Regulation of insulin receptor functions by a peptide derived from a major histocompatibility complex class I antigen

Open MHC Class I Conformers: A Look through the Looking Glass

Studies carried out during the last few decades have consistently shown that cell surface MHC class I (MHC-I) molecules are endowed with functions unrelated with antigen presentation. These include cis–trans-interactions with inhibitory and activating KIR and LILR, and cis-interactions with receptors for hormones, growth factors, cytokines, and neurotransmitters. The mounting body of evidence indicates that these non-immunological MHC-I functions impact clinical and biomedical settings, including autoimmune responses, tumor escape, transplantation, and neuronal development. Notably, most of these functions appear to rely on the presence in hematopoietic and non-hematopoietic cells of heavy chains not associated with β2m and the peptide at the plasma membrane; these are known as open MHC-I conformers. Nowadays, open conformers are viewed as functional cis-trans structures capable of establishing physical associations with themselves, with other surface receptors, and being shed into the extracellular milieu. We review past and recent developments, strengthening the view that open conformers are multifunctional structures capable of fine-tuning cell signaling, growth, differentiation, and cell communication.

Zinc-α2-glycoprotein relieved seizure-Induced neuronal glucose uptake impairment via insulin-like growth factor 1 receptor-regulated glucose transporter 3 expression

Glucose hypometabolism is observed in epilepsy and promotes epileptogenesis. Glucose hypometabolism in epilepsy may be attributed to decreased neuronal glucose uptake, but its molecular mechanism remains unclear. Zinc-α2-glycoprotein (ZAG) is related to glucose metabolism and is reported to suppress seizures. The anti-epileptic effect of ZAG may be attributed to its regulation of neuronal glucose metabolism. This study explored the effect of ZAG on neuronal glucose uptake and its molecular mechanism via insulin-like growth factor 1 receptor (IGF1R)-regulated glucose transporter 3 (GLUT-3) expression. The ZAG level was modulated by lentivirus in primary culture neurons. Neuronal seizure models were induced by Mg²⁺ -free artificial cerebrospinal fluid. We assessed neuronal glucose uptake by the 2-NBDG method and Glucose Uptake Colorimetric Assay Kit. IGF1R was activated by IGF1 and blocked by AXL1717. The expression and distribution of IGF1R and GLUT-3, together with IGF1R phosphorylation, were measured by western blot. The binding between ZAG and IGF1R was determined by coimmunoprecipitation. Neuronal glucose uptake and GLUT-3 expression were significantly decreased by seizure or ZAG knockdown, whereas ZAG over-expression or IGF1 treatment reversed this decrease. The effect of ZAG on neuronal glucose uptake and GLUT-3 expression was blocked by AXL1717. ZAG increased IGF1R distribution and phosphorylation possibly by binding. Additionally, IGF1R increased GLUT-3 activity by increasing GLUT-3 expression. In epilepsy/seizure, neuronal glucose uptake suppression may be attributed to a decrease in ZAG, which suppresses neuronal GLUT-3 expression by regulating the activity of IGF1R. ZAG, IGF1R, and GLUT-3 may be novel potential therapeutic targets of glucose hypometabolism in epilepsy and seizures.

Potential Role of HLA Class I Antigens in the Glycolytic Metabolism and Motility of Melanoma Cells

Besides playing a crucial role in immune surveillance, human leukocyte antigens (HLA) possess numerous non-immune functions involved in cell communication. In the present study, screening of a panel of HLA class I- and HLA class II-specific monoclonal antibodies (mAbs) for their effects on the metabolism of human melanoma cells showed for the first time that the HLA-B,C-specific mAb B1.23.2 reduced the expression level of key glycolytic enzymes, but did not affect that of mitochondrial respiration effectors. As a result, the metabolism of melanoma cells shifted from a Warburg metabolism to a more oxidative phosphorylation. In addition, the HLA-B,C-specific mAb B1.23.2 downregulated the expression of glutamine transporter and glutaminase enzyme participating in the reduction of tricarboxylic acid cycle. The HLA-B,C-specific mAb B1.23.2-mediated reduction in energy production was associated with a reduction of melanoma cell motility. On the whole, the described results suggest that HLA class I antigens, and in particular the gene products of HLA-B and C loci play a role in the motility of melanoma cells by regulating their metabolism.

Insulin receptor activation with transmembrane domain ligands

Complementary surfaces are buried when peptide hormones, growth factors, or cytokines bind and activate cellular receptors. Although these extended surfaces provide high affinity and specificity to the interactions, they also present great challenges to the design of small molecules that might either mimic or antagonize the process. We show that the insulin receptor (IR) and downstream signals can be activated by targeting a site outside of its ligand-binding domain. A 24-residue peptide having the IR transmembrane (TM) domain sequence activates IR, but not related growth factor receptors, through specific interactions with the receptor TM domain. Like insulin-dependent activation, IR-TM requires that IR have a competent ATP-binding site and kinase activation loop. IR-TM also activates mutated receptors from patients with severe insulin resistance, which do not respond to insulin. These results show that IR can be activated through a pathway that bypasses its canonical ligand-binding domain.

