title: Calcium and P-glycoprotein independent synergism between schweinfurthins and verapamil.
abstract: Schweinfurthins are intriguing natural products with anti-cancer activities and as yet incompletely understood mechanisms of action. We investigated whether inhibitors of P-glycoprotein (Pgp), in a manner analogous to other natural products, might enhance schweinfurthins' growth inhibitory actions by increasing intracellular schweinfurthin levels. Both the schweinfurthin-sensitive glioblastoma multiforme cell line SF-295 and relatively insensitive lung carcinoma cell line A549 were treated with 2 schweinfurthin analogs: 3-deoxyschweinfurthin B-p-nitro bis-stilbene (3dSB-PNBS) and 5'-methylschweinfurthin G (methyl-G). There was a synergistic enhancement of growth inhibition with the combination of the Pgp inhibitor verapamil and both analogs in SF-295 cells. Methyl-G, verapamil, and the combination did not result in alterations to intracellular calcium concentration. Verapamil increased the intracellular concentration of 3dSB-PNBS in both SF-295 and A549 cells in a Pgp-independent manner. Methyl-G, verapamil, and the combination do not result in increased ER stress. Methyl-G increased the intracellular concentration of a known Pgp substrate, Rhodamine 123 in SF-295 cells. Reduction of cellular cholesterol leads to the accumulation of Pgp substrates, as Pgp requires cholesterol for proper function. Since 3dSB enhances lovastatin-induced upregulation of the cholesterol efflux pump ABCA1, it is intriguing that co-treatment with cholesterol rescued the methyl-G-induced increase in Rhodamine 123 intracellular concentration. These studies support the hypothesis that verapamil potentiates the schweinfurthin growth inhibitory effect by increasing its intracellular concentration.
authors.affiliation: a Department of Pharmacology ; University of Iowa ; Iowa City , IA USA.
authors.forename: Ryan M
authors.initials: RM
authors.lastname: Sheehy
authors.forename: Craig H
authors.initials: CH
authors.lastname: Kuder
authors.forename: Zoe
authors.initials: Z
authors.lastname: Bachman
authors.forename: Raymond J
authors.initials: RJ
authors.lastname: Hohl
mesh_terms: D064286:ATP Binding Cassette Transporter 1 / Q000378:metabolism; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000737:chemistry / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D045744:Cell Line, Tumor / Q000187:drug effects; D002784:Cholesterol / Q000378:metabolism; D004354:Drug Screening Assays, Antitumor; D004357:Drug Synergism; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins / Q000378:metabolism; D006801:Humans; D013267:Stilbenes / Q000737:chemistry / Q000494:pharmacology*; D014700:Verapamil / Q000008:administration & dosage / Q000494:pharmacology
chemical_list: C000607365:3-deoxyschweinfurthin B-p-nitro bis-stilbene; C000607366:5'-methylschweinfurthin G; C577836:ABCA1 protein, human; D064286:ATP Binding Cassette Transporter 1; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D013267:Stilbenes; C534360:schweinfurthin G; D002784:Cholesterol; D014700:Verapamil; D002118:Calcium
keywords: 3dSB, 3-deoxyschweinfurthin B; 3dSB-PNBS, 3-deoxyschweinfurthin B p-nitro bis-stilbene; BAPTA-AM, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; CI, combination index; DMP-PNBS, 3,4-dimethoxypheny-p-nitro bis-stilbene; ER, endoplasmic reticulum; GBM, Glioblastoma Multiforme; MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide; Methyl-G, 5'-methoxyschweinfurthin G; NCI, National Cancer Institute; PARP, poly-ADP-ribose polymerase; Pgp, P-glycoprotein drug efflux pump; cholesterol metabolism; drug efflux pump; glioblastoma multiforme; oxysterol binding protein; p-glycoprotein; schweinfurthin; verapamil