Insulin degradation by human skeletal muscle

Hydroxychloroquine use in the Baltimore Lupus Cohort: effects on lipids, glucose and thrombosis

Hydroxychloroquine is used by 35% of SLE patients enrolled in the Baltimore Lupus Cohort. Eighty per cent of patients who took hydroxychloroquine at cohort entry remain on it six years later. In addition to its role for disease manifestations of lupus, hydroxychloroquine may be indicated for the prevention of disease or treatment-induced complications, including hyperlipidemia, diabetes mellitus, liver function test elevation and thrombosis.

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.

Inverse relationship between peripheral insulin removal and action: studies with metformin

The interaction of insulin with metformin on muscle glucose metabolism was examined in the perfused rat hindquarter. Glucose, lactate, and insulin were measured at the inflow to and outflow from the hindquarter, which was perfused with human erythrocytes suspended in a Kreb's-Ringer albumin buffer for 120 min. Perfusions were performed with no additions (I) and with insulin infusions targeted to concentrations of 175 (II) and 350 pmol/l (III) as well as infusions targeted to levels of 0 (IV), 70 (V), and 175 pmol/l (VI) but in the presence of metformin (90 microg/ml). In the presence of metformin, identical infusion rates of insulin yielded higher insulin concentrations, namely 283 +/- 19 vs. 202 +/- 31 pmol/l for VI and II, respectively (P < 0.05). Glucose uptake (GU) increased correspondingly to 79.8 +/- 0.8 in VI from 60.8 +/- 2.1 for IV and 50.1 +/- 1.3 for II and 46.1 +/- 2.7 mg/120 min for I (P < 0.05). This enhanced GU was matched by increasing insulin levels using only a higher rate of its infusion (III): GU of 70.2 +/- 2.4 mg/120 min with insulin of 334 +/- 26 pmol/l (P > 0.05). The simple concurrent presence of metformin and insulin [matching insulin concentrations in II rather than infusion rates (IV)] demonstrated no additonal effect on GU above that of metformin. The synergistic effects of metformin and insulin could thus be explained by a metformin-mediated decrease in the extraction of insulin by the hindquarter (4.8 +/- 0.4% vs. 8.6 +/- 0.9%, P < 0.05). This increases interstitial insulin (and, in a closed system, perfusate insulin), which acts on cell surface receptors to increase glucose uptake. The results demonstrate that the extracellular insulin concentration, rather than insulin internalization and degradation, is the primary determinant of insulin action on GU in muscle and that changes in tissue insulin extraction may alter local concentrations and, therefore, systemic insulin sensitivity. This provides both a physiological mechanism and a possible therapeutic target for improving insulin sensitivity.

Prolonged elevation of insulin content in the unicellular tetrahymena after insulin treatment: induction of insulin production or storage?

In the unicellular organism, Tetrahymena, the first encounter with an exogeneously given hormone results in hormonal imprinting. This causes an increase of the binding capacity of receptors and the production of the appropriate hormone in the progeny generations of the treated cell. In the present experiments the quantity (using radioimmunoassay) and localization (using confocal laser scanning microscopy) of the immunologically insulin-like material (hereafter insulin) were studied for 10 days after 4 h or 24 h 10(-6) M insulin treatment (hormonal imprinting). Forty-eight hours after both insulin treatments a high quantity of insulin was present in the cells. This value was also significantly increased after 96 h. After 8 days the difference to the control was significant only in the 24 h treated group. Confocal microscopy (using antibody to pig insulin) localized insulin in the cell body. The oral field contained extremely high quantities of the endogeneous hormone. Insulin treatment (after 48 and 96 h) caused an elevation of insulin content in general, and specific accumulation in the posterior sections of the cell, around the nucleus and in the periphery were observed. Ten days after both treatments only the peripheral region of the cell body and the ciliary row contained more insulin than the control. This means that after insulin treatment the quantity of insulin increases for a lengthy time period which is followed by the expression of insulin in the peripheral region. Insulin contained by Tetrahymena 48 h after imprinting stimulated glucose uptake of rat diaphragm.

