Chloroquine extends the lifetime of the activated insulin receptor complex in endosomes

Transmembrane stem cell factor protein therapeutics enhance revascularization in ischemia without mast cell activation

Stem cell factor (SCF) is a cytokine that regulates hematopoiesis and other biological processes. While clinical treatments using SCF would be highly beneficial, these have been limited by toxicity related to mast cell activation. Transmembrane SCF (tmSCF) has differential activity from soluble SCF and has not been explored as a therapeutic agent. We created novel therapeutics using tmSCF embedded in proteoliposomes or lipid nanodiscs. Mouse models of anaphylaxis and ischemia revealed the tmSCF-based therapies did not activate mast cells and improved the revascularization in the ischemic hind limb. Proteoliposomal tmSCF preferentially acted on endothelial cells to induce angiogenesis while tmSCF nanodiscs had greater activity in inducing stem cell mobilization and recruitment to the site of injury. The type of lipid nanocarrier used altered the relative cellular uptake pathways and signaling in a cell type dependent manner. Overall, we found that tmSCF-based therapies can provide therapeutic benefits without off target effects.

The effects of hydroxychloroquine on insulin sensitivity, insulin clearance and inflammation in insulin-resistant adults: A randomized trial

AIM

To determine the effect of hydroxychloroquine (HCQ) on skeletal muscle and liver insulin sensitivity, insulin clearance, inflammation and adipokines.

METHODS

Insulin-resistant adults without rheumatic disease were randomized to 13 weeks of HCQ (400 mg/day) versus placebo (double-blinded). Primary outcomes were changes in skeletal muscle and liver insulin sensitivity assessed by hyperinsulinaemic-euglycaemic clamp and stable-isotope tracer methods. Secondary outcomes included insulin clearance, inflammation biomarkers and adipokines.

RESULTS

Compared with placebo, HCQ significantly improved skeletal muscle insulin sensitivity by 26% (p = .019) and enhanced systemic glucose clearance (p = .025). By contrast, HCQ had no effect on hepatic insulin sensitivity. HCQ did not affect insulin clearance but decreased circulating IL-6 (p = .01) and increased adiponectin (p = .045). There were no effects on leptin, RBP-4, FGF-21 or C-reactive protein.

CONCLUSIONS

HCQ selectively enhances insulin sensitivity and glucose disposal in skeletal muscle, without affecting hepatic insulin sensitivity or insulin clearance. These findings offer a mechanistic explanation for the antidiabetic properties of HCQ and suggest that this medication might be useful in conditions linked to insulin resistance such as type 2 diabetes.

Repurposing Antiviral Protease Inhibitors Using Extracellular Vesicles for Potential Therapy of COVID-19

In January 2020, Chinese health agencies reported an outbreak of a novel coronavirus-2 (CoV-2) which can lead to severe acute respiratory syndrome (SARS). The virus, which belongs to the coronavirus family (SARS-CoV-2), was named coronavirus disease 2019 (COVID-19) and declared a pandemic by the World Health Organization (WHO). Full-length genome sequences of SARS-CoV-2 showed 79.6% sequence identity to SARS-CoV, with 96% identity to a bat coronavirus at the whole-genome level. COVID-19 has caused over 133,000 deaths and there are over 2 million total confirmed cases as of April 15th, 2020. Current treatment plans are still under investigation due to a lack of understanding of COVID-19. One potential mechanism to slow disease progression is the use of antiviral drugs to either block the entry of the virus or interfere with viral replication and maturation. Currently, antiviral drugs, including chloroquine/hydroxychloroquine, remdesivir, and lopinavir/ritonavir, have shown effective inhibition of SARS-CoV-2 in vitro. Due to the high dose needed and narrow therapeutic window, many patients are experiencing severe side effects with the above drugs. Hence, repurposing these drugs with a proper formulation is needed to improve the safety and efficacy for COVID-19 treatment. Extracellular vesicles (EVs) are a family of natural carriers in the human body. They play a critical role in cell-to-cell communications. EVs can be used as unique drug carriers to deliver protease inhibitors to treat COVID-19. EVs may provide targeted delivery of protease inhibitors, with fewer systemic side effects. More importantly, EVs are eligible for major aseptic processing and can be upscaled for mass production. Currently, the FDA is facilitating applications to treat COVID-19, which provides a very good chance to use EVs to contribute in this combat.

Revisiting the Cardiotoxic Effect of Chloroquine

PURPOSE

Cardiotoxicity is a well-known side effect of chloroquine. Several studies have proposed chloroquine as a potential anti-diabetic treatment but do not address this problem. The current study investigated the effect of ex vivo chloroquine treatment on (1) heart function and glucose uptake, (2) mitochondrial function and (3) in vivo treatment on heart function.

METHODS

Control or obese male Wistar rats were used throughout. Dose responses of increasing chloroquine concentrations versus vehicle on cardiac function were measured using isolated, Langendorff-perfused hearts whilst glucose uptake and cell viability were determined in ventricular cardiomyocytes. Mitochondrial function was assessed with a Clark-type oxygraph (Hansatech) after ex vivo perfusion with 30 μM chloroquine versus vehicle. Animals were treated orally with 5 mg/kg/day chloroquine for 6 weeks.

RESULTS

Acute chloroquine treatment of 10 μM was sufficient to significantly decrease heart function (p < 0.05) whilst 30 μM significantly reduced heart rate (p < 0.05). Chloroquine became toxic to isolated cardiomyocytes at high concentrations (100 μM), and had no effect on cardiomyocyte glucose uptake. Ex vivo treatment did not affect mitochondrial function, but chronic low-dose in vivo chloroquine treatment significantly decreased aortic output and total work in hearts (p < 0.005).

CONCLUSION

Low and intermediate chloroquine doses administered either chronically or acutely are sufficient to result in myocardial dysfunction.

Alterations in Cellular Processes Involving Vesicular Trafficking and Implications in Drug Delivery

Endocytosis and vesicular trafficking are cellular processes that regulate numerous functions required to sustain life. From a translational perspective, they offer avenues to improve the access of therapeutic drugs across cellular barriers that separate body compartments and into diseased cells. However, the fact that many factors have the potential to alter these routes, impacting our ability to effectively exploit them, is often overlooked. Altered vesicular transport may arise from the molecular defects underlying the pathological syndrome which we aim to treat, the activity of the drugs being used, or side effects derived from the drug carriers employed. In addition, most cellular models currently available do not properly reflect key physiological parameters of the biological environment in the body, hindering translational progress. This article offers a critical overview of these topics, discussing current achievements, limitations and future perspectives on the use of vesicular transport for drug delivery applications.

Lipopolysaccharide Potentiates Insulin-Driven Hypoglycemic Shock

Critically ill patients typically present with hyperglycemia. Treatment with conventional insulin therapy (targeting 144-180 mg/dl) improves patient survival; however, intensive insulin therapy (IIT) targeting normal blood glucose levels (81-108 mg/dl) increases the incidence of moderate and severe hypoglycemia, and increases mortality. Septic patients are especially prone to IIT-induced hypoglycemia, but the mechanism remains unknown. Here, we show that codelivery of insulin with otherwise sublethal doses of LPS induced hypoglycemic shock in mice within 1-2 h. LPS impaired clearance of insulin, which amplified insulin receptor signaling. These effects were mediated by caspase-11, TLR4, and complement, each of which trigger eicosanoid production that potentiates insulin signaling. Finally, in an animal model of sepsis, we observed that Salmonella typhimurium-infected mice exhibited simultaneous impaired insulin clearance coexisting with insulin resistance. Our results raise the possibility that septic patients have impaired insulin clearance, which could increase their susceptibility to hypoglycemia during IIT, contraindicating its use.

