Core modification of substituted piperidines as Novel inhibitors of HDM2–p53 protein–protein interaction

The discovery of 3,3-disubstituted piperidine 1 as novel p53–HDM2 inhibitors prompted us to implement subsequent SAR follow up directed towards piperidine core modifications. Conformational restrictions and further functionalization of the piperidine core were investigated as a strategy to gain additional interactions with HDM2. Substitutions at positions 4, 5 and 6 of the piperidine ring were explored. Although some substitutions were tolerated, no significant improvement in potency was observed compared to 1. Incorporation of an allyl side chain at position 2 provided a drastic improvement in binding potency.     

Negative regulation of HDM2 to attenuate p53 degradation by ribosomal protein L26

HDM2 is a p53-specific E3 ubiquitin ligase. Its overexpression leads to excessive inactivation of tumor protein p53, diminishing its tumor suppressor function. HDM2 also affects the cell cycle, apoptosis and tumorigenesis through interacting with other molecules, including several ribosomal proteins. To identify novel HDM2 regulators, we performed a yeast two-hybrid screening using HDM2 as bait. Among the candidates, ribosomal protein L26 (RPL26) was characterized as a novel HDM2-interactor. The interaction between HDM2 and RPL26 was further validated by in vivo and in vitro assays. RPL26 modulates the HDM2–p53 interaction by forming a ternary complex among RPL26, HDM2 and p53, which stabilize p53 through inhibiting the ubiquitin ligase activity of HDM2. The ribosomal stress caused by a low dose of Act D enhances RPL26–HDM2 interaction and activates p53. Overexpression of RPL26 results in activating of p53, inhibits cell proliferation and induces a p53-dependent cell cycle arrest. These results provide a novel regulatory mechanism of RPL26 to activate p53 by inhibiting HDM2.     

p53 promotes its own polyubiquitination by enhancing the HDM2 and HDMX interaction

HDM2 and HDMX are two homologs essential for controlling p53 tumor suppressor activity under normal conditions. Both proteins bind different sites on the p53 N‐terminus, and while HDM2 has E3 ubiquitin ligase activity towards p53, HDMX does not. Nevertheless, HDMX is required for p53 polyubiquitination and degradation, but the underlying molecular mechanism remains unclear. Alone, HDMX and HDM2 interact via their respective C‐terminal RING domains but here we show that the presence of p53 induces an N‐terminal interface under normal cellular conditions. This results in an increase in HDM2‐mediated p53 polyubiquitination and degradation. The HDM2 inhibitor Nutlin‐3 binds the N‐terminal p53 binding pocket and is sufficient to induce the HDM2‐HDMX interaction, suggesting that the mechanism depends on allosteric changes that control the multiprotein complex formation. These results demonstrate an allosteric interchange between three different proteins (HDMX‐HDM2‐p53) and help to explain the molecular mechanisms of HDM2‐inhibitory drugs.     

Long isoform of ErbB3 binding protein, p48, mediates protein kinase B/Akt-dependent HDM2 stabilization and nuclear localization

p48 is a long isoform of the ErbB3 binding protein that has oncogenic functions including promotion of carcinogenesis and induction of malignant transformation through negative regulation of tumor suppressor p53. Here, we show that high level of p48 protein expression leads to enhance HDM2 phosphorylation by Akt and inhibits the self-ubiquitination of HDM2 by up-regulation of Akt activity, thereby promoting its protein stability. Moreover, p48 expression leads to accumulated nuclear localization of HDM2, whereas p48 depletion disturbs its nuclear localization. Hence, higher expression of p48 in cancer cells reduces p53 levels through modulation of HDM2 nuclear localization and protein stability via regulation of its Akt-mediated phosphorylation.     

