Submitted on November 15, 2008
Revised on March 30, 2009
Accepted on April 13, 2009
Dysregulated glutathione metabolism links to impaired insulin action in adipocytes
Hironori Kobayashi1, Morihiro Matsuda1*, Atsunori Fukuhara2, Ryutaro Komuro1, and Iichiro Shimomura1
1 Osaka University
2 Osaka University Graduate School of Medicine
* To whom correspondence should be addressed. E-mail: mmatsuda{at}imed2.med.osaka-u.ac.jp.
Oxidative stress plays an important role in obesity-related metabolic diseases. Glutathione peroxidase (GPx) is an antioxidant enzyme down-regulated in adipose tissue of obese mice. However, the role of GPx in adipocytes remains elusive. The objective of this study was to clarify the pathophysiological changes in GPx activity and glutathione metabolism, and their roles in the pathogenesis of insulin resistance in adipocytes. To achieve this goal, we measured cellular GPx activity, glutathione (GSH) contents, GSH/GSSG ratio, and mRNA expression of 
-glutamylcysteine synthetase (-GCS), a rate-limiting enzyme for de novo GSH synthesis, in adipose tissue of control and ob/ob mice, and in 3T3-L1 adipocytes treated with insulin, H2O2, free fatty acid (FFA) or TNF-
. Furthermore, we investigated the effects of GPx inhibition with a specific GPx inhibitor or RNA interference against GPx, H2O2 and reduced GSH on insulin signaling in 3T3-L1 adipocytes. Ob/ob mice showed not only a decrease in cellular activity of GPxs (GPx1, 4 and 7), but also an increase in -GCS expression, resulting in increased GSH contents in adipose tissue. These alterations in glutathione metabolism were also observed during differentiation of 3T3-L1 cells and their exposure to insulin, FFA or H2O2. Inhibition of GPx activity, addition of GSH, and H2O2, resulted in impaired insulin signaling in 3T3-L1 adipocytes. These results suggest that decreased GPx activity and increased -GCS expression lead to over-accumulation of GSH, which might be involved in the pathogenesis of insulin resistance in obesity.
