The beta-cell sulfonylurea receptor

Signaling pathways and intervention for therapy of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) represents one of the fastest growing epidemic metabolic disorders worldwide and is a strong contributor for a broad range of comorbidities, including vascular, visual, neurological, kidney, and liver diseases. Moreover, recent data suggest a mutual interplay between T2DM and Corona Virus Disease 2019 (COVID-19). T2DM is characterized by insulin resistance (IR) and pancreatic β cell dysfunction. Pioneering discoveries throughout the past few decades have established notable links between signaling pathways and T2DM pathogenesis and therapy. Importantly, a number of signaling pathways substantially control the advancement of core pathological changes in T2DM, including IR and β cell dysfunction, as well as additional pathogenic disturbances. Accordingly, an improved understanding of these signaling pathways sheds light on tractable targets and strategies for developing and repurposing critical therapies to treat T2DM and its complications. In this review, we provide a brief overview of the history of T2DM and signaling pathways, and offer a systematic update on the role and mechanism of key signaling pathways underlying the onset, development, and progression of T2DM. In this content, we also summarize current therapeutic drugs/agents associated with signaling pathways for the treatment of T2DM and its complications, and discuss some implications and directions to the future of this field.

Mechanisms and Characteristics of Sulfonylureas and Glinides

BACKGROUND

Type 2 diabetes mellitus is a complex progressive endocrine disease characterized by hyperglycemia and life-threatening complications. It is the most common disorder of pancreatic cell function that causes insulin deficiency. Sulfonylurea is a class of oral hypoglycemic drugs. Over the past half century, these drugs, together with the subsequent non-sulfonylureas (glinides), have been the main oral drugs for insulin secretion.

OBJECTIVE

Through in-depth study, the medical profession considers it as an important drug for improving blood sugar control.

METHODS

The mechanism, characteristics, efficacy and side effects of sulfonylureas and glinides were reviewed in detail.

RESULTS

Sulfonylureas and glinides not only stimulated the release of insulin from pancreatic cells, but also had many extrapanular hypoglycemic effect, such as reducing the clearance rate of insulin in liver, reducing the secretion of glucagon, and enhancing the sensitivity of peripheral tissues to insulin in type 2 diabetes mellitus.

CONCLUSION

Sulfonylureas and glinides are effective first-line drugs for the treatment of diabetes mellitus. Although they have the risk of hypoglycemia, weight gain and cardiovascular disease, their clinical practicability and safety can be guaranteed as long as they are reasonably used.

Persistent hyperinsulinemic hypoglycemia presenting with a rare complication: West syndrome

BACKGROUND

Persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is characterized by disproportional secretion of insulin from pancreatic beta-cells. Although one of the manifestations of hypoglycemia is West syndrome, it is rarely reported in PHHI.

PATIENT REPORT

A 6 month-old girl who was followed up with the diagnosis of PHHI was admitted to hospital with the complaint of jerky movements at her extremities. EEG revealed the typical pattern of hypsarrhythmia leading to the diagnosis of West syndrome.

CONCLUSION

To our knowledge, there is only one report in the literature of West syndrome as a manifestation of PHHI, and that was the hyperammoniemic form of the disease. The present report is the first of normoammoniemic PHHI leading to West syndrome. We wish to highlight the potential risks of PHHI, especially in inadequately treated patients, and to emphasize that close neurological follow-up is very important in children who suffer from PHHI.

Synthesis and evaluation of fluorine-18 labeled glyburide analogs as beta-cell imaging agents

Glyburide is a prescribed hypoglycemic drug for the treatment of type 2 diabetic patients. We have synthesized two of its analogs, namely N-[4-[beta-(2-(2'-fluoroethoxy)-5-chlorobenzenecarboxamido)ethyl]benzenesulfonyl]-N'-cyclohexylurea (2-fluoroethoxyglyburide, 8b) and N-[4-[beta-(2-(2'-fluoroethoxy)-5-iodobenzenecarboxamido)ethyl]benzenesulfonyl]-N'-cyclohexylurea (2-fluoroethoxy-5-deschloro-5-iodoglyburide, 8a), and their fluorine-18 labeled analogs as beta-cell imaging agents. Both F-18 labeled compound 8a and compound 8b were synthesized by alkylation of the corresponding multistep synthesized hydroxy precursor 4a and 4b with 2-[(18)F]fluoroethyl tosylate in DMSO at 120 degrees C for 20 minutes followed by HPLC purification in an overall radiochemical yield of 5-10% with a synthesis time of 100 minutes from EOB. The octanol/water partition coefficients of compounds 8a and 8b were 141.21 +/- 27.77 (n = 8) and 124.33 +/- 21.61 (n = 8), respectively. Insulin secretion experiments of compounds 8a and 8b on rat islets showed that both compounds have a similar stimulating effect on insulin secretion as that of glyburide. In vitro binding studies showed that approximately 2% of compounds 8a and 8b bound to beta TC3 and Min6 cells and that the binding was saturable. Preliminary biodistribution studies in mice showed that the uptake of both compounds 8a and 8b in liver and small intestine were high, whereas the uptake in other organs studied including pancreas were low. Additionally, the uptake of compound 8b in vivo was nonsaturable. These results tend to suggest that compounds 8a and 8b may not be the ideal beta-cell imaging agents.

