Activation of phospholipase D is not mediated by direct phosphorylation on tyrosine residues

The uncovering of a novel regulatory mechanism for PLD2: formation of a ternary complex with protein tyrosine phosphatase PTP1B and growth factor receptor-bound protein GRB2

The regulation of PLD2 activation is poorly understood at present. Transient transfection of COS-7 with a mycPLD2 construct results in elevated levels of PLD2 enzymatic activity and tyrosyl phosphorylation. To investigate whether this phosphorylation affects PLD2 enzymatic activity, anti-myc immunoprecipitates were treated with recombinant protein tyrosine phosphatase PTP1B. Surprisingly, lipase activity and PY levels both increased over a range of PTP1B concentrations. These increases occurred in parallel to a measurable PTP1B-associated phosphatase activity. Inhibitor studies demonstrated that an EGF-receptor type kinase is involved in phosphorylation. In a COS-7 cell line created in the laboratory that stably expressed myc-PLD2, PTP1B induced a robust (>6-fold) augmentation of myc-PLD2 phosphotyrosine content. The addition of growth factor receptor-bound protein 2 (Grb2) to cell extracts also elevated PY levels of myc-PLD (>10-fold). Systematic co-immunoprecipitation-immunoblotting experiments pointed at a physical association between PLD2, Grb2, and PTP1B in both physiological conditions and in overexpressed cells. This is the first report of a demonstration of the mammalian isoform PLD2 existing in a ternary complex with a protein tyrosine phosphatase, PTP1b, and the docking protein Grb2 which greatly enhances tyrosyl phosphorylation of the lipase.

Tyrosine phosphorylation of phospholipase D1 by v-Src does not per se result in activation

The relationship between tyrosine phosphorylation and activation of phospholipase D1 (PLD1) by v-Src was examined. Co-expression of v-Src and PLD1 in COS-7 cells resulted in increased activity and marked tyrosine phosphorylation of PLD1. PLD activity was increased in membranes or immunoprecipitates prepared from these cells. Dephosphorylation of the immunoprecipitated enzyme by tyrosine phosphatase or phosphorylation by c-Src produced no changes in its activity. Tyrosine phosphorylation induced by v-Src caused a shift of the enzyme from the Triton-soluble to the Triton-insoluble fraction. v-Src and PLD1 could be co-immunoprecipitated from cells co-expressing these and were co-localized in the perinuclear region as assessed by immunofluorescence. Mutation of the palmitoylation sites of PLD1 significantly reduced tyrosine phosphorylation by v-Src. It is concluded that tyrosine phosphorylation of PLD1 by v-Src does not per se alter its activity. It is proposed that activation of PLD1 by v-Src in vivo may involve association/colocalization of the two proteins.

Implication of phospholipase D2 in oxidant-induced phosphoinositide 3-kinase signaling via Pyk2 activation in PC12 cells

The role of phospholipase D (PLD) activation in hydrogen peroxide (H(2)O(2))-induced signal transduction and cellular responses is not completely understood. Here we present evidence that Ca(2+)-dependent tyrosine kinase, Pyk2, requires PLD activation to mediate survival pathways in rat pheochromocytoma PC12 cells under oxidative stress. The H(2)O(2)-induced phosphorylation of two Pyk2 sites (Tyr(580), and Tyr(881)) was suppressed by 1-butanol, an inhibitor of transphosphatidylation by PLD, and also by transfection of catalytically negative mouse PLD2K758R (PLD2KR). Furthermore, we found that PLD2 was associated with Pyk2 and Src, and that activation of PLD2 was required for H(2)O(2)-enhanced association of Src with Pyk2 leading to full activation of Pyk2. H(2)O(2)-induced phosphorylation of Akt and p70S6K was dependent on phosphatidylinositol 3-kinase (PI3K) activity and was abolished by 1-butanol but not t-butanol. Furthermore, the PI3K/Akt activation in response to H(2)O(2) was reduced by transfection of either PLD2KR or the dominant negative Pyk2DN. This study is the first demonstration that PLD2 activation is implicated in Src-dependent phosphorylation of Pyk2 (Tyr(580) and Tyr(881)) by promoting the complex formation between Pyk2 and activated Src in PC12 cells exposed to H(2)O(2), thereby resulting in activation of the survival signaling pathway PI3K/Akt/p70S6K.

Regulation of acetylcholine-induced phosphorylation of PLD1 in porcine tracheal smooth muscle

The muscarinic agonist, acetylcholine (ACh), stimulates phospholipase D (PLD) activity in tracheal smooth muscle cells. Direct activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate (PMA) also stimulates PLD in this tissue. Activation of ACh-induced PLD was inhibited by the tyrosine kinase inhibitor genistein in a concentration-dependent manner. Presently known isoforms of PLD, PLD1 and PLD2, were identified in tracheal smooth muscle and their activation-induced phosphorylation status studied. Both ACh and PMA increased phosphorylation of PLD1 that was significantly blocked by genistein or the PKC inhibitor calphostin C. PLD2 phosphorylation was not detected in the present experiments. Western blots probed with an anti-phosphotyrosine antibody indicate that PLD1 in this tissue is phosphorylated on tyrosine residues after ACh or PMA stimulation. Tyrosine phosphorylation of PLD1 was blocked by genistein and calphostin C. No tyrosine residues were phosphorylated on PLD2. Taken together, these results demonstrate that porcine tracheal smooth muscle cells express both isoforms PLD1 and PLD2. However, on muscarinic activation only PLD1 in this tissue is phosphorylated by PKC via a tyrosine-kinase-dependent pathway.