The Effects of Transdermally Delivered Oleanolic Acid on Malaria Parasites and Blood Glucose Homeostasis in P. berghei-Infected Male Sprague-Dawley Rats

The present study investigated the effects of transdermally delivered oleanolic acid (OA) monotherapy and in combination with chloroquine (CHQ) on malaria parasites and glucose homeostasis of P. berghei-infected male Sprague-Dawley rats. Oral glucose test (OGT) responses to OA-pectin patch and CHQ-OA combination matrix patch were monitored in non-infected and infected rats. To evaluate the short-term effects of treatment, percentage parasitaemia, blood glucose, glycogen and plasma insulin were monitored in separate groups of animals treated with either OA-patch monotherapy or CHQ-OA combination pectin patch over a 21-days period. Animals treated with drug-free pectin and CHQ acted as untreated and treated positive controls, respectively. Infected control rats exhibited significantly increased parasitaemia which was accompanied by hypoglycaemia. Both OA monotherapy and CHQ-OA combination therapy reduced and cleared the malaria parasites within a period of 4 and 3 days, respectively. Compared to respective controls groups, OGT responses of animals treated with OA monotherapy or CHQ-OA combination therapy exhibited lower blood glucose levels at all time points. A once-off transdermal application of OA-patch or CHQ-OA combination patch significantly improved blood glucose concentrations inducing any changes in insulin concentration. Transdermal OA used as a monotherapy or in combination with CHQ is able to clear and reduce the malaria parasites within a shorter period of time without eliciting any adverse effects on glucose homeostasis of P. berghei-infected rats.

The influence of other drugs on coronary heart disease (CHD) risk in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) patients currently are exposed to a number of agents to control various manifestations of their disease. In particular the degree of immunosuppression employed is strongly related to the underlying disease severity. This causes problems when evaluating the longterm associations between drugs and outcomes such as cardiovascular disease. Nevertheless, most studies do suggest that increased and prolonged exposure to steroids is harmful to the vasculature and that antimalarial drugs may have several potential beneficial effects in reducing CHD risk in SLE. Further work is now needed to assess if there are doses or regimes of steroid therapy that can optimize their anti-inflammatory effects and minimise their adverse metabolic effects. With regard to other drugs used, it should not be assumed that all immunosuppressive drugs are equal with regard to the cardiovascular system. There is some emerging data that azathioprine may have an association with atherosclerosis development. Whether this is independent of the clinical phenotype remains an open question. Studies of MTX in RA are controversial and in SLE the specific cardiovascular effects of MTX requires direct study.

With several novel agents currently in development, their potential effects on the vasculature need specific study to unravel the relative effects of the inflammatory burden, specific organ involvement and therapy on vascular risk.

Antidiabetogenic effects of hydroxychloroquine on insulin sensitivity and beta cell function: a randomised trial

AIMS/HYPOTHESIS

Hydroxychloroquine (HCQ), an antimalarial drug with anti-inflammatory properties, is employed in rheumatic diseases. In observational studies, patients with rheumatic diseases treated with HCQ have a lower risk of developing diabetes. However, the physiological mechanisms remain unexplained. We hypothesised that HCQ may have favourable effects on insulin sensitivity and/or beta cell function.

METHODS

This was a randomised, double-blind, parallel-arm (placebo vs HCQ 400 mg/day) trial at the University of Pittsburgh. Randomisation was conducted by a computer system with concealment by sealed envelopes. Treatment duration was 13 ± 1 weeks. Randomised participants (HCQ n = 17; placebo n = 15) were non-diabetic volunteers, age >18, overweight or obese, with one or more markers of insulin resistance. All participants were included in intention-to-treat analysis. Outcomes were changes in insulin sensitivity and beta cell function measured by intravenous glucose tolerance tests and minimal model analysis.

RESULTS

There was a positive change in insulin sensitivity with HCQ but not placebo (mean ± SEM: +20.0% ± 7.1% vs -18.4% ± 7.9%, respectively; p < 0.01; difference: 38.3% ± 10.6%; 95% CI: 17%, 60%). Improvement in beta cell function was also observed with HCQ but not placebo (+45.4% ± 12.3% vs -19.7% ± 13.6%; p < 0.01; difference: 65% ± 19%; 95% CI: 27%, 103%). There were modest treatment effects on fasting plasma glucose and HbA(1c) (p < 0.05) but circulating markers of inflammation (IL-6, IL-1, TNF-α, soluble intercellular adhesion molecule) were not affected in either group. In contrast, adiponectin levels increased after HCQ treatment but not after placebo (+18.7% vs +0.7%, respectively; p < 0.001). Both low- and high-molecular-weight adiponectin forms accounted for the increase. There were no serious or unexpected adverse effects.

CONCLUSIONS/INTERPRETATION

HCQ improves both beta cell function and insulin sensitivity in non-diabetic individuals. These metabolic effects may explain why HCQ treatment is associated with a lower risk of type 2 diabetes. An additional novel observation is that HCQ improves adiponectin levels, possibly being a mediator of the favourable effects on glucose metabolism. Our findings suggest that HCQ is a drug with considerable metabolic effects that warrant further exploration in disorders of glucose metabolism.

TRIAL REGISTRATION

Clinicaltrials.gov NCT01326533 FUNDING: This study was funded by National Institutes of Health no. 5R21DK082878, UL1-RR024153 and UL-1TR000005.

Metabolic syndrome is not only a risk factor for cardiovascular diseases in systemic lupus erythematosus but is also associated with cumulative organ damage: a cross-sectional analysis of 311 patients

BACKGROUND/PURPOSE

Patients with systemic lupus erythematosus (SLE) have increased rates of cardiovascular disease (CVD) that are one of the major causes of mortality. The aim of this study was to determine the frequencies of metabolic syndrome (MetS) and CVD in SLE patients and investigate the link between these and clinical features of SLE.

METHODS

A total of 311 SLE patients were consecutively assessed for cumulative organ damage (SDI/SLICC scores), history of CVD and MetS as defined by the National Cholesterol Educational Program Adult Treatment Panel III (NCEP ATP III). Clinical data of SLE patients were collected from the records.

RESULTS

The mean age of the patients was 40.2 ± 13.4 years and 89% were female. The frequencies of CVD and MetS were 15.2% and 19%, respectively. In this SLE cohort increased age, cumulative damage, disease duration and CVD were associated with MetS. CVD was associated with disease duration, cumulative damage, pericarditis, hematologic involvement, lymphopenia, thrombocytopenia, neurological involvement and antiphospholipid antibody (aPL) positivity. Hydroxychloroquine (HCQ) use was found as a protective factor for CVD.

CONCLUSION

In SLE patients, MetS was associated with CVD and both increased with disease duration. Patients who developed MetS and/or CVD had increased cumulative organ damage. Certain clinical features of SLE and the presence of aPL were also associated with CVD. There was a significant protective effect of HCQ from CVD. The prevention of MetS and long-term use of HCQ may be beneficial in improving the prognosis of SLE.

[Involvement of the endosomal compartment in cellular insulin signaling]

The insulin receptor and insulin signaling proteins downstream the receptor reside in different subcellular compartments and undergo redistribution within the cell upon insulin activation. Endocytosis of the insulin-receptor complex, by mediating ligand degradation and receptor dephosphorylation, is generally viewed as a mechanism which attenuates or arrests insulin signal transduction. However, several observations suggest that insulin receptor endocytosis and/or recruitement of insulin signaling proteins to endosomes are also involved in a positive regulation of insulin signaling: (1) upon internalization, the insulin receptor remains transiently phosphorylated and activated; (2) in insulin-stimulated cells or tissues, signaling proteins of the PI3K/Akt and Ras/Raf/Mek/Erk pathways are recruited to endosomes or other intracellular compartments, in which they undergo phosphorylation and/or activation; and (3) depletion or overexpression of proteins involved in the regulation of membrane trafficking and endocytosis interfere with insulin signaling. These observations support a spatial and temporal regulation of insulin signal transduction and reinforce the concept that, as for other membrane signaling receptors, endocytosis and signaling are functionally linked.

