QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

Vol. 16, Issue 12, 5719-5735, December 2005

This Article Abstract Full Text (PDF) Supplemental Material All Versions of this Article: E05-06-0572v1 16/12/5719    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Taranger, C. K. Articles by Collas, P. Search for Related Content PubMed PubMed Citation Articles by Taranger, C. K. Articles by Collas, P.

Induction of Dedifferentiation, Genomewide Transcriptional Programming, and Epigenetic Reprogramming by Extracts of Carcinoma and Embryonic Stem Cells

Department of Biochemistry, Institute of Basic Medical Sciences, University of Oslo, 0317 Oslo, Norway

Submitted June 28, 2005; Revised September 15, 2005; Accepted September 16, 2005
Monitoring Editor: Carl-Henrik Heldin

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
Functional reprogramming of a differentiated cell toward pluripotency may have long-term applications in regenerative medicine. We report the induction of dedifferentiation, associated with genomewide programming of gene expression and epigenetic reprogramming of an embryonic gene, in epithelial 293T cells treated with an extract of undifferentiated human NCCIT carcinoma cells. 293T cells exposed for 1 h to extract of NCCIT cells, but not of 293T or Jurkat T-cells, form defined colonies that are maintained for at least 23 passages in culture. Microarray and quantitative analyses of gene expression reveal that the transition from a 293T to a pluripotent cell phenotype involves a dynamic up-regulation of hundreds of NCCIT genes, concomitant with down-regulation of 293T genes and of indicators of differentiation such as A-type lamins. Up-regulated genes encompass embryonic and stem cell markers, including OCT4, SOX2, NANOG, and Oct4-responsive genes. OCT4 activation is associated with DNA demethylation in the OCT4 promoter and nuclear targeting of Oct4 protein. In fibroblasts exposed to extract of mouse embryonic stem cells, Oct4 activation is biphasic and RNA-PolII dependent, with the first transient rise of Oct4 up-regulation being necessary for the second, long-term activation of Oct4. Genes characteristic of multilineage differentiation potential are also up-regulated in NCCIT extract-treated cells, suggesting the establishment of "multilineage priming." Retinoic acid triggers Oct4 down-regulation, de novo activation of A-type lamins, and nestin. Furthermore, the cells can be induced to differentiate toward neurogenic, adipogenic, osteogenic, and endothelial lineages. The data provide a proof-of-concept that an extract of undifferentiated carcinoma cells can elicit differentiation plasticity in an otherwise more developmentally restricted cell type.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
Differentiated cells are thought to be stably committed to their fate; however, there is evidence to indicate that dedifferentiation events can take place. Urodele amphibians and teleost fish can replace lost anatomical parts by a process of migration, dedifferentiation, proliferation, and redifferentiation of epithelial cells in the wounded area. Functional reprogramming of differentiated cell nuclei has also been illustrated by the derivation of pluripotent embryonic stem (ES) cells (ESCs) (Cibelli et al., 1998; Munsie et al., 2000; Wakayama et al., 2001; Hwang et al., 2004) and by the live birth of cloned animals (Wilmut et al., 2002; Gurdon and Byrne, 2003) after nuclear transplantation into unfertilized eggs. Notably, Xenopus eggs can reprogram mammalian somatic nuclei to express the POU family member homeodomain transcription factor gene Oct4 (Byrne et al., 2003) by a process requiring DNA demethylation (Simonsson and Gurdon, 2004). DNA demethylation also occurs after fusion of mouse thymocytes with embryonic germ (EG) cells (EGCs) or ESCs (Tada et al., 1997, 2001; but interestingly, only EG cells are capable of demethylating imprinted genes (Tada et al., 1997). Similarly, fusion of neuronal progenitor cells (Pells et al., 2002; Ying et al., 2002) or bone marrow-derived cells (Terada et al., 2002) with ESCs results in hybrids that express markers of pluripotency (Pells et al., 2002), contribute to chimeras (Ying et al., 2002), and form teratomas (Terada et al., 2002). Similar observations resulted from fusing human fibroblasts with ESCs (Cowan et al., 2005). Fusion of embryonal carcinoma (EC) cells (ECCs) with T-lymphoma cells also promotes the formation of colonies expressing pluripotent cell transcripts from the lymphoma genome (Flasza et al., 2003). Thus, components of pluripotent EG, ES, or EC cells have the potential of eliciting reprogramming events in a somatic genome.

As an alternative to fusion, somatic nuclear function may also be altered using nuclear and cytoplasmic extracts, with the rationale that extracts provide the necessary regulatory components. Notably, extracts of regenerating newt limbs promote cell cycle reentry and down-regulation of myogenic markers in differentiated myotubes (McGann et al., 2001). Furthermore, we have shown that kidney epithelial 293T cells permeabilized with streptolysin O (SLO) and briefly exposed to an extract of Jurkat T-cells take on T-cell properties, including growth in aggregates, chromatin remodeling, expression of T-cell-specific genes and surface receptors, secretion of interleukin-2, and stimulation-dependent assembly of the interleukin-2 receptor (Håkelien et al., 2002, 2005; Landsverk et al., 2002). Similarly, lysates of cardiomyocytes or insulinoma cells elicit expression of cardiomyocyte or -cell markers in adipose stem cells (Gaustad et al., 2004) and fibroblasts (Håkelien et al., 2004), and a pneumocyte extract was recently shown to induce differentiation of ESCs into a pneumocyte phenotype (Qin et al., 2005). 293T cells were also shown to express pluripotency markers such OCT4 and GCAP and down-regulate a kidney marker after coculture with extract of Xenopus eggs (Hansis et al., 2004). Despite these observations, evidence for induction of epigenetic reprogramming events in large numbers of cells by extracts is lacking.

Teratocarcinomas are a particular type of germ cell tumors that contain undifferentiated stem cells and differentiated derivatives that can include endoderm, mesoderm, and ectoderm germ layers (Chambers and Smith, 2004). Undifferentiated carcinoma cells can be cultured to give rise to lines of ECCs. ECCs form malignant teratocarcinomas when transplanted into ectopic sites; however, some ECC lines can also contribute to tissues of the developing fetus when introduced into a blastocyst (Blelloch et al., 2004). Undifferentiated human teratocarcinoma NCCIT cells were established from a mediastinal mixed germ cell tumor (Teshima et al., 1988). NCCIT is at a stage intermediate between a seminoma (a precursor of germ cell tumors) and an embryonal carcinoma (Damjanov et al., 1993). NCCIT is a developmentally pluripotent cell line that can differentiate into derivatives of all three embryonic germ layers and extraembryonic cell lineages (Damjanov et al., 1993).

