Shunting of insulin from a retroendocytotic pathway to a degradative pathway by sodium vanadate

PTP1B-dependent insulin receptor phosphorylation/residency in the endocytic recycling compartment of CHO-IR cells

Insulin binds to the alpha subunit of the insulin receptor (IR) on the cell surface. The insulin-IR complex is subsequently internalized and trafficked within the cell. Endocytosed receptors, devoid of insulin, recycle back to the plasma membrane through the endocytic recycling compartment (ERC). Using a high content screening system, we investigate the intracellular trafficking of the IR and its phosphorylation state, within the ERC, in response to protein tyrosine phosphatase-1B (PTP1B) inhibition. Insulin stimulates, in a time- and dose-dependent manner, the accumulation of phosphorylated IR (pY(1158,1162,1163 IR) in the ERC of CHO-IR cells. Treatment of CHO-IR cells with PTP1B-specific inhibitors or siRNA leads to dose-dependent increases in IR residency and phosphorylation within the ERC. The results also demonstrate that PTP1B redistributes within CHO-IR cells upon insulin challenge. The established system will allow for efficient screening of candidate inhibitors for the modulation of PTP1B activity.

Protein Kinase C-ζ and Protein Kinase B Regulate Distinct Steps of Insulin Endocytosis and Intracellular Sorting

We have investigated the molecular mechanisms regulating insulin internalization and intracellular sorting. Insulin internalization was decreased by 50% upon incubation of the cells with the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY294002. PI3K inhibition also reduced insulin degradation and intact insulin release by 50 and 75%, respectively. Insulin internalization was reduced by antisense inhibition of protein kinase C-zeta (PKCzeta) expression and by overexpression of a dominant negative PKCzeta mutant (DN-PKCzeta). Conversely, overexpression of PKCzeta increased insulin internalization as a function of the PKCzeta levels achieved in the cells. Expression of wild-type protein kinase B (PKB)-alpha or of a constitutively active form (myr-PKB) did not significantly alter insulin internalization and degradation but produced a 100% increase of intact insulin release. Inhibition of PKB by a dominant negative mutant (DN-PKB) or by the pharmacological inhibitor ML-9 reduced intact insulin release by 75% with no effect on internalization and degradation. In addition, overexpression of Rab5 completely rescued the effect of PKCzeta inhibition on insulin internalization but not that of PKB inhibition on intact insulin recycling. Indeed, PKCzeta bound to and activated Rab5. Thus, PI3K controls different steps within the insulin endocytic itinerary. PKCzeta appears to mediate the PI3K effect on insulin internalization in a Rab5-dependent manner, whereas PKB directs intracellular sorting toward intact insulin release.

A small reduction in glomerular filtration is accompanied by insulin resistance in type I diabetes patients with diabetic nephrophathy

BACKGROUND

Insulin sensitivity and insulin clearance are compromised in end-stage renal disease but it has not been fully established whether they are altered in earlier stages of diabetic nephropathy.

DESIGN

We studied three groups of patients with type 1 diabetes; 10 with no sign of nephropathy, 11 with albuminuria (> 20 microg min-1) but normal glomerular filtration rate (GFR) and eight with a small reduction in GFR, (43-73 mL min-1 1.73 m-2). The groups were matched for age (range 36-61 years), body mass index (BMI), diabetes duration and glycaemic control. The euglycaemic hyperinsulinaemic clamp technique was utilized to study insulin sensitivity (M-value) and metabolic clearance rate for insulin. Needle biopsies from abdominal subcutaneous fat tissue were obtained to study insulin binding, insulin degradation, insulin-stimulated glucose uptake and anti-lipolysis in adipocytes in vitro.

RESULTS

Patients with reduced GFR were more insulin-resistant (M-value 5.7 +/- 0.7 mg kg LBM-1 min-1) than those without nephropathy (9.6 +/- 0.7, P = 0.001) and those with only albuminuria (8.9 +/- 1.2, P = 0.044). In all subjects taken together there was a strong association between insulin sensitivity and GFR (r = 0.46, P = 0.012). Patients with reduced GFR displayed no significant difference in insulin clearance (12.2 +/- 1.6 mL kg-1 min-1) compared to controls (13.8 +/- 1.3) but a slightly lower insulin clearance than patients with only albuminuria (16.6 +/- 1.0, P = 0.027). There were no significant differences between patient groups in the adipocyte studies in vitro, i.e. with respect to insulin binding, insulin degradation and the effects of insulin on glucose uptake and lipolysis. This is compatible with humoral factors causing whole-body insulin resistance and in the group with reduced GFR, we found that serum parathyroid hormone, interleukin-6 and tumour necrosis factor-alpha levels were elevated whereas the morning cortisol was decreased.

