TuAg.1 is the liver isoform of the rat colon tumor-associated antigen pE4 and a member of the immunoglobulin-like supergene family

TuAg.1 is a tumor-associated membrane glycoprotein first identified in rat hepatocellular carcinoma by monoclonal antibodies (mAbs) 324.5 and 324.9. This oncofetal antigen is also expressed by hepatocytes in cell culture but not normal adult hepatocytes in vivo. Affinity chromatography and preparative continuous elution slab-gel electrophoresis were used to separate TuAg.1 from co-purified actin and immunoglobulin. TuAg.1 was recovered as a series of bands Mr 82,000-90,000, which were pooled and subjected to CNBr digestion for primary amino acid sequence analysis. Computer database analysis of TuAg.1 peptide sequence revealed homology to the rat colon carcinoma-associated antigen pE4, a member of the immunoglobulin gene superfamily. Oligonucleotide primers derived from sequences shared by TuAg.1 and pE4 were used in reverse transcription-PCR to amplify tumor-specific products corresponding to TuAg.1 cDNA. Northern blot analysis with one of these products confirmed the oncofetal expression of transcripts related to TuAg.1/pE4 and indicated an RNA species of different size expressed only in normal liver. Identity between TuAg.1 and pE4 was further confirmed by immunochemical analysis with mAb 324.5 and mAb E4. Both antibodies were reactive with the same protein on transplantable hepatocellular carcinoma AS30D but recognized different epitopes. The reactivity of human tumor cells with mAb 324.5 and 324.9 indicates the presence of a related TuAg.1 molecule expressed in human neoplasia as well.