Insulin-induced surface redistribution regulates internalization of the insulin receptor and requires its autophosphorylation

An overview of receptor endocytosis and signaling

Endocytosis is a cellular process which mediates receptor internalization, nutrient uptake, and the regulation of cell signaling. Microorganisms (many bacteria and viruses) and toxins also use the same process and enter the cells. Generally, endocytosis is considered in the three forms such as phagocytosis (cell eating), pinocytosis (cell drinking), and highly selective receptor-mediated endocytosis (clathrin-dependent and independent). Several endocytic routes exist in an analogous, achieving diverse functions. Most studies on endocytosis have used transformed cells in culture. To visualize the receptor internalization, trafficking, and signaling in subcellular organelles, a green fluorescent protein-tagged receptor has been utilized. It also helps to visualize the endocytosis effects in live-cell imaging. Confocal laser microscopy increases our understanding of receptor endocytosis and signaling. Site-directed mutagenesis studies demonstrated that many short-sequence motifs of the cytoplasmic domain of receptors significantly play a vital role in receptor internalization, subcellular trafficking, and signaling. However, other factors also regulate receptor internalization through clathrin-coated vesicles. Receptor endocytosis can occur through clathrin-dependent and clathrin-independent pathways. This chapter briefly discusses the internalization, trafficking, and signaling of various receptors in normal conditions. In addition, it also highlights the malfunction of the receptor in disease conditions.

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.

A covalently linked probe to monitor local membrane properties surrounding plasma membrane proteins

Functional membrane proteins in the plasma membrane are suggested to have specific membrane environments that play important roles to maintain and regulate their function. However, the local membrane environments of membrane proteins remain largely unexplored due to the lack of available techniques. We have developed a method to probe the local membrane environment surrounding membrane proteins in the plasma membrane by covalently tethering a solvatochromic, environment-sensitive dye, Nile Red, to a GPI-anchored protein and the insulin receptor through a flexible linker. The fluidity of the membrane environment of the GPI-anchored protein depended upon the saturation of the acyl chains of the lipid anchor. The local environment of the insulin receptor was distinct from the average plasma membrane fluidity and was quite dynamic and heterogeneous. Upon addition of insulin, the local membrane environment surrounding the receptor specifically increased in fluidity in an insulin receptor-kinase dependent manner and on the distance between the dye and the receptor.

Spatial dynamics of the insulin receptor in living neurons

Insulin signaling through the insulin receptor has long been studied in classic target organs, such as adipose tissue and skeletal muscle, where one of its effects is to increase glucose uptake. Insulin and insulin receptor are present in many areas of the brain, but the functions of brain insulin signaling outside feeding circuits are not well defined. It has been proposed that hippocampal insulin signaling is important for memory, that brain insulin signaling is deficient in Alzheimer's disease, and that intranasal insulin treatment improves cognition, but the mechanisms remain unclear and do not seem to involve increased glucose uptake by neurons. The molecular behavior of the insulin receptor itself is not well known in living neurons; therefore, we investigated the spatial dynamics of the insulin receptor on somatodendritic membranes of live rat hippocampal neurons in culture. Using single-molecule tracking of quantum dot-tagged insulin receptors and single-particle tracking photoactivation localization microscopy, we show that the insulin receptor is distributed over both dendritic shafts and spines. Using colocalization with synaptic markers, we also show that in contrast to the glutamate receptor subunit glutamate receptor subunit A1, the dynamics of the insulin receptor are not affected by association with excitatory synapses; however, the insulin receptor is immobilized by components of inhibitory synapses. The mobility of the insulin receptor is reduced both by low concentrations of the pro-inflammatory cytokine tumor necrosis factor α and by cholesterol depletion, suggesting an association with sphingolipid-rich membrane domains. On the other hand, the insulin receptor dynamics in hippocampal neurons are not affected by increased excitatory signaling. Finally, using real-time single-event quantification, we find evidence of strong insulin receptor exocytosis on dendritic shafts. Our results suggest an association of the neuronal insulin receptor with specific elements of the dendritic shaft, rather than excitatory synapses.

The cell biology of systemic insulin function

Tokarz et al. review the cell biology of insulin physiology throughout the body, from synthesis to the delivery, action, and final degradation of insulin.

Insulin is the paramount anabolic hormone, promoting carbon energy deposition in the body. Its synthesis, quality control, delivery, and action are exquisitely regulated by highly orchestrated intracellular mechanisms in different organs or “stations” of its bodily journey. In this Beyond the Cell review, we focus on these five stages of the journey of insulin through the body and the captivating cell biology that underlies the interaction of insulin with each organ. We first analyze insulin’s biosynthesis in and export from the β-cells of the pancreas. Next, we focus on its first pass and partial clearance in the liver with its temporality and periodicity linked to secretion. Continuing the journey, we briefly describe insulin’s action on the blood vasculature and its still-debated mechanisms of exit from the capillary beds. Once in the parenchymal interstitium of muscle and adipose tissue, insulin promotes glucose uptake into myofibers and adipocytes, and we elaborate on the intricate signaling and vesicle traffic mechanisms that underlie this fundamental function. Finally, we touch upon the renal degradation of insulin to end its action. Cellular discernment of insulin’s availability and action should prove critical to understanding its pivotal physiological functions and how their failure leads to diabetes.

