Stimulation of incretin secretion by dietary lipid: is it dose dependent?

After the ingestion of nutrients, secretion of the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) by the enteroendocrine cells increases rapidly. Previous studies have shown that oral ingestion of fat stimulates secretion of both incretins; however, it is unclear whether there is a dose-dependent relationship between the amount of lipid ingested and the secretion of the hormones in vivo. Recently, we found a higher concentration of the incretin hormones in intestinal lymph than in peripheral or portal plasma. We therefore used the lymph fistula rat model to test for a dose-dependent relationship between the secretion of GIP and GLP-1 and dietary lipid. Under isoflurane anesthesia, the major mesenteric lymphatic duct of male Sprague-Dawley rats was cannulated. Each animal received a single, intraduodenal bolus of saline or varying amounts of the fat emulsion Liposyn II (0.275, 0.55, 1.1, 2.2, and 4.4 kcal). Lymph was continuously collected for 3 h and analyzed for triglyceride, GIP, and GLP-1 content. In response to increasing lipid calories, secretion of triglyceride, GIP, and GLP-1 into lymph increased dose dependently. Interestingly, the response to changes in intraluminal lipid content was greater in GLP-1- than in GIP-secreting cells. The different sensitivities of the two cell types to changes in intestinal lipid support the concept that separate mechanisms may underlie lipid-induced GIP and GLP-1 secretion. Furthermore, we speculate that the increased sensitivity of GLP-1 to intestinal lipid content reflects the hormone's role in the ileal brake reflex. As lipid reaches the distal portion of the gut, GLP-1 is secreted in a dose-dependent manner to reduce intestinal motility and enhance proximal fat absorption.

A cell-based approach for diabetes treatment using engineered non-beta cells

BACKGROUND

Implantation of insulin-secreting cells has the potential to provide tight glycemic regulation in diabetic subjects. Implantation of cadaveric human islets in immunosuppressed human patients is currently applied at a very small scale. To overcome the limitations of tissue availability and recipient immunosuppression, encapsulation of nonautologous cells and use of potentially autologous nonislet cells, the latter engineered for insulin secretion, are being pursued. This article reports on recent findings with the implantation of tissue constructs containing enteroendocrine cells stably expressing recombinant insulin in diabetic mice. The concept of a dual recombinant hepatic and enteroendocrine cell system, which may better approximate the secretory response of islets, is discussed.

METHODS

Mouse GLUTag-INS cells engineered to secrete human insulin were developed and incorporated in tissue constructs as reported previously. Constructs were implanted intraperitoneally in diabetic mice, and blood glucose levels, animal weights, and plasma insulin levels were measured at various time points.

RESULTS

GLUTag-INS-containing tissue constructs secreted insulin preimplantation and postexplantation, and human insulin was detected in the plasma of diabetic mice. However, normoglycemia was not restored.

CONCLUSIONS

A variety of cell types and of encapsulation methods to enhance immune acceptance of insulin-secreting grafts are being pursued. Recombinant enteroendocrine cells show promise, but it is likely that they need to be combined with recombinant hepatic cells to achieve glycemic normalization.

Comparison of immunoreactivity to serotonin, FMRFamide and SCPb in the gut and visceral nervous system of larvae, pupae and adults of the yellow fever mosquito Aedes aegypti

In all life stages, the gut of the mosquito is innervated by a small number (typically 4) of central neurons immunoreactive to serotonin (SI). The serotonergic system appears to pass through metamorphosis largely intact, despite extensive remodeling of the gut. Axons immunoreactive to antibodies raised against molluscan FMRFamide (RF-I) constitute peptidergic innervation that anatomically parallels the serotonergic system. In the larva, two clusters of 3 neurons project to the anterior regions of the gut, whereas in the pupa and adult, typically two large RF-I neurons located next to the esophagus send several processes posteriorly. In adults, these neurons branch throughout the diverticula and anterior stomach. In pupae, but not in larvae or adults, the gut RF-l system coexpresses reactivity to antibodies raised against a member of another peptide family, molluscan small cardioactive peptide b (SCP-I). SCP-I immunoreactivity is localized independently of RF-l immunoreactivity in the ganglia of all stages and in neurons that project along the gut of the adult. We did not find any colocalization of S-I and the peptide markers. Distinct populations of enteroendocrine cells populate different regions of the gut at different life stages. Changes in staining pattern suggest that these cells are replaced at metamorphosis along with the other gut cells during the extensive remodeling of the tract. Distributed in the gut epithelium are subpopulations that express either RF-I or SCP-I; a small fraction of these cells bind antibodies to both peptides. The stomachs of adult females are larger than those of males, and the numbers of SCP-I and RF-I enteroendocrine cells are proportionately greater in females. In all the life stages, the junctions between different regions of the gut are the focus of regulatory input. The larval cardiac valve possesses a ring of cells, the necklace cells, which appear to receive extensive synaptic inputs from both the serotonergic system and the peptidergic system. Another focus of control is the pyloric valve, which is encircled by axon-like processes. The immunoreactive pattern of this region differs across life stages, expressing SCP-I in larvae, S-I in pupae, and both SCP-I and RF-I in adults.

