The secretory pathway - the key for unlocking the potential of Chinese hamster ovary cell factories for manufacturing therapeutic proteins

Mammalian cell factories (in particular the CHO cell system) have been crucial in the rise of biopharmaceuticals. Mammalian cells have compartmentalized organelles where intricate networks of proteins manufacture highly sophisticated biopharmaceuticals in a specialized production pipeline - the secretory pathway. In the bioproduction context, the secretory pathway functioning is key for the effectiveness of cell factories to manufacture these life-changing medicines. This review describes the molecular components and events involved in the secretory pathway, and provides a comprehensive summary of the intracellular steps limiting the production of therapeutic proteins as well as the achievements in engineering CHO cell secretory machinery. We also consider antibody-producing plasma cells (so called "professional" secretory cells) to explore the mechanisms underpinning their unique secretory function/features. Such understandings offer the potential to further enhancement of the current CHO cell production platforms for manufacturing next generation of biopharmaceuticals.

Endoplasmic reticulum stress causes insulin resistance by inhibiting delivery of newly synthesized insulin receptors to the cell surface

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER stress and activates a signaling network known as the unfolded protein response (UPR). Here we characterize how ER stress and the UPR inhibit insulin signaling. We find that ER stress inhibits insulin signaling by depleting the cell surface population of the insulin receptor. ER stress inhibits proteolytic maturation of insulin proreceptors by interfering with transport of newly synthesized insulin proreceptors from the ER to the plasma membrane. Activation of AKT, a major target of the insulin signaling pathway, by a cytosolic, membrane-bound chimera between the AP20187-inducible FV2E dimerization domain and the cytosolic protein tyrosine kinase domain of the insulin receptor was not affected by ER stress. Hence, signaling events in the UPR, such as activation of the JNK mitogen-activated protein (MAP) kinases or the pseudokinase TRB3 by the ER stress sensors IRE1α and PERK, do not contribute to inhibition of signal transduction in the insulin signaling pathway. Indeed, pharmacologic inhibition and genetic ablation of JNKs, as well as silencing of expression of TRB3, did not restore insulin sensitivity or rescue processing of newly synthesized insulin receptors in ER-stressed cells. [Media: see text].

Elevated thrombin generation in patients with congenital disorder of glycosylation and combined coagulation factor deficiencies

BACKGROUND

Congenital disorders of glycosylation are rare inherited diseases affecting many different proteins. The lack of glycosylation notably affects the hemostatic system and leads to deficiencies of both procoagulant and anticoagulant factors.

OBJECTIVE

To assess the hemostatic balance in patients with multiple coagulation disorders by using a thrombin generation assay.

METHOD

We performed conventional coagulation assays and a thrombin generation assay on samples from patients with congenital disorder of glycosylation. The thrombin generation assay was performed before and after activation of the protein C system by the addition of soluble thrombomodulin.

RESULTS

A total of 35 patients were included: 71% and 57% had low antithrombin and factor XI levels, respectively. Protein C and protein S levels were abnormally low in 29% and 26% of the patients, respectively, whereas only 11% displayed low factor IX levels. Under baseline conditions, the thrombin generation assay revealed a significantly higher endogenous thrombin potential and thrombin peak in patients, relative to controls. After spiking with thrombomodulin, we observed impaired involvement of the protein C system. Hence, 54% of patients displayed a hypercoagulant phenotype in vitro. All the patients with a history of stroke-like episodes or thrombosis displayed this hypercoagulant phenotype.

CONCLUSION

A thrombin generation assay revealed a hypercoagulant in vitro phenotype under baseline condition; this was accentuated by impaired involvement of the protein C system. This procoagulant phenotype may thus reflect the risk of severe vascular complications. Further research will have to determine whether the thrombin generation assay is predictive of vascular events.

Knocking out Ornithine Decarboxylase Antizyme 1 (OAZ1) Improves Recombinant Protein Expression in the HEK293 Cell Line

Creating efficient cell lines is a priority for the biopharmaceutical industry, which produces biologicals for various uses. A recent approach to achieving this goal is the use of non-coding RNAs, microRNA (miRNA) and small interfering RNA (siRNA), to identify key genes that can potentially improve production or growth. The ornithine decarboxylase antizyme 1 (OAZ1) gene, a negative regulator of polyamine biosynthesis, was identified in a genome-wide siRNA screen as a potential engineering target, because its knock down by siRNA increased recombinant protein expression from human embryonic kidney 293 (HEK293) cells by two-fold. To investigate this further, the OAZ1 gene in HEK293 cells was knocked out using CRISPR genome editing. The OAZ1 knockout cell lines displayed up to four-fold higher expression of both stably and transiently expressed proteins, with comparable growth and metabolic activity to the parental cell line; and an approximately three-fold increase in intracellular polyamine content. The results indicate that genetic inactivation of OAZ1 in HEK293 cells is an effective strategy to improve recombinant protein expression in HEK293 cells.

