Specific and Redundant Functions of Retinoid X Receptor/Retinoic Acid Receptor Heterodimers in Differentiation, Proliferation, and Apoptosis of F9 Embryonal Carcinoma Cells

Hideki Chiba, John Clifford, Daniel Metzger, and Pierre Chambon

Institut de Génétique et de Biologie Moléculaire et Cellulaire, Centre National de la Recherche Scientifique/Institut National de la Santé et de la Recherche Médicale/Université Louis Pasteur, Collège de France, 67404 Illkirch-Cedex, France

Abstract

Materials and Methods

Results

Discussion

Footnotes

Acknowledgements

Abbreviations used in this paper

References

Abstract

RETINOIDS exert their pleiotropic effects on vertebrate development, cellular differentiation, proliferation, and homeostasis through two classes of ligand-dependent transactivators: the retinoic acid receptors (RARs)¹, and the retinoid X receptors (RXRs) (for reviews see De Luca, 1991; Blomhoff, 1994; Chambon, 1994, 1996; Kastner et al., 1995; Mangelsdorf and Evans, 1995). RARs are activated by all-trans retinoic acid (tRA) and by 9-cis retinoic acid (9C-RA), whereas RXRs are activated by 9C-RA only. The various RAR (RAR, , and ) and RXR (RXR, , and ) isotypes are encoded by different genes, and their isoforms, which differ in their NH₂-terminal regions, are generated by differential promoter usage and alternative splicing. The multiple RAR and RXR isotypes and isoforms are conserved in vertebrate evolution, and display distinct spatiotemporal expression patterns in developing embryos and adult tissues, suggesting that each receptor performs some unique functions (for reviews see Leid et al., 1992a; Chambon, 1994; Kastner et al., 1994). RXR/RAR heterodimers bind much more efficiently to retinoic acid response elements (RAREs) than their respective homodimers in vitro (for reviews see Leid et al., 1992a; Chambon, 1994, 1996; Giguère, 1994; Glass, 1994; Kastner et al., 1994; Mangelsdorf et al., 1994; Gronemeyer and Laudet, 1995; Keaveney and Stunnenberg, 1995; Mangelsdorf and Evans, 1995), and several lines of evidence support the idea that these heterodimers represent the functional units transducing the retinoid signal in vivo (Kastner et al., 1995; Chambon, 1996).

The F9 murine embryonal carcinoma (EC) cell line expresses all types of RARs and RXRs (Zelent et al., 1989; Martin et al., 1990; Wan et al., 1994), and upon retinoic acid treatment, it differentiates into cells resembling three distinct extraembryonic endoderm (primitive, parietal, and visceral), depending on the culture conditions (for reviews see Strickland, 1981; Hogan et al., 1983; Gudas et al., 1994). Retinoid-induced differentiation is accompanied by an apoptotic response and a dramatic decrease in the rate of proliferation (Sleigh, 1992; Atencia et al., 1994; Clifford et al., 1996). Thus, F9 cells provide an attractive system for the analysis of retinoid signaling in vivo.

To further investigate the roles of RXRs and RARs in differentiation, proliferation, and apoptosis, we have now generated F9 cells lacking either RXR and RAR, or RXR and RAR, and then compared their phenotypes with those of wild-type (WT), RXR^/ (Clifford et al., 1996), RAR^/ (Boylan et al., 1995), and RAR^/ (Boylan et al., 1993; Taneja et al., 1995) F9 cells. Multiple gene targeting in a given cell has been achieved by using a Cre/loxP system (Sauer and Henderson, 1990; Metzger et al., 1995), which allows removal of the antibiotic resistance gene from a targeted locus, and therefore subsequent mutagenesis of the second allele of a given gene with the same targeting construct, as well as the targeting of additional genes. We demonstrate that tRA-treated RXR^// RAR^/ cells differentiate poorly into primitive and parietal endodermlike cells and are impaired in both antiproliferative and apoptotic responses, whereas they fully differentiate into visceral endoderm (VE)-like cells, as previously observed for RXR^/ cells (Clifford et al., 1996). In contrast, RXR^//RAR^/ cells are defective for all three types of endodermal differentiation, as well as for the antiproliferative and apoptotic responses, indicating that the absence of both RXR and RAR cannot be functionally compensated by the other retinoid receptors in these cells. Taken together with results obtained by treatment of WT and mutant F9 cells with panRXR- and RAR isotype- selective retinoids, our findings support the conclusion that RXR/RAR heterodimers are the functional units mediating the retinoid signal in vivo. Furthermore our results indicate that RXR/RAR heterodimers can exert both specific and redundant functions in differentiation, proliferation, and apoptosis. We also show that functional redundancy between RXR isotypes and between RAR isotypes can be artifactually generated by gene knockouts.

