In contrast to native low density lipoprotein (LDL), mildly oxidized LDL (mox-LDL) induced platelet shape change and stimulated during shape change the tyrosine phosphorylation of specific proteins including Syk; the translocation of Src, Fyn, and Syk to the cytoskeleton; and the increase of cytosolic Ca 2؉ due to mainly Ca 2؉ entry. The stimulation of these early signal pathways by mox-LDL was inhibited by desensitization of the lysophosphatidic acid (LPA) receptor and specific LPA receptor antagonists, was independent of the ␣ IIb ␤ 3 -integrin, and was mimicked by LPA. Stimulation of tyrosine phosphorylation and Syk activation were independent of the increase of cytosolic Ca 2؉ and were suppressed by genistein and two specific inhibitors of the Src family tyrosine kinases, PP1 and PD173956. In contrast to PP1 and PD 173956, genistein prevented shape change by mox-LDL. The results indicate that mox-LDL, through activation of the LPA receptor, stimulates two separate early signal pathways, (a) Src family and Syk tyrosine kinases, and (b) Ca 2؉ entry. The activation of these early signaling pathways by mox-LDL probably plays a role in platelet responses subsequent to shape change. The inhibition of mox-LDL-induced platelet activation by LPA receptor antagonists or dietary isoflavonoids such as genistein could have implications in the prevention and therapy of cardiovascular diseases.Oxidative modification of LDL 1 plays an important role in the pathogenesis of atherosclerosis. Soft, lipid-rich plaques containing LDL and oxidized LDL (1, 2) are vulnerable and upon rupture may expose thrombogenic LDL particles that activate circulating platelets, causing them to aggregate and to form an intravascular plug that eventually leads to stroke and myocardial infarction (3,4). Indeed, previous studies indicate that oxidatively modified LDL stimulates platelets, mildly oxidized LDL (mox-LDL) being more active than heavily oxidized LDL (ox-LDL) (5, 6).mox-LDL or minimally modified LDL (mm-LDL) could also be present in the circulation. A subfraction of LDL that consists of electronegative, small dense, slightly oxidized LDL particles has been found in peripheral blood (7). An interaction of platelets with circulating mm-LDL may explain the enhanced platelet aggregation often observed in patients with cardiovascular disease. Indeed, a higher prevalence of small dense circulating LDL particles in coronary heart disease and in conditions commonly associated with atherogenesis has been described (8). The interaction of circulating mm-LDL with platelets may favor intravascular thrombus formation at hemodynamically critical sites of stenotic coronary and carotid arteries.Whereas ox-LDL has often toxic effects on cells (9, 10), mm-LDL and mox-LDL seem to alter the function of cells of the vessel wall and platelets through stimulation of specific signal transduction pathways (1,6,11). The elucidation of signal transduction mechanisms of mox-LDL and mm-LDL may provide a basis for new preventive and therapeutic strategies in atheroscl...