Induction of cyclic AMP synthesis by forskolin is followed by a reduction in the expression of c-myc messenger RNA and inhibition of 3H-thymidine incorporation in human fibroblasts

A rapid and versatile method for the isolation, purification and cryogenic storage of Schwann cells from adult rodent nerves

We herein developed a protocol for the rapid procurement of adult nerve-derived Schwann cells (SCs) that was optimized to implement an immediate enzymatic dissociation of fresh nerve tissue while maintaining high cell viability, improving yields and minimizing fibroblast and myelin contamination. This protocol introduces: (1) an efficient method for enzymatic cell release immediately after removal of the epineurium and extensive teasing of the nerve fibers; (2) an adaptable drop-plating method for selective cell attachment, removal of myelin debris, and expansion of the initial SC population in chemically defined medium; (3) a magnetic-activated cell sorting purification protocol for rapid and effective fibroblast elimination; and (4) an optional step of cryopreservation for the storage of the excess of cells. Highly proliferative SC cultures devoid of myelin and fibroblast growth were obtained within three days of nerve processing. Characterization of the initial, expanded, and cryopreserved cell products confirmed maintenance of SC identity, viability and growth rates throughout the process. Most importantly, SCs retained their sensitivity to mitogens and potential for differentiation even after cryopreservation. To conclude, this easy-to-implement and clinically relevant protocol allows for the preparation of expandable homogeneous SC cultures while minimizing time, manipulation of the cells, and exposure to culture variables.

Potential benefits of triethylamine as n-electron donor in the estimation of forskolin by electronic absorption and emission spectroscopy

Diterpenoid forskolin was isolated from Coleus forskolii. The electronic absorption and emission studies of forskolin were investigated in various solvents with an aim to improve its detection limits. The two chromophores present in the diterpenoid are not conjugated leading to the poor absorption and emission of UV light. The absorption and fluorescence spectra were solvent specific. In the presence of a monodentate ligand, triethylamine the detection of forskolin is improved by 3.63 times in ethanol with the fluorescence method and 3.36 times in DMSO by the absorption spectral method. The longer wavelength absorption maximum is blue shifted while the lower energy fluorescence maximum is red shifted in the presence of triethylamine. From the wavelength of fluorescence maxima of the exciplex formed between excited forskolin and triethylamine it is concluded that the order of reactivity of hydroxyl groups in the excited state forskolin is in the reverse order to that of the order of the reactivity of hydroxyl groups in its ground state.

Ccdc6 knock-in mice develop thyroid hyperplasia associated to an enhanced CREB1 activity

CCDC6 was originally identified upon rearrangement with RET in human thyroid papillary carcinomas generating the RET/PTC1 oncogene. We have previously reported that CCDC6 interacts with CREB1 and represses its transcriptional activity. Since the function of at least one allele of CCDC6 is lost following RET/PTC1 rearrangements, we aimed at the generation of mice, carrying a CCDC6 mutant gene. Previous studies suggested that the coiled-coil domain of CCDC6, mainly encoded by human exon 2, is required for the protein function. Therefore, we engineered a murine Ccdc6 construct, carrying a deletion of the exon 2, that was able to exert only a mild repression on CREB1 transcriptional activity, with respect to the wild type Ccdc6. Subsequently, we generated Ccdc6-ex2 knock-in mice. These mice developed thyroid hyperplasia associated with an enhanced CREB1 activity and an increased expression of the CREB-1 regulated genes. These results strongly support a CCDC6 promoting role, ascribed to its functional impairment, in the development of thyroid papillary carcinomas harboring the RET/PTC1 oncogene.

EPAC proteins transduce diverse cellular actions of cAMP

It has now been over 10 years since efforts to completely understand the signalling actions of cAMP (3'-5'-cyclic adenosine monophosphate) led to the discovery of exchange protein directly activated by cAMP (EPAC) proteins. In the current review we will highlight important advances in the understanding of EPAC structure and function and demonstrate that EPAC proteins mediate multiple actions of cAMP in cells, revealing future targets for pharmaceutical intervention. It has been known for some time that drugs that elevate intracellular cAMP levels have proven therapeutic benefit for diseases ranging from depression to inflammation. The challenge now is to determine which of these positive actions of cAMP involve activation of EPAC-regulated signal transduction pathways. EPACs are specific guanine nucleotide exchange factors for the Ras GTPase homologues, Rap1 and Rap2, which they activate independently of the classical routes for cAMP signalling, cyclic nucleotide-gated ion channels and protein kinase A. Rather, EPAC activation is triggered by internal conformational changes induced by direct interaction with cAMP. Leading from this has been the development of EPAC-specific agonists, which has helped to delineate numerous cellular actions of cAMP that rely on subsequent activation of EPAC. These include regulation of exocytosis and the control of cell adhesion, growth, division and differentiation. Recent work also implicates EPAC in the regulation of anti-inflammatory signalling in the vascular endothelium, namely negative regulation of pro-inflammatory cytokine signalling and positive support of barrier function. Further elucidation of these important signalling mechanisms will no doubt support the development of the next generation of anti-inflammatory drugs.

Cyclic AMP inhibits the proliferation of thyroid carcinoma cell lines through regulation of CDK4 phosphorylation

How cyclic AMP (cAMP) could positively or negatively regulate G1 phase progression in different cell types or in cancer cells versus normal differentiated counterparts has remained an intriguing question for decades. At variance with the cAMP-dependent mitogenesis of normal thyroid epithelial cells, we show here that cAMP and cAMP-dependent protein kinase activation inhibit S-phase entry in four thyroid carcinoma cell lines that harbor a permanent activation of the Raf/ERK pathway by different oncogenes. Only in Ret/PTC1-positive TPC-1 cells did cAMP markedly inhibit the Raf/ERK cascade, leading to mTOR pathway inhibition, repression of cyclin D1 and p21 and p27 accumulation. p27 knockdown did not prevent the DNA synthesis inhibition. In the other cells, cAMP little affected these signaling cascades and levels of cyclins D or CDK inhibitors. However, cAMP differentially inhibited the pRb-kinase activity and T172-phosphorylation of CDK4 complexed to cyclin D1 or cyclin D3, whereas CDK-activating kinase activity remained unaffected. At variance with current conceptions, our studies in thyroid carcinoma cell lines and previously in normal thyrocytes identify the activating phosphorylation of CDK4 as a common target of opposite cell cycle regulations by cAMP, irrespective of its impact on classical mitogenic signaling cascades and expression of CDK4 regulatory partners.

Growth factor-like action of lysophosphatidic acid: mitogenic signalling mediated by G proteins

Several classes of growth factors can be distinguished that act through different signal transduction pathways. One class is constituted by the peptide growth factors that bind to receptors with ligand-dependent protein tyrosine kinase activity. Another class of mitogens activates a phosphoinositide-specific phospholipase C via a receptor-linked G protein. An intriguing member of this class is lysophosphatidic acid (LPA). LPA mitogenicity is not dependent on other mitogens and is blocked by pertussis toxin. LPA evokes at least three separate signalling cascades: (i) activation of a pertussis toxin-insensitive G protein mediating phosphoinositide hydrolysis; (ii) release of arachidonic acid in a GTP-dependent manner, but independent of prior phosphoinositide hydrolysis; and (iii) activation of a pertussis toxin-sensitive Gi protein mediating inhibition of adenylate cyclase. The peptide bradykinin mimics LPA in inducing responses (i) and (ii), but fails to activate Gi and to stimulate DNA synthesis. Our results suggest that the mitogenic action of LPA occurs through Gi or a related pertussis toxin substrate and that, unexpectedly, the phosphoinositide hydrolysis pathway is neither required nor sufficient, by itself, for mitogenesis.

