Uncoordinate synthesis of histone H1 in cells arrested in the G1 phase

Differential cytoplasmic and whole cellular expression of histone H1 within the avian bursa of Fabricius

Bursal anti-steroidogenic peptide (BASP), purified from the chicken bursa of Fabricius (BF), has been previously demonstrated to be a potent and efficacious inhibitor of steroid hormone biosynthesis from chicken ovarian, and both mammalian and avian adrenal cells in vitro. Other studies have demonstrated that BASP can markedly reduce avian and mammalian mitogen-stimulated lymphocyte proliferation. Recent studies have indicated that BASP has a structural and functional relationship with histone H1. Immunohistochemical studies using a monoclonal antibody, which is known to recognize a common histone H1 epitope from several plant and animal species identified the protein within the cytoplasm and nucleus of distinct cells within both the cortex and medulla of all BF follicles. Additionally, epithelial cells within the BF expressed the protein strongly in the cytoplasm with reduced nuclear staining. In contrast, the same antibody did not recognize the protein in thymus of the same animals. The differential expression of histone H1 immunoreactivity within selected cells of the BF may support a previous proposed role of histone H1 in extranuclear and extracellular signaling in chickens and possibly other species.

Characterization of the histone H1-binding protein, NASP, as a cell cycle-regulated somatic protein

Nuclear autoantigenic sperm protein (NASP), initially described as a highly autoimmunogenic testis and sperm-specific protein, is a histone-binding protein that is a homologue of the N1/N2 gene expressed in oocytes of Xenopus laevis. Here, we report a somatic form of NASP (sNASP) present in all mitotic cells examined, including mouse embryonic cells and several mouse and human tissue culture cell lines. Affinity chromatography and histone isolation demonstrate that NASP from myeloma cells is complexed only with H1, linker histones. Somatic NASP is a shorter version of testicular NASP (tNASP) with two deletions in the coding region arising from alternative splicing and differs from tNASP in its 5' untranslated regions. We examined the relationship between NASP mRNA expression and the cell cycle and report that in cultures of synchronized mouse 3T3 cells and HeLa cells sNASP mRNA levels increase during S-phase and decline in G(2), concomitant with histone mRNA levels. NASP protein levels remain stable in these cells but become undetectable in confluent cultures of nondividing CV-1 cells and in nonmitotic cells in various body tissues. Expression of sNASP mRNA is regulated during the cell cycle and, consistent with a role as a histone transport protein, NASP mRNA expression parallels histone mRNA expression.

Expression of murine H1 histone genes during postnatal development

Murine genes encoding the seven H1 histone isoforms H1.1-H1.5, H1(o) and H1t have been isolated and sequenced. We have established expression patterns of these genes in several tissues during postnatal development. For that analysis, RNase protection assay rather than Northern blot hybridization was used, since the sequences of these genes are highly similar and would cross-hybridize under Northern blot conditions. Expression patterns of H1.1 to H1.5 and H1(o) were determined in tissues of animals at days 5, 9 and 20 after birth and of adult mice. In addition, RNA was analyzed in three mouse cell lines (NIH3T3, P19, TM4). Transcription of the subtype genes H1.2 and H1.4 was found in all tissues and cell lines studied. The most varied expression patterns were obtained with the H1.1 subtype. H1.1 mRNA was found at high concentrations in thymus and spleen throughout development and in testis beginning with a low expression in 5-day-old animals and increasing levels in testis RNA from 9- and 20-day-old and adult mice. H1(o) mRNA was found primarily in highly differentiated tissues with concentrations decreasing from 5-day-old to adult animals.

The mouse histone H1 genes: gene organization and differential regulation

There are six mouse histone H1 genes present in the histone gene cluster on mouse chromosome 13. These genes encode five histone H1 variants expressed in somatic cells, H1a to H1e, and the testis-specific H1t histone. Two of the genes that have not been assigned previously to the five somatic H1 subtypes have been identified as encoding the H1b and H1d subtypes. Three of the H1 genes, H1a, H1c and H1t, are present on an 80 kb segment of DNA that contains nine core histone genes. Two others, H1d and H1e, are present in a second patch, while the H1b gene is at least 500 kb away in a patch containing 14 core histone genes. The histone H1 genes are differentially expressed. All five genes for the somatic histone H1 proteins are expressed in exponentially growing cells. However, the levels of H1a, H1b and H1d mRNAs are greatly reduced in cells that are terminally differentiated or arrested in G0, while the H1c and H1e mRNAs continue to be expressed. In addition to the major RNA that ends at the stem-loop, the H1c gene expresses a longer, polyadenylated mRNA in differentiated cells, although in varying amounts. None of the other histone H1 genes encodes detectable amounts of polyadenylated mRNAs.

