Cyclic AMP, calcium and control of cell growth

Calcium signaling and cell cycle: Progression or death

Cytosolic Ca²⁺ concentration levels fluctuate in an ordered manner along the cell cycle, in line with the fact that Ca²⁺ is involved in the regulation of cell proliferation. Cell proliferation should be an error-free process, yet is endangered by mistakes. In fact, a complex network of proteins ensures that cell cycle does not progress until the previous phase has been successfully completed. Occasionally, errors occur during the cell cycle leading to cell cycle arrest. If the error is severe, and the cell cycle checkpoints work perfectly, this results into cellular demise by activation of apoptotic or non-apoptotic cell death programs. Cancer is characterized by deregulated proliferation and resistance against cell death. Ca²⁺ is a central key to these phenomena as it modulates signaling pathways that control oncogenesis and cancer progression. Here, we discuss how Ca²⁺ participates in the exogenous and endogenous signals controlling cell proliferation, as well as in the mechanisms by which cells die if irreparable cell cycle damage occurs. Moreover, we summarize how Ca²⁺ homeostasis remodeling observed in cancer cells contributes to deregulated cell proliferation and resistance to cell death. Finally, we discuss the possibility to target specific components of Ca²⁺ signal pathways to obtain cytostatic or cytotoxic effects.

Antiproliferative effect of brief exposure to cholera toxin in vascular smooth muscle cells: role of cAMP and protein kinase A

The effect of cholera toxin (CTX), an activator of the adenylate cyclase-coupled G protein αS subunit, was studied on cultured vascular smooth muscle cell (VSMC) proliferation. Continuous exposure (48 h) to CTX as well as 2-min pretreatment of VSMC with CTX led to the same level of cAMP production, inhibition of DNA synthesis, and arrest in the G1 phase without induction of necrosis or apoptosis in VSMC. Protein kinase A (PKA) activity in CTX-pretreated cells was transiently elevated by 3-fold after 3 h of incubation, whereas after 48 h it was reduced by 2-fold compared with baseline values without modulation of the expression of its catalytic α subunit. The PKA inhibitors H89 and KT 5720 did not protect VSMC from the antiproliferative effect of CTX. Two-dimensional electrophoresis was used to analyze the influence of CTX on protein phosphorylation. After 3 h of incubation of CTX-pretreated cells, we observed both newly-phosphorylated and dephosphorylated proteins (77 and 50 protein species, respectively). After 24 h of incubation, the number of phosphorylated proteins in CTX-treated cells was decreased to 39, whereas the number of dephosphorylated proteins was increased to 106. In conclusion, brief exposure to CTX leads to full-scale activation of cAMP signaling and evokes VSMC arrest in the G1 phase.Key words: vascular smooth muscle, proliferation, cholera toxin, cAMP, protein kinase A.

Induction of apoptosis by an inhibitor of cAMP-specific PDE in malignant murine carcinoma cells overexpressing PDE activity in comparison to their nonmalignant counterparts

In order to study potential changes in phosphodiesterase (PDE) activity associated with malignant transformation, normal primary keratinocytes and cells corresponding to different stages of epidermal tumor development in mouse skin were analyzed with respect to their 3',5'-cyclic adenosine monophosphate (cAMP) hydrolyzing activity. Expression of cAMP-specific PDE-4, intracellular cAMP content, and the sensitivity to the growth inhibitory effect of the PDE-4-specific inhibitor 7-benzylamino-6-chloro-2 piperazino-4-pyrrolidino-pteridine (DC-TA-46) were studied in the two papilloma cell lines, MSCP6 and 308, and in the highly malignant carcinoma cell line CarB. No significant difference in soluble PDE activity and in intracellular cAMP was found in the two papilloma cell lines when compared to primary keratinocytes. In contrast, the spindle-cell carcinoma cell line CarB exhibited significantly higher PDE activity, concomitant with the lowest cAMP level. In all cell lines and also in the primary keratinocytes, rolipram-sensitive PDE-4 activity accounted for the major cAMP-hydrolyzing activity. In primary keratinocytes and in MSCP6 cells, the PDE-4 inhibitor DC-TA-46 induced at best marginal growth inhibition, whereas cell growth of 308 cells was markedly affected at concentrations > 2 microM. The carcinoma cell line CarB showed the highest sensitivity to DC-TA-46 (IC50 = 0.8 +/- 0.3 microM). Treatment of CarB cells with DC-TA-46 strongly inhibits intracellular PDE activity, resulting in a marked and long-lasting rise of cAMP. After 24 h of treatment, arrest in the G0/G1 phase of the cell cycle is induced. Treatment with concentrations > 2 microM of this highly effective PDE inhibitor results in induction of apoptotic cell death, as detected by fluorescence microscopy, flow cytometry, and ELISA-based determination of fragmented DNA in intact cells.

