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Abstract

Here we evaluated the ability of l-theanine in preventing experimental hepatic cirrhosis and investigated the roles of nuclear factor-κB (NF-κB) activation as well as transforming growth factor β (TGF-β) and connective tissue growth factor (CTGF) regulation. Experimental hepatic cirrhosis was established by the administration of carbon tetrachloride (CCl₄) to rats (0.4 g/kg, intraperitoneally, three times per week, for 8 weeks), and at the same time, adding l-theanine (8.0 mg/kg) to the drinking water. Rats had ad libitum access to water and food throughout the treatment period. CCl₄ treatment promoted NF-κB activation and increased the expression of both TGF-β and CTGF. CCl₄ increased the serum activities of alanine aminotransferase and γ-glutamyl transpeptidase and the degree of lipid peroxidation, and it also induced a decrease in the glutathione and glutathione disulfide ratio. l-Theanine prevented increased expression of NF-κB and down-regulated the pro-inflammatory (interleukin (IL)-1β and IL-6) and profibrotic (TGF-β and CTGF) cytokines. Furthermore, the levels of messenger RNA encoding these proteins decreased in agreement with the expression levels. l-Theanine promoted the expression of the anti-inflammatory cytokine IL-10 and the fibrolytic enzyme metalloproteinase-13. Liver hydroxyproline contents and histopathological analysis demonstrated the anti-fibrotic effect of l-theanine. In conclusion, l-theanine prevents CCl₄-induced experimental hepatic cirrhosis in rats by blocking the main pro-inflammatory and pro-fibrogenic signals.

Keywords

liver damage necrosis oxidative stress NF-κB cytokines cirrhosis carbon tetrachloride

Introduction

Cirrhosis is one of the most frequent and serious liver diseases and is the end stage of progressive fibrosis, which causes major disruptions to metabolic function and blood circulation.¹ Cirrhosis is characterized by the accumulation of extracellular matrix (ECM) proteins, and it causes the perturbation of liver homeostasis, leading to the intracellular release of cytokines. In particular, the fibrogenic cytokine, transforming growth factor β (TGF-β), is a key factor in cirrhosis development as it stimulates the generation of ECM and inhibits matrix protein removal.

Excessive activation of cellular signalling pathways by nuclear factor κB (NF-κB) has important roles in innate immunity, liver inflammation, fibrosis and the prevention of apoptosis. NF-κB constitutes a critical target for manipulating pathophysiological processes in different hepatic diseases. The activation of NF-κB is related to oxidative stress-induced cell death.

l-Theanine is the primary amino acid in green tea and accounts for 1–2% of the leaf dry weight. There are two isomers of l-theanine, and the l isomer is the predominant form. Its systematic IUPAC name is 2-amino-4-(ethylcarbamoyl) butyric acid and its chemical structure is shown in Figure 1. The structure of l-theanine is very similar to that of glutamic acid, which is a precursor of the main endogenous antioxidant glutathione. l-Theanine is synthesized in the roots of the green tea plant and is concentrated in the leaves.^2,3 Previous studies have shown that l-theanine protects cells from damage through its antioxidant activity⁴ that maintains normal cellular levels of GSH,¹ protecting against cancer^5,6 and neurotoxicity.⁷

Figure 1.

Chemical structure of l-theanine.

These observations prompted us to investigate whether l-theanine could play a protective role in cirrhosis induced in a murine experimental model.

Materials and methods

Chemicals

l-Theanine, carboxymethylcellulose, thiobarbituric acid, bovine serum albumin, γ-glutamyl-p-nitroanilide, l-γ-glutamyl-p-nitroaniline, chloramine-T, p-dimethylaminobenzaldehyde and thioacetamide were purchased from Sigma Chemical Company (St Louis, Missouri, USA). Carbon tetrachloride (CCl₄), sodium acetate, sodium hydroxide, glacial acetic acid, hydrochloric acid, ethanol, methanol and formaldehyde were obtained from J.T. Baker (Xalostoc, Mexico). All reagents were of analytical quality.

