Possible involvement of cyclic GMP in growth control of cultured mouse cells

Oral administration of inducible nitric oxide synthase inhibitors reduces nitric oxide synthesis but has no effect on the severity of experimental colitis

BACKGROUND

Increased concentrations of nitrate and nitrite (the breakdown products of nitric oxide) in the serum and faeces of patients with inflammatory bowel disease (IBD) suggests that increased synthesis of nitric oxide occurs in IBD. The aim of this study was to assess aminoguanidine (AMG), a selective inhibitor of inducible nitric oxide synthase, with regard to its effectiveness as a nitric oxide inhibitor and as a modulator of inflammation in trinitrobenzene sulfonic acid (TNBS)-induced colitis.

MATERIALS AND METHODS

Colitis was induced in Wistar rats. Selective (AMG) and non-selective (1-nitroso-arginine methyl ester (1-NAME)) inhibitors of nitric oxide synthase were given in the drinking water. Colonic citrulline and arginine concentrations were assessed using high-performance liquid chromatography. The severity of colitis was assessed by a macroscopic scoring system.

RESULTS

Both 1-NAME and AMG successfully reduced nitric oxide synthesis. There was no evidence of substrate depletion in the colonic wall. Neither of the agents reduced the severity of colonic inflammation.

CONCLUSIONS

Oral administration of nitric oxide synthase inhibitors reduced nitric oxide synthesis in the colonic wall. This study does not provide evidence to support a role for nitric oxide in the pathogenesis of colonic inflammation in TNBS colitis.

Nitric oxide donors, sodium nitroprusside and S-nitroso-N-acetylpencillamine, stimulate myoblast proliferation in vitro

Nitric oxide (NO) is an inter- and intracellular messenger involved in a variety of physiologic and pathophysiologic conditions. The effect of two NO donors, sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP) and their effect on myoblast proliferation was examined. Both donors stimulated an increase in myoblast cell number over a range (1-10 microM) of donor concentrations. However, 50 microM SNAP inhibited myoblast proliferation. Cell numbers from cultures treated with degraded 10 microM SNAP were equivalent to the control. Therefore, it appears NO can stimulate as well as inhibit myoblast proliferation.

Effects of intracellular cyclic AMP and cyclic GMP levels on DNA synthesis of young-adult rat hepatocytes in primary culture

Possible roles of dibutyryladenosine 3',5'-cyclic monophosphate (cAMP) and dibutyryl-guanosine 3',5'-cyclic monophosphate (cGMP) in regulation of hepatocyte DNA synthesis were examined using primary cultures of young-adult rat hepatocytes maintained in arginine-free medium. Throughout the experimental period, nonparenchymal cells were hardly observed in the selective medium. When epidermal growth factor (EGF) was added to the cultures, a transient increase in the intracellular cAMP level preceded the elevation of hepatocyte DNA synthesis. EGF-stimulated hepatocyte DNA synthesis was remarkably enhanced by the elevation of the intracellular cAMP level induced by treatment with cAMP alone or a combination of cAMP and theophylline, an inhibitor of cyclic nucleotide phosphodiesterase. Furthermore, the early elevation of intracellular cAMP alone, which was induced by treatment with the combination of cAMP and theophylline, caused a remarkable increase in hepatocyte DNA synthesis. On the other hand, addition of EGF to the cultures caused a rapid decrease in the intracellular cGMP level followed by an increase in hepatocyte DNA synthesis. EGF-stimulated hepatocyte DNA synthesis was severely suppressed or completely inhibited by the elevation of the intracellular cGMP level induced by treatment with cGMP alone or a combination of cGMP and dipyridamole, a specific inhibitor of cGMP phosphodiesterase. These findings indicate that cAMP and cGMP act oppositely on the regulation of DNA synthesis of young-adult rat hepatocytes in primary culture: cAMP plays a positive role, whereas cGMP plays a negative role. Also it is strongly suggested that an early elevation of the intracellular cAMP level is essential for the onset of DNA synthesis in hepatocyte primary cultures.

Neurotensin stimulates growth of colon cancer

Neurotensin (NT), a peptide from the distal gut that is released by fat ingestion, stimulates the growth of normal small bowel and colonic mucosa. The purpose of this study was to determine whether chronic administration of NT would affect the growth of a mouse colon cancer (MC-26) and a human colon cancer (LoVo) in vivo. In experiment 1, male Balb/c mice were inoculated with MC-26 cells (5 x 10(4)) and then randomized to four treatment groups receiving either saline (control) or NT (150, 300 or 600 micrograms kg-1) administered subcutaneously (s.c.) every 8 h for 21 days. In experiment 2, 60 mice with MC-26 tumours were randomized to receive saline (control) or NT (300 or 600 micrograms kg-1) for 28 days, and survival was then assessed. In experiment 3, 16 athymic nude mice with LoVo tumour xenografts were randomized to receive either saline (control) or NT (600 micrograms kg-1). We found that administration of NT (300 and 600 micrograms kg-1) significantly stimulated mean tumour area, weight and DNA, RNA and protein content of MC-26 tumours. In addition, the survival rate of mice bearing MC-26 tumours and treated with either dose of NT was significantly decreased compared with the control group given saline injections. Similarly, NT (600 micrograms kg-1) stimulated growth (tumour area, weight and nucleic acid contents) of the human colon cancer, LoVo. We conclude that NT acts as a tropic factor for the colon cancer cell lines MC-26 and LoVo in vivo. NT may play an important role in growth regulation of certain colon cancers.

