Deregulation of Internal Ribosome Entry Site-Mediated p53 Translation in Cancer Cells with Defective p53 Response to DNA Damage

Abstract

Synthesis of the p53 tumor suppressor and its subsequent activation following DNA damage are critical for its protection against tumorigenesis. We previously discovered an internal ribosome entry site (IRES) at the 5= untranslated region of the p53 mRNA. However, the connection between IRES-mediated p53 translation and p53's tumor suppressive function is unknown. In this study, we identified two p53 IRES trans-acting factors, translational control protein 80 (TCP80), and RNA helicase A (RHA), which positively regulate p53 IRES activity. Overexpression of TCP80 and RHA also leads to increased expression and synthesis of p53. Furthermore, we discovered two breast cancer cell lines that retain wild-type p53 but exhibit defective p53 induction and synthesis following DNA damage. The levels of TCP80 and RHA are extremely low in both cell lines, and expression of both proteins is required to significantly increase the p53 IRES activity in these cells. Moreover, we found cancer cells transfected with a shRNA against TCP80 not only exhibit decreased expression of TCP80 and RHA but also display defective p53 induction and diminished ability to induce senescence following DNA damage. Therefore, our findings reveal a novel mechanism of p53 inactivation that links deregulation of IRES-mediated p53 translation with tumorigenesis.O ne of the most important tumor suppressors identified thus far is p53. Under normal conditions, p53 is inactive, and its cellular levels are low. In response to various genotoxic or cytotoxic stress, including DNA damage, p53 becomes activated (1). The activated p53 causes cell growth arrest or apoptosis, allowing damaged cells to self-repair or be eliminated. In this manner, p53 protects against malignant transformation of normal cells (2, 3). Activation of p53 involves both accumulation and posttranslational modification. It is thought that the control of p53 induction or accumulation occurs mainly at the translational and posttranslational levels (1). Although levels of p53 are known to be regulated by the ubiquitin ligase mouse double minute 2 (MDM2), now there is clear evidence showing that enhanced p53 translation is essential for its induction following DNA damage (4). More specifically, the 5= untranslated region (5= UTR) of p53 mRNA was found to be a major site for regulation of p53 translation (5, 6). The mechanism underlying translational regulation of p53 induction via its 5= UTR has started to emerge.Cap-dependent initiation of protein translation is used by the majority of mRNAs, since almost all eukaryotic mRNAs have an N 7 -methylguanosine cap structure at their 5= ends. Eukaryotic translation initiation factor 4E (eIF-4E), a translation initiation protein that binds to the cap structure, initiates the process (7). In situations where cap-dependent translation is impaired, including apoptosis or DNA damage, cap-independent protein translation, mediated by an internal ribosomal entry site (IRES) that does not require the participation of eIF-4E, is needed in eukaryotes for the synthe...

Translation acrobatics: how cancer cells exploit alternate modes of translational initiation

“…In a recent study, a novel mechanism of TP53 inactivation in cancer cells was discovered. The study found that in various cancer cell lines, the IRES‐dependent translation of TP53 was inhibited due to reduced levels of ITAFs RHA and TCP80 .…”

“…In a recent study, a novel mechanism of TP53 inactivation in cancer cells was discovered. The study found that in various cancer cell lines, the IRES-dependent translation of TP53 was inhibited due to reduced levels of ITAFs RHA and TCP80 [230]. XIAP: One oncogene that is extensively studied for its tumor initiating properties is XIAP (X-linked inhibitor of apoptosis protein), a protein that inhibits apoptosis.…”

The biology of DHX9 and its potential as a therapeutic target

“…This stimulation is significantly enhanced upon exposure to DNA damage as a result of increased binding of TCP80 to the p53 IRES and improved interaction between DHX9 and TCP80. It is predicted that DHX9 likely helps unwind the p53 5′IRES, thereby promoting efficient translation [157, 158]. …”

“…Specifically, the breast cancer cell lines, ZR75-1 and MDA-MB-175, which express wild-type p53 but do not exhibit p53 induction following DNA damage, contain extremely low levels of both DHX9 and TCP80 compared to MCF-7 cells (which show a normal p53 response), and exhibit low p53-IRES activity when exposed to DNA damaging agents. IRES-mediated p53 translation was rescued by overexpression of DHX9 and TCP80 [158]. This shows that DHX9 levels can have a direct effect on the ability of p53 to suppress tumourigenesis.…”

Long non-coding RNA Gm15441 attenuates hepatic inflammasome activation in response to metabolic stress

“…An internal ribosome entry site (IRES) is an RNA element that allows for translation initiation (Lampe et al, 2018). Deregulation of IRES-mediated p53 translation promoted the defective p53 response against DNA damage in human cancer cells (Halaby et al, 2015). ER stress, serum deprivation, and hypoxia-induced human acetyl-CoA carboxylase 1 (hACC1) are controlled by IRES sequences (Damiano et al, 2018).…”