Substrate-selective inhibitors that reprogram the activity of insulin-degrading enzyme

The Insulin-Degrading Enzyme from Structure to Allosteric Modulation: New Perspectives for Drug Design

The insulin-degrading enzyme (IDE) is a Zn2+ peptidase originally discovered as the main enzyme involved in the degradation of insulin and other amyloidogenic peptides, such as the β-amyloid (Aβ) peptide. Therefore, a role for the IDE in the cure of diabetes and Alzheimer's disease (AD) has been long envisaged. Anyway, its role in degrading amyloidogenic proteins remains not clearly defined and, more recently, novel non-proteolytic functions of the IDE have been proposed. From a structural point of view, the IDE presents an atypical clamshell structure, underscoring unique enigmatic enzymological properties. A better understanding of the structure-function relationship may contribute to solving some existing paradoxes of IDE biology and, in light of its multifunctional activity, might lead to novel therapeutic approaches.

Substrate-selective small-molecule modulators of enzymes: Mechanisms and opportunities

Small-molecule inhibitors of enzymes are widely used tools in reverse chemical genetics to probe biology and explore therapeutic opportunities. They are often compared with genetic knockdown or knockout and are expected to produce phenotypes similar to the genetic perturbations. This review aims to highlight that small molecule inhibitors of enzymes and genetic perturbations may not necessarily produce the same phenotype due to the possibility of substrate-selective or substrate-dependent effects of the inhibitors. Examples of substrate-selective inhibitors and the mechanisms for the substrate-selective effects are discussed. Substrate-selective modulators of enzymes have distinct advantages and cannot be easily replaced with biologics. Thus, they present an exciting opportunity for chemical biologists and medicinal chemists.

Cysteine cathepsins: A long and winding road towards clinics

Biomedical research often focuses on properties that differentiate between diseased and healthy tissue; one of the current focuses is elevated expression and altered localisation of proteases. Among these proteases, dysregulation of cysteine cathepsins can frequently be observed in inflammation-associated diseases, which tips the functional balance from normal physiological to pathological manifestations. Their overexpression and secretion regularly exhibit a strong correlation with the development and progression of such diseases, making them attractive pharmacological targets. But beyond their mostly detrimental role in inflammation-associated diseases, cysteine cathepsins are physiologically highly important enzymes involved in various biological processes crucial for maintaining homeostasis and responding to different stimuli. Consequently, several challenges have emerged during the efforts made to translate basic research data into clinical applications. In this review, we present both physiological and pathological roles of cysteine cathepsins and discuss the clinical potential of cysteine cathepsin-targeting strategies for disease management and diagnosis.

The Role of Insulin Signaling in Hippocampal-Related Diseases: A Focus on Alzheimer's Disease

Alzheimer's disease (AD) is a global concern and has become a major public health event affecting human health. Insulin is a metabolic hormone secreted mainly by the peripheral tissue pancreas. In recent years, more and more evidence has proved that insulin regulates various functions of the brain. The hippocampus, one of the earliest brain regions affected by AD, is widely distributed with insulin receptors. Studies have shown that type 2 diabetes mellitus, characterized by insulin resistance, is closely related to AD, which has drawn extensive attention to the relationship between hippocampal insulin signaling and AD. Therefore, we provide an overview of intranasal insulin administration on memory and its underlying mechanism. We also highlight the molecular link between hippocampal insulin resistance and AD and provide a theoretical basis for finding new therapeutic targets for AD in clinical practice.

New Insights on the Regulation of the Insulin-Degrading Enzyme: Role of microRNAs and RBPs

The evident implication of the insulin-degrading enzyme (IDE) in Alzheimer’s disease (AD) and type 2 diabetes mellitus (T2DM), among its capacity to degrade insulin and amyloid-β peptide (Aβ), suggests that IDE could be an essential link in the relation between hyperinsulinemia, insulin resistance and AD. However, little is known about the cellular and molecular regulation of IDE expression, and even less has been explored regarding the post-transcriptional regulation of IDE, although it represents a great molecular target of interest for therapeutic treatments. We recently described that miR-7, a novel candidate for linking AD and T2DM at the molecular level, regulates IDE and other key genes in both pathologies, including some key genes involved in the insulin signaling pathway. Here, we explored whether other miRNAs as well as other post-transcriptional regulators, such as RNA binding proteins (RBP), could potentially participate in the regulation of IDE expression in vitro. Our data showed that in addition to miR-7, miR-125, miR-490 and miR-199 regulate IDE expression at the post-transcriptional level. Moreover, we also found that IDE contains multiple potential binding sites for several RBPs, and a narrow-down prediction analysis led us to speculate on a novel regulation of IDE by RALY and HuD. Taken together, these results demonstrate the novel players controlling IDE expression that could represent potential therapeutical targets to treat several metabolic diseases with a high impact on human health, including AD and T2DM.

