Intranucleolar localization of the RNA polymerase A activity in isolated nuclei of regenerating rat liver

Ultrastructural autoradiography was used to visualize RNA polymerase A activity in parenchymal cell nuclei isolated from normal and regenerating (3, 24, 36 and 48 h after partial hepatectomy) rat liver. High resolution autoradiography showed that the activity of RNA polymerase A which was not inhibited by α-amanitin in a concentration of 0.8 μg/ml, was restricted to the nucleolus. Both the distribution pattern and number of grains were similar in control liver and regenerating liver 3 h after hepatectomy. Twentyfour, 36, and 48 h after hepatectomy nucleoli were enlarged and labeling was distinctly increased. In all experimental groups the activity of RNA polymerase A was located within fibrillar components of the nucleolus. The association of enzyme activity with this component was especially distinct in later stages (36 and 48 h) of liver regeneration. These results suggest that the fibrillar component of the nucleolus contains the active template for ribosomal RNA (rRNA) synthesis in rat liver cell nuclei.     

Isolation and characterization of different-sized nucleoli from Ehrlich ascites tumor cells : Evidence of different nucleolar ribosomal cistrons

A procedure was developed for isolation of variously sized nucleoli in order to study the mechanism of nucleolar formation from multiple nucleolar organizers and to compare the compositions of different-sized nucleoli from Ehrlich ascites tumor cells. Relatively small nucleoli and large nucleoli from Ehrlich ascites tumor cells were separated by centrifugation at 400 g for 5 min in a layer of 0.34 M sucrose over 0.88 M sucrose. Small nucleoli remained in the 0.34 M sucrose layer while the large nucleoli accumulated in the 0.88 M sucrose.Three fractions, provisionally named small, intermediate and large nucleoli, containing 0.33, 0.41 and 0.84 pg DNA/nucleolus, respectively, were separated. Unfractionated nucleoli contained 0.59 pg DNA/nucleolus. The RNA content also increased with the size of the nucleolus and no significant difference was observed in the RNA/DNA ratios in the three fractions. Large nucleoli incorporated more [³H]uridine and [³²P]orthophosphate into RNA than did small nucleoli, but the base compositions of the RNAs extracted from the different-sized nucleoli were similar. No significant fragmentation occurred on sonication of large nucleoli for 3 min, so the observed difference in the DNA contents was not due to mechanical damage of the nucleoli.The DNAs of these different-sized nucleoli were analysed on CsCl gradients. The nucleoli contained similar percentages of satellite DNA (20–22%) which were also similar to those of total, unfractionated nucleoli. Approx. 10% of the extranucleolar DNA is satellite DNA—thus the nucleolar fractions were probably not appreciably contaminated with extranucleolar DNA. The DNA of small nucleoli contained a slightly lower percentage (0.058%) of ribosomal cistrons than large nucleoli (0.081%). This means that the higher content of DNA in the large nucleoli is not merely due to longer sized chromatin with extra regions of the vicinity of nucleolar organizers. Thus these results suggest that the total content of ribosomal cistrons/nucleolus is roughly proportional to the DNA content of the nucleoli, at least in Ehrlich ascites tumor cells. Namely, the number of ribosomal cistrons per nucleolus for small, intermediate and large nucleoli is 40, 60 and 130, respectively.     

Localization of chromatin in "persistent" nucleoli in okadaic acid treated HeLa cells.

In okadaic acid treated HeLa cells, the chromosomes sometimes condense without being accompanied by nuclear envelope breakdown. These cells show "persistent" nucleoli. Within these "persistent" nucleoli the intranucleolar chromatin condenses and can be observed in the region of the dense nucleolar component (DNC) of the nucleoli. Other nucleolar components, namely the fibrillar centre (FC) and the granular component (GC) remain unchanged. These observations strongly speak for the localization of nucleolar chromatin (ribosomal cistrons) within the dense nucleolar component of the interphase nucleolus.     

