c-myc regulation during retinoic acid-induced differentiation of F9 cells is posttranscriptional and associated with growth arrest

Retinoid X receptor-mediated transdifferentiation cascade in budding tunicates

In the budding tunicate, Polyandrocarpa misakiensis, retinoic acid (RA) applied to buds promotes transdifferentiation of somatic cells to form the secondary body axis. This study investigated the gene cascade regulating such RA-triggered transdifferentiation in tunicates. Genes encoding retinoic acid receptor (RAR) and retinoid X receptor (RXR) were induced during transdifferentiation, and they responded to all-trans RA or 13-cis RA in vivo, whereas 9-cis RA had the least effects, demonstrating differences in the ligand preference between budding tunicates and vertebrates. In contrast to RAR mRNA, RXR mRNA could induce transdifferentiation-related genes such as RXR itself, ERK, and MYC in an RA-dependent manner and also induced β-catenin (β-CTN) RA-independently when it was introduced in vitro into tunicate cell lines that do not express endogenous RAR or RXR. Small interfering RNA (siRNA) of RXR dramatically attenuated not only RXR but also ERK and β-CTN gene activities. An ERK inhibitor severely blocked wound healing and dedifferentiation. β-CTN siRNA suppressed morphogenesis and redifferentiation, similar to RXR siRNA. These results indicate that in P. misakiensis, the main function of RA is to trigger positive feedback regulation of RXR rather than to activate RAR for unlocking downstream pathways for transdifferentiation. Our results may reflect an ancient mode of RA signaling in chordates.

Regulation of TFIIIB during F9 cell differentiation

BACKGROUND

Differentiation of F9 embryonal carcinoma (EC) cells into parietal endoderm (PE) provides a tractable model system for studying molecular events during early and inaccessible stages of murine development. PE formation is accompanied by extensive changes in gene expression both in vivo and in culture. One of the most dramatic is the ~10-fold decrease in transcriptional output by RNA polymerase (pol) III. This has been attributed to changes in activity of TFIIIB, a factor that is necessary and sufficient to recruit pol III to promoters. The goal of this study was to identify molecular changes that can account for the low activity of TFIIIB following F9 cell differentiation.

RESULTS

Three essential subunits of TFIIIB decrease in abundance as F9 cells differentiate; these are Brf1 and Bdp1, which are pol III-specific, and TBP, which is also used by pols I and II. The decreased levels of Brf1 and Bdp1 proteins can be explained by reduced expression of the corresponding mRNAs. However, this is not the case for TBP, which is regulated post-transcriptionally. In proliferating cells, pol III transcription is stimulated by the proto-oncogene product c-Myc and the mitogen-activated protein kinase Erk, both of which bind to TFIIIB. However, c-Myc levels fall during differentiation and Erk becomes inactive through dephosphorylation. The diminished abundance of TFIIIB is therefore likely to be compounded by changes to these positive regulators that are required for its full activity. In addition, PE cells have elevated levels of the retinoblastoma protein RB, which is known to bind and repress TFIIIB.

CONCLUSION

The low activity of TFIIIB in PE can be attributed to a combination of changes, any one of which could be sufficient to inhibit pol III transcription. Declining levels of essential TFIIIB subunits and of activators that are required for maximal TFIIIB activity are accompanied by an increase in a potent repressor of TFIIIB. These events provide fail-safe guarantees to ensure that pol III output is appropriate to the diminished metabolic requirements of terminally differentiated cells.

Overexpression of CRABPI in suprabasal keratinocytes enhances the proliferation of epidermal basal keratinocytes in mouse skin topically treated with all-trans retinoic acid

We investigated whether ectopic expression of CRABPI, a cellular retinoic acid binding protein, influenced the actions of all-trans retinoic acid (ATRA) in transgenic (TG) mice. We targeted CRABPI to the basal vs. suprabasal layers of mouse epidermis by using the keratin 14 (K14) and keratin 10 (K10) promoters, respectively. Greater CRABPI protein levels were detected in the epidermis of adult transgenic(+) mice than in transgenic(-) mice for both transgenes. In adult mouse skin CRABPI overexpression in the basal or suprabasal keratinocytes did not cause morphological abnormalities, but did result in decreased CRABPII mRNA levels. Ectopically overexpressed CRABPI in suprabasal keratinocytes, but not in basal keratinocytes, enhanced the thickening of the epidermis induced by topical ATRA treatments (10 microM, 400 microl for 4 days) by 1.59+/-0.2-fold (p<0.05). ATRA treatment (10 microM) resulted in a 59.9+/-9.8% increase (p<0.05) in the BrdU labeling index in K10/FLAG-CRABPI TG(+) mice vs. TG(-) mice. Retinoid topical treatments reduced p27 and CYP26A1 mRNA levels in TG(+) and TG(-) mouse skin in K14 and K10/FLAG-CRABPI transgenic mice. As epidermal basal keratinocyte proliferation is stimulated by paracrine growth factors secreted by ATRA activated suprabasal keratinocytes, our results indicate that CRABPI overexpression in suprabasal keratinocytes enhances the physiological functions of ATRA.

Evidence of extra-telomeric effects of hTERT and its regulation involving a feedback loop

The human telomerase reverse transcriptase (hTERT) is the catalytic subunit of the enzyme telomerase which is responsible for telomeric maintenance and extension. Using RNA interference to knock down hTERT mRNA expression, we provide evidence that hTERT exerts extra-telomeric effects on the cell cycle and on its own regulatory proteins, specifically: p53 and p21. We tested our hypothesis that hTERT regulates its own expression through effects on upstream regulatory genes using transformed human embryonic kidney (HEK 293) cells, p53 and p16(INK4a) null human ovarian cancer SKOV-3 cells, and p53-null MDA-MB-157 human mammary cancer cells. In HEK 293 cells, hTERT knockdown resulted in elevated p53 and p21 transcription and a decrease in cellular proliferation. Similar results were observed in the MDA-MB-157 cell line where p21 was upregulated, correlating with cell growth inhibition. In contrast, we observed a decrease in expression of p21 in SKOV-3 cells with hTERT knockdown and cell growth appeared to be unaffected. These findings suggest that hTERT may be involved in a feedback loop system, thereby playing a role in its own regulation.