Effects of unidirectional permeability in asymmetric poly(DL-lactic acid-co-glycolic acid) conduits on peripheral nerve regeneration: Anin vitro andin vivo study

The high outflow permeability of the nerve conduit used to emit the drained waste generated from the traumatized host nerve stump is critical in peripheral nerve regeneration. Our earlier studies have established that asymmetric conduits fulfill the basic requirements for use as nerve guide conduits. In this study, the inflow characteristics of optimal nerve conduits were further examined using in vivo and in vitro trials. Various asymmetric poly(DL-lactic acid-co-glycolic acid) (PLGA) conduits were controlled by modifying precipitation baths using 0, 20, and 95% isopropyl alcohol, with high-porosity (permeability), medium-porosity (high outflow and low inflow), and low-porosity (permeability), respectively. In the in vitro trial, the Schwann cells and fibroblasts were seeded on either side of the asymmetric PLGA films in a newly designed coculture system that simulated the repaired nerve conduit environment. The results of the directional permeable films indicated the statistically significant proliferation of Schwann cells and the inhibition of the division of fibroblasts in lactate dehydrogenase release and inhibition of 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl-tetrazolium bromide (MTT) reduction, compared with the other films. In the in vivo trial, the PLGA conduits seeded with Schwann cells were implanted into 10 mm right sciatic nerve defects in rats. After 6 weeks, implanted conduits were harvested. Histological examination verified that directional permeable conduits had markedly more A-type and B-type myelin fibers in the midconduit and distal nerve. In this work, the directional transport characteristics were established as an extremely important factor to the design and development of optimal nerve guide conduits in peripheral nerve regeneration.

Biphasic regulation by bile acids of dermal fibroblast proliferation through regulation of cAMP production and COX-2 expression level

We have previously reported that the bile acids chenodeoxycholate (CDCA) and ursodeoxycholate (UDCA) decreased PGE1-induced cAMP production in a time- and dose-dependent manner not only in hepatocytes but also in nonhepatic cells, including dermal fibroblasts. In the present study, we investigated the physiological relevance of this cAMP modulatory action of bile acids. PGE1 induced cAMP production in a time- and dose-dependent manner. Moreover, PGE1 (1 microM), forskolin (1-10 microM), and the membrane-permeable cAMP analog CPT-cAMP (0.1-10 microM) decreased dermal fibroblast proliferation in a dose-dependent manner with a maximum inhibition of approximately 80%. CDCA alone had no significant effect on cell proliferation at a concentration up to 25 microM. However, CDCA significantly reduced PGE1-induced cAMP production by 80-90% with an EC(50) of approximately 20 microM. Furthermore, at concentrations < or =25 microM, CDCA significantly attenuated the PGE-1-induced decreased cell proliferation. However, at concentrations of 50 microM and above, while still able to almost completely inhibit PGE-1-induced cAMP production, CDCA, at least in part through an increased cyclooxygenase-2 (COX-2) expression level and PGE2 synthesis, produced a direct and significant decrease in cell proliferation. Indeed, the CDCA effect was partially blocked by approximately 50-70% by both indomethacin and dexamethasone. In addition, overexpression of COX-2 cDNA wild type resulted in an increased efficacy of CDCA to block cell proliferation. The effects of CDCA on both cAMP production and cell proliferation were similar to those of UDCA and under the same conditions cholate had no effect. Results of the present study underline pathophysiological consequences of cholestatic hepatobiliary disorders, in which cells outside of the enterohepatic circulation can be exposed to elevated bile acid concentrations. Under these conditions, low bile acid concentrations can attenuate the negative hormonal control on cell proliferation, resulting in the stimulation of cell growth, while at high concentrations these bile acids provide for a profound and prolonged inhibition of cell proliferation.

A new method of selecting Schwann cells from adult mouse sciatic nerve

We describe a method of using laminin for the selection and purification of Schwann cells in vitro. We also studied the viability of the selected cells suspended in alginate beads both in vitro and in vivo. We observed that the homogeneity of the Schwann cell culture increased with each round of laminin selection and reached 85-90% after five passages. The viability of cells after incubation within an alginate bead in vivo was between 73 and 76% compared with greater than 90% viability for cells that were maintained in monolayer culture. This new method of serial selection using laminin-coated surfaces has optimized the purification of a Schwann cell culture expanded from cells harvested from the adult sciatic nerve of a mouse. This method has the advantage of being technically easier than other methods described and results in a Schwann cell culture that is 80-90% homogenous.

The expression of the thyroid-stimulating hormone (TSH) receptor and the cAMP-dependent protein kinase RII beta regulatory subunit confers TSH-cAMP-dependent growth to mouse fibroblasts

TSH activates its specific receptor in thyroid cells and induces cAMP, a robust stimulator of thyroid cell proliferation. Conversely, cAMP is a potent inhibitor of growth in mouse fibroblasts. To dissect the signals mediating cAMP-dependent growth, we have expressed in mouse fibroblasts the human thyrotropin receptor (TSHR) or a constitutively active mutant, under the control of the tetracyclin promoter. Both TSHR and cAMP levels were modulated by tetracyclin. In the presence of serum, activation of TSHR by TSH induced growth arrest. In the absence of serum, cells expressing TSHR stimulated with TSH, replicated their DNA, but underwent apoptosis. Co-expression of cAMP-dependent protein kinase (PKA) regulatory subunit type II (RIIbeta) inhibited apoptosis and stimulated the growth of cells only in the presence of TSH. Expression of RIIbeta-PKA, in the absence of TSHR, induced apoptosis, which was reversed by cAMP. Growth, stimulated by TSHR-RIIbeta-PKA in mouse fibroblasts, was also dependent on Rap1 activity, indicating cAMP-dependent growth in thyroid cells. As for the molecular mechanism underlying these effects, we found that in normal fibroblasts, TSH induced AKT and ERK1/2 only in cells expressing TSHR and RII. Similarly, activation of TSHR increased cAMP levels greatly, but was unable to stimulate CREB phosphorylation and transcription of cAMP-induced genes in the absence of RII. These data provide a simple explanation for the anti-proliferative and proliferative effects of cAMP in different cell types and indicate that RII-PKAII complements TSHR action by stably propagating robust cAMP signals in cell compartments.

