Exposure to an obesity-inducing diet early affects the pattern of expression of peroxisome proliferator, retinoic acid, and triiodothyronine nuclear receptors in the rat

Abstract

Since evidence has appeared that alpha and gamma isoforms of the peroxisome proliferator receptors (PPARs) are involved in the regulation of triglyceride homeostasis and in the control of the differentiation of adipocytes that is required for the development of obesity, a large number of studies have investigated the physiologic role of nuclear receptors in the control of energy balance. The aim of this study was to determine the early effects of an obesity-inducing diet on the expression of PPAR alpha and gamma and other nuclear receptors such as all-trans retinoic acid receptor (RAR) and triiodothyronine receptor (TR), which all form functional heterodimers with a common partner, the 9-cis retinoic acid receptor (RXR). The experiment used a cafeteria diet where 60% of the energy was supplied as lipids. This diet was offered to young rats for 8 and 28 days and the expression of nuclear receptors was determined at the end of each experimental time period (1) in the liver by assaying the binding properties of RAR and TR and by quantifying mRNA levels of RAR beta, TR alpha(1)beta(1), RXR alpha, and PPAR alpha, and (2) in the white adipose tissue (WAT) by quantifying mRNA levels of RAR alpha, RXR alpha, TR alpha(1)beta(1), and PPAR gamma(2). After 8 days of cafeteria diet a significant decrease of RAR and TR maximal binding capacity (MBC) was observed in the liver (-20.1% and -35.0%, respectively, P <.05) and the level of the mRNA of RAR beta was significantly decreased (-17.4%, P <.05). After 28 days of cafeteria diet, the level of the mRNA of PPAR alpha and acyl-CoA oxidase (ACOX) was significantly increased (+54.5% and +37.8%, P <.01 and P <.05, respectively), whereas the MBC of RAR and TR was significantly decreased (-16.0% and -23.4%, P <.01), as were the mRNA levels of RAR beta and TR alpha(1) beta(1) (-28.5% and -32.0%, P <.05). The level of RXR alpha mRNA was unchanged. In WAT, the mRNA level of PPAR gamma(2) was significantly increased after 28 days of cafeteria diet (+49.5%, P <.05) and the mRNA levels of RAR alpha and TR alpha(1) beta(1) significantly decreased (-22.3% and -31.0%, P <.05). These results as a whole showed that a high-fat diet can induce early modifications in the pattern of expression of nuclear receptors in the liver and the WAT. These modifications could be compatible with an early adaptive phenomenon. Further investigations are necessary to better understanding the link between the modifications of the pattern of expression of these receptors and plasticity of adipose tissue leading to the onset of obesity.