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

Well differentiated nonmetastasizing fibrosarcoma (aggressive fibromatosis) of mandible: a rare case report and a literature review

Fibromatosis in the maxillofacial region is a very rare occurrence among diverse pathologic conditions, and because of the rarity of this tumor, definite treatment regimen is not established, which may be a contributing factor for a high recurrence rate. Fibromatosis may attain a large size and cause compression, infiltration, and destruction of adjacent structures. Such growth behavior presents severe management problems, especially in the head and neck region, where the presence of many vital structures within a small space makes the patient susceptible to the effects of the fibromatosis, likewise making complete excision difficult. We report the case of a 28-year-old female patient with aggressive desmoid fibromatosis involving the mandible, with a literature review of postoperative pharmacologic management to prevent recurrence.

Mitogenic signaling pathways induced by G protein-coupled receptors

G protein-coupled receptor (GPCR) agonists, including neurotransmitters, hormones, chemokines, and bioactive lipids, act as potent cellular growth factors and have been implicated in a variety of normal and abnormal processes, including development, inflammation, and malignant transformation. Typically, the binding of an agonistic ligand to its cognate GPCR triggers the activation of multiple signal transduction pathways that act in a synergistic and combinatorial fashion to relay the mitogenic signal to the nucleus and promote cell proliferation. A rapid increase in the activity of phospholipases C, D, and A2 leading to the synthesis of lipid-derived second messengers, Ca2+ fluxes and subsequent activation of protein phosphorylation cascades, including PKC/PKD, Raf/MEK/ERK, and Akt/mTOR/p70S6K is an important early response to mitogenic GPCR agonists. The EGF receptor (EGFR) tyrosine kinase has emerged as a transducer in the signaling by GPCRs, a process termed transactivation. GPCR signal transduction also induces striking morphological changes and rapid tyrosine phosphorylation of multiple cellular proteins, including the non-receptor tyrosine kinases Src, focal adhesion kinase (FAK), and the adaptor proteins CAS and paxillin. The pathways stimulated by GPCRs are extensively interconnected by synergistic and antagonistic crosstalks that play a critical role in signal transmission, integration, and dissemination. The purpose of this article is to review recent advances in defining the pathways that play a role in transducing mitogenic responses induced by GPCR agonists.

The pharmacological treatment of aggressive fibromatosis: a systematic review

BACKGROUND

Despite the use of surgery and radiotherapy, 20-35% of patients with aggressive fibromatosis (AF) will have local recurrence. The purpose of this review was to collect and analyze all available information regarding the role of non-cytotoxic and cytotoxic chemotherapy in AF that has been accumulated over the past few decades.

PATIENTS AND METHODS

A systematic review of published clinical trials, studies and case series was carried out using the Medline Express Databases and the Cochrane Collaboration Database from 1970 to October 2000.

RESULTS

Most studies published in the literature are in the form of successful case reports and single-arm series with small patient numbers. Most commonly used agents include hormonal agents, non-steroidal anti-inflammatory drugs (NSAIDs), interferons and cytotoxics. The literature data support the use of hormonal agents. Several questions, however, remain unresolved, such as which is the most suitable endocrine manipulation and what is the optimal dose and duration of treatment. NSAIDs and interferons have demonstrated activity against AF either alone or in combination with hormone therapy or chemotherapy but the precise mechanism of action is still unknown. Finally, there is growing evidence in the literature that chemotherapy is effective against AF with almost one in two patients being likely to respond.

CONCLUSIONS

The evidence in the literature supports the opinion that both non-cytotoxic and cytotoxic chemotherapies are effective against AF. However, the lack of sufficient patient numbers and randomized trials compromises the validity of the reported results and mandates further investigation with properly designed prospective studies including larger patient numbers, with main end points to include not only tumor response rate and survival but also quality-of-life issues.

Cell cycle regulation in a dinoflagellate, Amphidinium operculatum: identification of the diel entraining cue and a possible role for cyclic AMP

This research describes the diel phasing of the cell cycle in the dinoflagellate, Amphidinium operculatum Claparéde and Lachmann, and investigates the mechanisms that serve to link the cell cycle to the diel cycle. Unlike many dinoflagellates, A. operculatum has a naturally high division rate of approximately 1 division day(-1), which yields a nearly synchronous population, making it useful for population studies. When grown on a 16:8 h light/dark cycle, S-phase begins 10 h and mitosis 14-16 h after the onset of light, as determined by flow cytometry. Alterations in the timing of the dark/light and light/dark transitions showed that the cell cycle is entrained by the dark/light transition, with the light/dark cue being uninvolved. Cells in logarithmic phase growth also undergo diel changes in cell size (9-14 &mgr;m), reaching a maximum size late in the light phase, concurrent with mitosis. Stationary phase cells or cells blocked in G1 of the cell cycle with a cell cycle inhibitor, olomoucine, showed no size changes or reduced size changes over the diel cycle, suggesting a coupling of cell size to the cell division cycle. In Euglena, cAMP-dependent signaling appears to mediate diel phasing of the cell cycle. Therefore, the role of cAMP in cell cycle control in A. operculatum was investigated. Measurement of intracellular cAMP by radioimmunoassay (RIA) revealed that cAMP concentrations varied on a diel basis, but increases observed appeared to correlate with cell size increases, and did not correlate with light cues at the dark/light or light/dark transition. However, when cells were treated with the cAMP phosphodiesterase inhibitor, IBMX, cell cycle progression was inhibited at both the G1/S and the G2/M phase transitions. This supports a role for cAMP-dependent signaling in the dinoflagellate cell cycle and is in agreement with the documented role of cAMP in the cell cycle control of higher eukaryotes.

Protein kinase A-dependent and -independent signaling pathways contribute to cyclic AMP-stimulated proliferation

The effects of cyclic AMP (cAMP) on cell proliferation are cell type specific. Although the growth-inhibitory effects of cAMP have been well studied, much less is known regarding how cAMP stimulates proliferation. We report that cAMP stimulates proliferation through both protein kinase A (PKA)-dependent and PKA-independent signaling pathways and that phosphatidylinositol 3-kinase (PI3K) is required for cAMP-stimulated mitogenesis. In cells where cAMP is a mitogen, cAMP-elevating agents stimulate membrane ruffling, Akt phosphorylation, and p70 ribosomal S6 protein kinase (p70s6k) activity. cAMP effects on ruffle formation and Akt were PKA independent but sensitive to wortmannin. In contrast, cAMP-stimulated p70s6k activity was repressed by PKA inhibitors but not by wortmannin or microinjection of the N-terminal SH2 domain of the p85 regulatory subunit of PI3K, indicating that p70s6k and Akt can be regulated independently. Microinjection of highly specific inhibitors of PI3K or Rac1, or treatment with the p70s6k inhibitor rapamycin, impaired cAMP-stimulated DNA synthesis, demonstrating that PKA-dependent and -independent pathways contribute to cAMP-mediated mitogenesis. Direct elevation of PI3K activity through microinjection of an antibody that stimulates PI3K activity or stable expression of membrane-localized p110 was sufficient to confer hormone-independent DNA synthesis when accompanied by elevations in p70s6k activity. These findings indicate that multiple pathways contribute to cAMP-stimulated mitogenesis, only some of which are PKA dependent. Furthermore, they demonstrate that the ability of cAMP to stimulate both p70s6k- and PI3K-dependent pathways is an important facet of cAMP-regulated cell cycle progression.

