Inhibitory effect of flavonoids on the efflux of N-acetyl 5-aminosalicylic acid intracellularly formed in Caco-2 cells.

N-acetyl 5-aminosalicylic acid (5-AcASA) that was intracellularly formed from 5-aminosalicylic acid (5-ASA) at 200 microM was discharged 5.3, 7.1, and 8.1-fold higher into the apical site than into the basolateral site during 1, 2, and 4-hour incubations, respectively, in Caco-2 cells grown in Transwells. The addition of flavonols (100 microM) such as fisetin and quercetin with 5-ASA remarkably decreased the apically directed efflux of 5-AcASA. When 5-ASA (200 microM) was added to Caco-2 cells grown in tissue culture dishes, the formation of 5-AcASA decreased, and, in addition, the formed 5-AcASA was found to be accumulated within the cells in the presence of such flavonols. Thus, the decrease in 5-AcASA efflux by such flavonols was attributed not only to the inhibition of N-acetyl-conjugation of 5-ASA but to the predominant cellular accumulation of 5-AcASA. Various flavonoids also had both of the effects with potencies that depend on their specific structures. The essential structure of flavonoids was an absence of a hydroxyl substitution at the C5 position on the A-ring of flavone structure for the inhibitory effect on the N-acetyl-conjugation of 5-ASA, and a presence of hydroxyl substitutions at the C3' or C4' position on the B-ring of flavone structure for the promoting effect on the cellular accumulation of 5-AcASA. Both the decrease in 5-AcASA apical efflux and the increase in 5-AcASA cellular accumulation were also caused by MK571 and indomethacin, inhibitors of MRPs, but not by quinidine, cyclosporin A, P-glycoprotein inhibitors, and mitoxantrone, a BCRP substrate. These results suggest that certain flavonoids suppress the apical efflux of 5-AcASA possibly by inhibiting MRPs pumps located on apical membranes in Caco-2 cells.

Presence or absence of a gallate moiety on catechins affects their cellular transport.

The accumulation of (-)-epicatechin (EC), a non-gallate catechin, was significantly lower than that of (-)-epicatechin gallate (ECG), a gallate catechin, in Caco-2 cells. Using Caco-2 cell monolayers cultured in transwells, the transport of catechins in the basolateral-to-apical direction was much higher than that in the apical-to-basolateral direction, suggesting the involvement of an efflux transporter. Moreover, the results suggest that involvement of a transporter in EC efflux is greater than that for ECG. Treatment with transporter inhibitors MK571, quinidine or mitoxantrone, which inhibit MRP2, P-glycoprotein (P-gp) and BCRP, respectively, led to an increase in the accumulation of EC into Caco-2 cells and a decrease in the Papp ratio (Papp B-->A/Papp A-->B) for EC. These transporters seemed to be involved in EC efflux. BCRP was not an efflux transporter for ECG, and the influences of MRP2 and P-gp on ECG efflux were lower than for EC. Thus, efflux transporters appear to be responsible for the difference in cellular accumulation of EC versus ECG, suggesting that the presence or absence of a gallate moiety in the catechin structure influences the transporters.

Effects of benzo(e)pyrene and benzo(a)pyrene on P-glycoprotein-mediated transport in Caco-2 cell monolayer: a comparative approach.

The previous studies from our laboratory reported that benzo(a)pyrene (Bap) influenced efflux transport of rhodamine 123 (Rho-123) by induction of P-glycoprotein (P-gp) in Caco-2 cells. The present study investigated whether induction of P-gp and the enhanced efflux transport of Rho-123 were caused by benzo(e)pyrene (Bep), which has a structure similar to Bap, but is not a carcinogenic compound. In Caco-2 monolayer exposed to 50 microM Bep for 72 h, the ratio of the apparent permeability coefficient (P(app)) of Rho-123 efflux increased significantly compared to that of the control monolayer. Similarly, a significant increase in expression of MDR1 mRNA and of P-gp at the protein level were detected by RT-PCR and by Western blot analysis, respectively, in Caco-2 cells exposed to Bep, compared to that of the control. Caco-2 cells exposed to Bep showed oxidative stress that was detected by fluorescence microscopy using aminophenyl fluorescein. However, the oxidative stress was weaker compared with that of Bap. The cellular GSH content was decreased to 80% or 59% of control cells, respectively, in Caco-2 cells exposed to either Bep or Bap. Our results further show that Bep or Bap-induced P-gp in Caco-2 cells might have been the result of oxidative stress rather than DNA damage.

