Increased membrane permeability for an antitumoral alkyl lysophospholipid in sensitive tumor cells

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Abstract
We have investigated cellular sensitivity to the antitumoral alkyl lysophospholipid (ALP) 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) in vitro. The permeation of this lipid into the cell was not influenced by metabolic inhibitors of ATP biosynthesis. ET-18-OCH3 uptake was not saturable within sublytic concentrations, but could be inhibited in part by cytochalasin B (CB) and dipyridamole. The activation energy of the CB-sensitive uptake process was increased up to threefold compared to CB-insensitive uptake. ET-18-OCH3 influx and equilibrium binding of ET-18-OCH3 were decreased in a fibrosarcoma cell variant (MethA) selected for ET-18-OCH3 resistance. The resistant MethA cells were also less sensitive to cytolysis by lysophosphatidylcholine and other ALP. After 72 hr, the resistant MethA cells had metabolized only 11.8% more of the absorbed ET-18-OCH3 than sensitive MethA cells. However, they tolerated at least a 30-fold concentration of this ALP. The uptake mechanism, which could be inhibited by CB, was less active in resistant MethA cells and several other ALP-resistant cell lines. The concentration of CB, required for maximal uptake inhibition, was increased more than four times in the ALP-sensitive tumor cell lines. CB-specific ET-18-OCH3 uptake was also enhanced after virus transformation of 3T3 fibroblasts by SV 40. Dipyridamole retarded the ET-18-OCH3-mediated cell destruction.