Inhibition of voltage-gated potassium channels mediates uncarboxylated osteocalcin-regulated insulin secretion in rat pancreatic β cells

Hyperglycemia from Diabetes Potentiates Uncarboxylated Osteocalcin-Stimulated Insulin Secretion in Rat INS-1 Pancreatic β-Cells

Uncarboxylated osteocalcin (ucOC) is a hormone secreted by osteoblasts that strengthens bone during mineralization and is a biomarker for ongoing bone formation. It also regulates glucose homeostasis by stimulating insulin secretion from pancreatic β-cells. However, its effect on β-cells under hyperglycemic diabetic conditions is unclear. The objective of this study was to investigate ucOC's effect on insulin secretion in β-cells maintained under high glucose conditions. We hypothesized that hyperglycemia potentiates insulin secretion in response to ucOC stimulation. Using INS-1 cells, we performed insulin secretion experiments, intracellular calcium recordings, and RT-qPCR to determine ucOC's effect on glucose-stimulated insulin secretion (GSIS)-related genes. The results reveal that ucOC significantly increased insulin secretion under hyperglycemic conditions compared to lower glucose levels. High glucose conditions also potentiated the effect of ucOC on calcium signals, which enhanced insulin secretion. The increase in intracellular calcium was due to an influx from the extracellular space via voltage-dependent calcium channels (VDCCs). Interestingly, the treatment of cells with NPS-2143, a GPRC6A blocker, failed to abolish the calcium signals. Uncarboxylated osteocalcin upregulated the expression of GSIS-related genes under high glucose conditions (450 mg/dL) compared to cells under standard culture conditions (200 mg/dL). In conclusion, hyperglycemia potentiates ucOC-induced insulin secretion in β-cells by opening VDCCs and upregulating GSIS genes. These findings provide a better understanding of ucOC's mechanism in the diabetic state and could lead to alternative treatments to stimulate insulin secretion.

Berberine is an insulin secretagogue targeting the KCNH6 potassium channel

Coptis chinensis is an ancient Chinese herb treating diabetes in China for thousands of years. However, its underlying mechanism remains poorly understood. Here, we report the effects of its main active component, berberine (BBR), on stimulating insulin secretion. In mice with hyperglycemia induced by a high-fat diet, BBR significantly increases insulin secretion and reduced blood glucose levels. However, in mice with hyperglycemia induced by global or pancreatic islet β-cell-specific Kcnh6 knockout, BBR does not exert beneficial effects. BBR directly binds KCNH6 potassium channels, significantly accelerates channel closure, and subsequently reduces KCNH6 currents. Consequently, blocking KCNH6 currents prolongs high glucose-dependent cell membrane depolarization and increases insulin secretion. Finally, to assess the effect of BBR on insulin secretion in humans, a randomized, double-blind, placebo-controlled, two-period crossover, single-dose, phase 1 clinical trial (NCT03972215) including 15 healthy men receiving a 160-min hyperglycemic clamp experiment is performed. The pre-specified primary outcomes are assessment of the differences of serum insulin and C-peptide levels between BBR and placebo treatment groups during the hyperglycemic clamp study. BBR significantly promotes insulin secretion under hyperglycemic state comparing with placebo treatment, while does not affect basal insulin secretion in humans. All subjects tolerate BBR well, and we observe no side effects in the 14-day follow up period. In this study, we identify BBR as a glucose-dependent insulin secretagogue for treating diabetes without causing hypoglycemia that targets KCNH6 channels.

Bone-Derived Modulators That Regulate Brain Function: Emerging Therapeutic Targets for Neurological Disorders

Bone has traditionally been regarded as a structural organ that supports and protects the various organs of the body. Recent studies suggest that bone also acts as an endocrine organ to regulate whole-body metabolism. Particularly, homeostasis of the bone is shown to be necessary for brain development and function. Abnormal bone metabolism is associated with the onset and progression of neurological disorders. Recently, multiple bone-derived modulators have been shown to participate in brain function and neurological disorders, including osteocalcin, lipocalin 2, and osteopontin, as have bone marrow-derived cells such as mesenchymal stem cells, hematopoietic stem cells, and microglia-like cells. This review summarizes current findings regarding the roles of these bone-derived modulators in the brain, and also follows their involvement in the pathogenesis of neurological disorders. The content of this review may aide in the development of promising therapeutic strategies for neurological disorders via targeting bone.

Antidiabetic compounds 8a, 8b, 8k, and 9h enhance insulin secretion: activity and mechanism

PURPOSE

This study primarily investigated the effects of hypoglycemic compounds (Imeglimin derivatives) on insulin secretion in type 2 diabetes mellitus (T2DM), and further explored the possible mechanism underlying these effects.

