Identification of the metal associated with the insulin degrading enzyme

Type 2 diabetes as a protein misfolding disease

Type 2 diabetes (T2D) is a highly prevalent and chronic metabolic disorder. Recent evidence suggests that formation of toxic aggregates of the islet amyloid polypeptide (IAPP) might contribute to β-cell dysfunction and disease. However, the mechanism of protein aggregation and associated toxicity remains unclear. Misfolding, aggregation, and accumulation of diverse proteins in various organs is the hallmark of the group of protein misfolding disorders (PMDs), including highly prevalent illnesses affecting the central nervous system (CNS) such as Alzheimer's disease (AD) and Parkinson's disease (PD). In this review we discuss the current understanding of the mechanisms implicated in the formation of protein aggregates in the endocrine pancreas and associated toxicity in the light of the long-standing knowledge from neurodegenerative disorders associated with protein misfolding.

Gut microbiota in health and disease

Gut microbiota is an assortment of microorganisms inhabiting the length and width of the mammalian gastrointestinal tract. The composition of this microbial community is host specific, evolving throughout an individual's lifetime and susceptible to both exogenous and endogenous modifications. Recent renewed interest in the structure and function of this "organ" has illuminated its central position in health and disease. The microbiota is intimately involved in numerous aspects of normal host physiology, from nutritional status to behavior and stress response. Additionally, they can be a central or a contributing cause of many diseases, affecting both near and far organ systems. The overall balance in the composition of the gut microbial community, as well as the presence or absence of key species capable of effecting specific responses, is important in ensuring homeostasis or lack thereof at the intestinal mucosa and beyond. The mechanisms through which microbiota exerts its beneficial or detrimental influences remain largely undefined, but include elaboration of signaling molecules and recognition of bacterial epitopes by both intestinal epithelial and mucosal immune cells. The advances in modeling and analysis of gut microbiota will further our knowledge of their role in health and disease, allowing customization of existing and future therapeutic and prophylactic modalities.

Yeast Ste23p shares functional similarities with mammalian insulin-degrading enzymes

The S. cerevisiae genome encodes two M16A enzymes: Axl1p and Ste23p. Of the two, Ste23p shares significantly higher sequence identity with M16A enzymes from other species, including mammalian insulin-degrading enzymes (IDEs). In this study, recombinant Ste23p and R. norvegicus IDE (RnIDE) were isolated from E. coli, and their enzymatic properties compared. Ste23p was found to cleave established RnIDE substrates, including the amyloid-beta peptide (Abeta1-40) and insulin B-chain. A novel internally quenched fluorogenic substrate (Abz-SEKKDNYIIKGV-nitroY-OH) based on the polypeptide sequence of the yeast P2 a-factor mating propheromone was determined to be a suitable substrate for both Ste23p and RnIDE, and was used to conduct comparative enzymological studies. Both enzymes were most active at 37 degrees C, in alkaline buffers and in high salt environments. In addition, the proteolytic activities of both enzymes towards the fluorogenic substrate were inhibited by metal chelators, thiol modifiers, inhibitors of cysteine protease activity and insulin. Characteristics of STE23 expression were also evaluated. Our analysis indicates that the 5' terminus of the STE23 gene has been mischaracterized, with the physiologically relevant initiator corresponding to residue M53 of the publicly annotated protein sequence. Finally, we demonstrate that, unlike haploid-specific Axl1p, Ste23p is expressed in both haploid and diploid cell types. Our study presents the first comprehensive biochemical analysis of a yeast M16A enzyme, and provides evidence that S. cerevisiae Ste23p has enzymatic properties that are highly consistent with mammalian IDEs and other M16A enzymes.

