Helenalin triggers a CD95 death receptor-independent apoptosis that is not affected by overexpression of Bcl-x(L) or Bcl-2.

Abstract

Apoptosis is required for proper tissue homeostasis. Defects in apoptosis signaling pathways, thus, contribute to carcinogenesis and chemoresistance. A major goal in chemotherapy is, therefore, to find cytotoxic agents that restore the ability of tumor cells to undergo apoptosis. We show here that the sesquiterpene lactone helenalin (10-50 microM) induces apoptosis in leukemia Jurkat T cells even if they lack the CD95 death receptor or overexpress the antiapoptotic proteins Bcl-x(L) or Bcl-2. Activated peripheral blood mononuclear cells, however, are not affected (10-50 microM helenalin). Helenalin led to a time-dependent (0-24 h) cleavage of the specific caspase-3-like substrate Asp-Glu-Val-Asp-7-amino-4-trifluoromethylcoumarin as well as to the proteolytic processing of procaspase-3 and -8. Caspase activation was a necessary requirement for apoptosis because the broad-spectrum caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone (zVAD-fmk, 50 microM) completely abrogated helenalin-induced DNA fragmentation as well as phosphatidylserin translocation. Although the initiator caspase-8 was activated, the helenalin-induced signaling pathway did not require the CD95 death receptor as shown using cells without or with an antibody (ZB4)-blocked CD95 receptor. Helenalin also did not induce CD95 or CD95-ligand expression. On the other hand, helenalin was found to induce the release of cytochrome c from mitochondria that was not inhibited by the caspase inhibitor zVAD-fmk, which indicated that cytochrome c release precedes caspase activation. Cytochrome c release was accompanied by dissipation of the mitochondrial transmembrane potential (DeltaPsi(m)), which was partly inhibited by zVAD-fmk, which suggests that caspases are involved in loss of DeltaPsi(m). Most importantly, overexpression of the mitochondria protecting proteins Bcl-x(L) or Bcl-2 failed to confer resistance to helenalin-induced apoptosis, although the data presented here suggest that helenalin induces a mitochondria-dependent pathway. Thus, helenalin is a promising experimental cytotoxic agent that possibly points to new strategies to overcome apoptosis resistance attributable to overexpression of antiapoptotic Bcl-2 proteins.

• Authors: V M Dirsch, H Stuppner, A M Vollmar
• Journal: Cancer research (Cancer Res) Vol. 61 Issue 15 Pg. 5817-23 (Aug 01 2001) ISSN: 0008-5472 [Print] United States
• PMID: 11479221 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• Antineoplastic Agents, Phytogenic
• BCL2L1 protein, human
• FASLG protein, human
• Fas Ligand Protein
• Membrane Glycoproteins
• Proto-Oncogene Proteins c-bcl-2
• Sesquiterpenes
• Sesquiterpenes, Guaiane
• bcl-X Protein
• fas Receptor
• helenalin
• CASP3 protein, human
• CASP8 protein, human
• CASP9 protein, human
• Caspase 3
• Caspase 8
• Caspase 9
• Caspases
• Matrix Metalloproteinases

Topics

• Antineoplastic Agents, Phytogenic (pharmacology)
• Apoptosis (drug effects, physiology)
• Caspase 3
• Caspase 8
• Caspase 9
• Caspases (metabolism)
• Enzyme Activation
• Enzyme Induction (drug effects)
• Fas Ligand Protein
• Humans
• Jurkat Cells (drug effects, metabolism, pathology)
• Matrix Metalloproteinases (biosynthesis)
• Membrane Glycoproteins (physiology)
• Proto-Oncogene Proteins c-bcl-2 (biosynthesis, physiology)
• Sesquiterpenes (pharmacology)
• Sesquiterpenes, Guaiane
• bcl-X Protein
• fas Receptor (physiology)