Retinoid target gene activation during induced tumor cell differentiation: human embryonal carcinoma as a model

Heterogeneous Differentiation of Highly Proliferative Embryonal Carcinoma PCC4 Cells Induced by Curcumin: An In Vitro Study

Curcumin, the yellow pigment derived from turmeric rhizomes, exhibits antioxidant, anti-inflammatory, antimicrobial, and anticancer properties. We have previously reported in a study that curcumin could induce differentiation in embryonal carcinoma cell (EC). EC cells are the primary constituents of teratocarcinoma tumors, and hence differentiating them to a non-proliferative cell type may be useful in anticancer therapies. Here, we conducted a detailed study using various molecular approaches to characterize this differentiation at the cellular and molecular levels. The cells were treated with 20 µM curcumin, which was the optimal concentration to produce the highest amount of differentiated cells. Changes in protein and RNA expression, membrane dynamics, and migration of these cells after treatment with curcumin were then studied in a time-dependent manner. The differentiated cells were morphologically distinct from the precursor cells, and gene expression profiles were altered in curcumin-treated cells. Curcumin promoted cell motility and cell adhesion. Curcumin also induced changes in membrane fluidity and the lateral mobility of lipids in the plasma membrane. The findings of this study suggest that curcumin might have therapeutic potential in differentiation therapy for the treatment of teratocarcinomas or germ cell tumors (GCTs) such as testicular and ovarian GCTs.

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.

All-trans-retinoic acid antagonizes the Hedgehog pathway by inducing patched

Male germ cell tumors (GCTs) are a model for a curable solid tumor. GCTs can differentiate into mature teratomas. Embryonal carcinomas (ECs) represent the stem cell compartment of GCTs and are the malignant counterpart to embryonic stem (ES) cells. GCTs and EC cells are useful to investigate differentiation therapy and chemotherapy response. This study explored mechanistic interactions between all-trans-retinoic acid (RA), which induces differentiation of EC and ES cells, and the Hedgehog (Hh) pathway, a regulator of self-renewal and proliferation. RA was found to induce mRNA and protein expression of Patched 1 (Ptch1), the Hh ligand receptor and negative regulator of this pathway. PTCH1 is also a target gene of Hh signaling through Smoothened (Smo) activation. Yet, this observed RA-mediated Ptch1 induction was independent of Smo. It occurred despite co-treatment with RA and Smo inhibitors. Retinoid induction of Ptch1 also occurred in other RA-responsive cancer cell lines and in normal ES cells. Notably, this enhanced Ptch1 expression was preceded by induction of the homeobox transcription factor Meis1, a direct RA target. Direct interaction between Meis1 and Ptch1 was confirmed using chromatin immunoprecipitation assays. To establish the translational relevance of this work, Ptch1 expression was shown to be deregulated in human ECs relative to mature teratoma and the normal seminiferous tubule. Taken together, these findings reveal a previously unrecognized mechanism through which RA can inhibit the Hh pathway via Ptch1 induction. Engaging this pathway is a new way to repress the Hh pathway that can be translated into the cancer clinic.

Headway and hurdles in the clinical development of dietary phytochemicals for cancer therapy and prevention: lessons learned from vitamin A derivatives

Accumulating epidemiologic and preclinical evidence support the pharmacologic use of a variety of dietary chemicals for the prevention and treatment of cancer. However, it will be challenging to translate these findings into routine clinical practice since phytochemicals have pleiotropic biological activities that have to be balanced for optimal efficacy without unacceptable and potentially unanticipated toxicities. Correctly matching patient populations and settings with optimal, natural product-based phytochemical therapies will require a greater understanding of the specific mechanisms underlying the efficacy, toxicity, and resistance of each agent in a variety of normal, premalignant, and malignant settings. This, in turn, necessitates continued commitment from the basic research community to guide carefully designed and informed clinical trials. The most developed class of anticancer phytochemicals consists of the derivatives of vitamin A called retinoids. Unlike other natural product chemicals currently under study, the retinoids have been extensively tested in humans. Over 30 years of clinical investigation has resulted in several disappointments, but there were some spectacular successes where certain retinoid-based protocols are now FDA-approved standard of care therapies to treat specific malignancies. Furthermore, retinoids are one of the most evaluated pharmacologic agents in the ultra-challenging setting of interventional cancer prevention. This review will summarize the development of retinoids in cancer therapy and prevention with an emphasis on currently proposed mechanisms mediating their efficacy, toxicity, and resistance.

