Methylmercury effects on cell cycle kinetics

Methylmercury (MeHg) effects on cell cycle kinetics were investigated to help identify its mechanisms of action. Flow cytometric analysis of normal human fibroblasts grown in vitro in the presence of BrdU allowed quantitation of the proportion of cells in G1, S, G2 and the next G1 phase. This technique provides a rapid and easily performed method of characterizing phase lengths and transition rates for the complete cell cycle. After first exposure to MeHg the cell cycle time was lengthened due to a prolonged G1. At 3 microM MeHg the G1 phase length was 25% longer than the control. The G1/S transition rate was also decreased in a dose-related manner. Confluent cells exposed to MeHg and replated with MeHg respond in the same way as cells which have not been exposed to MeHg before replating. Cells exposed for long times to MeHg lost a detectable G1 effect, and instead showed an increase in the G2 percentage, which was directly related to MeHg concentration and length of exposure. After 8 days at 5 microM MeHg, 45% of the population was in G2. The G2 accumulation was reversible up to 3 days, but at 6 days the cells remained in G2 when the MeHg was removed. Cell counts and viability indicated that there was not a selective loss of cells from the MeHg. MeHg has multiple effects on the cell cycle which include a lengthened G1 and decreased transition probability after short term exposure of cycling cells, and a G2 accumulation after a longer term exposure. There were no detectable S phase effects. It appears that mitosis (the G2 accumulation) and probably synthesis of some macromolecules in G1 (the lengthened G1 and lowered transition probability) are particularly susceptible to MeHg.     

Effect of covalent attachment of immunoglobulin fragments on liposomal integrity

Liposome stability during and after covalent coupling of Fab' antibody fragments was investigated. Large unilamellar vesicles containing entrapped 5(6)-carboxyfluorescein (CF) as a marker for liposomal integrity were prepared by extrusion through polycarbonate membranes. N-[4-(p-Maleimidophenyl)-butyryl]phosphatidylethanolamine (MPB-PE) was employed as a liposomal anchor for the covalent coupling of Fab' fragments. We observed that coupling of Fab' fragments to liposomes containing 5 mol % MPB-PE caused a concentration-dependent increase in size and polydispersity of the liposomes. Dependent on the concentration of the MPB-PE anchor in the membrane and the concentration of Fab' added, coupling was associated with the release of up to 95% of the entrapped CF. Rupture of the liposomes was identified as the primary mechanism of CF release during Fab' coupling. Reduction of the MPB-PE concentration to 1 mol % resulted in liposomes that were stable during and after Fab' coupling. The increased stability of these liposomes was due to the lower MPB-PE concentration and not to the lower number of attached Fab' fragments. By proper adjustment of the experimental conditions for coupling, the number of Fab' fragments attached to the 1 mol % MPB-PE liposomes could be increased without affecting the stability of the resulting liposomes. These stable liposomes, made by an extrusion method that avoids the use of organic solvents, detergents, or sonication, are therefore suitable for entrapment of labile compounds and can be used for immunotargeting or immunoassays.     

Monoclonal antibody-mediated inhibition of interferon-gamma-induced macrophage antiviral resistance and surface antigen expression

A monoclonal antibody (MAb) with specificity for murine interferon-gamma (IFN-gamma) was used as a probe for studying the effect of recombinant IFN-gamma (rIFN-gamma) on antiviral activity, Fc receptor expression, and Ia antigen induction in macrophages. Cultures of C3H/HeJ peritoneal exudate macrophages were used to allow direct comparison of all three functions in the same target cell system. Our data provide two major findings: the efficacy of the MAb is very different depending on whether murine fibroblasts or macrophages are used as the target cell in the antiviral assay, i.e., greater than 20 to 100 times more MAb was required to block antiviral activity in macrophage cultures; and 10 to 50 times more MAb was required to inhibit Fc receptor vs Ia antigen expression in response to rIFN-gamma. These latter findings confirm and extend previous observations, which indicate that the induction pathways of two important differentiation markers by IFN-gamma may be dissociable.     

