Identification of a cloned sequence activated during multi-stage carcinogenesis in mouse skin

Differential screening of cDNA libraries made from chemically induced malignant mouse skin squamous cell carcinomas (SCC) identified three sequences, including one called mal2, that were upregulated in their expression at both the benign papilloma and malignant SCC stages. The mal2 plasmid cDNA clone (containing a 350 bp insert) was used to screen lambda phage cDNA libraries made from chemically induced SCCs. Two of the largest mal2-related cDNA inserts obtained from the phage libraries were sequenced. In addition a mal2-related genomic clone was obtained by hybridization probing of a mouse spleen genomic DNA library. The sequence of the genomic clone overlapped and was identical with both the mal2 plasmid and lambda cDNA clones. Identity was found between the mal2 cDNAs, the mal2 genomic sequence and the cDNA sequence for a mouse hyperproliferative keratin called K6. A synthetic oligonucleotide specific for the 3' untranslated region of the mal2 or keratin K6 gene was used in Northern analyses to demonstrate elevated steady-state levels of K6 keratin transcripts in SCCs induced by various protocols involving both chemical and ionizing radiation initiation of tumors as well as complete chemical and radiation carcinogenesis protocols. Metastatic lung lesions derived from SCCs generated by repeated doses of benzo[a]pyrene showed moderate levels of K6 keratin transcripts, whereas normal lung showed very low levels of K6 transcripts. The overexpression of the mal2 or keratin K6 gene in malignant SCCs was independent of the protocol, either chemical or radiation, that was used to induce the tumors.