Mitf-PU.1 interactions with the tartrate-resistant acid phosphatase gene promoter during osteoclast differentiation.

It has been postulated that the transcription factors micropthalmia associated factor (Mitf) and PU.1 interact with the tartrate-resistant acid phosphatase (TRAP) gene promoter and activate TRAP gene expression in osteoclasts. However, studies on the interaction of these factors with the TRAP promoter employing nuclear extracts from osteoclasts and osteoclast precursors have not been reported. We therefore treated murine mononuclear phagocyte cells with various cytokines to generate cultures of osteoclasts and macrophagic cells with high or low potential to form osteoclasts. The presence of Mitf and PU.1 in nuclear extracts from these cultures and the ability of these factors to bind to the TRAP promoter was then assessed. We demonstrate that Mitf and a related factor, TFE3, are present in nuclear extracts from all cultures and bind the TRAP promoter. While PU.1 is present in nuclear extracts from all cultures, it does not significantly interact with a putative binding site in the TRAP promoter. These results suggest Mitf and PU.1 interactions with the TRAP promoter are not responsible for the specific activation of TRAP gene expression in osteoclasts.

Tissue-specific activity of the proximal human calcitonin receptor promoter is mediated by Sp1 and an epigenetic phenomenon.

To identify cis-acting sequences transcriptionally regulating the human calcitonin receptor (hCTR) gene, hCTR promoter/luciferase gene constructs were transiently or stably transfected into hCTR-positive and -negative cell lines. Luciferase assays demonstrated that the proximal hCTR promoter (hCTRP1) was transcriptionally active in all cell lines tested. High-level hCTRP1 activity depended on an 11 bp Sp1/Sp3 binding site. Electrophoretic mobility shift assay showed that this region bound the transcription factors Sp1 and Sp3. We further showed that hCTRP1 was strongly activated by the 11 bp Sp1/Sp3 binding site in hCTRP1/luciferase-, Sp1-transfected Drosophila S2 cells. Bisulphite-mediated sequencing of genomic DNA from hCTR-expressing and -non-expressing cell lines demonstrated that the endogenous hCTRP1 was hypomethylated in all cell lines tested. These results suggest that the hCTRP1 is activated by the tissue-ubiquitous transcription factor Sp1 and that an epigenetic process unrelated to CpG methylation represses its activity in hCTR-negative tissues.

Phosphorylation of GATA-1 increases its DNA-binding affinity and is correlated with induction of human K562 erythroleukaemia cells.

We have investigated by electrophoretic mobility shift assay (EMSA) the level of GATA-1 DNA-binding activity in nuclear extracts prepared from the human erythroleukaemic cell line, K562, after erythroid induction by hemin, sodium butyrate (NaB) or Trichostatin A or treatment with N -acetylcysteine (NAC). Relative to extract from untreated cells, GATA-1 binding activity increased markedly in all cases. However, immunoblot analysis revealed unchanged levels of GATA-1 protein after induction. Incubation of induced but not uninduced K562 extracts with phosphatase prior to EMSA weakened the binding activity, suggesting that the increase in GATA-1 binding following induction of K562 cells was a consequence of phosphorylation. When the mouse erythroleukaemic cell line MEL was induced with dimethylsulphoxide (DMSO), NaB or NAC, GATA-1 binding activity fell with DMSO, rose significantly with NaB and remained at about the same level in NAC-induced cells. In this case immunoblotting revealed that GATA-1 protein levels were in accord with the EMSA data. The DNA-binding activities of induced and uninduced MEL cell nuclear extracts were decreased by incubation with phosphatase, showing that phosphoryl-ation and DNA binding of GATA-1 are already optimalin these cells. The DNA-binding activity of affinity-purified GATA-1 from MEL cells was also reduced by phosphatase treatment, showing that phosphorylation/dephosphorylation is directly affecting the factor. Furthermore, when a comparison was made by EMSA of nuclear extracts prepared from K562 and MEL cells untreated or incubated with okadaic acid, a phosphatase inhibitor, GATA-1 binding was seen to increase with K562 cells, whereas with MEL cells there was no change in GATA-1 binding. Overall the results suggest that the level of GATA-1 phosphorylation increases after the induction of K562, but not MEL cells, where GATA-1 is already highly phosphorylated. Furthermore, phosphorylation increases the binding affinity of GATA-1 for a canonical binding site.

Factor binding to the human gamma-globin gene distal CCAAT site: candidates for repression of the normal gene or activation of HPFH mutants.