Do the frequencies of sister chromatid exchanges in endoreduplieated mitoses provide a measure for lesion persistence and repair?

Endoreduplication was induced in V 79 cells using Colcemid. The concentration of Colcemid necessary to induce endoreduplication is about 1000 times higher than that needed to arrest mitoses or to induce ordinary tetraploid cells. Diplochromosomes with sister chromatid differentiation were obtained by adding BrdU for the duration of one cell cycle prior to the induction of endoreduplication. The induction of endoreduplication with Colcemid had no influence on the frequency of sister chromatid exchanges (SCEs). Treating the cultures with mitomycin C (MMC) before adding BrdU increased the percentage of endoreduplieated mitoses and also led to marked SCE induction. In the diplochromosomes, the frequencies of both twin SCEs (first cycle) as well as single SCEs (second cycle) were increased. It was also found that the SCE frequencies in mitoses after endoreduplication were lower than the values found in diploid and ordinary tetraploid metaphases of the same preparation. The possible conclusions concerning the lifetime of SCE-inducing lesions and the influence of repair processes are discussed.     

Bone marrow-derived macrophages: development and regulation of differentiation markers by colony-stimulating factor and interferons

To investigate the role of specific cytokines in the development of the fully mature macrophage, we have employed murine bone marrow cells that were grown in the presence of CSF-1, a colony-stimulating factor that has been shown to induce the proliferation and differentiation of macrophages from their precursor cells. The CSF-1 employed in these studies was partially purified to ensure removal of contaminating interferon (IFN) from the preparations. After 1 to 2 wk in the presence of the partially purified CSF-1, the adherent macrophages were removed from flasks enzymatically and were recultured at known densities in the absence of CSF-1. Cell surface antigens (Mac-1 and Ia) and Fc receptor capacity (as assessed by Fc-mediated phagocytosis) were examined as markers of macrophage differentiation. Basal levels of Fc receptor capacity and Mac-1 antigen were markedly influenced by exposure to CSF-1, and appear to be modulated by CSF-induced, macrophage-derived IFN. When the bone marrow-derived macrophages were exposed to exogenous IFN in the absence of CSF-1, they proved to be extremely inducible with respect to Fc-mediated phagocytosis (IFN-beta and rIFN-gamma) and Ia antigen expression (rIFN-gamma) when compared with thioglycollate-elicited macrophages. Thus, macrophage growth factors, such as CSF-1, promote macrophage maturation by inducing the production of autostimulatory signals, such as macrophage-derived IFN. In addition, exogenous cytokine stimuli, such as IFN-gamma, further amplify the differentiative potential of these cells. Bone marrow-derived macrophages, propagated under well-defined conditions and never exposed to eliciting agents, provide a powerful model for studying the role of cytokines, such as CSF-1 and IFN, in the differentiative pathway of macrophages.     

Antagonistic effect of interferon-beta on the interferon-gamma-induced expression of Ia antigen in murine macrophages

The cell surface expression of I region-associated (Ia) antigens by murine and human macrophages has been shown by investigators from a number of laboratories to be induced in a dose-dependent fashion by IFN-gamma, which is free of other lymphokines. The experiments described in this report demonstrate that fibroblast-derived IFN-beta exerts an antagonistic effect on IFN-gamma induced Ia expression in murine macrophages. Simultaneous addition of IFN-beta and IFN-gamma to peritoneal exudate macrophages results in decreased Ia expression when compared with macrophages treated with IFN-gamma only. Different sources of highly purified IFN-beta, as well as a recombinant human IFN-alpha (A/D Bgl; shown previously to be as active as IFN-beta in several other murine systems) acted in a similar antagonistic fashion to IFN-gamma-induced Ia induction. The down-regulation of Ia expression by IFN-beta is dose-dependent over a concentration range up to 100 U/ml. Time-course experiments indicated that for IFN-beta to down-regulate IFN-gamma-induced Ia, it had to be present either before stimulation with IFN-gamma or during the first 24 hr of simultaneous stimulation. Further experiments in which a highly specific antibody against IFN-alpha/beta was added to the cultures confirmed the findings of the time-course experiments. Inhibitors of the arachidonic acid pathway failed to reverse the effect of IFN-beta to reduce Ia antigen expression, which suggests that this inhibition is not prostaglandin mediated. Thus, these findings support a role for type I IFN as naturally occurring substances that negatively regulate the expression of class II molecules.     