Effects of insulin-like growth factor I and II on DNA synthesis and proliferation in primary cultures of adult rat hepatocytes

We compared the effects of insulin-like growth factor I (IGF-I) and II (IGF-II) on DNA synthesis and proliferation and investigated various signal transduction mechanisms involved in insulin-like growth factor-induced mitogenesis in primary cultures of adult rat hepatocytes. IGF-I stimulated hepatocyte DNA synthesis and proliferation with an EC50 of 75 ng/ml within 4 h of culture. These effects were sensitive to the IGF-I concentration and cell density. Hepatocyte proliferation induced by IGF-I was potentiated by metaproterenol (10(-6) M) as well as by 8-bromo-cAMP, phorbol 12-myristate 13-acetate (PMA; 10(-8) M) and was inhibited by U-73122 (1-(-[[17beta-3-methoxyestra-1,3,5(10)-triene-17-yl]amino]hexyl]-+ ++1Hpyrrol-2,5-dione)), genistein, wortmannin, PD98059 (2'-amino-3'-methoxyflavone) and rapamycin. The IGF-I effect was independent of pertussis toxin (100 ng/ml). IGF-II also dose dependently stimulated hepatocyte DNA synthesis and proliferation with an EC50 of 0.75 ng/ml within 4 h of culture. However, these effects were not dependent on the initial plating density. The stimulatory effects of IGF-II were potentiated by UK-14304 (5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline) (10(-5) M) and inhibited by phenylephrine, PMA, metaproterenol, 8-bromo-cAMP, PD98059, rapamycin, and pertussis toxin. The IGF-II effects were not affected by genistein, U-73122, and wortmannin. These results suggest that IGF-I and IGF-II rapidly stimulate the DNA synthesis and proliferation of adult rat hepatocytes by separate mechanisms.

The inhibition of DNA synthesis by prostaglandin E2 in human gingival fibroblasts is independent of the cyclic AMP-protein kinase A signal transduction pathway

In this study we attempted to clarify the mechanism of the inhibitory effects of PGE2 on DNA synthesis in Gin-1 (fibroblasts derived from healthy human gingiva) from the aspect of the cyclic AMP-dependent protein kinase signal transduction pathway. PGE2 upregulated intracellular cyclic AMP accumulation and inhibited DNA synthesis in Gin-1 in a dose-dependent manner. When the PGE2-induced intracellular cyclic AMP accumulation was further enhanced by treatment with the cyclic AMP-phosphodiesterase inhibitor, IBMX, the inhibitory effect of PGE2 on DNA synthesis was also enhanced. Furthermore, when we examined the effects of forskolin, an activator of cyclic AMP production, on intracellular cyclic AMP accumulation and DNA synthesis, similar results were obtained. However, inhibitors of cyclic AMP-dependent protein kinase (protein kinase A) such as HA1004 did not diminish the inhibitory effect of PGE2 on DNA synthesis in Gin-1. These results suggest that in Gin-1, PGE2-induced cyclic AMP accumulation may not lead to the activation of protein kinase A or protein kinase A activity may not relate directly to the growth inhibitory effect of PGE2, and that PGE2 does not inhibit DNA synthesis through the cyclic AMP-protein kinase A signal transduction pathway in Gin-1.

Density-dependent proliferation of adult rat hepatocytes in primary culture induced by epidermal growth factor is potentiated by cAMP-elevating agents

We investigated whether or not epidermal growth factor (EGF) and cAMP-elevating agents induce the proliferation of adult rat hepatocytes during the early (4 h after adding EGF) and late phases (21 h after adding EGF) of primary cultures. Adult rat hepatocytes did not significantly proliferate after culture with 20 ng/ml EGF for 4 h at a density of 1 X 10(5) cells/cm2. In contrast, when the density was decreased by about one-third to 3.3 X 10(4) cells/cm2, the number of nuclei increased about 1.2-fold after culture with 10-20 ng/ml EGF for 4 h. Under these culture conditions, DNA synthesis began within 2-4 h of exposure to 20 ng/ml of EGF, although at the high cell density, DNA was not synthesized during this period. The beta-adrenoceptor agonists, metaproterenol and isoproterenol, and other cAMP-elevating agents, such as glucagon, forskolin, and dibutyryl cAMP, potentiated both hepatocyte DNA synthesis and proliferation about 1.4-fold when cultured in combination with 20 ng/ml EGF. The stimulatory effects of metaproterenol and other cAMP-elevating agents were specifically blocked by the cAMP-dependent protein kinase inhibitor, H-89 (10(-7) M). The effect of EGF was almost completely suppressed by genistein (5 X 10(-6) M) and rapamycin (10 ng/ml), but it was unaffected by wortmannin (10(-7) M). These results demonstrate that mature rat hepatocytes can proliferate very rapidly in low-density cultures with EGF, the effects of which were potentiated by beta-adrenoceptor agonists and cAMP-elevating agents. In addition, the activation of receptor tyrosine kinase and p70 ribosomal protein S6 kinase may be involved in EGF-induced hepatocyte DNA synthesis and proliferation.

Positive and negative regulation of cell proliferation through prostaglandin receptors in NIH-3T3 cells

Among major eicosanoids and their analogs, prostaglandin (PG) F2 alpha > PGD2 > PGE1 > or = PGE2 > iloprost, a stable agonist of PGI2, dose-dependently stimulated DNA synthesis in quiescent NIH-3T3 cells. PGF2 alpha, PGD2, and PGE2, in that order, formed inositol phosphates and elevated intracellular Ca2+ ([Ca2+]i) but did not form cAMP nor inhibit forskolin-induced cAMP formation. Iloprost, PGI2, and PGE1 induced cAMP formation dose dependently with an ED50 of around 10(-7) M, and PGE2 at more than 10(-6) M did it. [3H]PGF2 alpha and [3H]PGD2 bindings membranes from NIH-3T3 cells were displaced in the order of PGF2 alpha > PGD2 > or = PGE2, while [3H]PGE2 binding was displaced by PGE2 > PGD2 > or = PGF2 alpha. Expression of mRNA encoding EP1 and EP4 (EP2) subtypes could be detected by reverse transcription- polymerase chain reaction using primers specific for EP1 and EP4 (EP2) cDNAs, but not that of EP3 subtype mRNA. The dose dependence of cAMP formation on iloprost and PGI2 and that of [Ca2+]i elevation on PGF2 alpha, D2, and E2 were similar to that of [3H]thymidine incorporation on the corresponding agonists. Fluprostenol (1 microM), a PGF2 alpha receptor agonist > 17-phenyl-trinor-PGE2 (1 microM), an EP1 receptor agonist stimulated [3H]thymidine incorporation, but an EP3 receptor agonist, ONO-AP-324 nor an EP4 (EP2) receptor agonist, 11-deoxy-PGE1 (1 microM) did not. Iloprost, dibutyryl cAMP, forskolin, or cholera toxin, when applied alone, enhanced [3H]thymidine incorporation, while they inhibited [3H]thymidine incorporation induced by submaximal concentrations of PGF2 alpha or epidermal growth factor (EGF), when applied within 12 hr after agonist stimulation. These results suggest that the proliferation of NIH-3T3 cells is stimulated by PGs via the PGF2 alpha receptor, EP1 subtype of PGE receptor, and the PGI2/PGE1 receptor through [Ca2+]i- and cAMP-dependent pathways, and that cAMP pathway negatively cross-talks with [Ca2+]i-or receptor tyrosine kinase-mediated DNA synthesis in a cell cycle-dependent manner.

