Activation of four homeobox gene clusters in human embryonal carcinoma cells induced to differentiate by retinoic acid

All-trans-retinoic acid and hexamethylene bisacetamide (HMBA) regulate TGF-alpha and Hst-1/kFGF expression in differentiation sensitive but not in resistant human teratocarcinomas

The multipotent human teratocarcinoma (TC) cell NTera-2 clone D1 (abbreviated NT2/D1) differentiates into a neuronal lineage after retinoic acid (RA) treatment and a distinct phenotype after hexamethylene bisacetamide (HMBA) treatment. We previously reported that RA treatment of NT2/D1 cells reduces cellular cloning efficiency and nude mouse tumorigenicity. This accompanied a loss of mRNA expression of transforming growth factor-alpha (TGF-alpha) and the fibroblast growth factor kFGF, also known as hst-1 (abbreviated hst-1/kFGF). This study extends prior work by reporting that the distinct phenotype induced by HMBA also decreases cloning efficiency, tumorigenicity, and TGF-alpha and hst-1/kFGF mRNA expression in NT2/D1 cells. These RNA findings were confirmed by measurements of growth factor protein in the conditioned media of inducer-treated and untreated NT2/D1 cells. In two established human TC lines refractory to the actions of RA, N2102ep and Tera-1, RA fails to decrease expression of either growth factor despite induction of its nuclear receptor, RAR-beta. However, HMBA induces morphologic maturation and down-regulation of these growth factors in N2102ep cells. This indicates that the loss of TGF-alpha and hst-1/kFGF expression serves as a new marker of differentiation in human TCs. To explore the effects of these growth factors on growth and differentiation of NT2/D1 cells, TGF-alpha or hst-1/kFGF protein was added following inducer treatment or no treatment. Neither growth factor blocked immunophenotypic differentiation, but both promoted the growth of uninduced NT2/D1 cells in cloning assays.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of the human HOXD4 gene by retinoids

Hox genes are developmentally regulated in mammalian embryogenesis, according to temporally and spatially restricted patterns which are affected by retinoids, vitamin A derivatives which have a function as, or at least mimic the action of, axis-specifying morphogens. In the human embryonal carcinoma cell line NT2/D1, HOX gene clusters are activated by at least two retinoids, all-trans- and 9-cis-retinoic acid (RA), in a 3' to 5' sequential cascade which reproduces the activation pattern observed in early embryogenesis. We have studied the regulation of the early activated HOXD4 gene, which is expressed in human embryogenesis in multiple transcripts generated by the developmentally controlled use of alternative transcription start sites and polyadenylation signals. Transfection of a 2.9 kb HOXD4 upstream genomic region linked to a reporter gene in NT2/D1 cells, allowed the identification of two different promoters and a distal enhancer element necessary for RA-dependent gene activation. This element confers to a heterologous promoter the ability to be induced by RA in NT2/D1 cells, and transactivated by alpha, beta and gamma retinoic acid receptors (RARs), but not retinoid X receptor (RXR), in COS-7 cells. DNase I footprinting analysis allowed the identification of four sequences which bind nuclear factors from both RA-induced NT2/D1 cells and embryonic tissues with similar patterns. The use of specific antibodies allowed the identification of at least RAR beta in some of the DNA-protein complexes, although the four sequences bind single RARs transfected in COS cells much less efficiently, or not at all, when compared to a canonical RAR responsive element. Induction of the HOXD4 promoter-enhancer in the presence of a selective RAR alpha antagonist indicated that the RAR alpha-dependent RAR beta activation is nevertheless a necessary step in HOX gene activation. Our results indicate that the ligand-dependent activity of RARs upon specific, cis-acting regulatory elements may have a key role in the induction of early activated HOX genes in response to retinoids. However, RARs represent only a fraction of the transcription factors interacting with the RA-responsive HOXD4 enhancer, which appears to be a complex element requiring specific combinations of nuclear factors for its proper function.

EGF abrogation-induced fusilli-form dysmorphogenesis of Meckel's cartilage during embryonic mouse mandibular morphogenesis in vitro

Mutations associated with genes of the EGF superfamily are implicated in facial malformations arising from abnormal development of the first branchial arch. EGF and EGF receptor (EGFr) transcripts are expressed in the mouse embryonic first branchial arch and derivatives from E9 through E15. EGF transcripts are localized to ectomesenchymal cells associated with precartilage, cartilage, bone and tooth-forming cells. EGF and EGFr proteins co-localize to the same cells suggesting an autocrine regulation. To test whether EGF effects the timing and positional information required for Meckel's cartilage (MC) and tooth development, we cultured E10 mandibular explants in serumless, chemically defined medium with either antisense or sense EGF oligodeoxynucleotides. Antisense inhibition of EGF expression produces bilaterally symmetrical Fusilli-form dysmorphogenesis of MC and decreases tooth bud size; these effects are reversed by the addition of exogenous EGF to the culture medium. Tyrphostin RG 50864, which inhibits EGF receptor kinase activity, inhibits EGF stimulation of tyrosine phosphorylation in a concentration-dependent manner and severely retards mandibular development yet increases tooth size. These findings support the hypothesis that endogenous EGF and EGF-like proteins provide signalling to regulate the size and shape both of cartilage and tooth formation during craniofacial morphogenesis.

