Evidence for non-neurotensin receptor-mediated effects of xenin (1-25)--focus on intestinal microcirculation

Minimum biological domain of xenin-25 required to induce anion secretion in the rat ileum

Xenin-25 has a variety of physiological functions in the gastrointestinal tract, including ion transport and motility. Xenin-25 and neurotensin show sequence homology, especially near their C-terminal regions. The sequence similarity between xenin-25 and neurotensin indicates that the effects of xenin-25 is mediated by the neurotensin receptor but some biological actions of xenin-25 are independent. We have previously reported that xenin-25 modulates intestinal ion transport and colonic smooth muscle activity. However, minimal biological domain of xenin-25 to induce ion transport was not clear. To improve the mechanistic understanding of xenin-25 and to gain additional insights into the functions of xenin-25, the present study was designed to determine the minimal biological domain of xenin-25 required for ion transport in the rat ileum using various truncated xenin fragments and analogues in an Ussing chamber system. The present results demonstrate that the minimum biological domain of xenin-25 to induce Cl-/HCO3- secretion in the ileum contains the C-terminal pentapeptide. Furthermore, Arg at position 21 is important to retain the biological activity of xenin-25 and induces Cl-/HCO3- secretion in the rat ileum.

Xenin and Related Peptides: Potential Therapeutic Role in Diabetes and Related Metabolic Disorders

Xenin bioactivity and its role in normal physiology has been investigated by several research groups since its discovery in 1992. The 25 amino acid peptide hormone is secreted from the same enteroendocrine K-cells as the incretin hormone glucose-dependent insulinotropic polypeptide (GIP), with early studies highlighting the biological significance of xenin in the gastrointestinal tract, along with effects on satiety. Recently there has been more focus directed towards the role of xenin in insulin secretion and potential for diabetes therapies, especially through its ability to potentiate the insulinotropic actions of GIP as well as utilisation in dual/triple acting gut hormone therapeutic approaches. Currently, there is a lack of clinically approved therapies aimed at restoring GIP bioactivity in type 2 diabetes mellitus, thus xenin could hold real promise as a diabetes therapy. The biological actions of xenin, including its ability to augment insulin secretion, induce satiety effects, as well as restoring GIP sensitivity, earmark this peptide as an attractive antidiabetic candidate. This minireview will focus on the multiple biological actions of xenin, together with its proposed mechanism of action and potential benefits for the treatment of metabolic diseases such as diabetes.

The effect of Xenin25 on spontaneous circular muscle contractions of rat distal colon in vitro

Xenin25 has a variety of physiological functions in the Gastrointestinal (GI) tract, including ion transport and motility. However, the motility responses in the colon induced by Xenin25 remain poorly understood. Therefore, the effect of Xenin25 on the spontaneous circular muscle contractions of the rat distal colon was investigated using organ bath chambers and immunohistochemistry. Xenin25 induced the inhibition followed by postinhibitory spontaneous contractions with a higher frequency in the rat distal colon. This inhibitory effect of Xenin25 was significantly suppressed by TTX but not by atropine. The inhibitory time (the duration of inhibition) caused by Xenin25 was shortened by the NTSR1 antagonist SR48692, the NK1R antagonist CP96345, the VPAC2 receptor antagonist PG99-465, the nitric oxide-sensitive guanylate-cyclase inhibitor ODQ, and the Ca2+ -dependent K+ channel blocker apamin. The higher frequency of postinhibitory spontaneous contractions induced by Xenin25 was also attenuated by ODQ and apamin. SP-, NOS-, and VIP-immunoreactive neurons were detected in the myenteric plexus (MP) of the rat distal colon. Small subsets of the SP-positive neurons were also Calbindin positive. Most of the VIP-positive neurons were also NOS positive, and small subsets of the NK1R-positive neurons were also VIP positive. Based on the present results, we propose the following mechanism. Xenin25 activates neuronal NTSR1 on the SP neurons of IPANs, and transmitters from the VIP and apamin-sensitive NO neurons synergistically inhibit the spontaneous circular muscle contractions via NK1R. Subsequently, the postinhibitory spontaneous contractions are induced by the offset of apamin-sensitive NO neuron activation via the interstitial cells of Cajal. In addition, Xenin25 also activates the muscular NTSR1 to induce relaxation. Thus, Xenin25 is considered to be an important modulator of post prandial circular muscle contraction of distal colon since the release of Xenin25 from enteroendocrine cells is stimulated by food intake.