Deregulation in trans or c-myc expression in immortalized human urothelial cells and in T24 bladder carcinoma cells

The expression of a number of cellular oncogenes was investigated in human urothelial cell lines with different in vitro growth properties. Constitutively elevated levels of expression of c-myc RNA were found in Hu609, an immortalized, nontumorigenic cell line that was derived from normal urothelium, and in the bladder carcinoma cell line T24. Potential mechanisms that might underlie deregulation of c-myc expression in these cells were investigated. It was found that the c-myc gene was apparently intact and not amplified in Hu609 and T24. No increased stability of c-myc RNA was detected. A c-myc-CAT fusion construct containing 2.5 kb of normal c-myc 5' sequences showed levels of expression that paralleled the overexpression of the endogenous gene, indicating that the high constitutive levels of c-myc expression were due, at least in part, to alterations in the activities of cellular trans-acting transcriptional regulators.

The c-MYC oncoprotein as a treatment target in cancer and other disorders of cell growth

The c-MYC proto-oncogene is essential for cellular proliferation but, paradoxically, may also promote cell death. Deregulated expression of c-MYC is present in most, if not all, human cancers, and is associated with a poor prognosis. However, given that human tumours at diagnosis generally carry multiple genetic lesions that have accumulated during (although they are not necessarily essential for) tumour progression, it has proved difficult to attribute a specific role to any given single factor or indeed to explore the therapeutic potential of selectively mitigating their biological functions. Regulatable transgenic mouse models of oncogenesis have shed light on these issues, influenced our thinking about cancer and provided encouragement for the future development of cancer therapies based on targeting individual oncogenes such as c-MYC. Although still in its infancy, encouraging results have been reported using antisense oligodeoxynucleotide-based methods, as well as other approaches to interfere with MYC expression both in vitro and in vivo.

Regulation of Ras-MAPK pathway mitogenic activity by restricting nuclear entry of activated MAPK in endoderm differentiation of embryonic carcinoma and stem cells

In response to retinoic acid, embryonic stem and carcinoma cells undergo differentiation to embryonic primitive endoderm cells, accompanied by a reduction in cell proliferation. Differentiation does not reduce the activation of cellular MAPK/Erk, but does uncouple mitogen-activated protein kinase (MAPK) activation from phosphorylation/activation of Elk-1 and results in inhibition of c-Fos expression, whereas phosphorylation of the cytoplasmic substrate p90RSK remains unaltered. Cell fractionation and confocal immunofluorescence microscopy demonstrated that activated MAPK is restricted to the cytoplasmic compartment after differentiation. An intact actin and microtubule cytoskeleton appears to be required for the restriction of MAPK nuclear entry induced by retinoic acid treatment because the cytoskeletal disrupting agents nocodazole, colchicine, and cytochalasin D are able to revert the suppression of c-Fos expression. Thus, suppression of cell proliferation after retinoic acid–induced endoderm differentiation of embryonic stem and carcinoma cells is achieved by restricting nuclear entry of activated MAPK, and an intact cytoskeleton is required for the restraint.

Transcriptional regulation of pulmonary elastin gene expression in elastase-induced injury

Previously we have shown that treatment of confluent, pulmonary fibroblast cultures with elastase results in upregulation of elastin mRNA and protein levels. In the present study we focused on determining the level at which elastin expression is upregulated after elastase exposure. We examined as models for this investigation elastin gene expression in primary pulmonary fibroblast cells during the transition from subconfluent to confluent cultures and in confluent, matrix-laden cultures treated briefly with elastase. In addition, we extended our studies to mice that were given an intratracheal dose of elastase; the effects on lung elastin mRNA and elastin promoter activity levels were measured and compared with results from in vitro cell models. The results demonstrate that upregulation of elastin gene expression during the transition of subconfluent to confluent cultures and after elastase injury is associated with an increase in the level of transcription both in vitro and in vivo. Furthermore, intratracheal administration of elastase to transgenic mice illustrates that the increased levels of elastin mRNA are accompanied by increased activity of the elastin gene promoter in cells spatially positioned near major sites of tissue injury.

Basic fibroblast growth factor decreases elastin gene transcription in aortic smooth muscle cells

The extracellular matrix (ECM) protein elastin plays an essential role in the cardiovascular system by imparting elasticity to blood vessel wall. In this study, we examined the effect of basic fibroblast growth factor (bFGF) on the expression of elastin in aortic smooth muscle cells (SMC) to gain insight into events associated with cardiovascular diseases. The results show that bFGF treatment of SMC causes a significant decrease in elastin mRNA and secreted tropoelastin levels. Nuclear run-on analyses demonstrate that the downregulation is due to a decrease in the level of elastin gene transcription. Transient transfections of SMC with wild-type and mutated elastin gene promoter/chloramphenicol acetyl transferase (CAT) constructs show that a previously identified activator protein-1-cAMP response element (AP1/CRE) (-564 to -558-bp) within the elastin promoter mediates the bFGF-dependent downregulation of elastin gene transcription in SMC. Addition of bFGF to SMC activates the extracellular signal-regulated kinases 1/2 (ERK1/2) resulting in their translocation into the nucleus and subsequent induction of Fra-1. The addition of PD-98059, an inhibitor of ERK1/2 kinase, abrogates the bFGF-dependent decrease of elastin mRNA in SMC. The described inhibitory effect of bFGF on elastin gene expression in SMC may significantly contribute to the inefficient repair of elastin in early stages of vascular wall injury.

c-MYC: more than just a matter of life and death

Deregulated expression of c-MYC occurs in a broad range of human cancers and is often associated with poor prognosis, indicating a key role for this oncogene in tumour progression. However, as established human tumours often bear multiple genetic lesions, it is difficult to determine whether c-MYC is instrumental in the initiation/progression of the tumour, or indeed whether inactivating c-MYC would lead to tumour regression. Regulatable transgenic mouse models of oncogenesis have shed light on these issues and provide hope for effective cancer therapies.