Okadaic acid, cAMP, and selected nutrients inhibit hepatocyte proliferation at different stages in G1: modulation of the cAMP effect by phosphatase inhibitors and nutrients

The protein phosphatase inhibitor okadaic acid (> 100 nM) caused an abrupt and complete cessation of primary rat hepatocyte cell cycle progression at the restriction point in late G1. A decline in the G1/S transition rate was observed in response to elevated cAMP, excess selected nutrients, and okadaic acid (< 100 nM). Excess nutrients (40 mM glucose +/- 5 mM dihydroxyacetone) acted by imposing an incomplete block in early G1. The cAMP action was potentiated by the phosphatase inhibitor microcystin, which in itself did not affect DNA replication. This suggests that cAMP acted by phosphorylating substrate(s) that is dephosphorylated by a microcystin-sensitive phosphatase. The additive effects of submaximal concentrations of okadaic acid and cAMP analogs indicated that okadaic acid and cAMP acted via different pathways. In conclusion, okadaic acid, cAMP, and excess nutrients, acting through distinct pathways, inhibited hepatocytes in different parts of the G1 phase.

cAMP and PMA enhance the effects of IGF-I in the proliferation of endometrial adenocarcinoma cell line HEC-1-A by acting at the G1 phase of the cell cycle

The present study was undertaken to determine whether endometrial cancer cell line HEC-1-A differ from nontransformed cells, in that the cAMP and protein kinase C pathways may enhance IGF-I effects in mitogenesis by acting at the G1 phase of the cell cycle instead of G0. Immunofluorescence staining of HEC-1-A cells using the proliferating cell nuclear antigen (PCNA) monoclonal antibody and flow cytometric analysis determined that HEC-1-A cells do not enter the G0 phase of the cell cycle when incubated in a serum-free medium. Approximately 51% of the cells were in G1, 12% were in S and 37% in G2 phase of the cell cycle prior to treatment. Forskolin and phorbol-12-myristate 13-acetate (PMA) were used to stimulate cAMP production and protein kinase C activity, respectively. IGF-I, forskolin and PMA each increased (P < 0.01) [3H]-thymidine incorporation in a dose and time dependent manner. The interaction of forskolin and PMA with IGF-I was then determined. Cells preincubated with forskolin or PMA followed by incubation with IFG-I incorporated significantly more (P < 0.01) [3H]-thymidine into DNA than controls or any treatment alone. It is concluded that forskolin and, to a lesser extent, PMA exert their effect at the G1 phase of the cycle to enhance IGF-I effects in cell proliferation.

Cyclic AMP distinguishes between two functions of acidic FGF in a rat bladder carcinoma cell line

The rat bladder carcinoma cell line NBT-II exhibits two completely different responses to acidic FGF (aFGF): at high cell density, aFGF is a potent mitogen whereas at low cell density, aFGF acts as a scattering agent that can convert the epithelial NBT-II cells into fibroblastic-like, motile cells. The basis of the dual action of aFGF has been approached by using substances interfering with the transducing pathways known to be activated by growth factors. Genistein and tyrphostin, two inhibitors of tyrosine kinases, inhibit both cell scattering and mitogenesis induced by aFGF. Conversely, sodium orthovanadate, a potent inhibitor of tyrosine phosphatases can reproduce the two effects of aFGF, indicating that protein tyrosine phosphorylations are determinant in the two pathways. In contrast, transforming growth factor (TGF)-beta 1 is a strong inhibitor of DNA synthesis induced by aFGF but has no effect on cell scattering, providing evidence that the two pathways are divergent. In an attempt to determine the specificity of the pathways of aFGF we found that the level of cAMP, which can be externally elevated, is of pivotal importance in distinguishing between the two transducing pathways leading to either DNA replication or cell dispersion. Forskolin, 8-bromo cAMP, dibutyryl-cAMP, and cholera toxin are all capable of potentiating the mitogenic effect of aFGF while strongly inhibiting its scattering action. Moreover, addition of any of these substances to NBT-II cells converted into fibroblasts immediately induces their reversion towards an epithelial phenotype. These findings support a role for cAMP as a modulator of the effects of aFGF. Moreover, basal cAMP synthesis, which is not affected by aFGF, is higher in sparse than in dense cultures indicating that the level of cAMP depends on the status of the cell. Altogether, these results suggest that establishment and maintenance of the epithelial state require a precise regulation of cAMP level.

Cholera toxin potentiates TPA-induced mitogenesis and c-fos expression in BALB/c-3T3-derived proadipocytes

Treatment of quiescent density-arrested A31T6 proadipocytes with medium supplemented with either 12-O-tetradecanoylphorbol-13-acetate (TPA), insulin, or cholera toxin alone did not stimulate G0/G1 traverse and initiation of DNA synthesis. Combinations of either TPA and cholera toxin or insulin and cholera toxin caused a small stimulation of proliferation. Addition of medium supplemented with TPA and insulin caused a marked stimulation of cell cycle traverse which was significantly potentiated by the coaddition of cholera toxin. The actions of cholera toxin were mimicked by forskolin. Expression of c-fos was regulated in a manner that reflected the results of the mitogenic experiments. TPA caused a marked induction of expression, while only a small increase in transcript levels was seen after treatment with cholera toxin. Addition of a combination of cholera toxin and TPA caused a synergistic induction of c-fos expression. The model system described in this paper allows a detailed analysis of the regulation, by independent second messenger systems, of the transcription of a gene in a mitogenically relevant manner.

Synergistic and coordinate expression of the genes encoding ribonucleotide reductase subunits in Swiss 3T3 cells: effect of multiple signal-transduction pathways

Ribonucleoside-diphosphate reductase (ribonucleotide reductase, EC 1.17.4.1) is the enzyme responsible for the in vivo production of deoxyribonucleotides for DNA synthesis and is essential for cell proliferation. We examined the signal transduction pathways leading to expression of the M1 and M2 subunits of this enzyme in Swiss 3T3 mouse fibroblasts by Northern blot analysis. Stimulation of quiescent cells resulted in coordinate expression of both subunits, beginning at 8 hr after serum addition, in late G1 phase, and peaking at 18-24 hr. Serum increased M2 message to 30 to 50 times that of quiescent cells, in contrast with M1 message, which was increased 10 times. Agents that elevated cAMP, including forskolin, and the cAMP analogue 8-bromo-cAMP modestly stimulated gene expression. Each of these agents was synergistic with insulin, and these combinations induced expression equivalent to that induced by serum stimulation. Likewise, agents that activate protein kinase C such as phorbol 12,13-dibutyrate, bombesin, and vasopressin were also synergistic with insulin with respect to ribonucleotide reductase gene expression, as was epidermal growth factor, which stimulates receptor tyrosine kinase activity. The time course for induction of mRNA expression by each of these agents alone or in combination was identical to that for induction stimulated by serum. Finally, the synergistic effects apparent in Northern analysis of ribonucleotide reductase gene expression were mirrored in parallel determinations of DNA synthesis. Thus, the combinatorial nature of signal transduction pathways resulting in proliferation of Swiss 3T3 cells is expressed at the level of ribonucleotide reductase gene expression.

