Smac modulates chemosensitivity in head and neck cancer cells through the mitochondrial apoptotic pathway.
OBJECTIVE
Overexpression of inhibitors of apoptosis proteins (IAP) contributes to therapeutic resistance. Second mitochondria-derived activator of caspase (Smac) promotes caspase activation by binding to IAPs upon release from the mitochondria. IAP antagonists, also called SMAC mimetics, are promising anticancer agents modeled after this mechanism. We investigated the role and mechanisms of Smac- and Smac mimetic-mediated chemosensitization in head and neck squamous cell carcinoma (HNSCC) cells.


METHODS
The effects of SMAC knockdown, SMAC overexpression, and a small molecule Smac mimetic on the chemosensitivities of HNSCC cells were determined. The mechanisms of Smac- and Smac mimetic-mediated chemosensitization were investigated by analyzing growth suppression, the mitochondrial apoptotic pathway, caspase activation, and IAP proteins. The therapeutic responses of HNSCC cells with different levels of Smac were compared in xenograft models.


RESULTS
We found that Smac mediates apoptosis induced by several classes of therapeutic agents through the mitochondrial pathway. SMAC knockdown led to impaired caspase activation, mitochondrial membrane depolarization, and release of cytochrome c. A small molecule Smac mimetic, at nanomolar concentrations, significantly sensitized HNSCC cells to gemcitabine-induced apoptosis and restored gemcitabine sensitivity in SMAC knockdown cells, through caspase activation, X-linked IAP dissociation, and mitochondria-associated events, but not the TNF-α pathway. Furthermore, Smac levels modulated the therapeutic response of HNSCC cells to gemcitabine in xenograft models.


CONCLUSIONS
Our results establish a critical role of Smac in mediating therapeutic responses of HNSCC cells and provide a strong rationale for combining Smac mimetics with other anticancer agents to treat HNSCC.
University of Pittsburgh Cancer Institute, Hillman Cancer Center, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, USA.
Quanhong
Q
Sun
Xingnan
X
Zheng
Lin
L
Zhang
Jian
J
Yu
D000818:Animals; D000964:Antimetabolites, Antineoplastic / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects*; D051017:Apoptosis Regulatory Proteins; D040761:Biomimetic Materials / Q000494:pharmacology; D015153:Blotting, Western; D053148:Caspase 3 / Q000378:metabolism; D045744:Cell Line, Tumor; D045304:Cytochromes c / Q000378:metabolism; D003841:Deoxycytidine / Q000031:analogs & derivatives* / Q000494:pharmacology; D004357:Drug Synergism; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D006258:Head and Neck Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008819:Mice, Nude; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism; D024101:Mitochondrial Proteins / Q000235:genetics / Q000378:metabolism*; D034622:RNA Interference; D015398:Signal Transduction / Q000187:drug effects; D023041:Xenograft Model Antitumor Assays; D000093542:Gemcitabine
D000964:Antimetabolites, Antineoplastic; D051017:Apoptosis Regulatory Proteins; C412403:DIABLO protein, human; D047908:Intracellular Signaling Peptides and Proteins; D024101:Mitochondrial Proteins; D003841:Deoxycytidine; D045304:Cytochromes c; D053148:Caspase 3; D000093542:Gemcitabine
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