Heparin induces dimerization and confers proliferative activity onto the hepatocyte growth factor antagonists NK1 and NK2.

Hepatocyte growth factor (HGF) is a potent epithelial mitogen whose actions are mediated through its receptor, the proto-oncogene c-Met. Two truncated variants of HGF known as NK1 and NK2 have been reported to be competitive inhibitors of HGF binding to c-Met, and to function as HGF antagonists (Lokker, N.A., and P.J. Godowski. 1993. J. Biol. Chem. 268: 17145-17150; Chan, A.M., J.S. Rubin, D.P. Bottaro, D.W. Hirschfield, M. Chedid, and S.A. Aaronson. 1991. Science (Wash. DC). 254:1382-1387). We show here, however, that NK1 acts as a partial agonist in mink lung cells. Interestingly, NK1, which is an HGF antagonist in hepatocytes in normal conditions, was converted to a partial agonist by adding heparin to the culture medium. The interaction of NK1 and heparin was further studied in BaF3 cells, which express little or no cell surface heparan sulfate proteoglycans. In BaF3 cells transfected with a plasmid encoding human c-Met, heparin and NK1 synergized to stimulate DNA synthesis and cell proliferation. There was no effect of heparin on the IL-3 sensitivity of BaF3-hMet cells, and no effect of NK1 plus heparin in control BaF3 cells, indicating that the response was specific and mediated through c-Met. The naturally occurring HGF splice variant NK2 also stimulated DNA synthesis in mink lung cells and exerted a heparin-dependent effect on BaF3-hMet cells, but not on BaF3-neo cells. The activating effect of heparin was mimicked by a variety of sulfated glycosaminoglycans. Mechanistic studies revealed that heparin increased the binding of NK1 to BaF3-hMet cells, stabilized NK1, and induced dimerization of NK1. Based on these studies, we propose that the normal agonist activity of NK1 and NK2 in mink lung cells is due to an activating interaction with an endogenous glycosaminoglycan. Consistent with that model, a large portion of the NK1 binding to mink lung cells could be blocked by heparin. Moreover, a preparation of glycosaminoglycans from the surface of mink lung cells induced dimerization of NK1. These data show that the activity of NK1 and NK2 can be modulated by heparin and other related glycosaminoglycans to induce proliferation in cells expressing c-Met.

Signal transduction and transcriptional regulation by glucocorticoid receptor-LexA fusion proteins.

The glucocorticoid receptor regulates transcriptional initiation upon binding to its cognate hormone. A series of fusion genes was constructed to examine the mechanism of hormone-regulated transcriptional enhancement. The DNA binding domain of the bacterial LexA repressor was fused to receptor derivatives lacking the region that is necessary and sufficient for specific DNA binding and transcriptional enhancement at glucocorticoid response elements (GRE's). The resultant hybrid proteins activated transcription from promoters linked to the lex operator. Enhancement still required hormone binding by the hybrid receptor regardless of the exact positioning of the LexA binding domain within the protein. Thus, the unliganded hormone binding domain of the receptor acts as a strong but reversible inhibitor of receptor activity in a manner that is independent of the means by which the receptor recognizes DNA. The results also show directly that the receptor contains at least one "enhancement domain" other than that overlapping the GRE binding region; the second domain, enh2, occupies a region near the receptor amino terminus.

Glucocorticoid receptor mutants that define a small region sufficient for enhancer activation.

Transcriptional enhancement is a general mechanism for regulation of gene expression in which particular proteins bound to specific DNA sequences stimulate the efficiency of initiation from linked promoters. One such protein, the glucocorticoid receptor, mediates enhancement in a glucocorticoid hormone-dependent manner. In this study, a region of the 795-amino acid rat glucocorticoid receptor that is active in transcriptional enhancement was identified. The active region was defined by expressing various receptor deletion mutants in stably and transiently transfected cells and examining the regulated transcription of hormone-responsive genes. Mutant receptors lacking as many as 439 amino-terminal amino acids retained activity, as did those with as many as 270 carboxyl-terminal amino acids deleted. This suggests that the 86-amino acid segment between the most extensive terminal deletions, which also includes sequences required for specific DNA binding in vitro, is sufficient for enhancer activation. In fact, a 150-amino acid receptor fragment that encompasses this segment mediates constitutive enhancement.