We have examined factor binding to the distal human gamma-globin CCAAT site and three naturally occurring hereditary persistence of fetal haemoglobin (HPFH) mutations of this site. Factor binding was examined using nuclear extracts from the erythroleukaemic cell lines K562 and MEL, and from A4 cells, a non-transformed mouse bone marrow stem cell line, using the electrophoretic mobility shift assay. Under standard binding conditions, in addition to the previously reported binding by a CCAAT factor (CP1) and GATA-1, the wild-type (wt) sequence bound high mobility factors which appeared to be GATA-2 isoforms. However, when the non-specific competitor conditions were varied, the binding profile with K562, but not MEL nuclear extract, was substantially altered. CP1 and GATA-1 were absent, and two new factors were detected, one of which bound preferentially to the Greek and Japanese non-deletion HPFH mutants. However, binding by the GATA-2 isoforms to the wt sequence was maintained with both cell types, as it was using the A4 cell line. With modified binding conditions, in A4 cells the two non-deletion and the Black deletion HPFH mutants each had a different protein binding profile which was lost on erythroid induction of the cells. We discuss the possibility that the GATA-2 isoforms bound to the wt sequence may function to suppress wt gamma gene expression in the bone marrow. Additionally, those factors which bind preferentially either to the deletion or non-deletion HPFH mutants may play positive roles in establishing an active chromatin structure.

GATA-2 is a maternal transcription factor present in Xenopus oocytes as a nuclear complex which is maintained throughout early development.

We show that Xenopus oocytes and embryos contain GATA-2, stored in nuclei as a non-chromatin-bound complex. Its binding site specificity is different from that of GATA-1, having a much higher affinity for the motif with a core GATC sequence. This binding site preference was markedly reduced by either release of the factor with deoxycholate or purification on a DNA affinity column, suggesting a role for a cofactor(s). The identity of the maternal GATA factor was established as GATA-2 in two ways: (1) binding to an oligonucleotide probe was abolished by inclusion of either of two GATA-2 monoclonal antibodies, and (2) a protein of correct molecular weight for GATA-2 was detected by immunoblotting with a polyclonal antibody raised against a Xenopus GATA-2-specific peptide. Although predominantly complexed, some of the oocyte GATA-2 is functional as a transcription factor because the transcriptional activity of the chicken betaH-globin promoter injected into oocytes was reduced by mutation of either of two GATA binding sites. This effect was more pronounced when the stronger of the two sites was mutated. Butyrate treatment of oocytes stimulated cap-site initiation by up to 17-fold with both normal promoter and GATA site mutant constructs, showing that the mechanism of butyrate stimulation is not via GATA-2. The possible significance of regulating the availability of maternal GATA-2 during early development is discussed.

Nuclear translocation of a maternal CCAAT factor at the start of gastrulation activates Xenopus GATA-2 transcription.

The transcription factor GATA-2 is present in blood cell precursors and plays a pivotal role in the control of erythroid differentiation. In Xenopus embryos, low levels of GATA-2 mRNA are maternally derived, while the onset of zygotic GATA-2 expression coincides with commitment to haematopoietic lineages. However, its initial transcriptional activation is not restricted to the presumptive blood islands, but occurs throughout ventral and lateral regions, in all three germ layers. In order to determine how this expression pattern is controlled, we have isolated and characterized the Xenopus GATA-2 gene. We show that 1.65 kb of 5' flanking sequences are sufficient to direct both correct transcriptional initiation in oocytes and appropriate temporal and spatial gene expression in early embryos. The transgene is activated during gastrulation and by neurula stages in predominantly expressed in the ventral hemisphere. We demonstrate that a CCAAT element is necessary for gene activity in both systems and that extracts prepared from oocytes and embryos contain a factor which specifically recognizes this element. We also show that cytoplasmic localization inhibits the function of this CCAAT factor until the beginning of gastrulation, when the zygotic GATA-2 gene is activated. These observations extend our understanding of the mechanisms by which maternal factors control the temporal activation of transcription in early vertebrate embryos.

Human globin gene transcription in injected Xenopus oocytes: enhancement by sodium butyrate.

We have studied the expression of the human adult (beta) and foetal (gamma) globin genes by injecting a cosmid containing the complete -G gamma-A gamma-psi beta-delta-beta cluster into Xenopus oocytes. The transcripts from the gamma and beta genes were characterised with respect to their 5' termini by S1 nuclease mapping using probes which extend 5' to the cap site and 3' into the first exon. The only beta transcripts detectable in injected oocytes spanned from either -231 or -177 (cap site +1) into the first exon. The sensitivity of this transcription to inhibition by a low concentration of alpha-amanitin indicated that polymerase II was responsible. S1 analysis of gamma gene transcripts showed that the predominant transcript mapped from a site at -227, but a low level of transcription from the cap site was also detectable. Incubation of oocytes in saline solution containing sodium butyrate prior to injection resulted in a significant stimulation of transcription from the gamma gene cap site. In contrast, transcription from the beta gene upstream sites was only slightly increased. The possible role of sodium butyrate in promoting this effect is discussed in the context of its known property as a modifier of cellular chromatin.

