Degradation of insulin by isolated rat liver cells

Insulin-degrading neutral cysteine proteinase activity of adipose tissue and liver of nondiabetic, streptozotocin-diabetic, and insulin-treated diabetic rats

The activity of the insulin-degrading enzyme neutral cysteine proteinase (EC 3.4.22.11, insulinase) was studied in adipose tissue and in liver of nondiabetic, streptozotocin-diabetic, and insulin-treated diabetic rats. Proteinase activity was found to be significantly decreased during diabetes and was restored to near normal levels in both tissues following insulin treatment. The insulin-mediated increase of proteinase activity in both tissues was partially or completely blocked by actinomycin D (an inhibitor of RNA synthesis) and by cyclohexamide (an inhibitor of protein synthesis). Kinetic analysis showed that the changes in proteinase activity of both liver and adipose tissues were accompanied by a change in Vmax (i.e., maximal enzyme activity) without a change in Km (i.e., substrate affinity). These data indicate that insulin functions as an inducer for neutral cysteine proteinase in both tissues. These alterations in the proteinase activity paralleled the alterations in the activity of a second insulin-degrading enzyme, glutathione-insulin transhydrogenase in adipose tissue (this paper) and in liver (previously published papers) under the same physiological conditions.

Differences in insulin sensitivity between normal men and women

An incremental intravenous low-dose insulin infusion has been used to examine differences in insulin sensitivity between normal young men and women. Fasting blood glucose concentration did not differ significantly at the start of the infusion but women had significantly higher plasma insulin and C-peptide concentrations. Similar changes in blood glucose occurred during insulin infusion but insulin concentrations were higher in women. Blood total ketone bodies and alanine were lower in women over the four hours of infusion. Significant differences were found between normal men and women for the effect of insulin upon blood glucose concentration.

Diurnal changes in the rate of cholesterogenesis in hepatocytes from fed and starved rats: effects of precursors and pancreatic hormones in vitro

The activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) varied with a diurnal periodicity in hepatocytes prepared at different times from rats accustomed to a controlled feeding and lighting schedule. The rates of sterol synthesis varied in a similar manner but the maximum rate was not synchronous with maximum HMG-CoA reductase activity. The diurnal increase in HMG-CoA reductase activity and sterol synthesis rate started before food was offered to donor animals. Neither insulin nor glucagon had any effect on the diurnal pattern of hepatic sterol synthesis in vitro. Pyruvate inhibited sterol synthesis in hepatocytes prepared during the feeding period but had no effect at other times of day. When food was withheld from donor animals at the beginning of the normal feeding period both HMG-CoA reductase activity and the rate of sterol synthesis rapidly decreased. During this period neither insulin nor lipogenic substrates, alone or in combination, were able to restore the rates of sterol synthesis to normal values. In hepatocytes prepared from animals starved for a longer period (43 h) the decrease in the activity of HMG-CoA reductase was much less than that in the rate of sterol synthesis. In contrast to hepatocytes from fed or short-term-starved animals, the rate of sterol synthesis in these hepatocytes could be increased by glucose or pyruvate.

Insulin degradation in human erythrocyte: effects of cations

Insulin degradation by human erythrocyte fractions was studied using the TCA-precipitation method. Hemolysate exhibited an insulin degrading activity higher than membranes. Triton X-100 treatment of membranes led to the appraisal of Triton-soluble degrading activity and of a more efficient Triton-not-soluble degrading activity. Monovalent cations (Na+, K+, Li+) did not modify the insulin degradation by any of the erythrocyte fractions. Divalent cations, Ca++ and Zn++ selectively enhanced insulin degradation by the membranous fractions, and Cu++ and Zn++ strongly inhibited insulin degradation by all the erythrocyte fractions. The results supported the hypothesis of the existence of at least two different degrading systems in human erythrocytes: soluble (cytosolic) Ca++ and Mg++ insensitive system(s) and membrane associated Ca++ and Mg++ sensitive system(s).

