Involvement of guanylin and GC-C in rat mesenteric macrophages in resistance to a high-fat diet

Abstract

The worldwide incidence of obesity has accelerated during the last decade. Obesity is caused by excessive accumulation of white adipose tissue, often as the result of ingesting calories in excess of daily requirements ( 1, 2 ). In investigations of disease outcome, excess adipose tissue is defi ned by using the body mass index (BMI), which is calculated as weight (kg)/height (m 2 ). A BMI of 25.0-25.9 kg/m 2 corresponds to an overweight condition, whereas obesity is defi ned as a BMI of 30 kg/m 2 or greater ( 3, 4 ). Adipose depots in mammals are distributed throughout the body and include the fat surrounding the heart and the subcutaneous, retroperitoneal, and mesenteric fat. The amount of mesenteric fat is thought to be correlated most strongly with morbidity rate in obesity ( 5 ). Therefore, investigating the system regulating adipocytes in the mesenteric fat may yield insight into target molecules for treating or preventing obesity-related diseases.Both humans and animals vary in their body-weight responses to high-fat diets (HFDs). When animals are fed HFDs, most of them increase in body weight, with higher levels of adiposity than occur when standard chow is fed. However, a few subjects fed HFDs show less weight gain than do control animals fed standard chow or obesity-prone (diet-induced obesity, DIO) animals. These animals that do not become obese even when fed HFDs are categorized as being "diet resistant" (DR) ( 6, 7 ). To investigate the characteristics of genes expressed in the mesenteric fat tissues, we Abstract A high-fat diet (HFD) is a well-known contributing factor in the development of obesity. Most rats fed HFDs become obese. Those that avoid obesity when fed HFDs are considered diet resistant (DR). We performed a microarray screen to identify genes specifi c to the mesenteric fat of DR rats and revealed high expression of guanylin and guanylyl cyclase C (GC-C) in some subjects. Our histologic studies revealed that the cellular source of guanylin and GC-C is macrophages. Therefore, we developed double-transgenic (Tg) rats overexpressing guanylin and GC-C in macrophages and found that they were resistant to the effects of HFDs. In the mesenteric fat of HFD-fed Tg rats, Fas and perilipin mRNAs were downregulated, and those of genes involved in fatty acid oxidation were upregulated, compared with the levels in HFD-fed wild-type rats. In vitro studies demonstrated that lipid accumulation was markedly inhibited in adipocytes cocultured with macrophages expressing guanylin and GC-C and that this inhibition was reduced after treatment with guanylin-and GC-C-specifi c siRNAs. Our results suggest that the macrophagic guanylin-GC-C system contributes to the altered expression of genes involved in lipid metabolism, leading to resistance to obesity.

Guanylin-Guanylyl cyclase-C signaling in macrophages regulates mesenteric fat inflammation induced by high-fat diet

“…## , P < 0.01 compared with value for WT rats on the same diet. SR-A promotor to drive the overexpression of Gn and GC-C in the macrophages of our dTg rats [5]. Given that SR-A is known to increase when animals are fed a HFD [19,20], we postulate that the HFD activated the Gn-GC-C system in the mesenteric fat of dTg rats, resulting in the activation of the cGMP-PKG pathway.…”

“…We recently developed dTg rats that overexpress Gn and GC-C in macrophages by using the SR-A promotor and characterized their phenotype [5]. dTg and WT rats (age, 6 wk; sex, male) were housed individually in plastic cages in a room under controlled conditions of temperature and a 12:12-h light:dark cycle (lights on, 08:00 to 20:00).…”

“…Peritoneal macrophages were obtained from peritoneal exudates of dTg rats as described [5]. Briefly, dTg rats were injected intraperitoneally with 5 mL of 4% thioglycollate.…”

“…mesenteric macrophages of some diet-resistant rats [4,5]. Although Gn and GC-C are known to be present mainly in the intestine and to contribute to maintaining the homeostasis of body fluids [6][7][8][9][10][11] through their effects on intracellular cyclic guanosine monophosphate (cGMP), the roles of these two molecules in macrophages were unclear.…”

“…Although Gn and GC-C are known to be present mainly in the intestine and to contribute to maintaining the homeostasis of body fluids [6][7][8][9][10][11] through their effects on intracellular cyclic guanosine monophosphate (cGMP), the roles of these two molecules in macrophages were unclear. Therefore, we generated rats transgenic for both Gn and GC-C (that is, double-transgenic [dTg] rats) by using the promotor of scavenger receptor A (SR-A) and found that these rats demonstrate an anti-obesity phenotype [5]. Considering that a HFD induces obesity and that infiltrating macrophages cause unnecessary inflammation in adipose tissue through their secretion of proinflammatory cytokines, we postulated that the Gn-GC-C system in macrophages may function as a regulatory system for inflammation.…”