Exchange Broadening Underlies the Enhancement of IDE-Dependent Degradation of Insulin by Anionic Membranes

Insulin-degrading enzyme (IDE) is an evolutionarily conserved ubiquitous zinc metalloprotease implicated in the efficient degradation of insulin monomer. However, IDE also degrades monomers of amyloidogenic peptides associated with disease, complicating the development of IDE inhibitors. In this work, we investigated the effects of the lipid composition of membranes on the IDE-dependent degradation of insulin. Kinetic analysis based on chromatography and insulin's helical circular dichroic signal showed that the presence of anionic lipids in membranes enhances IDE's activity toward insulin. Using NMR spectroscopy, we discovered that exchange broadening underlies the enhancement of IDE's activity. These findings, together with the adverse effects of anionic membranes in the self-assembly of IDE's amyloidogenic substrates, suggest that the lipid composition of membranes is a key determinant of IDE's ability to balance the levels of its physiologically and pathologically relevant substrates and achieve proteostasis.

Characterization of Dense Granule Metalloproteinase INS-16 in Cryptosporidium parvum

The protozoan pathogen Cryptosporidium parvum infects intestinal epithelial cells and causes diarrhea in humans and young animals. Among the more than 20 genes encoding insulinase-like metalloproteinases (INS), two are paralogs with high sequence identity. In this study, one of them, INS-16 encoded by the cgd3_4270 gene, was expressed and characterized in a comparative study of its sibling, INS-15 encoded by the cgd3_4260 gene. A full-length INS-16 protein and its active domain I were expressed in Escherichia coli, and antibodies against the domain I and an INS-16-specific peptide were produced in rabbits. In the analysis of the crude extract of oocysts, a ~60 kDa fragment of INS-16 rather than the full protein was recognized by polyclonal antibodies against the specific peptide, indicating that INS-16 undergoes proteolytic cleavage before maturation. The expression of the ins-16 gene peaked at the invasion phase of in vitro C. parvum culture, with the documented expression of the protein in both sporozoites and merozoites. Localization studies with antibodies showed significant differences in the distribution of the native INS-15 and INS-16 proteins in sporozoites and merozoites. INS-16 was identified as a dense granule protein in sporozoites and macrogamonts but was mostly expressed at the apical end of merozoites. We screened 48 candidate INS-16 inhibitors from the molecular docking of INS-16. Among them, two inhibited the growth of C. parvum in vitro (EC₅₀ = 1.058 µM and 2.089 µM). The results of this study suggest that INS-16 may have important roles in the development of C. parvum and could be a valid target for the development of effective treatments.

Phenotypic Screening for Small Molecules that Protect β-Cells from Glucolipotoxicity

Type 2 diabetes is marked by progressive β-cell failure, leading to loss of β-cell mass. Increased levels of circulating glucose and free fatty acids associated with obesity lead to β-cell glucolipotoxicity. There are currently no therapeutic options to address this facet of β-cell loss in obese type 2 diabetes patients. To identify small molecules capable of protecting β-cells, we performed a high-throughput screen of 20,876 compounds in the rat insulinoma cell line INS-1E in the presence of elevated glucose and palmitate. We found 312 glucolipotoxicity-protective small molecules (1.49% hit rate) capable of restoring INS-1E viability, and we focused on 17 with known biological targets. 16 of the 17 compounds were kinase inhibitors with activity against specific families including but not limited to cyclin-dependent kinases (CDK), PI-3 kinase (PI3K), Janus kinase (JAK), and Rho-associated kinase 2 (ROCK2). 7 of the 16 kinase inhibitors were PI3K inhibitors. Validation studies in dissociated human islets identified 10 of the 17 compounds, namely, KD025, ETP-45658, BMS-536924, AT-9283, PF-03814735, torin-2, AZD5438, CP-640186, ETP-46464, and GSK2126458 that reduced glucolipotoxicity-induced β-cell death. These 10 compounds decreased markers of glucolipotoxicity including caspase activation, mitochondrial depolarization, and increased calcium flux. Together, these results provide a path forward toward identifying novel treatments to preserve β-cell viability in the face of glucolipotoxicity.