The proteolytic activity of insulin-degrading enzyme: a mass spectrometry study

The prominent role that insulin-degrading enzyme (IDE) has on amyloidogenic peptides degradation has recently boosted a lot of attention toward this enzyme. Although many substrates are known to be degraded by IDE, little is known about the changes in the proteolytic activity of the enzyme upon modification of environmental factors. In a previous work we have already shown the great potentiality of atmospheric pressure/laser desorption ionization-mass spectrometry (AP/MALDI-MS) for studying the interaction between IDE and insulin. Here, the activity of IDE was investigated regarding cleavage sites' preferentiality upon modification of environmental factors by AP/MALDI-MS. The roles that IDE/insulin concentration ratio, reaction time, adenosine 5'-triphosphate (ATP) and metal ions (Zn and Cu) have on the insulin cleavage pattern produced by IDE are investigated and a plausible interpretation involving the proteolytic action of the different IDE oligomeric forms is proposed.

Association of polymorphisms in the insulin-degrading enzyme gene with type 2 diabetes in the Korean population

Insulin-degrading enzyme (IDE) is a metalloproteinase which degrades insulin and terminates its action. Homologous deletion of IDE gene resulted in hyperinsulinemia and glucose intolerance in a rat model of type 2 diabetes mellitus. Several genetic association studies examined IDE as a susceptibility gene for type 2 diabetes in European descents. Here we investigated the genetic association of IDE polymorphisms with the risk of type 2 diabetes and its related phenotypes in the Korean population. Among six single nucleotide polymorphisms analyzed, g.-179T>C (OR=1.73, P=0.04), and g.IVS18+99G>A (OR=1.23, P=0.02) revealed borderline association with increased risk of type 2 diabetes. Combining our results with previous data obtained from the European population, g.-179T>C (OR=1.11, P=0.03), and g.IVS24-64A>T (OR=1.18, P=0.005) showed significant association with type 2 diabetes. Haplotype consisting of common alleles of the six polymorphisms was associated with decreased risk of type 2 diabetes (OR=0.82, P=0.02). However, none of the polymorphisms was significantly associated with metabolic phenotypes. We can conclude that variations in IDE might contribute to diabetes susceptibility in the Korean population.

Inhalation, deposition, and fate of insulin and other therapeutic proteins

In contrast to other interfaces between the outer and inner environment, such as the skin and gastrointestinal tract, the lungs are ideally suited for the delivery of polypeptides and proteins to the systemic circulation. There is a wealth of experience from aerosol physics and lung biology to support the idea that insulin delivery via the lungs is an effective strategy. The elaborate branching network of airways, the extensive surface area of the deep lung, and the thin air-blood barrier make it possible to deliver insulin and other proteins to the parenchyma, where it can be absorbed into the pulmonary circulation and become available throughout the body. Protein-containing aerosols in inspired air deposit throughout the respiratory tract because of the following forces: gravity, diffusion, and inertial impaction. Lung anatomy, breathing pattern, and particle size determine the influence of these forces. Once deposited in the lungs, most protein-containing formulations quickly dissolve in airway or alveolar lining fluid. Then, a significant fraction of insulin is transported across the air-blood barrier into the blood, where it continues to be biologically active. A substantial fraction--usually the majority--is cleared from the lungs or degraded within the lungs.

Enhanced peptide secretion by gene disruption of CYM1, a novel protease in Saccharomyces cerevisiae

Saccharomyces cerevisiae is a widely used host in the production of therapeutic peptides and proteins. Here we report the identification of a novel endoprotease in S. cerevisiae. It is encoded by the CYM1 gene and is specific for the C-terminus of basic residues of heterologously expressed peptides. Gene disruption of CYM1 not only reduced the intracellular proteolysis, but also enhanced the secretion of heterologously expressed peptides such as growth hormone, pro-B-type natriuretic peptide and pro-cholecystokinin. Cym1p resembles metalloendoproteases of the pitrilysin family with the HXXEH(X)E(71-77) catalytic domain as seen in insulysin, nardilysin and human metalloprotease 1. It is a nuclear encoded protease that localizes to mitochondria without a hydrophobic N-terminal signal sequence or a C-terminal tail-anchor. The protease does not require post-translational processing prior to activation and it contains cytosolic activity that processes peptides designated for the secretory pathway prior to translocation into the endoplasmic reticulum.

