The effect of osmolarity on metabolism and morphology in adhesion and suspension chinese hamster ovary cells producing tissue plasminogen activator

The effects of constant osmolarity, between 300 and500 mOsm/kg, on the metabolism of Chinese HamsterOvary (CHO) cells producing tissue plasminogenactivator (tPA) were compared between adhesion andsuspension cultures. In both suspension and adhesionculture, the specific rates of glucose consumption(_G), lactate production (q_L), and tPAproduction (q_tPA) increased as osmolarityincreased, while these rates decreased when osmolaritywas higher than the respective critical levels. However, specific growth rate () decreased withincrease in osmolarity and this slope grew steeper inthe osmolarity range higher than the critical level. The decrease in in the adhesion culture was morerapid than that in the suspension culture. Thecritical osmolarity for adhesion culture (400 mOsm/kg)was lower than that for suspension culture (450 mOsm/kg). These results indicated that the adhesionculture was more sensitive to increase of osmolaritythan the suspension culture, while the specific ratesobtained from the adhesion cultures were in general1.5- to 3-fold higher than those obtained from thesuspension cultures. Cell volume increased asosmolarity increased in both the suspension andadhesion cultures, as reported previously forsuspension culture of hybridoma cells, but there wasno morphological change in the suspension culture. Incontrast, cell height decreased and cell adhesion areamarkedly increased as osmolarity increased in theadhesion culture. This morphological change inadhesion cultures may be one reason for the highersensitivity of adherent cells to the increase ofosmolarity than suspended cells.     

Overexpression of mutant cell division cycle 25 homolog B (CDC25B) enhances the efficiency of selection in Chinese hamster ovary cells

The effects of mutant cell division cycle 25 homolog B (CDC25B) overexpression on the generation of cells producing a monoclonal antibody were investigated in Chinese hamster ovary (CHO) cells. Mutant CDC25B (m-CDC25B) expression plasmids were transfected into CHO DG44-derived cells producing a monoclonal antibody, and the frequency of highly producing cells was assessed following gene amplification in the presence of 250 nM methotrexate. Most of the clones obtained from the m-CDC25B-overexpressing cells had higher antibody titers than did mock-transfected control cells. This arose from either higher transgene copy numbers or higher mRNA expression levels for the antibody. However, the high mRNA expression levels were not always accompanied by increases in transgene copy numbers. Our results suggest that cells producing high levels of a monoclonal antibody can be selected efficiently using m-CDC25B overexpression.     

An optimized method for extraction and quantification of nucleotides and nucleotide sugars from mammalian cells

Glycosylation is a critical attribute of therapeutic proteins given its impact on the clinical safety and efficacy of these molecules. The biochemical process of glycosylation is inextricably dependent on metabolism and ensuing availability of nucleotides and nucleotide sugars (NSs) during cell culture. Herein, we present a comprehensive methodology to extract and quantify these metabolites from cultured cells. To establish the full protocol, two methods for the extraction of these compounds were evaluated for efficiency, and the requirement for quenching and washing the sample was assessed. A chromatographic method based on anion exchange has been optimized to separate and quantify eight nucleotides and nine NSs in less than 30 min. Degradation of nucleotides and NSs under extraction conditions was evaluated to aid in selection of the most efficient extraction protocol. We conclude that the optimized chromatographic method is quick, robust, and sensitive for quantifying nucleotides and NSs. Furthermore, our results show that samples taken from cell culture should be treated with 50% v/v acetonitrile and do not require quenching or washing for reliable extraction of nucleotides and NSs. This comprehensive protocol should prove useful in determining the impact of nucleotide and NS metabolism on protein glycosylation.     

Transient transfection of mammalian cells with DNA of the plant-pathogenic Ti-plasmid and expression of marker and resident sequences

Summary The large Ti-plasmid from Agrobacterium tumefaciens strain C58 has been used for transfection experiments with mammalian cells. In DNA from Tupaia baby fibroblasts Ti-plasmid sequences could be identified by filter hybridization as long as four weeks after transfection including two cell passages. The hybridization signals decreased rapidly after addition of the Ti-plasmid DNA-coprecipitate to the cells. The signals were often not detected any more after the first day, but were visible one week after transfection. Nuclei prepared from Ti-plasmid-transfected cells hybridized to pTi-specific RNA. With the chloramphenicol acetyl transferase-gene as marker no discrimination in DNA uptake was found between the Ti-plasmid and much smaller plasmids. According to the number of nuclei with homology to pTi-sequences it is assumed that about 0.2% of the cells carry Ti-plasmid DNA in the nucleus. Analysis of RNA isolated from cells transfected with cloned segments of the Ti-plasmid revealed that the TDNA region of the Ti-plasmid was predominantly transcribed.Abbreviations CAT Chloramphenicol Acetyl Transferase - NPT Neomycin Phosphotransferase - SDS Sodium Dodecylsulfate - TK Thymidine Kinase     

