Bombesin stimulation of c-fos expression and mitogenesis in Swiss 3T3 cells: the role of prostaglandin E2-mediated cyclic AMP accumulation

Cytotoxic effects of ellagitannins isolated from walnuts in human cancer cells

Walnuts contain many bioactive components that may slow cancer growth. A previous report showed that a diet supplemented with walnuts decreased the tumor size formed by MDA-MB-231 human cancer cells injected into nude mice. However, the mechanism of action was never determined. We characterized the effects of a methanol extract prepared from walnuts on human MDA-MB-231, MCF7, and HeLa cells. The extract was cytotoxic to all cancer cells. We identified compounds from the methanol extract that induced this cytotoxicity. The predominant compounds were Tellimagrandin I and Tellimagrandin II, members of the ellagitannin family. We also show a walnut extract decreases the intracellular pH, depolarizes the mitochondrial membrane with release of cytochrome c and phosphatidylserine flipping. The antimitogenic effects of walnut extract were associated with a twofold reduction of mitochondria respiration. These results suggest impairment of mitochondrial function and apoptosis as relevant mechanism of anticancer effects of the walnut extract.

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.

Gastrin and EGF synergistically induce cyclooxygenase-2 expression in Swiss 3T3 fibroblasts that express the CCK2 receptor

Over-expression of cyclooxygenase-2 (COX-2) has been demonstrated to be tumorigenic in transgenic mice. Chronic treatment with NSAIDs is chemoprotective for colorectal cancer. Gastrin is a growth factor for gastric mucosa and has been shown to promote proliferation of colorectal cells. Recent studies suggest that COX-2 expression levels could mediate the growth effects of gastrin. Here, we report that gastrin increased PGE2 secretion in Swiss 3T3 cells expressing the CCK2 receptor. Gastrin dose dependently induced COX-2 protein levels in a time dependent manner. COX-2 mRNA levels were rapidly induced by a dose dependent increase in gastrin. Prior treatment of the cells with the CCK2 receptor specific antagonist, L365,260, inhibited gastrin-induced COX-2 protein and mRNA expression. Pretreatment with L364,714, the CCK1 receptor specific antagonist did not block COX-2 induction by gastrin. Inhibition of de novo protein synthesis by cycloheximide did not block COX-2 mRNA induction by gastrin. Also, gastrin-dependent COX-2 expression did not require PKC activity, activation of ERK, or transactivation of EGFR. However, co-stimulation with EGF and gastrin synergistically induced COX-2 protein and mRNA expression and PGE2 secretion. Measurements of COX-2 mRNA stability and COX-2 gene transcription reveal that EGF significantly increased the half-life of COX-2 mRNA with only a slight increase in the COX-2 transcription rate. Conversely, gastrin significantly increased COX-2 gene transcription rates but did not enhance COX-2 mRNA stability.

Combined effects of prostaglandin E receptor subtype EP1 and subtype EP4 antagonists on intestinal tumorigenesis in adenomatous polyposis coli gene knockout mice

Previous studies have shown that prostaglandin E(2) (PGE(2)) is involved in intestinal carcinogenesis through its binding to the PGE(2) receptor subtypes EP(1) and EP(4) and activation of downstream pathways. ONO-8711 and ONO-AE2-227, prostaglandin E receptor subtype EP(1)- and EP(4)-selective antagonists, respectively, are known to suppress formation of intestinal polyps in adenomatous polyposis coli gene-deficient mice. The present study was designed to investigate the combined effects of EP(1) and EP(4) antagonists on spontaneous polyp formation in APC1309 mice in order to determine the contribution of each receptor to intestinal tumorigenesis. APC1309 mice were treated with 400 ppm of ONO-8711 alone, 400 ppm of ONO-AE2-227 alone or both in combination in the diet for 6 weeks. The mean area of polyps found in the intestine, calculated as the longer diameter x the shorter diameter x pi, was reduced by 12%, 43% (P < 0.01) and 56% (P < 0.01) of the mean control value (8.8 mm(2)) in the ONO-8711 alone, ONO-AE2-227 alone and combination treatment groups, respectively, suggesting clear additive effects of the combination. The same additive tendency for suppression was also observed with respect to the numbers of polyps in the intestine. Polyp size reduction was more remarkable with the EP(4) antagonist, while the number reduction was more pronounced with the EP(1) antagonist. Our results indicate that EP(1) and EP(4) may have separate intrinsic roles and, to some extent, contribute to polyp formation independently. Thus, combination treatment has potential for the chemoprevention of colon carcinogenesis.

