No abstract

Low density lipoprotein (LDL), at concentrations high enough for receptor binding but not high enough to saturate the receptor, induces activation of phosphatidylinositot (PtdIns) turnover in a variety of cell types with various biological functions. Using both biochemical and electron microscopic studies, we have shown that blood platelets take up and degrade LDL in a manner reminiscent of phagocytic cell types. The activation of both PtdlIns turnover and LDL metabolism is inhibited by high density lipoprotein. Thus, LDL at hormonal concentrations causes general cellular activation.Since all cell types studied responded to LDL with increased PtdIns turnover and uptake of LDL cholesterol, the PtdIns cycle may also be involved in the cellular regulation of LDL cholesterol metabolism.It has recently been demonstrated that low density lipoprotein (LDL), at concentrations in the range of its dissociation constant (Kd) for receptor binding, =1 nM, rapidly affects human platelets in several ways: (i) their shape and ultrastructural morphology are transiently altered; (ii) phosphatidylinositol (PtdIns) turnover and the molar concentration of intracellular free calcium, [Ca2+]i, are increased; and (iii) thromboxane B2 formation is enhanced (1). All of these effects are inhibited by high density lipoprotein fraction 3 (HDL3), which is known to interfere with LDL binding in platelets (2).Studies with fibroblasts have shown that Ca2+-and phospholipid-dependent protein kinase C, which is activated by stimulation ofPtdIns turnover, controls the activity ofcertain enzymes involved in cellular LDL cholesterol (LDL-Chol) metabolism (3-5). This suggests an interrelationship between the PtdIns response and LDL-Chol metabolism, but it is not known whether low concentrations of LDL stimulate the PtdIns cycle in cells other than platelets. Furthermore, it remains to be clarified whether platelets, like other cells, are capable of metabolizing LDL-Chol. Although platelets, which freely exchange cholesterol with plasma under normocholesteremic conditions, are unable to synthesize cholesterol, there is an unexplained increase in the cholesterol-phospholipid ratio in familial and experimental hypercholesterolemia (6-9). Also HDL fractions are known to be internalized and degraded by platelets (10). These findings suggested to us that there might be a catabolic pathway for LDL-Chol in the platelets.We show here that the LDL-induced activation of the PtdIns cycle occurs not only in platelets but also in various cell types that metabolize LDL-Chol, including arterial smooth muscle cells, lung fibroblasts, lymphocytes, and vascular endothelial cells. We also establish that the catabolism of lipoproteins occurs not only in these cell types but also in platelets. Thus, LDL induces general cellular activation at concentrations near the Kd for receptor binding, which appears to be comparable to hormonal effects. MATERIALS AND METHODSLipoprotein Isolation. LDL (density, 1.019-1.063 g/ml) and HDL3 (density, 1.125-1.3 g/ml) were isolate...

The interaction of low-density lipoprotein (LDL) with the human platelet was investigated with regard to saturable high-affinity binding, shape change, cytosolic free Ca2+ concentration, phosphatidylinositol (PtdIns) turnover, and thromboxane Bz biosynthesis. The experiments show that LDL, at a concentration approximately 100 times lower than in plasma, causes platelet activation concomitantly with stimulation of the PtdIns cycle and thromboxane B2 formation, similarly to other activators of platelets. The effects of LDL were inhibited by highdensity lipoprotein. The results suggest that activation of platelets by low concentrations of LDL may play a role in pathophysiological conditions and that platelet can serve as a model for studying the influence of LDL on various target cells.Low-density lipoprotein (LDL) binds with high affinity to blood platelets; the Kd of this binding is in the nanomolar range, whereas the concentrations of the lipoprotein in plasma are one or two orders of magnitude higher [l]. LDL exhibits a synergistic action on thrombin-induced platelet activation [2] and plasma levels of LDL have been correlated with enhanced sensitivity of platelets to aggregation induced by adrenaline [3]. Platelets prepared from patients with various types of hypercholesterolemia also show increased responsiveness to thrombin [4,5).A recent review indicates the importance of phosphoinositol-derived messenger molecules during platelet activation associated with the elevation of intracellular free Ca2 + concentration [Ca2+Ii [6]. Two major questions arise from these studies. a) Do LDL and other common agonists share the same activation pathways in platelets leading to elevation of [Ca2+Ii and shape change? Is it possible that there is an analogy between the mechanism of action of LDL and other platelet agonists? b) What are the concentrations of LDL required for optimal activation of platelets and how do these levels compare with physiological levels?In this context it should be considered that LDL is found not only in plasma but also in the extravascular fluids, where it is present at a much lower concentration owing to the diffusion barrier offered by the endothelium [7,8]. In addition, Therefore, it is of interest to know whether small concentrations of the lipoprotein, comparable to those occurring in the extravascular space, can cause cellular activation. Platelets were chosen since they are thought to be a valuable model for less-accessible target cells in the investigation of certain clinical disorders [l, 12-141. In this study we investigated the events associated with the activation of platelets by LDL. We show that LDL initiates the cascade of platelet activation in the same way as other platelet agonists such as thrombin; more importantly these effects were found to occur at LDL levels approximately 100-fold lower than the physiological concentrations in plasma. MATERIALS AND METHODS Materialsmy0-[2-~H] Inositol(l6.3 Ci/mmol), 32Pi (1 mCi/ml), 1251-thromboxane B2 and thromboxane B2 antiserum were pu...

The present investigation strongly supports the hypothesis that PDGF-isoforms are major stimulators of proliferation of Tenon's capsule fibroblasts after glaucoma filtering surgery while TGF-beta-isoforms are essential for the transformation of Tenon's capsule fibroblasts into myofibroblasts.