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 malig nant mouse skin squamous cell carcinomas (SCCs) identified sequences, including one called mall, that were up-regulated in their expression at both the benign papilloma and the malignant SCC stages during tumor development. The mall plasmid cDNA clone was used to screen lambda ph age cDNA libraries made from chemically induced papillomas and SCCs. T wo size classes (655 and 933 nucleotides excluding the poly(A) tail) o f full-length cDNAs were isolated. The corresponding mRNAs differ in t heir 3'-untranslated region by 278 nucleotides as a result of utilizin g two alternative polyadenylation signals. Both transcripts were expre ssed simultaneously, showing the same expression patterns, with the sm aller one being the predominant species. Most tissues examined showed a weak expression of mall mRNA. High levels of mall transcripts could be detected in adipose and mammary tissues and tongue epithelia and pr edominantly in epidermis. The expression observed in epidermis was up- regulated dramatically during tumor formation. Computer-assisted seque nce analysis revealed one open reading frame that encoded a protein of 135 amino acid residues with extensive homology to members of the lip id-binding protein family. Residues determining the proposed beta-clam structure of these proteins and the structure of the lipid-binding re gion were shown to be conserved in the mal1 gene. In vitro translation of mall RNA yielded a polypeptide of the predicted size of 15 kDa tha t was immunoprecipitable with an anti-rat liver fatty acid-binding pro tein antiserum. Based on the sequence analysis and antigenic propertie s of mall, we conclude that it encodes a novel member of the lipid-bin ding protein family.