Effects of beta-endorphin, met-enkephalin, and dynorphin A on basal and stimulated insulin secretion in the mouse

The role of mu-opioid receptors in pancreatic islet alpha cells

30% of people in the United States have diabetes or pre-diabetes. Many of these individuals will develop diabetic neuropathy as a comorbidity, which is often treated with exogenous opioids like morphine, oxycodone, or tramadol. Although these opioids are effective analgesics, growing evidence indicates that they may directly impact the endocrine pancreas function in human and preclinical models. One common feature of these exogenous opioid ligands is their preference for the mu opioid receptor (MOPR), so we aimed to determine if endogenous MOPRs directly regulate pancreatic islet metabolism and hormone secretion. We show that pharmacological antagonism of MOPRs enhances glucagon secretion, but not insulin secretion, from human islets under high glucose conditions. This increased secretion is accompanied by increased cAMP signaling. mRNA expression of MOPRs is enriched in human islet α-cells, but downregulated in T2D islet donors, suggesting a link between metabolism and MOPR expression. Conditional genetic knockout of MOPRs in murine α-cells increases glucagon secretion in high glucose conditions without increasing glucagon content. Consistent with downregulation of MOPRs during metabolic disease, conditional MOPR knockout mice treated with a high fat diet show impaired glucose tolerance, increased glucagon secretion, increased insulin content, and increased islet size. Finally, we show that MOPR-mediated changes in glucagon secretion are driven, in part, by KATP channel activity. Together, these results demonstrate a direct mechanism of action for endogenous opioid regulation of endocrine pancreas.

Type 2 diabetes candidate genes, including PAX5, cause impaired insulin secretion in human pancreatic islets

Type 2 diabetes (T2D) is caused by insufficient insulin secretion from pancreatic β cells. To identify candidate genes contributing to T2D pathophysiology, we studied human pancreatic islets from approximately 300 individuals. We found 395 differentially expressed genes (DEGs) in islets from individuals with T2D, including, to our knowledge, novel (OPRD1, PAX5, TET1) and previously identified (CHL1, GLRA1, IAPP) candidates. A third of the identified expression changes in islets may predispose to diabetes, as expression of these genes associated with HbA1c in individuals not previously diagnosed with T2D. Most DEGs were expressed in human β cells, based on single-cell RNA-Seq data. Additionally, DEGs displayed alterations in open chromatin and associated with T2D SNPs. Mouse KO strains demonstrated that the identified T2D-associated candidate genes regulate glucose homeostasis and body composition in vivo. Functional validation showed that mimicking T2D-associated changes for OPRD1, PAX5, and SLC2A2 impaired insulin secretion. Impairments in Pax5-overexpressing β cells were due to severe mitochondrial dysfunction. Finally, we discovered PAX5 as a potential transcriptional regulator of many T2D-associated DEGs in human islets. Overall, we have identified molecular alterations in human pancreatic islets that contribute to β cell dysfunction in T2D pathophysiology.

Methionine enkephalin (MENK) regulates the immune pathogenesis of type 2 diabetes mellitus via the IL-33/ST2 pathway

The incidence and mortality of type 2 diabetes mellitus (T2DM) rank among the top ten worldwide. Emerging studies indicate pathological roles for the immune system in inflammation, insulin resistance and islet β-cell damage in subjects with T2DM. Methionine enkephalin (MENK) is present in endocrine cells of the pancreas and has been suggested to be an important mediator between the immune and neuroendocrine systems. Therefore, it may play a role in modulating insulin secretion from islet cells. Since little is known about the effect of MENK on T2DM, therefore it was the aim of this study to characterize the role and possible mechanism of action of MENK on plasma glucose and serum insulin levels in T2DM rats and INS-1 cells in vivo and in vitro. MENK significantly decreased the plasma glucose level and increased the serum insulin concentration in T2DM rats. It also increased the serum levels of the cytokines IL-5 and IL-10, while decreased TNF-α and IL-2 levels. We further confirmed that MENK regulated glucose metabolism by upregulating opioid receptor expression and modulating the IL-33/ST2 and MyD88-TRAF6-NF-κB p65 signaling pathways. Based on these results, an intraperitoneal injection of MENK represents a potentially new approach for T2DM.

