Neuroprotective Role of Selected Antioxidant Agents in Preventing Cisplatin-Induced Damage of Human Neurons In Vitro.

Chemotherapy-induced peripheral neuropathy (CIPN) is a side effect of platinum-based chemotherapy and decreases the quality of life of cancer patients. We compared neuroprotective properties of several agents using an in vitro model of terminally differentiated human cells NT2-N derived from cell line NT2/D1. Sodium azide and an active metabolite of amifostine (WR1065) increase cell viability in simultaneous treatment with cisplatin. In addition, WR1065 protects the non-dividing neurons by decreasing cisplatin caused oxidative stress and apoptosis. Accumulation of Pt in cisplatin-treated cells was heterogeneous, but the frequency and concentration of Pt in cells were lowered in the presence of WR1065 as shown by X-ray fluorescence microscopy (XFM). Transition metals accumulation accompanied Pt increase in cells; this effect was equally diminished in the presence of WR1065. To analyze possible chemical modulation of Pt-DNA bonds, we examined the platinum LIII near edge spectrum by X-ray absorption spectroscopy. The spectrum found in cisplatin-DNA samples is altered differently by the addition of either WR1065 or sodium azide. Importantly, a similar change in Pt edge spectra was noted in cells treated with cisplatin and WR1065. Therefore, amifostine should be reconsidered as a candidate for treatments that reduce or prevent CIPN.

The overexpression of SOX2 affects the migration of human teratocarcinoma cell line NT2/D1.

The altered expression of the SOX2 transcription factor is associated with oncogenic or tumor suppressor functions in human cancers. This factor regulates the migration and invasion of different cancer cells. In this study we investigated the effect of constitutive SOX2 overexpression on the migration and adhesion capacity of embryonal teratocarcinoma NT2/D1 cells derived from a metastasis of a human testicular germ cell tumor. We detected that increased SOX2 expression changed the speed, mode and path of cell migration, but not the adhesion ability of NT2/D1 cells. Additionally, we demonstrated that SOX2 overexpression increased the expression of the tumor suppressor protein p53 and the HDM2 oncogene. Our results contribute to the better understanding of the effect of SOX2 on the behavior of tumor cells originating from a human testicular germ cell tumor. Considering that NT2/D1 cells resemble cancer stem cells in many features, our results could contribute to the elucidation of the role of SOX2 in cancer stem cells behavior and the process of metastasis.

Human embryonal carcinoma cells in serum-free conditions as an in vitro model system of neural differentiation.

Serum is generally regarded as an essential component of many eukaryotic cell culture media, despite the fact that serum composition varies greatly and may be the source of a wide range of artefacts. The objective of this study was to assess serum-free growth conditions for the human embryonal carcinoma cell line, NT2/D1. These cells greatly resemble embryonic stem cells. In the presence of retinoic acid (RA), NT2/D1 cells irreversibly differentiate along the neuronal lineage. We have previously shown that the early phases of neural induction of these cells by RA involve the up-regulation of SOX3 gene expression. Our goal was to compare RA-induced differentiation of NT2/D1 cells in serum-containing and serum-free media, by using SOX3 protein levels as a marker of differentiation. We found that NT2/D1 cells can be successfully grown under serum-free conditions, and that the presence or absence of serum does not affect the level of SOX3 protein after a 48-hour RA induction. However, six days of RA treatment resulted in a marked increase in SOX3 protein levels in serum-free media compared to serum-containing media, indicating that serum might have an inhibitory effect on the expression of this neural differentiation marker. This finding is important for both basic and translational studies that hope to exploit cell culture conditions that are free of animal-derived products.

Expression analysis of SOX14 during retinoic acid induced neural differentiation of embryonal carcinoma cells and assessment of the effect of its ectopic expression on SOXB members in HeLa cells.

SOX14 is a member of the SOXB2 subgroup of transcription factors implicated in neural development. Although the first SOX14 gene in vertebrates was cloned and characterized more than a decade ago and its expression profile during development was revealed in various animal model systems, the role of this gene during neural development is largely unknown. In the present study we analyzed the expression of SOX14 in human NT2/D1 and mouse P19 pluripotent embryonal carcinoma cells. We demonstrated that it is expressed in both cell lines and upregulated during retinoic acid induced neural differentiation. We showed that SOX14 was expressed in both neuronal and non-neuronal differentiated derivatives, as revealed by immunocytochemistry. Since it was previously proposed that increased SOXB2 proteins level interfere with the activity of SOXB1 counteracting partners, we compared expression patterns of SOXB members during retinoic acid induction of embryonal carcinoma cells. We revealed that upregulation of SOX14 expression is accompanied by alterations in the expression patterns of SOXB1 members. In order to analyze the potential cross-talk between them, we generated SOX14 expression construct. The ectopic expression of SOX14 was demonstrated at the mRNA level in NT2/D1, P19 and HeLa cells, while an increased level of SOX14 protein was detected in HeLa cells only. By transient transfection experiments in HeLa cells we showed for the first time that ectopic expression of SOX14 repressed SOX1 expression, whereas no significant effect on SOX2, SOX3 and SOX21 was observed. Data presented here provide an insight into SOX14 expression during in vitro neural differentiation of embryonal carcinoma cells and demonstrate the effect of its ectopic expression on protein levels of SOXB members in HeLa cells. Obtained results contribute to better understanding the role of one of the most conserved SOX proteins.

