Biochemical Mechanisms Of Action A Detoxification Antioxidant Actions

1. Induction of Detoxifying Enzymes

The hepatoprotection afforded by Sch B and other lignans could at least in part be attributed to the induction of hepatic cytochrome P-450-dependent (phase I) and GST (phase II) drug-metabolizing enzymes for detoxification reactions (6,7). Gom A increased the hepatic levels of microsomal cytochrome b5 and P-450, and activities of NADPH cytochrome C reductase, aminopy-rine N-demethylase, and 7-ethoxycoumarin O-deethylase (11). As regards the CCl4 hepatotoxicity, Sch B and Gom A could inhibit the CCl4-induced lipid peroxidation as well as the binding of CCl4 metabolites to the liver microsomal lipids (6,70,71). The ability of Sch B/Gom A to inhibit peroxidation of membrane lipids and hence maintain membrane stability of hepatocytes under oxidative stress conditions may also contribute to the hepatoprotective action against toxins that can generate reactive metabolites in the liver (72,73).

Sch B and Gom A inhibited demethylase activity induced by PB in liver microsomes in a similar manner as metyrone (70). Dual induction of Gom A and PB decreased the mutagenicity of benzo[a]pyrene (BP) by inhibiting the covalent binding of BP metabolites to DNA. Gom A also decreased the capacity of BP-induced rat microsomes to activate BP to its mutagenic metabolites (70).

2. In Vitro Antioxidant Activities

Lignans isolated from FS were found to possess antioxidant properties (74,75). Their inhibitory effect on lipid peroxidation reaction has been extensively investigated in a number of in vitro assay systems using micro-somes and mitochondria prepared from brain, liver, and kidney cells/tissues as the lipid source (76-81). In all cases, the lignans, including Sch B and Gom A, were found to be more potent than a-tocopherol or its analogues, in the inhibition of lipid peroxidation. Using electron spin resonance measurement, lignans with different structures and configurations were investigated for scavenging activity on reactive oxy-radicals generated from human polymor-phonuclear leukocytes stimulated by phorbol myristate acetate (82). The free-radical-scavenging activity was found to be dependent on the stereo-configurations of the lignans, in that S(-) Sch B produced a stronger effect than that of R( + ) Sch B. Interestingly, the scavenging effect of S,R (±) Sch B was stronger than either that of S(-) or R( + )-Sch B (82). In this regard, a recent study in our laboratory indicated that the enantiomers of Sch B also produced differential effects on activities of hepatic glutathione antioxidant enzymes in mice (83).

3. In Vivo Antioxidant Potential

Antioxidant Actions of the Lignan-enriched FS Extract. CCl4-induced hepatotoxicity is a commonly used model for investigating lipid peroxidation-related tissue injury (84). The involvement of free-radical-mediated reactions in the development of CCl4-induced hepatic injury has been implicated in various in vitro and in vivo studies (85,86). The use of CCl4 hepatotoxicity as an in vivo model for screening herbal extracts with antioxidant activities would be desirable (87). Early study examining the effect of the lignan-enriched FS extract on hepatic glutathione status in both control and CCl4-treated rats has shown its ability to enhance hepatic glutathione status, as evidenced by increases in hepatic reduced glutathione (GSH) level and activities of hepatic glucose-6-phosphate dehydrogenase (G6PDH) and glutathione reductase (GRD), as well as a decreased susceptibility of hepatic tissue homogenates to in vitro peroxide-induced GSH depletion (88). The beneficial effect on hepatic glutathione status became more evident after CCl4 challenge. Exposure of liver homogenates to an in vitro tert-butyl hydroperoxide challenge can be used as a means for measuring the GSH regeneration capacity (GRC) of hepatic tissues (89). Pretreatment of rats with the lignan extract caused a moderate enhancement of hepatic GRC in control rats, but the GRC-enhancing effect of the lignan pretreatment on hepatic tissues was greatly exaggerated after CCl4 challenge (89). These results suggest that the mechanism of hepatoprotection afforded by the lignan extract may involve the facilitation of GSH regeneration via the GRD-catalyzed and NADPH-mediated reaction.

When examining the effect on rats subject to intoxication by aflatoxin B1 or cadmium chloride, which can produce hepatocellular damage through biochemical mechanisms different from that of CCl4, the hepatoprotective action of the lignan-enriched FS extract was found to be nonhepatotoxin-specific and more effective than that of a-tocopherol (90). This supports the fundamental role of glutathione-related antioxidant and detoxification processes in the liver, which are effectively enhanced by the lignan extract treatment.

