Molecular organization of hepatocyte peroxisomes

Dietary controlled carcinogenicity study of chloral hydrate in male B6C3F1 mice

Chloral hydrate, which is used as a sedative in pediatric medicine and is a by-product of water chlorination, is hepatocarcinogenic in B6C3F1 mice, a strain that can exhibit high rates of background liver tumor incidence, which are associated with increased body weight. In this study, dietary control was used to manipulate body growth in male B6C3F1 mice in a 2-year bioassay of chloral hydrate. Male B6C3F1 mice were treated with water or 25, 50, or 100 mg/kg chloral hydrate by gavage. The study compared ad libitum-fed mice with dietary controlled mice. The latter received variably restricted feed allocations to maintain their body weights on a predetermined "idealized" weight curve predictive of a terminal background liver tumor incidence of 15-20%. These mice exhibited less individual body weight variation than did their ad libitum-fed counterparts. This was associated with a decreased variation in liver to body weight ratios, which allowed the demonstration of a statistically significant dose response to chloral hydrate in the dietary controlled, but not the ad libitum-fed, test groups. Chloral hydrate increased terminally adjusted liver tumor incidence in both dietary controlled (23.4, 23.9, 29.7, and 38.6% for the four dose groups, respectively) and ad libitum-fed mice (33.4, 52.6, 50.6, and 46.2%), but a statistically significant dose response was observed only in the dietary controlled mice. This dose response positively correlated with markers of peroxisomal proliferation in the dietary controlled mice only. The study suggests that dietary control not only improves terminal survival and decreases interassay variation, but also can increase assay sensitivity by decreasing intra-assay variation.

Peroxisome biogenesis

Peroxisome biogenesis conceptually consists of the (a) formation of the peroxisomal membrane, (b) import of proteins into the peroxisomal matrix and (c) proliferation of the organelles. Combined genetic and biochemical approaches led to the identification of 25 PEX genes-encoding proteins required for the biogenesis of peroxisomes, so-called peroxins. Peroxisomal matrix and membrane proteins are synthesized on free ribosomes in the cytosol and posttranslationally imported into the organelle in an unknown fashion. The protein import into the peroxisomal matrix and the targeting and insertion of peroxisomal membrane proteins is performed by distinct machineries. At least three peroxins have been shown to be involved in the topogenesis of peroxisomal membrane proteins. Elaborate peroxin complexes form the machinery which in a concerted action of the components transports folded, even oligomeric matrix proteins across the peroxisomal membrane. The past decade has significantly improved our knowledge of the involvement of certain peroxins in the distinct steps of the import process, like cargo recognition, docking of cargo-receptor complexes to the peroxisomal membrane, translocation, and receptor recycling. This review summarizes our knowledge of the functional role the known peroxins play in the biogenesis and maintenance of peroxisomes. Ideas on the involvement of preperoxisomal structures in the biogenesis of the peroxisomal membrane are highlighted and special attention is paid to the concept of cargo protein aggregation as a presupposition for peroxisomal matrix protein import.

Cytochemical and biochemical demonstration of an ATPase in membranes of human peroxisomes

We demonstrated a neutral Mg-ATPase activity in human peroxisomal membranes. To establish the precise experimental conditions for detection of this ATPase, both cytochemical and biochemical characterizations were first carried out in liver peroxisomes from control and cipofibrate-treated rats. The results demonstrated an Mg-ATPase reaction in both normal and proliferated peroxisomes. The nucleotidase activity, with marked preference for ATP, was sensitive to the inhibitors N-ethylmaleimide and 7-chloro-4-nitro-benzo-2-oxadiazole (NBDCl). An ultrastructural cytochemical analysis was developed to evaluate the peroxisomal localization, which localized the reaction product to the peroxisomal membrane. These characteristics can help to differentiate the peroxisomal ATPase from the activity found in mitochondria and endoplasmic reticulum. The conditions established for detecting the rat peroxisomal ATPase were then applied to human peroxisomes isolated from liver and skin fibroblasts in culture. A similar Mg-ATPase activity was readily shown, both cytochemically and biochemically, in the membranes of human peroxisomes. These results, together with previous evidence, strongly support the presence of a specific ATPase in the human peroxisomal membrane. This ATPase may play a crucial role in peroxisome biogenesis.

Morphometry of fine structural alterations of hepatocytes of Japanese monkeys under fasting stress

The fine structural alterations of hepatocytes of Japanese monkeys under 4 days of fasting stress were analyzed morphometrically. One of the conspicuous alterations was the enlargement of mitochondria. The average diameter of mitochondria in fasting group increased to approximately 1.89-fold of that in control group, though their number did not change. The number of peroxisomes was 1.36-fold of that in control, though their area did not change. In addition, many of r-ER were swollen and were vesiculated. The appearance of bundle of actin-like stress fiber also increased in the fasting animals. The glycogen area as well as liver weight decreased in fasting group.

Morphometry of abnormal peroxisomes induced by withdrawal of bezafibrate, a hypolipidemic drug in male rat hepatocytes

Peroxisomes containing fibrillar structures were induced after 1 week withdrawal of bezafibrate, a peroxisome proliferator. In this report, the relation between the duration of bezafibrate treatment and the induction of abnormal peroxisomes in rat hepatocytes was studied morphometrically. The abnormal peroxisomes did not appear during 3 to 90 days of treatment with bezafibrate, but they appeared after 1 week withdrawal of it. The number and frequency of abnormal peroxisomes were prominent in 3, 7, and 14 days of treatment followed by 1 week of withdrawal of bezafibrate. It was evident that the frequency of abnormal peroxisomes decreased with 30-90 days administration of bezafibrate. This means that long-term (30-90 days) treatment with bezafibrate suppresses the induction of abnormal peroxisomes.

Inhibition of peroxisomal degradation by 3-methyladenine (3MA) in primary cultures of rat hepatocytes

BACKGROUND

A significant reduction in peroxisomes has been demonstrated in primary cultures of rat hepatocytes. This report demonstrates that 3-methyladenine (3MA), a potent inhibitor of autophagy, inhibits this effect.

METHODS

Hepatocytes from male Wistar rats were isolated by a two-step in situ perfusion technique using collagenase and were cultured in Williams E medium. After a 2-hr attachment period (day 0 of culture), the cells were treated with 200 microM bezafibrate (BF), a peroxisome proliferator, and 5 mM 3MA for 3 days. The cells in the culture dish were fixed in situ, stained for catalase, and embedded in Poly/Bed 812. The number and size of peroxisomes in electron micrographs were analyzed morphometrically.

RESULTS

After 3 days of culture, the number of peroxisomes had decreased to 30% of the day 0 level. However, the day 0 level was maintained by treatment with 3MA. In BF-treated cells, many autophagosomes were observed, and peroxisomes had proliferated significantly, although they did not exceed the day 0 level. In cells treated with a combination of 3MA and BF, the number and size of peroxisomes had increased remarkably.

CONCLUSIONS

These results suggest that 3MA is effective in maintaining both the number and size of peroxisomes in the course of primary cultures of rat hepatocytes. Suppression of peroxisome proliferation by treatment with BF may be regulated by autophagic/lysosomal degradation.