This study tests the hypothesis that an extract of undifferentiated somatic cells can elicit dedifferentiation in a somatic cell line. Based on morphological and immunolabeling observations, gene expression profiling, DNA methylation assays, and functional assessments, we show that 293T and NIH3T3 cells can be programmed by extracts of undifferentiated NCCIT cells or mouse ES cells to acquire characteristics of pluripotency.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
Cells
NCCIT, Jurkat (clone E6-1), and 293T cells (American Type Culture Collection, Vanassas, MD) were cultured in RPMI 1640 medium (Sigma-Aldrich, St. Louis, MO) with 10% fetal calf serum (FCS), 2 mM L-glutamine, 1 mM sodium pyruvate, and nonessential amino acids (complete RPMI 1640 medium). NIH3T3 Swiss-Albino fibroblasts (American Type Culture Collection) were cultured in DMEM (Sigma-Aldrich) with 10% FCS, L-glutamine, and 0.1 mM -mercaptoethanol. Mouse ESCs were isolated from inner cell masses of strain sv129 blastocysts and plated on mouse fibroblast -irradiated feeder layers in ESC medium (DMEM, 15% FCS, 0.1 mM -mercaptoethanol, nonessential amino acids, and 1% penicillin/streptomycin) supplemented with 1000 U/ml (10 ng/ml) recombinant leukemia inhibitory factor (LIF; Sigma-Aldrich) on gelatin-coated plates. Before harvesting for preparing extracts, ESCs were passaged and cultured under feeder-free conditions in RPMI 1640 medium containing 10 ng/ml LIF. Cells treated with NCCIT or 293T extract were seeded at 100,000 cells per well in a 48-well plate and cultured in 250 µl of complete RMPI 1640 medium with antibiotics. Cells exposed to ESC extract were cultured as ESCs with 10 ng/ml LIF under feeder-free conditions.

Cell Extracts
To prepare NCCIT extracts, cells were washed in phosphate-buffered saline (PBS) and in cell lysis buffer (100 mM HEPES, pH 8.2, 50 mM NaCl, 5 mM MgCl₂, 1 mM dithiothreitol, and protease inhibitors), sedimented at 400 x g, resuspended in 1 volume of cold cell lysis buffer, and incubated for 30–45 min on ice. Cells were sonicated on ice in 200-µl aliquots using a Labsonic-M pulse sonicator fitted with a 3-mm-diameter probe (B. Braun Biotech, Melsungen, Germany) until all cells and nuclei were lysed, as judged by microscopy (our unpublished data). The lysate was sedimented at 15,000 x g for 15 min at 4°C to pellet the coarse material. The supernatant was aliquoted, frozen in liquid nitrogen, and stored for up to 9 mo at -80°C. Lysate of 95,583 ± 10,966 NCCIT cells was used to generate 1 µl of extract. Protein concentration of the NCCIT extract was 29.5 ± 4.6 mg/ml (Bradford) and pH was 7.0 ± 0.0 (4 batches). ESC extracts (25–30 mg/ml protein) were similarly prepared from LIF-adapted ESC cultures. 293T, Jurkat, and NIH3T3 extracts were also prepared as described above. If necessary, extracts were diluted with H₂O before use to adjust osmolarity to 300 mOsM.

SLO-mediated Permeabilization and Cell Extract Treatment
293T and 3T3 cells were washed in cold PBS and in cold Ca²⁺- and Mg²⁺-free Hank's balanced salt solution (HBSS) (Invitrogen, Carlsbad, CA). Cells were resuspended in aliquots of 100,000 cells/100 µl of HBSS, or multiples thereof; placed in 1.5-ml tubes; and centrifuged at 120 x g for 5 min at 4°C in a swing-out rotor. Sedimented cells were suspended in 97.7 µl of cold HBSS, tubes were placed in a H₂O bath at 37°C for 2 min, and 2.3 µl of SLO (Sigma-Aldrich) (100 µg/ml stock diluted 1:10 in cold HBSS) was added to a final SLO concentration of 230 ng/ml. Samples were incubated horizontally in a H₂O bath for 50 min at 37°C with occasional agitation and set on ice. Samples were diluted with 200 µl of cold HBSS, and cells were sedimented at 120 x g for 5 min at 4°C. Permeabilization was assessed by monitoring uptake of a 70,000-M_r Texas Red-conjugated dextran (50 µg/ml; Invitrogen) in a separate sample 24 h after resealing and replating the cells (our unpublished data). Permeabilization efficiency under these conditions was 80%.

After permeabilization, cells were suspended at 1000 cells/µl in 100 µl of NCCIT, ESC or indicated control extract (or multiples thereof) containing an ATP-regenerating system (1 mM ATP, 10 mM creatine phosphate, and 25 µg/ml creatine kinase; Sigma-Aldrich), 100 µM GTP (Sigma-Aldrich), and 1 mM each nucleotide triphosphate (NTP; Roche Diagnostics, Mannheim, Germany). The tube containing cells was incubated horizontally for 1 h at 37°C in a H₂O bath with occasional agitation. To reseal plasma membranes, the extract was diluted with complete RPMI 1640 medium containing 2 mM CaCl₂ and antibiotics, and cells were seeded at 100,000 cells per well on a 48-well plate. After 2 h, floating cells were removed, and plated cells were cultured in complete RPMI 1640 medium.

Microarray Analysis of Gene Expression
Microarrays. Gene expression analysis was performed using Human Genome Affymetrix U133A GeneChips as described previously (Ji et al., 2004). Total RNA was isolated using a Stratagene RNA Nanoprep isolation kit, treated with DNAse I, and purified with RNeasy Mini columns (QIAGEN, Valencia, CA).

PCRcDNA. First-strand cDNA was prepared as described previously (Ji et al., 2004) using a SMART PCRcDNA synthesis kit (BE Biosciences Clontech, Palo Alto, CA). Briefly, total RNA was reverse transcribed using Superscript II reverse transcriptase (Invitrogen). cDNA was amplified by PCR as follows: 95°C for 1 min, 20–30 cycles of 95°C for 15 s, 65°C for 30 s, and 68°C for 3 min. PCRcDNA was purified with QIAquick columns (QIAGEN) and ethanol precipitation and dissolved in H₂O.

Biotin-labeling of cRNA. Biotin-labeled cRNA was prepared from PCRcDNA using a T7 RNA polymerase MEGAscript T7 kit (Ambion, Austin, TX), purified with an RNeasy mini kit and fragmented as described (Ji et al., 2004). Fragmented cRNA was used for hybridization or stored at -80°C.

Hybridization to GeneChips, Labeling, and Scanning. Hybridization was performed using 50 µg of fragmented cRNA at 45°C for 16 h as described previously (Ji et al., 2004). Chips were washed, stained at 35°C for 15 min with a phycoerythrin-strepavidin conjugate (Invitrogen), washed, and scanned on an HP GeneArray scanner Hewlett Packard (Palo Alto, CA).

GeneChip Image Quantification and Data Processing. GeneChip images were quantified and gene expression values calculated using the Affymetrix Microarray suite version 5.0 (MAS 5.0; Affymetrix, Santa Clara, CA). Expression ratios were calculated relative to mean hybridization level of three glyceraldehyde-3-phosphate dehydrogenase (GAPDH) oligonucleotide spots on the arrays, and plots were drawn using Microsoft Excel 2002 (Microsoft, Redmond, WA).

