Netrin-1 feedforward mechanism promotes pancreatic cancer liver metastasis via hepatic stellate cell activation, retinoid, and ELF3 signaling

The biology of metastatic pancreatic ductal adenocarcinoma (PDAC) is distinct from that of the primary tumor due to changes in cell plasticity governed by a distinct transcriptome. Therapeutic strategies that target this distinct biology are needed. We detect an upregulation of the neuronal axon guidance molecule Netrin-1 in PDAC liver metastases that signals through its dependence receptor (DR), uncoordinated-5b (Unc5b), to facilitate metastasis in vitro and in vivo. The mechanism of Netrin-1 induction involves a feedforward loop whereby Netrin-1 on the surface of PDAC-secreted extracellular vesicles prepares the metastatic niche by inducing hepatic stellate cell activation and retinoic acid secretion that in turn upregulates Netrin-1 in disseminated tumor cells via RAR/RXR and Elf3 signaling. While this mechanism promotes PDAC liver metastasis, it also identifies a therapeutic vulnerability, as it can be targeted using anti-Netrin-1 therapy to inhibit metastasis using the Unc5b DR cell death mechanism.

Netrin-1 blockade inhibits tumor associated Myeloid-derived suppressor cells, cancer stemness and alleviates resistance to chemotherapy and immune checkpoint inhibitor

Drug resistance and cancer relapse represent significant therapeutic challenges after chemotherapy or immunotherapy, and a major limiting factor for long-term cancer survival. Netrin-1 was initially identified as a neuronal navigation cue but has more recently emerged as an interesting target for cancer therapy, which is currently clinically investigated. We show here that netrin-1 is an independent prognostic marker for clinical progression of breast and ovary cancers. Cancer stem cells (CSCs)/Tumor initiating cells (TICs) are hypothesized to be involved in clinical progression, tumor relapse and resistance. We found a significant correlation between netrin-1 expression and cancer stem cell (CSC) markers levels. We also show in different mice models of resistance to chemotherapies that netrin-1 interference using a therapeutic netrin-1 blocking antibody alleviates resistance to chemotherapy and triggers an efficient delay in tumor relapse and this effect is associated with CSCs loss. We also demonstrate that netrin-1 interference limits tumor resistance to immune checkpoint inhibitor and provide evidence linking this enhanced anti-tumor efficacy to a decreased recruitment of a subtype of myeloid-derived suppressor cells (MDSCs) called polymorphonuclear (PMN)-MDSCs. We have functionally demonstrated that these immune cells promote CSCs features and, consequently, resistance to anti-cancer treatments. Together, these data support the view of both a direct and indirect contribution of netrin-1 to cancer stemness and we propose that this may lead to therapeutic opportunities by combining conventional chemotherapies and immunotherapies with netrin-1 interfering drugs.

Ultrasound molecular imaging as a non-invasive companion diagnostic for netrin-1 interference therapy in breast cancer

