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

Cytotoxic ether lipid analogues have been studied for their ability to inhibit growth factor-dependent [Ca2+] signaling in Swiss 3T3 fibroblasts. l-Octadecyl-2-methyl-roc-glycero-3-phosphocholine (ET-18-OCH3) inhibited 45Ca2+ uptake and inositol(1,4,5)trisphosphate-induced 45Ca2+ release in saponin permeabilized cells with concentration producing 50% inhibition values of 55 and 360 mm, respectively. When cells were exposed to ET-I8-OCH3 for 18 h before permeabilization there was selective inhibition of inositol(1,4,5)tr is phosphate-induced 45Ca2+ release with a concentration producing 50% inhibition value of 20 μm, but no effect on 45Ca2+ uptake, or on 45Ca2+ release by arachidonic acid. The concentration of ET-18-OCH3 with continuous exposure to inhibit cell growth 50% was 19 μm. The ether lipid analogues l-hexadecylthio-2-ethyl-rac-glycero-3-phosphocholine and 1 -S-octadecyl-2-O-methylthiopropyl-3-N,N-di-methyl-γ-hydroxypropyl ammonium iodide had effects similar to those of ET-I8-OCH3 but the noncytotoxic analogue l-alkyl-2-hydroxy-sn-glycero-3-phosphocholine was without effect. Exposure of cells to 10 mm ET-I8-OCH3 produced 81% inhibition of platelet-derived growth factor-stimulated inositol phosphate formation and 66% inhibition of fluoroa-luminate anion-stimulated inositol phosphate formation. Addition of ET-I8-OCH3 to cells in medium with 10% fetal calf serum gave a transient increase in [Ca2+l without causing an increase in resting [Ca2+k, while the addition of ET-I8-OCH3 to cells in medium without serum gave a sustained increase in resting |Ca2+]|. Cells exposed to 5 mm ET-I8-OCH3 for 18 h showed no increase in resting [Ca2+] but there was 95% inhibition of the [Ca2+|i 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 phosphate-mediated [Ca2+]i signaling suggests a mechanism for preventing the action of growth factors that could contribute to the inhibition of cell proliferation by the agents.