Lens epithelial cell mRNA, II. Expression of a mRNA encoding a lipid-binding protein in rat lens epithelial cells

A lens epithelial (LE) cell cDNA clone, designated pLELBP, was isolated by subtraction-hybridization methods between a 4-week-old rat LE cell and rat lens fiber cell lambda ZAP cDNA libraries. The cDNA contained 683 bp, an ATG at bp 42, and an open reading frame (ORF) encoding a protein of 135 amino acids (aa), and a poly(A) signal at bp 641 (GenBank/EMBL accession No. U13253). Northern blot analysis showed that the lens mRNA was at a concentration that exceeded 100-fold that found in non-ocular tissues examined, except in skin it was found at levels equal to about 1/15 of the lens levels. In the retina, it was also found at about 1/15 of that present in the lens. By in situ hybridization analysis, the sense RNA was localized to the LE cells and to the glial cells of the retina, and negligibly to the lens fiber cells. Search of GenBank, EMBL and SwissProt data bases revealed that the LE cell mRNA and its aa encoded protein showed extensive sequence homology to members of the cytosolic lipid-binding protein (LBP) family. It matched to 91% in aa sequence homology to the protein encoded by ORF of mal1 cDNA, isolated from mouse skin squamous cell carcinoma [Krieg et al., J. Biol. Chem. 268 (1993) 17362-17369], and 99% in aa sequence homology to the protein encoded by ORF of rat skin LBP mRNA [Watanabe et al., Biochem. Biophys. Res. Commun. 200 (1994) 253-259]. Occurrence of high levels of mRNA for a specific LBP in the LE cells relative to other non-ocular tissues is consistent with a hypothesis that the encoded protein may be involved in several lens epithelial cell-specific mechanisms, possibly including differentiation, protective and nutritional processes.