Insulin-Degrading Enzyme, an Under-Estimated Potential Target to Treat Cancer?

Insulin-degrading enzyme (IDE) is a multifunctional protease due to the variety of its substrates, its various cellular locations, its conservation between species and its many non-proteolytic functions. Numerous studies have successfully demonstrated its implication in two main therapeutic areas: metabolic and neuronal diseases. In recent years, several reports have underlined the overexpression of this enzyme in different cancers. Still, the exact role of IDE in the physiopathology of cancer remains to be elucidated. Known as the main enzyme responsible for the degradation of insulin, an essential growth factor for healthy cells and cancer cells, IDE has also been shown to behave like a chaperone and interact with the proteasome. The pharmacological modulation of IDE (siRNA, chemical compounds, etc.) has demonstrated interesting results in cancer models. All these results point towards IDE as a potential target in cancer. In this review, we will discuss evidence of links between IDE and cancer development or resistance, IDE's functions, catalytic or non-catalytic, in the context of cell proliferation, cancer development and the impact of the pharmacomodulation of IDE via cancer therapeutics.

Structural basis for the mechanisms of human presequence protease conformational switch and substrate recognition

Presequence protease (PreP), a 117 kDa mitochondrial M16C metalloprotease vital for mitochondrial proteostasis, degrades presequence peptides cleaved off from nuclear-encoded proteins and other aggregation-prone peptides, such as amyloid β (Aβ). PreP structures have only been determined in a closed conformation; thus, the mechanisms of substrate binding and selectivity remain elusive. Here, we leverage advanced vitrification techniques to overcome the preferential denaturation of one of two ~55 kDa homologous domains of PreP caused by air-water interface adsorption. Thereby, we elucidate cryoEM structures of three apo-PreP open states along with Aβ- and citrate synthase presequence-bound PreP at 3.3–4.6 Å resolution. Together with integrative biophysical and pharmacological approaches, these structures reveal the key stages of the PreP catalytic cycle and how the binding of substrates or PreP inhibitor drives a rigid body motion of the protein for substrate binding and catalysis. Together, our studies provide key mechanistic insights into M16C metalloproteases for future therapeutic innovations.

Presequence protease (PreP) is essential to mitochondrial proteostasis. This study leverages advanced vitrification techniques to solve cryoEM structures of apo- and substrate-bound PreP and integrates these data with other analysis to reveal key stages and mechanistic insights of the PreP catalytic cycle.

Inhibition of Insulin Degrading Enzyme to Control Diabetes Mellitus and its Applications on some Other Chronic Disease: a Critical Review

PURPOSE

This review aims to provide a precise perceptive of the insulin-degrading enzyme (IDE) and its relationship to type 2 diabetes (T2D), Alzheimer's disease (AD), obesity, and cardiovascular diseases. The purpose of the current study was to provide clear idea of treating prevalent diseases such as T2D, and AD by molecular pharmacological therapeutics rather than conventional medicinal therapy.

METHODS

To achieve the aims, molecular docking was performed using several softwares such as LIGPLOT+, Python, and Protein-Ligand Interaction Profiler with corresponding tools.

RESULTS

The IDE is a large zinc-metalloprotease that breakdown numerous pathophysiologically important extracellular substrates, comprising amyloid β-protein (Aβ) and insulin. Recent studies demonstrated that dysregulation of IDE leads to develop AD and T2D. Specifically, IDE regulates circulating insulin in a variety of organs via a degradation-dependent clearance mechanism. IDE is unique because it was subjected to allosteric activation and mediated via an oligomer structure.

CONCLUSION

In this review, we summarised the factors that modulate insulin reformation by IDE and interaction of IDE and some recent reports on IDE inhibitors against AD and T2D. We also highlighted the latest signs of progress of the function of IDE and challenges in advancing IDE- targetted therapies against T2D and AD.

