Phosphorylation of hepatic AMP-activated protein kinase and liver kinase B1 is increased after a single oral dose of green tea extract to mice

Abstract

We have previously shown that green and black tea extracts increase the phosphorylation of AMP-activated protein kinase (AMPK) and HMG-CoA reductase in rat hepatoma cells in culture, concomitant with a decrease in cholesterol synthesis. In the present study, we evaluated the ability of a single oral dose of green or black tea extract to promote the phosphorylation of AMPK, liver kinase B1 (LKB1, an AMPK-kinase), and HMG-CoA reductase in mouse liver. Green tea extract administered by gavage at 50 and 100 mg/kg caused a 2- to 3-fold increase in hepatic AMPK phosphorylation at 3 and 6 hours after dosing and a 1.5- to 2-fold increase in LKB1 phosphorylation at these same time points. The phosphorylation of HMG-CoA reductase at these and later time points was not significantly increased. Black tea administered by gavage at up to 250 mg/kg was ineffective in increasing hepatic AMPK phosphorylation. Both green and black tea extracts increased LKB1 phosphorylation in hepatoma cells in culture at 15 μg/mL, and black tea also increased the phosphorylation of protein kinase A in hepatoma cells. These results suggest that compounds in both tea extracts activate AMPK by activating its upstream kinase, LKB1, and that black tea may do so by first activating protein kinase A, a known kinase for LKB1. Only green tea, at 50 and 100 mg/kg, was able to activate AMPK and LKB1 in mouse liver after oral dosing, suggesting that the polymerized catechins present in black tea do not reach the liver in sufficient concentration to affect AMPK activity.

Fig. 1

Activation of LKB1,PKA, and PKCζ by green and black tea extracts. McA-RH7777 rat hepatoma cells were treated with 15 μg/mL of green or black tea extract or 1 mM AICAR, metformin, dibutyryl-cAMP or 20 ng/mL of PDGF for 3 hours and levels of phosphorylated LKB1 (A), PKA (B), or PKCζ (C) were determined by immunodetection; representative immunoblot images are shown on the right. Means and SEs (n = 3); statistical significance determined by ANOVA with Dunnett’s post hoc test, P < .05.

Fig. 2

Activation of hepatic AMPK and LKB1 by a single oral dose of green tea extract in mice. Mice received 50 mg/kg (left) or 100 mg/kg (right) of green tea extract by gavage and levels of phosphorylated AMPK, LKB1, and HMG-CoA reductase (HMGR) in liver homogenates were determined at various time points (0-24 hours) after dosing by immunodetection; representative immunoblot images are shown below the graphs. Open bars indicate control animals (gavaged with water). Means and SEs (n = 3); statistical significance determined by ANOVA with Dunnett’s post hoc test, P < .05.

Fig. 3

Lack of activation of hepatic AMPK by a single oral dose of black tea extract in mice. Mice received 50 mg/kg (upper), 100 mg/kg (middle), or 250 mg/kg (lower) of black tea extract by gavage and levels of phosphorylated AMPK in liver homogenates were determined at various time points (0-24 hours) after dosing by immunodetection; representative immunoblot images are shown below the graphs. Open bars indicate control animals (gavaged with water). Means and SEs (n = 4); statistical significance determined by ANOVA with Dunnett’s post hoc test, P < .05.