Bypass of Activation Loop Phosphorylation by Aspartate 836 in Activation of the Endoribonuclease Activity of Ire1

The bifunctional protein kinase-endoribonuclease Ire1 initiates splicing of the mRNA for the transcription factor Hac1 when unfolded proteins accumulate in the endoplasmic reticulum. Activation of Saccharomyces cerevisiae Ire1 coincides with autophosphorylation of its activation loop at S840, S841, T844, and S850. Mass spectrometric analysis of Ire1 expressed in Escherichia coli identified S837 as another potential phosphorylation site in vivo. Mutation of all five potential phosphorylation sites in the activation loop decreased, but did not completely abolish, splicing of HAC1 mRNA, induction of KAR2 and PDI1 mRNAs, and expression of a β-galactosidase reporter activated by Hac1ⁱ. Phosphorylation site mutants survive low levels of endoplasmic reticulum stress better than IRE1 deletions strains. In vivo clustering and inactivation of Ire1 are not affected by phosphorylation site mutants. Mutation of D836 to alanine in the activation loop of phosphorylation site mutants nearly completely abolished HAC1 splicing, induction of KAR2, PDI1, and β-galactosidase reporters, and survival of ER stress, but it had no effect on clustering of Ire1. By itself, the D836A mutation does not confer a phenotype. These data argue that D836 can partially substitute for activation loop phosphorylation in activation of the endoribonuclease domain of Ire1.

Improved antibody production in Chinese hamster ovary cells by ATF4 overexpression

To improve antibody production in Chinese hamster ovary (CHO) cells, the humanized antibody-producing CHO DP-12-SF cell line was transfected with the gene encoding activating transcription factor 4 (ATF4), a central factor in the unfolded protein response. Overexpression of ATF4 significantly enhanced the production of antibody in the CHO DP-12-SF cell line. The specific IgG production rate of in the ATF4-overexpressing CHO-ATF4-16 cells was approximately 2.4 times that of the parental host cell line. Clone CHO-ATF4-16 did not show any change in growth rate compared with the parental cells or mock-transfected CHO-DP12-SF cells. The expression levels of mRNAs encoding both the antibody heavy and light chains in the CHO-ATF4-16 clone were analyzed. This analysis showed that ATF4 overexpression improved the total production and specific production rate of antibody without affecting the mRNA transcription level. These results indicate that ATF4 overexpression is a promising method for improving recombinant IgG production in CHO cells.

The role of endoplasmic reticulum stress in autoimmune-mediated beta-cell destruction in type 1 diabetes

Unlike type 2 diabetes which is caused by the loss of insulin sensitivity, type 1 diabetes (T1D) is manifested by the absolute deficiency of insulin secretion due to the loss of β mass by autoimmune response against β-cell self-antigens. Although significant advancement has been made in understanding the pathoetiology for type 1 diabetes, the exact mechanisms underlying autoimmune-mediated β-cell destruction, however, are yet to be fully addressed. Accumulated evidence demonstrates that endoplasmic reticulum (ER) stress plays an essential role in autoimmune-mediated β-cell destruction. There is also evidence supporting that ER stress regulates the functionality of immune cells relevant to autoimmune progression during T1D development. In this paper, we intend to address the role of ER stress in autoimmune-mediated β-cell destruction during the course of type 1 diabetes. The potential implication of ER stress in modulating autoimmune response will be also discussed. We will further dissect the possible pathways implicated in the induction of ER stress and summarize the potential mechanisms underlying ER stress for mediation of β-cell destruction. A better understanding of the role for ER stress in T1D pathoetiology would have great potential aimed at developing effective therapeutic approaches for the prevention/intervention of this devastating disorder.

