p63 mediates survival in squamous cell carcinoma by suppression of p73-dependent apoptosis

Abstract

We demonstrate that deltaNp63alpha is an essential survival factor in head and neck squamous cell carcinoma (HNSCC) through its ability to suppress p73-dependent apoptosis. Inhibition of endogenous p63 expression by RNAi induces apoptosis selectively in HNSCC cells that overexpress deltaNp63alpha. Knockdown of p63 induces the proapoptotic bcl-2 family members Puma and Noxa, and both their induction and subsequent cell death are p53 independent but require transactivating isoforms of p73. Inhibition of p73-dependent transcription by deltaNp63alpha involves both direct promoter binding and physical interaction with p73. In HNSCC cells lacking endogenous deltaNp63alpha expression, bcl-2 is instead upregulated and can suppress p73-mediated death. Together, these data define a pathway whereby deltaNp63alpha promotes survival in squamous epithelial malignancy by repressing a p73-dependent proapoptotic transcriptional program.

Mechanism of TAp73 inhibition by ΔNp63 and structural basis of p63/p73 hetero-tetramerization

“…Direct interaction between p63 and p73 has also been reported in HNSCCs. 8 This tumor type often shows a high expression level of ΔNp63α that inhibits the tumor-suppressor function of TAp73. Our analysis of the potential mechanism of this inhibition, however, suggests that most of it is probably due to promoter squelching although the formation of hetero-complexes between transactivating p73 isoforms and inactive p63 isoforms on promoter sites with a reduced number of TA domains may contribute as well.…”

“…We used this specific combination as it has been shown that direct interaction between these two isoforms inhibits the tumor-suppressor activity of TAp73β in HNSCC cells. 8 Introduction of the homotetramer mutations (E370K, K377E, E380R and R397E) in ΔNp63α led to a marked decrease in co-IP efficiency with TAp73β, indicating that hetero-tetramer formation was repressed by these mutations also in a cellular context ( Figure 5). Interaction between the hetero-tetramer mutants of TAp73β (E363K) and of ΔNp63α (E377E) again showed a strong co-IP efficiency in all tested combinations.…”

“…In addition to this indirect effect via promoter squelching, it was shown that p73 directly interacts with ΔNp63α, which may also contributes to inhibiting p73's function as a tumor suppressor. 8 In previous studies, along with others, we have shown that p63 and p73 can form mixed tetramers via their tetramerization domains (TDs). 9,10 Surprisingly, hetero-tetramers consisting of one homo p63 dimer and one homo p73 dimer are thermodynamically more stable than both homo-tetramers.…”

“…Overexpression of the ΔNp63α isoform, however, is observed in squamous cell carcinomas, particularly in head and neck squamous cell carcinomas (HNSCCs), a group of malignancies derived from cells of the basal epithelia of the aerodigestive mucosa. 8 Although it has been shown that ΔNp63α acts as a transcription factor for a certain set of genes, it inhibits the p53 and p73-induced activation of prototypical apoptosis-related genes by binding to and blocking the promoter. In addition to this indirect effect via promoter squelching, it was shown that p73 directly interacts with ΔNp63α, which may also contributes to inhibiting p73's function as a tumor suppressor.…”

Are interactions with p63 and p73 involved in mutant p53 gain of oncogenic function?

“…If this were the case then one would have to take into account the fact that DN p63/p73 proteins can have pro-proliferative and possibly oncogenic activity. In fact, DNp63, like some p53 mutants, promotes the survival of squamous cell carcinoma (SCC) by blocking the TAp73 pathway (Rocco et al, 2006). In addition, many SCC cells express both mutant p53 and DNp63 (Hibi et al, 2000).…”