A mouse is not a rat is not a human: species differences exist.

The observation that similar treatments produce unique outcomes in different sexes or different species often provides a model for mechanistic studies. As an example unrelated to the present paper in this issue, the observation was made that male and female rats exhibited highly different responses to chronic chemical exposure in the kidney, which allowed scientists to characterize the sex-specific responses to chemicals that induce 2u-globulin nephropathy. This syndrome is produced by a variety of chemicals and is manifested by accumulation of 2u-globulin in the kidney, resulting in compensatory cell proliferation and renal tubular tumors. 2u-globulin is a low molecular weight protein (18,700 daltons) synthesized in the liver of male but not female rats. Following secretion from the liver, 2u-globulin is filtered in the glomerulus and slowly hydrolyzed in the proximal tubule. There is a strict requirement that a chemical or a metabolite physically bind to the 2uglobulin to produce nephropathy, although covalent, irreversible binding is not required. When the chemical2u-globulin complex is filtered by the kidney, it accumulates in phagolysosomes in the proximal tubule region and produces cytotoxicity, which results in subsequent regenerative hyperplasia (Lehman-McKeeman, 1997). Several nongenotoxic chemicals, including d-limonene, unleaded gasoline, jet fuels, and 1,4dichlorobenzene produce sustained nephrotoxicity, which appears to be the causative factor in their induction of renal cancer (Borghoff et al., 1990; Borghoff and Lagarde, 1993; Swenberg et al., 1992). Female rats, mice, and NBR rats do not accumulate 2u-globulin in their kidneys, do not produce nephrotoxicity and regenerative hyperplasia, and do not develop renal tumors following chemical exposure. These data are now accepted by regulatory agencies to discount the risk to humans by chemicals that are shown to induce 2u-globulin nephropathy (Baetcke, 1991; IARC, 1998). An observation that two species of rodents differ dramatically in their responses to chemical treatment also offers a unique opportunity to study mechanisms of chemical toxicity across species and allows insight into the use of such data for human risk assessment. The manuscript highlighted in this issue exploits the differential response of rats and mice to chronic 2-butoxyethanol (ethylene glycol monobutyl ether) exposure (Siesky et al., 2002). Male mice were found to be highly sensitive to hepatocarcinogenesis following chronic exposure to 2-butoxyethanol. Male mice exposed to 2-butoxyethanol by inhalation for 2 years had significantly increased incidence of hemangiosarcoma of the liver relative to controls and exceeded the range in historical controls. In addition, there were possible exposure-related increases in the incidence of hepatocellular carcinoma. Incidences of hemosiderin (iron deposition) pigmentation in the Kupffer cells were significantly increased in mice. The incidences of splenic hematopoietic cell proliferation and hemosiderin pigmentation were generally increased, and the incidences of bone marrow hyperplasia were also increased in mice (NTP, 2000). These data lead the authors to hypothesize that the increased incidence of liver tumors in mice was a result of increased oxidative stress and Kupffer cell activation arising from iron accumulation in the liver secondary to red blood cell hemolysis. This is a testable hypothesis since rats were found to be resistant to the hepatocarcinogenicity of 2-butoxyethanol and should, therefore, respond less to the possible prooxidant effects of 2-butoxyethanol than the more sensitive mice species. The hypothesis tested by these researchers is that 2-butoxyethanol, via the action of its primary metabolite 2-butoxyacetic acid, induces hemolysis with concomitant accumulation of iron in the liver. Iron is a prooxidant, and high levels of iron produce oxidant stress either directly by producing reactive oxygen species by the Fenton reaction or by activation of Kupffer cells, which in turn produce reactive oxygen species and release cytokines that may suppress apoptosis and increase cell proliferation, both of which are shown to be associated with increased susceptibility for cancer development. This is a testable hypothesis due to the species specificity of the carcinogenicity assay — for the hypothesis to be confirmed, the 1 To whom correspondence should be addressed. Fax: (919) 541-4632. E-mail: cunning1@niehs.nih.gov. TOXICOLOGICAL SCIENCES 70, 157–158 (2002) Copyright © 2002 by the Society of Toxicology