Expression of homeobox-containing genes in primary and metastatic colorectal cancer

Homeobox genes are a network of genes encoding nuclear proteins functioning as transcriptional regulators. Human and murine homeobox genes of the HOX family are organised in four clusters on different chromosomes. Gene order within each cluster is highly conserved, perhaps in direct relation to their expression. Homeobox genes have recently been involved in normal development and oncogenesis. We have analysed HOX gene expression in normal human colon and in primary and metastatic colorectal carcinomas. The majority of HOX genes are active in normal adult colon and their overall expression pattern is characteristic of this organ. Furthermore, the expression of some HOX genes is identical in normal and neoplastic colon indicating that these genes may exert an organ-specific function. In contrast, other HOX genes exhibit altered expression in primary colon cancers and their hepatic metastases which may suggest an association with colon cancer progression.

Differential distribution patterns of CRABP I and CRABP II transcripts during mouse embryogenesis

We have compared the transcript distribution of cellular retinoic acid binding protein (CRABP) I and II genes in mouse embryos at various stages of development. Both CRABP transcripts are present in embryonic structures from the earliest stages studied and exhibit specific patterns of distribution, suggesting that the two retinoic acid (RA) binding proteins perform different functions during mouse embryogenesis. The CRABP I transcript distribution correlates well with structures known to be targets of excess retinoid-induced teratogenesis (e.g. neural crest cells and hindbrain), suggesting that cells expressing CRABP I are those that cannot tolerate high levels of RA for their normal developmental function. The embryonic structures expressing CRABP II transcripts include those structures that have been shown to be adversely affected by excess of retinoids, such as limbs and hindbrain, but CRABP II transcripts are also found in structures not known to be specifically vulnerable to raised RA levels. The CRABP II gene is coexpressed with retinoic acid receptor (RAR)-beta and cellular retinol binding protein (CRBP) I genes in a number of tissues such as the gut endoderm, hypophysis and interdigital mesenchyme, all of which are devoid of CRABP I transcripts. Interestingly, the expression of the three genes, RAR-beta, CRABP II and CRBP I, is induced by retinoic acid, which suggests a link between the synthesis of RA from retinol and the control of expression of subsets of RA-responsive genes. The transcript distribution of CRABP I and II is discussed in relation to the teratogenic effects of RA, and compared to the RA-sensitive pattern of expression of other important developmental genes.

HOX gene expression in normal and neoplastic human kidney

As a consequence of transformation, cancer cells generally lose some of their differentiative properties. Thus, alterations interfering with the genetic mechanisms required to maintain embryonic determination could lead to tumorigenesis. Homeobox genes are a network of genes encoding nuclear proteins containing DNA-binding homeodomains that are highly conserved throughout evolution. They are expressed in a stage-related fashion in the developing embryo and, in adult life, in normal tissues. In mice and humans, homeobox genes of the HOX family are organized in 4 clusters on different chromosomes which have presumably evolved by duplication of a primordial gene cluster. Strikingly, the order of genes within each cluster is also highly conserved throughout evolution, suggesting that the physical organization of HOX genes might be essential for their expression. Recent reports indicate that homeobox mutant mice display morphological abnormalities or show neoplastic alterations, and that growth factors can turn on homeobox genes. We have studied the expression of the Antennapedia-like HOX genes in normal human kidney and in renal carcinomas. The great majority of the HOX genes analyzed are expressed in a peculiar manner in normal kidney: blocks of genes, even entire HOX loci, are coordinately regulated. Alterations in HOX gene expression in renal carcinoma can be observed in 2 genes of the HOX-2 locus, HOX-2A and HOX-2E, which are actively expressed in normal kidney and silent in cancer biopsies. The HOX-3H gene is not expressed in normal kidney whereas the HOX-3H transcripts are present in renal carcinomas. Homeobox genes within the 4 HOX loci can be aligned on the basis of the maximal sequence homology of their homeodomains: this alignment defines 13 paralogous gene groups. In renal carcinomas, genes of group 10 (HOX-1D, 2F, 3E, 4B) display a marked difference in their transcript classes when compared to those of normal kidney. Our findings suggest an association between altered HOX gene expression and kidney cancer.

Update of sporadic microphthalmos and coloboma. Non-inherited anomalies

The majority of patients with microphthalmos and colobomas have genetic disorders. This is a survey of non-inherited, sporadic microphthalmos. Such cases may occur in the Goldenhar, CHARGE and VATER associations; it may also be due to teratological agents, for instance maternal ingestion of drugs, maternal infection, fever or irradiation. In these cases it is possible that neural crest cell development is abnormal. Some drugs, for instance retinoic acid are regulators of Hox genes which control an ordered sequence of differentiation; coloboma or microphthalmos may appear if deregulation occurs. Large choristomas of the anterior segment of the eye may be associated with microphthalmos or anophthalmos. Encephaloceles or orbital tumours may deform the growing eye and are another cause of non-inherited microphthalmos.

