An Endoluminal Sleeve Induces Substantial Weight Loss and Normalizes Glucose Homeostasis in Rats with Diet-Induced Obesity

Introduction

The prevalence of obesity is increasing worldwide with profound health and economic consequences (1,2). Patients with obesity have substantially increased morbidity and mortality from obesity-related complications, including type 2 diabetes, cardiovascular disease, and several types of cancer (3,4,5). Although there are many therapeutic options for obesity, including pharmacologic, dietary, and behavioral approaches, the weight loss from these interventions is seldom durable. In contrast, surgical therapies commonly result in substantial and sustained weight loss, as well as improvement, and even resolution, of obesity-related comorbidities. Among the available gastrointestinal (GI) weight loss surgical procedures, Roux-en-Y gastric bypass (RYGB) is currently the operation of choice. Surgical reconstruction of the GI tract during RYGB likely alters mucosal exposure to ingested macronutrients so as to differentially modulate efferent gut signals that govern energy regulation and metabolism. The result is weight loss and improved metabolic control. The precise mechanisms of these effects are unknown, although evidence is accumulating that RYGB alters neuroendocrine function in the GI tract at multiple levels (6,7,8,9,10,11,12,13,14,15,16).

In recent years, several animal models, including ileal transposition, jejuno-ileal bypass, RYGB, and others, have been designed to investigate the physiologic effects of GI tract reconstruction during bariatric procedures (9,10,14,17,18,19,20,21,22,23). None of these models includes the entire complement of GI manipulations present in the RYGB, and some incorporate changes not present in the standard gastric bypass. Each surgical model is effective in reducing body weight and improving metabolism, although specific effects vary from model to model. These studies suggest that different components of RYGB contribute to its effects on energy balance and metabolism. To define the mechanisms by which RYGB regulates these processes, it is useful to consider this operation as comprising five GI manipulations or “components,” some combination of which results in the physiologic effects of the procedure: (1) isolation of the gastric cardia, (2) exclusion of the distal stomach from the alimentary flow, (3) exclusion of the proximal intestine from the alimentary flow, (4) exposure of the jejunum to partially digested nutrients, and (5) partial vagotomy (1). Identification of the component or combination of components responsible for the effects of RYGB on weight loss and metabolic control would help define the role of the GI tract in these functions and facilitate the development of more targeted and less invasive therapies.

To explore the relative contributions of selective components of RYGB to body weight and metabolic control, we developed an indwelling, endoluminal device that mimics two of them in isolation. The endoluminal sleeve (ELS) is a nutrient-impermeable, flexible tube designed to be anchored in the duodenal bulb and to extend into the proximal jejunum (1). After implantation, ingested nutrients pass from the pylorus directly into and through the lumen of the ELS, isolating the mucosa of the proximal intestine from the nutrient-rich chyme (exclusion of the proximal intestine). As the flow of nutrients exits from the distal end of the ELS, the mucosa of the proximal jejunum is exposed to nutrients that have not been modified by either pancreaticobiliary secretions or the digestive and absorptive functions of the proximal gut (exposure of the jejunum to partially digested nutrients). Early experience with a similar device in a swine model and in human patients suggests efficacy of this approach for the treatment of obesity and diabetes (24,25,26,27,28). To explore the effects of ELS implantation on body weight and metabolic function in detail, we developed a rat model of the ELS device and a procedure for its safe and stable implantation. In this paper, we report that ELS implantation induces substantial weight loss and improves metabolic function in rats with diet-induced obesity, suggesting an important role of exclusion of the proximal small intestine and enhanced jejunal delivery of partially digested nutrients in the mechanism of action of RYGB.

Methods and Procedures

Results

ELS induces weight loss in a rat model of diet-induced obesity

To determine the effect of the ELS on body weight, ELS devices were surgically implanted in the duodenum of obese Sprague-Dawley rats (1). A second group of animals underwent the same procedure omitting the ELS device (SO). Regardless of treatment (ELS or SO), all rodents exhibited an immediate postoperative loss of ∼15–20% of their preoperative body weight (2). By POW 2, ELS-treated animals weighed an average of 8% less than SO (583 ± 9.5 (ELS) vs. 616 ± 27 g (SO); P = 0.0001), a difference that increased to 20% (552 ± 12 (ELS) vs. 671 ± 27 g (SO); P = 0.001) by POW 7 (2). This difference persisted through the end of the experiment at POW 16 (P = 0.001; 2).