Major histocompatibility complex class I proteins in brain development and plasticity

Proper development of the central nervous system (CNS) requires the establishment of appropriate connections between neurons. Recent work suggests that this process is controlled by a balance between synaptogenic molecules and proteins that negatively regulate synapse formation and plasticity. Surprisingly, many of these newly identified synapse-limiting molecules are classic 'immune' proteins. In particular, major histocompatibility complex class I (MHCI) molecules regulate neurite outgrowth, the establishment and function of cortical connections, activity-dependent refinement in the visual system, and long-term and homeostatic plasticity. This review summarizes our current understanding of MHCI expression and function in the CNS, as well as the potential mechanisms used by MHCI to regulate brain development and plasticity.

Killing tumor cells through their surface beta(2)-microglobulin or major histocompatibility complex class I molecules

Targeted antibody-based therapy has been used successfully to treat cancers. Recent studies have demonstrated that tumor cells treated with antibodies specific for beta(2)-microglobulin (beta(2)M) or major histocompatibility complex (MHC) class I molecules undergo apoptosis in vitro and in vivo (mouse models). Antibodies against beta(2)M or MHC class I induce tumor cell apoptosis by 1) recruiting MHC class I molecules to lipid rafts and activating LYN kinase and the signal-transducing enzyme phospholipase C-gamma2-dependent c-Jun N-terminal kinase signaling pathway and 2) expelling interleukin 6 and insulin-like growth factor 1 receptors out of lipid rafts and inhibiting the growth and survival factor-induced activation of the phosphatidylinositol 3-kinase/Akt and extracellular signal-related kinase pathways. Consequently, mitochondrial integrity is compromised, and the caspase-9-dependent cascade is activated in treated tumor cells. However, although beta(2)M and MHC class I are expressed on normal hematopoietic cells, which is a potential safety concern, the monoclonal antibodies were selective to tumor cells and did not damage normal cells in vitro or in human-like mouse models. These findings suggest that targeting beta(2)M or MHC class I by using antibodies or other agents offers a potential therapeutic approach for beta(2)M/MHC class I-expressing malignancies. Cancer 2010. (c) 2010 American Cancer Society.

Open conformers: the hidden face of MHC-I molecules

A pool of MHC-I molecules present at the plasma membrane can dissociate from the peptide and/or the light chain, becoming open MHC-I conformers. Whereas peptide-bound MHC-I molecules have an important role in regulating adaptive and innate immune responses, through trans-interactions with T cell and NK cell receptors, the function of the open MHC-I conformers is less clear but seems to be related to their inherent ability to cis-associate, both with themselves and with other receptors. Here, we review data indicating the open MHC-I conformers as regulators of ligand-receptor interactions and discuss the biological implications for immune and non-immune cells. The likelihood that the MHC-I heavy chains have hidden functions that are determined by the amino acid sequence of the alpha1 and alpha2 domains are discussed.

The export of major histocompatibility complex class I molecules from the endoplasmic reticulum of rat brown adipose cells is acutely stimulated by insulin

Major histocompatibility complex class I (MHC-I) molecules have been implicated in several nonimmunological functions including the regulation and intracellular trafficking of the insulin-responsive glucose transporter GLUT4. We have used confocal microscopy to compare the effects of insulin on the intracellular trafficking of MHC-I and GLUT4 in freshly isolated rat brown adipose cells. We also used a recombinant vaccinia virus (rVV) to express influenza virus hemagglutinin (HA) as a generic integral membrane glycoprotein to distinguish global versus specific enhancement of protein export from the endoplasmic reticulum (ER) in response to insulin. In the absence of insulin, MHC-I molecules largely colocalize with the ER-resident protein calnexin and remain distinct from intracellular pools of GLUT4. Surprisingly, insulin induces the rapid export of MHC-I molecules from the ER with a concomitant approximately three-fold increase in their level on the cell surface. This ER export is blocked by brefeldin A and wortmannin but is unaffected by cytochalasin D, indicating that insulin stimulates the rapid transport of MHC-I molecules from the ER to the plasma membrane via the Golgi complex in a phosphatidyl-inositol 3-kinase-dependent and actin-independent manner. We further show that the effect of insulin on MHC-I molecules is selective, because insulin does not affect the intracellular distribution or cell-surface localization of rVV-expressed HA. These results demonstrate that in rat brown adipose cells MHC-I molecule export from the ER is stimulated by insulin and provide the first evidence that the trafficking of MHC-I molecules is acutely regulated by a hormone.

Crystal structure of the hereditary haemochromatosis protein HFE complexed with transferrin receptor

HFE is related to major histocompatibility complex (MHC) class I proteins and is mutated in the iron-overload disease hereditary haemochromatosis. HFE binds to the transferrin receptor (TfR), a receptor by which cells acquire iron-loaded transferrin. The 2.8 A crystal structure of a complex between the extracellular portions of HFE and TfR shows two HFE molecules which grasp each side of a twofold symmetric TfR dimer. On a cell membrane containing both proteins, HFE would 'lie down' parallel to the membrane, such that the HFE helices that delineate the counterpart of the MHC peptide-binding groove make extensive contacts with helices in the TfR dimerization domain. The structures of TfR alone and complexed with HFE differ in their domain arrangement and dimer interfaces, providing a mechanism for communicating binding events between TfR chains. The HFE-TfR complex suggests a binding site for transferrin on TfR and sheds light upon the function of HFE in regulating iron homeostasis.