Cell differentiation induces TIF1β association with centromeric heterochromatin via an HP1 interaction

The transcriptional intermediary factor 1 (TIF1) family protein TIF1βis a corepressor for Krüppel-associated box (KRAB)-domain-containing zinc finger proteins and plays a critical role in early embryogenesis. Here, we examined TIF1β distribution in the nucleus of mouse embryonic carcinoma F9 cells during retinoic-acid-induced primitive endodermal differentiation. Using confocal immunofluorescence microscopy, we show that, although TIF1β is diffusely distributed throughout the nucleoplasm of undifferentiated cells, it relocates and concentrates into distinct foci of centromeric heterochromatin in differentiated cells characterized by a low proliferation rate and a well developed cytokeratin network. This relocation was not observed in isoleucine-deprived cells, which are growth arrested, or in compound RXRα-/-/RARγ-/- null mutant cells, which are resistant to RA-induced differentiation. Amino-acid substitutions in the PxVxL motif of TIF1β, which abolish interaction with members of the heterochromatin protein 1 (HP1) family, prevent its centromeric localization in differentiated cells. Collectively, these data provide compelling evidence for a dynamic nuclear compartmentalization of TIF1βthat is regulated during cell differentiation through a mechanism that requires HP1 interaction.

Retinoic acid (RA) receptor transcriptional activation correlates with inhibition of 12-O-tetradecanoylphorbol-13-acetate-induced ornithine decarboxylase (ODC) activity by retinoids: a potential role for trans-RA-induced ZBP-89 in ODC inhibition

Evaluation of retinoic acid receptor (RAR) subtype-selective alpha and gamma agonists and antagonists and a retinoid X receptor (RXR) class-selective agonist for efficacy at inhibiting both induction of ornithine decarboxylase (ODC) by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse epidermis and rat tracheal epithelial cells and the appearance of papillomas in mouse epidermis treated in the 2-stage tumor initiation-promotion model indicated that (i) RXR class-selective transcriptional agonists, such as MM11246, were not involved in ODC inhibition; (ii) RAR-selective agonists that induce gene transcription from RA-responsive elements (RAREs) were active at low concentrations; (iii) RAR-selective antagonists that bind RARs and inhibit AP-1 activation on the collagenase promoter but do not activate RAREs to induce gene transcription were less effective inhibitors; and (iv) RARgamma-selective retinoid agonists were more effective inhibitors of TPA-induced ODC activity than RARalpha-selective agonists. These results suggest that RARE activation has a more important role in inhibition of ODC activity than RXR activation or AP-1 inhibition and that RARgamma-selective agonists would be the most useful inhibitors of epithelial cell proliferation induced by tumor promoters. The natural retinoid all-trans-RA induced expression of transcription factor ZBP-89, which represses activation of the GC box in the ODC promoter by the transcription factor Sp1.

Differentiation-related changes in the cell cycle traverse

This review examines recent developments relating to the interface between cell proliferation and differentiation. It is suggested that the mechanism responsible for this transition is more akin to a "dimmer" than to a "switch," that it is more useful to refer to early and late stages of differentiation rather than to "terminal" differentiation, and examples of the reversibility of differentiation are provided. An outline of the established paradigm of cell cycle regulation is followed by summaries of recent studies that suggest that this paradigm is overly simplified and should be interpreted in the context of different cell types. The role of inhibitors of cyclin-dependent kinases in differentiation is discussed, but the data are still inconclusive. An increasing interest in the changes in G2/M transition during differentiation is illustrated by examples of polyploidization during differentiation, such as megakaryocyte maturation. Although the retinoblastoma protein is currently maintaining its prominent role in control of proliferation and differentiation, it is anticipated that equally important regulators will be discovered and provide an explanation at the molecular level for the gradual transition from proliferation to differentiation.

Apoptosis induced by oxysterol in CEM cells is associated with negative regulation of c-myc

Previously we have demonstrated that treatment of the human lymphoblastic leukemic CEM cells with 25-hydroxycholesterol (25OHC) induces apoptosis. In the present study, we show that both c-myc mRNA and c-Myc protein levels are reduced only in oxysterol-sensitive and not in oxysterol-resistant cells after treatment with concentrations of 25OHC that kill the sensitive CEM cells. The repression of c-Myc protein precedes c-myc mRNA reduction, and both events occur before the onset of cell death. Our data suggest that 25OHC-induced suppression of c-myc gene expression in CEM cells results from posttranscriptional regulation. These results demonstrate the regulation by an oxysterol of a gene/gene product important for cell growth and viability and an association between oxysterol-induced apoptosis of CEM cells and the negative regulation of c-myc.