Treatment of animals

In this work, male Wistar rats (3 weeks old at the beginning of treatments) were used and were maintained on a standard rat chow diet with free access to drinking water. Eight animals were housed per polycarbonate cage and kept under controlled conditions at 22 ± 2°C with 50–60% relative humidity and 12-h light/12-h dark cycles. All animals received humane care, and the study complied with the institution’s guidelines, official Mexican regulations (NOM-062-ZOO-1999) and the criteria outlined in the Guide for the Care and Use of Laboratory Animals prepared by the National Academy of Sciences and published by the National Institutes of Health (NIH publication 86-23, revised 1985).

Cirrhosis induction

Rats initially weighing 100–110 g were divided into four groups of eight rats each. Animals in the control group were given mineral oil (0.25 mL, intraperitoneally (i.p.)) three times per week for 8 weeks. Cirrhosis was induced in the CCl₄ group by the administration of CCl₄ (0.4 g/kg, i.p.) three times per week for 8 weeks.^8,9 The CCl₄-l-theanine-treated (THE) group rats were treated as in group CCl₄, and they also received daily doses of l-theanine (8.0 mg/kg). Since gavage administration induce stress in animals, and drinking water is an administration route more similar to the human condition, where l-theanine was first consumed as an infusion, and then dissolved in drinking water. Water consumption was measured daily to warrant dosage. The usual dose given to humans in drinking water was 8.0 mg/kg. A dose–response study was not necessary since the dose chosen was effective and safe.The THE group rats received l-theanine only as described for group III. CCl₄ and mineral oil were administered in the morning. Animals were killed by exsanguination 72 h following the last dose of CCl₄ or mineral oil.

Biochemical assays

Blood sample was collected by cardiac puncture, and the liver was rapidly removed. Serum was obtained to assess liver damage by measuring the enzymatic activities of alanine aminotransferase (ALT)¹⁰ and γ-glutamyl transpeptidase (γ-GTP).¹¹

Assessment of lipid peroxidation

The extent of lipid peroxidation in liver homogenates was determined by quantification of malonyldialdehyde (MDA)¹² formation by the thiobarbituric acid method. Total protein contents were determined by the Bradford method, using bovine serum albumin as standard.¹³

Determinations of liver GSH and GSSG

Glutathione (GSH) and glutathione disulfide (GSSG) levels were quantified as described by Hissin and Hilf.¹⁴ GSH and GSSG assays were performed, on the same day when the animals were killed, in 0.1 M sodium phosphate, 0.005 M ethylenediaminetetraacetic acid buffer (pH 8.0) and kept on ice until used.

Collagen quantification

Collagen concentrations were determined by measuring the hydroxyproline contents of fresh liver samples digested with hydrochloric acid as previously described.^15,16

Histology

Liver samples were taken from all animals and fixed with 10% formaldehyde in phosphate-buffered saline for 24 h. Tissue pieces were then washed with tap water, dehydrated in alcohol and embedded in paraffin. Five micrometre-thick sections were mounted on glass slides and covered with silane. Staining was performed using haematoxylin and eosin and Masson’s trichromic stain.

Molecular biology assays

Total protein isolation

Trizol^® reagent (Invitrogen™, Carlsbad, California, USA) was used to isolate the total protein content of liver tissue, and the total protein levels were determined by the bicinchoninic acid method.

Western blot assays

Samples containing 50 μg total protein were separated on 10% polyacrylamide gels, and the proteins were then transferred onto Immuno-Blot™ polyvinylidene difluoride membranes (Bio-Rad, Hercules, California, USA).¹⁷ The blots were subsequently blocked with 5% skim milk and 0.05% Tween-20 for 1 h at room temperature and independently incubated at 4°C overnight with antibodies (Abs) selective against each protein: NF-κB (p65), TGF-β, Metalloproteinase 13 (MMP-13) and interleukin (IL)-1β Abs (MAB3026, MAB1032, MAB13426 and AB1832, respectively) were from Millipore Corp. (Billerica, Massachusetts, USA). IL-6, IL-10 and connective tissue growth factor (CTGF) Abs (SC-57315, SC-57245 and SC14939, respectively) were from Santa Cruz Biotechnology Inc. (Santa Cruz, California, USA). Membranes were then washed and exposed to a secondary peroxidase-labelled Ab (Zymed, San Francisco, California, USA) diluted to 1:1500 in blocking solution for 1 h at room temperature.