High-resolution imaging and quantification of plasma membrane cholesterol by NanoSIMS

Cholesterol is a crucial lipid within the plasma membrane of mammalian cells. Recent biochemical studies showed that one pool of cholesterol in the plasma membrane is "accessible" to binding by a modified version of the cytolysin perfringolysin O (PFO*), whereas another pool is sequestered by sphingomyelin and cannot be bound by PFO* unless the sphingomyelin is destroyed with sphingomyelinase (SMase). Thus far, it has been unclear whether PFO* and related cholesterol-binding proteins bind uniformly to the plasma membrane or bind preferentially to specific domains or morphologic features on the plasma membrane. Here, we used nanoscale secondary ion mass spectrometry (NanoSIMS) imaging, in combination with ¹⁵N-labeled cholesterol-binding proteins (PFO* and ALO-D4, a modified anthrolysin O), to generate high-resolution images of cholesterol distribution in the plasma membrane of Chinese hamster ovary (CHO) cells. The NanoSIMS images revealed preferential binding of PFO* and ALO-D4 to microvilli on the plasma membrane; lower amounts of binding were detectable in regions of the plasma membrane lacking microvilli. The binding of ALO-D4 to the plasma membrane was virtually eliminated when cholesterol stores were depleted with methyl-β-cyclodextrin. When cells were treated with SMase, the binding of ALO-D4 to cells increased, largely due to increased binding to microvilli. Remarkably, lysenin (a sphingomyelin-binding protein) also bound preferentially to microvilli. Thus, high-resolution images of lipid-binding proteins on CHO cells can be acquired with NanoSIMS imaging. These images demonstrate that accessible cholesterol, as judged by PFO* or ALO-D4 binding, is not evenly distributed over the entire plasma membrane but instead is highly enriched on microvilli.

Fenofibrate Decreases Insulin Clearance and Insulin Secretion to Maintain Insulin Sensitivity

High fat diet reduces the expression of CEACAM1 (carcinoembryonic antigen-related cell adhesion molecule 1), a transmembrane glycoprotein that promotes insulin clearance and down-regulates fatty acid synthase activity in the liver upon its phosphorylation by the insulin receptor. Because peroxisome proliferator-activated receptor α (PPARα) transcriptionally suppresses CEACAM1 expression, we herein examined whether high fat down-regulates CEACAM1 expression in a PPARα-dependent mechanism. By activating PPARα, the lipid-lowering drug fenofibrate reverses dyslipidemia and improves insulin sensitivity in type 2 diabetes in part by promoting fatty acid oxidation. Despite reducing glucose-stimulated insulin secretion, fenofibrate treatment does not result in insulin insufficiency. To examine whether this is mediated by a parallel decrease in CEACAM1-dependent hepatic insulin clearance pathways, we fed wild-type and Pparα^-/- null mice a high fat diet supplemented with either fenofibrate or Wy14643, a selective PPARα agonist, and examined their effect on insulin metabolism and action. We demonstrated that the decrease in insulin secretion by fenofibrate and Wy14643 is accompanied by reduction in insulin clearance in wild-type but not Pparα^-/- mice, thereby maintaining normoinsulinemia and insulin sensitivity despite continuous high fat intake. Intact insulin secretion in L-CC1 mice with protected hepatic insulin clearance and CEACAM1 levels provides in vivo evidence that insulin secretion responds to changes in insulin clearance to maintain physiologic insulin and glucose homeostasis. These results also emphasize the relevant role of hepatic insulin extraction in regulating insulin sensitivity.

Insulin Signaling in the Control of Glucose and Lipid Homeostasis

A continuous supply of glucose is necessary to ensure proper function and survival of all organs. Plasma glucose levels are thus maintained in a narrow range around 5 mM, which is considered the physiological set point. Glucose homeostasis is controlled primarily by the liver, fat, and skeletal muscle. Following a meal, most glucose disposals occur in the skeletal muscle, whereas fasting plasma glucose levels are determined primarily by glucose output from the liver. The balance between the utilization and production of glucose is primarily maintained at equilibrium by two opposing hormones, insulin and glucagon. In response to an elevation in plasma glucose and amino acids (after consumption of a meal), insulin is released from the beta cells of the islets of Langerhans in the pancreas. When plasma glucose falls (during fasting or exercise), glucagon is secreted by α cells, which surround the beta cells in the pancreas. Both cell types are extremely sensitive to glucose concentrations, can regulate hormone synthesis, and are released in response to small changes in plasma glucose levels. At the same time, insulin serves as the major physiological anabolic agent, promoting the synthesis and storage of glucose, lipids, and proteins and inhibiting their degradation and release back into the circulation. This chapter will focus mainly on signal transduction mechanisms by which insulin exerts its plethora of effects in liver, muscle, and fat cells, focusing on those pathways that are crucial in the control of glucose and lipid homeostasis.