Characterization of genes which exhibit reduced expression during the retinoic acid-induced differentiation of F9 teratocarcinoma cells: involvement of cyclin D3 in RA-mediated growth arrest

In the presence of retinoic acid (RA), F9 murine teratocarcinoma cells differentiate into cells resembling the extra-embryonic endoderm of the early mouse embryo. Using differential hybridization, we have cloned and characterized six cDNAs corresponding to mRNAs that exhibit reduced expression in F9 cells following RA treatment. Two of these cDNAs encode novel genes (REX-2 and REX-3). The other isolated cDNAs encode genes that have been previously described in other contexts: 1-4 (cyclin D3); 2-10 (pyruvate kinase); 2-12 (glutathione S-transferase); and 2-17 (GLUT 3). The mRNA levels of these genes are reduced by RA or RA plus theophylline and cAMP (RACT) only after 48 h of treatment, and continue to decrease at 96 h. The half-lives of these mRNAs are not changed by RA treatment, indicating that these mRNAs may be regulated through a transcriptional mechanism. In isoleucine-deprived cells, which are growth arrested but do not differentiate, the steady state mRNA levels of genes Rex 2, Rex 3, pyruvate kinase and GLUT 3 are not reduced, in contrast to cyclin D3 and glutathione S-transferase. The expression of the REX-2, REX-3, pyruvate kinase, glutathione S-transferase and GLUT 3 genes is reduced by RACT to the same extent in F9 RARgamma-/- and RARalpha-/- lines as in F9-Wt. In contrast, cyclin D3 exhibits lower mRNA expression in F9 RARgamma-/- and RARalpha-/- stem cells, and this mRNA is not decreased by RACT treatment. Overexpression of cyclin D3 blocks the RA-induced growth arrest of F9 cells, indicating that the downregulation of this gene following RA treatment may constitute a necessary step in the cascade of events leading to growth inhibition by RA.

c-myc mRNA is down-regulated during myogenic differentiation by accelerated decay that depends on translation of regulatory coding elements

Murine C2C12 myoblasts induced to differentiate into multinucleated myotubes decrease their levels of c-myc mRNA 3-10-fold through posttranscriptional mechanisms that recognize regulatory elements contained in protein-coding sequences in exons 2 and 3 of the mRNA. To determine the mechanism by which these elements mediate c-myc mRNA down-regulation, we examined the regulation of mutant MYC and human beta-globin-MYC fusion mRNAs. Regulation of mRNAs containing MYC exon 2 or 3 is abolished by insertion of an upstream termination codon indicating that regulatory function depends on their translation. Exploiting this translation dependence, we show that pharmacologic inhibition of translation with cycloheximide abolishes the down-regulation of regulated MYC and globin-MYC mRNAs and induces their levels in differentiating C2C12 cells. We exclude the possibility that this induction in mRNA levels results from cycloheximide effects on transcription or processing of parts of the RNA other than the regulatory elements, leading to the conclusion that cycloheximide induction results from mRNA stabilization. We show that the magnitude of cycloheximide induction can be used to estimate turnover rates of mRNAs whose decay is translation-dependent. By using cycloheximide inducibility to examine turnover rates of MYC and globin-MYC mRNAs, we show that the MYC exon 2 and exon 3 regulatory elements, but not MYC 3'-untranslated region or chloramphenicol acetyltransferase coding sequences, mediate accelerated mRNA decay in differentiating, but not undifferentiated, C2C12 cells. We show that these regulatory elements must be translated to confer accelerated mRNA decay and that increased turnover occurs in the cytoplasm and not in the nucleus. Finally, using cycloheximide induction to examine mRNA half-lives, we show that mRNA turnover is increased sufficiently by mechanisms targeting the exon 2 and 3 regulatory elements to account for the magnitude of c-myc mRNA down-regulation during differentiation. We conclude from these results that c-myc mRNA down-regulation during myogenic differentiation is due to translation-dependent mechanisms that target mRNAs containing myc exon 2 and 3 regulatory elements for accelerated decay.

A differential display strategy identifies Cryptic, a novel EGF-related gene expressed in the axial and lateral mesoderm during mouse gastrulation

We have developed a differential display screening approach to identify mesoderm-specific genes, relying upon the differentiation of embryonic stem (ES) cells in vitro. Using this strategy, we have isolated a novel murine gene that encodes a secreted molecule containing a variant epidermal growth factor-like (EGF) motif. We named this gene Cryptic, based on its predicted protein sequence similarity with Cripto, which encodes an EGF-related growth factor. Based on their strong sequence similarities, we propose that Cryptic, Cripto, and the Xenopus FRL-1 gene define a new family of growth factor-like molecules, which we name the ‘CFC’ (Cripto, Frl-1, and Cryptic) family. Analysis of Cryptic expression by in situ hybridization shows that it is expressed during gastrulation in two spatial domains that correspond to the axial and lateral mesoderm. In the first domain of expression, Cryptic expression is progressively localized to the anterior primitive streak, the head process, and the node and notochordal plate. In the second domain, Cryptic expression is initially concentrated in the lateral region of the egg cylinder, and is later found circumferentially in the intermediate and lateral plate mesoderm. Furthermore, Cryptic expression can also be detected at the early head-fold stage in the midline neuroectoderm, and consequently is an early marker for the prospective floor plate of the neural tube. Expression of Cryptic ceases at the end of gastrulation, and has not been observed in later embryonic stages or in adult tissues. Thus, Cryptic encodes a putative signaling molecule whose expression suggests potential roles in mesoderm and/or neural patterning during gastrulation.

Insulin-like growth factor-I regulates transcription of the elastin gene through a putative retinoblastoma control element. A role for Sp3 acting as a repressor of elastin gene transcription

Previous studies have demonstrated that insulin-like growth factor-I (IGF-I) increases elastin gene transcription in aortic smooth muscle cells and that this up-regulation is accompanied by a loss of protein binding to the proximal promoter. Sp1 has been identified as one of the factors whose binding is lost, and in the present study we show that Sp3 binding is also abrogated by IGF-I, but in a selected manner. In functional analyses using Drosophila SL-2 cells, Sp1 expression can drive transcription from the elastin proximal promoter, while co-expression of Sp3 results in a repression of Sp1 activity. Footprint and gel shift analyses position the IGF-I responsive sequences to a putative retinoblastoma control element (RCE). Mutation of the putative RCE sequence as assessed by transient transfection of smooth muscle cells results in an increase in reporter activity equal in magnitude to that conferred by IGF-I on the wild type promoter. Together these results support the hypothesis that IGF-I-mediated increase in elastin transcription occurs via a mechanism of derepression involving the abrogation of a repressor that appears to be Sp3 binding to the RCE.

