Deoxyhypusine hydroxylase: A novel therapeutic target differentially expressed in short-term vs long-term survivors of glioblastoma

Glioblastoma (GB) is the most aggressive neoplasm of the brain. Poor prognosis is mainly attributed to tumor heterogeneity, invasiveness and drug resistance. Only a small fraction of GB patients survives longer than 24 months from the time of diagnosis (ie, long-term survivors [LTS]). In our study, we aimed to identify molecular markers associated with favorable GB prognosis as a basis to develop therapeutic applications to improve patients' outcome. We have recently assembled a proteogenomic dataset of 87 GB clinical samples of varying survival rates. Following RNA-seq and mass spectrometry (MS)-based proteomics analysis, we identified several differentially expressed genes and proteins, including some known cancer-related pathways and some less established that showed higher expression in short-term (<6 months) survivors (STS) compared to LTS. One such target found was deoxyhypusine hydroxylase (DOHH), which is known to be involved in the biosynthesis of hypusine, an unusual amino acid essential for the function of the eukaryotic translation initiation factor 5A (eIF5A), which promotes tumor growth. We consequently validated DOHH overexpression in STS samples by quantitative polymerase chain reaction (qPCR) and immunohistochemistry. We further showed robust inhibition of proliferation, migration and invasion of GB cells following silencing of DOHH with short hairpin RNA (shRNA) or inhibition of its activity with small molecules, ciclopirox and deferiprone. Moreover, DOHH silencing led to significant inhibition of tumor progression and prolonged survival in GB mouse models. Searching for a potential mechanism by which DOHH promotes tumor aggressiveness, we found that it supports the transition of GB cells to a more invasive phenotype via epithelial-mesenchymal transition (EMT)-related pathways.

MCP-1/CCR2 axis inhibition sensitizes the brain microenvironment against melanoma brain metastasis progression

Development of resistance to chemo- and immunotherapies often occurs following treatment of melanoma brain metastasis (MBM). The brain microenvironment (BME), particularly astrocytes, cooperate toward MBM progression by upregulating secreted factors, among which we found that monocyte chemoattractant protein-1 (MCP-1) and its receptors, CCR2 and CCR4, were overexpressed in MBM compared with primary lesions. Among other sources of MCP-1 in the brain, we show that melanoma cells altered astrocyte secretome and evoked MCP-1 expression and secretion, which in turn induced CCR2 expression in melanoma cells, enhancing in vitro tumorigenic properties, such as proliferation, migration, and invasion of melanoma cells. In vivo pharmacological blockade of MCP-1 or molecular knockout of CCR2/CCR4 increased the infiltration of cytotoxic CD8⁺ T cells and attenuated the immunosuppressive phenotype of the BME as shown by decreased infiltration of Tregs and tumor-associated macrophages/microglia in several models of intracranially injected MBM. These in vivo strategies led to decreased MBM outgrowth and prolonged the overall survival of the mice. Our findings highlight the therapeutic potential of inhibiting interactions between BME and melanoma cells for the treatment of this disease.

Abstract 266: Adaptation of colorectal cancer cells to the brain microenvironment: The role of IRS2

Colorectal cancer (CRC) reflects the fourth most frequent etiology of brain metastasis (BM), with rising incidence. Yet, molecular mechanisms supporting the formation of these lesions from CRC are unknown. We aimed to explore drivers enabling tropism and adaptation of CRC cells to the brain environment and decipher mechanisms facilitating the process. We analyzed the FoundationOne database, which contains genomic alterations data of cancer-related genes in over 16,000 human CRC primary and metastasis samples. Increased prevalence of IRS2 gene amplification was observed in 13% of BM, compared to only 3% of primary tumors or other metastatic sites. IRS2 is a cytoplasmic adaptor mediating effects of insulin and IGF-1 receptors and is involved in more aggressive behavior of different cancer types. In agreement with the genomic data, immunohistochemistry of human clinical samples showed increased expression of IRS2 protein in BM. We constructed an in vitro system mimicking the brain microenvironment using cultured human astrocytes or their conditioned media. Under these conditions, IRS2-overexpressed CRC cells survived better and formed larger 3D spheres. IRS2-silenced CRC cells showed a mirror image. Moreover, in an intracranial CRC BM mouse model, IRS2-overexpressed cells generated larger brain lesions, while silencing IRS2 dramatically decreased tumor outgrowth and extended survival. Interestingly, transcriptomic analysis revealed enrichment of oxidative phosphorylation (OXPHOS) and Wnt/β-catenin pathways by IRS2. Indeed, IRS2-expressing cells showed increased mitochondrial activity and glycolysis-independent viability. Furthermore, IRS2-expressing cells had increased β-catenin transcriptional activity. Interestingly, β-catenin inhibition (using ICG-001) or IRS2 inhibition (using NT219) in IRS2-expressing cells decreased their viability, β-catenin transcriptional activity, and mitochondrial activity, suggesting involvement of IRS2 in modulating OXPHOS through β-catenin. β-catenin is known to confer 5-FU resistance; consequently, we showed that combination of 5-FU and NT219 worked in synergy, inhibited the formation of BM, and extended animal survival. These data reveal, for the first time, the unique genomic profile of CRC BM and imply the IRS2 role in promoting CRC BM. These effects may be mediated, at least in part, by modulation of the β-catenin and OXPHOS pathway. Given the molecular signature described, the approach to patients with BM may be significantly impacted by agents such as NT219.

Citation Format: Inbal Greenberg, Anat Klein, Rachel Grossman, Ethan Sokol, Eilam Yeini, Paula Ofek, Ronit Satchi-Fainaro, Bertrand Liang, Hadas Reuveni, Tami Rubinek, Ido Wolf. Adaptation of colorectal cancer cells to the brain microenvironment: The role of IRS2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 266.

BSCI-03. Adaptation of colorectal cancer cells to the brain microenvironment: The role of IRS2

Colorectal cancer (CRC) reflects the fourth most frequent etiology of brain metastasis (BM). Yet, molecular mechanisms supporting it are unknown. We aimed to explore drivers enabling adaptation of CRC cells to the brain and decipher mechanisms facilitating the process.

