Retinoids in neuroblastoma therapy: distinct biological properties of 9-cis- and all-trans-retinoic acid

An in vitro developmental neurotoxicity screening assay for retinoic acid-induced neuronal differentiation using the human NT2/D1 cell line

Traditional approaches (e.g., neurobehavior, neuropathology) can detect alterations in apical endpoints indicative of developmental neurotoxicity (DNT). However, there is an increasing desire to understand mode-of-action (MOA) for DNT effects; thus, this short communication describes initial work on a neuronal differentiation assay. Basically, our laboratory used the human NT2/D1 cell line to develop an assay to evaluate toxicants for effects on all-trans retinoic acid (RA)-induced neuronal differentiation. Based on literature reports, we selected a neuronal protein, neuronal class III β-tubulin (β3-tubulin), as a marker of differentiation. For this assay, cultured RA-treated NT2 cells were trypsinized to individual cells, methanol fixed, and labeled with a β3-tubulin specific monoclonal antibody (TUJ1). Characterization studies using 100,000 cells/sample showed that NT2 cells had appreciable expression of β3-tubulin starting around day 7 of the differentiation process with a peak expression noted around day 12. Methylmercury, 22(R)-hydroxycholesterol, N-(4-hydroxyphenol)retinamide (4HPR), and 9-cis retinoic acid were selected as initial test compounds. Of these, only 9-cis RA, which is known to affect the RA pathway, was positive for specific impacts on differentiation. These results demonstrate the feasibility of using a flow cytometry method targeting specific cellular biomarkers for evaluating effects on neuronal differentiation. Additional assays are needed to detect compounds targeting other (non-RA) neuronal differentiation pathways. Ultimately, a battery of in vitro assays would be needed to evaluate the potential MOAs involved in altered neuronal differentiation.

The atypical cell cycle regulator Spy1 suppresses differentiation of the neuroblastoma stem cell population

Neuroblastoma is an aggressive pediatric cancer originating embryonically from the neural crest. The heterogeneity of the disease, as most solid tumors, complicates diagnosis and treatment. In neuroblastoma this heterogeneity is well represented in both primary tumours and derived cell lines and has been shown to be driven by a population of stem-like tumour initiating cells. Resolving the molecular mediators driving the division of this population of cells may indicate effective therapeutic options for neuroblastoma patients. This study has determined that the atypical cyclin-like protein Spy1, recently indicated in driving symmetric division of glioma stem cells, is a critical factor in the stem-like properties of neuroblastoma tumor initiating cell populations. Spy1 activates Cyclin Dependent Kinases (CDK) in a manner that is unique from classical cyclins. Hence this discovery may represent an important opportunity to design CDK inhibitor drugs to uniquely target subpopulations of cells within these aggressive neural tumours.

Changes in gene expression profiling of apoptotic genes in neuroblastoma cell lines upon retinoic acid treatment

To determine the effect of retinoic acid (RA) in neuroblastoma we treated RA sensitive neuroblastoma cell lines with 9-cis RA or ATRA for 9 days, or for 5 days followed by absence of RA for another 4 days. Both isomers induced apoptosis and reduced cell density as a result of cell differentiation and/or apoptosis. Flow cytometry revealed that 9-cis RA induced apoptosis more effectively than ATRA. The expression profile of apoptosis and survival pathways was cell line specific and depended on the isomer used.

Histone deacetylase inhibitors regulate retinoic acid receptor beta expression in neuroblastoma cells by both transcriptional and posttranscriptional mechanisms

The retinoic acid receptor beta (RARbeta) is a retinoic acid (RA)-inducible tumor suppressor, which plays an important role in the arrest of neuroblastoma cell growth. Using human neuroblastoma SH-SY5Y cells, we have examined the regulation of RARbeta expression by histone deacetylase inhibitors (HDACi), considered to be promising agents in anticancer therapy. Our results show that HDACi cooperated with RA to increase RARbeta mRNA levels and to activate the RARbeta2 promoter in transient transfection assays. Chromatin immunoprecipitation assays showed that the basal RARbeta2 promoter that contains the RA response element was refractory to acetylation by both HDACi and RA. In addition, HDACi caused a transient increase in acetylation of a downstream RARbeta2 region, even though global histones remain hyperacetylated after a prolonged treatment with the inhibitors. RA potentiated this response and maintained acetylation for a longer period. Despite the cooperation of RA with HDACi to increase transcription of the RARbeta gene, these inhibitors caused a paradoxical reduction of the cellular levels of the RARbeta protein in cells treated with the retinoid. This reduction is secondary to a change in the protein half-life that is decreased by the HDACi due to increased ubiquitin-independent proteasomal degradation. These results show that HDACi regulate expression of the tumor suppressor gene RARbeta by both transcriptional and posttranscriptional mechanisms and might then modulate sensitivity to the retinoid in neuroblastoma cells.