Polymerase Chain Reaction
PCR amplification of the simian virus SV40 large T antigen was performed using primers 5'-GTGGCTATGGGAACTGGAG-3' and 5'-CTCTACAGATGTGATATGGCTG-3', which cover nucleotides 39–265 of GenBank locus AF168998 [GenBank] . PCR conditions were 95°C for 3 min and 30 cycles of 95°C for 45 s, 60°C for 45 s, and 72°C for 45 s followed by 10 min at 72°C. PCR products were visualized by ethidium bromide staining in a 2% agarose gel.

Reverse transcription (RT)-PCR reactions were carried from 200 to 1000 ng of total RNA using the Iscript cDNA synthesis kit (Bio-Rad, Hercules, CA). Quantitative RT-PCR reactions were performed in triplicates on a MyiQ real-time PCR detection system using either IQ SYBR Green (Bio-Rad) or ProbeLibrary probes (Exiqon, Vedbæk, Denmark) as indicated in Table S1. SYBR Green PCR conditions were 95°C for 4.5 min and 40 cycles of 95°C for 30 s, 60°C for 30 s, and 72°C for 30 s, using GAPDH as normalization control. ProbeLibrary PCR conditions were 95°C for 7 min and 40 cycles of 94°C for 20 s and 60°C for 1 min using ACTB as standard.

Bisulfite Sequencing
DNA was purified by phenol-chloroform-isoamylalcohol extraction or by using the GenElute mammalian genomic DNA miniprep kit (Sigma-Aldrich). Bisulfite conversion was performed using the MethylEasy DNA bisulphite modification kit (Human Genetic Signatures, Sydney, Australia) as described by the manufacturer. Converted DNA was amplified by seminested PCR using primers (Human Genetic Signatures) specific for the human OCT4, LMNA, and LMNB1 genes (see Results), and PCR products were sequenced. PCR conditions were, for each of the nested PCRs, 95°C for 3 min and 30 cycles of 95°C for 1 min, 50°C for 2 min, and 72°C for 2 min, followed by 10 min at 72°C.

Induction of Neuronal Differentiation
To generate neuronal derivatives (Stewart et al., 2003), cells were seeded in complete RPMI 1640 medium at 5 x 10⁵ cells per 90-mm sterile bacterial culture dish. Suspension cultures were maintained for 24 h before adding 10 µM all-trans-retinoic acid (Sigma-Aldrich). Cells were cultured for 3–5 wk in retinoic acid, replacing the medium every 2–3 d. Subsequently, cell aggregates were washed in complete RPMI 1640 medium and plated onto poly-L-lysine (10 µg/ml; Sigma-Aldrich)-coated plates in complete RPMI 1640 medium containing the mitotic inhibitors fluorodeoxyuridine (10 µM; Sigma-Aldrich), cytosine arabinosine (1 µM; Sigma-Aldrich), and uridine (10 µM; Sigma-Aldrich).

Mesodermal Lineage Differentiation
Cells were cultured for 21 d in complete RPMI 1640 medium containing 10 µM all-trans-retinoic acid and washed in complete RPMI 1640 medium. For adipogenic differentiation, cells were cultured for a further 21 d in DMEM/Ham's F-12 supplemented with 10% FCS, dexamethasone, insulin, and indomethacin (Boquest et al., 2005). Cells were fixed with 4% formalin, washed in 5% isopropanol, and stained for 15 min with Oil-Red-O (Sigma-Aldrich). For osteogenic differentiation, cells were cultured for 21 d in DMEM containing 10% FCS, dexamethasone, -glycerophosphate, and L-ascorbate-2-phosphate (Boquest et al., 2005). Extracellular matrix mineralization nodules were visualized by Alzarin red staining. Endothelial differentiation was performed as described previously (Planat-Benard et al., 2004). Briefly, NCCIT extract-treated cells and controls were harvested by flask shaking and plated at 2 x 10⁵ cells per milliliter in 3 ml of methylcellulose (Methocult GF H4434; Stem Cell Technologies, Vancouver, British Columbia, Canada) and cultured for 7 d.

Immunological Procedures
For immunofluorescence, cells were seeded onto coverslips, fixed with 3% paraformaldehyde, permeabilized with 0.1% Triton X-100, blocked with bovine serum albumin, and hybridized with relevant antibodies. Antibodies used were a rabbit polyclonal anti-Oct4 (1:1000 dilution; Santa Cruz Biotechnology, Santa Cruz, CA), an anti-lamin A/C monoclonal antibody (mAb) (1:50 dilution; mAb XB10; BAbCO, Covance Research Products, Grand Rapids, MI), and a rabbit polyclonal antibody against a peptide of human B-type lamins (1:1000) (Chaudhary and Courvalin, 1993). Rabbit polyclonal antibodies (1:200) against neurofilament NF200 were from Sigma-Aldrich, and anti-NeuN (mAb377) and anti-nestin antibodies (mAb5362; 1:200) were from Chemicon International (Temecula, CA). Secondary antibodies were Cy2- and Cy3-conjugated anti-mouse and anti-rabbit antibodies and Cy3-conjugated anti-rabbit antibodies (1:1000 dilution; Jackson ImmunoResearch Laboratories, West Grove, PA). For direct immunolabeling, cells (300,000) were incubated with fluorescein isothiocyanate-conjugated mouse anti-human CD31 or rabbit anti-human CD144 antibodies (1:10 dilution; Serotec, Oxford, United Kingdom) in 100 µl of PBS. After extensive washing in PBS, cells were fixed with 3% paraformaldehyde before viewing. For Western blotting, antibodies used were anti-Oct4 (1:250), anti-lamin A/C (1:500), anti-lamin B (1:5000), and anti-tubulin (1:250; Santa Cruz Biotechnology). Immunodepletion of Oct4 from mouse ESC extract was performed using anti-Oct4 antibodies (1:50 dilution) bound to protein A/G-Sepharose beads. After 1-h incubation in extract at 4°C, bead complexes were removed by sedimentation at 4000 x g for 5 min, and a second round of immunoprecipitation was carried out for 30 min at 4°C. After sedimentation of the beads, an aliquot of the supernatant was removed for Western blotting, whereas the extract was used for cell treatment.

Alkaline Phosphatase Assay
Relative intracellular ALP levels were determined using a dot-blot assay. Two microliters of soluble lysate (15,000 x g supernatant at 20 µg/µl protein) from indicated cell types were spotted on a dry 45-µm nitrocellulose membrane (Bio-Rad). The membrane was wetted in 50 mM NaCl, 10 mM Tris-HCl, pH 7.0, and ALP was revealed by applying an Alk-Phos Direct detection solution (GE Healthcare, Piscataway, NJ). Light emission on film was quantified by densitometry within a linear signal range.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
Treatment of 293T Cells with NCCIT Extract Promotes Colony Formation
293T cells were permeabilized with SLO and exposed for 1 h to an extract of undifferentiated NCCIT cells. In vitro culture of extract-treated cells was accompanied by morphological, immunological, gene expression, and functional analyses over 8–12 wk to evaluate induction of dedifferentiation in two to six independent experiments. As controls, permeabilized 293T cells were treated with extract of 293T or Jurkat T-cells.

