Screening for genes that reprogram cancer cells for the tumor reversion switch identified TCTP (encoding translationally controlled tumor protein) as a crucial regulator of apoptosis. Here we report a negative feedback loop between P53 and TCTP. TCTP promotes P53 degradation by competing with NUMB for binding to P53-MDM2-containing complexes. TCTP inhibits MDM2 auto-ubiquitination and promotes MDM2-mediated ubiquitination and degradation of P53. Notably, Tctp haploinsufficient mice are sensitized to P53-dependent apoptosis. In addition, P53 directly represses TCTP transcription. In 508 breast cancers, high-TCTP status associates with poorly differentiated, aggressive G3-grade tumors, predicting poor prognosis (P < 0.0005). Tctp knockdown in primary mammary tumor cells from ErbB2 transgenic mice results in increased P53 expression and a decreased number of stem-like cancer cells. The pharmacological compounds sertraline and thioridazine increase the amount of P53 by neutralizing TCTP's action on the MDM2-P53 axis. This study links TCTP and P53 in a previously unidentified regulatory circuitry that may underlie the relevance of TCTP in cancer.

Overexpression of MDM4 (also known as MDMX or HDMX) is thought to promote tumorigenesis by decreasing p53 tumor suppressor function. Even modest decrease in Mdm4 levels affects tumorigenesis in mice, suggesting that genetic variants of MDM4 might have similar effects in humans. We sequenced the MDM4 gene in a series of ovarian cancer cell lines and carcinomas to identify mutations and/or single nucleotide polymorphisms (SNPs). We identified an SNP (SNP34091) in the 3 0 -UTR of MDM4 that creates a putative target site for hsa-miR-191, a microRNA that is highly expressed in normal and tumor tissues. Biochemical evidence supports specific miR-191-dependent regulation of the MDM4-C, but not MDM4-A, variant. Consistently, the A-allele was associated with statistically significant increased expression of MDM4 mRNA and protein levels in ovarian carcinomas. Importantly, the wild-type genotype (A/A) is more frequent (57.8% vs. 42.2% for A/C and C/C, respectively) in patients with high-grade carcinomas than in patients with low-grade carcinomas (47.2% vs. 52.5% for A/A and A/C þ C/C, respectively). Moreover, A/A patients who do not express the estrogen receptor had a 4.2-fold [95% confidence interval (CI) ¼ 1.2-13.5; P ¼ 0.02] increased risk of recurrence and 5.5-fold (95% CI ¼ 1.5-20.5; P ¼ 0.01) increased risk of tumor-related death. Unexpectedly, the frequency of p53 mutations was not significantly lower in A/A patients. We conclude that acquisition of an illegitimate miR-191 target site causes downregulation of MDM4 expression, thereby significantly delaying ovarian carcinoma progression and tumor-related death. Importantly, these effects appear to be, at least partly, independent of p53. Cancer Res; 70(23); 9641-9. Ó2010 AACR.