[Construction of CHO-IVB, A serum-independent, apoptosis-resistant cell line that can grow in adherence].

Without serum to provide adherent factors, CHO-dhfr- cells grow in suspension when cultured in serum-free medium. Although this offers advantages in some applications, in most production systems adherent cell growth is preferable. Gene transfection, clonal selection and amplification can be easier for adherent cells; the density of immobilized cells is often higher than those in suspension culture, which results in a higher protein productivity; washout of cells by perfused medium during continuous fermentation can be avoided; for high-throughput microplate assays, adherent cells are preferred to facilitate medium changes and cell washing. It has been proved that purified vitronectin alone was able to mediate attachment and spreading of CHO cells in serum-free medium. So we constructed a tricistronic expression vector expressing Igf-1, Vitronectin and Bel-2 at the same time. The vector was transfected into CHO-dhfr- cells and one clone, namely CHO-IVB2, expressing high level of the three proteins was screened out by Western blot. The cell line showed similar apoptosis-resistant and serum-independent properties to CHO-IB, an engineered cell line constructed before. When cultured in IMEM protein-free medium without any components supplemented, CHO-IVB can grow adherently. The viable cell numbers and growth rate of CHO-IVB were much higher than CHO-IB, making CHO-IVB an apoptosis-resistant host for production of recombinant proteins which can grow adherently in protein-free medium.

[Construction of two robust CHO cell lines resistant to apoptosis and adapted to protein-free medium by over-expression of Igf-1/bcl-2 or bcl-2/cyclin E genes].

Serum used widely in mammalian cell culture is also a potential source of bacterial, mycoplasmal and viral contaminations. In addition, the complex biological components in serum make harder the subsequent product recovery process. High cost, high batch variation and potential source limitation are among the other shortcomings. So serum-free or even protein-free medium are preferable for recombinant protein production. However, without serum to provide essential components such as hormones, growth factors and binding proteins, cells are easy to die. In this study, CHO-dhfr- cells were genetically engineered to make them adapted to IMEM, a protein-free medium, and resistant to apoptosis. The genes in choice are insulin-like factor (Igf-1), Bcl-2 and cyclin E. Bcl-2 is a mitochondrial membrane-integrated protein. It can block the release of cytochrome c by maintaining the integrity of mitochondrial membrane, and thus inhibit apoptosis. Igf-1 is similar both in structure and function to insulin, a growth factor added to serum-free medium to promote cell growth and is the only protein component in many currently used serum-free media. cyclin E is a cell cycle protein expressed continuously in G1 phase. When cyclin E accumulates to certain amount, cell cycle was driven to S phase. So cyclin E is a proliferation-promoting protein. By co-express Igf-1/Bcl-2 or Bcl-2/ cyclin E in CHO-dhfr- cells with a dicistronic expression vector, we constructed two cell lines: CHO-IB and CHO-BC. The high expression of each protein was confirmed by Western blot and flow cytometry. Apoptosis was analyzed by flow cytometry and DNA ladder detection, and the two cell lines were both found much more resistant to apoptosis induced by withdrawal of serum or addition of actinomycin D than the CHO-dhfr- parent cell. Cell proliferation assay by MTT method showed that the two cell lines proliferated much faster than CHO-dhfr- in IMDM medium without serum. Continuously culture assay proved that the two cell lines grow very well in IMEM protein-free medium supplemented with fibronectin and vitronectin to ease adherence. When compared to CHO-dhfr-, the two cell lines exhibited much more viable cell numbers and faster growth rate.

[Construction of an anti-apoptosis CHO cell line for biopharmaceutical production].

Mammalian cells are prone to apoptosis when cultured in large scale for production of biopharmaceuticals. And this will reduce production duration and result in high cost of production. Apoptosis is triggered by various factors, and delicately regulated by a set of genes. Bcl-2, a component integrated in mitochondria membrane, is an important member of these genes. By maintaining the integrity of mitochondria membrane, Bcl-2 keeps cytochrome C from releasing into cytoplasm, and thus blocks the activation of caspases, and subsequent onset of apoptosis. Over-expression of Bcl-2 has proven to be useful in blocking apoptosis in various cell lines, including CHO, hybridoma, myeloma, lymphoma and insect cells. Ammonia, a metabolite of cultured cells, however, showed apparent pro-apoptosis activity. In living cells, ammonia can be utilized by glutamine synthetase (GS) to synthesize glutamine, and thus lower the concentration of ammonia in medium, and its negative effects. Glutamine is essential to living cells. If not added into medium, glutamine can only be synthesized by GS, which makes GS a qualified selection marker. This marker can be used for gene amplification by adding into medium increased concentration of MSX, an inhibitor of GS. In this study, we over-expressed Bcl-2 using GS amplification in a recombinant CHO cell line stably expressing human interferon-beta. The modified cell line, with higher expression of Bcl-2 and lower production of ammonia, exhibited good anti-apoptosis quality and higher interferon-beta production in continuous culture.