Circadian-Stage Dependent Acth1-17 Effect on DNA Synthesis in Murine Duodenum, Colon and Recum

The objective was to determine the effect of adrenocorticotropin (ACTH 1-17) on the incorporation of [³H]TdR into DNA (DNA synthesis) in the duodenum, colon and rectum of CD2F, mice standardised to 12hr of light alternating with 12hr of darkness. A question asked was whether the difference in times of administration along the 24 -hr time scale influenced any response found. The response was complex as ACTH 1-17 was capable of bringing about statistically significant increases in the incorporation of [³H]TdR into DNA at certain times, decreases at other times, or no response at still another time. A generalization that can be made from all these tissues is that ACTH 1-17 had a greater influence in bringing about a decrease in DNA synthesis when it was administered around the time of transition from dark to light. A similar finding was made earlier for the ACTH 1-17 effect upon the tongue, esophagus and stomach.

A 2- and 3-way analysis of variance supports our conclusion that the kind-of-treatment, time-of-treatment and the interval-to-kill (Sampling time) as well as their interactions are important factors when determining any response of ACTH 1-17 or placebo.     

Effect of ACTH 1-17 at different circadian stages on [3H]TdR incorporation into DNA

The objective was to determine the effect of adrenocorticotropin (ACTH 1-17) on the incorporation of [3H]TdR into DNA (DNA synthesis) in the tongue, esophagus and stomach of CD2F1 mice standardized to 12 hours of light alternating with 12 hours of darkness. A question asked was whether the time of administration along the 24-hour time scale influenced any response found. The response was complex as ACTH 1-17 was capable of bringing about statistically significant increases in the incorporation of [3H]TdR into DNA at certain times, decreases at other times, or no response at still another time. In general the most marked effects of 20 IU/kg of ACTH 1-17 when compared to controls, was to decrease DNA synthesis of as much as 60% 4 hours after administration at the end of the dark or beginning of the light span. A 2- and 3-way analysis of variance supported the conclusion that the kind-of-treatment, time-of-treatment and the interval-to-kill (Sampling time) as well as their interactions are important factors when determining any response of ACTH 1-17 or placebo.     

Circadian ACTH 1-17 effects on murine tritiated thymidine incorporation into DNA of thymus, bone marrow and spleen; also on total splenic RNA and DNA

The importance of administration time along the 24-h scale is shown for a potent corticosteroidogenic adrenocorticotropin analogue, ACTH 1-17 (Synchrodyn 1-17). This molecule affects the incorporation of [3H]TdR into DNA (DNA synthesis) in the thymus, bone marrow and spleen, and total RNA and DNA of spleen in CD2F1 mice, standardized in light alternating with darkness at 12-h intervals. As a function of timing, the same dose of ACTH 1-17 at one time increases, at another time decreases (in each case with statistical significance) and at still another time elicits no response in DNA synthesis or in total RNA and DNA of spleen. Effects upon DNA synthesis are recorded with doses of 0.02 IU/kg body weight. The most marked effect with 20 IU/kg body weight is a decrease of DNA synthesis seen (4 h) after administration of ACTH 1-17 late in the dark span and early in the light span. The effect of ACTH 1-17 on the thymus is more prominent than that on bone marrow and spleen. Time-dependence also characterizes placebo effects by comparison to values in untreated controls. At the cellular level responses to ACTH 1-17 or placebo are characterized by critical interactions of treatment kind with treatment timing as well as interval-to-kill-time. The study documents the need to time-specify, in several ways, responses to ACTH 1-17 and suggests more broadly that 'increases' and 'decreases' may have to be complemented by changes in endpoints of rhythms in all those endocrine studies that involve rhythmic variables and rhythm-dependent effects upon these variables.     

LOW LEVEL INCORPORATION OF TRITIATED THYMIDINE INTO THE NUCLEAR DNA OF PURKINJE NEURONS OF ADULT MICE

Adult mice were pulse labeled with tritiated thymidine [³H]TdR and killed 9 hr later. A low level incorporation of [³H]TdR into the nuclear DNA of Purkinje neurons was found in autoradiographs. Enzymatic digestions with DNase and with RNase in combination with autoradiographic grain counts indicate that a portion of nuclear DNA is not stable in the Purkinje nucleus. These results are discussed in light of reports of the stable nature of DNA in Purkinje neurons of adult mice.     

