Vascular Medial Hyperplasia Following Chronic, Intermittent Exposure to 4,4′-Methylenedianiline

4,4′-Methylenedianiline (DAPM) is an aromatic amine used in the synthesis of polyurethanes and epoxy resins. Acute exposure to DAPM produces hepatobiliary toxicity in humans as well as animal models. However, the toxic effects of intermittent DAPM exposure have not been explored. We treated male and female rats with 25 mg DAPM/kg or vehicle once per week for 17-22 wk. Though concentric fibrosis around bile ducts of the liver was noted, vascular medial hyperplasia was also prominent. Morphometric analysis of histologic sections revealed that in male rats, vessel wall area increased relative to lumen area in hepatic arteries by 22 wk. However, in female rats, wall areas of both hepatic and pulmonary arteries increased relative to lumen area by 17 wk. In both male and female rats, increased wall thickness was localized to the medial layer; no intimal changes were noted. In vitro treatment of vascular smooth muscle cells (VSMC) with 25-100 μM DAPM resulted in increased DNA synthesis and VSMC proliferation. To test whether the observed alterations in cell cycle control involved VSMC-mediated metabolism of DAPM to electrophilic intermediates, cells were treated with DAPM or DAPM plus 50 μM N-acetylcysteine (NAC). Coincubation with NAC afforded dramatic protection against DAPM-induced VSMC proliferation. Though DAPM had no appreciable effect on levels of reduced glutathione, oxidized glutathione, or oxidant production, DAPM increased glutathione-S-transferase activity in VSMC. These data indicate that DAPM can initiate VSMC proliferation, possibly via VSMC-mediated metabolism of DAPM to reactive intermediates.