Requirement for HDM2 activity in the rapid degradation of p53 in neuroblastoma

The wild type p53 tumor suppressor protein is rapidly degraded in normal cells by MDM2, the ubiquitin ligase that serves as the key regulator of p53 function by modulating protein stability. Cellular exposure to genotoxic stress triggers the stabilization of p53 by multiple pathways that converge upon interference with MDM2 function. In this study, we first investigated the ability of HDM2 (MDM2 human homologue) to degrade endogenous p53 in neuroblastoma (NB). Although the p53 protein in NB has been reported to be constitutively stabilized, we find that HDM2 in NB is functional and facilitates the rapid turnover of p53 in nonstressed cells via the proteasome pathway. Second, we examined the relationship between p53 and HDM2 in the adriamycin-mediated stabilization of p53 in NB. We demonstrate that while p53 stabilization depends neither upon the phosphorylation of specific N-terminal sites nor upon dissociation from HDM2, it requires inactivation of functional HDM2. In support of this notion, p53 stabilization following adriamycin resulted in an inhibition of both p53 ubiquitination and HDM2 ligase activity. Taken together, these data implicate a requirement for enzymatic inactivation of HDM2 as a novel mechanism for p53 stabilization in the DNA damage response pathway.     

A Spiroligomer α-Helix Mimic That Binds HDM2, Penetrates Human Cells and Stabilizes HDM2 in Cell Culture

We demonstrate functionalized spiroligomers that mimic the HDM2-bound conformation of the p53 activation domain. Spiroligomers are stereochemically defined, functionalized, spirocyclic monomers coupled through pairs of amide bonds to create spiro-ladder oligomers [1]. Two series of spiroligomers were synthesized, one of structural analogs and one of stereochemical analogs, from which we identified compound 1, that binds HDM2 with a Kd value of 400 nM. The spiroligomer 1 penetrates human liver cancer cells through passive diffusion and in a dose-dependent and time-dependent manner increases the levels of HDM2 more than 30-fold in Huh7 cells in which the p53/HDM2 negative feed-back loop is inoperative. This is a biological effect that is not seen with the HDM2 ligand nutlin-3a. We propose that compound 1 modulates the levels of HDM2 by stabilizing it to proteolysis, allowing it to accumulate in the absence of a p53/HDM2 feedback loop.     

Cryptic in vitro ubiquitin ligase activity of HDMX towards p53 is probably regulated by an induced fit mechanism

HDMX and its homologue HDM2 are two essential proteins for the cell; after genotoxic stress, both are phosphorylated near to their RING domain, specifically at serine 403 and 395, respectively. Once phosphorylated, both can bind the p53 mRNA and enhance its translation; however, both recognize p53 protein and provoke its degradation under normal conditions. HDM2 has been well-recognized as an E3 ubiquitin ligase, whereas it has been reported that even with the high similarity between the RING domains of the two homologs, HDMX does not have the E3 ligase activity. Despite this, HDMX is needed for the proper p53 poly-ubiquitination. Phosphorylation at serine 395 changes the conformation of HDM2, helping to explain the switch in its activity, but no information on HDMX has been published. Here, we study the conformation of HDMX and its phospho-mimetic mutant S403D, investigate its E3 ligase activity and dissect its binding with p53. We show that phospho-mutation does not change the conformation of the protein, but HDMX is indeed an E3 ubiquitin ligase in vitro; however, in vivo, no activity was found. We speculated that HDMX is regulated by induced fit, being able to switch activity accordingly to the specific partner as p53 protein, p53 mRNA or HDM2. Our results aim to contribute to the elucidation of the contribution of the HDMX to p53 regulation.     

Proteasome inhibitors suppress the protein expression of mutant p53

Tumor suppressor p53 is one of the most frequently mutated genes in cancer, with almost 50% of all types of cancer expressing a mutant form of p53. p53 transactivates the expression of its primary negative regulator, HDM2. HDM2 is a ubiquitin ligase, which initiates the proteasomal degradation of p53 following ubiquitination. Proteasome inhibitors, by targeting the ubiquitin proteasome pathway inhibit the degradation of the majority of cellular proteins including wild-type p53. In contrast, in this study we found that the protein expression of mutant p53 was suppressed following treatment with established or novel proteasome inhibitors. Furthermore, for the first time we demonstrated that Arsenic trioxide, which was previously shown to suppress mutant p53 protein level, exhibits proteasome inhibitory activity. Proteasome inhibitor-mediated suppression of mutant p53 was partially rescued by the knockdown of HDM2, suggesting that the stabilization of HDM2 by proteasome inhibitors might be responsible for mutant p53 suppression to some extent. This study suggests that suppression of mutant p53 is a general property of proteasome inhibitors and it provides additional rationale to use proteasome inhibitors for the treatment of tumors with mutant p53.     