The effect of UDP-glucuronosyltransferase 1A1 expression on the mutagenicity and metabolism of the cooked-food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in CHO cells

UDP-glucuronosyltransferase proteins (UGT) catalyze the glucuronidation of both endogenous and xenobiotic compounds. In previous studies, UGT1A1 has been implicated in the detoxification of certain food-borne carcinogenic-heterocyclic amines. To determine the importance of UDP-glucuronosyltransferase 1A1 (UGT1A1) in the biotransformation of the cooked-food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), genetically modified CHO cells that are nucleotide excision repair-deficient, and express cytochrome P4501A2 (UV5P3 cell line) were transfected with a cDNA plasmid of human UGT1A1 to establish the UDP-glucuronosyltransferase 1A1 expressing 5P3hUGT1A1 cell line. Expression of the UGT1A1 gene was verified by screening neo gene expressing clonal isolates (G-418 resistant) for their sensitivity to cell killing from PhIP exposure. Five of 11 clones were chosen for further analysis due to their resistance to cell killing. Western blot analysis was used to confirm the presence of the UGT1A1 and CYP1A2 proteins. All five clones displayed a 52-kDa protein band, which corresponded to a UGT1A1 control protein. Only four of the clones had a protein band that corresponded to the CYP1A2 control protein. Correct fragment size of the cDNAs in the remaining four clones was confirmed by RT-PCR and quantification of the mRNA product was accomplished by real-time RT-PCR. Expression of UGT1A1 in the transfected cells was 10⁴–10⁵-fold higher relative to the UV5P3 parental cells. One clone (#14) had a 10-fold higher increase in expression at 1.47 × 10⁵ over the other three clones. This clone was also the most active in converting N-hydroxy-PhIP to N-hydroxy-PhIP glucuronide conjugates in microsomal metabolism assays. Based on the D₅₀ values, the cytotoxic effect of PhIP was decreased 350-fold in the 5P3hUGT1A1 cells compared to the UV5P3 control cells. In addition, no significant increase in mutation frequency was observed in the transfected cells. These results clearly indicate that UGT1A1 plays a critical role in PhIP biotransformation, providing protection against PhIP-mediated cytotoxicity and mutagenicity.     

Use of a Plackett-Burman statistical design to determine the effect of selected amino acids on monoclonal antibody production in CHO cells

Culture media design is central to the optimization of monoclonal antibody (mAb) production. Although general strategies do not currently exist for optimization of culture media, the combined use of statistical design and analysis of experiments and strategies based on simple material balances can facilitate culture media design. In this study, we evaluate the effect of selected amino acids on the growth rate and monoclonal antibody production of a Chinese hamster ovary DG-44 (CHO-DG44) cell line. These amino acids were selected based on their relative mass fraction in the specific mAb produced in this study, their consumption rate during bioreactor experiments, and also through a literature review. A Plackett-Burman statistical design was conducted to minimize the number of experiments needed to obtain statistically relevant information. The effect of this set of amino acids was evaluated during exponential cell culture (considering viable cell concentration and the specific growth rate as main output variables) and during the high cell-density stage (considering mAb final concentration and specific productivity as relevant output variables). For this particular cell line, leucine (Leu) and arginine (Arg) had the highest negative and positive effects on cell viability, respectively; Leu and threonine (Thr) had the highest negative effect on growth rate, and valine (Val) and Arg demonstrated the highest positive impact on mAb final concentration. Results suggest the pertinence of a two-stage strategy for amino acid supplementation, with a mixture optimized for cell growth and a different amino acid mixture for mAb production at high density.     

