The production of foreign proteins in mammalian cells

Expression systems for producing foreign proteins in mammalian cells are built from two components. One component is the DNA expression vector and the other is the mammalian host-cell line. Functional elements of DNA are better understood and generally easier to manipulate than complex mammalian cells. The standard approach, therefore, has been to manipulate the expression vectors to work well in convenient host cell lines. A wide variety of DNA regulatory signals for efficient transcription and translation have been tested in mammalian-cell-expression vectors. Many of the most successful and widely used regulatory signals are derived from eukaryotic viral DNAs. In addition to optimizing the vectors to give efficient transcription and translation of the foreign protein, higher expression levels can be achieved by increasing the number of foreign gene copies per cell. High gene copy number is usually attained by including an amplifiable gene, such as dhfr, in the expression vector, introducing the vector DNA into the host-cell lines, and then using a toxic agent to select for resistant cell lines containing high copy numbers of the amplifiable gene. Cells with amplified copy numbers of the selected gene generally also contain high copy numbers of the foreign gene and thus produce elevated levels of the foreign protein product. Although gene amplification has been most successful in creating cell lines producing high levels of foreign proteins, there are inherent instability problems with cell lines forced to carry extremely high copy numbers of foreign genes. One means of avoiding instability problems due to continuous high gene copy and continuous high foreign protein production, is to develop regulatable expression systems. The regulatable expression systems being developed are based either on regulating the gene copy number by regulating DNA replication or on regulating the level of transcription by using a regulatable promoter to transcribe the foreign protein coding cDNA. In addition to designing a good expression vector, it is important to consider the mammalian host cell. Although most potential mammalian host-cell lines are capable of post-translational processing and secretion, certain processing steps, such as gamma-carboxylation, may be done efficiently only in specialized cell types. It is also important to estimate how much of the foreign protein will be needed and to decide whether the proposed host-cell line can be easily and economically grown to produce that amount. Regulatory considerations are also important in choosing a host-cell line for commercial production. Many of the potential host-cell lines are tumorigenic and carry retroviruses.(ABSTRACT TRUNCATED AT 400 WORDS)