Characterization of murine Flt4 ligand/VEGF-C.

Flt4 is a receptor protein tyrosine kinase that is expressed in the adult lymphatic endothelium and high endothelial venules. We have used a BIAcore assay to identify rodent and human cell conditioned media containing the ligand of Flt4 (Flt4-L). Receptor-based affinity chromatography was used to purify this growth factor, followed by amino acid sequencing and molecular cloning of the murine cDNA, the orthologue of human vascular endothelial growth factor-C and vascular endothelial growth factor related protein. The murine flt4-L gene was localized to chromosome 8 and demonstrated to be widely expressed. Flt4-L was found to have a hydrophobic signal sequence and a pro-peptide-like sequence that is removed to generate the mature N-terminus. In addition, the C-terminal region of Flt4-L has four repeats of a cysteine-rich motif that is presumably also proteolytically processed to generate the 21000 Mr polypeptide subunit of the Flt4-L homodimer. Recombinant Flt4-L activated Flt4 as judged by induction of tyrosyl phosphorylation, and induced mitogenesis in vitro of lymphatic endothelial cells.

Structural identification of the hematopoietic progenitor antigen ER-MP12 as the vascular endothelial adhesion molecule PECAM-1 (CD31).

The monoclonal antibody ER-MP12 was recently described to recognize an antigen present on cell subpopulations of adult mouse bone marrow, including pluripotent hematopoietic stem cells. In an effort to understand the function of ER-MP12 antigen in hematopoiesis, we used biochemical and physical methods to determine its identity. ER-MP12 antigen was isolated by immunoprecipitation from FDCP-1 cell membrane proteins, yielding a glycosylated 113-kDa band upon analysis by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Thirteen peptides derived from trypsinized ER-MP12 antigen were analyzed by electrospray ionization mass spectrometry and compared to a protein sequence database. The search revealed the identity of the ER-MP12 antigen as platelet endothelial cell adhesion molecule-1, CD31 (PECAM-1). This result was subsequently confirmed by Edman sequencing of a single ER-MP12 peptide fragment followed by comparison with PECAM-1 sequence. In addition, flow cytometric analysis of bone marrow and embryonic stem cells revealed highly similar profiles between ER-MP12 and CD31 (MEC 13.3 antibody)-stained cells. The presence of PECAM-1 on primitive hematopoietic stem cells supports the theory for the interaction of hematopoietic progenitor and stem cells with bone marrow stroma and transendothelial migration.

Synergistic interaction between interleukin-12 and steel factor in support of proliferation of murine lymphohematopoietic progenitors in culture.

We have investigated the effects of interleukin (IL)-12 (natural killer cell stimulatory factor/cytotoxic lymphocyte maturation factor) on the proliferation of murine myeloid and lymphohematopoietic progenitors in methylcellulose culture. In the presence of erythropoietin (Ep), IL-12 alone failed to support colony formation by mononuclear and enriched marrow cells of normal mice. Steel factor (SF) alone supported primarily formation of granulocyte/macrophage (GM) colony formation. However, the combination of the two cytokines yielded a significant number of multilineage colonies. When tested on marrow cells from 5-fluorouracil (5-FU)-treated mice, the combination of IL-12 and SF, but not the single factors, was effective in support of formation of various types of colonies. Approximately 25% of these colonies yielded pre-B-cell colonies when replated in secondary culture containing SF and IL-7, indicating that IL-12 can interact with SF in supporting the development of primitive lymphohematopoietic progenitors. These results demonstrate that IL-12, a cytokine believed to be involved in the development of cell-mediated immune responses, has a wider range of activity, including committed myeloid and multipotent lymphohematopoietic progenitors.

Antichymotrypsin interaction with chymotrypsin. Partitioning of the complex.