Host defense mechanisms triggered by microbial lipoproteins through toll-like receptors.

The generation of cell-mediated immunity against many infectious pathogens involves the production of interleukin-12 (IL-12), a key signal of the innate immune system. Yet, for many pathogens, the molecules that induce IL-12 production by macrophages and the mechanisms by which they do so remain undefined. Here it is shown that microbial lipoproteins are potent stimulators of IL-12 production by human macrophages, and that induction is mediated by Toll-like receptors (TLRs). Several lipoproteins stimulated TLR-dependent transcription of inducible nitric oxide synthase and the production of nitric oxide, a powerful microbicidal pathway. Activation of TLRs by microbial lipoproteins may initiate innate defense mechanisms against infectious pathogens.

Induction of direct antimicrobial activity through mammalian toll-like receptors.

The mammalian innate immune system retains from Drosophila a family of homologous Toll-like receptors (TLRs) that mediate responses to microbial ligands. Here, we show that TLR2 activation leads to killing of intracellular Mycobacterium tuberculosis in both mouse and human macrophages, through distinct mechanisms. In mouse macrophages, bacterial lipoprotein activation of TLR2 leads to a nitric oxide-dependent killing of intracellular tubercle bacilli, but in human monocytes and alveolar macrophages, this pathway was nitric oxide-independent. Thus, mammalian TLRs respond (as Drosophila Toll receptors do) to microbial ligands and also have the ability to activate antimicrobial effector pathways at the site of infection.

Identification of a new member of the tumor necrosis factor family and its receptor, a human ortholog of mouse GITR.

The tumor necrosis factor (TNF) and TNF receptor (TNFR) gene superfamilies regulate diverse biological functions, including cell proliferation, differentiation, and survival [1] [2] [3]. We have identified a new TNF-related ligand, designated human GITR ligand (hGITRL), and its human receptor (hGITR), an ortholog of the recently discovered murine glucocorticoid-induced TNFR-related (mGITR) protein [4]. The hGITRL gene mapped to chromosome 1q23, near the gene for the TNF homolog Fas/CD95 ligand [5]. The hGITR gene mapped to chromosome 1p36, near a cluster of five genes encoding TNFR homologs [1] [6]. We found hGITRL mRNA in several peripheral tissues, and detected hGITRL protein on cultured vascular endothelial cells. The levels of hGITR mRNA in tissues were generally low; in peripheral blood T cells, however, antigen-receptor stimulation led to a substantial induction of hGITR transcripts. Cotransfection of hGITRL and hGITR in embryonic kidney 293 cells activated the anti-apoptotic transcription factor NF-kappaB, via a pathway that appeared to involve TNFR-associated factor 2 (TRAF2) [7] and NF-kappaB-inducing kinase (NIK) [8]. Cotransfection of hGITRL and hGITR in Jurkat T leukemia cells inhibited antigen-receptor-induced cell death. Thus, hGITRL and hGITR may modulate T lymphocyte survival in peripheral tissues.

Hepatocyte growth factor/scatter factor effects on epithelia. Regulation of intercellular junctions in transformed and nontransformed cell lines, basolateral polarization of c-met receptor in transformed and natural intestinal epithelia, and induction of rapid wound repair in a transformed model epithelium.

