Uptake and metabolic fate of [HisA8,HisB4,GluB10,HisB27]insulin in rat liver in vivo

Spatial and Temporal Regulation of Receptor Tyrosine Kinase Activation and Intracellular Signal Transduction

Epidermal growth factor (EGF) and insulin receptor tyrosine kinases (RTKs) exemplify how receptor location is coupled to signal transduction. Extracellular binding of ligands to these RTKs triggers their concentration into vesicles that bud off from the cell surface to generate intracellular signaling endosomes. On the exposed cytosolic surface of these endosomes, RTK autophosphorylation selects the downstream signaling proteins and lipids to effect growth factor and polypeptide hormone action. This selection is followed by the recruitment of protein tyrosine phosphatases that inactivate the RTKs and deliver them by membrane fusion and fission to late endosomes. Coincidentally, proteinases inside the endosome cleave the EGF and insulin ligands. Subsequent inward budding of the endosomal membrane generates multivesicular endosomes. Fusion with lysosomes then results in RTK degradation and downregulation. Through the spatial positioning of RTKs in target cells for EGF and insulin action, the temporal extent of signaling, attenuation, and downregulation is regulated.

[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.

Improved glucose metabolism in vitro and in vivo by an allosteric monoclonal antibody that increases insulin receptor binding affinity

Previously we reported studies of XMetA, an agonist antibody to the insulin receptor (INSR). We have now utilized phage display to identify XMetS, a novel monoclonal antibody to the INSR. Biophysical studies demonstrated that XMetS bound to the human and mouse INSR with picomolar affinity. Unlike monoclonal antibody XMetA, XMetS alone had little or no agonist effect on the INSR. However, XMetS was a strong positive allosteric modulator of the INSR that increased the binding affinity for insulin nearly 20-fold. XMetS potentiated insulin-stimulated INSR signaling ∼15-fold or greater including; autophosphorylation of the INSR, phosphorylation of Akt, a major enzyme in the metabolic pathway, and phosphorylation of Erk, a major enzyme in the growth pathway. The enhanced signaling effects of XMetS were more pronounced with Akt than with Erk. In cultured cells, XMetS also enhanced insulin-stimulated glucose transport. In contrast to its effects on the INSR, XMetS did not potentiate IGF-1 activation of the IGF-1 receptor. We studied the effect of XMetS treatment in two mouse models of insulin resistance and diabetes. The first was the diet induced obesity mouse, a hyperinsulinemic, insulin resistant animal, and the second was the multi-low dose streptozotocin/high-fat diet mouse, an insulinopenic, insulin resistant animal. In both models, XMetS normalized fasting blood glucose levels and glucose tolerance. In concert with its ability to potentiate insulin action at the INSR, XMetS reduced insulin and C-peptide levels in both mouse models. XMetS improved the response to exogenous insulin without causing hypoglycemia. These data indicate that an allosteric monoclonal antibody can be generated that markedly enhances the binding affinity of insulin to the INSR. These data also suggest that an INSR monoclonal antibody with these characteristics may have the potential to both improve glucose metabolism in insulinopenic type 2 diabetes mellitus and correct compensatory hyperinsulinism in insulin resistant conditions.

Proteolysis of Pseudomonas exotoxin A within hepatic endosomes by cathepsins B and D produces fragments displaying in vitro ADP-ribosylating and apoptotic effects

To assess Pseudomonas exotoxin A (ETA) compartmentalization, processing and cytotoxicity in vivo, we have studied the fate of internalized ETA with the use of the in vivo rodent liver model following toxin administration, cell-free hepatic endosomes, and pure in vitro protease assays. ETA taken up into rat liver in vivo was rapidly associated with plasma membranes (5-30 min), internalized within endosomes (15-60 min), and later translocated into the cytosolic compartment (30-90 min). Coincident with endocytosis of intact ETA, in vivo association of the catalytic ETA-A subunit and low molecular mass ETA-A fragments was observed in the endosomal apparatus. After an in vitro proteolytic assay with an endosomal lysate and pure proteases, the ETA-degrading activity was attributed to the luminal species of endosomal acidic cathepsins B and D, with the major cleavages generated in vitro occurring mainly within domain III of ETA-A. Cell-free endosomes preloaded in vivo with ETA intraluminally processed and extraluminally released intact ETA and ETA-A in vitro in a pH-dependent and ATP-dependent manner. Rat hepatic cells underwent in vivo intrinsic apoptosis at a late stage of ETA infection, as assessed by the mitochondrial release of cytochrome c, caspase-9 and caspase-3 activation, and DNA fragmentation. In an in vitro assay, intact ETA induced ADP-ribosylation of EF-2 and mitochondrial release of cytochrome c, with the former effect being efficiently increased by a cathepsin B/cathepsin D pretreatment. The data show a novel processing pathway for internalized ETA, involving cathepsins B and D, resulting in the production of ETA fragments that may participate in cytotoxicity and mitochondrial dysfunction.