Effect of benzo[a]pyrene on P-glycoprotein-mediated transport in Caco-2 cell monolayer.

The main exposure pathway of benzo[a]pyrene (Bap) for humans is considered to be via the daily diet. The purpose of this study was to investigate the effect of BaP on the intestinal transport of chemicals mediated by P-glycoprotein (P-gp). The intestinal epithelial membrane transport of rhodamine-123 (Rho-123), a substrate of P-gp, was examined using a monolayer of the human Caco-2 cell line grown in transwells. In the monolayer exposed to Bap for 72 h before transport experiments, the ratio of the apparent permeability coefficients (P(app)) of Rho-123 efflux increased compared to that of the control. The permeability of rhodamine-B (Rho-B), not a substrate of P-gp, showed no difference between the monolayers. Treatment with quinidine or cyclosporine A, which are P-gp inhibitors, decreased the P(app) of Rho-123 to the same degree in both monolayers. The transport of Rho-123 was not influenced by the presence of Bap. Thus, Bap seemed not to act directly on the efflux activity of P-gp and be a binding site competitor of Rho-123. In the Caco-2 cells that enhanced the efflux of Rho-123 by the treatment with Bap, an increase in mRNA expression of MDR 1 (P-gp) was confirmed compared to that of control by RT-PCR. Furthermore, Western blot analysis using a monoclonal antibody, C219, demonstrated the increase of P-gp in Caco-2 cells exposed to Bap, compared with controls. It was inferred that Bap exposure induced the expression of P-gp, which led to the observed increase in efflux transport of Rho-123. The possibility was suggested that Bap might affect the disposition of medicines by increasing P-gp expression.

Inhibitory effects of catechin gallates on o-methyltranslation of protocatechuic acid in rat liver cytosolic preparations and cultured hepatocytes.

Flavonoids including tea catechins and gallic acid esters were characterized for their ability to inhibit o-methyltranslation of protocatechuic acid (PCA) to form vanillic acid (VA) in rat liver cytosolic preparations and cultured hepatocytes. Flavonols and flavones exhibited different behaviors in inhibiting the formation of VA between the cell-free enzymatic preparations and the intact cells. The underlying mechanism of the inhibitory effects of flavonols and flavones on PCA o-methylation in cultured hepatocytes may not be due to the inhibition of the enzyme activity of catechol o-methyl transferase (COMT). Catechin gallates inhibited PCA o-methylation in liver cytosolic preparations with markedly higher potency than other flavonoids. As compared with catechin gallates, ungallated catechins had two to three orders of magnitude lower efficiency in inhibiting cytosolic PCA o-methylation. Gallic acid esters inhibited cytosolic PCA o-methylation with strong potency almost equal to that of catechin gallates. These results suggest that the COMT-inhibitory activity of catechin gallates is derived from the presence of the galloyl moiety at the C3 position in the C-ring. Catechin gallates and gallic acid esters inhibited PCA o-methylation in cultured hepatocytes with two orders of magnitude lower efficacy than that in cytosolic preparations. The inhibitory effects of catechin gallates and gallic acid esters on cellular PCA o-methylation appear to be due to the direct inhibition of COMT activity.

Synergistic effects of flavonoids and ascorbate on enhancement in DNA degradation induced by a bleomycin-Fe complex.