Inhibition of insulin metabolism by hydroxychloroquine and its enantiomers in cytosolic fraction of liver homogenates from healthy and diabetic rats

To elucidate the mechanism by which hydroxychloroquine (HCQ) affects glucose metabolism, the effect of this drug and its enantiomers on insulin metabolism was studied using the cytosolic fraction of liver homogenates from healthy and diabetic rats. Eadie-Hofstee plots were monophasic suggesting that only a one-component enzyme system is involved in insulin degradation in the fraction used. Reaction velocity (V) vs substrate concentration plots were consistent with a Vmax model. HCQ caused a significant reduction in Vmax and Vmax/Km values in both healthy (Vmax, 3.63 +/- 0.46 vs 1.97 +/- 0.13, ng/min/mg; protein P < 0.001; and Vmax/Km 0.265 +/- 0.015 vs 0.112 +/- 0.004, ml/min/g protein) and diabetic rats (Vmax, 0.718 +/- 0.06 vs 0.360 +/- 0.024, ng/min/mg protein; and Vmax/Km, 0.05 +/- 0.002 vs 0.023 +/- 0.001, ml/min/g protein). Significant reduction in the V was observed in the presence of racemic (rac)-, R-, or S-HCQ. Ranking of the inhibitory potency was HCQ > S = R except at highest examined concentration (20 mg/mL) which was HCQ > S > R. In conclusion, the effect of HCQ on insulin degradation appears to be, in part, through inhibition of cytosolic insulin metabolizing enzyme. The effect is not stereoselective except at high concentrations. The R- and S-HCQ may have synergistic effects on inhibition of insulin degradation.

Characterization of an intracellular insulin-degrading enzyme in human erythrocytes

Using conventional techniques of ammonium sulfate fractionation and Sephadex gel column chromatography, insulin-degrading enzyme was partially purified from lysate of human erythrocytes. The enzymatic activity was measured by the trichloroacetic acid precipitation method. Compared to trypsin, the enzyme was highly specific for insulin. The apparent molecular weight of the enzyme was 160,000 Da, the optimum pH was the 7.4 to 7.8 range, and the Km value for insulin for the partially purified enzyme was 162 nM. Bacitracin and N-ethylmaleimide were potent inhibitors, while chloroquine, ethylenediaminetetraacetate, antipain, and soybean trypsin inhibitor failed to inhibit the activity of the enzyme. Like most nucleated cells, human erythrocytes not only have the membranal insulin receptors, but also possess the cytosolic specific insulin-degrading enzyme. Insulin internalization and degradation are shown to be due to the receptor and the enzyme acting in concert as in many nucleated cells. Anucleated erythrocytes have both these entities for possible internalization and degradation of insulin.

Impaired glucose handling in active rheumatoid arthritis: effects of corticosteroids and antirheumatic treatment

Forty-two patients with active rheumatoid arthritis were studied serially with respect to glucose metabolism after the institution of different anti-inflammatory and antirheumatic therapies. Sixteen patients received 20 mg of prednisolone daily. After 1 week of treatment the mean k value in glucose tolerance tests increased from 1.0 +/- 0.1 (SEM) to 1.6 +/- 0.1 (P less than .001). The corticosteroid therapy thus restored the glucose tolerance to normal and significantly enhanced the insulin response (P less than .01). Corticosteroids also normalized the growth hormone response to glucose infusion but had no effect on plasma glucagon. Treatment with nonsteroidal anti-inflammatory drugs did not affect the k values nor the hormonal pattern either after short-term treatment or after three months of therapy, except for causing a minor increase in the plasma glucagon levels both before and after glucose infusion. The long-term effects of treatment with penicillamine (n = 4), chloroquine (n = 7), and immunosuppressive agents [corticosteroids combined with azathioprine or cyclophosphamide (n = 7)], were an improvement of the clinical state, a reduction of the inflammatory activity, and a reversal of the glucose handling to normal.

Inhibition of insulin degradation by hepatoma cells after microinjection of monoclonal antibodies to a specific cytosolic protease

Four monoclonal antibodies were identified by their ability to bind to 125I-labeled insulin covalently linked to a cytosolic insulin-degrading enzyme from human erythrocytes. All four antibodies were also found to remove more than 90% of the insulin-degrading activity from erythrocyte extracts. These antibodies were shown to be directed to different sites on the enzyme by mapping studies and by their various properties. Two antibodies recognized the insulin-degrading enzyme from rat liver; one inhibited the erythrocyte enzyme directly; and two recognized the enzyme after gel electrophoresis and transfer to nitrocellulose filters. By this latter procedure and immunoprecipitation from metabolically labeled cells, the enzyme from a variety of tissues was shown to be composed of a single polypeptide chain of apparent Mr 110,000. Finally, these monoclonal antibodies were microinjected into the cytoplasm of a human hepatoma cell line to assess the contribution of this enzyme to insulin degradation in the intact cell. In five separate experiments, preloading of cells with these monoclonal antibodies resulted in an inhibition of insulin degradation of 18-54% (average 39%) and increased the amount of 125I-labeled insulin associated with the cells. In contrast, microinjection of control antibody or an extraneous monoclonal antibody had no effect on insulin degradation or on the amount of insulin associated with the cells. Moreover, the monoclonal antibodies to the insulin-degrading enzyme caused no significant inhibition of degradation of another molecule, low density lipoprotein. Thus, these results support a role for this enzyme in insulin degradation in the intact cell.

