2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly(ADP-ribose) polymerase (TiPARP/ARTD14) is a member of the ARTD family and is regulated by the aryl hydrocarbon receptor (AHR); however, little is known about TiPARP function. In this study we examined the catalytic function of TiPARP and determined its role in AHR transactivation. We observed that TiPARP exhibited auto-mono-ADP-ribosyltransferase activity and ribosylated core histones. RNAi-mediated knockdown of TiPARP in T47D breast cancer and HuH7 hepatoma cells increased TCDD-dependent cytochrome P450 1A1 (CYP1A1) and CYP1B1 mRNA expression and recruitment of AHR to both genes. Overexpression of TiPARP reduced AHR-dependent increases in CYP1A1-reporter gene activity, which was restored by overexpression of AHR, but not ARNT. Deletion and mutagenesis studies showed that TiPARP-mediated inhibition of AHR required the zinc finger and catalytic domains. TiPARP and AHR co-localized in the nucleus, directly interacted and both were recruited to CYP1A1 in response to TCDD. Overexpression of TiPARP enhanced whereas RNAi-mediated knockdown of TiPARP reduced TCDD-dependent AHR proteolytic degradation. TCDD-dependent induction of AHR target genes was also enhanced in Tiparp-/- mouse embryonic fibroblasts compared to wildtype controls. Moreover, livers excised from TCDD-treated Tiparp-/- mice displayed significantly greater AHR target gene expression compared with wildtype or heterozygous mice. Comparison of TiPARP to known negative regulator, AHR repressor (AHRR) revealed TiPARP and AHRR some notable similarities and differences between their mechanisms of repression. Similar to TiPARP, the AHRR was recruited to AHR target regulatory regions in response to TCDD and its overexpression repressed reporter gene activity. However unlike TiPARP, knockdown of the AHRR did not affect AHR transactivation or its proteasomal degradation. Despite some mechanistic similarities, our data suggest that TiPARP and AHRR independently repress AHR transactivation. Overall, our findings show that TiPARP is a mono-ADP-ribosyltransferase and a transcriptional repressor of AHR, revealing a novel negative feedback loop controlling AHR transcriptional regulation.

Family, twin, and adoption studies have indicated that genetic and environmental factors contribute to the development of alcohol dependence (AD). We conducted a low-density genome-wide association analysis to identify genetic variants influencing AD. We used 11,120 SNPs from the Affymetrix 10K Genechips genotyped in 116 Caucasian pedigrees (272 nuclear families) from Genetic Analysis Workshop 14, a subset from the Collaborative Study on the Genetics of Alcoholism (COGA). Family-based association analyses for AD were performed by the PBAT program for autosomal SNPs and by the FBAT program for X-chromosome SNPs. We identified 37 SNPs associated with AD (P < 10 -3), thirteen of which were located in known genes. The most significant association with AD was observed with SNP rs1986644 (P = 8.51 × 10 -6) at 13q22 near EDNRB gene. The next best signal was at 1q41 in USH2A (rs532342, P = 1.07 × 10 -5) and the third region was at 3q25.31 in TIPARP (rs1367311, P = 2.31 × 10 -5). Furthermore, we found support for association of MAOA gene (P = 4.14 × 10 -4 for rs979606). Six of the 37 AD associated SNPs were confirmed to be associated with AD in Australian twin-family study sample (P < 0.05). Interestingly, four SNPs in DSCAML1 at 11q23 reached the genome-wide significance (the top SNP is rs10892169 with P = 5.31 × 10 -9), while rs637547 in NKAIN2 at 6q21 showed strong association with AD (P = 5.11 × 10 -7) in the replication sample. These findings offer the potential for new insights into the pathogenesis of AD and will serve as a resource for replication in other populations to elucidate the potential role of these genetic variants in AD.

TiPARP is a PARP-like mART that is induced by and negatively regulates AHR transactivation. Despite these insights, not much is known about TiPARP. This study aimed to characterize the regulation of TiPARP by AHR, PDGFR, and ERα, and investigate potential receptor interplay. Gene expression studies revealed that coactivation of AHR and PDGFR can enhance TiPARP expression after 3 h relative to activation of either receptor pathway alone. Gene expression and ChIP studies demonstrated that while co-activation of AHR and ER enhanced AHR, ARNT, and ERα recruitment to the regulatory region of TiPARP, TiPARP mRNA levels were not potentiated by co-activation relative to activation of either pathway. Dissection of the 5’ regulatory region of TiPARP using reporter gene assays revealed that a putative AHRE cluster and an ERE half-site were functional. Lastly, overexpression of TiPARP with an estrogen-responsive reporter revealed that TiPARP can repress ERα signalling and requires its catalytic activity.

TiPARP is a PARP-like mART that is induced by and negatively regulates AHR transactivation. Despite these insights, not much is known about TiPARP. This study aimed to characterize the regulation of TiPARP by AHR, PDGFR, and ERα, and investigate potential receptor interplay. Gene expression studies revealed that coactivation of AHR and PDGFR can enhance TiPARP expression after 3 h relative to activation of either receptor pathway alone. Gene expression and ChIP studies demonstrated that while co-activation of AHR and ER enhanced AHR, ARNT, and ERα recruitment to the regulatory region of TiPARP, TiPARP mRNA levels were not potentiated by co-activation relative to activation of either pathway. Dissection of the 5’ regulatory region of TiPARP using reporter gene assays revealed that a putative AHRE cluster and an ERE half-site were functional. Lastly, overexpression of TiPARP with an estrogen-responsive reporter revealed that TiPARP can repress ERα signalling and requires its catalytic activity.

Yes / 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly-ADP-ribose polymerase (TIPARP) is an enzyme that adds a single ADP-ribose moiety to itself or other proteins. Tiparp is highly expressed in the brain; however, its function in this organ is unknown. Here, we used Tiparp–/– mice to determine Tiparp’s role in the development of the prefrontal cortex. Loss of Tiparp resulted in an aberrant organization of the mouse cortex, where the upper layers presented increased cell density in the knock-out mice compared with wild type. Tiparp loss predominantly affected the correct distribution and number of GABAergic neurons. Furthermore, neural progenitor cell proliferation was significantly reduced. Neural stem cells (NSCs) derived from Tiparp–/– mice showed a slower rate of migration. Cytoskeletal components, such as α-tubulin are key regulators of neuronal differentiation and cortical development. α-tubulin mono-ADP ribosylation (MAR) levels were reduced in Tiparp–/– cells, suggesting that Tiparp plays a role in the MAR of α-tubulin. Despite the mild phenotype presented by Tiparp–/– mice, our findings reveal an important function for Tiparp and MAR in the correct development of the cortex. Unravelling Tiparp’s role in the cortex, could pave the way to a better understanding of a wide spectrum of neurological diseases which are known to have increased expression of TIPARP. / European Union Seventh Framework Program (FP7-PEOPLE-2013-COFUND) Grant n609020-Scientia Fellows (to G.G.) and by the Johan Throne Holst Foundation and the University of Oslo (J.M.).

No / TCDD-inducible poly-ADP-ribose polymerase (TIPARP; also known as PARP7 and ARTD14) is a mono-ADP- ribosyltransferase that has emerged as an important regulator of innate immunity, stem cell pluripotency, and transcription factor regulation. Characterizing TIPARP’s catalytic activity and identifying its target proteins are critical to understanding its cellular function. Here we describe methods that we use to characterize TIPARP catalytic activity and its mono-ADP-ribosylation of its target proteins.