Mechanisms of nuclear translocation of insulin

Insulin (Ins) and various other hormones and growth factors have been shown to be rapidly internalized and translocated to the cell nucleus. This review summarizes the mechanisms that are involved in the translocation of Ins to the nucleus, and discusses its possible role in Ins action, based on observations by the authors and others. Ins is internalized to endosomes by both receptor-mediated and fluid-phase endocytosis, the latter occurring only at high Ins concentrations. The authors recently demonstrated the caveolae are the primary cell membrane locations responsible for initiating the signal transduction cascade induced by Ins. Once Ins is internalized, Ins dissociates from the Ins receptor in the endosome, and is translocated to the cytoplasm, where most Ins is degraded by Ins-degrading enzyme (IDE), although how the polypeptides cross the lipid bilayer is unknown. Some Ins escapes the degradation and binds to cytosolic Ins-binding proteins (CIBPs), in addition to IDE. IDE and some CIBPs are known to be binding proteins for other hormones or their receptors, and are involved in gene regulation, suggesting physiological relevance of CIBPs in the signaling of Ins and other hormones. Ins is eventually translocated through the nuclear pore to the nucleus, where Ins tightly associates with nuclear matrix. The role of Ins internalization and translocation to the nucleus is still controversial, although there is substantial evidence to support its role in cellular responses caused by Ins. Many studies indicate that nuclear translocation of various growth factors and hormones plays an important role in cell proliferation or DNA synthesis. It would be reasonable to suggest that Ins internalization, its association with CIBPs, and its translocation to the nucleus may be essential for the regulation of nuclear events by Ins.

Angiotensin II inhibits insulin-induced egr-1 expression in mesangial cells

The gene early growth response gene-1 (egr-1) encodes a zinc transcription factor involved in cell proliferation. Increased expression of egr-1 has been linked to heart and kidney disease. In mouse mesangial cells, insulin stimulated egr-1 expression more than angiotensin II, suggesting that insulin may play an important role in stimulating cell proliferation, leading to glomerulonephritis and diabetic nephropathy. Angiotensin II inhibited insulin-induced egr-1 expression but not c-fos expression, and the decrease in egr-1 expression was concurrent with a decrease in insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation. These results suggest that insulin-induced egr-1 expression in mouse mesangial cells is downstream of tyrosine phosphorylation of IRS-1 and activation of the MAP kinase pathway and that crosstalk between angiotensin II and insulin signaling pathways led to an inhibition of IRS-1 tyrosine phosphorylation and egr-1 expression.

Differential modulation of the tyrosine phosphorylation state of the insulin receptor by IRS (insulin receptor subunit) proteins

In response to insulin, tyrosine kinase activity of the insulin receptor is stimulated, leading to autophosphorylation and tyrosine phosphorylation of proteins including insulin receptor subunit (IRS)-1, IRS-2, and Shc. Phosphorylation of these proteins leads to activation of downstream events that mediate insulin action. Insulin receptor kinase activity is requisite for the biological effects of insulin, and understanding regulation of insulin receptor phosphorylation and kinase activity is essential to understanding insulin action. Receptor tyrosine kinase activity may be altered by direct changes in tyrosine kinase activity, itself, or by dephosphorylation of the insulin receptor by protein-tyrosine phosphatases. After 1 min of insulin stimulation, the insulin receptor was tyrosine phosphorylated 8-fold more and Shc was phosphorylated 50% less in 32D cells containing both IRS-1 and insulin receptors (32D/IR+IRS-1) than in 32D cells containing only insulin receptors (32D/IR), insulin receptors and IRS-2 (32D/IR+IRS-2), or insulin receptors and a form of IRS-1 that cannot be phosphorylated on tyrosine residues (32D/IR+IRS-1F18). Therefore, IRS-1 and IRS-2 appeared to have different effects on insulin receptor phosphorylation and downstream signaling. Preincubation of cells with pervanadate greatly decreased protein-tyrosine phosphatase activity in all four cell lines. After pervanadate treatment, tyrosine phosphorylation of insulin receptors in insulin-treated 32D/IR, 32D/ IR+IRS-2, and 32D/IR+IRS-1F18 cells was markedly increased, but pervanadate had no effect on insulin receptor phosphorylation in 32D/IR+IRS-1 cells. The presence of tyrosine-phosphorylated IRS-1 appears to increase insulin receptor tyrosine phosphorylation and potentially tyrosine kinase activity via inhibition of protein-tyrosine phosphatase(s). This effect of IRS-1 on insulin receptor phosphorylation is unique to IRS-1, as IRS-2 had no effect on insulin receptor tyrosine phosphorylation. Therefore, IRS-1 and IRS-2 appear to function differently in their effects on signaling downstream of the insulin receptor. IRS-1 may play a major role in regulating insulin receptor phosphorylation and enhancing downstream signaling after insulin stimulation.

Insulin-induced protein tyrosine phosphorylation cascade and signalling molecules are localized in a caveolin-enriched cell membrane domain

The cellular localisation of time- and temperature-dependent 125I-insulin binding, insulin-sensitive signalling proteins and the insulin-induced protein tyrosine phosphorylation cascade were assessed in subcellular fractions isolated on Iodixanol gradients from control and insulin-treated H35 hepatoma cells. Western blot analysis demonstrated that the concentrations of IRS-1, Shc, GRB-2, SOS, Syp, PI 3-kinase, MAP kinase and Gi alpha were at least 10-fold higher in cell surface-derived, caveolin-enriched fraction than in a cell surface-derived, caveolin-poor fraction (i.e., the plasma membranes). Insulin treatment caused a 15-fold increase in tyrosine phosphorylation of IRS-1 in the caveolin-enriched fraction in 5 min at 37 degrees C compared with a 3-fold increase in plasma membranes and a 6-fold increases in the cytosol and endosomes. Insulin also increased tyrosine phosphorylation of both a 72-kDa protein and the 46-kDa Shc isoform only in the caveolin-enriched fraction. Insulin treatment did not change the concentrations of insulin receptors or Shc but increased IRS-1 in the caveolin-enriched fraction, possibly recruited from the cytosolic pool. Insulin also increased the concentrations of insulin receptors, IRS-1 and Shc in endosomes, suggesting insulin-induced internalization of the insulin receptors and proteins activated with them. Electron microscopic analysis, with the use of a combination of colloidal gold-labelled insulin to label the insulin receptor and immunolabelling to detect caveolin or IRS-1, demonstrated the co-localisation of insulin receptors in caveolin- and IRS-1 containing vesicular structures. Differences in the insulin-induced protein tyrosine phosphorylation and concentrations of these proximal signalling proteins in the caveolin-enriched fraction, plasma membranes, and cytosol suggest that insulin receptors in the caveolae play a major role in initiating insulin's signal transduction processes.

Receptor-mediated cellular entry of nuclear localizing anti-DNA antibodies via myosin 1

A unique subset of anti-DNA antibodies enters living cells, interacts with DNase 1, and inhibits endonuclease activity, before their nuclear localization and subsequent attenuation of apoptosis. We now report that endocytosis of these immunoglobulins is mediated by cell surface binding to brush border myosin (myosin 1). Cellular entry and internalization via this unique receptor provides initial contact for entry and sorting these immunoglobulins to translocate to the nuclear pore and enter the nucleus, interact with DNase 1 within the cytoplasm, or recycle back to the cell surface. This internalization pathway provides clues to the translocation of large proteins across cell membranes and the functional effects of intracellular antibodies on cytopathology. This is the first demonstration that brush border myosin functions as a specific cell surface receptor for internalization of large proteins.

Insulin internalization and other signaling pathways in the pleiotropic effects of insulin

Insulin is the major anabolic hormone in humans and affects multiple cellular processes. Insulin rapidly regulates short-term effects on carbohydrate, lipid, and protein metabolism and is also a potent growth factor controlling cell proliferation and differentiation. The metabolic and growth-related effects require insulin binding to its receptor and receptor phosphorylation. Evidence suggests these events result in subsequent substrate phosphorylation and activation of multiple signaling pathways involving Src homology domain-containing proteins and the internalization of the insulin:receptor complex. The role of insulin internalization in insulin action is largely speculative. For more than two decades, extensive investigation has been carried out by numerous laboratories of the mechanisms by which insulin causes its pleiotropic responses and the cellular processing of insulin receptors. This chapter reviews our current knowledge of the phosphorylation signaling pathways activated by insulin and presents evidence that substrates other than insulin receptor substrate-1 are involved in insulin's regulation of immediate-early gene expression. We also review the mechanisms involved in insulin internalization and present evidence that internalization may play a key role in insulin action through both signal transduction processes and translocation of insulin to the cell cytoplasm and nucleus.

