Antithrombin and fibrinogen levels as predictors for plasma L-asparaginase activity in children with acute lymphoblastic leukemia.

BACKGROUND: L-asparaginase is a cornerstone treatment for children with acute lymphoblastic leukemia (ALL). However, immune reaction to the drug may increase the clearance or impair the function of L-asparaginase and reduces its therapeutic efficacy. The objective of this study was to identify potential plasma proteins that could be used as proxies for L-asparaginase activity. METHODS: Fibrinogen, von Willebrand factor antigen (VWF:Ag), total protein, and albumin levels as well as antithrombin (AT) and L-asparaginase activities were measured in 97 children with ALL treated for prolonged period of time with L-asparaginase. Binary logistic regression and a receiver operating characteristic (ROC) curve analysis were performed to evaluate the predictive value of plasma proteins for L-asparaginase activity. RESULTS: Median E. coli L-asparaginase activity was 220 IU/L (range, 0-1308) throughout the treatment period. L-asparaginase activity was below 100 IU/L in 23% of measured samples. L-asparaginase activity was inversely associated with AT activity, fibrinogen, total protein, and albumin levels (r = -0.63, -0.62, -0.57, and -0.45, respectively; P < 0.0001), but not with VWF:Ag. ROC curve analyses showed an intermediate accuracy of AT activity (area under the ROC curve [AUC] = 0.77) to detect specimens with subtherapeutic level of L-asparaginase. An optimal accuracy was found when AT and fibrinogen were combined (AUC = 0.82; sensitivity = 75%; specificity = 82%; positive predictive value = 55%; negative predictive value = 92%) with cutoff values of 0.73 IU/mL and 1.85 g/L, respectively. CONCLUSIONS: AT combined with fibrinogen levels could be used as a proxy to identify patients with therapeutic level of L-asparaginase activity in the absence of real-time asparaginase measurement during prolonged exposure to L-asparaginase.

L-Asparaginase lowers plasma antithrombin and mannan-binding-lectin levels: Impact on thrombotic and infectious events in children with acute lymphoblastic leukemia.

BACKGROUND: L-asparaginase, a key therapeutic agent in the management of patients with acute lymphoblastic leukemia (ALL), dramatically impairs hepatic protein synthesis. We investigated the effects of prolonged exposure to L-asparaginase on antithrombin (AT), fibrinogen and mannan-binding-lectin (MBL) levels, and on the occurrence of thrombotic events (TE) and febrile neutropenia episodes (FN) in pediatric patients. PROCEDURE: Protein levels were measured in 97 children during 30 weeks of chemotherapy with L-asparaginase and up to 1 year following remission. TE and FN episodes were recorded during this period. RESULTS: Median AT level decreased from 0.96 IU/mL prior to treatment (range: 0.69-1.38) to 0.55 IU/mL (0.37-0.76) during therapy. Fibrinogen and MBL decreased from 3.18 g/L (1.29-7.28) and 1,177 ng/mL (57-5,343) to 1.56 g/L (0.84-2.13) and 193 ng/mL (57-544), respectively. All three proteins had recovered 1-4 weeks after L-asparaginase cessation. TE were reported in 22 (23%) patients. Of these, 11 occurred after a median of 10 administrations of L-asparaginase. Fifty-one FN were associated with infections, of which 36 occurred during treatment with L-asparaginase. Patients with low levels of MBL at diagnosis were at higher risk of FN associated with infections (RR = 1.59, 95%CI: 1.026-2.474). Both AT and MBL decreases were moderately correlated with fibrinogen (r = 0.51 and 0.58, respectively). CONCLUSIONS: Children with ALL are exposed to significant decrease in AT, fibrinogen and MBL levels, and concomitant increased risk of thrombosis and FN with infection during L-asparaginase treatment. Measuring plasma levels of these liver-derived proteins could help predict the occurrence of adverse events.

Understanding the impact of nuclear-localized GPCRs on cellular signalling.

G protein-coupled receptors (GPCRs) have historically been associated with signalling events driven from the plasma membrane. More recently, signalling from endosomes has been recognized as a feature of internalizing receptors. However, there was little consideration given to the notion that GPCRs can be targeted to distinct subcellular locations that did not involve an initial trafficking to the cell surface. Here, we focus on the evidence for and the potential impact of GPCR signalling specifically initiated from the nuclear membrane. We also discuss the possibilities for selectively targeting this and other internal pools of receptors as novel venues for drug discovery.

