Role of cyclic nucleotides in growth control

Cellular levels, excretion, and synthesis rates of cyclic AMP in Escherichia coli grown in continuous culture

Changes in dilution rate did not elicit large and systematic changes in cellular cyclic AMP levels in Escherichia coli grown in a chemostat under carbon or phosphate limitation. However, the technical difficulties of measuring low levels of cellular cyclic AMP in the presence of a large background of extracellular cyclic AMP precluded firm conclusions in this point. The net rate of cyclic AMP synthesis increased exponentially with increasing dilution rate through either the entire range of dilution rates examined (phosphate limitation) or a substantial part of the range (lactose and glucose limitations). Thus, it is probable that growth rate regulates the synthesis of adenylate cyclase. The maximum rate of net cyclic AMP synthesis was greater under lactose than under glucose limitation, which is consistent with the notion that the uptake of phosphotransferase sugars is more inhibitory to adenylate cyclase than the uptake of other carbon substrates. Phosphate-limited cultures exhibited the lowest rate of net cyclic AMP synthesis, which could be due to the role of phosphorylated metabolites in the regulation of adenylate cyclase activity. Under all growth conditions examined, greater than 99.9% of the cyclic AMP synthesized was found in the culture medium. The function of this excretion, which consumed up to 9% of the total energy available to the cell and which evidently resulted from elaborate regulatory mechanisms, remains entirely unknown.

Cyclic AMP-mediated control of meiosis: effects of progesterone, cholera toxin, and membrane-active drugs in Xenopus laevis oocytes

Progesterone depressed rapidly (50% at 1 min) and persistently cyclic AMP (cAMP) concentration that had been elevated by cholera toxin in Xenopus laevis oocytes. cAMP remained below 1 pmol per oocyte (mean basal level) for approximately 1 hr and thereafter rose to approximately 120% of control values, while germinal vesicle (nucleus) breakdown did not occur. In the absence of cholera toxin, progesterone treatment for 6 hr maintained cAMP concentration below the basal level (but not lower than 80%), and germinal vesicle breakdown occurred. Experiments in the presence of phosphodiesterase inhibitors suggested that progesterone modulates adenylate cyclase activity. The maturation promoting factor, which is formed after 3-5 hr of progesterone treatment and provokes germinal vesicle breakdown after its injection into untreated oocytes, also decreased cAMP concentration, an observation that may explain its "autoamplification." Nonsteroidal inducers of meiosis reinitiation (e.g., propranolol, methoxyverapamil, mersalyl) diminished the cholera toxin-mediated accumulation of cAMP, in contrast to compounds devoid of meiotic-inducing capacity and antagonist to progesterone action, such as gammexane (an inositol analogue) and 5'-deoxy-S-(2-methylpropyl)-5'-thioadenosine (a methylase inhibitor), that increased the nucleotide level. The fine control, suggested by the effects of small changes in cAMP levels, gives evidence of great sensitivity to a critical determinant governing meiotic cell division.

Development of beta-adrenergic receptors and the in vitro accumulation of cyclic AMP in the chick spinal cord

To clarify the functional development of the descending monoaminergic input to the chick spinal cord we have studied the ontogeny of beta-adrenergic receptors by measuring the specific binding the tritiated dihydroalprenolol (DHA). In addition, we examined the ability of isoproterenol to stimulate the accumulation of cyclic AMP in slices of developing chick spinal cord. Results show that the chick spinal cord contains a high density of beta-adrenergic receptors that are apparently linked to adenylate cyclase. During development, both the density of beta-receptors, as determined by the specific binding of DHA, and the response of tissue slices to isoproterenol underwent marked changes. beta-Adrenergic receptors (approximately 4 fmol/mg tissue) were first detected on the fourteenth day in ovo. Receptor density increased to approximately 20 fmol/mg by day 20. Between day 20 and the time of hatching, a sharp increase in receptor density, to approximately 50 fmol/mg, was seen. The density of receptors remained high until the second day after hatching, fell off to approximately 30 fmol/mg by the fourth day, and remained relatively unchanged through day 30. The response of spinal cord slices to isoproterenol showed a similar pattern of development with the peak response (7-fold increase in levels of cyclic AMP) occurring at or near the time of hatching. During the period between day 18 in ovo and the time of hatching, when both the response of tissue slices to isoproterenol and the density of beta-receptors increased markedly, the activity of phosphodiesterase did not change. Therefore, the pronounced changes in adrenergic responsiveness that occurred near the time of hatching appear to be related primarily to changes in the density of beta-adrenergic receptors coupled to adenylate cyclase. Such developmental changes in the density of beta-adrenergic receptors and adrenergic responsiveness may play an important role in determining the functional state of the descending monoaminergic systems in the chick spinal cord.