Assessment of insulin proteolysis in rat liver endosomes: its relationship to intracellular insulin signaling

Insulin binding to insulin receptor (IR) at the cell surface results in the activation of IR kinase and initiates the translocation of insulin-IR complexes to clathrin-coated pits and to early endosomes containing internalized but still active receptors. In liver parenchyma, several mechanisms are involved in the regulation of endosomal IR tyrosine kinase activity. Two of these regulatory mechanisms are at the level of intraendosomal ligand. First, a progressive decrease in endosomal pH mediated by the vacuolar H(+)-ATPase proton pump promotes dissociation of the insulin-IR complex. Second, free dissociated insulin is degraded by a soluble endosomal acidic insulinase, which has been identified as aspartic acid protease cathepsin D. This enzyme catalyzes the cleavage of insulin at the Phe(B24)-Phe(B25) bond, generating a major clipped molecule, A(1-21)-B(1-24) insulin, that can no longer bind to IR within endosomes. Concomitant with, or shortly after, the tyrosine-phosphorylated IR is deactivated by two independent processes: its rapid dephosphorylation by endosome-associated phosphotyrosine phosphatase(s) and its association with the molecular adaptor Grb14, with resulting inhibition of IR catalytic activity. By mediating the removal and degradation of circulating insulin, as well as the deactivation of the activated IR, internalization of the insulin-receptor complex into endosomes represents a major mechanism involved in the negative regulation of insulin signaling.

Acute pretreatment with chloroquine attenuates renal I/R injury in rats

BACKGROUND

Acute kidney injury (AKI) still remains an unresolved problem in pharmacotherapy and renal inflammation is a major factor in its development. Chloroquine, a well-known antimalarial drug, posses pleitropic effects as well: antiinflammatory, anticoagulant and vascular actions. The effects of chloroquine on renal function may involve significant increase in urine flow rate, glomerular filtration rate and sodium excretion, as well as stimulation of nitric oxide synthase. However, its role in experimental models of renal I/R injury is unknown. We aimed to analyze the acute effects of a single-dose intravenous chloroquine administered at three different times in the experimental model of I/R injury in rat.

METHODS

Rats were subjected to bilateral renal ischemia (45 min) followed by reperfusion with saline lasting 4 hours. Chloroquine was administered in doses of 0.3 mg/kg i.v. and 3 mg/kg i.v. 30 min before ischemia, 30 min before reperfusion and 5 min before reperfusion. Selected a hemodynamic, biochemical and morphological parameters were followed in the Sham-operated animals and rats subjected to I/R injury and pretreated with saline or chloroquine.

RESULTS

Chloroquine (0.3 and 3 mg/kg, i.v.) protected the I/R injured kidney in an U-shaped manner. Both doses were protective regarding biochemical and histological markers of the I/R injury (serum urea, creatinine and fractional excretion of sodium, as well as total histological score, tubular necrosis score and KIM-1 staining score) (P<0.05 vs. corresponding controls, i.e. rats subjected to I/R injury and treated with saline only). The protective effects of the lower dose of chloroquine were more profound. Time-related differences between pretreatments were not observed (P>0.05, all).

CONCLUSION

Our study shows for the first time that a single dose of chloroquine (0.3 mg/kg i.v.) could afford significant protection of the injured rat kidney.

Metabolic syndrome in Chinese patients with systemic lupus erythematosus: no association with plasma cortisol level

Our objective was to determine metabolic syndrome (MS) prevalence in Chinese patients with systemic lupus erythematosus (SLE) and to investigate the conditions that contribute to its development. 116 patients with SLE classified according to the American College of Rheumatology (ACR) classification criteria, and 115 controls were enrolled. MS was defined by the joint interim statement of the International Diabetes Federation Task Force on Epidemiology and Prevention; National Heart, Lung, and Blood Institute; American Heart Association; World Heart Federation; International Atherosclerosis Society; and International Association for the Study of Obesity (IDF/NHLBI/AHA/WHF/IAS/IASO). SLE features and treatment of SLE were assessed. Fasting insulin and cortisol levels of 30 newly diagnosed, untreated patients and 33 age and sex-matched controls were detected. MS prevalence was 34.2% in patients with SLE and 14.8% in controls ( p = 0.002). Lupus patients with MS had less frequency of hydroxychloroquine (HCQ) intake (16.0% vs 45.8%; p = 0.012). Untreated patients with SLE had higher levels of fasting insulin (10.92 ± 13.53 vs 5.48 ± 5.43 uU/mL, p < 0.001) and plasma cortisol at 16:00 (257.22 ± 177.98 vs 139.84 ± 63.46 nmol/L, p = 0.001), but lower plasma cortisol at 08:00 (195.51 ± 149.84 vs 278.95 ± 136.27 nmol/L, p = 0.024). Comparisons regarding steroid therapy, levels of insulin and cortisol were not statistically significant between patients with MS and without MS. The Chinese patients with SLE presented a higher MS prevalence and fasting insulin than controls. MS was not associated with the steroid therapy and plasma cortisol. HCQ use proved to be protective against MS. The circadian rhythm of cortisol may differ in patients with SLE.

Impairment of lysosomal functions by azithromycin and chloroquine contributes to anti-inflammatory phenotype

Azithromycin and chloroquine have been shown to exhibit anti-inflammatory activities in a number of cellular systems, but the mechanisms of these activities have still not been clarified unequivocally. Since both drugs are cationic, accumulate in acidic cellular compartments and bind to phospholipids with a consequent increase in lysosomal pH and induce phospholipidosis, we examined the relevance of these common properties to their anti-inflammatory activities. We compared also these effects with effects of concanamycin A, compound which inhibits acidification of lysosomes. All three compounds increased lysosomal pH, accumulation of autophagic vacuoles and ubiquitinated proteins and impaired recycling of TLR4 receptor with consequences in downstream signaling in LPS-stimulated J774A.1 cells. Azithromycin and chloroquine additionally inhibited arachidonic acid release and prostaglandin E2 synthesis. Therefore, impairment of lysosomal functions by azithromycin and chloroquine deregulate TLR4 recycling and signaling and phospholipases activation and lead to anti-inflammatory phenotype in LPS-stimulated J774A.1 cells.

Systemic lupus erythematosus and cardiovascular disease: prediction and potential for therapeutic intervention

Patients with systemic lupus erythematosus have a significantly increased risk of cardiovascular events due to atherosclerosis. Traditional cardiac risk factors cannot fully explain this increased risk. Recent evidence strongly suggests that atherosclerotic plaque is largely driven by inflammation and an active immunological response, in contrast to the long-held belief that plaque is a passive accumulation of lipids in the arterial wall. Current approaches to the prevention of atherosclerosis in systemic lupus erythematosus involve targeting modifiable cardiac risk factors. Future preventive strategies may include therapies that counteract the immunologic responses that lead to plaque formation.