Insulin-induced egr-1 and c-fos expression in 32D cells requires insulin receptor, Shc, and mitogen-activated protein kinase, but not insulin receptor substrate-1 and phosphatidylinositol 3-kinase activation

Many studies suggest that insulin utilizes multiple signal transduction pathways. Insulin's effects are initiated by insulin binding to the insulin receptor, resulting in tyrosine phosphorylation of insulin receptor and intracellular substrates, such as insulin receptor substrate-1 (IRS-1), IRS-2, or Shc. We recently demonstrated that immediate-early gene egr-1 transcription was fully induced without phosphorylation of IRS-1 in Chinese hamster ovary cells (Harada, S., Smith, R. M., Smith, J. A., Shah, N. , Hu, D.-Q. & Jarett, L. (1995) J. Biol. Chem. 270, 26632-26638). In the present study, we examined the effects of insulin on immediate-early gene egr-1 and c-fos expression in 32D cells overexpressing the insulin receptor (32D/IR), IRS-1 (32D/IRS), or both (32D/IR+IRS) and compared these effects with insulin-induced tyrosine phosphorylation. Insulin (17 nM) increased egr-1 and c-fos expression in 32D/IR and 32D/IR+IRS cells, but not in parental cells or 32D/IRS cells, as determined by Northern blot analysis. Insulin treatment (5 min at 37 degrees C) markedly increased tyrosine phosphorylation of several proteins, including the insulin receptor, IRS-1, and Shc, in 32D/IR+IRS cells as determined by immunoprecipitation and Western blot analysis with anti-phosphotyrosine antibody. In contrast, only two tyrosine-phosphorylated proteins, i.e. insulin receptor and Shc, were detected in 32D/IR cells. These data suggest that insulin receptor and Shc phosphorylation is necessary for insulin-induced egr-1 and c-fos expression, but IRS-1 phosphorylation is not necessary or sufficient for the expression of these genes. Furthermore, the effect of specific inhibitors on insulin-induced egr-1 expression was examined. Wortmannin (25 nM), a phosphatidylinositol 3-kinase inhibitor, had no effect on insulin-induced egr-1 expression. In contrast, PD 98059 (30 microM), a mitogen-activated protein kinase kinase inhibitor, totally blocked egr-1 expression induced by insulin. These data indicate that mitogen-activated protein kinase activation, but not phosphatidylinositol 3-kinase activation, is involved in insulin-induced egr-1 expression. Taken together, insulin receptor tyrosine phosphorylation, Shc tyrosine phosphorylation, and mitogen-activated protein kinase activation appear to be the signal transduction pathway responsible for insulin-induced egr-1 expression in 32D cells. These data demonstrate that insulin has multiple signal transduction pathways that vary from cell to cell.

The role of receptor kinase activity and the NPEY960 motif in insulin-accelerated receptor-mediated insulin internalization

This study used biochemical and quantitative ultrastructural approaches to examine the roles that insulin receptor beta subunit kinase activity, the NPEY motif in the juxtamembrane region, and tyrosine phosphorylation within that domain plays in insulin-accelerated receptor-mediated insulin internalization in CHO cells. Internalization of insulin in cells that expressed kinase-deficient receptors (CHOA1018) or receptors lacking the NPEY Ala954-Asp965 domain (CHO delta 960) was reduced by 80% compared to cells expressing wild-type human insulin receptors (CHOHIRc). Ultrastructural analysis revealed that the decreased internalization in CHOA1018 cells was due to the reduced ability of the kinase deficient receptor to migrate from the microvilli of cultured cells and aggregate on the cell surface. Deletion of the NPEY motif in the juxtamembrane region of the beta subunit severely reduced receptor migration, interfered with the normal aggregation of receptors on the cell surface, and virtually eliminated accumulation of the occupied receptors in the coated invaginations. Replacement of Tyr960 in the NPEY domain/with phenylalanine (CHOF960) had no significant effect on insulin internalization, receptor mobility, aggregation or accumulation in coated invaginations. In contrast, replacement of Tyr960 with alanine (CHOA960) decreased insulin internalization, slowed migration, receptor aggregation and accumulation in coated invaginations. These studies document that kinase activity is required, but not sufficient, for receptor movement from the microvilli and aggregation of occupied receptors on the non-villous surface. An intact NPEY motif or surrounding amino acids, but not the phosphorylation of Tyr960 plays a role in receptor mobility and aggregation and is essential for the accumulation of insulin receptors in coated invaginations.

The expression of and insulin binding to cellular thyroid hormone binding protein, but not insulin degrading enzyme, is increased during 3T3-L1 adipocytes differentiation

Specific insulin binding to several proteins (cytosolic insulin binding proteins; CIBPs) in the isolated cytoplasm of numerous cell types has been demonstrated. CIBPs include insulin degrading enzyme (IDE), CIBP p55 (identified as cellular thyroid hormone binding protein (CTHBP), which is also known as protein disulfide isomerase, or glutathione insulin transhydrogenase). To assess the possible role of CIBP p55/CTHBP in insulin action, we compared 125I-insulin binding to CIBP in cytosol isolated from 3T3-L1 cells at various time points during differentiation of the adipocytes. Insulin did not bind to CTHBP in fibroblasts, but the labeling was markedly increased during adipocyte differentiation. In contrast, insulin binding to IDE did not change during differentiation. Protein expression level of CTHBP in the cytosol fraction increased gradually during the differentiation of adipocytes, whereas that of IDE did not change throughout the period. These data indicate that CTHBP, but not IDE, was up-regulated during differentiation of the adipocytes, suggesting that CIBP p55/CTHBP may play a role in regulating some insulin action, especially the counter regulation between insulin and other hormones during adipocyte differentiation.

Report of the American Diabetes Association's Task Force on standardization of the insulin assay

Recent large-scale epidemiological studies demonstrate that blood concentrations of immunoreactive insulin predict the development of NIDDM and IDDM and are associated with the risk of several degenerative diseases, such as coronary and peripheral vessel atherosclerosis, hypertension, and dyslipidemia. The reliability of these measurements is dependent on a biological assay that has not been well standardized between laboratories. Recognizing this, the American Diabetes Association organized a task force to assess comparability of blood insulin measurements between laboratories and to suggest techniques to improve comparability. The task force found that identical serum and plasma samples measured in different laboratories produced widely disparate values that were unacceptable for population comparisons. Use of a single reference standard did little to improve comparability. Assay characteristics such as linearity, recovery, accuracy, and cross-reactivity to proinsulin and its primary conversion intermediates varied among the laboratories, and they did not readily explain differences in the measurements made from assay to assay. Use of the same assay kit in different laboratories did not always ensure comparable measurements. Linear regression of assay results from one laboratory to an arbitrarily chosen reference assay greatly improved comparability and demonstrated the potential value in comparing each assay to a reference method. The task force report defines acceptable assay characteristics and proposes a three-step process of insulin assay proficiency and comparability. A central reference assay and ongoing sample exchange will be needed to allow reliable comparisons of insulin measurements made in different laboratories. Rigorous quality control and continuous quality improvement are needed to maintain reliability of the insulin measurement.

Dexamethasone inhibits insulin binding to insulin-degrading enzyme and cytosolic insulin-binding protein p82

We recently demonstrated that insulin specifically binds to several cytosolic insulin-binding proteins (CIBPs) including insulin-degrading enzyme (IDE) and CIBP p82 in cytosol isolated from H35 rat hepatoma cells. Insulin binding to these CIBPs was regulated by culture conditions, such as serum or insulin. In the present study, we examined the effect of dexamethasone on insulin binding to CIBPs in H35 cells. When the cells were treated with 100 nM dexamethasone for 24 hrs, insulin binding to IDE and CIBP p82 was decreased by about 50% without decreasing the expression level of IDE. Insulin added with the dexamethasone prevented the steroid's effect. Furthermore, dexamethasone directly blocked insulin binding to CIBPs in isolated cytosol. These results suggest that dexamethasone, directly or as a complex with other proteins, binds to IDE and CIBP p82 and changes their ability to bind insulin, possibly by inducing a conformational change or by blocking insulin binding sites. IDE was recently identified as a receptor accessory factor for androgen and glucocorticoid receptors and plays an important role in the regulation of gene transcriptional responses. Combined with previous reports, our findings suggest IDE and other CIBPs such as CIBP p82 may play a role in the cross-talk between insulin and the signal transduction pathways of steroid hormones.