Identification of indole-based activators of insulin degrading enzyme

Insulin degrading enzyme (IDE) is a zinc metalloprotease that cleaves numerous substrates among which amyloid-β and insulin. It has been linked through genetic studies to the risk of type-2 diabetes (T2D) or Alzheimer's disease (AD). Pharmacological activation of IDE is an attractive therapeutic strategy in AD. While IDE inhibition gave paradoxal activity in glucose homeostasis, recent studies, in particular in the liver suggest that IDE activators could be also of interest in diabetes. Here we describe the discovery of an original series of IDE activators by screening and structure-activity relationships. Early cellular studies show that hit 1 decreases glucose-stimulating insulin secretion. Docking studies revealed it has an unprecedented extended binding to the polyanion-binding site of IDE. These indole-based pharmacological tools are activators of both Aβ and insulin hydrolysis by IDE and could be helpful to explore the multiple roles of IDE.

Macrocyclic DNA-encoded chemical libraries: a historical perspective

While macrocyclic peptides are extensively researched for therapeutically relevant protein targets, DNA-encoded chemical libraries (DELs) are developed at a quick pace to discover novel small molecule binders. The combination of both fields has been explored since 2004 and the number of macrocyclic peptide DELs is steadily increasing. Macrocycles with high affinity and potency were identified for diverse classes of proteins, revealing DEL's huge potential. By giving a historical perspective, we would like to review the methods which permitted the rise of macrocyclic peptide DELs, describe the different DELs which were created and discuss the achievements and challenges of this emerging field.

Highly selective acid-catalyzed olefin isomerization of limonene to terpinolene by kinetic suppression of the overreactions in a confined space of porous metal-macrocycle framework

Natural enzymes control the intrinsic reactivity of chemical reactions in the natural environment, giving only the necessary products. In recent years, challenging research on the reactivity control of terpenes with structural diversity using artificial host compounds that mimic such enzymatic reactions has been actively pursued. A typical example is the acid-catalyzed olefin isomerization of (+)-limonene, which generally gives a complex mixture due to over-isomerization to thermodynamically favored isomers. Herein we report a highly controlled conversion of (+)-limonene by kinetic suppression of over-isomerization in a confined space of a porous metal–macrocycle framework (MMF) equipped with a Brønsted acid catalyst. The terminal double bond of (+)-limonene migrated to one neighbor, preferentially producing terpinolene. This reaction selectivity was in stark contrast to the homogeneous acid-catalyzed reaction in bulk solution and to previously reported catalytic reactions. X-ray structural analysis and examination of the reaction with adsorption inhibitors suggest that the reactive substrates may bind non-covalently to specific positions in the confined space of the MMF, thereby inhibiting the over-isomerization reaction. The nanospaces of the MMF with substrate binding ability are expected to enable highly selective synthesis of a variety of terpene compounds.

A porous metal–macrocycle framework (MMF) equipped with a Brønsted acid catalyst in nanochannels enables highly selective isomerization of limonene to terpinolene by kinetically suppressing over-isomerization at confined acid sites.

Recent advances in enzyme-enhanced immunosensors

Among the products for rapid detection in different fields, enzyme-based immunosensors have received considerable attention. Recently, great efforts have been devoted to enhancing the output signals of enzymes through different strategies that can significantly improve the sensitivity of enzyme-based immunosensors for the need of practical applications. In this manuscript, the significance of enzyme-based signal transduction patterns in immunoassay and the central role of enzymes in achieving precise control of reaction systems are systematically described. In view of the rapid development of this field, we classify these strategies based on the combination of immune recognition and enzyme amplification into three categories, namely enzyme-based enhancement strategies, combination of the catalytic amplification of enzymes with other signal amplification methods, and substrate-based enhancement strategies. The current focus and future direction of enzyme-based immunoassays are also discussed. This article is not exhaustive, but focuses on the latest advances in different signal generation methods based on enzyme-initiated catalytic reactions and their applications in the detection field, which could provide an accessible introduction of enzyme-based immunosensors for the community with a view to further improving its application efficiency.