Design of a peptide-based vector, PepFect6, for efficient delivery of siRNA in cell culture and systemically in vivo

While small interfering RNAs (siRNAs) have been rapidly appreciated to silence genes, efficient and non-toxic vectors for primary cells and for systemic in vivo delivery are lacking. Several siRNA-delivery vehicles, including cell-penetrating peptides (CPPs), have been developed but their utility is often restricted by entrapment following endocytosis. Hence, developing CPPs that promote endosomal escape is a prerequisite for successful siRNA implementation. We here present a novel CPP, PepFect 6 (PF6), comprising the previously reported stearyl-TP10 peptide, having pH titratable trifluoromethylquinoline moieties covalently incorporated to facilitate endosomal release. Stable PF6/siRNA nanoparticles enter entire cell populations and rapidly promote endosomal escape, resulting in robust RNAi responses in various cell types (including primary cells), with minimal associated transcriptomic or proteomic changes. Furthermore, PF6-mediated delivery is independent of cell confluence and, in most cases, not significantly hampered by serum proteins. Finally, these nanoparticles promote strong RNAi responses in different organs following systemic delivery in mice without any associated toxicity. Strikingly, similar knockdown in liver is achieved by PF6/siRNA nanoparticles and siRNA injected by hydrodynamic infusion, a golden standard technique for liver transfection. These results imply that the peptide, in addition to having utility for RNAi screens in vitro, displays therapeutic potential.

Characterization of cellular internalization pathways for CPP-mediated oligonucleotide delivery

The methods for evaluating internalization pathways of cellular CPP-mediated ON delivery utilizing a pre-mRNA splice correction assay and fluorescence-based quantification are described. Examples for characterization of CPP uptake routes, employing various endocytosis inhibitors, and special treatment conditions are demonstrated. The methods are developed to characterize cellular delivery of pre-mRNA splice switching peptide nucleic acids conjugated to CPPs by disulfide bond.

The lysosome among targets of metformin: new anti-inflammatory uses for an old drug?

BACKGROUND

Rheumatoid arthritis and type-2 diabetes exhibit progressive co-morbidity. Chloroquine (CQ) reportedly improves both. CQ inhibits lysosomal function in cultured cells at supra-therapeutic concentration; however, this is doubted as target mechanism. Some anti-diabetic biguanides are metal-interactive lysosomal inhibitors; and all bind Zn(2+).

OBJECTIVES

i) To bioassay the potency of CQ using (3)H-leucine release from perfused myocardial tissue. ii) To determine whether metformin (MET) is CQ-mimetic, and interactive with Zn(2+).

RESULTS

Therapeutic CQ concentration (0.1 - 0.5 microM) clearly does cause lysosomal inhibition although delayed and submaximal. MET alone (10 microM) caused sub-maximal inhibition. Supra-physiological extracellular Zn(2+) (5 - 50 microM) alone increased tissue Zn(2+) content, and inhibited lysosomal proteolysis. Physiological equivalent Zn(2+) (approximately 1 microM) had no effect. MET (<or= 25 microM) and Zn(2+) (<or= 1 microM) exhibited astounding 10 - 100 fold anti-lysosomal synergy. Cathepsin B was 50% inhibited by 1 muM Zn(2+), and is reportedly inhibited by gold agents.

INTERPRETATION

MET somehow increases the natural inhibitory action of action of Zn(2+) against cysteinyl proteases. TNF-alpha activates lysosomal function; and CatB is among post-receptor players. MET might decrease antigen processing in specialized cells, and lysosomal hyper-catabolism in other cells.

CONCLUSIONS

Trials of MET for new use as an anti-inflammatory agent are suggested. Guanidylguanidine is a practical pharmacophore for synthesis of future anti-lysosomal agents.

Elucidating cell-penetrating peptide mechanisms of action for membrane interaction, cellular uptake, and translocation utilizing the hydrophobic counter-anion pyrenebutyrate

Cell-penetrating peptides (CPPs) are membrane permeable vectors recognized for their intrinsic ability to gain access to the cell interior. The hydrophobic counter-anion, pyrenebutyrate, enhances cellular uptake of oligoarginine CPPs. To elucidate CPP uptake mechanisms, the effect of pyrenebutyrate on well-recognized CPPs with varying hydrophobicity and arginine content is investigated. The cellular CPP uptake and CPP-mediated oligonucleotide delivery is analyzed by fluorescence activated cell sorting, confocal microscopy, and a cell-based splice-switching assay. The splice-switching oligonucleotide is a mixmer of 2'-O-methyl RNA and locked nucleic acids delivered as a non-covalent complex with 10-fold molar CPP excess. CPP-induced membrane perturbation on large unilamellar vesicles is investigated in calcein release experiments. We observed that pyrenebutyrate facilitates cellular uptake and translocation of oligonucleotide mediated by oligoarginine nonamer while limited effect of pyrenebutyrate on more hydrophobic CPPs was observed. By combining the different experimental results we conclude that the pathway for cellular uptake of oligoarginine is dominated by direct membrane translocation, whereas the pathway for oligoarginine-mediated oligonucleotide translocation is dominated by endocytosis. Both mechanisms are promoted by pyrenebutyrate and we suggest that pyrenebutyrate has different sites of action for the two uptake and translocation mechanisms.

Rapid insulin-dependent endocytosis of the insulin receptor by caveolae in primary adipocytes

Background

The insulin receptor is localized in caveolae and is dependent on caveolae or cholesterol for signaling in adipocytes. When stimulated with insulin, the receptor is internalized.

Methodology/Principal Findings

We examined primary rat adipocytes by subcellular fractionation to examine if the insulin receptor was internalized in a caveolae-mediated process. Insulin induced a rapid, t_1/2<3 min, endocytosis of the insulin receptor in parallel with receptor tyrosine autophosphorylation. Concomitantly, caveolin-1 was phosphorylated at tyrosine(14) and endocytosed. Vanadate increased the phosphorylation of caveolin-1 without affecting insulin receptor phosphorylation or endocytosis. Immunocapture of endosomal vesicles with antibodies against the insulin receptor co-captured caveolin-1 and immunocapture with antibodies against tyrosine(14)-phosphorylated caveolin-1 co-captured the insulin receptor, demonstrating that the insulin receptor was endocytosed together with tyrosine(14)-phosphorylated caveolin-1. By immunogold electron microscopy the insulin receptor and caveolin-1 were colocalized in endosome vesicles that resembled caveosomes. Clathrin was not endocytosed with the insulin receptor and the inhibitor of clathrin-coated pit-mediated endocytosis, chlorpromazine, did not inhibit internalization of the insulin receptor, while transferrin receptor internalization was inhibited.

Conclusion

It is concluded that in response to insulin stimulation the autophosphorylated insulin receptor in primary adipocytes is rapidly endocytosed in a caveolae-mediated process, involving tyrosine phosphorylation of caveolin-1.

Protective effect of hydroxychloroquine on renal damage in patients with lupus nephritis: LXV, data from a multiethnic US cohort

OBJECTIVE

To assess whether hydroxychloroquine can delay renal damage development in lupus nephritis patients.

METHODS

Lupus nephritis patients (n = 256) from the LUpus in MInorities, NAture versus nurture study (n = 635), a multiethnic cohort of African Americans, Hispanics, and Caucasians, age > or =16 years with disease duration < or =5 years at baseline (T0) were studied. Renal damage was defined using the Systemic Lupus International Collaborating Clinics Damage Index (> or =1 of the following lasting at least 6 months: estimated/measured glomerular filtration rate <50%, 24-hour proteinuria > or =3.5 gm and/or end-stage renal disease, regardless of dialysis or transplantation). Patients with renal damage before T0 were excluded (n = 53). The association between hydroxychloroquine use and renal damage (as defined, or omitting proteinuria) was estimated using Cox proportional regression analyses adjusting for potential confounders. Kaplan-Meier survival curves based on hydroxychloroquine intake or the World Health Organization (WHO) class glomerulonephritis were also derived.

