Bariatric procedures, such as Roux-en-Y gastric bypass (RYGB) or vertical sleeve gastrectomy (VSG), are the most effective approaches to resolve type 2 diabetes in obese individuals. Alimentary glucose absorption and intestinal disposal of blood glucose have not been directly compared between individuals or animals that underwent RYGB vs VSG. We evaluated in rats and humans how the gut epithelium adapts after surgery and the consequences on alimentary glucose absorption and intestinal disposal of blood glucose. Obese male rats underwent RYGB, VSG, or sham (control) operations. We collected intestine segments from all rats; we performed histologic analyses and measured levels of messenger RNAs encoding the sugar transporters SGLT1, GLUT1, GLUT2, GLUT3, GLUT4, and GLUT5. Glucose transport and consumption were assayed using ex vivo jejunal loops. Histologic analyses were also performed on Roux limb sections from patients who underwent RYGB 1-5 years after surgery. Roux limb glucose consumption was assayed after surgery by positron emission and computed tomography imaging. In rats and humans that underwent RYGB, the Roux limb became hyperplasic, with an increased number of incretin-producing cells compared with the corresponding jejunal segment of controls. Furthermore, expression of sugar transporters and hypoxia-related genes increased and the nonintestinal glucose transporter GLUT1 appeared at the basolateral membrane of enterocytes. Ingested and circulating glucose was trapped within the intestinal epithelial cells of rats and humans that underwent RYGB. By contrast, there was no hyperplasia of the intestine after VSG, but the intestinal absorption of alimentary glucose was reduced and density of endocrine cells secreting glucagon-like peptide-1 increased. The intestine adapts differently to RYGB vs VSG. RYGB increases intestinal glucose disposal and VSG delays glucose absorption; both contribute to observed improvements in glycemia.

Cloned 20 years ago, GLUT2 is a facilitative glucose transporter in the liver, pancreas, intestine, kidney, and brain. It ensures large bidirectional fluxes of glucose in and out the cell due to its low affinity and high capacity. It also transports other dietary sugars, such as fructose and galactose, within the range of physiological concentrations. Sugars and hormones regulate its gene expression. The contribution of GLUT2 to human metabolic diseases previously appeared modest. However, in the past decade, three major features of the GLUT2 protein have been revealed. First, GLUT2 mutations cause the severe but rare Fanconi-Bickel syndrome, mainly characterized by glycogenosis. Recently, a GLUT2 polymorphism has been associated with preferences for sugary food. Second, the GLUT2 location at the cell surface is regulated; this governs cellular activities dependent on glucose in the intestine and possibly those in the liver and pancreas. For instance, GLUT2 translocation from an intracellular pool to the apical membrane after a sugar meal transiently increases sugar uptake by enterocytes (reviewed in 32). Third, GLUT2 functions as a membrane receptor of sugar. Independently of glucose metabolism, GLUT2 detects the presence of extracellular sugar and transduces a signal to modulate cell functions, including β-cell insulin secretion, renal reabsorption, and intestinal absorption according to the sugar environment. These recent developments are examined here in heath and metabolic disease, highlighting various unanswered questions.

OBJECTIVE: In healthy rodents, intestinal sugar absorption in response to sugar-rich meals and insulin is regulated by GLUT2 in enterocyte plasma membranes. Loss of insulin action maintains apical GLUT2 location. In human enterocytes, apical GLUT2 location has not been reported but may be revealed under conditions of insulin resistance. RESEARCH DESIGN AND METHODS: Subcellular location of GLUT2 in jejunal enterocytes was analyzed by confocal and electron microscopy imaging and Western blot in 62 well-phenotyped morbidly obese subjects and 7 lean human subjects. GLUT2 locations were assayed in ob/ob and ob/+ mice receiving oral metformin or in high-fat low-carbohydrate diet–fed C57Bl/6 mice. Glucose absorption and secretion were respectively estimated by oral glucose tolerance test and secretion of [U-14C]-3-O-methyl glucose into lumen. RESULTS: In human enterocytes, GLUT2 was consistently located in basolateral membranes. Apical GLUT2 location was absent in lean subjects but was observed in 76% of obese subjects and correlated with insulin resistance and glycemia. In addition, intracellular accumulation of GLUT2 with early endosome antigen 1 (EEA1) was associated with reduced MGAT4a activity (glycosylation) in 39% of obese subjects on a low-carbohydrate/high-fat diet. Mice on a low-carbohydrate/high-fat diet for 12 months also exhibited endosomal GLUT2 accumulation and reduced glucose absorption. In ob/ob mice, metformin promoted apical GLUT2 and improved glucose homeostasis. Apical GLUT2 in fasting hyperglycemic ob/ob mice tripled glucose release into intestinal lumen. CONCLUSIONS: In morbidly obese insulin-resistant subjects, GLUT2 was accumulated in apical and/or endosomal membranes of enterocytes. Functionally, apical GLUT2 favored and endosomal GLUT2 reduced glucose transepithelial exchanges. Thus, altered GLUT2 locations in enterocytes are a sign of intestinal adaptations to human metabolic pathology.

Anchorage removal like growth factor removal induces apoptosis. In the present study we have characterized signaling pathways that can prevent this cell death using a highly growth factor– and anchorage-dependent line of lung fibroblasts (CCL39). After anchorage removal from exponentially growing cells, annexin V-FITC labeling can be detected after 8 h. Apoptosis was confirmed by analysis of sub-G1 DNA content and Western blotting of the caspase substrate poly (ADP-ribose) polymerase. Growth factor withdrawal accelerates and potentiates suspension-induced cell death. Activation of Raf-1 kinase in suspension cultures of CCL39 or Madin–Darby canine kidney cells stably expressing an estrogen-inducible activated–Raf-1 construct (ΔRaf-1:ER) suppresses apoptosis induced by growth factor and/or anchorage removal. This protective effect appears to be mediated by the Raf, mitogen- or extracellular signal–regulated kinase kinase (MEK), and mitogen-activated protein kinase module because it is sensitive to pharmacological inhibition of MEK-1 and it can be mimicked by expression of constitutively active MEK-1 in CCL39 cells. Finally, apoptosis induced by disruption of the actin cytoskeleton with the Rho-directed toxin B (Clostridium difficile) is prevented by activation of the ΔRaf-1:ER chimeric construct. These findings highlight the ability of p42/p44 mitogen-activated protein kinase to generate survival signals that counteract cell death induced by loss of matrix contact, cytoskeletal integrity, and extracellular mitogenic factors.