In ultrasound molecular imaging (USMI), ligand-functionalized microbubbles (MBs) are used to visualize vascular endothelial targets. Netrin-1 is upregulated in 60% of metastatic breast cancers and promotes tumor progression. A novel netrin-1 interference therapy requires the assessment of netrin-1 expression prior to treatment. In this study, we studied netrin-1 as a target for USMI and its potential as a companion diagnostic in breast cancer models. Methods: To verify netrin-1 expression and localization, an in vivo immuno-localization approach was applied, in which anti-netrin-1 antibody was injected into living mice 24 h before tumor collection, and revealed with secondary fluorescent antibody for immunofluorescence analysis. Netrin-1 interactions with the cell surface were studied by flow cytometry. Netrin-1-targeted MBs were prepared using MicroMarker Target-Ready (VisualSonics), and validated in in vitro binding assays in static conditions or in a flow chamber using purified netrin-1 protein or netrin-1-expressing cancer cells. In vivo USMI of netrin-1 was validated in nude mice bearing human netrin-1-positive SKBR7 tumors or weakly netrin-1-expressing MDA-MB-231 tumors using the Vevo 2100 small animal imaging device (VisualSonics). USMI feasibility was further tested in transgenic murine FVB/N Tg(MMTV/PyMT634Mul) (MMTV-PyMT) mammary tumors. Results: Netrin-1 co-localized with endothelial CD31 in netrin-1-positive breast tumors. Netrin-1 binding to the surface of endothelial HUVEC and cancer cells was partially mediated by heparan sulfate proteoglycans. MBs targeted with humanized monoclonal anti-netrin-1 antibody bound to netrin-1-expressing cancer cells in static and dynamic conditions. USMI signal was significantly increased with anti-netrin-1 MBs in human SKBR7 breast tumors and transgenic murine MMTV-PyMT mammary tumors compared to signals recorded with either isotype control MBs or after blocking of netrin-1 with humanized monoclonal anti-netrin-1 antibody. In weakly netrin-1-expressing human tumors and normal mammary glands, no difference in imaging signal was observed with anti-netrin-1- and isotype control MBs. Ex vivo analysis confirmed netrin-1 expression in MMTV-PyMT tumors. Conclusions: These results show that USMI allowed reliable detection of netrin-1 on the endothelium of netrin-1-positive human and murine tumors. Significant differences in USMI signal for netrin-1 reflected the significant differences in netrin-1 mRNA & protein expression observed between different breast tumor models. The imaging approach was non-invasive and safe, and provided the netrin-1 expression status in near real-time. Thus, USMI of netrin-1 has the potential to become a companion diagnostic for the stratification of patients for netrin-1 interference therapy in future clinical trials.

Structural Decoding of the Netrin-1/UNC5 Interaction and its Therapeutical Implications in Cancers

Netrin-1 has been shown to be up-regulated in a fraction of human cancers as a mechanism to allow these tumors to escape the pro-apoptotic activity of some of its main dependence receptors, the UNC5 homologs (UNC5H). Here we identify the V-2 domain of netrin-1 to be important for its interaction with the Ig1/Ig2 domains of UNC5H2. We generate a humanized anti-netrin-1 antibody that disrupts the interaction between netrin-1 and UNC5H2 and triggers death of netrin-1-expressing tumor cells in vitro. We also present evidence that combining the anti-netrin-1 antibody with epidrugs such as decitabine could be effective in treating tumors showing no or modest netrin-1 expression. These results support that this antibody is a promising drug candidate.

Abstract 2921: Preclinical characteristics of NP137, a first-in-class monoclonal antibody directed against netrin-1 and inducing dependence receptors-mediated cell death

Background: Some receptors are active in the absence of ligand and actively trigger cell death through apoptosis. These receptors are called “dependence receptors” (DRs) as their expression at the cell surface renders cells critically dependent for their survival on ligand availability. Deleted in Colorectal Carcinoma (DCC) and UNC5H are prototypic DRs which induce apoptosis unless their ligand netrin-1 is present. It has been shown that netrin-1 is up-regulated in a large fraction of tumors and that interference with netrin-1/receptors interaction is associated with inhibition of tumor growth and metastasis in various preclinical models. We have generated the first humanized netrin-1 antibody, NP137, and characterized its antitumor activity.

Preclinical results: NP137 is a humanized monoclonal IgG1 antibody, which binds netrin-1 in the nanomolar range affinity and efficiently inhibits the binding of netrin-1 to its receptor UNC5H2 (IC50: 0.5 nM). NP137 recognizes a netrin-1 specific epitope located in the second laminin-type EGF-like repeat. A crystal structure analysis shows that this region is crucial for the binding of netrin-1 to its receptor, providing a structural basis for the biological activity of NP137. In vitro, NP137 induced apoptosis in netrin-1-dependent cell lines as assessed by cell density measurement and caspase 3 activation. Ex vivo analysis performed on fresh human breast tumor slices (3D cultures) showed a specific and significant induction of apoptosis in tumor cells in approximately half of cases. In vivo, NP137 exhibited a significant anti-tumor effect associated with good pharmacokinetic properties in models of both solid tumors and hematologic malignancies. A dose-effect relationship was observed with an optimal pharmacologically active dose established at 10 mg/kg twice a week. We also showed that NP137 potentiates cancer cell death induced by cytotoxic drugs like doxorubicin or platinum derivatives as well as demethylating agents, which induce upregulation of netrin-1 and its receptors. In this context, a strong synergy with doxorubicin was shown in a rat syngeneic model of osteosarcoma, reversing its chemoresistant phenotype. Toxicological studies are ongoing with first results suggesting a high therapeutic margin.