Dual REST-dependence of L1CAM: from gene expression to alternative splicing governed by Nova2 in neural cells.

L1 cell adhesion molecule (L1CAM), an adhesion/signaling protein encoded by a gene target of the transcription repressor RE-1-Silencing Transcription factor (REST), is expressed in two alternatively spliced isoforms. The full-length isoform, typical of low-REST neural cells, plays key roles in survival/migration, outgrowth/fasciculation/regeneration of axons, synaptic plasticity; the isoform missing two mini-exons, abundant in a few high-REST non-neural cells, maintains some effect on migration and proliferation. To investigate whether and how L1CAM alternative splicing depends on REST we used neural cell models expressing low or high levels of REST (PC12, SH-SY5Y, differentiated NT2/D1 and primary neurons transduced or not with REST). The short isoform was found to rise when the low-REST levels of neural cells were experimentally increased, while the full-length isoform increased in high-REST cells when the repressor tone was attenuated. These results were due to Nova2, a neural cell-specific splicing factor shown here to be repressed by REST. REST control of L1CAM occurs therefore by two mechanisms, transcription and alternative splicing. The splicing mechanism, affecting not only L1CAM but all Nova2 targets (∼7% of brain-specific splicing, including the mRNAs of other adhesion and synaptic proteins) is expected to be critical during development and important also for the structure and function of mature neural cells.

A signaling loop of REST, TSC2 and ß-catenin governs proliferation and function of PC12 neural cells.

The RE-1-specific silencing transcription factor (REST or NRSF) is a transcription repressor that orchestrates differentiation and also operates in differentiated neurons and neurosecretory cells (neural cells). Its role in proliferation has been investigated so far only in rapidly growing tumors, with conflicting results: suppression in non-neural tumors, stimulation in medulloblastomas. Working with two clones of chromaffin-neuronal PC12 cells, which express different levels of REST, and using genetic complementation and knockdown approaches, we show that REST also promotes proliferation in differentiated neural cells. Mechanistically, this occurs by a signaling pathway involving REST, the GTPase-activating protein tuberin (TSC2) and the transcription co-factor ß-catenin. In PC12 cells, raised expression of REST correlates with reduced TSC2 levels, nuclear accumulation and co-transcriptional activation of ß-catenin, and increased expression of its target oncogenes Myc and Ccnd1, which might account for the proliferation advantage and the distinct morphology. Rest transcription is also increased, unveiling the existence of a self-sustaining, feed-forward REST-TSC2-ß-catenin signaling loop that is also operative in another neural cell model, NT2/D1 cells. Transfection of REST, knockdown of TSC2 or forced expression of active ß-catenin recapitulated the biochemical, functional and morphological properties of the high-expressing REST clone in wild-type PC12 cells. Upregulation of REST promoted proliferation and phenotypic changes, thus hindering neurosecretion. The new REST-TSC2-ß-catenin signaling paradigm might have an important role in various aspects of neural cell physiology and pathology, including the regulation of proliferation and neurosecretion.

Tissue-specific Forkhead protein FOXA2 up-regulates SOX14 gene expression.

The expression of Sox14 gene in spinal cord explants was found to be regulated by Sonic hedgehog (SHH) in a dose-dependent manner, indicating that this signaling molecule might act as a regulator of Sox14-expressing interneuron differentiation. In the present study we identified the positive control element and provided the first evidence that FOXA2 is involved in up-regulation of SOX14 expression in HepG2 and U87MG cell lines. By functional analysis we demonstrated that mutation in FOXA2 binding site reduced the SOX14 reporter construct activity, and that FOXA2 over-expression increased endogenous SOX14 protein expression. Further, we have shown that human SOX14 expression is GLI1 dependent in U87MG cells and SHH-N dependent in U87MG and HepG2 cell lines. By applying siRNA silencing of FOXA2, we have demonstrated that upregulation of endogenous SOX14 gene expression by SHH is, at least in part, mediated by FOXA2. However, our data revealed that a positive regulatory region, containing functional FOXA2 site analyzed in this study, is not involved in mediation of SHH dependent SOX14 activation. Data presented here provide the initial insight into molecular mechanism underlying tissue and developmentally specific regulation of the SOX14 gene expression.

The rest repression of the neurosecretory phenotype is negatively modulated by BHC80, a protein of the BRAF/HDAC complex.