Expression of MHC class I heavy chain and beta2-microglobulin in rat brainstem motoneurons and nigral dopaminergic neurons

We demonstrate here that motoneurons and nigral dopaminergic neurons in the brainstem of the adult rat, with the exception of motoneurons innervating ocular muscles, display high levels of both MHC class I heavy chain and beta2-microglobulin mRNAs. These neurons also display interferon-gamma receptor mRNA. We find it striking that these particular neurons are those which are vulnerable to neurodegeneration in diseases such as Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS).

Activation of erythropoietin receptor in the absence of hormone by a peptide that binds to a domain different from the hormone binding site

Applying a homology search method previously described, we identified a sequence in the extracellular dimerization site of the erythropoietin receptor, distant from the hormone binding site. A peptide identical to that sequence was synthesized. Remarkably, it activated receptor signaling in the absence of erythropoietin. Neither the peptide nor the hormone altered the affinity of the other for the receptor; thus, the peptide does not bind to the hormone binding site. The combined activation of signal transduction by hormone and peptide was strongly synergistic. In mice, the peptide acted like the hormone, protecting against the decrease in hematocrit caused by carboplatin.

The transferrin receptor binding site on HFE, the class I MHC-related protein mutated in hereditary hemochromatosis

HFE is a class I major histocompatibility complex (MHC)-related protein that is mutated in patients with the iron storage disease hereditary hemochromatosis. HFE binds tightly to transferrin receptor (TfR), the receptor that mediates uptake of iron-loaded transferrin. The binding affinities for TfR of HFE mutants, designed using the HFE crystal structure, were measured using biosensor assays. The results allow localization of the TfR binding site on HFE to the C-terminal portion of the alpha1 domain helix and an adjacent loop, a region distinct from the ligand binding sites on class I MHC and related proteins. A biosensor-derived pH-dependent affinity profile for the HFE-TfR interaction is discussed in terms of HFE's hypothesized role in intracellular trafficking.

Inhibition of IRS-1 phosphorylation and the alterations of GLUT4 in isolated adipocytes from cachectic tumor-bearing rats

Cellular and molecular mechanisms of insulin resistance in isolated adipocytes from methylcholanthrene-induced sarcoma-bearing rats were investigated by measuring 3-O-[14C]methyl glucose transport activity, glucose transporter-4 (GLUT4) protein in both plasma membrane and low-density microsomes, and insulin-stimulated tyrosine phosphorylation of the insulin receptor (IR) and insulin receptor substrate-1 (IRS-1). Compared to both pair-fed and freely fed controls, tumor-bearing rats (TBR) had a decreased insulin-stimulated glucose transport activity with a lower Vmax and a higher EC50. GLUT4 protein in low-density microsomes from adipocytes maintained at the basal state was less in TBR than in controls. In insulin-stimulated adipocytes, GLUT4 protein in plasma membranes was also less in tumor-bearing rats than in controls. Insulin-induced tyrosine phosphorylation of IRS-1 was less in TBR than controls, but that of the IR was similar among the three groups. These data suggest that the insulin resistance seen in adipose cells of these tumor-bearing rats was caused in part by a decreased amount of GLUT4 protein in both basal and insulin-stimulated states resulting from the selective inhibition of insulin-stimulated phosphorylation of IRS-1.

Class I MHC Molecule-Mediated Inhibition of Sindbis Virus Replication

The threshold for systemic viral infection relies on the amplification of virus at a primary infection site. We have identified that class I MHC molecules can trigger the inhibition of replication of Sindbis virus in a haplotype- and allele-specific manner. Class I MHC molecules of H-2d haplotypes exhibit a strong inhibitory effect whereas H-2k haplotypes show minimal inhibition of Sindbis viral replication. By a single gene transfection of H-2d class I MHC molecules, into cells that express class I MHC molecules of H-2k haplotype and are susceptible to viral replication, these cells became resistant to viral replication. The inhibition of viral replication by class I MHC molecules occurs neither during the stage of virus entry/endocytosis nor during virus maturation. Rather, viral-specific RNA replication, as well as viral gene expression, are inhibited in cells expressing inhibitory class I MHC molecules. This class I MHC molecule-mediated inhibition requires newly synthesized host gene products, implying the activation of an intracellular signaling mechanism that is triggered by specific class I MHC molecules.

Interferons enhance HLA-G mRNA and protein in transfected mouse fibroblasts

The HLA class Ib gene, HLA-G, has a 16-bp deletion in its Enhancer A/interferon response element (IRE). We used a model system consisting of mouse fibroblasts transfected with 6.0 kb of HLA-G DNA, the S14/8 cells, to test the postulate that this deletion prevents interferons (IFNs) from enhancing transcription. Northern blot hybridization experiments showed that after 48 h of treatment with IFN-alpha, IFN-beta or IFN-gamma, steady-state levels of HLA-G mRNA in the S14/8 cell line were doubled. Proteins were also increased by IFNs as demonstrated in flow cytometry and immunocytochemical experiments that used monoclonal antibodies to all HLA class I antigens (W6/32), HLA-G heavy chains (87G) and light chains (beta2m). Thus, interferons enhance expression of HLA-G and would be expected to improve host defense at the maternal-fetal interface by increasing the ability of maternal immune cells to recognize and destroy infected HLA-G+ cells.

