Anti-apoptotic Bcl-2 proteins are upregulated in different cancers, including diffuse large B-cell lymphoma (DLBCL) and
chronic lymphocytic leukemia (CLL), enabling survival by inhibiting pro-apoptotic Bcl-2-family members and inositol
1,4,5-trisphosphate (IP3) receptor (IP3R)-mediated Ca2+-signaling. A peptide tool (Bcl-2/IP3R Disruptor-2; BIRD-2) was
developed to abrogate the interaction of Bcl-2 with IP3Rs by targeting Bcl-2′s BH4 domain. BIRD-2 triggers cell death in
primary CLL cells and in DLBCL cell lines. Particularly, DLBCL cells with high levels of IP3R2 were sensitive to BIRD-2.
Here, we report that BIRD-2-induced cell death in DLBCL cells does not only depend on high IP3R2-expression levels, but
also on constitutive IP3 signaling, downstream of the tonically active B-cell receptor. The basal Ca2+ level in SU-DHL-4
DLBCL cells was significantly elevated due to the constitutive IP3 production. This constitutive IP3 signaling fulfilled a prosurvival role, since inhibition of phospholipase C (PLC) using U73122 (2.5 µM) caused cell death in SU-DHL-4 cells.
Milder inhibition of IP3 signaling using a lower U73122 concentration (1 µM) or expression of an IP3 sponge suppressed
both BIRD-2-induced Ca2+ elevation and apoptosis in SU-DHL-4 cells. Basal PLC/IP3 signaling also fulfilled a pro-survival
role in other DLBCL cell lines, including Karpas 422, RI-1 and SU-DHL-6 cells, whereas PLC inhibition protected these
cells against BIRD-2-evoked apoptosis. Finally, U73122 treatment also suppressed BIRD-2-induced cell death in primary
CLL, both in unsupported systems and in co-cultures with CD40L-expressing fibroblasts. Thus, constitutive IP3 signaling in
lymphoma and leukemia cells is not only important for cancer cell survival, but also represents a vulnerability, rendering
cancer cells dependent on Bcl-2 to limit IP3R activity. BIRD-2 seems to switch constitutive IP3 signaling from pro-survival
into pro-death, presenting a plausible therapeutic strategy