Caspase-9 is the upstream caspase activated by 8-methoxypsoralen and ultraviolet-A radiation treatment of Jurkat T leukemia cells and normal T lymphocytes

Haematologica. 2004 Apr;89(4):471-9.


Background and objectives: A combination of 8-methoxypsoralen and ultraviolet-A radiation (PUVA) is used for the treatment of T cell-mediated disorders, including chronic graft-versus-host disease. The mechanisms of action of this therapy, referred to as extracorporeal phototherapy, have not been fully elucidated. PUVA is known to induce apoptosis in T lymphocytes collected by apheresis, however scarce information is available concerning the apoptotic pathways activated by PUVA.

Design and methods: We used Jurkat human T leukemia cells and normal T lymphocytes to analyze the PUVA-triggered caspase activation pattern by means of immunoblot analysis, in vitro caspase activity assays, and selective caspase inhibitors coupled to flow cytometric analysis.

Results: PUVA treatment induced activation of apical caspases-9 and -8, and of effector caspases-3 and -7 in Jurkat cells and human T lymphocytes. While activation of caspase-9 occurred as early as 1 h after PUVA treatment of Jurkat cells, procaspase-8 cleavage was delayed and was detected 6 h after the exposure. Also in normal T lymphocytes, cleavage of caspase-8 was subsequent to activation of caspase-9. PUVA-dependent proteolytic cleavage of procaspase-8 was blocked by inhibitors selective for either caspase-9 or -3. Moreover, procaspase-8 was cleaved in vitro by activated caspase-3, which gave rise to proteolytic fragments equivalent to those generated in vivo.

Interpretation and conclusions: Activation of caspase-8 in PUVA-treated Jurkat cells and normal T lymphocytes is secondary to up-regulation of caspase-9. Overall, our results identify caspase-9 as the critical upstream caspase initiating apoptosis by PUVA in Jurkat T-cells and human T lymphocytes.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Apoptosis / drug effects
  • Apoptosis / radiation effects
  • Caspase 8
  • Caspase 9
  • Caspases / metabolism*
  • Enzyme Activation / drug effects
  • Enzyme Activation / radiation effects
  • Humans
  • Jurkat Cells
  • Leukemia, T-Cell / pathology
  • Leukemia, T-Cell / therapy
  • Methoxsalen / pharmacology
  • T-Lymphocytes / drug effects
  • T-Lymphocytes / radiation effects
  • Ultraviolet Therapy / methods*


  • CASP8 protein, human
  • CASP9 protein, human
  • Caspase 8
  • Caspase 9
  • Caspases
  • Methoxsalen