Methyl isobutyl ketone exposure-related increases in specific measures of α2u-globulin (α2u) nephropathy in male rats along with in vitro evidence of reversible protein binding

Chronic exposure to methyl isobutyl ketone (MIBK) resulted in an increase in the incidence of renal tubule adenomas and occurrence of renal tubule carcinomas in male, but not female Fischer 344 rats. Since a number of chemicals have been shown to cause male rat renal tumors through the alpha 2u nephropathy-mediated mode of action, the objective of this study is to evaluate the ability of MIBK to induce measures of eau nephropathy including renal cell proliferation in male and female F344 rats following exposure to the same inhalation concentrations used in the National Toxicology Program (NTP) cancer bioassay (0, 450, 900, or 1800 ppm). Rats were exposed 6 h/day for 1 or 4 weeks and kidneys excised approximately 18 h post exposure to evaluate hyaline droplet accumulation (HDA), alpha 2u staining of hyaline droplets, renal cell proliferation, and to quantitate renal alpha 2u concentration. There was an exposure-related increase in all measures of alpha 2u nephropathy in male, but not female rat kidneys. The hyaline droplets present in male rat kidney stained positively for alpha 2u. The changes in HDA and eau concentration were comparable to D-limonene, an acknowledged inducer of alpha 2u nephropathy. In a separate in vitro study using a two-compartment vial equilibration model to assess the interaction between MIBK and alpha 2u, the dissociation constant (k(d)) was estimated to be 1.27 x 10(-5)M. This K-d is within the range of other chemicals known to bind to alpha 2u and cause nephropathy. Together, the exposure-related increase in measures of eau nephropathy, sustained increase in renal cell proliferation along with an indication of reversible binding of MIBK to alpha 2u, support the inclusion of MIBK in the category of chemicals exerting renal effects through a protein droplet alpha 2u nephropathy-mediated mode of action (MoA). (C) 2015 Elsevier Ireland Ltd. All rights reserved.