Bcl-B and Nur77/TR3: a deadly combination

The antiapoptotic activity of Bcl-2 can be converted to proapoptotic by virtue of its interaction with the orphan member of nuclear receptor family Nur77/TR3. In this issue of Blood, Luciano and colleagues demonstrate that Nur77/TR3 can also alter the prosurvival trait of other members of Bcl-2 family of proteins to prodeath, thereby providing the rationale for therapeutic targeting of Nur77/TR3 in cancers with Bcl-2 aberrations.

Defects of apoptotic signaling pathways confer chemoresistance.^1,2  Bcl-2 family proteins are key regulators of apoptosis and have been divided into 2 groups based on their function: proapoptotic (Bax, Bak, Bad, Bid, Bik, Bim, Nip3, Nix, Bok) and antiapoptotic (Bcl-2, Bcl-X_L, Mcl-1, Bfl-1/A1, Bcl-W, Bcl-B). Specifically, Bcl-2 and Bcl-X_L are overexpressed in many cancers and mediate tumorigenesis and drug resistance. A recent study by Lin et al showed for the first time that Nur77/TR3 binds to Bcl-2 and reverses Bcl-2 function from protector to killer.³  Now, Luciano and colleagues demonstrate here that besides Bcl-2, Nu77/TR3 also converts Bcl-B from an antiapoptotic to proapoptotic phenotype.

In a series of elegant experiments using GST fusion protein-binding assays, coimmunoprecipitation assays, and confocal microscopy, the authors show selective association of Nur77/TR3 with Bfl-1 and Bcl-B, but not Bcl-X_L, Mcl-1, or Bcl-w. Interaction of Nur77/TR3 with Bcl-B led to caspase activation and apoptosis. Immunohistochemical analysis of various human tissues shows robust Bcl-B expression in plasma cells and multiple myeloma (MM) cells (Bcl-B expressed in 2 of 6 MM cell lines and 73 of 165 patient MM specimens). Variable expression levels of Bcl-B were noted in MM cell lines; however, lower Bcl-B correlated with higher Bcl-2 expression. Most MM cells do not contain Nur77/TR3 under normal conditions, but treatment of these cells with phorbol ester (TPA) triggered Nur77/TR3 expression and its accumulation in the cytosol. Coimmunoprecipitation showed association of endogenous Nur77/TR3 with endogenous Bcl-B. Although TPA induces interaction of Nur77/TR3 and Bcl-B, this in itself does not cause apoptosis since induced levels of endogenous Nur77/TR3 are lower than is required to initiate apoptosis. Overexpression of Nur77/TR3 resulted in apoptosis in RPMI-8226 MM cells in a Bcl-B–dependent manner, evidenced by specific blockade using Bcl-B siRNA. Of importance, synthetic peptide mimicking human Nur77/TR3 triggered apoptosis in RPMI-8226 MM cells in a dose-dependent fashion.

The current findings further support the potential for therapeutic targeting of Bcl-2 family proteins in cancer. In the context of MM, clinical strategies to block Bcl-2 activity have not been successful due to half-life/stability and/or specificity of Bcl-2 antagonists. Moreover, the heterogeneous expression of Bcl-2 family proteins in MM adds complexity in designing therapeutic strategies (ie, lack of expression of one family member of antiapoptotic Bcl-2 family protein is compensated by higher levels of another). A similar observation was noted in the current study (ie, lower Bcl-B levels correlated with higher Bcl-2 expression in various MM cell lines). The ability of Nur77/TR3 to target not only Bcl-B, but also Bcl-2 or Bcl-X_L, makes it a more viable therapeutic target. Preclinical studies with a novel Bcl-2/Bcl-X_L/Bcl-w inhibitor ABT-737 showed promising anti-MM activity,⁴  further supporting this view. However, Mcl-1 is also expressed in MM and its expression correlates with shorter survival.^4,5  The finding that Nur77/TR3 targets Bcl-2, Bcl-X_L, and Bfl-1, but not Mcl-1, suggests that combining either Nur77/TR3 mimetic or ABT-737 with agents that neutralize Mcl-1 may enhance anti-MM activity in these patients.

The identification of Nur77/TR3 mimetics mediating conversion of highly expressed endogenous Bcl-2 or Bcl-B from a savior to a killer therefore provides the framework for promising novel therapeutics.