Cholera toxin inhibits signal transduction by several mitogens and the in vitro growth of human small-cell lung cancer

A distinct role for interleukin-6 as a major mediator of cellular adjustment to an altered culture condition

Tissue microenvironment adjusts biological properties of different cells by modulating signaling pathways and cell to cell interactions. This study showed that epithelial–mesenchymal transition (EMT)/ mesenchymal–epithelial transition (MET) can be modulated by altering culture conditions. HPV E6/E7‐transfected immortalized oral keratinocytes (IHOK) cultured in different media displayed reversible EMT/MET accompanied by changes in cell phenotype, proliferation, gene expression at transcriptional, and translational level, and migratory and invasive activities. Cholera toxin, a major supplement to culture medium, was responsible for inducing the morphological and biological changes of IHOK. Cholera toxin per se induced EMT by triggering the secretion of interleukin 6 (IL‐6) from IHOK. We found IL‐6 to be a central molecule that modulates the reversibility of EMT based not only on the mRNA level but also on the level of secretion. Taken together, our results demonstrate that IL‐6, a cytokine whose transcription is activated by alterations in culture conditions, is a key molecule for regulating reversible EMT/MET. This study will contribute to understand one way of cellular adjustment for surviving in unfamiliar conditions. J. Cell. Biochem. 116: 2552–2562, 2015. © 2015 The Authors. Journal of Cellular Biochemistry Published by Wiley Periodicals, Inc.

Cholera toxin inhibits human hepatocarcinoma cell proliferation in vitro via suppressing ATX/LPA axis

AIM

To investigate the antitumor effect of cholera toxin (CT) in hepatocellular carcinoma (HCC) in vitro and the mechanisms underlying the effect.

METHODS

Human hepatocellular carcinoma cell lines Hep3B and Huh7, which expressed moderate and high level of autotaxin (ATX), respectively, were used. Cytokine level in the cells was evaluated using ELISA assay, and cell proliferation was investigated using MTT assay. ATX expression was determined using Western blot. ATX/lyso-PLD activity in the conditioned medium was measured using FS-3, a fluorescent lysophosphatidylcholine (LPC) analogue, as substrate.

RESULTS

Exposure to CT (7.5 and 10 ng/mL) significantly inhibited the cell growth, decreased secretion of proinflammatory cytokine TNF-α and promoted secretion of anti-inflammatory cytokines IL-4 and IL-10. CT at 10 ng/mL markedly suppressed ATX expression in Hep3B and Huh7 cells. Furthermore, ATX and lysophosphatidic acid (LPA) were found to be crucial for growth of the cancer cells. CT could inhibit TNF-α-induced expression and secretion of ATX that led to decreased activity of lysophospholipase D, thus decreasing the conversion of LPC to LPA.

CONCLUSION

CT inhibits hepatocellular carcinoma cell growth in vitro via regulating the ATX-LPA pathway.

Bcl-2 down modulation in WEHI-3B/CTRES cells resistant to Cholera Toxin (CT)-induced apoptosis

The very different effects of Cholera Toxin (CT) on cell growth and proliferation may depend on the type of ganglioside receptors in cell membranes and different signal transduction mechanisms triggered, but other functions related to the drug resistance mechanisms can not be excluded. The effect of CT treatment on the "in vitro" clonogenicity, the Population Doubling Time (PDT), apoptosis, PKA activation and Bax and Bcl-2 expression was evaluated in WEHI-3B cell line and its CT-resistant subclone (WEHI-3B/CTRES). In WEHI-3B parental cells the dramatic accumulation of cAMP induced by CT correlated well with PKA activation, increased PDT value, inhibition of clonogenicity and apoptosis. H-89 treatment inhibited PKA activation by CT but did not protect the cells from apoptosis and growth inhibition. In WEHI-3B/CTRES no significant CT-dependent accumulation of cAMP occurred with any increase of PKA activity and PDT. In CT resistant cells (WEHI-3B/CTRES), Bcl-2 expression was down regulated by both CT or drug treatment (eg., ciprofloxacin, CPX) although these cells were protected from CT-dependent apoptosis but not from drug-induced apoptosis. Differently from other cell models described, down regulation of Bcl-2 is proved to be independent on cAMP accumulation and PKA activation. Our observations support the implication of cAMP dependent kinase (PKA) in the inhibition of WEHI-3B cells growth and suggest that, in WEHI-3B/CTRES, Bcl-2 expression could be modulated by CT in the absence of cAMP accumulation. Also in consideration of many contradictory data reported in literature, our cell models (of one sensitive parental cell strain and two clones with different uncrossed specific resistance to CT and CPX) provides a new and interesting tool for better investigating the relationship between the CT signal transduction mechanisms and Bcl-2 expression and function.