The interactions of bovine pancreatic chymotrypsin (Chtr) and recombinant alpha 1-antichymotrypsin (rACT) and rACT variants were studied by kinetic and gel electrophoretic analyses, leading to the formulation of a general kinetic scheme that accounts for all known results concerning this serpin-protease pair, as well as for results obtained with other such pairs. Incubation of rACT and Chtr leads rapidly to the formation of an inhibited complex, Chtr.rACT*, that is stable toward sodium dodecyl sulfate denaturation and boiling. The extent of release of active Chtr from this complex increases markedly as ionic strength, mu, is raised. The kinetic scheme quantitatively accounts for this effect on the basis of a partitioning of Chtr.rACT* between dissociation of the complex to yield active enzyme and cleaved rACT, and Chtr-catalyzed conversion of the complex to a form that is much more resistant to release of active enzyme. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses of reaction mixtures of rACT and Chtr are consistent with the scheme. Also consistent are the results of experiments measuring the effects of 1) Chtr.rACT* concentration, 2) uncomplexed Chtr, and 3) added alpha 2-macroglobulin on active Chtr release from Chtr.rACT*. Proteolysis of Chtr.rACT* to give a resistant complex is also catalyzed by human neutrophil elastase, a process with potential physiological relevance. Comparison of the rates of Chtr dissociation from the complexes formed with rACT and with rACT variants mutated at the P1 site suggests that such rates are more sensitive to P1 substitution at low mu than at high mu. Several equivalents of the L358R-rACT variant are required for full inhibition of Chtr. This observation is also quantitatively accounted for by the proposed kinetic scheme, on the basis of another partitioning step between L358R-rACT acting as a substrate or as an inhibitor toward Chtr.

Production of natural killer cell stimulatory factor (interleukin 12) by peripheral blood mononuclear cells.

Natural killer cell stimulatory factor (NKSF), or interleukin 12 (IL-12), is a 70-kD heterodimeric cytokine composed of two covalently linked chains, p40 and p35. NKSF/IL-12 has multiple effects on T and NK cells and was originally identified and purified from the supernatant fluid of Epstein-Barr virus (EBV)-transformed human B lymphoblastoid cell lines. We have produced a panel of monoclonal antibodies against both chains of NKSF/IL-12. Some of these antibodies have neutralizing activity, and several combinations of them have been used to establish sensitive radioimmunoassays detecting the free p40 chain, the free p35 chain, or the p70 heterodimer. Using these reagents, we have determined that most EBV-transformed human B lymphoblastoid cell lines constitutively produce low levels of the p70 heterodimer and an excess of the free p40 chain, whereas Burkitt lymphoma-derived, T, myeloid, and many solid tumor-derived cell lines produce neither. Production of both p40 and p70 is increased several-fold upon stimulation of the EBV-transformed cell lines with phorbol diesters. The ability of supernatant fluids from unstimulated and phorbol diester-stimulated cell lines to induce interferon gamma (IFN-gamma) production from T and NK cells, one of the effects of NKSF/IL-12, parallels the levels of production of the p70 heterodimer, known to be the biologically active form of NKSF/IL-12. Staphylococcus aureus Cowan I strain (SAC) and other stimuli induce accumulation of p40 mRNA and production of both p40 and p70 by peripheral blood mononuclear cells (PBMC). The producer cells appear to include both adherent cells and nonadherent lymphocytes, possibly B cells. The supernatant fluids from SAC-stimulated PBMC mediate the typical functions of NKSF/IL-12 (i.e., IFN-gamma induction, mitogenic effects on T/NK blasts, enhancement of NK cell cytotoxicity) at concentrations of p70 similar to those at which recombinant NKSF/IL-12 mediates the same functions. Moreover, these activities are significantly inhibited by anti-NKSF/IL-12 antibodies. The neutralizing anti-NKSF/IL-12 antibodies also inhibit 85% of the IFN-gamma production in response to SAC, an NKSF/IL-12 inducer, and approximately 50% of the IFN-gamma production in response to non-NKSF/IL-12-inducers such as IL-2, phytohemagglutinin, and anti-CD3 antibodies. These results indicate that induced or constitutively produced NKSF/IL-12 has a major role in facilitating IFN-gamma production by peripheral blood lymphocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Natural killer (NK) cell stimulatory factor increases the cytotoxic activity of NK cells from both healthy donors and human immunodeficiency virus-infected patients.

