THE EFFECT OF BILE SALTS AND SOME BILE-SALT ANALOGUES ON THE OXIDATION OF CHOLESTEROL BY LIVER MITOCHONDRIA

Exposure to bile acids alters the intracellular location and function of MnSOD in Barrett's esophagus

BACKGROUND

Oxidative stress secondary to bile-acid exposure has been associated with metaplastic degeneration of normal esophageal mucosa into Barrett's esophagus (BE) cells and eventually esophageal adenocarcinoma. We previously reported that the macromolecular response of BE cells to this stress was largely regulated by the expression of manganese-dependent mitochondrial superoxide dismutase (MnSOD). As the mitochondrion plays a vital role in MnSOD activation, this study sought to determine the location and activity of MnSOD within BE cells after exposure to oxidative stress.

METHODS

A human BE cell line, BAR-T cell, was exposed 0.4 mM concentrations of taurocholic acid (Tau) or a 0.4 mM 1:1 mixture of bile salts for 4 h. Cell viability was performed with 3-(4, 5-dimthyl-thiazol-2-yl)-2, 5-diphenyltetrazolium bromide assays. Proteins were extracted and separated into mitochondrial, nuclear, and cytoplasmic fractions followed by analysis by a western blot and enzymatic activities.

RESULTS

BAR-T cell showed resistance to the bile-salt insults. Expression of MnSOD was significantly increased in the cells exposed to a mixture of bile acids and Tau versus control. Mitochondria MnSOD is abundant and highly active. Nuclear fraction displayed presence of both MnSOD and Cu/zinc superoxide dismutase secondary to bile-acid exposure; however, the MnSOD was inactive in nuclear fraction.

CONCLUSIONS

This is the first study to specifically evaluate cellular fraction MnSOD expression, increased in BE cells in response to the oxidative stress of bile exposure. Mitochondrial MnSOD contributes to resistance of BAR-T cells to the bile-salt insults. Further investigation is required to determine the potential correlation between bile exposure and BE to adenocarcinoma progression via MnSOD-mediated cell signaling.

Colocalization of MnSOD expression in response to oxidative stress

The loss of manganese superoxide dismutase function has been associated with increased incidence of Barrett's esophagus and esophageal adenocarcinoma. In previous studies, we have demonstrated that loss of MnSOD resulted in severe esophageal damage by both endogenous and exogenous bile. However, the alterative manner of MnSOD in esophageal epithelium is largely unknown. In this study, we investigated the expression and localization of MnSOD in response to the exposure to bile salts in an esophageal epithelial cell line. Het-1A cells were seeded at 5 x 10(5) and 10(7) and incubated with taurocholate, cholate, glycocholate, deoxycholate, and the mixture of these bile salts. Mitochondria and cytoplasma were separated, and the expression and localization of MnSOD was determined by Western blot and immunocytochemical assay. Proliferation rates were strongly inhibited in the groups with taurocholate and bile salts mixture at 4 h, with 0.367 +/- 0.042 and 0.396 +/- 0.046, respectively, compared to 0.684 +/- 0.054 in untreated groups (P < 0.05). An increased apoptotic rate compared to untreated group (3.65 +/- 0.59) were significantly increased in taurocholate group and in bile salts mixture group were 33.62 +/- 10.25 and 31.52 +/- 8.97 at 4 h, respectively (P < 0.05). The protein level of MnSOD in mitochondria was increased at 4 h, but with a decreased enzymatic activity after bile salts treatment. Cytoplasmic MnSOD was detected in the cells with bile salts treatment. Immunocytochemical staining demonstrated that esophageal epithelial cell underwent morphological alteration and MnSOD relocalization after bile salts treatment. This is the first study to demonstrate cellular cytosolic MnSOD expression and that this relocalization to the cytosol is a cause for decreased MnSOD enzymatic activity. This suggests that bile salts may contribute to the dysfunction of mitochondria, by enzymatically inhibiting of MnSOD localization and thus activation in the mitochondria.