The last enzyme of the de novo purine synthesis pathway 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase (ATIC) plays a central role in insulin signaling and the Golgi/endosomes protein network

Insulin is internalized with its cognate receptor into the endosomal apparatus rapidly after binding to hepatocytes. We performed a bioinformatic screen of Golgi/endosome hepatic protein fractions and found that ATIC, which is a rate-limiting enzyme in the de novo purine biosynthesis pathway, and PTPLAD1 are associated with insulin receptor (IR) internalization. The IR interactome (IRGEN) connects ATIC to AMPK within the Golgi/endosome protein network (GEN). Forty-five percent of the IR Golgi/endosome protein network have common heritable variants associated with type 2 diabetes, including ATIC and AMPK. We show that PTPLAD1 and AMPK are rapidly compartmentalized within the plasma membrane (PM) and Golgi/endosome fractions after insulin stimulation and that ATIC later accumulates in the Golgi/endosome fraction. Using an in vitro reconstitution system and siRNA-mediated partial knockdown of ATIC and PTPLAD1 in HEK293 cells, we show that both ATIC and PTPLAD1 affect IR tyrosine phosphorylation and endocytosis. We further show that insulin stimulation and ATIC knockdown readily increase the level of AMPK-Thr172 phosphorylation in IR complexes. We observed that IR internalization was markedly decreased after AMPKα2 knockdown, and treatment with the ATIC substrate AICAR, which is an allosteric activator of AMPK, increased IR endocytosis in cultured cells and in the liver. These results suggest the presence of a signaling mechanism that senses adenylate synthesis, ATP levels, and IR activation states and that acts in regulating IR autophosphorylation and endocytosis.

Inhibition of Rab5 Activation During Insulin Receptor-Mediated Endocytosis

Activation of receptor tyrosine kinases is a key feature in receptor signaling and membrane trafficking processes. In this study, we found that the insulin receptor tyrosine kinase activity is required for fusion between early endosomes. AG1024, a receptor tyrosine kinase inhibitor, blocked the in vitro endosome fusion in a concentration-dependent manner. We observed that Rab5: wild type partially rescued the fusion reaction, whereas Rab5: Q79L mutant fully rescued it. We also observed that treatment of cells with insulin receptor kinase inhibitor HNMPA-(AM)₃ blocked the formation of Rab5-positive endosomes as well as the activation of Rab5 upon addition of insulin in intact cells. HNMPA-(AM)₃ inhibitor also affected the endosomal co-localization of Rab5 and insulin receptor. However, the formation of Rab5: Q79L mutant-positive endosomes were not affected by the HNMPA-(AM)₃ inhibitor. In addition, HNMPA-(AM)₃ inhibitor affected the association of Rin1 to membrane upon insulin stimulation. Furthermore, Rin1 did not fully support endosome fusion in the presence of the AG1024 inhibitor. These results constitute the first evidence that, at least in part, the enzymatic activity of insulin receptor is required for the fusion events via the activation of Rab5.

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

Background

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

Methodology/Principal Findings

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

Conclusion

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

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

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

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

Protein kinase C-alpha regulates insulin action and degradation by interacting with insulin receptor substrate-1 and 14-3-3 epsilon

Protein kinase C (PKC)-alpha exerts a regulatory function on insulin action. We showed by overlay blot that PKCalpha directly binds a 180-kDa protein, corresponding to IRS-1, and a 30-kDa molecular species, identified as 14-3-3epsilon. In intact NIH-3T3 cells overexpressing insulin receptors (3T3-hIR), insulin selectively increased PKCalpha co-precipitation with IRS-1, but not with IRS-2, and with 14-3-3epsilon, but not with other 14-3-3 isoforms. Overexpression of 14-3-3epsilon in 3T3-hIR cells significantly reduced IRS-1-bound PKCalpha activity, without altering IRS-1/PKCalpha co-precipitation. 14-3-3epsilon overexpression also increased insulin-stimulated insulin receptor and IRS-1 tyrosine phosphorylation, followed by increased activation of Raf1, ERK1/2, and Akt/protein kinase B. Insulin-induced glycogen synthase activity and thymidine incorporation were also augmented. Consistently, selective depletion of 14-3-3epsilon by antisense oligonucleotides caused a 3-fold increase of IRS-1-bound PKCalpha activity and a similarly sized reduction of insulin receptor and IRS-1 tyrosine phosphorylation and signaling. In turn, selective inhibition of PKCalpha expression by antisense oligonucleotides reverted the negative effect of 14-3-3epsilon depletion on insulin signaling. Moreover, PKCalpha inhibition was accompanied by a >2-fold decrease of insulin degradation. Similar results were also obtained by overexpressing 14-3-3epsilon. Thus, in NIH-3T3 cells, insulin induces the formation of multimolecular complexes, including IRS-1, PKCalpha, and 14-3-3epsilon. The presence of 14-3-3epsilon in the complex is not necessary for IRS-1/PKCalpha interaction but modulates PKCalpha activity, thereby regulating insulin signaling and degradation.

Lipid microdomains and insulin resistance: is there a connection?