Intestinal epithelial cells rest on a fibroblast sheath. Thus, factors produced by these fibroblasts may influence epithelial function in a paracrine fashion. We examined modulation of intestinal epithelial function by one such fibroblast product, scatter factor/hepatocyte growth factor (HGF/SF). This effect was studied in vitro by using model T84 intestinal epithelial cells. When applied to confluent T84 monolayers, HGF/SF attenuates transepithelial resistance to passive ion flow in a dose-dependent manner (maximum fall at 300 ng/ml, 28% control monolayer resistance, P < 0.001, ED50 of 1.2 nM), t1/2 of 20 h. This functional effect of HGF/SF and distribution of its receptor, c-met, are polarized to the basolateral membranes of T84 intestinal epithelial cells. HGF/SF effects on resistance are not attributable to altered transcellular resistance (opening of Cl- and/or basolateral K+ channels), cytotoxicity, or enhanced cell proliferation; they therefore represent specific regulation of paracellular tight junction resistance. Analysis with biochemically purified rodent HGF/SF and Madin-Darby canine kidney cells reveals that effects on paracellular tight junctions also occur in other nontransformed epithelia. Binding of HGF/SF to its receptor in T84 intestinal epithelial cells is accompanied by tyrosine phosphorylation of the receptor. Because loosening of intercellular junctions between cells could facilitate separation, spreading, and migration of epithelial cells during physiologic processes such as wound resealing, we determined the effects of HGF/SF on intestinal epithelial wound resealing using our previously published in vitro model (Nusrat, A., C. Delp, and J. L. Madara. 1992. J. Clin. Invest. 89:1501-1511). HGF/SF markedly enhanced wound closure (> 450% increase in rate, P < 0.001) by influencing the migratory and spreading response in not only cells adjoining the wound but also cells many positions removed from the wound. We thus speculate that HGF/SF may serve as an important cytokine that influences epithelial parameters such as transepithelial resistance and wound resealing. Further pharmacological approaches to manipulate HGF/SF signaling pathways may provide novel therapeutic strategies to enhance repair of intestinal epithelial erosions/ulcerations.

Crystal structure of the NK1 fragment of human hepatocyte growth factor at 2.0 A resolution.

BACKGROUND: Hepatocyte growth factor (HGF) is a mitogen for hepatocytes and has also been implicated as an epithelial morphogen in tumor invasion. HGF activates its specific cellular receptor, c-met, through an aggregation mechanism potentiated by heparan sulfate glycosaminoglycans. HGF consists of an N-terminal (N) domain, four kringle domains (the first of which carries receptor-binding determinants), and an inactive serine-protease-like domain. NK1, a naturally occurring fragment of HGF, acts as an antagonist of HGF in the absence of heparin. RESULTS: The N domain of NK1 consists of a central five-stranded antiparallel beta sheet flanked by an alpha helix and a two-stranded beta ribbon. The overall N domain structure in the context of the NK1 fragment is similar to the structure of the isolated domain; two lysines and an arginine residue coordinate a bound sulfate ion. The NK1 kringle domain is homologous to kringle 4 from plasminogen, except that the lysine-binding pocket is altered by the insertion of a glycine residue. Here, a HEPES molecule is bound in the pocket. The asymmetric unit of the crystal contains a 'head-to-tail' NK1 dimer. We use this dimer to propose a model of the NK2 fragment of HGF. CONCLUSIONS: A cluster of exposed lysine and arginine residues in or near the hairpin-loop region of the N domain might form part of the NK1 heparin-binding site. In our NK2 model, both kringle domains pack loosely against the N domain, and a long, positively charged groove lines the interface. This groove might be involved in glycosaminoglycan binding. The HGF receptor-binding determinants are clustered near the binding pocket of the first kringle domain, opposite the N domain.

Identification of Gas6 as a ligand for Mer, a neural cell adhesion molecule related receptor tyrosine kinase implicated in cellular transformation.

Mer/Nyk/Eyk is an orphan receptor tyrosine kinase expressed at high levels in monocytes and cells derived from epithelial and reproductive tissues. Overexpression of Mer has been associated with lymphoid malignancies. Here we identify Gas6, the product of a growth arrest specific gene, as a ligand for Mer. Gas6 has previously been shown to activate both Axl and Rse/Tyro3, two other receptor tyrosine kinases in the same family as Mer. The apparent relative association and dissociation rate constants of Gas6 for soluble Axl, Rse/Tyro3 and Mer were compared using surface plasmon resonance. Gas6 was shown to induce rapid phosphorylation of Mer expressed in several different types of cells. We also observed a transient activation of p42 MAP kinase following activation of Mer by Gas6. Thus, Gas6 exerts its biological effects through multiple receptor tyrosine kinases.

Reduction of alpha-naphthylisothiocyanate-induced hepatotoxicity by recombinant human hepatocyte growth factor.

