Early retinoic acid-induced F9 teratocarcinoma stem cell gene ERA-1: alternate splicing creates transcripts for a homeobox-containing protein and one lacking the homeobox

A dominant negative mutation of the alpha retinoic acid receptor gene in a retinoic acid-nonresponsive embryonal carcinoma cell

Pluripotential embryonal carcinoma cells such as those of the P19 line differentiate when exposed to retinoic acid (RA). The RAC65 cell line is a mutant clone of P19 cells selected to be RA nonresponsive. RAC65 cells carry a rearrangement affecting one of the genes encoding a nuclear retinoic acid receptor (RAR alpha). The mutant gene encodes a protein, RAR alpha', that has lost its 70 C-terminal amino acids, thus truncating the RA-binding domain. The RAR alpha' was found to be a dominant repressor of transcription from an RA-responsive target gene; however, expression of RAR alpha' was insufficient to confer RA nonresponsiveness, suggesting that RAC65 cells carry an additional mutation(s) affecting RA-induced genes.

Retinoic acid promotes transcription of the platelet-derived growth factor alpha-receptor gene

Retinoic acid together with dibutyryl cyclic AMP stimulated transcription of the platelet-derived growth factor alpha-receptor gene in embryonal carcinoma cells (line F9). Processed mRNA transcripts appeared within 4 h after exposure to these agents, and functional alpha:alpha homodimers appeared within 24 h.

Characterization of a functional promoter for the human retinoic acid receptor-alpha (hRAR-alpha)

The three retinoic acid receptors RAR-alpha, beta and gamma are thought to mediate the effects of RA in vivo. We have determined here the exon organisation in the 5' region of the human RAR-alpha (hRAR-alpha) gene, and have identified its promoter. This promoter drives the expression of promoterless beta-globin or CAT reporter genes when transfected into HeLa, Cos-1 or mouse embryonal carcinoma (EC) P19.6 cells in culture. There are no TATA or CCAAT-box elements in this promoter, which appears to belong to the class of promoters made up of an initiator element preceded by several putative binding sites for the transcription factor Sp1. In addition, the hRAR-alpha promoter region contains a number of sequences that are similar to known enhancer elements. Notably, the hRAR-alpha promoter contains a sequence identical to a binding site for the Krox-20 transcription factors, a zinc finger-containing protein which is thought to play a role in the early development of the mouse central nervous system.

A dominant negative mutation of the alpha retinoic acid receptor gene in a retinoic acid-nonresponsive embryonal carcinoma cell

Pluripotential embryonal carcinoma cells such as those of the P19 line differentiate when exposed to retinoic acid (RA). The RAC65 cell line is a mutant clone of P19 cells selected to be RA nonresponsive. RAC65 cells carry a rearrangement affecting one of the genes encoding a nuclear retinoic acid receptor (RAR alpha). The mutant gene encodes a protein, RAR alpha', that has lost its 70 C-terminal amino acids, thus truncating the RA-binding domain. The RAR alpha' was found to be a dominant repressor of transcription from an RA-responsive target gene; however, expression of RAR alpha' was insufficient to confer RA nonresponsiveness, suggesting that RAC65 cells carry an additional mutation(s) affecting RA-induced genes.

Retinoic acid promotes transcription of the platelet-derived growth factor alpha-receptor gene

Retinoic acid together with dibutyryl cyclic AMP stimulated transcription of the platelet-derived growth factor alpha-receptor gene in embryonal carcinoma cells (line F9). Processed mRNA transcripts appeared within 4 h after exposure to these agents, and functional alpha:alpha homodimers appeared within 24 h.

Eukaryotic transcription factors

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Most of the homeobox-containing Xhox 36 transcripts in early Xenopus embryos cannot encode a homeodomain protein

Multiple Xhox 36 transcripts accumulate in Xenopus embryos from gastrula to early tadpole stages. The transcripts were characterized by sequencing cDNA clones and by S1 protection and Northern (RNA) blotting of embryonic RNA with probes derived from the cDNAs. The Xhox 36 RNAs included unspliced precursor transcripts that accumulated in the embryonic nuclei, spliced transcripts that contained multiple stop codons in frame with the homeobox, and less abundant coding mRNAs. These transcripts were generated either by alternative splicing or multiple initiations from a single Xhox 36 gene. The sequence of a cDNA clone of the unspliced transcript showed that the intron contained a noncanonical 3' splice site. However, the intron was spliced efficiently when expressed from a plasmid injected into Xenopus embryos, suggesting that the inefficient splicing of the endogenous RNA is not due to the unusual 3' splice site. The accumulation of noncoding and unspliced transcripts suggests multiple levels of regulation in the embryonic expression of the Xhox 36 gene.