The effects of brefeldin A on the glucose transport system in rat adipocytes. Implications regarding the intracellular locus of insulin-sensitive Glut4

Insulin activates glucose transport by recruiting Glut4 glucose transporters from an intracellular pool to plasma membrane (PM). To localize intracellular translocating Glut4, we studied the effects of brefeldin A (BFA), which disassembles Golgi and prevents trans-Golgi vesicular budding, on the glucose transport system. Isolated rat adipocytes were treated with and without both BFA (10 micrograms/ml) and insulin. BFA did not affect maximal rates of either 2-deoxyglucose or 3-O-methyl-glucose transport or the insulin:glucose transport dose-response curve but did increase basal transport by approximately 2-fold (p < 0.05). We also measured Glut4 in PM, low (LDM) and high density microsome subfractions. In basal cells, BFA increased PM Glut4 by 58% concomitant with a 18% decrease in LDM (p < 0.05). Insulin alone increased PM Glut4 by 3-fold concomitant with a 56% decrease in LDM. BFA did not affect insulin-induced changes in Glut4 levels in PM or LDM. Most intracellular Glut4 was localized to sub-PM vesicles by immunoelectron microscopy in basal cells, and BFA did not affect insulin-mediated recruitment of immunogold-labeled Glut4 to PM. In summary, 1) in basal cells, BFA led to a small increase in glucose transport activity and redistribution of a limited number of transporters from LDM to PM; 2) BFA did not affect insulin's ability to stimulate glucose transport or recruit normal numbers of LDM Glut4 to PM; and 3) insulin action is predominantly mediated by a BFA-insensitive pool of intracellular Glut4, which localizes to sub-PM vesicles. Thus, the major translocating pool of Glut4 in rat adipocytes does not involve trans-Golgi.

Insulin-induced egr-1 expression in Chinese hamster ovary cells is insulin receptor and insulin receptor substrate-1 phosphorylation-independent. Evidence of an alternative signal transduction pathway

Insulin's effects primarily are initiated by insulin binding to its plasma membrane receptor and the sequential tyrosine phosphorylation of the insulin receptor and intracellular substrates, such as insulin receptor substrate-1 (IRS-1). However, studies suggest some insulin effects, including those at the nucleus, may not be regulated by this pathway. The present study compared the levels of insulin binding, insulin receptor and IRS-1 tyrosine phosphorylation, and phosphatidylinositol 3'-kinase activity to immediate early gene c-fos and egr-1 mRNA expression in Chinese hamster ovary (CHO) cells expressing only neomycin-resistant plasmid (CHONEO), overexpressing wild type human insulin receptor (CHOHIRc) or ATP binding site-mutated insulin receptors (CHOA1018K). Insulin binding in CHONEO cells was markedly lower than that in other cell types. 10 nM insulin significantly increased tyrosine phosphorylation of insulin receptor and IRS-1 in CHOHIRc cells. Phosphorylation of insulin receptor and IRS-1 in CHONEO and CHOA1018K cells was not detected in the presence or absence of insulin. Similarly, insulin increased phosphatidylinositol 3-kinase activity only in CHOHIRc cells. As determined by Northern blot, nuclear run-on analysis, and in situ hybridization, insulin induced c-fos mRNA expression, through transcription, in CHOHIRc cells but not in CHONEO and CHOA1018K cells, consistent with previous reports. In contrast, all three cell types showed a similar insulin dose-dependent increase of egr-1 mRNA expression through transcription. These data indicated that insulin-induced egr-1 mRNA expression did not correlate with the levels of insulin binding to insulin receptor or phosphorylation of insulin receptor and IRS-1. These results suggest that different mechanisms are involved in induction of c-fos and egr-1 mRNA expression by insulin, the former by the more classic insulin receptor tyrosine kinase pathway and the latter by a yet to be determined alternative signal transduction pathway.

Electron microscopic visualization of insulin translocation into the cytoplasm and nuclei of intact H35 hepatoma cells using covalently linked Nanogold-insulin

Insulin affects numerous metabolic processes as well as nuclear events such as gene transcription. Our previous ultrastructural and biochemical studies demonstrated insulin accumulation in nuclei of cultured and rapidly proliferating cells, and biochemical evidence suggested that insulin entered the cell cytoplasm before accumulating in the nucleus. The present study was undertaken to develop a covalently linked electron-dense insulin complex that could be used to visualize the intracellular translocation of insulin and confirm that insulin enters the cytoplasm of cells. Insulin was cross-linked to 1.4-nm diameter Nanogold particles. The complex binds to the plasma membrane insulin receptor, is biologically active, and is degraded by cellular insulin-degradative enzymes. Ultrastructural analysis after silver intensification of the gold particles confirmed that insulin internalization culminates in the translocation of some internalized insulin to the cytoplasm and nuclei. When cytoplasmic insulin-degrading enzyme (IDE) activity was inhibited with 1,10-phenanthroline, an increase in the number of cytoplasmic and nuclear Nanogold-insulin particles was observed. The results of this and previous studies suggest that 1) the translocation of insulin to the cytoplasm, 2) the regulation of insulin degradation in the cytoplasm by IDE, 3) the possible interaction of insulin with cytoplasmic proteins other than IDE, and 4) the subsequent accumulation of intact insulin or insulin complexed with cytoplasmic proteins in nuclei may play a role in insulin's regulation of gene transcription and cell proliferation.

The heterotrimeric G-protein Gi is localized to the insulin secretory granules of beta-cells and is involved in insulin exocytosis

Mastoparan, a tetradecapeptide found in wasp venom that stimulates G-proteins, increases insulin secretion from beta-cells. In this study, we have examined the role of heterotrimeric G-proteins in mastoparan-induced insulin secretion from the insulin-secreting beta-cell line beta-TC3. Mastoparan stimulated insulin secretion in a dose-dependent manner from digitonin-permeabilized beta-TC3 cells. Active mastoparan analogues mastoparan 7, mastoparan 8, and mastoparan X also stimulated secretion. Mastoparan 17, an inactive analogue of mastoparan, did not increase insulin secretion from permeabilized beta-TC3 cells. Mastoparan-induced insulin secretion from permeabilized beta-TC3 cells was inhibited by pretreatment of the cells with pertussis toxin, suggesting that mastoparan-induced insulin secretion is mediated through a pertussis toxin-sensitive G-protein present distally in exocytosis. Enriched insulin secretory granules (ISG) were prepared by sucrose/nycodenz ultracentrifugation. Western immunoblotting performed on beta-TC3 homogenate and ISG demonstrated that G alpha i was dramatically enriched in ISG. Levels of G alpha o and G alpha q were comparable in homogenate and ISG. Mastoparan stimulated ISG GTPase activity in a pertussis toxin-sensitive manner. Mastoparan 7 and mastoparan 8 also stimulated GTPase activity in the ISG, while the inactive analogue mastoparan 17 had no effect. Selective localization of G alpha i to ISG was confirmed with electron microscopic immunocytochemistry in beta-TC3 cells and beta-cells from rat pancreas. In contrast to G alpha o and G alpha q, G alpha was clearly localized to the ISG. Together, these data suggest that mastoparan may act through the heterotrimeric G-protein G alpha i located in the ISG of beta-cells to stimulate insulin secretion.

Binding, uptake, and intracellular trafficking of phosphorothioate-modified oligodeoxynucleotides

An enhanced appreciation of uptake mechanisms and intracellular trafficking of phosphorothioate modified oligodeoxynucleotides (P-ODN) might facilitate the use of these compounds for experimental and therapeutic purposes. We addressed these issues by identifying cell surface proteins with which P-ODN specifically interact, studying P-ODN internalization mechanisms, and by tracking internalized P-ODN through the cell using immunochemical and ultrastructural techniques. Chemical cross-linking studies with a biotin-labeled P-ODN (bP-ODN), revealed the existence of five major cell surface P-ODN binding protein groups ranging in size from approximately 20-143 kD. Binding to these proteins was competitively inhibited with unlabeled P-ODN, but not free biotin, suggesting specificity of the interactions. Additional experiments suggested that binding proteins likely exist as single chain structures, and that carbohydrate moieties may play a role in P-ODN binding. Uptake studies with 35S-labeled P-ODN revealed that endocytosis, mediated by a receptor-like mechanism, predominated at P-ODN concentrations < 1 microM, whereas fluid-phase endocytosis prevailed at higher concentrations. Cell fractionation and ultrastructural analysis demonstrated the presence of ODN in clathrin coated pits, and in vesicular structures consistent with endosomes and lysosomes. Labeled ODN were also found in significant amounts in the nucleus, while none was associated with ribosomes, or ribosomes associated with rough endoplasmic reticulum (ER). Since nuclear uptake was not blocked by wheat germ agglutinin or concanavalin A, a nucleoporin independent, perhaps diffusion driven, import process is suggested. These data imply that antisense DNA may exert their effect in the nucleus. They also suggest rational ways to design ODN which might increase their efficiency.