The effects of oncogene transfection on growth and antibody production of human-mouse heterohybridomas

Human-mouse heterohybridomas producing human monoclonal antibodies show slower growth rates and lower peak cell densities than murine hybridomas. In order to improve the growth properties we transfected a heterohybridoma cell line with expression plasmids containing the oncogenes v-src, c-Ha-ras and SV40largeT. The plasmids were transferred by electroporation. Growth promoting activities of the plasmids were proven in NIH3T3 cells whereby a doubling of the maximum cell densities of this cell type was observed. The oncogene products were analyzed by means of northern blotting and immunofluorescence. After transfection of c-src and c-ras, a heterohybridoma cell line was derived which showed improved growth characteristics compared to the original cell line. Although specific antibody production was lower, antibody concentrations which accumulated in batch culture were higher due to increased maximum cell densities.     

mRNA transfection of dendritic cells: synergistic effect of ARCA mRNA capping with Poly(A) chains in cis and in trans for a high protein expression level

The occurrence of translation mechanism in the cytosol offers advantages to mRNA transfer over DNA-based transfection in non-dividing cells. Here, we sought to optimize mRNA constructs allowing a high level of protein upon lipofection. We found that luciferase into mouse dendritic cells (JAWSII cells) was approximately 20-fold higher when the luciferase mRNA was capped with 3'-O-methyl-m7(5')Gppp5'G (anti-reverse cap analogue; ARCA) than with m7(5')Gppp5'G (CAP). Adding a Poly(A) tail of 100 instead of 64 adenosines in cis increased by approximately 35-fold more the protein level. Finally, ARCA-Luc-mRNA-A100 construct was 700-fold better efficient than the CAP-Luc-mRNA-A64 one. Moreover, co-transfection with free Poly(A) chains in trans enhanced by 100% the luciferase level. The efficiency of ARCA-mRNA-A100 construct was validated in immature and mature human CD34-derived dendritic cells. Such mRNA construct was also successful to obtain high level of MART-1 tumor antigen.     

Induction and expression of mutations in mammalian cells in the absence of DNA synthesis and cell division

The induction of mutations by the alkylating agent ethyl methanesulfonate (EMS) was determined with Chinese hamster ovary cells maintained in serum-free medium to arrest DNA synthesis and cell division. The arrested cultures were treated with EMS and maintained in serum-free medium for various time intervals post-treatment before serum containing medium was added to initiate DNA synthesis and cell division. The concentration-dependent increase in 6-thioguanine-resistant mutants in the arrested cultures was similar to that found with exponentially dividing cultures when serum was added to the arrested cultures immediately after the EMS treatment; the time course of phenotypic expression was also similar with both cultures. In addition, maintenance of the arrested cultures in serum-free medium for up to 18 days post-treatment resulted in no change in the mutant frequency. This suggests that the mutagenic damage is not removed in these arrested cultures. Furthermore, maintenance of the arrested state for increasing time intervals before serum addition results in decreases in the time necessary for maximum phenotypic expression. Cultures maintained in serum-free medium for 16 days after mutation treatment show complete expression of the mutations with no need for subculture. This last result suggests that the mutagenic damage induced by EMS in Chinese hamster ovary cells is not removed and that this damage results in both the induction and expression of mutation in the absence of DNA replication.     

Development and manufacturability assessment of chemically‐defined medium for the production of protein therapeutics in CHO cells

Advantages of using internally developed chemically‐defined (CD) media for cell culture‐based therapeutic protein production over commercial media include better raw material control and medium vendor options, and most importantly, flexibility for process development and subsequent optimization needed for therapeutic protein production. Through several rounds of design of experiment (DOE) screening, and medium component supplementation and optimization studies, we successfully developed a CD basal medium (CDM) for CHO cell culture. The internally prepared liquid CDM demonstrated comparable cell culture performance to that from a commercially available control medium. However, when the same CDM formulation was transferred to two major commercial medium suppliers for manufacturing, cell culture performance utilizing these newly prepared media was significantly reduced compared with the in‐house prepared counterpart. An investigation was launched to assess whether key medium components were sensitive to large‐scale preparation of the final bulk media by the vendors. Further work necessitated the reformulation of the original CDM formulation into a core medium that was suitable for large‐scale media manufacturing. The modified preparation of the core medium with two separate supplements to generate the final CDM was able to recover the expected cell culture performance and monoclonal antibody (mAb) productivity. Confirmation of cell culture robustness in cell growth and production was corroborated in two additional mAb‐expressing cell lines. This work demonstrates that a robust CD medium is not only one that performs during the development stage, but also one that must be reproducible by commercial media vendors. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:1163–1171, 2015     