Tumour cell activity markers in epithelial ovarian cancer: are biochemical and cytometric indices complementary?

Flow cytometry has enabled the objective assessment of cellular morphology and activity, which can also be biochemically evaluated by measuring products of cellular metabolism, such as cyclic 3'5' guanosine monophosphate (cGMP). Using paraffin-embedded formalin-fixed material obtained from the primary operation, an analysis of the correlation between nuclear ploidy and the proliferative index (PI) as quantified by flow cytometry with pre-treatment urinary cGMP was performed in 40 epithelial ovarian cancer (EOC) patients. The majority of the study group had advanced disease (28 FIGO III/IV) and residual disease (31). All but three (stage I) patients received single agent high dose cisplatinum as first-line therapy (100 mg m-2 x 5); in patients with evaluable disease there was a response rate of 64%. Thirty-one patients have died; the median survival of the study population being 27 months. There was a significant association between cGMP and PI. Significantly more aneuploid tumours had elevated PI values (P = 0.02). No variable predicted response. An initial univariate log rank analysis identified stage, the amount of residual disease, cGMP and PI as prognostic factors. Because of the interrelation between these and other factors and because PI did not conform to the proportional hazards model, a multivariate stepwise discriminant analysis was performed using survival at 36 months (the minimum follow-up for surviving patients) as the end-point. On the basis of this analysis, stage and residual disease were the most important prognostic factors, but cyclic GMP continued to have prognostic value even when these other factors were entered into the predictive model. However, the additional information gained has little clinical relevance.

1 alpha,25-dihydroxyvitamin D3-induced increments in hepatocyte cytosolic calcium and lysophosphatidylinositol: inhibition by pertussis toxin and 1 beta,25-dihydroxyvitamin D3

1 alpha,25-Dihydroxyvitamin D3 rapidly increases cytosolic calcium and alters membrane phospholipid metabolism in hepatocytes. To define the causal relationship between these events, we examined the effects of 1 alpha,25-dihydroxyvitamin D3 on 32P-labeled lysophosphatidylinositol levels and cytosolic calcium as affected by pertussis toxin and 1 beta,25-dihydroxyvitamin D3, the biologically inactive analog. 32P-labeled lysophosphatidylinositol was determined by two-dimensional thin-layer chromatography. Cytosolic calcium was measured in cells loaded with quin-2AM. Within 5 min, 1 alpha,25-dihydroxyvitamin D3 increased hepatocyte cytosolic calcium by 31% (p less than 0.05) and 32P-labeled lysophosphatidylinositol by 38% (p less than 0.05). Pertussis toxin inhibited the hormone-induced rise in cytosolic calcium but not the increase in 32P-labeled lysophosphatidylinositol. Exposure to exogenous lysophosphatidylinositol for 5 min increased cytosolic calcium by 40% (p less than 0.05), an effect that was also inhibited by pertussis toxin. 1 beta,25-Dihydroxyvitamin D3 had no effect on either hepatocyte cytosolic calcium or 32P-labeled lysophosphatidylinositol but prevented the 1 alpha,25-dihydroxyvitamin D3-induced increments. The results suggest that a G protein sensitive to pertussis toxin is required for the transduction of the lysophosphatidylinositol signal but not the generation of the signal. The ability of 1 beta,25-dihydroxyvitamin D3 to inhibit the 1 alpha,25-dihydroxyvitamin D3-induced changes in phospholipids suggests that the epimer may compete with 1 alpha,25-dihydroxyvitamin D3 for an initiating receptor.

Thrombin-receptor occupancy initiates a transient increase in cAMP levels in mitogenically responsive hamster (NIL) fibroblasts

Previous studies from our laboratory have shown that thrombin mitogenesis requires both high-affinity receptor occupancy and enzymic activity. Combined addition of DIP-inactivated-thrombin, which retains the ability to bind to thrombin receptors, and enzymically active gamma-thrombin generates a complete set of signals sufficient to initiate cell proliferation. Several possible signals, including stimulation of ion fluxes and phosphoinositide turnover, appear to be stimulated by thrombin's enzymic activity, but not by receptor occupancy. We now report that alpha-thrombin and DIP-thrombin stimulate an early, transient increase of 60 to 200% in intracellular levels of cAMP. This stimulation occurs at low mitogenic concentrations of alpha-thrombin where less than half the receptors are occupied. Enzymically active gamma-thrombin, which stimulates other types of signals, has no stimulatory effects on cAMP. Thus, this effect appears to be generated by high-affinity interaction of thrombin with its cell-surface receptors. Artificially increasing cAMP levels within these cells, however, cannot replace the requirement for thrombin-receptor occupancy in completing the mitogenic stimulation. Therefore, thrombin-receptor occupancy may generate additional, as yet unidentified, required signals.