The potential contribution of lipids to insulin signaling has excited interest because of the notion that cholesterol and sphingolipids form functional microdomains-lipid rafts-in cell membranes and that these domains may affect signal transduction. In this Perspective, we discuss the evidence suggesting that cholesterol-sphingolipid rafts play a role in the pathogenesis of insulin resistance. The data relating insulin signaling to lipid rafts in the main insulin target tissues are briefly summarized, including partially controversial findings on the role of caveolae versus other types of rafts. In addition, recent results pointing toward the importance of raft perturbations in the pathogenesis of insulin resistance are discussed. Notably, several studies suggest a correlation between membrane lipid composition and insulin sensitivity. We put forward the idea that the dyslipidemic changes typically associated with insulin resistance and metabolic syndrome may impair the functionality of rafts in insulin target cells, thereby promoting insulin resistance.

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

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

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.

p56Lck anchors CD4 to distinct microdomains on microvilli

Cell-surface microvilli play a central role in adhesion, fusion, and signaling processes. Some adhesion and signaling receptors segregate on microvilli but the determinants of this localization remain mostly unknown. In this study, we considered CD4, a receptor involved in immune response and HIV infection, and p56(Lck), a CD4-associated tyrosine kinase. Analysis of CD4 trafficking reveals that p56(Lck) binds tightly to CD4 independently of its activation state and inhibits CD4 internalization. Electron microscopy analysis established that p56(Lck) mediates CD4 association with microvilli whereas biochemical data indicate that p56(Lck) expression renders CD4 insoluble by the nonionic detergent Triton X-100. In addition, cytoskeleton-disrupting agent increased CD4 solubility, suggesting the involvement of cytoskeletal elements in CD4 anchoring to microvilli. This concept was supported further by the observation that the lateral mobility of CD4 within the plasma membrane was decreased in cells expressing p56(Lck). Finally, isolation of detergent-resistant membranes revealed that the complex CD4-p56(Lck) is enriched within these domains as opposed to conditions in which CD4 does not interact with p56(Lck). In conclusion, our results show that p56(Lck) targets CD4 to specialized lipid microdomains preferentially localized on microvilli. This localization, which prevents CD4 internalization, might facilitate CD4-mediated adhesion processes and could correspond to the signaling site of the receptor.

Dynamic association of human insulin receptor with lipid rafts in cells lacking caveolae

Cholesterol-sphingolipid rich plasma membrane domains, known as rafts, have emerged as important regulators of signal transduction. The adipocyte insulin receptor (IR) is localized to and signals via caveolae that are formed by polymerization of caveolins. Caveolin binds to IR and stimulates signalling. We report that, in liver-derived cells lacking caveolae, autophosphorylation of the endogenous IR is dependent on raft lipids, being compromised by acute cyclodextrin-mediated cholesterol depletion or by antibody clustering of glycosphingolipids. Moreover, we provide evidence that IR becomes recruited to detergent-resistant domains upon ligand binding and that clustering of GM2 ganglioside inhibits IR signalling apparently by excluding the ligand-bound IR from these domains. Our results indicate that, in cells derived from liver, an important insulin target tissue, caveolae are not required for insulin signalling. Rather, the dynamic recruitment of the ligand-bound IR into rafts may serve to regulate interactions in the initiation of the IR signalling cascade.

Role of microvillar cell surfaces in the regulation of glucose uptake and organization of energy metabolism

Experimental evidence suggesting a type of glucose uptake regulation prevailing in resting and differentiated cells was surveyed. This type of regulation is characterized by transport-limited glucose metabolism and depends on segregation of glucose transporters on microvilli of differentiated or resting cells. Earlier studies on glucose transport regulation and a recently presented general concept of influx regulation for ions and metabolic substrates via microvillar structures provide the basic framework for this theory. According to this concept, glucose uptake via transporters on microvilli is regulated by changes in the structural organization of the microfilament bundle, which is acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. Both microvilli formation and the switch of glucose metabolism from "metabolic regulation" to "transport limitation" occur during differentiation. The formation of microvillar cell surfaces creates the essential preconditions to establish the characteristic functions of specialized tissue cells including the coordination between glycolysis and oxidative phosphorylation, regulation of cellular functions by external signals, and Ca(2+) signaling. The proposed concept integrates various aspects of glucose uptake regulation into a ubiquitous cellular mechanism involved in regulation of transmembrane ion and substrate fluxes.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Thyroid hormone regulation of hepatic genes in vivo detected by complementary DNA microarray

The liver is an important target organ of thyroid hormone. However, only a limited number of hepatic target genes have been identified, and little is known about the pattern of their regulation by thyroid hormone. We used a quantitative fluorescent cDNA microarray to identify novel hepatic genes regulated by thyroid hormone. Fluorescent-labeled cDNA prepared from hepatic RNA of T3-treated and hypothyroid mice was hybridized to a cDNA microarray, representing 2225 different mouse genes, followed by computer analysis to compare relative changes in gene expression. Fifty five genes, 45 not previously known to be thyroid hormone-responsive genes, were found to be regulated by thyroid hormone. Among them, 14 were positively regulated by thyroid hormone, and unexpectedly, 41 were negatively regulated. The expression of 8 of these genes was confirmed by Northern blot analyses. Thyroid hormone affected gene expression for a diverse range of cellular pathways and functions, including gluconeogenesis, lipogenesis, insulin signaling, adenylate cyclase signaling, cell proliferation, and apoptosis. This is the first application of the microarray technique to study hormonal regulation of gene expression in vivo and should prove to be a powerful tool for future studies of hormone and drug action.

