Rapid activation of human platelets by low concentrations of low‐density lipoprotein via phosphatidylinositol cycle

Abstract

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 influence of plasma on basal and ACTH-stimulated in vitro adrenocortical steroidogenesis

“…A 5-fold increase in ACTH-stimulated (50 nM) cortisol production was recorded with human LDL after prolonged stimulation but comparison with human HDL was not made (Rainey et al 1992). In addition, HDL, but not LDL, has also been reported to increase basal and AII-stimulated mineralocorticoid production by bovine zona glomerulosa cells (Knorr et al 1988). Our results from direct comparisons between LDL and HDL on basal and ACTH-stimulated steroid production are broadly in agreement with these studies (Koper et al 1985, Knorr et al 1988.…”

“…In addition, HDL, but not LDL, has also been reported to increase basal and AII-stimulated mineralocorticoid production by bovine zona glomerulosa cells (Knorr et al 1988). Our results from direct comparisons between LDL and HDL on basal and ACTH-stimulated steroid production are broadly in agreement with these studies (Koper et al 1985, Knorr et al 1988. We observed that both LDL and HDL increased basal secretion but with LDL being more potent, whilst opposite effects were observed on ACTHstimulated steroidogenesis with HDL having a greater effect.…”

Regulation of signal transduction by HDL

“…Platelet aggregation stimulated by a variety of means, including by thrombin and ADP, is reduced by HDL, and the lipoprotein also decreases thromboxane A2 and 12-hydroxy-eicosatetraenoic acid release from platelets ( 85,86 ). HDL activates p38MAPK in platelets ( 87 ), and it attenuates intracellular calcium mobilization invoked by LDL cholesterol ( 88 ). ApoA-Irich HDL may exert its actions on platelets via SR-BI, whereas responses to apoE-rich HDL are likely mediated by a splice variant of the LDL receptor family member apoER2 or Lrp8 ( 85,89 ).…”

Low Density Lipoprotein Phosphorylates the Focal Adhesion-associated Kinase p125FAK in Human Platelets Independent of Integrin αIIbβ3

“…Among the signal transducing elements that are activated by LDL are protein kinase C (PKC) (21,22), Ca 2ϩ mobi-lization (20,23,24) and phosphoinositide turnover (20,21,24,25). The faster collagen-induced secretion in LDL-treated platelets critically depends on ligand-induced outside-in signaling via integrin ␣ IIb ␤ 3 .…”