The intracellular domain of p75NTR as a determinant of cellular reducing potential and response to oxidant stress

The Tangential Dialogue Between Science and Medicine: A Case in Point

The road between a hypothesis about a disease or condition and its cure or palliation is never simply linear. There are many tantalizing tangents to be chased and many seemingly obvious truths with countless exceptions; this is usually a feature, not a bug, as they say in computer programming. In the tangents and exceptions are clues and alternative roads to science and medicine that can provide cures and palliative measures, sometimes for diseases or conditions other than the one being studied. The narrative that follows uses the author's scientific experience in childhood nervous system cancer to illustrate the importance of a robust, bidirectional interaction between the laboratory bench and the clinic bedside in the quest for solutions to problems of health, longevity, and quality of life.

Is there a role for the p75 neurotrophin receptor in mediating degeneration during oxidative stress and after hypoxia?

Cholinergic basal forebrain (cBF) neurons are particularly vulnerable to degeneration following trauma and in neurodegenerative conditions. One reason for this is their characteristic expression of the p75 neurotrophin receptor (p75^NTR ), which is up-regulated and mediates neuronal death in a range of neurological and neurodegenerative conditions, including dementia, stroke and ischaemia. The signalling pathway by which p75^NTR signals cell death is incompletely characterised, but typically involves activation by neurotrophic ligands and signalling through c-Jun kinase, resulting in caspase activation via mitochondrial apoptotic signalling pathways. Less well appreciated is the link between conditions of oxidative stress and p75^NTR death signalling. Here, we review the literature describing what is currently known regarding p75^NTR death signalling in environments of oxidative stress and hypoxia to highlight the overlap in signalling pathways and the implications for p75^NTR signalling in cBF neurons. We propose that there is a causal relationship and define key questions to test this assertion.

p75NTR-/- mice exhibit an alveolar bone loss phenotype and inhibited PI3K/Akt/β-catenin pathway

OBJECTIVES

The aim of this study was to investigate the role of p75 neurotrophin receptor (p75NTR) in regulating the mouse alveolar bone development and the mineralization potential of murine ectomesenchymal stem cells (EMSCs). Moreover, we tried to explore the underlying mechanisms associated with the PI3K/Akt/β-catenin pathway.

MATERIALS AND METHODS

p75NTR knockout (p75NTR-/- ) mice and wild-type (WT) littermates were used. E12.5d p75NTR-/- and WT EMSCs were isolated in the same pregnant p75NTR-/+ mice from embryonic maxillofacial processes separately. Mouse alveolar bone mass was evaluated using micro-CT. Differential osteogenic differentiation pathways between p75NTR-/- and WT EMSCs were analysed by RNA-sequencing. The PI3K inhibitor LY294002 and PI3K agonist 740Y-P were used to regulate the PI3K/Akt pathway in EMSCs. p75NTR overexpression lentiviruses, p75NTR knock-down lentiviruses and recombined mouse NGF were used to transfect cells.

RESULTS

The alveolar bone mass was found reduced in the p75NTR knockout mouse comparing to the WT mouse. During mineralization induction, p75NTR-/- EMSCs displayed decreased osteogenic capacity and downregulated PI3K/Akt/β-catenin signalling. The PI3K/Akt/β-catenin pathway positively regulates the potential of differential mineralization in EMSCs. The promotive effect of p75NTR overexpression can be attenuated by LY294002, while the inhibitory effect of p75NTR knock-down on Runx2 and Col1 expression can be reversed by 740Y-P.

CONCLUSION

Deletion of p75NTR reduced alveolar bone mass in mice. P75NTR positively regulated the osteogenic differentiation of EMSCs via enhancing the PI3K/Akt/β-catenin pathway.

