Phospholipase D in platelets and other cells.

Phospholipase D (PLD) hydrolyses phosphatidylcholine to phosphatidic acid (PA) and choline, where PA is considered to be the main effector of PLD's functions in cells. PA can act as a second messenger itself or as a precursor for Diacylglycerols (DAG) and lyso-PA. PA is reported to be involved in protein recruitment in membranes and membrane fusion processes, and PLD is proposed to play a role in signalling, intracellular transport and cytoskeletal rearrangements in cells. Protein kinase C (PKC), small G proteins and phosphatidylinositol-(4,5)-bis-phosphate (PIP(2)) are all central in the regulation of PLD activity; however, PLD has also been shown to be regulated by Ca(2+), protein tyrosine kinases and other kinases. Two isoforms of PLD have been cloned, PLD1 and 2, which are also present in platelets. In vitro PLD1 has a low basal activity and is readily activated by PKC, Adenosine diphosphate(ADP)-ribosylation factor (ARF) and Rho family members, while in contrast PLD2 shows a constitutive high basal activity and is not as easily activated by the factors mentioned above. The two PLD isoforms may have different localization and play different roles in cells. The role and regulation of PLD in platelets are largely unknown. However, PLD in platelets is activated by physiological activators like thrombin and collagen and inhibited by PKA, implying that PLD is involved in established signalling pathways in these cells. Activation by thrombin is stimulated by extracellular Ca(2+) and accompanied by translocation from cytosol to the plasma membrane area. Thrombin-induced PLD activity is dependent of autocrine stimulation. Possible roles for PLD in platelets include lysosomal secretion and actin polymerization. In this review we present the knowledge of PLD from other cells together with findings from platelets and demonstrate that PLD in platelets seems to have much of the same properties as in other cells, which implies that knowledge on PLD from other cells can be used in identifying activation mechanisms and roles in platelets.

Expression of a peptide binding to receptor for activated C-kinase (RACK1) inhibits phorbol myristoyl acetate-stimulated phospholipase D activity in C3H/10T1/2 cells: dissociation of phospholipase D-mediated phosphatidylcholine breakdown from its synthesis.

The C3H/10T1/2 Cl8 HAbetaC2-1 cells used in this study express a peptide with a sequence shown to bind receptor for activated C-kinase (RACK1) and inhibit cPKC-mediated cell functions. Phorbol myristoyl acetate (PMA) strongly stimulated phosphatidylcholine (PtdCho)-specific phospholipase D (PLD) activity in the C3H/10T1/2 Cl8 parental cell line, but not in Cl8 HAbetaC2-1 cells, indicating that full PLD activity in PMA-treated Cl8 cells is dependent on a functional interaction of alpha/betaPKC with RACK1. In contrast, the PMA-stimulated uptake of choline and its subsequent incorporation into PtdCho, were not inhibited in Cl8 HAbetaC2-1 cells as compared to Cl8 cells, indicating a RACK1-independent but PKC-mediated process. Increased incorporation of labelled choline into PtdCho upon PMA treatment was not associated with changes of either CDP-choline: 1,2-diacylglycerol cholinephosphotransferase activity or the CTP:phosphocholine cytidylyltransferase distribution between cytosol and membrane fractions in Cl8 and Cl8 HAbetaC2-1 cells. The major effect of PMA on the PtdCho synthesis in C3H/10T1/2 fibroblasts was to increase the cellular uptake of choline. As a supporting experiment, we inhibited PMA-stimulated PtdH formation by PLD, and also putatively PtdH-derived DAG, in Cl8 cells with 1-butanol. Butanol did not influence the incorporation of [(14)C]choline into PtdCho. The present study shows: (1) PMA-stimulated PLD activity is dependent on a functional interaction between alpha/betaPKC and RACK1 in C3H/10T1/2 Cl8 fibroblasts; and (2) inhibition of PLD activity and PtdH formation did not reduce the cellular uptake and incorporation of labelled choline into PtdCho, indicating that these processes are not directly regulated by PtdCho-PLD activity in PMA-treated C3H/10T1/2 Cl8 fibroblasts.

Choline derived from the phosphatidylcholine specific phospholipase D is not directly available for the CDP choline pathway in phorbol ester-treated C3H10T1/2 Cl 8 fibroblasts.

We have shown that 12-O-tetradecanoylphorbol 13-acetate (TPA) increases protein kinase C (PKC)-mediated choline transport, incorporation of choline into phosphatidylcholine (PtdCho) and PtdCho degradation by phospholipase D (PLD) in C3H10T1/2 Cl 8 cells. Dual prelabeling experiment using [3H]/[14C]choline indicated that intracellular choline generated from the PLD reaction was not directly recycled to PtdCho synthesis within the cell, and that a large fraction of the choline was transported out of the TPA-treated cells. In contrast, medium derived choline was preferably channeled to PtdCho synthesis. These results indicate that in TPA-treated cells, the choline derived from the PKC-mediated increased PLD activity and the choline newly taken up by the cell behave as two distinctly different metabolic pools.