Modification of cell response to insulin by membrane-acting agents in rat white adipocytes: analysis of structural...

The effect of membrane-acting agents, biscoclaurine alkaloids (cepharanthine, tetrandrine, isotetrandrine), carbobenzoxy-D-Phe-L-Phe-Gly (z-FFG), and tyrphostin AG17, on the insulin-involved fatty acid synthesis by an beta-agonist (e.g., isoproterenol) in adipocytes was examined. The alkaloids dose-dependently enhanced the insulin-involved fatty acid synthesis in rat white adipocytes, stabilized the C(6)-NBD-PC (1-acyl-2-[6-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-caproyl]-sn-glycero-3-phosphatidylcholine) model membrane, and suppressed the phospholipase A(2)-induced phospholipid degradation. In contrast, z-FFG had no effect on the fatty acid synthesis or the membrane stability. Tyrphostin AG17 suppressed insulin action, but promoted the model membrane stabilization. In the same culture conditions as for the fatty acid synthesis assay, cepharanthine, z-FFG and tyrphostin AG17 had no effect on the transcript levels of glucose transporter isoforms (GLUT 1, 4) and hexokinase isozymes (HK I, II) in rat white adipocytes. Thus, these membrane-acting agents modify the insulin action via a change in the cell membrane condition, and do not directly act on the insulin-involved glucose metabolism. Then we analyzed the structural conformation of these membrane-acting agents by computational simulations. The alkaloids had an elliptic macrocyclic structure, and the order of ellipticity (cepharanthine>tetrandrine>isotetrandrine) agreed with that of the modifying ability for insulin action. The distribution of electrostatic potential fields of these alkaloids was essentially equal by turn in surrounding with the dipole moments. Both in z-FFG and tyrphostin AG17, the distribution pattern of electrostatic potential fields was different from that of the alkaloids. Judging from these results, we concluded that the electrostatic potential field is a good index of the modification of insulin action, and the elliptic structure in these alkaloids is regarded with the modification of insulin action.