Mechanism of the lack of induction of UDP-glucuronosyltransferase activity in Gunn rats by 3-methylcholanthrene. Identification of a truncated enzyme

Gunn rats lack UDP-glucuronosyltransferase (UDPGT) activity toward bilirubin. 4-Nitrophenol glucuronidation is mediated by several UDPGT isoforms that are distinct from bilirubin-UDPGT, one of which is induced after 3-methylcholanthrene (3-MC) administration in normal, but not in Gunn rats. In normal rats, 3-MC-inducible UDPGT mRNA concentration increased 15-fold in the liver and 3-fold in kidney after 3-MC (40 mg/kg) administration. Concentration of this mRNA is much lower in Gunn rat liver and kidney compared to normal. However, this mRNA was normally induced after 3-MC administration. By RNA blot hybridization, the mRNA in Gunn rat liver and kidney appeared to be of normal size. Nuclear run-on studies showed that the transcription rate for 3-MC-inducible UDPGT was 3-fold higher in Gunn rat liver and kidney than in normal and increased 3- to 5-fold after 3-MC administration. Immunotransblot studies revealed an M(r) = 56,000 3-MC-inducible UDPGT in liver and kidney of normal, but not in Gunn rats. However, a new immunoreactive UDPGT band (M(r) = 43,000) was present in Gunn, but not in normal rats. Cell-free translation of kidney mRNA from 3-MC-treated Gunn rats showed that the M(r) = 43,000 UDPGT is synthesized as an M(r) = 45,000 protein. Prior hybridization of the mRNA with an isoform-specific oligonucleotide spanning the initiation codon abolishes synthesis of this protein. These results suggest that a sequence abnormality in the 3-MC-inducible UDPGT mRNA in Gunn rats results in reduced mRNA concentration and synthesis of a truncated, enzymatically inactive UDPGT.