Constitutive expression of class 3 aldehyde dehydrogenase in cultured rat corneal epithelium

Mammalian Class 3 aldehyde dehydrogenase (ALDH) is normally associated with neoplastic transformation or xenobiotic induction by aromatic hydrocarbons in liver. However, Class 3 ALDH is constitutively expressed at it's highest specific activity in corneal epithelium. Tissue-specific, differential gene expression is often controlled by alternative, independent molecular pathways. We report here the development of an in vitro corneal epithelium culture system that retains constitutive high expression of the ALDH3 gene. This model system was used to establish, by enzymatic assays, Western and Northern analyses, histochemical and immunocytochemical staining, and 5'3' RACE methodologies that constitutive and xenobiotic induction of Class 3 ALDHs occurs from a single gene. Our results also provide a plausible explanation for the very high Class 3 ALDH activity in mammalian cornea, as the primary mechanism of oxidation of lipid peroxidation-derived aldehydes. Further studies with corneal epithelium suggest the presence of additional mechanisms, other than Ah-receptor-mediated, by which the ALDH3 gene can be differentially regulated in a tissue-specific manner.