Nanoencapsulation of MDM2 Inhibitor RG7388 and Class-I HDAC Inhibitor Entinostat Enhances their Therapeutic Potential Through Synergistic Antitumor Effects and Reduction of Systemic Toxicity

Inhibitors of the p53-MDM2 interaction such as RG7388 have been developed to exploit latent tumor suppressive properties in p53 in 50% of tumors in which p53 is wild-type. However, these agents for the most part activate cell cycle arrest rather than death, and high doses in patients elicit on-target dose-limiting neutropenia. Recent work from our group indicates that combination of p53-MDM2 inhibitors with the class-I HDAC inhibitor Entinostat (which itself has dose-limiting toxicity issues) has the potential to significantly augment cell death in p53 wild-type colorectal cancer cells. We investigated whether coencapsulation of RG7388 and Entinostat within polymeric nanoparticles (NPs) could overcome efficacy and toxicity limitations of this drug combination. Combinations of RG7388 and Entinostat across a range of different molar ratios resulted in synergistic increases in cell death when delivered in both free drug and nanoencapsulated formats in all colorectal cell lines tested. Importantly, we also explored the in vivo impact of the drug combination on murine blood leukocytes, showing that the leukopenia induced by the free drugs could be significantly mitigated by nanoencapsulation. Taken together, this study demonstrates that formulating these agents within a single nanoparticle delivery platform may provide clinical utility beyond use as nonencapsulated agents.

Interruption of p53-MDM2 Interaction by Nutlin-3a in Human Lymphoma Cell Models Initiates a Cell-Dependent Global Effect on Transcriptome and Proteome Level

In most lymphomas, p53 signaling pathway is inactivated by various mechanisms independent to p53 gene mutations or deletions. In many cases, p53 function is largely regulated by alterations in the protein abundance levels by the action of E3 ubiquitin-protein ligase MDM2, targeting p53 to proteasome-mediated degradation. In the present study, an integrating transcriptomics and proteomics analysis was employed to investigate the effect of p53 activation by a small-molecule MDM2-antagonist, nutlin-3a, on three lymphoma cell models following p53 activation. Our analysis revealed a system-wide nutlin-3a-associated effect in all examined lymphoma types, identifying in total of 4037 differentially affected proteins involved in a plethora of pathways, with significant heterogeneity among lymphomas. Our findings include known p53-targets and novel p53 activation effects, involving transcription, translation, or degradation of protein components of pathways, such as a decrease in key members of PI3K/mTOR pathway, heat-shock response, and glycolysis, and an increase in key members of oxidative phoshosphorylation, autophagy and mitochondrial translation. Combined inhibition of HSP90 or PI3K/mTOR pathway with nutlin-3a-mediated p53-activation enhanced the apoptotic effects suggesting a promising strategy against human lymphomas. Integrated omic profiling after p53 activation offered novel insights on the regulatory role specific proteins and pathways may have in lymphomagenesis.

p52-ZER6: a determinant of tumor cell sensitivity to MDM2-p53 binding inhibitors

Targeting MDM2-p53 interaction has emerged as a promising antitumor therapeutic strategy. Several MDM2-p53 inhibitors have advanced into clinical trials, but results are not favorable. The lack of appropriate biomarkers for selecting patients has been assumed as the critical reason for this failure. We previously identified ZER6 isoform p52-ZER6 as an oncogene upregulated in tumor tissues. In this study we investigated whether p52-ZER6 acted as a blocker of MDM2-p53 binding inhibitors, and whether p52-ZER6 could be used as a biomarker of MDM2-p53 binding inhibitors. In p53 wild-type colorectal carcinoma HCT116, hepatocarcinoma HepG2 and breast cancer MCF-7 cells, overexpression of p52-ZER6 enhanced MDM2-p53 binding and promoted p53 ubiquitination/proteasomal degradation. Furthermore, overexpression of p52-ZER6 in the tumor cells dose-dependently reduced their sensitivity to both nutlin and non-nutlin class MDM2-p53 binding inhibitors. We showed that p52-ZER6 restored tumor cell viability, which was suppressed by nutlin-3, through restoring their proliferation potential while suppressing their apoptotic rate, suggesting that MDM2-p53 binding inhibitors might not be effective for patients with high p52-ZER6 levels. We found that nutlin-3 treatment or p52-ZER6 knockdown alone promoted the accumulation of p53 protein in the tumor cells, and their combinatorial treatment significantly increased the accumulation of p53 protein. In HCT116 cell xenograft nude mouse model, administration of shp52-ZER6 combined with an MDM2-p53 binding inhibitor nutlin-3 exerted synergistic antitumor response. In conclusion, this study reveals that p52-ZER6 might be a potential biomarker for determining patients appropriate for MDM2-p53 binding inhibition-based antitumor therapy, and demonstrates the potential of combinatorial therapy using MDM2-p53 binding inhibitors and p52-ZER6 inhibition.

