Cytoplasmic mutant p53 increases Bcl-2 expression in estrogen receptor-positive breast cancer cells.

The Bcl-2 gene is positively regulated by estrogen (E2) primarily through E2-response elements in the coding region and a putative p53 negative regulatory element (NRE) containing a short upstream open reading frame (uORF). The ability of mutant p53 to repress or induce Bcl-2 expression is controversial. In this study E2-receptor positive (ER(+))/wild-type p53 MCF-7cells were transfected with p53Delta291, which lacks a nuclear localization signal or a DNA binding domain mutant, p53(173L). Both p53 mutants but especially p53Delta291 increased Bcl-2 protein expression from a CMV-NRE-Bcl-2 cDNA construct in an NRE-position/orientation independent manner as well as from a 1.7 kb Bcl-2 promoter reporter gene. Bcl-2 protein expression prevented the p53Delta291-mediated increase in Bcl-2 promoter activity although immunoprecipitation demonstrated that only a small proportion of the wild-type p53 but not p53Delta91 protein interacts with Bcl-2. Unless levels of ectopically expressed mutant p53 were extremely high, stable expression of mutant p53 in MCF-7 cells moderately increased Bcl-2 protein levels. Expression of mutant p53 did not alter E2 regulation of Bcl-2, however, mutation of the uORF prevented regulation by both mutant p53 and E2. Adenovirus-mediated overexpression of WT p53 strongly reduced Bcl-2 expression in ER(-)/mut p53 MDA-MB-231 cells. Taken together these data support the position that mutant p53 behaves in a dominant "positive" manner relieving repression by WT p53 or another Bcl-2 transcriptional inhibitor in a manner independent of nuclear translocation.