Reversal of multidrug resistance by calcium channel blocker SR33557 without photoaffinity labeling of P-glycoprotein.
The altered pharmacology of drugs in multidrug-resistant cells (decreased accumulation and retention) appears to be mediated by a high molecular weight integral membrane protein, called P-glycogprotein (P-gp). Agents known to reverse this pleiotropic drug resistance (chemosensitizers) have been shown to interact with P-gp; and as such, the inhibition of photoaffinity labeling by P-gp probes (such as [3H]azidopine) has been proposed as a basis for mass screening of chemosensitizers. In this study, we provide direct evidence that a novel calcium channel blocker (SR33557), which was 4.5 times more potent in sensitizing P388/ADR cells to doxorubicin as compared to verapamil (while inducing a similar increase in uptake and decrease in efflux of [14C]doxorubicin, did not compete for the [3H]azidopine-binding site on P-gp, whereas verapamil did. Moreover, SR33557, which is inherently photoactivable, did not photolabel P-gp, but a 65-kDa protein did appear to be an acceptor; and this binding was displaced by diltiazem and nifedipine, but not by verapamil. Finally, the implication for the participation of a sphingomyelin/sphingosine cycle (as a potential lipid second messenger system) in the chemosensitization of P388/ADR cells was investigated. 30 microM SR33557 induced a 72% inhibition in acid lysosomal sphingomyelinase activity, a 5-fold increase in sphingosine levels, and a 75% inhibition in intracellular protein kinase C activity. Although no direct link is established between these observations and P-gp activity, further studies on a possible sphingosine-mediated regulation of P-gp may yield information on the involvement of this second messenger system in the action of SR33557.
Jaffrézou
J P
JP
Laboratoire de Pharmacologie et de Toxicologie Fondamentales, Centre National de la Recherche Scientifique 8221, Toulouse, France.
Herbert
J M
JM
Levade
T
T
Gau
M N
MN
Chatelain
P
P
Laurent
G
G
D000818:Animals; D002121:Calcium Channel Blockers / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D004351:Drug Resistance; D007212:Indolizines / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D007942:Leukemia, Experimental / Q000201:enzymology / Q000473:pathology; D051379:Mice; D010627:Phenethylamines / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D011493:Protein Kinase C / Q000378:metabolism; D015290:Second Messenger Systems; D013110:Sphingosine / Q000032:analysis; D013329:Structure-Activity Relationship; D014407:Tumor Cells, Cultured; D014700:Verapamil / Q000494:pharmacology
D002121:Calcium Channel Blockers; D007212:Indolizines; D010627:Phenethylamines; D014700:Verapamil; D011493:Protein Kinase C; C059901:fantofarone; D013110:Sphingosine