Identification of the Aryl Hydrocarbon Receptor Target Gene TiPARP as a Mediator of Suppression of Hepatic Gluconeogenesis by 2,3,7,8-Tetrachlorodibenzo-p-dioxin and of Nicotinamide as a Corrective Agent for This Effect

Abstract

The environmental toxin TCDD (2,3,7,8-tetrachlorodibenzop-dioxin, dioxin) produces diverse toxic effects including a lethal wasting syndrome whose hallmark is suppressed hepatic gluconeogenesis. All TCDD toxicities require activation of the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor. Whereas the mechanism for AHR induction of target genes is well understood, it is not known how AHR activation produces any TCDD toxicity. This report identifies for the first time an AHR target gene, TiPARP (TCDD-inducible poly(ADPribose) polymerase, PARP7) that can mediate a TCDD toxicity, i.e. suppression of hepatic gluconeogenesis. TCDD suppressed hepatic glucose production, expression of key gluconeogenic genes, phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase (G6Pase), and NAD ؉ levels, and increased PARP activity and

Aryl Hydrocarbon Receptor Activation by Dioxin Targets Phosphoenolpyruvate Carboxykinase (PEPCK) for ADP-ribosylation via 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-inducible Poly(ADP-ribose) Polymerase (TiPARP)

“…AHR Suppression and Inhibition of PARP Activity Increase PEPCK Levels-We expected that TCDD effects on PEPCK levels would be dependent on the AHR given that TiPARP, an AHR activated gene, suppressed PEPCK transcription (9). Consistent with that expectation, AHR siRNA prevented the decrease in PEPCK-C levels by TCDD (Fig.…”

“…Hepatocyte suspensions were mixed with 5 g of a pcDNA-TiPARP-FLAG construct generated in our laboratory (9) or with an empty plasmid, pcDNA (Invitrogen/Invitrogen), or with pmaxGFP (Lonza). The following sequences were used for silencing TiPARP and the AHR: for TiPARP, SMARTpool dsRNA (Dharmacon/ThermoScientific, Waltham, MA) (5Ј-ggagatacaccgtgggcaa-3Ј, 5Ј-gctatgaagttatacgaaa-3Ј, 5Ј-ggatctgggaccttggata-3Ј, and 5Ј-gccagaaaggcaagctatt-3Ј); for AHR, a dsRNA targeting AHR sequence, 5Ј-gctcttactgcaggcatta-3Ј (Invitrogen); for control, siGENOME Non-Targeting siRNA #3 (Dharmacon, Lafayette CO).…”

“…It has been noted that lethal exposure to TCDD causes rats to "behave as if they were satiated although they suffer from severe deficiency of energy" (7). TCDD treatment impairs hepatic glucose production and utilization in both mammalian and avian species (4,5,8,9). The mechanism(s) by which TCDD produces its toxic effects and the relationship of TCDD toxicities to enhancement of AHR transcriptional effects remain central questions in AHR biology.…”

“…As a result TiPARP is now included as a member of the AHR gene battery, a canonical group of genes co-induced by AHR activation in diverse cell types and species (CYP1A1, 1A2, 1B1, NQOI, ALHD3A1, UGT1A6, GSTA1 and now TiPARP (16 -18)). We previously reported that TCDD-induced TiPARP suppresses PEPCK transcription by a mechanism in which utilization and consumption of NAD ϩ by TiPARP leads to decreased activity of the NAD ϩ -dependent deacetylase sirtuin 1 and, in turn, to decreased deacetylation (diminished activation) of peroxisome proliferator-activated receptor ␥ coactivator 1␣ (PGC1␣) (9), a transcriptional coactivator of PEPCK. The findings established a connection between an AHR-activated gene (TiPARP) and a particular effect of TCDD (suppression of gluconeogenesis).…”

“…We recently reported that the AHR target gene, TiPARP (TCDD-inducible poly(ADP-ribose) polymerase, PARP7, ARTD14), contributes to TCDD suppression of PEPCK mRNA levels and hepatic glucose production (9). TiPARP is one of 17 members of the poly(ADP-ribose) polymerase (PARP) family of enzymes that catalyze poly-or mono-ADP-ribosylation of target proteins by NAD ϩ -dependent transfer of ADP-ribose to specific amino acids (10).…”