Alteration of tumor suppressors changes the endometrial tumor spectrum

The most common gynecological cancer in Europe and the United States is endometrial. Like most cancers, early-stage endometrial cancer has a more favorable prognosis, while high-grade, including endometrioid and nonendometrioid, has the worst prognosis. In endometrioid human tumors, the tumor suppressor genes PTEN and p53 (Trp53) are frequently altered or lost, as identified in datasets from The Cancer Genome Atlas. These suppressors' somatic mutations or loss of gene expression can lead to neoplastic development, tumor progression, and therapeutic resistance. In addition, somatic missense mutations are prevalent in another tumor suppressor, the F-box and WD repeats containing 7 (FBXW7). FBXW7 is part of the SCF-βTrCP ubiquitin complex that signals protein destruction. Specifically, FBXW7 is responsible for binding and facilitating the destabilization of proteins involved in proliferation and migration. Losing the function of multiple tumor suppressors could activate pathways involved in neoplastic progression, malignancy, therapeutic resistance, and formation of different tumor subtypes. The study by Brown et al in this issue of EMBO Mol Med (Brown et al, 2023) provides insight into the complexity of tumor suppressor mutations in malignant endometrial cancer.

Mdm2-mediated neddylation of pVHL blocks the induction of antiangiogenic factors

Mutations in the tumor suppressor TP53 are rare in renal cell carcinomas. p53 is a key factor for inducing antiangiogenic genes and RCC are highly vascularized, which suggests that p53 is inactive in these tumors. One regulator of p53 is the Mdm2 oncogene, which is correlated with high-grade, metastatic tumors. However, the sole activity of Mdm2 is not just to regulate p53, but it can also function independent of p53 to regulate the early stages of metastasis. Here, we report that the oncoprotein Mdm2 can bind directly to the tumor suppressor VHL, and conjugate nedd8 to VHL within a region that is important for the p53-VHL interaction. Nedd8 conjugated VHL is unable to bind to p53 thereby preventing the induction of antiangiogenic factors. These results highlight a previously unknown oncogenic function of Mdm2 during the progression of cancer to promote angiogenesis through the regulation of VHL. Thus, the Mdm2-VHL interaction represents a pathway that impacts tumor angiogenesis.

Mutant p53 drives clonal hematopoiesis through modulating epigenetic pathway

Clonal hematopoiesis of indeterminate potential (CHIP) increases with age and is associated with increased risks of hematological malignancies. While TP53 mutations have been identified in CHIP, the molecular mechanisms by which mutant p53 promotes hematopoietic stem and progenitor cell (HSPC) expansion are largely unknown. Here we discover that mutant p53 confers a competitive advantage to HSPCs following transplantation and promotes HSPC expansion after radiation-induced stress. Mechanistically, mutant p53 interacts with EZH2 and enhances its association with the chromatin, thereby increasing the levels of H3K27me3 in genes regulating HSPC self-renewal and differentiation. Furthermore, genetic and pharmacological inhibition of EZH2 decreases the repopulating potential of p53 mutant HSPCs. Thus, we uncover an epigenetic mechanism by which mutant p53 drives clonal hematopoiesis. Our work will likely establish epigenetic regulator EZH2 as a novel therapeutic target for preventing CHIP progression and treating hematological malignancies with TP53 mutations.

Protein phosphatase 5 and the tumor suppressor p53 down-regulate each other's activities in mice

Protein phosphatase 5 (PP5), a serine/threonine phosphatase, has a wide range of biological functions and exhibits elevated expression in tumor cells. We previously reported that pp5-deficient mice have altered ataxia-telangiectasia mutated (ATM)-mediated signaling and function. However, this regulation was likely indirect, as ATM is not a known PP5 substrate. In the current study, we found that pp5-deficient mice are hypersensitive to genotoxic stress. This hypersensitivity was associated with the marked up-regulation of the tumor suppressor tumor protein p53 and its downstream targets cyclin-dependent kinase inhibitor 1A (p21), MDM2 proto-oncogene (MDM2), and phosphatase and tensin homolog (PTEN) in pp5-deficient tissues and cells. These observations suggested that PP5 plays a role in regulating p53 stability and function. Experiments conducted with p53 ^+/- pp5 ^+/- or p53 ^+/- pp5 ^-/- mice revealed that complete loss of PP5 reduces tumorigenesis in the p53 ^+/- mice. Biochemical analyses further revealed that PP5 directly interacts with and dephosphorylates p53 at multiple serine/threonine residues, resulting in inhibition of p53-mediated transcriptional activity. Interestingly, PP5 expression was significantly up-regulated in p53-deficient cells, and further analysis of pp5 promoter activity revealed that p53 strongly represses PP5 transcription. Our results suggest a reciprocal regulatory interplay between PP5 and p53, providing an important feedback mechanism for the cellular response to genotoxic stress.

The proto-oncogene function of Mdm2 in bone

Mouse double minute 2 (Mdm2) is a multifaceted oncoprotein that is highly regulated with distinct domains capable of cellular transformation. Loss of Mdm2 is embryonically lethal, making it difficult to study in a mouse model without additional genetic alterations. Global overexpression through increased Mdm2 gene copy number (Mdm2^Tg ) results in the development of hematopoietic neoplasms and sarcomas in adult animals. In these mice, we found an increase in osteoblastogenesis, differentiation, and a high bone mass phenotype. Since it was difficult to discern the cell lineage that generated this phenotype, we generated osteoblast-specific Mdm2 overexpressing (Mdm2^TgOb ) mice in 2 different strains, C57BL/6 and DBA. These mice did not develop malignancies; however, these animals and the MG63 human osteosarcoma cell line with high levels of Mdm2 showed an increase in bone mineralization. Importantly, overexpression of Mdm2 corrected age-related bone loss in mice, providing a role for the proto-oncogenic activity of Mdm2 in bone health of adult animals.

Nuclear PTEN enhances the maturation of a microRNA regulon to limit MyD88-dependent susceptibility to sepsis

Sepsis-induced organ damage is caused by systemic inflammatory response syndrome (SIRS), which results in substantial comorbidities. Therefore, it is of medical importance to identify molecular brakes that can be exploited to dampen inflammation and prevent the development of SIRS. We investigated the role of phosphatase and tensin homolog (PTEN) in suppressing SIRS, increasing microbial clearance, and preventing lung damage. Septic patients and mice with sepsis exhibited increased PTEN expression in leukocytes. Myeloid-specific Pten deletion in an animal model of sepsis increased bacterial loads and cytokine production, which depended on enhanced myeloid differentiation primary response gene 88 (MyD88) abundance and resulted in mortality. PTEN-mediated induction of the microRNAs (miRNAs) miR125b and miR203b reduced the abundance of MyD88. Loss- and gain-of-function assays demonstrated that PTEN induced miRNA production by associating with and facilitating the nuclear localization of Drosha-Dgcr8, part of the miRNA-processing complex. Reconstitution of PTEN-deficient mouse embryonic fibroblasts with a mutant form of PTEN that does not localize to the nucleus resulted in retention of Drosha-Dgcr8 in the cytoplasm and impaired production of mature miRNAs. Thus, we identified a regulatory pathway involving nuclear PTEN-mediated miRNA generation that limits the production of MyD88 and thereby limits sepsis-associated mortality.

Mutant and wild-type p53 form complexes with p73 upon phosphorylation by the kinase JNK

The transcription factors p53 and p73 are critical to the induction of apoptotic cell death, particularly in response to cell stress that activates c-Jun N-terminal kinase (JNK). Mutations in the DNA-binding domain of p53, which are commonly seen in cancers, result in conformational changes that enable p53 to interact with and inhibit p73, thereby suppressing apoptosis. In contrast, wild-type p53 reportedly does not interact with p73. We found that JNK-mediated phosphorylation of Thr⁸¹ in the proline-rich domain (PRD) of p53 enabled wild-type p53, as well as mutant p53, to form a complex with p73. Structural algorithms predicted that phosphorylation of Thr⁸¹ exposes the DNA-binding domain in p53 to enable its binding to p73. The dimerization of wild-type p53 with p73 facilitated the expression of apoptotic target genes [such as those encoding p53-up-regulated modulator of apoptosis (PUMA) and Bcl-2-associated X protein (BAX)] and, subsequently, the induction of apoptosis in response to JNK activation by cell stress in various cells. Thus, JNK phosphorylation of mutant and wild-type p53 promotes the formation of a p53/p73 complex that determines cell fate: apoptosis in the context of wild-type p53 or cell survival in the context of the mutant. These findings refine our current understanding of both the mechanistic links between p53 and p73 and the functional role for Thr⁸¹ phosphorylation.