Insulin-degrading enzyme: embarking on amyloid destruction

Several human disorders are caused by or associated with the deposition of protein aggregates known as amyloid fibrils. Despite the lack of sequence homology among amyloidogenic proteins, all amyloid fibrils share a common morphology, are insoluble under physiological conditions and are resistant to proteolytic degradation. Because amyloidogenic proteins are being produced continuously, eukaryotic organisms must have developed a form of proteolytic machinery capable of controlling these aggregation-prone species before their fibrillization. This article suggests that an intracellular metalloprotease called insulin-degrading enzyme (IDE) is responsible for the elimination of proteins with amyloidogenic potential and proposes a mechanism for the selectivity of the enzyme. In this respect, IDE can also be referred to as ADE: amyloid-degrading enzyme.

Mechanisms of nuclear translocation of insulin

Insulin (Ins) and various other hormones and growth factors have been shown to be rapidly internalized and translocated to the cell nucleus. This review summarizes the mechanisms that are involved in the translocation of Ins to the nucleus, and discusses its possible role in Ins action, based on observations by the authors and others. Ins is internalized to endosomes by both receptor-mediated and fluid-phase endocytosis, the latter occurring only at high Ins concentrations. The authors recently demonstrated the caveolae are the primary cell membrane locations responsible for initiating the signal transduction cascade induced by Ins. Once Ins is internalized, Ins dissociates from the Ins receptor in the endosome, and is translocated to the cytoplasm, where most Ins is degraded by Ins-degrading enzyme (IDE), although how the polypeptides cross the lipid bilayer is unknown. Some Ins escapes the degradation and binds to cytosolic Ins-binding proteins (CIBPs), in addition to IDE. IDE and some CIBPs are known to be binding proteins for other hormones or their receptors, and are involved in gene regulation, suggesting physiological relevance of CIBPs in the signaling of Ins and other hormones. Ins is eventually translocated through the nuclear pore to the nucleus, where Ins tightly associates with nuclear matrix. The role of Ins internalization and translocation to the nucleus is still controversial, although there is substantial evidence to support its role in cellular responses caused by Ins. Many studies indicate that nuclear translocation of various growth factors and hormones plays an important role in cell proliferation or DNA synthesis. It would be reasonable to suggest that Ins internalization, its association with CIBPs, and its translocation to the nucleus may be essential for the regulation of nuclear events by Ins.

Complementation between mitochondrial processing peptidase (MPP) subunits from different species

Mitochondrial processing peptidase (MPP), a dimer of nonidentical subunits, is the primary peptidase responsible for the removal of leader peptides from nuclearly encoded mitochondrial proteins. Alignments of the alpha and beta subunits of MPP (alpha- and beta-MPP) from different species show strong protein sequence similarity in certain regions, including a highly negatively charged region as well as a domain containing a putative metal ion binding site. In this report, we describe experiments in which we combine the subunits of MPP from yeast, rat, and Neurospora crassa, both in vivo and in vitro and mesure the resultant processing activity. For in vivo complementation, we used the temperature sensitive mif1 and mif2 yeast mutants, which lack MPP activity at the nonpermissive temperature (37 degrees C). We found that the defective alpha-MPP of mif2 cannot be substituted for by the alpha-MPP from rat or Neurospora. On the other hand, the beta-MPP from rat and Neurospora can fully substitute for the defective beta-MPP in the mif1 mutant. These results were confirmed in in vitro experiments in which individually expressed subunits were combined. Only combinations of the alpha-MPP from yeast with the beta-MPP from rat or Neurospora produced active MPP.

