Multidrug resistance (MDR1) P-glycoprotein enhances esterification of plasma membrane cholesterol

Class I P-glycoproteins (Pgp) confer multidrug resistance in tumors, but th e physiologic function of Pgp in normal tissues remains uncertain. In cells derived from tissues that normally express Pgp, recent data suggest a poss ible role for Pgp in cholesterol trafficking from the plasma membrane to th e endoplasmic reticulum, We investigated the esterification of plasma membr ane cholesterol under basal conditions and in response to sphingomyelinase treatment in transfected and drug-selected cell lines expressing differing amounts of functional class I Pgp. Compared with parental NIH 3T3 fibroblas ts, cells transfected with human multidrug resistance (MDR1) Pgp esterified more cholesterol both without and with sphingomyelinase. Esterification al so was greater in drug-selected Dox 6 myeloma cells than parental 8226 cell s, which express low and non-immunodetectable amounts of Pgp, respectively. However, no differences in total plasma membrane cholesterol were detected . Transfection of fibroblasts with the multidrug resistance-associated prot ein (MRP) did not alter esterification, showing that cholesterol traffickin g was not generally affected by ATP-binding cassette transporters. Steroida l (progesterone, dehydroepiandrosterone) and non-steroidal antagonists (ver apamil, PSC 833, LY335979, and GF120918) were evaluated for effects on both cholesterol trafficking and the net content of Tc-99m-Sestamibi, a reporte r of drug transport activity mediated by Pgp, In Pgp-expressing cells treat ed with nonselective and selective inhibitors, both the kinetics and effica cy of inhibition of cholesterol esterification differed from the antagonism of drug transport mediated by Pgp, Thus, although the data show that great er expression of class I Pgp within a given cell type is associated with en hanced esterification of plasma membrane cholesterol in support of a physio logic function for Pgp in facilitating cholesterol trafficking, the molecul ar mechanism is dissociated from the conventional drug transport activity o f Pgp.