Targeted inhibition of mammalian target of rapamycin signaling inhibits tumorigenesis of colorectal cancer.
OBJECTIVE
The mammalian target of rapamycin (mTOR) kinase acts downstream of phosphoinositide 3-kinase/Akt to regulate cellular growth, metabolism, and cytoskeleton. Because approximately 60% of sporadic colorectal cancers (CRC) exhibit high levels of activated Akt, we determined whether downstream mTOR signaling pathway components are overexpressed and activated in CRCs.


METHODS
HCT116, KM20, Caco-2, and SW480 human CRC cells were used to determine the effects of pharmacologic (using rapamycin) or genetic (using RNAi) blockade of mTOR signaling on cell proliferation, apoptosis, cell cycle progression, and subcutaneous growth in vivo.


RESULTS
We show that the mTOR complex proteins mTOR, Raptor, and Rictor are overexpressed in CRC. Treatment with rapamycin significantly decreased proliferation of certain CRC cell lines (rapamycin sensitive), whereas other cell lines were resistant to its effects (rapamycin resistant). Transient siRNA-mediated knockdown of the mTORC2 protein, Rictor, significantly decreased proliferation of both rapamycin-sensitive and rapamycin-resistant CRC cells. Stable shRNA-mediated knockdown of both mTORC1 and mTORC2 decreased proliferation, increased apoptosis, and attenuated cell cycle progression in rapamycin-sensitive CRCs. Moreover, stable knockdown of both mTORC1 and mTORC2 decreased proliferation and attenuated cell cycle progression, whereas only mTORC2 knockdown increased apoptosis in rapamycin-resistant CRCs. Finally, knockdown of both mTORC1 and mTORC2 inhibited growth of rapamycin-sensitive and rapamycin-resistant CRCs in vivo when implanted as tumor xenografts.


CONCLUSIONS
Targeted inhibition of the mTORC2 protein, Rictor, leads to growth inhibition and induces apoptosis in both rapamycin-sensitive and rapamycin-resistant CRCs, suggesting that selective targeting of mTORC2 may represent a novel therapeutic strategy for treatment of CRC.
Gulhati
Pat
P
Department of Surgery, University of Kentucky, Lexington, Kentucky 40536-0093, USA.
Cai
Qingsong
Q
Li
Jing
J
Liu
Jianyu
J
Rychahou
Piotr G
PG
Qiu
Suimin
S
Lee
Eun Y
EY
Silva
Scott R
SR
Bowen
Kanika A
KA
Gao
Tianyan
T
Evers
B Mark
BM
D000818:Animals; D017209:Apoptosis; D018938:Caco-2 Cells; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000502:physiology; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D015179:Colorectal Neoplasms / Q000188:drug therapy* / Q000378:metabolism*; D006801:Humans; D007150:Immunohistochemistry / Q000379:methods; D047908:Intracellular Signaling Peptides and Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008297:Male; D051379:Mice; D008819:Mice, Nude; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D000076226:Rapamycin-Insensitive Companion of mTOR Protein; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases
D002352:Carrier Proteins; D047908:Intracellular Signaling Peptides and Proteins; C492571:RICTOR protein, human; D000076226:Rapamycin-Insensitive Companion of mTOR Protein; C546842:MTOR protein, human; C546843:mTOR protein, mouse; C494918:AKT1 protein, human; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases