The role of intracellular free calcium mobilization in the mechanism of action of antitumour ether lipids SRI 62-834 and ET18-OMe.

Membrane-active antitumour ether lipids such as ET18-OMe (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine) and SRI 62-834 ((+-)-2-(Hydroxy[tetrahydro-2-(octadecyloxy) methylfuran-2-yl] methoxyl phosphinyloxy)-N,N,N-trimethylethaniminium hydroxide) are selectively cytotoxic to tumour cells in vitro. Their precise mechanisms of action are unclear, but they are known to have effects on cell membranes and cell signalling. A previous report suggested that ether lipids cause a biphasic sustained rise in intracellular free calcium [Lazenby et al., Cancer Res 50: 3327-3330, 1990]. We show here that the second phase is an experimental artefact due to cell membrane permeabilization by ether lipids in serum-free buffers. In serum-free medium, the membrane toxicity of antitumour ether lipids was increased 50-60 fold, when compared to medium containing 10% serum. Membrane disruption was neither dependent on extracellular calcium, nor modulated by preloading cells with the calcium chelators bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or 2-[[2-[bis(carboxymethyl)amino]-5-methylphenoxy]methyl]-6- methoxy-8-[bis(carboxymethyl)amino]quinoline. This indicates that the mechanism of membrane damage by ether lipids does not involve changes in calcium homeostasis. Using indo-1 and fura-2 as calcium probes, we established that lower concentrations of antitumour ether lipids do elicit a genuine monophasic and transient rise in intracellular free calcium, predominantly mobilized from internal stores. This acute calcium agonist activity of ether lipids is distinct from the inhibitory effects on cell signalling reported previously after more prolonged exposure. It appears that the calcium elevation induced by antitumour ether lipids is unlikely to be instrumental in their selective and potent antitumour activity.