We analyzed the FoundationOne database, which contains genomic alterations data of cancer-related genes in over 16,000 human CRC primary and metastasis samples. Increased prevalence of IRS2 gene amplification was observed in 13% of BM, compared to only 3% of primary tumors or other metastatic sites. IRS2 is a cytoplasmic adaptor mediating effects of insulin and IGF-1 receptors and is involved in more aggressive behavior of different cancer types. In agreement with the genomic data, immunohistochemistry of human clinical samples showed increased expression of IRS2 protein in BM. We constructed an in-vitro system mimicking the brain microenvironment using cultured human astrocytes or their conditioned media. Under these conditions, IRS2-overexpressed CRC cells survived better and formed larger 3D spheres. IRS2-silenced CRC cells showed a mirror image. Moreover, in an intracranial CRC BM mouse model, IRS2-overexpressed cells generated larger brain lesions, while silencing IRS2 dramatically decreased tumor outgrowth and extended survival. Interestingly, transcriptomic analysis revealed enrichment of oxidative phosphorylation (OXPHOS) and Wnt/β-catenin pathways by IRS2. Indeed, IRS2-expressing cells showed increased mitochondrial activity and glycolysis-independent viability. Furthermore, IRS2-expressing cells had increased β-catenin transcriptional activity. Interestingly, β-catenin or IRS2 inhibition (using NT219) in IRS2-expressing cells decreased their viability, β-catenin transcriptional activity, and OXPHOS gene expression, suggesting involvement of IRS2 in modulating OXPHOS through β-catenin. β-catenin is known to confer 5-FU resistance; consequently, we showed that combination of 5-FU and NT219 worked in synergy, inhibited the formation of BM, and extended animal survival.

These data reveal the unique genomic profile of CRC BM and suggest IRS2 inhibition as a novel target for treatment of these patients.

Meet me halfway: Are in vitro 3D cancer models on the way to replace in vivo models for nanomedicine development?

The complexity and diversity of the biochemical processes that occur during tumorigenesis and metastasis are frequently over-simplified in the traditional in vitro cell cultures. Two-dimensional cultures limit researchers' experimental observations and frequently give rise to misleading and contradictory results. Therefore, in order to overcome the limitations of in vitro studies and bridge the translational gap to in vivo applications, 3D models of cancer were developed in the last decades. The three dimensions of the tumor, including its cellular and extracellular microenvironment, are recreated by combining co-cultures of cancer and stromal cells in 3D hydrogel-based growth factors-inclusive scaffolds. More complex 3D cultures, containing functional blood vasculature, can integrate in the system external stimuli (e.g. oxygen and nutrient deprivation, cytokines, growth factors) along with drugs, or other therapeutic compounds. In this scenario, cell signaling pathways, metastatic cascade steps, cell differentiation and self-renewal, tumor-microenvironment interactions, and precision and personalized medicine, are among the wide range of biological applications that can be studied. Here, we discuss a broad variety of strategies exploited by scientists to create in vitro 3D cancer models that resemble as much as possible the biology and patho-physiology of in vivo tumors and predict faithfully the treatment outcome.

Abstract 2716: P-selectin axis plays a key role in microglia immunophenotype and glioblastoma progression

Glioblastoma (GB) is an aggressive type of brain cancer with high mortality rate. It is a highly angiogenic tumor exhibiting an extremely invasive nature. As such, its brain microenvironment plays a crucial role in its progression. Microglia are the brain resident immune cells which have been shown to facilitate GB cell invasion and immune suppression. The mechanism by which GB cells alter microglia behavior is yet to be fully understood. One proposed mechanism involves adhesion molecules such as the Selectins family of proteins which are expressed on the surface of endothelial and immune cells and are involved in immune modulation and cancer immunity. We have previously shown that P-Selectin (SELP) is expressed by GB cells. Here, we investigated the factional role of SELP in GB-microglia interactions. First, we found that microglia cells facilitate the expression and secretion of SELP by GB cells, and that GB cells facilitate the expression of P-Selectin ligand by microglia. We then showed that SELP mediates microglia-enhanced GB invasion and proliferation in 2D and 3D in vitro models and has a role in microglia activation state. These findings were validated in vivo, showing that inhibition or downregulation of SELP leads to reduced tumor growth, increased overall survival and improved immune response. Single-Cells RNA-seq analysis of the tumors revealed an increase in pro-inflammatory microglia signature, reduction in cancer cell tumorigenesis potential and improved T cell activation. Our results indicated that SELP has an important role in GB progression and microenvironment activation. This work can improve our understanding of tumor-associated microglia function and the mechanisms by which GB cells suppress the immune system and invade the brain tissue.

Citation Format: Eilam Yeini, Paula Ofek, Sabina Pozzi, Nitzan Albeck, Dikla Ben-Shushan, Galia Tiram, Sapir Golan, Ron Kleiner, Ron Sheinin, Shlomit Reich-Zeliger, Rachel Grossman, Zvi Ram, Henry Brem, Thomas Hyde, Prerna Magod, Dinorah Friedmann-Morvinski, Asaf Madi, Ronit Satchi-Fainaro. P-selectin axis plays a key role in microglia immunophenotype and glioblastoma progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2716.