CONCLUSIONS

In type 1 diabetes, the appearance of albuminuria does not seem to alter insulin sensitivity and clearance. A marked insulin resistance but no consistent impairment in insulin clearance seems to accompany progression to a stage with a slight reduction in GFR. These alterations are not accompanied by general defects in insulin target cells. Instead, alterations in the regulation of insulin-antagonistic hormones and cytokines could potentially contribute to the development of insulin resistance in diabetic nephropathy.

Effects of peroxovanadate and vanadate on insulin binding, degradation and sensitivity in rat adipocytes

The effects of vanadate and the stable peroxovanadate compound bpV(pic) on insulin binding and degradation were investigated in rat adipocytes under conditions of ongoing receptor cycling. Both bpV(pic) and vanadate increased 125I-insulin binding to intact cells through an increase in apparent receptor affinity. The maximal effect of bpV(pic) was to increase binding approximately 4-fold (EC50 0.06 +/- 0.01 mM), whereas vanadate increased binding approximately 2-fold (EC50 1.4 +/- 0.2 mM). Removal of cell surface insulin-receptor complexes with trypsin showed that the effects on binding exerted by bpV(pic) and vanadate were due to a similar increase in both cell surface binding and intracellular accumulation of radioactivity. Both bpV(pic) and vanadate inhibited the degradation of 125I-insulin in medium containing 1% bovine serum albumin. The ratio of degraded/intact intracellular 125I-insulin was also markedly reduced by these agents, suggesting that they inhibit intracellular insulin-degrading proteases. Similar to previous findings with vanadate, bpV(pic) stimulated glucose transport and, at low concentrations, enhanced insulin sensitivity. Taken together, these data demonstrate that both bpV(pic) and vanadate inhibit insulin degradation. In addition, they significantly enhance cell surface insulin binding in rat fat cells and this is associated with an improved insulin sensitivity.

Mechanisms of inotropic responses of the isolated rat hearts to vanadate

In view of the invariable development of insulin resistance in different types of cardiovascular diseases, considerable attention has been focused on vanadate because of its ability to exert insulin-like effects in the body. Since vanadate, like insulin, has been shown to exert a beneficial effect in diabetic cardiomyopathy, this study was undertaken to examine the mechanisms of its action on the heart. Vanadate, at 5-10 microM concentrations, produced a positive inotropic effect in the isolated perfused rat heart, whereas at higher concentrations (20 microM), it decreased the contractile force development. The positive inotropic effect of 10 microM vanadate was not affected by the pretreatment of animals with reserpine as well as the presence of propranolol or phenoxybenzamine in the perfusion medium. The increase in contractile force development due to vanadate at low (0.3-0.6 mM) concentrations of Ca2+ was markedly augmented, but this agent produced a negative inotropic action at high concentrations of Ca2+ (2.0-3.0 mM). Preperfusion of hearts with verapamil enhanced the positive inotropic effect of vanadate whereas hearts preperfused with ouabain, low sodium or amiloride showed negative inotropic effects of vanadate. Vanadate was found to inhibit sarcoplasmic reticular Ca(2+)-pump and sarcolemmal Ca(2+)-pump as well as Na(+)-K(+)-ATPase activities but the sarcolemmal effects were evident at lower concentrations in comparison to that on the sarcoplasmic reticulum. The actions of vanadate on membrane Ca2+ transport and ATPase systems were specific since this agent exerted no effect on sarcolemmal Na(+)-Ca2+ exchange or myofibrillar ATPase activities. In isolated cardiomyocytes suspended in buffer containing 0.5 or 1.0 mM Ca2+, vanadate increased the intracellular concentration of Ca2+; this increase in intracellular Ca2+ was more pronounced at 0.5 mM Ca2+. These results indicate that increased intracellular concentration of Ca2+ due to inhibition of sarcolemmal Na(+)-K(+)-ATPase and sarcolemmal Ca(2+)-pump may be the primary mechanism of the positive inotropic action of vanadate in the heart. It is suggested that vanadate may serve as an inotropic agent and that this mechanism may contribute towards its beneficial effects on cardiac dysfunction in different cardiovascular diseases.

