Herbicide metolachlor effects on liver and thyroid endocrinology compared with CYP2B1/2-inducer phenobarbital and hepatotoxicant alachlor

Metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide) meets several prioritization criteria outlined by the Safe Water Drinking Act Amendment, Endocrine Disruptor Screening and Testing Advisory Committee, and combined risk assessment with other chloroacetanilide herbicides mandated by the Food Quality Protection Act. Metolachlor is a nongenotoxic rat hepatocarcinogen. Chronic bioassays suggests that metolachlor is a phenobarbital (PB)-like rat liver tumor-promoter. Reduced epidermal growth factor (EGF) binding and EGF-induced mitogenesis in hepatocytes have been associated with PB-treatment. We demonstrated that hepatocyte EGF receptor (EGFr) protein levels were unchanged in PB-treated rats and that in vivo PB treatment sensitized cultured hepatocytes to PB concentrations attainable in plasma. Although EGFr down-regulation was correlated with tumor promoting activity and CYP2B1/2 induction of several barbiturates, involvement of CYP2B1/2 activity was discounted. These data suggest that PB-induced EGFr down-regulation results from effects on existing protein independent of CYP2B1/2 activity.;Metolachlor induced CYP2B1/2 and CYP3A1/2, suggesting a PB-like mode of action. PB activates the thyroid gland through increased thyroid hormone clearance. Thyroid glands of metolachlor-treated rats were also activated, evidenced by increased hyperplasia and hypertrophy of thyroid follicular cells. Unlike PB, serum thyroid hormone and thyroid stimulating hormone levels were unaffected by metolachlor and hepatic UDP-glucuronosyltransferase activity toward thyroxine was significantly decreased. Thus, metolachlor disrupts thyroid gland activity, but mechanistically differs from PB.;Alachlor (2-chloro-2',6'-diethyl-N-(methoxymethyl)-acetanilide), another chloroacetanilide herbicide, is nonliepatocarcinogenic but hepatotoxic in a different rat strain than used in the metolachlor bioassays. Alachlor, but not metolachlor, is readily metabolized to a reactive quinone imine (QI), the proposed mediator of alachlor's hepatotoxic effects. The rat strains from the bioassays were used to compare liver responses of these herbicides to address the role of QI hepatotoxicity and CYP2B1/2 induction. Metolachlor comparably affected several responses previously associated with oxidant stress-mediated toxicity of alachlor suggesting antioxidant glutathione displacement of chloroacetanilide chloride mediates these effects. Further, CYP2B1/2 was comparably induced by both herbicides suggesting that their different hepatocarcinogenicities were not due to strain differences nor dependent upon QI formation.;These studies provide mechanistic information on the toxic endpoints of metolachlor and alachlor and should be of immediate importance to the anticipated review of these compounds.