Modulation of type I and type III collagen production in normal and mutant human skin fibroblasts by cell density, prostaglandin E2 and epidermal growth factor

Previously, we found that, at high cell density, human skin fibroblasts produce a higher proportion of type III to type I collagen. Treatment of sparse cultures with prostaglandin E2 also increases this ratio, while treatment of dense cultures with epidermal growth factor reduces it. Similar changes were observed using some strains of fibroblasts from patients with the Ehlers-Danlos syndrome type IV and osteogenesis imperfecta. Studies on the extent of intracellular degradation suggest that the changes observed could be caused in part by changes in the breakdown of collagen inside the cell with preferential loss of type I collagen and/or changes in synthesis of the two proteins.

Cyclic nucleotides in the denervated rat diaphragm and the effect of cyclic AMP on ornithine and adenosylmethionine decarboxylases

The concentrations of cyclic AMP and cyclic GMP were measured in the denervated rat diaphragm at various times following unilateral phrenicectomy. Cyclic AMP concentration was raised by the second day after operation, reached a peak by the third day, followed by another increase at around 10 days. By contrast, cyclic GMP concentration was decreased within a day after denervation and remained below control levels at all subsequent times studied. Epinephrine in vitro produced a comparable increase in the concentration of cyclic AMP in both normal and denervated tissue. The concentration of adenosine appeared unchanged in the denervated diaphragm by comparison with its innervated control. Activity of ornithine decarboxylase was elevated in the diaphragms of rats treated with dibutyryl cyclic AMP, but this effect could also be achieved with sodium butyrate alone. Adenosylmethionine decarboxylase activity, was unaffected after treatment with either compound. These observations and others discussed are taken to indicate a lack of direct relationship between cyclic AMP concentrations and the activity of the rate-limiting enzymes of polyamine biosynthesis in the rat diaphragm.

Adenosine 3',5'-phosphate phosphodiesterase and pheromone response in the yeast Saccharomyces cerevisiae

Theophylline, aminophylline, and isobutylmethylxanthine, compounds reported to be inhibitors of adenosine 3',5'-phosphate (cAMP) phosphodiesterase, prevented the alpha-factor-induced cell cycle arrest of Saccharomyces cerevisiae a cells. To determine whether the in vivo effect of these methylxanthines on yeast pheromone response was related to their known biochemical mode of action, two assays for cAMP phosphodiesterase based on affinity of the product of the reaction (5'-AMP) for boronate groups were developed and were used to monitor the activity of the low Km cAMP phosphodiesterase present in yeast extracts. It was found that the relative efficacy of the methylxanthines as inhibitors of this enzyme in vitro was correlated with the degree to which they antagonized alpha-factor action in vivo. These results were consistent with our previous proposal that pheromone action involves a lowering of cAMP level in the target cell.

Multiple mechanisms of growth inhibition by cyclic AMP derivatives in rat GH1 pituitary cells: isolation of an adenylate cyclase-deficient variant

GH pituitary cells have been widely utilized for studies of hormone response mechanisms. Studies reported here were motivated by the desirability of isolating characterized GH clones defective in cyclic AMP synthesis or action. Spontaneously occurring GH1 cell variants resistant to the growth-inhibitory effects of cyclic AMP analogs were isolated. Characterization of four variants showed that these were deficient in adenosine kinase and had acquired resistance to the cytotoxic effects of purine nucleoside derivatives formed in the culture medium. A second-stage selection was undertaken with mutagenized adenosine kinase-deficient cells. One 8 Br cAMP-resistant variant was found to have normal cyclic AMP-dependent protein kinase activity but exhibited altered adenylate cyclase activity. Activation of cyclase activity by fluoride, guanyl nucleotides, cholera toxin, and hormone (VIP) was subnormal in the variant. Mn-dependent cyclase activity was also subnormal, suggesting that the 8 Br cAMP-resistant variant may have a deficiency in the catalytic moiety of adenylate cyclase. Surprisingly, adenosine 3':5'-monophosphate and 5'-monophosphate derivatives were found to be equally potent in growth-inhibiting adenosine kinase-deficient cells. Cross-resistance to 8 Br AMP was observed in the 8 Br cAMP-resistant variant. We conclude that cyclic AMP derivatives inhibit growth of GH cells by an unanticipated mechanism that is, nonetheless, related to endogenous cyclic AMP synthesis.