Differential expression of membrane and soluble adenylyl cyclase isoforms in cytotrophoblast cells and syncytiotrophoblasts of human placenta

Adenylyl cyclase (AC) activity is ubiquitous in mammalian cells, and various forms of this enzyme exist that widely differ with regard to tissue distribution, abundance, and modes of regulation. Human placenta is made, among others, of cytotrophoblast cells and syncytiotrophoblasts. This latter is a polynucleate structure that originates from the differentiation of proliferative mononucleated cytotrophoblast cells, the placental stem cell, into syncytiotrophoblasts. In vitro, this differentiation process is associated with a concomitant increase in cellular levels of cAMP and enhanced expression of genes representative of syncytiotrophoblasts endocrine activity. Thus, in this study we evaluated the differential distribution of AC isoforms in cytotrophoblast cells and syncytiotrophoblasts by reverse transcription-polymerase chain reaction (RT-PCR) using total RNA or purified mRNA. Our results demonstrate that all membrane and soluble AC mRNA isoforms are present in both cell types. Interestingly in syncytiotrophoblasts, AC4 and AC8 mRNA are highly expressed, while AC1, AC2 mRNA are less abundant when compared to cytotrophoblast cells. Additionally, the soluble AC is expressed in both trophoblast cells, but its expression is greatly reduced in differentiated cells, syncytiotrophoblasts. The presence of these AC proteins in both cells was confirmed by Western blotting. Taken together, these data help us to characterize the different AC isoforms in human cytotrophoblast cells and syncytiotrophoblasts, and demonstrate that the AC isoforms expression seems to be mainly modulated in groups 1 and 2. Moreover, the important decrease of the soluble AC isoform in syncytiotrophoblasts as compared to cytotrophoblast cells could suggest an important role of this AC in the extravillous trophoblast formation.

In vitro and in vivo inhibition of SK-N-MC neuroblastoma growth using cyclic nucleotide phosphodiesterase inhibitors

The effect of cyclic nucleotide phosphodiesterase (PDE) inhibitors Zaprinast and DC-TA-46 has been tested on SK-N-MC neuroblastoma growth. Antiproliferative activity of the tested drugs was assayed both in vitro and in the xenograft model of nude mice. In clonal density experiments, the IC50 value was 3.3 microM for Zaprinast and 1.9 microM for DC-TA-46, while 7.5 microM BCNU alkylating agent was required to obtain the same effect. SK-N-MC cells xenografted in the nude mouse showed that the administration of Zaprinast and DC-TA-46 caused a significant 50% decrease of the tumour weight. These data demonstrate that PDE inhibitors may be useful for at least reducing tumour growth; they may be of interest for further evaluation as alternative molecules in the design of multiple agent protocols for neuroblastoma treatment.

Platelet-derived growth factor stimulates phospholipase C-gamma 1, extracellular signal-regulated kinase, and arachidonic acid release in rat myometrial cells: contribution to cyclic 3',5'-adenosine monophosphate production and effect on cell proliferation

In the present study, we examined downstream signaling events that followed exposure of cultured rat myometrial cells to platelet-derived growth factor (PDGF) and their effect on cell proliferation. PDGF-BB induced tyrosine phosphorylation of PDGF-beta receptors and increased inositol trisphosphate production via the tyrosine phosphorylation of phospholipase (PL)C-gamma 1. PDGF-BB also increased cAMP synthesis. This increase was potentiated by forskolin and reduced by indomethacin, a cyclooxygenase inhibitor, reflecting a Gs protein-mediated process via prostaglandin biosynthesis. The prostaglandin produced by PDGF was characterized as prostacyclin (PGI(2)). PDGF-BB increased arachidonic acid (AA) release, which, similarly to cAMP accumulation, was abolished in the presence of AACOCF3, a cytosolic PLA(2) inhibitor, and in the absence of Ca(2+). U-73122, a potent inhibitor of PLC activity, blocked both the production of inositol phosphates and the AA release triggered by PDGF-BB. Extracellular signal-regulated kinases (ERKs) 1 and 2 are expressed in myometrial cells, and PDGF-BB selectively activated ERK2. PD98059, an inhibitor of the ERK-activating kinase, blocked PDGF-BB-mediated ERK2 activation, AA release, and cAMP production. The results demonstrate that PDGF-BB stimulated cAMP formation through both PLC activation and ERK-dependent AA release and PGI(2) biosynthesis. PDGF-BB also increased cell proliferation and [(3)H]thymidine incorporation. This was abolished by PD98059, demonstrating that the ERK cascade is required for the mitogenic effect of PDGF-BB. Forskolin, which potentiated the cAMP response to PDGF-BB, attenuated both DNA synthesis and ERK activation triggered by PDGF-BB, suggesting the presence of a negative feedback regulation.

Labdane type diterpenes down-regulate the expression of c-Myc protein, but not of Bcl-2, in human leukemia T-cells undergoing apoptosis

Sclareol (1) and ent-3beta-hydroxy-13-epi-manoyl oxide (2) belong to the labdane type diterpenes. They were isolated from the leaves and from the fruits of Cistus creticus subsp. creticus, and were found to be active against human leukemic cell lines. Compound 2 was converted to its thiomidazolide derivative (3). Compounds 1 and 3 were found to induce apoptotic cell death in human T-cell leukemia lines and to interfere with their cell cycle, arresting cells at G(0/1) phase. Apoptosis can involve the activation and/or suppression of critical genes such as c-myc whose reduction or its inappropriate expression can be associated with induction of cell death and bcl-2 whose activation prevents apoptosis in the latter case. In order to detect any concomitant effect (1 and 3) upon c-myc and bcl-2 oncogene expression, we performed Western blot analysis to determine the levels of expression of these two genes upon treatment with the above compounds. Western blot analysis showed that of c-myc proto-oncogene levels were markedly reduced before massive apoptosis ensued in H33AJ-JA1 and MOLT3 cells, while bcl-2 expression remained unaffected. Thus, induction of apoptosis due to compounds 1 and 3 in these T-cell leukemic cell lines is preceded by c-myc down regulation and furthermore sustained bcl-2 expression does not rescue cells from apoptosis under the conditions used.

C1q arrests the cell cycle progression of fibroblasts in G(1) phase: role of the cAMP/PKA-I pathway

C1q may participate in the loss of connective tissue occurring in chronic inflammatory lesions. The hypothesis of a detrimental role of C1q on cell proliferation was tested on primary cultures of human fibroblasts (HFs). C1q suppressed the DNA synthesis of HF in response to platelet-derived growth factor (PDGF) with an IC(50) of 20 microg/ml, and blocked 78% of the cycling cells in G(1) phase. The C1q block did not involve production of inhibitory prostaglandin by the cells. Given that C1q elicits signals of the adenylyl cyclase pathway in HF, we examined cAMP-dependent mechanisms to understand how C1q inhibited the PDGF response. Whereas the C1q block was enhanced by agonist dibutyryl-adenosine 3', 5'-cyclic mono-phosphate (db-cAMP), antagonist adenosine 3', 5'-cyclic monophosphorotioate triethylammonium salt (Rp-cAMP) minimized it. C1q increased the level of cAMP-dependent protein kinase I (PKA-I) 4.5-fold, without altering the activation of the extracellular-regulated protein kinase (ERK) pathway. These results demonstrate that the interactions of C1q with HF cause growth arrest at the G(1) phase through mechanisms associated with a PKA-I dependent pathway.