Endosomal proteolysis of internalised [ArgA0]-human insulin at neutral pH generates the mature insulin peptide in rat liver in vivo

AIMS/HYPOTHESIS

A proteolysis study of human monoarginyl-insulin ([Arg(A0)]-HI) and diarginyl-insulin ([Arg(B31)-Arg(B32)]-HI) within hepatic endosomes was undertaken to determine whether the endosomal compartment represents a physiological site for the removal of Arg residues and conversion of Arg-extended insulins into fully processed human insulin.

METHODS

The metabolic fate of arginyl-insulins has been studied using the in situ rat liver model system following ligand administration to rats and cell-free hepatic endosomes.

RESULTS

While the kinetics of insulin receptor endocytosis after the administration of arginyl-insulins were similar to those observed using human insulin, a more prolonged concentration of endosomal insulin receptor was observed in response to [Arg(A0)]-HI. [Arg(A0)]-HI induced a marked increase in the phosphotyrosine content of endosomal insulin receptor, coinciding with a more sustained endosomal association of growth factor receptor-bound protein 14 (GRB14), and a higher and prolonged activation of mitogen-activated protein kinase pathways. At acidic pH, the endosomal cathepsin D rapidly degraded insulin peptides with similar binding affinity, and generated comparable intermediates for both arginyl-insulins without affecting amino and carboxyl arginyl-peptide bonds. At neutral pH, hepatic endosomes fully processed [Arg(A0)]-HI into mature human insulin while no conversion was observed with [Arg(B31)-Arg(B32)]-HI. The neutral endosomal Arg-convertase was sensitive to bestatin, immunologically distinct from insulin-degrading enzyme, nardilysin or furin, and was potentially related to aminopeptidase-B-type enzyme.

CONCLUSIONS/INTERPRETATION

The data describe a unique processing pathway for the endosomal proteolysis of [Arg(A0)]-HI which involves the removal of Arg(A0) and subsequent generation of mature human insulin through an uncovered neutral Arg-aminopeptidase activity. The endosomal conversion of [Arg(A0)]-HI into human insulin might extend the insulin receptor signalling at this locus.

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.

Proteolytic activation of internalized cholera toxin within hepatic endosomes by cathepsin D

We have defined the in vivo and in vitro metabolic fate of internalized cholera toxin (CT) in the endosomal apparatus of rat liver. In vivo, CT was internalized and accumulated in endosomes where it underwent degradation in a pH-dependent manner. In vitro proteolysis of CT using an endosomal lysate required an acidic pH and was sensitive to pepstatin A, an inhibitor of aspartic acid proteases. By nondenaturating immunoprecipitation, the acidic CT-degrading activity was attributed to the luminal form of endosomal cathepsin D. The rate of toxin hydrolysis using an endosomal lysate or pure cathepsin D was found to be high for native CT and free CT-B subunit, and low for free CT-A subunit. On the basis of IC(50) values, competition studies revealed that CT-A and CT-B subunits share a common binding site on the cathepsin D enzyme, with native CT and free CT-B subunit displaying the highest affinity for the protease. By immunofluorescence, partial colocalization of internalized CT with cathepsin D was confirmed at early times of endocytosis in both hepatoma HepG2 and intestinal Caco-2 cells. Hydrolysates of CT generated at low pH by bovine cathepsin D displayed ADP-ribosyltransferase activity towards exogenous Gsalpha protein suggesting that CT cytotoxicity, at least in part, may be related to proteolytic events within endocytic vesicles. Together, these data identify the endocytic apparatus as a critical subcellular site for the accumulation and proteolytic degradation of endocytosed CT, and define endosomal cathepsin D an enzyme potentially responsible for CT cytotoxic activation.

