Murine chromosomal location of four hepatocyte-enriched transcription factors: HNF-3 alpha, HNF-3 beta, HNF-3 gamma, and HNF-4

Suppression of FOXM1 Transcriptional Activities via a Single-Stranded DNA Aptamer Generated by SELEX

The transcription factor FOXM1 binds to its consensus sequence at promoters through its DNA binding domain (DBD) and activates proliferation-associated genes. The aberrant overexpression of FOXM1 correlates with tumorigenesis and progression of many cancers. Inhibiting FOXM1 transcriptional activities is proposed as a potential therapeutic strategy for cancer treatment. In this study, we obtained a FOXM1-specific single stranded DNA aptamer (FOXM1 Apt) by SELEX with a recombinant FOXM1 DBD protein as the target of selection. The binding of FOXM1 Apt to FOXM1 proteins were confirmed with electrophoretic mobility shift assays (EMSAs) and fluorescence polarization (FP) assays. Phosphorthioate-modified FOXM1 Apt (M-FOXM1 Apt) bound to FOXM1 as wild type FOXM1 Apt, and co-localized with FOXM1 in nucleus. M-FOXM1-Apt abolished the binding of FOXM1 on its consensus binding sites and suppressed FOXM1 transcriptional activities. Compared with the RNA interference of FOXM1 in cancer cells, M-FOXM1 Apt repressed cell proliferation and the expression of FOXM1 target genes without changing FOXM1 levels. Our results suggest that the obtained FOXM1 Apt could be used as a probe for FOXM1 detection and an inhibitor of FOXM1 transcriptional functions in cancer cells at the same time, providing a potential reagent for cancer diagnosis and treatment in the future.

Activation of the signal transducers and activators of the transcription 3 pathway in alveolar epithelial cells induces inflammation and adenocarcinomas in mouse lung

The lung is an organ for host defense to clear up pathogens through innate and adaptive immunity. This process involves up-regulation of proinflammatory cytokines and chemokines that lead to activation of the signal transducers and activators of the transcription 3 (Stat3) signaling pathway. Overexpression of Stat3C in alveolar type II epithelial cells of CCSP-rtTA/(tetO)(7)-Stat3C bitransgenic mice leads to severe pulmonary inflammation, including immune cell infiltration and up-regulation of proinflammatory cytokines and chemokines in the lung. As a consequence, spontaneous lung bronchoalveolar adenocarcinoma was observed in bitransgenic mice. Aberrantly expressed genes in the bitransgenic model were identified and served as biomarkers for human bronchoalveolar adenocarcinoma. During tumorigenesis, genes that are critical to epithelial cell proliferation in lung development were reactivated. Therefore, Stat3 is a potent proinflammatory molecule that directly causes spontaneous lung cancer in vivo.

International Union of Pharmacology. LXVI. Orphan nuclear receptors

Half of the members of the nuclear receptors superfamily are so-called "orphan" receptors because the identity of their ligand, if any, is unknown. Because of their important biological roles, the study of orphan receptors has attracted much attention recently and has resulted in rapid advances that have helped in the discovery of novel signaling pathways. In this review we present the main features of orphan receptors, discuss the structure of their ligand-binding domains and their biological functions. The paradoxical existence of a pharmacology of orphan receptors, a rapidly growing and innovative field, is highlighted.

Winged-helix transcription factors and pancreatic development

The forkhead gene family, named after the founding gene member in Drosophila, is characterized by a unique DNA-binding domain. This so-called forkhead box encodes a winged-helix DNA-binding motif, the name of which describes the structure of the domain when bound to DNA. The three Fox (forkhead box) group A genes, Foxa1, Foxa2 and Foxa3, are expressed in embryonic endoderm, the germ layer that gives rise to the digestive system, and contribute to the specification of the pancreas and the regulation of glucose homoeostasis. Deletion of the Foxa2 gene in pancreatic beta-cells in mice results in a phenotype resembling PHHI (persistent hyperinsulinaemic hypoglycaemia of infancy). Molecular analyses have demonstrated that Foxa2 is an important regulator of the genes encoding Sur1, Kir6.2 and Schad (short chain L-3-hydroxyacyl-CoA dehydrogenase), mutation of which causes PHHI in humans. Foxa1 was shown to be an essential activator of glucagon gene expression in vivo. An additional winged-helix protein, Foxo1, contributes to pancreatic beta-cell function by regulating the Pdx1 gene, which is required for pancreatic development in cooperation with Foxa2.