An intact HDM2 RING-finger domain is required for nuclear exclusion of p53

The p53 tumour-suppressor protein is negatively regulated by HDM2. Recent reports indicate that the leucine-rich nuclear-export sequence (NES) of HDM2 enables it to shuttle to the cytoplasm, and that this activity is required for degradation of p53. However, it is unclear whether HDM2 is involved in nuclear export of p53, partly because p53 has itself been shown to contain a functional NES within its tetramerization domain. Here we show that co-expression of HDM2 with green fluorescent protein (GFP)-tagged p53 causes redistribution of p53 from the nucleus to the cytoplasm of the cell. This activity is dependent on binding of p53 to HDM2, and requires an intact p53 NES, but is independent of the HDM2 NES. A mutant of the HDM2 RING-finger domain that is unable to ubiquitinate p53 does not cause relocalization of p53, indicating that ubiquitin ligation or other activities of this region of HDM2 may be necessary for its regulation of p53 localization.     

2-(4-Chlorophenyl)-2-oxoethyl 4-benzamidobenzoate derivatives,a novel class of SENP1 inhibitors: Virtual screening,synthesis and biological evaluation

Prostate cancer is one of the most prevalent types of malignant cancers in men and has a high mortality rate among all male cancers. Previous studies have demonstrated that Sentrin/SUMO-specific protease 1 (SENP1) plays an important role in the occurrence and development of prostate cancer, and has been identified as a novel drug target for development of small molecule drugs against prostate cancer. In this paper, we used virtual screening and docking to identify compound J5 as a novel lead compound inhibiting SENP1, from SPECS library. We further investigated the SAR (structure–activity relationship) of the benzoate substituent of compound J5, and discovered compounds 8d and 8e as better small molecule inhibitors of SENP1. Both compounds are the high potent SENP1 small molecule inhibitors discovered up to date, and further lead optimization may lead to a series of novel anti-SENP1 agents. Further SAR studies are in process and will be reported in due course.     

Binding of Translationally Controlled Tumour Protein to the N-Terminal Domain of HDM2 Is Inhibited by Nutlin-3

Translationally Controlled Tumour Protein (TCTP), a highly conserved protein present in all eukaryotic organisms, has a number of intracellular and extracellular functions including an anti-apoptotic role. TCTP was recently shown to interact with both p53 and HDM2, inhibiting auto-ubiquitination of the latter and thereby promoting p53 degradation. In this study, we further investigated the interaction between TCTP and HDM2, mapping the reciprocal binding sites of TCTP and HDM2. TCTP primarily interacts with the N-terminal, p53-binding region of HDM2 through its highly basic domain 2. Furthermore, we discovered that Nutlin-3, a small molecule known to promote apoptosis and cell cycle arrest by blocking binding between HDM2 and p53, has a similar inhibitory effect on the interaction of HDM2 and TCTP. This result may provide an additional explanation of how Nutlin-derived compounds currently in clinical trials function to promote apoptosis in cancer cells.     

Synthesis and activity evaluation of benzoylurea derivatives as potential antiproliferative agents

3-Haloacylamino benzoylureas (3-HBUs) consist of a new family of tubulin ligands that kill cancer cells through mitotic arrest. In exploring the structure–activity relationship (SAR), 17 analogues defined through variations of formylurea at the 1-position of the aromatic ring were synthesized. SAR analysis revealed that (i) the p–π conjugation between the aromatic ring and formylurea was essential; (ii) suitable aryl substitutions at the N′-end increased anticancer activity with a mechanism different from that of parent compounds; and (iii) introduction of pyridyl at the N′-end provided an opportunity of making soluble salts to improve bioavailability. Among the analogues, 16c bearing 3,4,5-trimethoxyphenyl and 16g bearing 2-pyridyl at the N′-end showed an enhanced activity and were active in hepatoma cells that were resistant to tubulin ligands including the parent compounds. Furthermore, 16c and 16g killed cancer cells with a mechanism independent of mitotic arrest, indicating a change of action mode.     