Demonstration of specific insulin binding to cytosolic proteins in H35 hepatoma cells, rat liver and skeletal muscle

We previously demonstrated that internalized insulin enters the cytoplasm before accumulating in nuclei of H35 rat hepatoma cells. This finding raises the possibility that insulin may interact with cytosolic proteins in addition to insulin-degrading enzyme (IDE). In the present study, cytosol from H35 hepatoma cells, rat liver or muscle was incubated with A14- or B26-125I-insulin at 4 degrees C for 5-120 min in the absence or presence of 25 micrograms/ml unlabelled insulin. 125I-insulin was cross-linked to cytosolic proteins by disuccinimidyl suberate and analysed by reducing or non-reducing SDS/PAGE and autoradiography. Our results demonstrate the presence of both tissue-specific and common cytosolic proteins which specifically bind insulin. In muscle cytosol, only two proteins of 27 and 110 kDa were specifically labelled with B26-125I-insulin. Seven major bands, of 27, 45, 55, 60, 76, 82 and 110 kDa, were labelled in rat liver cytosol. Detection of cytosolic insulin-binding proteins in H35-cell cytosol was dependent on cell-culture conditions. Labelling in cytosol from serum-deprived cells was decreased or absent compared with cytosol prepared from serum-fed or serum-deprived cells treated with 100 ng/ml insulin for 1 h before preparation of the cytosol, in which six bands, of 32, 41, 45, 55, 82 and 110 kDa, were specifically labelled with B26-125I-insulin. This result suggests that the concentration or binding activity of some cytosolic insulin-binding proteins is rapidly regulated. Labelling of both rat liver and H35 cytosolic insulin-binding proteins was time-dependent, and decreased or disappeared at 120 min in parallel with the degradation of labelled insulin. Fewer bands were specifically labelled with A14-125I-insulin than with B26-125I-insulin. The number of labelled bands observed under reducing and non-reducing conditions was not different in any of the cytosols. The 110 kDa band in all cytosols was identified as IDE by Western-blot analysis; the other proteins did not react with anti-IDE antibody and remain unidentified. 1,10-Phenanthroline (2 mM) increased IDE labelling, but decreased the labelling of 82 and 27 kDa bands. The marked difference in the number of cytosolic insulin-binding proteins in muscle and either H35 cells or liver suggests both that the labelling is specific and that these proteins serve a function and may be involved in some heretofore unknown mechanism of the signalling pathway by which insulin regulates cell growth or differentiation.

A subgroup of murine monoclonal anti-deoxyribonucleic acid antibodies traverse the cytoplasm and enter the nucleus in a time-and temperature- dependent manner

BACKGROUND

The capacity of lupus autoantibodies to enter living cells and bind to molecules for which they have intrinsic affinity is not well appreciated. In previous studies, we identified a subgroup of three murine monoclonal IgG anti-DNA antibodies, derived from lupus-prone MRL-lpr/lpr mice, that localized within nuclei of cells in multiple organs and induced functional perturbations, in vivo, after passive transfer to normal mice. To examine the mechanisms of this phenomenon, we now extend these observations, using the same monoclonal anti-DNA antibodies and cultured cell lines.

EXPERIMENTAL DESIGN

Multiple experimental approaches were utilized to track nuclear localization of anti-DNA antibodies, including direct immunofluorescence, confocal microscopy and immunoelectron microscopy. The requirements for nuclear localization were further evaluated quantitatively, in nuclei isolated from co-cultures of cells and 125I-Ig, under varying experimental conditions.

RESULTS

Nuclear localization was observed with the same subset of anti-DNA antibodies that localized within nuclei in vivo; it was dependent on the antigen-binding region of the molecule; and it was not found with other anti-DNA antibodies. At progressive intervals, the Ig were observed: at the cell surface, within the cytoplasm, clustered at the nuclear pore, and within the nucleus. Nuclear localization of Ig was found to be a time- and temperature- dependent process, specific for a subset of anti-DNA antibodies and dependent on the antigen binding region of the Ig.

CONCLUSIONS

This is the first demonstration that monoclonal autoantibodies can traverse both the cell and nuclear membranes to localize within the nuclei of cultured cells. Furthermore, nuclear localization of Ig was regulated in a manner analogous to that of other large cytoplasmic proteins that enter the nucleus. This confirms and extends our results using the same antibodies in whole animals, and it provides the basis to further examine the underlying mechanisms and consequences of this phenomenon.

1,10-Phenanthroline increases nuclear accumulation of insulin in response to inhibiting insulin degradation but has a biphasic effect on insulin's ability to increase mRNA levels

Previous reports demonstrated that insulin is translocated through the cytoplasm to the nucleus of H35 hepatoma cells and suggested that nuclear insulin may be involved in stimulating transcription of immediate-early genes. In a recent study, inhibition of insulin-degrading enzyme with 1,10-phenanthroline, a Zn2+ chelator, caused a significant increase in the nuclear accumulation of insulin. The present study characterized the effects of 1,10-phenanthroline and its nonchelating isomer, 1,7-phenanthroline, on insulin degradation, nuclear accumulation, and stimulation of immediate-early gene expression. 1,10- but not 1,7-phenanthroline inhibited insulin degradation and increased nuclear accumulation of insulin in a dose-dependent manner. 1,7-phenanthroline caused a dose-dependent decrease in the expression of insulin-stimulated immediate-early genes, but had no significant effect on alpha-tubulin mRNA levels. In the presence of insulin, Northern analysis revealed that 1,10-phenanthroline at all concentrations tested increased alpha-tubulin mRNA levels, but had a biphasic effect on insulin-stimulated immediate-early gene expression. At low concentrations (5-200 microM), 1,10-phenanthroline increased the expression of insulin-stimulated g33, c-fos, and Egr-1 mRNA. At concentrations greater than 1 mM, insulin-stimulated immediate-early gene expression was decreased similar to the effect seen with 1,7-phenanthroline. Nuclear run-on analysis demonstrated that high concentrations of 1,10-phenanthroline decreased insulin-stimulated immediate-early gene transcription but had no effect on transcription of alpha-tubulin. However, low concentrations of 1,10-phenanthroline did not increase transcription of any genes.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration by two-color flow cytometry that tyrosine kinase activity is required for down-modulation of the oncogenic neu receptor

Expression of rat oncogenic neu receptor, p185T-neu (a growth factor receptor with constitutive tyrosine kinase activity), causes cells to become transformed. Treatment with anti-neu receptor monoclonal antibodies reverts the transformed phenotype by down-modulation of p185T-neu. Monoclonal antibody treatment of cells expressing normal neu receptor, p185C-neu (which lacks constitutive tyrosine kinase activity), does not result in down-modulation of p185C-neu. To understand further the role the biochemical activity of p185T-neu plays in transformation and endocytosis, we created a series of mutations in p185T-neu. We found that fibroblasts expressing the tyrosine kinase-defective mutants cannot form foci in culture, colonies in soft agar, or tumors in immunocompromised mice. To follow the antibody-induced endocytosis of neu receptors expressed in these transfectants, we developed a novel two-color flow cytometric assay and confirmed receptor localization by electron microscopy. Cells were treated with mAb7.16.4 over time. After 4 hr of antibody treatment, less than 50% of full-length p185T-neu and of mutant T691 remained on the cell surface, whereas internal expression of the neu receptors within these cells initially increased and then decreased to the original internal receptor level. In contrast, the level of kinase-deficient mutated neu receptors remaining on the cell surface initially decreased by 35%, but, after 4 hr of antibody treatment, the cell surface expression level returned to approximately the original level. Concurrently, fluctuations in expression levels were seen internally over time as well. These cell lines were also treated with gold-conjugated mAb7.16.4. Using electron microscopy, we consistently found the gold particles within multivesicular bodies of cell lines expressing full-length or mutated neu receptor. These data strongly suggest that the fate of the neu receptor, once internalized, is directed by its tyrosine kinase activity. When the kinase activity of the neu receptor is disrupted, the receptor is internalized but recycled to the cell surface, whereas neu receptors which have constitutive kinase activity are internalized and presumably degraded when engaged with anti-neu receptor mAb. Understanding the regulation of receptor endocytosis, degradation, and recycling will contribute to the development of novel therapeutic protocols to combat human malignancies, particularly those associated with the overexpression of the human homologue of the neu receptor, c-erbB2.

