Please can you give a brief introduction to p53 and the important role it plays in the cell cycle and in cancer?
Before we talk about p53, it is important to understand why cancer cells are a problem for the body. Cancer cells multiply recklessly, refuse to die and blithely metastasize to set up shop in places where they don’t belong.
And one protein that keeps healthy cells from behaving this way is a tumor suppressor named p53. This protein stops potentially precancerous cells from dividing and induces suicide in those that are damaged beyond repair.
Not surprisingly, p53’s critical function is disrupted or silenced in many cancers.
Your research team first identified iASPP as an inhibitor of p53 back in 2003. Please can you explain what iASPP is and what your recent research revealed about the mechanism by which it inactivates p53?
Our current research looked at how p53 is silenced in advanced melanomas by a protein named iASPP.
iASPP is a member of the ASPP family of proteins and is one of the most evolutionarily conserved inhibitors of p53. It binds p53 and inhibits the transcription and the apoptotic function of p53.
We showed that a protein complex named cyclin B1/cdk1, which is expressed at high levels in the cytoplasm of advanced melanomas, induces a pair of precise chemical modifications on iASPP to activate the protein.
When activated, iASPP is shuttled into the nucleus, binds to p53 and ultimately inhibits its ability to induce cell suicide.
This is the first time that such a mechanism of p53 inactivation has been described.
Your current study also explored whether p53 function could be restored by scuttling iASPP activation. How did you attempt to do this and were you successful in restoring p53 function?
To uncover how to restore p53, we treated melanoma cells with a panel of small molecules and identified JNJ-7706621 (JNJ) as the best inhibitor of cyclinB1/cdk1.
We showed that p53 is inhibited by two proteins in melanoma cells, iASPP and MDM2. The activity of the latter protein is known to be blocked by a small molecule called Nutlin-3.
When JNJ and the Nutlin-3 were combined, the full function of p53 was restored in metastatic melanoma cells. Such treatment significantly suppressed tumor growth in mice.
Were the results of your research surprising?
We have been looking for p53 inhibitors in addition to MDM2 for some time. It is, therefore, gratifying to uncover that functional p53 in melanoma is normally inhibited by two different factors, iASPP and MDM2, instead of one, as previously thought.
Our results also provide a proof of principle that both of those factors need to be blocked if p53 is to be successfully reactivated in cancer cells.
Do you think it would be possible to use the restoration of p53 function to improve existing cancer therapies?
Yes, that is what we are working toward. To find out, we treated advanced melanomas with JNJ, Nutlin-3 and a chemotherapeutic drug used in the clinic today named vemurafenib. This drug specifically inhibits BRAFV600E, a mutated protein that drives cancer cell proliferation.
With such treatment, advanced melanoma tumors in preclinical mouse models shrank by a full 75% after 28 days of treatment. This has notable implications for the treatment of cancers in which p53 is not mutated but is instead functionally silenced—roughly half of all cancer cases.
Based on our results, the best strategy in such cases might be to use drug combinations that target multiple, parallel pathways involved in tumor development and maintenance.
Such combinations of drugs that normally have short-term efficacy could achieve an additive, if not a long-term synergistic effect, helping improve existing cancer therapies.
What further research needs to be carried out on p53?
It will be important to elucidate how common the identified pathway is in other tumour types that express structurally wild-type yet functionally silent p53.
How important do you think research on p53 will be in the future of improving melanoma therapies?
It is important for readers to understand that metastatic melanoma is resistant to treatment and accounts for 80% of skin cancer deaths. Restoring p53 function in melanoma is an attractive therapeutic strategy and important for the future of improving cancer therapies, as nearly 90% of human melanomas express functionally defective wild-type p53.
Nutlin-3, an agonist of p53, is currently in clinical trials but often fails to reactivate p53 function when used alone.
We found that concurrent inhibition of MDM2 and iASPP with small molecules resulted in p53-dependent apoptosis and growth suppression of melanoma cells.
Reactivation of p53 together with BRAFV600E inhibition induced apoptosis and suppressed melanoma growth, presenting a crucial alternative strategy for melanoma therapy.
Recent research at the University of Dundee showed that the drug Tenovin kills cancer cells in chronic lymphocytic leukaemia (CLL) patients without affecting the level of p53. What are your thoughts on this research?
Tenovin reactivates p53 by targeting SIRT1 and SIRT2 deacetylases, and the drug prevents MDM2 from inhibiting p53. So Tenovin functions via a similar pathway as Nutlin-3.
Based on our findings, Tenovin may work together with JNJ to reactivate p53. Future work is needed to test this hypothesis.
How was your research funded?
This work was mainly funded by the Ludwig Institute for Cancer Research. Our collaborators are funded by the Oxford NIHR Biomedical Research Centre, the Structural Genomics Consortium and the Medical Research Council.
Where can readers find more information?
The paper has been published in the April 25 issue of Cancer Cell.
Restoring p53 Function in Human Melanoma Cells by Inhibiting MDM2 and Cyclin B1/CDK1-Phosphorylated Nuclear iASPP
Min Lu, Hilde Breyssens, Victoria Salter, Shan Zhong, Ying Hu, Caroline Baer, Indrika Ratnayaka, Alex Sullivan, Nicholas R. Brown, Jane Endicott, Stefan Knapp, Benedikt M. Kessler, Mark R. Middleton, Christian Siebold, E. Yvonne Jones, Elena V. Sviderskaya, Jonathan Cebon, Thomas John, Otavia L. Caballero, Colin R. Goding, Xin Lu. Cancer Cell. 2013 April 25.
About Dr. Xin Lu
Dr. Xin Lu is a Member of the Ludwig Institute for Cancer Research and Ludwig's Director in Oxford. She is also Professor of Cancer Biology at the University of Oxford and Director.
Dr Lu’s research is focused on understanding tumour suppression. Dr Lu was among the first researchers to show that the tumour suppressor protein p53 responds to both oncogene activation and DNA damaging signals.
Subsequently, she investigated cross-talk between the p53 and retinoblastoma (RB) tumour-suppressor pathways, demonstrating how alterations in the RB pathway could sensitize tumour cells to p53-induced apoptosis.
Her lab was also one of the first to show how to selectively activate p53 to kill cancer cells, through the identification and characterization of the evolutionarily conserved ASPP family of proteins.
Current studies are focused on investigating the potential of the ASPPs as biomarkers and targets for the development of novel anti-cancer therapies.