Insulin inhibits transcription of IRS-2 gene in rat liver through an insulin response element (IRE) that resembles IREs of other insulin-repressed genes

Recent data indicate that sustained elevations in plasma insulin suppress the mRNA for IRS-2, a component of the insulin signaling pathway in liver, and that this deficiency contributes to hepatic insulin resistance and inappropriate gluconeogenesis. Here, we use nuclear run-on assays to show that insulin inhibits transcription of the IRS-2 gene in the livers of intact rats. Insulin also inhibited transcription of a reporter gene driven by the human IRS-2 promoter that was transfected into freshly isolated rat hepatocytes. The human promoter contains a heptanucleotide sequence, TGTTTTG, that is identical to the insulin response element (IRE) identified previously in the promoters of insulin-repressed genes. Single base pair substitutions in this IRE decreased transcription of the IRS-2-driven reporter in the absence of insulin and abolished insulin-mediated repression. We conclude that insulin represses transcription of the IRS-2 gene by blocking the action of a positive factor that binds to the IRE. Sustained repression of IRS-2, as occurs in chronic hyperinsulinemia, contributes to hepatic insulin resistance and accelerates the development of the diabetic state.

Vitamin B12 and hepatitis C: Molecular biology and human pathology

Cobalamins are stored in high concentrations in the human liver and thus are available to participate in the regulation of hepatotropic virus functions. We show that cyanocobalamin (vitamin B12) inhibited the HCV internal ribosome entry site (IRES)-dependent translation of a reporter gene in vitro in a dose-dependent manner without significantly affecting the cap-dependent mechanism. Vitamin B12 failed to inhibit translation by IRES elements from encephalomyocarditis virus (EMCV) or classical swine fever virus (CSFV). We also demonstrate a relationship between the total cobalamin concentration in human sera and HCV viral load (a measure of viral replication in the host). The mean viral load was two orders of magnitude greater when the serum cobalamin concentration was above 200 pM (P < 0.003), suggesting that the total cobalamin concentration in an HCV-infected liver is biologically significant in HCV replication.

Hepatitis B virus transactivator protein, HBx, associates with the components of TFIIH and stimulates the DNA helicase activity of TFIIH.

Human hepatitis B virus genome encodes a protein, termed HBx, that is widely recognized as a transcriptional transactivator. While HBx does not directly bind cis-acting transcriptional control elements, it has been shown to associate with cellular proteins that bind DNA. Because HBx transactivated a large number of viral/cellular transcriptional control elements, we looked for its targets within the components of the basal transcriptional machinery. This search led to the identification of its interactions with TFIIH. Here, we show that HBx interacts with yeast and mammalian TFIIH complexes both in vitro and in vivo. These interactions between HBx and the components of TFIIH are supported by several lines of evidence including results from immunoprocedures and direct methods of measuring interactions. We have identified ERCC3 and ERCC2 DNA helicase subunits of holoenzyme TFIIH as targets of HBx interactions. Furthermore, the DNA helicase activity of purified TFIIH from rat liver and, individually, the ERCC2 component of TFIIH is stimulated in the presence of HBx. These observations suggest a role for HBx in transcription and DNA repair.

Defective propagation of signals generated by sympathetic nerve stimulation in the liver of connexin32-deficient mice.

The gap junctional protein connexin32 is expressed in hepatocytes, exocrine pancreatic cells, Schwann cells, and other cell types. We have inactivated the connexin32 gene by homologous recombination in the mouse genome and have generated homozygous connexin32-deficient mice that were viable and fertile but weighed on the average approximately 17% less than wild-type controls. Electrical stimulation of sympathetic nerves in connexin32-deficient liver triggered a 78% lower amount of glucose mobilization from glycogen stores, when compared with wild-type liver. Thus, connexin32-containing gap junctions are essential in mouse liver for maximal intercellular propagation of the noradrenaline signal from the periportal (upstream) area, where it is received from sympathetic nerve endings, to perivenous (downstream) hepatocytes. In connexin32-defective liver, the amount of connexin26 protein expressed was found to be lower than in wild-type liver, and the total area of gap junction plaques was approximately 1000-fold smaller than in wild-type liver. In contrast to patients with connexin32 defects suffering from X chromosome-linked Charcot-Marie-Tooth disease (CMTX) due to demyelination in Schwann cells of peripheral nerves, connexin32-deficient mice did not show neurological abnormalities when analyzed at 3 months of age. It is possible, however, that they may develop neurodegenerative symptoms at older age.

