Phorbol esters and vasopressin stimulate DNA synthesis by a common mechanism

Arginine vasopressin: Direct and indirect action on metabolism

From its identification and isolation in 1954, arginine vasopressin (AVP) has attracted attention, not only for its peripheral functions such as vasoconstriction and reabsorption of water from kidney, but also for its central effects. As there is now considerable evidence that AVP plays a crucial role in feeding behavior and energy balance, it has become a promising therapeutic target for treating obesity or other obesity-related metabolic disorders. However, the underlying mechanisms for AVP regulation of these central processes still remain largely unknown. In this review, we will provide a brief overview of the current knowledge concerning how AVP controls energy balance and feeding behavior, focusing on physiological aspects including the relationship between AVP, circadian rhythmicity, and glucocorticoids.

KRAS, YAP, and obesity in pancreatic cancer: A signaling network with multiple loops

Pancreatic ductal adenocarcinoma (PDAC) continues to be a lethal disease with no efficacious treatment modalities. The incidence of PDAC is expected to increase, at least partially because of the obesity epidemic. Increased efforts to prevent or intercept this disease are clearly needed. Mutations in KRAS are initiating events in pancreatic carcinogenesis supported by genetically engineered mouse models of the disease. However, oncogenic KRAS is not entirely sufficient for the development of fully invasive PDAC. Additional genetic mutations and/or environmental, nutritional, and metabolic stressors, e.g. inflammation and obesity, are required for efficient PDAC formation with activation of KRAS downstream effectors. Multiple factors "upstream" of KRAS associated with obesity, including insulin resistance, inflammation, changes in gut microbiota and GI peptides, can enhance/modulate downstream signals. Multiple signaling networks and feedback loops "downstream" of KRAS have been described that respond to obesogenic diets. We propose that KRAS mutations potentiate a signaling network that is promoted by environmental factors. Specifically, we envisage that KRAS mutations increase the intensity and duration of the growth-promoting signaling network. As the transcriptional activator YAP plays a critical role in the network, we conclude that the rationale for targeting the network (at different points), e.g. with FDA approved drugs such as statins and metformin, is therefore compelling.

Pharmacological lineage analysis revealed the binding affinity of broad-spectrum substance P antagonists to receptors for gonadotropin-releasing peptide

A group of synthetic substance P (SP) antagonists, such as [Arg(6),D-Trp(7,9),N(Me)Phe(8)]-substance P(6-11) and [D-Arg(1),D-Phe(5),D-Trp(7,9),Leu(11)]-substance P, bind to a range of distinct G-protein-coupled receptor (GPCR) family members, including V1a vasopressin receptors, and they competitively inhibit agonist binding. This extended accessibility enabled us to identify a GPCR subset with a partially conserved binding site structure. By combining pharmacological data and amino acid sequence homology matrices, a pharmacological lineage of GPCRs that are sensitive to these two SP antagonists was constructed. We found that sensitivity to the SP antagonists was not limited to the Gq-protein-coupled V1a and V1b receptors; Gs-coupled V2 receptors and oxytocin receptors, which couple with both Gq and Gi, also demonstrated sensitivity. Unexpectedly, a dendrogram based on the amino acid sequences of 222 known GPCRs showed that a group of receptors sensitive to the SP antagonists are located in close proximity to vasopressin/oxytocin receptors. Gonadotropin-releasing peptide receptors, located near the vasopressin receptors in the dendrogram, were also sensitive to the SP analogs, whereas α1B adrenergic receptors, located more distantly from the vasopressin receptors, were not sensitive. Our finding suggests that pharmacological lineage analysis is useful in selecting subsets of candidate receptors that contain a conserved binding site for a ligand with broad-spectrum binding abilities. The knowledge that the binding site of the two broad-spectrum SP analogs partially overlaps with that of distinct peptide agonists is valuable for understanding the specificity/broadness of peptide ligands.

Plasminogen-stimulated airway smooth muscle cell proliferation is mediated by urokinase and annexin A2, involving plasmin-activated cell signalling

BACKGROUND AND PURPOSE

The conversion of plasminogen into plasmin by interstitial urokinase plasminogen activator (uPA) is potentially important in asthma pathophysiology. In this study, the effect of uPA-mediated plasminogen activation on airway smooth muscle (ASM) cell proliferation was investigated.

EXPERIMENTAL APPROACH

Human ASM cells were incubated with plasminogen (0.5-50 μg·mL(-1) ) or plasmin (0.5-50 mU·mL(-1) ) in the presence of pharmacological inhibitors, including UK122, an inhibitor of uPA. Proliferation was assessed by increases in cell number or MTT reduction after 48 h incubation with plasmin(ogen), and by earlier increases in [(3) H]-thymidine incorporation and cyclin D1 expression.

KEY RESULTS

Plasminogen (5 μg·mL(-1) )-stimulated increases in cell proliferation were attenuated by UK122 (10 μM) or by transfection with uPA gene-specific siRNA. Exogenous plasmin (5 mU·mL(-1) ) also stimulated increases in cell proliferation. Inhibition of plasmin-stimulated ERK1/2 or PI3K/Akt signalling attenuated plasmin-stimulated increases in ASM proliferation. Furthermore, pharmacological inhibition of cell signalling mediated by the EGF receptor, a receptor trans-activated by plasmin, also reduced plasmin(ogen)-stimulated cell proliferation. Knock down of annexin A2, which has dual roles in both plasminogen activation and plasmin-signal transduction, also attenuated ASM cell proliferation following incubation with either plasminogen or plasmin.

CONCLUSIONS AND IMPLICATIONS

Plasminogen stimulates ASM cell proliferation in a manner mediated by uPA and involving multiple signalling pathways downstream of plasmin. Targeting mediators of plasminogen-evoked ASM responses, such as uPA or annexin A2, may be useful in the treatment of asthma.

Spatial intensity distribution analysis (SpIDA): a new tool for receptor tyrosine kinase activation and transactivation quantification

This chapter presents a general approach for the application of spatial intensity distribution analysis (SpIDA) to pharmacodynamic quantification of receptor tyrosine kinase homodimerization in response to direct ligand activation or transactivation by G protein-coupled receptors. A custom graphical user interface developed for MATLAB is used to extract quantal brightness and receptor density information from intensity histograms calculated from single fluorescence microscopy images. This approach allows measurement of monomer/oligomer protein mixtures within subcellular compartments using conventional confocal laser scanning microscopy. Application of quantitative pharmacological analysis to data obtained using SpIDA provides a universal method for comparing studies between cell lines and receptor systems. In addition, because of its compatibility with conventional immunostaining approaches, SpIDA is suitable not only for use in recombinant systems but also for the characterization of mechanisms involving endogenous proteins. Therefore, SpIDA enables these biological processes to be monitored directly in their native cellular environment.

Vasopressin V1a and V1b Receptors: From Molecules to Physiological Systems

The neurohypophysial hormone arginine vasopressin (AVP) is essential for a wide range of physiological functions, including water reabsorption, cardiovascular homeostasis, hormone secretion, and social behavior. These and other actions of AVP are mediated by at least three distinct receptor subtypes: V1a, V1b, and V2. Although the antidiuretic action of AVP and V2 receptor in renal distal tubules and collecting ducts is relatively well understood, recent years have seen an increasing understanding of the physiological roles of V1a and V1b receptors. The V1a receptor is originally found in the vascular smooth muscle and the V1b receptor in the anterior pituitary. Deletion of V1a or V1b receptor genes in mice revealed that the contributions of these receptors extend far beyond cardiovascular or hormone-secreting functions. Together with extensively developed pharmacological tools, genetically altered rodent models have advanced the understanding of a variety of AVP systems. Our report reviews the findings in this important field by covering a wide range of research, from the molecular physiology of V1a and V1b receptors to studies on whole animals, including gene knockout/knockdown studies.

