Synergistic cooperation between thapsigargin and phorbol ester for induction of nitric oxide synthesis in murine peritoneal macrophages

From Plant to Patient: Thapsigargin, a Tool for Understanding Natural Product Chemistry, Total Syntheses, Biosynthesis, Taxonomy, ATPases, Cell Death, and Drug Development

Thapsigargin, the first representative of the hexaoxygenated guaianolides, was isolated 40 years ago in order to understand the skin-irritant principles of the resin of the umbelliferous plant Thapsia garganica. The pronounced cytotoxicity of thapsigargin is caused by highly selective inhibition of the intracellular sarco-endoplasmic Ca²⁺-ATPase (SERCA) situated on the membrane of the endo- or sarcoplasmic reticulum. Thapsigargin is selective to the SERCA pump and to a minor extent the secretory pathway Ca²⁺/Mn²⁺ ATPase (SPCA) pump. Thapsigargin has become a tool for investigation of the importance of SERCA in intracellular calcium homeostasis. In addition, complex formation of thapsigargin with SERCA has enabled crystallization and structure determination of calcium-free states by X-ray crystallography. These results led to descriptions of the mechanism of action and kinetic properties of SERCA and other ATPases. Inhibition of SERCA depletes Ca²⁺ from the sarco- and endoplasmic reticulum provoking the unfolded protein response, and thereby has enabled new studies on the mechanism of cell death. Development of protocols for selective transformation of thapsigargin disclosed the chemistry and facilitated total synthesis of the molecule. Conversion of trilobolide into thapsigargin offered an economically feasible sustainable source of thapsigargin, which enables a future drug production. Principles for prodrug development were used by conjugating a payload derived from thapsigargin with a hydrophilic peptide selectively cleaved by proteases in the tumor. Mipsagargin was developed in order to obtain a drug for treatment of cancer diseases characterized by the presence of prostate specific membrane antigen (PSMA) in the neovascular tissue of the tumors. Even though mipsagargin showed interesting clinical effects the results did not encourage funding and consequently the attempt to register the drug has been abandoned. In spite of this disappointing fact, the research performed to develop the drug has resulted in important scientific discoveries concerning the chemistry, biosynthesis and biochemistry of sesquiterpene lactones, the mechanism of action of ATPases including SERCA, mechanisms for cell death caused by the unfolded protein response, and the use of prodrugs for cancer-targeting cytotoxins. The presence of toxins in only some species belonging to Thapsia also led to a major revision of the taxonomy of the genus.

Molecular Mechanism of Crocin Induced Caspase Mediated MCF-7 Cell Death: In Vivo Toxicity Profiling and Ex Vivo Macrophage Activation

BACKGROUND

Crocus sativus and its major constituent crocin are well established to have anti-cancer properties in breast cancer cells (MCF-7). However the role of C. sativus extract (CSE) and crocin on caspase signaling mediated MCF-7 cell death at molecular level is remains unclear. In this study, we tried to unravel role of CSE and crocin on caspase mediated MCF-7 cells death and their in vivo preclinical toxicity profiling and immune stimulatory effect.

MATERIALS AND METHODS

CSE extract was fractionated by HPLC and crocin was isolated and characterized by NMR, IR, and MS. MCF-7 cells were treated with both CSE and crocin and expression of Bcl-2 and Bax was assessed after 24 and 36 hours. Furthermore, caspase 3, caspase 8 and caspase 9 expression was determined by Western blotting after 24 hours of treatment. DNA fragmentation analysis was performed for genotoxicity of CSE and crocin in MCF-7 cells. The in vivo toxicity profile of CSE (300 mg/kg of b.wt) was investigated in normal Swiss albino mice. In addition, peritoneal macrophages were collected from crocin (1, 1.5 and 2 mg/kg body weight) treated mice and analyzed for ex vivo yeast phagocytosis.

RESULTS

Immunoblot analysis revealed that there was time dependent decline in anti-apoptotic Bcl-2 with simultaneous upregulation of Bax in CSE and crocin treated MCF-7 cells. Further CSE and crocin treatment downregulated caspase 8 and 9 and cleaved the caspase 3 after 24 hours. Both CSE and crocin elicited considerable DNA damage in MCF-7 cells at each concentration tested. In vivo toxicity profile by histological studies revealed no observable histopathologic differences in the liver, kidney, spleen, lungs and heart in CSE treated and untreated groups. Crocin treatment elicited significant dose and time dependent ex vivo yeast phagocytosis by peritoneal macrophages.

