Up to 100-Fold Increase of Apparent Gene Expression in the Presence of Epstein-Barr Virus oriP Sequences and EBNA1: Implications of the Nuclear Import of Plasmids

A 100-fold increase in luciferase activity was observed in 293 cells, stably expressing Epstein-Barr nuclear antigen 1 (EBNA1; 293-EBNA1 cells), that had been transiently transfected with plasmids carrying Epstein-Barr virus (EBV) oriP sequences. This increase was observed in comparison to reporter gene activity obtained after transfection with a plasmid carrying no oriP sequences. The luciferase gene on these plasmids was under the control of either the cytomegalovirus immediate-early 1 gene enhancer-promoter (CMV IE1) or the Rous sarcoma virus promoter. The increase of reporter gene activity was not due to plasmid replication, since a similar enhancement was observed in the presence of aphidicolin, an inhibitor of replicative DNA synthesis, or after deletion of the dyad symmetry (DS) element within oriP. Luciferase production was not increased in the presence of only the DS element. Microinjection of plasmids carrying the CMV IE1 promoter-driven luciferase gene with or without oriP sequences into the nuclei of 293-EBNA1 cells resulted in a 17-fold increase in luciferase activity. Cytoplasmic injection of these plasmids led to an enhancement of luciferase activity of up to 100-fold. This difference in the factor of activation after nuclear or cytoplasmic injection could be ascribed to increased transport of plasmids carrying oriP from the cytoplasm to the nucleus in the presence of EBNA1. These data suggest the possibility of substantially increasing the apparent expression of a gene under the control of a strong constitutive promoter in the presence of oriP sequences and EBNA1. This improvement in expression is due to intranuclear enhancement of gene expression. oriP-specific transport of plasmid DNA from the cytoplasm of 293-EBNA1 cells to the nucleus seems to contribute to the observed effect.     

Characterization of morphogenetic and invasive abilities of human mammary epithelial cells: Correlation with variations of urokinase-type plasminogen activator activity and type-1 plasminogen activator inhibitor level

Urokinase-type plasminogen activator (uPA) and one of its inhibitors, the PAI-1, are involved in the proteolytic cascade of matrix degradation during in vivo morphogenesis or metastasis. In the present study, we have characterized the in vitro morphological behavior of human normal and malignant mammary epithelial cells and determined the levels of uPA activity and PAI-1 during these events. Two-dimensional cultures in the presence of inductive fibroblast-conditioned medium (CM) allowed migration of HBL-100 cells and MDA-MB-231 cells. Normal human mammary epithelial cells (HMEC) and MCF-7 cells failed to migrate under these conditions. The epithelial cell migration correlated with an increase in the uPA activity whereas their immobility correlated with both increases in uPA activity and PAI-1 level. In three-dimensional cultures in collagen gel, fibroblasts or fibroblast CM induced branching tubular morphogenesis to HMEC, cord-like extensions to HBL-100 cells and a greater invasiveness ability to MDA-MB-231 cells. These events correlated with an increased uPA activity. In contrast, no morphological rearrangement was observed in MCF-7 cells and this correlated with both increases in uPA activity and PAI-1 level. Altogether, these results show that the in vitro mammary epithelial behavior is under the influence of mesenchymal inductive signals and is in agreement with modifications of uPA activity and PAI-1 levels. Our culture system gives a suitable model to study the mechanisms of mammary development and metastasis and to highlight the involvement of proteases and their inhibitors in cell-cell positioning and cell-matrix reorganization.