Viability of cultured nerve grafts: An assessment of proliferation of Schwann cells and fibroblasts

Previous studies demonstrated that the viability of nerve grafts had a positive effect on nerve regeneration, while the cold storage of nerve grafts obtained few viable cells at the later stage. The purpose of this study was to examine the cellular activities of Schwann cells and fibroblasts in cultured nerve grafts prior to transplantation. 2.5-cm long sciatic nerve grafts were harvested from 75 male Lewis rats. Two different media were utilized to culture the nerve grafts up to 3 weeks: Dulbecco's modified eagle medium (DMEM) only or DMEM supplemented with 2 microM forskolin and 10 microg/ml pituitary exact (mitogen medium for Schwann cells). In vivo predegenerated and normal nerve grafts were used as positive and negative controls, respectively. We employed a 5-bromo-2'-deoxyuridine (BrdU) incorporation method to evaluate the proliferating cells in the cultured nerve grafts. S-100 and vimentin immunostaining were used to estimate the presence of Schwann cells and fibroblasts in all nerve grafts at different intervals. The results showed that the proliferating cells increased progressively under culture conditions. The proliferating cells distributed evenly in small fascicles (average diameter 251 +/- 71.5 microm), whereas they appeared mainly in the margin of large fascicles (average diameter 624 +/- 87.3 microm). The mitogen medium stimulated Schwann cell multiplication more significantly in comparison with DMEM after 3 days of culture (P < 0.01), however, there were fewer fibroblasts present in the mitogen medium than in DMEM after 2 days of culture (P < 0.01). It is suggested that the viability of nerve grafts can be preserved under culture conditions. Furthermore, the cellular activity of the Schwann cells and fibroblasts in nerve grafts can be manipulated in in vitro Wallerian degeneration.

Immediate early gene expression in dog thyrocytes in response to growth, proliferation, and differentiation stimuli

In dog thyroid cells, insulin or IGF-1 induces cell growth and is required for the mitogenic action of TSH through cyclic AMP, of EGF, and of phorbol esters. HGF per se stimulates cell proliferation and is thus the only full mitogenic agent. TSH and cAMP enhance, whereas EGF phorbol esters and HGF repress differentiation expression. In this study, we have investigated for each factor and regulatory cascade of the intermediate step of immediate early gene induction, that is, c-myc, c-jun, jun D, jun B, c-fos, fos B, fra-1, fra-2, and egr1; fra-1 and fra-2 expressions were very low. TSH or forskolin increased the levels of c-myc, jun B, jun D, c-fos, and fos B while decreasing those of c-jun and egr1. Phorbol myristate ester stimulated the expression of all the genes. EGF and HGF stimulated the expression of all the genes except jun D and for EGF fos B. All these effects were obtained in the presence and in the absence of insulin, which shows that insulin is not necessary for the effects of the mitogens on immediate early gene expression. The definition of the repertoire of early immediate genes inductible by the various growth cascades provides a framework for the analysis of gene expression in tumors. (1) Insulin was able to induce all the protooncogenes investigated except fos B. This suggests that fos B could be the factor missing for insulin to induce mitogenesis. (2) No characteristic pattern of immediate early gene expression has been observed for insulin, which induces cell hypertrophy and is permissive for the action of the other growth factors. These effects are therefore not accounted for by a specific immediate early gene expression. On the other hand, insulin clearly enhances the effects of TSH, phorbol ester, and EGF on c-myc, junB, and c-fos expression. This suggests that the effect of insulin on mitogenesis might result from quantitative differences in the transcription complexes formed. (3) c-myc, c-fos, and jun B mRNA induction by all stimulating agents, whether inducing cell hypertrophy, or growth and dedifferentiation, or growth and differentiation, suggests that, although these expressions are not sufficient, they may be necessary for the various growth responses of thyroid cells. (4) The inhibition of c-jun and egr1 mRNA expression, and the marked induction of jun D mRNA appear to be specific features of the TSH cAMP pathway. They might be related to its differentiating action. (5) fos B, which is induced by TSH, forskolin, phorbol ester, and HGF but not by insulin, could be involved in the mitogenic action of the former factors.

Effects of adrenomedullin on cultured rat cardiac myocytes and fibroblasts

The direct effects of adrenomedullin, a novel vasorelaxant peptide, on protein synthesis and atrial natriuretic peptide release in myocytes and on DNA and collagen syntheses in fibroblasts were examined using cultured ventricular cardiocytes. The protein synthesis of cardiac myocytes was not affected by adrenomedullin under non-stimulated conditions. Endothelin-1-induced protein synthesis in myocytes was slightly but significantly elevated by adrenomedullin. Likewise, the secretion of atrial natriuretic peptide from myocytes stimulated by endothelin-1 was increased by adrenomedullin. In cardiac fibroblasts, adrenomedullin clearly inhibited DNA synthesis and collagen production in a dose-dependent manner under both basal and angiotensin II-stimulated conditions. DNA and collagen syntheses by cardiac fibroblasts were suppressed by both 8-bromo cAMP and forskolin. Furthermore, a cAMP-specific phosphodiesterase inhibitor decreased DNA and collagen syntheses in fibroblasts and enhanced the inhibitory effects of adrenomedullin on these syntheses. Our observations suggest that adrenomedullin has opposite effects on cultured cardiac myocytes and fibroblasts and that the effects of adrenomedullin at least on fibroblasts are probably mediated through a cAMP-dependent pathway. As adrenomedullin is produced and secreted from both types of cardiac cells, adrenomedullin may play a role as an autocrine/paracrine modulator in the process of cardiac remodeling, mainly by suppressing mitogenesis and collagen synthesis in fibroblasts.

Identity of adenylyl cyclase isoform determines the rate of cell cycle progression in NIH 3T3 cells

Cell cycle progression is regulated by cAMP in several cell types. Cellular cAMP levels depend on the activity of different adenylyl cyclases (ACs), which have varied signal-receiving capabilities. The role of individual ACs in regulating proliferative responses was investigated. Native NIH 3T3 cells contain AC6, an isoform that is inhibited by a variety of signals. Proliferation of exogenous AC6-expressing cells was the same as in control cells. In contrast, expression of AC2, an isoform stimulated by protein kinase C (PKC), resulted in inhibition of cell cycle progression and increased doubling time. In AC2-expressing cells, platelet-derived growth factor (PDGF) elevated cAMP levels in a PKC-dependent manner. PDGF stimulation of mitogen-activated protein kinases 1 and 2 (MAPK 1,2), DNA synthesis, and cyclin D1 expression was reduced in AC2-expressing cells as compared with control cells. Dominant negative protein kinase A relieved the AC2 inhibition of PDGF-induced DNA synthesis. Expression of AC2 also blocked H-ras-induced transformation of NIH 3T3 cells. These observations indicate that, because AC2 is stimulated by PKC, it can be activated by PDGF concurrently with the stimulation of MAPK 1,2. The elevation in cAMP results in inhibition of signal flow from the PDGF receptor to MAPK 1,2 and a significant reduction in the proliferative response to PDGF. Thus, the molecular identity and signal receiving capability of the AC isoforms in a cell could be important for proliferative homeostasis.

Increased c-fos mRNA expression by human fibroblasts contracting stressed collagen matrices

We studied early changes in gene expression during fibroblast contraction of stressed collagen matrices. The level of c-fos mRNA increased dramatically and peaked 50 to 60 min after matrix contraction was initiated. This response did not require serum and could not be accounted for simply by disruption of the actin cytoskeleton. Increased c-fos mRNA levels required Ca2+ influx but not the cyclic AMP or extracellular signal-regulated kinase (ERK 1/2) signaling pathways, both of which are activated when fibroblasts contract stressed collagen matrices. The levels of two other immediate-early genes, fosb and c-jun, also increased transiently after fibroblast contraction, whereas the levels of fra-1, fra-2, c-myc, and the transcription factor NF-kappaB remained the same, indicating that fibroblast contraction caused changes in a selective group of genes. The increase in c-fos mRNA during contraction of stressed collagen matrices may reflect a unique role for c-fos in mechanoregulated events at the end of wound repair.