Mouse preimplantation embryos exhibit receptor-mediated binding and transcytosis of maternal insulin-like growth factor I

High-resolution microscopy in conjunction with colloidal gold-labeled insulin-like growth factor I (IGF-I) has been used to provide evidence that the IGF-I receptor is first detected in 8-cell-stage mouse embryos, confirming the results of previous reverse transcriptase polymerase chain reaction (RT-PCR) studies. Specificity for the IGF-I receptor was demonstrated by displacement with unlabeled IGF-I and dual-labeling experiments with colloidal gold-labeled or unlabeled insulin. Labeled IGF-I ligand is internalized by means of receptor-mediated endocytosis following its concentration in coated pits, and it can be visualized within cytoplasmic organelles. Immunocytochemical analyses at the blastocyst stage, using gold-labeled antibodies to the receptor, confirmed the expression of IGF-I receptors on all cells of the embryo. Similar studies with antibodies directed against the ligand demonstrated that IGF-I internalized by the embryo in vivo is maternally derived. Approximately 40% of blastocysts showed apical plasma membrane binding of gold-labeled ligand ("responders"), while approximately 60% did not demonstrate binding ("nonresponders"); however, both classes of embryo expressed receptors on basolateral membranes of trophectoderm cells and on the surface of inner masses. Functional studies show that incubating embryos in physiological levels of IGF-I (40 ng/ml) results in increased numbers of cells in the inner cell mass (p < 0.05), but not the trophectoderm, as compared to controls.

Inhibition of insulin-degrading enzyme increases translocation of insulin to the nucleus in H35 rat hepatoma cells: evidence of a cytosolic pathway

We previously demonstrated the translocation of insulin to the nucleus in several cell types and partially characterized the uptake mechanisms and pathways in H35 rat hepatoma cells. Nuclear accumulation of insulin was energy independent, time and temperature dependent, and apparently was not saturable at insulin concentrations which resulted in full receptor occupancy. We also have shown insulin could be internalized by both receptor-mediated and fluid-phase endocytosis. This study investigated subsequent steps involved in the nuclear accumulation of insulin following internalization. We examined the effects of inhibiting insulin degrading enzyme (IDE) with 1,10-phenanthroline on the nuclear accumulation of insulin in H35 cells. 1,10-phenanthroline (2 mM) which markedly inhibited insulin degradation, significantly increased nuclear accumulation of insulin without having any effects on total cell-associated and intracellular insulin. This reagent increased 125I-insulin on the cellular membrane and decreased 125iodine (125I-insulin and 125I-insulin degradation products) in the cytosolic fractions. Chemical extraction and Sephadex G-50 chromatography revealed the insulin associated with the nucleus in 1,10-phenanthroline-treated cells formed the same complex(es) with the nuclear matrix as in control cells. These results suggested that inhibition of cytosolic IDE activity resulted in increased insulin translocation from the cytosol to the nucleus. Furthermore, when IDE activity was inhibited by high cytosolic insulin concentrations, the amount of 125I-insulin in the nucleus was significantly increased. Our study suggests internalized insulin is probably released from endosomes into the cytosol where modulation of IDE activity could have significant effects on the accumulation of insulin, or insulin-cytoplasmic protein complexes, in nuclei. The IDE regulatory mechanism, by controlling the translocation of insulin to the cell nucleus, could play a crucial role in insulin's regulation of gene expression and cell proliferation.

Activation and inhibition of insulin receptor autophosphorylation by trypsin treatment of intact H35 cells

1. Treatment of intact cultured H35 cells with trypsin (1 mg/ml) for 15 min at low temperature (4 degrees C) or for 30 sec at 37 degrees C causes activation of the insulin receptor subsequently isolated from the cells. 2. Receptor activation was assessed by increased phosphotyrosine content of the beta-subunit of the receptor, and increased autophosphorylation using [32P]-ATP. 3. Treatment of the cells for 15 min at 37 degrees C however completely abolished insulin binding and all insulin receptor kinase activity. 4. These data demonstrate that proteolytic damage of the extracellular domain of the insulin receptor can render the receptor kinase inactive and lead to a cell which is unresponsive to insulin.

Insulin induces an unmasking of the carboxyl terminus of G(i) proteins in rat adipocytes

Several groups have shown a relationship between the insulin receptor and inhibitory G proteins, G(i). An antisera, 8729, to a peptide sequence (KNNLKDCGLF) corresponding to the carboxyl termini of G(i)alpha subunits was used to investigate this relationship by immunoelectron microscopy. Rat adipocytes were incubated in the absence or presence of 100 ng/ml insulin for 1 h and fixed for immunoelectron microscopy. Insulin-treated adipocytes stained with 8729 were labeled at the cell surface at a much higher density than control adipocytes. Subcellular fractionation of insulin-treated and control cells was followed by PAGE and Western blots of the plasma membrane and low-density microsomes with 8729. The density of the bands did not change in response to insulin treatment. Antibodies to noncarboxyl terminus sequences of the alpha subunit were used for immunoelectron microscopy and no difference was noted between insulin-treated and control adipocytes. These results indicated that 8729 was detecting a conformational change in the structure of G(i)alpha subunit in the plasma membrane in response to insulin. This unmasking of the carboxyl terminus was also seen in response to treatment with phenylisopropyladenosine and prostaglandin E2. Pertussis toxin-catalyzed ADP ribosylation also unmasked the carboxyl terminus. In contrast, isoproterenol, an agonist of stimulatory G proteins (Gs), did not induce an unmasking of the carboxyl terminus. These results support the hypothesis that some of insulin's effects are mediated through G(i) proteins in adipocytes.

A truncated human insulin receptor missing the COOH-terminal 365 amino acid residues does not undergo insulin-mediated receptor migration or aggregation

A previous study of tyrosine kinase-defective insulin receptors demonstrated that receptor autophosphorylation or tyrosine kinase activity was required for concentrating insulin receptors in coated pits, but not for their migration or aggregation on the cell surface. Furthermore, receptor migration and aggregation on the cell surface were not sufficient to cause internalization of the occupied receptors in coated pits. In the present study, biochemical and ultrastructural techniques were used to compare insulin receptor mobility and internalization in Rat 1 fibroblasts expressing wild-type human insulin receptors (HIRc) with those in cells expressing receptors truncated at residues 978 (HIR delta 978) or 1301 of the carboxyl-terminus (HIR delta CT). There were no significant differences in the mobility or internalization of insulin receptors on HIR delta CT cells compared to those of insulin receptors on HIRc cells. Ultrastructural analysis revealed that truncated insulin receptors on HIR delta 978 cells failed to migrate from their initial location on the microvilli, move to the plasma membrane, and aggregate in coated pits. Receptor-mediated insulin internalization in HIR delta 978 cells was markedly decreased due entirely to a decrease in ATP-dependent, coated pit-mediated internalization. ATP-independent endocytosis in non-coated pinocytotic invaginations was not affected by receptor truncations. These results provide evidence of the roles that regions of the beta-subunit play in the processing of occupied insulin receptors. 1) The carboxyl-terminus of the insulin receptor is not involved in the events leading to receptor internalization, i.e. migration, aggregation, and concentration in coated pits. 2) Internalization of insulin receptors by the ATP-independent noncoated invagination pathway is not regulated by residues in the insulin receptor beta-subunit distal to 978. 3) Sequences in the beta-subunit between 978-1300, but not the autophosphorylation and kinase domains, are involved in insulin-induced receptor migration and aggregation.

Genistein inhibits insulin-stimulated glucose transport and decreases immunocytochemical labeling of GLUT4 carboxyl-terminus without affecting translocation of GLUT4 in isolated rat adipocytes: additional evidence of GLUT4 activation by insulin

A recent study from this laboratory (Abler et al., J. Biol. Chem. 267, 18172-18179, 1992) showed genistein blocked insulin-stimulated glucose oxidation without affecting receptor autophosphorylation or tyrosine kinase activity. The mechanism by which genistein inhibited insulin-stimulated glucose metabolism was investigated in the present study. Insulin caused a approximately 12-fold increase in 3-O-methyl-D-glucose (3OMG) uptake compared to that of control cells. Basal and insulin-stimulated 3OMG transport was inhibited 40-60% by genistein in a concentration-dependent manner (10-100 micrograms/ml). Genistein had no effect on insulin-stimulated GLUT4 translocation from low density microsomes to plasma membranes as determined by Western blotting. These results suggested that genistein inhibited glucose transport in adipocytes by decreasing the intrinsic activity, rather than the number, of the plasma membrane-associated glucose transporters. We also previously reported that insulin treatment of adipocytes resulted in the immunocytochemically visualized unmasking of the carboxyl-terminus of plasma membrane-associated GLUT4 and suggested the unmasking might be related to an insulin-induced increase in the intrinsic activity of the glucose transporter (Smith et al., Proc. Natl. Acad. Sci. USA 88, 6893-6897, 1991). In the present study, genistein decreased immunocytochemical labeling of plasma membrane-associated GLUT4 by approximately 50% in control and insulin-treated adipocytes by carboxyl-terminus antibodies but had no effect on labeling observed in an amino-terminus antibody. Since genistein did not affect the number of plasma membrane-associated GLUT4 transporters, this result supports the hypothesis that conformational changes in the glucose transporter, reflected by the ability of anti-carboxyl-terminus antibodies to bind to the transporter, may be an indication of the intrinsic activity of the plasma membrane-associated transporter. We therefore conclude that conformational changes in and activation of glucose transporters, in addition to insulin-stimulated GLUT4 translocation, play an important role in insulin-regulated glucose transport in adipocytes.