Isolation and mapping of EVX1, a human homeobox gene homologous to even-skipped, localized at the 5' end of HOX1 locus on chromosome 7

We isolated and mapped the human homeobox gene EVX1. This gene encodes a protein of 407 amino acid residues containing a homeodomain closely related to the Drosophila even-skipped (eve) segmentation gene of the pair-rule class. EVX1 belongs to a small family of vertebrate eve-related homeobox genes including human EVX1 and EVX2 genes, their murine homologs, Evx 1 and Evx 2, and the frog Xhox-3 gene. We previously reported that EVX2 is localized at the 5' end of the HOX4 locus on chromosome 2. We show here that EVX1 is localized at the 5' end of the HOX1 locus on chromosome 7, 48 kb upstream from the most 5' of the eleven HOX1 genes, namely HOX1J. Both EVX genes are transcribed in an opposite orientation as compared to that of adjacent HOX genes. Human HOX1 and HOX4 complex loci appear to be both closely linked to a homeobox gene of the EVX family.

The expression of murine Hox-2 genes is dependent on the differentiation pathway and displays a collinear sensitivity to retinoic acid in F9 cells and Xenopus embryos

In this paper we describe experiments that detail the response of murine Hox-2 genes to cellular differentiation and retinoic acid in cell culture. Hox-2 genes are transiently activated in differentiating ES cells even in the absence of retinoic acid (RA), indicating that their induction is a normal aspect of differentiation. Furthermore, in the continuous presence of RA F9 teratocarcinoma cells show a differential ability to maintain Hox-2 expression depending upon whether the cells follow a visceral or parietal endoderm pathway. These data suggest a clear dependence of Hox-2 expression on the degree and type of differentiation in different cells. However, RA also has dramatic differentiation independent effects on Hox-2 regulation. In ES cells the levels of Hox expression are greatly enhanced by exposure to RA, and in F9 cells of the visceral or parietal phenotype the continuous presence of RA is required to maintain these high levels. Nuclear run-on experiments illustrate that Hox-2 genes are active in F9 stem cells and that a large portion of the RA induction is mediated by post-transcriptional mechanisms. Therefore RA exerts its effects on Hox-2 expression by upregulating or modulating genes which are already active, rather than by turning-on silent genes. All nine Hox-2 genes are induced in F9 cells by RA and there is a direct correlation (collinearity) between gene order and the relative dose response of each gene to RA. In Xenopus embryos treated with RA, homologues of the Hox-2 genes also displayed a temporal and dose response collinearity with gene organisation. Together these findings suggest that the collinear response to RA is highly conserved in vertebrates and combined with the ability of RA to modify expression during cellular differentiation could be an important feature of the Hox-2 cluster itself used to generate the spatially-restricted patterns of gene expression in embryogenesis.

Homeotic transformations of murine vertebrae and concomitant alteration of Hox codes induced by retinoic acid

Exposure of murine embryos to teratogenic doses of retinoic acid (RA) induced homeotic transformations of vertebrae. Posterior transformations occurred along the complete body axis after RA administration on day 7 of gestation and were accompanied by anterior shifts of Hox gene expression domains in embryos. Anterior transformations of vertebrae in the caudal half of the vertebral column were induced on day 8.5. We suggest that the identity of a vertebral segment is specified by a combination of functionally active Hox genes, a "Hox code." In this concept the sequential activation of Hox genes defines sequentially more posterior axial levels, while mesodermal cells leave the primitive streak. Exogenous RA interferes with the normal establishment of Hox codes and thus with axial specification.

Alteration of homeobox gene expression by N-ras transformation of PA-1 human teratocarcinoma cells

We used a series of cell clones from a human teratocarcinoma cell line, PA-1, to study the effect of transformation by an activated N-ras oncogene on the expression of genes involved in retinoic acid (RA)-induced differentiation and growth regulation. Recently, it has been shown that expression of human HOX 2 genes is sequentially activated by RA beginning from Hox 2.9 at the 3' end of the HOX 2 cluster (A. Simeone, D. Acampora, L. Arcioni, P. W. Andrews, E. Boncinelli, and F. Mavilio, Nature [London] 346:763-766, 1990). We now report that six different genes of the cluster HOX 1 are sequentially induced by RA in a similar temporal pattern, beginning with genes at the 3' end of the cluster. However, in N-ras-transformed cell clones, RA-induced expression of these homeobox genes is delayed. Hox 1.4 and Hox 1.3, genes abundantly induced in nontransformed clones after 3 days of RA treatment, are expressed in N-ras-transformed cells only after 10 days of RA treatment. At this time, the cells' growth is arrested at very high density, and no differentiated morphologic characteristics are observed. Constitutive expression of a transfected Hox 1.4 gene under the control of a simian virus 40 promotor leads to differentiated cell morphology similar to that of the RA-induced phenotype and restores the growth-inhibitory effects of RA in N-ras-transformed cells. These observations provide evidence that enhanced proliferation in N-ras-transformed cells compromises teratocarcinoma cell differentiation by a mechanism that transiently suppresses homeobox gene induction and implies a central role for homeobox genes in RA-induced cell differentiation. We conclude that stimulation of a putative growth factor signal pathway, associated with ras-induced proliferation, transiently suppresses the induction of transcription factors functionally involved in cell growth and differentiation.