Regulation of multicatalytic enzyme activity by insulin and the insulin-degrading enzyme

The insulin-degrading enzyme (IDE) plays an important role in the cellular metabolism of insulin. Recent studies have also suggested a regulatory role for this protein in controlling the activity of cytoplasmic protein complexes, including the proteasome [multicatalytic proteinase (MCP)] and the glucocorticoid and androgen receptors. Binding of IDE to these complexes increases their activity, whereas the addition of substrates for IDE inhibits activity. This provides a potential mechanism of action for internalized insulin and other IDE substrates in the control of protein turnover. To examine further the interactions, partially purified IDE-MCP complex was treated with EDTA or EGTA, and activity was measured in the absence and presence of various divalent cations (Ca2+, Mn2+, Co2+, and Zn2+) and insulin. EDTA treatment reduced MCP activity and eliminated the effect of insulin on the complex. Divalent cations partially or completely restored MCP activity, but did not restore the effect of insulin. EGTA treatment had a lesser effect on MCP activity, but abolished insulin inhibition of activity. Divalent cations restored the insulin effect. Inhibitors of IDE also blocked the insulin effect on MCP activity, as did treatment with SDS. These findings suggest that conformational changes in the complex may play a role in the insulin control of MCP activity.

Insulin degradation: progress and potential

Insulin degradation is a regulated process that plays a role in controlling insulin action by removing and inactivating the hormone. Abnormalities in insulin clearance and degradation are present in various pathological conditions including type 2 diabetes and obesity and may be important in producing clinical problems. The uptake, processing, and degradation of insulin by cells is a complex process with multiple intracellular pathways. Most evidence supports IDE as the primary degradative mechanism, but other systems (PDI, lysosomes, and other enzymes) undoubtedly contribute to insulin metabolism. Recent studies support a multifunctional role for IDE, as an intracellular binding, regulatory, and degradative protein. IDE increases proteasome and steroid hormone receptor activity, and this activation is reversed by insulin. This raises the possibility of a direct intracellular interaction of insulin with IDE that could modulate protein and fat metabolism. The recent findings would place intracellular insulin-IDE interaction into the insulin signal transduction pathway for mediating the intermediate effects of insulin on fat and protein turnover.

The mitochondrial processing peptidase behaves as a zinc-metallopeptidase

The yeast mitochondrial processing peptidase (MPP) and its subunits were purified in Escherichia coli under conditions for which the enzyme retains most of its processing activity in the absence of externally added divalent cation. The holoenzyme exhibited a Km value of 1.35 microM and a Vmax value of 0.25 microM/min and was inhibited by metal chelators in a time-dependent manner. Measurement of the metal content showed that both, MPP and beta-MPP, contained 0.86 and 1.05 atoms of Zn2+ per molecule, respectively. An enzymatically inactive MPP mutant carrying a mutation of the first histidine of the putative metal-ion binding HXXEH motif in beta-MPP retained less than 0.2 atom of Zn2+ per molecule. A metal-free enzyme (apoenzyme) was prepared from the holoenzyme and shown to be devoid of any processing activity. Incubation of the apoenzyme with 50 nM and 500 nM Zn2+ restored 50% and 80% of the processing activity, respectively. However, no reactivation occurred at concentrations of Zn2+ higher than 1 microM. Addition of 500 nM Mn2+ or higher concentrations (up to 50 microM) reactivated only 50% of the processing activity. The holoenzyme was competitively inhibited by molar excess of Zn2+ (Ki of 3.1 microM) but not by molar excess of Mn2+. Taken together, our data suggest that the authentic MPP is a Zn2+ rather than a Mn2+ metallopeptidase.

Identification of the cleavage sites of transforming growth factor alpha by insulin-degrading enzymes

Insulin-degrading enzyme (IDE) is a sulfhydryl-dependent metalloproteinase with a zinc binding site unique to a new class of proteinases. The enzyme is relatively specific for a number of hormones/growth factors, such as insulin, atrial natriuretic peptide, IGF-II, and proinsulin. In this study we have identified the amino-acid bonds cleaved by IDE in transforming growth factor-alpha. High-performance liquid chromatography was used to separate the peptides generated by the degradation of 125I-TGF-alpha. The peptides were then submitted to sequential Edman degradation to determine the peptide bond broken. Cleavage sites were found at amino acids, 10-11 (Asp-Ser), 25-26 (Val-Gln), 28-29 (Asp-Lys), and 30-31 (Pro-Ala). In agreement with studies of cleavage sites of other hormones by this enzyme, no clear amino-acid specificity was seen. However, examination of the sites on a three-dimensional model of TGF-alpha suggest the primary mechanism used by IDE for determining cleavage sites is the tertiary structure of the substrate.