The fate of murine double minute X (MdmX) is dictated by distinct signaling pathways through murine double minute 2 (Mdm2)

Mouse double minute 2 (Mdm2) and MdmX dimerize in response to low levels of genotoxic stress to function in a ubiquitinating complex, which signals for destabilization of p53. Under growth conditions, Mdm2 functions as a neddylating ligase, but the importance and extent of MdmX involvement in this process are largely unknown. Here we show that when Mdm2 functions as a neddylating enzyme, MdmX is stabilized. Furthermore, we demonstrate that under growth conditions, MdmX enhances the neddylation activity of Mdm2 on p53 and is a substrate for neddylation itself. Importantly, MdmX knockdown in MCF-7 breast cancer cells resulted in diminished neddylated p53, suggesting that MdmX is important for Mdm2-mediated neddylation. Supporting this finding, the lack of MdmX in transient assays or in p53/MdmX-/- MEFs results in decreased or altered neddylation of p53 respectively; therefore, MdmX is a critical component of the Mdm2-mediated neddylating complex. c-Src is the upstream activator of this Mdm2-MdmX neddylating pathway and loss of Src signaling leads to the destabilization of MdmX that is dependent on the RING (Really Interesting New Gene) domain of MdmX. Treatment with a small molecule inhibitor of neddylation, MLN4924, results in the activation of Ataxia Telangiectasia Mutated (ATM). ATM phosphorylates Mdm2, converting Mdm2 to a ubiquitinating enzyme which leads to the destabilization of MdmX. These data show how distinct signaling pathways engage neddylating or ubiquitinating activities and impact the Mdm2-MdmX axis.

Early-Stage Metastasis Requires Mdm2 and Not p53 Gain of Function

Metastasis of cancer cells to distant organ systems is a complex process that is initiated with the programming of cells in the primary tumor. The formation of distant metastatic foci is correlated with poor prognosis and limited effective treatment options. We and others have correlated Mouse double minute 2 (Mdm2) with metastasis; however, the mechanisms involved have not been elucidated. Here, it is reported that shRNA-mediated silencing of Mdm2 inhibits epithelial-mesenchymal transition (EMT) and cell migration. In vivo analysis demonstrates that silencing Mdm2 in both post-EMT and basal/triple-negative breast cancers resulted in decreased primary tumor vasculature, circulating tumor cells, and metastatic lung foci. Combined, these results demonstrate the importance of Mdm2 in orchestrating the initial stages of migration and metastasis.Implication: Mdm2 is the major factor in the initiation of metastasis. Mol Cancer Res; 15(11); 1598-607. ©2017 AACR.

Abstract 2552: MDM2 induces cell fusion in breast cancer and osteosarcoma

Cell fusion is a physiological process utilized by a variety of biological systems during fertilization, embryogenesis, and cellular differentiation and morphogenesis. During the malignant transformation of breast and bone tissues, normal and malignant cells become fused to form hybrid structures. These enhanced phenotypes acquire genetic characteristics which support initiation, progression, and oncogenesis. In breast cancer and osteosarcoma cell lines, cell fusion generates chromosomal instability, aneuploidy, and DNA damage while maintaining the characteristics of enhanced migration, invasion, and chemoresistance. We have found that mouse double minute 2 (Mdm2) significantly induces cell fusion and the generation of large multinucleated cells in MDA468, MDA231, MDA157, T47D breast cancer and MG63 osteosarcoma cell lines under normoxia when compared to silenced Mdm2 controls (P<0.0005). Interestingly, silencing mutant p53 in the MDA468 cell line resulted in significantly higher fusion events (P<0.0004) suggesting mutant p53 may play a regulatory role in reducing Mdm2-associated cell fusion. Silenced Mdm2 MDA231 cells showed a three-fold increase under hypoxia whereas MG63 cells showed similar levels of cell fusion events as in normoxic conditions suggesting cell specific dependency to environmental stimulus. The variation in proportions of cell fusion among each cell type suggests this phenomenon is dependent on Mdm2 expression and cell type. CD133+ MDA468 cells have increased fractions of polyploidal cell events suggesting stemness may influence the polyploidal giant cancer cell (PGCC) dynamics. The mechanism by which cell fusion occurs in breast and osteosarcoma is poorly understood. Collectively these data suggest the interplay of Mdm2, p53, and stemness in supporting the formation of fused polypoidal cells and may serve as relevant targets for cancer therapy.

Citation Format: David J. Olivos, Mikhail Chtcherbinine, Eric R. Wolf, Ciarán McAtarsney, Lindsey D. Mayo. MDM2 induces cell fusion in breast cancer and osteosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2552. doi:10.1158/1538-7445.AM2017-2552

Emerging Non-Canonical Functions and Regulation by p53: p53 and Stemness

Since its discovery nearly 40 years ago, p53 has ascended to the forefront of investigated genes and proteins across diverse research disciplines and is recognized most exclusively for its role in cancer as a tumor suppressor. Levine and Oren (2009) reviewed the evolution of p53 detailing the significant discoveries of each decade since its first report in 1979. In this review, we will highlight the emerging non-canonical functions and regulation of p53 in stem cells. We will focus on general themes shared among p53's functions in non-malignant stem cells and cancer stem-like cells (CSCs) and the influence of p53 on the microenvironment and CSC niche. We will also examine p53 gain of function (GOF) roles in stemness. Mutant p53 (mutp53) GOFs that lead to survival, drug resistance and colonization are reviewed in the context of the acquisition of advantageous transformation processes, such as differentiation and dedifferentiation, epithelial-to-mesenchymal transition (EMT) and stem cell senescence and quiescence. Finally, we will conclude with therapeutic strategies that restore wild-type p53 (wtp53) function in cancer and CSCs, including RING finger E3 ligases and CSC maintenance. The mechanisms by which wtp53 and mutp53 influence stemness in non-malignant stem cells and CSCs or tumor-initiating cells (TICs) are poorly understood thus far. Further elucidation of p53's effects on stemness could lead to novel therapeutic strategies in cancer research.

Abstract 3702: Src phosphorylated Mdm2 requires MdmX to act as a neddylating ligase

Mdm2 is an oncoprotein and, along with its family member MdmX, have been shown to be elevated in human cancers. Like Mdm2, MdmX, is a RING containing protein, yet it lacks intrinsic E3 ligase function. Mdm2 has been shown to facilitate ubiquitination as a monomer, a homodimer, and as a heterodimer with MdmX depending on the cellular stress. Our recent report establishes that under growth conditions that activate Src, Mdm2 functions as a neddylating enzyme. Whether MdmX is necessary for Mdm2 to neddylate p53 has yet to be shown. Here we demonstrate that Src phosphorylation results in increased levels of MdmX, increased binding between Mdm2 and MdmX, and increased neddylation of MdmX and p53. Interestingly, the lack of MdmX (in transient assays or in shRNA cell lines) results in decreased neddylation of p53. These data support a critical role for MdmX as part of the Mdm2 neddylating complex.

Citation Format: Paula M. Hauck, Lindsey D. Mayo. Src phosphorylated Mdm2 requires MdmX to act as a neddylating ligase. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3702.

Abstract 5109: MDM2 regulates intracellular pathways in TNBC and osteosarcoma that affect osteoclastogenesis

Advanced cancer cells infiltrate and colonize new organ microenvironments through transformation of the dynamic metastatic niche. In bone, tumor cells secrete soluble factors that distort and manipulate normal bone remodeling leading to osteolytic lesions and inflammation. Clinically, primary high-grade bone sarcomas and secondary metastatic cancers exhibit elevated levels of murine double minute 2 (Mdm2), the principal cellular antagonist and E3 ubiquitin ligase of tumor suppressor p53. We demonstrate a novel function of mdm2, independent of p53, to drive osteolytic disease influenced by the signaling cascade of RANKL, ICAM1, and RANTES. To elucidate the direct influence of Mdm2 in cancer cells on osteoclast differentiation, several cell lines were created to overexpress or silence Mdm2. We found that osteoblast-like osteosarcoma (MG63, Saos2) and metastatic breast cancer cell lines (MDA468, MDA231, TMD-231, T47D, and BT474) lead to significant increased osteoclast differentiation, proliferation, and bone resorption. Co-cultures of Mdm2 over-expressing cancers and normal monocytes resulted in increased numbers of osteoclasts. In cancer cell lines where Mdm2 is silenced, we observed lower number of osteoclasts. The presence of Mdm2 in cancer cells also increased osteoclast resorption in an in vitro organ model. To determine the effects of ICAM1, we used an ICAM antibody to block ICAM1 in the media and showed a decrease in osteoclastogenesis. Understanding the role of Mdm2 in osteoclastogenesis may be amendable to translational benefit in disease models of advanced bone resorption and bone metastases.

Citation Format: David J. Olivos, Lindsey D. Mayo. MDM2 regulates intracellular pathways in TNBC and osteosarcoma that affect osteoclastogenesis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 5109.

