EGCG regulation of non-insulin-responsive endosomal compartments in insulin-resistant skeletal muscle

Tea consumption reduces the risk of type 2 diabetes mellitus, but how (-)-epigallocatechin-3-gallate (EGCG), the most abundant catechin in tea, mediates glucose transporter (GLUT)- containing vesicles in muscle cells is still unknown. This study investigated the hypothesis that EGCG can stimulate glucose transport by regulating endosome- specific signaling control systems, thus enhancing glucose uptake by skeletal muscle cells. To understand the relationship between the endosomal system and the EGCG-regulated pathway in skeletal muscle, the insulin-resistance L6 myotubes, induced by palmitate treatment, were treated with vesicle ablation, actin filament disruption and signaling inhibition and results were analyzed. The horseradish peroxidase-conjugated human transferrin was localized to endosomes in the cells, followed by incubation with diaminobenzidine/H2O2. The unablated cell-associated glucose uptake was determined using quantitative fluorescence assay. Ablation of endosomal compartments showed that insulin had little effect on the endosomal recycling pathway, and it mainly affected GLUT4 storage vesicles. Conversely, EGCG stimulated endosomal compartments to facilitate glucose transport using a mechanism independent of the actin-based cytoskeletal transit system. Both Akt and atypical protein kinase C, the downstream effectors of phosphatidylinositol-3-kinase (PI3K), participated in the development of palmitate-induced muscle cells insulin resistance. However, EGCG-stimulated glucose uptake is mediated using a PI3K-independent mechanism that involves silent information regulator 1. Thus, glucose transport is regulated by at least two signal cascades and by two types of vesicles, and EGCG can improve glucose uptake in insulin-resistant skeletal muscle through the non-insulin-responsive endosomal pathway.