METHODS

Firstly, Metformin was used as the initiating compound to synthesize three sets of derivatives which contained Imeglimin structure core. At the cellular level, we screened compounds with better effect on the activity of insulin receptor tyrosine protein kinase (IFcTPK) after the islet β cells were treated with the compounds of different concentrations. The insulin secretion was assessed using radioimmunoassay and the cytotoxicity to islet β cells was evaluated by means of MTT assay following treatment with the compounds. The Ca²⁺-related mechanism by which these compounds promote insulin secretion was elucidated with whole cell recordings from current-clamp mode.

RESULTS

Totally, 48 synthesized compounds were generated, wherein 10 compounds could increase the activity of IFcTPK in HIT-T15 cells better among these compounds. The modified Imeglimin, especially in the structure of hydrophilic hydroxyl or piperidine rings, could improve the activity of the compound to promote insulin secretion. Furthermore, the compounds 8a, 8b, 8k, and 9h revealed high insulin secretion-promoting activity. These compounds enhanced insulin secretion in islet β cells by repressing the ATP-sensitive K(+) and voltage-gated K+ pathway.

CONCLUSIONS

Our findings indicate that the hypoglycemic compounds 8a, 8b, 8k, and 9h confer better promotive effect on insulin secretion, which provides a reference for the development of drugs with better hypoglycemic activity.

Bone Regulation of Insulin Secretion and Glucose Homeostasis

For centuries our image of the skeleton has been one of an inert structure playing a supporting role for muscles and a protective role for inner organs like the brain. Cell biology and physiology modified this view in the 20st century by defining the constant interplay between bone-forming and bone resorbing cells that take place during bone growth and remodeling, therefore demonstrating that bone is as alive as any other tissues in the body. During the past 40 years human and, most important, mouse genetics, have allowed not only the refinement of this notion by identifying the many genes and regulatory networks responsible for the crosstalk existing between bone cells, but have redefined the role of bone by showing that its influence goes way beyond its own physiology. Among its newly identified functions is the regulation of energy metabolism by 2 bone-derived hormones, osteocalcin and lipocalin-2. Their biology and respective roles in this process are the topic of this review.

D2-Like Receptors Mediate Dopamine-Inhibited Insulin Secretion via Ion Channels in Rat Pancreatic β-Cells

Dopamine (DA) has a vital role in the central nervous system and also modulates lipid and glucose metabolism. The present study aimed to investigate the effect of dopamine on insulin secretion and the underlying mechanisms in rat pancreatic β-cells. Data from the radioimmunoassay indicated that dopamine inhibited insulin secretion in a glucose- and dose-dependent manner. This inhibitory effect of dopamine was mediated mainly by D2-like receptors, but not D1-like receptors. Whole-cell patch-clamp recordings showed that dopamine decreased voltage-dependent Ca²⁺ channel currents, which could be reversed by inhibition of the D2-like receptor. Dopamine increased voltage-dependent potassium (K_V) channel currents and shortened action potential duration, which was antagonized by inhibition of D2-like receptors. Further experiments showed that D2-like receptor activation by quinpirole increased K_V channel currents. In addition, using calcium imaging techniques, we found that dopamine reduced intracellular Ca²⁺ concentration, which was also reversed by D2-like receptor antagonists. Similarly, quinpirole was found to decrease intracellular Ca²⁺ levels. Taken together, these findings demonstrate that dopamine inhibits insulin secretion mainly by acting on D2-like receptors, inhibiting Ca²⁺ channels, and activating Kv channels. This process results in shortened action potential duration and decreased intracellular Ca²⁺ levels in β-cells. This work offers new insights into a glucose-dependent mechanism whereby dopamine regulates insulin secretion.

Insulinotropic Activity of Standardized Methanolic Extracts of Ficus deltoidea from Seven Varieties

Ficus deltoidea is a traditional medicinal plant that has been proven to show antidiabetic effects. This study focus is to assess the insulin secretion activity of Ficus deltoidea standardized methanolic extracts from seven independent varieties and mechanisms that underlie the insulin secretion action of the extracts. The cytotoxicity of Ficus deltoidea extracts was tested using viability assay. The insulin secretion assay was carried out by treating clonal BRIN BD11 cell line with standardized methanolic Ficus deltoidea extracts or glybenclamide. The clonal BRIN BD11 cell was also treated with insulin agonist and antagonist to elucidate the insulin secretion mechanism. Only the viability percentage for Ficus deltoidea var. kunstleri and intermedia was identified to be toxic at 500 and 1000 μg/ml (P<0.001). The insulin secretion for Ficus deltoidea var. deltoidea, angustifolia, and motleyana was dose-dependent; further evaluation suggested that Ficus deltoidea var. trengganuensis was involved in K_ATP-independent pathway. This study suggests that standardized methanolic extracts of Ficus deltoidea varieties have an insulinotropic effect on clonal BRIN BD11 cell line and can be utilized as a modern candidate of antidiabetic agents targeting the escalation for insulin secretion from pancreatic beta cells.

