Control of p53 expression by adenovirus 12 early region 1A and early region 1B 54K proteins.

The level of p53 is markedly increased in human cells in response to expression of the Ad12 E1A proteins and, quite separately, to the Ad12 E1B 54K protein. The behaviour of p53 in these two circumstances has been examined using A549 cells infected with Ad12 dl620 (a mutant virus which does not express the larger E1B protein and is replication-defective) and human skin fibroblasts expressing the Ad12 E1B 54K protein (HSF 54K). In normal and E1A-expressing A549 cells, p53 is located predominantly in the nucleus, whereas in the HSF 54K cells it is primarily cytoplasmic as is the Ad12 E1B 54K protein. The half-life of p53 is increased in Ad12 dl620-infected A549 cells from about 10 min (in uninfected cells) to 2 hr. The half-life of p53 in HSF 54K cells is even longer-probably in excess of 48 hr. The capacity of p53 to regulate transcription was assessed using a transfected CAT construct linked to p53-responsive elements. p53 transcriptional activity was very low in the HSF 54K cells and in human embryo kidney cells expressing the Ad12 E1B 54K protein (and p53) at high level. It was, however, dramatically increased in response to the p53 expressed as a result of E1A expression. Additionally, MDM2 was present at low level in the HSF 54K cell lines, whilst, as we have previously shown, it is overexpressed in response to infection with Ad12 dl620. We conclude that there are two distinct mechanisms for up-regulation of p53 attributable to the adenovirus E1 proteins. When E1A only is present the p53 is nuclear and transcriptionally active and can probably induce apoptosis in the absence of the E1B 19K protein. When the E1B 54K protein is present, however, p53 is transcriptionally inactive and does not induce apoptosis.