Use of high affinity insulin analogues to assess the functional relationships between insulin receptor trafficking, mitogenic signaling and mRNA expression in rat liver

We have investigated the functional relationships between insulin receptor (IR) trafficking, mitogenic signaling and mRNA expression in rat liver and primary hepatocytes. The low-K(d) insulin analogues [His(A8),His(B4), Glu(B10),His(B27)]-human insulin (-HI) (the H2-analogue), [Asp(B10)]HI and [Glu(A13),Glu(B10)]HI, were studied in liver parenchymal cells and compared with wild-type HI and epidermal growth factor (EGF), a mitogenic inducer. The extent and duration of IR endocytosis were markedly increased in response to the H2-analogue and [Asp(B10)]HI compared to wild-type HI, but similar to HI after [Glu(A13),Glu(B10)]HI administration. Importantly, the insulin analogues induced a higher and more prolonged tyrosine phosphorylation of the IR-beta subunit in endosomes compared to authentic HI. A low cell-free endosome-lysosome transfer of the internalized IR was only observed in response to HI and H2-analogue injection. Concomitant with the low endosome-lysosome transfer of the intact IR-beta subunit, 47 and 50 kDa fragments of the IR-beta subunit accumulated in lysosomal fractions. Neither HI nor the insulin analogues promoted the endosomal recruitment and tyrosine phosphorylation of Shc, whereas EGF accessed the Shc signaling pathway. Moreover, EGF induced a fast and prolonged activation of Raf-1 and MAP-kinase pathways whereas HI and insulin analogues displayed a moderate and transient effect. Finally, treatment of primary rat hepatocytes with HI and the protease-resistant H2-analogue did not affect the total level and relative expression of isotype A and B of IR mRNA regardless of time of exposure. These results suggest a lack of relationship between IR trafficking, endosomal tyrosine phosphorylation and mitogenic signaling in rat liver in vivo.

Cell itinerary and metabolic fate of proinsulin in rat liver: in vivo and in vitro studies

Proinsulin, the insulin precursor in pancreatic beta-cells, displays a slower hepatic clearance than insulin and exerts a more prolonged metabolic effect on liver in vivo. To elucidate the mechanisms underlying these differences, the cellular itinerary and processing of proinsulin and insulin in rat liver have been comparatively studied using cell fractionation. As [125I]-insulin, [125I]-proinsulin taken up into liver in vivo was internalized and accumulated in endosomes, in which it underwent dissociation from the insulin receptor and degradation in a pH- and ATP-dependent manner. However, relative to [125I]-insulin, [125I]-proinsulin showed a delayed and prolonged in vivo association with endosomes, a slower in vivo and cell-free endosomal processing, and a higher cell-free endosome-lysosome transfer. Endosomal extracts degraded to a lesser extent proinsulin than insulin at acidic pH; so did, and even proportionally less, at neutral pH, plasma membrane and cytosolic fractions. Proinsulin degradation products generated by soluble endosomal extracts were isolated by HPLC and characterized by mass spectrometry. Under conditions resulting in multiple cleavages in insulin, proinsulin was cleaved at eight bonds in the C peptide but only at the Phe24-Phe25 bond in the insulin moiety. As native insulin, native proinsulin induced a dose- and time-dependent endocytosis and tyrosine phosphorylation of the insulin receptor; but at an inframaximal dose, proinsulin effects on these processes were of longer duration. We conclude that a reduced proteolysis of proinsulin in endosomes, and probably also at the plasma membrane, accounts for its slower hepatic clearance and prolonged effects on insulin receptor endocytosis and tyrosine phosphorylation.

Endosomal proteolysis of internalized insulin at the C-terminal region of the B chain by cathepsin D

The endosomal compartment of hepatic parenchymal cells contains an acidic endopeptidase, endosomal acidic insulinase, which hydrolyzes internalized insulin and generates the major primary end product A(1--21)-B(1--24) insulin resulting from a major cleavage at residues Phe(B24)-Phe(B25). This study addresses the nature of the relevant endopeptidase activity in rat liver that is responsible for most receptor-mediated insulin degradation in vivo. The endosomal activity was shown to be aspartic acid protease cathepsin D (CD), based on biochemical similarities to purified CD in 1) the rate and site of substrate cleavage, 2) pH optimum, 3) sensitivity to pepstatin A, and 4) binding to pepstatin A-agarose. The identity of the protease was immunologically confirmed by removal of greater than 90% of the insulin-degrading activity associated with an endosomal lysate using polyclonal antibodies to CD. Moreover, the elution profile of the endosomal acidic insulinase activity on a gel-filtration TSK-GEL G3000 SW(XL) high performance liquid chromatography column corresponded exactly with the elution profile of the immunoreactive 45-kDa mature form of endosomal CD. Using nondenaturating immunoprecipitation and immunoblotting procedures, other endosomal aspartic acid proteases such as cathepsin E and beta-site amyloid precursor protein-cleaving enzyme (BACE) were ruled out as candidate enzymes for the endosomal degradation of internalized insulin. Immunofluorescence studies showed a largely vesicular staining pattern for internalized insulin in rat hepatocytes that colocalized partially with CD. In vivo pepstatin A treatment was without any observable effect on the insulin receptor content of endosomes but augmented the phosphotyrosine content of the endosomal insulin receptor after insulin injection. These results suggest that CD is the endosomal acidic insulinase activity which catalyzes the rate-limiting step of the in vivo cleavage at the Phe(B24)-Phe(B25) bond, generating the inactive A(1--21)-B(1--24) insulin intermediate.

