Amplification and expression of recombinant genes in serum‐independent Chinese hamster ovary cells

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Abstract
CHO SSF3 cells grow as a suspension culture in unmodified commercial medium with only low–molecular weight ingredients. Continuous serum–free culture unexpectedly induced expression of a low dihydrofolate reductase activity in the originally dhfr– CHO cells. Nevertheless, it was possible with methotrexate to induce amplification of a gene coding for the hybrid plasminogen activator K2tu–PA cotransfected with a dhfr gene. Expression of K2tu–PA expression was proportionally increased to that of dhfr, which was measured with fluorescent methotrexate. Because no serum proteases were present, secreted K2tu–PA was not converted to the enzymatically active form, but was exclusively recovered in proenzyme form.