Increased physical activity is accompanied by significantly high rates of oxygen consumption and metabolism, particularly in skeletal muscle (91). Much evidence has now accumulated suggesting the involvement of reactive oxygen radicals in the development of exercise-mediated tissue injury (92). Significant elevations in plasma CPK, aspartate aminotransferases, and LDH, which are indicative of muscle damage, were observed immediately after physical activities exercise both in humans (93) and in rats (94). It has been postulated that liver may supply GSH to skeletal muscle as a protective antioxidant (95). In this regard, the protective effect of the lignan-enriched extract on physical exercise-induced muscle damage may be related to the enhancement of hepatic GSH status, thereby providing sufficient GSH for effective antioxidant protection of skeletal muscle during exercise (64).

Antioxidant Actions of Sch B. The hepatoprotection afforded by Sch B pretreatment was found to be mainly attributed to the enhancement in the functioning of the hepatic glutathione antioxidant system, possibly through stimulating the activities of glutathione related enzymes (19). A later study indicated that Sch B protected against CCl4 toxicity by enhancing the mitochondrial glutathione redox status in mouse liver (96). However, treating mice with 1,3-bis(2-chloroethyl)-1-nitrosourea, an inhibitor of GRD, did not deplete hepatic GSH or abrogate the hepatoprotective action of Sch B in CCl4-treated mice (96). The hepatic G6PDH-catalyzed formation of NADPH, but not GRD activity, may therefore be a limiting factor in Sch B-induced enhancement in the regeneration of GSH. A comparison between the effects of Sch B and butylated hydroxytoluene (BHT), a synthetic phenolic antioxidant, was made to identify the critical antioxidant action of Sch B

involved in hepatoprotection in mice (97). The ability of Sch B, but not BHT, to sustain hepatic mitochondrial GSH level, as well as hepatic ascorbic acid and a-tocopherol levels, represents a crucial antioxidant action in protecting against CCl4 hepatotoxicity. In further defining the antioxidant mechanism of Sch B, the effects of Sch B and a-tocopherol on ferric-chloride-induced oxidation of erythrocyte membrane lipids in vitro and CCl4-induced lipid peroxidation in vivo were examined (98). The ability of Sch B to inhibit lipid peroxidation, while being in the absence of pro-oxidant activity as compared to a-tocopherol, would make it a more desirable antioxidant in vivo.

The antioxidant effect of Sch B can be extended to extrahepatic tissues. The myocardial protection afforded by Sch B pretreatment against myocar-dial IR injury was also associated with an enhancement in myocardial glutathione antioxidant status (39). In contrast, the inability of DDB to enhance myocardial glutathione antioxidant status resulted in a failure in preventing IR injury (40). Since the in vitro perfusion of isolated hearts with Sch B-containing perfusate did not protect against IR injury, the myocardial protective action of Sch B was unlikely owing to free-radical-scavenging action (39). Instead, the cardioprotection may be mainly mediated by the enhancement of myocardial glutathione antioxidant status, particularly under oxidative stress conditions. In addition, modulations in tissue level of nonenzymatic antioxidants such as ascorbic acid and a-tocopherol in response to IR challenge, which may be an effect secondary to the enhancement of myocardial glutathione status, were also observed in Sch B-pretreated hearts [99]. A recent study has shown that a single dose of Sch B treatment produced a time-dependent enhancement in myocardial mitochondrial glu-tathione antioxidant status (100). This effect was paralleled by the stimulation in mitochondrial ATP generation and protection against IR injury (91).

GSH plays an important role in the maintenance of cellular redox status and antioxidant defense (101). The ability of Sch B to enhance hepatic and myocardial glutathione status can offer an effective antioxidant protection by sustaining the fundamental cellular defense system against oxidative challenge. The nonenzymatic antioxidants such as GSH, ascorbic acid, and a-tocopherol work synergistically in cellular antioxidant defense (102,103), in that the enhanced cellular GSH status facilitates the regeneration of ascorbic acid and a-tocopherol from their oxidized forms (104,105). As a result, the functional integrity of the cellular antioxidant defense can be maintained even under conditions of increased oxidative stress.

Being the major site of oxygen free-radical production, mitochondrion requires strong antioxidant protection. The maintenance of mitochondrial glutathione redox status is critical for cell viability (106,107). The ability of Sch B to increase the mitochondrial GSH level therefore represents an ultimate defense against free-radical attack. In addition, liver can supply

GSH to other tissues by exporting GSH into the blood for subsequent uptake in extrahepatic tissues. The enhancement of hepatic glutathione status by Sch B may produce a generalized antioxidant effect on organs other than the liver. Sch B or the lignan extract was found to protect myocardial, brain, as well as skeletal muscle tissues against free-radical-induced damage. The generalized tissue-protective effect of Sch B may be related to its ability to produce a sustainable and GSH-mediated antioxidant effect on various tissues.