Primary structure, chromosomal mapping, expression and transcriptional activity of murine hepatocyte nuclear factor 4gamma

We demonstrate the presence of a new member of the orphan nuclear receptor hepatocyte nuclear factor 4 (HNF4) subfamily in mouse which is genetically distinct from the previously characterized mouse HNF4alpha gene. The new member of the HNF4 subfamily shows highest amino acid identity, similar tissue distribution and syntenous chromosomal localization to the recently described human HNF4gamma (NR2A2), we therefore classify it as mouse HNF4gamma (mHNF4gamma). A combination of RT-PCR and immunohistochemical analysis showed expression of mHNF4gamma mRNA and protein in the endocrine pancreas, testes, kidney and gut. By co-transfection experiments, we show that mHNF4gamma is able to activate transcription, acting through binding sites that have been previously characterized as HNF4alpha binding sites. The presence of HNFgamma in human and mouse implies that a complex transcriptional network exists in higher vertebrates involving a number of HNF4 members with overlapping yet distinct function and tissue distribution.

Mouse HNF-3/fork head homolog-1-like gene: structure, chromosomal location, and expression in adult and embryonic kidney

Screening of a mouse kidney cDNA library with a HNF-3/fork head domain probe revealed cDNA Hfh-1L containing the highly conserved fork head DNA-binding domain. The Hfh1L cDNA shows 92.7% homology at the nucleic acid level with the fork head gene HFH-1 from rat. Southern blot analyses demonstrated that the Hfh-1L gene is highly conserved in a wide variety of species, including goldfish and frog. Sequencing the corresponding genomic clone, we found that the Hfh-1L gene is most likely intronless. By interspecific back-cross analysis, the Hfh-1L gene was localized to mouse chromosome 13. In order to analyze the expression pattern of Hfh-1L, we performed Northern blot analyses and revealed a 2.7-kb transcript in adult kidney and stomach. In situ hybridization experiments of adult mouse kidney showed Hfh-1L expression in the outer medulla of the kidney and the transitional epithelium. In light of the significance of a number of fork head genes in early embryonic development, the pattern of expression during murine embryogenesis was examined by reverse transcriptase-polymerase chain reaction (RT-PCR), and Hfh-1L transcripts were detected in mouse embryos at every stage tested from day 10.5 to 16.5 postconception (p.c.) and in the developing metanephros of 14.5- and 15.5-day p.c. embryos. This expression pattern suggests that the Hfh-1L gene is involved in the development of the kidney.

A detailed physical and transcriptional map of the region of chromosome 20 that is deleted in myeloproliferative disorders and refinement of the common deleted region

Acquired deletions of the long arm of chromosome 20 are the most common chromosomal abnormality seen in polycythemia vera and are also associated with other myeloid malignancies. Such deletions are believed to mark the site of one or more tumor suppressor genes, loss of which perturbs normal hematopoiesis. A common deleted region (CDR) has previously been identified on 20q. We have now constructed the most detailed physical map of this region to date--a YAC contig that encompasses the entire CDR and spans 23 cM (11 Mb). This contig contains 140 DNA markers and 65 unique expressed sequences. Our data represent a first step toward a complete transcriptional map of the CDR. The high marker density within the physical map permitted two complementary approaches to reducing the size of the CDR. Microsatellite PCR refined the centromeric boundary of the CDR to D20S465 and was used to search for homozygous deletions in 28 patients using 32 markers. No such deletions were detected. Genetic changes on the remaining chromosome 20 may therefore be too small to be detected or may occur in a subpopulation of cells.

Isolation of the mouse (MFH-1) and human (FKHL 14) mesenchyme fork head-1 genes reveals conservation of their gene and protein structures

The very recently found evolutionarily conserved DNA-binding domain of 100 amino acids, termed the fork head domain, emerged from a sequence comparison of the rat hepatocyte transcription factor HNF-3 alpha and the homeotic gene fork head of Drosophila. We previously isolated a new member of this family, the mesenchyme fork head-1 (MFH-1) gene, which is expressed in developing mesenchyme. Here we describe the isolation of the mouse (MFH-1) and human (FKHL14) chromosomal MFH-1 genes and the determination of the gene and protein structures of MFH-1. We found that the MFH-1 gene has no introns and that the identity of the amino acid sequences of mouse and human MFH-1 proteins is 94%. We also investigated the transcriptional activity of the mouse and human MFH-1 proteins and found that both proteins act as positive transactivators.

