Bombesin and phorbol ester stimulate phosphatidylcholine hydrolysis by phospholipase C: Evidence for a role of protein kinase C

Bombesin caused a marked stimulation of 32 P i into phosphatidylinositol (Pl), with no apparent lag, and into phosphatidylcholine (PC), after a lag of about 20 min. Stimualtion was blocked by the bombesin receptor antagonist, [D‐Arg 1 , D‐Pro 2 , D‐Trp 7,9 , Leu 11 ] substance P, indicating that the effects on both Pl and PC were mediated through the same receptor. The tumor‐promoting phorbol ester 12‐0‐tetradecanoylphorbol‐13‐acetate (TPA) and dioctanoylglycerol (diC 8 ) both directly activate protein kinase C and in this report were shown to stimulate 32 P i incorporation into PC but not into Pl. In addition, TPA stimulated the release of [ 3 H]choline and [ 3 H]phosphocholine and the accumulation of [ 3 H]diacylglycerol from prelabelled cells. These results strongly suggest that TPA activates a phospholipase C specific for PC. Pretreatment of cells with phorbol‐12, 13‐dibutyrate (PDBu) for 24 h depleted cellular protein kinase C activity and inhibited the ability of TPA to induce these effects suggesting a direct involvement of protein kinase C. Similarly the bombesin stimulation of 32 P i into PC and of [ 3 H]choline and [ 3 H]phosphocholine release was inhibited by PDBu pretreatment. DiC 8 and, to a lesser extent, TPA stimulated the translocation of CTP:phosphocholine cytidylyltransferase from the cytosolic to the particulate fraction. DiC 8 also stimulated this translocation in cells depleted of protein kinase C. It was concluded that both bombesin and TPA activated protein kinase C leading to activation of a phospholipase C specific for PC.