Natural killer cell stimulatory factor (NKSF), or interleukin 12 (IL-12), is a heterodimeric lymphokine produced by B cells that has multiple effects on T and NK cell functions. NKSF at concentrations as low as 0.4 pM enhances the spontaneous cytotoxic activity of peripheral blood lymphocytes (PBL) against a variety of tumor-derived target cell lines and virus-infected target cells. The combined treatment of PBL with NKSF and IL-2 results in a less than additive enhancement of cytotoxicity. NKSF enhances the cytotoxic activity of spontaneously cytotoxic CD16+CD5- NK cells and does not confer cytotoxic activity to CD16-CD5+ T cells. PBL from patients infected with human immunodeficiency virus (HIV) have significantly lower cytotoxic activity against tumor-derived target cells and virus-infected target cells than PBL from control healthy donors. Treatment of PBL from HIV-infected patients with NKSF and/or IL-2 results in an increase of NK cell cytotoxicity against both types of target cells to levels similar to or higher than those of untreated PBL from healthy donors. PBL from HIV-infected patients produce interferon gamma in response to NKSF and/or IL-2, although at levels 5- or 10-fold lower than those produced by PBL from healthy donors. The multiple biological effects of NKSF, its activity at very low molar concentrations, and its ability to synergize with other physiological stimuli suggest that NKSF/IL-12 is a lymphokine likely to have physiological importance and considerable therapeutic potential.

Inhibition of human neutrophil superoxide generation by alpha 1-antichymotrypsin.

Reactive oxygen intermediates and serine proteases are important components of host defense systems but can produce host injury if not tightly regulated. To determine whether these components can be coordinately controlled, we investigated regulation of superoxide generation by physiologically relevant concentrations of a) highly purified serum-derived antichymotrypsin (ACT), b) recombinant, wild-type ACT, c) rACT in which amino acid substitutions were engineered into the reactive center, and d) chymotrypsin/ACT complexes. These proteins and protein complexes inhibited superoxide anion production in neutrophils stimulated by f-Met-Leu-Phe, Con A, or PMA. In contrast, ligand-stimulated degranulation was not inhibited. In addition, using the recombinants and complexes, the region of ACT involved in inhibiting superoxide anion production was shown to be structurally distinct from the reactive center of the protein. The results indicate that functional domains of ACT corresponding to different biological activities can be decoupled and suggest that three species of ACT (intact ACT, a complexed protease/ACT form, and a partially denatured or proteolyzed form of ACT) that can exist in the microenvironment of an activated neutrophil may play an important role in regulating neutrophil function.

Cloning, expression, purification, and biological activity of recombinant native and variant human alpha 1-antichymotrypsins.

Human alpha 1-antichymotrypsin has been cloned, sequenced and expressed in Escherichia coli and recombinant protein as well as point-specific mutants have been purified and characterized. The corrected gene-deduced amino acid sequence has 45% overall identity with alpha 1-protease inhibitor, which is higher than the 42% previously reported (Chandra, T., Stackhouse, R., Kidd, V. J., Robson, J. H., and Woo, S. L. C. (1983) Biochemistry 22, 5055-5060). Recombinant antichymotrypsin (rACT) is similar to natural antichymotrypsin with respect to the specificity of its interactions with proteases. Its second-order rate constant for association with bovine chymotrypsin is 6-8 x 10(5) M-1 s-1, which is identical to that of the serum-derived inhibitor. Site-specific mutagenesis has been used to produce two variants of rACT in which the P1 position has been changed from leucine to either methionine (L358M-rACT) or arginine (L358R-rACT). L358M-rACT has a specificity of inhibitory activity toward serine proteases closely similar to that of native rACT. By contrast, the specificity of L358R-rACT is quite different from that of native rACT, most notably in efficiently inhibiting trypsin and human thrombin while showing a decreased ability to inhibit chymotrypsin.

Triosephosphate isomerase: energetics of the reaction catalyzed by the yeast enzyme expressed in Escherichia coli.