Combination therapy in a xenograft model of glioblastoma: enhancement of the antitumor activity of temozolomide by an MDM2 antagonist

OBJECTIVE Improvement in treatment outcome for patients with glioblastoma multiforme (GBM) requires a multifaceted approach due to dysregulation of numerous signaling pathways. The murine double minute 2 (MDM2) protein may fulfill this requirement because it is involved in the regulation of growth, survival, and invasion. The objective of this study was to investigate the impact of modulating MDM2 function in combination with front-line temozolomide (TMZ) therapy in GBM. METHODS The combination of TMZ with the MDM2 protein-protein interaction inhibitor nutlin3a was evaluated for effects on cell growth, p53 pathway activation, expression of DNA repair proteins, and invasive properties. In vivo efficacy was assessed in xenograft models of human GBM. RESULTS In combination, TMZ/nutlin3a was additive to synergistic in decreasing growth of wild-type p53 GBM cells. Pharmacodynamic studies demonstrated that inhibition of cell growth following exposure to TMZ/nutlin3a correlated with: 1) activation of the p53 pathway, 2) downregulation of DNA repair proteins, 3) persistence of DNA damage, and 4) decreased invasion. Pharmacokinetic studies indicated that nutlin3a was detected in human intracranial tumor xenografts. To assess therapeutic potential, efficacy studies were conducted in a xenograft model of intracranial GBM by using GBM cells derived from a recurrent wild-type p53 GBM that is highly TMZ resistant (GBM10). Three 5-day cycles of TMZ/nutlin3a resulted in a significant increase in the survival of mice with GBM10 intracranial tumors compared with single-agent therapy. CONCLUSIONS Modulation of MDM2/p53-associated signaling pathways is a novel approach for decreasing TMZ resistance in GBM. To the authors' knowledge, this is the first study in a humanized intracranial patient-derived xenograft model to demonstrate the efficacy of combining front-line TMZ therapy and an inhibitor of MDM2 protein-protein interactions.

Potentiation of Carboplatin-Mediated DNA Damage by the Mdm2 Modulator Nutlin-3a in a Humanized Orthotopic Breast-to-Lung Metastatic Model

Triple-negative breast cancers (TNBC) are typically resistant to treatment, and strategies that build upon frontline therapy are needed. Targeting the murine double minute 2 (Mdm2) protein is an attractive approach, as Mdm2 levels are elevated in many therapy-refractive breast cancers. The Mdm2 protein-protein interaction inhibitor Nutlin-3a blocks the binding of Mdm2 to key signaling molecules such as p53 and p73α and can result in activation of cell death signaling pathways. In the present study, the therapeutic potential of carboplatin and Nutlin-3a to treat TNBC was investigated, as carboplatin is under evaluation in clinical trials for TNBC. In mutant p53 TMD231 TNBC cells, carboplatin and Nutlin-3a led to increased Mdm2 and was strongly synergistic in promoting cell death in vitro. Furthermore, sensitivity of TNBC cells to combination treatment was dependent on p73α. Following combination treatment, γH2AX increased and Mdm2 localized to a larger degree to chromatin compared with single-agent treatment, consistent with previous observations that Mdm2 binds to the Mre11/Rad50/Nbs1 complex associated with DNA and inhibits the DNA damage response. In vivo efficacy studies were conducted in the TMD231 orthotopic mammary fat pad model in NOD.Cg-Prkdc(scid)Il2rg(tm1Wjl)/SzJ (NSG) mice. Using an intermittent dosing schedule of combined carboplatin and Nutlin-3a, there was a significant reduction in primary tumor growth and lung metastases compared with vehicle and single-agent treatments. In addition, there was minimal toxicity to the bone marrow and normal tissues. These studies demonstrate that Mdm2 holds promise as a therapeutic target in combination with conventional therapy and may lead to new clinical therapies for TNBC.

Abstract 2241: MDM2 silencing promotes tumor-initiating cells in osteosarcoma and breast cancer

Chemotherapy has made remarkable strides in the past century to alleviate tumor burden across most cancer types, yet it remains ineffective at achieving complete tumor remission. To improve chemotherapy efficacy and patient survival, novel drugs will require a greater mechanistic understanding of tumor initiating cell (TIC) regulatory pathways. Intratumoral heterogeneity is a major obstacle for conventional targeted treatment modalities, which select for aggressive phenotypes with acquired drug resistance and TIC enrichment. Here we focus on elucidating the role of Mdm2 in mediating TIC levels in osteosarcoma (MG-63, Saos2) and breast cancer (MDA 231, MDA 468, and BT474) cell lines. We demonstrate Mdm2 silencing by ShRNA results in increased levels of Nanog, Oct4a, and Sox2, and diminished expression of ERα and ERβ by western blot. We observed an increase in the immunophenotypic expression of CD133 in ShMdm2 osteosarcoma and breast cancer cell lines by flow cytometry. Our data provides the first evidence that loss of Mdm2 results in increased TIC population. Further analysis will examine the affects of TIC using small molecular inhibitors to Mdm2. This insight will contribute to the molecular foundations defining the role of Mdm2 in TIC and will determine if targeting Mdm2 therapeutically is a prognostic indicator for recurrence.

Citation Format: David J. Olivos, Lindsey D. Mayo. MDM2 silencing promotes tumor-initiating cells in osteosarcoma and breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2241. doi:10.1158/1538-7445.AM2015-2241

Signaling pathways involved in megakaryocyte-mediated proliferation of osteoblast lineage cells

Recent studies suggest that megakaryocytes (MKs) may play a significant role in skeletal homeostasis, as evident by the occurrence of osteosclerosis in multiple MK related diseases (Lennert et al., 1975; Thiele et al., 1999; Chagraoui et al., 2006). We previously reported a novel interaction whereby MKs enhanced proliferation of osteoblast lineage/osteoprogenitor cells (OBs) by a mechanism requiring direct cell-cell contact. However, the signal transduction pathways and the downstream effector molecules involved in this process have not been characterized. Here we show that MKs contact with OBs, via beta1 integrin, activate the p38/MAPKAPK2/p90RSK kinase cascade in the bone cells, which causes Mdm2 to neutralizes p53/Rb-mediated check point and allows progression through the G1/S. Interestingly, activation of MAPK (ERK1/2) and AKT, collateral pathways that regulate the cell cycle, remained unchanged with MK stimulation of OBs. The MK-to-OB signaling ultimately results in significant increases in the expression of c-fos and cyclin A, necessary for sustaining the OB proliferation. Overall, our findings show that OBs respond to the presence of MKs, in part, via an integrin-mediated signaling mechanism, activating a novel response axis that de-represses cell cycle activity. Understanding the mechanisms by which MKs enhance OB proliferation will facilitate the development of novel anabolic therapies to treat bone loss associated with osteoporosis and other bone-related diseases.

Ferroxitosis: a cell death from modulation of oxidative phosphorylation and PKM2-dependent glycolysis in melanoma

Reliance on glycolysis is a characteristic of malignancy, yet the development of resistance to BRAF inhibitors in melanoma is associated with gain of mitochondrial function. Concurrent attenuation of oxidative phosphorylation and HIF-1α/PKM2-dependent glycolysis promotes a non-apoptotic, iron- and oxygen-dependent cell death that we term ferroxitosis. The redox cycling agent menadione causes a robust increase in oxygen consumption, accompanied by significant loss of intracellular ATP and rapid cell death. Conversely, either hypoxic adaptation or iron chelation prevents menadione-induced ferroxitosis. Ectopic expression of K213Q HIF-1α mutant blunts the effects of menadione. However, knockdown of HIF-1α or PKM2 restores menadione-induced cytotoxicity in hypoxia. Similarly, exposure of melanoma cells to shikonin, a menadione analog and a potential PKM2 inhibitor, is sufficient to induce ferroxitosis under hypoxic conditions. Collectively, our findings reveal that ferroxitosis curtails metabolic plasticity in melanoma.