The mitochondrial processing peptidase: function and specificity

Targeting signals of mitochondrial precursors are cleaved in the matrix during or after import by the mitochondrial processing peptidase (MPP). This enzyme consists of two nonidentical alpha- and beta-subunits each of molecular weight of about 50 kDa. In mammals and fungi, MPP is soluble in the matrix, whereas in plants the enzyme is part of the cytochrome bc1 complex. MPP is a metalloendopeptidase which has been classified as a member of the pitrilysin family on the basis of the HXXEHX76E zinc-binding motif present in beta-MPP. Both subunits of MPP are required for processing activity. The alpha-subunit of MPP, which probably recognizes a three-dimensional motif adopted by the presequence, presents the presequence to beta-MPP, which carries the catalytic active site. MPP acts as an endoprotease on chemically synthesized peptides corresponding to mitochondrial presequences. Matrix-targeting signals and MPP cleavage signals seem to be distinct, although the two signals may overlap within a given presequence. The structural element helix-turn-helix, that cleavable presequences adopt in a membrane mimetic environment, may be required for processing but is not sufficient for proteolysis. Binding of the presequence by alpha-MPP tolerates a high degree of mutations of the presequence. alpha-MPP may present a degenerated cleavage site motif to beta-MPP in an accessible conformation for processing. The conformation of mitochondrial presequences bound to MPP remains largely unknown.

Are the 'core' proteins of the mitochondrial bc1 complex evolutionary relics of a processing protease?

The so-called 'core' proteins of the respiratory cytochrome bc1 complex and the two subunits of the mitochondrial processing peptidase (MPP) are structurally similar but their evolutionary relationship remains a mystery. Here, we present a model suggesting that the core proteins originated from an ancient proteolytic enzyme that was integrated into the bc1 complex during early stages of endosymbiosis.

A periplasmic insulin-cleaving proteinase (ICP) from Acinetobacter calcoaceticus sharing properties with protease III from Escherichia coli and IDE from eucaryotes

A periplasmic insulin-cleaving proteinase (ICP), purified to its electrophoretic homogeneity in the SDS-PAGE from the Gram-negative bacterium Acinetobacter calcoaceticus, was examined and compared in its properties with the protease III (protease Pi, pitrilysin, EC 3.4.99.44) of Escherichia coli and the insulin-destroying proteinase (IDE, insulinase, EC 3.4.99.45) from eucaryotes. The enzyme was proven to be a metalloprotease like protease III and IDE, as was shown by the inhibitory effects exerted by EDTA and o-phenanthroline. Furthermore, dialysis against EDTA and o-phenanthroline led to a complete loss of activity, which could be restored by addition of Co2+, and, to a lesser extent, but at a lower metal ion concentration by Zn2+. Similar to protease III and IDE, ICP prefers the cleavage of small polypeptides (insulin, insulin B-chain, glucagon) to the cleavage of proteins (casein, human serum albumin, globin) and was inactive against synthetic amino acid derivates (esters, p-nitranilides, and furoylacroleyl substrates) of subtilisin, thermolysin, trypsin, and chymotrypsin. The peptide-bond-specificity of the ICP in the cleavage of the oxidized insulin B-chain was investigated and the results were compared to the specificity of protease III of E. coli, IDE, protease-24,11, and thermolysin. Cleavage sites in the oxidized insulin B-chain generated by ICP are Asn3-Gln4, His10-Leu11, Ala14-Leu15, Leu17-Val18, Gly23-Phe24, Phe24-Phe25, and Phe25-Tyr26. Principally, ICP cleaves between hydrophobic amino acids and amides. The ICP shares one of the only two cleavage sites with the protease III and four sites with the IDE.

