title: 4E-binding proteins, the suppressors of eukaryotic initiation factor 4E, are down-regulated in cells with acquired or intrinsic resistance to rapamycin.
abstract: To determine whether inhibition of either the ribosomal p70 S6 kinase or eukaryotic initiation factor (eIF) 4E pathways downstream of the mammalian target of rapamycin, mTOR, contributes to rapamycin-induced growth arrest, clones of Rh30 rhabdomyosarcoma cells were selected for rapamycin resistance. Expression of c-Myc and anchorage-independent growth were enhanced in resistant cells. Resistance was unstable in each of three clones characterized. In resistant cells, as compared with parental cells, approximately 10-fold less 4E-binding protein (4E-BP) was bound to eIF4E, and total cellular 4E-BP was markedly reduced. Levels of eIF4E were unchanged. Steady-state levels of 4E-BP transcript remained unaltered, but the rate of 4E-BP synthesis was reduced in resistant cells. In cells that reverted to rapamycin sensitivity, levels of total 4E-BP returned to those of parental cells. Compared with parental cells, resistant clones had either similar or lower levels and activity of ribosomal p70 S6 kinase, but c-Myc levels were elevated in both resistant and revertant clones. Several colon carcinoma cell lines with intrinsic rapamycin resistance were found to have low 4E-BP:eIF4E ratios. In stable clones of HCT8 carcinoma engineered to overexpress 4E-BP, rapamycin sensitivity increased markedly (>1000-fold) as 4E-BP expression increased. These results suggest that the 4E-BP:eIF4E ratio is an important determinant of rapamycin resistance and controls certain aspects of the malignant phenotype.
authors.affiliation: Department of Molecular Pharmacology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105-2794, USA.
authors.forename: Michael B
authors.initials: MB
authors.lastname: Dilling
authors.forename: Glen S
authors.initials: GS
authors.lastname: Germain
authors.forename: Lorina
authors.initials: L
authors.lastname: Dudkin
authors.forename: Arun L
authors.initials: AL
authors.lastname: Jayaraman
authors.forename: Xiongwen
authors.initials: X
authors.lastname: Zhang
authors.forename: Franklin C
authors.initials: FC
authors.lastname: Harwood
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Houghton
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000362:Agar / Q000494:pharmacology; D000903:Antibiotics, Antineoplastic / Q000494:pharmacology; D015152:Blotting, Northern; D015153:Blotting, Western; D002352:Carrier Proteins / Q000378:metabolism*; D018797:Cell Cycle Proteins; D002455:Cell Division; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation; D019008:Drug Resistance, Neoplasm; D039642:Eukaryotic Initiation Factors; D006801:Humans; D020128:Inhibitory Concentration 50; D009154:Mutation; D010641:Phenotype; D010750:Phosphoproteins / Q000378:metabolism*; D011485:Protein Binding; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology*; D013997:Time Factors; D014162:Transfection; D014407:Tumor Cells, Cultured
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D000903:Antibiotics, Antineoplastic; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C087000:EIF4EBP1 protein, human; C104151:EIF4EBP2 protein, human; D039642:Eukaryotic Initiation Factors; D010750:Phosphoproteins; D016271:Proto-Oncogene Proteins c-myc; D012333:RNA, Messenger; D000362:Agar; D019893:Ribosomal Protein S6 Kinases; D020123:Sirolimus