Lipid rafts and metabolic energy differentially determine uptake of anti-cancer alkylphospholipids in lymphoma versus carcinoma cells

Perifosine is a member of the class of synthetic alkylphospholipids (APLs) and is being evaluated as anti-cancer agent in several clinical trials. These single-chain APLs accumulate in cellular membranes and disturb lipid-dependent signal transduction, ultimately causing apoptosis in a variety of tumor cells. The APL prototype edelfosine was previously found to be endocytosed by S49 mouse lymphoma cells via lipid rafts. An edelfosine-resistant cell variant, S49AR, was found to be cross-resistant to other APLs, including perifosine. This resistance was due to defective synthesis of the raft constituent sphingomyelin, which abrogated APL cellular uptake. Sensitivity of S49 cells to edelfosine was higher than perifosine, which correlated with a relatively higher uptake. Human KB epidermal carcinoma cells were much more sensitive to APLs than S49 cells. Their much higher APL uptake was highly dependent on intracellular ATP and ambient temperature, and was blocked by chlorpromazine, independent of canonical endocytic pathways. We found no prominent role of lipid rafts for APL uptake in these KB cells; contrary to S49AR cells, perifosine-resistant KBr cells display normal sphingomyelin synthesis, whereas APL uptake by the responsive KB cells was insensitive to treatment with methyl-β-cyclodextrin, a cholesterol-sequestrator and inhibitor of raft-mediated endocytosis. In conclusion, different mechanisms determine APL uptake and consequent apoptotic toxicity in lymphoma versus carcinoma cells. In the latter cells, APL uptake is mainly determined by a raft- and endocytosis-independent process, but metabolic energy-dependent process, possibly by a lipid transporter.