Cell surface expression by adult rat hepatocytes of a non-collagen glycoprotein present in rat liver biomatrix

A rabbit antiserum raised against ACI rat liver biomatrix was used to identify components common to biomatrix and plasma membranes of adult hepatocytes. Biomatrix was isolated from intact rat livers by reverse perfusion via the inferior vena cava with sodium deoxycholate, nucleases and lipid extracting solvents. Immunoprecipitation analysis of detergent extracts of hepatocytes surface-labeled with 125I indicated that antibodies, purified from anti- biomatrix antiserum by adsorption and desorption from intact hepatocytes, showed reactivity with a single MW 105 kD component, designated Hep 105. Indirect immunofluorescence analysis showed that Hep 105 was expressed in some regions of the perisinusoidal space and in all three domains of the hepatocyte plasma membrane and was present on some but not all of the fibrous elements in frozen sections of biomatrix . The presence of Hep 105 on biomatrix was confirmed by immunoprecipitation analysis which showed that Hep 105 was present in components solubilized from biomatrix by sequential treatment with 0.5 M acetic acid, 0.05% collagenase and 4 M urea. Further characterization using immunoprecipitation analysis in combination with immobilized lectins and two-dimensional polyacrylamide gel electrophoresis (PAGE) indicated that Hep 105 was a non-collagen glycoprotein which showed charge heterogeneity and existed on the cell surface as a disulfide-linked heterodimer of apparent MW 125 kD. Two hybridomas, constructed by fusing P3 X 63Ag8 myeloma cells with spleen cells from mice immunized with intact hepatocytes, were shown by immunodepletion and two-dimensional gel electrophoretic analysis to be secreting monoclonal antibodies (Mab) against Hep 105. Examination of frozen sections of rat liver stained by indirect immunofluorescence showed that reactivity of both Mabs was concentrated in the bile canalicular domain of the hepatocyte plasma membrane, suggesting that the reactive epitopes were not accessible in the sinusoidal and intercellular membrane domains. Taken together, these results suggest that Hep 105 may play a role in the interactions between hepatocytes and extracellular matrix.     

Binding of complement component C3b to glycoprotein C is modulated by sialic acid on herpes simplex virus type 1-infected cells.

Neuraminidase treatment of cells infected with herpes simplex virus type 1 (HSV-1) markedly enhanced the binding of complement component C3b to HSV 1 glycoprotein C (gC). When HSV-1 was grown in BHK RicR14 cells in which glycoproteins had reduced amounts of N-linked complex oligosaccharides, including sialic acid, the binding of C3b to gC was markedly enhanced. We used neuraminidase treatment to demonstrate that cloning the gC gene from the HSV-1 F strain into an HSV-1 mutant which fails to express gC converted the mutant virus from C3b receptor negative to receptor positive. These results further support a role for gC as a C3b receptor and indicate that sialic acid modifies receptor activity.     

Acute-phase-response induction in rat hepatocytes co-cultured with rat liver epithelial cells.

The response of rat hepatocytes co-cultured with rat liver epithelial cells to conditioned medium (CM) from lipopolysaccharide (LPS)-activated monocytes was investigated by measuring the concentration of alpha 2-macroglobulin (alpha 2M), alpha 1-acid glycoprotein (AGP), albumin and transferrin, as well as the changes in glycosylation of alpha 1-acid glycoprotein. During an initial 8-day treatment with CM, concentrations of alpha 2M and AGP increased markedly over those of control culture, whereas concentrations of albumin and transferrin decreased. The glycosylation pattern of AGP indicated an important relative increase of the concanavalin A-strongly-reactive (SR) variant upon treatment. When CM addition to hepatocyte culture medium was stopped, the concentrations of the four proteins and the glycosylation pattern of AGP reverted to those of control cultures. Further addition (on day 15) to cultures of CM increased the concentration of alpha 2M and decreased albumin and transferrin concentrations. Although AGP concentrations did not increase above those of controls, the appearance of the SR variant was again stimulated by CM. These results show that, in co-culture, rat hepatocytes remain able to respond to repeated inflammatory stimuli.     