Abstract 2854: Adaptation of colorectal cancer cells to the brain microenvironment: the role of IRS2

Colorectal cancer (CRC) reflects the fourth most frequent etiology of brain metastasis (BM), with rising incidence. Yet, molecular mechanisms supporting the formation of these lesions from CRC are unknown. We aimed to explore drivers enabling tropism and adaptation of CRC cells to the brain environment and decipher mechanisms facilitating the process. We analyzed the FoundationOne database, which contains genomic alterations data of cancer-related genes in over 16,000 human CRC primary and metastasis samples. Increased prevalence of insulin receptor substrate 2 (IRS2) gene amplification was noted in BMs relative to primary tumors and other metastatic sites. IRS2 is a cytoplasmic adaptor mediating effects of insulin and IGF-1 receptors and is known to be involved in more aggressive behavior of different cancer types. In agreement with the genomic data, immunohistochemistry of human clinical samples showed increased expression of IRS2 protein in BMs. We subsequently constructed an in vitro system mimicking the brain microenvironment using cultured human astrocytes or their conditioned media (CM). Under these conditions using InSphero system, IRS2-overexpressed CRC cells survived better and formed larger 3D spheres. IRS2-silenced CRC cells showed a mirror image. Moreover, in an intracranial CRC surgical implantation BMs mouse model, IRS2-overexpressed CRC cells generated larger brain lesions, while silencing IRS2 in IRS2-amplified CRC cells dramatically decreased tumor outgrowth and survival. Interestingly, transcriptomic analysis revealed significant enrichment of the oxidative phosphorylation pathway by IRS2. Indeed, IRS2-expressing CRC cells showed increased mitochondrial activity using Seahorse extracellular flux analysis and glycolysis-independent viability. Furthermore, IRS2-expressing cells showed enhanced AKT phosphorylation and inhibition of PI3K or AKT using Alpelisib or iAKT, respectively, slightly decreased their proliferation in CM, suggesting that mechanisms additional to the AKT pathway may mediate IRS2 activity. The Wnt/β-catenin pathway was among the most significantly enriched pathways in the transcriptome. Indeed, IRS2-expressing cells had increased transcriptional activity of the β-catenin. In addition, iAKT or Alpelisib, and most significantly the IRS2 inhibitor (NT219), decreased the transcriptional activity of β-catenin in IRS2-expressing CRC cells, suggesting relevance of IRS2 in activating β-catenin. Moreover, NT219 demonstrated significant and dose-dependent inhibition of CRC cells viability. These data reveal, for the first time, the unique genomic profile of CRC BMs and imply the IRS2 role in promoting CRC BMs. These effects may be mediated, at least in part, by modulation of the AKT and β-catenin pathway. Given the molecular signature described, the approach to patients with BMs may be significantly impacted by agents such as NT219.

Citation Format: Inbal Greenberg, Anat Klein, Rachel Grossman, Ethan Sokol, Eilam Yeini, Paula Ofek, Ronit Satchi-Fainaro, Bertrand Liang, Hadas Reuveni, Tami Rubinek, Ido Wolf. Adaptation of colorectal cancer cells to the brain microenvironment: the role of IRS2 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2854.

P-selectin axis plays a key role in microglia immunophenotype and glioblastoma progression

Glioblastoma (GB) is a highly invasive type of brain cancer exhibiting poor prognosis. As such, its microenvironment plays a crucial role in its progression. Among the brain stromal cells, the microglia were shown to facilitate GB invasion and immunosuppression. However, the reciprocal mechanisms by which GB cells alter microglia/macrophages behavior are not fully understood. We propose that these mechanisms involve adhesion molecules such as the Selectins family. These proteins are involved in immune modulation and cancer immunity. We show that P-selectin mediates microglia-enhanced GB proliferation and invasion by altering microglia/macrophages activation state. We demonstrate these findings by pharmacological and molecular inhibition of P-selectin which leads to reduced tumor growth and increased survival in GB mouse models. Our work sheds light on tumor-associated microglia/macrophage function and the mechanisms by which GB cells suppress the immune system and invade the brain, paving the way to exploit P-selectin as a target for GB therapy.

Abstract PO-003: Adaptation of colorectal cancer cells to the brain microenvironment: The role of IRS2

Brain metastases (BMs) from colorectal cancer (CRC) are currently the fourths leading cause of BMs, and their incidence is on the rise. Yet, mechanisms mediating the formation of BMs from CRC are unknown. We aimed to explore genomic drivers enabling tropism and adaptation of CRC cells to the brain environment and decipher mechanisms facilitating the process. We analyzed FoundationOne database, which contains genomic alterations data of cancer-related genes in over 16,000 human CRC primary and metastasis samples. Increased prevalence was noted in the insulin receptor substrate 2 (IRS2) gene, which was significantly amplified in BMs relative to primary tumors and other metastatic sites. IRS2 is a cytoplasmic adaptor mediating effects of insulin and IGF-1 receptors and is known to be involved in more aggressive behavior of different cancer types. In agreement with the genomic data, immunohistochemistry of human clinical samples showed increased expression of IRS2 protein in BMs. Next, we created an in-vitro system mimicking the brain microenvironment using human astrocytes or their conditioned media (CM). Under these conditions, IRS2-overexpressed CRC cells survived better and formed larger 3D spheres. IRS2-silenced CRC cells showed a mirror image. Furthermore, IRS2-overexpressed CRC cells generated larger brain lesions in an intracranial CRC BMs mice model. Interestingly, a transcriptomic analysis revealed significant enrichment of the AKT and β-catenin pathways by IRS2. Indeed, IRS2-amplified cells showed enhanced AKT phosphorylation and inhibition of PI3K or AKT using Alpelisib or Akt1/2 kinase inhibitor, respectively, effectively decreased their proliferation in CM. Importantly, IRS2-amplified cells had increased expression and transcriptional activity of the β-catenin, and AKT pathway inhibitors or IRS2 inhibitor (NT-219) had a more pronounced effect on the activity of β-catenin on those cells. These data indicate, for the first time, the unique genomic profile of CRC BMs and imply the IRS2 role in promoting CRC BMs. These effects may be mediated, at least in part, by modulation of the AKT and β-catenin pathway. These findings may pave the way for the development of novel therapeutic strategies to prevent BMs formation.

Citation Format: Inbal Greenberg, Anat Klein, Rachel Grossman, Ethan Sokol, Eilam Yeini, Paula Ofek, Ronit Satchi-Fainaro, Hadas Reuveni, Tami Rubinek, Ido Wolf. Adaptation of colorectal cancer cells to the brain microenvironment: The role of IRS2 [abstract]. In: Abstracts: AACR Special Virtual Conference on Epigenetics and Metabolism; October 15-16, 2020; 2020 Oct 15-16. Philadelphia (PA): AACR; Cancer Res 2020;80(23 Suppl):Abstract nr PO-003.

