G1P3 (ISG 6-16), an interferon (IFN) stimulated survival factor antagonizes TRAIL/Apo2L induced apoptosis in myeloma cells

5152 TRAIL/Apo2L was one of the most potent pro-apoptotic genes induced (>100 fold) in myeloma cells following IFN-α2b. Yet 200 fold) and IFN-α1b, IFN-α2b and IFN-β markedly increased G1P3 expression in all myeloma cell lines tested. G1P3 encodes a 13 kD protein that has a putative mitochondrial localization sequence and belongs to the FAM protein family. Myeloma cells expressing G1P3 were resistant to TRAIL/Apo2L induced mitochondrial perturbation, cytochrome c release and apoptosis. TRAIL (50 ng/ml) for 24 hr markedly increased apoptosis (from 4% to 77.5%) of vector transduced cells and higher concentrations of TRAIL (100 ng/ml) resulted in a further increase to 93.6%. Compared to control cells only 4.3% of G1P3 cells underwent apoptosis with 50 ng/ml of TRAIL and 16.3% with 100 ng/ml of TRAIL. However, overexpression of G1P3 did not result in bortezomib (a proteasome inhibitor) resistance, suggesting specificity of G1P3 for the intrinsic apoptosis pathway. Further, 52% knockdown of IFN-α2b induced G1P3 mRNA with an siRNA resulted in a 10 fold increase in apoptosis (4.1% with control siRNA vs. 40.3% with G1P3 siRNA) of RPMI 8266 cells suggesting a pro-survival role for G1P3 upon IFNs stimulation. Data thus identified the direct role of a mitochondrial localized prosurvival ISG in antagonizing the effect of TRAIL/Apo2L, a pro-apoptotic ISG in myeloma. Ability to overcome G1P3 mediated anti-apoptotic signals could open new avenues for increasing the efficacy of therapies for myeloma or other malignancies.