Inhibitory effect of calcium channel blockers on human mesangial cell growth: evidence for actions independent of L-type Ca2+ channels

Calcium channel blockers (CCB) are known to affect the outcome of glomerulosclerosis in vivo and to suppress mesangial cell proliferation and cytokine production in vitro. It is uncertain, however, whether (i) human adult mesangial cells (HMC) express L-type Ca2+ channels and (ii) whether the effect of CCB on HMC is mediated by inhibition of L-type Ca2+ channels. In single cell preparations of HMC, the L-type Ca2+ channel agonist Bay K 8644 and K+-depolarization of the cell membrane caused a transient increase of cytosolic free Ca2+ ([Ca2+]i) in 60 to 80% of the cells. The CCB verapamil and nifedipine partially inhibited the effect of Bay K 8644 and K+-depolarization on [Ca2+]i. Binding experiments confirmed these functional studies by showing specific binding at the phenylalkylamine binding site of L-Type Ca2+ channels. Quiescent HMC were stimulated with fetal calf serum (FCS) or growth factors (platelet derived growth factor A/B, epidermal growth factor, angiotensin II, endothelin 1) in the presence of various concentrations (10(-10) to 10(-5) M) of different CCB: either (R)-verapamil, (S)-verapamil or the raceme of verapamil, and nifedipine or diltiazem, respectively. In addition, the enantiomers of devapamil were studied, because their action on the L-type Ca2+ channel is more stereoselective than that of the enantiomers of verapamil. At high concentrations (10(-6) to 10(-5) M) (R,S)-verapamil decreased cell numbers in cultures of quiescent HMC, increased LDH in the supernatant, and caused loss of trypan blue exclusion (cytotoxicity). At lower concentrations (R,S)-verapamil showed no cytotoxicity, but had two effects: (1.) concentration dependent (down to 10(-8) M) inhibition of indices of cell proliferation, that is, (i) stimulated (FCS or growth factor) 3H-thymidine incorporation and (ii) increment in cell number; and (2.) inhibition of indices of cell or matrix protein synthesis, that is, (i) stimulated 3H-methionine incorporation and (ii) 3H-proline incorporation. At equimolar concentrations the dihydropyridine nifedipine was equipotent with verapamil, whereas the benzothiazepine diltiazem was conspicuously less effective. Even at the lowest effective concentration (10(-8) M) comparison of (R)- and (S)-verapamil showed no significant difference between the enantiomer with weak or with strong effect on L-type Ca2+ channels, and this was true even when the more stereoselective enantiomers of devapamil were tested. These observations argue against the notion that effects of CCB result from specific interaction with L-type Ca2+ channels. The data are more consistent with the idea that interactions with targets other than L-type Ca2+ channels are involved.

