We investigated the gene expression of the nuclear transcription factors c/EBPalpha, GATA-2, and the silencer Oct-1 in conjunction with the gene expression of all major cytochrome P450 genes and of eNOS in cultures of endothelial cells of the rat. The purity of cultured endothelial cells was also confirmed by flow cytometry measurements of PECAM-1, a surface antigen of endothelial cells. Taken collectively, the gene expression and flow cytometry studies provide strong evidence for c/EBPalpha, GATA-2, and Oct-1 to play a key role in the cellular dedifferentiation of endothelial cells; gene expression of eight individual CYP genes in conjunction with protein activity could be significantly increased upon treatment with Aroclor 1254, a well-documented chemical inducer of a battery of genes. Nevertheless, the gene expression of c/EBPalpha, GATA-2, and most of the CYP genes was dramatically reduced (up to 90%) in cell cultures lacking PECAM-1 expression; in strong contrast, expression of the silencer Oct-1 was massively increased (approximately 14-fold). We thus conclude activation of the silencer Oct-1 to be strongly correlated with loss of PECAM-1 and eNOS gene expression, e.g., loss of cellular differentiation and endothelial function; in conjunction, gene expression of all major P450 isoforms was dramatically reduced in cultures of dedifferentiated endothelial cells. This process of cellular dedifferentiation and endothelial dysfunction was accompanied by down-regulation of endothelial specific transcription factors.