Inhibition of insulin-degrading enzyme increases translocation of insulin to the nucleus in H35 rat hepatoma cells: evidence of a cytosolic pathway

Insulin-Degrading Enzyme Regulates the Proliferation and Apoptosis of Porcine Skeletal Muscle Stem Cells via Myostatin/MYOD Pathway

Identifying the genes relevant for muscle development is pivotal to improve meat production and quality in pigs. Insulin-degrading enzyme (IDE), a thiol zinc-metalloendopeptidase, has been known to regulate the myogenic process of mouse and rat myoblast cell lines, while its myogenic role in pigs remained elusive. Therefore, the current study aimed to identify the effects of IDE on the proliferation and apoptosis of porcine skeletal muscle stem cells (PSMSCs) and underlying molecular mechanism. We found that IDE was widely expressed in porcine tissues, including kidney, lung, spleen, liver, heart, and skeletal muscle. Then, to explore the effects of IDE on the proliferation and apoptosis of PSMSCs, we subjected the cells to siRNA-mediated knockdown of IDE expression, which resulted in promoted cell proliferation and reduced apoptosis. As one of key transcription factors in myogenesis, MYOD, its expression was also decreased with IDE knockdown. To further elucidate the underlying molecular mechanism, RNA sequencing was performed. Among transcripts perturbed by the IDE knockdown after, a downregulated gene myostatin (MSTN) which is known as a negative regulator for muscle growth attracted our interest. Indeed, MSTN knockdown led to similar results as those of the IDE knockdown, with upregulation of cell cycle-related genes, downregulation of MYOD as well as apoptosis-related genes, and enhanced cell proliferation. Taken together, our findings suggest that IDE regulates the proliferation and apoptosis of PSMSCs via MSTN/MYOD pathway. Thus, we recruit IDE to the gene family of regulators for porcine skeletal muscle development and propose IDE as an example of gene to prioritize in order to improve pork production.

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.

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.

Knockout of insulin-degrading enzyme leads to mice testicular morphological changes and impaired sperm quality

Insulin-degrading enzyme (IDE) is a zinc metalloprotease responsible for degrading and inactivating several bioactive peptides, including insulin. Individuals without this enzyme or with a loss-of-function mutation in the gene that codifies it, present hyperinsulinemia. In addition, impairment of IDE-mediated insulin clearance is associated with the development of metabolic diseases, namely prediabetes. Although insulin regulates male fertility, the role of IDE on male reproductive function remains unknown. We proposed to study the influence of IDE in the reproductive potential of males. As insulin mediates key events for the normal occurrence of spermatogenesis, we hypothesized that IDE functioning might be linked with sperm quality. We used C57BL/6N mice that were divided in three groups according to its genotype: wild type (WT), heterozygous and knockout (KO) male mice for Ide. Spermatozoa were collected from the cauda of epididymis and sperm parameters were evaluated. Testicular tissue morphology was assessed through hematoxylin and eosin stain. Mitochondrial complex protein levels and lipid peroxidation were also evaluated in the testicular tissue. Our results show that KO mice present a 50% decrease in testes weight compared to WT mice as well as a decrease in seminiferous tubules diameter. Moreover, KO mice present impaired sperm quality, namely a decrease in both sperm viability and morphology. These results provide evidence that IDE plays an important role in determining the reproductive potential of males.

Effect of Passage Number and Culture Time on the Expression and Activity of Insulin-Degrading Enzyme in Caco-2 Cells

Background

Insulin-degrading enzyme (IDE) is a conserved zinc metallopeptidase. Here, we have evaluated the effect of passage number and culture time on IDE expression and activity in colorectal adenocarcinoma cell line (Caco-2).

Methods

Caco-2 cells were cultured with different passage ranges of 5-15, 25-35, 52-63 for 48, 72, and 120 hours. Subsequently, IDE expression and enzyme activity were assessed by Western blot analysis and fluorescent assay, respectively.

Results

Our results confirmed that the amount of IDE was higher in cell extract compared to supernatant, and different passage numbers and culture times had small effect on IDE expression. However, when cells were cultured in the passage number range of 5-15 for 72 hours, the IDE activity was 35% higher compared to other passage numbers (p < 0.05).