Synergistic effects of cyclic AMP and Ca2+ ionophore A23187 on de novo synthesis of histidine decarboxylase in mastocytoma P-815 cells

In the preceding paper (Kawai, H. et al. (1992) Biochim. Biophys. Acta 1133, 172-178), we reported that in mastocytoma P-815 cells dexamethasone and 12-O-tetradecanoylphorbol-13-acetate (TPA) synergistically enhanced the de novo synthesis of L-histidine decarboxylase (HDC). Here we found that Ca2+ acted synergistically with cAMP in the induction of HDC mRNA and HDC activity in mastocytoma P-815 cells, and that the mechanism underlying the enzyme induction by Ca2+ plus cAMP was distinguishable from that by dexamethasone plus TPA. Ca2+ ionophore A23187, itself having no significant activity, markedly enhanced the induction of HDC activity by N6,O2'-dibutyryl cAMP (db cAMP) or cAMP-inducible prostaglandins such as PGE1, PGE2 and PGI2 in the presence of the phosphodiesterase inhibitor, Ro201724. However, A23187 had little effect on increases in HDC activity induced by other known stimulants, such as TPA, dexamethasone and sodium butyrate. These results suggest that A23187 has a specific effect on the induction of HDC activity due to an increased level of cAMP. The finding that both A23187 and cAMP enhanced HDC activity suggests that both Ca2+/calmodulin and cyclic nucleotide dependent protein kinase play essential roles in the process of enhancement of HDC activity. To examine this possibility, we studied the effects of W-7, an inhibitor of calmodulin, removal of extracellular Ca2+, and H-8, an inhibitor of cAMP-dependent protein kinase, on the enhancing activity of A23187 plus db cAMP. The enhancement of HDC activity by A23187 plus db cAMP was inhibited by W-7, removal of extracellular Ca2+, and H-8. The increase in HDC activity was due to the de novo synthesis of the enzyme, since it was suppressed by the addition of cycloheximide or actinomycin D, and was well correlated with the marked accumulation of a 2.7 kilobase HDC mRNA. Furthermore, the mechanism underlying the induction of HDC by db cAMP plus A23187 is distinguishable from that in the case of dexamethasone plus TPA, since preexposure to dexamethasone plus TPA for 12 h, for a plateau level to be reached, did not affect the subsequent increase in HDC activity due to db cAMP plus A23187.

A unifying model of the cell proliferation emphasizing plasma membrane fluxes

The regulation of cellular growth and proliferation is perhaps the most investigated and elusive problem in cell biology and seems to be possible to solve from almost any angle of study chosen. Among the non-systemic factors that have been discussed are genetic damage, genomic control, regulation by stimulatory and inhibitory peptide factors such as EGF, chalones, and fibronectin, protein kinase activation with tyrosine phosphorylation, adenylylcyclase and cAMP, cGMP, membrane perturbations and specifically in tumours the failure of the Pasteur effect in control of glycolysis, excessive membrane ATPase activity, and excessive hydrolytic and proteolytic activities at the cell surface. This article focuses on the central role of fluxes within the plasma membrane and re-examines the possibility that changes of flux of metabolites, ions, and reducing equivalents may be the common denominator regulating cellular proliferation.

Cyclic AMP calcium and the growth of mastocytoma cells

Arresting P815 mastocytoma cell growth with N6, O2'-dibutyryladenosine 3':5' cyclic monophosphate (db cAMP) and theophylline increased 45Ca2+ uptake and efflux by the cells (i.e, Ca2+ cycling) without altering cytoplasmic free Ca2+ concentrations or the amount or distribution of protein kinase C in the cells. Attempts to identify the Ca2+ channels involved using a wide variety of drugs were unsuccessful. However, the inhibitory effect of db cAMP on growth was greatly increase in medium containing low Ca2+ concentrations, confirming that interactions between Ca2+ and cyclic AMP can affect mastocytoma cell growth.