Altered distribution of the nuclear receptor RAR beta accompanies proliferation and differentiation changes caused by retinoic acid in Caco-2 cells

All epithelial cells require retinoic acid for growth, maintenance, and differentiation. Although the epithelial cells that line the gastrointestinal tract are exposed to extreme retinoid concentration fluctuations in luminal fluid, whether proliferation and differentiation in these cells are significantly affected is not known. We have investigated this question using Caco-2 cells as a model because, although they are derived from a colon adenocarcinoma, they differentiate spontaneously in a manner similar to enterocytes in the small intestine. We found that retinoic acid caused maximum inhibition of cell growth and ornithine decarboxylase activity during the proliferative period. Retinoic acid increased brush border enzyme activities only in differentiating cells but stimulated transglutaminase activity in cells at all stages. In untreated proliferating cells, we found an early peak of transglutaminase activity that has not been reported before. Retinoic acid in intestinal cells acts through its nuclear receptor, RAR beta. The nuclear distribution of this receptor has not been demonstrated. In this study, we show that RAR beta responds to increasing concentrations of retinoic acid with a shift to the nuclear membrane in undifferentiated cells and progressive aggregation, diffusion, and loss in differentiated cells. We conclude that retinoic acid can inhibit proliferation and stimulate differentiation in Caco-2 cells depending on concentration and cell stage, and that these effects are accompanied by changes in distribution, as well as by the loss of RAR beta.

Prothymosin alpha mRNA levels vary with c-myc expression during tissue proliferation, viral infection and heat shock

Expression of prothymosin alpha, an acidic nuclear protein implicated in cellular proliferation, has been reported to be regulated by c-myc in vitro. We have studied the correlation of expression levels between prothymosin alpha and c-myc, using three different in vivo systems, viz. normal ontogenic process of placental development, lytic viral infection and heat shock treatment. The two genes have been found to share a similar expression pattern, although prothymosin alpha mRNA remains always detectable, indicating the existence of yet another mechanism, in addition to c-myc, which regulates its expression in vivo.

Retinoic acid induces cholinergic differentiation of NTera 2 human embryonal carcinoma cells

Retinoic acid (RA), a natural metabolite of vitamin A, influences the survival and neurotransmitter phenotype of several classes of vertebrate neurons during development. We now report that RA induces a subpopulation of NTera 2/clone D1 (NT2) human embryonal carcinoma cells to differentiate into postmitotic cells with cholinergic properties (NT2-N cells). After growth for 6 days in the presence of RA (10 microM) low levels of the acetylcholine-synthesizing enzyme choline acetyltransferase (ChAT) were detected in NT2 cell cultures. ChAT activity in the NT2 cell cultures continued to increase for at least an additional 22 days to a final activity of 50 pmol ACh synthesized/min/mg protein. Immunohistochemical staining of RA-treated cultures demonstrated that only those cells with a neuronal morphology (NT2-N cells) expressed the human ChAT protein. Since such cells comprised a small proportion (approximately 20%) of the population, the ChAT activity per neuronal cell was estimated to approach 250-300 pmol ACh/min/mg protein. Cultures composed of > 95% NT2-N cells had significantly lower ChAT specific activities and this could be increased by either ciliary neurotrophic factor or leukemia inhibitory factor, but not by nerve growth factor. We conclude that NT2 cells provide a system in which to study the molecular events that underlie neurotransmitter choice during the differentiation of human cholinergic neurons.

Transcriptional regulation of the elastin gene by insulin-like growth factor-I involves disruption of Sp1 binding. Evidence for the role of Rb in mediating Sp1 binding in aortic smooth muscle cells

We have recently identified a novel element (EFE 5/6) in the human elastin gene promoter that modulates the ability of insulin-like growth factor I (IGF-I) to up-regulate elastin gene transcription in aortic smooth muscle cells. In the present study, we have pursued the identification of those nuclear proteins binding to the EFE 5/6 element and affected by IGF-I treatment. Chelation inactivation and metal reactivation experiments together with supershift gel analyses demonstrated that Sp1 was one of the proteins affected by IGF-I. Southwestern and Western analyses showed that Sp1 was present in IGF-I nuclear extracts and capable of binding DNA after fractionation. Addition of retinoblastoma gene product (Rb) antibody mimicked the effect of IGF-I in gel shift analysis, suggesting that Sp1 binding may be regulated by an inhibitor normally associated with Rb. The fact that the phosphorylation state of Rb was affected by IGF-I was shown by Western blot analysis. The control smooth muscle cells transcribed the elastin gene at a high level without addition of IGF-I, so it is likely that disruption of Sp1 binding is the first step in allowing the binding of a more potent activating factor.

Changes of proliferative activity and phenotypes in spontaneous differentiation of a colon cancer cell line

We examined the alterations of proliferative activity and c-myc expression of a colon cancer cell line (Caco-2) during its spontaneous differentiation. Caco-2 cells were cultured in various types of media and the degree of differentiation was monitored in terms of dome formation in cell monolayers and expression of alkaline phosphatase (ALP) activity. In Caco-2 cells cultured with Eagle's minimum essential medium (EMEM) containing 10% fetal calf serum (FCS), dome formation was demonstrated and ALP activity was markedly increased after the cells reached confluence. Five-fold reduction of c-myc mRNA and a marked decrease in S-phase cells were observed in the differentiated cells. These changes were not induced in FCS-free EMEM. The addition of insulin and transferrin to FCS-free EMEM did not induce cell differentiation or reduction of c-myc mRNA expression. When Caco-2 cells were cultured with three different serum-free media, the induction of dome formation and the increase of ALP activity were observed to varying degrees. Expression of c-myc mRNA in the cells cultured with one serum-free medium decreased to a level similar to that in fully differentiated cells cultured with EMEM containing 10% FCS. These results suggest that a spontaneous switch from a proliferative state with high c-myc expression to differentiated phenotype occurs after cells reach confluence and depends on the culture conditions.

