Platelet-activating factor (PAF) is not an essential component of the cascade leading to smooth muscle cell proliferation following vascular injury

Increase of phosphoinositide hydrolysis and diacylglycerol production by PAF in isolated rat liver nuclei

When isolated rat liver nuclei were treated with platelet-activating factor (PAF), a rapid increase in the mass of diacylglycerol (DAG) occurred. This effect was dose- and time-dependent. The maximum effect was observed after 1 min of 10(-7) M PAF treatment. A concomitant decrease of polyphosphoinositides and phosphatidic acid (PA) levels was observed. PAF-induced DAG accumulation was inhibited by the treatment with WEB 2086 or PCA-4248, specific PAF-receptor antagonists. This result may suggest that PAF exerts its action in the nucleus through specific nuclear PAF binding sites. The findings described herein are due to the activation of phospholipase C, as the results from experiments using U73122, a phospholipase C inhibitor, indicate. These are the first data on the action of

Mitogenic effect of lipoproteins on human vascular smooth muscle cells: the impact of hydrolysis by gr II A phospholipase A(2)

Multifactorial interaction among lipoproteins, vascular wall cells, and inflammatory mediators has been recognized as the basis of atherogenesis. In the arterial wall high-density lipoprotein (HDL) and human secretory phospholipase A(2) (sPLA(2)) colocalize with vascular smooth muscle cells and concentrate in the atherosclerotic lesions. It has been shown that gr IIA sPLA(2) hydrolyzes lipoproteins, altering their structure and releasing active agents such as lyso-phosphatidylcholine (PtdCho) and free fatty acids. We investigated the impact of normal HDL(3) (NHDL(3)), acute phase HDL(3) (APHDL(3)), and low-density lipoprotein (LDL), both unhydrolyzed and sPLA(2)-hydrolyzed, and some products of hydrolysis, such as lyso-PtdCho, oleic and linoleic acid, on [(3)H] thymidine incorporation by DNA of cultured human vascular smooth muscle cells (VSMC). NHDL(3) markedly enhanced mitogenic activity of VSMC in a dose- and time-dependent manner. Doubling of thymidine incorporation was usually achieved by 40 microg/ml of NHDL(3) after 4 hours of incubation. APHDL(3) had invariably a stronger inducing effect on the mitogenic activity than NHDL(3); 40 microg/ml more than tripled [(3)H] thymidine incorporation after 4 hours of incubation. NHDL(3) preincubated with human apo serum amyloid A apolipoprotein-induced higher mitogenic activity in VSMC than NHDL(3) alone. Hydrolysis of NHDL(3), APHDL(3), or LDL by gr IIA sPLA(2) markedly enhanced mitogenic activity of VSMC as compared with unhydrolyzed lipoproteins. sPLA(2) concentrations that can be found in atherosclerotic vascular walls markedly enhanced lipoprotein-induced mitogenic activity of VSMC. sPLA(2) per se did not affect thymidine incorporation and VSMC did not release sPLA(2) into the medium. There was no evidence for hydrolysis of the wall of VSMC by gr IIA sPLA(2). The presence of the products of hydrolysis of lipoproteins such as oleic and linoleic acids and lyso-PtdCho or their combinations with NHDL(3) explains in part markedly enhanced mitogenic activity of VSMC. It is conceivable that sPLA(2,) which is known to colocalize with lipoproteins in the vascular wall in the domain of VSMC, is capable of induction of the mitogenic activity in these cells in vivo and should be considered as a proatherogenic enzyme.

Platelets and restenosis

Restenosis is currently the major limitation of percutaneous transluminal coronary angioplasty (PTCA). Factors such as elastic recoil, migration of vascular smooth muscle cells from media to intima, neointimal proliferation and vascular remodeling underly the restenotic process. Presently there is no effective therapy available for restenosis. The role of platelets in the development of thrombosis and abrupt closure after PTCA is well recognized. However, the effects of platelets in PTCA extend well beyond the early phase. Although antiplatelet agents such as glycoprotein IIb/IIIa antagonists have been reported to reduce target vessel revascularization, major unresolved controversies still exist. This report reviews the potential role of platelets in restenosis. Various drugs, successfully tested in experimental studies and in a small number of human studies, that inhibit the effect of platelets on the restenotic process are also reviewed.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.

Platelet-activating factor and antagonists modulate DNA synthesis in human bone marrow stromal cell cultures

Platelet-activating factor (PAF) is present in the human bone marrow. We have investigated the effect of PAF and antagonists (BN 52,021 and CV 3988) on the growth of human marrow stromal cells. PAF (1 microM) stimulates and PAF antagonists (0.1-1 microM) inhibit [3H]thymidine incorporation in cells grown in 5% serum. The catabolism of PAF by stromal cells was inhibited by CV 3988 suggesting the presence of specific PAF receptor on cells. PAF and antagonists (0.1 nM-10 microM) had no effect on cells cultured in high serum concentration (20%) or in low serum concentration (1%) with 0.5 ng/ml of basic fibroblast growth factor (bFGF). This study indicates for the first time that PAF modulates the serum-induced but not the bFGF-induced growth of marrow stromal cells. The interactions between PAF and stromal cells during inflammatory marrow events such as myelofibrosis deserve to be assessed.