Expression of MHC class I and beta2-microglobulin in rat spinal motoneurons: regulatory influences by IFN-gamma and axotomy

The low expression of MHC antigens is believed to be one factor of importance contributing to the immune-privileged status of CNS neurons. We here describe that motoneurons, in contrast to other nerve cells in the lumbar spinal cord of the adult rat, express both MHC class I and beta2-microglobulin mRNA. The motoneurons also display in situ hybridization signal for IFN-gamma receptor mRNA. After a peripheral axotomy, the motoneurons show a clear upregulation of beta2-microglobulin mRNA. IFN-gamma treatment of cultured rat embryonic spinal motoneurons causes a similar upregulation of especially beta2-microglobulin. Based on these facts, we propose that spinal motoneurons can be influenced by IFN-gamma and recognized by cytotoxic CD8+ T-cells. These findings could be of relevance in the search for pathogenetic mechanisms in motoneuron-specific diseases, such as ALS.

Requirement of major histocompatibility complex-compatible microenvironment for spleen colony formation (CFU-S on day 12 but not on day 8)

To clarify major histocompatibility complex (MHC) restriction between hematopoietic stem cells (HSCs) and microenvironments, T cell-depleted bone marrow cells (BMCs) were transplanted into MHC-compatible and MHC-incompatible recipients. A significantly larger number of spleen colony-forming units (CFU-S) on day 12 were noted in MHC-compatible recipients, while only a small number were observed in MHC-incompatible recipients. There was, however, no significant difference in CFU-S counts on day 8 between the two groups. A large number of CFU-S counts on day 12 were also observed in F1 hybrid recipients, as seen in syngeneic recipients. The decrease in CFU-S counts on day 12 in MHC-incompatible recipients was also observed even after in vivo abrogation of T and NK cells. The difference in CFU-S counts on day 12 became more prominent when HSC-enriched cells were transferred. These results suggest that an MHC restriction exists between pluripotent HSCs (P-HSCs) and spleen microenvironments. Furthermore, experiments using B10. A recombinant strains revealed that H-2D and S loci play a crucial role in the MHC restriction. The experiments of serial transplantation suggest that the differentiation and proliferation of P-HSCs are inhibited in MHC-incompatible microenvironments. It is therefore likely that the MHC-compatible microenvironment is essential to the differentiation and proliferation of P-HSCs.

Insulin resistance and the alterations of glucose transporter-4 in adipose cells from cachectic tumor-bearing rats

BACKGROUND

Insulin resistance may play an important role in cancer cachexia; however, its mechanisms remain to be clarified.

METHODS

Cellular mechanisms of insulin resistance in tumor-bearing rats (TBR) were investigated in isolated adipose cells by measuring 3-O-[14C]methyl glucose transport activity and glucose transporter-4 (GLUT4) protein in low-density microsomes at a basal state and in the plasma membrane at an insulin-stimulated state.

RESULTS

The insulin-stimulated glucose transport activity in adipose cells from TBR was significantly lower than that of control rats (CTR) (0.51 +/- 0.25 and 2.27 +/- 0.11 fmol/cell/min, respectively). The amount of GLUT4 in low-density microsomes at a basal state and in plasma membrane at an insulin-stimulated state was less in TBR than in CTR.

CONCLUSIONS

These data suggest that the insulin resistance seen in the adipose cells of these tumor-bearing rats was caused in part by both a decreased amount of GLUT4 protein in a basal state and a decreased translocation of GLUT4 in response to insulin stimulation.

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.

HLA class I expression on the materno-fetal interface

PROBLEM

The conditions that permit the genetically dissimilar (haplo-non-identical) human fetus to evade rejection by its mothers immune system have been the subject of intense interest for several years. As the placental cells, which are in contact with maternal blood or tissue, are devoid of HLA class II antigens, the interest has focused on the expression of HLA class I molecules.

METHOD OF STUDY

Recent findings on the constitutive, transcriptional, and translational expression of HLA class I molecules on anatomically and morphologically different subpopulations of trophoblast cells will form the basis of this article.

RESULTS

The expression of HLA class I molecules in the trophoblast cells, forming the materno-fetal junctional zone is inhomogeneous. It differs depending on the differentiation and location of trophoblast cells within the placenta and furthermore on the stage of gestation. On the transcriptional level HLA-A, -B, -C, -E, and -G could be detected on individual trophoblast populations, whereas only HLA-C and HLA-G seem to be translated to protein.

CONCLUSIONS

The expression of HLA class I antigens by trophoblast cells is not simply suppressed. Instead, less polymorphic HLA-G and HLA-C antigens are carefully selected from the class I multigene family. This gives rise to the assumption that these two HLA class I molecules play an important role in the maintenance of pregnancy.

Decreased cytotoxic activity of natural killer cells in kidney allograft recipients treated with human HLA-derived peptide

Peptides derived from a conserved region (aa 75-84) of HLA class I, overlapping the supertypic HLA-BW4/BW6 antigen region, have been shown to exhibit nonallele restricted immunosuppressive properties in rats and mice, prolonging survival of major histocompatibility complex-mismatched allografts. Furthermore, HLA-B7 peptides inhibit alloreactive cytotoxic cells, and both HLA-B7 and HLA-B2702 peptides inhibit natural killer (NK) cytotoxicity in vivo. In this article, we report on a randomized, controlled study of the safety and pharmacokinetics of HLA-B2702-derived peptide in human recipients of a first kidney allograft. Escalating doses of HLA-B2702 were compared with doses of placebo controls. No toxicity and no immunization against the peptide were noted. Although the study was not designed as an efficacy trial, patients who received the high-dose protocol (7 mg/kg) did experience more rejection episodes, but this was not statistically significant when compared with control patients. Interestingly, in human recipients, as previously observed in rodents, administration of the peptide was associated with a statistically significant decrease in the cytotoxicity of NK cells against K562 targets (P<0.001). As these peptides correspond to a region of the HLA class I molecule that interacts with the newly described NK receptors for class I, their mode of action through interaction with such receptors is discussed. As a peptide of the same sequence from HLA-B7 blocks both NK and alloreactive T cell cytotoxicity, it is possible that, in humans too, both types of cytotoxic cells are affected by this peptide. The biological significance of these observations should be confirmed in future controlled studies with a larger patient population.