Topology of glutathione-insulin transhydrogenase in rat liver microsomes

Glutathione-insulin transhydrogenase (EC 1.8.4.2) catalyzes the inactivation of insulin through scission of the disulfide bonds to form insulin A and B chains. In the liver, the transhydrogenase occurs primarily in the microsomal fraction where most of the enzyme is present in a latent ('inactive') state. We have isolated rat hepatic microsomes with latent transhydrogenase activity being an integral part of the vesicles. We have used these vesicles to study the topological location of glutathione-insulin transhydrogenase by investigating the effects of detergents (Triton X-100 and sodium deoxycholate), phospholipase A2 and proteinases (trypsin and thermolysin) on the latent enzyme activity. Treatment of intact vesicles with variable concentrations of detergents and phospholipase A2 resulted in the unmasking of latent transhydrogenase activity. The extent of unmasking of transhydrogenase activity is dependent upon the concentration of detergent or phospholipase used and is accompanied by a parallel release of the enzyme into the soluble fraction. Activation of the transhydrogenase by phospholipase A2 is partially inhibited by bovine serum albumin and the extent of inhibition is inversely proportional to the phospholipase concentration. In intact vesicles, latent transhydrogenase activity is resistant to proteolytic inactivation by both trypsin and thermolysin, while in semipermeable and permeable vesicles these proteases inactivate 60 and 25% of the total transhydrogenase activity, respectively. Together these results indicate that in microsomes transhydrogenase is probably weakly bound to membrane phospholipid components and that most of the enzyme is present on the cisternal surface (i.e., the luminal surface of the endoplasmic reticulum) of microsomes. Each detergent and phospholipase apparently unmasks glutathione-insulin transhydrogenase activity through disruption of the phospholipid-enzyme interaction followed by translocation of the enzyme to the soluble (cytoplasmic) fraction and not through increases in substrate availability.

Studies on the inhibitory effect of bacitracin on 125I-labelled insulin internalization in the rat hepatocyte

Previous studies have suggested that transglutaminase has a role in the internalization of some polypeptide hormones and is inhibited by the antibiotic, bacitracin. Bacitracin has been used in insulin-receptor studies to inhibit extracellular degradation of 125I-labelled insulin. The aim of this study was to investigate bacitracin's effect on 125I-labelled insulin-receptor interactions in isolated rat hepatocytes. 1 g/l bacitracin increased cell-associated 125I-labelled insulin insulin at 20, 30 and 37 degrees C (P less than 0.001, 0.0005 and 0.0005, respectively). At 5 and 15 degrees C (internalization does not occur), bacitracin did not affect cell-associated 125I-labelled insulin. The bacitracin effect was concentration dependent, increasing to 2 g/l. Scatchard analysis showed that bacitracin did not alter insulin receptor affinity or number. 1 g/l bacitracin abolished the effect of chloroquine. The increased cell-associated radioactivity with bacitracin was surface-bound in nature. 0.5 g/l bacitracin decreased 125I-labelled insulin degradation in hepatocyte suspensions (P less than 0.001) and in buffer previously incubated with hepatocytes (P less than 0.0005). More 125I-labelled insulin remained associated with cells during dissociation studies at 37 degrees C when the buffer contained 1 g/l bacitracin. Label that appeared in the buffer after 60 min was significantly more intact in the presence of bacitracin (P less than 0.025). These results suggest that bacitracin retards the internalization of 125I-labelled insulin in isolated rat hepatocytes.

The fate of insulin in cardiac muscle. Studies on isolated muscle cells from adult rat heart

Isolated muscle cells from adult rat heart were used to study myocardial degradation of insulin and the reactions after the initial binding event. After 60 min of association at 37 degrees C, 90% of specifically bound insulin could be dissociated from the cells; this fraction remained unaltered under steady-state conditions (up to 180 min). To assess the nature of cell-associated radioactivity, cardiocytes were solubilized and filtered on Sephadex G-50. After 5 min of association only intact insulin was observed, whereas under steady-state conditions 4% of 125I-labelled insulin bound to the cells was degraded to iodotyrosine-containing fragments. The Km for insulin degradation by isolated heart cells was estimated to be 1.75 x 10(-7)M. Receptor-mediated insulin degradation was studied by examination of the nature of radioactivity released by the cells after different times of association. After 5 min 83% of dissociating material consisted of intact insulin, whereas this fraction decreased to 50% under steady-state conditions. Treatment of cells with the lysosomotropic agent chloroquine (0.1 mM) significantly decreased the fraction that was eluted at the internal column volume. This study demonstrates that insulin degradation by the heart cell occurs by a receptor-independent and a receptor-dependent mechanism. The latter may involve internalization and a lysosomal pathway.

Impaired insulin degradation in a patient with insulin resistance and acanthosis nigricans

The kinetics of plasma insulin were studied in a 14 year old girl with the syndrome of insulin resistance and acanthosis nigricans. The clearance of plasma insulin was found to be strikingly reduced (135 ml/min . m2 versus 456 +/- 22 in 17 normal control subjects), whereas the basal systemic insulin delivery rate was increased about 10-fold (25.5 mU/min . m2 versus 2.6 +/- 0.3 in normal subjects). Thus, reduced insulin clearance and excessive posthepatic delivery of the hormone both contributed to the severe fasting hyperinsulinemia (218 microunits/ml) associated with the other clinical features of the syndrome (glucose intolerance, primary amenorrhea, polycystic ovaries, hirsutism). Following ovarian wedge resection, insulin clearance rose to 264 ml/min . m2, and insulin delivery fell to 9.8 microunits/ml min . m2. The resulting abatement of the patient's hyperinsulinism (fasting plasma insulin = 37 microunits/ml) was accompanied by the appearance of menses, normalization of glucose tolerance, and amelioration of the acanthosis. The improvement in menstrual function and acanthosis, however, was not sustained. This case provides evidence for interdependence of insulin action and insulin degradation in humans.

