Phospholipids regulate growth and function of MDCK cells in hormonally defined serum free medium

Dioleoyl phosphatidic acid induces morphological changes through an endogenous LPA receptor in C6 glioma cells

Previously, we suggested that dioleoyl phosphatidic acid (PA) and lysophosphatidic acid (LPA) increased [Ca(2+)](i) through endogenous LPA receptors coupled to pertussis toxin-sensitive G proteins in rat C6 glioma cells. In the present report, we investigated morphological changes and cytotoxicity induced by PA and LPA in C6 glioma cells. Isoproterenol treatment led to changes in the cell morphology of rat C6 glioma cells, which were reverted by the addition of PA and LPA. PA-and LPA-induced morphological reversions were inhibited by treatment with Ki16425, an LPA(1)/LPA(3) receptor antagonist. VPC32183, another LPA(1)/LPA(3) receptor antagonist with a different structure, only inhibited PA-induced morphological reversion but not LPA-induced reversion. However, the reversions were not inhibited by treatment with pertussis toxin, a specific inhibitor of G(i/o) proteins. In addition, cytotoxicity was only induced by LPA but not by PA in C6 glioma cells. Our results suggest that PA may act as a partial agonist at endogenous LPA receptors, which are sensitive to Ki16425 and coupled to PTX-insensitive G proteins, to evoke morphological changes in C6 glioma cells.

Dioleoyl phosphatidic acid increases intracellular Ca2+ through endogenous LPA receptors in C6 glioma and L2071 fibroblasts

Phosphatidic acid (PA) increased intracellular Ca(2+) concentration ([Ca(2+)](i)) in C6 rat glioma and L2071 mouse fibroblast cells. Dioleoyl PA (PA, 18:1) was the most efficacious, followed by dipalmitoyl PA (16:0 PA) and dimyristoyl PA (14:0 PA). Lysophosphatidic acid (LPA) also increased the [Ca(2+)](i) in the both cells. PA desensitized LPA-induced Ca(2+) response completely in C6 cells, but partly in L2071 cells. Treatment of pertussis toxin (PTX), a specific inhibitor of G(i/o)-type G proteins, completely ameliorated LPA- and PA-induced Ca(2+) response in C6 cells. However, in L2071 cells, PTX inhibited PA-induced Ca(2+) increase by 80% and LPA-induced one by 20%. Ki16425, a specific inhibitor of LPA(1)/LPA(3) receptors, completely inhibited both LPA- and PA-induced Ca(2+) responses in C6 cells. On the other hand, in L2071 cells, Ki16425 completely inhibited PA-induced Ca(2+) response, but partly LPA-induced one. VPC32183, another specific inhibitor of LPA(1)/LPA(3) receptors, completely inhibited LPA- and PA-induced Ca(2+) responses in both C6 and L2071 cells. Therefore, PA and LPA appear to increase [Ca(2+)](i) through Ki16425/VPC32183-sensitive LPA receptor coupled to PTX-sensitive G proteins in C6 cells. In L2071 cells, however, LPA increases [Ca(2+)](i) through Ki16425-insensitive LPA receptor coupled to PTX-insensitive G proteins and Ki16425-sensitive LPA receptor coupled to PTX-sensitive G protein, whereas PA utilized only the latter pathway. Our results suggest that PA acts as a partial agonist on endogenous LPA receptors, which are sensitive to Ki16425 and coupled to PTX-sensitive G protein, but not on LPA receptors, which are not sensitive to Ki16425 and coupled to PTX-insensitive G protein.

Phosphatidic acid has a potential to promote hair growth in vitro and in vivo, and activates mitogen-activated protein kinase/extracellular signal-regulated kinase kinase in hair epithelial cells

Phospholipids have recently been discovered to play an important role in cellular regulation. In this study, we focused on phosphatidic acid and lysophosphatidic acid, which are phospholipids known to possess growth-hormonal effects on several types of cells, and examined their growth-promoting effects on murine hair epithelial cells. We discovered that phosphatidic acid possesses intensive growth-promotional effects on hair epithelial cells and epidermal keratinocytes. In contrast, lyso-phosphatidic acid showed lower growth-promoting effects on hair epithelial cells relative to phosphatidic acid and showed minimal or no growth-promoting activity on epidermal keratinocytes. Phosphatidic acid was also shown to have hair-growing activity to induce the anagen phase of the hair cycle in the in vivo murine model. For the purpose of examining the hair-growing mechanisms of phosphatidic acid, we examined its relationship to the mitogen-activated protein kinase cascade linked to cell proliferation and the transforming growth factor beta signal pathway known to be a regulator of catagen induction. We confirmed that phosphatidic acid activates MEK-1/2 and upregulates the expression of MEK-1/2 in cultured murine hair epithelial cells. Addition of transforming growth factor beta1 to hair epithelial cell cultures concentration-dependently decreased cell growth and induced apoptosis; however, addition of phosphatidic acid to the culture neutralized the growth-inhibiting effects of transforming growth factor beta1 and protected the cells from apoptosis. We speculate that the hair-growing activity of phosphatidic acid is at least linked to its growth-promoting effects on hair epithelial cells that follow mitogen-activated protein kinase/extracellular signal-regulated kinase kinase activation and its protective action on transforming-growth-factor-beta1-induced apoptosis that is assumed to trigger catagen induction in the hair cycle.