A Prospective Study on Complement Activation Distinguishes Focal Segmental Glomerulosclerosis from Minimal Change Disease.

Introduction: Minimal change disease (MCD) and focal segmental glomerulosclerosis (FSGS) are related podocytopathies with distinct kidney outcomes. Surprisingly, elevated urinary activation fragments have been found in FSGS despite little complement deposition on immunofluorescence (IF) staining. Whether complement activation distinguishes FSGS from MCD, participating in the development of segmental lesions, remains unknown. Methods: We performed an observational study in patients with MCD and FSGS, and proteinuria ≥1 g/g of creatinine. We included both primary and secondary or unknown causes. We compared urinary fragments of terminal pathway activation, sC5b9, and C5a expressed as creatinine ratios, between MCD and FSGS. Results: Patients with FSGS (n = 41) had a serum albumin of 31±10 g/l and proteinuria of 5.1 (2.6-9.1) g/g at sampling, whereas those with MCD (n = 15) had a lower serum albumin (22 ± 9 g/l; P = 0.002), and a proteinuria of 3.8 (1.9-7.7) g/g (P = 0.40). Urinary sC5b9 and C5a were 8.7 (1.7-52.3) and 1.26 (0.45-1.84) µg/mmol of creatinine, respectively in patients with FSGS; compared to 0.8 (0.0-1.5) and 0.06 (0.01-0.15) µg/mmol of creatinine in MCD (P < 0.001), respectively. We found no association between urinary complement fragments and age, estimated glomerular filtration rate (eGFR), or chronic kidney lesions. When analyzing samples with proteinuria ≥ 3 g/g, the c-statistics for urinary sC5b9 and C5a were 0.96 and 1.00, respectively, in differentiating FSGS from MCD. Conclusion: We found no urinary complement activation fragments in MCD, in comparison to FSGS, despite similar levels of proteinuria. This suggests a role for complement activation in the pathogenesis of FSGS and provides an additional tool for distinguishing these 2 entities.

Regulation of cardiac nitric oxide signaling by nuclear ß-adrenergic and endothelin receptors.

At the cell surface, ßARs and endothelin receptors can regulate nitric oxide (NO) production. ß-adrenergic receptors (ßARs) and type B endothelin receptors (ETB) are present in cardiac nuclear membranes and regulate transcription. The present study investigated the role of the NO pathway in the regulation of gene transcription by these nuclear G protein-coupled receptors. Nitric oxide production and transcription initiation were measured in nuclei isolated from the adult rat heart. The cell-permeable fluorescent dye 4,5-diaminofluorescein diacetate (DAF2 DA) was used to provide a direct assessment of nitric oxide release. Both isoproterenol and endothelin increased NO production in isolated nuclei. Furthermore, a ß3AR-selective agonist, BRL 37344, increased NO synthesis whereas the ß1AR-selective agonist xamoterol did not. Isoproterenol increased, whereas ET-1 reduced, de novo transcription. The NO synthase inhibitor l-NAME prevented isoproterenol from increasing either NO production or de novo transcription. l-NAME also blocked ET-1-induced NO-production but did not alter the suppression of transcription initiation by ET-1. Inhibition of the cGMP-dependent protein kinase (PKG) using KT5823 also blocked the ability of isoproterenol to increase transcription initiation. Furthermore, immunoblotting revealed eNOS, but not nNOS, in isolated nuclei. Finally, caged, cell-permeable isoproterenol and endothelin-1 analogs were used to selectively activate intracellular ß-adrenergic and endothelin receptors in intact adult cardiomyocytes. Intracellular release of caged ET-1 or isoproterenol analogs increased NO production in intact adult cardiomyocytes. Hence, activation of the NO synthase/guanylyl cyclase/PKG pathway is necessary for nuclear ß3ARs to increase de novo transcription. Furthermore, we have demonstrated the potential utility of caged receptor ligands in selectively modulating signaling via endogenous intracellular G protein-coupled receptors.

Metabolic effects of glutamine on the heart: anaplerosis versus the hexosamine biosynthetic pathway.