Effects of epidermal growth factor on deoxyribonucleic acid synthesis and stomach weight in hypophysectomized adult mice

Epidermal growth factor (EGF) or saline was administered intraperitonally to hypophysectomized adult male CD2F₁ mice or intact controls at 0700 hr. Subgroups of mice were killed at 4, 8, or 12 hr after injection. EGF was shown to stimulate [³H]TdR incorporation into DNA into several organs as previously reported. The response to EGF was found to be enhanced in both hypophysectomized and fasted mice. Differences in [³H]TdR incorporation into DNA, corneal epithelium mitotic index, RNA in pancreas and kidney of hypophysectomized and intact mice are reported. EGF was shown to result in stomach enlargement due to increased luminal contents in both hypophysectomized and intact mice.     

Mirex-Induced Adaptive Liver Growth: A Corticosterone-Mediated Response

The adaptive liver growth response was investigated in intact and adrenalectomized rats. When adult male rats were given a single oral dose of mirex (100 mg/kg body weight) there was a 72% increase in relative liver weight (RLW) in 72 hr. Based on [³H]-thymidine ([³H]TdR) incorporation into hepatic DNA, there was also a wave of DNA synthesis which peaked at 48 hr and decreased to essentially control values by 96 hr post mirex dose. In mirex-dosed adrenalectomised (Adx) animals, the RLW was increased by only 38% and there was sustained DNA synthesis. When mirex-dosed Adx rats were given corticosterone supplements, the RLW response was similar to the RLW response in intact mirex-dosed rats. However, the 48-hr DNA synthesis peak seen in intact mirex-dosed rats was eliminated. From these data it is suggested that mirex-induced adaptive liver growth has two components: a hypertrophic component which is mediated by corticosterone, and a hyperplastic component which is independent of corticosterone.     

Effect of epidermal growth factor (EGF) on [3H]TdR incorporation into DNA in ad lib fed and fasted CD2F1 mice

The effect of EGF on the incorporation of [3H]TdR into DNA (DNA synthesis) was determined in the esophagus, liver, pancreas, and kidney in mice standardized to 12 hours (hr) of light alternating with 12 hr of darkness. A question asked was whether intraperitoneally administered EGF could alter the circadian patterns of DNA synthesis in these organs. The most marked effects of EGF were: an increase in DNA synthesis but only after a specific duration of time after treatment, ranging from 8 to 23 hr, which differed for each tissue, a similarity in the response of the esophagus in both ad lib fed and fasted mice, but not in the response of the liver, where the stimulatory effect of EGF observed in fed mice was dramatically reduced in fasted ones, and an advance in the phasing of the circadian rhythm in DNA synthesis of the esophagus by about 12 hr. In addition, no sex differences in fasted animals were found under the conditions of this study.     

Studies on the regulation of lymphocyte reactivity by normal and activated macrophages.

To determine the potential role of macrophages as regulators of the immune response, the effect of mouse peritoneal macrophages on transforming mouse spleen lymphocytes was investigated. Mitogen and antigen stimulated lymphocyte transformation, as measured by DNA synthesis, was enhanced by all concentrations of normal macrophages tested, but only by low concentrations of activated macrophages. High concentrations of activated macrophages markedly inhibited lymphocyte transformation. This inhibition occurred whether lymphocyte DNA synthesis was measured by incorporation of [³H]TdR or of ³²P. Activated macrophages cultured with lymphocytes within 4 hr of being removed from the peritoneal cavity inhibited lymphocyte transformation. When activated macrophages were cultured alone for 24 or more hours before addition of lymphocytes, enhancement of transformation was noted. Once lymphocytes were exposed to activated macrophages, they could not be induced to undergo transformation in the presence of Con A. Whereas heat-killed activated macrophages, which appeared intact morphologically, lost their capacity to inhibit lymphocyte transformation, macrophages treated with mitomycin C to inhibit DNA synthesis retained this capacity. Syngeneic and allogeneic macrophages had similar inhibitory ability. Supernatants from cultures of many cell types (including normal or activated macrophages, lymphocytes, lymphocytes plus macrophages, and L cells) inhibited [³H]TdR incorporation by both mitogen stimulated lymphocytes and tumor cells. These studies demonstrate the capacity of macrophages to regulate lymphocyte transformation in vitro and suggest a role for these cells as regulators of cell-mediated immunity in vivo.     