Mouse CD-RAP/MIA gene: structure, chromosomal localization, and expression in cartilage and chondrosarcoma

A cDNA encoding a novel protein has been previously isolated from two independent sources: melanoma cell cultures and chondrocytes. The protein from human melanoma cell lines and tumors is called melanoma inhibitory activity (MIA) (Blesch et al. [1994] Cancer Res. 54:5695-5701) and the protein from primary bovine chondrocytes and cartilaginous tissues is called cartilage-derived retinoic acid-sensitive protein (CD-RAP) (Dietz and Sandell [1996] J. Biol. Chem. 271:3311-3316). In order to investigate the gene regulation and function of CD-RAP/MIA, the mouse gene locus was isolated and analyzed. Developmental expression was determined by in situ hybridization to mouse embryos. Expression was limited to cartilaginous tissues and was initiated with the advent of chondrogenesis, remaining abundant throughout development. The mouse gene was isolated and sequenced from a 129Sv library and sequenced directly from an additional strain, B6C3Fe. The mouse CD-RAP/MIA gene is 1.5 kbp and consists of four exons. The promoter sequence of the gene contains many potential regulatory domains including 8 basic helix-loop-helix protein-binding domains and an AT-rich domain, both motifs shown to be present in the cartilage-specific enhancer of the type II procollagen gene. Other potential cis-acting motifs include binding sites for GATA-1, NF-IL6, PEA3, w-elements, NF kappa B, Zeste and Sp1. The gene, called cdrap, was localized to the end of an arm of chromosome 7 at the same site as the transforming growth factor beta 1 (Tgf-beta 1) and the glucose phosphate isomerase 1 (Gpi 1) genes. Potential mouse mutants that mapped to the same region of chromosome 7 were identified. Two of the potential mutants with skeletal phenotypes were sequenced, pudgy (pu) and extra toes with spotting (XsJ); however, no mutations were found in the coding sequence. To determine whether CD-RAP/MIA is associated with tumors of cartilage, mRNAs from a variety of rodent tissues and cell lines were screened. Expression was detected in a rodent tumor, the Swarm rat chondrosarcoma and a chondrosarcoma cell line derived from it, but not in other tissues or tumors of non-cartilage origin. Immunolocalization revealed CD-RAP/MIA protein localized in cartilage only. These results show that the normal expression of CD-RAP/MIA is limited to cartilage; however, pathologically, it is expressed both in melanoma and chondrosarcoma. The restricted expression of CD-RAP/MIA may provide an opportunity to monitor cartilage metabolic activity as well as the tumor activity of melanoma and chondrosarcoma.

Isolation and characterization of a third isoform of human hepatocyte nuclear factor 4

Hepatocyte nuclear factor 4 (HNF-4) is an essential positive regulator of a large number of liver-specific genes. We report here the isolation of three HNF-4 isoforms from a human liver cDNA library. hHNF-4A and hHNF-4B, differing by the insertion of 10 amino acids in the C-terminal region, have been previously identified in mouse, rat and human liver. The novel isoform, hHNF-4C, is identical to hHNF-4A and B in the regions encompassing the DNA-binding and dimerization domains, but contains a different C-terminal domain. Similar to the other isoforms, hHNF-4C is produced in a limited number of tissues and represents 2.6-13% of the total hHNF-4 mRNA population, depending on the cell type. The chromosomal origin of all three isoforms has been localized to human chromosome 20. hHNF-4C can form heterodimers with hHNF-4A and B in vitro, and exhibits similar transactivation potential as hHNF-4A or B in transient transfection assays, suggesting that the divergent C-terminal region is not part of the transactivation domain.