Annexin-1 signals mitogen-stimulated breast tumor cell proliferation by activation of the formyl peptide receptors (FPRs) 1 and 2

The role of the calcium- and phospholipid-binding protein annexin I (ANXA1) in cell cycle regulation has been investigated in estrogen receptor (ER)-positive MCF-7 and ER-negative MDA-MB-231 breast tumor cell lines. In MCF-7 cells, ANXA1-targeting small interfering RNA (siRNA) reduced ANXA1 mRNA and protein levels and attenuated cell proliferation induced by FCS, estradiol, or epidermal growth factor. Well-characterized agonists for the known ANXA1 receptor, FPR2, including the ANXA1 N-terminal proteolytic product ANXA1(2-26), lipoxin A(4) (LXA(4)), and the synthetic peptide, Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm), stimulated proliferation of MCF-7 and MDA-MB-231 cells that was attenuated by incubation with FPR2 antagonists WRW(4) (1 μM) or Boc2 (100 nM) or by siRNA against FPR2. FCS-induced mitogenic responses were attenuated by each of the FPR antagonists and by siRNA against FPR2 and, to a lesser extent, FPR1. LXA(4) increased phosphorylation of Akt, p70(S6K) but not ERK1/2. Increases in cyclin D1 protein induced by FCS or LXA(4) were blocked by the PI3 kinase inhibitor, LY294002, and attenuated by FPR2 antagonism using Boc2. In invasive breast cancer, immunohistochemistry revealed the presence of ANXA1 and its receptor, FPR2, in both tumor epithelium and stromal cells. These observations suggest a novel signaling role for ANXA1 in mitogen-activated proliferation of breast tumor epithelial cells that is mediated via activation of FPR1 and FPR2.

Signalling mitogenesis in 3T3 cells: role of Ca2+-sensitive, phospholipid-dependent protein kinase

Understanding the molecular mechanisms that control cell proliferation requires the identification of the early signals important for initiating a mitogenic response. In this context, the activation of Ca2+-sensitive, phospholipid-dependent protein kinase (protein kinase C), which is stimulated by diacylglycerols and serves as a major phorbol ester receptor, may play an important part in signalling mitogenesis. This conclusion is based on two main lines of evidence. Firstly, activation of protein kinase C in intact quiescent fibroblasts is one of the earliest events elicited by a variety of growth-promoting agents including serum, platelet-derived growth factor (PDGF), vasopressin and bombesin, as judged by the increase in the phosphorylation of a cellular protein characterized by an Mr of 80 000 and a pI of 5. Secondly, the synthetic diacylglycerol, 1-oleoyl-2-acetylglycerol, which directly competes with [3H]phorbol dibutyrate for binding to specific receptors in intact 3T3 cells and rapidly stimulates protein kinase C in these cells, is a potent mitogen for Swiss 3T3 cells, acting synergistically with other growth factors. We propose that activation of protein kinase C may be one of the early signals that mediate the mitogenic effects of a variety of growth factors and peptide hormones in quiescent fibroblastic cells.

Receptors for the phorbol ester tumour promoters

The phorbol esters are potent tumour promotors in mouse skin and have profound effects on a wide variety of biological systems. Using the derivative [20-3H]phorbol 12,13-dibutyrate ([3H]PDBu), we demonstrated that specific receptors for the phorbol esters exist which mediate many of these biological effects. The receptors represent a complex between phospholipids and protein kinase C, an enzyme first identified by Nishizuka and coworkers. Considerable evidence indicates heterogeneity in the pharmacology of the biological responses to the phorbol esters. Likewise, multiple subclasses of binding sites have been observed. Differences in the phospholipids associated with protein kinase C may account for this heterogeneity, affecting both absolute and relative affinities. An endogenous analogue of the phorbol esters had been predicted from the high evolutionary conservation of the receptor. Nishizuka and coworkers have reported that diacylglycerols appear to be natural activators of protein kinase C. We find that diacylglyerols competitively inhibit phorbol ester binding, consistent with their acting at the same site on the enzyme. Likewise, by binding analysis, we can demonstrate a 1:1 stoichiometry between D-1-2-diacylglycerol and the receptor. The potencies of diacylglycerols for binding reflect their local concentration in the phospholipids. The K1 for diolein is approximately 0.1%. Relative to the corresponding phorbol esters, the potencies of the diacylglycerols are somewhat (17-fold) to markedly (30 000-fold) lower. We conclude that factors affecting phospholipid-receptor interactions or diacylglycerol production are of potential importance in the promotion process.

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.

Histidine Residues at the Active Site of the Pasteurella multocida Toxin

We have investigated histidine residues near the active site of the mitogenic Pasteurella multocida toxin. Mutation of H1202 or H1228 had little effect, while the effect of mutation on H1223 depended on the amino acid substituted. Mutation of H1205 caused complete loss of activity, indicating its importance in PMT activity.

Quercetin Induces Necrosis and Apoptosis in SCC-9 Oral Cancer Cells

Evidence has accumulated that dietary polyphenols, in particular, flavonoids, have protective effects against oral cancer. In this study, we have examined the effects of quercetin, a major dietary flavonoid, on cell growth and necrosis/apoptosis and cell cycle regulation in human oral squamous carcinoma SCC-9 cells. Quercetin induced dose- and time-dependent, irreversible inhibition of cell growth and cellular DNA synthesis. Light microscopy and lactate dehydrogenase measurements showed modifications in the morphology and membrane integrity of these cells after quercetin treatment. Propidium iodide/annexin V staining showed that quercetin induced necrosis at 24 h and 48 h, whereas at 72 h cells underwent apoptosis, correlating with caspase-3 activation. Flow cytometry studies of the cell cycle distribution showed that quercetin induced mainly S-phase arrest. Thymidylate synthase (TS), a key S-phase enzyme, was inhibited in a time- and dose-dependent fashion by quercetin at the protein level. A lack of effect on TS mRNA suggested that TS down-regulation occurred at the translational level. In conclusion, our data support a view that quercetin initially induces a stress response, resulting in necrosis of these oral epithelial cells. Prolonged exposure of the surviving cells to quercetin causes apoptosis, presumably mediated by inhibition of TS protein.

Synthesis of new phorbol derivatives having ethereal side chain and evaluation of their anti-HIV activity

Several new phorbol derivatives having ethereal substituents at the 12-position were synthesized and subjected to biological evaluation to find new candidates of an anti-HIV agent. Among them, 12-O-(methoxymethyl)phorbol 13-decanoate showed potent inhibitory activity against infection of HIV-1 in MT-4 cells (EC50: 1.3 ng/mL) and relatively low cytotoxicity (CC50: 8.3 microg/mL). This compound was also found to have sufficient stability in mouse plasma compared with the corresponding 12-acetate derivative, which was an equipotent HIV-1 inhibitor, but with an activity that decreased considerably after plasma treatment.

Resistance to TGF-beta-induced apoptosis in regenerating hepatocytes

Treatment with transforming growth factor beta (TGF-beta) of hepatocytes from two different proliferative conditions, such as fetal development and adult liver regeneration, shows that regenerating cells respond to this cytokine in terms of growth inhibition, but are less sensitive than the fetal ones to the apoptosis induced by this factor. Regenerating TGF-beta treated cells show higher cell viability and lower percentage of apoptotic cells than the fetal treated ones. Furthermore, TGF-beta treated regenerating hepatocytes maintain a well-preserved parenchyma-like organization. Treatment with TGF-beta induces the loss of mitochondrial transmembrane potential in fetal but not in regenerating hepatocytes and activation of caspase-3 is lower in regenerating than in fetal cells. Regenerating hepatocytes show higher intracellular levels of some antiapoptotic proteins, such as Bcl-x(L) and c-IAP-1 and, interestingly, they present higher intracellular glutathione levels, which might provide of mechanisms to avoid potential dangerous effects of the oxidative stress-mediated apoptosis induced by TGF-beta. In fact, treatment with BSO (a glutathione synthesis inhibitor) restores the response of regenerating hepatocytes to TGF-beta in terms of cell death. In conclusion, increased levels of Bcl-x(L) and cIAP-1 and higher intracellular glutathione levels could confer resistance to the apoptosis induced by TGF-beta during liver regeneration.

Identification and characterization of the Pasteurella multocida toxin translocation domain

The Pasteurella multocida toxin (PMT) is a potent mitogen which enters the cytosol of eukaryotic cells via a low pH membrane translocation event. In common with the Escherichia coli cytotoxic necrotizing factor 1 (CNF1), the core of the PMT translocation domain is composed of two predicted hydrophobic helices (H1 - residues 402-423, H2 - 437-457) linked by a hydrophilic loop (PMT-TL - 424-436). The peptide loop contains three acidic residues (D425, D431 and E434), which may play a role equivalent to D373, D379 and E382/383 in CNF1. To test this hypothesis, a series of point mutants was generated in which acidic residues were mutated into the permanently charged positive residue lysine. Individual mutation of D425, D431 and E434 each caused a four- to sixfold reduction in toxin activity. Interestingly, mutation of D401 located immediately outside the predicted helix-loop-helix motif completely abolished toxin activity. Individual mutations did not affect cell binding nor greatly altered toxin structure, but did prevent translocation of the surface-bound proteins into the cytosol after a low pH pulse. Moreover, we demonstrate using an in vitro assay that PMT undergoes a pH-dependent membrane insertion.