Ultrastructural changes in vitro of rat liver nucleoli in response to polyamines

Summary Structural alterations of the nucleoli of rat liver cells were noted when these nuclei were isolated with spermidine or spermine rather than magnesium. When 5–10 mM spermidine or spermine were used to isolate the nuclei, the nucleoli were a) larger, b) contained numerous and sometimes large lacunae, and c) were less aggregated and had prominent chromatin caps. These chromatin caps gave the nucleolus a ring-shaped appearance in the light microscope. These findings, coupled with physiological data that indicate that polyamines enhance nucleolar RNA polymerase activity (Russell et al., 1971), suggest that spermidine and spermine may be involved in the control of ribosomal RNA synthesis. To our knowledge, this is the first instance of the direct stimulation of ribosomal RNA synthesis during nuclear isolation.Supported by USPHS Grant NS-07934.     

Calcium-binding protein regucalcin inhibits deoxyribonucleic acid synthesis in the nuclei of regenerating rat liver

The effect of regucalcin, a Ca²⁺-binding protein isolated from rat liver cytosol, on deoxyribonucleic acid (DNA) synthesis in the nuclei of regenerating rat liver was investigated. At 1 day after partial hepatectomy, the liver weight was increased about 50% of that of sham-operated rats, and it reached to the same levels as sham operation at 3 days after hepatectomy. Nuclear DNA synthesis was markedly increased at 1 day after hepatectomy, and this increase was also seen at 3 days. Nuclear DNA synthesis was clearly enhanced in the presence of EGTA (0.4 mM) in the incubation mixture. The presence of Ca²⁺ ( 1.0–25 M) caused a significant decrease in the nuclear DNA synthesis of normal rat liver. Regucalcin (0.25 and 0.5 M) clearly inhibited the nuclear DNA synthesis of normal rat liver. This inhibition was also seen in the presence of Ca²⁺ (1.0 M). Moreover, in the liver nuclei obtained at 1 day after partial hepatectomy, the presence of regucalcin (0.05–0.5 M) caused a remarkable inhibition of nuclear DNA synthesis. This effect was also revealed in the presence of EGTA (0.4 mM). Thus, the inhibitory effect of regucalcin was remarkable in regenerating rat liver nuclei in comparison with that of normal rat liver. The present results demonstrate that regucalcin can suppress nuclear DNA synthesis in regenerating rat liver. We suppose that regucalcin may have a role in the regulation of nuclear DNA synthesis in liver cell proliferation.     

Morphometry of nuclear and nucleolar structures in a CaCo-2 cell line

The number of the nucleoli in a CaCo-2 cell nucleus does not generally depend on the quantity of DNA in the nucleus, but nucleolar DNA content is directly proportional to total nuclear DNA. However, in multinucleolar cells (three or more nucleoli), the nucleolar DNA content increases after 96 h incubation in culture without concomitant quantitative changes in nuclear DNA. The percentage of multinucleolar cells and the average number of nucleoli per nucleus increase with increasing incubation time. After 72 and 96 h in culture, multinucleolar cells show distinctive morphologies. The ratio of the sum of nucleolar perimeters to the nuclear perimeter increases linearly when the number of nucleoli in a nucleus increases, but there is no concomitant increase in total nucleolar area or DNA content, except in the 72 and 96 h populations. When the number of nucleoli in CaCo-2 cells increases after 48 and 60 h in culture, the amount of DNA per nucleolus decreases.     

Some chemical properties of isolated pea nucleoli

Isolated nuclei and nucleoli of ungerminated pea embryos have been analyzed chemically for their content of DNA, RNA, zinc, iron, phosphorus, and protein sulfhydryl groups. The values obtained cannot be considered to represent the whole of the living nucleolar body as an undetermined amount of material is extracted from nucleoli in the course of their isolation. Only negligible amounts of DNA have been found in the isolated nucleoli; most of the DNA released on disruption of nuclei appears in a fraction showing very few structures under the light microscope. RNA is more concentrated in the nucleolus than in the nucleus or cytoplasm, but since nucleolar protein is 6 per cent of nuclear and less than 1 per cent of cytoplasmic protein, the total amount of nucleolar RNA is comparatively small. None of the other components listed occurs in high concentration in either nucleus or nucleolus.     

Some Chemical Properties of Isolated Pea Nucleoli

Isolated nuclei and nucleoli of ungerminated pea embryos have been analyzed chemically for their content of DNA, RNA, zinc, iron, phosphorus, and protein sulfhydryl groups. The values obtained cannot be considered to represent the whole of the living nucleolar body as an undetermined amount of material is extracted from nucleoli in the course of their isolation. Only negligible amounts of DNA have been found in the isolated nucleoli; most of the DNA released on disruption of nuclei appears in a fraction showing very few structures under the light microscope. RNA is more concentrated in the nucleolus than in the nucleus or cytoplasm, but since nucleolar protein is 6 per cent of nuclear and less than 1 per cent of cytoplasmic protein, the total amount of nucleolar RNA is comparatively small. None of the other components listed occurs in high concentration in either nucleus or nucleolus.     