The effects of G418 on the growth and metabolism of recombinant mammalian cell lines

It is widely reported that the growth of recombinant bacteria and yeast is adversely affected by increased metabolic load caused by the maintenance of plasmid copy number and recombinant protein expression. Reports suggest that recombinant mammalian systems are similarly affected by increased metabolic load. However, in comparison to bacterial systems relatively little information exists. It was the aim of this study to test the effects of recombinant gene expression on the growth and metabolism of two industrially important cell lines. A BHK and CHO cell line were stably transfected with the human gastric inhibitory peptide (h-GIP)and glucagon receptor respectively. Selection was by way of the neomycin resistance (neo (r)) gene using G418.The growth and metabolism of both cell lines was affected by the presence of G418 in a manner indicative of increased metabolic load and which appeared to be caused by over-expression of the neomycin resistance protein. The two cell lines differed in their metabolic response to G418, which suggested that some cell lines or clones may be better able to tolerate a metabolic load than others. Growth under increased metabolic load was affected by medium composition with serum, insulin and glutamine concentration as influencing factors. Implications for the use of G418 are discussed.

Characterisation of G418-induced metabolic load in recombinant CHO and BHK cells: effect on the activity and expression of central metabolic enzymes

In a previous article (Yallop and Svendsen 2001), recombinant CHO and BHK cell lines, expressing the human glucagon receptor and the gastric inhibitory peptide receptor, respectively, showed reduced growth rates and altered nutrient utilisation when grown with increasing concentrations of G418. This response was associated with an increased expression of the neo (r) protein, while expression of the recombinant membrane receptors remained unaltered. The metabolic response was characterised in both cell lines by an increase in the specific rate of glutamine utilisation and in CHO cells by a decrease in the yield of lactate from glucose, suggesting a change in the flux of glucose through central metabolism. The aim of this study was to further elucidate these metabolic changes by determining the activity and relative expression of key enzymes involved in glucose and glutamine metabolism. For both CHO and BHK cells, there was an increase in the activity of glutaminase, glutamate dehydrogenase and glutamine synthetase, suggesting an increased flux through the glutaminolysis pathway. The activity of glucose-6-phosphate dehydrogenase and pyruvate carboxylase in CHO cells was also increased whilst lactate dehydrogenase activity remained unaltered, suggesting an increased flux to the pentose phosphate pathway and TCA cycle, respectively. The activity of these enzymes in BHK cells was unchanged. Quantitative RT-PCR showed that expression levels of glutaminase and pyruvate carboxylase were the same with and without G418, indicating that the differences in activities were likely due to post-translational modifications.

Intraclonal protein expression heterogeneity in recombinant CHO cells

Therapeutic glycoproteins have played a major role in the commercial success of biotechnology in the post-genomic era. But isolating recombinant mammalian cell lines for large-scale production remains costly and time-consuming, due to substantial variation and unpredictable stability of expression amongst transfected cells, requiring extensive clone screening to identify suitable high producers. Streamlining this process is of considerable interest to industry yet the underlying phenomena are still not well understood. Here we examine an antibody-expressing Chinese hamster ovary (CHO) clone at single-cell resolution using flow cytometry and vectors, which couple light and heavy chain transcription to fluorescent markers. Expression variation has traditionally been attributed to genetic heterogeneity arising from random genomic integration of vector DNA. It follows that single cell cloning should yield a homogeneous cell population. We show, in fact, that expression in a clone can be surprisingly heterogeneous (standard deviation 50 to 70% of the mean), approaching the level of variation in mixed transfectant pools, and each antibody chain varies in tandem. Phenotypic variation is fully developed within just 18 days of cloning, yet is not entirely explained by measurement noise, cell size, or the cell cycle. By monitoring the dynamic response of subpopulations and subclones, we show that cells also undergo slow stochastic fluctuations in expression (half-life 2 to 11 generations). Non-genetic diversity may therefore play a greater role in clonal variation than previously thought. This also has unexpected implications for expression stability. Stochastic gene expression noise and selection bias lead to perturbations from steady state at the time of cloning. The resulting transient response as clones reestablish their expression distribution is not ordinarily accounted for but can contribute to declines in median expression over timescales of up to 50 days. Noise minimization may therefore be a novel strategy to reduce apparent expression instability and simplify cell line selection.

Enhancement of recombinant erythropoietin production in CHO cells in an incubator without CO(2) addition