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.

Identification of a family of sorting nexin molecules and characterization of their association with receptors

Sorting nexin 1 (SNX1) is a protein that binds to the epidermal growth factor (EGF) receptor and is proposed to play a role in directing EGF receptors to lysosomes for degradation (R. C. Kurten, D. L. Cadena, and G. N. Gill, Science 272:1008-1010, 1996). We have obtained full-length cDNAs and deduced the amino acid sequences of three novel homologous proteins, which were denoted human sorting nexins (SNX2, SNX3, and SNX4). In addition, we identified a presumed splice variant isoform of SNX1 (SNX1A). These molecules contain a conserved domain of approximately 100 amino acids, which was termed the phox homology (PX) domain. Human SNX1 (522 amino acids), SNX1A (457 amino acids), SNX2 (519 amino acids), SNX3 (162 amino acids), and SNX4 (450 amino acids) are part of a larger family of hydrophilic molecules including proteins identified in Caenorhabditis elegans and Saccharomyces cerevisiae. Despite their hydrophilic nature, the sorting nexins are found partially associated with cellular membranes. They are widely expressed, although the tissue distribution of each sorting nexin mRNA varies. When expressed in COS7 cells, epitope-tagged sorting nexins SNX1, SNX1A, SNX2, and SNX4 coimmunoprecipitated with receptor tyrosine kinases for EGF, platelet-derived growth factor, and insulin. These sorting nexins also associated with the long isoform of the leptin receptor but not with the short and medium isoforms. Interestingly, endogenous COS7 transferrin receptors associated exclusively with SNX1 and SNX1A, while SNX3 was not found to associate with any of the receptors studied. Our demonstration of a large conserved family of sorting nexins that interact with a variety of receptor types suggests that these proteins may be involved in several stages of intracellular trafficking in mammalian cells.

Redox priming of the insulin receptor beta-chain associated with altered tyrosine kinase activity and insulin responsiveness in the absence of tyrosine autophosphorylation

Induction of tyrosine kinase activity of the insulin receptor (IR) beta-chain is believed to require its autophosphorylation at Tyr1162, Tyr1163, and Tyr1158. However, the mechanism of the initial phosphorylation is poorly understood. We show that treatment of IR-transfected Chinese hamster ovary cells with antioxidants inhibits insulin responsiveness. Conversely, partial inhibition of glutathione biosynthesis by buthionine sulfoximine (BSO) and glutathione reductase by 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), i.e., procedures that intracellularly induce mildly oxidative conditions, caused a decrease in IR beta-chain sulfhydryl groups and enhanced synergistically the induction of IR tyrosine phosphorylation by insulin. The IR beta-chain from cells treated with BSO/BCNU in the absence of insulin was not detectably tyrosine phosphorylated, but nevertheless was functionally altered, as demonstrated in vitro by a moderate kinase activity at lowATP concentrations (5 nM) and a strong kinase activity at 25 microM ATP. This activity was found to be specific for tyrosine (not for serine or threonine), and tryptic peptide maps indicated that it is more selective than that induced by insulin. Moreover, the kinase activity from BSO/BCNU-treated cells showed a spontaneous decay that was not prevented by the phosphatase inhibitor vanadate. Together, these results suggest that optimal insulin responsiveness may require a process of 'redox priming' of the IR beta-chain that involves structural and functional changes in the absence of detectable tyrosine phosphorylation of the beta-chain.

Nef-mediated clathrin-coated pit formation

The sequence of events leading to clathrin-coated pit (CCP) nucleation on the cell surface and to the incorporation of receptors into these endocytic structures is still imperfectly understood. In particular, the question remains as to whether receptor tails initiate the assembly of the coat proteins or whether receptors migrate into preformed CCP. This question was approached through a dissection of the mechanisms implemented by Nef, an early protein of human and simian immunodeficiency virus (HIV and SIV, respectively), to accelerate the endocytosis of cluster of differentiation antigen type 4 (CD4), the major receptor for these viruses. Results collected showed that: (a) Nef promotes CD4 internalization via an increased association of CD4 with CCP; (b) the Nef-mediated increase of CD4 association with CCP is related to a doubling of the plasma membrane area occupied by clathrin-coated structures; (c) this increased CCP number at the plasma membrane has functional consequences preferentially on CD4 uptake and does not significantly affect transferrin receptor internalization or fluid-phase endocytosis; (d) the presence of a CD4 cytoplasmic tail including a critical dileucine motif is required to induce CCP formation via Nef; and (e) when directly anchored to the cytoplasmic side of the plasma membrane, Nef itself can promote CCP formation. Taken together, these observations lead us to propose that CD4 can promote CCP generation via the connector molecule Nef. In this model, Nef interacts on one side with CD4 through a dileucine-based motif present on CD4 cytoplasmic tail and on the other side with components of clathrin-coated surface domain (i.e., adaptins). These Nef-generated complexes would then initiate the nucleation of CCP.