Selection of a WEHI-3B leukemia cell subclone resistant to inhibition by cholera toxin

The studies on the inhibitory effect exerted by Cholera Toxin (CT) on cell growth and proliferation indicate a remarkable heterogeneity of cell response suggesting that the inhibition represents the final event of many different ways or mechanisms. After CT binding, cAMP accumulation may not occur (as in L1210 leukemia cells) or, when occurring (as in SR-4987 stromal cells), may not be coupled with the antiproliferative effect of CT. In WEHI-3B cells CT binds a Gal-GalNac-GM1b receptor and the anticlonogenic effect of CT seems correlated with cAMP accumulation. To demonstrate the central role of cAMP in WEHI-3B cells, starting from the sensitive cell strain we selected and established a clone of WEHI-3B resistant to CT. This revertant clone (WEHI-3B/CT/REV) is currently cultured in the absence of CT and in the proliferation assay shows a dramatic resistance (>46,000 than the parental cells). Stimulation ofWEHI-3B/CT/REV cells by cholera toxin failed to enhance cAMP and the ganglioside-CT binding studied on Thin Layer Chromatography (TLC) blots showed that the resistant cells lost the spot correspondent to the migration of Gal-GalNac-GM1b ganglioside. Both the lines respond at the same level to the adenylate cyclase stimulation by forskolin and the incorporation of GM1a did not decrease the resistance of WEHl-3B/CT/REV. These data confirm that Gal-GalNac-GM1b is the most important functional receptor for CT in WEHI-3B cells able to transduce the signal by enhancing cAMP which in turn inhibits cell proliferation (probably by cAMP dependent protein kinase activation). Our study describes the first cell line resistant to CT originated from a susceptible parental strain and provides a new interesting cell model for studying the cAMP dependent mechanisms involved in cell growth regulation.

Neuropeptide-induced androgen independence in prostate cancer cells: roles of nonreceptor tyrosine kinases Etk/Bmx, Src, and focal adhesion kinase

The bombesin/gastrin-releasing peptide (GRP) family of neuropeptides has been implicated in various in vitro and in vivo models of human malignancies including prostate cancers. It was previously shown that bombesin and/or neurotensin (NT) acts as a survival and migratory factor(s) for androgen-independent prostate cancers. However, a role in the transition from an androgen-dependent to -refractory state has not been addressed. In this study, we investigate the biological effects and signal pathways of bombesin and NT on LNCaP, a prostate cancer cell line which requires androgen for growth. We show that both neurotrophic factors can induce LNCaP growth in the absence of androgen. Concurrent transactivation of reporter genes driven by the prostate-specific antigen promoter or a promoter carrying an androgen-responsive element (ARE) indicate that growth stimulation is accompanied by androgen receptor (AR) activation. Furthermore, neurotrophic factor-induced gene activation was also present in PC3 cells transfected with the AR but not in the parental line which lacks the AR. Given that bombesin does not directly bind to the AR and is known to engage a G-protein-coupled receptor, we investigated downstream signaling events that could possibly interact with the AR pathway. We found that three nonreceptor tyrosine kinases, focal adhesion kinase (FAK), Src, and Etk/BMX play important parts in this process. Etk/Bmx activation requires FAK and Src and is critical for neurotrophic factor-induced growth, as LNCaP cells transfected with a dominant-negative Etk/BMX fail to respond to bombesin. Etk's activation requires FAK, Src, but not phosphatidylinositol 3-kinase. Likewise, bombesin-induced AR activation is inhibited by the dominant-negative mutant of either Src or FAK. Thus, in addition to defining a new G-protein pathway, this report makes the following points regarding prostate cancer. (i) Neurotrophic factors can activate the AR, thus circumventing the normal growth inhibition caused by androgen ablation. (ii) Tyrosine kinases are involved in neurotrophic factor-mediated AR activation and, as such, may serve as targets of future therapeutics, to be used in conjunction with current antihormone and antineuropeptide therapies.