B complex vitamins activate rat guanylate cyclase and increase cyclic GMP levels

Since B complex vitamins and the intracellular messenger cyclic guanosine monophosphate (GMP) have similar effects of promoting growth, DNA and protein synthesis, the present investigation was designed to determine if the B complex vitamins' mechanism of action might involve cyclic GMP. All of the B complex vitamins increased rat cyclic GMP tissue levels. The cause of these increased cyclic GMP levels was activation of the enzyme guanylate cyclase [E.C.4.6.1.2.] which was increased significantly (P less than 0.001) in a variety of tissues at the l nmol l-1 concentration of these vitamins. The maximal activation of this enzyme required the presence of manganese ion. The present investigation suggests that the guanylate cyclase-cyclic GMP system may play a role in the mechanism of action of B complex vitamins at the cellular level.

GTP stimulates fusion between homologous and heterologous nuclear membranes

The tissue and species specificity of GTP-stimulated nuclear membrane fusion has been examined. The fusion capacity of the membranes of nuclei isolated from two different tissue sources and three different animal species was determined. In all cases the incubation of isolated nuclei in the presence of 0.5 mM GTP led to the pairing of nuclei and formation of continuous outer membranes between the nuclei as a result of membrane fusion. Experiments using mixtures of nuclei from the different sources demonstrated that hybrid nuclear membranes could be formed as a result of the fusion between the outer membranes of heterologous nuclear pairs. The results suggest that the capacity for nuclear membranes to fuse in the presence of GTP is highly conserved when viewed on an evolutionary basis.

Cation-dependent prolactin activation of guanylate cyclase

Prolactin enhanced guanylate cyclase [E.C.4.6.1.2] two- to threefold in ovary, testis, mammary gland, liver and kidney. Dose response relationships revealed that maximal activation of this enzyme was at a concentration of one nanomolar and that increasing prolactin's concentration to the millimolar range caused no further increase in activity. There was an absolute cation requirement for prolactin's enhancement of guanylate cyclase. Calcium or manganese allowed prolactin to increase guanylate cyclase activity. Greater enhancement of this enzyme's activity by prolactin was observed when manganese was the co-factor. The data in this investigation suggest that guanylate cyclase may play a role in the mechanism of action of prolactin.

Characterization and metabolism of cyclic guanosine 3'5'-monophosphate in Mycobacterium smegmatis

Cyclic guanosine 3'5'-monophosphate was isolated and purified from Mycobacterium smegmatis TMC 1515 using ion exchange chromatography. It was characterized by thin layer chromatography and radioimmunoassay. Guanylate cyclase and cGMP phosphodiesterase were detected in the cytosolic and particulate (37,000 X g pellet) fractions respectively. On the basis of our observations, cGMP appears to play a dual role (i) at the time of induction of cell proliferation and (ii) protects the bacteria against unfavourable surroundings during stationary phase of the growth. This is the first report demonstrating presence of cGMP, guanylate cyclase and cGMP phosphodiesterase in mycobacteria.

Study on the growth promoting capacity of calf and fetal bovine serum for animal cells "in vitro". II. Electrophoretic study and survey on the antiproteolytic activity of pools of calf and fetal bovine serum

Calf serum and fetal bovine serum present great variability as to its growth promoting efficiency (GPE). As supplement of culture media to cultivate cells of animal origin they stimulate the "in vitro" multiplication and maintain cell viability. When fourteen lots of calf sera of variable GPE had the total protein contents as well as the percentages of serum fractions determined, no significant differences that could possibly explain the variability of the GPE were observed. Evaluation of the antiproteolytic activity of nineteen lots of calf serum and eighteen serum lots of younger calves showed that the former exhibited lower antiproteolytic titers (1:40 to 1:80) than the latter (1:80 to 1:160). Twelve lots of fetal bovine serum studied in parallel, showed the highest concentration of antiproteolytic factors, with titers equal to 1:320. Sera of bovine origin, but not fetal sera, are usually heat-inactivated, what was demonstrated to be responsible for the decrease of the antiproteolytic activity of 75% of the lots tested. This could explain the inability of certain heat-inactivated sera in promoting multiplication of some cells "in vitro", as verified with primary monkey kidney cells. The results obtained in this study indicated the convenience of submiting each lot of serum to be introduced in cell culture to previous determination of its characteristics, such as growth promoting efficiency, antiproteolytic activity and also toxicity, absence of extraneous agents, etc., in order to minimize the possibility of using serum lots of questionable quality, thus preventing not only the loss of cell lines, but also undesirable and sometimes expensive delays.

Cation-dependent vitamin D activation of human renal cortical guanylate cyclase

The objective of this investigation was to determine whether physiological levels of vitamin D and its metabolites have part of their mechanisms of action through stimulation of guanylate cyclase (EC 4.6.1.2). These sterols enhanced both soluble and particulate guanylate cyclase activities as well as cGMP levels two- to threefold in human and rat tissues. At a concentration of 1 nM, 1,25(OH)2D3 greater than 25(OH)D3 greater than vitamin D3 greater than 24,25(OH)2D3 = 25,26(OH)2D3 = vitamin D2. Dose-response curves revealed that maximal stimulation of guanylate cyclase by these sterols was at 1 nM and that there was no augmented guanylate cyclase activity at 0.01 nM. The precursors of vitamin D, cholesterol and 7-dehydrocholesterol, had no effect on guanylate cyclase activity. The activation of guanylate cyclase activity by the vitamin D sterols required the presence of manganese ion. Calcium was not as efficient as manganese in optimizing basal or hormone-stimulated guanylate cyclase activity. Vitamin D and its metabolites failed to stimulate adenylate cyclase (EC 4.6.1.1) activity. The data in this investigation suggest that guanylate cyclase may play a role in the mechanism of action of vitamin D at the cellular level.

