Magnolol induces cytosolic-free Ca2+ elevation in rat neutrophils primarily via inositol trisphosphate signalling pathway

Insights on the Multifunctional Activities of Magnolol

Over years, various biological constituents are isolated from Traditional Chinese Medicine and confirmed to show multifunctional activities. Magnolol, a hydroxylated biphenyl natural compound isolated from Magnolia officinalis, has been extensively documented and shows a range of biological activities. Many signaling pathways include, but are not limited to, NF-κB/MAPK, Nrf2/HO-1, and PI3K/Akt pathways, which are implicated in the biological functions mediated by magnolol. Thus, magnolol is considered as a promising therapeutic agent for clinic research. However, the low water solubility, the low bioavailability, and the rapid metabolism of magnolol dramatically limit its clinical application. In this review, we will comprehensively discuss the last five-year progress of the biological activities of magnolol, including anti-inflammatory, antimicroorganism, antioxidative, anticancer, neuroprotective, cardiovascular protection, metabolism regulation, and ion-mediating activity.

The effect of magnolol on Ca2+ homeostasis and its related physiology in human oral cancer cells

OBJECTIVE

Magnolol, a polyphenol compound from herbal medicines, was shown to alter physiology in various cell models. However, the effect of magnolol on Ca²⁺ homeostasis and its related physiology in oral cancer cells is unclear. This study examined whether magnolol altered Ca²⁺ signaling and cell viability in OC2 human oral cancer cells.

METHODS

Cytosolic Ca²⁺ concentrations ([Ca²⁺]_i) in suspended cells were measured by using the fluorescent Ca²⁺-sensitive dye fura-2. Cell viability was examined by 4-[3-[4-lodophenyl]-2-4(4-nitrophenyl)-2H-5-tetrazolio-1,3-benzene disulfonate] water soluble tetrazolium-1 (WST-1) assay.

RESULTS

Magnolol at concentrations of 20-100 μM induced [Ca²⁺]_i rises. Ca²⁺ removal reduced the signal by approximately 50%. Magnolol (100 μM) induced Mn²⁺ influx suggesting of Ca²⁺ entry. Magnolol-induced Ca²⁺ entry was partially suppressed by protein kinase C (PKC) regulators, and inhibitors of store-operated Ca²⁺ channels. In Ca²⁺-free medium, treatment with the endoplasmic reticulum Ca²⁺ pump inhibitor 2,5-di-tert-butylhydroquinone (BHQ) abolished magnolol-evoked [Ca²⁺]_i rises. Conversely, treatment with magnolol abolished BHQ-evoked [Ca²⁺]_i rises. Inhibition of phospholipase C (PLC) with U73122 partially inhibited magnolol-induced [Ca²⁺]_i rises. Magnolol at 20-100 μM decreased cell viability, which was not reversed by pretreatment with the Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA/AM).

CONCLUSIONS

Together, in OC2 cells, magnolol induced [Ca²⁺]_i rises by evoking partially PLC-dependent Ca²⁺ release from the endoplasmic reticulum and Ca²⁺ entry via PKC-sensitive store-operated Ca²⁺ entry. Magnolol also caused Ca²⁺-independent cell death. Therefore, magnolol-induced cytotoxicity may not be involved in activation mechanisms associated with intracellular Ca²⁺ mobilization in oral cancer cells.

Antagonism of human formyl peptide receptor 1 with natural compounds and their synthetic derivatives

Formyl peptide receptor 1 (FPR1) regulates a wide variety of neutrophil functional responses and plays an important role in inflammation and the pathogenesis of various diseases. To date, a variety of natural and synthetic molecules have been identified as FPR1 ligands. Here, we review current knowledge on natural products and natural product-inspired small molecules reported to antagonize and/or inhibit the FPR1-mediated responses. Based on this literature, additional screening of selected commercially available natural compounds for their ability to inhibit fMLF-induced Ca(2+) mobilization in human neutrophils and FPR1 transfected HL-60 cells, and pharmacophore modeling, natural products with potential as FPR1 antagonists are considered and discussed in this review. The identification and characterization of natural products that antagonize FPR1 activity may have potential for the development of novel therapeutics to limit or alter the outcome of inflammatory processes.