Flavonoids were examined for synergistic effects with ascorbate on enhancement of DNA degradation induced by a bleomycin(BLM)-Fe complex. The synergistic effects of flavonoids and ascorbate on DNA degradation induced by the BLM-Fe complex were observed to be greater with flavonoids such as isorhamnetin, kaempferol and morin, which accelerated oxidation more markedly in the presence, than in the absence of BLM. Conversely, myricetin and fisetin, which showed oxidation barely accelerated by the addition of BLM, inhibited DNA degradation promoted by ascorbate. Consequently, there was a good correlation between oxidation of flavonoids accelerated by BLM and the extent of DNA degradation promoted synergistically with ascorbate. Our previous studies indicated that oxidation of flavonoids accelerated by BLM and DNA degradation promoted by flavonoids were not correlated with Fe(III)-reducing activity of flavonoids. Those results suggest that Fe(III)-reducing activity of flavonoids is not the only factor determining DNA degradation-promoting activity induced by the BLM-Fe complex. On the other hand, in a Fenton reaction, degradation of 2-deoxy-d-ribose promoted by flavonoids was correlated to the Fe(III)-reducing activity of flavonoids. However, there was not a synergistic interaction between flavonoids and ascorbate in the degradation of 2-deoxy-d-ribose. Therefore, it is suggested that the synergistic DNA degradation caused by flavonoids and ascorbate in the BLM-Fe redox cycle arose from the difference in the reductive processes in which flavonoids and ascorbate mainly act.

Inhibitory effect of flavonoids on N-acetylation of 5-aminosalicylic acid in cultured rat hepatocytes.

5-Aminosalicylic acid (5-ASA) is an effective drug for the treatment of ulcerative colitis and Crohn's disease. A large group of flavonoids was investigated for their inhibitory effects on the N-acetyl-conjugation of 5-ASA in rat hepatocytes and subcellular preparations. When added to cultured hepatocytes, some flavonoids inhibited the production of N-acetyl-5-aminosalicylic acid (5-AcASA) with potencies that depended on the specific structure of flavonoids. Among the flavonols, quercetin, kaempferol and galangin had inhibitory activity with a tendency to be more effective at increasing the number of hydroxyl substitutions in the B-ring. Flavones such as luteolin, apigenin and chrysin were as effective as the corresponding three flavonols above. 7,3',4'-OH flavone was more effective than other simple flavones such as 7-, 5-, 3-, 7,3-, 7,4'- and 3',4'-OH flavones. Isoflavones were relatively weak inhibitors. Taxifolin and catechins had little or no inhibitory effect. These data suggest that the presence of C7 hydroxyl substitution on the A-ring and the catechol group on the B-ring in the flavone structure is required for effective inhibitory activity. The inhibitory effect of flavonoids on N-acetyl-conjugation of 5-ASA was also examined by incubating 5-ASA with isolated liver cytosolic preparations. The active flavonoids in the cells inhibited the N-acetylation of 5-ASA in the cell-free enzymatic preparations with a potency comparable to that for cultured rat hepatocytes.

Inhibitory effect of flavonoids on sulfo- and glucurono-conjugation of acetaminophen in rat cultured hepatocytes and liver subcellular preparations.

A large group of flavonoids was investigated for inhibitory effects on sulfo- and glucurono-conjugation of acetaminophen when added to rat cultured hepatocytes and liver subcellular preparations. The flavonoids inhibited the production of both sulfate and glucuronide conjugates in the cultured cells, with potencies that depended on the specific flavonoid. Among the flavonols, quercetin, kaempferol and galangin were much more effective than myricetin and morin. Flavones including luteolin, apigenin and chrysin were as effective as the corresponding three flavonols above. The inhibition of conjugation by other simple flavones such as 3-, 5-, 7- and 3',4'-OH flavones, and by catechins such as epicatechin and epigallocatechin, was very weak. These data suggest that the presence of both C5 and 7 hydroxyl substitutions on the A-ring in the flavone structure is required for effective inhibitory activity. The effect of flavonoids on sulfo- and glucurono-conjugation was also examined by incubating acetaminophen with isolated liver cytosolic and microsomal preparations, respectively. The active flavonoids in the cells remarkably inhibited the sulfation, but not glucuronidation, in cell-free enzymatic preparations in vitro. The mechanism of inhibition of conjugation by flavonoids in cultured hepatocytes is not likely to depend on the direct inhibition of sulfo- and glucurono-transferase activity by flavonoids.

Oxidation of flavonoids which promote DNA degradation induced by bleomycin-Fe complex.