Reversible inhibition by human serum lipoproteins of cell proliferation

Normal human serum or plasma was studied for the presence of inhibitors of cell proliferation by assaying inhibition of incorporation of labeled thymidine into acid-insoluble fraction using human FL cells. Lipoprotein fraction obtained by gel filtration through Sepharose 4B and by KBr density gradient centrifugation was found to play a major part of the inhibitory activity of the serum. It was also shown that the inhibitory activity resides in low-density lipoprotein (LDL). The addition of the lipoprotein fraction to growing FL cells caused an early decrease in the transport of uridine and thymidine across the membrane. This change in the permeability of membrane was followed by the preferential inhibition of DNA synthesis and a reduction in the percentage of mitotic cells in the cell population. The inhibition of the growth was reversible and was observed in various types of cells irrespective of species.

2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNPase) activity in cultured nerve cell lines from central nervous system: comparison of proliferating and resting growth states and cell cycle-dependent activity changes

1. The present communication is concerned with the expression and cell cycle-dependent regulation of the enzyme 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) in cultured nerve cell lines derived from the rat central nervous system (CNS). 2. The enzyme activity was measured in relation to two reversible serum-controlled growth states (exponentially growing/quiescent) including a comparison of the enzyme activities in cell lines of neuronal and glial origin as well as in fibroblasts. CNPase is present in all cell types tested, but the enzyme activity is very sensitive to changes in the cellular growth state. Nerve cell lines in exponentially growing cultures express a 3 to 15 times higher specific CNPase activity than the nonneural cell types. In serum-starved quiescent cultures, the differences in specific enzyme activity between the nerve cell lines and the fibroblasts are enlarged even more up to a ratio of about 50 to 150, indicating a specific function of this enzyme within the central nervous system. 3. Neuron-like B104 cells could be stimulated to synchronized growth by serum readdition to quiescent cultures. A series of ordered activity changes of CNPase has been observed after the reinitiation of cell growth. The enzyme is stimulated at two particular stages during the cell cycle, leading to a biphasic activity profile. Maximum stimulation of CNPase correlates with the G1 phase. 4. Hydroxyurea-induced blockage of synchronized B104 cells to traverse the S phase also prevents the subsequent stimulation of CNPase activity. Therefore, we conclude that a correlation exists between the periodic activity changes of CNPase and particular phases of the B104 cell cycle.

Synergistic stimulation of DNA synthesis by cyclic AMP derivatives and growth factors in mouse 3T3 cells

Addition of the cAMP derivatives butcAMP or 8BrcAMP to quiescent cultures of Swiss 3T3 causes synergistic stimulation of DNAk synthesis with insulin, phorbol esters, vasopressin, epidermal growth factor, or fetal bovine serum (2-5%). In the presence of insulin, 8BrcAMP, and butcAMP stimulate [3H]-thymidine incorporation into acid-precipitable material in a dose-dependent manner. The effect of these agents is specific since 8Br5'AMP, 5'AMP, butyrate, or 8BrcGMP fail to stimulate DNA synthesis under identical experimental conditions. Furthermore, the mitogenic effects of the cAMP derivatives were markedly potentiated by 1-methyl-3-isobutyl xanthine and 4-(3-butoxy-4-methoxy benzyl)-2-imidazolidine, both of which are potent inhibitors of cyclic nucleotide phosphodiesterase activity. The growth-promoting effects of the cAMP derivatives were demonstrated by [3H]-thymidine incorporation (either by scintillation counting or by autoradiography), by flow cytofluorometric analysis, and by increase in cell number. When quiescent Swiss 3T3 cells were exposed to butcAMP and insulin, DNA synthesis began after a lag of 17h. The result of sequential additions of cAMP derivatives and insulin to quiescent 3T3 cells suggest that these agents must act simultaneously in G0/G1 to stimulate entry into DNA synthesis in these cells. The findings support the proposition that an increase in cellular levels of cAMP (but not cGMP) act sas a mitogenic stimulus for confluent and quiescent Swiss 3T3 cells.