RESULTS

Sixty-three (31.0%) of the 203 patients included developed renal damage over a mean +/- SD disease duration of 5.2 +/- 3.5 years. The most frequent renal damage domain item was proteinuria. Patients who received hydroxychloroquine (79.3%) exhibited a lower frequency of WHO class IV glomerulonephritis, had lower disease activity, and received lower glucocorticoid doses than those who did not take hydroxychloroquine. After adjusting for confounders, hydroxychloroquine was protective of renal damage occurrence in full (hazard ratio [HR] 0.12, 95% confidence interval [95% CI] 0.02-0.97, P = 0.0464) and reduced (HR 0.29, 95% CI 0.13-0.68, P = 0.0043) models. Omitting proteinuria provided comparable results. The cumulative probability of renal damage occurrence was higher in those who did not take hydroxychloroquine and those classified as WHO class IV glomerulonephritis (P < 0.0001).

CONCLUSION

After adjusting for possible confounding factors, the protective effect of hydroxychloroquine in retarding renal damage occurrence in systemic lupus erythematosus is still evident.

Model-based hypothesis testing of key mechanisms in initial phase of insulin signaling

Type 2 diabetes is characterized by insulin resistance of target organs, which is due to impaired insulin signal transduction. The skeleton of signaling mediators that provide for normal insulin action has been established. However, the detailed kinetics, and their mechanistic generation, remain incompletely understood. We measured time-courses in primary human adipocytes for the short-term phosphorylation dynamics of the insulin receptor (IR) and the IR substrate-1 in response to a step increase in insulin concentration. Both proteins exhibited a rapid transient overshoot in tyrosine phosphorylation, reaching maximum within 1 min, followed by an intermediate steady-state level after approximately 10 min. We used model-based hypothesis testing to evaluate three mechanistic explanations for this behavior: (A) phosphorylation and dephosphorylation of IR at the plasma membrane only; (B) the additional possibility for IR endocytosis; (C) the alternative additional possibility of feedback signals to IR from downstream intermediates. We concluded that (A) is not a satisfactory explanation; that (B) may serve as an explanation only if both internalization, dephosphorylation, and subsequent recycling are permitted; and that (C) is acceptable. These mechanistic insights cannot be obtained by mere inspection of the datasets, and they are rejections and thus stronger and more final conclusions than ordinary model predictions.

Insulin is a central player in maintaining energy balance in our bodies and in type 2 diabetes, where the effect of insulin on its target tissues is diminished. Insulin acts on cells by binding to specific insulin receptors (IRs) at the cell surface. This triggers a series of events, including attachment of phosphate to IR, activation of downstream proteins that eventually mediate the signal to specific targets in the cell, and internalization of IR to the inner cytosolic part of the cell. The importance, time relations, and interactions between these events are not fully understood. We have collected experimental time-series and developed a novel analysis method based on mathematical modeling to gain insights into these initial aspects of how insulin controls cells. The main conclusion is that either IR internalization and the subsequent recycling back to the cell surface or feedbacks from downstream proteins (or both) must be significantly active during the first few minutes of insulin action. These conclusions could not have been reached from the experimental data through conventional biological reasoning, and this work thus illustrates the power of modeling to improve our understanding of biological systems.

Endosomal receptor kinetics determine the stability of intracellular growth factor signalling complexes

There is an emerging paradigm that growth factor signalling continues in the endosome and that cell response to a growth factor is defined by the integration of cell surface and endosomal events. As activated receptors in the endosome are exposed to a different set of binding partners, they probably elicit differential signals compared with when they are at the cell surface. As such, complete appreciation of growth factor signalling requires understanding of growth factor-receptor binding and trafficking kinetics both at the cell surface and in endosomes. Growth factor binding to surface receptors is well characterized, and endosomal binding is assumed to follow surface kinetics if one accounts for changes in pH. Yet, specific binding kinetics within the endosome has not been examined in detail. To parse the factors governing the binding state of endosomal receptors we analysed a whole-cell mathematical model of epidermal growth factor receptor trafficking and binding. We discovered that the stability of growth factor-receptor complexes within endosomes is governed by three primary independent factors: the endosomal dissociation constant, total endosomal volume and the number of endosomal receptors. These factors were combined into a single dimensionless parameter that determines the endosomal binding state of the growth factor-receptor complex and can distinguish different growth factors from each other and different cell states. Our findings indicate that growth factor binding within endosomal compartments cannot be appreciated solely on the basis of the pH-dependence of the dissociation constant and that the concentration of receptors in the endosomal compartment must also be considered.

The plecomacrolide vacuolar-ATPase inhibitor bafilomycin, alters insulin signaling in MIN6 beta-cells

Inhibition of endosomal acidification disturbs insulin signaling in both liver and adipose cells. In this study we used MIN6 beta cells to determine whether bafilomycin, a potent inhibitor of the proton-translocating vacuolar ATPase, disrupts insulin signaling in islet beta cells. Pretreatment of MIN6 cells with varying concentrations of bafilomycin according to a time course revealed concentration and time-dependent changes in phosphorylation of insulin receptor signaling components. Increased phosphorylation of insulin receptor (IR), IRS2 and Akt was prolonged at low bafilomycin concentrations (10 and 50 nmol/L), whereas at high concentrations (100 and 200 nmol/L) phosphorylation rapidly returned to basal levels or below. Akt activation was demonstrated by transient increases in phosphorylation of BAD, cytoplasmic retention of FoxO1 and increased preproinsulin mRNA. Bcl2 expression was also transiently increased but reduced after 30 min exposure to bafilomycin, and this coincided with reduced cell viability. Thus, in beta cells inhibition of endosomal acidification by low concentrations of bafilomycin transiently increases insulin signaling, whereas high concentrations promote cell death. Bafilomycin and other agents that interfere with insulin signaling may contribute to diabetes development through disturbing homeostatic control of beta cell growth.

Atherogenesis and autoimmune disease: the model of lupus

Accelerated atherosclerosis is a major cause of morbidity and mortality in patients with systemic lupus erythematosus (SLE). Certain 'classic' risk factors are associated with atherosclerosis risk in SLE. However, these factors alone do not fully explain the excess risk observed. Atherosclerosis is increasingly recognized as a chronic inflammatory condition and in SLE, complement activation and immune complex formation may promote atheroma development. Similarly, autoantibody production, especially those in the anticardiolipin (ACLA) family are gaining increasing attention. The role of steroids may not be completely straightforward, low doses may have a beneficial anti-inflammatory role whereas higher doses may exacerbate metabolic factors. In contrast, antimalarials have a beneficial effect on lipids as well as anti-inflammatory and anti-platelet effects. The aetiology of atherosclerosis in SLE is therefore multifactorial. A better understanding of the interface of autoimmunity and atherogenesis in the context of SLE will benefit lupus patients and will also help us better understand the pathogenesis of atherosclerosis in general.