Anti-HLA antibody ligation to HLA class I molecules expressed by endothelial cells stimulates tyrosine phosphorylation, inositol phosphate generation, and proliferation

The major threat to long-term survival of solid organ allografts is chronic rejection. Progressive narrowing and ultimate luminal occlusion of the arteries and arterioles of the transplanted organ are the hallmarks of the disease. The mechanism of chronic rejection is poorly understood, but it is suspected that the associated vascular changes are a result of anti-HLA antibody-mediated injury to the endothelium. We have postulated that anti-HLA antibodies initiate chronic rejection by binding to class I molecules on the endothelium and transducing signals that result in endothelial cell activation and proliferation. Our data demonstrate that anti-HLA class I antibodies transduce signals in endothelial cells stimulating increased tyrosine phosphorylation of intracellular proteins. Antibody binding to class I antigens also leads to the generation of inositol phosphate and endothelial cell proliferation. These results indicate that anti-HLA antibodies can deliver functionally important signals to endothelial cells, a finding that may be fundamental to an understanding of the mechanisms of chronic rejection.

Insulin promotes and cyclic adenosine 3',5'-monophosphate impairs functional insertion of insulin receptors in the plasma membrane of rat adipocytes: evidence for opposing effects of tyrosine and serine/threonine phosphorylation

The aim of the present study was to elucidate events in the plasma membrane (PM) associated with the previously described effect of insulin to rapidly enhance the number of cell surface insulin binding sites in rat adipocytes. [125I]insulin was cross-linked to cell surface insulin receptors of intact cells that had been preincubated with or without insulin. Subsequently prepared PM displayed a approximately 3-fold increase in bound [125I]insulin when cells had been pretreated with 6 nM insulin for 20 min compared to membranes from control cells, and SDS-PAGE with autoradiography showed that this occurred at the insulin receptor alpha-subunit. The magnitude of the effect was similar to that found for insulin binding to intact cells that had been preincubated with insulin. In contrast, the insulin binding capacity in the PM was not affected by prior treatment of cells with insulin when assessed with the addition of [125I]insulin directly to solubilized PM; this suggests an unchanged total number of PM receptors. Thus, the enhancement of cell surface insulin binding capacity produced by insulin is not due to the translocation of receptors, but instead appears to be confined to receptors already present in the PM. The addition of phospholipase C (from Clostridium perfringens), which cleaves PM phospholipids, mimicked the effect of insulin to enhance cell surface binding in adipocytes, and this suggests a pool of cryptic PM receptors. Both the nonmetabolizable cAMP analog N6-monobutyryl cAMP (N6-mbcAMP) and the serine/threonine phosphatase inhibitor okadaic acid abolished the effect of concomitant insulin treatment to increase binding capacity. In contrast, the tyrosine phosphatase inhibitor vanadate increased insulin binding even in the presence of okadaic acid or N6-mbcAMP. The effect of N6-mbcAMP to impair cell surface insulin binding was also evident in the presence of a peptide derived from the major histocompatibility complex type I that effectively impairs receptor internalization, but the amount of PM receptors assessed by immunoblot was unaltered. Taken together, the data suggest that insulin exposure leads to the uncovering of cryptic receptors associated with the PM. It is also suggested that tyrosine phosphorylation promotes this process, whereas enhanced serine phosphorylation, e.g. produced by cAMP, impairs the functional insertion of the receptors, rendering them unable to bind insulin.

A synthetic peptide corresponding to the Rab4 hypervariable carboxyl-terminal domain inhibits insulin action on glucose transport in rat adipocytes

The present study was conducted to examine the involvement of Rab4, a low molecular weight GTP-binding protein, in the action of insulin on glucose transport. A synthetic peptide corresponding to the Rab4 hypervariable carboxyl-terminal domain, Rab4-(191-210), was successfully transferred into rat adipocytes by electroporation and inhibited insulin-stimulated glucose transport by about 50% without affecting the basal transport activity. In contrast, synthetic peptides corresponding to the Rab3C and Rab3D carboxyl-terminal hypervariable domain had little effect on insulin action on glucose transport. The Rab4-(191-210) peptide also reduced insulin-induced GLUT4 translocation from the intracellular pool to the plasma membrane. Furthermore, the Rab4-(191-210) peptide reduced both insulin-induced glucose transport and GLUT4 translocation in the presence of a major histocompatibility complex class I antigen-derived peptide, D(k)-(62-85), which is a potent inhibitor of GLUT4 internalization, suggesting that the peptide inhibited exocytotic recruitment of GLUT4-containing vesicles. The Rab4-(191-210) peptide also inhibited GTP gamma S-stimulated glucose transport. In addition, insulin-stimulated glucose transport was inhibited by the addition of anti-Rab4 antibody. These results suggest that Rab4 protein plays a crucial role in insulin action on GLUT4 translocation, especially in exocytotic recruitment by the hormone of the glucose transporter to the plasma membrane from the intracellular retention pool.