Glutamine, the most abundant amino acid in plasma, has attracted considerable interest for its cardioprotective properties. The primary effect of glutamine in the heart is commonly believed to be mediated via its anaplerotic metabolism to citric acid cycle (CAC) intermediates; however, there is little direct evidence to support this concept. Another potential candidate is the hexosamine biosynthetic pathway (HBP), which has recently been shown to modulate cardiomyocyte function and metabolism. Therefore, the goal of this study was to evaluate the contribution of anaplerosis and the HBP to the acute metabolic effects of glutamine in the heart. Normoxic ex vivo working rat hearts were perfused with (13)C-labeled substrates to assess relevant metabolic fluxes either with a physiological mixture of carbohydrates and a fatty acid (control) or under conditions of restricted pyruvate anaplerosis. Addition of a physiological concentration of glutamine (0.5mM) had no effect on contractile function of hearts perfused under the control condition, but improved that of hearts perfused under restricted pyruvate anaplerosis. Changes in CAC intermediate concentrations as well as (13)C-enrichment from [U-(13)C]glutamine did not support a major role of glutamine anaplerosis under any conditions. Under the control condition, however, glutamine significantly increased the contribution of exogenous oleate to ß-oxidation, 1.6-fold, and triglyceride formation, 2.8-fold. Glutamine had no effect on malonyl-CoA or AMP kinase activity levels; however, it resulted in a higher plasma membrane level of the fatty acid transporter CD36. These metabolic effects of glutamine were reversed by azaserine, which inhibits glucose entry into the HPB. Our results reveal a metabolic role of physiological concentration of glutamine in the healthy working heart beyond anaplerosis. This role appears to involve the HBP and regulation of fatty acid entry and metabolism via CD36. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism".

Intracrine endothelin signaling evokes IP3-dependent increases in nucleoplasmic Ca²âº in adult cardiac myocytes.

Endothelin receptors are present on the nuclear membranes in adult cardiac ventricular myocytes. The objectives of the present study were to determine 1) which endothelin receptor subtype is in cardiac nuclear membranes, 2) if the receptor and ligand traffic from the cell surface to the nucleus, and 3) the effect of increased intracellular ET-1 on nuclear Ca(2+) signaling. Confocal microscopy using fluorescently-labeled endothelin analogs confirmed the presence of ETB at the nuclear membrane of rat cardiomyocytes in skinned-cells and isolated nuclei. Furthermore, in both cardiac myocytes and aortic endothelial cells, endocytosed ET:ETB complexes translocated to lysosomes and not the nuclear envelope. Although ETA and ETB can form heterodimers, the presence or absence of ETA did not alter ETB trafficking. Treatment of isolated nuclei with peptide: N-glycosidase F did not alter the electrophoretic mobility of ETB. The absence of N-glycosylation further indicates that these receptors did not originate at the cell surface. Intracellular photolysis of a caged ET-1 analog ([Trp-ODMNB(21)]ET-1) evoked an increase in nucleoplasmic Ca(2+) ([Ca(2+)]n) that was attenuated by inositol 1,4,5-trisphosphate receptor inhibitor 2-aminoethoxydiphenyl borate and prevented by pre-treatment with ryanodine. A caged cell-permeable analog of the ETB-selective antagonist IRL-2500 blocked the ability of intracellular cET-1 to increase [Ca(2+)]n whereas extracellular application of ETA and ETB receptor antagonists did not. These data suggest that 1) the endothelin receptor in the cardiac nuclear membranes is ETB, 2) ETB traffics directly to the nuclear membrane after biosynthesis, 3) exogenous endothelins are not ligands for ETB on nuclear membranes, and 4) ETB associated with the nuclear membranes regulates nuclear Ca(2+) signaling.

Comparison of Complement Pathway Activation in Autoimmune Glomerulonephritis.

Introduction: Studies on complement activation have implicated a combination of the classical pathway (CP), lectin pathway (LP), and alternative pathway (AP) in triggering the terminal pathway (TP) for each common autoimmune glomerulonephritis (GN). Evaluating different pathways simultaneously may help identify whether one is preferentially activated and, consequently, which is best to target for each disease. Methods: We followed 112 patients with focal segmental glomerular sclerosis (FSGS), membranous nephropathy (MN), IgA nephropathy (IgAN), lupus nephritis (LN), and antineutrophil cytoplasmic autoantibody-associated vasculitis (AAV) for a median duration of 22 (12-52) months. At the time of greatest clinical activity, we simultaneously evaluated urinary C3a (C3 convertase activity), C5a and sC5b-9 (TP), MASP-1 and MASP-2 (LP), C1q (CP), C4a (CP/LP), and Ba and Bb (AP). We evaluated the relation between activation fragments of the AP and CP/LP with the TP. Results: Urinary complement biomarkers for each pathway were associated with the severity of proteinuria. Fragments of the TP were higher among patients with FSGS and MN compared with patients with IgAN, LN, and AAV. For the AP, urinary Ba level was lower in those with IgAN and LN compared with those with FSGS. For the CP/LP, urinary C4a, MASP-1, and MASP-2 levels were similar between diseases whereas urinary C1q levels were lower in those with LN. For each GN, independent associations existed between the activation markers of the AP and CP/LP with the degree of TP activation, except for the AP in AAV, although perhaps underpowered. Conclusion: The AP and CP/LP contribute individually to the TP activation in autoimmune GN, and both seem to be valid potential therapeutic targets.