Materials and Methods

Cell Culture

F9 cells were cultured and induced to differentiate into primitive, parietal, and visceral endodermlike cells as previously described (Boylan et al., 1993; Clifford et al., 1996). The retinoids (tRA, Am80, BMS753, BMS453, BMS961, and BMS649) were dissolved in ethanol.

Targeting of the RAR or RAR Genes in RXR-null F9 Cells.

The RAR targeting vector, pRAR^(LNL), was previously described (Metzger et al., 1995). The RAR targeting vector, pRAR^(LPL), was derived from pD6.5A (a gift from D. Lohnes, IGBMC, CNRS/INSERM/ ULP, Illkirch, France), which contains a 6-kb genomic fragment including exons 5 and 8. A unique SmaI site, followed by stop codons in all three reading frames, was introduced into pD6.5A at the KpnI site located in exon 8 of RAR by inserting the oligonucleotides 5-CCCCGGGTAGGTAGATAGCGTAC-3 and 5-GCTATCTACCTACCCGGGGGTAC-3, yielding the pRART4 construct. An XhoI-BamHI fragment containing the phosphoglycerate kinase (PGK) promoter-driven, puromycin- resistance (puro) gene, flanked by loxP sites, was isolated from pHRLpuro1, and blunt ended with T4 DNA polymerase, followed by ligation into the SmaI site of pRART4. pHRLpuro1 was constructed from VS-1, a plasmid containing a loxP site-flanked PGKpuroA+ cassette, by mutating the SalI site. The PGKpuroA+ cassette was obtained by ligating the PGK promoter (a 500-bp EcoRI-PstI fragment isolated from pKJ-1 [Adra et al., 1987]) to the coding sequence of the puro gene (a 600-bp HindIII-ClaI fragment isolated from pLXPB [Imler et al., 1996]), and by inserting the SV-40 polyadenylation signal (a 160-bp BglII-XbaI fragment isolated from pSG5 [Green et al., 1988]) using synthetic oligonucleotides. This cloning resulted in the loss of the PstI, HindIII, and ClaI restriction sites, and the introduction of HindIII and EcoRI sites at 5 and 3 ends of the PGK promoter, respectively. KpnI, ApaI, XhoI, and BglII restriction sites, and BamHI and SacI sites are located at 5 and 3 ends of the loxP-flanked cassette, respectively.

Electroporation, selection of neomycin-resistant clones, Cre-mediated excision of the resistance genes, and Southern blot analysis were performed as previously described (Metzger et al., 1995; Clifford et al., 1996). Puromycin selection (500 ng/ml) was carried out for 10 d.

Western Blotting and Electrophoretic Mobility Shift Assays (EMSA)

Isolation of whole cell extracts from F9 cells and transfected COS-1 cells, Western blot analysis, and electrophoretic mobility shift assays were performed as previously described (Rochette-Egly et al., 1991; Gaub et al., 1992; Boylan et al., 1993).

Reverse Transcription (RT)-PCR

RNA preparation, RT-PCR, and Southern blotting were performed as previously described (Bouillet et al., 1995; Roy et al., 1995). The PCR primers and end-labeled oligonucleotide probes for collagen IV1, laminin B1, -fetoprotein (AFP), and 36B4, were described previously (Clifford et al., 1996). Transcript levels were quantified using a BAS 2000 bio-imaging analyzer (Fuji Ltd., Tokyo, Japan), and were normalized to the corresponding 36B4 signals.