Blots were then washed and developed using the Western Lightning™ Plus-ECL enhanced chemiluminescence detection system (NEN Life Sciences Products, Elmer LAS Inc., Boston, Massachusetts, USA). After stripping, the blots were incubated with a monoclonal Ab directed against β-actin,¹⁸ as a control for protein loading, and developed using the Western Lightning Plus-ECL kit. Images were digitally captured using a BioDoc-It imaging system (UVP, Upland, California, USA).¹⁹

RT-PCR analysis

The messenger RNA (mRNA) levels of the cytokine TGF-β in liver samples were measured using reverse transcriptase polymerase chain reaction (RT-PCR). Total RNA was isolated from rat liver tissue using the Trizol reagent (Invitrogen), and the concentration and purity of RNA samples were determined by measuring the A ₂₆₀–A ₂₈₀ nm absorbance ratios. Complementary DNA (cDNA) was obtained using poly dT primed RT (Invitrogen), and Taq polymerase (Invitrogen) was used for the PCR reactions.²⁰

The primer sequences and PCR conditions used to amplify TGF-β and CTGF are described in Table 1. As a positive control, β-actin cDNA was amplified as previously described. Images were digitally acquired using a BioDoc-It Imaging System (UVP).

Table 1.

Primers used for RT-PCR.^a

Gene	Gene bank accession number	Prime sequence	Number of cycles	T _m (°C)	Product size (bp)
TCF-β	NM_021578.2	F: 5′AACCCCCATTGCTGTCCCGT-3′ R: 5′TTCAGCCACTGCCGGACAAC-3′	35	58	190
CTGF	NM_022266.2	F: 5′CAAGGACCGCACAGTGGTT-3′ R: 5′AACTCTGCTTCTCCAGCCTGC-3′	35	58	196

TGF-β: transforming growth factor β; CTGF: connective tissue growth factor; RT-PCR: reverse transcriptase polymerase chain reaction.

^aPrimer sequences specific for the indicated rat genes and the sizes of the expected PCR products are shown.

Statistical analysis

Data are expressed as the mean values ± SE. Comparisons were performed using one-way analysis of variance (ANOVA), followed by Tukey’s test, using the GraphPad Prism 5.00 software. Differences were considered to be statistically significant when p < 0.05.

Results

The enzymatic activities of ALT (a marker of necrosis) and γ-GTP were increased approximately 1.5-fold in the group treated with CCl₄ for 8 weeks compared with the control group, and administration of l-theanine normalized the activities of both the enzymes (Table 2).

Table 2.

Enzyme activities determined in serum in groups of six rats each.^a

Parameters	Control	CCl₄	CCl₄ + l-theanine	l-Theanine
ALT (μmol/L/min)	27.07 ± 2.77	79.52 ± 2.29^b	69.86 ± 2.05^b,c	17.70 ± 0.96
γ-GTP (μmol/L/min)	9.98 ± 1.45	11.29 ± 0.70^b	7.75 ± 0.26^b,c	8.98 ± 0.70

CCl₄: carbon tetrachloride; γ-GTP: γ-glutamyl transpeptidase; ALT: alanine aminotransferase.

^aEffect of the chronic administration of CCl₄ on the activities of ALT and γ-GTP in control group, CCl₄-treated rats (CCl₄), CCl₄-treated rats plus l-theanine (CCl₄ + THE) and rats administered with l-theanine only (THE). Each result represents the mean value of experiments performed in triplicate assays ±SE (n = 6).

^bSignificantly different from control at p < 0.05.

^cSignificantly different from CCl₄ group at p < 0.05.

Chronic exposure to CCl₄ increased fibrosis nearly sixfold when compared with the control. This effect was partially, but significantly, inhibited by l-theanine (Figure 2).

Figure 2.

