Drug discovery in the p53 pathway
© Current Science Ltd 2000
Published: 12 March 2000
The activity of the tumour suppressor protein p53 is critically controlled by proteolysis. When cells are exposed to a variety of stress stimuli including hypoxia, DNA damage, or the action of certain oncogenes, this degradative pathway is inhibited and p53 protein levels rise, inducing cell-cycle arrest and apoptosis. The function of the p53 pathway is affected by many DNA tumour virus-derived oncogenes. In addition, two cellular proteins, Mdm2 and Arf, have been discovered to play a critical role in regulating the specific stability of p53. Mdm2 binds to the N terminus of p53, recognising a specific peptide motif, and targets p53 to the proteasome. The Mdm2 protein acts as a specific E3 ubiquitin ligase, and the Arf protein binds to Mdm2 and inhibits its ligase activity. Recently we have also discovered that p53 is modified by the small ubiquitin-like protein SUMO, and this modification may inhibit the degradation of p53. Microinjection of antibodies to the p53-binding domain of Mdm2 will activate the p53 response in normal cells as will mini-proteins displaying phage-optimised Mdm2 binding peptides that block p53 binding. The N terminal 64 amino acids of Arf are also a potent activator of the p53 response, and we have recently localised the Arf-Mdm2 interaction using pepscan libraries of Arf. An important feedback pathway exists because the Mdm2 gene is only transcribed in cells that contain normal p53. This explains why tumours stain strongly with anti-p53 antibodies if that p53 is inactive as a transcription factor. In a survey of small molecules we found that both the nuclear export inhibitor leptomycin B and the proteasome inhibitor Lactacystin caused the accumulation of p53 in normal cells. Strikingly, only Leptomycin-induced p53 was transcriptionally active. Using inducible cell-line systems, we established that Mdm2 targets p53 for nuclear export. The ability to induce the p53 response with non-genotoxic agents combined with the recognition that p53 mutant human tumours lack the Mdm2 dependant degradation pathway opens up many exciting new approaches to drug discovery in the p53 pathway.