Medium-Chain Length Fatty Acids Enhance Aβ Degradation by Affecting Insulin-Degrading Enzyme

The accumulation of amyloid β-protein (Aβ) is one of the major pathological hallmarks of Alzheimer's disease. Insulin-degrading enzyme (IDE), a zinc-metalloprotease, is a key enzyme involved in Aβ degradation, which, in addition to Aβ production, is critical for Aβ homeostasis. Here, we demonstrate that saturated medium-chain fatty acids (MCFAs) increase total Aβ degradation whereas longer saturated fatty acids result in an inhibition of its degradation, an effect which could not be detected in IDE knock-down cells. Further analysis of the underlying molecular mechanism revealed that MCFAs result in an increased exosomal IDE secretion, leading to an elevated extracellular and a decreased intracellular IDE level whereas gene expression of IDE was unaffected in dependence of the chain length. Additionally, MCFAs directly elevated the enzyme activity of recombinant IDE, while longer-chain length fatty acids resulted in an inhibited IDE activity. The effect of MCFAs on IDE activity could be confirmed in mice fed with a MCFA-enriched diet, revealing an increased IDE activity in serum. Our data underline that not only polyunsaturated fatty acids such as docosahexaenoic acid (DHA), but also short-chain fatty acids, highly enriched, for example in coconut oil, might be beneficial in preventing or treating Alzheimer's disease.

Insulin-Degrading Enzyme: Paradoxes and Possibilities

More than seven decades have passed since the discovery of a proteolytic activity within crude tissue extracts that would become known as insulin-degrading enzyme (IDE). Certainly much has been learned about this atypical zinc-metallopeptidase; at the same time, however, many quite fundamental gaps in our understanding remain. Herein, I outline what I consider to be among the most critical unresolved questions within the field, many presenting as intriguing paradoxes. For instance, where does IDE, a predominantly cytosolic protein with no signal peptide or clearly identified secretion mechanism, interact with insulin and other extracellular substrates? Where precisely is IDE localized within the cell, and what are its functional roles in these compartments? How does IDE, a bowl-shaped protein that completely encapsulates its substrates, manage to avoid getting “clogged” and thus rendered inactive virtually immediately? Although these paradoxes are by definition unresolved, I offer herein my personal insights and informed speculations based on two decades working on the biology and pharmacology of IDE and suggest specific experimental strategies for addressing these conundrums. I also offer what I believe to be especially fruitful avenues for investigation made possible by the development of new technologies and IDE-specific reagents. It is my hope that these thoughts will contribute to continued progress elucidating the physiology and pathophysiology of this important peptidase.

The Use of Informer Sets in Screening: Perspectives on an Efficient Strategy to Identify New Probes

Small-molecule discovery typically involves large-scale screening campaigns, spanning multiple compound collections. However, such activities can be cost- or time-prohibitive, especially when using complex assay systems, limiting the number of compounds tested. Further, low hit rates can make the process inefficient. Sparse coverage of chemical structure or biological activity space can lead to limited success in a primary screen and represents a missed opportunity by virtue of selecting the "wrong" compounds to test. Thus, the choice of screening collections becomes of paramount importance. In this perspective, we discuss the utility of generating "informer sets" for small-molecule discovery, and how this strategy can be leveraged to prioritize probe candidates. While many researchers may assume that informer sets are focused on particular targets (e.g., kinases) or processes (e.g., autophagy), efforts to assemble informer sets based on historical bioactivity or successful human exposure (e.g., repurposing collections) have shown promise as well. Another method for generating informer sets is based on chemical structure, particularly when the compounds have unknown activities and targets. We describe our efforts to screen an informer set representing a collection of 100,000 small molecules synthesized through diversity-oriented synthesis (DOS). This process enables researchers to identify activity early and more extensively screen only a few chemical scaffolds, rather than the entire collection. This elegant and economic outcome is a goal of the informer set approach. Here, we aim not only to shed light on this process, but also to promote the use of informer sets more widely in small-molecule discovery projects.

Scanning Protein Surfaces with DNA-Encoded Libraries

Understanding the ligandability of a target protein, defined as the capability of a protein to bind drug-like compounds on any site, can give important stimuli to drug-development projects. For instance, inhibition of protein-protein interactions usually depends on the identification of protein surface binders. DNA-encoded chemical libraries (DELs) allow scanning of protein surfaces with large chemical space. Encoded library selection screens uncovered several protein-protein interaction inhibitors and compounds binding to the surface of G protein-coupled receptors (GPCRs) and kinases. The protein surface-binding chemotypes from DELs are predominantly chemically modified and cyclized peptides, and functional small-molecule peptidomimetics. Peptoid libraries and structural peptidomimetics have been less studied in the DEL field, hinting at hitherto less populated chemical space and suggesting alternative library designs. Roughly a third of bioactive molecules evolved from smaller, target-focused libraries. They showcase the potential of encoded libraries to identify more potent molecules from weak, for example, fragment-like, starting points.

