Ninu Poulose, Aswathy Sheena, Suma Mohan, Gopalakrishnapillai Anilkumar
School of Biotechnology, Amrita Vishwa Vidyapeetham, Amritapuri P.O., Kollam, Kerala 690525, India.
Glucose transporter 4 (GLUT4) is a twelve transmembrane protein which plays a key role in the maintenance of whole body glucose homeostasis. It is mainly expressed in adipocytes and skeletal muscles which are the major sites of glucose disposal in the body. The majority of the GLUT4 expressed is localized to intracellular compartments in an unstimulated state and acutely redistributes to the plasma membrane upon insulin exposure to facilitate the bulk uptake of glucose. In the present study, we report that arjunolic acid, a pentacyclic triterpenoid, is an inhibitor of insulin stimulated glucose uptake in 3T3-L1 adipocytes. Arjunolic acid treatment did not inhibit insulin stimulated GLUT4 translocation to the cell surface. Nor was there any decrease in insulin induced p38 mitogen activated protein kinase (p38 MAPK) activation. Since glucose uptake was also inhibited when the compound was added to insulin stimulated cells at the time of glucose uptake assay, it suggested an existence of direct interaction of arjunolic acid with GLUT4. Molecular docking studies of arjunolic acid with GLUT4 homology model has revealed an interaction at the ATP binding motif (GRRTLHL) present in the loop connecting TM helices 8-9. However there was no binding at the QLS motif which is the substrate binding site present in GLUT4. The molecular dynamics simulation studies suggested distinct conformational changes in GLUT4 in the arjunolic acid-substrate bound form which favor an arjunolic acid dependent substrate occlusion similar to that of ATP. Formation of several inter-domain hydrogen bonds and ionic interactions contributed to this effect. Read more…