Growth arrest and morphological change of human breast cancer cells by dibutyryl cyclic AMP and L-arginine

The growth in vitro of human breast cancer cells, line MCF-7, was inhibited by a daily supplement of L-arginine (1 milligram per milliliter). Arginine acted synergistically with dibutyryl adenosine 3',5'-monophosphate (cyclic AMP) (10(-6) molar) to enhance the growth inhibitory effect: the cell replication ceased completely within 2 days after treatment. The growth arrest accompanied a change in cell morphology and was preceded by increases in the cellular concentration of cyclic AMP, adenylate cyclase, and type II cyclic AMP-dependent protein kinase activities as well as a decrease of estrogen binding activity. The results suggest that growth of human breast cancer cells is subject to cyclic AMP-mediated regulation and that arginine may play a specific role in this process.

[Physiological and pathophysiological significance of superoxide-radicals and the regulatory role of the enzyme superoxide dismutase (author's transl)]

The monovalent reduction of molecular oxygen, resulting in the formation of superoxide radicals (O(2)) is regarded as to be an ongoing physiological process involved in the respiration and other biological processes of aerobic cells. These reactive oxygen species have been reported to function as cofactors in many biosynthetic reaction steps. Thus, deviations from cellular steady state concentrations may lead to a multiplicity of clinical symptoms or may to a great deal determine the characteristic of a distinct malady. Decrease of cellular O(2)-concentration is discussed in connection with Trisomie 21 and various mental disorders. The role of O(2) in the biochemistry of inflammation, autoimmune diseases, various toxicological cases and the biological aging process is described. Hypothetical considerations concerning the involvement of O(2) in the pathogenetic mechanisms of Morbus Wilson, haemochromatosis, Parkinson syndrome, cataractogenesis and in carcinogenesis are presented. The physiological control of cellular O(2)-concentration is performed by formation rates of the various cellular O(2)-sources and the overall elimination rates of O(2)-consuming reaction steps. Superoxide dismutase (SOD) is of special interest within this cycle because it detoxifies O(2) radicals with velocity rates which are significantly faster than any other pathway involved in O(2) elimination. Thus attempts for a therapeutic interference on tissue levels of O(2)-radicals are mainly based on inhibition or activation of cellular SOD-activities depending on a supposed decrease or increase in cellular steady state concentrations of O(2). The availability of a drug version of SOD and of various synthetic SOD-active compounds allowing a therapeutic decrease of O(2)-tissue levels. Inhibition of cellular SOD is also possible, however, many still unknown toxic side effects should be expected because of unspecific action of the inhibitor available.

Growth inhibition by adenosine 3',5-monophosphate derivatives does not require 3',5' phosphodiester linkage

Analogs of adenosine 3',5'-monophosphate (cyclic AMP) inhibit the growth of cultured cell lines. The effects of 8-bromo- and N6-butyryl-substituted analogs of cyclic and noncyclic AMP on six cell lines were examined and were equally inhibitory. Variant cell lines with altered cyclic AMP-dependent protein kinase were more resistant to both cyclic and noncyclic nucleotides. We conclude that growth inhibition by analogs of cyclic AMP (i) does not require a 3',5' phosphodiester bond and (ii) may be mediated by a pathway involving endogenous cyclic AMP.

DNA synthesis in primary cultures of adult rat hepatocytes in a defined medium: effects of epidermal growth factor, insulin, glucagon, and cyclic-AMP

Epidermal growth factor (EGF) especially in combination with insulin and glucagon, has been shown to stimulate DNA synthesis in liver cells, both in the whole animal and in cell cultures. As a further development we have found that in primary monolayer cultures of freshly isolated adult rat liver parenchymal cells, in which contamination with nonparenchymal cells was negligible, DNA synthesis was substantially stimulated by these substances. In control cultures, incorporation of [3H]thymidine into DNA and labeling of nuclei in autoradiographs was low. The stimulation by EGF was enhanced by insulin and glucagon, whereas these hormones by themselves exhibited only limited activity. These observations were made in cultures of hepatocytes that were never exposed to serum, even during cell isolation and plating. Hence for stimulation of DNA synthesis under these conditions neither serum factors nor interactions with other types of cells or their products were required. The effects of glucagon were reproduced by substances that elevate intracellular concentration of cyclic-AMP, including cholera toxin, isoproterenol, and methylisobutylxanthine. These various substances, especially EGF, glucagon, or cyclic-AMP, altered the morphological characteristics of the cultures during early stages, promoting cellular spreading and aggregation.