Porphyrin−Retinamides: Synthesis and Cellular Studies

A series of four porphyrin-retinamides containing either all-trans- or 13-cis-retinoid acid residues, directly linked to the para-phenyl position of meso-tetraphenylporphyrin or via a low-molecular-weight PEG spacer, have been synthesized. The biological properties of these conjugates were evaluated in a model cell line, human HEp2, and in neuroblastoma SK-N-DZ cells, which exhibit moderate expression of retinoic acid receptors and retinoic acid-induced differentiation. The directly linked porphyrin-retinamides were taken up by a greater extent (20-50% more) in SK-N-DZ than in HEp2 cells. However, the PEG-containing conjugates accumulated maximally within both cell lines and approximately by the same amount, probably due to their increased amphiphilicity. Among all conjugates, the porphyrin-PEG-13-cis-retinamide accumulated the most in both cell lines (about 5 times more than the non-pegylated conjugates). None of the porphyrin-retinamide conjugates were toxic toward HEp2 cells at concentrations up to 100 microM, and only the hydrophobic non-pegylated conjugates were moderately toxic to SK-N-DZ cells [IC50 (dark) = 56-92 microM, and IC50 (at 1 J/cm2) = 6-8 microM]. All conjugates preferentially localized within cellular vesicles that correlated well to the lysosomes and, in addition, the PEG-containing porphyrin-retinamides were also found in the ER.

Cytodifferentiation by retinoids, a novel therapeutic option in oncology: rational combinations with other therapeutic agents

Retinoic acid (RA) and derivatives are promising antineoplastic agents endowed with both therapeutic and chemopreventive potential. Although the treatment of acute promyelocytic leukemia with all-trans retinoic acid is an outstanding example, the full potential of retinoids in oncology has not yet been explored and a more generalized use of these compounds is not yet a reality. One way to enhance the therapeutic and chemopreventive activity of RA and derivatives is to identify rational combinations between these compounds and other pharmacological agents. This is now possible given the information available on the biochemical and molecular mechanisms underlying the biological activity of retinoids. At the cellular level, the antileukemia and anticancer activity of retinoids is the result of three main actions, cytodifferentiation, growth inhibition, and apoptosis. Cytodifferentiation is a particularly attractive modality of treatment and differentiating agents promise to be less toxic and more specific than conventional chemotherapy. This is the result of the fact that cytotoxicity is not the primary aim of differentiation therapy. At the molecular level, retinoids act through the activation of nuclear retinoic acid receptor-dependent and -independent pathways. The cellular pathways and molecular networks relevant for retinoid activity are modulated by a panoply of other intracellular and extracellular pathways that may be targeted by known drugs and other experimental therapeutics. This chapter aims to summarize and critically discuss the available knowledge in the field.

Polynucleotide phosphorylase: an evolutionary conserved gene with an expanding repertoire of functions

RNA metabolism plays a seminal role in regulating diverse physiological processes. Polynucleotide phosphorylase (PNPase) is an evolutionary conserved 3',5' exoribonuclease, which plays a central role in RNA processing in bacteria and plants. Human polynucleotide phosphorylase (hPNPase old-35) was cloned using an inventive strategy designed to identify genes regulating the fundamental physiological processes of differentiation and senescence. Although hPNPase old-35 structurally and biochemically resembles PNPase of other species, targeted overexpression and inhibition studies reveal that hPNPase old-35 has evolved to serve more specialized functions in humans. The present review provides a global perspective on the structure and function of PNPase and then focuses on hPNPase old-35 in the contexts of differentiation and senescence.

