Action of insulin at the nuclear envelope

Insulin-induced translocation of IR to the nucleus in insulin responsive cells requires a nuclear translocation sequence

Insulin binding to its cell surface receptor (IR) activates a cascade of events leading to its biological effects. The Insulin-IR complex is rapidly internalized and then is either recycled back to the plasma membrane or sent to lysosomes for degradation. Although most of the receptor is recycled or degraded, a small amount may escape this pathway and migrate to the nucleus of the cell where it might be important in promulgation of receptor signals. In this study we explored the mechanism by which insulin induces IR translocation to the cell nucleus. Experiments were performed cultured L6 myoblasts, AML liver cells and 3T3-L1 adipocytes. Insulin treatment induced a rapid increase in nuclear IR protein levels within 2 to 5 min. Treatment with WGA, an inhibitor of nuclear import, reduced insulin-induced increases nuclear IR protein; IR was, however, translocated to a perinuclear location. Bioinformatics tools predicted a potential nuclear localization sequence (NLS) on IR. Immunofluorescence staining showed that a point mutation on the predicted NLS blocked insulin-induced IR nuclear translocation. In addition, blockade of nuclear IR activation in isolated nuclei by an IR blocking antibody abrogated insulin-induced increases in IR tyrosine phosphorylation and nuclear PKCδ levels. Furthermore, over expression of mutated IR reduced insulin-induced glucose uptake and PKB phosphorylation. When added to isolated nuclei, insulin induced IR phosphorylation but had no effect on nuclear IR protein levels. These results raise questions regarding the possible role of nuclear IR in IR signaling and insulin resistance.

Effects of voluntary physical exercise on high-fat diet-promoted pancreatic carcinogenesis in the hamster model

We examined the effect of voluntary physical exercise (running wheels) on pancreatic carcinogenicity of N-nitrosobis-(2-oxopropyl) amine (BOP) in groups of female Syrian hamsters fed a high-fat (HF) diet in which corn oil was 24.6% of the diet or a low-fat (LF) diet in which corn oil was 4.5% of the diet. Each group was divided into an exercising (EX) group (LF-EX and HF-EX) and a sedentary (S) group (LF-S and HF-S). All hamsters were treated with BOP (20 mg/kg body wt) weekly for two weeks beginning four weeks after the experimental diets, which were fed from weaning. A modified glucose tolerance test was performed before the BOP injections and then again at 20 and 40 weeks, and the levels of glucose, insulin-like growth factor I, and insulin were determined in the plasma samples. At the end of the experiment, serum levels of lipid metabolites were also examined in six hamsters from each group. The experiment was terminated 44 weeks after the BOP treatment. Pancreatic ductal/ductular adenocarcinoma incidence was significantly higher in hamsters fed the HF diet (HF-S and HF-EX) than in those fed the LF diet (LF-S and LF-EX). In all groups, glucose and insulin-like growth factor I levels remained within the normal range throughout the experiment, whereas insulin and lipid metabolite levels were significantly elevated in all hamsters fed the HF diet (HF-S and HF-EX). Exercise significantly reduced the insulin level in the group fed the HF diet but did not influence the cancer burden, possibly by the generation of reactive lipid metabolites. Overall, the results showed that voluntary physical exercise does not influence the promotional action of the HF diet on pancreatic carcinogenesis in hamsters. This action could be attributed to the ability of the HF diet to increase the secretion of insulin, which has a growth-promoting and mitogenic effect on pancreatic cells, and to the effect of an HF diet or physical exercise in producing excess free radicals.

Expression of biologically active procorticotrophin-releasing hormone (proCRH) in stably transfected CHO-K1 cells: characterization of nuclear proCRH

Corticotrophin-releasing hormone (CRH) is a 41 amino acid neuropeptide which is cleaved at a pair of dibasic amino acids from a larger precursor molecule (pre-proCRH) by the action of endopeptidases. In cells possessing a regulated secretory pathway, sorting of proneuropeptides and prohormones occurs within the trans-Golgi network, where they are finally packaged into secretory vesicles to be released in response to an external stimulus. Such cells also possess a constitutive secretory pathway, and neuropeptides are also translocated into this subcellular compartment. We have recently established stably transfected CHO-K1 cells expressing the rat pre-proCRH cDNA, and shown that proCRH was localized within the secretory pathway and the nucleus of transfected cells. Both the cytoplasmic and nuclear species of IR-CRH displayed an apparent molecular weight approximately 19 kDa, consistent with the size of the uncleaved CRH precursor molecule. In this paper, we further characterized the bitopological, i.e. nuclear and cytoplasmic localization of proCRH within transfected CHO-K1 cells. Immunoreactive nuclear CRH was not extractable using detergents (Triton X-100 and CHAPS), 10 mM salt washes or RNase digestion but could be abolished by digestion with DNase I. These results therefore suggest that nuclear proCRH is in close association with DNA/chromatin. Treatment of transfected cells with inhibitors of protein and RNA synthesis for up to 24 h had no effect upon immunoreactive nuclear CRH, indicating that it is very stable with a long half life. Brefeldin A treatment had no effect upon the nuclear translocation of newly synthesized proCRH, suggesting that late stages of the secretory pathway (i.e. post rough endoplasmic reticulum compartments) of the transfected cells do not play a role in proCRH nuclear transport. We also demonstrate that proCRH synthesized within stably transfected CHO-K1 cells is capable of stimulating ACTH release from primary cultures of anterior pituitary cells, therefore showing for the first time that the intact precursor is also biologically active and could act as an ACTH secretagogue in-vivo.