Transcriptional analysis of human zeta globin genes.

The human embryonic alpha-like globin gene (zeta) and a closely linked pseudogene (psi zeta) are located on chromosome 16. The psi zeta gene has a nonsense mutation in exon 1 but has identical promoter sequence and RNA processing sites to the zeta gene, raising the possibility that both psi zeta and zeta are transcriptionally active. We have studied transcription of the human zeta and psi zeta genes in a number of systems to examine their cell type specificity and enhancer requirement. (i) Cloned zeta and psi zeta genes transfected into human HeLa or monkey Cos7 tissue culture cells show no transcriptional activity. The presence of an SV40 enhancer does not activate the zeta promoter except at low levels when in very close proximity (less than 50 bp from the CCAAT box). (ii) In contrast to other tissue-specific genes tested to date, both zeta and psi zeta gene promoters initiate transcription efficiently when micro-injected into Xenopus oocyte nuclei. We suggest that embryonic-specific factors in the oocyte may permit efficient zeta gene transcription. Furthermore, the zeta promoter sequence from - 111 to + 38 bp is sufficient for transcription in this system.

Somatic nuclei in amphibian oocytes: evidence for selective gene expression.

Previous work has shown that multiple HeLa nuclei injected into Xenopus oocytes remain transcriptionally active for many days and that the expression of HeLa genes in oocytes can be detected by 2-D gel electrophoresis. We show here that of 25 proteins which have the electrophoretic properties of HeLa gene products, only 3 are expressed in injected oocytes. To test that these proteins are products of HeLa genes, and not products of activated oocyte genes, we have injected HeLa nuclei into enucleated oocytes. Three days later, several HeLa proteins were synthesized. The turning off of most HeLa genes in injected oocytes is apparently not at the translational level. This is indicated by the fact that adenovirus mRNA is efficiently translated when injected into Xenopus oocytes. When adenovirus-infected HeLa cells nuclei are injected into oocytes the adenovirus genes are not expressed, although some HeLa genes are expressed by the same nuclei. The same HeLa genes as are expressed or switched off in injected Xenopus oocytes are also preferentially expressed or switched off in injected oocytes of a Urodele amphibian, Pleurodeles. This suggests that conditions or molecules may exist in oocytes which selectively impose on injected nuclei a new programme of gene expression.

Injected amphibian oocytes: a living test tube for the study of eukaryotic gene transcription?

Somatic-cell nuclei injected in Xenopus laevis oocytes remain transcriptionally active for up to 4 weeks. The rate of RNA synthesis increases as the somatic-cell nuclei enlarge. The message activity of the RNA synthesized by the injected nuclei can be demonstrated, as new proteins are made a few days after injection of HeLa-cell nuclei. Several lines of evidence demonstrate that these proteins are coded for by the HeLa-cell genome and that they result from RNA synthesized within the oocytes in the course of several days. Furthermore gene expression by the somatic-cell nuclei is selective, since only a few gene products can be detected. The oocyte cytoplasm is able to reprogramme the injected nuclei in such a way that only genes of the type normally active in oocytes are expressed. We describe experiments which indicate that genes previously inactive in the somatic cells can be switched on by the oocyte cytoplasm. These experiments have involved the use of the frog somatic-cell nuclei injected into newt oocytes. In addition, we have also found that even purified DNA is transcribed when injected into the nucleus of Xenopus oocytes. The identification of the molecules presumed to regulate chromatin transcription has been hampered by the lack of a satisfactory experimental system for assaying the biological activity of purified macromolecules. Living oocytes injected with somatic-cell nuclei (or cloned DNA) may provide such a system, since these regulatory factors could be introduced by a second injection.

Injected nuclei in frog oocytes:RNA synthesis and protein exchange.

Nuclei from HeLa and other mammalian cells have been injected into Xenopus oocytes. The synthesis, uptake, and release of RNA and proteins by injected nuclei have been investigated by autoradiography. Injected nuclei which undergo enlargement synthesize RNA continuously for up to 28 days. When oocytes are incubated in [3H]uridine or [3H]guanosine, injected nuclei are labelled nearly as strongly as the nucleoli, but much more strongly than the nucleoplasm of the oocyte's germinal vesicle. Injected nuclei appear to increase their rate of RNA synthesis during incubation in oocytes. This apparent increase in the rate of RNA synthesis is correlated with nuclear enlargement, as well as with the loss of protein from injected nuclei and with their uptake of histone and nonhistone proteins from oocyte cytoplasm. Injected HeLa nuclei lose most of the previously synthesized RNA from their nucleoplasm, but little if any of the RNA from their remaining nucleoli.