Mammalian Sug1 and c-Fos in the nuclear 26S proteasome.

In a search for regulatory proteins that interact with the leucine zipper motif of c-Fos in the yeast two-hybrid screen, we have identified a protein (FZA-B) that has extensive sequence similarity to SUG1 of Saccharomyces cerevisiae. Here we show that FZA-B can functionally substitute for SUG1 in yeast and that FZA-B interacts with Fos proteins in vitro through their leucine zippers. In rat liver and in HeLa cells, FZA-B is present in the 26S proteasome complex, as is c-Fos. Immobilized antibody raised against an FZA-B-specific peptide depleted peptidase activity, proteasomal proteins, FZA-B, and c-Fos from a 26S proteasome preparation. FZA-B is found predominantly in the nuclear fraction of COS cells expressing an FZA-B transgene and in the nuclear 26S proteasome of HeLa cells. We conclude that FZA-B is the mammalian homolog of SUG1 (mSug1) and that it is present in the nuclear 26S proteasome of cells. Our results suggest that mSug1 may be involved in the degradation of c-Fos and other transcription factors.

Growth retardation and cysteine deficiency in gamma-glutamyl transpeptidase-deficient mice.

gamma-Glutamyl transpeptidase (GGT) is an ectoenzyme that catalyzes the first step in the cleavage of glutathione (GSH) and plays an essential role in the metabolism of GSH and GSH conjugates of carcinogens, toxins, and eicosanoids. To learn more about the role of GGT in metabolism in vivo, we used embryonic stem cell technology to generate GGT-deficient (GGTm1/GGTm1) mice. GGT-deficient mice appear normal at birth but grow slowly and by 6 weeks are about half the weight of wild-type mice. They are sexually immature, develop cataracts, and have coats with a gray cast. Most die between 10 and 18 weeks. Plasma and urine GSH levels in the GGTm1/GGTm1 mice are elevated 6-fold and 2500-fold, respectively, compared with wild-type mice. Tissue GSH levels are markedly reduced in eye, liver, and pancreas. Plasma cyst(e)ine levels in GGTm1/GGTm1 mice are reduced to approximately 20% of wild-type mice. Oral administration of N-acetylcysteine to GGTm1/GGTm1 mice results in normal growth rates and partially restores the normal agouti coat color. These findings demonstrate the importance of GGT and the gamma-glutamyl cycle in cysteine and GSH homeostasis.

Cloning and expression of human deoxyguanosine kinase cDNA.

A human cDNA sequence homologous to human deoxycytidine kinase (dCK; EC 2.7.1.74) was identified in the GenBank sequence data base. The longest open reading frame encoded a protein that was 48% identical to dCK at the amino acid level. The cDNA was expressed in Escherichia coli and shown to encode a protein with the same substrate specificity as described for the mitochondrial deoxyguanosine kinase (dGK; EC 2.7.1.113). The N terminus of the deduced amino acid sequence had properties characteristic for a mitochondrial translocation signal, and cleavage at a putative mitochondrial peptidase cleavage site would give a mature protein size of 28 kDa. Northern blot analysis determined the length of dGK mRNA to 1.3 kbp with no cross-hybridization to the 2.8-kbp dCK mRNA. dGK mRNA was detected in all tissues investigated with the highest expression levels in muscle, brain, liver, and lymphoid tissues. Alignment of the dGK and herpes simplex virus type 1 thymidine kinase amino acid sequences showed that five regions, including the substrate-binding pocket and the ATP-binding glycine loop, were also conserved in dGK. To our knowledge, this is the first report of a cloned mitochondrial nucleoside kinase and the first demonstration of a general sequence homology between two mammalian deoxyribonucleoside kinases. Our findings suggest that dCK and dGK are evolutionarily related, as well as related to the family of herpes virus thymidine kinases.