A Non-obese, Treatment-Naive Japanese Diabetic Patient With Elevated Insulin Clearance and Hyperglycemia Under Enhanced Insulin Sensitivity and Increased Insulin Secretion: Elevated Insulin Clearance as Type 2 Japanese Diabetes Mellitus (T2JDM)

Diabetes mellitus is a heterogeneous and complex metabolic disorder characterized by hyperglycemia secondary to either resistance to insulin actions on the liver and peripheral tissues, insufficient insulin secretion from pancreatic β-cells, or both. An integrated balance between blood insulin levels and whole-body insulin sensitivity could theoretically provide the clinical effectiveness of insulin action. Peripheral blood insulin concentrations might be determined by the capacity of endogenous pancreatic β-cell insulin secretion and the degree of the whole body insulin clearance. Here, we report a non-obese normoinsulinemic Japanese diabetic patient with elevated insulin clearance assessed by a hyperinsulinemic-euglycemic clamp examination and increased insulin secretion measured by daily urinary excretion of C-peptide immunoreactivity. We propose this unique pathogenic condition of diabetes with normoinsulinemia and elevated insulin clearance as "type 2 Japanese diabetes mellitus (T2JDM)."

Hydroxypyridinethione Inhibitors of Human Insulin-Degrading Enzyme

Insulin-degrading enzyme (IDE) is a human mononuclear Zn²⁺ -dependent metalloenzyme that is widely regarded as the primary peptidase responsible for insulin degradation. Despite its name, IDE is also critically involved in the hydrolysis of several other disparate peptide hormones, including glucagon, amylin, and the amyloid β-protein. As such, the study of IDE inhibition is highly relevant to deciphering the role of IDE in conditions such as type-2 diabetes mellitus and Alzheimer disease. There have been few reported IDE inhibitors, and of these, inhibitors that directly target the active-site Zn²⁺ ion have yet to be fully explored. In an effort to discover new, zinc-targeting inhibitors of IDE, a library of ∼350 metal-binding pharmacophores was screened against IDE, resulting in the identification of 1-hydroxypyridine-2-thione (1,2-HOPTO) as an effective Zn²⁺ -binding scaffold. Screening a focused library of HOPTO compounds identified 3-sulfonamide derivatives of 1,2-HOPTO as inhibitors of IDE (K_i values of ∼50 μM). Further structure-activity relationship studies yielded several thiophene-sulfonamide HOPTO derivatives with good, broad-spectrum activity against IDE that have the potential to be useful pharmacological tools for future studies of IDE.

Insulin Resistance in Congenital Adrenal Hyperplasia is Compensated for by Reduced Insulin Clearance

CONTEXT

Congenital adrenal hyperplasia (CAH) patients have potential normal longevity. However, a greater risk for cardiovascular disease has been reported. Insulin resistance and hyperinsulinemia have been described in CAH patients, whereas the prevalence of overt type 2 diabetes is not higher in CAH than in normal population.

OBJECTIVE

To examine the contributions of insulin secretion and of hepatic insulin clearance to compensatory hyperinsulinemia in young insulin-resistant adults with classic CAH due to 21-hydroxylase deficiency (21-OHD).

DESIGN

Cross-sectional.

SETTING

University outpatient clinics.

METHODS

Fifty-one participants: 21 controls, and 30 CAH (15 virilizing and 15 salt-wasting phenotypes), female/male (33/18), age (mean [SD]): 24.0 (3.6) years, body mass index: 24.6 (4.9)kg/m2 with normal glucose tolerance, were submitted to a hyperglycemic clamp study.

MAIN OUTCOME MEASURES

Insulin sensitivity, beta cell function, and hepatic insulin clearance using appropriate modeling.

RESULTS

We found an increased insulin resistance in 21-OHD. The systemic hyperinsulinemia (posthepatic insulin delivery) was elevated in CAH patients. No increases were observed in insulin secretory rate (beta cell function) in the first phase or during the hyperglycemic clamp. The increase in insulin concentrations was totally due to a ~33% reduction in insulin clearance.

