IP(3)R2 levels dictate the apoptotic sensitivity of diffuse large B-cell lymphoma cells to an IP(3)R-derived peptide targeting the BH4 domain of Bcl-2

Disrupting inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)/B-cell lymphoma 2 (Bcl-2) complexes using a cell-permeable peptide (stabilized TAT-fused IP(3)R-derived peptide (TAT-IDP(S))) that selectively targets the BH4 domain of Bcl-2 but not that of B-cell lymphoma 2-extra large (Bcl-Xl) potentiated pro-apoptotic Ca(2+) signaling in chronic lymphocytic leukemia cells. However, the molecular mechanisms rendering cancer cells but not normal cells particularly sensitive to disrupting IP(3)R/Bcl-2 complexes are poorly understood. Therefore, we studied the effect of TAT-IDP(S) in a more heterogeneous Bcl-2-dependent cancer model using a set of ‘primed to death' diffuse large B-cell lymphoma (DL-BCL) cell lines containing elevated Bcl-2 levels. We discovered a large heterogeneity in the apoptotic responses of these cells to TAT-IDP(S) with SU-DHL-4 being most sensitive and OCI-LY-1 being most resistant. This sensitivity strongly correlated with the ability of TAT-IDP(S) to promote IP(3)R-mediated Ca(2+) release. Although total IP(3)R-expression levels were very similar among SU-DHL-4 and OCI-LY-1, we discovered that the IP(3)R2-protein level was the highest for SU-DHL-4 and the lowest for OCI-LY-1. Strikingly, TAT-IDP(S)-induced Ca(2+) rise and apoptosis in the different DL-BCL cell lines strongly correlated with their IP(3)R2-protein level, but not with IP(3)R1-, IP(3)R3- or total IP(3)R-expression levels. Inhibiting or knocking down IP(3)R2 activity in SU-DHL-4-reduced TAT-IDP(S)-induced apoptosis, which is compatible with its ability to dissociate Bcl-2 from IP(3)R2 and to promote IP(3)-induced pro-apoptotic Ca(2+) signaling. Thus, certain chronically activated B-cell lymphoma cells are addicted to high Bcl-2 levels for their survival not only to neutralize pro-apoptotic Bcl-2-family members but also to suppress IP(3)R hyperactivity. In particular, cancer cells expressing high levels of IP(3)R2 are addicted to IP(3)R/Bcl-2 complex formation and disruption of these complexes using peptide tools results in pro-apoptotic Ca(2+) signaling and cell death.