Reverting the molecular fingerprint of tumor dormancy as a therapeutic strategy for glioblastoma

Glioblastoma is an aggressive and invasive brain malignancy with high mortality rates despite current treatment modalities. In this study, we show that a 7-gene signature, previously found to govern the switch of glioblastomas from dormancy to aggressive tumor growth, correlates with improved overall survival of patients with glioblastoma. Using glioblastoma dormancy models, we validated the role of 2 genes from the signature, thrombospondin-1 ( TSP-1) and epidermal growth factor receptor ( EGFR), as regulators of glioblastoma dormancy and explored their therapeutic potential. EGFR up-regulation was reversed using EGFR small interfering RNA polyplex, antibody, or small-molecule inhibitor. The diminished function of TSP-1 was augmented via a peptidomimetic. The combination of EGFR inhibition and TSP-1 restoration led to enhanced therapeutic efficacy in vitro, in 3-dimensional patient-derived spheroids, and in a subcutaneous human glioblastoma model in vivo. Systemic administration of the combination therapy to mice bearing intracranial murine glioblastoma resulted in marginal therapeutic outcomes, probably due to brain delivery challenges, p53 mutation status, and the aggressive nature of the selected cell line. Nevertheless, this study provides a proof of concept for exploiting regulators of tumor dormancy for glioblastoma therapy. This therapeutic strategy can be exploited for future investigations using a variety of therapeutic entities that manipulate the expression of dormancy-associated genes in glioblastoma as well as in other cancer types.-Tiram, G., Ferber, S., Ofek, P., Eldar-Boock, A., Ben-Shushan, D., Yeini, E., Krivitsky, A., Blatt, R., Almog, N., Henkin, J., Amsalem, O., Yavin, E., Cohen, G., Lazarovici, P., Lee, J. S., Ruppin, E., Milyavsky, M., Grossman, R., Ram, Z., Calderón, M., Haag, R., Satchi-Fainaro, R. Reverting the molecular fingerprint of tumor dormancy as a therapeutic strategy for glioblastoma.

Amphiphilic poly(α)glutamate polymeric micelles for systemic administration of siRNA to tumors

RNAi therapeutics carried a great promise to the area of personalized medicine: the ability to target "undruggable" oncogenic pathways. Nevertheless, their efficient tumor targeting via systemic administration had not been resolved yet. Amphiphilic alkylated poly(α)glutamate amine (APA) can serve as a cationic carrier to the negatively-charged oligonucleotides. APA polymers complexed with siRNA to form round-shaped, homogenous and reproducible nano-sized polyplexes bearing ~50 nm size and slightly negative charge. In addition, APA:siRNA polyplexes were shown to be potent gene regulators in vitro. In light of these preferred physico-chemical characteristics, their performance as systemically-administered siRNA nanocarriers was investigated. Intravenously-injected APA:siRNA polyplexes accumulated selectively in tumors and did not accumulate in the lungs, heart, liver or spleen. Nevertheless, the polyplexes failed to induce specific mRNA degradation, hence neither reduction in tumor volume nor prolonged mice survival was seen.

Amphiphilic nanocarrier-induced modulation of PLK1 and miR-34a leads to improved therapeutic response in pancreatic cancer

The heterogeneity of pancreatic ductal adenocarcinoma (PDAC) suggests that successful treatment might rely on simultaneous targeting of multiple genes, which can be achieved by RNA interference-based therapeutic strategies. Here we show a potent combination of microRNA and siRNA delivered by an efficient nanocarrier to PDAC tumors. Using proteomic-microRNA profiles and survival data of PDAC patients from TCGA, we found a novel signature for prolonged survival. Accordingly, we used a microRNA-mimic to increase miR-34a together with siRNA to silence PLK1 oncogene. For in vivo dual-targeting of this combination, we developed a biodegradable amphiphilic polyglutamate amine polymeric nanocarrier (APA). APA-miRNA-siRNA polyplexes systemically administered to orthotopically inoculated PDAC-bearing mice showed no toxicity and accumulated at the tumor, resulting in an enhanced antitumor effect due to inhibition of MYC oncogene, a common target of both miR-34a and PLK1. Taken together, our findings warrant this unique combined polyplex's potential as a novel nanotherapeutic for PDAC.

Co-targeting the tumor endothelium and P-selectin-expressing glioblastoma cells leads to a remarkable therapeutic outcome

Glioblastoma is a highly aggressive brain tumor. Current standard-of-care results in a marginal therapeutic outcome, partly due to acquirement of resistance and insufficient blood-brain barrier (BBB) penetration of chemotherapeutics. To circumvent these limitations, we conjugated the chemotherapy paclitaxel (PTX) to a dendritic polyglycerol sulfate (dPGS) nanocarrier. dPGS is able to cross the BBB, bind to P/L-selectins and accumulate selectively in intracranial tumors. We show that dPGS has dual targeting properties, as we found that P-selectin is not only expressed on tumor endothelium but also on glioblastoma cells. We delivered dPGS-PTX in combination with a peptidomimetic of the anti-angiogenic protein thrombospondin-1 (TSP-1 PM). This combination resulted in a remarkable synergistic anticancer effect on intracranial human and murine glioblastoma via induction of Fas and Fas-L, with no side effects compared to free PTX or temozolomide. This study shows that our unique therapeutic approach offers a viable alternative for the treatment of glioblastoma.

Angiogenesis regulation by nanocarriers bearing RNA interference

Since the approval of bevacizumab as anti-angiogenic therapy in 2004 by the FDA, an array of angiogenesis inhibitors have been developed and approved. However, results were disappointing with regard to their therapeutic efficacy. RNA interference approaches offer the possibility of rational design with high specificity, lacking in many current drug treatments for various diseases including cancer. However, in vivo delivery issues still represent a significant obstacle for widespread clinical applications. In the current review, we summarize the advances in the last decade in the field of angiogenesis-targeted RNA interference approaches, with special emphasis on oncology applications. We present pro-angiogenic and anti-angiogenic factors as potential targets, experimental evidence and clinical trials data on angiogenesis regulation by RNA interference. Consequent challenges and opportunities are discussed.

