Renin and angiotensinogen expression and functions in growth and apoptosis of human glioblastoma.

The expression and function in growth and apoptosis of the renin-angiotensin system (RAS) was evaluated in human glioblastoma. Renin and angiotensinogen (AGT) mRNAs and proteins were found by in situ hybridisation and immunohistochemistry in glioblastoma cells. Angiotensinogen was present in glioblastoma cystic fluids. Thus, human glioblastoma cells produce renin and AGT and secrete AGT. Human glioblastoma and glioblastoma cells expressed renin, AGT, renin receptor, AT(2) and/or AT(1) mRNAs and proteins determined by RT-PCR and/or Western blotting, respectively. The function of the RAS in glioblastoma was studied using human glioblastoma cells in culture. Angiotensinogen, des(Ang I)AGT, tetradecapaptide renin substrate (AGT1-14), Ang I, Ang II or Ang III, added to glioblastoma cells in culture, did not modulate their proliferation, survival or death. Angiotensin-converting enzyme inhibitors did not diminish glioblastoma cell proliferation. However, the addition of selective synthetic renin inhibitors to glioblastoma cells decreased DNA synthesis and viable tumour cell number, and induced apoptosis. This effect was not counterbalanced by concomitant addition of Ang II. In conclusion, the complete RAS is expressed by human glioblastomas and glioblastoma cells in culture. Inhibition of renin in glioblastoma cells may be a potential approach to control glioblastoma cell proliferation and survival, and glioblastoma progression in combination therapy.

Characterization of a human angiotensinogen cleaved in its reactive center loop by a proteolytic activity from Chinese hamster ovary cells.

Angiotensinogen, the renin (E.C. 3.4.23.15) substrate, belongs to the serpins superfamily and has been classified as a noninhibitory serpin. Using mass spectroscopy, angiotensinogen purified from Chinese hamster ovary cell supernatant shows a broad spectrum. The absence of protease inhibitors throughout the purification leads to an angiotensinogen cleaved within the reactive center loop. This cleavage does not affect the Ang I generation because kinetic parameters are similar to the values of the full-length angiotensinogen. Although cleavage is complete, the cleaved angiotensinogen migrates after deglycosylation on SDS-polyacrylamide gel electrophoresis as a doublet differing by 4 kDa. To test whether the circulating angiotensinogen is cleaved in the reactive center loop, it was purified from a pool of human plasma and was shown to be uncleaved. Its migration was obviously slower than of cleaved angiotensinogen but also consisted of two bands pointing to a so far unexplained residual heterogeneity. We then compared the heat-induced polymerization of full-length- and reactive center loop-cleaved angiotensinogens. Both monomers were able to aggregate, revealing a particular behavior of angiotensinogen distinct from that of reactive center loop-cleaved serpins. Lacking the three-dimensional structure of angiotensinogen, we propose and discuss a structural model of the serpin fold within the renin substrate.

Histidine 450 plays a critical role in catalysis and, with Ca2+, contributes to the substrate specificity of aminopeptidase A.

Aminopeptidase A (EC 3.4.11.7, APA) is a 130 kDa membrane-bound protease that contains the HEXXH consensus sequence found in the zinc metalloprotease family, the zincins. In addition to the catalytic zinc atom, APA contains a Ca2+ ion that increases its enzymatic activity. Aligning the sequences of the mouse APA, APN, and other monozinc aminopeptidases led to the identification of a conserved histidine (His 450 in mouse APA). Replacing this residue with a phenylalanine (Phe 450) by site-directed mutagenesis resulted in markedly lower levels of APA activity and in a change in the sensitivity of APA to Ca2+ (the EC50 for Ca2+ was 25 microM in the wild type and only 279 microM in the mutant). Kinetic studies, with a supramaximal Ca2+ concentration (4 mM), showed that the Km of the mutant enzyme for the substrate alpha-L-glutamyl-beta-naphthylamide was 25 times higher than that of the wild type, whereas the kcat value was much lower (factor of 22). Thus, overall, the wild-type enzyme had a cleavage efficiency that was 571 times higher than that of the mutant. The inhibitory potencies of two different classes of inhibitors, a glutamate thiol and a glutamate phosphonate compound, were significantly lower (factors of 19 and 22, respectively) for the mutated enzyme than for the wild-type enzyme. In contrast, inhibition by lysine thiol was unaffected. These data strongly suggest that His 450 is critical for catalytic activity and is involved in substrate binding via interaction with the P1 carboxylate side chain of the substrate. Furthermore, His 450, together with Ca2+, may contribute to the substrate specificity of APA for N-terminal acidic amino acid residues.

Angiotensinogen variants and human hypertension.