Incidence of thrombotic microangiopathies in Quebec: insight from a laboratory centralizing ADAMTS-13 testing.

BACKGROUND: Thrombotic microangiopathies (TMA) are serious medical conditions requiring a prompt diagnosis to adapt treatment. The determination of ADAMTS-13 activity enables discriminating thrombotic thrombocytopenic purpura (TTP) from other forms of TMA. The purpose of this study was to provide an estimate of the incidence of TTP and TMA in the Canadian Quebec province using data collected from a laboratory centralizing ADAMTS-13 testing for the whole province. RESULTS: From 2012 to 2019, 846 patients were evaluated for plasma ADAMTS-13 activity due to a suspicion of TMA. TTP was identified in 147 patients. Of these, 118 patients with a median age of 51.5 years and a male-female ratio of 1:1.4 had their first episode of TTP during the study period. The number of ADAMTS-13 tests performed and the number of patients with suspected TMA increased annually by 19% and 21% respectively. While the incidence of non-TTP TMA increased annually, that for TTP remained unchanged. This averaged 10.2 (95% CI 5.9-14.4) per million persons per year for suspected non-TTP TMA and 1.8 (95% CI 1.3-2.4) for confirmed TTP. The incidence rate of TMA other than TTP was higher in the age group 70-79 years (21.8; 95% CI 5.4-38.1) for females and in the age group 80-89 years (24.4; 95% CI 7.2-41.7) for males compared to other age groups. The incidence rate of TTP was higher in the age group 40-49 years (4.0; 95% CI 2.0-5.9) for women and in the age group 60-69 years (3.4; 95% CI 1.1-5.6) for men compared to other age groups. CONCLUSION: The analysis of centralized data measuring ADAMTS-13 activity allowed us to adequately establish the incidence rate and demographic characteristics of TMA, particularly TTP, in Quebec. TTP incidence remained stable while suspected non-TTP TMA steadily increased from 2012 to 2019.

Post-translational modifications, a key process in CD36 function: lessons from the spontaneously hypertensive rat heart.

CD36, a multifunctional protein, is involved in cardiac long chain fatty acid (LCFA) metabolism and in the etiology of heart diseases, yet the functional impact of Cd36 gene variants remains unclear. In 7-week-old spontaneously hypertensive rats (SHR), which, like humans, carry numerous mutations in Cd36, we tested the hypothesis that their restricted cardiac LCFA utilization occurs prior to hypertrophy due to defective CD36 post-translational modifications (PTM), as assessed by ex vivo perfusion of (13)C-labeled substrates and biochemical techniques. Compared to their controls, SHR hearts displayed a lower (i) contribution of LCFA to ß-oxidation (-40%) and triglycerides (+2.8 folds), which was not explained by transcriptional changes or malonyl-CoA level, a recognized ß-oxidation inhibitor, and (ii) membrane-associated CD36 protein level, but unchanged distribution. Other results demonstrate alterations in CD36 PTM in SHR hearts, specifically by N-glycosylation, and the importance of O-linked-ß-N-acetylglucosamine for its membrane recruitment and role in LCFA use in the heart.

Predictive significance of anti-FVIII immunoglobulin patterns on bleeding phenotype and outcomes in acquired hemophilia A: Results from the Quebec Reference Center for Inhibitors.

