Amino acid sequences Gly-Pro-Leu-Tyr and Asn-Pro-Glu-Tyr in the submembranous domain of the insulin receptor are required for normal endocytosis

The Activation Mechanism of the Insulin Receptor: A Structural Perspective

The insulin receptor (IR) is a type II receptor tyrosine kinase that plays essential roles in metabolism, growth, and proliferation. Dysregulation of IR signaling is linked to many human diseases, such as diabetes and cancers. The resolution revolution in cryo-electron microscopy has led to the determination of several structures of IR with different numbers of bound insulin molecules in recent years, which have tremendously improved our understanding of how IR is activated by insulin. Here, we review the insulin-induced activation mechanism of IR, including (a) the detailed binding modes and functions of insulin at site 1 and site 2 and (b) the insulin-induced structural transitions that are required for IR activation. We highlight several other key aspects of the activation and regulation of IR signaling and discuss the remaining gaps in our understanding of the IR activation mechanism and potential avenues of future research.

The insulin receptor endocytosis

Insulin signaling controls multiple aspects of animal physiology. At the cell surface, insulin binds and activates the insulin receptor (IR), a receptor tyrosine kinase. Insulin promotes a large conformational change of IR and stabilizes the active conformation. The insulin-activated IR triggers signaling cascades, thus controlling metabolism, growth, and proliferation. The activated IR undergoes internalization by clathrin- or caveolae-mediated endocytosis. The IR endocytosis plays important roles in insulin clearance from blood, and distribution and termination of the insulin signaling. Despite decades of extensive studies, the mechanism and regulation of IR endocytosis and its contribution to pathophysiology remain incompletely understood. Here we discuss recent findings that provide insights into the molecular mechanisms and regulatory pathways that mediate the IR endocytosis.

Comprehensive insulin receptor phosphorylation dynamics profiled by mass spectrometry

Insulin receptor (IR) phosphorylation is critical for the assessment of the extent of IR agonism and nuances in the downstream signaling cascade. A thorough identification and monitoring of the phosphorylation events is important for understanding the process of insulin signaling transduction and regulation. Although IR phosphorylation has been studied extensively in the past decades, only a handful of phosphorylation sites can be identified by either traditional antibody-based assays or recent large-scale mass spectrometry-based phosphoproteomics approaches. In the present study, the most exhaustive assessment of the IR phosphorylation was conducted using nano-liquid chromatography-tandem mass spectrometry, in which 13 IR phosphorylation sites and 22 combinations thereof were analyzed. The kinetic analysis included Y965, Y972, S968/969, and S974/976 in the juxtamembrane region; Y1158, Y1162, and Y1163 in the kinase domain; and Y1328, Y1334, S1278, S1320, S1321, and T1348 in the C-terminal region. Employing two different receptor agonists (i.e. insulin and an IR peptide agonist), the data revealed contrasting phosphorylation kinetics across these sites with dynamics far more diverse than expected for known IR agonists. Notably, cell trafficking experiments revealed that the IR peptide agonist was incapable of inducing IR to the early endosome, which is probably linked to a difference in IR phosphorylation. The present study provides a powerful tool for investigating IR signaling and trafficking that will benefit the design of IR agonists with improved therapeutic utility.

Insulin action at a molecular level - 100 years of progress

The discovery of insulin 100 years ago and its application to the treatment of human disease in the years since have marked a major turning point in the history of medicine. The availability of purified insulin allowed for the establishment of its physiological role in the regulation of blood glucose and ketones, the determination of its amino acid sequence, and the solving of its structure. Over the last 50 years, the function of insulin has been applied into the discovery of the insulin receptor and its signaling cascade to reveal the role of impaired insulin signaling-or resistance-in the progression of type 2 diabetes. It has also become clear that insulin signaling can impact not only classical insulin-sensitive tissues, but all tissues of the body, and that in many of these tissues the insulin signaling cascade regulates unexpected physiological functions. Despite these remarkable advances, much remains to be learned about both insulin signaling and how to use this molecular knowledge to advance the treatment of type 2 diabetes and other insulin-resistant states.

Endocytosis and Trafficking of Natriuretic Peptide Receptor-A: Potential Role of Short Sequence Motifs

The targeted endocytosis and redistribution of transmembrane receptors among membrane-bound subcellular organelles are vital for their correct signaling and physiological functions. Membrane receptors committed for internalization and trafficking pathways are sorted into coated vesicles. Cardiac hormones, atrial and brain natriuretic peptides (ANP and BNP) bind to guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) and elicit the generation of intracellular second messenger cyclic guanosine 3',5'-monophosphate (cGMP), which lowers blood pressure and incidence of heart failure. After ligand binding, the receptor is rapidly internalized, sequestrated, and redistributed into intracellular locations. Thus, NPRA is considered a dynamic cellular macromolecule that traverses different subcellular locations through its lifetime. The utilization of pharmacologic and molecular perturbants has helped in delineating the pathways of endocytosis, trafficking, down-regulation, and degradation of membrane receptors in intact cells. This review describes the investigation of the mechanisms of internalization, trafficking, and redistribution of NPRA compared with other cell surface receptors from the plasma membrane into the cell interior. The roles of different short-signal peptide sequence motifs in the internalization and trafficking of other membrane receptors have been briefly reviewed and their potential significance in the internalization and trafficking of NPRA is discussed.

Ligand-mediated endocytosis and intracellular sequestration of guanylyl cyclase/natriuretic peptide receptors: role of GDAY motif

The guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), also referred to as GC-A, is a single polypeptide molecule having a critical function in blood pressure regulation and cardiovascular homeostasis. GC-A/NPRA, which resides in the plasma membrane, consists of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular cytoplasmic region containing a protein kinase-like homology domain (KHD) and a guanylyl cyclase (GC) catalytic domain. After binding with atrial and brain natriuretic peptides (ANP and BNP), GC-A/NPRA is internalized and sequestered into intracellular compartments. Therefore, GC-A/NPRA is a dynamic cellular macromolecule that traverses different subcellular compartments through its lifetime. This review describes the roles of short-signal sequences in the internalization, trafficking, and intracellular redistribution of GC-A/NPRA from cell surface to cell interior. Evidence indicates that, after internalization, the ligand-receptor complexes dissociate inside the cell and a population of GC-A/NPRA recycles back to the plasma membrane. Subsequently, the disassociated ligands are degraded in the lysosomes. However, a small percentage of the ligand escapes the lysosomal degradative pathway, and is released intact into culture medium. Using pharmacologic and molecular perturbants, emphasis has been placed on the cellular regulation and processing of ligand-bound GC-A/NPRA in terms of receptor trafficking and down-regulation in intact cells. The discussion is concluded by examining the functions of short-signal sequence motifs in the cellular life-cycle of GC-A/NPRA, including endocytosis, trafficking, metabolic processing, inactivation, and/or down-regulation in model cell systems.