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

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

The galanin receptor type 2 initiates multiple signaling pathways in small cell lung cancer cells by coupling to G(q), G(i) and G(12) proteins

Neuropeptides like galanin produced and released by small cell lung cancer (SCLC) cells are considered principal mitogens in these tumors. We identified the galanin receptor type 2 (GALR2) as the only galanin receptor expressed in H69 and H510 cells. Photoaffinity labeling of G proteins in H69 cell membranes revealed that GALR2 activates G proteins of three subfamilies: G(q), G(i), and G(12). In H69 cells, galanin-induced Ca2+ mobilization was pertussis toxin-insensitive. While phorbol ester-induced extracellular signal-regulated kinase (ERK) activation required protein kinase C (PKC) activity, preincubation of H69 cells with the PKC-inhibitor GF109203X had no effect on galanin-dependent ERK activity. A rise of the intracellular calcium concentration was necessary and sufficient to mediate galanin-induced ERK activation. In support of G(i) coupling, stimulation of GALR2 expressed in HEK293 cells inhibited isoproterenol-induced cAMP accumulation and raised cAMP levels in COS-7 cells when coexpressed with a chimeric G alpha(S)-G alpha(i) protein In H69 cells, galanin activated the monomeric GTPase RhoA and induced stress fiber formation in Swiss 3T3 cells expressing GALR2. Thus, we provide the first direct evidence that in SCLC the mitogenic neuropeptide galanin, interacting with GALR2, simultaneously activates multiple classes of G proteins and signals through the G(q) phospholipase C/calcium sequence and a G(12)/Rho pathway. Oncogene (2000) 19, 4199 - 4209

The biphasic induction of p21 and p27 in breast cancer cells by modulators of cAMP is posttranscriptionally regulated and independent of the PKA pathway

Cyclic AMP (cAMP) elevation affects growth arrest and differentiation in a wide variety of breast cell lines; however, the mechanisms associated with this process are poorly understood. Previous studies linked cAMP-mediated growth arrest in breast tumor cells to increased levels of cyclin kinase inhibitor (CKI), p21. In the present study we examined the role of cAMP-dependent protein kinase (PKA) on p21 and p27 induction in the breast cancer cell line, MDA-MB-157. The induction of the CKIs by modulators of cAMP such as cholera toxin (CT) + 1-isobutyl-3-methylxanthine (IBMX) and lovastatin fluctuates with biphasic kinetics (although the kinetics of CKI induction with CT + IBMX treatment are different from that of lovastatin) and is depicted by the periodic accumulation of lower molecular weight forms of p21 and p27 which also correlate with fluctuations in CDK2 activity. Using three different approaches we show that the cAMP-mediated induction of CKIs is independent of PKA activity. In the first approach we treated MDA-MB-157 cells with a variety of cAMP modulators such as CT + IBMX, and forskolin in the presence or absence of H-89, a potent PKA inhibitor. This analysis revealed that the cAMP activators were capable of inducing p21 even though PKA activity was completely eliminated. In the second approach PKA dominant negative stable clones of MDA-MB-157 treated with CT + IBMX or forskolin also resulted in p21 induction, in the absence of any PKA activity. Last, treatment of MDA-MB-157 cells with lovastatin, another known cAMP modulator which also causes growth arrest, resulted in the induction of p21 and p27 without any increase in PKA activity. Collectively, the above results suggest that the induction of p21 by cAMP is through a novel pathway, independent of PKA activity.

Reduced DNA synthesis and cell viability in small cell lung carcinoma by treatment with cyclic AMP phosphodiesterase inhibitors

