Characterization of preptin-induced insulin secretion in pancreatic β-cells

Preptin: A New Bone Metabolic Parameter?

Preptin is a 34-aminoacid peptide derived from the E-peptide of pro-insulin-like growth factor 2 (pro-IGF2) that is co-secreted with insulin and upregulates glucose-mediated insulin secretion. High serum preptin levels were described in conditions associated with insulin resistance, such as polycystic ovary syndrome and type 2 diabetes mellitus (T2M). Insulin and also IGF2 are known to be anabolic bone hormones. The "sweet bone" in T2M usually associates increased density, but altered microarchitecture. Therefore, preptin was proposed to be one of the energy regulatory hormones that positively impacts bone health. Experimental data demonstrate a beneficial impact of preptin upon the osteoblasts. Preptin also appears to regulate osteocalcin secretion, which in turn regulates insulin sensitivity. Preptin is greatly influenced by the glucose tolerance status and the level of physical exercise, both influencing the bone mass. Clinical studies describe low serum preptin concentrations in osteoporosis in both men and women, therefore opening the way towards considering preptin a potential bone anabolic therapy. The current review addresses the relationship between preptin and bone mass and metabolism in the experimental and clinical setting, also considering the effects of preptin on carbohydrate metabolism and the pancreatic-bone loop.

Preptin Deficiency Does Not Protect against High-Fat Diet-Induced Metabolic Dysfunction or Bone Loss in Mice

Preptin is derived from the cleavage of the E-peptide of pro-insulin-like growth factor (IGF)-II and is an insulin secretagogue. Observational studies have linked elevated circulating preptin to metabolic dysfunction in humans; however, a causal role for preptin in metabolic dysfunction has not been established. Additionally, preptin can promote osteoblast proliferation and differentiation, suggesting a link with skeletal health. We previously described a global preptin knockout (KO) model. In this study, we sought to uncover the impact of preptin KO in mice on the response to a moderately high-fat diet (HFD) and low-fat diet (LFD). HFD groups had higher weight and fat mass gain, lower trabecular and cortical bone volume and fracture load, and higher liver triglycerides. In males, preptin deficiency led to lower blood glucose than wild-type (WT) mice under LFD conditions. This was accompanied by differences in bone microarchitecture, including lower trabecular bone volume fraction, trabecular number, and lower cortical thickness. These differences were absent in female mice, although KO females had a HFD-driven increase in fat mass and liver triglycerides that was absent in WT mice. Female WT mice had increased glucose-stimulated insulin secretion under HFD conditions that was absent in female KO mice. Overall, preptin may have a detrimental impact on metabolism and a positive impact on bone health in male mice and may protect against liver fat storage in females while enabling islet compensation under HFD conditions. When we consider that serum preptin levels are elevated in humans of both sexes in pathological states in which insulin levels are elevated, the impact of preptin on comorbidity risk needs to be better understood. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Plasma and aqueous levels of subfatin, preptin and betatrophin in patients with diabetic retinopathy

AIM

To examine subfatin, preptin and betatrophin levels in plasma and aqueous in patients with diabetes mellitus (DM) (with and without retinopathy).

MATERIAL AND METHOD

Sixty patients, who were similar in terms of age and gender, and were scheduled for operation due to cataract, were included in the study. The patients were divided into three groups as Group C (20 weeks without diabetes and comorbidity), Group DM (20 patients with DM but no retinopathy) and Group DR (20 patients with diabetic retinopathy). The preoperative body mass index (BMI), fasting plasma glucose, HbA1c, lipid profile levels of all patients in the groups were examined. Blood samples were also taken for plasma subfatin, preptin and betatrophin levels. At the beginning of the cataract surgery, 0.1 ml of aqueous fluid was taken from the anterior chamber. Plasma and aqueous subfatin, preptin and betatrophin levels were analyzed by ELISA (enzyme-linked immunosorbent assays) method.

RESULTS

In our study results, there was a significant difference in BMI, fasting plasma glucose and hemoglobin A1c levels (p < 0.05 for all parameters). Plasma and aqueous subfatin levels were higher in Group DR compared to Group C (p < 0.001, p = 0.036, respectively). Plasma and aqueous preptin levels were higher in group DR and group DM than in group C (p = 0.001, p = 0.002, p < 0.001, p = 0.001, respectively). Plasma and aqueous betatrophin levels were higher in Group DR compared to group C (p = 0.001, p = 0.010, respectively).