Systemic L-Buthionine -S-R-Sulfoximine Treatment Increases Plasma NGF and Upregulates L-cys/L-cys2 Transporter and γ-Glutamylcysteine Ligase mRNAs Through the NGF/TrkA/Akt/Nrf2 Pathway in the Striatum

Glutathione (GSH) is the most abundant intracellular antioxidant. GSH depletion leads to oxidative stress and neuronal damage in the central nervous system (CNS). In mice, the acute systemic inhibition of GSH synthesis by L-buthionine-S-R-sulfoximine (BSO) triggers a protective response and a subsequent increase in the CNS GSH content. This response might be modulated by a peripheral increment of circulating nerve growth factor (NGF). NGF is an important activator of antioxidant pathways mediated by tropomyosin-related kinase receptor A (TrkA). Here, we report that peripheral administration of BSO increased plasma NGF levels. Additionally, BSO increased NGF levels and activated the NGF/TrkA/Akt pathway in striatal neurons. Moreover, the response in the striatum included an increased transcription of nrf2, gclm, lat1, eaac1, and xct, all of which are involved in antioxidant responses, and L-cys/L-cys₂ and glutamate transporters. Using antibody against NGF confirmed that peripheral NGF activated the NGF/TrkA/Akt/Nrf2 pathway in the striatum and subsequently increased the transcription of gclm, nrf2, lat1, eaac1, and xct. These results provide evidence that the reduction of peripheral GSH pools increases peripheral NGF circulation that orchestrates a neuroprotective response in the CNS, at least in the striatum, through the NGF/TrkA/Akt/Nrf2 pathway.

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.

A role for the p75 neurotrophin receptor in axonal degeneration and apoptosis induced by oxidative stress

The p75 neurotrophin receptor (p75(NTR)) mediates the death of specific populations of neurons during the development of the nervous system or after cellular injury. The receptor has also been implicated as a contributor to neurodegeneration caused by numerous pathological conditions. Because many of these conditions are associated with increases in reactive oxygen species, we investigated whether p75(NTR) has a role in neurodegeneration in response to oxidative stress. Here we demonstrate that p75(NTR) signaling is activated by 4-hydroxynonenal (HNE), a lipid peroxidation product generated naturally during oxidative stress. Exposure of sympathetic neurons to HNE resulted in neurite degeneration and apoptosis. However, these effects were reduced markedly in neurons from p75(NTR-/-) mice. The neurodegenerative effects of HNE were not associated with production of neurotrophins and were unaffected by pretreatment with a receptor-blocking antibody, suggesting that oxidative stress activates p75(NTR) via a ligand-independent mechanism. Previous studies have established that proteolysis of p75(NTR) by the metalloprotease TNFα-converting enzyme and γ-secretase is necessary for p75(NTR)-mediated apoptotic signaling. Exposure of sympathetic neurons to HNE resulted in metalloprotease- and γ-secretase-dependent cleavage of p75(NTR). Pharmacological blockade of p75(NTR) proteolysis protected sympathetic neurons from HNE-induced neurite degeneration and apoptosis, suggesting that cleavage of p75(NTR) is necessary for oxidant-induced neurodegeneration. In vivo, p75(NTR-/-) mice exhibited resistance to axonal degeneration associated with oxidative injury following administration of the neurotoxin 6-hydroxydopamine. Together, these data suggest a novel mechanism linking oxidative stress to ligand-independent cleavage of p75(NTR), resulting in axonal fragmentation and neuronal death.

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.

Kidins220/ARMS is expressed in neuroblastoma tumors and stabilizes neurotrophic signaling in a human neuroblastoma cell line

BACKGROUND

Neurotrophic signaling is an important factor in the survival of developing neurons, and the expression of neurotrophic receptors correlates with prognosis in neuroblastoma. Kinase D-interacting substrate of 220 kDa (Kidins220) associates with neurotrophic receptors and stabilizes them, but the expression and function of Kidins220 in neuroblastoma are unknown.

METHODS

We study Kidins220 expression in human neuroblastoma cell lines and tumor samples by western blotting and microarray analyses. We determine the functional consequences of downregulation of Kidins220 for response of cell lines to oxidative stress, chemotherapeutic treatment, and neurotrophins using small interfering RNA silencing and by measuring cell survival, signaling, and migration.

RESULTS

Kidins220 is expressed in all neuroblastoma tumors and cell lines studied. Downregulation of Kidins220 leads to attenuation of nerve growth factor (NGF)-induced, but not brain-derived neurotrophic factor (BDNF)-induced, MAPK signaling. However, downregulation of Kidins220 does not alter the response to chemotherapeutic drugs or oxidative stress or affect cellular motility.

CONCLUSION

Kidins220 is expressed in neuroblastoma tumors and stabilizes NGF-induced, but not BDNF-induced, survival signaling in neuroblastoma cell lines.

