Synthesis and biological evaluation of substituted flavones as gastroprotective agents

Flavone (1) was found to protect against ethanol-induced gastric damage in rats; however, it is known that certain compounds in the flavone class, including flavone itself, are inducers of hepatic drug metabolizing enzymes. With the hope of identifying gastroprotective flavones that have minimal effects on drug metabolizing enzymes, we have synthesized and evaluated selected flavone analogs. Gastroprotective potency in the ethanol model was retained by methoxy substitution in the 5-position (4) and by methoxy (12) or methyl (14) substitution in the 7-position. A number of substituted analogs of the potent molecule 5-methoxyflavone (4) were also synthesized, and in many cases, these substitutions provided gastroprotective molecules. In order to assess liver enzyme induction potential, two of the gastroprotective flavones, 7-methoxyflavone (12) and 5-methoxy-4'-fluoroflavone (26), were examined for their effect on liver microsomal cytochrome P450 and 7-ethoxyresorufin O-dealkylase (CYP1A) activity. These two compounds caused minimal changes in the cytochrome P450 concentration and were considerably less potent than beta-naphthoflavone as inducers of CYP1A enzyme activity. Furthermore, following oral administration to rats, 5-methoxy-4'-fluoroflavone (26) was found to protect against indomethacin-induced gastric damage. These results indicate that, through appropriate substitution, flavones can be obtained that are gastroprotective but have minimal effects on drug-metabolizing enzymes.

In vitro and in vivo studies on the degradation of metallothionein

Degradation of metallothionein (MT) from rat liver was examined. Degradation of apo-MT by liver homogenate was greater than that by cytosol. At pH 5.5, degradation by homogenate was more than that at pH 7.2. These findings suggest that proteases that function at acidic pH are probably involved in MT degradation. Because lysosomes are the principal subcellular organelles that contain acid proteases (cathepsins), we compared the degradation of apo-MT by lysosomes and cytosol. Apo-MT was degraded about 400 times faster by lysosomal fraction than by cytosolic fraction. To determine the relative importance of different cathepsins, we used different inhibitors. Leupeptin, which inhibits cathepsins B and L, inhibited the degradation of apo-MT by 80%, implying that cathepsins B and/or L might be very important in the intracellular turnover of MT. Cathepsin D appeared to be the least significant, because apo-MT degradation was reduced by about 20% by inhibiting cathepsin D. When we extended this study with purified cathepsins, we obtained the same answer, i.e., the ability of different cathepsins to degrade apo-MT was in the following order: cathepsin B >> cathepsin C > cathepsin D. While apo-MT was susceptible to degradation, ZnMT and CdMT were highly resistant to degradation. Coincubation of ZnMT or CdMT with either lysosomal extract or purified cathepsins did not result in any appreciable degradation even after 16 hr. However, longer incubations did result in some degradation, especially by purified cathepsin B. Interestingly, CdMT degraded little faster than ZnMT by both lysosomal extract as well as purified cathepsin B.(ABSTRACT TRUNCATED AT 250 WORDS)

Cadmium-induced hepatic endothelial cell injury in inbred strains of mice

Susceptibility to cadmium (Cd) hepatotoxicity differs among inbred strains of mice. For example, C3H/HeJ mice are sensitive to Cd-induced hepatotoxicity, whereas DBA/2J mice are resistant. The mechanism of genetic predisposition to Cd hepatotoxicity is unknown. A contemporary theory for acute target organ intoxication maintains that Cd initially damages vascular endothelium and parenchymal cell injury is a secondary event that results from localized ischemia. In the present study, the hypothesis that hepatic endothelial cells (EC) of C3H mice are more susceptible to Cd toxicity than those of DBA mice was tested. Hepatic parenchymal and endothelial cells were grown separately on monolayer cultures for 22 h and subsequently treated with various concentrations of Cd. Hepatocellular toxicity was assessed by lactate dehydrogenase leakage and intracellular K+ loss, whereas endothelial cell injury was assessed by trypan blue exclusion and the inhibition of protein synthesis. The susceptibility of hepatocytes to the cytotoxic effects of Cd was identical between strains. In contrast, the vulnerability of EC to Cd intoxication was strain-dependent. When exposed to 2.5-10.0 microM Cd, EC of Cd-sensitive mice were more susceptible to the cytotoxic effects of Cd than those of Cd-resistant mice. Basal metallothionein (MT) levels as well as Cd uptake into EC were similar in the two strains. Following Cd exposure, EC of Cd-sensitive mice accumulated similar amounts of MT as EC of Cd-resistant mice. These observations suggest that the microvasculature in livers of inbred mice is the target tissue responsible for strain-dependent susceptibility to Cd-induced liver injury. The mechanisms that account for this genetic variation in endothelial cell response to Cd are unknown, but do not appear to be related to the cellular disposition of Cd nor to a defect in the metabolism of MT.