Analysis of Cellular Growth

Cells were plated in triplicate 3-cm culture wells (5 x 10² cells per well), and cell counting experiments were performed as previously described (Clifford et al., 1996). [³H]Thymidine incorporation assays were performed essentially as described (Clifford et al., 1996), with the following modifications. Cells were cultured for 4 d in six replicate 3-cm wells, in the presence or absence of 1 µM tRA, and three of the six wells were treated with 8 µCi/well [³H]methylthymidine (20.0 Ci/mmol; Dupont-NEN, Boston, MA) for 2 h before harvesting. The cell cycle profile of WT and mutant cells was determined as previously described (Clifford et al., 1996).

Analysis of Apoptosis

Apoptosis was analyzed by both Hoechst staining of nuclei and FACS^® analysis, as previously described (Clifford et al., 1996).

Results

Targeted Disruption of the RAR or RAR Genes in RXR Null F9 Cells

The RXR-null F9 cell line, C2RXR^(L)/(L), which constitutively expresses a ligand-dependent chimeric Cre- recombinase (Cre-ER; Metzger et al., 1995), was electroporated with the targeting constructs pRAR^(LNL) (Fig. 1 A) or pRAR^(LPL) (Fig. 2 A) to generate F9 cells disrupted in either the RXR and RAR genes or the RXR and RAR genes. These targeting constructs contain translation stop codons and loxP site-flanked neomycin- or puromycin-resistance genes in exon 9 of the RAR gene or exon 8 of the RAR gene, respectively (Materials and Methods; Figs. 1 A and 2 A). Since these exons encode the B region, which is common to all isoforms of a given RAR isotype (Kastner et al., 1990; Leroy et al., 1991; Chambon, 1994), the expression of RAR and RAR proteins is suppressed by these mutations. Homologous recombination (HR) and Cre-mediated excision of the resistance genes were verified by Southern blotting (Figs. 1 B and 2 B).

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Southern blot analysis using a 3 probe (a1) located outside of the pRAR^(LNL) targeting construct (Fig. 1 A), indicated that, after electroporation, 5 out of 96 C2RXR^(L)/(L) neomycin-resistant clones had one targeted RAR allele (Fig. 1, A, HR[I]; and B, compare lanes 3 and 4 with lanes 1 and 2; data not shown). One RXR^(L)/(L)/RAR^+/(LNL) cell line (clone nine) was treated with estradiol (E₂) to excise the loxP-flanked cassette ([LNL] and [L]) designate the targeted allele before and after Cre-mediated excision, respectively). Southern blot analysis using a1 and "neo" probes revealed that excision of the resistance gene occurred in two out of six subclones treated with E₂ (Fig. 1, A, HR[E]; and B, compare lane 5 with lanes 3 and 4; see also Metzger et al., 1995). The second allele of the RAR gene was targeted in the RXR^(L)/(L)/RAR^+/(L) cell line (Fig. 1 B, clone 9a) using the same targeting construct and strategy. 2 of 96 neomycin-resistant clones were positive for the desired recombination event, resulting in RXR^(L)/(L)/RAR^(L)/(LNL) cell lines (Fig. 1 B, clones 9a-10 and 9a-26; compare lanes 6 and 7 with lanes 1-5). No wild-type RAR transcripts were detected in RXR^(L)/(L)/ RAR^(L)/(LNL) cells by semi-quantitative RT-PCRs (data not shown). Similarly, no RAR protein could be detected in RXR^(L)/(L)/RAR^(L)/(LNL) cells (called hereafter RXR^//RAR^/) by Western blotting using the polyclonal antibody RP(F) (Gaub et al., 1992), directed against the F region of the RAR protein (Fig. 1 C, compare lanes 3 and 4 with lanes 2, 5, and 6).