This study investigated the effects of the adenosine 3',5'-cyclic monophosphate (cAMP) phosphodiesterase inhibitors caffeine, theophylline, and 3-isobutyl-1-methyl-xanthine (IBMX) on the proliferation and viability of the small cell lung carcinoma (SCLC) cell lines NCI-H345, NCI-H128, and SCC-9. These effects were correlated with the ability of the drugs to induce intracellular Ca2+ mobilization. Treatment of NCI-H345 cells with caffeine resulted in rapid mobilization of Ca2+, as indicated by Fura-2 fluorescence. Incubation of NCI-H345 cells with 6.25 mM caffeine resulted in a 62% inhibition of [3H]thymidine uptake after 2 hr, indicating reduced DNA synthesis. Incubation with 25 mM caffeine resulted in almost total inhibition of [3H]thymidine uptake after 2 hr. Similar effects on [3H]thymidine uptake were seen upon treatment of NCI-H128 and SCC-9 cells with caffeine; however, these cells did not exhibit caffeine-induced Ca2+ mobilization. Inhibition of DNA synthesis (66-93%) also occurred upon incubation of all cell lines with theophylline and IBMX, which did not mobilize Ca2+. Treatment of NCI-H345, NCI-H128, and SCC-9 cells with caffeine, theophylline, or IBMX markedly reduced cell viability. Levels of cAMP increased in the cells following treatment with caffeine, theophylline, or IBMX, reflecting the ability of these drugs to inhibit cAMP phosphodiesterase. These results suggest that the decrease in DNA synthesis and the subsequent cell death induced by these drugs are due to reduced cAMP phosphodiesterase activity, rather than to changes in intracellular Ca2+. These findings indicate that drugs that alter cAMP signaling pathways are potentially valuable agents to inhibit SCLC survival.

Anti-growth factor therapy for lung cancer

Lung cancers are the leading cause of cancer death in developed countries. In the USA, lung cancer accounts for 29% of all cancer deaths. The cure rate for lung cancer is low (14%) because the cancer spreads early and because chemotherapy cannot cure metastatic disease. In small cell lung cancer (SCLC) two-thirds of patients present with metastatic disease in a distant organ (stage IV). In non-small cell lung cancers (adenocarcinoma, squamous carcinoma, large cell carcinoma) one-third present with metastatic disease. Initial chemotherapy produces high response rates in both SCLC (85%-90% response rate) and NSCLC (50% response rate) but response duration is short and drug resistance develops rapidly. Growth factors play an important role in the pathogenesis and the progression of lung cancers. Knowledge of the role of these growth factors, their receptors and their signal pathways has produced new therapeutic targets. Compounds developed toward these targets have completed preclinical testing and are now in clinical trials. Some of these compounds are active in both drug sensitive and drug resistant lung cancers. They also produce synergistic growth inhibition when combined with cytotoxic chemotherapeutic agents. Thus, these compounds may provide a new way to overcome drug resistance in lung cancer.

Ganglioside expression in lung cancer cell lines

Gangliosides are complex glycolipid constituents of cell membranes. They are involved in many biological functions including cell-cell recognition, cell-matrix attachment, cell growth and cell differentiation. Analysis of tumor associated gangliosides may aid in the characterisation of tumour cells and their degree of malignant transformation. We have characterised a total of eight lung cancer cell lines (four small cell and four non-small cell lung cancer) with respect to ganglioside and alpha v integrin receptor expression. Ganglioside GD3 was detected using the monoclonal antibody R24. Ganglioside GM1 was detected using the beta-subunit of cholera toxin. Ganglioside 9-O-acetyl GD3 and the alpha v integrin receptor were measured using commercially available monoclonal antibodies. Our results indicate that small cell lung cancer cell lines express significant levels of GD3 and 9-O-acetyl GD3. Ganglioside GM1 and alpha v integrin receptor were not specific to any histological subtype. The expression of ganglioside GM1 and GD3 was independent of cell-cycle phase. We conclude that GD3 and 9-O-acetyl GD3 expression may be additional markers of the Small Cell Lung Cancer phenotype, but their significance is unknown. Therefore a characteristic ganglioside pattern cannot be defined according to histological subtype. alpha v integrin receptor expression is not unique to cells expressing GD3.

Beta-adrenergic signalling in neoplastic lung type 2 cells: glucocorticoid-dependent and -independent defects

Tumorigenic mouse lung-derived type 2 cell lines have large reductions in both beta-adrenergic-stimulated cAMP production and ligand binding to beta-adrenergic receptors. These tumorigenic cells are also relatively insensitive to glucocorticoids. Because glucocorticoids regulate both beta-adrenergic receptor expression and receptor coupling to the stimulatory guanine nucleotide binding protein Gs interactions between the glucocorticoid and beta-adrenergic signalling systems were examined. This study demonstrates that beta-adrenergic ligand binding and agonist sensitivity are increased in a tumorigenic cell line stably expressing a normal glucocorticoid receptor transgene. However, although the transfected tumour cells and non-tumorigenic cells have similar amounts and affinities of beta-adrenergic agonist and antagonist binding, similar amounts of Gs subunits and similar forskolin-stimulated adenylyl cyclase activities, the former remain much less isoproterenol responsive. Competition binding studies demonstrate that tumour cell beta-adrenergic receptors have both high- and low-affinity agonist binding but are functionally uncoupled from Gs. This uncoupling may involve an alteration in Gs, as guanine nucleotides exhibit a reduced ability to stimulate adenylyl cyclase. Thus, some aspects of tumorigenic cell dysfunction in beta-adrenergic signalling can be ameliorated by interactions with the glucocorticoid pathway, but additional defects are also involved.