Aiming at neuroblastoma and hitting other worthy targets

Neuroblastoma is, at once, the most common and deadly extracranial solid tumor of childhood. Efforts aimed at targeting the neural characteristics of these tumors have taught us much about neural crest cell biology, apoptosis induction in the nervous system, and neurotrophin receptor signaling and intracellular processing. But neuroblastoma remains a formidable enemy to the oncologist and an enigmatic target to the neuroscientist.

Structural characterization of the self-association of the death domain of p75(NTR.)

The neurotrophin receptor p75(NTR) conveys multiple signals via its intracellular death domain. However, how the death domain is activated and interacts with downstream adaptors remains unclear. Here, we report two crystal structures of the p75(NTR) death domain in the form of a non-covalent asymmetric dimer and a Cys379-Cys379 disulfide bond linked symmetric dimer, respectively. These two dimer arrangements have not previously been observed in other death domain-containing proteins. Further analysis shows that both the Cys379-Cys379 disulfide linked and non-covalent full-length p75(NTR) dimers are present on the cell surface. These observations suggest that various oligomers may exist simultaneously on the cell surface, and that p75(NTR) activation and signalling may be modulated by neurotrophins or other factors via inducing a shift of the equilibrium between different oligomeric states.

p75NTR: an enhancer of fenretinide toxicity in neuroblastoma

OBJECTIVE

Neuroblastoma is a common, frequently fatal, neural crest tumor of childhood. Chemotherapy-resistant neuroblastoma cells typically have Schwann cell-like ("S-type") morphology and express the p75 neurotrophin receptor (p75NTR). p75NTR has been previously shown to modulate the redox state of neural crest tumor cells. We, therefore, hypothesized that p75NTR expression level would influence the effects of the redox-active chemotherapeutic drug fenretinide on neuroblastoma cells.

METHODS

Transfection and lentiviral transduction were used to manipulate p75NTR expression in these cell lines. Sensitivity to fenretinide was determined by concentration- and time-cell survival studies. Apoptosis incidence was determined by morphological assessment and examination of cleavage of poly-ADP ribose polymerase and caspase-3. Generation and subcellular localization of reactive oxygen species were quantified using species- and site-specific stains and by examining the effects of site-selective antioxidants on cell survival after fenretinide treatment. Studies of mitochondrial electron transport employed specific inhibitors of individual proteins in the electron transport chain.

RESULTS

Knockdown of p75NTR attenuates fenretinide-induced accumulation of mitochondrial superoxide and apoptosis. Overexpression of p75NTR has the opposite effects. Pretreatment of cells with 2-thenoyltrifluoroacetone or dehydroascorbic acid uniquely prevents mitochondrial superoxide accumulation and cell death after fenretinide treatment, indicating that mitochondrial complex II is the likely site of fenretinide-induced superoxide generation and p75NTR-induced potentiation of these phenomena.

CONCLUSION

Modification of expression of p75NTR in a particular neuroblastoma cell line modifies its susceptibility to fenretinide. Enhancers of p75NTR expression or signaling could be potential drugs for use as adjuncts to chemotherapy of neural tumors.

Chopper is prodeath regardless of the effect of p75ICD on sensitivity to oxidative stress

BACKGROUND

The intracellular domain (ICD) of the neurotrophin receptor, p75NTR, exhibits variably pro- and antiapoptotic activity and has been implicated in neurodegenerative and neurodestructive disease. The molecular determinants of these cellular effects are not completely understood. The "Chopper" domain of p75ICD has been shown to be proapoptotic in in vitro systems in which p75ICD is proapoptotic. The effects of Chopper in systems in which p75ICD is antiapoptotic and, therefore, whether or not Chopper accounts for the variability of the cellular effects of p75ICD are not known. We therefore examined the effects of deletion of Chopper on the effects of p75ICD on in vitro cell culture systems in which p75ICD is pro- or antiapoptotic, respectively.

RESULTS

In HN33.11 murine neuroblastoma-hippocampal neuron hybrid cells, p75ICD is antiapoptotic. In NIH 3T3 cells, p75ICD is proapoptotic. In both cell lines deletion of the Chopper domain from p75ICD decreases the incidence of apoptosis resulting from oxidative stress. Thus, irrespective of the nature of the effects of p75ICD on the cell, its Chopper domain is proapoptotic.

