Characterization of upstream activation elements essential for the expression of germ cell alkaline phosphatase in human choriocarcinoma cells.

Expression of the germ cell alkaline phosphatase is a highly regulated process tied to malignant transformation of the human placenta. Human choriocarcinoma cells (malignant trophoblasts) express primarily the germ cell alkaline phosphatase gene and only low or nondetectable levels of the placental alkaline phosphatase normally found in the human placenta. Here, we show that nucleotides -156 to -1 region relative to the gene transcription start site (+1) contain cis-acting DNA elements that direct germ cell alkaline phosphatase expression in choriocarcinoma cells. Within the minimal activator region, at least three nuclear protein-binding sites, I (-63/-44), II (-87/-67), and III (-136/-103), were identified by DNase I footprinting analysis. All three sites are GC-rich. Sites I and II contain a sequence known to bind the transcription factor AP-2; the AP-2 site in site II overlaps a consensus motif for the transcription factor Sp1. Gel retardation experiments showed that similar nuclear protein factor(s) in JEG-3 choriocarcinoma cells bind to all three sites, with highest affinity to sites I and II. Site-directed mutagenesis that prevents binding of nuclear proteins to either site I or II, or both sites I and II, resulted in the loss of factor binding and reduced activator activity. The germ cell alkaline phosphatase promoter that contains an intact binding site III but altered sites I and II had little activator activity, suggesting that protein-protein interaction is important for germ cell alkaline phosphatase gene activation.