Clustered arrangement of winged helix genes fkh-6 and MFH-1: possible implications for mesoderm development

The ‘winged helix’ or ‘forkhead’ transcription factor gene family is defined by a common 100 amino acid DNA binding domain which is a variant of the helix-turn-helix motif. Here we describe the structure and expression of the mouse fkh-6 and MFH-1 genes. Both genes are expressed in embryonic mesoderm from the headfold stage onward. Transcripts for both genes are localised mainly to mesenchymal tissues, fkh-6 mRNA is enriched in the mesenchyme of the gut, lung, tongue and head, whereas MFH-1 is expressed in somitic mesoderm, in the endocardium and blood vessels as well as the condensing mesenchyme of the bones and kidney and in head mesenchyme. Both genes are located within a 10 kb region (in mouse chromosome 8 at 5.26 +/− 2.56 cM telomeric to Actsk1. The close physical linkage of these two winged helix genes is conserved in man, where the two genes map to chromosome 16q22-24. This tandem arrangement suggests the common use of regulatory mechanisms. The fkh-6/MFH-1 locus maps close to the mouse mutation amputated, which is characterised by abnormal development of somitic and facial mesoderm. Based on the expression patterns we suggest that a mutation in MFH-1, not fkh-6 is the possible cause for the amputated phenotype.

Five years on the wings of fork head

Since its discovery five years ago the conserved family of fork head/HNF-3-related transcription factors has gained increasing importance for the analysis of gene regulatory mechanisms during embryonic development and in differentiated cells. Different members of this family, which is defined by a conserved 110 amino acid residues encompassing DNA binding domain of winged helix structure, serve as regulatory keys in embryogenesis, in tumorigenesis or in the maintenance of differentiated cell states. The purpose of this review is to summarize the accumulating amount of data on structure, expression and function of fork head/HNF-3-related transcription factors.

Human hepatocyte nuclear factor 4 isoforms are encoded by distinct and differentially expressed genes

Hepatocyte nuclear factor 4 (HNF4) was first identified as a DNA binding activity in rat liver nuclear extracts. Protein purification had then led to the cDNA cloning of rat HNF4, which was found to be an orphan member of the nuclear receptor superfamily. Binding sites for this factor were identified in many tissue-specifically expressed genes, and the protein was found to be essential for early embryonic development in the mouse. We have now isolated cDNAs encoding the human homolog of the rat and mouse HNF4 splice variant HNF4 alpha 2, as well as a previously unknown splice variant of this protein, which we called HNF alpha 4. More importantly, we also cloned a novel HNF4 subtype (HNF4 gamma) derived from a different gene and showed that the genes encoding HNF 4 alpha and HNF4 gamma are located on human chromosomes 20 and 8, respectively. Northern (RNA) blot analysis revealed that HNF4 GAMMA is expressed in the kidney, pancreas, small intestine, testis, and colon but not in the liver, while HNF4 alpha RNA was found in all of these tissues. By cotransfection experiments in C2 and HeLa cells, we showed that HNF4 gamma is significantly less active than HNF4 alpha 2 and that the novel HNF4 alpha splice variant HNF4 alpha 4 has no detectable transactivation potential. Therefore, the differential expression of distinct HNF4 proteins may play a key role in the differential transcriptional regulation of HNF4-dependent genes.

Murine chromosomal location of eight members of the hepatocyte nuclear factor 3/fork head winged helix family of transcription factors

A 100-amino-acid DNA-binding motif, known as the winged helix, was first identified in the mammalian hepatocyte nuclear factor-3 (HNF-3) and Drosophila fork head family of transcription factors. Subsequently, more than 40 different genes that contain the winged helix motif have been identified. In the studies described here, we have determined the murine chromosomal location of eight members of this gene family, HFH-1, HFH-3, HFH-4, HFH-5, HFH-6, HFH-8, BF-1, and BF-2, by interspecific backcross analysis. These genes, designated HNF-3 fork head homolog 1 (Hfh1), Hfh3, Hfh4, Hfh5, Hfh6, Hfh8, Hfh9, and Hfh10, respectively, mapped to 6 different mouse autosomes and are thus well dispersed throughout the mouse genome. Based on this mapping information, we predict the chromosomal location of these genes in humans and discuss the potential of these genes as candidates for uncloned mouse mutations.

Identification of two splice isoforms of mRNA for mouse hepatocyte nuclear factor 4 (HNF-4)