125I-labelled insulin degradation by isolated rat hepatocytes: the roles of glutathione-insulin transhydrogenase and insulin-specific protease

Isolated rat hepatocytes degraded 125I-insulin with a Km of 150 nmol/l. Degradation was stimulated by the addition of glutathione and dithiothreitol. In cells incubated with diamide, glutathione was oxidised to the disulphide. Regeneration of reduced glutathione commenced after a further 30 min incubation at 37 degrees C. Diamide (1 mmol/l) significantly inhibited insulin degradation by hepatocytes (p less than 0.001). The 'apparent Vmax' for insulin degradation was decreased tenfold and the Km decreased to 25 nmol/l. The diamide-insensitive degrading activity was cell-associated and produced an intermediate of hormone degradation that was apparently of a higher molecular weight than insulin A chain. The biological activity of the intermediate was 0.03% of that of insulin. The diamide-insensitive activity was not due to release of protease into the medium by cell lysis. We conclude that there are at least two pathways capable of degrading insulin existing in rat hepatocytes.

Receptor-mediated insulin degradation and insulin-stimulated glycogenesis in cultured foetal hepatocytes

Insulin-stimulated glycogenesis and insulin degradation were studied simultaneously at 37 degrees C in cultured foetal hepatocytes grown for 2-3 days in the presence of cortisol. Degradation of cell-associated insulin, as measured by trichloroacetic acid precipitation, was significant after 4 min in the presence of 1-3 nM-125I-labelled insulin. This process became maximal (30% of insulin degraded) after 20 min, a time when binding-state conditions were achieved. No insulin-degradative activity was detected in a medium that had been exposed to cells. At steady-state, the appearance of insulin degradation products in the medium was linearly dependent on time (1.5 fmol/min per 10(6) cells at 1nM-125I-labelled insulin). Chloroquine (3-50 microM), bacitracin (0.1-10 mM) and NH4Cl (1-10 mM) inhibited insulin degradation as soon as this became detectable and caused an increase in the association of insulin to hepatocytes after 20 min. Lidocaine and dansylcadaverine had similar effects, whereas N-ethylmaleimide, aprotinin, phenylmethanesulphonyl fluoride and leupeptin were found to be ineffective. Chloroquine, and also bacitracin, at concentrations that inhibited insulin degradation, decreased the insulin-stimulated incorporation of [14C]glucose into glycogen over 2 h. This effect of chloroquine was specific, since it did not modify the basal glycogenesis, or the glycogenic effect of a glucose load in the absence of insulin. It therefore appears that the receptor-mediated insulin degradation (or some associated pathway) is functionally related to the glycogenic effect of insulin in foetal hepatocytes.

Zinc and insulin metabolism

A review of experimental studies of the effect of zinc nutrition on insulin metabolism is presented. In addition to a short introduction to the synthesis, secretion, and action of insulin, the effects of zinc deficiency-specifically on glucose tolerance, insulin secretion, insulin synthesis and storage, and on total insulin-like activity-are dealt with. The concentrations of zinc and chromium in serum, pancreas, and liver are compared to those of zinc-deficient animals and pair-fed controls.In contrast to pair-fed controls, zinc-deficient rats had unaltered proinsulin contents after glucose stimulation, but they showed a diminished glucose tolerance, lowered serum insulin content, and an elevated total insulin-like activity. The serum zinc concentration of the deficient animals was greatly reduced and did not change during glucose stimulation, whereas it rose in the case of the pair-fed controls. The serum chromium concentration increased in both groups in response to glucose stimulation. In the pancreas of the deficient animals, the zinc concentration was reduced 60% and it increased during the glucose tolerance test. In the liver there were no significant differences. The chromium concentrations were elevated in both the pancreas and liver of the zinc-deficient rats by 60 and 100%, respectively, and were not influenced by glucose injection.These studies show clearly that nutritional zinc deficiency influences insulin metabolism and action.