Cell cycle specific fluctuations of adenosine 3',5' -monophosphate and prostaglandin binding in synchronized mastocytoma P-815 cells

Endogenous adenosine 3',5' -monophosphate (cAMP) levels in mastocytoma P-815 cells, synchronized either at the G1/S transition by amethopterin- or double thymidine-block or in mitosis by colcemid block, were highest during late S and early G2 phases and lowest during mitosis. These cell cycle-dependent changes in cAMP levels were largely accounted for by changes in adenylate cyclase and phosphodiesterase activities. Similar fluctuations occurred simultaneously with specific prostaglandin E1 (PGE1) binding, histidine decarboxylase activity, histamine content, and [35S]SO-2(4) incorporation into glycosaminoglycans of the cells. In addition, endogenous levels of the E group of prostaglandins (PGEs) and "14C]carachiodonic acid incorporations into PGE, phosphatidylcholine and phosphatidylinositol also exhibited fluctuation patterns similar to that of cAMP levels. Since cAMP levels still fluctuated in a serum-depleted medium where DNA synthesis and cell division were inhibited, endogeneous levels of prostaglandin and cAMP appeared not to be regulated solely by serum factor(s). Exposure of cells at G1/S transition to 1-methyl-3-isobutylxanthine (MIX) resulted in 10-fold elevation of cAMP levels throughout the cell cycle without affecting DNA synthesis. On the other hand, PGE1 and/or MIX added at late S phase elevated cAMP levels, prolonged C2 phase and retarded the cell division, but these agents added at the beginning of mitosis elevated cAMP levels without affecting the cell division. These results suggest that prostaglandin newly synthesized by the increased metabolism of phospholipids promote the cAMP synthesis via their binding to the receptors and thereby control the division and phenotypic expression of mastocytoma P-815 cells.

Changes in cyclic nucleotide levels and phosphodiesterase and protein kinase activities in mitogenically stimulated 3T3 fibroblasts

Activities of cyclic-nucleotide-hydrolysing enzymes cAMP-, cCMP- and cGMP phosphodiesterase, the intracellular concentrations of cAMP, cCMP, and cGMP, and the activity of the cAMP-dependent protein kinase were studied in serum-starved 3T3 cells stimulated to proliferate by serum. Within 1 and 2 min after stimulation the activities of cAMP- and cGMP phosphodiesterase were unaffected while the concentration of cGMP was raised and that of cAMP lowered, suggesting increased synthesis of cGMP and simultaneously reduced synthesis of cAMP. 48 h after stimulation, when the cells multiplied rapidly, both the cAMP phosphodiesterase and the cCMP phosphodiesterase were reduced. Evidence was also obtained that cAMP-dependent protein kinase is important for expressing the cAMP effect in the 3T3 cells.

Guanylate cyclase activity in Escherichia coli mutants defective in adenylate cyclase

Guanylate cyclase, which catalyzes the synthesis of guanosine 3',5'-monophosphate, has been assayed in several strains of Escherichia coli. They include wild-type cells and mutants defective in adenylate cyclase, which is responsible for the synthesis of adenosine 3',5'-phosphate. Our results demonstrate that adenylate cyclase and guanylate cyclase are two different enzymes in E. coli and suggest that the gene that encodes adenylate cyclase also plays a regulatory role in the synthesis of guanylate cyclase.

Dependence of interfacial properties of normal and transformed 3T3 cell membranes on treatment with factors modifying proliferation

Interfacial properties of the outer cell membrane of normal and transformed in vitro cultures of mouse 3T3 cells have been investigated. The contact angles of sessile drops on dried cell preparations were measured and the interfacial tensions derived using the thermodynamic approach introduced by Neumann. Interfacial tensions were found to be within an order of magnitude of those determined for other cell and model membranes. Treatment of cells with calf serum, a stimulant to proliferation, resulted in a decrease in the interfacial tension of normal and transformed cells, whereas use of concanavalin A and its succinylated derivative lead to an increase of interfacial tensions of both cell types. These and further results show a detailed correlation between the growth-regulating effects and the effects on interfacial properties of these proliferation-modifying factors. An interpretation of the results of serum depression of the interfacial tension in terms of a binding equilibrium dependent on the concentration of humoral growth factors in the medium is attempted.

The involvement of cyclic GMP in tyrosine aminotransferase degradation in rat hepatoma tissue culture cells

Theophylline, a cyclic nucleotide phosphodiesterase inhibitor, increases the rate of tyrosine aminotransferase (TAT) degradation in rat hepatoma tissue culture (HTC) cells. Theophylline (0.1-10 mM) causes a two- to five-fold increase in intracellular cAMP concentration but a 30-60% decrease in cGMP concentration. The decrease in cGMP occurs at doses of theophylline which increase the rate of TAT degradation. When cGMP levels are increased by incubating the cells with either Mn2+, an activator of guanylate cyclase, or 8-bromo-cGMP, an analog of cGMP, the effect of theophylline is reversed and the rate of TAT degradation is slowed. Thus, the rate of TAT degradation is inversely related to the concentration of cGMP in HTC cells. This raises the possibility that a cGMP-dependent event is involved in the control of specific protein degradation.