CONCLUSIONS

Our study delineated involvement of pro-apoptotic and caspase mediated MCF-7 cell death by CSE and crocin at the molecular level accompanied with extensive DNA damage. Further we found that normal swiss albino mice can tolerate the maximum dose of CSE. Crocin enhanced ex vivo macrophage yeast phagocytic ability.

Inhibitor of sarco-endoplasmic reticulum Ca2+-ATPase thapsigargin stimulates production of nitric oxide and secretion of interferon-gamma

Thapsigargin is a sesquiterpene lactone of guaianolide type isolated from the Mediterranean plant Thapsia garganica L. It is widely used experimentally as a potent and selective inhibitor of sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) leading to rapid elevation of intracellular calcium [Ca2+]i. Several previous reports have shown that thapsigargin interferes with production of nitric oxide (NO) by mouse peritoneal macrophages and mouse macrophage cell lines. The present data confirm that thapsigargin is a modest inducer of NO in mouse macrophages, production of NO being slightly enhanced by lipopolysaccharide. However, thapsigargin on its own very potently induces NO in macrophages of rats under conditions in vitro. The highest effect was observed after the concentration of 0.25 microM thapsigargin, producing approximately 30 microM accumulation of nitrites in supernatants of cells cultured for 24 h. The aim of our experiments was to investigate immune mechanisms implicated in activation of high-output NO biosynthesis. It has been found that thapsigargin dose-dependently induces secretion of interferon-gamma (IFN-gamma) in macrophages of both rats and mice, and also in human peripheral blood mononuclear cells. The IFN-gamma production was rather low in macrophages of mice while relatively very high levels of IFN-gamma were found in cultures of rat macrophages and human peripheral blood mononuclear cells. The concentration of IFN-gamma produced by 5 microM thapsigargin within the interval of 24 h exceeded 3 ng/ml in rat macrophages and approached 2 ng/ml in cultures of human peripheral blood mononuclear cells. The effects are mediated by mitogen-activated protein kinases (MAPKs) such as p38 mitogen-activated protein kinase (p38) and extracellular signal-regulated kinases 1/2 (ERK1/2), and by nuclear transcriptional factor NF-kappaB. In summary, the original findings demonstrate immunostimulatory potential of thapsigargin and warrant more detailed preclinical studies.

Thapsigargin, a selective inhibitor of sarco-endoplasmic reticulum Ca2+ -ATPases, modulates nitric oxide production and cell death of primary rat hepatocytes in culture

Increased cytosolic calcium ([Ca2+]i) and nitric oxide (NO) are suggested to be associated with apoptosis that is a main feature of many liver diseases and is characterized by biochemical and morphological features. We sought to investigate the events of increase in [Ca2+]i and endoplasmic reticulum (ER) calcium depletion by thapsigargin (TG), a selective inhibitor of sarco-ER-Ca2+ -ATPases, in relation to NO production and apoptotic and necrotic markers of cell death in primary rat hepatocyte culture. Cultured hepatocytes were treated with TG (1 and 5 micromol/L) for 0-24 or 24-48 h. NO production and inducible NO synthase (iNOS) expression were determined as nitrite levels and by iNOS-specific antibody, respectively. Hepatocyte apoptosis was estimated by caspase-3 activity, cytosolic cytochrome c content and DNA fragmentation, and morphologically using Annexin-V/propidium iodide staining. Hepatocyte viability and mitochondrial activity were evaluated by ALT leakage and MTT test. Increasing basal [Ca2+]i by TG, NO production and apoptotic/necrotic parameters were altered in different ways, depending on TG concentration and incubation time. During 0-24 h, TG dose-dependently decreased iNOS-mediated spontaneous NO production and simultaneously enhanced hepatocyte apoptosis. In addition, TG 5 micromol/L produced secondary necrosis. During 24-48 h, TG dose-dependently enhanced basal NO production and rate of necrosis. TG 5 micromol/L further promoted mitochondrial damage as demonstrated by cytochrome c release. A selective iNOS inhibitor, aminoguanidine, suppressed TG-stimulated NO production and ALT leakage from hepatocytes after 24-48 h. Our data suggest that the extent of the [Ca2+]i increase and the modulation of NO production due to TG treatment contribute to hepatocyte apoptotic and/or necrotic events.

