Bone is permanently remodeled by a complex network of local, hormonal, and neuronal factors that affect osteoclast and osteoblast biology. Among these factors, a role for gastrointestinal hormones has been proposed based on the evidence that bone resorption dramatically falls after a meal. Glucagon-like peptide-1 (GLP1) is one of these gut hormones, and despite several reports suggesting an anabolic effect of GLP1, or its stable analogs, on bone mass, little is known about the effects of GLP1/GLP1 receptor on bone strength. In this study, we investigated by three-point bending, quantitative X-ray microradiography, microcomputed tomography, qBEI, and FTIRI bone strength and bone quality in male Glp1r knockout (Glp1r KO) mice when compared with control WT animals. Animals with a deletion of Glp1r presented with a significant reduction in ultimate load, yield load, stiffness, and total absorbed and post-yield energies when compared with WT animals. Furthermore, cortical thickness and bone outer diameter were significantly decreased in deficient animals. The mineral quantity and quality were not significantly different between Glp1r KO and WT animals. On the other hand, the maturity of the collagen matrix was significantly reduced in deficient animals and associated with lowered material properties. Taken together, these data support a positive effect of GLP1R on bone strength and quality.

Glucose-dependent insulinotropic polypeptide (GIP) is a 42 amino acid hormone secreted from intestinal K-cells, which exhibits a number of actions including stimulation of insulin release. A truncated form, GIP(1–30), has recently been demonstrated in intestine and islet α-cells. To evaluate the potential physiological significance of this naturally occurring form of GIP, the present study has examined and compared the bioactivity of enzymatically stabilised forms, [d-Ala²]GIP(1–30) and [d-Ala²]GIP(1–42), in high-fat fed mice. Twice-daily injection of GIP peptides for 42 days had no significant effect on food intake or body weight. However, non-fasting glucose levels were significantly lowered, and insulin levels were elevated in both treatment groups compared to saline controls. The glycaemic response to i.p. glucose was correspondingly improved (P<0.05) in [d-Ala²]GIP(1–30)- and [d-Ala²]GIP(1–42)-treated mice. Furthermore, glucose-stimulated plasma insulin levels were significantly elevated in both treatment groups compared to control mice. Insulin sensitivity was not significantly different between any of the groups. Similarly, plasma lipid profile, O₂ consumption, CO₂ production, respiratory exchange ratio, and energy expenditure were not altered by 42 days twice-daily treatment with [d-Ala²]GIP(1–30) or [d-Ala²]GIP(1–42). In contrast, ambulatory activity was significantly (P<0.05) elevated during the light phase in both GIP treatment groups compared to saline controls. The results reveal that sustained GIP receptor activation exerts a spectrum of beneficial metabolic effects in high-fat fed mice. However, no differences were discernable between the biological actions of the enzyme-resistant analogues of the naturally occurring forms, GIP(1–30) and GIP(1–42).

Recent studies have characterised the biological properties and glucose-dependent insulinotropic polypeptide (GIP) potentiating actions of an enzymatically stable, C-terminal hexapeptide fragment of the gut hormone xenin, namely Ψ-xenin-6. Given the primary therapeutic target of clinically approved dipeptidyl peptidase-4 (DPP-4) inhibitor drugs is augmentation of the incretin effect, the present study has assessed the capacity of Ψ-xenin-6 to enhance the antidiabetic efficacy of sitagliptin in high fat fed (HFF) mice. Individual administration of either sitagliptin or Ψ-xenin-6 alone for 18 days resulted in numerous metabolic benefits and positive effects on pancreatic islet architecture. As expected, sitagliptin therapy was associated with elevated circulating GIP and GLP-1 levels, with concurrent Ψ-xenin-6 not elevating these hormones or enhancing DPP-4 inhibitory activity of the drug. However, combined sitagliptin and Ψ-xenin-6 therapy in HFF mice was associated with further notable benefits, beyond that observed with either treatment alone. This included body weight change similar to lean controls, more pronounced and rapid benefits on circulating glucose and insulin as well as additional improvements in attenuating gluconeogenesis. Favourable effects on pancreatic islet architecture and peripheral insulin sensitivity were more apparent with combined therapy. Expression of hepatic genes involved in gluconeogenesis and insulin action were partially, or fully, restored to normal levels by the treatment regimens, with beneficial effects more prominent in the combination treatment group. These data demonstrate that combined treatment with Ψ-xenin-6 and sitagliptin did not alter glucose tolerance but does offer some metabolic advantages, which merit further consideration as a therapeutic option for type 2 diabetes.