CONCLUSION

Subfatin, preptin and betatrophin molecules may have an important role in the pathogenesis of diabetic retinopathy.

Genetic ablation of the preptin-coding portion of Igf2 impairs pancreatic function in female mice

Preptin is a 34-amino acid peptide derived from the E-peptide of pro-insulin-like growth factor 2 and is co-secreted with insulin from β-cells. Little is understood about the effects of endogenous preptin on whole body glucose metabolism. We developed a novel mouse model in which the preptin portion of Igf2 was genetically ablated in all tissues, hereafter referred to as preptin knockout (KO), and tested the hypothesis that the removal of preptin will lead to a decreased insulin response to a metabolic challenge. Preptin KO and wild-type (WT) mice underwent weekly fasting blood glucose measurements, intraperitoneal insulin tolerance tests (ITT) at 9, 29, and 44 wk of age, and an oral glucose tolerance test (GTT) at 45 wk of age. Preptin KO mice of both sexes had similar Igf2 exon 2-3 mRNA expression in the liver and kidney compared with WT mice, but Igf2 exon 3-4 (preptin) expression was not detectable. Western blot analysis of neonatal serum indicated that processing of pro-IGF2 translated from the KO allele may be altered. Preptin KO mice had similar body weight, body composition, β-cell area, and fasted glucose concentrations compared with WT mice in both sexes up to 47 wk of age. Female KO mice had a diminished ability to mount an insulin response following glucose stimulation in vivo. This effect was absent in male KO mice. Although preptin is not essential for glucose homeostasis, when combined with previous in vitro and ex vivo findings, these data show that preptin positively impacts β-cell function.NEW & NOTEWORTHY This is the first study to describe a model in which the preptin-coding portion of the Igf2 gene has been genetically ablated in mice. The mice do not show reduced size at birth associated with Igf2 knockout suggesting that IGF2 functionality is maintained, yet we demonstrate a change in the processing of mature Igf2. Female knockout mice have diminished glucose-stimulated insulin secretion, whereas the insulin response in males is not different to wild type.

Electrostatics drive the molecular chaperone BiP to preferentially bind oligomerized states of a client protein

Hsp70 chaperones bind short monomeric peptides with a weak characteristic affinity in the low micromolar range, but can also bind some aggregates, fibrils, and amyloids, with low nanomolar affinity. While this differential affinity enables Hsp70 to preferentially target potentially toxic aggregates, it is unknown how a chaperone can differentiate between monomeric and aggregated states of a client protein and why preferential binding is only observed for some aggregated clients but not others. Here we examine the interaction of BiP (the Hsp70 paralog in the endoplasmic reticulum) with the client proIGF2, the pro-protein form of IGF2 that includes a long and mostly disordered E-peptide region that promotes proIGF2 oligomerization. By dissecting the mechanism by which BiP targets proIGF2 and E-peptide oligomers we discover that electrostatic attraction is a powerful driving force for oligomer recognition. We identify the specific BiP binding sites on proIGF2 and as monomers they bind BiP with characteristically weak affinity in the low micromolar range, but electrostatic attraction to E-peptide oligomers boosts the affinity to the low nanomolar level. The dominant role of electrostatics is manifested kinetically as a steering force that accelerates the binding of BiP to E-peptide oligomers by approximately two orders of magnitude as compared against monomeric peptides. Electrostatic targeting of Hsp70 provides an explanation for why preferential binding has been observed for some aggregated clients but not others, as all the currently-documented cases in which Hsp70 binds aggregates with high-affinity involve clients that have an opposite charge to Hsp70.

