The neurogene BTG2^TIS21/PC3 is transactivated by Np73 via p53 specifically in neuroblastoma cells

The p53 gene and its homologue p73 are rarely mutated in neuroblastoma. In recent studies, we showed that overexpression of Np73, 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, Np73 inhibits Waf1/p21 gene expression, but equally importantly, it upregulates BTG2^TIS21/PC3, another p53 target gene. This effect is not observed in neuroblastoma cells that express a mutated p53. To better understand the Np73-mediated transactivation of the BTG2^TIS21/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 BTG2^TIS21/PC3 transactivation by Np73 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 Np73 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 BTG2^TIS21/PC3 promoter requires a complete Np73 C-terminus sequence as it is not observed with Np73, which lacks most of the C-terminal domain. We have previously shown that Np73 is the only p73 isoform expressed in undifferentiated neuroblastoma tumors. In light of all these findings, we propose that Np73 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 BTG2^TIS21/PC3 gene expression.