Discordant signal transduction and growth inhibition of small cell lung carcinomas induced by expression of GTPase-deficient G alpha 16

Small cell lung carcinoma (SCLC) accounts for 20-25% of primary lung cancers and is rapidly growing, widely metastatic, and rarely curable. Autocrine stimulation of multiple G protein-coupled neuropeptide receptor systems contributes to the transformed growth of SCLC. The ability of neuropeptide receptors to stimulate phospholipase C and mobilize intracellular Ca2+ indicates that Gq family members of heterotrimeric G proteins are a convergence point mediating autocrine signaling by multiple neuropeptides in SCLC. Expression of a GTPase-deficient, constitutive active form of an alpha q family member, alpha 16Q212L, in SCLC markedly inhibited growth of the cells in soft agar and tumor formation in nude mice. SCLC lines expressing alpha 16Q212L exhibited 2-4-fold elevated basal phospholipase C activity, but neuropeptide and hormone-regulated intracellular Ca2+ mobilization was nearly abolished. The data suggest that Ca2+ mobilization is an obligatory signal in neuropeptide-stimulated growth of SCLC. In addition, the proline-directed c-Jun NH2-terminal kinases/stress-activated protein kinases, which are members of the mitogen-activated protein kinase family, were stimulated approximately 2-fold in parental SCLC in response to exogenous neuropeptides and muscarinic agonists and were constitutively activated to the same degree in alpha 16Q212L-expressing SCLC. Thus, alpha 16Q212L expression induced desensitizaton of neuropeptide-stimulated Ca2+ signaling and persistent activation of the c-Jun NH2-terminal kinase/stress-activated protein kinase pathway. We propose that the induction of discordant signaling by selective perturbation of receptor-regulated effector systems leads to the inhibition of SCLC cell growth.

Involvement of signal transduction pathways in lung cancer biology

Pathways involved in the transduction of biological signals within cells overlap with those involved in oncogenesis. Previous studies have identified a number of discrete disturbances of some elements of these pathways in human lung cancer cells, by virtue of the overexpression or the mutation of certain key molecules. The sequence of biochemical events triggered by a mitogenic stimulus such as the exposure to bombesin-like peptides are being unravelled. The opportunity exists to identify additional changes involving regulatory proteins which may contribute to the regulation of these systems and which may function as suppressors of the malignant phenotype. Furthermore, the understanding of these pathways may identify targets for the pharmacological regulation of tumor cell response to mitogens which may be usable in the clinic.

The different inhibiting effect of cholera toxin on two leukemia cell lines does not correlate with their toxin binding capacity

The murine leukemia cell lines L1210 and WEHI-3B show a very different sensitivity to the cholera toxin (CT). The in vitro growth of L1210 is completely inhibited by 10(-8) M CT, while WEHI-3B growth shows the same inhibition at 10(-11) M. The analysis of membrane ganglioside pattern of the two cell lines shows that in L1210 cells the major component is the GM1a ganglioside while the monosialogangl oside fraction from WEHI-3B is entirely composed of gangliosides of the 'b' series among which GM1b is the more represented. The total cholera toxin binding capacity of the ganglioside extract from L1210 cells is more than hundred fold higher than that of WEHI-3B and this difference is also confirmed by the number of CT receptors/cell and by the binding of FITC-B subunit of CT on the cells. These surprising data are in conflict with the poor sensitivity to CT evidenced by L1210 compared to WEHI-3B cells. In order to clarify this discrepancy we investigated the cAMP accumulation, the cell viability and the clonogenicity of these two leukemia cell lines following the treatment with CT and forskolin (FRSK). The treatment of WEHI-3B cells with CT induces a dramatic increase of intracellular cAMP which highly correlates with cell death and the decrease of clonogenicity and this result is partially obtained by the treatment with FRSK. L1210 cells do not evidence significant cAMP accumulation neither with CT nor with FRSK treatment. These data suggest that the different inhibiting effect of CT on WEHI-3B and L1210 cells does not correlate with their different pattern of gangliosides and the related toxin binding capacity. Further they indicate that the growth inhibition of WEHI-3B cells is closely related with a cAMP-dependent cell killing mechanism, while the inhibition of L1210 growth (produced by high concentration of CT) is mediated by a cAMP independent mechanism.

