Physical and Functional Antagonism between Tumor Suppressor Protein p53 and Fortilin, an Anti-apoptotic Protein

Abstract

Tumor suppressor protein p53, our most critical defense against tumorigenesis, can be made powerless by mechanisms such as mutations and inhibitors. Fortilin, a 172-amino acid polypeptide with potent anti-apoptotic activity, is up-regulated in many human malignancies. However, the exact mechanism by which fortilin exerts its anti-apoptotic activity remains unknown. Here we present significant insight. Fortilin binds specifically to the sequence-specific DNA binding domain of p53. The interaction of fortilin with p53 blocks p53-induced transcriptional activation of Bax. In addition, fortilin, but not a double point mutant of fortilin lacking p53 binding, inhibits p53-dependent apoptosis. Furthermore, cells with wild-type p53 and fortilin, but not cells with wild-type p53 and the double point mutant of fortilin lacking p53 binding, fail to induce Bax gene and apoptosis, leading to the formation of large tumor in athymic mice. Our results suggest that fortilin is a novel p53-interacting molecule and p53 inhibitor and that it is a logical molecular target in cancer therapy.Tumor suppressor protein p53 keeps us free of cancer when it is functional. Mice lacking p53 (p53 Ϫ/Ϫ ) spontaneously develop numerous neoplasms within 6 months (1). Mutated p53 genes are seen in more than 50% of all human cancers, making them the most frequently observed genetic derangement in human cancer (2). At a molecular level, the ability of p53 to eliminate cancerous cells relies on its ability to induce apoptosis, through either the transcriptional activation of proapoptotic genes such as Noxa (3), PUMA 4 (4), and Bax (5) or the direct transcription-independent activation of Bax on mitochondria (6). Growing cancers manage to keep p53 in check either by mutating the p53 gene itself (7-9) or by expressing p53 inhibitors such as Mdm2 (9, 10). The function of fortilin, a ubiquitous, highly conserved, 172-amino acid polypeptide also known as "translationally controlled tumor protein," or TCTP, remained unknown (11,12). Investigation in our laboratory and others showed that fortilin possesses potent anti-apoptotic activity (13-15). Fortilin is overexpressed in human cancers (16,17), the depletion of which is associated with spontaneous death of cancerous cells (13, 18). Higher levels of fortilin are associated with more malignant cancer phenotypes (14). Although heterozygous fortilin-deficient mice (fortilin ϩ/Ϫ ) were normal in appearance and fertile, homozygous fortilin-deficient (fortilin Ϫ/Ϫ ) mice were embryonically lethal around 3.5 days postcoitus due to massive apoptosis, as reported by our laboratory and others (19 -21).The mechanism by which fortilin functions as an anti-apoptotic molecule has been under robust investigation. First, based on the fact that fortilin physically interacts with myeloid cell leukemia protein-1 (MCL1), an anti-apoptotic Bcl-2 family member, it was suggested that fortilin stabilizes and exerts its anti-apoptotic activity through MCL1 (22). However, fortilin is capable of protecting cells from apoptosis in the...

The Translational Controlled Tumour Protein TCTP: Biological Functions and Regulation

“…Indeed down-regulation of TCTP through activation of a (temperature-sensitive) mutant protein of p53 (Tuynder et al 2002;Bommer et al 2010) was an initial observation in this context. The mutual antagonism between p53 and TCTP was then described by three groups in brief succession (Amson et al 2012;Chen et al 2011;Rho et al 2011). The underlying mechanisms involved in this included the following:…”

“…However, one of these anti-apoptotic mechanisms ought to be mentioned here, i.e. the antagonism of TCTP to the tumour suppressor protein p53, which is crucial for TCTP to exert its cancer-promoting activity (Amson et al 2012;Chen et al 2011). The cytoprotective role of TCTP in cancer cells is well documented, for example in conditions of oxidative stress (Lucibello et al 2011), or of treatment with various anticancer drugs, the latter often resulting in chemoresistance of these cells (Li et al 2001;Jung et al 2014;Graidist et al 2004;Bommer et al 2017;He et al 2015), which is a frequent problem in cancer chemotherapy.…”

Cellular Function and Regulation of the Translationally Controlled Tumour Protein TCTP

“…Rho et al [57] showed that overexpression of TCTP in lung carcinoma cells reversed p53-mediated apoptosis, whereas TCTP knock-down increased apoptosis. Similarly, Chen and colleagues [58] observed TCTP (fortilin) interaction with p53 using GST-pull down assays; they showed that TCTP inhibits p53-dependent apoptosis. However, these two studies arrive at differing conclusions in the following two points: 1.…”

“…2. In the mechanism by which TCTP antagonises p53: Rho et al reported that TCTP is destabilising p53 in lung cancer cells, whereas Chen and colleagues did not observe p53 destabilisation, but instead proposed that TCTP blocks p53-induced transcriptional activation of Bax [58].…”

“…In the mapping of the domain in TCTP that is involved in p53 binding: Rho et al [57] found that an internal domain (corresponding to thehelical domain; Fig. 1) is involved in p53 binding, whereas Chen et al [58] observed that the N-and C-terminal parts of the molecule (the -sheet domain; Fig. 1) participate in this interaction.…”