Helping the Released Guardian: Drug Combinations for Supporting the Anticancer Activity of HDM2 (MDM2) Antagonists

The protein p53, known as the “Guardian of the Genome”, plays an important role in maintaining DNA integrity, providing protection against cancer-promoting mutations. Dysfunction of p53 is observed in almost every cancer, with 50% of cases bearing loss-of-function mutations/deletions in the TP53 gene. In the remaining 50% of cases the overexpression of HDM2 (mouse double minute 2, human homolog) protein, which is a natural inhibitor of p53, is the most common way of keeping p53 inactive. Disruption of HDM2-p53 interaction with the use of HDM2 antagonists leads to the release of p53 and expression of its target genes, engaged in the induction of cell cycle arrest, DNA repair, senescence, and apoptosis. The induction of apoptosis, however, is restricted to only a handful of p53^wt cells, and, generally, cancer cells treated with HDM2 antagonists are not efficiently eliminated. For this reason, HDM2 antagonists were tested in combinations with multiple other therapeutics in a search for synergy that would enhance the cancer eradication. This manuscript aims at reviewing the recent progress in developing strategies of combined cancer treatment with the use of HDM2 antagonists.

The histone demethylase JMJD2B is critical for p53-mediated autophagy and survival in Nutlin-treated cancer cells

Autophagy promotes cancer cell survival in response to p53 activation by the anticancer agent Nutlin-3a (Nutlin). We reported previously that Nutlin kills MDM2-amplified cancer cells and that this killing is associated with an inhibition of glucose metabolism, reduced α-ketoglutarate (α-KG) levels, and reduced autophagy. In the current report, using SJSA1, U2OS, A549, and MHM cells, we found that Nutlin alters histone methylation in an MDM2 proto-oncogene-dependent manner and that this, in turn, regulates autophagy-related gene (ATG) expression and cell death. In MDM2-amplified cells, Nutlin increased histone (H) 3 lysine (K) 9 and K36 trimethylation (me3) coincident with reduced autophagy and increased apoptosis. Blocking histone methylation restored autophagy and rescued these cells from Nutlin-induced killing. In MDM2-nonamplified cells, H3K9me3 and H3K36me3 levels were either reduced or not changed by the Nutlin treatment, and this coincided with increased autophagy and cell survival. Blocking histone demethylation reduced autophagy and sensitized these cells to Nutlin-induced killing. Further experiments suggested that MDM2 amplification increases histone methylation in Nutlin-treated cells by causing depletion of the histone demethylase Jumonji domain-containing protein 2B (JMJD2B). Finally, JMJD2B knockdown or inhibition increased H3K9/K36me3 levels, decreased ATG gene expression and autophagy, and sensitized MDM2-nonamplified cells to apoptosis. Together, these results support a model in which MDM2- and JMJD2B-regulated histone methylation levels modulate ATG gene expression, autophagy, and cell fate in response to the MDM2 antagonist Nutlin-3a.

MDM2-Recruiting PROTAC Offers Superior, Synergistic Antiproliferative Activity via Simultaneous Degradation of BRD4 and Stabilization of p53

Although the number of proteins effectively targeted for posttranslational degradation by PROTAC has grown steadily, the number of E3 ligases successfully exploited to accomplish this has been limited to the few for which small-molecule ligands have been discovered. Although the E3 ligase MDM2 is bound by the nutlin class of small-molecule ligands, there are few nutlin-based PROTAC. Because a nutlin-based PROTAC should both knockdown its target protein and upregulate the tumor suppressor p53, we examined the ability of such a PROTAC to decrease cancer cell viability. A nutlin-based, BRD4-degrading PROTAC, A1874, was able to degrade its target protein by 98% with nanomolar potency. Given the complementary ability of A1874 to stabilize p53, we discovered that the nutlin-based PROTAC was more effective in inhibiting proliferation of many cancer cell lines with wild-type p53 than was a corresponding VHL-utilizing PROTAC with similar potency and efficacy to degrade BRD4. This is the first report of a PROTAC in which the E3 ligase ligand and targeting warhead combine to exert a synergistic antiproliferative effect. Our study highlights the untapped potential that may be unlocked by expanding the repertoire of E3 ligases that can be recruited by PROTAC. SIGNIFICANCE: These findings present the first BRD4-targeting MDM2-based PROTAC that possesses potent, distinct, and synergistic biological activities associated with both ends of this heterobifunctional molecule.

