury can result in increase in the levels of various hepatic enzymes, traditionally sequestered within the liver cells, in the blood. Estimation of hepatic transaminases activities in serum is a useful tool for evaluating the hepatocyte integrity and liver function [41, 42]. Estimation of serum transaminases is also useful in detecting tissue/cellular damage that results from exposure to toxic chemicals prior to biopsies and histological evaluation. However, ALT enzyme is more specific to assess liver injury as AST is also increased following myocardial infraction and muscle injury [43, 44]. Another marker of liver disorder is GGT, which is an enzyme catalyzing the first step in GSH degradation [45]. Increase in serum GGT, as observed in animal treated with toxicants, could result in excessive free radical generation [46], deleterious to cells and cellular macromolecules.

In the present study, the significant increase in the plasma activities of hepatic transaminases and kidney function biomarkers (urea and creatinine) manifested in the rats treated with B[a]P alone relative to control cohort (Fig. 1). This is indicative of B[a]P-induced damage. This finding correlates with previous studies on B[a]P-induced toxicities in liver and kidney [47]. Hepatic transaminases activity was significantly reduced in rats treated with taurine compared with rats treated with B[a]P [48]. Previous studies have shown that taurine exhibits hepatoprotective and nephroprotective effect [9]. This may the reason for the significant difference of total protein for both liver and kidney between the B[a]P-induced rats and the group co-treated with B[a]P and taurine, corroborating previous studies by Adedara et al. [49]. Elevated plasma urea signifies decreased reabsorption at the renal epithelium whereas high plasma creatinine revealed impairment in the renal functions, mostly for glomerular filtration rate in the B[a]P-treated rats. Interestingly, co-treatment with taurine remarkably reversed the B[a]P-mediated increase in the serum levels of renal functional indices. The restoration of these biomarkers indicates a protective effect of taurine against renal toxicity resulting from B[a]P exposure in the experimental rats.

The antioxidant defense system in cells consists of enzymatic and non-enzymatic antioxidants which mitigates oxidative stress [50]. Antioxidant status has been used in estimating the risk of oxidative damage induced by chemical carcinogens. Endogenous enzymatic (SOD, Catalase, GPx and GST) and non-enzymatic (GSH) antioxidants efficiently eliminate intracellularly generated free radicals during normal metabolism or exposure to xenobiotics and consequently protect the cells against oxidative damage. However overwhelming of the antioxidant system by toxicants results in oxidative stress requiring support by exogenous antioxidants to restore homeostasis [51].

In the present study, decrease in SOD and CAT activities in the liver and kidney after exposure to B[a]P (Fig. 2) shows enzyme inhibition and concomitant increase in the levels of oxidants (superoxide radicals and H₂O₂) and hydroxyl radical generation through the Haber–Weiss reaction in the kidney and liver. Also, the decrease in GSH in the B[a]P-induced toxicity group (Fig. 5) shows the depletion of the store during the process of conjugating the toxic product of B[a]P. However, the significant difference in the GSH level, SOD activity and CAT activity between the B[a]P administered group and the B[a]P-taurine co-administered group shows the modulatory role of taurine in B[a]P-induced hepatorenal oxidative stress (Figs 3 and 5). This result supports previous studies [49, 52]. Furthermore, inhibition of antioxidant defense may be coupled with lowered total sulfhydryl (TSH) contents or depletion of GSH as shown in the present study (Figs 3 and 4) [53]. Although, taurine is not an AhR ligand taurine metabolism is perturbed, in PCB-126 treated cell [13] exhibiting AhR activation and increased oxidative stress. This is indicative of a role for taurine in reducing oxidative stress and perhaps downregulating AhR in cells exposed to chemical carcinogens including B[a]P. Although a report suggests that B[a]P toxicity maybe independent of the canonical AhR pathway [14], however increased oxidative stress is involved in AhR-dependent toxicity [15] in a cyclical manner.

The mechanism of protection against B[a]P-mediated toxicity is mainly through deactivation of the B[a]P epoxides to non-toxic products which can be easily excreted in urine. GST is one of key enzymes responsible for detoxifying B[a]P epoxides and is involved in the conjugation of epoxides with GSH which is then excreted as the mercapturic acid [54, 55]. This study shows that GST activity was remarkably decreased in liver and kidney of rats exposed to B[a]P, inferring the inhibition of GST activity and accumulation of toxic epoxides in the liver and kidney of the rat (Fig. 3). However, the activities of enzymes involved in the metabolizing of B[a]P were significantly increased in rat co-administered with taurine and B[a]P. The aforementioned findings strongly suggest that taurine may facilitate B[a]P detoxification through amplifying the activity of GST thereby preventing B[a]P-mediated hepatorenal toxicity.

In a previous study, Das and Sil [9] showed that taurine is capable of reducing xanthine oxidase (XO) activity in stressed kidney. This study shows significant difference between the control group and B[a]P-treated group in the kidney while there is a significant decrease in XO activity in group co-administered with B[a]P and taurine when compared with the group treated with B[a]P alone.