The effect of low levels of carbon dioxide (CO(2)) in the gas phase on the production of recombinant human erythropoietin (EPO)in CHO cells was explored. A T-flask culture in an incubator without CO(2) addition showed a slow cell growth initially followed by the cessation of growth, while other cultures incubated under 0.5-5% CO(2) concentrations grew normally at the same rate during the entire period of cultivation. Interestingly, the production of EPO in the culture incubated under no CO(2) supply was highest among the tested cultures. The cell specific secretion rate of EPO (q(EPO)) of the culture under no CO(2) supply was about 3 times higher than that of the culture under 5% CO(2) supply. Western blot analysis and in vivo bioassay of EPO showed no apparent changes in EPO quality between the two cases of different CO(2) environments (air vs. 5% CO(2)), suggesting robust glycosylation of EPO by CHO cells even under very reduced CO(2) environment. Various combinations of the two extreme cases, with 5% CO(2) supply (suitable for cell growth) and no CO(2) addition (better for EPO production), were made in order to maximize the volumetric productivity of EPO secretion (P(V)) in CHO cells. The P(V) of the cultures programmed with initial incubation under 5% CO(2) followed by no CO(2) supply was about 2 times superior to that of the culture incubated only under no CO(2) supply. The P(V) of the culture under no CO(2) supply was slightly lower than that of culture grown under 5% CO(2). However, the q(EPO) of the no CO(2) supply case was more than 5 times higher than that of the culture under 5% CO(2) supply. In conclusion, we have demonstrated that a simple programming of CO(2) supply to an incubator can enhance the production of EPO in CHO cells remarkably, without any apparent change of the EPO quality.

Engineering of chaperone systems and of the unfolded protein response

Production of recombinant proteins in mammalian cells is a successful technology that delivers protein pharmaceuticals for therapies and for diagnosis of human disorders. Cost effective production of protein biopharmaceuticals requires extensive optimization through cell and fermentation process engineering at the upstream and chemical engineering of purification processes at the downstream side of the production process. The majority of protein pharmaceuticals are secreted proteins. Accumulating evidence suggests that the folding and processing of these proteins in the endoplasmic reticulum (ER) is a general rate- and yield limiting step for their production. We will summarize our knowledge of protein folding in the ER and of signal transduction pathways activated by accumulation of unfolded proteins in the ER, collectively called the unfolded protein response (UPR). On the basis of this knowledge we will evaluate engineering approaches to increase cell specific productivities through engineering of the ER-resident protein folding machinery and of the UPR.

Choice of cellular protein expression system

Recombinant protein expression has become a standard laboratory tool, and a wide variety of systems and techniques are now in use. Because there are so many systems to choose from, the investigator has to be careful to use the combination that will give the best results for the protein being studied. This overview unit discusses expression and production choices, including post-translational modifications (e.g., glycosylation, acylation, sulfation, and removal of N-terminal methionine), in vivo and in vitro folding, and influence of downstream elements on expression.

Engineering eukaryotic protein factories

The biopharmaceuticals market is currently outperforming the pharmaceuticals market and is now valued at US$ 48 billion with an average annual growth of 19%. Behind this success is a 100-fold increase in productivities of eukaryotic expression systems. However, the productivity per cell has remained unchanged for more than 10 years. The engineering of the ER-resident protein folding machinery is discussed together with an overview of signal transduction pathways activated by heterologous protein overexpression to increase cell specific productivities.

Engineering mammalian cell factories for improved recombinant monoclonal antibody production: lessons from nature?

In this review we consider how cell specific recombinant monoclonal antibody (Mab) production by engineered mammalian cells can be improved. Whilst it is generally recognized that Mab production is limited post-transcriptionally at folding and assembly reactions, genetic engineering strategies based on overexpression of individual chaperones or foldases in mammalian cells have not reliably increased cell specific Mab production. Given that recent studies have established that chaperones and foldases themselves exist in a large multiprotein complex, which may coordinate the sequential processing of Mabs, we propose that global expansion of all components of the secretory pathway will likely be necessary to generically improve recombinant Mab production by mammalian cells. In this context, what can be learnt from nature? Important recent studies have delineated some of the main cellular pathways involved in the differentiation of B-cells into nature's own high level Mab producers, plasma cells. This is achieved by a dramatic re-programming of cellular function where the coordinated expansion of metabolic and secretory machinery precedes Ig production, then is maintained by induction of a key intracellular signaling pathway, the unfolded protein response (UPR). Here we review genetic engineering strategies to increase cell specific production rate and discuss whether manipulation of intracellular signaling systems such as the UPR will provide a novel means to engineer mammalian cells for high level recombinant Mab production.

eIF2alpha phosphorylation, stress perception, and the shutdown of global protein synthesis in cultured CHO cells