Conclusion: We are developing the first therapeutic antibody targeting netrin-1 and inducing dependence receptors-mediated tumor cell death. Preclinical results suggest a therapeutic potential both in solid tumors and hematological malignancies. A high therapeutic margin is expected, which may accelerate combination strategies. Based on these encouraging results, a first-in-human trial is planned at the end of 2015, with the support of EORTC.

Note: This abstract was not presented at the meeting.

Citation Format: Benjamen Ducarouge, Jean-Guy Delcros, Riad Abès, David Goldschneider, Benjamin Gibert, John Blachier, David Neves, Patrick Mehlen, Agnès Bernet, Stéphane Depil. Preclinical characteristics of NP137, a first-in-class monoclonal antibody directed against netrin-1 and inducing dependence receptors-mediated cell death. [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 2921. doi:10.1158/1538-7445.AM2015-2921

Nucleolar localization of a netrin-1 isoform enhances tumor cell proliferation

Netrin-1 displays proto-oncogenic activity in several cancers, which is thought to be due to the ability of this secreted cue to stimulate survival when bound to its receptors. We showed that in contrast to full-length, secreted netrin-1, some cancer cells produced a truncated intranuclear form of netrin-1 (ΔN-netrin-1) through an alternative internal promoter. Because of a nucleolar localization signal located in its carboxyl terminus, ΔN-netrin-1 was targeted to the nucleolus, where it interacted with nucleolar proteins, affected nucleolar ultrastructure, and interacted with the promoters of ribosomal genes. Moreover, ΔN-netrin-1 stimulated cell proliferation in vitro and tumor growth in vivo. Thus, some cancer cells produce not only a full-length, secreted form of netrin-1 that promotes cell survival but also a truncated netrin-1 that stimulates cell proliferation, potentially by enhancing ribosome biogenesis.

Amyloid precursor protein regulates netrin-1-mediated commissural axon outgrowth

The multifunctional protein netrin-1 was initially discovered as the main attractive cue for commissural axon guidance by acting through its receptor DCC. Recently, we have shown that netrin-1 also interacts with the orphan transmembrane receptor amyloid precursor protein (APP). APP is cleaved by proteases, generating amyloid-β peptide, the main component of the amyloid plaques that are associated with Alzheimer disease. Our previous work demonstrated that via its interaction with APP, netrin-1 is a negative regulator of amyloid-β production in adult brain, but the biological relevance of APP/netrin-1 interaction under non-pathological conditions was unknown. We show here that during commissural axon navigation, APP, expressed at the growth cone, is part of the DCC receptor complex mediating netrin-1-dependent axon guidance. APP interacts with DCC in the presence of netrin-1 and enhances netrin-1-mediated DCC intracellular signaling, such as MAPK activation. Inactivation of APP in mice is associated with reduced commissural axon outgrowth. Thus, APP functionally acts as a co-receptor for DCC to mediate axon guidance.

Targeting heat shock protein 27 (HspB1) interferes with bone metastasis and tumour formation in vivo

Background:

The small stress heat shock protein 27 (Hsp27) has recently turned as a promising target for cancer treatment. Hsp27 upregulation is associated with tumour growth and resistance to chemo- and radio-therapeutic treatments, and several ongoing drugs inhibiting Hsp27 expression are under clinical trial. Hsp27 is now well described to counteract apoptosis and its elevated expression is associated with increased aggressiveness of several primary tumours. However, its role in the later stage of tumour progression and, more specifically, in the later and most deadly stage of tumour metastasis is still unclear.

