Negative cross-talk between Nur77 and small heterodimer partner and its role in apoptotic cell death of hepatoma cells

Nur77, an orphan nuclear receptor, has been implicated in apoptosis of a variety of cell types, including hepatocytes. The small heterodimer partner (SHP) binds and inhibits the function of many nuclear receptors. Here, we investigated crosstalk between Nur77 and SHP during anti-Fas antibody (CH11)-mediated apoptosis of hepatic cells. Expression of SHP decreased, whereas antisense SHP enhanced, the transcriptional activity of Nur77 in HepG2 cells. SHP and Nur77 were physically associated in vivo and colocalized in the nucleus. SHP decreased the transactivation function of the N-terminal domain of Nur77 that recruits coactivators. Nur77 and SHP competitively bound to cAMP response element-binding protein-binding protein and the expression of coactivators, such as cAMP response element-binding protein-binding protein and activating signal cointegrator-2, recovered the decreased function of Nur77 caused by SHP. Finally, SHP was differentially expressed in hepatoma cell lines in that it was not detected in the interferon-gamma(IFN gamma)/CH11-sensitive SNU354, whereas it was significantly expressed in the IFN gamma/CH11-resistant HepG2. Interestingly, a stable SNU354 cell line that expressed SHP became resistant to the IFN gamma/CH11-induced apoptosis. Together, our results suggest that SHP plays a key role in the regulation of Nur77 activation and thereby in Nur77-mediated apoptosis in the liver.