title: Revealing Different Roles of the mTOR-Targets S6K1 and S6K2 in Breast Cancer by Expression Profiling and Structural Analysis.
abstract: BACKGROUND
  The AKT/mTORC1/S6K pathway is frequently overstimulated in breast cancer, constituting a promising therapeutic target. The benefit from mTOR inhibitors varies, likely as a consequence of tumour heterogeneity, and upregulation of several compensatory feed-back mechanisms. The mTORC1 downstream effectors S6K1, S6K2, and 4EBP1 are amplified and overexpressed in breast cancer, associated with a poor outcome and divergent endocrine treatment benefit. S6K1 and S6K2 share high sequence homology, but evidence of partly distinct biological functions is emerging. The aim of this work was to explore possible different roles and treatment target potentials of S6K1 and S6K2 in breast cancer.
  
  
  METHODS
  Whole-genome expression profiles were compared for breast tumours expressing high levels of S6K1, S6K2 or 4EBP1, using public datasets, as well as after in vitro siRNA downregulation of S6K1 and/or S6K2 in ZR751 breast cancer cells. In silico homology modelling of the S6K2 kinase domain was used to evaluate its possible structural divergences to S6K1.
  
  
  RESULTS
  Genome expression profiles were highly different in S6K1 and S6K2 high tumours, whereas S6K2 and 4EBP1 profiles showed significant overlaps, both correlated to genes involved in cell cycle progression, among these the master regulator E2F1. S6K2 and 4EBP1 were inversely associated with IGF1 levels, and their prognostic value was shown to be restricted to tumours positive for IGFR and/or HER2. In vitro, S6K1 and S6K2 silencing resulted in upregulation of genes in the mTORC1 and mTORC2 complexes. Isoform-specific silencing also showed distinct patterns, e.g. S6K2 downregulation lead to upregulation of several cell cycle associated genes. Structural analyses of the S6K2 kinase domain showed unique structure patterns, deviating from those of S6K1, facilitating the development of isoform-specific inhibitors. Our data support emerging proposals of distinct biological features of S6K1 and S6K2, suggesting their importance as separate oncogenes and clinical markers, where specific targeting in different breast cancer subtypes could facilitate further individualised therapies.
authors.affiliation: Department of Clinical and Experimental Medicine, and Department of Oncology, Linköping University, SE-58185, Linköping, Sweden.
authors.forename: Elin
authors.initials: E
authors.lastname: Karlsson
authors.affiliation: Department of Clinical and Experimental Medicine, and Department of Oncology, Linköping University, SE-58185, Linköping, Sweden.|Division of Chemistry, Department of Physics, Chemistry and Biology, Linköping University, SE-58183, Linköping, Sweden.
authors.forename: Ivana
authors.initials: I
authors.lastname: Magić
authors.affiliation: Department of Clinical and Experimental Medicine, and Department of Oncology, Linköping University, SE-58185, Linköping, Sweden.
authors.forename: Josefine
authors.initials: J
authors.lastname: Bostner
authors.affiliation: Division of Chemistry, Department of Physics, Chemistry and Biology, Linköping University, SE-58183, Linköping, Sweden.
authors.forename: Christine
authors.initials: C
authors.lastname: Dyrager
authors.affiliation: Division of Bioinformatics and SeRC (Swedish e-Science Research Centre), Department of Physics, Chemistry and Biology, Linköping University, SE-581 83, Linköping, Sweden.
authors.forename: Fredrik
authors.initials: F
authors.lastname: Lysholm
authors.affiliation: Department of Clinical and Experimental Medicine, and Department of Oncology, Linköping University, SE-58185, Linköping, Sweden.
authors.forename: Anna-Lotta
authors.initials: AL
authors.lastname: Hallbeck
authors.affiliation: Department of Clinical and Experimental Medicine, and Department of Oncology, Linköping University, SE-58185, Linköping, Sweden.
authors.forename: Olle
authors.initials: O
authors.lastname: Stål
authors.affiliation: Division of Chemistry, Department of Physics, Chemistry and Biology, Linköping University, SE-58183, Linköping, Sweden.
authors.forename: Patrik
authors.initials: P
authors.lastname: Lundström
mesh_terms: D014408:Biomarkers, Tumor / Q000235:genetics*; D001943:Breast Neoplasms / Q000235:genetics* / Q000401:mortality / Q000473:pathology; D005260:Female; D020869:Gene Expression Profiling; D059014:High-Throughput Nucleotide Sequencing; D006801:Humans; D008958:Models, Molecular; D011487:Protein Conformation; D012333:RNA, Messenger / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000737:chemistry / Q000235:genetics*; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000737:chemistry / Q000235:genetics*; D015996:Survival Rate; D058570:TOR Serine-Threonine Kinases / Q000235:genetics*; D014407:Tumor Cells, Cultured
chemical_list: D014408:Biomarkers, Tumor; D012333:RNA, Messenger; C546842:MTOR protein, human; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D038744:Ribosomal Protein S6 Kinases, 90-kDa; D058570:TOR Serine-Threonine Kinases; C448901:ribosomal protein S6 kinase, 70kD, polypeptide 1; C069330:ribosomal protein S6 kinase, 90kDa, polypeptide 3