p53 promotes AKT and SP1-dependent metabolism through the pentose phosphate pathway that inhibits apoptosis in response to Nutlin-3a

Nutlin-3a is a MDM2 antagonist and preclinical drug that activates p53. Cells with MDM2 gene amplification are especially prone to Nutlin-3a-induced apoptosis, though the basis for this is unclear. Glucose metabolism can inhibit apoptosis in response to Nutlin-3a through mechanisms that are incompletely understood. Glucose metabolism through the pentose phosphate pathway (PPP) produces NADPH that can protect cells from potentially lethal reactive oxygen species (ROS). We compared apoptosis and glucose metabolism in cancer cells with and without MDM2 gene amplification treated with Nutlin-3a. Apoptosis in MDM2-amplified cells was associated with a reduction in glycolysis and the PPP, reduced NADPH, increased ROS, and depletion of the transcription factor SP1, which normally promotes PPP gene expression. In contrast, glycolysis and the PPP were maintained or increased in MDM2 non-amplified cells treated with Nutlin-3a. This was dependent on p53-mediated AKT activation and was associated with maintenance of SP1 and continued expression of PPP genes. Knockdown or inhibition of AKT, SP1, or the PPP sensitized MDM2-non-amplified cells to apoptosis. The data indicate that p53 promotes AKT and SP1-dependent activation of the PPP that protects cells from Nutlin-3a-induced apoptosis. These findings provide insight into how glucose metabolism reduces Nutlin-3a-induced apoptosis, and also provide a mechanism for the heightened sensitivity of MDM2-amplified cells to apoptosis in response to Nutlin-3a.

Autophagy Inhibition Mediates Apoptosis Sensitization in Cancer Therapy by Relieving FOXO3a Turnover

Macroautophagy (autophagy) is intimately linked with cell death and allows cells to evade apoptosis. This has prompted clinical trials to combine autophagy inhibitors with other drugs with the aim of increasing the likelihood of cancer cells dying. However, the molecular basis for such effects is unknown. Here, we describe a transcriptional mechanism that connects autophagy to apoptosis. The autophagy-regulating transcription factor, FOXO3a, is itself turned over by basal autophagy creating a potential feedback loop. Increased FOXO3a upon autophagy inhibition stimulates transcription of the pro-apoptotic BBC3/PUMA gene to cause apoptosis sensitization. This mechanism explains how autophagy inhibition can sensitize tumor cells to chemotherapy drugs and allows an autophagy inhibitor to change the action of an MDM2-targeted drug from growth inhibition to apoptosis, reducing tumor burden in vivo. Thus, a link between two processes mediated via a single transcription factor binding site in the genome can be leveraged to improve anti-cancer therapies.

A Kinase-Independent Role for Cyclin-Dependent Kinase 19 in p53 Response

The human Mediator complex regulates RNA polymerase II transcription genome-wide. A general factor that regulates Mediator function is the four-subunit kinase module, which contains either cyclin-dependent kinase 8 (CDK8) or CDK19. Whereas CDK8 is linked to specific signaling cascades and oncogenesis, the cellular roles of its paralog, CDK19, are poorly studied. We discovered that osteosarcoma cells (SJSA) are naturally depleted of CDK8 protein. Whereas stable CDK19 knockdown was tolerated in SJSA cells, proliferation was reduced. Notably, proliferation defects were rescued upon the reexpression of wild-type or kinase-dead CDK19. Comparative RNA sequencing analyses showed reduced expression of mitotic genes and activation of genes associated with cholesterol metabolism and the p53 pathway in CDK19 knockdown cells. SJSA cells treated with 5-fluorouracil, which induces metabolic and genotoxic stress and activates p53, further implicated CDK19 in p53 target gene expression. To better probe the p53 response, SJSA cells (shCDK19 versus shCTRL) were treated with the p53 activator nutlin-3. Remarkably, CDK19 was required for SJSA cells to return to a proliferative state after nutlin-3 treatment, and this effect was kinase independent. These results implicate CDK19 as a regulator of p53 stress responses and suggest a role for CDK19 in cellular resistance to nutlin-3.

