title: Partial inhibition of multidrug resistance by safingol is independent of modulation of P-glycoprotein substrate activities and correlated with inhibition of protein kinase C.
abstract: Safingol is a lysosphingolipid protein kinase C (PKC) inhibitor that competitively interacts at the regulatory phorbol binding domain of PKC. We investigated the effects of safingol on antineoplastic drug sensitivity and PKC activity of MCF-7 tumor cell lines. Safingol treatment of 32P-labeled MCF-7 WT and MCF-7 DOXR cells inhibited phosphorylation of the myristoylated alanine-rich protein kinase C substrate in both cell lines, suggesting inhibition of cellular PKC. However, only in MCF-7 DOXR cells did safingol treatment increase accumulation of [3H]vinblastine and enhance toxicity of Vinca alkaloids and anthracyclines. Drug accumulation changes in MCF-7 DOXR cells treated with safingol were accompanied by inhibition of basal and phorbol 12,13-dibutyrate-stimulated phosphorylation of P-glycoprotein (P-gp). Expression of P-gp and levels of mdr1 message in MCF-7 DOXR cells were not altered by safingol treatment alone or in combination with vinblastine. Treatment of MCF-7 DOXR cell membranes with safingol did not inhibit [3H]vinblastine binding or [3H]azidopine photoaffinity labeling of P-gp. Furthermore, safingol did not stimulate P-gp ATPase activity in membranes prepared from MCF-7 DOXR cells. We conclude that enhanced drug accumulation and sensitivity in MCF-7 DOXR cells treated with safingol are correlated with inhibition of PKC rather than competitive interference with P-gp drug binding through direct interaction with P-glycoprotein.
authors.affiliation: Division of Hematology-Oncology, Duke University Medical Center, Durham, North Carolina 27705, USA.
authors.forename: C W
authors.initials: CW
authors.lastname: Sachs
authors.forename: A R
authors.initials: AR
authors.lastname: Safa
authors.forename: S D
authors.initials: SD
authors.lastname: Harrison
authors.forename: R L
authors.initials: RL
authors.lastname: Fine
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism*; D000345:Affinity Labels; D000818:Animals; D000970:Antineoplastic Agents / Q000633:toxicity*; D001921:Brain / Q000201:enzymology; D001943:Breast Neoplasms; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002470:Cell Survival / Q000187:drug effects; D003630:Daunorubicin / Q000633:toxicity; D004317:Doxorubicin / Q000633:toxicity; D018432:Drug Resistance, Multiple / Q000502:physiology*; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005260:Female; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D000076250:Myristoylated Alanine-Rich C Kinase Substrate; D015240:Phorbol 12,13-Dibutyrate / Q000378:metabolism / Q000494:pharmacology; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011506:Proteins / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D013110:Sphingosine / Q000031:analogs & derivatives* / Q000494:pharmacology; D013379:Substrate Specificity; D014407:Tumor Cells, Cultured; D014700:Verapamil / Q000494:pharmacology; D014747:Vinblastine / Q000378:metabolism / Q000633:toxicity
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000345:Affinity Labels; D000970:Antineoplastic Agents; D004791:Enzyme Inhibitors; D047908:Intracellular Signaling Peptides and Proteins; C000619710:MARCKS protein, human; C000619712:Marcks protein, rat; D008565:Membrane Proteins; D011506:Proteins; D011994:Recombinant Proteins; D000076250:Myristoylated Alanine-Rich C Kinase Substrate; D015240:Phorbol 12,13-Dibutyrate; D014747:Vinblastine; D004317:Doxorubicin; D014700:Verapamil; D011493:Protein Kinase C; D013110:Sphingosine; C005682:safingol; D003630:Daunorubicin