AT-101, a small molecule inhibitor of anti-apoptotic Bcl-2 family members, activates the SAPK/JNK pathway and enhances radiation-induced apoptosis.
BACKGROUND
Gossypol, a naturally occurring polyphenolic compound has been identified as a small molecule inhibitor of anti-apoptotic Bcl-2 family proteins. It induces apoptosis in a wide range of tumor cell lines and enhances chemotherapy- and radiation-induced cytotoxicity both in vitro and in vivo. Bcl-2 and related proteins are important inhibitors of apoptosis and frequently overexpressed in human tumors. Increased levels of these proteins confer radio- and chemoresistance and may be associated with poor prognosis. Consequently, inhibition of the anti-apoptotic functions of Bcl-2 family members represents a promising strategy to overcome resistance to anticancer therapies.


METHODS
We tested the effect of (-)-gossypol, also denominated as AT-101, radiation and the combination of both on apoptosis induction in human leukemic cells, Jurkat T and U937. Because activation of the SAPK/JNK pathway is important for apoptosis induction by many different stress stimuli, and Bcl-X(L) is known to inhibit activation of SAPK/JNK, we also investigated the role of this signaling cascade in AT-101-induced apoptosis using a pharmacologic and genetic approach.


RESULTS
AT-101 induced apoptosis in a time- and dose-dependent fashion, with ED50 values of 1.9 and 2.4 microM in Jurkat T and U937 cells, respectively. Isobolographic analysis revealed a synergistic interaction between AT-101 and radiation, which also appeared to be sequence-dependent. Like radiation, AT-101 activated SAPK/JNK which was blocked by the kinase inhibitor SP600125. In cells overexpressing a dominant-negative mutant of c-Jun, AT-101-induced apoptosis was significantly reduced.


CONCLUSIONS
Our data show that AT-101 strongly enhances radiation-induced apoptosis in human leukemic cells and indicate a requirement for the SAPK/JNK pathway in AT-101-induced apoptosis. This type of apoptosis modulation may overcome treatment resistance and lead to the development of new effective combination therapies.
Department of Radiation Oncology, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands.
Shuraila F
SF
Zerp
Rianne
R
Stoter
Gitta
G
Kuipers
Dajun
D
Yang
Marc E
ME
Lippman
Wim J
WJ
van Blitterswijk
Harry
H
Bartelink
Rogier
R
Rooswinkel
Vincent
V
Lafleur
Marcel
M
Verheij
D000972:Antineoplastic Agents, Phytogenic / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects* / Q000528:radiation effects; D015153:Blotting, Western; D045744:Cell Line, Tumor; D003131:Combined Modality Therapy; D004307:Dose-Response Relationship, Radiation; D006072:Gossypol / Q000031:analogs & derivatives* / Q000494:pharmacology; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases / Q000187:drug effects* / Q000528:radiation effects*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000037:antagonists & inhibitors; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology; D011878:Radiotherapy; D015398:Signal Transduction / Q000187:drug effects / Q000528:radiation effects
D000972:Antineoplastic Agents, Phytogenic; D019253:Proto-Oncogene Proteins c-bcl-2; D011838:Radiation-Sensitizing Agents; D048031:JNK Mitogen-Activated Protein Kinases; D006072:Gossypol; C028178:gossypol acetic acid