New research conducted to evaluate the mechanisms of methyl isobutyl ketone (CAS No. 108-10-1) carcinogenicity
MIBK-Induced Kidney Tumors in Male Rats
Borghoff SJ, Poet TS, Green S, Davis J, Hughes B, Mensing T, Sarang SS, Lynch AM, Hard GC. 2015. Methyl isobutyl ketone exposure-related increases in specific measures of a2u-globulin (a2u) nephropathy in male rats along with in vitro evidence of reversible protein binding. Toxicology 333:1–13.
Borghoff et al. (2015) provided data that fills the gaps in knowledge concerning the role of MIBK-induced a2u-globulin accumulation and nephropathy associated with the male rat kidney tumor response, a mechanism that IARC has acknowledged is not relevant to humans (IARC, 1999; Swenberg and Lehman-McKeeman, 1999). Under the same exposure conditions as the 2-year MIBK chronic bioassay, this study found:
- Induction of the selected characteristic histopathological changes associated with a2u-globulin nephropathy, with demonstration of protein droplet accumulation.
- a2u-Globulin-associated nephropathy was identified in male, but not female rats, similar to the renal tumor response reported by the National Toxicology Program.
- The protein droplets that accumulate with exposure to MIBK in the proximal tubule cells of male rats were stained specifically for a2u-globulin.
- There was increased renal cell proliferation that was shown to be sustained over the 4 weeks of exposure in male, but not female, rats.
- A concentrated related correlation between increased protein droplets and renal a2u accumulation, as well as renal-cell proliferation and increased protein droplets, was observed in the male, but not the female, rat kidney.
- In an in vitro binding assay, there was evidence that MIBK binds reversibly to a2u globulin.
MIBK-Induced Liver Tumors in Mice
Hughes BJ, Thomas J, Lynch AM, Borghoff SJ, Green S, Mensing T, Sarang SS, LeBaron MJ. 2016. Methyl isobutyl ketone-induced hepatocellular carcinogenesis in B6C3F1 mice: A constitutive androstane receptor (CAR)-mediated mode of action. Regul Appl Toxicol (81):421429.
In this publication, Hughes et al. hypothesized that MIBK caused hepatocellular tumors in mice through a CAR/PXR nuclear receptor activation mechanism. The approach for testing this hypothesis was to evaluate key events that have been established for this mechanism following MIBK inhalation exposure, at the concentration that caused liver tumors in mice, by comparing the CAR/PXR response in the parental wild type (WT) C57BL/6 and B6C3F1 mice to that in the CAR/PXR knockout mice.
In summary, MIBK exposure for 2 weeks at the concentration that produced liver tumors in mice exposed for 2 years (1800 ppm) resulted in:
- Increased liver weights that corresponded with hepatocellular hypertrophy and increased mitotic figures in the livers of exposed mice, with a greater increase in the B6C3F1 mice compared to the CAR/PXR KO mice.
- Hepatocellular proliferation induced by evaluating increases in S-phase DNA synthesis in B6C3F1 and C57BL/6 mice exposed to 1800 ppm MIBK, compared to controls, with no increase observed in the CAR/PXR KO mice.
- Liver gene expression changes that indicated a maximally induced Cyp2b10 (CAR-associated) transcript and a slight increase in Cyp3a11(PXR-associated) transcript in B6C3F1 and C57BL/6 mice exposed to 1800 ppm MIBK, compared to controls, but not in Cyp1a1 (AhR associated) or Cyp4a10 (PPAR-α-associated) transcripts.
- No evidence of activation of AhR, CAR, PXR, or PPAR-α nuclear receptors via their associated transcripts in MIBK exposed CAR/PXR KO.
This study provides data to support the identification of key events, such as liver hypotrophy, hyperplasia, and induction of Cyb2b10 transcripts, and that they are dependent on CAR/PXR, because these responses were present in the WT mice, versus the lack of these responses in the KO mice (with the exception of a very slight hypertrophy). Together, the responses identified above are similar to other known CAR activators such as phenobarbital, and are consistent with a CAR-mediated hepatocarcinogenic mechanism, a mechanism that is not relevant to humans.