Transdifferentiation of beta- to alpha-cells has been implicated in the pathogenesis of diabetes. To investigate the impact of contrasting aetiologies of beta-cell stress, as well as clinically approved incretin therapies on this process, lineage tracing of beta-cells in transgenic Ins1 ^Cre/+/Rosa26-eYFP mice was investigated. Diabetes-like syndromes were induced by streptozotocin (STZ), high fat feeding (HFF) or hydrocortisone (HC), and effects of treatment with liraglutide or sitagliptin were investigated. Mice developed the characteristic metabolic features associated with beta-cell destruction or development of insulin resistance. Liraglutide was effective in preventing weight gain in HFF mice, with both treatments decreasing energy intake in STZ and HC mice. Treatment intervention also significantly reduced blood glucose levels in STZ and HC mice, as well as increasing either plasma or pancreatic insulin while decreasing circulating or pancreatic glucagon in all models. The recognised changes in pancreatic morphology induced by STZ, HFF or HC were partially, or fully, reversed by liraglutide and sitagliptin, and related to advantageous effects on alpha- and beta-cell growth and survival. More interestingly, induction of diabetes-like phenotype, regardless of pathogenesis, led to increased numbers of beta-cells losing their identity, as well as decreased expression of Pdx1 within beta-cells. Both treatment interventions, and especially liraglutide, countered detrimental islet cell transitioning effects in STZ and HFF mice. Only liraglutide imparted benefits on beta- to alpha-cell transdifferentiation in HC mice. These data demonstrate that beta- to alpha-cell transdifferentiation is a common consequence of beta-cell destruction or insulin resistance and that clinically approved incretin-based drugs effectively limit this.

Cholecystokinin (CCK) is a hormone that has important physiological effects on energy balance. This study has used a stable CCK₁ receptor agonist, (pGlu-Gln)-CCK-8, to evaluate the metabolic effects of prolonged administration in normal mice. Twice-daily injection of (pGlu-Gln)-CCK-8 for 28 days resulted in significantly lowered body weights (P<0.05) on days 24 and 28, which was associated with decreased accumulated calorie intake (P<0.01) from day 12 onward. Nonfasting plasma glucose was significantly reduced (P<0.05) on day 28, while plasma insulin concentrations were increased (P<0.05). After 28 days, glucose tolerance and glucose-mediated insulin secretion were not significantly different in (pGlu-Gln)-CCK-8-treated mice. However, following a 15-min refeeding period in 18-h fasted mice, glucose levels were significantly (P<0.05) decreased by (pGlu-Gln)-CCK-8 despite similar food intake and nutrient-induced insulin levels. Insulin sensitivity in (pGlu-Gln)-CCK-8-treated mice was significantly (P<0.01) improved compared with controls. Accumulation of triacylglycerol in liver was reduced (P<0.01) but there were no differences in circulating cholesterol and triacylglycerol concentrations, as well as triacylglycerol content of pancreatic, muscle, and adipose tissue in (pGlu-Gln)-CCK-8 mice. These data highlight the beneficial metabolic effects of prolonged (pGlu-Gln)-CCK-8 administration and confirm a lack of detrimental effects.

