Novel Allosteric Mechanism of Dual p53/MDM2 and p53/MDM4 Inhibition by a Small Molecule

Restoration of the p53 tumor suppressor for personalised cancer therapy is a promising treatment strategy. However, several high-affinity MDM2 inhibitors have shown substantial side effects in clinical trials. Thus, elucidation of the molecular mechanisms of action of p53 reactivating molecules with alternative functional principle is of the utmost importance. Here, we report a discovery of a novel allosteric mechanism of p53 reactivation through targeting the p53 N-terminus which promotes inhibition of both p53/MDM2 (murine double minute 2) and p53/MDM4 interactions. Using biochemical assays and molecular docking, we identified the binding site of two p53 reactivating molecules, RITA (reactivation of p53 and induction of tumor cell apoptosis) and protoporphyrin IX (PpIX). Ion mobility-mass spectrometry revealed that the binding of RITA to serine 33 and serine 37 is responsible for inducing the allosteric shift in p53, which shields the MDM2 binding residues of p53 and prevents its interactions with MDM2 and MDM4. Our results point to an alternative mechanism of blocking p53 interaction with MDM2 and MDM4 and may pave the way for the development of novel allosteric inhibitors of p53/MDM2 and p53/MDM4 interactions.

The scaffold protein WRAP53β orchestrates the ubiquitin response critical for DNA double-strand break repair

The WD40 domain-containing protein WRAP53β controls trafficking of splicing factors and telomerase to Cajal bodies. Here, Henriksson et al. demonstrate that WRAP53β rapidly localizes to double-strand breaks (DSBs) in an ATM-, H2AX-, and MDC1-dependent manner. WRAP53β targets the E3 ligase RNF8 to DNA lesions by facilitating the interaction between RNF8 and its upstream partner, MDC1, in response to DNA damage. Loss of WRAP53β impairs DSB repair by both homologous recombination and nonhomologous end-joining, causes accumulation of spontaneous DNA breaks, and delays recovery from radiation-induced cell cycle arrest.

The WD40 domain-containing protein WRAP53β (WD40 encoding RNA antisense to p53; also referred to as WDR79/TCAB1) controls trafficking of splicing factors and the telomerase enzyme to Cajal bodies, and its functional loss has been linked to carcinogenesis, premature aging, and neurodegeneration. Here, we identify WRAP53β as an essential regulator of DNA double-strand break (DSB) repair. WRAP53β rapidly localizes to DSBs in an ATM-, H2AX-, and MDC1-dependent manner. We show that WRAP53β targets the E3 ligase RNF8 to DNA lesions by facilitating the interaction between RNF8 and its upstream partner, MDC1, in response to DNA damage. Simultaneous binding of MDC1 and RNF8 to the highly conserved WD40 scaffold domain of WRAP53β facilitates their interaction and accumulation of RNF8 at DSBs. In this manner, WRAP53β controls proper ubiquitylation at DNA damage sites and the downstream assembly of 53BP1, BRCA1, and RAD51. Furthermore, we reveal that knockdown of WRAP53β impairs DSB repair by both homologous recombination (HR) and nonhomologous end-joining (NHEJ), causes accumulation of spontaneous DNA breaks, and delays recovery from radiation-induced cell cycle arrest. Our findings establish WRAP53β as a novel regulator of DSB repair by providing a scaffold for DNA repair factors.

The conserved Trp114 residue of thioredoxin reductase 1 has a redox sensor-like function triggering oligomerization and crosslinking upon oxidative stress related to cell death

The selenoprotein thioredoxin reductase 1 (TrxR1) has several key roles in cellular redox systems and reductive pathways. Here we discovered that an evolutionarily conserved and surface-exposed tryptophan residue of the enzyme (Trp114) is excessively reactive to oxidation and exerts regulatory functions. The results indicate that it serves as an electron relay communicating with the FAD moiety of the enzyme, and, when oxidized, it facilitates oligomerization of TrxR1 into tetramers and higher multimers of dimers. A covalent link can also be formed between two oxidized Trp114 residues of two subunits from two separate TrxR1 dimers, as found both in cell extracts and in a crystal structure of tetrameric TrxR1. Formation of covalently linked TrxR1 subunits became exaggerated in cells on treatment with the pro-oxidant p53-reactivating anticancer compound RITA, in direct correlation with triggering of a cell death that could be prevented by antioxidant treatment. These results collectively suggest that Trp114 of TrxR1 serves a function reminiscent of an irreversible sensor for excessive oxidation, thereby presenting a previously unrecognized level of regulation of TrxR1 function in relation to cellular redox state and cell death induction.

ROS-dependent activation of JNK converts p53 into an efficient inhibitor of oncogenes leading to robust apoptosis

Rescue of the p53 tumor suppressor is an attractive cancer therapy approach. However, pharmacologically activated p53 can induce diverse responses ranging from cell death to growth arrest and DNA repair, which limits the efficient application of p53-reactivating drugs in clinic. Elucidation of the molecular mechanisms defining the biological outcome upon p53 activation remains a grand challenge in the p53 field. Here, we report that concurrent pharmacological activation of p53 and inhibition of thioredoxin reductase followed by generation of reactive oxygen species (ROS), result in the synthetic lethality in cancer cells. ROS promote the activation of c-Jun N-terminal kinase (JNK) and DNA damage response, which establishes a positive feedback loop with p53. This converts the p53-induced growth arrest/senescence to apoptosis. We identified several survival oncogenes inhibited by p53 in JNK-dependent manner, including Mcl1, PI3K, eIF4E, as well as p53 inhibitors Wip1 and MdmX. Further, we show that Wip1 is one of the crucial executors downstream of JNK whose ablation confers the enhanced and sustained p53 transcriptional response contributing to cell death. Our study provides novel insights for manipulating p53 response in a controlled way. Further, our results may enable new pharmacological strategy to exploit abnormally high ROS level, often linked with higher aggressiveness in cancer, to selectively kill cancer cells upon pharmacological reactivation of p53.