Clinical Overview of MDM2/X-Targeted Therapies

MDM2 and MDMX are the primary negative regulators of p53, which under normal conditions maintain low intracellular levels of p53 by targeting it to the proteasome for rapid degradation and inhibiting its transcriptional activity. Both MDM2 and MDMX function as powerful oncogenes and are commonly over-expressed in some cancers, including sarcoma (~20%) and breast cancer (~15%). In contrast to tumors that are p53 mutant, whereby the current therapeutic strategy restores the normal active conformation of p53, MDM2 and MDMX represent logical therapeutic targets in cancer for increasing wild-type (WT) p53 expression and activities. Recent preclinical studies suggest that there may also be situations that MDM2/X inhibitors could be used in p53 mutant tumors. Since the discovery of nutlin-3a, the first in a class of small molecule MDM2 inhibitors that binds to the hydrophobic cleft in the N-terminus of MDM2, preventing its association with p53, there is now an extensive list of related compounds. In addition, a new class of stapled peptides that can target both MDM2 and MDMX have also been developed. Importantly, preclinical modeling, which has demonstrated effective in vitro and in vivo killing of WT p53 cancer cells, has now been translated into early clinical trials allowing better assessment of their biological effects and toxicities in patients. In this overview, we will review the current MDM2- and MDMX-targeted therapies in development, focusing particularly on compounds that have entered into early phase clinical trials. We will highlight the challenges pertaining to predictive biomarkers for and toxicities associated with these compounds, as well as identify potential combinatorial strategies to enhance its anti-cancer efficacy.

Interaction between p53 and estradiol pathways in transcriptional responses to chemotherapeutics

Estrogen receptors (ERs) and p53 can interact via cis-elements to regulate the angiogenesis-related VEGFR-1 (FLT1) gene, as we reported previously. Here, we address cooperation between these transcription factors on a global scale. Human breast adenocarcinoma MCF7 cells were exposed to single or combinatorial treatments with the chemotherapeutic agent doxorubicin and the ER ligand 17β-estradiol (E2). Whole-genome transcriptome changes were measured by expression microarrays. Nearly 200 differentially expressed genes were identified that showed limited responsiveness to either doxorubicin treatment or ER ligand alone but were upregulated in a greater than additive manner following combined treatment. Based on exposure to 5-fuorouracil and nutlin-3a, the combined responses were treatment-specific. Among 16 genes chosen for validation using quantitative real-time PCR, seven (INPP5D, TLR5, KRT15, EPHA2, GDNF, NOTCH1, SOX9) were confirmed to be novel direct targets of p53, based on responses in MCF7 cells silenced for p53 or cooperative targets of p53 and ER. Promoter pattern searches and chromatin IP experiments for the INPP5D, TLR5, KRT15 genes supported direct, cis-mediated p53 and/or ER regulation through canonical and noncanonical p53 and ER response elements. Collectively, we establish that combinatorial activation of p53 and ER can induce novel gene expression programs that have implications for cell-cell communications, adhesion, cell differentiation, development and inflammatory responses as well as cancer treatments.

On the interaction mechanisms of a p53 peptide and nutlin with the MDM2 and MDMX proteins: a Brownian dynamics study

The interaction of p53 with its regulators MDM2 and MDMX plays a major role in regulating the cell cycle. Inhibition of this interaction has become an important therapeutic strategy in oncology. Although MDM2 and MDMX share a very high degree of sequence/structural similarity, the small-molecule inhibitor nutlin appears to be an efficient inhibitor only of the p53-MDM2 interaction. Here, we investigate the mechanism of interaction of nutlin with these two proteins and contrast it with that of p53 using Brownian dynamics simulations. In contrast to earlier attempts to examine the bound states of the partners, here we locate initial reaction events in these interactions by identifying the regions of space around MDM2/MDMX, where p53/nutlin experience associative encounters with prolonged residence times relative to that in bulk solution. We find that the initial interaction of p53 with MDM2 is long-lived relative to nutlin, but, unlike nutlin, it takes place at the N- and C termini of the MDM2 protein, away from the binding site, suggestive of an allosteric mechanism of action. In contrast, nutlin initially interacts with MDM2 directly at the clefts of the binding site. The interaction of nutlin with MDMX, however, is very short-lived compared with MDM2 and does not show such direct initial interactions with the binding site. Comparison of the topology of the electrostatic potentials of MDM2 and MDMX and the locations of the initial encounters with p53/nutlin in tandem with structure-based sequence alignment revealed that the origin of the diminished activity of nutlin toward MDMX relative to MDM2 may stem partly from the differing topologies of the electrostatic potentials of the two proteins. Glu25 and Lys51 residues underpin these topological differences and appear to collectively play a key role in channelling nutlin directly toward the binding site on the MDM2 surface and are absent in MDMX. The results, therefore, provide new insight into the mechanism of p53/nutlin interactions with MDM2 and MDMX and could potentially have a broader impact on anticancer drug optimization strategies.

