Hexadecylphosphocholine Inhibits Inositol Phosphate Formation and Protein Kinase C Activity¹

Hexadecylphosphocholine (HePC) inhibits protein kinase C (PKC) from NIH3T3 cells in cell-free extracts with a 50% inhibitory concentration of about 7 µm. Inhibition is competitive with regard to phosphatidylserine with a K₁ of 0.59 µm. In order to determine whether HePC affects PKC in intact cells, the bombesin or tetradecanoylphorbolacetate-induced, PKC-mediated activation of the Na⁺/H⁺-antiporter was determined. It is demonstrated that HePC causes a drastic inhibition of this enzyme indicating a similar sensitivity of PKC to HePC in intact cells compared to cell-free extracts. In addition to the effects on PKC, treatment of NIH3T3 cells with HePC depresses the bombesin-induced formation of inositol 1,4,5-trisphosphate and the concomitant mobilization of intracellular Ca²⁺. Dose-response curves for the inhibition of inositol 1,4,5-trisphosphate formation and Ca²⁺ mobilization reveal 50% inhibitory concentrations of 2 or 5 µm, respectively. Polyphosphorylated phosphoinositides accumulate in HePC-treated cells indicating that the depression of inositol 1,4,5-trisphosphate generation is not caused by an inhibition of phosphoinositide kinases. Addition of bombesin to HePC-treated cells in the presence of LlCl revealed no evidence for an accelerated rate of inositol 1,4,5-trisphosphate turnover by the phospholipid analogue. It is concluded that HePC inhibits phosphoinositidase C in intact cells. The data strongly suggest that the growth-inhibitory effect of HePC is at least in part explained by the interference with mitogenic signal transduction.