To investigate the mechanism of the observed weight loss, we measured daily caloric intake and fecal caloric content during POW 8. ELS treatment reduced daily caloric intake by 28% (63.7 ± 11.4 (ELS) vs. 88.8 ± 6.2 kcal/d (SO); P = 0.001; 3, left). Stool quantity and bowel function was similar in ELS-treated and SO animals (data not shown). Daily stool caloric content was similar in the two groups (1.88 ± 0.1 (ELS) vs. 1.49 ± 0.3 kcal/d (SO); NS; 3, right). Ninety-eight percent of ingested calories were absorbed by the GI tract in both ELS-treated and SO animals, and animals in both groups absorbed >99% of ingested fat.

Ghrelin, the most potent peripherally active orexigenic hormone identified to date, has been implicated in regulating premeal hunger and meal initiation (6,34,35). Fasting ghrelin is unchanged or decreased after RYGB-induced weight loss, suggesting that this operation blocks or bypasses a compensatory hormonal (ghrelin) response that would tend to promote weight regain (6). Fasting ghrelin levels were unchanged after ELS-induced weight loss (4).

ELS prevents acute weight gain in a rat model of diet-induced obesity

To determine whether the ELS can prevent acute diet-induced weight gain, ELS devices were implanted in 15 lean, OM rats. After postsurgical introduction of a HFD, a subgroup of ELS-treated rats (n = 8) failed to gain as much weight as the SO group, as demonstrated by a 12% lower body weight at POW 4 (P = 0.001 vs. SO; 5). Weekly fluoroscopy demonstrated that the ELS device was intact in all animals of this subgroup for the duration of the study. Necropsy confirmed maintenance of ELS position and patency. Interestingly, several ELS-treated rats (n = 7) exhibited initial weight loss followed by regain of weight to the same level as SO animals (5). Fluoroscopic evaluation during POW 2 revealed that the ELS had dislodged from the duodenal wall, migrated distally into the GI tract, and was eventually passed in the stool. Necropsy at the end of the study confirmed that the device had been extruded from in all animals in this subgroup. Presumably, the observed weight gain in these animals was due to ELS failure and early migration.

ELS improves glucose homeostasis through peripheral insulin sensitization

ELS treatment substantially decreased fasting blood glucose (78 ± 8 (ELS) vs. 90 ± 2 mg/dl (SO); P = 0.001; 6) and insulin levels (0.57 ± 0.06 (ELS) vs. 1.11 ± 0.13 ng/ml (SO); P = 0.02; 6). Calculation of the HOMA-IR demonstrated a 55% improvement of peripheral IR by ELS treatment (2.67 ± 0.46 (ELS) vs. 5.94 ± 0.80 (SO) HOMA-IR units; P = 0.025; 6). Consistent with this observation, β-cell activity, as predicted by HOMA-%B, was decreased 36% after ELS treatment (236.8 ± 25 (ELS) vs. 367.2 ± 33 HOMA-%B units (SO); P = 0.028; 6). ELS-treated rats also exhibited improved oral glucose tolerance, as evidenced by a 40% decrease in the AUC analysis of glucose excursion (317.8 ± 25 (ELS) vs. 522 ± 37 (SO) AUC units; P = 0.008; 7, inset). Although the acute insulin response was unchanged after oral glucose administration in ELS-treated animals (7), glucose-stimulated insulin levels were significantly decreased (P = 0.006; 7. Notably, insulin tolerance did not differ between ad libitum-fed ELS and SO rats (7.

ELS improves glucose homeostasis independent of intraluminal nutrient passage

To determine whether the passage of endoluminal nutrients is required for the effect of ELS on glucose homeostasis, ELS-treated and SO animals underwent IP glucose tolerance testing. IP administration of glucose bypasses GI absorption and, presumably, the secretion of incretins and other neuroendocrine signals induced by luminal nutrients. ELS-treated animals demonstrated dramatically improved IP glucose tolerance, with an overall 43% decrease in the AUC of glucose excursion (368 ± 62 (ELS) vs. 643 ± 111 mg/dl AUC units (SO); P = 0.027; 8, inset).

