Amino acids and control of nucleolar size, the activity of RNA polymerase I, and DNA synthesis in liver

Genetic control of nucleolar size: An evolutionary perspective

Exploiting a C. elegans mutant (ncl-1) exhibiting nucleolar abnormalities, we recently identified the let-7/ncl-1/fib-1 genetic cascade underlying proper rRNA abundance and nucleolar size. These 3 factors, let-7 (a miRNA), NCL-1 (a member of the TRIM-NHL family), and fibrillarin (a nucleolar methyltransferase), are evolutionarily conserved across metazoans. In this article, we provide several lines of bioinformatic evidence showing that human and Drosophila homologues of C. elegans NCL-1, TRIM-71 and Brat, respectively, likely act as translational suppressors of fibrillarin. Moreover, since their 3'-UTRs contain putative target sites, they may also be under the control of the let-7 miRNA. We hypothesize that let-7, TRIM and fibrillarin contribute activities in concert, and constitute a conserved network controlling nucleolar size in eukaryotes. We provide an in-depth literature review of various molecular pathways, including the let-7/ncl-1/fib-1 genetic cascade, implicated in the regulation of nucleolar size.

Ultrastructural morphometrical and immunocytochemical analyses of hepatocyte nuclei from mice fed on genetically modified soybean

No direct evidence that genetically modified (GM) food may represent a possible danger for health has been reported so far; however, the scientific literature in this field is still quite poor. Therefore, we carried out an ultrastructural morphometrical and immunocytochemical study on hepatocytes from mice fed on GM soybean, in order to investigate eventual modifications of nuclear components of these cells involved in multiple metabolic pathways related to food processing. Our observations demonstrate significant modifications of some nuclear features in GM-fed mice. In particular, GM fed-mice show irregularly shaped nuclei, which generally represents an index of high metabolic rate, and a higher number of nuclear pores, suggestive of intense molecular trafficking. Moreover, the roundish nucleoli of control animals change in more irregular nucleoli with numerous small fibrillar centres and abundant dense fibrillar component in GM-fed mice, modifications typical of increased metabolic rate. Accordingly, nucleoplasmic (snRNPs and SC-35) and nucleolar (fibrillarin) splicing factors are more abundant in hepatocyte nuclei of GM-fed than in control mice. In conclusion, our data suggest that GM soybean intake can influence hepatocyte nuclear features in young and adult mice; however, the mechanisms responsible for such alterations remain unknown.

Incorporation of 5-fluorouracil into hepatoma and normal tissue RNA at protein depletion in the rat

BACKGROUND AND OBJECTIVES

5-fluorouracil (5-FU) exerts its effects mainly by its incorporation into RNA and inhibition of DNA synthesis. Its toxicity may therefore be estimated by measuring its incorporation into RNA. Protein malnutrition has been considered to increase the toxicity of 5-FU.

METHODS

Rats with a hepatoma implanted into the liver were fed on either a 25% or a 0% casein diet for 1 week. On the last day, they were infused via the hepatic artery with a therapeutic dose of 3H-5-FU. Its incorporation into RNA was measured in hepatoma and several normal tissues.

RESULTS

Protein deprivation increased the incorporation of 5-FU into liver and intestinal RNA. Incorporation into hepatoma RNA did not increase significantly, but the ratio, liver/hepatoma RNA incorporation, remained unchanged.

CONCLUSIONS

Protein deprivation might increase the toxicity of 5-FU on liver and intestine.

Enhanced phosphorylation of a nucleolar 110-kDa protein in rat liver by dietary manipulation

RNA polymerase 1 activity and nucleolar volume have been reported to increase in hepatocytes from rats fed a protein-free diet. Phosphorylation in vitro of a 110-kDa protein was enhanced in nuclei and nucleoli from livers of rats fed a protein-free diet. In nuclear extracts the 110-kDa protein in heat-treated nuclei was much more phosphorylated than from control liver. In contrast, casein kinase activity in the nuclear extract from control liver was comparable to that from livers of rats fed a protein-free diet. Nuclear extracts from control rat liver and livers of rats fed a protein-free diet were fractionated by DEAE-cellulose column chromatography. Casein kinase II (NII) eluted at around 0.17 M NaCl scarcely phosphorylates the 110-kDa protein. Chromatography of the nuclear extract from livers of rats fed a protein-free diet, but not from control liver, yielded fractions which eluted at 0.21-0.25 M NaCl and predominantly phosphorylated the 110-kDa protein. The phosphorylation of 110-kDa protein was not appreciably affected by a heparin concentration of 5 micrograms/ml, which completely inhibited casein kinase II. In addition, phosphorylation of the 110-kDa protein in liver nucleoli from rats fed a protein-free diet showed a lower sensitivity to heparin than that in control rat liver nucleoli. These results suggest that enhanced phosphorylation of the nuclear 110-kDa protein in livers from rats fed a protein-free diet is due to the induction of a 110-kDa protein kinase distinct from casein kinase II.

