Introduction
The aryl hydrocarbon receptor (AHR) has historically been associated with mediating the toxic effects of environmental pollutants like 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; commonly known as dioxin) and regulating cytochrome P450 genes (CYP1A1 and CYP1B1). TCDD has been shown to cause numerous toxic effects in laboratory animals, including wasting syndrome, which is characterized by altered energy metabolism, body weight loss, and death. However, the precise mechanisms underlying these effects remain unclear. Beyond its role in mediating the effects of exogenous pollutants, AHR is involved in several biological pathways, including development, cell cycle regulation, and immune response. The transient activation of AHR by endogenous or dietary ligands can explain many of these effects. An emerging area of research focuses on understanding how AHR activity is controlled and shedding light on TCDD-inducible poly-ADP-ribose polymerase (TIPARP) and ADP-ribosylation in regulating AHR signaling and TCDD-dependent toxicity.
Fig. 1 TIPARP is a negative regulator of AHR signaling. (Matthews J. 2017)
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The Aryl Hydrocarbon Receptor Mechanism of Action
AHR belongs to the basic helix-loop-helix Per-ARNT-Sim (bHLH-PAS) family, a group of transcription factors that respond to extracellular signals and environmental stresses to modify cellular function. While AHR is well-known for its ability to bind and mediate the toxic effects of halogenated and polycyclic aromatic hydrocarbons, it also interacts with various non-toxic ligands derived from diet, the gut microbiome, and endogenous sources. The molecular mechanisms through which AHR operates are context-dependent and multifaceted.
The canonical AHR pathway begins when a ligand binds to the cytosolic, chaperone-bound form of AHR, resulting in its nuclear translocation and heterodimerization with the AHR nuclear translocator (ARNT). The AHR:ARNT complex then attaches to AHR response elements (AHREs) found in regulatory regions, enhancers, and intronic sequences of an array of AHR target genes involved in many cellular processes. Notable AHR target genes include CYP1A1, CYP1B1, AHR repressor (AHRR), and TIPARP (also known as PARP7/ARTD14).
As with all transcription factors, the activity of AHR must be precisely regulated to prevent over-activation. This regulation occurs through several mechanisms, such as the metabolism and inactivation of AHR ligands by the CYP1A1/CYP1B1 enzymes. Additionally, ligand-activated AHR undergoes proteolytic degradation via the 26S proteasome. AHR is also regulated via a negative feedback loop involving AHRR, which competes with ARNT for AHR binding and tethers to the AHR-ARNT complex, thereby inhibiting AHR activity. Recent studies suggest that AHRR operates as a context- and tissue-specific negative regulator of AHR, hinting at other control mechanisms yet to be understood.
AHR Mediates TCDD Toxicity and Wasting Syndrome
TCDD acts as a potent AHR ligand, inducing severe toxicological effects in laboratory animals, including developmental abnormalities, liver steatosis, immune dysfunction, and a fatal wasting syndrome characterized by body weight loss and metabolic disruption. The sensitivity to TCDD significantly varies among species and strains, with some showing resistance to its effects.
Research reveals that the canonical AHR signaling pathway is crucial for TCDD-induced toxicity. Mice deficient in AHR or with mutations that prevent AHR from binding to AHREs display resistance to TCDD toxicity. Conversely, overexpression of AHR exacerbates symptoms such as hepatic steatosis, underscoring AHR's role in lipid homeostasis. However, the precise downstream target genes and mechanisms mediating these toxic effects are not fully elucidated.
TCDD-Inducible Poly-ADP-Ribose Polymerase (TIPARP)
TIPARP (PARP7/ARTD14) is an AHR target gene recently recognized for its regulatory role in AHR activity and downstream cellular responses. TIPARP belongs to the poly ADP-ribose polymerase (PARP) family, which catalyzes the transfer of ADP-ribose from NAD+ to specific amino acid residues on target proteins, releasing nicotinamide (NAM) in the process. The majority of PARPs catalyze mono-ADP-ribosylation rather than poly-ADP-ribosylation.
ADP-ribosylation significantly alters target protein activity and plays a role in cellular stress responses, including DNA repair, oxidative stress, immune responses, transcription, protein degradation, and metabolism. TIPARP was first identified as a TCDD-responsive gene in mouse hepatoma cells and is expressed widely across tissues and cell lines. TIPARP is not solely regulated by AHR but also by other transcription factors and signaling pathways, indicating the broad range of cellular roles TIPARP may play.
Evolutionarily, TIPARP is most closely related to PARP12 and PARP13, which also belong to the ARTD family. Like its relatives, TIPARP is involved in antiviral defense mechanisms, suggesting that it plays a role beyond AHR signaling. Tiparp-null mice show numerous phenotypes, including vascular defects and infertility, highlighting TIPARP's importance in development and cellular differentiation.
The AHR-TIPARP Negative Feedback Axis
TIPARP acts as a negative regulator of AHR activity, as demonstrated by studies involving human TIPARP. TIPARP recruitment to AHR target genes is TCDD-dependent, and TIPARP overexpression decreases, while its knockdown or knockout increases, AHR-mediated transcription. The ADP-ribosyl transferase activity of TIPARP, which selectively ADP-ribosylates AHR, is crucial for its repressive function. Moreover, MACROD1, which reverses TIPARP's effects, highlights the regulatory interplay between TIPARP and ADP-ribosylation in transcriptional AHR regulation.
AHR undergoes multiple posttranslational modifications, including phosphorylation, sulfonation, ubiquitination, and SUMOylation, and now, ADP-ribosylation. ADP-ribosylation's exact impact on AHR activity remains to be elucidated, but it is likely to influence the recruitment and composition of coactivator/corepressor complexes, thereby affecting the regulation of AHR target genes.
TIPARP Protects Against TCDD Toxicity and Wasting Syndrome
Tiparp-deficient mice exhibit heightened AHR responsiveness and sensitivity to TCDD-induced wasting syndrome. Higher doses of TCDD, which are typically non-lethal in wildtype mice, are lethal in Tiparp-deficient mice, manifesting in rapid weight loss and other toxic effects such as hepatosteatosis and immune cell infiltration in the liver. These observations indicate that TIPARP, through ADP-ribosylation, regulates AHR activity and provides protection against TCDD toxicity.
TIPARP Modulates Endogenously Regulated AHR Activity in Innate Immunity
TIPARP not only modulates AHR activity in response to TCDD but also in the presence of endogenous ligands like kynurenine (KYN). KYN, derived from tryptophan metabolism, activates AHR and subsequently represses type-I-IFN responses during viral infection. This repression is contingent on TIPARP, which ADP-ribosylates TBK1, reducing its phosphorylation activity and thus decreasing interferon-beta levels. These findings underscore the potential of targeting the AHR-TIPARP axis for antiviral therapies and reveal TIPARP's broader role in regulating immune responses.
Conclusion
The intricate interplay between AHR and TIPARP reveals a complex regulatory network crucial for maintaining cellular homeostasis and mitigating environmental toxicities. TIPARP's role in ADP-ribosylating AHR and other proteins underscores its significance as a modulator of AHR activity and TCDD toxicity.
References
- Matthews J. AHR toxicity and signaling: Role of TIPARP and ADP-ribosylation. Current Opinion in Toxicology. 2017, 2: 50-57.
- Ma Q., et al. TCDD-inducible poly (ADP-ribose) polymerase: a novel response to 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin. Biochemical and Biophysical Research Communications. 2001, 289 (2): 499-506.
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