title: Trastuzumab stimulation of ribosomal protein S6 kinase 1 (S6K1) predicts de novo trastuzumab resistance.
abstract: Breast cancer is a complex disease with at least five different molecular subtypes identified. The breast tumor molecular subtypes guide stratification of patients for specific targeted therapy regimens and each subtype is associated with significantly different patient outcomes. For example, patients with the HER2 positive molecular subtype benefit from the HER2 targeted therapy trastuzumab. Unfortunately, women with the HER2 positive molecular subtype have the worst overall prognosis and nearly 70% of women with HER2 positive breast cancer exhibit de novo or acquired resistance to trastuzumab. Identification of tumor markers predicting trastuzumab response can be used to further stratify patients for life-saving personalized therapeutic options. The aim of this study was to identify clinically useful tumor markers predicting de novo tumor cell resistance to trastuzumab treatment. To identify oncogenic signaling pathways activated in response to trastuzumab treatment, we performed a Human Phospho-Kinase Proteome Profiler Array analysis comparing trastuzumab sensitive MCF-7/HER2.2 and trastuzumab resistant MCF-7/HER2Δ16H cells following acute treatment with 20 μg/ml of trastuzumab for 2 h. We found that of the 43 phosphorylation activated human kinases represented on the array, S6K1 was the only kinase altered greater than 1.5-fold in response to trastuzumab treatment of the trastuzumab resistant MCF-7/HER2Δ16H cells. Trastuzumab activation of S6K1 was confirmed in the two trastuzumab resistant SUM190 and SUM225 cell lines. Significantly, trastuzumab failed to stimulate S6K1 activation in the trastuzumab sensitive MCF-7/HER2.2, BT474, and SKBR3 cell lines suggesting that trastuzumab activation of S6K1 is a tumor cell marker for trastuzumab resistance. Consistent with a role for mTORC1/S6K1 signaling promoting trastuzumab resistance, all cell lines were sensitive to S6K1 inactivation with significant growth inhibition following treatment with the mTORC1 inhibitor rapamycin. In conclusion, characterizing rapid trastuzumab induced molecular alterations resulted in the identification of activated S6K1 as an early breast tumor cell marker for trastuzumab resistance. Our results further suggest that trastuzumab resistant breast tumor cells are addicted to mTORC1/S6K1 oncogenic signaling and targeting mTORC1 with rapamycin reverses trastuzumab resistance.
authors.affiliation: Department of Cell and Molecular Biology, Tulane University, New Orleans, LA, 70118, USA.
authors.forename: Felicia C
authors.initials: FC
authors.lastname: Huynh
authors.affiliation: Department of Cell and Molecular Biology, Tulane University, New Orleans, LA, 70118, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Nguyen
authors.affiliation: Department of Cell and Molecular Biology, Tulane University, New Orleans, LA, 70118, USA. Electronic address: fjones3@tulane.edu.
authors.forename: Frank E
authors.initials: FE
authors.lastname: Jones
mesh_terms: D000970:Antineoplastic Agents / Q000627:therapeutic use*; D014408:Biomarkers, Tumor / Q000378:metabolism*; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism; D019008:Drug Resistance, Neoplasm / Q000187:drug effects / Q000235:genetics; D006801:Humans; D061986:MCF-7 Cells; D000076222:Mechanistic Target of Rapamycin Complex 1; D046912:Multiprotein Complexes / Q000037:antagonists & inhibitors / Q000378:metabolism; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism*; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D000068878:Trastuzumab / Q000627:therapeutic use*
chemical_list: D000970:Antineoplastic Agents; D014408:Biomarkers, Tumor; D046912:Multiprotein Complexes; D000076222:Mechanistic Target of Rapamycin Complex 1; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases; C448901:ribosomal protein S6 kinase, 70kD, polypeptide 1; D000068878:Trastuzumab; D020123:Sirolimus
keywords: Breast cancer; EGFR-family; Rapamycin; S6K1; Therapeutic resistance; Trastuzumab; mTORC1