Tamoxifen inhibits the release of arachidonic acid stimulated by thapsigargin in estrogen receptor-negative A549 cells

Thapsigargin induces apoptosis by impairing cytoskeleton dynamics in human lung adenocarcinoma cells

The objective of this study was performed to investigate the effects of thapsigargin on apoptosis, actin cytoskeletal dynamics, and actin cytoskeletal proteins in human lung adenocarcinoma cell. Thapsigargin is a specific irreversible inhibitor of ER calcium-ATPase, which may promote ER stress by depletion of lumenal calcium stores and show potential to induce cell death. The effects of thapsigargin on the apoptosis in A549 cells were assayed by Hoechst staining. Moreover, the F-actin staining by Rhodamine-phalloidin and RhoA antibody for cytoskeleton organizations were applied to A549 cells. To confirm the impairment of cytoskeletal dynamics treated with thapsigargin, western blots were applied to analyze the protein levels of p-Cofilin-1 (Ser3), Cofilin-1, and pPaxillin (Tyr118), as well as RhoA and pS6 (S240/244). Results suggest that thapsigargin may induce cell death in A549 cells with a time- and dose-dependent manner. The F-actin fibers and RhoA signals are also reduced with a time- and dose-dependent manner by thapsigargin treatment. The phosphorylation forms of Cofilin-1 and paxillin are attenuated by 1 μM thapsigargin treatment for 24 h. These alternations may be caused by the inhibition of of mTORC1 activities (indicated by pS6 (Ser240/244)) and RhoA pathways after thapsigargin treatment. The present findings highlight important roles of calcium entry in cytoskeleton organization and apoptosis in human lung adenocarcinoma cells and will help to set a stage to the clinical treatment of cancer cell metastasis.

Regulation of connexin36 gap junction channels by n-alkanols and arachidonic acid

We examined junctional conductance (gj) and its dependence on transjunctional voltage in gap junction (GJ) channels formed of wild-type connexin36 (Cx36) or its fusion form with green fluorescent protein (Cx36-EGFP) transfected in HeLa cells or endogenously expressed in primary culture of pancreatic β-cells. Only a very small fraction (∼0.8%) of Cx36-EGFP channels assembled into junctional plaques of GJs were open under control conditions. We found that short carbon chain n-alkanols (SCCAs) increased gj, while long carbon chain n-alkanols resulted in full uncoupling; cutoff is between heptanol and octanol. The fraction of functional channels and gj increased several fold under an exposure to SCCAs, or during reduction of endogenous levels of arachidonic acid (AA) by exposure to fatty acid-free BSA or cytosolic phospholipase A2 inhibitors. Moreover, uncoupling caused by exogenously applied AA can be rescued by BSA, which binds AA and other polyunsaturated fatty acids (PUFAs), but not by BSA modified with 1,2-cyclohexanedione, which does not bind AA and other PUFAs. We propose that under control conditions, Cx36 GJ channels in HeLa transfectants and β-cells are inhibited by endogenous AA, which stabilizes a closed conformational state of the channel that leads to extremely low fraction of functional channels. In addition, SCCAs increase gj by interfering with endogenous AA-dependent inhibition, increasing open probability and the fraction of functional channels.

Polychlorinated biphenyls induce arachidonic acid release in human platelets in a tamoxifen sensitive manner via activation of group IVA cytosolic phospholipase A2-alpha