Most of the homeobox-containing Xhox 36 transcripts in early Xenopus embryos cannot encode a homeodomain protein

Multiple Xhox 36 transcripts accumulate in Xenopus embryos from gastrula to early tadpole stages. The transcripts were characterized by sequencing cDNA clones and by S1 protection and Northern (RNA) blotting of embryonic RNA with probes derived from the cDNAs. The Xhox 36 RNAs included unspliced precursor transcripts that accumulated in the embryonic nuclei, spliced transcripts that contained multiple stop codons in frame with the homeobox, and less abundant coding mRNAs. These transcripts were generated either by alternative splicing or multiple initiations from a single Xhox 36 gene. The sequence of a cDNA clone of the unspliced transcript showed that the intron contained a noncanonical 3' splice site. However, the intron was spliced efficiently when expressed from a plasmid injected into Xenopus embryos, suggesting that the inefficient splicing of the endogenous RNA is not due to the unusual 3' splice site. The accumulation of noncoding and unspliced transcripts suggests multiple levels of regulation in the embryonic expression of the Xhox 36 gene.

Retinoic acid-induced granulocytic differentiation of HL-60 myeloid leukemia cells is mediated directly through the retinoic acid receptor (RAR-alpha)

Retinoic acid (RA) induces terminal granulocytic differentiation of the HL-60 promyelocytic leukemia cell line as well as certain other human myeloid leukemias. Specific RA receptors that are members of the steroid-thyroid hormone superfamily of nuclear transcription factors have recently been identified. We developed an HL-60 subclone that was relatively resistant to RA-induced differentiation. Specific nuclear RA receptors in this RA-resistant subclone had a decreased affinity for RA and exhibited a lower molecular weight compared with nuclear RA receptors from the RA-sensitive parental HL-60 cells. Retroviral vector-mediated transduction of a single copy of the RA receptor (RAR-alpha) into this RA-resistant HL-60 subclone restored the sensitivity of these cells to RA. These observations indicate that RAR-alpha plays a critical and central role in mediating RA-induced terminal differentiation of HL-60 leukemia cells.

Gene expression in visceral endoderm: a comparison of mutant and wild-type F9 embryonal carcinoma cell differentiation

We have examined the abundance and cell specificity of several mRNAs that are regulated during the retinoic acid (RA)-induced differentiation of F9 embryonal carcinoma cells to visceral endoderm. The experiments confirmed the multistep nature of this process by demonstrating the expression of the ERA-1/Hox 1.6 message within 6 h after RA addition; the expression of messages specific for the extracellular matrix proteins laminin B1 and B2, and collagen IV(alpha 1) between days 4 and 12; and the expression of two visceral endoderm markers, alpha-fetoprotein (AFP) and H19, by days 8-15. In situ hybridization experiments revealed that the collagen IV(alpha 1) mRNA is restricted to the outer cell layer of F9 cell aggregates regardless of the presence or absence of RA. Laminin B1 and B2 mRNAs are concentrated in the outer cell layer of RA-treated aggregates although significant levels of message are also observed within the interior cells of the aggregates. Unexpectedly, AFP mRNA is detectable in only a subset of the outer cells of F9 cell aggregates grown 15 d in the presence of RA. The results obtained from wild-type F9 cells were compared with those from a mutant F9 cell line, RA-5-1, which was previously shown to synthesize collagen IV containing six- to ninefold less 4-hydroxyproline than that in wild-type F9 cells. RA-5-1 cells exhibit four- to sixfold less of the mRNAs encoding two visceral endoderm proteins, AFP and H19, than wild-type F9 cells after RA treatment of RA-5-1 aggregates. RA-5-1 cells, however, do exhibit an RA-associated increase in the level of ERA-1/Hox 1.6 mRNA within 6 h after adding RA. Although the collagen IV protein level is similar in wild-type F9 and RA-5-1 aggregates, the collagen IV(alpha 1) message level is 6-20-fold greater in aggregates of mutant cells than in aggregates of wild-type cells. Moreover, in situ hybridizations showed that this message is evenly distributed throughout the RA-5-1 aggregates rather than restricted to the outer cell layers as it is in wild-type F9 aggregates. These results suggest that abnormal collagen IV expression and localization are associated with decreased expression of the visceral endoderm markers, AFP and H19, in RA-5-1 cell aggregates.

