Examination of major tea catechins on GLUT4 translocation in L6 skeletal muscle cells

Obesity and its manifested pathophysiological effects (insulin-resistance, metabolic disease, type 2 diabetes, cardiovascular disease, etc.) are reaching epidemic proportions in the developed world. Roughly one-third of adults twenty years and over are considered overweight or obese, therefore interventional approaches have been of interest in the scientific community. Besides its palatable popularity, tea brewed from the leaves of the plant Camellia sinensis has become increasingly popular due to its possible effects on human health in alleviating complications that arise from diseases such as metabolic syndrome, diabetes, cardiovascular disease, and various types of cancers. These beneficial qualities have mainly been attributed to catechins, compounds found in tea leaves that account for 30-42% of the water-extractable solids. In the present study, I investigated major tea catechins epigallocatechin-3-O-gallate (EGCG), epigallocatechin (EGC), epicatechin-3-O-gallate (ECG), and epicatechin (EC), as well as gallic acid (GA) on L6 rat skeletal muscle cells in order to test their ultimate effects on glucose transporter 4 (GLUT4) translocation via activation of two distinct signaling pathways implicated in the process: (1) the insulin-mediated phosphoinositide 3-kinase (PI3K) pathway and (2) the insulin-independent AMPK pathway. Treatment of cells with the catechins resulted in Thr308 phosphorylation of Akt (a key protein in the PI3K pathway that facilitates GLUT4 translocation). Activation was stronger with the gallated catechins (EGCG and ECG) than the non-gallated catechins (EGC and EC) and gallic acid. Similar results were observed in AMPK activation. Furthermore, GLUT4 translocation from the cytosol to the plasma membrane was higher in samples treated with gallated catechins than cells treated with other polyphenols. Together, these data suggest that major polyphenols found in tea promote translocation of the GLUT4 protein in skeletal muscle. The effectiveness of the catechins on GLUT4 translocation may be attributable to their abilities to generate reactive oxygen species (ROS), decrease intracellular ATP levels, or their abilities to interact with components on the outer leaflet of the plasma membrane, but these possible mechanisms remain to be investigated.M.S.Includes bibliographical referencesby Nicole Hamli