1,25-Dihydroxyvitamin D-3 destabilizes c-myc mRNA in HL-60 leukemic cells

The differentiation process is accompanied by alterations in the expression of a variety of genes. Monocytic maturation of hematopoietic cells (HL-60) induced by 1,25-dihydroxyvitamin D-3 (1,25(OH)2D3), results in a decrease in steady state c-myc mRNA levels. To elucidate the mechanism by which 1,25(OH)2D3 regulates c-myc mRNA expression, transcriptional and post-transcriptional modes of regulation were investigated. No transcriptional regulation was identified, however, 1,25(OH)2D3 appeared to decrease steady state c-myc mRNA levels by increasing its turnover rate. Using actinomycin D to block transcription, the half-life of c-myc mRNA was shown to decrease from 20 min in the absence of 1,25(OH)2D3 to < 5 min in the presence of 1,25(OH)2D3. Cycloheximide reversed the instability induced by 1,25(OH)2D3, prolonging the half-life of c-myc mRNA in both uninduced and 1,25(OH)2D3-induced HL-60 cells to > 60 min, indicating a translational requirement for the destabilization process. Additionally, the c-myc mRNA instability induced by 1,25(OH)2D3 in HL-60 appears to be a specific result of this agent, as indicated by the inability of other monocytic and granulocytic differentiation inducing agents to destabilize c-myc mRNA.

Reduced expression of PLC-gamma during the differentiation of mouse F9 teratocarcinoma cells

Expression of phosphoinositide-specific phospholipase C (PLC)-gamma during the retinoic acid (RA)-induced differentiation of F9 teratocarcinoma stem cells was investigated. PLC-gamma activity and PLC-gamma protein content were decreased during the differentiation of F9 teratocarcinoma stem cells induced by a combination (RACT) of RA, dibutyryl cyclic AMP (C) and theophylline (T). However, PLC-gamma activity and protein levels in RA- or C- and T-treated F9 cells remained the same as that of the F9 stem cells. Northern analysis of the 5.4-kb PLC-gamma transcript in differentiated F9 cells by RACT revealed that RACT decreased the PLC-gamma gene expression. These results suggest that the expression of PLC-gamma is negatively controlled by RACT treatment in the differentiation of F9 teratocarcinoma stem cells.

Evidence that filopodia outgrowth is a common final pathway for fibroblast growth inhibition in vitro

To identify events associated with fibroblast growth inhibition, the effect of two known inhibitors, interferon-alpha and all-trans retinoic acid, on the growth and surface morphology of cultured fibroblasts was examined. Interferon-alpha administered at seeding reduced both growth rate and saturation density; all-trans retinoic acid reduced only saturation density. However, both negative growth modulators were associated with an increase in filopodia outgrowth and an increase in intracellular filamentous actin in a time course corresponding to onset of growth inhibition by these agents. In combination with earlier findings, these data suggested that, regardless of etiology, cultured fibroblast growth restriction is mediated in part by an actin-dependent outgrowth of filopodia that augment intercellular contact.

Modulation of mRNA levels during human keratinocyte differentiation

Cultures of human keratinocytes provide an excellent model system in which to study differentiation. Using the phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) and calcium, two agents known to induce keratinocyte differentiation in vitro, we examined the expression of the genes encoding c-fos, c-myc, and c-jun; involucrin, a protein precursor of the keratinocyte cornified envelope; and L-7, a ribosomal protein. Overall, at the doses studied, TPA induced a more rapid and profound differentiation than did calcium, as evaluated by culture morphology and northern blot analysis. Our studies showed a constant low level of c-fos and c-jun expression in unstimulated cells with no significant change after addition of either TPA or calcium except when transcript breakdown was inhibited by cycloheximide. The c-myc proto-oncogene, known to have a high constitutive expression in actively proliferating cells, was strongly downregulated by TPA, but calcium had no effect over a 32 hour period, consistent with the greater growth inhibition of TPA in this system. Involucrin was induced about ninefold by both TPA and calcium as early as 8 hours after stimulation, suggesting transcriptional regulation of this gene during differentiation. L-7, recently demonstrated to be downregulated in late passage human fibroblasts in an in vitro model of senescence, was also strongly downregulated by either TPA or calcium, consistent with an interrelationship between the basic cellular processes of aging and differentiation. These finding expand our knowledge of the differentiation process in human keratinocytes.

High expression of stathmin in multipotential teratocarcinoma and normal embryonic cells versus their early differentiated derivatives

Stathmin is a ubiquitous cytoplasmic protein, phosphorylated in response to agents regulating the proliferation, the differentiation and the specialized functions of cells, in a way possibly integrating the actions of diverse concomitant regulatory signals. Its expression is also regulated in relation with cell proliferation and differentiation and reaches a peak at the neonatal stage. To assess the possible role of stathmin at earlier stages of development, we examined its expression and regulation in embryonal carcinoma (EC) and derived cell lines as well as in the early mouse embryo. Interestingly, stathmin is highly abundant in the undifferentiated, multipotential cells of the F9, 1003 and 1009 EC cell lines. Its high expression markedly decreased, both at the protein and mRNA levels, when F9 cells were induced to differentiate into endodermal-like cells with retinoic acid and dibutyryl-cAMP. Stathmin was also much less abundant in differentiated cell lines such as the trophectodermal line TDM-1, as well as in several F9- and 1003-derived cell lines committed to differentiate towards the mesodermal and neuroectodermal lineages but still proliferating. Therefore, the observed decrease of stathmin expression is not related to the reduced proliferation rate but rather to the differentiation of the multipotential EC cells. The immunocytochemical pattern of stathmin expression during early mouse development indicated that stathmin is also highly abundant in the multipotential cells of the inner cell mass of the blastula, whereas it is much lower in the differentiated trophectodermal cells. These results confirm the physiological relevance of the observations with EC cells, and suggest that stathmin, in addition to its high expression at later stages of development and in the adult nervous system, may be considered as a new marker of the multipotential cells of the early mouse embryo.

