Disruption of Clock Gene Expression Alters Responses of the Aryl Hydrocarbon Receptor Signaling Pathway in the Mouse Mammary Gland

Aug 24, 2007Molecular pharmacology

Changes in body clock genes affect the aryl hydrocarbon receptor pathway in the mouse breast tissue

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Abstract

TCDD induced Cyp1A1 expression in the mammary glands of mutant mice 17.9- and 5.9-fold more than in wild-type animals.

Targeted disruption of clock genes Per1 and Per2 affects toxin-induced expression of biological markers in the AhR signaling pathway.
In Per1(ldc) and Per1(ldc)/Per2(ldc) mice, TCDD increased Cyp1B1 expression compared to wild-type mice.
In primary cultures of mammary gland tissue, TCDD induced greater expression of Cyp1A1 and Cyp1B1 in mutant cells than in wild-type cells.
AhR mRNA levels were higher in all mutant genotypes, but decreased in both wild-type and mutant cultures after TCDD treatment.
The effects of TCDD on Cyp1A1 and Cyp1B1 expression in wild-type mice varied significantly over time, being greater during the night.

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The biological effects of many environmental toxins are mediated by genes containing Per-Arnt-Sim (PAS) domains, the aryl hydrocarbon receptor (AhR), and AhR nuclear translocator. Because these transcription factors interact with other PAS genes that form the circadian clockworks in mammals, we determined whether targeted disruption of the clock genes, Per1 and/or Per2, alters toxin-induced expression of known biological markers in the AhR signaling pathway. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a prototypical Ahr agonist, had an inductive effect on mammary gland expression of cytochrome P450, subfamily I, polypeptide 1 (Cyp1A1) mRNA regardless of genotype. However, TCDD-mediated Cyp1A1 induction in the mammary glands of Per1(ldc) and Per1(ldc)/Per2(ldc) mice was significantly (17.9- and 5.9-fold) greater than that in wild-type (WT) animals. In addition, TCDD-induced Cyp1B1 expression in Per1(ldc) and Per1(ldc)/Per2(ldc) mammary glands was significantly increased relative to that in WT mice. Similar to in vivo observations, experiments using primary cultures of mammary gland tissue demonstrated that TCDD-induced Cyp1A1 and Cyp1B1 expression in Per1(ldc) and Per1(ldc)/Per2(ldc) mutant cells was significantly greater than that in WT cultures. AhR mRNA levels were distinctively elevated in cells derived from all mutant genotypes, but they were commonly decreased in WT and mutant cultures after TCDD treatment. In WT mice, an interesting corollary is that the inductive effects of TCDD on mammary gland expression of Cyp1A1 and Cyp1B1 vary over time and are significantly greater during the night. These findings suggest that clock genes, especially Per1, may be involved in TCDD activation of AhR signaling pathways.

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Disruption of Clock Gene Expression Alters Responses of the Aryl Hydrocarbon Receptor Signaling Pathway in the Mouse Mammary Gland

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