Increased formation and keeping of ribosomes during dietary recovery of mouse kidney

In the kidney of 5-day protein-depleted mice there is a decrease of 23% in the rRNA mass. When these animals are fed with a complete diet, rRNA content is restored to its normal value after 24 h of refeeding. The mechanisms that underlie this phenomenon were studied. It was found that the activity of rRNA polymerase I in the nuclei of kidneys from refed mice showed an increase of about twofold compared with the activity in normal and protein-depleted nuclei. The in vivo incorporation of a large dose (nontrace) of [14C]orotic acid into rRNA was also twofold enhanced in kidneys from refed mice. Ribosome degradation (measured by the disappearance of radioactivity from either ribosomal proteins or rRNA previously labeled by the injection of NaH14CO3 and [14C]orotic acid to the mice, respectively) stopped during the 1st day after refeeding. The estimation of the difference between the rRNA synthesis rate and the net rRNA increase also demonstrated a decrease in the rRNA degradation rate in refed mice.

Determination of the rate of rRNA synthesis in Xenopus laevis triploid embryos produced by low-temperature treatment

Triploid embryos of Xenopus laevis were obtained by cold-temperature shocking of the fertilized eggs, and the rate of the ribosomal RNA (rRNA) synthesis was determined for comparison with that in diploid embryos. For this purpose, both triploid and diploid embryos were dissociated into cells at the neurula stage, and then labeled with (3H)uridine for varying lengths of time. The rate of rRNA synthesis, as estimated after determination of (3H)UTP specific radioactivity and the total label incorporation into the purified rRNA, was about 0.1 pg/cell/hr for both diploid and triploid embryo cells. Nuclei of triploid embryo cells contained three nucleoli of apparently similar sizes--an indication of the functioning of all the three rRNA gene clusters to a more or less similar extent. Also, rates of synthesis of 4S RNA and 5S RNA were determined: Both rates did not change appreciably between triploid and diploid embryo cells. Based on these results, it appears that transcription of these redundant genes occurs at a constant rate on a per cell basis irrespective of the presence of 1.5 times as many genes as the control.

Increased transcription and decreased degradation control and recovery of liver ribosomes after a period of protein starvation

In the livers of 5-days-protein-depleted mice there is a decrease of 47% of the ribosome mass. When these animals are fed with an adequate diet, ribosome content is restored to the normal value after 1 day of re-feeding. The mechanisms underlying this phenomenon were studied. It was found that: (1) the activity of RNA polymerase I in the nuclei of livers from re-fed animals showed an enhancement of about 2-fold compared with the activity in normal and protein-depleted liver nuclei; (2) ribosome degradation, measured by the disappearance of radioactivity from ribosomal proteins previously labelled by the administration of NaH14CO3 to the mice, stopped during the first day after re-feeding.

Modification of rat thymocyte membrane properties by hyperthermia and ionizing radiation

Thymocytes are one the most widely used cell models for the study of radiation-induced interphase death. This cell-type was chosen for the study of hyperthermic and radiation effects on two membrane-related processes implicated in the interphase death of cells: Na+-dependent 2-aminoisobutyric acid (AIB) transport and cyclic 3'-5' adenosine monophsophate formation. The response of AIB transport to heat is dose-dependent, but the biphasic thermal response curve (AIB uptake versus time) differs fom the sigmoidal radiation response curve. Heating thymocytes for 20-30 min at 43 degrees C stimulates AIB uptake. Additional heating at 43 degrees C, however, markedly reduces AIB uptake. Despite the immediate stimulating effect of heat (30 min at 43 degrees C), the thymocyte has already developed irrepairable impairments, as demonstrated by the fractionated heating experiments. The heat-induced impairment of AIB uptake is mainly on the Na+-dependent component of neutral amino-acid transport, affecting primarily the maximal rate of uptake, i.e. Vmax. Additional evidence for heat-induced plasma membrane damage is the alteration in cAMP levels. Heating thymocytes for 30 min or longer at 43 degrees C causes a massive rise in cAMP level within the cell. This differs from thymocytes exposed to radiation where no rise in cAMP is observed.