Alteration of homeobox gene expression by N-ras transformation of PA-1 human teratocarcinoma cells

We used a series of cell clones from a human teratocarcinoma cell line, PA-1, to study the effect of transformation by an activated N-ras oncogene on the expression of genes involved in retinoic acid (RA)-induced differentiation and growth regulation. Recently, it has been shown that expression of human HOX 2 genes is sequentially activated by RA beginning from Hox 2.9 at the 3' end of the HOX 2 cluster (A. Simeone, D. Acampora, L. Arcioni, P. W. Andrews, E. Boncinelli, and F. Mavilio, Nature [London] 346:763-766, 1990). We now report that six different genes of the cluster HOX 1 are sequentially induced by RA in a similar temporal pattern, beginning with genes at the 3' end of the cluster. However, in N-ras-transformed cell clones, RA-induced expression of these homeobox genes is delayed. Hox 1.4 and Hox 1.3, genes abundantly induced in nontransformed clones after 3 days of RA treatment, are expressed in N-ras-transformed cells only after 10 days of RA treatment. At this time, the cells' growth is arrested at very high density, and no differentiated morphologic characteristics are observed. Constitutive expression of a transfected Hox 1.4 gene under the control of a simian virus 40 promotor leads to differentiated cell morphology similar to that of the RA-induced phenotype and restores the growth-inhibitory effects of RA in N-ras-transformed cells. These observations provide evidence that enhanced proliferation in N-ras-transformed cells compromises teratocarcinoma cell differentiation by a mechanism that transiently suppresses homeobox gene induction and implies a central role for homeobox genes in RA-induced cell differentiation. We conclude that stimulation of a putative growth factor signal pathway, associated with ras-induced proliferation, transiently suppresses the induction of transcription factors functionally involved in cell growth and differentiation.

The oncofetal gene Pem encodes a homeodomain and is regulated in primordial and pre-muscle stem cells

The oncofetal gene, Pem, is expressed in a stage specific manner during murine ontogeny. The carboxy terminal portion of the predicted Pem protein has significant similarity to homeodomains of the Drosophila prd family. The Pem gene is expressed in undifferentiated embryonal stem (ES) and embryonal carcinoma (EC) cell lines. Pem mRNA is induced 35-fold in ES cells differentiated in the absence of retinoic acid. Pem mRNA is increased in EC cells differentiated towards parietal or visceral endoderm, consistent with the abundant Pem expression in embryonic yolk sac. In 10T mesenchymal stem cells committed to muscle cell differentiation, Pem mRNA expression is dramatically increased. The elevation in Pem expression preceded the induction of the muscle master regulatory gene, myoD. We conclude that the Pem gene encodes a candidate transcription factor which is developmentally regulated.

EVX2, a human homeobox gene homologous to the even-skipped segmentation gene, is localized at the 5' end of HOX4 locus on chromosome 2

We isolated and mapped three new human homeoboxes located on chromosome 2 upstream from the reported seven HOX4 homeobox sequences. Two of them, HOX41 and HOX4H, clearly belong to the HOX gene family, in particular to homology groups 1 and 2, and possibly represent the most 5' HOX4 homeoboxes. A third homeobox 13 kb upstream from HOX41 was identified. Sequencing data show that this is the human homolog of the murine Evx-2 homeobox. Both homeoboxes are closely related to the murine Evx-1 and to the frog Xhox-3 homeoboxes. The four genes represent vertebrate homologs of Drosophila even-skipped (eve), a segmentation gene of the pair-rule class. Human EVX2 sequences belong to an active gene because they are transcribed and properly processed in cells and tissues. We have identified for the first time a homeogene of a different class at a HOX locus. These findings are relevant to the understanding of the evolution of HOX gene clusters and their regulation.

Identification and characterization of novel human endogenous retroviral sequences prefentially expressed in undifferentiated embryonal carcinoma cells

A novel endogenous retroviral sequence (ERV-9) has been isolated from a human embryonal carcinoma cDNA library by hybridization to a probe containing a recently described human repetitive element. DNA sequence analysis of the 4kb cDNA insert (pHE.1) revealed the presence of ORFs potentially coding for putative retrovirus-related gag, pol and env proteins. Northern blot and RNase protection experiments showed that RNA homologous to the pHE.1 insert is detected only in embryonal carcinoma cells as a 8 kb mRNA, and its expression is negatively regulated during retinoic acid induced differentiation of the human teratocarcinoma cell line NT2/D1. Using a pol specific probe we have isolated a genomic locus containing the ERV-9 sequences. Characterization by restriction enzyme analysis and DNA sequencing allowed us to define LTR-like sequences, that are composed by a complex array of subrepetitive elements. In addition we show that ERV-9 LTR sequences are capable to drive expression of linked CAT gene in a cell specific manner as LTR promoter activity has been detected only in NT2/D1 cells.