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.

Second-messenger regulation of receptor association with clathrin-coated pits: a novel and selective mechanism in the control of CD4 endocytosis

CD4, a member of the immunoglobulin superfamily, is not only expressed in T4 helper lymphocytes but also in myeloid cells. Receptor-mediated endocytosis plays a crucial role in the regulation of surface expression of adhesion molecules such as CD4. In T lymphocytes p56lck, a CD4-associated tyrosine kinase, prevents CD4 internalization, but in myeloid cells p56lck is not expressed and CD4 is constitutively internalized. In this study, we have investigated the role of cyclic AMP (cAMP) in the regulation of CD4 endocytosis in the myeloid cell line HL-60. Elevations of cellular cAMP were elicited by 1) cholera toxin, 2) pertussis toxin, 3) forskolin and IBMX, 4) NaF, or 5) the physiological receptor agonist prostaglandin E1. All five interventions led to an inhibition of CD4 internalization. Increased cAMP levels did not inhibit endocytosis per se, because internalization of insulin receptors and transferrin receptors and fluid phase endocytosis were either unchanged or slightly enhanced. The mechanism of cAMP inhibition was further analyzed at the ultrastructural level. CD4 internalization, followed either by quantitative electron microscopy autoradiography or by immunogold labeling, showed a rapid and temperature-dependent association of CD4 with clathrin-coated pits in control cells. This association was markedly inhibited in cells with elevated cAMP levels. Thus these findings suggest a second-messenger regulation of CD4 internalization through an inhibition of CD4 association with clathrin-coated pits in p56lck-negative cells.

Wortmannin-sensitive trafficking pathways in Chinese hamster ovary cells. Differential effects on endocytosis and lysosomal sorting

Phosphatidylinositol (PI) 3'-kinases are a family of lipid kinases implicated in the regulation of cell growth by oncogene products and tyrosine kinase growth factor receptors. The catalytic subunit of the p85/p110 PI 3'-kinase is homologous to VPS-34, a phosphatidylinositol-specific lipid kinase involved in the sorting of newly synthesized hydrolases to the yeast vacuole. This suggests that PI 3'-kinases may play analogous roles in mammalian cells. We have measured a number of secretory and endocytic trafficking events in Chinese hamster ovary cells in the presence of wortmannin, a potent inhibitor of PI 3'-kinase. Wortmannin caused a 40-50% down-regulation of surface transferrin receptors, with a dose dependence identical to that required for maximal inhibition of the p85/p110 PI 3'-kinase in intact cells. The redistribution of transferrin receptors reflected a 60% increase in the internalization rate and a 35% decrease in the recycling rate. Experiments with fluorescent transferrin showed that entry of transferrin receptors into the recycling compartment and efflux of receptors out of the compartment were slowed by wortmannin. Wortmannin altered the morphology of the recycling compartment, which was more vesiculated than in untreated cells. Using Semliki Forest virus as a probe, we also found that delivery of the endocytosed virus to its lysosomal site of degradation was slowed by wortmannin, whereas endosomal acidification was unaffected. In contrast to these effects on endocytosis and recycling, wortmannin did not affect intracellular processing of newly synthesized viral spike proteins. Wortmannin did induce missorting of the lysosomal enzyme cathepsin D to the secretory pathway, but only at a dose 20-fold greater than that required to inhibit p85/p110 PI 3'-kinase activity or to redistribute transferrin receptors. Our data demonstrate the presence of wortmannin-sensitive enzymes at three distinct steps of the endocytic cycle in Chinese hamster ovary cells: internalization, transit from early endosomes to the recycling and degradative compartments, and transit from the recycling compartment back to the cell surface. The wortmannin-sensitive enzymes critical for endocytosis and recycling are distinct from those involved in sorting newly synthesized lysosomal enzymes.

Coat proteins and vesicle budding

The trafficking of proteins within eukaryotic cells is achieved by the capture of cargo and targeting molecules into vesicles that bud from a donor membrane and deliver their contents to a receiving department. This process is bidirectional and may involve multiple organelles within a cell. Distinct coat proteins mediate each budding event, serving both to shape the transport vesicle and to select by direct or indirect interaction the desired set of cargo molecules. Secretion, which has been viewed as a default pathway, may require sorting and packaging signals on transported molecules to ensure their rapid delivery to the cell surface.

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.

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.