Effect of cloned gene dosage on cell growth and hepatitis B surface antigen synthesis and secretion in recombinant CHO cells

The effect of cloned gene copy number on growth and product formation has been studied in sufficient detail using a Chinese hamster overy (CHO) cell line producing recombinant hepatits B surface antigen (HbsAg). Batch culture experiments were carried out in T flasks in order to characterize cell growth and HbsAg secretion in various clones carrying different numbers of HbsAg gene copies integrated into CHO cell chromosomes. Specific growth rates were found to decrease with increasing gane copy number. Secreted HbsAg concentration and specific HbsAg secretion rates were found to increase with increase in gene copy number. Gene copy numbers in each clone determined using Southern hybridizations were positively correlated with intracellular dihydrofolate reductase (dhfr) content using a flow cytometric assay. The mRNA levels quantitated using Northern hybridization followed by autroadiography and densitometry also gave the same trends. The flow cytometry experiments show that while parental cells were quite homogeneous with respect to intracellular dhfr content, the amplified clones exhibit a great deal of heterogeneity in dhfr content. Pulse-chase experiments show that the efficiency of HbsAg secretion (defined here as the fraction of initially labeled HbsAg that is secreted into the extracellular medium at the end of a 23.5-h chase) decreases and also that the intracellular HbsAg degradation increases with increasing gene copy number.     

Establishment of a signal peptide with cross-species compatibility for functional antibody expression in both Escherichia coli and Chinese hamster ovary cells

Signal peptides are short peptides located at the N-terminus of secreted proteins. They characteristically have three domains; a basic region at the N-terminus (n-region), a central hydrophobic core (h-region) and a carboxy-terminal cleavage region (c-region). Although hundreds of different signal peptides have been identified, it has not been completely understood how their features enable signal peptides to influence protein expression. Antibody-derived signal peptides are often used to prepare recombinant antibodies expressed by eukaryotic cells, especially Chinese hamster ovary (CHO) cells. However, when prokaryotic Escherichia coli (E. coli) are utilized in drug discovery processes, such as for phage display selection or antibody humanization, signal peptides have been selected separately due to the differences in the expression systems between the species. In this study, we successfully established a signal peptide that enables a functional antibody to be expressed in both prokaryotic and eukaryotic cells by focusing on the importance of having an Ala residue in the c-region of the signal sequence. We found that changing Ser to Ala at only two positions significantly augmented the anti-HER2 antigen binding fragment (Fab) expression in E. coli. In addition, this altered signal peptide also retained the ability to express functional anti-HER2 antibody in CHO cells. Taken together, the present findings indicate that the signal peptide can promote functional antibody expression in both prokaryotic E. coli and eukaryotic CHO cells. This finding will contribute to the understanding of signal peptides and accelerate therapeutic antibody research.     

Rapid production of a chimeric antibody-antigen fusion protein based on 2A-peptide cleavage and green fluorescent protein expression in CHO cells

To enable large-scale antibody production, the creation of a stable, high producer cell line is essential. This process often takes longer than 6 months using standard limited dilution techniques and is very labor intensive. The use of a tri-cistronic vector expressing green fluorescent protein (GFP) and both antibody chains, separated by a GT2A peptide sequence, allows expression of all proteins under a single promotor in equimolar ratios. By combining the advantages of 2A peptide cleavage and single cell sorting, a chimeric antibody-antigen fusion protein that contained the variable domains of mouse IgG with a porcine IgA constant domain fused to the FedF antigen could be produced in CHO-K1 cells. After transfection, a strong correlation was found between antibody production and GFP expression (r = 0.69) using image analysis of formed monolayer patches. This enables the rapid selection of GFP-positive clones using automated image analysis for the selection of high producer clones. This vector design allowed the rapid selection of high producer clones within a time-frame of 4 weeks after transfection. The highest producing clone had a specific antibody productivity of 2.32 pg/cell/day. Concentrations of 34 mg/L were obtained using shake-flask batch culture. The produced recombinant antibody showed stable expression, binding and minimal degradation. In the future, this antibody will be assessed for its effectiveness as an oral vaccine antigen.     