Mechanistic and functional changes in Ca2+ entry after retinoic acid-induced differentiation of neuroblastoma cells

We have investigated effects of neuronal differentiation on hormone-induced Ca2+ entry. Fura-2 fluorescence measurements of undifferentiated SH-SY5Y neuroblastoma cells, stimulated with methacholine, revealed the presence of voltage-operated Ca2+-permeable, Mn2+-impermeable entry pathways, and at least two voltage-independent Ca2+- and Mn2+-permeable entry pathways, all of which apparently contribute to both peak and plateau phases of the Ca2+ signal. Similar experiments using 9-cis retinoic acid-differentiated cells, however, revealed voltage-operated Ca2+-permeable, Mn2+-impermeable channels, and, more significantly, the absence or down-regulation of the most predominant of the voltage-independent entry pathways. This down-regulated pathway is probably due to CCE (capacitative Ca2+ entry), since thapsigargin also stimulated Ca2+ and Mn2+ entry in undifferentiated but not differentiated cells. The Ca2+ entry components remaining in methacholine-stimulated differentiated cells contributed to only the plateau phase of the Ca2+ signal. We conclude that differentiation of SH-SY5Y cells results in a mechanistic and functional change in hormone-stimulated Ca2+ entry. In undifferentiated cells, voltage-operated Ca2+ channels, CCE and NCCE (non-CCE) pathways are present. Of the voltage-independent pathways, the predominant one appears to be CCE. These pathways contribute to both peak and plateau phases of the Ca2+ signal. In differentiated cells, CCE is either absent or down-regulated, whereas voltage-operated entry and NCCE remain active and contribute to only the plateau phase of the Ca2+ signal.

Expression analysis and genomic characterization of human melanoma differentiation associated gene-5, mda-5: a novel type I interferon-responsive apoptosis-inducing gene

Melanoma differentiation associated gene-5 (mda-5) was identified by subtraction hybridization as a novel upregulated gene in HO-1 human melanoma cells induced to terminally differentiate by treatment with IFN-beta+MEZ. Considering its unique structure, consisting of a caspase recruitment domain (CARD) and an RNA helicase domain, it was hypothesized that mda-5 contributes to apoptosis occurring during terminal differentiation. We have currently examined the expression pattern of mda-5 in normal tissues, during induction of terminal differentiation and after treatment with type I IFNs. In addition, we have defined its genomic structure and chromosomal location. IFN-beta, a type I IFN, induces mda-5 expression in a biphasic and dose-dependent manner. Based on its temporal kinetics of induction and lack of requirement for prior protein synthesis mda-5 is an early type I IFN-responsive gene. The level of mda-5 mRNA is in low abundance in normal tissues, whereas expression is induced in a spectrum of normal and cancer cells by IFN-beta. Expression of mda-5 by means of a replication incompetent adenovirus, Ad.mda-5, induces apoptosis in HO-1 cells as confirmed by morphologic, biochemical and molecular assays. Additionally, the combination of Ad.mda-5+MEZ further augments apoptosis as observed in Ad.null or uninfected HO-1 cells induced to terminally differentiate by treatment with IFN-beta+MEZ. The mda-5 gene is located on human chromosome 2q24 and consists of 16 exons, without pseudogenes, and is conserved in the mouse genome. Present data documents that mda-5 is a novel type I IFN-inducible gene, which may contribute to apoptosis induction during terminal differentiation and during IFN treatment. The conserved genomic and protein structure of mda-5 in human and mouse will permit analysis of the evolution and developmental aspects of this gene.

Differential effects of 9-cis, 13-cis and all-trans retinoic acids on the neuronal differentiation of human neuroblastoma cells

A human neuroblastoma cell line IMR-32 was used as an in vitro model to examine three naturally occurring retinoic acid (RA) isomers, 9-cis (9c), 13-cis (13c) and all-trans (AT) RA, in mediating growth differentiation and neuronal differentiation. All RA isomers inhibited cellular proliferation, with 13c-RA being most effective. Cyclic AMP-responsive-element-binding-protein (CREB) was activated during RA treatment. AT-RA was a better differentiating agent in inducing the highest expression of the neurotrophic factor receptor TrkA. After prolonged RA treatment, the expression of RA receptors (RARs) was comparable for the three isomers, but retinoid X receptors (RXRs) were differentially regulated. These results imply that distinctive molecular pathways might be involved in the in vitro differentiation of neuroblastoma with different RA isomers.