α-Endosulfine, a potential regulator of insulin secretion, is required for adult tissue growth control in Drosophila

Alpha-endosulfine is a small protein that has been proposed to regulate ion channel activity and insulin secretion, but in vivo studies have been lacking. We have previously established the Drosophila ovary as a model system in which to study adult tissue growth regulation, and demonstrated a role of the insulin pathway in the proliferative response of ovarian cells to nutritional changes. Here, we find that the Drosophila alpha-endosulfine (dendos) gene, whose protein is expressed in germline and somatic cells of the ovary, as well as in the brain and certain regions of the intestine, is also required for this response. This requirement is non-cell autonomous, which is consistent with a role of dendos in secretion of Drosophila insulin-like peptides (DILPs), required for the proliferative response to nutritional changes. Our results show that dendos is also required for a distinct process in oogenesis, namely, the osmotic regulation of stage 14 oocytes, and that this requirement is cell autonomous, consistent with the role in ion channel regulation suggested by studies of the mammalian homologues.

Gene expression of alpha-endosulfine in the rat brain: correlative changes with aging, learning and stress

Endosulfine (EDSF) belongs to a highly conserved cAMP-regulated phosphoprotein (ARPP) family and was first isolated from ovine brain as a possible endogenous ligand for sulfonylurea receptors. To explore its involvement in brain functions, we investigated regional distribution of alpha-EDSF gene expression in the rat brain, and its regulation under physiological and pathological conditions. The majority of alpha-EDSF gene was expressed in the pyramidal neurons, which represent the principal excitatory neurons in various brain regions. Down-regulation of alpha-EDSF mRNA was detected in the rat hippocampus during long-term memory consolidation following a spatial learning experience, whereas swimming-related stress caused persistent up-regulation of alpha-EDSF gene expression in several brain regions. These changes, however, were absent from brains of diabetic rats that were subjected to the same behavioral treatments. Intracerebroventricular injection of streptozocin with a toxic dose induced severe learning deficits and brain structure alteration accompanied by a massive increase of alpha-EDSF mRNA in the somatosensory cortex. These results suggest that alpha-EDSF gene expression is differentially regulated by distinct brain processes involving excitatory neuronal activities.

Ion channels in secretory granules of the pancreas and their role in exocytosis and release of secretory proteins

Regulated secretion in exocrine and neuroendocrine cells occurs through exocytosis of secretory granules and the subsequent release of stored small molecules and proteins. The introduction of biophysical techniques with high temporal and spatial resolution, and the identification of Ca(2+)-dependent and -independent "docking" and "fusion" proteins, has greatly enhanced our understanding of exocytosis. The cloning of families of ion channel proteins, including intracellular ion channels, has also revived interest in the role of secretory granule ion channels in exocytotic secretion. Thus secretory granules of pancreatic acinar cell express a ClC-2 Cl(-) channel, a HCO-permeable member of the CLCA Ca(2+)-dependent anion channel family, and a KCNQ1 K(+) channel. Evidence suggests that these channels may facilitate the release of digestive enzymes and/or prevent exocytosed granules from collapsing during "kiss and run" recycling. In pancreatic beta-cells, a granular ClC-3 Cl(-) channel provides a shunt pathway for a vacuolar-type H(+)-ATPase. Acidification "primes" the granules for Ca(2+)-dependent exocytosis and release of insulin. In summary, secretory granules are equipped with specific sets of ion channels, which modulate regulated exocytosis and the release of macromolecules. These channels could represent excellent targets for therapeutic interventions to control exocytotic secretion in relevant diseases, such as pancreatitis, cystic fibrosis, or diabetes mellitus.