Functional stapled fragments of human preptin of minimised length

Preptin is a 34-amino-acid-long peptide derived from the E-domain of a precursor of insulin-like growth factor 2 (pro-IGF2) with bone-anabolic and insulin secretion amplifying properties. Here, we describe the synthesis, structures, and biological activities of six shortened analogues of human preptin. Eight- and nine-amino-acid-long peptide amides corresponding to the C-terminal part of human preptin were stabilised by two types of staples to induce a higher proportion of helicity in their secondary structure. We monitored the secondary structure of the stapled peptides using circular dichroism. The biological effect of the structural changes was determined afterwards by the ability of peptides to stimulate the release of intracellular calcium ions. We confirmed the previous observation that the stabilisation of the disordered conformation of human preptin has a deleterious effect on biological potency. However, surprisingly, one of our preptin analogues, a nonapeptide stabilised by olefin metathesis between positions 3 and 7 of the amino acid chain, had a similar ability to stimulate calcium ions' release to the full-length human preptin. Our findings could open up new ways to design new preptin analogues, which may have potential as drugs for the treatment of diabetes and osteoporosis.

New Peptides as Potential Players in the Crosstalk Between the Brain and Obesity, Metabolic and Cardiovascular Diseases

According to the World Health Organization report published in 2016, 650 million people worldwide suffer from obesity, almost three times more than in 1975. Obesity is defined as excessive fat accumulation which may impair health with non-communicable diseases such as diabetes, cardiovascular diseases (hypertension, coronary artery disease, stroke), and some cancers. Despite medical advances, cardiovascular complications are still the leading causes of death arising from obesity. Excessive fat accumulation is caused by the imbalance between energy intake and expenditure. The pathogenesis of this process is complex and not fully understood, but current research is focused on the role of the complex crosstalk between the central nervous system (CNS), neuroendocrine and immune system including the autonomic nervous system, adipose tissue, digestive and cardiovascular systems. Additionally, special attention has been paid to newly discovered substances: neuropeptide 26RFa, preptin, and adropin. It was shown that the above peptides are synthesized both in numerous structures of the CNS and in many peripheral organs and tissues, such as the heart, adipose tissue, and the gastrointestinal tract. Recently, particular attention has been paid to the role of the presented peptides in the pathogenesis of obesity, metabolic and cardiovascular system diseases. This review summarizes the role of newly investigated peptides in the crosstalk between brain and peripheral organs in the pathogenesis of obesity, metabolic, and cardiovascular diseases.

High-Intensity Interval vs. Continuous Endurance Training: Preventive Effects on Hormonal Changes and Physiological Adaptations in Prediabetes Patients

ABSTRACT

Safarimosavi, S, Mohebbi, H, and Rohani, H. High-intensity interval vs. continuous endurance training: Preventive effects on hormonal changes and physiological adaptations in prediabetes patients. J Strength Cond Res 35(3): 731-738, 2021-The aim of this study was to examine the effects of a 12-week high-intensity interval training (HIIT) intervention, or an isocaloric continuous endurance training (CET) intervention on insulin resistance indices and change in irisin and preptin in patients with prediabetes. Thirty-two prediabetic male patients (age = 38.7 ± 4; body mass index = 26.9 ± 1.4 kg·m-2; and V̇o2peak = 2.49 ± 0.22 L·min-1) were randomly assigned into 3 training groups (N = 8). These groups were matched based on the required energy expenditure (EE) for completing each protocol: (a) HIIT (10 × 60 seconds at 90% peak oxygen uptake [V̇o2peak], 1: 1 work to recovery at 50 W), (b) CET at an intensity equivalent to maximal fat oxidation (Fatmax) (CETFAT) (pedaling for a duration that expends an equivalent EE to an HIIT session [E ≈ HIIT]), (c) CET at an intensity equivalent to anaerobic threshold (CETAT) (E ≈ HIIT), and (d) the control group (CON): continued to perform their daily activities. After intervention, blood glucose levels were significantly (p < 0.05) lower in the HIIT group compared with CETAT group. Exercise training improved the insulin resistance index by 35, 28, and 37% in CETFAT, CETAT, and HIIT groups, respectively. Irisin concentrations in the HIIT and CETAT groups was significantly (p < 0.05) decreased compared with the pre-training values. Also, HIIT and CETFAT resulted in significant (p < 0.05) changes in preptin concentration compared with baseline. This study demonstrated that both HIIT and CETFAT protocols had similar effects on the insulin resistance index of prediabetic patients. Also, the intensity and type of exercise were effective factors in changing irisin and preptin concentrations.

