Retrospective evaluation of alpha 2u-globulin accumulation in male rat kidneys following high doses of diisononyl phthalate

Diisononyl phthalate (DINP), a widely used plasticizer, has been evaluated in two chronic studies in rats and one in mice. In the early 1980s, Exxon found no carcinogenic potential at the estimated maximum tolerated dose (MTD) of 0.6% (307 mg/kg/day for male rats) administered in the diet of rats for 2 years. A recent study conducted at dietary levels up to 1.2% DINP (733 mg/kg/d for male rats) reported kidney tumors in male rats at the high treatment level, but not in female rats nor mice of either sex. Because these tumors occurred only in male rats, and only at high doses, the male rat-specific alpha 2u-globulin (alpha 2UG) mechanism of action was investigated. Technological advances in immunohistochemical staining and computerized image analysis techniques permitted measuring the accumulation of alpha 2UG in archived kidneys from the earlier Exxon study. Using archived tissue obtained at the la-month interim sacrifice, we identified a dose-dependent accumulation of alpha 2UG in specific regions of male rat kidneys only. An increase in cell proliferation was confirmed by immunohistochemical detection of proliferating-cell nuclear antigen (PCNA) and was confined to the areas of alpha 2UG accumulation. H and E-stained sections revealed tubular epithelial hypertrophy and regeneration, consistent with the immunohistopathology findings. These findings are consistent with the alpha 2UG mechanism of tumorigenesis, which is not regarded as relevant for humans. Thus, exposure to DINP produced a dose-dependent alpha 2UG accumulation in male rat kidneys, significant at a dietary level of 0.6% and a likely mechanism for the kidney tumors seen only in male rats administered higher dietary levels of DINP.