Comparison of the properties of the five soluble guanylyl cyclase subunits in Drosophila melanogaster

The Drosophila melanogaster genome contains 5 genes that code for soluble guanylyl cyclase subunits. Two of these genes code for subunits, Gycalpha-99B and Gycbeta-100B, which form a conventional NO-sensitive guanylyl cyclase and the other three code for atypical subunits, Gyc-88E, Gyc-89Da and Gyc-89Db. The properties and distribution of Gyc-88E and Gyc-89Db have previously been described and here Gyc-89Da is described. Gyc-89Da only forms an active guanylyl cyclase when co-expressed with Gyc-88E. The three atypical subunits probably form two different heterodimers in vivo: Gyc-88E/89Da and Gyc-88E/89Db. Both of these heterodimers were slightly stimulated by NO donors and Gyc-88E/89Da showed a greater activation by Mn2+, with an increase in Vmax and a decrease in K(m), compared to Gyc-88E/89Db. Both Gyc-88E/89Da and Gyc-88E/89Db were expressed in neurons in both the peripheral and central nervous system. Although all three heterodimeric soluble guanylyl cyclases in D. melanogaster can be activated by NO and inhibited by ODQ, the atypical enzymes can be distinguished from the conventional soluble guanylyl cyclase by their sensitivity to the NO-independent activators YC-1 and BAY 41-2272, which will only activate the conventional enzyme.

Autocrine growth inhibition by transforming growth factor beta-1 (TGFbeta-1) in human neuroendocrine tumour cells

BACKGROUND AND AIM

The role of transforming growth factor beta-1 (TGFbeta-1) in neuroendocrine tumour biology is currently unknown. We therefore examined the expression and biological significance of TGFbeta signalling components in neuroendocrine tumours (NETs) of the gastroenteropancreatic (GEP) tract.

METHODS

Expression of TGFbeta-1 and its receptors, Smads and Smad regulated proteins, was examined in surgically resected NET specimens and human NET cell lines by immunohistochemistry, reverse transcriptase-polymerase chain reaction, immunoblotting, and ELISA. Activation of TGFbeta-1 dependent promoters was tested by transactivation assays. Growth regulation was evaluated by cell numbers, soft agar assays, and cell cycle analysis using flow cytometry. The role of endogenous TGFbeta was assessed by a TGFbeta neutralising antibody and stable transfection of a dominant negative TGFbetaR II receptor construct.

RESULTS

Coexpression of TGFbeta-1 and its receptors TGFbetaR I and TGFbetaR II was detected in 67% of human NETs and in all three NET cell lines examined. NET cell lines expressed the TGFbeta signal transducers Smad 2, 3, and 4. In two of the three cell lines, TGFbeta-1 treatment resulted in transactivation of a TGFbeta responsive reporter construct as well as inhibition of c-myc and induction of p21((WAF1)) expression. TGFbeta-1 inhibited anchorage dependent and independent growth in a time and dose dependent manner in TGFbeta-1 responsive cell lines. TGFbeta-1 mediated growth inhibition was due to G1 arrest without evidence of induction of apoptosis. Functional inactivation of endogenous TGFbeta revealed the existence of an autocrine antiproliferative loop in NET cells.

CONCLUSIONS

Neuroendocrine tumour cells of the gastroenteropancreatic tract are subject to paracrine and autocrine growth inhibition by TGFbeta-1, which may account in part for the low proliferative index of this tumour entity.

Neurogenin 3 is essential for the proper specification of gastric enteroendocrine cells and the maintenance of gastric epithelial cell identity

The notch signaling pathway is essential for the endocrine cell fate in various tissues including the enteroendocrine system of the gastrointestinal tract. Enteroendocrine cells are one of the four major cell types found in the gastric epithelium of the glandular stomach. To understand the molecular basis of enteroendocrine cell development, we have used gene targeting in mouse embryonic stem cells to derive an EGFP-marked null allele of the bHLH transcription factor, neurogenin 3 (ngn3). In ngn3(-/-) mice, glucagon secreting A-cells, somatostatin secreting D-cells, and gastrin secreting G-cells are absent from the epithelium of the glandular stomach, whereas the number of serotonin-expressing enterochromaffin (EC) cells is decreased dramatically. In addition, ngn3(-/-) mice display intestinal metaplasia of the gastric epithelium. Thus, ngn3 is required for the differentiation of enteroendocrine cells in the stomach and the maintenance of gastric epithelial cell identity.

Postnatal development of intestinal endocrine cell populations in the water buffalo

The frequency and distribution of 11 endocrine cell populations were studied in the intestine of differently aged buffalo, grouped on the basis of diet: 2-d-olds (suckling), 5-mo-olds (weaning) and 5-y-olds (ruminant adult diet). The endocrine cell populations were identified immunocytochemically using antisera against 5-hydroxytryptamine (5-HT), somatostatin, gastrin, cholecystokinin (CCK), COOH-terminal octapeptide of gastrin/CCK, neurotensin, motilin, gastric inhibitory polypeptide (GIP), secretin, glucagon/glicentin (GLU/GLI) and polypeptide YY (PYY). In adult buffalos the regional distribution of endocrine cells is similar to that of other adult ruminants. During postnatal development, these cell types showed the following changes in their frequency and distribution: (1) 5-HT, neurotensin and gastrin/CCK immunoreactive cells (i.c.) showed a decrease in frequency with age; (2) somatostatin i.c. frequency remained stable with age; (3) motilin, GIP, secretin and CCK i.c. showed a slight increase in frequency with age; (4) GLU/GLI and PYY i.c. decreased in frequency with age in the small intestine, caecum and proximal colon and an increase in frequency in the rectum. It was hypothesised that the endocrine cell types, whose presence and localisation is substantially stable in all examined ages, probably contain substances that are strictly necessary for intestinal function. In contrast the hormones contained in the cell populations that decreased with age, are probably involved in physiological needs during the milk and weaning diet or play a role in intestinal growth.