The cytoplasmic tail of the G-protein-coupled receptor for parathyroid hormone and parathyroid hormone-related protein contains positive and negative signals for endocytosis

The present studies were done to evaluate the role of the cytoplasmic tail of the G-protein-coupled receptor for parathyroid hormone (PTH) and PTH-related protein (PTHrP) in the endocytosis of agonist-occupied receptors. PTH/PTHrP receptor mutants progressively truncated from the C terminus were expressed in COS-7 cells, and their ability to internalize 125I-PTHrP(1-34)amide was determined. Most of the C-terminal tail (91 of 127 residues) could be deleted without affecting internalization. However, further truncation removing residues 475-494 resulted in a 50-60% decrease in ligand internalization. A mutant with an internal deletion of these 20 amino acids showed a similar reduction in internalization, confirming the presence of a positive endocytic signal. No additional positive signals were found in the membrane-proximal region of the tail. However, alanine mutagenesis of the membrane-proximal residues 459-461 (EVQ-->AAA) resulted in a mutant PTH/PTHrP receptor displaying a 40% increase in ligand endocytosis, indicating that EVQ functions as a negative signal. Treatment of COS-7 cells with hypertonic sucrose (to disrupt clathrin lattices) markedly suppressed (by > 80%) PTH/PTHrP receptor internalization. These results demonstrate the presence of both positive and negative endocytic signals in the membrane-proximal cytoplasmic tail of the PTH/PTHrP receptor and suggest that these signals regulate the ability of the receptor to accumulate in clathrin-coated pits.

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.

Deletion of 343 amino acids from the carboxyl terminus of the beta-subunit of the insulin receptor inhibits insulin signaling

Naturally occurring mutations in the insulin receptor gene that impair the receptor tyrosine kinase activity cause insulin resistance in vivo in a dominant fashion. Previously, two unrelated families have been described that express an insulin receptor with a truncation due to a premature chain termination at codon 1000 (delta 1000), thereby deleting 343 amino acids from the carboxyl terminus of the beta-subunit. While clinical findings suggest that the truncated receptor does not mediate insulin action in vivo, a recent study suggested that a similarly truncated receptor enhanced insulin sensitivity in transfected cells by augmenting the signaling by endogenous receptors [Sasaoka, T., Takata, Y., Kusari, J., Anderson, C. M., Langlois, W. J., & Olefsky, J. M. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 4379-4383]. To investigate these paradoxical data, we studied the structure and function of delta 1000 truncated insulin receptors when expressed in NIH-3T3 cells. We found that, despite the deletion of most of the tyrosine kinase domain and all of the C-terminal domain of the beta-subunit of the insulin receptor, the delta 1000 mutant receptors were processed normally and were transported to the plasma membrane where they bind insulin with high affinity. Following ligand addition, the truncated receptors are degraded with a normal half-life. However, they fail to undergo insulin-stimulated internalization, do not regulate the phosphorylation of insulin receptor substrate 1, and are unable to mediate an insulin-stimulated increase in DNA synthesis and c-jun and c-fos expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific factors are required for kinase-dependent endocytosis of insulin receptors

Mouse B82 cells that support high affinity saturable endocytosis of epidermal growth factor receptors (EGFR) exhibited only low rates of nonsaturable internalization of insulin receptors (InsR). To investigate the defect in endocytosis of InsR in B82 cells, we examined the role of sequence motifs and tyrosine kinase, the two receptor components shown to be required for efficient saturable endocytosis of InsR in Rat 1 cells. Placement of residues encoded by exon 16 of the InsR onto an EGFR truncated to residue 958 restored EGF-induced internalization of this mutant receptor indicating that the sequence codes in exon 16 are recognized by B82 cells. To determine whether the kinase function could be provided in trans, a B82 cell expressing both receptors was established. EGF-activated EGFR kinase was not able to restore insulin-dependent rapid endocytosis to InsR. However, fusion of untransfected Rat1 cells with InsR-expressing B82 cells enabled rapid endocytosis of InsR, indicating that the internalization defect can be complemented. These results indicate that, although internalization codes can function in the context of other receptors, activation of tyrosine kinase receptors requires an additional specific component.

Mechanisms of hormone resistance: lessons from insulin-resistant patients

Hormones are secreted by endocrine glands and transported to the target cell at which the hormone acts. The hormone binds to its receptor, thereby eliciting various biological responses within the target cell. Examples of disease mechanisms that function at the different stages in the development of the insulin receptor, and result in insulin resistance, are discussed in this review. Antibodies to insulin can impair delivery of the hormone to the target cell, and can desensitize that target cell to insulin action. In recent years, several genetic diseases have been identified that result from mutations in the genes encoding the relevant receptors. Studies of syndromes of insulin resistance provide illustrations of the multiple types of defects in receptor function that can generally cause hormone resistance (12, 13). For example, mutations in the receptor can decrease the number of receptors on the cell surface by inhibiting receptor biosynthesis, impairing receptor transport to the cell surface, or accelerating the rate of receptor degradation. Alternatively, mutations have been identified that decrease the affinity of insulin binding or inhibit receptor tyrosine kinase activity. In recent years, there has been considerable progress toward elucidating post-receptor mechanisms in the biochemical pathways of hormone action. At present, there are a limited number of examples of mutations in genes encoding proteins that function in this part of the pathway, but it seems likely that additional examples will be discovered in the future. It is likely that these insights into biochemical mechanisms of disease will ultimately lead to an improvement in our ability to treat human disease.

