Replication-linked histone acetylation in rat liver tissue is sensitive to alkylating agents

Replication factors MCM2 and ORC1 interact with the histone acetyltransferase HBO1

The minichromosome maintenance (MCM) proteins, together with the origin recognition complex (ORC) proteins and Cdc6, play an essential role in eukaryotic DNA replication through the formation of a pre-replication complex at origins of replication. We used a yeast two-hybrid screen to identify MCM2-interacting proteins. One of the proteins we identified is identical to the ORC1-interacting protein termed HBO1. HBO1 belongs to the MYST family, characterized by a highly conserved C2HC zinc finger and a putative histone acetyltransferase domain. Biochemical studies confirmed the interaction between MCM2 and HBO1 in vitro and in vivo. An N-terminal domain of MCM2 is necessary for binding to HBO1, and a C2HC zinc finger of HBO1 is essential for binding to MCM2. A reverse yeast two-hybrid selection was performed to isolate an allele of MCM2 that is defective for interaction with HBO1; this allele was then used to isolate a suppressor mutant of HBO1 that restores the interaction with the mutant MCM2. This suppressor mutation was located in the HBO1 zinc finger. Taken together, these findings strongly suggest that the interaction between MCM2 and HBO1 is direct and mediated by the C2HC zinc finger of HBO1. The biochemical and genetic interactions of MYST family protein HBO1 with two components of the replication apparatus, MCM2 and ORC1, suggest that HBO1-associated HAT activity may play a direct role in the process of DNA replication.

Identification of mouse histone deacetylase 1 as a growth factor-inducible gene

Reversible acetylation of core histones plays an important role in transcriptional regulation, cell cycle progression, and developmental events. The acetylation state of histones is controlled by the activities of acetylating and deacetylating enzymes. By using differential mRNA display, we have identified a mouse histone deacetylase gene, HD1, as an interleukin-2-inducible gene in murine T cells. Sequence alignments revealed that murine HD1 is highly homologous to the yeast RPD3 pleiotropic transcriptional regulator. Indirect immunofluorescence microscopy proved that mouse HD1 is a nuclear protein. When expressed in yeast, murine HD1 was also detected in the nucleus, although it failed to complement the rpd3delta deletion phenotype. HD1 mRNA expression was low in G0 mouse cells but increased when the cells crossed the G1/S boundary after growth stimulation. Immunoprecipitation experiments and functional in vitro assays showed that HD1 protein is associated with histone deacetylase activity. Both HD1 protein levels and total histone deacetylase activity increased upon interleukin-2 stimulation of resting B6.1 cells. When coexpressed with a luciferase reporter construct, HD1 acted as a negative regulator of the Rous sarcoma virus enhancer/promoter. HD1 overexpression in stably transfected Swiss 3T3 cells caused a severe delay during the G2/M phases of the cell cycle. Our results indicate that balanced histone acetylation/deacetylation is crucial for normal cell cycle progression of mammalian cells.

Effect of aging and gamma radiation on acetylation of rat liver histones

We studied age-related and radiation induced changes in histone acetyltransferase activity and acetylation of histone fractions in normal and regenerating rat livers. Male Wistar rats aged 1-28 months were used through the studies. They were exposed to whole-body doses of 5.7 Gy gamma radiation from a 60Co source and partial hepatectomy was carried out 30 min after irradiation. Within nuclei isolated from normal and regenerating livers of rats, acetyltransferase activity of histones increased with age. An age-dependent decrease in histone acetyltransferase activity in normal and regenerating rat livers was also obvious at the time of maximum activity (i.e. at the 4th min of incubation). The radiation-induced drop in acetyltransferase activity was mild in normal livers. Twenty-four hours after partial hepatectomy, acetyltransferase activity in regenerating livers was almost completely inhibited by irradiation. The acetylation of histone fractions and subfractions was not equal. The highest activity among all histone fractions was found in fraction H4. In regenerating livers of non-irradiated and irradiated rats, there was a greater rate of acetylation in tetraacetylated subfractions of histone H4 compared with intact livers.