Rat plasma levels of amino acids and related compounds during stress

Forty-one amino acids and related compounds were measured (using an HPLC physiological amino acid analysis procedure fully validated for plasma studies) in rat plasma obtained through an indwelling jugular catheter before, during and following a 30 min period of immobilization. Taurine, phosphoethanolamine, aspartic acid, glutamic acid, alanine, cystine, tyrosine, beta-alanine and ethanolamine were increased during the period of stress; whereas, valine, tryptophan and arginine were decreased. Most of these alterations were restored toward normal during the 30 min of rest following the stress period. However, cystine, ethanolamine and beta-alanine remained significantly elevated, and valine, tryptophan and arginine remained significantly reduced. Serine, isoleucine, leucine and glutamine were not significantly altered during the stress period, but became significantly reduced during the 30 min following the stress period. While the patterns of amino acid alterations were generally consistent from animal to animal, the magnitude of the responses were variable with some rats demonstrating much larger responses than others. These results may implicate amino acids as important markers for stress related pathologies. The individual differences noticed may explain why some individuals show more stress effects than others.     

The binding of copper(II) and zinc(II) to oxidized glutathione

1H and 13C NMR studies of Zn(II) binding to oxidized glutathione (GSSG) in aqueous solution over the pH range 4-11 show that it forms a complex with a 1:1 Zn:GSSG stoichiometry. At pH values between 6 and 11 the metal ligands are the COO- and NH2 groups of the glutamate residues. Below pH 5 the glycine end of the molecule also binds to the metal ions. EPR and visible absorption spectra of Cu(II) GSSG solutions suggest that similar complexes are formed with Cu(II). The solid products obtained from these solutions are shown by analysis and EPR to be primarily binuclear with Cu2GSSG stoichiometry, although the structures depend on the pH and stoichiometry of the solution from which they were obtained.     

The structure of the lactose permease derived from Raman spectroscopy and prediction methods.

The secondary structure of the lactose permease of Escherichia coli reconstituted in lipid membranes was determined by Raman spectroscopy. The alpha-helix content is approximately 70%, the beta-strand content below 10% and beta-turns contribute 15%. About 1/3 of the residues in alpha-helices and most other residues are exposed to water. Employing a method for structural prediction which accounts for amphipathic helices, 10 membrane-spanning helices are predicted which are either hydrophobic or amphipathic. They are expected to form an outer ring of helices in the membrane. The interior of the ring would be made of residues which are predominantly hydrophilic and, evoking the analogy to sugar-binding proteins, suited to provide the sugar binding site.     

Characterization of mitoses with sister chromatid differentiation (SCD) and consequences for the analysis of proliferation kinetics and sister chromatid exchanges in asynchronously growing cells

Summary Labeling cells with bromodeoxyuridine (BrdU) permits the differentiation of mitoses of the first, second, and third generation after the addition of BrdU. The term second mitoses is used for those cells which have incorporated BrdU for two-S-phases and which exhibit sister chromatid differentiation (SCD). However, SCD can also be obtained if the cell was in S-phase at the time of BrdU-addition and had already replicated part of its DNA. Such cells with incomplete BrdU-substitution in the first S-phase can only be differentiated from completely substituted ones by the quality of the SCD and are usually also grouped as second mitoses in the evaluation of experiments. Due to the heterogeneity of the evaluated second mitoses, the determination of proliferation delay and the incidence of sister chromatid exchange-induction can depend on the time of chromosome preparation.