Cell cycle changes in the adenylate cyclase of C6 glioma cells

The adenylate cyclase of C6 glioma cell cultures was characterized for sensitivity to the beta-adrenergic agonist isoproterenol, as well as fluoride, and GTP as a function of the cell cycle. The mitotic phase of the cell cycle was emphasized because both the basal cellular cyclic AMP level and the intact C6 cell's capacity to accumulate cyclic AMP in response to isoproterenol decreased during mitosis. Basal and stimulated adenylate cyclase activities in mitotic cells were decreased relative to the enzyme activities in the G1, S, and G2 phases of the cell cycle. Analysis of the beta-adrenergic receptor using the radioligand(-)[3H]dihydroalprenolol showed that neither ligand affinity nor receptor density changed during the cell cycle, indicating that the reduced adenylate cyclase activity of the mitotic C6 cell was not caused by alterations in this hormone receptor. The reduction in the mitotic cell's basal adenylate cyclase activity was more prominent than the decrease in isoproterenol-, fluoride, or GTP-stimulated activities suggesting that the effectiveness of these enzymes activators (i.e., the efficiency of the coupling mechanism) was not attenuated during mitosis. These studies indicate that the intrinsic catalytic capacity (not the beta-adrenergic receptor or the coupling mechanism) of the C6 adenylate cyclase complex is reduced during mitosis and contributes to the mitotic cell's inability to accumulate and maintain the cyclic AMP concentration at the interphase level.

Cyclic GMP metabolism in psoriasis: increased activity of soluble epidermal cyclic GMP phosphodiesterase and its modulation by calcium

Soluble cyclic GMP-phosphodiesterase was measured in normal and psoriatic epidermis. The specific activity of the enzyme was increased almost four-fold in involved compared with normal epidermis, and two- to three-fold in involved compared with uninvolved epidermis. The enzyme activity from all three sources was inhibited by 40-50% by ethylene glycol tetraacetic acid (EGTA). These results indicate that in addition to the reported enhanced capacity of psoriatic epidermis to generate cGMP, it has an increased ability to hydrolyse this nucleotide, although to a lesser degree than the augmentation found in soluble guanylate activity from psoriatic epidermis. These observations are compatible with the elevated steady-state levels of this nucleotide observed in the involved epidermis of psoriasis.

Human and rat growth hormones enhance guanylate cyclase activity

The objective of this investigation was to determine whether physiological levels of growth hormone have part of their mechanism of action through stimulation of guanylate cyclase (EC 4.6.1.2.). Rat and human growth hormones enhanced the activity of soluble guanylate cyclase two- to fourfold in rat gracilis anticus skeletal muscle, liver, lung, heart, pancreas, and kidney cortex at a concentration of 10 nM. Dose-response relationships revealed that more than half-maximal stimulation of guanylate cyclase activity was seen at a concentration as low as 10 nM and nonstimulation of guanylate cyclase activity was seen when the concentration was decreased to 1 nM. Maximal enhancement was seen at 100 nM of growth hormone, and there was no further enhancement when the concentration was increased to the micromolar or millimolar range. Thus, the data in this investigation indicate that at concentrations at which growth hormone is known to cause its growth-promoting effects, growth hormone does cause an enhancement of the activity of the guanylate cyclase-cyclic GMP system.

Epidermal cyclic GMP is increased in psoriasis lesions

Cyclic GMP levels in epidermis of normal subjects and of psoriatic patients were measured with a highly sensitive radioimmunoassay method. Technical improvements for the assay are 2-fold: (1) skin samples were frozen in vivo before biopsy and local injection of any anesthetic was avoided to overcome ischemia effect which could lower cyclic GMP artificially; (2) epidermis was microdissected to avoid contamination of dermis and keratin layers. The results show that on a per mg tissue dry weight basis the cyclic GMP levels are about 200 fmol in the involved lesional epidermis and 70 fmol in the uninvolved or normal epidermis. Similarly increases in the cyclic GMP levels in the lesional epidermis are observed when the data are expressed either on a DNA or protein basis. The cyclic GMP level in normal epidermis from nonpsoriatic subjects is the same as that in the uninvolved epidermis of psoriasis patients.

The effect of histamine on epidermal outgrowth: its possible dual role as an inhibitor and stimulator

We have investigated the effect of histamine on pig epidermal cell outgrowths in vitro. Histamine inhibited the epidermal cell outgrowths (and also mitosis). This inhibition was partially counteracted by a specific H2 antagonist, cimetidine. Inhibition was maximal at a histamine concentration of 10(-4) M and was less at 10(-3) M. These histamine concentrations respectively coincide with the optimal concentrations for accumulating intracellular cyclic AMP (via H2 receptors) and cyclic GMP (via H1 receptors) in the same pig epidermal slice system. 4-Methyl-histamine, a pure H2 agonist, which only increased the intracellular cyclic AMP level but not the cyclic GMP level, caused a maximal outgrowth inhibition at 10(-3) M. Attempts to counteract the histamine effects due to cyclic GMP accumulation by various H1 antagonists (so that 10(-3) M histamine would have caused maximal outgrowth inhibition) were unsuccessful, since the addition of each H1 antagonist alone strongly inhibited the outgrowth. These data strongly suggest a dual role of histamine through the cyclic nucleotide system; i.e., histamine inhibits epidermal cell growth by elevating the intracellular cyclic AMP level via an H2 receptor, while histamine at high concentrations (10(-3) M) partially counteracts the inhibition by increasing cyclic GMP via an H1 receptor.