Regulation of the SOX3 gene expression by retinoid receptors

Sox3/SOX3 gene is considered to be one of the earliest neural markers in vertebrates. Despite the mounting evidence that Sox3/SOX3 is one of the key players in the development of the nervous system, limited data are available regarding the transcriptional regulation of its expression. This review is focused on the retinoic acid induced regulation of SOX3 gene expression, with particular emphasis on the involvement of retinoid receptors. Experiments with human embryonal carcinoma cells identified two response elements involved in retinoic acid/retinoid X receptor-dependent activation of the SOX3 gene expression: distal atypical retinoic acid-response element, consisting of two unique G-rich boxes separated by 49 bp, and proximal element comprising DR-3-like motif, composed of two imperfect hexameric half-sites. Importantly, the retinoic acid-induced SOX3 gene expression could be significantly down-regulated by a synthetic antagonist of retinoid receptors. This cell model provides a solid base for further studies on mechanism(s) underlying regulation of expression of SOX3 gene, which could improve the understanding of molecular signals that induce neurogenesis in the stem/progenitor cells both during development and in adulthood.

Testing potential developmental toxicants with a cytotoxicity assay based on human embryonic stem cells

Since the differentiation of embryonic stem cells mimics early development, these cells could potentially permit the detection of embryotoxicants which interfere with this process. Although reliable tests based on murine embryonic stem cells exist, no such methods are available for human embryonic stem (hES) cells. Nonetheless, to avoid the false classification of substances due to inter-species differences, human-relevant toxicity tests are needed. We therefore developed an assay based on three human cell types, representing different degrees of developmental maturation, namely, human foreskin fibroblasts, hES cell-derived progenitor cells, and pluripotent hES cells. A set of embryotoxicants for which existing in vivo data were available, namely, all-trans retinoic acid (ATRA), 13-cis retinoic acid (13CRA), valproic acid (VPA) and dimethyl sulphoxide (DMSO), were tested. 5-fluorouracil (5-FU) was used as a positive control, and saccharin as a negative control. Two methods were compared for the assessment of cell viability -- the determination of intracellular ATP content and of resazurin reduction. In addition, the protective capacity of basic fibroblast growth factor (bFGF) against retinoid-induced toxicity was investigated. This novel assay system reliably detected the embryotoxic potentials of the test substances, 5-FU, ATRA, 13-CRA (a substance that displays inter-species differences in its effects) and VPA. This was possible due to the apparent differences in the sensitivities of the human cell types used in the assay system. Thus, our results clearly indicate the advantages and relevance of using hES cells in in vitro developmental toxicity testing.

Identification of a novel antiapoptotic human protein kinase C delta isoform, PKCdeltaVIII in NT2 cells

Protein kinase C (PKC) delta plays an important role in cellular proliferation and apoptosis where it is involved in the caspase-3 mediated apoptotic pathway. Cleavage of PKCdeltaI by caspase-3 releases a catalytically active C-terminal fragment that is sufficient to induce apoptosis. In this paper, we identified a novel human PKCdelta isozyme, PKCdeltaVIII (Genbank accession number DQ516383) in human teratocarcinoma (NT2) cells that differentiate into hNT neurons upon retinoic acid (RA) treatment. Expression of PKCdeltaVIII was confirmed by real-time RT-PCR analysis, and we observed that after an initial peak at 24 h following RA treatment, its expression gradually declined with prolonged RA treatment. PKCdeltaVIII is generated via the utilization of an alternative 5' splice site, and this results in an insertion of 31 amino acids in the caspase-3 recognition sequence DMQD. The function of PKCdeltaVIII was examined by subcloning it into an expression vector and raising an antibody specific to PKCdeltaVIII. Using in vivo and in vitro assays, we demonstrated that PKCdeltaVIII is resistant to caspase-3 cleavage. Next, we sought to determine the role of PKCdeltaVIII in apoptosis in NT2 cells. Overexpression of PKCdeltaVIII and knockdown using PKCdeltaVIII siRNA suggest an antiapoptotic function for the PKCdeltaVIII isozyme. We demonstrate that antisense oligonucleotides (ASO) directed toward the 5' splice site I promote the expression of the PKCdeltaVIII isozyme. Our results indicated that ASO mediated PKCdeltaVIII expression rescued NT2 cells from etoposide-induced apoptosis. We conclude that the novel human PKCdeltaVIII splice variant functions as an antiapoptotic protein in NT2 cells.

