Inhibition of phosphatidylcholine and phosphatidylethanolamine biosynthesis by cytochalasin B in cultured glioma cells: Potential regulation of biosynthesis by Ca 2+-dependent mechanisms

Abstract

The major route of phosphatidylcholine (PtdCho) biosynthesis in mammalian cells is the sequence: choline (Cho)----phosphocholine (PCho)----cytidinediphosphate choline (CDP-Cho)----PtdCho. Recently, we have found that intermediates of this pathway are not freely diffusible in cultured rat glioma (C6) cells but are channeled towards PtdCho biosynthesis (George et al. (1989). Biochim. Biophys. Acta. 1004, 283-291). Channeling of intermediates in other mammalian systems is thought to be mediated through adsorption of enzymes to membranes and cytoskeletal elements to form multienzyme complexes. In this study, agents which perturb the structure and function of cytoskeletal elements were tested for effects on phospholipid metabolism in glioma cells. The filament-disrupting agent cytochalasin B (CB), but not other cytochalasins or the microtubule depolymerizer colchicine inhibited PtdCho and phosphatidylethanolamine (PtdEtn) biosynthesis as judged by dose-dependent reduction of labeling from [3H]Cho and [14C]ethanolamine (Etn). 32Pi pulse-labeling indicated that CB selectively decreased PtdCho and PtdEtn biosynthesis without affecting synthesis of other phospholipids. Synthesis of water-soluble intermediates of PtdCho metabolism was unaffected but the conversion of phosphoethanolamine to CDP-ethanolamine was reduced by CB. Effects of CB on phospholipid biosynthesis were not due to inhibition of glucose uptake as shown by experiments with 2-deoxyglucose, glucose-starved cells and other cytochalasins. Experiments with Ca(2+)-EGTA buffers and digitonin-permeabilized cells, and the Ca(2+)-channel blocker verapamil suggest that effects of CB on PtdCho and PtdEtn biosynthesis are due to alteration of intracellular Ca2+. Taken together, these results suggest that CB acts at sites distinct from glucose transport and cellular microfilaments to specifically inhibit PtdCho and PtdEtn biosynthesis by mechanisms dependent on intracellular Ca2+.