gamma-Glutamyltranspeptidase-negative phenotypic property of preneoplastic and neoplastic liver lesions induced by ciprofibrate does not change following 2-acetylaminofluorene administration

Hepatic peroxisome proliferation in rodents and its significance for humans

Peroxisomes are subcellular organelles found in all eukaryotic cells. In the liver they are usually round and measure about 0.5-1.0 microns; in rodents they contain a prominent crystalloid core, but this may be absent in newly formed rodent peroxisomes as well as in human peroxisomes. A major role of the peroxisomes is the breakdown of long-chain fatty acids, thereby complementing mitochondrial fatty-acid metabolism. Many chemicals are known to increase the number of peroxisomes in rat and mouse hepatocytes. This peroxisome proliferation is accompanied by replicative DNA synthesis and liver growth. No clear structure-activity relationships are apparent. Many of these peroxisome proliferators contain acid functions that can modulate fatty acid metabolism. Two mechanisms have been proposed for the induction of peroxisome proliferation. One is based on the existence of one or several specific cytosolic receptors that bind the peroxisome proliferator, facilitating its translocation to the cell nucleus and the activation of the expression of specific genes. The second, perhaps more general, hypothesis involves chemically mediated perturbation of lipid metabolism. These two hypotheses are not mutually exclusive. Many peroxisome proliferators have been shown to induce hepatocellular tumours, despite being uniformly non-genotoxic, when administered at high dose levels to rats and mice for long periods. Three mechanisms have been proposed to explain the induction of tumours. One is based on increased production of active oxygen species due to imbalanced production of peroxisomal enzymes; it has been proposed that these reactive oxygen species cause indirect DNA damage with subsequent tumour formation. In rodents, an alternative mechanism is the promotion of endogenous lesions by sustained DNA synthesis and hyperplasia. Thirdly, it is conceivable that sustained growth stimulation may be sufficient for tumour formation. Marked species differences are apparent in response to peroxisome proliferations. Rats and mice are extremely sensitive, and hamsters show an intermediate response while guinea pigs, monkeys and humans appear to be relatively insensitive or non-responsive at dose levels that produce a marked response in rodents. These species differences may be reproduced in vitro using primary culture hepatocytes isolated from a variety of species including humans. The available experimental evidence suggests a strong association and a probable casual link between peroxisome-proliferator-elicited liver growth and the subsequent development of liver tumours in rats and mice. Since humans are insensitive or unresponsive, at therapeutic dose levels, to peroxisome-proliferator-induced hepatic effects, it is reasonable to conclude that the encountered levels of exposure to these non-genotoxic agents do not present a hepatocarcinogenic hazard to humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Biochemical markers associated with the stages of promotion and progression during hepatocarcinogenesis in the rat

Specific biochemical changes occurring during hepatocarcinogenesis have been sought by many investigators. The development of multistage models for hepatocarcinogenesis in the rodent has renewed interest in such marker alterations in preneoplastic as well as neoplastic hepatocytes. Preneoplastic altered hepatic foci (AHF) exhibit specific histomorphologic changes as viewed with tinctorial stains and show a variety of biochemical changes as evidenced by enzyme and immunohistochemistry and by other histochemical markers. During the reversible stage of promotion when AHF are scored by multiple markers, the distribution of markers within these lesions differs with the use of different promoting agents. One interpretation of this finding is that each promoting agent stimulates the replication of a set of initiated cells exhibiting the phenotypic characteristics of a specific programmed phenotype. The same markers score AHF during the stage of progression, but many AHF in this stage are phenotypically heterogeneous, exhibiting in tissue sections a "focus-in-focus" pattern of marker alteration. These latter changes can be correlated with the appearance of karyotypic alterations in preneoplastic hepatocytes. On the other hand, it has been difficult to demonstrate the activation, either mutational or transcriptional, of proto-oncogenes until this stage of progression in rat hepatocarcinogenesis. Thus, a study of biochemical and molecular markers during the stages of hepatocarcinogenesis may lead to a better understanding of potential mechanisms involved in the development of neoplasia through the stages of initiation, promotion, and progression.

Tumor promotion as a target for estrogen/antiestrogen effects in rat hepatocarcinogenesis

Tamoxifen is a well-tolerated palliative and adjuvant treatment for human breast cancer and requires continuous, long-term administration for optimal therapeutic effectiveness. A two-stage model of experimental hepatocarcinogenesis, based upon the natural history of cancer development, has been employed to assess the carcinogenic potential of tamoxifen. In this study, the effectiveness of tamoxifen both as an initiator and a promoter in hepatocarcinogenesis was assessed in female F-344 rats. Tamoxifen was tested as an initiator at a single intragastric dose of 40 mg/kg, followed by promotion with 0.05% phenobarbital. The number and size of the resulting altered hepatic lesions were quantified, and tamoxifen was found to lack initiating action at the dose tested. Other groups of animals were initiated with a nonnecrogenic, subcarcinogenic dose of diethylnitrosamine (10 mg/kg) and were fed tamoxifen at either 250 or 500 mg/kg in the AIN-76A purified diet for 6 months. The livers of these animals showed an increase in the size and number of altered hepatic lesions compared with those animals that were initiated but not exposed to tamoxifen; this indicates that tamoxifen acts as a tumor promoter in the rat liver. The promotion index of tamoxifen, a measure of relative potency, was less than one-tenth that of ethinyl estradiol and more than four times that of phenobarbital, an agent commonly employed as a representative promoting agent in experimental carcinogenesis. Since tamoxifen lacked initiating activity in the rat liver at the dose tested, the mechanism of tumor induction in long-term feeding studies by tamoxifen may be due to its promotion of spontaneously initiated hepatocytes. The chronic therapeutic use of tamoxifen should therefore be limited by the potential carcinogenic risk of this agent as an effective tumor promoter.

Oral administration of chlordiazepoxide plus sodium nitrite investigated for tumor initiating activity in the rat liver

Chlordiazepoxide (CDE) reacts with sodium nitrite at acid pH yielding the genotoxic derivative N-nitrosochlordiazepoxide (NO-CDE). In the present study oral administration of CDE plus NaNO2, previously found to produce DNA fragmentation in the rat liver, was examined for its ability to initiate hepatocarcinogenesis. The oral treatment for 6 successive weeks with CDE + NaNO2, added to the diet at the levels of 290 + 270 and 870 + 800 ppm, did not significantly increase the number or volume of gamma-glutamyltranspeptidase-positive foci (putative preneoplastic lesions). These findings are in agreement with the negative results previously obtained in rodent carcinogenesis assays and indicate that NO-CDE belongs to the progressively expanding list of genotoxic non-carcinogens.