Regulation of Energy Metabolism by Bone-Derived Hormones

Like many other organs, bone can act as an endocrine organ through the secretion of bone-specific hormones or "osteokines." At least two osteokines are implicated in the control of glucose and energy metabolism: osteocalcin (OCN) and lipocalin-2 (LCN2). OCN stimulates the production and secretion of insulin by the pancreatic β-cells, but also favors adaptation to exercise by stimulating glucose and fatty acid (FA) utilization by the muscle. Both of these OCN functions are mediated by the G-protein-coupled receptor GPRC6A. In contrast, LCN2 influences energy metabolism by activating appetite-suppressing signaling in the brain. This action of LCN2 occurs through its binding to the melanocortin 4 receptor (MC4R) in the paraventricular nucleus of the hypothalamus (PVN) and ventromedial neurons of the hypothalamus.

Osteocalcin in the brain: from embryonic development to age-related decline in cognition

A remarkable, unexpected aspect of the bone-derived hormone osteocalcin is that it is necessary for both brain development and brain function in the mouse, as its absence results in a profound deficit in spatial learning and memory and an exacerbation of anxiety-like behaviour. The regulation of cognitive function by osteocalcin, together with the fact that its circulating levels decrease in midlife compared with adolescence in all species tested, raised the prospect that osteocalcin might be an anti-geronic hormone that could prevent age-related cognitive decline. As presented in this Review, recent data indicate that this is indeed the case and that osteocalcin is necessary for the anti-geronic activity recently ascribed to the plasma of young wild-type mice. The diversity and amplitude of the functions of osteocalcin in the brain, during development and postnatally, had long called for the identification of its receptor in the brain, which was also recently achieved. This Review presents our current understanding of the biology of osteocalcin in the brain, highlighting the bony vertebrate specificity of the regulation of cognitive function and pointing toward where therapeutic opportunities might exist.

Influence of Flavonoids on Mechanism of Modulation of Insulin Secretion

BACKGROUND

The development of alternatives for insulin secretion control in vivo or in vitro represents an important aspect to be investigated. In this direction, natural products have been progressively explored with this aim. In particular, flavonoids are potential candidates to act as insulin secretagogue.

OBJECTIVE

To study the influence of flavonoid on overall modulation mechanisms of insulin secretion.

METHODS

The research was conducted in the following databases and platforms: PubMed, Scopus, ISI Web of Knowledge, SciELO, LILACS, and ScienceDirect, and the MeSH terms used for the search were flavonoids, flavones, islets of Langerhans, and insulin-secreting cells.

RESULTS

Twelve articles were included and represent the basis of discussion on mechanisms of insulin secretion of flavonoids. Papers in ISI Web of Knowledge were in number of 1, Scopus 44, PubMed 264, ScienceDirect 511, and no papers from LILACS and SciELO databases.

CONCLUSION

According to the literature, the majority of flavonoid subclasses can modulate insulin secretion through several pathways, in an indication that corresponding molecule is a potential candidate for active materials to be applied in the treatment of diabetes.

SUMMARY

The action of natural products on insulin secretion represents an important investigation topic due to their importance in the diabetes controlIn addition to their typical antioxidant properties, flavonoids contribute to the insulin secretionThe modulation of insulin secretion is induced by flavonoids according to different mechanisms. Abbreviations used: KATP channels: ATP-sensitive K+ channels, GLUT4: Glucose transporter 4, ERK1/2: Extracellular signal-regulated protein kinases 1 and 2, L-VDCCs: L-type voltage-dependent Ca+2 channels, GLUT1: Glucose transporter 1, AMPK: Adenosine monophosphate-activated protein kinase, PTP1B: Protein tyrosine phosphatase 1B, GLUT2: Glucose transporter 2, cAMP: Cyclic adenosine monophosphate, PKA: Protein kinase A, PTK: Protein tyrosine kinase, CaMK II: Ca2+/calmodulin-dependent protein kinase II, GSIS: Glucose-stimulated insulin secretion, Insig-1: Insulin-induced gene 1, IRS-2: Insulin receptor substrate 2, PDX-1: Pancreatic and duodenal homeobox 1, SREBP-1c: Sterol regulatory element binding protein-1c, DMC: Dihydroxy-6'-methoxy-3',5'-dimethylchalcone, GLP-1: Glucagon-like peptide-1, GLP-1R: Glucagon-like peptide 1 receptor.