Maintenance and reversibility of active albumin secretion by adult rat hepatocytes co-cultured with another liver epithelial cell type

When adult rat hepatocytes were co-cultured with another liver epithelial cell type in a medium supplemented or not with fetal calf serum (FCS), it was found that 1. They survived for more than 2 months 2. Albumin secretion levels remained high over the whole culture period 3. Decreased secretion might be reversed 4. This protein secretion activity appeared to be dependent upon both the presence of cell-cell contacts and the production of an extracellular material. The results demonstrate for the first time long-term stabilization and reversibility of a specific function (albumin secretion) at high levels by adult hepatocytes cultured in serum-free medium and suggest that both the presence of other liver cell type(s) and the production of an extracellular matrix are needed for the maintenance of specific functions in cultured hepatocytes.     

Recombinant rat IL-1beta and IL-6 synergistically enhance C3 mRNA levels and complement component C3 secretion by H-35 rat hepatoma cells

Hepatic synthesis of complement component C3 is regulated in part by inflammatory cytokines. Rat models are frequently employed to investigate pathogenic roles of complement and cytokines. However, cytokines obtained from species other than the rat were used in previous studies of cytokine regulation of C3 synthesis in rat hepatocytes or hepatoma cells. It is not known whether these prior reports predict hepatocellular responses evoked by rat cytokines. Therefore, H-35 rat hepatoma cells were employed to measure the effect of recombinant rat IL-1beta, IL-6, IFN-gamma, and TNF-alpha on C3 protein secretion and C3 mRNA levels quantified by ELISA and quantitative RT-PCR. Compared to untreated control cells, H-35 cells treated with IL-1beta, IL-6, and IFN-gamma increased C3 secretion approximately 10-, 4-, and 2-fold, respectively. TNF-alpha was toxic, precluding further analysis. IL-1beta and IL-6 demonstrated synergy with respect to the quantity and rate of increase of C3 mRNA measured and the magnitude of C3 protein secretion. Previous reports using non-rat cytokines did not consistently predict H-35 responses to rat cytokines. Consequently, we recommend the use of rat cytokines in rat models that include analysis of cytokine-mediated events.     

Monoglucosylated glycans in the secreted human complement component C3: implications for protein biosynthesis and structure

The monoglucosylated oligomannose N-linked oligosaccharide (Glc(1)Man(9)GlcNAc(2)) is a retention signal for the calnexin-calreticulin quality control pathway in the endoplasmic reticulum. We report here the presence of such monoglucosylated N-glycans on the human complement serum glycoprotein C3. This finding represents the first report of monoglucosylated glycans on a human serum glycoprotein from non-diseased individuals. The presence of the glucose moiety in 5% of the human C3 glycoprotein suggests that this glycosylation site is sequestered within the protein and is consistent with previous studies identifying a cryptic conglutinin binding site on C3 that becomes exposed upon its conversion to iC3b.     

Structure of compstatin in complex with complement component C3c reveals a new mechanism of complement inhibition

Undesired complement activation is a major cause of tissue injury in various pathological conditions and contributes to several immune complex diseases. Compstatin, a 13-residue peptide, is an effective inhibitor of the activation of complement component C3 and thus blocks a central and crucial step in the complement cascade. The precise binding site on C3, the structure in the bound form, and the exact mode of action of compstatin are unknown. Here we present the crystal structure of compstatin in complex with C3c, a major proteolytic fragment of C3. The structure reveals that the compstatin-binding site is formed by the macroglobulin (MG) domains 4 and 5. This binding site is part of the structurally stable MG-ring formed by domains MG 1-6 and is far away from any other known binding site on C3. Compstatin does not alter the conformation of C3c, whereas compstatin itself undergoes a large conformational change upon binding. We propose a model in which compstatin sterically hinders the access of the substrate C3 to the convertase complexes, thus blocking complement activation and amplification. These insights are instrumental for further development of compstatin as a potential therapeutic.     