Measurements of membrane potential, transmembrane 45Ca fluxes, cytoplasmic free Ca2+ concentration and insulin release by transplantable rat insulinoma cells maintained in tissue culture

Regulation of insulin release, membrane potential, transmembrane 45Ca fluxes and cytoplasmic free Ca2+ concentration, [Ca2+]i, was examined using suspensions of transplantable NEDH rat insulinoma cells previously cultured for 2-3 days to eliminate necrotic tumour cells and counter prior hypoglycaemia. Insulinoma cells displayed a resting [Ca2+]i of 94 +/- 8 nM (n = 17) and released 104 +/- 15 ng insulin 10(-6) cells (n = 7) during 60 min incubations with uptake of 2.7 +/- 0.2 nmol 45Ca 10(-6) cells (n = 7). High concentrations of glucose did not affect membrane potential, transmembrane 45Ca fluxes, [Ca2+]i or insulin release by insulinoma cells. K+ at 25 mM depolarised the plasma membrane, induced a small increase in 45Ca efflux and increased [Ca2+]i by 65%. This modest action was not associated with demonstrable effects on 45Ca uptake and insulin release. The effect of 25 mMK+ on [Ca2+]i was counteracted by D-600, but this blocker of voltage-activated Ca2+ channels and verapamil lacked effects on transmembrane 45Ca fluxes and insulin release. The Ca2+-calmodulin antagonist, trifluoroperazine, was also without effect on 45Ca fluxes and insulin release. Ca2+ ionophore ionomycin increased [Ca2+]i, whereas A23187 and X537A did not affect transmembrane 45Ca fluxes. Moreover, insulin release was independent of extracellular Ca2+ over the range 0-20.4 mM despite marked affects on transmembrane 45Ca fluxes and a greater than 4-fold change of [Ca2+]i. Dibutyryl cyclic AMP increased insulin release by 55% without affecting transmembrane 45Ca fluxes or [Ca2+]i. The phosphodiesterase inhibitor, theophylline, also enhanced insulin release by 10-36% with no change of 45Ca uptake. The effectiveness of theophylline was independent of extracellular Ca2+ over the range 0-10.2 mM. These results indicate that inappropriate Ca2+ regulation is a key pathogenic feature underlying the inappropriate insulin secretion of rat insulinoma cells.

Effects of cationic modification on 45Ca uptake and insulin release by transplantable rat insulinoma cells maintained in tissue culture

1. Acute effects of cations on 45Ca uptake and insulin release by transplantable rat insulinoma cells were examined after 2-3 days culture in RPMI-1640 containing 11.1 mM glucose. 2. At 2.6 mM Ca2+, rat insulinoma cells (greater than 95% viability) released 78-158 ng insulin/10(6) cells during 60 min incubation with uptake at 2.19-3.24 nmol 45Ca/10(6) cells. 3. Addition of 2 mM La3+, Co2+, Mn2+, Zn2+ or Ba2+ did not affect 45Ca uptake. Insulin release was also unaffected by these cations with the exception of 87% inhibition in the presence of La3+ or Zn2+. 4. Omission of 5.9 mM K+, 1.2 mM Mg2+, 115 mM Na+ or H+ (pH 8.5) did not affect 45Ca uptake or insulin release, irrespective of osmotic compensation using choline chloride or sucrose. Rat insulinoma cells were similarly unresponsive to addition of 30.9 mM K+, 12 mM Mg2+ or H+ (pH 6.3). 5. Omission of 2.6 mM Ca2+ (with or without addition of 1 mM EGTA) or addition of 20.5 mM Ca2+ did not affect insulin release. 6. The results indicate that rat insulinoma cells are little affected by cationic modifications which have profound effects on Ca2+ handling and insulin release by pancreatic beta-cells. Dysregulation of insulin release by insulinoma cells is associated with marked irregularities in the control of transmembrane Ca2+ fluxes and sensitivity to extracellular Ca2+.

Metabolic iteration, evolution and cognition in cellular proliferation

A model for cellular proliferation is described according to which proliferation ensues when metabolism evolves towards commitment to DNA synthesis, and inhibition of proliferation occurs when enzymic interactions are iterated within a few metabolic pathways, another limiting factor being the supply of metabolites. The model successfully describes cellular growth and division as a 'cognitive process' based on interaction within enzymic elements and the genome, and affords an explanation in these terms of some empirical phenomena which have previously been understood only as isolated observations.