CELL PROLIFERATION KINETICS OF EPIDERMIS AND SEBACEOUS GLANDS IN RELATION TO CHALONE ACTION

Median S-phase lengths of pinna epidermis and sebaceous glands, and of epithelia from the oesophagus and under surface of the tongue of Albino Swiss S mice were estimated by the percentage labelled mitoses method (PLM). The 18.4 and 18.8 hr for the median length of S-phase for pinna epidermis and sebaceous glands respectively made it possible for these two tissues to be used experimentally for testing tissue specificity in chalone assay experiments. The 10.0 and 11.5 hr for oesophagus and tongue epithelium respectively made experimental design for chalone assay difficult when pinna epidermis was the target tissue. The results of the Labelling Index measured each hour throughout a 24-hr period showed no distinct single peaked diurnal rhythm for pinna epidermis and sebaceous glands. Instead a circadian rhythm with several small peaks occurred which would be expected if an S-phase of approximately 18 hr was imposed on the diurnal rhythm. This indicates that there may be very little change in the rate of DNA synthesis. The results are given for the assay in vivo of purified epidermal G₁ and G₂ chalones, and the 72–81% ethanol precipitate of pig skin from which they could be isolated. These experiments were performed over a time period which took into account the diurnal rhythm of activity of the mice as well as the S-phase lengths. Extrapolating the results with time of action of the chalone shows that the G₁ chalone acts at the point of entry into DNA synthesis and that the S-phase length was approximately 17 hr for both the pinna epidermis and sebaceous glands. This may be a more correct value since the PLM method overestimates the median S-phase length as it is known that in pinna skin the [³H]TdR is available to the tissues for 2 hr and true flash labelling does not take place. The previous reports that epidermal G₁ chalone acts some hours prior to entry into S-phase resulted from experiments on back skin where the S-phase is shorter and there is a pronounceddiurnal rhythm which could mask the chalone effect. The epidermal G, chalone had no effect on DNA synthesis even at different times in the circadian rhythm. Thus the circadian rhythms and S-phase lengths of the test tissues need to be considered when experiments are performed with chalones. Ideally, the target tissues selected for cell line specificity tests should have the same cell kinetics for the easier and more accurate assessment and interpretation of results. When the tissues have markedly different cell kinetics, experimental procedures and results need to be evaluated accordingly. The point of action of G, chalone can only be assessed if the effect is measured over the peak of incorporation of 13H]TdR into DNA. The results of the effects of skin extracts are analysed in relation to changes in the availability of i3H]TdR for the incorporation into DNA and to the possibility of there being two distinct populations of proliferating cells.     

Proliferation of PHA-stimulated lymphocytes measured by combined autoradiography and sister chromatid differential staining

Lymphocyte proliferation in culture was studied by combined [³H]TdR incorporation and sister chromatid differential staining. The majority of 1st division metaphases in a 72 h culture commenced DNA synthesis after 48 h and had a cell cycle of less than 24 h. A small proportion of cells from some donors commenced DNA synthesis between 24–30 h and had cell cycle times of up to 48 h. Although many cells entered DNA synthesis at the same time, they showed marked asynchrony in the length of their cell cycle, with some completing one, some two and others three cell cycles in the 72 h culture period. The time taken for cells to enter S following stimulation with PHA ranged from 24 to 48 h and there was considerable variation between donors in the number of fast and slow responding cells.     