Conclusion

The use of Caco-2 cells at passage number range of 5-15 and culture time of 72 hours provides proper conditions for IDE-related studies.

Effects of ageing and experimental diabetes on insulin-degrading enzyme expression in male rat tissues

Due to an increasing life expectancy in developing countries, cases of type 2 diabetes and Alzheimer's disease (AD) in the elderly are growing exponentially. Despite a causative link between diabetes and AD, general molecular mechanisms underlying pathogenesis of these disorders are still far from being understood. One of the factors leading to cell death and cognitive impairment characteristic of AD is accumulation in the brain of toxic aggregates of amyloid-β peptide (Aβ). In the normally functioning brain Aβ catabolism is regulated by a cohort of proteolytic enzymes including insulin-degrading enzyme (IDE) and their deficit with ageing can result in Aβ accumulation and increased risk of AD. The aim of this study was a comparative analysis of IDE expression in the brain structures involved in AD, as well as in peripheral organs (the liver and kidney) of rats, during natural ageing and after experimentally-induced diabetes. It was found that ageing is accompanied by a significant decrease of IDE mRNA and protein content in the liver (by 32 and 81%) and brain structures (in the cortex by 58 and 47% and in the striatum by 53 and 68%, respectively). In diabetic animals, IDE protein level was increased in the liver (by 36%) and in the striatum (by 42%) while in the brain cortex and hippocampus it was 20-30% lower than in control animals. No significant IDE protein changes were observed in the kidney of diabetic rats. These data testify that ageing and diabetes are accompanied by a deficit of IDE in the brain structures where accumulation of Aβ was reported in AD patients, which might be one of the factors predisposing to development of the sporadic form of AD in the elderly, and especially in diabetics.

Intracellular insulin in human tumors: examples and implications

Insulin is one of the major metabolic hormones regulating glucose homeostasis in the organism and a key growth factor for normal and neoplastic cells. Work conducted primarily over the past 3 decades has unravelled the presence of insulin in human breast cancer tissues and, more recently, in human non-small cell lung carcinomas (NSCLC). These findings have suggested that intracellular insulin is involved in the development of these highly prevalent human tumors. A potential mechanism for such involvement is insulin's binding and inactivation of the retinoblastoma tumor suppressor protein (RB) which in turn is likely controlled by insulin-degrading enzyme (IDE). This model and its supporting data are collectively covered in this survey in order to provide further insight into insulin-driven oncogenesis and its reversal through future anticancer therapeutics.

Insulin receptor dysfunction impairs cellular clearance of neurotoxic oligomeric a{beta}

Accumulation of amyloid beta (Abeta) oligomers in the brain is toxic to synapses and may play an important role in memory loss in Alzheimer disease. However, how these toxins are built up in the brain is not understood. In this study we investigate whether impairments of insulin and insulin-like growth factor-1 (IGF-1) receptors play a role in aggregation of Abeta. Using primary neuronal culture and immortal cell line models, we show that expression of normal insulin or IGF-1 receptors confers cells with abilities to reduce exogenously applied Abeta oligomers (also known as ADDLs) to monomers. In contrast, transfection of malfunctioning human insulin receptor mutants, identified originally from patient with insulin resistance syndrome, or inhibition of insulin and IGF-1 receptors via pharmacological reagents increases ADDL levels by exacerbating their aggregation. In healthy cells, activation of insulin and IGF-1 receptor reduces the extracellular ADDLs applied to cells via seemingly the insulin-degrading enzyme activity. Although insulin triggers ADDL internalization, IGF-1 appears to keep ADDLs on the cell surface. Nevertheless, both insulin and IGF-1 reduce ADDL binding, protect synapses from ADDL synaptotoxic effects, and prevent the ADDL-induced surface insulin receptor loss. Our results suggest that dysfunctions of brain insulin and IGF-1 receptors contribute to Abeta aggregation and subsequent synaptic loss.