Dual targeting of wild-type and mutant p53 by small molecule RITA results in the inhibition of N-Myc and key survival oncogenes and kills neuroblastoma cells in vivo and in vitro

PURPOSE

Restoration of the p53 function in tumors is a promising therapeutic strategy due to the high potential of p53 as tumor suppressor and the fact that established tumors depend on p53 inactivation for their survival. Here, we addressed the question whether small molecule RITA can reactivate p53 in neuroblastoma and suppress the growth of neuroblastoma cells in vitro and in vivo.

EXPERIMENTAL DESIGN

The ability of RITA to inhibit growth and to induce apoptosis was shown in seven neuroblastoma cell lines. Mechanistic studies were carried out to determine the p53 dependence and the molecular mechanism of RITA-induced apoptosis in neuroblastoma, using cell viability assays, RNAi silencing, co-immunoprecipitation, qPCR, and Western blotting analysis. In vivo experiments were conducted to study the effect of RITA on human neuroblastoma xenografts in mice.

RESULTS

RITA induced p53-dependent apoptosis in a set of seven neuroblastoma cell lines, carrying wild-type or mutant p53; it activated p53 and triggered the expression of proapoptotic p53 target genes. Importantly, p53 activated by RITA inhibited several key oncogenes that are high-priority targets for pharmacologic anticancer strategies in neuroblastoma, including N-Myc, Aurora kinase, Mcl-1, Bcl-2, Wip-1, MDM2, and MDMX. Moreover, RITA had a strong antitumor effect in vivo.

CONCLUSIONS

Reactivation of wild-type and mutant p53 resulting in the induction of proapoptotic factors along with ablation of key oncogenes by compounds such as RITA may be a highly effective strategy to treat neuroblastoma.

p53-dependent inhibition of TrxR1 contributes to the tumor-specific induction of apoptosis by RITA

Thioredoxin reductase 1 (TrxR1) is a key regulator in many redox-dependent cellular pathways, and is often overexpressed in cancer. Several studies have identified TrxR1 as a potentially important target for anticancer therapy. The low molecular weight compound RITA (NSC 652287) binds p53 and induces p53-dependent apoptosis. Here we found that RITA also targets TrxR1 by non-covalent binding, followed by inhibition of its activity in vitro and by inhibition of TrxR activity in cancer cells. Interestingly, a novel approximately 130 kDa form of TrxR1, presumably representing a stable covalently linked dimer, and an increased generation of reactive oxygen species (ROS) were induced by RITA in cancer cells in a p53-dependent manner. Similarly, the gold-based TrxR inhibitor auranofin induced apoptosis related to oxidative stress, but independently of p53 and without apparent induction of the approximately 130 kDa form of TrxR1. In contrast to the effects observed in cancer cells, RITA did not inhibit TrxR or ROS formation in normal fibroblasts (NHDF). The inhibition of TrxR1 can sensitize tumor cells to agents that induce oxidative stress and may directly trigger cell death. Thus, our results suggest that a unique p53-dependent effect of RITA on TrxR1 in cancer cells might synergize with p53-dependent induction of pro-apoptotic genes and oxidative stress, thereby leading to a robust induction of cancer cell death, without affecting non-transformed cells.

MDM2-dependent downregulation of p21 and hnRNP K provides a switch between apoptosis and growth arrest induced by pharmacologically activated p53

We have previously identified the p53-reactivating compound RITA in a cell-based screen. Here, using microarray analysis, we show that the global transcriptional response of tumor cells to RITA is p53 dependent. Pathway analysis revealed induction of the p53 apoptosis pathway, consistent with apoptosis being the major response to RITA in cancer cells. We uncovered that MDM2 released from p53 by RITA promotes degradation of p21 and the p53 cofactor hnRNP K, required for p21 transcription. Functional studies revealed MDM2-dependent inhibition of p21 as a key switch regulating cell fate decisions upon p53 reactivation. Our results emphasize the utility of targeting wild-type p53 protein itself as a promising approach for anticancer therapy.

Tumor-specific induction of apoptosis by a p53-reactivating compound

The tumor suppressor function of p53 is disabled in the majority of tumors, either by a point mutation of the p53 gene, or via MDM2-dependent proteasomal degradation. We have screened a chemical library using a cell-based assay and identified a low molecular weight compound named MITA which induced wild-type p53-dependent cell death in a variety of different types of human tumor cells, such as lung, colon and breast carcinoma cells, as well as in osteosarcoma and fibrosarcoma-derived cells. MITA inhibited p53-MDM2 interaction in vitro and in cells, which in turn prevented MDM2-mediated ubiquitination of p53 and resulted in a prolonged half-life and accumulation of p53 in tumor cells. Notably, p53 induction by MITA resulted in upregulated expression of p53 target genes MDM2, Bax, Gadd45 and PUMA, on protein and mRNA level. Importantly, neither p53 nor these target genes were induced in normal human fibroblasts (HDFs), which correlated with the absence of growth suppression in fibroblasts after treatment with MITA. However, upon activation of oncogenes in fibroblasts an induction and activation of p53 was observed, suggesting that activation of p53 by MITA occurs predominantly in tumor cells.