To establish F9 cells in which both RXR and RAR genes are inactivated, C2RXR^(L)/(L) cells were electroporated with the pRAR^(LPL) targeting construct (Fig. 2 A). Southern blot analysis using the r1 probe, located 5 to the pRAR^(LPL) sequence (Fig. 2 A), revealed that 3 out of 96 puromycin-resistant clones had one targeted RAR allele (Fig. 2, A, HR[I]; and B, compare lanes 3 and 4 with lanes 1 and 2; and data not shown). One RXR^(L)/(L)/ RAR^+/(LPL) cell line (clone 25) was transiently transfected with a Cre recombinase expression construct (pPGK-Cre) (Clifford et al., 1996), since for unknown reasons the loxP-flanked, puromycin-resistance gene was not excised by treatment of the cells with E₂ (data not shown; [LPL] and [L] designate the targeted allele before and after Cre-mediated excision, respectively). The pattern obtained by Southern blot analysis, using r1, r2, and puro probes, clearly indicated that 2 out of 96 subclones had lost the puromycin-resistance cassette (Fig. 2, A, HR[E]; and B, compare lane 5 with lane 4; and data not shown). The second allele of the RAR gene was inactivated in one RXR^(L)/(L)/RAR^+/(L) cell line (Fig. 2 B, clone 25a) using the same targeting construct and strategy, yielding a RXR^(L)/(L)/RAR^(L)/(LPL) cell line (Fig. 2 B, clone 25a-3, compare lane 6 with lanes 1-5). No wild-type RAR RNA was detected in RXR^(L)/(L)/RAR^(L)/(LPL) cells (data not shown). The absence of RAR protein was verified by EMSA using the monoclonal antibodies Ab2(F) and Ab10(A2) (Rochette-Egly et al., 1991), directed against the F and A2 regions of the RAR protein, respectively. No antibody-shifted complex was observed in RXR^(L)/(L)/ RAR^(L)/(LPL) cells (called hereafter RXR^//RAR^/) (Fig. 2 C, compare lane 5 with lanes 4, 6, and 10). Note that the RXR loci, which contain loxP sites, were not rearranged during excision of the resistance genes at the RAR or RAR loci (data not shown). Note also that the knockout of a given receptor(s) did not result in major variations of those remaining (Chiba et al., 1997).

Function of RXRs and RARs in the Retinoid-induced Differentiation of F9 Cells into Primitive and Parietal Endodermlike Cells

WT F9 cells differentiate into primitive and parietal endodermlike cells, when grown in monolayer culture in the presence of tRA alone and tRA in combination with dibutyryl cAMP (bt₂cAMP), respectively (Strickland, 1981; Hogan et al., 1983). Previous studies have shown that these two types of differentiation are severely impaired in RAR^/ and RXR^/ cells (Boylan et al., 1993; Clifford et al., 1996). The differentiation patterns of RXR^// RAR^/ and RXR^//RAR^/ cells were compared with those of WT and single knockout cell lines. After 4 d of treatment, <10% of RXR^//RAR^/ cells became flatter and irregular in shape, or rounded with long cell processes, which are the morphological characteristics of primitive or parietal endodermal differentiation of WT F9 cells, respectively. The same results were obtained with 9a-10 and 9a-26 RXR^//RAR^/ cell lines (Fig. 3 A, compare g-i with a-f, and data not shown). No morphological differentiation at all was observed in RXR^// RAR^/ cells after 4 d of treatment, and <0.1% of the cells exhibited a differentiated morphology after 6 d of treatment (Fig. 3 A, j-l, and data not shown). This undifferentiated phenotype persisted after 10 d of treatment.

