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The p53 gene and its homologue p73 are rarely mutated in neuroblastoma. In recent studies, we showed that overexpression of DeltaNp73alpha, an isoform lacking the N-terminal transactivation (TA) domain, surprisingly induces p53 protein accumulation in the wild-type (wt) p53 human neuroblastoma line SH-SY5Y. As can be expected owing to its dominant-negative effect, DeltaNp73alpha inhibits Waf1/p21 gene expression, but equally importantly, it upregulates BTG2TIS21/PC3, another p53 target gene. This effect is not observed in neuroblastoma cells that express a mutated p53. To better understand the DeltaNp73-mediated transactivation of the BTG2TIS21/PC3 gene we performed luciferase assays with two reporter plasmids harboring long and short BTG2 promoter sequences in three human neuroblastoma cell lines and one breast cancer cell line. Our results demonstrate that BTG2TIS21/PC3 transactivation by DeltaNp73alpha depends on both p53 status (as it is not observed in a p53-/- neuroblastoma cell line) and cellular context (as it occurs in a p53+/+ neuroblastoma cell line but not in a p53+/+ breast tumor cell line). The fact that DeltaNp73alpha may either inhibit or stimulate wt-p53 transcriptional activity, depending on both the p53 target gene and the cellular context, was confirmed by real-time quantitative PCR. Moreover, transactivation of the BTG2TIS21/PC3 promoter requires a complete DeltaNp73alpha C-terminus sequence as it is not observed with DeltaNp73beta, which lacks most of the C-terminal domain. We have previously shown that DeltaNp73alpha is the only p73 isoform expressed in undifferentiated neuroblastoma tumors. In light of all these findings, we propose that DeltaNp73alpha not only acts as an inhibitor of p53/TAp73 functions in neuroblastoma tumors, but also cooperates with wt-p53 in playing a physiological role through the activation of BTG2TIS21/PC3 gene expression.

p73, the first p53 gene homologue, encodes an array of p73 proteins including p73 alpha full-length (TAp73 alpha) and amino-truncated isoforms (Delta Np73 alpha), two proteins with opposite biological functions. TAp73 alpha can induce tumor suppressive properties, while Delta Np73 alpha antagonizes p53 as well as TAp73 in a dominant-negative manner. In human malignant neuroblasts, p53 protein is wild-type but known to be excluded from the nucleus, therefore disabling its function as a tumor suppressor. The present study investigates whether there is a functional link between p73 isoforms and p53 in neuroblastoma. Experiments were performed on two neuroblastoma cell lines differing in their p53 status, e.g. wild-type p53 SH-5Y5Y cells and mutated p53 IGR-N-91 cells. Data indicate that (i) both TA- and Delta N-p73 alpha enhance p53 protein level in SH-SY5Y cells, whereas level remains unchanged in IGR-N-91 cells; (ii) only in SH-SY5Y cells does forced TAp73 alpha overexpression markedly induce nuclear accumulation of p53 protein; (iii) p21 protein expression is increased in both cell lines infected with TAp73, suggesting that, in IGR-N-91 cells, p21 is induced by p73 through a p53-independent pathway; (iv) in the SHSY5Y cell line, Btg2 expression is strongly enhanced in cells overexpressing TA, and to a lesser extent in cells overexpressing Delta N. Taken together our results suggest that TAp73 may restore p53 function in NB with wild-type nonfunctional p53, but not in NB with mutated p53.