Age-related differences in oxidative protein-damage in young and senescent fibroblasts

Aging is accompanied by an accumulation of oxidized proteins and cross-linked modified protein material. The intracellular formation and accumulation of highly oxidized and cross-linked proteins, the so-called lipofuscin, is a typical sign of senescence. However, little is known whether the lipofuscin accumulation during aging is related to environmental conditions, as oxidative stress, and whether the accumulation of oxidized proteins and lipofuscin is preferentially taking place in the cytosol or the nucleus and finally, what is the role of lysosomes in this process. Therefore, we investigated human skin fibroblasts in an early stage of proliferation ("young cells") and in a late stage ("senescent cells"). Such cells were compared for the amount of protein carbonyls and lipofuscin and their distribution within the cytosol and the nucleus. Furthermore, cells were exposed to single and repeated doses of hydrogen peroxide and paraquat, measuring the same set of parameters. In addition to that the role of the proteasome to degrade oxidized proteins in young and senescent cells was tested. Furthermore, detailed microscopic analysis was performed testing the intracellular distribution of lipofuscin. The results clearly demonstrated that repeated/chronic oxidative stress induces a senescence-like phenotype of the distribution of oxidized proteins as well as of lipofuscin. It could be demonstrated that most of the lipofuscin is located in lysosomes and that senescent cells contain less lysosomes not lipofuscin-laden in comparison to young cells.

Immunohistochemical evidence of ubiquitous distribution of the metalloendoprotease insulin-degrading enzyme (IDE; insulysin) in human non-malignant tissues and tumor cell lines

Immunohistochemical evidence of ubiquitous distribution of the metalloprotease insulin-degrading enzyme (IDE; insulysin) in human non-malignant tissues and tumor cells is presented. Immunohistochemical staining was performed on a multi-organ tissue microarray (pancreas, lung, kidney, central/peripheral nervous system, liver, breast, placenta, myocardium, striated muscle, bone marrow, thymus, and spleen) and on a cell microarray of 31 tumor cell lines of different origin, as well as trophoblast cells and normal blood lymphocytes and granulocytes. IDE protein was expressed in all the tissues assessed and all the tumor cell lines except for Raji and HL-60. Trophoblast cells and granulocytes, but not normal lymphocytes, were also IDE-positive.

Insulin degrading enzyme is localized predominantly at the cell surface of polarized and unpolarized human cerebrovascular endothelial cell cultures

Insulin degrading enzyme (IDE) is expressed in the brain and may play an important role there in the degradation of the amyloid beta peptide (Abeta). Our results show that cultured human cerebrovascular endothelial cells (HCECs), a primary component of the blood-brain barrier, express IDE and may respond to exposure to low levels of Abeta by upregulating its expression. When radiolabeled Abeta is introduced to the medium of cultured HCECs, it is rapidly degraded to smaller fragments. We believe that this degradation is largely the result of the action of IDE, as it can be substantially blocked by the presence of insulin in the medium, a competitive substrate of IDE. No inhibition is seen when an inhibitor of neprilysin, another protease that may degrade Abeta, is present in the medium. Our evidence suggests that the action of IDE occurs outside the cell, as inhibitors of internalization fail to affect the rate of the observed degradation. Further, our evidence suggests that degradation by IDE occurs on the plasma membrane, as much of the IDE present in HCECs was biotin-labeled by a plasma membrane impermeable reagent. This activity seems to be polarity dependent, as measurement of Abeta degradation by each surface of differentiated HCECs shows greater degradation on the basolateral (brain-facing) surface. Thus, IDE could be an important therapeutic target to decrease the amount of Abeta in the cerebrovasculature.

Study on the possibility of insulin as a carrier of IUdR for hepatocellular carcinoma-targeted therapy

AIM: To evaluate the possibility of using insulin as a carrier for carcinoma-targeted therapy mediated by receptor, and to investigate the expression of insulin receptor in human hepatocellular carcinoma and the receptor binding characteristics of insulin-IUdR (iododeoxyuridine).

METHODS: IUdR was covalently conjugated to insulin. Receptor binding assays of ¹²⁵I-insulin to human hepatocellular carcinoma and its adjacent tissue were performed. Competitive displacements of ¹²⁵I-insulin by insulin and insulin-IUdR to bind to insulin receptor were respectively carried out. Statistical comparisons between the means were made with paired t-test at a confidence level of 95%.

