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Description
Methyl tertiary-butyl ether (MTBE) is a gasoline additive that was blended into fuel formulas and reached its peak of use during the 1990s. MTBE is a gasoline oxygenate intended to decrease carbon monoxide and other toxic emissions from combustion engines. In 2006 MTBE was banned in the United States because of environmental persistence and concerns for public health associated with MTBE contamination is also found in other countries that have ceased using MTBE, but some countries continue to use it. MTBE persistence within the environment and possible ingestion along with other routes of exposure by people resulted in its regulation in both ground and surface water by the EPA. The effects in humans of long-term, low dose exposure are unknown at this time. Extensive research has been conducted in rodents, including long-term cancer bioassays using high concentrations of MTBE--much higher than contamination levels found in nature--that showed increased incidences of tumors at various organ sites. The experiment performed in this thesis is one of many studies that have been conducted at San Diego State University in order to get a better comprehension of the mechanism behind the increase in the rat Leydig cell tumors. The increase in Leydig tumors has been hypothesized to be the result of endocrine disruption. MTBE is an inducer of cytochrome P450, a group of enzymes responsible for both endogenous steroid synthesis and catabolism and detoxifying xenobiotics and making them more water soluble, resulting in easier elimination from the body. Aromatase is a specific P450 isozyme (CYP19). It can be found in the smooth endoplasmic reticulum of many organs throughout the body, and is more specifically responsible for the conversion of androgens to estrogens. An earlier preliminary single dose group study in adult male rats gavaged for 14 days with 1200 mg/kg MTBE reported significantly lower serum testosterone and increased serum estradiol, but also found decreased aromatase activity, which was puzzling. Altered conversion of testosterone to estradiol could explain the Leydig tumor increase in rats, and is the focus of the more extensive research here on aromatase. Examining both total P450 and aromatase activity in a multi-dose response study was expected to give an indication to whether aromatase plays a significant role in the Leydig tumors cell development, following high chronic doses of MTBE. Adult Sprague-Dawley male rats were gavaged with 0 (corn oil vehicle), 600 or 1200 mg/kg MTBE daily for 14 consecutive days. A dose-related increase in total P450 was seen in both the liver and testes microsomes, but these changes did not reach statistical significance (ANOVE, p>0.05). Aromatase activity was decreased in a dose-dependent way in both liver and testes, but like the P450 results the p-value of an ANOVA was >0.05. The variability within the groups was high relative to differences in group means when comparing treated and control animals. This experiment confirmed that MTBE affects aromatase activity. On the other hand the effects seen in this experiment were not dramatic and were accompanied by evidence of generalized toxicity; specifically body weight loss, enlarged adrenal glands and serum corticosterone, early deaths before scheduled necropsy in the high dose group. Inhibited catalytic activity of aromatase and other related changes in the male reproductive endocrine system observed may have contributed to the mechanism of Leydig cell cancers reported in male rats exposed to high chronic doses of MTBE; however, these endocrine system-related changes are not always detected in the absence of other significant toxicity. This means the endocrine system is not necessarily an exclusive or even a primary target of MTBE. The cause of increased Leydig tumor incidence reportedly seen in male rats exposed to high doses of MTBE continues to be elusive and is probably multifactorial