Cyclic AMP-mediated inhibition of angiotensin II-induced protein synthesis is associated with suppression of tyrosine phosphorylation signaling in vascular smooth muscle cells

In the present study, we have examined the effect of increased cyclic AMP (cAMP) levels on the stimulatory action of angiotensin II (Ang II) on protein synthesis. Treatment with cAMP-elevating agents potently inhibited Ang II-induced protein synthesis in rat aortic smooth muscle cells and in rat fibroblasts expressing the human AT1 receptor. The inhibition was dose-dependent and was observed at all concentrations of the peptide. To explore the mechanism of cAMP action, we have analyzed the effects of forskolin and 3-isobutyl-1-methylxanthine on various receptor-mediated responses. Elevation of cAMP did not alter the binding properties of the AT1 receptor and did not interfere with the activation of phospholipase C or the induction of early growth response genes by Ang II. Likewise, Ang II-dependent activation of the mitogen-activated protein kinases ERK1/ERK2 and p70 S6 kinase was unaffected by cAMP. In contrast, we found that increased concentration of cAMP strongly inhibited the stimulatory effect of Ang II on protein tyrosine phosphorylation. Specifically, cAMP abolished Ang II-induced tyrosine phosphorylation of the focal adhesion-associated protein paxillin and of the tyrosine kinase Tyk2. These results identify a novel mechanism by which the cAMP signaling system may exert growth-inhibitory effects in specific cell types.

1,25-dihydroxyvitamin D3 and agents that increase intracellular adenosine 3',5'-monophosphate synergistically inhibit fibroblast proliferation

Agents that increase intracellular cAMP (cAMP elevating agents) and 1, 25(OH)2D3 inhibit the proliferation of many cell types. We investigated the combined effect of 1,25(OH)2D3 and cAMP elevating agents on exponentially growing mouse 3T3 fibroblasts. The following cAMP elevating agents were used: theophylline and pentoxyfilline, which inhibit cAMP-dependent phosphodiesterase; prostaglandin E2 which activates adenylate cyclase by a receptor-mediated mechanism; forskolin, which directly stimulates adenylate cyclase; and the cell permeable cAMP analogs 8-bromo cAMP and N6 benzoyl cAMP. 1,25(OH)2D3 and cAMP elevating agents were added to exponentially growing fibroblasts cultured in 96-well microtiter plates and cell number was monitored 3-7 d later. 1,25(OH)2D3 and the cAMP elevating agents as single agents inhibited the growth of the 3T3 cells. The combined treatment of the fibroblasts with 1,25(OH)2D3 and the cAMP elevating agents resulted in an antiproliferative effect that was more than additive. The synergistic interaction depended on the dose of 1,25(OH)2D3 and was apparent already at 10(-8) M of the hormone. The specificity of the effect of 1,25(OH)2D3 was demonstrated by the finding that 24,25-dihydroxyvitamin D3, a vitamin D metabolite with low affinity for the vitamin D receptor, did not affect the antiproliferative effect of cAMP elevating agents. From the synergistic interaction between 1,25(OH)2D3 and the cell permeable cAMP analogs, we infer that the site of interaction between the two signaling pathways is distal to the cAMP generating and degrading machinery.

cAMP suppresses p21ras and Raf-1 responses but not the Erk-1 response to granulocyte-colony-stimulating factor: possible Raf-1-independent activation of Erk-1

The cAMP analogue 8-bromo-cAMP (8BrcAMP) inhibits granulocyte-colony-stimulating factor (G-CSF)-stimulated DNA synthesis in myeloid NFS-60 cells. We examined the effect of 8BrcAMP addition on the G-CSF-stimulated extracellular signal-related protein kinase 1 (Erk-1), p21ras and Raf-1 activation. The Erk-1 activity was not down-regulated by the increase in intracellular cAMP levels, whereas p21ras and Raf-1 activities were, suggesting that Erk-1 activity might not be dependent on upstream p21ras and/or Raf-1 activity in this system. To explore this possibility further, we sought to determine whether there were downstream substrates of Raf-1 that were distinguishable from those of Erk-1 by using two-dimensional SDS/PAGE analysis of the protein phosphorylation patterns of NFS-60 cell cytosolic extracts treated with exogenous Raf-1 or Erk-1 in the presence of [gamma-32P]ATP. The two phosphorylation patterns were found to have many differences. To gain further insights into the possible relevance of these phosphorylation patterns and as an approach to exploring in more detail the inhibitory effect of 8BrcAMP, two-dimensional SDS/PAGE analysis was performed on the cytosolic extracts of 32P-labelled NFS-60 cells treated with G-CSF, in the absence or presence of 8BrcAMP. It was found that the phosphorylated proteins whose appearance was specific to the action of exogenous Raf-1 were sensitive to the action of 8BrcAMP in vivo, whereas those whose appearance was specific to the action of exogenous Erk-1 alone, or common to the actions of Raf-1 and Erk-1, were 8BrcAMP-insensitive. The results are consistent with a Raf-1-independent pathway for Erk-1 activation in G-CSF treated myeloid cells, and a number of potential downstream substrates of these kinases have been identified for further characterization.

Decreased growth rate and tumour formation of human anaplastic thyroid carcinoma cells transfected with a human thyrotropin receptor cDNA in NMRI nude mice treated with propylthiouracil

The effect of the human TSH-receptor (TSHR) on the growth of human anaplastic thyroid carcinoma cells lacking the endogenous expression of TSHR, was studied both in vitro and in vivo in NMRI nude mice. Cells from a human anaplastic thyroid carcinoma cell line (C643) were transfected with a TSHR cDNA, and clones were isolated after neomycin selection. The expression of a functional receptor protein was ensured by analysis of the specific binding of 125I-TSH and measurement of TSH-induced cAMP. Incorporation of [3H]thymidine and increase in cell number was slightly inhibited by TSH in TSHR-expressing cells in vitro. In order to investigate whether the regained expression of a functional TSHR protein in C643 cells could influence the in vivo growth, cells were injected subcutaneously into NMRI nude mice. To manipulate the endogenous level of TSH, animals were given 6n-propyl-2-thiouracil (PTU; resulting in a high TSH level), T4 (a low TSH level) or no treatment (as a control). There seemed to be a TSH induced inhibition of tumour growth, since tumours in mice treated with PTU grew after a longer take rate and with a slower growth rate. The present results suggest a TSH-mediated growth inhibition in the TSHR-transfected C 643 anaplastic thyroid carcinoma cells.