Cd-metallothionein nephrotoxicity in inbred strains of mice

Genetic differences in the acute hepatic and testicular toxicity of Cd occur among different strains of mice. However, it is not known whether genetic variation to the renal damage caused by Cd-metallothionein (CdMT) exists. Therefore, male mice of the C3H/HeJ, C57/Bl10, CBA/CA, and DBA/2J strains, previously shown to differ in hepatic and testicular injury due to Cd, were treated with CdMT at dosages of 0.2, 0.4, 0.8, and 1.6 mg/kg (sc). For all strains of mice, tissue accumulation of Cd occurred predominantly in kidney, which had two to three times as much Cd as liver, while testes had no measurable amounts of Cd. Hepatic and renal metallothionein (MT) concentrations were increased with increasing dosage of CdMT, and no differences between strains were demonstrated. Urinary glucose was increased significantly at the three highest dosages of CdMT, with no differences between strains. At each dose level, light microscopic manifestations of CdMT nephropathy did not differ between strains. In summary, all CdMT-treated strains of mice responded similarly with respect to all measured renal parameters (accumulation of Cd and MT and nephrotoxicity). Unlike the strain differences in hepatic and testicular injury from Cd in these strains of mice, CdMT nephrotoxicity shows no such genetic variation.

Degradation and metal composition of hepatic isometallothioneins in rats

Recently, our laboratory demonstrated that metallothionein-1 (MT-1) is degraded faster than metallothionein-2 (MT-2) in liver of Zn-treated adult rats; however, it is not clear whether this phenomenon is unique to Zn treatment or the age of the animal. Furthermore, many investigators maintain that the degradation of MT is regulated by its metal composition. The objective of this study was twofold: (1) to determine if MT-1 is more susceptible than MT-2 to proteolytic breakdown regardless of age or chemical pretreatment and (2) to examine the hypothesis that the amount and type of metals bound to MT influences its resistance to degradation. Pulse-labeling experiments were conducted to determine the half-lives of MT-1 and MT-2 in liver of adult rats (75-day-old), immature rats (1-day-old), and mature rats treated with single dosages of Zn (1 mmol/kg, sc), Cd (10 mumol/kg, sc), or ethanol (109 mmol/kg, po). Atomic absorption spectrometry was utilized to measure the Zn, Cu, and Cd contents of MT-1 and MT-2 obtained in selected experimental groups. MT-1 had a shorter half-life than MT-2 in Zn-treated adults (21 vs 33 hr) and in nontreated immature rats (49 vs 73 hr). In contrast, the half-life values of MT-1 and MT-2 were identical in nontreated adults (4 hr) and ethanol-treated adults (9 hr) and nearly identical in Cd-treated adults (58 and 61 hr, respectively). Both isoforms obtained from immature rats and adults treated with Zn or ethanol contained approximately 6.0 g atoms Zn/mol MT, trace levels of Cu, and nondetectable quantities of Cd. In Cd-treated rats, both isoforms contained approximately equal amounts of Zn and Cd (3.2 g atoms metal/mol MT) and trace levels of Cu. These results indicate that MT-1 is either as susceptible or more susceptible than MT-2 to intracellular degradation depending on age or chemical pretreatment. Furthermore, factors unrelated to the metal composition of MT appear to regulate the degradation of MT-1 and MT-2.

