Cytokeratin expression in simple epithelia: III. Detection of mRNAs encoding human cytokeratins nos. 8 and 18 in normal and tumor cells by hybridization with cDNA sequences in vitro and in situ

Abstract. We describe cDNA clones of mRNAs encoding human cytokeratins nos. 8 and 18, and the amino acid sequences deduced from their nucleotide sequences. Human cytokeratin no. 8 is a typical cytokeratin of the basic (type II) subfamily, which is highly homologous to the corresponding bovine and amphibian (Xenopus laevis) proteins; however, unlike the amphibian protein, it does not contain glycine‐rich oligopeptide repeats in its carboxyterminal‘tail’ domain. Comparison with the reported amino acid sequences of two fragments of human‘tissue polypeptide antigen’ (TPA), a widely used serodiagnostic carcinoma marker, revealed sequence identity, indicating that this serum component is derived from the intracellular cytokeratin no. 8 present in diverse kinds of epithelia and epitheliumderived tumors. Human cytokeratin no. 18 is very similar to the corresponding murine protein but contains two additional blocks of 4 and 5 amino acids in the ‘head’ portion. These cDNA clones and the RNA probes derived therefrom were used to detect specifically mRNAs by Northern‐blot assays of RNAs from various carcinomas and cultured carcinoma cells. Using in situ hybridization on frozen sections of tumor‐containing tissues, notably lymph nodes containing metastatic breast carcinoma, we were able to demonstrate the specificity and sensitivity of this procedure. The potential value for cell‐biological research and pathology of being able to detect a mRNA encoding a given cytokeratin polypeptide in situ is discussed.