Insulin receptor isoforms and insulin receptor/insulin-like growth factor receptor hybrids in physiology and disease

In mammals, the insulin receptor (IR) gene has acquired an additional exon, exon 11. This exon may be skipped in a developmental and tissue-specific manner. The IR, therefore, occurs in two isoforms (exon 11 minus IR-A and exon 11 plus IR-B). The most relevant functional difference between these two isoforms is the high affinity of IR-A for IGF-II. IR-A is predominantly expressed during prenatal life. It enhances the effects of IGF-II during embryogenesis and fetal development. It is also significantly expressed in adult tissues, especially in the brain. Conversely, IR-B is predominantly expressed in adult, well-differentiated tissues, including the liver, where it enhances the metabolic effects of insulin. Dysregulation of IR splicing in insulin target tissues may occur in patients with insulin resistance; however, its role in type 2 diabetes is unclear. IR-A is often aberrantly expressed in cancer cells, thus increasing their responsiveness to IGF-II and to insulin and explaining the cancer-promoting effect of hyperinsulinemia observed in obese and type 2 diabetic patients. Aberrant IR-A expression may favor cancer resistance to both conventional and targeted therapies by a variety of mechanisms. Finally, IR isoforms form heterodimers, IR-A/IR-B, and hybrid IR/IGF-IR receptors (HR-A and HR-B). The functional characteristics of such hybrid receptors and their role in physiology, in diabetes, and in malignant cells are not yet fully understood. These receptors seem to enhance cell responsiveness to IGFs.

Internalization and trafficking of guanylyl (guanylate) cyclase/natriuretic peptide receptor A is regulated by an acidic tyrosine-based cytoplasmic motif GDAY

We have identified a GDAY motif in the C-terminal domain of guanylyl cyclase (guanylate cyclase)/NPRA (natriuretic peptide receptor A) sequence, which serves a dual role as an internalization signal and a recycling signal. To delineate the role of the GDAY motif in receptor internalization and sequestration, we mutated Gly920, Asp921 and Tyr923 to alanine residues (GDAY/AAAA) in the NPRA cDNA sequence. The cDNAs encoding wild-type and mutant receptors were transfected in HEK-293 cells (human embryonic kidney 293 cells). The internalization studies of ligand-receptor complexes revealed that endocytosis of 125I-ANP by HEK-293 cells expressing G920A, Y923A or GDAY/AAAA mutant receptor was decreased by almost 50% (P<0.001) when compared with cells expressing the wild-type receptor. However, the effect of D921A mutation on receptor internalization was minimal. Ligand-mediated down-regulation of G920A, Y923A and GDAY/AAAA mutant receptors was decreased by 35-40% when compared with wild-type NPRA. Subsequently, the recycling of internalized D921A and GDAY/AAAA mutant receptors from the intracellular pool was decreased by more than 40+/-4% when compared with wild-type NPRA. Recycling of G920A and Y923A mutant receptors was also decreased, but to a significantly lesser extent compared with the D921A or GDAY/AAAA mutant receptors. We conclude that the Gly920 and Tyr923 residues within the GDAY consensus motif are necessary for internalization, and that residue Asp921 is important for recycling of NPRA. The current results provide new evidence for a dual role of the GDAY sequence motif in ligand-mediated internalization, recycling and down-regulation of a single-transmembrane receptor protein NPRA.

Autoinhibition of the insulin-like growth factor I receptor by the juxtamembrane region

The juxtamembrane (JM) regions of several receptor tyrosine kinases are involved in autoinhibitory interactions that maintain the low basal activity of the receptors; mutations can give rise to constitutive kinase activity and signaling. In this report, we show that the JM region of the human insulin-like growth factor I receptor (IGF1R) plays a role in kinase regulation. We mutated JM residues that were conserved in this subfamily of receptor tyrosine kinases, and expressed and purified the cytoplasmic domains using the Sf9/baculovirus system. We show that a kinase-proximal mutation (Y957F) and (to a lesser extent) a mutation in the central part of the JM region (N947A) increase the autophosphorylation activity of the kinase. Steady-state kinetic measurements show the mutations cause an increase in V(max) for phosphorylation of peptide substrates. When the holoreceptors were expressed in fibroblasts derived from IGF1R-deficient mice, the Y957F mutation led to a large increase in basal and in IGF1-stimulated receptor autophosphorylation. Together, these data demonstrate that the JM region of IGF1R plays an important role in limiting the basal activity of the receptor.

Selective gene activation by spatial segregation of insulin receptor B signaling

Insulin exerts pleiotropic effects at the cellular level. Signaling via the two isoforms of the insulin receptor (IR) may explain the activation of different signaling cascades, while it remains to be explored how selectivity is achieved when utilizing the same IR isoform. We now demonstrate that insulin-stimulated transcription of c-fos and glucokinase genes is activated simultaneously in the insulin-producing beta-cell via IR-B localized in different cellular compartments. Insulin activates the glucokinase gene from plasma membrane-standing IR-B, while c-fos gene activation is dependent on clathrin-mediated IR-B-endocytosis and signaling from early endosomes. Moreover, glucokinase gene up-regulation requires the integrity of the juxtamembrane IR-B NPEY-motif and signaling via PI3K-C2alpha-like/PDK1/PKB, while c-fos gene activation requires the intact C-terminal YTHM-motif and signaling via PI3K Ia/Shc/MEK1/ERK. By using IR-B as an example it is thus possible to demonstrate how spatial segregation allows simultaneous and selective signaling via the same receptor isoform in the same cell.