Multiple growth factor independence in rat mammary carcinoma cells

In previous studies we demonstrated that rat mammary tumor (RMT) cells that are serially transplantable consist of cells that are independent of growth factors strictly required by normal rat mammary epithelial (RME) cells for growth in serum-free culture. The present studies were designed to determine the extent of the growth factor independence of several cell lines derived from these tumors and to determine if the cells that expressed growth factor independence in vitro are also tumorigenic in vivo. Cells from a transplantable mammary carcinoma (8-12 RMT) were seeded into culture in serum-free medium in the absence of either insulin (IN), epidermal growth factor (EGF), or cholera toxin (CT), and cell populations independent of the individual factors were developed. Next, the three growth factor independent populations were tested for their ability to grow in the absence of multiple growth factors. 8-12 RMT cells did not lose proliferative potential when multiple growth factors were deleted from the medium. Indeed, 8-12 RMT cells could be serially propagated in serum-free medium supplemented solely with bovine serum albumin (BSA) and ethanolamine. Cell lines independent of single growth factors were also developed from two other transplantable tumors (1-9 RMT and 7-15 RMT). In contrast to the 8-12 RMT-derived cell lines, deletion of additional growth factors from the media of the 1-9 RMT and 7-15 RMT-derived cells resulted in dramatic losses in growth potential. These results suggest that independence of individual growth factors is mediated by different mechanisms, since cells from different tumors can stably express independence of one, two, or three or more factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Testolactone, sulindac, warfarin, and vitamin K1 for unresectable desmoid tumors

Ten patients with large inoperable desmoid tumors in various body locations were treated with testolactone. Four tumors (40%) responded with major regressions, i.e., more than 50% reduction in volume. Eight patients received nonsteroid anti-inflammatory drugs (indomethacin, sulindac, or sulindac with warfarin and vitamin K1 [Mephyton]) for periods of 2 to 91 months. There was one major regression, one partial regression, and three instances of tumor growth arrest over periods up to 8 years. Seven patients were treated with nonsteroid anti-inflammatory drugs concurrent with or after testolactone or tamoxifen. There were five major regressions and one partial regression with extensive central necrosis of an enormous intra-abdominal tumor. The last patient has been treated for only 12 months, with no change in tumor volume. It appears that estrogens function as growth factors for desmoid tumors, and that minimization of these effects inhibits tumor growth in some, but not all, cases. In those instances where antiestrogens were not effective as single agents, the tumors usually responded to subsequent nonsteroid anti-inflammatory drug therapy. Withdrawal of estrogen may be followed by inhibition of transcription of genes that support tumor cell proliferation, and sulindac and indomethacin may augment these effects by inhibiting prostaglandin and cyclic AMP synthesis and the activity of protein kinase C. Warfarin may function as a protonophore to acidify the cytoplasm and prevent the alkalinization that is necessary to initiate DNA synthesis and cell cycle progression, again an impairment of the transcription process.

Alternative patterns of mitogenesis and cell scattering induced by acidic FGF as a function of cell density in a rat bladder carcinoma cell line

The dual function exerted by acidic fibroblast growth factor (aFGF) in a rat bladder carcinoma cell line has been explored under two different conditions of culture density. At low cell density, aFGF promotes the epithelium-to-mesenchyme transition of NBT-II cells characterized by cell dissociation, morphological changes toward a fibroblastic-like phenotype, and acquisition of cell motility. Under these conditions, NBT-II cells are unresponsive to the growth-promoting effect of aFGF. At high cell density, aFGF is a potent mitogenic factor, but its scattering activity is essentially abrogated. Slight modifications in the binding of aFGF to its specific receptors were observed at high cell density; these changes correlated with a downregulation of receptors with no apparent change in their molecular form. NBT-II cells located at the edge of artificial wounds mimicked the behavior of subconfluent cells, because they did not proliferate upon aFGF treatment. Furthermore, in large-sized NBT-II colonies, peripheral cells were the first to dissociate in response to aFGF. Altogether, our results suggest that the cellular response to multifunctional growth factors might depend on the localization within the responding cell population.

Dual effects of glucagon and cyclic AMP on DNA synthesis in cultured rat hepatocytes: stimulatory regulation in early G1 and inhibition shortly before the S phase entry

Although several lines of evidence implicate cyclic AMP in the humoral control of liver growth, its precise role is still not clear. To explore further the role of cyclic AMP in hepatocyte proliferation, we have examined the effects of glucagon and other cyclic AMP-elevating agents on the DNA synthesis in primary cultures of adult rat hepatocytes, with particular focus on the temporal aspects. The cells were cultured in a serum-free, defined medium and treated with epidermal growth factor (EGF), insulin, and dexamethasone. Exposure of the hepatocytes to low concentrations (10 pM-1 nM) of glucagon in the early stages of culturing (usually within 6 h from plating) enhanced the initial rate of S phase entry without affecting the lag time from the plating to the onset of DNA synthesis, whereas higher concentrations inhibited it. In contrast, glucagon addition at later stages (24-45 h after plating) produced only the inhibition. Thus, if glucagon was added at a time when there was a continuous EGF/insulin-induced recruitment of cells to S phase, the rate of G1-S transition was markedly decreased within 1-3 h. This inhibitory effect occurred at low glucagon concentrations (ID50 less than 1 nM) and was mimicked by cholera toxin, forskolin, isobutyl methylxanthine, and 8-bromo cyclic AMP. The results indicate that cyclic AMP has dual effects on hepatocyte proliferation with a stimulatory modulation early in the prereplicative period (G0 or early G1), and a marked inhibition exerted immediately before the transition from G1 to S phase.

Cyclic adenosine monophosphate acts synergistically with dexamethasone to inhibit the entrance of cultured adult rat hepatocytes into S-phase: with a note on the use of nucleolar and extranucleolar [3H]-thymidine labelling patterns to determine rapid changes in the rate of onset of DNA replication

Analogs of cyclic adenosine monophosphate (cAMP) (N6benzoyl cAMP and N6monobutyryl cAMP) as well as agents that increased the intracellular level of cAMP (glucagon and isobutylmethylxanthine) inhibited the EGF-stimulated DNA replication of adult rat hepatocytes in primary culture independently of cell density. This inhibition was strongly potentiated by the glucocorticoid dexamethasone. The effect of cAMP (and dexamethasone) was not due to toxicity, because the inhibition was reversible and the cell ultrastructure preserved. cAMP acted by decreasing the rate of transition from G1- to S-phase, the duration of G2- and S-phase of the hepatocyte cell cycle being unaffected. DNA replication started in the extranucleolar compartment of the nucleus and ended in the nucleolar compartment as described earlier for cells grown in the absence of cAMP (O.K. Vintermyr and S.O. Døskeland, J. Cell. Physiol., 1987, 132:12-21). The action of cAMP was very rapid: significant inhibition of the transition was noted 2 hr after the addition of glucagon/IBMX and half-maximal inhibition after 4 hours. The determination of extranucleolarly labelled nuclei in cells pulse-labelled with [3H]thymidine allowed precise analysis of rapid changes in the probability of transition from G1- to S-phase. The extranucleolar labelling index could also be determined in cells continuously exposed to [3H]thymidine.