Activity and kinetic properties of basal aryl hydrocarbon hydroxylase during proliferation in the transformable C3H 10T1/2 CL8 cell line

Basal aryl hydrocarbon hydroxylase (AHH) activity and its kinetic properties were studied as a function of proliferation in C3H mouse embryo 10T1/2 CL8 cells. Activity was low in freshly plated cells, increased during exponential growth, peaked at confluency, and then declined. The apparent Km-values for benzo[a]pyrene (BP) and NADPH were less in proliferating (approx. 0.37 microM BP, 3.3 nM NADPH) than in confluent cells (0.74-1.39 microM BP, 33.4-53.4 nM NADPH). Cells at different growth states responded differently to benz[a]anthracene (BA) and aminophylline, an inhibitor of cyclic nucleotide phosphodiesterases. When cells were harvested at the mid log phase of growth, 12 h of exposure to aminophylline caused maximum induction, while 24 h of BA treatment were required. In contrast, at early confluence, 12 h of BA treatment gave the greatest levels of activity, while exposure to aminophylline did not induce AHH. In fact, decreases in activity were observed. These differences are indicative of different regulatory mechanisms for BA and aminophylline induction. They also suggest the regulation of basal AHH by cyclic nucleotides changes during growth. The exposure times giving maximum activity were used to determine the kinetic properties of BA-induced activity. As with basal AHH, the Km-value for BP was less in log phase (0.2-0.4 microM BP) than in confluent cells (0.64-1.05 microM BP). Moreover, the Km-values for BP and NADPH in control cultures at confluency (0.10-0.14 microM BP, 15.4-23.2 nM NADPH) were less than those for BA-treated cells (0.64 microM BP, 37.9-54.8 nM NADPH) under the same nutritional conditions. The finding that the Km-values for BP is lower in rapidly dividing cells than in confluent cells may help to explain why proliferating cells are more susceptible to transforming agents.

Induction of growth in kidney epithelial cells in culture by Na+

The role of Na+ in the regulation of cell growth was examined in quiescent, high-density cultures of kidney epithelial cells of the BSC-1 line. The addition of NaCl to the medium increased the number of cells initiating DNA synthesis in a concentration-dependent manner after serum stimulation. In the presence of added NaCl, cells in confluent cultures grew to high density at an increased rate, whereas growth in sparse cultures was retarded. These results suggest that, in the presence of serum, Na+ can act as a mediator of the events that initiate cell proliferation.

Cigarette smoke exposure in vivo increases cyclic GMP in rat lung

The enzyme guanylate cyclase is stimulated to produce cycle guanosine 3',5'-monophosphate (GMP) when lung tissue is exposed to cigarette smoke in vitro. These experiments tested whether in vivo exposure in rats to cigarette smoke produces a similar response. Adult rats were anaesthetized with pentobarbital and ventilation with mixtures of air and cigarette smoke at 10 cm H2O inspiratory pressure was achieved after a tracheotomy was performed. Lung tissue samples were taken at intervals during 20 min exposure period and analyzed for levels of cyclic adenosine 3', 5'-monophosphate (AMP) and cycle GMP. Blood carboxyhemoglobin (COHb) levels at 5 min and 15 min of exposure showed high, but sublethal levels of COHb. lung tissue cAMP was unchanged with this exposure, but cGMP levels rose dramatically. Rat lungs showed no changes related to ventilation under similar conditions in the absence of smoke. This observed response of cGMP to cigarette smoke may represent an important pulmonary defense mechanism.

Regulation of 40S ribosomal protein S6 phosphorylation in Swiss mouse 3T3 cells

Addition of serum to resting cultures of Swiss mouse 3T3 cells causes an immediate multiple phosphorylation of 40S ribosomal protein S6. After 60 min of stimulation, changing to medium containing no serum led to the net dephosphorylation of S6. During this same period, a second protein, as yet unidentified, became increasingly phosphorylated. Incubation of cells with cycloheximide prior to the addition of serum almost completely blocked the activation of protein synthesis. There was no effect on the serum-induced phosphorylation of S6. If cells were stimulated in the presence of cAMP phosphodiesterase inhibitors theophylline or SQ 20006, both S6 phosphorylation and the activation of protein synthesis were inhibited. Stimulation of cells with serum also led to an immediate drop in total intracellular cAMP levels. This was blocked by prostaglandin E1 (PGE1), which caused a 10 fold increase in total intracellular cyclic AMP. However, PGE1 had no effect on protein synthesis or S6 phosphorylation.

The effects of epidermal growth factor on the cyclic nucleotide system in pig epidermis

Incubation of pig skin slices with epidermal growth factor (EGF) caused an increase in intracellular cyclic GMP concentration. A significant increase was found after 1 hr of incubation and reached a peak by 6 hr. EGF caused no change in the cyclic AMP level nor did it affect epinephrine-induced cyclic AMP responses.