Inhibition of down regulation by chloroquine in cultured lymphocytes (RPMI-1788 line)

We studied the effect of chronic exposure to insulin on insulin receptors of cultured lymphocytes (RPMI-1788 line). The cells treated with insulin (2 micrograms/ml) at 37 degrees C for 12 h, showed a 36.5% decrease in the number of binding sites. Solubilized extract from the cells treated with insulin showed a 35.9% decrease of binding capacity, suggesting that insulin exposure induced the loss of total (cell surface and intracellular) receptors. Insulin-induced loss of receptors was blocked by chloroquine, suggesting that receptor loss is mediated by a chloroquine sensitive pathway. Bacitracin, which inhibited the insulin degradation, had no effect on insulin-induced receptor loss in this cell line. We also found that vitamin K5, one of the insulin mimickers, induced a 31.5% loss of insulin receptors. Therefore, the post-receptor process appeared to mediate down regulation. In cultured lymphocytes, insulin exposure caused a significant loss of total receptors, suggesting that insulin-induced receptor loss may be due to receptor degradation. Insulin-induced receptor loss is mediated by a chloroquine sensitive pathway, and is related to the post-binding process stimulated by vitamin K5.

Characterization of an insulin degrading enzyme from cultured human lymphocytes

An insulin degrading enzyme from cultured human lymphocytes, IM-9 cells, has been purified and characterized. The biochemical, enzymatic and immunological characteristics of this enzyme were all found to be similar to the characteristics of insulin degrading enzymes previously isolated from rat and pig skeletal muscle. Furthermore, this insulin degrading enzyme was found to have no effect on the structure of the insulin receptor nor to be linked to the insulin receptor either on the plasma membrane of cells or when they are shed into the media. The present studies suggest that the IM-9 lymphocytes, which have been extensively used to study the human insulin receptor, may also be a good system for studying human insulin degrading enzymes.

Purification and characterization of a rat liver cytosol neutral thiol peptidase that degrades glucagon, insulin, and isolated insulin A and B chains

A thiol peptidase that catalyzes at near neutral pH the hydrolysis of insulin, the isolated A and B chains of insulin, and glucagon was purified from rat liver cytosol by fractionation on Sephadex G-200, Affi-Gel Blue, and Spherogel TSK-G 3000 SW. The purified enzyme showed a single component by chromatography on a Spherogel TSK column and by gel filtration on a Sephadex G-200 column. The native enzyme has a molecular weight of approximately 180,000 and consists of two subunits having pI's of 5.9 and 6.3. Studies on its substrate specificity showed that the purified enzyme degrades glucagon, insulin, insulin B chain, and insulin A chain, but it does not degrade proinsulin, ACTH, or denatured hemoglobin. Kinetic analyses were performed on three substrates. The Km values were: 34 nM for insulin, 276 nM for insulin B chain, and 3.5 microM for glucagon. The kcat and Vm/Km values were glucagon greater than B chain greater than insulin. Thus, the enzyme has the highest affinity/lowest efficiency for insulin, an intermediate affinity/intermediate efficiency for B chain of insulin and the lowest affinity/highest efficiency for glucagon. The effect of several potential activators and inhibitors on the enzyme's activity was investigated. The enzyme activity was markedly inhibited by N-ethylmaleimide, p-chloromercuribenzoic acid, iodoacetamide, and Np-tosyl-L-phenylalanine chloromethyl ketone (TPCK), and was partially inhibited by dithiothreitol, by the chelating agents EDTA and EGTA, and by phenylmethylsulfonyl fluoride (PMSF). Bacitracin inhibited the activity of the enzyme, but the protease inhibitors aprotinin, leupeptin, pepstatin, and phosphoramidon had little or no effect. Reduced glutathione, iodoacetate, and N alpha,p-tosyl-L-lysine chloromethyl ketone (TLCK) also had little or no effect on the enzyme activity.