RESULTS: The data indicated that there were high- and low- affinity binding sites for ¹²⁵I-insulin on both hepatocellular carcinoma and its adjacent tissue. Hepatocellular carcinoma had a significantly higher Bmax for high affinity binding site than its adjacent liver tissue (P < 0.05, t = 2.275). Insulin-IUdR competed as effectively as insulin with ¹²⁵I-insulin for binding to insulin receptor. Values of IC₅₀1, C₅₀2, KI1 and KI2 for insulin-IUdR were 11.50 ± 2.83 nmol·L^-1, 19.35 ± 5.11 nmol·L^-1, 11.26 ± 2.65 nmol·L^-1 and 19.30 ± 5.02 nmol·L^-1 respectively, and for insulin were 5.01 ± 1.24 nmol·L^-1,17.75 ± 4.86 nmol·L^-1, 4.85 ± 1.12 nmol·L^-1 and 17.69 ± 4.81 nmol·L^-1, respectively. Values of IC₅₀1 and KI1 for insulin-IUdR were significantly higher than that for insulin (P < 0.01, t = 4.537 and 4.813).

CONCLUSION: It is possible to use insulin as a carrier for carcinoma-targeted therapy mediated by receptor.

Nuclear matrix association of insulin receptor and IRS-1 by insulin in osteoblast-like UMR-106 cells

In the present study, we explored to determine whether insulin has any effect on the nuclear translocation of insulin receptor and IRS-1 in the nucleus of UMR-106 cells. Following insulin treatment, cells were subjected to subcellular fractionation. Each fraction containing equal amount of protein was subjected to Western blot analysis using antibodies against IR and IRS-1. Significant amounts of insulin receptors and IRS-1 were detected in the nucleus. Insulin receptor and IRS-1 appeared to be translocated to the nucleus in a time dependent manner by insulin whereas Akt levels remained unchanged by insulin treatment. The majority of insulin receptor and IRS-1 translocated to the nucleus appeared to associate with nuclear matrix. Tyrosine phosphorylation of a number of proteins with a molecular mass of 180, 95, 85, or 70 kDa in the nucleus was significantly stimulated by insulin, suggesting insulin signals to the nucleus and could regulate nuclear proteins. Confocal laser scanning microscope (CLSM) analysis also supports the nuclear translocation of insulin receptor and IRS-1. The nuclear insulin signaling may play an important role in the transcription control, differentiation, and growth of osteoblast cells.

Differential regulation of early growth response gene-1 expression by insulin and glucose in vascular endothelial cells

OBJECTIVE

Early growth response gene (Egr)-1 is a key transcription factor involved in vascular pathophysiology. Its role in diabetic vascular complications, however, remains unclear. Because hyperinsulinemia and hyperglycemia are major risk factors leading to diabetic vascular complications, we examined the effect of insulin and glucose on Egr-1 expression in murine glomerular vascular endothelial cells.

METHODS AND RESULTS

Insulin or glucose, when added separately, increased egr-1 mRNA levels and promoter activity, as well as Egr-1 protein levels in nuclear extracts. When insulin was added to cells preincubated with glucose, the two had an additive effect on Egr-1 expression. Furthermore, vascular endothelial growth factor receptor-1 (flt-1) and plasminogen activator inhibitor-1, two known Egr-1-responsive genes, were also upregulated in the presence of insulin or glucose. An investigation into the underlying molecular mechanisms demonstrated that insulin, but not glucose, increased Egr-1 expression through extracellular signal-regulated kinase 1/2 activation, which is consistent with our previous reports. In contrast, inhibition of protein kinase C by phorbol ester or by the specific protein kinase C inhibitor chelerythrine chloride downregulated glucose-induced, but not insulin-induced, Egr-1 expression.

CONCLUSIONS

Differential regulation of Egr-1 expression by insulin and glucose in vascular cells may be one of the initial key events that plays a crucial role in the development of diabetic vascular complications.

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.