Mapping of the RXRalpha binding elements involved in retinoic acid induced transcriptional activation of the human SOX3 gene

Sox3/SOX3 gene is implicated in the control of nervous system development and is considered to be one of the earliest neural markers. Expression of human SOX3 gene is modulated during the RA-induced neuronal differentiation cascade of NT2/D1 cells. Our present results demonstrate that the sequences responsible for RA-induced activation of SOX3 gene are localized within the 0.4 kb of its 5'-flanking region and implicate RXRalpha involvement in this regulation. The active RA/RXRalpha responsive region is pinned down to two regulatory elements. Only in the presence of both elements full RA/RXRalpha inducibility is achieved, suggesting they act synergistically. These elements comprise two unique G-rich boxes, separated by 49 bp, that could be considered as a novel, atypical RA-response element. Here, for the first time, we have demonstrated direct interaction of RXRalpha and SOX3 control elements. Furthermore, the functional in vivo analysis revealed that liganded RXRalpha is a potent activator of endogenous SOX3 protein expression. Since it is proven that Sox3 is critical determinant of neurogenesis our data may help in providing new insight into complex regulatory networks involved in retinoic acid induced neural differentiation of NT2/D1 cells.

Overview of retinoid metabolism and function

Retinoids (vitamin A) are crucial for most forms of life. In chordates, they have important roles in the developing nervous system and notochord and many other embryonic structures, as well as in maintenance of epithelial surfaces, immune competence, and reproduction. The ability of all-trans retinoic acid to regulate expression of several hundred genes through binding to nuclear transcription factors is believed to mediate most of these functions. The role of all-trans retinoic may extend beyond the regulation of gene transcription because a large number of noncoding RNAs also are regulated by retinoic acid. Additionally, extra-nuclear mechanisms of action of retinoids are also being identified. In organisms ranging from prokaryotes to humans, retinal is covalently linked to G protein-coupled transmembrane receptors called opsins. These receptors function as light-driven ion pumps, mediators of phototaxis, or photosensory pigments. In vertebrates phototransduction is initiated by a photochemical reaction where opsin-bound 11-cis-retinal is isomerized to all-trans-retinal. The photosensitive receptor is restored via the retinoid visual cycle. Multiple genes encoding components of this cycle have been identified and linked to many human retinal diseases. Central aspects of vitamin A absorption, enzymatic oxidation of all-trans retinol to all-trans retinal and all-trans retinoic acid, and esterification of all-trans retinol have been clarified. Furthermore, specific binding proteins are involved in several of these enzymatic processes as well as in delivery of all-trans retinoic acid to nuclear receptors. Thus, substantial progress has been made in our understanding of retinoid metabolism and function. This insight has improved our view of retinoids as critical molecules in vision, normal embryonic development, and in control of cellular growth, differentiation, and death throughout life.

Testicular Germ Cell Tumors: A Paradigm for the Successful Treatment of Solid Tumor Stem Cells

Treatment of testicular germ cell tumors (TGCTs) has been a success primarily due to the exquisite responsiveness of this solid tumor to cisplatin-based therapy. Despite the promise of cure for the majority of TGCT patients, the effectiveness of therapy for some patients is limited by toxicity and the problem of resistance. There is compelling rationale to further understand the biology of TGCTs in order to better treat other solid tumors and to address the shortcomings of present TGCT therapies. TGCTs contain undifferentiated pluripotent stem cells, known as embryonal carcinoma, that share many properties with human embryonic stem cells. The importance of cancer stem cells in the initiation, progression and treatment of solid tumors is beginning to emerge. We discuss TGCTs in the context of solid tumor curability and targeted cancer stem cell therapy.

A p53-dominant transcriptional response to cisplatin in testicular germ cell tumor-derived human embryonal carcinoma

Testicular germ cell cancers remain one of the few solid tumors routinely cured in advanced stages with conventional cisplatin-based chemotherapy. The mechanisms remain largely unknown. Through use of gene-expression array profiling we define immediate transcriptional targets in response to cisplatin in testicular germ cell-derived human embryonal carcinoma cells. We report 46 genes upregulated and five genes repressed by cisplatin. Several of these gene products, including FAS, TRAILR3, PHLDA3, LRDD, and IER3 are previously implicated in the apoptotic death receptor pathway, while others including SESN1, FDXR, PLK3, and DDIT4 are known mediators of reactive oxygen species generation. Approximately 54% of the upregulated genes are established or suspected downstream targets of p53. Specific siRNA to p53 prevents cisplatin-mediated activation of p53 and p53 pathway genes and renders embryonal carcinoma cells relatively resistant to cisplatin cytotoxicity. Interestingly, in p53 knockdown cells nearly the entire set of identified cisplatin targets fail to respond or have a diminished response to cisplatin, suggesting that many are new direct or indirect targets of p53 including GPR87, STK17A, INPP5D, FLJ11259, and EPS8L2. The data indicate that robust transcriptional activation of p53 is linked to the known hypersensitivity of testicular germ cell tumors to chemotherapy. Many of the gene products may participate in the unique curability of this disease.

