Introduction
Glucagon-like peptide-1 (GLP-1) receptor (GLP-1R) agonists, such as exendin-4, have been used in the treatment of type 2 diabetes; they increase glucose-dependent insulin secretion, regulate gastric emptying and reduce food intake and body weight [
1]. At least some of these actions are mediated through neuroendocrine mechanisms, as shown by rodent studies, consistent with the notion that GLP-1R is highly expressed in the hypothalamus. Intracerebroventricular (i.c.v.) administration of GLP-1 reduces food intake and body weight [
2,
3]. Moreover, the central GLP-1R signalling system is linked to the control of peripheral glucose metabolism by inhibiting non-insulin-mediated glucose uptake by muscle and increasing insulin secretion from the pancreas [
4,
5]. Previously, we have shown that subcutaneous GLP-1 treatment reduces hepatic glucose production in mice, partly through central GLP-1R signalling [
6]. Hence, one of the functions of the central GLP-1R system is the modulation of the metabolic activity in peripheral organs that are crucial for the maintenance of energy homeostasis.
Brown adipose tissue (BAT) is a regulator of overall energy homeostasis by combusting triacylglycerol (TG) and glucose into heat. Enhancement of the thermogenic capacity of BAT induces weight loss through increased energy expenditure [
7,
8] and lowers the plasma levels of TG and glucose. In fact, BAT activation can correct hyperlipidaemia [
9] and hyperglycaemia [
10] by increased uptake of TG-derived fatty acids [
11] and glucose from plasma. Recently, it has been shown that activation of central GLP-1R increases thermogenesis in BAT and induces browning within white adipose tissue (WAT), and that this correlates with increased expression of genes required for thermogenesis, including uncoupling protein-1 [
12,
13]. While these findings suggest an important role of the central GLP-1R system in BAT activation, the effect of central GLP-1R activation on lipid and glucose control by BAT has not yet been investigated. Furthermore, although GLP-1R agonists are implemented to treat type 2 diabetes, the responsiveness of the central GLP-1R system to activate BAT to take up TG-derived fatty acids and glucose under central insulin resistance [
14] is unknown.
The aim of this study was to evaluate the potential of activating central GLP-1R by exendin-4 to facilitate clearance of lipids and glucose from the circulation by activating BAT, under both insulin-sensitive and insulin-resistant conditions in diet-induced obese (DIO) C57Bl/6J mice.
Discussion
In the present study we show that central GLP-1R activation by exendin-4 increases the plasma clearance of TG and glucose in both lean and DIO C57Bl/6J mice via increased uptake of TG-derived fatty acids and glucose by BAT, accompanied by activation of BAT and browning of WAT.
First, we showed that continuous central infusion of the GLP-1R analogue exendin-4 (5 days, 0.75 nmol/day) evokes the well-known effect of reducing food intake and body weight [
3,
17]. In addition, central administration of exendin-4 increased SNS activity towards BAT and WAT, as evidenced by increased TH and UCP-1 protein content and lowered lipid content. This corroborates previous findings demonstrating the essential role of the SNS in BAT and WAT activation by central GLP-1R signalling [
12,
18]. Exendin-4 also caused a robustly accelerated clearance of plasma TG and glucose, which likely contributed to reduced plasma TG and glucose concentrations observed in DIO mice. Strikingly, the uptake of TG-derived activity, presumably [
3H]oleate liberated by lipoprotein lipase (LPL) [
11], was not only increased by BAT but also by WAT, together with the increase in UCP-1 content indicative of so-called browning. Although activation of BAT and WAT are likely involved in the observed reduction of plasma TG and glucose by central exendin-4, further studies are warranted to investigate the quantitative contribution of GLP-1R signalling towards BAT and WAT to the overall metabolic improvements on GLP-1R agonism.
It has been reported that central GLP-1 infusion reduces the lipid content of WAT in lean mice but not DIO mice [
18], suggesting that some resistance to the actions of central GLP-1 is induced in DIO mice. Clearly, this effect is not desirable for a drug to treat obesity. Therefore, we explored the effects of chronic central exendin-4 infusion after 12 weeks of high-fat feeding, sufficient to induce obesity and insulin resistance in this mouse model [
21,
22]. Our results show that under these conditions, chronic central GLP-1R signalling still improves the clearance of plasma TG and glucose via a robustly increased uptake by BAT. However, central GLP-1R agonism did not alter UCP-1 protein content or lipid droplet content in WAT in DIO mice, consistent with previous findings by Nogueiras et al [
18]. It is interesting to speculate on why, during obesity, WAT is not susceptible to browning on exendin-4 treatment. Previous studies reported a decreased sensitivity of white adipocytes to adrenergic stimulation in obese individuals [
23]. However, in our study we also describe the absence of exendin-4-induced TH expression in WAT in DIO mice. It is possible that different brain areas modulate specific BAT and WAT functions. While multiple tissues are simultaneously sympathetically stimulated during cold exposure, there are many examples of treatments that result in differential sympathetic outflow to various types of tissues (e.g. WAT vs BAT) and even within a type of tissue (e.g. different WAT pads) [
24]. The GLP-1R is widely expressed through the hypothalamus [
25], and exerts effects via different nuclei. For example, GLP-1R signalling in the arcuate nucleus regulates glucose metabolism, while it modulates feeding via the paraventricular nucleus [
5]. Altogether, it is likely that GLP-1R signalling and subsequent sympathetic outflow differ during obesity and lean conditions.
In apparent contrast to the notion that increased BAT activity is generally correlated with an enhanced total energy expenditure [
26], central GLP-1R activation in fact reduced total energy expenditure, an effect we have observed before during chronic peripheral exendin-4 treatment [
27]. Possibly, the reduction in energy intake is compensated by a relative reduction in energy expenditure. In addition, it appears that chronic central GLP-1R activation results in a shift from using carbohydrates to fatty acids as an energy source, which is consistent with previous studies that showed that chronic i.c.v. GLP-1 treatment decreased the respiratory quotient, indicative of a higher level of fat oxidation by BAT [
18,
28]. The exendin-4-induced decrease in physical activity, resulting from a diminished food-seeking behaviour [
29], is less likely to contribute to the change in total energy expenditure [
30].
From a clinical perspective, the possibility that BAT activity in humans may be amenable to pharmacological manipulation by GLP-1R agonism to control insulin sensitivity and body weight is attractive but, as yet, undemonstrated. Drugs targeting the GLP-1R system are already widely prescribed for their incretin properties to treat type 2 diabetes, but they may be useful in a wider context related to energy balance. The resting energy expenditure of obese individuals with type 2 diabetes increases with 1 year of treatment with a combination of metformin and exenatide or liraglutide [
13]. It is tempting to speculate that BAT may be activated in these patients. Increasing our knowledge about the mechanism of action of exendin-4 may add to the (further) development of peptidomimetics in our battle against obesity and type 2 diabetes.
In conclusion, our results show that chronic central infusion of exendin-4 increases SNS output to enhance BAT activity in both lean and DIO C57Bl/6J mice. Via highly active BAT, the plasma clearance of TG and glucose is accelerated and body fat content is decreased, which, together with reduced food intake, leads to a decrease in body weight. Therefore, we suggest that GLP-1R agonists via BAT activation reduce both hyperlipidaemia and hyperglycaemia, and possibly even atherosclerosis [
31], in addition to the effects of BAT activation on obesity.
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