Identification of Ebp1 as a component of cytoplasmic bcl-2 mRNP (messenger ribonucleoprotein particle) complexes

The 3'-UTR (untranslated region) of bcl-2 mRNA contains an ARE (AU-rich element) that potentially regulates the stability of bcl-2 mRNA in a cell specific fashion. Previous studies have demonstrated that multiple proteins interact with bcl-2 mRNA in HL-60 (human leukaemia-60) cells, potentially contributing to the overexpression of Bcl-2 protein. Treatment of HL-60 cells with taxol or okadaic acid has been shown to induce destabilization of bcl-2 mRNA, which was associated with decreased binding of trans-acting factors to bcl-2 mRNA. Nucleolin has been identified as one of the bcl-2 mRNA-binding proteins [Sengupta, Bandyopadhyay, Fernandes and Spicer (2004) J. Biol. Chem. 279, 10855-10863]. In an effort to identify additional bcl-2 mRNA-binding proteins, two polypeptides of approx. 45 kDa and 60 kDa were isolated from HL-60 cells by ARE(bcl-2) (transcripts that contain bcl-2 AREs) RNA affinity chromatography. These proteins were identified as the human proliferation associated protein, Ebp1, and human DRBP76 (double stranded RNA-binding protein 76) respectively, by MALDI (matrix-assisted laser-desorption ionization)-MS. RNA electrophoretic mobility shift assays indicated that recombinant Ebp1 binds to ARE(bcl-2) RNA but not to the group 1 ARE present in GM-CSF (granulocyte macrophage-colony stimulating factor) mRNA in vitro. Antibody supershift assays demonstrated that Ebp1 is present in protein-ARE(bcl-2) RNA complexes formed with cytosolic HL-60 extracts. The interaction of Ebp1 with bcl-2 mRNA in HL-60 cells was also demonstrated by RNA co-immunoprecipitation assays. This interaction was not detected in extracts of taxol-treated HL-60 cells. Immunoprecipitation assays further revealed that Ebp1 co-precipitates with nucleolin from HL-60 cytoplasmic extracts. The observation that co-precipitation was decreased when extracts were treated with RNase suggests that Ebp1 and nucleolin are present in the same bcl-2 mRNP (messenger ribonucleoprotein particle) complexes. RNA-decay assays further demonstrated that Ebp1 decreased the rate of decay of beta-globin-ARE(bcl-2) transcripts in HL-60 cell extracts. Collectively, these results indicate a novel function for Ebp1 in contributing to the regulation of bcl-2 expression in HL-60 cells.