Low concentration of oxidized low density lipoprotein suppresses platelet reactivity in vitro: An intracellular study

Abstract

The intracellular mechanisms underlying oxidized low density lipoprotein (oxLDL)-signaling pathways in platelets remain obscure and findings have been controversial. Therefore, we examined the influence of oxLDL in washed human platelets. In this study, oxLDL concentration-dependently (20-100 microg/mL) inhibited platelet aggregation in human platelets stimulated by collagen (1 microg/mL) and arachidonic acid (60 microM), but not by thrombin (0.02 U/mL). The activity of oxLDL was greater at 24 h in inhibiting platelet aggregation than at 12 h. At 24 h, oxLDL concentration-dependently inhibited intracellular Ca2+ mobilization and thromboxane B2 formation in human platelets stimulated by collagen. In addition, at 24 h oxLDL (40 and 80 microg/mL) significantly increased the formation of cyclic AMP, but not cyclic GMP or nitrate. In an ESR study, 24 h-oxLDL (40 microg/mL) markedly reduced the ESR signal intensity of hydroxyl radicals (OH(-)) in both collagen (2 microg/mL)-activated platelets and Fenton reaction (H2O2 + Fe2+). The inhibitory effect of oxLDL may induce radical-radical termination reactions by oxLDL-derived lipid radical interactions with free radicals (such as hydroxyl radicals) released from activated platelets, with a resultant lowering of intracellular Ca2+ mobilization, followed by inhibition of thromboxane A2 formation, thereby leading to increased cyclic AMP formation and finally inhibited platelet aggregation. This study provides new insights concerning the effect of oxLDL in platelet aggregation.

Inhibitory mechanisms of low concentrations of oxidized low-density lipoprotein on platelet aggregation

“…The IC 50 value for platelet aggregation stimulated by collagen in PRP was about 83.2±9.5 lg/ml (Figure 1b), a little higher than that of the IC 50 value obtained in washed platelets [16], indicating that oxLDL seems to be bound by plasma protein, thus lowers its antiplatelet activity. In addition, the solvent control (Tyrode's solution) did not significantly affect the platelet aggregation stimulated by collagen (1 lg/ml) in PRP (data not shown).…”

“…In our previous study [16], we demonstrated that oxLDL, but not native LDL (nLDL), concentration-dependently (20-100 lg/ ml) inhibited platelet aggregation stimulated by collagen (1 lg/ml) and arachidonic acid (60 lM) in washed human platelets [16]. The IC 50 value of the oxLDL for platelet aggregation induced by collagen was estimated to be 55.2±1.0 lg/ml [16]. In this study, we further examined whether oxLDL also exhibits a similar inhibitory activity in PRP.…”

“…In that study, we found that the 12 h-and 24 h-oxLDL, but not nLDL, concentration-dependently (20-100 lg/ml) inhibited platelet aggregation stimulated by collagen (1 lg/ml) and AA (60 lM) in human platelet suspensions. The IC 50 values of the 12 h-and 24 h-oxLDL for platelet aggregation induced by collagen were estimated to be about 46.3±1.1 and 55.2±1.0 lg/ml, respectively [16]. We suggested that the inhibitory effect of oxLDL may induce radical-radical termination reactions by oxLDL-derived lipid radical interactions with free radicals (such as hydroxyl radicals) released from activated platelets, with a resultant lowering of intracellular Ca 2+ mobilization, followed by inhibition of thromboxane A 2 formation, thereby leading to increased cyclic AMP formation, and finally inhibited platelet aggregation [16].…”

“…The IC 50 values of the 12 h-and 24 h-oxLDL for platelet aggregation induced by collagen were estimated to be about 46.3±1.1 and 55.2±1.0 lg/ml, respectively [16]. We suggested that the inhibitory effect of oxLDL may induce radical-radical termination reactions by oxLDL-derived lipid radical interactions with free radicals (such as hydroxyl radicals) released from activated platelets, with a resultant lowering of intracellular Ca 2+ mobilization, followed by inhibition of thromboxane A 2 formation, thereby leading to increased cyclic AMP formation, and finally inhibited platelet aggregation [16]. In that study, we partially resolved the mechanisms of oxLDL on platelet aggregation, however detailed intracellular mechanisms underlying oxLDLplatelet interaction are still not yet completely understood.…”

“…In our previous studies, we found that oxLDL (LDL oxidized with copper), at lower concentrations (40-80 lg/ml), significantly inhibited agonist-induced platelet aggregation [16]. In that study, we found that the 12 h-and 24 h-oxLDL, but not nLDL, concentration-dependently (20-100 lg/ml) inhibited platelet aggregation stimulated by collagen (1 lg/ml) and AA (60 lM) in human platelet suspensions.…”