Ubiquitin-like protein MNSFβ covalently binds to Bcl-G and enhances lipopolysaccharide/interferon γ-induced apoptosis in macrophages.

Monoclonal non-specific suppressor factor β (MNSFβ) is a ubiquitously expressed member of the ubiquitin-like family that is involved in various biological functions. Previous studies have demonstrated that MNSFβ covalently binds to intracellular pro-apoptotic protein Bcl-G and regulates the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) cascade in the mouse macrophage cell line Raw264.7. In this study, we demonstrate that MNSFβ promotes lipopolysaccharide (LPS)/interferon γ (IFNγ)-induced apoptosis of Raw264.7 macrophages. In Raw264.7 cells treated with MNSFβ small interfering RNA (siRNA), LPS/IFNγ- or NO donor S-nitrosoglutathione-induced apoptosis was inhibited. siRNA-mediated knockdown of MNSFβ did not affect inducible nitric-oxide synthase (iNOS) expression in LPS/IFNγ-stimulated Raw264.7 cells. Conversely, co-transfection with MNSFβ and Bcl-G greatly enhanced LPS/IFNγ- induced apoptosis in Raw264.7 cells, accompanied by increased expression of p53 and decreased Cox-2 activity. Unlike co-transfection with wild-type MNSFβ, co-transfection of a mutant MNSFβ (G74A) and Bcl-G did not result in enhancement of LPS/IFNγ-induced apoptosis. Co-over-expression of MNSFβ and Bcl-G reduced S-nitrosoglutathione-induced ERK1/2 phosphorylation. Furthermore, electrophoretic mobility shift assay experiments revealed that MNSFβ down-regulates the ERK/activator protein 1 (AP-1) signaling cascade which leads to Cox-2 activation. We also observed that MNSFβ-Bcl-G promotes LPS/IFNγ-induced apoptosis of mouse peritoneal macrophages, together with a decrease in Cox-2 expression. Taken together, our data indicate an apoptosis-enhancing effect of MNSFβ-Bcl-G is due in part to down-regulation of Cox-2 activation in macrophages.