Selective inhibition of phosphatidylinositol phospholipase C by cytotoxic ether lipid analogues.

The ether lipid analogue 1-octadecyl-2-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) has been shown to be a direct inhibitor of Swiss 3T3 fibroblast and BG1 ovarian adenocarcinoma cell cytosolic phosphoinositide selective phospholipase C (PIPLC) using [3H]-phosphatidylinositol-(4, 5)-bisphosphate ([3H]PIP2) as the substrate. The inhibition occurred when ET-18-OCH3 was incorporated into the [3H]PIP2 substrate micelles, with 50% inhibition (IC50) occurring at a ET-18-OCH3: [3H]PIP2 ratio of 0.04, or an assay concentration of 0.4 microM, and when ET-18-OCH3 was added directly to the incubation, with an IC50 of 9.6 microM. Lipid prepared from cells exposed to cytotoxic concentrations of ET-18-OCH3 for 18 h also inhibited PIPLC with an IC50 less than 1 microM. The noncytotoxic analogue 1-O-alkyl-2-hydroxy-sn-glycero-3-phosphocholine inhibited PIPLC when incorporated into the [3H]PIP2 substrate micelles, but lipid from cells grown with 5 microM 1-O-alkyl-2-hydroxy-sn-glycero-3-phosphocholine did not inhibit PIPLC. BG1 cells, which were more sensitive than Swiss 3T3 fibroblasts to growth inhibition by ET-18-OCH3, had a cytosolic PIPLC activity one-third that of Swiss 3T3 cells. NIH 3T3 cells exhibited the same sensitivity to growth inhibition by ET-18-OCH3 as Swiss 3T3 cells and had a similar level of PIPLC. v-sis NIH 3T3 cells were relatively resistant (greater than 3-fold) to growth inhibition by ET-18-OCH3 and had a cytosolic PIPLC activity more than twice that of the wild type cells. ET-18-OCH3 was a weak inhibitor, IC50 greater than 100 microM, of phospholipase D activity in NIH 3T3 cell membranes. In intact NIH 3T3 cells ET-18-OCH3 at cytotoxic concentrations did not inhibit phospholipase D or phosphatidylcholine-selective phospholipase C activity. The results show that the ether lipid analogues at cytotoxic concentrations are selective inhibitors of PIPLC and that the inhibition of PIPLC may be related to the growth inhibitory activity of the ether lipid analogues.