Internalization and trafficking of guanylyl cyclase/natriuretic peptide receptor-A

One of the principal loci involved in the regulatory action of atrial and brain natriuretic peptides (ANP and BNP) is guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), whose ligand-binding efficiency and GC catalytic activity vary remarkably in different target cells and tissues. In its mature form, NPRA resides in the plasma membrane and contains an extracellular ligand-binding domain, a single transmembrane region, and the intracellular protein kinase-like homology domain (KHD) and guanylyl cyclase (GC) catalytic domain. NPRA is a dynamic cellular macromolecule that traverses through different compartments of the cell through its lifetime. Binding of ligand to NPRA triggers a complex array of signal transduction events and accelerates the endocytosis. The endocytic transport is important in regulating signal transduction, formation of specialized signaling complexes, and modulation of specific components of internalization events. The present review describes the experiments which reveal the internalization of ligand-receptor complexes of NPRA, receptor trafficking and recycling, and delivery of both ligand-receptor molecules into subcellular compartments. The ligand-receptor complexes of NPRA are finally degraded within the lysosomes. The experimental evidence provides a consensus forum, which establishes the endocytosis, cellular trafficking, sequestration, and metabolic processing of ANP/NPRA complexes in the intact cells. The discussion is afforded to address the experimental insights into the mechanisms that cells utilize in modulating the delivery and metabolic processing of ligand-bound NPRA into the cell interior.

Sorting motifs in receptor trafficking

A variety of receptors have been analyzed in sufficient detail to identify sorting motifs. Initial studies focused on the identification of sequences in the cytoplasmic tails of the LDL and transferrin receptors that mediated their internalization. These motifs have since been found in the cytoplasmic domains of a wide variety of receptors and provide for numerous sorting functions. This review will outline the early studies on LDL and transferrin receptors and will then focus on two classes of signaling receptors, receptor tyrosine kinases (EGF and the insulin receptors) and heterotrimeric G-protein coupled receptors (beta2-adrenergic receptors). The identification of sorting motifs and proteins that bind these motifs will be discussed. Importantly, the studies identify a variety of potential targets for modulating the sorting and hence activity of these medically important receptors.

Structural and biochemical evidence for an autoinhibitory role for tyrosine 984 in the juxtamembrane region of the insulin receptor

Tyrosine 984 in the juxtamembrane region of the insulin receptor, between the transmembrane helix and the cytoplasmic tyrosine kinase domain, is conserved among all insulin receptor-like proteins from hydra to humans. Crystallographic studies of the tyrosine kinase domain and proximal juxtamembrane region reveal that Tyr-984 interacts with several other conserved residues in the N-terminal lobe of the kinase domain, stabilizing a catalytically nonproductive position of alpha-helix C. Steady-state kinetics measurements on the soluble kinase domain demonstrate that replacement of Tyr-984 with phenylalanine results in a 4-fold increase in kcat in the unphosphorylated (basal state) enzyme. Moreover, mutation of Tyr-984 in the full-length insulin receptor results in significantly elevated receptor phosphorylation levels in cells, both in the absence of insulin and following insulin stimulation. These data demonstrate that Tyr-984 plays an important structural role in maintaining the quiescent, basal state of the insulin receptor. In addition, the structural studies suggest a possible target site for small molecule activators of the insulin receptor, with potential use in the treatment of noninsulin-dependent diabetes mellitus.

Role of two dileucine-like motifs in insulin receptor anchoring to microvilli

In the absence of ligand, the insulin receptor is maintained on microvilli on the cell surface. A dileucine motif (LL(986-987)) is necessary but not sufficient for this anchoring, which also required the presence of additional sequence(s) downstream of position 1000. The aim of the present study was to identify this (these) additional sequence(s). First, exons 16 or 17 were fused to the extracellular and transmembrane domains of complement receptor 1 and stably expressed in Chinese hamster ovary cells. Results obtained indicate that exon 17 is sufficient for anchoring to microvilli. Second, analysis of insulin receptor mutants truncated within exon 17 demonstrated that whereas receptors truncated at position 1000 showed no preferential association with microvilli, receptors truncated at position 1012 displayed a level of association identical to that of the full-length insulin receptor. Third, mutation of a diisoleucine motif (II(1006-1007)) present within this 12-amino acid stretch abrogated the preferential association of the receptor with microvilli. These results indicate that the domain required for association of insulin receptor with microvilli is contained within the region encoded by exon 17 and that, within this sequence, two dileucine-like motifs (LL(986-987) and II(1006-1007)) play a crucial role.

Synergistic role of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase cascade in the regulation of insulin receptor trafficking

To examine the molecular mechanism of insulin receptor trafficking, we investigated the intracellular signaling molecules that regulate this process in Rat1 fibroblasts overexpressing insulin receptors. Cellular localization of insulin receptors was assessed by confocal laser microscopy with indirect immunofluorescence staining. Insulin receptors were visualized diffusely in the basal state. Insulin treatment induced the change of insulin receptor localization to perinuclear compartment. This insulin-induced insulin receptor trafficking was not affected by treatment of the cells with PI3-kinase inhibitor (wortmannin), whereas treatment with MEK [mitogen-activated protein (MAP) kinase-Erk kinase] inhibitor (PD98059) partly inhibited the process in a dose-dependent manner. Interestingly, treatment with both wortmannin and PD98059 almost completely inhibited insulin receptor trafficking. The functional importance of PI3-kinase and MAP kinase in the trafficking process was directly assessed by using single cell microinjection analysis. Microinjection of p85-SH2 and/or catalytically inactive MAP kinase ([K71A]Erk1) GST fusion protein gave the same results as treatment with wortmannin and PD98059. Furthermore, to determine the crucial step for the requirement of PI3-kinase and MAP kinase pathways, the effect of wortmannin and PD98059 on insulin receptor endocytosis was studied. Insulin internalization from the plasma membrane and subsequent insulin degradation were not affected by treatment with wortmannin and PD98059. In contrast, insulin receptor down-regulation from the cell surface and insulin receptor degradation, after prolonged incubation with insulin, were markedly impaired by the treatment. These results suggest that PI3-kinase and MAP kinase pathways synergistically regulate insulin receptor trafficking at a step subsequent to the receptor internalization.

Interactions of the cytoplasmic domains of human and simian retroviral transmembrane proteins with components of the clathrin adaptor complexes modulate intracellular and cell surface expression of envelope glycoproteins

The cytoplasmic domains of the transmembrane (TM) envelope proteins (TM-CDs) of most retroviruses have a Tyr-based motif, YXXO, in their membrane-proximal regions. This signal is involved in the trafficking and endocytosis of membrane receptors via clathrin-associated AP-1 and AP-2 adaptor complexes. We have used CD8-TM-CD chimeras to investigate the role of the Tyr-based motif of human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency virus (SIV), and human T-leukemia virus type 1 (HTLV-1) TM-CDs in the cell surface expression of the envelope glycoprotein. Flow cytometry and confocal microscopy studies showed that this motif is a major determinant of the cell surface expression of the CD8-HTLV chimera. The YXXO motif also plays a key role in subcellular distribution of the envelope of lentiviruses HIV-1 and SIV. However, these viruses, which encode TM proteins with a long cytoplasmic domain, have additional determinants distal to the YXXO motif that participate in regulating cell surface expression. We have also used the yeast two-hybrid system and in vitro binding assays to demonstrate that all three retroviral YXXO motifs interact with the micro1 and micro2 subunits of AP complexes and that the C-terminal regions of HIV-1 and SIV TM proteins interact with the beta2 adaptin subunit. The TM-CDs of HTLV-1, HIV-1, and SIV also interact with the whole AP complexes. These results clearly demonstrate that the cell surface expression of retroviral envelope glycoproteins is governed by interactions with adaptor complexes. The YXXO-based signal is the major determinant of this interaction for the HTLV-1 TM, which contains a short cytoplasmic domain, whereas the lentiviruses HIV-1 and SIV have additional determinants distal to this signal that are also involved.