Cyclic nucleotide metabolism in differentiated and undifferentiated epithelial thyroid cells in culture

A highly differentiated thyroid cell line (FR-RL) was compared with a less differentiated (FR-T Cl1) and an undifferentiated (1-5G) cell line. FR-TL is modulated in vivo and in vitro by thyrotropin and has the lowest adenylate cyclase and guanylate cyclase and the highest phosphodiesterase activities. In contrast, 1-5G tumor cells do not respond to thyrotropin and have the highest adenylate cyclase guanylate cyclase and lowest hydrolyzing enzyme activities. Intermediate enzyme activities were found in FR-T Cl1 cells. The differences between the two normal rat thyroid cell lines are not due to differences in the composition of the growth medium.

Adenosine 3',5'-monophosphate receptor proteins in HeLa cells

The adenosine 3',5'-monophosphate receptor proteins of HeLa cells have been characterized. Using the Millipore filter assay, in the presence of 5'AMP and a phosphodiesterase inhibitor, specific [3H]cyclic AMP binding was detected in cytosol and in a nuclear-free particulate fraction, but not in nuclei. Both preparations exhibited biphasic Scatchard plots. 8-Azido[32P]cyclic AMP was used as a photoaffinity probe to covalently link ligand with receptor proteins. Proteins were then separated on denaturing gels and analyzed by autoradiography. The cytosol exhibited four specific binding proteins, with molecular weights of 46 000, 50 000, 52 000 and approx. 120 000. The 50 000/52 000 doublet could not be interconverted by phosphorylation-dephosphorylation reactions. On DEAE-cellulose, the 50 000-dalton protein eluted with peak II cyclic AMP-dependent protein kinase. The other proteins eluted with Peak I and with a binding peak not associated with kinase activity. Only the 50 000 protein was precipitated by type II protein kinase antibody from bovine heart. In the particulate fraction, the 120 000 protein was not detectable, but 8-azido[32P]cyclic AMP treatment revealed the other three proteins, with a relative increase in the 50 000-dalton protein. The results suggest that HeLa cells have four binding proteins which can associate with catalytic subunit and that the Peak I enzyme is heterogeneous, consisting of several distinct regulatory subunits.

Cyclic AMP: a mitogenic signal for Swiss 3T3 cells

Addition of cholera toxin (100 ng/ml) to quiescent cultures of Swiss 3T3 cells acts synergistically with serum (2-4%), insulin, phorbol esters, epidermal growth factor, and fibroblast-derived growth factor to stimulate DNA synthesis. In the presence of insulin, cholera toxin caused a dose-dependent increase in cumulative [3H]thymidine incorporation into acid-insoluble material and in the intracellular cyclic AMP (cAMP) level. The dose--response curves for the two processes were similar. Furthermore, addition of 1-methyl-3-isobutylxanthine (15--500 microM) or of 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (5--100 microM), both of which are potent inhibitors of cyclic nucleotide phosphodiesterase which are potent inhibitors of cyclic nucleotide phosphodiesterase activity, stimulated DNA synthesis and increased cAMP levels in Swiss 3T3 cells. These compounds strikingly potentiated the effect of cholera toxin on DNA synthesis and on cAMP levels. When quiescent Swiss 3T3 cells were exposed to cholera toxin (100 ng/ml) and insulin at 10 micrograms/ml (4- to 7-fold increase in cAMP level) or to these agents and 1-methyl-3-isobutyl xanthine at 50 microM (35-fold increase in cAMP level), DNA synthesis began after a lag of 16 hr. These results indicate that cAMP acts as a mitogenic signal for Swiss 3T3 cells and differ from the widely held view that cyclic AMP inhibits the proliferation of fibroblast cells.

[Cell division and cell cycle]

The paper gives a short review of biochemical and genetic analyses of the eukaryotic cell cycle and cell division. Emphasis is placed on the interrelationship of macromolecular syntheses during chromosome replication, the possible involvement of protein phosphorylation in chromosome condensation, the function of contractile proteins in mitosis and cytokinesis and on mechanisms which trigger cell proliferation.

Modification of steroidogenesis in a mouse adrenal cell line (Y-1) transformed by simian adenovirus SA-7

Transformation of a steroidogenic mouse adrenal cell line (Y-1) by simian adenovirus SA7 produced a cell line with low apparent steroidogenic activity. The effect of ACTH and cholera toxin on cyclic AMP production was similar in both not transformed and virus-transformed cells and activity of cyclic AMP-dependent protein kinase was also similar in both cells. In transformed cells, cholesterol was metabolized to delta 5-3 beta-hydroxysteroids, mainly 20 alpha-dihydropregnenolone while in not transformed cells, the major metabolites were delta 4-3 ketosteroids (20 alpha-dihydro- and 11 beta-hydroxy-20 alpha-dihydroprogesterone). In both cell lines ACTH increased the metabolism of cholesterol. Further studies with labelled pregnenolone and progesterone revealed a loss of delta 5-3 beta-hydroxysteroid dehydrogenase/isomerase and 11 beta-hydroxylase activity in the transformed cells.