Cardiovascular protection of magnolol: cell-type specificity and dose-related effects

Magnolia officinalis has been widely used in traditional Chinese medicine. Magnolol, an active component isolated from Magnolia officinalis, is known to be a cardiovascular protector since 1994. The multiplex mechanisms of magnolol on cardiovascular protection depends on cell types and dosages, and will be reviewed and discussed in this article. Magnolol under low and moderate dosage possesses the ability to protect heart from ischemic/reperfusion injury, reduces atherosclerotic change, protects endothelial cell against apoptosis and inhibits neutrophil-endothelial adhesion. The moderate to high concentration of magnolol mainly acts on smooth muscle cells and platelets. Magnolol induces apoptosis in vascular smooth muscle cells at moderate concentration and inhibits proliferation at moderate and high concentration. High concentration of magnolol also abrogates platelet activation, aggregation and thrombus formation. Magnolol also serves as an smooth muscle relaxant only upon the high concentration. Oral intake of magnolol to reach the therapeutic level for cardiovascular protection is applicable, thus makes magnolol an agent of great potential for preventing cardiovascular diseases in high-risk patients.

Honokiol and magnolol as multifunctional antioxidative molecules for dermatologic disorders

Chinese herbs have been and still are widely used as important remedies in Oriental medicine. Over the recent years, a variety of biologically active constituents have been isolated from these sources and confirmed to have multifunctional activity in experimental studies. Honokiol is a small-molecule polyphenol isolated from the genus Magnolia. It is accompanied by other related polyphenols, including magnolol, with which it shares certain biological properties. Recently, honokiol and magnolol have been found to have anti-oxidative, anti-inflammatory, anti-tumor, and anti-microbial properties in preclinical models, without appreciable toxicity. These findings have increased interest in bringing honokiol and magnolol to the clinic as novel therapeutic agents in dermatology. In this review, the findings concerning the major mechanisms of action of honokiol and magnolol are described. Knowledge of the multiple activities of honokiol and magnolol can assist with the development of honokiol and magnolol derivatives and the design of clinical trials that will maximize the potential benefit of honokiol and magnolol in the patient setting for dermatologic disorders.

Magnolol suppresses NF-kappaB activation and NF-kappaB regulated gene expression through inhibition of IkappaB kinase activation

The mis-regulation of nuclear factor-kappa B (NF-kappaB) signal pathway is involved in a variety of inflammatory diseases that leds to the production of inflammatory mediators. Our studies using human U937 promonocytes cells suggested that magnolol, a low molecular weight lignan isolated from the medicinal plant Magnolia officinalis, differentially down-regulated the pharmacologically induced expression of NF-kappaB-regulated inflammatory gene products MMP-9, IL-8, MCP-1, MIP-1alpha, TNF-alpha. Pre-treatment of magnolol blocked TNF-alpha-induced NF-kappaB activation in different cell types as evidenced by EMSA. Magnolol did not directly affect the binding of p65/p50 heterodimer to DNA. Immunoblot analysis demonstrated that magnolol inhibited the TNF-alpha-stimulated phosphorylation and degradation of the cytosolic NF-kappaB inhibitor IkappaBalpha and the effects were dose-dependent. Mechanistically, a non-radioactive IkappaB kinases (IKK) assay using immunoprecipitated IKKs protein demonstrated that magnolol inhibited both intrinsic and TNF-alpha-stimulated IKK activity, thus suggesting a critical role of magnolol in abrogating the phosphorylation and degradation of IkappaBalpha. The involvement of IKK was further verified in a HeLa cell NF-kappaB-dependent luciferase reporter system. In this system magnolol suppressed luciferase expression stimulated by TNF-alpha and by the transient transfection and expression of NIK (NF-kappaB-inducing kinase), wild type IKKbeta, constitutively active IKKalpha and IKKbeta, or the p65 subunit. Magnolol was also found to inhibit the nuclear translocation and phosphorylation of p65 subunit of NF-kappaB. In line with the observation that NF-kappaB activation may up-regulate anti-apoptotic genes, it was shown in U937 cells that magnolol enhanced TNF-alpha-induced apoptotic cell death. Our results suggest that magnolol or its derivatives may have potential anti-inflammatory actions through IKK inactivation.