Abstract 2741: Sensitization of temozolomide-mediated glioblastoma cell death by targeting MDM2: Assessment of PD biomarkers, brain penetration, and efficacy in humanized orthotopic xenograft models

Treatment of glioblastoma multiforme (GBM) continues to be a challenge due to its infiltrative nature, tumor heterogeneity, and lack of therapeutic agents that penetrate the blood-brain barrier (BBB). While the primary tumor is permeable to some degree, the ability of GBM cells to invade areas of parenchyma with an intact BBB indicates development of BBB-penetrable compounds is a necessity. The multifunctional protein MDM2 holds promise as a therapeutic target in a variety of cancers and plays a critical role in controlling cell survival, invasion, and DNA repair. MDM2 antagonists such as nutlin3a and RG7112 are being used to interrogate the impact of modulating MDM2 function in combination with front-line therapy. Our objective was to investigate whether MDM2 antagonists, alone or in combination with temozolomide (TMZ), can augment cell death in orthotopic GBM xenograft models. In vitro data indicate that TMZ and nutlin3a are synergistic in decreasing cell viability in wild type (wt) p53 U87-MG, primary human wt p53 GBM10 and mutant (mt) p53 GBM43 cells. Pharmacodynamic studies demonstrated that the mechanism of action for promoting cell death following exposure to TMZ/nutlin3a was multifactorial. Comet assays indicated that repair of TMZ-mediated DNA damage was significantly delayed in wt and mt p53 GBM cells treated with TMZ/nutlin3a compared to TMZ alone and the base excision repair protein Ape1 was downregulated in cells treated with TMZ/nutlin3a. Pharmacokinetic studies guided development of rational dosing regimens in which 2-3 five-day cycles of TMZ followed by nutlin3a 4 hours later were investigated. In ectopic U87-MG xenografts, nutlin3a sensitized xenografts to TMZ-mediated cell death. Orthotopic studies employing U87-MG, GBM10, and GBM43 tumors are being utilized to determine if nutlin3a levels detectable in the brain via HPLC-MS/MS(API 4000) are sufficient to modulate MDM2 function in the context of TMZ and increase survival. In the TMZ-resistant GBM10 (wtp53, MGMTpos, and PTENnull) orthotopic model, there was a modest increase in median survival from 63 days with TMZ to 73 days with TMZ/nutlin3a. Higher and more sustained brain levels of MDM2 antagonists will likely be necessary to improve survival. The MDM2 antagonist RG7112 has an improved PK profile and structural analysis of RG7112 and nutlin3a via QikProp 3.0 (www.schrodinger.com) indicates that RG7112 also has an improved predicted brain/blood partition coefficient (plog BB) compared to nutlin3a (RG7112 = -0.17; nutlin3a = -0.415). Studies are in progress to assess the brain penetration of RG7112 and its effect on GBM growth in vivo. Taken together, our data suggest that modulation of MDM2 function in the context of cytotoxic therapy has the potential to alter mechanisms involved in DNA repair that can promote cell death and improve survival.

Citation Format: Haiyan Wang, Shanbao Cai, Barbara J. Bailey, Lawrence M. Gelbert, M. Reza Saadatzadeh, Aaron A. Cohen-Gadol, Jann N. Sarkaria, Paul Territo, Taxiarchis M. Georgiadis, T. Zachary Gunter, Samy Meroueh, Eric C. Long, David R. Jones, Lindsey D. Mayo, Shannon Harlan, Karen E. Pollok. Sensitization of temozolomide-mediated glioblastoma cell death by targeting MDM2: Assessment of PD biomarkers, brain penetration, and efficacy in humanized orthotopic xenograft models. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2741. doi:10.1158/1538-7445.AM2014-2741

Abstract 1680: Modulation of MDM2 in context of DNA damage enhances cell death in a metastatic breast-to-lung xenograft model

Metastatic breast cancer is highly refractive to current treatment strategies, and new multi-targeted treatments need to be elucidated. In metastatic disease, inhibiting key protein-protein interactions with the murine double minute 2 (MDM2) could be beneficial for developing new treatment modalities since this signaling pathway is a critical regulatory point in cancer progression. Inhibition of protein binding to the hydrophobic pocket of MDM2 by Nutlin-3a can activate pro-apoptotic proteins such p73 and E2F1 as well as decrease pro-angiogenic Hif-1α. Since the DNA damaging agent carboplatin is currently being studied in clinical trials of triple-negative breast cancers (TNBCs), our objective was to evaluate the effects of carboplatin and Nutlin-3a in combination in TNBC in a mutant p53 background. Using a TNBC cell line TMD231 derived from the MDA-MB-231 human breast cancer cell line, we performed combination studies using different ratios of carboplatin to Nutlin-3a in vitro to evaluate the range of carboplatin-mediated DNA damage required to obtain synergism with inhibition of MDM2 function. A fixed ratio of 1:1 carboplatin:Nutlin-3a was strongly synergistic with a combination index of <0.5. In cell proliferation assays there was increased sensitivity to the drugs when given in combination (p<0.05). TMD231 cells implanted into the mammary fat pad of NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice showed enhanced tumor growth, and metastasis was evident in the lungs. Dose-finding studies were performed to determine an optimal carboplatin dosing schema. NSG mice were randomized based on fluorescent imaging of E2-crimson expressing TMD231 cells allowing for a sensitive measurement of early tumor burden. Following Nutlin-3a and carboplatin combination treatment in vivo, there was a statistically significant reduction in tumor volume and lung metastases compared to vehicle and single drug treated mice (p<0.001). Following Kaplan-Meier analysis, the combination treated mice had a significant increase in survival, (54.3±1.5 days) compared to the vehicle (39.3±0.6 days) and each single drug (Nutlin-3a: 39±1 and carboplatin: 47.5±1.8 days) (p<0.001). While there was a decrease in bone-marrow cellularity, this did not lead to bone-marrow aplasia, and body weights recovered to normal levels within 7 days post-treatment. Pharmacodynamic studies are ongoing to further understand at the molecular level how the DNA damage response and repair is modulated by MDM2 resulting in a robust synergistic response. These studies will lead to a better understanding of how to potentiate DNA damage and may lead to new clinical therapies in the future for metastatic breast cancer.

Citation Format: Eva Tonsing-Carter, Harlan E. Shannon, Barbara J. Bailey, Anthony L. Sinn, Kacie M. Peterman, Lindsey D. Mayo, Karen E. Pollok. Modulation of MDM2 in context of DNA damage enhances cell death in a metastatic breast-to-lung xenograft model. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1680. doi:10.1158/1538-7445.AM2014-1680

Pyk2 regulates megakaryocyte-induced increases in osteoblast number and bone formation

Preclinical and clinical evidence from megakaryocyte (MK)-related diseases suggests that MKs play a significant role in maintaining bone homeostasis. Findings from our laboratories reveal that MKs significantly increase osteoblast (OB) number through direct MK-OB contact and the activation of integrins. We, therefore, examined the role of Pyk2, a tyrosine kinase known to be regulated downstream of integrins, in the MK-mediated enhancement of OBs. When OBs were co-cultured with MKs, total Pyk2 levels in OBs were significantly enhanced primarily because of increased Pyk2 gene transcription. Additionally, p53 and Mdm2 were both decreased in OBs upon MK stimulation, which would be permissive of cell cycle entry. We then demonstrated that OB number was markedly reduced when Pyk2-/- OBs, as opposed to wild-type (WT) OBs, were co-cultured with MKs. We also determined that MKs inhibit OB differentiation in the presence and absence of Pyk2 expression. Finally, given that MK-replete spleen cells from GATA-1-deficient mice can robustly stimulate OB proliferation and bone formation in WT mice, we adoptively transferred spleen cells from these mice into Pyk2-/- recipient mice. Importantly, GATA-1-deficient spleen cells failed to stimulate an increase in bone formation in Pyk2-/- mice, suggesting in vivo the important role of Pyk2 in the MK-induced increase in bone volume. Further understanding of the signaling pathways involved in the MK-mediated enhancement of OB number and bone formation will facilitate the development of novel anabolic therapies to treat bone loss diseases.

Abstract 3435: MDM2 as a therapeutic target: improving upon front-line chemotherapy for nervous system tumors

There is a significant unmet medical need to develop more effective treatments for cancers of the central and peripheral nervous systems. Glioblastoma multiforme (GBM) is the most common form of brain cancer, and successful long-term treatment has not been realized. Neuroblastomas (NB) arise in the sympathetic nervous system and are the most common extracranial solid tumor of childhood. Survival of children with disseminated, high-risk NB tumors is less than 50%, even with intensive multimodal treatment. To improve treatment outcome, inhibition of murine double minute 2 (MDM2) function in the context of front-line therapy is being explored. MDM2 is a multi-functional protein that plays a critical role in controlling cell growth and migration. MDM2 is a negative regulator of p53 and p73 and, under low stress conditions, sequesters them in the cytoplasm. However, inhibition of MDM2 interactions with p53 and p73 by the MDM2 antagonist nutlin3a can lead to activation of p53/p73-mediated cell-death in tumor cells. The objective of the present study was to evaluate the extent to which blockade of MDM2-p53/p73 interactions by nutlin3a, alone or in combination with front-line therapies, could augment cell death in GBM and NB cells with a variety of molecular profiles. Using the median effect model, we found that temozolomide (TMZ) and nutlin3a were synergistic in increasing cell death in wtp53 U87 and U373 but were predominantly additive in mtp53 U118 cells. Primary patient GBM cells were typically resistant to TMZ relative to established cell lines. However, resistance to TMZ was decreased in the presence of nutlin3a. Further, TMZ and nutlin3a were synergistic in primary GBM10 and GBM43 but not in recurrent primary IU-GBM16, IU-GBM23, IU-GBM27, and IU-GBM32 cells. Moreover, in NB cells with wtp53, nutlin3a and cisplatin were synergistic in MYCN nonamplified SK-N-SH and additive in MYCN amplified IMR5. In mtp53 NB cells, nutlin3a and cisplatin were synergistic in MYCN nonamplified SK-N-FI as well as in MYCN amplified SK-N-DZ. Nutlin3a alone produced dose-related increases in cell death of all GBM and NB cell lines irrespective of p53 status, suggesting that p53-independent mechanisms play a role in inhibition of cell growth produced by blockade of MDM2-mediated signaling. Consistent with this hypothesis, nutlin3a increased p73 protein levels in mtp53 GBM43 cells that were exposed to TMZ. Inhibition of MDM2 function also inhibited repair of TMZ-mediated DNA damage in GBM. In comet assays, nutlin3a+TMZ decreased DNA repair relative to TMZ alone in wtp53 U87 and GBM10 and in mtp53 GBM43 cells. The present results demonstrate that MDM2 is a valid therapeutic target in GBM and NB cells with varying genetic profiles. In conclusion, inhibition of MDM2 signaling by nutlin3a is generally synergistic to additive in combination with front-line chemotherapeutics used to treat cancers of the central and peripheral nervous systems.

