Phorbol diester enhances calcium ionophore A23187‐induced [3H]acetate incorporation into platelet‐activating factor in murine macrophages: Predominant incorporation into 1‐O‐acyl‐2‐acetyl‐sn‐glycero‐3‐phosphocholine

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Abstract
Pretreatment of macrophages with 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) has been shown to enhance the release of arachidonic acid from cell phospholipids in response to agonist stimulation. This study describes the ability of TPA to also alter calcium ionophore A23187‐induced incorporation of [3H]acetate into platelet activating factor (PAF). Cultured murine peritoneal macrophages were preincubated with [3H]acetate (25 μCi) and TPA (10 ng/ml) for 10 min, and subsequently incubated with 0.1 μM A23187 for 0.5–10 min. Buffer and cells were then extracted and PAF resolved by normal‐phase HPLC. Sequential exposure to TPA and A23187 resulted in a greatly enhanced incorporation (11,861 dpm/106 cells) of [3H]acetate into PAF compared to TPA alone, which did not significantly influence [3H]acetate incorporation into PAF, and 0.1 μM A23187, which induced minimal incorporation (688 dpm/106 cells). Macrophage‐produced [3H]PAF was resolved by HPLC, extracted, treated with phospholipase‐C, and acetylated to facilitate quantitation of 1‐O‐alkyl‐2‐acetyl‐GPC (PAF) from 1‐O‐acyl‐2‐acetyl‐GPC (acylPAF). A23187 alone (1 μM) produced 72% 1‐O‐acyl‐2‐[3H]acetyl‐GPC, and A23187 (0.1 μM) following TPA pretreatment produced 81% 1‐O‐acyl‐2‐[3H]acetyl‐GPC. Less than 2% of the radioactivity of acylPAF was in the acyl moiety. These data support a role for protein kinase C in modulating agonist‐induced PAF synthesis. The results also suggest that acetyltransferase of murine macrophages does not possess specificity for 1‐O‐alkyl‐2‐lyso‐GPC, and that availability of specific species of lyso‐phospholipid may determine the type of PAF produced.

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