Involvement of protein kinase C in the inhibition of lipopolysaccharide-induced nitric oxide production by thapsigargin in RAW 264.7 macrophages

This study explored the effects of inhibition of endoplasmic reticulum (ER) Ca(2+)-ATPase on lipopolysaccharide (LPS)-induced protein kinase C (PKC) activation, nuclear factor-kappaB (NF-kappaB) translocation, inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) production in RAW 264.7 macrophages. Thapsigargin (TG) irreversibly inhibits ER Ca(2+)-ATPase and LPS-induced NO production is reduced even after washout. TG also attenuated LPS-stimulated iNOS expression by using immunoblot analysis. However, another distinct fully reversible ER Ca(2+)-ATPase inhibitor, 2,5-di-tert-butylhydroquinone (DBHQ), ionophore A23187 and ionomycin could exert a similar effect to TG in increasing intracellular calcium concentration; however, these agents could not mimic TG in reducing iNOS expression and NO production. LPS increased PKC-alpha and -beta activation, and TG pretreatment attenuated LPS-stimulated PKC activation. Not did pretreatment with DBHQ, A23187 and ionomycin reduce LPS-stimulated PKC activation. Furthermore, NF-kappaB-specific DNA-protein-binding activity in the nuclear extracts was enhanced by treatment with LPS, and TG pretreatment attenuated LPS-stimulated NF-kappaB activation. None of DBHQ, A23187 and ionomycin pretreatment reduced LPS-stimulated NF-kappaB activation. These data suggest that persistent inhibition of ER Ca(2+)-ATPase by TG would influence calcium release from ER Ca2+ pools that was stimulated by the LPS activated signal processes, and might be the main mechanism for attenuating PKC and NF-kappaB activation that induces iNOS expression and NO production.

LAAE-14, a new in vitro inhibitor of intracellular calcium mobilization, modulates acute and chronic inflammation

A new lipidic acid-amido ether derivative (LAAE-14) able to reduce dose-dependently the calcium increases mediated either by calcium ionophore ionomycin, by the endoplasmic reticular Ca(2+)-ATPase inhibitor thapsigargin, or by the chemotactic tripeptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), in human neutrophils as well as in murine peritoneal macrophages, but not ATP, has been evaluated as a potential anti-inflammatory drug. This compound attenuated leukocyte activation by means of its inhibitory effect on the respiratory burst elicited in both types of cells by 12-O-tetradecanoyl phorbol 13-acetate, by inhibition of the degranulation process induced by cytochalasin B+fMLP or cytochalasin B+platelet activating factor, as well as by reduction of leukotriene B(4) synthesis induced by the calcium ionophore A23187. In addition, in zymosan-stimulated mouse peritoneal macrophages LAAE-14 caused a potent inhibition of nitrite and prostaglandin E(2) production. This compound exerted acute and chronic anti-inflammatory effects by oral route, that may be related with several mechanisms such as attenuation of leukocyte activation, inhibition of inducible nitric oxide synthase, cyclo-oxygenase-2 and cytosolic phospholipase A(2) expression as well as reduction in tumour necrosis factor-alpha production. Its anti-inflammatory profile is clearly correlated with its behavior as inhibitor of intracellular calcium mobilization. The profile and potency of this compound may have relevance for the inhibition of the inflammatory response at different levels and may represent a new approach to the development of new anti-inflammatory drugs.

VCAM-1-induced inwardly rectifying K(+) current enhances Ca(2+) entry in human THP-1 monocytes

Hyperpolarization in human leukemia THP-1 monocytes adherent to vascular cell adhesion molecule (VCAM)-1 is due to an induction of inwardly rectifying K(+) currents (I(ir)) (Colden-Stanfield M and Gallin EK, Am J Physiol Cell Physiol 275: C267-C277, 1998). We determined whether the VCAM-1-induced hyperpolarization is sufficient to augment the increase in intracellular free calcium ([Ca(2+)](i)) produced by Ca(2+) store depletion with thapsigargin (TG) and readdition of external CaCl(2) in fura 2-loaded THP-1 monocytes. Whereas there was a 2.1-fold increase in [Ca(2+)](i) in monocytes bound to glass for 5 h in response to TG and CaCl(2) addition, adherence to VCAM-1 produced a 5-fold increase in [Ca(2+)](i). Depolarization of monocytes adherent to VCAM-1 by I(ir) blockade or exposure to high [K(+)] abolished the enhancement of the peak [Ca(2+)](i) response. In monocytes bound to glass, hyperpolarization of the membrane potential with valinomycin, a K(+) ionophore, to the level of hyperpolarization seen in cells adherent to VCAM-1 produced similar changes in peak [Ca(2+)](i). Adherence of monocytes to E-selectin produced a similar peak [Ca(2+)](i) to cells bound to glass. Thus monocyte adherence to the physiological substrate VCAM-1 produces a hyperpolarization that is sufficient to enhance Ca(2+) entry and may impact Ca(2+)-dependent monocyte function.