The ErbB2/Neu/HER2 receptor is a new calmodulin-binding protein

We have demonstrated previously that the EGFR (epidermal growth factor receptor) is a calmodulin (CaM)-binding protein. To establish whether or not the related receptor ErbB2/Neu/HER2 also binds CaM, we used human breast adenocarcinoma SK-BR-3 cells, because these cells overexpress this receptor thus facilitating the detection of this interaction. In the present paper, we show that ErbB2 could be pulled-down using CaM-agarose beads in a Ca2+-dependent manner, as detected by Western blot analysis using an anti-ErbB2 antibody. ErbB2 was also isolated by Ca2+-dependent CaM-affinity chromatography. We also demonstrate using an overlay technique with biotinylated CaM that CaM binds directly to the immunoprecipitated ErbB2. The binding of biotinylated CaM to ErbB2 depends strictly on the presence of Ca2+, since it was prevented by the presence of EGTA. Moreover, the addition of an excess of free CaM prevents the binding of its biotinylated form, demonstrating that this was a specific process. We excluded any interference with the EGFR, as SK-BR-3 cells express considerably lower levels of this receptor, and no detectable EGFR signal was observed by Western blot analysis in the immunoprecipitated ErbB2 preparations used to perform the overlay assays with biotinylated CaM. We also demonstrate that treating living cells with W7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide], a cell-permeant CaM antagonist, down-regulates ErbB2 phosphorylation, and show that W7 does not interfere non-specifically with the activity of ErbB tyrosine kinases. We also show that W7 inhibits the phosphorylation (activation) of both ERK1/2 (extracellular-signal-regulated kinases 1 and 2) and Akt/PKB (protein kinase B), in accordance with the inhibition observed in ErbB2 phosphorylation. In contrast, W7 treatment increased the phosphorylation (activation) of CREB (cAMP-response-element-binding protein) and ATF1 (activating transcription factor-1), two Ca2+-sensitive transcription factors that operate downstream of these ErbB2 signalling pathways, most likely because of the absence of calcineurin activity. We conclude that ErbB2 is a new CaM-binding protein, and that CaM plays a role in the regulation of this receptor and its downstream signalling pathways in vivo.

Pasteurella multocida toxin facilitates inositol phosphate formation by bombesin through tyrosine phosphorylation of G alpha q

The intracellularly acting Pasteurella multocida toxin (PMT) is a potent mitogen that stimulates Gq-dependent formation of inositol trisphosphate. We show that PMT, a nontoxic mutant of PMT (PMTC1165S), and bombesin each stimulate time-dependent phosphorylation of G alpha q at tyrosine 349. Although PMT and PMTC1165S each cause phosphorylation of G alpha q, only the wild-type toxin activates Gq. Pretreatment of cells with wild-type or mutant PMT potentiated the formation of inositol phosphates stimulated by bombesin equally. These data show that PMT potentiates bombesin receptor signaling through tyrosine phosphorylation of Gq and distinguishes between the two proposed models of Gq activation, showing that tyrosine phosphorylation is not linked to receptor uncoupling.

Nitric oxide-induced epidermal growth factor-dependent phosphorylations in A431 tumour cells

Nitric oxide (NO*) strongly inhibits the proliferation of human A431 tumour cells. It also inhibits tyrosine phosphorylation of a 170-kDa band corresponding to the epidermal growth factor receptor (EGFR) and induces the phosphorylation at tyrosine residue(s) of a 58-kDa protein which we have denoted NOIPP-58 (nitric oxide-induced 58-kDa phosphoprotein). The NO*-induced phosphorylation of NOIPP-58 is strictly dependent on the presence of EGF. Phosphorylation of NOIPP-58 and inhibition of the phosphorylation of the band corresponding to EGFR are both cGMP-independent processes. We also demonstrate that the p38 mitogen-activated protein kinase (p38MAPK) pathway is activated by NO* in the absence and presence of EGF, whereas the activity of the extracellular signal-regulated protein kinase 1/2 (ERK1/2) and the c-Jun N-terminal kinase 1/2 (JNK1/2) pathways are not significantly affected or are slightly decreased, respectively, on addition of this agent. Moreover, we show that the p38MAPK inhibitor, SB202190, induces rapid vanadate/peroxovanadate-sensitive dephosphorylation of prephosphorylated EGFR and NOIPP-58. We propose that the dephosphorylation of both NOIPP-58 and EGFR are mediated by a p38MAPK-controlled phosphotyrosine-protein phosphatase (PYPP). Activation of the p38MAPK pathway during nitrosative stress probably prevents the operation of this PYPP, allowing NOIPP-58, and in part EGFR, to remain phosphorylated and therefore capable of generating signalling events.

The 3'-UTR of the mRNA coding for the major protein kinase C substrate MARCKS contains a novel CU-rich element interacting with the mRNA stabilizing factors HuD and HuR

The expression of the major protein kinase C substrate MARCKS (myristoylated alanine-rich C kinase substrate) is controlled by the stability of its mRNA. While the MARCKS mRNA is long living in quiescent fibroblasts (t1/2 = 14 h), its half-life time is drastically reduced (t1/2 = 2 h) in cells treated with phorbol esters to activate protein kinase C (PKC) or treated with growth factors. In a first step to study the underlying mechanism we identified both a cis-element on the MARCKS mRNA and the corresponding trans-acting factors. Fusing the complete 3'-UTR or specific regions of the 3'-UTR of the MARCKS gene to a luciferase reporter gene caused a drastic decrease in luciferase expression to as low as 5-10% of controls. This down-regulation was a result of destabilization of the chimeric transcript as shown by RNA run-off and Northern blot-assays. By RNase/EMSA and UV-cross-linking experiments, we identified a stretch of 52 nucleotides [(CUUU)11(U)8] in the 3'-UTR of the MARCKS mRNA specifically recognized by two RNA-binding proteins, HuD and HuR. These trans-acting factors are members of the ELAV gene family and bind the MARCKS CU-rich sequence with high affinity. Overexpression of HuD and HuR in murine fibroblasts caused a striking stabilization of the endogenous MARCKS mRNA even under conditions when the MARCKS mRNA is normally actively degraded, i.e. after treating cells with phorbol ester. These data imply, that the identified CU-rich cis-element of the MARCKS 3'-UTR is involved in conferring instability to mRNAs and that members of the ELAV gene family oppose this effect. Based on its structural and functional properties, the (CUUU)11(U)8 sequence described here can be grouped into class III of AU-rich elements.

Localization of functional domains of the mitogenic toxin of Pasteurella multocida

The locations of the catalytic and receptor-binding domains of the Pasteurella multocida toxin (PMT) were investigated. N- and C-terminal fragments of PMT were cloned and expressed as fusion proteins with affinity tags. Purified fusion proteins were assessed in suitable assays for catalytic activity and cell-binding ability. A C-terminal fragment (amino acids 681 to 1285) was catalytically active. When microinjected into quiescent Swiss 3T3 cells, it induced changes in cell morphology typical of toxin-treated cells and stimulated DNA synthesis. An N-terminal fragment with a His tag at the C terminus (amino acids 1 to 506) competed with full-length toxin for binding to surface receptors and therefore contains the cell-binding domain. The inactive mutant containing a mutation near the C terminus (C1165S) also bound to cells in this assay. Polyclonal antibodies raised to the N-terminal PMT region bound efficiently to full-length native toxin, suggesting that the N terminus is surface located. Antibodies to the C terminus of PMT were microinjected into cells and inhibited the activity of toxin added subsequently to the medium, confirming that the C terminus contains the active site. Analysis of the PMT sequence predicted a putative transmembrane domain with predicted hydrophobic and amphipathic helices near the N terminus over the region of homology to the cytotoxic necrotizing factors. The C-terminal end of PMT was predicted to be a mixed alpha/beta domain, a structure commonly found in catalytic domains. Homology to proteins of known structure and threading calculations supported these assignments.