The control of growth of mouse mastocytoma cells by N6,O2'-dibutyryladenosine cyclic 3',5'-monophosphate

Addition of N6,O2'-Dibutyryladenosine cyclic 3',5'-monophosphate (DB cyclic AMP) plus theophylline or transfer to medium containing 0.2% serum slowed the growth of cultured mouse mastocytoma cells and eventually arrested their growth in G1 phase. Examination of the properties of cells arrested by either procedure suggested that the drugs arrested cells in G1 phase 1.5-2 h after the point of low serum arrest. Cycloheximide prevented the recovery of cell growth after low serum or drug-induced arrest demonstrating that protein synthesis was necessary to pass either growth restriction point. Cordycepin also prevented drug-arrested cells from progressing into cycle indicating a requirement for RNA synthesis to overcome the drug-induced growth arrest. Evidence is also presented that DB cyclic AMP prevented the cells receiving a pulse of calcium necessary to proceed past the DB cyclic AMP-sensitive growth restriction point. It is suggested that high cyclic AMP levels prevent mastocytoma cells from receiving a surge of calcium in G1 phase that is necessary if the cells are to proceed to S phase and eventually divide.

Effects of nucleoside antivirals and their metabolites on the corneal endothelium

The endothelial surface of rabbit corneas were perfused in vitro with bicarbonate Ringer's containing 5 mM glucose, 0.3 mM reduced glutathione, and various concentrations of nucleoside antivirals or their metabolites. During three hour perfusions, the swelling rates of corneas perfused with buffer containing either antivirals or metabolites were not significantly different from controls. Scanning electron microscopy of the endothelial cell layer revealed no structural abnormalities in any treatment group. One metabolite, fluoride ion, reduced endothelial glucose oxidation by about 60 percent when incubated with corneal tissue in vitro. The inhibition of glucose metabolism by fluoride ions was observed only at concentrations at least sixty times greater than would be anticipated in the anterior chamber of patients receiving topical F3TdR therapy. These studies indicate that 5-trifluoromethyl-2'-deoxyuridine, 5-iodo-2'-deoxyuridine, 9-(2-hydroxyethoxymethyl)-guanine, and their metabolites do not alter endothelial function when studied at physiological concentrations over a short term of exposure.

Alterations in growth requirements of kidney epithelial cells in defined medium associated with malignant transformation

The possibility has been investigated that 1) the supplements required for the growth of the Madin Darby Canine Kidney (MDCK) cell line in serum-free Medium K-1 are indeed requirements for the growth of normal kidney cells in vitro, and 2) that alterations in these growth requirements are associated with malignant transformation. Consistent with the hypothesis that MDCK cells resemble normal kidney cells in culture, primary cultures of baby mouse kidney epithelial cells grow in Medium K-1 and respond to the 5 components in the medium. The growth properties of Moloney sarcoma virus (MSV)-transformed MDCK cells in defined media have been examined. Unlike MDCK cells, MSV-transformed MDCK cells form tumors in adult nude mice. Although they still respond to the 5 factors in Medium K1, the optimal dosage for insulin is lower for the MSV transformants than for MDCK cells. The MSV transformants also have an additional requirement for growth in Medium K-1-fibronectin. Variants of MDCK cells have been isolated that have lost the PGE1 requirement for growth in defined medium. These variant cells have acquired 1) the ability to form tumors in adult nude mice and 2) an alteration affecting cAMP metabolism, in addition to PGE1 independence.

Role of steroid hormones and prostaglandins in the regulation of DNA synthesis by decidual cells in culture