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The extent of differentiation of WT and mutant F9 cells was further investigated biochemically by determining the mRNA levels of collagen type IV1 and laminin B1, two markers of endodermal differentiation (Fig. 3, B and C). After 48 h of 1 µM tRA treatment, the induction of collagen type IV1 and laminin B1 was reduced in RAR^/ cells (10-fold and 6-fold lower levels, respectively) and in RXR^/ cells (3-fold and 6-fold lower levels, respectively) when compared to WT cells, whereas these inductions were not altered in RAR^/ cells (Boylan et al., 1993, 1995; Clifford et al., 1996; Taneja et al., 1996). The induction of both transcripts was also impaired in RXR^// RAR^/ cells (fivefold and sevenfold lower levels, respectively), whereas it was fully abrogated in RXR^// RAR^/ cells. Thus, RA-induced primitive and parietal endodermal differentiation of WT F9 cells appears to be mainly mediated by the RXR/RAR pair, whereas it cannot be mediated by combinations of RXR(+) with either RAR or RAR (see Table IV).

Table IV. Summary of the Involvement of the Various RARs and RXRs in the Transduction of the Retinoid Signal in F9 Cells, as Deduced from the Present and Previous Studies of RAR and RXR Mutant Cells and the Use of Receptor-specific Retinoids [View Table]

Function of RXRs and RARs in the Retinoid-induced Differentiation of F9 Cells into VE-like Cells

When F9 cells are grown in suspension as aggregates, low levels of tRA induce a VE phenotype in the outermost layer of cells, which display an irregular surface (Strickland, 1981; Hogan et al., 1983; see also Fig. 4 A, WT, a and b, brackets). We have previously shown that, in contrast to primitive and parietal endodermal differentiation, VE differentiation can be induced by tRA in RXR^/ F9 cells (Clifford et al., 1996). Similarly, after 10 d of treatment, >80% of the outer layer of RAR^/ and RXR^// RAR^/ cell aggregates differentiated into VE-like cells, which were indistinguishable from those of WT and RXR^/ cells (Fig. 4 A, compare f with panels b and d; and Table I). In contrast, <10% of the outer layer of RAR^/ cells exhibited VE conversion after 10 d of treatment, whereas full VE differentiation was eventually achieved after 14 d of treatment (Fig. 4 A, compare h with g and b; and Table I). In RXR^//RAR^/ cells, the surface of the aggregates was as smooth after 10 d of treatment as in untreated controls (Fig. 4 A, compare j with a and i), and <10% of the aggregates displayed only a spotty VE conversion after 12 or 18 d of treatment (Table I). To exclude the possibility that this very poor differentiation of the RXR^//RAR^/ cells could be due to some clonal variation, rather than the presence of the RXR-null mutation in the RAR-null background, we expressed the RXR cDNA in RXR^//RAR^/ cells. As expected, cells expressing the RXR cDNA exhibited a phenotype identical to that of RAR^/ cells, i.e., the RA- induced VE differentiation was restored at late time (14 d) of RA treatment (data not shown).

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Table I. Effect of Various Retinoids on Morphological Differentiation of WT and Mutant F9 Cells into Visceral Endoderm (VE)-like Cells [View Table]

We also analyzed the mRNA levels of collagen type IV1, laminin B1 and AFP in WT and mutant F9 cells (Fig. 4 B). After 10 d of aggregate culture in the presence of 50 nM tRA, the three markers were similarly induced in WT, RXR^/, RAR^/, and RXR^//RAR^/cells. In RAR^/ cells, the induction of laminin B1 was similar to that of WT cells, whereas the induction of collagen IV1 was slightly reduced (twofold lower than in WT cells). In contrast, the induction of AFP, a specific marker of VE differentiation, was hardly detectable in RAR^/ cells after 10 d of RA treatment (Fig. 4 B). There was no induction of either collagen IV1, laminin B1 or AFP in RXR^// RAR^/ cells, in agreement with their lack of morphological differentiation into VE. Thus, RXR and RAR play an essential role in VE differentiation of WT F9 cells.