Effects of peroxovanadate and vanadate on insulin binding, degradation and sensitivity in rat adipocytes

The effects of vanadate and the stable peroxovanadate compound bpV(pic) on insulin binding and degradation were investigated in rat adipocytes under conditions of ongoing receptor cycling. Both bpV(pic) and vanadate increased 125I-insulin binding to intact cells through an increase in apparent receptor affinity. The maximal effect of bpV(pic) was to increase binding approximately 4-fold (EC50 0.06 +/- 0.01 mM), whereas vanadate increased binding approximately 2-fold (EC50 1.4 +/- 0.2 mM). Removal of cell surface insulin-receptor complexes with trypsin showed that the effects on binding exerted by bpV(pic) and vanadate were due to a similar increase in both cell surface binding and intracellular accumulation of radioactivity. Both bpV(pic) and vanadate inhibited the degradation of 125I-insulin in medium containing 1% bovine serum albumin. The ratio of degraded/intact intracellular 125I-insulin was also markedly reduced by these agents, suggesting that they inhibit intracellular insulin-degrading proteases. Similar to previous findings with vanadate, bpV(pic) stimulated glucose transport and, at low concentrations, enhanced insulin sensitivity. Taken together, these data demonstrate that both bpV(pic) and vanadate inhibit insulin degradation. In addition, they significantly enhance cell surface insulin binding in rat fat cells and this is associated with an improved insulin sensitivity.

Inhibitory effects of cerulenin on protein palmitoylation and insulin internalization in rat adipocytes

Protein acylation by long-chain fatty acids has been suggested as a necessary step in membrane trafficking. Because several insulin effects are dependent upon membrane trafficking, the cellular effects of the protein acylation inhibitor cerulenin were examined. Cerulenin blocked palmitoylation of selected rat adipocyte proteins including CD36, the dominant marker for palmitoylation in adipocytes. To measure cerulenin's effects on insulin internalization, rat adipocytes were incubated with 125I-insulin at 37 degrees C in the presence or absence of cerulenin. Surface-bound and intracellular insulin were discriminated by the sensitivity of the former to rapid dissociation by a pH 3 buffer at 4 degrees C. Insulin internalization was inhibited 85% by 0.3 mM cerulenin. Inhibition required preincubation with the agent, was irreversible, was not dependent upon protein synthesis, and was not the result of ATP depletion. Cerulenin was also found to inhibit insulin-stimulated glucose uptake and acetyl-CoA carboxylase activity. Cerulenin had no effect on basal glucose uptake and utilization or on the uptake and retention of fatty acids. In summary, protein acylation may be an important step in insulin-regulated cellular functions dependent upon membrane trafficking, such as insulin internalization.

Engagement of MHC class I proteins on natural killer cells inhibits their killing capacity

We have studied whether engagement of MHC class I (MHC-I) molecules on natural killer (NK) cells can influence the NK killing activity. Human NK effector cells, enriched by nylon wool passage, were incubated with monoclonal antibodies (MoAb) to MHC-I followed by cross-linking with secondary rabbit anti mouse Ig or streptavidin. Cross linking of MHC-I molecules on NK cells resulted in a clear inhibition of the NK activity against the target cells K562, Molt-4 and U937. The inhibitory effect was selective for MHC-I and was not seen with MoAb to MHC-II or CD56 molecules. The inhibition was not mediated via Fc receptors since F(ab)2 fragments of the MHC-I MoAb W6/32 were as effective as the intact antibody. The best inhibition of NK activity was obtained using biotin-labelled F(ab)2 fragments of W6/32 and streptavidin as a cross-linker, where up to 70% reduction in NK cell activity was observed. Antibody dependent cellular cytotoxicity (ADCC) was also inhibited by cross-linking MHC-I molecules on the effector cells. The results show that antibody mediated cross-linking of MHC-I proteins on NK cells can inhibit their killing capacity. This indicates that MHC-I molecules on NK cells can be involved in the regulation of NK cytotoxicity, perhaps by transmitting inhibitory signals into the NK cell.

Dissection of GLUT4 recycling pathway into exocytosis and endocytosis in rat adipocytes. Evidence that GTP-binding proteins are involved in both processes

The effects of guanine nucleotides on either exocytosis or endocytosis of GLUT4 were examined in electrically permeabilized rat adipocytes by using Dk-(62-85), a major histocompatibility complex class I-derived peptide. Reversal of glucose transport activity that had been stimulated with insulin was completely blocked by Dk-(62-85). Likewise, endocytosis of the trypsin-cleaved 35-kDa fragment of GLUT4 was almost completely inhibited by the peptide. Insulin-stimulated glucose transport activity was enhanced about 50% by Dk-(62-85), whereas the basal transport activity was stimulated only slightly. Although guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) augmented glucose transport to the same extent as insulin in the absence of the peptide, glucose transport stimulated by GTP gamma S was only 60% of the insulin effect in the presence of the peptide; the effect of insulin was markedly enhanced by Dk-(62-85), whereas GTP gamma S-induced glucose transport was not affected, suggesting that GTP gamma S has an effect similar to that of the peptide. In fact, endocytosis of the 35-kDa fragment of GLUT4 was markedly inhibited by GTP gamma S. Additionally, GLUT4 endocytosis was accelerated by GTP but was inhibited by guanosine 5'-O-(2-thiodiphosphate). These results indicate that GTP gamma S induces translocation of GLUT4 by both stimulating exocytosis and inhibiting endocytosis. With respect to the dependence on GTP hydrolysis, distinct types of GTP-binding proteins are involved in exocytosis and endocytosis of GLUT4.