B. Enhancement/Protection of Liver Function

It has also been suggested that Gom A can enhance liver function under normal or injured conditions, in that the prevention of CCl4-induced cholestasis can be attributed to its sustained stimulatory action on secretion of the bile-acid-independent fraction from the liver (12). Sch B and Gom A induced hypertrophy and mild hyperplasia of the liver, resulting in the increase in liver weight (108). Hepatic [14C] phenylalanine incorporation, protein content, and microsomal cytochrome P-450 content were also increased (108,109). Gom A helps to regenerate the liver tissue after partial hepatectomy by enhancing ornithine decarboxylase activity, an important biochemical event in the early stages of liver regeneration in rats (110). Gom A was also found to promote hepatocyte growth after mitosis during regeneration of partially resected rat liver, and enhanced directly or indirectly the proliferative process of non-parenchymal cells, which was likely mediated by an increase expression of c-myc gene preceding DNA replication in proliferating cells (111).

Leukotrienes are potent inflammatory agents that are thought to play a role in inflammatory liver diseases (112). In immunologically induced hepatic failure, mononuclear cells are the predominant cells producing leukotrienes. The ability of Gom A to inhibit the biosynthesis of leukotrienes induced in rat peritoneal macrophages by Ca2+ ionophore A2318 may also be related to its antihepatotoxic effect (113). Furthermore, liver cells could be injured by antibody-dependent cell-mediated cytotoxicity (ADCC) reaction or marco-phage activation. The inhibition of liver cell injury induced by ADCC or macrophage activation in vitro by Gom A may be related to the protective effect on immunologically induced liver failure (32).

Liver plays an important role in glucose synthesis by converting lactic acid or other keto-acids arising from anaerobic metabolism occurring in muscle during physical exercise back to glucose. The increase in activity of hepatic transaminase enzymes, which can deplete the level of glucose precursors by converting them into amino acids, was found to be associated with the impairment in physical performance in horses (62). An association between poor performance and high serum activities of these hepatic enzymes has been demonstrated (62). The lignans derived from FS, which can inhibit liver transaminase activities (26), can therefore sustain the blood glucose level through hepatic gluconeogenetic process during physical exercise, thereby improving the endurance and performance.

C. Heat Shock Proteins Induction

Heat shock proteins (Hsps) are a family of inducible and constitutively expressed gene products that collectively function to maintain cellular protein conformation during stress conditions. The synthesis of Hsps is induced by a variety of mild stresses, including oxidants, heat, hypoxia, and low pH, all of which can affect protein conformation (114). The synthesis of Hsps allows cells to adapt to gradual changes in their environment and to survive in otherwise lethal conditions (115). Increased production of Hsps by heat shock or other forms of stress, or gene overexpression, was found to be associated with tissue protection against noxious stimuli (116-120). The ability of Sch B to induce Hsp 70 production and protect against TNF-a-induced hepatic apoptosis in mice suggests the involvement of Hsp induction as a fundamental protective mechanism (20), in addition to the enhancement of mitochondrial glutathione antioxidant status (97), in its generalized tissue-protective action. Investigation of the correlation between induction of Hsps and enhancement of cellular/tissue antioxidant status afforded by Sch B treatment in respect to its hepato- and cardioprotective actions is currently underway in our laboratory. Preliminary results indicated that the hepatoprotective action of Sch B against CCl4 toxicity seemed to be mediated by both enhancement of mitochondrial glutathione antioxidant status and induction of Hsp 25/70 (121).

D. Other Biochemical Actions

The effect of gomisin C (Gom C, Fig. 1h) on the respiratory burst induced by the peptide (formyl Met-Leu-Phe, FMLP) was investigated in rat neutrophils (122). Gom C was found to inhibit FMLP-induced superoxide radical formation and oxygen consumption in a concentration-dependent manner. The inhibitory action of Gom C on the respiratory burst may be mediated partly by the suppression of NADPH oxidase activity on neutrophils and partly by the decrease of cytosolic Ca2 + released from an agonist-sensitive intracellular store (122). This activity may contribute to the anti-inflammatory action of lignans.

The effect of lignans on the binding of platelet-activating factor on rabbit platelets was examined (123). Among the three lignans, Sch A showed the most potent antagonistic activity, and Sch C produced only marginal activity. It is therefore suggested that Sch A and Sch B may be responsible, at least in part (together with other active compounds), for the antitussive effect ofFS (123).

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