Regulating the p53 pathway: in vitro hypotheses, in vivo veritas

Abstract

In vitro and transfection studies have suggested a p53 regulation model that emphasizes the importance of phosphorylation to produce structural changes in p53 to enable competition between MDM2 and p300 for binding the N-terminal p53 transactivation domain and inducing competing modifications in the p53 C-terminal regulatory domain. In unstressed cells, MDM2 binding in the N terminus would inhibit p53 activity and the MDM2-mediated ubiquitylation of the C terminus would promote p53 degradation; after stress, the phosphorylation of the p53 N terminus increases p300 binding, and the p300-mediated acetylation of the C terminus stabilizes and activates p53. The above model is not supported by recent in vivo studies, because mouse mutants that express different point mutations in the N terminus and C terminus of p53 do not have the predicted phenotypes. Analysis of mutations found in human tumours also suggest that modifiable serine, threonine and lysine residues in the N-terminal and C-terminal domains do not provide on–off switches for p53. Recent mouse mutants confirm the importance of MDM2 in p53 regulation, and show the separate contribution of the MDM2-related protein, MDM4 (also called MDMX) for p53 regulation: in vivo data now indicate that MDM2 mainly regulates p53 stability, whereas MDM4 contributes significantly to regulating p53 activity. These and other data suggest that a switch from MDM2 degradation of p53 to degradation of itself and MDM4 is responsible for p53 accumulation and activation after stress. These results indicate the importance of developing drugs that antagonize MDM2–p53 and MDM4–p53 interactions. Candidate MDM2 antagonists have been developed, but not MDM4 antagonists. Importantly, MDM2 and MDM4 antagonists could cooperate to activate p53 in two to three million patients diagnosed with cancer each year. As p53, MDM2 and MDM4 interact with many proteins, further analyses of these interactions might also lead to new and broadly useful anticancer strategies.