Mammalian phospholipase D: phosphatidylethanolamine as an essential component.

Bovine kidney phospholipase D (PLD) was assayed by measuring the formation of phosphatidylethanol from added radioactive phosphatidylcholine (PtdCho) in the presence of ethanol, guanosine 5'-[gamma-thio]triphosphate, ammonium sulfate, and cytosol factor that contained small GTP-binding regulatory proteins. The PLD enzyme associated with particulate fractions was solubilized by deoxycholate and partially purified by chromatography on a heparin-Sepharose column. This PLD preferentially used PtdCho as substrate. After purification, the enzyme per se showed little or practically no activity but required an additional factor for the enzymatic reaction. This factor was extracted with chloroform/methanol directly from particulate fractions of various tissues, including kidney, liver, and brain, and identified as phosphatidylethanolamine (PtdEtn), although this phospholipid did not serve as a good substrate. Plasmalogen-rich PtdEtn, dioleoyl-PtdEtn, and L-alpha-palmitoyl-beta-linoleoyl-PtdEtn were effective, but dipalmitoyl-PtdEtn was inert. Sphingomyelin was 30% as active as PtdEtn. The results suggest that mammalian PLD reacts nearly selectively with PtdCho in the form of mixed micelles or membranes with other phospholipids, especially PtdEtn.

Targeted disruption of gp130, a common signal transducer for the interleukin 6 family of cytokines, leads to myocardial and hematological disorders.

gp130 is a ubiquitously expressed signal-transducing receptor component shared by interleukin 6, interleukin 11, leukemia inhibitory factor, oncostatin M, ciliary neurotrophic factor, and cardiotrophin 1. To investigate physiological roles of gp130 and to examine pathological consequences of a lack of gp130, mice deficient for gp130 have been prepared. Embryos homozygous for the gp130 mutation progressively die between 12.5 days postcoitum and term. On 16.5 days postcoitum and later, they show hypoplastic ventricular myocardium without septal and trabecular defect. The subcellular ultrastructures in gp130-/- cardiomyocytes appear normal. The mutant embryos have greatly reduced numbers of pluripotential and committed hematopoietic progenitors in the liver and differentiated lineages such as T cells in the thymus. Some gp130-/- embryos show anemia due to impaired development of erythroid lineage cells. These results indicate that gp130 plays a crucial role in myocardial development and hematopoiesis during embryogenesis.

Improved biodistribution, tumor targeting, and reduced immunogenicity in mice with a gamma 4 variant of Campath-1H.

Radiolabeled antibodies have shown promise for the treatment of lymphoma and for solid tumor targeting. Campath-1H is a humanized monoclonal antibody that reacts with the CD52 antigen present on human lymphoid and myeloid cells. Campath-1H is a gamma1 (G1) isotype that induces lymphopenia via an Fc-mediated mechanism(s). Isotype switches were engineered, and the resulting antibodies were expressed in NS0 mouse myeloma cells and biosynthetically radiolabeled with [35S]methionine. The forms included G1, G4, and a G4 variant that contained alanine substitutions at (EU numbering) Leu-235, Gly-237, and Glu-318. All isotypes bound antigen equivalently as assessed by target cell binding in vitro. The G4 variant had a greatly reduced capacity to interact with Fc receptor by virtue of reduced binding to THP-1 human myeloid cells and by a 1000-fold increase in EC50 to intermediate antibody-dependent cellular cytotoxicity. The pharmacokinetics of the isotypes were compared in CD-1 (nu/nu) mice bearing an experimental antigen-expressing tumor. The plasma half-life and tumor uptake were increased for the G4 variant. The G4 variant showed significantly less spleen, liver, and bone uptake but similar uptake in the lung, kidney, and stomach and lower tissue-to-blood ratios. Immunogenicity was assessed after repeated monthly administrations of unlabeled antibody in BALB/c mice. A 50% reduction in the incidence of anti-globulin response was observed for the G4 variant. These properties suggest that antibodies with reduced Fc receptor interaction merit additional study as potential targeting vehicles relative to other isotypes for radioimmunotherapy or situations where diminished normal tissue binding contributes to efficacy.

