Dissociation of the AhR/ARNT complex by TGF-β/Smad signaling represses CYP1A1 gene expression and inhibits benze[a]pyrene-mediated cytotoxicity

Abstract

<jats:p>Cytochrome P450 1A1 (CYP1A1) catalyzes the metabolic activation of polycyclic aromatic hydrocarbons (PAHs) such as benzo[a]pyrene (B[a]P) and is transcriptionally regulated by the aryl hydrocarbon receptor (AhR)/AhR nuclear translocator (ARNT) complex upon exposure to PAHs. Accordingly, inhibition of CYP1A1 expression reduces production of carcinogens from PAHs. Although transcription of the <jats:italic>CYP1A1</jats:italic> gene is known to be repressed by transforming growth factor-β (TGF-β), how TGF-β signaling is involved in the suppression of <jats:italic>CYP1A1</jats:italic> gene expression has yet to be clarified. In this study, using mammalian cell lines, along with shRNA-mediated gene silencing, CRISPR/Cas9-based genome editing, and reporter gene and quantitative RT-PCR assays, we found that TGF-β signaling dissociates the B[a]P-mediated AhR/ARNT heteromeric complex. Among the examined Smads, Smad family member 3 (Smad3) strongly interacted with both AhR and ARNT via its MH2 domain. Moreover, hypoxia-inducible factor 1α (HIF-1α), which is stabilized upon TGF-β stimulation, also inhibited AhR/ARNT complex formation in the presence of B[a]P. Thus, TGF-β signaling negatively regulated the transcription of the <jats:italic>CYP1A1</jats:italic> gene in at least two different ways. Of note, TGF-β abrogated DNA damage in B[a]P-exposed cells. We therefore conclude that TGF-β may protect cells against carcinogenesis because it inhibits CYP1A1-mediated metabolic activation of PAHs as part of its anti-tumorigenic activities.</jats:p>

Effects of Low-Dose Gestational TCDD Exposure on Behavior and on Hippocampal Neuron Morphology and Gene Expression in Mice

“…Our data suggested that TCDD exposure during gestation led to the altered expression of genes regulated by SMAD3, providing further evidence for significant interactions previously identified between the AHR and TGFb-SMAD3 pathways, both during normal development and disease contexts (Gramatzki et al 2009;Cheng et al 2020;Nakano et al 2020;Sarić et al 2020). Most of the 36 SMAD3 targets identified in our data set were downregulated in the TCDD-exposed hippocampi, suggesting that TGFb signaling may be down-regulated by TCDD exposure.…”

TGF-β-induced cell motility requires downregulation of ARHGAPs to sustain Rac1 activity

“…We previously established SMAD3 knockout A549 (A549-S3KO) cells using CRISPR/Cas9-mediated genome editing (5). The endogenous expression and TGF-β-induced phosphorylation of Smad3 were abrogated while those of Smad2 were not, thus ensuring specific effects of the knockout (Fig.…”

“…Smad cofactors are thought to assist in selective as well as stable binding of Smad proteins to genomic DNA, thereby enabling context-dependent Smad-mediated transcription in various target cells (2). Alternatively, activated Smad proteins can regulate gene expression indirectly by interacting physically with other transcription factors, thereby either repressing or derepressing their functions (3)(4)(5). Smad proteins also promote miRNA processing upon their activation by TGF-β (6).…”

Epigenetic Regulations of AhR in the Aspect of Immunomodulation

“…Considering the particular role of AhR in immune responses, increasing data reveals its cross-roads on the other significant in immunology pathways such as NF-κB [ 10 ], TGFβ/SMAD3 [ 11 , 12 , 13 ] and TLRs [ 14 ]. For example, the AHR seems to regulate innate inflammatory signaling not only through binding to its cognate response element in association with ARNT but also through direct binding to RELA (also known as p65) and RELB, which are members of the NF-κB family of transcription factors [ 15 , 16 ].…”