Hepatic gene expression profiles in a long-term high-fat diet-induced obesity mouse model

Abstract

To understand the molecular mechanisms underlying alterations in the pathophysiologic status of dietary obesity, we examined hepatic genes differentially expressed in a long-term high-fat intake-induced obesity mouse model. C57BL/6J male mice were fed with two kinds of diets for 12 weeks; a low-fat diet (LFD), a high-fat diet (HFD; n=8), and the expression levels of ∼10,000 transcripts in liver tissues from the two groups were assessed using cDNA microarray analysis. Twelve-week feeding with the HFD resulted in significant increase in body weight, visceral fat accumulation and circulating cholesterol concentration, compared with the LFD group. The cDNA microarray analysis revealed marked differences in the expressions of 97 hepatic genes. These genes were categorized into seven groups:

• metabolism;

• defense, stress, and inflammation responses;

• signal transduction, apoptosis, and cell cycle;

• transcription regulation;

• protein synthesis and modification;

• transport; and

• cellular adhesion, cytoskeleton and trafficking.

The expression of genes involved in fatty acid catabolism and ketone body synthesis, such as acyl–CoA oxidase1 (Acox1) and HMG–CoA lyase (Hmgcl), was significantly increased, and expression of genes involved in lipogenesis and cholesterol synthesis, such as acetyl–CoA synthetase2 (Acs2), fatty acid synthase (Fasn), and squalene epoxidase (Sqle), was drastically decreased in the HFD group. Interestingly, the genes implicated in defense and stress responses, such as glutathione S-transferases (GSTs) and heat shock proteins (Hsps), were also highly represented in the HFD group. Besides, a number of previously unappreciated regulatory molecules were changed by the HFD. These results revealed a transcriptional adaptation to long-term HFD and provided interesting information about the molecules involved in the development and maintenance of the obesity phenotype in vivo.

Introduction

Obesity usually results from excessive energy storage over a prolonged period of time. The development of obesity is believed to be influenced by a number of factors, including genes and environment. Studies on animal models have clearly demonstrated two distinct types of obesity; the first type is genetic obesity, as seen in rodent strains such as the Zucker fatty (fa/fa) rat and the leptin-deficient obese (Lep^ob/Lep^ob) mouse, which becomes obese under various experimental conditions Phillips et al., 1996, Zhang et al., 1994. The second type of obesity, which reflects more closely the human condition, results from a combination of genetic and environmental factors. Among environmental factors, long-term high-fat intake has been most intensively studied because of its contribution to the development of both obesity and diabetes in humans and rodents Olefsky et al., 1974, Lin et al., 2000, Murase et al., 2001. The C57BL/6J mouse has especially been used as a human obesity model because this strain develops obesity, hyperglycemia, and hyperlipidemia when raised on a high-fat and high-sucrose diet; however, it remains lean if the fat content of the diet is limited (Lin et al., 2000).

It is generally accepted that the majority of the pleiotropic effects of long-term high-fat diet (HFD) is accompanied with changes in gene expression profiles. Several genes which encode enzymes or signal mediators involved in lipid and glucose metabolism have been shown to respond to long-term HFD Murase et al., 2001, Yu et al., 2000. For example, acyl–CoA oxidase (Acox) and uncoupling protein-2 genes have been found to be altered in livers of long-term HFD mice, accompanied with an increase in the mRNA level of sterol regulatory element binding protein1 (SREBP1), the major transcriptional regulator for lipogenic genes (Murase et al., 2001). Investigations of such changes have unraveled many insights into the molecular mechanisms of metabolic and/or endocrine adaptations to the long-term HFD; however, the results in most cases have been obtained in a “gene by gene” manner. In living organisms, the mechanisms are much more complex. For this reason, a global analysis of gene expression in response to changes in nutritional status appears to be essential for understanding the biological mechanisms. To obtain a more comprehensive picture of the diet-induced hepatic transcriptional adaptation in the C57BL/6J mouse, we used cDNA microarray, containing ∼10,000 mouse transcripts.

In the present study, comparisons of mice fed with a HFD and a low-fat diet (LFD) revealed apparent differences in the mRNA expression of 97 genes. Some of them have been known to be sensitive to nutritional status; however, the majority was newly identified. The interesting findings, which are related to metabolism, defense, and stress responses, are described in detail.