EGF receptor function is required in late G(1) for cell cycle progression induced by bombesin and bradykinin

We examined the role of epidermal growth factor (EGF) receptor (EGFR) tyrosine kinase activation in G protein-coupled receptor (GPCR) agonist-induced mitogenesis in Swiss 3T3 and Rat-1 cells. Addition of EGFR tyrosine kinase inhibitors (e.g., tyrphostin AG-1478) abrogated bombesin-induced extracellular signal-regulated kinase (ERK) activation in Rat-1 cells but not in Swiss 3T3 cells, indicating the importance of cell context in determining the role of EGFR in ERK activation. In striking contrast, treatment with tyrphostin AG-1478 markedly (~70%) inhibited DNA synthesis induced by bombesin in both Swiss 3T3 and Rat-1 cells. Similar inhibition of bombesin-induced DNA synthesis in Swiss 3T3 cells was obtained using four structurally different inhibitors of EGFR tyrosine kinase. Furthermore, kinetic analysis indicates that EGFR function is necessary for bombesin-induced mitogenesis in mid-late G(1) in both Swiss 3T3 and Rat-1 cells. Our results indicate that EGFR kinase activity is necessary in mid-late G(1) for promoting the accumulation of cyclins D1 and E and implicate EGFR function in the coupling of GPCR signaling to the activation of the cell cycle.

Tetrahydrobiopterin enhances apoptotic PC12 cell death following withdrawal of trophic support

(6R)-Tetrahydro-l-biopterin (BH(4)) is the rate-limiting cofactor in the production of catecholamine and indoleamine neurotransmitters and is also essential for the synthesis of nitric oxide by nitric-oxide synthase. We have previously reported that BH(4) administration induces PC12 cell proliferation and that nerve growth factor- or epidermal growth factor-induced PC12 cell proliferation requires the elevation of intracellular BH(4) levels. We show here that BH(4) accelerates apoptosis in undifferentiated PC12 cells deprived of serum and in differentiated neuron-like PC12 cells after nerve growth factor withdrawal. Increased production of catecholamines or nitric oxide cannot account for the enhancement of apoptosis by BH(4). Furthermore, increased calcium influx by exogenous BH(4) administration is not involved in the BH(4) proapoptotic effect. Our data also argue against the possibility that increased oxidative stress, due to BH(4) autoxidation, is responsible for the observed BH(4) effects. Instead, they are consistent with the hypothesis that BH(4) induces apoptosis by increasing cell cycle progression. Elevation of intracellular BH(4) during serum withdrawal increased c-Myc (and especially Myc S) expression earlier than serum withdrawal alone. Furthermore, N-acetylcysteine and the cyclin-dependent kinase inhibitor olomoucine ameliorated the BH(4) proapoptotic effect. These data suggest that BH(4) affects c-Myc expression and cell cycle-dependent events, possibly accounting for its effects on promoting cell cycle progression or apoptosis.

Requirement of cortical actin organization for bombesin, endothelin, and EGF receptor internalization

The role of actin organization in occupancy-induced receptor internalization remains poorly defined. Here we report that treatment of mouse Swiss 3T3 cells with latrunculin A, a potent inhibitor of actin polymerization (including cortical actin), inhibited the internalization of the endogenous bombesin/gastrin-releasing peptide (GRP) receptor, as judged by uptake of (125)I-labeled GRP or fluorescent Cy3-labeled bombesin. In contrast, cells pretreated with cytochalasin D showed minimal inhibition of bombesin/GRP receptor internalization. Similarly, pretreatment of Swiss 3T3 cells with the potent Rho-kinase inhibitor HA-1077, at concentrations (10-20 microM) that abrogated bombesin-mediated stress fiber formation, did not significantly alter receptor-mediated internalization of (125)I-GRP. These results indicate that bombesin/GRP receptor internalization depends on latrunculin A-sensitive cortical actin rather than on rapidly turning over actin stress fibers that are disrupted by either cytochalasin D or HA-1077. The rates and total levels of internalization of the endogenously expressed endothelin A receptor and epidermal growth factor receptor were also markedly reduced by latrunculin A in Swiss 3T3 cells. The potency of latrunculin A for inhibiting G protein-coupled receptor endocytosis was comparable to that for reducing internalization of the epidermal growth factor tyrosine kinase receptor. We conclude that cortical actin structures, disrupted by latrunculin A, are necessary for occupancy-induced receptor internalization in animal cells.

Effects of tetrandrine on growth factor-induced DNA synthesis and proliferative response of rat pulmonary artery smooth muscle cells

The objective of the present study was to investigate the effect of tetrandrine (a plant alkaloid isolated from Stephenia tetrandra) on growth factor-induced DNA synthesis and proliferative responses of rat pulmonary artery smooth muscle cells. Male rat and bovine pulmonary artery smooth muscle cells (PASMC) were cultured in Medium 199 containing FBS (10%). DNA synthesis was monitored from [(3)H]-thymidine uptake and cell proliferation by direct cell counting. In the present study FBS (1% v/v) caused a small increase in DNA synthesis above basal levels in rat and bovine PASMC (6% and 11% respectively). Platelet-derived growth factor (PDGF, 50 ng/ml), fibroblast growth factor (FGF, 50 ng/ml) or interleukin-1alpha (IL-1alpha, 100 pg/ml) alone increased rat PASMC proliferation (69-85%) and DNA synthesis above basal levels (76-92%). The addition of these growth factors in combination with FBS (1%) resulted in higher increases in DNA synthesis above basal levels (rat PASMC:PDGF, 465%; FGF, 421%; IL-1alpha, 406%; bovine PASMC:PDGF, 279%). Tetrandrine (10(-5) M) inhibited FBS (10%)-induced rat PASMC proliferation (90.5%) and DNA synthesis (89.0%). Tetrandrine significantly inhibited cell proliferation (86.5-98.5%) and DNA synthesis (79.9-89.0%) induced by FBS (1%) in combination with one of the following mitogens; PDGF (50 ng/ml), FGF (50 ng/ml), IL-1alpha (100 pg/ml). The inhibitory effects of tetrandrine were observed between 10(-6) and 10(-5)M and PASMC viability was not affected by tetrandrine below 3x10(-5) m. In summary, these results suggest that tetrandrine can exert anti-proliferative effects against a range of mitogenic stimuli for vascular smooth muscle cells in vitro. Such effects may contribute to the inhibitory effect of tetrandrine on pulmonary vascular remodelling associated with pulmonary hypertension.

Tumor necrosis factor-alpha mediates both apoptotic cell death and cell proliferation in a human hematopoietic cell line dependent on mitotic activity and receptor subtype expression

The TF-1 human erythroleukemic cell line exhibits opposing physiological responses toward tumor necrosis factor-alpha (TNF) treatment, dependent upon the mitotic state of the cells. Mitotically active cells in log growth respond to TNF by rapidly undergoing apoptosis whereas TNF exposure stimulates cellular proliferation in mitotically quiescent cells. The concentration-dependent TNF-induced apoptosis was monitored by cellular metabolic activity and confirmed by both DNA epifluorescence and DNA fragmentation. Moreover, these responses could be detected by measuring extracellular acidification activity, enabling rapid prediction (within approximately 1.5 h of TNF treatment) of the fate of the cell in response to TNF. Growth factor resupplementation of quiescent cells, resulting in reactivation of cell cycling, altered TNF action from a proliferative stimulus to an apoptotic signal. Expression levels of the type II TNF receptor subtype (p75TNFR) were found to correlate with sensitivity to TNF-induced apoptosis. Pretreatment of log growth TF-1 cells with a neutralizing anti-p75TNFR monoclonal antibody inhibited TNF-induced apoptosis by greater than 80%. Studies utilizing TNF receptor subtype-specific TNF mutants and neutralizing antisera implicated p75TNFR in TNF-dependent apoptotic signaling. These data show a bifunctional physiological role for TNF in TF-1 cells that is dependent on mitotic activity and controlled by the p75TNFR.