Paradoxical accumulation of the cyclin-dependent kinase inhibitor p27kip1 during the cAMP-dependent mitogenic stimulation of thyroid epithelial cells

In different systems, cAMP either blocks or promotes cell cycle progression in mid to late G1 phase. Dog thyroid epithelial cells in primary culture constitute a model of positive control of DNA synthesis initiation and G0-S pre-replicative phase progression by cyclic AMP (cAMP) as a second messenger for thyrotropin (TSH). We report here that TSH markedly increases the expression of p27kip1, the inhibitor of the cell cycle and cyclin-dependent kinases. This effect was prevented by the concomitant administration of the cAMP-independent mitogens, epidermal growth factor (EGF)+serum. EGF+serum also slightly inhibited the weak basal accumulation of p27kip1. Nevertheless, in the case of stimulation by TSH alone, the cAMP-dependent cell cycle progression was fully compatible with the enhanced expression of p27kip1. This observation is paradoxical since a decrease of p27kip1 is generally associated with growth stimulation in other systems, and since a similar cAMP-dependent increase of p27kip1 in macrophages has been found responsible for mid-G1 cell cycle arrest. The opposite regulation of p27kip1 in response to TSH or EGF+serum in dog thyroid epithelial cells suggests a major difference at mid to late G1 stages between cAMP-dependent and cAMP-independent mitogenic pathways.

c-Myc expression is controlled by the mitogenic cAMP-cascade in thyrocytes

In dog thyroid epithelial cells in primary culture, thyrotropin (TSH), acting through cAMP, induces proliferation and differentiation expression, whereas epidermal growth factor (EGF) and phorbol esters induce proliferation and dedifferentiation. In these cells, we have detailed the regulation by cAMP of the c-myc protooncogene mRNA and protein. The cAMP signaling pathway induces a biphasic increase of c-myc mRNA and protein. c-Myc protein accumulation follows the abundance and kinetics of its mRNA expression. Using in vitro elongation of nascent transcripts to measure transcription and actinomycin D (AcD) chase experiments to study mRNA stability, we have shown that in the first phase cAMP releases a transcriptional elongation block. No modification of transcriptional initiation was observed. After 30 min of treatment with TSH, c-myc mRNA was also stabilized. During the second phase, cAMP stabilization of the mRNA disappears and transcription is again shut off. Thus, in a tissue in which it stimulates proliferation and specific gene expression, cAMP regulates biphasically c-myc expression by mechanisms operating at the transcriptional and posttranscriptional levels.

Effect of adenosine 3':5'-cyclic monophosphate and inhibition of protein kinase A on heat sensitivity in H35 hepatoma cells

PURPOSE

To investigate the role of the cyclic adenosine 3':5'-monophosphate (AMP) signal transductions pathway in heat-induced cell death and the development of thermotolerance.

METHODS AND MATERIALS

Reuber H35 rat hepatoma cells were heated after preincubation with various compounds known to modulate the cyclic AMP signal transduction pathway. Cell survival was determined by colony-forming ability.

RESULTS

Preincubation of H35 cells with forskolin, a stimulator of adenylate cyclase, in combination with IBMX (3-isobutyl-1-methylxanthine), an inhibitor of cyclic AMP phosphodiesterase, results in thermosensitization. Similar results are obtained with various cyclic AMP analogs. Maximum thermosensitization occurs with 0.5 mM dibutyryl cyclic AMP (DBcAMP) after a preincubation period of 5 h and heating in the presence of the drug. The same relative degree of thermosensitization is found with 8-Cl-cAMP, but at a 10-fold lower concentration. Thermosensitization by DBcAMP is prevented by H89, a specific inhibitor of cyclic AMP-dependent protein kinase (PKA). Without additional cyclic AMP-inducing factors, H89 induces thermoprotection. None of the drug treatments are cytotoxic at 37 degrees C. DBcAMP does not affect the development of heat-induced thermotolerance but it reduces its expression to an extent similarly found in the observed thermosensitization in nonthermotolerant cells.

CONCLUSION

The results strongly indicate that the cyclic AMP signal transduction pathway is involved in the process of heat-induced cell death. DBcAMP reduces the expression of thermotolerance, but does not affect its induction.

Mitogen induced proliferation of isolated adult mouse Schwann cells

The proliferation of neonatal Schwann cells (SCs) in response to mitogenic agents has been well analyzed in vitro, but a limited range of mitogens have been defined. We investigated whether three identified neonatal SC mitogens [glial growth factor (GGF), platelet-derived growth factor BB (PDGF-BB), and basic fibroblast growth factor (bFGF)] are required to stimulate mitosis of adult SCs. Adult SCs were isolated from mouse sciatic nerves by mechanical and chemical dissociation, following three experimental steps: 1) culturing the dissociated cells for 24 hr in 10% FCS-F12 medium, 2) culturing these cells in serum-free medium for the next 48 hr, and 3) purifying adult SCs by differential adhesion. We describe a new method for preparation of SCs from peripheral nerves of adult mouse that provides 99.5% pure SCs populations at cell yields of greater than 3 x 10(3) cells/mg of starting nerve wet weight within 5 culture days. Although mitosis of SCs in culture in response to mitogens requires the presence of serum, the complex nature of serum renders difficult a complete analysis of mitogens required for SCs DNA synthesis, so we examined the proliferating response of adult SCs to GGF, PDGF-BB, and bFGF in serum-free medium. GGF alone had mitogenicity for adult SCs in a dose-dependent manner, and synergistic activation coupling with forskolin was not observed. Neither PDGF-BB nor bFGF was mitogenic for adult SCs when used alone or with forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)

Perturbation of the platelet-derived growth factor receptor signaling by dibutyryl-cAMP in human astrocytoma cells

It has been shown that cAMP may perturb the polypeptide growth factor-induced nuclear events. However, the possible interactions of the cAMP-protein kinase A (cAMP-PKA) and receptor tyrosine kinase pathways in the cytosol have not been fully elucidated. In this study, we use human astrocytoma cells as a model to investigate this issue. The results show that platelet-derived growth factor (PDGF)-induced receptor autophosphorylation in human astrocytoma cells is suppressed by dibutyryl-cAMP pretreatment and such suppression is not due to changes in the ligand-receptor binding properties. Further studies show that PDGF-induced tyrosine phosphorylation of phospholipase C-gamma 1 (PLC-gamma 1) and phosphatidylinositol 3-kinase (PI 3-kinase) are also suppressed in dibutyryl-cAMP-pretreated cells. The suppression of PLC-gamma 1 tyrosine phosphorylation was accompanied by a decreased production of water soluble inositol phosphates. In contrast, similar treatment with normal human astrocytes potentiates the tyrosine phosphorylation of PLC-gamma 1 and PI 3-kinase. The results indicate that cAMP can either negatively or positively modulate the PDGF receptor tyrosine kinase activity depending on the cell types examined.

Stimulation of hyaluronan biosynthesis by platelet-derived growth factor-BB and transforming growth factor-beta 1 involves activation of protein kinase C

The intracellular signal transduction pathways that mediate the stimulatory effects of platelet-derived growth factor (PDGF)-BB and transforming growth factor (TGF)-beta on hyaluronan biosynthesis in human fibroblasts were investigated. The stimulatory effects of both PDGF-BB and TGF-beta 1 were dependent on protein kinase C (PKC), since the PKC inhibitor calphostin C inhibited the stimulation by the growth factors. Direct activation of PKC by phorbol 12-myristate 13-acetate (PMA) also stimulated hyaluronan production, and the combination of either PDGF-BB or TGF-beta 1 and PMA gave an increased effect. One possible mechanism for activation of PKC is via induction of phospholipase C (PLC) activity; U-17322, an inhibitor of PLC-gamma, was found to inhibit partially PDGF-BB-stimulated hyaluronan synthesis. PDGF-BB is known to activate PLC-gamma through tyrosine phosphorylation; however, a PDGF beta-receptor mutant unable to interact with and activate PLC-gamma was still able to mediate induction of hyaluronan biosynthesis, indicating that PDGF-mediated stimulation is not entirely dependent on PLC-gamma. The stimulations by PDGF-BB and TGF-beta 1 were partly dependent on protein synthesis, since parts of the effects were inhibited by cycloheximide; in contrast, the effects mediated by PMA were not. Our results indicate that PKC is involved in the transduction of the effects of growth factors on hyaluronan biosynthesis, and that the effects involve direct or indirect activation of existing hyaluronan synthetase molecules, as well as induction of new enzyme molecules.