Citation Format: Harlan E. Shannon, Haiyan Wang, Shanbao Cai, Wei Michael Liu, Lawrence M. Gelbert, Aaron A. Cohen-Gadol, Jann N. Sarkaria, Lindsey D. Mayo, Karen E. Pollok. MDM2 as a therapeutic target: improving upon front-line chemotherapy for nervous system tumors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3435. doi:10.1158/1538-7445.AM2013-3435

Abstract 4639: Blockade of MDM2-mediated signaling in context of DNA damage increases E2F1 expression and enhances cell death in triple-negative breast cancer cells

Metastatic breast cancer is highly refractive to current treatment strategies and new multi-targeted treatments need to be elucidated. In metastatic disease, blocking key protein-protein interactions with the murine double minute 2 (MDM2) could be beneficial for developing new treatment modalities since this signaling pathway is a critical regulatory point in cancer progression. Inhibition of protein binding to the hydrophobic pocket of MDM2 by Nutlin-3a can result in increases in pro-apoptotic proteins such as p53, p73, and E2F1. Since the DNA damaging agent carboplatin is being studied in clinical trials of triple-negative breast cancers (TNBCs), our objective was to evaluate the effects of carboplatin and Nutlin-3a in combination in TNBC cancer in a mutant p53 background. We utilized TMD231 breast cancer cells derived from the MDA-MB-231 human TNBC cancer line. TMD231 cells are highly metastatic in vivo and readily metastasize from the primary tumor to the lung. Combination studies were performed using different ratios of carboplatin to Nutlin-3a to evaluate the range of carboplatin-mediated DNA damage required to obtain synergism with inhibition of MDM2 function. A fixed ratio of 1:1 carboplatin:Nutlin-3a was strongly synergistic with a combination index of < 0.5. In cell proliferation assays there was increased sensitivity to the drugs when given together and a significant reduction in number of colonies in colony formation assays. In order to understand how DNA damage and inhibition of MDM2 signaling leads to enhanced cell death, dose-response and time-course studies were performed. Western analyses demonstrated increases in E2F1 when treated with carboplatin and Nutlin-3a alone, and a time-dependent increase in E2F1 following combination treatment at a fixed 1:1 ratio. Following DNA damage, E2F1 has been shown to promote the transcription of pro-apoptotic genes and repression of survival genes once E2F1 is released from interaction with Rb. Here, E2F1 levels correlated with Rb phosphorylation at serine 780, a phosphorylation site which inhibits binding of E2F1 to Rb. The E3 ubiquitin ligase Itch, which ubiquitinates p73, increased following treatment with carboplatin alone or in combination with Nutlin-3a in a dose- and time-dependent manner but appeared not to regulate p73 since p73 levels remained unchanged following combination treatment. Studies are ongoing to further understand how modulation of DNA damage by blockade of MDM2 results in a robust synergistic response and how upregulation of E2F1 affects downstream targets. The impact of the combination of DNA damaging carboplatin and MDM2 blockade by Nutlin-3a on DNA repair kinetics is currently being evaluated in an orthotopic TMD231 model. These studies will lead to a better understanding of how to potentiate DNA damage and may lead to new clinical therapies in the future.

Citation Format: Eva Tonsing-Carter, Harlan E. Shannon, Barbara J. Bailey, Lindsey D. Mayo, Karen E. Pollok. Blockade of MDM2-mediated signaling in context of DNA damage increases E2F1 expression and enhances cell death in triple-negative breast cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4639. doi:10.1158/1538-7445.AM2013-4639

Abstract 5380: Detection of OvCa1 in human malignancies

Immunohistochemistry biomarkers are currently being developed to target specific proteins found in cancer cells, which could contribute to a better understanding of specific types of cancers and provide information to develop better treatments for cancer patients. The biomarker and putative tumor suppressor, OvCa1, has a function that is not well characterized. Due to a lack of staining reagents, we developed monoclonal antibodies of OvCa1 to examine multiple human malignancies. The goal of this project was to gain a better understanding of OvCa1 and its role in cancer. Primary cancers with different histological grades as well as metastatic lesions were examined with the monoclonal antibodies. Ovarian cancer tissue samples from the Indiana University Simon Cancer Center Tissue Bank were used for this study. The samples were fixed in neutral buffered formalin and processed into paraffin blocks. The tissues were cut using a microtome and placed on slides. Staining of the OvCa1 antibody was completed using immunohistochemistry (IHC) techniques. Thirty-one samples were evaluated including low grade tumors, medium grade tumors, high grade tumors, and metastatic ovarian carcinomas. All cases revealed a range of staining intensity with the OvCa1 antibody. The results indicated that OvCa1 for the high grade, stage 3/4, ovarian carcinomas had the highest staining intensity. Medium grade tumors had less OvCa1 expression. The metastatic tumors had less staining than any of the other three grades. Immunostaining was observed primarily in the cytoplasm and nucleus of the tumor cells. In addition, we evaluated approximately 20 tumors from various different organs. These included prostate, breast, spleen, lung, colon, stomach, and kidney tumors, which were positive for immunostaining with the OvCa1 antibody. In summary, the results indicated that all histological grades expressed the biomarker, OvCa1, and the staining intensity was highest in the high grade, stage 3/4, tumors. Our preliminary studies demonstrated a further need to delineate OvCa1 as a potential biomarker, which could be used for early detection and diagnosis of ovarian cancer.

Citation Format: Jessica L. Jackson, Kristen A. Grothaus, Connie J. Temm, Lindsey D. Mayo, George E. Sanduksy. Detection of OvCa1 in human malignancies. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5380. doi:10.1158/1538-7445.AM2013-5380

Temozolomide-mediated DNA methylation in human myeloid precursor cells: differential involvement of intrinsic and extrinsic apoptotic pathways

PURPOSE

An understanding of how hematopoietic cells respond to therapy that causes myelosuppression will help develop approaches to prevent this potentially life-threatening toxicity. The goal of this study was to determine how human myeloid precursor cells respond to temozolomide (TMZ)-induced DNA damage.

EXPERIMENTAL DESIGN

We developed an ex vivo primary human myeloid precursor cells model system to investigate the involvement of cell-death pathways using a known myelosuppressive regimen of O(6)-benzylguanine (6BG) and TMZ.

RESULTS

Exposure to 6BG/TMZ led to increases in p53, p21, γ-H2AX, and mitochondrial DNA damage. Increases in mitochondrial membrane depolarization correlated with increased caspase-9 and -3 activities following 6BG/TMZ treatment. These events correlated with decreases in activated AKT, downregulation of the DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT), and increased cell death. During myeloid precursor cell expansion, FAS/CD95/APO1(FAS) expression increased over time and was present on approximately 100% of the cells following exposure to 6BG/TMZ. Although c-flipshort, an endogenous inhibitor of FAS-mediated signaling, was decreased in 6BG/TMZ-treated versus control, 6BG-, or TMZ alone-treated cells, there were no changes in caspase-8 activity. In addition, there were no changes in the extent of cell death in myeloid precursor cells exposed to 6BG/TMZ in the presence of neutralizing or agonistic anti-FAS antibodies, indicating that FAS-mediated signaling was not operative.

CONCLUSIONS

In human myeloid precursor cells, 6BG/TMZ-initiated apoptosis occurred by intrinsic, mitochondrial-mediated and not extrinsic, FAS-mediated apoptosis. Human myeloid precursor cells represent a clinically relevant model system for gaining insight into how hematopoietic cells respond to chemotherapeutics and offer an approach for selecting effective chemotherapeutic regimens with limited hematopoietic toxicity.