Methods/results:

In the present study, we showed by qRT–PCR that Hsp27 gene is overexpressed in a large fraction of the metastatic breast cancer area in 53 patients. We further analysed the role of this protein in mice during bone metastasis invasion and establishment by using Hsp27 genetically depleted MDA-MB231/B02 human breast cancer cell line as a model. We demonstrate that Hsp27 silencing led to reduced cell migration and invasion in vitro and that in vivo it correlated with a decreased ability of breast cancer cells to metastasise and grow in the skeleton.

Conclusion:

Altogether, these data characterised Hsp27 as a potent therapeutic target in breast cancer bone metastasis and skeletal tumour growth.

The dependence receptor UNC5H2/B triggers apoptosis via PP2A-mediated dephosphorylation of DAP kinase

The UNC5H dependence receptors promote apoptosis in the absence of their ligand, netrin-1, and this is important for neuronal and vascular development and for limitation of cancer progression. UNC5H2 (also called UNC5B) triggers cell death through the activation of the serine-threonine protein kinase DAPk. While performing a siRNA screen to identify genes implicated in UNC5H-induced apoptosis, we identified the structural subunit PR65β of the holoenzyme protein phosphatase 2A (PP2A). We show that UNC5H2/B recruits a protein complex that includes PR65β and DAPk and retains PP2A activity. PP2A activity is required for UNC5H2/B-induced apoptosis, since it activates DAPk by triggering its dephosphorylation. Moreover, netrin-1 binding to UNC5H2/B prevents this effect through interaction of the PP2A inhibitor CIP2A to UNC5H2/B. Thus we show here that, in the absence of netrin-1, recruitment of PP2A to UNC5H2/B allows the activation of DAPk via a PP2A-mediated dephosphorylation and that this mechanism is involved in angiogenesis regulation.

p73alpha isoforms drive opposite transcriptional and post-transcriptional regulation of MYCN expression in neuroblastoma cells

MYCN activation, mainly by gene amplification, is one of the most frequent molecular events in neuroblastoma (NB) oncogenesis, and is associated with increased malignancy and decreased neuronal differentiation propensity. The frequency of concomitant loss of heterozygosity at the 1p36.3 locus, which harbours the p53 anti-oncogene homologue TP73, indicates that MYCN and p73 alterations may cooperate in the pathogenesis of NB. We have previously shown that p73 isoforms are deregulated in NB tumours and that TAp73 co-operates synergistically with p53 for apoptosis of NB cells, whereas ΔNp73 activates the expression of neuronal differentiation genes such as BTG2. Herein, using both ectopic expression and RNA interference-mediated silencing of p73 in MYCN amplified NB cells, we show that p73α isoforms inhibit MYCN expression at both transcript and protein levels, in spite of transactivator effects on MYCN promoter. To explain this paradox, we found that TAp73α exerts negative post-transcriptional effects leading to reduced MYCN mRNA stability. RNA immunoprecipitation experiments suggest that this dominant inhibitory post-transcriptional effect could be due to an interaction between the p73 protein and MYCN mRNA, a hypothesis also raised for the regulation of certain genes by the p53 protein.

Expression of C-terminal deleted p53 isoforms in neuroblastoma

The tumor suppressor gene, p53, is rarely mutated in neuroblastomas (NB) at the time of diagnosis, but its dysfunction could result from a nonfunctional conformation or cytoplasmic sequestration of the wild-type p53 protein. However, p53 mutation, when it occurs, is found in NB tumors with drug resistance acquired over the course of chemotherapy. As yet, no study has been devoted to the function of the specific p53 mutants identified in NB cells. This study includes characterization and functional analysis of p53 expressed in eight cell lines: three wild-type cell lines and five cell lines harboring mutations. We identified two transcription-inactive p53 variants truncated in the C-terminus, one of which corresponded to the p53β isoform recently identified in normal tissue by Bourdon et al. [J. C. Bourdon, K. Fernandes, F. Murray-Zmijewski, G. Liu, A. Diot, D. P. Xirodimas, M. K. Saville and D. P. Lane (2005) Genes Dev., 19, 2122–2137]. Our results show, for the first time, that the p53β isoform is the only p53 species to be endogenously expressed in the human NB cell line SK-N-AS, suggesting that the C-terminus truncated p53 isoforms may play an important role in NB tumor development.