Liver collagen as measured by hepatic hydroxyproline content in livers from rats following chronic administration of CCl₄ for 8 weeks. Control (CONTROL), CCl₄-treated (CCl₄), CCl₄ plus l-theanine-treated (CCl₄ + THE) and l-theanine-treated (THE) rats. Data represent the mean values of experiments performed in triplicate ±SE (n = 6). ^a p < 0.05: significant difference from the control; ^b p < 0.05: significant difference from CCl₄-treated rats. CCl₄: carbon tetrachloride.

Sections of rat livers from the different treatment groups were stained with haematoxylin and eosin and are presented in Figure 3. The control group showed a normal appearance (Figure 3(a)), whilst samples from CCl₄-treated group showed pronounced nodular fibrosis of the parenchyma with hypertrophic hepatocytes, steatosis and neoformed acidophilic cells (Figure 3(b)). Liver sections from the CCl₄ + l-theanine-treated group revealed that fibrotic regions were diminished and the number of neoformed cells was increased (figure 3(c)), whilst the control group treated with only l-theanine showed a liver parenchyma with normal appearance (Figure 3(d)).

Figure 3.

Haematoxylin and eosin staining of rat liver samples. (a) Untreated control, (b) CCl₄ treated, (c) CCL₄ + l-theanine treated and (d) theanine treated. Bar scale = 50 μm. H: hepatocytes; NH: neoformed hepatocytes; PS: portal space; CCl₄: carbon tetrachloride.

Moreover, Masson’s trichromic staining (Figure 4) revealed that small collagen fibres appeared only in portal areas within the livers of the control group (Figure 4(a)). Liver cirrhotic sections as well as nodular fibrosis surrounding hypertrophic hepatocytes were found in tissue samples from the CCl₄-treated group (Figure 4(b)). The CCl₄ + l-theanine-treated group showed a reduced level of fibrous tissue (Figure 4(c)) and presented a normal appearance of the liver parenchyma (Figure 4(d)).

Figure 4.

Masson’s trichromic stain of rat liver samples. (a) Untreated control, (b) CCl₄ treated, (c) CCL₄ + l-theanine treated and (d) theanine treated. Bar scale = 50 μm. CV: central vein; F: fibrous tissue; H: hepatocytes; NH: neoformed hepatocytes; PS: portal space.

Membrane oxidative stress was evaluated by determining liver MDA levels. Lipid peroxidation was significantly increased by CCl₄ treatment, and this increase was partially prevented by l-theanine (Figure 5).

Figure 5.

Liver lipid peroxidation as determined by MDA contents of samples from control (CONTROL), CCl₄-treated (CCl₄), CCl₄ plus l-theanine-treated (CCl₄ + THE) and l-theanine-treated (THE) rats. Results represent the mean value of experiments performed in triplicate ±SE (n = 6). ^a p < 0.05: significant difference from the control; ^b p < 0.05: significant difference from CCl₄-treated rats. MDA: malondialdehyde; CCl₄: carbon tetrachloride.

Cytosolic oxidative stress was evaluated by the quantification of liver GSH levels (Figure 6). Decreases in the GSH levels of the CCl₄-treated group were observed, and GSSG levels tended to increase in the CCl₄-treated group compared with the control group.

Figure 6.

Reduced GSH and oxidized GSSG, GSH/GSSG ratio and total glutathione (GSH + GSSG) determined in livers from control (CONTROL), CCl₄-treated (CCl₄), CCl₄ plus l-theanine-treated (CCl₄ + THE) and l-theanine-treated (THE) rats. Each bar represents the mean value of experiments performed in triplicate ±SE (n = 6). ^a p < 0.05: significant difference from the control; ^b p < 0.05: significant difference from CCl₄-treated rats. CCl₄: carbon tetrachloride; GSH: glutathione; GSSH: glutathione disulfide.

The administration of CCl₄ increased p65 levels compared with the control group and l-theanine partially inhibited this increase (Figure 7).

Figure 7.

Expression of NF-κB-p65 as determined by Western blot analysis of hepatic extracts. (a) A representative Western blot is shown. (b) Control (CONTROL), CCl₄-treated (CCl₄), CCl₄ plus l-theanine-treated (CCl₄ + THE) and l-theanine-treated (THE) rats. β-actin was used as a control. Signal intensities were determined by densitometric analysis, and values were calculated as the ratio of p65/β-actin. Each bar represents the mean values determined from three rats ±SE in triplicate. ^a p < 0.05: significant difference from the control; ^b p < 0.05: significant difference from CCl₄-treated rats. CCl₄: carbon tetrachloride; NF-κB: nuclear factor κB.