Insulin-like growth factor-II and bioactive proteins containing a part of the E-domain of pro-insulin-like growth factor-II

Insulin-like growth factor (IGF)-II is considered to function as an important fetal growth factor, which is structurally and functionally related to IGF-I and proinsulin. At least in vitro, IGF-II actions are mediated through the IGF-I receptor and to a lesser extent the insulin receptor. After birth, the function of IGF-II is less clear although in adults the serum level of IGF-II exceeds that of IGF-I several fold. The IGF-II gene is maternally imprinted, with exception of the liver and several parts of the brain, where it is expressed from both alleles. The regulation, organization, and translation of the IGF-II gene is complex, with five different putative promotors leading to a range of noncoding and coding mRNAs. The 180-amino acid pre-pro-IGF-II translation product can be divided into five domains and include a N-terminal signal peptide of 24 amino acid residues, the 67 amino acid long mature protein, and an 89 residues extension at the COOH terminus, designated as the E-domain. After removal of the signal peptide, the processing of pro-IGF-II into mature IGF-II requires various steps including glycosylation of the E-domain followed by the action of endo-proteases. Several of these processing intermediates can be found in the human circulation. There is increasing evidence that, besides IGF-II, several incompletely processed precursor forms of the protein, and even a 34-amino acid peptide (preptin) derived from the E-domain of pro-IGF-II, exhibit distinct biological activities. This review will focus on the current insights regarding the specific roles of the latter proteins in cancer, glucose homeostasis, and bone physiology. To address this topic clearly in the right context, a concise overview of the biological and biochemical properties of IGF-II and several relevant aspects of the IGF system will be provided.

Association of polymorphisms of preptin, irisin and adropin genes with susceptibility to coronary artery disease and hypertension

OBJECTIVES

Preptin, irisin and adropin are 3 new players in energy regulation that are related body mass index, lipids, glucose and insulin levels which may affect incidence of cardiovascular diseases. The aim of the present study was to evaluate eight single nucleotide polymorphisms (SNPs) of preptin genes (rs1003483, rs1004446, rs2239681, rs680, and rs3741204), irisin (rs16835198 and rs3480) and adropin (rs2281997) gene in patients with coronary artery disease (CAD) and hypertension.

METHODS

This case-control study was carried out on 372 volunteers, which were divided into 3 subgroups including: CAD patients with hypertension (CAD+H+), CAD patients with no hypertension (CAD+H-), and non-hypertensive non-CAD subjects as control group (CAD-H-) as health control. Genomic DNA from whole blood was extracted and eight SNPs were assessed using polymerase chain reaction- ligase detection reaction method.

RESULTS

A significant difference was found in the genotype and allele frequency of preptin rs1003483 gene in CAD+H+ compared to CAD+H- groups (P = .019 and P = .018, respectively). Allele frequency of rs1003483 was significantly different between CAD+H- groups and healthy control groups (P = .043). There also existed a significant difference the genotype frequency of rs1004446 gene in CAD+H+ compared to CAD+H- groups (P = .027).

CONCLUSIONS

The findings of present study revealed that the preptin rs1003483 and rs1004446 gene polymorphism might serve as predisposing factor in CAD and hypertension.

Effects of exercise training on serum preptin, undercarboxylated osteocalcin and high molecular weight adiponectin in adults with metabolic syndrome

NEW FINDINGS

What is the central question of this study? Are the advantages of aerobic interval exercise, resistance exercise and concurrent exercise on the metabolic profile mediated in part through preptin and undercarboxylated osteocalcin (ucOCN)? What is the main finding and its importance? Glucose was significantly lowered after concurrent exercise and aerobic interval exercise, but serum preptin and insulin were significantly lowered in all three training groups. By contrast, ucOCN and high molecular weight adiponectin increased significantly in all three training groups. These findings support the possible cross-talk between bone, pancreatic β-cells and energy metabolism in humans and suggest that preptin and ucOCN may potentially serve as markers of exercise-induced improvement of metabolism.