Nonreceptor mediated nuclear accumulation of insulin in H35 rat hepatoma cells

We previously demonstrated that insulin accumulated in the nucleus in several cell types and partially characterized the uptake mechanisms and pathways in H35 rat hepatoma cells. Nuclear accumulation of insulin was energy independent, time, temperature, and insulin concentration dependent, but apparently nonsaturable. This study investigated further the initial endocytotic pathways that contribute to the nuclear accumulation of insulin using trypsin treatment of the cells to prevent insulin binding to its plasma membrane receptor. Total cell-associated, intracellular, and nuclear insulin were compared in control and trypsin-treated H35 hepatoma cells. Trypsin treatment markedly decreased total cell-associated and intracellular insulin as well as the nuclear accumulation of insulin when cells were incubated with 2.8 ng/ml insulin. When the cells were incubated with 100 ng/ml insulin, trypsin treatment totally inhibited insulin binding to the plasma membrane for at least 90 min. However, intracellular accumulation of insulin was reduced by only 50% at 60 min, and trypsin treatment failed to inhibit the nuclear accumulation of insulin. Chemical extraction and Sephadex G-50 chromatography revealed nuclear associated insulin in trypsin-treated cells was identical to that in control cells incubated with either 2.8 or 100 ng/ml insulin. These results suggest that a nonreceptor mediated uptake pathway, i.e., fluid-phase endocytosis, contributed significantly to the nuclear accumulation of insulin at high insulin concentrations, but at lower insulin concentrations the receptor-mediated pathway predominated. No matter which initial endocytotic route was used to internalize insulin, the insulin apparently associated with the same nuclear matrix proteins. This association of insulin with the nuclear matrix may be involved in regulation of nuclear events such as cell growth and differentiation or gene transcription.

Direct stimulation of immediate-early genes by intranuclear insulin in trypsin-treated H35 hepatoma cells

H35 hepatoma cells were treated with trypsin to abolish insulin binding and insulin-stimulated receptor kinase activity. Insulin was, however, internalized by fluid-phase endocytosis in trypsin-treated cells. Furthermore, nuclear accumulation of insulin was similar in control and trypsin-treated hepatoma cells. Northern blot analysis revealed insulin increased g33 and c-fos mRNA concentrations identically in control and trypsin-treated cells but had no effect on beta 2-microglobulin mRNA. Actinomycin D treatment prior to or after insulin addition demonstrated that insulin increased gene transcription and had no effect on mRNA degradation. These studies suggest that the accumulation of intact insulin in cell nuclei may be directly involved in the increased transcription of immediate-early genes.

Insulin and IGF1 receptors in a model renal epithelium: receptor localization and characterization

Insulin and IGF1 stimulate transepithelial Na+ transport in the urinary bladder of the toad Bufo marinus, a model renal epithelium. The signal transduction mechanisms for the natriferic action are unknown. Ultrastructural techniques were used to localize both receptors and ligands in the epithelium. Electron microscopy using gold-labelled anti-insulin or anti-IGF1 receptor antibodies demonstrated the majority of the insulin receptors were associated with the basolateral membrane while IGF1 receptors were found on basolateral and apical membranes. Both insulin and IGF1 receptors were found in endosomes and on the membranes surrounding subapical granules. In intact tissues incubated with iodinated IGF1 or insulin, both ligands were associated with the basolateral membrane. IGF1 was internalized to a greater extent than insulin and only IGF1 accumulated in cell nuclei.

Developmental expression and cellular localization of glucose transporter molecules during mouse preimplantation development

Two general mechanisms mediate glucose transport, one is a sodium-coupled glucose transporter found in the apical border of intestinal and kidney epithelia, while the other is a sodium-independent transport system. Of the latter, several facilitated transporters have been identified, including GLUT1 (erythrocyte/brain), GLUT2 (liver) and GLUT4 (adipose/muscle) isoforms. In this study, we used Western-blot analysis and high resolution immunoelectron microscopy (IEM) to investigate the stage-related expression and cellular localization of GLUT1, 2 and 4. The Western blot results demonstrate that GLUT1 is detectable in the oocyte and throughout preimplantation development. GLUT2 isoforms were not detectable until the blastocyst stage, while the GLUT4 isoform was undetectable in the oocyte through blastocyst stages. The present findings confirm previous studies at the molecular level which demonstrated that mRNAs encoding the same GLUT isoforms are detectable at corresponding developmental stages. GLUT1 and GLUT2 display different cellular distributions at the blastocyst stage as shown by IEM studies. GLUT1 has a widespread distribution in both trophectoderm and inner cell mass cells, while GLUT2 is located on trophectoderm membranes facing the blastocyst cavity. This observation suggests a different functional significance for these isoforms during mouse preimplantation development.

Genistein differentially inhibits postreceptor effects of insulin in rat adipocytes without inhibiting the insulin receptor kinase

Genistein, an isoflavone putative tyrosine kinase inhibitor, was used to investigate the coupling of insulin receptor tyrosine kinase activation to four metabolic effects of insulin in the isolated rat adipocyte. Genistein inhibited insulin-stimulated glucose oxidation in a concentration-dependent manner with an ID50 of 25 micrograms/ml and complete inhibition at 100 micrograms/ml. Genistein also prevented insulin's (10(-9) M) inhibition of isoproterenol-stimulated lipolysis with an ID50 of 15 micrograms/ml and a complete effect at 50 micrograms/ml. The effect of genistein (25 micrograms/ml) was not reversed by supraphysiological (10(-7) M) insulin levels. In contrast, genistein up to 100 micrograms/ml had no effect on insulin's (10(-9) M) stimulation of either pyruvate dehydrogenase or glycogen synthase activity. We determined whether genistein influenced insulin receptor beta-subunit autophosphorylation or tyrosine kinase substrate phosphorylation either in vivo or in vitro by anti-phosphotyrosine immunoblotting. Genistein at 100 micrograms/ml did not inhibit insulin's (10(-7) M) stimulation of insulin receptor tyrosine autophosphorylation or tyrosine phosphorylation of the cellular substrates pp185 and pp60. Also, genistein did not prevent insulin-stimulated autophosphorylation of partially purified human insulin receptors from NIH 3T3/HIR 3.5 cells or the phosphorylation of histones by the activated receptor tyrosine kinase. In control experiments using either NIH 3T3 fibroblasts or partially purified membranes from these cells, genistein did inhibit platelet-derived growth factor's stimulation of its receptor autophosphorylation. These findings indicate the following: (a) Genistein can inhibit certain responses to insulin without blocking insulin's stimulation of its receptor tyrosine autophosphorylation or of the receptor kinase substrate tyrosine phosphorylation. (b) In adipocytes genistein must block the stimulation of glucose oxidation and the antilipolytic effects of insulin at site(s) downstream from the insulin receptor tyrosine kinase. (c) The inhibitory effects of genistein on hormonal signal transduction cannot necessarily be attributed to inhibition of tyrosine kinase activity, unless specifically demonstrated.

Insulin stimulates accumulation and efflux of macromolecules in isolated nuclei from H35 hepatoma cells

This study used 10-nm gold particles with 5-7 insulin molecules attached (Au10-Ins) to investigate the site of interaction of insulin with the nuclear envelope during insulin uptake into intact isolated nuclei. Despite its size, and in the absence of ATP, Au10-Ins entered nuclei through the nuclear pore and associated with the heterochromatin. Because Au10-Ins is essentially gold-bovine serum albumin (Au-BSA) with a few insulin molecules attached, the effect of insulin and other growth factors on the nuclear accumulation of BSA coupled to 10-, 15-, and 24-nm-diam colloidal gold particles (Au10-BSA, Au15-BSA, and Au24-BSA) was determined. The Au-BSA complexes were excluded from nuclei in the absence of insulin. Insulin (0.5-100 ng/ml) caused a dose-dependent accumulation of Au10-BSA in the nucleus. The nuclear membrane was shown to be intact by several criteria, therefore, accumulation of Au-BSA occurred via the nuclear pore and was not due to leakage across or through the membrane. Uptake of 15- and 24-nm Au-BSA molecules was not affected by insulin, suggesting the hormone had a limited effect in increasing the functional diameter of the nuclear pores. Glucagon, epidermal growth factor, platelet-derived growth factor, insulinlike growth factor I, and insulin A or B chains did not stimulate the accumulation of Au10-BSA. The insulin-stimulated accumulation of Au10-BSA was blocked by concanavalin A, mimicked by wheat-germ agglutinin, and did not require ATP. The Au10-BSA in the nucleus was associated with heterochromatin, suggesting it bound to a nuclear element.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of orthovanadate on phosphoinositide metabolism in NIH 3T3 fibroblasts