Extracellular ATP is a mitogen for 3T3, 3T6, and A431 cells and acts synergistically with other growth factors

Extracellular ATP in concentrations of 5-50 microM displayed very little mitogenic activity by itself but it caused synergistic stimulation of [3H]thymidine incorporation in the presence of phorbol 12-tetradecanoate 13-acetate, epidermal growth factor, platelet-derived growth factor, insulin, adenosine, or 5'-(N-ethyl)carboxamidoadenosine. Cultures of Swiss 3T3, Swiss 3T6, A431, DDT1-MF2, and HFF cells were used. The percent of cell nuclei labeled with [3H]thymidine and cell number were also increased. ADP was equally mitogenic, while UTP and ITP were much less active. The effect of ATP was not due to hydrolysis by ectoenzymes to form adenosine, a known growth factor. Thus, the nonhydrolyzable analogue adenosine 5'-[beta, gamma-imido]triphosphate was mitogenic. In addition, it was found that ATP showed synergism in 3T6 and 3T3 cells when present for only the first hour of an incorporation assay, during which time no significant hydrolysis occurred. Furthermore, prolonged preincubation of cells with ATP reduced the mitogenic response to ATP but not to adenosine; preincubation with adenosine or N6-(R-phenylisopropyl)adenosine had the reverse effect. Finally, the effect of adenosine, but not of ATP, was inhibited by aminophylline. We conclude that extracellular ATP is a mitogen that interacts with P2 purinoceptors on the plasma membrane.

cAMP levels in proliferating rat mammary epithelial cells in vitro and in vivo

Agents that elevate intracellular cAMP levels are required for growth of many cell types in culture including normal rat mammary epithelial (RME) cells. To determine if the intracellular levels of cAMP that result from stimulation by agents such as cholera toxin (CT) or prostaglandin E-1 (PGE-1) are within the physiological range, cAMP levels were determined in RME cells growing in primary culture and compared to levels measured in freshly isolated mammary epithelium. The results indicate that the cAMP levels of mammary epithelial organoids obtained from 45-day-old virgin rats are 4 to 6 pmol/10(6) cells. Growth of RME cells in primary culture in the presence of CT results in cAMP levels of approximately 15 to 20 pmol/10(6) cells early in culture when cells are proliferating rapidly. As cells approach confluence, cAMP concentrations decrease to levels observed in fresh organoids. CT-stimulated cAMP levels appear to be within the range of those found in pregnant mammary epithelium in vivo. Growth of RME cells in medium supplemented with PGE-1 instead of CT results in cAMP levels equivalent to those found in fresh mammary epithelial organoids and under these conditions the growth rate is approximately half that found in CT-stimulated cells. These results indicate cAMP to be a positive regulator of cell growth in vivo at levels that are within the physiological range.

Regulation of osteosarcoma EGF receptor affinity by phorbol ester and cyclic AMP

We studied the binding and degradation of 125I-labeled epidermal growth factor (EGF) by UMR-106 osteosarcoma cells and the regulation of EGF receptor affinity for EGF by the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and by treatments that raise intracellular levels of cyclic AMP. Cell surface binding of [125I]EGF to A431 cells reached a plateau after a 30 minute incubation at 37 degrees C but was undetectable in UMR-106 cells. Degradation of [125I]EGF proceeded at a 50-fold higher rate in A431 cells on a per cell basis, but receptor-bound [125I]EGF was internalized and degraded at a 3.5-fold higher rate by UMR-106 cells on a per receptor basis. At 4 degrees C, [125I]EGF labeled a single class of surface binding sites in the UMR-106 cell. Treatment with TPA at 37 degrees C reduced subsequent cell surface binding of [125I]EGF at 4 degrees C a maximum of 80% with an IC50 of 1.25 ng/ml. Maximal TPA reduction of [125I]EGF binding was observed within 5-15 minutes and was due to a reduction in the affinity of cell surface receptors of [125I]EGF without a change in receptor density. Pretreatment of the cells for 4 h with 30 microM forskolin, 1 mM isobutylmethylxanthine (IBMX) plus 30 microM forskolin, or 1 mM IBMX plus 100 ng/ml parathyroid hormone (PTH) attenuated the loss in [125I]EGF binding caused by a subsequent dose of 10 ng/ml of TPA by 17% (p less than 0.0005), 39% (p less than 0.0002), and 35% (p less than 0.002), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Different growth responses to agents which elevate cAMP in human melanoma cell lines of high and low experimental metastatic capacity

Human melanoma variants of low and high experimental metastatic activity, which had been derived from the same parental line, showed markedly different growth responses to agents which elevated intracellular cAMP. The high metastatic line had a significant decrease in in vitro proliferation following treatment with cholera toxin (10(-9) M) and forskolin (100 microM), with both agents causing virtual cessation of cell growth after 3-5 days incubation. Pre-treatment with 10(-9) M cholera toxin reduced colony forming ability to 11-15 per cent of control values, saturation densities were decreased to 10-25 per cent of controls and these cytostatic responses were accompanied by changes in cellular morphology. Lung colonising capacity of this cell line after i.v. injection into athymic mice was reduced significantly by prior exposure to cholera toxin (a median of 2 lung nodules versus 26 lung nodules for untreated, control cells). In contrast, low metastatic cell lines showed no significant growth inhibition in the presence of these agents. Cholera toxin (10(-9) M) reduced colony forming ability of these cells to only 74 per cent of control values and there were no significant decreases in growth rate nor any morphological changes in response to either cholera toxin or forskolin. The variable response obtained in the cell lines appeared neither to be a consequence of variation in induced levels of intracellular cAMP nor in differences between the cell lines in response to the same agent; forskolin (100 microM) induced a maximal 25-fold elevation and cholera toxin (10(-9) M) a 2.5-fold elevation increase in cAMP. These data show that highly metastatic variants of a human melanoma cell line differ from their less metastatic counterparts in the way they respond to agents which elevate the second messenger molecule cAMP.