‘Not only…but also’: factors that contribute to accelerated atherosclerosis and premature coronary heart disease in systemic lupus erythematosus

Premature coronary heart disease (CHD) has emerged as a major cause of morbidity and mortality in patients with systemic lupus erythematosus (SLE). Overall SLE patients have a 5-6-fold increased risk of CHD and this excess risk is especially pronounced in younger women where the excess risk may be >50-fold. Studies from our group and others have also demonstrated that SLE patients have a higher prevalence of subclinical atherosclerosis compared with controls, with approximately 30% having evidence of subclinical involvement. It is important to consider what factors may underlie this excess risk. We have found that certain 'classic' risk factors, i.e. hypertension and diabetes mellitus, are more prevalent in SLE and that persistent hypercholesterolaemia independently predicts patients who will develop CHD. These risk factors alone do not completely explain the excess risk observed, and after adjusting for classic risk factors SLE remains independently associated with both clinical and subclinical outcomes. Certain other metabolic changes also occur more frequently in SLE, namely premature menopause, renal impairment, high triglycerides and higher plasma homocysteine. In addition, insulin resistance is more pronounced in patients with SLE, and approximately 18% have the metabolic syndrome. It is also increasingly accepted that atherosclerosis is a chronic inflammatory condition, and in SLE systemic complement activation as well as immune complex formation can result in changes that promote the development of atheroma. Similarly, autoantibody production, especially antibodies directed against lipoprotein subtypes and those in the antiphospholipid (APLA) family, are gaining increasing attention. The role of the latter are particularly controversial as different subtypes have been shown to both promote and protect against atherogenesis. In a study looking at carotid plaque in SLE, we found that APLA was independently associated with the presence of plaque; this study also found that patients with plaque had higher white cell counts, suggesting ongoing chronic inflammation. We have also noted a negative correlation between activation of transforming growth factor beta-1 and carotid intima-medial thickness. This cytokine, which is known to be a potent anti-inflammatory molecule, has also been shown to be protective against atherogenesis. With regard to therapy, steroids may be a true double-edged sword, with low doses exerting a beneficial anti-inflammatory role whereas higher doses may be detrimental through exacerbation of metabolic risk factors. In contrast, we have found that antimalarials have a beneficial effect on lipids especially when co-prescribed with steroids, and this, along with anti-inflammatory and proposed antiplatelet effects, may confer protection against CHD in lupus. The risk of premature CHD in SLE is therefore mediated by a number of factors that involve not only classic risk factors but also a range of factors associated with SLE itself. Preventative strategies will therefore need to address all potential risk factors of relevance. A more through understanding of the interplay between autoimmunity and atherogenesis should be possible by the study of SLE, and this may not only benefit lupus patients but also may have implications for our understanding of atherosclerosis in general.

Cardiovascular disease in lupus patients: Should all patients be treated with statins and aspirin?

Premature coronary heart disease (CHD) is a major cause of morbidity and mortality in patients with systemic lupus erythematosus (SLE). In certain age groups, the risk can be >50 times that of an age-matched population. This population also has an increased prevalence of several key classic risk factors that contribute to the CHD development. Chronic inflammation, anti-phospholipid antibodies and exposure to steroid therapy are also likely to have an impact. We have adopted a proactive approach to classic risk factor management with 'ideal targets' based on viewing SLE as a CHD equivalent condition. In this context, a significant proportion of SLE patients (approximately 30%) will require statins and the majority would be treated with aspirin prophylaxis. Better control of the underlying inflammatory disease is also likely to play an important role and the relative safety of anti-malarials allows their consideration as an adjunct in a large proportion of patients. Well-conducted clinical trials are now needed to advance beyond these initial recommendations.

Systemic lupus erythematosus in three ethnic groups: XVI. Association of hydroxychloroquine use with reduced risk of damage accrual

OBJECTIVE

To examine whether hydroxychloroquine (HCQ) usage is associated with a reduced risk of damage accrual in patients with systemic lupus erythematosus (SLE).

METHODS

Patients (n = 518) meeting the American College of Rheumatology criteria for diagnosis of SLE and with </=5 years disease duration at study entry were followed up annually. Socioeconomic, demographic, clinical, and serologic manifestations as well as disease activity (by the Systemic Lupus Activity Measure [SLAM]) and damage (by the Systemic Lupus International Collaborating Clinics damage index [SDI]) were measured. Propensity scores were calculated to adjust for confounding factors affecting treatment assignment. A Cox proportional hazards model was used to compare the risk of developing new damage according to HCQ use at enrollment into the study.

RESULTS

Fifty-six percent of the patients were treated with HCQ at the time of study enrollment. Patients who were not treated with HCQ on enrollment had higher SLAM and SDI scores than patients who were treated. Untreated patients were significantly more likely to have major organ involvement such as renal disease (P < 0.0001) or central nervous system disease (P < 0.0025). Results of unadjusted analysis suggested that treated patients were less likely to accrue damage (hazard ratio [HR] 0.68). With adjustment for differences in treatment assignment, HCQ usage was still associated with a reduced risk of developing new damage, with an HR of 0.68 (95% confidence interval [95% CI] 0.53-0.93) (P = 0.014). With adjustment for differences in treatment assignment, HCQ usage was still associated with a reduced risk of developing new damage (HR 0.73 [95% CI 0.52-1.00]) (P = 0.05). However, patients receiving HCQ who had no damage at study entry had a statistically significant decrease in the risk of damage accrual (HR 0.55 [95% CI 0.34-0.87]) (P = 0.0111), whereas those receiving HCQ who had damage at study entry did not (HR 1.106 [95% CI 0.70-1.74]) (P = 0.6630).

CONCLUSION

These findings indicate that, after adjustment for propensity to receive HCQ, HCQ usage is independently associated with a reduced risk of damage accrual in SLE patients who had not yet accrued damage at the time of treatment initiation.

Effect of inhibiting vacuolar acidification on insulin signaling in hepatocytes

Previous studies have shown that the endosomal apparatus plays an important role in insulin signaling. Inhibition of endosomal acidification leads to a decrease in insulin-insulin receptor kinase (IRK) dissociation and insulin degradation. Thus, vacuolar pH could function as a modulator of insulin signaling in endosomes. In the present study we show that in primary hepatocytes pretreated with bafilomycin, there is an inhibition of vacuolar acidification. Incubation of these cells with insulin was followed by an augmentation of IRK activity but an inhibition of phosphatidylinositol 3-kinase/Akt activity and a decrease in insulin-induced DNA and glycogen synthesis. Bafilomycin treatment inhibited IRK recycling to the plasma membrane without affecting IRK internalization. Impaired IRK recycling correlated with a decrease in insulin signaling. We suggest that inhibiting vacuolar acidification sequesters activated IRKs in an intracellular compartment(s) where signaling is inhibited. This implies that endosomal receptor trafficking plays a role in regulating signal transduction.

Increase in epidermal growth factor receptor protein induced in osteoblastic cells after exposure to flow of culture media

To investigate how bone cells respond to mechanical stimuli, we subjected osteoblastic cells to fluid flow. We and others already reported that in a culture system of osteoblast-like cells, ERK1/2, Shc, and other proteins were tyrosine-phosphorylated by medium flow and the early response gene, egr-1 or c-fos mRNA, increased. These are the same as events found after stimulation by various growth factors. Moreover, because there were also reports suggesting that growth factor signaling is involved in the responses to mechanical stimuli, we examined the change in epidermal growth factor (EGF) receptor in the cells exposed to medium flow. The results demonstrated that EGF receptor protein increased after exposure to medium flow. This increase did not occur without serum in media, and the addition of EGF restored it. Furthermore, leupeptin blocked this increase. These results suggest that degradation of EGF-occupied EGF receptor by leupeptin-sensitive protease(s) in endosomes decreased with exposure to medium flow. This was presumed to participate, at least in part, in signaling of fluid flow.