Differential requirement for type I and type II transforming growth factor beta receptor kinase activity in ligand-mediated receptor endocytosis

Transforming growth factor beta (TGFbeta) superfamily polypeptides regulate cell growth and differentiation by binding to single pass serine/threonine kinases referred to as TGFbeta type I and type II receptors. Signal propagation is dependent upon heteromeric (type I-type II) complex formation and transphosphorylation of the type I receptor by the type II receptor. While many of the phosphorylation events necessary for receptor signaling have recently been characterized, the role of TGFbeta receptor kinase activity in modulating receptor endocytosis has not been addressed. To that end, we have used chimeric receptors consisting of the extracellular domain of the granulocyte/macrophage colony-stimulating factor alpha and beta receptors spliced to the TGFbeta type I and type II transmembrane and cytoplasmic domains to address the specific role of type I and/or type II receptor kinase activity in TGFbeta receptor internalization, down-regulation, and signaling. To inactivate chimeric receptor kinase activity, point mutations in the ATP binding site were made at amino acids 232 and 277 in the type I and type II receptor, respectively. Either of these mutations abolished plasminogen activator inhibitor 1 protein expression stimulated by granulocyte/macrophage colony-stimulating factor activation of chimeric heteromeric type I-type II TGFbeta receptors. They did not, however, modulate TGFbeta signaling stimulated through the endogenous TGFbeta receptor. Although TGFbeta receptor signaling was dependent upon the kinase activity of both chimeric receptors, the initial endocytic response was distinctly regulated by type I and/or type II receptor kinase activity. For instance, while heteromeric receptor complexes containing a kinase-inactive type I receptor were endocytosed similarly to wild type complexes, the kinase activity of the type II TGFbeta receptor was necessary for optimal internalization and receptor down-regulation. Furthermore, these responses were shown to occur independently of type II receptor autophosphorylation but require a type II receptor capable of transphosphorylation.

Partial activation of the insulin receptor kinase domain by juxtamembrane autophosphorylation

Increased enzymatic activity of receptor tyrosine kinases occurs after trans-phosphorylation of one or two tyrosines in the activation loop, located near the catalytic cleft. Partial activation of the insulin receptor's kinase domain was observed at dilute concentrations of kinase, suggesting that cis-autophosphorylation was occurring. Autophosphorylation during partial activation mapped to the juxtamembrane (JM) tyrosines and not to activation loop tyrosines. Furthermore, a double JM Tyr-to-Phe mutant kinase (JMY2F) did not undergo partial activation but catalyzed substrate phosphorylation at a very low rate. Steady-state kinetics of peptide phosphorylation were determined with and without JM autophosphorylation. The JMY2F mutant was used to prevent concurrent cis-autophosphorylation and therefore to approximate the basal state apoenzyme in the kinetic analysis. Partial activation was dominated by a decreased Michaelis constant for peptide substrate, from KM,PEP >/= 2.5 mM in the basal state to 0.2 mM in the partially activated state; the KM,ATP remained virtually unchanged at approximately 1 mM, and kcat increased from 180 to 600 min-1. The high KM,PEP suggests weak binding of peptide substrates to the apoenzyme. This was confirmed by Ki > 1 mM for peptide substrates used as inhibitors of JM autophosphorylation. The absence of comparably large changes in kcat and KM,ATP suggests that the JM region is primarily a strong barrier to the peptide entry step of trans-phosphorylation reactions. The JM region therefore functions as an intrasteric inhibitor in the basal state of the insulin receptor's kinase domain.

A noninternalized nondesensitized truncated AT1A receptor transduces an amplified ANG II signal

The structural determinants of the rat angiotensin (ANG) II AT1A receptor involved in receptor internalization, desensitization, and activation are investigated by producing six mutants that had progressively larger deletions of the cytoplasmic tail (-13, -19, -24, -31, -46, and -56 residues, respectively). After stable transfection of the cDNAs into Chinese hamster ovary cells, all mutants, except the most truncated, exhibit normal [Sar1]ANG II affinities [dissociation constant (Kd) = 0.19-0.70 nM] compared with the wild-type (WT) receptor (Kd = 0.62 nM) and are able to activate a Gq/11 protein and a phospholipase C as measured by the ANG II-induced inositol phosphate (IP) turnover in the different clones. However, one of these mutants, delta 329 (deletion of 31 residues), exhibits a peculiar phenotype. This mutant shows a reduced ligand-induced internalization as measured by the acid-washing procedure (only 32% of receptors are internalized vs. 83% for WT). Moreover, the delta 329 mutant is less desensitized by a pretreatment with either ANG II (15% desensitization of ANG II-stimulated IP turnover vs. 60% for WT receptor) or the phorbol ester phorbol 12-myristate 13-acetate (no desensitization vs. 29% for WT receptor). These functional modifications of the delta 329 mutant are associated with the transduction of an amplified signal as demonstrated on both IP turnover and an integrated physiological effect of ANG II. Taken together, these data indicate that the sequence 329SLSTKMS335 of the rat AT1A receptor is involved in both receptor internalization and desensitization. This is the first demonstration that a desensitization- and internalization-defective AT1A receptor mutant is also hyperreactive and mediates augmented cellular responses.

Structure and function of the insulin-like growth factor I receptor

Insulin-like growth factors I and II (IGF-I, IGF-II) were originally identified as potent mitogens and as the mediators of growth hormone action. Besides being mitogenic, however, these polypeptide growth factors play a crucial role in cell survival, and contribute to transformation and to maintenance of the malignant phenotype. Here we will discuss signaling by the IGFs, focusing specifically on the structure and function of the IGF-I receptor and the domains of this receptor responsible for distinct IGF functions: mitogenesis, transformation, and protection from apoptosis. We will also compare the structural domains of the related but functionally distinct receptor for insulin.

