Circadian clock disruption in the mouse ovary in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Abstract

Activation of the aryl hydrocarbon receptor (AhR) by the highly toxic, prototypical ligand, 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD) or other dioxin-like compounds compromises ovarian function by altering follicle maturation and steroid synthesis. Although alteration of transcription after nuclear translocation and heterodimerization of AhR with its binding partner, aryl hydrocarbon nuclear transporter (ARNT), is often cited as a primary mechanism for mediating the toxic effects of dioxins, recent evidence indicates that crosstalk between AhR and several other signaling pathways also occurs. Like the circadian clock genes, AhR is a member of the basic helix-loop-helix, Per-ARNT-SIM (bHLH-PAS) domain family of proteins. Thus, these studies tested the hypothesis that TCDD can act to alter circadian clock regulation in the ovary. Adult female c57bl6/J mice entrained to a typical 12 h light/12 h dark cycle were exposed to a single 1 µg/kg dose of TCDD by gavage. Six days after exposure, animals were released into constant darkness and ovaries were collected every 4 h over a 24 h period. Quantitative real-time PCR and immunoblot analysis demonstrated that TCDD exposure alters expression of the canonical clock genes, Bmal1 and Per2 in the ovary. AhR transcript and protein, which displayed a circadian pattern of expression in the ovaries of control mice, were also altered after TCDD treatment. Immunohistochemistry studies revealed co-localization of AhR with BMAL1 in various ovarian cell types. Furthermore, co-immunoprecipitation demonstrated time-of-day dependent interactions of AhR with BMAL1 that were enhanced after TCDD treatment. Collectively these studies suggest that crosstalk between classical AhR signaling and the molecular circadian clockworks may be responsible for altered ovarian function after TCDD exposure.

The Mammalian Circadian System Is Resistant to Dioxin

“…The percentage of DMSO in vehicle- and TCDD-treated mice was 1%. A single 1-µg/kg dose of TCDD administered by oral gavage in corn oil was chosen because this dose and route of administration have been shown to induce Cyp1a1 mRNA expression in liver, alter circadian gene expression, and alter physiological processes such as oxidative stress and immunosuppression (Inouye et al 2005; Mukai et al 2008; Narasimhan et al 1994; Slezak et al 2000; Tischkau et al 2011; Xu et al 2010). Either 4 or 14 days later, tissues were cultured and livers were collected for preparation of RNA and analysis of Cyp1a1 expression.…”

“…One of AhR’s functions may be a role in circadian rhythms since the bHLH-PAS proteins, BMAL1 and AhR, have been shown to interact in vitro and in vivo (Hogenesch et al 1997; Tischkau et al 2011; Xu et al 2010). AhR mRNA is expressed in the mouse and rat SCN (Mukai et al 2008; Petersen et al 2000) and in many peripheral oscillators, including the liver, lung, and thymus, so it is anatomically poised to participate in circadian functions (Li et al 1994; Okey 2007).…”

“…AhR mRNA is expressed in the mouse and rat SCN (Mukai et al 2008; Petersen et al 2000) and in many peripheral oscillators, including the liver, lung, and thymus, so it is anatomically poised to participate in circadian functions (Li et al 1994; Okey 2007). Circadian gene expression is altered in the liver of AhR -deficient mice, and treatment of wild-type mice with TCDD affects the phase of their locomotor activity rhythms and alters circadian gene expression in the SCN, ovary, liver, and bone marrow (Garrett and Gasiewicz 2006; Miller et al 1999; Mukai et al 2008; Mukai and Tischkau 2007; Tischkau et al 2011; Xu et al 2010). Together these data suggest that there is crosstalk between the circadian system and AhR signaling pathways.…”

Back to the future: transgenerational transmission of xenobiotic-induced epigenetic remodeling

“…122 Specifically, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters the clock genes Bmal1 and Per2 in the ovary, disrupting in the process aryl hydrocarbon receptor (AhR) circadian expression. 123 Other examples include diethylnitrosamine (DEN)-induced cytotoxicity on mouse primary hepatocytes, which is very dependent on the Clock gene system, 124 and a role for hypoxia and HIF-1a in regulating AhR signaling via aryl hydrocarbon receptor nuclear translocator (ARNT) downstream of polychlorinated biphenyl (PCB) 126 exposure. 125 Like Clock and other periodregulating proteins, HIF-1a, ARNT and AhR are all PAS (period circadian protein-ARNT-single-minded protein) domain-containing proteins.…”

Persistent polar depletion of stratospheric ozone and emergent mechanisms of ultraviolet radiation-mediated health dysregulation

“…Together these findings further validate light as a cue in the SCN and highlight the coactivation of AhR and circadian PAS genes including CLOCK, BMAL, and Per. Follow-up studies confirming interactions between circadian disruptions and AhR have primarily utilized treatments of TCDD, further implicating atypical time activation of PAS genes as a decoupling mechanism between master and peripheral clocks via modulations of Per gene expression (169, 170). Treatment of Hepa-1c1c7 cells in vitro and extracted mice liver cells treated in vivo with TCDD collectively demonstrate that AhR activation causes a downstream repression of Per1 through competitive heterodimerization of AhR/BMAL (versus CLOCK/BMAL) (170).…”