Loss of cytochrome P-450 from hepatic nuclear membranes of rats fed 2-acetylaminofluorene

Effect of dietary butylated hydroxytoluene on nuclear envelope cytochrome P-450 during the initiation and promotion stages of hepatocarcinogenesis

The anticarcinogenic effect of the dietary antioxidant butylated hydroxytoluene (BHT) correlates with a preservation of nuclear envelope (NE) cytochrome P-450 in rats undergoing chemically induced hepatocarcinogenesis. This effect of BHT on NE cytochrome P-450 was observed during both the initiation and the promotion stages of hepatocarcinogenesis. Complex interactions between the two different mechanisms of action of BHT (i.e., enzyme induction and antioxidant activity) may account for some of the differences between the patterns of response to BHT observed during initiation and promotion.

Prevention of 2-acetylaminofluorene-induced loss of nuclear envelope cytochrome P450 by the simultaneous administration of 3-methylcholanthrene

Rats fed a basal diet containing 0.05% (w/w) 2-acetylaminofluorene (AAF) for 3 weeks showed a 50% loss of hepatic nuclear envelope cytochrome P450, whereas microsomal P450 remained at control levels. A similar dietary treatment with 0.004% (w/w) 3-methylcholanthrene (MC) caused moderate losses (20-25%) of cytochrome P450 in both nuclear envelopes and microsomes. Administration of the basal diet supplemented with a mixture of AAF (0.05%) plus MC (0.004%) resulted in a preservation of control levels of nuclear envelope cytochrome P450 and a 30% elevation of microsomal P450. Immunoblot analysis revealed that AAF alone, or in concert with MC, induced comparable levels of the P450d form. Induction of cytochrome P450c by dietary MC was detected only when MC was fed together with AAF. As previously found for butylated hydroxytoluene (BHT), the protective effect of dietary MC against hepatocarcinogenesis in AAF-fed rats correlated with a preservation of nuclear envelope cytochrome P450 content and with the induction of cytochrome P450c.

Induction of rat liver microsomal and nuclear cytochrome P-450 by dietary 2-acetylaminofluorene and butylated hydroxytoluene

The influence of dietary 2-acetylaminofluorene (AAF) on the cytochrome P-450 content of rat liver microsomal and nuclear fractions was immunochemically probed with monoclonal and polyclonal antibodies to cytochromes P-450c and P-450d. Cytochrome P-450d but not P-450c was immunodetected in microsomes, nuclear envelopes, and nuclei from untreated rats. The levels of both cytochromes P-450c and P-450d were elevated after a diet of either 0.1% AAF for 1 week or 0.05% AAF for 3 weeks. However, the level of cytochrome P-450c relative to P-450d was lower after the more prolonged AAF feeding. Supplementation of AAF-containing diets with 0.3% butylated hydroxytoluene (BHT), which affords protection against AAF hepatocarcinogenesis in high-fat fed rats, protected and/or induced total (spectral) nuclear envelope cytochrome P-450 content. Immunochemical studies of liver fractions showed that BHT enhanced the AAF-dependent induction of cytochrome P-450c, but not of P-450d. This was a concerted effect of AAF + BHT since dietary BHT by itself did not affect the levels of cytochrome P-450c or P-450d as compared to control rats. Since 1- to 3-week dietary AAF had little effect on total (spectral analyses) microsomal cytochrome P-450 but markedly reduced total P-450 in nuclear envelopes, the coordinated induction of specific cytochrome P-450s in the different fractions suggests selective induction and depression of different forms of cytochrome P-450 and provides additional evidence for independent regulation of the drug-metabolizing system in nuclear envelope and microsomes. In addition, these results suggest that regulation of cytochrome P-450 may play a crucial role in the nutritional modulation of AAF hepatocarcinogenesis.

