Green Tea Polyphenols Upregulate the Nrf2 Signaling Pathway and Suppress Oxidative Stress and Inflammation Markers in D-Galactose-Induced Liver Aging in Mice

Abstract

The beneficial effects of green tea polyphenols (GTPs) on D-galactose (D-Gal)-induced liver aging in male Kunming mice were investigated. For this purpose, 40 adult male Kunming mice were divided into four groups. All animals, except for the normal control and GTPs control, were intraperitoneally injected with D-galactose (D-Gal; 300 mg/kg/day for 5 days a week) for 12 consecutive weeks, and the D-Gal-treated mice were allowed free access to 0.05% GTPs (w/w) diet or normal diet for 12 consecutive weeks. Results showed that GTP administration improved the liver index and decreased transaminases and total bilirubin levels. Furthermore, GTPs significantly increased hepatic glutathione and total antioxidant levels, and the activities of superoxide dismutase, catalase, and glutathione S-transferase (GST). Furthermore, GTPs downregulated 8-hydroxy-2-deoxyguanosine, advanced glycation end products, and hepatic oxidative stress markers, such as malondialdehyde and nitric oxide. Additionally, GTPs abrogated dysregulation in hepatic Kelch-like ECH-associated protein 1 and nuclear factor erythroid 2-related factor 2 (Nrf2) and its downstream target gene expression [heme oxygenase 1, NAD(P)H:quinone oxidoreductase 1, and GST] and inhibited tumor necrosis factor-α, transforming growth factor-β, and interleukin (IL)-1β and IL-6 in the liver of treated mice. Finally, GTPs effectively attenuated D-Gal-induced edema, vacuole formation, and inflammatory cell infiltration. In conclusion, GTPs showed antioxidant and anti-inflammatory properties in D-Gal-induced aging mice, and may be considered a natural alternative to the effects of hepatic aging.

Keywords: D-galactose; Nrf2 signaling pathway; aging; antioxidants; green tea polyphenols (GTPs); inflammatory cytokines.

Figure 1

Effects of GTPs on liver damage indexes and antioxidant markers of D-gal treated mice. (A) Plasma levels of Alb. (B) Plasma levels of TBiL. (C) Plasma levels of ALP, ALT, and AST. (D) Hepatic contents of T-SOD, GST, and CAT. (E) Hepatic levels of GSH. (F) Hepatic levels of T-AOC. ^*P < 0.05 and ^**P < 0.001 vs. control group; ^#P < 0.05 and ^##P < 0.01 vs. D-Gal model group. Alb, albumin; ALP, alkaline phosphatase; ALT, alanine aminotransferase; AST, aspartate aminotransferase; CAT, catalase; D-Gal, D-galactose; GSH, reduced glutathione; GST, glutathione S-transferase; GTPs, green tea polyphenols; T-SOD, total superoxide dismutase; T-AOC, total antioxidant capacity; TBil, total bilirubin.

Figure 2

Effects of GTPs on liver oxidative stress markers and inflammatory mediators of D-gal-treated mice. (A) Plasma level of NO. (B) Hepatic contents of MDA. (C) Plasma levels of AGEs and 8-OHdG. (D) Hepatic contents of TNF-α, TGF-β, IL-1β, and IL-6. ^*P < 0.05 and ^**P < 0.001 vs. control group; ^#P < 0.05 and ^##P < 0.01 vs. D-Gal model group.

Figure 3

Effects of GTPs on hepatic histopathological alterations in D-Gal-treated mice. Representative HE-stained sections of the liver tissues from rats in each group (100× and 400×). Green arrow indicates edema, vacuoles, and cytoplasmic porosity. Yellow arrow indicates inflammatory cell infiltration. Red arrow indicates degeneration. Scale bars (200 and 40 μm).

Figure 4

Effects of GTPs on the expressions of Nrf2 pathway in the liver of D-Gal-treated mice. (A) Hepatic protein levels of Keap1. (B) Hepatic protein levels of cytoplasmic and nuclear Nrf2. (C) Hepatic protein levels of HO1 and NQO1. (D) Hepatic mRNA expressions of Nrf2-targeted genes. ^***P < 0.001 vs. control group; ^###P < 0.001 vs. the D-Gal model group.

Figure 5

Mechanism of action of GTPs in D-galactose-induced aging in mice liver. GTPs, green tea polyphenols; ROS, reactive oxygen species; MDA, malondialdehyde; NO, nitric oxide; 8-OhdG, 8-hydroxydeoxyguanosine; AGEs, advanced glycation end products; Keap1, Kelch-like ECH-associated protein 1; Nrf2, NF-E2-related factor 2; NF-kB, nuclear factor kappa B; ARE, antioxidant response element; HO-1, hemeoxygenase; NQO1, NAD(P)H:quinone oxidoreductase 1; GST, glutathione S-transferase; SOD, superoxide dismutase; CAT, catalase; GSH, reduced glutathione; T-AOC, total antioxidant capacity; TNF, tumor necrosis factor; TGF, transforming growth factor; IL, interleukin.