Activation of Brown Adipose Tissue by Oral Administration of Leptin

Special Article - Oral Administration of Leptin

Austin J Endocrinol Diabetes. 2016; 3(4): 1051.

Activation of Brown Adipose Tissue by Oral Administration of Leptin

Bendayan M* and Cammisotto PG

Department of Pathology and Cell Biology, University of Montreal, Canada

*Corresponding author: Bendayan M, Department of Pathology and Cell Biology, University of Montreal, C.P. 6128 Succ. Centre Ville 2900 Edouard Mont Petit, Montreal, Quebec, H3C 3J7, Canada

Received: August 18, 2016; Accepted: October 03, 2016; Published: October 04, 2016


Upon demonstrating in previous studies that daily oral administration of leptin is able to regulate food intake and reduce body weight in small rodents while controlling appetite in dogs, we assessed in the present study the action of oral leptin on brown adipose tissue. This thermogenic tissue is stimulated by leptin through activation of UCP1. Leptin increases peripheral lipid oxidation with increased lipolysis and decreased fat synthesis leading to reduction of adiposity. Upon force-feeding leptin to gerbils once a day, the animals reduced their food intake and started to lose weight. Morphological and biochemical examination of the interscapular brown adipose tissue revealed significant activation by oral leptin. By the third day of treatment, the brown adipocytes demonstrated a breakdown of the lipid droplets into smaller ones, an increase in the number of mitochondria and their cristae, while nuclei with extended dispersed chromatin moved towards the center of the cells. By day ten of treatment, the number of lipid droplets decreased being degrade by phagolysosomal structures. Mitochondrial UCP1, key protein in thermogenesis, as well as several enzymes essential to sustain energy metabolism of activated BAT were significantly increased. Thus oral leptin mimics all activities of the endogenous one; it reduces food intake and activates BAT thermogenic activity, thus contributing to the rapid reduction in body weight and adiposity.

Keywords: Oral leptin; Brown adipose tissue; UCP1; Lipolysis


In previous series of studies we have demonstrated that in response to food intake, leptin secretion originating from the gastric mucosa is stimulated. At mealtime, gastric chief cells process leptin through regulated secretion [1]. The release of leptin takes place concurrently with other gastric proteins such as pepsinogen and lipase and corresponds to its exocrine secretion into the gastric cavity. As an integral part of the gastric juice, leptin requires protection from early degradation. This is carried out by a chaperon, the soluble isoform of its own receptor [2,3]. A complex leptin-leptin receptor is formed in the chief cell secretory granules prior its discharge into the gastric cavity. The complex is then vehiculated towards the duodenal lumen by the gastric juice. Transmembrane leptin receptors, present on the enterocyte luminal brush border membrane, bind leptin and internalize it towards the Golgi apparatus [3,4]. Upon an intricate transcytotic path, a leptin-leptin receptor complex is released towards the baso-lateral space of the duodenal mucosa and reaches circulation to be carried to the hypothalamic target cells where it acts as a satiety factor [4,5]. On the other hand and concurrently, leptin is continuously synthesized and secreted through a constitutive pathway by white adipocytes [5-7]. This adipocyte-secreted leptin plays important roles not only for the control of appetite but also for regulating energy expenditure [8-13].

Presence of leptin in the gastric cavity led us to propose the oral administration of leptin. Indeed leptin given orally should be very effective in triggering early satiety and could thus control our tendency for overeating. Obesity has become a very serious problem in our western society leading to major health problems [14-16]. Several physiological and psychological issues bring humans to the trend of overeating which in turn gives rise to leptin resistance that exacerbates our system leading to morbid obesity [17,18]. There is a crucial need for safe and efficient approaches to control our food intake. Our studies on leptin secretion by the gastric mucosa led us to envision the prospect of having an oral medication of leptin. Exogenous leptin would undertake the same physiological path as the endogenous one from the gastric cavity into circulation and would reach the target hypothalamic cells to exert its physiological actions [19].