Opioid Receptor Activation Impairs Hypoglycemic Counterregulation in Humans

Although intensive glycemic control improves outcomes in type 1 diabetes mellitus (T1DM), iatrogenic hypoglycemia limits its attainment. Recurrent and/or antecedent hypoglycemia causes blunting of protective counterregulatory responses, known as hypoglycemia-associated autonomic failure (HAAF). To determine whether and how opioid receptor activation induces HAAF in humans, 12 healthy subjects without diabetes (7 men, age 32.3 ± 2.2 years, BMI 25.1 ± 1.0 kg/m²) participated in two study protocols in random order over two consecutive days. On day 1, subjects received two 120-min infusions of either saline or morphine (0.1 μg/kg/min), separated by a 120-min break (all euglycemic). On day 2, subjects underwent stepped hypoglycemic clamps (nadir 60 mg/dL) with evaluation of counterregulatory hormonal responses, endogenous glucose production (EGP, using 6,6-D2-glucose), and hypoglycemic symptoms. Morphine induced an ∼30% reduction in plasma epinephrine response together with reduced EGP and hypoglycemia-associated symptoms on day 2. Therefore, we report the first studies in humans demonstrating that pharmacologic opioid receptor activation induces some of the clinical and biochemical features of HAAF, thus elucidating the individual roles of various receptors involved in HAAF's development and suggesting novel pharmacologic approaches for safer intensive glycemic control in T1DM.

Mice lacking δ-opioid receptors resist the development of diet-induced obesity

Pharmacological manipulation of opioid receptors alters feeding behavior. However, the individual contributions of each opioid receptor subtype on energy balance remain largely unknown. Herein, we investigated whether genetic disruption of the δ-opioid receptor (DOR) also controls energy homeostasis. Mice lacking DOR and wild-type mice were fed with standard diet and high-energy diet (HED). Mice were analyzed in vivo with the indirect calorimetry system, and tissues were analyzed by real-time PCR and Western blot analysis. DOR-knockout (KO) mice gained less weight (P<0.01) and had lower fat mass (P<0.01) when compared to WT mice fed an HED. Although DOR-KO mice were hyperphagic, they showed higher energy expenditure (P<0.05), which was the result of an increased activation of the thermogenic program in brown adipose tissue. The increased nonshivering thermogenesis involved the stimulation of uncoupling protein 1 (UCP1; P<0.01), peroxisome proliferator-activated receptor γ coactivator (PGC1α; P<0.05), and fibroblast growth factor 21 (FGF21; P<0.01). DOR deficiency also led to an attenuation of triglyceride content in the liver (P<0.05) in response to an HED. These findings reveal a novel role of DOR in the control of thermogenic markers and energy expenditure, and they provide a potential new therapeutic approach for the treatment of obesity.

Nonopioid effect of β-endorphin

This review presents the generalized literature data and the results of our own research of the nonopioid effect of β-endorphin, an opioid neuropeptide interacting not only with opioid but also with nonopioid (insensitive to the opioid antagonist naloxone) receptors. The roles of the hormone and its receptors in regulation of the immune, nervous, and endocrine systems are discussed. The effect of neuromediator on the immune system mediated by both opioid and nonopioid receptors is considered in detail. The data on distribution and function of the nonopioid β-endorphin receptor in human and animal organisms are presented. All available data on the characteristics of the nonopioid β-endorphin receptor obtained by means of radioligand analysis are given. The discussed information is supposed to extend our conceptions of the role of β-endorphin in mammals and to be of extensive use in medicine and pharmacology.

Fentanyl inhibits glucose-stimulated insulin release from β-cells in rat pancreatic islets

AIM: To explore the effects of fentanyl on insulin release from freshly isolated rat pancreatic islets in static culture.