Cellular membrane type-1 matrix metalloproteinase (MT1-MMP) cleaves C3b, an essential component of the complement system

Neoplasms have developed numerous strategies to protect themselves against the host immune system. Membrane type-1 matrix metalloproteinase (MT1-MMP) is strongly associated with many cancer types and is up-regulated in the aggressive, metastatic neoplasms. During the past few years, there has been an increasing appreciation of the important, albeit incompletely understood, role of MT1-MMP in cancer. We have discovered, using cell-free and cell-based assays in vitro, that MT1-MMP proteolysis specifically targets C3b, an essential component of the complement propagation pathway. MT1-MMP proteolysis liberates the deposited C3 activation fragments from the cell surface. The shedding of these cell-deposited opsonins by MT1-MMP inhibits the complement cascade and protects breast carcinoma MCF7 cells from direct complement-mediated injury in the in vitro tests. The functional link associating MT1-MMP with the host immune system, heretofore unrecognized, may empower tumors with an escape mechanism that contributes to the protection against the host anti-tumor immunity as well as to the survival of invading and metastatic malignant cells in the bloodstream.     

Synthesis of antithrombin III and alpha-1-antitrypsin by the perfused rat liver.

Livers isolated from control or turpentine-injected rats were perfused for 3 h with human red cells suspended in Krebs-Henseleit solution containing bovine serum albumin, dextran, glucose, heparin, cortisol, insulin, a mixture of 20 amino acids and [3H]leucine. Changes in the concentrations of antithrombin III and alpha-1-antitrypsin were evaluated by rocket immunoelectrophoresis using specific antisera, and incorporation of the 3H radioactivity into the total protein, albumin, antitrhombin III and alpha-1-antitrypsin in the perfusate was measured. The results indicate that both antithrombin III and alpha-1-antitrypsin are synthesized in the liver. Local inflammation induced in the liver donors moderately stimulated the synthesis of alpha-1-antitrypsin but it affected only marginally that of antithrombin III.     

Glucagon stimulates the A system for neutral amino acid transport in isolated hepatocytes of adult rat.

It has been found that both the peroxidase and synthetase activity of sheep vesicular gland microsomes catalyze the oxygenation of singlet oxygen trapping or quenching agents. Furthermore the synthetase was also readily inactivated by these agents, particularly bilirubin, and suggests that singlet oxygen formed by the peroxidase activity may initiate prostaglandin biosynthesis. The singlet oxygen agents also protected the synthetase from self-catalyzed destruction or inactivation by peroxides and suggest that singlet oxygen may also be responsible for the inactivation.     

Stimulation of sphingomyelin biosynthesis by brefeldin A and sphingomyelin breakdown by okadaic acid treatment of rat hepatocytes.