A first result of NCCIT extract exposure was a change in morphology of 293T cells. Within 2 wk, colonies with defined edges developed and resembled NCCIT colonies (Figure 1A and B, a–e). This phenotype was maintained for at least 12 wk in culture, corresponding to >50 population doublings and 23 passages (Figure 1B, a–e). The phenotype was not a mere consequence of treatment with any extract as 293T cells incubated in their own extract did not form colonies (Figure 1B, f–j), and 293T cells treated with an extract of Jurkat cells formed clearly morphologically distinct aggregates (Figure 1C). The latter were reminiscent of Jurkat T-cell clusters (Håkelien et al., 2005).

View larger version (92K): [in this window] [in a new window]   Figure 1. Morphology of 293T cells treated with NCCIT extract. (A) Untreated 293T and NCCIT cells. (B) 293T cells at indicated time points after exposure to NCCIT (a–e) or 293T (f–j) extract. (C) 293T cells 10 d after exposure to Jurkat extract and cultured under T-cell growth conditions. Dark spots are Dynal Biotech (Montebello, Norway) magnetic beads bearing antibodies against CD3 and CD28 surface antigens and used to promote T-cell expansion. Bars, 30 µm.

View larger version (33K): [in this window] [in a new window]   Figure 2. Immunofluorescence analysis of Oct4, lamin A/C and B-type lamin expression in 293T cells exposed to NCCIT extract. Untreated NCCIT and 293T cells (A) and 293T cells (B) treated with NCCIT or 293T extract were immunolabeled with antibodies against Oct4, lamin A/C, and B-type lamins (B, 1 wk after extract treatment). Bars, 20 µm. (C) Proportions (mean ± SD) of untreated NCCIT and 293T cells and of extract-treated cells expressing Oct4, lamin A/C, and B-type lamins. Three sets of 200 cells were examined for each marker. *p < 0.05 compared with 293T cells (t test); **p < 0.001 compared with 293T cells and 293T cells treated with 293T extract (t test).

Induction of OCT4 (POU5F1) transcription and loss of LMNA (lamin A) gene expression over time were demonstrated by quantitative RT-PCR analysis 4 wk after extract treatment (Table 1). Several target genes of Oct4 were also up-regulated, including UTF1, OXT2, REX1, and NANOG. Interestingly, Oct4 is known to act in cooperation with Sox2 (Avilion et al., 2003), which was also found to be induced in extract-treated cells. Additional markers of pluripotentiality (Hoffman and Carpenter, 2005) up-regulated in NCCIT extract-treated cells and verified by quantitative RT-PCR included ALP 1 (APL), STELLA, AC133, CD9, DMNT3B, and DNMT3L (Table 1). Expression of these genes was examined and confirmed at 2, 4, and 6 wk after extract treatment but not examined thereafter (our unpublished data). However, genes such as PDGFR, FGF2, LEFTY1, LEFTY2, CD135, or CD117 were not expressed in any cell type or were not altered by extract treatment (our unpublished data). As expected, transcripts for the ubiquitously expressed lamin B1 (LMNB1) were not altered by exposure to either extract (Table 1), supporting our immunofluorescence observations. None of the pluripotency marker transcripts examined by real time RT-PCR were elicited in 293T cells treated with their own extract (our unpublished data).

To verify that expression of Oct4 and reduction of A-type lamin expression did take place in 293T cells, 293T cells stably expressing an enhanced green fluorescent protein (EGFP) and a geneticin-resistance gene (293T-EGFP-Gen^R cells) were treated with NCCIT extract. After 2 wk of culture with 700 µg/ml geneticin, which kills NCCIT cells (our unpublished data), the majority of 293T-EGFP-Gen^R cells stained positive for Oct4 (Figure 3A). Strong up-regulation of OCT4, SOX2, and APL and moderate up-regulation of STELLA (2-fold) gene expression were also detected in these cells, whereas LMNA was repressed (Figure 3B). We also took advantage of the large T antigen marker carried by 293T cells. PCR analysis indicated that 293T-EGFP-Gen^R cells, but not NCCIT cells, contained the SV40 large T antigen transgene (Figure 3C), confirming that cells expressing Oct4 were of 293T origin. Of note, karyotyping analysis of 293T cells before and after extract treatment was inconclusive due to the severe aneuploidy and genomic instability of untreated 293T cells (our unpublished data). 293T cell aneuploidy was expected to occur as a result of large T antigen transformation, which is known to cause endoreplication (Wu et al., 2004).

View larger version (33K): [in this window] [in a new window]   Figure 3. Oct4 expression in EGFP-labeled 293T cells. (A) 293T cells stably expressing EGFP and a geneticin resistance (GenR) gene were treated with 293T or NCCIT extract and cultured for 2 wk with 700 ng/ml geneticin before immunolabeling with anti-Oct4 antibodies. NCCIT extract was also treated with 500 µg/ml DNAse I before incubating cells (bottom row). Bar, 20 µm. (B) Quantitative RT-PCR analysis of expression of indicated genes in 293T-EGFP-GenR cells 2 wk after incubation in intact or DNAse I-treated NCCIT extract (relative to 293T extract-treated controls). (C) PCR analysis of the presence of SV40 large T antigen in 293T, NCCIT, and extract-treated cells. Ladder is a 123-base pair DNA ladder.

The NCCIT Extract Promotes OCT4 DNA Demethylation
Transfer of mammalian cells into the germinal vesicle of Xenopus oocytes elicits DNA demethylation in the OCT4 promoter, a prerequisite for OCT4 expression in this system (Simonsson and Gurdon, 2004). The OCT4 region analyzed in our study was from nucleotide 1433–1671 (GenBank sequence AJ297527 [GenBank] ), encompassing eight potentially methylated cytosines in CpG dinucleotides between conserved regions CR2 and CR3 in the OCT4 promoter (Nordhoff et al., 2001) (Figure 4A). Bisulfite sequencing showed that this region was unmethylated in NCCIT but methylated in 293T cells (Figure 4A). In 293T cells exposed to their own extract, OCT4 remained methylated. However, in NCCIT extract-treated cells, OCT4 demethylation was evident after 4 and 9 wk of culture (Figure 4A) and provided molecular support for long-term transcriptional activation of OCT4. Two CpGs showed apparent partial demethylation (Figure 4A), which was interpreted as the expected presence of a mixed cell population in which cytosines -1686 and -1676 did not undergo demethylation. We concluded therefore that the NCCIT extract was capable of eliciting OCT4 demethylation in 293T cells. OCT4 demethylation was specific for the NCCIT extract and it did not occur in 293T extract (Figure 4A).