Retinoic acid modulates the cell-cycle in fetal rat hepatocytes and HepG2 cells by regulating cyclin-cdk activities

Retinoic acid (RA), the most biologically active metabolite of vitamin A, is known to modulate cell proliferation, apoptosis and differentiation, with different effects depending on the cellular context. Retinoic acid can exert its effects by directly or indirectly influencing the expression of genes involved in the control of cell proliferation. In the present report we investigate the possible correlation between the antiproliferative, differentiative and apoptotic effects previously observed on rat hepatocytes and HepG2 cells, with a possible modulation of cell-cycle regulators. We demonstrate that RA induces growth arrest and differentiation in HepG2 cells by influencing the activities of cyclin-cdk complexes involved in the regulation of G1/S transition and S-phase progression, in particular by modifying the binding of these complexes to p21 and p27 inhibitors. In fetal cells, however, the induction of apoptosis and differentiation by RA was obtained via inhibition of cyclin D1-cdk4 activity, as result of an increased binding to the p16 inhibitor. Retinoic acid also modulates c-myc and Bcl-2 expression. In conclusion, our data suggest that RA could be useful to regulate the reversion of transformed phenotype and could also be utilized as a chemiopreventive agent in cells of hepatic origin.

Retinoid X receptors and retinoid response in neuroblastoma cells

Retinoic acid (RA) modulates differentiation and apoptosis of neural cells via RA receptors (RARs) and retinoid X receptors (RXRs). Neuroblastoma cells are potentially useful models for elucidating the molecular mechanisms of RA in neural cells, and responses to different isomers of RA have been interpreted in terms of differential homo- and heterodimerization of RXRs. The aim of this study was to identify the RXR types expressed in neuroblast and substrate-adherent neuroblastoma cells, and to study the participation of these RXRs in RAR heterodimers. RXRbeta was the predominant RXR type in N-type SH SY 5Y cells and S-type SH EP cells. Gel shift and supershift assays demonstrated that RARbeta and RARgamma predominantly heterodimerize with RXRbeta. In SH SY 5Y cells, RARgamma/RXRbeta was the predominant heterodimer binding to the DR5 RARE in the absence of 9-cis RA (9C), whereas the balance shifted in favor of RARbeta/RXRbeta in the presence of ligand. There was a marked difference between the N- and S-type neuroblastoma cells in retinoid receptor-DNA interactions, and this may underlie the differential effects of retinoids in these neuroblastoma cell types. There was no evidence to indicate that 9C functions via RXR homodimers in either SH SY 5Y or SH EP neuroblastoma cells. The results of this study suggest that interactions between retinoid receptors and other nuclear proteins may be critical determinants of retinoid responses in neural cells.

Retinoic acid reduces the cytotoxicity of cyclopentenyl cytosine in neuroblastoma cells

In this paper, it is demonstrated that all-trans, 9-cis and 13-cis retinoic acid (RA) decreased the sensitivity of SK-N-BE(2)c neuroblastoma cells towards the chemotherapeutic agent cyclopentenyl cytosine (CPEC), a potent inhibitor of cytosine-5'-triphosphate synthetase. Retinoic acid attenuated CPEC-induced apoptosis as reflected by a decreased caspase-3 induction. Retinoic acid decreased the accumulation of CPEC, whereas the salvage of cytidine was strongly increased. Metabolic labeling studies using [(3)H]uridine showed a strongly decreased biosynthesis of CTP via CTP synthetase. Retinoic acid likely confers resistance of neuroblastoma cells to CPEC in part by slowing down proliferation, and in part by shifting the synthesis of CTP towards the salvage of cytidine, thereby bypassing CTP synthetase.