Polychlorinated biphenyls (PCBs) are stable compounds commonly found in nature as environmental pollutants. PCBs can affect the endocrine function of hormones such as steroid-hormones. Also, PCBs are known to be inducers of arachidonic acid release in various cells. We report, here, the effects of PCBs on eicosanoid formation, arachidonic acid release and cytosolic phospholipase A2-alpha (cPLA2-alpha) activation in human platelets. Ortho-substituted PCBs induced a time and dose-dependent release of arachidonic acid and the concomitant formation of 12(S)-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid (12-HETE) and 12(S)-hydroxy-5-cis-8,10-trans-heptadecatrienoic acid (12-HHT) in human platelets. The release of arachidonic acid and the formation of 12-HETE was completely blocked by the cPLA2-alpha inhibitors AACOCF3 or pyrrolidine-1. PCB-treatment of platelets demonstrated that the cPLA2-alpha protein as well as PLA2 activity translocated to the membrane fraction, independent of a rise in intracellular Ca2+. Furthermore, electrophoretic gel mobility shift analysis of cPLA2-alpha on SDS-PAGE demonstrated a PCB-dependent phosphorylation of cPLA2-alpha. The effects of 17beta-estradiol and two structurally unrelated anti-estrogens, nafoxidin and tamoxifen on PCB-induced arachidonic acid release in platelets were also investigated. Both nafoxidin and tamoxifen inhibited PCB-induced arachidonic acid release as well as 12-HETE and 12-HHT formation. Interestingly, platelets incubated with PCBs did not aggregate despite the fact that robust release of arachidonic acid was observed. In summary, these results demonstrate that certain PCBs induce activation of cPLA2-alpha independent of a rise in intracellular calcium and a robust release of arachidonic acid release with resulting eicosanoid formation in human platelets.

Anti-inflammatory effects of resveratrol in lung epithelial cells: molecular mechanisms

Resveratrol (3,4′,5-trihydroxystilbene) is a polyphenolic stilbene found in the skins of red fruits, including grapes, that may be responsible for some of the health benefits ascribed to consumption of red wine. Resveratrol has been shown to have antioxidant properties and can act as an estrogen agonist. This study examined the anti-inflammatory effects of resveratrol on human airway epithelial cells. Resveratrol and the related molecule quercetin, but not deoxyrhapontin, inhibited IL-8 and granulocyte-macrophage colony-stimulating factor release from A549 cells. Neither the estrogen receptor antagonist tamoxifen nor the glucocorticoid antagonist mifepristone altered the inhibitory effect of resveratrol. The mechanism of resveratrol action was investigated further using luciferase reporter genes stably transfected into A549 cells. Resveratrol and quercetin inhibited NF-κB-, activator protein-1-, and cAMP response element binding protein-dependent transcription to a greater extent than the glucocorticosteroid dexamethasone. These compounds also had no significant effect on acetylation or deacetylation of core histones. Resveratrol, but not estradiol or N-acetyl cysteine, inhibited cytokine-stimulated inducible nitric oxide synthase expression and nitrite production (IC50 = 3.6 ± 2.9 μM) in human primary airway epithelial cells. Resveratrol also inhibited granulocyte-macrophage colony-stimulating factor release (IC50 = 0.44 ± 0.17 μM), IL-8 release (IC50 = 4.7 ± 3.3 μM), and cyclooxygenase-2 expression in these cells. This study demonstrates that resveratrol and quercetin have novel nonsteroidal anti-inflammatory activity that may have applications for the treatment of inflammatory diseases.

Tamoxifen and the Rafoxifene analog LY117018: their effects on arachidonic acid release from cells in culture and on prostaglandin I2 production by rat liver cells

BACKGROUND

Tamoxifen is being used successfully to treat breast cancer. However, tamoxifen also increases the risk of developing endometrial cancer in postmenopausal women. Raloxifene also decreases breast cancer in women at high risk and may have a lower risk at developing cancer of the uterus. Tamoxifen has been shown to stimulate arachidonic acid release from rat liver cells. I have postulated that arachidonic acid release from cells may be associated with cancer chemoprevention.

METHODS

Rat liver, rat glial, human colon carcinoma and human breast carcinoma cells were labelled with [3H] arachidonic acid. The release of the radiolabel from these cells during incubation with tamoxifen and the raloxifene analog LY117018 was measured. The prostaglandin I2 produced during incubation of the rat liver cells with microM concentrations of tamoxifen and the raloxifene analog was quantitatively estimated.

RESULTS

Tamoxifen is about 5 times more effective than LY117018 at releasing arachidonic acid from all the cells tested. In rat liver cells only tamoxifen stimulates basal prostaglandin I2 production and that induced by lactacystin and 12-O-tetradecanoyl-phorbol-13-acetate. LY117018, however, blocks the tamoxifen stimulated prostaglandin production. The stimulated prostaglandin I2 production is rapid and not affected either by preincubation of the cells with actinomycin or by incubation with the estrogen antagonist ICI-182,780.

