Troglitazone Not Only Increases GLUT4 but Also Induces Its Translocation in Rat Adipocytes

Abstract

Thiazolidinediones, insulin-sensitizing agents, have been reported to increase glucose uptake along with the expression of glucose transporters in adipocytes and cardiomyocytes. Recently, we have further suggested that the translocation of GLUT4 is stimulated by thiazolidinediones in L6 myocytes. However, the direct effects of thiazolidinediones on translocation of glucose transporters have not yet been determined. In this study, using hemagglutinin epitope-tagged GLUT4 (GLUT4-HA), we provide direct evidence of the effect of troglitazone on the translocation of GLUT4 in rat epididymal adipocytes. Primary cultures of rat adipocytes were transiently transfected with GLUT4-HA and overexpressed eightfold compared with endogenous GLUT4 in transfected cells. A total of 24 h of treatment with troglitazone (10 ؊4 mol/l) increased the cell surface level of GLUT4-HA by 1.5 ؎ 0.03-fold (P < 0.01) without changing the total amount of GLUT4-HA, whereas it increased the protein level of endogenous GLUT4 (1.4-fold) without changing that of GLUT1. Thus, the direct effect on the translocation can be detected apart from the increase in endogenous GLUT4 content using GLUT4-HA. Troglitazone not only increased the translocation of GLUT4-HA on the cell surface in the basal state but also caused a leftward shift in the doseresponse relations between GLUT4-HA translocation and insulin concentration in the medium (ED 50 : from ϳ0.1 to 0.03 nmol/l). These effects may partly contribute to the antidiabetic activity of troglitazone in patients with obesity and type 2 diabetes.

Rosiglitazone increases cell surface GLUT4 levels in 3T3-L1 adipocytes through an enhancement of endosomal recycling

“…In support of this, the rosiglitazoneinduced effects on GLUT4 required several days of treatment. In contrast, troglitazone has been shown to largely alter cell surface GLUT4 levels in muscle and fat cells within 15 and 24 hours of treatment, respectively [30,31]. Troglitazone concentrations that were found to increase cell surface GLUT4 levels in these studies were much higher than the concentrations of rosiglitazone we found to be effective, but this correlates well with the lower affinity of troglitazone for PPARγ [32,33].…”

“…Troglitazone concentrations that were found to increase cell surface GLUT4 levels in these studies were much higher than the concentrations of rosiglitazone we found to be effective, but this correlates well with the lower affinity of troglitazone for PPARγ [32,33]. Neither in muscle cells [30] nor in adipocytes [31,34] (Fig. 3A, C) were the effects of glitazones and high concentrations of insulin additive.…”

“…However, rosiglitazone treatment of ob/ob mice has been reported to increase glucose uptake and cell surface GLUT4 levels in primary adipocytes stimulated with insulin ex vivo, but this was most probably due to an increase in GLUT4 expression [16]. Basal cell surface GLUT4 levels were found to be increased by troglitazone in L6 muscle cells [30] and primary adipocytes [31], and by rosiglitazone and troglitazone in 3T3-L1 adipocytes (this study). Furthermore, troglitazone was found to induce a significant (50%) increase in GLUT4 expression levels in adipocytes [31], while this was not the case for rosiglitazone in our studies.…”

“…Basal cell surface GLUT4 levels were found to be increased by troglitazone in L6 muscle cells [30] and primary adipocytes [31], and by rosiglitazone and troglitazone in 3T3-L1 adipocytes (this study). Furthermore, troglitazone was found to induce a significant (50%) increase in GLUT4 expression levels in adipocytes [31], while this was not the case for rosiglitazone in our studies.…”

Phenyllactic Acid from Lactobacillus plantarum PromotesAdipogenic Activity in 3T3-L1 Adipocyte via Up-Regulationof PPAR-γ2

“…PPAR-γ activation leads to stimulated glucose uptake, storage of triglycerides and production of adiponectin. It also induces increased GLUT-4 mRNA levels in adipocytes [31]. Muscle and adipose tissue contain at least two kinds of glucose transporter, GLUT-1 and GLUT-4, among which GLUT-4 is an important insulin regulator [32].…”