Characterization of skeletal muscle insulin receptor

The insulin receptor from rat skeletal muscle was characterized. Treatment of muscle membranes with the photoactive insulin analog, 125I[N-epsilonB29-monoazidobenzoyl]-insulin revealed a single protein band of 135,000 Da, the alpha subunit. Iodination of total membrane protein followed by Triton X-100 solubilization and immunoprecipitation demonstrated the presence of a protein band of 90,000 Da, the beta subunit, together with a protein band of 190,000 Da, which may be the receptor precursor. In partially purified receptor preparations, the beta subunit exhibited dose-dependent, insulin-stimulated phosphorylation with incorporation of phosphate solely into tyrosine residues, which was also observed in the 190,000-Da receptor precursor. Purified plasma membranes contained a large amount of insulin-degrading activity which had to be inactivated prior to performing insulin-binding studies. If degradation of insulin was not prevented, apparent enhanced binding in the presence of unlabeled insulin was observed.

Insulin degrading enzyme activity and insulin binding of erythrocytes in normal subjects and Type 2 (non-insulin-dependent) diabetic patients

Specific insulin degrading enzyme activity of erythrocytes was determined in relation to erythrocyte insulin binding in 16 healthy subjects, 14 Type 1 (insulin-dependent) and various groups of Type 2 (non-insulin-dependent) diabetic patients (n = 39). Degrading activity was increased in Type 2 diabetic patients on sulphonylureas, as well as in a subgroup with good metabolic control (p less than 0.001) and in patients with secondary failure to oral therapy (p less than 0.02); degrading activity returned to normal in the latter patients after 1 week of insulin treatment. Highest degrading activity was found in insulin-treated, yet insulin-insensitive patients (daily insulin dose greater than 80 U). Degrading activity was significantly correlated in healthy subjects both with circulating insulin concentrations and maximal specific insulin binding. In contrast, in Type 2 diabetic subjects, degrading activity was inversely correlated with serum insulin with no apparent association with maximal specific insulin binding except in those patients given 1 week of insulin treatment. High erythrocyte insulin degrading enzyme activity might be a common feature in the insulin-insensitive Type 2 diabetic patient and might occur subsequent to some aspect of insulin deficiency at the tissue level.

Degradation of insulin-like growth factors I and II by a human insulin degrading enzyme

A human insulin degrading enzyme purified from IM-9 lymphocytes was tested for its ability to degrade insulin-like growth factor I (IGF-I) and insulin-like growth factor II (IGF-II). Degradation of these molecules was assessed by trichloroacetic acid precipitation, binding to specific receptors and chromatography on Sephadex G-50. All three techniques indicated that the enzyme readily degraded IGF-II and slightly degraded IGF-I. The IGF-II degrading activity chromatofocused with the insulin degrading activity and was absorbed by specific antibodies to the insulin degrading enzyme. These studies indicate, therefore, that a human insulin degrading enzyme can degrade IGF-II and, to a lesser extent, IGF-I.

Glucagon degradation by human mononuclear cells

Degradation of 125I-iodoglucagon by human mononuclear cell preparations including one containing 18%-27% monocytes, one consisting of 97% pure monocytes and one consisting of 98% lymphocytes was examined. Intact cells were incubated with 125I-iodoglucagon and degradation assessed by measuring an increase in trichloroacetic acid soluble products or in non-immunoprecipitable products. The preparation consisting of intact lymphocytes did not degrade glucagon. Glucagon was degraded by preparations containing monocytes and this degradation increased with time. No difference between monocyte degradation as measured by trichloroacetic acid or immunoprecipitation was found. Degradation by intact monocytes and by mononuclear homogenates increased sixfold from 4 degrees C to 37 degrees C. Subcellular fractionation demonstrated that the majority of the neutral glucagon degrading activity was in the 100,000 g supernatant (cytosol). Kinetic analyses gave Km values of 1.1 x 10(-5) mol/l, 7.5 x 10(-6) mol/l, and 1.2 x 10(-5) mol/l for glucagon degradation by intact mononuclear cells, homogenates, and cytosol, respectively. Inhibitor studies indicated a sulphydryl dependent enzyme was involved in glucagon degradation by both intact cells and cytosol. The monocyte appeared to be the cell responsible for degradation of glucagon by mononuclear cell preparations. The degradation of glucagon under physiological conditions by intact monocytes was mediated by a neutral proteolytic enzyme, primarily localized in the cytosol.