title: Alterations of ceramide/sphingosine 1-phosphate rheostat involved in the regulation of resistance to imatinib-induced apoptosis in K562 human chronic myeloid leukemia cells.
abstract: In this study, mechanisms of resistance to imatinib-induced apoptosis in human K562 cells were examined. Continuous exposure to stepwise increasing concentrations of imatinib resulted in the selection of K562/IMA-0.2 and -1 cells, which expressed approximately 2.3- and 19-fold resistance, respectively. Measurement of endogenous ceramides by high performance liquid chromatography/mass spectroscopy showed that treatment with imatinib increased the generation of ceramide, mainly C18-ceramide, which is generated by the human longevity assurance gene 1 (hLASS1), in sensitive, but not in resistant cells. Inhibition of hLASS1 by small interfering RNA partially prevented imatinib-induced cell death in sensitive cells. In reciprocal experiments, overexpression of hLASS1, and not hLASS6, in drug-resistant cells caused a marked increase in imatinib-induced C18-ceramide generation, and enhanced apoptosis. Interestingly, there were no defects in the levels of mRNA and enzyme activity levels of hLASS1 for ceramide generation in K562/IMA-1 cells. However, expression levels of sphingosine kinase-1 (SK1) and generation of sphingosine 1-phosphate (S1P) were increased significantly in K562/IMA-1 cells, channeling sphingoid bases to the sphingosine kinase pathway. The partial inhibition of SK1 expression by small interference RNA modulated S1P levels and increased sensitivity to imatinib-induced apoptosis in resistant cells. On the other hand, forced expression of SK1 in K562 cells increased the ratio between total S1P/C18-ceramide levels approximately 6-fold and prevented apoptosis significantly in response to imatinib. Additional data indicated a role for SK1/S1P signaling in the up-regulation of the Bcr-Abl expression at the post-transcriptional level, which suggested a possible mechanism for resistance to imatinib-mediated apoptosis. In conclusion, these data suggest a role for endogenous C18-ceramide synthesis mainly via hLASS1 in imatinib-induced apoptosis in sensitive cells, whereas in resistant cells, alterations of the balance between the levels of ceramide and S1P by overexpression of SK1 result in resistance to imatinib-induced apoptosis.
authors.affiliation: Department of Biochemistry and Molecular Biology, and Hollings Cancer Center, Ralph H. Johnson Veterans Administration Hospital, and Department of Medicine, Medical University of South Carolina, Charleston 29425, USA.
authors.forename: Yusuf
authors.initials: Y
authors.lastname: Baran
authors.forename: Arelis
authors.initials: A
authors.lastname: Salas
authors.forename: Can E
authors.initials: CE
authors.lastname: Senkal
authors.forename: Ufuk
authors.initials: U
authors.lastname: Gunduz
authors.forename: Jacek
authors.initials: J
authors.lastname: Bielawski
authors.forename: Lina M
authors.initials: LM
authors.lastname: Obeid
authors.forename: Besim
authors.initials: B
authors.lastname: Ogretmen
mesh_terms: D017209:Apoptosis / Q000187:drug effects*; D001549:Benzamides; D002518:Ceramides / Q000378:metabolism*; D019008:Drug Resistance, Neoplasm / Q000187:drug effects*; D016044:Fusion Proteins, bcr-abl / Q000378:metabolism; D006801:Humans; D000068877:Imatinib Mesylate; D020014:K562 Cells; D015464:Leukemia, Myelogenous, Chronic, BCR-ABL Positive / Q000235:genetics / Q000378:metabolism* / Q000473:pathology*; D008246:Lysophospholipids / Q000378:metabolism*; D053078:Membrane Potential, Mitochondrial / Q000187:drug effects; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D010879:Piperazines / Q000494:pharmacology*; D011743:Pyrimidines / Q000494:pharmacology*; D034741:RNA, Small Interfering / Q000235:genetics; D013110:Sphingosine / Q000031:analogs & derivatives* / Q000378:metabolism; D051049:Sphingosine N-Acyltransferase; D013997:Time Factors; D015854:Up-Regulation
chemical_list: D001549:Benzamides; D002518:Ceramides; D008246:Lysophospholipids; D008565:Membrane Proteins; D010879:Piperazines; D011743:Pyrimidines; D034741:RNA, Small Interfering; C060506:sphingosine 1-phosphate; D000068877:Imatinib Mesylate; C493790:CERS1 protein, human; D051049:Sphingosine N-Acyltransferase; D016044:Fusion Proteins, bcr-abl; D013110:Sphingosine