Xenin: the oldest after insulin?

Effects of xenin-25 on insulin and glucagon secretions in healthy conscious sheep

The aim of present study was to determine effect of an intravenous injection of xenin-25 on insulin and glucagon secretion in healthy conscious sheep. After feeding once at 17:00, the experiment was started from 9:00 on the next day. Xenin-25 was intravenously (i.v.) injected at a dose of 100 to 1000 pmol/kg with and without the simultaneous injection of glucose at a dose of 200 μmol/kg, and blood was withdrawn before and after the injections. A single xenin-25 injection at 100 and 300 pmol/kg significantly increased the plasma insulin concentration, whereas the 1000 pmol/kg dose did not elicit significantly enhanced insulin response. Plasma glucose and glucagon concentrations did not significantly change after a single xenin-25 injection. Xenin-25 injection significantly and dose-dependently augmented the glucose-induced insulin secretion. However, the changes in the plasma glucose and glucagon level after the glucose injection were not altered by xenin injection. A prior intravenous injection of the neurotensin receptor subtype-1 (NTR-1) antagonist SR 48692 at 100 nmol/kg did not modify the glucose-induced change in plasma insulin caused by xenin-25 at 300 pmol/kg, and intravenous injection of the NTR-2 agonist levocabastine at 1000 pmol/kg did not augment the insulin response to the glucose injection. On the other hand, no xenin-25 immunopositive cells were detected in the ovine pancreas. The mRNAs of the three NTR subtypes were highly expressed in the ovine pancreas in comparison with the expression in the abomasum. These results suggest that xenin-25 released from the upper gastrointestinal tract plays a role of an insulinotropic factor in sheep, possibly through NTRs in the pancreatic islets, but not via NTR-2.

Mitochondrial Dysfunction and Chronic Inflammation in Polycystic Ovary Syndrome

Polycystic ovarian syndrome (PCOS) is the most common endocrine-metabolic disorder affecting a vast population worldwide; it is linked with anovulation, mitochondrial dysfunctions and hormonal disbalance. Mutations in mtDNA have been identified in PCOS patients and likely play an important role in PCOS aetiology and pathogenesis; however, their causative role in PCOS development requires further investigation. As a low-grade chronic inflammation disease, PCOS patients have permanently elevated levels of inflammatory markers (TNF-α, CRP, IL-6, IL-8, IL-18). In this review, we summarise recent data regarding the role of mtDNA mutations and mitochondrial malfunctions in PCOS pathogenesis. Furthermore, we discuss recent papers dedicated to the identification of novel biomarkers for early PCOS diagnosis. Finally, traditional and new mitochondria-targeted treatments are discussed. This review intends to emphasise the key role of oxidative stress and chronic inflammation in PCOS pathogenesis; however, the exact molecular mechanism is mostly unknown and requires further investigation.

Enzymatically stable analogue of the gut-derived peptide xenin on beta-cell transdifferentiation in high fat fed and insulin-deficient Ins1Cre/+ ;Rosa26-eYFP mice

BACKGROUND

The antidiabetic effects of the gut hormone xenin include augmenting insulin secretion and positively affecting pancreatic islet architecture.

METHODS

The current study has further probed pancreatic effects through sub-chronic administration of the long-acting xenin analogue, xenin-25[Lys¹³ PAL], in both high fat fed (HFF) and streptozotocin (STZ)-induced insulin-deficient Ins1^Cre/+ ;Rosa26-eYFP transgenic mice. Parallel effects on metabolic control and pancreatic islet morphology, including islet beta-cell lineage tracing were also assessed.

