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

PLD2 is enriched on exosomes and its activity is correlated to the release of exosomes

Exosomes are small vesicles secreted by different immune cells and which display anti-tumoral properties. Stimulation of RBL-2H3 cells with ionomycin triggered phospholipase D2 (PLD2) translocation from plasma membrane to intracellular compartments and the release of exosomes. Although exosomes carry the two isoforms of PLD, PLD2 was enriched and specifically sorted on exosomes when overexpressed in cells. PLD activity present on exosomes was clearly increased following PLD2 overexpression. PLD2 activity in cells was correlated to the amount of exosome released, as measured by FACS. Therefore, the present work indicates that exosomes can vehicle signaling enzymes.

Participation of phospholipase D and alpha/beta-protein kinase C in growth factor-induced signalling in C3H10T1/2 fibroblasts

We have studied phospholipase D (PLD) activation in relation to protein kinase C (PKC) and the involvement of PLD in extracellularly regulated kinase 1 (MAPK) (ERK1) activation and c-fos mRNA expression in C3H/10T1/2 (Cl8) fibroblasts. In these cells, the PLD activity was significantly increased by porcine platelet-derived growth factor (PDGF-BB), phorbol 12-myristate 13-acetate (PMA), and epidermal growth factor (EGF). PLD activation by PDGF-BB and PMA, but not EGF, was inhibited in Cl8 cells expressing the HAbetaC2-1 peptide (Cl8 HAbetaC2-1 cells), with a sequence (betaC2-1) shown to bind receptor for activated C kinase 1 (RACK1) and inhibit c-PKC-mediated cell functions [Science 268 (1995) 247]. A role of alpha-PKC in PLD activation is further underscored by co-immunoprecipitation of alpha-PKC with PLD1 and PLD2 in non-stimulated as well as PMA- and PDGF-BB-stimulated Cl8 cells. However, only PKC in PLD1 precipitates was activated by these agonists, while the PKC in the PLD2 precipitates was constitutively activated. The c-fos mRNA levels in Cl8 cells increased more than 30-fold in response to either PDGF-BB, EGF, or PMA. Approximately 60% inhibition of this increase in c-fos mRNA levels was observed in Cl8 HAbetaC2-1 cells. Formation of phosphatidylbutanol (PtdBut) at the expense of phosphatidic acid (PtdH) in the presence of n-butanol inhibited ERK1 activation and c-fos mRNA expression in PDGF-BB-treated Cl8 cells. ERK activation by PMA was unaffected by n-butanol in Cl8 cells but almost abolished by n-butanol in Cl8 HAbetaC2-1 cells, showing that ERK activation by PMA is heavily dependent on PKC and PLD1. In contrast, ERK activation by EGF in both cell types was not sensitive to n-butanol. These results indicate (1) a role of a functional interaction between the RACK1 scaffolding protein and a alphaPKC-PLD complex for achieving full PLD activity in PDGF-BB- and PMA-stimulated Cl8 cells; (2) PLD-mediated PtdH formation is needed for optimal ERK1 activation by PDGF-BB and maximal increase in c-fos mRNA expression. These findings place PLD as an important component in PDGF-BB- and PMA-stimulated intracellular signalling leading to gene activation in Cl8 cells, while EGF does not require PLD.

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.