• A novel chemically modified analogue of xenin-25 exhibits improved glucose-lowering and insulin-releasing properties.

      Parthsarathy, Vadivel; Irwin, Nigel; Hasib, Annie; Martin, Christine M.; McClean, Stephen; Bhat, Vikas K.; NG, Ming T.; Flatt, Peter R.; Gault, Victor A. (Elsevier, 2016-01-21)
      BACKGROUND: Xenin-25 is a K-cell derived gut peptide with insulin-releasing activity which is rapidly degraded following release into the circulation. We hypothesized that substitution of all naturally-occurring Lys and Arg residues with Gln would lead to prolonged enzyme resistance and enhanced biological efficacy.METHODS: Peptide stability was assessed using murine plasma, in vitro insulin-releasing actions evaluated in BRIN-BD11 cells and acute glucose-lowering and insulin-releasing actions examined in high fat fed mice. For sub-chronic studies, a range of metabolic parameters and pancreatic histology were assessed in high fat fed mice which had received saline vehicle or xenin-25(gln) twice-daily for 21days.RESULTS: In contrast to native xenin-25, xenin-25(gln) was resistant to plasma-mediated degradation and significantly stimulated insulin secretion in BRIN-BD11 cells. Acute administration of xenin-25(gln) in high fat fed mice significantly reduced blood glucose and increased plasma insulin concentrations. Twice-daily administration of xenin-25(gln) in high fat fed mice did not affect food intake, body weight or circulating insulin concentrations but significantly decreased blood glucose from day 9 onwards. Furthermore, glucose tolerance, glucose-mediated insulin secretion, insulin sensitivity and GIP-stimulated insulin-release were significantly enhanced in xenin-25(gln)-treated mice. Pancreatic immunohistochemistry revealed decreased alpha cell area with increased beta cell area and beta-to-alpha cell ratio in xenin-25(gln)-treated mice. In addition, xenin-25(gln) exerted similar beneficial actions in ob/ob mice as demonstrated by reduced blood glucose, superior glycaemic response and glucose-mediated insulin release.CONCLUSIONS: Xenin-25(gln) is resistant to plasma-mediated degradation and exerts sustained and beneficial metabolic actions in high fat fed and ob/ob mice.GENERAL SIGNIFICANCE: Glutamine (gln)-modified analogues of xenin may represent an attractive therapeutic approach for type 2 diabetes.
    • An enzymatically stable GIP/xenin hybrid peptide restores GIP sensitivity, enhances beta cell function and improves glucose homeostasis in high-fat-fed mice

      Hasib, Annie; Ng, Tony; Gault, Victor A.; Khan, Dawood; Parthsarathy, Vadivel; Flatt, Peter; Irwin, Nigel (Springer, 2017-03-01)
      AIMS/HYPOTHESIS: Glucose-dependent insulinotropic polypeptide (GIP) and xenin, regulatory gut hormones secreted from enteroendocrine K cells, exert important effects on metabolism. In addition, xenin potentiates the biological actions of GIP. The present study assessed the actions and therapeutic utility of a (DAla2)GIP/xenin-8-Gln hybrid peptide, in comparison with the parent peptides (DAla2)GIP and xenin-8-Gln.METHODS: Following confirmation of enzymatic stability, insulin secretory activity of (DAla2)GIP/xenin-8-Gln was assessed in BRIN-BD11 beta cells. Acute and persistent glucose-lowering and insulin-releasing effects were then examined in vivo. Finally, the metabolic benefits of twice daily injection of (DAla2)GIP/xenin-8-Gln was determined in high-fat-fed mice.RESULTS: All peptides significantly (p < 0.05 to p < 0.001) enhanced in vitro insulin secretion from pancreatic clonal BRIN-BD11 cells, with xenin (and particularly GIP)-related signalling pathways, being important for this action. Administration of (DAla2)GIP or (DAla2)GIP/xenin-8-Gln in combination with glucose significantly (p < 0.05) lowered blood glucose and increased plasma insulin in mice, with a protracted response of up to 4 h. All treatments elicited appetite-suppressive effects (p < 0.05), particularly (DAla2)GIP/xenin-8-Gln and xenin-8-Gln at elevated doses of 250 nmol/kg. Twice-daily administration of (DAla2)GIP/xenin-8-Gln or (DAla2)GIP for 21 days to high-fat-fed mice returned circulating blood glucose to lean control levels. In addition, (DAla2)GIP/xenin-8-Gln treatment significantly (p < 0.05) reduced glycaemic levels during a 24 h glucose profile assessment. Neither of the treatment regimens had an effect on body weight, energy intake or circulating insulin concentrations. However, insulin sensitivity was significantly (p < 0.001) improved by both treatments. Interestingly, GIP-mediated glucose-lowering (p < 0.05) and insulin-releasing (p < 0.05 to p < 0.01) effects were substantially improved by (DAla2)GIP and (DAla2)GIP/xenin-8-Gln treatment. Pancreatic islet and beta cell area (p < 0.001), as well as pancreatic insulin content (p < 0.05), were augmented in (DAla2)GIP/xenin-8-Gln-treated mice, related to enhanced proliferation and decreased apoptosis of beta cells, whereas (DAla2)GIP evoked increases (p < 0.05 to p < 0.01) in islet number.CONCLUSIONS/INTERPRETATION: These studies highlight the clear potential of GIP/xenin hybrids for the treatment of type 2 diabetes.
    • Biological Activity and Antidiabetic Potential of C-Terminal Octapeptide Fragments of the Gut-Derived Hormone Xenin