CONCLUSIONS

Tamoxifen and the raloxifene analog, LY117018, may prevent estrogen-independent as well as estrogen-dependent breast cancer by stimulating phospholipase activity and initiating arachidonic acid release. The release of arachidonic acid and/or molecular reactions that accompany that release may initiate pathways that prevent tumor growth. Oxygenation of the intracellularly released arachidonic acid and its metabolic products may mediate some of the pharmacological actions of tamoxifen and raloxifene.

Tamoxifen stimulates arachidonic acid release from rat liver cells by an estrogen receptor-independent, non-genomic mechanism

Background

Tamoxifen is widely prescribed for the treatment of breast cancer. Its success has been attributed to the modulation of the estrogen receptor. I have previously proposed that the release of arachidonic acid from cells may also mediate cancer prevention.

Methods

Rat liver cells were radiolabelled with arachidonic acid. The release of [³H] arachidonic acid after various times of incubation of the cells with tamoxifen was measured.

Results

Tamoxifen, at micromolar concentrations, stimulates arachidonic acid release. The stimulation is rapid and is not affected by pre-incubation of the cells with actinomycin or the estrogen antagonist ICI-182,780.

Conclusions

The stimulation of AA release by tamoxifen is not mediated by estrogen receptor occupancy and is non-genomic.

Tamoxifen-induced growth arrest and apoptosis in pituitary tumor cells in vitro via a protein kinase C-independent pathway

Protein kinase C (PKC), a kinase family involved in cell signal transduction, is overexpressed in most pituitary adenoma cells. We studied the effect of tamoxifen, an estrogen receptor antagonist and also a protein kinase inhibitor, on pituitary tumor cell proliferation and the induction of apoptosis; and we compared its effects with those of another PKC inhibitor, staurosporine. Tamoxifen induced growth arrest and apoptosis in a mouse pituitary adenoma cell line, AtT20, and in low-passage human primary pituitary tumor cell cultures. Staurosporine also inhibited pituitary tumor cell growth. PKC activity in AtT20 cells was inhibited by staurosporine and by prolonged treatment with phorbol myristate acetate, which down-regulates PKC activity, but not by tamoxifen, at the dosages used to induce apoptosis. Our findings suggest that tamoxifen induces apoptosis in AtT20 cells independent of a classical PKC isozyme pathway.

A combination of lactacystin and 12-O-tetradecanoylphorbol-13-acetate enhances the release of arachidonic acid from cells in cultures: release is reduced in confluent and serum deprived cells

Lactacystin, a selective inhibitor of some proteasome activities, in the presence of 12-tetradecanoyl phorbol-13-acetate stimulates the release of arachidonic acid from the phospholipids of bovine pulmonary artery endothelial, bovine embryonic thoracic aorta smooth muscle, rat liver and rat glial cells. The enhanced release from rat liver cells is inhibited by dexamethasone, aspirin, Na salicylate and the immunosuppressive fungal metabolite, gliotoxin. The effects of aspirin plus dexamethasone are not additive, but those of gliotoxin plus dexamethasone are additive. The stimulated and unstimulated release from all four cells are decreased when these cells are grown to confluence or when they are growth arrested by serum deprivation. The unstimulated as well as the stimulated release is partially inhibited by the cytosolic phospholipase A2 inhibitor, methylarachidonyl fluorophosphonate. The enhanced release is reduced in the presence of the translation and transcription inhibitors, actinomycin and cycloheximide.

Tamoxifen inhibits acidification in cells independent of the estrogen receptor

Tamoxifen has been reported to have numerous physiological effects that are independent of the estrogen receptor, including sensitization of resistant tumor cells to many chemotherapeutic agents. Drug-resistant cells sequester weak base chemotherapeutics in acidic organelles away from their sites of action in the cytosol and nucleus. This work reports that tamoxifen causes redistribution of weak base chemotherapeutics from acidic organelles to the nucleus in drug-resistant cells. Agents that disrupt organelle acidification (e.g., monensin, bafilomycin A1) cause a similar redistribution. Measurement of cellular pH in several cell lines reveals that tamoxifen inhibits acidification of endosomes and lysosomes without affecting cytoplasmic pH. Similar to monensin, tamoxifen decreased the rate of vesicular transport though the recycling and secretory pathways. Organellar acidification is required for many cellular functions, and its disruption could account for many of the side effects of tamoxifen.