Inauhzin and Nutlin3 synergistically activate p53 and suppress tumor growth

Several proteins have been suggested in promoting tumor formation in numerous human tissues by inactivating the tumor suppressor p53. This has generated interest in the development of small molecules to block these inhibitors of p53 and to regain p53 activity. Recently, we identified a small molecule, Inauhzin, which can inhibit SIRT1 activity and activate p53. SIRT1 is a deacetylase that deacetylates p53 and facilitates Mdm2 mediated p53 destabilization. In this study, we tested if combining Inauhzin with Nutlin-3, an inhibitor of MDM2-p53 binding, might synergistically activate p53 to suppress tumor growth. Indeed, at lower doses, combination of Inauhzin and Nutlin-3 exhibited a synergistic effect on inhibiting cell growth and promoting apoptosis in human colon and lung cancer cell lines in a p53-dependent fashion. Minimal effects were observed with treatment of either compound alone. Using a xenograft tumor model, we also showed a synergistic effect with both compounds. Thus, to fully regain p53 activity, targeting its multiple inhibitory proteins might be a better approach. Our study provides evidence supporting this concept for achieving better therapeutic efficacy in tumors that possess wild type p53.

Megakaryocytes regulate expression of Pyk2 isoforms and caspase-mediated cleavage of actin in osteoblasts

The proliferation and differentiation of osteoblast (OB) precursors are essential for elaborating the bone-forming activity of mature OBs. However, the mechanisms regulating OB proliferation and function are largely unknown. We reported that OB proliferation is enhanced by megakaryocytes (MKs) via a process that is regulated in part by integrin signaling. The tyrosine kinase Pyk2 has been shown to regulate cell proliferation and survival in a variety of cells. Pyk2 is also activated by integrin signaling and regulates actin remodeling in bone-resorbing osteoclasts. In this study, we examined the role of Pyk2 and actin in the MK-mediated increase in OB proliferation. Calvarial OBs were cultured in the presence of MKs for various times, and Pyk2 signaling cascades in OBs were examined by Western blotting, subcellular fractionation, and microscopy. We found that MKs regulate the temporal expression of Pyk2 and its subcellular localization. We also found that MKs regulate the expression of two alternatively spliced isoforms of Pyk2 in OBs, which may regulate OB differentiation and proliferation. MKs also induced cytoskeletal reorganization in OBs, which was associated with the caspase-mediated cleavage of actin, an increase in focal adhesions, and the formation of apical membrane ruffles. Moreover, BrdU incorporation in MK-stimulated OBs was blocked by the actin-polymerizing agent, jasplakinolide. Collectively, our studies reveal that Pyk2 and actin play an important role in MK-regulated signaling cascades that control OB proliferation and may be important for therapeutic interventions aimed at increasing bone formation in metabolic diseases of the skeleton.

Induction of apoptotic genes by a p73-phosphatase and tensin homolog (p73-PTEN) protein complex in response to genotoxic stress

The p53 family member, p73, has been characterized as a tumor suppressor and functions in a similar manner as p53 to induce cellular death. The phosphatase and tensin homolog (PTEN) can function as a dual specificity lipid/protein phosphatase. However, recent data have described multiple roles for nuclear PTEN independent of its lipid phosphatase activity. PTEN can directly or indirectly activate p53 to promote apoptosis. We examined whether PTEN would interact and regulate p73 independent of p53. Co-localization in the nucleus and complex formation of p73/PTEN were observed after DNA damage. Furthermore, we also demonstrate that p73α/PTEN proteins directly bind one another. Both overexpressed and endogenous p73-PTEN interactions were determined to be the strongest in the nuclear fraction after DNA damage, which suggested formation of a transcriptional complex. We employed chromatin immunoprecipitation (ChIP) and found that p73 and PTEN were associated with the PUMA promoter after genotoxic stress in TP53-null cells. We found that another p73 target, BAX, had an increased expression in the presence of p73 and PTEN. In addition, in virus-transduced cell lines stably expressing p73, PTEN, or both p73/PTEN, we found that the p73/PTEN cells were more sensitive to genotoxic stress and cellular death as measured by increased poly(ADP-ribose) polymerase cleavage and PUMA/Bax induction. Conversely, knockdown of PTEN dramatically reduced Bax and PUMA levels. Thus, a p73-PTEN protein complex is engaged to induce apoptosis independent of p53 in response to DNA damage.

Abstract 2543: Humanized bone-marrow mouse model as a pre-clinical tool to assess therapy-mediated hematotoxicity

Murine xenograft models are one of the primary tools used for screening of new therapeutic compounds and regimens. However, one major limitation of utilizing murine xenograft studies to assess therapeutic efficacy is that significant inter-species differences in drug sensitivity can exist between mice and humans. It is possible that the levels of a therapeutic compound reached in a mouse xenograft model may not be achievable in humans due to differential toxicity profiles. There is a growing need for in vivo models that can simulate efficacy of promising therapeutic regimens but at the same time can evaluate the potential for off-target human toxicity. Since bone-marrow toxicity can be a major life-threatening side effect of treatment, models to screen for the impact of treatment on human hematopoiesis would improve our ability to select compounds with decreased off-target toxicities. In this study, a xenograft model containing humanized bone marrow is utilized as an in vivo assay to monitor hematotoxicity. As a proof-of-concept, the impact of a combination therapy consisting of O6-benzylguanine and temozolomide (O6-BG/TMZ) on human hematopoiesis in vivo was investigated since this regimen is currently being evaluated in clinical trials and the main dose-limiting toxicity in these patients is myelosuppression. A dose-intensive O6-BG/TMZ-dosing regimen that requires stem-cell rescue was developed that significantly inhibits the growth of human-glioblastoma xenografts in NOD/SCID/γchainnull mice. To monitor human hematotoxicity profiles, NOD/SCID/gammanull mice were next transplanted with human CD34+ cells and reconstitution confirmed one month post-transplantation by peripheral blood analysis. The dose-intensive regimen was then administered to NOD/SCID/gammanull mice reconstituted with human hematopoietic cells and the impact of treatment on human hematopoiesis evaluated. Flow cytometric analyses indicated that the human bone-marrow cells were significantly more sensitive to treatment than the murine bone-marrow cells in vivo, and that the regimen was highly toxic to human-derived hematopoietic cells of all lineages (CD34+ progenitor, CD45+CD19+ lymphoid, and CD33+ myeloid). This proof-of-concept study indicates that use of NOD/SCID/γchainnull mice with humanized bone marrow can be used as an in vivo toxicity measure of human hematopoiesis. This model holds promise as a new approach for monitoring the impact of anti-cancer therapies on human hematopoiesis and could lead to subsequent refinement of therapies prior to clinical evaluation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2543. doi:10.1158/1538-7445.AM2011-2543

Humanized bone marrow mouse model as a preclinical tool to assess therapy-mediated hematotoxicity

PURPOSE

Preclinical in vivo studies can help guide the selection of agents and regimens for clinical testing. However, one of the challenges in screening anticancer therapies is the assessment of off-target human toxicity. There is a need for in vivo models that can simulate efficacy and toxicities of promising therapeutic regimens. For example, hematopoietic cells of human origin are particularly sensitive to a variety of chemotherapeutic regimens, but in vivo models to assess potential toxicities have not been developed. In this study, a xenograft model containing humanized bone marrow is utilized as an in vivo assay to monitor hematotoxicity.

EXPERIMENTAL DESIGN

A proof-of-concept, temozolomide-based regimen was developed that inhibits tumor xenograft growth. This regimen was selected for testing because it has been previously shown to cause myelosuppression in mice and humans. The dose-intensive regimen was administered to NOD.Cg-Prkdc(scid)IL2rg(tm1Wjl)/Sz (NOD/SCID/γchain(null)), reconstituted with human hematopoietic cells, and the impact of treatment on human hematopoiesis was evaluated.

RESULTS

The dose-intensive regimen resulted in significant decreases in growth of human glioblastoma xenografts. When this regimen was administered to mice containing humanized bone marrow, flow cytometric analyses indicated that the human bone marrow cells were significantly more sensitive to treatment than the murine bone marrow cells and that the regimen was highly toxic to human-derived hematopoietic cells of all lineages (progenitor, lymphoid, and myeloid).

CONCLUSIONS

The humanized bone marrow xenograft model described has the potential to be used as a platform for monitoring the impact of anticancer therapies on human hematopoiesis and could lead to subsequent refinement of therapies prior to clinical evaluation.

c-Abl phosphorylation of Mdm2 facilitates Mdm2-Mdmx complex formation

Mdm2 and Mdmx are oncoproteins that have essential yet nonredundant roles in development and function as part of a multicomponent ubiquitinating complex that targets p53 for proteasomal degradation. However, in response to DNA damage, Mdm2 and Mdmx are phosphorylated and protect p53 through various mechanisms. It has been predicted that Mdm2-Mdmx complex formation modulates Mdm2 ligase activity, yet the mechanism that promotes formation of Mdm2-Mdmx complexes is unknown. Here, we show that optimal Mdm2-Mdmx complex formation requires c-Abl phosphorylation of Mdm2 both in vitro and in vivo. In addition, Abl phosphorylation of Mdm2 is required for efficient ubiquitination of Mdmx in vitro, and eliminating c-Abl signaling, using c-Abl(-/-) knock-out murine embryonic fibroblasts, led to a decrease in Mdmx ubiquitination. Further, p53 levels are not induced as efficiently in c-Abl(-/-) murine embryonic fibroblasts following DNA damage. Overall, these results define a direct link between genotoxic stress-activated c-Abl kinase signaling and Mdm2-Mdmx complex formation. Our results add an important regulatory mechanism for the activation of p53 in response to DNA damage.

