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


RESULTS
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
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.
Keuling
Angela M
AM
Department of Medical Genetics, University of Alberta, Edmonton, Alberta, Canada.
Felton
Kathleen E A
KE
Parker
Arabesque A M
AA
Akbari
Majid
M
Andrew
Susan E
SE
Tron
Victor A
VA
D017209:Apoptosis / Q000187:drug effects*; D001713:Biphenyl Compounds / Q000494:pharmacology*; D053181:Caspase 8 / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D055785:Gene Knockdown Techniques; D006801:Humans; D008545:Melanoma / Q000201:enzymology / Q000235:genetics / Q000473:pathology*; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009596:Nitrophenols / Q000494:pharmacology*; D010879:Piperazines / Q000494:pharmacology; D011110:Polymorphism, Genetic; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics*; D034622:RNA Interference; D013449:Sulfonamides / Q000494:pharmacology*
C501332:ABT-737; D001713:Biphenyl Compounds; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D009596:Nitrophenols; D010879:Piperazines; D019253:Proto-Oncogene Proteins c-bcl-2; D013449:Sulfonamides; D053181:Caspase 8
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