by ecancer reporter Clare Sansom

The tumour suppressor p53 is one of the most commonly mutated genes in human tumours, and it has therefore attracted much attention as a drug target. Cancers of the lung, which kill more people worldwide every year than those at any other site, very frequently contain mutated, inactivated p53. Furthermore, restoration of p53 activity has been shown to be able to trigger tumour regression in animal models of lung cancer. However, two papers published back-to-back in the 25 November issue of Nature now suggest that there are limitations of a p53 restoration strategy in treating lung cancer. Both suggest that restoration of p53 induces regression only in aggressive tumours, with no effect on early-stage cancers or on the more indolent cells found in tumours at all stages.

Gerard Evan and his colleagues at the University of California San Francisco, CF, USA, with co-workers at the Cancer Research UK Cambridge Research Institute, Cambridge, UK, studied the p53 pathway in a mouse model with mutant Kras and induced, multiple non-small cell lung carcinomas [1]. These mice were crossed with a mouse model in which p53 can be repeatedly induced and inactivated by treatment with tanoxifen. Crossed mice with tamoxifen-dependent p53 had tumours that were more aggressive and more quickly growing than those with normal p53 levels.

When the researchers restored p53 activity for a short period in the tamoxifen-dependent mice, they found, unexpectedly, that the tumours overall did not show significant regression. Individual tumours had a wide variety of responses, with some stable or growing but others shrinking. All tamoxifen-treated tumours, however, were shown to contain active p53, indicating that this variability was caused by differences in response to p53 rather than by differences in its induction. A more detailed micrographic examination of individual tumours indicated that p53 was activated selectively only in the higher-grade, more aggressive cells in each tumour. Immunohistochemistry also revealed that expression of the oncogenic signalling protein p19^ARF, which triggers p53 activation, occurs only in the same high-grade tumour cells. These results imply that there is a minimum level of oncogenic Kras signalling below which a tumour will be insensitive to p53 restoration.

The second study, by Tyler Jacks and colleagues from the Koch Institute of Cancer Biology and Howard Hughes Medical Institute, Cambridge, MA, USA, also involved an analysis of the effect of p53 restoration on tumours at different stages. He used a similar Kras-driven mouse model of lung cancer, where p53 activity could be restored using tamoxifen. By four weeks of age, these cancer-prone mice had developed lung adenomas and adenocarcinomas that showed slight variations in histology. At ten weeks, the tumours had become more heterogeneous with a larger proportion of high-grade lesions. Mice given a two-week treatment with tamoxifen to restore p53 activity had stable tumours during that period, whereas the tumours of control mice continued to grow. However, the proportion of high grade tumours (adenocarcinomas) and the proportion of aggressive cells in all tumours shrank during tamoxifen treatment.

The activity of signal transduction pathways in treated and untreated mice during the adenoma – adenocarcinoma transition was explored using immunohistochemistry. Phosphorylation levels of kinases in the MAPK pathway were found to be higher in adenocarcinomas than in adenomas, indicating that cancer progression in this model may be driven by MAPK signalling, and this result was confirmed using fluorescence in situ hybridization (FISH) analysis. Gene expression profiles from adenocarcinomas were analysed before and seven days after p53 restoration and the p53-restored adeno-carcinomas were found to have expression profiles that were more characteristic of adenomas.

The MAPK signaling pathway is known to activate the expression of the Arf tumour-suppressor, which is itself an activator of p53. These results indicate that restoring p53 activity can cause tumour regression in this mouse model, but only when the tumours have progressed enough for MAPK signaling to activate Arf.

All tumours, even the most aggressive ones, are heterogeneous, containing cells at different grades and stages. Both these studies show that only high-grade cells respond to p53 activation, which implies that drugs that restore the activity of this crucial tumour suppressor are unlikely to be completely effective alone. They may, however, play an important part in containing tumours at an indolent stage and, therefore, buying time for other treatments to work. 

References

[1] Junttila, M.R., Karnezis, A.N., Garcia, D., Madriles, F., Kortlever, R.M., Rostker, F.,

Swigart, L.B., Pham, D.M., Seo, Y., Evan, G.I. & Martins, C.P. (2010). Selective activation of p53-mediated tumour suppression in high-grade tumours. Nature 468, 567-571. doi:10.1038/nature09526

[2] Feldser, D.M., Kostova, K.K., Winslow, M.M., Taylor, S.E., Cashman, C., Whittaker, C.A., Sanchez-Rivera, F.J., Resnick, R., Bronson, R., Hemann, M.T. & Jacks, T. (2010). Stage-specific sensitivity to p53 restoration during lung cancer progression. Nature 468, 572-575. doi:10.1038/nature09535