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High-level expression of human tissue-type plasminogen activator was accomplished in endothelial cells by a novel approach to dihydrofolate reductase (DHFR) coamplification in DHFR+ cells. A tripartite mammalian expression vector coding for DHFR, neomycin phosphotransferase, and the t-PA gene was introduced into bovine endothelial cells by transfection and selection for G418 resistance. Upon methotrexate selection of these transformants, we obtained endothelial cells that had amplified the plasmid-encoded DHFR and t-PA genes. As a result, cell lines were isolated that efficiently produced t-PA (greater than 4 pg/cell.day). This t-PA was purified and compared with recombinant t-PA produced in Chinese hamster ovary cells. These two t-PA samples differed in carbohydrate composition, and amounts of 530 and 527 amino acid forms but had similar in vitro activity.

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