Dioctanoylglycerol stimulates accumulation of [ methyl- 14C]choline and its incorporation into acetylcholine and phosphatidylcholine in a human cholinergic neuroblastoma cell line

Abstract

Dioctanoylglycerol, a synthetic diacylglycerol, stimulated [ 14C]choline uptake in cultured human neuroblastoma (LA-N-2) cells. As this effect has not, to our knowledge, been reported before, it was of interest to characterize it in more detail. In the presence of 500 μM dioctanoylglycerol the levels of [ 14C]choline attained during a 2 hour labeling period were elevated by 78 ± 12%, while [ 14C]acetylcholine and long fatty acyl chain [ 14C]phosphatidylcholine levels increased by 26 ± 2% and 19 ± 5%, respectively (mean ± S.E.M.). Total (long chain plus dioctanoyl-1) [ 14C]phosphatidylcholine was increased b 198 ± 33%. Kinetic analysis showed that dioctanoylglycerol reduced the apparent K m for choline uptake to 56 ± 9% of control ( n = 4). The V max was not significantly altered. The stimulation of [ 14C]choline accumulation by dioctanoylglycerol was not dependent on protein kinase C activation; the effect was not mimicked by phorbol ester or by 1-oleoyl-2-acetylglycerol, and was not inhibited by the protein kinase C inhibitors H-7 or staurosporine, or by prolonged pretreatment with phorbol 12-myristate 13-acetate. The effect of dioctanoylglycerol was slightly (but not significantly) reduced by EGTA and strongly inhibited by the cell-permeant calcium chelator bis( o-aminophenoxy)-ethane- N, N, N, N′ N′-tetraacetic acid, tetra(acetoxymethyl)ester. Although these results implicate elevated intracellular calcium in the response, dioctanoylglycerol did not increase phosphatidylinositol hydrolysis in LA-N-2 cells, and its effect was not inhibited by the diacylglycerol kinase inhibitor R 59 022 (which blocks the conversion of diacylglycerol to phosphatidic acid, a known stimulator of phosphatidylinositol hydrolysis). The enhancement of choline uptake by diacylglycerol, a phospholipid metabolite, may represent a mechanism by which precursor levels can be replinished more rapidly when membrane turnover is increased.