Hepatocyte growth factor (HGF) is a potent stimulator of DNA synthesis in cultured hepatocytes. To determine whether HGF has any activity in vivo, we have tested HGF in rats in which intrahepatic cholestasis was induced by acute administration of alpha-naphthylisothiocyanate (ANIT). The hepatotoxic effects of a single injection of ANIT were manifested 48 h later as large increases in serum bilirubin, alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase. These biochemical changes were accompanied by widespread periportal edema, hypertrophy of bile duct epithelium, and randomly scattered areas of liquifaction necrosis in the hepatic parenchyma. The increases in bilirubin, alanine aminotransferase, aspartate aminotransferase and alkaline phosphatase were markedly attenuated when HGF was administered 30 min before ANIT and again at 6, 12, 24, 30, and 36 h after ANIT. In addition, this HGF dosing regimen completely prevented the occurrence of parenchymal lesions, although it had no effect on periportal histopathology. The effect of ANIT was dose dependent; a maximal response was observed at 320 micrograms/kg per injection, with an intermediate response at 105 micrograms/kg. Delaying the administration of HGF until 12 h after ANIT was as effective as when administration was begun 30 min before ANIT. Taken together these results show that HGF can prevent some aspects of ANIT hepatotoxicity.

The pharmacokinetics, tissue localization, and metabolic processing of recombinant human hepatocyte growth factor after intravenous administration in rats.

Hepatocyte growth factor (HGF) is a pluripotent mitogen thought to be involved in liver regeneration. It is synthesized as a single chain promitogen and requires proteolytic processing to a two-chain heterodimeric form for biological activity. The pharmacokinetics and tissue distribution of radioiodinated single chain recombinant human HGF ([125I]rhuHGF) were studied in male Sprague-Dawley rats after an iv bolus dose. Pharmacokinetic parameters were determined from trichloroacetic acid-precipitable radioactivity in serum samples. There was a rapid distribution phase (t1/2 alpha = 3.1 min) and a slower elimination phase (t1/2 beta = 114 min). Tissue distribution was assessed by whole body autoradiography 5, 60, and 1440 min after an iv bolus dose. rhuHGF rapidly distributed to the liver, kidney, adrenal gland, and spleen. The importance of the liver in the rapid clearance and subsequent conversion of single chain pro-rhuHGF to the mitogenically active two-chain form was demonstrated using an isolated rat liver perfusion system. TCA-precipitable radioactivity excreted into the bile (1.0 +/- 0.1%) and released into the venous effluent (38.9 +/- 6.1%) was monitored for 60 min after a portal vein injection. The appearance of radioactivity in both the bile and venous effluent was maximal between 20-35 min. Further characterization of the reduced samples by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed that the two-chain form of [125I]rhuHGF was the predominant form after hepatic perfusion. These studies suggest that the liver plays a major role in the rapid clearance and subsequent activation of pro-rhuHGF in vivo.

Identification of a herpes simplex virus function that represses late gene expression from parental viral genomes.

The expression of herpes simplex virus gamma 2 (late) genes is inhibited before the onset of viral DNA replication. We report that the block in the expression of certain gamma 2 genes is relieved, at least in part, by defects in the beta ICP8 protein. We have examined the expression of the gamma 2 gene encoding glycoprotein C (gC) in cells infected with a temperature-sensitive ICP8 mutant. Under conditions in which viral DNA replication is inhibited, cells infected with the ICP8 mutant overproduce the gC family of mRNAs relative to the level observed in cells infected with a wild-type virus. The gC mRNA synthesized in cells infected with the ICP8 mutant virus is correctly initiated and spliced and is translated with the same relative efficiency as in cells infected with a replicating wild-type virus. These results suggest that ICP8 is involved in the negative regulation of gamma 2 genes expressed from parental viral genomes. The level of gC expression was greatest in cells infected with a replicating wild-type virus. These data suggest that DNA replication and genome amplification are not absolute requirements for gamma 2 gene expression but may facilitate full-level expression of these genes.

Transcriptional control of herpesvirus gene expression: gene functions required for positive and negative regulation.