The effect of oestrogen and progesterone on prostaglandin synthesis and on DNA synthesis by rat decidual cells was studied in a culture system. The cells were explanted from deciduoma either during the proliferation phase (namely on the 5th day of leukocytic smear, Day L5:"L5 cells") or during the maintenance phase ("L8 cells") and examined on the second day of culture. Oestradiol-17 beta (7 X 10(-11) M) and progesterone (6 X 10(-8) M) significantly inhibited accumulation of PGE by cells explanted on Day L5: L8-cell cultures showed no significant response to oestradiol and the progesterone effect was markedly reduced. Progesterone stimulated [3H]thymidine incorporation into cells explanted on Day L5, but had no effect on L8-cultures. Other inhibitors of PG synthesis, namely cortisol, flufenamic acid and indomethacin, also had a stimulatory effect on DNA synthesis by L5 cells. PGE2 (5-10 micrograms/ml) inhibited DNA synthesis in control, indomethacin-treated and progesterone-treated L5-cell cultures, suggesting that the progesterone-induced stimulation of DNA synthesis may be in part be due to its inhibitory effect on PGE accumulation by decidual cells. The possibility is discussed that during the proliferation phase of decidual development in vivo, the rate of DNA synthesis may be influenced by steroid-induced changes in PGE content of the tissue.

Effects of beta adrenergic agents and prostaglandin E1 on erythroid colony (CFU-E) growth and cyclic AMP formation in Friend erythroleukemic cells

The formation of erythroid colonies from bone marrow and spleen cells infected with the polycythemic strain of the Friend virus (FV-P) was characterized in an in vitro methyl cellulose colony-forming system in response to prostaglandin E1 and the beta-2 adrenergic agonist, albuterol. Both drugs markedly inhibited the formation of CFU-E colonies of FV-P-infected bone marrow and spleen in the absence or presence of erythropoietin. The albuterol-mediated inhibition of CFU-E colonies (FV-P-infected) was selectively blocked by butoxamine, a beta-2 antagonist. Adenylate cyclase (AC) activity was also determined in FV-P spleen membrane preparations in response to albuterol and PGE1. Both agents stimulated enzyme activity, and butoxamine blocked the stimulation seen with albuterol. The ability of albuterol and PGE1 to stimulate AC activity in the FV-P-infected cells suggests that the effects of these agents on CFU-E formation may be mediated by specific beta-2 adrenergic and PG receptors through the adenylate cyclase-cyclic AMP system.

Growth of functional primary cultures of kidney epithelial cells in defined medium

Primary cultures of baby mouse kidney epithelial cells can grow without fibroblast overgrowth in a hormone-supplemented serum-free medium (Medium K-1) designed for an established kidney epithelial cell line, MDCK. The five supplements in Medium K-1 are insulin, transferrin, PGE1, T3, and hydrocortisone. Medium K-1 also supports the growth of kidney epithelial cell cultures from a number of animals, including man, without fibroblast overgrowth. Outgrowth of kidney epithelial cells from kidney explants was also observed with Medium K-1. Thus, the medium appears to be selective for epithelial cell growth. The physiological properties of primary cultures of baby mouse kidney epithelial cells were studied in detail. Baby mouse kidney epithelial cells grew at equal rates (0.5 doublings/day) in Medium K-1 and serum-supplemented medium. Medium K-1 also supported the formation of baby mouse kidney epithelial colonies at low cell densities. The dependence of baby mouse kidney epithelial colony formation upon the five factors in Medium K-1 was examined. These studies indicated that the formation of baby mouse kidney epithelial colonies in defined medium depended upon all the five supplements in Medium K-1, in a manner similar, although not identical, to MDCK colonies. Primary cultures of baby mouse kidney epithelial cells grown in Medium K-1 retained kidney cell-associated properties, including the ability to form multicellular domes, a phenomenon associated with transepithelial salt transport. Amiloride-sensitive Na+ uptake and the mucosal surface enzyme leucine aminopeptidase were also observed in baby mouse kidney cultures. Similar functions were observed in MDCK monolayers.

[Variations in cyclic AMP level and specific activities of adenylate cyclase and cyclic AMP phosphodiesterase during the cell cycle of an Actinomycete (author's transl)]

The variations in the concentrations of intra- and extracellular cyclic AMP and in he specific activities of adenylate cyclase (EC 4.6.1.1) and cyclic AMP phosphodiesterase (EC 3.1.4.17) have been monitored in synchronized cultures of Nocardia restricta, a prokaryote belonging to the group of Actinomycetes. At the beginning of the cell cycle, during a first period of RNA and protein synthesis, there is an increasing synthesis of adenylate cyclase which can be suppressed in the presence of chloramphenicol or rifampicin. Simultaneously, the specific activity of cyclic AMP phosphodiesterase decreases and the concentrations of intra- and extracellular cyclic AMP rise. After the end of DNA replication, during a second period of RNA and protein synthesis, the specific activity of cyclic AMP phosphodiesterase increases; during the same time, the specific activity of adenylate cyclase and the level of intracellular cyclic AMP drop. It appears that the overall metabolism of cyclic AMP is coordinated so that the cyclic AMP level will be high at the beginning of DNA replication and will fall thereafter. The results are discussed in comparison with known data about the variations of cyclic AMP during the cell cycle of mammalian cells in cultures.