Characteristics and mechanisms of tachyphylaxis of cardiac contractile response to insulin

Although insulin is known to cause internalization of its own receptors, the physiological significance of this phenomenon is not clear. In the isolated rat heart we observed that the positive inotropic effect of 25 munits/ml insulin was completely abolished if the heart was preperfused with insulin for 10 min. This tachyphylactic response to insulin began to appear 3-4 min after starting preperfusion with insulin and was partially reversible after 30 min of washing. Preperfusion with insulin did not affect the action of vanadate, which has insulin-like effect on glucose transport, or the actions of the other positive inotropic agents, isoproterenol and ouabain. The presence of propranolol in the perfusion medium, unlike atenolol, phenoxybenzamine, guanethidine, verapamil or quinidine, modified the inotropic as well as tachyphylactic responses to insulin. The positive inotropic and tachyphylactic responses to insulin were not altered in hearts from reserpine-treated animals. Perfusion of heart with glucose-free solution abolished the tachyphylaxis due to insulin. Likewise, no tachyphylactic response to insulin was evident when iodoacetate, but not sodium fluoride, was added in medium containing glucose. These results suggest that ATP formed during glycolysis may play an important role in insulin-induced tachyphylaxis with respect to cardiac contractile activity.

Distribution of insulin binding sites on Leydig cells of rat testes using insulin-coated gold particles

The distribution of insulin binding sites in Leydig cells dispersed with collagenase from rat testes was studied using insulin-coated gold particles as an electron opaque ligand. Using electron microscope is convenient to distinguish Leydig cells among a variety of cells in crude preparations by their ultrastructural characteristics. Leydig cells were shown to possess insulin-binding sites on their plasma membranes. Initial binding sites of insulin were located to the microvillous surfaces. Following binding, receptor-ligand complexes seemed to move to the intermicrovillous plasma membrane, then to be internalized. Two modes of the internalization were confirmed. Most of the receptor-ligand complexes on Leydig cells appeared to be internalized via large, uncoated plasma membrane invaginations, while the remainder became internalized via small pits into vesicles. The receptor-ligand complexes were subsequently transferred to large subsurface vacuoles with electron-lucent lumens believed to correspond to endosomes. The reason why IGCs on the postendosomal pathway moving toward lysosomes was also discussed.

Vanadate modulates the activity of a subpopulation of asialoglycoprotein receptors on isolated rat hepatocytes: active surface receptors are internalized and replaced by inactive receptors

In the absence of ligand, sodium vanadate causes a time- and dose-dependent loss of up to approximately 50% of the surface galactosyl receptor (GalR) activity in rat hepatocytes at 37 degrees C. The effect on total (surface plus intracellular) GalR activity is also dependent on exposure time and vanadate concentration. At less than 1 mM, vanadate induces a transient decrease and then partial recovery of cell surface GalR activity. At greater than 3 mM vanadate, surface GalR activity decreases rapidly (t1/2 approximately 2 min). Lost surface activity is initially recovered in digitonin-permeabilized cells, indicating that active surface GalRs redistribute to the cell interior. However, an antibody assay for GalR protein showed that although surface activity decreased, there was no decrease in surface receptor protein. The active intracellular GalRs then slowly inactivate over 30-60 min. With 8 mM vanadate, the loss of both surface and total cellular GalR activity is more rapid and coincident; no lag is observed. Maximal activity loss, however, was still only approximately 50%. Again, no net change was seen in the distribution of GalR protein between the cell surface and the interior. These results indicate that vanadate causes active GalRs to move from the surface to the inside and be replaced by inactive receptors moving from the inside to the cell surface. The Gal receptor system is comprised of two functionally different receptor subpopulations that operate via two distinct intracellular pathways. Only the State 2 GalRs, which recycle constitutively, are sensitive to modulation by vanadate. Consistent with this, vanadate inhibits the endocytosis of 125I-asialoorosomucoid (ASOR) only partially. The rate of uptake and the steady state level of ASOR intracellular accumulation were maximally inhibited by 50 and 70%, respectively, at 0.2 mM vanadate. The rate and extent of degradation of 125I-ASOR were also inhibited by 50-70%. Residual ASOR uptake and degradation is accounted for by the minor vanadate-resistant State 1 Gal receptor pathway.

