Characterization of an HL-60 cell variant resistant to the antineoplastic ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine

A resistant cell line (HL-60R) was selected by incubating HL-60 cells with increasing concentrations of 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) and used to examine the mechanism of resistance to the antineoplastic ether-linked lipid. The HL-60R cells exhibited a >10-fold increase in resistance when measured by [H-3]- thymidine incorporation in comparison to the HL-60 cell line. ET-18-OCH3 binding occurred at 4 degrees C and was not saturable at the concentrations tested (1-100 mu M), indicating that the binding was receptor-independent. At 4 degrees C, association of ET-18-OCH3, was low for each cell line. At 37 degrees C, uptake in the HL-60 cells was approximately 5-fold greater in comparison to HL-GOR cells at each concentration tested. However, when the cellular content of ET-18-OCH3 was equal, both cell lines experienced similar declines in cell growth, Cellular incorporation of ether lipid was determined using serum-free media and in the presence of serum albumin or lipoproteins. Reduced uptake by the resistant cell line was observed only in the presence of albumin. A greater proportion of ether lipid could be removed from prelabeled HL-60R cells than from HL-60 cells, by an albumin wash procedure, indicating an increased rate of internalization and retention by the sensitive cell line. ET-18-OCH3 uptake in the HL-60 cell line was also more sensitive to treatment with endocytic (chloroquine, monensin) or metabolic (NaF, KCN) inhibitors. These results suggest that uptake is the principal determinant influencing sensitivity of the resistant cell line and consists of receptor-independent binding followed by internalization. Differential uptake requires the presence of serum albumin and is dependent on the energy-dependent endocytosis of the ether lipid.