Differential regulation by retinoic acid of the homeobox genes of the four HOX loci in human embryonal carcinoma cells

We studied the expression of 38 human homeobox genes belonging to the four HOX complex loci in embryonal carcinoma (EC) cells induced to differentiate by culturing them in a medium containing retinoic acid (RA). Genes located at the 3' end of each one of the four HOX loci are activated by RA in a sequential order colinear with their 3' to 5' arrangement in the cluster: 3' HOX genes respond early to the drug while upstream genes respond progressively later. Among the genes located at the 5' end of HOX loci RNase protection analysis reveals that one HOX3 gene and four HOX4 genes are weakly expressed in EC stem cells and downregulated upon treatment with 10(-5) M RA. While activation of early responding genes does not require continuous protein synthesis, the observed timing and polarity of gene activation is disrupted in the absence of protein synthesis.

Retinoic acid modifies mesodermal patterning in early Xenopus embryos

Treatment of early Xenopus embryos with retinoic acid (RA) produces a concentration-dependent series of defects in anterior axial structures that range from small deletions to embryos lacking heads. The graded series of axial defects obtained after RA administration to early embryos appears to result, in part, from actions of RA on embryonic mesoderm. RA modifies the differentiation of anterior dorsal mesoderm from animal cap ectoderm induced by mesoderm-inducing peptide growth factors (PGFs). Concentrations of RA that suppress anterior dorsal mesoderm result in the differentiation of mesoderm of more posterior or ventral character. The suppression of anterior dorsal mesoderm may account for the absence of anterior neural ectoderm after RA treatment. Although RA changes the character of mesoderm, it does not seem to affect mesodermal induction by PGFs or the levels of Xhox3 mRNA induced in the mesoderm by PGFs. RA therefore appears to affect steps downstream from those involved in the initial induction of mesoderm. In experiments to examine the possible physiological role of RA in early Xenopus development, dorsal and ventral ectoderm were found to respond differently to identical concentrations of PGFs. One potential basis for this heterogeneity is the existence of a localized inhibitor, possibly RA, in the early Xenopus embryo. RA could therefore contribute to axial patterning by inhibiting the development of mesoderm of different character induced by PGFs.

Retinoic acid regulates thymosin beta 10 levels in rat neuroblastoma cells

A small acidic polypeptide, termed thymosin beta 10, has been identified and is present in the nervous system of the rat by the ninth day of gestation. Thymosin beta 10 levels rise during the remaining days of life in utero, and then decline to nearly undetectable values between the second and fourth week post partum. The present study investigates the possible developmental signals and mechanisms that might regulate the expression of thymosin beta 10 during neuroembryogenesis. Many cell lines derived from tumors of the central nervous system express thymosin beta 10, as well as its homologue gene product, thymosin beta 4. Because some of these cell lines respond to exogenously applied agents by increasing their apparent state of differentiation, we have determined whether thymosin beta 10 levels are coordinately modulated. In several neuroblastomas, including the B103 and B104 lines, retinoic acid elicits a time- and dose-dependent increase in the content of thymosin beta 10, but not that of thymosin beta 4. The increase in thymosin beta 10 polypeptide is associated with a marked increase in the specific mRNA encoding this molecule. The mRNA for thymosin beta 4 is unaffected by retinoic acid. This is in contrast with the situation in vivo, where the expression of both genes decreases after birth. Other agents that influence the morphology of B104 cells, such as phorbol esters and dibutyryl cyclic AMP, have no influence on beta-thymosin levels. A range of steroids, which like retinoids act upon nuclear receptors, was also inactive. The stimulatory action of retinoic acid is detectable within 4 h, and thymosin beta 10 peptide levels continue to rise for at least 4 days. The influence of the isoprenoid is fully reversible and exhibits structural specificity. We believe that this culture system is mimicking the early rising phase of thymosin beta 10 levels in brain and that endogenous retinoids may be candidate physiological regulators of this gene.

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.

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.

Differential activation of Xenopus homeo box genes by mesoderm-inducing growth factors and retinoic acid

What is the nature of positional information during embryogenesis? By using Xenopus homeo box genes as anteroposterior (A-P) markers, we confirm the findings of others that mesoderm-inducing growth factors and retinoic acid (RA) can provide positional information along the axis of the body. Xenopus tissue culture-mesoderm-inducing factor (XTC-MIF) selectively activates an anteriorly expressed homeo box gene (XlHbox 1), while basic fibroblast growth factor (bFGF) activates selectively a posteriorly expressed homeo box gene (XlHbox 6). RA activates expression of the posterior gene XlHbox 6, but not of XlHbox 1. This activation, however, requires exposure to growth factors. The data suggest that growth factors and RA may cooperate with each other to provide positional information in vertebrates.

