Two separate research teams claim that activity of nuclear receptor peroxisome proliferator-activated receptor-γ (PPAR-γ) in the brain plays a role in the mechanisms that cause weight gain among patients receiving thiazolidinedione (TZD) treatment for type 2 diabetes. The two sets of animal-based research, one led by a team at the University of California, San Diego (UCSD), and the other carried out by scientists at the University of Cincinnati College of Medicine, have identified a previously unknown role for central nervous system PPAR-γ in the regulation of energy balance, leptin sensitivity, and at least some of the weight gain associated with administering PPAR-γ–modulating drugs. Both research teams published their results in Nature Medicine.
The nuclear receptor PPAR-γ is activated by lipids to induce the expression of genes involved in lipid and glucose metabolism, explains the Cincinnati team. In adipose tissue, muscle, liver, and macrophages, signalling by PPAR-γ is a determinant of insulin sensitivity and mediates the insulin-sensitizing effects of TZDs, the UCSD researchers add.
PPAR-γ is also the target of TZDs including rosiglitazone for the treatment of type 2 diabetes, and chronic peripheral administration of such drugs improves glycemic control but at the expense of increased caloric intake, body weight, and body fat gain. The traditional view has been that these changes in energy balance are mediated by the actions of PPAR-γ to induce adipogenesis in white adipose tissue, note Randy J. Seeley, M.D., at the University of Cincinnati’s Department of Internal Medicine, Division of Endocrinology, and colleagues. However, as both research teams point out, the receptor is also expressed in key brain areas involved in energy homeostasis and glucose metabolism, and this suggests that the CNS might represent a previously unrecognized site for TZD activity.
To investigate this possibility further, Jerrold M. Olefsky, Ph.D., at UCSD’s Department of Medicine, and colleagues, generated mice in which the gene for PPAR-γ was knocked out only in the brain (Pparg brain knockout [BKO] mice), to determine whether neuronal PPAR-γ signalling contributes to either weight gain or insulin sensitivity.
When they fed control and PParg-BKO mice a high-fat diet (HFD), the knockout animals gained significantly less weight than the control animals, had a lower body fat percentage, and were also more active and ate less. The finding that Pparg-BKO mice are protected against excess weight gain on a HFD led the team to hypothesize that neuronal PPAR-γ signalling may also contribute to leptin resistance in animals fed a HFD. They found leptin sensitivity was increased in Pparg-BKO mice when measured as a function of the effect on food intake by leptin administration for 48 hours. Leptin induced a larger decrease in food intake in Pparg-BKO mice fed their normal food (chow) compared with control animals, despite the fact there was no difference in body weight between two groups.
The UCSD team then added TZDs into the equation. They fed both control and knockout mice with a HFD to induce obesity and insulin resistance, and then supplemented the HFD with rosiglitazone. Addition of the drug led to 50% less weight gain and food intake by the Pparg-BKO animals than in the control mice.
Importantly, when mice given a HFD were treated with rosiglitazone for a number of weeks, glucose tolerance was markedly improved in control animals but not in the Pparg-BKO mice, despite equally reduced fasting free fatty acid concentrations. The authors claim the impaired response to rosiglitazone in the knockout animals appeared to be liver specific, “which implies that the effect of rosiglitazone to improve insulin sensitivity in liver, but not in other tissues, involves a CNS mechanism.” Dr. Olefsky’s team’s results are published in a paper titled “Brain PPAR-γ promotes obesity and is required for the insulin-sensitizing effect of thiazolidinediones.”
To test the effects of direct activation of CNS PPAR-γ by rosiglitazone, meanwhile, Dr. Seeley’s team in Cincinnati administered small doses of the drug directly into the hypothalamic regions of the brains of male rats. They found the drug triggered a 50% higher caloric intake over 24 hours and led to a corresponding weight change. They also demonstrated that administering a single bolus of rosiglitazone to the brain led to significantly greater food intake for up to three days, and body fat gain was still higher seven days after the single injection.
The researchers then investigated the effects of chronically activating brain PPAR-γ, using a fusion protein comprising a viral transcriptional activator tagged to PPAR-γ that has previously been used to explore the effects of chronic tissue-specific PPAR-γ activation in the absence of ligand. When the fusion protein was expressed specifically in the hypothalamus of male rats for four weeks, the animals consumed more calories, gained more weight, and accumulated twice as much body fat as control-treated rats. “Collectively, these data suggest a potential role for hypothalamic PPAR-γ in both acute and chronic regulation of food intake and adiposity,” the authors write.
To test whether TZD drugs might increase weight gain by activating CNS PPAR-γ, the team gave rats oral rosiglitazone and then some animals were given brain infusions of the PPAR-γ antagonist. The results showed that while a single oral dose of rosiglitazone induced overeating and greater body weight gain within 24 hours, this effect was completely blocked by the brain-delivered PPAR-γ antagonist. The same attenuating effect on weight gain and overeating was achieved by injecting the hypothalamus of rosiglitazone-treated rats with a lentivector-based shRNA to block PPAR-γ expression.
Separate studies were carried out to see whether inhibiting endogenous CNS PPAR-γ would lead to negative energy balance. In obese rats fed a HFD, the inhibitor led to a 40% reduction in calorie intake compared with control-treated obese rats, whereas the same dose of inhibitor had no effect on caloric intake in animals fed a normal chow diet.
Interestingly, the researchers found that plasma thyroid-stimulating hormone concentration was more than fivefold higher among diet-induced obese rats acutely treated with the PPAR-γ inhibitor than in control rats. “These data agree with previous reports of cross-talk between PPAR-γ and the hypothalamic-pituitary-thyroid (HPT) axis,” the authors point out.
Studies by Dr. Seeley’s team also concurred with those of the UCSD team with regard to the effects of CNS PPAR-γ on leptin signalling. Leptin signalling in the hypothalamus is blunted in rats fed a HFD, and this leptin resistance is thought to contribute to the continued accumulation of body fat. The Cincinnati team hypothesized that hypothalamic PPAR-γ specifically may contribute to the development of HFD-induced leptin resistance, and that chronic antagonism of CNS PPAR-γ would restore leptin sensitivity these animals. To test this, they administered the PPAR-γ antagonist into the lateral ventricle of HFD-fed rats, at a dose that had no effect on body weight but that did result in significantly lower hypothalamic expression of PPAR-γ’s target gene lipoprotein lipase. After two weeks of inhibitor treatment, the animals were then challenged with exogenous intraperitoneal leptin at a dose that would normally reduce caloric intake and body weight in lean animals. The leptin- and PPAR-γ antagonist-treated HFD rats demonstrated lower 24 hour food intake and body weight changes than animals receiving just the inhibitor, whereas the weight-matched control rats remained leptin-resistant. “These findings implicate PPAR-γ as a potential molecular link between fatty acids and leptin action to regulate food intake,” the authors remark.
The Cincinnati team’s research is published in a paper titled “A role for central nervous system PPAR-γ in the regulation of energy balance.” Dr Seeley says the findings have implications both for the design of future diabetes therapies that don’t induce weight gain, but also highlight the need for better understanding of the interaction between the brain’s PPAR-γ system and micronutrients in the diet. “We know that one way to activate PPAR-γ is by exposing cells to fatty acids. If we know which ones activate PPAR-γ, we could find ways to alter diets so as to limit their ability to turn on this system that drives increased food intake, making it easier for people to avoid weight gain.”