Functionalized nanogels carrying an anticancer microRNA for glioblastoma therapy

Glioblastoma Multiforme (GBM) is one of the most aggressive forms of all cancers. The median survival with current standard-of-care radiation and chemotherapy is about 14months. GBM is difficult to treat due to heterogeneity in cancer cell population. MicroRNA-based drugs have rapidly become a vast and burgeoning field due to the ability of a microRNA (miRNA) to target many genes involved in key cellular pathways. However, in vivo delivery of miRNA remains a crucial challenge for its therapeutic success. To bypass this shortcoming, we designed polymeric nanogels (NGs), which are based on a polyglycerol-scaffold, as a new strategy of miRNA delivery for GBM therapy. We focused on miR-34a, which is known for its key role in important oncogenic pathways and its tumor suppression ability in GBM and other cancers. We evaluated the capability of six NG derivatives to complex with miR-34a, neutralize its negative charge and deliver active miRNA to the cell cytoplasm. Human U-87 MG GBM cells treated with our NG-miR-34a nano-polyplexes showed remarkable downregulation of miR-34a target genes, which play key roles in the regulation of apoptosis and cell cycle arrest, and induce inhibition of cells proliferation and migration. Administration of NG-miR-34a nano-polyplexes to human U-87 MG GBM-bearing SCID mice significantly inhibited tumor growth as opposed to treatment with NG-negative control miR polyplex or saline. The comparison between different polyplexes highlighted the key features for the rational design of polymeric delivery systems for oligonucleotides. Taken together, we expect that this new therapeutic approach will pave the way for safe and efficient therapies for GBM.

Abstract B05: Patient-derived GBM tumors versus patient-derived primary cells and GBM cell lines: lessons from microRNA profiling

Glioblastoma multiforme is a very heterogeneous tumor, highly infiltrative, angiogenic and resistant to standard therapeutic intervention. Experimental models currently used in research are mostly based on cancer cell lines, which are grown in culture for many generations and have lost many essential biological characteristics. In consequence, the xenograft tumor models derived from those cells do not maintain genomic and phenotypic characteristics present in the original tumor. It is questionable whether these artificial preclinical models can serve as reliable platforms to select the lead candidate for a novel therapeutic approach.

We utilized miRNA expression patterns to evaluate the clinical relevance of some of the currently available experimental models of GBM, searching for similarities and differences in miRNA expression levels between freshly isolated tumors, patient-derived primary cells and GBM cell lines grown in culture. The study included 22 formalin-fixed paraffin-embedded (FFPE) tumors from glioblastoma resections. Those were divided into two groups of Short Term Survivors (STS, survived up to 3 months from initial diagnosis; n=12) and Long Term Survivors (LTS, survived more than 3 years from initial diagnosis; n=10).We further tested 6 patient-derived primary cells, and 5 ATCC human GBM cell lines widely used in preclinical research. Two of the cell lines included dormant and fast-growing variants previously developed in our laboratory to resemble LTS and STS phenotypes, respectively. Samples were loaded on custom microRNA (miR) arrays, including 2172 known miRNAs (miRBase 19).

We found several miRNAs that were upregulated in the STS samples while others exhibited higher expression levels in the LTS samples.

Both GBM cell lines and patient-derived primary cells exhibited a miRNA profile which was very different from patient samples. Brain miRNAs used as brain markers, such as hsa-miR-124-3p), show relatively low expression levels in both patient-derived primary cells and commercial cell lines but are highly expressed in tumors.

We concluded that GBM experimental models must be reevaluated and pre-clinical findings derived from long-established and commonly used in vitro and in vivo models should be further validated in patient-derived models that are more suitable to evaluate the potential clinical relevance of new therapeutics.

Citation Format: Paula Ofek, Nir Dromi, Noga Yerushalmi, Sharon Kredo-Russo, Rachel Grossman, Zvi Ram, Ronit Satchi-Fainaro. Patient-derived GBM tumors versus patient-derived primary cells and GBM cell lines: lessons from microRNA profiling. [abstract]. In: Proceedings of the AACR Special Conference: Patient-Derived Cancer Models: Present and Future Applications from Basic Science to the Clinic; Feb 11-14, 2016; New Orleans, LA. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(16_Suppl):Abstract nr B05.

Abstract B42: Dysregulation of key microRNAs controlling tumor-host interactions triggers escape from osteosarcoma dormancy

The presence of dormant, microscopic cancerous lesions possesses a major obstacle for the treatment of metastatic and recurrent cancers. While it is well-established that microRNAs play a major role in tumorigenesis, their involvement in tumor dormancy has yet to be fully elucidated. We established and comprehensively characterized a human osteosarcoma dormancy model of a pair of cells originating from the same parental tissue; one that remains avascular and non-palpable a year following inoculation into mice and another that generates vascularized palpable tumors one month following inoculation. Using this model of cell lines generating dormant or fast-growing osteosarcomas, we identified three novel regulators of osteosarcoma dormancy: miR-34a, miR-93 and miR-200c. This is the first report showing that loss of these three microRNAs occurs during the switch from dormant avascular into fast-growing angiogenic phenotype. Furthermore, we validated their downregulation in patients' tumor samples compared to normal bone, making them attractive candidates for osteosarcoma therapy. Reconstitution of these microRNAs into Soas-2 and MG-63 cells, which generate fast-growing osteosarcomas, reduced the levels of their target genes, MET proto-oncogene, hypoxia-inducible factor 1α, and moesin, critical to cancer angiogenesis and cancer cells' migration. We further demonstrate that these miRNAs attenuate the angiogenic capabilities of fast-growing osteosarcoma in vitro and in vivo. Moreover, treatment with each of these microRNAs using our novel polyglycerol dendritic nanocarrier significantly prolonged their dormancy period. Taken together, these findings suggest that miR-34a, miR-93 and miR-200c have a key role in osteosarcoma progression, and provide the rationale for the development of novel diagnostic and therapeutic tools for osteosarcoma and other malignancies.

Citation Format: Galia Tiram, Paula Ofek, Taturo Udagawa, Noam Shomron, Maayan Roniger, Batsheva Kerem, Yuval Shaked, Sarit Aviel-Ronen, Iris Barshack, Marcelo Calderon, Rainer Haag, Ronit Satchi-Fainaro. Dysregulation of key microRNAs controlling tumor-host interactions triggers escape from osteosarcoma dormancy. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr B42.