Cyclic AMP increasing agents rapidly stimulate vimentin phosphorylation in quiescent cultures of Swiss 3T3 cells

The results presented here demonstrate that an elevation in the cellular levels of cyclic AMP (cAMP) increases the phosphorylation of an Mr = 58,000 cellular protein in quiescent cultures of Swiss 3T3 cells. The enhancement of 32Pi incorporation into the Mr 58,000 cellular protein was detected as early as 1 min and reached a maximum after 20 min of treatment. The role of cAMP in the phosphorylation of Mr = 58,000 protein is substantiated by the following lines of evidence: a) a variety of agents that cause cAMP accumulation in 3T3 cells, including cholera toxin, 5'-N-ethylcarboxamideadenosine (NECA), PGE1, and 3-isobutyl-1-methyl-xanthine (IBMX) increased the phosphorylation of the same Mr 58,000 cellular protein as demonstrated by peptide mapping; b) inhibitors of cyclic nucleotide phosphodiesterase potentiated the ability of low concentrations of the adenylate cyclase activators NECA, PGE1, and forskolin to increase Mr 58,000 phosphorylation; and c) permeable derivatives of cAMP such as 8BrcAMP were also effective and specific in promoting Mr 58,000 phosphorylation. Detergent extraction, immunoblotting, and immunoprecipitation identified the Mr = 58,000 phosphoprotein as vimentin, the main protein subunit of the intermediate filaments of mesenchymal cells including Swiss 3T3 cells. Studies with intact 3T3 cells revealed that an increase in the intracellular level of cAMP induced a marked redistribution and collapse of the intermediate filaments. These results raise the possibility that an intact intermediate filament network may restrict the reinitiation of DNA synthesis.

Bombesin enhancement of cAMP accumulation in Swiss 3T3 cells: evidence of a dual mechanism of action

Addition of bombesin in the presence of either forskolin or cholera toxin caused a marked (4-6 fold) enhancement of cAMP accumulation in Swiss 3T3 cells. This effect was time and concentration dependent, induced by various bombesin-like peptides and blocked by a bombesin antagonist. Enhancement of cAMP accumulation by bombesin was diminished by chronic pretreatment with phorbol dibutyrate implicating the involvement of protein kinase C in the activation. Pretreatment with pertussis toxin, which uncouples protein kinase C activation from cAMP accumulation (Proc. Natl. Acad. Sci. U.S.A., 84:2282, 1987) also inhibited bombesin enhancement of cAMP. Bombesin was also shown to release E type prostaglandins into the medium, an effect which was abolished by the cyclooxygenase inhibitor indomethacin. Low concentrations (100 nM) of indomethacin partially inhibited the accumulation of cAMP by bombesin in the presence of forskolin indicating that the release of E type prostaglandins into the medium is also involved in the accumulation of cAMP by bombesin. The additive nature of PBt2-mediated down-regulation and treatment with indomethacin suggests that activation of protein kinase C and the release of E type prostaglandins provide two distinct pathways involved in the enhancement of cAMP accumulation by bombesin. Finally, bombesin in the presence of forskolin stimulated the phosphorylation of the intermediate filament component vimentin, identified in the accompanying paper as a substrate for a cAMP dependent protein kinase in intact Swiss 3T3 cells.

Vasoactive intestinal peptide synergistically stimulates DNA synthesis in mouse 3T3 cells: role of cAMP, Ca2+, and protein kinase C

Vasoactive intestinal peptide synergistically stimulated initiation of DNA synthesis in Swiss 3T3 cells. The peptide stimulated [3H]thymidine incorporation in the presence of insulin and either forskolin or an inhibitor of cAMP phosphodiesterase in a concentration-dependent manner. Half-maximal effect was obtained at 1 nM. At mitogenic concentrations, VIP stimulated a marked accumulation (eightfold) of cAMP. In contrast to other growth-promoting neuropeptides, VIP did not induce an increase in cytosolic free Ca2+ or an activation of protein kinase C. We conclude that neuropeptides can modulate long-term cell proliferation through multiple signaling pathways.

Epidermal growth factor potentiates the induction of ornithine decarboxylase activity by prostaglandins in embryonic palate mesenchymal cells: effects on cell proliferation and glycosaminoglycan synthesis

Epidermal growth factor (EGF) and prostaglandins (PGs) have been implicated in the regulation of a number of developmental processes in the mammalian embryonic palate. Normal palatal ontogenesis is dependent on the presence and quite possibly on the interaction of various hormones and growth factors. The interaction between EGF and PGs in regulation of murine embryonic palate mesenchymal (MEPM) cell growth and differentiation was therefore investigated by monitoring the activity of ornithine decarboxylase (ODC), the principle and rate limiting enzyme of polyamine biosynthesis. ODC activity is tightly coupled to the proliferative and differentiative state of eukaryotic cells and therefore serves as a reliable indicator of such cellular functions. Treatment of confluent cultures of MEPM cells with EGF (1-50 ng/ml) resulted in a dose-related increase in ODC activity, while similar treatment with either PGE2 or PGF2 alpha (at concentrations up to 1 microM) did not elicit a dose-dependent increase in enzyme activity. Concurrent treatment of MEPM cells with EGF (20 ng/ml) and either PGE2 or PGF2 alpha (0.1-10000 nM) resulted in a marked prostaglandin dose-dependent induction of ODC activity, suggesting a strong cooperative interaction between these factors. ODC activity was maximal by 4 to 8 hr and could be completely inhibited by preincubation of the cells with actinomycin D or cycloheximide, indicating that de novo synthesis of RNA and protein is necessary for enzyme induction. Stimulation of ODC activity by EGF and PGE2 in these cells was not positively correlated with the level of cellular DNA synthesis but did result in a ninefold increase in the synthesis of extracellular glycosaminoglycans (GAGs), a key macromolecular family implicated in palatal morphogenesis. Stimulation of GAG synthesis was significantly inhibited by the administration of 5 mM DFMO (an irreversible inhibitor of ODC), indicating that the marked increase in GAG production was dependent, in part, on the induction of ODC activity by EGF and PGE2. Qualitative analysis of the palatal GAGs indicated that synthesis of several major classes of GAGs was stimulated. Collectively these data demonstrate a cooperative interaction between EGF and PGs in the induction of ODC activity. Such activity may serve to regulate the synthesis of GAGs, which are instrumental in mammalian palatal ontogenesis.

Influence of hormone and growth factor interactions on the proliferative potential of normal rat mammary epithelial cells in vitro

These experiments were aimed at using a recently developed serum-free culture system for growth of normal rat mammary epithelial (RME) cells in vitro to examine the interactions of specific hormones and growth factors on the proliferative potential of these cells. RME cells were obtained by enzymatic dissociation of mammary tissues of Lewis rats. Primary cultures were started by plating 2 X 10(5) RME cells per 60-mm type I collagen-coated tissue culture dish. Cultures were maintained in a basal medium that consisted of Ham's F-12 medium supplemented with bovine serum albumin (BSA), ethanolamine (EA), and transferrin (Tf), which, by itself, did not support RME cell proliferation. Insulin (I), hydrocortisone (HC), and epidermal growth factor (EGF), when added to the basal medium interacted synergistically to stimulate RME cell proliferation, but this effect was dependent on the additional presence of cholera toxin (CT). Under these conditions a greater-than-tenfold increase in cell number over a 10-day culture period was obtained. Insulin could be replaced by physiological levels of insulin-like growth factor-I (IGF-I). CT could be replaced by other agents that elevate intracellular levels of cyclic adenosine 3':5' monophosphate (cAMP) such as dibutyryl-cAMP (db-cAMP), prostaglandin E1 (PGE-1), and/or isobutylmethylxanthine (IBMX). Prolactin (M) or progesterone (P) potentiated the effect of I, HC, EGF, and CT, resulting in an additional twofold increase in cell number over that found in their absence. However, addition of both hormones was no more effective than either one alone. Furthermore, addition of M or P in the absence of EGF had no effect on RME cell proliferation. Addition of 17-B-estradiol (E2) to the I-, HC-, EGF-, and CT-containing medium also resulted in enhanced RME cell proliferation. These results point to a number of hormone and growth factor interactions that influence the proliferation of normal RME cells in vitro.

