p75NTR: an enhancer of fenretinide toxicity in neuroblastoma

Upregulation of p75NTR by Histone Deacetylase Inhibitors Sensitizes Human Neuroblastoma Cells to Targeted Immunotoxin-Induced Apoptosis

Histone deacetylase (HDAC) inhibitors are novel chemotherapy agents with potential utility in the treatment of neuroblastoma, the most frequent solid tumor of childhood. Previous studies have shown that the exposure of human neuroblastoma cells to some HDAC inhibitors enhanced the expression of the common neurotrophin receptor p75NTR. In the present study we investigated whether the upregulation of p75NTR could be exploited to render neuroblastoma cells susceptible to the cytotoxic action of an anti-p75NTR antibody conjugated to the toxin saporin-S6 (p75IgG-Sap). We found that two well-characterized HDAC inhibitors, valproic acid (VPA) and entinostat, were able to induce a strong expression of p75NTR in different human neuroblastoma cell lines but not in other cells, with entinostat, displaying a greater efficacy than VPA. Cell pretreatment with entinostat enhanced p75NTR internalization and intracellular saporin-S6 delivery following p75IgG-Sap exposure. The addition of p75IgG-Sap had no effect on vehicle-pretreated cells but potentiated the apoptotic cell death that was induced by entinostat. In three-dimensional neuroblastoma cell cultures, the subsequent treatment with p75IgG-Sap enhanced the inhibition of spheroid growth and the impairment of cell viability that was produced by entinostat. In athymic mice bearing neuroblastoma xenografts, chronic treatment with entinostat increased the expression of p75NTR in tumors but not in liver, kidney, heart, and cerebellum. The administration of p75IgG-Sap induced apoptosis only in tumors of mice that were pretreated with entinostat. These findings define a novel experimental strategy to selectively eliminate neuroblastoma cells based on the sequential treatment with entinostat and a toxin-conjugated anti-p75NTR antibody.

Neurotrophin Signaling in Medulloblastoma

Neurotrophins are a family of secreted proteins that act by binding to tropomyosin receptor kinase (Trk) or p75NTR receptors to regulate nervous system development and plasticity. Increasing evidence indicates that neurotrophins and their receptors in cancer cells play a role in tumor growth and resistance to treatment. In this review, we summarize evidence indicating that neurotrophin signaling influences medulloblastoma (MB), the most common type of malignant brain cancer afflicting children. We discuss the potential of neurotrophin receptors as new therapeutic targets for the treatment of MB. Overall, activation of TrkA and TrkC types of receptors seem to promote cell death, whereas TrkB might stimulate MB growth, and TrkB inhibition displays antitumor effects. Importantly, we show analyses of the gene expression profile of neurotrophins and their receptors in MB primary tumors, which indicate, among other findings, that higher levels of NTRK1 or NTRK2 are associated with reduced overall survival (OS) of patients with SHH MB tumors.

Using Chemistry to Target Neuroblastoma

Neuroblastoma is a cancer of the neural crest almost exclusively seen in childhood. While children with single, small primary tumors are often cured with surgery alone, the 65% of children with neuroblastoma whose disease has metastasized have less than a 50% chance of surviving five years after diagnosis. Innovative pharmacological strategies are critically needed for these children. Efforts to identify novel targets that afford ablation of neuroblastoma with minimal toxicity to normal tissues are underway. Developing approaches to neuroblastoma include those that target the catecholamine transporter, ubiquitin E3 ligase, the ganglioside GD2, the retinoic acid receptor, the protein kinases ALK and Aurora, and protein arginine N-methyltransferases. Here, as examples of the use of chemistry to combat neuroblastoma, we describe targeting of the protein arginine N-methyltransferases and their role in prolonging the half-life of the neuroblastoma oncoprotein N-Myc, redox signaling in neuroblastoma, and developmentally regulated proteins expressed in primitive neuroblastoma cells but not in mature neural crest elements.

Cell Line-Dependent Variability of Coordinate Expression of p75NTR and CRABP1 and Modulation of Effects of Fenretinide on Neuroblastoma Cells

Neuroblastoma is a childhood neural crest tumor. Fenretinide, a retinoic acid analogue, induces accumulation of mitochondrial reactive oxygen species and consequent apoptosis in neuroblastoma cells. The p75 neurotrophin receptor (p75NTR) enhances the antineuroblastoma cell efficacy of fenretinide in vitro. We examined the role of the retinoid binding protein, CRABP1, in p75NTR-mediated potentiation of the efficacy of fenretinide. Knockdown and overexpression, respectively, of either p75NTR or CRABP1 were effected in neuroblastoma cell lines using standard techniques. Expression was determined by qRT-PCR and confirmed at the protein level by Western blot. Metabolic viability was determined by Alamar blue assay. While protein content of CRABP1 correlated roughly with that of p75NTR in the three neuroblastoid or epithelioid human neuroblastoma cell lines studied, manipulation of p75NTR expression resulted in cell line-dependent, variable change in CRABP1 expression. Furthermore, in some cell lines, induced expression of CRABP1 in the absence of p75NTR did not alter cell sensitivity to fenretinide treatment. The effects of manipulation of p75NTR expression on CRABP1 expression and the effects of CRABP1 expression on fenretinide efficacy are therefore neuroblastoma cell line-dependent. Potentiation of the antineuroblastoma cell effects of fenretinide by p75NTR is not mediated solely through CRABP1.