CONCLUSION

21-OHD nonobese subjects have reduced insulin sensitivity and beta cell response unable to compensate for the insulin resistance, probably due to overexposure to glucocorticoids. Compensatory hyperinsulinemia is most related with reduced hepatic insulin clearance. The exclusive adaptation of the liver acts as a gating mechanism to regulate the access of insulin to insulin-sensitive tissues to maintain glucose homeostasis.

Targeting Insulin-Degrading Enzyme in Insulin Clearance

Hepatic insulin clearance, a physiological process that in response to nutritional cues clears ~50–80% of circulating insulin, is emerging as an important factor in our understanding of the pathogenesis of type 2 diabetes mellitus (T2DM). Insulin-degrading enzyme (IDE) is a highly conserved Zn²⁺-metalloprotease that degrades insulin and several other intermediate-size peptides. Both, insulin clearance and IDE activity are reduced in diabetic patients, albeit the cause-effect relationship in humans remains unproven. Because historically IDE has been proposed as the main enzyme involved in insulin degradation, efforts in the development of IDE inhibitors as therapeutics in diabetic patients has attracted attention during the last decades. In this review, we retrace the path from Mirsky’s seminal discovery of IDE to the present, highlighting the pros and cons of the development of IDE inhibitors as a pharmacological approach to treating diabetic patients.

Modulation of Insulin Sensitivity by Insulin-Degrading Enzyme

Insulin-degrading enzyme (IDE) is a highly conserved and ubiquitously expressed metalloprotease that degrades insulin and several other intermediate-size peptides. For many decades, IDE had been assumed to be involved primarily in hepatic insulin clearance, a key process that regulates availability of circulating insulin levels for peripheral tissues. Emerging evidence, however, suggests that IDE has several other important physiological functions relevant to glucose and insulin homeostasis, including the regulation of insulin secretion from pancreatic β-cells. Investigation of mice with tissue-specific genetic deletion of Ide in the liver and pancreatic β-cells (L-IDE-KO and B-IDE-KO mice, respectively) has revealed additional roles for IDE in the regulation of hepatic insulin action and sensitivity. In this review, we discuss current knowledge about IDE’s function as a regulator of insulin secretion and hepatic insulin sensitivity, both evaluating the classical view of IDE as an insulin protease and also exploring evidence for several non-proteolytic functions. Insulin proteostasis and insulin sensitivity have both been highlighted as targets controlling blood sugar levels in type 2 diabetes, so a clearer understanding the physiological functions of IDE in pancreas and liver could led to the development of novel therapeutics for the treatment of this disease.

Quantitative, High-Throughput Assays for Proteolytic Degradation of Amylin

Amylin is a pancreatic peptide hormone that regulates glucose homeostasis but also aggregates to form islet amyloid in type-2 diabetes. Given its role in both health and disease, there is renewed interest in proteolytic degradation of amylin by insulin-degrading enzyme (IDE) and other proteases. Here, we describe the development and detailed characterization of three novel assays for amylin degradation, two based on a fluoresceinated and biotinylated form of rodent amylin (fluorescein-rodent amylin-biotin, FrAB), which can be used for any amylin protease, and another based on an internally quenched fluorogenic substrate (FRET-based amylin, FRAM), which is more specific for IDE. The FrAB-based substrate can be used in a readily implemented fluorescence-based protocol or in a fluorescence polarization (FP)-based protocol that is more amenable to high-throughput screening (HTS), whereas the FRAM substrate has the advantage of permitting continuous monitoring of proteolytic activity. All three assays yield highly quantitative data and are resistant to DMSO, and the FRAM and FP-based FrAB assay are ideally suited to HTS applications.

Regio- and Diastereoselective Synthesis of 2-Arylazetidines: Quantum Chemical Explanation of Baldwin's Rules for the Ring-Formation Reactions of Oxiranes†

A general, scalable two-step regio- and diastereoselective method has been described for the synthesis of versatile alkaloid-type azetidines from simple building blocks with excellent overall yields. In the kinetically controlled reaction, only the formation of the strained four-membered ring can be achieved instead of the thermodynamically favorable five-membered rings under appropriate conditions. Remarkable functional group tolerance has also been demonstrated. In this paper, we give a new scope of Baldwin’s rules by density functional theory (DFT) calculations with an explicit solvent model, confirming the proposed reaction mechanisms and the role of kinetic controls in the stereochemical outcome of the reported transition-metal-free carbon–carbon bond formation reactions.