ABSTRACT

Preptin is a peptide hormone that plays an important role in the development of obesity by regulation of carbohydrate metabolism. Undercarboxylated osteocalcin (ucOCN) is also linked to the regulation of body energy in that it modulates fat and glucose metabolism. This research aimed to examine the impact of aerobic interval, resistance and concurrent exercise on serum preptin, ucOCN and high molecular weight adiponectin (HMW-APN) in obese adults with metabolic syndrome (MetS). Forty-four obese men with MetS were randomized to receive aerobic interval exercise (AIEX, n = 10), resistance exercise (REX, n = 10), or concurrent aerobic interval and resistance exercise (CEX, n = 10), or to act as a non-exercise control (CON, n = 10) three times a week for 12 weeks. Preptin was reduced more after AIEX and CEX than after REX (89.1% and 87.1% versus 9.6%; P = 0.028 and 0.030, respectively). ucOCN increased significantly only in the CEX (27.5%, P = 0.009) and AIEX (25%, P = 0.025) groups, but HMW-APN increased significantly in all three training groups (AIEX 145.1%, P < 0.001; CEX 137%, P < 0.001; and REX 59.8%, P = 0.041). After the intervention, the improvement of peak oxygen uptake ( V ̇ O 2 peak ) in the AIEX group (73%) was greater than in the CEX (29.3%) and REX (3.8%) groups. On the other hand, CEX exhibited a greater reduction in glucose, insulin, insulin resistance index and HbA1c than did AIEX and REX. Our study indicates that the reduction in glucose after exercise training (especially AIEX and CEX) may be, somewhat, linked to decreased preptin and raised ucOCN and HMW-APN.

The Neglected Insulin: IGF-II, a Metabolic Regulator with Implications for Diabetes, Obesity, and Cancer

When originally discovered, one of the initial observations was that, when all of the insulin peptide was depleted from serum, the vast majority of the insulin activity remained and this was due to a single additional peptide, IGF-II. The IGF-II gene is adjacent to the insulin gene, which is a result of gene duplication, but has evolved to be considerably more complicated. It was one of the first genes recognised to be imprinted and expressed in a parent-of-origin specific manner. The gene codes for IGF-II mRNA, but, in addition, also codes for antisense RNA, long non-coding RNA, and several micro RNA. Recent evidence suggests that each of these have important independent roles in metabolic regulation. It has also become clear that an alternatively spliced form of the insulin receptor may be the principle IGF-II receptor. These recent discoveries have important implications for metabolic disorders and also for cancer, for which there is renewed acknowledgement of the importance of metabolic reprogramming.

Elevated serum preptin concentrations in patients with diabetic nephropathy

Preptin is a peptide synthesized and secreted accompanied with insulin from pancreatic β cells. Here, we tested the hypothesis that serum preptin concentrations are correlated with diabetic nephropathy (DN). Our study was performed in a population of 234 patients with type 2 diabetes mellitus (T2DM) and 78 healthy subjects. Patients with T2DM were divided into three groups: normoalbuminuria group (DN0, n=106), microalbuminuria group (DN1, n=90), and macroalbuminuria group (DN2, n=38) according to urine albumin to creatinine ratio (ACR). Serum preptin concentrations were significantly increased in the three T2DM subgroups than those in the controls. DN2 group showed significantly higher serum preptin concentrations compared with DN0 and DN1 groups. Moreover, DN1 group had higher serum preptin concentrations than DN0 group. Serum preptin was correlated with a higher risk of T2DM and DN after logistic regression analysis. Simply linear regression analysis demonstrated a positive correlation between serum preptin and gender, body mass index (BMI), blood urea nitrogen, creatinine, ACR, and a negative correlation between serum preptin and glomerular filtration rate, metformin, acarbose treatment. Gender, BMI, and ACR were still positively correlated with serum preptin after multiple linear regression analysis. Our findings indicate that serum preptin concentrations are associated with renal function and DN.

Lupin seed hydrolysate promotes G-protein-coupled receptor, intracellular Ca2+ and enhanced glycolytic metabolism-mediated insulin secretion from BRIN-BD11 pancreatic beta cells

Lupin seed proteins have been reported to exhibit hypoglycaemic effects in animals and humans following oral administration, however little is known about its mechanism of action. This study investigated the signalling pathway(s) responsible for the insulinotropic effect of the hydrolysate obtained from lupin (Lupinus angustifolius L.) seed extracts utilizing BRIN-BD11 β-cells. The extract was treated with digestive enzymes to give a hydrolysate rich in biomolecules ≤7 kDa. Cells exhibited hydrolysate induced dose-dependent stimulation of insulin secretion and enhanced intracellular Ca²⁺ and glucose metabolism. The stimulatory effect of the hydrolysate was potentiated by depolarizing concentrations of KCl and was blocked by inhibitors of the ATP sensitive K⁺ channel, Gα_q protein, phospholipase C (PLC) and protein kinase C (PKC). These findings reveal a novel mechanism for lupin hydrolysate stimulated insulin secretion via Gα_q mediated signal transduction (Gα_q/PLC/PKC) in the β-cells. Thus, lupin hydrolysates may have potential for nutraceutical treatment in type 2 diabetes.