Impaired renal sensory responses after renal ischemia in the rat

Renal sensory responses and reflex function were examined in rats 24 h after 45 min of ischemic injury caused by unilateral renal arterial occlusion (RAO). The integrity of renal pelvic mechanoreceptor (MRu)-mediated renorenal reflex was examined. An increase in ipsilateral afferent renal nerve activity (ARNA) and a reflex decrease in efferent renal nerve activity (ERNA) and contralateral diuresis and natriuresis produced by increasing the intrapelvic pressure were seen in sham-operated (Sham) rats, but it was largely attenuated in RAO rats. Using single-fiber recordings of the renal MRu discharge, graded increases in intrapelvic pressure or renal pelvic administration of substance P (SP) resulted in pressure- or concentration-dependent increases in ARNA in the control kidney of Sham rats, whereas attenuated responses were seen in the postischemic kidney of RAO rats. The unresponsiveness of renal MRus in RAO rats was accompanied by an insufficient release of SP. However, the baseline SP release is higher in RAO kidneys due to a reduced neutral endopeptidase (NEP) activity in the renal pelvis of the postischemic kidney. No changes in NK-1 receptor mRNA levels were demonstrated; however, the expression of NK-1 receptors in the plasma membrane of RAO pelvis were decreased, possibly resulting from the internalization of the receptors associated with beta-arrestin trafficking. Renal excretory responses after saline loading were significantly lower in the postischemic kidney of RAO rats than in Sham rats. Responses of ARNA and ERNA were also lower. It is concluded that the defective activation of renal sensory mechanoreceptors in the postischemic kidney results from an inadequate release of SP after mechanostimulation and the reduced functional NK-1 receptors.

The inhibition of gluconeogenesis by chloroquine contributes to its hypoglycaemic action

The effect of chloroquine on gluconeogenesis in isolated hepatocytes and kidney-cortex tubules of rabbit has been studied. The inhibitory action of 200 microM chloroquine was the highest in hepatocytes and renal tubules incubated with glutamine and glutamate+glycerol+octanoate, respectively, while in the presence of other substrates the drug action was less pronounced. With amino acids as substrates, the inhibition of gluconeogenesis was accompanied by a decreased glutamine production, resulting from a decline of glutamate dehydrogenase activity. A decrease in the urea production by hepatocytes incubated with chloroquine in the presence of glutamine but not NH4Cl as the source of ammonium is in agreement with this suggestion. The degree of inhibition by chloroquine of the rate of gluconeogenesis in renal tubules isolated from control rabbits was similar to that determined in diabetic animals. Chloroquine-induced changes in levels of intracellular gluconeogenic intermediates indicate a decrease in phosphoenolpyruvate carboxykinase and glucose-6-phosphatase activities probably due to increased concentration of 2-oxoglutarate, an inhibitor of these two enzymes. In view of the data, it is likely that inhibition by chloroquine of glucose formation in liver and kidney may contribute to the hypoglycaemic action of this drug. The importance of the inhibitory effect of chloroquine on glutamate dehydrogenase activity in the antihyperglycaemic action of the drug is discussed.

Transforming Growth Factor (TGF)-β1 Internalization

Transforming growth factor-beta (TGF-beta) internalization was studied by monitoring the uptake of (125)I-TGF-beta1 in Mv1Lu cells, which endogenously express TGF-beta receptors types I (RI), II (RII), and III (RIII), and 293 cells transfected with RI and RII. At 37 degrees C internalization occurred rapidly, within 10 min of ligand addition. Internalization was optimal in 293 cells expressing both RI and RII. Internalization was prevented by phenylarsine oxide, a nonspecific inhibitor of receptor internalization, but was not affected by reagents that interfere with clathrin-mediated endocytosis such as monodansylcadaverine, K44A dynamin, and inhibitors of endosomal acidification. Electron microscopic examination of Mv1Lu cells treated with (125)I- TGF-beta1 at 37 degrees C indicated that internalization occurred via a noncoated vesicular mechanism. Internalization was prevented by prebinding cells with TGF-beta1 at 4 degrees C for 2 h prior to switching the cells to 37 degrees C. This was attributed to a loss of receptor binding, as indicated by a rapid decrease in the amount of TGF-beta1 bound to the cell surface at 37 degrees C and by a reduction in the labeling intensities of RI and RII in (125)I-TGF-beta1-cross-linking experiments. Mv1Lu or 293 (RI+RII) cells, prebound with TGF-beta1 at 4 degrees C and subsequently stripped of ligand by an acid wash, nevertheless initiated a signaling response upon transfer to 37 degrees C, suggesting that prebinding promotes formation of stable RI.RII complexes that can signal independently of ligand.

Compartmentalization and insulin-induced translocations of insulin receptor substrates, phosphatidylinositol 3-kinase, and protein kinase B in rat liver

Physiological doses of insulin in rats resulted in a rapid redistribution of key signaling proteins between subcellular compartments in rat liver. In plasma membranes (PM) and microsomes, insulin induced a rapid decrease in insulin receptor substrate-1/2 (IRS1/2) within 30 sec and an increase in these proteins in endosomes (EN) and cytosol. The level of p85 in PM increased 2.3-fold at 30 sec after insulin stimulation followed by a decrease at 2 min. In this interval, 60-85% and 10-20% of p85 in PM was associated with IRS1 and IRS2, respectively. Thus, in PM, IRS1/2 accounts for almost all of the protein involved in phosphatidylinositol 3-kinase activation. In ENs insulin induced a maximal increase of 40% in p85 recruitment. As in PM, almost all p85 was associated with IRS1/2. The greater level of p85 recruitment to PM was associated with a higher level of insulin-induced recruitment of Akt1 to this compartment (4.0-fold in PM vs. 2.4-fold in EN). There was a close correlation between Akt1 activity and Akt1 phosphorylation at Thr308 and Ser473 in PM and cytosol. However, in ENs the level of Akt1 activity per unit of phosphorylated Akt1 was significantly greater than in PM, indicating that in addition to phosphorylation, another factor(s) modulates Akt1 activation by insulin in rat liver. Our results demonstrate that activation of the insulin receptor kinase and modulation of key components of the insulin signaling cascade occur at the cell surface and within the endosomal system. These data provide further support for the role of the endocytic process in cell signaling.

Growth factor receptor signalling: location, location, location

Ligand binding to plasma membrane receptors initiates a series of events culminating in a variety of changes in cellular phenotypes. Although numerous publications have documented the activation/inactivation of signalling molecules following receptor binding, relatively few investigations have focused on the cellular compartment responsible for either initiating or selecting the particular pathway that mediates the response. Specifically, does receptor signalling occur only at the plasma membrane; is signalling dependent upon the location of defined endosome populations; or are components of both plasma membrane and endosomal activity operative depending upon the particular signalling pathway or cell type? This review addresses aspects of these questions by discussing the evidence supporting or contrasting the interplay between the endocytic and signalling systems for a subset of tyrosine kinase, serine/threonine kinase and G-protein-coupled receptors.

The role of insulin dissociation from its endosomal receptor in insulin degradation

Mechanisms that terminate signals from activated receptors have potential to influence the magnitude and nature of cellular responses to insulin. The aims of this study were to determine in rat liver endosomes (the subcellular site of insulin signal termination) whether dissociation of insulin from its receptor was a pre-requisite for ligand degradation and whether the state of receptor phosphorylation influenced the dissociation and hence endosomal degradation of insulin and/or receptor recycling. Following in vivo administration of 125I-[A14]-insulin or analogues (native, X10 or H2, relative binding affinities 1:7:67) livers were removed and endosomes prepared. In the endosomal preparations a significantly greater percentage of both analogues were receptor-bound than native insulin with concomitantly less ligand degradation. When rats were injected with protein-tyrosine phosphatase inhibitors (peroxovanadium compounds bpV(phen) or bpV(pic)) before insulin, endosomal insulin receptor phosphotyrosine content, assessed by Western blotting, was increased as was receptor-bound 125I-[A14]-insulin, whilst insulin degradation was decreased. Peroxovanadiums also completely inhibited recycling of insulin receptors from endosomes. However, treatment of freshly isolated endosomes with acid phosphatase which completely dephosphorylated the insulin receptor, did not return the rate of insulin dissociation and degradation to control values, suggesting that peroxovanadium compounds elicit their effect on binding and degradation via a mechanism other than as protein-tyrosine phosphatase inhibitors. We conclude that promotion of sustained receptor binding decreases endosomal insulin degradation and extends the half-life of the activated endosomal receptor, which in turn would be expected to potentiate insulin signalling from this intracellular compartment.

