Xue W, Zender L, Miething C, Dickins RA, Hernando E, Krizhanovsky V, et al. Senescence and tumour clearance is triggered by p53 restoration in murine liver carcinomas. Nature 2007;445:656-660. (Reproduced by permission.)
Although cancer arises from a combination of mutations in oncogenes and tumour suppressor genes, the extent to which tumour suppressor gene loss is required for maintaining established tumours is poorly understood. p53 is an important tumour suppressor that acts to restrict proliferation in response to DNA damage or deregulation of mitogenic oncogenes, by leading to the induction of various cell cycle checkpoints, apoptosis or cellular senescence. Consequently, p53 mutations increase cell proliferation and survival, and in some settings promote genomic instability and resistance to certain chemotherapies. To determine the consequences of reactivating the p53 pathway in tumours, we used RNA interference (RNAi) to conditionally regulate endogenous p53 expression in a mosaic mouse model of liver carcinoma. We show that even brief reactivation of endogenous p53 in p53-deficient tumours can produce complete tumour regressions. The primary response to p53 was not apoptosis, but instead involved the induction of a cellular senescence program that was associated with differentiation and the upregulation of inflammatory cytokines. This program, although producing only cell cycle arrest in vitro, also triggered an innate immune response that targeted the tumour cells in vivo, thereby contributing to tumour clearance. Our study indicates that p53 loss can be required for the maintenance of aggressive carcinomas, and illustrates how the cellular senescence program can act together with the innate immune system to potently limit tumour growth.
The transcription factor p53 is one of the main tumor suppressor proteins known to date. Upon a wide variety of cellular stress stimuli, p53 undergoes nuclear accumulation and activation that implicates posttranscriptional modifications, increased genomic stability, transcriptional activity, and protein-protein interaction with other molecules.1 Activation of p53 induces cell growth arrest, cellular senescence, differentiation, DNA repair, or apoptosis (Fig. 1) in order to prevent damaged cells from proliferating or undergoing oncogenic transformation.1, 2 Inactivation or mutation of p53 results in loss of apoptotic function and impairment of DNA-binding ability that derive in tumorigenic transformation and increased genetic instability (Fig. 1).3–5 Additionally, maintenance of a certain p53 expression level (gene dosage) is essential to preserve its tumor repressor function.6
Along with p53 inactivation, ras and p16 mutations are highly prevalent in human tumor development. Serrano et al. described how retroviral transfection with an activated ras allele (HrasV12) induced cell cycle arrest characterized by cellular senescence in the presence of abundant p53 expression. However, disruption of p53 activation favored ras-induced cell transformation, because they could efficiently escape from p53-induced cell cycle arrest.7
From this line of evidence, the recent study by Xue et al.8 efficiently showed tumor regression upon p53 restoration. They transplanted embryonic liver progenitor cells (hepatoblasts) transduced with oncogenic Ras and with a tet-responsive (tet-off system) p53 short hairpin RNA (shp53mir) in athymic nude mice. Through this approach, they induced rapid development of ras-expressing hepatocellular carcinomas. This was performed in the absence of doxycycline (Dox), which resulted in a lack of wild-type (wt) p53 expression. After Dox treatment, ras-tumors exhibited a complete regression as endogenous p53 expression was restored by tet-off–induced shp53mir silencing.
Tumor regression mediated by p53 did not involve apoptosis or necrosis but cellular senescence and activation of the innate immune response. The authors observed a strong production of proinflammatory cytokines which triggers neutrophil, macrophage, and natural killer cell infiltration in the peritumor and tumor areas after p53 restoration. This observation was associated with a strong expression of cellular senescence markers in tumors shortly after p53 restoration. Senescent cells express inflammatory cytokines, chemoattractants, and adhesion molecules9, 10 in order to facilitate recruitment and tumor targeting of immune cells.
These data suggest that p53 expression coordinates the link between cellular senescence and the innate immune response in order to eliminate tumor cells by phagocytosis and cytotoxic mechanisms. This finding was further strengthened by the observation that depletion of macrophages, neutrophils, or natural killer cells by gadolinium or neutralizing antibodies delayed tumor clearance after p53 restoration. Additionally, in NOD/SCID (nonobese diabetic, severe combined immunodeficient) mice, no tumor regression upon p53 reactivation was observed although cellular senescence was induced. These results confirm the tight coordination between the senescence cell program and the innate immune system in eliminating ras-induced tumors. Interestingly, only transient p53 restoration was sufficient to irreversibly activate the cellular senescence program.
In a second study published in the same issue of Nature, Ventura et al.11 also demonstrate that p53 restoration results in tumor regression. However, in their work, induction of programmed cell death (apoptosis) was essential to trigger tumor clearance. The different p53-dependent mechanisms observed in these 2 studies might be best explained by the different nature of the applied tumor models and potential tissue-specific differences. Xue et al.8 induced hepatocellular carcinomas via oncogenic ras, whereas Ventura et al.11 accelerated autochthonous lymphoma and sarcoma formation, characteristic for p53 null mice after radiation shortly after birth. In their study, tumor regression was tissue dependent. Restoration of p53 in lymphomas triggered apoptosis, whereas in sarcomas cell cycle arrest was induced.11
These 2 reports highlight the critical role of wt p53 in regulating both apoptosis and cellular senescence during tumor regression, suggesting p53 restoration in tumor cells as a potential therapeutic target. However, before directly addressing this approach in humans it has to be stressed that both experimental models are artificial. In many human tumors, p53 expression is not deleted and mutant p53 versions are expressed, especially in hepatocellular carcinomas. At present, it is unclear if wt p53 expression in these tumors would really lead to senescence and activation of the innate immune response in order to trigger tumor regression. Therefore, the data presented in both studies are very promising for development of new treatment options, because transient activation of a single gene can reverse tumor growth by different cellular mechanisms. However, future studies are essential to test if this concept can be applied to a broader spectrum of tumors also in humans.