Preptin in women with polycystic ovary syndrome

INTRODUCTION

Polycystic ovary syndrome (PCOS) patients, frequently develop metabolic complications, such as insulin resistance (IR), impaired carbohydrate metabolism, dyslipidemia, obesity. Among the new markers responsible for metabolic disorders, preptin seems to be of great significance.

MATERIAL

One hundred and thirty-four women aged 17-45 were enrolled. PCOS was diagnosed in 73 women on the basis of ESHRE-ASRM criteria. Non-PCOS group consisted of 61 women with regular menstruation matched for nutritional status.

METHODS

All women underwent anamnesis, physical examination, anthropometric measurements, the abdominal ultrasound examination, and dual energy X-ray absorptiometry (DXA). Serum adropin levels were determined by ELISA. Biochemical and hormonal (testosterone, androstenedione, LH, FSH, estradiol) measurements were also performed. Insulin resistance indices (HOMA, QUICKI, Matsuda) and free androgen index (FAI) were calculated with the test results according to the standard formula. For all comparisons, statistical significance was defined by p ≤ .05.

RESULTS

Serum preptin levels were significantly higher in the PCOS group. No significant correlations between preptin level and metabolic and hormonal markers were observed. The logistic regression analysis demonstrated that serum preptin level was an independent factor differentiating the two groups.

CONCLUSIONS

Serum preptin levels were significantly higher in women with PCOS compared with controls. This peptide might be an independent predictor of PCOS in the future.

Investigation of triamterene as an inhibitor of the TGR5 receptor: identification in cells and animals

Background

G-protein-coupled bile acid receptor 1 (GPBAR1, also known as TGR5) has been shown to participate in glucose homeostasis. In animal models, a TGR5 agonist increases incretin secretion to reduce hyperglycemia. Many agonists have been developed for clinical use. However, the effects of TGR5 blockade have not been studied extensively, with the exception of studies using TGR5 knockout mice. Therefore, we investigated the potential effect of triamterene on TGR5.

Methods

We transfected the TGR5 gene into cultured Chinese hamster ovary cells (CHO-K1 cells) to express TGR5. Then, we applied a fluorescent indicator to examine the glucose uptake of these transfected cells. In addition, NCI-H716 cells that secrete incretin were also evaluated. Fura-2, a fluorescence indicator, was applied to determine the changes in calcium concentrations. The levels of cyclic adenosine monophosphate (cAMP) and glucagon-like peptide (GLP-1) were estimated using enzyme-linked immunosorbent assay kits. Moreover, rats with streptozotocin (STZ)-induced type 1-like diabetes were used to investigate the effects in vivo.

Results

Triamterene dose dependently inhibits the increase in glucose uptake induced by TGR5 agonists in CHO-K1 cells expressing the TGR5 gene. In cultured NCI-H716 cells, TGR5 activation also increases GLP-1 secretion by increasing calcium levels. Triamterene inhibits the increased calcium levels by TGR5 activation through competitive antagonism. Moreover, the GLP-1 secretion and increased cAMP levels induced by TGR5 activation are both dose dependently reduced by triamterene. However, treatment with KB-R7943 at a dose sufficient to block the Na⁺/Ca²⁺ exchanger (NCX) failed to modify the responses to TGR5 activation in NCI-H716 cells or CHO-K1 cells expressing TGR5. Therefore, the inhibitory effects of triamterene on TGR5 activation do not appear to be related to NCX inhibition. Blockade of TGR5 activation by triamterene was further characterized in vivo using the STZ-induced diabetic rats.

Conclusion

Based on the obtained data, we identified triamterene as a reliable inhibitor of TGR5. Therefore, triamterene can be developed as a clinical inhibitor of TGR5 activation in future studies.