Recent approval of the dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist, tirzepatide, for the management of type 2 diabetes mellitus (T2DM) has reinvigorated interest in exploitation of GIP receptor (GIPR) pathways as a means of metabolic disease management. However, debate has long surrounded the use of the GIPR as a therapeutic target and whether agonism or antagonism is of most benefit in management of obesity/diabetes. This controversy appears to be partly resolved by the success of tirzepatide. However, emerging studies indicate that prolonged GIPR agonism may desensitise the GIPR to essentially induce receptor antagonism, with this phenomenon suggested to be more pronounced in the human than rodent setting. Thus, deliberation continues to rage in relation to benefits of GIPR agonism vs antagonism. That said, as with GIPR agonism, it is clear that the metabolic advantages of sustained GIPR antagonism in obesity and obesity-driven forms of diabetes can be enhanced by concurrent GLP-1 receptor (GLP-1R) activation. This narrative review discusses various approaches of pharmacological GIPR antagonism including small molecule, peptide, monoclonal antibody and peptide-antibody conjugates, indicating stage of development and significance to the field. Taken together, there is little doubt that interesting times lie ahead for GIPR agonism and antagonism, either alone or when combined with GLP-1R agonists, as a therapeutic intervention for the management of obesity and associated metabolic disease.

Discerning modification to the amino acid sequence of native glucagon can generate specific glucagon receptor (GCGR) antagonists, that include desHis¹Pro⁴Glu⁹-glucagon and the acylated form desHis¹Pro⁴Glu⁹(Lys¹²PAL)-glucagon. In the current study, we have evaluated the metabolic benefits of once-daily injection of these peptide-based GCGR antagonists for 18 days in insulin-resistant high-fat-fed (HFF) mice with streptozotocin (STZ)-induced insulin deficiency, namely HFF-STZ mice. Administration of desHis¹Pro⁴Glu⁹-glucagon moderately (P < 0.05) decreased STZ-induced elevations of food intake. Body weight was not different between groups of HFF-STZ mice and both treatment interventions delayed (P < 0.05) the onset of hyperglycaemia. The treatments reduced (P < 0.05–P < 0.001) circulating and pancreatic glucagon, whilst desHis¹Pro⁴Glu⁹(Lys¹²PAL)-glucagon also substantially increased (P < 0.001) pancreatic insulin stores. Oral glucose tolerance was appreciably improved (P < 0.05) by both antagonists, despite the lack of augmentation of glucose-stimulated insulin release. Interestingly, positive effects on i.p. glucose tolerance were less obvious suggesting important beneficial effects on gut function. Metabolic benefits were accompanied by decreased (P < 0.05–P < 0.01) locomotor activity and increases (P < 0.001) in energy expenditure and respiratory exchange ratio in both treatment groups. In addition, desHis¹Pro⁴Glu⁹-glucagon increased (P < 0.01–P < 0.001) O₂ consumption and CO₂ production. Together, these data provide further evidence that peptidic GCGR antagonists are effective treatment options for obesity-driven forms of diabetes, even when accompanied by insulin deficiency.

Xenin-25, a peptide co-secreted with the incretin hormone glucose-dependent insulinotropic polypeptide (GIP), possesses promising therapeutic actions for obesity-diabetes. However, native xenin-25 is rapidly degraded by serum enzymes to yield the truncated metabolites: xenin 9–25, xenin 11–25, xenin 14–25 and xenin 18–25. This study has examined the biological activities of these fragment peptides. In vitro studies using BRIN-BD11 cells demonstrated that native xenin-25 and xenin 18–25 possessed significant (P<0.05 to P<0.001) insulin-releasing actions at 5.6 and 16.7 mM glucose, respectively, but not at 1.1 mM glucose. In addition, xenin 18–25 significantly (P<0.05) potentiated the insulin-releasing action of the stable GIP mimetic (d-Ala²)GIP. In contrast, xenin 9–25, xenin 11–25 and xenin 14–25 displayed neither insulinotropic nor GIP-potentiating actions. Moreover, xenin 9–25, xenin 11–25 and xenin 14–25 significantly (P<0.05 to P<0.001) inhibited xenin-25 (10⁻⁶ M)-induced insulin release in vitro. I.p. administration of xenin-based peptides in combination with glucose to high fat-fed mice did not significantly affect the glycaemic excursion or glucose-induced insulin release compared with controls. However, when combined with (d-Ala²)GIP, all xenin peptides significantly (P<0.01 to P<0.001) reduced the overall glycaemic excursion, albeit to a similar extent as (d-Ala²)GIP alone. Xenin-25 and xenin 18–25 also imparted a potential synergistic effect on (d-Ala²)GIP-induced insulin release in high fat-fed mice. All xenin-based peptides lacked significant satiety effects in normal mice. These data demonstrate that the C-terminally derived fragment peptide of xenin-25, xenin 18–25, exhibits significant biological actions that could have therapeutic utility for obesity-diabetes.