Xenin-25 induces anion secretion by activating noncholinergic secretomotor neurons in the rat ileum

Xenin-25 is a neurotensin-like peptide that is secreted by enteroendocrine cells in the small intestine. Xenin-8 is reported to augment duodenal anion secretion by activating afferent neural pathways. The intrinsic neuronal circuits mediating the xenin-25-induced anion secretion were characterized using the Ussing-chambered, mucosa-submucosa preparation from the rat ileum. Serosal application of xenin-25 increased the short-circuit current in a concentration-dependent manner. The responses were abolished by the combination of Cl^--free and HCO3- -free solutions. The responses were almost completely blocked by TTX (10^-6 M) but not by atropine (10^-5 M) or hexamethonium (10^-4 M). The selective antagonists for neurotensin receptor 1 (NTSR1), neurokinin 1 (NK1), vasoactive intestinal polypeptide (VIP) receptors 1 and 2 (VPAC1 and VPAC2, respectively), and capsaicin, but not 5-hydroxyltryptamine receptors 3 and 4 (5-HT₃ and 5-HT₄), NTSR2, and A803467, inhibited the responses to xenin-25. The expression of VIP receptors (Vipr) in rat ileum was examined using RT-PCR. The Vipr1 PCR products were detected in the submucosal plexus and mucosa. Immunohistochemical staining showed the colocalization of NTSR1 and NK1 with substance P (SP)- and calbindin-immunoreactive neurons in the submucosal plexus, respectively. In addition, NK1 was colocalized with noncholinergic VIP secretomotor neurons. Based on the results from the present study, xenin-25-induced Cl^-/ HCO3- secretion is involved in NTSR1 activation on intrinsic and extrinsic afferent neurons, followed by the release of SP and subsequent activation of NK1 expressed on noncholinergic VIP secretomotor neurons. Finally, the secreted VIP may activate VPAC1 on epithelial cells to induce Cl^-/ HCO3- secretion in the rat ileum. Activation of noncholinergic VIP secretomotor neurons by intrinsic primary afferent neurons and extrinsic afferent neurons by postprandially released xenin-25 may account for most of the neurogenic secretory response induced by xenin-25. NEW & NOTEWORTHY This study is the first to investigate the intrinsic neuronal circuit responsible for xenin-25-induced anion secretion in the rat small intestine. We have found that nutrient-stimulated xenin-25 release may activate noncholinergic vasoactive intestinal polypeptide (VIP) secretomotor neurons to promote Cl^-/ HCO3- secretion through the activation of VIP receptor 1 on epithelial cells. Moreover, the xenin-25-induced secretory responses are mainly linked with intrinsic primary afferent neurons, which are involved in the activation of neurotensin receptor 1 and neurokinin 1 receptor.

Emerging therapeutic potential for xenin and related peptides in obesity and diabetes

Xenin-25 is a 25-amino acid peptide hormone co-secreted from the same enteroendocrine K-cell as the incretin peptide glucose-dependent insulinotropic polypeptide. There is no known specific receptor for xenin-25, but studies suggest that at least some biological actions may be mediated through interaction with the neurotensin receptor. Original investigation into the physiological significance of xenin-25 focussed on effects related to gastrointestinal transit and satiety. However, xenin-25 has been demonstrated in pancreatic islets and recently shown to possess actions in relation to the regulation of insulin and glucagon secretion, as well as promoting beta-cell survival. Accordingly, the beneficial impact of xenin-25, and related analogues, has been assessed in animal models of diabetes-obesity. In addition, studies have demonstrated that metabolically active fragment peptides of xenin-25, particularly xenin-8, possess independent therapeutic promise for diabetes, as well as serving as bioactive components for the generation of multi-acting hybrid peptides with antidiabetic potential. This review focuses on continuing developments with xenin compounds in relation to new therapeutic approaches for diabetes-obesity.

Locally produced xenin and the neurotensinergic system in pancreatic islet function and β-cell survival

Modulation of neuropeptide receptors is important for pancreatic β-cell function. Here, islet distribution and effects of the neurotensin (NT) receptor modulators, xenin and NT, was examined. Xenin, but not NT, significantly improved glucose disposal and insulin secretion, in mice. However, both peptides stimulated insulin secretion from rodent β-cells at 5.6 mmglucose, with xenin having similar insulinotropic actions at 16.7 mmglucose. In contrast, NT inhibited glucose-induced insulin secretion. Similar observations were made in human 1.1B4 β-cells and isolated mouse islets. Interestingly, similar xenin levels were recorded in pancreatic and small intestinal tissue. Arginine and glucose stimulated xenin release from islets. Streptozotocin treatment decreased and hydrocortisone treatment increased β-cell mass in mice. Xenin co-localisation with glucagon was increased by streptozotocin, but unaltered in hydrocortisone mice. This corresponded to elevated plasma xenin levels in streptozotocin mice. In addition, co-localisation of xenin with insulin was increased by hydrocortisone, and decreased by streptozotocin. Furtherin vitroinvestigations revealed that xenin and NT protected β-cells against streptozotocin-induced cytotoxicity. Xenin augmented rodent and human β-cell proliferation, whereas NT displayed proliferative actions only in human β-cells. These data highlight the involvement of NT signalling pathways for the possible modulation of β-cell function.