Inhibition of endosomal insulin-like growth factor-I processing by cysteine proteinase inhibitors blocks receptor-mediated functions

The receptor for the type 1 insulin-like growth factor (IGF-I) has been implicated in cellular transformation and the acquisition of an invasive/metastatic phenotype in various tumors. Following ligand binding, the IGF-I receptor is internalized, and the receptor.ligand complex dissociates as the ligand is degraded by endosomal proteinases. In the present study we show that the inhibition of endosomal IGF-I-degrading enzymes in human breast and murine lung carcinoma cells by the cysteine proteinase inhibitors, E-64 and CA074-methyl ester, profoundly altered receptor trafficking and signaling. In treated cells, intracellular ligand degradation was blocked, and although the receptor and two substrates, Shc and Insulin receptor substrate, were hyperphosphorylated on tyrosine, IGF-I-induced DNA synthesis, anchorage-independent growth, and matrix metalloproteinase synthesis were inhibited. The results suggest that ligand processing by endosomal proteinases is a key step in receptor signaling and function and a potential target for therapy.

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.

Negative regulation of epidermal growth factor signaling by selective proteolytic mechanisms in the endosome mediated by cathepsin B

We have investigated the relevant protease activity in rat liver, which is responsible for most of the receptor-mediated epidermal growth factor (EGF) degradation in vivo. EGF was sequentially cleaved by endosomal proteases at a limited number of sites, which were identified by high performance liquid chromatography and mass spectrometry. EGF proteolysis is initiated by hydrolysis at the C-terminal Glu(51)-Leu(52) bond. Three additional minor cleavage sites were identified at positions Arg(48)-Trp(49), Trp(49)-Trp(50), and Trp(50)-Glu(51) after prolonged incubation. Using nondenaturating immunoprecipitation and cross-linking procedures, the major proteolytic activity was identified as that of the cysteine protease cathepsin-B. The effect of injected EGF on subsequent endosomal EGF receptor (EGFR) proteolysis was further evaluated by immunoblotting. Using endosomal fractions prepared from EGF-injected rats and incubated in vitro, the EGFR was lost with a time course superimposable with the loss of phosphotyrosine content. The cathepsin-B proinhibitor CA074-Me inhibited both in vivo and in vitro the endosomal degradation of the EGFR and increased the tyrosine phosphorylation states of the EGFR protein and the molecule SHC within endosomes. The data, therefore, describe a unique pathway for the endosomal processing of internalized EGF receptor complexes, which involves the sequential function of cathepsin-B through selective degradation of both the ligand and receptor.

In vitro endosome-lysosome transfer of dephosphorylated EGF receptor and Shc in rat liver

We have studied the endosome-lysosome transfer of internalized epidermal growth factor receptor (EGFR) complexes in a cell-free system from rat liver. Analytical subfractionation of a postmitochondrial supernatant fraction showed that a pulse of internalized [(125)I]EGF was largely associated with a light endosomal fraction devoid of lysosomal markers. After an additional 30 min incubation in vitro in the presence of an ATP-regenerating system, the amount of [(125)I]EGF in this compartment decreased by 39%, with an increase in [(125)I]EGF in lysosomes. No transfer of [(125)I]EGF to the cytosol was detected. To assess the fate of the internalized EGFR protein over the time course of the endo-lysosomal transfer of the ligand, the effect of a saturating dose of native EGF on subsequent lysosomal targeting of the EGFR was evaluated by immunoblotting. A massive translocation of the EGFR to the endosomal compartment was observed in response to ligand injection coincident with its tyrosine phosphorylation and receptor recruitment of the tyrosine-phosphorylated adaptor protein Shc. During cell-free endosome-lysosome fusion, a time-dependent increase in the content of the EGFR and the two 55- and 46-kDa Shc isoforms was observed in lysosomal fractions with a time course superimposable with the lysosomal transfer of the ligand; no transfer of the 66-kDa Shc isoform was detected. The relationship between EGFR tyrosine kinase activity and EGFR sorting in endosomes investigated by immunoblot studies with anti-phosphotyrosine antibodies revealed that endosomal dephosphorylation of EGFR and Shc preceded lysosomal transfer. These results support the view that a lysosomal targeting machinery distinct from the endosomal receptor kinase activity, such as the recruitment of the signaling molecule Shc, may regulate this sorting event in the endosome.