Modifications in molecular mechanisms associated with control of cell cycle regulated human histone gene expression during differentiation

Histone proteins are preferentially synthesized during the S-phase of the cell cycle, and the temporal and functional coupling of histone gene expression with DNA replication is mediated at both the transcriptional and posttranscriptional levels. The genes are transcribed throughout the cell cycle, and a 3-5-fold enhancement in the rate of transcription occurs during the first 2 h following initiation of DNA synthesis. Control of histone mRNA stability also accounts for some of the 20-100fold increase in cellular histone mRNA levels during S-phase and for the rapid and selective degradation of the mRNAs at the natural completion of DNA replication or when DNA synthesis is inhibited. Two segments of the proximal promoter, designated Sites I and II, influence the specificity and rate of histone gene transcription. Occupancy of Sites I and II during all periods of the cell cycle by three transacting factors (HiNF-A, HiNF-C, and HiNF-D) suggests that these protein-DNA interactions are responsible for the constitutive transcription of histone genes. Binding of HiNF-D in Site II is selectively lost, whereas occupancy of Site I by HiNF-A and -C persists when histone gene transcription is down regulated when cells terminally differentiate. These results are consistent with a primary role for interactions of HiNF-D with a proximal promoter element in rendering cell growth regulated human histone genes transcribable in proliferating cells.

Changes in the H-1 histone complement during myogenesis. I. Establishment by differential coupling of H-1 species synthesis to DNA replication

Proportions of the four major chicken H-1 histones (referred to as H-1's a-d) change during in vitro skeletal myogenesis. As myoblasts fuse and differentiate into myotubes, the relative amount of H-1c increases dramatically. The change occurs primarily because synthesis of the H-1 species is coupled to DNA synthesis to different extents. H-1c synthesis is least tightly coupled to DNA replication in precursor myoblasts and in differentiated myotubes. Thus H-1c synthesis predominates after dividing myoblasts fuse into postmitotic myotubes. This results in the replacement of pre-existing H-1 and therefore increases the relative amount of H-1c. Differences in the stability of the H-1's are also involved in changing H-1 proportions. The results show that changes in H-1 proportions during myogenesis are a consequence of withdrawal from the cell cycle. The data provides a general mechanistic explanation of how tissue-specific H-1 proportions are established.

G1- and S-phase syntheses of histones H1 and H1o in mitotically selected CHO cells: utilization of high-performance liquid chromatography

We have employed high-performance liquid chromatography (HPLC) to investigate the syntheses of histones H1 and H1o as synchronized cells traverse from mitosis to S phase. Chinese hamster (line CHO) cells were synchronized by mitotic selection, and, at appropriate times, they were pulse labeled for 1 h with [3H]lysine. Histones H1 and H1o were extracted by blending radiolabeled and carrier cells directly in 0.83 M HC1O4; the total HC1O4-soluble, Cl3CCO2H-precipitable proteins were then separated by a modification of an HPLC system employing three mu Bondapak reversed-phase columns [Gurley, L. R., D'Anna, J. A., Blumenfeld, M., Valdez, J. G., Sebring, R. J., Donahue, D. K., Prentice, D. A., & Spall, W. D. (1984) J. Chromatogr. 297, 147-165]. These procedures (1) produce minimally perturbed populations of synchronized proliferating cells and (2) maximize the recovery of radiolabeled histones during isolation and analysis. Measurements of rates of synthesis indicate that the rate of H1 synthesis increases (3.6 +/- 0.5)-fold as cells traverse from early to mid G1; as cells enter S phase, the rate of H1 synthesis increases an additional congruent to 22-fold and is proportional to the number of S-phase cells. In contrast to H1, the rate of H1o synthesis is nearly constant throughout G1. As cells progress into S phase, the rate of H1o synthesis increases (3.1 +/- 0.2)-fold so that it also appears to be proportional to the number of S-phase cells. Except for the first 1-2 h after mitotic selection, these results are similar to those obtained when cells are synchronized in G1 with the isoleucine deprivation procedure.

Effect of adenovirus infection on expression of human histone genes

The influence of adenovirus type 2 infection of HeLa cells upon expression of human histone genes was examined as a function of the period of infection. Histone RNA synthesis was assayed after run-off transcription in nuclei isolated from mock-infected cells and after various periods of adenovirus infection. Histone protein synthesis was measured by [3H]leucine labeling of intact cells and fluorography of electrophoretically fractionated nuclear and cytoplasmic proteins. The cellular representation of RNA species complementary to more than 13 different human histone genes was determined by RNA blot analysis of total cellular, nuclear or cytoplasmic RNA by using a series of 32P-labeled cloned human histone genes as hybridization probes and also by analysis of 3H-labeled histone mRNA species synthesized in intact cells. By 18 h after infection, HeLa cell DNA synthesis and all parameters of histone gene expression, including transcription and the nuclear and cytoplasmic concentrations of core and H1 mRNA species, were reduced to less than 5 to 10% of the control values. By contrast, transcription and processing of other cellular mRNA sequences have been shown to continue throughout this period of infection. The early period of adenovirus infection was marked by an inhibition of transcription of histone genes that accompanied the reduction in rate of HeLa cell DNA synthesis. These results suggest that the adenovirus-induced inhibition of histone gene expression is mediated in part at the transcriptional level. However, the persistence of histone mRNA species at concentrations comparable to those of mock-infected control cells during the early phase of the infection, despite a reduction in histone gene transcription and histone protein synthesis, implies that histone gene expression is also regulated post-transcriptionally in adenovirus-infected cells. These results suggest that the tight coupling between histone mRNA concentrations and the rate of cellular DNA synthesis, observed when DNA replication is inhibited by a variety of drugs, is not maintained after adenovirus infection.