Anti-tumor mechanisms of valproate: a novel role for an old drug

Valproic acid (VPA, 2-propylpentanoic acid) is an established drug in the long-term therapy of epilepsy. During the past years, it has become evident that VPA is also associated with anti-cancer activity. VPA not only suppresses tumor growth and metastasis, but also induces tumor differentiation in vitro and in vivo. Several modes of action might be relevant for the biological activity of VPA: (1) VPA increases the DNA binding of activating protein-1 (AP-1) transcription factor, and the expression of genes regulated by the extracellular-regulated kinase (ERK)-AP-1 pathway; (2) VPA downregulates protein kinase C (PKC) activity; (3) VPA inhibits glycogen synthase kinase-3beta (GSK-3beta), a negative regulator of the Wnt signaling pathway; (4) VPA activates the peroxisome proliferator-activated receptors PPARgamma and delta; (5) VPA blocks HDAC (histone deacetylase), causing hyperacetylation. The findings elucidate an important role of VPA for cancer therapy. VPA might also be useful as low toxicity agent given over long time periods for chemoprevention and/or for control of residual minimal disease.

RB2/p130 ectopic gene expression in neuroblastoma stem cells: evidence of cell-fate restriction and induction of differentiation

The activity of the RB2/p130 gene, which is a member of the retinoblastoma gene family, is cell-cycle-regulated and plays a key role in growth inhibition and differentiation. We used neuroblastoma cell lines as a model for studies on neural crest progenitor cell differentiation. We show that Rb2/p130 ectopic protein expression induces morphological and molecular modifications, promoting differentiation of intermediate (I) phenotype SK-N-BE(2)-C neuroblastoma cells towards a neuroblastic (N) rather than a Schwann/glial/melanocytic (S) phenotype. These modifications are stable as they persist even after treatment with an S-phenotype inducer. Rb2/p130 ectopic expression also induces a more differentiated phenotype in N-type SH-SY-5Y cells. Further, this function appears to be independent of cell-cycle withdrawal. The data reported suggest that the Rb2/p130 protein is able to induce neuronal lineage specification and differentiation in neural crest stem and committed neuroblastoma cells, respectively. Thus, the Rb2/p130 protein seems to be required throughout the full neural maturation process.

Differential gene regulation by 9-cis and all-trans retinoic acid in neuroblastoma cells

BACKGROUND

9-cis retinoic acid (RA) is more effective than all-trans RA at inducing neuroblastoma differentiation in vitro, and has distinct biological properties with respect to its ability to promote apoptosis in N-type neuroblastoma cells. The cellular effects of 9-cis RA may, in part, result from activation of retinoid X receptor (RXR) homodimers. If this hypothesis is correct, 9-cis RA may control the expression of a different subset of retinoid-regulated genes compared to all-trans RA.

PROCEDURE

We have therefore used differential mRNA display to identify genes differentially expressed in neuroblastoma cells in response to all-trans and 9-cis RA.

RESULTS

The majority of cDNAs differentially expressed in response to all-trans or 9-cis RA matched to nonredundant Genbank sequences or EST database sequences. Differential-display profiles were similar in SH SY 5Y and SH S EP cells, clonal derivatives of the mixed neuroblastoma cell line SK N SH, although there were apparent differences between these cell lines with respect to the retinoid-regulation of specific RT-PCR cDNA fragments.

CONCLUSIONS

These data support the view that 9-cis and all-trans RA act via different receptor mechanisms.

Differential gene regulation by 9-cis and all-trans retinoic acid in neuroblastoma cells

BACKGROUND

9-cis retinoic acid (RA) is more effective than all-trans RA at inducing neuroblastoma differentiation in vitro, and has distinct biological properties with respect to its ability to promote apoptosis in N-type neuroblastoma cells. The cellular effects of 9-cis RA may, in part, result from activation of retinoid X receptor (RXR) homodimers. If this hypothesis is correct, 9-cis RA may control the expression of a different subset of retinoid-regulated genes compared to all-trans RA.

PROCEDURE

We have therefore used differential mRNA display to identify genes differentially expressed in neuroblastoma cells in response to all-trans and 9-cis RA.

RESULTS

The majority of cDNAs differentially expressed in response to all-trans or 9-cis RA matched to nonredundant Genbank sequences or EST database sequences. Differential-display profiles were similar in SH SY 5Y and SH S EP cells, clonal derivatives of the mixed neuroblastoma cell line SK N SH, although there were apparent differences between these cell lines with respect to the retinoid-regulation of specific RT-PCR cDNA fragments.

CONCLUSIONS

These data support the view that 9-cis and all-trans RA act via different receptor mechanisms.