Ribosomal protein L11 negatively regulates oncoprotein MDM2 and mediates a p53-dependent ribosomal-stress checkpoint pathway

The gene encoding p53 mediates a major tumor suppression pathway that is frequently altered in human cancers. p53 function is kept at a low level during normal cell growth and is activated in response to various cellular stresses. The MDM2 oncoprotein plays a key role in negatively regulating p53 activity by either direct repression of p53 transactivation activity in the nucleus or promotion of p53 degradation in the cytoplasm. DNA damage and oncogenic insults, the two best-characterized p53-dependent checkpoint pathways, both activate p53 through inhibition of MDM2. Here we report that the human homologue of MDM2, HDM2, binds to ribosomal protein L11. L11 binds a central region in HDM2 that is distinct from the ARF binding site. We show that the functional consequence of L11-HDM2 association, like that with ARF, results in the prevention of HDM2-mediated p53 ubiquitination and degradation, subsequently restoring p53-mediated transactivation, accumulating p21 protein levels, and inducing a p53-dependent cell cycle arrest by canceling the inhibitory function of HDM2. Interference with ribosomal biogenesis by a low concentration of actinomycin D is associated with an increased L11-HDM2 interaction and subsequent p53 stabilization. We suggest that L11 functions as a negative regulator of HDM2 and that there might exist in vivo an L11-HDM2-p53 pathway for monitoring ribosomal integrity.     

Design and synthesis of new substituted spirooxindoles as potential inhibitors of the MDM2–p53 interaction

The designed compounds, 4a–p, were synthesized using a simple and smooth method with an asymmetric 1,3-dipolar reaction as the key step. The chemical structures for all synthesized compounds were elucidated and confirmed by spectral analysis. The molecular complexity and the absolute stereochemistry of 4b and 4e designed analogs were determined by X-ray crystallographic analysis. The anticancer activities of the synthesized compounds were tested against colon (HCT-116), prostate (PC-3), and hepatocellular (HepG-2) cancer cell lines. Molecular modeling revealed that the compound 4d binds through hydrophobic–hydrophobic interactions with the essential amino acids (LEU: 57, GLY: 58, ILE: 61, and HIS: 96) in the p53-binding cleft, as a standard p53-MDM2 inhibitor (6SJ). The mechanism underlying the anticancer activity of compound 4d was further evaluated, and the study showed that compound 4d inhibited colony formation, cell migration, arrested cancer cell growth at G2/M, and induced apoptosis through intrinsic and extrinsic pathways. Transactivation of p53 was confirmed by flow cytometry, where compound 4d increased the level of activated p53 and induced mRNA levels of cell cycle inhibitor, p21.     

Renal cell carcinoma escapes death by p53 depletion through transglutaminase 2-chaperoned autophagy