The role of cAMP and calcium in the stimulation of proliferation of immature erythroblasts by erythropoietin

The hypothesis that cAMP or calcium are the second messengers of erythropoietin (Epo) was tested on fractionated, Epo-responsive immature erythroblasts from anemic rabbit bone marrow by examining whether the proliferative effects of the hormone could be mimicked by agents that increase the intracellular concentration of cAMP or Ca2+. None of the compounds tested (including 10(-6)-10(-4) M db-cAMP, forskolin, isoprenaline or 10(-7)-10(-6) M of the calcium ionophore A23187) alone or in combination could either initiate or potentiate the mitogenic action of the hormone. Furthermore, addition of 0.2 U/ml erythropoietin produced no permanent or transient increase in the uptake of 45Ca2+ by erythroblasts at 37 degrees C. However, cells cultured with imidazole or cordycepin (which reduce the level of intracellular cAMP), or with the calcium chelator EGTA, or the drugs verapamil or TMB-8 (which interfere with the utilization of extracellular or intracellular calcium) showed a decreased stimulation of DNA synthesis by Epo. Finally, the tumour promoter phorbol ester TPA could partially mimic the action of Epo when added to cultures containing more immature progenitor cells. We conclude then that an artificial increase in the cytoplasmic concentration of either cAMP or Ca2+ is not sufficient to elicit the proliferation of Epo-responsive cells.

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.

The action of pulsed magnetic fields on cyclic AMP levels in cultured fibroblasts

Pulsed magnetic fields, similar to those used clinically to promote bone repair, have been applied to cultured fibroblasts obtained from chick embryo tendons and rabbit bone marrow. Cyclic adenosine monophosphate (cAMP) levels were found to be lower in field-treated cultures in response to hormones such as prostaglandin E2 and isoproterenol, and the fibroblasts appear less sensitive to environmental perturbation prior to hormone incubation. We propose that the adenylate cyclase complex is temporarily inactivated by prolonged exposure to pulsed magnetic fields, and that this effect might be analogous to desensitisation phenomena.

Vasoactive intestinal peptide actions on cyclic AMP levels in cultured striatal neurons

The actions of vasoactive intestinal peptide (VIP) on intracellular cyclic AMP, in primary cultures of striatal neurons, were examined. VIP stimulated cyclic AMP formation five-fold over basal levels in neurons after 6 days in vitro (DIV); half maximal activation (EC50) was obtained with 10 nM of the peptide. VIP stimulation was both more potent and effective than those due to adrenocorticotropin (ACTH), dopamine (DA) or serotonin (5-HT). VIP efficacy was augmented to 15-20-fold in the presence of 0.1 microM forskolin, which had virtually no effect on cyclic AMP production alone; VIP potency was unaffected. At saturating concentrations of VIP (0.1-1.0 microM), no other agonist can further activate cyclic AMP production. Under these conditions, the interaction with opiate, DA D2 and 5-HT1 receptors, whose activation results in the inhibition of cyclic AMP production, was shown. During the differentiation of striatal neurons, VIP stimulation of cyclic AMP over basal levels, in the presence of 0.1 microM forskolin, decreases progressively from 30-fold after 3 DIV to 11-fold after 10-13 DIV.

The calcium antagonist nifedipine inhibits arterial smooth muscle cell proliferation

Migration of smooth muscle cells from the media to the intima of the arterial wall and proliferation of intimal smooth muscle are major early events in the formation of an atherosclerotic lesion. The start of proliferation requires that the cells have passed through a modulation from contractile to synthetic phenotype and that they are stimulated with growth factors. Here, we have examined the effects of the calcium antagonist nifedipine on phenotypic modulation and growth of isolated rat arterial smooth muscle cells cultivated in vitro. The results indicate that micromolar concentrations of nifedipine slow down the rate of transformation of the cells from a contractile to a synthetic phenotype and inhibit initiation of DNA synthesis as well as cellular proliferation. The inhibitory effect on DNA synthesis was seen both in cells stimulated with whole blood serum and with purified platelet-derived growth factor. The results raise the possibility that nifedipine may be used to prevent atherogenesis and to inhibit progression of fibromuscular lesions by interfering with the proliferation of arterial smooth muscle cells.

