An enhancer with cell-type dependent activity is located between the myeloid and epithelial aminopeptidase N (CD 13) promoters.

The 5' flanking region of the gene encoding the small intestinal brush-border peptidase aminopeptidase N (APN) was screened for the presence of enhancer regions. A 300 bp region with enhancer activity was identified 2.7 kb upstream of the transcriptional start site which is used in epithelial cells. The enhancer stimulated transcription from a heterologous promoter (the simian virus 40 early promoter) in a position- and orientation-independent manner. The activity of the enhancer is cell-type dependent and it is active in liver (HepG2), intestinal (Caco-2) and myeloid (K562) cells. As the epithelial APN promoter is active in the first two cell-types and the myeloid APN promoter in the last, the results may suggest that the enhancer, through a cooperation with either of the promoters, is important for the tissue-specific expression of APN. A detailed analysis of the enhancer led to the identification of four functionally important regions that are protected against DNase I digestion by Caco-2 nuclear extract. Sequence analysis suggests that two of the regions may interact with members of the Ets transcription factor family (Ets is a transformation-specific protein first discovered in the E26 avian erythroblastosis virus), one region with a CCAAT enhancer-binding protein and one region with Sp1, a transcriptional activator first described as a factor binding to the simian virus 40 early promoter.