Development of betulinic acid as an agonist of TGR5 receptor using a new in vitro assay

Background

G-protein-coupled bile acid receptor 1, also known as TGR5 is known to be involved in glucose homeostasis. In animal models, treatment with a TGR5 agonist induces incretin secretion to reduce hyperglycemia. Betulinic acid, a triterpenoid present in the leaves of white birch, has been introduced as a selective TGR5 agonist. However, direct activation of TGR5 by betulinic acid has not yet been reported.

Methods

Transfection of TGR5 into cultured Chinese hamster ovary (CHO-K1) cells was performed to establish the presence of TGR5. Additionally, TGR5-specific small interfering RNA was employed to silence TGR5 in cells (NCI-H716 cells) that secreted incretins. Uptake of glucose by CHO-K1 cells was evaluated using a fluorescent indicator. Amounts of cyclic adenosine monophosphate and glucagon-like peptide were quantified using enzyme-linked immunosorbent assay kits.

Results

Betulinic acid dose-dependently increases glucose uptake by CHO-K1 cells transfected with TGR5 only, which can be considered an alternative method instead of radioligand binding assay. Additionally, signals coupled to TGR5 activation are also increased by betulinic acid in cells transfected with TGR5. In NCI-H716 cells, which endogenously express TGR5, betulinic acid induces glucagon-like peptide secretion via increasing calcium levels. However, the actions of betulinic acid were markedly reduced in NCI-H716 cells that received TGR5-silencing treatment. Therefore, the present study demonstrates the activation of TGR5 by betulinic acid for the first time.

Conclusion

Similar to the positive control lithocholic acid, which is the established agonist of TGR5, betulinic acid has been characterized as a useful agonist of TGR5 and can be used to activate TGR5 in the future.

Activation of imidazoline-I3 receptors ameliorates pancreatic damage

Agmatine, an endogenous ligand of imidazoline receptors, is reported to exhibit anti-hyperglycaemic and many other effects. It has been established that the imidazoline I3 receptor is involved in insulin secretion. The current study characterizes the role of the imidazoline I3 receptor in the protection of pancreatic islets. The activity effect of agmatine against on streptozotocin (STZ)-induced (5 mmol/L) rat β cell apoptosis was examined by using ApoTox-Glo triplex assay, live/dead cell double staining assay, flow cytometric analysis, and western blot. Imidazoline I3 receptors antagonist KU14R and the phospholipase C inhibitor named U73122 were treated in β cells to investigate the potential signalling pathways. The serum glucose and recovery of insulin secretion were measured in STZ-treated rats after continuously injected agmatine. The apoptosis in rat β cells was reduced by agmatine in a dose-dependent manner, cell viability was improved after treatment with agmatine and these effects were suppressed after the blockade of KU14R and U73122. Western blot analysis confirmed that agmatine could decrease caspase-3 expression and increase the p-BAD levels. In STZ-treated rats, injection of agmatine for 4 weeks may significantly lower the serum glucose and recovery of insulin secretion. This improvement of pancreatic islets induced by agmatine was deleted by KU14R in vivo. Agmatine can activate the imidazoline I3 receptor linked with the phospholipase C pathway to induce cell protection against apoptosis induced by a low dose of STZ. This finding provides new insight into the prevention of early stage pancreatic islet damage.

Insulinotropic effect of Chikusetsu saponin IVa in diabetic rats and pancreatic β-cells

ETHNOPHARMACOLOGICAL RELEVANCE

As a well-known traditional Chinese medicine the root bark of Aralia taibaiensis has traditionally been used as the medicine considered alleviating several disorders including diabetes mellitus (DM). Chikusetsu saponin IVa (CHS) has been defined as a major active ingredient of triterpenoid saponins extracted from Aralia taibaiensis. The scientific evidence of anti-diabetic effect for CHS remains unknown and the purpose of our study was to study its hypoglycemic and insulin secretagogue activities.

MATERIALS AND METHODS

In vivo studies were performed on type 2 diabetic mellitus (T2DM) rats given CHS for 28 days to test the antihyperglycemic activity. The in vitro effects and possible mechanisms of CHS on the insulin secretion in pancreatic β-cell line βTC3 were determined.