Mutagenesis of Phe381 and Phe382 in the extracellular domain of the insulin receptor: effects on receptor biosynthesis, processing, and ligand-dependent internalization

Mutations of the extracellular domain of the insulin receptor impair processing and transport of receptors to the plasma membrane. We have previously reported that a mutation substituting Val for Phe382 in the alpha-subunit of the insulin receptor impairs intracellular processing and insulin-induced autophosphorylation of the mutant receptor. In this investigation, we have generated two independent mutations of amino acids Phe381 and Phe382 of the insulin receptor: Val for Phe381 and Leu for Phe382. These substitutions cause a slight impairment of intracellular processing and transport of the mutant receptors. Furthermore, insulin-dependent internalization of the mutant receptors is unaffected by these mutations. Thus, of the three substitutions studied to date, Val for Phe382 is the only mutation of the Phe381-Phe382 sequence that causes a major defect in post-translational processing of the receptor.

Two steps of insulin receptor internalization depend on different domains of the beta-subunit

The internalization of signaling receptors such as the insulin receptor is a complex, multi-step process. The aim of the present work was to determine the various steps in internalization of the insulin receptor and to establish which receptor domains are implicated in each of these by the use of receptors possessing in vitro mutations. We find that kinase activation and autophosphorylation of all three regulatory tyrosines 1146, 1150, and 1151, but not tyrosines 1316 and 1322 in the COOH-terminal domain, are required for the ligand-specific stage of the internalization process; i.e., the surface redistribution of the receptor from microvilli where initial binding occurs to the nonvillous domain of the cell. Early intracellular steps in insulin signal transduction involving the activation of phosphatidylinositol 3'-kinase are not required for this redistribution. The second step of internalization consists in the anchoring of the receptors in clathrin-coated pits. In contrast to the first ligand specific step, this step is common to many receptors including those for transport proteins and occurs in the absence of kinase activation and receptor autophosphorylation, but requires a juxta-membrane cytoplasmic segment of the beta-subunit of the receptor including a NPXY sequence. Thus, there are two independent mechanisms controlling insulin receptor internalization which depend on different domains of the beta-subunit.

Robert Feulgen Prize Lecture 1993. The journey of the insulin receptor into the cell: from cellular biology to pathophysiology

The data that we have reviewed indicate that insulin binds to a specific cell-surface receptor. The complex then becomes involved in a series of steps which lead the insulin-receptor complex to be internalized and rapidly delivered to endosomes. From this sorting station, the hormone is targeted to lysosomes to be degraded while the receptor is recycled back to the cell surface. This sequence of events presents two degrees of ligand specificity: (a) The first step is ligand-dependent and requires insulin-induced receptor phosphorylation of specific tyrosine residues. It consists in the surface redistribution of the receptor from microvilli where it preferentially localizes in its unoccupied form. (b) The second step is more general and consists in the association with clathrin-coated pits which represents the internalization gate common to many receptors. This sequence of events participates in the regulation of the biological action of the hormone and can thus be implicated in the pathophysiology of diabetes mellitus and various extreme insulin resistance syndromes, including type A extreme insulin resistance, leprechaunism, and Rabson-Mendehall syndrome. Alterations of the internalization process can result either from intrinsic abnormalities of the receptor or from more general alteration of the plasma membrane or of the cell metabolism. Type I diabetes is an example of the latter possibility, since general impairment of endocytosis could contribute to extracellular matrix accumulation and to an increase in blood cholesterol. Thus, better characterization of the molecular and cellular biology of the insulin receptor and of its journey inside the cell definitely leads to better understanding of disease states, including diabetes.

Insulin uptake and processing by cultured mouse glomerular endothelial cells

Endothelial cells isolated from a variety of vascular beds bind and transport insulin but exhibit relatively low insulin degrading activity. Because endothelial cells exhibit heterogeneity and since kidney is a major site of insulin degradation, we studied the processing of insulin by glomerular endothelial cells (GEC). When exposed to 2 x 10(-10) M 125I-labeled insulin, GEC associated with the hormone in a specific manner. This interaction was inhibited by insulin but not by a number of unrelated peptide hormones. Over a 90-min period, GEC degraded 42 +/- 3% of the 125I-insulin, as measured by solubility in trichloroacetic acid (TCA). Degradation was inhibited 90% by an excess of insulin or adrenocorticotropic hormone (10(-6) M) and 57% by glucagon, whereas growth hormone and calcitonin were without effect. Separation of plasma membrane bound from internalized insulin was achieved by decreasing extracellular pH. In the steady state, 43% of cell-associated insulin was membrane bound and 57% internalized. The fate of the internalized 125I-insulin was examined by incubating acid-washed cells at 37 degrees C for 60 min. Over this time 18% of the radioactivity was released as TCA insoluble- and 72% as TCA-soluble radioactivity. Release was increased by insulin (10(-6) M) but not by unrelated peptide hormones. In the presence of chloroquine, 125I-insulin release increased by one third while degradation fell. High-performance liquid chromatography revealed that GEC released both intact insulin and large intermediates and that chloroquine inhibited intermediate formation.(ABSTRACT TRUNCATED AT 250 WORDS)