Inhibition of cyclooxygenase and prostaglandin E2 synthesis by γ-mangostin, a xanthone derivative in mangosteen, in C6 rat glioma cells1☆,
Introduction
Mangosteen, Garcinia mangostana L. (Guttifereae), is a tree that is fairly widespread in Thai, India, Srilanka, and Myanmar and is known for its medicinal properties. The fruit hull of this plant has been in use in Thai indigenous medicine for the treatment of skin infections, wounds, and diarrhea for many years [1]. Thus, mangosteen may be expected to contain an anti-inflammatory drug. γ-Mangostin, a constituent of the fruit hull, is a tetraoxygenated diprenylated xanthone derivative. Xanthone derivative has been reported to possess several pharmacological activities, such as antimalarial activity [2], antimicrobial activity against methicillin-resistant Staphylococcus aureus (MRSA) [3], and inhibitory activity for monoamine oxidase [4].
In the brain, prostaglandin E2 (PGE2) levels are very low or undetectable in normal conditions, but can rise during inflammatory processes, multiple sclerosis, and AIDS-associated dementia [5], [6]. High levels of PGE2 can affect the activities of several cell types, including neurons, glial, and endothelial cells, and can regulate microglia/macrophage and lymphocyte functions during inflammatory and immune processes [7]. Therefore, the interplay between PGE2 and other local factors, including pro- and anti-inflammatory cytokines, is likely to influence the outcome of inflammatory and immune responses in the central nervous system (CNS). There is considerable evidence linking the generation of prostaglandins (PGs) with inflammation, pain, and fever. Glial cells, which outnumber neurons by about 10 to 1 in the brain, provide both mechanical and metabolic support for neurons. Glial cells are assumed to be an important source of PGs in the CNS [8]. It is suggested that regulation of arachidonic acid (AA) metabolism, particularly PGE2 production, appears beneficial in patients with inflammatory conditions [9].
Phospholipase A2 (PLA2) is subdivided into several groups based on their structures and enzymatic characteristics [10], [11], [12]. Secretory (s)PLA2 is a family of low-molecular-mass (∼14 kDa) enzymes that require millimolar concentration of Ca2+ for enzymatic activity. Cytosolic (cPLA2), or group IV PLA2, is a ubiquitously distributed 85 kDa enzyme, the activation of which is tightly regulated by postreceptor transmembrane signaling. cPLA2 is activated by p42/p44 extracellular signal regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) under cytosolic Ca2+ concentration of the submicromolar or micromolar range, and p42/p44 ERK/MAPK was activated by dual phosphorylation of tyrosine/threonine residues [11], [13].
Cyclooxygenase (COX) is the rate-limiting enzyme in PG synthesis and exists as two isoforms, constitutive (COX-1) and inducible (COX-2). These isoforms originate from distinct genes, but are structurally conserved [14], [15]. COX-1 is regarded as a constitutive enzyme whose expression is developmentally regulated. PGs produced by COX-1 primarily function in fluid and electrolyte homeostasis, gastric acid secretion, and platelet aggregation. In contrast, COX-2 is expressed in response to inflammatory stimuli and is active in physiological responses to growth factors and glucocorticoids [16].
In this study, we examined the effect of γ-mangostin (Fig. 1), a tetraoxygenated diprenylated xanthone contained in mangosteen, on AA cascade in C6 rat glioma cells. The results suggest that γ-mangostin, a xanthone derivative, directly inhibits COX activity.
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Materials
Fetal bovine serum was obtained from Cell Culture Laboratory (Cleveland, OH, USA); horse serum was purchased from Dainippon Pharmaceutical Co. Ltd.; F-10 (Nutrient Mixture: Ham) was obtained from GIBCO BRL; γ-Mangostin was purified from the fruit hull of G. mangostana L., with its purity of more than 90% determined by an high-performance liquid chromatography analysis. It was dissolved in dimethyl sufoxide to make a concentration of 20 mM and used after its dilution. Eagle’s minimum essential
Effect of γ-mangostin on PGE2 synthesis stimulated by A23187 in C6 cells
A23187, a Ca2+ ionophore, is known to stimulate PG synthesis mediated through an activation of cPLA2 following by AA liberation in glial cells [23]. In C6 rat glioma cells, A23187 potently stimulated PGE2 release (2.3 ng/well from 0.05 ng/well), and γ-mangostin inhibited the release in a concentration-dependent manner, with the IC50 value of about 5 μM (Fig. 2).
Effect of γ-mangostin on p42/p44 ERK/MAPK phosphorylation and AA liberation
To determine the site of action of γ-mangostin, we first examined upstream of AA liberation. The p42/p44 ERK/MAPK is known to be
Discussion
The fruit hull of G. mangostana L., mangosteen, has been widely used as an anti-inflammatory agent for the treatment of skin infections, wounds, and diarrhea for many years in Southeast Asia [1]. The crude extract of the fruit hull has been reported to possess several pharmacological activities, such as inhibitory activity against HIV-1 protease [26] and antimicrobial activity [27]. Several constituents contained in the fruit hull of mangosteen, such as α-mangostin, γ-mangostin, or another
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This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sport and Culture of Japan.
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Abbreviations: AA, arachidonic acid; BSA, bovine serum albumin; COX, cyclooxygenase; COX-1, constitutive COX; COX-2, inducible COX; cPLA2, cytosolic PLA2; EMEM, Eagle’s minimum essential medium; ERK, extracellular signal regulated kinase; HEPES, 2-[4-(2-Hydroxyethyl)-1-piperazinyl]ethanesulfonic acid; MAPK, mitogen-activated protein kinase;PGE2, prostaglandin E2; sPLA2, secretory phospholipase A2; TBST, Tris-buffered saline containing 0.05% Tween 20.