Hepatic nuclear envelope cytochrome P-450 in rats fed 2-acetylaminofluorene. Effect of dietary fats and butylated hydroxytoluene

Early losses of nuclear envelope (NE) cytochrome P-450 occur during 2-acetylaminofluorene (AAF)-hepatocarcinogenesis. Butylated hydroxytoluene (BHT), which protects against AAF in rats fed high fat diets, preserves NE P-450 suggesting that the latter may be required for AAF detoxification. AAF-hepatocarcinogenesis is enhanced by increasing the amount or degree of unsaturation of dietary fats. The present studies show that in rats fed AAF in high fat diets BHT preservation of NE P-450 is considerably shorter (3 weeks) with polyunsaturated than with saturated fat (9 weeks). In rats fed AAF in a low fat diet, where BHT does not modify the low tumor incidence, there is a bimodal effect on NE P-450. In controls and AAF-fed rats, BHT causes significant induction of NE P-450 at 1-3 weeks. At 9-16 weeks BHT has no effect on controls and only partially preserves NE P-450 in AAF-fed rats. Due to time constraints, livers could not be utilized fresh, but they were kept frozen at -80 degrees C until processed. Thus, specimens from all groups underwent identical treatment for purposes of comparative studies.

Increased cholesterol epoxide hydrolase activity in clofibrate-fed animals

Cholesterol epoxide hydrolase (mCE) is a microsomal enzyme that hydrolyzes cholesterol-5,6-epoxides (CE) to cholestanetriol (CT). In the present study, hepatic mCE activity was measured in mice pretreated with several different xenobiotics known to induce a variety of hepatic drug-metabolizing enzymes. Only the phenoxyacetate hypolipidemics (clofibrate and ciprofibrate, included in the diet for 14 days) were found to increase hepatic mCE activity (1.8-fold increase). Clofibrate administration also increased rabbit hepatic (2.9-fold increase) and rat and rabbit renal mCE activity (1.5- and 2.3-fold increase respectively). On a subcellular level, mCE activities in rabbit nuclear and light mitochondrial fractions were increased (3.3- and 1.8-fold increase respectively), whereas activities in the cytosolic and heavy mitochondrial fractions were unchanged. In rabbits, clofibrate administration enhanced the hepatic microsomal hydrolysis of CE without increasing the hydrolysis of arene epoxides (benzopyrene-4,5-oxide) or fatty acid epoxides (methyl cis-9,10-epoxystearate). Increase of tissue mCE activity may significantly enhance tissue CT levels. In this light, it is worth noting similarities in the mechanisms of hypocholesterolemic action caused by clofibrate or CT administration.

Dietary butylated hydroxytoluene protects cytochrome P-450 in hepatic nuclear membranes of rats fed 2-acetylaminofluorene

We previously reported that during hepatocarcinogenesis which is induced by feeding 2-acetylaminofluorene (AAF) there is an early loss of cytochrome P-450 in the nuclear envelope (Carubelli et al., Chem. Biol. Interact. 58, 125-136, 1986). Cytochrome P-450 participates in the activation in addition to the detoxification of xenobiotics; therefore, these findings suggested that AAF may cause the loss of an important defense for the protection of the genetic material of the nucleus against carcinogenic metabolites of AAF generated by microsomal P-450 which, in contrast to the nuclear envelope cytochrome P-450, remains essentially undiminished during early stages of AAF feeding. Because dietary butylated hydroxytoluene (BHT) affords good protection against AAF carcinogenicity, we decided to investigate the possibility that the BHT effect could be mediated through the preservation of nuclear envelope cytochrome P-450. AAF (0.05% wt/wt) was administered in a purified diet with a high content of polyunsaturated fat (20% wt/wt corn oil), which is known to enhance AAF carcinogenicity. These studies showed that BHT supplementation (0.3% wt/wt) of control in addition to the AAF-containing diets resulted in higher levels of nuclear envelope cytochrome P-450. After 16 weeks of AAF feeding, nuclear envelope cytochrome P-450 could not be detected in rats fed BHT-free diet, whereas in the rats fed the diet containing AAF and BHT, measurable amounts of nuclear envelope P-450 were observed. These results are compatible with the hypothesis that nuclear envelope cytochrome P-450 is needed for protection against AAF and that BHT protects nuclear envelope cytochrome P-450.