p53-mediated suppression of BiP triggers BIK-induced apoptosis during prolonged endoplasmic reticulum stress.

Physiological and pathological conditions that affect the folding capacity of the endoplasmic reticulum (ER) provoke ER stress and trigger the unfolded protein response (UPR). The UPR aims to either restore the balance between newly synthesized and misfolded proteins or if the damage is severe, to trigger cell death. However, the molecular events underlying the switch between repair and cell death are not well understood. The ER-resident chaperone BiP governs the UPR by sensing misfolded proteins and thereby releasing and activating the three mediators of the UPR: PERK, IRE1 and ATF6. PERK promotes G2 cell cycle arrest and cellular repair by inducing the alternative translated p53 isoform p53ΔN40 (p53/47), which activates 14-3-3σ via suppression of p21CDKN1A. Here we show that prolonged ER stress promotes apoptosis via a p53-dependent inhibition of BiP expression. This leads to the release of the pro-apoptotic BH3-only BIK from BiP and activation of apoptosis. Suppression of bip mRNA translation is mediated via the specific binding of p53 to the first 346-nt of the bip mRNA and via a p53 trans-suppression domain located within the first seven N-terminal amino acids of p53ΔN40. This work shows how p53 targets BiP to promote apoptosis during severe ER stress and further illustrates how regulation of mRNA translation has a key role in p53-mediated regulation of gene expression during the UPR.

p53-mediated control of gene expression via mRNA translation during Endoplasmic Reticulum stress.

p53 is activated by different stress and damage pathways and regulates cell biological responses including cell cycle arrest, repair pathways, apoptosis and senescence. Following DNA damage, the levels of p53 increase and via binding to target gene promoters, p53 induces expression of multiple genes including p21(CDKN1A) and mdm2. The effects of p53 on gene expression during the DNA damage response are well mimicked by overexpressing p53 under normal conditions. However, stress to the Endoplasmic Reticulum (ER) and the consequent Unfolded Protein Response (UPR) leads to the induction of the p53/47 isoform that lacks the first 40 aa of p53 and to an active suppression of p21(CDKN1A) transcription and mRNA translation. We now show that during ER stress p53 also suppresses MDM2 protein levels via a similar mechanism. These observations not only raise questions about the physiological role of MDM2 during ER stress but it also reveals a new facet of p53 as a repressor toward 2 of its major target genes during the UPR. As suppression of p21(CDKN1A) and MDM2 protein synthesis is mediated via their coding sequences, it raises the possibility that p53 controls mRNA translation via a common mechanism that might play an important role in how p53 regulates gene expression during the UPR, as compared to the transcription-dependent gene regulation taking place during the DNA damage response.

The alternative translated MDMX(p60) isoform regulates MDM2 activity.

Isoforms derived from alternative splicing, mRNA translation initiation or promoter usage extend the functional repertoire of the p53, p63 and p73 genes family and of their regulators MDM2 and MDMX. Here we show cap-independent translation of an N-terminal truncated isoform of hMDMX, hMDMX(p60), which is initiated at the 7th AUG codon downstream of the initiation site for full length hMDMX(FL) at position +384. hMDMX(p60) lacks the p53 binding motif but retains the RING domain and interacts with hMDM2 and hMDMX(FL). hMDMX(p60) shows higher affinity for hMDM2, as compared to hMDMX(FL). In vitro data reveal a positive cooperative interaction between hMDMX(p60) and hMDM2 and in cellulo data show that low levels of hMDMX(p60) promote degradation of hMDM2 whereas higher levels stabilize hMDM2 and prevent hMDM2-mediated degradation of hMDMX(FL). These results describe a novel alternatively translated hMDMX isoform that exhibits unique regulatory activity toward hMDM2 autoubiquitination. The data illustrate how the N-terminus of hMDMX regulates its C-terminal RING domain and the hMDM2 activity.

HDMX folds the nascent p53 mRNA following activation by the ATM kinase.

Regulated protein synthesis via changes in mRNA structures forms an important part of how prokaryotic cells adapt protein expression in response to changes in the environment. Little is known regarding how this concept has adapted to regulate mRNA translation via signaling pathways in mammalian cells. Here, we show that following phosphorylation by the ataxia telangiectasia mutated (ATM) kinase at serine 403, the C-terminal RING domain of HDMX binds the nascent p53 mRNA to promote a conformation that supports the p53 mRNA-HDM2 interaction and the induction of p53 synthesis. HDMX and its homolog HDM2 bind the same p53 internal ribosome entry sequences (IRES) structure but with different specificity and function. The results show how HDMX and HDM2 act as nonredundant IRES trans-acting factors (ITAFs) to bring a positive synergistic effect on p53 expression during genotoxic stress by first altering the structure of the newly synthesized p53 mRNA followed by stimulation of translation.