Digestive Tract Cell Proliferation and Food Consumption Patterns of Ha/Icr Mice

The relationship between the daily pattern of food consumption and the proliferation rate of the qesophagus, stomach, forestomach, small intestine and colon of Ha/ICR mice was examined. Proliferative activity was determined by [³H]TdR incorporation on a wet weight tissue basis, along with selective counting of labelled nuclei. Under conditions of ad libitum feeding with a 12 hr light cycle (lights on at 0600) mice eat most of their food during the dark period. A distinct circadian rhythm was observed in the oesophagus, stomach, forestomach and colon with the peak of [³H]TdR incorporation between 0400 and 0600 and the nadir between 1600 and 1800. Although a circadian fluctuation was observed in the small intestine, its amplitude was much less than in other areas. This rhythmic change in proliferation rate could be phase shifted by allowing the mice to feed only between 0800 and 1600 for 14 days. Under these conditions the peak in proliferative activity occurred between 1800 and 2000. Fasting reduced the daily level of proliferative activity in all of the digestive tract sites studied, and for all areas except the oesophagus greatly reduced or eliminated the circadian fluctuation. the forestomach and colon were the most influenced by fasting with 24 hr [³H]TdR incorporation reduced to 30–40% of the control value. Refeeding following a 48 hr fast produced a rapid increase in proliferative activity peaking at levels well above the control value at 16 hr after the onset of refeeding. the major exception to this was the small intestine which slowly returned to the control value during the first 24 hr. Partial refeeding produced a diminished refeeding response. Once the normal pattern of food consumption was re-established following refeeding the normal proliferative fluctuations were again observed.     

Biogenesis of mammalian mitochondria in the renoprival kidney. II. Aspects of mitochondrial DNA synthesis

Mitochondrial DNA (m-DNA) content and factors which might control its concentration were investigated in the renoprival kidney at various times after unilateral nephrectomy. On the basis of mitochondrial protein, m-DNA increased 30% in the renoprival kidney at 24 hr and returned to normal by 48 hr. The total tissue content of m-DNA was also increased at 24 hr. The specific activity of [³H]thymidine incorporated into m-DNA in vivo was decreased markedly at 24 hr after mononephrectomy; at the same time there was a threefold increase of [³H]thymidine incorporation into total cellular DNA. The incorporation into m-DNA was above normal at 48 hr. The mitochondrial specific DNase was decreased 60% at 24 and 36 hr post-mononephrectomy. There was no significant difference in the total radioactivity or total optical density at 260 nm of the acid soluble extract from mitochondria isolated at various times after mononephrectomy. The incorporation of [³H]thymidine into TMP and TDP in the renoprival kidney was not different from normal but there was a decrease in the incorporation into TTP. It is suggested that the increase in mitochondrial DNA could be due to a decrease in the rate of degradation rather than an increase in synthesis.     

DNA synthesis in cultured human fibroblasts: Regulation by 3′ : 5′-cyclic amp

The addition of serum to density-inhibited human fibroblast cultures induced a wave of DNA synthesis, measured as [³H] thymidine incorporation into acid-precipitable material, beginning after 8–12 hr and reaching maximum levels at 16–24 hr. Addition of dibutyryl-3′ : 5′-cyclic AMP (DBcAMP) together with serum inhibited [³H] thymidine incorporation by 75–95%. When DBcAMP was added for the first 4 hr of serum stimulation and then removed, the wave of DNA synthesis was not delayed. This suggested that serum could induce DNA synthesis even though cyclic AMP concentrations were maintained at high levels by DBcAMP during this initial period. These results are inconsistent with the hypothesis that it is the immediate transient reduction in 3′ : 5′-cyclic AMP concentration following the addition of serum that triggers DNA synthesis. By contrast, DBcAMP added 8 hr after serum inhibited [³H] thymidine incorporation to the same extent as DBcAMP added at the same time as serum. This indicated that a step essential for DNA synthesis and occurring late in G₁ was inhibited by high concentrations of 3′ : 5′-cyclic AMP.     

Endogenous Regulation of Endothelial Cell Proliferation

Bovine aortic endothelial cells (BAEC) in culture have the ability to regulate their own proliferation. We have found that a fraction below 100,000 daltons obtained from the media of confluent cultures of BAEC inhibits tritiated thymidine [³H]TdR incorporation as well as their proliferation. the inhibition is dose- and time-dependent; maximum inhibition of [³H]TdR incorporation occurs 8 hr after cells are released from synchronization and the inhibitory fraction is added. Inhibition is evident at concentrations as low as 50 μg/ml and reaches a maximum at 600 μg/ml. the blockage of [³H]TdR incorporation is reflected in the inhibition of cell proliferation. In the presence of 400 μg of endogenous inhibitor per ml of media, added at the time of plating, the average population doubling time increases from 19 to 41 hr. These findings indicate that, in culture, BAEC can regulate their own proliferation by synthesizing an endogenous inhibitor(s) of proliferation.     