The inhibitory effect of magnolol from Magnolia officinalis on glucosyltransferase

Dental caries has been an intractable disease in spite of intense dental research. Glucosyltransferase (GTF) enzyme plays the most important role in the development of dental caries. In our previous studies, magnolol, a compound from Magnolia officinalis Rehder et Wilson (Magnoliaceae), was shown to possess a strong anti-GTF activity. The purpose of the present study was to examine the effect of magnolol on the functional domains of GTF for the purpose of defining its anti-GTF activity mechanism. GTF-I which was prepared from Streptococcus milleri transformant KSB8 cells expressing the gtfB gene was used. The results demonstrated magnolol reduced total glucan synthesis, depending on the magnolol concentration. There were no significant differences in Michaelis constant (K(m)) values between the presence and absence of magnolol as determined by Lineweaver-Burk plot, and maximum velocity (V(m)) in the presence of magnolol was lower than that in its absence. Magnolol significantly inhibited both sucrose hydrolysis and glucosyl transfer to glucan by GTF-I. Free glucose in the presence of magnolol was reduced by 33-48% as compared to in its absence, while the quantity of glucan was reduced by 75-82%. These findings suggested that magnolol inhibited both two sequential reaction phases of GTF non-competitively by operating on the glucan-binding domain, but not on the catalytic domain. Magnolol could be a valuable resource for the exploration of novel bioactive compounds in natural products.

Magnolol and honokiol enhance HL-60 human leukemia cell differentiation induced by 1,25-dihydroxyvitamin D3 and retinoic acid

Magnolol (MG) and honokiol (HK), two lignans showing anti-inflammatory and anti-oxidant properties and abundantly available in the medicinal plants Magnolia officinalis and M. obovata, were found to enhance HL-60 cell differentiation initiated by low doses of 1,25-dihydroxyvitamin D3 (VD3) and all-trans-retinoic acid (ATRA). Cells expressing membrane differentiation markers CD11b and CD14 were increased from 4% in non-treated control to 8-16% after being treated with 10-30 microM MG or HK. When added to 1 nM VD3, MG or HK increased markers expressing cells from approximately 30% to 50-80%. When either MG or HK was added to 20 nM ATRA, only CD11b, but not CD14, expressing cells were increased from 9% to 24-70%. Under the same conditions, adding MG or HK to VD3 or ATRA treatment further enlarged the G0/G1 cell population and increased the expression of p27(Kip1), a cyclin-dependent kinase inhibitor. Pharmacological studies using PD098059 (a MEK inhibitor), SB203580 (a p38 MAPK inhibitor) and SP600125 (a JNK inhibitor) suggested that the MEK pathway was important for VD3 and ATRA-induced differentiation and also its enhancement by MG or HK, the p38 MAPK pathway had a inhibitory effect and the JNK pathway had little influence. It is evident that MG and HK are potential differentiation enhancing agents which may allow the use of low doses of VD3 and ATRA in the treatment for acute promyelocytic leukemia.

Mechanisms of the influence of magnolol on eicosanoid metabolism in neutrophils

We have demonstrated that magnolol suppressed thromboxane B2 (TXB2) and leukotriene B4 (LTB4) formation in A23187-stimulated rat neutrophils. Maximum inhibition was obtained with about 10 microM magnolol. Magnolol was more effective in the inhibition of cyclooxygenase (COX) activity than in the inhibition of 5-lipoxygenase (5-LO) activity as assessed by means of enzyme activity determination in vitro and COX and 5-LO metabolic capacity analyses in vivo. Magnolol alone stimulated cytosolic phospholipase A2 (cPLA2) phosphorylation and the translocation of 5-LO and cPLA2 to the membrane, and evoked arachidonic acid (AA) release. Recruitment of both 5-LO and cPLA2 to the membranes was suppressed by EGTA. Arachidonyl trifluoromethyl ketone (AACOCF3), a PLA2 inhibitor, bromoenol lactone (BEL), a Ca2+-independent PLA2 (iPLA2) inhibitor, and EGTA suppressed the magnolol-induced AA release. However, none of the follows affected magnolol-induced AA-release: 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580), a p38 mitogen-activated protein kinase (MAPK) inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene (U0126), a MAPK kinase (MEK) inhibitor, or 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide (GF109203X), a protein kinase C (PKC) inhibitor. In addition, magnolol at 30 microM did not stimulate the p38 MAPK and extracellular signal-regulated kinase 2 (ERK2) enzyme activities. These results indicated that magnolol inhibits the formation of prostaglandins and leukotrienes in A23187-stimulated rat neutrophils, probably through a direct blockade of COX and 5-LO activities. The stimulatory effects of magnolol at high concentration on the membrane association of 5-LO and cPLA2 are attributable to the elevation of [Ca2+]i, and on the AA release is likely via activation of cPLA2 and iPLA2.