Phospholipase D, tumor promoters, proliferation and prostaglandins

Phosphatidylcholine hydrolysis by phospholipase D is a widespread response to cellular stimulation. However, the downstream signaling events subsequent to phosphatidylcholine hydrolysis are just beginning to be determined. Initially it was proposed that diglyceride formation by phospholipase D and phosphatidate phosphohydrolase resulted in long-term stimulation of protein kinase C. However, recent studies indicate that phosphatidic acid is the relevant signaling molecule in some signaling pathways. The present review will summarize studies of phospholipase D in the response of cells to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate, which causes cells to mimic the phenotype of oncogenic transformation. The role of phospholipase D in stimulation of Raf-1 and prostaglandin H synthase type-2 is emphasized.

Effects of lysophosphatidic acid on proliferation and cytosolic Ca++ of human adult vascular smooth muscle cells in culture

Lysophosphatidic acid (LPA) is a lipid mediator generated by activated platelets and having various effects on numerous cell types. We investigated some effects of 1-oleyl LPA on vascular smooth muscle cells cultured from adult human normal arteries. At micromolar concentrations, LPA induced a mitogenic effect ([3H]-thymidine incorporation and cell proliferation) on quiescent cells, without an additional growth factor being required. This effect was equipotent to that of 10% fetal calf serum, and it was accompanied by early (5 minutes) and late (1-3 hours) phosphorylation of mitogenactivated protein kinase. LPA inhibited cell migration through collagen coated membranes, with or without platelet-derived growth factor BB as chemoattractant. LPA induced a typical biphasic Ca2+ signal response made up of a rapid first phase due to Ca2+ release from intracellular stores followed by a second wave due to external Ca2+ influx. These findings support the proposal that LPA released from activated platelets is a mediator for smooth muscle cell response at the site of vessel injury in humans.

Modulation of renal tubular cell function by RGS3

The recently discovered family of regulators of G protein signaling (RGS) accelerates the intrinsic GTPase activity of certain Galpha subunits, thereby terminating G protein signaling. Particularly high mRNA levels of one family member, RGS3, are found in the adult kidney. To establish the temporal and spatial renal expression pattern of RGS3, a polyclonal antiserum was raised against the COOH terminus of RGS3. Staining of mouse renal tissue at different gestational stages revealed high levels of RGS3 within the developing and mature tubular epithelial cells. We tested whether RGS3 can modulate tubular migration, an important aspect of tubular development, in response to G protein-mediated signaling. Several mouse intermedullary collecting duct (mIMCD-3) cell lines were generated that expressed RGS3 under the control of an inducible promoter. Lysophosphatidic acid (LPA) is a potent chemoattractant that mediates its effects through heterotrimeric G proteins. We found that induction of RGS3 significantly reduced LPA-mediated cell migration in RGS3-expressing mIMCD-3 clones, whereas chemotaxis induced by hepatocyte growth factor remained unaffected by RGS3. Our findings suggest that RGS3 modulates tubular functions during renal development and in the adult kidney.

Phosphatidic acid effects on cytosolic calcium and proliferation in osteoblastic cells

Our previous studies show that epidermal growth factor (EGF) stimulates phospholipase D (PLD)-induced phosphatidic acid (PA) formation in rat calvarial osteoblastic cells. This study investigated the effects of PA on cytosolic calcium ([Ca2+]i) and proliferation, and the possible involvement of the PLD pathway in EGF effects on [Ca2+]i and proliferation in rat calvarial osteoblastic cells. PA markedly increased [Ca2+]i. This response was unaffected by thapsigargin, which depletes [Ca2+]i pools, blocked by verapamil, a calcium channel blocker, and enhanced by propanolol, an inhibitor of PA-phosphohydrolase. PA also reduced the EGF dependent-[Ca2+]i increase by 60%, while a PLD inhibitor blocked these effects. Furthermore, PA significantly increased cell proliferation (P < 0.05) which was inhibited by verapamil and enhanced by H-7 (PKC inhibitor). The PLD inhibitor significantly (P < 0.05) reduced the EGF-induced increase in proliferation. In summary, PA stimulates rat calvarial osteoblastic cell proliferation and mobilization of [Ca2+]i using extracellular pools, and EGF's mitogenic effect on these cells requires activation of PLD.