Retinoic acid-induced apoptosis of the CHP134 neuroblastoma cell line is associated with nuclear accumulation of p53 and is rescued by the GDNF/Ret signal

BACKGROUND

Neuroblastoma (NBL) is one of the most common solid malignancies in childhood and is derived from the sympathetic precursor cells. Although p53, a tumor suppressor, has been reported to be rarely mutated in NBLs, it is sequestered abnormally in the cytoplasm of the NBL cell. The mechanism and functional role of the abnormal intracellular localization of p53 remain unclear.

PROCEDURE

Here, we established an in vitro system of apoptosis model using a NBL cell line CHP134 which also showed a cytoplasmic sequestration of p53. The treatment of the cells with 1 or 5 microM all-trans retinoic acid (RA) induced moderate neurite outgrowth followed by massive death of CHP134 cells by days 5 to 6.

RESULTS

TUNEL staining showed that the cell death was due to apoptosis. Immunofluorescent stain demonstrated that p53 was strongly positive in the nucleus on day 5, which was accompanied with induction of p21WAF1. In addition, expression of caspase-3 was also increased during the cell death. Intriguingly, the RA treatment induced expression of Ret tyrosine kinase receptor in CHP134 cells.

CONCLUSIONS

The addition of ligands, glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN), inhibited apoptosis as well as nuclear accumulation of p53 in the cell. The present results suggest that the RA-induced apoptosis of NBL cells is associated with activation of both the caspase cascade and the p53-mediated pathway with its nuclear translocation. The neurotrophic signal through the GDNF-Ret system may prevent the neuronal cell death.

Human renal mesangial cells are a target for the anti-inflammatory action of 9-cis retinoic acid

Mesangial cells play an active role in the inflammatory response to glomerular injury. We have studied in cultured human mesangial cells (CHMC) several effects of 9-cis retinoic acid (9-cRA), an activator of both retinoic acid receptors (RARs) and retinoid X receptors (RXRs). 9-cRA inhibited foetal calf serum-induced CHMC proliferation. It also prevented CHMC death induced by the inflammatory mediator H(2)O(2). This preventive effect was not due to any increase in H(2)O(2) catabolism and it persisted even when both catalase and glutathione synthesis were inhibited. Finally, 9-cRA diminished monocyte adhesion to FCS-stimulated CHMC. Interestingly, the retinoid also inhibited in FCS-stimulated cells the protein expression of two mesangial adhesion molecules, fibronectin and osteopontin, but it did not modify the protein expression of intercellular adhesion molecule-1 and vascular adhesion molecule-1. All major RARs and RXRs isotypes were expressed in CHMC regardless of the presence or absence of 9-cRA. Transcripts to RAR-alpha, RAR-beta and RXR-alpha increased after incubation with 9-cRA whereas RXR-gamma was inhibited, suggesting a major role for RARs and RXRs in 9-cRA-anti-inflammatory effects. 9-cRA was toxic only at 50 microM (a concentration 50 - 5000 times higher than required for the effects above). Cell death occurred by apoptosis, whose onset was associated with a pronounced increase in catalase activity and reduced glutathione content, being more effectively induced by all-trans retinoic acid. Modulation of the oxidant/antioxidant balance failed to inhibit apoptosis. We conclude that mesangial cells might be a target for the treatment of inflammatory glomerulopathies with 9-cRA.

Recent advances in the use of vitamin A (retinoids) in the prevention and treatment of cancer

Vitamin A, its physiologic metabolites, and synthetic derivatives (retinoids) have been shown to have protective effects against the development of certain types of cancer. In addition, pharmacologic amounts of retinoids have been used with some success in the treatment of a few human tumors. The chemoprevention effect of retinoids is most likely exerted at the tumor-promotion phase of carcinogenesis. Retinoids block tumor promotion by inhibiting proliferation, inducing apoptosis, inducing differentiation, or a combination of these actions. Clinically, isotretinoin (13-cis-retinoic acid) significantly decreases the incidence of second primary tumors in patients with head-and-neck cancer and reduces appearance of non-melanoma skin cancer in patients with xeroderma pigmentosum. Retinoic acid has proved to be an effective treatment for promyelocytic leukemia. However, retinoid resistance limits its use as a single agent. Clinical trials are in progress to determine the efficacy of retinoids in treating other types of cancer such as neuroblastoma and breast carcinoma. The development of receptor-selective retinoids and selective inhibitors of retinoid metabolism may lead to further use of retinoids in both chemoprevention and treatment of cancer.