Protective effects of zinc on cultured rat primary hepatocytes to metals with low affinity for metallothionein

The purpose of this study was to determine if Zn pretreatment could protect rat primary hepatocyte cultures from the cytotoxicity of five metals that have little or no affinity for metallothionein (MT). Hepatocytes were grown in monolayer cultures for 22 h and subsequently treated with ZnCl2 (100 microM) for 24 h; which increased the MT concentration 15-fold. Following Zn pretreatment, hepatocytes were exposed to various concentrations of Mn, V, Cr, Se, or Fe for an additional 24 h. Cytotoxicity was assessed by enzyme leakage and loss of intracellular K+. The toxicity of all five metals was significantly reduced in the Zn-pretreated cells. Zn pretreatment had no appreciable effect on the hepatocellular uptake (1-24 h) of Mn or Se. Zn pretreatment also did not increase the distribution of Mn or Se to the cytosol and neither metal was bound to MT, suggesting the protection was not due to their binding to MT. However, Zn pretreatment significantly decreased Mn-, Cr-, and V-induced cellular glutathione depletion. In summary, Zn pretreatment of rat primary hepatocyte cultures protects against Cr-, Mn-, Fe-, Se-, or V-induced hepatotoxicity. This protection does not appear to be related to MT induction but may be due to Zn-induced thiol or membrane stabilization and/or other biological changes produced by Zn.

Cadmium-induced elevation of hepatic isometallothionein concentrations in inbred strains of mice

Susceptibility to Cd toxicity differs among inbred strains of mice. For example, C3H/He mice are sensitive to Cd-induced hepatotoxicity while DBA/2 mice are resistant. Metallothionein (MT), which in rodents exists predominantly as two isoproteins (MT-I and MT-II), is an important endogenous protein in the detoxication of Cd. The present investigation examines the possibility that strain-dependent susceptibility to Cd-induced liver injury is mediated by an inherited inability to accumulate a specific isoform of MT in response to Cd exposure. Hepatic concentrations of MT-I and MT-II were measured in C3H/He (Cd-sensitive) and DBA/2 (Cd-resistant) mice at various times after the administration of non-toxic (2.5 mumol Cd/kg) to hepatototoxic (80 mumol Cd/kg) dosages of Cd. The concentration of MT-I and MT-II in these strains was similar 24 h after injection of non-hepatotoxic dosages of Cd (10 mumol Cd/kg or less) as well as 6-12 h after a mildly hepatotoxic dose of Cd (20 mumol Cd/kg). The concentration of total MT in liver of Cd-sensitive mice was greater than that present in resistant mice 24-72 h after 20 mumol Cd/kg injection. The data indicates that susceptibility to Cd-induced hepatotoxicity observed in C3H/He mice is not due to a deficit in the induction of a particular isoform of MT.

The protective effect of metallothionein on the toxicity of various metals in rat primary hepatocyte culture

Metallothionein (MT), a low-molecular-weight, cysteine-rich, metal-binding protein, has been implicated in the detoxification of Cd. However, whether MT protects against the cellular toxicity of other metals has not been examined thoroughly. This study was therefore designed to determine the effects of Zn-induced MT on the toxicity of seven metals in rat primary hepatocyte cultures. Hepatocytes were grown in monolayer culture for 22 hr and subsequently treated with ZnCl2 (100 microM) for 24 hr which produced a 15-fold increase in MT concentration. Following Zn pretreatment, hepatocytes were exposed to various concentrations of Ag, Co, Cu, Hg, Ni, Pb, or Zn for 24 hr. Cytotoxicity was assessed by enzyme leakage and loss of intracellular K+. The toxicity of all seven metals was significantly less in the Zn-pretreated cells. Zn pretreatment had no appreciable effect on the hepatocellular uptake (1-24 hr) of 110Ag or 203Hg, but markedly altered their subcellular distribution, with metals accumulating more in the cytosol and less in the nuclear, mitochondrial, and microsomal fractions. In the cytosol of control cells, the metals were bound mainly to high-molecular-weight proteins whereas in the Zn-pretreated cells, the metals were mainly associated with MT. In summary, Zn-induced MT in rat primary hepatocyte cultures protects against Ag-, Co-, Cu-, Hg-, Ni-, Pb-, and Zn-induced cytotoxicity. This protection appears to be due to the binding of metals to MT with a concomitant reduction of metal content in critical organelles and proteins.