GRP receptor-mediated immediate early gene expression and transcription factor Elk-1 activation in prostate cancer cells

Bombesin (BN) and its mammalian homologue gastrin-releasing peptide (GRP) have been shown to play an important role in human cancer as autocrine and paracrine growth factors. Prostatic neuroendocrine cells are thought to secrete these regulatory peptides and they may therefore interact with their specific, aberrantly expressed GRP receptor (GRP-R) in prostate cancer. In this study, we investigated the effect of BN on immediate early gene expression in two androgen-independent prostate cancer cell lines DU-145 and PC-3 with functional GRP receptor. We found that BN induced c-fos mRNA expression in both cell lines in a time-dependent manner. In contrast, c-jun mRNA was only modestly induced in DU-145 cells but not at all in PC-3 cells. On the protein level, we detected BN-induced stimulation of the c-fos gene product but not of c-jun protein. Sustained increase of the c-myc gene product was detectable in PC-3 but not in DU-145 cells. Concurrently, we demonstrated BN-dependent activation of the transcription factor Elk-1 and significant increase of cell proliferation in both prostate cancer cell lines. Taken together, these data suggest that BN acts as a mitogen in prostate cancer and this might be associated with the activation of the transcription factor Elk-1 and the immediate early gene c-fos.

Prostaglandin E2 and the protein kinase A pathway mediate arachidonic acid induction of c-fos in human prostate cancer cells

Arachidonic acid (AA) is the precursor for prostaglandin E2 (PGE2) synthesis and increases growth of prostate cancer cells. To further elucidate the mechanisms involved in AA-induced prostate cell growth, induction of c-fos expression by AA was investigated in a human prostate cancer cell line, PC-3. c-fos mRNA was induced shortly after addition of AA, along with a remarkable increase in PGE2 production. c-fos expression and PGE2 production induced by AA was blocked by a cyclo-oxygenase inhibitor, flurbiprofen, suggesting that PGE2 mediated c-fos induction. Protein kinase A (PKA) inhibitor H-89 abolished induction of c-fos expression by AA, and partially inhibited PGE2 production. Protein kinase C (PKC) inhibitor GF109203X had no significant effect on c-fos expression or PGE2 production. Expression of prostaglandin (EP) receptors, which mediate signal transduction from PGE2 to the cells, was examined by reverse transcription polymerase chain reaction in several human prostate cell lines. EP4 and EP2, which are coupled to the PKA signalling pathway, were expressed in all cells tested. Expression of EP1, which activates the PKC pathway, was not detected. The current study showed that induction of the immediate early gene c-fos by AA is mediated by PGE2, which activates the PKA pathway via the EP2/4 receptor in the PC-3 cells.

Distinct signaling pathways mediate induction of c-fos by PGE2 in glomerular mesangial cells

Prostanoids induce expression of several immediate-early genes but the molecular mechanisms underlying these responses remain poorly characterized. We have studied induction of the proto-oncogenc c-fos by PGE2 in mesangial cells as a model of gene regulation by prostanoids. PGE2 induced marked and transient accumulation of c-fos mRNA. Induction of c-fos by PGE2 and TxA2 did not correlate with induction of phospholipase C. Addition of exogenous cAMP failed to induce c-fos mRNA, suggesting that activation of an EP2 receptor linked to adenylate cyclase did not account for induction of c-fos by PGE2. These data contrast with previous experiments in NIH 3T3 cells where PGE2 induced c-fos by a cAMP-dependent mechanism. We further showed that PGE2 induces the c-fos gene by direct activation of the serum response element. Taken together these experiments provide evidence for a pathway linking a PGE2 receptor on the plasma membrane to transcriptional activation in the nucleus.

Gastrin-releasing peptide-induced expression of prostaglandin synthase-2 in Swiss 3T3 cells

Prostaglandins, produced in response to mitogens and cytokines, are potent modulators of gastrointestinal physiology and pathophysiology. We investigated modulation of Prostaglandin synthase 2 (PGS-2) expression by the gastrin-releasing peptide (GRP) receptor in Swiss 3T3 cells. PGS-2 mRNA expression in Swiss 3T3 cells was determined by Northern blot analysis. PGS-2 protein expression in Swiss 3T3 cells was measured by Western blot analysis. GRP caused a transient induction of PGS-2 mRNA in Swiss 3T3 cells that resulted in GRP-dependent expression of PGS-2 protein. Transcriptional activation of PGS-2 by GRP was independent of de novo protein synthesis and was not affected by pertussis toxin. Comparison of signaling pathways used by PMA or EGF to those used by GRP showed that PGS-2 induction by GRP increased under conditions that inhibit PKC activity. Dexamethasone, which blocks PMA and EGF induction of PGS-2, also inhibited GRP-induced accumulation of PGS-2 mRNA. These results show that PGS-2 expression in Swiss 3T3 cells is not only controlled by PKC and receptor tyrosine kinase pathways but also by G-protein coupled receptor signaling pathways.