Restoring the oncosuppressor activity of microRNA-34a in glioblastoma using a polyglycerol-based polyplex

Glioblastoma multiforme (GBM) is the most common and aggressive primary neoplasm of the brain. Poor prognosis is mainly attributed to tumor heterogeneity, invasiveness, and drug resistance. microRNA-based therapeutics represent a promising approach due to their ability to inhibit multiple targets. In this work, we aim to restore the oncosuppressor activity of microRNA-34a (miR-34a) in GBM. We developed a cationic carrier system, dendritic polyglycerolamine (dPG-NH₂), which remarkably improves miRNA stability, intracellular trafficking, and activity. dPG-NH₂ carrying mature miR-34a targets C-MET, CDK6, Notch1 and BCL-2, consequently inhibiting cell cycle progression, proliferation and migration of GBM cells. Following complexation with dPG-NH₂, miRNA is stable in plasma and able to cross the blood–brain barrier. We further show inhibition of tumor growth following treatment with dPG-NH₂–miR-34a in a human glioblastoma mouse model. We hereby present a promising technology using dPG-NH₂–miR-34a polyplex for brain-tumor treatment, with enhanced efficacy and no apparent signs of toxicity.

Identification of Dormancy-Associated MicroRNAs for the Design of Osteosarcoma-Targeted Dendritic Polyglycerol Nanopolyplexes

The presence of dormant, microscopic cancerous lesions poses a major obstacle for the treatment of metastatic and recurrent cancers. While it is well-established that microRNAs play a major role in tumorigenesis, their involvement in tumor dormancy has yet to be fully elucidated. We established and comprehensively characterized pairs of dormant and fast-growing human osteosarcoma models. Using these pairs of mouse tumor models, we identified three novel regulators of osteosarcoma dormancy: miR-34a, miR-93, and miR-200c. This report shows that loss of these microRNAs occurs during the switch from dormant avascular into fast-growing angiogenic phenotype. We validated their downregulation in patients' tumor samples compared to normal bone, making them attractive candidates for osteosarcoma therapy. Successful delivery of miRNAs is a challenge; hence, we synthesized an aminated polyglycerol dendritic nanocarrier, dPG-NH2, and designed dPG-NH2-microRNA polyplexes to target cancer. Reconstitution of these microRNAs using dPG-NH2 polyplexes into Saos-2 and MG-63 cells, which generate fast-growing osteosarcomas, reduced the levels of their target genes, MET proto-oncogene, hypoxia-inducible factor 1α, and moesin, critical to cancer angiogenesis and cancer cells' migration. We further demonstrate that these microRNAs attenuate the angiogenic capabilities of fast-growing osteosarcomas in vitro and in vivo. Treatment with each of these microRNAs using dPG-NH2 significantly prolonged the dormancy period of fast-growing osteosarcomas in vivo. Taken together, these findings suggest that nanocarrier-mediated delivery of microRNAs involved in osteosarcoma tumor-host interactions can induce a dormant-like state.

Abstract 4391: Multi-modal nanomedicine for glioblastoma

Glioblastoma multiforme (GBM) is an aggressive primary neoplasm of the brain that exhibit notable refractivity to standard treatment regimens. Recent large-scale molecular profiling has revealed deregulated molecular networks as potential targets for therapeutic development. MicroRNAs (miRNAs) are noncoding RNA molecules which act as post-transcriptional regulators of specific messenger RNA transcripts. miRNAs play major roles in normal developmental processes, and their deregulation significantly contributes to various aspects of carcinogenesis.

Nevertheless, in vivo delivery of small interfering RNA (siRNA) and miRNA remains a crucial challenge for their therapeutic success. siRNAs and miRNAs on their own are not taken-up by most mammalian cells in a way that preserves their activity. In order to circumvent these limitations, we developed a cationic carrier system, which can strongly improve its stability, intracellular trafficking and silencing efficacy. Polyglycerol (PG)-Amine, a water-soluble polyglycerol-based hyperbranched polymer accumulates in the tumor microenvironment due to the enhanced permeability and retention (EPR) effect, and therefore, represents an ideal nanocarrier for antitumor biological agents.

Using our novel nanocarrier, we have studied the expression targets and functional effects of miR-34a in several human glioblastoma cell lines and human tissue samples. miR-34a levels inversely correlated to their target gene levels measured in the same cell lines or tissue. Transient transfection of PG-NH2-miR-34a polyplex into glioblastoma cells strongly inhibited cell proliferation, cell cycle progression, and cell migration. Consequently, we performed an in vivo experiment and achieved a significant tumor growth inhibition effect following treatment with PG-NH2-miR-34a polyplex in a human glioblastoma mouse model.

We further characterized the synergistic effect of combining PG-NH2-miR-34a polyplex with chemotherapy and achieved promising results. Together, our findings show that PG-NH2 efficiently delivers anticancer miRNAs to glioblastoma cells and suppresses brain tumor growth. These results suggest that our polyplex could serve as a potential nanomedicine for glioblastoma.

Citation Format: Paula Ofek, Marcelo Calderon, Fatemeh Sheikhi-Mehrabadi, Shiran Ferber, Rainer Haag, Ronit Satchi-Fainaro. Multi-modal nanomedicine for glioblastoma. [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 4391. doi:10.1158/1538-7445.AM2014-4391

Abstract 4219: miR-34a, miR-93 and miR-200c - novel molecular regulators of osteosarcoma dormancy

There is an increasing interest in elucidating the mechanisms of the earlier stages in tumor progression, the point at which a dormant tumor acquires the ability to grow and metastasize. Although the tumor dormancy phenomenon has important implications for early detection and treatment of cancer, its biology and genetic characteristics are poorly understood. MicroRNAs (miRs), small non-coding RNA molecules which regulate gene expression at the post-transcriptional level, are emerging as key regulators in physiological and pathological processes, including tumorigenesis. We hypothesized that specific miRs may also play a key role in the regulation of tumor dormancy and the “angiogenic switch”. We therefore established a pair of cell lines that generate dormant avascular or fast-growing angiogenic osteosarcomas in SCID mice (Saos-2-D and Saos-2-E respectively). Although these cells share similar growth rate in vitro, when inoculated into mice, Saos-2-E cells generate vascularized and palpable tumors within one month, while Saos-2-D cells remain avascular and non-palpable for a year.