Characterization of two novel forms of the rat sulphonylurea receptor SUR1A2 and SUR1BDelta31

1. The ATP-sensitive potassium channel (K(ATP)) of pancreatic beta-cells is composed of the sulphonylurea-binding protein, SUR1, and the inwardly rectifying K(+) channel subunit, Kir6.2. We have characterized two novel isoforms of rat SUR1 in the RINm5F insulin-secreting cell line. 2. SUR1A2 is an allelic variant with a single amino acid change in the first nucleotide-binding domain. Coinjection of SUR1A2 plus Kir6.2 into Xenopus oocytes or expression of a SUR1A2-Kir6.2 tandem in HEK-293 cells yielded large currents with characteristics similar to the wild-type K(ATP) channel. 3. SUR1BDelta31, detected in several human tissues, is a splice variant of the rat SUR1 gene that lacks exon 31 of the corresponding human SUR1 gene. SUR1BDelta31 lacks the TM16-TM17 transmembrane-spanning helices leading to a protein with a different transmembrane topology. Coinjection of SUR1BDelta31 plus Kir6.2 into Xenopus oocytes or expression of the Kir6.2/SUR1BDelta31 tandem construct in HEK-293 cells did not result in any current, and a surface expression assay indicated that this channel does not reach the plasma membrane. 4. SUR1A2 and SUR1A1 proteins expressed in HEK-293 cells display similar binding affinities for [(3)H]-glibenclamide, while SUR1BDelta31 shows a 500-fold lower affinity. 5. These findings confirm that TM16-TM17 of SUR1 are important for high-affinity glibenclamide binding and that their deletion impairs trafficking of the K(ATP) channel to the surface membrane.

Membrane targeting of ATP-sensitive potassium channel. Effects of glycosylation on surface expression

Oligosaccharides play significant roles in trafficking, folding, and sorting of membrane proteins. Sulfonylurea receptors (SURx), members of the ATP binding cassette family of proteins, associate with the inward rectifier Kir6.x to form ATP-sensitive potassium channels (K(ATP)). These channels are found on the plasma membrane in many tissues and play a pivotal role in synchronizing electrical excitability with cell metabolic state. Trafficking defects resulting from three independent SUR1 mutations involved in the disease persistent hyperinsulinemic hypoglycemia of infancy have been described. Two of these mutations displayed notable decreases in glycosylation. Here we have investigated the relationship between the two N-linked glycosylation sites (Asn(10) and Asn(1050)) and SUR1 trafficking. Using patch clamp analysis, surface biotinylation, and immunofluorescence microscopy, we demonstrate a significant decrease in surface expression of SUR1 single or double glycosylation site mutants (N10Q,N1050Q) when co-expressed with Kir6.2. Additionally, we show prominent retention within the ER of the SUR1 double glycosylation mutant under the same conditions. Further investigation revealed that mutation of the ER retention signal was able to partially restore surface expression of the SUR1 double glycosylation mutant. These studies suggest that SUR1 glycosylation is a key element for the proper trafficking and surface expression of K(ATP) channels.

Investigation of the molecular assembly of beta-cell K(ATP) channels

We have investigated the protein interactions involved in the assembly of pancreatic beta-cell ATP-sensitive potassium channels. The channels are a heterooligomeric complex of pore-forming Kir6.2 subunits and sulfonylurea receptor (SUR1) subunits. SUR1 belongs to the ATP binding cassette (ABC) family of proteins and has two nucleotide binding domains (NBD1 and NBD2) and 17 putative transmembrane (TM) sequences. Previously we showed that co-expression in a baculovirus expression system of two parts of SUR1 divided at Pro1042 between TM12 and 13 leads to restoration of glibenclamide binding activity, whereas expression of either individual N- or C-terminal domain alone gave no glibenclamide binding activity [M.V. Mikhailov and S.J.H. Ashcroft (2000) J. Biol. Chem. 275, 3360-3364]. Here we show that the two half-molecules formed by division of SUR1 between NBD1 and TM12 or between TM13 and 14 also self-assemble to give glibenclamide binding activity. However, deletion of NBD1 from the N-part of SUR1 abolished SUR1 assembly, indicating a critical role for NBD1 in SUR1 assembly. We found that differences in glibenclamide binding activity obtained after co-expression of different half-molecules are attributable to different amounts of binding sites, but the binding affinities remained nearly the same. Simultaneous expression of Kir6.2 resulted in enhanced glibenclamide binding activity only when the N-half of SUR1 included TM12. We conclude that TM12 and 13 are not essential for SUR1 assembly whereas TM12 takes part in SUR1 Kir6.2 interaction. This interaction is specific for Kir 6.2 because no enhancement of glibenclamide binding was observed when half-molecules were expressed together with Kir4.1. We propose a model of K(ATP) channel organisation based on these data.