Pattern of glycosaminoglycans and glycoproteins associated with nuclei of regenerating liver of rat.

1. Hyaluronic acid was detected as the largest glycosaminoglycan component in the glycosaminoglycan fraction from purified nuclei of regenerating livers as in the case of normal livers (Furukawa, K. and Tarayama, H. (1977) Biochim. Biophys. Acta 499, 278--289). However, the nuclear content of glycosaminoglycans tended to decrease after partial hepatectomy, reaching one-third of the normal liver level at 24--30 h after partial hepatectomy. On the other hand, two new polyanionic components were detected in the glycosaminoglycan fraction from regenerating liver nuclei. 2. One of these new components seems to be a sulfated glycopeptide. The 35SO4 incorporation into this component was stimulated biphasically after partial hepatectomy; the first stimulation occurring immediately after partial hepatectomy and the second stimulation occurring almost in parallel to the DNA synthesis. 3. Another polyacnionic component which also increases in the nuclear content after partial hepatectomy lacks hexuronic acid, sialic acid and 35SO4 and yet it is intensely stained by Alcian Blue. Preliminary investigations revealed the presence of hexose, ribose and phosphate as the major components. 4. In contrast to the primary localization of hyaluronic acid in the chromatin fraction and also in the nonhistone chromosomal protein fraction from it, these new polyanionic components were detected mainly in the karyosol fraction.     

Changes in nucleolar proteins and their phosphorylation patterns during liver regeneration.

Two-dimensional polyacrylamide gel electrophoresis of nucleolar proteins of rat liver revealed marked changes at various times after partial hepatectomy. Some of the non-histone protein spots including A11, A24, A25, C13, and C14 decreased in size and density. Others, including A15, B13, B16, B18, B24-25, B27, B33, B34, C1, C2, C8, C11, C17, and C23-24 were markedly increased in size and density. In vitro labeling of the nucleoli using [gamma-32P]ATP and subsequent analysis of the proteins by two-dimensional gel electrophoresis and autoradiography indicated that the uptake of 32P into proteins increased greatly during regeneration. In addition, the relative labelling of various spots changed throughout the regeneration process. With spot B33 as a reference, there was increased labeling of spots A1P, B2, B5, B24-25, C2, and C23-24 at 8 hours after hepatectomy. On the other hand, the labeling of spots A8P, A16, A20, B9, and B13 decreased at 8 hours after hepatectomy. Since the primary role of the nucleolus is the snythesis of ribosomal precursors, these changes in protein content and phosphorylation are presumably primarily associated with the increased processing and transport of peribosomal ribonucleoproteins in regenerating liver.     

Suppressive role of endogenous regucalcin in the enhancement of deoxyribonucleic acid synthesis activity in the nucleus of regenerating rat liver

The role of endogenous regucalcin in the regulation of deoxyribonucleic acid (DNA) synthesis activity in the nuclei of regenerating rat liver after partial hepatectomy was investigated. The addition of regucalcin (0.25 and 0.5 microM) in the reaction mixture caused a significant decrease in the nuclear DNA synthesis activity of normal rat liver. This decrease was also seen in the presence of Ca2+-chelator EGTA (0.4 mM), indicating that the effect of regucalcin is not related to nuclear Ca2+. Nuclear DNA activity was significantly increased in the presence of anti-regucalcin monoclonal antibody (10-50 ng/ml) in the reaction mixture. The effect was completely abolished by the addition of regucalcin (0.5 microM). Nuclear DNA synthesis activity was significantly increased at 24, 48, and 72 h after partial heptectomy. The effect of anti-regucalcin monoclonal antibody (25 ng/ml) in increasing nuclear DNA synthesis activity was significantly enhanced at 24 and 48 h after partial hepatectomy. The presence of staurospone (10(-6) M), trifluoperazine (2 x 10(-5) M), or PD98059 (10(-5) M) in the reaction mixture caused a significant decrease in DNA synthesis activity in the nuclei obtained at 24 after partial hepateactomy. The effect of these inhibitors in the presence of anti-regucalcin monoclonal antibody (25 ng/ml) was greater than that in the absence of the antibody. The present study suggests that endogenous regucalcin plays a suppressive role in the enhancement of nuclear DNA synthesis activity in regenerating liver with cell proliferation after partial hepatectomy in rats.     