Concomitant increases in IL-1β and IL-6 protein levels in liver samples obtained from CCl₄-treated rats compared with the control group are shown in Figures 8 and 9, respectively; furthermore, the levels of these cytokines were reduced by co-administration of l-theanine. In contrast, the production of anti-inflammatory cytokine IL-10¹⁹ was increased by CCl₄ and was further increased by l-theanine (Figure 10).

Figure 8.

Expression of IL-1β as determined by Western blot analysis of hepatic extracts. (a) A representative Western blot is shown. (b) Control (CONTROL), CCl₄-treated (CCl₄), CCl₄ plus l-theanine-treated (CCl₄ + THE) and l-theanine treated (THE) rats. β-actin was used as a control. Signal intensities were determined by densitometric analysis, and values were calculated as the ratio of IL-1β/β-actin. Each bar represents the mean values determined from three rats ±SE in triplicate. ^a p < 0.05: significant difference from the control; ^b p < 0.05: significant difference from CCl₄-treated rats. IL: interleukin; CCl₄: carbon tetrachloride.

Figure 9.

Expression of IL-6 as determined by Western blot analysis of hepatic extracts. (a) A representative Western blot is shown. (b) Control (CONTROL), CCl₄-treated (CCl₄), CCl₄ plus l-theanine-treated (CCl₄ + THE) and l-theanine-treated (THE) rats. β-actin was used as a control. Signal intensities were determined by densitometric analysis and values were calculated as the ratio of IL-1β/β-actin. Each bar represents the mean values from three rats ±SE in triplicate. ^a p < 0.05: significant difference from the control; ^b p < 0.05: significant difference from CCl₄-treated rats. IL: interleukin; CCl₄: carbon tetrachloride.

Figure 10.

Expression of IL-10 as determined by Western blot analysis of hepatic extracts. (a) A representative Western blot is shown. (b) Control (CONTROL), CCl₄-treated (CCl₄), CCl₄ plus l-theanine-treated (CCl₄ + THE) and l-theanine-treated (THE) rats. β-actin was used as a control. Signal intensities were determined by densitometric analysis and values were calculated as the ratio of IL-1β/β-actin. Each bar represents the mean values from three rats ±SE in triplicate. ^a p < 0.05: significant difference from the control; ^b p < 0.05: significant difference from CCl₄-treated rats. IL: interleukin; CCl₄: carbon tetrachloride.

Following the administration of CCl₄ over 8 weeks, significant increases in the expression of TGF-β (Figure 11) and CTGF (Figure 12) were detected when compared with the control group, and l-theanine diminished the expression of these pro-fibrotic molecules.

Figure 11.

Expression of TGF-β as determined by Western blot analysis of hepatic extracts. (a) A representative Western blot is shown. (b) Control (CONTROL), CCl₄-treated (CCl₄), CCl₄ plus l-theanine-treated (CCl₄ + THE) and l-theanine-treated (THE) rats. β-actin was used as a control. Signal intensities were determined by densitometric analysis and values were calculated as the ratio of TGF-β/β-actin. Each bar represents the mean values from three rats ±SE in triplicate. ^a p < 0.05: significant difference from the control; ^b p < 0.05: significant difference from CCl₄-treated rats. TGF-β:transforming growth factor β; CCl₄: carbon tetrachloride.

Figure 12.

Expression of CTGF as determined by Western blot analysis of hepatic extracts. (a) A representative Western blot is shown. (b) Control (CONTROL), CCl₄-treated (CCl₄), CCl₄ plus l-theanine-treated (CCl₄ + THE) and l-theanine-treated (THE) rats. β-actin was used as a control. Signal intensities were determined by densitometric analysis and values were calculated as the ratio of CTGF/β-actin. Each bar represents the mean values from three rats ±SE in triplicate. ^a p < 0.05: significant difference from the control; ^b p < 0.05: significant difference from CCl₄-treated rats. CTGF: connective transforming growth factor; CCl₄: carbon tetrachloride.