Discussion

Implantation of an ELS device in the duodenum and proximal jejunum mimics many of the effects of RYGB. In Sprague-Dawley rats with diet-induced obesity, ELS implantation causes modest weight loss and substantially improves glucose homeostasis. Presence of this device in the proximal intestine also prevents the development of diet-induced obesity in otherwise susceptible animals. Despite creating a barrier between ingested nutrients and the duodenal mucosa, the ELS does not cause significant malabsorption. Rather, the observed weight loss results primarily from reduced calorie intake. In addition, ELS implantation appears to improve glucose homeostasis by multiple mechanisms. Both fasting blood glucose and insulin levels decrease after this procedure, which is reflected in a 55% decrease in HOMA-IR and is consistent with significant improvement of basal peripheral IR. Because hepatic glucose output is the major determinant of the fasting glucose level, these data suggest that ELS implantation leads to decreased hepatic glucose output, likely as a result of improved insulin signaling in hepatocytes. Improved oral glucose tolerance with a concurrent decrease in glucose-stimulated insulin levels further underscores an overall improvement in peripheral insulin sensitivity. The similar effect of ELS implantation on IP glucose tolerance suggests a global improvement in glucose homeostasis that is independent of postprandial signals generated by endoluminal nutrient passage and provides additional evidence of overall improvement of insulin sensitivity. The improvement in glucose tolerance suggests that ELS implantation also enhances glucose disposal. We observed no effect of the device on insulin tolerance, which is largely a reflection of insulin-stimulated glucose disposal in muscle and adipose tissue. However, insulin tolerance testing is a relatively insensitive measure of peripheral insulin sensitivity, so the data are consistent with a modest effect of the device on glucose disposal in these tissues.

Although the ELS induces only about two-thirds as much weight loss as RYGB in the same strain of rats, the effect on glucose homeostasis is similar to that observed after RYGB (9,10,13,14,20,22). This observation suggests that manipulations of the small bowel leading to duodenal exclusion and enhanced delivery of partially digested nutrients to the jejunum are primary mechanisms by which RYGB alters glucose metabolism. In contrast, the more limited effect of ELS on body weight suggests that other components of RYGB (e.g., gastric manipulation and/or partial vagotomy) likely make a significant contribution to the weight loss induced by this procedure. The differential ability of ELS implantation to mimic some of the effects of RYGB on body weight and glucose homeostasis underscores the complexity of GI regulation of these metabolic functions. It suggests that different components of RYGB contribute to the myriad effects of this operation and that multiple regions of the GI tract participate in the normal regulation of metabolic function. By examining selected components of this complex operation in isolation, we have attempted to dissect the contributions of these manipulations to the regulation of body weight, food intake, and glucose homeostasis. The preferential effect of ELS on glucose homeostasis underscores the basic validity of this approach.

Of course, ELS is not merely a component of RYGB, and there are fundamental differences in the manner in which these two manipulations may affect GI anatomy and function. Although the ELS device prevents interaction between the nutrient stream and the mucosa of the proximal intestine, these regions of the gut are not completely bypassed as they are after RYGB. With ELS, nutrients pass through the lumen of the device and retain the opportunity to affect local mechanoreceptors and motor function. In addition, the presence of an endoluminal device in the duodenum (including the fixed anchor at the proximal end) may affect gastric emptying and gastric motility, two biological processes known to acutely regulate satiety (36,37). During fluoroscopic assessment of ELS position and patency, we have observed that gastric emptying appears somewhat slowed after implantation of this device (data not shown). Thus, a possible contribution of gastric motility effects to the observed weight loss, decreased food intake and improved glucose homeostasis cannot be excluded. Similarly, it is not known whether increased energy expenditure contributes to ELS-induced weight loss. We have observed that resting energy expenditure is increased after RYGB in rats, and pair-feeding studies suggest that this effect is a significant contributor to RYGB-induced weight loss (N. Stylopoulous and L.M. Kaplan, unpublished observations). Given the similar effects of ELS and RYGB on metabolic function, ELS implantation may also regulate energy expenditure.