View larger version (45K): [in this window] [in a new window]   Figure 4. Bisulfite sequencing analysis of DNA methylation changes in extract-treated cells. 293T cells, NCCIT cells, and cells treated with 293T or NCCIT extract were examined for cytosine methylation in underlined CpG dinucleotides within shown genomic regions of the human OCT4 (A), LMNA (B), and LMNB1 (C) genes. Diagrams show localization (nucleotide numbers) of regions examined relative to the ATG translation start (+1).

Transcriptional Profiling of NCCIT Cells Relative to 293T Cells
To evaluate the extent of transcriptional alterations elicited by the NCCIT extract, an Affymetrix U133A GeneChip microarray analysis of 293T cell gene expression was carried out for 8 wk after extract treatment. We took advantage of a SMART PCRcDNA approach that combines PCR amplification and T7 RNA polymerase to amplify submicrogram RNA samples (Ji et al., 2004). Although some distortion of within-sample stoichiometry occurs with this method, one can assume the same distortion between samples, thus maintenance of between-sample stoichiometry allows comparative analyses.

We first assessed genes significantly (p < 0.001) up- or down-regulated at a more than threefold difference level in NCCIT compared with 293T cells. A total of 2950 genes were up-regulated, whereas 2528 genes were down-regulated in NCCIT cells, in two independent analyses performed in duplicate. Distributions of up- and down-regulated genes into functional classes were similar, with most genes encoding elements involved in transcription regulation (22 and 19%, respectively), cytoskeletal organization (6 and 3%), metabolism (4 and 10%), protein synthesis and processing (6 and 4%), cell signaling (10 and 8%), and chromatin organization (6 and 4%) (Figure S1).

NCCIT Extract Induces Expression of NCCIT-specific Genes and Down-Regulation of 293T Genes
The NCCIT extract elicited up- and down-regulation of 1700–2000 and 1650–1800 genes, respectively, on any given week relative to 293T cells (Figure 5B, green and red bars). Of these, 70 and 34%, respectively, were shared with NCCIT cells and qualified as "NCCIT genes" (Figure 5, A and B, yellow bars; E). Furthermore, the likelihood that expression of these genes was altered by chance rather than as a result of extract treatment was extremely low (p < 10^-5 and p < 10^-4, respectively; t tests), indicating that changes were elicited by the NCCIT extract.

View larger version (36K): [in this window] [in a new window]   Figure 5. Microarray analysis of gene expression in extract-treated 293T cells. (A) Venn diagram identifying "NCCIT-specific" genes (yellow area). Numbers of genes up- or down-regulated more than threefold (relative to input 293T cells) in cells incubated in extract of NCCIT (B), 293T (C), and Jurkat (D) cells (mean ± SD of two [B and C] and four [D] experiments). Yellow bars indicate genes up- or down-regulated in extract-treated cells and shared with NCCIT cells. In B, the likelihood that NCCIT genes are up- or down-regulated by chance rather than by extract treatment is extremely low (p < 10-5 and p < 10-4, respectively; t tests). By contrast, in C and D these probabilities are relatively high (p > 0.07 and p > 0.08, respectively; t tests). (E) Percentage of NCCIT genes up- or down-regulated in extract-treated cells (percentage of total up- or down-regulated genes). (F) Number of NCCIT genes specifically up- or down-regulated by treatment with NCCIT extract, over time. (G) Consistency of gene up- or down-regulation over time after treatment with NCCIT extract. Numbers of up- and down-regulated NCCIT genes in cells exposed to NCCIT extract are shown in green and red. Gray bars represent genes consistently up- or down-regulated at weeks 1 and 2 (gray bars at week 2), weeks 1, 2, and 4 (gray bars at week 4), etc., and shared between the two experiments. (H) Functional class distribution of genes consistently up- or down-regulated over 8 wk in two experiments (gray bars in G). These genes are listed in Table S3.

The consistency of NCCIT gene expression changes in NCCIT extract-treated cells over time was subsequently assessed. Figure 5G (gray bars) shows that 686 genes were consistently up-regulated from weeks 1–8, whereas 161 genes were consistently down-regulated (these genes were shared between both experiments). These genes are listed in Table S3. The remaining affected genes included those with an onset of up- or down-regulation later than week 1, or those with a more fluctuating expression level. For example, a large number of genes were up-regulated from week 2 onward, and thus they were not taken into account in the above-mentioned analysis. Functional class distribution of the consistently up- or down-regulated genes (Figure 5H) shows that most annotated up-regulated genes encoded elements involved in transcription, cytoskeletal organization, metabolism, signaling, and chromatin remodeling, whereas down-regulated genes were more evenly distributed across functional classes.

Treatment with NCCIT Extract Up-Regulates Markers of Pluripotency and Genes Indicative of Multilineage Priming
Table 2 lists markers of pluripotency represented in the array and that were up-regulated in extract-treated cells. In agreement with our immunolabeling and DNA methylation data, OCT4 (POU5F1) was up-regulated from week 2 onward. Notably, the Oct4-responsive genes UTF1 and REX1/DRN3 (Hosler et al., 1989; Okuda et al., 1998) were also up-regulated together with SOX2, suggesting the induction of Oct4-dependent functions in reprogrammed cells. Telomerase (TERT) and telomerase-associated factor 1 (TERF1) were also increasingly up-regulated. Other pluripotency markers up-regulated were the CD44 stem cell antigen, placental ALP (APL1), LIF, stem cell growth factor (SCGF), germ cell nuclear factor (GCNF), and integrin 6 (INTA6), a putative marker of "stemness" shared between three gene expression profile analyses of mouse ESCs (Fortunel et al., 2003). Remarkably, except for Dikkopf2 (DKK2), none of the stem cell marker genes listed in Table 2 was affected by treatment with 293T or Jurkat extract, illustrating the extract specificity of changes elicited (Table S4). In parallel, LMNA was essentially repressed, whereas expression of B-type lamins (LMNB1 and LMNB2) persisted, consistent with our RT-PCR and immunolabeling data. The kidney-derived 293T cell marker natriuretic peptide receptor C (NPR3) was also strongly down-regulated (Hansis et al., 2004).

View larger version (20K): [in this window] [in a new window]   Figure 6. Quantitative RT-PCR analysis of expression of indicated multilineage priming genes in 293T cells treated with NCCIT extract relative to transcript levels in 293T cells exposed to 293T extract. Expression levels were adjusted to those of GAPDH in triplicate samples. Single data points show mean expression level in NCCIT cells.