Distinct endocytic responses of heteromeric and homomeric transforming growth factor beta receptors

Transforming growth factor beta (TGF beta) family ligands initiate a cascade of events capable of modulating cellular growth and differentiation. The receptors responsible for transducing these cellular signals are referred to as the type I and type II TGF beta receptors. Ligand binding to the type II receptor results in the transphosphorylation and activation of the type I receptor. This heteromeric complex then propagates the signal(s) to downstream effectors. There is presently little data concerning the fate of TGF beta receptors after ligand binding, with conflicting reports indicating no change or decreasing cell surface receptor numbers. To address the fate of ligand-activated receptors, we have used our previously characterized chimeric receptors consisting of the ligand binding domain from the granulocyte/macrophage colony-stimulating factor alpha or beta receptor fused to the transmembrane and cytoplasmic domain of the type I or type II TGF beta receptor. This system not only provides the necessary sensitivity and specificity to address these types of questions but also permits the differentiation of endocytic responses to either homomeric or heteromeric intracellular TGF beta receptor oligomerization. Data are presented that show, within minutes of ligand binding, chimeric TGF beta receptors are internalized. However, although all the chimeric receptor combinations show similar internalization rates, receptor down-regulation occurs only after activation of heteromeric TGF beta receptors. These results indicate that effective receptor down-regulation requires cross-talk between the type I and type II TGF beta receptors and that TGF beta receptor heteromers and homomers show distinct trafficking behavior.

A peptide derived from an extracellular domain selectively inhibits receptor internalization: target sequences on insulin and insulin-like growth factor 1 receptors

Certain peptides derived from the alpha1 domain of the major histocompatibility class I antigen complex (MHC-I) inhibit receptor internalization, increasing the steady-state number of active receptors on the cell surface and thereby enhancing the sensitivity to hormones and other agonists. These peptides self-assemble, and they also bind to MHC-I at the same site from which they are derived, suggesting that they could bind to receptor sites with significant sequence similarity. Receptors affected by MHC-I peptides do, indeed, have such sequence similarity, as illustrated here by insulin receptor (IR) and insulin-like growth factor-1 receptor. A synthetic peptide with sequence identical to a certain extracellular receptor domain binds to that receptor in a ligand-dependent manner and inhibits receptor internalization. Moreover, each such peptide is selective for its cognate receptor. An antibody to the IR peptide not only binds to IR and competes with the peptide but also inhibits insulin-dependent internalization of IR. These observations, and binding studies with deletion mutants of IR, indicate that the sequence QILKELEESSF encoded by exon 10 plays a key role in IR internalization. Our results illustrate a principle for identifying receptor-specific sites of importance for receptor internalization, and for enhancing sensitivity to hormones and other agonists.

Insulin signal transduction by a mutant human insulin receptor lacking the NPEY sequence. Evidence for an alternate mitogenic signaling pathway that is independent of Shc phosphorylation

The cytoplasmic juxtamembrane domain of the human insulin receptor (hIR) contains a single copy of the tetrameric amino acid sequence Asn-Pro-Glu-Tyr (NPEY) (residues 969-972 in the exon 11-containing B-isoform), which exists in the context of NPXY. In this study, we examined the role of NPEY972 in mediating insulin signal transduction and cellular biological effects. Transfected Chinese hamster ovary cell lines expressing either the wild-type hIR-B isoform (hIR.WT) or a mutant receptor lacking the NPEY972 sequence (hIRDeltaNPEY) and control Chinese hamster ovary.Neo cells were used to comparatively analyze the following insulin effects: in vivo receptor tyrosine autophosphorylation and kinase activity, signal transduction to downstream signaling molecules, and stimulation of glycogen and DNA synthesis. The results showed that in comparison to hIR.WT, the hIRDeltaNPEY mutant demonstrated the following: (a) normal insulin-mediated receptor tyrosine phosphorylation, but approximately 50% reduction in phosphorylation of p185-(insulin receptor substrate-1) and binding of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase), (b) an enhanced stimulation of PI 3-kinase enzymatic activity, (c) a complete inability to phosphorylate Shc, (d) minimal impairment of insulin sensitivity for glycogen synthesis, and (e) an augmented response to insulin-stimulated DNA synthesis via a high capacity, low sensitivity pathway. These results demonstrate the following: 1) the NPEY972 sequence is important but not absolutely essential for coupling of hIR kinase to insulin receptor substrate-1 and p85 or for mediating insulin's metabolic and mitogenic effects, 2) the NPEY972 sequence is necessary for Shc phosphorylation, and 3) the absence of Shc phosphorylation releases the constraints on maximal insulin-stimulated mitogenic response, thus indicating that alternate signaling pathway(s) exist for this insulin action. This alternate pathway appears to be associated with enhanced activation of PI 3-kinase and is of high capacity and low sensitivity.

Dual role of a dileucine motif in insulin receptor endocytosis

Two leucines (Leu986 and Leu987) have recently been shown to take part in the control of human insulin receptor (HIR) internalization (Renfrew-Haft, C., Klausner, R. D., and Taylor, S. I. (1994) J. Biol. Chem. 269, 26286-26294). The aim of the present study was to further investigate the exact mechanism of this control process. Constitutive and insulin-induced HIR internalizations were studied biochemically and morphologically in NIH 3T3 cells overexpressing either a double alanine (amino acid residues 986-987) mutant HIR (HIR AA1) or HIR truncated at either amino acid residue 981 (HIR Delta981) or 1000 (HIR Delta1000). Data collected indicate that: (a) the three mutant HIR show a reduced association with microvilli as compared with HIR wild-type; (b) the two receptors containing the dileucine motif (HIR WT and HIR Delta1000) show the highest propensity to associate with clathrin-coated pits, independently of kinase activation; (c) the two receptors lacking the dileucine motif but containing two tyrosine-based motifs, previously described as participating in clathrin-coated pit segregation, associate with these surface domains with a lower affinity than the two others, (d) in the presence of the kinase domain, an unmasking of the tyrosine-based motifs mediated by kinase activation is required. These results indicate that the dileucine motif is not sufficient by itself, but participates in anchoring HIR on microvilli and that another sequence, located downstream from position 1000 is crucial for this event. This dileucine motif also plays a role in HIR segregation in clathrin-coated pits. This latter function is additive with that of the tyrosine-based motifs but the role of the dileucine motif predominates. Eventually, the clathrin-coated pit anchoring function of the dileucine motif is independent of receptor kinase activation in contrast to the tyrosine-based motifs.