Controlling the Mdm2-Mdmx-p53 Circuit

The p53 tumor suppressor is a key protein in maintaining the integrity of the genome by inducing either cell cycle arrest or apoptosis following cellular stress signals. Two human family members, Mdm2 and Mdmx, are primarily responsible for inactivating p53 transcription and targeting p53 protein for ubiquitin-mediated degradation. In response to genotoxic stress, post-translational modifications to p53, Mdm2 and Mdmx stabilize and activate p53. The role that phosphorylation of these molecules plays in the cellular response to genotoxic agents has been extensively studied with respect to cancer biology. In this review, we discuss the main phosphorylation events of p53, Mdm2 and Mdmx in response to DNA damage that are important for p53 stability and activity. In tumors that harbor wild-type p53, reactivation of p53 by modulating both Mdm2 and Mdmx signaling is well suited as a therapeutic strategy. However, the rationale for development of kinase inhibitors that target the Mdm2-Mdmx-p53 axis must be carefully considered since modulation of certain kinase signaling pathways has the potential to destabilize and inactivate p53.

Abstract 5373: Modulation of temozolomide-mediated DNA damage in glioblastoma multiforme by the HDM2 antagonist, nutlin3

Development of efficacious strategies for eradication of glioblastoma multiforme (GBM) remains a significant challenge. While treatment of GBM with radiation and temozolomide (TMZ) has improved clinical outcome, lack of long-term efficacy can be due to the ability of GBM to acquire resistance to therapy by modulation of signaling pathways that control DNA repair and cell survival. The HDM2 antagonist, Nutlin3, blocks interactions of HDM2 with key signaling molecules such as p53, p73α, and HIF1α, and modulates their downstream effector function. A panel of human GBM cell lines is currently being used to monitor the extent to which Nutlin3 can sensitize cells to TMZ. In vitro survival assays indicate that Nutlin3 potentiates TMZ-mediated cell death; analysis of cell survival data using Dose Effect CalcuSyn software indicates that the combination index value was < 1 and indicative of a synergistic effect between Nutlin3 and TMZ. Western analyses of p53 wild-type GBM indicated increased p53 stabilization and activation of down-stream targets in cells treated with the combination of Nutlin3/TMZ compared to vehicle, TMZ, or Nutlin3. In addition, time-course studies indicated that U87-MG cells treated with Nutlin3/TMZ in comparison to single agent resulted in increased γ-H2AX and DNA-strand breaks. Efficacy studies using ectopic U87-MG xenografts indicated that 2 cycles of 5-consecutive days of Nutlin3/TMZ led to a significant decrease in tumor growth compared to vehicle or single agent (p < 0.001) with no signs of off-target toxicity. Regulation of DNA repair was further analyzed using the primary GBM.10 line that when grown as an ectopic xenograft, expresses the direct-reversal repair protein, O6-methylguanine DNA methyltransferase (MGMT) and proteins involved in base-excision repair (BER). In vivo target-validation studies using GBM.10 xenografts confirmed that repair of TMZ-mediated DNA damage is modulated in the presence of Nutlin3. GBM.10 xenografts were treated with vehicle, TMZ, nutlin3, or TMZ/Nutlin3 and modulation of critical targets determined by Western analyses. Increases in total p53, p53 phosphorylated at residue serine16, p73α, and substantial decreases in MGMT and the BER protein, APE-1, were observed following treatment with Nutlin3/TMZ compared to vehicle or single-agent exposure. Busso et al (Oncogene 2009) recently reported that HDM2 can monoubiquitinate APE-1 leading to proteosomal-mediated degradation. In addition, recombinant activity assays in our laboratory indicate that HDM2 directly monoubiquitinates recombinant MGMT and may play a role in its degradation. These data suggest that DNA repair proteins required for repair of TMZ-mediated DNA damage can be downregulated by altering HDM2-mediated signaling. Combination therapy that targets the p53-HDM2 E3 ligase network and DNA repair represents a novel approach towards improving treatment of GBM.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5373.

Abstract 5069: c-Abl contributes to Mdm2-Mdmx complex formation protecting p53 levels

This study was designed to better understand the signaling events through c-Abl kinase that impact p53 activity. The p53 tumor suppressor protein is a key protein in maintaining the integrity of the genome by inducing either cell cycle arrest or apoptosis following cellular stress signals. Two human family members, Mdm2 and Mdmx, are primarily responsible for inactivating transcription initiation of p53 or destabilizing p53 through ubiquitination. In response to genotoxic stress, post-translational modifications to Mdm2 and Mdmx relieve inhibition on p53, which is in part mediated through the c-Abl kinase. c-Abl is necessary for maintaining p53 expression levels and for maximal accumulation of p53 following DNA damage. While p53 is not a direct substrate of c-Abl there are c-Abl phosphorylation sites present in Mdm2 and Mdmx. Considering that c-Abl regulates both Mdm2 and Mdmx, we designed this study to understand the role of c-Abl phosphorylation of Mdm2 in protecting p53 activity. We hypothesized that protection of p53 would be linked to Mdm2-Mdmx complex formation and modulated by c-Abl phosphorylation. Here we present a mechanism of p53 stabilization whereby c-Abl promotes Mdm2-Mdmx heterodimer complex formation. For this study we used multiple complementary approaches including: recombinant in vitro protein assays, therapeutic drug treatments of cells and targeted mutagenesis of Mdm2 phosphorylation sites to show the role of c-Abl in protecting p53. Additionally, blocking c-Abl phosphorylation has therapeutic implications in treatment of disease through modulation of p53 activity. There are several small molecule inhibitors that affect c-Abl activity, such as imatinib, and a better understanding of the mechanistic role of c-Abl in the stoichiometric balance between Mdm2 and Mdmx may lead to better approaches for engaging p53 activity. This could lead to improvements in current disease treatment and the development of novel therapeutics for cancers harboring wild-type p53.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5069.

Inactivation of p53 signaling by p73 or PTEN ablation results in a transformed phenotype that remains susceptible to Nutlin-3 mediated apoptosis

The p53 signaling pathway is frequently disrupted in carcinogenesis. However, roughly 50% of all cancers express wild-type p53 and have alterations in accessory signaling components required for p53 activity. Using the well described E1A/RAS transformation model, in which p53 activity must be suppressed for transformation, we show here that p53 is inactive and unable to suppress transformation following ablation of p73 or PTEN. However, despite the transformed phenotype conferred by p53 inactivation following p73 or PTEN loss, p53 could be fully activated by Nutlin-3, resulting in efficient caspase-mediated apoptosis. Our novel and unexpected finding provides important information regarding the efficacy of Nutlin-3 and indicates that patients with tumors deficient in p53 function due to p73 or PTEN loss may benefit from Nutlin-3 treatment.

TGF-beta1-induced expression of human Mdm2 correlates with late-stage metastatic breast cancer

The E3 ubiquitin ligase human murine double minute (HDM2) is overexpressed in 40%-80% of late-stage metastatic cancers in the absence of gene amplification. Hdm2 regulates p53 stability via ubiquitination and has also been implicated in altering the sensitivity of cells to TGF-beta1. Whether TGF-beta1 signaling induces Hdm2 expression leading to HDM2-mediated destabilization of p53 has not been investigated. In this study, we report that TGF-beta1-activated SMA- and MAD3 (Smad3/4) transcription factors specifically bound to the second promoter region of HDM2, leading to increased HDM2 protein expression and destabilization of p53 in human cancer cell lines. Additionally, TGF-beta1 expression led to Smad3 activation and murine double minute 2 (Mdm2) expression in murine mammary epithelial cells during epithelial-to-mesenchymal transition (EMT). Furthermore, histological analyses of human breast cancer samples demonstrated that approximately 65% of late-stage carcinomas were positive for activated Smad3 and HDM2, indicating a strong correlation between TGF-beta1-mediated induction of HDM2 and late-stage tumor progression. Identification of Hdm2 as a downstream target of TGF-beta1 represents a critical prosurvival mechanism in cancer progression and provides another point for therapeutic intervention in late-stage cancer.

Up-regulation of hepatitis C virus replication and production by inhibition of MEK/ERK signaling

BACKGROUND

Viruses interact with and exploit the host cellular machinery for their multiplication and propagation. The MEK/ERK signaling pathway positively regulates replication of many RNA viruses. However, whether and how this signaling pathway affects hepatitis C virus (HCV) replication and production is not well understood.