Retinoic acid-induced granulocytic differentiation of HL-60 myeloid leukemia cells is mediated directly through the retinoic acid receptor (RAR-alpha)

Retinoic acid (RA) induces terminal granulocytic differentiation of the HL-60 promyelocytic leukemia cell line as well as certain other human myeloid leukemias. Specific RA receptors that are members of the steroid-thyroid hormone superfamily of nuclear transcription factors have recently been identified. We developed an HL-60 subclone that was relatively resistant to RA-induced differentiation. Specific nuclear RA receptors in this RA-resistant subclone had a decreased affinity for RA and exhibited a lower molecular weight compared with nuclear RA receptors from the RA-sensitive parental HL-60 cells. Retroviral vector-mediated transduction of a single copy of the RA receptor (RAR-alpha) into this RA-resistant HL-60 subclone restored the sensitivity of these cells to RA. These observations indicate that RAR-alpha plays a critical and central role in mediating RA-induced terminal differentiation of HL-60 leukemia cells.

A retinoic acid responsive gene MK found in the teratocarcinoma system is expressed in spatially and temporally controlled manner during mouse embryogenesis

A newly identified gene MK is transiently expressed in early stages of retinoic acid-induced differentiation of embryonal carcinoma cells (Kadomatsu, K., M. Tomomura, and T. Muramatsu, 1988. Biochem. Biophys. Res. Commun. 151:1312-1318). MK gene has been predicted to code a polypeptide that is rich in basic amino acids and cysteine and is not related to any other peptides so far reported. In the present study, we investigated MK expression during mouse embryogenesis by in situ hybridization. The MK transcript was detected all over the embryo proper of the 7-d embryo, while it was not detectable in the 5-d embryo. The ubiquitous expression continued in the 9-d embryo proper. On the 11th-13th d of gestation, the sites where MK gene was intensely expressed became progressively restricted; these sites were the brain ectoderm around the lens and brain ventricles, the anterior lobe of the pituitary gland, the upper and lower jaw, the caudal sclerotomic half of vertebral column, the limbs, the stomach, and the epithelial tissues of the lung, the pancreas, the small intestine, and the metanephros. These areas include the region where secondary embryonic induction is prominent. In the 15-d embryo, only the kidney expressed MK significantly. These data suggest that MK gene plays a fundamental role in the differentiation of a wide variety of cells; MK gene may also play some specific roles in generation of epithelial tissues, and remodeling of mesoderm.

Rapid and transient decrease of N-myc expression in retinoic acid-induced differentiation of OTF9 teratocarcinoma stem cells

The level of expression of N-myc in mouse teratocarcinoma stem cells is very high. Previous studies have shown that N-myc expression significantly decreases when the stem cells are subjected to long-term induction for differentiation by retinoic acid (RA). We found that in a stem cell line, OTF9, a steep yet transient decrease of N-myc expression takes place much earlier, immediately after induction by RA. To examine whether this decrease is responsible for differentiation, we constructed a gene, miwNmyc, to express N-myc cDNA constitutively and transformed OTF9 cells with this gene construct. Transformants under the constitutive expression of miwNmyc differentiated normally, as judged by morphological changes and by modulation of c-myc, Hox1.1, and laminin B1 expression. Therefore, transient decrease of N-myc expression may be the consequence of RA-induced differentiation, even though it occurs very early in the process. Alternatively, in addition to N-myc decrease, there may be redundant mechanisms which lead to OTF9 differentiation after induction by RA, so that suppression of N-myc decrease is bypassed by at least one other mechanism.

Rapid and transient decrease of N-myc expression in retinoic acid-induced differentiation of OTF9 teratocarcinoma stem cells

The level of expression of N-myc in mouse teratocarcinoma stem cells is very high. Previous studies have shown that N-myc expression significantly decreases when the stem cells are subjected to long-term induction for differentiation by retinoic acid (RA). We found that in a stem cell line, OTF9, a steep yet transient decrease of N-myc expression takes place much earlier, immediately after induction by RA. To examine whether this decrease is responsible for differentiation, we constructed a gene, miwNmyc, to express N-myc cDNA constitutively and transformed OTF9 cells with this gene construct. Transformants under the constitutive expression of miwNmyc differentiated normally, as judged by morphological changes and by modulation of c-myc, Hox1.1, and laminin B1 expression. Therefore, transient decrease of N-myc expression may be the consequence of RA-induced differentiation, even though it occurs very early in the process. Alternatively, in addition to N-myc decrease, there may be redundant mechanisms which lead to OTF9 differentiation after induction by RA, so that suppression of N-myc decrease is bypassed by at least one other mechanism.