Antiproliferative effects of A02011-1, an adenylyl cyclase activator, in cultured vascular smooth muscle cells of rat

1. The effects of A02011-1, a pyrazole derivative, on the proliferation of rat vascular smooth muscle cells (VSMCs) were examined. 2. A02011-1 (1-100 microM) concentration-dependently inhibited [3H]-thymidine incorporation into DNA in rat VSMCs that were synchronized by 48 h serum depletion and then re-stimulated by addition of foetal calf serum (FCS, 10%), platelet-derived growth factor (PDGF, 10 ng ml-1), 5-hydroxytryptamine (10 microM) or ADP (10 microM). The inhibitory effect of A02011-1 was fully reversible. However, FCS-induced [3H]-thymidine incorporation into rat endothelial cells was unaffected by A02011-1. 3. The concentration of A02011-1 necessary for inhibition of the FCS-induced proliferation was similar to that necessary for adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation. Adenylyl cyclase activity was increased in A02011-1-treated VSMCs, whereas cyclic AMP-specific phosphodiesterase activity was unchanged. 4. A02011-1 was equipotent with forskolin but was more potent than 8-bromo-cyclic AMP against FCS (10%)-induced proliferation. 5. The antiproliferative action of A02011-1 was mimicked by 8-bromo-cyclic AMP, a membrane-permeable cyclic AMP analogue and was antagonized by 2',5'-dideoxyadenosine, an adenylyl cyclase inhibitor and by Rp-cyclic AMPS, a competitive inhibitor of cyclic AMP-dependent protein kinase (PKA) type I and II. 3-Isobutyl-1-methylxanthine (IBMX) caused significant potentiation of the antiproliferative activity of A02011-1. However, Rp-8-bromo-cyclic GMPS and staurosporine did not affect the antiproliferative activity of A02011-1. 6. A02011-1 still inhibited the FCS-induced DNA synthesis even when added 10-18h after restimulation of the serum-starved VSMCs with 10% FCS. Flow cytometry in synchronized cells revealed an acute blockade of FCS-inducible cell cycle progression at a point in the G,/S phase in A02011-1-treated cells. The inhibition of proliferation by A0201 1-1 was shown to be independent of cell damage,as documented by several criteria of cell viability.7. These results indicate that A0201 1-1 inhibition of VSMC proliferation was mediated by cyclic AMP and was due to a delay in the progression from the G1 into S phase of the cell cycle. A02011-1 did not cause cell toxicity and may thus hold promising potential for the prevention of atherosclerosis or vascular diseases.

Increased intracellular cyclic AMP levels suppress the mitogenic responses of human astrocytoma cells to growth factors

It has been shown that the intracellular cAMP levels were decreased in human malignant astrocytomas. On the other hand, various growth factors and their receptors were found to be overexpressed in these tumors. It is therefore intriguing as to whether there is interplay between the two phenomena in the modulation of the astrocytoma cell growth. In a basal medium consisting of 75% DMEM, 25% Ham's F-12 supplemented with 2% FBS, we show that the mitogenic effects of platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), and epidermal growth factor (EGF) on human astrocytoma cells were suppressed by dibutyryl-cAMP. Dibutyryl-cAMP alone neither potentiated nor inhibited the tumor cell growth. Further studies show that PDGF-induced receptor autophosphorylation in human astrocytoma cells is suppressed by increased intracellular cAMP levels as measured by immunoprecipitation with anti-PDGF receptor and antiphosphotyrosine antibodies. Our results indicate that there is antagonistic interplay between the receptor tyrosine kinase pathway and cAMP-dependent protein kinase pathway in the control of the malignantly transformed glial cells. A reduced cAMP level seen in many human astrocytoma cells may favor their response to growth factor mitogenesis.

Signal transduction in atherosclerosis: second messengers and regulation of cellular cholesterol trafficking

The data summarized in this review demonstrate that the regulation of intracellular cholesterol trafficking is mediated not only by extracellular lipoprotein concentrations and transcriptional responses to alterations in intracellular free cholesterol content. Rather, the modulation of cholesterol trafficking is also regulated by the products synthesized following activation of signal transduction pathways originating at the cell surface. Furthermore, we have identified those cell-derived factors which utilize these signal transduction pathways to elicit alterations in cholesterol trafficking, and demonstrated the importance of the generation of second messengers, most notably eicosanoids, and cyclic AMP in promoting a modulatory influence on specific pro-atherogenic effects of mitogens.

Lysophosphatidic acid and bradykinin have opposite effects on phenotypic transformation of normal rat kidney cells

The bioactive lipid lysophosphatidic acid is besides a strong mitogen for quiescent fibroblasts, a potent inducer of phenotypic transformation of normal rat kidney cells. The lysophosphatidic acid induced loss of density-arrest is strongly inhibited by bradykinin. Although their effects on normal rat kidney cell proliferation are opposite, bradykinin mimics many of the intracellular effects induced upon lysophosphatidic acid receptor activation, including phosphoinositide turnover, Ca(2+)-mobilization and arachidonic acid release. Bradykinin does not counteract the lysophosphatidic acid induced reduction of cAMP levels in normal rat kidney cells. However, bradykinin inhibits the lysophosphatidic acid and other growth factor induced phenotypic transformation through the induction of a so far uncharacterized prostaglandin G/H synthase product. The growth inhibitory effect of bradykinin is limited to density-arrested cells, while upon prolonged treatment bradykinin itself is capable to induce the loss of density-dependent growth control. It is concluded that bradykinin is a bifunctional regulator of normal rat kidney cell proliferation and that its inhibitory effects are mediated via the induction of a prostaglandin derivative.

Mitogenic effects of platelet-derived growth factor, fibroblast growth factor, transforming growth factor-beta, and heparin-binding serum factor for adult mouse Schwann cells

Mitogenic effects of fetal calf serum (FCS), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), transforming growth factor-beta (TGF-beta), and forskolin to adult mouse Schwann cells were examined by bromodeoxyuridine (BrdU) incorporation and double immunofluorescence for S100 and BrdU. PDGF-BB, basic FGF, and TGF-beta 1 and beta 2 were all mitogenic for Schwann cells in media containing FCS. Forskolin suppressed the mitogenic activity of these factors. In serum-free media, PDGF-BB and bFGF were also mitogenic, but TGF-beta 1 and beta 2 were not. Heparin-binding fractions of FCS obtained by heparin-Sepharose chromatography synergized with TGF-beta 1 and beta 2 to produce a mitogenic response. Since PDGF-BB, acidic FGF, and basic FGF were not detected in these fractions by immunoabsorption and immunoblot assays, the presence of unidentified heparin-binding molecules in FCS bioactive for adult mouse Schwann cells is suggested.