Metalloendopeptidase EC 3.4.24.15 is necessary for Alzheimer's amyloid-beta peptide degradation

We have investigated the functional relationship between metalloendopeptidase EC 3.4.24.15 (MP24.15) and the amyloid precursor protein involved in Alzheimer's disease (AD) and discovered that the enzyme promotes Abeta degradation. We show here that conditioned medium (CM) of MP24.15 antisense-transfected SKNMC neuroblastoma has significantly higher levels of Abeta. Furthermore, synthetic-Abeta degradation was increased or decreased following incubation with CM of sense or antisense-transfected cells, respectively. Soluble Abeta1-42 was degraded more slowly than soluble Abeta1-40, while aggregated Abeta1-42 showed almost no degradation. Pretreatment of CM with serine proteinase inhibitors 4-(2-aminoethyl)benzenesulfonyl fluoride and diisopropyl fluorophosphate completely inhibited Abeta degradation. Additionally, alpha1-antichymotrypsin (ACT), a serpin family inhibitor tightly associated with plaques and elevated in AD brain, blocked up to 60% of Abeta degradation. Interestingly, incubation of CM of MP24. 15-overexpressing cells with ACT formed an SDS-resistant ACT complex, suggesting an ACT-serine proteinase interaction. Recombinant MP24. 15 alone did not degrade Abeta. 14C-Diisopropyl fluorophosphate-radiolabeled CM from MP24.15-overexpressing cells contained increased levels of several active serine proteinases, suggesting that MP24.15 activates one or more Abeta-degrading serine proteases. Thus, ACT may cause Abeta accumulation by inhibiting an Abeta-degrading enzyme or by direct binding to Abeta, rendering it degradation-resistant. Identification of the Abeta-degrading enzyme and MP24.15's role in its activation is underway. Pharmacological modulation of either enzyme may provide a means of regulating Abeta in the brain.

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.

Investigation into the presence of insulin-degrading enzyme in cultured type II alveolar cells and the effects of enzyme inhibitors on pulmonary bioavailability of insulin in rats

The purpose of this study was to investigate the role of insulin-degrading enzyme (IDE, EC 3.4.22.11) in insulin degradation in alveolar epithelium. The primary culture of isolated rat type-II pneumocytes was used for the in-vitro characterization of IDE. Insulin was then administered intratracheally with various inhibitors to assess the improvement in its pulmonary bioavailability. In cultured type-II pneumocytes, the cytosolic insulin-degrading activity contributed 81% of total insulin degradation, reached a maximum at pH 7.5 and had an apparent Michaelis-Menten constant (Km) of 135 nM. N-Ethylmaleimide, p-chloromercuribenzoic acid and 1,10-phenanthroline inhibited insulin-degrading activity almost completely in both crude homogenate and cytosol. An immunoprecipitation study showed that IDE contributed 74% of cytosolic insulin-degrading activity. Western blot analysis showing a single band of 110 kDa on reduced SDS (sodium dodecylsulphate) gels confirmed the presence of IDE in cultured type-II cells. When given intratracheally with insulin, inhibitors including N-ethylmaleimide, p-chloromercuribenzoic acid, and 1,10-phenanthroline significantly enhanced the absolute bioavailability of insulin and the compound's hypoglycaemic effects. These results suggest that IDE is present in alveolar epithelium and might be involved in limiting insulin absorption in the lung.

Insulin-induced protein tyrosine phosphorylation cascade and signalling molecules are localized in a caveolin-enriched cell membrane domain