We have used an in vitro nuclear run-off assay to measure the levels of transcription of specific herpes simplex virus genes at different times during a lytic infection. We analyzed the effects of inhibition of DNA replication and of defects in two herpes simplex virus regulatory proteins on the transcription of these genes. We present evidence that the transcription of the alpha ICP4 gene is negatively regulated during a lytic infection. The regulation of ICP4 gene transcription requires the beta protein ICP8 (where ICP = infected cell polypeptide). Transcription of the beta ICP8, gamma 1 ICP5, and gamma 2 glycoprotein C (gC) genes was dependent on ICP4, and transcription of the gamma 2gC gene was strongly inhibited when DNA replication was blocked. Defects in ICP8 also resulted in increased levels of transcription of the ICP4, ICP8, ICP5, and gC genes from parental viral genomes. Our results suggest that ICP8 may be important in maintaining the highly ordered cascade of viral gene expression.

Generation and characterization of a competitive antagonist of human hepatocyte growth factor, HGF/NK1.

Our previous studies have suggested that a derivative of hepatocyte growth factor (HGF), HGF/NK2, containing the coding sequences for the N-terminal hairpin and first two kringle domains, is sufficient to mediate high affinity binding to the HGF receptor. Here, we wished to test directly whether HGF/NK1 (N-terminal hairpin and first kringle domains) could bind the receptor and/or mediate receptor signaling. HGF/NK1 was expressed in Escherichia coli and purified to homogeneity using heparin-affinity and fast protein liquid cation-exchange chromatography. Biological characterization of HGF/NK1 showed that it can compete for binding to the HGF receptor on human lung carcinoma A549 cells and to a soluble form of the HGF receptor. HGF/NK1 is inefficient at promoting autophosphorylation of the HGF receptor, although some activity was detected at very high concentrations. HGF/NK1 fails to exhibit mitogenic properties even at very high concentrations. However, HGF/NK1 can act as a potent competitive antagonist in this assay. Our data demonstrate directly that a receptor binding determinant of HGF is located within the N-terminal 32-212 residues of HGF. HGF/NK1 will serve as a powerful tool for (i) generating neutralizing antibodies, (ii) in determining x-ray crystallographic and nuclear magnetic resonance structures, and (iii) for in vivo studies as an HGF antagonist.

Mutations in the major DNA-binding protein gene of herpes simplex virus type 1 result in increased levels of viral gene expression.

We have examined the effect of temperature-sensitive mutations in the herpes simplex virus 1 DNA-binding protein gene on viral gene expression. We have found that at the nonpermissive temperature, the synthesis of certain immediate early, early, and late viral polypeptides was greater in cells infected with the temperature-sensitive mutants than in cells infected with the wild-type virus. This effect was independent of the requirement for this viral protein for viral DNA replication. The altered rate of synthesis of viral proteins was due to a thermolabile gene product. Cells infected with these mutants at the permissive temperature and then shifted to the nonpermissive temperature exhibited enhanced levels of viral gene expression. The addition of actinomycin D at the time of the temperature shift prevented the alteration in viral protein synthesis. Therefore, continuing transcription is required for this change in gene expression. Northern blot analysis of cytoplasmic RNA showed that the steady-state level of specific viral transcripts expressed from parental virus genomes was greater in cells infected by these mutants at the nonpermissive temperature. These results indicate that the major DNA-binding protein of herpes simplex virus type 1 acts as a negative regulator of viral gene expression by affecting the abundance of cytoplasmic viral mRNAs.

In vivo response of hepatocytes to growth factors requires an initial priming stimulus.

Although growth factor effects have been studied in cultured hepatocytes, little information exists as to whether these factors can trigger hepatocyte replication in vivo. In this study we infused epidermal growth factor, transforming growth factor-alpha and hepatocyte growth factor directly into the portal vein of rats for 24 hr to see whether they could induce DNA synthesis in normal livers or in livers subjected to one-third hepatectomy. Infusion of transforming growth factor-alpha or epidermal growth factor at doses up to 80 micrograms/24 hr had little effect on hepatic DNA synthesis in normal liver, whereas the monomeric and heterodimeric forms of hepatocyte growth factor generally produced increases of less than threefold in hepatic DNA synthesis. In contrast, after one-third hepatectomy infusion of epidermal growth factor, transforming growth factor-alpha or hepatocyte growth factor produced dose-dependent increases in hepatic DNA synthesis. At a dose of 40 micrograms/24 hr, epidermal growth factor increased DNA synthesis threefold, whereas transforming growth factor-alpha or hepatocyte growth factor increased DNA synthesis to greater than six times that in rats that had undergone hepatectomy alone. Furthermore, infusion of these growth factors, with or without one third-hepatectomy, induced the expression of transforming growth factor-alpha mRNA in the liver. The pattern of protooncogene expression induced by one-third hepatectomy was studied to determine the effect of this procedure in sensitizing the liver to the growth factors. Compared with the well-characterized two-thirds hepatectomy system, there was a similar but smaller increase in c-myc expression but no induction of c-jun expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Mutational analysis and molecular modeling of the N-terminal kringle-containing domain of hepatocyte growth factor identifies amino acid side chains important for interaction with the c-Met receptor.