Activity of the mitogenic Pasteurella multocida toxin requires an essential C-terminal residue

Pasteurella multocida toxin (PMT) is a potent mitogen that also affects bone resorption. PMT acts intracellularly and is therefore postulated to have several domains involved in different aspects of its function. The toxin contains eight cysteine residues. Mutants with individual substitutions for each of these residues were constructed, and the effects of these on the biological activity of the toxin were determined by cultured-cell assays. Only the most C-terminal of the eight cysteines (C1165) was essential for full activity, although mutation of the cysteine residue at position 1159 caused a slight but reproducible loss of potency. In animal challenge experiments, mutant toxin (C1165S) was not toxic to piglets, even at doses exceeding a lethal dose of active PMT 1, 000-fold. The mutant and wild-type toxins displayed identical purification characteristics, similar susceptibility to proteolytic digestion, and circular dichroism profiles, which indicated that no gross structural changes had taken place. The function of the essential C1165 residue is not yet known, although its most likely role is an enzymatic one at or near the catalytic center of the toxin.

The roles of protein kinase C beta I and beta II in vascular smooth muscle cell proliferation

The role of protein kinase C (PKC) on proliferation of A10 vascular smooth muscle cells (VSMC) was studied by overexpressing specific PKC-beta I and -beta II isozymes. PKC-beta I and -beta II are derived from alternative splicing of the exon encoding the carboxy-terminal (C-terminal) 50 or 52 amino acids, respectively. The differential functions of the two isozymes with regard to cell proliferation, DNA synthesis, and the cell cycle were investigated in A10 cells, a clonal cell line of VSMC from rat aorta, and in A10 cells overexpressing PKC-beta I and PKC-beta II (beta I-A10 and beta II-A10). PKC levels were increased three- to fourfold in heterogeneous cultures of stably transfected cells. Although doubling time of A10 cells was 36 h, the cell doubling time in beta I-A10 cells decreased by 12 h, and, in contrast, the doubling time of beta II-A10 cells increased by 12 h compared to A10 cells. The increase of [3H]thymidine (TdR) incorporation was accelerated and increased in beta I-A10 cells, but slowed and diminished in beta II-A10 cells compared to A10 and control cells transfected with empty vector. Cell cycle analysis of beta I-A10 cells showed an acceleration of S phase entry while beta II-A10 cells slowed S phase entry. These results suggest that PKC-beta I and PKC-beta II regulate cell proliferation bidirectionally and that PKC-beta I and PKC-beta II may have distinct and opposing functions as cell cycle check point mediators during late G1 phase and may regulate S phase entry in A10 VSMC.

Phorbol esters down-regulate alpha-fetoprotein gene expression without affecting growth in fetal hepatocytes in primary culture

The effects of phorbol esters (phorbol-12,13-dibutyrate, PDB) on alpha-fetoprotein expression and cell growth were assayed by using fetal hepatocytes in primary culture. PDB acts synergistically with epidermal growth factor (EGF) to specifically decrease alpha-fetoprotein (AFP) mRNA levels, without affecting the expression of other genes of the same family, such as albumin and Vitamin D-binding protein (DBP). This effect is PDB-dose dependent, maximal effects being at 10 ng/ml. The implication of protein kinase C (PKC) in this effect seems clear since bisindolylmaleimide (BIS), a specific PKC inhibitor, completely blocks the PDB effect on AFP expression. Nuclear run-on experiments show that the decrease in AFP mRNA levels is mainly due to an inhibition in the transcription rate of the gene. Determination of PKC activities shows that fetal hepatocytes contain mainly Ca(2+)-independent isoenzymes, which patterns of activation was not modified by EGF plus PDB treatment with respect to PDB treatment. We have found that MAPK and JNK activities, c-jun and c-fos mRNA levels and AP-1 binding activity are notably increased when cells are incubated with both EGF and PDB, PDB does not stimulate growth of fetal hepatocytes, measured either as [3H]-thymidine incorporation into DNA or by cell cycle analysis using flow cytometry. All these results suggest that activation of PKC may affect liver gene expression rather than cell growth in fetal hepatocytes.

Nitric oxide reversibly inhibits the epidermal growth factor receptor tyrosine kinase

Although it has been demonstrated that NO inhibits the proliferation of different cell types, the mechanisms of its anti-mitotic action are not well understood. In this work we have studied the possible interaction of NO with the epidermal growth factor receptor (EGFR), using transfected fibroblasts which overexpress the human EGFR. The NO donors S-nitroso-N-acetylpenicillamine (SNAP), 1,1-diethyl-2-hydroxy-2-nitrosohydrazine (DEA-NO) and N-{4-[1-(3-aminopropyl)-2-hydroxy-2-nitrosohydrazino]butyl}propane -1, 3-diamine (DETA-NO) inhibited DNA synthesis of fibroblasts growing in the presence of fetal calf serum, epidermal growth factor (EGF) or EGF plus insulin, as assessed by [methyl-3H]thymidine incorporation. Neither 8-bromo-cGMP nor the cGMP-phosphodiesterase inhibitor zaprinast mimicked this effect, suggesting that NO is unlikely to inhibit cell proliferation via a cGMP-dependent pathway. SNAP, DEA-NO and DETA-NO also inhibited the transphosphorylation of the EGFR and its tyrosine kinase activity toward the exogenous substrate poly-l-(Glu-Tyr), as measured in permeabilized cells using [gamma-32P]ATP as phosphate donor. In contrast, 3-[morpholinosydnonimine hydrochloride] (SIN-1), a peroxynitrite-forming compound, did not significantly inhibit either DNA synthesis or the EGFR tyrosine kinase activity. The inhibitory action of DEA-NO on the EGFR tyrosine kinase was prevented by haemoglobin, an NO scavenger, but not by superoxide dismutase, and was reversed by dithiothreitol. The binding of EGF to its receptor was unaffected by DEA-NO. The inhibitory action of DEA-NO on the EGF-dependent transphosphorylation of the receptor was also demonstrated in intact cells by immunoblot analysis using an anti-phosphotyrosine antibody. Taken together, these results suggest that NO, but not peroxynitrite, inhibits in a reversible manner the EGFR tyrosine kinase activity by S-nitrosylation of the receptor.

The involvement of reactive oxygen species and arachidonic acid in alpha 1-adrenoceptor-induced smooth muscle cell proliferation and migration

1. In a previous study, we demonstrated phenylephrine-stimulated arachidonic acid (AA) release in rabbit cultured aortic smooth muscle cells. Therefore, we have investigated the functional implications of AA which are involved in the cellular response to phenylephrine, particularly proliferation and migration of rabbit cultured aortic smooth muscle cells. 2. First, to determine whether AA directly modifies proliferation and mobility of vascular smooth muscle cells (VSMCs), we exposed the cells to AA. AA induced proliferation and migration of the cells in a dose-dependent fashion. Concomitantly added catalase inhibited the proliferation and chemotaxis induced by AA of VSMCs. Conversely, aminotriazole enhanced the proliferation and migration induced by AA. 3. Secondly, we investigated whether the proliferation and migration of VSMCs by phenylephrine were related to AA and hydrogen peroxide (H2O2). The proliferation and chemotaxis of VSMCs by phenylephrine were inhibited by a phospholipase A2 (PLA2) inhibitor, or catalase. 4. Lastly, we investigated the effects of AA and phenylephrine on the content of H2O2 in VSMCs. AA and phenylephrine treatment led to an increase of H2O2 in a dose-dependent manner. 5. These results suggest that the addition of phenylephrine to the cells caused the enhancement of proliferation and migration, probably by mediating AA release and reactive oxygen species (ROS) production.

Ehrlich ascites tumor cells produce a transforming growth factor-beta (TGFbeta)-like activity but lack receptors with TGFbeta-binding capacity

Ehrlich ascites tumor cells incorporate [methyl-3H]thymidine into DNA independently of exogenous growth factors or fetal calf serum. Using an acid/ethanol extraction procedure we have obtained from these tumor cells a fraction that induces both the proliferation and the formation of cell foci by Swiss 3T3 mouse fibroblasts in the presence of insulin; inhibits the proliferation of Mv1Lu mink lung epithelial cells; and stimulates the growth of NRK rat kidney fibroblasts in soft-agar in the presence of epidermal growth factor. An antibody against transforming growth factor-beta (TGFbeta) prevents both the tumor extract-induced proliferation of Swiss 3T3 fibroblasts and the tumor extract-induced proliferative arrest of Mv1Lu cells. The tumor cells secrete a TGF beta-like activity to the extracellular medium in a partially-activated form. However, authentic TGFbeta does not affect their proliferation, and no TGFbeta receptors were detected using [125I]TGFbeta as a ligand. Therefore, the absence of TGFbeta receptors with ligand-binding capacity in these tumor cells may bypass the negative control that this factor exerts on the proliferation of their normal cell counterparts.