The perception of environmental stress in animal cells engineered to produce heterologous protein leads to the induction of stress signaling pathways and ultimately apoptosis and cell death. Protein synthesis is regulated in response to various environmental stresses by phosphorylation of the alpha subunit of the eukaryotic initiation factor 2 (eIF2). In this study we have utilized a model system of Chinese hamster ovary cells engineered to secrete recombinant TIMP-1 protein to investigate the relationship between the cellular rate of protein synthesis, eIF2alpha phosphorylation, cellular stress perception, and the rate of cell specific recombinant protein synthesis. The rate of total protein synthesis was maximal after 48 hours of culture, remaining relatively high until 96 hours of culture, after which a decline was observed. Towards the end of culture a marked increase in labeled secreted protein was observed. Total eIF2alpha expression levels were high during the exponential growth phase and decreased slightly towards the end of culture. On the other hand, the relative expression of phosphorylated eIF2alpha showed a bi-phasic response with a small increase in phosphorylated eIF2alpha observed at 48 hours of culture, and a significant increase at 120 hours post-inoculation. The large increase in phosphorylated eIF2alpha coincided with the observed increase in labeled secreted protein and the decline in total cellular protein synthesis. A marked increase in ubiquitination was also observed at 120 hours post-inoculation that coincided with reduced rates of cellular protein synthesis and mRNA translation attenuation. We suggest that eIF2alpha phosphorylation is an indicator of cellular stress perception, which could be exploited in recombinant protein manufacturing to commence feeding and engineering strategies.

ER stress and the unfolded protein response

Conformational diseases are caused by mutations altering the folding pathway or final conformation of a protein. Many conformational diseases are caused by mutations in secretory proteins and reach from metabolic diseases, e.g. diabetes, to developmental and neurological diseases, e.g. Alzheimer's disease. Expression of mutant proteins disrupts protein folding in the endoplasmic reticulum (ER), causes ER stress, and activates a signaling network called the unfolded protein response (UPR). The UPR increases the biosynthetic capacity of the secretory pathway through upregulation of ER chaperone and foldase expression. In addition, the UPR decreases the biosynthetic burden of the secretory pathway by downregulating expression of genes encoding secreted proteins. Here we review our current understanding of how an unfolded protein signal is generated, sensed, transmitted across the ER membrane, and how downstream events in this stress response are regulated. We propose a model in which the activity of UPR signaling pathways reflects the biosynthetic activity of the ER. We summarize data that shows that this information is integrated into control of cellular events, which were previously not considered to be under control of ER signaling pathways, e.g. execution of differentiation and starvation programs.

High-pressure refolding of bikunin: efficacy and thermodynamics

Bikunin is a glycosylated protein that aggregates extensively during mammalian cell culture, resulting in loss of activity, loss of native secondary structure, and the formation of nonnative disulfide bonds. We investigated the use of high hydrostatic pressure (1000-3000 bar) for the refolding of bikunin aggregates. The refolding yield obtained with pressure-modulated refolding at 2000 bar was 70 (+/-5%) by reverse-phase chromatography (RP-HPLC), significantly higher than the value of 55 (+/-6%) (RP-HPLC) obtained with traditional guanidine HCl "dilution-refolding." In addition, we determined the thermodynamics of pressure-modulated refolding. The change in volume for the transition of aggregate to monomer DeltaV(refolding) was calculated to be -28 (+/-5) mL/mole. Refolding was accompanied by a loss of hydrophobic exposure, resulting in a positive contribution to the DeltaV(refolding). These findings suggest that the disruption of electro-static interactions or the differences in size of solvent-free cavities between the aggregate and the monomer are the prevailing contributions to the negative DeltaV(refolding).

Serum- and protein-free media formulations for the Chinese hamster ovary cell line DUKXB11

The production of therapeutic proteins in mammalian cell lines is of outstanding importance. The maintenance of most mammalian cell lines in culture requires the addition of serum to the culture medium. The elimination of serum from mammalian cell culture is desirable since serum is expensive and a source of contaminants, e.g. viruses, mycoplasma or prions. Here we describe the composition of serum- and protein-free media for the Chinese hamster ovary (CHO) cell line DUKXB11. The serum-free formulation supports excellent growth of CHO DUKXB11 cells at low (23cells/cm2) and high (2 x 10(4) cells/cm2) seeding densities characterized by a generation time of 10-12h, and, after addition of 0.2% pluronic F-68, the growth of a recombinant suspension cell line derived from DUKXB11. In addition, this formulation also allowed us to adapt recombinant cell lines expressing various amounts of human antithrombin ATIII (ATIII) to serum-free conditions. Secretion of ATIII was readily observed in the serum-free medium. Minor changes to the serum-free formulation resulted in a protein free formulation that supported growth of CHO DUKXB11 cells, growth of recombinant CHO cells expressing ATIII, and production of ATIII.