Spontaneous apoptosis and retinoic acid receptor incidence in neuroblastomas and peripheral neuroectodermal tumors

Twelve cases of neuroblastoma (NB) (7 boys and 5 girls) and 4 cases of primitive peripheral neuroectodermal tumor (PNET) (3 boys and 1 girl) were investigated for the presence of apoptosis and retinoic acid receptor (RAR) by immunhistochemical method. The apoptotic index in NB was zero or 1% in 8 children and relatively low (2-4.8%) in the other 4 cases, while it was higher (4.1-10.5%) in PNET. The RAR index determined by immunoperoxidase reaction in NB was zero or 3% in 5 cases and 9-34% in 7 children. RAR index in PNET was 16-68% in all the 4 cases. Good correlation (r = .47 according to Pearson-Bravis) was found between the number of RAR and spontaneous apoptosis. These results suggest that the RAR index in untreated NB and PNET shows great individual variation since its determination is necessary for the evaluation of the efficacy of retinoic acid treatment.

Differentiation, proliferation and adhesion of human neuroblastoma cells after treatment with retinoic acid

Because of the known property of spontaneous regression in stage IVS of neuroblastoma all attempts are made to elucidate whether differentiation inducers possibly could be applied for neuroblastoma therapy. Here we examined the influence of retinoic acid (RA) in vitro on differentiation, proliferation and adhesion of 10 permanent and 4 primary cell lines as well as of several SCID-mouse tumour transplants. In general, after RA treatment morphologically different cell types which are characteristic for neuroblastoma cells have changed. N (neuronal)-type cells prolonged their neuronal processes, whereas S (epithelial, substrate-adherent, Schwann cell-like)-type cells lost their adherence to substratum and became apoptotic. Additionally, the reactions of all neuroblastoma cell lines with monoclonal antibodies against beta-tubulin (for neuronal cells) and glial fibrillary acidic protein (for epithelial cells) were determined. The anti-proliferative effect of all-trans-RA as well as 13-cis-RA was more profound in S-type cells (up to 40% in primary cell lines). To elucidate the role of adhesion molecules during neuronal cell differentiation, we have analysed the adhesion of neuroblastoma cells on poly-D-lysin-precoated plates under RA influence. While N-type cells displayed an increased adhesion, all S-type cell lines as well as all primary cell lines exhibited a reduced adhesion (IMR-5 and IMR-32: p < 0.001; JW, SR and PM: p < 0.05). RA treatment increased predominantly the tested antigens (HCAM, ICAM-1, NCAM, PECAM-1, VCAM-1, cadherin, FGF-R, IGF-R, NGF-R, TGF-beta/1, NF200, NF160, NF68, NSE, HLA-ABC) in all cell lines independently of their phenotypes (TGF-beta/1: p < 0.001; NF68: p < 0.01; PECAM-1 and NGF-R: p < 0.05). In recultured SCID-mouse-passaged tumour cells antigens were down-regulated (FGF-R: p < 0.01), but increased again after RA influence (TGF-beta/1: p < 0.05). In summary, the RA differentiation model demonstrates the possibility to interfere in cell adhesion and to diminish growth potential both in N-type as well as S-type neuroblastoma cells.

Differentiation of neuroblastoma cells by phorbol esters and insulin-like growth factor 1 is associated with induction of retinoic acid receptor beta gene expression

The retinoic acid (RA) receptor beta isoform (RARbeta) plays an important role in RA-induced differentiation of human neuroblastoma. In this study we show that insulin-like growth factor 1 (IGF-1) and tetradecanoyl phorbol acetate (TPA) induce RARbeta gene expression in neuroblastoma SH-SY5Y cells. IGF-1 and TPA caused a marked induction of RARbeta2 promoter activity and had a synergistic effect with RA that also upregulates transcription. The effect of RA is mediated by two RA responsive elements (RAREs), whereas the IGF-1 and TPA actions are independent of the RAREs and map to sequences that overlap the TATA box. These results suggest that the signaling pathways stimulated by TPA and IGF-1 could modify the components assembled at the core RARbeta2 promoter and activate transcription. Expression of RasVal12 mimics the effect of IGF-1 and TPA on the promoter, and a dominant negative Ras mutant abrogates activation. A dominant negative Raf also blocks activation showing that the Ras-Raf pathway mediates stimulation of the RARbeta2 promoter. Our results show that neuronal differentiation induced by non-retinoid agents that activate Ras is accompanied by increased transcription of the RARbeta gene.