Recently, glucagon-like peptide 1 (GLP1) and glucose-dependent insulinotropic polypeptide (GIP) have received much attention regarding possible roles in aetiology and treatment of type 2 diabetes. However, peptides co-secreted from the same enteroendocrine cells are less well studied. The present investigation was designed to characterise the in vitro and in vivo effects of xenin, a peptide co-secreted with GIP from intestinal K-cells. We examined the enzymatic stability, insulin-releasing activity and associated cAMP production capability of xenin in vitro. In addition, the effects of xenin on satiety, glucose homoeostasis and insulin secretion were examined in vivo. Xenin was time dependently degraded (t _1/2=162±6 min) in plasma in vitro. In clonal BRIN-BD11 cells, xenin stimulated insulin secretion at 5.6 mM (P<0.05) and 16.7 mM (P<0.05 to P<0.001) glucose levels compared to respective controls. Xenin also exerted an additive effect on GIP, GLP1 and neurotensin-mediated insulin secretion. In clonal β-cells, xenin did not stimulate cellular cAMP production, alter membrane potential or elevate intra-cellular Ca^{2 +}. In normal mice, xenin exhibited a short-acting (P<0.01) satiety effect at high dosage (500 nmol/kg). In overnight fasted mice, acute injection of xenin enhanced glucose-lowering and elevated insulin secretion when injected concomitantly or 30 min before glucose. These effects were not observed when xenin was administered 60 min before the glucose challenge, reflecting the short half-life of the native peptide in vivo. Overall, these data demonstrate that xenin may have significant metabolic effects on glucose control, which merit further study.

The present study examines differences in metabolic and pancreatic islet adaptative responses following streptozotocin (STZ) and hydrocortisone (HC) administration in male and female transgenic Glu^CreERT2/Rosa26-eYFP mice. Mice received five daily doses of STZ (50 mg/kg, i.p.) or 10 daily doses of HC (70 mg/kg, i.p.), with parameters assessed on day 11. STZ-induced hyperglycaemia was evident in both sexes, alongside impaired glucose tolerance and reduced insulin concentrations. HC also had similar metabolic effects in male and female mice resulting in classical increases of circulating insulin indicative of insulin resistance. Control male mice had larger pancreatic islets than females and displayed a greater reduction of islet and beta-cell area in response to STZ insult. In addition, female STZ mice had lower levels of beta-cell apoptosis than male counterparts. Following HC administration, female mouse islets contained a greater proportion of alpha cells when compared to males. All HC mice presented with relatively comparable increases in beta- and alpha-cell turnover rates, with female mice being slightly more susceptible to HC-induced beta-cell apoptosis. Interestingly, healthy control female mice had inherently increased alpha-to-beta-cell transdifferentiation rates, which was decreased by HC treatment. The number of glucagon-positive alpha cells altering their lineage to insulin-positive beta cells was increased in male, but not female, STZ mice. Taken together, although there was no obvious sex-specific alteration of metabolic profile in STZ or HC mice, subtle differences in pancreatic islet morphology emphasises the impact of sex hormones on islets and importance of taking care when interpreting observations between males and females.