Signaling pathway of magnolol-stimulated lipolysis in sterol ester-loaded 3T3-L1 preadipocyes

The aims of the present study were to examine the effect of magnolol on lipolysis in sterol ester (SE)-loaded 3T3-L1 preadipocytes and to determine the signaling mechanism involved. We demonstrate that magnolol treatment resulted in a decreased number and surface area of lipid droplets, accompanied by release of glycerol. The lipolytic effect of magnolol was not mediated by PKA based on the facts that magnolol did not induce an elevation of intracellular cAMP levels, and protein kinase A (PKA) inhibitor KT5720 did not block magnolol-induced lipolysis. Calcium/calmodulin-dependent protein kinase (CaMK) was involved in this signaling pathway, since magnolol-induced a transient rise of intracellular [Ca(2+)] and Ca(2+) influx across the plasma membrane, and CaMK inhibitor significantly abolished magnolol-induced lipolysis. Moreover, magnolol increased the relative levels of phosphorylated extracellular signal-related kinases (ERK1 and ERK2). In support of the involvement ERK, we demonstrated that magnolol-induced lipolysis was inhibited by PD98059, an inhibitor of mitogen-activated protein kinase kinase (MEK), and PD98059 reversed magnolol-induced ERK phosphorylation. Further, the relationship between CaMK and ERK was connected by the finding that CaMK inhibitor also blocked magnolol-induced ERK phosphorylation. Taken together, these findings suggest that magnolol-induced lipolysis is both CaMK- and ERK-dependent, and this lipolysis signaling pathway is distinct from the traditional PKA pathway. ERK phosphorylation is reported to enhance lipolysis by direct activation of hormone sensitive lipase (HSL), thus magnolol may likely activate HSL through ERK and increase lipolysis of adipocytes.

Honokiol and magnolol induce Ca2+ mobilization in rat cortical neurons and human neuroblastoma SH-SY5Y cells

We examined the intracellular Ca(2+) response in primary cultured rat cortical neurons and human neuroblastoma SH-SY5Y cells by Fluo 3 fluorescence imaging analysis. In these two kinds of neuronal cells, honokiol and magnolol increased cytoplasmic free Ca(2+) with a characteristic lag phase. The cytoplasmic free Ca(2+) increase was independent of extracellular Ca(2+), but dependent on activation of phospholipase C and inositol 1,4,5-triphosphate (IP(3)) receptors. These results suggest that honokiol and magnolol increase cytoplasmic free Ca(2+) through a phospholipase C-mediated pathway, and that the release of Ca(2+) from intracellular stores mainly contributes to the increase in cytoplasmic free Ca(2+). Thus, honokiol and magnolol may be involved in a new activation mechanism closely associated with intracellular Ca(2+) mobilization.

Magnolol has the ability to induce apoptosis in tumor cells

We previously found that multiple intraperitoneal administration of magnolol from Magnolia obovata inhibited tumor metastasis and growth in vivo, and that the anti-metastatic effect of magnolol was due to the inhibition of the tumor cell invasion. The purpose of this study was to clarify the inhibitory mechanism of magnolol on the growth of tumor cells, and we expect that magnolol may have the ability to induce apoptosis in tumor cells. In an in vitro proliferation assay, 100 microM of magnolol inhibited the proliferation of B16-BL6, THP-1, BAE and HT-1080 cells, but 30 microM of magnolol did not affected cells proliferation. In addition, 100 microM of magnolol induced apoptotic cell death within 24 h in three tumor cell lines, B16-BL6, THP-1 and HT-1080, not BAE cells, and then up-regulated the activity of caspase-3 and caspase-8. The up-regulation of caspases activity by 100 microM of magnolol was suppressed by the inhibitor of all caspases, z-VAD-fmk. These data suggest that magnolol possesses ability to inhibit tumor growth, and the ability is due to the induction of apoptosis with the activation of caspases.