Nucleocytoplasmic transport

Images

Internalization and nuclear localization of peptide hormones

The presence of neuropeptide receptors on the plasma membrane is well accepted, as is its internalization and down-regulation. The analysis of the fate of these peptides within their target-cells is difficult. Endogenous peptides or administered native peptides are visualized in these cells using immunocytology after cryoultramicrotomy. Labelled peptides can be injected and their internalization kinetics studied using ultrastructural autoradiography. The pituitary gland is a suitable model for the study of the neuropeptide mechanism, with the lactotroph function being taken as an example in the present case. Prolactin (PRL) release depends on two main neuropeptides: thyrotropin-releasing hormone (TRH) and somatostatin (SS). The TRH immunoreactivity obtained from endogenous as well as injected material was restricted to the plasma membrane, secretory granules, cytoplasmic matrix and nucleus. The internalization kinetics of exogenous native TRH showed an increase of immunoreactive material in all compartments including the nucleus. The endogenous SSs (SS14 and SS28) were detected in the same subcellular lactotroph compartments. Injection of 125I-SS showed a rapid binding of SS at the plasma membrane level before internalization. For 60 min of in vivo uptake, 125I-SS28, the large SS molecule, was detected in the cytoplasm only, while 125I-SS14 was found in the nuclear matrix. In vitro 125I-SS28 was restricted to the nuclear membrane. Under physiological conditions the endogenous neuropeptides were visualized in the nucleus, but after injection of labelled peptides only small molecules were found in the nucleus. The significance of the presence of these neuropeptides is discussed.

A 46 kDa NTPase common to rat liver nuclear envelope, mitochondria, plasma membrane, and endoplasmic reticulum

A 46 kDa ATP binding polypeptide of the nuclear envelope, virtually identical to the nuclear envelope NTPase putatively involved in mRNA efflux [6], is present in all rat liver cell membranes. Its presence in nuclear envelope is not the result of cross contamination during isolation.

Light and electron microscopic localization of retrogradely transported neurotensin in rat nigrostriatal dopaminergic neurons

We previously demonstrated the existence of a retrograde axonal transport of radioactivity to the substantia nigra pars compacta following injection of mono-iodinated neurotensin in rat neostriatum. In the present study, the topographical and cellular distribution of this retrogradely transported material was examined by light and electron microscopic autoradiography. Four and a half hours after unilateral injection of [125I]neurotensin in the caudoputamen, retrogradely labelled neuronal cell bodies were detected by light microscopic autoradiography throughout the ipsilateral substantia nigra pars compacta as well as within the ventral tegmental area and retrorubral field. In semithin sections, silver grains were prevalent over the perinuclear cytoplasm of neuronal cell bodies but were also detected over neuronal nuclei. Analysis of electron microscopic autoradiographs revealed that the vast majority (greater than 85%) were associated with neuronal perikarya, unmyelinated and myelinated axons, dendrites and terminals. Within the soma, a significant proportion of silver grains (16% of somatic grains) was detected over the nucleus. However, the majority were identified over the cytoplasm where they often encompassed cytoplasmic organelles, including rough endoplasmic reticulum, mitochondria, Golgi apparatus, lysosomes, and multi-vesicular bodies. In dendrites and axons, a substantial percentage of silver grains (63-89%) was localized over the plasma membrane. A minor proportion (13% of total) of the autoradiographic labelling was detected over myelin sheaths, astrocytes, and oligodendrocytes. The present results are consistent with previous light-microscopic evidence for internalization and retrograde transport of intrastriatal neurotensin within nigrostriatal dopaminergic neurons. They further suggest that retrogradely transported neurotensin may be processed along a variety of intracellular pathways including those mediating degradation in lysosomes and recycling in rough endoplasmic reticulum. The detection of specific autoradiographic labelling in the nucleus supports the concept that neurotensin alone, or complexed to its receptor, might be involved in the regulation of gene expression through direct or indirect interactions with nuclear DNA. Consequently, the retrograde transport of neurotensin in nigrostriatal dopaminergic neurons might provide a vehicle through which events occurring at the level of the axon terminal may initiate long-term biological responses.