Dual effect of lysophosphatidic acid on proliferation of glomerular mesangial cells

Among the variety of factors able to contribute to mesangial hypertrophy by altering mesangial cell growth, lysophosphatidic acid (LPA) is the focus of increasing attention. It is produced in plasma following platelet activation, as well as by mesangial cells stimulated by secretory phospholipase A2. As mitogenic/antimitogenic properties of LPA are already described in a variety of cells, knowledge of its specific actions on mesangial cells is of potential interest regarding the pathophysiology of glomerulus damage in situ. We tested the effect of LPA on cultured rat mesangial cell growth. At 10 to 20 microM, LPA stimulated thymidine incorporation as well as phosphorylation of mitogen-activated protein kinases (MAP-kinases) p42-p44 in dose- and time-dependent manner, which demonstrated a positive effect on cell proliferation. However, higher concentrations of LPA (100 microM) were unable to stimulate thymidine incorporation and partly inhibited the proliferative effect as well as p42-p44 phosphorylation evoked by serum. Finally, whereas lysophosphatidylcholine (10 to 20 microM) was lytic for mesangial cells, no cell lysis could be detected at the highest concentrations of LPA. Taken together, these results suggest that LPA exerts a dual effect on mesangial cell proliferation, which could be due to activation of distinct specific signaling pathways, in dose-dependent fashion. Specific actions of LPA able to modify mesangial cell proliferation in a positive or negative manner are also likely to influence the pathophysiological processes involved in the progression of glomerulosclerosis in the kidney.

Phosphatidic acid: a lipid messenger involved in intracellular and extracellular signalling

Generated during the initial phases of cell signalling, phosphatidic acid has been implicated as a messenger involved in the activation of cellular kinases and phospholipases as well as certain proto-oncogene products and low-molecular-weight G-proteins. Although many of the reported effects of phosphatidic acid can be attributed to metabolites generated by cellular hydrolases, the parent compound clearly possesses important biological activities. However, instead of acting as a ubiquitous second messenger mediating signalling events shared by a wide variety of cells, in many systems the phospholipid seems to function specifically, regulating unique functions confined to specialized groupings of cells. One such function is neutrophil superoxide generation, which is induced when phosphatidic acid, generated by activated phospholipase D (PLD), facilitates the interaction of a cytoplasmic low-molecular-weight G-protein with dormant, membrane-bound reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Positioned on the outer surface of the plasma membrane of triggering cells, phosphatidic acid potentially mediates the "juxtacrine" stimulation of cells in direct contact. This review critically evaluates the known biological effects of phosphatidic acid as opposed to functions induced by its metabolites and addresses the mechanisms by which these effects are specifically induced by this phospholipid.

The level of pancreatic PLA2 receptor is closely associated with the proliferative state of rat uterine stromal cells

Rat uterine stromal cells (U(III)) express pancreatic type PLA2 (PLA2-I) receptor and internalize the enzyme bound to receptors. Here, we investigate the proliferating effect and alterations in binding of PLA2-I. There is a dramatic decline in PLA2-I binding in U(III) cells as they progress from a non-confluent proliferating state (40,000 sites/cell) to a confluent state (1300 sites/cell). Intracellular concentration of PLA2-I changed with the alteration in binding, suggesting that regulation in the PLA2 binding capacity may have important implications in growth control mechanisms.

Messenger functions of phosphatidic acid

Under physiological conditions, phosphatidic acid (PA) is an anionic phospholipid with moderate biological reactivity. Some of its biological effects can be attributed to lyso-PA and diacylglycerol generated by the action of cellular hydrolases. However, it is clear that the parent compound exhibits biological activities of its own. Early studies implicated PA in the transport of Ca++ across plasma membranes as well as in the mobilization of intracellular stored calcium. Both responses may be induced as a consequence of other cellular processes activated by PA, as opposed to being directly mediated by the lipid. PA may be involved in the activation of certain functions confined to specialized groupings of cells, such as the neutrophil superoxide-generating enzyme or actin polymerization. Recent studies implicate PA as an activator of intracellular protein kinases, and a PA-dependent superfamily of kinases involved in cellular signalling has been hypothesized. Deployed on the outer surface of the plasma membrane, PA potentially provides a method of communication between cells in direct contact. This review will explore the known functions of PA as an intracellular mediator and extracellular messenger of biological activities and address ways in which these functions are potentially regulated by cellular enzymes which hydrolyse the phospholipid.

Phospholipase A2 inhibitors regulate the proliferation of normal uterine cells

The effects of inhibitors of phospholipase A2, cyclooxygenase, lipoxygenase and cytochrome P450 activity on the proliferation of normal rat uterine stromal cells (UIII) were studied. At non-cytotoxic doses, inhibitors of cyclooxygenase, lipoxygenase and cytochrome P450 activity had no effect; UIII cells did not lose their ability to synthesize and secrete arachidonic acid metabolites, mainly prostaglandin I2 and prostaglandin E2, after successive passages. Inhibition of prostaglandin production did not affect their proliferation. In contrast, phospholipase A2 inhibitors significantly reduced UIII cell proliferation in a reversible and dose-dependent manner. Aristolochic acid was the most potent inhibitor with an IC50 of 0.3 mumol/l on day 7 of culture. Moreover, low doses of arachidonic acid stimulated UIII cell proliferation. Thus the proliferation of normal uterine stromal cells appears to be independent of arachidonic acid oxygenated metabolites, contrary to what is observed in tumor cells, but requires an intact phospholipase A2 pathway.