Differential regulation of p73 variants in response to cisplatin treatment in SH-SY5Y neuroblastoma cells

The present study aims to investigate the role of p73 in response to cisplatin treatment in p53 wild-type neuroblastoma SH-SY5Y cells. Results showed that cisplatin induced a dose-dependent up-regulation of p53, p73, and a number of p53-responsive genes. Interestingly, endogenous Deltaexon2p73-expression was down-regulated by cisplatin treatment. Neither p21 nor GADD45 induction was observed in p53-deficient Lan-1 cells, although endogenous TAp73 expression was markedly induced. In the presence of cisplatin, exogenous TAp73 overexpression in SH-SY5Y cells induced p21 up-regulation without altering the apoptotic sub-G1 cell population. Moreover, siRNA-mediated suppression of TAp73 expression did not alter the sub-G1 population. Collectively, our results suggest that wt-p53 SH-SY5Y cells respond to cisplatin by inducing p73 isoform regulation and sustaining p53-dependent apoptosis that is independent of TAp73alpha.

Wnt-5a gene expression in malignant human neuroblasts

Neuroblastoma (NB), an embryonal malignancy, poses a major challenge in pediatric oncology for the treatment of disseminated forms. Here, we report the decrease of Wnt-5a gene expression in high-risk NB (HR-NB) as well as in cultured metastatic neuroblasts. Wnt-5a is a member of the Wnt signaling pathway which is mainly associated with patterning decisions in the embryonic nervous system. Moreover, Wnt-5a has been involved in metastatic melanoma progression and invasive ductal breast cancer via adhesion and migration alterations. As retinoic acid (RA) plays a major role in the neural crest induction and differentiation, we showed that RA reverses the aberrant negative regulation of Wnt-5a in metastatic neuroblasts. While beta-catenin expression remained unchanged, PKC-theta, a protein kinase C isoform, was evidenced to increase and parallel Wnt-5a level. For the first time, the involvement of Wnt-5a through the Wnt/calcium signaling is highlighted in the pathogenesis of a pediatric embryonal malignancy, NB.

The neurogene BTG2TIS21/PC3 is transactivated by DeltaNp73alpha via p53 specifically in neuroblastoma cells

The p53 gene and its homologue p73 are rarely mutated in neuroblastoma. In recent studies, we showed that overexpression of DeltaNp73alpha, an isoform lacking the N-terminal transactivation (TA) domain, surprisingly induces p53 protein accumulation in the wild-type (wt) p53 human neuroblastoma line SH-SY5Y. As can be expected owing to its dominant-negative effect, DeltaNp73alpha inhibits Waf1/p21 gene expression, but equally importantly, it upregulates BTG2TIS21/PC3, another p53 target gene. This effect is not observed in neuroblastoma cells that express a mutated p53. To better understand the DeltaNp73-mediated transactivation of the BTG2TIS21/PC3 gene we performed luciferase assays with two reporter plasmids harboring long and short BTG2 promoter sequences in three human neuroblastoma cell lines and one breast cancer cell line. Our results demonstrate that BTG2TIS21/PC3 transactivation by DeltaNp73alpha depends on both p53 status (as it is not observed in a p53-/- neuroblastoma cell line) and cellular context (as it occurs in a p53+/+ neuroblastoma cell line but not in a p53+/+ breast tumor cell line). The fact that DeltaNp73alpha may either inhibit or stimulate wt-p53 transcriptional activity, depending on both the p53 target gene and the cellular context, was confirmed by real-time quantitative PCR. Moreover, transactivation of the BTG2TIS21/PC3 promoter requires a complete DeltaNp73alpha C-terminus sequence as it is not observed with DeltaNp73beta, which lacks most of the C-terminal domain. We have previously shown that DeltaNp73alpha is the only p73 isoform expressed in undifferentiated neuroblastoma tumors. In light of all these findings, we propose that DeltaNp73alpha not only acts as an inhibitor of p53/TAp73 functions in neuroblastoma tumors, but also cooperates with wt-p53 in playing a physiological role through the activation of BTG2TIS21/PC3 gene expression.

