RNA silencing of Mcl-1 enhances ABT-737-mediated apoptosis in melanoma: role for a caspase-8-dependent pathway

PLoS One. 2009 Aug 17;4(8):e6651. doi: 10.1371/journal.pone.0006651.


Background: Malignant melanoma is resistant to almost all conventional forms of chemotherapy. Recent evidence suggests that anti-apoptotic proteins of the Bcl-2 family are overexpressed in melanoma and may contribute to melanoma's striking resistance to apoptosis. ABT-737, a small-molecule inhibitor of Bcl-2, Bcl-xl and Bcl-w, has demonstrated efficacy in several forms of leukemia, lymphoma as well as solid tumors. However, overexpression of Mcl-1, a frequent observance in melanoma, is known to confer ABT-737 resistance.

Methodology/principal findings: Here we report that knockdown of Mcl-1 greatly reduces cell viability in combination with ABT-737 in six different melanoma cell lines. We demonstrate that the cytotoxic effect of this combination treatment is due to apoptotic cell death involving not only caspase-9 activation but also activation of caspase-8, caspase-10 and Bid, which are normally associated with the extrinsic pathway of apoptosis. Caspase-8 (and caspase-10) activation is abrogated by inhibition of caspase-9 but not by inhibitors of the death receptor pathways. Furthermore, while caspase-8/-10 activity is required for the full induction of cell death with treatment, the death receptor pathways are not. Finally, we demonstrate that basal levels of caspase-8 and Bid correlate with treatment sensitivity.

Conclusions/significance: Our findings suggest that the combination of ABT-737 and Mcl-1 knockdown represents a promising, new treatment strategy for malignant melanoma. We also report a death receptor-independent role for extrinsic pathway proteins in treatment response and suggest that caspase-8 and Bid may represent potential markers of treatment sensitivity.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Apoptosis / drug effects*
  • Biphenyl Compounds / pharmacology*
  • Caspase 8 / genetics
  • Caspase 8 / metabolism*
  • Cell Line, Tumor
  • Gene Knockdown Techniques
  • Humans
  • Melanoma / enzymology
  • Melanoma / genetics
  • Melanoma / pathology*
  • Myeloid Cell Leukemia Sequence 1 Protein
  • Nitrophenols / pharmacology*
  • Piperazines / pharmacology
  • Polymorphism, Genetic
  • Proto-Oncogene Proteins c-bcl-2 / genetics*
  • RNA Interference*
  • Sulfonamides / pharmacology*


  • ABT-737
  • Biphenyl Compounds
  • Myeloid Cell Leukemia Sequence 1 Protein
  • Nitrophenols
  • Piperazines
  • Proto-Oncogene Proteins c-bcl-2
  • Sulfonamides
  • Caspase 8