Human fatty acid synthase: properties and molecular cloning.

Fatty acid synthase (FAS; EC 2.3.1.85) was purified to near homogeneity from a human hepatoma cell line, HepG2. The HepG2 FAS has a specific activity of 600 nmol of NADPH oxidized per min per mg, which is about half that of chicken liver FAS. All the partial activities of human FAS are comparable to those of other animal FASs, except for the beta-ketoacyl synthase, whose significantly lower activity is attributable to the low 4'-phosphopantetheine content of HepG2 FAS. We cloned the human brain FAS cDNA. The cDNA sequence has an open reading frame of 7512 bp that encodes 2504 amino acids (M(r), 272,516). The amino acid sequence of the human FAS has 79% and 63% identity, respectively, with the sequences of the rat and chicken enzymes. Northern analysis revealed that human FAS mRNA was about 9.3 kb in size and that its level varied among human tissues, with brain, lung, and liver tissues showing prominent expression. The nucleotide sequence of a segment of the HepG2 FAS cDNA (bases 2327-3964) was identical to that of the cDNA from normal human liver and brain tissues, except for a 53-bp sequence (bases 3892-3944) that does not alter the reading frame. This altered sequence is also present in HepG2 genomic DNA. The origin and significance of this sequence variance in the HepG2 FAS gene are unclear, but the variance apparently does not contribute to the lower activity of HepG2 FAS.

Mouse Col18a1 is expressed in a tissue-specific manner as three alternative variants and is localized in basement membrane zones.

We have isolated overlapping cDNAs encoding the N-terminal non-triple-helical region of mouse alpha 1(XVIII) collagen and shown that three different variants of alpha 1(XVIII) collagen exist. Each of the three variants shows characteristic tissue-specific expression patterns. Immunohistochemical studies show positive staining for alpha 1(XVIII) collagen along the basement membrane zones of vessels in the intestinal villi, the choroid plexus, skin, liver, and kidney. Thus, we conclude that alpha 1(XVIII) collagen may interact (directly or indirectly) with components in basement membrane zones or on the basal surface of endothelial/epithelial cells.

Host insulin stimulates Echinococcus multilocularis insulin signalling pathways and larval development.

BACKGROUND The metacestode of the tapeworm Echinococcus multilocularis is the causative agent of alveolar echinococcosis, a lethal zoonosis. Infections are initiated through establishment of parasite larvae within the intermediate host's liver, where high concentrations of insulin are present, followed by tumour-like growth of the metacestode in host organs. The molecular mechanisms determining the organ tropism of E. multilocularis or the influences of host hormones on parasite proliferation are poorly understood. RESULTS Using in vitro cultivation systems for parasite larvae we show that physiological concentrations (10 nM) of human insulin significantly stimulate the formation of metacestode larvae from parasite stem cells and promote asexual growth of the metacestode. Addition of human insulin to parasite larvae led to increased glucose uptake and enhanced phosphorylation of Echinococcus insulin signalling components, including an insulin receptor-like kinase, EmIR1, for which we demonstrate predominant expression in the parasite's glycogen storage cells. We also characterized a second insulin receptor family member, EmIR2, and demonstrated interaction of its ligand binding domain with human insulin in the yeast two-hybrid system. Addition of an insulin receptor inhibitor resulted in metacestode killing, prevented metacestode development from parasite stem cells, and impaired the activation of insulin signalling pathways through host insulin. CONCLUSIONS Our data indicate that host insulin acts as a stimulant for parasite development within the host liver and that E. multilocularis senses the host hormone through an evolutionarily conserved insulin signalling pathway. Hormonal host-parasite cross-communication, facilitated by the relatively close phylogenetic relationship between E. multilocularis and its mammalian hosts, thus appears to be important in the pathology of alveolar echinococcosis. This contributes to a closer understanding of organ tropism and parasite persistence in larval cestode infections. Furthermore, our data show that Echinococcus insulin signalling pathways are promising targets for the development of novel drugs.