Varied protocols of cadmium exposure produce different effects on nitric oxide production in macrophages

Different protocols of cadmium (Cd) exposure in non-cytotoxic conditions (i.e. 10 microM Cd for 18 h), and their effect on nitric oxide (NO) generation induced by NO inductor agents (NOIA) in peritoneal macrophages (pM) were studied. In all cases, NOIA (i.e. bacterial lipopolysaccharide [LPS], phorbol ester [PMA], okadaic acid [OA] or their combinations [LPS/OA] and [LPS/PMA]) were added at the beginning of the first incubation, only. Simultaneously exposure with 10 microM Cd enhanced NO generation and inducible NO synthase (iNOS) expression evoked by LPS, OA, PMA; those induced by LPS/PMA were not modified; and those caused by LPS/OA in relation to culture without Cd (medium) decreased. Double incubation, either with or without Cd (Cd+Cd or medium+medium), or Cd added at the start of the first or second incubation only (Cd+medium or medium+Cd), were tested. After the second incubation, medium+Cd protocol produced the highest NO generation in relation to other exposure protocols. When NO production was measured at the end of the second incubation, Cd+medium protocol enhanced NO production induced by OA, and LPS/OA, while medium+Cd protocol enhanced the response to LPS, PMA, and LPS/OA, in both cases in relation to the first incubation. Cd+Cd incubation protocol decreases the response to all NOIA in relation to another protocols. Cd effect on NO generation in macrophages is dependent on protocol and timing of exposure.

Treatment of the macrophage-like P388D.1 cells with bacterial lipopolysaccharide and interferon-gamma causes long-term alterations in calcium metabolism

The intracellular calcium concentrations ([Ca2+]i) of P338D.1 macrophage-like cells, activated with interferon-gamma (IFN-gamma) and/or bacterial lipopolysaccharide (LPS) were determined using fura-2/AM and ratiometric imaging techniques. Treatment of macrophages with IFN-gamma and LPS resulted in significant downward shift in [Ca2+]i, 8, 16 and 24 h but not at 1 and 4 h after treatment. The decrease in [Ca2+]i also occurred when macrophages were treated with LPS only, but not after exposure of the cells to recombinant IFN-gamma, indicating that LPS was an essential signal in the observed changes in [Ca2+]i of activated macrophages. The IFN-gamma and/or LPS alteration in the [Ca2+]i, paralleled the in vitro nitric oxide production of the activated macrophages, 8, 16 and 24 h after treatment. The decrease in the [Ca2+]i may be caused by vigorous buffering and storing of Ca2+ by macrophages to below the normal resting quantities, following the reported transient increase in Ca2+ during the priming stage of macrophage activation. Thus, the downward shift in [Ca2+]I may play a physiological role in the activation processes of macrophages for antimicrobial responses.

Induction of nitric oxide (NO) synthesis in murine macrophages requires potassium channel activity

The activation of macrophages for antimicrobial responses is a multistage event involving numerous intracellular signalling cascades that makes possible target cell destruction by these effector cells. This study examined the effects of different potassium channel inhibitors and activators on the NO production of murine macrophage-like cell lines P388D.1 and B10-4(S). We found that the potassium channel inhibitors tetraethylammonium, 4-aminopyridine, and quinine caused dose-dependent reductions in the NO production of macrophages, and that the potassium channel activator, minoxidol, caused a dose-dependent enhancement of NO production. The inhibition of NO production was due to involvement of potassium channels in the priming stage of macrophage activation, since pretreatment with the priming agent interferon-gamma partially restored the NO response of the macrophages. The results of this study demonstrate a link between potassium channel activity and the activation of anitimicrobial functions of murine macrophages.

ZR-75-1 human breast cancer cells: expression of inducible nitric oxide synthase and effect of tamoxifen and phorbol ester on nitric oxide production

The existence of the L-arginine-nitric oxide pathway was investigated in ZR-75-1 human breast cancer cells. The presence of inducible nitric oxide synthase in these cells was confirmed by staining with an anti-iNOS antibody. ZR-75-1 cells spontaneously produced nitric oxide (NO) and this production could be significantly (P < 0.001) enhanced by L-arginine (0.01-10 mM) and was inhibited by L-NAME (2 mM). Stimulating the cells with phorbol 12-myristate 13-acetate (PMA) (200-1000 nM) resulted in a significant (P < 0.001) increase in NO2- secreted into the medium. Although treatment of the same cells with tamoxifen (10(-10)-10(-6) M) had no effect on NO production, tamoxifen was able to significantly (P < 0.001) downregulate PMA-enhanced nitrite production. Our results suggest that tamoxifen could play a role in the biology of nitric oxide in breast tumours.