Bcl-2 is the prominent member of a family of proteins responsible for dysregulation of apoptosis, and resistance to chemotherapy. It has been shown that reduction in Bcl-2 protein levels could ultimately induce a lower apoptotic threshold and restore chemosensitivity in a variety of malignancies. Short interfering RNA (siRNA) has been evaluated as an attractive and effective tool for suppressing a target protein by specifically digesting its mRNA. In our lab, we have identified a siRNA targeting against Bcl-2 could effectively down-regulate Bcl-2 protein. In this study, we investigated the effect of gamma radiation combined with the siRNA targeting Bcl-2 on proliferation and apoptosis in B-lymphoma Raji cells. The siRNA was introduced into cells using Oligofectamine transfection. Cells were treated with Bcl-2 siRNA alone or with 2–8 Gy dose of (60)Co gamma rays. Expression of Bcl-2 mRNA and protein was assayed by quantitative reverse transcriptase-polymerase chain reaction and Western blotting analysis. Radiosensitivity was determined by clonogenic cell survival assay. Apoptosis was determined by Giemsa staining, Annexin-V binding assay and flow cytomertry. Furthermore caspase-3 activity and poly (ADP-ribose) polymerase (PARP) cleavage were evaluated. Transfection of Raji cells with 100 nmol/L siRNA targeted against Bcl-2 resulted in reduction of Bcl-2 mRNA by 75% compared with control-siRNA treated group and the vehicle control group(p<0.05). The levels of Bcl-2 protein were significantly reduced by 70% compared with the two control groups (p<0.05). There was significant difference in the radiosensitivity of Raji cells in which Bcl-2 was silenced compared with the cells transfected vehicle or control siRNA.Apoptosis index of the Raji cells treated with Bcl-2 siRNA combined with radiation was significantly increased (p<0.05), compared with either control siRNA / radiation combination or radiation-treatment cells alone, or Bcl-2 siRNA-treatment cells alone.Raji cells treated with Bcl-2 siRNA combined with radiation revealed enhanced caspase-3 and PARP cleavage as compared to Bcl-2 siRNA treated cells alone or only irradiated cells. These findings show that Bcl-2 siRNA synergistically enhances radiation-induced apoptosis through the expression of proteins involved in the programmed cell death.

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