Cis-autophosphorylation of juxtamembrane tyrosines in the insulin receptor kinase domain

Receptor tyrosine kinases undergo ligand-induced dimerization that promotes kinase domain trans-autophosphorylation. However, the kinase domains of the insulin receptor are effectively dimerized because of the covalent alpha2beta2 holomeric structure. This fact has made it difficult to determine the molecular mechanism of intraholomeric autophosphorylation, but there is evidence for both cis- and trans-autophosphorylation in the absence and presence of insulin. Here, using the cytoplasmic kinase domain (CKD) of the human insulin receptor, we demonstrate that autophosphorylation in the juxtamembrane (JM) subdomain follows a cis-reaction pathway. JM autophosphorylation was independent of CKD concentration over the range 6 nM-3 microM and was characterized kinetically: Half-saturation (K(ATP)) was observed at 75 microM ATP [5 mM Mn(CH3CO2)2] with a maximal rate of 0.24 mol of PO4 (mol of CKD)(-1) min(-1). Pairwise substitutions of Phe for Tyr in the other two autophosphorylation subdomains, generated by site-directed mutagenesis, altered the kinetics of JM autophosphorylation but did not change the pathway from a cis-reaction. Tyr(1328,1334) to Phe (in the carboxy-terminal subdomain) yielded <2-fold increase in the efficiency of JM autophosphorylation, whereas Tyr(1162,1163) to Phe (in the activation loop subdomain) yielded approximately 38-fold increased efficiency of JM autophosphorylation, due predominantly to a 23-fold decreased K(ATP). These findings demonstrate basal state binding of ATP to the CKD leading to cis-autophosphorylation and novel basal state regulatory interactions among the subdomains of the insulin receptor kinase. On the basis of these results and the crystal structure of the conserved catalytic core of this kinase [Hubbard, S. R., et al. (1994) Nature 372, 746], a model is proposed which reconciles the JM cis-reaction and the activation loop cis-inhibition/trans-reaction with the complex kinetics of insulin receptor autophosphorylation [Kohanski, R. A. (1993) Biochemistry 32, 5766].

Identification of cytoplasmic motifs required for short prolactin receptor internalization

Cloning of rat prolactin receptor (PRLR) cDNAs revealed the existence of two isoforms, termed short and long according to the length of their cytoplasmic domain. Internalization studies show, first, that PRLR internalization is hormone-dependent and, second, that ligand-receptor complexes of the short PRLR are internalized to a larger extent compared to the long form. In order to identify regions within the cytoplasmic domain of the short PRLR required for efficient internalization, serial truncations of the cytoplasmic tail were performed by inserting a stop codon in place of those encoding residues 282, 273, 262, 253, 244, or 237 (wild type short PRLR contains 291 amino acids). Our data show that two motifs, lying within residues 253-261 and 273-281, are involved in internalization. Both regions contain a consensus feature identified within other receptors as internalization signals, namely a di-leucine peptide (amino acids 259-260) and a tetrapeptide predicted to adopt a beta-turn structure (amino acids 276-279). We propose these two motifs are involved in PRLR endocytosis. Finally, we show that alpha-adaptin, a component of adaptor protein AP-2, coprecipitates with short PRLR complexes upon PRL stimulation, which strongly suggests that PRLR internalization is mediated by the clathrin-coated pits endocytotic pathway.

Specificity of LIM domain interactions with receptor tyrosine kinases

LIM domains, Cys-rich motifs containing approximately 50 amino acids found in a variety of proteins, are proposed to direct protein*protein interactions. To identify structural targets recognized by LIM domains, we have utilized random peptide library selection, the yeast two-hybrid system, and glutathione S-transferase fusions. Enigma contains three LIM domains within its carboxyl terminus and LIM3 of Enigma specifically recognizes active but not mutant endocytic codes of the insulin receptor (InsR) (Wu, R. Y., and Gill, G. N. (1994) J. Biol. Chem. 269, 25085-25090). Interaction of two random peptide libraries with glutathione S-transferase-LIM3 of Enigma indicated specific binding to Gly-Pro-Hyd-Gly-Pro-Hyd-Tyr-Ala corresponding to the major endocytic code of InsR. Peptide competition demonstrated that both Pro and Tyr residues were required for specific interaction of InsR with Enigma. In contrast to LIM3 of Enigma binding to InsR, LIM2 of Enigma associated specifically with the receptor tyrosine kinase, Ret. Ret was specific for LIM2 of Enigma and did not bind other LIM domains tested. Mutational analysis indicated that the residues responsible for binding to Enigma were localized to the carboxyl-terminal 61 amino acids of Ret. A peptide corresponding to the carboxyl-terminal 20 amino acids of Ret dissociated Enigma and Ret complexes, while a mutant that changed Asn-Lys-Leu-Tyr in the peptide to Ala-Lys-Leu-Ala or a peptide corresponding to exon16 of InsR failed to disrupt the complexes, indicating the Asn-Lys-Leu-Tyr sequence of Ret is essential to the recognition motif for LIM2 of Enigma. We conclude that LIM domains of Enigma recognize tyrosine-containing motifs with specificity residing in both the LIM domains and in the target structures.

Ubiquitination of a yeast plasma membrane receptor signals its ligand-stimulated endocytosis

Binding of alpha factor to Ste2p, a G protein-coupled plasma membrane receptor, activates a signal transduction pathway and stimulates endocytosis of the receptor-ligand complex. Ligand binding also induces ubiquitination of the Ste2p cytoplasmic tail. Protein ubiquitination is required for stimulated endocytosis of Ste2p, as internalization is 5- to 15-fold slower in ubc mutants that lack multiple ubiquitin-conjugating enzymes. In a C-terminal truncated form of Ste2p that is rapidly ubiquitinated and endocytosed in response to ligand binding, a single lysine to arginine substitution in its cytoplasmic tail eliminates both ubiquitination and internalization. Thus, ubiquitination of Ste2p itself is required for ligand-stimulated endocytosis. We propose that ubiquitination mediates degradation of receptor-ligand complexes, not via the proteasome, but by acting as a signal for endocytosis leading to subsequent degradation in the lysosome/vacuole.

