The increased CAR-dependent metabolism of thyroid hormones in mice with high cancer susceptibility.
OBJECTIVE
our aim was to compare activation of the constitutive androstane receptor (CAR), hepatic expression of its target genes, and the serum thyroid hormone levels in C3H/He, C57BL/6J, and CC57BR/Mv mice following phenobarbital treatment. These differences, if present, could help to explain the different susceptibility to phenobarbital-induced liver tumor promotion among these strains of mice.


METHODS
CAR DNA-binding activity and CAR content in nuclear protein extracts from mouse livers were assessed using the electrophoretic mobility shift assay and immunoblotting. Serum thyroid hormone concentrations were determined by radioimmunoassay. Real-time PCR was used to measure the hepatic expression level of CAR target genes.


RESULTS
we found a 2.3-fold increase of CAR DNA-binding activity in response to phenobarbital in the sensitive C3H/He mice, but no change in the relatively resistant C57BL/6J and CC57BR/Mv mice. Phenobarbital treatment caused a significant decrease in triiodothyronine and free thyroxine concentrations (17% and 40%, respectively) in the sensitive C3H/He mice by the end of 60-day treatment, while in the resistant mice, these changes were not observed. In the sensitive C3H/He mice only, the expression of a CAR target gene encoding sulfotransferase Sult2a1, the thyroid hormone inactivation enzyme, increased by 260-fold after phenobarbital administration. The expression of another CAR target gene, Mdm2, was also increased by phenobarbital treatment in C3H/He mice.


CONCLUSIONS
we have shown that phenobarbital activates CAR and increases the expression of its target genes thereby accelerating the metabolism of thyroid hormones only in mice susceptible to liver tumor promotion by phenobarbital, but not in relatively resistant animals.
Pakharukova
Maria
M
Institute of Cytology and Genetics SB RAS, Novosibirsk, Russia. pmaria@yandex.ru
Smetanina
Mariya
M
Kaledin
Vasily
V
Obut
Timofey
T
Merkulova
Tatiana
T
D000818:Animals; D000090702:Constitutive Androstane Receptor; D004247:DNA / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects; D020022:Genetic Predisposition to Disease; D008099:Liver / Q000187:drug effects / Q000473:pathology; D008113:Liver Neoplasms / Q000139:chemically induced* / Q000235:genetics*; D008297:Male; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D010634:Phenobarbital / Q000009:adverse effects / Q000494:pharmacology*; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D013045:Species Specificity; D013963:Thyroid Hormones / Q000097:blood*
D000090702:Constitutive Androstane Receptor; D018160:Receptors, Cytoplasmic and Nuclear; D013963:Thyroid Hormones; D004247:DNA; D010634:Phenobarbital