Janardan K. Reddy

Hepatic peroxisome proliferation: induction by two novel compounds structurally unrelated to clofibrate

Two hypolipidemic compounds [ 4-chloro-6(2,3-xylidino)-2-pyrimidinyl-thio] acetic acid, and 2-chloro-5(3,5-dimethylpiperidinosufony)benzoic acid (tibric acid) greatly increased the number of peroxisomes (microbodies) in liver cells of rats and mice. This augmented peroxisome population was accompanied by significant elevation of liver catalase activity. These two hypolipidemic peroxisome proliferators are structurally different from ethyl a-p-chlorophenozyisobutyrate (clofibrate) and other hypolipidemic, arylocyisobutyrate derivatives which cause hepatic peroxisome proliferation. Induction of peroxisome proliferation by these structurally unrelated hypolipidemic compounds suggests a possible relation between hepatic peroxisome proliferation and hypolipidemia.

Di-(2-ethylhexyl)phthalate: An industrial plasticizer induces hypolipidemia and enhances hepatic catalase and carnitine acetyltransferase activities in rats and mice

Abstract Di-(2-ethylhexyl)phthalate (DOP), a plasticizer, when administered to rats and mice at a dietary concentration of 0.5 percent, 2 percent or 4 percent, caused a substantial reduction in serum cholesterol and triglyceride levels. The hypocholesterolemic and hypotriglyceridemic effects were more pronounced in rats fed DOP at 2 and 4 percent level than in mice. The hypolipidemic effect of this compound is associated with marked hepatomegaly in both species. The liver cells of both rats and mice fed this plasticizer for 1–4 weeks, revealed a considerable increase in peroxisome population. The elevations of hepatic catalase and carnitine acetyltransferase activities appeared to parallel the peroxisome proliferation induced by this compound. The hypolipidemic and peroxisome proliferative effects of this compound strongly suggest that all peroxisome proliferators possess hypolipidemic properties.

POSSIBLE PROPERTIES OF MICROBODIES (PEROXISOMES) MICROBODY PROLIFERATION AND HYPOLIPIDEMIC DRUGS

M icrobodies (peroxisomes), the cytoplasmic constituents identified originally in kidney and liver, have only recently been recognized as ubiquitous structures in animal cells (13, 14, 16, 20, 22, 31). De Duve and collaborators investigated the biochemical composition of hepatic microbodies and showed that these organelles contain catalase and several oxidative enzymes (8, 9, 17, 23). It appears that catalase is the only constant component of peroxisomes derived from different sources, whereas the hydrogen peroxide-producing oxidative enzymes that accompany catalase differ in number and in specificity (8). Identification of microbodies in tissues other than liver and kidney was based on the cytochemical localization of catalase with the alkaline 3, 3’-diaminobenzidine medium described by Novikoff and Goldfischer (19). It remains to be determined whether these catalase-containing cytoplasm ic structures contain “at least one hydrogen peroxide producing oxidase” (8) in order to regard them as peroxi-

Microbody proliferation in liver induced by nafenopin, a new hypolipidemic drug: comparison with CPIB.

Abstract Nafenopin (2-methyl-2[P-(1,2,3,4-tetrahydro-1 naphthyl) phenoxy]- propionic acid, a phenolic ether with hypolipidemic properties, when administered by gavage at 100 mg/kg b wt daily for 1 to 2 weeks, caused a significant increase in the number of microbody profiles and simultaneous increase in catalase activity in livers of male rats. The concentration of catalase protein and the rate of incorporation of H3-δ-aminolevulinic acid into catalase fraction, as determined by immunochemical methods were approximately twice that of controls. The microbody proliferation resulting from nafenopin treatment was comparable to that induced by CPIB.

Stimulation of liver catalase synthesis in rats by ethyl-α-p-chlorophenoxyisobutyrate

Abstract In male rats fed 0.25% CPIB in diet 1 to 2 weeks, the concentration of catalase protein in liver extract was 2.1 times that of controls. With immunochemical methods, it was shown that CPIB enhances the rate of synthesis of hepatic catalase by more than 80% in three days and maintains this enhanced synthesis throughout the duration of its administration.

Mitogenic effect in mouse liver induced by a hypolipidemic drug, nafenopin

SummaryThe mitogenic effect of Nafenopin, a hypolipidemic hepatic peroxisome proliferator, in mouse liver has been studied in acute and chronically treated mice. After 1, 6 and 32 weeks of treatment, the total hepatic DNA was increased 1.5–2.0-fold over controls. Mitotic and labeling indices were also increased 3–4 fold after 5 days, 6 weeks and 32 weeks of treatment. The increased mitotic activity in nafenopin fed animals was not associated with liver cell necrosis. The nafenopin induced hepatomegaly therefore appears to arise from a combination of cell proliferation, as well as, cellular hypertrophy, which is associated with peroxisome proliferation.

Some Effects of Chemical Carcinogens on Cell Organelles

There is no single ultrastructural feature that distinguishes the malignant cell from its normal counterpart, and, with some exceptions, the premalignant state, although conceptually useful, has not been identifiable morphologically. Nevertheless, ultrastructural studies of tissues and cells from animals given chemical carcinogens have disclosed many early, intermediate, and late alterations in cell structure that have helped to clarify the biological interaction between a carcinogen or its metabolite and specific organelles. In addition, such studies have made possible comparisons between the cellular responses to carcinogenic injury and other forms of cell damage, thereby placing the problem of carcinogenesis in an appropriately larger biological perspective. Although morphological description of the responses of cells to carcinogens cannot substitute for biochemical analysis of the significance of such responses, nonetheless descriptive morphology serves as a useful step preliminary to other forms of investigation.

Microbody (Peroxisome) matrix: Transformation into tubular structures

SummaryFollowing CPIB treatment one or more double-walled tubular structures, measuring 110 μ in diameter, were observed in the hepatic microbody profiles in hypophysectomized rats, as well in rats bearing liver tumors. These tubules appeared as straight rigid rods of considerable length and were oriented irregularly within the microbody profiles. Examination of several microbody profiles containing these tubules suggests that these structures may be formed as a result of transformation of microbody matrix protein(s). Varying degrees of electron lucency of microbody matrix and the size and shape of the channels containing these straight tubules are considered as further evidence to indicate that microbody proteins circulate within the endoplasmic reticulum channels.

Hepatic catalase is not essential for the hypolipidemic action of peroxisome proliferators.

Summary To evaluate the role of peroxi-somal catalase in lipid metabolism the hypo-lipidemic drugs clofibrate, methyl clofena-pate, and nafenopin were administered to rats in combination with allylisopropylace-tamide, an agent which inhibits catalase synthesis. The number of peroxisomes in liver cells was increased, but these organelles did not contain cytochemically demonstrable catalase. A marked reduction in liver catalase activity was also noted. Although the proliferated peroxisomes were deficient in catalase activity, a substantial hypolipidemic effect was observed in these animals, suggesting that peroxisome catalase is not necessary for the hypolipidemic effect. The increase in carnitine acetyltransferase activity in these animals emphasizes the need to delineate other peroxisomal enzymes which may be involved in lipid metabolism.

Natural segregation of nucleolar components of liver cells in newts.

The granular and fibrillar ribonucleoprotein components of the liver cell nucleoli of Triturus viridescens, T. cristatus, T. pyrroghaster , and Ambystoma tigrinum were found to be segregated naturally. The significance of this phenomenon needs to be elucidated.