Secretion of enzymatically active human renin from mammalian cells using an avian retroviral vector

Enzymatic characterization of purified recombinant human renin

Renin is a highly specific aspartyl protease of the renin-angiotensin system initially synthesized as preprorenin. Recombinant human prorenin was produced in cell factories from stably transfected DAMP cells, a dog epithelial cell line. The equivalent of 10-15 mg of recombinant human renin was secreted in the supernatant from each cell factory. Following a single affinity chromatography step using a renin inhibitor as the ligand, a 181-fold purification was achieved with 81% recovery of the renin activity. This highly pure recombinant enzyme having a specific activity of 3.44 mg angiotensin I.mg protein-1.h-1 was used for kinetic analysis. The kinetic parameters were determined with the natural substrate angiotensinogen and a tetradecapeptide substrate corresponding to the amino terminus of angiotensinogen, Asp1-Asn14, at their respective optimum pH of 5.5 and 6.8. Although there was a six-fold increase in both Km and kcat values for the peptidic substrate (13.3 microM and 8.1 s-1, respectively), when compared with values for the natural substrate (2.04 microM and 1.41 s-1), the catalytic efficiency (0.69 microM-1.s-1) of the enzyme for both substrates was the same. However, the kcat/Km value with angiotensinogen at the physiological pH 7.4 was 30% lower than that observed at the optimum pH 5.5. The recombinant human renin displayed similar optimum pH and kinetic parameters with angiotensinogen and the tetradecapeptide substrate when compared with human kidney renin.

Denaturation/refolding of purified recombinant HIV reverse transcriptase yields monomeric enzyme with high enzymatic activity

We engineered a prokaryotic expression vector encoding the HIV reverse transcriptase (RT). We grew Escherichia coli JM109 carrying the vector in a 250-liter stirred tank fermentor and purified RT (p66) under native conditions to apparent homogeneity. Purified p66 (greater than or equal to 5 mg/ml) was not stable, and was rapidly processed to its 51 kD derivative (p51), until p66:p51 levels were approximately 1:1. These latter RT preparations were chromatographed as heterodimers and had approximately fivefold higher specific RT enzymatic activities compared with those containing predominantly p66. P66 purified under dilute concentrations (less than or equal to 0.5 mg/ml) was monomeric in solution, resistant to p51 processing for weeks at 4 degrees C, but also had low specific RT enzymatic activities. To attempt the preparation of homogeneous p66 with specific RT enzymatic activities equivalent to p66:p51 heterodimers, purified heterodimers were denatured and p66 was purified and refolded during extensive dialysis (refolded p66). Refolded p66 (less than or equal to 0.5 mg/ml) was monomeric in solution and had identical specific RT enzymatic activities, Km for dTTP, and inhibition by 3'-azido-3'-deoxythymidine triphosphate compared with heterodimeric p66:p51 RT. The data indicates that HIV RT obtained from recombinant E. coli under native conditions is extensively processed at concentrations promoting dimerization. Moreover, RT denaturation and refolding yields apparently homogeneous monomeric p66, with specific RT enzymatic activities equivalent to heterodimeric RT.

Development of new cell lines for animal cell biotechnology

Mammalian cell culture has been an important technique in laboratory-scale experimentation for many decades. Developments in large-scale culture have been due to the need to grow large numbers of cells to support the growth of viruses for vaccine production, and more recently, for growing hybridoma cells as a source of monoclonal antibody. Increasingly, however, pharmaceutical products such as hormones, enzymes, growth factors, and clotting factors are being produced from cell lines which have been manipulated by recombinant DNA techniques. It is clear, therefore, that the high cost of growing mammalian cells on a large scale does not necessarily prohibit their use for biotechnology, and indeed there is considerable evidence to suggest that animal cell biotechnology will continue to be a major growth area in the future.

A rapid, quantitative bioassay based on the human immunodeficiency virus trans-activator

We constructed a human immunodeficiency virus (HIV) trans-activator cDNA (tat) encoding the N-terminal 76 amino acids of the viral trans-activator followed by two additional amino acids (val and pro). This cDNA encoded a functional trans-activator (TAT) as shown by cotransfection into murine cells with a HIV promoter-chloramphenicol acetyltransferase DNA construct. The tat cDNA was cloned into an avian retroviral expression vector, a modified spleen necrosis virus (SNV), and high-titer infectious stocks of recombinant virus (SNV-tat) were recovered from dog cells. Hybridization analyses indicated that SNV-tat was stably propagated in these cells for months. We also prepared recombinant cells that stably carry reporter genes, either a human gene encoding a soluble CD4 receptor (sCD4) or the human preprorenin gene, under the transcriptional control of the HIV promoter. Medium obtained from these cell cultures after infection with control viruses or an SNV carrying an antisense tat contained only low background levels of sCD4 or prorenin (HRN) as determined by specific immunoassays (1-10 ng protein per 10(6) cells per ml medium). In contrast, cells infected with SNV carrying tat in the transcriptional sense orientation secreted 75 +/- 7 ng sCD4 and 73 +/- 4 ng HRN per 10(6) cells per ml medium. Moreover, these proteins were constitutively secreted at these levels during months of subculturing. The data indicate that sCD4 and HRN are secreted from these cells because of a TAT-mediated trans-activation of the HIV reporter gene DNA and/or RNA. This combination of recombinant cells, SNV-tat, and specific immunoassays provide a rapid, quantitative, and safe bioassay to seek inhibitors of TAT.

The pro-peptide is not necessary for active renin secretion from transfected mammalian cells

Cultured mouse myeloma cells were transfected with expression vectors encoding the aspartyl proteinase, human renin. The full construct, encoding the renin precursor prorenin, allows transfected cells to secrete the enzymically inactive pro-protein. Activity is detectable only following trypsin treatment which mimics the physiological activation step. Accordingly, it appears that myeloma cells do not contain detectable levels of an appropriate activating proteinase. However, when these cells are transfected with a construct from which the pro-peptide coding sequence has been deleted, they secrete an apparently fully active enzyme which is indistinguishable from mature renin. We conclude that expression of the pro-peptide is not necessary to allow correct folding of the molecule and its passage through the secretory pathway.

Renin activity determination using human plasma as a substrate

A method for human renin activity determination using human plasma as the substrate is described. Angiotensin I is generated by incubating renin with human plasma, which is available along with the commercial radioimmunoassay kit for angiotensin I. The method obviates the need to isolate and purify the substrate, human angiotensinogen, from human plasma. In addition, the assay is highly renin specific, sensitive, and convenient to use for the routine determination of active human renin during its isolation and purification from tissue extracts or from genetically engineered bacterial and nonbacterial expression systems.

A bacteriophage transcription terminator permits the cloning of a mammalian expression vector carrying the human preprorenin gene in E. coli

We constructed a plasmid-based expression vector carrying the murine metallothionein gene promoter, the human preprorenin gene, the Tn5 neomycin phosphotransferase II gene, and a complete bovine papilloma virus genome. We were unsuccessful in cloning the preprorenin gene in E. coli when it was inserted into a plasmid vector 3' to the metallothionein gene promoter. This result was consistent with our previous data suggesting that expression of the preprorenin gene is toxic to E. coli (Weighous, T.F. et al; (1986) Gene 45: 121-129). We then inserted a DNA fragment from bacteriophage lambda carrying the t1 transcriptional terminator between the promoter and the preprorenin gene; this vector was stably maintained in E. coli. Prior to introducing the vector DNA into mouse cells for expression studies the terminator was removed. Mouse cells carrying the vector DNA secreted high levels of human prorenin for greater than six weeks.