Novel role of hnRNP- A2/B1 in modulating aryl hydrocarbon receptor ligand sensitivity

The aryl hydrocarbon receptor (AHR) is responsible for susceptibility to its ligand-dependent responses. However, the effect of non-AHR factors is less clear. To explore the non-AHR factors, we used two mouse strains with different AHR genetic variants, namely C3H/ lpr and MRL/ lpr strains with Ala and Val as the 375th amino acid residue, respectively. To assess the contribution of AHR alone, COS-7 cells transiently expressing AHR from each strain were treated with 6-formylindolo[3,2-b]carbazole (FICZ) and 2,3,7,8-tetrachlorodibenzo- p -dioxin (TCDD), and xenobiotic-responsive element (XRE)-driven reporter gene activities were measured. FICZ-EC 50 values for the C3H/ lpr and MRL/ lpr AHR-mediated transactivation were 0.023 and 0.046 nM, respectively, indicating a similar susceptibility in both AHR genotypes. In contrast, C3H/ lpr AHR was fourfold more sensitive to TCDD than MRL/ lpr AHR. By a pull-down assay using a XRE-containing PCR product as bait and the hepatic nuclear extracts of both FICZ-treated mouse strains, we identified two interacting proteins as heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNP- A2 ) and its splicing variant (hnRNP- A2b ). Immunoprecipitation assays demonstrated the AHR interaction with hnRNP- A2/B1 . When hnRNP- A2 was co-expressed with the MRL/ lpr or C3H/ lpr AHR in COS-7, FICZ treatment decreased EC 50 to about threefold in both AHR genotypes, compared with EC 50 in AHR alone. Similarly, hnRNP- A2b co-expression also lowered the FICZ-EC 50 values. In TCDD-treated COS-7, responses depended on the AHR genotype; while no change in TCDD-EC 50 was observed for C3H/ lpr AHR when hnRNP- A2 was co-expressed, the value was reduced to nearly tenfold for MRL/ lpr AHR. Co-transfection with hnRNP- A2b attenuated the AHR sensitivity to TCDD. In conclusion, the hnRNP- A2 / B1 interacting with AHR may be a modulator of the AHR ligand sensitivity.