Retinoic acid represses a cassette of candidate pluripotency chromosome 12p genes during induced loss of human embryonal carcinoma tumorigenicity

Testicular germ cell tumors (TGCTs) are the most common carcinomas of young men aged 15-35. The molecular events involved in TGCT genesis are poorly understood. TGCTs have near universal amplification of the short arm of chromosome 12, however positional cloning efforts have not identified causative genes on 12p involved in formation or progression of TGCTs. Human embryonal carcinoma (EC) are the stem cells of TGCTs and are pluripotent. EC cells terminally differentiate toward a neuronal lineage with all-trans retinoic acid (RA) treatment resulting in a concomitant G1 cell cycle arrest and loss of tumorigenicity. Our efforts to define the molecular mechanisms of RA-mediated tumor cell differentiation at a critical "commitment to differentiate" window has identified a cassette of genes on 12p that are repressed with RA precisely as EC cells lose tumorigenic potential. These are Nanog, CD9, EDR1 (PHC1), SCNN1A, GDF3, Glut3 and Stella. The master pluripotency regulator Oct4 is located on chromosome 6 and is also repressed by RA. Notably, knockdown of Oct4 with siRNA results in repression of basal Nanog, EDR1, GDF3 and Stella gene expression. Nanog has recently been identified to play a role in maintenance of the pluripotency of mouse embryonic stem cells and CD9, EDR1, GDF3, and Stella have each been implicated as stem cell markers. Since RA suppresses the tumorigenicity of EC cells, these genes may have a critical role in the etiology of TGCTs, suggesting a link between enforced pluripotency and transformation.

p53 in human embryonal carcinoma: identification of a transferable, transcriptional repression domain in the N-terminal region of p53

Testicular germ cell tumors (TGCTs) arise despite possessing high levels of wild-type p53, suggesting p53 latency. We have previously shown that p53 repression in TGCT-derived human embryonal carcinoma (EC) is relieved upon treatment with all-trans retinoic acid (RA), resulting in enhanced p53 transactivation activity. To further investigate p53 repression in EC, a series of gal4-p53 truncation constructs were generated. Deletion of the core DNA-binding region, residues 117-274, had no effect on basal or RA-induced p53 activity. Progressively, larger truncations were made in the C- or N-terminal direction. Deletion of residues toward the C-terminus of p53 as far as residue 354 did not affect either the basal or RA-inducible activity of gal4-p53. When a small region in the N-terminus was deleted (residues 105-116), relief of the basal repression of p53 activity characteristic of EC was observed. Fusion of this region to the VP16 activation domain (VPAD) resulted in a 10-20-fold repression of VPAD activity in NT2/D1 human EC cells, indicating that this region acts as a heterologous repressor. Owing to its location in the N-terminal half of p53, we have named this region the p53 N-terminal Repression Domain (p53-NRD). The p53-NRD mediated repression in a variety of cell lines, with the most prominent repression observed in human EC cells. While RA alone had no effect on p53-NRD activity, cotreatment with RA and the histone deacetylase inhibitor trichostatin-A (TSA) completely relieved p53-NRD-mediated repression. In contrast, NRD-mediated repression was not sensitive to RA and TSA in a derived RA-resistant cell line with a retinoic acid receptor gamma (RARgamma) defect, but sensitivity could be restored with transfection of RARgamma. These data indicate that a unique repressor domain resides in p53 at residues 90-116 whose activity can be modulated in the presence of 'differentiation therapy' and 'transcription therapy' agents.

Reevaluation of in vitro differentiation protocols for bone marrow stromal cells: disruption of actin cytoskeleton induces rapid morphological changes and mimics neuronal phenotype