Effects of transforming growth factor-beta 1 and phorbol ester on PAI-1 and PA genes in human lung cells

Transforming growth factor-beta (TGF-beta) mediates the production of extracellular matrix proteins, proteases and protease inhibitors in epithelial cells. Both TGF-beta and phorbol-12-myristate-13-acetate (PMA) exert both positive and negative effects on mitogenesis in these as well as other cell types. Phorbol esters act through stimulation of protein kinase C (PKC) and are among the most potent tumor promoters known. The present study was conducted to determine whether the effect of TGF-beta in human non-small cell lung cancer (NSCLC) and normal human bronchial epithelial (NHBE) cells parallels that of the phorbol esters and whether this effect of TGF-beta involves PKC. TGF-beta 1 and PMA increased expression of TGF-beta 1 mRNA 24 hr after their addition to both NSCLC and NHBE cells. The effects of these agents on expression of the mRNAs for TGF-beta 2 and TGF-beta 3 were more complex; while TGF-beta 2 and TGF-beta 3 mRNAs increased transiently in response to TGF-beta 1 in NHBE cells and TGF-beta 3 mRNA increased transiently in some NSCLC cells, expression of these mRNAs decreased in most of these cells in response to PMA with the exception of the carcinoid NCI-H727 where TGF-beta 2 mRNA increased dramatically, TGF-beta 1 and PMA both caused a persistent increase in expression of the mRNAs for both plasminogen activator inhibitor-1 (PAI-1) and plasminogen activator (PA) up to 24 hr in most NSCLC cells, with the increase in PAI-1 mRNA beginning several hours before that of PA mRNA. In contrast, while TGF-beta 1 also increased expression of PAI-1 mRNA in NHBE cells, the expression of PA mRNA decreased simultaneously. The effect of PMA on PAI-1 and PA mRNAs was opposite of TGF-beta 1 in these cells, with expression of PAI-1 mRNA decreasing and PA mRNA increasing after addition of PMA. These data show that there is parallel regulation of the genes for TGF-beta 1, PAI-1 and PA by TGF-beta 1 and PMA in NSCLC, but differential regulation of the genes for PAI-1 and PA by these agents in NHBE cells. The responses of the mRNAs and proteins of TGF-beta 1, PAI-1 and PA to TGF-beta 1 and PMA were inhibited by the serine/ threonine kinase inhibitor H7 in NSCLC cells. Treatment of NSCLC cells with TGF-beta 1 and PMA resulted in a persistent increase in the expression of fibronectin mRNA and protein. This response was blocked by the addition of H7. Inhibition of these effects by H7 in NSCLC cells suggests that H7 blocks TGF-beta responses by inhibiting a protein serine/threonine kinase(s). Because the effects of TGF-beta and PMA on the different TGF-beta isoforms, PA, PAI and fibronectin in NHBE and NSCLC cells are complex, our data suggest that there are distinct mechanisms for controlling the different TGF-beta isoforms, PA, PAI and extracellular matrix proteins in normal lung and lung cancer cells.

[D-Arg1,D-Trp5,7,9,Leu11]Substance P coordinately and reversibly inhibits bombesin- and vasopressin-induced signal transduction pathways in Swiss 3T3 cells

The novel substance P (SP) analogue, [D-Arg1,D-Trp5,7,9,Leu11]SP like [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP inhibited DNA synthesis induced by bombesin, vasopressin, and bradykinin, but did not interfere with the mitogenic response induced by other growth factors or pharmacological agents in Swiss 3T3 cells. [D-Arg1,D-Trp5, 7,9,Leu11]SP reversibly inhibited bombesin-induced DNA synthesis, causing a 6-fold greater rightward shift in the bombesin dose response than [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP at identical concentrations (10 microM). We found that the new, more potent, SP analogue coordinately and reversibly inhibited bombesin-induced Ca2+ mobilization and protein kinase C (PKC) and mitogen-activated protein (MAP) kinase activation. The dose-response curves for bombesin-induced Ca2+ mobilization and MAP kinase activation were similarly displaced (51- and 40-fold, respectively) by [D-Arg1, D-Trp5,7,9,Leu11]SP. In addition, [D-Arg1,D-Trp5,7,9,Leu11]SP reversibly inhibited bombesin-induced tyrosine phosphorylation of Mr 110,000-130,000 and 70,000-80,000 bands as well as p125 focal adhesion kinase. [D-Arg1,D-Trp5,7,9,Leu11]SP also reversibly and coordinately inhibited vasopressin-induced Ca2+ mobilization, PKC stimulation, MAP kinase activation, tyrosine phosphorylation, and DNA synthesis in Swiss 3T3 cells. Surprisingly, deletion of the terminal Leu of [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP to yield [D-Arg1, D-Phe5,D-Trp7,9]SP1-10 resulted in a selective loss of inhibitory activity of this analogue against bombesin- but not vasopressin-stimulated DNA synthesis, Ca2+ mobilization, and MAP kinase activation. Collectively, these results suggest that SP analogues act at the receptor level to coordinately and reversibly antagonize bombesin- or vasopressin-induced signal transduction in Swiss 3T3 cells.

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.

Protein kinase C alpha expression is required for heparin inhibition of rat smooth muscle cell proliferation in vitro and in vivo

Heparin is a complex glycosaminoglycan that inhibits vascular smooth muscle cell (SMC) growth in vitro and in vivo. To define the mechanism by which heparin exerts its antiproliferative effects, we asked whether heparin interferes with the activity of intracellular protein kinase C (PKC). The membrane-associated intracellular PKC activity increased following stimulation of cultured rat SMCs with fetal calf serum and was suppressed by heparin in a time- and dose-dependent manner. Heparin acted through a selective inhibition of the PKC-alpha since preincubation of the cells with a 20-mer phosphorothioate PKC-alpha antisense oligodeoxynucleotide (ODN) eliminated the heparin effect. In vivo, following balloon injury of the rat carotid artery, particulate fraction PKC content increased with a time course and to an extent comparable with the observed changes in vitro. Heparin, administered at the time of injury or shortly thereafter, inhibited the activity of the particulate PKC and suppressed the in situ phosphorylation of an 80-kDa myristoylated alanine-rich protein kinase C substrate (MARCKS), a substrate of PKC. The topical application of the phosphorothioate antisense ODN selectively suppressed the expression of the PKC-alpha isoenzyme in vivo but did not affect injury-induced myointimal proliferation. Topical application of the ODN also eliminated the antiproliferative activity of heparin. These results therefore suggest that heparin might block SMC proliferation by interfering with the PKC pathway through a selective direct inhibition of the PKC-alpha isoenzyme.

Cloning, expression, and molecular characterization of the dermonecrotic toxin gene of Bordetella spp

A cosmid library of random fragments of Bordetella bronchiseptica genomic DNA was prepared and screened with oligonucleotides designed from the sequence of the B. pertussis dermonecrotic toxin (DNT) gene. Two cosmid clones which apparently contained the complete B. bronchiseptica DNT gene were identified, but they did not express the toxin. A 5-kb fragment containing the DNT gene was subcloned from one of the cosmid clones onto a high-copy-number plasmid, and this resulted in low-level expression of the toxin. The expression level was increased by deletion of a small region upstream of the coding sequence. Assays for biological activity, including the infant mouse dermonecrosis assay, confirmed that the product of the cloned gene was DNT. The complete sequence of the B. bronchiseptica DNT gene was determined and was more than 99% homologous to the DNT gene of B. pertussis. A putative purine nucleotide-binding motif was shown to be important for toxic activity. Extracts containing the recombinant or the native toxin induced DNA synthesis in Swiss 3T3 cells but inhibited cell division leading to binucleation.

Anticoagulant factor protein S inhibits the proliferation of rat astrocytes after injury

The actions of protein S (PS) on the scratch injury-induced proliferation of rat astrocytes (AC) were studied. PS (10-300 nM) markedly inhibited [3H]thymidine incorporation into injured AC. The effect of 100 nM PS was comparable with that of transforming growth factor-beta 1 (TGF-beta 1; 20 ng/ml). The incorporation of bromodeoxyuridine, which is usually detectable in AC along the border of the wound, was undetectable in the presence of 300 nM PS. The level of PS mRNA in the injured AC was slightly increased 15 h after the injury, although the level of its receptor, Tyro 3 mRNA was not changed significantly. The results of the present study suggest that PS plays an important role in tissue repair processes in the central nervous system (CNS) by suppressing the proliferation of AC as in the case of TGF-beta 1.

