Low-density (LD) lipoproteins inhibit phytohaemagglutinin-enhanced turnover of phosphatidylinositol in human peripheral lymphocytes. Turnover was assessed by 32P incorporation into phospholipids and by loss of 32P from [32P]phosphatidylinositol. Inhibition of lipid turnover by LD lipoproteins is not the result of a change in the amount of phytohaemagglutinin required for maximum cellular response. Neither phytohaemagglutinin nor LD lipoproteins influence 32P incorporation into phosphatidylethanolamine and phosphatidylcholine during the first 60min after mitogenic challenge. The extent of inhibition of phosphatidylinositol turnover by LD lipoproteins depends on the concentration of LD lipoproteins present in the incubation medium: 50% of maximum inhibition occurs at a low-density-lipoprotein protein concentration of 33μg/ml and maximum inhibition occurs at low-density-lipoprotein protein concentrations above 100μg/ml. Phytohaemagglutinin stimulates 32P incorporation into phosphatidylinositol, phosphatidylinositol phosphate and phosphatidylinositol bisphosphate. However, LD lipoproteins abolish 32P incorporation into phosphatidylinositol without affecting incorporation into phosphatidylinositol phosphate and phosphatidylinositol bisphosphate. The ability of LD lipoproteins to inhibit phytohaemagglutinin-induced phosphatidylinositol turnover is mimicked by EGTA. Furthermore, inhibition of LD lipoproteins by phytohaemagglutinin-induced 32P incorporation into phosphatidylinositol correlates directly with inhibition by LD lipoproteins of Ca2+ accumulation. These results suggest that Ca2+ accumulation and turnover of phosphatidylinositol are coupled responses in lymphocytes challenged by mitogens. The step in phosphatidylinositol metabolism that is sensitive to LD lipoproteins and, by inference, that is coupled to Ca2+ accumulation is release of [32P]phosphoinositol from phosphatidylinositol.

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