Hepatocyte nuclear factor 4 (HNF-4) is a liver-enriched transcription factor involved in the expression of many liver-specific genes. In the preceding communication (Hata, S., Tsukamoto, T. and Osumi, T. (1992) Biochim. Biophys. Acta 1131, 211-213), we reported the presence of two isoforms of mRNA for HNF-4 in rat liver and kidney. The longer isoform contained a segment of 30 bases which was not present in the shorter one. As an initial step to determine whether or not other mammals have these mRNA isoforms, we isolated a cDNA for mouse HNF-4 using the rat HNF-4 gene as a probe. The cDNA had an open reading frame for a 465 amino acid polypeptide. The deduced amino acid sequence was remarkably conserved between mouse HNF-4 and rat HNF-4 (99.6% identical). Moreover, like the cDNA for the larger rat isoform, the mouse cDNA contained an extra segment of 30 bp in the coding region near the C-terminus. Blotting analyses showed that the mRNA is about 3.7 kb in size and that a single copy of the gene is present in the mouse genome. Next we carried out the polymerase chain reaction (PCR) using primers located just upstream and downstream of the extra segment. Two PCR products were amplified from a mouse liver cDNA library. Determination of their nucleotide sequences proved that they exactly corresponded to the two rat isoforms. Finally, we amplified a DNA fragment (1.1 kb in size) from mouse genomic DNA using the same PCR primers as above. Its nucleotide sequence unequivocally confirmed that different splice donor sites were used to generate the two isoforms.

Fine genetic mapping defines the genetic order of Pax9, Tcf3a, and Acrodysplasia (Adp)

We present here the fine genetic mapping of the proximal part of mouse Chromosome (Chr) 12 between D12Mit54 and D12Mit4. This chromosomal region contains three loci, Pax9, Tcf3a, and Acrodysplasia (Adp), which seem to play an important role in pattern formation during mouse embryogenesis. The Adp mutation, which was created by transgene integration, causes skull, paw, and tail deformities. Pax9, which is expressed in the face, paws, and tail, once qualified as a possible candidate for the Adp locus. We analyzed 997 interspecific backcross progeny for recombination between the markers D12Mit54 and D12Mit4; we recovered 117 recombinants, which were further typed for Pax9, Tcf3a, Adp, D12Mit88, D12Nds1, D12Mit36, and D12Mit34. This study represents the first instance in which all the above loci have been included in a single analysis, thereby allowing unambiguous determination of the genetic order and distance between D12Mit54 and D12Mit4. From our results, we conclude that the Adp locus is distinct from either Pax9 or Tcf3a.

Tissue-specific regulation of mouse hepatocyte nuclear factor 4 expression

Hepatocyte nuclear factor 4 (HNF-4) is a liver-enriched transcription factor and a member of the steroid hormone receptor superfamily. HNF-4 is required for the hepatoma-specific expression of HNF-1 alpha, another liver-enriched transcription factor, suggesting the early participation of HNF-4 in development. To prepare for further study of HNF-4 in development, the tissue-specific expression of the mouse HNF-4 gene was studied by analyzing the promoter region for required DNA elements. DNase-hypersensitive sites in the gene in liver and kidney tissues were found in regions both distal and proximal to the RNA start that were absent in tissues in which HNF-4 expression did not occur. By use of reporter constructs in transient-transfection assays and with transgenic mice, a region sufficient to drive liver-specific expression of HNF-4 was identified. While an HNF-1 binding site between bp -98 and -68 played an important role in the hepatoma-specific promoter activity of HNF-4 in transient-transfection assays, it was not sufficient for the liver-specific expression of a reporter gene in transgenic mice. Distal enhancer elements indicated by the presence of DNase I-hypersensitive sites at kb -5.5 and -6.5, while not functional in transient-transfection assays, were required for the correct expression of the mouse HNF-4 gene in animals.

Tissue-specific regulation of mouse hepatocyte nuclear factor 4 expression

Hepatocyte nuclear factor 4 (HNF-4) is a liver-enriched transcription factor and a member of the steroid hormone receptor superfamily. HNF-4 is required for the hepatoma-specific expression of HNF-1 alpha, another liver-enriched transcription factor, suggesting the early participation of HNF-4 in development. To prepare for further study of HNF-4 in development, the tissue-specific expression of the mouse HNF-4 gene was studied by analyzing the promoter region for required DNA elements. DNase-hypersensitive sites in the gene in liver and kidney tissues were found in regions both distal and proximal to the RNA start that were absent in tissues in which HNF-4 expression did not occur. By use of reporter constructs in transient-transfection assays and with transgenic mice, a region sufficient to drive liver-specific expression of HNF-4 was identified. While an HNF-1 binding site between bp -98 and -68 played an important role in the hepatoma-specific promoter activity of HNF-4 in transient-transfection assays, it was not sufficient for the liver-specific expression of a reporter gene in transgenic mice. Distal enhancer elements indicated by the presence of DNase I-hypersensitive sites at kb -5.5 and -6.5, while not functional in transient-transfection assays, were required for the correct expression of the mouse HNF-4 gene in animals.

