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R van Wijk

Publications and source records attributed to R van Wijk.

At least 73 records · Page 4Linked to original sources
PubMed1982

Variations in some molecular events during the early phases of the Reuber H35 hepatoma cell cycle. III. Role of protein synthesis in the initiation of DNA synthesis and the mechanism of stimulation of protein synthesis by serum.

Induction of DNA synthesis by serum and amino acids has been investigated in cultured Reuber H35 hepatoma cells. Commitment of DNA synthesis was found to occur 6-8 hours before the actual start of this synthesis. The rate of initiation of DNA synthesis is proportional to the stimulation of protein synthesis by serum and/or amino acids. The increased protein synthesis is important for the proliferation only during the early period after serum addition. The withdrawal of serum and the inhibition by cycloheximide confirm this finding. Actinomycin D hardly influenced the early effect of serum on protein synthesis and it is concluded that the serum-stimulated protein synthesis is carried out on pre-existing mRNA's. The mechanism of stimulation of protein synthesis by serum has been studied by determination of the polyribosome size, the number of growing polypeptide chains, and the ribosomal transit time. The rate of the initiation of translation has been found to be specifically enhanced while the rate of elongation remained unchanged. Two-dimensional gel electrophoresis showed that the early stimulation of protein synthesis by serum involves all types of major cellular proteins, and no new proteins could be detected.

Amino Acids↗
PubMed1982

Positive and negative cAMP-mediated control of tyrosine aminotransferase synthesis in Reuber H35 hepatoma cells.

Induction of L-tyrosine:2-oxoglutarate aminotransferase (EC 2.6.1.5) by N6,O2'-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) in Reuber H35 hepatoma cells reaches a maximum value between 3-5 h after addition of Bt2cAMP and subsequently decreases in the continuous presence of Bt2cAMP. We have investigated the kinetics of the increase, i.e. induction, and the decrease, i.e. the repressed state, of the tyrosine-aminotransferase-synthesizing system under these conditions. Our experimental results are as follows. 1. The repressed state of the tyrosine-aminotransferase-synthesizing system is not caused by a decrease in the intracellular cAMP concentration. 2. The repressed state is inhibited by actinomycin D (while induction is not inhibited). 3. During the repressed state no effect of dexamethasone on tyrosine aminotransferase synthesis is found, while during induction Bt2cAMP and dexamethasone act synergistically. 4. Longer starvation of the cells in serum-free medium has no influence on the kinetics of the induction/repressed state curve. From these results we have concluded that the mechanism of the transition to the repressed state of the tyrosine-aminotransferase-synthesizing system is essentially different from the mechanism of deinduction which occurs after removal of the inducer. Moreover, the repressed state of the system is a phenomenon which is induced by Bt2cAMP separately from induction at a different level of protein synthesis.

Animals↗
PubMed1981

The induction of ornithine decarboxylase and DNA synthesis in rat hepatocytes after a single administration of diethylnitrosamine.

In primary monolayer culture of adult rat liver cells, the presence of a mixture of hormones stimulates DNA synthesis and proliferation. Under conditions of suboptimal hormone stimulation diethylnitrosamine was found to be effective in stimulation of DNA synthesis. Diethylnitrosamine also influences ornithine decarboxylase activity. The effect of some carcinogens and their structural related non-carcinogenic analogs on ornithine decarboxylase activity in primary cultured rat hepatocytes was studied. The addition of the carcinogens diethylnitrosamine, benzo[a]pyrene, methylmethanesulphonate and ethylmethanesulphonate caused significant increases in ornithine decarboxylase activity. Under the same conditions the non-carcinogenic analogs diethylformamide and pyrene did not show this change in enzyme activity.

Animals↗
PubMed1981

Induction of rat hepatic ornithine decarboxylase by the tumor promotors 12-O-tetradecanoylphorbol-13-acetate and phenobarbital in vivo; effect of retinyl-acetate.

A single i.p. injection in rats of 20 microgram 12-O-tetradecanoylphorbol-13-acetate (TPA) or 100 mg phenobarbital (PB)/kg body weight resulted in a transient increase in liver ornithine decarboxylase (ODC) activity. Maximal stimulation of ODC activity was observed approximately 4 h after application of TPA and 4-6 h after treatment of PB. Six hours after TPA injection and greater than 8 h after treatment with PB the ODC activity had returned to control level. ODC activity induction occurred in a dose dependent manner, maximal stimulation was obtained with 20-100 microgram TPA and 100 mg PB/kg body weight, respectively. At higher doses the tumor promotors became increasingly inhibitory. The in vivo induction of rat liver ODC activity by TPA appeared to be under transcriptional control since administration of 2 mg actinomycin D/kg body weight or 50 mg cycloheximide/kg body weight 1 h prior to application of the tumor promotor prevented the increase of ODC activity. Furthermore the TPA-stimulated ODC induction was shown to be very sensitive to retinyl-acetate (RA). An i.p. dose of RA of 0.2 microgram/kg body weight applied 1 h before TPA inhibited ODC induction by 35%. Increasing the dose of RA to 20 microgram/kg body weight completely prevented the stimulation of ODC activity by TPA.