Oncogenes and Tumor Suppressor Genes Are Not Affected by NCCIT Extract Treatment
NCCIT is a tumor cell line that bears genetic mutations required for its expansion and phenotypic characteristics. We determined whether mRNA levels for oncogenes and tumor suppressor genes were altered in 293T cells exposed to NCCIT extract, relative to untreated cells. We did not observe any up-regulation or induction of oncogene expression in NCCIT extract-treated cells. Genes such c-MYC, c-MYC-responsive genes, genes encoding Myc-interacting or Myc-regulated proteins, and genes encoding RAB and RAF isoforms were not significantly expressed in any of the cell types examined nor were they altered by NCCIT extract. Among tumor suppressor genes, P53 was strongly up-regulated in NCCIT compared with 293T (p < 10^-4) but remained unaltered in NCCIT extract-treated cells (p > 0.05). Other tumor suppressor genes, however, were either not significantly expressed in 293T or NCCIT [RB1, TSC1, TSC22, BRCA1, BRCA2, CDKN2A (p16), CDKN1A (p21, Cip1), CDKN1C (P57, Kip2), MSH2, STK11, MEN1, and MEN2] or were expressed at similar levels in both cell types (PTCH, PTEN, and WT1D). These genes remained unaltered by treatment with NCCIT extract (p > 0.05). Similarly, genes encoding enzymes involved in DNA repair (XPA, ERCC5, FANCA, -C, -E, -F, and -G) were not significantly expressed in either cell types nor altered by extract treatment. ATM was highly expressed in 293T and NCCIT cells and remained unchanged in extract-treated cells. Because it is unlikely that NCCIT genetic lesions are passed onto the 293T cell genome through extract treatment, acquisition of an NCCIT phenotype by 293T cells implies that either the phenotype obtained is independent of NCCIT lesions or that genetic mutations that gave rise to the NCCIT tumor phenotype are dispensable for the maintenance of this state.

Retinoic Acid Stimulation of NCCIT Extract-treated Cells Induces Neuronal Differentiation
To determine whether NCCIT extract-treated 293T cells acquired a potential for pluripotency, we attempted to induce neuronal differentiation in vitro with retinoic acid (Stewart et al., 2003). 293T, NCCIT (our unpublished data), and extract-treated cells were exposed to 10 µM all-trans-retinoic acid and were maintained as suspended aggregation cultures (Figure 7A). Suspensions of all cell types formed disorganized aggregates in bacteriological dishes but after 2 wk in retinoic acid, the cells formed spheres that sometimes fused with one another (Figure 7A). This was particularly evident for NCCIT extract-treated 293T cells (Figure 7A, top). After washing and replating, cells adhered to poly-L-lysine-coated coverslips. However, only NCCIT and NCCIT extract-treated cells showed evidence of neurite outgrowth already after culture for 2 d in the absence of retinoic acid but in the presence of mitotic inhibitors (Figure 7A, bottom, and S2A). This suggested that neuronal progenitor cells emanated from NCCIT extract-treated 293T cells.

View larger version (65K): [in this window] [in a new window]   Figure 7. Neuronal differentiation of 293T cells treated with 293T or NCCIT extract. (A) Top, induction of differentiation. Suspended aggregates after 2 wk of culture with 10 µM all-trans-retinoic acid. Bottom, differentiation. Cells were plated onto poly-L-lysine-coated coverslips after 2 d of culture in the absence of retinoic acid but with mitotic inhibitors. Note neurite extensions in NCCIT extract-treated cells. (B) NCCIT extract-treated cells either treated with retinoic acid (+RA) or not (-RA) for 3 wk were immunolabeled using antibodies against Oct4 and nestin. Graph shows proportions (mean ± SD) of cells immunolabeled with anti-Oct4 and anti-nestin antibodies (n = 200 cells in each of a triplicate analysis for each treatment). (C) Quantitative RT-PCR analysis of expression of OCT4, NES (nestin), LMNA and LMNB1 in 293T cells treated with 293T or NCCIT extract, in absence (-RA) or presence (+RA) or retinoic acid for 3 wk. (D) NeuN and NF200 immunofluorescence analysis of indicated cell types induced to differentiate as described in A. Insets, DNA labeled with Hoechst 33342. Bars, 400 µm (A, top); 40 µm (A, bottom); 40 µm (B and D).

NCCIT Extract Enhances Differentiation Potential toward Adipogenic, Osteogenic, and Endothelial Lineages
To provide additional evidence for induction of differentiation potential in 293T cells treated with NCCIT extract, we determined whether the cells would acquire phenotypic characteristics of adipocytes and osteoblasts. After 3 wk of retinoic acid treatment and 3 wk of stimulation in appropriate differentiation medium (see Materials and Methods), a proportion of 293T cells, NCCIT cells, and of cells treated with 293T or NCCIT extract were induced to differentiate toward adipogenic and osteogenic pathways (Figures 8, A and B, and S2D and S2E). Intracellular lipid staining with Oil-Red-O showed enhanced differentiation of NCCIT extract-treated cells relative to any other cell type toward the adipogenic pathway (Figure 8A). Moreover, significantly more Alzarin red-stained mineralized nodules were detected in NCCIT extract-treated 293T cells compared with any other cell type (Figure 8B, arrows; p < 10^-6; t tests). Additionally, induction of endothelial differentiation of NCCIT extract-treated cells in methylcellulose triggered the appearance of an endothelial cell phenotype after 7 d. With cells forming elongated "tracks" in methylcellulose (Figure 8C; see controls in Figure S2F). Morphological changes were supported by immunoreactivity to CD31 and CD144, two endothelial cell surface markers (Boquest et al., 2005) (Figure 8D), and by induction of expression of CD31 (2 ± 0.15-fold up-regulation compared with undifferentiated, extract-treated cells) and CD144 (347 ± 97.6-fold up-regulation). Collectively, these results indicate that treatment with NCCIT extract enhances the ability of 293T cells to differentiate into ectoderm and several mesoderm lineages.

View larger version (61K): [in this window] [in a new window]   Figure 8. Induction of adipogenic, osteogenic, and endothelial differentiation of 293T cells treated with 293T or NCCIT extract. (A and B) Cells were exposed to 10 µM retinoic acid for 21 d, washed, and cultured in the absence of retinoic acid in adipocyte (A) or osteoblast (B) differentiation medium for 21 d. (A) Cells were stained with Oil-Red-O to reveal lipid droplets. Images on the right are enlargements of two areas framed in white in the adjacent panel (top right-hand quadrant). Arrows point to strongly stained lipid droplets. (B) Cells were stained with Alzarin red to visualize mineralized nodules (arrows). Graph shows mean ± SD number of distinct strongly mineralized nodules (arrows) per unit area (area shown in B). Twenty-four to 27 areas were analyzed within wells of six-well culture plates. *p < 10-6 relative to other treatments (ANOVA). (C) 293T and NCCIT extract-treated cells were passaged onto methylcellulose for 7 d to elicit endothelial differentiation. Note the formation of a track phenotype characteristic of cultured endothelial cells (right). (D) Direct immunofluorescence labeling of CD31 and CD144 surface antigens in extract-treated cells induced to differentiate as described in C. After differentiation, cells were loosened from the methylcellulose semisolid substrate by dilution with PBS and thus lost their elongated phenotype. Bars, 40 µm (A), 200 µm (B and C), and 40 µm (D).