Expression of HOX homeogenes in human neuroblastoma cell culture lines

Mammalian genes containing a class-I homeobox (HOX genes) are highly expressed in the embryonic nervous system. As a first step towards the molecular analysis of the role these genes play in neural cells, we studied the expression of four human HOX genes in five neuroblastoma (NB) cell lines - SK-N-BE, CHP-134, IMR-32, SK-N-SH and LAN-1 - during the process of differentiation induced by treatment with retinoic acid (RA). The four genes, HOX1D, 2F, 3E and 4B, located at corresponding positions in the four HOX loci, share a high degree of sequence similarity with the Drosophila Deformed homeotic gene and constitute a homology group, group 10. One of these genes, HOX1D, is not expressed in the cells used, whereas the other three are highly expressed in untreated and RA-induced NB cells, even though the expression pattern in the various lines is slightly different for the three genes. Our analysis reveals a complex and specific expression pattern in these lines, paving the way to an identification of different NB-cell populations by means of specific HOX gene expression schemes. On the other hand, in every line studied, morphological maturation toward a neuronal differentiated phenotype appears to be associated with increased HOX gene expression.

Sequential activation of HOX2 homeobox genes by retinoic acid in human embryonal carcinoma cells

RETINOIC acid had been implicated as a natural morphogen in chicken and frog embryogenesis, and is presumed to act through the gene regulatory activity of a family of nuclear receptors. Homeobox genes, which specify positional information in Drosophila and possibly in vertebrate embryogenesis, are among the candidate responsive genes. We previously reported that retinoic acid specifically induces human homeobox gene (HOX) expression in the embryonal carcinoma cell line NT2/D1. We now show that the nine genes of the HOX2 cluster are differentially activated in NT2/D1 cells exposed to retinoic acid concentrations ranging from 10(-8) to 10(-5) M. Genes located in the 3' half of the cluster are induced at peak levels by 10(-8) M retinoic acid, whereas a concentration of 10(-6) to 10(-5) M is required to fully activate 5' genes. At both high and low retinoic acid concentrations, HOX2 genes are sequentially activated in embryonal carcinoma cells in the 3' to 5' direction.

Molecular mechanisms underlying the expression of the human HOX-5.1 gene

The complex mechanisms underlying homeobox genes expression involve regulation at transcriptional, post-transcriptional and translational levels. The multiple transcripts of the human HOX-5.1 gene are expressed differentially in tissue- and stage-specific patterns during embryogenesis, and differentially induced by retinoic acid (RA) in human embryonal carcinoma (EC) NT2/D1 cells. We have sequenced 6.3 Kb of the genomic region containing the HOX-5.1 gene and analyzed its mechanisms of expression. Two alternative promoters underlie the transcription of two classes of HOX-5.1-specific mRNAs. These classes differ in tissue and subcellular distribution, induction by RA, structure of the 5'-UT region and mRNA stability: these features are compatible with a differential function of the two classes of transcripts in embryogenesis.

Human HOX genes are differentially activated by retinoic acid in embryonal carcinoma cells according to their position within the four loci

We studied the expression of 33 human homeobox genes belonging to four complex HOX loci in embryonal carcinoma NT2/D1 cells. These cells can be induced to differentiate by culturing them in media containing retinoic acid. Northern blot analysis reveals that no expression of these genes was detectable in NT2/D1 stem cells, whereas 22 HOX genes are well expressed in NT2/D1 cells treated with 10 microM retinoic acid for 14 days. The 11 HOX genes the expression of which remained undetectable in NT2/D1 cells after this treatment are located at the 5' end of their loci: four in HOX1, five in HOX3 and two in HOX4. The boundary between induced and silent genes roughly corresponds to the HOX genes constituting the homology group 5, related to the Abdominal-B homeotic gene of Drosophila. All nine identified HOX2 genes are well expressed in fully induced NT2/D1 cells and none of them maps 5' genes of this homology group. We conclude that HOX genes are differentially activated by retinoic acid in these cells according to their physical location within the four chromosomal loci.

Murine developmental control genes

Various strategies have been used to isolate genes that participate in the regulation of mouse development. Gene families that have been identified on the basis of their homology to motifs within Drosophila control genes or human transcription factor genes, namely homeobox (Hox), paired-box (Pax), and POU genes, can be compared with respect to gene organization, structure, and expression patterns. The functions of these genes can be analyzed molecularly in vitro and in vivo with the use of available mouse mutants or transgenic mice. In addition, it has been possible to generate gain- or loss-of-function mutations by random or targeted introduction of transgenes. Models derived from these studies can reveal the successive steps of developmental control on a genetic level.

Thymosin beta 10 levels in developing human brain and its regulation by retinoic acid in the HTB-10 neuroblastoma

Human fetal brain expresses high levels of a polypeptide identified by protein biochemistry and molecular cloning as thymosin beta 10. Within the first 18 months after birth, the thymosin beta 10 content of human brain falls to undetectable levels. In order to establish the molecular basis of this process we screened a number of human tumor cell lines derived from the nervous system for the presence of thymosin beta 10. All of the cell line expressed authentic thymosin beta 10. However, in the HTB-10 neuroblastoma, retinoic acid caused a reduction in the level of thymosin beta 10. This effect of the retinoid was conditional upon its continual presence in the tissue culture medium and was not evident in the other cell lines examined. These results suggest that the thymosin beta 10 gene may be a target for retinoids in the developing nervous system.

