Inhibition of Growth Factor-dependent Inositol Phosphate Ca2+ Signaling by Antitumor Ether Lipid Analogues1

Cytotoxic ether lipid analogues have been studied for their ability to inhibit growth factor-dependent |< ¡r'*| signaling in Swiss 3T3 fibroblasts. l -Octadecyl^-methyl-rac-glycero-S-phosphocholine (ET-18-OCH3) in hibited 4<<V+ uptake and inositol(l,4,5)trisphosphate-induced "Ça2* release in saponin permeabilized cells with concentration producing 50% inhibition values of 55 and 360 UM,respectively. When cells were exposed to ET-18-OCH3 for 18 h before permeabilization there was selective inhibition of inositol(l,4,5)trisphosphate-induced 45Ca2*release with a concentration producing 50% inhibition value of 20 UM,but no effect on 45Ca2*uptake, or on 45Ca2*release by arachidonic acid. The concentration of ET-18-OCH;) with continuous exposure to inhibit cell growth 50% was 19 MM.The ether lipid analogues l-hexadecylthio-2-ethyl-rac-glycero3-phosphocholine and 1-S-octadecyl-Z-O-methylthiopropyl-S-A'.A'-dimethyl-T-hydroxypropyl ammonium iodide had effects similar to those of ET-18-OCHj but the noncytotoxic analogue l-alkyl-2-hydroxy-înglycero-3-phosphocholine was without effect. Exposure of cells to 10 MM ET-18-OCHj produced 81% inhibition of platelet-derived growth factorstimulated inositol phosphate formation and 66% inhibition of fluoroaluminate anion-stimulated inositol phosphate formation. Addition of II18-OCHj to cells in medium with 10% fetal calf serum gave a transient increase in |Ca2+|,without causing an increase in resting |Ca2*]i,while the addition of ET-18-OCH3 to cells in medium without serum gave a sustained increase in resting [Ça2*],. Cells exposed to 5 MMET-18-OCH3 for 18 h showed no increase in resting |( a '% but there was 95% inhibition of the |<'a2+|¡ response to platelet-derived growth factor, 63% inhibition of the response to bradykinin, and 55% inhibition of the response to vasopressin. The block by ether lipid analogues of inositol phosphatemediated |( ¡i'* |, signaling suggests a mechanism for preventing the action of growth factors that could contribute to the inhibition of cell prolifera tion by the agents.