Heterologous regulation of EGF receptors in fibroblastic cells

Platelet-derived growth factor (PDGF) increases the mitogenic activity of epidermal growth factor (EGF) in several cells lines, including BALB/C-3T3. PDGF-treated BALB/C-3T3 cells manifest a reduced capacity to bind 125I-labeled EGF due to a loss of high affinity EGF receptors. Cholera toxin potentiates the ability of PDGF to both decrease EGF binding and initiate mitogenesis. Whether PDGF increases EGF sensitivity via its effects on EGF receptors is not known and requires a more complete understanding of the mechanism by which PDGF decreases EGF binding. 12-O-tetradecanoylphorbol 13-acetate (TPA) also reduces EGF binding in BALB/C-3T3 and other cells, presumably by activating protein kinase C and, consequently, inducing the phosphorylation of EGF receptors at threonine-654. PDGF indirectly activates protein kinase C, and EGF receptors in PDGF-treated WI-38 cells are phosphorylated at threonine-654. Thus, the effects of PDGF on EGF binding may also be mediated by protein kinase C. We investigated this hypothesis by comparing the actions of PDGF and TPA on EGF binding in density-arrested BALB/C-3T3 cells. Both PDGF and TPA caused a rapid, transient, cycloheximide-independent loss of 125I-EGF binding capacity. The actions of both agents were potentiated by cholera toxin. However, whereas TPA allowed EGF binding to recover, PDGF induced a secondary and cycloheximide-dependent loss of binding capacity. Most importantly, PDGF effectively reduced binding in cells refractory to TPA and devoid of detectable protein kinase C activity. These findings indicate that PDGF decreases EGF binding by a mechanism that involves protein synthesis and is distinct from that of TPA.

Induction of DNA synthesis in dog thyrocytes in primary culture: synergistic effects of thyrotropin and cyclic AMP with epidermal growth factor and insulin

We have investigated the growth effects of thyrotropin (TSH) (mimicked by forskolin and acting through cyclic AMP), epidermal growth factor (EGF), serum (10%) and insulin on quiescent dog thyroid epithelial cells in primary culture in a serum-free defined medium. These cells were previously shown to retain the capacity to express major thyroid differentiation markers. In the presence of insulin and after a similar prereplicative phase of 18 +/- 2h, TSH, EGF, and serum promoted DNA synthesis in such quiescent cells only a minority of which had proliferated in vitro before stimulation. The combination of these factors induced more than 90% of the cells to enter S phase within 48 h and near exponetial proliferation. Analysis of the cell cycle parameters of the stimulated cells revealed that the G1 period duration was similar to the length of the prereplicative phase of quiescent thyroid cells; this might indicate that they were in fact in an early G1 stage rather than in G0 prior to stimulation. TSH and EGF action depended on or was potentiated by insulin. Strikingly, nanomolar concentrations of insulin were sufficient to support stimulation of DNA synthesis by TSH, while micromolar concentrations of insulin were required for the action of EGF. This suggests that insulin supported the action of TSH by acting on its own high affinity receptors, whereas its effect on EGF action would be related to its somatomedinlike effects at high supraphysiological concentrations. Insulin stimulated the progression in the prereplicative phase initiated by TSH or forskolin. In addition, in some primary cultures TSH must act together with insulin to stimulate early events of the prereplicative phase. In the presence of insulin, EGF, and forskolin, an adenylate cyclase activator, markedly synergized to induce DNA synthesis. Addition of forskolin 24 h after EGF or EGF 24 h after forskolin also resulted in amplification of the growth response but with a lag equal to the prereplicative period observed with the single compound. This indicates that events induced by the second factor can no longer be integrated during the prereplicative phase set by the first factor. These findings demonstrate the importance of synergistic cooperation between hormones and growth factors for the induction of DNA synthesis in epithelial thyroid cells and support the proposal that essentially different mitogenic pathways--cyclic AMP-dependent or independent--may coexist in one cell.

The effect of cyclic AMP on the accumulation of newly synthesized protein in the extracellular matrix of PFHR-9 teratocarcinoma cells

The effect of elevated cyclic AMP levels on the accumulation of newly synthesized extracellular matrix protein was examined in PFHR-9 cells producing Type IV collagen. The effect of dbcAMP, 8-BrcAMP, IBMX and forskolin on the synthesis of total protein, non-collagen protein and collagen were compared. DbcAMP increased the accumulation of total protein but did not affect the distribution of collagen and non-collagen protein. 8-BrcAMP, IBMX and forskolin also increased collagen and non-collagen accumulation. However, the effect on collagen was significantly greater with 8-BrcAMP and IBMX. Consequently, 8-BrcAMP and IBMX resulted in an increased percent collagen synthesis in the extracellular matrix. Elevated cAMP levels had no effect on cell proliferation or DNA synthesis but did produce a significant effect on cell morphology.

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.

Possible involvement of RAS-encoded proteins in glucose-induced inositolphospholipid turnover in Saccharomyces cerevisiae

Incubation of yeast Saccharomyces cerevisiae at very low (0.02%) glucose levels led to arrest of the cell cycle at the G0/G1 phase. Readdition of glucose to these "starved" yeast resulted in cell proliferation. In glucose-starved yeast, glucose stimulated 32P incorporation into phosphatidic acid, phosphatidylinositol, phosphatidylinositol monophosphate, and phosphatidylinositol bisphosphate but not into phosphatidylethanolamine and phosphatidylcholine. Preincubation of yeast with [3H]inositol and subsequent exposure to glucose resulted in rapid formation of [3H]inositol monophosphate and [3H]inositol trisphosphate, presumably derived from phosphatidylinositol and phosphatidylinositol bisphosphate. Under similar conditions, glucose elicited both efflux and influx of Ca2+ in yeast. Glucose-induced 32P incorporation into inositolphospholipids and formation of [3H]inositol phosphates were more pronounced in RAS-related mutants such as ras1, ras1 ras2 bcy1, and RAS2Val19 than in the wild-type strain. These results strongly suggest that glucose stimulates inositolphospholipid turnover, Ca2+ mobilization, and subsequent cell proliferation in a manner similar to that of growth factors with mammalian cells, and that RAS-encoded proteins are involved in regulation of this glucose-induced inositolphospholipid turnover in yeast.

Early signals in the mitogenic response

Polypeptide growth factors, regulatory peptides, and a variety of pharmacological agents acting alone or synergistically induce mitogenesis in cultured fibroblasts. The early signals in the membrane, cytosol, and nucleus promoted by these extracellular factors, together with their mitogenic effectiveness, are integrated in a unified hypothesis for the regulation of fibroblast growth.