Insulin resistance and growth retardation in mice lacking insulin receptor substrate-1

Insulin receptor substrate-1 (IRS-1) is the major substrate of insulin receptor and IGF-1 receptor tyrosine kinases; it has an apparent relative molecular mass of 160-190,000 (M(r), 160-190K) on SDS polyacrylamide gel. Tyrosine-phosphorylated IRS-1 binds the 85K subunit of phosphatidylinositol 3-kinase which may be involved in the translocation of glucose transporters and the abundant src homology protein (ASH)/Grb2 which may be involved in activation of p21ras and MAP kinase cascade. IRS-1 also has binding sites for Syp and Nck and other src homology 2 (SH2) signalling molecules. To clarify the physiological roles of IRS-1 in vivo, we made mice with a targeted disruption of the IRS-1 gene locus. Mice homozygous for targeted disruption of the IRS-1 gene were born alive but were retarded in embryonal and postnatal growth. They also had resistance to the glucose-lowering effects of insulin, IGF-1 and IGF-2. These data suggest the existence of both IRS-1-dependent and IRS-1-independent pathways for signal transduction of insulin and IGFs.

Regulation of receptor internalization by the major histocompatibility complex class I molecule

We showed previously that peptides derived from the alpha 1 domain of the major histocompatibility complex class I protein (MHC-I) inhibit internalization of some receptors, thereby increasing the steady-state number of active receptors on the cell surface. In consequence, sensitivity to hormone (e.g., insulin) is enhanced, transport (e.g., of glucose by GLUT-4) is increased, and carrier proteins (e.g., transferrin) operate less efficiently. Now we report that a bioactive peptide (but not closely related inactive ones) binds to MHC-I on the cell surface, not in the groove but apparently to the alpha 1 helix. The binding is saturable, and the number of peptide binding sites on the cell surface approximately equals the number of MHC-I molecules. Antibodies to MHC-I inhibit peptide binding. Most significant, antibodies to MHC-I mimic the effect of a bioactive peptide, inhibiting receptor internalization. These results indicate that MHC-I participates in the regulation of cell surface receptor activity.

Trypanosoma brucei and the nervous system

African sleeping sickness, characterized by a peculiar pain syndrome and prominent neuropsychiatric symptoms, is caused by the parasite Trypanosoma brucei (T.b.). In experimental T.b. infections, a molecule released from the trypanosomes has been isolated that binds to the CD8 molecule of T cells, whereby T cells are activated to secrete interferon gamma. This cytokine binds to the parasites and triggers them to proliferate, establishing a peculiar bidirectional activating signal system. The hypothesis is presented that the molecules involved in these bidirectional signals might also interact with neurons, thus causing brain dysfunctions. Studies on the molecular interactions between parasites and the nervous system in sleeping sickness might reveal basic mechanisms underlying other neuropsychiatric diseases.

Insulin receptors in syncytiotrophoblast and fetal endothelium of human placenta. Immunohistochemical evidence for developmental changes in distribution pattern

The localisation of insulin receptors (IR) was investigated on cryosections of human non-pathologic first trimester and full term placentae by indirect immunohistochemistry with three different monoclonal antibodies (MABS). In placentae from 6 to 10 weeks postmenstruation (p-m.), only syncytiotrophoblast was stained, predominantly that of mesenchymal villi and syncytial sprouts, which are areas of high proliferative activity. In placentae from 11 to 14 weeks p-m., endothelial cells commenced to react with the IR MABS and the syncytiotrophoblast was less intensely labelled than at weeks 6 to 10 p-m. In term placentae, the microvillous membrane of the syncytiotrophoblast showed only patches of weak immunoreactivity. In contrast, the endothelial cells in the placenta but not in the umbilical cord were strongly stained. The amniotic epithelium in the chorionic plate and fibroblasts in the stroma were conspicuously labelled. The data indicate: (1) the receptor density on villous syncytiotrophoblast decreases and that of fetal endothelium increases' throughout gestation; (2) syncytiotrophoblast of human term placentae expresses a low level per unit area of surface IR; and (3) the majority of IR in human term placentae is located in fetal endothelium. Apart from yet unknown functional effects of maternal and fetal insulin at the placental barrier, the results suggest a growth promoting effect on the trophoblast of maternal insulin in first trimester as well as developmental effects of fetal insulin on the feto-placental vessels at term.

CD8 and beta 2-microglobulin-free MHC class I molecules in T cell immunoregulation

Intracellular assembly of MHC class I heavy chains with beta 2-microglobulin occurs prior to the expression of the antigen-presenting complex on the cell surface. The association of beta 2-microglobulin with newly synthesized class I heavy chains is thought to be a strict prerequisite for their transport to the cell surface. However, MHC class I molecules not associated with beta 2-microglobulin (beta 2-microglobulin-free class I heavy chains) have been detected on the surface of activated lymphoid cells. These molecules have different conformations. Therefore, their interactions with other membrane proteins and biological functions may be different from those assigned to beta 2-microglobulin-associated MHC class I molecules. The two forms of MHC class I molecules on the surface of activated cells can self-associate and also form complexes with distinct proteins. Upon interaction with the appropriate ligands these molecular complexes transduce signals regulating cell activation. The ligand for beta 2-microglobulin-free class I heavy chains appears to be soluble CD8. A model is presented describing a novel mechanism of immunoregulation mediated by both soluble and membrane-bound forms of CD8 and beta 2-microglobulin-free class I heavy chains.