Pasteurella multocida toxin stimulates mitogenesis and cytoskeleton reorganization in Swiss 3T3 fibroblasts

Pasteurella multocida toxin (PMT) causes cytoplasmic retraction in epithelial cells, activates osteoclast neoformation, and is a potent mitogen for Swiss 3T3 fibroblasts. In the present study designed to further investigate the effects of PMT on cell shape and proliferation, we report that the mitogenic effect of affinitypurified PMT on quiescent 3T3 cells was even superior at 5 ng/ml to that of fetal bovine serum or bombesin. This positive effect was inhibited by heat denaturation and methylamine treatment (this agent blocks internalization). Preincubation of PMT with gangliosides GM1, GM2, or GM3 counteracted its effect on DNA synthesis, suggesting that the toxin binds to GM-type ceramides on target cells. The distribution of F-actin was analyzed in control/treated cells using FITC-conjugated phalloidin. In comparison with FBS and bombesin, PMT triggered a more rapid and profound reorganization of cortical actin into prominent stress fibers after only 5-10 min. This event lead to the retraction of cells after only 30 min and ultimately to the induction of mitotic figures. Interestingly, methylamine blocked the effects of PMT on stress fiber formation and cell retraction but not the ruffling response, suggesting that some early events may not require toxin internalization. In summary, these findings indicate that PMT concomitantly exerts a strong mitogenic activity and a rapid stimulation of cytoskeletal rearrangements, possibly after binding to membrane gangliosides and subsequent internalization. We propose that this toxin could be used in the future as a defined inducer of transduction signals involved in cellular proliferation and control of cell shape.

Phorbol ester inhibits the phosphorylation of the retinoblastoma protein without suppressing cyclin D-associated kinase in vascular smooth muscle cells

To elucidate the role of protein kinase C in vascular smooth muscle cell proliferation, we examined the effects of phorbol 12-myristate 13-acetate (PMA) on G1 events in human arterial cells. About 15 h after G0 cells were stimulated with fetal bovine serum and basic fibroblast growth factor, [3H]thymidine incorporation started. PMA (10 nM) inhibited the incorporation over 90% when added earlier than 3 h after stimulation, but had no effect when added 12 h or later. PMA inhibited the phosphorylation of the retinoblastoma protein (pRb), which normally began at about 9 h. PMA did not inhibit the gene expression of Cdk2, Cdk3, Cdk4, Cdk5, and cyclins G, C, and D, all of which began at 0-3 h. However, PMA reduced the expression of cyclins E and A, which usually began at 3-9 h and about 15 h, respectively. PMA inhibited the histone H1 kinase activity of Cdk2, which increased from about 9 h, whereas PMA did not inhibit the pRb kinase activities of cyclin D-associated kinase(s) and Cdk4, detectable from 0-3 h. These results suggested that the PMA-induced inhibition of pRb phosphorylation is not mediated by suppressing cyclin D-associated kinase(s) including Cdk4, but involves the suppression of Cdk2 activity that results from the reduced expression of cyclins E and A.

Requirement for phosphatidylinositol 3'-kinase activity in platelet-derived growth factor-stimulated tyrosine phosphorylation of p125 focal adhesion kinase and paxillin

The role of phosphatidylinositol 3'-kinase (PI 3'-kinase) activity in platelet-derived growth factor (PDGF)-stimulated tyrosine phosphorylation of focal adhesion kinase (p125FAK) and paxillin has been examined. The tyrosine phosphorylation of p125FAK and paxillin in response to PDGF was markedly inhibited by wortmannin in a dose-dependent manner. PDGF-stimulated PI 3'-kinase activity, membrane ruffle formation, and tyrosine phosphorylation of p125FAK and paxillin were all inhibited by the same low concentrations of wortmannin (>90% inhibition at 40nM). In contrast, tyrosine phosphorylation of p125FAK and paxillin in response to bombesin, endothelin, and phorbol 12,13-dibutyrate was not inhibited by wortmannin in these cells. Furthermore, LY294002, an inhibitor of PI 3'-kinase structurally unrelated to wortmannin, also inhibited PDGF-stimulated p125FAK tyrosine phosphorylation. PDGF was shown to stimulate the tyrosine phosphorylation of p125FAK in porcine aortic endothelial (PAE) cells transfected with the wild type PDGF-beta receptors, but not in PAE cells transfected with PDGF-beta receptors in which the PI 3'-kinase binding sites (Tyr-740/751) were replaced by phenylalanine. PDGF-stimulated, PI 3'-kinase-dependent tyrosine phosphorylation of p125FAK was not inhibited by rapamycin, and thus it was dissociated from the activation of p70 S6 kinase, previously identified as a molecular downstream target of PI 3'-kinase. Thus, we have identified a PI 3'-kinase-dependent signal transduction pathway in the action of PDGF, which leads to the phosphorylation of p125FAK and paxillin.

Antiproliferative effect in rat vascular smooth muscle cells by osthole, isolated from Angelica pubescens

The antiproliferative effect of osthole on rat vascular smooth muscle cells was examined in this study. A number of mitogenic agents, e.g., foetal-calf serum (10%, v/v) and platelet-derived growth factor (20 ng/ml), and pharmacological agents, e.g., serotonin (10 microM), ionomycin (3 nM), phorbol 12,13-dibutyrate (20 nM) and phorbol myristate acetate (200 nM), were used to induce DNA synthesis in rat vascular smooth muscle cells; these effects were concentration dependently inhibited by osthole and the half-maximal inhibition (IC50) occurred at 13.6 +/- 1.8, 11.8 +/- 1.3, 7.9 +/- 0.9, 7.1 +/- 0.2, 7.8 +/- 0.2 and 8.6 +/- 0.4 microM, respectively. Osthole itself increased the cyclic AMP and cyclic GMP formations in a concentration-dependent manner; it synergistically increase cyclic AMP and cyclic GMP levels induced by forskolin and sodium nitroprusside, respectively. After 48 h deprivation of serum, cells were re-stimulated with serum and the cell cycle was observed by flow cytometry; treatment of cells with osthole (100 microM) caused a block of serum-inducible cell cycle progression at a point before the G1-S boundary. The addition of osthole (100 microM) at various times after serum addition to serum-deprived cells showed full inhibition of DNA synthesis even when added 6 h after serum. The cell cycle progression block was gradually lost as the delay from serum to osthole application was increased from 6 to 18 h. The effect of osthole on serum-stimulated [3H]thymidine incorporation into endothelial cells was examined and the IC50 value (158.7 +/- 2.7 microM, n = 6) was obtained; it exhibited greater potency (12-fold) for vascular smooth muscle cells as compared with endothelial cells as an antiproliferative agent. These results suggest that osthole is a selective antiproliferative agent in vascular smooth muscle cells. The antiproliferative effect occurs at the early G1 phase of the cell cycle and is due to the increase in cyclic AMP and cyclic GMP contents.

Pasteurella multocida toxin is a mitogen for bone cells in primary culture

The effect of recombinant Pasteurella multocida toxin (PMT) on primary cultures of embryonic chick bone-derived osteoblastic cells was investigated. It was found that PMT was a potent mitogen for primary derived chicken osteoblasts. The toxin stimulated DNA synthesis and cell proliferation in quiescent osteoblasts at the first passage and accelerated cell growth in subconfluent cultures. Cell viability was not affected by PMT, even at relatively high concentrations. Osteoblast numbers increased in a dose-dependent manner in response to PMT. Intracellular inositol phosphates were elevated in response to PMT, but no elevation in cyclic AMP (cAMP) levels was evident. Indeed, PMT inhibited cAMP elevation in osteoblasts in response to cholera toxin at a stage before other PMT-mediated events take place. In addition to increased cell turnover, PMT down-regulated the expression of several markers of osteoblast differentiation. Both alkaline phosphatase and type I collagen were reduced, but osteonectin was not affected. The in vitro deposition of mineral in cultures of primary osteoblasts and osteoblast-like osteosarcoma cells was also inhibited by the presence of PMT. This suggests that PMT interferes with differentiation at a preosteoblastic stage.

