The state of cellular differentiation determines the activity of the adenovirus E1A enhancer element: evidence for negative regulation of enhancer function.

Most of the eucaryotic enhancer elements so far described consist of multiple DNA binding sites for proteins that act either synergistically or antagonistically to modulate the rate of transcription. In this report, we show that the activity of the adenovirus E1A enhancer element is suppressed in virus-infected undifferentiated rodent fetal fibroblast cells (CREF and F111 cells) and primary rat liver hepatocytes that have lost their fully differentiated phenotype (dedifferentiated). This contrasts with the results obtained for virus-infected differentiated or partially dedifferentiated rodent hepatocytes or hepatoma cell lines and human HeLa cells, in which deletion of the E1A enhancer domain greatly reduces the rate of E1A gene transcription. An in vitro quantitation of the nuclear proteins (from HeLa and CREF cells) that interact with and modulate the activity of the E1A enhancer revealed similar binding activities for the E2f and ATF proteins. However, an AP3-like (phi AP3) activity was present at a 10- to 20-fold higher concentration in CREF cells than in HeLa cells, and removal of this phi AP3-binding site on the viral genome resulted in an increase in the rate of E1A gene transcription in virus-infected CREF cells. Together, these results demonstrated that the factors which positively regulate enhancer function were present in CREF cells and that the phi AP3 factor was acting to suppress the activity of the E1A enhancer. Furthermore, the level of this factor was found to increase to even higher levels in CREF cells treated with 12-O-tetradecanoylphorbol-13-acetate, and this induction resulted in a further suppression in the rate of E1A gene transcription. On the basis of these observations, we propose that E1A expression is negatively regulated by the phi AP3 factor in undifferentiated rodent fetal fibroblast cells and that this could be an important mechanism that distinguishes between establishment of the differentiated cell versus transformed cell phenotypes.