BACKGROUND: Acquired hemophilia A (AHA) is a potentially life-threatening bleeding disorder caused by factor VIII (FVIII) autoantibodies, involving various immunoglobulin (Ig) isotypes and IgG subclasses. OBJECTIVES: We analyzed the profile of Ig against FVIII in patients with AHA to identify Ig patterns predictive of bleeding phenotype and outcomes. PATIENTS/METHODS: Ig detection and titration were determined by enzyme-linked immunosorbent assay (ELISA) at disease presentation in a cohort of 66 subjects from the Quebec Reference Centre for Inhibitors registry. RESULTS: Most of plasma samples analyzed (97%) contained multiple anti-FVIII Ig isotypes and IgG subclasses, IgG(1,2,3,4) (24.2%), [IgG(1,2,3,4),IgA] (16.7%) and IgG(2.4) (13.6%) being the most prevalent combinations of Ig detected. AHA patients who presented with IgA antibodies were more likely to have an associated auto-immune disease (p = .049). The presence of IgG4-was associated with bleeding symptoms at presentation (p = .002). IgG1-positive patients were more likely to require transfusions with red packed cell (p = .014) whereas IgM detection was associated with a higher probability of death linked to AHA (p = .011). CONCLUSION: The Ig pattern of AHA patients at diagnosis is widely heterogeneous and is at least partially associated with some underlying conditions. Our data supports the differential predictive significance for IgG1, IgG4 and IgM on bleeding severity and suggests that the early determination of Ig profile may help to identify AHA patients at higher risk of poorer outcomes.

Effect of ABO blood group on haemostatic parameters in severe haemophilia A patients performing acute moderate-intensity exercise.

: The primary objective was to assess the effect of ABO blood group on von Willebrand factor (VWF) rise induced by four bouts of moderate-intensity physical activity, on pharmacokinetics of a B-domain-deleted recombinant FVIII (BDD-rFVIII), and haemostatic parameters in severe haemophilia A patients with a null mutation. The secondary objective was to compare the response to exercise according to infused product type in a subgroup of patients who previously participated to the same exercise protocol, while treated with a full length recombinant FVIII (FL-rFVIII). Twenty patients had two visits (rest and exercise). Blood samples were drawn before administration of BDD-rFVIII and at 6 time points, until 24 h postinfusion. FVIII activity increased transiently by 1.1-fold, but only after the first exercise session, as compared to rest. VWF:Ag and platelet count were significantly elevated after each session. Mean FVIII half-life and thromboelastography measurements were unchanged with exercise. However, 14 participants had a slight variation of FVIII half-life with exercise compared to rest (from -3.42 h to +2.51 h). Seven patients demonstrated a longer FVIII half-life (four with O blood group), whereas the remainders had a reduced half-life (three with O blood group). FVIII half-life correlated with baseline VWF:Ag at rest (r = 0.70, P < 0.001) and with exercise (r = 0.67, P < 0.002). Recovery was different between FL-rFVIII and BDD-rFVIII at rest (P = 0.032), but no significant differences were observed between half-life of products at rest and with exercise. ABO blood group and the type of rFVIII administered did not influence the response to exercise.

Role of receptor-mediated endocytosis, endosomal acidification and cathepsin D in cholera toxin cytotoxicity.

Using the in situ liver model system, we have recently shown that, after cholera toxin binding to hepatic cells, cholera toxin accumulates in a low-density endosomal compartment, and then undergoes endosomal proteolysis by the aspartic acid protease cathepsin-D [Merlen C, Fayol-Messaoudi D, Fabrega S, El Hage T, Servin A, Authier F (2005) FEBS J272, 4385-4397]. Here, we have used a subcellular fractionation approach to address the in vivo compartmentalization and cytotoxic action of cholera toxin in rat liver parenchyma. Following administration of a saturating dose of cholera toxin to rats, rapid endocytosis of both cholera toxin subunits was observed, coincident with massive internalization of both the 45 kDa and 47 kDa Gsalpha proteins. These events coincided with the endosomal recruitment of ADP-ribosylation factor proteins, especially ADP-ribosylation factor-6, with a time course identical to that of toxin and the A subunit of the stimulatory G protein (Gsalpha) translocation. After an initial lag phase of 30 min, these constituents were linked to NAD-dependent ADP-ribosylation of endogenous Gsalpha, with maximum accumulation observed at 30-60 min postinjection. Assessment of the subsequent postendosomal fate of internalized Gsalpha revealed sustained endolysosomal transfer of the two Gsalpha isoforms. Concomitantly, cholera toxin increased in vivo endosome acidification rates driven by the ATP-dependent H(+)-ATPase pump and in vitro vacuolar acidification in hepatoma HepG2 cells. The vacuolar H(+)-ATPase inhibitor bafilomycin and the cathepsin D inhibitor pepstatin A partially inhibited, both in vivo and in vitro, the cAMP response to cholera toxin. This cathepsin D-dependent action of cholera toxin under the control of endosomal acidity was confirmed using cellular systems in which modification of the expression levels of cathepsin D, either by transfection of the cathepsin D gene or small interfering RNA, was followed by parallel changes in the cytotoxic response to cholera toxin. Thus, in hepatic cells, a unique endocytic pathway was revealed following cholera toxin administration, with regulation specificity most probably occurring at the locus of the endosome and implicating endosomal proteases, such as cathepsin D, as well as organelle acidification.

