Influence of the human endogenous retrovirus-like element HERV-E.PTN on the expression of growth factor pleiotrophin: a critical role of a retroviralSp1-binding site

Germ line insertion of a human endogenous retrovirus-like element (HERV-E.P TN) into the growth factor pleiotrophin (PTN) gene generated a phylogenetic ally new promoter driving the expression of functional HERV-PTN fusion tran scripts. Here we show by in situ hybridization, that HERV-PTN fusion transc ripts are expressed in malignant trophoblasts (i.e. choriocarcinoma) and in the proliferative and in the invasive trophoblasts of gestational trophobl astic tissue. Additionally, a 1.9 kb fragment of the HERV-derived PTN promo ter was analysed which has strong activity when transiently transfected int o choriocarcinoma JEG-3 cells in contrast to HeLa cells. Deletion of the re trovirally-derived promoter portion abolished its activity and an enhancer (+443 to +486) was identified in this region. Electrophoretic mobility shif t and supershift experiments identified a Spl binding site in this enhancer and site specific mutation of this site abolished its activity in chorioca rcinoma cells. Sp1 overexpression in Drosophila SL2 cells showed that the e nhancer activity is mediated via Sp1 binding in vivo. Furthermore, mutation of the Sp1 binding site reduced the activity of a promoter test fragment i n choriocarcinoma cells by 80%. Our result shows that a retroviral Sp1 bind ing site in the PTN promoter is important for the expression of growth fact or pleiotrophin in human choriocarcinoma cells.