Mutant insulin receptors in syndromes of insulin resistance

To date, mutations of the insulin receptor remain the only well-established causes of severe insulin resistance. There is a broad correlation between the extent of impairment of signal transduction seen when the mutant receptors are expressed in vitro with the severity of the clinical phenotype. Thus leprechaunism, Rabson-Mendenhall syndrome and Type A insulin resistance appear to represent points on a continuum of severity of receptor dysfunction, rather than completely distinct syndromes. In other syndromes of insulin resistance, insulin receptor abnormalities remain the exception. However, functional studies of expressed naturally occurring insulin receptor mutations have acted as experiments of nature and greatly aided attempts to dissect the structure-function relationships of the receptor. The next few years will no doubt begin to reveal the contributions made by defects in the post-receptor signalling cascade to the syndromes of insulin resistance in man.

The regions of the Fe65 protein homologous to the phosphotyrosine interaction/phosphotyrosine binding domain of Shc bind the intracellular domain of the Alzheimer's amyloid precursor protein

Fe65 is a protein mainly expressed in several districts of the mammalian nervous system. The search of protein sequence data banks revealed that Fe65 contains two phosphotyrosine interaction (PID) or phosphotyrosine binding (PTB) domains, previously identified in the Shc adaptor molecule. The two putative PID/PTB domains of Fe65 were used to construct glutathione S-transferase-Fe65 fusion proteins. Co-precipitation experiments demonstrated that the Fe65 PID/PTB domains interacted with several proteins of apparent molecular mass 135, 115, 105, and 51 kDa. The region of Fe65 containing the PID/PTB domains was used as a bait to screen a human brain cDNA library in yeast by the two-hybrid system. Three different cDNA clones were isolated, two of which contain overlapping segments of the cDNA encoding the COOH terminus of the Alzheimer's beta-amyloid-precursor protein (APP), that represents the short intracellular domain of this membrane protein. The third clone contains a cDNA fragment coding for the COOH terminus of the human counterpart of a mouse beta-amyloid-like precursor protein. The alignment of the three APP encoding cDNA fragments found in the screening suggests that the region of APP involved in the binding is centered on the NPTY sequence, which is analogous to that present in the intracellular domains of the growth factor receptors interacting with the PID/PTB domain of Shc.

Receptor-mediated internalization of insulin. Potential role of pp120/HA4, a substrate of the insulin receptor kinase

pp120/HA4 is a hepatocyte membrane glycoprotein phosphorylated by the insulin receptor tyrosine kinase. In this study, we have investigated the role of pp120/HA4 in insulin action. Transfection of antisense pp120/HA4 cDNA in H35 hepatoma cells resulted in inhibition of pp120/HA4 expression and was associated with a 2-3-fold decrease in the rate of insulin internalization. Furthermore, insulin internalization in NIH 3T3 fibroblasts co-transfected with insulin receptors and pp120/HA4 was increased 2-fold compared with cells expressing insulin receptors alone. In contrast, no effect on internalization was observed in cells overexpressing a naturally occurring splice variant of pp120/HA4 that lacks the phosphorylation sites in the intracellular domain. Insulin internalization was also unaffected in cells expressing three site-directed mutants of pp120/HA4 in which the sites of phosphorylation by the insulin receptor kinase had been removed (Y488F, Y488F/Y513F, and S503A). Our data suggest that pp120/HA4 is part of a complex of proteins required for receptor-mediated internalization of insulin. It is possible that this function is regulated by insulin-induced phosphorylation of the intracellular domain of pp120/HA4.

Thyrotropin internalization is directed by a highly conserved motif in the seventh transmembrane region of its receptor

The thyrotropin (TSH) receptor is a member of G protein-coupled seven-transmembrane-segment receptors. It is characterized by a large extracellular domain linked to the seven transmembrane segments and ending with a cytoplasmic tail. Sequence alignment shows that a highly conserved motif, NPXXY where X is any amino acid, exists at the boundary between the seventh transmembrane domain and proximal part of the cytoplasmic tail of virtually all G protein-coupled receptors. This motif has been implicated as an internalization signal for several cell surface receptors, such as the low density lipoprotein (LDL), insulin and insulin-like growth factor-1 (IGF-1) receptors. The potential effects of this motif on the TSH receptor signal transduction and receptor-mediated TSH internalization was analysed by replacement of the tyrosine(678) residue with an alanine residue. This mutation does not impair high affinity TSH binding, but completely abolishes the ability of cAMP response upon TSH stimulation. It also significantly reduces TSH internalization. The role of the cytoplasmic tail of the TSH receptor in receptor-mediated internalization was also assessed. Deletion of up to 56 amino acids from the C-terminus of the cytoplasmic tail enhances TSH internalization as compared to the wild-type receptor. We conclude that tyrosine(678) in the NPXXY motif is required for efficient receptor-mediated TSH internalization and G protein coupling. The cytoplasmic tail of the TSH receptor may contain sequence domains which could modulate the effects of the NPXXY internalization signal.

Interaction of atrial natriuretic peptide with its receptors in bovine lung membranes

In bovine lung membranes, atrial natriuretic peptide (ANP) showed temperature-dependent binding to guanylate cyclase-natriuretic peptide receptor (NPR-GC). Photoaffinity labeling of the receptors with 4-azidobenzoyl (AZB)-125I-ANP and competitive binding studies with 125I-ANP, ANP, and atriopeptin I (API) revealed that NPR-GC was detected as the predominant ANP-binding protein at 0 degrees C, whereas at 37 degrees C natriuretic peptide clearance receptor (NPR-C) was detected as the predominant protein. The ratio of NPR-GC and NPR-C was 89:11 at 0 degrees C for 40 min, respectively, whereas 6:94 at 37 degrees C. AZB-125I-ANP bound to NPR-GC dissociated from the binding site within 5 min at 37 degrees C but not at 0 degrees C, whereas ANP bound to NPR-C did not dissociate from the binding site at 0 and 37 degrees C. The dissociated AZB-125I-ANP rapidly rebound to NPR-GC at 37 degrees C but not to NPR-C, and the dissociated NPR-GC was capable of binding. Some AZB-125I-ANP was hydrolyzed by a membrane-bound proteinase(s). Phosphoramidon inhibited the hydrolysis of AZB-125I-ANP. Thus, the dissociated AZB-125I-ANP rebound to NPR-GC and NPR-C. These results suggest that usually intact ANP repeatedly binds to NPR-GC until hydrolysis. Furthermore, the majority of ANP bind to NPR-GC before binding to NPR-C under physiological temperature.