CONCLUSIONS

Expression of p75ICD can enhance or attenuate oxidative induction of apoptosis. Variability of the effects of p75ICD is not related to variability of the effects of its Chopper domain.

Pharmacologic management of high-risk neuroblastoma in children

Neuroblastoma is the most common extracranial solid tumor of childhood. It accounts for 15% of pediatric cancer deaths. Children with high-risk disease have a 3-year event-free survival rate of only 20%. Chemotherapy is the mainstay of treatment in children with advanced neuroblastoma. The aim of this article was to review and critically evaluate the pharmacotherapy of neuroblastoma, using peer reviewed and review literature from 2000-11. All peer reviewed, published human subject studies of therapy for neuroblastoma in children were included. Animal model and in vitro studies were included only if they added to the understanding of the mechanism of a proposed or existing human neuroblastoma therapy. Current therapeutic options for neuroblastoma involve insufficient differentiation of normal from neoplastic tissue. Critically needed new approaches will increasingly exploit targeting of therapy for unique characteristics of the neuroblastoma cell. Pharmacotherapy for neuroblastoma still suffers from an inadequate therapeutic window. Enhancement of toxicity for tumor and safety for normal tissues will entail innovation in targeting neuroblastoma cells and rescuing or protecting normal tissue elements.

Necdin and neurotrophin receptors: interactors of relevance for neuronal resistance to oxidant stress

Necdin is a protein known to interact with the neurotrophin receptors, neurotrophic tyrosine kinase receptor type 1 (TrkA) and 75 kD low-affinity neurotrophin receptor (p75NTR). TrkA and p75NTR play roles in development and disease of the nervous system and chemoresistance of nervous system tumors. Necdin deletion is associated with Prader-Willi syndrome. The present studies demonstrate that the effects of necdin on the susceptibility of neuroblastoma cells to oxidant stress are dependent on the ratio of p75NTR to TrkA in the cell. In low p75NTR:TrkA ratio cells, necdin down-regulation decreases sensitivity to oxidant stress and expression of and signaling through TrkA. In high p75NTR:TrkA cells, necdin down-regulation is without effect. The effects of necdin deletion on the developing nervous system may depend on the relative expression of p75NTR and TrkA in the cells of particular regions of the nervous system.

Role of tyrosine phosphorylation in the antioxidant effects of the p75 neurotrophin receptor

The p75 neurotrophin receptor (p75NTR) is an α-and γ-secretase substrate expressed preferentially in the cholinergic neurons of the nucleus basalis of Meynert, the hippocampus, and the cerebellum of the adult brain. Mutations of the γ-secretase, presenilin, have been implicated in familial Alzheimer's disease. Furthermore, oxidative and inflammatory injury to the cholinergic neurons of the nucleus basalis of Meynert and hippocampus plays a critical role in the pathology of Alzheimer's disease. The intracellular domain of p75NTR (p75ICD) is the α- and γ-secretase cleavage fragment of the holoreceptor that functions as an antioxidant in PC12 rat pheochromocytoma cells. Phosphorylation of the receptor is thought to be necessary for many of its functions, and two tyrosines in p75ICD have been among the functionally important phosphorylation sites. Site-directed mutagenesis was used to generate three p75ICD mutants that cannot be phosphorylated at either or both tyrosines, respectively. Each of these mutants was expressed in p75NTR-deficient PC12 cells to determine the effects of blocking phosphorylation at specific sites on the antioxidant activity of p75ICD. Interfering with phosphorylation at tyrosine-337 impairs antioxidant function, while interfering with phosphorylation at tyrosine-366 does not, and may in fact impart protection from oxidant stress. Neither MAPK (i.e., p38, ERK1, ERK2) content nor NF-κB activation accounts for the differential sensitivity to oxidant stress among the differentially phosphorylated p75NTR cell lines. However, differences in the time course of ERK1,2 phosphorylation among the lines account in large measure for their differential oxidant sensitivity. The phosphorylation state of specific sites on p75ICD may modulate the resistance of neurons in Alzheimer's disease-relevant brain regions to oxidant stress.