Comprehensive metabolic characterization of serum osteocalcin action in a large non-diabetic sample

Recent research suggested a metabolic implication of osteocalcin (OCN) in e.g. insulin sensitivity or steroid production. We used an untargeted metabolomics approach by analyzing plasma and urine samples of 931 participants using mass spectrometry to reveal further metabolic actions of OCN. Several detected relations between OCN and metabolites were strongly linked to renal function, however, a number of associations remained significant after adjustment for renal function. Intermediates of proline catabolism were associated with OCN reflecting the implication in bone metabolism. The association to kynurenine points towards a pro-inflammatory state with increasing OCN. Inverse relations with intermediates of branch-chained amino acid metabolism suggest a link to energy metabolism. Finally, urinary surrogate markers of smoking highlight its adverse effect on OCN metabolism. In conclusion, the present study provides a read-out of metabolic actions of OCN. However, most of the associations were weak arguing for a limited role of OCN in whole-body metabolism.

Multifaceted interaction of bone, muscle, lifestyle interventions and metabolic and cardiovascular disease: role of osteocalcin

Undercarboxylated osteocalcin (ucOC) may play a role in glucose homeostasis and cardiometabolic health. This review examines the epidemiological and interventional evidence associating osteocalcin (OC) and ucOC with metabolic risk and cardiovascular disease. The complexity in assessing such correlations, due to the observational nature of human studies, is discussed. Several studies have reported that higher levels of ucOC and OC are correlated with lower fat mass and HbA1c. In addition, improved measures of glycaemic control via pharmacological and non-pharmacological (e.g. exercise or diet) interventions are often associated with increased circulating levels of OC and/or ucOC. There is also a relationship between lower circulating OC and ucOC and increased measures of vascular calcification and cardiovascular disease. However, not all studies have reported such relationship, some with contradictory findings. Equivocal findings may arise because of the observational nature of the studies and the inability to directly assess the relationship between OC and ucOC on glycaemic control and cardiovascular health in humans. Studying OC and ucOC in humans is further complicated due to numerous confounding factors such as sex differences, menopausal status, vitamin K status, physical activity level, body mass index, insulin sensitivity (normal/insulin resistance/T2DM), tissue-specific effects and renal function among others. Current observational and indirect interventional evidence appears to support a relationship between ucOC with metabolic and cardiovascular disease. There is also emerging evidence to suggest a direct role of ucOC in human metabolism. Further mechanistic studies are required to (a) clarify causality, (b) explore mechanisms involved and

The PLC/PKC/Ras/MEK/Kv channel pathway is involved in uncarboxylated osteocalcin-regulated insulin secretion in rats

Uncarboxylated osteocalcin, a bone matrix protein, has been proposed to regulate glucose metabolism by increasing insulin secretion, improving insulin sensitivity and stimulating β cell proliferation. Our previous study also indicated that uncarboxylated osteocalcin stimulates insulin secretion by inhibiting voltage-gated potassium (K_V) channels. The goal of this study is to further investigate the underlying mechanisms for the regulation of Kv channels and insulin secretion by uncarboxylated osteocalcin. Insulin secretion and Kv channel currents were examined by radioimmunoassay and patch-clamp technique, respectively. Calcium imaging system was applied to measure intracellular Ca²⁺ concentration ([Ca²⁺]_i). The protein levels were detected by western blot. The results showed that uncarboxylated osteocalcin potentiated insulin secretion, inhibited Kv channels and increased [Ca²⁺]_i compared to control. These effects were suppressed by phospholipase-C (PLC)/protein kinase C (PKC)/Ras/MAPK-ERK kinase (MEK) signaling pathway, indicating that this signaling pathway plays an important role in uncarboxylated osteocalcin-regulated insulinotropic effect. In addition, the results also showed that adenylyl cyclase (AC) did not influence the effect of uncarboxylated osteocalcin on insulin secretion and Kv channels, suggesting that AC is not involved in uncarboxylated osteocalcin-stimulated insulin secretion. These findings provide new insight into the mechanism of uncarboxylated osteocalcin-regulated insulin secretion.