The effects of l-theanine on MMP-13 activation were evaluated (Figure 13). The latent form (Figure 13(a)) increased in the group treated chronically with CCl₄, whilst l-theanine co-administration partially prevented this effect. The cirrhotic group had significantly increased levels of the active form of MMP-13, and l-theanine further increased it. Administration of l-theanine to normal rats did not produce alterations in the expression of either the latent or active forms of this enzyme.

Figure 13.

Expression of the latent (a) and active (b) forms of MMP-13 as determined by Western blot analysis of hepatic extracts. A representative Western blot is shown (c). Control (CONTROL), CCl₄-treated (CCl₄), CCl₄ plus l-theanine-treated (CCl₄ + THE) and l-theanine-treated (THE) rats. β-actin was used as a control. Signal intensities were determined by densitometric analysis and values were calculated as the ratio of MMP-13/β-actin. Each bar represents the mean values from three rats ±SE in triplicate. ^a p < 0.05: significant difference from the control; ^b p < 0.05: significant difference from CCl₄-treated rats. MMP-13: metalloproteinase-13; CCl₄: carbon tetrachloride.

Chronic CCl₄ intoxication increased both TGF-β and CTGF mRNA levels. Interestingly, co-treatment with l-theanine decreased the levels of mRNA encoding these proteins (Figure 14).

Figure 14.

mRNA levels of TGF-β and CTGF. RT-PCR products were obtained using specific primers for each gene and β-actin as described in the text. Control (CONTROL), CCl₄-treated (CCl₄), CCl₄-treated plus l-theanine (CCl₄ + THE) and l-theanine-treated (THE) rats. mRNA : messenger RNS; TGF-β: transforming growth factor β; CTGF: connective tissue growth factor; RT-PCR: reverse transcriptase polymerase chain reaction; CCl₄: carbon tetrachloride.

Discussion

In the present study, we demonstrated that l-theanine, an amino acid present exclusively in the tea plant (Camellia sinensis), inhibited the necrosis, fibrosis and cirrhosis induced by chronic CCl₄ intoxication.

l-Theanine is consumed in Asia as a flavouring and relaxing agent in beverages, foods and dietary supplements. l-Theanine toxicity was evaluated by administration of up to 4000 mg/kg daily as a dietary supplement to male and female rats, and no adverse effects were observed.²¹ The typical doses administered to animals are 2.0 or 4.0 mg/kg; in this study, we administered 8.0 mg/kg similar to the dose given to humans.²²

We hypothesized that l-theanine might protect the liver against CCl₄-induced injury and fibrosis in part by attenuating oxidative stress. The GSH/GSSG ratio was statistically decreased in the CCl₄-treated group in agreement with previous observations.²³ l-Theanine effectively preserved the GSH/GSSG ratio, and as oxidative stress is directly associated with hepatic fibrosis,²⁴ the effect of l-theanine on the GSH/GSSG ratio may explain the observed reduction in collagen accumulation.

Randle et al.²⁵ reported acute intoxication with xenobiotics including CCl₄, increased glutamate cysteine ligase (GCL) activity, and the rate-limiting enzyme in the GSH biosynthetic pathway. However, free radicals produced by chronic CCl₄ intoxication reduced hepatic GSH contents as a result of its metabolism (Figure 6); in fact, oxidative stress, including lipid peroxidation and GSH consumption, is one of the most common mechanisms leading to liver damage.^26

–29 Although inhibition of NF-κB could decrease GSH biosynthesis by regulating GCL expression,^30,31 the antioxidant properties of l-theanine² were capable of preventing GSH oxidation (Figure 6) similar to other natural antioxidants.³² Total glutathione levels (GSH + GSSG) were decreased as a result of GSH reduction in the CCl₄-treated group; however, l-theanine prevented the drop in total glutathione, possibly by inhibiting GSH oxidation. Redox homeostasis following 8 weeks of chronic treatment is likely responsible for maintaining glutathione contents without dramatic alterations.