Beta-actin mRNA localization is regulated by signal transduction mechanisms

Beta-actin mRNA is localized in the leading lamellae of chicken embryo fibroblasts (CEFs) (Lawrence, J., and R. Singer. 1986. Cell. 45:407-415), close to where actin polymerization in the lamellipodia drives cellular motility. During serum starvation beta-actin mRNA becomes diffuse and non-localized. Addition of FCS induces a rapid (within 2-5 min) redistribution of beta-actin mRNA into the leading lamellae. A similar redistribution was seen with PDGF, a fibroblast chemotactic factor. PDGF-induced beta-actin mRNA redistribution was inhibited by the tyrosine kinase inhibitor herbimycin, indicating that this process requires intact tyrosine kinase activity, similar to actin filament polymerization and chemotaxis. Lysophosphatidic acid, which has been shown to rapidly induce actin stress fiber formation (Ridley, A., and A. Hall. 1992. Cell. 790:389-399), also increases peripheral beta-actin mRNA localization within minutes. This suggests that actin polymerization and mRNA localization may be regulated by similar signaling pathways. Additionally, activators or inhibitors of kinase A or C can also delocalize steady-state beta-actin mRNA in cells grown in serum, and can inhibit the serum induction of peripherally localized beta-actin mRNA in serum-starved CEFs. These data show that physiologically relevant extracellular factors operating through a signal transduction pathway can regulate spatial sites of actin protein synthesis, which may in turn affect cellular polarity and motility.

Cyclic AMP downregulates c-myc expression by inhibition of transcript initiation in human B-precursor Reh cells

In the human pre-B cell line Reh, activation of the cyclic AMP signal transduction pathway induces a rapid, transient 10-fold down-regulation of steady-state c-myc mRNA. We have investigated the mechanisms involved in this cAMP-mediated regulation of c-myc expression. Forskolin did not alter c-myc mRNA stability. Initiation of c-myc transcripts was strongly inhibited after 1 h of forskolin treatment, as measured by nuclear run-on assays. Reinitiation of c-myc transcription was apparent after 3-4 h, and full transcriptional elongation was detected after 8 h of forskolin treatment. These data suggest that cyclic AMP downregulates c-myc expression by inhibition of transcriptional initiation.

Protein kinase A antagonizes platelet-derived growth factor-induced signaling by mitogen-activated protein kinase in human arterial smooth muscle cells

Stimulation of aortic smooth muscle cells with platelet-derived growth factor BB homodimer (PDGF-BB) leads to the rapid activation of mitogen-activated protein kinase (MAPK) and MAPK kinase (MAPKK). Compounds that increase cAMP and activate protein kinase A (PKA)--prostaglandin E2, isoproterenol, cholera toxin, and forskolin--were found to inhibit the PDGF-BB-induced activation of MAPKK and MAPK. Forskolin, but not the inactive analogue 1,9-dideoxyforskolin, inhibited PDGF-BB-stimulated MAPKK and MAPK activation in a dose-dependent manner. PKA antagonism of MAPK signaling was observed at all doses of PDGF-BB or PDGF-AA. PKA did not inhibit MAPKK and MAPK activity in vitro, and MAPKK and MAPK from extracts of forskolin-treated cells could be activated normally with purified Raf-1 and MAPKK, respectively, suggesting that PKA blocked signaling upstream of MAPKK. Neither PDGF-BB-stimulated tyrosine autophosphorylation of the PDGF receptor beta subunit nor inositol monophosphate accumulation was affected by increased PKA activity, suggesting that PKA inhibits events downstream of the PDGF receptor. This study provides an example of cross talk between two important signaling systems activated by physiological stimuli in smooth muscle cells--namely, the PKA pathway and the growth factor-activated MAPK cascade.

Stimulation of interleukin-6 production by either calcitonin gene-related peptide or parathyroid hormone in two phenotypically distinct bone marrow-derived murine stromal cell lines

It was recently shown that interleukin-6 (IL-6) is produced by bone and bone marrow-derived stromal cells and that it plays an important role in osteoclast development. Here we examined whether parathyroid hormone (PTH), calcitonin (CT), or the calcitonin gene-related peptide (CGRP) influence IL-6 production by two murine bone marrow-derived stromal cell lines: the preadipocyte-like stromal cell line +/+ LDA11 and the fibroendothelial stromal cell line MBA 13.2. We found that CGRP (but not PTH or CT) exerted a dose-dependent increase in cAMP and IL-6 production in the +/+ LDA11 cells. In addition, CGRP had an inhibiting effect on the proliferation of this stromal cell line. CGRP, however, did not affect cAMP or IL-6 in the rat osteogenic sarcoma cell line UMR-106-06, which exhibits CT receptors, whereas CT stimulated both cAMP and IL-6 by the UMR-106-01 cells. In contrast to the specificity of the IL-6 response of the +/+ LDA11 cells to CGRP, IL-6 production by the MBA 13.2 stromal cells was stimulated by PTH whereas CGRP or CT had no effect. These data suggest that bone marrow-derived stromal cells express receptors for either CGRP or PTH in a phenotype-specific manner and that, acting via these receptors, CGRP and PTH stimulate IL-6 production by stromal cells. In addition, the evidence for specific receptors for the neuropeptide CGRP in bone marrow stromal cells and an effect of CGRP on IL-6 raises the possibility for a role of cytokines in a putative interplay between neuronal stimuli and bone.

Ruffling and locomotion: role in cell resistance to growth factor-induced proliferation

It has long been known that the growth rate of cells in vitro can be retarded by providing substrates of restricted area. Such experiments were performed with adhesive islets, made by depositing metals onto agarose layers through templates of various sizes. Since normal cells are unable to adhere to agarose, they become confined to the metallic surface. Using such haptotactic islets, we have studied the role of membrane ruffling and cell locomotion in the resistance of AG1523 human fibroblasts to growth factor-induced mitogenesis. Cells plated on small substrates, i.e., 2,150 microns 2 in area, initially showed vigorous ruffling, which was suppressed by 8 h after plating but had resumed again by 12 h. In contrast, cells on larger-size islets showed a rapid decline and stabilization of ruffling activity. When the growth rate was measured for single cells cultured on haptotactic islets, it was found to increase linearly from areas of 4,280 microns 2 up to 425,000 microns 2. Since the area needed to saturate the growth response was approximately 50-fold larger than the area occupied by a single cell, the growth inhibition was attributed in part to an interference with locomotion. The implication that locomotion provided positive input into growth control mechanisms was subjected to a direct test by evaluating the effect of nine polypeptide growth factors on the motility of serum-starved cells. All except TGF-beta 1 stimulated movement. Finally, the mitogenic effect of growth factors was measured by [3H]thymidine incorporation and found to be proportional to motile activities, as quantitatively assayed. We conclude that locomotion suppression is a factor in AG1523 cell resistance to growth factor-induced mitogenesis.

Selective culture of mitotically active human Schwann cells from adult sural nerves

We devised a simple method to isolate mitotically active human Schwann cells from sural nerve biopsy specimens and expand the population in culture. Nerve fascicles were treated with cholera toxin for 7 days in culture before dissociation, which increased the cell yield at least twenty-five-fold over immediated tissue dissociation. Digesting the tissue completely with enzymes in serum-containing medium resulted in the highest cell viability, and released 2 to 6 x 10(4) cells/mg of tissue. Seeding the cells on a poly-L-lysine substrate in a small volume of serum-free medium optimized the plating efficiency. Although Schwann cells comprised 90% of the initial culture population, their numbers declined over time due to a faster mitotic rate of the fibroblasts in the presence of cholera toxin alone. However, treating the cultures with a combination of cholera toxin and forskolin, which act synergistically to elevate cyclic AMP levels, inhibited fibroblast growth without causing Schwann cell toxicity. Adding glial growth factor to the adenyl cyclase activators maximized Schwann cell proliferation, and the population rapidly and selectively expanded. Therefore, it should be possible to generate large numbers of Schwann cells from diseased nerves to study defects in cell function or from normal nerves to study the effects of Schwann cell grafts on neuronal regeneration.