Temporal relationships between induced changes in c-myc mRNA abundance, proliferation, and differentiation in HL60 cells

Changes in the relative abundances of c-myc mRNA have been related to changes in other parameters of differentiation (histochemical, clonogenic) during the course of the differentiation of HL60 cells to monocytes/macrophages or to granulocytes. Induction of differentiation to monocytes/macrophages was marked by a rapid rate of appearance of committed cells (80 to 90% in 24 hours) and a concomitant rapid loss of c-myc mRNA. Induction of granulocytic differentiation resulted in a much slower rate of appearance of committed cells (50% in 48 hours), and a much faster rate of loss of c-myc mRNA (tenfold in 1 hour). These data are consistent with there being a direct link between down-regulation of the expression of c-myc and the onset of proliferation arrest and monocytic differentiation, but show there is no such association of c-myc mRNA abundance and proliferation or differentiation during the maturation of HL60 granulocytes.

Expression of SPARC is correlated with altered morphologies in transfected F9 embryonal carcinoma cells

SPARC (secreted protein, acidic and rich in cysteine) is a Ca(2+)-binding glycoprotein that has recently been identified as a member of a group of proteins that exert antispreading effects on various cultured cells. In addition, SPARC is induced during the later stages of F9 stem cell differentiation to parietal endoderm (PE). When treated with retinoic acid and dibutyryl cAMP, F9 cells differentiate into PE and SPARC mRNA is increased approximately 20-fold. To determine whether the chronic overexpression or inhibition of expression of SPARC would affect the morphology, attachment, or differentiation of F9 cells, we transfected undifferentiated F9 cells with cDNA encoding SPARC or anti-sense SPARC and cloned lines that expressed either elevated or reduced levels of SPARC protein. The transfected F9 cells displayed altered morphologies in culture: cells of four overexpressing lines appeared clumped and rounded, whereas those of three underexpressing lines were spread and flat, in comparison to controls. Moreover, the morphological differences persisted during differentiation of the lines to PE. The altered morphology was not due to an increased expression of collagenases and did not affect the ability of the cells to attach and adhere to tissue culture plastic. The altered phenotype of the transfected F9 cells appeared to be directly related to the level of extracellular SPARC. Since overexpression of SPARC induced rounding and aggregation of F9 cells in culture, we propose that SPARC facilitates modulation of cell-cell or cell-substrate interactions in vivo.

Proteins associated with the messenger ribonucleoprotein particle for the estrogen-regulated apolipoprotein II mRNA

The stability of the mRNA for apolipoprotein (apo) II is regulated by estrogen [Gordon et al. (1988) J. Biol. Chem. 263, 2625-2631]. On the hypothesis tha estrogen regulation of apoII mRNA stability is mediated through mRNA-protein interaction, we have examined the messenger ribonucleoprotein particle (mRNP) for apoII mRNA following release from chicken liver polyribosomes. Polyribosomes containing undegraded apoII mRNA were obtained when tissue was homogenized without detergent, and polyribosomes were isolated following simultaneous addition of detergent and magnesium to a 20000g supernatant. ApoII mRNP released by EDTA sedimented at 12-18 S in sucrose gradients, and banded at rho = 1.4 g/mL in CsCl isopycnic centrifugation, indicative of a 3:1 ratio of protein to mRNA. A fraction in which apoII mRNP was enriched to 40-50% of total mRNP was prepared by successive size fractionation steps on sucrose gradients. Proteins associated with sucrose gradient enriched apoII mRNP were examined by iodination of UV-cross-linked proteins followed by SDS-polyacrylamide gel electrophoresis. Comparisons of proteins in highly enriched apoII mRNP to proteins in mRNP from non-estrogen-treated rooster liver did not reveal any differences. This result suggests that the major proteins associated with apoII mRNA are mRNP proteins also associated with the bulk of liver mRNAs.

Induction of c-fos but not c-myc in S-91 cells by melanization signals

Synthesis of the pigment melanin is a complex differentiated function performed by pigment cells in response to a variety of stimuli. The possible roles of the proto-oncogenes c-fos and c-myc in the control of pigmentation were studied using subconfluent, actively proliferating Cloudman S-91 murine melanoma cells stimulated to synthesize melanin by melanocyte stimulating hormone (MSH) or forskolin. Stimulation caused a significant increase in melanin synthesis when compared to control cells, but had no effect on cell growth. Northern analysis of total cellular RNA demonstrated rapid, transient induction of c-fos mRNA as early as 30 min after stimulation with MSH or forskolin. In contrast, there was no effect on the high constitutive expression of c-myc in these actively proliferating cells. These data strongly suggest that the induction of c-fos mRNA is an early genetic event in stimulation of melanin synthesis and thus this proto-oncogene may play a major role in the regulation of this differentiated function, as reported for other forms of cellular differentiation. In contrast, c-myc expression is unaffected and instead correlated with cellular proliferative capacity. These results are consistent with the hypothesis that the down-regulation of c-myc frequently observed during cell differentiation is not a necessary event, but rather reflects an associated decrease in cell growth rate. The S-91 melanoma system appears to provide a convenient model for study of the regulation for a single, well defined differentiated function that is independent of growth rate.

