Adiponutrin Gene Expression in 3T3-L1 Adipocytes is Downregulated by Troglitazone

Introduction

Thiazolidinedione compounds (TZDs) are increasingly useful in the treatment of type 2 diabetes [1]. TZDs are able to diminish insulin resistance and improve many of the associated symptoms. It is thought that the improvement of glucose homeostasis may be brought about by TZDs acting as ligands of PPARγ in adipose tissue and changing either plasma concentrations of adipocyte-derived metabolites such as fatty acids or secreted adipocytokines. PPARγ comprises transcription factors that modulate the expression of a cohort of target genes involved in adipocyte differentiation, glucose metabolism and lipid metabolism. The majority of genes that have so far been confirmed to be direct targets of TZD-activated PPARγ in adipose tissue encode proteins involved in lipid metabolism and its regulation, including adipocytokines [2]. It is not clear whether the antidiabetic potential of TZDs is related to their adipogenic actions or to their inhibitory or stimulatory effects on expression of specific genes. Identification of new target genes will provide further insights into the molecular mechanism of TZDs and in addition may reveal novel genes that are involved in glucose and lipid homeostasis. This paper reports that the recently identified gene adiponutrin [5] is downregulated by TZDs.

Adiponutrin was first identified by differential display as being expressed in mature 3T3-L1 adipocytes [5]. Its 3.2 kb mRNA codes for a 413 amino acid protein showing some sequence homology to a putative membrane-bound vesicle transport protein, TTS-2. The localisation of adiponutrin to the plasma membrane was confirmed in cells transiently transfected with epitope-tagged adiponutrin [5]. Adiponutrin shows many features characteristic of the cohort of genes involved in lipid metabolism. It is expressed exclusively in adipose tissues and induced early in differentiation along with many of the other adipose-specific genes [5]. In mature adipocytes, the level of adiponutrin mRNA changes in concert with fasting and feeding, decreasing to barely detectable levels upon fasting and increasing rapidly and markedly upon feeding [5] [6]. Adipocytokines leptin and resistin are also decreased by fasting and restored by feeding [4] [6]. The expression of adiponutrin in response to acute induction by meal-feeding most closely resembles that of leptin but not any of the other adipocytokines or lipogenic enzymes [6]. Another feature shared by adiponutrin and leptin is that their expression is markedly elevated in white adipose tissue of obese (fa/fa) Zucker rats [5] [7] indicating an association with the obese state. The function of adiponutrin is unknown, but these shared features suggest that adiponutrin might be regulated by similar factors to leptin and may contribute to the regulatory role that adipose tissue plays in overall glucose and lipid homeostasis.

In order to determine whether TZDs could directly regulate the expression of adiponutrin, 3T3-L1 adipocytes were exposed to troglitazone, one of the original members of the TZD family of antidiabetic drugs, and mRNA levels determined and compared with other genes that are known to be induced or repressed by TZDs.

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