Specific alterations in the pattern of histone-3 synthesis during conversion of human leukemic cells to terminally differentiated cells in culture

The presence of nano- to micromolar concentrations of 12-0-tetradecanoyl-phorbol-13-acetate (TPA) in suspension cultures of human promyelocytic leukemia cells, HL-60, or human monocytic leukemia cells, THP-1, resulted in the appearance of macrophage-like cells attached to the substratum. The terminally TPA-differentiated cells continued to synthesize histones at a low rate even though DNA replication had ceased. The pattern of synthesis of histone variants in differentiated cells differed from that in undifferentiated cells and resembled that of quiescent or density-arrested cells. In undifferentiated cells, all three histone-H3 variants are synthesized, while in quiescent cells, only the H3.3 variant is synthesized. When TPA-differentiated macrophages were placed in normal medium, the pattern of histone synthesis was not altered, thus substantiating previous findings that the differentiation is irreversible. Further, TPA-differentiated macrophages and macrophages isolated from a normal human donor exhibited identical pattern of histone synthesis. Altogether, the results indicate that changes in the synthetic rates of histones during the TPA-induced maturation of human leukemic cells is not directly due to TPA or terminal cell differentiation per se but is due to the cessation of cell proliferation and DNA replication.

Is human histone gene expression autogenously regulated?

It has been well documented that core and H1 histone mRNAs accumulate in a manner which closely parallels the initiation of DNA synthesis and histone protein synthesis, suggesting that the onset of histone gene expression early during S phase is at least in part transcriptionally mediated. In fact, it appears that throughout S phase the synthesis of histone proteins is modulated by the availability of histone mRNAs. On the other hand, the stability of histone mRNAs and the destabilization of histone mRNAs when DNA replication is completed or inhibited are highly selective, tightly coupled and largely post-transcriptionally controlled. We present a model to account for histone mRNA turnover whereby the natural or inhibitor-induced termination of DNA replication results in an immediate loss of high affinity binding sites for newly synthesized histone proteins which in turn brings about a transient accumulation of unbound histones. These unbound histones could modify the histone translation complex, via interactions with polysomal histone mRNAs, in such a manner as to render histone mRNAs accessible to cellular ribonucleases. This type of mechanism would be operative solely at the post-transcriptional level and would be compatible with the rapid, RNA synthesis-independent destabilization of histone mRNAs which occurs following inhibition of DNA replication, as well as with the requirement for protein synthesis for histone mRNA destabilization to be initiated.

Effect of adenovirus infection on expression of human histone genes

The influence of adenovirus type 2 infection of HeLa cells upon expression of human histone genes was examined as a function of the period of infection. Histone RNA synthesis was assayed after run-off transcription in nuclei isolated from mock-infected cells and after various periods of adenovirus infection. Histone protein synthesis was measured by [3H]leucine labeling of intact cells and fluorography of electrophoretically fractionated nuclear and cytoplasmic proteins. The cellular representation of RNA species complementary to more than 13 different human histone genes was determined by RNA blot analysis of total cellular, nuclear or cytoplasmic RNA by using a series of 32P-labeled cloned human histone genes as hybridization probes and also by analysis of 3H-labeled histone mRNA species synthesized in intact cells. By 18 h after infection, HeLa cell DNA synthesis and all parameters of histone gene expression, including transcription and the nuclear and cytoplasmic concentrations of core and H1 mRNA species, were reduced to less than 5 to 10% of the control values. By contrast, transcription and processing of other cellular mRNA sequences have been shown to continue throughout this period of infection. The early period of adenovirus infection was marked by an inhibition of transcription of histone genes that accompanied the reduction in rate of HeLa cell DNA synthesis. These results suggest that the adenovirus-induced inhibition of histone gene expression is mediated in part at the transcriptional level. However, the persistence of histone mRNA species at concentrations comparable to those of mock-infected control cells during the early phase of the infection, despite a reduction in histone gene transcription and histone protein synthesis, implies that histone gene expression is also regulated post-transcriptionally in adenovirus-infected cells. These results suggest that the tight coupling between histone mRNA concentrations and the rate of cellular DNA synthesis, observed when DNA replication is inhibited by a variety of drugs, is not maintained after adenovirus infection.