Cyclic AMP analogs and retinoic acid influence the expression of retinoic acid receptor alpha, beta, and gamma mRNAs in F9 teratocarcinoma cells

Retinoic acid (RA) receptor alpha (RAR alpha) and RAR gamma steady-state mRNA levels remained relatively constant over time after the addition of RA to F9 teratocarcinoma stem cells. In contrast, the steady-state RAR beta mRNA level started to increase within 12 h after the addition of RA and reached a 20-fold-higher level by 48 h. This RA-associated RAR beta mRNA increase was not prevented by protein synthesis inhibitors but was prevented by the addition of cyclic AMP analogs. In the presence of RA, cyclic AMP analogs also greatly reduced the RAR alpha and RAR gamma mRNA levels, even though cyclic AMP analogs alone did not alter these mRNA levels. The addition of either RA or RA plus cyclic AMP analogs did not result in changes in the three RAR mRNA half-lives. These results suggest that agents which elevate the internal cyclic AMP concentration may also affect the cellular response to RA by altering the expression of the RARs.

Expression of REX-1, a gene containing zinc finger motifs, is rapidly reduced by retinoic acid in F9 teratocarcinoma cells

In the presence of retinoic acid (RA), cultured F9 murine teratocarcinoma stem cells differentiate into nontumorigenic cells resembling the extraembryonic endoderm of the early mouse embryo. By differential hybridization screening of an F9 cell cDNA library, we isolated a 1,745-nucleotide cDNA for a gene, REX-1 (for reduced expression), whose steady-state mRNA level began to decline in F9 cells in monolayer culture within 12 h after the addition of RA. By 48 to 96 h after RA treatment of F9 cells in monolayer culture, the REX-1 steady-state mRNA level was more than sevenfold lower than the level in undifferentiated F9 stem cells. The REX-1 mRNA decrease did not result from the reduction in cell growth rate associated with the differentiation process, since the REX-1 mRNA level did not decline in F9 cells that were partially growth arrested after 48 h of isoleucine deprivation. The RA-associated REX-1 mRNA decrease resulted primarily from a reduction in the transcription rate of the REX-1 gene in the presence of RA. In contrast to results in F9 cells, we have been unable thus far to detect REX-1 mRNA in day 7.5 to 12.5 mouse embryo RNA samples or in the P19 teratocarcinoma stem cell line. The putative REX-1 protein identified by DNA sequence analysis contains four repeats of the zinc finger nucleic acid-binding motif and a potential acidic activator domain, suggesting that REX-1 encodes a regulatory protein. The REX-1 gene is not identical to the previously reported murine genes that encode zinc finger-containing proteins.

Cyclic AMP analogs and retinoic acid influence the expression of retinoic acid receptor alpha, beta, and gamma mRNAs in F9 teratocarcinoma cells

Retinoic acid (RA) receptor alpha (RAR alpha) and RAR gamma steady-state mRNA levels remained relatively constant over time after the addition of RA to F9 teratocarcinoma stem cells. In contrast, the steady-state RAR beta mRNA level started to increase within 12 h after the addition of RA and reached a 20-fold-higher level by 48 h. This RA-associated RAR beta mRNA increase was not prevented by protein synthesis inhibitors but was prevented by the addition of cyclic AMP analogs. In the presence of RA, cyclic AMP analogs also greatly reduced the RAR alpha and RAR gamma mRNA levels, even though cyclic AMP analogs alone did not alter these mRNA levels. The addition of either RA or RA plus cyclic AMP analogs did not result in changes in the three RAR mRNA half-lives. These results suggest that agents which elevate the internal cyclic AMP concentration may also affect the cellular response to RA by altering the expression of the RARs.

The human HOX gene family

We report the identification of 10 new human homeobox sequences. Altogether, we have isolated and sequenced 30 human homeoboxes clustered in 4 chromosomal regions called HOX loci. HOX1 includes 8 homeoboxes in 90 kb of DNA on chromosome 7. HOX2 includes 9 homeoboxes in 180 kb on chromosome 17. HOX3 contains at least 7 homeoboxes in 160 kb on chromosome 12. Finally, HOX4 includes 6 homeoboxes in 70 kb on chromosome 2. Homeodomains obtained from the conceptual translation of the isolated homeoboxes can be attributed to 13 homology groups on the basis of their primary peptide sequence. Moreover, it is possible to align the 4 HOX loci so that corresponding homeodomains in all loci share the maximal sequence identity. The complex of these observations supports and extends an evolutionary hypothesis concerning the origin of mammalian and fly homeobox gene complexes. We also determined the coding region present in 3 HOX2 cDNA clones corresponding to HOX2G, HOX2H and HOX2I.