[125I]-insulin metabolism by the rat liver in vivo: evidence that a neutral thiol-protease mediates rapid intracellular insulin degradation

The subcellular site where insulin is degraded by rat hepatocytes in vivo is controversial. While several potential insulin-degrading enzyme systems, each with its own characteristic cellular location, are known to exist in the liver, questions remain about which of them participates in the degradation of physiologic doses of insulin. These studies examine the proteases that degrade physiologic doses of [125I]-insulin in vivo to determine (1) when and where initial degradation occurs, and (2) which of the potential degradative enzymes is active. Following injection into the mesenteric veins of male rats, intact [125I]-insulin and its labeled degradation products were analysed by reverse-phase high-performance liquid chromatography (RP-HPLC) of biopsy homogenates. [125I]-insulin was rapidly degraded in vivo; the t 1/2 of degradation was approximately 2.7 minutes. To test for extracellular protease activity, an isolated perfused liver system was employed. [125I]-insulin (or [125I]-glucagon) uptake was controlled by changing the temperature of the perfusion medium. Five minutes after [125I]-insulin injection, surface-bound label was recovered in an acidic (pH 3.5) wash. In perfusion at 15 degrees C, both the internalization and degradation of [125I]-insulin were inhibited; 7.2% of unbound hormone was degraded and 5.1% of surface-bound insulin was degraded. Only 11.4% of unbound insulin and 17.4% of surface-bound insulin were degraded at 35 degrees C. In contrast, 95.5% of unbound glucagon and 89.9% of surface-bound glucagon were degraded at 35 degrees C. Thus, although glucagon degradation occurs at the sinusoidal plasmalemma of perfused livers, the same membrane does not mediate the rapid degradation of insulin observed in vivo. Analysis of the RP-HPLC [125I]-insulin elution profiles from liver biopsy homogenates, and comparison of them to profiles produced by purified proteases, suggested that insulin protease is responsible for most hepatic degradation of physiologic doses of insulin. Some cathepsin D-like activity was also observed in vivo, confirming that two pathways exist for insulin metabolism. The time course over which insulin was degraded was more rapid than previous studies in vitro would have predicted. This suggests that more insulin was receptor-bound at the time of its initial degradation, and that the active protease was soluble and was introduced into endocytic peripheral endosomes within seconds after their formation.

Vanadate mimics effects of fungal cell wall in eliciting gene activation in plant cell cultures

Cell-suspension cultures of peanut (Arachis hypogaea L.) can be used as a very sensitive and rapidly responding physiological system for monitoring extracellular signals. Elicitors effect the activation of the genes that code for a set of enzymes synthesizing stilbenes. Within 2-6 h after administering micromolar, concentrations of orthovanadate to the suspended cells, the enzyme activities of phenylalanine ammonia-lyase, stilbene synthase, and cinnamate 4-hydroxylase increased 10-to 100-fold. The transient time course of induction, and the quality and quantity of gene expression found with vanadate as artificial elicitor were very similar to those observed after biotic stress generated by fungal cell walls. The dose-response of vanadate as an elicitor of gene expression in intact cells matched precisely its inhibitory effect on the ATPase activity of isolated plasma membrane. By concentrating, on the profiles of cinnamate 4-hydroxylase activity, we observed differences between the effects elicited by fungal cell wall or vanadate when different stages of cell development were analyzed. Unlike the fungal elicitor, vanadate did not induce the hydroxylase activity when cells at the stationary phase of the cell cycle were used. This lack of response was not the result of a decrease in membrane biosynthesis. The finding, that the effects of vanadate and fungal elicitor are additive indicates that vanadate does not interfere negatively with the perception of the biotic signal but rather addresses the same intracellular intermediate of the signalling process. We hypothesize that membrane potentials created or modulated by ATPases may be intermediates in the signal chain, starting with the recognition process at the plasma membrane and eventually leading to the production of stilbenes as low-molecular-weight plant-defence products.

Subchronic treatment with vanadate does not potentiate the toxicity of cardiac glycosides

Since it has been claimed that vanadate is an endogenous regulator of Na/K-ATPase activity and that it potentiates the toxicity of cardiac glycosides, we were alarmed to discover that certain Finnish physicians were prescribing vanadate in combination with other trace minerals to elderly patients for many different chronic diseases (e.g., cancer, rheumatism). To study the interaction of vanadate and cardiac glycosides, we fed vanadate in the drinking water (25 micrograms/mL) to guinea pigs for 20 d, and studied either their sensitivity to the acute toxicity of the cardiac glycoside ouabain or whether the vanadate would influence the subacute toxicity of ouabain. Vanadate had no influence on the toxicity of ouabain either acute or subchronically administered, nor was there any sign of inhibition of Na/K-ATPase activity as measured by 86Rb-uptake into intact erythrocytes (RBCs), RBC content of sodium or potassium or Na/K-ATPase activity in RBC membranes prepared from the vanadate-treated guinea pigs. Vanadate had been absorbed in substantial quantities from the gastrointestinal tract, since serum, heart, liver, and especially kidney contained measurable amounts of vanadium in contrast to controls, but it is concluded that this vanadate is not in a biologically active form.