RESULTS

Xenin-25[Lys¹³ PAL] treatment reversed body weight loss induced by STZ, increased plasma insulin and decreased blood glucose levels. There were less obvious effects on these parameters in HFF mice, but all xenin-25[Lys¹³ PAL] treated mice exhibited decreased pancreatic alpha-cell areas and circulating glucagon. Xenin-25[Lys¹³ PAL] treatment fully, or partially, returned overall islet and beta-cell areas in STZ- and HFF mice to those of lean control animals, respectively, and was consistently associated with decreased beta-cell apoptosis. Interestingly, xenin-25[Lys¹³ PAL] also increased beta-cell proliferation and decreased alpha-cell apoptosis in STZ mice, with reduced alpha-cell growth noted in HFF mice. Lineage tracing studies revealed that xenin-25[Lys¹³ PAL] reduced the number of insulin positive pancreatic islet cells that lost their beta-cell identity, in keeping with a decreased transition of insulin positive to glucagon positive cells. These beneficial effects on islet cell differentiation were linked to maintained expression of Pdx1 within beta-cells. Xenin-25[Lys¹³ PAL] treatment was also associated with increased numbers of smaller sized islets in both models.

CONCLUSIONS

Benefits of xenin-25[Lys¹³ PAL] on diabetes includes positive modulation of islet cell differentiation, in addition to promoting beta-cell growth and survival.

Messenger RNA expression and localization of xenin in the gastrointestinal tract in sheep

The present study aimed to determine the primary sequence of ovine xenin and clarify the mRNA expression and peptide localization of xenin in the gastrointestinal tract in sheep. The colocalization of xenin and glucose-dependent insulinotropic polypeptide was also compared in the antrum and duodenum. Analysis of the nucleotide sequence of ovine xenin revealed a high degree (97.9%) of sequence homology of the sequence between sheep and cattle, and the amino acids sequence determined for ovine xenin coincided (100%) with that of other mammalian species. Real-time quantitative PCR for ovine xenin did not show regional difference in the mRNA expression ratio of xenin. In contrast to the real-time quantitative PCR results, anti-xenin positive cells were abundantly localized in the abomasal antrum (P < 0.01) and at a lesser amount in the duodenum, but no antixenin positive cells were observed in the other regions. Anti-xenin single-positive cells were in a majority in the abomasal antrum, whereas anti-xenin single-positive cells, and anti-GIP single-positive cells, and double-positive cells were even colocalized in the duodenum. These results suggest that abomasal antrum is a major source of xenin in the ovine gastrointestinal tract.

Effects of an enzymatically stable C-terminal hexapseudopeptide fragment peptide of xenin-25, ψ-xenin-6, on pancreatic islet function and metabolism

Xenin-25 undergoes rapid enzyme metabolism following secretion. Early studies demonstrated bioactivity of a C-terminal hexapeptide fragment of xenin-25, namely xenin-6, which were enhanced through introduction of a reduced N-terminal peptide bond, to yield Ψ-xenin-6. The present study was undertaken to define the biological actions and potential antidiabetic properties of Ψ-xenin-6. In vitro enzymatic stability, insulin and glucagon secretory activity, as well as effects on beta-cell survival were determined. Studies in mice were used to assess the impact of Ψ-xenin-6 on glucose homeostasis and satiety. Ψ-xenin-6 was resistant to murine plasma degradation. In BRIN-BD11 cells and isolated murine islets, Ψ-xenin-6 significantly stimulated insulin secretion, and prominently enhanced the insulinotropic actions of GIP. Xenin-6 and Ψ-xenin-6 had no impact on glucagon secretion, although xenin-6 partially reversed the glucagonotropic action of GIP. Further in vitro investigations revealed that, similar to GLP-1, Ψ-xenin-6 significantly augmented proliferation of human and rodent clonal beta-cells, whilst also fully protecting against cytokine-induced beta-cell cytotoxicity, with greater potency than xenin-25 and xenin-6. When administered to mice in combination with glucose, Ψ-xenin-6 significantly reduced glucose levels and enhanced glucose-induced insulin release, with a duration of biological action beyond 8 h. Ψ-xenin-6 also significantly enhanced the glucose-lowering action of GIP in vivo. In overnight fasted mice, Ψ-xenin-6 exhibited satiety actions at both 25 and 250 nmol/kg. These data demonstrates that Ψ-xenin-6 is a metabolically stable C-terminal fragment analogue of xenin-25, with a metabolic action profile that merits further study as a potential antidiabetic compound.