Mouse F9 teratocarcinoma stem cells expressing the stably transfected homeobox gene Hox 1.6 exhibit an altered morphology

Expression of the murine homeobox gene Hox 1.6 rapidly increases in F9 teratocarcinoma cells when these cells are induced with retinoic acid to differentiate into primitive and parietal endoderm. Hox 1.6 encodes a putative transcriptional regulatory protein which may function as a secondary regulator of gene expression during the differentiation process. To examine the role of the Hox 1.6 gene, we have stably transfected F9 stem cells with a cDNA containing the complete coding sequence of Hox 1.6 under the control of the mouse metallothionein promoter. Two clonally distinct cell lines that express high levels of the transfected Hox 1.6 gene have been isolated and characterized. We show that expression of the transfected Hox 1.6 gene in F9 cells dramatically alters the stem cell morphology. However, the transfected cells do not differentiate in the absence of retinoic acid treatment, nor are they prevented from differentiating in response to such treatments. We therefore suggest that the Hox 1.6 gene controls the expression of genes which influence changes in F9 cell morphology during RA-induced differentiation.

A transient decrease in N-myc expression and its biological role during differentiation of human embryonal carcinoma cells

The human embryonal carcinoma (EC) cell line, NEC14 can be induced to morphologically differentiate by the addition of 10(-2) M N,N'-hexamethylene-bis-acetamide (HMBA) in vitro. The expression of several cellular oncogenes (c-onc) in NEC14 cells was examined after induction of differentiation by HMBA. The level of N-myc expression was the highest in undifferentiated cells but decreased transiently to less than 1/10 of the original level shortly after the induction of differentiation. To investigate the role of the transient decrease in N-myc level on NEC14 cell differentiation, a chimeric human N-myc gene in which transcription is initiated at the human beta-actin gene promoter was constructed and introduced into NEC14 cells. Several transformants expressing the exogenous N-myc gene constitutively were established. These transformants showed 10- to 70-fold increases in plating efficiency and shorter population doubling times as compared with the parental NEC14 cells. The transformants were hard to induce, spontaneously differentiated cells on the periphery of cell clusters in culture, unlike parental NEC14 cells, and took longer for HMBA-induced morphological differentiation. The populations of the cells expressing HLA and SSEA-1 antigens increased from 10%-20% to nearly 100% in NEC14 cells after the induction of differentiation, while the populations expressing these antigens increased only to 50%-60% in one of the transformants, S11. The transformants gained an increased tumorigenic potential in nude mice, and the tumors produced consisted exclusively of EC stem cells. These results suggest that the additional expression of the exogenous N-myc gene (increased about two-fold) confers the more transformed state on the cells.

Effect of vitamin E succinate and a cAMP-stimulating agent on the expression of c-myc and N-myc and H-ras in murine neuroblastoma cells

D-Alpha-tocopheryl succinate (vitamin E succinate) at a concentration of 11.3 microM inhibited growth and reduced the expression of c-myc, N-myc and H-ras specific mRNAs in murine neuroblastoma cells (NBP2) in culture. R020-1724 [4-(3-butoxy-4-methoxybenzyl)-2- imidazolidinone], an inhibitor of cyclic AMP phosphodiesterase, also inhibited growth and reduced the expression of these oncogenes. Vitamin E succinate treatment caused the formation of two c-myc related transcript of 1.9 and 3.7 kb; however, R020-1724 treatment did not. These results suggest that the inhibition of growth is sufficient to reduce the expression of c-myc, N-myc and H-ras in NB cells in culture, but it is not sufficient to produce two c-myc related transcripts.

Reversible effects of sodium butyrate on the differentiation of F9 embryonal carcinoma cells

We have studied effects of sodium butyrate on embryonal carcinoma F9 cell differentiation. In the presence of sodium butyrate, F9 cells underwent rapid and drastic morphological changes and expressed marked increases in mRNA levels of various differentiation markers. When sodium butyrate was removed from the cultures, all the examined phenotypes of F9 cell differentiation rapidly reverted to the characteristics of undifferentiated stem cells. However, under the same conditions, when cycloheximide or actinomycin D was added to the cultures, such phenotypic reversion was not observed, but high mRNA levels of the differentiation markers as well as altered cell morphology were retained. These results indicated that the effects of sodium butyrate on induction of teratocarcinoma cell differentiation were reversible and that de novo syntheses of some mRNA(s) and protein(s) were necessary for the reversion of differentiated cells to stem cells.

Activation of the interferon system during myogenesis in vitro

Differentiation of skeletal muscle involves withdrawal of myoblasts from cell replication, fusion to form multinucleated myotubes, coordinate appearance of a variety of muscle-specific proteins and the disappearance of a set of other proteins responsible for cell growth. The possible activation of the interferon (IFN) system in this process was studied. Thus, the activity of two IFN-induced enzymes known to be part of the system-(2'-5') oligoadenylate synthetase (2-5A synthetase) and double-stranded RNA-activated protein kinase as well as the expression of 2-5A synthetase coding genes were examined during myogenesis. It is demonstrated that the activity of the enzymes is transiently increased in cultured myoblasts, reaching a peak activity on the 3rd day in culture and then declining to a basal level. This peak activity precedes both cell fusion and the appearance of muscle-specific proteins--acetylcholine receptors (AChR) and creatine kinase. The same kinetics of 2-5A synthetase activity was evident in myoblasts from chick, rat or mouse origin. The enzymatic product appears to be primarily the trimer form of 2-5A, rather than a set of oligomers observed in enzymatic reactions performed on IFN-treated cells, including muscle cultures. The kinetics of 2-5A synthetase gene expression revealed that the largest amount of specific RNA transcripts appeared on the 1st day after seeding, followed by a reduction thereafter. In addition, a decrease was also observed in expression of c-myc, a cell-growth-associated protooncogene. However, an increase towards the 2nd day of both AChR and myosin light chain gene expression was evident, indicating selective regulation of gene expression during myogenesis.