Attachment of DNA to nucleolar and nuclear skeletal structures as visualized by Kleinschmidt molecular spreading

Co-isolated residual nuclear shells and residual nucleoli from membrane-depleted rat liver nuclei were spread according to Kleinschmidt's method. Comparison of the spread residual structures isolated from nuclear shells and spread pore complex-lamina isolated from nuclear envelopes showed that these residual structures are morphologically identical. Furthermore, our nuclear shell isolation procedure allowed visualization of DNA strands bound to a granular component of the lamina. The fragmentation of nuclear shells allowed us to obtain well-spread nucleolar remnants, in which we observed DNA strands anchored on a residual nucleolar network attached to the lamina. The different molecular features revealed by the spreading of residual nucleolar structures suggest that both non-transcribing nucleolar DNA and active ribosomal genes are linked to the nucleolar network. Although the exact nature of this network remains to be defined, the results of the present study strongly suggest that the DNA molecules of the chromosomes bearing ribosomal genes have many sites of attachment to a non-chromatin nucleolar network which can be referred to as a nucleolar skeletal complex.     

Enhancement of nuclear Ca2+-ATPase activity in regenerating rat liver: involvement of nuclear DNA increase

The alteration of calcium content, Ca²⁺-ATPase activity, DNA content and DNA fragmentation in the nuclei of regenerating rat liver was investigated. Liver was surgically removed about 70% of that of sham-operated rats. the reduced liver weight by partial hepatectomy was completely restored at 3 days after the surgery. Regenerating liver significantly increased Ca²⁺-ATPase activity and DNA content in the nuclei between 1 and 5 days after hepatectomy. The nuclear calcium content was clearly increased from 2 days after hepatectomy. The increase of Ca²⁺-ATPase activity in regenerating liver was clearly inhibited by the presence of trifluoperazine (10 M), staurosporine (2.5 M) and dibucaine (10 M), which are inhibitors of calmodulin and protein kinase, in the enzyme reaction mixture. However, the nuclear enzyme activity in normal rat liver was not significantly altered by these inhibitors. Meanwhile, the increase of nuclear DNA content in regenerating liver was completely blocked by the administration of trifluoperazine (2.5 mg/100 g body weight), suggesting an involvement of calmodulin. Now, the nuclear DNA fragmentation was significantly decreased in regenerating liver, suggesting that this decrease is partly contributed to the increase in nuclear DNA content. The present study clearly demonstrates that regenerating liver enhances nuclear Ca²⁺-ATPase activity and induces a corresponding elevation of nuclear calcium content. This Ca²⁺-signaling system may be involved in the regulation of nuclear DNA functions in regenerating rat liver.     

Studies on satellite nucleoli in rat hepatocytes and Novikoff hepatoma cells

Summary The present study was undertaken to provide information on the presence and frequency of satellite nucleoli in cells with increased nucleolar biosynthetic activity. The number of hepatocytes containing satellite nucleoli was analyzed in rat liver, regenerating liver 18 h after partial hepatectomy and in Novikoff hepatoma ascites cells. In comparison with hepatocytes of normal liver, the number of both stimulated hepatocytes and malignant hepatoma cells containing satellite nucleoli was significantly reduced. The results also indicated that whereas most satellite nucleoli contain protein C23, a smaller percentage contain protein B23.     

Dense distribution of nuclear alkaline protease to nucleoli with increase in rapidly growing cells in rats

Neutral and alkaline proteases (chymotrypsin-like serine proteases) are tightly bound to chromatin in nuclei of various tissues of rats, and rapidly growing cells are abundant in the latter enzyme. Activities per mg DNA of these enzymes were measured with fractions of euchromatin, heterochromatin, nucleoli and extranucleoli from normal and regenerating livers, and Rhodamine sarcoma. With normal liver, both enzymes, respectively, were almost evenly distributed among the fractions, except that alkaline protease was significantly higher in nucleoli. The nucleolar alkaline protease increased by 30-60% with regenerating liver and by higher than 100% with the tumor.