Intracellular Angiotensin-II Interacts With Nuclear Angiotensin Receptors in Cardiac Fibroblasts and Regulates RNA Synthesis, Cell Proliferation, and Collagen Secretion.

BACKGROUND: Cardiac fibroblasts play important functional and pathophysiological roles. Intracellular ("intracrine") angiotensin-II (Ang-II) signaling regulates intercellular communication, excitability, and gene expression in cardiomyocytes; however, the existence and role of intracrine Ang-II signaling in cardiac fibroblasts is unstudied. Here, we evaluated the localization of Ang-II receptors on atrial fibroblast nuclei and associated intracrine effects of potential functional significance. METHODS AND RESULTS: Immunoblots of subcellular protein-fractions from isolated canine atrial fibroblasts indicated the presence of nuclear Ang-II type 1 receptors (AT1Rs) and Ang-II type 2 receptors (AT2Rs). Fluorescein isothiocyanate-Ang-II binding displaceable by AT1R- and AT2R-blockers was present on isolated fibroblast nuclei. G-protein subunits, including Gαq/11, Gαi/3, and Gß, were observed in purified fibroblast nuclear fractions by immunoblotting and intact-fibroblast nuclei by confocal immunocytofluorescence microscopy. Nuclear AT1Rs and AT2Rs regulated de novo RNA synthesis ([α32P]UTP incorporation) via IP3R- and NO-dependent pathways, respectively. In intact cultured fibroblasts, intracellular Ang-II release by photolysis of a membrane-permeable caged Ang-II analog led to IP3R-dependent nucleoplasmic Ca2+-liberation, with IP3R3 being the predominant nuclear isoform. Intracellular Ang-II regulated fibroblast proliferation ([3H]thymidine incorporation), collagen-1A1 mRNA-expression, and collagen secretion. Intracellular Ang-II and nuclear AT1R protein levels were significantly increased in a heart failure model in which atrial fibrosis underlies atrial fibrillation. CONCLUSIONS: Fibroblast nuclei possess AT1R and AT2R binding sites that are coupled to intranuclear Ca2+-mobilization and NO liberation, respectively. Intracellular Ang-II signaling regulates fibroblast proliferation, collagen gene expression, and collagen secretion. Heart failure upregulates Ang-II intracrine signaling-components in atrial fibroblasts. These results show for the first time that nuclear angiotensin-II receptor activation and intracrine Ang-II signaling control fibroblast function and may have pathophysiological significance.

Glucagon-mediated internalization of serine-phosphorylated glucagon receptor and Gsalpha in rat liver.

To assess glucagon receptor compartmentalization and signal transduction in liver parenchyma, we have studied the functional relationship between glucagon receptor endocytosis, phosphorylation and coupling to the adenylate cyclase system. Following administration of a saturating dose of glucagon to rats, a rapid internalization of glucagon receptor was observed coincident with its serine phosphorylation both at the plasma membrane and within endosomes. Co-incident with glucagon receptor endocytosis, a massive internalization of both the 45- and 47-kDa Gsalpha proteins was also observed. In contrast, no change in the subcellular distribution of adenylate cyclase or beta-arrestin 1 and 2 was observed. In response to des-His(1)-[Glu(9)]glucagon amide, a glucagon receptor antagonist, the extent and rate of glucagon receptor endocytosis and Gsalpha shift were markedly reduced compared with wild-type glucagon. However, while the glucagon analog exhibited a wild-type affinity for endosomal acidic glucagonase activity and was processed at low pH with similar kinetics and rates, its proteolysis at neutral pH was 3-fold lower. In response to tetraiodoglucagon, a glucagon receptor agonist of enhanced biological potency, glucagon receptor endocytosis and Gsalpha shift were of higher magnitude and of longer duration, and a marked and prolonged activation of adenylate cyclase both at the plasma membrane and in endosomes was observed. The subsequent post-endosomal fate of internalized Gsalpha was evaluated in a cell-free rat liver endosome-lysosome fusion system following glucagon injection. A sustained endo-lysosomal transfer of the two 45- and 47-kDa Gsalpha isoforms was observed. Therefore, these results reveal that within hepatic target cells and consequent to glucagon-mediated internalization of the serine-phosphorylated glucagon receptor and the Gsalpha protein, extended signal transduction may occur in vivo at the locus of the endo-lysosomal apparatus.