Activation of keratin 19 gene expression by a 3' enhancer containing an AP1 site

We previously reported that the human keratin 19 (K19) gene was expressed in nonkeratinizing oral epithelial subtypes, and that the steady state K19 mRNA levels in different normal epithelial subtypes correlated with the levels of the retinoic acid receptor (RAR) beta-mRNA (Hu, L., Crowe, D. L., Rheinwald, J. G., Chambon, P., and Gudas, L. J. (1991) Cancer Res. 51, 3972-3981). To elucidate the mechanisms by which the K19 gene is differentially expressed in various epithelial subtypes, we isolated phage containing human K19 genomic DNA from a human placental library. Through transient transfection assays with various K19/CAT constructs that contain different portions of K19 genomic DNA, an enhancer sequence has been identified in the K19 3'-flanking region. In normal human epithelial cell strains, the activity of this enhancer correlates with K19 mRNA abundance. This enhancer activates both the K19 and TK basal promoters in HeLa cells. A high level of K19/CAT fusion mRNA was detected when this K19 3' enhancer sequence was present at the 3' end of the fusion gene whereas no K19/CAT fusion transcript was detected if this 3' K19 enhancer sequence was absent, suggesting that the 3' K19 enhancer is crucial for the positive regulation of K19 expression. Deletion analysis has permitted the localization of the enhancer to a 19-base pair sequence which contains an AP1 binding site (AGTCATCT). Point mutations within this AP1 site completely abolished K19 enhancer activity in HeLa cells. Co-transfections of a c-jun expression vector with K19/CAT reporter constructs demonstrated that c-jun was able to activate the K19 promoter via the 3' K19 enhancer. Collectively, these data indicate that a cis-acting AP1 element and its associated trans-acting effector proteins regulate expression of lineage-specific genes in epithelial cells.