Following our hypothesis, miR arrays of Saos-2-D and Saos-2-E cells were performed. Several miRs that were differentially-expressed in the dormant versus fast-growing-tumor-generating cells were selected for further evaluation. Out of those miRs, miR-34a, miR-93 and miR-200c, when administered complexed with a polymeric nanocarrier, reverted several tumor cell lines forming fast-growing tumors to a dormant non-angiogenic phenotype, both in vitro and in vivo. We therefore propose that these three miRs play a role in the switch from dormancy to progressive disease in human osteosarcoma tumors. Uncovering new molecular targets involved in the tumor dormancy phenomenon could provide important tools for novel dormancy-directed tumor therapy strategies.

Citation Format: Galia Tiram, Shiran Ferber, Paula Ofek, Noam Shomron, Taturo Udagawa, Sarit Aviel-Ronen, Iris Barshack, Ronit Satchi-Fainaro. miR-34a, miR-93 and miR-200c - novel molecular regulators of osteosarcoma dormancy. [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 4219. doi:10.1158/1538-7445.AM2014-4219

Interfering cancer with polymeric siRNA nanomedicines

The ability to specifically silence genes using RNA interference (RNAi) has wide therapeutic applications for the treatment of disease. Numerous studies have demonstrated global gene and protein signatures distinguishing malignant and nonmalignant tissues. This worldwide pursuit of optimal cancer targets has so far provided a wide list of potential targets for each cancer type and for each patient, for which RNAi-based therapies can be applied. Nevertheless, due to poor stability of RNAi molecules in physiological conditions and their inability to cross cellular membranes, the delivery of siRNA and microRNA (miRNA) in vivo holds a great challenge and remains a crucial issue for their therapeutic success. Supramolecular carriers are often used in order to improve the physicochemical and biopharmaceutical properties of RNAi. Nano-sized delivery systems enable the accumulation of drugs and oligonucleotides (ONTs) in angiogenesis-dependent areas due to the enhanced permeability and retention (EPR) effect, and are able to cross cellular membranes and release the siRNA/miRNA only inside the target cell. In addition, a targeting moiety can increase the selectivity and specific uptake in the target tissue. Several vehicles (dendrimers, nanoparticles, liposomes, polyplex, lipoplex, polymeric nanoconjugates) are being developed for siRNA/miRNA delivery. These vehicles provide an important tool for exploiting the full potential of ONTs as therapeutic agents. In this review we will focus on the polymer-based approaches to deliver siRNA to cancer in vivo.

Abstract 4643: Shedding light on the molecular basis of tumor dormancy using antiangiogenic nanomedicines

Based on clinical necessity for improved anticancer drugs and the importance of understanding the mechanism underlying the angiogenic switch phenomenon, we designed a targeted drug delivery system for the treatment of bone-related angiogenesis-dependent malignances. Using multidisciplinary approaches involving novel chemistry and molecular imaging, we synthesized polymer-drug nano-conjugates of HPMA copolymer-alendronate bound to TNP-470 or paclitaxel designed to target the angiogenic switch of bone neoplasms such as osteosarcoma and bone metastases. Antitumor efficacy of the conjugates was evaluated in osteosarcoma- and breast cancer bone metastases-bearing mice revealing improved anti-angiogenic and antitumor activity with reduced toxicity compared with the free drugs. To obtain data regarding the pharmacokinetic profiles of the drugs, we developed a novel prodrug system that enables real-time monitoring of drug release by different fluorescence imaging modalities. To further analyze host-tumor interactions in response to treatment, we developed a model of tumor dormancy based on mCherry-infected human osteosarcoma cells that allows deep tissue signal penetration with an optical readout. This pair of tumor cells that originate from the same parental cell line, enable intravital non-invasive detection of early stages of cancer and evaluation of treatment efficacy with the nanoconjugate. The pair has similar growth rate in vitro. However, when inoculated into mice, Saos-2-E cells established a vascularized and palpable tumor within one month, while Saos-2-D cells remained avascular and non-palpable for a year until they spontaneously escape. Phenotypic and molecular (microRNA-based) differences between these two cell lines were evaluated by in vitro and in vivo methods, with emphasis on changes related to the angiogenesis process. Although slight differences between the Saos-2-D and Saos-2-E cells were shown in vitro, when inoculated in vivo Saos-2-E cells showed increased angiogenic potential compared with the Saos-2-D cells. Our nanoconjugate was able to regress fast growing-angiogenic human tumors to a dormant avascular phenotype.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4643. doi:1538-7445.AM2012-4643

Abstract 5235: Uncovering tumor-host molecular and cellular interactions involved in tumor dormancy using microRNAs

There is an increasing interest in elucidating the mechanisms of the earlier stages in tumor progression, the point at which a dormant tumor acquires the ability to grow and metastasize. Although the tumor dormancy phenomenon has important implications for early detection and treatment of cancer, its biology and genetic characteristics are poorly understood. microRNAs (miRs), small non-coding RNA molecules which regulate gene expression at the post transcriptional level, are emerging as key regulators in physiological and pathological processes, including tumorigenesis. We hypothesized that specific miRs may also play a key role in the regulation of tumor dormancy and the angiogenic switch. We therefore established a pair of cell lines that generate dormant avascular and fast-growing angiogenic osteosarcomas in SCID mice (Saos-2-D and Saos-2-E respectively). Although these cells share similar growth rate in vitro, when inoculated into mice, Saos-2-E cells establish vascularized and palpable tumors within one month, while Saos-2-D cells remain avascular and non-palpable for a year. Following our hypothesis, miR array of Saos-2-D and Saos-2-E cells, was performed. Several miRs that were differentially-expressed in the dormant versus fast-growing-tumor-forming cells were selected for further evaluation. Out of those miRs, miR200c and miR93 (representing either a mild or a substantial difference in the expression profile respectively) reverted cells forming fast-growing tumors to a dormant non-angiogenic phenotype, both in vitro and in vivo. Preliminary in vitro analysis of human brain, breast and prostate cancer cell lines revealed a similar phenotypic effect following overexpression of those miRs. We therefore propose that miR200c and miR93 play a role in the switch from dormancy. Further analysis on the target genes of those miRs is required in order to fully elucidate their role in tumor dormancy. Uncovering new molecular targets involved in the tumor dormancy phenomenon could provide important tools for novel dormancy-directed tumor therapy strategies.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5235. doi:1538-7445.AM2012-5235