Les récepteurs à dépendance DCC et UNC5H : rôle de l'apoptose dans le contrôle de la tumorigenèse

Recent studies have led a different view about membrane receptors. While a receptor used to be considered as inactive until bound by its ligand, it has been proposed that some receptors may also be active in the absence of their ligand. These so-called dependence receptors induce a specific death signal when the ligand is absent from the cell. Therefore, the expression of one of these receptors drives the cell to become dependent on the presence of the ligand for its survival. We have hypothesized that this mechanism allows inhibition of tumor growth, by inducing apoptosis of "abnormal" cells that would usually grow when ligand are unavailable--i.e., during local growth of tumor cells or growth beyond primary tumor site -. Along this line, back in the early 90s, Vogelstein and colleagues suggested that a gene called DCC (for "deleted in colorectal cancer") could be a tumor suppressor gene because it was found to be deleted in more than 70 % of colorectal cancers, as well as in many other cancers. During the last fifteen years, controversial data have failed to firmly establish whether DCC is indeed a tumor suppressor gene. However, our observation that DCC behaves as a dependence receptor that induces cell death unless its ligand netrin-1 is present, together with the fact that mice engineered to block DCC-induced cell death by overexpressing netrin-1 are predisposed to develop colorectal tumors, strengthen the role of dependence receptors as tumor suppressors. In this review, we will describe the implication of the netrin-1/receptor pairs as novel negative regulators of tumor development.

Differential response of p53 target genes to p73 overexpression in SH-SY5Y neuroblastoma cell line

p73, the first p53 gene homologue, encodes an array of p73 proteins including p73 alpha full-length (TAp73 alpha) and amino-truncated isoforms (Delta Np73 alpha), two proteins with opposite biological functions. TAp73 alpha can induce tumor suppressive properties, while Delta Np73 alpha antagonizes p53 as well as TAp73 in a dominant-negative manner. In human malignant neuroblasts, p53 protein is wild-type but known to be excluded from the nucleus, therefore disabling its function as a tumor suppressor. The present study investigates whether there is a functional link between p73 isoforms and p53 in neuroblastoma. Experiments were performed on two neuroblastoma cell lines differing in their p53 status, e.g. wild-type p53 SH-5Y5Y cells and mutated p53 IGR-N-91 cells. Data indicate that (i) both TA- and Delta N-p73 alpha enhance p53 protein level in SH-SY5Y cells, whereas level remains unchanged in IGR-N-91 cells; (ii) only in SH-SY5Y cells does forced TAp73 alpha overexpression markedly induce nuclear accumulation of p53 protein; (iii) p21 protein expression is increased in both cell lines infected with TAp73, suggesting that, in IGR-N-91 cells, p21 is induced by p73 through a p53-independent pathway; (iv) in the SHSY5Y cell line, Btg2 expression is strongly enhanced in cells overexpressing TA, and to a lesser extent in cells overexpressing Delta N. Taken together our results suggest that TAp73 may restore p53 function in NB with wild-type nonfunctional p53, but not in NB with mutated p53.

[Interrelations between p73 and p53: a model, neuroblastoma]

Homologies in sequence and gene organization of p53 and their relatives, p73 and p63, suggest similar biological functions. However differences exist between the p53 family members. Indeed in human tumors p53 is often mutated while p63 and p73 are very rarely mutated. In addition, in contrast to p53 which is transcribed in a unique mRNA species spanning all gene exons, each homologue expresses two types of isoforms: some with transactivation domain (TAD) showing tumor suppressive properties, the others deprived of TAD, with oncogenic properties. If p53 responds to immediate genotoxic stress, its homologues participate to the cell homeostasis of specific tissues along their development and differentiation, neuronal tissue for p73, epithelial for p63. However a collaboration between the three p53 family members has been shown to occur in response to cell genotoxic damages. Neuroblastic tumors characterized by a large spectrum of neuronal differentiation constitute a good model to study relationship between p73 and p53 as well as the regulation of their respective expression.