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.

Intracellular hyperinsulinism: a metabolic characteristic of obesity with and without Type 2 diabetes: intracellular insulin in obesity and Type 2 diabetes

There is evidence that intracellular insulin may carry out some insulin mediated actions, including glucose transport. As intracellular insulin has never been quantitatively assessed in human cells, we evaluated its concentrations in monocytes from normal subjects (n = 7) and obese patients without (n = 9) and with Type 2 diabetes mellitus (n = 10). After the incubation of cells with labeled insulin for 60 min at 37 degrees C, intracellular intact insulin concentrations were measured by HPLC and expressed as pmol x 10(-6). Insulin concentrations were higher (ANOVA P < 0.01) within cells from obese (115.4 +/- 26.4 pmol x 10(-6)/2 x 10(5) cells) and obese diabetic patients (93.2 +/- 36.3 pmol x 10(-6)/2 x 10(5) cells) compared with normal cells (28.5 +/- 13.1 pmol x 10(-6)/2 x 10(5) cells). Moreover, after insulin was removed from the incubation medium the decrease of intracellular insulin was significantly lower (P < 0.01) in cells from both obese and obese diabetic patients than in normal subjects. Intracellular undissociated insulin-insulin receptor complexes on average, increased 2-fold (P < 0.01) in cells from insulin resistant patients compared with normal cells. Finally, in downregulated cells from obese and obese diabetic patients, the recycling of the internalized insulin receptor was completely disrupted. In conclusion, monocytes from obese patients with and without Type 2 diabetes mellitus, present increased intracellular insulin concentrations and these conditions are associated with a significant impairment of insulin receptor processing. Increased intracellular insulin concentration in cells from these patients may be necessary in order to overcome insulin resistance.

Visualization of time-dependent redistribution of delta-opioid receptors in neuronal cells during prolonged agonist exposure

To date, the visualization of delta-opioid receptor (DOR) internalization has been largely focused on the events of short-term agonist treatment in transfected non-neuronal cells. In this study, we followed DOR trafficking upon prolonged agonist exposure in the neuronally derived neuro2a cells, stably transfected with the fusion DOR (HA-DOR) cDNA. Internalization of surface DOR was clearly visualized in 5 min of exposure to agonist (100 nM DADLE), and the cell surface DOR remained low throughout the entire 24 h agonist exposure. Significant intracellular accumulation was visible at 20 min exposure, and increased to a maximum at 4 h, after which intracellular DOR staining gradually diminished. DOR intracellular staining was enhanced in the presence of agonist and chloroquine, a lysosomotropic agent, suggesting that internalized receptors were targeted to lysosomes and degraded upon prolonged treatment. Time-dependent colocalization of DOR with transferrin and LAMP-2 following short-term and prolonged agonist exposure further confirmed that receptor was distributed to early endosomes (sequestration) and subjected to lysosomes for degradation (down-regulation), respectively.

Expression of a dominant interfering dynamin mutant in 3T3L1 adipocytes inhibits GLUT4 endocytosis without affecting insulin signaling

To examine the role of clathrin-coated vesicle endocytosis in insulin receptor signaling and GLUT4 trafficking, we used recombinant adenovirus to express a dominant interfering mutant of dynamin (K44A/dynamin) in 3T3L1 adipocytes. Functional expression of K44A/dynamin, as measured by inhibition of transferrin receptor internalization, did not affect insulin-stimulated insulin receptor autophosphorylation, Shc tyrosine phosphorylation, or mitogen-activated protein kinase activation. Although the tyrosine phosphorylation of insulin receptor substrate-1 was slightly reduced, correlating with a 25% decrease in insulin receptor substrate-1-associated phosphatidylinositol 3-kinase activity, insulin-stimulated Akt kinase activation was unaffected. In contrast, expression of K44A/dynamin resulted in the cell-surface accumulation of GLUT4 under basal conditions and an inhibition of GLUT4 endocytosis without affecting insulin-stimulated GLUT4 exocytosis. These data demonstrate that disruption of clathrin-mediated endocytosis does not significantly perturb insulin receptor signal transduction pathways. Furthermore, K44A/dynamin expression causes an accumulation of GLUT4 at the cell surface, suggesting that GLUT4 vesicles exist in at least two distinct intracellular compartments, one that undergoes continuous recycling and a second that is responsive to insulin.

ATP-dependent desensitization of insulin binding and tyrosine kinase activity of the insulin receptor kinase. The role of endosomal acidification

Incubating endosomes with ATP decreased binding of 125I-insulin but not 125I-labeled human growth hormone. Increasing ATP concentrations from 0.1 to 1 mM increased beta-subunit tyrosine phosphorylation and insulin receptor kinase (IRK) activity assayed after partial purification. At higher (5 mM) ATP concentrations beta-subunit tyrosine phosphorylation and IRK activity were markedly decreased. This was not observed with nonhydrolyzable analogs of ATP, nor with plasma membrane IRK, nor with endosomal epidermal growth factor receptor kinase autophosphorylation. The inhibition of endosomal IRK tyrosine phosphorylation and activity was completely reversed by bafilomycin A1, indicating a role for endosomal proton pump(s). The inhibition of IRK was not due to serine/threonine phosphorylation nor was it influenced by the inhibition of phosphotyrosyl phosphatase using bisperoxo(1,10-phenanthroline)oxovanadate anion. Prior phosphorylation of the beta-subunit with 1 mM ATP did not prevent the inhibition of IRK activity on incubating with 5 mM ATP. To evaluate conformational change we incubated endosomes with dithiothreitol (DTT) followed by SDS-polyacrylamide gel electrophoresis under nonreducing conditions. Without DTT the predominant species of IRK observed was alpha2 beta2. With DTT the alpha beta dimer predominated but on co-incubation with 5 mM ATP the alpha2 beta2 form predominated. Thus, ATP-dependent endosomal acidification contributes to the termination of transmembrane signaling by, among other processes, effecting a deactivating conformational change of the IRK.

Insulin signaling in the yeast Saccharomyces cerevisiae. 2. Interaction of human insulin with a putative binding protein

A putative insulin-binding protein (Kd = 0.5 +/- 0.2 microM for human insulin) was partially purified from solubilized plasma membranes of Saccharomyces cerevisiaeby wheat germ agglutinin and insulin affinity chromatographies. The binding affinities of various mutant insulin analogues correlated well with their capacities to activate glycogen synthase and SNF1 kinase in glucose-induced yeast spheroplasts, the ranking of their relative efficacies in yeast and in isolated rat adipocytes being similar. Using a bifunctional cross-linker and two different experimental protocols, a 53-kDa polypeptide contained in the insulin-binding protein preparation was specifically affinity cross-linked to [125I]monoiodo[B26]insulin. The relative rankings of the insulin analogues with respect to inhibition of cross-linking and binding to the partially purified insulin-binding protein were identical. Incubation of intact yeast spheroplasts with [125I]monoiodo[AI4]insulin led to specific and time-dependent association of the radiolabeled insulin with the cell surface followed by its internalization and degradation. These processes were considerably delayed by low temperature and energy depletion of the spheroplasts, suggesting involvement of the ATP-dependent endosomal apparatus. These data provide evidence for the existence of a low-affinity insulin-binding protein in the plasma membrane of Saccharomyces cerevisiae.