Abstract 5650: Targeting siRNA to tumors and their stroma as a dual anticancer and anti-angiogenic therapy

New targets for RNA interference (RNAi)-based cancer therapy are constantly emerging from the increasing knowledge on the molecular pathways involved in carcinogenesis. Nevertheless, in vivo delivery of small interfering RNA (siRNA) remains a crucial challenge for its therapeutic success. SiRNAs on their own are not taken-up by most mammalian cells in a way that preserves their activity. In order to circumvent these limitations, we developed a cationic carrier system, which can strongly improve its stability, intracellular trafficking and silencing efficacy. We have recently developed a polyglycerol (PG)-Amine, a water-soluble polyglycerol-based dendrimer that accumulates in the tumor environment due to the enhanced permeability and retention (EPR) effect, and therefore, represents an ideal delivery vehicle for antitumor biological agents. PG-Amine entrapment of siRNA neutralized its negative charge in a dose-dependent manner and significantly improved its cellular uptake. The luciferase gene, ectopically overexpressed in U87 human glioblastoma cell line was used as a model system and its silencing efficacy was extensively evaluated both in vitro and in vivo. Following encouraging results obtained from several cancer cell lines, the silencing efficiency of the nanocarrier luciferase-siRNA polyplexes was followed-up in vivo by non-invasive intravital bioluminescence imaging. A significant gene silencing effect was accomplished in vivo in both human glioblastoma and murine mammary adenocarcinoma mouse models following intravenous administration of our polyplexes. We further characterized the effect of silencing a few selected key proteins (e.g. Notch1) in vitro and achieved significant effects on the proliferation and migration of glioblastoma and neuroblastoma cell lines. In conclusion, we predict that specific delivery of siRNA to tumor epithelial and endothelial cells in vivo, and silencing of an important cell growth and angiogenesis regulator will warrant this approach as a successful anticancer therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5650. doi:1538-7445.AM2012-5650

Targeting angiogenesis-dependent calcified neoplasms using combined polymer therapeutics

BACKGROUND

There is an immense clinical need for novel therapeutics for the treatment of angiogenesis-dependent calcified neoplasms such as osteosarcomas and bone metastases. We developed a new therapeutic strategy to target bone metastases and calcified neoplasms using combined polymer-bound angiogenesis inhibitors. Using an advanced "living polymerization" technique, the reversible addition-fragmentation chain transfer (RAFT), we conjugated the aminobisphosphonate alendronate (ALN), and the potent anti-angiogenic agent TNP-470 with N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer through a Glycine-Glycine-Proline-Norleucine linker, cleaved by cathepsin K, a cysteine protease overexpressed at resorption sites in bone tissues. In this approach, dual targeting is achieved. Passive accumulation is possible due to the increase in molecular weight following polymer conjugation of the drugs, thus extravasating from the tumor leaky vessels and not from normal healthy vessels. Active targeting to the calcified tissues is achieved by ALN's affinity to bone mineral.

METHODS AND FINDING

The anti-angiogenic and antitumor potency of HPMA copolymer-ALN-TNP-470 conjugate was evaluated both in vitro and in vivo. We show that free and conjugated ALN-TNP-470 have synergistic anti-angiogenic and antitumor activity by inhibiting proliferation, migration and capillary-like tube formation of endothelial and human osteosarcoma cells in vitro. Evaluation of anti-angiogenic, antitumor activity and body distribution of HPMA copolymer-ALN-TNP-470 conjugate was performed on severe combined immunodeficiency (SCID) male mice inoculated with mCherry-labeled MG-63-Ras human osteosarcoma and by modified Miles permeability assay. Our targeted bi-specific conjugate reduced VEGF-induced vascular hyperpermeability by 92% and remarkably inhibited osteosarcoma growth in mice by 96%.

CONCLUSIONS

This is the first report to describe a new concept of a narrowly-dispersed combined polymer therapeutic designed to target both tumor and endothelial compartments of bone metastases and calcified neoplasms at a single administration. This new approach of co-delivery of two synergistic drugs may have clinical utility as a potential therapy for angiogenesis-dependent cancers such as osteosarcoma and bone metastases.

Cell cycle regulation and p53 activation by protein phosphatase 2C alpha

Protein phosphatase 2C (PP2C) dephosphorylates a broad range of substrates, regulating stress response and growth-related pathways in both prokaryotes and eukaryotes. We now demonstrate that PP2C alpha, a major mammalian isoform, inhibits cell growth and activates the p53 pathway. In 293 cell clones, in which PP2C alpha expression is regulated by a tetracycline-inducible promoter, PP2C alpha overexpression led to G(2)/M cell cycle arrest and apoptosis. Furthermore, PP2C alpha induced the expression of endogenous p53 and the p53-responsive gene p21. Activation of the p53 pathway by PP2C alpha took place both in cells harboring endogenous p53, as well as in p53-null cells transfected with exogenous p53. Induction of PP2C alpha resulted in an increase in the overall levels of p53 protein as well as an augmentation of p53 transcription activity. The dephosphorylation activity of PP2C alpha is essential to the described phenomena, as none of these effects was detected when an enzymatically inactive PP2C alpha mutant was overexpressed. p53 plays an important role in PP2C alpha-directed cell cycle arrest and apoptosis because perturbation of p53 expression in human 293 cells by human papillomavirus E6 led to a significant increase in cell survival. The role of PP2C alpha in p53 activation is discussed.