Adenosine receptor-mediated changes in cyclic AMP production and DNA synthesis in cultured arterial smooth muscle cells

The effects of adenosine and two analogs, L-phenylisopropyladenosine (L-PIA) and 5'-N-ethylcarboxamidoadenosine (NECA), on cAMP production and on platelet-derived growth factor (PDGF)-stimulated initiation of DNA synthesis in growth-arrested cultures of rat arterial smooth muscle cells (SMC) were studied. The intracellular cAMP concentration was dose-dependently enhanced by micromolar concentrations of adenosine and its analogs, with the potency order NECA greater than adenosine greater than L-PIA. The effect was antagonized, in a competitive manner, by the adenosine receptor antagonist 8-phenyltheophylline (8-PT). The stimulatory effect of adenosine was enhanced by 3 microM dipyridamole an adenosine-uptake blocker. DNA synthesis was inhibited in a parallel manner, showing the same potency order. The inhibition was antagonized by 8-PT. Forskolin, a diterpene with the ability to stimulate the catalytic unit of adenylate cyclase and thereby cAMP formation, potentiated the effects of micromolar concentrations of NECA and L-PIA. Forskolin, by itself, stimulated cAMP production and inhibited DNA synthesis. The forskolin-stimulated increase in cAMP was inhibited by L-PIA at nanomolar concentrations. L-PIA in the nanomolar concentration range also stimulated DNA synthesis when initiation was stimulated with suboptimal concentrations of PDGF. These findings suggest the presence of adenosine receptors of both the A1- and A2-subtype on SM-mediating bidirectional changes of cAMP and DNA synthesis.

Cyclic AMP and c-myc gene expression in PY815 mouse mastocytoma cells

The possibility was examined that inhibition of growth of PY815 mouse mastocytoma cells by N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate (DB cyclic AMP) results from inhibition of c-myc gene expression. Temporary increases in c-myc RNA which occurred soon after DB cyclic AMP treatment and upon removal of the drug were not consistent with direct inhibition of c-myc gene expression by DB cyclic AMP. The increases in c-myc RNA coincided with the passage through, or accumulation of cells in late G1-early S phase. It is proposed that cyclic AMP may stimulate c-myc gene expression which normally occurs only in late G1-early S phase in PY815 cells and that cyclic AMP prevents c-myc expression in cells at other phases of the cell cycle by inhibiting their progression past a cyclic AMP-sensitive restriction point in early G1 phase.

Elevated rate of DNA synthesis and its correlation to cAMP-phosphodiesterase activity during induction of polydactyly in mouse embryos heterozygous for Hemimelia-extra toe (Hmx)

The induction of polydactyly in mouse embryos heterozygous for Hemimelia-extra toe (Hmx) is associated with aberrant outgrowth of the developing autopod on day 12 of gestation. We have quantitated the rate of DNA synthesis and the activity of cAMP-phosphodiesterase (PDE) that is characteristic of the prospective polydactylous region. Mid-stage 18 hind-limb buds were labeled with [3H]dThd either in situ using whole embryo culture, or as isolated preaxial autopod fragments cultured on a membrane substratum. The mean specific activities of incorporation were compared for normal (+/+) and mutant (Hmx/+) genotypes. A significant (P less than or equal to 0.01) 19% increase, peculiar to the prospective polydactylous region, was measured after 4 hours in embryo culture. The same increment was detected after 4 hours in organ culture, but was amplified linearly to 55% when incubation was extended to 20 hours. During this period, continuous exposure to 1.0 mM IBMX (3-isobutyl-1-methyl xanthine), an inhibitor of cAMP-PDE activity, "slowed down" the rate of DNA synthesis to untreated +/+ proportions. When cAMP-PDE activity was assayed in uncultured autopods, a significant (P less than or equal to 0.01) 18% increase was detected within the prospective polydactylous region specifically on stage 18 of gestation. This is the developmental phase during which polydactylous outgrowth is induced in situ. Thus, uncontrolled cAMP-PDE activity may, in part, provoke the enhanced rate of cell proliferation.