RESULTS

Oral administration of CHS dose-dependently increased the level of serum insulin and decreased the rise in blood glucose level in an in vivo treatment. In vitro, CHS potently stimulated the release of insulin from βTC3 cells at both basal and stimulatory glucose concentrations, the effect which was changed by the removal of extracellular Ca(2+). Two methods showed that CHS enhanced the intracellular calcium levels in βTC3 cells. CHS was capable of enhancing the phosphorylation of extracellular signal-regulated protein kinases C (PKC), which could be reversed by a PKC inhibitor (RO320432), and the insulin secretion induced by CHS was also inhibited by RO320432. Further study also showed that the insulinotropic effect, intracellular calcium levels and the phosphorylation of PKC were reduced by inhibiting G protein-coupled receptor 40 (GPR40) by a GPR40 inhibitor (DC126026).

CONCLUSION

These observations suggest that the signaling of CHS-induced insulin secretion from βTC3 cells via GPR40 mediated calcium and PKC pathways and thus CHS might be developed into a new potential for therapeutic agent used in T2DM patients.

Relationships between preptin and osteocalcin in obese, overweight, and normal weight adults

Preptin is an endocrine peptide secreted from pancreatic β cells; it acts as a physiological amplifier of glucose-mediated insulin secretion. Osteocalcin, secreted by osteoblasts, is also involved in the regulation of body energy in that it modulates fat and glucose metabolism. The aim of this study was to investigate circulating preptin and osteocalcin levels and the possible association between them in obese and overweight adults. A total of 100 obese-overweight adults and 50 healthy controls matched for age and sex were enrolled in this study. Anthropometric measurements were taken and fasting glucose, insulin, homeostasis model assessment of insulin resistance (HOMA-IR), lipid profile, preptin, and osteocalcin were assessed. Preptin was significantly higher in obese and overweight subjects than in controls, whereas osteocalcin was significantly lower. In multiple regression analysis, preptin was independently influenced by waist circumference (WC) (β = 3.75, P = 0.03), fasting insulin (β = 4.77, P = 0.01), HOMA-IR (β = 8.21, P = 0.003), and osteocalcin (β = -28.41, P = 0.04). Osteocalcin was independently influenced by body mass index (β = -1.46, P = 0.05), WC (β = -2.04, P = 0.03), fasting insulin (β = -2.69, P = 0.02), HOMA-IR (β = -0.19, P = 0.006), and triglyceride (β = -4.54, P = 0.001). In conclusion, elevated serum preptin and decreased osteocalcin concentrations, together with insulin resistance, are associated with obesity and overweight. Serum preptin is also independently associated with osteocalcin, which may explain, at least in part, the crosstalk between bone and the β cells of the pancreas.

Three new players in energy regulation: preptin, adropin and irisin

Homeostasis of energy is regulated by genetic factors, food intake, and energy expenditure. When energy input is greater than expenditure, the balance is positive, which can lead to weight gain and obesity. When the balance is negative, weight is lost. Regulation of this homeostasis is multi-factorial, involving many orexigenic (appetite-stimulating) and anorexigenic (appetite-suppressing) peptide hormones. Peripheral tissues are now known to be involved in weight regulation and research on its endocrine characteristics proceeds apace. Preptin with 34 amino acids (MW 3948 Da), adropin with 43 amino acids and a molecular weight of (4999 Da), and irisin with 112 amino acids (12587 Da), are three newly discovered peptides critical for regulating energy metabolism. Preptin is synthesized primarily in pancreatic beta cells, and adropin mainly in the liver and brain, and many peripheral tissues. Irisin, however, is synthesized principally in the heart muscle, along with peripheral tissues, including salivary glands, kidney and liver. The prime functions of preptin and adropin include regulating carbohydrate, lipid and protein metabolisms by moderating glucose-mediated insulin release. Irisin is an anti-obesitic and anti-diabetic hormone regulating adipose tissue metabolism and glucose homeostasis by converting white to brown adipose tissue. This review offers a historical account of these discovery and function of these peptides, including their structure, and physiological and biochemical properties. Their roles in energy regulation will be discussed. Their measurement in biological fluids will be considered, which will lead to further discussion of their possible clinical value.