View larger version (47K): [in this window] [in a new window]   Figure 9. Induction of Oct4 and ALP expression in 3T3 cells exposed to mouse ESC extract. (A) 3T3 cells 10 d after treatment with ESC extract (ES ex.) or 3T3 extract (3T3 ex.). Bar, 20 µm. (B) RT-PCR analysis of expression of indicated genes in ESCs, 3T3 cells and 3T3 cells at 2 wk after treatment with ESC extract. (C) ALP expression in cells treated as described in B (2 wk after extract treatment). (D) Relative ALP level in ESCs, 3T3 cells, and in 3T3 cells treated in 3T3 extract or in ESC extract alone or with 25 µM DRB or 10 µg/ml CHX, as indicated. Cells were analyzed 2 wk after extract treatment. (E) Indicated cell types (as in B) were analyzed by Western blotting for expression of Oct4, lamin A/C, B-type lamins, and -tubulin, 2 wk after extract treatment. (F) Relative Oct4 level in 3T3 cells treated with ESC extract containing decreasing concentrations of NTPs (1 wk after extract treatment). (G) Relative Oct4 level in 3T3 cells exposed to ESC extract containing indicated ATP or GTP analogues. ATP-RS, ATP-regenerating system (2 wk after extract treatment).

Embryoid-like bodies derived from ESC extract-treated cells expressed ALP, another embryonic and ESC marker, after 8 d of culture (Figure 9, C and D; 1 wk after extract exposure). ALP expression was inhibited by a 24-h exposure to 25 µM RNA PolII inhibitor 5,6-dichloro-1-D-ribofuranosyl benzimidazole (DRB) or to 10 µg/ml protein synthesis inhibitor cycloheximide (CHX) (Figure 9D). Thus, ALP expression was endogenous to the extract-treated cells. No expression was detected in control cells (Figure 9, C and D). Furthermore, immunofluorescence analysis showed that 90% of embryoid-like body-derived cells expressed intranuclear Oct-4, whereas in the same cells lamin A/C expression was reduced to undetectable levels (our unpublished data). Immunoblots confirmed this observation (Figure 9E) and indicated that ESC extract-treated cells expressed Oct4 to levels similar to ESCs. Oct4 not was detected in 3T3 cells incubated in their own extract or in intact (nonpermeabilized) 3T3 cells treated with ESC extract (Figure 9E). This ruled out the detection of an unspecific anti-Oct4 immunoreactive product and of any extract-derived Oct4 protein that would stick to the cell surface. Detection of Oct4 protein in ESC extract-treated cells required a threshold (10 µM) concentration of NTPs in the extract (Figure 9F), providing additional evidence for the lack of unspecific detection of Oct4 in these cells.

Closer examination of the need for exogenous nucleotides to promote Oct4 detection indicated a requirement for ATP and GTP hydrolysis (Figure 9G). Indeed, ATP depletion from the extract with glucose and hexokinase, replacement of exogenous ATP with adenosine-5'-O-(3-thio)triphosphate or adenyl-5'-yl imidodiphosphate, or GTP removal or substitution with guanosine 5'-O-(3-thio)triphosphate or guanosine 5'-[,-imido]triphosphate abrogated Oct4 detection (Figure 9G). This is consistent with a role of active nuclear import for transcription in cell extracts (Håkelien et al., 2002). Furthermore, heat treatment (95°C; 5 min), trypsinization, or proteinase K treatment of the ESC extract abolished Oct4 detection in 3T3 cells (Figure 10). However, DNAse I (100 mg/ml) or RNAse A (50 mg/ml) did not affect Oct4 levels in extract-treated cells (see below; Figure 10), ruling out a significant contribution of mRNA or DNA of extract origin.

View larger version (21K): [in this window] [in a new window]   Figure 10. Relative Oct4 protein levels in 3T3 cells exposed to ESC extract pretreated as indicated in each panel. Cells were cultured for 1–96 h after extract treatment, lysed, and analyzed by Western blotting. Oct4 protein levels were determined by densitometric analysis of duplicate blots for each time point under each condition. Level 1 indicates Oct4 level at 1 h after removal of cells from an untreated ESC extract (top left).

View larger version (42K): [in this window] [in a new window]   Figure 11. Oct4 expression in ESC extract-treated cells is biphasic and RNA PolII dependent. (A) Immunoblotting analysis of intracellular Oct4 levels in 3T3 cells treated with ESC extract and cultured for indicated time periods. (B) Cells were exposed to 25 µM DRB at 1–24 h (top), 36–60 h (middle), and 60–84 h (bottom) of culture. (C) Cells were exposed to 10 µg/ml CHX at 1–24 h (left) or 60–84 h (right) of culture before immunoblotting. (D) Real-time RT-PCR analysis of Oct4 expression in 3T3 cells exposed to ESC extract. Cells were cultured with 0 or 25 µM DRB at indicated time periods as described in B. Reference level (level 1) is Oct4 mRNA level in 3T3 cells immediately upon plating cells after recovery from extract. Data show a representative set from two experiments.

View larger version (26K): [in this window] [in a new window]   Figure 12. Immunodepletion of Oct4 from ESC extract maintains Oct4 expression profile. (A) Immunoprecipitation of Oct4 from ESC extract. Numbers indicate rounds of immunoprecipitation (IP). Resulting extracts were immunoblotted using anti-Oct4 antibodies. (B) 3T3 cells were exposed to immunodepleted ESC extract, cultured for indicated time periods, and blotted using anti-Oct4 antibodies. (C) Relative intracellular Oct4 protein level in 3T3 cells exposed to control (no antibody), mock-depleted (IgG), or anti-Oct4-depleted ESC extract and cultured for indicated time periods. *p < 0.001 relative to the 1-h time point in other treatments (ANOVA). Data from two experiments. (D) Real-time RT-PCR analysis of Oct4 expression in 3T3 cells exposed to intact, mock-depleted, or Oct4-depleted ESC extract. Reference level (level 1) is Oct4 transcript level in 3T3 cells immediately upon plating cells after recovery from extract. Data show a representative set of two experiments.

DRB exposure from 36 to 60 h after extract treatment, during the dip in Oct4 mRNA and protein levels, did not affect early Oct4 transcript and protein levels and only slightly delayed the second rise of Oct4 (Figure 11B, middle). This delay was also noticed at the transcript level, but it did not significantly affect Oct4 mRNA or protein level at 7 d (168 h; Figure 11D). Moreover, DRB applied from 55 to 84 h after extract treatment completely abrogated Oct4 transcription and translation by 72 h, but removal of the drug ultimately restored control Oct4 levels (Figure 11B, bottom, and D). Thus, there is a requirement for PolII activity for de novo transcription of Oct4 3 d after extract treatment. Last, both Oct4 elevations were abolished upon incubation with 10 µg/ml CHX, a protein synthesis inhibitor (Figure 11C). We concluded that whereas uptake of limited amounts of short-lived extract-derived Oct4 protein probably occurs, the two phases of Oct4 induction result from transcriptional and translational activity in extract-treated cells.