Magnolol suppresses proliferation of cultured human colon and liver cancer cells by inhibiting DNA synthesis and activating apoptosis

Magnolol, a hydroxylated biphenyl compound isolated from the Chinese herb Hou p'u of Magnolia officinalis, has been reported to have anti-cancer activity. In the present study, magnolol at very low concentrations of 3-10 microM inhibited DNA synthesis and decreased cell number in cultured human cancer cells (COLO-205 and Hep-G2) in a dose-dependent manner, but not in human untransformed cells such as keratinocytes, fibroblasts, and human umbilical vein endothelial cells (HUVEC). Magnolol was not cytotoxic at these concentrations and this indicates that it may have an inhibitory effect on cell proliferation in the subcultured cancer cell lines. [(3)H] thymidine incorporation and flow cytometry analyses revealed that magnolol treatment decreased DNA synthesis and arrested the cells at the G0/G1 phase of the cell cycle. Moreover, the magnolol-induced cell cycle arrest occurred when the cyclin-CDK system was inhibited, just as p21 protein expression was augmented. When magnolol concentration was increased to 100 microM, apoptosis was observed in COLO-205 and Hep-G2 cells, but not in cultured human fibroblasts and HUVEC. COLO-205 cells implanted subcutaneously in nude mice formed solid tumors; subsequent daily i.p.-injections of magnolol led to profound regression of these tumors of up to 85%. In these tumors, an increase in the expression of p21 protein level and the occurrence of apoptosis were observed. These findings demonstrate for the first time that magnolol can inhibit the proliferation of tumor cells in vitro and in vivo.

Stimulation of the BK(Ca) channel in cultured smooth muscle cells of human trachea by magnolol

BACKGROUND

Magnolol, a compound isolated from the cortex of Magnolia officinalis, has been found to possess anti-allergic and anti-asthmatic activity.

METHODS

The effect of magnolol on ionic currents was studied in cultured smooth muscle cells of human trachea with the aid of the patch clamp technique.

RESULTS

In whole cell current recordings magnolol reversibly increased the amplitude of K+ outward currents. The increase in outward current caused by magnolol was sensitive to inhibition by iberiotoxin (200 nM) or paxilline (1 microM) but not by glibenclamide (10 microM). In inside out patches, magnolol added to the bath did not modify single channel conductance but effectively enhanced the activity of large conductance Ca2+ activated K+ (BK(Ca)) channels. Magnolol increased the probability of these channel openings in a concentration dependent manner with an EC50 value of 1.5 microM. The magnolol stimulated increase in the probability of channels opening was independent of internal Ca2+. The application of magnolol also shifted the activation curve of BK(Ca) channels to less positive membrane potentials. The change in the kinetic behaviour of BK(Ca) channels caused by magnolol in these cells is the result of an increase in dissociation and gating constants.

CONCLUSIONS

These results provide evidence that, in addition to the presence of antioxidative activity, magnolol is potent in stimulating BK(Ca) channel activity in tracheal smooth muscle cells. The direct stimulation of these BK(Ca) channels by magnolol may contribute to the underlying mechanism by which it acts as an anti-asthmatic compound.

Molecular mechanisms of apoptosis induced by magnolol in colon and liver cancer cells

