Diethylhexylphthalate (DEHP) is a phthalate plasticizer that belongs to the peroxisome proliferator (PP) class of rodent nongenotoxic hepatocarcinogens. Previously, we have shown that MEHP (a principal metabolite of DEHP and the proximal PP) induced DNA synthesis and suppressed apoptosis in rat but not in human hepatocytes in vitro. Here, we present further studies of species differences in response to DEHP. In rats, 4 days of exposure to DEHP (950 mg/kg per day by gavage) induced peroxisomal beta-oxidation, DNA synthesis and suppressed apoptosis. In contrast, there was no response of guinea pig liver to DEHP. In rat hepatocytes in vitro, MEHP (250, 500 and 750 microM) induced peroxisomal beta-oxidation, DNA synthesis and suppressed apoptosis. In contrast to the pleiotropic response noted in rat hepatocytes, there was no response of human hepatocytes to 250, 500 or 750 microM MEHP. PPs activate the peroxisome proliferator activated receptor alpha (PPARalpha) that binds to DNA at peroxisome proliferator response elements (PPREs) within the promoters of PP-responsive genes such as rat acyl CoA oxidase (ACO). However, the human ACO gene promoter differs at three bases within the PPRE from the rat ACO promoter and appears refractory to PPs. To address species differences in response to DEHP at the molecular level, we used promoter-reporter gene assays to compare the ability of MEHP to induce gene expression from the rat or the human ACO promoter. MEHP gave a concentration-dependent increase in reporter gene expression from the rat ACO gene promoter with either mouse or human PPARalpha. In contrast, the human ACO promoter was unable to drive MEHP-induced gene transcription irrespective of the species origin of PPARalpha. These data provide further weight of evidence at the cellular and molecular levels for a lack of risk to human health from the phthalate DEHP.

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We studied nine presumed nongenotoxic rodent carcinogens, as defined by the U.S. National Toxicology Program (NTP), to determine their ability to induce acute or subacute biochemical and tissue changes that may act as useful predictors of nongenotoxic rodent carcinogenesis. The chemicals selected included six liver carcinogens (two of which are peroxisome proliferators), three thyroid gland carcinogens, and four kidney carcinogens. We administered the chemicals (diethylhexyl phthalate, cinnamyl anthranilate, chlorendic acid, 1,4-dichlorobenzene, monuron, ethylene thiourea, diethyl thiourea, trimethyl thiourea, and d-limonene to the same strains of mice and rats used in the original NTP bioassays (nine chemicals to rats and seven to mice). Selected tissues (liver, thyroid gland, and kidney) were collected from groups of animals at 7, 28, and 90 days for evaluation. Tissue changes selected for study were monitored for all of the test groups, irrespective of the specificity of the carcinogenic responses observed in those tissues. This allowed us to assess both the carcinogen specificity and the carcinogen sensitivity of the events being monitored. We studied relative weight, cell labeling indices, and pathologic changes such as hypertrophy in all tissues; a range of cytochrome P450 enzymes and palmitoyl coenzyme A oxidase in the liver; changes in the levels of plasma total triiodothyronine, total thyroxine, and thyroid-stimulating hormone (TSH) as markers of thyroid gland function; and hyaline droplet formation, tubular basophilia, and the formation of granular casts in the kidney. There were no single measurements that alerted specifically to the carcinogenicity of the agents to the rodent liver, thyroid gland, or kidney. However, in the majority of cases, the chemical induction of cancer in a tissue was preceded by a range of biochemical/morphologic changes, most of which were moderately specific for a carcinogenic outcome, and some of which were highly specific for it (e.g., increases in TSH in the thyroid gland and increases in relative liver weight in the mouse). The only measurements that failed to correlate usefully with carcinogenicity were the induction of liver enzymes (with the exception of the enzymes associated with peroxisome proliferation). Most of the useful markers were evident at the early times studied (7 days and 28 days), but no overall best time for the measurement of all markers was identified. The judicious choice of markers and evaluation times can aid the detection of potential nongenotoxic rodent carcinogens.

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Diethylhexylphthalate (DEHP) and diisononylphthalate (DINP) are plasticizers with many important commercial, industrial and medical applications. However, both DEHP and DINP are rodent peroxisome proliferators (PPs), a class of compounds that cause rodent liver tumours associated with peroxisome proliferation, induction of hepatic DNA synthesis and the suppression of apoptosis. Despite these effects in the rodent, humans appear to be nonresponsive to the adverse effects of PPs. Previously, we have shown that the fibrate hypolipidaemic peroxisome proliferator, nafenopin, induced DNA synthesis and suppressed apoptosis in rat but not in human hepatocytes. In this work, we have examined species differences in the response of rat and human hepatocytes to DEHP and DINP in vitro. In rat hepatocytes in vitro, both DINP and MEHP (a principle metabolite of DEHP and the proximal peroxisome proliferator) caused a concentration-dependent induction of DNA synthesis and suppression of both spontaneous and transforming growth factor beta1 (TGFbeta1)-induced apoptosis. Similarly, both MEHP and DINP caused a concentration-dependent induction of peroxisomal beta-oxidation although the response to DINP was less robust. In contrast to the pleiotropic response noted in rat hepatocytes, neither DINP nor MEHP caused an induction of beta-oxidation, stimulation of DNA synthesis and suppression of apoptosis in human hepatocytes cultured from three separate donors. These data provide evidence for species differences in the hepatic response to the phthalates DEHP and DINP, confirming that human hepatocytes appear to be refractory to the hepatocarcinogenic effects of PPs first noted in rodents.