IRE1- and HAC1-independent transcriptional regulation in the unfolded protein response of yeast

The unfolded protein response (UPR) is a signalling pathway leading to transcriptional activation of genes that protect cells from accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER). In yeast, the only known ER stress signalling pathway originates at the type I transmembrane protein kinase/endoribonuclease Ire1p. Ire1p regulates synthesis of the basic leucine-zipper (bZIP)-containing transcription factor Hac1p by controlling splicing of HAC1 mRNA. Only spliced HAC1 mRNA (HAC1i) is translated, and Hac1ip activates transcription of genes that contain a conserved UPR element (UPRE) in their promoters. Here, we demonstrate that in addition to this well-understood ER stress signalling pathway, a second, IRE1, HAC1 and UPRE-independent mechanism for transcriptional activation upon ER stress, exists in yeast. A genetic screen identified recessive SIN4 alleles as suppressors of a defective UPR in ire1 Delta strains. Elevation of basal transcription in sin4 strains or by tethering the RNA polymerase II holoenzyme with LexAp-holoenzyme component fusion proteins to a promoter allowed for activation of the promoter by ER stress in an IRE1, HAC1 and UPRE-independent manner. We propose that this novel second ER-to-nucleus signal transduction pathway culminates in core promoter activation (CPA) through stimulation of RNA polymerase II holoenzyme activity. Core promoter activation was observed upon diverse cellular stresses, suggesting it represents a primordial stress-induced gene activation mechanism.

Endoplasmic reticulum signaling as a determinant of recombinant protein expression

Generation of functional recombinant proteins requires efficient and undisturbed functioning of the ER-Golgi secretory pathway in host cells. In large-scale production, where target proteins are highly overexpressed, this pathway can be easily congested with unfolded or misfolded proteins. Accumulating evidence suggests that, in addition to responsibility for protein processing, ER is also an important signaling compartment and a sensor of cellular stress. Two ER responses have been described to arise from the overaccumulation of proteins: unfolded protein response (UPR) and ER overload response (EOR). UPR and EOR employ various mechanisms at the transcriptional and the translational levels to deal efficiently and appropriately with encountered stress. This review will outline the molecular bases of ER functioning and stress response, highlight the relevance of ER signaling to the large-scale cell culture productivity and discuss possible approaches to the improvement of the secretion capacities of recombinant cells.

Induction of protein aggregation in an early secretory compartment by elevation of expression level

A variety of valuable therapeutic proteins are expressed in mammalian cells. Currently, rate-limiting for secretion of recombinant glycoproteins are activities in the secretory pathway of eukaryotic cells, i.e., folding and glycosylation of the naked polypeptide chain. In this paper we provide evidence that elevation of expression level alone is sufficient to cause intracellular aggregation of a structurally relatively simple glycoprotein, antithrombin III (ATIII). Elevation of expression level by selection for increased drug resistance in Chinese hamster ovary cells stably expressing ATIII resulted in formation of disulfide-bonded aggregates of ATIII. Aggregated ATIII displayed incomplete sialylation and Endo H-sensitivity and located to the endoplasmic reticulum and the cis-Golgi compartment in subcellular fractionations. To explore possible causes for aggregation of ATIII at elevated expression levels we investigated the influence of the two major energy sources of cultured mammalian cells, D-glucose and L-glutamine, on the ATIII-yield. We found that utilization of D-glucose was not limiting for synthesis of ATIII at elevated expression levels. However, the amount of ATIII-synthesized per L-glutamine consumed did not seem to increase steadily with expression level for ATIII, indicating that secretion of ATIII may be limited by the capacity of the cell to utilize L-glutamine.

Mutation of any site of N-linked glycosylation accelerates the in vivo clearance of recombinant rabbit antithrombin

