Role of Angiotensin II Type 2 Receptor in Regulation of Lipid Metabolism and Adiposity
Obesity is becoming increasingly common in the United States and worldwide. Abundant supplies of cheap foods and sedentary life styles are predisposing human beings to obesity which is an underlying risk factor for metabolic syndrome and cardiovascular diseases. Adipose tissue is the obligatory site of fat accumulation which causes obesity. Moreover, adipose tissue expresses local renin angiotensin system (RAS) which has been implicated in obesity. Recent studies using angiotensin type 2 receptor (AT2R) knockout mice suggested that AT2R contributes to adiposity and obesity in male mice while prevents adiposity and obesity in female mice. On the other hand, deficiency of AT2R in Apolipoprotein E knockout mice (a model of atherosclerosis) exaggerated adiposity in male mice suggesting that AT2R prevents adiposity in this animal model. Thus, role of AT2R on adiposity in knockout studies was in-conclusive and also indicated a gender specific role of AT2R. Furthermore, the studies showed that the relative expression of AT2R deceases after birth through adolescence and adulthood. Also, there are reports suggesting that estrogen (E2) positively regulates AT2R expression and AT2R affects levels of urinary E2 which is known regulator of adiposity. Hence, we designed the present study to investigate the effect of pharmacological activation of AT2R on adiposity and to study whether the said effect is reduced at adolescence. Male C57BL/6 mice (5 and 12 weeks old) were pretreated with the AT2R agonist C21 (0.3 mg/kg, daily i.p.) for 4 days. Thereafter the animals were placed on normal chow diet (ND) or high-fat diet (HFD) with concurrent drug treatment for the next 10 days. The HFD significantly increased epididymal WAT (eWAT) weight, adipocyte size, plasma free fatty acid (FFA) and triglyceride (TAG) in these mice. The pharmacological activation of AT2R reduced the HFD-induced increase in eWAT weight, adipocyte size, plasma FFA and TAG. Thus, we observed that the pharmacological activation of AT2R reduced the HFD-induced adiposity and plasma parameters in male mice, both in young and adults. To explore the effects of C21 on lipid metabolism we investigated the adipose lipid metabolism (lipogenic and lipolytic) regulators in the eWAT of 5 weeks old male mice only. We observed that HFD increased the protein expressions (measured by western blot) of eWAT lipogenic regulators, namely adipose fatty acid binding protein (FABP4) and fatty acid synthase (FASN). On the other hand, HFD caused a decrease in the protein expression of eWAT adipose triglyceride lipase (ATGL) and an increase in hormone-sensitive lipase (HSL) protein expression 5 weeks old male mice. Interestingly, C21 treatment altered HFD-induced changes in lipogenic and lipolytic regulators. To explore further whether C21 directly impacts eWAT lipogenic/lipolytic regulators or impacts them due to its effect on fatty acid uptake in adipocytes, we performed an in vitro study using isolated primary epididymal adipocytes from 5 weeks old male mice. We observed that C21 acting on epididymal adipocyte AT2R via a NOS/GC/PKG-dependent pathway reduced fatty acid transport in adipocytes. Thus, pharmacological activation of AT2R reduced the fatty acid transport in adipocytes and possibly in consequence prevented HFD-induced adiposity in the male mice.
Our studies with AT2R knock out female mice demonstrated that genetic deletion of AT2R causes an increase in adiposity with a parallel decrease in urinary E2 in female mice. Thus to explore whether the pharmacological activation of AT2R using C21 prevents adiposity in female mice and whether the E2 plays a role in C21-mediated effect on adiposity, female C57BL/6 ovary-intact (Ovi) and ovariectomized (Ovx) mice were used. These mice were treated with C21 (0.3 mg/kg, daily i.p.) with(out) E2 (5 µg/day) supplementation and placed on HFD for 10 days. We observed that the C21 treatment prevented the HFD-induced adiposity and plasma parameters, namely FFA, TAG and insulin in female mice independent of E2. In the liver, C21 treatment increased the CPT1A expression (an index of β-oxidation) and prevented liver weight increase under HFD. Interestingly, we also observed that C21 treatment increased the physical activity of female mice. Thus, this study demonstrated that pharmacological activation of AT2R increased physical activity of the female mice and prevented the HFD-induced adiposity in these mice. Overall these studies demonstrate that the pharmacological activation of AT2R reduces fatty acid uptake in adipocytes. This reduction in fatty acid uptake possibly prevents HFD-induced changes in lipid metabolism, adiposity and plasma parameters in mice. Moreover, pharmacological activation of AT2R also increases physical activity which may increase utilization of the fatty acid via oxidation and thus remove fatty acid from the circulation. In light of these studies, we propose that AT2R activation may serve as a potential therapeutic target to control obesity which in turn protects against obesity-associated metabolic disorders.