Metabotropic glutamate receptor modulation of glutamate responses in the suprachiasmatic nucleus

Glutamate is the primary excitatory transmitter in the suprachiasmatic nucleus (SCN). Ionotropic glutamate receptors (iGluRs) mediate transduction of light information from the retina to the SCN, an important circadian clock phase shifting pathway. Metabotropic glutamate receptors (mGluRs) may play a significant modulatory role. mGluR modulation of SCN responses to glutamate was investigated with fura-2 calcium imaging in SCN explant cultures. SCN neurons showed reproducible calcium responses to glutamate, kainate, and N-methyl-D-aspartate (NMDA). Although the type I/II mGluR agonists L-CCG-I and t-ACPD did not evoke calcium responses, they did inhibit kainate- and NMDA-evoked calcium rises. This interaction was insensitive to pertussis toxin. Protein kinase A (PKA) activation by 8-bromo-cAMP significantly reduced iGluR inhibition by mGluR agonists. The inhibitory effect of mGluRs was enhanced by activating protein kinase C (PKC) and significantly reduced in the presence of the PKC inhibitor H7. Previous reports show that L-type calcium channels can be modulated by PKC and PKA. In SCN cells, about one-half of the calcium rise evoked by kainate or NMDA was blocked by the L-type calcium channel antagonist nimodipine. Calcium rises evoked by K+ were used to test whether mGluR inhibition of iGluR calcium rises involved calcium channel modulation. These calcium rises were primarily attributable to activation of voltage-activated calcium channels. PKC activation inhibited K+-evoked calcium rises, but PKC inhibition did not affect L-CCG-I inhibition of these rises. In contrast, 8Br-cAMP had no effect alone but blocked L-CCG-I inhibition. Taken together, these results suggest that activation of mGluRs, likely type II, modulates glutamate-evoked calcium responses in SCN neurons. mGluR inhibition of iGluR calcium rises can be differentially influenced by PKC or PKA activation. Regulation of glutamate-mediated calcium influx could occur at L-type calcium channels, K+ channels, or at GluRs. It is proposed that mGluRs may be important regulators of glutamate responsivity in the circadian system.

Tamoxifen induces selective membrane association of protein kinase C epsilon in MCF-7 human breast cancer cells

Tamoxifen, a synthetic antiestrogen, is known for its antitumoral action in vivo; however, it is well accepted that many tamoxifen effects are elicited via estrogen receptor-independent routes. Previously, we reported that tamoxifen induces PKC translocation in fibroblasts. In the present study, we investigated the influence of tamoxifen, and several triphenylethylene derivatives, on protein kinase C (PKC) in MCF-7 human breast cancer cells. As measured by Western blot analysis, tamoxifen elicited isozyme-specific membrane association of PKC-epsilon, which was time-dependent (as early as 5 min post-treatment) and dose-dependent (5.0-20 microM). Tamoxifen did not influence translocation of alpha, beta, gamma, delta or zeta PKC isoforms. Structure-activity relationship studies demonstrated chemical requirements for PKC-epsilon translocation, with tamoxifen, 3-OH-tamoxifen and clomiphene being active. Compounds without the basic amino side chain, such as triphenylethylene, or minus a phenyl group, such as N,N-dimethyl-2-[(4-phenylmethyl)phenoxy]ethanamine, were not active. In vitro cell growth assays showed a correlation between agent-induced PKC-epsilon translocation and inhibition of cell growth. Exposure of cells to clomiphene resulted in apoptosis. Since PKC-epsilon has been associated with cell differentiation and cellular growth-related processes, the antiproliferative influence of tamoxifen on MCF-7 cells may be related to the interaction with PKC-epsilon.