The cellular localisation of time- and temperature-dependent 125I-insulin binding, insulin-sensitive signalling proteins and the insulin-induced protein tyrosine phosphorylation cascade were assessed in subcellular fractions isolated on Iodixanol gradients from control and insulin-treated H35 hepatoma cells. Western blot analysis demonstrated that the concentrations of IRS-1, Shc, GRB-2, SOS, Syp, PI 3-kinase, MAP kinase and Gi alpha were at least 10-fold higher in cell surface-derived, caveolin-enriched fraction than in a cell surface-derived, caveolin-poor fraction (i.e., the plasma membranes). Insulin treatment caused a 15-fold increase in tyrosine phosphorylation of IRS-1 in the caveolin-enriched fraction in 5 min at 37 degrees C compared with a 3-fold increase in plasma membranes and a 6-fold increases in the cytosol and endosomes. Insulin also increased tyrosine phosphorylation of both a 72-kDa protein and the 46-kDa Shc isoform only in the caveolin-enriched fraction. Insulin treatment did not change the concentrations of insulin receptors or Shc but increased IRS-1 in the caveolin-enriched fraction, possibly recruited from the cytosolic pool. Insulin also increased the concentrations of insulin receptors, IRS-1 and Shc in endosomes, suggesting insulin-induced internalization of the insulin receptors and proteins activated with them. Electron microscopic analysis, with the use of a combination of colloidal gold-labelled insulin to label the insulin receptor and immunolabelling to detect caveolin or IRS-1, demonstrated the co-localisation of insulin receptors in caveolin- and IRS-1 containing vesicular structures. Differences in the insulin-induced protein tyrosine phosphorylation and concentrations of these proximal signalling proteins in the caveolin-enriched fraction, plasma membranes, and cytosol suggest that insulin receptors in the caveolae play a major role in initiating insulin's signal transduction processes.

Two pathways for insulin metabolism in adipocytes

Using selected conditions, the appropriate collagenase, albumin and cell treatment, a preparation of isolated adipocytes was developed with no extracellular insulin degrading activity. Cell mediated insulin degradation rates were 0.68% +/- 0.05%/100,000 cell/h using trichloracetic acid precipitability as a measure. Chloroquine (CQ) increased cell-associated radioactivity and decreased degradation while dansylcadaverine (DC), PCMBS and bacitracin (BAC) decreased degradation with no effect on binding. Extraction and chromatography of the cell-associated radioactivity showed 3 peaks, a large molecular weight peak, a small molecular weight peak and an insulin-sized peak. CQ, DC and BAC all decreased the small molecular weight peak while CQ and DC also increased the peak of large molecular weight radioactivity. Cell mediated insulin degradation in the presence of combinations of inhibitors suggested two pathways in adipocytes, one affected by inhibitors of the insulin degrading enzyme (IDE) (bacitracin and PCMBS) and the other altered by cell processing inhibitors (DC, CQ and phenylarsenoxide). Chloroquine altered the pattern of the insulin-sized cell-associated HPLC assayed degradation products, further supporting two pathways of degradation; one a chloroquine-sensitive and one a chloroquine-insensitive pathway.

Insulin-degrading enzyme does not require peroxisomal localization for insulin degradation

Although considerable evidence implicates insulin-degrading enzyme (IDE) in the cellular metabolism of insulin in many cell types, its mechanism and site of action are not clear. In this study, we have examined the relationship between insulin-degrading enzyme's peroxisomal location and its ability to degrade insulin by mutation of its peroxisomal targeting signal (PTS), the carboxy terminal A/S-K-L tripeptide. Site-directed mutagenesis was used to destroy the peroxisomal targeting signal of human insulin-degrading enzyme by changing alanine to leucine (AL.pts), leucine to valine (LV.pts), or by deleting the entire tripeptide (DEL.pts). The alanine or leucine mutants, when expressed in COS cells, were indistinguishable from wild-type insulin-degrading enzyme with respect to size (110 kDa), amount of immunoreactive material, ability to bind insulin, in vitro activity, and cellular degradation of insulin. In contrast, the deletion mutant was shorter in size (approximately 0 kDa) and unable to bind the hormone. Thus, although the tripeptide at insulin-degrading enzyme's carboxy terminus appeared to confer enzyme stability, the conserved sequence was not required for insulin degradation. Finally, an immunocytofluorescence study showed that, whereas a significant amount of the wild-type protein was localized in peroxisomes, none of the peroxisomal targeting mutants could be detected in these organelles. These findings indicate that insulin-degrading enzyme does not require peroxisomal localization for insulin degradation and suggest that this enzyme has multiple cellular functions.