Whole liver tissue was used to measure p65, the main subunit of NF-κB RelA.³³ As we used a specific Ab recognizing p65 that does not cross-react with NF-κB when bound to IκB, but does when activated, the p65 increase shown in Figure 7 represents the activation of this transcription factor. The CCl₄-induced NF-κB activation is highly dependent on oxidative stress and liver inflammation is closely associated with the development of hepatic fibrosis and cirrhosis.^34,35 In this study, we showed that l-theanine significantly reduced the increase in NF-κB expression.

Our results are supported by histopathological assays showing that following CCl₄ intoxication, the hepatic parenchyma is distorted and fibrosis increased significantly as assessed by hydroxyproline content and visualized by staining with haematoxylin and eosin and trichromic stain. All of these effects were also partially prevented by l-theanine. As demonstrated biochemically and histologically, l-theanine showed anti-fibrotic activity by significantly decreasing the ECM accumulation.

Cirrhosis is strongly associated with oxidative stress, chronic inflammation and increased TGF-β expression.^{31,34

–39} The fibrogenic cytokine TGF-β plays a pivotal role in the activation of hepatic stellate cells to generate myofibroblasts, which in turn increase the production of ECM proteins leading to the progression of fibrosis;³⁷ is a prominent pro-fibrogenic cytokine with antiproliferative effects; and can up-regulate the deposition of ECM. In this study, l-theanine inhibited TGF-β expression and thereby prevented liver fibrosis.

CTGF is a downstream mediator of TGF-β signalling in fibroblast cells and is widely thought to promote the development of fibrosis in collaboration with TGF-β. The crucial role of CTGF in fibrogenesis was evidenced by a significant up-regulation of ECM in fibrotic livers.^39
–41 In this study, we evaluated CTGF protein and mRNA levels, finding that the l-theanine treatment reduced CTGF expression. Furthermore, we found that l-theanine prevented the increases in IL-1β and IL-6 expression.

Degradation of the interstitial matrix is regulated by inhibiting the synthesis of the metalloproteinases and increasing the production of the tissue inhibitor of metalloproteinases. Nevertheless, our results revealed a moderate presence of activated MMP-13 following 8 weeks of CCl₄ treatment and a remarkable increase when CCl₄-rats were treated with l-theanine. The induction of MMP-13 to degrade the accumulated ECM in liver fibrosis represents an important mechanism of l-theanine action for improving medical treatments of this pathology.

Conclusions

The anti-fibrotic properties of l-theanine are associated with its capacity to decrease TGF-β and CTGF levels and induce the expression of the active form of MMP-13. The anti-inflammatory and anti-necrotic properties of l-theanine are associated with its ability to inactivate NF-κB, a well-known target for liver injury³³ and thus decrease pro-inflammatory cytokines (e.g. IL-1β and IL-6) and increase anti-inflammatory IL-10. Moreover, l-theanine has low toxicity and can thus be used in human trials.

Footnotes

Acknowledgements

The authors express their gratitude to Biol. Mario Gil Moreno, Mr Ramón Hernández and QFB Silvia Galindo Gomez for their excellent technical assistance and to M.V.Z. Rafael Leyva Muñoz, MVZ Benjamin E Chavez and M.V.Z. Ricardo Gaxiola for animal handling and care. The authors also acknowledge the Animal Lab Facility, UPEAL-Cinvestav.. The authors also acknowledge the support from Conacyt-PNPC, 2015.

Funding

JEP-V was a Conacyt fellow (225157) and JS was partially supported by Conacyt grant 127357.

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l -Theanine prevents carbon tetrachloride-induced liver fibrosis via inhibition of nuclear factor κB and down-regulation of transforming growth factor β and connective tissue growth factor

Abstract

Keywords

Introduction

Materials and methods

Chemicals

Treatment of animals

Cirrhosis induction

Biochemical assays

Assessment of lipid peroxidation

Determinations of liver GSH and GSSG

Collagen quantification

Histology

Molecular biology assays

Total protein isolation

Western blot assays

RT-PCR analysis

Statistical analysis

Results

Discussion

Conclusions

Footnotes

Acknowledgements

Funding

References