Antithrombin (AT) is a plasma protein with four sites of N-linked glycosylation. Asn 135 is incompletely glycosylated, and the resulting 3-glycan AT is cleared more rapidly in vivo than the 4-glycan form. The Asn codons in each of the four sites of glycosylation were altered in turn, to create four mutant rabbit AT cDNAs. Permanently transfected CHO cell lines were generated following transfection of the resulting constructs, encoding either the wild-type rabbit AT (AT-WT) or one of the four underglycosylated variants (AT-N96Q, AT-N135Q, AT-N155Q, and AT-N155Q). Comparison of the five resulting recombinant AT proteins revealed that the major AT species of each variant co-migrated on SDS gels, and migrated more rapidly than the major form of AT-WT. The shift in mobility, from 60 to 57 kDa, was consistent with the loss of one fully sialylated complex N-linked glycan. Neither the amount of AT secreted (range: 1.25 to 4.2 microg/10(6) cells/day) nor the kinetics of secretion differed significantly between cell lines expressing AT-WT or any of the AT variants. All forms of recombinant rabbit AT were capable of forming denaturation-resistant complexes with thrombin. Purification and radioiodination of each of the five recombinant AT proteins permitted pharmacokinetic analysis of their individual clearance in rabbits. While neither the equilibration half-life (t(0.5)alpha) nor the terminal catabolic half-life (t(0. 5)beta) differed significantly between plasma-derived rabbit AT and AT-WT, the t(0.5)beta of all the underglycosylated variants was decreased relative to that of AT-WT (maximum reduction in mean: from 70.1+/-3.2 h to 52.4+/-2.5 h). These results suggest that the overall extent of glycosylation, rather than the location within AT of the glycan chains, is a primary determinant of AT clearance.

Limitations to the amplification and stability of human tissue-type plasminogen activator expression by Chinese hamster ovary cells

Chinese hamster ovary cell production of recombinant tissue-type plasminogen activator (t-PA) was increased by amplification of cotransfected dihydrofolate reductase cDNA using stepwise adaptation to increasing methotrexate (MTX) concentrations. The highest producing clones were isolated at 5 microM MTX and yielded 26,000 U/10(6) cells/day t-PA (43 microgram/10(6) cells/day). Above 25 microM MTX, cell specific t-PA production rates became increasingly variable and the cDNA copynumbers decreased. No apparent correlation between the cell specific t-PA production rate and the growth rate was observed upon subcloning of the amplified cells. When MTX selection was removed, the t-PA production rate decreased up to tenfold within 40 days; this was accompanied by an up to 60% drop in cDNA copynumber. Subclones isolated after 108 days of culture in the absence of MTX were, on average, sixfold more stable than their parental cells. In culture without MTX, the maximum stable t-PA production rate obtained (over 250 days) was 7000 +/- 750 U/10(6) cells/day (approximately 12 microgram/10(6) cells/day), approximately threefold lower than the maximum unstable levels of production reached under selective pressure. Taken together, these results define a wide range of the highest t-PA expression rates obtained under MTX selection, for which stable expression without selection has not been reported.

Constraints on the transport and glycosylation of recombinant IFN-gamma in Chinese hamster ovary and insect cells

In this study we compare intracellular transport and processing of a recombinant glycoprotein in mammalian and insect cells. Detailed analysis of the N-glycosylation of recombinant human IFN-gamma by matrix-assisted laser-desorption mass spectrometry showed that the protein secreted by Chinese hamster ovary and baculovirus-infected insect Sf9 cells was associated with complex sialylated or truncated tri-mannosyl core glycans, respectively. However, the intracellular proteins were predominantly associated with high-mannose type oligosaccharides (Man-6 to Man-9) in both cases, indicating that endoplasmic reticulum to cis-Golgi transport is a predominant rate-limiting step in both expression systems. In CHO cells, although there was a minor intracellular subpopulation of sialylated IFN-gamma glycoforms identical to the secreted product (therefore associated with late-Golgi compartments or secretory vesicles), no other intermediates were evident. Therefore, anterograde transport processes in the Golgi stack do not limit secretion. In Sf9 insect cells, there was no direct evidence of post-ER glycan-processing events other than core fucosylation and de-mannosylation, both of which were glycosylation site-specific. To investigate the influence of nucleotide-sugar availability on cell-specific glycosylation, the cellular content of nucleotide-sugar substrates in both mammalian and insect cells was quantitatively determined by anion-exchange HPLC. In both host cell types, UDP-hexose and UDP-N-acetylhexosamine were in greater abundance relative to other substrates. However, unlike CHO cells, sialyltransferase activity and CMP-NeuAc substrate were not present in uninfected or baculovirus-infected Sf9 cells. Similar data were obtained for other insect cell hosts, Sf21 and Ea4. We conclude that although the limitations on intracellular transport and secretion of recombinant proteins in mammalian and insect cells are similar, N-glycan processing in Sf insect cells is limited, and that genetic modification of N-glycan processing in these insect cell lines will be constrained by substrate availability to terminal galactosylation.