Ethanol decreases cadmium hepatotoxicity in rats: possible role of hepatic metallothionein induction

The present investigation examines the possibility that Cd and ethanol have a significant toxicological interaction. This examination was warranted as exposure to either chemical is known to compromise human health. Inasmuch as both chemicals affect the morphology, biochemistry, and physiology of liver, it seemed reasonable to consider liver as a possible site of interaction. Specifically, the hypothesis that ethanol alters the hepatotoxic action of Cd was evaluated. Accordingly, male rats were injected iv with hepatotoxic (3.0 mg/kg) or lethal (4.5 mg/kg) dosages of Cd, 24 hr after single-dose ethanol administration (7 g/kg, po). Cd-induced hepatotoxicity was assessed by measuring the activities of alanine aminotransferase, aspartate aminotransferase, and sorbitol dehydrogenase in serum collected 10 hr after Cd injection. Lethality was assessed by recording the number of survivors over a 7-day period. Prior exposure to ethanol substantially reduced the lethal and hepatotoxic properties of Cd. Two mechanisms were evaluated in an effort to explain ethanol-induced suppression of Cd hepatotoxicity. Ethanol pretreatment was postulated to: (1) enhance Cd excretion in bile thereby decreasing hepatic Cd content and/or (2) reduce the interaction between Cd and target sites in liver such as organelles and cytosolic high-molecular-weight (HMW) proteins. The first proposed mechanism was incorrect as the biliary excretion of Cd was nearly abolished and the concentration of Cd in whole liver increased (33%) as a result of ethanol exposure. The second proposed mechanism was a plausible explanation of ethanol-induced suppression of Cd hepatotoxicity because ethanol pretreatment decreased (approximately 60%) the content of Cd in nuclei, mitochondria, and endoplasmic reticulum, and nearly eliminated the association of Cd with cytosolic HMW proteins. Reduction in the concentration of Cd in potential target sites of intoxication was caused by a metallothionein-promoted sequestration of Cd in cytosol.

Hepatic isometallothioneins in mice: induction in adults and postnatal ontogeny

The purpose of this study was to quantitate hepatic metallothionein-I (MT-I) and metallothionein-II (MT-II) in adult mice pretreated with various dosages of selected inorganic and organic compounds and in nonchemically treated neonatal mice. Male CF-1 mice received Zn (0.38-6.0 mmol/kg, sc), Cd (5-80 mumol/kg, sc), dexamethasone (10-1000 mumol/kg, sc), or ethanol (60-180 mmol/kg, po). Liver cytosol was prepared 24 hr after the administration of each compound. In another experiment, liver cytosols were prepared from male and female neonates 1 to 35 days after parturition. MT-I and MT-II in liver cytosols were isolated by high-performance anion-exchange chromatography and quantitated by atomic absorption spectrometry. Hepatic MT-I and MT-II concentrations in adult controls were 5.1 +/- 1.3 and 3.7 +/- 1.0 micrograms/g liver, respectively. All compounds increased hepatic MT levels in a dose-dependent manner over a narrow range of dosages. The lowest dosages of Zn, Cd, dexamethasone, and ethanol that produced a significant increase in total MT content (MT-I plus MT-II) were 0.38, 0.005, 0.3, and 90 mmol/kg, respectively. Maximal induction of total MT following the highest dosages of Zn, Cd, ethanol, and dexamethasone was 58, 34, 24, and 13 times the control value (8.8 +/- 2.4 micrograms total MT/g liver), respectively. The relationship between dose and hepatic MT content was linear following ethanol administration and log-linear following Zn, Cd, and dexamethasone administration. The ratio of MT-I/MT-II was approximately 2.4 following all dosages of metals. Following low and high dosages of organic compounds, the ratio of MT-I/MT-II was approximately 1.0 and 1.5, respectively. Total MT concentration in livers of 1- to 14-day-old mice was approximately 40 times that observed in adult liver (5.5 +/- 1.6 micrograms total MT/g liver) and returned toward adult levels 21 days after parturition. The ratio of MT-I/MT-II was approximately 1.8 during Postpartum Days 1 through 14 and thereafter decreased to approximately 1.0. These results indicate that MT-I is more abundant than MT-II in mouse liver following chemical exposure and during neonatal development.

