Protective Effect and Mechanism of Theanine on Lipopolysaccharide-Induced Inflammation and Acute Liver Injury in Mice

Theanine, a unique bioactive constituent from tea (Camellia sinensis) leaves, is widely used as a functional ingredient and dietary supplement. To evaluate the anti-inflammatory and hepatoprotective effects of theanine and its molecular mechanism, the lipopolysaccharide (LPS)-induced inflammation mouse model was employed in this study. The survival rate of mice in the theanine-treated group increased significantly compared with that of LPS-only group mice. Furthermore, ICR male mice were randomly divided into three or four groups: control, LPS (LPS treatment only), LPS + theanine (20 mg/kg/day), and theanine (theanine treatment only). The results showed that compared with the LPS group, the liver damage and oxidative stress of the theanine-treated group decreased significantly, based on plasma alanine aminotransferase (ALT) and aspartate aminotransferase (AST) concentrations, hepatic total superoxide dismutase (T-SOD), and malondialdehyde (MDA) levels, and histological scores and apoptosis [terminal deoxynucleotide transferase-mediated deoxyuridine triphosphate nick end-labeling (TUNEL) staining and caspase-3 activity] in the liver tissues. Furthermore, compared with no treatment, pretreatment with theanine significantly decreased the release of interleukin (IL)-1 beta and tumor necrosis factor (TNF)-alpha, inhibited the expression of several inflammatory factors (including IL-1 beta, TNF-alpha, and IL-6), and increased the IL-10/interferon (IFN)-gamma ratio in the hepatic tissues. In the LPS-induced inflammation model, theanine inhibited the expression of proinflammatory mediators involved in the nuclear factor-kappa B (NF-kB) pathway, such as inducible nitric oxide synthase (iNOS) and matrix metalloproteinase-3 (MMP-3), and attenuated the phosphorylation of NF-kB in the hepatic tissues. Moreover, theanine suppressed the acute-phase response (elevated nitric oxide and C-reactive protein levels). Furthermore, theanine suppressed the LPS-induced inflammatory state by normalizing hypothalamic-pituitary-adrenal (HPA) axis hyperactivity. Taken together, the results suggest that theanine potentially ameliorates LPS-induced inflammation and acute liver injury; molecular mechanism of action may involve normalization of HPA axis hyperactivity and inactivation of the NF-kB signaling pathway.