The child is father to the man: developmental roles for proteins of importance for neurodegenerative disease

Although Alzheimer's and Parkinson's diseases predominately affect elderly adults, the proteins that play a role in the pathogenesis of these diseases are expressed throughout life. In fact, many of the proteins hypothesized to be important in the progression of neurodegeneration play direct or indirect roles in the development of the central nervous system. The systems affected by these proteins include neural stem cell fate decisions, neuronal differentiation, cellular migration, protection from oxidative stress, and programmed cell death. Insights into the developmental roles of these proteins may ultimately impact the understanding of neurodegenerative diseases and lead to the discovery of novel treatments.

Necdin and TrkA contribute to modulation by p75NTR of resistance to oxidant stress

The neurotrophin receptor p75NTR provides protection from oxidant stress induced by 6-hydroxydopamine (6-OHDA) and resultant cell death. In the absence of p75NTR, TrkA is upregulated and its signaling pathway effectors are increasingly activated. Necdin, a MAGE protein and known interactor of p75NTR and TrkA, is a potential mediator of this phenomenon. Decreased expression of necdin protein in p75NTR-deficient PC12 cells decreased TrkA expression and increased PC12 cell resistance to 6-OHDA. Inhibition of JNK phosphorylation by SP600125 also resulted in increased resistance to 6-OHDA, suggesting that TrkA signaling underlies the susceptibility of these cells to oxidant stress.

p75NTR-dependent modulation of cellular handling of reactive oxygen species

Our previous studies demonstrated that p75NTR confers protection against oxidative stress-induced apoptosis upon PC12 cells; however, the mechanisms responsible for this effect are not known. The present studies reveal decreased mitochondrion membrane potential and increased generation of reactive oxygen species (ROS) in p75NTR-deficient PC12 cells as well as diminution of ROS generation after transfection of a full-length p75NTR construct into these cells. They also show that p75NTR deficiency attenuates activation of the phosphatidylinositol 3-kinase --> phospho-Akt/protein kinase B pathway in PC12 cells by oxidative stress or neurotrophic ligands and inhibition of Akt phosphorylation decreases the glutathione (GSH) content in PC12 cells. In addition, decreased de novo GSH synthesis and increased GSH consumption are observed in p75NTR-deficient cells. These findings indicate that p75NTR regulates cellular handling of ROS to effect a survival response to oxidative stress.

Neurotrophic support and oxidative stress: converging effects in the normal and diseased nervous system

Oxidative stress and loss of neurotrophic support play major roles in the development of various diseases of the central and peripheral nervous systems. In disorders of the central nervous system such as Alzheimer's, Parkinson's, and Huntington's diseases, oxidative stress appears inextricably linked to the loss of neurotrophic support. A similar situation is seen in the peripheral nervous system in diseases of olfaction, hearing, and vision. Neurotrophic factors act to up-regulate antioxidant enzymes and promote the expression of antioxidant proteins. On the other hand, oxidative stress can cause down-regulation of neurotrophic factors. We propose that normal functioning of the nervous systems involves a positive feedback loop between antioxidant processes and neurotrophic support. Breakdown of this feedback loop in disease states leads to increased oxidative stress and reduced neurotrophic support.

Molecular predictors of human nervous system cancer responsiveness to enediyne chemotherapy

PURPOSE

To identify and mathematically model molecular predictors of response to the enediyne chemotherapeutic agent, neocarzinostatin, in nervous system cancer cell lines.

METHODS

Human neuroblastoma, breast cancer, glioma, and medulloblastoma cell lines were maintained in culture. Content of caspase-3 and Bcl-2, respectively, was determined relative to actin content for each cell line by Western blotting and optical densitometry. For each cell line, sensitivity to neocarzinostatin was determined. Brain tumor cell lines were stably transfected with human Bcl-2 cDNA cloned into the pcDNA3 plasmid vector.

RESULTS

In human tumor cell lines of different tissue origins, sensitivity to neocarzinostatin is proportional to the product of the relative contents of Bcl-2 and caspase-3 (r (2) = 0.9; P < 0.01). Neuroblastoma and brain tumor cell lines are particularly sensitive to neocarzinostatin; the sensitivity of brain tumor lines to neocarzinostatin is enhanced by transfection with an expression construct for Bcl-2 and is proportional in transfected cells to the product of the relative contents of Bcl-2 and caspase-3 (r (2) = 0.7).