Studies on sphingomyelin metabolism in rat hepatocytes were facilitated by the use of choline-deficient cells which allowed for the rapid labeling of phosphatidylcholine and as a result sphingomyelin. Pulse and pulse-chase studies with [methyl-3H]choline and [methyl-3H]methionine demonstrated that both compounds were effectively used for sphingomyelin biosynthesis and that newly made and pre-existing phosphatidylcholine could be used for sphingomyelin biosynthesis. When hepatocytes were incubated with brefeldin A, there was a 2.4-fold stimulation of the conversion of phosphatidylcholine into sphingomyelin. Since brefeldin A causes collapse of the cis/medial Golgi into the endoplasmic reticulum the stimulation of sphingomyelin biosynthesis could be due to more rapid access of the labeled phosphatidylcholine in the endoplasmic reticulum to sphingomyelin synthase in the collapsed Golgi. Forskolin inhibited the brefeldin A-induced stimulation of sphingomyelin biosynthesis. To investigate whether or not phosphorylation reactions regulate sphingomyelin metabolism, hepatocytes were incubated with okadaic acid, a potent inhibitor of protein phosphatases 1 and 2A. Rather than stimulating sphingomyelin biosynthesis, okadaic acid enhanced the catabolism of sphingomyelin. In contrast, a cyclic AMP analogue and forskolin had no effect on sphingomyelin biosynthesis or catabolism. Surprisingly, other pulse-chase studies demonstrated that okadaic acid stimulated the catabolism of only newly made sphingomyelin. The brefeldin A and okadaic acid effects were independent of lysosomal involvement. Subcellular fractionation studies revealed that brefeldin A and okadaic acid effects were generalized in all sphingomyelin containing membranes. The brefeldin A studies suggest that the rate of transfer of phosphatidylcholine from the endoplasmic reticulum to the Golgi might be limiting for sphingomyelin biosynthesis. The okadaic acid studies indicate that the catabolism of sphingomyelin by a sphingomyelinase is regulated by an unidentified protein kinase and by either protein phosphatase 1 and/or 2A activity in hepatocytes.     

Transient induction of cytochrome P450 1A1 mRNA by culture medium component in primary cultures of adult rat hepatocytes

Summary Because the metabolic environment can alter gene expression in cultured cells, we examined the effects of change of medium on the levels of several cytochrome P450 mRNAs in primary cultures of rat hepatocytes maintained on Matrigel. The amounts of P450 1A2, 2B1/2, or 3A1 mRNA were unaffected by changing the medium. In contrast, P450 1A1 mRNA levels were increased 1 to 2 h after media change, reached maximum levels by 6 h, and declined to baseline by 24 h. Supplementing day-old media with components of the medium revealed that only addition of amino acids resulted in 1A1 mRNA induction. From the results of direct additions and omissions, we showed that tryptophan, but not histidine, was largely responsible for the 1A1 mRNA induction. Moreover, mild photoactivation of the tryptophan resulted in a substantially increased magnitude of 1A1 mRNA induction. The time course for 1A1 mRNA induction by treatment with photoactivated tryptophan was identical to that observed after medium change. Treatment of hepatocyte cultures with β-naphthoflavone, which is metabolized by 1A1, also resulted in a transient 1A1 mRNA induction time-course profile over a 24-h period, whereas treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin, which is relatively stable to metabolic transformation, produced sustained elevations of 1A1 mRNA, suggesting that the transient response to tryptophan also may involve metabolism of the inducer. Our results extend previous data showing that oxidized products of tryptophan induce 1A1, and suggest that the transient nature of the induction may be due to elimination of the activated tryptophan molecule.     

Structural homologies of component C5 of human complement with components C3 and C4 by neutron scattering

The complement component C5 is one of a family of structurally related plasma proteins that includes components C3 and C4. Activation of C5 is the initial step in the formation of the membrane attack complex of complement. Analysis of the solution structure of C5 and comparisons with similar analyses of the structures of C3 and C4 are reported here. Neutron solution scattering gave an Mr for C5 of 201,000, which demonstrates that C5 is monomeric in solution. The radius of gyration RG of C5 at infinite contrast is 4.87 nm and corresponds to an elongated structure. The longest length of C5 was determined to be at least 15-16 nm from three calculations on the basis of the RG, the scattering intensity at zero angle I(0), and the indirect transformation of the scattering curve into real space. Comparison of the RG and contrast variation data and indirect transformations of the scattering curves for C3, C4, and C5 show that these have very similar structures. Comparisons of the C5 scattering curve with Debye small-sphere models previously employed for C4 and C3 show that good curve fits could be obtained. Unlike previous studies that have suggested significant differences, these experiments indicate that, while C5 differs from C3 and C4 in its activation and inactivation pathways, significant structural homology exists between the native proteins, as might be predicted from their high (and similar) sequence homology.