Bone marrow stromal cells (MSC), which represent a population of multipotential mesenchymal stem cells, have been reported to undergo rapid and robust transformation into neuron-like phenotypes in vitro following treatment with chemical induction medium including dimethyl sulfoxide (DMSO; Woodbury et al. [2002] J. Neurosci. Res. 96:908). In this study, we confirmed the ability of cultured rat MSC to undergo in vitro osteogenesis, chondrogenesis, and adipogenesis, demonstrating differentiation of these cells to three mesenchymal cell fates. We then evaluated the potential for in vitro neuronal differentiation of these MSC, finding that changes in morphology upon addition of the chemical induction medium were caused by rapid disruption of the actin cytoskeleton. Retraction of the cytoplasm left behind long processes, which, although strikingly resembling neurites, showed essentially no motility and no further elaboration during time-lapse studies. Similar neurite-like processes were induced by treating MSC with DMSO only or with actin filament-depolymerizing agents. Although process formation was accompanied by rapid expression of some neuronal and glial markers, the absence of other essential neuronal proteins pointed toward aberrantly induced gene expression rather than toward a sequence of gene expression as is required for neurogenesis. Moreover, rat dermal fibroblasts responded to neuronal induction by forming similar processes and expressing similar markers. These studies do not rule out the possibility that MSC can differentiate into neurons; however, we do want to caution that in vitro differentiation protocols may have unexpected, misleading effects. A dissection of molecular signaling and commitment events may be necessary to verify the ability of MSC transdifferentiation to neuronal lineages.

Growth suppression by acute promyelocytic leukemia-associated protein PLZF is mediated by repression of c-myc expression

The transcriptional repressor PLZF was identified by its translocation with retinoic acid receptor alpha in t(11;17) acute promyelocytic leukemia (APL). Ectopic expression of PLZF leads to cell cycle arrest and growth suppression, while disruption of normal PLZF function is implicated in the development of APL. To clarify the function of PLZF in cell growth and survival, we used an inducible PLZF cell line in a microarray analysis to identify the target genes repressed by PLZF. One prominent gene identified was c-myc. The array analysis demonstrated that repression of c-myc by PLZF led to a reduction in c-myc-activated transcripts and an increase in c-myc-repressed transcripts. Regulation of c-myc by PLZF was shown to be both direct and reversible. An interaction between PLZF and the c-myc promoter could be detected both in vitro and in vivo. PLZF repressed the wild-type c-myc promoter in a reporter assay, dependent on the integrity of the binding site identified in vitro. PLZF binding in vivo was coincident with a decrease in RNA polymerase occupation of the c-myc promoter, indicating that repression occurred via a reduction in the initiation of transcription. Finally, expression of c-myc reversed the cell cycle arrest induced by PLZF. These data suggest that PLZF expression maintains a cell in a quiescent state by repressing c-myc expression and preventing cell cycle progression. Loss of this repression through the translocation that occurs in t(11;17) would have serious consequences for cell growth control.

Negative Feedback at the Level of Nuclear Receptor Coregulation

Nuclear receptor-mediated gene expression is proposed to be regulated by the ordered recruitment of large protein complexes in which activity depends on mutual interactions and posttranslational modifications. In contrast, relatively little attention has been given to mechanisms regulating the expression of the coregulator proteins themselves. Previously we have shown that the ligand-dependent corepressor, RIP140, is a direct transcriptional target of all-trans retinoic acid (RA). Here we demonstrate that RA induction of RIP140 constitutes a rate-limiting step in the regulation of retinoic acid receptor signaling. Silencing of the RA induction of RIP140 dramatically enhances and accelerates retinoid receptor transactivation, endogenous expression of other RA target genes, and RA-induced neuronal differentiation and cell cycle arrest in human embryonal carcinoma cells. The data suggest that RA induction of RIP140 constitutes a functional negative feedback loop that limits activation of retinoid receptors in the continued presence of RA and that acutely regulated expression of coregulators may be a general regulatory mechanism in hormonal signaling.

Anticarcinogenic properties of a daidzein-rich fraction isolated from soybean

In a previous study, we demonstrated that the methanol extract of soybean powder contains an active component(s) that promotes the differentiation of HL-60 cells. Partial purification of the extract, using solvent fractionation and silica gel chromatography, produced an active fraction rich in daidzein. The daidzein-rich fraction (DRF) was evaluated for its cancer preventive potential by assessing its cytotoxic activity and effect on the expression of the transforming growth factor beta (TGF-beta) family of cytokines and their receptors. DRF appeared to exert cytotoxic activity via an apoptotic pathway as evaluated by a DNA fragmentation assay and caspase-3 induction. DRF also increased the expression of TGF-beta2, but had little effect on the expression of other members of the TGF-beta family of cytokines and their receptors, or on the expression of the vascular endothelial growth factor gene. In conclusion, the DRF isolated from the methanol extract of soybean may have the potential to prevent tumorigenesis and, therefore, deserves to undergo further evaluation of its active component(s) and in vivo evaluation for anticarcinogenic efficacy.