Irradiation of mitochondria initiates apoptosis in a cell free system

Oncogene. 2001 Jan 11;20(2):167-77. doi: 10.1038/sj.onc.1204054.


The ability to modulate the sensitivity of mammalian cells to ionizing radiation (IR) (e.g. using chemotherapeutics) is dependent on our understanding of the primary target and biochemical pathway that leads to IR-induced apoptosis. We demonstrate using a cell free assay that irradiation of mitochondria is a primary event that initiates IR-induced apoptosis. IR results in loss of mitochondrial membrane potential, opening of the permeability transition pore (PTP) and the release of cytochrome c (cyto c). Apaf-1 and ATP were required to initiate apoptosis upon release of cyto c from mitochondria. The importance of mitochondrial events in the initiation of IR-induced apoptosis was also supported by the observation that inhibition of caspase-9 by the over-expression of dominant negative mutants resulted in the inhibition of IR-induced apoptosis. In contrast, inhibition of caspase-8 had only a minor impact on IR-induced apoptosis. Over-expression of Bcl-X(L) inhibited the initiation of IR-induced apoptosis due to its ability to prevent the loss of mitochondrial membrane potential, PTP opening and cytochrome c release. In a cell free assay for apoptosis, mitochondria as well as cytosol derived from Bcl-X(L) over-expressing cells were less efficient at supporting apoptosis in response to IR suggesting multiple roles for Bcl-X(L) in the regulation of apoptosis.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adenosine Triphosphate / metabolism
  • Amino Acid Chloromethyl Ketones / pharmacology
  • Apoptosis / drug effects
  • Apoptosis / physiology*
  • Apoptosis / radiation effects*
  • Apoptotic Protease-Activating Factor 1
  • Biological Transport
  • Caspase 8
  • Caspase 9
  • Caspase Inhibitors
  • Caspases / genetics
  • Caspases / metabolism*
  • Cell Extracts / pharmacology
  • Cell Membrane / radiation effects
  • Cell Nucleus / drug effects
  • Cell Nucleus / radiation effects
  • Cell-Free System
  • Cyclosporine / pharmacology
  • Cysteine Proteinase Inhibitors / pharmacology
  • Cytochrome c Group / metabolism
  • Cytosol / radiation effects
  • Electrons
  • Humans
  • Jurkat Cells / drug effects
  • Mitochondria / radiation effects*
  • Proteins / metabolism
  • Proto-Oncogene Proteins c-bcl-2 / genetics
  • Proto-Oncogene Proteins c-bcl-2 / metabolism
  • Proto-Oncogene Proteins c-bcl-2 / radiation effects
  • Reactive Oxygen Species / metabolism
  • bcl-X Protein


  • APAF1 protein, human
  • Amino Acid Chloromethyl Ketones
  • Apoptotic Protease-Activating Factor 1
  • BCL2L1 protein, human
  • Caspase Inhibitors
  • Cell Extracts
  • Cysteine Proteinase Inhibitors
  • Cytochrome c Group
  • Proteins
  • Proto-Oncogene Proteins c-bcl-2
  • Reactive Oxygen Species
  • bcl-X Protein
  • benzyloxycarbonylvalyl-alanyl-aspartyl fluoromethyl ketone
  • Cyclosporine
  • Adenosine Triphosphate
  • CASP8 protein, human
  • CASP9 protein, human
  • Caspase 8
  • Caspase 9
  • Caspases