The effect of serum and multiplication stimulating activity on L6 myoblast growth: the lack of correlation with cyclic nucleotide changes

The role of cAMP and cGMP in triggering proliferation of rat myoblasts was evaluated by: (1) measuring effects of mitogens on intracellular cyclic nucleotide levels, and (2) observing effects of agents which altered cyclic nucleotide levels on cell proliferation. Multiplication stimulating activity (MSA, 1 microgram/ml), a member of the somatomedin family, stimulated cell proliferation after 48 hr. It had little effect on cellular cyclic nucleotide levels, measured by radioimmunoassay. Horse serum (HS) and fetal bovine serum (FBS) stimulated cell proliferation approximately equally. Neither affected cAMP levels; FBS reduced cGMP to 33% of control values, but HS had no effect. Thus, there was no simple correlation between mitogenic action and cyclic nucleotide levels at any time from 5 min to 24 hr after addition of a purified mitogen or serum. Furthermore, agents which caused substantial changes in cyclic nucleotide levels had no effect on cell proliferation. Prostaglandin E1 (5 microM) elevated cAMP 440% without affecting cGMP levels or cell growth. A potent phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine (MIX, 0.1 mM), when added alone or in the presence of MSA, HS, or FBS, elevated cAMP 200% and cGMP 167%, but it had little effect on their mitogenic action. Lastly, a purified mitogen such as MSA, unlike serum, must be present for extended periods of time in order to stimulate cell proliferation. This makes it unlikely that a trigger mechanism functions in initiating cell division. We conclude that neither cAMP nor cGMP appear to be second messengers for the mitogenic action of MSA or serum on muscle cells.

Patterns of cyclic nucleotides in normal and leukaemic human leucocytes

Because recent observations indicate that metabolism of cyclic nucleotides may be altered in neoplastic cells, the intracellular levels of cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) were measured in mononuclear leukaemic and normal human leucocytes. The activities of adenylate cyclase, guanylate cyclase and cyclic nucleotide phosphodiesterases were also determined. Under basal conditions, cAMP levels were always higher in the normal leucocytes, whilst cGMP levels were of the same order of magnitude in both normal and leukaemic cells, causing the cAMP/cGMP ratios to be significantly lower in leukaemic leucocytes. Leukaemic cells significantly increased cyclic nucleotide levels in response to theophylline, but did not respond to serotonin, carbamylcholine or D,L-isoproterenol. Preincubation of these leucocytes with theophylline produced a detectable cAMP response to D,L-isoproterenol but no cGMP response to serotonin or carbamylcholine was found. Adenylate cyclase and guanylate cyclase were significantly lower in leukaemic than in normal cells, which could largely explain the abnormal cyclic nucleotide pattern found in human leukaemic leucocytes. In our experiments, cAMP phosphodiesterase activity was comparable in normal and leukaemic cells, whereas cGMP phosphodiesterase activity was undetectable inall mononuclear-leucocyte preparations with the methods used.

Sponge cell aggregation

Dissociated sponge cell system has proved to be a useful model to study the process of cell aggregation both on cellular and subcellular level. The purpose of this review is to discuss recent results obtained from experiments with the marine sponge Geodia cydonium. Dissociated cells form functional aggregates during a process which can be sub-divided into three phases: first, formation of small primary aggregates in the presence of Ca2+; second, formation of secondary aggregates in the presence of an aggregation factor and third, reconstitution of a functional system of water-containing channels by rearrangement in the secondary aggregates. On subcellular level a series of macromolecules are known which are involved in the control of aggregation and separation of sponge cells: Aggregation factor, aggregation receptor, anti-aggregation receptor, beta-glucuronidase, beta-glucuronosyltransferase, beta-galactosyltransferase, beta-galactosidase and a lectin. These components might be linked in the following sequence: (a) Activation of the aggregation receptor by its enzymic glucuronylation; (b) Adhesive recognition of the cells, mediated by the aggregation factor and the glucuronylated aggregation receptor; (c) Inactivation of the aggregation receptor by its deglucuronylation with the membrane-associated beta-glucuronidase; (d) Cell separation due to either the loss of the recognition site (glucuronic acid) of the aggregation receptor for the aggregation factor or to an inactivation of the aggregation factor by the anti-aggregation receptor. The activity of the anti-aggregation receptor is most likely controlled by the Geodia lectin. The events leading to cell-cell recognition cause a change in the following metabolic events: Increase of oxygen uptake, decrease of cyclic AMP level, increase of cyclic GMP level and stimulation of programmed syntheses.

Control of animal cell proliferation

Present understanding of the control of animal cell proliferation is summarized briefly. Major gaps in present knowledge are listed. Models of growth control are discussed.

Electric currents, bone remodeling, and orthodontic tooth movement. I. The effect of electric currents on periodontal cyclic nucleotides

Osteogenesis has been found to occur in response to the application of electric currents to bone. The objective of this experiment was to study the effects of D.C. electric currents on periodontal tissues in cats. Cyclic nucleotides, compounds known to be involved in cellular activation, were studied by immunohistochemistry in the involved tissues. Three groups of three young adult cats each were treated for 1, 3, and 7 days, respectively, by a device delivering 15 microamperes of direct current to bone osteoblasts and PDL cells stained intensely for cAMP and cGMP were observed adjacent to the cathode and anode, and bone apposition was found near the cathode. These results suggest that electric stimulation enhances cellular enzymatic phosphorylation activities in periodontal tissues and may be a potent tool in accelerating alveolar bone turnover.

Early changes of cyclic nucleotide levels in a mitogenic reaction in the rat mesentery

By measuring simultaneously cAMP and cGMP we found a biphasic time course with regard to cGMP and the cGMP/cAMP ratio very early in a mitogenic reaction in vivo. This is a new finding.