Expression of REX-1, a gene containing zinc finger motifs, is rapidly reduced by retinoic acid in F9 teratocarcinoma cells

In the presence of retinoic acid (RA), cultured F9 murine teratocarcinoma stem cells differentiate into nontumorigenic cells resembling the extraembryonic endoderm of the early mouse embryo. By differential hybridization screening of an F9 cell cDNA library, we isolated a 1,745-nucleotide cDNA for a gene, REX-1 (for reduced expression), whose steady-state mRNA level began to decline in F9 cells in monolayer culture within 12 h after the addition of RA. By 48 to 96 h after RA treatment of F9 cells in monolayer culture, the REX-1 steady-state mRNA level was more than sevenfold lower than the level in undifferentiated F9 stem cells. The REX-1 mRNA decrease did not result from the reduction in cell growth rate associated with the differentiation process, since the REX-1 mRNA level did not decline in F9 cells that were partially growth arrested after 48 h of isoleucine deprivation. The RA-associated REX-1 mRNA decrease resulted primarily from a reduction in the transcription rate of the REX-1 gene in the presence of RA. In contrast to results in F9 cells, we have been unable thus far to detect REX-1 mRNA in day 7.5 to 12.5 mouse embryo RNA samples or in the P19 teratocarcinoma stem cell line. The putative REX-1 protein identified by DNA sequence analysis contains four repeats of the zinc finger nucleic acid-binding motif and a potential acidic activator domain, suggesting that REX-1 encodes a regulatory protein. The REX-1 gene is not identical to the previously reported murine genes that encode zinc finger-containing proteins.

Differentiation-responsive elements in the 5' region of the mouse tissue plasminogen activator gene confer two-stage regulation by retinoic acid and cyclic AMP in teratocarcinoma cells

F9 cells induced to differentiate with retinoic acid (RA) increase transcription of the tissue plasminogen activator (t-PA) gene. Further treatment of these cells with cyclic AMP (cAMP) results in an additional stimulation of t-PA gene transcription. To investigate the mechanism of this two-stage regulation, 4 kilobase pairs (kbp) of 5'-flanking sequence from the murine t-PA gene was isolated. Two major start sites for transcription were found, neither of which depended on a classical TATA motif for correct initiation. By using transient transfection assays, it was determined that 4-kbp of flanking sequence could confer on reporter genes the same two-stage differentiation-specific expression as was observed for the endogenous t-PA gene. Deletion analyses of this 4-kbp fragment showed that 190 bp of flanking sequence was sufficient to bestow the same degree of two-stage regulation on reporter gene constructs. Within this region of DNA, sequence analysis revealed a possible cAMP regulatory element, a CTF/NF-1 recognition sequence, two potential Sp1 sites, and five potential binding sites for transcription factor AP-2. The deletion experiments, coupled with the positions of these potential cis-acting elements, suggest that multiple transcription factors, including those that bind to cAMP regulatory element, CTF/NF-1, Sp1, and AP-2 sites, may be involved in regulation of the t-PA gene during F9 cell differentiation.

Differentiation-responsive elements in the 5' region of the mouse tissue plasminogen activator gene confer two-stage regulation by retinoic acid and cyclic AMP in teratocarcinoma cells

F9 cells induced to differentiate with retinoic acid (RA) increase transcription of the tissue plasminogen activator (t-PA) gene. Further treatment of these cells with cyclic AMP (cAMP) results in an additional stimulation of t-PA gene transcription. To investigate the mechanism of this two-stage regulation, 4 kilobase pairs (kbp) of 5'-flanking sequence from the murine t-PA gene was isolated. Two major start sites for transcription were found, neither of which depended on a classical TATA motif for correct initiation. By using transient transfection assays, it was determined that 4-kbp of flanking sequence could confer on reporter genes the same two-stage differentiation-specific expression as was observed for the endogenous t-PA gene. Deletion analyses of this 4-kbp fragment showed that 190 bp of flanking sequence was sufficient to bestow the same degree of two-stage regulation on reporter gene constructs. Within this region of DNA, sequence analysis revealed a possible cAMP regulatory element, a CTF/NF-1 recognition sequence, two potential Sp1 sites, and five potential binding sites for transcription factor AP-2. The deletion experiments, coupled with the positions of these potential cis-acting elements, suggest that multiple transcription factors, including those that bind to cAMP regulatory element, CTF/NF-1, Sp1, and AP-2 sites, may be involved in regulation of the t-PA gene during F9 cell differentiation.