In summary, this study provides transcriptional and functional evidence that an extract of undifferentiated EC or ES cells can induce markers of dedifferentiation and signs of differentiation plasticity in an otherwise more developmentally restricted cell type. Furthermore, the NCCIT extract induces DNA demethylation of OCT4, indicative of an epigenetic reprogramming event at this locus.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
Target Cell Type-specific Programming of Gene Expression
The induction of a transcriptional profile of, according to our data, uncertain stability, suggests the establishment of a program of gene expression, as opposed to a complete functional reprogramming such as that occurring after nuclear transfer. Of the hundreds of NCCIT-specific genes up- or down-regulated on any given week after NCCIT extract treatment, only 5–7% are altered by Jurkat extract, and most of these genes overlap with genes altered (statistically by chance) in cells exposed to their own extract. This argues for some cell-type specificity in the nature of genes modulated by extract treatment. Despite some stability in the expression profile of specific genes after extract exposure, not all changes seem to be heritable. Genes with unstable expression pattern may include "passive bystanders" that generate a transcriptional noise (Paulsson, 2004) and result from more specific alterations in the transcriptional network. Perturbation in the network, however, would be expected to lead to changes trickling down the network until a transcriptional equilibrium is reached (Miklos and Maleszka, 2004). Fluctuations in the gene expression profile therefore may result from incomplete reprogramming and from heterogeneity in the transcriptional response to extract treatment. Nevertheless, the dynamics of gene expression may also illustrate a temporal compartmentalization, in terms of timing, duration, and periodicity of gene activity required to establish a heritable transcriptional network (Klevecz et al., 2004).

Evidence of Induction of Potential for Pluripotency
The gene expression program elicited by NCCIT extract suggests the establishment of a potential for multilineage differentiation in otherwise more developmentally restricted cells. An indicator of dedifferentiation is the down-regulation of genes indicative of a differentiated state. This is exemplified by the down-regulation of many 293T cell genes, including the kidney natriuretic peptide receptor C (NPR3), and the repression of lamin A (LMNA), a marker of differentiated cells (Hutchison and Worman, 2004). LMNA down-regulation seems specific for extracts of undifferentiated cells that do not express lamin A/C. In contrast, cardiomyocyte extracts can up-regulate LMNA expression in adipose stem cells, an event that correlates with differentiation toward a cardiomyocyte phenotype (Gaustad et al., 2004), and LMNA is reactivated upon retinoic acid-mediated differentiation of NCCIT extract-treated cells. Thus, the transcriptional status of LMNA provides a direct assessment of (de)differentiation transitions mediated by cell extracts. The mechanism of gene inactivation in extracts is unclear. However, evidence for the down-regulation of many genes by single small interfering RNAs (Mathieu and Bender, 2004), possibly through a control of DNA methylation (Matzke and Birchler, 2005), raises the hypothesis of a contribution of small RNAs in extract-based nuclear (re)programming.

An indicator of pluripotency is the up-regulation of genes characteristic of undifferentiated EC (NCCIT) or ES cells. Several embryonic, germ cell, and stem cell genes are activated to levels similar to those of NCCIT cells. Of note, Oct4 is expressed in ESCs to maintain pluripotency and acts in cooperation with SOX2 (Avilion et al., 2003). The latter is also expressed in extract-treated cells. The Oct4 transcription factor acts on a subset of target genes, including UTF1, REX1, OCT2, and NANOG (see Hoffman and Carpenter, 2005 for an updated review of human embryonic and ESC genes). These were found to be up-regulated by NCCIT extract. Furthermore, because UTF1 expression requires synergistic activities of Oct4 and Sox2 (Nishimoto et al., 1999), our results suggest the formation of a functional transcriptional complex between these factors.

Another feature of NCCIT extract-treated cells is the expression of genes suggestive of a potential for multiple lineage differentiation and acquisition of neurogenic, adipogenic, osteogenic, and endothelial differentiation ability. Differentiation potential toward other lineages was not investigated. Multilineage priming is a hallmark of hematopoietic stem cells (Akashi et al., 2003) and mesenchymal stem cells from bone marrow (Woodbury et al., 2002) and adipose tissue (Boquest et al., 2005). It may reflect their ability to promptly differentiate into a specific cell type in the tissue in which they reside, in response to simulation. Thus, similarly to somatic stem cells, the transcriptional signature of NCCIT extract-treated cells extends across germ layer boundaries. Additionally, because they also express embryonic and ESC markers, these cells display characteristics of a perhaps more precursor cell than the starting epithelial cell type.

Chromatin Remodeling Associated with Nuclear Reprogramming
The NCCIT extract retains the ability to elicit epigenetic reprogramming of OCT4 in 293T cells. Our data illustrate the demethylation of six of eight cytosines in CpG dinucleotides between CR2 and CR3 in the OCT4 promoter (Nordhoff et al., 2001). Oct4 DNA demethylation in thymocyte nuclei has been reported after fusion with EG or ES cells (Tada et al., 1997, 2001) and is required for Oct4 transcription after nuclear transplantation into Xenopus oocytes (Simonsson and Gurdon, 2004). The process driving OCT4 DNA demethylation remains unclear but seems to require deproteinization (Simonsson and Gurdon, 2004), and it may involve cleavage of methyl groups (Ramchandani et al., 1999) or cytosine deamination (Morgan et al., 2004). The ability to induce DNA demethylation in bulk cells or nuclei incubated in extracts raises the possibility of isolating the DNA demethylation activity involved.

A transient induction of Oct4 transcription and translation—independent of uptake of residual Oct4 protein from the extract—is triggered within the first hours after extract exposure. This early Oct4 up-regulation may be explained by nuclear uptake of extract-derived transcription factors and chromatin remodelers that target the Oct4 promoter (Nordhoff et al., 2001). This possibility is supported by the inhibitory effect of removing proteins from the extract (Figure 10) and of immunodepleting BRG1 from mouse ESC extracts on Oct4 transcription in 3T3 cells (our unpublished data). The transient (24- to 48-h) nature of this first wave of Oct4 activation presumably results from depletion of factors (most transcription factors have a half-life of hours). This suggests that transcription factor synthesis and targeting are not optimally sustained during the first hours after extract treatment. The second wave of Oct4 up-regulation, however, is sustained for several days and weeks. Long-term Oct4 expression is consistent with DNA demethylation taking place in our system. Timing of long-term Oct4 activation by ESC extract is consistent with the time interval observed between introduction of nuclei into oocytes and Oct4 demethylation in Xenopus oocytes (Simonsson a