Neurotensin and its receptors in the control of glucose homeostasis

The pharmacological roles of the neuropeptide neurotensin through its three known receptors are various and complex. Neurotensin is involved in several important biological functions including analgesia and hypothermia in the central nervous system and also food intake and glucose homeostasis in the periphery. This review focuses on recent works dealing with molecular mechanisms regulating blood glucose level and insulin secretion upon neurotensin action. Investigations on crucial cellular components involved in the protective effect of the peptide on beta cells are also detailed. The role of xenin, a neurotensin-related peptide, on the regulation of insulin release by glucose-dependent insulinotropic polypeptide is summarized. The last section comments on the future research areas which should be developed to address the function of new effectors of the neurotensinergic system in the endocrine pancreas.

Glucose-dependent insulinotropic polypeptide: from pathophysiology to therapeutic opportunities in obesity-associated disorders

Glucose-dependent insulinotropic polypeptide (GIP) is a hormone secreted from the intestinal K-cells with established insulin-releasing actions. However, the GIP receptor is widely distributed in peripheral organs, including the adipose tissue, gut, bone and brain, where GIP modulates energy intake, cell metabolism and proliferation, and lipid and glucose metabolism, eventually promoting lipid and glucose storage. In diabetes and obesity, the incretin effect of GIP is blunted, while the extrapancreatic tissues keep a normal sensitivity to this hormone. As GIP levels are normal or elevated in obesity and diabetes, mounting evidence from chemical or genetic GIP deletion in animal models of obesity-related diabetes suggests that GIP may have a pro-obesogenic action and that a strategy antagonizing GIP action may be beneficial in these conditions, clearing triglyceride deposits from adipose tissue, liver and muscle, and restoring normal insulin sensitivity. Emerging evidence also suggests that the metabolic benefits of bypass surgery are mediated, at least in part, by surgical removal of GIP-secreting K-cells in the upper small intestine.

Targeted ablation of glucose-dependent insulinotropic polypeptide-producing cells in transgenic mice reduces obesity and insulin resistance induced by a high fat diet

The K cell is a specific sub-type of enteroendocrine cell located in the proximal small intestine that produces glucose-dependent insulinotropic polypeptide (GIP), xenin, and potentially other unknown hormones. Because GIP promotes weight gain and insulin resistance, reducing hormone release from K cells could lead to weight loss and increased insulin sensitivity. However, the consequences of coordinately reducing circulating levels of all K cell-derived hormones are unknown. To reduce the number of functioning K cells, regulatory elements from the rat GIP promoter/gene were used to express an attenuated diphtheria toxin A chain in transgenic mice. K cell number, GIP transcripts, and plasma GIP levels were profoundly reduced in the GIP/DT transgenic mice. Other enteroendocrine cell types were not ablated. Food intake, body weight, and blood glucose levels in response to insulin or intraperitoneal glucose were similar in control and GIP/DT mice fed standard chow. In contrast to single or double incretin receptor knock-out mice, the incretin response was absent in GIP/DT animals suggesting K cells produce GIP plus an additional incretin hormone. Following high fat feeding for 21-35 weeks, the incretin response was partially restored in GIP/DT mice. Transgenic versus wild-type mice demonstrated significantly reduced body weight (25%), plasma leptin levels (77%), and daily food intake (16%) plus enhanced energy expenditure (10%) and insulin sensitivity. Regardless of diet, long term glucose homeostasis was not grossly perturbed in the transgenic animals. In conclusion, studies using GIP/DT mice demonstrate an important role for K cells in the regulation of body weight and insulin sensitivity.

Stimulatory effect of xenin-8 on insulin and glucagon secretion in the perfused rat pancreas

Xenin is a 25-amino acid peptide of the neurotensin/xenopsin family identified in gastric mucosa as well as in a number of tissues, including the pancreas of various mammals. In healthy subjects, plasma xenin immunoreactivity increases after meals. Infusion of the synthetic peptide in dogs evokes a rise in plasma insulin and glucagon levels and stimulates exocrine pancreatic secretion. The latter effect has also been demonstrated for xenin-8, the C-terminal octapeptide of xenin. We have investigated the effect of xenin-8 on insulin, glucagon and somatostatin secretion in the perfused rat pancreas. Xenin-8 stimulated basal insulin secretion and potentiated the insulin response to glucose in a dose-dependent manner (EC(50)=0.16 nM; R(2)=0.9955). Arginine-induced insulin release was also augmented by xenin-8 (by 40%; p<0.05). Xenin-8 potentiated the glucagon responses to both arginine (by 60%; p<0.05) and carbachol (by 50%; p<0.05) and counteracted the inhibition of glucagon release induced by increasing the glucose concentration. No effect of xenin-8 on somatostatin output was observed. Our observations indicate that the reported increases in plasma insulin and glucagon levels induced by xenin represent a direct influence of this peptide on the pancreatic B and A cells.