Manipulation of host hepatocytes by the malaria parasite for delivery into liver sinusoids.

The merozoite stage of the malaria parasite that infects erythrocytes and causes the symptoms of the disease is initially formed inside host hepatocytes. However, the mechanism by which hepatic merozoites reach blood vessels (sinusoids) in the liver and escape the host immune system before invading erythrocytes remains unknown. Here, we show that parasites induce the death and the detachment of their host hepatocytes, followed by the budding of parasite-filled vesicles (merosomes) into the sinusoid lumen. Parasites simultaneously inhibit the exposure of phosphatidylserine on the outer leaflet of host plasma membranes, which act as "eat me" signals to phagocytes. Thus, the hepatocyte-derived merosomes appear to ensure both the migration of parasites into the bloodstream and their protection from host immunity.

(Table 1) Neutral and phenolic organohalogen compounds in blubber samples of ringed seals (Pusa hispida) from East Greenland

We report on the comparative bioaccumulation, biotransformation and/or biomagnification from East Greenland ringed seal (Pusa hispida) blubber to polar bear (Ursus maritimus) tissues (adipose, liver and brain) of various classes and congeners of persistent chlorinated and brominated contaminants and metabolic by-products: polychlorinated biphenyls (PCBs), chlordanes (CHLs), hydroxyl (OH-) and methylsulfonyl (MeSO2-) PCBs, polybrominated biphenyls (PBBs), OH-PBBs, polybrominated diphenyl ether (PBDE) and hexabromocyclododecane (HBCD) flame retardants and OH- and methoxyl (MeO-) PBDEs, 2,2-dichloro-bis(4-chlorophenyl)ethene (p,p'-DDE), 3-MeSO2-p,p'-DDE, pentachlorophenol (PCP) and 4-OH-heptachlorostyrene (4-OH-HpCS). We detected all of the investigated contaminants in ringed seal blubber with high frequency, the main diet of East Greenland bears, with the exception of OH-PCBs and 4-OH-HpCS, which indicated that these phenolic contaminants were likely of metabolic origin and formed in the bears from accumulated PCBs and octachlorostyrene (OCS), respectively, rather than being bioaccumulated from a seal blubber diet. For all of the detectable sum of classes or individual organohalogens, in general, the ringed seal to polar bear mean BMFs for SumPCBs, p,p'-DDE, SumCHLs, SumMeSO2-PCBs, 3-MeSO2-p,p'-DDE, PCP, SumPBDEs, total-(alpha)-HBCD, SumOH-PBDEs, SumMeO-PBDEs and SumOH-PBBs indicated that these organohalogens bioaccumulate, and in some cases there was tissue-specific biomagnification, e.g., BMFs for bear adipose and liver ranged from 2 to 570. The blood-brain barrier appeared to be effective in minimizing brain accumulation as BMFs were <= 1 in the brain, with the exception of SumOH-PBBs (mean BMF = 93±54). Unlike OH-PCB metabolites, OH-PBDEs in the bear tissues appeared to be mainly accumulated from the seal blubber rather than being metabolic formed from PBDEs in the bears. In vitro PBDE depletion assays using polar bear hepatic microsomes, wherein the rate of oxidative metabolism of PBDE congeners was very slow, supported the probability that accumulation from seals is the main source of OH-PBDEs in the bear tissues. Our findings demonstrated from ringed seal to polar bears that organohalogen biotransformation, bioaccumulation and/or biomagnification varied widely and depended on the contaminant in question. Our results show the increasing complexity of bioaccumulated and in some cases biomagnified, chlorinated and brominated contaminants and/or metabolites from the diet may be a contributing stress factor in the health of East Greenland polar bears.