title: Studies on the mechanism of resistance to rapamycin in human cancer cells.
abstract: Rapamycin is a potent cytostatic agent that arrests cells in the G1 phase of the cell cycle. The relationships between cellular sensitivity to rapamycin, drug accumulation, expression of mammalian target of rapamycin (mTOR), and inhibition of growth factor activation of ribosomal p70S6 kinase (p70(S6k)) and dephosphorylation of pH acid stable protein I (eukaryotic initiation factor 4E binding protein) were examined. We show that some cell lines derived from childhood tumors are highly sensitive to growth inhibition by rapamycin, whereas others have high intrinsic resistance (>1000-fold). Accumulation and retention of [14C]rapamycin were similar in sensitive and resistant cells, with all cells examined demonstrating a stable tight binding component. Western analysis showed levels of mTOR were similar in each cell line (<2-fold variation). The activity of p70(S6k), activated downstream of mTOR, was similar in four cell lines (range, 11.75-41. 8 pmol/2 x 10(6) cells/30 min), but activity was equally inhibited in cells that were highly resistant to rapamycin-induced growth arrest. Rapamycin equally inhibited serum-induced phosphorylation of pH acid stable protein I in Rh1 (intrinsically resistant) and sensitive Rh30 cells. In serum-fasted Rh30 and Rh1 cells, the addition of serum rapidly induced c-MYC (protein) levels. Rapamycin blocked induction in Rh30 cells but not in Rh1 cells. Serum-fasted Rh30/rapa10K cells, selected for high level acquired resistance to rapamycin, showed >/=10-fold increased c-MYC compared with Rh30. These results suggest that the ability of rapamycin to inhibit c-MYC induction correlates with intrinsic sensitivity, whereas failure of rapamycin to inhibit induction or overexpression of c-MYC correlates with intrinsic and acquired resistance, respectively.
authors.affiliation: Department of Molecular Pharmacology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, USA.
authors.forename: H
authors.initials: H
authors.lastname: Hosoi
authors.forename: M B
authors.initials: MB
authors.lastname: Dilling
authors.forename: L N
authors.initials: LN
authors.lastname: Liu
authors.forename: M K
authors.initials: MK
authors.lastname: Danks
authors.forename: T
authors.initials: T
authors.lastname: Shikata
authors.forename: A
authors.initials: A
authors.lastname: Sekulic
authors.forename: R T
authors.initials: RT
authors.lastname: Abraham
authors.forename: J C
authors.initials: JC
authors.lastname: Lawrence
authors.forename: P J
authors.initials: PJ
authors.lastname: Houghton
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000903:Antibiotics, Antineoplastic / Q000493:pharmacokinetics / Q000494:pharmacology*; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D002648:Child; D019008:Drug Resistance, Neoplasm; D004354:Drug Screening Assays, Antitumor; D005260:Female; D005909:Glioblastoma / Q000188:drug therapy* / Q000201:enzymology / Q000378:metabolism; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D009363:Neoplasm Proteins / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000378:metabolism; D011494:Protein Kinases; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis; D012208:Rhabdomyosarcoma / Q000188:drug therapy* / Q000201:enzymology / Q000378:metabolism; D019893:Ribosomal Protein S6 Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D020123:Sirolimus / Q000493:pharmacokinetics / Q000494:pharmacology*; D058570:TOR Serine-Threonine Kinases; 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; D009363:Neoplasm Proteins; D010750:Phosphoproteins; D016271:Proto-Oncogene Proteins c-myc; D007334:Insulin-Like Growth Factor I; D011494:Protein Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); C546842:MTOR protein, human; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases; D020123:Sirolimus