1,10-Phenanthroline increases nuclear accumulation of insulin in response to inhibiting insulin degradation but has a biphasic effect on insulin's ability to increase mRNA levels

Previous reports demonstrated that insulin is translocated through the cytoplasm to the nucleus of H35 hepatoma cells and suggested that nuclear insulin may be involved in stimulating transcription of immediate-early genes. In a recent study, inhibition of insulin-degrading enzyme with 1,10-phenanthroline, a Zn2+ chelator, caused a significant increase in the nuclear accumulation of insulin. The present study characterized the effects of 1,10-phenanthroline and its nonchelating isomer, 1,7-phenanthroline, on insulin degradation, nuclear accumulation, and stimulation of immediate-early gene expression. 1,10- but not 1,7-phenanthroline inhibited insulin degradation and increased nuclear accumulation of insulin in a dose-dependent manner. 1,7-phenanthroline caused a dose-dependent decrease in the expression of insulin-stimulated immediate-early genes, but had no significant effect on alpha-tubulin mRNA levels. In the presence of insulin, Northern analysis revealed that 1,10-phenanthroline at all concentrations tested increased alpha-tubulin mRNA levels, but had a biphasic effect on insulin-stimulated immediate-early gene expression. At low concentrations (5-200 microM), 1,10-phenanthroline increased the expression of insulin-stimulated g33, c-fos, and Egr-1 mRNA. At concentrations greater than 1 mM, insulin-stimulated immediate-early gene expression was decreased similar to the effect seen with 1,7-phenanthroline. Nuclear run-on analysis demonstrated that high concentrations of 1,10-phenanthroline decreased insulin-stimulated immediate-early gene transcription but had no effect on transcription of alpha-tubulin. However, low concentrations of 1,10-phenanthroline did not increase transcription of any genes.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of glutamate-169 as the third zinc-binding residue in proteinase III, a member of the family of insulin-degrading enzymes

A novel active site has been identified in a family of zinc-dependent metalloendopeptidases that includes bacterial proteinase III, the human and Drosophila insulin-degrading enzymes, and the processing-enhancing protein subunit of the mitochondrial processing proteinase. None of these enzymes contains the conserved active site described in most other metalloendopeptidases, HEXXH; instead, all four contain an inversion of this motif, HXXEH. Prior mutagenesis studies of proteinase III indicate that the two histidines are essential for co-ordinating the zinc atom, while all three residues are required for enzyme activity. To identify the third zinc-binding residue in this protein family, three glutamates downstream from the active site were mutated to glutamine in proteinase III. The mutant proteins were expressed and their ability to degrade insulin was compared with the wild-type enzyme. The glutamate-204 mutant was as active as the wild-type protein, the glutamate-162 mutant retained 20% of the activity of the wild-type enzyme and the glutamate-169 mutant was completely devoid of insulin-degrading activity. The purified wild-type and glutamate-204 mutant enzymes were found to contain nearly stoichiometric levels of zinc by atomic absorption spectrophotometry, whereas the glutamate-162 mutant had a slight reduction in the level of zinc, and the glutamate-169 mutant retained less than 0.3 mol of zinc/mol of enzyme. These findings are consistent with glutamate-169 being the third zinc-binding residue in proteinase III.

The rat insulin-degrading enzyme. Molecular cloning and characterization of tissue-specific transcripts

The primary structure of the rat insulin-degrading enzyme (IDE) was determined by cDNA analysis. Rat IDE, as well as the previously characterized homologs from human and Drosophila, contain the carboxyl-terminal consensus sequence A/S-K-L, for peroxisome targeting. A stretch of 43 bp surrounding an alternatively used polyadenylation site is highly conserved between rat and human, suggesting that it may contain important regulatory information. Northern blot analysis revealed two IDE transcripts of 3.7 and 5.5 kb in various tissues. Testis was found to be exceptional in having three different RNAs (3.7, 4.1 and 6.1 kb) at a relatively high abundance. The expression of the IDE gene in testis is correlated with sexual maturation.