Within the lumen of the proximal intestine, duodenal, pancreatic, and biliary secretions further digest nutrients previously emulsified in the stomach. The macromolecular components of this chyme are then capable of interacting with chemoreceptors and mechanoreceptors in the mucosa and bowel wall to generate neurohumoral signals of nutrient intake, composition, and availability (6,8,11,15,16,38,39). Candidate pathways mediating this communication include: (i) peptide hormones secreted by mucosal enteroendocrine cells; (ii) nonpeptide signals (e.g., lipids and carbohydrates) transported across the mucosal epithelium and released into the portal and lymphatic circulation; (iii) enteric neuronal circuits that communicate with other regions of the gut directly or via reflex arcs; and (iv) autonomic nervous pathways that communicate with the central nervous system. End-organ targets including other regions of the GI tract, the central nervous system (e.g., hypothalamus, brainstem, and reward centers), liver and pancreas normally coordinate a biological response to these gut-derived signals through the appropriate, compensatory regulation of energy intake, storage, and utilization (8,11,15,38,39). Implantation of the ELS device appears to alter the luminal microenvironment of the duodenum and proximal jejunum by segregating ingested nutrients both from luminal secretions and from the mucosa of the proximal intestine. Minimally digested nutrients and GI secretions are then delivered to the jejunum. As a result, nutrient binding to chemoreceptors of the proximal intestine is prevented, ingested nutrients passed beyond the region of the ELS can interact with regulatory chemoreceptors of the jejunum, and GI secretions untitrated by their usual substrates are exposed to the mucosa of the proximal small bowel. These alterations of the luminal environment likely modulate efferent gut signals to end-organ targets, thereby contributing to the observed effects of the device on feeding behavior, body weight, and glucose homeostasis.

The specific gut pathways activated in response to ELS-treatment are unknown. We and others have demonstrated that bypass surgery affects gut hormone secretion, particularly hormones implicated in the regulation of feeding behavior, satiety, glucose homeostasis, and the so-called “ileal brake” mechanism (9,10,17,19,20). Neutralizing antibodies to peptide YY reverse the inhibitory effects of jejuno-ileal bypass on food intake in thin rodents, suggesting a causal role for gut hormones in at least some of the effects of GI weight loss surgery (10). In addition, ghrelin, a potent peripherally active orexigenic hormone, has been implicated in a compensatory hormonal response to weight loss induced by food restriction or restrictive bariatric procedures (6). As with RYGB (6), ELS treatment does not lead to increased ghrelin levels. This finding suggests that weight loss induced by intestinal exclusion and/or accelerated jejunal delivery of ingested nutrients does not stimulate a compensatory stimulus to ghrelin secretion. To date, the roles of specific gut hormones in the physiological effects of RYGB or ELS have not been established. Because of the limited nature of ELS implantation, exploration of the relative effects of ELS and RYGB on hormonal and neuronal signaling from the gut will reveal specific contributions of the proximal small bowel to the regulation of appetite, body weight, and metabolic function.

In summary, exclusion of the proximal intestine from ingested nutrients and accelerated nutrient delivery to the jejunum induced by ELS mimic many of the effects of RYGB on body weight, food intake, and glucose homeostasis. The dramatic effects of ELS on glucose homeostasis suggest an important role for the duodenum and jejunum in the regulation of this metabolic function. The more limited effect of ELS implantation on food intake and body weight suggests that other components of RYGB, particularly manipulation of the stomach, likely provide a substantial contribution to GI regulation of body weight and ingestive behavior. The degree to which all of these regulatory effects are mediated through the pancreas, liver, or central nervous system remains unknown but is an exciting and interesting area for further study. In the long term, understanding these mechanisms will likely lead to new targets and approaches for the treatment of obesity and diabetes. More immediately, the efficacy of the ELS device in this rat model brightens the prospect that its endoscopically placed human analog will contribute to the treatment of these important disorders.

Disclosure

The authors declared no conflict of interest.