The research on molecular genetics of human hypertension aims to identify the loci involved in the regulation of blood pressure, detect gene variants within the identified loci, associate them with intermediate phenotypes, and ultimately estimate their quantitative effects on blood pressure level and their interaction with main environmental factors. So far, the angiotensinogen (AGT) gene is one of the few candidate genes that has been investigated using these multiple statistical, clinical, and biochemical strategies. A highly polymorphic dinucleotide GT repeat (80% heterozygosity) has been used in several linkage studies. Other diallelic polymorphisms, located in the 5' regulatory region of the gene in intronic and exonic sequences, have been described, which were then used in association studies in different clinical settings. Positive associations between the M235T and the G-6A polymorphisms and plasma angiotensinogen levels indicates a pathway by which the AGT locus could be involved in essential hypertension.

Role of cysteine residues in human angiotensinogen. Cys232 is required for angiotensinogen-pro major basic protein complex formation.

The M235T polymorphism of human angiotensinogen is associated with essential and pregnancy-induced hypertension. A covalent complex is formed between angiotensinogen and the proform of the eosinophil major basic protein (proMBP) during pregnancy. The sequence of human angiotensinogen contains four cysteines. Their function was analyzed. Presence of free cysteines was demonstrated by their alkylation with iodo[14C]acetic acid. A disulfide bond between Cys18 and Cys138 using a fully N-deglycosylated mutant of human angiotensinogen was identified by tryptic digestion and mass spectrometry. We produced angiotensinogen. proMBP complex by co-transfection of COS-7 cells and by co-culturing transfected CHO-K1 cells. Experiments with 8 mutated recombinant angiotensinogen, in which one or more of the four cysteines were replaced by alanine, demonstrated that Cys232 is involved in complex formation and could interact with the M235T variant. The angiotensinogen.proMBP complex was isolated by molecular sieving. Hydrolysis of the complex by human renin was 7 times slower than hydrolysis of monomeric form, whatever the M235T genotype. The complex:monomeric angiotensinogen ratio was greater for Met235 (72%) than for Thr235 (58%) angiotensinogen. These data suggest a new pathophysiological explanation for the genetic association between M235T angiotensinogen polymorphism and pregnancy-induced hypertension.

Role of N-glycosylation in human angiotensinogen.

Human angiotensinogen, the specific substrate of renin, is a heterogeneous glycoprotein constitutively secreted by the liver. Different glycosylation levels may be responsible for its heterogeneity. It contains four putative asparagine-linked glycosylation sites (Asn-X-Ser/Thr). Systematic site-directed mutagenesis (Asn replaced with Gln) of these four sites was undertaken, and 11 (single, double, triple, and quadruple (N-4)) mutants were produced in COS-7 and/or CHO-K1 cells and characterized. All of the sites were N-glycosylated with preferential glycosylation of the Asn14 and the Asn271. The suppression of the Asn14 glycosylation site led to 5 times lower Km and a 10 times lower kcat. Angiotensinogen heterogeneity was much lower for the N-4 mutant protein, which produced a single form at 48 kDa. Pulse-chase experiments showed slight intracellular retention (15%) of the deglycosylated protein after 24 h. Interestingly, the N-4 mutant had a higher catalytic efficiency (kcat/Km = 5.0 versus 1.6 microM-1 . s-1) than the wild-type protein. The thermal stability of the N-4 protein was unaffected by deglycosylation, suggesting that it was correctly folded. This deglycosylated recombinant human angiotensinogen could be of value for x-ray crystallography studies.

The natural mutation Y248C of human angiotensinogen leads to abnormal glycosylation and altered immunological recognition of the protein.

Common molecular variants of the angiotensinogen gene have been associated with human hypertension. The rare Tyr to Cys change at residue 248 of mature angiotensinogen was identified in one pedigree. Heterozygous individuals (Y248C) had a 40% decrease in plasma angiotensinogen concentration and a 35% reduction of the angiotensin I production rate. Recombinant wild-type (Tyr-248) and mutant (Cys-248) proteins were stably expressed in Chinese hamster ovary cells. Angiotensinogen monoclonal antibodies revealed marked differences in the epitope recognition of the mutant protein and allowed the demonstration of its presence in plasma of Y248C individuals. Similar kinetic constants of angiotensin I production with human renin were observed for both proteins. Western blot analysis showed similar heterogeneities; however, a 3-kDa increase in molecular mass for the Cys-248 protein was observed after immunopurification. Metabolic labeling of the intracellular Cys-248 protein showed a 61-kDa band in addition to the 55.5- and 58-kDa bands observed for the Tyr-248 protein, with all bands being sensitive to endoglycosidase H. In addition, pulse-chase studies revealed a slower intracellular processing for the Cys-248 protein. In conclusion, the Cys-248 mutation alters the structure, glycosylation, and secretion of angiotensinogen in Chinese hamster ovary cells and is accompanied by a decrease in plasma angiotensinogen concentration in Y248C individuals.