Recent progress in interferon research: molecular mechanisms of regulation, action, and virus circumvention

A complex system of cis regulatory elements exists by which induction of IFN gene expression is initiated in response to a variety of inducers; cis elements also appear to be involved in the down-regulation of IFN production. IFN gene activation or inhibition of expression may be tightly regulated by the specific binding of newly synthesized or modified proteins to be regulatory regions of the IFN genes. IFN itself acts as a potent modulator of multiple cellular activities. By binding to specific cell surface receptors and probable internalization via receptor-mediated endocytosis and transport into the dense chromatin, IFN treatment leads to activation of numerous genes, some of which possess known antiviral or immunoregulatory functions, whereas the function of others remains to be identified. As with the IFN genes themselves, many of the IFN-inducible genes appear to possess complex regulatory mechanisms, including domains for binding of specific trans-acting proteins. To add to this molecular complexity some viruses have successfully developed methods to circumvent, among other mechanisms, the 2',5'-A-mediated system and the P1 protein kinase system.

Interferon receptors and their role in interferon action

Interferon (IFN) proteins interact with cells through specific cell surface receptors, some of which have been purified and cloned. The alpha-IFNs and beta-IFN bind to a common receptor (type I), whereas gamma-IFN binds to a separate receptor (type II). Both types of high-affinity receptors have been demonstrated on a variety of receptors and the ways in which IFNs may affect cellular physiology and gene expression is discussed.

Ultrastructural evidence for the accumulation of insulin in nuclei of intact 3T3-L1 adipocytes by an insulin-receptor mediated process

Monomeric ferritin-labeled insulin (Fm-Ins), a biologically active, electron-dense marker of occupied insulin receptors, was used to characterize the internalization of insulin in 3T3-L1 adipocytes. Fm-Ins bound specifically to insulin receptors and was internalized in a time- and temperature-dependent manner. Fm-Ins was found in cytoplasmic vesicles within 5-10 min at 37 degrees C and subsequently was observed in multivesicular bodies and lysosomes. In addition, small amounts of Fm-Ins were associated with nuclei after 30 min. The number of Fm-Ins particles observed in nuclei continued to increase in a time-dependent manner until at least 90 min. In the nucleus, several Fm-Ins particles usually were found in the same general location--near nuclear pores, associated with the periphery of the condensed chromatin. Addition of a 250-fold excess of unlabeled insulin or incubation at 15 degrees C reduced the number of Fm-Ins particles found in nuclei after 90 min by 99% or 92%, respectively. Nuclear accumulation of unlabeled ferritin was only 2% of that found with Fm-Ins after 90 min at 37 degrees C. Biochemical experiments utilizing 125I-labeled insulin and subcellular fractionation indicated that intact 3T3-L1 adipocytes internalized insulin rapidly and that approximately equal to 3% of the internalized ligand accumulated in nuclei after 1 hr. These data provide biochemical and high-resolution ultrastructural evidence that 3T3-L1 adipocytes accumulate potentially significant amounts of insulin in nuclei by an insulin receptor-mediated process. The transport of insulin or the insulin-receptor complex to nuclei in this cell or in others may be directly involved in the long-term biological effects of insulin--in particular, in the control of DNA and RNA synthesis.

Proteins from rat liver cytosol which stimulate mRNA transport. Purification and interactions with the nuclear envelope mRNA translocation system

Two polysome-associated proteins with particular affinities for poly(A) have been purified from rat liver. These proteins stimulate the efflux of mRNA from isolated nuclei in conditions under which such efflux closely stimulates mRNA transport in vivo, and they are therefore considered as mRNA-transport-stimulatory proteins. Their interaction with the mRNA-translocation system in isolated nuclear envelopes has been studied. The results are generally consistent with the most recently proposed kinetic model of mRNA translocation. One protein, P58, has not been described previously. It inhibits the protein kinase that down-regulates the NTPase, it enhances the NTPase activity in both the presence and the absence of poly(A) and it seems to increase poly(A) binding in unphosphorylated, but not in phosphorylated, envelopes. The other protein, P31, which probably corresponds to the 35,000-Mr factor described by Webb and his colleagues, enhances the binding of poly(A) to the mRNA-binding site in the envelope, thus stimulating the phosphoprotein phosphatase and, in consequence, the NTPase. The possible physiological significance of these two proteins is discussed.