Retinoic acid, a developmental signalling molecule

Retinoic acid has been used as a tool both by embryologists studying the spatial organization of cells in the embryo and by molecular biologists studying the control of gene expression in the nucleus. Embryologists have shown that retinoic acid can modify the pattern of cell differentiation so as to duplicate complete parts of the embryo in a well-organized way; molecular biologists have shown that retinoic acid can act as the switch starting the sequence of differential gene expression that results in cell differentiation. In the past year these two approaches have converged so that there now seems a real possibility that we may soon for the first time understand how a particular vertebrate development system works.

Age-related changes in the response of chick lens cells during long-term culture to insulin, cyclic AMP, retinoic acid and a bovine retinal extract

We have reported that 1-day-old post-hatch chick lens epithelial cells lose the capacity for lentoid body formation and delta-crystallin expression during long-term serial subculture, although they continue to synthesize, but not to accumulate, alpha- and beta-crystallins, even in cells with a transformed phenotype. Here we present evidence that dedifferentiation may reflect an age-related change in the capacity for response to regulatory signals. We have tested the capacity of these cells in serial subcultures to respond to agencies which affect lens cell growth and differentiation in primary culture: retinoic acid (RA), insulin, cAMP and bovine retinal extract (BRE). Secondary cultures responded only to RA and BRE, by an increase in lentoid formation and by alpha- and beta-accumulation, while RA also restored delta-crystallin expression. Later cultures showed no such responses. The results suggest that the process of lens cell dedifferentiation may, at first, be reversible but later becomes irreversible, despite the continuing persistence of low levels of crystallin expression.

Different patterns of glycolipid antigens are expressed following differentiation of TERA-2 human embryonal carcinoma cells induced by retinoic acid, hexamethylene bisacetamide (HMBA) or bromodeoxyuridine (BUdR)

NTERA-2 cl.D1 human embryonal carcinoma (EC) cells were induced to differentiate by either bromodeoxyuridine (BUdR) or hexamethylene bisacetamide (HMBA), and also by retinoic acid. Following exposure to each of these inducers, the globoseries glycolipid antigens stage-specific embryonic antigens -3 and -4 (SSEA-3 and -4) and the glycoprotein antigen TRA-1-60, all characteristic of the human EC cell surface, underwent a marked reduction in expression within about 7 days. At the same time, the lactoseries glycolipid antigen SSEA-1, and ganglioseries antigens A2B5 (GT3) and ME311 (9-0-acetyl GD3) were induced in BUdR- and retinoic acid-treated cells. However, these antigens did not appear during the first 7-14 days of HMBA-induced differentiation. The observations of cell surface antigen expression were paralleled by analysis of glycolipids isolated from the cells by thin-layer chromatography. This analysis, in which the new monoclonal antibodies VINIS-56 and VIN-2PB-22 were included, also revealed expression of gangliosides GD3 and GD2 in all differentiated cultures, albeit at much lower levels following HMBA exposure than following retinoic acid or BUdR-exposure. Further, disialylparagloboside was detected in retinoic acid and BUdR-induced, but not HMBA-induced, cultures. Taken with morphological observations, the results suggest that HMBA induces differentiation of NTERA-2 cl.D1 EC cells along a pathway distinct from the pathway(s) induced by retinoic acid and BUdR.

Effects of retinoic acid on the distribution of glycoconjugates during mouse tail bud development

Retinoic acid (RA), a potent teratogen of caudal axial development in rodents, has been shown to alter glycoconjugates in a variety of embryonic tissues and teratocarcinomas. In this study, we examined its effects on the expression of cell surface and extracellular matrix glycoconjugates during tail bud development in mouse embryos by using lectin histochemistry. The lectins WGA, sWGA, and PNA showed striking differences in binding between RA-exposed and control embryos. Computer-assisted densitometry revealed a significant increase in binding of all three lectins to the extracellular material of the luminal and abluminal borders of the secondary neural tube and surrounding the notochord in RA-exposed embryos. RA-treated embryos also showed an increased binding affinity for the lectins sWGA and PNA to the cells of the notochord, while WGA showed increased binding to the neuroepithelial cells of the secondary neural tube. The results suggest that RA affects the expression of lectin binding sites during the early development of RA-induced caudal axial defects.

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.

Identification of new human repetitive sequences: characterization of the corresponding cDNAs and their expression in embryonal carcinoma cells

We have identified new repeated interspersed DNA sequences by analysis of homologous RNA transcripts from a human teratocarcinoma cell line (NTERA-2 clone D1). The abundance of transcripts varies upon retinoic acid induced differentiation of NTERA-2/D1 cells, and it is highest when the cells display the embryonal carcinoma phenotype. The expression of these novel repeated sequences appears to be tissue specific as no detectable expression was found in various cell lines of different embryological derivation. Characterization of the RNA transcripts by analysis of recombinant cDNA clones indicated that transcripts of different genomic units are present in undifferentiated embryonal teratocarcinoma cells. Nucleotide sequencing of the cloned cDNAs reveals a complex structure composed by unique and tandemly repeated sub-elements.