Expression in Escherichia coli of the flavin and the haem domains of Hansenula anomala flavocytochrome b2 (flavodehydrogenase and b2 core) and characterization of the recombinant proteins.

The flavin and haem domains of Hansenula anomala flavocytochrome b2 have been independently expressed in Escherichia coli. The flavin domain activity, studied only in the total cellular extract, owing to its instability, has characteristics very similar to those of the flavin domain obtained by proteolysis. The haem domain (r-core) has been purified to homogeneity and characterized in detail from spectroscopic and functional points of view. Spectral differences with respect to the domain produced by proteolysis (p-core) were found using resonance Raman and c.d. spectroscopy and have been interpreted in terms of changes in haem-protein interactions. However, this structural difference is functionally silent, since the r-core is able to reduce cytochrome c with the same efficiency as the proteolytic domain.

Purification and identification of a CD25 expression inhibitory protein from cell-free supernatants of chronically HIV-infected promonocytic cells.

The CD25 (IL-2-R alpha) cell surface glycoprotein expressed transiently during T-cell activation is implicated in the high affinity IL-2 receptor. This paper shows that cell-free supernatants from chronically HIV-infected promonocytic cells spontaneously produce a soluble factor which inhibits CD25 expression on PHA-activated human PBMC. We purified the CD25 expression inhibitory activity by a factor 12,350, using XM50 ultrafiltration, Superose 12 molecular sieving chromatography and MonoQ anion-exchange chromatography. Then we associated this activity to one single spot (M(r) 29,000, pI 6.8) on an O'Farrell two-dimensional gel. Our data demonstrate that this protein (M(r) 29,000, pI 6.8) is released from HIV-infected promonocytic cells and suggest that this factor is a new monokine regulating the T-cell activation process.

Isolation of the flavodehydrogenase domain of Hansenula anomala flavocytochrome b2 after mild proteolysis by an H. anomala proteinase.

The protomeric chain of Hansenula anomala flavocytochrome b2 was previously shown to be built as the covalent association of two functional domains: an L-lactate dehydrogenase domain and a cytochrome c reductase domain, joined together by a proteolytically sensitive zone. This paper concerns the specific cleavage of this latter zone with a H. anomala proteinase(s) preparation and the purification of the resulting L-lactate dehydrogenase moiety of the molecule with at least 25% recovery, (i.e. one order of magnitude more than for the previously published method). A preliminary characterization of this dehydrogenase domain indicates that it is a tetramer (Mr = 4 x 39000) containing FMN as expected and not heme. It has high L-lactate:ferricyanide oxidoreductase activity (about 70% that of the whole flavocytochrome b2) and the same Km for L(+)-lactate as flavocytochrome b2, but it has no L-lactate:cytochrome c oxidoreductase activity. Its flavin semiquinone is stabilized in the presence of pyruvate as in flavocytochrome b2. The subcellular origin of the H. anomala proteinase in the preparation has not yet been elucidated.

Identification of psiB genes of plasmids F and R6-5. Molecular basis for psiB enhanced expression in plasmid R6-5.

PsiB protein of plasmid R6-5 inhibits the induction of the SOS pathway. The F sex factor also carries a psiB gene homologous to that of R6-5. Yet, it fails to inhibit SOS induction. In order to solve this difference, we characterized the psiB genes of R6-5 and F. We found that (i) the sequences of the two psiB genes share extensive homology the predicted amino acid sequences of the two proteins differing by 5 residues, (ii) the expression of R6-5 psiB is 4 times higher than F psiB gene, (iii) in plasmid R6-5, a Tn10 transposon upstream from the psiB gene enhances psiB expression. Hence, the F sex factor may be unable to prevent SOS induction for two non-exclusive reasons: (i) F PsiB protein, being slightly different from R6-5, may be less active, (ii) the level of synthesis of F PsiB protein may be insufficient to prevent SOS induction.

PsiB polypeptide prevents activation of RecA protein in Escherichia coli.

We further characterize a novel plasmid function preventing SOS induction called Psi (Plasmid SOS Inhibition). We show that Psi function is expressed by psiB, a gene located at coordinate 54.9 of plasmid R6-5 and near oriT, the origin of conjugal transfer. Deletions and amber mutations of the psiB gene permitted us to demonstrate that PsiB polypeptide (apparent molecular weight, 12 kDa) is responsible for Psi function. PsiB protein prevents recA730-promoted mutagenesis and intra-chromosomal recombination but not recombination following conjugation. Overproduction of PsiB protein sensitizes the host cell to UV irradiation. We propose that PsiB polypeptide has an anti-SOS action by inhibiting activation of RecA protein, thus preventing the occurrence of LexA-controlled functions.