B[a]P is a very effective carcinogen with ability to generate free radicals during its bioactivation, which eventually reacts with polyunsaturated fatty acid component of the membrane lipids leading to formation of MDA, an end product of LPO [56, 57]. LPO is a critical event to cell death and has been shown to cause deleterious impairment of membrane functions via increased membrane permeability and membrane distortion, cytotoxicity and finally cell death [58]. Previous studies have shown taurine lowers the LPO induced by various hepatotoxins thereby positioning taurine as an antioxidant and anti-inflammatory agent in different animal models [6, 59–63].

The increase in the liver and kidney MDA level, observed in the B[a]P-treated rats in the present investigation (Fig. 5), evidenced induction of LPO in the animals. However, the increase in GSH level with concomitant decrease in the MDA level in the kidney and liver of rats treated with taurine may be related to the antioxidant and anti-lipid peroxidative activities of taurine (Figs 3 and 5).

NO is a very reactive free radical that induces nitrosative stress and cellular damage through reaction with biomolecules, including lipids, proteins and nucleic acids, following the depletion of the antioxidant defense system [64–66]. Elevation of intracellular RONS, NO and other reactive species in the kidney and liver contributes to oxidative stress and induction of several inflammatory signaling cascades through the activation of nuclear factor-kB (NF-kB) [67, 68]. In the present study, B[a]P-treated rats showed marked diminution in renal and hepatic activities of antioxidant enzymes with significant elevation in MPO activity, NO, RONS and LPO levels (Figs 5 and 6). These observations clearly indicate a compromised antioxidant defense system, inflammation and a state of oxidative stress in the renal tissues of B[a]P-exposed rats. The present observations are in agreement with the previous reports that B[a]P exposure induced oxidative stress and inflammation in rodents [49, 69, 70].

Hepatic and renal MPO, NO, IL-1β and IL-10, assayed in the present study, are indices of inflammation. Several studies have shown that several cytokines (e.g., TNFα, IL-1β, MCP-1 and IL-6) are upregulated in the kidney and liver in the inflammatory cascade induced by toxicants while cytokines, such as IL-10, inhibit the increase in other pro-inflammatory cytokines so as to prevent tissue injury [71–73]. Neutrophils, through the action of MPO which removes H₂O₂ and catalyzes the formation of toxic hypochlorous acid that leads to neutrophil infiltration and oxidative damage, are known to contribute to the development of inflammatory responses and oxidative stress and release of a large variety of pro-inflammatory cytokines [74–77]. Our result showed a significant difference in hepatic and renal IL-10 between B[a]P alone and B[a]P and taurine co-treated groups (Fig. 6). Besides, hepatic and renal IL-1β level in the B[a]P alone treated group is significantly higher relative to control and taurine alone treated groups, furthermore there was significant reduction in IL-1β level at both doses of B[a]P and taurine co-treated group, indicative of an anti-inflammatory role for taurine.

Conclusion

Herein, we evaluated the effects of taurine on the hepatorenal toxicities induced by exposure of rats to B[a]P. Our study clearly showed that taurine is a potent hepatorenal protective agent. It is proposed that taurine attenuated the alterations in the biochemical parameters partly by alleviating oxidative stress, augmenting the activities of the antioxidant enzymes, modulating inflammatory response (Fig. 8) and by exhibiting protective effects on the liver and kidneys, consequently improving their function. Given that taurine is an inert amino acid widely available in the diet, it could be useful to use taurine supplementation to neutralize the toxic effects of B[a]P and other PAHs that are ubiquitous in grilled food, cigarette smoke and other industrial exposure. Thus, the results of the present study encourage new experimental and clinical studies to evaluate the efficacy of taurine as an adjunctive agent to ameliorate the toxic effects of B[a]P and other PAHs that could result in oxidative tissue injury. The limitation of this investigation includes the absence of data on taurine and the possible interaction with the B[a]P-activated transcription factor AhR. To the best of our knowledge, there is no evidence of taurine with B[a]P-activated AhR interaction, it is necessary to further study these possibilities so as to fully elucidate the role of taurine in abating B[a]P-mediated toxicity.

Figure 8.

Proposed mechanism of taurine-mediated resolution of B[a]P-induced hepatorenal toxicities. Red and purple arrows indicate the effect of B[a]P while blue arrows indicate the effect of co-administration of taurine and B[a]P on markers of oxidative and inflammatory stress.

Authors’ Contributions

All authors partook in the design, interpretation and analysis of the data generated from the study. SEO: supervised the investigation. GA: carried out the research and preliminary data analysis. SEO and AKO: conceptualized the experiments and were involved in proof checking the data for error. The manuscript was written and revised by all authors.

Conflict of Interest

The authors declared no conflicts of interest concerning this study, publication and authorship of this manuscript.

Funding

The authors privately funded this research by the author’s contribution and received no external grant from funding agencies in the commercial, not-for-profit, or public sectors.