Enhancing TRAIL-induced apoptosis by Bcl-X(L) siRNA

Cancer Biol Ther. 2005 Apr;4(4):393-7. doi: 10.4161/cbt.4.4.1616. Epub 2005 Apr 21.


We previously found that a change in the balance between mitochondrial pro- and anti-apoptotic proteins caused by ectopic expression of the Bax gene led to increased induction of apoptosis by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). To investigate whether a similar effect can be elicited by down-regulating Bcl-X(L), an anti-apoptotic protein, we tested the effects of a small interfering RNA (siRNA) specific for Bcl-X(L) in TRAIL-resistant cells. The down-regulation of Bcl-X(L) by siRNA inhibited cell proliferation and sensitized TRAIL-induced apoptosis in human cancer cells with both acquired and intrinsic TRAIL resistance. Combining the Bcl-X(L) siRNA with TRAIL protein treatment resulted in an increase in the percentage of apoptotic cells and increased cleavage of caspase-8, caspase-9, caspase-3 and PARP. Furthermore, the release of cytochrome c but not Smac from mitochondria was induced by Bcl-X(L) siRNA alone, and this release was dramatically amplified by combining the Bcl-X(L) siRNA and TRAIL protein treatment. Together, our data suggest that simultaneous triggering of the death receptor and mitochondrial apoptotic pathways leads to enhanced induction of apoptosis, which makes it potentially useful for the treatment of resistant cancers.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Apoptosis / drug effects*
  • Apoptosis Regulatory Proteins / pharmacology*
  • Caspases / metabolism
  • Cell Line, Tumor
  • Cell Proliferation / drug effects
  • Cell Survival / drug effects
  • Collagen Type XI / metabolism
  • Colonic Neoplasms / drug therapy*
  • Colonic Neoplasms / genetics
  • Colonic Neoplasms / metabolism
  • Colonic Neoplasms / pathology
  • Cytochromes c / metabolism
  • Down-Regulation
  • Drug Synergism
  • Enzyme Activation / drug effects
  • Female
  • Humans
  • Ligands
  • Membrane Glycoproteins / pharmacology*
  • Mitochondria / metabolism
  • RNA, Small Interfering / metabolism
  • RNA, Small Interfering / pharmacology*
  • TNF-Related Apoptosis-Inducing Ligand
  • Tumor Necrosis Factor-alpha / pharmacology*
  • bcl-X Protein / metabolism*


  • Apoptosis Regulatory Proteins
  • COL11A2 protein, human
  • Collagen Type XI
  • Ligands
  • Membrane Glycoproteins
  • RNA, Small Interfering
  • TNF-Related Apoptosis-Inducing Ligand
  • TNFSF10 protein, human
  • Tumor Necrosis Factor-alpha
  • bcl-X Protein
  • Cytochromes c
  • Caspases