Evidence for the presence of insulin binding sites in isolated rat intestinal epithelial cells

Insulin receptors have been demonstrated in isolated rat intestinal epithelial cells. The specific binding of 125I-insulin was time--and temperature--dependent, the optimal temperature of study being 15 degrees. Dissociation of bound 125I-insulin by an excess of unlabelled hormone was rapid and attained 66 +/- 2% in 2 h. When initiated by dilution, the dissociation attained 35 +/- 4% in 2 h, and 72 +/- 1% in 2 h when 10(-7) mol/l unlabelled insulin was added. The pH optimum for the binding process was between 7.5 and 8, and the binding increased proportionally to cell protein concentration up to 1.5 mg/ml. Under standard conditions (2 h at 15 degrees) the degradation of the labelled hormone in the medium accounted for 20--50% of total tracer, depending on the concentration of cells. At apparent equilibrium (2 h at 15 degrees), unlabelled insulin in the range of 10(-10) to 10(-7) mol/l inhibited competitively the binding of 4.3--7 X 10(-11) mol/l 125I-insulin; fifty per cent inhibition was obtained with 3 X 10(-9) mol/l native insulin. Scatchard analysis, after correction for degradation, gave curvilinear plots, that may be explained by two orders of binding sites, with 2,000 +/- 200 sites/cell of high affinity (Ka = 2.2 +/- 0.2 X 10(9) l/mol) and 39,000 +/- 3,000 sites/cell of low affinity (Ka = 5.6 +/- 1.6 X 10(7) l/mol). The potency of proinsulin to compete with 125I-insulin for the binding site was 3% that of insulin, unrelated peptides were inactive. Such results give a molecular basis to different reports suggesting that the intestine could be a target-tissue for insulin.

Mechanisms of acute hyperinsulinemia after Kupffer cell phagocytosis

Blockade of hepatic Kupffer cells by prior phagocytosis of a variety of particulate materials caused acute hyperinsulinemia in glucose-stimulated fasted rats under pentobarbital anesthesia. At 4-h postblockade a 125-250% increase in peripheral plasma insulin levels occurred due to a combination of enhanced pancreatic insulin secretion and depressed hepatic insulin extraction. Enhanced pancreatic insulin secretion was confirmed by a 36-54% elevation of portal venous insulin levels. Depressed hepatic insulin extraction was indicated by a 37-47% reduction in insulin uptake by in situ perfused livers as well as alterations in portal-hepatic venous insulin differences and intravenous insulin tolerance tests in vivo. All parameters began to return toward control values at 24 and 48 h postblockade. Return was slow after inert carbon phagocytosis and rapid after degradable bacteria phagocytosis. Peripheral plasma insulin levels were very highly correlated with glucose clearance rates in all groups both control and experimental. Mechanisms are proposed to explain these findings based on the release of lysosomal enzymes and endogenous pyrogens by phagocytizing Kupffer cells as well as the presence of insulin receptors on hepatocytes and Kupffer cells.

Apparent insulin resistance due to abnormal enzymatic insulin degradation: a new mechanism for insulin resistance

A 16-year-old girl presented with severe, prolonged insulin resistance. Insulin antibodies, initially thought to be responsible for the insulin resistance, were suppressed using monocomponent insulin and immunosuppressive therapy; however insulin resistance persisted. Insulin kinetic studies suggested abnormal metabolism of a bolus injection of 125I insulin and the reappearance in the circulation of radioactive products, demonstrated by chromatography to be of different molecular weight to insulin. These products were of similar molecular weight to material obtained by incubating 125I insulin with protease. Trasylol significantly reduced the patient's insulin requirements and normalised the disappearance of 125I insulin from the circulation. Prolonged treatment with Trasylol resulted in a fall in insulin requirement to non "insulin-resistance" levels. The insulin requirement remained static when Trasylol was ceased. We propose abnormally rapid insulin degradation to be a new mechanism of resistance to insulin therapy.

Insulin receptor binding and degradation in IM-9 cultured human lymphocytes - importance of extracellular degradation

Insulin degradation in IM-9 cultured human lymphocytes occurs extracellularly. In the presence of 0.1% bovine serum albumin, insulin degradation products are formed that are soluble in 5% trichloroacetic acid. In the presence of 5% bovine serum albumin, the major products of insulin degradation are insoluble in 5% trichloroacetic acid and have little or no biological activity. Inhibition of binding with Concanavalin A or antireceptor antibody does not affect the rate of insulin degradation.

Relationship of binding to internatlization of 125I-insulin in isolated rat hepatocytes

By quantitative electron microscopic autoradiographic technique, we have previously shown that 125I-insulin initially localizes to the plasma membrane of isolated rat hepatocytes and is subsequently internalized in a limited region of the peripheral cytoplasm. In the present study, we have shown that when cells are incubated at 20 degrees C, steady state binding is reached by 60 minutes and maintained up until 120 minutes of incubation while at 37 degrees C steady state binding is reached by 10 minutes and maintained for 30 minutes. Under both of these conditions, internalization of the labelled material occurs as a constant function of the binding. These data suggest that under normal conditions the binding of the ligand is an important rate limiting determinant of the internalization process.