Inhibition of mammalian target of rapamycin is required for optimal antitumor effect of HER2 inhibitors against HER2-overexpressing cancer cells.
OBJECTIVE
A significant fraction of HER2-overexpressing breast cancers exhibit resistance to the HER2 antibody trastuzumab. Hyperactivity of the phosphatidylinositol 3-kinase (PI3K)/AKT pathway confers trastuzumab resistance, and mammalian target of rapamycin (mTOR) is a major downstream effector of PI3K/AKT. Therefore, we examined whether mTOR inhibitors synergize with trastuzumab.


METHODS
Immunocompetent mice bearing HER2(+) mammary tumors were treated with trastuzumab, the mTOR inhibitor rapamycin, or the combination. Mice were imaged for tumor cell death using an optical Annexin-V probe and with [(18)F]FDG positron emission tomography. The signaling and growth effects of the mTOR inhibitor RAD001 on HER2(+) cells treated with trastuzumab or lapatinib were evaluated.


RESULTS
Treatment of mice with trastuzumab plus rapamycin was more effective than single-agent treatments, inducing complete regression of 26 of 26 tumors. The combination induced tumor cell death (Annexin-V binding) and inhibited FDG uptake. Rapamycin inhibited mTOR and tumor cell proliferation as determined by phosphorylated S6 and Ki-67 immunohistochemistry, respectively. In culture, the combination of RAD001 plus trastuzumab inhibited cell growth more effectively than either drug alone. Trastuzumab partially decreased PI3K but not mTOR activity. Knockdown of TSC2 resulted in HER2-independent activation of mTOR and dampened the response to trastuzumab and lapatinib. Treatment with the HER2 inhibitor lapatinib decreased phosphorylated S6 and growth in TSC2-expressing cells but not in TSC2-knockdown cells.


CONCLUSIONS
Inhibition of PI3K and mTOR are required for the growth-inhibitory effect of HER2 antagonists. These findings collectively support the combined use of trastuzumab and mTOR inhibitors for the treatment of HER2(+) breast cancer.
Miller
Todd W
TW
Department of Medicine, Vanderbilt University, Nashville, Tennessee, USA.
Forbes
James T
JT
Shah
Chirayu
C
Wyatt
Shelby K
SK
Manning
H Charles
HC
Olivares
Maria G
MG
Sanchez
Violeta
V
Dugger
Teresa C
TC
de Matos Granja
Nara
N
Narasanna
Archana
A
Cook
Rebecca S
RS
Kennedy
J Phillip
JP
Lindsley
Craig W
CW
Arteaga
Carlos L
CL
D000818:Animals; D000911:Antibodies, Monoclonal / Q000494:pharmacology; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents / Q000494:pharmacology*; D001943:Breast Neoplasms / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D019008:Drug Resistance, Neoplasm; D005260:Female; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D051379:Mice; D016513:Mice, SCID; D009369:Neoplasms / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000378:metabolism*; D058570:TOR Serine-Threonine Kinases; D000068878:Trastuzumab
D000911:Antibodies, Monoclonal; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents; D047908:Intracellular Signaling Peptides and Proteins; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D018719:Erb-b2 Receptor Tyrosine Kinases; D058570:TOR Serine-Threonine Kinases; D000068878:Trastuzumab; C546842:MTOR protein, human; C546843:mTOR protein, mouse; C508053:ERBB2 protein, human