METHODS AND RESULTS

In this study, we took advantage of two well-characterized MEK/ERK inhibitors and MEK/ERK dominant negative mutants and investigated the roles of the MEK/ERK signaling pathway in HCV gene expression and replication. We showed that inhibition of MEK/ERK signaling enhanced HCV gene expression, plus- and minus-strand RNA synthesis, and virus production. In addition, we showed that this enhancement was independent of interferon-alpha (IFN-alpha) antiviral activity and did not require prior activation of the MEK/ERK signaling pathway. Furthermore, we showed that only MEK and ERK-2 but not ERK-1 was involved in HCV replication, likely through regulation of HCV RNA translation.

CONCLUSIONS

Taken together, these results demonstrate a negative regulatory role of the MEK/ERK signaling pathway in HCV replication and suggest a potential risk in targeting this signaling pathway to treat and prevent neoplastic transformation of HCV-infected liver cells.

Ribosomal protein S3: A multi-functional protein that interacts with both p53 and MDM2 through its KH domain

The p53 protein responds to cellular stress and regulates genes involved in cell cycle, apoptosis, and DNA repair. Under normal conditions, p53 levels are kept low through MDM2-mediated ubiquitination and proteosomal degradation. In search for novel proteins that participate in this regulatory loop, we performed an MDM2 peptide pull-down assay and mass spectrometry to screen for potential interacting partners of MDM2. We identified ribosomal protein S3 (RPS3), whose interaction with MDM2, and notably p53, was further established by His and GST pull-down assays, fluorescence resonance energy transfer and an in situ proximity ligation assay. Additionally, in cells exposed to oxidative stress, p53 levels increased slightly over 24h, whereas MDM2 levels declined after 6h exposure, but rose over the next 18h of exposure. Conversely, in cells exposed to oxidative stress and harboring siRNA to knockdown RPS3 expression, decreased p53 levels and loss of the E3 ubiquitin ligase domain possessed by MDM2 were observed. DNA pull-down assays using a 7,8-dihydro-8-oxoguanine duplex oligonucleotide as a substrate found that RPS3 acted as a scaffold for the additional binding of MDM2 and p53, suggesting that RPS3 interacts with important proteins involved in maintaining genomic integrity.

Hdm2 is a ubiquitin ligase of Ku70-Akt promotes cell survival by inhibiting Hdm2-dependent Ku70 destabilization

Earlier, we have reported that 70 kDa subunit of Ku protein heterodimer (Ku70) binds and inhibits Bax activity in the cytosol and that ubiquitin (Ub)-dependent proteolysis of cytosolic Ku70 facilitates Bax-mediated apoptosis. We found that Hdm2 (human homolog of murine double minute) has an ability to ubiquitinate Ku70 and that Hdm2 overexpression in cultured cells causes a decrease in Ku70 expression levels. An interaction between Ku70 and Hdm2 was shown by means of immunoprecipitation, whereas none could be shown between 80 kDa subunit of Ku protein heterodimer and Hdm2. Vascular endothelial growth factor (VEGF) is known to inhibit endothelial cell (EC) apoptosis through an Akt-mediated survival kinase signal; however, the mechanism underlying this inhibition of apoptosis has not been fully elucidated. We found that VEGF inhibited cytosolic Ku70 degradation induced by apoptotic stress. It is known that Akt-dependent phosphorylation of Hdm2 causes nuclear translocation of Hdm2 followed by Hdm2-mediated inactivation of p53. We found that VEGF stimulated nuclear translocation of Hdm2 in EC and efficiently inhibited Ku70 degradation. We also found that constitutively active Akt, but not kinase-dead Akt, inhibited Ku70 degradation in the cytosol. Furthermore, Ku70 knockdown diminished antiapoptotic activity of Akt. Taken together, we propose that Hdm2 is a Ku70 Ub ligase and that Akt inhibits Bax-mediated apoptosis, at least in part, by maintaining Ku70 levels through the promotion of Hdm2 nuclear translocation.

Therapeutic considerations for Mdm2: not just a one trick pony

BACKGROUND: The mdm2 proto-oncogene is elevated in numerous late stage cancers. The Mdm2 protein manifests its oncogenic properties in part through inactivation of the tumor suppressor protein p53. Recent efforts in anti-cancer drug design have focused on the identification of small molecules that disrupt the Mdm2-p53 interaction, in hopes of re-engaging the p53 pathway. OBJECTIVE: In addition to binding p53, Mdm2 complexes with numerous proteins involved in DNA repair, translation, metabolic activities, tumor growth and apoptosis. Additional biochemical analysis is required to understand how Mdm2 integrates into all of these cellular processes. Post-translational modifications to Mdm2 can alter its ability to associate with numerous proteins. Changes in protein structure may also affect the ability of small molecule inhibitors to effectively antagonize Mdm2. CONCLUSION: The complexity of Mdm2 modification has been largely neglected during the development of previous Mdm2 inhibitors. Future high-throughput or in silico screening efforts will need to recognize the importance of post-translational modifications to Mdm2. Furthermore, the identification of molecules that target other domains in Mdm2 may provide a tool to prevent other pivotal p53-independent functions of Mdm2. These aims provide a useful roadmap for the discovery of new Mdm2 binding compounds with therapeutic potency that may exceed its predecessors.

Nutlin3 blocks vascular endothelial growth factor induction by preventing the interaction between hypoxia inducible factor 1alpha and Hdm2

Hdm2 is elevated in numerous types of malignancies and is thought to impede the function of wild-type p53. Reactivation of p53 by disrupting the association with Hdm2 was the impetus for the development of Nutlin3. Although regulation of p53 has been the central focus of Hdm2 activity, it also binds other proteins through its p53-binding domain. Here, we show that hypoxia-inducible factor 1alpha (HIF1alpha) binds to Hdm2 in the domain designated to bind p53. HIF1alpha and p53 share a conserved motif that is required to bind Hdm2. Distinct complexes form between Hdm2-HIF1alpha and Hdm2-p53 as determined by immunoprecipitation of nuclear extracts and in vitro. The Hdm2 antagonist Nutlin3 prevents the association between Hdm2 and HIF1alpha. The vascular endothelial growth factor (VEGF) gene is a transcriptional target of HIF1alpha, and under normoxic or hypoxic conditions, Hdm2 increases HIF1alpha activity to induce VEGF production. Blocking the association of Hdm2 and HIF1alpha by Nutlin3, or ablating Hdm2 expression, diminished the level of VEGF under conditions of normoxia or hypoxia. Our findings establish a unique role for Nutlin3 in attenuating VEGF induction by preventing the association of Hdm2 with HIF1alpha.

Levels of HdmX expression dictate the sensitivity of normal and transformed cells to Nutlin-3

Hdm2 and HdmX coordinately regulate the stability and function of p53. Each is overexpressed in subsets of many different types of malignancy, and most of these subsets maintain wild-type p53. Nutlins, newly discovered small-molecule inhibitors of the Hdm2-p53 interaction, offer a novel strategy for therapy of tumors with wild-type p53. We now show that Nutlin-3 efficiently induces apoptosis and diminishes long-term survival of human fibroblasts transformed in vitro by Hdm2 but not HdmX. The resistance of cells overexpressing HdmX to Nutlin-3 is due to its inability to disrupt the p53-HdmX interaction, resulting in continued suppression of p53 activity. Although HdmX overexpression yielded cells resistant to Nutlin-3, ablation of HdmX expression by short hairpin RNA sensitized tumor cells to Nutlin-3-mediated cell death or arrest. Furthermore, deletion of the COOH-terminal RING finger domain of HdmX completely reversed the resistance to Nutlin-3, probably reflecting the requirement of the RING finger for interaction with Hdm2. Thus, the relative abundance of Hdm2 and HdmX and the specificity of Nutlin-3 for Hdm2 influence the sensitivity of cells to p53-dependent apoptosis or arrest in response to Nutlin-3. Our findings establish Hdm2 and HdmX as independent therapeutic targets with respect to reactivating wild-type p53 as a means for cancer therapy.

Hdm2 nuclear export, regulated by insulin-like growth factor-I/MAPK/p90Rsk signaling, mediates the transformation of human cells

Insulin-like growth factor (IGF)-I receptor activation leads to enhanced proliferation and cell survival via the MAP kinase and phosphatidylinositol 3-kinase-signaling pathways. Upon stimulation by IGF-I, the Hdm2 oncoprotein is phosphorylated by AKT, leading to its rapid nuclear translocation and subsequent inhibition of p53. We now show that IGF-I stimulation regulates the nuclear export of Hdm2 and p53 via the MAP kinase pathway. Inhibition of p38 MAPK or MEK via pharmacological means or expression of dominant negative proteins inhibited the cytoplasmic accumulation of Hdm2 and increased Hdm2 and p53 protein levels, whereas constitutively active p90Rsk promoted the nuclear export of Hdm2. Expression of constitutively active p90Rsk with E1A, oncogenic H-Ras, and hTERT resulted in the anchorage-independent growth of normal human fibroblasts. Our findings link p90Rsk-mediated modulation of Hdm2 nuclear to cytoplasmic shuttling with the diminished ability of p53 to regulate cell cycle checkpoints that ultimately leads to transformation.