Relationship between recombinant activated protein C secretion rates and mRNA levels in baby hamster kidney cells

Analysis of 12 baby hamster kidney (BHK) clones in exponential growth revealed a linear relationship between cell-specific recombinant activated protein C (APC) production rates and APC mRNA levels. This correlation indicated that mRNA levels limited APC productivity. Two strategies were employed to increase APC mRNA levels and APC productivity. First, sodium butyrate was added to increase mRNA levels by two- to sixfold in five APC-producing clones to obtain up to 2.7-fold increase in APC production rate. The second strategy was to retransfect an APC-producing BHK cell line with a vector containing additional APC cDNA and a mutant DHFR. This mutant DHFR gene allowed the selection of retransfected clones in higher MTX concentrations. Over two-fold higher mRNA levels were obtained in these retransfected clones and the cell-specific APC production rate increased twofold. At the highest level of APC secretion, increases in mRNA levels did not result in higher rates of APC production. Analysis of the intracellular APC content revealed a possible saturation in the secretory pathway at high mRNA levels. The relation between mRNA level and APC secretion rate was also investigated in batch culture. The levels of total cellular RNA, APC mRNA, and beta-actin mRNA were relatively stable while cells were in the exponential growth phase, but rapidly decreased when cells reached the stationary phase. The decline of cell-specific APC mRNA levels correlated with a decline in APC secretion rates, which indicated that the mRNA levels continued to limit the rates beyond the exponential phase and into the declining growth and stationary phases of batch APC production.

Changes during subclone development and ageing of human antibody-producing recombinant CHO cells

Some of the problems encountered with human or human-mouse heterohybridomas, such as low growth rates and high serum requirements, have led to the increased use of recombinant cell lines for production of human antibodies. To evaluate the suitability of such alternative cell lines for the production of human antibodies we have analysed several subclones with differing specific production rates of a recombinant CHO cell line. Gene copy number and site of chromosomal integration for the light and heavy chain and the dhfr gene were determined by in-situ hybridisation. Specific mRNA content was analysed by Northern blot. In addition the intracellular content in light and heavy chain was measured by flow cytometry and the specific secretion rates were determined. The stability of gene expression was followed in the highest producing subclone for over a year. As previously seen in heterohybridoma cells a high expression rate of light chain is beneficial in speeding up secretion rates of whole antibody. When grown in the presence of G418 and methotrexate the amplified gene copies in the genome of recombinant CHO cells were stable over more than 100 passages. However, the expression of light chain, and with it the secretion rate, decreased with time. The low intracellular concentration of light chain resulted in accumulation of heavy chain in the endoplasmic reticulum due to retention by chaperones. The specific secretion rate decreased by 50% after 100 passages. When no G418 or methotrexate were present 75% of the gene copies were lost after 100 passages.

Quantitative analysis of transcription and translation in gene amplified Chinese hamster ovary cells on the basis of a kinetic model

The elevation of expression levels for secreted glycoproteins by gene amplification in mammalian cells shows a saturation behavior at high levels of gene amplification. At high expression levels a drop in the secretion efficiency for the recombinant protein occurs (Schröder and Friedl, 1997), coinciding with the appearance of misfolded protein in the cell. In this communication we investigated whether additional limitations exist at the levels of transcription and translation. Four Chinese hamster ovary (CHO) cell lines expressing different amounts of human antithrombin III (ATIII) were used as a model system. A tenfold increase in the ATIII cDNA copy number from the lowest to the highest producing cell line coincided with a 38-fold increase in ATIII mRNA levels, and an 80-fold increase in the amount of intracellular ATIII levels. The data was analyzed using a simple kinetic model. The following conclusions were derived: I. The transcriptional activity for the recombinant protein is not saturated. II. Translation itself is not saturated either, but may be downregulated as secretion efficiency drops. III. Two explanations for the previously reported drop in secretion efficiency for the recombinant protein with increasing expression level are possible: A. Protein degradation is an alternative fate for translated ATIII and the fraction of ATIII degraded after translation increases as expression level is increased. B. Translation is downregulated as the secretory apparatus becomes exhausted to maintain cell viability.

Determination of recombinant protein in animal cell culture supernatant by near-infrared spectroscopy

A method is described for the measurement of recombinant protein in animal cell culture supernatants with near-infrared (NIR) absorbance spectroscopy. Human antithrombin III is determined in CHO (Chinese hamster ovary)-T1 medium, both fresh and taken from CHO cell culture. NIR spectra are collected over the frequency range of 1000-2500 nm. A multivariate calibration model is generated by correlating spectral features with concentration values using partial least-squares regression. Separate test sets are used to evaluate the computed model. Protein measurement is established over the range 0.1-5 micrograms/mL with a standard error of prediction of less than 0.5 microgram/mL. This result underlines the analytical applicability of NIR spectroscopy for animal cell culture process monitoring.