To further investigate the functions of RARs and RXRs in VE differentiation, WT and mutant F9 cells were treated for 10-18 d with tRA or synthetic retinoid agonist selective for RAR (BMS188,753 [BMS753]; Taneja et al., 1996), RAR (BMS189,453 [BMS453]; Chen et al., 1995), RAR (BMS188,961 [BMS961]; Taneja et al., 1996) and all three RXRs (panRXR, BMS188,649 [BMS649]; also known as SR11237; Lehmann et al., 1992; see Roy et al., 1995) (Table I). In WT cells, VE differentiation was triggered by 100 nM of the RAR agonist as effectively as by 50 nM tRA, and it was synergistically induced by a combination of 10 nM RAR and 1 µM panRXR agonists (Table I). In contrast, VE differentiation of RAR^/ cells was triggered by 10 nM RAR agonist as efficiently as by 50 nM tRA, and it was even synergistically induced by 1 nM of the RAR agonist in combination with 1 µM panRXR agonist, indicating that RAR partially hinders the RAR function in WT cells. The RAR agonist alone, or together with the panRXR agonist, was more efficient in RXR^/ and RXR^//RAR^/ cells than in WT cells, but weaker than in RAR^/ cells, demonstrating that RXR prevents an efficient synergism between RXR(+) and RAR (Table I; see Table IV). As expected, no VE differentiation was observed in RAR^/ and RXR^// RAR^/ cells treated with the RAR/panRXR agonist combination.

The RAR agonist, BMS753, on its own did not trigger VE differentiation of WT and RXR^/ cells, whereas the combination of 100 nM RAR and 1 µM panRXR agonists was much less efficient than the RAR/panRXR agonist combination. As expected, no VE differentiation was seen in RAR^/ and RXR^//RAR^/ cells upon treatment with the RAR/panRXR agonist combination. In contrast, RAR^/ cells weakly differentiated into VE-like cells upon treatment with 100 nM RAR agonist alone, and this differentiation was markedly enhanced by addition of 1 µM panRXR agonist. This synergistic stimulation was almost abrogated in RXR^//RAR^/ cells.

A combination of RAR (BMS453) and panRXR agonists did not trigger VE differentiation in WT, RXR^/, RAR^/, and RXR^//RAR^/ cells. Interestingly, this combination synergistically induced VE differentiation of RAR^/ cells, and this effect was almost absent in RXR^//RAR^/ cells, as in the case of the RAR/panRXR agonist combination (Table I). Thus, RAR strongly prevents RAR and RAR to synergize with RXR, and mutation of RAR artefactually generates functional redundancy between RARs for VE differentiation of F9 cells (see Table IV).

Function of RXRs and RARs in the Retinoid-induced Antiproliferative Response of F9 Cells and Retinoid-induced Proliferation of RXR/RAR Null F9 Cells

The effect of tRA on proliferation of WT, RXR^/, RXR^//RAR^/, and RXR^//RAR^/ F9 cells was investigated (Fig. 5 A). After 6 d of 1 µM tRA treatment, the inhibition of growth as determined by cell counting, was lower for RXR^/ than for WT cells (58 and 79% inhibition relative to untreated control cells, respectively) (Clifford et al., 1996). The antiproliferative effect of tRA was decreased to the same extent for RXR^//RAR^/ cells (54% inhibition) and RXR^/ cells. On the other hand, tRA did not reduce, but slightly increased the number of RXR^//RAR^/ cells. The rate of DNA synthesis was also compared for WT and mutant cell lines by measuring [³H]thymidine incorporation during the anti-proliferative response to tRA (Fig. 5 B). After 4 d of 1 µM tRA treatment, [³H]thymidine incorporation was reduced by 54% in WT cells relative to vehicle-treated control cells, and only by 20% in RXR^/ and RXR^//RAR^/ cells. In contrast, there was no inhibition of [³H]thymidine incorporation in RXR^//RAR^/ cells.

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FACS^® analysis has previously shown that tRA treatment of WT F9 cells results in an accumulation of cells in the G0 and G1 phases of the cell cycle, and that this accumulation was decreased in RXR^/ cells (Clifford et al., 1996) (Fig. 5 C). The cell cycle profile of untreated RXR^//RAR^/ and RXR^//RAR^/ cells was the same as that of WT and RXR^/ cells. After 5 d of 1 µM tRA treatment, the proportion of cells in the G0 and G1 phases, was 71% for WT cells, whereas it was lower for RXR^/ and RXR^//RAR^/ cells (38 and 40%, respectively; Fig. 5 C, compare b, d, and f). Interestingly, the cell cycle profile of RXR^//RAR^/ cells was not significantly affected by tRA treatment, and was almost identical to that of untreated WT cells (Fig. 5 C, compare h with g and a).