Induction of Expression of p75 Neurotrophin Receptor Intracellular Domain Does Not Induce Expression or Enhance Activity of Mitochondrial Complex II

Fenretinide is a chemotherapeutic agent in clinical trials for the treatment of neuroblastoma, among the most common and most deadly cancers of childhood. Fenretinide induces apoptosis in neuroblastoma cells through accumulation of mitochondrial reactive oxygen species released from Complex II. The neurotrophin receptor, p75NTR, potentiates this effect. The signaling activity of p75NTR is dependent upon its cleavage to its intracellular domain, p75ICD, trafficking of p75ICD to the nucleus, and functioning of p75ICD as a transcription factor. Mitochondrial Complex II comprises 4 subunits, all of which are encoded by nuclear DNA. We therefore hypothesized that the fenretinide-potentiating effects of p75NTR are the result of transcriptional enrichment of Complex II by p75ICD. However, the present studies demonstrate that neither induced expression of p75ICD or its active fragments nor overexpression of p75NTR results in altered expression or activity of Complex II.

Sciatic nerve regeneration using a nerve growth factor-containing fibrin glue membrane

Our previous findings confirmed that the nerve growth factor-containing fibrin glue membrane provides a good microenvironment for peripheral nerve regeneration; however, the precise mechanism remains unclear. p75 neurotrophin receptor (p75(NTR)) plays an important role in the regulation of peripheral nerve regeneration. We hypothesized that a nerve growth factor-containing fibrin glue membrane can promote neural regeneration by up-regulating p75(NTR) expression. In this study, we used a silicon nerve conduit to bridge a 15 mm-long sciatic nerve defect and injected a mixture of nerve growth factor and fibrin glue at the anastomotic site of the nerve conduit and the sciatic nerve. Through RT-PCR and western blot analysis, nerve growth factor-containing fibrin glue membrane significantly increased p75(NTR) mRNA and protein expression in the Schwann cells at the anastomotic site, in particular at 8 weeks after injection of the nerve growth factor/fibrin glue mixture. These results indicate that nerve growth factor-containing fibrin glue membrane can promote peripheral nerve regeneration by up-regulating p75(NTR) expression in Schwann cells.

Cerebellar Purkinje cell p75 neurotrophin receptor and autistic behavior

The p75 neurotrophin receptor (p75NTR) is normally expressed in cerebellar Purkinje cells throughout the lifespan. Children with autism spectrum behavior exhibit apparent cerebellar Purkinje cell loss. Cerebellar transcriptome changes seen in the murine prenatal valproate exposure model of autism include all of the proteins known to constitute the p75NTR interactome. p75NTR is a modulator of cytoplasmic and mitochondrial redox potential, and others have suggested that aberrant response to oxidant stress has a major role in the pathogenesis of autism. We have created Purkinje cell-selective p75NTR knockout mice that are the progeny of hemizygous Cre-Purkinje cell protein 2 C57Bl mice and p75NTR floxed C57Bl mice. These Cre-loxP mice exhibit complete knockout of p75NTR in ~50% of the cerebellar Purkinje cells. Relative to Cre-only mice and wild-type C57Bl mice, this results in a behavioral phenotype characterized by less allogrooming of (P<0.05; one-way analysis of variance) and socialization or fighting with (each P<0.05) other mice; less (1.2-fold) non-ambulatory exploration of their environment than wild-type (P<0.01) or Cre only (P<0.01) mice; and almost twofold more stereotyped jumping behavior than wild-type (P<0.05) or Cre (P<0.02) mice of the same strain. Wild-type mice have more complex dendritic arborization than Cre-loxP mice, with more neurites per unit area (P<0.025, Student's t-test), more perpendicular branches per unit area (P<0.025) and more short branches/long neurite (P<0.0005). Aberrant developmental regulation of expression of p75NTR in cerebellar Purkinje cells may contribute to the pathogenesis of autism.

p75 neurotrophin receptor and fenretinide-induced signaling in neuroblastoma

PURPOSE

Neuroblastoma is the most common extracranial solid tumor of childhood. The retinoic acid analogue, fenretinide (4-hydroxyphenyl retinamide; 4-HPR), induces apoptosis in neuroblastoma cells in vitro and is currently in clinical trials for children with refractory neuroblastoma. We have previously shown that expression of the p75 neurotrophin receptor (p75NTR) enhances apoptosis induction and mitochondrial accumulation of reactive oxygen species by 4-HPR in neuroblastoma cells. We now examine the signaling events that underlie this effect.

METHODS

Systematic examination of pro- and anti-apoptotic signaling effectors was performed by Western blot. Specific inhibitors of JNK phosphorylation and scavengers of mitochondrial reactive oxygen species were used to demonstrate the roles of these phenomena in the enhancement of fenretinide efficacy.

RESULTS

The present studies demonstrate that enhancement of 4-HPR-induced apoptosis by p75NTR is dependent upon p38MAPK phosphorylation, JNK phosphorylation, caspase 3 activation, Akt cleavage, and decreased Akt phosphorylation. In addition, treatment with 4-HPR results in upregulation of MKK4 and MEKK1, and phosphorylation of MKK3/6. Efforts to enhance the efficacy of 4-HPR and to identify those tumors most likely to respond to it might exploit these effectors of 4-HPR-induced apoptosis.

CONCLUSIONS

Pharmacological agents that enhance MKK4 or MEKK1 expression or JNK expression or phosphorylation may enhance efficacy of 4-HPR in neuroblastomas that do not express high levels of p75NTR.

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