Post-transcriptional regulation of the human inducible nitric oxide synthase (iNOS) expression by the cytosolic poly(A)-binding protein (PABP)

Abstract

Affinity purification using the 3'-untranslated region (3'-UTR) of the human inducible nitric oxide synthase (iNOS) mRNA identified the cytosolic poly(A)-binding protein (PABP) as a protein interacting with the human iNOS 3'-UTR. Downregulation of PABP expression by RNA interference resulted in a marked reduction of cytokine-induced iNOS mRNA expression without changes in the expression of mRNAs coding for the major subunit of the RNA polymerase II (Pol 2A) or β2-microglobuline (β2M). Along with the mRNA also iNOS protein expression was reduced by siPABP-treatment, whereas in the same cells protein expression of STAT-1α, NF-κB p65, or GAPDH was not altered. Reporter gene analyses showed no change of the inducibility of the human 16kb iNOS promoter in siPABP cells. In contrast, the siPABP-mediated decline of iNOS expression correlated with a reduction in the stability of the iNOS mRNA. As the stability of the Pol 2A and β2M mRNA was not changed, siPABP-treatment seems to have a specific effect on iNOS mRNA decay. UV-crosslinking experiments revealed that PABP interacts with one binding site in the 5'-UTR and two different binding sites in the 3'-UTR of the human iNOS mRNA. Mutation or deletion of the binding site in the 5'-UTR but not in the 3'-UTR reduced luciferase expression in DLD-1 cells transfected with iNOS-5'-UTR or iNOS-3'-UTR luciferase reporter constructs. In summary, our data demonstrate that PABP by binding to specific sequence elements in the 5'-UTR post-transcriptionally enhances human iNOS mRNA stability and thereby iNOS expression.

Keywords: 3′-UTR; 3′-untranslated region; 5′-UTR; 5′-untranslated region; 6-dichloro-1-ribofuranosylbenzimidazole; ARE; ARE binding protein; ARE-BP; AU-rich element; AU-rich element RNA binding protein 1; AUF1; CM; Co; DRB; EGFP; ELAV; FACS; GAPDH; GST; HuR; IFN-γ; IL-1β; IP; KH-type splicing regulatory protein; KSRP; Luc; NO; PABP; Pol 2A; RNA polymerase II great subunit; TNF-α; TTP; UTR; WB; Western blot; Y box factor-1; YB-1; cds; coding sequence; control cells; cytokine mixture; eRF3; embryonic lethal abnormal vision; enhanced green fluorescence protein; eukaryotic release factor 3; fluorescent activated cell sorting; glutathione S-transferase; glyceraldehyde-3-phosphate dehydrogenase; heteronuclear ribonucleoprotein; hnRNP; human antigen R; iNOS; immunoprecipitation; inducible NO-synthase; interferon-γ; interleukin-1β; luciferase; mRNA stability; nitric oxide; poly(A)binding protein; qRT-PCR; quantitative reverse transcription polymerase chain reaction; shRNA; siRNA; small hairpin RNA; small interfering RNA; tristetraprolin; tumor necrosis factor-α; β-tub.; β-tubulin; β2-microglobulin; β2M.