To further dissect the roles of RARs and RXRs in the control of proliferation, WT and mutant F9 cells were treated for 6 d with tRA or receptor-selective retinoids, and cell numbers were counted (Table II). In WT cells, 100 nM RAR agonist efficiently reduced the proliferation, and 10 nM of the same agonist, which had no effect on its own, synergized with 1 µM panRXR agonist. The effect of these retinoids on proliferation was reduced, while not abolished, in RXR^/ and RXR^//RAR^/ cells, indicating that RXR can be partially replaced by RXR(+) for synergizing with RAR (see Table IV). Interestingly, the RAR/panRXR agonist combination was more efficient in RAR^/ cells than in WT cells, revealing that RAR partially hinders the antiproliferative effect of RAR in WT cells (Table II; see Table IV). As expected, this combination did not inhibit the proliferation of RAR^/ and RXR^//RAR^/ cells. The combination of 100 nM RAR and 1 µM panRXR agonists, which reduced the proliferation of WT cells less efficiently than the RAR/panRXR agonist combination, was more efficient in RAR^/ cells than in WT cells (Table II), indicating that RAR partially hinders the antiproliferative effect of RAR in WT cells (see Table IV). The RAR/panRXR agonist combination had no effect on the proliferation of RXR^/ cells, showing that RAR can only synergize with RXR to inhibit proliferation (see Table IV).

Table II. Effect of Various Retinoids on Proliferation of WT and Mutant F9 Cells [View Table]

Surprisingly, a treatment with 10 and 100 nM RAR agonist increased the number of RXR^//RAR^/ cells, indicating that RAR can mediate a proliferative effect in the absence of both RXR and RAR. As expected, the RAR/panRXR agonist combination did not affect proliferation of RAR^/ and RXR^//RAR^/ cells. Neither the RAR agonist alone nor in combination with the panRXR agonist affected the proliferation of WT, RXR^/, RAR^/, and RXR^//RAR^/ cells, whereas this combination synergistically reduced the proliferation of RAR^/ cells. Note that 500 nM RAR agonist alone increased the cell number of RXR^//RAR^/ cells, and that this effect was enhanced by addition of 1 µM panRXR agonist. Thus, the presence of RAR not only hinders the antiproliferative effect of the RXR/RAR pair, but also the proliferation-promoting effects of the combinations of RXR(+) with RAR, showing again that knockouts generate artifactual effects not observed under WT conditions, as already seen above in the case of F9 cell differentiation (see Table IV).

Function of RXRs and RARs for the Retinoid-induced Apoptotic Response of F9 Cells

Since retinoids can induce apoptosis, which also contributes to the decrease in cell number in retinoid-treated F9 cells (Atencia et al., 1994), we determined the extent of the tRA-induced apoptotic response of WT and mutant F9 cells by FACS^® analysis. Sub-2N-size, DNA-containing particles corresponding to "apoptotic bodies" appeared in WT cells after 5 d of tRA treatment, whereas they were not detected in tRA-treated RXR^/ (see also Clifford et al., 1996), RXR^//RAR^/, and RXR^//RAR^/ cells (Fig. 5 C, compare d, f, and h with b [arrow]). Apoptosis was confirmed by staining with the DNA-binding fluorochrome Hoechst 33258. Apoptotic particles and condensed chromatin were similarly observed in tRA-treated WT and RAR^/ cells (Fig. 6, a, b, e, and f; Table III). In contrast, tRA-induced apoptosis was reduced in RAR^/ cells, rarely seen in RXR^/ (Clifford et al., 1996) and RXR^//RAR^/ cells, and abolished in RXR^// RAR^/ cells (Fig. 6, c, d, and g-l; Table III). Note that a background level of apoptosis occurred at high cell density even in the absence of