Expression of neurosecretion by nerve cells requires the levels of the transcription repressor element-1 silencing transcription factor (REST) to be very low. However, when high-REST clones of PC12 cells, defective of neurosecretion, were fused to other high-REST, non-neurosecretory cells, some neurosecretion was recovered. To clarify the mechanism of this recovery, we fused defective PC12 cells with human lymphocytes. A cytogenetic analysis revealed all hybrid clones that recovered neurosecretion to contain a fragment of chromosome 11 including the gene encoding BHC80, a protein of one of the complexes that mediate REST repression. In these clones, REST levels were as high as in defective PC12, whereas BHC80, localized in the nucleus, was 4- to 5-fold higher. Transient transfection of defective PC12 with various amounts of BHC80 cDNA induced (1) in defective PC12, the reexpression of only neurosecretion mRNAs; (2) in defective PC12 cotransfected with the REST negative construct DNA-binding domain (to attenuate gene repression), the recovery of a weak, but complete neurosecretory phenotype, including dense-core granules and their regulated exocytosis. Chromatin immunoprecipitation and immunodepletion analyses revealed the extensive BHC80 association with REST at the genes of two neurosecretion proteins, chromograninB and SNAP25, however only in the low-REST PC12, whereas in high-REST defective PC12 no association was appreciable. In defective PC12 transfected with BHC80 some association was reestablished. Therefore, the recovery of neurosecretion observed after fusion/transfection of defective PC12 depends on the reciprocal level of BHC80 and REST, with BHC80 working as a negative modulator of REST repression. This role appears of possible cell physiological and pathological importance.

Expression of dense-core vesicles and of their exocytosis are governed by the repressive transcription factor NRSF/REST.

The mechanism by which neurons and neurosecretory cells govern the expression and the exocytic discharge of their clear and dense-core vesicles had remained unclear until recently when studies in the neurosecretory cell model PC12 revealed these processes to be orchestrated by the transcriptional repressor neuron restrictive silencer factor (NRSF)/repressor element-1 silencing transcription factor (REST). In wild-type PC12 fully competent for neurosecretion, NRSF/REST is low. The genes of the proteins involved in neurosecretion [from the secretory to vesicle membrane and plasma membrane proteins, including the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) of exocytosis] were all repressed by increases of NRSF/REST expression to various extents when the increase was only a fewfold but were completely or almost completely repressed when the increase was large, as in spontaneously defective PC12 clones. In the first case the dense-core vesicles were still competent for exocytosis but were smaller and less dense than in wild-type cells; in the second they were no longer visible but did reappear when the repression was attenuated by transfection of a dominant-negative construct of NRSF/REST combined with a secretory chromogranin or strengthened by treatment with a blocker of NRSF/REST-associated enzymes, the histone deacetylases.

Expression of the neurosecretory process in PC12 cells is governed by REST.

The neurosecretory process is acquired during differentiation and can be lost en block by differentiated cells. To investigate the role of REST/NRSF, a transcription repressor, in the maintenance of the process we studied two PC12 clones, one wt and one defective, expressing low and high levels of endogenous RE-1 silencing transcription (factor) (REST), respectively. Stable transfection of constructs demonstrated that REST represses 10 genes coding for proteins of neurosecretory vesicles and their exocytosis, eight including and two lacking the REST-binding sequence, RE-1. Of these genes, those of chromogranins were strongly repressed by fewfold increases of REST, those of VAMP2 and syntaxin1a required much higher levels. Moreover, in wt cells transfected with an active construct the dense-core vesicles, still competent for regulated exocytosis, were much smaller, with lighter cores; in defective cells, the dominant-negative construct induced the rescue of many vesicle/exocytosis genes but not of those of chromogranins. Small dense-core vesicles, exocytized upon stimulation, were rescued when the construct-transfected defective cells were transfected also with chromograninA or treated with trichostatinA, a blocker of histone deacetylases. Our results identify REST, working by direct and indirect mechanisms, as the factor governing the maintenance of the neurosecretory process and the properties of dense-core vesicles in PC12 cells.

Non-secretory exocytoses in the brain.

Regulated exocytosis, the process by which the membrane of specific cytoplasmic organelles fuse with the plasma membrane in response to adequate stimulation, is most often considered to serve only for the discharge of secretory products, in the brain especially neurotransmitters and peptides. Growing evidence demonstrates however that non-secretory exocytoses, aimed at the insertion at the cell surface of the organelle membrane, are of great physiological importance and may also have critical roles in specific diseases. Recently, two groups of non-secretory exocytoses have been identified: those aimed at the transfer to the cell surface of specific proteins, that we have proposed to be called the protein-exposing exocytoses; and those aimed at the enlargement of the surface itself, the expansive exocytoses. Here we present the existing knowledge about three types of non-secretory exocytoses that occur in the brain: the protein-exposing exocytoses that transfer ionic receptors to the postsynaptic membrane, the best known example being that of the glutamatergic AMPA receptor, a main actor of synaptic plasticity; the expansive exocytosis necessary for the growth of nerve fibres; and the rapid exocytosis of enlargeosomes, that can induce considerable expansion of the cell surface area in a variety of cells types, including the astrocytes.