Preclinical pharmacology of cholera toxin

Cholera toxin was selected for pharmacologic evaluation by the National Cancer Institute on the basis of antiproliferative activity against small-cell and non-small-cell lung-cancer cell lines. A feature common to the sensitive cell lines was abundant expression of GM1 ganglioside, the cellular receptor for cholera toxin. A sandwich enzyme-linked immunosorbent assay (ELISA) was developed to quantitate cholera toxin in biological fluids. A sigmoidal relationship was observed between the cholera toxin plasma concentration and the absorbance at 490 nm (OD490) of the product of horseradish peroxidase-catalyzed oxidation of o-phenylenediamine over the range of 6.25-1,600 ng/ml. Logit transformation of the OD490 data was linear over the entire concentration range and assay variability was less than 25%. Cholera toxin was stable in murine and human whole blood and plasma. Following i.v. administration of 1,500 micrograms/kg to male CD2F1 mice, cholera toxin plasma elimination was described by a two-compartment open model. The half-lives (t1/2 alpha, t1/2 beta), plasma clearance, and steady-state volume of distribution were 0.7 min, 49 min, 24 ml min-1 kg-1 912 ml/kg, respectively. Cholera toxin was not detected in plasma following an s.c. dose of 1,500 micrograms/kg. Urinary recovery following intravenous drug administration was less than 0.1%.

G proteins in rat liver proliferation during cholestasis

Liver proliferation appears to be dually regulated, in part by cyclic AMP levels. Here we studied the alterations in the stimulatory action of cholera toxin and other agents on the adenylyl cyclase system, as well as the status of Gs and Gi protein subunits during the liver proliferation that follows bile duct ligation in rats. The stimulatory effects of glucagon and vasoactive intestinal peptide (which act through membrane receptors) or guanosine 5'-[beta gamma-imido]triphosphate (which interacts with G proteins) and forskolin (which directly activates the adenylyl cyclase catalytic subunit) on liver adenylyl cyclase activity were blunted in cholestasis. The results indicated an impairment in the stimulatory interaction between the alpha s subunit of Gs protein and the adenylyl cyclase catalytic subunit. Indeed, we observed an important decrease in the stimulation of adenylyl cyclase activity by cholera toxin in cholestasis that was accompanied by a reduced extent of [32P]ADP ribosylation of alpha s protein catalyzed by cholera toxin, as revealed by the poor labeling of the 42,000 Da band in liver membranes from cholestatic rats. However, there was no change in the amount of alpha s or beta proteins as measured with immunoblotting techniques. Experiments on [32P]ADP ribosylation of alpha i subunits of Gi proteins indicated an impairment in liver membranes from cholestatic rats, whereas Western blotting for the detection of alpha i subunits showed decreased alpha i3 and increased alpha i2 levels in this condition. Further efforts are needed to better understand the molecular mechanisms underlying the relationship between the observed divergent expression of Gs and Gi proteins and liver cell proliferation in the cholestatic liver.

Inhibition of human pancreatic cancer cell (MIA PaCa-2) growth by cholera toxin and 8-chloro-cAMP in vitro