Retention of labelled DNA precursor by murine cells after a single administration of tritium labelled thymidine

The central zone of the rat lens epithelium, extending half way from the centre to the periphery of a whole mount preparation, normally has less than 1% of the cells in the cell cycle at any given time. Mechanical wounding initiates a burst of proliferation in the central zone. DNA synthesis begins 14 hr after wounding followed by mitosis 10 hr later. When [3H]TdR was applied at 2 hr prior to S phase, some moderately heavy and some light labelling was observed after the onset of S phase. When [3H]TdR was applied 5 hr before S phase (9 hr after wounding), all the cells were lightly labelled. Only small amounts of the label were available to these cells 5 hr after application. It is significant that there was labelling in this group because it indicates the persistence of relatively small intracellular pools of [3H]TdR for several hours after the initial 'pulse' labelling of cells. Determinations of the duration of S phase were based on the assumption that pulse labelling may be affected by the persistence of the pools of [3H]TdR and consequent light labelling of the cells.     

Modulation of serum-stimulated DNA synthesis in cultured human fibroblasts by cAMP

Density-dependent inhibition of growth of cultured human fibroblasts was associated with a 3- to 4-fold rise in the intracellular concentration of cyclic AMP (cAMP). Serum lowered cAMP levels in 2–5 min, with the low levels persisting for several hours. When quiescent fibroblast cultures were treated with 10% serum, the incorporation of [³H]TdR into DNA increased after a 10–16 h lag, reaching a peak by 20–24 h. Dibutyryl cyclic AMP (db-cAMP), when present throughout serum treatment, produced a dose-dependent inhibition of [³H]TdR incorporation. Half-maximal inhibition was seen with 0.1 mM db-cAMP. When db-cAMP or another cyclic nucleotide phosphodiesterase inhibitor, l-methyl-3-isobutylxanthine (SC-2964), was added together with serum to maintain elevated cAMP levels and after 4 h was replaced with fresh serum-containing medium, the wave of DNA synthesis induced by serum was not delayed. This implied that stimulation by serum could occur without an initial decrease in cAMP concentration. In contrast, db-cAMP added 8 h later than serum and not removed, inhibited [³H]TdR incorporation at the peak to the same extent as db-cAMP added together with serum. The inhibition decreased progressively when db-cAMP was added more than 8 h after serum. These results suggested that a cAMP-sensitive step occurred approx. 8 h after the addition of serum in mid-G1 of the cell cycle. Results obtained using fibroblasts synchronized at the G1/S boundary with hydroxyurea or exposed to db-cAMP for 24 h suggested that db-cAMP also inhibited TdR incorporation at the G1/S interphase or during S phase. Thus, whereas reduced cAMP concentrations did not appear to serve as an initial trigger for serum-stimulated DNA synthesis in human fibroblasts, db-cAMP and SC-2964, presumably by elevating cAMP levels, appeared to act in mid-G1 and possibly at the G1/S boundary or within S phase to inhibit thymidine incorporation.     

SYNCHRONIZATION OF MITOCHONDRIAL DNA SYNTHESIS IN CHINESE HAMSTER CELLS (LINE CHO) DEPRIVED OF ISOLEUCINE

Mitochondrial DNA (mit-DNA) synthesis was compared in suspension cultures of Chinese hamster cells (line CHO) whose cell cycle events had been synchronized by isoleucine deprivation or mitotic selection. At hourly intervals during cell cycle progression, synchronized cells were exposed to tritiated thymidine ([³H]TdR), homogenized, and nuclei and mitochondria isolated by differential centrifugation. Mit-DNA and nuclear DNA were isolated and incorporation of radioisotope measured as counts per minute ([³H]TdR) per microgram DNA. Mit-DNA synthesis in cells synchronized by mitotic selection began after 4 h and continued for approximately 9 h. This time-course pattern resembled that of nuclear DNA synthesis. In contrast, mit-DNA synthesis in cells synchronized by isoleucine deprivation did not begin until 9–12 h after addition of isoleucine and virtually all [³H]TdR was incorporated during a 3-h interval. We have concluded from these results that mit-DNA synthesis is inhibited in CHO cells which are arrested in G₁ because of isoleucine deprivation and that addition of isoleucine stimulates synchronous synthesis of mit-DNA. We believe this method of synchronizing mit-DNA synthesis may be of value in studies of factors which regulate synthesis of mit-DNA.     