CONCLUSION

These studies underscore the potential of molecular profiling in identifying effective chemotherapeutic paradigms for cancer in general and tumors of the nervous system in particular.

p75NTR enhances PC12 cell tumor growth by a non-receptor mechanism involving downregulation of cyclin D2

p75NTR is a member of the tumor necrosis superfamily of proteins which is variably associated with induction of apoptosis and proliferation. Cyclin D2 is one of the mediators of cellular progression through G1 phase of the cell cycle. The present study demonstrates the inverse relationship between expression of cyclin D2 and expression of p75NTR in PC12 cells. Induction of p75NTR expression in p75NTR-negative PC12 cells results in downregulation of cyclin D2; suppression of p75NTR expression with siRNA in native PC12 cells results in upregulation of cyclin D2. The effects of p75NTR on cyclin D2 expression are mimicked in p75NTR-negative cells by transfection with the intracellular domain of p75NTR. Cyclin-D2-positive PC12 cell cultures grow more slowly than cyclin-D2-negative cultures, and induction of expression of cyclin D2 slows the culture growth rate of cyclin-D2-negative cells. Finally, subcutaneous murine xenografts of cyclin-D2-negative, p75NTR-positive PC12 cells more frequently and more rapidly produce tumors than the analogous xenografts of cyclin-D2-positive, p75NTR-negative cells. These results suggest that p75NTR suppresses cyclin D2 expression in PC12 cells by a mechanism distinct from its function as a nerve growth factor receptor and that cyclin D2 expression decreases cell culture and xenografted tumor growth.

Cholesterol biosynthesis and the pro-apoptotic effects of the p75 nerve growth factor receptor in PC12 pheochromocytoma cells

Neocarzinostatin (NCS), an enediyne antimitotic agent, induces cell death in both p75NTR neurotrophin receptor (NTR)-positive and p75NTR-negative PC12 cells in a concentration-dependent fashion. However, p75NTR-positive cells demonstrate a higher susceptibility to NCS-induced cell damage. Furthermore, treatment of p75NTR-positive cells with the p75NTR-specific ligand, MC192, resulted in apoptosis, while treatment of these cells with the TrkA-specific ligand, NGF-mAbNGF30, protected them from NCS-induced death, implying that both the naked and liganded p75NTR receptors have a pro-apoptotic effect on PC12 cells. Microarray studies aimed at examining differential gene expression between p75NTR-positive and p75NTR-negative cells suggested that enzymes of the cholesterol biosynthetic pathway are differentially expressed. We therefore tested the hypothesis that altered cholesterol biosynthesis contributes directly to the pro-apoptotic effects of p75NTR in this PC12 cell-NCS model. Subsequent Northern blotting studies confirmed that the expression of p75NTR is associated with the upregulation of cholesterol biosynthetic enzymes including 3-hydroxy-3-methylglutaryl CoA reductase (HMG CoA reductase), farnesyl-diphosphate synthase, and 7-dehydro-cholesterol reductase. Mevastatin, an HMG CoA reductase inhibitor, converts the apoptosis susceptibility of p75NTR-positive cells to that of p75NTR-negative cells. It does so at concentrations that do not themselves alter cell survival. These studies provide evidence that the pro-apoptotic effects of p75NTR in PC12 cells are related to the upregulation of cholesterol biosynthetic enzymes and consequent increased cholesterol biosynthesis.

The p75 neurotrophin receptor in human development and disease

The functional effects of nerve growth factor (NGF) and its precursor, pro-NGF, are thought to be mediated through binding of these ligands to one or both of their receptors, TrkA and p75NTR. While the signaling pathways and downstream effects of NGF binding to TrkA are reasonably well known, those related to the binding of NGF and pro-NGF to p75NTR are less well understood. Furthermore, p75NTR appears to play functional roles that are unrelated to its ability to bind NGF and pro-NGF, some of which are ligand-independent and others of which are dependent upon binding to other neurotrophins. As these functional roles and their biochemical mechanisms become better known, the importance of p75NTR, related receptors, and both extracellular ligands and intracellular interactors and effectors for human development and health has become increasingly apparent. A complete understanding of p75NTR and its cellular partners is best served by approaching the remaining questions from both sides, with studies of function in normal states and studies of dysfunction in aberrant states mutually informing one another.