Amino acid residues essential for biological activity of a peptide derived from a major histocompatibility complex class I antigen

The stimulatory activity of peptides from the alpha 1 domain of the major histocompatibility complex (MHC) class I antigen on adipose cell glucose transport was previously shown to require a preformed, ordered conformation of the peptide. The two peptides studied previously were Dk-(61-85) (ERETQIAKGNEQSFRVDLRTLLRYY) and Dk-(69-85). We now show that systematic alanine substitution in Dk-(69-85) identifies residues that are essential for biological activity. Ordered structure of the peptides, estimated by circular dichroism, was found in all peptides with activity, but with a complex variety of spectra. Inactive peptides were in either a random coil or an ordered structure. Ordered structure, therefore, is not sufficient for activity. The peptides self-interact in the absence of cells and form aggregates that precipitate upon centrifugation. The tendency to aggregate is correlated with biological potency. Only MHC class I molecules have significant homology to the peptides studied here. The peptide self-interaction suggests that the biological effects in cells, which result from inhibition of receptor and transporter internalization, may be due to the binding (tantamount to self-interaction) of the peptide to the homologous sequences in the alpha 1 domain of the MHC class I molecule.

Interferon-gamma promotes proliferation of rat skeletal muscle cells in vitro and alters their AChR distribution

Recombinant interferon-gamma (IFN-gamma) caused a dose-dependent increase in the proliferation of myoblasts in cultures of rat skeletal muscles as determined by bromodeoxyuridine incorporation. As a result of this proliferation the number of myotubes increased in the cultures, while the fusion index was unchanged. In myotubes exposed to IFN-gamma there was a change in the distribution of nicotinic acetylcholine receptors (nAChR) as detected by binding of FITC-conjugated alpha-bungarotoxin with a significant decrease in the number of linear aggregates and an increase in diffusely distributed receptors.

Bidirectional signals between Trypanosoma brucei brucei and dorsal root ganglion neurons

The extracellular haemoflagellate Trypanosoma brucei brucei releases a factor, which can induce CD8+ T-cells to produce interferon-gamma. Interferon-gamma derived from these cells promotes proliferation of the trypanosomes. We now report that these trypanosomes can interact with small neurons in cultures of rat dorsal root ganglia, which contain an interferon-gamma like immunoreactive molecule. Cultures of dorsal root ganglia were able to promote the proliferation and survival of the trypanosomes and this growth promoting effect was blocked by monoclonal antibodies against the trypanosome-derived lymphocyte triggering factor, interferon-gamma and CD8+ and by Fab-fragments of these antibodies. Living trypanosomes and the factor induced an increase in expression of major histocompatibility complex class I antigens in the cultures; this induction was blocked by Fab fragments of the interferon-gamma antibody, indicating that it is mediated by release of the neuronal interferon-gamma-like immunoreactive molecule. These data suggest, that the interferon-gamma-like immunoreactive molecule in small dorsal root ganglion neurons mimics certain physiological effects of lymphocyte derived interferon-gamma. Furthermore, the trypanosomes and small sensory neurons seem to interact bidirectionally by release of the trypanosome-derived lymphocyte triggering factor and the neuronally derived interferon-gamma-like molecule, whereby, perhaps, neuronal disturbances may be elicited and trypanosome proliferation regulated.

Malarial toxic antigens synergistically enhance insulin signalling

Hypoglycaemia is a major complication of severe malaria [(1990) Trans. Roy. Soc. Trop. Med. 84 (suppl. 2) 1-65], especially cerebral malaria, in which it is associated with increased mortality [(1990) Lancet 336, 1039-1043; (1989) Quart. J. Med. (New series) 71, 441-459]; however, the mechanisms responsible have not been fully explained. Preparations containing toxic malaria antigens (TMA) released by blood stage Plasmodium yoelii malaria parasites have been shown to induce hypoglycaemia in mice lasting at least 8 h [(1992) Clin. Exp. Immunol. (in press)]. Here we report that TMAs can act synergistically with insulin in both stimulating lipogenesis and inhibiting lipolysis in rat adipocytes in vitro, and, furthermore, that they act synergistically with insulin in the induction of hypoglycaemia in vivo.

Altered proliferative kinetics in PHA-activated human T-lymphocytes treated with the anti-HLA class I monoclonal antibody 01.65

Anti-HLA class I monoclonal antibody (mAb) 01.65 inhibited phytohaemagglutinin (PHA)-induced human lymphocyte proliferation. The inhibitory effect was inversely correlated to the strength of the proliferative response. It was increased when lymphocytes were stimulated with suboptimal doses of PHA but it disappeared with supraoptimal doses. Proliferation inhibition was achieved by prolonging the cell cycle time and by slowing down its recruitment rate. The former effect was not restricted to the G1-phase but also included the S phase. These results support the idea that HLA class I molecules are important in the PHA-induced proliferation of human T-lymphocytes.