A Novel Peptide Containing a Nine Amino Acid Sequence from Nur77 Induces Bcl-2-Dependent Apoptosis in AML.

Nur77 (also known as TR3 and NGFI-B) is a nuclear orphan receptor that in response to various stimuli can translocate from the nucleus to the mitochondria, bind Bcl-2, and induce a conformational change in Bcl-2 that exposes its BH3 domain. This conformational change of Bcl-2 transforms this protein into a pro-apoptotic molecule that can induce cytochrome c release and apoptosis (Cell 116:527, 2004). Interestingly, in acute myeloid leukemia (AML) cell lines and blasts, Nur77 is absent. We have recently generated a nine amino acid peptide from the Nur77 protein (TR3) that is capable of inducing the same pro-apoptotic conformational change in Bcl-2 that Nur77 elicits (manuscript submitted). We tested the hypothesis that a cell-permeable version of this peptide can induce Bcl-2-dependent apoptosis in AML cells and AML stem cells. Primary AML samples (n=6) were incubated with 20 μM control or TR3 peptide for 24 hrs. Induction of apoptosis was measured as CD34⁺38⁻123⁺ PS/Annexin V⁺ by multiparametric flow cytometry. Our results demonstrate that the L-enantiomer form of TR3 indeed induces apoptosis in total AML cells (60%, p=0.001) and CD34⁺38⁻123⁺ stem cells (67%, p=0.001), as compared to control peptide. Bax expression did not affect the peptide’s ability to induce apoptosis, as wild-type HCT116 cells and HCT116 cells deficient in Bax showed similar levels of apoptosis after treatment with the TR3 peptide. However, Bcl-2 expression was shown to be critical since lentiviral shRNA ablation of Bcl-2 in KG1 cells completely prevented induction of apoptosis by TR3 peptides. We conclude that the TR3 peptide is a potent inducer of apoptosis that mimics the action of Nur77 on Bcl-2 at the mitochondrial level. These results suggest that the Nur77 peptide or compounds that mimic this peptide may have utility as a novel therapeutic agent against Bcl-2 expressing cancers and leukemias.