Properties of the yeast nuclear histone deacetylase

A nuclear histone deacetylase from yeast was partially purified and some of its characteristics were studied. Histone deacetylase activity was stimulated in vitro by high-mobility-group nonhistone chromatin proteins 1 and 2 and ubiquitin and inhibited by spermine and spermidine, whereas n-butyrate had no significant inhibitory effect. Like the mammalian enzyme, partially purified histone deacetylase from yeast was strongly inhibited by trichostatin A. However, in crude extract preparations the yeast enzyme was not inhibited and treatment with trichostatin in vivo did not show any effect, either on the histone acetylation level or on cell viability. At low ionic strength, the enzyme can be isolated as a complex of high molecular mass that is much less inhibited by trichostatin A than is partially purified histone deacetylase activity. Furthermore, radiolabelled oligonucleosomes were more efficiently deacetylated by the complex than by the low-molecular-mass form of the enzyme. The histone deacetylase activity was separated from a polyamine deacetylase activity and its specificity studied. Using h.p.l.c.-purified core histone species as substrate, histone deacetylase from yeast is able to deacetylate all core histones with a slight preference for H3. Our results support the idea that the yeast histone deacetylase may act as a high-molecular-mass complex in vivo.

Cytosolic and nuclear spermidine acetyltransferases in growing NIH 3T3 fibroblasts stimulated with serum or polyamines: relationship to polyamine-biosynthetic decarboxylases and histone acetyltransferase

The expression (mRNA level of enzymic activity) of cytosolic and nuclear spermidine acetyltransferases was studied in NIH 3T3 fibroblasts, either (1) serum-starved and stimulated to grow by serum refeeding, or (2) treated with inhibitors of ornithine decarboxylase (ODC) (MDL 72.175) and S-adenosylmethionine decarboxylase (AdoMetDC) (MDL 73.811) and stimulated to grow by spermidine. Expression of the known growth-regulated genes for ODC, AdoMetDC and histone acetyltransferase was also examined. The mRNA for spermidine/spermine N1-acetyltransferase (SAT) accumulated after serum refeeding (between 6 and 16 h) and even more after spermidine addition (16 h). Histone acetyltransferase activity increased after both growth stimuli, whereas spermidine N8-acetyltransferase activity remained unchanged. After serum stimulation, the ODC mRNA level and activity rose between 6 and 16 h, whereas AdoMetDC mRNA accumulation occurred later (16 h) than the increase in enzyme activity (6 h). Stimulation of ODC and AdoMetDC activities was suppressed by the inhibitors added alone or in combination with spermidine, whereas mRNA accumulation was down-regulated by spermidine. These results indicate that the expression of SAT was growth-controlled and that SAT mRNA level was regulated by polyamines.

Possible role of histone acetylation and histone H1(0) replacement for the initiation of replication in regenerating rat liver

The role of histone acetylation and DNA synthesis has been investigated extensively in the regenerating rat liver system in the presence and absence of the cyclophosphamide derivative mafosfamide. We demonstrate a mafosfamide-induced inhibition of maximum histone acetyltransferase activity followed by a second elevation of enzyme activity and an accompanying total suppression of DNA synthesis for 7-8 h. The maximum of histone acetyltransferase activity, in parallel with an elevated acetylation in vivo, the consecutive replacement of histone H1(0) amd initiation of replication occur sequentially in the presence and absence of mafosfamide, but with a temporary delay of 7-8 h. Our data indicate that modifications of histone acetyltransferase (EC 2.3.1.48) activity do not significantly influence the acetylation patterns of histones H3 and H4. The mafosfamide-induced change of histone acetyltransferase activity and acetylation in vivo, the shift of histone H1(0) exchange and the consecutive transition of initiation of replication suggest that these three events might be functionally related.