Tumor-specific overexpression of a novel keratinocyte lipid-binding protein. Identification and characterization of a cloned sequence activated during multistage carcinogenesis in mouse skin

Differential screening of cDNA libraries from chemically induced malignant mouse skin squamous cell carcinomas (SCCs) identified sequences, including one called mal1, that were up-regulated in their expression at both the benign papilloma and the malignant SCC stages during tumor development. The mal1 plasmid cDNA clone was used to screen lambda phage cDNA libraries made from chemically induced papillomas and SCCs. Two size classes (655 and 933 nucleotides excluding the poly(A) tail) of full-length cDNAs were isolated. The corresponding mRNAs differ in their 3'-untranslated region by 278 nucleotides as a result of utilizing two alternative polyadenylation signals. Both transcripts were expressed simultaneously, showing the same expression patterns, with the smaller one being the predominant species. Most tissues examined showed a weak expression of mal1 mRNA. High levels of mal1 transcripts could be detected in adipose and mammary tissues and tongue epithelia and predominantly in epidermis. The expression observed in epidermis was up-regulated dramatically during tumor formation. Computer-assisted sequence analysis revealed one open reading frame that encoded a protein of 135 amino acid residues with extensive homology to members of the lipid-binding protein family. Residues determining the proposed beta-clam structure of these proteins and the structure of the lipid-binding region were shown to be conserved in the mal1 gene. In vitro translation of mal1 RNA yielded a polypeptide of the predicted size of 15 kDa that was immunoprecipitable with an anti-rat liver fatty acid-binding protein antiserum. Based on the sequence analysis and antigenic properties of mal1, we conclude that it encodes a novel member of the lipid-binding protein family.