METHODS: Islets were isolated from the pancreas of mature Sprague Dawley rats by common bile duct intraductal collagenase V digestion and were purified by discontinuous Ficoll density gradient centrifugation. The islets were divided into four groups according to the fentanyl concentration: control group (0 ng/mL), group I (0.3 ng/mL), group II (3.0 ng/mL), and group III (30 ng/mL). In each group, the islets were co-cultured for 48 h with drugs under static conditions with fentanyl alone, fentanyl + 0.1 μg/mL naloxone or fentanyl + 1.0 μg/mL naloxone. Cell viability was assessed by the MTT assay. Insulin release in response to low and high concentrations (2.8 mmol/L and 16.7 mmol/L, respectively) of glucose was investigated and electron microscopy morphological assessment was performed.

RESULTS: Low- and high-glucose-stimulated insulin release in the control group was significantly higher than in groups II and III (62.33 ± 9.67 μIU vs 47.75 ± 8.47 μIU, 39.67 ± 6.18 μIU and 125.5 ± 22.04 μIU vs 96.17 ± 14.17 μIU, 75.17 ± 13.57 μIU, respectively, P < 0.01) and was lowest in group III (P < 0.01). After adding 1 μg/mL naloxone, insulin release in groups II and III was not different from the control group. Electron microscopy studies showed that the islets were damaged by 30 ng/mL fentanyl.

CONCLUSION: Fentanyl inhibited glucose-stimulated insulin release from rat islets, which could be prevented by naloxone. Higher concentrations of fentanyl significantly damaged β-cells of rat islets.

The MOR-1 opioid receptor regulates glucose homeostasis by modulating insulin secretion

In addition to producing analgesia, opioids have also been proposed to regulate glucose homeostasis by altering insulin secretion. A considerable controversy exists, however, regarding the contribution of the mu-opioid receptor (MOR-1) to insulin secretion dynamics. We employed congenic C57BL/6J MOR-1 knockout (KO) mice to clarify the role of MOR in glucose homeostasis. We first found that both sexes of MOR-1 KO mice weigh more than wild-type mice throughout postnatal life and that this increase includes preferentially increased fat deposition. We also found that MOR-1 KO mice exhibit enhanced glucose tolerance that results from insulin hypersecretion that reflects increased beta-cell mass and increased secretory dynamics in the MOR-1 mutant mice compared with wild type. Analysis of the isolated islets indicated that islet insulin hypersecretion is mediated directly by MOR expressed on islet cells via a mechanism downstream of ATP-sensitive K(+) channel activation by glucose. These findings indicate that MOR-1 regulates body weight by a mechanism that involves insulin secretion and thus may represent a novel target for new diabetes therapies.

Glucose stimulation of pancreatic beta-cell lines induces expression and secretion of dynorphin

To investigate adaptive responses of pancreatic beta-cells to hyperglycemia, genes induced by glucose stimulation were identified by subtraction cloning. Among 53 clones representing differentially expressed genes, 20 encoded the endogenous opioid precursor, prodynorphin. The amino acid sequence of murine prodynorphin is identical to the rat protein in sequences comprising the opioid peptides and 86% identical in the remainder of the molecule. Stimulation of MIN6 cells increased prodynorphin RNA levels to more than 20-fold in proportion to physiological glucose concentrations. Similar induction levels were observed in murine betaTC3 and rat Rinm5F beta-cell lines. Prodynorphin RNA expression increased within 1 h of glucose stimulation, achieved maximal levels by 4 h, and remained elevated for at least 24 h. By using RIA, MIN6 cells were shown to contain and secrete increased amounts of dynorphin-A following glucose stimulation. Treatment of MIN6 cells with KCl, forskolin, or isobutyl-methyl-xanthine strongly induced prodynorphin RNA expression, suggesting that induction may be related to secretion-coupled signaling pathways. The induction of prodynorphin in several beta-cell lines is consistent with previous demonstrations of beta-cell synthesis of other endogenous opioids, including beta-endorphin, and suggests that opioids may have a potentially significant role in regulating beta-cell secretion.