We have previously designed a vehicule capable to administer leptin orally [20]. This vehicle protects the hormone while in transit in the gastric cavity and promotes its absorption by the duodenal mucosa [20]. We reported that once administrated orally to mice, exogenous leptin is transferred very efficiently to blood circulation in a matter of minutes [20]. Oral leptin is able to control food intake and to induce loss of body weight [20]. The efficiency of oral leptin was found to be more striking when given to leptin-deficient ob/ob obese mice [20]. In addition to significantly reducing their food intake, the animals were losing so much body weight that we had to end the experiments [20]. The fact that the ob/ob mice lack leptin but do express leptin receptors at the surface of their cells [21-23], allowed for oral leptin to restore circulating levels of leptin and correct the over-eating problem in these leptin-deficient animals [20]. On the other hand, experiments performed with db/db mice lacking the leptin receptor [21-23], demonstrated that oral leptin administration is not effective [20]. For oral leptin to be efficient, expression of the corresponding leptin-receptor at the plasma membrane of the target cells is essential.

Further work confirmed the efficiency of oral administered leptin. Experiments were carried out on large animals, namely dogs [24]. For such studies, a leptin pill was designed. Human leptin was inserted in a capsule together with components that protect leptin from early degradation during transit in the digestive track and promote its absorption by intestinal cells [24]. As it occurred in rodents, leptin given orally to dogs was able to significantly reduce amounts of food ingested by the animals. Efficiency of the treatment reached 60% in some animals. Treatments were by far more efficient in the mornings that in the afternoons. Correlations between amounts of circulating oral leptin and amounts of food intake were highly significant which indicates that the effectiveness of the treatment is related to the efficiency for leptin to cross the intestinal wall rather than the leptin action by itself [24].

Leptin, a 16kd peptide, regulates adiposity and body weight by controlling food intake and energy homeostasis through its membrane receptor at the level of the target hypothalamic cells and those of some peripheral tissues such as the Brown Adipose Tissue (BAT) [25]. Brown adipocytes play crucial roles in thermogenesis; property conferred by a particular mitochondrial protein, the Uncoupling Protein-1 (UCP1) [26-30]. BAT is found in abundance in particular anatomical sites mainly in small mammals, in hibernating animals, in newborns of large animals and humans [30-33]. In addition to the large number of small lipid droplets the activated brown adipocyte displays numerous mitochondria packed with well developed cristae. Leptin stimulates brown adipocytes through activation of UCP1 [31,34]. The activation of this protein represents a key player in the nonshivering thermogenesis activity for cold acclimation. Leptin increases peripheral lipid oxidation with increased lipolysis and decreased fat synthesis leading to a rapid reduction in body weight and adiposity [35].

In the present study we have evaluated the response of BAT to oral administrated leptin. Gerbils were force-fed daily with leptin and their brown adipose tissue was examined periodically by morphological approach and analysed for the expression of various enzymes and mitochondrial proteins. Results have shown that oral leptin not only reduces food intake in the gerbil but it also activates its brown adipocytes with raise in the number of mitochondria cristae, increases in the expression of various mitochondrial proteins and fragmentation followed by disappearance of the lipids droplets.

Oral leptin mimics all physiological activities of endogenous leptin ; not only it reduces food intake leading to loss of body weight but it also acts on adipose tissue by stimulating lipid oxidation increasing lipolysis and decreasing fat synthesis, overall contributing to a rapid reduction of adiposity.

Material and Methods and Results

Thirteen adult male gerbils weighing an average of 75g were used for this study. After a few days of acclimation 8 animals were force-fed with 100 μg of leptin in the appropriate vehicle, once a day for a period of 11 days. Five control animals were force-fed with the vehicule alone. Based on previous results obtained on mice [20], 100 μg of mouse recombinant leptin (R&D Systems Inc. Minneapolis, MN, USA) were administered orally, dissolved in 0.5 ml of vehicle which composition included 2000 Kallikrein Inhibitor Units (KIU) of aprotinin (Trasylol, Bayer, Leverkusen, FRG) and 5 mg of sodium deoxycholate (22mmol/l) (Sigma St-Louis, Mo, USA) in bicarbonate buffer pH 9. Aprotinin reduces leptin degradation in the gastric cavity while sodium deoxycholate enhances duodenal absorption [36-38]. Food intake and body weight were monitored daily. Results are reported in Figures 1&2. Figure 1 demonstrates that oral leptin reduces the daily food intake by 55% as soon as the leptin treatment was initiated. Body weight show steady decreases when compared to vehicle-fed control animals already by the second day of treatment (Figure 2). Body weight continued to decrease as long as the treatment with oral leptin was maintained (Figure 2). These results are similar to those previously obtained on mice [20].