The hepatocyte growth factor receptor (HGFr) transduces a wide range of biological signals, including mitogenesis, motogenesis and morphogenesis. We recently localized a region within the N-terminal 175 amino acids of hepatocyte growth factor (HGF), termed HGF/NK1, that is necessary and sufficient for binding to the HGFr. HGF/NK1 contains an as-yet structurally undefined N-terminal region followed by the first of four HGF kringles. We have used a combination of molecular modeling and mutagenesis to dissect the function of this region of HGF. Two mutation-sensitive patches on the proposed surface regions of HGF kringle one (K1) were identified. The first patch consists of residues E159, S161, E195 and R197, all of which are predicted to be close to each other in the tertiary structure of K1. The second patch, lying on the opposite side of the kringle, consists of residues D171 and Q173. Mutational analysis of the N-terminal region of HGF identified residue D117 which also appeared to influence receptor binding. We also investigated the properties of a naturally occurring HGF variant (delta 5-HGF) that arises from an alternatively spliced transcript and therefore lacks five residues within K1. Our data suggest that in wild-type HGF, F162 is crucial in maintaining the hydrophobic core of the kringle. In delta 5-HGF, the loss of this residue is compensated for by a functional substitution of F162 with Y167, which is predicted to occupy the delta 5-HGF K1 core. Comparison of the models of wild-type and delta 5 kringles reveals that the positions of the presumed receptor binding determinants remain unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of herpes simplex virus 2 temperature-sensitive mutants whose lesions map in or near the coding sequences for the major DNA-binding protein.

By marker rescue with cloned herpes simplex virus 2 DNA fragments, we have mapped the temperature-sensitive mutations of a series of herpes simplex virus 2 mutants to a region of the herpes simplex virus 2 genome that lies within or near the coding sequences for the major DNA-binding protein, ICP8. In cells infected with certain of these mutants at the nonpermissive temperature, the association of the major DNA-binding protein with the cell nucleus was defective. In these cells, the DNA-binding protein accumulated in the cytoplasmic and the crude nuclear detergent wash fractions. At the permissive temperature, the maturation of the mutant ICP8 was similar to that of the wild-type viral protein. With the remainder of the mutants, the nuclear maturation of ICP8 was similar to that encoded by the wild-type virus at the nonpermissive and permissive temperatures as assayed by cell fractionation.

Glucocorticoid receptor mutants that are constitutive activators of transcriptional enhancement.

Glucocorticoids, a class of steroid hormones, associate specifically with intracellular receptors, facilitating a conformational change that converts the receptor in vitro to a DNA-binding protein and in vivo to a nuclear species that activates a class of transcriptional enhancers termed glucocorticoid response elements (GREs). The DNA sequences recognized specifically by the hormone-receptor complex correspond directly to those required for GRE enhancement. The structural transition that accompanies steroid binding, 'receptor transformation', has been monitored by changes in receptor chromatographic properties, accessibility to monoclonal antibodies, association with other receptor subunits or with heterologous proteins, and aqueous two-phase partition coefficient. However, the significance of the structural change for the biological activity of the receptor is not understood. We have used cloned rat glucocorticoid-receptor coding sequences to produce and characterize a novel class of receptor mutants that elicit GRE enhancer function in transfected cells even in the absence of hormone. The constitutive activity of those receptor derivatives, together with mapping studies that distinguish between the DNA- and hormone-binding domains of the receptor, imply that the conformational change corresponding to receptor transformation may simply unmask pre-existing functional domains for DNA binding, enhancer activation, or both.