Effect of Methotrexate On Cells In A Keratinized Epithelium With an Active Thymidine Salvage Pathway Assessed By Autoradiography*

In the partially synchronized cell system of the hamster cheek pouch epithelium, the inhibitory effect of a bolus injection of methotrexate (Mtx) (2 g/m², injected at 1200 hr) was analysed by means of both autoradiography and flow cytometry (FCM) in a 21-hr experiment. For autoradiography [³H]TdR and [³H]UdR were used as tracers for salvage and de nouo pathways of thymidylate (TMP) synthesis, respectively. For FCM no tracers were injected. the autoradiographic studies demonstrated an active TdR salvage pathway for DNA synthesis, not affected by the impaired de novo TMP synthesis. the blocked de novo TMP synthesis was partially released 7 hr after Mtx injection, but it had not totally recovered at the end of the experiment. the decrease in the fraction of S-phase cells detected about 10 hr after Mtx injection by autoradiographic labelling with [³H]TdR and by FCM was found to be caused by a decrease in the number of cells entering S phase. However, Mtx did not influence the salvage TMP synthesis rate of cells entering S phase.)     

Stimulation by tumor necrosis factor of HL-60 thymidine salvage pathway metabolism dissociated from proliferation

The effect of human tumor necrosis factor (TNF) on early-passage HL-60 cells was studied. A transient phase of increased [3H]thymidine (TdR) incorporation was noted at 20-24 hr of exposure to TNF. This increase was disproportionate to the much slighter stimulation of the percentage of S-phase cells, which was measured by flow cytometry. Evidence for increased metabolic trapping of [3H]TdR following TNF treatment was apparent from whole cell uptake experiments. The salvage pathway enzyme TdR kinase was therefore measured and was found to be elevated comparably to [3H]TdR uptake. The mechanism of TNF regulation of TdR kinase was further investigated by a series of combination treatment experiments using other biologic factors and pharmacologic inhibitors of various intracellular steps. The response to TNF was not potentiated or reproduced by IL-1, IL-2, IL-3, IL-4, G-CSF, M-CSF, GM-CSF or alpha- or gamma-interferon. Blockers of early signal transduction steps, including H7, W7, sphingosine, and pertussis toxin, failed to inhibit TNF stimulation of [3H]TdR incorporation. mRNA synthesis inhibition with alpha-amanitin blocked this TNF effect, as did cAMP but not cGMP analogues. A sensitizing effect was noted with amiloride or cytochalasin B, characterized by greater relative increases of [3H]TdR incorporation and TdR kinase activity in response to TNF. In the presence of cytochalasin B, TNF treatment resulted in no change or slight decreases in the percentage of S-phase cells. Regulation of TdR kinase could thereby be dissociated from the usual cell cycle control. This study thus documents a unique example of stimulation of thymidine salvage pathway metabolism by a biologic factor, dissociable from overall cell cycle regulation.     

Inorganic Phosphorus Stimulation of Bacterioplankton Production in a Meso-Eutrophic Lake

Experiments were conducted to determine whether production of heterotrophic bacterioplankton in a small meso-eutrophic lake was influenced by the dissolved inorganic phosphorus (DIP) supply. DIP may indirectly limit bacterial production by limiting phytoplankton, which in turn may limit the carbon available to bacteria. Direct DIP limitation of bacteria occurs where the availability of DIP for bacteria is insufficient to maintain growth. This work examined direct DIP limitation of bacteria by removing phytoplankton and incubating flasks with or without added P in the dark. Bacterial production was measured via the rate of incorporation of [³H]thymidine ([³H]TdR) into DNA. Bacterial abundance was followed with epifluorescent direct counts. Rates of [³H]TdR incorporation were significantly greater in flasks with added DIP, and changes in cell abundances generally paralleled increases in [³H]TdR incorporation. Even very small additions of P (0.05 μM) were sufficient to stimulate production. DIP addition to whole lakewater also stimulated [³H]TdR incorporation relative to that in zero-addition controls, but there was not a concurrent increase in bacterial cell numbers. The stimulation of [³H]TdR incorporation after DIP addition to whole lakewater was significantly less than the stimulation due to DIP addition to 1-μm-pore-size-filtered lakewater. In this study, addition of DIP caused as much as an eightfold stimulation of [³H]TdR incorporation.