Differential effects of retinoic acid isomers on the expression of nuclear receptor co-regulators in neuroblastoma

Retinoic acid modulates growth and induces differentiation and apoptosis of neuroblastoma cells in vitro, with the all-trans and 9-cis isomers having different biological properties. Transcriptional activation in response to retinoic acid isomers is mediated by retinoic acid receptors and retinoid X receptors. The differential expression of co-activators and co-repressors which preferentially interact with retinoic acid receptors or retinoid X receptors may be a mechanism leading to different cellular responses to 9-cis and all-trans retinoic acid. To test this hypothesis, we have studied the expression of the nuclear receptor co-regulators TIF1alpha, TIF1beta, SUG1 and SMRT in the N-type and S-type neuroblastoma cell lines SH SY 5Y and SH S EP. Transcripts for all four co-regulators were expressed in these neuroblastoma cells. The expression of TIF1alpha, TIF1beta and SUG1 did not change in response to retinoic acid; however, SMRT was induced in both neuroblastoma cell lines, but particularly by all-trans retinoic acid in SH S EP cells. An additional co-activator, Trip3, was isolated by differential mRNA display and shown to be preferentially induced by 9-cis retinoic acid in SH SY 5Y and SH S EP cells. These data suggest that retinoic acid isomer-specific induction of nuclear receptor co-regulators may determine, in part, the differential biological effects of retinoic acid isomers.

Retinoid-induced differentiation of neuroblastoma: comparison between LG69, an RXR-selective analogue and 9-cis retinoic acid

The aim of this study was to investigate in vitro the effects of all-trans retinoic acid (RA), 9-cis RA and the RXR-selective analogue, LG69, on the morphological differentiation, proliferation and gene expression of neuroblastoma cells. Three different cell lines were cultured with the retinoid for either 9 continuous days or for 5 days followed by 4 days without the retinoid and morphological differentiation was assessed both qualitatively and quantitatively. SH SY 5Y cell proliferation was examined by measuring cell numbers after exposure to the retinoids and RAR-beta gene expression was examined by Northern blot analysis. Morphological differentiation was more effectively induced by all-trans and 9-cis RA than by LG69. SH SY 5Y cells, when treated with 9-cis RA for only 5 of the 9 days of culture, underwent apoptosis, but this was not seen with 9 days continuous exposure nor with LG69. Inhibition of SH SY 5Y cell proliferation by all-trans or 9-cis RA was dose-dependent, but LG69 had little effect. Conversely, LG69 induced higher expression of RAR-beta than all-trans RA, but less than that produced by 9-cis RA. These data suggest that 9-cis RA as a single agent is the most effective modulator of neuroblastoma behaviour and may be the most appropriate therapeutic agent.

Expression of co-factors (SMRT and Trip-1) for retinoic acid receptors in human neuroectodermal cell lines

Retinoic acid (RA) induces growth inhibition, differentiation or cell death in many human neuroblastoma cell lines. Recently, the transactivation activity of nuclear retinoids receptors has been shown to be modulated through physical association with other proteins that act as co-activators or as co-repressors. We investigated the expression of the co-repressor (SMRT) and co-activator (Trip 1) for retinoid and thyroid-hormone receptors in several neuroectodermal tumour cell lines, and its modulation by all-trans-retinoic acid, as well as by synthetic agonists, for RAR alpha, RAR beta, RAR gamma and RXR. We demonstrate that (i) SMRT and Trip-1 mRNAs are expressed in many human neuroblastoma and melanoma cell lines in basal conditions, (ii) SMRT mRNA expression in human neuroblastoma cell line SK-N-BE(2) increases after 48 hours of incubation with 1 microM RA and RARs specific agonists, (iii) Trip-1 mRNA in the same cell line does not change during incubation with RA or selective synthetic agonists for RARs and RXR.