Mitogenic inhibition by phorbol esters is associated with decreased phosphatidylinositol-3 kinase activation

In contrast to their role as potent tumor promoters, phorbol esters can cause inhibition of cell growth. Because the effect of phorbol esters occurs through activation of protein kinase C (PKC) and because activated PKC is translocated to the membrane placing it in a position to act on the intracellular portion of the growth factor receptor, we asked whether this inhibitory effect is mediated through the action of phorbol 12-myristate 13-acetate (PMA) on receptor association with the signal transfer proteins. When added to rat vascular smooth muscle (VSM) cells concurrently with basic fibroblast growth factor (bFGF), PMA at 100 ng/ml completely inhibits bFGF-stimulated DNA synthesis. Under the same growth-inhibitory conditions of PMA addition, aggregation of phosphatidylinositol 3-kinase (PI3K) to the fibroblast growth factor receptor and tyrosine phosphorylation of the 85-kDa regulatory component of the signal transfer protein PI3K are reduced by 94 and 79%, respectively. PI3K catalytic activity, as measured by conversion of phosphatidylinositol to phosphatidylinositol 3-phosphate, is decreased 88% by PMA addition. This effect is not specific to PI3K, since aggregation of phospholipase C-gamma 1 to the activated bFGF receptor is also decreased by PMA treatment. In addition, the PI3K inhibitor wortmannin markedly attenuates bFGF-stimulated VSM cell growth in a dose-dependent manner. These data suggest that the site of growth inhibition by PMA in VSM cells lies upstream of signal transfer particle aggregation and that such growth arrest may be mediated through inhibition of activation of PI3K.

Effects of pancreatic spasmolytic Polypeptide (PSP) on epithelial cell function

Trefoil peptides are expressed near endodermal ulcerations and may modulate epithelial repair. The trefoil pancreatic spasmolytic polypeptide (PSP) was tested for growth activity in vitro on epithelial cells and in vivo following intragastric or intravenous infusion in parenterally fed intact rats. Ion transport was assessed as changes in short-circuit current in rat intestine and adenocarcinoma cells in Ussing chambers. PSP stimulated growth of MCF-7 and Colo-357 cells, but only in the presence of extracellular glutathione (GSH). The effect was attenuated by GSH depletion with buthionine sulphoximine, even in GSH-containing media. When GSH-reduced PSP was carboxymethylated with iodoacetic acid, it still depended on extracellular GSH for its growth effect. Intestinal epithelial proliferation in rats was not affected by either intravenous or intraluminal infusion. PSP had no effect on basal or stimulated ion flux in rat jejunum or epithelial monolayers. The peptide did not compete with 125I-labeled epidermal growth factor for its receptor. [14C]Iodoacetamide treatment of PSP, followed by prolonged tryptic digestion yielded predominantly a 14C-labeled tetrapeptide fragment containing Cys1O4, with a lesser quantity of a 14C-labeled 15-amino-acid peptide containing Cys95 (molar ratio 15:1). GSH may predominantly reduce the Cys6-Cys1O4 terminal disulphide bond in PSP. We conclude that some epithelia may exhibit a growth response to PSP if extracellular GSH is present. Reduction of PSP by GSH is not necessary for this response, suggesting that the trefoil receptor or its signal transduction is GSH sensitive. PSP could assist wound healing by interactions with epithelial cells exposed concurrently to a local high GSH concentration.

Transforming growth factor beta modulates growth and differentiation of fetal hepatocytes in primary culture

Fetal hepatocytes in primary culture are cells capable to carry out both proliferation and differentiation processes simultaneously. Previous studies have shown that these cells respond to mitogens, such as hepatocyte growth factor (HGF) or epidermal growth factor (EGF), inducing the expression of early genes, such as fos and myc. The transforming growth factor-beta (TGF-beta) family is one of the most influential groups of growth and differentiation factors. In this report, we show that TGF-beta 1 inhibits fetal hepatocyte proliferation, arresting these cells at G1 phase of the cell cycle. In addition, TGF-beta down-regulates the mitogen-induced myc early expression. However, TGF-beta has no effect on the expression of other protooncogenes, such as fos and H-ras. In addition to its inhibitory role on fetal hepatocyte growth, TGF-beta increases the mRNA levels of fibronectin, an extracellular matrix protein, and maintains the expression of some liver specific genes, such as albumin and alfafetoprotein, above control values. The analysis of the expression of some hepatocyte transcriptional factors has shown that TGF-beta increases HNF1 alpha and HNF1 beta mRNA levels. We conclude that TGF-beta may modulate liver growth and differentiation throughout fetal development.

TGF-beta and phorbol esters inhibit mitogenesis utilizing parallel protein kinase C-dependent pathways

Transforming growth factor (TGF)-beta mediates matrix production in both mesangial and vascular smooth muscle cells. Both TGF-beta and phorbol-12-myristate-13-acetate (PMA) exert both positive and negative effects on mitogenesis in these as well as other cell types. Phorbol esters act through stimulation of protein kinase C (PKC) and are among the most potent tumor promoters known. The present study was conducted to determine if the growth inhibitory effect of TGF-beta parallels that of the phorbol esters and whether this effect of TGF-beta is dependent on activation of PKC. We show that, in vascular smooth muscle cells stimulated to divide by the addition of the serum component basic fibroblast growth factor (bFGF), TGF-beta1 inhibits mitogenesis in a dose-dependent manner, by a maximum of 79% when applied at a concentration of 1 ng/ml. Furthermore, the inhibitory effect on mitogenesis of either TGF-beta1 or PMA, when added four hours after bFGF, are 71% and 84% respectively. Both TGF-beta1 and PMA cause translocation of celllular PKC with similar time courses, while neither PKC-alpha nor PKC-betaII are increased in quantity in response to TGF-beta1. In addition, down-regulation of PKC by 24 hours incubation with PMA abolishes TGF-beta's inhibitory effect in bFGF-stimulated cells. We conclude that (i) the signaling pathway utilized by TGF-beta resulting in inhibition of mitogenesis parallels that of PMA, and (ii) the inhibitory effect of TGF-beta1 on bFGF-induced mitogenesis is partially due to activation of PKC. These data suggest that TGF-beta may be an endogenous activator of the growth-inhibitory pathway of PKC, and, since cellular differentiated functions generally occur when the cells are proliferation-inhibited, PKC may be a modulator of extracellular matrix deposition.

Salbutamol inhibits the proliferation of human airway smooth muscle cells grown in culture: relationship to elevated cAMP levels

The link between increased usage of beta-adrenoceptor agonists and worsening of asthma symptoms has raised interest in the effects of agents such as salbutamol on airway wall remodelling, and particularly airway smooth muscle proliferation. In the present study we have investigated the role of increases in intracellular cAMP in the inhibitory effect of salbutamol on airway smooth muscle proliferation. The inhibitory effects of a combination of submaximally effective concentrations of salbutamol (10 nM) and the non-selective phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX, 100 microM) on thrombin (0.3 U/mL)-induced mitogenesis in human cultured airway smooth muscle cells was greater than that for either agent alone. In addition, agents known to increase cAMP-dependent protein kinase activity including forskolin (10 microM), 8-bromoadenosine-3',5'-cyclic monophosphate (100 microM), and prostaglandin E2 (1 microM) have an inhibitory effect on thrombin (0.3 U/mL)-induced induced proliferation. Furthermore, the cAMP antagonist, 8-bromoadenosine-3',5'-cyclic monophosphorothioate, Rp-isomer (300 microM) significantly reduced the inhibitory effect of salbutamol (10 nM) on thrombin (0.3 U/mL)-induced DNA synthesis. In IBMX (100 microM)-pretreated cells, salbutamol (100 nM) increased intracellular cAMP levels via stimulation of a beta 2-adrenoceptor. Salbutamol (10 microM), at concentrations supramaximally effective for inhibition of mitogenesis, had no effect on thrombin (0.3 U/mL)-induced increases in intracellular calcium levels. Therefore, our results suggest that the previously reported inhibition of mitogen-induced proliferation in human cultured airway smooth muscle cells by the beta 2-adrenoceptor agonist, salbutamol (100 nM), is at least partly due to elevation of intracellular cAMP, while there is no effect of salbutamol on initial mitogen-induced increases in intracellular calcium.