Induction of tumor-specific cytotoxic T lymphocytes and natural killer cells by tumor cells transfected with the interleukin-2 gene

To study the antitumor effect of local production of interleukin-2 (IL-2) from tumor cells, the poorly immunogenic murine colon cancer cells, colon26, was transfected with murine IL-2 cDNA in a bovine papilloma virus vector. IL-2 gene transfectants (mIL2+colon26) did not alter their growth rate compared with parental colon26 cells in vitro, but reduced their tumorigenicity in vivo. Immunization with mIL2+colon26 cells could induce protective immunity against parental colon26 cells. Following intravenous challenges, the colonies of lung metastasis were also inhibited. Moreover, inoculation of mIL2+ colon26 cells slowed the growth of challenged renal cell carcinoma cells, RenCa. Intraperitoneal inoculation of IL-2 gene transfectants generated a large number of peritoneal exudate cells and these cells had a highly cytolytic activity against colon26 and YAC-1. These results suggest that inoculation with IL-2 transfected tumor cells can stimulate not only cytotoxic T lymphocytes but also natural killer cells, and that these cells will act as antitumor effector cells in host animals.     

The effect of gene copy number and co-expression of chaperone on production of albumin fusion proteins in Pichia pastoris

Interleukin-1 receptor antagonist (IL1ra) is known to treat a number of diseases such as rheumatoid arthritis and type 2 diabetes. However, the biological half-life of IL1ra is very short due to its rapid renal clearance. Our present study aimed to increase the biological half-life of IL1ra through fusion with human serum albumin (HSA), and then augmented expression of the IL1ra and HSA fusion protein (IH) in Pichia pastoris strain by increasing IH gene copy number or was co-expressed with chaperone. By comparing clones containing varying copy numbers of IH fusion gene, it was observed that higher levels of secretory IH fusion protein was produced in strain with higher IH gene copy number. In addition, IH protein yield was further improved after being co-expressed with protein disulfide isomerase (PDI). Conversely, it was significantly decreased (i.e., secretory IH in the culture medium) by co-expression of immunoglobulin binding protein. We have also discussed whether the multi-copy strain and co-expressed of PDI could enhance the levels of other secretory albumin fusion protein (e.g., HSA and human growth hormone fusion protein). Interestingly, the level of this fusion protein was apparently also increased by these approaches. In conclusion, our results have demonstrated that increasing copy number and co-expression of PDI may raise yield of albumin fusion protein in P. pastoris, which might probably contribute to the industry for the development of proteinous drugs.     

Further evidence for the genetic origin of mutations in mammalian somatic cells: the effects of ploidy level and selection stringency on dose-dependent chemical mutagenesis to purine analogue resistance in Chinese hamster ovary cells.

Whether resistance to purine analogues 8-azaguanine (AG) and 6-thioguanine (TG) in mammalian cells is due to gene mutation or to epigenetic changes was investigated by an ethyl methanesulfonate (EMS) dose-dependent induced “resistance” to these analogues in two near-diploid (2N) and one tetraploid (4N) Chinese hamster ovary (CHO) cells. EMS produced higher cell killing in 2N than in 4N cells. In the 2N cells, EMS-induced mutations to TG (1.7 μg/ml) resistance increased approximately as a linear function of the dose from 0–400 μg/ml. However, EMS was ineffective in inducing such mutation in the 4N cells. These observations are consistent with the notion that the induced TG resistance arose as a result of mutation at the gene or chromosome level. In each cell type, both the “observed” spontaneous and the EMS-induced frequency to purine analogue resistance decreased with increasing concentration of purine analogues. However, among the “resistant” clones a high proportion of those selected at 1.2 and 3.0 μg/ml of AG, a small portion selected at 7.5 μg/ml of AG, and virtually none at 1.7 and 6.0 μg/ml of TG are capable of growth in medium containing aminopterin (10 μM). This suggests that, under less stringent selective conditions, some resistant variants were being selected through mechanisms not yet defined.     

Mapping of the mAb73 epitope on Ad2 E1A proteins with random peptide libraries and deletion mutants

Abstract mAb73, a monoclonal antibody against adenoviruses type 2 and 5 E1A, recognises an epitope within the C-terminal part of this protein. To identify the epitope we used random peptide libraries expressed on the surface of filamentous phages (Fd, M13). We found a consensus sequence homologous to the nuclear transport signal KRPRP at the C-terminus of Ad2 and Ad5 E1A. An E1A mutant deleted for these residues failed to be immunoprecipitated by mAb73, confirming that the nuclear transport signal of E1A is the epitope recognised by mAb73.