Cyclic AMP-independent processes mediate Kirsten sarcoma virus-induced changes in collagen production and other properties of cultured cells

Previous studies suggested that the decreased collagen production observed in Kirsten sarcoma virus (Ki-MSV)-transformed BALB 3T3 cells could be reversed by treating cells with Bt2cAMP. We examined the relationship between intracellular cAMP, collagen production, and other properties in NRK and BALB 3T3 cells transformed by Ki-MSV. Two 3T3 transformants (Ki-3T3-234 and Ki-3T3Cl1) had lower cAMP levels than nontransformed cells. The level in a temperature-sensitive transformant, tsKi-3T3-714, was similar to the level in its parent, 3T3-714, and when it was shifted to a temperature nonpermissive for transformation (40 degrees C), intracellular cAMP did not increase although the growth and morphological properties were normal. The relative rate of collagen production also increased to the normal level. These results indicate that transformation-induced changes were regulated independently of cAMP. Further observations supported this conclusion. Intracellular cAMP in a flat revertant of Ki-3T3Cl1 was lower, rather than higher, than in the transformant, although the relative rate of collagen production was higher. Treatment of Ki-3T3-234 and tsKi-3T3-714 with cholera toxin plus isobutylmethylxanthine increased intracellular cAMP concentrations to 2-20 times the level in untreated cells, levels much higher than in nontransformed 3T3. In spite of this, collagen production was not increased by these agents in tsKi-3T3-714 and it was only partially restored in Ki-3T3-234 relative to the level in the nontransformed cells. In contrast, these agents inhibited growth on a substratum or in soft agar and produced a flattened morphology in both lines. Similarly, collagen production in transformed NRK cells (K-NRK) was only 3% of normal but treatment with Bt2cAMP or cholera toxin plus isobutylmethylxanthine increased production to only one-third the normal level while increasing cAMP to four times the normal level. We conclude that in Ki-MSV-transformed BALB 3T3 cells, changes in cAMP may be secondary effects and not related to maintenance of the transformed phenotype. The high levels of cAMP induced by exogenous agents may act on similar targets as those affected by transformation, but reversal of the transformed phenotype by these agents probably occurs by a different mechanism than that originally used to impose the changes.

Chlorpromazine: a potential anticancer agent?

The antipsychotic drug chlorpromazine causes scission of the DNA in PY815 mouse mastocytoma cells or isolated PY815 cell nuclei and the broken DNA reseals when chlorpromazine is removed from nuclei. These properties suggest that chlorpromazine interferes with topoisomerase action as do several other DNA-intercalating anti-cancer drugs. However, protein is not associated with the broken DNA after chlorpromazine treatment suggesting a different mode of action on the topoisomerase. Reasons why chlorpromazine may have potential as anti-cancer agent are considered.

Prostaglandin E1 inhibits DNA synthesis in arterial smooth muscle cells stimulated with platelet-derived growth factor

The effect of prostaglandins (PG) on initiation of DNA synthesis in arterial smooth muscle cells (SMC) stimulated with platelet-derived growth factor (PDGF) was examined. A concentration of 10 ng/ml PGE1 inhibited DNA synthesis, measured as autoradiographically labeled nuclei, by about 70%. Similar results were obtained with PGE2 and PGD2 but at concentrations 10-20 times higher, whereas PGF2 alpha lacked effect. The inhibitory action of the prostaglandins was restricted to the first 6 h of the lag phase. Treatment with PGE1 also raised the intracellular concentration of cyclic AMP, indicating that the inhibition may be mediated via changes in the levels of cyclic nucleotides.