Association of epidermal growth factor receptors with coated pit adaptins via a tyrosine phosphorylation-regulated mechanism

We investigated the mechanism by which ligand-activated epidermal growth factor receptors (EGFR) associate with coated pit adaptor protein (AP) complexes. In vivo association, assayed by coimmunoprecipitation of AP with mutant EGFR, required tyrosine kinase activity, intact autophosphorylation sites, and the regulatory carboxyl terminus of EGFR. The role of autophosphorylation of EGFR in interaction with AP was examined in vitro using a BIAcore instrument. Purified active EGFR, immobilized on the biosensor surface, was reversibly autophosphorylated or dephosphorylated by treatment with ATP or phosphatase. Autophosphorylation of EGFR significantly increased AP binding. Once formed, EGFR AP complexes were resistant to disassembly by dephosphorylation of EGFR or competition with phosphotyrosine, indicating that phosphorylated tyrosine residues do not directly participate in AP binding. Induction of conformational changes in EGFR by treatment with urea increased AP binding up to 10-fold in the absence of EGFR autophosphorylation. A recombinant EGFR carboxyl terminus specifically bound the AP complex and each of the isolated alpha- and beta-subunits of AP2. We conclude that tyrosine autophosphorylation of EGFR exposes structural motif(s) in the carboxyl terminus of EGFR that interact specifically with AP2.

Insulin receptor kinase activation releases a constraint maintaining the receptor on microvilli

To examine whether the surface redistribution of the insulin receptor from microvilli, where it sits in its unoccupied form, to the nonvillous domain, where it is internalized through clathrin-coated pits, is an active movement or a passive redistribution linked to the release of a restraint maintaining it on microvilli, we have generated a mutated insulin receptor with a truncation of exons 17-22 and tracked it biochemically and morphologically. Biochemical analysis indicates that this mutated receptor is constitutively internalized and recycled even in the absence of ligand. Quantitative electron microscope autoradiography analysis reveals that it does not preferentially associate with microvilli in its unoccupied form but is normally segregated in clathrin-coated pits through the preserved signal sequence(s) of exon 16. We conclude that (a) insulin receptor internalization initiated through receptor kinase activation and autophosphorylation, which free the receptor from constraints maintaining it on microvilli; (b) the signal sequences contained in exon 16 are entirely sufficient to promote clathrin-coated pit-mediated internalization of insulin receptors; (c) these sequences are not uncovered by kinase activation; and (d) the "code" maintaining the unoccupied receptors on microvilli is contained within exons 17-21 of the receptor.

Role of the transmembrane domain and flanking amino acids in internalization and down-regulation of the insulin receptor

We have characterized the internalization and down-regulation of the insulin receptor and nine receptors with mutations in the transmembrane (TM) domain and/or flanking charged amino acids to define the role of this domain in receptor cycling. When expressed in Chinese hamster ovary cells, all had normal tetrameric structure and normal insulin-stimulated autophosphorylation/kinase activity. Replacement of the TM domain with that of the platelet-derived growth factor receptor, insertion of 3 amino acids, and substitution of Asp for Val938 or of Ala for either Gly933 or Pro934 had no effect on internalization. Replacement of the TM domain with that of c-neu or conversion of the charged amino acids on the cytoplasmic flank to uncharged amino acids, on the other hand, resulted in a 40-60% decrease in insulin-dependent internalization rate constants. By contrast, substitution of Ala for both Gly933 and Pro934 increases lateral diffusion mobility and accelerates internalization rate. These changes in internalization were due to decreased or increased rates of redistribution of receptors from microvilli to the nonvillous cell surface. In all cases, receptor down-regulation and receptor-mediated insulin degradation paralleled the changes in internalization. Thus, the structure of the transmembrane domain of the insulin receptor and flanking amino acids are major determinants of receptor internalization, insulin degradation, and receptor down-regulation.

An irregularity in the transmembrane domain helix correlates with the rate of insulin receptor internalization

Internalization of insulin and its receptor via receptor-mediated endocytosis is an important step in insulin-induced signal transduction. To investigate the structural determinants underlying the enhanced internalization rate observed for the insulin receptor transmembrane mutant Gly933-Pro934-->Ala-Ala (GP-->AA), we have designed and chemically synthesized two peptides, IR(TM)-GP and IR-(TM)-AA, corresponding respectively to the N-terminal portion of the wild-type and the mutant insulin receptor TM segment containing these sites. Conformational studies by circular dichroism (CD) spectroscopy on these two peptides in their monomeric states revealed that peptide IR(TM)-GP forms an irregular helix in the membrane-mimetic environments of sodium dodecyl sulfate (SDS) micelles with a possible "kink" in the helix imposed by its Gly-Pro sequence, while peptide IR(TM)-AA assumes largely classical alpha-helical structure under corresponding conditions. The helical pattern of peptide IR(TM)-AA was maintained at elevated temperatures, while the shape of the CD curve for peptide IR(TM)-GP was found to alter as a function of temperature. At higher concentrations, both peptides formed high molecular weight aggregates in SDS micelles, as demonstrated by SDS-PAGE gels, but peptide IR(TM)-AA was shown to aggregate more readily and more extensively than peptide IR(TM)-GP. Fluorescent dye-leakage experiments indicated that peptide IR(TM)-GP produces an enhanced disruption of the membrane bilayer in phosphatidylglycerol vesicles vs that induced by IR(TM)-AA.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin receptor internalization: molecular mechanisms and physiopathological implications

The initial interaction between insulin and its receptor on target cell surface is followed by a series of surface and intracellular steps which participate in the control of insulin action. Abnormalities of any of these steps could result in mishandling of the receptor leading to defective modulation of receptor number on the cell surface and to inappropriate cell sensitivity to the hormone. Thus, the identification of each of these steps as well as understanding the mechanisms governing them is obligatory to unravel some aspects of the pathogenesis of insulin resistance states. This was the goal of the studies we have carried out during recent years using combined molecular and cellular biology as well as biochemical techniques. These studies allowed us to propose the following ordered sequence of events: 1) insulin binds to receptors preferentially associated with microvilli on the cell surface; 2) insulin triggers receptor kinase activation and autophosphorylation which not only results in initiation of the various biological signals leading to insulin action but also in redistribution of the hormone-receptor complex in the plane of the membrane; 3) on the non-villous domain of the cell surface, insulin receptors anchor to clathrin-coated pits through specific "internalization sequences" present in their cytoplasmic juxtamembrane domain; 4) insulin-receptor complexes are internalized together with other receptors present in the same clathrin-coated pits through the formation of clathrin-coated vesicles; 5) the complexes are delivered to endosomes, the acidic pH of which induces the dissociation of insulin molecules from insulin receptors and their sorting in different directions; 6) insulin molecules are targetted to late endosomes and lysosomes where they are degraded; 7) receptors are recycled back to the cell surface in order to be reused.