When p53 needs p73 to be functional - forced p73 expression induces nuclear accumulation of endogenous p53 protein

In human neuroblastoma (NB), wild type p53 protein does not elicit its archetypal human tumor suppressive activity so far described. To elucidate this alteration, substantial investigations using NB cell lines have underscored p53 protein nuclear localization defect and/or inappropriate conformation, but no definitive evidence has been provided so far. p73, the first homologue of the p53 gene, locates at the 1p36.3 locus, which is known to be deleted in various human tumors including NB. Unlike p53 mRNA, which specifies a single protein, p73alpha mRNAs encode two types of isoform (TAp73alpha and DeltaNp73alpha) resulting from the use of two different promoters, and eliciting or lacking NH(2)-terminal transactivation domain, respectively. DeltaNp73alpha inhibits p53 pro-apoptotic function in murine developing neurons and is abundantly expressed in human undifferentiated NB tumors. However, critical issues have been raised regarding p73alpha isoform roles, and their possible link to p53 are yet to be clarified in human NB using adenoviral infection approach.

MYCN enhances P-gp/MDR1 gene expression in the human metastatic neuroblastoma IGR-N-91 model

Despite intensive high-dose chemotherapy and autologous hematopoietic stem cell transplantation, disseminated neuroblastoma (NB) frequently proves to be chemosensitive but not chemocurable, and more often so in NB-presenting MYCN amplification. To assess the direct relationship between the MYCN oncogene and chemoresistance acquisition during NB metastatic dissemination, we have studied MYCN and MDR1 genes using the human IGR-N-91 ectopic xenograft metastatic model. This characterized experimental in vitro model includes human neuroblasts derived from a subcutaneous primary tumor xenograft, disseminated blood cells, myocardium, and bone marrow (BM) metastatic cells. All IGR-N-91-derived neuroblasts harbor a consistent MYCN genomic content but, unlike primary tumor xenograft, BM, and myocardium, human neuroblasts elicit a concomitant increase in MYCN and MDR1 transcripts levels, consistent with chemoresistance phenotype and active P-gp. In contrast, no variation of MRP1 transcript level was associated with the metastatic process in this model. Using an MDR1 promoter-CAT construct, we have shown that the MycN protein activates MDR1 transcription both in exogenous transient MYCN-transfected SK-N-SH cells and in endogenous BM metastatic neuroblasts with an increase in the MYCN transcript level. Band-shift experiments indicate that IGR-N-91 cells enriched with the MycN transcription factor do bind to two E-box motifs localized within the MDR1 promoter. Overall, our data indicate that MYCN overexpression increment contributes to the acquired drug resistance that occurs throughout the NB metastatic process.

[A functional gene map is required to adapt therapy of metastatic neuroblastoma]

Neuroblastoma is a very common solid tumor which arises in childhood and shows an extreme heterogeneity at the clinical, histological and genetic levels. Besides age and stage, N-myc amplification and 1p deletion are prognostic factors of the disease: in Europe, these genetic markers are used to conduct therapy. In France, N-myc amplification is a factor of bad prognosis which leads, in all forms of the disease including localised forms and metastatic forms of children aged of less than 1 year, to a myeloablative treatment with autologous hematopoietic stem cells transplantation. By contrast, N-myc amplification has no impact on the survival of children aged of more than 1 year with a poor prognosis (30% overall survival, 5 years) but this genetic abnormality is taken into account to treat primary tumor of these patients. In an attempt to find out prognostic factors of these aggressive forms of the disease, various pathways (apoptosis, differentiation angiogenesis, detoxication, immune response) have been recently surveyed, but studies have been carried out on a limited number of genes. Moreover, experimental models of human metastatic neuroblastoma have been obtained in which variations of genes transcript levels involved in these pathways, are observed. The current break-through of cDNA microarrays allows to develop a dynamic transcriptomic scanning of these models as well as of tumors and bone marrows from patients upon conventional chemotherapy. This technology will enable: i) to define molecular entities of the metastatic disease; ii) to apply adapted treatment; iii) to develop new therapeutic strategies.