Sixteen flavonoids including quercetin and kaempferol and their relatives were examined for their ability to promote DNA degradation induced by the bleomycin (BLM)-Fe complex. Three hydroxyl groups in the flavonoidal nucleus were proposed as a crucial structural requirement for effectively promoting DNA degradation: 1). the C7-hydroxyl substitution in the A-ring; 2). the C4'-hydroxyl substitution in the B-ring; and 3). the C3-hydroxyl substitution in the C-ring. Flavonoids, which lack even one of these hydroxyl substitutions, showed remarkably diminished activity. There was a good correlation (r=0.920, p<0.001) between activity to promote DNA degradation and oxidizability, which was measured following the Fe(III)-induced oxidation of flavonoids themselves, among the 16 flavonoids. The oxidizability of flavonoids which have the crucial hydroxyl substitutions, was remarkably enhanced in the presence compared with the absence of BLM. On the other hand, the extent of oxidation of flavonoids lacking these substitutions was enhanced little or not at all by BLM. No correlation between the Fe(III)-reducing activity and DNA degradation-promoting activity was found among flavonoids satisfying the crucial structural requirements. Furthermore, the correlation between the extent of oxidation of flavonoids and the Fe(III)-reducing activity was not confirmed among these flavonoids. Therefore, it was suggested that Fe(III)-reducing activity was not the only factor determining DNA degradation-promoting activity in flavonoids having the three hydroxyl groups necessary for effectively promoting DNA degradation induced by BLM-Fe complex.

Glucurono- and sulfo-conjugation of kaempferol in rat liver subcellular preparations and cultured hepatocytes.

Glucurono- and sulfo-conjugation of kaempferol in rat liver preparations and cultured hepatocytes were studied using high-performance liquid chromatography (HPLC) with two distinctly different elution solvents. Kaempferol glucuronides and sulfates were produced by treating kaempferol with microsomes plus UDPGA or with cytosol plus PAPS, respectively. HPLC analysis of the conjugates revealed one major and three minor glucuronides with solvent A and one sulfate with solvent B. Kaempferol metabolites produced by cultured hepatocytes also consisted of four glucuronides and one minor sulfate, all of which corresponded to their respective in vitro-produced conjugates in the liver subcellular preparations. The relative proportion of kaempferol sulfate accounted for about 9% of the total conjugates in the cultured hepatocytes. The kinetic data on glucurono- and sulfo-conjugation of kaempferol by the liver subcellular preparations correlated well with the preferential production of kaempferol glucuronides in the cultured hepatocytes. Glucurono- and sulfo-conjugation of 3-, 5- and 7-OH flavones in the liver subcellular preparations were also kinetically characterized. 7-OH flavone was predominantly conjugated to form a glucuronide compared to 3- and 5-OH flavones. These data suggest that glucuronidation at the 7-OH position on the A-ring is a major metabolic pathway of kaempferol in hepatic cells.

The contribution of the pyrogallol moiety to the superoxide radical scavenging activity of flavonoids.

Sixteen flavonoids including flavonols, flavones, flavanonol and catechins, and five aromatic compounds were examined for their ability to scavenge superoxide radical (O2-*) generated enzymatically in a xanthin-xanthinoxidase system and non-enzymatically in a phenazine methosulfate-NADH system. Pyrogallol, gallic acid and its ester, were much more efficient in scavenging O2-* than catechol. The superiority of pyrogallol over catechol in the flavonoidal nucleus is apparent from the much higher O2-* scavenging activity of myricetin and epigallocatechin, which contain 3',4',5'-trihydroxyl substitution in the B-ring, compared to quercetin and epicatechin, which contain 3',4'-dihydroxyl substitution, respectively. The strong O2-* scavenging ability of pyrogallol appears to function even in the A-ring, as in baicalein, and also in the form of a pyrogalloyl ester at the C-3 position in the C-ring, as in epicatechin gallate and epigallocatechin gallate. It can be concluded that the pyrogallol moiety is an active component of flavonoids for displaying high O2-* scavenging activity. Flavonoids and aromatics were also examined to correlate their O2-* scavenging activity with their oxidizability, which was measured on the basis of electrochemical redox potential and the reducing ability of the Cu2+ ion. Aromatics such as pyrogallol, gallic acid and its ester, and flavonoids such as baicalein, epicatechin gallate and epigallocatechin gallate, in which the O2-* scavenging activity is enhanced by the presence of a pyrogallol moiety which does not belong to the B-ring, reduced the correlation between the higher O2-* scavenging activity and the lower redox potential. The O2-* scavenging activity was well correlated with the Cu2+ reducing ability of flavonoids and aromatics.