Insulin internalization and other signaling pathways in the pleiotropic effects of insulin

Insulin is the major anabolic hormone in humans and affects multiple cellular processes. Insulin rapidly regulates short-term effects on carbohydrate, lipid, and protein metabolism and is also a potent growth factor controlling cell proliferation and differentiation. The metabolic and growth-related effects require insulin binding to its receptor and receptor phosphorylation. Evidence suggests these events result in subsequent substrate phosphorylation and activation of multiple signaling pathways involving Src homology domain-containing proteins and the internalization of the insulin:receptor complex. The role of insulin internalization in insulin action is largely speculative. For more than two decades, extensive investigation has been carried out by numerous laboratories of the mechanisms by which insulin causes its pleiotropic responses and the cellular processing of insulin receptors. This chapter reviews our current knowledge of the phosphorylation signaling pathways activated by insulin and presents evidence that substrates other than insulin receptor substrate-1 are involved in insulin's regulation of immediate-early gene expression. We also review the mechanisms involved in insulin internalization and present evidence that internalization may play a key role in insulin action through both signal transduction processes and translocation of insulin to the cell cytoplasm and nucleus.

Immunohistochemical localization of insulin-degrading enzyme along the rat intestine, in the human colon adenocarcinoma cell line (Caco-2), and in human ileum

Insulin-degrading enzyme (IDE) has been implicated in the intracellular degradation of insulin in insulin target cells. Knowledge of the existence of this enzyme in the intestine will be beneficial to the achievement of clinical oral efficacy of insulin. A comparative study was conducted with rat intestine, human colon adenocarcinoma (Caco-2) cells, and human ileum. Confocal microscopy analysis using the anti-IDE antibody showed that IDE was localized in the mucosal cells of rat and human intestines, as well as in Caco-2 cells. Immunostaining of this enzyme was homogeneous throughout the cell excluding nucleus, indicating a typical cytosolic distribution in rat and human enterocytes and in Caco-2 cells.

Insulin-degrading enzyme in a human colon adenocarcinoma cell line (Caco-2)

The activity of insulin-degrading enzyme (IDE), a thiol metalloprotease degrading insulin in many insulin target cells, was determined in human colon adenocarcinoma (Caco-2) cells. Insulin-degrading activity was localized in the cytosol of Caco-2 cells, accounting for 88% of total activity. Western blots and immunoprecipitation showed that IDE was present in the cytosol of Caco-2 cells and contributed to more than 93% cytosolic insulin-degrading activity. Cytosolic insulin degradation was strongly inhibited by IDE inhibitors, including N-ethylmaleimide, 1,10-phenanthroline, p-chloromericuribenzoate, and EDTA, but was not significantly or not as extensively inhibited by strong inhibitors of proteasome, i.e., chymostatin, soybean trypsin inhibitor, leupeptin, and Dip-F. These results suggest that IDE is present in Caco-2 cells, that Caco-2 IDE has properties similar to those of its counterparts in insulin-target tissues, and that it significantly contributes to intracellular insulin degradation.

Demonstration of specific insulin binding to cytosolic proteins in H35 hepatoma cells, rat liver and skeletal muscle

We previously demonstrated that internalized insulin enters the cytoplasm before accumulating in nuclei of H35 rat hepatoma cells. This finding raises the possibility that insulin may interact with cytosolic proteins in addition to insulin-degrading enzyme (IDE). In the present study, cytosol from H35 hepatoma cells, rat liver or muscle was incubated with A14- or B26-125I-insulin at 4 degrees C for 5-120 min in the absence or presence of 25 micrograms/ml unlabelled insulin. 125I-insulin was cross-linked to cytosolic proteins by disuccinimidyl suberate and analysed by reducing or non-reducing SDS/PAGE and autoradiography. Our results demonstrate the presence of both tissue-specific and common cytosolic proteins which specifically bind insulin. In muscle cytosol, only two proteins of 27 and 110 kDa were specifically labelled with B26-125I-insulin. Seven major bands, of 27, 45, 55, 60, 76, 82 and 110 kDa, were labelled in rat liver cytosol. Detection of cytosolic insulin-binding proteins in H35-cell cytosol was dependent on cell-culture conditions. Labelling in cytosol from serum-deprived cells was decreased or absent compared with cytosol prepared from serum-fed or serum-deprived cells treated with 100 ng/ml insulin for 1 h before preparation of the cytosol, in which six bands, of 32, 41, 45, 55, 82 and 110 kDa, were specifically labelled with B26-125I-insulin. This result suggests that the concentration or binding activity of some cytosolic insulin-binding proteins is rapidly regulated. Labelling of both rat liver and H35 cytosolic insulin-binding proteins was time-dependent, and decreased or disappeared at 120 min in parallel with the degradation of labelled insulin. Fewer bands were specifically labelled with A14-125I-insulin than with B26-125I-insulin. The number of labelled bands observed under reducing and non-reducing conditions was not different in any of the cytosols. The 110 kDa band in all cytosols was identified as IDE by Western-blot analysis; the other proteins did not react with anti-IDE antibody and remain unidentified. 1,10-Phenanthroline (2 mM) increased IDE labelling, but decreased the labelling of 82 and 27 kDa bands. The marked difference in the number of cytosolic insulin-binding proteins in muscle and either H35 cells or liver suggests both that the labelling is specific and that these proteins serve a function and may be involved in some heretofore unknown mechanism of the signalling pathway by which insulin regulates cell growth or differentiation.