Orthovanadate is an agent known to stimulate cell growth and mimic insulin action. The effects of this compound on phosphoinositides in NIH 3T3 cells were examined. Both 100 and 1000 microM orthovanadate were found to increase the cellular content of inositol phosphate secondary to the activation of phosphatidylinositol-specific phospholipase C (PtdIns-PLC). The time course, dependence on orthovanadate concentration, and sensitivity to the isoflavone genistein were similar for orthovanadate-induced accumulation of inositol phosphate and protein tyrosine phosphate, indicating that there is a correlation between cellular protein tyrosine phosphate levels and PtdIns-PLC activity. Increased phosphatidylinositol phosphate (PtdInsP) content also occurred when cells were incubated with orthovanadate and appeared to result from the activation of PtdIns kinase. This effect was not correlated with cellular protein tyrosine phosphate content. Hence, orthovanadate is shown to affect phosphoinositide metabolism at a minimum of two sites by both tyrosine phosphate-dependent and -independent mechanisms.

Subcellular distribution of the alpha subunit(s) of Gi: visualization by immunofluorescent and immunogold labeling

The subcellular distribution of the alpha subunit(s) of Gi has an obvious bearing on the ability of this protein to interact with receptors and targets and on its potential to serve in still unexplored capacities. In this study, we have examined the distribution of Gi alpha by means of light and electron microscopy. The cells employed were mouse 3T3 fibroblasts, normal rat kidney fibroblasts, rat C6 glioma cells, human umbilical vein endothelial cells, and human 293 kidney fibroblasts. By indirect immunofluorescence, two patterns of Gi alpha were evident. The more prominent was that associated with phase-dense, cytoplasmic structures exhibiting a tubule-like morphology. A similar distribution was noted for mitochondria, indicating attachment to a subset of microtubules. The second pattern appeared as a diffuse, particulate fluorescence associated with the plasma membrane. By immunogold labeling and electron microscopy, two populations of Gi alpha were again evident. In this instance, labeling of the plasma membrane was the more prominent. Gold particles were most often evenly distributed along the plasma membrane and were concentrated along microspikes. The second, less abundant population of Gi alpha represented the subunit (or fragments) within lysosomes. Specificity in immunolabeling was confirmed in all instances by immunotransfer blotting, the use of antibodies differing in specificities for epitopes within Gi alpha, the absence of labeling with preimmune sera, and the decrease in labeling after preincubation of antisera with appropriate peptides. These results support the proposal that several populations of Gi alpha exist: those evident within the cytoplasm by immunofluorescence, those present at the plasma membrane, and those evident within lysosomes by immunogold labeling.

Tyrosine kinase-defective insulin receptors undergo insulin-induced microaggregation but do not concentrate in coated pits

Biologically active colloid-gold complexes were used to compare ligand-induced microaggregation, redistribution, and internalization of insulin receptors on Rat 1 fibroblasts expressing wild type (HIRc) or tyrosine kinase-defective (HIR A/K1018) human insulin receptors. Insulin-like growth factor I (IGF I) and alpha 2-macroglobulin receptors also were compared. On both cell types, all four unoccupied receptor types occurred predominantly as single receptors. Ligand binding caused receptor microaggregation. Microaggregation of wild type or kinase-defective insulin receptors or IGF I receptors was not different. alpha 2-Macroglobulin receptors formed larger microaggregates. Compared to wild type insulin or IGF I receptors, accumulation of kinase-defective insulin receptor microaggregates in endocytic structures was decreased, and the size of microaggregates in coated pits was significantly smaller. As a result, receptor-mediated internalization of gold-insulin by HIR A/K1018 cells was less than 6% of the cell-associated particles compared to approximately 60% of the particles in HIRc cells. On HIR A/K1018 cells, alpha 2-macroglobulin and IGF I were internalized via coated pits demonstrating that those structures were functional. These results suggest that: 1) ATP binding, receptor autophosphorylation, and activation of receptor kinase activity are not required for receptor microaggregation; 2) receptor microaggregation per se is not sufficient to cause ligand-induced receptor-mediated internalization or the biological effects of insulin; and 3) autophosphorylation of the beta-subunit or activation of the receptor kinase activity is required for the insulin-induced concentration of occupied receptors in coated pits.

Immunoelectron microscopic demonstration of insulin-stimulated translocation of glucose transporters to the plasma membrane of isolated rat adipocytes and masking of the carboxyl-terminal epitope of intracellular GLUT4

Polyclonal antibodies to the amino- or carboxyl-terminated peptide sequences of the GLUT4 transporter protein were used in immunoelectron microscopic studies to demonstrate the location and insulin-induced translocation of GLUT4 in intact isolated rat adipocytes. Labeling of untreated adipocytes with the amino-terminal antibody revealed 95% of GLUT4 was intracellular, associated with plasma membrane invaginations or vesicles contiguous with or within 75 nm of the cell membrane. Insulin treatment increased plasma membrane labeling approximately 13-fold, to 52% of the total transporters, and decreased intracellular labeling proportionately. In contrast, labeling of untreated adipocytes with the carboxyl-terminal antibody or with a monoclonal antibody (1F8) that binds to the carboxyl terminus of GLUT4 detected fewer transporters, only approximately 40% of which were intracellular. In insulin-treated cells, plasma membrane labeling increased approximately 20-fold, but the total number of labeled transporters also increased approximately 13-fold. The number of intracellular transporters was not changed. The insulin-induced increase in plasma membrane labeling was reversible. Thus, the vast majority of GLUT4 transporters in untreated adipocytes are intracellular in invaginations or vesicles attached or close to the plasma membrane. Insulin treatment causes translocation of transporters to the plasma membrane, which involves flow of transporters from invaginations to the cell surface and possible fusion of subplasma membrane vesicles with the plasma membrane. Differences in the labeling of intracellular transporters by peptide antibodies suggested the carboxyl-terminal epitope of intracellular transporters was masked. The unmasking of the carboxyl terminus during translocation to the plasma membrane may be part of the mechanism by which insulin stimulates glucose transport in rat adipocytes.

Embryonic chicken lens cells cultured in reconstituted basement membrane: an experimental model to maintain the epithelial phenotype in culture

The action of a reconstituted basement membrane has been studied on primary cultures of embryonic lens cells. When a solution of this matrix (Matrigel) was included in the culture medium, a high percentage of cells maintained the epithelial phenotype, judged by electron microscopy criteria, in contrast to the differentiated state induced by serum. Complete matrix stimulated by 6-fold the incorporation of 3H-thymidine into the cells, while one of its defined components, laminin, only had a 2-fold stimulatory effect. Thus, the basement membrane may stimulate mitogenesis and play a role complementary to that of growth factors in development.

Characterization of the growth of murine fibroblasts that express human insulin receptors. I. The effect of insulin in the absence of other growth factors

The effect of insulin on the growth of murine fibroblasts transfected with an expression vector containing human insulin receptor cDNA (NIH 3T3/HIR) and the parental cells (NIH/3T3) was characterized. Insulin in the absence of other mitogens increased the rate of incorporation of thymidine into NIH 3T3/HIR cells with a half-maximal response occurring at an insulin concentration of 35 ng/ml and a maximal response that was equivalent to that elicited by 10% fetal calf serum. The thymidine incorporation rate was increased by 12 h, was maximal at approximately 16 h, and returned to basal rates at 24 h after the addition of insulin. Insulin induced a maximum of 65% of cells to incorporate thymidine. The increased DNA synthesis was accompanied by net growth. Addition of insulin to the NIH 3T3/HIR cells resulted in increased DNA content with a half-maximal response occurring at approximately 30 ng/ml insulin and a maximal response equivalent to that elicited by serum. An increase in cell number detected after the addition of insulin to the NIH 3T3/HIR suggests that the cells had progressed through mitosis. Insulin did not increase the rate of thymidine incorporation, DNA content, or number of the parental NIH 3T3 cells. These data show that insulin, in the absence of a second mitogen, is able to induce NIH 3T3/HIR fibroblasts to traverse the cell cycle.