Investigations on the mechanism of the hypocholesterolemic action of 1-ethoxysilatrane

Intraperitoneal administration of the nontoxic silicon compound, 1-ethoxysilatrane, to the rat did not cause proliferation of hepatic mitochondria or of endoplasmic reticulum, nor did it affect mitochondrial oxidative phosphorylation. The activities of cholesterol 7 alpha-hydroxylase in hepatic microsomes and of cholesterol oxidase in mitochondria respectively were unaffected by silatrane treatment. The rate of release of bile, whose composition remained unchanged, also was not increased in silatrane-treated animals. The results indicated that the compound did not affect the pathway of cholesterol degradation. A progressive decrease in the activity of hepatic microsomal 3-hydroxy-3-methylglutaryl coenzyme A (HMGCoA) reductase was observed on administration of the compound over a period of three weeks. Consistent with this, cholesterol biosynthesis in liver as measured by incorporation of radioactive precursors, acetate and water but not mevalonate, was significantly decreased in silatrane-treated animals. However, enzyme-linked immunosorbent assay revealed that the concentration of HMGCoA reductase protein was not decreased by the treatment indicating that inactivated enzyme was also present in such microsomes. Addition of silatrane to microsomes in the assay system did not cause inhibition indicating that the inactivation is by an indirect mechanism. It is concluded that the hypocholesterolemic action of the compound rested entirely on the inhibition of cholesterol biosynthesis in vivo by inactivation of the rate-limiting enzyme HMGCoA reductase.

Investigations on the mechanism of the hypocholesterolemic action of diethylhexyl phthalate in rats

The plasticizer di(2-ethylhexyl)phthalate (DEHP), used widely in the manufacture of food packages and medical plastic devices, depressed serum cholesterol (40%) and proliferated hepatic mitochondria (100%) when administered in the diet (2%, w/w) to the rat. Microsomes isolated from the livers of animals administered DEHP showed lowered specific activity of 3-hydroxy-3-methyl glutaryl coenzyme A reductase (50%). The incorporation of acetate but not of mevalonate into hepatic cholesterol was decreased (52%) in these animals. The release of bile acids was greatly enhanced (100%), and the activity of cholesterol 7 alpha-hydroxylase, the regulatory enzyme in the pathway of bile acid formation, was stimulated (70%) on DEHP administration. Even though the capacity of mitochondria to oxidize the side chain of cholesterol was not enhanced in DEHP-fed animals, the larger population of mitochondria would ensure that the amount of cholesterol oxidized per gram of liver was significantly higher.

Functional changes in rat liver mitochondria on administration of 2-methyl-4-dimethylaminoazobenzene

Administration of 2-methyl-4-dimethylaminobenzene in the diet (0.1%, w/w) for 85-90 days doubled the content of mitochondria in the livers of rats. The azodye was covalently bound to liver proteins, and about 15% of the amount found in liver was associated with the mitochondrial fraction. Mitochondria isolated from the livers of azodye-fed animals showed drastically lowered ability to oxidize NAD+-linked substrates. The inhibited electron-transfer step was the reduction of ubiquinone. The organelles showed a large increase in succinate oxidase activity. The activity of cytochrome oxidase and the content of cytochrome aa3 were substantially higher in these organelles. Azodye-fed animals showed depressed serum cholesterol concentrations. The content of ubiquinone in liver also registered a small increase.

Increase of hepatic mitochondria on administration of ethyl alpha-p-chlorophenoxyisobutyrate to the rat

1. The antihypercholesterolaemic drug ethyl alpha-p-chlorophenoxyisobutyrate when fed to the rat orally or mixed with the diet increased the content of mitochondria in the liver by 50-100%. Other subcellular fractions did not show any significant change. 2. In oxidative activity, respiratory control and phosphorylating ability no significant difference was observed between the mitochondria isolated from the livers of the drug-treated rats and those from normal animals. 3. In agreement with earlier reports, administration of the drug depressed the concentration of serum cholesterol and increased liver weight and the liver content of ubiquinone. However, the increase of ubiquinone was greater in the nuclear than in the mitochondrial protein.