Craniofacial abnormalities induced by ectopic expression of the homeobox gene Hox-1.1 in transgenic mice

Hox-1.1 is a murine homeobox-containing gene expressed in a time- and cell-specific manner during embryogenesis. We have generated transgenic mice that ectopically express Hox-1.1 from the chicken beta-actin promoter. In these mice Hox-1.1 expression was changed to an almost ubiquitous pattern. Ectopic expression of Hox-1.1 leads to death of the transgenic animals shortly after birth and is associated with multiple craniofacial anomalies, such as cleft palate, open eyes at birth, and nonfused pinnae. This phenotype is similar to the effects seen after systemic administration of retinoic acid during gestation. This suggests that retinoic acid embryopathy and the specific developmental defects caused by ectopic expression of a potential developmental control gene share a common pathogenic mechanism.

Retinoid receptor antisense DNAs inhibit alkaline phosphatase induction and clonogenicity in malignant keratinocytes

Antisense oligodeoxynucleotides [oligo(dN)s] corresponding to human cellular retinol-binding protein I (cRBP) and human nuclear retinoic acid receptor alpha (hnRAR) were synthesized. Exposure of human malignant keratinocytes to these oligo(dN)s significantly attenuated the level of cytoplasmic cRBP and hnRAR in a concentration- and time-dependent manner. Further, the induction of alkaline phosphatase by retinol in these cells was blocked by treatment with 30 microM antisense oligo(dN) to cRBP or hnRAR but not by 30 microM of sense oligo(dN) to cRBP. Antisense oligo(dN) treatments concomitantly induced cell rounding, loss of cell-cell attachment, and cell adhesion to the substratum. By contrast, treatment of cells with an anticytokinetic agent, cytochalasin B, or with a cytostatic concentration of sodium azide failed to reduce cytoplasmic cRBP or hnRAR from nuclear extracts, even though antisense oligo(dN)-like changes in cell morphology were observed. Treatment of the cells for greater than 2.75 hr with 20-40 microM of either antisense oligo(dN) also led to the loss of clonogenic potential. These results show that both cytoplasmic and nuclear receptors for retinoids are important in the transduction of a retinoid signal response critical to cellular growth and differentiation. Our findings also suggest that defined genes, which are specified by retinoids and their receptors, may account for the pleiotropic effect of vitamin A compounds.

Vertebrate homeodomain proteins: families of region-specific transcription factors

Vertebrate homeodomain proteins are transcription factors whose genes can be isolated via a conserved DNA-binding domain called the homeobox. We review recent studies suggesting that one function of these genes is the early subdivision of the embryo along the antero-posterior axis into 'fields' of cells with different developmental potential.

Posttranscriptional control of human homeobox gene expression in induced NTERA-2 embryonal carcinoma cells

We have studied the expression of four human homeobox genes representative of four different clusters (i.e., HOX-1, HOX-2, HOX-3 and HOX-5) in the embryonal carcinoma (EC) cell line NT2/D1. Following treatment with retinoic acid (RA), these cells differentiate into several cell types, including neurons, and steadily accumulate polyadenylated transcripts derived from the genes in a period ranging from 18 hr to 14 days of RA treatment. The sizes of major transcripts in differentiated EC cells coincide with those previously detected by the same probes in human embryos. Nuclear run-on transcriptional analysis showed no difference in the transcription rate of the four homeobox genes in differentiated vs. undifferentiated EC cells. Inhibition of protein synthesis by 5-18 hr of treatment of undifferentiated cells with cycloeximide causes accumulation of some homeobox transcripts at levels comparable to those observed after 18 hr of RA induction, although it does not cause superinduction in fully differentiated cells. These data suggest that the activation of homeobox gene expression in RA-induced EC cells is controlled, at least in part, by posttranscriptional mechanisms.

Selective stimulation of in vitro limb-bud chondrogenesis by retinoic acid

Embryonic exposure to pharmacologic doses of vitamin A analogs (retinoids) is a well-known cause of limb-skeletal deletions, limb truncation and other skeletal malformations. The exclusively inhibitory effect of retinoic acid (RA) on chondrogenesis in standard serum-containing cultures of limb-bud mesenchymal cells is equally well known and has provided a means to explore the cellular basis for RA-mediated skeletal teratogenesis. Recent studies showing that lower RA concentrations can cause skeletal duplication when applied directly to the anterior border of a developing limb, suggest that RA may have a role in normal limb development as a diffusible morphogen capable of regulating skeletal pattern. While RA treatment causes both, skeletal deletions and duplications are clearly different (if not opposing) effects, the latter of which is difficult to reconcile with RA's heretofore exclusively inhibitory effect on in vitro chondrogenesis. In the present study. RA's effects on chondrogenesis and myogenesis were examined in serum-free cultures of chick limb-bud mesenchymal cells and compared with its effects on similar cultures grown in serum-containing medium. When added to serum-free medium, concentrations of RA known to cause skeletal duplication in vivo dramatically enhanced in vitro chondrogenesis (to over 200% of control values) as judged by both Alcian-blue staining and [35S]sulfate incorporation, while having little effect on myogenesis. Higher concentrations inhibited both chondrogenesis and myogenesis. The results indicate that at physiological concentrations. RA can selectively modulate chondrogenic expression and suggest that at higher concentrations, RA's inhibitory effects are less specific.(ABSTRACT TRUNCATED AT 250 WORDS)