Characterization of the growth of murine fibroblasts that express human insulin receptors. II. Interaction of insulin with other growth factors

The effects of insulin-like growth factor-1 (IGF-1), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and insulin on DNA synthesis were studied in murine fibroblasts transfected with an expression vector containing human insulin receptor cDNA (NIH 3T3/HIR) and the parental NIH 3T3 cells. In NIH 3T3/HIR cells, individual growth factors in serum-free medium stimulated DNA synthesis with the following relative efficacies: insulin greater than or equal to 10% fetal calf serum greater than PDGF greater than IGF-1 much greater than EGF. In comparison, the relative efficacies of these factors in stimulating DNA synthesis by NIH 3T3 cells were 10% fetal calf serum greater than PDGF greater than EGF much greater than IGF-1 = insulin. In NIH 3T3/HIR cells, EGF was synergistic with 1-10 ng/ml insulin but not with 100 ng/ml insulin or more. Synergy of PDGF or IGF-1 with insulin was not detected. In the parental NIH 3T3 cells, insulin and IGF-1 were found to be synergistic with EGF (1 ng/ml), PDGF (100 ng/ml), and PDGF plus EGF. In NIH 3T3/HIR cells, the lack of interaction of insulin with other growth factors was also observed when the percentage of cells synthesizing DNA was examined. Despite insulin's inducing only 60% of NIH 3T3/HIR cells to incorporate thymidine, addition of PDGF, EGF, or PDGF plus EGF had no further effect. In contrast, combinations of growth factors resulted in 95% of the parental NIH 3T3 cells synthesizing DNA. The independence of insulin-stimulated DNA synthesis from other mitogens in the NIH 3T3/HIR cells is atypical for progression factor-stimulated DNA synthesis and is thought to be partly the result of insulin receptor expression in an inappropriate context or quantity.

The state of differentiation of embryonic chicken lens cells determines insulin-like growth factor I internalization

Microdissected epithelial cells from chicken embryonic lens differentiate into fiber cells during primary culture. Binding of IGF-I to both cell types has been documented biochemically. This report describes differences in the internalization of insulin-like growth factor I (IGF-I) in the two cell types using three electron microscopic approaches. Cells were incubated with a biologically active colloidal gold-labeled-IGF-I complex or [125I]IGF-I and prepared for electron microscopy. Other cells were incubated with unlabeled IGF-I and prepared for immunoelectron microscopy, using a colloidal gold-labeled anti-IGF-I antibody to detect the IGF-I. Each technique demonstrated binding of IGF-I on the surface of epithelial and differentiated fiber cells. IGF-I was internalized in epithelial cells. In contrast, the ligand exhibited little endocytosis in fiber cells during a 5 h incubation. Intracellular IGF-I was observed in the endosomes, Golgi apparatus, and lysosomes of epithelial cells, in addition, it was shown for the first time that IGF-I was translocated to the nucleus of epithelial cells. The differences between epithelial and fiber cells regarding internalization and nuclear translocation of IGF-I suggest that there are cell-specific itineraries of the hormone, depending on the differentiation stage of the cell. These differences may relate to the specific biological actions of the growth factor on each of these cells, particularly in allowing proliferation and differentiation of the epithelial cells.

Partial characterization of mechanism of insulin accumulation in H35 hepatoma cell nuclei

The mechanism controlling insulin accumulation in nuclei of H35 hepatoma cells was investigated by incubating intact cells with 125I-labeled insulin in the presence or absence of agents that perturb different intracellular sites involved in the processing of ligand-receptor complexes. Purified nuclei were isolated, and nuclear-associated 125I-insulin was determined. Insulin accumulation in the nuclei was time and temperature dependent. Nuclear accumulation was linear and insulin-concentration dependent between 5 and 50 ng insulin/ml. However, pharmacological concentrations of insulin increased the amount of insulin translocated to the nucleus to a far greater extent than it increased total cell-associated insulin. Chloroquine, an acidotrophic agent, increased total cell-associated and intracellular insulin but had no effect on nuclear accumulation. The monovalent ionophores monensin and nigericin inhibited nuclear accumulation of insulin at low concentrations (0.5-5.0 microM) without affecting total insulin binding or intracellular accumulation. At 10 or 25 microM, monensin and nigericin also acted as acidotrophic agents and increased total insulin binding and intracellular accumulation but inhibited nuclear accumulation by a maximum of 50%. Low concentrations of monensin and nigericin were additive; maximal concentrations were not. A 23187 and valinomycin did not affect insulin binding or intracellular and nuclear accumulation of insulin. Neither depletion of ATP by sodium azide, 2,4-dinitrophenol, sodium cyanide, or oligomycin nor disruption of cytoskeletal elements by cytochalasin D or colchicine had any effect on nuclear accumulation of insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in adenosine triphosphate dependency of receptor-mediated endocytosis of alpha 2-macroglobulin and insulin correlate with separate routes of ligand-receptor complex internalization

Biochemical and morphological studies compared the ATP requirements for and the internalization routes of alpha 2-macroglobulin and insulin in H35 hepatoma cells. Cellular ATP concentrations were decreased more than 94% by 1 mM 2,4-dinitrophenol or 10 mM sodium azide, potassium cyanide, or oligomycin. ATP depletion decreased total cell-associated alpha 2-macroglobulin 70-90% by inhibiting binding 67-77% and receptor-mediated internalization 90-96%. Under the same conditions, insulin binding was decreased less than 10%, and endocytosis and intracellular accumulation were not affected. Quantitative electron microscopic analysis of the distribution of occupied receptors on the surface of control and treated cells was performed using colloidal gold-labeled alpha 2-macroglobulin or insulin. alpha 2-Macroglobulin concentrated in and was internalized almost exclusively by coated pits. Insulin was rarely associated with coated pits, but was found in and internalized by noncoated invaginations. ATP depletion did not affect receptor mobility or ligand-induced aggregation of either receptor. There was an increase in the amount of alpha 2-macroglobulin found in coated pit-like structures. The coat underlying pits in ATP-depleted cells was poorly defined and may account for the inability of coated pits to form and/or internalize. These results showed that receptor-mediated internalization via coated pits was ATP dependent, whereas internalization via pinocytotic invaginations was energy independent, which explained the difference in the ATP dependency of uptake for the two ligands. These observations suggested that autophosphorylation of the insulin receptor may not be involved in either the aggregation or internalization of the insulin-receptor complex, since ATP depletion did not affect either process. This study provided evidence that specialized mechanisms exist for the internalization of insulin which may be related to some of its intracellular effects.

Intranuclear localization of insulin in rat hepatoma cells: insulin/matrix association

Previous studies have documented nuclear insulin accumulation in a variety of cell types. The present investigation extends these observations by demonstrating that insulin associates with the matrix fraction of H35 rat hepatoma cell nuclei. Nuclei were isolated from [125I]insulin-loaded cells and extracted with DNase I, RNase A and high salt. The resulting matrix fraction was found to contain greater than 75% of the radiolabel initially present. Ultrastructural studies to confirm these findings were carried out using an agarose-encapsulated nuclear matrix preparation. Electron microscopic immunocytochemistry specifically detected insulin in matrices prepared from insulin-treated cells. No reaction was observed in matrices obtained from non-insulin-treated (control) cells. Further biochemical analysis revealed that matrix-associated insulin could be solubilized with 1% sodium dodecyl sulfate (SDS) or in the presence of high urea concentrations. Gel filtration analysis of urea-solubilized matrix material revealed the presence of apparently intact [125I]insulin and a higher molecular weight peak. It is hypothesized that the latter may represent a tightly associated complex of insulin with some matrix protein(s).

Immunological demonstration of the accumulation of insulin, but not insulin receptors, in nuclei of insulin-treated cells

Although insulin is known to regulate nuclear-related processes, such as cell growth and gene transcription, the mechanisms involved are poorly understood. Previous studies suggested that translocation of insulin or its receptor to cell nuclei might be involved in some of these processes. The present investigation demonstrated that intact insulin, but not the insulin receptor, accumulated in nuclei of insulin-treated cells. Cell fractionation studies demonstrated that the nuclear accumulation of 125I-labeled insulin was time-, temperature-, and insulin-concentration-dependent. Electron microscopic immunocytochemistry demonstrated that the insulin that accumulated in the nucleus was immunologically intact and associated with the heterochromatin. Only 1% of the 125I-labeled insulin extracted from isolated nuclei was eluted from a Sephadex G-50 column as 125I-labeled tyrosine. Plasma membrane insulin receptors were not detected in the nucleus by immuno electron microscopy or when wheat germ agglutinin-purified extracts of the nuclei were subjected to PAGE, electrotransfer, and immunoblotting with anti-insulin receptor antibodies. These results suggested that internalized insulin dissociated from its receptor and accumulated in the nucleus without its membrane receptor. We propose that some of insulin's effects on nuclear function may be caused by the translocation of the intact and biologically active hormone to the nucleus and its binding to nuclear components in the heterochromatin.