Magnolol has been reported to have anticancer activity. In this study we found that treatment with 100 microm magnolol induced apoptosis in cultured human hepatoma (Hep G2) and colon cancer (COLO 205) cell lines but not in human untransformed gingival fibroblasts and human umbilical vein endothelial cells. Our investigation of apoptosis in Hep G2 cells showed a sequence of associated intracellular events that included (a) increased cytosolic free Ca(2+); (b) increased translocation of cytochrome c (Cyto c) from mitochondria to cytosol; (c) activation of caspase 3, caspase 8, and caspase 9; and (d) downregulation of bcl-2 protein. Pretreatment of the cells with the phospholipase C inhibitor 1-[6-[[(17 beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1 H-pyrrole-2,5-dione (U73122) or the intracellular chelator of Ca(2+) 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA/AM) inhibited the subsequent magnolol augmentation of [Ca(2+)](i) and also the activation of caspase-8 and caspase-9, so that the occurrence of apoptosis in those cells was greatly reduced. Pretreatment of the cells with ZB4 (which disrupts the Fas response mechanism) also decreased the subsequent magnolol-induced caspase-8 activation and reduced the occurrence of apoptosis. We interpreted these findings to indicate that the above-listed sequence of intracellular events led to the apoptosis seen in Hep G2 cells and that [Ca(2+)](i), Cyto c, and Fas function as intracellular signals to coordinate those events.

Activation of p38 mitogen-activated protein kinase by formyl-methionyl-leucyl-phenylalanine in rat neutrophils

The signaling pathways leading to p38 mitogen-activated protein kinase (MAPK) activation in formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated rat neutrophils were examined. Immunoblot analysis with antibodies against a phosphorylated form of p38 MAPK showed that fMLP-stimulated p38 MAPK activation was dependent on a pertussis toxin-sensitive G protein. Two phosphatidylinositol 3-kinase inhibitors, wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002), did not affect the p38 MAPK activation. Phosphorylation of p38 MAPK was concentration dependently attenuated by a tyrosine kinase inhibitor, genistein, and by a Ca(2+)-dependent protein kinase C inhibitor, 13-cyanoethyl-12-methyl-6,7,12,13-tetrahydroindolo[2,3-a]pyrrolo[3 , 4-c]carbazole-7-one (Gö6976). However, the protein kinase C inhibitors with a broader spectrum, 2-[1-(3-dimethylaminopropyl)-5-methoxy-1H-indol-3-yl]-3-(1H-indol-3-y l)-maleimide (Gö6983) and 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimi de (GF109203X), had no inhibitory effect. fMLP-stimulated p38 MAPK phosphorylation was also reduced in cells pretreated with a phospholipase C inhibitor, 1-[6-((17beta-3-methoxyestra-1,3, 5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione (U73122), or preloaded with an intracellular Ca(2+) chelator, 1, 2-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA). We conclude that phosphorylation of p38 MAPK by fMLP stimulation in rat neutrophils is dependent on G(i/o) protein, nonreceptor tyrosine kinase, phospholipase C/Ca(2+), and probably Ca(2+)-dependent protein kinase C pathways.

Inhibition by magnolol of formylmethionyl-leucyl-phenyl alanine-induced respiratory burst in rat neutrophils

The influence of the plant product magnolol on neutrophil superoxide anion (O2-*) generation has been investigated in the rat. Intraperitoneal injection of magnolol (30mg kg(-1)) significantly inhibited the formylmethionyl-leucyl-phenylalanine (fMLP)-induced respiratory burst in rat whole blood ex-vivo. Magnolol also inhibited the 02-* generation with an IC50 (concentration resulting in 50% inhibition) of 15.4+/-1.6 microM and O2 consumption in rat neutrophils in-vitro. Magnolol weakly inhibited the O2-* generation in the xanthine-xanthine oxidase system, decreased cellular cyclic AMP level and had no effect on cyclic GMP levels. It weakly inhibited neutrophil cytosolic protein kinase C activity but did not alter porcine heart protein kinase A activity. Magnolol attenuated fMLP-induced protein tyrosine phosphorylation with an IC50 of 24.0+/-1.9 microM and the phosphorylation of mitogen-activated protein kinase p42/44 with an IC50 of 28.5+/-4.5 microM. However, magnolol alone activated neutrophil phospholipase D activity as determined by the formation of phosphatidic acid and phosphatidyl-ethanol in the presence of ethanol. In the presence of NADPH, the arachidonate-activated NADPH oxidase activity in a cell-free system was weakly suppressed by magnolol. These results suggest that the inhibition of respiratory burst in fMLP-activated neutrophils by magnolol is probably attributable mainly to the attenuation of protein tyrosine phosphorylation and p42/44 mitogen-activated protein kinase activation, and partly to the suppression of protein kinase C and NADPH oxidase activities.