AMPK and mTOR play principal roles in governing metabolic programs; however, mechanisms underlying the coordination of the two inversely regulated kinases remain unclear. In this study we found, most surprisingly, that the late endosomal/lysosomal protein complex v-ATPase-Ragulator, essential for activation of mTORC1, is also required for AMPK activation. We also uncovered that AMPK is a residential protein of late endosome/lysosome. Under glucose starvation, the v-ATPase-Ragulator complex is accessible to AXIN/LKB1 for AMPK activation. Concurrently, the guanine nucleotide exchange factor (GEF) activity of Ragulator toward RAG is inhibited by AXIN, causing dissociation from endosome and inactivation of mTORC1. We have thus revealed that the v-ATPase-Ragulator complex is also an initiating sensor for energy stress and meanwhile serves as an endosomal docking site for LKB1-mediated AMPK activation by forming the v-ATPase-Ragulator-AXIN/LKB1-AMPK complex, thereby providing a switch between catabolism and anabolism. Our current study also emphasizes a general role of late endosome/lysosome in controlling metabolic programs.

No abstract

The AMP-activated protein kinase (AMPK) is a master regulator of metabolic homeostasis by sensing cellular energy status. AMPK is mainly activated via phosphorylation by LKB1 when cellular AMP/ADP levels are increased. However, how AMP/ADP brings about AMPK phosphorylation remains unclear. Here, we show that it is AMP, but not ADP, that drives AXIN to directly tether LKB1 to phosphorylate AMPK. The complex formation of AXIN-AMPK-LKB1 is greatly enhanced in glucose-starved or AICAR-treated cells and in cell-free systems supplemented with exogenous AMP. Depletion of AXIN abrogated starvation-induced AMPK-LKB1 colocalization. Importantly, adenovirus-based knockdown of AXIN in the mouse liver impaired AMPK activation and caused exacerbated fatty liver after starvation, underscoring an essential role of AXIN in AMPK activation. These findings demonstrate an initiating role of AMP and demonstrate that AXIN directly transmits AMP binding of AMPK to its activation by LKB1, uncovering the mechanistic route for AMP to elicit AMPK activation by LKB1.

Fatty acids (FAs) are essential nutrients, but how they are transported into cells remains unclear. Here, we show that FAs trigger caveolae-dependent CD36 internalization, which in turn delivers FAs into adipocytes. During the process, binding of FAs to CD36 activates its downstream kinase LYN, which phosphorylates DHHC5, the palmitoyl acyltransferase of CD36, at Tyr91 and inactivates it. CD36 then gets depalmitoylated by APT1 and recruits another tyrosine kinase SYK to phosphorylate JNK and VAVs to initiate endocytic uptake of FAs. Blocking CD36 internalization by inhibiting APT1, LYN or SYK abolishes CD36-dependent FA uptake. Restricting CD36 at either palmitoylated or depalmitoylated state eliminates its FA uptake activity, indicating an essential role of dynamic palmitoylation of CD36. Furthermore, blocking endocytosis by targeting LYN or SYK inhibits CD36-dependent lipid droplet growth in adipocytes and high-fat-diet induced weight gain in mice. Our study has uncovered a dynamic palmitoylation-regulated endocytic pathway to take up FAs.

Obesity is a major health problem in the developed and developing world. Many “functional” foods and ingredients are advocated for their effects on body composition but few have consistent scientific support for their efficacy. However, an increasing amount of mechanistic and clinical evidence is building for green tea (GT). This experiment was therefore undertaken to study the effects of a high‐catechin GT on body composition in a moderately overweight Chinese population. In a randomized placebo‐controlled trial, 182 moderately overweight Chinese subjects, consumed either two servings of a control drink (C; 30 mg catechins, 10 mg caffeine/day), one serving of the control drink and one serving of an extra high‐catechin GT1 (458 mg catechins, 104 mg caffeine/day), two servings of a high‐catechin GT2 (468 mg catechins, 126 mg caffeine/day) or two servings of the extra high‐catechin GT3 (886 mg catechins, 198 mg caffeine/day) for 90 days. Data were collected at 0, 30, 60, and 90 days. We observed a decrease in estimated intra‐abdominal fat (IAF) area of 5.6 cm2 in the GT3 group. In addition, we found decreases of 1.9 cm in waist circumference and 1.2 kg body weight in the GT3 group vs. C (P < 0.05). We also observed reductions in total body fat (GT2, 0.7 kg, P < 0.05) and body fat % (GT1, 0.6%, P < 0.05). We conclude that consumption of two servings of an extra high‐catechin GT leads to improvements in body composition and reduces abdominal fatness in moderately overweight Chinese subjects.