Rat primary hepatocyte cultures are a good model for examining metallothionein-induced tolerance to cadmium toxicity

The effect of Zn-induced metallothionein (MT) on the toxicity, uptake, and subcellular distribution of cadmium (Cd) was examined in rat primary hepatocyte cultures and compared to results obtained earlier in this laboratory from intact animals. Hepatocytes were isolated and grown in monolayer culture for 22 h and subsequently treated with ZnCl2 (100 microM) for 24 h, which increased MT concentration about 15-fold. After Zn pretreatment, hepatocytes were exposed to Cd for 24 h. Cytotoxicity was assessed by enzyme leakage, intracellular potassium loss, and cellular glutathione content. The toxicity of Cd was much less in Zn-pretreated cells than in control cells, similar to that previously demonstrated in the intact animal. Zn pretreatment had no appreciable effect on the hepatocellular uptake of 109Cd, but markedly altered its subcellular distribution, with more Cd accumulating in the cytosol and less in the nuclear, mitochondrial, and microsomal fractions. In the cytosol of Zn-pretreated cells, Cd was associated mainly with MT; in contrast, cytosolic Cd in control cells was mainly associated with non-MT macromolecules. Zn-induced changes in the subcellular distribution of Cd in vitro are identical to those observed in vivo in Zn-pretreated rats challenged with Cd. In summary, Zn pretreatment of rat primary hepatocyte cultures protects cells against Cd toxicity. Protection seems to be due to MT-promoted sequestration of Cd and reduction of the amount of Cd associated with critical organelles and proteins. These observations are similar to those noted in the whole animal. These results indicate that cultured hepatocytes are an ideal model for examining MT-induced tolerance to Cd hepatotoxicity.

Methoxyflurane enhances allyl alcohol hepatotoxicity in rats. Possible involvement of increased acrolein formation

The effect of methoxyflurane anesthesia on allyl alcohol-induced hepatotoxicity and the metabolism of allyl alcohol was studied in male rats. Hepatotoxicity was assessed by the measurement of serum alanine aminotransferase activity and histopathological examination. Allyl alcohol-induced hepatotoxicity was enhanced when allyl alcohol (32 mg/kg) was administered 4 hr before or up to 8 days after a single 10-min exposure to methoxyflurane vapors. The possibility that methoxyflurane increases alcohol dehydrogenase-dependent oxidation of allyl alcohol to acrolein, the proposed toxic metabolite, was evaluated by measuring the rate of acrolein formation in the presence of allyl alcohol and liver cytosol. The effect of methoxyflurane on alcohol dehydrogenase activity in liver cytosol was also assessed by measuring the rate of NAD+ utilization in the presence of ethyl alcohol or allyl alcohol. Alcohol dehydrogenase activity and rate of acrolein formation were elevated in methoxyflurane-pretreated rats. The results suggest that a modest increase in alcohol dehydrogenase activity and rate of acrolein formation markedly enhances allyl alcohol-induced hepatotoxicity.

In vitro digitoxin metabolism. Rate-limiting step and alteration following spironolactone pretreatment

The biotransformation of digitoxin was considered as a linear transformation comprised of dual deoxy sugar cleavages culminating with product conjugation to glucuronic acid. Digitoxin and two metabolites, digitoxigenin bisdigitoxoside and digitoxigenin monodigitoxoside, were each incubated for 1 hr with rat liver slices. Measurements of amounts of various metabolites allowed determination of rate constants, showing the cleavage of the terminal sugar from digitoxigenin bisdigitoxoside to be the rate-limiting step. The inductive action of spironolactone predominately affected the rate-limiting step.