Pioglitazone acutely stimulates adiponectin secretion from mouse and human adipocytes via activation of the phosphatidylinositol 3′-kinase

Abstract

Aims

Thiazolidinediones increase circulating adiponectin. We have previously demonstrated the involvement of the phosphatidylinositol 3-kinase (PI3K) signaling pathway in insulin-stimulated adiponectin secretion. We therefore investigated the effects of the thiazolidinedione pioglitazone on acute adiponectin secretion, and the involvement of the PI3K signaling pathway in this action.

Main methods

We treated murine 3T3-L1 and human primary adipocytes with 1–10 uM pioglitazone for 2 h, +/− PI3K inhibition by Wortmannin (WT). Secreted adiponectin was measured by Western blot. PI3K activity following 15-minute treatments with 1–10 uM pioglitazone was measured by thin layer chromatography. Pioglitazone's effect on adiponectin synthesis and on secretion of newly synthesized adiponectin was studied in 3T3-L1 adipocytes using a pulse-chase technique.

Key findings

Pioglitazone was found to increase adiponectin secretion and PI3K activity in a dose-dependent manner from 3T3-L1 and human adipocytes. In 3T3-L1 adipocytes, 10 uM pioglitazone increased adiponectin secretion by 84 +/− 14% (p < 0.0001) at 2 h. Similarly, in human adipocytes there was a 56 +/− 18% (p < 0.02) increase in secretion. WT blocked the pioglitazone effect and decreased adiponectin secretion at 2 h (47% of pioglitazone treated, p < 0.006). Pioglitazone increased PI3K activity in a dose-dependent manner in both 3T3-L1 (1.7 vs. 2.7-fold increase over control at 2 uM vs. 10 uM dose, p = 0.02) and human adipocytes.

Significance

Our data show that pioglitazone acutely stimulates adiponectin secretion from both 3T3-L1 and human adipocytes. This acute effect of pioglitazone is PI3K-dependent.

Introduction

States of insulin resistance including obesity, diabetes, and the metabolic syndrome are associated with decreased circulating levels of the adipokine adiponectin (Hu et al., 1996, Arita et al., 1999, Hotta et al., 2000). Adiponectin has been shown to have insulin-sensitizing actions in muscle (Fruebis et al., 2001) and liver (Berg et al., 2001), and anti-atherogenic actions in vascular endothelium (Ouchi et al., 1999, Ouchi et al., 2000, Okamoto et al., 2002). Furthermore, decreased adiponectin in insulin-resistant individuals seems to contribute to increased risk of diabetes (Choi et al., 2004, Cruz et al., 2004) and atherosclerosis (Cesari et al., 2006, Kojima et al., 2007), making therapeutic interventions that raise adiponectin levels attractive options in the prevention of these diseases. One such promising intervention is treatment with insulin-sensitizing agents in the thiazolidinedione (TZD) class, which has been reported to result in increased plasma adiponectin levels in humans (Yang et al., 2002, Yu et al., 2002, Phillips et al., 2003, Miyazaki et al., 2004). However, the mechanisms through which TZD treatment increases circulating adiponectin are still poorly understood.

Insulin has been shown to be a hormonal regulator of many secreted proteins, including adiponectin (Bogan and Lodish, 1999, Fasshauer et al., 2002, Mohlig et al., 2002). The phosphatidylinositol 3-kinase (PI3K) signaling pathway is responsible for most of insulin's metabolic actions. We have previously shown that reduction in the strength of PI3K signaling (a hallmark of insulin resistance) leads to a significant reduction in adiponectin production and secretion in 3T3-L1 adipocytes (Pereira and Draznin, 2005). Herein, we provide additional support for the involvement of the PI3K pathway in the regulation of adiponectin secretion, and examine this pathway's role in the acute stimulation of adiponectin secretion by pioglitazone, an insulin-sensitizing agent of the TZD class.

TZDs activate the gamma isoform of nuclear receptor transcription factor PPAR (peroxisome proliferator-activated receptor), resulting in target gene transcription. However, this class of agents has also been shown to have more rapid “non-genomic” effects (Takeda et al., 2001, Wynne et al., 2005). In the present study, we demonstrate direct and significant activation of PI3K in both 3T3-L1 and human adipocytes by the TZD pioglitazone, and examine pioglitazone's acute effect on adiponectin synthesis and secretion, and the involvement of the PI3K pathway in this action. To our knowledge, ours is the first demonstration of acute TZD activation of PI3K in adipocytes.