Effect of the antitumour agent 5-(3,3-dimethyl-1-triazeno imidazole-4-carboxamide (DTIC) on cyclic AMP levels in intact MH1C1 hepatoma cells

Addition of the cytostatic agent 5-(3,3-dimethyl-1-triazeno)-imidazole-carboxamide (DTIC, dacarbazin) to MH1C1 hepatoma cells, alone or in combination with adrenaline, was shown to elevate intracellular cyclic AMP. The cyclic AMP level was increased from basal values of about 0.40 to about 0.75 pmol/mg protein 10 min. after DTIC addition. Accumulation of cyclic AMP in response to a supramaximal concentration of adrenaline is amplified in a dose-dependent way by 0.1-2 mM DTIC. The effect is apparently due to inhibition of cyclic AMP breakdown, since DTIC inhibits the low-Km form of the cyclic AMP phosphodiesterase without significantly affecting the adenylate cyclase activity of MH1C1 homogenates.

The general immunopharmacology of levamisole

The need for treatments to correct an immunological defect, or to restore an impaired immune response asociated with disease or ageing, has led to the development of nonspecific immunoactive agents. Levamisole, a synthetic low molecular weight compound, is the first member of a new class of drugs which can increase the functions of cellular immunity in normal, healthy laboratory animals. The properties of levamisole have contributed to improved understanding of the molecular events which mediate or trigger immune responses. Levamisole can act either as an immunostimulant agent or an immunosuppressive agent. These apparently paradoxical effects depend upon the dose administered, the timing of its administration, the experimental assay used to measure effects, and the host genetic background. Levamisole's potential for opposite effects explains certain apparent inconsistencies observed in experimental or clinical assays. The drug's actions are modulated by the interaction between the T-cell recruiting efficacy of the sulphur moiety and the cholinergic effects of the imidazole ring. The clinical implications resulting from the immunopharmacological properties of levamisole are obvious: one should avoid its use in diseases without known association with an immune defect, and always attempt to correlate clinical data with modifications of immune parameters, since the therapeutic usefulness of correctly administered levamisole parallels improvement in tests of cellular immunity. Immunomodulators act by modifying the functions of the host cells involved in defences against invaders, and the effectiveness of an immunotherapeutic drug is dependent upon characteristics of the individual host. Thus, therapy with such drugs must be individualised; the appropriate agent and dosage should be chosen according to the immune capabilities of individual patients.

Kinetics of cAMP inhibition of DNA synthesis in bovine adrenocortical cells

cAMP-treated bovine adrenocortical cells are arrested in the G1 phase of the cell cycle. Removal of serum also arrests bovine adrenocortical cells in G1. In the presence of cAMP, serum and fibroblast growth factor stimulate increases in medium cell volume, but DNA synthesis is not initiated. Under these conditions cAMP increases steroidogenic capacity 7- to 10-fold as assessed by metabolism of pregnenolone to fluorogenic steroids. When the kinetics of entry of cells into S phase are quantitated, serum- and FGF-treated cells initiate DNA synthesis at an exponential rate after a 12-h lag. In contrast when cAMP is removed, cells immediately initiate DNA synthesis without a lag at a similar exponential rate (6.3 and 5.3% of the cells entering S/h). In the presence of growth factors, cAMP-treated bovine adrenocortical cells are thus hypertrophied with increased steroidogenic capacity, but are reversibly arrested at the G1/S boundary. These findings suggest that cAMP arrests cell replication by mechanisms distinct from those of serum deprivation.

Phosphorylation of endogenous proteins by adenosine 3':5'-monophosphate-dependent protein kinase in mouse neuroblastoma cells

Increased intracellular adenosine 3':5'-monophosphate (cAMP) levels and activation of cAMP-dependent protein kinases (ATP:protein phosphotransferase, EC 2.7.1.37) in vivo were correlated in mouse neuroblastoma cells grown in the presence of 1 mM-6N,O2'-dibutyryl 3':5'-monophosphate (Bt2cAMP). The time course for activation showed that cAMP-dependent protein kinases were activated by 30 min. A heat-stable inhibitor protein inhibited a majority of activated cAMP-dependent protein kinase. Activation of cAMP-dependent protein kinase caused additional phosphorylation of proteins when compared with untreated control cells, as demonstrated by endogenous phosphorylation of proteins in vitro using [gamma-32P]ATP and analysis by two-dimensional polyacrylamide gel electrophoresis. The phosphorylation data show selective phosphorylation of specific proteins by cAMP-independent and cAMP-dependent protein kinase. Among the proteins in the postmitochondrial supernatant fraction phosphorylated by cAMP-dependent protein kinases, two proteins with a molecular weight of 43,000 were heavily phosphorylated. It is suggested that phosphorylation of cellular proteins by cAMP-dependent protein kinases might be involved in the cAMP-modulated biochemical changes in neuroblastoma cells.