Inhibition of human immunodeficiency virus type 1 integrase by a hydrophobic cation: the phenanthroline-cuprous complex

The human immunodeficiency virus type 1 integrase (HIV-1 integrase) is required for integration of a double-stranded DNA copy of the viral RNA genome into a host chromosome and for HIV replication. We have examined the effects of 2:1 1,10-phenanthroline-cuprous complexes on purified HIV-1 integrase. Although the uncomplexed phenanthrolines are not active below 100 microM, four of the cuprous complexes (neocuproine, 4-phenyl neocuproine, 2,3,4,7,8,9-hexamethyl phenanthroline, and 2,3,4,7,8-pentamethyl phenanthroline) have a 50% inhibitory concentration (IC50) for integration ranging between 1 and 10 microM. Disintegration is also inhibited by these phenanthroline-cuprous complexes at slightly higher concentrations (between 10 and 40 microM). Dialysis experiments showed that the inhibition is reversible and kinetic analyses revealed that the mode of inhibition by these cuprous complexes appears to be noncompetitive with respect to the substrate DNA. Consistent with these findings, binding assays demonstrate that, although these complexes can inhibit binding to DNA at high concentrations, they do not inhibit binding of integrase to the DNA substrate at their IC50 values. Because these complexes do not bind to B-DNA below 50 microM, inhibition via binding to a specific region on the enzyme was examined. Using deletion mutants of integrase, it was determined that neither the amino-terminal (zinc finger) nor the carboxy-terminal (DNA-binding) integrase domain is required for inhibition by the phenanthroline-cuprous complexes. Therefore, inhibition via binding to the enzyme catalytic core or to the interface between the enzyme and a noncanonical DNA structure generated during the enzymatic reaction is the probable mechanism. These results suggest the utility of neocuproine-cuprous complexes in developing inhibitors of HIV-1 integrase as well as probes for drug-binding sites and enzymatic reaction mechanism.

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)

Cellular distribution of insulin-degrading enzyme gene expression. Comparison with insulin and insulin-like growth factor receptors

Insulin-degrading enzyme (IDE) hydrolyzes both insulin and IGFs and has been proposed to play a role in signal termination after binding of these peptides to their receptors. In situ hybridization was used to investigate the cellular distribution of IDE mRNA and to compare it with insulin receptor (IR) and IGF-I receptor (IGFR) gene expression in serial thin sections from a variety of tissues in embryonic and adult rats. IDE mRNA is highly abundant in kidney and liver, tissues known to play a role in insulin degradation. IDE and IR mRNAs are highly coexpressed in brown fat and liver. The highest level IDE gene expression, on a per cell basis, is found in germinal epithelium. IDE and IGFR mRNAs are colocalized in oocytes, while IDE is colocalized with the IGF-II receptor in spermatocytes, suggesting that IDE may be involved with degradation of IGF-II in the testis. In summary, IDE expression demonstrates significant anatomical correlation with insulin/IGF receptors. These data are compatible with a role for IDE in degrading insulin and IGFs after they bind to and are internalized with their respective receptors and may also suggest a novel role for IDE in germ cells.