Browsing by Author "Li, D."

Missense mutations in p53 generate aberrant proteins with abrogated tumour suppressor functions that can also acquire oncogenic gain-of-function activities that promote malignant progression, invasion, metastasis and chemoresistance(1-5). Mutant p53 (mutp53) proteins undergo massive constitutive stabilization specifically in tumours, which is the key requisite for the acquisition of gain-of-functions activities(6-8). Although currently 11 million patients worldwide live with tumours expressing highly stabilized mutp53, it is unknown whether mutp53 is a therapeutic target in vivo. Here we use a novel mutp53 mouse model expressing an inactivatable R248Q hotspot mutation (floxQ) to show that tumours depend on sustained mutp53 expression. Upon tamoxifen-induced mutp53 ablation, allotransplanted and autochthonous tumours curb their growth, thus extending animal survival by 37%, and advanced tumours undergo apoptosis and tumour regression or stagnation. The HSP90/HDAC6 chaperone machinery, which is significantly upregulated in cancer compared with normal tissues, is a major determinant of mutp53 stabilization(9-12). We show that long-term HSP90 inhibition significantly extends the survival of mutp53 Q/- (R248Q allele(2)) and H/H (R172H allele(3)) mice by 59% and 48%, respectively, but not their corresponding p53(-/-) (also known as Trp53(-/-)) littermates. This mutp53-dependent drug effect occurs in H/H mice treated with 17DMAG+SAHA and in H/H and Q/- mice treated with the potent Hsp90 inhibitor ganetespib. Notably, drug activity correlates with induction of mutp53 degradation, tumour apoptosis and prevention of T-cell lymphomagenesis. These proof-of-principle data identify mutp53 as an actionable cancer-specific drug target.

The activity of p53 as an inducible transcription factor depends on its rapid nuclear stabilization after stress. However, surprisingly, mechanism(s) that regulate nuclear p53 accumulation are not well understood. The current model of stress-induced nuclear accumulation holds that a decrease in p53 nuclear export leads to its nuclear stabilization. We show here that regulated nuclear import of p53 also has a critical function. p53 import is mediated by binding to the importin-alpha 3 adapter and is negatively regulated by ubiquitination. p53 harbors several nuclear localization signals (NLS), with the major NLS I located at amino-acids 305-322. We find that direct binding of p53 to importin-alpha 3 depends on the positive charge contributed by lysine residues 319-321 within NLS I. The same lysines are also targets of MDM2-mediated ubiquitination. p53 ubiquitination occurs primarily in unstressed cells, but decreases dramatically after stress. Importin-alpha 3 preferentially interacts with non-ubiquitinated p53. Thus, under normal growth conditions, ubiquitination of Lys 319-321 negatively regulates p53-importin-alpha 3 binding, thereby restraining p53 import. Conversely, stress-induced accumulation of non-ubiquitinated p53 in the cytoplasm promotes interaction with importin-alpha 3 and rapid import. In later phases of the stress response, blocked nuclear export also takes effect. We propose that p53 nuclear import defines an important novel level of regulation in the p53-mediated stress response. Cell Death and Differentiation (2010) 17, 255-267; doi:10.1038/cdd.2009.173; published online 20 November 2009