Loss of tuberous sclerosis complex-2 function and activation of mammalian target of rapamycin signaling in endometrial carcinoma.
OBJECTIVE
The involvement of phosphatase and tensin homologue deleted on chromosome ten (PTEN) in endometrial carcinoma has implicated phosphatidylinositol 3-kinase signaling and mammalian target of rapamycin (mTOR) activation in this disease. Understanding the extent of mTOR involvement and the mechanism responsible for activation is important, as mTOR inhibitors are currently being evaluated in clinical trials for endometrial carcinoma. Although tuberous sclerosis complex 2 (TSC2) is the "gatekeeper" for mTOR activation, little is known about defects in the TSC2 tumor suppressor or signaling pathways that regulate TSC2, such as LKB1/AMP-activated protein kinase, in the development of endometrial carcinoma.


METHODS
We determined the frequency of mTOR activation in endometrial carcinoma (primary tumors and cell lines) and investigated PTEN, LKB1, and TSC2 defects as underlying cause(s) of mTOR activation, and determined the ability of rapamycin to reverse these signaling defects in endometrial carcinoma cells.


RESULTS
Activation of mTOR was a consistent feature in endometrial carcinomas and cell lines. In addition to PTEN, loss of TSC2 and LKB1 expression occurred in a significant fraction of primary tumors (13% and 21%, respectively). In tumors that retained TSC2 expression, phosphorylation of tuberin at S939 was observed with a high frequency, indicating that mTOR repression by TSC2 had been relieved via AKT phosphorylation of this tumor suppressor. In PTEN-null and LKB1-null endometrial carcinoma cell lines with functional inactivation of TSC2, phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY294002 were able to inhibit AKT and mTOR signaling and reverse TSC2 phosphorylation. In contrast, although rapamycin inhibited mTOR signaling, it did not relieve phosphorylation of TSC2 at S939.


CONCLUSIONS
Inactivation of TSC2 via loss of expression or phosphorylation occurred frequently in endometrial carcinoma to activate mTOR signaling. High-frequency mTOR activation supports mTOR as a rational therapeutic target for endometrial carcinoma. However, whereas rapamycin and its analogues may be efficacious at inhibiting mTOR activity, these drugs do not reverse the functional inactivation of TSC2 that occurs in these tumors.
Lu
Karen H
KH
Department of Gynecologic Oncology, The University of Texas M. D. Anderson Cancer Center, Smithville, Texas 78957, USA.
Wu
Weiguo
W
Dave
Bhuvanesh
B
Slomovitz
Brian M
BM
Burke
Thomas W
TW
Munsell
Mark F
MF
Broaddus
Russell R
RR
Walker
Cheryl Lyn
CL
D000091162:AMP-Activated Protein Kinase Kinases; D000730:Androstadienes / Q000494:pharmacology; D045744:Cell Line, Tumor; D002867:Chromones / Q000494:pharmacology; D016889:Endometrial Neoplasms / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006801:Humans; D009025:Morpholines / Q000494:pharmacology; D051059:PTEN Phosphohydrolase / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D000081082:Phosphoinositide-3 Kinase Inhibitors; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D011494:Protein Kinases / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D014402:Tuberous Sclerosis; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins / Q000378:metabolism*; D000077191:Wortmannin
D000091162:AMP-Activated Protein Kinase Kinases; D000730:Androstadienes; D002867:Chromones; D004791:Enzyme Inhibitors; D009025:Morpholines; D051059:PTEN Phosphohydrolase; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D020123:Sirolimus; D058570:TOR Serine-Threonine Kinases; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins; D000077191:Wortmannin; C000624653:TSC2 protein, human; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; C546842:MTOR protein, human; C109790:STK11 protein, human; C494929:PTEN protein, human