Triosephosphate isomerase from bakers' yeast, expressed in Escherichia coli strain DF502(p12), has been purified to homogeneity. The kinetics of the reaction in each direction have been determined at pH 7.5 and 30 degrees C. Deuterium substitution at the C-2 position of substrate (R)-glyceraldehyde phosphate and at the 1-pro-R position of substrate dihydroxyacetone phosphate results in kinetic isotope effects on kcat of 1.6 and 3.4, respectively. The extent of transfer of tritium from [1(R)-3H]dihydroxyacetone phosphate to product (R)-glyceraldehyde phosphate during the catalyzed reaction is only 3% after 66% conversion to product, indicating that the enzymic base that mediates proton transfer is in rapid exchange with solvent protons. When the isomerase-catalyzed reaction is run in tritiated water in each direction, radioactivity is incorporated both into the remaining substrate and into the product. In the "exchange-conversion" experiment with dihydroxyacetone phosphate as substrate, the specific radioactivity of remaining dihydroxyacetone phosphate rises as a function of the extent of reaction with a slope of about 0.3, while the specific radioactivity of the products is 54% that of the solvent. In the reverse direction with (R)-glyceraldehyde phosphate as substrate, the specific radioactivity of the product formed is only 11% that of the solvent, while the radioactivity incorporated into the remaining substrate (R)-glyceraldehyde phosphate also rises as a function of the extent of reaction with a slope of 0.3. These results have been analyzed according to the protocol described earlier to yield the free energy profile of the reaction catalyzed by the yeast isomerase.(ABSTRACT TRUNCATED AT 250 WORDS)

Triosephosphate isomerase: removal of a putatively electrophilic histidine residue results in a subtle change in catalytic mechanism.

An important active-site residue in the glycolytic enzyme triosephosphate isomerase is His-95, which appears to act as an electrophilic component in catalyzing the enolization of the substrates. With the techniques of site-directed mutagenesis, His-95 has been replaced by Gln in the isomerase from Saccharomyces cerevisiae. The mutant isomerase has been expressed in Escherichia coli strain DF502 and purified to homogeneity. The specific catalytic activity of the mutant enzyme is less than that of wild type by a factor of nearly 400. The mutant enzyme can be resolved from the wild-type isomerase on nondenaturing isoelectric focusing gels, and an isomerase activity stain shows that the observed catalytic activity indeed derives from the mutant protein. The inhibition constants for arsenate and for glycerol phosphate with the mutant enzyme are similar to those with the wild-type isomerase, but the substrate analogues 2-phosphoglycolate and phosphoglycolohydroxamate bind 8- and 35-fold, respectively, more weakly to the mutant isomerase. The mutant enzyme shows the same stereospecificity of proton transfer as the wild type. Tritium exchange experiments similar to those used to define the free energy profile for the wild-type yeast isomerase, together with a new method of analysis involving 14C and 3H doubly labeled substrates, have been used to investigate the energetics of the mutant enzyme catalyzed reaction. When the enzymatic reaction is conducted in tritiated solvent, the mutant isomerase does not catalyze any appreciable exchange between protons of the remaining substrate and those of the solvent either in the forward reaction direction (using dihydroxyacetone phosphate as substrate) or in the reverse direction (using glyceraldehyde phosphate as substrate). However, the specific radioactivity of the product glyceraldehyde phosphate formed in the forward reaction is 31% that of the solvent, while that of the product dihydroxyacetone phosphate formed in the reverse reaction is 24% that of the solvent. The deuterium kinetic isotope effects observed with the mutant isomerase using [1(R)-2H]dihydroxyacetone phosphate and [2-2H]glyceraldehyde 3-phosphate are 2.15 +/- 0.04 and 2.4 +/- 0.1, respectively. These results lead to the conclusion that substitution of Gln for His-95 so impairs the ability of the enzyme to stabilize the reaction intermediate that there is a change in the pathways of proton transfer mediated by the mutant enzyme. The data allow us more closely to define the role of His-95 in the reaction catalyzed by the wild-type enzyme, while forcing us to be alert to subtle changes in mechanistic pathways when mutant enzymes are generated.