The effect of cAMP on tumour promoter responses mediated by C-kinase

The phorbol ester tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induced several rapid changes in the HEL-37 mouse epidermal cell line. These included an alteration in cell morphology, inhibition of cell-cell communication, inhibition of epidermal growth factor (EGF) binding and a stimulation of phosphatidylcholine (PC) synthesis. The synthetic diacylglycerol sn 1-oleoyl-2-acetylglycerol (OAG) and sn 1,2-dioctanoylglycerol (diC8) caused similar changes, implying an involvement of the Ca2+- and phospholipid-dependent protein kinase (C-kinase). Treatment of the cells with the cAMP-generating agents db-cAMP and isoproterenol together with the phosphodiesterase inhibitors aminophylline and isobutyl-methylxanthine (IBMX) prior to and during TPA, OAG or diC8 treatment protected the cells against the inhibition of both junctional communication and EGF binding. TPA-induced morphological changes and enhanced PC synthesis, however, were unaffected by elevated levels of intracellular cAMP. These experiments provide evidence for the existence of a dual regulatory system controlling some (but not all) tumour promoter effects.

Effects of cholera toxin and isobutylmethylxanthine on growth of human fibroblasts

Cholera toxin produced a dose-dependent decrease in the restimulation of G0/G1 traverse in density-arrested human fibroblasts but did not inhibit the stimulation of cells arrested in G0 after serum starvation at low density. In addition, cholera toxin did not inhibit the proliferation of sparse logarithmically growing human fibroblasts, even when low concentrations of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) were also present. However, the final density to which sparse cells grew was limited by cholera toxin, when added either alone or together with low concentrations of IBMX. In contrast, high concentrations of the phosphodiesterase inhibitor alone produced a profound inhibition in the growth of sparse human fibroblasts. IBMX produced an inhibition both in the G1 and in the G2 phases of the cell cycle by a mechanism(s) that was not related to the magnitude of the increases in adenosine 3',5'-cyclic monophosphate concentrations.

Interactions between cyclic AMP- and phorbol ester-dependent phosphorylation systems in S49 mouse lymphoma cells

High-resolution two-dimensional gel electrophoresis of proteins labeled with either 32Pi or [35S]methionine was used to study interactions between cyclic AMP and tetradecanoyl phorbol acetate (TPA) at the level of intracellular protein phosphorylation. Cultured S49 mouse lymphoma cells were used as a model system, and mutant sublines lacking either the catalytic subunit of cyclic AMP-dependent protein kinase or the guanyl nucleotide-binding "Ns" factor of adenylate cyclase provided tools to probe mechanisms underlying the interactions observed. Three sets of phosphoproteins responded differently to TPA treatment of wild-type and mutant cells: Phosphorylations shown previously to be responsive to activation of intracellular cyclic AMP-dependent protein kinase were stimulated by TPA in wild-type cells but not in mutant cells, a subset of phosphorylations stimulated strongly by TPA in mutant cells was inhibited in wild-type cells, and two novel phosphoprotein species appeared in response to TPA only in wild-type cells. The latter two classes of TPA-mediated responses specific to wild-type cells could be evoked in adenylate cyclase-deficient cells by treating concomitantly with TPA and either forskolin or an analog of cyclic AMP. Three conclusions are drawn from our results: 1) TPA stimulates adenylate cyclase in wild-type cells causing increased phosphorylation of endogenous substrates by cyclic AMP-dependent protein kinase, 2) activated cyclic AMP-dependent protein kinase inhibits phosphorylation (or enhances dephosphorylation) of a specific subset of TPA-dependent phosphoproteins, and 3) cyclic AMP-dependent events facilitate TPA-dependent phosphorylation of some substrate proteins.

Inhibitory effects of parathyroid hormone on growth of osteogenic sarcoma cells

The effects of the bone resorbing hormone, parathyroid hormone (PTH), on the growth of malignant osteoblastic cells have been examined. The malignant osteoblastic cells were a clonal line (UMR 106) derived from a transplantable rat osteogenic sarcoma. The predominant effect of PTH at doses above 10(-10) M was an inhibition of replication and DNA synthesis. Replication was decreased by PTH in both the presence or absence of serum and at various cell seeding densities. Both bovine PTH (1-84) and the synthetic hormone, human PTH (1-34), inhibited replication, but with bovine hormone being an order of magnitude more potent. The effects could be observed in as short a time as 6 hours with DNA synthesis and 24 hours with replication.

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.

Thyrotropin modulates EGF receptor function in porcine thyroid follicle cells

Porcine thyroid follicle cells in monolayer cultures were shown to contain one single class of high-affinity EGF receptors with Kd = 4.5 X 10(-10) M and approximately 20 000-25 000 receptors per cell. Suspension cultures of aggregated follicle cells, exposed to TSH for 3 days, showed a 3-fold increase in [125I]EGF binding. Scatchard analysis demonstrated that this was due to an increase in receptor number. Other cAMP-elevating agents (cholera toxin, dibutyryl cAMP, forskolin) induced a similar effect. In suspension cultures, preincubation with TSH or cholera toxin for 2 days reduced the subsequent [3H]thymidine incorporation. This inhibition was overcome by a low concentration of EGF (0.1 ng/ml). At higher concentrations of EGF (1-10 ng/ml) the incorporation of [3H]thymidine was potentiated 2-3-fold in cultures preexposed to TSH or cholera toxin. The results demonstrate the presence of a high-affinity EGF receptor in porcine thyroid follicle cells. Receptor expression, as well as responsiveness to the mitogenic action of EGF, is modulated in vitro by TSH, through a cAMP-dependent process.

Stimulating the proliferation of quiescent 3T3 fibroblasts by peptide growth factors or by agents which elevate cellular cyclic AMP level has opposite effects on motility

The effects of some chemically defined growth factors on the locomotion of quiescent Swiss 3T3 fibroblasts have been studied. A computer digitiser has been used to facilitate recording the paths followed by cells in time-lapse films; this method allows 500 cell-hours to be recorded in 1 h of real time. Individual cells in the same culture vary widely in speed. This variation is not associated with the positions of the cells in the cell cycle; a small deceleration which seems to occur in G2 cannot account for any significant part of the variation seen. Nor is it related to the time elapsing before the cell divides, although this is equally variable; the speed and age at division of particular cells appear to be entirely independent of one another. Nevertheless, good reproducibility is seen between the mean speeds of large numbers of cells from the same type of culture. The mean speed of quiescent cells is less than 2 microns/h. A mixture of epidermal growth factor (EGF) and vasopressin, in the presence of insulin, is known to be a potent promoter of proliferation in this system. We have found it to increase speed to 30 microns/h. Agents which stimulate the cellular level of cAMP are also known to be potent promoters of proliferation in the presence of insulin. We have found these agents to be inhibitors of locomotion; several cycles of cell division take place while the cells move at a speed no greater than that seen in the presence of cytochalasin B (CB) or colchicine. These findings therefore give further support to the idea that there may be two different classes of growth-promoting factors, with major differences in their mode of action. They show that some members of these two different classes have opposite effects on motility.