Deleted chromosome 20 from a patient with Alagille syndrome isolated in a cell hybrid through leucine transport selection: study of three candidate genes

Alagille syndrome (AGS) is a well-defined genetic entity assigned to the short arm of Chromosome (Chr) 20 by a series of observations of AGS patients associated with microdeletions in this region. By fusing lymphoblastoid cells of an AGS patient that exhibited a microdeletion in the short arm of Chr 20 encompassing bands p11.23 to p12.3 with rodent thermosensitive mutant cells (CHOtsH1-1) deficient in-leucyl-tRNA synthetase, we isolated a somatic cell hybrid segregating the deleted human Chr 20. This hybrid clone, designated NR2, was characterized by several methods, including PCR, with eight pairs of oligonucleotides mapped to Chr 20: D20S5, D20S41, D20S42, D20S56, D20S57, D20S58, adenosine deaminase (ADA), and Prion protein (PRIP); Restriction Fragment Length Polymorphism (RFLP) analyses with four genomic anonymous probes (D20S5, cD3H12, D20S17, D20S18); and fluorescent in situ hybridization (FISH) with total human DNA and D20Z1, a sequence specific to the human Chr 20 centromere, as probes. The NR2 hybrid allowed us to exclude three candidate genes for AGS: hepatic nuclear factor 3 beta (HNF3 beta), paired box 1 (PAX1), and cystatin C (CST3) as shown by their localization outside of the deletion. The NR2 hybrid is a powerful tool for the mapping of new probes of this region, as well as for obtaining new informative probes specific for the deletion by subtractive cloning of the region. Such markers will be useful for linkage analysis and screening of cDNA libraries.

Molecular cloning of a diverged homeobox gene that is rapidly down-regulated during the G0/G1 transition in vascular smooth muscle cells

Adult vascular smooth muscle cells dedifferentiate and reenter the cell cycle in response to growth factor stimulation. Here we describe the molecular cloning from vascular smooth muscle, the structure, and the chromosomal location of a diverged homeobox gene, Gax, whose expression is largely confined to the cardiovascular tissues of the adult. In quiescent adult rat vascular smooth muscle cells, Gax mRNA levels are down-regulated as much as 15-fold within 2 h when these cells are induced to proliferate with platelet-derived growth factor (PDGF) or serum growth factors. This reduction in Gax mRNA is transient, with levels beginning to rise between 8 and 24 h after mitogen stimulation and returning to near normal by 24 to 48 h. The Gax down-regulation is dose dependent and can be correlated with the mitogen's ability to stimulate DNA synthesis. PDGF-AA, a weak mitogen for rat vascular smooth muscle cells, did not affect Gax transcript levels, while PDGF-AB and -BB, potent mitogens for these cells, were nearly as effective as fetal bovine serum. The removal of serum from growing cells induced Gax expression fivefold within 24 h. These data suggest that Gax is likely to have a regulatory function in the G0-to-G1 transition of the cell cycle in vascular smooth muscle cells.

Molecular cloning of a diverged homeobox gene that is rapidly down-regulated during the G0/G1 transition in vascular smooth muscle cells

Adult vascular smooth muscle cells dedifferentiate and reenter the cell cycle in response to growth factor stimulation. Here we describe the molecular cloning from vascular smooth muscle, the structure, and the chromosomal location of a diverged homeobox gene, Gax, whose expression is largely confined to the cardiovascular tissues of the adult. In quiescent adult rat vascular smooth muscle cells, Gax mRNA levels are down-regulated as much as 15-fold within 2 h when these cells are induced to proliferate with platelet-derived growth factor (PDGF) or serum growth factors. This reduction in Gax mRNA is transient, with levels beginning to rise between 8 and 24 h after mitogen stimulation and returning to near normal by 24 to 48 h. The Gax down-regulation is dose dependent and can be correlated with the mitogen's ability to stimulate DNA synthesis. PDGF-AA, a weak mitogen for rat vascular smooth muscle cells, did not affect Gax transcript levels, while PDGF-AB and -BB, potent mitogens for these cells, were nearly as effective as fetal bovine serum. The removal of serum from growing cells induced Gax expression fivefold within 24 h. These data suggest that Gax is likely to have a regulatory function in the G0-to-G1 transition of the cell cycle in vascular smooth muscle cells.