Trypsin modifies the activity of adenylate cyclase from normal, malignant and hybrid mammalian cells

Adenylate cyclase (ATP pyrophosphate-lyase (cylizing), EC 4.6.1.1) activity, measured in homogenates of normal, malignant and hybrid mammalian cell lines, is enhanced and subsequently inhibited by increasing concentrations of trypsin (EC 3.4.21.4). Treatment of intact cells with trypsin appears to cause latent activation of adenylate cyclase (i.e. activation which is only expressed after homogenization of the cells). Conversely, adenylate cyclase activity of a normal Chinese hamster fibroblast cell line is inhibited in intact cells by trypsin through the degradation of some site on the outer surface of the plasma membrane. The prostaglandin E1 receptor is not affected by trypsinization of cells.

Studies on the activities, kinetic properties and subcellular localisation of cyclic AMP phosphodiesterase in human neutrophil leukocytes

A one-step radioassay for cyclic AMP phosphodiesterase was optimised for human polymorphonuclear leukocytes. Kinetic studies indicated the presence of two forms of phosphodiesterase activity with apparent Km values of 0.015 mmol/l and 0.98 mmol/l for cyclic AMP. Control neutrophils were homogenised in isotonic sucrose and, after low speed centrifugation, the supernatant was subjected to analytical subcellular fractionation. Gradient fractions were assayed for principal marker enzymes and for cyclic AMP phosphodiesterase. Both forms of phosphodiesterase activity were located in the cytosol. Polymorphonuclear leukocyte homogenates were isolated from control subjects, patients with chronic granulocytic leukaemia and patients in the third trimester of pregnancy. A portion of each homogenate was used for enzyme analysis and the remainder assayed for cyclic AMP content. The specific activity (mUnits/mg protein) of the low Km phosphodiesterase was reduced in both patient groups compared with control values, whilst that of the high Km phosphodiesterase was unchanged. Leukocytes from patients with chronic granulocytic leukaemia had only a fifth of the cyclic AMP content of control neutrophils, whilst leukocytes from patients in the third trimester of pregnancy had an elevated cyclic AMP content.

Yeast mating pheromone alpha factor inhibits adenylate cyclase

The pheromone alpha factor, secreted by Saccharomyces cerevisiae cells of the alpha mating type, serves to synchronize the opposite mating type (a cells) at G1 as a prelude to fusion of the two cell types. We found that, in vitro, alpha factor inhibited the membrane-bound adenylate cyclase of these cells in a dose-dependent manner. Moreover, one class (ste5) of a cell mutants that grow normally at either 23 degrees or 34 degrees C but that are unable to respond to alpha factor or to mate at the higher temperature possessed an adenylate cyclase activity that was not inhibited by alpha factor at 34 degrees C but was fully sensitive to inhibition at 23 degrees C. Furthermore, addition of cyclic AMP to a cell culture medium shortened the period of pheromone-induced G1 arrest. We conclude that inhibition of adenylate cyclase activity by alpha factor may constitute, at least in part, the biochemical mode of action of the pheromone in vivo.

Inhibitory effect of cyclic adenosine 2',3'-monophosphate on leucine incorporation by L5178Y cells

The specificity of the reported stimulation of protein synthesis affected by dibutyryl adenosine cyclic 3',5'-monophosphate in L5178Y cells was tested by assaying the effect of adenosine cyclic 2',3'-monophosphate. Protein synthesis was assayed as total acid precipitable 3H-leucine. Leucine incorporation was inhibited by adenosine cyclic 2',3'-monophosphate. Alone, and in combination with actinomycin, 2',3'-cAMP inhibited leucine incorporation more rapidly and to a greater degree than did actinomycin alone. Since a similar potentiating effect was not observed with cordycepin, the hypothesis has been suggested that adenosine cyclic 2',3'-monophosphate inhibits protein synthesis by interfering with some post transcriptional event, perhaps modification of existing RNA. The results further suggest that 2,3 cyclic AMP may be antagonistic of the effects of 3,5 cyclic AMP, and as such may be useful as an inhibitor of that nucleotide's action.