      Martin, Christine M.; Parthsarathy, Vadivel; Hasib, Annie; NG, Ming T.; McClean, Stephen; Flatt, Peter R.; Gault, Victor A.; Irwin, Nigel (Public Library of Science (PLoS), 2016-03-31)
      Xenin is a peptide that is co-secreted with the incretin hormone, glucose-dependent insulinotropic polypeptide (GIP), from intestinal K-cells in response to feeding. Studies demonstrate that xenin has appetite suppressive effects and modulates glucose-induced insulin secretion. The present study was undertaken to determine the bioactivity and antidiabetic properties of two C-terminal fragment xenin peptides, namely xenin 18-25 and xenin 18-25 Gln. In BRIN-BD11 cells, both xenin fragment peptides concentration-dependently stimulated insulin secretion, with similar efficacy as the parent peptide. Neither fragment peptide had any effect on acute feeding behaviour at elevated doses of 500 nmol/kg bw. When administered together with glucose to normal mice at 25 nmol/kg bw, the overall insulin secretory effect was significantly enhanced in both xenin 18-25 and xenin 18-25 Gln treated mice, with better moderation of blood glucose levels. Twice daily administration of xenin 18-25 or xenin 18-25 Gln for 21 days in high fat fed mice did not affect energy intake, body weight, circulating blood glucose or body fat stores. However, circulating plasma insulin concentrations had a tendency to be elevated, particularly in xenin 18-25 Gln mice. Both treatment regimens significantly improved insulin sensitivity by the end of the treatment period. In addition, sustained treatment with xenin 18-25 Gln significantly reduced the overall glycaemic excursion and augmented the insulinotropic response to an exogenous glucose challenge on day 21. In harmony with this, GIP-mediated glucose-lowering and insulin-releasing effects were substantially improved by twice daily xenin 18-25 Gln treatment. Overall, these data provide evidence that C-terminal octapeptide fragments of xenin, such as xenin 18-25 Gln, have potential therapeutic utility for type 2 diabetes.
    • Xenin-25[Lys(13)PAL]: a novel long-acting acylated analogue of xenin-25 with promising antidiabetic potential

      Gault, Victor A.; Martin, Christine M.; Flatt, Peter R.; Parthsarathy, Vadivel; Irwin, Nigel (Springer, 2015-06-01)
      AIMS: Xenin-25 is co-secreted with glucose-dependent insulinotropic polypeptide (GIP) from intestinal K-cells following a meal. Xenin-25 is believed to play a key role in glucose homoeostasis and potentiate the insulinotropic effect of GIP.METHODS: This study investigated the effects of sub-chronic administration of the stable and longer-acting xenin-25 analogue, xenin-25[Lys(13)PAL] (25 nmol/kg), in diabetic mice fed with a high-fat diet.RESULTS: Initial studies confirmed the significant persistent glucose-lowering (p < 0.05) and insulin-releasing (p < 0.05) actions of xenin-25[Lys(13)PAL] compared with native xenin-25. Interestingly, xenin-25 retained significant glucose-lowering activity in GIP receptor knockout mice. Twice-daily intraperitoneal (i.p.) injection of xenin-25[Lys(13)PAL] for 14 days had no significant effect on food intake or body weight in high-fat-fed mice. Non-fasting glucose and insulin levels were also unchanged, but overall glucose levels during an i.p. glucose tolerance and oral nutrient challenge were significantly (p < 0.05) lowered by xenin-25[Lys(13)PAL] treatment. These changes were accompanied by significant improvements in i.p. (p < 0.05) and oral (p < 0.001) nutrient-stimulated insulin concentrations. No appreciable changes in insulin sensitivity were observed between xenin-25[Lys(13)PAL] and saline-treated high-fat mice. However, xenin-25[Lys(13)PAL] treatment restored notable sensitivity to the biological actions of exogenous GIP injection. Consumption of O2, production of CO2, respiratory exchange ratio and energy expenditure were not altered by 14-day twice-daily treatment with xenin-25[Lys(13)PAL]. In contrast, ambulatory activity was significantly (p < 0.05 to p < 0.001) increased during the dark phase in xenin-25[Lys(13)PAL] mice compared with high-fat controls.