The effects of cholera toxin (CT) and 8-chloro-cAMP (8-Cl-cAMP) on cell growth were investigated using two human pancreatic carcinoma cell lines (MIA PaCa-2, Panc-1). CT, which catalyses the ADP ribosylation of Gs, suppresses the proliferation of MIA PaCa-2(PC) cells. CT at the low dose of 0.1 pg ml-1 was inhibitory of PC cell growth, and the maximum suppression (70%) was achieved at a CT concentration of 100 pg ml-1. This phenomenon was reversible. The production of cAMP by CT (100 pg ml-1) in PC cells was enhanced 320-fold compared with the control. In addition, cAMP analogues (8-Cl-cAMP, 8-Br-cAMP) and forskolin decreased the growth rate of PC cells in a dose-dependent manner. These results support the view that CT suppresses PC cell growth by stimulating cAMP production. Conversely, Panc-1 cells were far less sensitive to CT in cell growth and cAMP production. 8-Cl-cAMP was also less effective on Panc-1 cell growth. The binding of an insulin-like growth factor (IGF)-I and transforming growth factor (TGF)-alpha, which has been shown to stimulate PC cell growth in an autocrine manner, to PC cells was not modified in cells treated with CT or 8-Cl-cAMP. The results suggest that the inhibitory actions of these substances do not occur at the level of the receptor for IGF-I or EGF/TGF-alpha. We have previously shown that phorbol esters, which decrease the binding of TGF-alpha to PC cells, has an anti-proliferative activity on these tumour cells. Inhibited cell growth by maximum suppressive dose of CT or 8-Cl-cAMP was further inhibited by TPA. In addition, an oncogene product of K-ras which is commonly activated in pancreatic cancer, was increased by CT and 8-Cl-cAMP. It is concluded that CT and 8-Cl-cAMP inhibit PC cell growth, presumably in a similar manner, and their mechanism(s) of action may be different from that of TPA. The anti-proliferative effect of CT or 8-Cl-cAMP was enhanced by TPA, implying that the combination of these substances results in increased inhibition of the PC cell growth.

Differential responsiveness to agents which stimulate cAMP production in normal versus neoplastic mouse lung epithelial cells

We examined the responsiveness of normal and neoplastic lung cells to agents which stimulate cAMP production. While their basal cAMP levels were similar, spontaneous in vitro transformant E9 cells and tumor-derived PCC4 cells produced much less cAMP in response to 1 microM isoproterenol compared to non-tumorigenic C10 cells derived from normal mouse lung epithelium. Iodocyanopindolol binding studies indicated that both neoplastic lines contained fewer beta-adrenergic receptors than normal C10 cells. When receptors were bypassed via treatment with 10 pM cholera toxin, the pattern of cAMP-responsiveness was reversed; both neoplastic cell lines produced more cAMP than C10 cells. Direct stimulation of adenylate cyclase with 100 microM forskolin greatly increased cAMP concentrations in all three cell lines. These anomalies at both the receptor and G-protein levels in neoplastic lung epithelial cells may contribute to their deregulated growth.

Dual regulation by cAMP of beta-hexosaminidase-induced mitogenesis in bovine tracheal myocytes

beta-Hexosaminidases, potent mitogens in bovine tracheal myocytes (BTM), stimulate a rapid and transient increase in intracellular cyclic adenosine monophosphate (cAMP) accumulation. The objective of this study was to elucidate the contribution of cAMP in hexosaminidase-induced airway muscle proliferation. Rate of DNA synthesis was measured by 3H-thymidine incorporation in quiescent cells prepared by a low-serum treatment (0.4%) for 48 h after reaching confluency in microtiter wells. cAMP accumulation was measured in acetylated cell extracts in the presence of isobutyl methylxanthine (100 microM) by radioimmunoassay using 125I-cAMP as tracer. Exposure of quiescent cells to purified human placental hexosaminidase B (5 micrograms/ml, 50 nM) caused a significant transient increase in cAMP accumulation (49 to 107 fmol/micrograms protein, or a 20- to 70-fold increase from basal level). Maximum increase occurred at 15 min followed by a rapid decline in cAMP accumulation within 30 min after exposure to hexosaminidase. Similar results were obtained in cells treated with neoglycoprotein mannose bovine serum albumin (100 to 500 nM). The increase in cAMP accumulation was inhibited by mannan (mannose receptor blocker, 0.1 mg/ml), as well as phenylisopropyladenosine (PIA; A1 receptor agonist that inhibits adenylyl cyclase, 0.1 to 1.0 microM). The increase in 3H-thymidine incorporation induced by hexosaminidase B was also inhibited by mannan and PIA. Exposure to 8-(4-chlorophenylthio)-cAMP (cpt-cAMP; a cell-permeable analog of cAMP, 100 microM) or forskolin (a direct activator of catalytic subunit of adenylyl cyclase, 24 microM) up to 6 h enhanced 3H-thymidine incorporation. In contrast, a prolonged exposure (18 to 30 h) to these agents inhibited 3H-thymidine incorporation.(ABSTRACT TRUNCATED AT 250 WORDS)