Interactions of the sulfonylurea receptor 1 subunit in the molecular assembly of beta-cell K(ATP) channels

We have investigated protein interactions involved in pancreatic beta-cell ATP-sensitive potassium channel assembly. These channels, which are of key importance for control of insulin release, are a hetero-oligomeric complex of pore-forming Kir6.2 subunits and sulfonylurea receptor (SUR1) subunits with two nucleotide-binding domains (NBD1 and NBD2). We divided SUR1 into two halves at Pro-1042. Expression of either the individual N- or C-terminal domain in a baculovirus expression system did not lead to glibenclamide binding activity, although studies with green fluorescent protein fusion proteins showed that both half-molecules were inserted into the plasma membrane. However, significant glibenclamide binding activity was observed when the half-molecules were co-expressed (even when NBD2 was deleted from the C-terminal half-molecule). Simultaneous expression of Kir6.2 resulted in enhanced glibenclamide binding activity. We conclude that the glibenclamide-binding site includes amino acid residues from both halves of the molecule, that there is strong interaction between different regions of SUR1, that NBD2 is not essential for glibenclamide binding, and that interactions between Kir6.2 and SUR1 participate in ATP-sensitive potassium channel assembly. Investigation of NBD1-green fluorescent protein fusion protein distribution inside insect cells expressing C-terminal halves of SUR1 demonstrated strong interaction between NBD1 and NBD2. We also expressed and purified NBD1 from Escherichia coli. Purified NBD1 was found to exist as a tetramer indicating strong homomeric attractions and a possible role for NBD1 in SUR1 assembly.

Toward understanding the assembly and structure of KATP channels

Adenosine 5'-triphosphate-sensitive potassium (KATP) channels couple metabolic events to membrane electrical activity in a variety of cell types. The cloning and reconstitution of the subunits of these channels demonstrate they are heteromultimers of inwardly rectifying potassium channel subunits (KIR6.x) and sulfonylurea receptors (SUR), members of the ATP-binding cassette (ABC) superfamily. Recent studies indicate that SUR and KIR6.x associate with 1:1 stoichiometry to assemble a large tetrameric channel, (SUR/KIR6.x)4. The KIR6.x subunits form the channel pore, whereas SUR is required for activation and regulation. Two KIR6.x genes and two SUR genes have been identified, and combinations of subunits give rise to KATP channel subtypes found in pancreatic beta-cells, neurons, and cardiac, skeletal, and smooth muscle. Mutations in both the SUR1 and KIR6.2 genes have been shown to cause familial hyperinsulinism, indicating the importance of the pancreatic beta-cell channel in the regulation of insulin secretion. The availability of cloned KATP channel genes opens the way for characterization of this family of ion channels and identification of additional genetic defects.

Is 95% pancreatectomy the procedure of choice for treatment of persistent hyperinsulinemic hypoglycemia of the neonate?

A 95% pancreatectomy became the treatment of choice for persistent hyperinsulinemic hypoglycemia of the neonate (PHHN, Nesidioblastosis) at the author's institution, when lesser resections failed to prevent hypoglycemia in 25% to 50% of cases. With few outcome data available in the literature, the authors reviewed their 25-year experience to assess the efficacy and the long-term consequences of this procedure. Since 1971, 27 infants underwent a 95% pancreatectomy for the treatment of PHHN. None had responded to medical treatment (glucose infusion, glucagon, octreotide, diazoxide), and two had 85% pancreatectomy that failed. The procedure consisted of resecting the pancreas including the uncinate process, leaving only the gland lying between the common bile duct (CBD) and the duodenum and a small rim of pancreas along the duodenal sweep. Hyperinsulinemia and hypoglycemia recurred in nine children (33%), all within 2 to 5 days. Seven of them were subsequently cured with near-total pancreatic resection. Partial pancreatic regrowth was evident at reoperation. In two cases hypoglycemia was controlled with diazoxide and frequent feedings because reoperation was refused. The gross anatomic findings and the histopathology were not predictive of treatment failure. Perioperative complications occurred in four of 27 children (15%) after 95% pancreatectomy and in four of seven children (57%) after near-total pancreatectomy. Clinical follow-up ranged from 0.5 to 18 years (mean, 8 years; median, 8 years). To date, diabetes has developed in 15 children (56%), nine of 20 (45%) after 95% pancreatectomy (mean age, 9.7 years) and six of seven (86%) after a near-total pancreatectomy (mean age, 1.7 years). After 95% pancreatectomy, the incidence of diabetes increased with age, developing in nine of the 13 (69%) children followed up for more than 4 years. The failure of 95% pancreatectomy to prevent hypoglycemia in one third of children with PHHN and the ultimate development of diabetes in a minimum of two-thirds, indicates that an alternative treatment strategy is needed for this disease.