Abstract
The p53 tumour-suppressor protein plays a critical role in the cellular response to
environmental and intracellular stresses that threaten DNA integrity. Inactivation of
p53 represents an important step during carcinogenesis and is associated with
genomic instability and tumour development. A key transcriptional target of p53 is
the cyclin-dependent kinase inhibitor, p21WAF1/CIP1 (hereafter referred to as p21),
which mediates p53-dependent G1 arrest. The role of p21 in tumour development
remains contentious. Early studies showed that p21 mutations are rare in human
cancers however there is a growing list of human carcinomas that have aberrant p21
expression. p21-null animals also have elevated tumour incidence, but the
mechanism underlying this is not yet defined.
Our data identifies p21 as a component of a positive feedback loop that
maintains the p53 transcriptional response. Three model systems were used to
characterise this novel mechanism of p53 regulation. In the human colon carcinoma
cell line HCT116 with targeted inactivation of p21, p53 stabilisation is uncoupled
from its activity as a transcription factor and shows defects in the p53 response to
DNA damage and double stranded RNA, indicating that a common mechanism
prevents p53 activation by distinct stresses in the absence of p21. The p53
transcriptional programme in response to cellular damage can be reactivated after
complementation of the p21 gene into the HCT116 p21-null cells. In B-cells from
mice lacking the p21 gene, a striking loss of the p53-dependent transcription
programme was identified using p53-specific microarray screens. Gene dosage
effects indicate a progressive loss of p53 function in B-cells heterozygous or
homozygous null for p21. Similarly, siRNA to p21 can attenuate the p53-dependent
transcription response in normal human fibroblasts. In all three model systems,
deletion of the p21 gene results in p53 nuclear export and eliminates the p53
transcriptional response. This data indicates that p53 has evolved a co-ordinated
transcription mechanism to control its own function: a positive feedback loop
maintained by p21 and a negative feedback loop maintained by MDM2, whose
balance controls the specific activity of p53.