Animals↗
PubMed1981

Studies on the posttranscriptional site of cAMP action in the regulation of the synthesis of tyrosine aminotransferase.

Synthesis of L-tyrosine:2-oxoglutarate aminotransferase (EC 2.6.1.5) can be induced by N6,O2'-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) in Reuber H35 cell cultures. Actinomycin D fails to block this induction which indicates a target for Bt2cAMP at a posttranscriptional level. We have determined the influence of Bt2cAMP on several translational events during the tyrosine aminotransferase synthesis with the following results. (1) The number of nascent tyrosine aminotransferase chains increased, whereas no effect was measured on the number of nascent total protein chains. (2) The rate of elongation along the tyrosine aminotransferase mRNA and total mRNA is not enhanced by Bt2cAMP. (3) The induced synthesis of tyrosine aminotransferase is more sensitive to the inhibition of elongation. We conclude from our results that Bt2cAMP induces the synthesis of tyrosine aminotransferase by an increase in the rate of initiation on the tyrosine aminotransferase mRNA.

Animals↗
PubMed1979

Variability of cell generation times in a hepatoma cell pedigree.

Time-lapse cinematographic analysis of a clone of HTC rat hepatoma cells showed variations in interdivision time within the clone. A positive correlation was found between the interdivision times of mother and daughter cells. The variability of the differences between interdivision times of cells of sister, cousin, second cousin or second-second cousin relationship was calculated. The proportion of cells with large differences in intermitotic times was found to increase with decreasing relationship. The clonal division pattern observed suggests strongly that 'inherited' factors goven the process leading to cell division but that their effects can be modified.

Animals↗
PubMed1979

Comparative studies of glucose metabolism in HTC, RLC, MH1C1, and Reuber H35 rat hepatoma cells.

Carbohydrate metabolism by four rat hepatoma cell lines in culture, namely, Reuber H35, MH1C1, RLC, and HTC, has been investigated. Glucose utilization by H35 and MH1C1 cells is lower than that by RLC and HTC cells. The four cell lines also differ with respect to the accumulation of lactic acid in the growth medium; in particular, H35 cells show uptake of lactic acid, rather than accumulation in the medium. Specific activities of a number of enzymes involved in glycolysis, gluconeogenesis, pentose phosphate pathway, and glycogen formation were determined in the four cell lines. A direct relationship between the differences was found for the activities of some enzymes belonging to carbohydrate metabolism, namely, hexokinase, pyruvate kinase, aldolases A and B, glucose-6-phosphate dehydrogenase, and phosphogluconate dehydrogenase and the differences found for glucose utilization by the different cell lines.

Animals↗
PubMed1974

Regulation of phosphoenolpyruvate carboxykinase and tyrosine transaminase in hepatoma cell cultures. 3. Comparative studies in H35, HTC, MH1C1 and RLC cells.

The ability of N(6), O(2)'-dibutyryl cyclic AMP (DBcAMP) to regulate a number of metabolic events in four lines of cultured rat hepatomas has been examined. Although dexamethasone induces tyrosine transaminase in all four lines, DBcAMP induces this enzyme normally only in H35 cells. A slight increase in transaminase activity was seen with MH(1)C(1) cells and HTC cells, but no effect was detectable in RLC cells. In contrast, phosphoenolpyruvate carboxykinase activity is increased by both agents in H35 and MH(1)C(1) cells, but neither had any effect in HTC or RLC cells. DBcAMP caused a rapid inhibition of the growth rate and DNA synthesis and an increase in protein content in both H35 and MH(1)C(1) cells but not in HTC or RLC cells. The effect of DBcAMP on DNA synthesis in MH(1)C(1) cells could be reversed by deoxycytidine as is also the case with H35 cells. The resistance of HTC and RLC cells to DBcAMP was not due to reduced uptake or deacylation as judged by studies with [(3)H]DBcAMP. The cyclic nucleotide appears to enter the cells by passive diffusion as the intracellular concentration approaches that in the medium within 30-60 min. Possible explanations for the differential responses observed are discussed.

Animals↗