Phospholipase D mimics platelet-derived growth factor as a competence factor in vascular smooth muscle cells

Lysophosphatidic acids induce proliferation of cultured vascular smooth muscle cells from rat aorta

Lysophosphatidic acids (LPA) with a C18 fatty acyl group accelerated thymidine incorporation into cultured rat aortic vascular smooth muscle cells and stimulated their cell division. LPA acted synergistically with epidermal growth factor and fibroblast growth factor but additively with platelet-derived growth factor. The stimulatory actions of LPA were suggested to be rather specific from the following findings: 1) their stimulation of DNA synthesis increased with an increase in their acyl moiety; 2) lysophosphatidylcholine, a neutral lysophospholipid, had no mitogenic action but was cytotoxic at high concentrations; and 3) LPA induced a rapid external Ca(2+)-independent increase in intracellular Ca2+ concentration ([Ca2+]i) in single fura 2-loaded cells that resembled the receptor-mediated increases in [Ca2+]i triggered by different agonists, whereas lysophosphatidylcholine provoked a slow sustained increase in [Ca2+]i in an external Ca(2+)-dependent manner. These results are discussed in relation to the possible pathophysiological role of LPA.

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

A role for phospholipase D in control of mitogenesis

How do growth factors that act on G protein-coupled cell-surface receptors communicate with the nucleus? These receptors commonly activate phospholipase C, and it has been assumed that the consequent rise in cytosolic Ca2+ concentrations and activation of protein kinase C mediates the mitogenic response. Recent evidence has demonstrated that phospholipase D (PLD) might be capable of eliciting mitogenesis. This enzyme is stimulated by a variety of growth factors, including those that act on receptors that possess intrinsic tyrosine kinase activity as well as those acting on G protein-coupled receptors. In this review, Michael Boarder considers the evidence that PLD, activated downstream of tyrosine protein kinases by both classes of cell-surface growth factor receptor, is implicated in the mitogenic response. This evidence is related to the possibility of PLD involvement in the regulation of vascular smooth muscle cell proliferation by endothelin-1 and platelet-derived growth factor.

Enhanced phospholipase D activity in vascular smooth muscle cells derived from spontaneously hypertensive rats

When cultured in the presence of fetal calf serum, aortic vascular smooth muscle cells (VSMC) derived from spontaneously hypertensive rats (SHR) grow faster than those from normotensive control Wistar-Kyoto (WKY) rats. In order to investigate the mechanism underlying this growth abnormality, we measured phospholipase D (PLD) activity in VSMC taken from both SHR and WKY rats. Upon stimulation with serum, platelet-derived growth factor (PDGF) and porbol 12-myristate 13-acetate (TPA), phosphatidylethanol (PEt) was produced in the presence of ethanol. The responses of the VSMC from SHR (SHR-cells) to all stimuli were significantly greater than those of the VSMC from WKY rats (WKY-cells), which suggests an enhanced PLD activity in the SHR-cells. Since PLD is regarded as an enzyme involved in signal transduction leading to cell proliferation, this PLD hyper-reactivity in the SHR-cells may account at least partially for the growth abnormality in the SHR-cells.

Protein kinase C zeta isoform is critical for mitogenic signal transduction

The requirement of protein kinase C zeta (zeta PKC) for maturation of X. laevis oocytes in response to insulin, p21ras, and phosphatidylcholine-hydrolyzing phospholipase C has recently been shown. Here we present experimental evidence demonstrating that activation of zeta PKC is not only necessary but also sufficient by itself to activate maturation in oocytes and to produce deregulation of growth control in mouse fibroblasts. Furthermore, by using a dominant kinase-defective mutant of zeta PKC, we confirm that this kinase is required for mitogenic activation in oocytes and fibroblasts. These results permit us to propose zeta PKC as a critical step downstream of p21ras in mitogenic signal transduction.

Enhancement of phospholipase A2 activation by phosphatidic acid endogenously formed through phospholipase D action in rat peritoneal mast cell

Contribution of phosphatidic acid (PA) generated by activated phospholipase (PL) D to PLA2 activation was studied in rat peritoneal mast cells. Exogenous didecanoyl PA induced arachidonate liberation in the permeabilized cells which was inhibited by p-bromophenacyl bromide. Upon exposure of the cells to ethanol in a high enough concentration to prevent PA formation, A23187-induced arachidonate liberation was suppressed by 50% and the rest was completely inhibited by p-bromophenacyl bromide. In contrast, propranolol, which enhanced PA accumulation, significantly increased the arachidonate liberation. These results suggest that A23187-induced PLA2 activation may be potentiated, at least in part, by PA generated through PLD action.