Blockade of Hsp90 by 17AAG antagonizes MDMX and synergizes with Nutlin to induce p53-mediated apoptosis in solid tumors

Strategies to induce p53 activation in wtp53-retaining tumors carry high potential in cancer therapy. Nutlin, a potent highly selective MDM2 inhibitor, induces non-genotoxic p53 activation. Although Nutlin shows promise in promoting cell death in hematopoietic malignancies, a major roadblock is that most solid cancers do not undergo apoptosis but merely reversible growth arrest. p53 inhibition by unopposed MDMX is one major cause for apoptosis resistance to Nutlin. The Hsp90 chaperone is ubiquitously activated in cancer cells and supports oncogenic survival pathways, many of which antagonize p53. The Hsp90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17AAG) is known to induce p53-dependent apoptosis. We show here that in multiple difficult-to-kill solid tumor cells 17AAG modulates several critical components that synergize with Nutlin-activated p53 signaling to convert Nutlin's transient cytostatic response into a cytotoxic killing response in vitro and in xenografts. Combined with Nutlin, 17AAG destabilizes MDMX, reduces MDM2, induces PUMA and inhibits oncogenic survival pathways, such as PI3K/AKT, which counteract p53 signaling at multiple levels. Mechanistically, 17AAG interferes with the repressive MDMX-p53 axis by inducing robust MDMX degradation, thereby markedly increasing p53 transcription compared with Nutlin alone. To our knowledge Nutlin+17AAG represents the first effective pharmacologic knockdown of MDMX. Our study identifies 17AAG as a promising synthetic lethal partner for a more efficient Nutlin-based therapy.

Inhibition of the p53 E3 ligase HDM-2 induces apoptosis and DNA damage--independent p53 phosphorylation in mantle cell lymphoma

PURPOSE

The ubiquitin-proteasome pathway has been validated as a target in non-Hodgkin's lymphoma through demonstration of the activity of the proteasome inhibitor bortezomib.

EXPERIMENTAL DESIGN

Another potentially attractive target is the human homologue of the murine double minute-2 protein, HDM-2, which serves as the major p53 E3 ubiquitin ligase; we therefore evaluated the activity of a novel agent, MI-63, which disrupts the HDM-2/p53 interaction.

RESULTS

Treatment of wild-type p53 mantle cell lymphoma (MCL) cell lines with MI-63 resulted in a dose- and time-dependent inhibition of proliferation, with an IC(50) in the 0.5 to 5.0 micromol/L range. MI-63 induced p53 and HDM-2 accumulation, as well as other downstream p53 targets such as p53 up-regulated modulator of apoptosis and p21(Cip1). This was associated with cell cycle arrest at G(1)-S; activation of caspase-3, caspase-8, and caspase-9; cleavage of poly-(ADP-ribose) polymerase; and loss of E2F1. HDM-2 inhibition caused phosphorylation of p53 at multiple serine residues, including 15, 37, and 392, which coincided with low levels of DNA strand breaks. DNA damage occurred in a small percentage of cells and did not induce phosphorylation of the DNA damage marker H2A.X(Ser139). Combinations of MI-63 with the molecularly targeted agents bortezomib and rapamycin showed synergistic, sequence-dependent antiproliferative effects. Treatment of primary MCL patient samples resulted in apoptosis and induction of p53 and p21, which was not seen in normal controls.

CONCLUSIONS

These findings support the hypothesis that inhibition of the HDM-2/p53 interaction may be a promising approach both by itself and in combination with currently used chemotherapeutics against lymphoid malignancies.

Radiosensitization of prostate cancer by priming the wild-type p53-dependent cellular senescence pathway

A dramatic stage-migration in diagnosis of prostate cancer has led to earlier detection of clinically localized carcinoma and an increased use of radiation therapy. The p53 protein responds to irradiation-induced DNA damage by removing critically damaged cells from the proliferative pool. This review will focus on the dominant role that p53-dependent cellular senescence, rather than cell death, plays in determining the radiosensitivity of human prostate cancer cells in vitro. The finding that senescence is a primary mechanism of tumor regression indicates that p53 activators or downstream effectors may prove effective in radiosensitizing some carcinoma of the prostate.