Rat Tumor Necrosis Factor-α. Transcription in Rat Kupffer Cells and in vitro Posttranslational Processing Based on a PCR-Derived cDNA

A DNA fragment with a reading frame of 708 basepairs coding for TNF-alpha from rat liver was cloned by the polymerase chain reaction. Using this species-specific cDNA a biotin-labelled antisense (-)RNA was transcribed. This probe was used for Northern blot analysis of TNF-alpha gene activation. Exposure of rat Kupffer cells to LPS led to a time-dependent change of TNF-alpha-mRNA expression with a maximum between one and two hours after stimulation. In vitro translation was carried out with sense (+)RNA in the presence of microsomes. SDS gel electrophoresis of the immunoprecipitated proteins revealed the formation of polypeptides with estimated molar masses of 26 and 17 kDa. They correspond to the estimated molar masses of a precursor of TNF-alpha encoded by the entire reading frame and of the mature form of TNF-alpha, respectively. A sequence of the cDNA assumed to code for the leader peptide was deleted and the remainder used for the construction of an expression plasmid. Using this construct, biologically active mature TNF-alpha was expressed in E. coli (10(6) units TNF-alpha per liter of culture medium). The propeptide as well as the biologically active TNF-alpha possess a homology of 92 and 76% to mouse and human TNF-alpha, respectively.