Interleukin-2 inhibits 3T3 fibroblast proliferation

In order to clarify the role played by interleukin-2 (IL-2) in the regulation of fibroblast function, we investigated the effect of rat IL-2 and human recombinant IL-2 on 3T3 fibroblast proliferation and collagen synthesis. Fibroblasts were incubated with various concentrations of IL-2 for different periods of time. IL-2 was found to decrease in time- and dose-dependent manner the proliferation of 3T3 fibroblasts. This effect correlated with ability of IL-2 to enhance PGE2 production by 3T3 fibroblasts. When 3T3 fibroblasts were cocultured with rat peritoneal mast cells (MC), the growth-inhibiting effect of IL-2 was significantly less pronounced. Treatment of the cultures with IL-2 had no effect on collagen production by both 3T3 fibroblasts and fibroblasts cocultured with MC. In conclusion, in this study we provide evidence that IL-2, the key cytokine in T-cell growth and differentiation, can affect fibroblast functions.

Neural and biochemical mediators of endotoxin and stress-induced c-fos expression in the rat brain

We and others have reported that c-fos protein is induced in the hypothalamus and brain stem of the rat following central and peripheral injections of endotoxin (lipopolysaccharide; LPS). We have now examined possible mechanisms through which LPS induces c-fos protein. The cyclooxygenase inhibitor indomethacin and the glutamate NMDA antagonist MK801 inhibited c-fos protein in the paraventricular nucleus (PVN), supraoptic nucleus (SON), and the A1/A2 regions of the brain stem induced by IP or IV injections of LPS (40 micrograms). The H1 histamine antagonist diphenhydramine, but not the H2 histamine antagonist cimetidine, reduced the amount of c-fos labeling. MK801 also attenuated the effects of stress (foot shock) on c-fos protein; however, indomethacin had no effect on c-fos protein induced by stress. We next examined the importance of visceral afferent innervation on the response to LPS or stress. Subdiaphragmatic vagotomy completely blocked the induction of c-fos protein following IP injections of LPS; however, vagotomy had a minimal effect on c-fos protein induced in the PVN and SON following IV injections of LPS, but potentiated c-fos induction following foot shock. Thus, prostaglandin synthesis, glutamate release, histamine receptors, and visceral afferents represent functional biochemical and neural pathways through which endotoxin activates c-fos protein in specific autonomic and neuroendocrine regulatory nuclei. Activation of NMDA glutamate receptors may represent a final common pathway for the induction of c-fos protein in the brain induced by both endotoxin and stress.

Inhibitory effect of bombesin/gastrin-releasing peptide (GRP) antagonists RC-3950-II and RC-3095 on MCF-7 MIII human breast cancer xenografts in nude mice

Bombesin/gastrin-releasing peptide (GRP) may be involved in the growth of human breast cancers. Nude mice bearing xenografts of MCF-7 MIII human breast cancer cell line were treated for 7 weeks with bombesin/GRP antagonists RC-3950-II and RC-3095. RC-3950-II, administered sc twice daily at a dose of 10 micrograms, produced significant inhibitory effects on tumor growth after 2 weeks of administration. RC-3095 acetate (D 22213), injected sc twice daily at the same dose of 10 micrograms, suppressed tumor growth after 4 weeks. Both RC-3950-II and RC-3095 significantly decreased the final tumor volume and tumor weights. RC-3950-II appeared to be somewhat more efficacious than RC-3095 in inhibiting the growth of MCF-7 MIII breast cancers. Chronic treatment with either bombesin/GRP antagonist caused down-regulation of receptors for epidermal growth factor (EGF) in tumor cell membranes, which might be related to inhibition of tumor growth. These findings suggest that bombesin/GRP antagonists should be considered for a new endocrine therapy of breast cancer.

[The effect of static magnetic fields on prostaglandin synthesis in L-929 and 3T3 mouse fibroblasts. An in-vitro study]

The aim of this study was to examine the influence of static, inhomogeneous magnetic fields on prostaglandin synthesis in L-929 and 3T3 mouse fibroblasts. Two pairs of magnets, one CoSm and the other NdFeB, were attached 10 mm apart under the culture chamber. One pair was in the attracting position and the other in the repelling position. The maximum magnetic induction measured was 85 respectively 120 mT and the incubation time was 72 hours. Control groups consisted of untreated cells and cells under the influence of demagnetized magnets. The following measurements were taken: prostaglandin synthesis (PGE2), protein content (GP), and the number (Z) of cells (only L-929 mouse fibroblasts). The data were analyzed statistically using the Tukey test and the significance niveau was set at alpha = 0.05. The results using this method demonstrated that the measurable effects of in vitro treatment of mouse fibroblasts with static magnetic fields of varying induction did not significantly differ from the control groups. The sensitivity of our method was verified beforehand with a recombinant, human TNF alpha stimulated reference group.