In renal cell carcinoma, transglutaminase 2 (TGase 2) crosslinks p53 in autophagosomes, resulting in p53 depletion and the tumor''s evasion of apoptosis. Inhibition of TGase 2 stabilizes p53 and induces tumor cells to enter apoptosis. This study explored the mechanism of TGase 2-dependent p53 degradation. We found that TGase 2 competes with human double minute 2 homolog (HDM2) for binding to p53; promotes autophagy-dependent p53 degradation in renal cell carcinoma (RCC) cell lines under starvation; and binds to p53 and p62 simultaneously without ubiquitin-dependent recognition of p62. The bound complex does not have crosslinking activity. A binding assay using a series of deletion mutants of p62, p53 and TGase 2 revealed that the PB1 (Phox and Bem1p-1) domain of p62 (residues 85–110) directly interacts with the β-barrel domains of TGase 2 (residues 592–687), whereas the HDM2-binding domain (transactivation domain, residues 15–26) of p53 interacts with the N terminus of TGase 2 (residues 1–139). In addition to the increase in p53 stability due to TGase 2 inhibition, the administration of a DNA-damaging anti-cancer drug such as doxorubicin-induced apoptosis in RCC cell lines and synergistically reduced tumor volume in a xenograft model. Combination therapy with a TGase 2 inhibitor and a DNA-damaging agent may represent an effective therapeutic approach for treating RCC.Transglutaminase 2 (TGase 2, E.C. 2.1.2.13) is an enzyme that catalyzes an isopeptide bond between protein glutamine and lysine residues, resulting in a covalent crosslink.¹ Previously, we found that, in renal cell carcinoma (RCC) cell lines, TGase 2 crosslinks p53 into aggregates in the autophagosome, resulting in p53 degradation by autophagy.² This p53 instability allows tumor cells to evade apoptosis and grow. Instability of p53 in RCC is not associated with mutations because only 4% of samples of clear cell RCC in the COSMIC database present p53 mutations (Supplementary Figure 1). We recently reported that monotherapy using the TGase 2 inhibitor GK921 in a xenograft tumor model abrogated RCC growth through p53 stabilization.³ These results suggest the possibility of using TGase 2 inhibitors as a cancer therapy.³p53 Regulation by human double minute 2 homolog (HDM2) involves proteasomal degradation through ubiquitination, whereas TGase 2-mediated p53 regulation involves autophagosome degradation. However, how TGase 2 depletes p53 in the context of the central regulation system of p53 under HDM2 surveillance is unknown. The half-life of p53 is very short due to its interaction with HDM2. HDM2 depletes p53 by binding to a region of its transactivation domain,⁴ which facilitates rapid turnover through ubiquitination.⁵ DNA damage-induced phosphorylation of the N terminus of p53 blocks its interaction with HDM2,⁶ which stabilizes p53 by inhibiting proteasome-mediated degradation.⁷In this study, we found that silencing TGM2, the gene encoding TGase 2, in RCC cell lines induced cell death under starvation conditions through p53 stabilization to the same extent as did silencing of HDM2. This result suggests that the general instability of p53 in RCC largely depends on TGase 2-mediated autophagy. In fact, p53 depletion induced by TGase 2 has no energy cost to cancer cells because TGase 2 only uses calcium to catalyze covalent crosslinks. However, the role of TGase 2 in autophagy is not clearly understood in cancer, although it is known that TGase 2 knockout mice present impaired autophagy, which increases ubiquitinated protein aggregates upon starvation and deceases p62-dependent peroxisome degradation.⁸ The binding mechanism between TGase 2 and p62 is unknown, although TGase 2 is able to bind directly with p62, which often localizes in the autophagosome.^{2, 8} p62 (also known as sequestosome 1) is an adapter protein in the degradation of ubiquitinated proteins in autophagosomes through interaction with microtubule-associated protein 1 light chain 3 alpha (LC3).⁹ The N terminus of p62 is a PB1 (Phox and Bem1p-1) domain that binds to the PB1 region in the atypical protein kinase C, the MAPK kinase, and the NBR1 (neighbor of BRCA1 gene) protein.¹⁰ The C terminus of p62 harbors a ubiquitin-associated (UBA) domain¹¹ that interacts with ubiquitin and polyubiquitin chains, and an LC3-interacting region domain that interacts with LC3 in phagophore membranes.¹² The part of the p53–TGase 2 complex that binds to p62 is unknown, but could be a part of either p53 or TGase 2. We observed that p53 does not bind to p62 directly but is transferred to p62 through association with TGase 2.Here, we explored the possibility that TGase 2 is a chaperone in autophagy by analyzing the domains of p53 and p62 that bind to TGase 2 as well as the domain of TGase 2 that binds to p62. Based on the finding that TGase 2 inhibition increases p53 stability, we also tested the possibility that DNA-damaging reagents such as doxorubicin expand the damage of p53-mediated cell death when combined with TGase 2 inhibition in a preclinical model of RCC.