[Regulation of acinar cell receptors of the pancreas by peptides]

Peptides may act on the same receptor they regulate or on another receptor by causing regulations via receptor interactions. These receptor regulations include changes of receptor affinity and capacity. Receptor capacity is regulated by internalization, recycling, degradation, synthesis, and modification of bioavailability without migration of the receptor. Examples for those regulations, mostly based on experiments with isolated pancreatic acini from the rat, mouse, or guinea pig, are given. For the CCK receptor these examples include complex regulations of this receptor by CCK itself, bringing into discussion the hypothesis of negative cooperativity and the two-site receptor model, desensitization of the receptor by CCK, in vivo CCK influences on its receptor, and insulin receptor/CCK receptor interactions. For the insulin receptor the physiological significance of "up and down regulation" of this receptor by insulin itself is discussed. For the IGF receptors and the EGF receptor CCK-induced, Ca2+-mediated regulation of receptor internalization are another type of regulation with unknown physiological and pathophysiological significance. Finally CCK-induced, Ca2+-mediated regulation of somatostatin receptor capacity and affinity are mentioned. It is postulated that those regulations play an important role in influencing the biological effect of hormones and that knowledge about them may improve our understanding of pathophysiology.

Nucleocytoplasmic RNA transport

A number of closely related post-transcriptional facets of RNA metabolism show nuclear compartmentation, including capping, methylation, splicing reactions, and packaging in ribonucleoprotein particles (RNP). These nuclear 'processing' events are followed by the translocation of the finished product across the nuclear envelope. Due to the inherent complexity of these interrelated events, in vitro systems have been designed to examine the processes separately, particularly so with regard to translocation. A few studies have utilized nuclear transplantation/microinjection techniques and specialized systems to show that RNA transport occurs as a regulated phenomenon. While isolated nuclei swell in aqueous media and dramatic loss of nuclear protein is associated with this swelling, loss of RNA is not substantial, and most studies on RNA translocation have employed isolated nuclei. The quantity of RNA transported from isolated nuclei is related to hydrolysis of high-energy phosphate bonds in nucleotide additives. The RNA is released predominantly in RNP: messenger-like RNA is released in RNP which have buoyant density and polypeptide composition similar to cytoplasmic messenger RNP, but which have distinctly different composition from those in heterogeneous nuclear RNP. Mature 18 and 28S ribosomal RNA is released in 40 and 60S RNP which represent mature ribosomal subunits. RNA transport proceeds with characteristics of an energy-requiring process, and proceeds independently of the presence or state of fluidity of nuclear membranes. The energy for transport appears to be utilized by a nucleoside triphosphatase (NTPase) which is distributed mainly within heterochromatin at the peripheral lamina. Photoaffinity labeling has identified the pertinent NTPase as a 46 kD polypeptide which is associated with nuclear envelope and matrix preparations. The NTPase does not appear to be modulated via direct phosphorylation or to reflect kinase-phosphatase activities. A large number of additives (including RNA and insulin) produce parallel effects upon RNA transport and nuclear envelope NTPase, strengthening the correlative relationship between these activities. Of particular interest has been the finding that carcinogens induce specific, long-lasting increases in nuclear envelope (and matrix) NTPase; this derangement may underlie the alterations in RNA transport associated with cancer and carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Murine interferon-beta receptor-mediated endocytosis and nuclear membrane binding

Radioiodinated mouse interferon-beta (125I-MuIFN-beta) bound with high affinity (Kd = 9.8 X 10(-10) M) to plasma membrane of L929 murine fibroblasts (4-6 X 10(3) receptor sites per cell). The binding was saturable and inhibited by a 100-fold excess of unlabeled MuIFN-beta but not by excess mouse IFN-gamma (MuIFN-gamma). MuIFN-beta bound at 4 degrees C was very rapidly internalized upon warming of the cells to 37 degrees C (t 1/2 = 1.5 min). Indirect immunoferritin labeling indicated that MuIFN-beta was initially located in coated pits and subsequently internalized by receptor-mediated endocytosis. Isolated L929 cell nuclei bound 125I-MuIFN-beta with a 7-fold higher affinity (Kd = 1.4 X 10(-10) M) and higher receptor density (about 10(4) per nucleus) than that for the plasma membrane. Binding to the nuclear membrane was inhibited by a 100-fold excess of unlabeled MuIFN-beta but not by excess MuIFN-gamma. Trypsin treatment of nuclei decreased IFN binding by 80%, suggesting that the putative nuclear receptors are protein. Specific binding of MuIFN-beta to nuclei was also shown by fluorescence and electron microscopy. We propose that the very rapid internalization of MuIFN-beta by receptor-mediated endocytosis is important in the cellular processing of IFN and that its high-affinity binding to the nuclear membrane suggests the nucleus as an intracellular site of IFN action.